U.S. patent application number 14/471687 was filed with the patent office on 2014-12-18 for high fidelity doping paste and methods thereof.
This patent application is currently assigned to Innovalight, Inc.. The applicant listed for this patent is Innovalight, Inc.. Invention is credited to Maxim Kelman, Elena Rogojina, Giuseppe Scardera.
Application Number | 20140370640 14/471687 |
Document ID | / |
Family ID | 45491769 |
Filed Date | 2014-12-18 |
United States Patent
Application |
20140370640 |
Kind Code |
A1 |
Rogojina; Elena ; et
al. |
December 18, 2014 |
HIGH FIDELITY DOPING PASTE AND METHODS THEREOF
Abstract
A high-fidelity dopant paste is disclosed. The high-fidelity
dopant paste includes a solvent, a set of non-glass matrix
particles dispersed into the solvent, and a dopant.
Inventors: |
Rogojina; Elena; (Los Altos,
CA) ; Kelman; Maxim; (Mountain View, CA) ;
Scardera; Giuseppe; (Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Innovalight, Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
Innovalight, Inc.
Sunnyvale
CA
|
Family ID: |
45491769 |
Appl. No.: |
14/471687 |
Filed: |
August 28, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12967654 |
Dec 14, 2010 |
8858843 |
|
|
14471687 |
|
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Current U.S.
Class: |
438/57 |
Current CPC
Class: |
H01L 31/1804 20130101;
H01L 31/022425 20130101; H01L 21/2225 20130101; Y02P 70/50
20151101; H01B 1/06 20130101; H01L 31/0288 20130101; H01B 1/12
20130101; H01L 31/18 20130101; Y02P 70/521 20151101; H01L 31/0321
20130101; Y02E 10/547 20130101 |
Class at
Publication: |
438/57 |
International
Class: |
H01L 31/18 20060101
H01L031/18 |
Claims
1. A method of doping a semiconductor substrate, comprising:
depositing on a surface of a semiconductor substrate a dopant paste
comprising a solvent, a set of non-glass matrix particles dispersed
into the solvent, and a dopant, wherein the set of non-glass matrix
particles is a set of ceramic particles selected from the group
consisting of Al.sub.2O.sub.3, MgO, CeO.sub.2, TiO.sub.2, ZnO,
ZrO.sub.2, ZrO.sub.2-3, and Y.sub.2O.sub.3, wherein the dopant is
selected from the group consisting of phosphorous dopant, arsenic
dopant, antimony dopant, boron dopant and gallium dopant; and
heating the dopant paste on the surface of the semiconductor
substrate to diffuse the dopant into the semiconductor
substrate.
2. A method of claim 1, wherein the dopant paste further comprises
a binder.
3. A method of claim 1, wherein the dopant constitutes 10-19 wt %
of the paste.
4. A method of claim 1, wherein the heating temperature is about
800.degree. C. to 1050.degree. C.
5. A method of claim 1, wherein the solvent is an organic solvent
with boiling point greater than about 200.degree. C.
6. A method of claim 1, wherein the solvent is selected from the
group consisting of a solvent with a linear or cyclic structure, a
solvent with saturated or unsaturated hydrocarbon parts, a
hydrocarbon-based solvent, an alcohol, a thiol, an ether, an ester,
an aldehyde, a ketone, and combinations thereof.
7. A method of claim 5, wherein the binder is a polymer soluble in
the organic solvent.
8. A method of claim 5, wherein the binder is one of a
polyacrylate, a polyacetal, a polyvinyl, a cellulose, and
copolymers thereof.
9. A method of claim 1, wherein the dopant is an n-type dopant
precursor or a p-type dopant precursor.
10. A method of claim 9, wherein the n-type dopant precursor is one
of an n-type liquid, an n-type solid, and an n-type polymer.
11. A method of claim 10, where in n-type liquid is one of
H.sub.3PO.sub.4 and organophosphate.
12. A method of claim 10, wherein n-type solid is one of
P.sub.2O.sub.5, Na.sub.3PO.sub.4, AlPO.sub.4, AlP, and
Na.sub.3P.
13. A method of claim 10, wherein n-type polymer is one of a
polyphosphonate and a polyphosphazene.
14. A method of claim 9, wherein the p-type dopant precursor is one
of a p-type liquid, a p-type solid, a p-type binary compound, and a
p-type polymer.
15. A method of claim 14, wherein the p-type liquid is
B(OR).sub.3.
16. A method of claim 14, wherein the p-type solid is one of
B(OH).sub.3, NaBO.sub.2, Na.sub.2B.sub.4O.sub.7, and
B.sub.2O.sub.3.
17. A method of claim 14, wherein the p-type binary compound is one
of boronitride, boron carbide, boron silicide and elementary
boron.
18. A method of claim 14, wherein the p-type polymer is one of a
polyborazole, and a organoboron-silicon.
19. A method of claim 1, wherein the average diameter of the set of
the non-glass matrix particles is less than 25 microns.
20. A method of claim 1, wherein the set of non-glass matrix
particles is a set of ceramic particles selected from the group
consisting of TiO.sub.2 and ZrO.sub.2.
21. A method of claim 1, wherein the semiconductor substrate is a
silicon substrate of a solar cell.
22. A method of claim 1, wherein the paste is deposited in such a
way that upon said heating a doped pattern is formed on the
substrate.
23. A method of claim 22, wherein said doped pattern is a solar
cell doped pattern.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 12/967,654, filed December 14, 2010.
FIELD OF DISCLOSURE
[0002] This disclosure relates in general to semiconductors and in
particular to a high fidelity doping paste and methods thereof.
BACKGROUND
[0003] Semiconductors form the basis of modern electronics.
Possessing physical properties that can be selectively modified and
controlled between conduction and insulation, semiconductors are
essential in most modern electrical devices (e.g. computers,
cellular phones, photovoltaic cells, etc.).
[0004] Typical solar cells are formed on a silicon substrate doped
with a first dopant (the absorber region), upon which a second
counter dopant is diffused using a gas or liquid process (the
emitter region) completing the p-n junction. After the addition of
passivation and antireflection coatings, metal contacts (fingers
and busbar on the emitter and pads on the back of the absorber) may
be added in order to extract generated charge carriers. Emitter
dopant concentration, in particular, must be optimized for both
carrier collection and for contact with the metal electrodes.
[0005] Electrons on the p-type side of the junction within the
electric field (or built-in potential) tend to be attracted to the
n-type region (usually doped with phosphorous) and repelled from
the p-type region (usually doped with boron), whereas holes within
the electric field on the n-type side of the junction may then be
attracted to the p-type region and repelled from the n-type region.
Generally, the n-type region and/or the p-type region can each
respectively be comprised of varying levels of relative dopant
concentration (e.g., phosphorous, arsenic, antimony, boron,
aluminum, gallium, etc.) often shown as n-, n+, n++, p-, p+, p++,
etc. The built-in potential and thus magnitude of electric field
generally depend on the level of doping between the adjacent
layers.
[0006] In some solar cell architectures, it may be beneficial to
alter the type and concentration of a dopant as a function of
substrate position. For example, for a selective emitter solar
cell, a low concentration of (substitutional) dopant atoms within
an emitter region will result in both low recombination (thus
higher solar cell efficiencies), and poor electrical contact to
metal electrodes. Conversely, a high concentration of
(substitutional) dopant atoms will result in both high
recombination (thus reducing solar cell efficiency), and low
resistance ohmic contacts to metal electrodes. One solution,
typically called a dual-doped or selective emitter, is generally to
configure the solar cell substrate with a relatively high dopant
concentration in the emitter region beneath the set of front metal
contacts, and a relatively low dopant concentration in the emitter
region not beneath the set of front metal contacts. Differential
doping may also be beneficial to other solar cell architectures
where the dopant needs to be localized, such as a backside contact
solar cell.
[0007] Referring now to FIG. 1, a simplified diagram of a
conventional solar cell is shown. In general, a moderately doped
diffused emitter region 108 is generally formed above a relatively
light and counter-doped diffused region absorber region 110. In
addition, prior to the deposition of silicon nitride (SiN.sub.x)
layer 104 on the front of the substrate, the set of metal contacts,
comprising front-metal contact 102 and back surface field
(BSF)/back metal contact 116, are formed on and fired into silicon
substrate 110.
[0008] In a common configuration, a light n-type diffused region
108 (generally called the emitter or field), is formed by exposing
the boron-doped substrate to POCl.sub.3 (phosphorus oxychloride)
ambient to form phosphosilicate glass (PSG) on the surface of the
wafer. The reduction of phosphorus pentoxide by silicon releases
phosphorus into the bulk of the substrate and dopes it. The
reaction is typically:
4POCl.sub.3(g)+3O.sub.2(g).fwdarw.2P.sub.2O.sub.5(l)6Cl.sub.2(g)
[Equation 1A]
2P.sub.2O.sub.5(l)+5Si.sub.(s).fwdarw.5SiO.sub.2(s)+4P.sub.(s)
[Equation 1B]
Si+O.sub.2.fwdarw.SiO.sub.2 [Equation 2]
[0009] The POCl.sub.3 ambient typically includes nitrogen gas
(N.sub.2 gas) which is flowed through a bubbler filled with liquid
POCl.sub.3, and a reactive oxygen gas (reactive O.sub.2 gas)
configured to react with the vaporized POCl.sub.3 to form the
deposition (processing) gas. In general, the reduction of
P.sub.2O.sub.5 to free phosphorous is directly proportional to the
availability of Si atoms.
[0010] Referring now to FIG. 2, a simplified diagram of a selective
emitter is shown. In general, a relatively heavy n-type diffused
region (high dopant concentration) 214 is generally formed in
emitter areas beneath the set of front-metal contacts 202, while a
relatively light n-type diffused region (low dopant concentration)
208 is generally formed in emitter areas not beneath the set of
front-metal contacts 202. In addition, prior to the deposition of
silicon nitride (SiN.sub.x) layer 204 on the front of the
substrate, the set of metal contacts, comprising front-metal
contact 202 and back surface field (BSF)/back metal contact 216,
are formed on and fired into silicon substrate 210. In a common
configuration, light n-type diffused region 208 (generally called
the emitter or field), is formed by exposing the boron-doped
substrate to POCl.sub.3 as previously described.
[0011] In an alternate configuration to those in FIGS. 1 & 2,
the diffusion may be formed (or partially formed) using a doping
paste directly deposited on the surface of the substrate, instead
of through an ambient gas source. In general, an n-type or p-type
dopant source is combined with some type of matrix material,
preferably printable, that both provides the dopant source in a
deposited pattern during the diffusion process, and is subsequently
easily removed once the diffusion process has completed.
[0012] N-type doping pastes may include dopant precursors such as
n-type liquids (i.e., phosphoric acid [H.sub.3PO.sub.4],
organophosphates [O.dbd.P(OR).sub.x(OH).sub.3-x], etc.), n-type
solids (i.e., P.sub.2O.sub.5, inorganic phosphates
[Na.sub.3PO.sub.4, AlPO.sub.4, etc.] and phosphides [AlP,
Na.sub.3P, etc.]), and n-type polymers (i.e., polyphosphonates,
polyphosphazenes, etc.).
[0013] P-type doping pastes may include dopant precursors such as
p-type liquids (i.e., borate esters [B(OR).sub.3]), p-type solids
(i.e., boric acid [B(OH).sub.3], borates [NaBO.sub.2,
Na.sub.2B.sub.4O.sub.7, B.sub.2O.sub.3]), p-type binary compounds
(i.e., boronitride, boron carbide, boron silicides and elementary
boron), and p-type polymers (i.e., polyborazoles,
organoboron-silicon polymers, etc.).
[0014] An example of a common matrix material is a silica sol-gel.
A "sol" is typically a stable suspension of colloidal particles
within a liquid (2-200 nm), and a "gel" is a porous 3-dimensional
interconnected solid network that expands in a stable fashion
throughout a liquid medium and is limited by the size of the
container.
[0015] In general, the sol-gel derived glass formation process
involves first the hydrolysis of the alkoxide (sol formation), and
second the polycondensation of hydroxyl groups (gelation). For a
given silicon alkoxide of general formula Si(OR).sub.4, R being an
alkyl chain, these reactions can be written as follows:
[0016] Hydrolysis
Si(OR).sub.4+H.sub.2O.fwdarw.(HO)Si(OR).sub.3+R--OH [Equation
3]
[0017] Condensation
(H.sub.O)Si(OR)3+Si(OR)4.fwdarw.(RO).sub.3Si--O--Si(OR)3+R--OH
[Equation 4]
(OR).sub.3Si(OH)+(HO)Si(OR).sub.3.fwdarw.(RO).sub.3Si--O--Si(OR).sub.3+H-
.sub.2O [Equation 5]
[0018] For example, a sol-gel suspension (comprising silicon
alkoxide) may be combined with an n-type precursor of phosphorus
pentoxide (P.sub.2O.sub.5), like phosphoric acid (H.sub.3PO.sub.4),
an organophosphate (O.dbd.P(OR).sub.x(OH).sub.3,) etc. Likewise,
p-type doping, the sol-gel suspension may be combined with a p-type
precursor of boron trioxide (B.sub.2O.sub.3), like boric acid
(B(OH).sub.3), boron alkoxides (B(OR).sub.3), etc.. The resulting
doped silicon glass (phosphoro-silicate glass (PSG) and
boro-silicate glass (BSG) for n-type and p-type doping
respectively) formed by condensation reaction during high
temperature bake (200.degree. C.<T.sub.bake<500.degree. C.)
is used for subsequent dopant diffusion process.
[0019] However, the use of a sol-gel doping paste may be
problematic for selective doping due to relatively low glass
transition temperature of doped silicon glasses. Additionally, the
glass transition temperature tends to decrease significantly with
an increasing dopant contencentration corresponding to increasing
atomic disorder of the silica layer. See J. W. Morris, Jr., Chapter
5: Glasses, Engineering 45 Notes, Fall 1995, UC Berkeley.
[0020] The glass transition temperature of doped silica glass
formed from a typical doping paste is substantially below
temperature needed to drive the dopant into the silicon substrate.
As a result, the doped silica glass tends to reflow during high
temperature processing resulting in spreading of the dopant source
on the surface. While not problematic (and perhaps even beneficial)
for the blanket doping of large substrates surfaces, the use of a
doping process that produces a silicon glass is problematic for the
forming of high-fidelity doping regions, such as would be required
under the front metal fingers to form an ohmic contact.
[0021] In addition, many typical sol-gel doping pastes have
sub-optimal screen printing characteristics. In general, in order
to be commercially viable in high-volume solar cell production with
a high printing resolution, a paste used in a screen printer must
be a non-Newtonian shear-thinning fluid. Non-Newtonian fluid refers
to a fluid whose flow properties are not described by a single
constant value of viscosity. Shear thinning refers to a fluid whose
viscosity decreases with increasing rate of shear stress.
[0022] Consequently, the viscosity of the paste must be relatively
low at high shear rates in order to pass through a screen pattern,
but must be relatively high prior to and after deposition (at low
or zero shear rates), in order not to run through the screen or on
the substrate surface respectively. However, many typical sol-gel
doping pastes exhibit a near-Newtonian behavior, which means that
they are either too viscous to effectively pass through a screen,
or not viscous enough to prevent running, which corresponds to a
low fidelity deposited pattern.
[0023] In view of the foregoing, there is desired a doping paste
with a glass transition temperature substantially greater than the
relevant doping temperature.
SUMMARY
[0024] The invention relates, in one embodiment, to a high-fidelity
dopant paste. The high-fidelity dopant paste includes a solvent, a
set of non-glass matrix particles dispersed into the solvent, and a
dopant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The present invention is illustrated by way of example, and
not by way of limitation, in the figures of the accompanying
drawings and in which like reference numerals refer to similar
elements and in which:
[0026] FIG. 1 shows a simplified diagram of a conventional solar
cell;
[0027] FIG. 2 shows a simplified diagram of a selective emitter
solar cell;
[0028] FIGS. 3A-D show a set of simplified diagrams comparing the
reflow on a silicon substrate of a set of doped glasses to a
silicon ink;
[0029] FIGS. 4A-B compare the viscosity vs. shear rate, and the
resulting line width after deposition with the same screen, for
both a n-type conventional doping paste and a n-type high fidelity
doping paste, in accordance with the invention;
[0030] FIG. 5 compares line width between a conventional doping
paste and an n-type high-fidelity doping paste, in accordance with
the invention;
[0031] FIG. 6 shows a simplified diagram of a back-to-back dopant
diffusion configuration for use with the high fidelity doping
paste, in accordance with the invention;
[0032] FIG. 7 compares the sheet resistance of a HF doping paste to
a conventional n-type doping paste on a set of p-type silicon
substrates, in accordance with the invention;
[0033] FIG. 8 compares the sheet resistance of various HF
(phosphorous) doping paste configurations on a set of p-type
silicon substrates, in accordance with the invention; and,
[0034] FIGS. 9A-B compare the sheet resistance of various HF
(boron) doping paste configurations on a set of n-type silicon
substrates, in accordance with the invention.
DETAILED DESCRIPTION
[0035] The present invention will now be described in detail with
reference to a few preferred embodiments thereof as illustrated in
the accompanying drawings. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. It will be apparent,
however, to one skilled in the art, that the present invention may
be practiced without some or all of these specific details. In
other instances, well known process steps and/or structures have
not been described in detail in order to not unnecessarily obscure
the present invention.
[0036] In an advantageous manner, a high-fidelity doped pattern may
be formed on the substrate by a high fidelity doping paste that
includes a dopant source(precursor) and a set of matrix particles
with a high melting temperature (i.e., substantially greater than
the diffusion temperature). In one configuration, the set of matrix
particles comprises non-glass forming particles.
[0037] In another configuration, the set of matrix particles is
dispersed in a solvent with a boiling point above 200.degree. C.
Examples of such solvents include, solvents with a linear or cyclic
structures, solvents with saturated or unsaturated hydrocarbon
parts, hydrocarbon-based solvents (i.e. alkane, alkene, alkyne),
alcohols, thiols, ethers, esters, aldehydes, ketones, or a solvent
with combinations of thereof.
[0038] In another configuration, the average diameter of the set of
matrix particles is less than 25 microns. In another configuration,
a binder is also added to the solvent. In another configuration,
the binder is one of a polyacrylate, a polyacetal, a polyvinyl, a
cellulose (including its ethers and esters), and copolymers
thereof.
[0039] In general, a typical dopant drive in temperature is between
about 800.degree. C. and about 1050.degree. C. (i.e., the
temperature at which the corresponding dopant is driven into the
substrate for substitutional bonding in the crystalline silicon).
Consequently, the melting temperature of the non-glass matrix
material should be greater than 1050.degree. C. in order to
minimize any change in shape or resolution of the deposited pattern
during the diffusion process. Examples of non-glass matrix
particles include ceramics (i.e., Al.sub.2O.sub.3, MgO, CeO.sub.2,
TiO2, Y.sub.2O.sub.3, ZnO, ZrO.sub.2, ZrO.sub.2-3,Y.sub.2O.sub.3),
W and WC, and elemental compounds like Carbon and Silicon.
[0040] The addition of a dopant precursor, as previously described,
has minimal effects on the melting temperature of non-glass matrix
particles. For example, crystalline silicon has high melting
temperature of about 1440.degree. C., and the incorporation of
boron up to the solid solubility limit can only reduce the melting
point to between 50-80.degree. C. [R. W. Olesinski and G. J.
Abbaschian, The B-Si System, Bull. Alloy. Phase Diagrams, 5 (no.
5), 1984, p478-484; A, I. Zaitsev and A. A. Koentsov, Thermodynamic
Properties and Phase Equilibria in the Si-B System J. Phase
Equilib. 22 (no. 2), 2001, p126-135]. Likewise, incorporating
phosphorous atoms into the silicon substrate matrix (again up to
the solid solubility limit) tends to reduce the melting temperature
to 1180.degree. C. [R. W. Olesinski, N. Kanani, G. J. Abbaschian,
The P-Si System, Bull. Alloy Phase Diagrams 6 (no. 3), 1985].
[0041] In addition, unlike typical sol-gel doping pastes, a
high-fidelity doping paste that comprises micron and sub-micron
particle sizes also tends to exhibit strong shear-thinning
(non-Newtonian) behavior. A previously described, non-Newtonian
fluid refers to a fluid whose flow properties are not described by
a single constant value of viscosity. Shear thinning refers to a
fluid whose viscosity decreases with increasing rate of shear
[0042] Referring now to FIGS. 3A-D, a set of simplified diagrams is
shown, comparing the reflow on a silicon substrate of a set of
doped glasses (as used in a conventional doping paste) to a
high-fidelity doping paste comprising a silicon ink, in accordance
with the invention. In general, a silicon ink, is a non-Newtonian
silicon nanoparticle colloidal dispersion. More detailed
information is described in U.S. patent application Ser. No.
12/493,946 entitled Sub-Critical Shear Thinning Group IV Based
Nanoparticle Fluid, filed on Jun,. 29, 2009, the entire disclosure
of which is incorporated by reference.
[0043] FIG. 3A compares the reflow angle .phi. to the reflow
temperature .degree. C. for a set of doped glasses. Reflow
temperature in .degree. C. is shown along the horizontal axis 302,
while reflow angle .phi. is shown along the vertical axis. Upon
deposition, a reflow angle .phi. is formed between the air-glass
boundary and the glass-substrate boundary of the fluid. By
definition, as a fluid spreads out, the corresponding reflow angle
.phi. decreases.
[0044] Doped silicon glasses are deposited and then heated from
about 810.degree. C. to about 890.degree. C. A first silicon glass
308 is comprised of 5% phosphorous and 3% boron. A second silicon
glass 310 is comprised of 5% phosphorous and 4% boron. A third
silicon glass 312 is comprised of 5% phosphorous and 5% boron. As
can be seen, for any given temperature in the flow range of
810.degree. C. to about 890.degree. C., a higher dopant
concentration corresponds to a smaller reflow angle .phi.. That is,
to higher wetting on the substrate surface and degradation of
pattern fidelity.
[0045] FIG. 3B compares the normalized line width for the same set
of doped glasses with doped silicon ink, in accordance with the
invention. The derivation of normalized line width from reflow
angle is described in FIG. 3C below.
[0046] A first silicon glass 328 is comprised of 5% phosphorous and
3% boron. A second silicon glass 330 is comprised of 5% phosphorous
and 4% boron. A third silicon glass 332 is comprised of 5%
phosphorous and 5% boron. In addition, a silicon ink 326 is
comprised of a 10% phosphorous dopant concentration.
[0047] As in FIG. 3A, for the set of glasses across any given
temperature in the flow range of 810.degree. C. to about
890.degree. C., a higher dopant concentration corresponds to a
higher difference in normalized line width. However, in an
advantageous manner, the silicon ink 326 shows no substantive
change in normalized line width across the same temperature
range.
[0048] FIGS. 3C-D derive the conversion from reflow angle to
normalized line width. See J. E. Tong, et al., Solid State Tech.,
January 1984, p161. In general, a deposited fluid droplet 344, such
as a deposited paste or silicon ink, may be modeled as a lateral
slice 342 of a cylinder. The following derivation shows the
conversion of reflow angle .phi. in FIG. 3A to nominal line width
314 in FIG. 3B.
[0049] Modeling the shape of the deposited fluid as a slice of a
cylinder, radius R may be calculated.
A = .pi. R 2 ( 2 .alpha. 2 .pi. ) = .alpha. R 2 [ Equation 6 A ] A
= x ( R - h ) 2 x = R sin .alpha. [ Equation 6 B ] A = ( R sin
.alpha. ( R - h ) 2 ) = R 2 sin .alpha. cos .alpha. 2 [ Equation 6
C ] A = R 2 sin .varies. cos .varies. [ Equation 6 D ] A = .varies.
R 2 - R 2 sin .varies. cos .varies. [ Equation 6 E ] R = ( 1
.varies. - sin .varies. cos .varies. ) 1 / 2 [ Equation 6 F ] X = R
sin .alpha. = ( 1 .varies. - sin .varies. cos .varies. ) 1 / 2 sin
.alpha. [ Equation 6 G ] ##EQU00001##
Rheology Comparison
[0050] Experiment 1
[0051] Referring now to FIGS. 4A-B, set of simplified diagrams
compare a conventional doping paste to the high-fidelity doping
paste, in accordance with the invention.
[0052] FIG. 4A shows viscosity vs. shear rate for both an n-type
Ferro doping paste 406 (glass) and a n-type silicon ink based
high-fidelity doping paste 408 (non-glass), in accordance with the
invention. Log of Shear rate in 1/sec 402 is shown along the
horizontal axis, while Log of Viscosity 404 measured in cP at
25.degree. C., is shown along the vertical axis. Viscosity was
measured as a function of shear rate to show the influence of
silicon nanoparticles on the flow behavior of the paste. As can be
seen, conventional doping paste 406 shows typical near-Newtonian
behavior due to low surface interaction of the sol particles. Thus,
the viscosity of the fluid change slightly under differing shear
rate. However, silicon ink based high-fidelity doping paste 408,
characterized by significant particle-particle interaction, shows a
much stronger shear thinning behavior. Increased shear thinning
behavior results in better ink flow through the screen with reduced
spreading of printed features on the target surface, as it can be
seen in FIG. 4B.
[0053] FIG. 4B. shows line width 403 for conventional Ferro n-type
paste and silicon ink based high-fidelity doping paste after
deposition with the same screen. Two p-type substrates were each
cleaned with a hydrofluoric acid/hydrochloric acid mixture prior to
paste deposition. Both pastes were deposited with a screen mask
opening of 175 .mu.m, and were then baked at 200.degree. C. for a
time period of about 3 minutes in order to remove solvents and
densify the deposited paste. As can be seen, the high fidelity
doping paste can be deposited with a smaller absolute line width
than the conventional doping paste due to stronger shear thinning
behavior. The median line width is about 365 .mu.m for the
conventional doping paste vs. a median line width of 224 .mu.m for
high fidelity doping paste. As compared to the finger opening of
175 .mu.m, the high fidelity paste spreads by .about.50 .mu.m as
compared to 190 .mu.m for the conventional doping paste.
[0054] Experiment 2
[0055] FIG. 5 compares the average line width at different stages
of processing between a conventional (Ferro n-type) doping paste
(see Matthew Edwards, y, Jonathan Bocking, Jeffrey E. Cotter and
Neil Bennett; Prog. Photovolt: Res. Appl. 16 (1) pp 31-45, 2008)
and an n-type high-fidelity doping paste on an ISO textured
substrate, in accordance with the invention.
[0056] The conventional dopant paste was printed through a screen
opening of 400 .mu.m (504a) resulting in a printed line width of
.about.520 um (506a), then baked at 300.degree. C. for a time
period of 1-2 min in order to remove solvents and densify the
deposited paste, resulting line width of about 570 .mu.m (508a), an
increase from the screen mask opening of about 40%. The
conventional dopant paste was heated to a temperature of
950.degree. C. for a time period of 90 min in order to diffuse the
dopant into the substrate, resulting in a dopant width of about 870
.mu.m (510a), or 217.5% of the original screen opening.
[0057] For the high fidelity doping paste, a screen mask opening of
175 .mu.m (504b) was used to deposit the paste, resulting in a
deposited width of about 220 .mu.m (506b), an increase of about
20%. The conventional dopant paste was then baked at 200.degree. C.
for a time period of about 3 minutes in order to remove solvents
and densify the deposited paste. However, the resulting line width
remains about 20% larger than the screen opening (508b). The high
fidelity dopant paste was then heated to a temperature of
950.degree. C. for a time period of 90 min in order to diffuse the
dopant into the substrate. As before, and unlike the conventional
doping paste, the resulting line width remains at about 120% of the
screen make opening (510b).
Doping Comparison
[0058] FIG. 6 shows a simplified diagram of a back-to-back dopant
diffusion configuration for use with the high-fidelity doping
paste, in accordance with the invention
[0059] Silicon substrates 604 are vertically positioned in a
back-to-back configuration in order to minimize the effect of
ambient dopant that becomes volatile from the doping paste during
the doping environment. The p-type silicon substrates 604 were
vertically placed back-to-back within a quartz tube in a horizontal
diffusion furnace in order to cover the deposited Ferro doping
paste and high-fidelity doping paste 606 (as appropriate) with a
corresponding substrate in an with an N.sub.2 ambient.
[0060] Experiment 3
[0061] Referring now to FIG.7, a simplified diagram comparing the
sheet resistance of a HF doping paste to a Ferro n-type doping
paste on a set of (2 ohm-cm/180 .mu.m/saw damage etched) p-type
silicon substrates, in accordance with the invention. The inventors
believe that the doping profile Ferro doping paste is substantially
similar to most doping pastes.
[0062] The high-fidelity doping paste was prepared by addition of
10% of phosphoric acid to Si nanoparticle paste containing 1.5 wt %
ethyl cellulose binder and 8 wt % silicon nanoparticles in a
terpineol solvent, followed by thorough mixing with a planetary
mixer. The conventional (Ferro) doping paste was used unmodified.
The set of p-type silicon substrates were each cleaned with a
hydrofluoric acid/hydrochloric acid mixture prior to paste
deposition.
[0063] The Ferro paste and the high-fidelity doping paste were each
deposited on three separate substrate subsets. The substrate subset
with the high-fidelity doping paste was then baked in N.sub.2
ambient at 200.degree. C. for 3 minutes to densify the film and to
dehydrate the phosphoric acid. All substrate subsets where then
heated in a quartz tube with an N.sub.2 ambient for 30 minutes to
drive in the phosphorous dopant: a first subset was heated to
860.degree. C., a second subset was heated to 900.degree. C., and a
third subset was heated to 1000.degree. C. All substrate subsets
were then cleaned with a 10 minute BOE and the sheet resistance
under ink regions was measured with a 4-point probe. In general, a
4-point probe determines the sample resistivity by supplying a high
impedance current source through the outer two probes, and
measuring voltage across the inner two probes.
[0064] At a drive-in temperature of 860.degree. C., the sheet
resistance of the substrate with Ferro paste is about 330 ohm/sq,
while the sheet resistance of the substrate with HF doping paste is
about 237 ohm/sq. At a drive-in temperature of 900 .degree. C., the
sheet resistance of the substrate with Ferro paste is about 372
ohm/sq, while the sheet resistance of the substrate with HF doping
paste is about 161 ohm/sq. And at a drive-in temperature of
1000.degree. C., the sheet resistance of the substrate with Ferro
paste is about 224 ohm/sq, while the sheet resistance of the
substrate with HF doping paste is about 75 ohm/sq. As can be seen,
for any given temperature, a lower sheet resistance and thus a
higher dopant concentration is driven into the substrate.
[0065] Experiment 4
[0066] Referring now to FIG. 8, a simplified diagram comparing the
sheet resistance of various n-type high-fidelity doping paste
configurations on a set of (2 ohm-cm/180 .mu.m/saw damage etched)
p-type silicon substrates, in accordance with the invention.
[0067] Sheet resistance (ohm/sq) 802 is shown along the vertical
axis, while exposure 804, doping concentration % 806, and drive-in
temperature (.degree. C.) 808, are shown along the horizontal
axis.
[0068] As previously described, high-fidelity doping paste was
prepared by addition of 5, 10 or 18% of phosphoric acid to Si
nanoparticles paste containing 1.5 wt % ethyl cellulose binder and
8 wt % silicon nanoparticles in a terpineol solvent, followed by
thorough mixing with a planetary mixer. The conventional (Ferro)
doping paste was used unmodified.
[0069] The set of p-type silicon substrates were each cleaned with
a hydrofluoric acid/hydrochloric acid mixture prior to paste
deposition. The substrate subsets were then baked at 200.degree. C.
for 3 minutes to remove solvent. All substrate subsets were then
heated in a quartz tube with an N.sub.2 ambient to dehydrate the
phosphoric acid and to drive in the phosphorous dopant. Covered
wafers were placed in the heated quartz tube using the back-to-back
configuration described in FIG. 6. Exposed wafers were placed in
the heated quartz tube with ink areas directly exposed to the
N.sub.2 ambient.
[0070] At a drive-in temperature of 860.degree. C., and a
phosphorous doping concentration of 5%, the exposed sheet
resistance of the substrate is about 267 ohm/sq, while the covered
sheet resistance of the substrate is about 162 ohm/sq.
[0071] At a drive-in temperature of 860.degree. C., and a
phosphorous doping concentration of 10%, the exposed sheet
resistance of the substrate is about 237 ohm/sq, while the covered
sheet resistance of the substrate is about 127 ohm/sq.
[0072] At a drive-in temperature of 860.degree. C., and a
phosphorous doping concentration of 18%, the exposed sheet
resistance of the substrate is about 126 ohm/sq, while the covered
sheet resistance of the substrate is about 83 ohm/sq.
[0073] At a drive-in temperature of 900 .degree. C., and a
phosphorous doping concentration of 5%, the exposed sheet
resistance of the substrate is about 198 ohm/sq, while the covered
sheet resistance of the substrate is about 112 ohm/sq.
[0074] At a drive-in temperature of 900.degree. C., and a
phosphorous doping concentration of 10%, the exposed sheet
resistance of the substrate is about 160 ohm/sq, while the covered
sheet resistance of the substrate is about 89 ohm/sq.
[0075] At a drive-in temperature of 900.degree. C., and a
phosphorous doping concentration of 18%, the exposed sheet
resistance of the substrate is about 89 ohm/sq, while the covered
sheet resistance of the substrate is about 57 ohm/sq.
[0076] At a drive-in temperature of 1000.degree. C., and a
phosphorous doping concentration of 5%, the exposed sheet
resistance of the substrate is about 114 ohm/sq, while the covered
sheet resistance of the substrate is about 42 ohm/sq.
[0077] At a drive-in temperature of 1000.degree. C., and a
phosphorous doping concentration of 10%, the exposed sheet
resistance of the substrate is about 75 ohm/sq, while the covered
sheet resistance of the substrate is about 52 ohm/sq.
[0078] At a drive-in temperature of 1000.degree. C., and a
phosphorous doping concentration of 18%, the exposed sheet
resistance of the substrate is about 36 ohm/sq, while the covered
sheet resistance of the substrate is about 41 ohm/sq.
[0079] As can be seen, for any given temperature, a lower sheet
resistance and thus a higher dopant concentration is driven into
the substrate.
[0080] Experiment 5
[0081] Referring now to FIGS. 9A-B, a set of simplified diagrams
comparing the sheet resistance of various p-type (boron)
high-fidelity doping paste configurations on a set of (2 ohm-cm/180
.mu.m/saw damage etched) n-type silicon substrates, in accordance
with the invention.
[0082] FIG. 9A displays the data in on a logarithmic scale, while
FIG. 9B displays the data on a linear scale.
[0083] Sheet resistance (ohm/sq) 902 is shown along the vertical
axis, while exposure 904, doping concentration % 906, and drive-in
temperature (.degree. C.) 908, are shown along the horizontal
axis.
[0084] The high-fidelity doping paste was prepared by addition of
5, 10 or 19% of Triethyl borate to Si nanoparticles paste
containing 1.5 wt % ethyl cellulose binder and 8 wt % silicon
nanoparticles in a terpineol solvent, followed by thorough mixing
with a planetary mixer.
[0085] Three high-fidelity doping paste boron concentrations where
prepared (5%, 10%, and 19%). The set of n-type silicon substrates
were each cleaned with a hydrofluoric acid/hydrochloric acid
mixture prior to paste deposition. The substrate subsets where then
baked in N.sub.2 ambient at 200.degree. C. for 3 minutes to densify
the films. All substrate subsets where then heated in a quartz tube
with an N.sub.2 ambient for 30 minutes to drive in the boron
dopant.
[0086] At a drive-in temperature of 860.degree. C., and a boron
doping concentration of 5%, the exposed sheet resistance of the
substrate is about 1588 ohm/sq, while the covered sheet resistance
of the substrate is about 431 ohm/sq.
[0087] At a drive-in temperature of 860.degree. C., and a boron
doping concentration of 10%, the exposed sheet resistance of the
substrate is about 889 ohm/sq, while the covered sheet resistance
of the substrate is about 268 ohm/sq.
[0088] At a drive-in temperature of 860.degree. C., and a boron
doping concentration of 19%, the exposed sheet resistance of the
substrate is about 629 ohm/sq, while the covered sheet resistance
of the substrate is about 247 ohm/sq.
[0089] At a drive-in temperature of 900.degree. C., and a boron
doping concentration of 5%, the exposed sheet resistance of the
substrate is about 1232 ohm/sq, while the covered sheet resistance
of the substrate is about 231 ohm/sq.
[0090] At a drive-in temperature of 900.degree. C., and a boron
doping concentration of 10%, the exposed sheet resistance of the
substrate is about 603 ohm/sq, while the covered sheet resistance
of the substrate is about 171 ohm/sq.
[0091] At a drive-in temperature of 900.degree. C., and a boron
doping concentration of 19%, the exposed sheet resistance of the
substrate is about 520 ohm/sq, while the covered sheet resistance
of the substrate is about 154 ohm/sq.
[0092] At a drive-in temperature of 1000.degree. C., and a boron
doping concentration of 5%, the exposed sheet resistance of the
substrate is about 653 ohm/sq, while the covered sheet resistance
of the substrate is about 58 ohm/sq.
[0093] At a drive-in temperature of 1000.degree. C., and a boron
doping concentration of 10%, the exposed sheet resistance of the
substrate is about 297 ohm/sq, while the covered sheet resistance
of the substrate is about 43 ohm/sq.
[0094] At a drive-in temperature of 1000.degree. C., and a boron
doping concentration of 19%, the exposed sheet resistance of the
substrate is about 105 ohm/sq, while the covered sheet resistance
of the substrate is about 43 ohm/sq.
[0095] As can be seen, for any given temperature, a lower sheet
resistance and thus a higher dopant concentration is driven into
the substrate.
[0096] The inventions illustratively described herein may suitably
be practiced in the absence of any element or elements, limitation
or limitations, not specifically disclosed herein. Thus, for
example, the terms "comprising," "including," "containing," etc.
shall be read expansively and without limitation. Additionally, the
terms and expressions employed herein have been used as terms of
description and not of limitation, and there is no intention in the
use of such terms and expressions of excluding any equivalents of
the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
[0097] Thus, it should be understood that although the present
invention has been specifically disclosed by preferred embodiments
and optional features, modification, improvement and variation of
the inventions herein disclosed may be resorted to by those skilled
in the art, and that such modifications, improvements and
variations are considered to be within the scope of this invention.
The materials, methods, and examples provided here are
representative of preferred embodiments, are exemplary, and are not
intended as limitations on the scope of the invention.
[0098] As will be understood by one skilled in the art, for any and
all purposes, particularly in terms of providing a written
description, all ranges disclosed herein also encompass any and all
possible sub-ranges and combinations of sub-ranges thereof. Any
listed range can be easily recognized as sufficiently describing
and enabling the same range being broken down into at least equal
halves, thirds, quarters, fifths, tenths, etc. As a non-limiting
example, each range discussed herein can be readily broken down
into a lower third, middle third and upper third, etc. As will also
be understood by one skilled in the art all language such as "up
to," "at least," "greater than," "less than," and the like include
the number recited and refer to ranges which can be subsequently
broken down into sub-ranges as discussed above. In addition, the
terms "dopant or doped" and "counter-dopant or counter-doped" refer
to a set of dopants of opposite types. That is, if the dopant is
p-type, then the counter-dopant is n-type. Furthermore, unless
otherwise dopant-types may be switched. In addition, the silicon
substrate may be either mono-crystalline or multi-crystalline. In
addition, "undoped" refers to a material with a lack of dopant. As
described herein, the ketone molecules and the alcohol molecules
may be cyclic, straight, or branched.
[0099] Furthermore, this invention may be applied to other solar
cell structures as described in U.S. patent application Ser. No.
12/029,838, entitled Methods and Apparatus for Creating Junctions
on a Substrate, filed Feb. 12, 2008, the entire disclosure of which
is incorporated by reference.
[0100] All publications, patent applications, issued patents, and
other documents referred to in this specification are herein
incorporated by reference as if each individual publication, patent
application, issued patent or other document were specifically and
individually indicated to be incorporated by reference in its
entirety. Definitions that are contained in text incorporated by
reference are excluded to the extent that they contradict
definitions in this disclosure.
[0101] For the purposes of this disclosure and unless otherwise
specified, "a" or "an" means "one or more." All patents,
applications, references and publications cited herein are
incorporated by reference in their entirety to the same extent as
if they were individually incorporated by reference. In addition,
the word set refers to a collection of one or more items or
objects.
[0102] Advantages of the invention include a high fidelity doping
paste, optimized for screen printing in the high-volume manufacture
of solar cells.
[0103] Having disclosed exemplary embodiments and the best mode,
modifications and variations may be made to the disclosed
embodiments while remaining within the subject and spirit of the
invention as defined by the following claims.
* * * * *