U.S. patent number RE37,512 [Application Number 09/525,334] was granted by the patent office on 2002-01-15 for method of preparing solar cell front contacts.
This patent grant is currently assigned to Interuniversitair Microelektronica Centrum (IMEC) VZW. Invention is credited to Roland Jozef Fick, Johan Nijs, Jozef Szlufcik.
United States Patent |
RE37,512 |
Szlufcik , et al. |
January 15, 2002 |
Method of preparing solar cell front contacts
Abstract
Method of preparing on a solar cell the top contact pattern
which consists of a set of parallel narrow finger lines and wide
collector lines deposited essentially at right angles to the finger
lines on the semiconductor substrate, characterized in that it
comprises at least the following steps: (a) screen printing and
drying the set of contact finger lines; (b) printing and drying the
wide collector lines on the top of the set of finger lines in a
subsequent step; (c) firing both finger lines and collector lines
in a single final step in order to form an ohmic contact between
the finger lines and the semiconductor substrate and between the
finger lines and the wide collector lines.
Inventors: |
Szlufcik; Jozef (Kessel-Lo,
BE), Nijs; Johan (Linden-Lubbeek, BE),
Fick; Roland Jozef (Oud-Turnhout, BE) |
Assignee: |
Interuniversitair Microelektronica
Centrum (IMEC) VZW (Leuven, BE)
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Family
ID: |
26140789 |
Appl.
No.: |
09/525,334 |
Filed: |
March 10, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
604666 |
Feb 21, 1996 |
05726065 |
Mar 10, 1998 |
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Foreign Application Priority Data
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Feb 21, 1995 [EP] |
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95870012 |
Dec 22, 1995 [EP] |
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95870135 |
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Current U.S.
Class: |
438/57; 136/256;
438/256; 438/98 |
Current CPC
Class: |
H01L
31/022425 (20130101); H01L 31/18 (20130101); Y02E
10/50 (20130101) |
Current International
Class: |
H01L
31/0224 (20060101); H01L 031/18 () |
Field of
Search: |
;136/256
;438/98,57,256 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2550 |
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Jun 1979 |
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EP |
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729189 |
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Aug 1996 |
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EP |
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62-156881 |
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Jul 1987 |
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JP |
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WO 92/22928 |
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Dec 1992 |
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WO |
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Other References
IR. Lawrence, Conference Record, 14th IEEE PVSC (1980), pp.
541-544.* .
K.F. Teng et al., Conference Record, 19th IEEE PVSC (1987), pp.
1430-1434.* .
Sabo et al., "Silver Thick Film Metallization for Photovoltaics
Fired at 300.degree. C", Proceedings 1985 International Symposium
on Microelectronics, pp. 59-66, Nov. 11, 1985.* .
Nakatani et al., "A New Proces for High Efficiency Silicon Solar
Cells," 17th IEEE Photovoltaic Specialists Conference, pp.
1352-1356, May 1, 1984.* .
Nunoi et al., "High Performance BSF Silicon Solar Cell With Fire
Through Contacts Printed on AR Coating," 14th IEEE Photovoltaic
Specialists Conference, pp. 805-810, Jan. 7, 1980..
|
Primary Examiner: Diamond; Alan
Attorney, Agent or Firm: Knobbe, Martens, Olson, and Bear
LLP
Claims
We claim:
1. A method of preparing a contact pattern on a semiconductor
substrate of a solar cell, said patterning comprising a set of
narrow finger lines and wide collector lines that intersect and
that are in electrical contact, said method comprising the steps
of:
(1) screen printing a masking paste on top of a front surface of
said semiconductor substrate using a screen with a pattern
structure, thereby forming a printed pattern;
(2) depositing a coating over said front surface;
(3) dissolving said masking paste whereby selectively lifting-off
the part of the coating deposited on top of the masking paste;
.Iadd.and .Iaddend.thereafter
(4) .[.etching off oxide layers from areas of said front surface
exposed through openings in the coating, wherein said openings are
formed through the dissolving and lift-off step (3); and
thereafter
(5).]. screen printing the set of finger lines in .[.said.].
openings formed in said coating using a screen with said pattern
structure and drying the set of finger lines.
2. The method of claim 1, wherein the step of depositing a coating
over the front surface comprises depositing an antireflection
layer.
3. The method of claim 1 further comprising the steps of:
printing and drying the collector lines on top of the set of finger
lines; and
firing both the finger lines and the collector lines.
4. The method of claim 1 further comprising the steps of:
subsequently, printing and drying the collector lines on top of the
set of finger lines; and
firing both the finger lines and the collector lines in a single
final step.
5. The method of claim 4, wherein .[.substrates.]. .Iadd.a
substrate .Iaddend.already having a rear ohmic contact .[.are.].
.Iadd.is .Iaddend.used.
6. The method of claim 4, wherein said step of screen printing the
set of finger lines comprises the use of a screen made of a solid
metal mask.
7. The method of claim 6, wherein the set of finger lines and the
collector lines are made with silver paste.
8. The method of claim 4, wherein the collector lines are printed
by a process selected from the group consisting of screen printing,
ink-jet printing, and off-set printing.
9. The method of claim 8 wherein the set of finger lines and the
collector lines are made with silver paste.
10. The method as recited in claim 1 further comprising the steps
of:
forming an oxide layer on top of said substrate thereby forming
said front surface;
thereafter executing steps (1) to (3),
etching of said oxide layer through openings in the coating;
and thereafter .[.executing step (4).]. .Iadd.etching off oxide
layers from areas of said front surface exposed through openings in
the coating, wherein said openings are formed through the
dissolving and lift-off step (3).Iaddend..
11. The method of claim 1, wherein said masking paste comprises
metal powders or powders of silicon oxide or powders of titanium
oxide or chalk powder, mixed with an organic material.
12. The method of claim 1 wherein the step of dissolving the
masking paste is executed by immersing said substrate in an organic
solvent.
13. The method of claim 1, wherein the step of dissolving the
masking paste is executed by immersing said substrate in a solution
of sulfuric acid and hydrogen peroxide.
Description
OBJECT OF THE INVENTION
The present invention is related to a method of preparing contacts
on the surface of semiconductor substrates. The present invention
is also related to products obtained by this method and more
particularly to a solar cell.
STATE OF THE ART
Conventional screen printing is currently used in a mass scale
production of solar cells. Typically, the top contact pattern of a
solar cell consists of a set of parallel narrow finger lines and
wide collector lines deposited essentially at a right angle to the
finger lines on the semiconductor substrate or wafer.
Such front contact formation of crystalline solar cells is
performed with standard screen printing techniques. It has
advantages in terms of production simplicity, automation, and low
production cost.
Low series resistance and low metal coverage (low front surface
shadowing) are basic requirements for the front surface
metallization.
According to the document Hybrid Circuit No. 30, January 1993,
"Thick-film Fine-line Fabrication Technique--Application to Front
Metallization of Solar Cells," by A. Dziedzic, J. Nijs, and J.
Szlufcik, minimum metallization widths of 100-150 .mu.m are
obtained using conventional screen printing. This causes a
relatively high shading of the front solar cell surface. In order
to decrease the shading a large distance between the contact lines,
i.e., 2 to 3 mm is required. On the other hand, this implies the
use of a highly doped, conductive emitter layer. However, the heavy
emitter doping induces a poor response of the solar cell to short
wavelength light. Narrower conductive lines can be printed using
ultra-thin stainless steel wire screens with a high mesh density of
325 or 400. A thin masking emulsion with a thickness of 5-15 .mu.m
is required to produce a line definition on the screen of at least
50 .nu.m.
Although a line width of 50 .mu.m can be achieved, the line
thickness decreases below 10 .mu.m measured after the firing
process. This gives rise to increased line resistance causing high
power dissipation, particularly in the main collector lines.
The fact that the fingers are ultra-thin can result in the
interruption of such fingers.
Another main disadvantage of the ultra-thin screens is their higher
cost and lower durability and/or reliability.
An alternative technique to the standard screen printing is the
application of an etched or electroformed metal mask. The
manufacturing process of such mask involves etching of a cavity
pattern on the one side of the metal foil and a mesh pattern on the
reverse side. Photoresist masking and precise mask positioning are
necessary for double-sided etching of the metal foil. This implies
a complicated design and a very high screen cost.
In the case of conventional wire mesh screens as well as in the
case of the metal etched screens, the open area (mesh openings) is
usually not higher than 50% of the pattern. The open area defines
the maximum amount of paste transferred to the substrates and at
the same time the wet line thickness. Another important point is
that a small mesh aperture requires utilization of special inks
formulated for fine line printing. This is in conflict with most of
the commercially available silver pastes for solar cell front
contact metallization. Silver powder has a tendency to create
agglomerates of particles in the paste. In addition, a flake-shaped
silver powder, usually used in the paste formulation for a solar
metallization, increases the tendencies to create agglomerates of
particles in the paste.
The modern solar cell processing includes growing of thin thermal
oxide (50-250 .ANG.) on the top emitter surface using methods well
known in microelectronics. Such an oxide layer passivates defects
and recombination centers always present on the semiconductor
surface. This process leads to an improvement of cell response to
solar short wavelength radiation that in effect gives rise to a
higher cell efficiency. Although commercially available screen
printed pastes produce good contact to non-oxidized silicon
surfaces, the firing through thermal oxide gives difficulties in
obtaining high quality contacts with low resistance.
It should also be noted that the solar cell manufacturing process
includes in most cases a step of applying an antireflection (AR)
coating which can be deposited before or after the contact
formation. If the AR layer is deposited before contact printing, it
often gives rise to the problem of high contact resistance between
silicon and printed contacts. This problem occurs particularly when
silicon nitride is used as an antireflection coating.
If an AR layer is deposited after the contact formation, another
problem is raised which is the soldering of the collector lines
during the module fabrication.
The solution to this problem brings the "firing through" method
described in PCT Document WO 89/12312, wherein the authors apply
the commercially available silver paste "Ferro #3349" to "fire
through" a silicon nitride ARC. A "fired through" TiO.sub.2 AR
layer is described in the paper by Nunoi, et al. "High performance
BSF silicon solar cell with fired through contacts printed on AR
coating", 14th IEEE PV Specialists Conference--1980, San Diego,
USA, pp. 805-810.
PCT Document WO 92/22928 describes a solar cell and a method to
make it wherein an antireflective coating is deposited on a
semiconductor substrate before a first set of narrow elongated
parallel electrodes are printed thereon and wherein finally a
second set of elongated electrodes are affixed to each of the first
electrodes.
It should be noted that the paste or the ink used in order to form
the array of narrow elongated parallel electrodes is such that it
penetrates said antireflective material and forms mechanically
adherent and low electrical resistance contact with the front
surface of the semiconductor substrate. This means that not all the
conventional pastes can be used. Furthermore, in order to have such
good contact between the semiconductor substrate and the narrow
elongated parallel electrodes, a step of "firing through" is
necessary.
The firing at the same time through the thermally grown silicon
dioxide and antireflection coating (particularly silicon nitride)
layers, although described in the technical literature, usually
gives problems of high contact resistance and is difficult to
achieve with commercial pastes.
E.P.O. Document EP-A-0002550 describes a method of forming a
contact configuration for soldering a metal connection on a region
of the surface of a semiconductor body comprising the provision by
serigraphy, on at least a part of said region, of a conductive
paste which comprises at least a principal metal, said paste then
being vitrified thermally such that the dopant migrates into at
least a surface part on the region of a surface of the
semiconductor body.
OBJECTS OF THE INVENTION
The present invention has an object to provide improved
semiconductor devices such as solar cells which do not have the
drawbacks of the prior art.
More particularly, the present invention aims to form semiconductor
devices such as solar cells wherein the electrical contacts exhibit
a low series of resistance and a low metal coverage which also
provides a low front surface shadowing.
Many other advantages will be mentioned hereunder in the
description of the main characteristics of the present
invention.
SUMMARY OF THE INVENTION
The present invention provides a method of forming the top contact
pattern of a solar cell, which consists of a set of parallel narrow
finger lines and wide collector lines deposited essentially at the
right angles to the finger lines on a semiconductor substrate,
characterized in that it comprises the following steps:
(a) screen printing and drying the set of narrow finger lines;
(b) printing and drying the wide collector lines on top of the set
of finger lines in a subsequent step;
(c) firing both finger lines and collector lines in a single final
step in order to form an ohmic contact between the finger lines and
the semiconductor substrate and between the finger lines and the
wide collector lines.
According to a first preferred embodiment, the following steps are
performed before the screen printing step of the contact finger
lines:
(1) screen printing a pattern of masking paste on the front surface
of the semiconductor substrate, so that the printed pattern will
form the pattern for the set of parallel finger lines;
(2) depositing an antireflection coating over the whole front
surface;
(3) dissolving the masking paste and selectively lifting-off the
portions of the antireflection coating which have been deposited on
the masking paste;
(4) etching-off the oxide layers from the openings in the
antireflection coating;
(5) performing the steps (a), (b), and (c) as described
hereabove.
According to another possible embodiment of the present invention,
an antireflection coating is deposited in an intermediate step
after printing and drying the front contact finger lines and before
the collector lines are printed and dried.
The several methods described hereabove can be applied to
substrates already having a rear ohmic contact or a back contact
can be formed during the front contact formation or after the front
contact has been already fabricated.
It should be noted that according to the method of the present
invention, the last step is only a co-firing step and not a step of
firing through.
The screen for printing the set of narrow parallel finger lines is
preferably made from a solid metal foil in which the set of
parallel lines which form the finger contact pattern can be
chemically etched or cut by a laser or an electron beam.
However, in some particular embodiments wherein bridges over the
openings are allowed, other masks besides metal stencils can be
used, such as a mesh screen.
The screen used for printing the collector lines is preferably made
of a conventional mesh screen or a metal stencil screen.
Other techniques such as ink-jet printing or off-set printing can
also be used in the present invention for printing the collector
lines. The proposed invention results in many advantages over using
conventional screen printing techniques.
Concerning Finger Lines
1. The pattern of parallel finger lines is formed in a solid metal
foil which means that it has an open area equal to 100%. No meshes
are present in the pattern openings. This increases the volume of
the paste transferred to the substrate in the printing process. It
should be noted that when using a standard wire mesh screen, the
open area is only between 40%-60%.
2. The absence of the meshes in the openings reduces the
requirements for good screen printability of pastes used for front
contact printing. Pastes with a high solids content and high
viscosity can be used.
3. Using a laser or an electron or ion beam for metal mask cutting
gives the possibility of obtaining pattern definition down to a few
micrometers. This depends on the metal foil thickness and the
quality of the cutting system. In practice, the line width is
limited by the requirement of high line thickness. The thicker the
metal mask the higher is the thickness of the printed lines. On the
other hand, the ratio of the cut line width to the mask thickness
should be above 0.5. A lower ratio leads to difficulties in paste
transfer through the mask openings during printing. It has been
demonstrated that laser cutting can fabricate a finger pattern of
30 .mu.m wide lines cut in a 50-60 .mu.m thick stainless steel
foil.
The result of the above advantages 1-3 gives the possibility of
printing very narrow lines with a high aspect ratio and no
interruption. Lines with width of 40 .mu.m and up to 25-30 .mu.m
thickness have been measured after printing and drying. This
corresponds to 13-16 .mu.m thick lines after firing. A metal sheet
resistance of 1-2 mohm/sq. was measured with most commercially
available specialized pastes for solar cell front contacts.
4. Furthermore, using solid stainless steel stencils instead of
wire mesh screens for printing the finger lines increases the
durability of the screens.
5. Cutting the continuous and completely open lines by a laser or
electron beam simplifies the screen fabrication process and
strongly decreases the screen cost.
Concerning Collector Lines
1. A collector pattern is preferably prepared with a conventional
wire mesh screen or with solid metal masks. A durable screen with a
total (screen+emulsion) thickness above 100 .mu.m can be used. A
standard screen with a mesh density of 200 or 180 per inch covered
with a 20 .mu.m thick emulsion is typical for collector
printing.
2. The thick collector lines with a sheet resistance below 1
mohm/sq. will be easily obtained with most of the silver pastes for
front contact metallization. The width of the collector lines can
be decreased, giving lower shading. Using the preferred embodiment
of the present invention wherein prior to the screen printing of
the finger lines, a screen printing of a masking paste is performed
with the deposition of antireflection coating, the following
advantages can be noted:
Concerning the Masking Paste
The role of the masking paste is to provide a selective mask for an
antireflection coating (ARC) deposition at those regions of
oxidized silicon substrates where the front contact finger lines
are going to be printed. The masking paste after the drying or
curing process should stay intact during the ARC deposition and be
easily removed, later lifting off of the ARC layer deposited on top
of it. Pastes containing fine metal powders or powders of silicon
oxide, titanium oxide, or chalk powder mixed with an organic
vehicle fulfill the task. These pastes are easily removed in
organic solvents.
Concerning the Front Finger Contact Paste
Since there is no intermediate layer between the printed front
finger contact and silicon substrate and since the applied laser
cut stencil screens have no blocking meshes, the requirements for
the front contact silver pastes specialized for the front contact
formation can be applied in the present invention.
Concerning the Front Collector Paste
The paste applied for the collector lines can be the same silver
paste as for the front fingers or any other high conductivity paste
which gives a good adherence to an antireflection coating layer,
does not penetrate completely through the ARC, and provides a
perfect low resistance ohmic contact to finger lines.
Furthermore, in the case when an antireflection coating is used,
the following advantage could be mentioned:
1. Both finger and collector lines are co-fired in the same firing
process. As a result, the finger lines are in good electrical
contact with the substrate, and the collector lines are in good
electrical contact with the finger lines. In any case, the
collector lines are not covered by an ARC layer. This gives no
problem with soldering of collector lines during module
fabrication.
2. Separation of the collector lines from the direct contact with
silicon substrate reduces carrier recombination losses existing at
the metal contact-silicon interface. Selection of material used for
the ARC coating and of the deposition technique are crucial for
achieving a separation. Most top contact silver pastes penetrate
through an ARC layer of titanium dioxide deposited by Atmospheric
Pressure Chemical Vapor Deposition (APCVD). In the case of using a
silicon nitride AR layer deposited by Plasma Enhanced CVD, such ARC
layer can be a very good barrier between silicon and most screen
printed silver pastes.
3. Solar cell contacts prepared according to the present invention
can have contact finger lines placed much more closely without
additional shadowing. Solar cells with lightly doped emitter and a
higher sheet resistance can be fabricated by a screen printing
solar cell process, which results in an improvement of solar cell
response to short wavelength light.
Accordingly, the solar cells having electrical contacts prepared
with the method according to the present invention exhibit:
low sheet resistance of fingers;
lower sheet resistance of collectors;
lower contact resistance of finger/substrate interface;
lower series of solar cell;
lower shadowing losses caused by fingers;
lower shadowing losses caused by collector lines;
lower solar cell total shadowing losses;
lower carrier recombination losses at contact-silicon
interfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described using a solar cell contact
pattern as an example, presented in the drawings wherein:
FIG. 1 is a schematic view of the top contact pattern containing
both contact collector lines and contact finger lines;
FIG. 2 is a schematic view of the top contact finger pattern
obtained by a separation of finger printing from collector
printing;
FIGS. 3a-3i show the steps of an entire manufacturing process of a
solar cell for one preferred embodiment of the invention; and
DETAILED DESCRIPTION OF THE PRESENT INVENTION
I. Preparation of Screens
1. A standard fabric screen is stretched and glued to a frame
adequate to the screen printer used. Typical parameters are metal
screen of 80 UT, orientation of wires to the frame at an angle of
90.degree., and tension of the screen 30N.
2. An emulsion typical for screen patterning is deposited over the
screen and dried.
3. A solid metal foil with thickness of 40-60 .mu.m is bonded at
its peripheries to the standard fabric screen stretched to the
frame. The meshes of the fabric screen are cut away from the middle
region of the foil where the pattern will be formed.
4. A set of parallel lines reflecting the finger pattern of solar
cell contacts is cut by a laser beam. An electron or iron beam can
also be used for the cutting process. The width of the cut lines is
regulated by the beam diameter, the power, and the cutting speed. A
typical contact flager screen consists of lines with width of 40-50
.mu.m and a distance between them of 1.2-1.5 mm cut in a stainless
steel foil with a thickness of 50-60 .mu.m. The area of finger
lines is between 3% and 4% of the total top surface.
5. The collector lines screen is prepared by standard techniques
using a wire mesh screen or by laser cutting of a metal foil. A
standard screen with a mesh density of 165 is typical for this
application. The metal foil bonded to a fabric screen as described
in I.1 is used for the preparation of a laser cut screen. Typical
collector line width is between 1-1.5 mm.
Other techniques such as ink-jet printing or off-set printing can
be used in the present invention for printing the collector
lines.
II. Description of the Manufacturing Processes of Solar Cells
The starting material which is represented in FIG. 3a as an "as
cut" Cz monocrystalline or multicrystalline silicon substrate (1)
and is subjected to the following preliminary steps:
1. Saw Damage Etching
Saw damage etching can be performed in an acid or caustic solution.
Hot sodium hydroxide or potassium hydroxide is used more often for
removal of a surface damaged layer. Typically a concentration of
20-30% NaOH solution in water and at a temperature of
90.degree.-95.degree. is used. A time of five minutes is sufficient
to etch away 20 .mu.m from each side of the wafer. This is followed
by a through rinsing in DI water.
2. Texturing
Texturing is done according to the well known process used in solar
cell technology:
a 2% weight solution of sodium hydroxide in 90% DI and 10%
isopropanol volume solution is heated to a temperature of
75-80.degree. C.
wafers are immersed in the solution for 15-30 minutes;
rinsing in DI water.
3. Chemical Cleaning
Phosphorus diffusion is usually preceded by chemical cleaning. In
the present case, it is dipped in a 4:1 solution of sulfuric acid
and hydrogen peroxide, followed by rinsing in DI water. Next, the
wafers are dipped in a 1% solution of hydrofluoric acid and rinsed
in DI water. Other cleaning method such as a RCA-cleaning or
rinsing in HCl solution can also be used.
4. Phosphorus Diffusion (FIG. 3b)
Phosphorus diffusion can be done by any means known in
microelectronics: gaseous source, or spin-on solution or screen
printing on a phosphorus paste. More information concerning this
step can be found in EP-B-0108065.
In the present embodiment, the diffusion is done by screen printing
a phosphorus paste on the top substrate surface (1) in order to
create an n+ layer (2). Diffusion was carried out in a conveyor
belt furnace at a peak temperature of 910.degree. C. Diffusion
glass was removed by dipping in wafers in 10%-25% hydrofluoric acid
for about 30 sec. The sheet resistance of the diffused layer is in
the range of 45-50 ohm/sq.
5. Dry Silicon Dioxide Growth (FIG. 3c)
The process of dry silicon dioxide growth is well known and widely
described in microelectronic literature. The thin passivation layer
(3) of silicon dioxide is usually grown in an open tube furnace in
a dry oxygen atmosphere at a temperature in the range from
800.degree. C. to 900.degree. C. In the present embodiment, a
temperature of 800.degree. C. and time of 15 minutes is used for
growing a 150 .ANG. thick silicon dioxide layer (3).
6. Masking Paste Printing and Drying (FIG. 3d)
A front contact metal stencil screen with a finger pattern as shown
in FIG. 2 is used for printing of a masking paste (4). The paste is
subsequently dried at a temperature of around
100.degree.-300.degree. C. In this embodiment of the present
invention, a paste comprising 60% wt. titanium dioxide powder and
40% wt. butyl carbitol is used.
7. Antireflection Coating Deposition (FIG. 3e)
Antireflection coating (5) can be fabricated by means and use of
any material known in microelectronics for antireflection coating
deposition. However, the properties of the AR layer influence the
next processing steps and solar cell characteristics. In this
embodiment of the present invention PECVD silicon nitride is
applied.
8. Masking Paste Removal and Selective ARC Lift-Off (FIG. 3f)
The substrates are then immersed in an organic solvent (isopropyl
alcohol, acetone or butyl carbitol or others) which can dissolve
the masking paste and leave the ARC layer intact. A solution of
sulfuric acid (4 vol. parts) and hydrogen peroxide (1 vol. part)
can also be used when the ARC is fabricated of silicon nitride.
Dissolving the masking paste lifts-off the ARC layer deposited on
its top and creates openings in the ARC layer. The ARC layer can be
used now as a mask for a thermal oxide etching in the openings.
This selectively uncovers the silicon surface in the areas where
the front finger pattern will be printed. It should be mentioned,
however, that if an applied paste fires through a thermal oxide,
the etching step can be omitted.
9. Printing and Drying a Front Contact Finger Pattern (FIG. 3g)
A front metal stencil screen with the same finger pattern (6) as in
step 6 is used for front contact screen printing. The silicon
surface in the contact areas is not covered by any layer (oxide or
ARC) so all problems related to a high contact resistance created
by intermediate layers between screen printed metal layers and
silicon are solved. As a result, any silver paste suitable for a
solar cell front contact metallization can be applied. Modern
screen printers equipped with an optical alignment system can be
employed. The pattern printing is followed by drying in an IR-dryer
at a temperature of 125.degree. C.-150.degree. C. A line width as
low as 40 .mu.m has been oriented.
10. Top Contact Collector Printing and Drying (FIG. 3h)
Top contact collector lines (7) are printed and dried in a
temperature range 125.degree.C.-150.degree. C. A thick standard
mesh screen or stencil screen can be used which in effect gives the
possibility of printing very thick collector lines with a low
resistance. The complete top contact pattern as shown in FIG. 1 is
obtained. The collector lines (7) are not covered by an ARC layer
(5), which causes no problems in soldering during module
fabrication. The paste used for collector line printing can be the
same silver paste as for the front finger pattern or different.
11. Back Contact Printing and Drying and All Contacts Firing (FIG.
3i)
A contact (9) is then formed on the back side of the wafer which is
covered by silver-aluminum paste or aluminum paste with small
apertures where silver paste slightly overlapping the adjacent
aluminum layer can be later printed. The silver areas are used for
tab attachment during module fabrication.
All pastes are co-fired in one step, preferably in an IR furnace.
During the firing process silver finger lines (6) are sintered
together with the n+ silicon surface, thereby creating a good
electrical contact. At the same time, collector lines (7) and
finger lines (6) are sintered together, also crating a good
electrical contact. There are intermediate ARC (5) and SiO.sub.2
(3) layers between the collector lines and silicon substrate (1).
Depending upon which materials are used for the ARC and collectors
lines, the collector lines are in contact with silicon or isolated.
The best results are obtained where the pastes do not penetrate to
the silicon surface. This reduces carrier recombination losses at
the metal-silicon interface. A p.sup.+ region (8) is formed in the
silicon substrate adjacent the back contact (9) as a result of the
firing process.
12. Edge Isolation
In the embodiment of the present invention, edge isolation is
carried out by scribing and cleaving off the cell edges. There are
many known techniques which can be also applied: plasma etching,
chemical etching, laser scribing, etc.
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