INVESTIGATION OF LASER ABLATION OF SILICON NITRIDE ... - Parc

INVESTIGATION OF LASER ABLATION OF SILICON NITRIDE PASSIVATION WITH
SELF-DOPING PASTE FOR SOLAR CELL CONTACTS
Adam M. Payne 1 , Kalyan Rapolu 1 , Preston Davis 1 , Vinodh Chandrasekaran 1 , Daniel Meier 1 , Baomin Xu 2 , Jim Zesch 2 ,
and Karl Littau 2
1 Suniva, Inc, Norcross, GA, USA
2 Palo Alto Research Center, Palo Alto, CA, USA
ABSTRACT
We have fabricated solar cells using a standard POCl 3
process along with a PECVD SiN x AR coating. In
particular, we have also used a self-doping paste with belt
furnace firing in conjunction with laser ablation to remove
the silicon nitride passivating layer to minimize the metal
contact area. Use of laser ablation to create holes in the
nitride passivation is coupled with use of a silver selfdoping
paste to create a selective emitter. Cell results with
an efficiency of 13% were obtained. Discussion of the
fabrication steps needed and the analysis of the data are
given below. The cells suffer from low open circuit voltage
as well as modest fill factors. Increased shading losses
and unoptimized AR coating limit J sc. The self-doping
paste cells had a very high contact resistance, the root
causes of which are discussed.
BACKGROUND & RATIONALE
Self-doping pastes have been used in the fabrication of
solar cells in the past [1-2]. The benefits of self-doping
pastes were out-weighed by problems with doping
uniformity and within wafer temperature uniformity in a belt
furnace. In addition to these two problems adding a frit to
the paste, and thus promoting the consumption of the AR
coating and adhesion to the wafer, inhibits the dopant
from leaving the paste and being driven in to the
underlying silicon layers.
Recently it has been shown that high speed picosecond
and nanosecond lasers can be used to ablate the nitride
passivation layer with minimal damage to the underlying
silicon. The technique of laser ablation to remove the
passivating dielectric on one side of a solar cell has been
demonstrated by various research groups [3] and is also
documented in the patent literature [4]. In this work we
combine the two concepts of laser ablation of the antireflection
coating (& passivating layer) to create vias into
which a self-doping paste is printed.
EXPERIMENT
Cells were fabricated following a standard sequence using
boron-doped (p-type) 125mm pseudo-square samples of
1-3 Ω·cm resistivity and originally ~200µm thick. A galvoscan
head was used to guide the 266nm nanosecond
laser.
The process sequence was:
Step
Standard POCl 3
Self-doping
process Paste/Laser Ablation
1 Saw damage etch, texture, clean
2 Diffusion in POCl 3 to ~50-80 Ω/□
3 PSG removal
4 SiN x deposition by PECVD
5 Print front Ag paste
Laser ablation of
6 Dry
nitride
Print proprietary selfdoping
paste
7 Fire self-doping paste
8 Print rear aluminum paste
9 Fire in a belt furnace
10 Laser junction isolation scribe
Table 1 Process flow comparison
The self-doping paste/laser ablation process has two more
steps (laser ablation and separate firing of the self-doping
paste) than the standard process, but we believe that it will
enable us to create a selective emitter, and it may over
time be reduced to a single extra step of laser ablation.
After fabrication the cells were tested under static 1 Sun
illumination as well as Suns-Voc measurement and the
components of series resistance. Earlier work had shown
that it is possible to fire the self-doping paste to the silversilicon
eutectic point of 877°C in a rapid thermal
processing unit (RTP), but no work to our knowledge has
yet utilized a belt furnace to the same effect [1]. After
some optimization it was possible to fire the paste, without
causing the metal lines to melt completely and form balls
on the surface of the sample. In addition it was observed
that the laser processing in air inadvertently melted some
silicon which formed a thin layer of SiO 2. This oxide
capped the vias with a layer impervious to the fritless selfdoping
paste. A 2 minute dip in 5% HF removed this oxide
and enabled contacts to be formed between the metal and
the semiconductor; however, the HF in some cases
removed the standard 78nm SiN x anti-reflection coating
and front passivation layer as well. Therefore a new
process was developed in which thicker SiN x layers were
deposited, ablated by the laser underneath the gridline
locations, and then dipped the 5% HF solution. This
solution completely removed the thin layer of oxide
blocking the vias, but only partially etched away the SiN x.
The cell efficiency results are shown in Figure 1 below.
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Eff(%)
RESULTS & DISCUSSION
The thicker nitride has had an adverse impact on the cell
open circuit voltage. These samples did not have their
nitride deposited in one continuous run, but were removed
from the reactor in between shorter nitride depositions.
Overall the cell open circuit voltage is much lower than our
standard process which yields ~620mV. We hypothesize
that the ~5 minutes the samples spend at 950°C during
the self-doping paste firing is killing the sample bulk
lifetime which accounts for this observed low V oc. A
cleaner firing environment (e.g. RTA unit or cleaner
furnace) should mitigate this problem.
Figure 1 shows the solar cell efficiency as a function
of different nitride thicknesses (in nm). Plus signs
represent 45µJ/pulse and circles represent
30µJ/pulse.
FF(%)
Comparing these cells’ efficiency to those of the baseline
cells (see Table 2 below) shows the inadequacy of the
current procedure.
Cell
Type
Eff.
(%)
J sc
(mA/cm 2 )
V oc
(mV)
FF
(%)
R series
(Ω·cm 2 )
Std. 17.58 36.75 624 76.7 0.80
New 13.19 32.53 566 71.7 1.80
Table 2 best cell efficiency
Figure 3 shows the fill factor of the cells. Control cells
fabricated using a fritted paste had FF ~77% (see
Table 2).
Some of the difficulties encountered included aligning the
screen-printed paste to the vias in the SiN x. This task was
made easier by using a screen with wider than normal
openings to print lines ~220µm wide instead of the
standard ~110µm. This increase in line width caused
increased shading and a resulting decrease in the short
circuit current density by approximately 2 mA/cm 2 . In
addition we observed an extra loss of another 2 mA/cm 2
due to an unoptimized AR coating after the HF dip.
Rser att (Ohm*cm2)
Voc(V)
Figure 4 shows the cell series resistance.
Figure 2 shows the change in open circuit voltage
with nitride thickness
These values are coming from the higher than normal
contact resistance. We have measured the other
components of the series resistance, emitter sheet &
gridlines being the two main ones. These components
together contribute only ~0.5Ω·cm 2 to the normalized
series resistance meaning that the contact resistance is
still surprisingly large—larger than for the standard POCl 3
process.
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Pseudo-FF
more exact formulation from equation 1 does not change
the results substantially. We have further correlated this
indirect measurement of contact resistance with a
measurement of the transfer length method. We cut up a
finished cell into a 10mm wide strip by laser cutting on the
rear and cleaving. The resistance was measured between
adjacent gridlines and then plotted as a function of the
number of gridline spacings.
100
TLM
R = 65 Ohm/sq. (by 4 pt. probe)
sheet
R = 760 mOhm*cm 2 (metal area)
contact
80
Figure 5 Pseudo-fill factor from the Suns-Voc
measurement.
60
No HF strip
y = 76 + 1.9x R= 0.4615
Some slight decrease in PFF is observed for thicker
nitrides, but in general these values are not so small
compared to POCl 3 diffused cells with full Al rear.
∆FF = -(J sc/V oc)·R series,tot·FF ideal (1)
Equation (1) describes the loss in fill factor for a given cell
with known J sc, V oc, and total area normalized series
resistance [5]. For the cells described in this paper FF ideal
≈ 0.8 in the best case. ∆FF = 0.083 for the self-doping
paste cells and 0.038 for the control samples.
The major cause of loss of fill factor is the increase in
series resistance from 0.80 Ω·cm 2 up to 1.80Ω·cm 2 .
Components
Std. Self-doping
Process Paste
Grid 0.325 0.231
Emitter 0.312 0.312
Back Al 0.036 0.036
Within busbar 0.004 0.004
Substrate 0.032 0.032
Sum of components 0.71 0.615
Total (from I-V curve) 0.80 1.80
Contact = Total – Sum 0.09 1.185
Metal area specific
contact resistance
7.2 mΩ·cm 2 ~95 mΩ·cm 2
Table 3 Components of Series Resistance in Ω·cm 2
(cell area) except the last row.
From Table 3 we observe that although the major
components of series resistance are the grid and the
emitter sheet resistance. For the standard, fritted paste the
contact resistance is small relative to these two
components. For the self-doping paste in contrast, the
contact resistance is much larger than anticipated. This
result implies a metal contact area specific contact
resistance of nearly 100mΩ·cm 2 . Taking into account the
40
20
5 min HF
y = 1.85 + 12.342x R= 0.99968
R = 51 Ohm/sq..
sheet
R = 18.5 mOhm*cm 2 (metal area)
contact
0
0 1 2 3 4 5
Spacing (Gridlines)
Figure 6 plots resistance between gridlines as a
function of spacing.
The inset equations to Figure 6 are derived from linear
fitting to the measured data. The slope of the equation
confirms the emitter sheet resistance and the intercept is
twice the contact resistance. The contact resistance has
been normalized to the metal contact area for the two
cases shown. The sample depicted in red did not have the
laser-grown oxide stripped by HF and has a
correspondingly high contact resistance. The sample
depicted in blue had its nitride completely removed by 5
minute HF dip and has a relatively low contact resistance.
The indirectly calculated contact resistance values
reported in Table 3 were confirmed to by this method.
We have two interpretations of these results. Our first
thought is that the laser may have damaged the diffused
emitter layer. Ablation of a significant portion of the emitter
means that the silver paste is trying to form an electrical
contact with less heavily doped silicon which is known to
be difficult. Furthermore because the paste is fritless, only
that portion of the paste which is printed onto the via can
contact the underlying silicon. The contact area of the selfdoping
paste is less than that of the standard, fritted paste
because the silver sinters only in those locations where
the laser ablated the silicon nitride. Since this contact area
is smaller than for the standard paste the contact
resistance, other things being equal, should be greater.
The entire point of the experiment is that the self-doping
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paste should allow lower contact resistance due to the
presence of extra dopant underneath the metal contacts
which can enable a selective emitter by decreasing the
doping in the space between the gridlines. After confirming
that the contact resistance was indeed very large, we were
forced to ask the question whether the self-doping paste
was functioning as named. To confirm the operation of the
paste, we used the SIMS technique.
alloying reaction of the Ag-Si on the mirror polished wafers
did not take place uniformly as it did on the textured
wafers. Micrographs showing the results are shown in
Figure 7 after most of the paste was removed by a nitric
acid solution followed by HF/HCl. For some portions of the
sample all the silver was removed (upper photo), but for
others the silver remained (lower photo).
Figure 7 shows also the difficulty of obtaining accurate
SIMS results given that the size of the alloyed areas is
only ~25-50µm per side—still smaller than the SIMS ion
beam. The alloying appears to take place only along the
crystal planes. Ref 1 showed that the alloying resulted in
triangles when the paste was printed on (111) oriented
silicon. Here on the (100) silicon it alloys preferentially in
rectangles.
In Figure 8 below the concentration of phosphorus and
silver are both shown as a function of depth. To our
surprise the phosphorus concentration is lower than
expected which can explain the large contact resistances
observed. If there were truly an eutectic formation then we
would expect a deep junction as as is observed for the
eutectic formation with Al and Si on the rear of the cell.
The phosphorus and silver are present only in a very
shallow region indicating that we have not achieved true
alloying in this case. The silver concentration is also
significantly greater than anticipated in the silicon and
might cause problems with shunting the device. Based on
this data, coupled with the high measured contact
resistance we conclude that we were unsuccessful in
getting the silver-silicon interface to form the eutectic, or if
so, only in small portions.
SIMS Data for Self-doping Paste
31-P Conc (cm -3 )
Ag Conc (cm -3 )
Figure 7 shows surface after firing and removal of the
silver by acid etch. The area shown in both cases is
225µm by 250µm. The upper photo also contains
rough depth data with some pits as deep as 19 µm. In
the lower photo the silver paste was not completely
removed even after ultrasonic agitation in nitric acid.
Secondary ion mass spectroscopy (SIMS) is not well
suited to analyzing textured substrates since the size of
the pyramids is smaller than the ion beam size in the
SIMS measurement making it impossible to achieve any
depth resolution. To overcome this difficulty (100) oriented
mirror polished silicon wafers were processed similarly to
the solar cells including SiN x deposition, laser ablation, HF
dip, screen printing of the self-doping paste, and firing in a
belt furnace. These samples were sent out for SIMS
analysis. Diffusion was explicitly excluded on these
samples so as to make the analysis easier. Any
phosphorus measured in these samples must be
attributed to the paste. Unfortunately after firing the
10 21
10 20
10 19
10 18
10 17
0 20 40 60 80 100
Depth (nm)
Figure 8 shows the concentration of phosphorus and
silver as a function of depth. The silver is going
deeper into the silicon and causing some shunting of
the devices describe elsewhere in this report.
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CONCLUSIONS
While the cells fabricated for this project were not better
than the standard process and the contact resistance is
still higher due to lack of significant phosphorus diffusion,
we successfully used a laser ablation process to fabricate
selective emitter solar cells on 125mm pseudo-square
wafers. A peak cell efficiency of 13% was achieved. Cell
efficiency was limited by lower open circuit voltage, most
likely caused by decreased bulk lifetime, as well as low fill
factor caused by increased series resistance. The
decrease in J sc is explained well by the extra shading and
unoptimized AR coating thickness. The analysis of the
components of the series resistance indicates that contact
resistance approximately 100 times greater for this
process than for the standard fritted paste process. The
root causes of this higher contact resistance are ablation
of part of the emitter lowering the phosphorus
concentration under the laser-ablated vias, decreased
contact area due to the laser ablated holes being the only
contact points and mainly that too little phosphorus
diffused into the silicon underneath the gridlines.
ACKNOWLEDGEMENTS
This work is partially supported by the DOE under contract
No. DE-EE0000584. We thank Alan Carroll of Dupont for
a helpful discussion. Thanks also go to Don Tremblay for
some of the measurements.
REFERENCES
[1] D. L. Meier et al., “Production of Thin (70-100 µm)
Crystalline Silicon Cells for Conformable Modules,”
29 th IEEE PVSC, 2002, pp. 110-113.
[2] M. Hilali et al., “Optimization of Self-Doping Ag Paste
Firing to Achieve High Fill Factors on Screen-Printed
Silicon Solar Cells with a 100 Ω/sq. Emitter,” 29 th
IEEE PVSC, 2002, pp. 356-359.
[3] Peter Engelhart et al., “Laser Ablation of SiO 2 for
Locally Contacted Si Solar Cells With Ultra-short
Pulses,” Prog. Photovolt. Res. Appl. 15, 2007, pp.
521–527.
[4] Stuart Wenham and Martin Green, US Pat. 6,429,037
B1.
[5] D.L. Meier, E. A. Good, R. A. Garcia, B.L. Bingham,
S. Yamanaka, V. Chandrasekaran, and C. Bucher,
“Determining components of series resistance from
measurements on a finished cell”, 32 th IEEE PVSC
2006, pp. 1315-1318.
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