U.S. patent application number 14/719443 was filed with the patent office on 2015-09-24 for process for making silver powder particles with very small size crystallites.
The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to ROBERTO IRIZARRY-RIVERA, HAIXIN YANG.
Application Number | 20150266098 14/719443 |
Document ID | / |
Family ID | 45888484 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150266098 |
Kind Code |
A1 |
IRIZARRY-RIVERA; ROBERTO ;
et al. |
September 24, 2015 |
PROCESS FOR MAKING SILVER POWDER PARTICLES WITH VERY SMALL SIZE
CRYSTALLITES
Abstract
The process for making silver powder particles with very small
size crystallites uses a combination of gum arabic and maleic acid
with the reduction of a silver salt with ascorbic acid. Silver
thick film paste containing these silver powder particles can be
used in electronic applications to form electrodes for
semiconductor devices and, in particular, solar cells.
Inventors: |
IRIZARRY-RIVERA; ROBERTO;
(RALEIGH, NC) ; YANG; HAIXIN; (DURHAM,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY |
Wilmington |
DE |
US |
|
|
Family ID: |
45888484 |
Appl. No.: |
14/719443 |
Filed: |
May 22, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13407950 |
Feb 29, 2012 |
9067261 |
|
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14719443 |
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61450263 |
Mar 8, 2011 |
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Current U.S.
Class: |
428/546 ;
75/370 |
Current CPC
Class: |
B22F 1/0011 20130101;
C22B 11/04 20130101; C22C 5/06 20130101; H01L 21/288 20130101; B22F
1/0044 20130101; B22F 9/24 20130101; Y02E 10/50 20130101; H01L
31/022425 20130101; B22F 1/0007 20130101; Y10T 428/12014 20150115;
H01B 1/02 20130101; H01L 31/0224 20130101 |
International
Class: |
B22F 9/24 20060101
B22F009/24; C22C 5/06 20060101 C22C005/06; H01L 31/0224 20060101
H01L031/0224; C22B 3/00 20060101 C22B003/00; H01L 21/288 20060101
H01L021/288; H01B 1/02 20060101 H01B001/02; B22F 1/00 20060101
B22F001/00 |
Claims
1. Silver powder particles made by a process comprising: (a)
preparing an acidic aqueous silver salt solution comprising a water
soluble silver salt dissolved in deionized water; (b) preparing an
acidic aqueous reducing and particle modifier solution comprising:
(i) a reducing agent selected from the group consisting of ascorbic
acid, ascorbates and mixtures thereof; (ii) nitric acid; (iii)
maleic acid; and (iv) deionized water, wherein the pH of said
acidic aqueous reducing and particle modifier solution is adjusted
by the addition of a base to between 2.5 and 6; (c) maintaining
said acidic aqueous silver salt solution and said acidic reducing
and surface morphology modifier solution at the same temperature,
wherein said temperature is in the range of 10.degree. C. to
65.degree. C., while stirring each said solution during and after
preparation; (d) adding said acidic aqueous silver salt solution to
said acidic aqueous reducing and particle modifier solution while
stirring to make a reaction mixture and maintaining the temperature
of said reaction mixture at said temperature of (c), wherein said
silver powder particles precipitate and are contained within the
resulting final aqueous solution; and (e) increasing the
temperature of said final aqueous solution to a temperature in the
range of 65.degree. C. to 80.degree. C. while continuing stirring;
one or both of said acidic aqueous silver salt solution and said
acidic aqueous reducing and particle modifier solution further
comprising gum arabic and said silver powder particles comprising
crystallites of size less than or equal to 32 nm as determined by
X-ray diffraction and the Scherrer formula; and said silver powder
particles having a d.sub.50 in the range of 0.5 to 3.5 .mu.m.
2. The silver powder particles of claim 1, said process further
comprising: (f) separating said silver powder particles from said
final aqueous solution; (g) washing said silver powder particles
with deionized water; and (h) drying said silver powder
particles.
3. The silver powder particles of claim 1, wherein in said process
said base used to adjust the pH of said acidic aqueous reducing and
particle modifier solution is NaOH.
4. The silver powder particles of claim 1, wherein in said process
said water soluble silver salt is silver nitrate and said reducing
agent is ascorbic acid.
5. The silver powder particles of claim 1, wherein in said process
said pH of said acidic aqueous reducing and particle modifier
solution is adjusted to between 3 and 5.
6. The silver powder particles of claim 1, wherein in said process
said acidic aqueous reducing and particle modifier solution further
comprising a metal colloid selected from the group consisting of
gold colloid and silver colloid.
7. The silver powder particles of claim 1, wherein in said process
said temperature in step (c) is in the range of 10.degree. C. to
35.degree. C. and the temperature in step (e) is in the range of
65.degree. C. to 75.degree. C.
8. A silver thick film paste comprising silver powder particles
made by a process comprising: (a) preparing an acidic aqueous
silver salt solution comprising a water soluble silver salt
dissolved in deionized water; (b) preparing an acidic aqueous
reducing and particle modifier solution comprising: (i) a reducing
agent selected from the group consisting of ascorbic acid,
ascorbates and mixtures thereof; (ii) nitric acid; (iii) maleic
acid; and (iv) deionized water, wherein the pH of said acidic
aqueous reducing and particle modifier solution is adjusted by the
addition of a base to between 2.5 and 6; (c) maintaining said
acidic aqueous silver salt solution and said acidic reducing and
surface morphology modifier solution at the same temperature,
wherein said temperature is in the range of 10.degree. C. to
65.degree. C., while stirring each said solution during and after
preparation; (d) adding said acidic aqueous silver salt solution to
said acidic aqueous reducing and particle modifier solution while
stirring to make a reaction mixture and maintaining the temperature
of said reaction mixture at said temperature of (c), wherein said
silver powder particles precipitate and are contained within the
resulting final aqueous solution; and (e) increasing the
temperature of said final aqueous solution to a temperature in the
range of 65.degree. C. to 80.degree. C. while continuing stirring;
one or both of said acidic aqueous silver salt solution and said
acidic aqueous reducing and particle modifier solution further
comprising gum arabic and said silver powder particles comprising
crystallites of size less than or equal to 32 nm as determined by
X-ray diffraction and the Scherrer formula; and said silver powder
particles having a d.sub.50 in the range of 0.5 to 3.5 .mu.m.
9. The silver thick film paste of claim 8, said process further
comprising: (f) separating said silver powder particles from said
final aqueous solution; (g) washing said silver powder particles
with deionized water; and (h) drying said silver powder
particles.
10. The silver thick film paste of claim 8, wherein in said process
said base used to adjust the pH of said acidic aqueous reducing and
particle modifier solution is NaOH.
11. The silver thick film paste of claim 8, wherein in said process
said water soluble silver salt is silver nitrate and said reducing
agent is ascorbic acid.
12. The silver thick film paste of claim 8, wherein in said process
said pH of said acidic aqueous reducing and particle modifier
solution is adjusted to between 3 and 5.
13. The silver thick film paste of claim 8, wherein in said process
said acidic aqueous reducing and particle modifier solution further
comprising a metal colloid selected from the group consisting of
gold colloid and silver colloid.
14. The silver thick film paste of claim 8, wherein in said process
said temperature in step (c) is in the range of 10.degree. C. to
35.degree. C. and the temperature in step (e) is in the range of
65.degree. C. to 75.degree. C.
15. A semiconductor device comprising an electrode formed by firing
a silver thick film paste containing silver particles, said silver
powder particles made by a process comprising: (a) preparing an
acidic aqueous silver salt solution comprising a water soluble
silver salt dissolved in deionized water; (b) preparing an acidic
aqueous reducing and particle modifier solution comprising: (i) a
reducing agent selected from the group consisting of ascorbic acid,
ascorbates and mixtures thereof; (ii) nitric acid; (iii) maleic
acid; and (iv) deionized water, wherein the pH of said acidic
aqueous reducing and particle modifier solution is adjusted by the
addition of a base to between 2.5 and 6; (c) maintaining said
acidic aqueous silver salt solution and said acidic reducing and
surface morphology modifier solution at the same temperature,
wherein said temperature is in the range of 10.degree. C. to
65.degree. C., while stirring each said solution during and after
preparation; (d) adding said acidic aqueous silver salt solution to
said acidic aqueous reducing and particle modifier solution while
stirring to make a reaction mixture and maintaining the temperature
of said reaction mixture at said temperature of (c), wherein said
silver powder particles precipitate and are contained within the
resulting final aqueous solution; and (e) increasing the
temperature of said final aqueous solution to a temperature in the
range of 65.degree. C. to 80.degree. C. while continuing stirring;
one or both of said acidic aqueous silver salt solution and said
acidic aqueous reducing and particle modifier solution further
comprising gum arabic and said silver powder particles comprising
crystallites of size less than or equal to 32 nm as determined by
X-ray diffraction and the Scherrer formula; and said silver powder
particles having a d.sub.50 in the range of 0.5 to 3.5 .mu.m.
16. The semiconductor device of claim 15, said process further
comprising: (f) separating said silver powder particles from said
final aqueous solution; (g) washing said silver powder particles
with deionized water; and (h) drying said silver powder
particles.
17. The semiconductor device of claim 15, wherein in said process
said base used to adjust the pH of said acidic aqueous reducing and
particle modifier solution is NaOH.
18. The semiconductor device of claim 15, wherein in said process
said water soluble silver salt is silver nitrate and said reducing
agent is ascorbic acid.
19. The semiconductor device of claim 15, wherein in said process
said pH of said acidic aqueous reducing and particle modifier
solution is adjusted to between 3 and 5.
20. The semiconductor device of claim 15, wherein in said process
said acidic aqueous reducing and particle modifier solution further
comprising a metal colloid selected from the group consisting of
gold colloid and silver colloid.
21. The semiconductor device of claim 15, wherein in said process
said temperature in step (c) is in the range of 10.degree. C. to
35.degree. C. and the temperature in step (e) is in the range of
65.degree. C. to 75.degree. C.
Description
FIELD OF THE INVENTION
[0001] The invention is directed to a process for making silver
powder particles with very small size crystallites. These silver
powder particles are particularly useful in electronic
applications.
BACKGROUND OF THE INVENTION
[0002] Silver powder is used in the electronics industry for the
manufacture of conductor thick film pastes. The thick film pastes
are screen printed onto substrates forming conductive circuit
patterns. These circuits are then dried and fired to volatilize the
liquid organic vehicle and sinter the silver particles.
[0003] Many processes currently used to manufacture metal powders
can be applied to the production of silver powders. For example,
thermal decomposition processes, electrochemical processes,
physical processes such as atomization or milling and chemical
reduction processes can be used. Thermal decomposition processes
tend to produce powders that are spongy, agglomerated, and very
porous whereas electrochemical processes produce powders that are
crystalline in shape and very large. Physical processes are
generally used to make flaked materials or very large spherical
particles. Chemical precipitation processes produce silver powders
with a range of sizes and shapes.
[0004] Silver powders used in electronic applications are generally
manufactured using chemical precipitation processes. Silver powder
is produced by chemical reduction in which an aqueous solution of a
soluble salt of silver is reacted with an appropriate reducing
agent under conditions such that silver powder can be precipitated.
Inorganic reducing agents including hydrazine, sulfite salts and
formate salts can produce powders which are very coarse in size,
are irregularly shaped and have a large particle size distribution
due to aggregation. Organic reducing agents such as alcohols,
sugars or aldehydes are used with alkali hydroxides to reduce
silver nitrate. The reduction reaction is very fast; hard to
control and produces a powder contaminated with residual alkali
ions. Although small in size (<1 .mu.m), these powders tend to
have an irregular shape with a wide distribution of particle sizes
that do not pack well. It is difficult to control the sintering of
these types of silver powders and they do not provide adequate line
resolution in thick film conductor circuits.
[0005] Therefore, there is a need for a process to produce silver
powder spherical particles comprising very small size crystallites
and having a d.sub.50 particle size in the range of 0.5 to 3.5
.mu.m to provide silver thick film paste with improved sintering
properties.
SUMMARY OF THE INVENTION
[0006] This invention provides a process for making silver powder
particles, the process comprising: [0007] (a) preparing an acidic
aqueous silver salt solution comprising a water soluble silver salt
dissolved in deionized water; [0008] (b) preparing an acidic
aqueous reducing and particle modifier solution comprising: [0009]
(i) a reducing agent selected from the group consisting of ascorbic
acid, ascorbates and mixtures; [0010] (ii) nitric acid; [0011]
(iii) maleic acid; and [0012] (iv) deionized water, wherein the pH
of the acidic aqueous reducing and particle modifier solution is
adjusted by the addition of a base to between 2.5 and 6; [0013] (c)
maintaining the acidic aqueous silver salt solution and the acidic
reducing and surface morphology modifier solution at the same
temperature, wherein the temperature is in the range of 10.degree.
C. to 65.degree. C., while stirring each the solution during and
after preparation; [0014] (d) adding the acidic aqueous silver salt
solution to the acidic aqueous reducing and particle modifier
solution while stirring to make a reaction mixture and maintaining
the temperature of the reaction mixture at the temperature of (c),
wherein the silver powder particles precipitate and are contained
within the resulting final aqueous solution; and [0015] (e)
increasing the temperature of the final aqueous solution to a
temperature in the range of 65.degree. C. to 80.degree. C. while
continuing stirring; one or both of the acidic aqueous silver salt
solution and the acidic aqueous reducing and particle modifier
solution further comprising gum arabic and the silver powder
particles comprising crystallites of size less than or equal to 32
nm as determined by X-ray diffraction and the Scherrer formula; and
the silver powder particles having a d.sub.50 in the range of 0.5
to 3.5 .mu.m.
[0016] Also provided is the above process further comprising:
[0017] (f) separating the silver powder particles from the final
aqueous solution; [0018] (g) washing the silver powder particles
with deionized water; and [0019] (h) drying the silver powder
particles.
[0020] Also provided are the silver powder particles made by the
above process, silver thick film paste made with the silver powder
particles and a semiconductor device comprising an electrode that
prior to firing comprises the silver thick film paste.
BRIEF DESCRIPTION OF THE FIGURES
[0021] FIG. 1 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in the
Example. The d.sub.50 particle size is 1.6 .mu.m.
[0022] FIG. 2 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in the
Comparative Experiment. The d.sub.50 particle size is 8.7
.mu.m.
DETAILED DESCRIPTION OF THE INVENTION
[0023] This invention provides a process for making
spherically-shaped silver powder particles with very small size
crystallites. A combination of gum arabic and maleic acid is used
with the reduction of a silver salt with ascorbic acid to control
the crystallite size. The process is a reductive process in which
silver particles comprising crystallites less than 30 nm in size as
determined by X-ray diffraction and the Scherrer formula are
precipitated by adding together an acidic aqueous solution of a
water soluble silver salt and an acidic aqueous reducing and
particle modifier solution containing a reducing agent, nitric
acid, gum arabic, and maleic acid. In addition, the pH of the
acidic aqueous reducing and particle modifier solution is adjusted
to between 2.5 and 6 by the addition of a base, preferably NaOH. In
one group of embodiments the pH of the acidic aqueous reducing and
particle modifier solution is adjusted to between 3 and 5 and in
another group of embodiments the pH is adjusted to 4.
[0024] The acidic aqueous silver salt solution is prepared by
adding a water soluble silver salt to deionized water. Any water
soluble silver salt, e.g., silver nitrate, silver phosphate, and
silver sulfate, can be used. Silver nitrate is preferred. No
complexing agents are used which could provide side reactions that
affect the reduction and type of particles produced. Nitric acid
can be added to increase the acidity.
[0025] The process can be run at concentrations up to 1.2 moles of
silver per liter of final aqueous solution. It is preferred to run
the process at concentrations of 0.47 to 0.8 moles of silver per
liter of final aqueous solution. These relatively high
concentrations of silver make the manufacturing process cost
effective.
[0026] The acidic aqueous reducing and particle modifier solution
is prepared by adding the various components, i.e., a reducing
agent, maleic acid and nitric acid, to deionized water. Suitable
reducing agents for the process are ascorbic acids such L-ascorbic
acid and D-ascorbic acid and related ascorbates such as sodium
ascorbate. Ascorbic acid is preferred.
[0027] One or both of the acidic aqueous silver salt solution and
the acidic aqueous reducing and particle modifier solution further
contain gum arabic.
[0028] In one embodiment, the acidic aqueous reducing and particle
modifier solution also contains a metal colloid selected from the
group consisting of gold colloid and silver colloid. A suitable
metal colloid is gold colloid or silver colloid. Gold colloid is
preferred.
[0029] The components can be added to the deionized water in
various orders. In one embodiment, gum arabic is the first
component added to the deionized water whether it is in the
preparation of the acidic aqueous silver salt solution or the
acidic aqueous reducing and particle modifier solution. The gum
arabic is added to the deionized water and then stirred for a
period of time before adding the other components. In the Examples,
for the acidic aqueous silver salt solution 10 g of gum arabic was
added to 125 g or 250 g of deionized water and for the acidic
aqueous reducing and particle modifier solution 14 g of gum arabic
was added to 375 g of deionized water and the mixture stirred for
an hour before adding any other component. Times for stirring
smaller or larger quantities could be adjusted accordingly. In this
embodiment nitric acid acid, maleic acid, ascorbic acid and gold
colloid solution are added in that order to form the acidic aqueous
reducing and particle modifier solution. Stirring for a time as
indicated above should occur following addition of the gum arabic
no matter the order of adding the components.
[0030] The order of preparing the acidic aqueous silver salt
solution and the acidic reducing and particle modifier solution is
no important. The acidic aqueous silver salt solution can be
prepared before, after, or contemporaneously with the acidic
reducing and particle modifier solution. The acidic aqueous silver
salt solution is added to the acidic reducing and particle modifier
solution. The addition is carried out slowly. For example, with the
quantities of solutions used in the Examples, i.e., 125 and 250 g
of deionized water for the acidic aqueous silver salt solution and
375 and 750 g of deionized water for the acidic aqueous reducing
and particle modifier solution, the acidic aqueous silver salt
solution was added to the acidic reducing and particle modifier
solution over a period of one hour. Smaller quantities could be
mixed over a shorter time interval and larger quantities over a
longer time period. The reaction mixture is stirred during the
addition.
[0031] In this process the acidic aqueous silver salt solution and
the acidic reducing and particle modifier solution are both
maintained at the same temperature, i.e., a temperature in the
range of 10.degree. C. to 65.degree. C. and each solution is
stirred until they are mixed. In some embodiments the solutions are
maintained at a temperature in the range of 10.degree. C. to
35.degree. C. In one such embodiment the solutions are maintained
at 25.degree. C. When the two solutions are mixed to form the
reaction mixture, the reaction mixture is maintained at that same
temperature.
[0032] The mixture of the acidic aqueous silver salt solution and
the acidic reducing and particle modifier solution results in the
precipitation of the silver particles that are contained within the
resulting final aqueous solution. Following precipitation, the
temperature of the final aqueous solution is heated to a
temperature in the range of 65.degree. C. to 80.degree. C. In some
embodiments the temperature is in the range of 65.degree. C. to
75.degree. C. In one such embodiment the temperature is 70.degree.
C. The final aqueous solution containing the silver particles is
maintained at this temperature for about an hour and is stirred
during that time.
[0033] The silver particles are then separated from the final
aqueous solution by filtration or other suitable liquid-solid
separation operation and the solids are washed with deionized water
until the conductivity of the wash water is 100 microSiemans or
less. The silver particles are then dried.
[0034] The silver powder particles made by the process of the
invention have a particular set of physical characteristics. The
silver particles are described herein as spherically-shaped. It can
be seen from the scanning electron microscope (SEM) image of FIG. 1
that the particles are generally spherical in shape but are not
perfect spheres. The average particle size determined from the SEM
image is in the range of 1 to 2.5 .mu.m. The silver powder
particles are comprised of crystallites less than or equal to 32 nm
in size as determined by X-ray diffraction and the Scherrer
formula. In one group of embodiments the crystallites are less than
or equal to 30 nm in size. The silver powder particles have a
d.sub.50 in the range of 0.5 to 3.5 .mu.m and a size distribution
such that (d.sub.90-d.sub.10)/d.sub.50<2.2. The quantity
(d.sub.90-d.sub.10)/d.sub.50 is a measure of the size distribution
of the particles. Since (d.sub.90-d.sub.10)/d.sub.50<2.2, it
indicates the narrow size distribution of the particles, i.e., the
uniformity of particle size and the correspondly highly dispersible
nature of the silver powders comprising these particles. The silver
particles have a solids content of in excess of 99% as determined
by thermogravimetric analysis. Thermomechanical analysis (TMA) is
used herein as an indicator of sinterability. A pellet was formed
from the silver particles and subjected to a temperature regime.
The observed per cent shrinkage, i.e., the dimensional change of
the pellet, is a measure of the sinterability of the particles. The
results are presented herein as TMA equal to the percent
dimensional change, a negative per cent change since it represents
a shrinkage. The silver particles of the invention showed TMA
dimensional changes, i.e., shrinkages, of at least -10%.
[0035] The silver powder particles made by this invention can be
used in thick film paste applications, including thick films for
front side metallization of photovoltaic solar cells and thick
films for other semiconductor devices.
[0036] This invention also provides the silver powder particles
made by the process of this invention. These silver powder
particles can be used in thick film paste. The structure of these
silver particles will lend them to be more readily sintered and
provide improved thick film conductors. Also provided is a
semiconductor device, e.g., a solar cell, comprising an electrode
that prior to firing comprises the silver thick film paste.
Silver Thick Film Paste
[0037] This invention also provides a silver thick film paste
comprised of the silver powder particles made by the process of
this invention and glass frit dispersed in an organic medium. As
used herein, "thick film paste" refers to a composition which after
being deposited on a substrate and fired has a thickness of 1 to
100 .mu.m.
[0038] The glass frit compositions are described herein as
including percentages of certain components. The percentages are
the percentages of the components used in the starling material
that was subsequently processed as described herein to form a glass
composition. The composition contains certain components and the
percentages of those components are expressed as a percentage of
the corresponding oxide or fluoride form. The weight percentages of
the glass frit components are based on the total weight of the
glass composition. A certain portion of volatile species may be
released during the process of making the glass. An example of a
volatile species is oxygen.
[0039] Various glass frit compositions are useful in the silver
thick film pastes of the invention. The glass frit used has a
softening point of 300 to 600.degree. C. The glass frit
compositions described herein are not limiting. Minor substitutions
of additional ingredients can be made without substantially
changing the desired properties of the glass composition. For
example, substitutions of glass formers such as 0-3 wt %
P.sub.2O.sub.5, 0-3 wt % GeO.sub.2 and 0-3 wt % V.sub.2O.sub.5 can
be used either individually or in combination to achieve similar
performance.
[0040] The glass frit compositions can also contain one or more
fluorine-containing components such as salts of fluorine, fluorides
and metal oxyfluoride compounds. Such fluorine-containing
components include, but are not limited to BiF.sub.3, AlF.sub.3,
NaF, LiF, KF, CsF, PbF.sub.2, ZrF.sub.4, TiF.sub.4 and
ZnF.sub.2.
[0041] Exemplary lead free glass compositions contain one or more
of SiO.sub.2, B.sub.2O.sub.3, Al.sub.2O.sub.3, Bi.sub.2O.sub.3,
BiF.sub.3, ZnO, ZrO.sub.2, CuO, Na.sub.2O, NaF, Li.sub.2O, LiF,
K.sub.2O, and KF. In various embodiments the compositions comprise
the following oxide constituents in the compositional ranges, the
SiO.sub.2 is 17 to 26 wt %, 19 to 24 wt %, or 20 to 22 wt %; the
B.sub.2O.sub.3 is 2 to 9 wt %, 3 to 7 wt %; or 3 to 4 wt %; the
Al.sub.2O.sub.3 is 0.1 to 5 wt %, 0.2 to 2.5 wt %, or 0.2 to 0.3 wt
%; the Bi.sub.2O.sub.3 is 0 to 65 wt %, 25 to 64 wt %, or 46 to 64
wt %; the BiF.sub.3 is 0 to 67 wt %, 0 to 43 wt %, or 0 to 19 wt %;
the ZrO.sub.2 is 0 to 5 wt %, 2 to 5 wt %, or 4 to 5 wt %; the
TiO.sub.2 is 1 to 7 wt %, 1 to 5 wt %, or 1 to 3 wt %; CuO is 0 to
3 wt % or 2 to 3 wt %; Na.sub.2O is 0 to 2 wt % or 1 to 2 wt %; NaF
is 0 to 3 wt % or 2 to 3 wt %; Li.sub.2O is 0 to 2 wt % or 1 to 2
wt %; and LiF is 0 to 3 wt % or 2 to 3 wt %. Some or all of the
Na.sub.2O or Li.sub.2O can be replaced with K.sub.2O and some or
all of the NaF or LiF can be replaced with KF to create a glass
with properties similar to the compositions listed above.
[0042] In other embodiments, the glass frit compositions can
include one or more of a third set of components: CeO.sub.2,
SnO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, MoO.sub.3,
WO.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, FeO,
HfO.sub.2, Cr.sub.2O.sub.3, CdO, Nb.sub.2O.sub.5, Ag.sub.2O,
Sb.sub.2O.sub.3, and metal halides (e.g. NaCl, KBr, NaI).
[0043] Exemplary lead containing glass compositions comprise the
following oxide constituents in the compositional range of 0-36 wt
% SiO.sub.2, 0-9 wt % Al.sub.2O.sub.3, 0-19 wt % B.sub.2O.sub.3,
16-84 wt % PbO, 0-4 wt % CuO, 0-24 wt % ZnO, 0-52 wt %
Bi.sub.2O.sub.3, 0-8 wt % ZrO.sub.2, 0-20 wt % TiO.sub.2, 0-5 wt %
P.sub.2O.sub.5, and 3-34 wt % PbF.sub.2. In other embodiments
relating to glasses containing bismuth oxide, the glass frit
composition contains 4-26 wt % SiO.sub.2, 0-1 wt % Al.sub.2O.sub.3,
0-8 wt % B.sub.2O.sub.3, 20-52 wt % PbO, 0-4 wt % ZnO, 6-52 wt %
Bi.sub.2O.sub.3, 2-7 wt % TiO.sub.2, 5-29 wt % PbF.sub.2, 0-1 wt %
Na.sub.2O and 0-1 wt % Li.sub.2O. In still other embodiments
relating to glasses containing 15-25 wt % ZnO, the glass frit
comprises 5-36 wt % SiO.sub.2, 0-9 wt % Al.sub.2O.sub.3, 0-19 wt %
B.sub.2O.sub.3, 17-64 wt % PbO, 0-39 wt % Bi.sub.2O.sub.3, 0-6 wt %
TiO.sub.2, 0-5 wt % P.sub.2O.sub.5 and 6-29 wt % PbF.sub.2. In
various of these embodiments containing ZnO, the glass frit
compositions comprises 5-15 wt % SiO.sub.2 and/or 20-29 wt %
PbF.sub.2 and/or 0-3 wt % ZrO.sub.2 or 0.1-2.5 wt % ZrO.sub.2.
Embodiments containing copper oxide and/or alkali modifiers
comprise 25-35 wt % SiO.sub.2, 0-4 wt % Al.sub.2O.sub.3, 3-19 wt %
B.sub.2O.sub.3, 17-52 wt % PbO, 0-12 wt % ZnO, 0-7 wt %
Bi.sub.2O.sub.3, 0-5 wt % TiO.sub.2, 7-22 wt % PbF.sub.2, 0-3 wt %
CuO, 0-4 wt % Na.sub.2O and 0-1 wt % Li.sub.2O.
[0044] An exemplary method for producing the glass frits described
herein is by conventional glass making techniques. Ingredients are
weighed then mixed in the desired proportions and heated in a
furnace to form a melt in platinum alloy crucibles or other
suitable metal or ceramic crucibles. As indicated above, oxides as
well as fluoride or oxyfluoride salts can be used as raw materials.
Alternatively, salts, such as nitrate, nitrites, carbonate, or
hydrates, which decompose into oxide, fluorides, or oxyfluorides at
temperature below the glass melting temperature can be used as raw
materials. Heating is conducted to a peak temperature of typically
800-1400.degree. C. and for a time such that the melt becomes
entirely liquid, homogeneous, and free of any residual
decomposition products of the raw materials. The molten glass is
then quenched between counter rotating stainless steel rollers to
form a 10-15 mil thick platelet of glass. The resulting glass
platelet was then milled to form a glass frit powder with its 50%
volume distribution set between to a desired target (e.g. 0.8-1.5
.mu.m). Alternative synthesis techniques such as water quenching,
sol-gel, spray pyrolysis, or others appropriate for making powder
forms of glass can be employed.
[0045] The organic medium used in the silver hick film paste is a
solution of a polymer in a solvent. The organic medium can also
contain thickeners, stabilizers, surfactants and/or other common
additives. In one embodiment, the polymer is ethyl cellulose. Other
exemplary polymers include ethylhydroxyethyl cellulose, wood rosin,
mixtures of ethyl cellulose and phenolic resins, polymethacrylates
of lower alcohols, and monobutyl ether of ethylene glycol
monoacetate, or mixtures thereof. The solvents useful in the
organic medium of the silver thick film paste compositions include
ester alcohols and terpenes such as alpha- or beta-terpineol or
mixtures thereof with other solvents such as kerosene,
dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene
glycol and high boiling alcohols and alcohol esters. The organic
medium can also contain volatile liquids for promoting rapid
hardening after application on the substrate.
[0046] The thick film silver composition is adjusted to a
predetermined, screen-printable viscosity with the organic
medium.
[0047] The inorganic components, i.e., the silver powder particles
and the glass frit are typically mixed with the organic medium by
mechanical mixing to form a viscous paste composition.
[0048] The ratio of organic medium in the silver thick film paste
composition to the inorganic components in the dispersion is
dependent on the method of applying the paste and the kind of
organic medium used, and it can vary. The dispersion will typically
contain 70 to 95 wt % of inorganic components and 5 to 30 wt % of
organic medium in order to obtain good wetting. The weight percents
(wt %) used herein are based on the total weight of the silver
thick film paste composition. Typically, the polymer present in the
organic medium is in the range of 8 wt % to 11 wt % of the weight
of the total composition.
[0049] In one embodiment, the silver thick film paste contains 65
to 90 wt % silver powder, 0.1 to 8 wt % glass frit and 5 to 30 wt %
organic medium. In another embodiment the silver thick film paste
contains 70 to 85 wt % silver powder, 1 to 6 wt % glass frit and 10
to 25 wt % organic medium. In still another embodiment the silver
thick film paste contains 78 to 83 wt % silver powder, 2 to 5 wt %
glass frit and 13 to 20 wt % organic medium.
Semiconductor Device; Solar Cell
[0050] The invention also provides a method of making a
semiconductor device, e.g., a solar cell or a photodiode. The
semiconductor device has an electrode, e.g., a front side electrode
of a solar cell or a photodiode, wherein prior to firing the
electrode is comprised of a silver thick film paste of the
invention.
[0051] The method of manufacturing a semiconductor device,
comprises the steps of: [0052] (a) providing a semiconductor
substrate, one or more insulating films, and the silver thick film
paste of the invention; [0053] (b) applying the insulating film to
the semiconductor substrate, [0054] (c) applying the silver thick
film paste to the insulating film on the semiconductor substrate,
and [0055] (d) firing the semiconductor substrate, the insulating
film and the silver thick film paste composition.
[0056] Exemplary semiconductor substrates useful in the methods and
devices described herein include, but are not limited to,
single-crystal silicon, multicrystalline silicon, and ribbon
silicon. The semiconductor substrate may be doped with phosphorus
and boron to form a p/n junction.
[0057] The semiconductor substrates can vary in size
(length.times.width) and thickness. As an example, the thickness of
the semiconductor substrate is 50 to 500 .mu.m; 100 to 300 .mu.m;
or 140 to 200 .mu.m. The length and width of the semiconductor
substrate are each 100 to 250 mm; 125 to 200 mm; or 125 to 156
mm.
[0058] Typically an anti-reflection coating is formed on the front
side of a solar cell. Exemplary anti-refection coating materials
useful in the methods and devices described herein include, but are
not limited to: silicon nitride, silicon oxide, titanium oxide,
SiN.sub.x:H, hydrogenated amorphous silicon nitride, and silicon
oxide/titanium oxide film. The coating can be formed by plasma
enhanced chemical vapor deposition (PECVD), CVD, and/or other known
techniques known. In an embodiment in which the coating is silicon
nitride, the silicon nitride film can be formed by PECVD, thermal
CVD, or physical vapor deposition (PVD). In an embodiment in which
the insulating film is silicon oxide, the silicon oxide film can be
formed by thermal oxidation, thermal CVD, plasma CVD, or PVD.
[0059] The silver thick film paste of the invention can be applied
to the anti-reflective coated semiconductor substrate by a variety
of methods such as screen-printing, ink-jet printing, coextrusion,
syringe dispensing, direct writing, and aerosol ink jet printing.
The paste can be applied in a pattern and in a predetermined shape
and at a predetermined position. In one embodiment, the paste is
used to form both the conductive fingers and busbars of the
front-side electrode. In such an embodiment, the width of the lines
of the conductive fingers are 20 to 200 .mu.m, 40 to 150 .mu.m, or
60 to 100 .mu.m and the thickness of the lines of the conductive
fingers are 5 to 50 .mu.m, 10 to 35 .mu.m, or 15 to 30 .mu.m.
[0060] The paste coated on the ARC-coated semiconductor substrate
can be dried, for example, for 0.5 to 10 minutes during which time
the volatile solvents and organics of the organic medium are
removed.
[0061] The dried paste is fired by heating to a maximum temperature
of between 500 and 940.degree. C. for a duration of 1 second to 2
minutes. In one embodiment, the maximum silicon wafer temperature
reached during firing ranges from 650 to 80.degree. C. for a
duration of 1 to 10 seconds. In a further embodiment, the electrode
formed from the silver thick film paste is fired in an atmosphere
composed of a mixed gas of oxygen and nitrogen. In another
embodiment, the electrode formed from the conductive thick film
paste is fired above the organic medium removal temperature in an
inert atmosphere not containing oxygen. This firing process removes
any remaining organic medium and sinters the glass frit with the
silver powder and any metal oxide present to form an electrode.
Typically, the burnout and firing is carried out in a belt furnace.
The temperature range in the burnout zone, during which time the
remaining organic medium is removed, is between 500 and 700.degree.
C. The temperature in the firing zone is between 860 and
940.degree. C.
[0062] During firing, the fired electrode, preferably the fingers,
reacts with and penetrates the anti-reflective coating, thereby
making electrical contact with the silicon substrate.
[0063] In a further embodiment, prior to firing, other conductive
and device enhancing materials are applied to the back side of the
semiconductor device and cofired or sequentially fired with the
paste compositions of the invention. The materials serve as
electrical contacts, passivating layers, and solderable tabbing
areas.
[0064] In one embodiment, the back side conductive material
contains aluminum or aluminum and silver.
[0065] In a still further embodiment the materials applied to the
opposite type region of the device are adjacent to the materials
described herein due to the p and n region being formed side by
side. Such devices place all metal contact materials on the non
illuminated back side of the device to maximize incident light on
the illuminated front side.
Measurements
[0066] The particle size distribution numbers (d.sub.10, d.sub.50,
d.sub.90) used herein are based on a volume (mass) distribution.
The particle sizes were measured using a Microtrac.RTM. Particle
Size Analyzer from Leeds and Northrup. The d.sub.10, d.sub.50 and
d.sub.90 are the equivalent diameters that represent the 10th
percentile, the median or 50th percentile and the 90th percentile
of the particle sizes, respectively, as measured by volume. That
is, the d.sub.50 (d.sub.10, d.sub.90) is a value on the
distribution such that 50 volume % (10 volume %, 90 volume %) of
the particles have an equivalent diameter of this value or
less.
[0067] The solids content of the silver particles was determined by
thermogravimetric analysis, i.e., by a weight loss method after
heating at 850.degree. C. for 10 minutes.
[0068] The size of the crystallites making up the silver powder
particles was determined by X-ray diffraction and the Scherrer
formula. The X-ray diffractometer used was a Rigaku Rint RAD-rb.
The Cu target provided a wavelength of 0.15405 nm. The Bragg plane
was the (111). In the Scherrer formula:
L (111)=0.94.lamda./(.beta. cos .theta.),
L is the crystallite size, .lamda. is the wavelength, .beta. is the
line broadening at half the peak maximum intensity (full width-half
maximum) in radians and .theta. is the Bragg angle.
[0069] The TMA was carried out as follows. A pellet was formed by
pouring 0.9-1.1 g of silver powder into a die and onto a
cylindrical metal insert serving as the bottom. A second
cylindrical metal is inserted into the die on top of the silver
powder so that the powder is sandwiched between the cylindrical
metal inserts. The top insert is then firmly tapped 6 times with a
mallet. The pellet is placed in a Bargal TMA:Q400 analyzer which
measured the dimensional change as a function of temperature. The
temperature was raised from room temperature at a rate of
10.degree. C./min to 650.degree. C. and the pellet is maintained at
that temperature for 10 minutes. The final per cent dimensional
change is reported as the TMA for that powder pellet. A negative
value indicates shrinkage and a positive value expansion of the
pellet.
EXAMPLES
[0070] The following Examples and discussion are offered to further
illustrate, but not limit the process of this invention.
Example 1
[0071] The acidic aqueous silver salt solution was prepared by
adding 10 g of gum arabic to 250 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 80 g of silver nitrate was then added. This solution was
kept at 25.degree. C. while continuously stirring.
[0072] The acidic reducing and particle modifier solution was
prepared by adding 5 g of nitric acid (70% w/w), 5 g maleic acid,
45 g ascorbic acid and 1 g of gold colloid solution to 750 g of
deionized water in a separate container from the acidic aqueous
silver salt solution. The pH was then adjusted to 4 by adding NaOH.
This solution was kept at 25.degree. C. while continuously
stirring.
[0073] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0074] The final aqueous solution was filtered and the silver
powder collected. The silver powder was washed with deionized water
until a conductivity of the wash water was less than or equal to
100 microSiemans. The silver powder was dried for 24 hours at
35.degree. C. in a freeze dryer.
[0075] As shown in the scanning electron microscope image of FIG.
1, the silver powder was comprised of spherically-shaped silver
particles. The size L of the crystallites in the particles was 27.1
nm. d.sub.10, d.sub.50, and d.sub.90 were 0.6 mm, 1.6 mm and 3.7
mm, respectively. The silver particles were 99.6% solids. The
TMA=-18.
Example 2
[0076] The acidic aqueous silver salt solution was prepared by
adding 10 g of gum arabic to 125 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 80 g of silver nitrate was then added. This solution was
kept at 25.degree. C. while continuously stirring.
[0077] The acidic reducing and particle modifier solution was
prepared by adding 14 g of gum arabic to 375 g of deionized water
in a separate container from the silver salt solution. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 3 g of nitric acid (70% w/w), 5 g maleic acid, 45 g
ascorbic acid and 1 g of gold colloid solution was then added to
the reducing and particle modifier solution. The pH was then
adjusted to 4 by adding NaOH. This solution was kept at 25.degree.
C. while continuously stirring.
[0078] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0079] The final aqueous solution was filtered and the silver
powder collected. The silver powder was washed with deionized water
until a conductivity of the wash water was less than or equal to
100 microSiemans. The silver powder was dried for 24 hours at
35.degree. C. in a freeze dryer.
[0080] The size L of the crystallites in the particles was 30.2 nm.
d.sub.10, d.sub.50, and d.sub.90 were 0.5 mm, 1.0 mm and 2.0 mm,
respectively. The silver particles were 99.6% solids. The
TMA=-17.
Example 3
[0081] The acidic aqueous silver salt solution was prepared by
adding 10 g of gum arabic to 125 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 80 g of silver nitrate was then added. This solution was
kept at 25.degree. C. while continuously stirring.
[0082] The acidic reducing and particle modifier solution was
prepared by adding 14 g of gum arabic to 375 g of deionized water
in a separate container from the silver salt solution. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 3 g of nitric acid (70% w/w), 1 g maleic acid, 45 g
ascorbic acid and 1 g of gold colloid solution was then added to
the reducing and particle modifier solution. The pH was then
adjusted to 4 by adding NaOH. This solution was kept at 25.degree.
C. while continuously stirring.
[0083] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0084] The final aqueous solution was filtered and the silver
powder collected. The silver powder was washed with deionized water
until a conductivity of the wash water was less than or equal to
100 microSiemans. The silver powder was dried for 24 hours at
35.degree. C. in a freeze dryer.
[0085] The size L of the crystallites in the particles was 26.2 nm.
d.sub.10, d.sub.50, and d.sub.90 were 0.3 mm, 0.7 mm and 1.8 mm,
respectively. The silver particles were 99.6% solids. The
TMA=-17.
Comparative Experiment
[0086] An acidic aqueous silver salt solution was prepared by
adding 14 g of gum arabic to 250 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. Then 80 g of silver nitrate was added. This solution was
kept at 25.degree. C. while continuously stirring.
[0087] An acidic reducing and particle modifier solution was
prepared by adding 5 g of nitric acid (70% w/w), 5 g maleic acid,
45 g ascorbic acid and 1 g of gold colloid solution to 750 g of
deionized water in a separate container from the silver nitrate
solution. This solution was kept at 25.degree. C. while
continuously stirring. The pH was not adjusted to between 3 and
5.
[0088] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0089] The silver particles were recovered as described in Example
1.
[0090] As shown in the scanning electron microscope image of FIG.
2, the silver powder was comprised of very irregular-shaped
relatively large silver particles. The size L of the crystallites
in the particles was 47.6 nm. d.sub.10, d.sub.50, and d.sub.90 were
5.4 .mu.m, 8.7 .mu.m and 14.0 .mu.m, respectively. The
TMA=-7.5%.
* * * * *