U.S. patent application number 14/968855 was filed with the patent office on 2016-11-17 for annealing for damage free laser processing for high efficiency solar cells.
The applicant listed for this patent is Solexel, Inc.. Invention is credited to Solene Coutant, Heather Deshazer, Pawan Kapur, Swaroop Kommera, Mehrdad M. Moslehi, Virendra V. Rana, Benjamin Rattle.
Application Number | 20160336473 14/968855 |
Document ID | / |
Family ID | 52480727 |
Filed Date | 2016-11-17 |
United States Patent
Application |
20160336473 |
Kind Code |
A1 |
Rana; Virendra V. ; et
al. |
November 17, 2016 |
ANNEALING FOR DAMAGE FREE LASER PROCESSING FOR HIGH EFFICIENCY
SOLAR CELLS
Abstract
Annealing solutions providing damage-free laser patterning
utilizing auxiliary heating to anneal laser damaged ablation
regions are provided herein. Ablation spots on an underlying
semiconductor substrate are annealed during or after pulsed laser
ablation patterning of overlying transparent passivation
layers.
Inventors: |
Rana; Virendra V.;
(Milpitas, CA) ; Moslehi; Mehrdad M.; (Milpitas,
CA) ; Kapur; Pawan; (Milpitas, CA) ; Rattle;
Benjamin; (Milpitas, CA) ; Deshazer; Heather;
(Milpitas, CA) ; Coutant; Solene; (Milpitas,
CA) ; Kommera; Swaroop; (Milpitas, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Solexel, Inc. |
Milpitas |
CA |
US |
|
|
Family ID: |
52480727 |
Appl. No.: |
14/968855 |
Filed: |
December 14, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14265331 |
Apr 29, 2014 |
9214585 |
|
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14968855 |
|
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61816830 |
Apr 29, 2013 |
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61827252 |
May 24, 2013 |
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61859166 |
Jul 26, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/1868 20130101;
H01L 31/0445 20141201; H01L 33/0095 20130101; Y02E 10/52 20130101;
H01L 31/186 20130101; H01L 31/0682 20130101; Y02P 70/521 20151101;
H01L 31/035281 20130101; Y02P 70/50 20151101; H01L 31/02167
20130101; H01L 31/068 20130101; H01L 31/056 20141201; H01L 31/1896
20130101; H01L 31/1864 20130101; H01L 31/022441 20130101; Y02E
10/547 20130101; H01L 31/02363 20130101; H01L 31/1804 20130101 |
International
Class: |
H01L 31/068 20060101
H01L031/068; H01L 31/18 20060101 H01L031/18; H01L 31/0236 20060101
H01L031/0236; H01L 31/0224 20060101 H01L031/0224; H01L 31/0216
20060101 H01L031/0216 |
Claims
1. A method for patterning an electrically insulating layer on a
semiconductor substrate, said method comprising: providing a
semiconductor substrate having n-type doping; depositing a first
layer of borosilicate glass or a borosilicate/undoped glass stack
on said semiconductor substrate; selectively ablating said first
layer of borosilicate glass or borosilicate/undoped glass stack
with a pulsed laser in a first emitter ablation pattern comprising
a plurality of first ablation spots; annealing said first ablation
spots; depositing a second layer of borosilicate glass or a
borosilicate/undoped glass stack on said first layer of
borosilicate glass or a borosilicate/undoped glass stack;
selectively ablating said second layer of borosilicate glass or
borosilicate/undoped glass stack with a pulsed laser in a second
base ablation pattern comprising a plurality of second ablation
spots; and, annealing said second ablation spots.
2. The method of claim 1, wherein said annealing of said first
ablation spots and said annealing of said second ablation spots are
formed as overlapping ablation spots.
3. The method of claim 1, wherein said annealing of said first
ablation spots and said annealing of said second ablation spots are
formed as non-overlapping ablation spots
4. The method of claim 1, wherein said annealing of said first
ablation spots and said annealing of said second ablation spots
anneals the wafer surface.
5. The method of claim 1, wherein said annealing of said first
ablation spots and said annealing of said second ablation spots
melts a thickness of the surface of the wafer.
6. The method of claim 1, wherein said annealing of said first
ablation spots forms the high-low selective emitter junction.
7. The method of claim 1, wherein said annealing of said second
ablation spots forms the high-low selective base junction.
8. The method of claim 1, wherein said ablation is performed using
a pulsed picoseconds laser and said annealing is performed using
pulsed nanosecond laser.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/265,331 filed Apr. 29, 2014 which claims
the benefit of U.S. provisional patent applications 61/816,830
filed on Apr. 29, 2013, 61/827,252 filed May 24, 2013, and
61/859,166 filed Jul. 26, 2013, which are hereby incorporated by
reference in their entirety. U.S. patent application Ser. No.
13/905,113 filed May 29, 2013 and Ser. No. 14/137,172 filed Dec.
20, 2013 are hereby incorporated by reference in their
entirety.
FIELD
[0002] The present disclosure relates in general to the fields of
solar photovoltaic (PV) cells, and more particularly to laser
processing of photovoltaic solar cell substrates.
BACKGROUND
[0003] Laser processing offers several advantages in terms of
efficiency enhancement and manufacturing cost reduction for
high-performance, high-efficiency solar cell processing. Firstly,
advanced crystalline silicon solar cells may benefit from having
the dimensions of the critical features such as electrical contacts
be much smaller than the current industrial practice. For front
contacted solar cells the contact area of the front metallization
to the emitter as well as the contact area of the back metal to the
base needs to be low (or the contact area ratios should be fairly
small, preferably much below 10%). For an all back-contact,
back-junction solar cell, where the emitter and base regions
forming the p/n junction and the metallization are on the same side
(the cell backside opposite the sunny side), the dimensions of the
various features are typically small for high efficiency. In these
cells where typically the emitter and base regions form alternate
stripes, the width of these regions (in particular the width of the
base contact) tends to be small. Also, the dimensions of the metal
contacts to these regions tend to be proportionally small. The
metallization connecting to the emitter and base regions then needs
to be patterned to a correspondingly finer scale. Generally,
lithography and laser processing are the technologies that have the
relatively fine resolution capability to provide the small
dimensions and the control required. Of these techniques, only
laser processing offers the low cost advantage required in solar
cell making. While lithography requires consumables such as
photoresist and subsequent resist developer and stripper (which add
to the process cost and complexity), laser processing is a
non-contact, dry, direct write method and does not require any
material consumables, making it a simpler and lower cost process
for solar cell fabrication. Moreover, laser processing is an
excellent choice for environmentally benign manufacturing since it
is an all-dry process which does not use any consumables such as
chemicals.
[0004] Further, to reduce the cost of solar cells there is a push
to reduce the thickness of the crystalline silicon used and also at
the same time increase the cell area for more power per cell and
lower manufacturing cost per watt. Laser processing is suitable for
these thin wafers and thin-film cell substrates as it is a
completely non-contact, dry process and can be easily scaled to
larger cell sizes.
[0005] Laser processing is also attractive as it is generally a
"green" and environmentally benign process, not requiring or using
poisonous chemicals or gases. With suitable selection of the laser
and the processing system, laser processing presents the
possibility of very high productivity with a very low cost of
ownership.
[0006] Despite these advantages, the use of laser processing in
crystalline silicon solar cell making has been limited because
laser processes that provide high performance cells have not been
developed. Disclosed here are laser processes using schemes that
are tailored for each key application to produce solar cells with
high efficiency. Specific embodiments are also disclosed for
applications of laser processing in manufacturing thin-film
crystalline silicon solar cells, such as those manufactured using
sub-50-micron silicon substrates formed by epitaxial silicon
growth.
SUMMARY
[0007] Various laser processing schemes are disclosed herein for
producing hetero-junction and homo-junction solar cells. The
methods include base and emitter contact opening, front and back
surface field formation, selective doping, metal ablation,
annealing, and passivation. In particular, annealing solutions
providing damage-free laser patterning utilizing auxiliary heating
to anneal laser damaged ablation regions are provided herein. Also,
laser processing schemes are disclosed that are suitable for
selective amorphous silicon ablation and selective doping for
hetero-junction solar cells. These laser processing techniques may
be applied to semiconductor substrates, including crystalline
silicon substrates, and further including crystalline silicon
substrates which are manufactured either through wire saw wafering
methods or via epitaxial deposition processes, that are either
planar or textured/three-dimensional. These techniques are highly
suited to thin crystalline semiconductor, including thin
crystalline silicon films.
[0008] Laser processing schemes are disclosed that meet the
requirements of base to emitter isolation (including but not
limited to shallow trench isolation) for all back-contact
homo-junction emitter solar cells (such as high-efficiency
back-contact crystalline silicon solar cells), opening for base
doping, and base and emitter contact opening (with controlled small
contact area ratios, for instance substantially below 10% contact
area ratio, for reduced contact recombination losses and increased
cell efficiency), selective doping (such as for base and/or emitter
contact doping), and metal ablation (formation of patterned
metallization layers such as creating the patterned metallization
seed layer on a thin-film monocrystalline silicon solar cell prior
to subsequent attachment of a backplane to the cell and its release
from a reusable host template) for both front-contact and all
back-contact/back-junction homo-junction emitter solar cells. Also,
laser processing schemes are disclosed that are suitable for
selective amorphous silicon ablation and oxide (such as a
transparent conductive oxide (TCO)) ablation, and metal ablation
for metal patterning for hetero-junction solar cells (such as
back-contact solar cells comprising hetero-junction amorphous
silicon emitter on monocrystalline silicon base). These laser
processing techniques may be applied to semiconductor substrates,
including crystalline silicon substrates, and further including
crystalline silicon substrates which are manufactured either
through wire saw wafering methods or using epitaxial deposition
processes, which may be either planar or
textured/three-dimensional, where the three-dimensional substrates
may be obtained using epitaxial silicon lift-off techniques using
porous silicon seed/release layers or other types of sacrificial
release layers. These techniques are highly suited to thin
crystalline semiconductor, including thin crystalline silicon films
obtained using epitaxial silicon deposition on a template
comprising a porous silicon release layer or other techniques known
in the industry.
[0009] An all back-contact homo-junction solar cell may be formed
in the crystalline silicon substrate, wherein laser processing is
used to perform one or a combination of the following: micromachine
or pattern the emitter and base regions including base to emitter
isolation as well as openings for base, provide selective doping of
emitter and base, make openings to base and emitter for metal
contacts, provide metal patterning, provide annealing, and provide
passivation. A front contacted homo-junction (emitter) solar cell
may be made using laser processing for selective doping of emitter
and making openings for metal contacts for both frontside and
backside metallization. A hetero-junction all back-contact
back-contact solar cell may be made using laser processing for
defining the base region and conductive oxide isolation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The features, nature, and advantages of the disclosed
subject matter will become more apparent from the detailed
description set forth below when taken in conjunction with the
drawings, in which like reference numerals indicate like features
and wherein:
[0011] FIG. 1 shows a scanning electron microscope (SEM) image of a
shallow trench made in silicon for application in an all back
contact back-junction solar cell, in accordance with the present
disclosure;
[0012] FIG. 2 shows a profile of a shallow trench in silicon for
application in all back contact back junction solar cells;
[0013] FIGS. 3A-3D show the procedure for selecting the laser
fluence to obtain reduced damage silicon dioxide (or oxide)
ablation. FIG. 3A shows the dependence of the size of the ablation
spot on the laser fluence; FIG. 3B shows irregular delamination of
oxide; FIG. 3C shows a damage-free spot; and FIG. 3D shows highly
damaged silicon in the spot opening;
[0014] FIG. 4 shows substantially parallel rows of contacts opened
in oxide using pulsed laser ablation in accordance with the present
disclosure;
[0015] FIG. 5 shows a screenshot with oxide ablation spots for
metal contacts;
[0016] FIGS. 6A and 6B show the laser-ablated area formed by making
ablation spots that are overlapped in both the x and y-direction;
FIG. 6A shows a 180 micron wide strip opened in 1000 A BSG
(boron-doped oxide)/500 A USG (undoped oxide) for base isolation
region; and
[0017] FIG. 6B shows a .about.90 micron wide stripe opened in 1000
A USG (undoped oxide) for base region;
[0018] FIG. 7A shows the threshold for oxide damage, below which
metal can be removed without metal penetration of the oxide
layer;
[0019] FIG. 7B shows that after 20 scans the metal runners are
fully isolated;
[0020] FIG. 7C shows an optical micrograph of the trench formed in
this metal stack;
[0021] FIGS. 8A and 8B show a top view and a cross-sectional view
of a pyramidal TFSC;
[0022] FIGS. 9A and 9B show a top view and a cross-sectional view
of a prism TFSC;
[0023] FIGS. 10A and 10B show a process flow for creation and
release of a planar epitaxial thin film silicon solar cell
substrate (TFSS);
[0024] FIGS. 11A and 11B show a process flow for planar epitaxial
thin film silicon solar cell substrate in case the TFSS is too thin
to be free standing or self-supporting;
[0025] FIGS. 12A and 12B show a process flow for micromold template
(or reusable template) creation for making a 3-D TFSS;
[0026] FIGS. 12C and 12D show a process flow for 3-D TFSS creation
using the reusable micromold template;
[0027] FIG. 13 shows a process flow for making a planar front
contacted solar cell where the TFSS is thick enough to be free
standing and self-supporting (e.g. thicker than approximately 50
microns for smaller 100 mm.times.100 mm substrates and thicker than
approximately 80 microns for 156 mm.times.156 mm substrates), in
accordance with the present disclosure;
[0028] FIG. 14 shows a process flow for making a planar front
contact solar cell where the TFSS is too thin to be self
supporting, in accordance with the present disclosure;
[0029] FIG. 15 shows a process flow for making a 3-D front contact
solar cell in accordance with the present disclosure;
[0030] FIGS. 16A-16D show a process flow for making an
interdigitated back contact back-junction solar cell where the TFSS
is thick enough to be self supporting, in accordance with the
present disclosure;
[0031] FIG. 17 shows a process flow for making an interdigitated
back-contact back-junction solar cell using thick TFSS where the
in-situ emitter is not deposited. Instead, a BSG (boron-doped
oxide) layer is deposited on the epitaxial silicon film and
patterned to open the base isolation region, in accordance with the
present disclosure;
[0032] FIG. 18 shows a process flow for making an interdigitated
back-contact back-junction solar cell where the TFSS is not thick
enough to be self supporting, where in-situ emitter and laser
ablation of silicon is used to form the base isolation opening, in
accordance with the present disclosure;
[0033] FIGS. 19A-19H show a process flow for making an
interdigitated back-contact back-junction solar cell where the TFSS
is not thick enough to be self supporting, and where instead of
in-situ emitter BSG (boron-doped oxide) deposition and selective
laser etchback is used to form the base isolation opening, in
accordance with the present disclosure;
[0034] FIG. 20 shows a process flow for making an interdigitated
back-contact back-junction solar cell using a 3-D TFSS, in
accordance with the present disclosure;
[0035] FIG. 21 shows a process flow for making an interdigitated
back-contact back-junction hetero-junction solar cell, in
accordance with the present disclosure;
[0036] FIGS. 22 through 30 are not found in U.S. patent application
Ser. No. 13/118,295 "LASER PROCESSING FOR HIGH-EFFICIENCY THIN
CRYSTALLINE SILICON SOLAR CELL FABRICATION" by Virendra V. Rana and
filed on May 27, 2011;
[0037] FIGS. 22A and 22B are schematics showing the profile of a
Gaussian beam and a flat top beam, respectively;
[0038] FIG. 23 is a cross-sectional diagram of a
back-contact/back-junction cell;
[0039] FIGS. 24A-24F are rear/backside views of a back contact
solar cell during fabrication;
[0040] FIG. 25 is a rear/backside view of the back contact solar
cell of FIG. 24A with alternating metal lines contacting the
emitter and base regions;
[0041] FIGS. 26A-26C are diagrams illustrating three ways a
flat-top beam profile may be created;
[0042] FIGS. 27A and 27B are schematics showing the profile of a
Gaussian beam and a flat top beam highlighting the ablation
threshold;
[0043] FIGS. 28A and 28B are diagrams showing a Gaussian beam and a
flat top beam ablate region profile/footprint, respectively;
[0044] FIG. 28C is a graph of overlap and scan speed;
[0045] FIGS. 29A and 29B are diagrams illustrating a beam alignment
window of a Gassian beam and flat top beam, respectively;
[0046] FIGS. 29C and 29D are diagrams showing a Gaussian beam
region profile and a flat top beam region profile, respectively;
and
[0047] FIG. 29E graphically depicts the results of Table 1;
[0048] FIG. 30 shows a process flow for an NBLAC cell;
[0049] FIG. 31 shows a schematic cross section of an NBLAC
cell;
[0050] FIG. 32 shows a graph of minority carrier lifetime with and
without laser annealing;
[0051] FIGS. 33A and 33B show process flows for all back contact
solar cells with oxide ablation;
[0052] FIGS. 33C and 33D show process flows for all back contact
solar cells with oxide ablation;
[0053] FIGS. 34A and 34B show an oxide ablation process;
[0054] FIGS. 35A and 35B show an oxide ablation process using an
amorphous silicon layer;
[0055] FIG. 36 outlines the process flow to form back-junction,
back-contact solar cell using a starting wafer;
[0056] FIGS. 37A and 37B are cross-sectional diagrams of a solar
cell having an interdigitated orthogonal back contact metallization
pattern;
[0057] FIGS. 38A through 38E are scanning electron microscope
(SEMS) images highlighting damage to an underlying silicon
substrate during oxide ablation;
[0058] FIG. 39A is a scanning electron microscope (SEMS) image of
overlapping ablation spots;
[0059] FIG. 39B is an expanded view of the image of FIG. 39A;
[0060] FIG. 39C is a scanning electron microscope (SEMS) image of
nonoverlapping ablation spots;
[0061] FIG. 39D is an expanded view of the image of FIG. 39C;
[0062] FIGS. 40A and 40B are schematic diagrams showing two laser
patterning opening and contact schemes;
[0063] FIGS. 41A and 41B are scanning electron micrographs of a
spot-in-spot laser pattern;
[0064] FIGS. 42A and 42B are schematic diagrams showing laser
annealing of damaged silicon in selective emitter and selective
base ablations, respectively;
[0065] FIGS. 42C and 42D are schematic diagrams showing laser
annealing spot overlap of contacts in selective emitter and
selective base ablations, respectively;
[0066] FIGS. 43A and 43B are schematic diagrams showing laser
annealing of damaged silicon by spot in selective emitter and
selective base ablations, respectively;
[0067] FIGS. 43C and 43D are schematic diagrams showing spot by
spot laser annealing of damaged silicon in ablations in a
spot-in-spot patterning scheme;
[0068] FIGS. 44A and 44B are optical micrographs showing laser
ablation spots before laser anneal and the same ablation spots
after annealing;
[0069] FIG. 45 is a minority carrier lifetime map of a silicon
substrate after oxide ablation;
[0070] FIG. 46 is process flow for the formation of a back contact
back junction solar cell utilizing laser annealing of laser
ablation damage;
[0071] FIG. 47 is an alternative process flow for the formation of
a back contact back junction solar cell utilizing laser annealing
of laser ablation damage;
[0072] FIG. 48 is an alternative process flow for the formation of
a back contact back junction solar cell utilizing laser
annealing;
[0073] FIGS. 49A and 49B are schematic diagrams of a laser tool
configuration a highly cost-effective multi-station platform that
provides for high-throughput parallel processing at different
stations;
[0074] FIG. 50 is a schematic diagram of a high throughput laser
ablation laser annealing system;
[0075] FIG. 51 is a schematic diagram outlining same station laser
ablation and corresponding laser annealing using collinear
(coaxial) laser beams;
[0076] FIG. 52 is a schematic diagram showing a same station
ablation and annealing using separate laser beams;
[0077] FIG. 53 is a process flow for the formation of a back
contact back junction solar cell showing oxide ablation carried out
in the presence of a heating beam;
[0078] FIG. 54 is a schematic diagram showing a same station
ablation and annealing using wafer heating;
[0079] FIG. 55 is a process flow for the formation of a back
contact back junction solar cell showing oxide ablation carried out
in the presence of a heating light;
[0080] FIG. 56 is a process flow for the formation of a back
contact back junction solar cell using flash annealing; and
[0081] FIG. 57 shows representative/idealized profiles of a
Gaussian laser beam and a flat-top (top-hat) laser beam.
DETAILED DESCRIPTION
[0082] Although the present disclosure is described with reference
to specific embodiments, one skilled in the art could apply the
principles discussed herein to other areas and/or embodiments
without undue experimentation.
[0083] We disclose here laser processing, more specifically pulsed
laser processing, schemes that have been developed to address the
varying requirements of different processes.
[0084] The disclosed methods may be useful in the area of
semiconductor device ablation, particularly crystalline silicon
ablation. Typically removal of silicon with a laser involves
silicon melting and evaporation that leaves undesirable residual
damage in the silicon substrate. This damage causes minority
carrier lifetime degradation and increased surface recombination
velocity (SRV) that reduces the solar cell efficiency. Hence, wet
cleaning of the silicon substrate is typically used to remove this
damage layer. We present a scheme to reduce this damage to a level
acceptable for high efficiency solar cell manufacturing that does
not require post-laser-processing wet cleaning, hence simplifying
the process flow and reducing the manufacturing cost.
[0085] The damage remaining in the silicon substrate upon ablating
a certain thickness of it using a laser is related to the amount of
laser energy absorbed in the substrate that is not used by the
ablated material. If it can be managed that most of the laser
energy is used in removing the material then the fraction of the
incident energy that seeps into the silicon substrate is minimized,
thus minimizing the laser-induced substrate damage and SRV
degradation. The penetration of laser energy into silicon depends
on the laser pulse length (also called pulse width) and wavelength.
The infrared (IR) laser beam, wavelength 1.06 microns, has a long
penetration depth in silicon, up to about 1000 microns, while a
green laser beam, with a wavelength of 532 nm, penetrates only to a
depth of approximately 3 to 4 microns. The penetration of UV laser
beam, with a wavelength of 355 nm, is even shorter, only about 10
nm. It is clear that using ultra-short pulses of UV or EUV
wavelength limits the penetration of the laser energy into silicon.
Additionally, shorter laser pulse length results in shorter
diffusion of heat into silicon. While a nanoseconds pulse can lead
to heat diffusion in silicon to approximately 3 to 4 microns range,
the picoseconds pulse reduces it to about 80 to 100 nm, while a
femtoseconds pulse is so short that typically there is no heat
diffusion into silicon during the laser ablation process. Hence
going to shorter pulses with a shorter wavelength lead to
diminishing damage to the laser-ablated substrate. For higher
production throughput, green or IR wavelengths can be used
depending on the extent of laser damage acceptable. Since even
under ideal conditions a certain fraction of the energy would still
seep into the substrate, this absorption and its undesirable side
effects can be further reduced by reducing the laser power.
However, this results in a smaller thickness of silicon being
ablated (or a lower silicon ablation rate or lower throughput). It
has been found that reducing the pulse energy but causing the
silicon removal by increasing the overlap of the laser pulses makes
the silicon shallow isolation trench smoother. This is an
indication of low silicon surface damage. At very low pulse
energies the thickness of silicon removed may be small. The desired
depth may then be obtained by using multiple overlapped scans of
the pulsed laser beam.
[0086] A pulsed laser beam with pulse length in the picoseconds
range and a wavelength of approximately 355 nm or below is suitable
for silicon ablation with low damage enabling low surface
recombination velocity (SRV) for passivated ablated surfaces. FIG.
1 shows a 2.25 micron deep and nearly 100 micron wide trench made
in a silicon substrate using a picoseconds UV laser beam of
Gaussian profile (M.sup.2.ltoreq.1.3), nearly 110 microns in
diameter with 4 microjoule pulse energy, with the laser spots
overlapped nearly 15 times. This depth of ablation was obtained
using twenty overlapped scans of the laser with each scan removing
about 112 nm of silicon. FIG. 2 shows the smooth profile of a 4
micron deep and 110 micron wide trench in silicon obtained using
the same picoseconds laser beam with the UV wavelength. The
smoothness of the profile should be noted. Such an ablation of
silicon is used in all back-contact back-junction solar cells to
form regions that isolate base regions from emitter regions. Use of
a femtoseconds laser may provide further reduction of laser damage
during silicon ablation.
[0087] The embodiments of this disclosure are also applicable to
the ablation of amorphous silicon. A similar scheme may be used to
ablate a desired thickness of amorphous silicon using a pulsed
laser beam with femtoseconds pulse length and in some embodiments a
UV or green wavelength. Since ablation of amorphous silicon
requires much lower energy than crystalline silicon, such a scheme
may effectively be used to selectively ablate amorphous silicon
films from the crystalline silicon surface for application to
hetero-junction solar cells.
[0088] This disclosure is also applicable to oxide ablation
selective to the underlying substrate, which may be crystalline or
amorphous silicon. The oxide film is transparent to laser beams of
wavelength down to UV. If a nanoseconds pulse length laser is used
to remove the overlying oxide, the removal of oxide takes place by
heating and melting of silicon underneath. Because of the pressure
from the ablated silicon underneath, the overlying oxide is cracked
and removed. This however, creates heavy damage in the silicon
substrate so that a wet cleaning treatment is typically used to
remove this damaged layer for use in high efficiency cells.
[0089] We present here a scheme where the oxide layer is
selectively removed from the silicon surface without any
appreciable damage to the silicon surface. During the laser
ablation, besides heating the material to melt or evaporate it,
other effects such as plasma formation take place. Sometimes
complex processes can take place at an interface. Using a laser
with picoseconds pulse length, the oxide to silicon interface is
affected. Using a picoseconds laser with a UV wavelength, the
interface effects are enhanced so that separation and delamination
of the oxide film takes place from the silicon surface. The silicon
surface left behind is virtually free of damage. Picoseconds laser
radiation with green or infra-red (IR) wavelength can also be used
depending on how much penetration damage of silicon substrate is
acceptable. This disclosure will outline the procedure to obtain
damage free selective ablation of oxide from the silicon
surface.
[0090] FIGS. 3A-3D disclose the procedure for obtaining damage-free
ablation of oxide. FIG. 3A shows the variation of laser spot
opening in a 1000 A PSG (phosphorus-doped oxide)/500 A USG (undoped
oxide) stack on a 35 micron thick epitaxial silicon film on a
template, using a picoseconds UV laser beam. The oxide layers were
deposited using APCVD (atmospheric-pressure CVD) technique. For a
given thickness of oxide the spot size depends on the laser fluence
(J/cm.sup.2). The laser fluence is the laser pulse energy divided
by the area of the laser beam. In this case, the laser beam was
about 100 microns in diameter with a Gaussian profile
(M.sup.2<1.3). At very low fluence, the spots are irregular and
there is irregular delamination of oxide from the silicon surface
as shown in FIG. 3B, while at very high fluence there is extensive
damage of silicon as shown in FIG. 3D. The range of fluence shown
by line a-a' indicates the optimum range where the damage to the
silicon substrate is minimal as seen in FIG. 3C.
[0091] FIG. 4 shows rows of cell contact openings that are
selectively opened in the oxide for application in all back-contact
(and back-junction) solar cells. FIG. 5 is a close-up of these
contacts. The laser ablation spots can be overlapped in both x and
y direction to open up an area of any desired length and width on
the wafer as shown in FIGS. 6A and 6B. FIG. 6A shows a 180 micron
wide opening made by selectively removing the BSG (boron-doped
oxide) for base isolation region using picoseconds UV laser beam
with ablation spots overlapping in both x and y-direction.
Similarly, FIG. 6B shows a 90 micron wide area opened up in USG
(undoped oxide) for forming the base region.
[0092] The selective ablation of oxide from a silicon surface as
disclosed here can be used in solar cell making in several ways. In
one application, when using in-situ emitter for back-contact cells,
this process is used to open tracks in an oxide film to expose the
underlying emitter. The emitter so exposed may be removed using wet
etching. This region is then used for base to emitter isolation and
with base formed inside it.
[0093] In another application, this process is used to open regions
that are then used for making metal contacts. For front contacted
cells, the oxide passivation can be used on the backside of the
cells. The scheme described here is then used to open contacts for
the metal that is subsequently deposited on these contacts. In this
manner, the metal has localized contact that is conducive to high
cell efficiency. For back contacted cells, contacts for both base
and emitter may be opened using this scheme.
[0094] In a solar cell process flow, a doped oxide may need to be
removed without causing any doping of the silicon underneath (i.e.,
without any appreciable heating of the doped oxide and silicon
structure). Since, as described above, the oxide is removed by
separation at the oxide/silicon substrate interface when using a
picoseconds laser beam, the removal of oxide happens with limited
pickup of the dopant from the oxide film being ablated.
[0095] The selective ablation of silicon nitride (SiN.sub.x) is
used for front contacted solar cells. Using laser ablation, the
contact area to the emitter surface can be reduced thereby
minimizing the area where the SiN passivation is removed. This
leads to higher Voc. Picosecond lasers with either UV or green
wavelength are suitable for this application, although nanoseconds
UV lasers can also be used.
[0096] Selective metal ablation from the oxide surface has
historically been difficult using lasers. This is because at the
high pulse energies needed to ablate metal, the energy is high
enough to damage the oxide underneath and cause penetration of
metal into oxide. In fact, this is the basis for the process of
"laser fired contacts" (LFC) used in solar cells.
[0097] We disclose three schemes for selectively removing metal
from the oxide (or another dielectric) surface with no metal
penetration of oxide (or other dielectrics such as silicon nitride)
and breaking or cracking of oxide. In all these schemes, aluminum
is the first metal in contact with base and emitter (aluminum being
used as the contact and light trapping rear mirror layer). A laser
with picoseconds pulse length is suitable for this application. For
high metal removal rate the IR wavelength is quite suitable.
According to the first scheme, metal is ablated at a pulse energy
that is lower than the threshold for oxide ablation. If the
thickness of metal removed in one scan is lower than the desired
thickness, multiple overlapping scans are used to remove the full
thickness of metal. Since the pulse energy is below the oxide
ablation threshold, a clean removal of metal from the oxide surface
is obtained. However, the exact recipe used highly depends on the
type of metal in the stack, their thickness and surface roughness,
etc.
[0098] FIGS. 7A-7C shows the ablation results when patterning a
PVD-deposited bi-layer stack of 2400 A of NiV on 1200 A of Al on
oxide. It is desired that the metal be removed completely between
the runners without breaking through the oxide layer underneath (to
prevent shunts in the cell). FIG. 7A shows the threshold for pulse
energy, below which this metal stack can be removed without
penetration of oxide. This threshold, besides depending on the
metal stack characteristics described above, depends on the laser
parameters such as spot overlap obtained using a certain pulse
repetition rate of the laser as well as the scan speed. With
increasing pulse overlap the threshold pulse energy would decrease,
because of the energy accumulation in the metal. FIG. 7B shows that
using a pulse energy below the threshold for oxide damage, more
than twenty scans provided complete isolation of metal runners as
determined by the 100M-ohm resistance between parallel lines. FIG.
7C shows a clean 75 micron trench formed in the 2400 A NiV/1200 Al
metal stack.
[0099] According to the second, high-throughput scheme higher pulse
energies are used, since a substantial part of the incident energy
is absorbed as it is being ablated thereby reducing damage to the
oxide. This approach makes the laser ablation of metal a very high
throughput process. Using this scheme we have ablated 1250 A
Al/100-250 A of NiV, with or without a tin (Sn) overlayer up to a
thickness of 2500 A successfully using a two step process. In the
first step the softer metal is removed using 15 microjoule pulses,
followed by 30 microjoule pulses both overlapped fifteen times. For
thicker aluminum such as 2000 A the second step can be carried out
at 50 microjoules with the same number of overlapping of
pulses.
[0100] The third scheme of metal ablation is applicable to highly
reflective films, for example Al/Ag stack (with Al in contact with
the cell and Ag on top of Al), such that most of the incident
energy of the picoseconds laser is reflected and ablation is
drastically reduced. In that case the surface of the reflective
metal (Ag) is first dented using a long pulse length nanoseconds
laser, pulse length from 10 to 800 nanoseconds, followed by
picoseconds cleanup of the aluminum underneath.
[0101] This disclosure is also applicable to the selective doping
of a substrate. For successful doping of silicon using an overlying
layer of the dopant-containing material, the pulse energy should be
high enough to melt the silicon but not high enough to ablate it or
the dopant layer above it. As the silicon melts, the dopant is
dissolved into it. Upon recrystallization of this silicon layer, a
doped layer is obtained. For this application a nanoseconds pulse
length laser with green wavelength is quite suitable because of its
limited penetration into silicon.
[0102] The laser processing techniques described above are
applicable to planar and 3-D thin-film crystalline silicon
substrates. The laser processes described here are suitable for any
thickness of the silicon substrate. These include the current
standard wafer thickness of .gtoreq.150 microns used for
crystalline silicon solar cells. However, they become even more
advantageous for thin, fragile wafers or substrates as the process
in carried out without any contact with the substrate. These
include the wafers thinner than 150 micron obtained from
monocrystalline CZ ingots or multi-crystalline bricks using
advanced wire sawing techniques or by other techniques such as
hydrogen implantation followed by annealing to separate the desired
thickness of wafer, or thin-film monocrystalline substrates (such
as in the thickness range of from a few microns up to 80 microns)
obtained using epitaxial deposition of silicon on a sacrificial
separation/release layer such as porous silicon and its subsequent
lift off.
[0103] The laser processing is particularly suited to three
dimensional substrates obtained using pre-structuring of reusable
templates and silicon micromachining techniques. One such method is
described in the '713 Application (published as U.S. Pat. Pub.
US2010/0304522 on Dec. 2, 2010). FIGS. 8A through 9B show the 3-D
thin film silicon substrates obtained using the technique described
in that publication. FIG. 8A shows the top view while FIG. 8B shows
the cross-section of the TFSS so obtained. For pyramidal
substrates, the tips may be flat or may end in a sharp point. FIGS.
9A and 9B show the TFSS with prism structure obtained using a
reusable pre-structured 3D template described in the reference
above.
[0104] Although the laser processes and the process flows described
here are applicable to any thickness of the silicon substrate (from
less than one micron to over 100 microns), we disclose here their
application to solar cells made using thin silicon substrates in
the thickness range of from less than 1 micron to about 80 microns,
including but not limited to those that are obtained using
epitaxial silicon on porous silicon (or other sacrificial layer)
surface of a reusable template as described in the '713
Application. To facilitate the understanding of our application,
the process flow for obtaining a desired thickness (e.g. from about
less than 10 microns up to about 120 microns) of planar
monocrystalline TFSSs according to that publication is shown in
FIGS. 10A and 10B for planar TFSS that are typically greater than
about 50 microns so that they can be handled as self supporting
substrates during cell processing, and FIGS. 11A and 11B for planar
TFSS that are typically thinner than about 50 microns so that they
are not self supporting during cell processing (and hence, are
reinforced prior to separation from their host templates). FIGS.
12A-12D show the process flow for obtaining three-dimensional
pyramidal silicon substrates. Three-dimensional prism-shaped
substrates can be obtained with similar processes, but using a
lithography or screen printed pattern that provides for that
structure.
[0105] The thin planar substrate obtained using the process flow of
FIGS. 10A and 10B may be processed according to the process flow of
FIG. 13 to obtain high efficiency front contacted solar cells. It
should be noted for self-supporting TFSSs it is advantageous to
process the template side of the TFSS first before proceeding to
the other side. Since the template side of the TFSS is textured
during the removal of the quasi-monocrystalline silicon remaining
on the TFSS after its separation from the template it is preferably
the frontside or sunnyside of the solar cell. The laser processes
of selective ablation of silicon oxide and silicon nitride (SiN)
are used to advantage in making this front contacted solar
cell.
[0106] FIG. 14 shows the application of various laser processes for
making high efficiency front contacted solar cells using planar
TFSSs where the TFSS is too thin to be free standing or self
supporting during cell processing. It should be noted that in this
case the non-template side surface is processed first with the TFSS
on the template. Once this processing is complete the TFSS is first
attached to a reinforcement plate or sheet (also called a
backplane) on the exposed processed side and then separated from
the template. After separation of the backplane-attached (or
backplane-laminated) thin-film crystalline silicon solar cell,
removal of residual porous silicon, texture etch, and SiN
passivation/ARC deposition, and forming-gas anneal (FGA) operation
processes are carried out on the released face of TFSS (which will
end up being the front surface of the solar cell).
[0107] FIG. 15 shows the application of various laser processes for
making high efficiency front contacted solar cells using 3-D front
TFSS. For this application it is advantageous to have pyramid tips
on the template side not be sharp but end in flat ledges.
[0108] The processes described here are further uniquely suited to
simplifying the all back-contact cell process flow.
[0109] FIGS. 16A-16D show the laser processes used on the planar
epitaxial substrate to make a back-contact/back-junction solar cell
where the TFSS is self supporting (i.e., no backplane attachment to
the cell). In this application the epitaxial emitter is deposited
in-situ during silicon epitaxy following the deposition of the
epitaxial silicon base. The ablation of silicon is then used to
remove the emitter from the base isolation regions. At the same
time four fiducials are etched into oxide to align subsequent
ablation to this pattern. Next, a thermal oxide is grown to
passivate the silicon surface that will become the back surface of
the back-contact back-junction solar cell. The epitaxial silicon
film is then disconnected or released from the template (by
mechanical release from the porous silicon interface). Next, the
residual porous silicon layer is wet etched and the surface is
textured (both can be done using an alkaline etch process). This
will become the textured front surface or the sunnyside of the
solar cell. Now, the thermal oxide is ablated using a picoseconds
UV laser to form base openings inside the base isolation region.
The base opening is aligned inside the base isolation region
(trench) formed by silicon ablation earlier using the fiducials
that were etched in silicon earlier as mentioned above. Next a
phosphorous containing oxide layer (PSG) is blanket deposited on
the surface. Scanning with a nanosecond green or IR laser aligned
to base opening using the fiducials in silicon causes the base to
be doped. Also, the region that will have the contact openings to
emitter is also doped in a similar manner using the aligned scans
of nanosecond green or IR laser. Next, contact opening are made to
these doped base and emitter areas using a picoseconds UV laser.
Again, the alignment of these contact openings is made using
fiducials in silicon. Now, a metal stack layer comprising aluminum
as its first layer in contact with the cell (e.g., a stack of 1250
A Al/100-250 A NiV/2250 Sn) is deposited using a suitable method
such as a PVD (physical vapor deposition) technique. Next, this
layer is patterned using a picoseconds IR laser so that the metal
runners are separately connected to the base and emitter regions.
After an optional forming gas anneal (FGA), the cell is connected
to and reinforced with a backplane with either embedded (Al or Cu)
high-conductivity interconnects or no embedded interconnects (in
the latter, the final cell metallization can be formed by a copper
plating process). The cell is now ready for test and use. FIG. 17
shows the laser processes used on the planar epitaxial substrate to
make a back-contact solar cell where epitaxial silicon base is not
deposited with an emitter layer. Instead, a boron containing oxide
(BSG) layer is deposited and patterned to open the base isolation
region. A similar process to that described above is followed
except that now the emitter and base are formed simultaneously
during a thermal oxidation step according to the process flow
outlined in FIG. 17.
[0110] FIG. 18 shows a process flow using laser processes on the
epitaxial substrate to make a planar back-contact/back-junction
solar cell where the TFSS is not self-supporting (hence, a
backplane is used). This flow uses the silicon ablation of in-situ
doped emitter to form the base isolation region.
[0111] FIG. 19A-19H show a process flow using laser processes on
the epitaxial substrate to make a planar back contact solar cell
where the TFSS is not self-supporting. In this flow, instead of an
in-situ emitter layer, the BSG deposition and selective laser
ablation followed by thermal oxidation (or a thermal anneal or a
thermal oxidizing anneal) is used to form the emitter as well as
the base isolation region.
[0112] FIG. 20 shows a process flow for making back contacted 3-D
solar cells, it is advantageous to have the template side of
pyramids end in relatively sharp points. Since the 3-D TFSS can be
self-supporting to relatively low thickness (e.g., silicon as thin
as about 25 microns), the process flow is similar to that shown in
FIG. 16. It should be clear that we again have a choice of using
the in-situ emitter followed by laser ablation of silicon, or BSG
deposition and selective laser ablation followed by thermal
oxidation (or thermal anneal, or thermal oxidizing anneal).
[0113] For applications in hetero-junction solar cells, a
hetero-junction emitter may be formed by a doped amorphous silicon
layer in contact with an oppositely doped crystalline silicon base.
For interdigitated back contact solar cells we pattern the
amorphous silicon layer and the transparent conducting oxide (TCO)
using laser ablation that is selective to the crystalline layer.
Femtoseconds pulsewidth lasers with either UV or green wavelength
are suitable for this application. A process flow is described in
FIG. 21. Several variations of this process flow are possible.
[0114] Various embodiments and methods of this disclosure include
at least one of the following aspects: the process to obtain
silicon ablation of crystalline and amorphous silicon with reduced
damage; the process to obtain oxide ablation for both doped and
undoped oxides with no or reduced damage to silicon; the process to
obtain fully isolated metal patterns on a dielectric surface for
solar cell metallization; the process to selectively dope the
emitter and base contact regions; the use of pulsed laser
processing on very thin wafers, including planar and 3-D silicon
substrate; the use of pulsed laser processing on substrates
obtained using epitaxial deposition on a reusable template made
using template pre-structuring techniques; the use of various
pulsed laser processes in making front contacted homo-junction
solar cells; the use of various pulsed laser processes in making
all-back contacted homo-junction solar cells; and the use of
various pulsed laser processes in making hetero-junction solar
cells.
[0115] Although the front contact solar cells are described with
p-type base and back-contact back-junction solar cells are
described with n-type base, the laser processes described here are
equally suited to the substrate with opposite doping, i.e., n-type
for front contact solar cell with P.sup.+ emitter, and p-type base
for back-contact back-junction solar cells with p-type base and
n.sup.+ emitter.
[0116] The following description, tables, and figures disclose the
application of flat top laser beams to laser processing methods for
interdigitated back-contact cells (IBC). The description following
is directed towards methods for the formation of back contact solar
cells utilizing flat top laser beams as compared to traditional
Gaussian laser beams. Further, the implementation of flat top laser
beams to the laser processing methods described throughout this
application provides substantial reduction in damage to silicon,
improvement in solar cell fabrication throughput, and a bigger
alignment window for defining patterns (e.g. patterns of emitter
and base regions) that are inset inside another pattern.
[0117] FIGS. 22A and 22B are schematics showing the profile of a
Gaussian beam, FIG. 22A, and a flat top beam, FIG. 22B. The beam
intensity of the Gaussian beam has a smooth decrease from a maximum
at the beam center to the outside of the beam. In contrast, the
intensity is "flat" or uniform for the flat top beam through most
of its profile (center to outside).
[0118] As disclosed herein, high-efficiency back-contacted,
back-junction cells with interdigitated back contact (IBC)
metallization benefits from the use of at least one or several
steps of pulsed laser processing. Laser processing may be utilized
in several processing throughout the formation of the back contact
cell, including: defining emitter and base regions (or
base-to-emitter isolation), defining back-surface field (BSF)
regions, doping to form back surface fields, opening contacts in
the dielectric to base and emitter, and metal patterning. Some of
these steps require laser processing of wide areas that are
typically produced by overlapping Gaussian beam laser spots.
Overlapping severely reduces cell processing speed and may cause
silicon damage, resulting in degradation of cell performance and
yield. By replacing smaller diameter Gaussian spots with a
relatively wide flat top laser beam, substantial improvement in
throughput is obtained. And because the overlapping of spots is
dramatically reduced, the semiconductor (e.g., crystalline silicon)
substrate damage is reduced significantly. FIGS. 23-25 illustrate
embodiments of back contact solar cells that may be formed
according to the disclosed flat top laser beam processing
methods.
[0119] FIG. 23 is a cross-sectional diagram of a
back-contact/back-junction cell with interdigitated back-contact
(IBC) metallization formed from an n-type substrate, such as that
disclosed herein. As shown in FIG. 23, alternating emitter and base
regions are separated by relatively lightly n-doped substrate
regions (the n-type base). The rear/backside surface is covered by
a surface passivation layer that provides good surface passivation
with low back surface recombination velocity, made of, for example:
thermal silicon dioxide, deposited silicon dioxide, or silicon
oxide/silicon nitride layers which may be deposited using
techniques such as PECVD or APCVD (and/or aluminum oxide deposited
by atomic layer deposition or ALD). This surface passivation
process may then be followed by making openings in this passivation
layer which act as `localized contacts` to the emitter and base
regions. Then conductor deposition and patterning (e.g., aluminum
as shown in FIG. 23) may be performed to separately connect the
emitter and base regions.
[0120] FIG. 24A is a rear/backside view of a back contact solar
cell illustrating an interdigitated back contact base and emitter
design with the emitter and base regions laid out in alternating
parallel rows. This backside may be formed, for example, by
starting with a surface that is completely covered by an emitter
region, then delineating a base region resulting in the formation
of the patterned emitter regions. Then doping base contact regions
with phosphorous is carried out and contacts are opened to the base
and emitter regions in preparation for metallization.
[0121] FIGS. 24B-24F are rear/backside views of a back contact
solar cell illustrating the back contact cell after key processing
steps, wherein any one step or combination of steps may be
performed according to a laser process which may or may not utilize
a flat top beam. The various laser patterning steps of this
particular exemplary method are outlined in FIGS. 24B-24E. Starting
with an n-type silicon substrate, a BSG layer is deposited over the
whole surface. Next, the emitter to BSF isolation region is defined
using laser ablation of the BSG as shown in FIG. 24B. This step,
the delineation of base and emitter regions, is referred to herein
as the "BSG Opening" step. Alternatively, an in-situ boron doped
layer may be deposited during silicon epitaxy and the BSF region
defined using laser ablation of silicon.
[0122] After the emitter to BSF isolation region is defined in the
BSG Open step, a USG layer is deposited on the wafer followed by
laser ablation of this layer in patterns that are inlaid to the BSG
Open region, as shown in FIG. 24C. This patterning step is referred
to herein as the BSF Opening step or base opening step. The BSF
openings should be isolated from the edges of the BSG Openings to
prevent shunt formation as shunts are deleterious to the solar cell
efficiency.
[0123] Next, a PSG layer is deposited on the wafer and the silicon
exposed to PSG in the BSF opening is doped using selective laser
scans of this area. The doped BSF regions (base regions) are
outlined in FIG. 24D
[0124] Next, the contacts to base and emitter are made using laser
ablation as shown in FIG. 24E. It should be noted that the contacts
may be point contacts as shown in FIG. 24E or line contacts as
shown in FIG. 24F. Also, the number of contacts or the number of
lines should be optimized for minimum series resistance of the
current conduction path for the solar cell--thus the designs and
methods of the disclosed subject matter are not limited to the
exemplary embodiments shown herein. It is also important that the
contact openings are properly aligned inside the particular doped
area so that there is no current leakage.
[0125] As disclosed previously, a picoseconds pulse length laser
may be used for oxide ablation processes of BSG open, BSF opening,
and contact opening, although a nanoseconds pulse length laser may
also be used. Further, although IR wavelength may be used, green or
UV or smaller wavelengths are more suitable because of their
reduced penetration into silicon.
[0126] For BSF doping particularly, a nanoseconds pulse length
laser may be more suitable because of its penetration into silicon.
And although IR wavelength may be used, green wavelength, because
of its reduced penetration compared to IR, may be more suitable for
the depth of doping typically desired.
[0127] FIG. 25 is a rear/backside view of the back contact solar
cell of FIG. 24A with alternating metal lines contacting the
emitter and base regions. Note that the metal lines for the emitter
and base regions are separately connected to busbars not shown in
FIG. 25 for simplicity of the figure. This metal pattern may be
formed by blanket deposition of a metal followed by laser ablation
of the metal to isolate base contacts from emitter contacts.
Because relatively thick metal lines are required for good current
conduction (usually lines 20 .mu.m thick or thicker), a thinner
metal stack such as aluminum/nickel-vanadium/Tin may be first
deposited and patterned by lasers, followed by the selective
deposition of a thicker metal such as copper using electro or
electroless plating. Alternatively, a backplane with relatively
thick conductors may be applied and attached to the cell with thin
conductor lines. A picoseconds pulse length laser with IR
wavelength may be most suitable for ablating the metal stack with
good selectivity to the underlying oxide layer.
[0128] The disclosed flat top laser beam processing steps that may
be utilized to make this structure possible include, but are not
limited to: delineation of emitter and base regions (BSF and
emitter to BSF isolation) by laser ablation of an emitter or
deposited boron doping dielectric (such as boro-silicate glass BSG
deposited by APCVD); delineation of the BSF region by opening the
dielectric covering the opening made in the BSG; N+ doping of the
base (e.g., with phosphorus); opening of metallization contacts to
base and emitter regions; and metal patterning using metal laser
ablation to isolate base and emitter contacts. FIGS. 26A-26C are
diagrams illustrating three ways a flat-top beam profile may be
created (diagrams reproduced from F. M. Dickey and S. C. Holswade,
"Laser Beam Shaping: Theory and Techniques", Mercel Dekker Inc.,
NY, which is hereby incorporated by reference in its entirety).
FIG. 26A illustrates one technique for creating a flat top beam
profile, the so-called "aperturing of the beam." Using this method,
the Gaussian beam is made flatter by expanding it and an aperture
is used to select a reasonable flat portion of the beam and to
cut-out the gradually decreasing sidewall' areas of the beam. Using
this method, however, may cause a significant loss of beam
power.
[0129] A second example method for creating a flat top beam, as
shown FIG. 26B, uses beam integration wherein multiple-aperture
optical elements, such as a micro-lens array, break the beam into
many smaller beams and recombine them at a fixed plane. This beam
integration method may work very well with beams of high M.sup.2
value.
[0130] A third beam shaping system for creating a flat top beam, as
shown FIG. 26C, uses a diffractive grating or a refractive lens to
redistribute the energy and map it to the output plane. Any known
method, including the three example techniques disclosed in FIGS.
26A-26C, may be used obtain the flat top beam profile for
applications described herein. The suitability and choice of a flat
top laser beam formation method depends on a variety of factors
including the available beam characteristics and the results
desired.
[0131] FIGS. 27A and 27B are schematics showing the profile of a
Gaussian beam and a flat top beam highlighting the ablation
threshold. As shown in FIGS. 27A and 27B, a flat top laser beam,
particularly as compared to a Gaussian beam, can substantially
reduce the laser damage during ablation and doping processing. For
Gaussian beams there is substantial excessive laser intensity above
that required for ablation, particularly in the center of the beam,
that can cause damage of silicon (as shown in FIG. 27A). The flat
top beam can be configured so the peak intensity is only slightly
above that required to ablate the material (the ablation threshold
as shown in FIG. 27B) and the damage that may be caused by the high
intensity of the Gaussian beam is avoided.
[0132] A flat top beam, whether having a square or rectangular
cross section, offers throughput advantages particularly as
compared to a Gaussian beam. FIG. 28A is diagram showing a Gaussian
beam ablated region profile/footprint. The circular shaped spots of
a Gaussian beam are required to overlap substantially to minimize
the zigzag outline of the pattern, typically as much as 50% overlap
(FIG. 28A). FIG. 28B is diagram showing a flat top beam ablate
region profile/footprint. Since the square or rectangular flat top
beam have flat edges, thus creating a flat outline, the overlap can
be significantly reduced (FIG. 28B). FIG. 28C is a graph showing
the improvement in scan speed as beam overlap is reduced. Note that
even for an overlap of 30%, a scan speed increase of 33% may be
realized.
[0133] FIG. 29A is a diagram illustrating a beam alignment window
of a Gaussian beam and FIG. 29B is a diagram illustrating a beam
alignment window of a flat top beam. As can be seen in FIGS. 29A
and 29B, yet another advantage of using a flat top beam for making
inlaid patterns is the larger alignment window the flap top beam
provides. The circular shaped spots obtained from a Gaussian beam
create zigzag edges of the ablated regions (FIG. 29A). The
alignment margin of M as shown in FIG. 29A is reduced and limited
to M-a-b due to the waviness of the zigzag edge profile.
[0134] However, the ablation region edges created using a flat top
beam are straight allowing the alignment margin to stay at M. For
the back contact back junction solar cells described herein, BSF
openings are formed inside the BSG Open regions, and contact
openings are formed inside the BSF region. Hence, a larger
alignment margin is important as it allows for smaller BGS Open,
BSF, and contact regions. Thus reducing the electrical shading and
improving solar cell performance.
[0135] Since the overlap of square or rectangular flat top beam can
be reduced in both x and y direction while making a large area
ablation or doping, the throughput is significantly enhanced. Also,
since the size of the square or rectangular flat top can be
increased without causing excessive zigzagging of the perimeter,
throughput is further increased. Table 1 shows the reduction in the
number of scans needed to open a 150 um wide line, such as used for
delineating the base area by ablating the BSG film.
[0136] Table 1 below shows the throughput of Gaussian vs. Flat Top
laser beams for creating a 90 .mu.m wide base opening. The results
of Table 1 are shown graphically in FIG. 29E.
TABLE-US-00001 TABLE 1 Width Spot Pitch of Number of line Size
scans of scans PROCESS (um) (um) Overlap % (um) per line BSG
Ablation with 150 30 50 15 9 Gaussian BSG Ablation with Flat 150 30
20 24 6 Top BSG Ablation with Flat 150 60 20 48 3 Top
[0137] FIG. 29E shows the throughput advantage of flat top beams
(the 60 .mu.m flat top beam region profile is depicted in FIG. 29D)
as compared to the Gaussian beam (the 30 .mu.m flat top beam region
profile is depicted in FIG. 29C), for a high productivity laser
system that can process four wafers at a time. To further reduce
cost, for example, two lasers may be utilized with each laser beam
further split into two. However, many variations of this flat top
laser beam hardware and fabrication scheme are possible.
[0138] Also, because overlap is significantly reduced in both x and
y directions when using a flat top beam, the laser induced damage
of silicon is greatly reduced as compared to the Gaussian beam.
[0139] Similar throughput advantages may also result when utilizing
a flat top beam for opening the oxide region for BSF, doping the
BSF region using the overlying PSG, forming base and metal contact
openings if they are line contacts, and the metal ablation
isolation lines--all with the concurrent advantage of reduced
silicon damage. Additionally, utilizing a flat top beam provides
the advantage of increased alignment window for BSF opening inside
the BSG opening and contact opening inside the BSF. Flat top laser
processing methods may also increase throughput for forming a back
surface field. For example, the back surface field may be formed by
doping the base region, opened as described, with an n-type dopant
such as phosphorous. For this process the base is covered with a
phosphorus-doped silicon oxide (PSG) layer and the doping may be
performed by irradiating this region with a laser beam. While
uniformly doping this region using Gaussian laser beams requires
overlapping, overlapping is minimized or may be completely reduced
using a flat top beam. And as with the base and emitter region
delineation and back surface field delineation described herein,
utilizing a flat top laser beam provides a substantial throughput
and reduced damage advantage as required overlapping is decreased.
It should be noted that for forming a back surface field, the beam
need to be flat top beam only in one direction--normal to the scan,
whereas it may be Gaussian in the direction of the scan. This type
of beam is called a hybrid flat top beam.
[0140] Importantly, for forming isolated base or emitter contacts,
although overlap is not an issue, the silicon damage is still
reduced using a flat top beam because of the absence, unlike
Gaussian, of a high intensity peak in the center of the beam (as
shown in FIGS. 27A and 27B).
[0141] Another aspect of this disclosure relates to the use of
laser annealing to improve the conversion efficiency performance of
crystalline semiconductor solar cells in general, and crystalline
silicon solar cells in particular, by improving the passivation
properties of dielectric-coated surfaces, and more specifically
silicon nitride (SiN)-coated surfaces. The improved front surface
passivation properties are manifested as reduced Front-Surface
Recombination Velocity (or reduced FSRV) and increased effective
minority carrier lifetime. This technique is especially
advantageous for high-efficiency back-junction, back-contacted
cells with interdigitated metallization (IBC) where annealing of
SiN-coated front surface may also be used to concurrently result in
the annealing of emitter and base metal contacts on the solar cell
back surface, thereby, lowering the specific contact resistivity
and improving the solar cell fill factor (FF). The laser annealing
methods of this disclosure are applicable to crystalline
semiconductor solar cells using semiconductor absorber layers over
a wide range of thicknesses, i.e., thick wafer-based solar cells
such as crystalline silicon wafer solar cells with wafer
thicknesses of 10's to 100's of microns. Moreover and more
specifically, the non-contact laser annealing process and methods
of this disclosure are applicable to extremely thin (e.g.,
crystalline semiconductor layers from a few microns to .about.50
microns thick) crystalline silicon solar cells where unsupported
cell mechanical handling can result in cell breakage. It is also an
in-line replacement for the batch furnace annealing processes. The
laser annealing process and methods can be used as the last step in
the cell manufacturing process flow or immediately after deposition
of the front-surface passivation and anti-reflection coating (ARC)
layer. The processes and methods of this disclosure enable
formation of high-quality surface passivation and ARC layers using
low-temperature, low-thermal budget deposition processes for
passivation & ARC layers such as silicon nitride deposited by
low-temperature PECVD.
[0142] The passivation of the surface of phosphorous-rich N.sup.+
emitter with silicon nitride for standard front contact solar cells
with p-type silicon bulk (or p-type base), is well known and widely
utilized in the solar industry. While the SiN film acts as an
antireflection coating to reduce the optical reflection losses and
to increase sunlight trapping, it also serves a very important task
of passivating the surface of the phosphorous-rich N.sup.+ emitter
by the well-known hydrogenation process. The hydrogen released from
the hydrogen-containing SiN layer satisfies the open bond on the
silicon surface (or silicon dangling bonds causing surface states
and traps), thereby reducing the surface recombination velocity or
rate of minority carriers by these dangling bond sites. For cells
made from multi-crystalline or polycrystalline silicon, this
hydrogen provided by the SiN layer further reacts with the
impurities and defects in the bulk of the silicon wafer as well as
removes the grain boundary trap sites, thereby reducing the overall
minority carrier recombination and increasing the effective
minority carrier lifetime in the bulk of the material.
[0143] The release of hydrogen and hence the surface and bulk
passivation of silicon is typically obtained during the so-called
"metal firing" process in the standard front-junction/front-contact
solar cell manufacturing process flow, currently widely used in the
solar cell manufacturing industry. The screen-printed metal firing
process consists of multiple-step heating of the solar cell using a
carefully designed temperature and time sequence with a final dwell
at about 850-900.degree. C. before a desired cooling sequence. This
firing cycle is optimized after careful experimentation. Since
hydrogen is a small atom it can diffuse out of the wafer if the
wafer temperature is too high or the annealing times are too long.
On the other hand, the hydrogen passivation may be unsatisfactory
if the temperature is too low or annealing times are too short.
Hence, the hydrogen-passivation phenomenon has been a subject of
intense investigation and research in the solar cell industry and
is considered not just science but also an art by many (since there
are still many areas yet to be fully understood). It is clear that
a process that can provide a high degree of control is thus
desired.
[0144] For the standard mainstream front-contact solar cell with
p-type silicon bulk (or p-type boron-doped base) and n.sup.+
phosphorus-doped emitter, the front contact surface is contacted by
silver while the back surface is contacted by aluminum--which may
be screen printed as a blanket layer or make selective contacts
through openings made in the backside dielectric surface. To obtain
low resistance contacts, the intermixing of silver with silicon in
the front and aluminum in the back is promoted during the metal
firing process that has been described above. Based on the
description of the metal firing process above, the practice of
obtaining low resistance contacts and hence high FF in the solar
cell is complicated. Again, a process that can provide a high
degree of control is desired.
[0145] Additionally, the all back-contact, back-junction solar
cells that use the same metal, aluminum, in contact with both
n.sup.+ and p.sup.+ contacts on the back side cannot be heated too
high as the doping of n.sup.+ contact by aluminum, a p-type dopant,
will increase the contact resistance, thereby lowering the fill
factor of the cell. Moreover, overheating of aluminum much above
450.degree. C. can result in degradation of optical reflectance of
aluminum (and thus increased optical losses of the infrared photons
in the cell). A controlled low-temperature heating, preferably in
the range of 200-450.degree. C., of the contacts where aluminum
makes intimate contact with silicon by reducing and absorbing the
oxide at the silicon surface, is highly desirable.
[0146] We disclose here a process where the front surface or
sunnyside of the solar cell is substantially uniformly or in
selected areas irradiated with the laser beam, selectively heating
the semiconductor (e.g., silicon) such that hydrogen atoms are
released from SiN thereby effectively passivating the silicon
surface, reducing the surface state density, reducing the
front-surface recombination velocity (FSRV), and increasing the
effective minority carrier lifetime of the solar cell. The
processes and methods of this disclosure may also reduce the bulk
trap density and enhance the bulk minority carrier lifetime. One
embodiment of the disclosed method is based on using a pulsed laser
source with a wavelength smaller than that of the semiconductor
(e.g., silicon) bandgap. In this embodiment (for instance, using a
pulsed green or UV laser source for crystalline silicon surface
annealing), the front-surface is selectively heated using pulsed
laser source irradiation, while the backside of the cell remains
substantially cooler than the frontside of the cell. Another
embodiment of the disclosed method is based on using a pulsed laser
source with a wavelength near to or larger than that of the
semiconductor bandgap. In this embodiment (for instance, using a
pulsed IR laser source for crystalline silicon surface annealing)
while the front-surface is heated using pulsed laser source
irradiation, the backside of the cell is also heated and annealed.
Using this alternative embodiment, at the same time the laser beam
penetrates to the back of the solar cell heating the Al/silicon
contacts to decrease the contact resistance and to improve the
overall cell fill factor and efficiency. The laser annealing
process and methods of this disclosure may be performed at the end
of the solar cell fabrication process flow or immediately after
formation of the passivation/ARC layer and before the cells are
tested and sorted for module packaging. Alternatively, the laser
annealing process and methods of this disclosure may be performed
after assembling and packaging the cells in a PV module and through
the front glass cover of the module assembly. In this case
wavelengths need to be used that can go through the glass, such as
infrared.
[0147] It is important that the laser anneal process should be
optimized (including the laser source wavelength, pulse width,
power, etc.) such that the passivation layer (e.g., the PECVD SiN
layer) is not degraded during this process so that the sunlight can
pass through this antireflection coating without significant
optical absorption losses. Also, the surface texture should not be
affected so that the light trapping is not reduced. It is clear
that the type of the pulsed laser source and the laser process
parameters should be carefully chosen to meet all these
requirements.
[0148] The laser pulse length should be long enough so that there
is no non-linear optical interaction with the passivation/ARC layer
(e.g., SiN layer) so that the passivation/ARC layer) is unaffected.
Although, lasers with pulse length from 1 nanosecond to
microseconds or continuous wave can be used for this application,
the choice depends on the depth to which the heat penetration is
desired. Using shorter pulse length the heat is limited to shallow
depths. Wavelength also should be chosen based upon the depth of
semiconductor (e.g., crystalline silicon) that is required to be
heated. For applications to single crystal solar cells where only
front surface passivation is required to be improved, green
wavelength may be more suitable. For applications where improved
bulk silicon passivation is required and/or back contact annealing
is desired, IR wavelength may be more suited. It should be clear
that based on the desired application a range of laser pulse length
and wavelengths can be used.
[0149] Processes for back contacted cells with interdigitated
metallization, called NBLAC cells, have been described in related
applications (see, e.g., U.S. patent application Ser. No.
13/057,104 filed Aug. 13, 2012).
[0150] FIG. 30 outlines one of the embodiments of the NBLAC process
flow, while FIG. 31 is the schematic of the cross section of the
cell (the backplane is not shown for clarity). The low-temperature
front-surface passivation/ARC: PECVD (silicon nitride)+laser anneal
process step in FIG. 30 involves the deposition of SiN at lower
temperatures than is used in the industry (<350C). The surface
is then subjected to pulsed laser irradiation causing preferential
silicon frontside annealing that results in improved passivation of
the silicon surface with hydrogen from the SiN. In particular, the
laser annealing processes and methods of this invention enable
formation of high-quality passivation and ARC layers (like single
layer SiN and bilayer SiN with amorphous silicon) deposited at low
temperature as low as 90.degree. C., and more typically in the
deposition temperature range of 90.degree. C. to 250.degree. C.
[0151] In some embodiments, the SiN being annealed may contain a
desired amount of phosphorus dopant. In this case, the annealing
step also causes silicon doping with phosphorus. This process is
discussed in connection with FIG. 36 below.
[0152] Besides SiN, silicon oxynitride (Si.sub.xO.sub.yN.sub.z), or
silicon carbide (Si.sub.xC.sub.y) single layers or a bilayer stack
with SiN on amorphous silicon (.alpha.-Si), a bilayer stack with
SiN on silicon oxide (SiO.sub.2), or a bilayer stack with SiN on
silicon oxynitride, can also be used for silicon surface
passivation. For example, it is known that an amorphous silicon
layer can passivate the silicon surface quite well. However, for
the current industrial process, significant surface cleaning of
silicon and process optimization of the .alpha.-Si deposition
process is required. Laser annealing of .alpha.-Si films covered
with hydrogenated SiN can activate the hydrogen in SiN and lead to
dramatic enhancement of passivation, as measured by substantially
increased effective minority carrier lifetime and substantially
reduced front-surface recombination velocity.
[0153] The PVD Al/NiV/Sn contact & backside reinforcement BSR
step and the pulsed picosecond laser ablation of Al for
interdigitated cell base & emitter Al lines step in FIG. 30
form the metal contacts to the base and emitter on the back surface
of the solar cell. These contacts are shown in the cross section in
FIG. 31. It should clear that the laser beam that penetrates to the
back of the silicon film will concurrently anneal the back
contacts, resulting in reduced contact resistance and increased
fill factor of the solar cell.
[0154] Results obtained using laser annealing are shown in FIG. 32.
It is seen that up to 100 times effective lifetime improvement is
obtained on low-temperature-deposited passivation layer of SiN
without resorting to high temperature metal firing. In the NBLAC
process the thin epitaxial silicon is supported on a backplane. In
case this backplane cannot withstand a high temperature, the SiN
deposition temperature is reduced to facilitate thin
epitaxial/backplane assembly processing and process integration
accommodating the heat sensitive backplane assembly. For such heat
sensitive backplanes the laser annealing is highly suitable since
with a suitable selection of laser pulse length and wavelength, the
heat can be limited to the front side of the silicon while keeping
the backside of the silicon within the acceptable value for the
backplane.
[0155] The non-contact laser annealing process is highly suitable
for NBLAC cells that use epitaxial films having thickness
approximately in the range of a few to 50 microns, which are
fragile to handle.
[0156] For enhanced throughput and improved process control, the
laser source used for these applications may have top-hat profile
(with relatively uniform beam power over at least 100 micron or
more) in order to reduce the overall surface irradiation scan time.
This also eliminates the chance of damage in beam overlapping
areas.
[0157] This laser annealing process is an attractive alternative to
furnace annealing as it can be an in-line cost effective
process.
[0158] According to another aspect of the present disclosure, the
selective laser ablation and patterning of electrically insulating
layers, such as thermally grown or chemical-vapor-deposited silicon
oxide on silicon is used in crystalline silicon solar cell process
flows for obtaining relatively high cell efficiency values. In such
applications it is advantageous that no or at most negligible
damage is introduced in the underlying silicon substrate, since any
substantial ablation-induced damage can lead to increased minority
carrier recombination loss, resulting in further loss of cell
conversion efficiency. We present here a novel scheme that ensures
that the solar cell semiconductor (e.g., silicon) surface will not
be damaged during the pattern-selective ablation of the dielectric
(e.g., silicon oxide) overlayers. This disclosure involves
introducing a thin intermediate layer of silicon that stops the
laser beam from reaching the silicon substrate. This thin
intermediate silicon layer may be placed closer to the underlying
silicon surface, separated only with a thin buffer layer of silicon
oxide. The layer of oxide above this intermediate silicon layer is
ablated by the laser beam interacting and separating the silicon
oxide-intermediate silicon layer interface. A very thin (for
example, 3 nm to 100 nm or in some embodiments 3 to 30 nm) layer of
silicon oxide under this intermediate silicon layer prevents any
significant damage-causing effect of laser action at this interface
from reaching the silicon substrate. The intermediate silicon layer
is subsequently oxidized (using either a thermal oxidation process
or an oxidizing anneal process), thereby eliminating any unwanted
interaction in subsequent laser processing. This scheme is
particularly suited for application in an all-back-contact
back-junction solar cell design where laser ablation of dielectric
layers such as silicon oxide is utilized several times, such as the
NBLAC solar cell.
[0159] In one embodiment of a process flow, the oxide ablation
process is used three times to form oxide patterns, namely BSG (or
BSG/USG stack) ablation to delineate emitter and base regions, USG
(or PSG/USG stack) ablation to define the base regions, and finally
ablation of PSG (phosphosilicate glass--oxide) to open contacts to
base and the ablation of BSG/USG/PSG ablation to open contacts to
the emitter regions. The technique described herein can be
advantageously used in the first step of ablation of the BSG layer
to define the patterned emitter and base regions (for solar cells
using n-type base). If desired, this technique can be further used
during the ablation of USG for defining the openings for N.sup.+
base regions. (These polarities would be reversed for solar cells
using p-type base.)
[0160] FIG. 33A shows a process flow for an all back contact solar
cell that involves oxide ablation at three different steps. FIG.
33B shows the slight modification to the BSG/USG (USG is undoped
silicate glass or undoped silicon oxide) deposition step where a
very thin .alpha.-Si layer is deposited on top of a thin USG layer
(in some embodiments in situ within the same APCVD BSG deposition
equipment) before the deposition of the remaining BSG/USG stack.
During the laser ablation process, the laser beam separates the
BSG/.alpha.-Si interface, thereby removing the BSG/USG stack. This
thin layer of silicon is oxidized during the subsequent steps as
described in FIG. 33B.
[0161] FIGS. 33C and 33D show a further modification to the process
flow of FIGS. 33A and 33B, where the USG deposition step is
modified to include the deposition of the very thin .alpha.-Si
layer on top of a very thin USG layer before the deposition of the
thicker USG layer. During the laser ablation the laser beam
separates the top USG/.alpha.-Si layer, thereby removing the top
USG layer. As before, this thin layer of silicon is oxidized along
with the previously deposited .alpha.-Si as described above during
the subsequent step as shown in FIG. 33D.
[0162] FIG. 34 shows schematically a standard oxide ablation
process using a laser beam with pulse width in the range of a few
picoseconds. It can be seen that the interface being acted upon by
the laser is the surface of the silicon substrate that may be
damaged if the correct pulse energy is not used. FIG. 35 shows the
scheme where a very thin amorphous silicon layer is deposited after
the deposition of a very thin USG layer. As shown in FIG. 35B, the
interface for laser action is the BSG/amorphous silicon interface.
This interface acts as an ablation stopping layer and shields the
crystalline silicon surface from laser irradiation thereby
preventing or suppressing any possible crystalline silicon surface
damage, resulting in higher cell efficiency.
[0163] The complete stack USG/.alpha.-Si/BSG/USG may be deposited
in situ using APCVD for solar cell fabrication. The APCVD equipment
may be high-productivity in-line APCVD equipment with multiple
sequential in-line deposition zones to enable deposition of the
entire stack in a single piece of APCVD equipment. Using APCVD
equipment, the thin undoped silicon layer may be deposited in one
of the APCVD deposition zones (the second zone after deposition of
the initial USG layer) using e.g. silane and argon (or silane and
nitrogen) at a temperature of less than approximately 500.degree.
C. Alternatively, it can be deposited using a PECVD technique. A
wide range of thicknesses of thin USG and thin .alpha.-Si can be
used based upon the particular process flow. Typically, the USG in
contact with the crystalline silicon surface may be in the range of
3 nm to 100 nm, while the amorphous silicon layer may be in the
range of 3 nm to 30 nm. However, as mentioned above, thicknesses
outside of these ranges will also work if the rest of the process
flow is changed to accommodate the thickness of these films.
[0164] The same scheme can also be used, if so desired, to open
oxide layer for base regions that will be subjected to phosphorous
doping to form N.sup.+ layer. In that case the process flow is
modified to ensure oxidation of this .alpha.-Si layer.
[0165] Various aspects of the laser processing innovations and
corresponding semiconductor, passivation, doping, metallization
materials disclosed herein may be used singularly or in combination
to improve solar cell efficiency.
[0166] The following laser patterning methods utilize auxiliary
heat on the wafer (e.g., laser) in combination with wet etching to
form patterned solar cell doped regions which may further improve
cell efficiency by completely avoiding laser ablation and
associated laser damage of the semiconductor substrate associated
with ablation of an overlying passivation/oxide layer. Further, the
passivation materials and associated laser oxide ablation methods
as well as laser doping parameters disclosed above may be used in
conjunction with the laser mask method described for solar cell
efficiency improvement.
[0167] High efficiency back-junction, back contact solar cells with
interdigitated metallization over alternating or distributed base
and emitter region highly benefit from very fine patterning of
passivation layers in order to obtain higher solar cell conversion
efficiencies. Pulsed laser ablation patterning of these layers,
particularly using pulsed pico-seconds and/or femto-seconds laser
processing, may be advantageous to obtain the smallest size pattern
dimensions with minimal damage. However, dielectric passivation
layers often used (e.g., patterned silicon oxide, aluminum oxide,
etc.) in conjunction with solar cells are transparent to laser
wavelengths down to ultraviolet (UV). Thus, it is not possible to
selectively ablate these dielectric passivation layers without
damaging the underlying silicon substrate or sensitive solar cell
semiconductor absorber layer to some degree. The solution disclosed
herein provides damage-free laser patterning utilizing auxiliary
heating to anneal laser patterned regions. The auxiliary heating
source may be a laser or a radiant heater. In the case of laser
source heating, such laser heating may be performed either in a
separate laser processing tool or within the laser ablation
processing tool incorporating both the ablation and annealing laser
sources.
[0168] Aspects of the process flow disclosed in FIG. 30 describe
the laser patterning of passivation layers to form high efficiency
three dimensional solar cells using epitaxially deposited thin
crystalline silicon films. These same laser patterning schemes are
applicable to standard back-junction, back-contact solar cells
using planar crystalline silicon wafers. FIG. 36 outlines the
process flow to form back-junction, back-contact solar cell using
laser ablation of oxide using a picoseconds laser with UV (355 nm)
wavelength to form selective emitter (i.e., a lightly doped emitter
junction with heavily doped emitter contact regions), patterned
base, and metallization contact openings on a wafer. It should be
noted that the use of a backplane to support a thin semiconductor
film (such as a thin layer of crystalline silicon) may make this
process flow suitable for thin silicon films having a thickness as
small as a few microns (e.g., in the range of approximately 100 to
5 microns).
[0169] Passivation layers such as silicon oxide, aluminum oxide,
silicon oxynitrides, and silicon nitride are typically transparent
to wavelengths as short as 355 nm (UV). To some degree laser beams
with wavelengths down to UV pass through these passivation layers
and attack and damage the silicon substrate. This damage may be
mitigated and reduced, for example using the methods and structures
outlined above, to have minimal effect on solar cell efficiency.
For example, using shorter wavelengths reduces the penetration of
the laser beam into silicon. And shorter pulse laser beams limit
the heat penetration into silicon. Thus, it is advantageous to go
to shorter wavelengths (e.g., green or UV) and shorter pulse
lengths (e.g., picoseconds or femtoseconds) for the ablation of
these transparent dielectric layers--a substantially reduced heat
affected zone (HAZ) may be obtained when going from, for example,
1064 nm (IR) to 355 nm (UV), and going from nanoseconds laser pulse
to picoseconds and femtoseconds pulse laser beams.
[0170] These solutions may lead to reduced semiconductor layer
(e.g., silicon) damage when ablating an overlying oxide layer
(i.e., reducing damage to a lower and/or negligible impact on cell
efficiency). FIGS. 38A through 38E are scanning electron microscope
(SEMS) images highlighting damage to an underlying silicon
substrate during oxide ablation--specifically laser damage found
when ablating using a Gaussian laser beam having an approximately
10 picoseconds pulse width and 355 nm wavelength. FIG. 38A, is a
SEMS image of an ablation spot in Si formed using a laser at a high
laser fluence. It may be noted there is extensive damage in the
center of the spot due to the high power at the Gaussian peak.
Additionally, there are ripples extending towards the ablation
edge. It is clear that this crystalline lattice damage may be
reduced by lowering the laser fluence to the minimum required for
ablation. FIG. 38B, is a SEMS image of an ablation spot in Si
formed using a laser at an optimized laser fluence. The ripple
damage is substantially reduced, however minimal ripples may still
be observed in the ablated spots. FIGS. 38C and 38D are two SEMS
images showing a magnified view of ripples and silicon melting near
the ablation spot in Si formed using a laser at an optimized laser
fluence (e.g., that shown in FIG. 38B). Droplets of silicon may be
observed near the oxide ablation edge in FIGS. 38C and 38D. FIG.
38E is a Transmission Electron Micrograph of the ablation edge
showing the creation of amorphous silicon by the picoseconds UV
laser ablation of an overlying oxide. Amorphous silicon formation
may be observed in the open spot as well as some distance under the
overlying oxide that is present outside the ablation area. This
amorphous silicon is typically seen in the optical microscope as a
gray halo around the ablation spot. As silicon oxide is transparent
to the picoseconds UV beam, oxide ablation occurs due to the
melting and subsequent evaporation of molten silicon. Any portion
of the melted and/or evaporated silicon not able to escape (e.g.,
at the ablation edge or the ripples seen in FIG. 38) solidifies as
amorphous silicon because of the extremely rapid heating and
cooling rates associated with ultrafast picoseconds laser beam
irradiation. Although, this amorphous (and/or nanocrystalline)
silicon may crystallize during subsequent process steps, for
example during furnace annealing such as that shown in FIG. 36, the
passivation of the silicon/oxide interface may be degraded and may
cause minority carrier recombination at these sites leading to
lowered solar cell efficiency. And while this damage may be reduced
and minimized to a lower and/or negligible impact utilizing the
methods herein, in some instances it may be desired to further
repair and/or avoid this damage.
[0171] FIGS. 37A and 37B are cross-sectional diagrams of a solar
cell having an interdigitated orthogonal back contact metallization
pattern with two levels metallization (e.g., on-cell metallization
metal 1 patterned orthogonally to metal 2, metal 1 and metal 2
separated by an insulating backplane), and having a non-overlapped
laser spots for selective emitter and base with the contact made in
each spot, referred to herein as a spot-in-in spot laser pattern,
such as may be formed using the process flow of FIG. 36. FIG. 37A
is a cell cross-section showing metal 1 and metal 2 emitter contact
and FIG. 37B is a cell cross-section showing metal 1 and metal 2
base contact. The backside passivation layer may comprise, for
example, an APCVD deposited BSG/USG/PSG/USG dielectric layer stack.
The cell frontside may be textured and coated with a passivation
layer (e.g., PECVD hydrogenated silicon nitride, AlOx/hydrogenated
silicon nitride, or amorphous silicon).
[0172] In order to open up the desired amount of area for selective
emitter (i.e., lightly doped emitter junctions in conjunction with
heavily doped emitter contact) and selective base (i.e., lightly
doped base region in conjunction with heavily doped base contact)
regions, the passivation layer may be ablated by using overlapped
pulsed laser ablation spots. FIG. 39A is an optical microscope
image showing four overlapped ablation rows with each ablation spot
overlapping the next along the row. FIG. 39B is an expanded view of
the image of FIG. 39A. As may be observed in FIGS. 39A and 39B, the
overlapping laser spots cause noticeable damage to the silicon
substrate--thus resulting in minority carrier lifetime drop.
Laser-induced damage created by the beam overlap may be reduced by
patterning isolated/non-overlapped ablation spots. FIG. 39C is an
optical microscope image showing isolated/non-overlapped ablation
spots formed using the same laser fluence as the ablation spots in
FIG. 39A. FIG. 39D is an expanded view of the image of FIG. 39C. As
may be observed in FIGS. 39C and 39D, isolated/non-overlapped
ablation spots may reduce laser damage as compared to overlapping
laser spots under the same laser fluence. However, damage to the
underlying semiconductor (e.g., silicon) substrate--areas of high
minority carrier recombination that reduce the resulting solar cell
efficiency--may still be observed in FIGS. 39C and 39D.
[0173] FIGS. 40A and 40B are schematic diagrams showing two laser
patterning opening and contact schemes. FIG. 40A show a laser
patterning scheme having overlapped ablation spots. In FIG. 40A the
selective emitter (SE) and selective base (SB) regions
opened/exposed by the laser ablation are doped with an emitter
dopant (e.g., p-type emitter such as boron-doped emitter for
n-base) and a base dopant (e.g., n-type base such as
phosphorus-doped base for n-base), respectively. The contacts to
these selective emitter and base regions may then be formed by a
subsequent laser ablation step, such as that as outlined in the
process flow of FIG. 36.
[0174] FIG. 40B shows a "spot-in-spot" laser patterning scheme
where the SE and SB openings are not overlapped (i.e., isolated
openings or islands) and the contact openings are aligned inside
and isolated within the SE and Base openings (e.g., having a single
base contact opening per discrete base island).
[0175] FIGS. 41A and 41B are scanning electron micrographs of a
spot-in-spot laser pattern. FIG. 40A shows the selective emitter
(SE) and selective base (SB) openings and FIG. 40B, shows emitter
and base contacts centrally located inside the SE and SB openings,
respectively. As can be observed, although the spot-in-spot
technique reduces laser-induced damage as compared to overlapped
spots, some crystalline lattice damage is still present and may
reduce solar cell efficiency due to recombination losses. In some
instances, this efficiency loss may be negligible. However, in
other cases it may be desired to further reduce and/or eliminate
this damage and improve the solar cell efficiency.
[0176] The presently claimed subject matter provides auxiliary
heating to anneal out or eliminate laser ablation damage. In one
embodiment, laser annealing is utilized. In this scheme, after the
pulsed laser ablation is complete (e.g., pulsed ablation of an
overlying oxide layer), the ablated area is annealed using another
suitable pulsed laser beam that anneals out the damage. If needed,
but not required, this laser annealing may result in the melting
and solid phase epitaxy of the amorphous silicon layer (i.e., the
damaged laser damaged silicon, as shown in FIG. 38E) yielding a
mono-crystalline silicon surface that may be adequately passivated
so there is no increase in minority carrier recombination at the
surface. Alternatively, pulsed laser annealing may not melt the
silicon surface layer and may simply promote and drive damage
removal and re-crystallization of silicon by pulsed laser heating.
For ablated areas with overlapped ablation spots, the laser
annealing spots may also be overlapped (or alternatively, the laser
annealing spot is larger to cover the overlapped ablation spots).
In spot-in-spot laser patterning scheme, each ablation spot may be
annealed using synchronized laser triggering from the annealing
laser. For annealing, a suitable laser may have pulse length in the
long nanoseconds range, for example in the range of approximately
10 to 900 nanoseconds (and in some instances pulse lengths in the
range of microseconds, as high as 100 microseconds, or even
femtoseconds may be used), and a wavelength in the range of IR
(1064 nm) to UV (355 nm). However, other lasers with shorter or
longer pulse length and alternative wavelengths may be used
depending on the extent of the ablation laser damage to be
annealed, e.g., a pulse length 10 to 500 nanoseconds, and a
wavelength of 532 nm. During laser anneal the amorphous silicon
formed during laser ablation may be melted (or heated) and
crystallized to obtain a damage free or damage reduced silicon
surface that may be suitably passivated and resulting in increased
effective minority carrier recombination lifetimes.
[0177] FIGS. 42A and 42B are schematic diagrams showing laser
annealing of damaged silicon in selective emitter and selective
base ablations, respectively. FIG. 42A is a schematic diagram
showing laser annealing of overlapped selective emitter ablations
and FIG. 42B is a schematic diagram showing laser annealing of
overlapped selective base ablations. FIGS. 42C and 42D are
schematic diagrams showing laser annealing of contacts formed by
ablation in the selective emitter and base regions of FIGS. 42A and
42B, respectively. FIG. 42C is a schematic diagram showing laser
annealing of contacts to selective emitter and FIG. 42D is a
schematic diagram showing laser annealing of contacts to selective
base.
[0178] FIGS. 43A and 43B are schematic diagrams showing spot by
spot laser annealing of damaged silicon in selective emitter (SE)
and selective base (SB) ablations in a spot-in-spot patterning
scheme. FIG. 43A is a schematic diagram showing laser annealing of
damage in selective emitter ablations and FIG. 43B is a schematic
diagram showing laser annealing of damage in selective base
ablations. FIGS. 43C and 43D are schematic diagrams showing spot by
spot laser annealing of laser damage in the contact ablation areas
formed in the selective emitter openings and selective base
openings of FIGS. 43A and 43B, respectively. FIG. 43C is a
schematic diagram showing laser annealing of damage in contacts in
selective emitter openings and FIG. 43D is a schematic diagram
showing laser annealing in contacts in selective base openings.
FIGS. 44A and 44B are optical micrographs showing laser ablation
spots before laser anneal and the same ablation spots after
annealing with a 30 nanoseconds UV laser, respectively. The
elimination of amorphous silicon (and/or nanocrystalline silicon)
in the ablation spots by laser annealing may clearly be observed in
FIG. 44B.
[0179] FIG. 45 is a minority carrier lifetime map of a silicon
substrate after oxide ablation. FIG. 45 shows the improved
effective minority carrier lifetime (MCL) obtained upon laser
annealing by comparing the wafer bottom half patterned by laser
ablation followed by laser annealing to the wafer top half
patterned by laser ablation which did not receive laser
annealing.
[0180] FIG. 46 is process flow for the formation of a back contact
back junction solar cell utilizing laser annealing of laser
ablation damage. The process flow of FIG. 46 is similar to that of
FIG. 36 except after each ablation (steps 3, 5, and 8 in FIG. 46)
laser annealing is carried out to reduce or eliminate the damaging
effect of laser. FIG. 47 is an alternative process flow for the
formation of a back contact back junction solar cell utilizing
laser annealing of laser ablation damage similar to FIG. 36 except
using a single anneal after contact ablation only (step 8 in FIG.
47) which may obtain equivalent laser damage reduction due to laser
ablation as that of the multi-anneal of FIG. 46. The embodiments
disclosed herein may utilize only one laser anneal process after at
least one or multiple pulsed laser ablation processes or multiple
laser anneal steps interspersed after multiple laser ablation
processes dependent on desired process flow requirements and
resulting cell structures.
[0181] The laser annealing embodiments disclosed herein may also be
integrated in a back contact back junction process flow to improve
the dopant profiles in junctions. In one scheme the laser anneal of
the ablated regions is carried out with at least one doped oxide
layer covering the ablated regions. FIG. 48 is a flow for the
formation of a back contact back junction solar cell utilizing
laser annealing to improve junction dopant profile. The process
flow of FIG. 48 is similar to that of FIG. 47 and also including a
laser anneal after step 7. As shown in FIG. 48 the melting on
silicon during laser annealing (shown as the laser anneal after
step 7 in FIG. 48) results in melt incorporation or absorption of
p-type (e.g., boron) and n-type dopant (e.g., phosphorous) from the
overlying BSG and PSG films into molten silicon, respectively. This
provides a high concentration of these dopants in the vicinity of
the surface in addition to the dopants that are driven in by the
furnace anneal and have a diffused error function profile from a
relatively fixed dopant source. This results in a high-low junction
for these doped junctions and may reduce carrier absorption at the
silicon surface thereby improving the cell efficiency. In this
instance, the annealing laser spot may be larger than ablation spot
and the laser fluence may be below the threshold level for melting
silicon. For example, using a Gaussian beam with a higher fluence
in the center of the ablation spot and a lower fluence below the
threshold level for melting silicon outside the ablation spot.
[0182] As described above, after laser ablation patterning the
wafers may be annealed in a separate laser processing system (or
chambers within the same laser platform) to recover the minority
carrier lifetime (MCL). However, this may require an additional
tool for laser annealing and result in increased production cost. A
cost-effective and economical solution is to perform
high-throughput annealing in the same tool as the laser ablation.
FIGS. 49A and 49B describes a laser tool configuration a highly
cost-effective multi-station platform that provides for
high-throughput parallel processing at different stations. FIG. 49A
is a diagram of a multi-station substrate laser processing tool and
FIG. 49B is a diagram of the tool of FIG. 49A holding multi-wafers.
FIGS. 49A and 49B show the configuration of a tool having four
stations although stations may be added or subtracted. The wafer is
rotated from one chuck to another where a different step of the
ablation/annealing process is carried out. As shown in FIGS. 49A
and 49B, the wafer is loaded in station 1, moved to station 2 for
fiducial detection for accuracy of laser ablation patterning and
aligned laser annealing. The laser ablation is carried out in
station 3 followed by annealing in station 4. It should be noted
that this scheme provides for parallel processing on different
chucks, the throughput being controlled by the slowest process in
this sequence. To improve the throughput the number of wafers on
the chuck can be increased with a concurrent increase in the laser
ablation and laser annealing to multi-wafer capability. FIG. 49B
shows such an embodiment having four wafers per chuck.
[0183] FIG. 50 is a schematic diagram of a high throughput laser
ablation laser annealing system. Additional wafers may be processed
by adjusting the wafer tray in the parallel processing tool of FIG.
50. As shown, the laser ablation and laser annealing systems have a
corresponding number of beams which may be calculated as the
optimal cost effective selection of the number lasers and the laser
beam subdivision from each laser.
[0184] In another embodiment, laser ablation and corresponding
laser annealing may be performed in the same station using
collinear (coaxial) laser beams. In other words, the laser ablation
and laser annealing beams are supplied to the same station thus
reducing the foot print and the cost of the laser system while
providing a high process throughput. In this embodiment, the laser
beams from the ablation laser and annealing laser are formed
collinear or coaxial using suitable optics before reaching the
scanner so the laser spots from the two lasers are continuously
aligned as the beams scan the wafer. Further, laser pulse
triggering may be synchronized for the two lasers using an external
trigger from the system electronics to trigger pulses in both
lasers. This approach is highly suitable for the spot-in-spot
patterning described earlier. FIG. 51 is a schematic diagram
outlining same station laser ablation and corresponding laser
annealing using collinear (coaxial) laser beams. A picoseconds
(ablation laser) laser based ablation of transparent passivation
layers in the presence of a nanoseconds (annealing laser) laser
beam incident on the wafer reduces the formation of amorphous
silicon and increase the ablation quality.
[0185] In yet another embodiment, laser ablation and corresponding
laser annealing may be performed in the same station using
collinear separate laser beams. FIG. 52 is a schematic diagram
showing a same station ablation and annealing using separate laser
beams. Specifically, FIG. 52 is a schematic diagram showing a
separate annealing laser beam incident on the area of the wafer
that is undergoing patterning by the ablation laser beam. Again,
the ablation pulse and the annealing pulse may be synchronized to
be simultaneously incident on the wafer. To reduce the alignment
accuracy requirement the annealing beam spot may be much wider
(oversized) as compared the ablation beam spot. As mentioned
earlier, the presence of a nanoseconds annealing beam (annealing
laser) during ablation by a picoseconds pulse beam (ablation laser)
improves the quality of ablation and reduced the MCL loss.
[0186] FIG. 53 is a process flow for the formation of a back
contact back junction solar cell showing oxide ablation carried out
in the presence of a heating beam (shown in steps 3, 5, and 8 of
FIG. 53) such as that provided by the tool shown in FIG. 52.
Importantly, the heating beam may be incident from the same beam
path or separate one.
[0187] In another embodiment, the wafer may be simultaneously
heated while carrying out laser ablation to reduce and in some
instances prevent the formation of crystalline damage (e.g.,
amorphous silicon and/or nanocrystalline silicon) in the
laser-ablated spot. This may be due to the increased efficiency of
silicon evaporation and resulting improved ablation quality of
simultaneous wafer heating as well as increased cooling time for
any molten silicon remaining on the wafer. A laser or a radiant
heat source such as an arc lamp may be used for this purpose. To
limit the heat penetration in the wafer the lamp may be selected to
have light output mostly in the green or UV region of the spectrum
and also may be pulsed. The heating area may be as large as the
area of the wafer. FIG. 54 is a schematic diagram showing a same
station ablation and annealing using wafer heating. Specifically,
FIG. 54 is a schematic diagram showing simultaneous wafer heating
during on the area of the wafer that is undergoing patterning by
the ablation laser beam.
[0188] FIG. 55 is a process flow for the formation of a back
contact back junction solar cell showing oxide ablation carried out
in the presence of a heating light (shown in steps 3, 5, and 8 of
FIG. 55) such as that provided by the tool shown in FIG. 54.
[0189] In another embodiment, the wafer may be flash heated for
annealing post ablation. Thin layers of amorphous silicon (and/or
nanocrystalline silicon) on top of mono-crystalline silicon have
been found to melt at lower temperature and then solidify into
mono-crystalline layers by liquid-phase epitaxy (or convert into
mono-crystalline layers by solid-phase epitaxy) on a
mono-crystalline substrate. Thus, flash annealing of the whole
wafer after ablation to a temperature above the melting of
amorphous silicon (or above the threshold temperature required for
solid phase epitaxy) and lower than the melting of crystalline
silicon may produce surface layers that would be well passivated
and have reduced defects. Further, at even lower temperatures
(e.g., below the melting point of damaged silicon) the amorphous
and/or nanocrystalline silicon may become mono-crystalline by solid
phase epitaxy. Again, for this application lamps producing
radiation in the green to UV wavelength may be used. Ideally, this
flash annealing may be carried out in a separate station in a
multi-station system, such as those depicted in FIGS. 49 and 50.
And with a suitable hardware modification the annealing may be
performed in the same station as the laser ablation without any
loss of wafer throughput. FIG. 56 is a process flow for the
formation of a back contact back junction solar cell using flash
annealing (shown after step 8 in FIG. 56) of a solar cell as
described above.
[0190] Importantly, the annealing processes disclosed herein may be
sub-melt annealing--in other words annealing below the
semiconductor substrate boiling point. The selective laser ablation
of transparent films (e.g., silicon oxide or aluminum oxide) using
laser ablation for use in manufacturing back junction, back contact
cells, is provided above. Further, the use of
supplementary/auxiliary heating of silicon substrates after or
during this laser ablation to reduce or eliminate the effects of
laser ablation damage are also provided. Heating the wafer during
laser ablation presents advantages that significantly reduce laser
damage to the wafer (e.g., silicon substrate). The nature of
supplementary/auxiliary heating suitable for annealing out the
laser damage from ablation processes such that the minority carrier
lifetime is maximized leading to a high efficiency of solar cell
are further detailed herein. And the advantages of sub-melt
annealing (i.e., annealing below the melting point for bulk
silicon) are highlighted.
[0191] Embodiments, including high-throughput and inexpensive
hardware configurations and the integration of annealing steps
outlined in exemplary process flows, for annealing laser during or
after the ablation are described in FIGS. 40A through 56. Below
additional laser parameter aspects are considered to maximize the
annealing benefits without adverse effects on other properties of
the solar cell.
[0192] Integration of laser annealing in the fabrication of
exemplary back contact back junction solar cells are described with
reference to FIGS. 36, 46, 47, 48, 53, 55, and 56. In some
instances it is important that the laser anneal be carried out
without disturbing, physically or chemically, the various layers
which may be present around the ablation spot. For example, too
high laser power during annealing can evaporate silicon from
contact, redistribute the dopant, and cause additional crystalline
defects. Also the dielectric around the ablation could be cracked,
wrinkled, and/or lifted up causing a loss of passivation of the
silicon/dielectric interface. Further, as shown in the exemplary
process flows provided herein, the silicon surface may be covered
with a dielectric film that is either boron or phosphorous doped.
In some instances, extensive melting or heating of the underlying
silicon may result in the undesired doping of the underlying
silicon with the dopant of the overlying film which may adversely
affect the solar cell performance.
[0193] To minimize or prevent adverse effect to the layers
surrounding the ablation spot the annealing beam may be highly
aligned to the ablation spot and pattern (i.e., not extend beyond
the ablation spot). This alignment may be relaxed if the part of
the beam falling outside of the ablation spot is of such low
intensity that no doping or disturbance of layers takes place.
Thus, appropriate laser fluence and the fluence distribution in the
laser beam is important.
[0194] And as noted previously, a key aspect of laser annealing is
the recrystallization of amorphous silicon (in the case of a
silicon solar cell wafer) left behind after ultra-short pulse laser
ablation. Melting and subsequent crystallization of this amorphous
silicon layer forms mono-crystalline silicon that is epitaxially
aligned to the mono-crystalline silicon bulk. Thus, there are
little or no recombination sites formed and the original high
minority carrier lifetime is maintained. However, the thin layers
of amorphous silicon present may melt at several hundred degrees
lower temperature than the bulk silicon. Therefore using a laser
fluence that is below the melting of bulk silicon but melts the
amorphous silicon results in the conversion of amorphous silicon
into an epitaxial mono-crystalline silicon while the laser fluence
is not high enough to cause substantial doping and disturbance of
the silicon and the dielectric layers. This sub-melt annealing,
below the melting point of the bulk silicon, provides process
margin improving the manufacturability of laser annealing.
[0195] Back junction back contact solar cells may have alternating
or distributed base and emitter regions that may be patterned using
laser ablation of transparent and passivating dielectrics such as
silicon oxide or aluminum oxide. The use of laser permits the
dimensions of the base and emitter regions to be minimized and lead
to high cell efficiency. To minimize the damage and heat
penetration into the silicon the ablation of transparent films may
be performed using ultra-short pulse lasers (e.g., picoseconds or
femtoseconds pulse length). The use of shorter wavelength (e.g., UV
as compared to IR) further limits the heat affected zone in the
silicon substrate however there still may be laser damage such that
the minority carrier lifetime is reduced.
[0196] The removal of transparent dielectrics by the picoseconds
laser beam takes place by the explosive evaporation of silicon, the
vapors cracking and dislodging the oxide film. Around the ablated
spot, the oxide film is lifted up causing a loss of passivation.
Vapors of silicon not able to escape because of low laser power
around the edge of the Gaussian beam, may condense back into an
amorphous silicon layer further causing a loss of passivation. In
some instances, the high laser fluence in the center of a Gaussian
beam causes melting and rippling of the silicon layer that is
difficult to passivate. These effects may be observed in the
photographs of FIGS. 38A to 39D.
[0197] FIG. 51 is a schematic diagram of a system for obtaining
high laser annealing spot to laser ablation spot alignment. FIGS.
52 and 54 show a laser annealing spot larger than the ablation
spot. In cases where the annealing spot is larger than the ablation
spot, the annealing laser fluence may be selected to be lower than
the fluence that could melt the bulk silicon. In other words, the
annealing fluence should be low enough (i.e., sub-melting) so there
is no undesired doping of silicon from overlying dielectric and the
silicon/passivation layer interface is not degraded. Thus, in some
cases sub-melt annealing (and the resulting prevention of adverse
effects) may allow for simplified hardware configurations.
[0198] The maximum permissible laser fluence for a specific solar
cell structure may obtained by scanning with the annealing laser a
control solar cell substrate containing all the appropriate device
layers but without ablation, and while ensuring there is no loss of
minority carrier lifetime or undesired dopant redistribution.
[0199] As a Gaussian beam has its energy peaks in the center and
the fluence decays dramatically towards the beam edge, in some
cases and dependent on additional cell considerations it may be
difficult to prevent melting of bulk silicon while obtaining the
required annealing of silicon using a Gaussian beam. In this case
the use of so-called flat top or top-hat beam may be advantageous.
FIG. 57 shows representative/idealized profiles of a Gaussian laser
beam and a flat-top (top-hat) laser beam. For a flat-top beam the
highest energy may be controlled over the whole exposure area to
stay below the melting threshold for the underlying semiconductor
substrate (e.g., bulk silicon) and still be high enough to anneal
and also melt the amorphous silicon if needed.
[0200] An advantage of heating the wafer either with an auxiliary
source, such as laser, lamps, or other means, is the reduction of
ablation threshold for the dielectric film. This reduces the power
required for ablation and lowers the damage introduced into
silicon. Additionally, for a heated wafer the cool down is slowed
even though the ablation laser pulse is ultra-short (e.g.,
picoseconds)--this allows for the silicon layer that is melted or
evaporated to condense back epitaxially into a mono-crystalline
silicon film without the generation of recombination sites for
minority carrier absorption.
[0201] In operation, various embodiments include: the application
of annealing laser fluence that is below what is needed to melt the
underlying semiconductor substrate (e.g., bulk silicon), referred
to as sub-melt annealing, but high enough to anneal the ablation
laser damage. In some instances the laser fluence may be high
enough to melt the amorphous silicon layer but still not melt the
bulk silicon; the application of flat top annealing laser beam
profile to maximize laser annealing benefits; the application of
heat during laser ablation so the fluence requirements for ablation
laser are reduced thereby reducing the damage introduced into
silicon; the application of heat during laser ablation so the
cooling rates on the silicon surface are reduced and the silicon
may condense back into a mono-crystalline silicon structure; and
applying auxiliary heating using a laser, lamp, resistance heating,
or other known method.
[0202] Those with ordinary skill in the art will recognize that the
disclosed embodiments have relevance to a wide variety of areas in
addition to those specific examples described above. The foregoing
description of the exemplary embodiments is provided to enable any
person skilled in the art to make or use the claimed subject
matter. Various modifications to these embodiments will be readily
apparent to those skilled in the art, and the generic principles
defined herein may be applied to other embodiments without the use
of the innovative faculty. Thus, the claimed subject matter is not
intended to be limited to the embodiments shown herein but is to be
accorded the widest scope consistent with the principles and novel
features disclosed herein.
[0203] It is intended that all such additional systems, methods,
features, and advantages that are included within this description
be within the scope of the claims.
* * * * *