low temperature processes for the front-side metallization ... - FreiDok

freidok.uni.freiburg.de

low temperature processes for the front-side metallization ... - FreiDok

LOW TEMPERATURE PROCESSES FOR

THE FRONT-SIDE METALLIZATION OF

CRYSTALLINE SILICON SOLAR CELLS

DISSERTATION

zur Erlangung des akademischen Grades des Doktors der

Ingenieurwissenschaften (Dr.- Ing.) der Technische Fakultät der

Albert-Ludwigs-Universität Freiburg im Breisgau

Vorgelegt von:

Mónica Alemán Martínez

Fraunhofer-Institut für Solare Energie Systeme

Freiburg im Breisgau

2013


Datum der Einreichung: Juni 2013

Dekan: Prof. Dr. Yiannos Manoli

Hauptreferent: Prof. Dr. Eicke Weber

Koreferent: Prof. Dr Leonhard Reindl

Tag der mündliche Prüfung: 08 November 2013


Table of Contents

i

Table of Contents

1 Introduction ................................................................................................... 1

2 Front-side metallization of silicon solar cells ............................................... 5

2.1 Working principle of a silicon solar cell ................................................. 5

2.1.1 Limits to the solar cell conversion efficiency ..................................... 5

2.2 State-of-the-art for the front side metallization....................................... 6

2.2.1 Industrial process ................................................................................. 6

2.2.2 High-efficiency laboratory process ..................................................... 7

2.3 Theory of metal-semiconductor contacts ................................................ 8

2.3.1 Ideal contacts: theory from Schottky and Mott ................................... 8

2.3.2 Impact of the surface states: theory from Bardeen ............................ 10

2.3.3 Current flow and formation of ohmic contacts ................................. 11

2.4 Front n + emitter ..................................................................................... 13

2.4.1 Junction formation ............................................................................. 13

2.4.2 Emitter design.................................................................................... 14

2.5 Front-side dielectric layer ..................................................................... 19

2.5.1 Features ............................................................................................. 20

2.5.2 Antireflection coating ........................................................................ 20

2.5.3 Materials ............................................................................................ 21

2.6 Two layer metallization approach ......................................................... 23

2.7 Technologies for the seed layer formation ............................................ 25

2.8 Silicon-metal interactions...................................................................... 26

2.8.1 Diffusion ............................................................................................ 26

2.8.2 Metal silicide formation .................................................................... 28

2.8.3 Outlook .............................................................................................. 31

3 Laser writing for the front metallization of silicon solar cells .................... 33

3.1 Introduction ........................................................................................... 33

3.2 Laser micro-sintering (LMS) ................................................................ 34

3.2.1 LMS for the metallization of silicon solar cells ................................ 35

3.2.2 Evaluation of the process feasibility ................................................. 36

3.2.3 Sintering through the ARC ................................................................ 39

3.2.4 Alternative coating mechanisms ....................................................... 41

3.2.5 Summary ........................................................................................... 42

3.3 Laser-induced forward transfer ............................................................. 43

3.4 Laser induced metal deposition from an electrolyte ............................. 44


ii

Table of Contents

3.4.1 Introduction ....................................................................................... 44

3.4.2 Processing with a wavelength of 1064 nm ........................................ 46

3.4.3 Optical evaluation of the metal based solutions ................................ 48

3.4.4 Processing with a wavelength of 532 nm .......................................... 49

3.4.5 Processing with nickel solutions ....................................................... 49

3.4.6 Tungsten-based solutions .................................................................. 54

3.5 Summary and outlook ........................................................................... 54

4 Patterning dielectrics for the front-side metallization ................................. 56

4.1 Structuring technologies........................................................................ 57

4.2 Masked patterning ................................................................................. 57

4.2.1 Chemical etching of SiN x dielectrics ................................................ 57

4.2.2 Photolithography: reference process for the front-side metallization

of high-efficiency cells .......................................................................................... 60

4.2.3 Screen-printing a mask ...................................................................... 61

4.2.4 Inkjet printing .................................................................................... 62

4.3 Mask-free patterning techniques: Direct writing .................................. 65

4.3.1 Laser grooving ................................................................................... 65

4.3.2 Mechanical grooving/trenching ......................................................... 66

4.3.3 Laser ablation .................................................................................... 67

4.3.4 Laser-chemical Processing ................................................................ 72

4.3.5 Printing an etching paste ................................................................... 74

4.3.6 Laser doping from a liquid source ..................................................... 74

4.4 Structured ARC deposition ................................................................... 75

4.4.1 Masked PVD deposition .................................................................... 75

4.4.2 Spray coating dielectrics ................................................................... 76

4.5 Discussion ............................................................................................. 76

4.6 Conclusions ........................................................................................... 77

5 Nickel plated contacts ................................................................................. 79

5.1 History of electroless nickel on silicon solar cells ................................ 80

5.2 Fundamentals of nickel plating ............................................................. 82

5.2.1 Plating definitions.............................................................................. 82

5.2.2 Electroless nickel deposition ............................................................. 83

5.2.3 Introduction to silicon electrochemistry ............................................ 87

5.2.4 Models for electroless plating on silicon........................................... 88

5.3 Process development ............................................................................. 93

5.3.1 Plating parameters ............................................................................. 94

5.3.2 Substrate influence .......................................................................... 100


Table of Contents

iii

5.4 Coupling temperature, pH and time .................................................... 106

6 Manufacturing of solar cells with nickel-plated contacts .......................... 109

6.1 Introduction ......................................................................................... 109

6.1.1 Basics for process development ...................................................... 110

6.2 Integration to Industrial Processing .................................................... 111

6.2.1 Potential integration paths ............................................................... 113

6.2.2 ARC patterning................................................................................ 115

6.2.3 Surface preparation.......................................................................... 115

6.2.4 Plating process ................................................................................. 117

6.2.5 Post-plating: thermal treatment ....................................................... 118

6.2.6 Optimal integration sequence industrial cells ................................. 130

6.3 Integration to High-Efficiency Processing .......................................... 132

6.3.1 Rear Aluminum ............................................................................... 134

6.3.2 Laser Fired Contacts ........................................................................ 137

6.3.3 Annealing and silicide formation .................................................... 139

6.4 Photo-assisted nickel plating for silicon solar cells ............................ 143

6.4.1 Process development ....................................................................... 144

6.4.2 Working principle............................................................................ 148

6.4.3 Photo-assisted plating applied to industrial silicon solar cells ........ 151

6.4.4 Photo-assisted plating applied to high efficiency cells ................... 153

6.5 Laser-ablated and Ni-plated solar cells with optimized sequence ...... 155

6.5.1 Laser ablated industrial solar cells .................................................. 155

6.5.2 High-efficiency silicon solar cells by photo-assisted plating .......... 162

6.6 Alternative plating mechanisms .......................................................... 169

6.6.1 Electroless plating with Ni alloys .................................................... 169

6.6.2 Light-Induced Nickel Electro-Plating ............................................. 171

7 Summary .................................................................................................... 175

Deutsche Zusammenfassung .............................................................................. 178

8 References ................................................................................................. 182

9 List of publications .................................................................................... 196

Acknowledgments .............................................................................................. 202


From everyone who has been given much,

much will be demanded; and from the one

who has been entrusted with much, much

more will be asked.

Luke 12:48


Personal motivation

There are many different reasons one could use to justify the meaning and

importance of the contribution to the development of renewable energies. For me, it’s

all about the responsibility we take for the choices we make.

There are over 7 billion people in this world. Only some of us have been given

the opportunity to shape the life we want to live in. So, how long will it take us to

profoundly change towards a sustainable way of life? Will it be soon enough?

I have been lucky to be granted lots of choices; therefore, I feel a great

responsibility towards my environment. It drives my dedication to this field of

research.


1 Introduction

The energy demand in our world is rising [BP12]. This is due to the increase of

the world’s population combined with a steady rise of the energy consumption per

capita. Currently, we cover most of our energetic needs by burning different fossil

sources (see figure 1-1). This creates two major threats. The first is related to global

warming, which is created by the release of greenhouse gases. The second and most

important is that these energy sources are limited. So, what will happen when we burn

everything?

Figure 1-1 World energy consumption [BP12]

Photovoltaic cells enable the direct conversion from the sunlight to electricity.

Together with the other renewable energy technologies they represent an alternative

for a sustainable future.

Nowadays, there are many different concepts and processes under development,

to improve the quality of solar cells. Presently, the most important material used for

photovoltaic production is silicon. Some of the research fields within silicon PV at this

moment are focusing on:

How to obtain cheaper silicon material and wafers while keeping a reasonable

quality?

Which cell structure delivers the optimal power/cost ratio?

How can we improve or change single step processes used during the device

fabrication to reduce costs or improve efficiencies?


2 Introduction

How can we improve the interconnections for modules, while keeping the

longest reliability?

How do we determine/characterize which factors affect the cell efficiency in the

most effective way?

Thesis Outline

The focus of this work is the development of alternative front-side metallization

techniques for double-side contacted silicon solar cells. This study has been performed

on cells formed on a p-type silicon substrate featuring a front junction obtained by

phosphorous diffusion. Several paths to reduce the gap between the high-quality

laboratory-based and industrial fabrication processes have been evaluated by

implementing techniques for the front-metallization which require either temperatures

below 400 o C, or local heating (applied by laser irradiation).

The basic concepts relevant for the development of alternative front-side

metallization techniques are presented in chapter 2: the functioning of a solar cell, the

metal-semiconductor theory, state-of-the-art front-side metallization technologies and

the cell manufacturing steps which are relevant for the front-side definition. These

include the emitter formation and the deposition of the anti-reflective coating (ARC).

Finally, the approach and the challenges evaluated during this work are introduced.

Innovative processes based on the direct laser writing of metal contacts onto the

solar cells are introduced in chapter 3. These include laser micro-sintering, in which a

fine metal powder is melted to form fine seed contacts, and laser induced metallization

from an electrolyte, where a chemical solution is used as a source for metal deposition

on a silicon wafer.

In chapter 4 we present different techniques for the patterning of the

antireflection layer. The focus in this work is to use these technologies as a

preparation step before nickel plating based metallization. Among these technologies

we find: masking and etching-based processes (e.g. inkjet printing a resist and etching

of the nitride), direct ablation (e.g. by laser), or even the patterned deposition (e.g. by

sputtering a SiN x layer). The advantages of each technology as well as the

experimental achievements realized during this work are summarized.

A brief history of the application of nickel plating to silicon solar cells is

introduced in Chapter 5. Then the theory of electroless nickel plating as well as its

application to silicon wafers is presented. We study the influence of the substrate and


Introduction 3

the impact of various plating parameters on the formation of an electroless plated

nickel coating on a silicon wafer.

Chapter 6 deals with the integration of the electroless plating process to silicon

solar cell fabrication. First, we evaluate the different alternatives for the integration of

this step into an industrial manufacturing line. Inkjet printing is used for the ARC

patterning, electroless nickel for the seed layer formation and silver light induced

plating for the contact thickening. Then we study and optimize the path to combine the

nickel electroless plating step to a high efficiency fabrication line. The light-assisted

electroless Ni plating is developed, taking advantage of the photo-electrical nature of

the silicon solar cell to form nickel seed contacts. This is combined with laser ablation

for the patterning of the ARC. Finally, alternative plating options with a potential for

further improvements of the plating-based front-side metallization are introduced.

Chapter 7 summarizes the work presented on this thesis, with a brief outlook

about the most interesting techniques considered by the author for the front

metallization of industrial p-type crystalline Si solar cells in the short and in the long

term.


2 Front-side metallization of silicon solar cells

In this chapter we present the basic concepts of silicon solar cells: with a

brief introduction of their functioning mechanism and their limitations. We discuss

the front side metallization theory and praxis. This information is used to define

the theoretical needs for alternative metallization techniques, mainly for the seed

layer formation and particularly for nickel- based contacts.

2.1 Working principle of a silicon solar cell

A solar cell is a device capable of converting sunlight into electricity by the

photovoltaic effect. When a photon with enough energy strikes a semiconductor, an

electron-hole pair is generated. Both (electrons and holes) are electrical carriers. A p-n

junction enables the separation of these carriers into areas with different polarities,

where their collection is performed in different terminals. On most commercially

available p-Si base cells with a front n + -p junction, a full metal layer acts as the rear

contact and a grid of metal fingers distributed over the full surface form the front

contacts (see figure 2-1). For this type of cells, front contacts carry the electrons while

the rear contacts conduct the holes.

Front-Contacts

Antireflective Coating

Emitter (n + -Si)

Base (p-Si)

Al-BSF

Rear-contacts (Full screen printed Al)

Figure 2-1 Schema of an industrial silicon solar cell

2.1.1 Limits to the solar cell conversion efficiency

The general aspects defining the main power losses on solar cells are presented

next:

• Optical losses

Non-absorption

Primary reflection at the front side

Parasitic absorption

• Recombination losses

Bulk (Auger, Radiative, SRH)

Surface (Semiconductor-Metal and Semiconductor-Insulator

interfaces)

Highly doped emitter and back surface field (BSF) regions

Edge

• Resistive losses


6 Front-side metallization of silicon solar cells

Emitter

Bulk

Contacts

Extensive literature explains in depth the different power loss mechanisms. An

overview of the theory for solar cells is provided in the books by Green or Götzberger

[Gre95, Goe97].

Researchers and industries are working hard to find solutions to reduce each of

these losses while keeping manufacturing costs as low as possible. This work focuses

on the front side metallization.

2.2 State-of-the-art for the front side metallization

There is a wide gap between the metallization technologies in laboratory

environments manufacturing high-efficiency silicon solar cells and most commercial

industrial cells. Throughout the investigations carried out during this thesis we

evaluated potential paths to reduce this gap.

2.2.1 Industrial process

Industrial solar cells are typically made by screen printing. This is a very robust

process, with a high throughput (with equipment available for 1000-2000 wafers/hour

for single or double lines respectively [Neu07]). The silver paste used for front

contacting is not as conductive as pure silver (see figure 2-2). The line conductivity is

~3x10 -6 cm, instead of 1.6 x10 -6 cm for pure Ag. The paste is formed by 70-80%

silver, 1-10% glass frit and 15-30% of organic components [Neu07]. The process is as

follows: a squeegee pushes the paste through the openings of a screen. The screen is

separated from the substrate by a small distance. Due to the pressure applied with the

squeegee, the screen lowers to the wafer, bringing the paste in contact with the

substrate. Once there is no more pressure applied on the screen, it lifts up again,

leaving behind the paste (see figure 2-3) [Hul98]. Then, the paste is dried in a furnace.

After this step an Al paste is printed over the rear side and dried, as well as the Ag

busbars which are also printed on top of the rear Al coating. Finally the cells are fired

in an inline IR furnace with temperatures typically ranging between 800-950 o C.

During the firing step the ARC is locally etched thanks to the glass frit, enabling the

formation of ohmic contacts on the front-side. Neuhaus and Münzer have presented a

detailed overview of the full industrial technology sequence applied nowadays on most

commercial solar cells [Neu07].


State-of-the-art for the front side metallization 7

Squeegee

Ag-paste

Screen

Substrate

Figure 2-2 SEM view of a screen-printed

silver finger on top of a silicon solar cell

Figure 2-3 schema of a part of the screenprinting

process: squeegee printing the paste on

the wafer [Hul98]

The specific contact resistance, or contact resistivity between the Ag paste and

the silicon lies between 1-3 m.cm 2 . The standard finger width is ~100 m, while the

finger height is typically between 10-15 m.

Different researchers have been dealing with understanding the contact formation

by screen printing [Bal03, Schu06, Hoe09]. Currently, one of the main limitations of

this technology is the need for highly doped surfaces, with a surface doping over

1x10 20 at/cm 3 , in order to form a good ohmic contact. Investigations have been

performed towards the contacting of highly resistive emitters, though these still have a

high surface doping concentration [Hil04, Ebo05].

2.2.2 High-efficiency laboratory process

The metallization of standard high-efficiency cells is performed using

photolithography for the patterning of the ARC. The resist is used as a lift-off mask

after an evaporation step. During the evaporation, a stack of metals is formed,

containing Ti/Pd/Ag with ~50/50/100 nm thickness corresponding to each layer. After

stripping the resist off from the front-side, the seed layer is thickened using a silver

light-induced electroplating step, up to a thickness ~10-15 m. The line conductivity

of the plated silver is as low as the pure silver. The contact resistance between the

metal and the silicon is typically around 0.3 m.cm 2 . The standard finger width is

between 15 to 30 m, while the finger height is typically between 10-15 m. So the

aspect ratio of this type of metal fingers is much better than for industrial screenprinted

fingers. The best silicon solar cells made so far have been manufactured using

this type of technology for the front side metallization [Zha99]. An interesting

evaluation of the contacting possibilities with different evaporated materials (including

Ti, Ni, Al, Cr, Pd, Ag) was presented by Mette [Met07].


8 Front-side metallization of silicon solar cells

Industrial: screen-printing

10 -15 µm

~100-120 µm

r c > 1-3 . m cm 2

r Finger ~ 3 . 10 -8 m

Anti-reflection layer

Emitter

n- Si

Base

p-Si

Laboratory: high-efficiency

~15-30 µm

r c < 0.3 . m cm 2

r Finger ~ 1.6 . 10 -8 m

Rear

contacts

Figure 2-4 Schema showing the relevant differences for the conversion efficiency resulting

from the use of the standard industrial metallization technology versus the high-efficiency

approach.

2.3 Theory of metal-semiconductor contacts

The main questions to answer in this section are: How does the current flow from

a semiconductor to a metal and how do we define which metal is more interesting for

contact formation?

Before discussing the practical issues related to the fabrication of alternative

front-side metallization technologies, an introduction into the physical models

describing the contact formation on silicon surfaces is provided. This will lead to a

theoretical guideline for materials that could be interesting for the seed layer formation

from an electrical point of view. An excellent review on contact formation for silicon

solar cells has been published by Schroeder and Meier in 1984 [Sch84]. It presents

answer to the questions “What is necessary to make a good ohmic contact, what does

contact resistivity mean, what contact resistivity values are required for solar cells,

and what is achievable today?” [Sch84]. An overview on the theory for the metalsemiconductor

contact formation can be found in the book by Rhoderick and Williams

[Rho88]. More information on this topic can be found in several books dealing with

semiconductors physics and device fabrication [Sze07, Sch98, Str00].

2.3.1 Ideal contacts: theory from Schottky and Mott

The work function is defined as the energy difference between the vacuum level

and the Fermi level of a material. For metals it can also be described as the energy

required to remove an electron from the Fermi level into the vacuum. It is represented

as m and s for metals and semiconductors respectively. The electron affinity () is

the energy required to extract an electron from the bottom of the conduction band of a

semiconductor into the vacuum level [Sze07]. When a metal and a semiconductor

come in contact at thermal equilibrium, charge transfer occurs between the two


Theory of metal-semiconductor contacts 9

materials until the Fermi levels of both are equal. For an n-type semiconductor, if

m > s , electrons travel from the semiconductor to the metal. The negative charge

transferred to the metal is then compensated by a positive charge in the semiconductor.

Due to the relatively low concentration of carriers in the semiconductors, this charge is

distributed over a region with a thickness W, which is comparable to a depletion

region in a p-n junction [Rho88].

The Schottky barrier ( Bn ) is the potential barrier formed by the establishment

of a close contact between a metal and a semiconductor. In the ideal case, when the

Schottky barrier is measured relative to the Fermi level, it is equal to Bn = m – χ and

the current flow occurs due to the thermal emission of carriers over the barrier, either

from the semiconductor to the metal (in forward bias) or from the metal to the

semiconductor (in reverse bias) [Sch84]. According to this theory, contacts to n-Si

(a)

(b)

(c)

(d)

Figure 2-5 Diagram of energy bands of a metal-semiconductor before (a,c) and after the

formation of an intimate contact (b,d). The formation of a Schottky contact is presented at the

top (a,b), while the formation of ohmic contacts is presented at the bottom (c,d). The Schottky

barrier is illustrated after the contact formation [after Str00]


10 Front-side metallization of silicon solar cells

semiconductors with m > s have a rectifying behavior, while if m < s , the

contacts have an ohmic behavior (the opposite is valid for p-Si semiconductors).

Figure 2-5 illustrates both cases. V bi is the potential difference between the bottom of

the conductive energy in the semiconductor and the top of the barrier height.

As an electron approaches the surface at a distance x from the interface, a

positive charge is induced in the surface of the metal with a force of attraction which

corresponds to the attraction between the electron and an equal positive “image”

charge located at a distance –x [Sze07]. So, the approach of an electron to the surface

induces an electrical field, which effectively lowers the barrier height, making it easier

for the electron to pass the barrier. This effect is called image-force-induced lowering

of the potential energy. It also implies that, when a bias voltage is applied to a

semiconductor/metal contact, the barrier height can be increased or reduced [Sze07].

2.3.2 Impact of the surface states: theory from Bardeen

For most real contacts the barrier height is relatively independent of the metal

work function (see figure 2-6) [Schro84]. This is due to various reasons including the

presence of an interface layer (~10 -20Å thick [Rho88]) between the metal and the

semiconductor and the existence of interface states at the surface of the semiconductor

[Sze07].

Figure 2-6 Semiconductor-metal interface according to the Bardeen model. Surface states are

distributed over the complete range of energies at the interface between the semiconductor

and the thin oxide ( ox ), but only the filled states are illustrated [taken from Schr84].

In 1947, Bardeen explained the influence of interface states in the contact

formation. He described the contacts as a function of the interface states between the

semiconductor and a thin insulating layer through which electrons could flow freely

[Schr84]. The distribution of surface states between the insulating layer and the silicon

is characterized by a neutral level 0 /E 0 [Rho88]. At zero Kelvin the interface states

are filled up to the corresponding level E 0 and empty above this level [Schr84]. “The

surface Fermi level must lie above E 0 [Schr84]” so that the charge neutrality remains.

The difference between the neutral level 0 and the Fermi level depends on the

concentration of defects on the surface D it . For surfaces with very high D it values,


Theory of metal-semiconductor contacts 11

there is a surface Fermi level pinning at a value q 0 /E 0 above the valence band “and

the barrier height is independent from the work function” [Sze07] (see figure 2-6). For

very low D it values the influence of surface states becomes negligible and the barrier

height follows the ideal Schottky behavior.

Both examples are illustrated in figure 2-7. The barrier heights are plotted against

the work function of both: metals and silicides. The dependence of the barrier height

on the work function is weak for pure metals (slope ~0.3), while it is strong for the

silicides (slope~1). A lower D it at the semiconductor silicide interface, due to the

chemical nature of the bond, is probably the cause for the silicide behavior closer to

ideality than for metals which are just deposited on top of the silicon [Schr84].

Figure 2-7 Barrier height for contacts formed with (1eft) different metals and n-Si, (right)

metal silicides and Si with both polarities [Schr84]

2.3.3 Current flow and formation of ohmic contacts

Ideally, ohmic contacts can be formed on n-Si by using a metal with a work

function smaller than the semiconductors’ work function (see figure 2-5.d). In reality,

most contacts show a rectifying behavior (see figure 2-5.b) [Schr84]. The definition of

ohmic contacts has been revised stating that ohmic contacts are formed when the

influence from the resistance of the contact is negligible in comparison with the

resistance of the semiconductor [Sze07]. “They should have a linear or quasi-linear

behavior” [Sch98].

There are different conduction paths for carriers traversing metal/semiconductor

interfaces. The diffusion theory from Schottky assumes that the width of the depletion

region is longer than the mean free path of the electrons (a low doping) so that the

current transport is the result from the diffusion of the carriers over the barrier height

and against the electrical field in the depletion region thanks to their thermal energy.


12 Front-side metallization of silicon solar cells

Figure 2-8 Current flow mechanisms between metal and semiconductors. From left to right

TE: thermionic emission, TFE Thermionic/Field Emission, FE: Field Emission [from Met07]

The thermionic emission theory by Bethe assumes that the width of the depletion

region is small compared to the mean free path of electrons (high doping), so that the

main barrier for the electrons is the barrier height. An electrical flow can be obtained

either through the overcoming of the barrier height by thermionic emission (for surface

doping concentrations N s < 10 17 cm -3 ) or by tunneling effects (for N s > 10 17 cm -3 ). The

tunneling effects can play a role in two different ways. The first is by field emission,

where the carriers tunnel through the complete barrier width at the conduction band

level (or very close), because the barrier is very thin (for N s > 10 19 cm -3 ). Another

possibility, for intermediate doping levels (10 17 cm -3 < N s < 10 19 cm -3 ) is that the

carriers get across the barrier after being thermally excited to a level where they can

tunnel through. This process is called thermionic/field emission [Schr84]. Figure 2-8

presents a schema showing each current flow mechanism.

An overview of the formulas corresponding to the current flow for each

mechanism has been summarized by Schroeder [Schr84]; a throughout description has

been published in the dissertation from Mette [Met07]. The most interesting aspect to

learn from these equations is that there is a strong dependence of the contact resistance

on the surface doping concentration (see figure 2-9).


Front n+ emitter 13

Figure 2-9 Specific contact resistance as a function of the doping level concentration in the

semiconductor and the barrier height [Schr84]

According to the theory, there are two ways to reduce the metal/semiconductor

contact resistance to make ohmic contacts: to implement high surface doping levels on

the semiconductor and/or to work with materials that provide a low barrier height (see

figure 2-9). Looking at the barrier height of the different metals to silicon (figure 2-7)

we can state that, if we leave aside rare earths and polluting materials, Mg, Ti, Mo, Ni,

Cr, Co, W, Al and some of their silicides would be interesting candidates for

contacting n-Si.

2.4 Front n + emitter

2.4.1 Junction formation

The front-side emitter of commercial and high efficiency silicon solar cells is

typically formed by the diffusion of phosphorus from a POCl 3 gas source [Gre98].

The conditions applied for the junction formation of industrial emitters uses

temperatures between 800-900 o C with processing times between 20 min to 1 h

[Gre98]. Alternative junction formation techniques applied in the industry are based on

the in-line diffusion of a doping source through a conveyor belt. The dopant can be

deposited by: spray-on [Ben06], screen-printing, or spin-off [Szlu06]. The result is

typically a shallow and homogeneous phosphorus emitter, with a strong field effect

passivation and sheet resistances ranging from 40-80 /sq (see figure 2-12).

The junction formation of high-efficiency cells features an additional step after

the thermal diffusion for the drive-in of the dopants. It typically combines the long

drive-in steps with a thermal oxidation at temperatures over 1000 o C. Such processes


14 Front-side metallization of silicon solar cells

last between 3 to 8 h. Figure 2-12 shows some narrow highly doped emitters, versus a

high-efficiency emitter, with a lower N s and a junction depth over 1 m.

Other techniques for the formation of n + junctions on silicon solar cells include

thermal diffusion from a doped source [Kin90], epitaxy, implantation or laser doping.

Even though epitaxy is interesting as a doping technique, one of its main drawbacks in

the application for the junction formation at the front-side is related to the difficulties

to form a homogeneous doping profile on textured surfaces and the lack of highthroughput

machines. In case of laser doping, some experiments have been performed

to form the full emitter by this technology [Est03]. Still, to this date laser doping is

mostly applied for the selective junction formation [Tja07]. Thanks to the

improvement of implanter tools, recently, this technology has demonstrated the

capability to form junctions and get an excellent surface passivation [Ben12].

Therefore, it has been gaining interest as a competitive alternative for tube diffusion,

capable of creating emitters with a low surface doping concentration in a

homogeneous way.

2.4.2 Emitter design

Observing figure 2-9, it is possible to conclude that front-side emitters with a

high surface doping concentration are ideal for obtaining low specific contact

resistance and relaxing the requirements in terms of barrier height for the

metallization. Nevertheless, just manufacturing contacts with the lowest contact

resistance, does not make excellent solar cells. In this section we present important

aspects affecting the definition of the optimal front-side n + emitter depending on the

surface recombination velocity.

As mentioned in section 2.1.1, the reduction of the total recombination current

losses is an important part in the achievement of high conversion efficiencies. The

front surface recombination has a strong impact on the device characteristics since

most of the carriers are generated very close to the front surface. Therefore,

understanding the sources for the recombination generated by the emitter located at the

front-side is a key aspect to reduce the total recombination current. One of the paths to

reduce the front-recombination is to reduce the number of dangling bonds by using

passivation layers on the top surface [Hon12]. Thanks to the work of scientists like

Richard King [Kin90], Andreas Cuevas [Cue96], Stefan Glunz [Glu95] and Mark Kerr

[Ker02], a better understanding of the effects of the passivation of highly-doped

phosphorus surfaces on solar cells has been achieved.

A graph from Kerr shows the influence of the surface doping concentration (N s )

on the surface recombination velocity (SRV) corresponding to different passivation

layers (see figure 2-10). An increase in the surface doping concentration results in an

increase in the surface recombination velocity. Additionally the recombination in the


Front n+ emitter 15

bulk of the emitter is strongly affected by Auger recombination which is increasing

with increasing doping level. On the presented phosphorus-doped surfaces, the thermal

SiO 2 coating delivers a lower SRV than the PECVD silicon nitride coating. Moreover,

the quality of thermal oxides can be further improved with the post-treatment. The best

passivation quality for laboratory cells with SiO 2 coatings is obtained by applying an

Alneal treatment after the oxidation. This consists in the evaporation of an aluminum

layer (~200 nm) on top of the oxide, sintering the samples at 400°C for 30 min and

etching the metal after annealing [Zha96]. After this step, there is an increase in the H 2

passivation at the Si-SiO 2 interface [Cue96].

The impact of the thermal post-treatment on the oxide passivation at the front

side can be viewed on figure 2-11 as published by Hoffman [Hof09]. He performed

different thermal treatments on silicon solar cells featuring a lowly doped POCl 3

diffused front side emitter passivated by thermal silicon oxide and a rear side

passivated by an amorphous Si/silicon oxide stack. The internal quantum efficiency of

the cells shows an improvement in the low-wavelength region corresponding to the

application of increasing sintering temperatures during an FGA step [Hof09].

Assuming that this increase is due to an improvement in the passivation quality of the

devices, this means that for thermal silicon oxide passivated samples, a temperature of

at least 400 o C is required during the FGA in order to obtain the best passivation

quality.

Figure 2-10 Extracted SRV values for the

different passivation schemes as a function of

the phosphorus surface doping concentration

(N s ) for oriented, mirror polished, FZ

silicon wafers [Ker02]

Figure 2-11 IQEs showing the influence of

the annealing temperature on the passivation

quality of the front and rear-side of high

efficiency wafers (120 /sq emitter on

0.5 cm FZ material with thermal oxide on

the front-side evaporated contacts Ti/Pd/Ag

with LIP on the front and evaporated Al with

LFC on the rear) [Hof09]


16 Front-side metallization of silicon solar cells

We have carried out a PC1D simulation to evaluate the trends showing the

influence of the SRV on the efficiency of solar cells featuring a dielectrically

passivated rear-structure similar to the one applied on high-efficiency cells

manufactured during this work. Four front-side emitter profiles have been used for the

simulation. These correspond to the standard diffused emitter used for high-efficiency

cells at the Fraunhofer ISE (R sheet =120 /sq) and 3 industrial-like profiles (R sheet =

110, 90 and 60 /sq). The SIMS measurements of these profiles are presented on

figure 2-12.

The properties which have been fixed for the simulation are presented in table

2-1. The optical and rear-recombination values correspond to the recommended data

provided by Hermle for LFC cells with a 105 nm thick thermal SiO 2 rear side

passivation [Her08]. Even though emitters with low sheet resistance would require a

fewer number of fingers on the front, which would translate into different optical

properties, a fixed value has been used for the reflection of all the cells (see table 2-1).

In this way, we can separate optical and recombination effects. The influence of the

surface doping on the contact resistance (thus on the series resistance) has also been

neglected. The results of these simulations are presented on figure 2-13.

Table 2-1Table 2-2 Parameters implemented for the PC1D simulation

Area 1 cm 2

Substrate

1 cm p-Si

Base doping (N a ) 1.5 . 10 16

Thickness 230 µm

Front surface texture depth 7 µm

R s emitter

0.5

Bulk recombination

1000 µs (FZ)

Rear-surface recombination velocity

(LFC)

140 cm/s

Front first bounce

Front subsequent

Internal reflectance

bounce

Rear first bounce

Rear subsequent

bounce

75% (Diffuse)

95.8%

94% (Specular)

95%

In a first approach, the simulation only shows the efficiency that could be

achieved with each emitter, assuming that any surface recombination velocity value

could be reached on any emitter (figure 2-13). The actual engineering SRV values

depend on the passivation technology and the surface doping concentration. Using the

combined model from Kerr for POCl-diffused surfaces presented on figure 2-10,

actual SRV values for 3 different passivation concepts are estimated: an ARC formed


Front n+ emitter 17

Table 2-3 Surface doping concentration estimated from the SIMS measurements for the 4

emitters used in the PC1D simulation and the corresponding SRV values from Kerr’s combined

parameterization model for different front-side passivation concepts (the SRV values have been

multiplied by a factor of 3 to account for texturing)

r sh

Chemical N s

from SIMS

Depth

/sq] [cm -3 ] [µm]

N s active

(used in Kerr’s

model)

SRV from Kerr’s parameterization

PECVD

SiN x

FGA SiO2

al- SiO2

[cm -3 ] [cm/s] [cm/s] [cm/s]

120 ~ 7 . 19 x 10 18 1.2 7 . 19 x 10 18 2.87 x 10 3 1.62 x 10 3 7.91 x 10 2

110 ~ 2 . 00 x 10 20 0.4 2 . 00 x 10 20 1.04 x 10 5 8.43 x 10 4 6.98 x 10 4

90 ~ 3 . 50 x 10 20 0.4 2 . 00 x 10 20 1.04 x 10 5 8.43 x 10 4 6.98 x 10 4

60 ~ 4 . 50 x 10 20 0.5 2 . 00 x 10 20 1.04 x 10 5 8.43 x 10 4 6.98 x 10 4

by a PECVD SiN x (n=1.95), and a thin SiO 2 coating (13 nm) annealed either by FGA

or by Alneal. The model takes into account the data from King and Cuevas [Ker02].

The parameterization is valid for doping concentrations between 10 13 – 2 x 10 20 at/cm 3 ;

in other words, only for the electrically active dopant atoms.

The SIMS profiles shown on figure 2-12 deliver a measurement of the chemical

content of phosphorus in the wafer. For a 60 /sq emitter N s reaches 4.5 x 10 20 at/cm 3 .

The solubility of phosphorous in Si reported in the literature, for the processing

10 21 120 /sq deep

Phosphorus concentration [cm -3 ]

10 20

10 19

10 18

10 17

110 /sq

90 /sq

60 /sq

10 16

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Depth [µm]

Figure 2-12 SIMS profiles for the emitters

used for the PC1D simulations

Figure 2-13 cell efficiency versus front

surface recombination velocity as calculated

with PC1D for the emitters profiles in figure

2-12. Simulation parameters in table 2-1.


18 Front-side metallization of silicon solar cells

temperatures applied during the cell manufacturing process, is around 2 x 10 20 at/cm 3

[Ben05]. Assuming that all soluble atoms are active, phosphorus atoms beyond the

max solubility level form an inactive layer on the front side. This is called the “dead

layer”. This layer is visible by SIMS, but not by electrical measurements such as

electrochemical capacitance-voltage (ECV) or spreading sheet resistance (SRP).

Because the model of Kerr only takes into account active dopants, the SRV estimation

for emitters with a dead layer (r sh = 60 and 90 /sq) has been done using the maximal

active doping concentration value N s ~2 x 10 20 cm -3 . It should be noted that in the case

of a dead layer strong recombination in the emitter bulk is the dominating effect. Table

2-2 presents the corresponding SRV values calculated with Kerr’s model for each

passivation concept.

The data points corresponding to the SRV for each passivation scheme on each

emitter and the corresponding efficiency are also reported on figure 2-13. The

maximal efficiency that could be achieved with lowly doped emitters

(N s ~ 1x10 19 at/cm 3 ) is much higher than the maximal efficiency that could be achieved

with the industrial emitters (N s > 2x10 20 at/cm 3 ) due to the improved surface

passivation on lowly doped surfaces. Highly-doped emitters are limited by the strong

Auger recombination.

The impact of the texture in the emitter saturation current, has been included

multiplying the SRV data by a factor of 3, taken from the literature as a reasonable

approximation to account for texturing [Dic92]. Still, the influence of inhomogeneous

doping concentrations over the pyramids surface, which could affect the surface

recombination velocity, as explained by Glunz, is not taken into account [Glu95].

Therefore, the engineering data plotted in figure 2-13 corresponding to real passivation

coatings should only be considered as a first estimation of the actual values.

Thanks to PC1D simulations and previous evaluations found in the literature for

emitter passivation concepts, the importance of the combination between emitter

definition and front-side passivation layers was revealed. Concentrating our attention

only on the effects of SRV, we could state that a lower surface doping enables a higher

current collection. However, there is a stronger dependence of the efficiency on the

SRV for emitters with a low surface doping concentration (N s ) than for emitters with a

high N s . The high surface doping concentration (N s ) results in a stronger electrical

field effect, which reduces the diffusion of minority carriers to the surface. For this

reason, emitters with a high surface doping concentration are less sensitive to the

surface passivation quality, thus more robust for industrial applications [Ker02].

Another aspect, which affects the total surface recombination, is related to the

SRV under the metalized areas (SRV met ). The SRV met is considered to be close to the

thermal limit of carrier recombination (v electrons ~ 10 7 cm/s). It can be reduced by

creating a local electrical field (called selective emitter), which should be strong


Front-side dielectric layer 19

enough to push minority carriers away from the surface underneath the contacts. Given

that there is no generation in those areas, the increase in Auger recombination due to

the local higher doping is not so critical. The use of selective emitters also contributes

to the reduction of the total contact resistance as shown on figure 2-9.

The final decision on the optimal process to manufacture the n + emitter should

include economic factors which are not considered in this work. From a physical point

of view the ideal emitter should feature a junction with the lowest surface

recombination velocity, while still being able to deliver the carriers at the terminals

with the lowest possible contact resistance. This means that for a front-side contacted

cell, the ideal front emitter features a low surface doping concentration over most of

the surface with a local high doping underneath the contacts.

A list with potential strategies to form a lowly doped emitter is provided next:

• High-temperature diffusion, followed by a long drive-in: This delivers the most

stable surface doping conditions featuring a deep emitter. As mentioned before, it

is used by high-efficiency cells, including drive-in steps, until the desired surface

doping concentration is achieved. A high thermal budget is applied.

• Short and low-temperature diffusion: in principle, through the optimal combination

of temperature and time: a shallow and lowly doped emitter could be formed.

However, this is not a trivial optimization. There is a high risk of homogeneity loss

over the distribution of the dopants.

• Emitter etch-back: the highly doped surfaces can be removed after diffusion, as for

example following the process developed by the University of Konstanz with the

porous silicon formation [Hav08].

• Implantation: Nowadays, this technique is been considered as one interesting

alternative for junction formation of solar cells [Roh12, Ben12]. This opens up new

processing windows which might enable the formation of homogeneous “lowly”

doped emitters applying processes with lower thermal budgets.

Depending on the technology chosen for the formation of the lowly doped

emitter, there are alternatives for the selective doping around the metalized areas.

Some examples include laser doping [Tja07, Wan12] or selective doping with inks

[Ant10]. An interesting overview on this topic was published by Hahn in 2010

[Hah10]. More information about these processes is presented in section 4.3.

2.5 Front-side dielectric layer

The front-side dielectric layer is important for the definition of the front-side

metallization since we need to be able to contact the silicon through this layer. The

screen-printing technology uses glass frits to locally etch the dielectric during the

firing step. The glass frits reduce the conductivity of the metal fingers. To avoid the

use of such components, the dielectric could be deposited after the front-side


20 Front-side metallization of silicon solar cells

metallization as typically done in the past [Gre98]; or it could be patterned before the

metallization as it is currently done for high-efficiency wafers. The lack of the

dielectric during the metallization can result in the potential contamination of the

surface during this step. Another advantage of depositing the dielectric prior to

metallizing with the current industrial technologies, includes the passivation of the

crystalline defects present in the material through the release of hydrogen from the

SiN x :H coating during firing. This is especially relevant for multicrystalline silicon.

For these reasons, the ARC is nowadays deposited prior to the metallization.

Chapter 4 deals with various techniques for the pattern formation on these

coatings. In this section, we introduce the basic requirements of the silicon solar cells

which define the characteristics of the front-side antireflective coatings.

2.5.1 Features

The main features for the dielectric layers used at the front side of a silicon solar

cell are:

• Reduce the total front reflection losses in order to increase the photon

absorption. A higher photon density reaching the silicon translates into higher

potential for current density. Typically 10% of the reflection losses can be

reduced by applying a standard ARC on commercial wafers [Gre98].

• Provide the front side passivation or at least be compatible with the coatings

that deliver surface passivation. Up to now, an alnealed thermal silicon oxide

layer delivers the best passivation quality on highly n-doped silicon surfaces

(see figure 2-10) [Cue96].

Other features of interest for the application of these coatings depend on the

structure and the process applied during manufacturing. One feature which has been

relevant to this thesis is their capability to act as plating masks and to provide chemical

resistance to certain treatments (like HF or KOH). In other cases, they are required to

act as diffusion masks (for selective doping steps) [Bru03] and/or provide stability

during high-temperature processes (like the firing step during screen printing) [Ric04].

For PV applications, they would also ideally be deposited with simple, cost-effective,

and high throughput machines.

2.5.2 Antireflection coating

The optimal design of the ARC is based on the search for the minimal reflection.

Since the refractive index is wavelength-dependent, it is common practice to optimize

the layers for a wavelength ~ 633 nm. This wavelength corresponds to the maximal

ratio of photon absorption by total photon flux for silicon solar cells under the


Front-side dielectric layer 21

AM 1.5G Spectrum 1 . It is important to mention that this is just a first approximation.

Actual silicon solar cells work under a wide band of wavelengths. The choice for the

optimal AR coating definition can be optimized by numerical calculations. Such an

example has been presented by Zhao [Zha91a].

To obtain cero reflectance

n

ARC

n0

n Si

where n ARC is the refractive index of the ARC layer, n 0 is the refractive index of the

medium for which the medium is optimized (1 for air or ~1.5 for the module

encapsulation), and n si is the refractive index of silicon in air (n Si ~ 3.85) [Gre98].

The thickness of a single AR coating is optimized by using the destructive

interferences to obtain a minimal reflection. The thickness of the layers should be an

odd multiple of:

t

where t is the layer thickness, 0 is the wavelength used for the optimization and n ARC

is the refractive index of the layer [Luq03].

2.5.3 Materials

In the industry, the best compromise between cost and efficiency is obtained with

a single ARC layer. As explained previously, typical industrial emitters are very highly

doped, so the surface passivation is not so significant in the achievement of good

devices. Therefore, the front dielectric is typically made by a ~75nm thick silicon

nitride coating with a low refractive index (n~2). In laboratories, the passivation of the

front emitter is crucial. Besides, most laboratory cells are only measured under air

conditions, so there is no need to optimize the layers for measurements under the

module glass. For this reason, the most common front dielectric used on laboratory

high-efficiency cells is made by a ~105 nm thick thermal silicon oxide layer. A more

sophisticated dielectric stack, which is compatible with module manufacturing, was

applied by Zhao on the manufacturing of the highest efficiency silicon solar cells made

to this date [Zha96]. He applied a thin thermal oxide for the surface passivation of the

emitter, underneath a double-layer AR coating. This stack was formed a material with

a high refractive index (ZnS) and a material with a lower refractive index (MgF)

[Zha96].


n ARC

0

4

1 AM 1.5 G: standard for the terrestrial solar spectrum under a global air mass of 1.5. According to the

latest standard (IEC 60904-3 ed. 2.0), it is equivalent to 997.47 W/m 2 .


22 Front-side metallization of silicon solar cells

Among some other materials interesting for their AR properties there are:

titanium oxide (TiO 2 ), titanium nitride, zinc sulfide, magnesium fluoride, hafnium

oxide (HfO 2 ), cerium dioxide (CeO 2 ) and indium tin oxide (ITO).

Table 2-4 Optical properties of different materials interesting for ARC [Ric04, Los01]

Material Symbol Coating technology

Refractive

index

n

Measured @

nm

Silicon Dioxide

SiO 2

SiO x

Thermal oxidation /

CVD

1.455

1.45-1.48

632.8

Silicon Nitride

a-SiN x :H PECVD/PVD 1.95 - 2.02 632.8

Si 3 N 4 LPCVD 2.0-2.2 300-1200

Magnesium fluoride MgF 2 Evaporation 1.38 632.8

Zinc Sulfide ZnS Evaporation 2.34 632.8

Titanium Dioxide TiO 2

Evaporation, ALD

(or screen-printing)

2.01-2.3 (2.88)

Aluminum Oxide Al 2 O 3 ALD 1.65 632.8

Haftnium Oxide HfO 2 ALD

Indium Tin Oxide

ITO

Evaporation or

sputtering

1.8 632.8

Cerium Dioxide CeO 2 Evaporation 2.21-2.7 632.8

Silicon Oxynitride SiON PECVD 1.7

Tantalum pentoxide Ta 2 O 5

Amorphous silicon a-Si PECVD 4.23

Silicon Carbide SiC x PECVD 2.73

EVA & Glass ~1.52 632.8

Silicon Si - 3.87 632.8

Aluminum oxide, silicon oxynitride, silicon carbide, amorphous silicon and

amorphous silicon nitride, are interesting for the surface passivation [Sch08]. The

main difficulty for the application of the last three on the front-side is that they have a

non-negligible optical absorption coefficient in the standard operation range of

terrestrial silicon solar cells. Considering that the photons reach the silicon surface by

the front-side, the inclusion of absorbing layers at the top of the cells is undesired.

Some research groups have solved this problem by depositing thin layers of the

absorbing materials, so that their negative impact is negligible [Lau98, Fer06].


Two layer metallization approach 23

An evaluation of the impact of the optical properties of layers like PECVD SiN x ,

porous SiO 2 , ZnS, MgF 2 and TiO x as well as their optimization in the application for

AR coatings was published by Nagel in 1999 [Nag99]. A comprehensive review of

various AR coatings for plating applications has been published by Richards in 2004

[Ric04].

In most of this work we have used the standard dielectric technologies for both

industrial and laboratory cells. That is a ~105 nm thermally grown silicon dioxide

layer for high efficiency cells and a ~75 nm SiN x layer for industrial cells. In chapter

4, two different technologies were tested for the deposition of the silicon nitride:

plasma-enhanced chemical vapor deposition and sputtering, due to their availability

and interest for industrial applications.

2.6 Two layer metallization approach

The theory defines which materials are interesting to obtain a low electrical

resistance between metal and silicon. Nevertheless, in the quest for alternatives for the

front-side metallization step, the first step is to define how do we determine which

technology is the best one?

In principle, the technological requirements for the ideal front-side metallization

process can be divided into 4 sections

Mechanical: the contacts should provide a strong adhesion to the silicon. The

process should not induce thermal or mechanical stress to the substrate.

Electrical: the contacts should have a negligible contact resistivity to the silicon

and high line conductivity, while avoiding shunt formation in the junction. If possible

they should provide a low contact resistance even with lowly doped surfaces

(N s ≤ 1 x 10 19 cm -3 )

Optical: the shadowing losses should be reduced as much as possible. Ideally

this should be done by forming fine line contacts with a high aspect ratio

(height/width).

Practical: in order to avoid material waste, selective deposition techniques are

desired. The working hazards should be minimized. The process should be compatible

with the manufacturing sequence of high quality devices.

For the industrialization of the technology, we would also need to take into

account the cost per Watt peak . This means that “simple” technologies with high

throughput potential are preferable. The cells would also need to be solderable and the

modules should last at least for 20-25 year. So the metallization process should be

reliable and stable.

High-efficiency metallization concepts have decoupled the contact formation and

the current conduction by using a stack metal layer, with a seed contact layer. The seed


24 Front-side metallization of silicon solar cells

layer delivers low contact resistance and good adhesion and the plated layer delivers

high finger conductivity [Glu09]. This work uses this approach to evaluate alternative

manufacturing sequences for the seed layer formation. This first approach has been

performed taking into account mainly the electrical and mechanical properties of the

alternative processes.

Seed

Growth

Figure 2-14 schema showing the seed and growth approach used in this work for the

development of alternative metallization technologies [Glu09]

The thickening of the contacts can

be performed in an inline light-induced

Ag plating tool or in a Cu plating tool.

This is performed by contacting the rear

side of the cells to the anode by an

external power supply. The illumination

induces a negative potential on the frontside

and the positively charged Ag ions

are attracted out of the bath into the

silicon front-contacts [Met06].

Figure 2-15 presents a schema of the

process. Even though it has been shown

that when enough defects are induced in

Figure 2-15 schema of the light induced

silver plating step

the front surface, it is possible to directly form an Ag layer on the front-side [Met07b].

But silver contacts directly deposited in this way, do not provide sufficient adhesion.

Consequently, the seed layer formation is critical for Ag plated contacts.

In case of Cu contacts, the main purpose for the seed layer is to provide a

diffusion barrier, to avoid the penetration of copper in the cell. This application for the

nickel plating process has been evaluated in the work of Jonas Bartsch [Bar11b].


Technologies for the seed layer formation 25

2.7 Technologies for the seed layer formation

An excellent overview of the different techniques for the front-side metallization

process for silicon solar cells and especially for the seed layer formation has already

been published by A. Mette in his dissertation [Met07]. A brief summary of the seed

layer formation techniques is presented next:

1. Physical Vapor Deposition:

a. Evaporation

b. Sputtering

These are typically combined with a lithography step, either by the

application of a lift-off process or by an etch-back step of the metal. Selfaligned

silicide formation step are currently under evaluation [Tou12]

2. Printing:

a. Fine line printing (down to 50 m) thanks to modifications of the

screen printing process (screens and pastes) [Her12].

b. Use of alternative printing techniques like:

i. inkjet printing

ii. pad printing

iii. aerosol printing

iv. stencil printing (down to 35um)

3. Laser-based metal deposition:

By this process, it is meant that the metal is deposited onto the silicon

directly due to the irradiation with the laser light. In principle, it can be

performed:

a. From a gas.

b. From a solid.

c. From a liquid.

4. Plating:

This step needs to be combined with patterning mechanisms to form the

front-grid on the plating mask

a. Electroless plating.

b. Light-assisted electroless plating.

c. Light induced electrolytic plating.

This work deals with techniques based on laser processing and plating combined

with the patterning of the plating masks. Chapter 3 treats direct writing laser based

metallization techniques. Chapter 4 introduces the different patterning techniques for

the plating masks. Chapter 5 and 6 are about the plating of nickel layers on silicon and

its application for the manufacturing of silicon solar cells.


26 Front-side metallization of silicon solar cells

A good overview of the current activities in the metallization of crystalline

silicon solar cells can be found in the summary of the metallization workshops

[Bea11]. Other scientists who have recently contributed to the improvement of the

understanding and application of diverse front-side metallization technologies include

J. Hoornstra for stencil printing [Hoo98], G. Schubert and G. Grupp for screen printing

[Schu06, Gru05], A. Mette and M. Hoerteis for fine line printing [Met07, Hor09], and

L. Tous and J. Bartsch for plated contacts [Tou11, Bar11].

2.8 Silicon-metal interactions

In this section we evaluate the diffusion properties of different metals in silicon,

and the silicide formation particularly for nickel contacts to try to understand how

these could result in junction degradation. Different mechanisms could have an impact

in the degradation of silicon devices as a result of the metallization. If the metallization

consists of silicides formed after a thermal treatment, the degradation could be due to:

1) the increase of recombination due to the diffusion of elemental metallic

impurities, forming deep level traps in the depletion region [Gam98].

2) the formation of Schottky diodes, due to silicides protrusions through the

junction [Gam98]

2.8.1 Diffusion

The movement of atoms into a

silicon wafer during thermal treatment,

and particularly of metal atoms, can be

described by the diffusion theory. The

diffusion length (L) as a function of time

(t) can be expressed as

L

Dt

,

were D is the diffusion coefficient, which

depends on the temperature as follows,

D D

0

e

E

a


kT


D 0 is the diffusion pre-exponential, E a is

the activation energy, k is the Boltzmann

constant and T is the temperature in

Kelvin.

At high temperatures, T > 0.7 T m

(T m is the melting temperature) [Web83],

the atoms can travel fast through the

lattice, and the diffusion is called

Figure 2-16 Diffusion coefficients as a

function of the temperature for 3d metals in

silicon [Web83]


Silicon-metal interactions 27

interstitial, since the atoms travel through the crystal. Figure 2-16 shows the diffusion

coefficient for different metals as a function of the temperature [Web83]. Nickel and

copper have the highest diffusion coefficients in silicon. Copper diffuses very fast

already at relatively low temperatures, while for nickel this behavior is only observed

for temperatures over 800 o C.

For diffusion processes at low temperatures, the metallic atoms substitute the

place of the silicon atoms in the crystal and the diffusion process is called

substitutional. In general, substitutional diffusion coefficients are several orders of

magnitude smaller than interstitial diffusion coefficients (see figure 2-17) [Web83].

Bonzel studied the substitutional diffusion coefficient of nickel at temperatures

between 400-800 o C, finding that the pre-exponential factor and the activation energy

in that temperature range are 0.1 cm 2 /s and 1.92 eV respectively [Bon67]. Berning

studied these parameters at temperatures ranging from 250 to 350 o C. In that range, a

diffusion coefficient of D = 10 -13 . exp(-0.27 eV/kT) cm 2 /s was determined [Ber78]. The

results from Weber show the interstitial diffusion coefficient of

D = 2 . 10 3 . exp(-0.47 eV/kT) cm 2 /s, for the temperature range between 800 and

1300 o C [Web83]. If we compare this data to the diffusion of tungsten in silicon

D=9 x 10 -6 exp(-2.2eV/kT) cm 2 /s at temperatures between 853 o C and 1303 o C [Gra95],

it is possible to imagine that using tungsten for the contact formation would make it

easier to avoid junction degradation due to metal diffusion (see figure 2-17).

Diffusion coefficient [cm/s]

10 -3 Ni interstitial

10 -5

Berning 78

Bonzel 67

10 -7

Weber 83

Graff 95

10 -9

Ni substitutional

W interstitial

Ni low Temp

10 -11

10 -13

10 -15

10 -17

0.75 1.00 1.25 1.50 1.75 2.00

1000/T [1000/K]

Figure 2-17 Substitutional vs. interstitial

diffusion coefficient of nickel and tungsten

in silicon as a function of the temperature

[Ber78, Bon67, Web83, Gra95]

Diffusion length [um]

10 6

10 5

10 4

10 3

10 2

10 1

10 0

10 -1

10 -2

10 -3

Industrial

emitter depth

D Low Temp

D subs

D interst.

1s

1 min

10 min

10 -4

200 300 400 500 600

Temperature [C]

Figure 2-18 diffusion length of elementary

nickel atoms after a thermal treatment lasting

1 s, 1 min or 10 min, corresponding to the

diffusion mechanisms presented in figure 2-17,

assuming that some nickel atoms could diffuse

interstitially even at 400 o C.

Figure 2-18 has been drawn to give an idea about the meaning of these values for

the manufacturing of silicon solar cells. A traditional emitter depth in industrial

manufacturing could be considered to be around 300 nm. So that if the metal doesn’t


28 Front-side metallization of silicon solar cells

reach to such depths, in principle there should be no issues with the degradation of the

junction. We have simulated a thermal treatment lasting 1 s, 1 min or 10 min for each

diffusion mechanism presented in figure 2- 17 in a range of temperatures relevant for

the front metallization. The relevance of these temperatures is determined taking into

account the silicide formation (see next section) and the oxide passivation (see section

2.4.7). Assuming that all metal diffusion occurs through substitutional or low

temperature diffusion we would have to reach a temperature of 600 o C for over 10 min

in order to get through the junction. If some nickel atoms would start to diffuse

interstitially already at lower temperatures (~400 o C), they could reach a depth of

100 m already after 1 s.

In chapter 5 we will see that there is a strong degradation of silicon solar cells

after processing nickel-based contacts at temperatures around 400 o C or higher, even

for junctions featuring ~1 m thick emitter. Those results cannot be explained by only

considering the substitutional diffusion theory. An interesting process simulation

suggested for future work should study how many active nickel atoms would be

required and how close from the depletion region could they get so that they would

have an impact in the electrical properties of the device.

Considering the metal diffusion, it is possible to argue that it would be better to

use tungsten instead of nickel for the seed contact formation on n-Si wafers. Tungsten

has a low diffusion coefficient (see figure 2-17) and it makes a good electrical contact

with silicon (see figure 2-7). Nevertheless, the standard techniques that could be used

for the selective deposition of tungsten contacts (evaporation, sputtering, printing) are

not easily compatible with solar cell processing. An alternative to form tungsten seed

contacts was evaluated through the melting of fine tungsten powders by laser microsintering

(see chapter 3).

2.8.2 Metal silicide formation

The chemical reaction between silicon and a metal can result in the formation of

a metal silicide (like for Ni, Ti, Mo and others) or the formation of an aggregate (like

for Ag). The main part of experimental work performed during this thesis deals with

nickel contacts. For this reason, this section is focused on the nickel silicide formation.

Different reviews on silicides can be found on the literature, particularly interesting are

the work by Murarka [Mur80, Mur95], Gambino [Gam98] and Maex [Mae95].

There are different ways to form a metal silicide. The most common one is by

evaporating or sputtering a metal on top of a silicon wafer and performing a

subsequent thermal treatment [May90]. During the nickel silicide formation, nickel is

the fastest diffusing material [Xu98], so it diffuses into the silicon; while for metals

like titanium and cobalt silicon is the fastest diffusing species [Schw06].


Silicon-metal interactions 29

The main phases are Ni 2 Si, NiSi and NiSi 2 with a resistivity of 24, 10.5 and

34 cm, respectively [Mae95]. The temperature required for the formation of each

silicide phase varies depending on the literature. Deng states that Ni 3 Si and Ni 2 Si are

formed at temperatures below 400 o C. NiSi is observed between 400 and 700 o C, while

NiSi 2 is formed around 750 o C [Den97]. Kim states that the nickel film is converted to

Ni 2 Si at temperatures between 200-300 o C, to NiSi at temperatures between 300-700 o C

and finally to NiSi 2 at temperatures between 700-900 o C [Kim05]. Lauwers obtained

similar results for the Ni 2 Si and NiSi formation [Lau04].

Actually even the nature of the interface plays a role in the determination of the

stoichiometry of the silicide: Berning observed that the difference in the evaporation

temperature applied during the nickel deposition resulted in a difference in the defect

density at the interface, which played a role in the silicide formation [Ber78]. Olowafe

studied the influence of the substrate on the silicide formation at low temperatures

(200-350 o C), concluding that the diffusion was slower on and poly-Si surfaces

than on Si surfaces [Olo76]. Lauwers observed an impact on the bulk doping

(arsenic or boron) on the silicide formation after spike or soak anneals (with a maximal

time of 30s) in a temperature range between 270 to 340 o C: epitaxial pyramids of NiSi 2

were formed on boron doped samples along the 111 planes [Lau04]. Interfacial oxides

and other impurities also play a role in the silicide formation [Gam98]. The impact of

the relative amount of metal vs. silicon is presented on figure 2-19 [Ott79].

Figure 2-19 Nickel silicide formation as a function of the temperature for thin films [Ott79]

Taking into account the volumetric data [Mae95], the amount of silicon that

would be consumed after the formation of each of the main silicides is presented on

figure 2-20. These values are only valid if the complete nickel layer is converted into a


30 Front-side metallization of silicon solar cells

silicide. Therefore, for very thin metal layers it would be possible to limit the distance

between the silicide/silicon interface and the junction by reducing the thickness of the

deposited metal layer. According to this theory, the silicon consumption during the

NiSi formation, could be reduced (for example to ~100 nm) by reducing the film

thickness of the metal (to 50 nm) (see figure 2-20). Such an approach was evaluated by

Foggiato, who deposited 10 nm thick Ni layers to reduce the leakage current on ~60-

80 nm thick junctions. He combined this strategy with a two-step thermal anneal

process: first annealing between 260-320 o C to form a Ni 2 Si, then removing the

remaining nickel and annealing again at 400-450 o C to form a NiSi, while reducing the

source for metal diffusion [Fog04].

Figure 2-20 Volumetric consideration of the silicon consumed as a function of the metal

thickness depending on the silicide formation (left) [Mae95], example of these values

considering actual nickel layer thicknesses (right).

Figure 2-21 XTEM micrograph, bright-field of a nickel silicide formed from an electroless

plated layer (pH4.4) after a 1h anneal at 400 o C [from Liu05]

Finally, evaluations of nickel diffusion from plated layers performed by Liu,

starting from acidic solutions have shown that the phosphorus, which is deposited with

the electroless plated nickel from hypophosphite-based-solutions, reacts with the

nickel forming a Ni 3 P layer and potentially contributing to the formation of a NiSi 2


Silicon-metal interactions 31

layer, even at temperatures as low as 400 o C. Figure 2-21 shows an example of such a

silicide formed after annealing 400 o C during 1 h [Liu05].

2.8.3 Outlook

As mentioned in the previous

section, a strong degradation of the

front junctions of silicon solar cells has

been observed, after applying a thermal

treatment on plated nickel contacts

(~120 nm thick) at temperatures

≤ 400 o C, even on emitters with a

junction depth down to 1 m (see

chapter 6). At such temperatures we

expect the formation of a mono silicide;

potentially combined with Ni 3 P, as

observed by Boulord [Bou10]. With the

current knowledge on silicide formation

or nickel diffusion the degradation

cannot be explained. There are several

aspects which could result in the

differences between the theory and the

praxis: the use of junctions formed on

textured surfaces, the

inhomogeneities in the junction profile

(shallower diffusions at the valley of

the pyramids and deeper at the tips). In

depth studies of the silicide formation

Figure 2-22 TEM view of nickel silicides

formed on textured silicon solar cells, after a

thermal treatment at 275 o C, during 150s (a,b)

or 350 o C during 30s. The silicides were formed

form a 40 nm Ni PVD source. [Tou11]

process for different nickel-based contacting schemes (PVD or plated nickel

deposition on wet-ecthed, laser ablated structures, treated by furnace or laser

annealing) are currently performed by Tous [Tou11, Tou12b]. Such an example is

shown on figure 2-22, with a silicide formed from a thin Ni PVD layer (40 nm thick)

on a textured silicon solar cell after different thermal treatments.

Even though avoiding the nickel diffusion through the junction during the

thermal treatment applied to form silicides is a challenging task, the use of nickel

contacts represents a very attractive option for the seed layer formation; because:

nickel can be directly deposited onto silicon by plating,

the ARC can be used as a plating or silicidation mask,

that nickel acts as a diffusion mask for copper contacts,

This approach has been evaluated in depth in chapters 5 and 6.


3 Laser writing for the front metallization of silicon solar

cells

This chapter presents two innovative processes for the laser-induced direct

writing of metal fingers, applied for the seed layer formation. One involves solidsolid

interactions: laser micro-sintering and the other involves liquid-solid

interactions: laser induced selective metal deposition from an electrolyte. Both

techniques have been developed for the front-side metallization of silicon solar

cells during this thesis.

3.1 Introduction

Laser processes have been evaluated and implemented in the manufacturing of

solar cells for over 20 years. One of their main applications at an industrial level was

based on the laser buried contacts [Wen86, Mas02]. Lasers are taking a wider role in

other areas of the fabrication of silicon solar cells, either as a standard or as a

developing technology; e.g. laser marking, laser texturing [Zol89], emitter formation

by laser doping [Ame05, Tja07, Suw10], laser ablation of dielectric layers for front

processing [Dub90, Kno09, Her10] or rear processing [Ago06], edge isolation

[Sch04], laser fired contacts (LFC) [Sch02], or in laser-written front-side metallization

[Roh85].

As it was already pointed out by Rohatgi in 1985, the main advantage of a

metallization technique based on direct-write patterning is the reduction of process

complexity and the in-situ sintering while providing a fine-line resolution [Roh85].

Laser-induced chemical processes occur through different excitation

mechanisms: pyrolytic, photolytic or a combination of both. Pyrolytic reactions use the

laser as a heat source, while in photolytic (or photochemical) reactions the photons

provided by the laser participate directly in the formation or breakage of chemical

bonds, either within the surface of the material or within the surrounding medium

[Bau86]. Figure 3-1 shows the different reactants that can be involved in a chemical

reaction induced by lasers.

Figure 3-1 Laser-induced chemical reactions at or near interfaces [redrawn from Bau88]


34 Laser writing for the front metallization of silicon solar cells

The interactions between a laser and a metal source in different phases are

studied in this chapter for the development of a laser-based front-metallization process.

Laser micro-sintering is presented as a technology involving the reaction between

solid-solid interfaces in the same manner as the laser induced forward transfer

technique (which has been evaluated by other groups). The laser induced metal

deposition from electrolytes, also presented here, involves reactions with solid-liquid

interfaces.

The use of gas-solid interfaces for the deposition of metals like Ni, Cr, W and

Mo on silicon has been demonstrated in the past [Bau88, Bau00]. Taking into account

the high toxicity of such gasses, and the potential complexity of building a tool that

would enable even just a proof-of-concept for our devices, this path has not been

further considered in this work. Interactions between lasers and adsorbate-solids

interfaces have also been evaluated by other groups in the past [Roh85].

3.2 Laser micro-sintering (LMS)

This process was developed by the Laser Institute Mittelsachsen e.V. (LIM) in

2003, as an application of selective laser sintering (SLS) for the formation of 3D

microstructures, using micro-metric and sub-micrometric metal powders. It has been

reoriented from its original application with metals towards the use of ceramic

powders and alternative materials, compatible with biological applications. [Str08]

The formation of structures by laser sintering is based on the reduction of the

total surface free energy by the reduction of the surface. The laser beam radiates

enough energy to create the particle fusion via the liquid phase. Not all the powder

needs to be melted during the process, mainly its surface.

The SLS process is as follows: A thin powder bed is spread on a substrate with a

squeegee. The squeegee is also used for the powder storage (see figure 3-2b). The

material is locally melted or sintered by the laser following a computer designed

pattern. The substrate is lowered to a new position. A subsequent coating is distributed

over the surface to keep the top metal at focus level, while filling the gaps.

(a) (b) (c)

Figure 3-2 LMS images: (a) Sintering chamber (b) Ring blade used as squeegee and powder

storage.(c)SEM view of a tungsten coil formed by LMS [Reg05]


Laser micro-sintering (LMS) 35

This sequence is repeated as many times as necessary until the desired structures

are finished. Figure 3-2a displays a photo of the chamber used for the experiments,

while figure 3-2c shows an example of a 3D structure formed by this technology.

The process can be performed under a controlled atmosphere or in normal air.

Steel holders are typically used as substrates, where the 3D form generation is

performed. The remaining powder is removed by cleaning the samples in water. In the

laboratory setup, the finished devices can be detached from the substrate by tearing

them off manually.

3.2.1 LMS for the metallization of silicon solar cells

In this work the LMS process was used in collaboration with LIM to create the

front contacts of silicon solar cells [Ale08]. Figure 3-3 shows the results of the firsts

tests performed with this technology for the potential contacting application. Full

metal lines with extraordinary aspect ratios can be formed by this technique.

Nevertheless, in such cases, considerable damage is created on the silicon substrates.

Besides, the metal deposition rate is slower than the metal deposition by electroplating.

Figure 3-3 View of structures formed by LMS on Si wafers. On the left 700 μm thick “walls”

with a width of 70 μm (aspect ratio 10), and on the right a 40 μm thick, 20 μm metal finger

(aspect ratio 2).

Therefore, we have chosen to use this technique as the seeding step in a seed and

plate strategy. Figure 3-4 shows a schema of the front metallization sequence by LMS:

After the formation of the thin seed layer (around ~100 nm thick) by LMS, the

remaining powder is removed in an ultrasonic water bath. The seeds are thickened in a

Ag-LIP bath up to 10-12 μm.


36 Laser writing for the front metallization of silicon solar cells

a) b) c) d)

Figure 3-4 Schema of laser micro-sintering for the formation silicon solar cells. (1) p- type

base material. (2) n-type emitter (3) dielectric layer (4) powder layer (5) LMS seed contacts

(6) Ag-plated fingers

are:

Compared to screen-printing and evaporation, some advantages of this process

No high-temperature steps required

Potential for lower shadowing losses with very thin lines and a better

height/width ratio as compared to screen-printed contacts

Contact resistance reduction by using pure metals instead of pastes, while

keeping a high conductivity by Ag or Cu LIP

Savings through a simple recycling of the pure metal powder which is left-over

Higher freedom degree in the front designing, due to the CAD pattern definition

No mechanical pressure applied on the sample during the metallization:

Compatibility with thinner substrates and lower breakage in general.

Possibility to use alternative materials for the metallization, like tungsten, due

to the high energies provided by the laser source.

3.2.2 Evaluation of the process feasibility

Solar cells of 1 cm 2

on p-type FZ (0.5 cm) material featuring a shallow

~20 /sq emitter are used. The rear is passivated by SiO 2 and contacted by Al via

LFC. The impact of the process on the junction is evaluated on different type of front

structures (see table 3-1).

The metallic powders, applied for the first evaluations, are chosen among the

ones with previous LMS processing knowledge at LIM, which are also be interesting

for the front-metallization of solar cells: silver, tungsten, and molybdenum.

The LMS is performed with a Nd:YAG laser at =1064 nm, at 4 Watt, 30 kHz,

and a velocity of 0.5 m/s. The chamber is filled with He and the process is performed

at a pressure of 500 mbar. After thickening the fingers by Ag-LIP up to 12-15 μm, an

ARC of TiO 2 is evaporated on the front. The characterization is done by IV and

SunsVoc, TLM, AFM and SEM.


Laser micro-sintering (LMS) 37

Table 3-1: Front side cell structures

Type # Type of texture Front side dielectric

1

17 nm of SiO 2

KOH (random pyramids)

2 Native oxide

3

17 nm of SiO 2

Non textured

4 Native oxide

SEM views of contacts formed with tungsten powder on polished and textured

solar cells are presented on figure 3-5: details of the W-Si interface are presented on

the left; while the full fingers after thickening with Ag-LIP fingers are shown on the

right. About 125 nm thick and 10 μm wide tungsten lines are formed on the cells.

(a)

(b)

(c)

(d)

Figure 3-5 SEM views of LMS tungsten contacts on silicon solar cells. At the top: contacts

formed on a polished surface (a) with a view of the cross section of the metal seed, (b) after

the thickening by silver plating,. At the bottom the contacts formed on a textured surface (c)

detail on the seed layer, (d) overview of the full thickened finger.

The cells contacted with molybdenum did not provide sufficient adhesion to be

finished with an Ag plating step. The best-cell results for tungsten and silver sintered

contacts after Ag-LIP are displayed in table 3-2. The reference data corresponds to


Silver

(Ag)

Tungsten (W)

38 Laser writing for the front metallization of silicon solar cells

SunsV oc measurements taken on the solar

cells prior to the front-metallization. Lines

made with tungsten on textured cells show

better adhesion than the ones with silver

contacts. This is observed by peeling of

some silver sintered contacts after Ag-LIP.

The laser reaction is affected by the

powder composition, its diameter, heat

conductivity and reflectance. This leads to

variations in mechanical adhesion,

homogeneity and electrical properties of

the deposited contacts.

The results, presented in table 3-2

and figure 3-6, show that tungsten is a

better contacting material than silver. The

Figure 3-6 Average V oc for 3 samples

treated with the same laser parameters.

Cells with front-seed layer contacts formed

by laser microsintering of W and Ag

powders followed by thickening by Ag-LIP.

textured cells provide a higher j sc than the polished cells, as expected. A higher FF is

obtained from cells that do not have a thin oxide protection on the front. This is

explained by a lower contact resistance on the cells without oxide. Nevertheless, for

textured cells the PFF is higher with the thin oxide. The V oc is higher for most of the

cells with the thin oxide emitter passivation. The comparison between the V oc values

obtained after metallization and the reference measurement shows that functional cells

are manufactured by this technique.

Table 3-2: I-V results of the best silicon solar cells with LMS contacts, Ag-plating and an ARC

on 18 /sq emitter with different front side (FS) structures (see table 3-1).

Cell type V oc j sc FF PFF

Table I [mV] [mA/cm 2 ] [%] [%] [%]

No metal* 1 622 84

1 609.9 33.18 60.5 12.2 75

2 595.5 34.41 62.9 12.9 63.5

3 608.9 29.90 61.5 11.2 60.2

4 617.7 28.47 71.3 12.5 76

1 622.0 34.12 73.6 14.0 78.1

2 615.9 34.58 65.6 14.0 75.6

3 626.7 31.68 73.3 14.5 71.9

4 628.2 31.66 64.9 13.1 66.0

* SunsVoc measured on a cell before the metallization


Laser micro-sintering (LMS) 39

The reduction of the Pseudo FF is induced by damage in the junction caused

during the laser processing. The specific contact resistances (ρ c ) measured by TLM

vary from sample to sample, in a range going from ~10 -5 to 10 -3 cm 2 .

Cells with a thin oxide passivation on a textured surface and contacted with W

provide the best PFF and FF. These contacts endure the scotch tape test.

AFM was used to characterize the surface of polished samples after the full

removal of the silver contacts. Damage to the silicon surface was observed in local

spots. In some cases it was going down to 500 nm below the surface. This explains the

local shunts generated through the emitter which results in the low PFF values

obtained with this process.

3.2.3 Sintering through the ARC

A strategy to reduce the loss of FF was tested in a second batch. A thick

dielectric, which also acts as the ARC is used to protect the emitter from the laser

damage. Part of the irradiation is absorbed by the metal before reaching the wafer

surface, so a mix between the melting effect of the metal and the ablation of the

dielectric can occur during this process. An emitter with a higher V oc potential

featuring a R sheet = 50 /sq and a thick thermally grown oxide layer passivation

(105 nm) was used in this experiment. Tungsten was used for the seed contact

formation by LMS. Several samples were manufactured by evaluating the impact of

various laser parameters. The laser velocity was varied from 0.25 to 0.8 m/s, the

frequency from 5 to 30 Hz, the laser intensity from 25 to 100% (from a max of

4 Watt), the focal distance from 150 to 700 μm, the number of laser firing cycles for

each squeegee cycle from 1 to 20, the atmosphere pressure from 0.5 bar to

1 atmosphere and the number of squeegee cycles was 50. SunsVoc measurements after

the metal deposition were used to assess the junction damage (determined by the

pseudo FF). The contacts were analyzed and characterized by light microscopy, SEM

and IV- measurements.

The fluctuation of the results is very high. No trend linking the laser parameters

evaluated in the test and the electrical properties of the devices could be found. After

the Ag-thickening step, the fill factors are very low, ranging from less than 30% to

54.4%. The pseudo fill factor of some cells reaches values up to 79.5%, proving that,

in some cases, there is a high potential to reduce the damage induced by the laser

irradiation. However, considering the strong variation of cell results, even for cells

processed with the same laser parameters, we think that the differences in the

distribution of the powder bed from sample to sample is the most likely cause for this

variation. Furthermore, this cannot be easily improved with the current setup. The best

cell results for different processing parameters are presented in table 3-3.


40 Laser writing for the front metallization of silicon solar cells

Table 3-3: Selected IV results for a LMS tungsten contacted solar cells

(50 /sq emitter) from the second batch with ARC

Laser

velocity

Laser

Frequency

Laser

intensity

V oc j sc FF

PFF

[m/s] [kHz] [%] [mV] [mA/cm 2 ] [%] [%] [%]

0.25 30 100 613.7 37.49 53.1 12.2 71

0.5 30 100 645.8 36.82 46.0 10.9 73

0.5 30 80 652.7 37.33 54.1 13.2 79.5

An SEM view for this type of samples is presented on figure 3-5c. The powder

does not cover the whole pyramid surface but just certain parts, leaving large areas

unconnected. The high series resistance can result from the combination of the poor

surface coverage, a poor contact through the oxide, and the lack of continuity in the

thickness of the plated Ag observed on some of these devices.

The laser microsintering setup was

also tested using an irradiation mode on

continuous wavelength, in order to reduce

the energy density and evaluate the impact

of the laser-induced damage on the opencircuit

voltage of the samples. The

resulting V oc data as a function of the laser

energy on tungsten and molybdenum

sintered seed contacts before Ag

thickening is presented on figure 3-7. As

mentioned earlier, molybdenum based

contacts did not supply sufficient adhesion

to finish the devices, during Ag-LIP. The

average IV data for the best cells resulting

from this experiment (tungsten seed

contacts sintered with 70% of the max.

Figure 3-7 V oc for silicon solar cells

measured by SunsVoc on silicon oxide

passivated wafers with seed contacts formed

by laser sintering of tungsten and

molybdenum powders. The sintering has

been performed with CW laser irradiation.

laser power) after Ag thickening are V oc = 611.6 mV, 30.7 mA/cm 2 , FF= 40% and a

conversion efficiency of 8.2% (compared to the best cell of 9.4%)


Laser micro-sintering (LMS) 41

3.2.4 Alternative coating mechanisms

An alternative setup was fabricated to perform the tests directly at the Fraunhofer

ISE, hoping to further evaluate the coating process and potentially enable the

manufacturing of large-area samples. Considering the small size of the powders, and

their toxic nature a special chamber was built for processing safety.

The metal powder distribution by the new coating process delivers a layer which

is not homogeneous (see Figure 3-8). There is a silicon wafer inside the chamber

underneath the long metal squeegee, which can be pulled or pushed with the metal stab

at the bottom of the photo. The squeegee is pushed to distribute a metal powder layer

over the surface, but some spots of the blue ARC layer can still be seen on the wafer

after the coating has occurred.

Figure 3-8 Chamber used for LMS tests at

ISE

Figure 3-9 Tribo-electric charged spray gun

[Kno94]

Different techniques for the deposition of powder coating were evaluated in the

search for higher homogeneity, with the contribution of Tino Rublack through his

master thesis [Rub08]. Among these techniques, triboelectric and corona charged

spray guns were tested (see figure 3-9). The metal layers coated with the tungsten

powder formed islands on the substrates. Ultrasonic screens placed on top of the

wafers were also tested, but the results were not better; in other words, the layers were

not homogeneous. Then, slurries were created by mixing different powders with

ethanol. These were spread over the wafers with a pipette or with a squeegee, while

placing the wafers in small containers to keep the solution before the complete drying

of the slurries. Thin layers were formed with a submicron tungsten powder, while

thick layers resulted from the use of a particle size around 1 -5 μm. The roughness of


42 Laser writing for the front metallization of silicon solar cells

the layers could be reduced by holding the samples in support holders in an ultrasonic

bath. The vibrations contribute to the homogenization of the slurries before the ethanol

completely dries out. The cells were sintered with a Nd:YAG laser at 1064 nm, at

0.5 m/s. V oc of the corresponding cells showed that none of these techniques worked in

a satisfactory way to reduce the impact of the metallization in the electrical properties

of the device.

An alternative coating solution is presented in the next section by working with

liquid/solid interfaces.

3.2.5 Summary

Laser micro-sintering together with light-induced plating has been evaluated as

an innovative alternative for the front side metallization of silicon solar cells. It is

possible to generate metallic lines with this technique, without requiring hightemperature

firing steps or the use of mechanical pressure on top of the breakable

silicon wafers. Solar cells have been manufactured in a two-step metallization process,

taking advantage of the low contact resistance of certain materials to silicon and the

good conductivity of silver. An excellent aspect ratio is possible with 40µm wide and

18µm thick fingers. Tungsten shows better mechanical and electrical properties than

the other materials evaluated.

Even with the high potential of this technique, the issues corresponding to the

formation of a very homogeneous coating on top of the cells to avoid local shunting

due to inhomogeneous irradiation make the stabilization and industrialization of this

technique unlikely.


Laser-induced forward transfer 43

3.3 Laser-induced forward transfer

Another alternative to form a

homogeneous and well controlled metal

coating is presented by the PVD of the

metal coating on an optically transparent

support from which it can be transferred

to a substrate

In this technology the metal film is

first deposited on an optically transparent

support from which it is transferred to the

substrate through the laser irradiation.

Typical supports are composed of glass,

Al 2 O 3 , SrTiO 3 . According to Bauerle’s

book typical metal thickness are between

10 and 300 nm and the laser fluences in

ns pulse between 0.1 and 10 J/cm 2 .

[Bau00]. It avoids the homogeneity issues

observed with laser micro-sintering.

Lately, Roeder, from the institute for

physical electronics (IPE) in Germany, has manufactured front electrical contacts for

silicon solar cells based on this technology. First a thin Ni layer is formed by the laser

transfer and it is then plated with Ni and Cu for the metal finishing (see figure 3-10)

[Roe10].

Another embodiment for this

process which allows a very soft transfer

is presented by the use of an intermediate

polymer, as a sacrificial layer. Such a

process is presented by EMPA, the Swiss

federal laboratory for materials testing

and research, using a resin formed by

triazene (specifically designed to absorb

the laser light). This ensures a transfer at

Figure 3-10 Laser Transferred nickel

Contact plated with Ni and Cu [Rod10]

Figure 3-11 laser-based patterning of

organic, inorganic and living materials.

[Dor06]

very low fluences, which has been proved to work even for living cells (see figure

3-11) [Dor06]. Such an approach could also be applied for the metallization of heterojunction

cells if a sufficient mechanical adhesion could be formed between the

contacts and the surface [Far07].


44 Laser writing for the front metallization of silicon solar cells

3.4 Laser induced metal deposition from an electrolyte

3.4.1 Introduction

As it was observed during the laser micro-sintering experiments, the main

challenge for working with a solid-metal source in the form of a powder or slurry

comes from the lack of homogeneity from the metal source on top of the wafer. The

application of the induced forward transfer is an interesting alternative. Another

option, which has been evaluated during this thesis, is the use of a liquid metal source.

This strategy was evaluated in depth within the master thesis of Dominik Rudolph for

nickel based solutions [Rud08] and Tino Rublack for tungsten based solutions

[Rub08]. Kuno Mayer collaborated with his expertise in chemistry.

It is possible to imagine different processing alternatives, like working from a

standing solution or from a moving solution. If the solution is moving, it could be

flowing parallel to the wafer surface or perpendicular, as part of the laser jet or even

from some sort of spraying mechanism. Setting up experiments with flowing solutions,

requires a more complex set of safety measures. Therefore, we have chosen to focus

on the processing from standing solutions.

Laser enhanced nickel deposition from a liquid solution was tested as early as

1979 by Von Gutfeld. He showed that the deposition rate for electroplated nickel on

pre-metallized substrates was increased by the laser enhancement. He concluded that

the thermal effect played the main role in the process [VGu79].

The basic idea behind the laser induced metal deposition from an electrolyte is

that a metal can be locally reduced on the silicon surface through the irradiation with a

laser light (see figure 3-12b). It was evaluated as early as 1981 [Mic81]. The liquid

source can be a metal salt dissolved in water (like for electroplating solutions) or an

electroless plating solution. This metal can be deposited in the form of thin lines to be

used as seeding contact for the metallization of silicon solar cells

The irradiation could act as a heat source for the metal reduction in a pyrolytic

process, as a source for carriers on the semiconductor-liquid interface through the

photonic activation in a photolytic process, or a mix of both. The energy absorption

could occur in the solution, the substrate or both.

Different metals can be used as a metal source. We chose to focus on nickel and

tungsten, considering the existing experience with nickel plating solutions, the

effectiveness of these two metals to make good contacts to silicon and the possibility

to dissolve them easily in water.

The first challenge is related to processing safety when working with toxic fluids.

As mentioned earlier, nickel solutions are carcinogenic. The second challenge, which

comes from the working combination of lasers and fluids, is the potential damage for


Laser induced metal deposition from an electrolyte 45

the laser optics due to the possible evaporation of the solution during the irradiation.

Thus, a special chamber was designed and built to contain the liquid in a closed

environment during the laser treatment. A photo of the chamber is shown on

figure 3-12a.

Laser

Glas

Contacts

Metal

salt

Wafer

(b)

(a)

Figure 3-12 (a) Chamber built for the laser induced metallization from an electrolyte (b)

schema for the laser induced metallization process

Considering the substrates and the process target, it would be possible to evaluate

the deposition directly on silicon or on top of the ARC coating. In the past, silicon

solar cells were made in a sequence where the front-metallization was performed

before the ARC deposition [Gre98], but with time the inverted sequence; i.e. the firethrough

process, has dominated the cell fabrication thanks to the better passivation

properties of the ARC layer on clean substrates and the use of less contaminated tools.

Therefore, to avoid issues with alignment it is desired to achieve the metal reduction

through the ARC layer. This means, that the processing window for the simultaneous

silicon nitride ablation and metal reduction needs to be found.

Considering that most solar cells use SiN x as an ARC, the use of metal solutions

containing phosphoric acid that could etch the nitride while being heated becomes a

very attractive option. Ideally, the phosphorus in the solution could even contribute in

the n + doping of the substrate.

The laser irradiation focused on a laser spot represents a localized heating source.

Taking into account that the temperature increase is one of the critical factors enabling

the electroless metal plating on silicon, the local increase of temperature could result in

the local deposition of the metal. This hypothesis was first tested by applying

commercial solutions used in previous electroless nickel plating experiments.


46 Laser writing for the front metallization of silicon solar cells

3.4.2 Processing with a wavelength of 1064 nm

Industrial silicon solar cells

featuring a 60 /sq emitter with an

Al-BSF on the rear, and a textured

surface on the front were used for the

development of this technology. For

the first tests, the ARC was fully

removed from the front side before

the metal deposition. In this way,

potential issues concerning the metal

deposition, on one side, and on the

nitride etching/ablation capabilities of

the process on the other could be

isolated. Two plating solutions were

tested: The alkaline solution Niposit

Figure 3-13 SEM view of a laser-treated solar

cell, after irradiation with a 1064 nm wavelength

on an electroless plating solution

980 with a pH of 9.2 controlled by ammonia addition; and the acidic solution

Duraposit SMT 88 with a pH of 4.8. Two different solution thicknesses over the

surface were evaluated: 3.81 mm and 1.9 mm. The laser processing was performed

using a Nd:YAG laser with 20-100 ns pulse and a 1064 nm wavelength. A frequency

of 10 kHz was used. The laser was focused on top of the silicon surface. The diode

current to pump the laser and the scanning velocity was tuned between 24 and 36 A

and 50 to 400 mm/s.

During these tests the solution was typically evaporated at the laser spot. In some

cases, only the silicon was melted within the spot, while on others, there was also

some Ni deposition around the edges of the melted silicon. SEM and EDX

measurements proved the presence of Ni around the edges of some samples, which had

only silicon in the region where the laser was focused. Figure 3-13 shows an example

of such lines.

The literature shows that following the Nernst equation, the laser could have an

impact on the electrochemical potential of a metal ( e ) either by the localized changes

in the temperature or by affecting the concentration of the ions in the solution [Bau00].

In other words, the increase of temperature in an electrolytic solution can result

in the formation of an electrolytic cell due the temperature difference created by the

laser. e is locally increased by the raise of temperature in the irradiated spot, which

can have temperature gradients ~10 2 K/cm along the solid-solution interface [Met98].

This temperature difference between the center of the laser spot and its radius creates

an electromotiv force, which is not very high in number (around 1 . 10 -3 V/K); but

delivers a strong electrical field considering the small features of the treated area

Ni

Melted silicon

Ni

Si

Si


Laser induced metal deposition from an electrolyte 47

[Bau00]. This electrical field creates a current in the metal from the center outwards

(see figure 3-14), which attracts the positive ions in the solution towards the center of

the laser spot. The result is a laser enhanced micro thermal battery [Bau00]. The laser

irradiation could also induce local ion concentration changes in the solution (e.g. due

to differences in the thermal diffusion of ions in the solution). These changes would

also have an impact in the electrochemical potential of the metal. In that case the

process is described as a concentration battery [Bau00].

Figure 3-14 schema for the thermal battery created by a laser on a metal (left) and an n-type

semiconductor (right) in an electrolyte [Bau00].

For semiconductors, the difference in the mobility of electrons and holes

(Dember effect) can result in the accumulation of carriers within the irradiated area.

For n-Si, holes are accumulated on the semiconductor side in the laser spot. This could

lead to the etching of the irradiated area and plating around it (see figure 3-14), while

for p-Si the opposite would be expected [Bau00].

Opposite to bare silicon, for the solar cells used during this work, the junction

formed by the doping of the n + diode on the p-Si base contributes to create a frontelectrical

field which pushes away the minority carriers (holes) from the front area and

into the bulk. This reduces the possibility of having enough holes available at the

semiconductor surface for the etching of the irradiated area.

Another more reasonable explanation for the lack of nickel within the melted

area and deposition around the edges of the ablated line, shown on figure 3-13, is that

the solution is evaporated during the irradiation in the laser spot. The temperature

increase of the substrate enables the metal deposition by electroless plating in the heataffected

zone, where the plating solution is still present.


48 Laser writing for the front metallization of silicon solar cells

3.4.3 Optical evaluation of the metal based solutions

The absorption spectra for two commercial nickel plating solutions, an alkaline

and an acidic, based on NiSO 4 are compared on figure 3-15 with a solution containing

only water and NiSO 4 . Both solutions show some absorption at 1064 nm. The nickel

complex alone already shows a strong absorption below 450 nm, around 730 nm and

over 1050 nm. For the other solutions, the absorption changes depending on their

additives and chemical composition.

The evaporation of the solution during the laser irradiation could be the result of

the direct liquid heating by photon absorption, or due to the heat transfer by

convection from the substrate to the solution. Either way, such a process is not

interesting for the front metallization of silicon solar cells, since it damages the frontemitter

by melting the surface.

Absorption

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

NiSO 4

Alkaline solution

Acidic solution

0.0

400 500 600 700 800 900 1000 1100

Wavelenght [nm]

Figure 3-15 Absorption spectra measured by UV-VIS using special quartz for e-less nickel

plating solutions.[Rud08a]

The lowering of pH from 9.5 to 7.5 by the loss of ammonia in the solution after

the process points out to the evaporation theory as the best explanation.

The evaluation of the silicon surface damage during ablation processes as a

function of the laser wavelength performed by Engelhart showed that using lasers with

shorter wavelengths, like 355 and 532 nm contributes to reduce the substrate damage,

as compared to lasers with a 1064 nm wavelength (see figure 4-13) [Eng07].

Taking into account Engelhart’s analysis and the absorption spectra shown on

figure 3-15 for the different plating solutions, we can state that the optimal working

wavelength to achieve a reduced surface damage and low absorption in the solution is

a laser with a wavelength around 500 nm.


Laser induced metal deposition from an electrolyte 49

3.4.4 Processing with a wavelength of 532 nm

The closest laser source available for this work within the range of interest is

achieved by using frequency doubling to switch from the infrared into a green laser at

532 nm on the Nd:YAG laser. In order to work with this wavelength, the scanner head

was changed, the optics were adapted and the power was measured for various

frequencies as a function of the pumping current. Figure 3-16 plots the results of these

measurements.

Power [W]

7

6

5

4

3

2

1

10kHz

30kHz

50kHz

0

24 26 28 30 32 34 36 38

Pumping diode current [A]

Figure 3-16 Laser power as a function of the pumping energy and frequency at 532 nm.

The deposition with commercial solutions did not succeed even using the 532 nm

wavelength. With the lack of detailed knowledge on the chemicals existing on such

electrolytes, it is difficult to answer why these experiments were unsuccessful. For this

reason, new solutions were mixed in-house to evaluate the metal deposition while

keeping a good control over the chemistry.

3.4.5 Processing with nickel solutions

As substrates, p-Si Cz wafers featuring a 50 /sq emitter, a SiN x ARC and full

Al-BSF for rear contacting were used in these tests. The first solutions mixed a nickel

salt formed from NiCl 2 and phosphorous acid (H 3 PO 3 ) to achieve the simultaneous

metal deposition, nitride ablation and potential phosphorous doping, all in one step

[May07]. Two different concentrations were evaluated for the nickel salt in the

solution: 0.3 M and 3 M. Different laser parameters were evaluated, with pumping

diode currents ranging from 25 to 33 A, frequencies between 10 and 50 kHz and

scanning velocities between 10 and 100 mm/s.

A thick nickel layer could be formed on the front-emitter by using the highly

concentrated solution (see figure 3-17). But no nickel could be deposited from the

lowly concentrated one. EDX measurements on the area shown in a red square on


50 Laser writing for the front metallization of silicon solar cells

figure 3-17 showed that the finger is formed by 91% nickel and 8.4% phosphorus. The

line profile demonstrated that the metal thickness goes up to 4.5 μm on the edges of

the lines and ~2.5 μm on the center. This difference in thickness is probably due to the

ion diffusion on non-metalized areas. The lines are not always continuous. It seems

like part of the metal was detached from the surface. This means that the adhesion for

this process should be optimized.

The strong acidity of the

phosphorous acid used in the solution as

a reducing agent (with a pH=1), results in

a strong chemical attack of the rear-side

aluminum, which completely removes

the rear contacts. Thus, this process is

not feasible for cells with a finished rear

metallization. It would be possible to

evaluate the use of such solutions by

altering the manufacturing sequence. But

alternative sequences have not been

evaluated within the time available for

this thesis. It would be interesting to see

the results of such evaluations on future

work.

EDX Analysis

Figure 3-17 SEM view of a nickel layer

formed from a 3M NiCl 2 solution mixed with

H 3 PO 3 .

To avoid the attack of the rear metal, simple solutions were mixed by dissolving

NiSO 4 , NiCl 2 and Ni(NO 3 ) 2 in water. Concentrations of 0.3 and 3M were tested for

these metal salts dissolved in water. The Nd:YAG laser with a frequency of 532 nm

was used. The pumping diode current, the scanning speed and the frequency were

evaluated within the following range: 25-31 A, 10-50 mm/s and 30-50 kHz,

respectively. The scanning repetitions were also tuned from 1 to 10 times. The pH of

the solutions was measured before and after processing.

The low concentrated NiSO 4 and NiCl 2 salt resulted in the formation of fine

nickel lines, while very poor or no nickel deposition was observed from the highly

concentrated solutions, or from any of the solutions containing Ni(NO 3 ) 2 as a metal

source. Figure 3-18 shows microscope and SEM views of nickel lines formed with the

different salts. For the same laser conditions the lines formed with the NiSO 4 salt are

more homogeneous than with NiCl 2 (see images a and b). An SEM view of a line

formed with NiSO 4 on figure 3-18c is presented to help visualize the nickel present

with different laser parameters. There, we can see that the nickel coverage for the

sulfate salt is higher than for the chlorine salt; while on the chlorine salt samples a

thicker coating are observed.


Laser induced metal deposition from an electrolyte 51

EDX measurements of these lines resulted in a total deposited nickel atomic

mass around 1-9 at% for the NiSO 4 solution, while a mass ranging between 6-25%

was measured for the NiCl 2 solution. Oxygen peaks were observed on the metalized

area. The oxygen mass measured by EDX increases directly with the nickel mass, the

oxygen atomic content is between 1.3 to 2 times higher than the nickel atomic content

in the deposited area. As will be seen on section 5.2.4, for nickel deposits formed from

nickel plating solutions working without a reducing agent, part of the electrons

required for the metal reduction can be provided by the oxidation of the silicon. The

proportional increase of the oxygen content with nickel indicates that this theory could

be applied as a potential explanation for the metal deposition for these experiments.

(a)

(b)

(c)

(d)

Figure 3-18 Microscope and SEM views of nickel lines directly deposited from an electrolyte

by laser writing. On the left the samples are made from a NiSO4 salt and on the right with a

NiCl2 salt. Both samples on top are made with the same laser parameters 29A, 50kHz,

50 mm/s and 2 repetitions. The samples at the bottom are (c) with 30A, 50kHz, 10mm/s, 1

repetition. (d)


52 Laser writing for the front metallization of silicon solar cells

V oc

[mV]

600

575

550

525

500

Scanning speed

50 mm/s

10 mm/s

2 4 6 8 20 25 30

Intensity [W/m 2 ]

V oc

[mV]

600

560

520

480

440

Before - After

NiSO 4

-

NiCl 2

-

0 5 10 15 20

Intensity [W/m 2 ]

Figure 3-19 Average V oc for test samples

made with NiSO 4 solution measured by

SunsVoc at different scanning speeds.

Figure 3-20 Average V oc values for solar cells

made with NiSO 4 and NiCl 2 before and after

Ag-LIP.

For the NiSO 4 based solution, an increase of the number of scanning repetitions

results in an increase of the percentage of total deposited Ni mass. However, this effect

is not as pronounced for the NiCl 2 based solution. In any case, the line width increases

with increasing repetition. This is undesired for the formation of thin metal lines for

the front side metallization. The finger width ranges from ~10 to 100 μm, depending

on the process parameters.

Silicon solar cells were manufactured with this process using the 0.3 M solutions

based on both salts: NiSO 4 and NiCl 2. After the nickel deposition, the cells were

thickened by Ag-LIP.

The average V oc values for cells made with NiSO 4 as a function of the laser

intensity before Ag plating for two different scanning speeds are presented on

figure 3-19. The laser intensity is determined from the power measurement presented

on figure 3-16 taking into account the line width resulting for each parameter at the

corresponding scanning speed. For a slower scanning speed, the increase of intensity

damages the front emitter.

Figure 3-20 shows the average V oc as a function of laser intensity before and

after thickening the fingers in the Ag-LIP bath for the nickel solutions based on NiSO 4

and NiCl 2 . V oc for all samples increases after Ag-LIP. This is most likely due to an

improvement in the contact between the measuring needle and the cell with the full

metal finger. Cells made with the NiCl 2 based solutions show lower V oc than cells

made with NiSO 4 based solutions.

The fill factor measured by light IV is presented on figure 3-21 as a function of

the series resistance for samples made with NiCl 2 and NiSO 4 . A very low FF is shown

for most cells made with NiCl 2 , regardless of the series resistance. Such behavior


Laser induced metal deposition from an electrolyte 53

corresponds most likely to the damage of the p-n junction during metallization. A

similar behavior (faster shunting) has been observed in the past for NiCl 2 based

electroless plating solutions, as compared to NiSO 4 based solutions. The overall fill

factor achieved by these samples is very low. The corresponding SunsVoc

measurements already show a degradation of the junction quality, with max PFF of

80% on some samples.

70

65

60

NiSO 4

NiCl 2

FF [%]

55

50

45

40

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Series Resistance [ cm 2 ]

Figure 3-21 Fill Factor as a function of the

series resistance for samples made by laser

writing from NiSO 4 and NiCl 2 based solutions

Figure 3-22 DLIT of a silicon solar cell

with laser written front contacts formed

from a dissolved metal salt solution and

thickened by Ag-LIP.

Dark Lock-In Thermography measurements presented in figure 3-22show that

the highest recombination centers are located around the bus bar area. (at the top of the

2.5 x 2.5 cm 2 cell).

The IV results for the best cells made with these solutions during the

development of this thesis are presented in table 3-4. As already seen with the

SunsVoc measurements on the test samples, higher V oc values were achieved with the

NiSO 4 based solutions, which also results in a higher efficiency for cells made using

this solution. The specific contact resistance was measured by TLM on test structures

specially designed for that purpose.

Table 3-4 IV characteristics for the best solar cells made by laser writing from an electrolyte for

NiSO 4 and NiCl 2 based solutions.

Metal salt

V oc j sc FF R s Width

[mV] [mA/cm 2 ] [%] [%] [cm 2 ] [μm]

NiSO 4 594.6 33.4 67.7 13.4 1.14 125

NiCl 2 589.6 31.1 47.7 8.8 3.24 86


54 Laser writing for the front metallization of silicon solar cells

Improved IV characteristics are observed with the NiSO 4 solution as compared to

the NiCl 2 solution. A conversion efficiency going up to 13.4% on Cz 5 x 5 cm 2 cells

featuring a 60 /sq emitter and a rear Al-BSF is not at the level of the screen-printed

references yet, (~17%). Nevertheless, these results already show that it is possible to

make solar cells with reasonable conversion efficiencies by applying this technique.

With the proper optimization and tuning of the processes it would be possible to

reduce this gap, and most likely to overcome it by better contacts than with Ag pastes.

Detailed information about the experiments performed on silicon solar cells with

nickel electrolytes and lasers, as well as a more detailed evaluation of the optical

properties of the nickel based solutions can be found on the diploma thesis of Dominik

Rudolph [Rud08].

3.4.6 Tungsten-based solutions

An attempt was made, within the diploma thesis of Tino Rublack, to use tungsten

based solutions for the seed layer formation. Tool limitations only allowed us to tryout

the deposition from a laser source with a higher power (Edgewave 532 nm, ~5 W).

The energy applied for the metal deposition with such solutions was too high, thus the

emitter showed strong damage and very poor pseudo fill factors. Lacking the

possibility to use a different laser source during the development of this thesis resulted

in the deferment of such tests for future work.

3.5 Summary and outlook

The evaluation of laser micro sintering of pure metal powders for the formation

of thin metal lines on silicon solar cells is very innovative. The main issue observed

for the positive application of this process is the high risk of shunting related to the

diffusion of the metal through the junction. One path, which could be evaluated in the

future, for the reduction of the emitter junction damage, is the designing of special

powders mixing low temperature melting components with pure metals. In this way,

the metal sintering could be performed at lower temperatures while the good electrical

contacting could still be performed.

For the laser-induced metal deposition from electrolytes the results from nickel

sulfate based solutions show a high potential. The distribution of the liquids over the

surface contributes to the performance of a homogeneous process. The optimization

challenges are related to the reduction of the damage to the space-charge region and

the line width. There is a wide room of choices to achieve such improvements: for

example reducing the local power density: by increasing the scanning speed, or just by

reducing the laser power; or by further working on the thickness or the chemistry of

the solution either the concentration of nickel sulfate or even using alternative metal


Summary and outlook 55

sources; like tungsten based solutions. Part of this research is currently followed by

Wehkamp [Weh12].

Time limitations did not allow further evaluation of the use of a reducing agent

like H 3 PO 3 in the solution during this work. The use of rear metallization concepts,

like the evaporation or sputtering of Al with subsequent laser fired contacting, would

enable the manufacturing of the rear-metallization after the front. With such a

sequence the evaluation of the simultaneous doping, nitride patterning and

metallization all in one step becomes possible. The author hopes that such evaluations

can be the topic further investigations at the Fraunhofer ISE.

The use of solid-gas interactions for the deposition of metals like Ni, Cr, W and

Mo on silicon has been already demonstrated in the past [Bau88, Bau00]. This path

has not been pursued during this work taking into account the high toxicity of such

gases, and the potential complexity of building a tool that would enable the proof-ofconcept

for our devices.


4 Patterning dielectrics for the front-side metallization

In this chapter we explore different techniques for the structuring of

dielectric layers, as for example, inkjet printing and laser ablation. These

technologies can be used for the substitution of photolithography in an

industrial-oriented manufacturing sequence for the front-side metallization of

high-efficiency silicon solar cells. Thus, the main focus of the chapter is the

patterning of the front-side anti-reflective coating for a metallization based on

electroless plating.

The standard Ag screen-printing technology uses metallic pastes containing a

glass frit that allows the contact formation through the dielectric. The glass frit, usually

made of PbO x or BiO x , etches part of the silicon nitride ARC during the firing step

[Hor08]. In this work we focus on a metallization technology based on nickel plating,

for this reason, in our approach an extra step for the selective etching of the dielectric

is required.

It is important to keep in focus the integration of the structuring step into a frontside

metallization sequence based on electroless plating. Figure 4-1 presents a

summary of the alternatives for the structuring of the ARC and how these could be

integrated. After the deposition of the metal seed, there’s a sintering step to create a

metal silicide. Finally, the contacts are thickened either by Ag light induced plating, or

by Cu plating. More details about the cell integration of selected structuring

technologies, like inkjet printing or laser ablation of the dielectric are provided in

chapter 6, together with solar cell analysis.

Masked patterning

Photolitho

Resist

coating

Development

ARC deposition

Etching

Stripping

Printing

Structured

resist coating

Mask-free

patterning

Direct

writing

Structured

dielectric

deposition

Structured ARC

deposition

Plating seed

Sintering

Thickening of the seed layer

Figure 4-1 Overview of the processing sequence of different structuring technologies for a

metallization based on plating


Chemical etching of SiNx dielectrics 57

4.1 Structuring technologies

The structuring technologies can be divided in three main groups:

Masked patterning: an etching mask, also called resist, contains the negative

image of the desired pattern. This resist is applied as an etching mask during a

chemical etching step and stripped afterwards. Enclosed in this group it is possible to

use technologies like photolithography, inkjet printing, screen-printing or pad-printing,

by combining their use with a resist.

Mask-free patterning: the structure is formed by the local removal of the

coating; for example, through laser ablation; or by techniques requiring a posttreatment

either for rinsing or for the removal of the surface damage, as with like laser

chemical doping, selective etching (by using etching printed pastes), laser grooving or

mechanical grooving...

Structured dielectric deposition: a bottom-up approach, where the pattern is

formed during the coating step. For example by using a mask during the PVD

deposition of the dielectric, or the screen printing of the dielectrics, like TiO 2 .

4.2 Masked patterning

A pattern transfer from a resist to a dielectric layer can be performed by wet or

dry chemistry. The dry etching of dielectrics is out of the scope of this thesis. Hence, a

suitable wet-etching process capable of removing the dielectric, while leaving the

resist untouched, is required for a masked pattern transfer.

4.2.1 Chemical etching of SiN x dielectrics

One of the most common wet chemical solutions used for the removal of silicon

nitride is hot phosphoric acid at a temperature of 165°C. The high temperature

required by this process makes it impossible to combine it with the standard resists

applied for patterning. Standard resists have a softening temperature between

110-130 °C [Mic09]. Therefore, an alternative for the wet removal of the dielectrics is

required.

Silicon oxide and silicon nitride can both be etched in hydrofluoric acid. A

commercial solution called SiO etch , buffered oxide-etch (BOE), or buffered HF (BHF),

is typically used for the removal of SiO 2 layers. It consists of a mix between HF (with

a concentration < 6% ) and NH 3 . This is a very well settled process; with an etching

rate ~80-120 nm/min at 20°C-25°C for the SiO 2 layer. The use of ammonia makes the

solution more suitable for its application with photoresists.

The etching of the SiN x depends on the chemical composition of the layers. Both

Lauinger and Kerr have shown that a higher Si content in PECVD SiN x leads to

smaller etching rates [Lau98, Ker02]. Typically, the nitride layers used as ARC have


58 Patterning dielectrics for the front-side metallization

low silicon content; while those optimized for the passivation of the bulk material on

the rear have a higher Si content [Lau98]. It is important to note that Kerr evaluated

the deposition of SiN x layers formed from a dilute silane source. In this way he was

able to decouple the stoichiometric composition of the nitride and its refractive index.

The etching rate of such layers is much higher than what has been observed by

Lauinger and in this research [Lau98].

In this work, the etching conditions of three different SiN x coatings have been

evaluated. Two of these layers were deposited by an in-line PECVD deposition tool

from Roth & Rau (SINA®). The first layer is typically used as ARC (n= 2.02), while

the second is a passivating nitride (n= 2.6). The third layer has been formed by

sputtering with a TWIN-MAG sputter coater manufactured by Applied Materials. It is

also interesting for ARC applications (n= 2.08) [Wol04]. The properties of these

layers relevant to this work are listed in table 4-1.For more information on the

properties of SiN x dielectric layers the author refers to the work of Lauinger, Schmidt,

Aberle, Kerr, Cuevas, among others [Lau96].

Damage-etched Cz p-type wafers were used as substrates. In a first experiment,

the wafers were introduced in BHF for a time lapse ranging from 1 to 15 min. The

layer thickness was measured with a single wavelength ellipsometer before and after

etching.

An etching rate ~5 nm/min was determined for our standard PECVD SiN x layers.

The typical thickness of the ARC is ~75 nm. Over 10 min are required for the full

layer removal. Such long times are not desired in production environments. The

etching rate for both the sputtered ARC nitride and the passivating SiN x layer was

even lower. The results are plotted in figure 4-2 and tabulated in table 4-1.

In a following experiment, the etching rate of the different coatings was

measured while etching the wafers for 3 min in different HF baths prepared with

increasing HF concentration, from 1 up to 25%. The thickness of the coatings was

again measured before and after etching.

The high refractive PECVD nitride delivers similar results as the ones measured

by Lauinger. Etching rates almost twice as high were observed for the low refractive

PECVD coating as compared to the ARC nitride (see figure 4-3).

Evaluating the precise time when the surface becomes completely free of

dielectrics is not always an easy task; especially since ellipsometry becomes difficult

for the remaining thin layers. A more precise confirmation of the full removal of the

dielectric layer can be obtained by a chemical proof: Nickel plating is very sensitive to

any dielectric layer: the presence of a native SiO 2 layer already hinders the metal

deposition. For this reason the substrates were immersed in an alkaline nickel plating

solution after etching. A metal layer appears only on the dielectric free surfaces.


Chemical etching of SiNx dielectrics 59

Nitride source

Table 4-1 Etching rates evaluated for different SiNx Coatings

Refractive

index n

Thickness

This

experiment

Etch rate in BHF

Lauinger

[Lau98]

Kerr / diluted

SiH4 deposition

Etch time

in 20% HF

[nm] [nm/min] [nm/min] [nm/min] [min]

PECVD ARC 2.02 75 5 ~1.3 ~30


60 Patterning dielectrics for the front-side metallization

Figure 4-4 SEM front-view of a silicon surface after the chemical etching of the silicon nitride

and electroless nickel deposition. The white surfaces are the plated areas, while the edges of

the pyramids, still dark represent non-plated areas, probably due to some residues of SiN x .

An important aspect for the integration of this step in the solar cell fabrication, is

that concentrated HF solutions (also when buffered) strongly attack the rear-side

aluminum pastes. The sequence applied for cell processing, as presented on the next

chapter, include the etching of the front-side ARC previous to the deposition of the Al

paste rear-side metallization.

4.2.2 Photolithography: reference process for the front-side

metallization of high-efficiency cells

A photosensitive resist is deposited on top of the ARC by spin coating. The resist

is exposed to UV radiation through a photolithography mask of the desired pattern.

The chemical composition of the resist is altered within the illuminated areas, allowing

the development of the film. After the development either the non-exposed resist or the

exposed areas remains on the wafer, depending on the type of resist used.

The structure is transferred to the ARC by wet etching. This treatment is damagefree.

Thin metal lines are required for the front-side metallization, therefore a lift-off

process is applied. Thus, after the evaporation of the metal, the resist and the undesired

metal are removed during the stripping step. Extensive literature on this topic can be

found in many semiconductor books [Men01].

As applied in the semiconductor industry, this process is not adapted to the highthroughput/low-cost

constraints of the PV industry. The minimal resolution for this

technology lies far below the requisites for standard silicon solar cells. Typical finger

openings for solar cells are of 5-8 µm on small area cells (4 cm 2 ), while 30-50 m

openings are used on large area cells (125 x125 mm 2 ).


Screen-printing a mask 61

After the seed metal deposition, there is a plating step, where the seed lines are

thickened up to 10-12 m height, widths of 25-30 m (for small area cells) and

12-15 m height, widths 50-80 m for large-area cells.

4.2.3 Screen-printing a mask

One of the alternatives to form a patterned structure with a resist on a surface is

based on screen-printing. A proof-of-concept was performed with a screen-printing

paste from the company Peters which is resistant to BHF [Pet12]. The stripping of the

paste takes place after a few seconds in a 1% KOH solution at room temperature. The

minimal resolution observed from this experiment was not as good as the one seen for

an inkjet printed process (~80 m). In addition, the pressure applied on the wafers

jeopardizes the compatibility with the performance of thinner substrates. Therefore,

this technology was discarded after the proof-of-concept. An example of lines etched

using this process is shown on figure 4-5.

Figure 4-5 Microscope view of a line made by screen printing a mask after printing (left)

and after etching the nitride and stripping the resist (right). The light gray layer on the left

is formed by the printing resist.


62 Patterning dielectrics for the front-side metallization

4.2.4 Inkjet printing

One of the most interesting techniques for the pattern transfer evaluated during

this work is based on inkjet printing. In order to develop a working process for the

solar cell manufacturing, as presented on figure 4-1, it is important to optimize the

deposition of the resist for a minimal width, but also to evaluate the resistance of the

inks to the etching process and the optimal resist removal for the following

metallization steps.

Resist deposition

A piezo drop-on-demand inkjet printer (DoD 300) from the Schmid Technology

GmbH was used to deposit a hotmelt-resist mask. A temperature above 70°C inside

the printing head keeps the ink in a liquid state. As the droplets reach the substrate

they solidify instantaneously, allowing a higher control over the width formation. A

standard resist thickness ~ 15-40 µm guarantees complete coverage of the texture,

even for high pyramids, which are sometimes difficult to coat with a spin-coating

process.

The optimization of printing specific issues has been performed at ISE in parallel

to this thesis as part of the research performed by Daniel Stuewe [Stue07]. He has

optimized printing parameters for aspects like print-head temperature, piezo voltage,

fire pulse width, printing strategy (e.g. direction of print), software and machine

compatibility. The printing direction affects the edge definition. A continuous line

edge is defined in the printing direction, while a juxtaposition of solidified droplets is

observed in the direction perpendicular to printing (see left of figure 4-6). The finest

openings are normally created when the position of the fingers is perpendicular to the

printing direction, as shown on the left side of figure 4-6. Using this technique, there is

a potential to create lines down to 10 µm wide before etching.

45 µm

Figure 4-6 Printed lines depending on the printing direction. When the lines are formed in

the printing direction (right), the droplets coalesce, whereas when they are formed

perpendicular to the printing direction they maintain the shape of the droplet [Spe08]. The

same effect is observed when a negative resist is printed (picture on the left).


Inkjet printing 63

The minimum finger width that can be formed by inkjet printing and etching also

depends on technological factors that go beyond printing parameters. As an example:

The texture has an impact on the distribution of the inks at the edges, so that

wafers with higher pyramids tend to have a higher deviation of the resist on

the edges than smaller pyramids (see figure 4-7).

Nitrides on the front side with a higher refractive index require longer etching

time, thus wider under-etching can be expected

Different thermal post-treatment can be applied to the inks, resulting in a

higher adhesion and less undercut

Resist endurance to the etching process

The ability of the resists to withstand an etching process up to 10 min in 20% HF

and still be stripped afterwards was evaluated.

From these experiments we concluded that the printing resolution affects the

adhesion of the ink to the substrate. If the resolution is too low, the resist is detached

from the surface during etching (below 750 dpi). If the chosen resolution is too high,

then it becomes difficult to remove the resist after etching (1016x1000 dpi). Therefore,

a printing resolution of 847x850 dpi was chosen as an optimum for this work [Stu07].

The position of the print-head can be controlled with 5 µm margin.

The undercut can be reduced by applying a thermal process on the inks after

printing, before etching. The samples are then annealed on a hot plate at a temperature

around 40-50 o C during 1-2 min. This treatment improves the adhesion of the resist to

the surface, without creating any visible change in the line opening before etching

[Spe08].

Resist stripping

The resist removal can be performed in aromatic solvents or using a 1-2% KOH

solution using an ultrasonic bath during 40s, which is compatible with industrial

applications. For laboratory scale experiments, a sequence similar to a standard

stripping process for photolithography cells is used, based on subsequent dips in

acetone and isopropanol, finishing with a DI water rinse.


64 Patterning dielectrics for the front-side metallization

Figure 4-7 Optical and scanning electron microscope views of a contact after Ni plating on a

line etched with HF through an inkjet mask on a PECVD SiN x layer. Observe that the

reduction of the nitride thickness induces a change in the color of the layer, which is also

considered while measuring the undercut, but doesn’t necessarily imply the deposition of

wider Ni contacts (the metal deposition occurs where the nitride has been completely

removed).

Technological aspects of inkjet printing

Inkjet printing for resist masking carries several advantages, among which:

• No mechanical pressure (contact-less): compatibility with thin substrates

• Limited influence of the surface conditioning

• Compatibility with high textures.

• Possible fast scaling-up from the tool manufacturers

• The technology also carries some drawbacks among which:

• The limitation for the minimal line size to ~20 µm (40 micron was the standard

chosen for this work)

• The higher number of steps required (compared to direct writing technologies)

• Higher material/consumable consumption (compared to direct writing

technologies), through the use of a mask for the indirect pattern formation.

The high etching rate of the ARC in 20% HF is interesting for the simultaneous

structuring of front and rear-side nitrides. A silicon nitride layer interesting for the

rear-side usually has a higher Si content, leading to slower etching rates. This

constraint imposes larger undercutting than a process optimized only for front-surface

dielectric opening.


Laser grooving 65

4.3 Mask-free patterning techniques: Direct writing

The selective removal of the surface coating through a direct writing process can

be performed by a number of technologies. By “direct writing”, we mean here that the

removal of the nitride is performed following a defined removal path.

4.3.1 Laser grooving

This technology was developed by the UNSW in the 80’s [Wen86]. Laser

grooves are formed on the surface by the removal of the ARC as well as part of the

silicon underneath. The remaining laser damage is etched with an alkaline etching

solution (KOH or NaOH). After a second phosphorus diffusion, the junction is rebuilt

in the laser-treated areas. This technology is known as Laser-Grooved Buried Contact

(LGBC) process. Cells manufactured by this technology were commercialized by BP

solar starting in 1992, under the name of Saturn modules. Large-area (147 cm 2 ) cells

with very good electrical values have been demonstrated, with an efficiency up to

18.3%, j sc = 36.28 mA/cm 2 , V oc = 625.1 mV and FF = 80.6%, on commercial Cz

material, and 19.5% efficiency, j sc = 37.3 mA/cm 2 , V oc = 660 mV and FF = 79.2% on

Fz wafers (140 cm 2 ) [Bru03]. The commercialization of the technology by BP solar

has stopped in 2008. The best laboratory results achieved so far by the LGBC (12 cm 2 )

cells on Fz-Si wafers were achieved in 1991, with an efficiency of 21.3% [Zha91b]

Figure 4-8 Scheme of a laser groove

buried contact cell [Bru03].

Figure 4-9: SEM Micrograph of groove with an

electroless nickel seed and partly filled with a Cu

plated material [Jen03]

The main advantage of this technology is the reduction of shadowing losses by

fine line formation. Thanks to their very high aspect ratios (width around 20-40 µm

and depth around 60-100 µm) the fingers keep very high conductance. Both

short-circuit current and fill factor are improved. In addition, the use of a selective

emitter on the front-side enables the combination of a lowly doped surface outside the

contacted areas, which improves the collection efficiency of the cell in the shortwavelength

region and the V oc potential, while keeping a low contact resistance and


66 Patterning dielectrics for the front-side metallization

improved recombination properties on the highly doped region underneath the

contacts. [Wen88].

The challenges for its industrialization include the need for a phosphorus

diffusion barrier, imposing an additional requirement for the ARC layer. Silicon oxide

layers with a thickness below 300 nm (as used for ARC applications), or industrial

standard PECVD SiN x layers do not satisfactorily comply with this constraint: they are

either too thin, or exhibit some pinholes. The approach used in the manufacturing of

the LGBC cells by BP solar included the formation of a low pressure CVD nitride.

Such nitrides are denser, so better to avoid ghost plating; but they do not provide a

good surface passivation [Rus12].

The laser buried contacts have been historically related to a metallization based

on Ni/Cu plating, although some research has been performed using other contacting

methods, like filling up the grooves with screen-printed pastes [Hau04], or as

presented by Kopecek using a Sn melt to fill-in the grooves [Kop01].

4.3.2 Mechanical grooving/trenching

An alternative to the use of lasers for the formation of grooves is presented by the

use of mechanical tools (e.g. dicing saw). It was patented by the UNSW in 1988

[Wen88]. The finger depth and width are defined arbitrarily, allowing an interesting

increase of the aspect ratio, and therefore reduction of the shadowing losses. The

minimal width for the fingers is determined by the minimal size of the dicing-saw.

McCann presented very good results with this technology on mc-Silicon in 2006, with

efficiencies up to 18.1% for large areas (137.7 cm 2 ) [MCa06].

An interesting embodiment of the groove formation also presented by McCann is

to create buried contacts under an angle, followed by a direction-dependent deposition

of the dielectric layer [MCa06]. The bottom and one of the sides of the grooves was

free from dielectrics and ready for plating, while the other side is coated with a

directionally deposited dielectric layer. Metal is then deposited on a region which does

not face the sun directly. As a result shadowing losses are minimized (see figure 4-11).

Figure 4-10 dicing saw groove [MCa05]

Figure 4-11 Angle buried contact on a mc-Si

wafer to reduce shadowing losses [MCa05]


Laser ablation 67

4.3.3 Laser ablation

The combination of the dielectric-layer laser-ablation process with nickel plating

corresponds, together with inkjet printing and laser-chemical processing, to one of the

three main technologies for the structuring of the front-side and nickel plating

metallization developed during this thesis. A detailed look into its principle and the

relevant characteristics of the substrate after the ablation will be presented here.

The implementation and development of the laser-ablation technology at

Fraunhofer ISE has been done thanks to the research from A. Grohe, A. Knorz and C.

Harmel, who have worked on its optimization and better understanding of the physics

behind this process. Other groups, which have also coincided in the evaluation of this

method for its application as the front-side patterning technique for silicon solar cells

are the ISFH, Germany [Eng07, Her10], Erfurt [Nec07], BP Solar [Mor08] and imec

[Tou11].

Principle

A possible explanation for the ARC removal process through laser ablation is

based in a lift-off / out-sputtering phenomenon. A thin silicon layer lying underneath

the ARC is heated so strongly that on its attempt to leave the surface, it lifts partly the

ARC. The presence of the nitride on top could impede the completion of this process

forcing the silicon to re-solidify on the surface.

An alternative explanation is based on a multi-photonic step. The absorption

coefficient of silicon at the wavelength used for ablation (both 532 nm and 355 nm) is

at least an order of magnitude higher than that of the front silicon nitride (see

figure 4-12). Therefore, when the ablation process begins, the photons go through the

nitride and the strongest light-matter interaction occurs in the Si-SiN x interface. As the

silicon is heated up, part of the temperature is transferred to the nitride. The

temperature rise increases the absorption coefficient of the coating. Consequently an

improvement of the ablation rate can be achieved. Grohe described this effect by

measuring the absorption coefficient of a PECVD SiN x layer at different temperatures,

up to 900°C (see figure 4-12) [Gro08]. The laser ablation of the nitride results most

likely from a combination of both effects.


68 Patterning dielectrics for the front-side metallization

Absorption coefficient [cm -1 ]

Si Room temperature

10 6 ARC SiN x

n~2.1

Room temperature

10 5

900°C

10 4

10 3

10 2

10 1

10 0

240 360 480 600 720

Wavelength [nm]

Figure 4-12 Absorption coefficient of the nitride layer depending on the temperature [Gro08]

Application to the front-side metallization of silicon solar cells

There are several aspects to be very closely considered while attempting to apply

this technology to front-side patterning for a front-side metallization, and in particular

one based on Ni plating.

1. Junction damage

The highest risk, for any laser-related technology applied for the front-side

metallization, consists in the damage of the p-n junction. Considering that the usual

thickness for emitters is less than 1 micron, if no “repairing” of the junction is applied

(either during laser processing or afterwards); the deposition of metals on electrically

damaged spots creates recombination paths, reducing the shunt resistance. This has an

impact on the FF, which could be directly translated into reduced efficiencies. In other

words, to avoid the need of a second P-diffusion step, the damage inflicted to the

junction must be minimized.

Engelhart measured the damage created on a silicon surface as a function of

various laser-based processes with ns-lasers of different wavelengths (1064 nm,

532 nm, 355 nm). He observed a degradation of the lifetime of the region affected by

the laser pulse. This degradation was quantified depending on the amount of silicon

that needed to be removed after the laser treatment in order to get back to the effective

lifetime measured before the laser treatment. The results are presented on figure 4-13:

the lifetime rate was determined as the effective lifetime of the damaged material

divided by the effective lifetime before the damage. Engelhart observed that the depth

of the damaged region depended on the laser wavelength [Eng07]. He concluded that

the optimal wavelength for the current laser technologies that should be used to obtain

the least surface damage is provided by the UV source (355 nm), which has the

smaller penetration depth.


Laser ablation 69

Lifetime rate rel

Trench depth [m]

Figure 4-13 Reduction in the lifetime of silicon samples after laser treatment with different

wavelengths concluding higher damage to the silicon from a higher laser wavelength [Eng07]

2. Influence of the laser process in the Emitter profile

Already in 1990 Dubé observed a reduction of the P concentration on the surface

after laser ablation [Dub90]. Neckermann evaluated this process using different laser

sources and wavelengths, by applying ns-laser pulses [Nec07]. This change of the

doping profile is potentially useful to the solar cell device due to the increase in the

depth of the junction directly underneath the contacts, resulting in a reduction of the

shunting risk. On the other hand, if the reduction of the doping on the surface would be

too high it could lead to the formation of Schottky contacts instead of ohmic contacts

and an increase in the contact resistance.

An evaluation performed at ISE by Annerose Knorz (see figure 4-14) shows the

influence of the ablation process on industrial emitters with a phosphorus surface

concentration ~10 21 cm -3 and a depth ~0.45 µm prior to ablation. When a ns-laser

pulse is applied to ablate the nitride layer, the surface doping concentration is reduced

to ~10 20 while the phosphorus is driven further into the silicon up to a depth of

~0.9 µm (depending on the applied laser process) [Kno09a]. This resulting profile

represents an excellent opportunity for a nickel plating metallization and will be used

as one of the starting materials for plating experiments with laser-ablated dielectrics.

The use of a laser with a ps-pulse length does not show the same drive-in effect

for the dopants, but it allows the ablation of thin oxide layers, without strong emitter

damage [Kno09a]. The ns-ablation process was performed using a frequency tripled

Nd:YVO laser, with a wavelength of 355 nm and a pulse length of approx. 20 ns.

Knorz has established a reference process for the ablation of an AR silicon nitride

coating, combined with evaporated Ti/Pd/Ag contacts, independently of the nickel


70 Patterning dielectrics for the front-side metallization

deposition. In this way, silicon solar cells with 2x2 cm 2 have been manufactured on

1 cm Fz p-type wafers, featuring an industrial emitter r sh =50 /sq, reaching 19.8 %

efficiency (j sc = 38 mA/cm 2 , V oc = 639 mV and FF=78.6%). These results constitute a

first proof-of-concept for the application of the ablation technology for the front-side

ARC structuring without harmful damage to the junction. One of the main

achievements of that work is related to the good fill factors observed, even after the

laser ablation (with similar results obtained for wafers structured by

photolithography) [Kno09a].

Figure 4-14 SIMS Profiles for different samples after a P diffusion (R s =50 /sq after PSG

etching) followed by the laser ablation of the ARC using two different lasers with different

laser parameters. = 355 nm and a ns-pulse (left) or a ps-pulse (right) [Kno09a]

The technology developed within this thesis combines the laser ablation process

with electroless plating as shown in figure 4-1. A more detailed explanation of the

solar cell results are presented in following chapters.

3. Fine line structuring

Dielectric-layer laser structuring technology enables the definition of ~7 to

20 µm-wide fingers thanks to the high resolution of the laser. Besides, its simplicity of

application facilitates a fast industrial implementation. Other advantages compared

with grooving techniques, include:

Reduction of the thermal budget because the second P-diffusion step is avoided

Possibility to increase the junction depth by driving in the phosphorus,

contributing to the stability of the process to avoid shunting during silicidation,

No mechanical pressure is applied to the substrates, which makes the process

compatible with thin wafers.

For the moment, the throughput is limited by the optics used during processing

(laser beam moves at 1 m/s). With the use of a scanner, faster velocities (and therefore

throughput) could be reached.


Laser ablation 71

As can be seen on figures 4-15 (left), the complete removal of the dielectric

coatings on textured surfaces is a challenge. On the positive side, if the metal layer is

good enough to deliver the required electrical and mechanical conditions, less direct

contact between silicon and metal translates into a reduction of the surface

recombination losses created by the metal in the device. On the negative side, there

could be an increase of contact resistance due to the reduction of the contacting area or

a reduction of the mechanical adhesion, if the metal coverage is not sufficient.

The most interesting technological aspects for the laser ablation process for the

front-side patterning are:

A resolution of 15 m

No mechanical pressure applied to the substrates: compatible with thin wafers

No need for sacrificial layers.

Potential increase of the junction depth when a ns-laser is applied, or very

low damage to the surface due to a reduced thermal impact with a ps-laser

Thanks to the high resolution of the laser ablation, contacts with a lower

metal fraction can be manufactured in a simple manner, increasing the

passivated surface and the potential for higher voltages.

Some negative aspects include

Impact of the texture on the laser density might result in differences in the

energy applied on the edge, tips or valleys of the pyramids (see figure 1-15)

Potential damage to the surface due to the laser process, which needs a very

well defined processing window. The voltages reached so far by laser

ablation have shown a lower value than those treated by photolithography for

a similar metalized area.

Ablated spots

Ni-Plated spots

Ag plated finger

Figure 4-15 Example of the sequence applied for (a) a laser-ablation, (b) nickel plating,

(c) Ag-LIP metallization technology

Experiments on laser-ablated surfaces are excellent proofs of the sensitivity of

the plating process to surface conditioning. As an illustration, figure 4-15 shows the

processing sequence typically applied for the front-metallization with Ni and Ag

plating. Nickel is only deposited on the areas where the nitride is removed.


72 Patterning dielectrics for the front-side metallization

4.3.4 Laser-chemical Processing

Damaging the emitter during the laser process is a limitation of the previously

described method. A possible way of relaxing the constraints related to the potential

laser damage, is to create a selective doped area while removing the nitride.

An interesting embodiment of dielectric layer laser structuring was presented by

Morilla [Mor08], who combined the ablation and laser doping in one step. She

compared the change in the R sheet of previously-diffused wafers after a laser treatment

with or without an additional spin-on doping source (like P 2 O 5 ). Working with a

Nd:YVO4 laser (= 1064 nm). Her results show that the implementation of an extra

dopant on top of the surface leads to a more pronounced change in the sheet resistance

than without it. The r sh of the samples was reduced, from 110 /sq (after diffusion) to

80-60 /sq without extra doping, and to 25-30 /sq with an extra dopant [Mor08].

Another possibility to combine the laser ablation, grooving of the Si, and laser

doping, is based on the use of a liquid-jet-guided laser. A pattern is created on the

front-side, while doping the surface at the same time with the solution contained in the

liquid jet (see figure 4-16) [Kra08]. For n-type emitters, phosphoric or phosphorous

acid can be used as precursors in the jet.

Figure 4-16 Principle of LCP selective

emitter fabrication. The opening of the SiN x

layer and local high doping is performed in

the first step. The deposition of a seed layer

and subsequent plating then follow to

complete the metallization [Kra08]

Figure 4-17 Top SEM view of a groove formed

by LCP after nickel plating [Hop07]

The characterization of this process was presented by Hopman [Hop07]. A SEM

view of a groove formed by this technology can be seen in figure 4-17. The groove

depth can be defined according to the laser parameters. It is then possible to structure


Laser-chemical Processing 73

the nitride and create a local surface doping without requiring the formation of a deep

groove.

Numerical simulations to understand the principle of the laser induced

phosphorus diffusion through LCP have been performed by Fell [Fel08]. The P

diffusion in the silicon during the laser treatment corresponds to the liquid diffusion

coefficient, which is independent of the temperature. The P liquid diffusion coefficient

in silicon is ~ 10 -4 cm 2 /s. This is at least 10 orders of magnitude higher than the

coefficients corresponding to solid-state diffusion (even when compared to a solidstate

diffusion at 1000°C). The pulse length determines the emitter depth.

The first solar cells made by LCP have already shown very good IV

characteristics, using evaporated contacts. Efficiencies up to 20.5% have been

achieved on solar cells featuring a 120 /sq emitter and an LFC rear-side, on 0.5 . cm

p-type Fz material. The laser process was applied using a frequency-doubled (532 nm)

Nd:YVO 4 laser with a ~10 ns pulse duration [Kra08].

Among the most interesting technological aspects for this structuring technology

we can cite:

A resolution around 50 µm.

No mechanical pressure is applied to the wafers during the process (contactless)

The increase of the junction depth guarantees a more robust nickelsilicidation

process. The homogeneity of the diffused layer on the edges of

the groove is still an open question. In case of evaporated contacts, there is no

direct contact of the metal to the silicon on the edge area. A nickel-plating

metallization includes the deposition over the complete dielectric-free area.

Therefore, the formation of a homogeneous doping layer is a requirement.

During this work, the maximal laser beam velocity has been limited by the

experimental setup. With further tool improvements, a velocity of 1 m/s is feasible.

Solar cells manufactured by combining this technology with a nickel plating

based metallization can be found in the work of Jonas Bartsch [Bar11].


74 Patterning dielectrics for the front-side metallization

4.3.5 Printing an etching paste

Pastes containing phosphoric acid or its derivatives have also been developed by

companies like Merck for the selective removal of SiN x and SiO 2 layers. After

printing, they are annealed on a hot-plate or in an IR belt furnace at ~350°C during

90 s. The etching of the ARC occurs during the thermal treatment. The paste stripping

can be performed in an 0.1 wt% KOH-based ultrasonic bath, at 40°C, for 90 s.

This process has been implemented by Bähr in combination with a further

screen-printed metallization using Ag pastes with only little or no glass frit [Bae07].

Book has also applied the screen-printed dielectric-etching paste process for the

formation of a SiN x mask for selective emitter diffusion. The process was followed by

a metallization based on screen-printing, achieving an increase of 0.5% absolute in

efficiency (best of 18.1%) and 0.4% abs. for the current density on 125 mm

semi-square cells [Boo08].

The minimal line width achieved with this technology so far is 85 and 50 µm on

textured and non-textured surfaces respectively.

Only a limited experiment was performed to evaluate this technology at the

Fraunhofer ISE, observing that complete removal of the etching paste required more

optimization than time allowed.

4.3.6 Laser doping from a liquid source

Another embodiment of the combination of laser grooving and laser doping

consists of laser doping simultaneously with laser dielectric etching/ablation while the

sample is immersed in a static fluid, or coated with a spin coated doping ink. The

difference between making grooves or not, is mainly based on the energy applied by

the laser.

It’s important to consider that a SiN x layer can be etched by hot phosphoric acid

H 3 PO 4 (reaction occuring at 155°C). Using a solution that contains a phosphorus

source enables the simultaneous chemical etching of the dielectric, combined with the

doping of the silicon substrate. In other words, the laser irradiation, which acts as a

source of energy and heat enables the thermal ablation of the dielectric, it can also act

as a thermal catalyzing agent for the chemical etching reaction, and a heat source for

the drive-in of the dopants.

In the latest experiments performed by the UNSW in Australia the use of such a

technology, called LDSE, has achieved the manufacturing of solar cells over 20%

efficiency [Wan12].

Other groups, like the IPE in Stuttgart, Germany, are trying to include phosphor

particles in the SiN x layer in order to achieve the ablation and the doping

simultaneously.


Masked PVD deposition 75

Following aspects are relevant and in some cases might be challenging for the

application of approach:

Achieve an excellent control over potential contamination sources in the

doping/etching solution, in other words, need for very clean solutions.

An excellent control of the laser process is required, to achieve the formation

of a new junction (which will be placed underneath the metal contacts)

without creating too much surface damage, and achieving the required emitter

profile.

Potential increase of emitter recombination depending on the laser parameters

used and the characteristics of the new junction

The advantages of this technique:

Local selective removal of the dielectric.

Low consumption of sacrificial material

Potential selective formation of a deeper junction, very compatible with

nickel plating

4.4 Structured ARC deposition

A bottom-up approach also tried during this thesis to fabricate patterned

dielectric layers is the masked PVD deposition. The main advantage of this technique

is that no additional structuring steps are required.

4.4.1 Masked PVD deposition

Preliminary experiments were

performed using a single-line mask

made of steel. The mask was placed

on top of the wafers to locally

protect the surface during the

sputtering of a silicon nitride coating.

Damage etched Cz material were

used during these tests. Figure 4-18

shows an optical microscope image

of such a sample after nickel plating.

The narrowest line obtained in our

case were ~50 µm. The metal is only

deposited on the areas where the

metal ions can build a direct contact

with the silicon. Figure 4-18 shows

Sputtered

Nitride

Figure 4-18 microscope view of a line structured

by implementing a deposition mask during silicon

nitride sputtering

an irregular nitride deposition along a width of ~100-150 µm, on the areas next to the

coil.

Ni


76 Patterning dielectrics for the front-side metallization

A process enabling the structured deposition of the dielectrics would eliminate all

the requirements for structuring steps. Nevertheless, the nitride deposition is usually

performed at temperatures ranging from 300-500 o C, so a good control of the potential

masks would be required. For this short test a metal coil was applied.

The potential contamination of the surface underneath the masked area during

sputtering or its consequences in the solar cell electrical behavior are vital issues that

require a deeper evaluation.

4.4.2 Spray coating dielectrics

Another option for the selective formation of dielectric layers is based on spray

coating. This has been already achieved with TiO layers. This alternative has not been

considered within the scope of this work, however more information on this topic can

be found on a paper by Cotter [Cot00]

4.5 Discussion

Each of the sequences evaluated here (see figure 4-1) have assets and

disadvantages. An overview of the most interesting features from the most relevant

technologies for this thesis is presented in table 4-2.

Table 4-2 Comparison of technological advantages for various standard and innovative

technologies for the structuring of ARC followed by nickel plating (=yes/good, = no/bad)

Mechanical

pressure

Local Increase

of emitter

depth

Process

duration

PL

Screenprintin

g

inkjet

Printing

Grooving

(Laser or

Mechanica

l)

Laser

ablation

LCP

Selective

etching

Laser

doping

Structured

PVD

deposition

- /

/

0 ? ?

Robust 0 ?

High

throughput

Possible bulk

damage

? ?


Cost ? ? ?

Potential use

for rear

structuring


The determination of the most interesting technology for the development of

front side contacts is still an open question. The robustness of technologies like screenprinting

or inkjet printing makes them more reliable from the high throughput point of


Spray coating dielectrics 77

view required by the PV industry. The mechanical pressure applied during screen

printing is an undesired companion for its application on thin substrates, making inkjet

printing thus more interesting between the two. The main disadvantage for the inkjet

printing process is the need for both a sacrificial layer and the regular cleaning of the

nozzles to avoid clogging.

Taking into account this points and in the search towards simple processes with

less number of steps laser ablation presents itself as an excellent option. With it, there

is no need for sacrificial layers, and the selective removal can be applied. The

applicability of this technology will most likely depend on the throughput.

4.6 Conclusions

In solar cell fabrication, the compatibility of the manufacturing steps for the

different parts of the device is not always a given. Defining the optimal manufacturing

route requires the implementation of specific process combinations for specific

efficiency goals and price constraints.

A pattern-transfer process using an inkjet printed mask for the patterning of

front-side SiNx was developed. First, an evaluation of the chemical etching of PECVD

SiN x layers in HF was performed. Then, the combination of this step with proper

etch-mask formation by inkjet printing was developed and optimized to achieve a

standard opening resolution ~ 50 µm width, though a minimum of ~ 20µm was

achieved.

As an alternative a direct writing method was analyzed. It consists in the ablation

of the coatings with different laser sources. Depending on the choice of the laser

wavelength and pulse length, a strong change in the emitter profile was observed.

These changes have a strong effect in the subsequent metallization process. The

application of more sophisticated concepts like LCP was also presented.

The strategy applied in this thesis is to evaluate industrial-oriented technologies,

starting from concepts which already allow a high-efficiency performance. The

developments presented in this chapter enable the substitution of expensive steps for

the patterning of dielectrics with industry-relevant processes, mainly inkjet printing

and laser ablation.

Most of the technologies presented here are contact-less, and therefore

compatible with the processing of very thin substrates. In addition, they allow fine line

formation. They can also be implemented for the formation of structures on the rearside

in alternative cell concepts.


5 Nickel plated contacts

In this chapter, we present the basics of electroless nickel deposition. Then,

we evaluate the impact of different plating parameters, like pH and temperature

on the properties of the deposited nickel layers, as well as the influence of the

substrate on the deposition.

Nickel and its silicides are an excellent alternative for the formation of electrical

contacts on silicon wafers. The selective deposition of Ni coatings from chemical

solutions represents a fast, low-cost and low-temperature metallization technology for

the PV industry, enabling large-scale, high-quality metallization concepts [Col80,

Gre78, And80, Dub90, Wen86]. The anti-reflection coating is used as a plating mask.

After the deposition, a thermal step is applied to form a metal silicide. The nickel

silicide delivers improved mechanical and electrical properties. A very low specific

contact resistance of 35 µ cm 2 has been reported for Si/Ni/Cu contacts formed by

electroless plating on silicon solar cells [Lee02]. The minimal resolution is determined

by the structuring process, which is realized prior to the metal deposition on the ARC.

As mentioned in the previous chapter, there are several alternatives to perform this

step, e.g. features with a resolution as low as 10 µm are possible by laser processing.

One of the main challenges for the application of nickel plating in the silicon PV

industry is related to the high risk of shunt formation due to the combination of a fast

metal diffusion during the silicidation step and the desire to apply high temp steps for

the improvement of the mechanical adhesion. Therefore, to this date, this technology

had been mostly applied on deep selective emitters formed underneath the metal

contacts [Weh86, McCa06]. However, the additional doping step required to form

selective emitters was increasing the process complexity considerably. Alternatives to

simplify the selective emitter formation are being currently studied by other groups

[Tja07, Kra08, Liv09]. An additional challenge lies in the high control required for the

stability of the bath chemistry.

Consequently, the goal of this and the following chapter is to prove that good

control over the Ni-electroless deposition allows a relaxation of the specifications in

the emitter design. In this chapter the focus is the evaluation of the technological

aspects related to the formation of thin electroless plated nickel coatings on silicon

wafers. In chapter 6, this step is integrated to the manufacturing sequence of industrial

and high-efficiency silicon solar cells and further plating options are evaluated.

Section 5.1 presents a literature overview for nickel plated contacts in solar cells

applications. In section 5.2 the current understanding of the plating principles,

concentrating on silicon surfaces is presented. Section 5.3 is related to the process


80 Nickel plated contacts

development and evaluation of the different issues which affect the formation of a

nickel coating on silicon; like plating parameters, substrate conditioning or surface

activation.

5.1 History of electroless nickel on silicon solar cells

The electroless deposition of nickel was first observed by Wurtz in 1844, with

the first patent being filed in 1916 by Roux for the plating of nickel on aluminum

surfaces [Rou16]. The stability of these solutions was very poor, leading to metal

deposition on any surface in contact with the electrolyte [Mal90]. Thanks to the work

of Brenner and Riddell, more reliable baths were developed in 1946 [Bre46]. Ever

since, a lot of effort has been undertaken to characterize and improve the quality of the

nickel depositions, mostly for applications in the automotive industry. A good

introduction to this process is provided by Riedel [Rie91].

Sullivan and Eigler were the firsts to use electroless plating for the contact

formation on silicon wafers in 1957 [Sul57]. By the end of the 70’s, this technology

gained interest for the metallization of terrestrial silicon solar cells. It was evaluated as

a diffusion barrier for copper-plated contacts [Col80, Tan80]; or combined with solder

coating [Col78, Col80, And80]; both cases including an activation step with PdCl 2 .

Patel and Gonsiorawski filed a patent in 1979 claiming a process that did not require

surface activation, thanks to the introduction of fluoride compounds in the plating

solution [Pat82]. All the solutions used until then, were heated to a certain

temperature (~80 - 90 o C) to enable the deposition. Saha presented a different approach

in 1979. By then, Saha showed that the selective nickel deposition was possible on

polished Si surfaces keeping the solution at room temperature, while illuminating the

substrate with a light source through a mask [Sah79].

The high potential of plating for high-efficiency silicon solar cells was first

demonstrated in combination with the laser-buried contacts by the group of Green and

Wenham, as described in the section 3.3.1 [Gre84, Zha91]. A simplified version of the

process was presented by Cotter, with a sequence consisting of a single diffusion step

for the emitter formation. His approach included the selective deposition of the

dielectric layer by spray coating of TiO 2 [Cot00].

Table 1-1 summarizes remarkable results found in the literature for various

metallization concepts based on nickel plating.


History of electroless nickel on silicon solar cells 81

Table 5-1 Literature survey of cells fabricated using electroless nickel plating until 2009

Pattern

ARC

Base

Material

Size V oc j sc FF

[cm 2 ] [mV] [mA/cm 2 ] [%] [%]

Note

Ref.

PL

PL

p-Si,

1.5cm,

p-Si,


2”)


3”)

587 28.3 79 12.9

Spin-on doping

rear. Eval.

Activation step.

[Coc84]

591 31.4 79.8 14.9 Pd/Ni/Sn system [Col78]

PL FZ-Si 4 664 38.1 79.8 20.2 PERC, 2

diffusions

PL Fz, 0.5 cm ~45 692 37.8 81.95 21.4 PERC, 2

diffusions

SBC

[Lee02]

[Kim05]

Cz 1 cm

50 cm 613 34.8 80.1 17.1 No front texture [Cot00]

LBC FZ-Si 12 686 38.0 81.6 21.3 PERL [Zha91]

LBC

Double

sided BC

MBC

Cz-Si

FZ-Si

n-Si, Cz

0.3-10 cm

Fz-Si,

0.5-1 cm

147

140


2”)

625

660

36.3

37.3

80.6

79.2

18.3

19.2

663 32.2 77.9 16.7

Industry.

Sputtered rear-

Al

Evaluation of

the activation

step.

[Bru03]

[Guo05]

4 622 37.1 80.0 18.5 Evap. Al [Yuw97]

MBC mc polix 144 633 35.85 77.7 17.6 [Joo03]

MBC mc ~138 636 36.9 77.0 18.5 [MCa06]

SLD

SLD

Laser

Ablation

p-Si

Cz 1 cm

633 37.2 75.1 17.7

n-Si, Cz

2.5Wcm 156 642 35.9 79.1 18.2

p-Si, EFG,

2.5 cm

50 594 32.5 76.9 14.85

ARC stack front

Al-BSF back

Al Rearjunction,

Front Surface

Field: 150 /sq

Diffusion of the

emitter during

ablation

[Tja07]

[Mai09]

[Dub90]

PL: photolithography, LBC: Laser Buried Contacts, MBC: Mechanical Buried Contacts, SBC: Single

Buried Contacts, SLD: Selective Laser Doping


82 Nickel plated contacts

5.2 Fundamentals of nickel plating

In this section, the basic electrochemical principles contributing to an

understanding of electroless plating processes are presented. A model, describing the

electron transfer occurring during the nickel deposition is provided. Finally, the theory

currently accepted for silicon-metal electrochemistry and some of the models for

electroless nickel deposition on silicon wafers are portrayed.

5.2.1 Plating definitions

Plating consists on the formation of a metal coating on a substrate by an

exchange of electrons which occurs through the immersion of the substrate in a

chemical solution. This can be performed by electrolytic, electroless, immersion or

contact plating.

In Electrolytic plating, typically called electroplating, the electrons are provided

by an external source, like a battery (see figure 5-1). At least two reactions occur

during an electrolytic deposition:

M z

R

ze

R



n ( nz)


0

M

ze


Cathodic reaction: reduction

Anodic reaction: oxidation

where M is the metal being reduced and R the reduced species, being oxidized.

Electroless plating is also called chemical or autocatalytic plating. In this case,

the electrons required for the reduction of the metal salt are provided by a reducing

agent. This occurs in an aqueous solution without the need for any external electron

source and it continues until the substrate is extracted from the solution or the required

reactants are depleted. Electroless reactions are typically described by the mixed

potential theory, using electrochemical reactions to model the deposition by a cathodic

partial reaction for the metal reduction and an anodic partial reaction for the reducing

agent in the solution. Both reactions occur at the same electrode, which is the catalytic

surface [Sch10].

The use of electroless plating solutions for the front-side metallization of silicon

solar cells is the main object of study of this and the following chapter.

In the Displacement or immersion plating the substrate is the electron source for

the reduction of the metallic ions from the solution. After the substrate is coated with a

thin metal layer the process stops. For this reason, only a very thin metal layer can be

formed by displacement [Sch10, Car07].

Another important definition within plating processes for noble metals is the

process called contact plating: the electrochemical potential of the noble material is


84 Nickel plated contacts

protection, magnetism, soldering and electrical conductivity. The layers deposited

from hypophosphite-based solutions always contain some percentage of phosphorus,

for this reason they are designated as NiP layers. The concentration of Phosphorous in

NiP layers depends on the pH of the solution [Rie91]. Alkaline baths create coatings

with a low P content (between 3-5%), while acidic baths show a P concentration from

10-15% in the NiP layer [Rie91]. Plating baths with reducing agents based on boron,

like borohydrides and amine boranes or hydrazine also exist. The boron-based

solutions deliver boron-containing coatings [Mon05], and the hydrazine baths are not

very stable, so they are not very common in the market [Mal90].

Extensive literature can be found on plating chemistry; particularly interesting

for electroless nickel, are the books by Pearlstein [Pea74], Riedel [Rie91], Mallory

[Mal90] and Kanani [Kan07].

Table 5-2 Main components of electroless nickel baths and their function [Rie91, Oki90, Kan07]

Component Function Example

Metal salt Source of metal ions Nickel chloride: NiCl 2

Nickel sulphate: NiSO 4

Nickel acetate: (CH 3 CO 2 )2Ni.4H 2 O

Reducing

agent

pH

regulator

Complexing

agent

accelerators

Source of electrons: required for the

metal reduction

For pH adjustment

Form Ni complexes which prevent

excess Ni ion concentration in the

solution.

Form the nickel complex, stabilizing

the solution thus preventing its

precipitation

Sometimes act also as pH buffers

Sodium hypophosphite: NaH 2 PO 2

Sodium borohydride: NaBH 4

Dimethylamine borane (DMBA):

(CH 3 )2NH:BH 3

Hydrazine: H 2 N-NH 2

Sulphuric acid and hydrochloric acids,

soda, caustic soda, ammonia

Monocarboxylic acids

Dicarboxylic acids

Hydroxycarboxylic acids

Ammonia

Alkanolamines

Fluorides, tartrate,

Other additives: accelerators (activate the action of the reducing agent-> act in the opposite of

the complexing agents), stabilizers (mask potential active seeds in the solution, to avoid the

decomposition of the solution), buffers and wetting agents (to improve the wetability of the

surface to be plated)


Fundamentals of nickel plating 85

Models for electroless nickel plating

Hypophosphite plating solutions were used to manufacture most of the cells

presented in this thesis. For this reason, the models presented in this section are

specific for such solutions. More information on chemical models for this or

alternative reducing agents can be found elsewhere [Mal90, Kan07].

Pourbaix diagrams show the most abundant ionic states of a material dissolved in

water at room temperature as a function of the pH and the applied potential. This

information helps to model the chemical reaction, taking into account that the ionic

states with a higher concentration are more likely to participate in the process. Such

diagrams can also be used to learn about the potential required to oxidize or reduce

ions of a given material at room temperature. The superposition of the Pourbaix

diagrams of Ni and hypophosphite is presented on figure 5-2. For acidic solutions, a

potential of -0.25 V is required to reduce Ni 2+ to nickel; while in alkaline solutions the

potential required to reduce the Ni(OH) 2 drops linearly with increasing pH down

to -0.6 V at pH 14.

Figure 5-2 Pourbaix-diagram Nickel-Phosphor [Bar81]

When no external voltage is applied, the most abundant phases of nickel and

hypophosphite existing in acidic solutions: are Ni 2+ and Ni 0 -

, H 2 PO 2 and H 2 PO - 3 ;

respectively. For alkaline solutions it is more likely to find Ni(OH) 2 , Ni 0 , H 2 PO - 2 and

HPO 2- 3 [Kan04]. The model to explain electroless nickel deposition should consider

two different reactions for the metal deposition: one for acidic solutions which

includes the reduction of Ni 2+ ions and one for alkaline solutions, which includes the

reduction of Ni(OH) 2 ions.


86 Nickel plated contacts

Actually, there are four models to describe the electroless process:

1. The first was proposed by Brenner and Riddell. It is based on the reduction of

the metal by atomic hydrogen [Bre46].

2. Hersch suggested a reaction based on hydride ions (H - ), which was modified

by Lukes in 1964 [Pea74]. The presence of H - ions in alkaline solutions is very

unlikely, making this model inconclusive for such solutions [Djo02].

3. Brenner and Riddell also proposed an electrochemical mechanism, based on

the idea that the electroless deposition is the result of simultaneous cathodic and

anodic reactions, leading to a combined mixed potential [Mal90]. This model has been

accepted by many researchers. The corresponding formula to describe the reactions is:

Eq. 5-1

Nevertheless, it does not explain the reduction of metal hydroxides (common for

alkaline solutions) [Mal90]. Paunovic has presented a mixed potential model adapted

for the reactions on alkaline media [Sch10].

4. Cavalotti and Salvago proposed a model based on the coordination of OH - ions

with nickel, which can also be applied to alkaline solutions [Mal90, Djo02, Mee80].

Our ability to predict and control the deposition process is limited to experience

and empirical developments. This is due to the lack of accurate understanding of the

details of the reaction kinetics. One of the most important factors which affects the

deposition and it is not explained by any of the formulas, is the meaning of the

catalytic nature of the surface.

Besides, the use of additives, which is common practice in the processing of

electroless solutions, complicates the modeling even further. For example “the

chemical properties of the ions are altered when combined with a complexing agent”

[Mal90]. The addition of ammonia to alkaline solutions introduces a solvation shell

around the nickel ions, which is formed by amine complexes, instead of water (see

figure 5-3). The color of the solution and the pH is affected by the complexation of the

nickel ions (see figure 5-4, the pH was changed from 6 to 9).

OH 2

H 2

O OH 2

Ni

H 2

O OH 2

+2 +2

NH 3

+ 6NH 3

=

H 3

N NH 3

Ni

H 3

N NH 3

+ 6H 2

O

OH 2

Green

Figure 5-3 Schematic representation of the formation of the blue ammonium complex through

the displacement of the 6 water molecules by ammonia [Mal90]

NH 3

Blue


Fundamentals of nickel plating 87

Figure 5-4 Hypophosphite based nickel solutions with increasing pH by addition of

ammonia. green solution to the left (pH 6) turning blue (middle) as the pH increases to 9

(right) where the solution turns dark blue.

This thesis does not focus on the development of a theoretical model for the

electroless deposition or even a settlement between the already existing ones. It has

been said in the literature that “a universal mechanism for electroless metal

deposition is not feasible [...] each model fails to account for some experimentally

observed characteristic of the plating reaction” [Mal90].

An in depth explanation of the models mentioned here can be found in the

literature [Pea74, Mal09, Mee80, Sch10]

5.2.3 Introduction to silicon electrochemistry

Electrochemical model for charge transfer

Libby and Randles observed in 1952 that in RedOx reactions the Franck-Condon

principle must be applied to the description of the electron transfer step. This principle

states that the electronic transitions are essentially instantaneous compared with the

time-scale of nuclear motions. Applied to electrochemistry, this means that the

electron transfer process is faster than the movement of ligands or the movement of the

solvation shell. [Ger90]. Thus, the charge transfer between an electrode and the

solution occurs without a change in the electron’s energy. Marcus developed a theory

stating that “the reorganization of the solvation shell is the rate-determining process

for bringing the redox species to a stage in which electron transfer can occur at a

constant energy for all the species involved” [Ger90].

If the electrons are transferred to the electrode without a change of energy levels, and

the solution provides them with a specific energy level, the electrode should provide

electrons (or empty energy states) at the level required by the reactants for the process

to happen.


88 Nickel plated contacts

Application to silicon electrodes

According to a model introduced by Gerischer for charge transfer processes

between semiconductors and electrolytes; the application of the Marcus principle to

electrode reactions means that only if the redox potential is close to one of the band

edges, then a significant current exchange can take place. If the redox potential lies in

the bandgap of the semiconductor only a small amount of current can be exchanged

[Ger90].

The situation as described by Gerischer, where electron transfer occurs through

the band edges of the semiconductor is often improbable [Zha01]. Often the surface

states play an important role in the charge transfer [Mor80]. On figure 5-5 we see the

three paths for electron transfer as represented by Oskam for the deposition of metals

on silicon, taking into account Gerischer’s model and the transfer through surface

states [Osk98]:

1) Electron transfer from the conduction band to the acceptor in the electrolyte

2) Surface states (with the concentration of surface states depending on the pH

for aqueous electrolytes)

3) Hole transfer through the valence band.

Figure 5-5 Three different mechanisms for the metal deposition on an n-type semiconductor

according to Gerischer’s model. (a) the metal/metal ion redox couple has enough energy for the

acceptor level (oxidized species) to overlap with the conduction band of the semiconductor, (b)

Transfer of electrons from surface states, (c) injection of holes into the valence band from a

metal/metal ion couple with sufficienty positive equilibrium potential [Osk98]

5.2.4 Models for electroless plating on silicon

Different groups have investigated the deposition of nickel on silicon from

chemical solutions, not only for its use in PV but also for other microelectronics

applications. The findings as well as the opinions and consensus within the scientific

community have differed over time:


Fundamentals of nickel plating 89

Ricaud claimed that plating nickel onto silicon from an acidic hypophosphite

containing bath was not possible, or that using strong alkaline solutions would lead to

the etching of the silicon [Ric85].

Takano tried to develop a model for the electroless nickel deposition on silicon.

His experiments included the removal of the reducing agent from the solution, making

the evaluation more about immersion plating than electroless plating. In any case, he

observed that a thin SiO 2 layer was formed between the silicon and the deposited Ni

layer on the FE-TEM measurements made on such samples (see figure 5-6) [Tak99].

Figure 5-6 Cross sectional FE-TEM image of a nickel dot plated with a solution without a

reducing agent for the fabrication of Ni dots on silicon [Tak99]

Three plausible models for the nickel reduction on Si substrates as suggested by

Takano are shown in figure 5-7. In the first case, the nickel is deposited by displacing

the H atoms which are passivating the silicon surface. In the second, there is a

photochemical reaction, which leads to the excitation of an electron from the valence

band to the conduction band. This electron becomes then available to reduce the nickel

ions in the solution. In the third case, the silicon is oxidized as the nickel is deposited.

Taking into account his observations of a SiO 2 layer at the Si/Ni interface, the third

mechanism is most likely the most reliable.

a. b. c.

Figure 5-7 alternative models for the nickel reduction on silicon substrates without a

reducing agent, as presented by Takano: (a) displacement of surface H-terminated Si surface,

(b) photochemical reaction, corresponding to the removal of the hypophosphite from the

solution (c) anodic oxidation of silicon and cathodic reduction of nickel ion. [Tak99]


90 Nickel plated contacts

Even though Takano’s results for electroless solutions were inconclusive

[Tak99], the possibility of a competing reaction of displacement plating is in place

while plating with reducing agents. This would result in the partial oxidation of the

hypophosphite and the partial oxidation of the silicon, at least at the beginning of the

process.

An interesting observation by Coleman in 1978 states that alkaline plating baths

could attack the silicon wafers, postulating that during the deposition a thin oxide layer

was formed during the deposition on bare silicon [Col78]. This supports the idea that

silicon can also be oxidized during the process.

Limitations of the theory

Measurement of the Fermi level of the Standard Hydrogen Electrode

As redox potentials are typically measured vs. the Standard Hydrogen Electrode,

a comparison between energy levels in semiconductors and redox potentials requires

the determination of the absolute energy for the reference electrode: the Standard

Hydrogen Electrode (SHE). Gerischer states that “the connection between the two

scales is given by the work function for the removal of an electron from the Fermi

level of the reference electrode to the vacuum level” [Ger90].

Table 5-3 presents a list of values obtained by different groups for the location of

the SHE compared to vacuum. Even though there is no final agreement about these

findings, Lohmann’s estimation of -4.5 eV has found the widest application in

electrochemical methods [Mem00]. The redox Fermi level of a redox couple is

calculated as follows:

E

Re dox_

abs

Eref

qU

redox

4.5eV

qU

red

4.

5eV

Ered

[Mem00] [Eq. 5.6]

Table 5-3 Different values for the absolute energy of the standard reference electrode [Mem00]

Reference

E ref [eV]

Lohmann (1967) -4.5

Trasatti -4.31

Gomer (1977) -4.73

Kötz (1986) -4.81

A difference of -0.3 eV between the value calculated by Lohmann for the

absolute position of the SHE versus the one calculated by Kötz is already higher than

the energy required to reduce nickel ions from an acidic solution (-0.25 eV) and close

to the value required for alkaline solutions (-0.49 eV). In other words, the

determination of the position of the energy levels of the substrate has higher degree of

uncertainty than the Redox Energy required by the electrolyte for the nickel reduction.


Fundamentals of nickel plating 91

Making a model for electron exchange with such a degree of uncertainty is very

difficult.

Surface interactions

When cations are adsorbed at the surface of an electrode, a counter charge is built

on the electrode, forming an electric field. This effect was first modeled by Helmholtz,

who observing the analogy with a capacitor defined an electric double layer, today also

called Helmholtz double layer at the liquid interface. In relatively concentrated

solutions, this Helmholtz layer or Helmholtz plane is the region of the electrolyte close

to the surface where the adsorbed ions create a linear electrostatic potential decay; it is

about ~10 -10 m thick [Zha01]. Instead, very dilute solutions with an ionic concentration

below 0.1 M, show an exponential potential decay over a wider region. Such solutions

are described by the Gouy-Chapman model [Men01]. In such cases we speak about a

diffuse region, called the diffuse layer, with a thickness about 10 -9 - 10 -6 m. Stern

combined both models into one, describing a linear potential drop next to the electrode

and an exponential fall with increasing distance. Grahame modified this model

introducing the concept of an inner Helmholtz plane formed by water dipoles and

specifically adsorbed ions followed by an outer shell consisting of the centered charges

of solvated ions. Bockris-Devanathan and Mueller (BDM) also refined Stern’s model,

considering that the interface is most likely formed by water molecules, neutral

atoms, dehydrated (or stripped) ions and even some ions with the same charge as the

electrode. Figure 5-8 schematically presents Stern’s model as refined by BDM

[Zhan01, Men01].

Figure 5-8: (a) Representation of the ionic distribution of an electric double layer formed on

an electrode-electrolyte interface according to the BDM model. (b) Corresponding potential

distribution in the metallic electrode and the electrolyte. [Men01]


92 Nickel plated contacts

Some semiconductors in contact with an aqueous electrolyte show stronger

interactions with OH - and H + ions existing in the solution than with the metal ions

involved. Silicon is such an example. The adsorption of H + /OH - ions at the surface

causes pinning of the Fermi level to a level that depends on the pH of the solution

leading to a band bending at the edges of the semiconductor [Mem00].

On one hand, the electrochemical theory states that the energy level at the surface

of the electrode needs to be at the same level as the one required by the electrolyte. On

the other hand the electroless plating theory states that the Redox reactions for the

nickel and the hypophosphite occur simultaneously at the same electrode. These two

reactions occur at different energy levels. So, an electron exchange occurs on the

silicon surface simultaneously at different energy levels.

The impact of the electrolyte as well as the surface preparation treatment is very

strong on the location of the energy levels of the silicon surface [Zha01]. The details

elucidating the chemical reaction governing the electroless process are still unknown.

Therefore, at the time of this work, no specific model detailing the charge transfer for

the electroless plating of nickel on silicon could be used.


Process development 93

5.3 Process development

The first important aspect related to the development of a front metallization

based on nickel plating is the choice of the right chemistry. A throughout evaluation of

the reactants for the optimal chemical mix of the solutions goes beyond the scope of

this thesis. Such evaluations started in 1957 when Sullivan managed to plate silicon

wafers with nickel for the first time. They have been ongoing ever since, both within

chemicals manufacturers and in other research groups [Sul57, Pea74, Mal90, Sch10].

Typically, the stability of the industrial solutions is the result of years of

empirical development. Some industrial manufacturers of these solutions in

alphabetical order are Atotech, Enthone (Cookson electronics), MacDermid, Rohm &

Haas, Technic Inc. and Transene.

Only some of these companies have developed special products for the

photovoltaic market. In this thesis we chose different commercial solutions for the

evaluation of the electroless process: Heliofab 200 from Enthone; SMT 88, PM988,

PM 980, and 468 from Rohm and Haas, Slotonip 30-3 from Schloetter; Techni Nickel

NiPX309 and TechniE Nickel AT 500 from Technic, and finally the ammonia and the

ammonia free baths from Transene.

In this section, technical results relevant to the development of the deposition

process are presented. First, the influence of the plating parameters in the coating

formation is evaluated. Then, an investigation of the right substrate conditioning is

performed. In the following chapter these results are used to develop a metallization

process for the front-side of silicon solar cells.

Process description and experimental setup

A nickel coating formed by electroless plating typically consists of small round

metal clusters, as presented in figure 5-9. The autocatalytic activity of this metal

allows the formation of thick coatings by increasing the plating time.

Figure 5-10 shows a scheme of the experimental setup used for these

experiments. A total of ~ 0.5 liters of plating solution heated with a water bath was

used during the evaluation of the plating parameters. Direct contact between the

beaker containing the plating solution and the hotplate was avoided by introducing

small quartz tubes in the water bath. Heating was provided by the heater/stirring plate.

The plating solution was exchanged after each experiment. Temperature and pH were

constantly monitored and controlled during the process.

The first observation was that the recommended operating conditions for

electroless plating on metal substrates usually did not deliver a good coating on silicon

surfaces: either there was no metal deposition, or the layers could be detached from the

surface.


94 Nickel plated contacts

Figure 5-9 Front-view of an electroless nickel

coating formed from a hypophosphite based

solution on top of a silicon solar cell at a high

bath temperature and high pH

Figure 5-10 schema of the experimental setup

used during the evaluation of the process

parameters.

5.3.1 Plating parameters

Table 5-4 summarizes the effect typically observed by the variation of different

deposition parameters during the electroless deposition on metal substrates. One of the

goals of this section is to evaluate the relevance of these effects on silicon electrodes

and to assess to which extent they could have an impact in the seed layer formation

and cell processing.

Table 5-4 Standard effect of plating parameters on the nickel deposition [Kan04]

Deposition Parameter

Bath temperature

pH

Deposition potential

Filtration

Work movement, bath

agitations

Influenced properties

deposition rate, bath stability, adhesion, appearance of deposit

deposition rate, initiation, stability, composition and other properties

deposition rate and initiation

bath stability and surface properties of deposit

structure, appearance and uniformity of the deposit

Plating time

For a first estimation of the plating speed of an electroless solution, solar cells

featuring a highly doped emitter (R sheet = 20 /sq) were used. The rear side of the

wafers is passivated by a 105 nm SiO 2 layer and coated with a 2 m evaporated Al

layer. The Al is contacted to the silicon by LFC [Sch02]. After a short dip in 1% HF

for the removal of the native oxide, the wafers are plated in an alkaline solution

(Niposit PM980). The solution was heated to different temperatures (from 40 o C up to

70 o C ±2 o C) by a water bath. The pH of the solution is manually controlled during by

addition of ammonia during the plating process. The pH value during plating is


Process development 95

between 9 and 9.05. The temperature control is done manually. All the wafers are

plated simultaneously.

The plating thickness is calculated by the weight difference of full-surface plated

samples (1 cm 2 ) before and after plating. The approximation used for the calculation is

that the density of the plated nickel equals the pure nickel density (8.91 mg/cm 3 ).

Nickel thickness [nm]

700

600

500

400

300

200

100

0

Temp.

70 o C

60 o C

2 4 6 8 10 12

Time [min]

Figure 5-11 Nickel thickness vs. time for an

alkaline bath pH 9 at 70 o C and at 60 o C

Deposition rate [nm/min]

50

40

30

20

10

0

40 50 60 70

Temperature [ o C]

Figure 5-12 Deposition rate vs. temperature

for an alkaline nickel plating solution at

pH 9

For the plating conditions evaluated during this experiment, the plating process

starts faster at higher temperatures and after the first metal deposition it continues

linearly with time. At 40 o C it takes 5 min before the deposition starts, 3 min at 60 o C

and only 30 s at 70 o C. In other words, the increase in temperature contributes to a

faster activation of the process. Figure 5-11 shows an example of plated thickness vs.

time at 70 o C and 60 o C. It is important to consider that the determination of the plated

thickness by measuring the weight of the samples (even for full surface plating)

always include a potential error which is higher for thin metal coatings. Figure 5-12

shows the exponential behavior of the deposition rate vs. temperature, using the data

from figure 5-11.

Depending on the mechanism controlling the plating process a different fit can be

done over the thickness vs. time data:

If process kinetics is controlled by the chemical reduction of the metal, a linear

increase of the thickness is observed with increasing time:

Thickness a

lin

t


96 Nickel plated contacts

If the process kinetics is controlled by the diffusion of the reactants to the

surface, then, according to Fick and Faraday’s laws, the coating grows as:

Thickness a

diff

t

where a lin and a diff are constants.

There is information in the literature for plating baths being diffusion-controlled,

as well as reaction-controlled [Knt06]. From the observations from this experiment,

there is a strong linear behavior at 70°C, meaning that the process at that temperature

is controlled by the reduction of the metal, while at 60°C this trend is less obvious. In

any case, the metal deposition rates have been estimated for each temperature and are

shown on figure 5-12.

A longer immersion results in the thickening of the plated layer. Depending on

the plating solution, increasing the plating time results in the peeling-off of the metal.

Bath pH

Acidic solutions require higher processing temperatures but are usually easier to

handle than alkaline solutions. Meanwhile, alkaline solutions are more active, forming

coatings even at lower temperatures, but usually show a lower bath lifetime [Rie91].

Figure 5-13 shows exemplary nickel layers formed from an alkaline (PM988)

and an acidic plating bath (SMT88), both from Rohm & Haas (now Dow) after 1 min

and 10 min of nickel plating. A closed Ni layer is formed already after 1 min in both

plating solutions.

Alkaline solutions

Alkaline baths are typically more reactive than acidic baths, so they can be used

at lower temperatures (starting at room temperature up to 70°C). There is a risk when

working with strong alkaline solutions at high temperatures: the silicon substrates

might be etched in these solutions. Processing with lower temperatures is also

desirable to avoid the high evaporation rates for the ammonia used in the solution to

control the pH. Very thin coatings of 80-150 nm can be formed using such alkaline

solutions (see figure 1-14).

The evaluation of the pH influence for a commercial alkaline solution was

performed on photolithography-structured samples with SiO 2 on the front side. The

rear-side of the samples was coated by an aluminum layer contacted to the base by

laser-fired contacts (LFC). The experiments performed on silicon wafers showed that

for alkaline solutions a minimum pH (above 8) is required to enable the metal

deposition [Ale07].

Another effect of the pH is observed in the size of the deposited metal clusters.

As the pH increases a higher homogeneity for the coatings is achieved [Gua04,

Mat06]. This effect can be applied for both alkaline and acidic solutions.


Process development 97

Acidic solutions

Coatings formed with these solutions are typically thicker than the alkaline

coatings, but also contain a higher percentage of phosphorus (see section 5.2.2). Acidic

baths are less reactive. Therefore, they require higher processing temperatures (around

70 - 90°C) to be effective. This introduces an undesired complication for industrial

manufacturing: the evaporation of the water contained in the solution needs to be

controlled.

Time 1 [min] 10 [min]

Acidic solution

Allkaline solution

Figure 5-13: SEM views for a silicon solar cell plated with nickel. Top: alkaline bath (Niposit

988, pH 9.2, 40 o C). Bottom: acidic plating solution (SMT 88 pH 4.6, 80 o C) after 1 or 10 min

plating time.

Figure 5-14 shows a detail in the cross section of plated substrates, indicating

that for the same plating time a thicker layer is formed with the acidic solution than

with the alkaline one. In this experiment, the deposition rates are: about 120 nm/min

for the acidic and 60 nm/min for alkaline solution, respectively. A minimal time of

about 10 to 15 s is required, in both cases before the reaction starts.


98 Nickel plated contacts

550 nm

Detached Ni finger

after breaking the

wafer

407 nm

Silicon

Figure 5-14 Cross section view of two different nickel coatings on top of a Si random

pyramid, underneath a Ag-LIP layer. (Left): alkaline solution (Niposit980 at 40 o C, pH 10, for

2 min). (Right): acidic solution (SMT88 pH 4.6, 85 o C, 2 min plating)

Temperature

According to Riedel the most important parameter affecting the deposition rate

for standard nickel plating deposits is the temperature [Rie91]. Experimentally, this

has been shown in figure 5-12.

The use of higher temperatures leads to an increase of the mobility of the ions in

a plating solution. The increase of the mobility facilitates the supply of ions close to

the interface, enabling the formation of a thicker deposit. Therefore, with an increase

in the temperature and the autocatalytic reaction taking place faster, the size of the

metal clusters formed on top of the silicon is bigger. This, results in the formation of a

thicker coating with bigger metal clusters (see figure 5-15). Another consequence of

the increase of the bath temperature is a higher instability in the solution. In other

words, baths working at higher temperatures can decompose more easily.

Figure 5-15 Scheme of the plated layer depending on the temperature of the plating solution.

The right combination of time, pH and temperature enables the formation of thin

metal coatings. A minimum time is required to get a fully covered surface, since the

activation does not occur all over the wafer simultaneously, but this time depends on

both pH and temperature of the solution. This effect can be seen on figure 5-14: the


Process development 99

places where the metal deposition started first have a thicker nickel coating, even when

the total layer is very thin (~80 nm)

Pretreatment / surface conditioning

The electroless deposition of nickel on silicon requires good surface

conditioning. The native SiO 2 layer is removed prior to the deposition in a 30 s dip in a

1% concentrated HF solution.

A PdCl 2 activation step can be applied after the removal of the native oxide,

enhancing adhesion and homogeneous plating, thanks to the creation of catalytic sites

on the substrate. Considering the driver of keeping costs as low as possible and

reducing the amount of equipment required in order to keep this metallization concept

as simple as possible, it was decided to avoid this type of pre-treatment in this work.

Surface conditioning: hydrophilic or hydrophobic?

The samples were dipped in an ethanol bath, between the HF dip and the nickel

plating, to evaluate the impact of the hydrophobicity of the surface on the plating

process. The Niposit PM 980 solution from Rohm and Haas (now Dow) was used as

the nickel plating solution in this experiment. The samples were plated at 38°C during

2 min at a pH of 10. Ammonia was added for pH control during plating. The samples

were evaluated with SEM after nickel plating.

It was observed that a dip in an ethanol solution between the native oxide

removal and the plating step results in the faster metal deposition with the formation of

a more adherent nickel coating. This observation was repeated on a mirror-polished

surface with the same result. This effect was also evaluated by Jai [Jai09]. He observed

that after the HF removal of the oxide the surface becomes hydrophobic, which is

detrimental for the wetting. The O-H bond in ethanol contributes to improve the

wetting of the surface.

Takano explained this behavior by stating that Nickel ions (Ni 2+ ) can approach

more easily a hydrophilic than a hydrophobic surface, thus enabling a faster reaction.

He applied different surface conditioning treatments, like ethanol or an HPM solution

(1 part 35% HCl, 1 part of 30% H 2 O 2 and five parts of H 2 O), observing that when the

surface was hydrophilic, higher quality Ni films were obtained [Tak99]. Niwa showed,

through X-ray photoelectron Spectroscopy (XPS) and electrochemical Open-Circuit

Potential (OCP) measurements, that the acceleration of the nickel reduction was due to

the faster oxidation of silicon and the formation of a thin hydrophilic surface [Niw03].

It’s important to consider that in Takano’s model for the plating process, the best

adhesion was obtained when the metal is deposited by the galvanic displacement of the

silicon atoms, e.g. the oxidation of the silicon. He considered that the chemical oxide

provided by the pretreatment in the HPM solution must be porous, which enables the


100 Nickel plated contacts

approach of the nickel in those parts of the surface where no oxide exists. He

reinforced his conclusion through the observation of the plating behavior of an

electrolytic solution. When the electrons for the reduction were provided by an

external electrical source, the metal could be easily peeled off the surface, thus

concluding that the oxidation of silicon was required for improved adhesion. However,

these observations are only valid for the electroless deposition from a solution without

a reducing agent [Tak99]. His evaluations showed that there is some discrepancy in the

deposition for electroless plating solutions containing a reducing agent [Tak99].

5.3.2 Substrate influence

The following section presents the assessment of the substrate’s influence in the

coating process. Considering that in our experimental setup the complete wafer is

introduced in the solution during the process, important parameters related to the

substrate which could affect the deposition are:

a) Silicon base resistivity

b) Emitter doping

c) Rear-side conditioning

d) Deposition area

Silicon base resistivity

The substrates of 4” Fz p- and n-type silicon wafers 250µm thick, with a doping

concentration ranging from 0.1 up to 100 cm were used in the experiments. The

wafers were cut with a laser, to make several samples with an area of 4 cm 2 . An

alkaline solution (PM980) was heated to 60°C. A pH >9 is sustained by ammonia

addition during the process. Three different plating times were tested, ranging from 1

to 10 minutes. No plating mask was applied on the substrates, therefore metal

deposition was observed on both sides and on the edges of the wafers. The samples

were weighted before and after plating to calculate the mass of the nickel coating.

Figure 5-16 shows the average mass deposited on the samples as a function of

the base resistivity of the material for the different plating times. No evident influence

of the base conductivity on the deposition rate can be determined.

The layer thickness is estimated from the weight measurements as in section

5.3.1 (the total plated area is equal to 8.2 cm 2 per wafer). The results as a function of

the plating time for all the samples from the experiment are presented in figure 5-17.

Two different fits are included in this figure: one for a kinetics controlled deposition

(linear fit) and another for a diffusion controlled deposition (Fick’s fit). It seems that

the closest fit for the data presented in figure 5-17, would correspond to a diffusioncontrolled

process, at least for the short plating times. Nevertheless, the spreading of


Process development 101

the data for longer plating periods is too high to give a conclusive answer on which

mechanism controls the process.

Deposited nickel [mg]

7

6

5

4

3

2

2 min

1

5 min

10 min

0

0.1 1 10 100

Resistivity [ cm]

Estimated thickness [nm]

900

800

700

600

500

400

300

200

Linear fit

Fick's fit

100

All data for R sheet

0

0 2 4 6 8 10 12

Plating time [min]

Figure 5-16 Mass deposited on both sides of

4 cm 2 samples for samples with different base

resistivity during three different plating times.

Figure 5-17 Nickel thickness calculated from

the data in figure 5-16. Two different fits have

been included, according to a linear or a

diffusion controlled process.

Through SEM measurements of some of these samples, we observed that using

this method to calculate the thickness leads to an overestimation for the coating (see

figure 5-18). This overestimation is attributed to the content of phosphorus in the

material.

Figure 5-18 Nickel coating on p-type silicon wafers with a resistivity of 0.1 . cm (left) and

100 . cm (right) processed during 2 minutes

When plating is performed separately on a p- or an n-type silicon wafer, both

material types can be plated (see figure 5-19). However, when both doping types


102 Nickel plated contacts

coexist in the same surface, there is a tendency for plating to occur only in the n-Si.

Guo showed that by modifying the plating chemistry, simultaneous plating of silicon

on both doping types is possible with the same solution [Guo05].

Figure 5-19 Nickel coating on n-type substrates with a resistivity of 1 .cm during 2 and

10 min.

Emitter doping

An experiment prepared in the same way, as for the previous one, was performed

on non-textured, p-type Fz wafers; with an area of 4 cm 2 after phosphorus diffusion.

Four different groups were formed, with emitter sheet resistances of 45 – 60 – 70 –

90 /sq, measured after P diffusion and PSG etching. The corresponding SIMS

measurements are presented in figure 5-20. The samples were then laser-cut, HFdipped

and plated with the alkaline electroless nickel plating solution PM980 at 60 o C

for 2 min with a pH ~9. Figure 5-21 shows some SEM views of the plated samples.

Dopant concentration [cm -3 ]

10 20

10 19

10 18

10 17

45 /sq

60 /sq

70 /sq

94 /sq

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Emitter depth [µm]

Figure 5-20 SIMS measurements for the different industrial emitters.

The metal deposition was not homogeneous for all the symmetrical samples. The

coating formed on some wafers occurred partly just on one side, leaving an uneven

deposition on the surface. An estimation of the layer thickness as the average of the


Process development 103

added weight would not deliver relevant results. The cause is difficult to elucidate. It

might be related to changes in the surface potential of the silicon wafers after doping

due to an uneven distribution of filled surface states or differences in the highly-doped

“dead layer” driving electrons away from the surface. But these effects would

contribute to a full-surface effect. In an attempt to reduce it, the pH of the solution was

changed from 9.0 to 9.4. In this way, the surface pinning, as well as the redox potential

of the solution were shifted to a point where enough electrons and empty surface states

would be available for the reaction to occur more homogeneously over the whole

surface.

Even though, it would be expected that the use of solutions with a higher pH

would result in more homogeneous Ni coatings, this conclusion could not be applied to

the samples from this experiment. Actually, no correlation was found between the

irregular coverage of the metal on the highly doped samples and the doping or the pH

level. So, for these wafers, the deposition is not fully controlled by the increase of the

pH or the temperature.

Figure 5-21 Plated samples featuring an double sided emitter with a R sh =70 /sq (left) and

45 /sq (right). Plated with an alkaline bath at 60°C and pH 9.2

Rear-side conditioning

The influence of the rear-side on the plating feasibility was evaluated for 5

different commercial plating solutions. All solutions are based on hypophosphite as a

reducing agent, with NiSO 4 as the source for metal ions. Four of these are alkaline and

one acidic. Four different substrate groups were formed depending on the rear-side

(see figure 5-22). The substrates consisted on Cz p-type wafers (1 cm), with a KOH

texture and exhibiting a thin emitter with a R sh =50 /sq as follows:

a. Emitter on both sides of the samples (no further processing)

b. Screen-printed Al paste and firing for Al-BSF formation on the rear.

c. Single-side etching of the phosphorous (with HNO 3 , HF): p-type base free

and in contact with the plating solution.

d. Deposition of a SiN x passivation coating on the rear after single-side etching.


104 Nickel plated contacts

a. b. c. d.

n-Si

n-Si

n-Si

n-Si

p-Si

p-Si

Al-BSF

Figure 5-22: Schema of the different samples implemented to evaluate the influence of the

rear-side condition on the plating process

Considering that each plating solution has an optimal plating range which

depends strongly on its chemistry, a direct comparison of the solutions under the same

conditions is not only difficult but also irrelevant. The optimal temperature and pH

couple for each solution was determined for samples of type b, which are closest to the

desired industrial application. The plating process was performed around the optimal

plating conditions for each solution (see table 5-5).

p-Si

p-Si

SiN x

Table 5-5 qualitative evaluation of the metal deposition on the different sample types depending on the

plating solution

Bath Bath name (Manufacturer pH

Temp

[C]

Type

A

Type

B

Type

C

1 PM 980 (Rohm & Haas) 9.3-9.9 40-60 - - ±

2 PM 988 (Rohm & Haas) 9.8 40 - ± -

3 HelioFab 200 (Enthone) 10 50

4 Nickelex ammonia (Transene) 8.7 75

5 SMT 88 (Rohm & Haas) 4.8 75-85 ± ±

Legend: - no deposition, metal deposition, ± very inhomogeneous deposition

Type

D

Strong differences were observed in the plated coatings. While two of the

alkaline solutions only delivered a good homogeneous layer on the samples of the type

B; the other two delivered good results for all wafer types. With the acidic solution the

deposition on the type A and B samples was very good, while it was sparse on samples

type C and D.

The electrical characterization performed during later experiments (with actual

solar cells) showed that the alkaline baths which formed a homogeneous nickel layer

on all surface-types (solutions 3 and 4) also showed a strong decrease of the open

circuit voltage on plated solar cells (see section 6.2.4). A plausible explanation is that

the chemical reaction taking place during the plating, from solutions 3 and 4, strongly

attacks the silicon surface by the oxidation of the silicon 2 . The V oc decay on the

2 Ammonia at high temperatures is a strong etchant for silicon.


Process development 105

samples treated with those solutions was obtained after nickel plating, before any

thermal treatment.

The introduction of aluminum in the plating solution contributes to a faster and

more homogeneous front-side deposition. This effect is possibly due to an increase in

the bath activity created by the simultaneous plating on the Al on the rear-side. A

detailed experiment to study this behavior is presented in the next chapter (see section

6.3.1).

Deposition area

The influence of the wafer size in the deposition process was evaluated by the

full surface plating of industrial-type silicon solar cells of three different sizes (6.25,

25 and 56.25 cm 2 ) as of type B from the prior section. The cells were manufactured by

KOH texturing of p-type 1 cm Cz material, featuring a 50 /sq emitter and a

screen-printed Al layer, fired on the rear side. No ARC was deposited on the frontside.

An acidic (SMT88) and an alkaline solution (PM980) were used, working with

the plating parameters corresponding to the best electrical results from prior

experiments for each solution.

Four groups of wafers were plated, alternating the order of the introduction of the

samples in the solution according to the size. For each group a fresh solution was used.

No clear evidence of the impact of the wafer size could be established for the

alkaline solution. It was observed that the low activity of this bath (working at low

temperature) had a strong influence on the process. Common practice of electroless

plating includes the introduction of metallic parts in the bath prior to the coating in

order to increase the activity of the solution. Still, in this experiment we observed that

the wafers introduced first in the bath delivered systematically a more homogeneous

coating than the following ones, without a significant influence of the size.

For both alkaline and acidic plating solutions, when narrow (fingers) and wide

areas (busbars) are simultaneously plated over the same silicon surface, there is a

tendency for the fingers to plate faster. This results in a thicker nickel deposit on the

fingers than on the busbars. This effect was observed by SEM for several samples. The

small features of the plated area for the fingers does not allow a simple comparison by

XRF measurements of the thickness of the fingers vs. busbars. The standard structure

of high-efficiency fingers applied during this project has a width of ~7 µm, while the

busbars are ~500 µm wide in the region close to the contacting pad. This could be

explained by an increase in the local current density for the fingers as compared to the

busbars.

Another observation is related to the adhesion. The following step (Ag LIP)

creates stress on the Ni seed layer. Since LIP is isotropic, this stress is even more


106 Nickel plated contacts

pronounced on narrow fingers. Combined with a smaller adhesion area, it can become

critical and lead to delamination of the fingers.

5.4 Coupling temperature, pH and time

Changes in the chemistry of the plating bath affect its processing window. But

for all solutions tested, pH and T are the two main drivers for process control, and

should therefore be considered together, as a couple.

When effective temperature and pH couples for solutions operating without

previous activation are plotted, a threshold process line can be drawn. The

effectiveness of the pairs is defined as those values resulting in the homogeneous

formation of a metal layer on silicon, see black squares (this thesis) and downward

green triangles (literature) in figure 5-23. By the application of certain reactants in the

solution like NaF the threshold/activation energy can be reduced, leading to lower T

vs. pH values, see red half-filled circles in figure 5-23 (literature). A possible cause for

this outcome is a reaction between the HF, that can be formed in the solution, and

loose bonds on the surface.

Another way to reduce the activation energy is by employing an activation step

before the nickel plating (generally including SnCl 2 and/or PdCl 2 ) as published by

different groups, see orange crossed circles in figure 5-23 [Liu05, Tsa04, Guo04].

When plating semiconductors, the option of light enhancement during the

process becomes available (blue upwards triangles). Through the photo-activation, the

catalytic activity of the substrate is increased, empty surface states lying at higher

energy levels are filled and the concentration of the minority carriers at the surface

increases. Then, processing at a lower pH and lower temperatures, which would not be

feasible otherwise, becomes possible. Other advantages of this process are the higher

homogeneity of the coating and the possibility to reduce the processing time. Process

development details corresponding to the photo-assisted electroless nickel plating are

presented in the next chapter, since this technology became an option thanks to the use

of silicon solar cells.

The data taken from the literature in figure 5-23, which corresponds to samples

treated without activation, was taken from the following references: [Sul57, Osa98,

Niw03, Ric85, Bou08]. Data for the samples activated with a PdCl 2 solution (orange

crossed diamonds) can be found in: [Tsa04, Ric85].


Coupling temperature, pH and time 107

100

Water evaporation

90

80

Temperature [C]

70

60

50

40

30

20

Std plating

+NaF

+ Light

Literature

Std plating

+ Activation

3 4 5 6 7 8 9 10 11 12

pH

High pH & T

= Si etch

Figure 5-23 Different pH and temperature conditions allowing the deposition of a nickel

coating by electroless plating on silicon for different commercial plating baths. All the data

presented above the black reference line shows points reached without the application of an

activation step pre-treatment (std. plating). The black squares correspond to experiments

performed during this thesis, while the green downwards triangles correspond to data taken

from the literature [Osa98, Niw03, Bou08]. Underneath the reference line, data

corresponding to a solution containing NaF in red circles, or blue triangles for the photoassisted

electroless plated cells (see next chapter). Data taken from the literature has been

included here, showing possible values reached when an activation step with PdCl 2 is applied

prior the nickel deposition.


108 Nickel plated contacts


6 Manufacturing of solar cells with nickel-plated contacts

In this chapter we’ll present the different aspects relevant for the integration

of the nickel plating step into solar cell manufacturing. We evaluate the optimal

processing sequence for both industrial and high efficiency solar cells. Finally, we

combine these processes with alternative structuring technologies like laser

ablation or laser chemical processing.

6.1 Introduction

One of the main drives of the PV industry is cost reduction. In that sense, copper

would be the best alternative for formation of conductive lines for the frontmetallization.

However, the use of this material introduces a challenge for the good

performance of the device. Copper has a high diffusion coefficient into silicon, already

at low temperatures, and it has a strong impact as a lifetime killer. For this reason, a

diffusion barrier for copper contacts is required. Nickel is an excellent option for this

task. The effectiveness of Ni as a diffusion barrier for copper has been evaluated in

more detail in the work of Jonas Bartsch [Bar11b]. In the present thesis we lay out the

basis for the use of nickel plated contacts with alternative patterning technologies.

The main issue opposing the use of nickel as metal seed is related to the risk of

shunt formation on the front junctions, which easily results in the fabrication of lowquality

devices. The source of this problem is the combination between the need to

form a nickel silicide through a thermal step, for an enhanced mechanical adhesion

versus the fast diffusion of the nickel through the emitter during the silicide formation.

To face this problem, solar cells featuring nickel contacts have been typically

manufactured in combination with a deep selective emitter. Such structures can

introduce significant complications for the device fabrication. An excellent example is

the Laser Groove Buried Contacts (LGBC) manufactured by BP/UNSW [Bru03]. A

different sequence, presented by Kim in 2005 and applied to high efficiency cells,

resulted in conversion efficiencies up to 21.4% [Kim05]. For an overview of cell

results manufactured by Ni plating until 2009 check table 5-1.

Alternative solutions for a simplified selective emitter formation are currently

under investigation, for example by laser doping [Wan12], or by laser-chemical

processing [Kra08]. Other approaches include junction narrowing, by a selective

etch-back process [Boo08].

Other approaches to reduce the shunting risk, which are investigated here,

include the reduction of the thermal budget applied for the silicide formation or the

reduction of the nickel thickness [Fog04] (see section 6.2.3).


110 Manufacturing of solar cells with nickel-plated contacts

6.1.1 Basics for process development

During this research, we have looked for the optimal combination between an

alternative patterning technique for the ARC and the introduction of the nickel plating

process for the seed contact formation. As it was mentioned in chapter 4, each

patterning technique has its advantages and drawbacks.

Electroless nickel plating is a surface-related process, and when changes are

applied to the structuring technique for the ARC patterning, strong changes can be

observed on the surface, thus on the plating result. We have chosen patterning

techniques based on the chemical etching of the ARC, assuming that these would have

the least impact and variation on the catalytic activity of the wafer surface.

Photolithography delivers the highest control over thin line formation through an

etching mask. For this reason, it has been used to create the front-side patterns of highefficiency

substrates (see section 6.3).

Ideally, it would also be the best technological choice as the patterning technique

used for industrial samples. However, with the experimental use of in-line tools for

surface texturing on industrial wafers, random pyramids with heights up to 20 m on

the front surface were observed. Such topologies represent a challenge for the

photolithography process available at the time of this work. The closest technique

available was inkjet printing. Therefore, we have optimized the plating process on

industrial samples by etching the ARC through an inkjet printed mask (see section

6.2).

Further developments on the plating process have included the use of the photoactivation

of the substrate, in order to achieve a more homogeneous metal deposition.

This process is presented in section 6.4.

The optimized plating sequence developed either with inkjet printed masks on

industrial samples or with photolithography on high-efficiency samples, has been

combined with alternative patterning technologies, like laser ablation. The results can

be found in section 6.5 of this chapter.

Finally alternative plating mechanisms, like the use of nickel alloys, or the direct

light-induced electroplating of nickel are introduced in section 6.6.


Integration to Industrial Processing 111

6.2 Integration to Industrial Processing

The differences between high-efficiency and industrial silicon solar cells are

related to fact that industrial manufacturing requires cost-effective processes and

materials, sometimes in detrimental of the efficiency. For example: the choice for

multicrystalline (mc-Si) or Cz material instead of float-zone wafers; even when, for a

given base resistivity, a lower defect concentration is expected on Fz material. A

performance comparison for different material types has been published by Glunz

[Glu99].

Phosphorous concentration [cm -3 ]

10 21 120 /sq (drive in)

60 /sq (industrial)

10 20

10 19

10 18

10 17

10 16

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Depth [µm]

Figure 6-1 SIMS measurements showing the main differences between high efficiency and

industrial emitters.

Another example is related to the doping and passivation schemes: Steps like

diffusion or thermal oxidation, which require over 1000 o C, and take a long period of

time (3 to 8 hours) are not frequently used in industrial manufacturing. Instead, shorter

diffusion steps (20 min to 1h) applied at temperatures between 800 - 900 o C [Gre98]

are good enough for industrial devices. An alternative industrial technique for junction

formation is based on the in-line diffusion of a doping source through a conveyor belt.

The dopant can be deposited by: spray-on [Ben06], screen-printing, or spin-off

[Szlu06]. The result is typically a shallow and homogeneous phosphorus emitter, with

a strong field effect passivation and sheet resistances ranging from 40-80 /sq.

Figure 6-1 shows an exemplary industrial emitter, featuring a shallow junction

with a high surface doping concentration (N s ), over 1x10 20 at/cm 3 versus a highefficiency

emitter, with a lower N s and a junction depth over 1 m.

The anti-reflective coating on industrial wafers is formed by a SiN x layer with a

refractive index between 1.9 and 2.4, deposited by PECVD [Slzu06].

The front metallization consists of a silver screen-printed paste which forms a

pattern of fingers and busbars, while the rear metallization consists of a full coating of

screen-printed Al paste. After printing and drying the pastes, the contacts are formed


112 Manufacturing of solar cells with nickel-plated contacts

during a firing step in a belt furnace. The firing step consists on the rise of the

temperature to 800-950 o C for a few seconds. Several things occur during this step. On

the front-side, the glass frit contained in the paste etches the SiN x layer, at around

700 o C and then the contacts are formed between the Ag paste and the silicon. The

mechanical adhesion is provided by the glass frit [Schu06b]. More information about

this step can be found on further literature [Schu06a, Pys08]. On the rear-side, the Al

paste forms an Al back surface field (Al-BSF), thanks to the eutectic transformation

between Al and Si at 577 o C [Man72, Loe95]. The rough surface provided by the Al-

BSF on the rear, has a strong impact in the optical properties of the device. An

interesting evaluation showing the impact of the different passivation and metallization

concepts available for the rear side processing has been published by Hermle [Her05].

The overall result is a much simpler fabrication process with a lower efficiency

potential. More information about industrial cell manufacturing can be found in the

literature [Tob06, Slzu06, Gre98].

In this section, we present the advantages and the challenges for the integration

of a nickel-based metallization on this type of devices.

Why use electroless nickel instead of Ag-screen printed pastes?

The theoretical advantages of using nickel as a contacting material for the front

metallization were presented in section 2.3. The reasons for using plating as the

deposition technique for nickel contacts were mentioned on section 5.1. Still, to this

date, most cells manufactured at industrial environments are made by Ag screenprinted

contacts. Therefore, it is interesting to present the added benefits of plated

nickel contacts vs. Ag screen printed contacts:

• No mechanical pressure is applied on the substrates during plating, reducing the

breakage risk when thinner substrates are used.

• A higher electrical conductivity can be reached thanks to the use of pure metals for

the thickening of the fingers, instead of the frit-containing pastes [Met07].

• It will be shown here that a good electrical contact can be achieved also on emitters

with a low surface doping concentration (N s = 7x10 18 at/cm 3 ). This gives a higher

degree of freedom in the emitter design.

• The nickel silicidation temperatures are compatible with alternative passivation

concepts, like a-Si [Glu05].

• Thanks to the functionality of Ni as a diffusion barrier for copper, Cu plating baths

can be used for the thickening of the fingers. Considering the price difference

between Ag and Cu, this represents a high potential for cost reduction. Excellent

results have been recently achieved combining Ni and Cu for the frontmetallization

[Bar11, Tou12a].


Integration to Industrial Processing 113

The sequence applied for a nickel-based metallization requires more steps than

the standard Ag printing technology. A comparative study of manufacturing costs for

both technologies would be interesting, but it is out of the scope of this thesis. An

interesting evaluation was made by Wolf in 1981. With the access to alternative

technologies for patterning, like laser ablation or the construction of high throughput

machines for manufacturing. That first price estimation is outdated. In any case, higher

efficiencies should be obtained by the nickel process, to justify the need for extra tools

and more handling steps.

The nickel compounds in plating solutions are carcinogenic and water-pollutant.

Therefore, like for any hazardous chemistry, the bath waste requires proper treatment

and the chemicals need to be handled with care.

6.2.1 Potential integration paths

In the following part of this chapter the impact of the manufacturing sequence

and the optimization of the steps related to the front-metallization of silicon solar cells

are presented. This information is used to determine the optimal integration path for

plated contacts on industrial cells with a front-side patterned by inkjet printing.

Silicon solar cells with an area of 5x5 cm 2 were manufactured on 1-3 cm

p-type Cz material of 125x125 mm 2 . The samples were cleaned and then textured in

KOH, forming random pyramids. POCl 3 gas diffusion in a tube furnace was used for

the junction formation. The cells feature different R sheet values depending on the

experiment.

After the PSG removal, a 75 nm thick SiN x coating, with a refraction index of

2.02, was deposited by PECVD as an ARC. From this point on, in principle various

integration sequences could be used to finish metallization of the devices (both front

and rear). Some of these paths are presented in figure 6-2.

Path A Path B Path C Path D

Inkjet printing

Al screen-printing

Inkjet printing

ARC etching

Resist removal

HF dip

Ni plating

Firing

Inkjet printing

ARC etching

Resist removal

ARC etching (HF)

Resist removal

Al screen-printing

Firing

Al screen-printing

Firing

Ag LIP

Short HF dip (native oxide removal)

Ni plating

Sintering

Ag LIP

Ag LIP

Sintering


114 Manufacturing of solar cells with nickel-plated contacts

Figure 6-2 potential metallization roadmaps to integrate nickel plating in the manufacturing

of industrial silicon solar cells:

Path A: patterning and nickel plating before the rear metallization

Path B: rear metallization prior to the front-side patterning and plating

Path C: rear metallization performed after the front patterning, before the nickel deposition

Path D: similar as path C, but the sintering is performed as the last step

Details about the different steps which need to be implemented to finish the cells

are presented next:

ARC patterning: the front structure is formed by inkjet printing and etching. A

DoD 3000 inkjet printer from the company Schmid used with a hotmelt ink is used to

form the front-side mask [Stu07]. Then, the wafers are introduced during 2 min in a

20% concentrated HF solution for the etching of the ARC (see section 4.2.1). The

resist is stripped off the surface either in a solution containing lowly concentrated

KOH (1-2%) or through a series of alcohol baths. More details about the inkjet

printing process can be found in section 4.2.4. The ARC patterning process is

illustrated on figure 6-3 (a and b), in combination with the relevant steps for the front

metallization.

Rear-side metallization: the rear side is coated with an Al paste. The cells are

fired at temperatures between 850 and 930°C for the Al-BSF formation. This

temperature was optimized using references with a screen-printed paste on the front.

Front-side metallization: the front-contacts are formed by an HF dip for the

native front-oxide removal followed by electroless nickel plating. A thermal step is

applied for nickel silicidation. The finger thickening occurs in a silver light-induced

plating step (Ag-LIP), for a thickness up to 10-12 m. An in-line machine, specially

developed by the company Schmid together with the Fraunhofer ISE, for the

thickening of screen-printed contacts was used for the Ag thickening step [Met06].

SiN x

Emitter Ink

SiN x

Emitter

Silver

Nickel

p- Si

p- Si

(b)

(c)

(a)

Figure 6-3 front side metallization process applied for industrial plated cells. (a) Printing an

ink on the SiN x layer, (b) ink removal, (c) electroless nickel plating and Ag LIP

Another possibility for contact thickening is the use of copper electroplating or

Cu-LIP. Excellent efficiencies have also been proven with Ni plated seeds and Cu

thickened contacts [Bar11, Tou12a].

p- Si


Integration to Industrial Processing 115

Characterization: finally, the edges of the wafers are isolated and the wafers are

diced into 4 samples of 5x5 cm 2 solar cells. The electrical characterization is

performed with a solar simulator, for light and dark IV; SunsVoc for the evaluation of

the pseudo fill factor [Sin00]; dark lock-in thermography (DLIT), for shunting

evaluation [Bre01] and transmission line method (TLM), for the contact resistance

analysis [Sch98]. Secondary electron microscopy (SEM) and secondary ion mass

spectroscopy (SIMS) are used for visual and material characterization, respectively.

6.2.2 ARC patterning

Some of the boundary conditions, which have an impact on the determination of

the integration sequence related to the ARC patterning, are:

• Screen printing pastes applied on the rear side are typically very rough, so coating

them with a resist is very challenging. This means that the screen printed Al

coating is exposed to the chemicals where the wafers are immersed.

• The resists applied during the patterning step, need to be off the wafer surface,

before going into any high temperature step (above 160 o C)

Considering that a 20% HF solution is used to remove the ARC, and that fully

protecting the screen printed metal from the HF solution is not an option, the pattern

formed by wet-chemistry on the ARC needs to be created before the rear metallization.

In other words, path B (on figure 6-2) is not compatible with a wet-etched ARC.

This also means that the areas to be metalized on the front-side are exposed to

potential contamination during firing; they do not have any dielectric protection. To

reduce risk of metal contamination from the conveyor belt, the wafers are positioned

face-up (path C and D in figure 6-2). It is interesting to note that for some samples,

marks of the conveyor belt could be observed after nickel plating as areas where the

metal deposition was thicker. The source for this local plating increase could be related

to a higher activation of the silicon on those areas due to metal contamination, or some

temperature related effects.

6.2.3 Surface preparation

The importance of the surface preparation for electroless nickel plating has been

presented in the previous chapter. Two steps were applied before plating: an HF dip

for the oxide removal and a dip in an ethanol solution for the surface activation (see

section 5.3.1). Since no influence on the process integration was observed from the

ethanol dip, we present the impact of the HF dip.

HF pre-treatment

A short HF dip is applied for the removal of the native oxide layer prior to the

electroless nickel deposition (section 5.3.1). The HF-containing solution also attacks


116 Manufacturing of solar cells with nickel-plated contacts

the Al contacts which are located at the rear-side of the solar cells. This presents an

integration issue:

It would be desired to form the rear metallization after the HF dip; that is, after

the front-metallization. Yet, a temperature above 577°C is required for the formation

of the Al-BSF on the rear-side [Loe95]. Such temperatures represent a great challenge

for the integration of a nickel-based front-metallization (see section 6.2.3). Therefore,

the rear-metallization for cells including a rear side formed by an Al-BSF needs to be

performed before the nickel deposition, which means that if the full wafer is immersed

in the solution, the Al comes in contact with the HF solution. In other words, path A

(see figure 6-2) is not compatible with nickel plated contacts.

Two alternatives to ease the chemical attack on the rear-side, when the wafers are

completely immersed in the bath were tested: the use of a shorter dipping time or the

reduction of the HF concentration. Silicon solar cells without the front-metallization,

prepared as described above, have been used for these experiments. After dipping the

wafers in the HF each sample was plated with an alkaline Ni bath at pH 9, 60°C during

2 min.

The minimum time was evaluated for a 1% concentrated HF solution. Different

samples were dipped in the HF solution during: 15, 30, 45, and 60 s consecutively. A

polished wafer with a 100 nm thermal oxide coating was introduced in the solution to

evaluate the impact of the etching step in the oxide. This wafer was left in the 1%

concentrated solution during 60 s. The oxide thickness was measured before and after

dipping with a single wave ellipsometer, setting the refractive index to n=1.46.

The adhesion of the fingers of the samples which were dipped only during 15 s is

very poor. After 30 s etching the plating process works well and the fingers stay on the

silicon surface. The ellipsometer results show that only 3 to 4 nm of the SiO 2 layer are

removed during the 60 s etch.

Then, the minimal HF concentration was tuned between 0.25% and 1% for a

1 min immersion time. The metal deposition occurs only on samples which are treated

with a 0.5% concentrated HF solution or higher. At the rear-side we observe the

formation of hydrogen bubbles regardless of the HF concentration. Nevertheless, this

reaction seems to be less pronounced for the lower HF concentrated solutions.

Screen-printed Al pastes vs. Al coatings by physical vapor deposition (PVD)

Al screen-printed pastes show a very strong reaction when compared to the

samples coated by an evaporated Al layer in the HF solution. The porosity of the paste

provides a wider area for the reaction. In addition, Al pastes contain some metal oxides

that also react strongly in reducing environments. Rinsing of rear screen-printed

samples takes longer time. This effect is likely to be due to the higher porosity of the

SP-paste compared to a homogeneous PVD coating.


Integration to Industrial Processing 117

6.2.4 Plating process

The optimization of the plating parameters, like pH and temperature, were

presented in chapter 5. Also, the influence of the base resistivity or doping

concentration of diffused surfaces can be found in chapter 5. The application of this

process to silicon solar cells involves further processing issues, which are presented

next.

Unless specified otherwise (like for the photo-assisted experiments), all samples

manufactured with the standard electroless plating process have been prepared under

the standard outlet light, without any direct illumination on the substrates.

Influence of the front texture

Figures 6-4 (a) to (c) show SEM images after electroless nickel plating. All

figures show a part of a metal finger with a border onto the ARC. Samples (a) and (b)

have been patterned by inkjet printing, and sample (c) by photolithography. In

figure (a) the edges and tips of the random pyramids are not plated, while on figure (b)

the contrary occurs: the tips and edges are plated and some areas on the lateral faces of

the pyramids are not. Figure (c) shows a thicker metal deposit starting from the tip of

the pyramids (and going downwards).

It is difficult to provide a satisfactory explanation which could clarify the

contradictory behavior of the deposition observed between figures 6-4 (a) and 6-4 (b)

for the same plating conditions. This effect could be related to differences in the

PECVD nitride which could result in the lack of homogeneity of the patterned areas

during the etching step.

(a) (b) (c)

Figure 6-4 SEM front view of a Ni plated pyramid: after 10 min plating, in an alkaline

solution, showing (a) the tip and the edges of the pyramids without metal, (b) the tip and

edges of the pyramids with metal. (c) a thicker metal deposition at the tip of the pyramids.

The formation of a thicker nickel deposit on the tips of the pyramids, as seen on

figure 6-4 (c) could be explained by two causes: The ions have higher access to these

spots, thus forming a thicker metal coating; or it could be the result of the higher

electrical field in these areas formed through the photovoltaic activity of the device,

which attracts more metal ions to these areas.


118 Manufacturing of solar cells with nickel-plated contacts

In any case, all these phenomena result in a non-homogeneous nickel deposition

over the wafer surface. The lack of control over the deposited metal thickness results

in the formation of local shunts through the junction during the thermal treatment (see

section 6.1.5).

A solution to the lack of homogeneity was developed in this thesis by the photoassisted

electroless nickel plating (see section 6.4).

6.2.5 Post-plating: thermal treatment

One of the main challenges of a nickel plating based metallization appears during

the sintering step, where the fast diffusing nickel metal can create paths contacting the

emitter and the base, which reduced the R p of the cells. Information found on the

literature points out to the advantage of using high sintering temperatures in order to

obtain improved adhesion [And80, Coc83]

Plating parameters and sintering time

The following experiments show the influence of the plating conditions and the

sintering time for a 300°C anneal on the electrical performance of the silicon solar

cells.

The sample preparation was performed following path C (see figure 6-2). The

patterning process for the AR coating was not fully optimized, which resulted in the

formation of ~150 m lines. So, strong shadowing losses are observed on all these

wafers. A shallow emitter with an R sheet =60 /sq was formed during the POCl 3

diffusion. The rear contacts were fired at 920°C. The surface preparation before

plating was performed by a 1% HF dip during 30 s. The samples have been

characterized by SunsVoc. The SIMS profile for the emitter corresponding to this

experiment can be seen on figure 6-10.

Table 6-1 Plating parameters applied for selected commercial solutions (without light

enhancement)

Commercial name Solution pH Temp Plating Time

SMT 88 Solution 1 (S1) Acidic 4.6 85 3

Eless Nickel, Ammonia Solution 2 (S2) Alkaline 9 85 3

type (ENPAT)

Niposit PM 980 Solution 3 (S3) Alkaline 9.5 40 3

[°C]

[min]

All the commercial solutions mentioned on section 5.3 have been tested, but only

the results of 3 selected solutions are detailed here. They have been chosen for their

exemplary behavior between an acidic and an alkaline pH, as well as for providing

interesting IV results. Information about the plating parameters applied in this


Integration to Industrial Processing 119

experiment can be found in table 6-1. Even though no curtains or masks were used to

protect the processing wet-bench from the standard room light, no special

enhancement by direct illumination of the samples was performed during these

experiments. A more detailed evaluation of all the commercial solutions was

performed during the diploma thesis of Barucha [Bar08].

After the electroless nickel plating, the wafers are thickened in the inline Ag-LIP

machine. Then, they are sintered at 300°C on a hotplate during a time ranging from 5

to 20 min, depending on the sample. SEM pictures of the deposited layers are

presented in figure 5-14. The evaluation of the SunsVoc results is presented on

figure 6-5. Each data point represents the average of three solar cells.

630

620

81.5

V oc

[mV]

610

600

590

580

570

S 1

S 2

S 3

0 5 10 15 20

Sintering time [min]

PFF [%]

81.0

80.5

80.0

79.5

S 1

S 3

0 5 10 15 20

Sintering time [min]

Figure 6-5 SunsVoc evaluation of the shunting behavior of 3 different commercial electroless

plating solutions depending on the sintering time for a process at 300°C.

The V oc values of the cells manufactured with the acidic and the low temp

alkaline solutions (S1 and S2) are stable even for a sintering process lasting 20 min.

Conversely, the high temperature, alkaline solution (S3) shows a V oc degradation even

before any thermal treatment. The metal deposition for the samples treated with the S3

solution starts after a very short period of time. This is detected by the formation of H 2

gas bubbles coming from the front surface, which correspond to the nickel deposition,

according to the reaction shown in Eq. 5-1.

The PFF of the acidic solution degrades already after 10 min annealing, while the

low temperature, alkaline bath (S3) only starts to degrade after sintering during

15 min. The PFF values for S2 are below the region of interest (around 72%).

Taking into account this data, we can state the following:

• The impact of the contact formation on the V oc is not only governed by the thermal

treatment, but also by the plating process in itself.

• Even for two solutions using a similar pH (alkaline ~ 9.5) a very different impact

on V oc is observed.


120 Manufacturing of solar cells with nickel-plated contacts

• The use of a high temp solution, with a strong alkalinity results in the faster

formation of a nickel coating. So, the plating time could be reduced under these

processing conditions. A potential cause for the strong reduction of V oc observed

while using high temperature baths containing ammonia could be the chemical

attack to the silicon surface during the plating step.

As presented in section 5.3.1, the homogeneity of the nickel layer from the acidic

baths is less controlled than for alkaline baths. This difference might explain the

formation of local shunts after a shorter sintering time with the acidic solution (even

with a sintering temperature as low as 300°C). Table 6-2 presents the most relevant IV

characteristics corresponding to the wafers in this experiment.

Table 6-2 IV characteristics for the plated samples after 10 min annealing at 300°C

Ni bath

V OC j SC FF

[mV] [mA/cm²] [%] [%]

Acidic S1 623 ± 0.9 35.2 ± 0.1 73.2 ± 0.8 16.0 ±0.2

Alkaline S3 621 ± 0.7 35.1 ± 0.2 72.6 ± 0.7 15.9 ± 0.1

This experiment was repeated after further optimization of the plating parameters

for each solution (see table 6-3). The cells featured an emitter with a sheet resistance

of 55 /sq and were fired at 900°C. With further development of the inkjet printing

process, ~90 μm wide lines were formed on the ARC at the time of these experiments,

enabling the achievement of higher short-circuit currents. The contacts were annealed

in a tube furnace under forming gas at 300°C during 10 min.

Table 6-3 Optimized plating parameters

Solution T pH Time Ni thickness P conc.

[°C] [min] [nm] [%]

S 1 85 4.7 ± 0.1 2 ~350 – 440 ~ 23

S 2 60 8.8 ± 0.2 1 ~300 ~ 10

S 3 40 10 ± 0.5 2 ~ 70-90 5 (estimated)

The resulting IV characteristics are presented in table 6-4, including the screenprinted

references. Each set of values corresponds to 5 plated solar cells, and 8 screenprinted

references. The average efficiency, including the best cell results for each

process is shown on figure 6-6. The contact resistance corresponding to each

metallization process was measured on test structures by TLM. These results are


Integration to Industrial Processing 121

shown on figure 6-7. The chemical composition of the two deposited layers was

measured by energy dispersive X-ray spectroscopy (EDX) (see table 6-3).

Table 6-4 IV characteristics of silicon solar cells with seed contacts formed by electroless nickel

versus screen-printed reference. The plating parameters are in table 6-3

Metallization

V OC j SC FF r spec, metal

[mV] [mA/cm²] [%] [%] [cm 2 ]

Screen-printed ref. 618 ± 0.8 36.2 ± 0.1 75.8 ± 0.3 17.0 ± 0.1 4.7 . 10 -3

S 1 613 ± 3.6 35.8 ± 0.7 71.0 ± 3.2 15.6 ± 0.6 4.1 . 10 -3

S 2 611 ± 5.8 36.0 ± 0.5 74.1 ± 1.5 16.3 ± 0.2 3.5 . 10 -5

S 3 614 ± 1.1 36.1 ± 0.7 75.0 ± 1.4 16.6 ± 0.3 1.2 . 10 -3

From these results, we observe:

• The IV characteristics for these plated cells are below the screen-printed

references.

• The most stable plating process is provided by working at low temperatures with a

very high pH between 9.5 and 10.5 (S3). Therefore this process has been chosen

for further experiments.

• The reduction of the plating time and the use of moderate temperatures (60°C

instead of 80°C) for the second solution, result in an improvement of the IV

characteristics. But the contact resistance is much higher than for the S3. It is

difficult to determine the nature of the difference of the specific contact resistance

for S2 and S3. It could be related to the P concentration or the thickness of the

deposited Ni metal. Additional investigations are required to elucidate this effect.

• A contact resistance as low as 35 μ.cm 2 is demonstrated with the nickel plated

contacts on highly doped wafers. This value depends strongly on the applied

chemistry. From the theoretical evaluation presented by Schroeder [Sch84], a

resistance in the same order of magnitude (13-30 μ.cm 2 ) is expected for a pure

nickel silicide.

• There seems to be a strong relation between the nickel thickness and the specific

contact resistance (see tables 6-3 and 6-4).

• A series resistance of 0.2 ± 0.02 cm 2 is obtained with the best plating conditions.

This value is lower than the 0.47 ± 0.01 cm 2 of the screen-printed reference.


122 Manufacturing of solar cells with nickel-plated contacts

18

10 -2

Efficiency [%]

17

16

15

r spec, metal

[cm 2 ]

10 -3

10 -4

14

SP ref. S 1 S 2 S 3

10 -5

SP ref. S 1 S 2 S 3

Figure 6-6 Average and best efficiency

values corresponding to the cells made with

optimized plating parameters presented in

table 6-3, versus the screen-printed reference

(SP)

Figure 6-7 Specific contact resistance

measured by TLM on test samples processed

with the solar cells. The data corresponds to

electroless Ni/Si contacts vs. the screen

printed reference (SP)

The dark IV characteristics of three selected cells manufactured by inkjet printing

with the solution S3 (see table 6-3), and sintered at 300°C during 10 min in forming

gas, are presented on figure 6-8. The evolution of the local ideality factor m local is also

presented for a better understanding of the loss mechanisms of these cells [McIn01].

These curves show some of the homogeneity issues found on nickel plated cells

on shallow emitters.

The reduction of the R p leads to the formation of a hump as seen on the m loc

curve for Cell A (with the red circles at the bottom of figure 6-8). The increase of the

recombination current in the low voltage region is most likely caused by the metal

diffusion close to the p-n junction.


Integration to Industrial Processing 123

Current density [A/cm²]

m local

10 -1

10 -2

10 -3

10 -4

10 -5

10 -6

10 -7

10 -8

10 -9

MPP region

Tag Voc Jsc FF Eta

[mV] [mA/ [%] [%]

A

B

C

615 36 76.1 16.8

614 36 73.6 16.2

613 36 76.3 16.8

7

6

5

4

3

2

1

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage [V]

Figure 6-8 Dark IV characteristics and m local for cells manufactured with a low temp, alkaline

solution (S3). A Different electrical behavior is observed for these 3 cells (Solution 3: Table

6-3 )

Another typical behavior is observed on the curve of cell B. In this case, local

shunts, which are isolated from the metal grid, create a hump at mid-to-high voltages.

These have a strong impact on the MPP. McIntosch also observed such behavior when

plating laser grooved buried solar cells. More information on his model can be found

in his thesis [McIn01].

The third cell shows the lowest recombination current loss (cell C in figure 6-8).

Nevertheless, with a FF of 76.3%, there is room for improvement. A DLIT

measurement of this cell is presented on figure 6-9. The strong edge recombination

and the local shunts, observed on the DLIT image, cause the limitation on the fill

factor. Such local shunts are most probably created by the presence of local nickel

clusters on the surface during the deposition. The strong edge recombination results

from the mechanical dicing of the wafers from the front-side.


124 Manufacturing of solar cells with nickel-plated contacts

High

Local shunts

Strong edge recombination

Low

Figure 6-9 DLIT of a NiP plated solar cell with low FF. The areas with a high recombination

current are created either by local shunts or a strong edge recombination

The full potential of the electroless nickel plating process is not exploited in these

experiments. As shown on figure 6-7, very low contact resistances can be achieved

with nickel silicides. Using GridSim, a simulation program developed at the

Fraunhofer ISE for the determination of the optimal contact pattern for a given solar

cell, we estimated the reduction of the power loss for a 50 /sq emitter contacted

either by a pattern of 25 μm wide Ni/Ag LIP fingers versus 100 μm wide screenprinted

fingers both with the same busbar size and height. A relative power loss

reduction of at least 4% could be achieved by applying an optimized front side

metallization grid.

Shallow junctions and sintering temperature

An evaluation of the shunting risk of shallow junctions depending on the surface

doping concentration and the junction depth was performed. The samples were

manufactured following the path C of the process baseline presented on figure 6-2 in

section 6.2. Three different emitter types, featuring a sheet resistance of 60, 70 and

95 Ω /sq, respectively, were evaluated. The wafers were plated with the Niposit

PM 980 solution at a pH of 9.5 and a temperature of 60°C.

The emitters with R sh = 60 and 95 /sq were formed by the constant flow of the

POCl 3 gas during diffusion, and a variation in the process temperature. The third

emitter with a R sheet of 70 /sq was formed in a different tube. SIMS profiles

corresponding to these three emitters are shown in figure 6-10.The 60 Ω/sq emitter

shows a surface doping concentration (N s ) at least three times higher than the 95 Ω/sq

emitter but with a similar junction depth. The N s of the 70 Ω/sq emitter is similar to N s

of the 95 Ω/sq emitter, but it is ~100 nm deeper.


Integration to Industrial Processing 125

Dopant Concentration [cm -3 ]

10 21

10 20

10 19

10 18

10 17

10 16

0.0 0.1 0.2 0.3 0.4 0.5

Emitter Depth [µm]

Figure 6-10 SIMS measurements of the 3 emitter profiles used in the evaluation of the nickel

shunting process

During the processing of this batch, the inkjet patterning step was still under

development. So, there was a strong under-etching of the ARC, which resulted in

nitride openings ~200 µm wide. Such wide fingers generate higher shadowing losses

and a smaller j sc on the finished devices. The resist printing process was improved in

further experiments, enabling the etching of lines down to 15 m wide thanks to an

excellent control over the processing conditions. These include precise wafer handling,

control over the time between resist printing and etching, over the temperature of the

HF solution, etc. About 50 m wide lines can be manufactured under conditions which

would be more suitable for industrial environments (including less waiting time)

[Ale08b].

After nickel plating, these wafers were thickened in the Ag-LIP machine. A

finger thickness of 10 m was expected, but only 2-2.5 m were obtained. The edges

were then isolated with sand paper. Sintering was performed on a hotplate during

1 min under a standard air atmosphere. The FF and the pseudo FF were sequentially

measured on both the standard IV tester and on a SunsVoc tool. The sintering

temperature was gradually increased using a 100°C step, starting from room

temperature up to 500 o C using the same samples for each temperature. The results are

presented on figure 6-11.

The emitter with biggest junction depth (60 /sq) is more resistant to sintering

at higher temperatures before showing a strong decrease of the PFF.

No evident influence from the surface doping concentration on the shunting

behavior can be extracted from these samples.

R sheet

~60 /sq: A

~70 /sq: B

~95 /sq: C

Up to 300°C there is an improvement of the FF (most likely due to a reduction

of the series resistance) with increasing sintering temperature. After a further


126 Manufacturing of solar cells with nickel-plated contacts

temperature increase, the FF decreases, as well as the pFF. This is caused by the

increase of the recombination due to the metal diffusion through the junction.

This effect turns into the driving force affecting the overall FF.

Therefore, it can be concluded that within the doping range evaluated in this

experiment, the parameter with the biggest influence leading to the formation of

shunts on the solar cells is not related to the surface doping concentration, but

to the depth of the junction.

The measurements show a very low FF, resulting from the high series resistance

created by the poor metal deposition during Ag-LIP. During the electroless nickel

plating of these samples, there are sharp non-isolated edges all over the wafer

perimeter. These edges act as a perfect catalytic surface for the nickel deposition.

Therefore, shunts are created on the edge of the devices. These affect the Ag-LIP

process, which uses the photovoltaic quality of the devices to obtain the silver

reduction on the front-side of the cell by illuminating it. When the cells are shunted,

the voltage on the front is not high enough to allow a standard silver deposition rate of

1 m/min. Large-area industrial cells do not have such sharp non-isolated edges during

the nickel plating process.

FF and Pseudo FF [%]

90

80

70

60

50

40

30

FF / PFF R sh

/ 60/sq

/ 70/sq

/ 95/sq

0 100 200 300 400 500

Increasing temperature [C]

Figure 6-11 SunsVoc and IV measurements of the PFF and the FF for cells featuring 3

different emitter profiles sintered at increasing temperatures. Note: the Ag thickness of these

cells was only ~2μm, resulting in a very high Rs, thus a low FF.

An alternative and elegant solution, developed by the tool manufacturers in the

industry, to avoid this issue is provided by the single-side processing of the wafers

through the plating baths, either via the CupPlating technology [Kue09] or by singleside

plating machines [Met06].


Integration to Industrial Processing 127

The specific contact resistance was measured by TLM. Each sample was sintered

at a different temperature during 1 min on a hotplate. The samples feature the 70 Ω/sq

emitter (figure 6-10). The results are presented on figure 6-12.

Specific contact resistance [.cm 2 ]

10 -2

10 -3

10 -4

10 -5

as deposited

100 200 300 400 500

Sintering termperature [C]

Figure 6-12 Specific contact resistance between a Ni electroless plated contact and a highly

doped silicon solar cell (N s >1 . 10 20 at/cm 3 ) depending on the sintering temperature.

Measurements performed by TLM.

On top of a lower contact resistance, a better mechanical adhesion should be

achieved by processing at higher temperatures [Coc84, And80] (at least for low

temperature processes T< 500 o C).

A specific contact resistance in an order of magnitude of 1x10 -4 Ω cm 2 is already

very good for the manufacturing of high-quality devices. So, the main compromise to

find for the sintering temperature lies between the use of higher temperatures to reach

an improved mechanical adhesion or lower temperatures to reduce the shunting risk.

Alternative sintering possibilities (RTP sintering)

A single wafer RTP system SHS10 from the company AST Electronics, with an

excellent control over the working environment, a conveyor belt from the company

RTC, USA (model L500) and a tube furnace sintering tool were used in the following

experiments for the evaluation of the silicidation of the nickel contacts.

Each one of these tools has its advantages and disadvantages. The SHS10 is a

single wafer tool, so no high throughput is possible on this machine. The conveyor belt

furnace RTC-L500 is heated with IR lamps and has 4 different zones; the total time is

controlled by the speed of the belt and the entrance and exit are in contact with air. The

tube furnace can process many wafers simultaneously, but not continuously, which

would be more interesting for the industry. Figure 6-13 shows some of the typical


128 Manufacturing of solar cells with nickel-plated contacts

profiles that were programmed on the different furnaces, depending on the tool

capabilities.

The first approach applied to evaluate the silicide formation was done in a tube

furnace, because of its stability in our lab and the excellent environment control. This

evaluation was performed with high-efficiency precursors, because they provided a

deeper emitter which could demonstrate a higher sensitivity for the process. The

results are found in section 6.1.2. The contacts were sintered at temperatures between

300, 350 and 400 o C during 10 min. It was observed that after 10 min sintering at

350°C was too high to avoid shunting even on deep junctions.

Alternative thermal profiles which could be performed during a shorter period of

time and allow peak temperatures were then evaluated. Exemplary profiles are shown

on figure 6-13. Process 1 corresponds to a sintering in the tube furnace, while

temperatures up to 600°C were tested with profiles as P2 (Process 2) and P3

(Process 3).

To give an idea of the possible variation of this process, all the relevant data

corresponding to the relative change of the V oc and the pseudo Fill Factor (PFF), as

measured by SunsVoc after sintering are presented on figure 6-14.

Samples treated with temperatures above 500°C show a strong degradation.

Meanwhile, samples treated at a temperature ~300°C can be sintered during longer

periods of time without directly creating shunts in the junctions. At temperatures

between 400-450°C an improvement in the process can be achieved by reducing the

peak temperature.

Temperature [ o C]

500 P1

P2

400

P3

300

200

100

0

0 1 2 3 4 5 6 7 8 9 10

Time [min]

Figure 6-13 Example of different temperature profiles applied for the contact sintering. P1

can be performed in any of the 3 furnaces described above, P2 only in the SHS10 and P3 is

possible in the SHS10 or RTC-L500.


Integration to Industrial Processing 129

A detailed look into this effect is shown on figure 6-15 for P2-type processes (see

figure 6-13) performed in the SHS10. As the time of the peak temperature is reduced,

the impact of the sintering step on the junction quality at 400°C is reduced. When a

temperature over 500°C is used, there is a wide spreading of the data, meaning the loss

of the process control. Even though it would have been interesting to apply such

temperatures during a shorter peak time, this was not possible with the tools at hand.

This means that the integration path A presented on figure 6-2 is not compatible with

our processing capabilities, because the required firing step at temperatures over 500 o C

would result in the damage of the p-n junction.

0

0

-20

-20

V oc_rel

[%]

-40

-60

-80

- 300- 350 o C

- 400- 450 o C

- 500- 600 o C

V oc_rel

[%]

-40

-60

-80

400 o C

500 o C

600 o C

PFF rel

[%]

0

-20

-40

0 1000 2000 3000 4000

Thermal budget [C*min]

Figure 6-14 Relative change in V oc after

sintering with different processes depending

on the total thermal budget applied to the

samples

PFF rel

[%]

0

-20

-40

0 10 20 30

Time peak [s]

Figure 6-15 Relative change of V oc

depending on the time applied at different

peak temperatures

It was observed that as the wafers come out of the conveyor belt furnace, an

oxidation of the nickel seeds occurs. The adhesion between the Ni and the Ag is

affected by this oxide with the Ag fingers detaching from the surface after Ag plating.

A step to etch away the Ni oxide before Ag plating would be required.

These approaches are of great interest for the development of an industrial

manufacturing line with plated contacts. Nevertheless, they have not been further

investigated within this thesis. Instead, either a well controlled environment provided

by the tube furnace was applied or the simple sintering of the contacts on a hotplate.

Short anneal processes in RTP furnaces with plated contacts have been further used by

Bartsch and Tous [Bar11b, Tou11]. Another interesting approach, currently under

evaluation by Tous is the application of an excimer laser for the annealing of the metal

contacts [Tou12b].


130 Manufacturing of solar cells with nickel-plated contacts

6.2.6 Optimal integration sequence industrial cells

Figure 6-2 presented different integration paths which could be implemented in

theory for the industrial manufacturing of solar cells by inkjet printing, etching and

plating. In practice, there are several integration issues which rule out some of the

paths. Some of the boundary conditions defining feasible integration sequences are the

following:

• In order to perform the thickening of the front contacts by light induced plating, the

rear metallization is required, to enable the rear contact formation.

• The standard process for the manufacturing of screen-printed cells applies an

Al-BSF at the rear, which requires a firing step with a temperature at least over

577 o C.

• Such high temperatures are not compatible with the processing of cells already

plated with nickel on the p-n junction, since that would lead to severe shunts.

Therefore path A (figure 6-2), where the metal is printed on the rear after nickel

plating, but before the thickening of the contacts is ruled out for standard screenprinted

cells featuring an Al-BSF.

o The firing of the Al screen printing needs to be performed before the nickel

plating.

• The etching solution that is used to attack the ARC would dissolve the rear metal,

if it could not be masked during the process. This means, that for standard screenprinted

rough surfaces, the path B (figure 6-2), where the rear metallization is

performed before the etching of the ARC is not possible, at least for cells with a

pattern formed by wet-processing.

o . For laser ablated cells, where the nitride can be patterned on the front without

affecting the rear metallization, path B is an interesting option.

• Taking into account the HF dip required for the front-side removal, a short and

lowly concentrated HF dip (0.5%) should be applied, to avoid a strong attack of the

screen printing paste.

• The last remaining question is whether the sintering should be performed before or

after the Ag-LIP (comparing paths C and D in figure 6-2).

Ideally the simplest handling would be done by combining all the “wetprocesses

in one tool (nickel plating and contact thickening), then drying the wafers

and introducing them in a furnace for silicidation (path D, figure 6-2). Such a sequence

was tested during the evaluation of the plating process on shallow junctions. However,

the Ag contacts had only achieved ¼ of their full thickness. Taking into account only


Integration to Industrial Processing 131

those results, it is difficult to assess whether this path could be applied for high quality

devices, which would require 10-15 m of metal instead of ~2.5 m.

So, the feasibility of an “all-wet-processing”, followed by the dry treatment was

evaluated. After a 30 s 1% HF dip, the electroless nickel deposition was performed

using the Niposit TM PM980 solution. A pH around 10 was controlled by ammonia

addition. The standard electroless metal deposition was performed at a temperature

between 40°C and 60°C during 2 min, depending on the sample. After plating, the

edges were isolated with sandpaper to remove the nickel which had been deposited

over the non passivated perimeter. The wafers were then introduced in the laboratory

setup for the thickening by Ag-LIP of single wafers.

The fingers peeled-off either during the Ag plating step or during the rinsing

step. No further electrical tests could be done on the samples. In other words, when an

attempt to achieve the full metal thickness of the Ag-LIP contacts on the Ni seeds was

applied with the plating conditions mentioned before, the mechanical adhesion was not

strong enough to avoid the detachment of the fingers. It is interesting to note, that the

only reason why this sequence could be applied for the experiment with the shallow

junctions, was because of the low amount of Ag deposition on those wafers.

So, the optimal integration sequence obtained in this work for inkjet printed cells

with electroless nickel plated contacts corresponds to path C on figure 6-2: after

cleaning, texturing, diffusing the POCl 3 emitter; the ARC is patterned, the resist

removed. An HF dip is applied for the removal of the native oxide on the front pattern,

the cells are plated with nickel, sintered and the contacts are thickened with silver.


132 Manufacturing of solar cells with nickel-plated contacts

6.3 Integration to High-Efficiency Processing

On the previous chapter we showed that it is possible to deposit a fine nickel line

on a silicon wafer with a thickness as low as to 80-120 nm combining the electroless

plating process with a structuring technique for the front side dielectric. At the

beginning of this chapter we showed how this step can be applied to the production of

industrial silicon solar cells.

Different type of integration related issues are faced when manufacturing highefficiency

solar cells. In this section we present the results from the search for the

optimal process sequence to combine a rear metallization based on the laser fired

contact process (LFC), which enables the achievement of high-efficiency PERC-type

devices [Sch02]; with a front-side metallization formed by a nickel plated seed

contact. So, one of the most important questions to solve is the following:

At which moment should the LFC-based rear metallization be performed

when combined with a front-side plating-based metallization?

The structure of high-efficiency cells made at the Fraunhofer ISE with the laser

fired technology is presented on figure 6-16. The rear-side of the devices is passivated

by a dielectric layer, typically a thermal silicon dioxide.

Front

emitter

Rear Al

Front

contacts

Laser fired contacts

Rear

passivation

Figure 6-16 High-efficiency solar cell structure used for references and nickel plated cells

The most relevant integration paths considered in this work are presented

schematically on Fig 6-17. The open issues related to process integration are:

1. Should the nickel plating be performed before or after the Al deposition?

What’s the influence of the Al presence on the rear during the plating process? Is it

necessary for a better functioning of the nickel plating? (path A vs. B)

2. Should the LFC be performed before or after plating? (path B vs. C)

3. Is the sintering temperature applied for the front-side suitable for the annealing

of the passivation and the LFC contacts? (path C vs. path D)


Integration to High-Efficiency Processing 133

Roadmaps for the metallization of high efficiency cells based on nickel plating

Path A Path B Path C Path D

Structuring front

Rear Al deposition -

Ni plating

Structuring front

Rear Al deposition

Ni plating

LFC

Sintering

LFC

Ni plating

Ni plating

Sintering

LFC

Thickening: Ag LIP

Sintering

Thickening: Ag LIP

Figure 6-17 presents a schema of 4 metallization roadmaps for the development of highly

efficient silicon solar cells combining plating for the front-side and a passivated rear-side

with LFC. A short description for each path is as follows:

Path A: Front-side plating before rear metallization

Path B: Rear metal deposition (without LFC) before Ni plating

Path C: rear metal deposition and LFC before plating

Path D: Same as C with an extra sintering step customized for the optimization of the rear

side.

In the following, we study the implications of each path in the achievement of

high efficiency devices

Sample preparation

Laboratory cells were made on high-quality p-type Fz material. The base

resistivity of the material ranged between 0.5 to 1 .cm depending on the experiment.

The cells are manufactured according to the following procedure: After proper

RCA cleaning, the cells are thermally oxidized. Emitter windows with an area of 4 cm 2

are defined on the front side of the device by a photolithography step. In this step, the

rear side is protected while the oxide on the emitter windows area is etched away.

Different emitter profiles are defined by POCl 3 diffusion at temperatures varying from

790°C to 880°C, depending on the experiment. Standard high efficiency cells typically

feature a 120 /sq emitter after thermal oxidation. The phosphor silicate glass (PSG)

and the remaining oxide are completely removed after 4 min in a commercial SiO etch

solution (containing up to 6% buffered HF). After cleaning, a second thermal step is

performed at 1050°C for ~ 1 hour (with 40 min under an oxidizing atmosphere and 20

min under argon). In this step, a 105 nm thick thermal silicon oxide layer is formed.

This layer has two functions: It acts as a passivation layer on the front and rear side; as


134 Manufacturing of solar cells with nickel-plated contacts

well as the front side ARC. The rear side metallization consists on a 2 µm thick Al

evaporated coating contacted to the base by laser fired contacts.

A photolithography step is applied for the formation of the front-pattern. Lines of

7 µm wide are etched in the 105 nm oxide, while the rest of the wafer is protected by

the resist. The reference cells are then made by the evaporation of a metal stack of

Ti/Pd/Ag (50/50/100 nm). This is followed by a lift-off process, leaving thin metal

lines on the substrate. After a forming gas anneal in a tube furnace at 425°C during

25 min, the fingers are thickened in a silver bath by light induced plating, up to

10-12 µm height and about 30 µm wide.

For the finished plated cells evaluated on this section the metal stack evaporation

step is substituted for a nickel electroless plating step, which doesn’t require a lift-off

process afterwards. The sintering process is also adapted to the nickel silicidation.

6.3.1 Rear Aluminum

It would be possible to perform the nickel plating step prior to the Al deposition

on high-efficiency devices (paths A vs. B of figure 6-17). The firing step required for

industrial wafers, at temperatures over 577°C, is not required for the manufacturing of

high efficiency devices. For this reason, it would be possible to apply the nickel

growth prior to the Al deposition (comparing paths A and B).

The test structure applied for this experiment consisted of silicon wafers

featuring a 50 /sq emitter diffused over the whole surface and passivated by a

105 nm thermal oxide. A split was made as follows: half of the wafers were coated

with a 2 μm evaporated Al layer before the nickel deposition (type A samples) and the

other half was coated with Al after the nickel deposition (type B samples). The front

oxide was fully removed of some wafers from each group, while the remaining wafers

were structured by photolithography to create the front pattern. After cutting the

wafers into 25 x 25 mm 2 samples, all the samples were dipped in a 1% HF solution

and then immersed in the plating solution during 2-3 min. An alkaline solution based

on hypophosphite as a reducing agent with a pH~9.5-10.5 and a working temperature

~60°C was used for this step.

The patterned wafers were further processed. Type B samples received the 2 μm

evaporated Al coating after Ni plating. Then LFC contacts with a 1 mm pitch were

made on the rear on type A and type B samples. The front-contacts were thickened in

the Ag-LIP solution.

The time for the plating process to start is determined by the observation of the

H 2 generation (see eq. 5.1). Using this technique the time for the deposition on

passivated wafers with an Al coating (type A) is compared with the time of the onlyoxide

wafers (type B). For an easier visibility of the H 2 generation, the wafers without


Integration to High-Efficiency Processing 135

any oxide on the front side were used. In this way, the complete wafer surface is

coated with nickel and it is easier to see the H 2 formation.

The nickel deposition on type A samples starts about 10-15 s after immersion,

while the reaction on samples without Al takes about 5-10 s longer.

It is doubtful that the photovoltaic nature of the devices could have an impact on

this difference in plating time, since at the time when the samples are introduced in the

solution no electrical connection between the Al coating on the rear and the silicon

solar cell has been made. A potential source for this time difference could be related to

the increase of the bath activity through the H 2 generation on the rear side. This

increase in bath activity could affect the nickel deposition on the silicon in its turn,

reducing the starting time for plating. In order to provide a more solid response to the

nature of this time difference further tests should be performed.

During the Ag-LIP, it was observed that the mechanical adhesion of both

structures was very poor. Nevertheless, the adhesion of the Al coated wafers was

higher than the oxide-only wafers. A complete detachment of the nickel fingers during

the silver plating was observed on type B samples, while type A samples could be

fully plated and only some fingers detached during the rinsing step. No electrical

characterization could be performed in either case.

The impact of an ethanol dip previous to the immersion in the nickel solution, as

mentioned on section 5.3.1, was tested to see if it could improve the adhesion of the

metal fingers. But, even then, the metal was detached from the oxide-only wafers

during Ag LIP.

The same test was repeated applying the thermal step for the silicide formation

before the thickening of the contacts in the Ag-LIP solution. The split on the sample

preparation before Ni plating was repeated between type A wafers with an Al coating

on the rear versus type B wafers with oxide-only on the rear. After the Ni deposition, a

2 μm Al coating was evaporated on type B wafers. Sintering was performed at 300°C

during 10 min after the LFC. The corresponding IV results are presented in table 6-5.

Figure 6-18 shows SEM front-views comparing the two samples types after electroless

nickel. Sample type A is presented on the left, plating after Al evaporation and type B

is on the right (plating before the Al evaporation). Bigger nickel clusters distributed

close to the top of the pyramids are observed when the plating is done on wafers with

Al on the rear, even though no electrical contact had been formed yet between the Al

and the substrate. The deposition on type B samples shows a very homogeneous

coating formation, but also with less metal.


136 Manufacturing of solar cells with nickel-plated contacts

Type B samples show improved IV data as compared to type A samples (see

table 6-5). This could be the result of the homogeneous nickel deposition obtained

when the rear metal was deposited after the nickel plating step (see figure 6-18).

Efficiencies up to 18.9 % with improved PFF and a small standard deviation are

demonstrated for type B samples.

Type A samples show a strong deviation for some of the IV characteristics,

which is very strong for the fill factor and the open-circuit voltage. The presence of the

thick nickel clusters on the top of the pyramids on this type of samples is the likeliest

cause for this effect.

Table 6-5 IV data for 4 cm 2 cells featuring a 50 /sq emitter (no edge passivation), with e-less Ni

plated contacts performed before or after rear Al deposition

V OC J SC FF PFF

Rear surface during plating

[mV] [mA/cm²] [%] [%] [%]

Al: Type A

643 ± 9 38.2 ± 0.2 66.7 ± 7.7 16.4 ± 1.9 76.7 ± 1.0

Average

SiO 2 : Type B 659 ± 2 38.6 ± 0.3 73.5 ± 0.5 18.7 ± 0.2 79.0 ± 0.5

Al: Type A

633 38.2 73.4 17.8 77.7

Best

SiO 2 : Type B 661 38.9 73.4 18.9 79.2

Still, adhesion represents a very critical aspect for nickel plated contacts. While

the contacts on type B samples could be peeled off with a scotch tape after IV

characterization, type A samples delivered a higher adhesion.

Thus, from this series of experiments we conclude that:

• It is possible to deposit nickel on silicon without any metal coating on the rear.

Good IV characteristics can be achieved by this process. However, the adhesion of

the nickel coating plated in this manner is very poor, even though the deposition

quality seems more homogeneous on the sample without metal (see figure 6-18).

Figure 6-18 SEM front view of a plated nickel contact on a sample with a rear Al coating

during plating on the left side (type A) versus no rear-Al on the right side (type B)


Integration to High-Efficiency Processing 137

• The presence of aluminum in the plating solution enhances the activation of the

bath, resulting in a reduction of the starting time for the nickel plating and the

improvement of the surface adhesion. So it is beneficial to perform the nickel

plating step after the Al deposition.

• The front metallization process for samples with an Al coating before the nickel

deposition requires further optimization, to avoid the deviation observed on the IV

data.

• The deposition of a homogeneous Ni coating contributes to the reduction of the

impact of the nickel during plating on the PFF.

6.3.2 Laser Fired Contacts

Knowing the positive effect of the Al presence in the Nickel plating solution, it is

interesting to consider the possibility of performing the LFC before plating. For this

reason, silicon solar cells on a non-textured surface, featuring a 120 /sq, with a

thermal oxide passivation and a 2 μm evaporated Al coating are prepared. After

patterning the front side, the LFC contacts are formed on the rear with a 1 mm pitch. A

1% concentrated HF dip is followed by an ethanol dip. Then the wafers are plated in

an alkaline solution at 70°C and a pH 9 during a time ranging between 1 and 10 min.

The HF dip attacks the Al layer on the rear side (as mentioned in section 6.2.3)

The impact on the LFC contacts is much stronger than on the full metal coating which

is in between the contact dots, even for a 30 s dipping time. Figures 6-19 and 6-20 help

to visualize the source of this problem. A standard laser-fired contact is formed by a

core region where the Al/Si legation occurs, forming a p + doped area in the crater

center. This zone is connected to the rest of the Al coating by a thin Al layer in the

border zone, see figure 6-19 [Sch04]. Figure 6-20 shows how the Al on this borderzone

is removed after the HF dip; leaving the sight of the underlying SiO 2 passivation

layer. In this way, the electrical contact to the silicon formed on the crater center is

partly isolated from the rear metallization.

In some cases, these zones are reconnected by the plated nickel to the Al coating

after the plating steps, though, such a process is not very stable. Figure 6-21 shows two

examples of wafers processed in the same manner with very different results. On the

left side the LFC contacts are isolated from the rest of the rear metal coating, while on

the right side, the LFC contacts are reconnected to the rest of the Al coating by the

plated metal. However, the electrical characteristics of the reconnected cells show a

very poor response when compared to other high efficiency cells made with nickel

plating. Fill factors between 30 and 50% were achieved on such samples.


138 Manufacturing of solar cells with nickel-plated contacts

Figure 6-19 SEM cross view of an LFC

contact with a 10 O angle [Sch04]

Figure 6-20 Microscope top view of a LFC

contact after an HF dip (before plating)

Figure 6-21 microscope views of two different cells with LFC contacts after: the HF dip, the

electroless Ni and Ag LIP

During the electroless deposition, a very thin NiP coating is also deposited on top

of the aluminum layer. In some cases even Ag is plated on some areas on the rear.

Figure 6-22 shows an SEM view of the rear side with the definition of the areas where

an EDX analysis has been performed. The EDX analysis presented on figure 6-23

shows how Ag has been plated only on some parts of the contacts, while Ni and P can

be found on top of the Al. The C and O peaks are probably due to the contamination of

the sample in the atmosphere.

When the rear contacts are made after the Ni plating step, a NiP layer is formed

on the aluminum. This layer is also present during the LFC formation. Nevertheless,

no strong impact on the device performance has been observed for any of cells made

in this way (see table 6-5, type A samples). For this reason, we assume that the

interaction between the nickel and the aluminum probably occurs away from the p +

formation zone for the BSF of the LFC process. Hence path B with the LFC after Ni

plating from figure 6-17 would be preferable to path C.


Integration to High-Efficiency Processing 139

Figure 6-22 SEM view of a finished rear

side, featuring LFC contacts coated with Ni

and Ag in some areas. The squares

represent the areas where EDX analysis

was performed

Figure 6-23 EDX analysis showing the

presence of Ni and Ag on top of the Al layer

6.3.3 Annealing and silicide formation

The standard ARC layer used for the high efficiency silicon solar cells in this

thesis consists of a thermal SiO 2 coating. This layer is also responsible for the

passivation of the front side. Hofmann showed the influence of the annealing

temperature on the passivation quality of the front side SiO 2 layer on high-efficiency

silicon solar cells, similar to the ones treated here, with a front metallization based on a

TiPdAg evaporated seed and thickened with Ag-LIP. As seen on figure 2-11, the

short-wavelength region shows an improved IQE when an annealing temperature

≥ 400°C is applied. Thus, an annealing temperature over 400°C would be desired for

the optimal front passivation.

Moreover, a study performed by Schneiderlöchner on the integration of rear

oxide passivated high-efficiency samples contacted by LFC showed that the optimal

conversion efficiencies were obtained when the cells were sintered at 425°C during

25 min. He also mentioned that this trend was not observed when the wafers were

passivated with alternative passivation coatings (like PECVD SiN x ); the best results

were obtained with a thermal SiO 2 passivation [Sch04].

Taking into account both studies, it is possible to state that the ideal sintering

process for the optimal passivation quality of LFC cells on both front and rear sides,

consists of a sintering step under forming gas at 425°C during 25 min.

It would be desired to combine this thermal step with the one required to form a

nickel silicide on the front.


Integration to High-Efficiency Processing 141

This can be explained, by the improvement of the hydrogen passivation at the Si/SiO 2

interface, as well as the annealing of the LFC. As the temperature increases up to

400 o C a competing effect is observed: The metal diffusion towards the emitter junction

deteriorates the junction quality, which results in a V oc reduction.

Voltage [mV]

660

650

640

630

620

610

600

590

580

300 350 400

Sintering temperature [°C]

Suns Voc after:

Ni

Sintering

Ag plating

Voc:IV

Figure 6-24 SunsVoc and IV data samples with a deep emitter sintered for 10 min. An

improvement of the V oc is expected after the first sintering step, corresponding to the

annealing of the LFC and the oxide on the rear side.

Even though a sintering temperature over 400 o C would be beneficial for an

improved passivation quality, sintering temperatures over 350°C for processes lasting

10-20 min are not recommended for the silicidation of nickel plated silicon solar cells.

The easiest solution to this issue on high efficiency processing consists on the

performance of two thermal anneals: the first applied before plating, at temperatures

> 400 o C for ~25 min as recommended for the improvement of the oxide passivation

and then the second, applied for the nickel silicidation at a temperature ~300°C.

Another option would be to use of a special rapid thermal process (RTP) with an

alternative type of furnace that would allow control of the sintering profile starting

from very low temperatures (~210°C) at least up to temperatures around 600°C for

very short time periods. Such experiments were performed on industrial wafers and the

results are presented on section 6.2.5.

An important observation about the results presented in table 6-6 is that the

plating process was still under optimization. For this reason, even with the use of

deeper emitters and a silicidation process at low temperatures, a strong impact was

afflicted on the emitter. This can be seen by the low pseudo FF measured by SunsVoc.

The highest pseudo-FF achieved was only 77%.


142 Manufacturing of solar cells with nickel-plated contacts

An optimal sequence for the manufacturing of a nickel-based metallization for

high-efficiency LFC cells has been determined. Even with all the different

improvements applied on the nickel plating process and contact sintering a very low

FF is still observed on the finished devices presented here (see tables 6-5 and 6-6).

These low fill factors are most probably caused by the high recombination observed on

the wafer edges. These cells had no emitter windows. The contact between the emitter

and the non-passivated wafer edge has a very strong impact on the FF of small area

cells [McIn01].

To minimize the losses due to edge recombination, high efficiency cells are

typically manufactured through emitter windows, defined in a SiO 2 mask which is

applied before the POCl 3 diffusion (see figure 6-25 a and b). However, when the

electroless plating was evaluated on such samples, no metal deposition could be

obtained. The process only started on broken samples. For this reason, all the results

presented previously on this section were manufactured on cells without any edge

passivation and a direct contact from the front-emitter to the edges (see figure 6-25 c).

(a) (b) (c) (d)

Figure 6-25 Schema with different structures interesting for the plating experiments.

(a) Front view high efficiency cells at ISE on a 4” FZ wafer, (b) Cross section of the same

wafer, (c) Cross section of a full diffused emitter, (d) Structure used for the evaluation of

junction-edge plating issue .

Test structures as shown on figure 6-25 (d) were used to evaluate the effect of the

junction-edge in closer detail. The substrates were mirror polished Fz wafers. The cell

on the left side has a junction edge which is in contact with the solution at least on two

sides, while for the solar cell on the right side of the wafer all the edges are fully

passivated by p-Si base. A POCl 3 diffusion featuring an emitter with a sheet resistance

~ 20 /sq was used in this experiment. The samples were introduced in the solution

after the Al evaporation in the dark. No oxide mask was applied on the n-Si diffused

areas and plating was performed at pH ~9.5-10, at 60°C, during 2-10 min.

The devices with a highly recombinative edge which creates a contact between

the p-n junction and the nickel bath could be plated very well (on the left of

figure 6-25 d). On the devices where the edges were fully passivated plating did not

occur. The photo-assisted plating process is presented as a solution to this problem,

enabling the electroless plating on high-efficiency devices.


Photo-assisted nickel plating for silicon solar cells 143

6.4 Photo-assisted nickel plating for silicon solar cells

On the previous sections it was demonstrated that it is possible to obtain a very

low contact resistance with electroless nickel plating on silicon solar cells, but the risk

of shunting the shallow junctions is high due to the presence of thicker nickel clusters

on some areas of the fingers.

It was observed, on the high efficiency devices, that plating could only be

achieved if at least one edge could provide a contact between the highly recombinative

surface, the junction and the solution. Such wafers have a limited potential for a high

fill factor (at least for the small samples). In this section, we demonstrate that thanks to

the assistance of a light source placed directly in front of the solar cells during the

plating process, a more homogeneous nickel deposit can be achieved on both highefficiency

devices and industrial cells. All the plated cells presented prior to this

section were treated under standard room illumination conditions, inside a wet-bench

without an access to direct solar light or any strong light sources.

The positive influence of illumination during the electroless deposition of nickel

on silicon solar cells was observed as early as 1979 by Saha [Sah79]. This approach

has been recently reconsidered for the plating of nickel on high-efficiency solar cells

[Ale07]. In Saha’s experiment the only possible way to plate polished n-Si surfaces

with an alkaline solution at 20°C was by applying a light source during plating (rough

surfaces could nonetheless be plated even without the contribution of light). He

suggested the selective metal deposition of nickel by the irradiation of wafers through

a glass mask coated with a silver layer containing the front-metallization pattern.

Moreover, in most of the literature concerning electroless plating for the frontmetallization

of silicon solar cells deals with processing at relatively high temperatures

and high pH [And80, Coc84, Vit96, Ara98, Jen03, Guo05, Bou08]. At such

temperatures the bath is more reactive, but the plating solution is less stable. Here, we

show that the advantageous use of the photovoltaic effect widens the processing

window for plating, enabling a high quality metal deposition at temperatures around

20-40°C. Working at such temperatures facilitates pH control for ammonia-based

solutions.

The first experimental setup applied for the evaluation of this process is rather

simple (see figure 6-26). No contacts are required on the wafer. The plating solution is

poured into a square glass beaker. The wafers are immersed in the solution while

holding them with tweezers. The edge, were the samples are held, does not come in

contact with the bath. The wafers are then rinsed and the metallization continues as for

the standard electroless plating.


144 Manufacturing of solar cells with nickel-plated contacts

Ni 2+

6.4.1 Process development

H 2 PO 2

-

High-efficiency structures

Heater

Figure 6-26 Schema of the experimental setup applied for the firsts photo-assisted electroless

nickel plating tests

Using the test devices presented in the last section (see figure 6-25 d). The

influence of the light in electroless nickel plating was evaluated. The structure consists

on a p-Si wafer featuring two devices with a diffused emitter (R sheet ~ 20 /sq). One

has with fully passivated edges and the other device has two passivated edges and two

edges where the junction comes in contact with the solution. The rear side is

passivated with 105 nm SiO 2 and coated with an evaporated Al layer. The plating

feasibility with or without light was compared on those structures. The photo-assisted

experiments were performed by placing a halogen lamp with a power up to 35 W in

front of the plating setup. The wafers were held in the solution with the front-side

facing the lamp during the total plating time. The electroless plating process for both

cases was performed at 60°C, pH ~ 9.5-10.5, depending on the sample, and a total

plating time between 2 to 10 min.

With the standard plating process, only the devices with a contact between a p-n

junction on a non-passivated edge and the plating solution do plate; while the devices

with fully passivated edges do not, even though both samples have the same features

and coexist on the same wafer. On the contrary, when the photo-assisted process is

applied, both devices can be plated. The light enhancement enables the metal

deposition on samples even with fully passivated edges.

One of the challenges of the plating process is that the pH of the solution is

controlled by ammonia addition. Ammonia evaporates quite fast at high temperatures.

For an improved control over the bath pH, it would be desirable to perform the plating

process at lower temperatures. In the previous chapter, it was shown that plating at

higher temperatures contributes to a faster activation of the plating process while for

lower temperatures longer plating times are required before the deposition starts (see


Photo-assisted nickel plating for silicon solar cells 145

section 5.3.1). The following experiment was performed within the diploma thesis of

Norbert Bay to evaluate whether the light enhancement would enable plating at low

temperatures for highly performing silicon solar cells [Bay07].

Textured silicon solar cells manufactured on 0.5 . cm Fz p-Si with a full surface

diffusion, featuring a 90 /sq emitter with a random-pyramid front-texture are used

for this test. The front-side is passivated by a thin SiO 2 layer (~10 nm thick) coated by

a sputtered nitride layer (~65 nm thick). The rear-side is passivated by a 105 nm thick

SiO 2 layer and coated with 2 μm evaporated Al. After the front patterning by

photolithography, plating is performed with the photo-assisted process. The power of

the lamp was tuned between 19 and 35 W. A temperature of 60°C was compared to

processing at room temperature (~22°C). A plating time between 2 min at 60°C was

compared with 2 or 4 min at room temperature. Three solar cells are made for each

plating condition and one reference with the standard plating process. The LFC are

formed after nickel plating. The wafers are sintered under forming gas at 300°C during

10 min and the front contacts are finally thickened by Ag-LIP. The IV characteristics

with their corresponding plating parameters are presented in table 6-7.

The first observation from the IV data presented in table 6-7 is that the

V oc ~ 630 mV achieved by the cells is lower than expected for such a structure

(~660 mV for oxide-passivated wafers). The cause for such values is the damage on

the front passivation after the sputtering of the SiN x ARC. An excellent alternative for

an improved V oc response is to deposit the SiN x layer on top of the thin oxide by

PECVD instead of sputtering. The low fill factor obtained for the reference cell is in

the expected range, considering the data presented in table 6-5 for cells processed in a

similar manner and demonstrates again that the electroless process needs

improvement.

Table 6-7 Plating parameters and IV data for the first photo-assisted Ni plated silicon solar cells,

featuring a 90 /sq emitter with a SiO 2 /SiN x ARC and LFC contacts compared to a standard

electroless plated reference.

Temp Time Light V OC J SC FF

[°C] [min] [W] [mV] [mA/cm²] [%] [%]

60 2 0/ ref. 623 38.7 65.8 15.8

22 2 20-35 625 ± 3 38.2 ± 0.4 70.0 ± 3.5 16.7 ± 0.8

22 4 20-35 622 ± 2 38.4 ± 0.3 60.1 ± 4.6 14.3 ± 1.1

60 2 20-35 627 ± 1 38.3 ± 0.3 75.8 ± 0.3 18.2 ± 0.2

SEM front-views of the plated nickel for each plating condition presented in

table 6-7 are displayed on figure 6-27. It is possible to plate nickel on silicon with a


146 Manufacturing of solar cells with nickel-plated contacts

photo-assisted process. Actually, a thick coating is observed after 2 min processing,

even at room temperatures. This indicates that it would be possible to reduce the total

plating time. The metal deposition after 4 min is already so thick, that it results in poor

IV results, similar to the standard electroless process without light. Figures 6-27 (c and

d) show a thicker nickel deposit on top of the pyramids for both the 22°C and 60°C

plated samples. This might be the result of the combined effect between the electrical

field generated during illumination, which can orient the ions in that direction, and the

higher access for ion diffusion around these areas. In any case, it is interesting to note

that the best IV results were obtained for 60°C process after 2 min and the contribution

of a halogen lamp.

(a)

(b)

(c)

(d)

Figure 6-27 SEM front views for the plated nickel with following plating conditions: (a) 2’

at 60°C, and with light enhancement at (b) 4’ at 22°C, (c) 2’ at 22°C, (d) 2’ at 60°C. The pH

evaluated was between 9.3 and 10.3.

From this experiment we learn that:

• Using light enhancement, it is possible to fully plate the contact fingers at room

temperature.

• The best IV results are obtained for the light-assisted plating process at 60°C.


Photo-assisted nickel plating for silicon solar cells 147

• In the search towards a more homogeneous nickel deposition, the total plating time

of 2 min can be reduced.

Epitaxial emitters

The first test to evaluate the impact of the reduction of the plating time on the

device characteristics was performed on a simple structure consisting on epitaxial n +

emitters with a depth between 0.8 and 1.07 μm manufactured on 0.5 . cm Fz p-Si

wafers with an Al-BSF rear contact and a thin thermal oxide on the front as a plating

mask. The reference wafers were plated with the standard electroless process at 60°C,

pH ~9.5-10 during 2 min while the others were plated with the photo-assisted process

at the same temperature and pH, but only during 30 s. The cells were then sintered at

temperatures between 300°C and 400°C during 10 min in a tube furnace under

forming gas. The electrical characterization was performed by SunsVoc before and

after sintering. The average results and the standard deviation for the V oc and the

Pseudo Fill Factor (PFF) after plating (“a.p”) and after sintering (“sinter”) are

presented in figure 6-28.

Regardless of the sintering temperature applied, the V oc of the light-assisted

plated cells improved after sintering, also showing a reduction in the standard

deviation of the voltage measurement. The PFF of the cells plated with the lightassisted

process is also higher than the cells plated with the standard process, reaching

values up to 77 % compared to 60% after sintering, respectively.

Even though this experiment has been useful in the development of the photoassisted

plating process, the application of a nickel-based metallization on epitaxial

emitters requires further development. More information regarding the growth and

fabrication of the epitaxial emitters combined with alternative metallization concepts

and IV characterization has been published by Schmich [Schm09].

700

600

Standard

Light assisted

80

Voc [mV]

500

400

60

40

PFF [%]

300

a.p.

Sinter

a.p.

Sinter

20

Figure 6-28 SunsVoc and PFF for epi-emitters with an Al-BSF as plated (a.p) and after

sintering (Sinter) for standard plated cells versus a shorter light assisted electroless plating

process


148 Manufacturing of solar cells with nickel-plated contacts

Optimization of the photo-assisted plating process

Further tests were performed reducing the illumination time, while keeping

constant the total plating time. In this way, the light is used as an activation source,

which enables the deposition of very thin nickel coatings by the photo-assisted

electroless process. The SEM views presented in figure 6-29 show a comparison of the

resulting thickness for the optimized standard plating versus the photo-assisted plating.

The optimized photo-assisted process consists on the illumination of the samples only

for a few seconds, and then the finishing of the metallization for 1-2 min. Very

homogeneous fingers with ~30-50 nm thick nickel deposits are formed on the silicon

surface. IV results corresponding to cells processed in this manner are presented in

sections 6.4.3 and 6.4.4.

Figure 6-29 SEM views of nickel coatings on chemically etched silicon solar cells. (a): 50-

120 nm thick layer processed without light enhancement. (b) 30-50 nm thick coatings grown

through photo-assisted plating.

6.4.2 Working principle

A good model to study the influence of the light in the electroless process would

require a reliable model for the electroless nickel deposition. It was shown in the

previous chapter that the current models for electroless nickel plating do not fully

elucidate the deposition process, even on highly conductive metal surfaces. This

makes the full description of the influence of light in the process very challenging.

One of the main contributors to the current understanding of the influence of

semiconductors in electrochemical processes is H. Gerischer [Ger83].

The chemical reduction of a nickel salt on silicon occurs through the e - transfer

from filled energetic states on the surface of the semiconductor to the positively

charged metal ions in the solution [Osk98]. This means that the silicon surface must

provide an electron with the energy level required by the Ni 2+ ion for the anodic

transfer resulting in the metal deposition (see figure 6-30).


Photo-assisted nickel plating for silicon solar cells 149

Figure 6-30 illustration of possible electron transfer processes as anodic and cathodic

current via the conduction and the valence bands (after [Zhan01])

At least three different effects are observed through the activation of the silicon

electrode by light enhancement:

• Empty surface states are filled with electrons, which are then transferred to the Ni 2+

ion.

• The surface conductivity is increased.

• A field effect concentrates the ions on the tips of the pyramids where the current

density is increased, leading to a thicker deposit on this region depending on the

plating conditions (see figures 6-27c and 6-27d).

The factor most likely to contribute to the nickel deposition on the illuminated

silicon samples is the filling of surface states existing in the forbidden area, up to an

energetic level which is higher than the one obtained only by thermal activation (see

figure 6-31). In other words, the sudden availability of electrons at the

semiconductor/electrolyte interface, with the energy corresponding to the level

required for the nickel reduction, contributes to a more effective metal deposition.

E

E S C

E

E

E S C

E S C

Metal ion

E S V

E S V

E S V

Dark

Illuminated

Illuminated in presence of

an electron acceptor

Figure 6-31 position of the energy bands at the surface of a semiconductor in the dark and

under illumination (after [Kel82])


150 Manufacturing of solar cells with nickel-plated contacts

A second factor which could influence the nickel deposition by the photoactivation

of the silicon is the shift of the pinned surface states towards a more anodic

potential (see section 5.2.4)

Either way, the illumination of the sample contributes to a faster surface

activation, enabling the reduction of the total plating time and the achievement of a

very homogeneous and thin Ni coating in less than one minute (see figure 6-28).The

result is the deposition of a very thin and homogeneous nickel coating by the photoassisted

electroless process.

Consequently, the photo-assisted deposition of thinner seeds contributes to the

reduction of the risk of shunting narrow junctions. In section 6.4.2 it is shown that with

this modification, an improvement in the fill factor of industrial cells with shallow

emitters is achieved, from an average of 76 % to 78 %. The corresponding Pseudo Fill

Factor (PFF) increased from 77.2 % to 80 % [Ale09]

Takano found that during nickel plating, the photo-activation of carriers in

silicon substrates results in the nickel reduction by the oxidation of the silicon. This

could be considered some sort of displacement plating. However, this statement could

only be demonstrated for baths without reducing agents (see figure 5-6) [Tak99].

When the reducing agent was added to the plating solution, a different mechanism,

which could not be clarified, governed the deposition process [Tak99].

For electroless solutions working under illumination, there is most probably a

competition at the surface between the oxidation of the reducing agent (in this case the

hypophosphite) and the oxidation of the silicon, to provide the electrons required for

the Ni +2 ion reduction. The hypophosphite has a reduction potential around 1.57 in

alakaline solutions vs. around ~0.5 eV for silicon in alkaline solutions (using the

standard hydrogen electrode as reference electrode). In principle, it is more likely for

the hypophosphite to give away its electrons to the nickel through the silicon surface,

than for the silicon to oxidize itself. This does not exclude the possibility of the local

oxidation of the silicon at sites which could be favorable for this process.

Further research with a focus on understanding the distribution of localized

surface potentials for textured silicon substrates and distribution of the ions throughout

the surface would be helpful. Nevertheless, it is important to consider that the basics of

the electroless nickel plating process even on fully conductive surfaces have not been

fully elucidated, which makes such an in depth analysis a big challenge.


Photo-assisted nickel plating for silicon solar cells 151

6.4.3 Photo-assisted plating applied to industrial silicon solar cells

Silicon solar cells were manufactured following the integration path resulting

from section 6.1.6. The same type of 5x5 cm 2 solar cells made on p-Si, Cz substrates

were used for these tests. The cells feature a 55 /sq emitter and a front-pattern with

narrower openings as compared to the cells from table 6-4. The front-metallization

based on the electroless process without light enhancement for three samples is

compared the photo-assisted electroless nickel deposition for nine samples. The

illumination conditions were tuned between 19 W and 30 W for a total plating time

ranging from 30 s to 60 s and an illumination time between 10 s and 30 s. Average IV

results for all samples are presented in table 6-8, as well as a standard plated reference

which was processed at the same time and the screen-printed reference also shown in

table 6-4.

The variations in the illumination conditions do not have a strong impact in the

standard deviation of the photo-assisted plated cells (see table 6-7). This means that

there is a wide processing window for the illuminated samples within this range of

parameters.

Figure 6-32 presents the dark IV curves comparing samples manufactured with

the photo-assisted electroless process vs. the standard plating process. The standard

electroless plating results on strong variations of the FF (see table 6-8). The photoassisted

plated cells show a lower recombination current with improved FF.

Table 6-8 IV characteristics of silicon solar cells with seed contacts formed by photo-assisted

electroless nickel versus the standard plated nickel and the screen-printed ref.

Metallization

V OC j SC FF

[mV] [mA/cm²] [%] [%]

Screen-printed ref. 618 ± 0.8 36.2 ± 0.1 75.8 ± 0.3 17.0 ± 0.1

Electroless Ni 614 ± 5.8 36.2 ± 0.4 71.5 ± 4.7 15.9 ± 1.3

Photo-assisted eless. Ni 617 ± 1.0 36.4 ± 0.2 77.9 ± 0.6 17.5 ± 0.1

Using the two diode model for a silicon solar cell with a I 01 =1x10 -12 A/cm 2 ,

I 02 =4x10 -7 A/cm 2 , R s =0.15 cm 2 , the dark IV curve and the local ideality factor (m local )

for cells with increasing R p values are simulated as described by McIntosch [McIn01].

The results are presented in figure 6-33. The behavior of these curves shows the strong

similarity with the experimental dark-IV and the m loc curves for the plated devices.

Thanks to the reduction of the recombination losses close to the junction, a high shunt

resistance (R p ) is obtained for the photo-assisted plated cells. R p values calculated from

the dark IV curve at low voltages (~50-70 mV) for the standard plating process vary

from 1400 to 9.9 cm 2 ; while the photo-assisted plated cells have a R p , with an

average of 1.5x10 4 cm 2 and a standard deviation of 1x10 4 cm 2 .


152 Manufacturing of solar cells with nickel-plated contacts

j [mA/cm 2 ]

m Local

10 0

10 -1

10 -2

10 -3

10 -4

10 -5

10 -6

10 -7

10 -8

12

10

8

6

4

2

Standard NiP

Photoassisted

NiP

R p

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage [V]

FF (%)

64.2

74.8

78.2

78.7

Figure 6-32 Dark IV curves of inkjet

structured solar cells comparing the

electroless vs. the photo-assisted nickel

plating process

j [mA/cm 2 ]

m Local

10 0

10 -1

10 -2

10 -3

10 -4

10 -5

10 -6

10 -7

10 -8

12

10

8

6

4

2

2Diode For R p

1.5 . 10 3

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage [V]

5.0 . 10 4

2.5 . 10 5

5.0 . 10 6

R p

Figure 6-33 Simulated dark IV and m local

curves using a two diode model with

following parameters. I 01 =1 . 10 -12 A/cm 2 ,

I 02 =4 . 10 -7 A/cm 2 , R s =0.15 cm 2 . The R p

values are presented on the legend

A reduction in the recombination losses in the low-to-mid voltages is observed for

the photo-assisted cells. This implies that there is also a reduction of the I 02 current for

these cells. It would be interesting to perform a two dimensional simulation to evaluate

the impact of the physical advance of recombination centers with energy levels

corresponding to both interstitial and substitutional nickel atoms, approaching the p-n

junction in different localized areas of the surface. Such an evaluation is out of the

scope of this thesis, but it could help to lay out the basis for the impact of the nickel

metallization in front emitters. A more specific approach would be required for

electroless nickel, considering that the metal is deposited with P and the concentration

of the P depends on the plating conditions. Further evaluations regarding the diffusion

of the NiP within the junction as a function of the P concentration in the nickel and the

surface conditioning of the silicon would be required. The work from Tous is heading

in this direction, by evaluating the formation of the contact for PVD Ni coatings vs.

plated coatings [Tous12c]. The impact of the P in the electrical behavior of the

recombination centers from NiP is another interesting domain for future work.


Photo-assisted nickel plating for silicon solar cells 153

6.4.4 Photo-assisted plating applied to high efficiency cells

Silicon solar cells are manufactured on 4 inch, 250 μm thick p-type Fz wafers

with a base resistivity of 0.5 cm. The samples are processed as described on section

6.3 for high-efficiency cells. The cells feature a 120 /sq emitter. The 2 μm thick Al

coating is deposited on the rear before Ni plating. The samples are plated with the

photo-assisted electroless nickel plating process. The LFC process is performed after

Ni plating. The samples are annealed and the front contacts are thickened in the Ag-

LIP bath up to 10-12 μm. Reference cells are manufactured by evaporating a Ti/Pd/Ag

stack on the front and sintering in forming gas anneal at 425°C during 25 min. The

corresponding IV results are presented in table 6-9. The contact resistance was

measured by TLM and the series resistance was calculated by comparing the SunsVoc

with IV data at the MPP.

Table 6-9 IV results for high efficiency silicon cells comparing the Ti/Pd/Ag contacts vs. photoassisted

Ni plated contacts

V OC j SC FF R S

[mV] [mA/cm²] [%] [%] [cm²]

Ti/Pd/Ag _ref 667.8 38.6 81.3 20.9 0.55

Ni_PL 668.1 38.0 79.9 20.3* 0.56

Calibrated measurements with solar spectrum AM1.5g (Ed 1, 1989)

Although efficiencies greater than 20% are achieved for the Ni-plated cells, there

is still a considerable difference in the current density of the references and the Niplated

samples. An evaluation of the internal quantum efficiency for both types

showed that the response for the plated samples in the long wavelengths does not

correspond to an optimal one (see figure 6-34). To achieve a better response from a

SiO 2 passivating layer, a sintering step at higher temperatures (>400°C) is required.

Such temperatures applied for the 10 min sintering time used as a standard during this

work is not compatible with plated contacts.

In order to achieve highly efficient Si solar cells with plated contacts it is

recommended to perform a sintering step at 425°C during 25 min after the Al

deposition and before the Ni metallization. The combination of all these improvements

was applied on the ARC-laser ablated samples, which also demonstrate efficiencies

over 20% with this process (see section 6.5).


154 Manufacturing of solar cells with nickel-plated contacts

10 -1 Ti/Pd/Ag ref

IQE, R

1.0

0.8

0.6

0.4

0.2

Current density [A/cm²]

10 -2

10 -3

10 -4

10 -5

10 -6

10 -7

IQE / R Metal Sintering

/ Ti/Pd/Ag 425°C 25 min

/ Nickel 300°C 10 min 10 -9

10 -8

3

MPP region

PL + NiP

R sh

700 800 900 1000 1100 1200

[nm]

Figure 6-34 Detail showing the difference in

the long wavelength response for two wafers

from the same bath passivated and contacted

using the same procedure, but with different

sintering parameters.

m local

2

1

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage [V]

Figure 6-35 Dark IV curve and local ideality

factor for the best cell by Photolithography +

Nickel plating compared to the best

evaporated reference.

The dark IV curves for the best cell from each process show a higher loss current

at low voltages for the nickel-plated cell compared to the reference cell (see figure 6-

35). As it was presented for the industrial cells, the analysis of the evolution of the

local ideality factor (m loc ) indicates that a reduction in the parallel resistance is the

cause for the V oc reduction; even though the junction is about 1 µm thick. The R p is

reduced from a value of ~10 8 cm 2 for the evaporated contacts to ~10 6 cm 2 for the

nickel plated contacts. Since this difference becomes irrelevant close to the maximal

power point (figure 6-35), excellent IV characteristics are achieved for the plated cells.

The series resistance is similar for both metallization concepts (see table 6-9). Specific

contact resistance values below 1 m cm 2 were measured for the plated nickel

contacts on the lowly doped silicon emitters.


Laser-ablated and Ni-plated solar cells with optimized sequence 155

6.5 Laser-ablated and Ni-plated solar cells with optimized

sequence

This section presents the most relevant results obtained during this thesis by

combining the laser ablation of a dielectric with the formation of a nickel plating seed

layer. One of the most interesting aspects of combining the laser ablation technology

with nickel plated contacts is the strong interaction between the ablation and the

surface. Plating processes are enhanced by strong defect density or high roughness,

like a scratch on a wafer. It represents a challenge when trying to avoid back-ground

plating. But it is a benefit when plating laser ablated surfaces.

This part of the thesis has been developed in close collaboration with Annerose

Knorz and Andreas Grohe, who focused on the ablation of dielectric layers [Gro06,

Gro08, Kno09a]. An introduction to the ablation process is presented in chapter 4 and

compared with other patterning techniques for ARC which are relevant for plating

experiments.

The evaluation of the ablated and plated industrial samples is performed with a

ns-pulse laser. First, we combined this process with the standard electroless plating

with the Niposit PM 980 solution and then with the photo-assisted electroless plating

using the same solution. Photo-assisted plated cells structured by laser ablation are

compared to the cells structured by inkjet printing and etching, which were presented

previously on this chapter.

Plating on high-efficiency substrates ablated by ns-pulse laser is compared to the

ps-pulse laser ablation and the photolithography structured samples. As described in

section 6.3, these cells could only be plated using the photo-assisted plating sequence.

Taking into account the potential differences on the ablated surface, a comparison

between the plating with an alkaline and an acid plating solution was also performed.

During the ablation of the ARC there is a restructuring of the silicon surface. This

restructuring is different for ns vs. ps lasers [Kno09a]. Surface conditioning is crucial

for the metal deposition. Hence, the deposition could be affected by this step.

6.5.1 Laser ablated industrial solar cells

The industrial-type cells by laser ablation and plated silicon solar cells are

manufactured as follows: after cleaning and texturing, the n + emitter is formed by

POCl 3 diffusion and the SiN x ARC is deposited on the front-side. The rear-side of the

cells is coated with an Al layer and fired at temperatures between 900-930°C, to form

the Al-BSF. The front-side of the cells is ablated using a UV laser with a ns-laser

pulse. A nickel seed coating is formed on the ablated areas by electroless plating.

Then, the nickel silicide is formed before or after thickening the contacts by Ag-LIP.


156 Manufacturing of solar cells with nickel-plated contacts

The cells used for the first test have an area of 5x5 cm 2 , featuring a 50 /sq

emitter. Considering the relevance of this technology for industrial manufacturing, the

evaluation has been performed simultaneously on 1-3 .cm p-Si Cz material and on

multi-crystalline (mc) wafers. Reference cells were made by Ag paste screen-printing

on the front before firing.

Figure 6-36 SEM view of a wafer after laser ablation with the ns laser (left) and after nickel

plating (right)

Figure 6-36 shows an example of the electroless nickel plating on laser ablated

areas. Nickel is only deposited on top of areas where the dielectric has been fully

removed. The view on the left side shows a laser ablated spot and on the right another

laser ablated spot after the nickel deposition.

The metallization sequence applied on these cells is as follows: after Ni and Ag

plating, the cells are annealed on a hotplate. The Ag shield on the nickel enables

sintering on a hotplate, without the Ni oxidation risk. Each sample is treated during

30 s at temperatures ranging from 150°C to 500°C. The cells are characterized by IV,

SunsVoc and DLIT. The relative change of the V oc , the FF and the PFF as a function

of the sintering temperature is calculated using the respective values before sintering

as the reference. The results are presented on figure 6-37. There is no strong V oc

degradation for the multi-crystalline wafers even after sintering at temperatures up to

500°C and that the mono-crystalline samples only show degradation at temperatures

over 450°C. These results illustrate yet another difference for the silicidation process

when comparing these cells to standard plated cells with patterns formed by inkjet

printing and etching, where a strong V oc degradation was observed already at 400°C

for the same sintering time or even shorter times (see figure 6-15).

At the bottom part of figure 6-37 we see that after sintering the FF of mono

wafers increases about 5% rel. for temperatures up to 300°C. It stays quite steady up to

400°C, and degrades strongly starting 450°C. The PFF of these wafers is steady up to


Laser-ablated and Ni-plated solar cells with optimized sequence 157

250°C, after which it starts to decrease slowly at temperatures over 300°C and strongly

at 450°C. mc wafers, on the other hand, show around 8% rel. increase of the FF,

starting at 250°C and which stays in that range up to 500°C. The PFF of mc wafers

only begins to degrade slowly at temperatures over 400°C.

V oc_rel

[%]

FF rel

& PFF rel

[%]

10

0

-10

10

0

-10

IV - SunsV oc

- mono

- multi

0 150 300 450 600

Sintering temperature [C]

Efficiency [%]

18

16

14

12

10

8

6

4

- R s

- Cz

- Multi

100 200 300 400 500

Temperature [C]

5

4

3

2

1

0

R s

[.cm 2 ]

Figure 6-37 Relative change of V oc , FF and

Pseudo FF as a function of the sintering

temperature for Cz and mc samples, for a

30 s sintering process on a hotplate after Ni

and Ag plating.

Figure 6-38 Efficiency and R s versus

sintering temperature for laser ablated and

nickel plated mono and mc wafers.

The increase of the FF with a moderate increase of the sintering temperature is

explained by a reduction of the R s , probably due to an improvement in the contact

resistance. The same behavior was observed on high efficiency wafers (see

figure 6-25). As the temperature increases, the reduction of the PFF governs the FF

evolution.

The lowest series resistance measured for these samples after sintering was about

0.9 cm 2 . In the same way as for the experiment shown in section 6.2.5 (shallow

junctions and sintering temperature), the deposition of the Ag on these substrates

resulted in only about 4 μm thick silver fingers. Therefore, the cells were plated by

Ag-LIP for a second time, reaching a thickness up to 10-12 μm. The electrical

properties of the cells were measured. The cells were sintered again for 1 min at

200°C. Figure 6-38 shows the efficiency and R series after the full process as a function

of the sintering temperature and table 6-10 shows a comparison of the IV results for


158 Manufacturing of solar cells with nickel-plated contacts

the plated cells, with a sintering temperature ranging between 200 and 400°C as well

as the best cell, compared to the screen-printed references.

The R s decreased down to 0.3-0.2 cm 2 , for mono or mc wafers respectively.

Even though no strong change of the V oc or the FF were observed after the first

sintering step at 450°C, the cells sintered at that temperature or higher degrade after

the second Ag plating step, and even further after the second sintering step. DLIT

measurements of such samples are shown on figure 6-39. The second sintering step

was applied to improve the contact between the two silver depositions. Such a

sequence would not be required when the full contact is plated in a single step. Pseudo

fill factors up to 81% have been obtained with this process.

Table 6-10 IV data for laser ablated cells plated with the standard electroless process with the

SMT88 solution.

Material Metal V OC J SC FF R s PFF

[mV] [mA/cm²] [%] [%] [ . cm 2 ] [%]

Cz

Mono

Multicrystalline

(mc)

SP 621 ±0.8 34.5 ±0.1 78.0 ±0.7 16.7 ±0.2 0.75 ±0.1 81.7 ±0.1

Nickel

(BEST)

618 ±1.7

621

34.1 ±0.4

34.2

77.1 ±0.8

77.8

16.3 ±0.2

16.5

0.5 ±0.2

0.3

79.7 ±0.9

80

SP 618 ±0.4 31.7 ±0.1 78.4 ±0.2 15.4 ±0.1 0.8 ±0.1 81.2 ±0.1

Nickel

(BEST)

609 ±1.5

609

31.8 ±0.3

32.4

77.0 ±2.2

79.3

14.9 ±0.5

15.5

0.4 ±0.4

0.2

79.9 ±0.5

80

The DLIT measurements after the full metallization and the second sintering step

show a strong increase of the recombination current on the mc sample sintered at

450°C, with high recombinative spots all over the surface. The high current loss,

measured at 0.5V, explains the low efficiency, shown on figure 6-38, for such samples.

Wafers sintered at lower temperatures only show a strong edge recombination (left of

figure 6-39). It is interesting that the strong shunts are only observed after the second

Ag plating step with an enhancement after the second sintering step.


Laser-ablated and Ni-plated solar cells with optimized sequence 159

Figure 6-39 DLIT measurements taken after the full metallization process on mc solar cells

made by laser ablation and plating. First sintering at 250°C (left) and 450°C (right)

From these results we learn that:

• Structuring the ARC by ns-laser ablation has an impact on the silicidation process.

A higher resistance to shunting is observed on these wafers when compared to the

inkjet printed and etched samples featuring a similar emitter profile after diffusion.

This can be caused by changes: on the emitter, on the roughness and defect

concentration of the area where the nickel is deposited or even by the difference in

the distribution of the plated surface.

• The impact of the sintering temperature is less pronounced on multicrystalline

wafers that on the monocrystalline material (see figure 6-37). This can be caused

either by the sintering process in itself, or by potential differences in the ablation

process on the surface of the multicrystalline wafers (isotexture vs. random

pyramids). It becomes less relevant after the full plating of the contacts and an

extra sintering step at 200 °C.

• The series resistance decreases with increasing sintering temperature (for

temperatures up to ~300°C). Then it increases, contributing to the deterioration of

the device quality.

• The density of shunts increases with increasing temperature, which is probably the

cause for the PFF reduction.

• For the same final finger width, the total area directly contacted with nickel on

laser ablated samples is smaller than for fully etched fingers. So, in principle if

there is no damage from the structuring process, a higher V oc should be expected

from ablated samples, due to the smaller front recombination underneath the metal

fingers. Thanks to the low resistivity between nickel and silicon, no impact should

be observed on the series resistance due to the reduction of the metal contacted

area.

Photo-assisted plating for laser ablation versus inkjet printing and

etching

The improvement of the nickel deposition process through the photo-activation

was also tested on cells patterned by laser ablation and compared to the patterning by

inkjet printed and etched samples and to screen-printed references.


160 Manufacturing of solar cells with nickel-plated contacts

REF

Type 1 T ype 2

Clean+ texture

Emitter formation: ~50 /sq

ARC dep osition: PECVD ARC SiN x

SP ref

IJ front mask

Etch openings

Strip resist

Al print rear + fire

Laser ablation

Ph oto-assisted E-less NiP

Sintering

Ag LIP

Figure 6-40 Schema for manufacturing of the photo-assisted plated cells

KI

The same substrates as used in the photo-assisted process development were used

for this comparison. Four cells with an area of 5x5 cm 2 were formed on 125x125 mm 2

wafers, featuring a 50 /sq emitter diffusion and a ~75 nm thick PECVD SiN x as the

ARC. The distance between the fingers is ~2 mm. A schema for the processing of

these cells is presented on figure 6-40.

The ablation of cells was performed using a UV laser with an ns-laser pulse. The

contact width for ablated samples is ~30 - 40 µm, while for the inkjet printed and

etched samples it is ~100-120 µm, same as the screen-printed reference. The cells

were characterized with light and dark IV, SunsVoc. The average results for the screen

printed and etched samples were already presented in table 6-8, and repeated in table

6-11 for a better comparison with the data corresponding to laser-ablated cells.

Very good results were achieved through the combination of alternative

patterning techniques and photo-assisted nickel plating. They illustrate the possibility

to obtain a better performance by applying alternative technologies (for the patterning

as well as for the metal deposition).

The series resistance was reduced down to 70% of the value for the Ag pastes

though the use of Ni and Ag plated contacts (see best ablated cell vs. ref.). An

improvement on j sc up to 0.5 mA/cm 2 was reached by the application of thinner lines

thanks to the front patterning by laser ablation and to the lower contact resistivity.


Laser-ablated and Ni-plated solar cells with optimized sequence 161

Table 6-11 IV data for photo-assisted plated cells compared to the SP reference

V oc jsc FF R s PFF

[mV] [mA/cm 2 ] [%] [%] [ . cm 2 ] [%]

Ref. Best 619 36.3 76.3 17.1 0.7 80.6

IJ Best 617 36.4 78.2 17.6 0.4 80.2

LA Best 610 36.5 78.3 17.5 0.2 78.0

Ref. 618 ± 0.8 36.2 ± 0.1 75.8 ± 0.3 17.0 ± 0.1 0.7 ± 0.1 80.3 ± 0.2

IJ 617 ± 1.0 36.4 ± 0.2 77.9 ± 0.6 17.5 ± 0.1 0.4 ± 0.1 80.0 ± 0.7

LA 614 ± 3.5 36.7 ± 0.1 75.0 ± 2.6 16.9 ± 0.5 0.5 ± 0.3 77.7 ± 0.6

Reasonable PFF were obtained with the plated nickel, though there is room for

optimization on the ablated cells. An average PFF of ~78% shows that there is still

some damage on the space charge region. For a good comparison, the same plating and

sintering parameters were used on the etched and ablated wafers. The process was

optimized for the etched cells. Considering the differences in the silicide formation

process for ablated samples, an optimization of the thermal step, applied to the laserbased

sequence might lead to better results.

Higher V oc values were obtained for both the reference cells and the printed and

etched samples. A potential explanation for this performance could be the formation of

alternative paths for metal diffusion on the ablated areas during silicidation. However,

the evaluation of photo-assisted vs. standard plated samples showed that when the

cells are shunted due to metal diffusion, this effect strongly affects the FF (with values

down to 71%, see figure 6-31). With a FF over 77%, such a strong effect is not

expected on these wafers, even though it cannot be fully excluded. An alternative

explanation is provided by the possible increase of the recombination current due to

the laser treatment on the front surface.

The m Loc curves illustrate the differences for the three cell types (see figure

6-41). The local ideality factor of the laser ablated cell at high injection levels

(voltages over 0.6 V) is lower than one. As mentioned by Sinton, Cuevas and Glunz

[Sin00, Glun07] such behavior is characteristic for Schottky contacts. However, this

behavior has not been observed in other experiments or in high efficiency samples.

The lack of more reliable IV results on the ablated wafers is probably related to the

low metal coverage on the busbars on most of these samples.

The lower short-circuit current observed for the inkjet printed and etched samples

when compared to the laser ablated cells is due to the high shadowing losses resulting

from fingers twice as wide. This difference is only related to a problem with underetching

of the ARC, which does not represent a standard in the processing of such

features.


162 Manufacturing of solar cells with nickel-plated contacts

The m loc curve shows how at maximal power point the ablated samples have a

lower performance than the other two. Besides, it presents a strong increase of current

in the high voltage region. The cause is not clear. It might be a consequence of poor

contacting on the busbars during the measurement. For all the cells, the overall level of

the local ideality factor goes beyond 2, but is still comparable to the screen-printed

cells, or even lower, as for the IJ cell.

Current density [A/cm²]

m local

10 0

10 -1

MPP region

10 -2

10 -3

10 -4

10 -5

10 -6

SP cell

10 -7

IJ cell

LA cell

10 -8

4

3

2

1

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage [V]

BB issue

Figure 6-41 Dark IV and m loc curves for the best cells for each techonology: SP: Screenprinting,

IJ: Inkjet print, etched and Ni-Ag plated, LA Laser ablated and Ni-Ag plated.

6.5.2 High-efficiency silicon solar cells by photo-assisted plating

Deep lowly doped emitters passivated by SiO 2

Considering the low absorption of the oxide layer, the ps-laser is required to

perform the ablation on the high efficiency samples.

Silicon solar cells were manufactured on 250 µm thick p-type Fz wafers. A thick

SiO 2 layer was used as a diffusion and etching mask. An active area of 4 cm 2 was

defined by photolithography on the front-side and textured with random pyramids.

This emitter type was formed by a POCl 3 diffusion at 790°C, followed by PSG and

oxide removal. The emitter was finished with a 2 nd oxidation and a P drive-in. The

further P diffusion reduces the surface doping concentration to N s = 7 x 10 18 cm -3 and

creates a junction with 1.2 µm depth. The final sheet resistance is R sh = 120 /sq. The


Laser-ablated and Ni-plated solar cells with optimized sequence 163

rear-side consists of a 105 nm SiO 2 passivation layer under a 2 µm thick Al coating.

The contacts between the Al and the Si base are formed by LFC. The 105 nm thick

SiO 2 layer was also used as ARC on the front-side. Structuring of the ARC was

performed by ps-laser ablation, with line openings ~ 20 µm wide. A reference cell was

structured by photolithography (PL) and wet-chemical etching with HF.

After this step, the wafers are dipped in 1 % HF for 30 s for native oxide

removal. Metal seeds are deposited by photo-assisted nickel plating, using a halogen

lamp. The wafers are irradiated only for a short time in order to activate the surface.

After the metal deposition, the silicidation is performed in forming gas at 300°C for 10

min. The contacts are thickened by light induced Ag plating. The series resistance (R s )

is calculated from the difference between light IV and SunsVoc measurements.

Considering the differences in the surface of the ablated cells, two different

plating solutions are evaluated on these samples, an alkaline solution (Niposit PM980)

working at pH 10 and 40°C and an acidic solution (SMT88) at 80°C. In both cases, the

photo-assisted deposition lasted 1 minute. The results are shown in table 6-12.

Table 6-12 IV characteristics of a deep, lowly-doped emitter (R sh = 120 /sq), contacted by

nickel plating, comparing the alkaline and acidic plating solutions

Solution

V oc j sc FF PFF R s_ Suns

[mV] [mA/cm 2 ] [%] [%] [%] [mcm 2 ]

Alkaline Best 659 38.43 80.6 20.4 * 2 83.4 0.5

Acidic Best 658 38.01 80.7 20.1 * 2 83.3 0.36

Alkaline Average 655.± 2 38.0 ± 0.2 79.9 ± 0.7 19.9 ± 0.2 83.1±1.2 0.5±0.2

Acidic Average 655 ± 1 38.0 ± 0.2 80.2 ± 0.5 19.9 ± 0.1 82.9±0.6 0.5±0.2

The results for the best plated wafers are compared with the photolithography

reference also metalized with Ni. The results are presented in table 6-13. The SR and

Reflection curves were measured for the determination of the internal quantum

efficiency.

A lower j sc is measured for the photolithography reference. This is caused by a

low response in the internal quantum efficiency (IQE) at long wavelengths. During the

process development, a single temperature step was used for: the annealing of the front

and rear passivation, the silicidation and LFC anneal. As it was explained in chapter 2;

this process is not sufficient for a high-quality passivation of the rear side with

thermally grown SiO 2 coatings [Hof09]. Therefore, further laser ablated cells were

annealed at 425°C for 25 min prior nickel plating and then annealed for a second time

after the metal deposition at 300°C for 10 min. The resulting IQE are shown on figure

6-42.


164 Manufacturing of solar cells with nickel-plated contacts

IQE, R

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

700 800 900 1000 1100 1200

All cells with Nickel

PL 300C 10min

LA 300C 10 min

LA 425C 25 min

[nm]

Figure 6-42 Internal quantum efficiency at long wavelenghts for nickel plated cells

The lower V oc obtained for the ablated cells when compared to the nickel plated

PL reference (see table 6-13) can be the result of some laser induced damage that

would increase the surface recombination velocity (SRV) on the front-side. A higher

SRV results in the reduction of the open-circuit voltage of the device. Another

potential cause for an increase in the SRV is a higher effective metal coverage for the

ablated fingers as compared to the PL fingers. Considering that the openings of the

ablated fingers are not continuous, and that they depend on the laser process applied

for the sample, the quantification of these differences is material for further research.

The PL fingers were 8 µm wide, while the max finger size of the ablated fingers was

~20 µm. A PC1D simulation was used to extract the SRV of the best PL treated and

ps-ablated cells obtaining SRVs ~ 4 x 10 3 and 8 x 10 3 cm/s, respectively.

Table 6-13 IV characteristics of a deep, lowly-doped emitter (R sh = 120 /sq), contacted by

nickel plating, comparing the best PL reference and the laser ablated cell, both plated with the

alkaline solution

Structuring

V oc j sc FF PFF R s_ Suns

[mV] [mA/cm 2 ] [%] [%] [%] [mcm 2 ]

PL + Ni best 668 38.04 79.9 20.3 * 1 83.2 0.7

ps-laser + Ni best 659 38.43 80.6 20.4 * 2 83.4 0.5

* Calibrated measurements

1

with old solar spectrum AM1.5g (Ed 1, 1989)

2 with new AM1.5g (Ed. 2, 2008) spectrum


Laser-ablated and Ni-plated solar cells with optimized sequence 165

The thickness of the Ni coatings on the laser ablated samples is measured at

different spots from SEM cross section views, like the one presented in figure 6-43.

Variations in the thickness are observed along the same wafer, with thicknesses

ranging from 0.5 µm up to 1.5 µm of nickel. We note that these wafers are plated

using the same process as for the wet-chemically treated samples, which had delivered

a thickness ~30-50 nm. So, the nickel seed on the laser ablated spots is thicker than on

wet-chemically treated wafers. This can be originated by the higher local current

density of the small laser openings combined with the improved passivation around the

spots, leading to a higher local photo-current. The reorganization of the surface after

the laser ablation can also have an impact on the surface conductivity, as well as on the

distribution of the surface states. In any case, considering the high FF and PFF

achieved with these cells, it is possible to assume that interface effects must play a

beneficial role during the silicidation, slowing down the metal diffusion. This diffusion

effect, as well as the impact of the local current density on the plating thickness for

laser ablated vs. wet-etched samples, require a throughout investigation.

(a)

(b)

(c)

Figure 6-43 SEM views of high efficiency samples after laser ablation and nickel plating.

(a) cross-section, (b) angle view, (c) after Ag LIP.


166 Manufacturing of solar cells with nickel-plated contacts

Shallow, highly doped, high sheet resistance emitters

These substrates are manufactured with slight changes compared to the deep

emitters presented previously. A higher temperature is applied during the diffusion to

form a 0.4 µm deep emitter with N s = 2 x 10 20 cm -3 and R sh = 110 /sq. No oxidation is

performed, to avoid the further diffusion of the P in the emitter. Instead of this, a

75 nm thick PECVD SiN x coating is deposited after PSG etching, as the ARC. The

rear-side is formed by an oxide layer, underneath 2 µm Al layer contacted by LFC. An

annealing step is performed at 425°C during 25 min in forming gas prior front-side

metallization to improve the rear-oxide passivation. Structuring of the SiN x coating is

performed by laser ablation using a ps- and a ns-laser sources. Details about the

ablation process have been presented elsewhere [Kno09b]. The front-metallization is

performed with the alkaline solution. The results for both lasers are presented in table

6-14. A comparison of the SR for this emitter and the deep emitter is presented on

figure 6-44. Figure 6-45 shows SEM views of the ps-laser ablated sample after

ablation, HF dip and after plating.

1.0

IQE, R

0.8

0.6

0.4

0.2

Type A: Deep 120/sq

Type B: Steep 110/sq

Laser ablated + Ni Plated cells

IQE / R

120/sq, SiO 2

110 /sq, SiN x

0.0

400 500 600 700 800 900 1000 1100 1200

[nm]

Figure 6-44 IQE and R for the deep emitter versus the shallow emitter both contacted by laser

ablation, nickel and Ag-LIP on the front and LFC on the rear.

Table 6-14 IV characteristics of steep emitters N s ~2 x 10 20 cm -3 , contacted by nickel plating

Structuring

V oc j sc FF

[mV] [mA/cm 2 ] [%] [%]

ns-laser 651 39.4 80.7 20.7

ps-laser 634 38.7 80.0 19.7


Laser-ablated and Ni-plated solar cells with optimized sequence 167

(a)

(b)

(c)

Figure 6-45 SEM views of the laser ablated samples after (a) laser ablation, (b) HF dip, (c)

Ni deposition

The high FF presented in table 6-14, lead to the assumption of very low damage

infringement on the narrow junction. This demonstrates a metallization concept where

the formation of a selective emitter is not necessary for the achievement of high

efficiencies with nickel plated contacts.

The difference in the current for both lasers is explained by a wider spot for the

ps laser than for the ns laser. This affects both the current, due to the formation of

wider fingers, as well as the voltage, due to the increased front-recombination.

The improved reflection for shallow emitters, with better AR properties thanks to

the use of the SiN x instead of the SiO 2 , also contributes to an increase in the short

circuit currents achieved by these cells when compared to the results presented in table

6-13.

Considering the process simplification for using a highly resistive shallow

emitter for the industry, it would be recommendable to keep on working towards such

emitter designs in the future.


168 Manufacturing of solar cells with nickel-plated contacts

Thanks to the improvement of the laser ablation, the nickel plating and the

sintering process, this possibility opens up the path towards high efficiencies with

industry relevant technologies!


Alternative plating mechanisms 169

6.6 Alternative plating mechanisms

We developed a base sequence process for the plating of both industrial and

high-efficiency solar cells. The reliability of this process is still strongly affected by

the adhesion of the nickel contacts on the silicon wafers. Therefore, by the end of this

thesis, using the optimal process sequence for industrial solar cells, further tests were

performed to investigate alternative plating mechanisms which might help to obtain an

improved adhesion.

6.6.1 Electroless plating with Ni alloys

Nickel alloys have been used in the past as an alternative to pure nickel coatings

in order to reduce the leakage current after the nickel silicide formation [Chi01].

Plating solutions capable of delivering a Co, NiW or NiCo deposits are available

on the market. They are mostly used for applications in the automotive industry. Both

cobalt and tungsten have diffusion coefficients which are a few orders of magnitude

smaller than nickel in silicon (see section 2.8.1). Thus, the deposition of cobalt or

nickel alloys represents an interesting approach in the search towards the reduction of

the fast metal diffusion through the junction during the sintering step. In addition, it

has been shown by Liu that the deposition of NiCoP coatings from hypophosphite

based baths is possible on n-type Si after activation with SnCl 2 and PdCl 2 [Liu07].

Considering that the electrochemical potential of cobalt is very similar to that of

nickel, in principle it would be possible to deposit just pure cobalt on silicon [Liu04].

Even though information was found on the literature showing that it is possible, the

tests performed in this direction in house, were not successful. The adhesion of the

layers was not strong enough to be combined with a full metallization process.

Table 6-15 Components and plating parameters applied for the alternative plating solutions

Component Formula NiCo bath

Cobaltsulfate Heptahydrate [g] CoSO 4 . 7H 2 O 16.6

Nickelsulfate Hexahydrate [g] NiSO 4 . 6 H 2 O 15.6

Sodium hypophosphite [g] NaPO 2 H 2 26.5

Ammoniumchloride [ml] NH 4 Cl 25

Tartrate [g]

C 4 H 4 O 6

2−

50

Water [ml] H 2 O 500

pH 8

T [C] 80

time [min] 4


170 Manufacturing of solar cells with nickel-plated contacts

Taking into account the results from Liu on the deposition of NiCoP alloys this

process was evaluated; using the solution is presented in table 6-15. The composition

of this mixture was taken from the book by Riedel, substituting the citric acid by

tartrate. Thanks to the collaboration with an industrial partner (MacDermid), further

metal alloy deposition tests were performed and compared to their electroless Ni

plating deposition.

Small-area industrial type solar cells were made on p-Si Cz wafers ~200 μm

thick, with a base resistivity of 1-3 .cm and an emitter featuring a sheet resistance of

55 /sq. The rear side was contacted by Al-BSF and the front-side was patterned by

inkjet printing. After the seed plating the cells were sintered at 300°C and thickened

with the Ag-LIP process. The cells were characterized with the IV tester and the

SunsVoc. The R s is calculated from the difference between the 2 measurements. The

results are presented in table 6-16.

After the IV characterization, a sintering test was performed on the best NiCoP

plated cell. Using a hotplate, the cell was annealed at 400°C for an extra minute, then

it was characterized, sintered at 500°C for another minute and characterized again. The

most relevant results for the sintering tests are also presented in table 6-16.

Table 6-16 IV small area industrial cells with NiP and NiCoP alloy plated contacts

Sintering V OC j SC FF R S PFF

[°C] – [min] [mV] [mA/cm²] [%] [%] [ . cm²] [%]

NiP (industrial) 300 - 10 610 35.6 80.2 17.4 0.1 82

NiCoP (Bath X) 300 - 10 598 35.0 71.9 15.0

300 - 10 622 36.8 73.8 16.9 0.8 78

NiCoP (industrial)

+ 400 - 1 622 37.0 75.2 17.3 0.6 79

+ 500 - 1 622 36.8 75.1 17.2 0.8 79

The first results worked well as a proof-of-concept of the alloy plating process.

But the poor V oc results show that the deposition process requires further optimization.

Through the results obtained from the industrial baths we demonstrate that the

electroless deposition of a NiCo alloy enables sintering at temperatures up to 500°C on

industrial emitters without the degradation of the electrical behavior.

These experiments provide a first look into the possibility of using electroless

plated alloys for sintering at higher temperatures, opening up a new research line for

the front-metallization of silicon solar cells. Further study is required for the

optimization and better understanding of such processes, but it goes beyond the extent


Alternative plating mechanisms 171

of this thesis. We focused further on the development of nickel based plating

processes.

Experiments with NiW alloys showed that it is possible to deposit this alloy on

the solar cells as well, but the cells could not be finished due to poor adhesion. During

the thickening step the contacts fell-off.

6.6.2 Light-Induced Nickel Electro-Plating

Electroless solutions enable the self aligned contact formation of nickel seeds on

silicon wafers. Nevertheless, their use requires excellent control of the chemicals.

They are more reactive than electroplating solutions, making the last easier to work

with. Taking advantage of the photovoltaic effect enables the electrochemical

deposition of nickel layers on silicon solar cells. To achieve the metal deposition, a

voltage is applied on the rear side of the cells and the electrochemical reduction is

realized on the front and the cell is illuminated during the process in order to get the

deposition on the front-side.

Experimental

A very simple experimental setup was used for these first evaluations. A halogen

lamp was placed in front of a square beaker containing the plating solution. The cells

were immersed in the solution in a vertical position and contacted on the top part of

the rear side. Random pyramid textured wafers featuring a 50 /sq emitter, patterned

by inkjet printing and etching and contacted by Al-BSF were used for these tests.

The composition of the plating

solution used in this first experiment was

suggested by Steffen and it can be found

in table 6-17 (Bath 1) [Ste08]. The

impact of the following processing

parameters was evaluated during this

test: rear-voltage, the lamp intensity, and

plating time. The nickel deposition is

evaluated at a bath temperature ~50°C.

The specific range of data applied for

each parameter is presented in table 6-18.

After plating, the wafers are sintered at

300°C during 5 min and the contacts are

Figure 6-46 Schema for the plating process

applied during LIP [Met06]

thickened by Ag-LIP. Three cells are manufactured for each plating condition. The

cells are measured with the IV tester, SunsVoc. The metal thickness is determined by

XRF.

Table 6-17 Bath composition for Ni electroplating solutions


172 Manufacturing of solar cells with nickel-plated contacts

Name of the chemical Formula Bath 1 Bath 2

Nickelsulfate Hexahydrate NiSO . 4 6H 2 O 155 g 1.55

Nickelchlorid NiCl 2 10 g 0.1

Borsäure H 3 BO 3 20 g 20g

Dihexyl sulfosuccinate 85 mg 85 mg

Octane sulfonic acid (Na-Salt Monohydrate) 175 mg 175 mg

Water 500 ml 500 ml

Table 6-18 Parameters used in the first test of Light-Induced Nickel Electro-Plating

Applied Rear-Voltage [V] -0.5 -1

Lamp [Volts]

7 to 11 Volts

Plating time [min

1 to 5 min

Figure 6-47 SEM views for electroplated contacts after the 1 st Ni-LIP tests (left), with an

improved pulse plating process (right)

From the first test we learned that

• Too strong negative rear potentials result in poor adhesion and the full detachment

of the nickel fingers: None of the cells with a rear voltage of -1V could be finished.

• For higher light intensity thicker Ni coatings were achieved. The homogeneity of

the illumination affected the nickel deposition, showing that a better setup is

required for the optimization of the process.

• Poor device characteristics were obtained for this test, but a Pseudo FF up to 80%

was observed.

With the implementation of the pulse-plating process which was patented by

Radtke [Rad09], the electrodeposition was further improved. Very homogeneous

coatings can be formed with this technique (right view on figure 6-47). Also a much


Alternative plating mechanisms 173

higher adhesion is achieved between the nickel and the silicon, enabling the thickening

of the contacts by Ag-LIP previous to sintering. As mentioned in section 6.2.6 such a

sequence would be helpful in the development of industrial tools, with the

combination of all-wet-processing followed by the thermal treatment.

The pulse plating process with the following cycle was performed: 50 ms

cathode potential, 10 ms anode potential and an off-time of 200 ms. A rear voltage of -

0.5 V was applied and the cells were illuminated throughout the whole process. The

impact of the concentration of the nickel ions in the solution is evaluated by diluting

the original solution (bath 1) to form the bath 2 presented in table 6-17. The total

number of cycle was tuned between 150, 300 and 600. This corresponds to a total

plating time of 39, 78 and 156 s, respectively. 3 samples are manufactured for each

plating time and for each solution. The nickel thickness is measured after the Ni LIP

by XRF (see figure 6-48).

As Fig 6-48 illustrates, the thickness of the deposited nickel layer depends on the

ion concentration in the solution. The use of the lower concentrated solution results in

the formation of a thinner coating. The thickness grows linear with time for the diluted

solution (~2 μm/h), while it increases with the square root function for the

concentrated bath with a deposition rate of (4.4 μm/h after 30 s).

Ni Thickness [nm]

500

400

300

200

100

Bath 1

Bath 2 (10x dilute)

0

20 40 60 80 100 120 140 160

Plating time [s]

Figure 6-48 Electroplated Ni thickness for

the concentrated and the dilute baths (table.

6-17), as a function of the plating time.

Efficiency [%]

PFF [%]

18

16

14

12

10

8

6

80

78

76

74

72

B1 - B2

- After Ag

- Sintered

0 100 200 300 400

Nickel Thickness [nm]

Figure 6-49 Average efficiency and Pseudo

FF depending on the nickel thickness for the

concentrated Ni solution (B1) and the diluted

solution B2 after Ag deposition and after full

sintering. The chemical composition for both

solutions is provided in table 6 -17


174 Manufacturing of solar cells with nickel-plated contacts

The contacts were then thickened by Ag-LIP. Sintering was performed on a

hotplate first at 200°C during 1 min and then at 300°C for a subsequent minute. The

electrical characterization on the full contacts is performed after each step. The most

relevant IV data is presented on figure 6-49.

The evolution of the efficiency as a function of the plated metal thickness shows

no direct relation between these two parameters. This behavior does not follow the

expectations for Ni diffusion in silicon as it has been observed in the semiconductor

industry for PVD layers [Fog00] or even for the electroless plated coatings presented

in this thesis. There are several potential reasons to explain this difference in

performance. As it was evaluated by Takano, when a nickel plating solution without a

reducing agent is applied on a silicon surface, the silicon oxidizes. So, it is possible

that the interfacial oxide formed during the electro LIP deposition contributes in the

reduction of the metal diffusion. This idea is in good agreement with the results

obtained for evaporated Ni coatings on high efficiency wafers, where no HF dip was

applied before the metal evaporation, so that there was an interfacial oxide [Met07].

Very good pseudo fill factors are observed with this process even after sintering.

Moreover, for all the cells the PFF increases after sintering. Taking into account that

the cells have an Al-BSF on the rear side, and that the front is formed by a fired SiNx

layer, no improvements due to sintering are expected either on the rear contact, rear

surface passivation or front passivation. Therefore, the lingering option to explain this

effect is an improvement of the front contact.

For the highly concentrated solution, the PFF increases with increasing nickel

thickness. The difference becomes less pronounced after sintering. The diluted

solution (Bath 2) does not show this behavior. Data for the best cell processed during

this work with this technique as well as more information on the process development

has been presented by Bay in 2009 [Bay09]. An efficiency up to 16.9% was achieved

on industrial-type devices manufactured with fully industrial techniques with a FF up

to 79%.

As mentioned earlier, the presence of P on the nickel deposit has an impact on

the formation of the electrical contact between Ni and silicon for electroless plated

metals [Tou12c]. It is conceivable that on top of the impact of the interfacial oxide, the

same effect affects the sintering of electroplated Ni coatings by LIP.

Outlook

The simple setup and first tests applied during this thesis on electroplating of Ni

with LIP have been improved much further in the work of J. Bartsch, with an

arrangement of LED illuminating the cell during plating. The deposition of plated

nickel contacts combined with a Cu metallization has been evaluated in depth in his

work [Bar11b].


7 Summary

This work deals with the front-side metallization of double-side contacted silicon

solar cells manufactured on p-type silicon substrates, featuring an n + emitter at the

front-side. It is based on the seed and plate technology: a fine-line metal contact is

deposited on the front-emitter and thickened in a light-induced plating bath, by using

the photoelectric effect of the cell. This approach enables a higher degree of freedom

in the design of the front-side of the cell.

Different techniques for the formation of seed metal contacts have been

developed and characterized. A screening of metals, based on the literature, showed

that additionally to the standard metals (Ti, Ag, and Ni), which are used for the frontcontacting

of high-efficiency and industrial lines, materials like, tungsten,

molybdenum, cobalt could also be of interest.

First technological approach: laser writing

We started from the idea that a metal could be selectively deposited on a silicon

wafer through local laser irradiation. In principle, different states of metal sources

(liquid, solid, gas) could be used as the metal source.

In collaboration with the laser institute of Mittweida, laser microsintering was

implemented to form metal lines on the silicon surface by melting fine solid metal

powders layer after layer. This technique enables the formation of thick metal lines,

but the degradation of the base is too strong with this approach. So, using the seed and

plate approach, seed contacts were sintered to form ~120 nm thick lines of metals like

tungsten, silver and molybdenum, with widths corresponding to the laser spot (down to

~10 m). While the adhesion of silver was very poor and molybdenum not fully

satisfactory, the contacts made of tungsten delivered very good adhesion. The highest

conversion efficiency reached solar cells featuring a front-side formed by a ~20 /sq

n + emitter and a thin SiO 2 layer was 14.5 % which is promising result taking account

the very early state of this technology

The next option was a process based on the use of liquids as metal source. The

focus was to use electrolytes based on nickel, taking into account not only the

electrical advantages provided by the use of this metal, but also the pre-existing inhouse

know-how on processing with nickel baths. A special setup was built and

different laser wavelengths were tested. After discovering that a 532 nm wavelength is

ideal to combine low silicon damage with reduced absorption in the solution, different

electrolytes were mixed by dissolving NiSO 4 , NiCl 2 and Ni(NO 3 ) 2 in water. The

impact of the nickel concentration in the solution, the laser frequency, power and

scanning speed were tested. Very promising results were achieved for the first tests

with ~13.8% conversion efficiency, on cells close to the current industrial


176 Summary

manufacturing sequence, featuring a screen-printed rear metallization, a SiN x front-

ARC and an emitter ~50 /sq. Further studies are required to fully assess the potential

of this technology. Such evaluations are currently being performed by N. Wehkamp

(3 rd Metallisation workshop, Charleroi 2011, published in Energy Procedia 21)

Second approach: patterning the front dielectric and plating

An approach based on the pattern formation of the dielectric and the direct metal

deposition on silicon by nickel plating was developed. Therefore, the different ways to

pattern the ARC were studied. The deposition process of electroless nickel on silicon

wafers and the integration of this step into the manufacturing of industrial and highefficiency

solar cells were optimized. Finally, the best processes were combined to

manufacture laser-ablated high-efficiency cells with the light-assisted electroless

nickel plating process.

ARC patterning

Different techniques were evaluated and developed for the formation of the frontside

grid on the antireflective coating. These include: masking (by photolithography,

inkjet or screen printing) and etching the dielectric with wet-chemistry; direct removal

by laser ablation or laser chemical doping, and masked PVD deposition. For general

reference, other interesting technologies which have been developed in various groups

are also presented. Among these, we find laser or mechanical grooving, laser doping,

printing etching pastes, or spray coating dielectrics. Photolithography, inkjet printing

and laser ablation have been further implemented in the manufacturing of electroless

plated nickel contacts on silicon solar cells in this thesis.

Nickel plating

In order to understand the chemical processes occurring during Ni deposition a

comprehensive discussion of the theoretical background was performed. The different

properties relevant for the electroless metal deposition of nickel on silicon were tested.

For the solutions, this means the evaluation of several commercial baths, both acidic

and alkaline tests on optimal pH, temperature, plating time. For the substrates, we

studied the substrate preparation and rear-side conditioning.

Process integration

The optimal integration sequences for both industrial and high-efficiency silicon

solar cells with a front-metallization based on nickel plating were established. The

impact of the sintering process on the cell properties was evaluated, observing a

decrease in the specific contact resistance with increasing temperature (up to 500 o C).


Summary 177

A specific contact resistance as low as 0.04 m.cm 2 was measured on standard

industrial emitters (N s >1x10 20 at/cm 3 ), after sintering at 300 o C.

The photo-assisted electroless plating process was then developed by profiting

from the surface photo-activation of the solar cell, to reduce the total plating time, thus

the plating thickness. In this way, very thin metal contacts ~30 nm thick were grown

on silicon solar cells. This technique was tested on both industrial and high-efficiency

cells. Inkjet printed and plated industrial cells delivered an average of 0.5% absolute

efficiency increase as compared to standard screen-printed cells. Laser ablated cells

with nickel plated contacts manufactured on high-efficiency substrates reached

conversion efficiencies up to 20.7%, , V oc = 657 mV, j sc = 39.4 mA/cm 2 and

FF = 80.7%, even with thicker nickel deposits. This excellent result shows that Ni

plating is a viable way to low-cost high-performance silicon solar cells.

Finally, alternative plating mechanisms were briefly evaluated, like the

electroless deposition of NiCoP alloys, or the application of a light induced

(electro)plating mechanism for the direct nickel deposition.

Overall conclusions

During this thesis the potential of different metallization technologies for the

seed layer formation have been evaluated. Laser writing was used to test innovative

ways to deposit metal contacts on silicon, and it was possible to discard those

technologies which did not provide sufficient stability. A better understanding of the

metal deposition by electroless plating on silicon wafers has been achieved and

excellent results were obtained with this technique. Further work is required to get

these processes up to industrial standards.

Outlook

From the various techniques for the front-side metallization that were presented

in this work, or even those currently around the market, I think that in the “short-term”

the combination of laser ablation and nickel plating is one of the likeliest to find its

way through towards industrial manufacturing. That is, assuming that the long term

stability and reliability of the contacts will prove to be good enough for module

makers. For the “long-term” formation of front metal contacts, the study of the direct

reduction of the metal from electrolytes remains of great interest. It combines the

metallization with the ablation of the nitride and even the potential doping of the

silicon, all three processes in one single step.


178 Deutsche Zusammenfassung

Deutsche Zusammenfassung

Diese Arbeit befasst sich mit der Vorderseitenmetallisierung von beidseitig

kontaktierten Silizium Solarzellen, die auf p-typ Silizium, mit einem n + Emitter auf

der Vorderseite hergestellt werden. Sie basiert auf der „seed and plate“ Technologie:

Ein dünner metallischer Kontakt wird auf dem Vorderseiten-Emitter abgeschieden

und in einer lichtinduzierten Galvanik nasschemisch verdickt, unter Ausnutzung des

photoelektrischen Effekts der Solarzelle. Ein weiterer Freiheitsgrad für die Definition

der Vorderseitenkontakte wird durch diese Vorgehensweise ermöglicht.

Unterschiedliche Technologien für die Herstellung metallischer Saatschichten

wurden entwickelt und charakterisiert. Eine Auswahl vielversprechender Metalle,

zusätzlich zum heutigen Standard für die Vorderseitenmetallisierung hocheffizienter

und industrieller Solarzellen (Ti, Ag, Ni), wurde aus der Literatur und Theorie zu

Metal/Halbleiter Kontakten zu n-typ Silizium identifiziert (zum Beispiel. Wolfram,

Molybdän und Kobalt).

Erste Methode: Laserbearbeitung

Mit der Idee, dass durch das Laserbestrahlen auf einen Siliziumwafer Metalle

auf eine selektive Art und Weise auf die Oberfläche abgeschieden werden könnten,

wurde diese Arbeit begonnen. Im Prinzip können die Metallquellen sich in

unterschiedlichen Zuständen befinden: Gas, Lösung oder Festkörper Phase.

In Zusammenarbeit mit dem Laser Institut Mittelsachsen in Mittweida wurde das

Laser Mikrosintering für die Herstellung metallischer Linien auf Siliziumwafern

entwickelt. Diese Technologie ist darauf basiert dass sehr feines metallisches Pulver

aufgeschmolzen wird. Dabei konnten sehr dicke metallische Linien (mit mehreren

Mikrometern Höhe) Schicht über Schicht hergestellt werden, allerdings ist dabei die

Degradation der Basis zu stark. Durch die „seed & plate“ Strategie wurden 120nm

dicke und ~10um breite Leiterbahnen aus Metallen wie Wolfram, Molybdän und

Silber hergestellt. Die Breite der Linien ist durch den Durchmesser des Laserspots

definiert. Während die Haftung der Silber Kontakte sehr schlecht war und die von

Molybdän nicht gut genug, haben Wolfram Kontakte sehr gute Haftung gezeigt. Der

höchste Wirkungsgrad, der auf eine Solarzellen mit einen n + Vorderseiten Emitter

(~20 /sq) erreicht wurde, war 14.5%. Dies ist ein ermutigendes Ergebnis, wenn man

den geringen Ausreifungsgrad dieser Technologie berücksichtigt.

Das Laserschreiben aus flüssigen Quellen wurde anschließend evaluiert. Da

Nickel gute elektrische Eigenschaften für Si Kontakte bietet und vorhandenes Knowhow

über Nickel Lösungen im Haus zu Verfügung stand, wurde der Fokus auf Nickel

basierte Elektrolyte gerichtet. Ein spezieller experimenteller Aufbau wurde gebaut und


Deutsche Zusammenfassung 179

unterschiedliche Laserwellenlänge wurden getestet. Eine Wellenlänge von 532 nm

wurde als ideal festgestellt um gleichzeitig die Schädigung der Silizium Oberfläche

klein zu halten und die niedrigste Absorption in den Nickel Lösungen auszunutzen.

Unterschiedliche Elektrolyte wurden durch das Auflösen von Metallsalzen wie NiSO 4 ,

NiCl 2 , und Ni(NO 3 ) 2 in Wasser hergestellt. Der Einfluss des Nickel Gehalts in der

Lösung, sowie der Einfluss der Laserfrequenz, -leistung und -scanning

Geschwindigkeit wurden getestet. Vielversprechende Ergebnisse wurden mit ~13.8%

Wirkungsgrad auf Zellen die sehr nahe an der Industrielle Herstellungssequenz sind,

mit einer siebgedruckten Rückseitenmetallisierung, SiN x als ARC und einem 50 /sq

Emitter gezeigt.

Zweite Methode: Strukturierung dielektrischer Schichten

und selektiv galvanisch abgeschiedene Kontakte

Ein Prozess zur Strukturierung dielektrischer Schichten und die direkte

Metallabscheidung auf Silizium durch Nickelplattieren wurde entwickelt. Dafür

wurden unterschiedliche Technologien zur Strukturierung der ARC evaluiert. Die

stromlose Nickel Abscheidung auf Silizium und die Integration dieses Schrittes in die

Herstellungssequenz industrieller und hocheffizienter Solarzellen wurden optimiert.

Schließlich, wurde die beste Prozesskombination benutzt, um laser-ablatierte

hocheffiziente Solarzellen mit lichtunterstütztem stromlosem Nickelplattieren

herzustellen.

Strukturierung von ARC Schichten

Unterschiedliche Technologien wurden evaluiert und entwickelt um die ARC-

Schicht in Form eines Vorderseitengitters zu entfernen. Die erste Technologie war das

Maskieren (mit Photolithographie, Inkjet- oder Siebdruck) und Ätzen des

Dielektrikums mit nasschemischen Lösungen. Die zweite Technologie war die direkte

Entfernung der Schicht durch Laser Ablation oder Laser Chemical Doping/Processing.

Die dritte Methode war eine maskierte PVD Abscheidung der ARC-Schicht. Andere

Technologien die nicht in dieser Arbeit untersucht wurden, werden zusätzlich

vorgestellt. Darunter finden sich auch das Laser- oder mechanische Grooving, das

Laserdotieren, das Drucken von ätzenden Pasten und das Spray Coating von

dielektrischen Schichten durch eine Maske. Photolithographie, Inkjet-Printing und

Laserablation wurden in dieser Arbeit implementiert um Si Solarzellen mit stromlos

Nickel Plattieren herzustellen.


180 Deutsche Zusammenfassung

Galvanische Nickelabscheidung

Die Natur der chemischen Bedingungen und Prozesse die die galvanische

Nickelabscheidung dominieren wurden zuerst im Rahmen einer Literaturstudie

ermittelt und werden in der Arbeit vorgestellt. Unterschiedliche Bedingungen, die für

die stromlose Abscheidung relevant sind, wurden getestet. Auf der Lösungsseite

bedeutet dies, dass mehrere kommerzielle (saure und basische) Bäder evaluiert

wurden, um die Optima von pH, Temperatur und Zeit festzustellen. Auf der

Substratseite, wurden unterschiedliche Vorbehandlungsmethoden und

Rückseitenkonditionierungen evaluiert.

Prozess Integration

Die optimierte Sequenz sowohl für industrielle als auch für hocheffiziente

Silizium Solarzellen mit einer auf Nickelplattieren basierten

Vorderseitenmetallisierung wurde ermittelt. Der Einfluss der Sinterbedingungen

wurde im Prozess evaluiert. Eine Erniedrigung des spezifischen Kontaktwiderstands

zwischen Ni und Silizium mit steigender Temperatur (bis 500˚C) wurde dabei

beobachtet. Ein spezifischer Kontaktwiderstand von 0.04 m.cm 2 wurde auf standard

industriellen Emittern (Ns >1x10 20 at/cm 3 ) nach einem Sinterschritt gemessen.

Die Lichtunterstützung der stromlosen Abscheidung wurde dann entwickelt.

Durch die homogene Aktivierung der Oberfläche der Solarzelle unter Licht Strahlung

wird eine Verringerung der Plattierzeit ermöglicht und dadurch eine Verringerung der

Dicke der Nickelkontakte. Auf diese Art und Weise wurden 30 nm dicke Kontakte auf

den Zellen hergestellt. Diese Technik wurde dann auf industriellen und

hocheffizienten Si-Solarzellen getestet. Inkjet gedruckte und plattierte Solarzellen

haben im Mittel 0.5% höhere Wirkungsgrade im Vergleich zur siebgedruckten

Referenz. Laserablatierte Zellen mit Nickelkontakten, die auf hocheffizienten

Solarzellen hergestellt wurden, haben Wirkungsgrade von bis zu 20.7% erreicht, mit

V oc = 657 mV, j sc = 39.4 mA/cm 2 und FF = 80.7%, auch mit dickeren

Nickelschichten. Diese Ergebnisse zeigen, dass durch die Ni Galvanik ein Weg zu

niedrigen Kosten bei gleichzeitig hocheffizienten Silizium Solarzellen möglich ist.

Zu guter Letzt wurden alternative galvanische Prozesse evaluiert, wie zum

Beispiel die stromlose Abscheidung von NiCoP Legierungen, oder die Nutzung von

licht-induzierten Electroplating Ni-Prozessen für die direkte Nickel Abscheidung ohne

Reduktionsmittel.

Gesamte Zusammenfassung

Während dieser Arbeit, wurde das Potential unterschiedlicher

Metallisierungstechnologien für die Herstellung einer Saatschicht auf Silizium


Deutsche Zusammenfassung 181

Solarzellen evaluiert. Das Laserschreiben wurde als innovatives Verfahren zur

Abscheidung von Metallkontakten auf Silizium genutzt. Ein besseres Verständnis der

Metallabscheidung durch stromloses Plattieren wurde erzielt und gute elektrische

Ergebnisse wurden mit diese Technologie erreicht. Zusätzliche Arbeit wird benötigt,

um diese Technologien auf ein industrietaugliches Niveau zu bringen.

Perspektive

Falls die Zuverlässigkeit und die Stabilität der Nickel Kontakte für die

Herstellung von Modulen bewiesen wird, besteht die Aussicht auf die kurzfristige

Einführung einer solchen Vorderseitenmetallisierung mit Ablation dielektrischer

Schichten und plattierten Kontakten in die industrielle Produktion. Auf lange Sicht

bleibt die weitere Evaluation der Prozesse, die auf der Reduktion von Metallionen aus

einem Elektrolyten mittels eines Lasers beruhen, sehr interessant. Diese Methode

kombiniert die Ablation dielektrischer Schichten mit der Metallabscheidung und der

Möglichkeit, die Oberfläche dabei zu dotieren

.


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196 List of publications

9 List of publications

Patents

1) M. Aleman; A. Mette, S. Glunz; R. Preu., Verfahren zum Aufbringen von

elektrischen Kontakten auf halbleitende Substrate, halbleitendes Substrat und

Verwendung des Verfahrens. Patent Number: DE 102006040352 A: 20060829

2) K. Mayer, M. Aleman, D. Kray, S. Glunz, A. Mette, R. Preu, A. Grohe,

“Verfahren zur Präzisionsbearbeitung von Substraten und dessen Verwendung.“

Patent Number DE 102007010872 A: 20070306

3) V. Radtke, N. Bay, M. Aleman, “Verfahren zur lichtinduzierten galvanischen

Pulsabscheidung zur Ausbildung einer Saatschicht fuer einen Metallkontakt einer

Solarzelle und zur nachfolgenden Verstaerkung dieser Saatschicht bzw. dieses

Metallkontakts sowie Anordnung zur Durchfuehrung des Verfahrens“. Patent

number DE 102009051688 A: 20091023

4) F. Granek, D. Kray, K. Mayer, M. Aleman, S. Hopman, “Solarzellen mit

Rueckseitenkontaktierung sowie Verfahren zu deren Herstellung“. Patent number

DE 102009011305 A: 20090302

5) F. Granek; D. Kray; K. Mayer; M. Aleman ,S. Hopmann, “Beidseitig kontaktierte

Solarzellen sowie Verfahren zu deren Herstellung”. Patent number DE

102009011306 A: 20090302

6) Filed patent at imec: IMEC780.001PRF. “Method for forming metal silicide

layers”

Journals

1) M. Aleman, N. Bay, D. Barucha, S. W. Glunz, R. Preu, “Front-side metallization

of silicon solar cells by nickel plating and light induced silver plating”. Published

on Galvanotechnik 100 (2009), No.2, pp.412-417 ISSN: 0016-4232 and Chemical

Vapor Deposition (2009) Volume: 100, Issue: 2, Pages: 412-417

2) X. Loozen, J. Larsen, F. Dross, M. Aleman, T. Bearda, B. O'Sullivan, I. Gordon,

and J. Poortmans,“Passivation of a metal contact with a tunneling layer”, 3 rd

Workshop on Metallization for Crystalline Silicon Solar Cells in Energy Procedia

(Elsevier). Vol. 21: 75-83, (2012).

3) L. Tous, D. van Dorp, R. Russell, J. Das, M. Aleman, H. Bender, J. Meersschaut,

K. Opsomer, J. Poortmans and R. Mertens, “Electroless nickel deposition and

silicide formation for advanced front side metallization of industrial silicon solar


List of publications 197

cells”. 3 rd Metallization Workshop for Crystalline Silicon Solar Cells, in Energy

Procedia (Elsevier); Vol. 21, (2012).

4) L. Tous, R. Russell, J. Das, R. Labie, M. Ngamo, J. Horzel, H. Philipsen, J.

Sniekers, K. Vandersmissen, J. van den Brekel, T. Janssens, M. Aleman, D. van

Dorp, J. Poortmans and R. Mertens, “Large area copper plated silicon solar cell

exceeding 19.5% efficiency”. 3 rd Metallization Workshop for Crystalline Silicon

Solar Cell in Energy Procedia (Elsevier); Vol. 21 (2012).

5) M Aleman, J. Das, T. Janssens, B. Pawlak, N. Posthuma, J. Robbelein, S. Singh, K.

Baert, J. Poortmans, J. Fernandez, K. Yoshikawa, P.J. Verlinden, “Development

and integration of a high efficiency baseline leading to 23% IBC cells”, 2 nd Silicon

PV conference in Energy Procedia (Elsevier), Vol 27, (2012)

6) P. Verlinden, M. Aleman, N. Posthuma, J. Fernandez, B. Pawlak, J. Robbelein, M.

Debucquoy, K. Van Wichelen, J. Poortmans, “Simple power-loss analysis method

for high efficiency interdigitated back contact (IBC) silicon solar cells”, SolMat

106 (2012) pp37-41.

7) B. Pawlak, P. Verlinden, K. Wostyn, M. Aleman, J. Robbelein, J. Fernandez, P.

Mertens, K. Baert and J. Poortmans, “TMAH versus KOH based texturing

chemistry for high efficiency interdigitated back-contact silicon solar cells”. 2 nd

Silicon PV conference in Energy Procedia (Elsevier), Vol. 27, (2012)

8) L. Tous, J. Lerat, T. Emeraud, R. Negru, K. Huet, A. Uruena De Castro, M.

Aleman, R. Russell, J. John, J. Poortmans and R. Mertens, “Nickel silicide

formation using excimer laser annealing”, 2 nd SiliconPV conference in Energy

Procedia (Elsevier); Vol. 27, pp.503-509, (2012).

9) L. Tous, J-F. Lerat, T. Emeraud, R. Negru, K. Huet, A. Uruena, M. Aleman, J.

Meersschaut, H. Bender, R. Russell, J. John, J. Poortmans, R. Mertens, “Nickel

silicide contacts formed by excimer laser annealing for high efficiency solar cells”,

Progress in Photovoltaics: Research and Applications, Vol. 21, 3, pp. 267–275,

(2013)

Conference Papers in chronological order

1) M. Aleman, M.; Streek, A.; Regenfuß, P.; Mette, A.; Ebert, R.; Exner, H.; Glunz,

S.W.; Willeke, G. “Laser micro-sintering as a new metallization technique for

silicon solar cells“ 21 st EUPVSEC. Dresden, Germany 2006.

2) Grohe, A.; Knorz, A.; Aleman, M.; Harmel, C.; Glunz, S.W.; Willeke, G.P. “Novel

low temperature front side metallisation scheme using selective laser ablation of


198 List of publications

anti-reflection coatings and electroless nickel plating”. 21st EUPVSEC. Dresden,

Germany 2006.

3) S.W. Glunz, A. Mette, M. Aleman, P. L. Richter, A. Filipovic, G. Willeke, “New

concepts for the front side metallization of silicon solar cells” 21 st EUPVSEC.

Dresden, Germany 2006.

4) S. Hopman, A. Fell,K. Mayer, M. Aleman, M. Mesec, R. Müller, D. Kray, G.

Willeke, “Characterization of laser doped silicon wafers with laser chemical

processing”, 22 nd EU PVSEC 2007.

5) M. Aleman, N. Bay, M. Fabritius, S.W. Glunz, “Characterization of Electroless

Nickel Plating on Silicon Solar Cells for the Front Side Metallization” 22 nd EU

PVSEC 2007.

6) M. Alemán, N. Bay, A.Grohe, A. Knorz, S.W Glunz, “Alternatives To Screen

Printing for the Front Side Metallization of Silicon Solar Cells” 17 th International

PVSEC, Fukuoka, Japan. 2007

7) D. Biro, D. Erath, U. Belledin, J. Specht, D. Stüwe, A. Lemke, M. Aleman, N.

Mingirulli, J. Rentsch, R. Preu, R. Schlosser, B. Bitnar, H. Neuhaus, “Inkjet

Printing for High Definition Industrial Masking Processes for Solar Cell

Production”, 17th International PVSEC, Fukuoka, Japan. 2007

8) M. Aleman, N. Bay, L. Gautero, J. Specht, D. Stüwe, R. Neubauer, D. Barucha, D.

Biro, J. Rentsch, S.W. Glunz, R. Preu “Industrially Feasible Front-Side

Metallization Based on Ink-Jet Masking and Nickel Plating” 23 rd EU PVSEC,

Valencia, Spain. 2008

9) T. Rublack, M. Aleman and S. W. Glunz , “Evaluation of the Laser Melting

Process of Different Materials for the Front-Side Metallisation of Silicon Solar

Cells”, 23 rd EU PVSEC, Valencia, Spain. 2008

10) D. Rudolph, M. Alemán, N. Bay, K. Mayer and S. W. Glunz, “Laser-Induced

Nickel Deposition from an Aqueous Electrolyte for the Front-Side Metallization of

Silicon Solar Cells” 23 rd EU PVSEC, Valencia, Spain. 2008

11) J. Specht, D. Biro, N. Mingirulli, M. Aleman, U. Belledin, R. Efinger, D. Erath, L.

Gautero, A. Lemke, D. Stüwe, J. Rentsch, R. Preu, “Using hotmelt-inkjet as a

structuring method for higher efficiency industrial silicon solar cells” 24 th IS&T

Digital Fabrication 2008 (NIP): Pittsburgh, USA, 2008 pp.912-91

12) S.W. Glunz, M. Aleman, J. Bartsch, N. Bay, K. Bayer, R. Bergander, A. Filipovic,

S. Greil, A. Grohe, M. Hoerteis, A. Knorz, M. Moenko, D. Pysch, V. Radtke, D.

Rudolph, T. Rublack, C. Schetter, D. Schmidt, D. Woehl, A. Mette, P. Richter, O.


List of publications 199

Schulz, “Progress in advanced metallization technology at Fraunhofer ISE”, 33 rd

IEEE PVSC 2008, San Diego, USA, 2008

13) D. Kray, M. Alemán, A. Fell, S. Hopman, K. Mayer, M. Mesec, R. Müller, G. P.