PLASMA IMMERSION ION IMPLANTATION AS AN ALTERNATIVE ...

PLASMA IMMERSION ION IMPLANTATION AS AN ALTERNATIVE TO DIFFUSION AND BEAM-LINE
IMPLANTATION FOR CREATING PHOSPHORUS DOPED EMITTERS AND SELECTIVE EMITTERS
A. Lawerenz 1 *, K. Neckermann 1 , G. Andrä 2
1 CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH, Konrad-Zuse-Str. 14,99099 Erfurt, Germany
2 Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany
*corresponding author: alawerenz@cismst.de; Tel: +49 - 361 - 663 1212, Fax: +49 - 361 - 663 1413
ABSTRACT: The plasma immersion ion implantation (PIII) can be used to implant phosphorus ions into the silicon
surface. Therefore it is a possible alternative to other doping techniques as phosphorus diffusion or beam
implantation. For a successful implantation into a solar cell processing line, some obstacles that are dealt with in this
contribution have to be overcome. We are investigating possible paths for the activation and annealing step that is
necessary for getting sufficient sheet resistances and we are correlating processing parameters to solar cell parameters
as doping profiles, emitter saturation currents and solar cell efficiencies, both for mono- and multicrystalline wafers.
Furthermore we are investigating the possibility of a laser assisted selective emitter process.
Keywords: Implantation, Selective Emitter, Crystalline Silicon Solar Cells
1 INTRODUCTION
In the last three years, ion beam line implantation has
shown its potential to replace the diffusion process by
implanting phosphorus and boron atoms into solar wafers
and to be integrated into the solar cell production [Gupta
2010, Yelundur 2011]. In our contribution we are
investigating whether an alternative implantation
technique – plasma immersion ion implantation (PIII or
simply plasma implantation) – is also feasible for the
solar cell processing. Compared to the established
diffusion technique, plasma implantation has the
following advantages. It is a single-side process (making
the removal or overcompensation of back-side doping
unnecessary), there is no phosphorus and boron silicate
glass formation (making its removal also unnecessary)
and it enables a conformal doping, which makes it
suitable for heavily textured surfaces as acid etched
multicrystalline silicon. On the other hand, implantation
causes a damage in the implanted region near to the
surface, which has to be cured, and the implanted dopants
have to be activated. This activation and annealing has to
be done at elevated temperatures.
Compared to the beam line implantation, large
surface areas can be faster implanted and the already
mentioned conformal doping might be easier, but the
energy distribution of the plasma excited implantation
ions is much broader. The latter effect is for the
application of solar cells still tolerable.
Until now, there are only very few publications
dealing with plasma implantated solar cells [3]. Therefore
it is not surprising that plasma implanted (p-type) solar
cells could not reach efficiencies comparable to POCl 3
diffused solar cells.
Our contribution focuses on phosphorus plasma
implantation parameters, the feasibility of different
activation and annealing steps and the possibility of
structuring implanted regions by a laser assisted
activation for creating selective emitters.
2 EXPERIMENTAL
2.1 Wafer material
For processing solar cells, solar grade silicon wafers
have been used (CZ, 3 - 6 Ωcm, 156 mm edge length,
alkaline textured and multicrystalline, 1.5 Ωcm, 156 mm
edge length, acid etched). For lifetime measurements
(including the determination of interstitial iron content)
the same sort of wafers has been used, but after shiny
etch treatment to obtain smooth surfaces. For the
determination of the emitter saturation current, high
resistance wafers (10 - 17 Ωcm) have been used, again
after a shiny etch treatment.
2.2 Plasma implantation
Plasma implantation was carried out at Ion Beam
Services (Peynier, France) and at Helmholtz-Zentrum
Dresden-Rossendorf e. V. (Dresden, Germany). Because
of the large scattering of the energy of the ions, a precise
number for the energies cannot be given. Doses varied
from 1.5 to 10 · 10 15 at/cm 2 .
2.3 Activation and annealing
The activation and annealing step (short: activation),
necessary for activating the implanted phosphorus atoms
and curing the implantation damage, was carried out by
different methods (s. Table I and II). The laser used was
an excimer laser with a top-hat profile and an
illumination area of 6 × 6 cm 2 . The flash lamp annealing
took place at and was carried out by Helmholtz-Zentrum
Dresden-Rossendorf e. V. (Dresden, Germany).
Table I: Furnaces applied for the activation step
Furnace Temperture Duration
range (°C) range
Walking string inline 880 - 950 20 - 50 min
furnace (Centrotherm)
Quarz tube furnace 950 - 1050 5 - 30 min
(Inotherm)
RTP furnace (Unitemp) 800 - 900 2 - 10 min
Sintering furnace 850 1 - 5 s
(Centrotherm)
Flash lamp annealing 1100 - 1200 20 ms
Table II: Laser applied for the activation step
Laser Wavelength Pulse Pulse
λ (nm) duration repetition
Excimer laser 248 25 ns 50 Hz

Solar cells were processed using the standard
processing line of the CiS Forschungsinstitut.
2.4 Measuring methods
Sheet resistances have been measured by a four point
probe (Napson RT-70 tester), and sheet resistance
mappings of the implanted and annealed wafers have
been carried out by two methods (SHR, “sheet
resistance” with a measuring head on a semilab WT2000
and SRI, “sheet resistance imaging” with a
Thermosensorik infrared camera system).
Carrier lifetime measurements were carried out on
samples whose emitter has been etched off past the
implantation and activation with a Semilab WT2000 tool
and a Sinton Consulting WCT-100. The latter has been
also used (on the same type of samples) for determining
the interstitial iron content, the emitter saturation currents
and for Illumination V oc-characteristics (“SunsVoc”).
Illumination V oc measurements were achieved on
samples having a screen printed aluminum rear side and
no metallisation at the front.
Phosphorus depth profiles were determined with a
Cameca sector field secondary ion mass spectrometre
(SIMS).
3 ANNEALING AND ACTIVATION
Because a drive-in of impurities at high temperatures
applied at the activation step has to be avoided, the wafer
surface has to be cleaned before the activation. A
standard clean like the RCA clean is not suitable in our
case, because it causes too large a removal of the surface
and thus a significant increase of the sheet resistance after
the activation step. Therefore a reduced RCA process
consisting of the SC2 step followed by an HF dip has
been used for the activation in the walking string inline
furnace.
3.1 Sheet resistance
With increasing annealing times, more and more
phosphorus atoms are becoming activated and at the
same time are diffusing deeper into the silicon bulk. The
latter causes a decrease in the concentration of
phosphorus atoms and therefore to an increase in the
carrier mobility. Both effects (activation and diffusion)
lead to a decrease in the sheet resistance with increasing
annealing times t. An example is shown in Fig. 1 for an
implantation with a bias voltage of 8 kV and a dosis of 5 ·
10 15 at/cm 2 and an activation in an inline diffusion
furnace (temperature: 880 °C). The fast decrease of the
sheet resistance (t ≤ 10 - 20 min) is attributed mainly to
the activation, and the slow decrease (t ≥ 20 min) to the
diffusion of the phosphorus atoms into the bulk.
All relevant sheet resistances R sh (30 Ω/sq < R sh <
120 Ω/sq) can easily be obtained for monocrystalline
wafers by a sufficient dose and activation temperature
(for example for R sh = 30 Ω/sq an implantation of 5 keV,
10 16 at/cm 2 and an activation of 1000 °C, 10 min is
sufficient). It is noteworthy that the activation for plasma
implanted samples is much shorter than the diffusion
time for phosphorus diffused samples. This holds also for
RTP processes, where 10 min at 600 °C and 2 min at
900 °C in N 2-atmosphere are enough to gain a sheet
resistance of 50 Ω/sq (again with the same implantation
parameters). Nevertheless the activation in a sintering
furnace with parameters sufficient for the contact
formation of screen printed metals is by far not sufficient
to reach reasonable sheet resistances. Therefore an
explicit activation step for the processing of standardtype
solar cells cannot be avoided.
For multicrystalline wafers, it is remarkable that for
the same implantation an activation with a higher
temperature or duration is needed to reach the same sheet
resistances as for the monocrystalline samples.
Homogeneities of the sheet resistance for the
implanted and activated samples are sufficient for the
processing of solar cells. An example is shown in Fig. 2
for the same implantation parameters and the same
furnace used (880 °C, 20 min). A sheet resistance of 53
± 2 Ω/sq measured with a four point probe at nine points
has been obtained (standard deviation of 4 %). (The high
sheet resistance values at the edge of the wafer (red
colour) in Fig. 2 are an artefact of the SHR measuring
technique due to the fact that the current path is inhibited
at the wafer edges.)
Sheet resistance [Ω/sq]
90
80
70
60
50
40
0 20 40 60 80 100
Annealing time [min]
Figure 1: Sheet resistances after activation using an
inline diffusion furnace
Figure 2: Sheet resistance (SHR) image of a walking
string inline furnace annealed wafer
3.2 Phosphorus depth profiles
There is a good correlation between implantation and
activation parameters and the phosphorus depth profiles
measured with SIMS. After implantation (and before

activation) an almost exponential depth profile can be
observed with a penetration depth (where the P
concentration decreases by a factor of e -1 ) of 7 to 8 nm
(dependent on the ion energy) and a surface
concentration of 5 to 50 · 10 21 at/cm 3 resulting in an
overall P content that is in good agreement with the
implanted dose.
After activation, depth profiles of the overall and the
activated phosphorus concentration have been measured
for a quartz tube furnace and an excimer laser activation
with SIMS and ECV (Electrochemical Capacitance
Voltage Profiling), respectively (s. Fig. 3). The observed
low differences in both profiles are indicating that at least
almost all phosphorus atoms are activated after these two
activation methods (implantation: 5 keV, 10 16 at/cm 2 ,
quartz tube furnace: 1000 °C, 10 min).
For a quartz tube furnace (and similarly for the
walking string inline furnace), SIMS profiles look
different from those that are known from POCl 3 or sprayon
inline diffusion processes (cf. eg. [4]). Especially the
kink that is found in diffused P samples (at about 2 · 10 19
cm -3 ), cannot be found in our cases. One reason for this
might be the lower phosphorus surface concentration.
Similarly to diffused samples, a hunch in the profile can
be detected at 2 - 5 · 10 18 cm -3 indicating the same
diffusion process that is dominated by interaction with
self-interstitials.
Profiles obtained from laser activated samples can
quite be varying, depending on the exact laser parameters
that are responsible for the fact whether the silicon
surface is molten or just heated. In our case, the surface is
molten (albeit a complete mixing of the recrystallised
silicon not reached for one laser pulse), leading to a
profile that shows the same characteristics as the quartz
tube furnace process, but on a much smaller scale (a
concentration of 5 · 10 17 cm -3 is reached at a depth of ca.
250 nm compared to 1000 nm for the quartz tube furnace
activation). This might be due to the much lower laser
impact time (25 ns).
4 ELECTRICAL CHARACTERISATION
4.1 Carrier lifetime
Carrier lifetimes were measured on samples whose
emitter has been etched off after the implantation and
activation and which were passivated afterwards by
silicon nitride coatings. Therefore the quoted
(“effective”) lifetimes are upper limits for the “true” bulk
lifetime. In our experiments, no influence of the
implantation parameters can be ascertained - in contrast
to the activation parameters. There is a tendency, that (in
the case of RTP furnace) higher temperatures yield lower
lifetimes which is possibly caused by some
contamination. For activation steps other than hightemperature-RTP,
effective lifetimes vary from 200 µs to
800 µs.
P Concentration [at./cm³]
P Concentration [at./cm³]
10 21
10 20
10 19
10 18
10 17
10 16
quartz tube furnace
SIMS
ECV
10
0,0 0,5 1,0 1,5
15
10 21
10 20
10 19
10 18
10 17
10 16
Depth [µm]
10
0,0 0,1 0,2 0,3 0,4
15
Depth [µm]
excimer laser
SIMS
ECV
Figure 3: SIMS and ECV depth profiles after activation
for quartz tube furnace and excimer laser
The WCT-100 tool has been used to determine the
interstitial iron (Fe i) content (by measuring lifetimes
before and after a light soaking of the passivated
samples). After the activation, we observe partly an
increase and partly a decrease of the interstitial iron
content in comparison to the Fe i content after the
implantation. A conclusion, whether a gettering effect or
contamination or an internal regrouping of iron atoms
(from the interstitial/boron pair state to iron containing
precipitates and vice versa) dominates, is not yet
possible.
4.2 Emitter saturation current
The same set of samples used for illustrating the
dependence of the sheet resistance R sh on the activation
time (Fig. 1) has been used for measuring the emitter
saturation current density (J 0e) (Fig. 4). Quite often, a
monotonic decrease of J 0e for increasing sheet resistances
is measured for phosphorus diffused samples. In our case,
such a decrease is observed only for sheet resistances
above 55 Ω/sq. The reason for the increase in J 0e for 45
Ω/sq < R sh < 55 Ω/sq is not known yet. The absolute
values we obtained are still somewhat high and do not
suffice for the processing of high efficiency solar cells.

J 0E [fA/cm²]
300
250
200
150
40 45 50 55 60 65 70 75 80 85
Sheet resistance [Ω/sq]
Figure 4: Emitter saturation current density in
dependence of the sheet resistance
4.3 Illumination V oc
Illumination V oc is a means to assess solar cell
parameters as pseudo fill factor (pseudo FF), pseudo
efficiency (pseudo η) and open circuit voltage V oc before
the final metallisation (in our case a screen printed front
metallisation). For different activation steps, the best
illumination V oc values are listed in Table III (the
combination of RTP and laser has been used for the
activation of selective emitter structures, s. below). The
relatively high pseudo FF are ensuring that shunting
problems are not relevant and cannot be responsible for
the low open circuit voltages. We assume the low V oc
values are caused by a high recombination in the bulk
and emitter region (hence the high j 01 and j 02 values).
Table III: V oc and pseudo cell parameters, measured
with illumination V oc
Activation inline RTP laser RTP +
furnace laser
pseudo η [%] 13.5 13.0 12.9 13.1
V OC [mV] 580 550 570 554
pseudo FF [%] 80 81 76 79
j 01 [pA/cm²] 51 18 52 8
j 02 [nA/cm²] 79 67 143 100
4.4 Solar cells
For different activation steps and both for CZ and
multicrystalline (mc) wafers, the best solar cell values are
listed in Table IV. In comparison to the illumination V oc
values, it is remarkable that V oc and (pseudo) FF values
are decreased. One possible reason for this (high series
resistances, R ser) can be excluded for most cases because
R ser values are tolerable (< 1 Ωcm 2 , not shown here).
Similar to illumination V oc, we assume that a high carrier
recombination is responsible for the rather bad solar cell
characteristics. This is confirmed by spectrally resolved
light beam induced current (SR-LBIC) mappings,
showing effective diffusion length values below 100 µm
(also not shown here).
IQE measurements show that the emitter region λ <
400 nm is reasonably well in contrast to the low values
for λ > 600 nm (Fig. 5). This again is a hint for a material
degradation.
Table IV: Results of plasma implanted solar cells
RTP inline quartz tube
furnace furnace
mc solar cells
JSC [mA/cm²] 31 30
VOC [mV] 561 564
FF [%] 72 74
η [%] 12.3 12.3
CZ solar cells
J SC [mA/cm²] 29 26 30
V OC [mV] 544 551 554
FF [%] 76 60 77
η [%] 12.2 8.8 13.0
EQE
1,0
0,8
0,6
0,4
0,2
0,0
400 600 800 1000 1200
wave length [nm]
RTP
quartz
tube
Figure 5: Internal quantum efficiency for two CZ solar
cells with a plasma implanted emitter annealed using
RTP or quartz tube furnace
5 SELECTIVE EMITTER
5.1 Laser treatment
For processing selective emitter structures, the areas
between front side fingers (‘high resistivity areas’) are
activated by annealing the implanted wafers in an RTP
furnace. The following parameters turned out to be best
suited to process the high resistivity areas: implantation
(3 kV, 1 x 10 15 at / cm 2 ) and RTP activation (600 °C,
10 min, obtaining R sh = 120 Ω / sq). The low resistivity
areas below the metallisation fingers were illuminated by
an excimer laser (details see section 2.3). Fig. 6 illustrates
the impact the laser has on the silicon surface. A slight
rounding of the otherwise sharp-edged surface texture is
observed.
Figure 6: SEM image of the acid etched silicon surface
of mc wafer before (left) and after (right) laser treatment
5.2 Results
Because the high resistivity area is activated by a low
temperature process for only a short time, there remains
the possibility to decrease further the resistivity locally

y a laser. This is visualised by SRI (Fig. 7), where the
red colour indicates a higher emittivity (and thus a higher
conductivity) of the silicon than the yellow colour. In
these low resistance areas a sheet resistance of 36 Ωcm
could be achieved.
Figure 7: Sheet resistance imaging (SRI) of selective
emitter (distance between fingers ca. 300 µm).
6 CONCLUSION
The reason why solar cells with plasma implanted
phosphorus emitters lack the efficiencies which are
standard to the phosphorus diffused emitter solar cells is
not yet understood. Until now it is only an assumption
that there is a degradation due to a reduction of the
effective lifetime during solar cell processing.
Contamination with iron is one candidate for this.
Although for low phosphorus concentrations a similar
depth profile to phosphorus diffused emitters can be
obtained, it is improbable that an effective phosphorus
diffusion gettering takes place during processing of
plasma implanted solar cells. Therefore the processing of
this kind of solar cells affords a special awareness in
respect to wafer qualities, clean process surroundings and
the interaction of impurities during processes.
On the other hand, we observed shunting or
contacting (series resistance) problems only in very few
cases. Homogeneity of emitters and the screen printing of
a silver based metallisation even for low phosphorus
surface concentrations does not seem a problem. The heat
necessary for the activation is by far not as large as the
heat needed for diffusion processes where the phosphorus
has to be driven into the silicon bulk from a surface
coating. Therefore we consider the plasma implantation
method still a very promising method for producing
commercially viable solar cells.
6 ACKNOWLEDGEMENTS
The autors like to thank Christian Schmiga and
Michael Rauer from Fraunhofer ISE, Freiburg, Germany
for providing the ECV measurements. The authors thank
their colleagues from Ion Beam Services (Yannick
Cizeron, Hasnaa Etienne, Thomas Michel) and at
Helmholtz-Zentrum Dresden-Rossendorf (Andreas
Kolitsch, Wolfgang Skorupa, Johannes von Borany, S.
Prucnal) for conducting the plasma implantation and the
flash lamp annealing and also for fruitful discussions.
The financial support by the German Bundesministerium
für Wirtschaft und Technologie (BMWi) is gratefully
acknowledged for funding the project Flaem under the
contract number VF 090004.
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[2] V. Yelundur et al: First implementation of ion
implantation to produce commercial silicon solar cells,
26 th EU PVSEC, 2011, p. 831
[3] Á. Németh, I. Pintér, Z. Lábadi, A. Tóth, S. Püspöki:
Study of Crystal Defects in Plasma Doped Silicon Solar
Cells, 21 st EU PVSEC, 2006, 1400 – 1403
[4] A. Bentzen, J.S. Christensen, B.G. Svensson, and A.
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