Perfect Line - Arnold Gruppe

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Perfect Line - Arnold Gruppe

Production & Inspection Production Technology Germany

Photo by Arnold Gruppe

Perfect Line

Reproducible Glueing Ensures Maximum

Yield in Wafer Fabrication

In the wafer production, the preliminary process step of adhesive glueing

of silicon bricks onto wire-saw beams is quite often severely neglected.

Not very logical: Conventional adhesive glueing is often found to be the

cause of high losses in the production of wafers.

By Wolfgang SchÜrgers

Photo 1. Automatically applied adhesive bead

80

June, 2012


Only a constantly

reproducible

and

in the cutting process

durable adhesive

bonded joint will

meet the growing demands

for greater

economic efficiency.

Wolfgang Schürgers

Massive changes in

the global photovoltaic

market are challenging the industry to

critically analyze their entire value chain for

saving potential.

Photo 2. High efficiency with automated glueing

Excessive Demand to Catch Up

A high variety of dedicated process steps

are required before ultra-thin wafers are produced

from silicon. Depending on the type

of mono- or multicrystalline feedstock material

silicon ingots must be─step by step

─squared, cropped, ground and chamfered

to obtain high-quality bricks for the following

wafer cutting process. While these process

steps including the later following

wafer cutting and cleaning process are operating

almost fully automatically, still about

90% of the preparatory adhesive glueing of

the brick to the wire saw beam is being executed

as manual work. Thus, this individualized

process step is highly depending on the

qualification of each individual worker.

Some manufacturers are trying to achieve a

certain uniform quality level by limiting this

job to few handpicked and experienced employees

only. Nevertheless, many manufacturers

are still a long way from standardization.

That is exactly the reason why this process

step is still operated with a high error

probability. Any minor mistake can increase

the wafer breakage rate during wafering or

the later de-glueing drastically and consequently

will lead to considerable additional

costs. Experience shows that the yield rate,

due to faulty adhesive glueing, is affected

between three up to five percent. For example,

a simple calculation: assuming a low

breakage rate of only 3% in a 100 megawatt

production line with a calculated output of

about 25 million wafers per year, this results

in 750,000 broken wafers. At a market value

of US$1.20 per each wafer, it consequently

means a financial loss of over US$0.9

Mio.─year by year.

It is worthwhile, especially in critical

times, to think outside the box and to copypaste

positive manufacturing approaches

from other industries. For example, the automotive

industry has been using the automated

adhesive glueing technique as joining

technology in the body assembly and final

assembly process for more than 20 years.

Thanks to innovative manufacturing technologies

and new developments in the

chemical industry, the adhesive glueing process

has been continuously improved and

thus becoming more efficient. Correspondingly

applied to the Photovoltaic (PV) industry

this means that an integral approach is

required─involving the various interests of

equipment manufacturers, adhesive material

suppliers and customer requirements─under

consideration of the specific requirements of

the wafer sawing process and the subsequent

wafer deglueing process from the wire saw

beam.

From a global point of view, only very

few manufacturers of PV production equipment

are offering automatic adhesive glueing

devices and machines. One is Germanybased

Arnold-Gruppe. Arnold not only offers

innovative and complete manufacturing

lines for the automatic brick production─but

now also production equipment for brick

glueing to wire saw beams based on automated

handling and glueing processes. As a

well-known automation specialist for the automotive

and photovoltaic industry, this

manufacturer is also capable to optimize the

individual process steps of the brick production

─from cropping and grinding up to the

automatic adhesive glueing─in one integrated

line.

Photo by Arnold Gruppe

Behind the Scenes

In the silicon wafer production, the finished

bricks are glued onto wire-saw beams

for the following wafer cutting process.

What seems to be quite trivial is in reality a

rather tricky process step. Why On the one

hand, every single silicon brick is─due to

the previous cropping process─a unique

kind of brick. On the other hand, the glueing

characteristics are differing strongly with

changing material- and environmental conditions.

In addition, customers have specific

requirements in order to customize the

placement of the bricks to the wire saw

beam for the following cutting process. One

may choose a maximum wire saw utilization,

others may decide for best throughput

or other criteria which may be selected by

the user individually. In any case, since

bonding strength and resistance of the adhesive

joint cannot be tested without destruction,

the adhesive glueing must be a technically

mature and absolutely reproducible

process step. For an efficient, automated adhesive

glueing, therefore, a comprehensive

overall process and system concept is needed

that addresses all requirements of the

wafer production. These include high-quality

and well-selected hardware components

such as, for example, adhesive dosing system,

industrial robots, assembly and transfer

equipment and intelligent software products

that analyze the process continuously and

allow easy optimization.

Basically, the bonding process already

starts with the commissioning of the cropped

bricks. Each brick piece─individually cut at

different lengths during the cropping

process─is first measured by the robot. After

that, ‘colleague robot’ assorts the finished

bricks in order to determine and assemble

the optimal batch for the wire saw. The actual

joining process of the brick─in preparation

for the wafer cutting process─includes

the following steps: brick cleaning, dosing

of the adhesive to the brick piece, positioning

of bricks on the glass plate and wire saw

beam, and finally curing of the adhesive

joint until the assembled beam can be handled

safely. All system components must be

of highest quality and optimized for the use

in automatic production systems because

they form the basis of a stable production

process. One more reason to take a closer

look to the adhesive glueing process and the

advantage of already available adhesive application

systems as a pre-step of wafer cutting.

The heart of the automated glueing center

(Photo 2) is the centrally positioned industrial

robot. The relevant process steps for

cleaning, glueing, joining and curing are positioned

optimally in the robot work space

and designed to operate fully automatically.

The ‘colleague’ robot can work efficiently.

Well Cleaned Is Half Glued

An adhesive joint is only as good as the

effective treatment of the joining zone. Adhesives

are very sensitive to contaminated

surfaces. It is obvious that at all those surface

areas which are not carefully cleaned,

June, 2012 81


Production & Inspection Production Technology Germany

Photo by Arnold Gruppe

Complete mobile glueing fixture consisting of wire saw beam, glass plate and brick

the adhesive bond between base material

and substrate is disconnected and thus weakening

the adhesive joint. As a result, wafers

can loosen during the wafer cutting process

in the wire-saw─which consequently means

production losses─and eventually will cause

harmful interference of production. A thorough

cleaning is, therefore, mandatory for a

durable adhesive joint during the wafer cutting

process. The joining zone must be absolutely

free from dust and grease.

The automated cleaning process is designed

to be simple but yet highly effective:

spraying-wiping-drying. The robot picks the

brick from the magazine or conveyor belt,

moves to the cleaning device and applies an

organic off-degradable cleaning solvent to

the bottom surface of the brick via spray applicator.

The surface is then automatically

wiped and dried using a specific fabric made

from continuous microfilament fiber to absorb

any dirt and moisture from the brick

surface. For heavily soiled bricks alternative,

but more costly cleaning processes such as

ultrasonic cleaning are possible within the

glueing cell.

Direct after, the cleaned brick piece is processed

by robot to the glueing process. In

preparation of the cutting process, the brick

is bonded on an intermediate glass plate

82

which is─prior to the adhesive glueing of

the brick pieces─also glued by the robot automation

to the wire saw beam. The glass

plate serves as a ‘lost plate’ to hold the wafer

in place until they are cleaned and separated

after cutting. Based on the exact length of

each brick piece─automatically measured

by the robot─the optimum number of bricks

as well as the position of the brick on the

wire saw beam is calculated. This has a particularly

advantageous effect on the most efficient

utilization of the wire saws. One or

─if applicable─more brick pieces, glass

plate and wire saw beam are embedded in a

mobile clamping fixture until the adhesive

joint is cured (Photo 3). Therefore, a minimum

of two reproducible adhesive glueing

steps are required for each glueing batch, all

of them are executed by the robot completely

automatically.

To start with, the robot applies─with support

of the adhesive dosing system─a predefined

adhesive bead on the glass plate

which will then be loaded to the wire saw

beam. After the glass plate is loaded, the

robot will then start the glueing process for

the bricks. Relative to the already known

length of the individual brick piece, the

robot will apply a precisely portioned adhesive

bead on the brick surface (Photo 1).

Thereafter, the brick is precisely positioned

on the glass plate according to the calculated

position data. Thanks to the optimal interaction

of automatically calculated brick length

with the accordingly exact applied adhesive

bead plus the following positioning and

pressing of the brick piece onto the glass

plate by robot, a complete layer film of adhesive

between brick surface and glass plate can

be achieved. Just enough to secure a constant

layer of adhesive between brick and glass

plate but not more than required in order to reduce

excessive over-amount, respectively

waste of glue. Step by step─according to programmed

instructions and brick by

brick─with reproducible quality.

After joining, bricks glued to glass plate

and wire saw beam must be resting for a

predefined time according to the instructions

of the supplier of the adhesive glue, free

from any mechanical shocks or vibration.

Only a robot can ensure an automatically

operated and utmost gentle handling of the

mobile clamping fixture together with the

completed wire saw beam to the storage

shelf for curing without mechanical stress.

Here the curing of adhesive can be performed

under continuous constant conditions.

Abrupt movements or other similar effects

could cause loosening of the adhesive

June, 2012


joint. When done manually, an almost unavoidable

problem. After curing, the glued

beams are available for the wafer cutting

process. In a later process step, after cutting

and wafer separation is completed, the glass

plate is loosened from the wire saw beam in

a separate process step.

Quickly Glued and Easily Released

For the bonding process, various parameters

are crucial and the desired result can

only be achieved by perfect interaction of all

parameters. These include the adhesive material

as such, the material mixing and dosing

system and the process automation.

Supplier of manufacturing equipment for

the photovoltaic industry orientate themselves

largely on the ITRPV Roadmap for

2011 and later 1) . Published by the ‘Crystalline

Cell Technology and Manufacturing’

(CTM), an interest group of the global industry

association for standardizing production

equipment SEMI, the roadmap

is─among others─focused on a severe reduction

of the future wafer thickness. While

today the commonly processed wafer are at

a thickness of 180 μ for a format of 156 mm

x 156 mm, it is already expected in 2017 to

achieve wafer thickness of not more than

120 μ in an industrial production environment.

These challenges have significant impact

on the entire manufacturing process

chain, including the adhesive glueing process.

It’s because the strength of the adhesive

joint is extremely depending on the

available surface contact area.

With the proposed thickness reduction by

one third, the surface contact area is reduced

accordingly and the demand on the strength

of the adhesive joint rises dramatically. The

thinner the wafer the less contact surface is

Intelligent intuitive software to optimize the glueing process

Dust and grease-free surface is essential.

Photo by Arnold Gruppe

in the joint. In addition, during cutting process,

more frictional heat arises because of

the lower distance between the individual

cuts with the result that the remaining silicon

wafer heats up much faster. But with increasing

temperature, the strength of the adhesive

joint is decreasing drastically. Rising

temperature not only means for the supplier

of the adhesive glue a big challenge, but also

Photo by Arnold Gruppe

for the wire saw supplier who is challenged

to bring the increased heat load received

during cutting under control by optimization

of the cooling process.

In view of the enormous cost pressure, the

sawing process of wafers must be executed

under high machine load at maximum speed

and full loading of the wire saw beams with

silicon bricks. Glueing with sophisticated,

specifically developed two-component adhesives,

thereby gains an even greater importance.

Especially in the wafering process,

the adhesive must fulfill two basic requirements:

high strength and temperature resistance

during the wafer cutting process and a

universal behavior during de-bonding of the

cut wafer from the wire saw beam with either

tempered water or weak acetic or lactic

acid. Of course, attributes like low viscosity

for easy automatic processing, sufficient pot

life for long processing before curing, yet

short curing time until the glueing batch can

leave the assembly, outstanding cutting

properties and easy removal of impurities

from wafers during the following pre-cleaning

and de-bonding process. These advanced

adhesives are only offered by few manufacturers

and are, therefore, quite cost-intensive.

Optimized and Economical Dosing

In the automatic glueing system, a PLCcontrolled

motor-driven volumetric dosing

unit is being used, which always provides─

independent of the viscosity of the adhesive

and the robot speed─the precise mixing and

dosing of the 2-component epoxy resin. For

this reason, the 2-component dosing system

is continuously providing a constant material

mixture from the beginning of the glueing

bead until the end (Photo 4). The mixture of

both components takes place immediately

prior to application in a static mixer unit.

The amount of glue applied along the bead

path is variable according process requirements.

Compared to other systems available

on the market, this dispenser scores with its

highly flexible, reproducible and always uniform

adhesive application.

The design of the dosing unit is based on a

‘first in, first out’ principle. This prevents

aging of adhesive material within the system.

The completely valveless design without

constriction or bottlenecks, supports the

most gentle and shearless material transport

within the system from pump to mixing

tube. Undesired degradation, which means

depolymerization of the adhesive due to

pressure peaks when the adhesive material

passes through, is avoided. In addition, the

bead size can rapidly be changed even along

a glueing bead or even switched to a point

application since the system is quickly reacting

to changing input parameters and operating

at very low material pressure. By means

of reverse motor movement, the adhesive

glue in the static mixing tube will be retracted

and such countervails the stringing behavior

of the adhesive. Air and gas bubbles

that may be present in the material container

are evacuated entirely with an inline de-gasing

device─if needed. The integrated stirring

unit, even operating during production stops,

June, 2012 83


Production & Inspection Production Technology Germany

Especially in the wafering

process, the adhesive

must fulfill two basic

requirements: high

strength and temperature

resistance during the

wafer cutting process

and a universal behavior

during de-bonding of

the cut wafer from

the wire saw beam

with either tempered

water or weak acetic or

lactic acid.

always keeps the adhesive homogeneous. It

is, therefore, always enough adhesive glue in

the system─even in case of changing

containers─and thus allowing continuous

production. There is no need to maintain rest

quantities in the container during container

change nor is it necessary to drain material

supply hoses. The additional material savings

of expensive adhesive substrate is a

considerable side effect.

The adhesive dosing system requires extremely

low maintenance, is flexible and

scalable for different requirements. Within a

few minutes, a simple and fast exchange of

the complete mixing head including the dosage

unit is possible. If necessary, a fast

change of the adhesive medium is possible.

The system may be operated with two cartridges

for 950 Cubic Centimeters (ccm)

each, which corresponds to a total of 1900

ccm. This is beneficial for small to medium

volume production. For example, this is suitable

for a production volume of 100 megawatt

with only one cartridge exchange required

per day. An enormous advantage over

conventional cartridge systems which

require─based on the identical production

volume─up to eight cartridge changeover

cycles per day. In case of large serial productions,

it is recommended to operate with

cost-effective container material with approximately

30 liter material content. In automated

bonding process there are savings

of at least 50% in glue consumption compared

with manual glueing by using larger

containers and linear reduction of the consumption

through the automatic application

of adhesive.

Fully Automated and Reproducible Glueing

The robot in the glueing system is programmed,

controlled and coordinated by intelligent

software tools. With the new developed

software tool AEPS (Arnold Easy Programming

System), customer’s own system

operators are now in the position to create

84

and edit movement and also process parameters

within the robot and glueing controller

intuitively to a certain admissible extent by

themselves (Photo 5). Expert knowledge

about the robot and PLC programing is not

required.

This software tool allows the optimization

of the characteristics of the adhesive bead,

changing of the robot velocity during adhesive

application, editing of the glueing bead

position as well as changing the bead thickness

and other parameters. Simply and safely

by just selecting the required position at

the central touch screen, process or movement

data can be added, deleted or edited.

But even more intuitive programing functions

are available via the Human Machine

Interface (HMI). These adhesive-specific

data include, for example, the required mixing

ratio of the two components, basic geometry

data of the wire saw beam and glass

plate, setting of the required distance between

brick pieces on the wire saw beam;

extended service functions such monitoring

the remaining amount of adhesive within the

system, adhesive-aging supervision, container

changeover cycles, status indication for all

system components, failure analysis and

many more.

The AEPS software tool simply can be

operated directly at the glueing system (System

OPC), or offline at any Windows-based

office-PC. The operator or the process expert

is able to adapt the adhesive process to

the changing production requirements without

time and effort. Program and process

changes are monitored automatically, thus

ensuring the traceability of changes.

By means of the process analysis tool

ARPAT (Arnold Remote Production Analysis

Tool), process data are collected within

the running production in real time, visualized

and recorded. These include not only

quality-related production data, but also machine

parameters and measurement data,

error messages, etc. In addition, there is an

interface for data transfer to higher-level

process control systems (SCADA, MES,

etc.).

Thanks to the wide variety of functionalities

of ARPAT, operator and quality control

department will receive important process

and production data. Based on data received

from the dispense unit during the adhesive

application, ARPAT creates a standard process

protocol from the actual processed part

but also provides diagnostic data in respect

to the dispense process from earlier process

cycles up to a period of minimum four

weeks in the past. For example, continuous

monitoring of the actual material pressure in

the dosing system prior to mixing can indicate

correct material feeding of the two components

but can be also an indication of material

blockages or even air bubble inclusions.

Based on the actual motor speed, the

correct material flow including the correct

mixing ratio during application can be monitored.

By counting all motor revolutions of

the doser during automatic glueing, the applied

material volume per brick is calculated.

For quality tracing purposes, there is

even the possibility to secure an image of the

bead of adhesive applied per brick.

The storage of process data, for example

material flow, pressure, etc., also serves the

purpose of later process analysis. Within a

closed loop of data receipt, data analysis and

parameter change, the operator or process

expert can react quickly as soon as changes

appear. Specifically for sensitive processes

like glueing, this appears to be extremely

important since batch-related material

changes, adhesive aging, and other changing

conditions will always require some handson

system for improvements. Thanks to

ARPAT and AEPS, this becomes an easy

task without expensive assistance from often

only externally available robot, or programing

specialists.

Small Cause a Big Effect

The level of automation in the wafer production

is already very high. With the aim to

reduce the costs while improving quality and

yield, upstream processes must also be reconsidered.

Solutions of highly automated

adhesive glueing systems are already existing

and an essential factor for optimization

of the Total Cost of Ownership (TCO). The

example of a 100 MW plant makes abundantly

clear that the investment is worthwhile

in an automated adhesive glueing system.

The enormous reduction in the glueing

material and labor costs combined with a

significant increase in yield by approximately

three percent show total savings of approximately

US$1.1 million based on a calculated

wafer price of US$1.20. The investment

in an automatic glueing system pays

for itself in less than a year. But critical to

sustainable success is not only a well-balanced

automated system equipped with

high-quality system components for each

process step and a high flexibility to ever

changing conditions, but also the ease of use

by locally available personnel.

Dipl.-Ing. Wolfgang Schürgers, Member of the

Board of the Arnold Group (www.arnold-gruppe.

de), is responsible for sales and marketing. Schurgers

has 28 years of international successful

experiences in the fields of automation, robotics

and glueing processes in the automotive and

photovoltaic industry. Within the range of responsibility

of Schürgers, Business Development

at Arnold Group, the automated glueing process

is considered as key technology in automated

brick processing lines.

REFERENCES

1) International Technology Roadmap for Photovoltaics (ITRPV.net), Results 2010; 4.1.2,

Material Cell Processing

June, 2012

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