U.S. patent application number 14/719534 was filed with the patent office on 2015-09-10 for method of in-line diffusion for solar cells.
The applicant listed for this patent is BTU INTERNATIONAL, INC.. Invention is credited to Frank J. Bottari, Paul J. Richter.
Application Number | 20150255662 14/719534 |
Document ID | / |
Family ID | 51687078 |
Filed Date | 2015-09-10 |
United States Patent
Application |
20150255662 |
Kind Code |
A1 |
Richter; Paul J. ; et
al. |
September 10, 2015 |
METHOD OF IN-LINE DIFFUSION FOR SOLAR CELLS
Abstract
A method is provided for the simultaneous diffusion of dopants
of different types on respective sides of a solar cell wafer in a
single stage process. The dopants are applied to respective sides
of the wafer in wet chemical form preferably by pad printing. The
doping materials can be applied to the entire wafer surface or
effective area thereof, or can be applied in a pattern to suit the
intended solar cell configuration. In a typical embodiment, the
dopants are boron and phosphorus.
Inventors: |
Richter; Paul J.;
(Chelmsford, MA) ; Bottari; Frank J.; (Acton,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BTU INTERNATIONAL, INC. |
No. Billerica |
MA |
US |
|
|
Family ID: |
51687078 |
Appl. No.: |
14/719534 |
Filed: |
May 22, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13861537 |
Apr 12, 2013 |
9093598 |
|
|
14719534 |
|
|
|
|
Current U.S.
Class: |
438/57 |
Current CPC
Class: |
H01L 21/2255 20130101;
Y02P 70/521 20151101; Y02P 70/50 20151101; H01L 31/18 20130101;
H01L 31/1804 20130101; H01L 31/0288 20130101; Y02E 10/547
20130101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/0288 20060101 H01L031/0288 |
Claims
1. A method of simultaneously diffusing dopants of different types
on respective sides of a silicon wafer comprising the steps of:
providing the silicon wafer having a first side and a second side
directly opposite the first side; applying a first liquid doping
material being of a first type to the first side of the wafer and
not applying the first liquid doping material to the second side of
the wafer; applying a second liquid doping material being of a
second type different from the first type to a second side of the
wafer and not applying the second liquid doping material to the
first side of the wafer; and heating the wafer to diffuse
simultaneously the first liquid doping material into the first side
of the wafer and the second liquid doping material into the second
side of the wafer.
2. The method of claim 1 wherein the first liquid doping material
includes a Group III dopant.
3. The method of claim 1 wherein the second liquid doping material
includes a Group V dopant.
4. The method of claim 2 wherein the dopant is a boron
composition.
5. The method of claim 3 wherein the dopant is a phosphorus
composition.
6. The method of claim 1 wherein the steps of applying the first
liquid doping material and the second liquid doping material
comprise one of pad printing, spray coating, dip coating, roll
coating, gravure coating, rod coating, ink-jet printing, and screen
printing.
7. The method of claim 1 wherein the steps of applying the first
and second liquid doping materials comprise pad printing the first
and second doping materials to the respective first and second
sides of the wafer.
8. The method of claim 1 including the step of applying a capping
coating over the first liquid doping material.
9. The method of claim 1 including the step of applying a capping
coating over the second liquid doping material.
10. The method of claim 1 including the steps of applying a capping
coating over each of the first and second liquid doping
materials.
11. The method of claim 1 wherein the heating step comprises
in-line heating the wafer during transport through a furnace
chamber.
12. The method of claim 1 further including the step of removing
diffusion glass layers formed on the first and second sides of the
wafer during heating and diffusion.
13. The method of claim 12 wherein the step of removing the
diffusion glass layers includes simultaneously removing the
diffusion glass layers from the first and second sides of the
wafer.
14. The method of claim 13 wherein simultaneously removing the
diffusion glass layers from the first and second sides of the wafer
is performed by etching both sides of the wafer with a solution of
hydrofluoric acid.
15. The method of claim 2 wherein the first liquid doping material
is a water-based boron solution.
16. The method of claim 2 wherein the second liquid doping material
is a water-based phosphorus solution.
17. The method of claim 1 further including the step of drying
either one or both of the first and second liquid doping
materials.
18. The method of claim 1, wherein the heating step comprises
directing a gas flow from a furnace in a direction away from the
first side of the wafer and away from the second side of the wafer,
thereby minimizing the cross-contamination of the first liquid
doping material on the second side of the wafer and the
cross-contamination of the second liquid doping material on the
first side of the wafer.
19. The method of claim 1, wherein the heating step comprises
directing a gas flow from a furnace at a velocity sufficient to
entrain at least one of gaseous dopants and particulate dopants
away from the first side of the wafer and away from the second side
of the wafer, thereby minimizing the cross-contamination of the
first liquid doping material on the second side of the wafer and
the cross-contamination of the second liquid doping material on the
first side of the wafer.
Description
BACKGROUND OF THE INVENTION
[0001] This invention generally relates to diffusion processes in
silicon. The invention is particularly applicable to crystalline
silicon solar cells.
[0002] Thermal diffusion is commonly used to form p-n junctions in
crystalline silicon solar cells. In p-type silicon, phosphorous is
diffused into the bulk silicon to form a junction. In n-type
silicon, boron is diffused into the silicon to form a junction.
Prior art methods of thermal diffusion include using tube furnaces
to provide the heat required to drive dopant atoms into the silicon
lattice structure. The dopant can be delivered to the wafer during
the diffusion heating using a vapor source such as phosphorous
oxychloride for a phosphorous diffusion or boron tribromide for a
boron diffusion. To produce a solar cell using n-type silicon, two
high-temperature diffusion processes are needed. A boron diffusion
is performed on the first surface of the wafer to produce a charge
separating emitter field. On the second surface of the wafer, a
phosphorous diffusion is carried out to form a deep junction to act
as a conductive back surface field (BSF). In conventional practice,
these two processes are performed sequentially with discrete
machinery which results in long cycle times, expensive capital
equipment and operating costs. Also, additional process steps, such
as glass removal etching and masking have to be performed during
each sequential process which adds to capital, materials and
operating expenses.
BRIEF SUMMARY OF THE INVENTION
[0003] The present invention provides for the simultaneous
diffusion of dopants of different types on respective sides of a
solar cell wafer in a single stage process. The dopants are applied
to respective sides of the wafer in wet chemical form preferably by
pad printing or alternatively by spray coating, dip coating, roll
coating, gravure coating, rod coating, ink jet printing, screen
printing or other known application techniques. The doping
materials can be applied to the entire wafer surface or effective
area thereof, or can be applied in a pattern to suit the intended
solar cell configuration. In a typical embodiment, the dopants are
boron and phosphorus. Other Group III and Group V dopants can also
be used as known in the art.
[0004] The wet chemical doping materials are inexpensive as are the
application techniques noted above. Moreover these wet-chemical
coating methods are versatile and can be used singularly, or
combined to form either full surface coatings, or patterned
depositions for conventional or advanced solar cell constructions.
For each coating method, basic dopant source chemistry can be the
same but factors such as rheology, drying characteristics, surface
wetting and flow control can be chemically altered to produce the
desired characteristics.
[0005] Further, the junction depths of the two simultaneous
diffusions can be controlled semi-independently by the formulation
of the dopant chemistries, such as by adjusting the concentration
of dopant source element in each dopant. Additionally, retarder
layers, such as silica can be disposed under the dopant source and
over the silicon wafer and act to impede the diffusion of the
dopant into the silicon wafer, thus affecting the depth of
diffusion.
[0006] After dopant application, both the boron and phosphorous
thermal diffusion steps are performed simultaneously in a single
in-line furnace. After diffusion, the resultant glass layers on
both the boron and phosphorous doped surfaces are then
simultaneously removed in a single acid etching process. In these
ways, capital equipment, materials, process steps and operating
costs are reduced from conventional two stage methods.
[0007] The invention is described herein for boron and phosphorus
diffusion in a silicon wafer or substrate, but is not limited to
these materials. The invention is broadly applicable to
simultaneous diffusion of other dopants in a semiconductor wafer
such as other Group III and Group V dopants, and to wafers other
than silicon.
[0008] To prevent the migration of a dopant to be diffused on one
side of a wafer to the opposite side on which a different dopant is
to be diffused, the invention in one aspect employs a barrier or
capping layer over the respective doping layers after they have
been dried and before the single diffusion step in which the
dopants are simultaneously diffused into respective sides of the
wafer.
[0009] In another aspect of the invention, the gas flow patterns of
the furnace are arranged to isolate the respective dopants and to
prevent migration of each dopant to the opposite side of the
wafer.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0010] The invention will be more fully understood from the
following detailed description read in conjunction with the
accompanying drawings in which:
[0011] FIG. 1 is a flowchart of a preferred embodiment of the
invention; and
[0012] FIG. 2 is a diagrammatic view of a furnace having isolated
gas flow paths.
DETAILED DESCRIPTION
[0013] A preferred embodiment of the invention is illustrated in
the flow chart of FIG. 1. As an initial step 10, the wafer is
etched clean in, for example, a 10% solution of hydrofluoric acid
for one minute followed by deionized water rinse and drying of the
wafer surfaces. In step 12, a Group III dopant is applied in wet
form to one side of the wafer and in step 14, the dopant is dried.
Optionally, in step 16 a barrier/capping layer is coated over the
dried Group III dopant and this barrier/capping layer is dried in
step 18. In step 20, a Group V dopant is applied to the opposite
side of the wafer and this dopant is dried in step 22. Optionally,
in step 24, a barrier/capping layer is provided over the dried
Group V dopant and this barrier/capping layer is dried in step 26.
In step 28, the wafer is heated in an inline furnace to
simultaneously diffuse the Group III and Group V dopants into the
wafer. In step 30, the diffusion glass layers which form on the
wafer surfaces during the heating and diffusion steps are
simultaneously removed from respective sides of the wafer. The
wafer can thereafter be further processed to form the solar
cells.
[0014] A pad printing method is particularly desirable for applying
wet-chemical doping source chemicals and coatings to silicon solar
wafers. This method uses a silicone print pad large enough to
completely transfer a full area, or patterned dopant deposition to
the solar wafer with a single pad impression. A metal, polymer, or
ceramic cliche plate is used to define the deposition image for
transfer. Cliche plate depths may be adjusted for the deposition
amount desired. Pad printing preferably with a polymer or ceramic
cliche plate provides a metal-free application system that can
rapidly deposit thin structured coatings of different materials
selectively to both wafer surfaces. Pad Printing also is able to
produce uniform thin coatings which are conformal to the surface
that they are disposed upon. In this way, coating voids are
minimized, junction depth uniformity is improved and shorting is
reduced.
[0015] Further, the junction depths of the two simultaneous
diffusions can be controlled semi-independently by the formulation
of the dopant chemistries, such as by adjusting the concentration
of dopant source element in each dopant. Additionally, retarder
layers, such as comprising silica can be disposed under the dopant
source and over the silicon wafer and act to impede the diffusion
of the dopant into the silicon wafer, thus affecting the depth of
diffusion.
[0016] Optionally, a barrier, or capping layer may be disposed over
either, or both dopant source coatings before co-diffusion to
prevent, or reduce contamination by the opposing dopant. These
barrier layers may consist of a sol gel derived silica sol, or nano
particles and may be applied by the same methods as the doping
materials. Another method to prevent cross-contamination of the
dopants is to join two, or more wafers together with like dopant
surfaces touching each other. These wafers can be temporarily
bonded together with a small quantity of an adhesive, such as a
silicate which can be dissolved by the post-diffusion glass etch
process. Alternately, a mechanical clip, or fixture could be used
to hold the wafers together during thermal processing.
[0017] Another way that dopant cross-contamination can be reduced
in an in-line furnace is to control the airflow within the furnace
in order to prevent, or minimize the deposit of airborne opposite
dopant contaminants onto wafer surfaces during diffusion. This
method uses a differential exhaust flow direction and rate so that
gas flow is always away from either side of the wafer. Flow
velocity is kept high enough to entrain gaseous, or particulate
dopants and direct them away from the wafer thus preventing
cross-contamination of phosphorous on the boron doped side and
boron on the phosphorous doped side.
[0018] Referring to FIG. 2, there is depicted diagrammatically a
furnace chamber 50 through which a conveyor 52 carries silicon
wafers 54. An upper gas flow path is illustrated by arrows 56 and
provides gas flow away from the top side of the wafer. A lower gas
flow path, illustrated by arrows 58, provides gas flow away from
the bottom side of the wafer. The techniques for producing such
differential gas flow are per as known in the art.
[0019] After dopant application, both the boron and phosphorous
thermal diffusion steps are performed simultaneously in a single
in-line furnace. This in-line furnace may use a metallic conveyor
belt made out of a high-temperature resistant material such as
Nichrome 5. A disadvantage of using a metallic conveyor belt is
that metallic elements from the belt may diffuse into the wafer
silicon during the diffusion process and produce carrier lifetime
reducing recombination centers. It has been shown that the
phosphorous coating on the surface of the wafer in contact with the
belt can act as a barrier layer to metallic belt elements before
they are diffused into the silicon, and also to getter metal
impurities out of the bulk silicon during diffusion. A preferred
method of conveying wafers through an in-line diffusion furnace is
to use a non-metal conveyance, instead of a metal belt. In this
way, no metal touches, or is in close proximity to the wafer which
could act as a metallic contamination source. Methods of non-metal
conveyances include conveying wafers through the furnace on ceramic
rollers and ceramic linked belts. Additionally, ceramic wafer
support elements may be conveyed through the furnace and linked
together with common drive elements, such as ceramic rope(s) within
the hot section of a furnace. If the ceramic wafer support elements
are linked together outside of the hot section of a furnace, lower
temperature resistant materials may be used as drive elements.
[0020] After diffusion, the resultant glass layers on both the
boron and phosphorous doped surfaces are then simultaneously
removed in a single acid etching process. In these ways, capital,
materials, process steps and operating costs are reduced from the
prior art method.
[0021] This invention offers several advantages, including the
following: [0022] 1.) Through-put - Improvement of through-put over
background art methods is attained by removing a process firing
step, or by eliminating a batch diffusion process step. Through-put
is further improved over background art methods by allowing wafers
to be conveyed through the production process as individual wafers,
not loaded into batch carriers, queued for processing and then
unloaded from batch carriers after processing. Through-put is even
further improved over the conventional method by combining the
borosilicate glass and phosphosilicate glass the removal steps into
one single process step. Furnace through-put can be improved by
using the method where two or more wafers are temporarily joined
together with like dopant surfaces touching each other, thus
doubling, or more furnace utilization.
[0023] 2.) Cost of energy usage--The use of in-line co-diffusion
can result in a significant cost savings over background art
methods. Current methods of tube furnace co-diffusion require long
process times. In-line sequential diffusion uses two separate
thermal drive-in steps. In-line co-diffusion is a lower cost method
requiring only one thermal drive-in step, resulting in lower
process energy costs.
[0024] 3.) Uniformity--Wafer areal uniformity is improved with
in-line co-diffusion processing over conventional tube furnace
processing. Tube furnace design factors of both thermal uniformity
and dopant gas flow are inherently difficult to control diffusion
uniformity across the whole area of the wafer. In-line co-diffusion
provides both a uniform application of the doping source material
and isothermal heating of the wafer which results in more uniform
diffusion depths.
[0025] 4.) Thin wafer processing--Mechanical yield loss
requirements are becoming more stringent and industry standard
wafer thickness is reducing. In-line co-diffusion processing
improves yield factors through reducing handling and process
steps.
[0026] 5.) Less hazardous chemicals--Boron tribromide and
phosphorous oxychloride used in traditional tube furnace diffusion
processes are hazardous chemicals that require elaborate
engineering control measures to provide a safe work environment in
the process area. In contrast, the wet chemical diffusion sources
used for in-line diffusion contain boron and phosphorus source
materials comprising acids, oxides, and precursors to oxides which
are inherently less hazardous than the materials commonly used for
tube furnace processing. Additionally, the quantities of chemicals
used in an in-line diffusion process are much less than used in a
traditional process.
[0027] 6.) Edge Isolation--Using a selective dopant printing method
such as: Pad Printing, Screen Printing, Ink Jet Printing, spray
coating over a resist pattern, etc., the dopants can be kept
sufficiently separated from the edge of a wafer so that no edge
isolation process step is required.
[0028] 7.) Less wafer damage (thermal)--In conventional diffusion
processing, two high-temperature thermal processes are used to
drive-in the boron and phosphorous dopants, respectively. For
conventional diffusion, relatively long process times are required
due to the thermal mass of the wafer load and carrier boats. An
advantage of using an in-line furnace for co-diffusion is that the
firing ramp profiles are easily controllable and there are no wafer
carriers used which would increase the system thermal mass. In-line
processing using high thermal ramp rates enable relatively short
thermal exposures to complete the diffusion drive-in.
[0029] 8.) Shaping of diffusion profile--Using an in-line furnace
for co-diffusion has the advantage of controlling different
segments of the thermal profile independently at any point, or
points throughout the thermal and cooling profiles. This flexible
capability is advantageous for the processing of some materials
that may require processing at specific temperatures during
different stages of the thermal exposure.
EXAMPLES
Example 1
[0030] One embodiment of the present invention includes using a pad
printing method to apply wet-chemical doping source chemicals to
silicon solar wafers. This method uses a silicone print pad large
enough to completely transfer a full area, or patterned dopant
image to the solar wafer with a single pad impression. A polymer or
ceramic cliche plate is used to define the image for transfer.
Cliche plate depths may be adjusted for the deposition desired.
[0031] A liquid boron diffusion source formulated for pad print
application was prepared by dissolving 22 grams of boric acid into
a mixture of 176 grams of 1-Methyl-2-pyrrolidinone, 4 grams of
Polyvinylpyrrolidone (360,000 M.W.) and 66 grams of 2-propanol.
This formulation was then pad printed on to a 156 mm.times.156 mm
pseudosquare n-type solar wafer using a 50 shore 00 silicone pad
and a 50 .mu.m cliche plate. After printing, the coated wafer
surface was dried with a hot air gun. Next, using the same pad
printer set-up, the opposite side of the wafer was pad print coated
with a liquid phosphorous diffusion source prepared by dissolving
13 grams of phosphoric acid into a mixture of 176 grams of
1-Methyl-2-pyrrolidinone, 4 grams of Polyvinylpyrrolidone (360,000
M.W.) and 66 grams of 2-propanol. This coating was then dried with
a hot air gun and both coatings were simultaneously diffused into
the wafer using a BTU International, Inc. in-line diffusion furnace
set for a thermal exposure of 980.degree. C. for 8 minutes. After
diffusion, the resultant borosilicate and phosphosilicate glasses
were simultaneously removed by etching in a solution of 50%
hydrofluoric acid for a period of 1 minute. After etching, sheet
resistance measurements were taken on the diffused wafer. The boron
diffused side measured 60.OMEGA./.quadrature..+-.2 and the
phosphorous diffused side measured 12
.OMEGA./.quadrature..+-.0.5.
Example 2
[0032] Another embodiment of the present invention includes using a
pad printing method to apply water based wet-chemical doping source
chemicals to silicon solar wafers. This method uses a silicone
print pad large enough to completely transfer a full area, or
patterned dopant image to the solar wafer with a single pad
impression. A polymer or ceramic cliche plate is used to define the
image for transfer. Cliche plate depths may be adjusted for the
deposition desired.
[0033] A water-based liquid boron diffusion source formulated for
pad print application was prepared by dissolving 20 grams of boric
acid into a mixture of 150 grams of hot distilled water and 120
grams of sucrose.
[0034] This formulation was then pad printed on to a 156
mm.times.156 mm pseudosquare n-type solar wafer using a 50 shore 00
silicone pad and a 50 .mu.m cliche plate. After printing, the
coated wafer surface was dried with a hot air gun. Next, using the
same pad printer set-up, the opposite side of the wafer was pad
print coated with a water-based liquid phosphorous diffusion source
formulated for pad print application and prepared by dissolving 15
grams of phosphoric acid into a mixture of 100 grams of distilled
water and 123 grams of sucrose.
[0035] This coating was then dried with a hot air gun and both
coatings were simultaneously diffused into the wafer using a BTU
International, Inc. in-line diffusion furnace set for a thermal
exposure of 980.degree. C. for 8 minutes. After diffusion, the
resultant borosilicate and phosphosilicate glasses were
simultaneously removed by etching in a solution of 50% hydrofluoric
acid for a period of 1 minute. After etching, sheet resistance
measurements were taken on the diffused wafer. The boron diffused
side measured 59.OMEGA./.quadrature..+-.2 and the phosphorous
diffused side measured 14 .OMEGA./.quadrature..+-.1.
Example 3
[0036] Another embodiment of the present invention includes using a
spray method to apply wet-chemical doping source chemicals to
silicon solar wafers. In this example, a SonoTek, Inc. ultrasonic
spray system was used to coat dopant materials on both surfaces of
the wafer.
[0037] A liquid diffusion source formulated for spray application
was prepared by dissolving 20 grams of boric acid into a solvent
mixture of 450 grams of 2-propanol and 50 grams of diethylene
glycol mono butyl ether, heated to about 80.degree. C.
[0038] This formulation was then spray coated on to a 156
mm.times.156 mm pseudosquare n-type solar wafer. After coating, the
coated wafer surface was dried with a hot air gun. Next, the
opposite side of the wafer was spray coated with a liquid
phosphorous diffusion source formulated for spray application and
prepared by mixing, by volume, 6 milliliters of phosphoric acid
into 200 milliliters of 2-propanol.
[0039] This coating was then dried with a hot air gun and both
coatings were simultaneously diffused into the wafer using a BTU
International, Inc. in-line diffusion furnace set for a thermal
exposure of 980.degree. C. for 8 minutes. After diffusion, the
resultant borosilicate and phosphosilicate glasses were
simultaneously removed by etching in a solution of 50% hydrofluoric
acid for a period of 1 minute. After etching, sheet resistance
measurements were taken on the diffused wafer. The boron diffused
side measured 60 .OMEGA./.quadrature..+-.2 and the phosphorous
diffused side measured 12 .OMEGA./.quadrature..+-.1.
[0040] The invention is not to be limited by what has been
particularly shown and described and is to embrace the spirit and
full scope of the appended claims.
* * * * *