U.S. patent application number 12/022113 was filed with the patent office on 2009-07-30 for indium electroplating baths for thin layer deposition.
Invention is credited to Serdar Aksu, Bulent M. Basol, Jiaxiong Wang.
Application Number | 20090188808 12/022113 |
Document ID | / |
Family ID | 40898114 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090188808 |
Kind Code |
A1 |
Wang; Jiaxiong ; et
al. |
July 30, 2009 |
INDIUM ELECTROPLATING BATHS FOR THIN LAYER DEPOSITION
Abstract
Indium (In) electroplating solutions which are used to deposit
compositionally pure, uniform, substantially defect free and smooth
In films with near 100% plating efficiency and repeatability. In
one embodiment the plating solution includes an In source, citric
acid and its conjugate pair salt and a solvent. At a pH value of
below 4.0, sub-micron thick In layers with close to 100% purity at
close to 100% plating efficiency are produced. Such In layers are
used in fabrication of electronic devices such as thin film solar
cells.
Inventors: |
Wang; Jiaxiong; (Castro
Valley, CA) ; Aksu; Serdar; (Milpitas, CA) ;
Basol; Bulent M.; (Manhattan Beach, CA) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Family ID: |
40898114 |
Appl. No.: |
12/022113 |
Filed: |
January 29, 2008 |
Current U.S.
Class: |
205/261 |
Current CPC
Class: |
H01L 31/0749 20130101;
Y02P 70/50 20151101; H01L 31/0322 20130101; Y02P 70/521 20151101;
Y02E 10/541 20130101; C25D 3/54 20130101 |
Class at
Publication: |
205/261 |
International
Class: |
C25D 3/02 20060101
C25D003/02 |
Claims
1. An electroplating solution for application of a substantially
pure indium (In) film onto a conductive surface at high plating
efficiency, comprising: a solvent; an In source providing In ions
to the solvent with an In ion concentration of at least about 0.05
M; and a weak acid and a conjugate pair salt of the weak acid,
wherein the pH value of the electroplating solution is below about
4.0, and wherein the weak acid is selected from the group
consisting of citric acid, acetic acid, formic acid, thioacetic
acid, glycolic acid, lactic acid, ascorbic acid, malic acid,
butanoic acid, and pentanoic acid.
2. The solution of claim 1 wherein the weak acid is citric acid and
the pH value is below 3.5.
3. The solution of claim 2, wherein the In source comprises at
least one of In-chloride, In-sulfate, In-acetate, In-carbonate,
In-nitrate, In-perchlorate, In-phosphate, In-oxide and
In-hydroxide.
4. The solution of claim 3, wherein the conjugate salt is at least
one of sodium citrate, lithium citrate, potassium citrate, and an
organically modified citrate, wherein the organically modified
citrate comprises a citrate moiety such that one or more organic
groups replaces (a) one or more hydrogens that are directly
connected to a carbon or oxygen bonded to carbon 2 of the citrate
moiety, or (b) the hydroxyl group bonded to carbon 2 of the citrate
moiety.
5. The solution of claim 2, wherein the concentration of the In
ions is at least 0.1M.
6. The solution of claim 5, wherein the pH value is below 2.5.
7. The solution of claim 5, wherein the solvent comprises
water.
8. The solution of claim 2 such that it comprises no organic
additives.
9. The solution of claim 1, wherein the solvent comprises
water.
10. The solution of claim 1, wherein the In source comprises at
least one of In-chloride, In-sulfate, In-acetate, In-carbonate,
In-nitrate, In-perchlorate, In-phosphate, In-oxide and
In-hydroxide.
11. A method of obtaining a substantially pure and substantially
defect-free In film on a surface of a conductor comprising the
steps of: providing a solution with a pH value below 4 that
includes a solvent, an In source providing In ions to the solvent
with an In ion concentration of at least 0.05 M, citric acid and a
conjugate pair salt of citric acid; applying the solution onto an
anode and the surface of the conductor, establishing a potential
difference between the anode and the conductor, and
electrodepositing the substantially pure In film on the surface of
the conductor.
12. The method of claim 11 wherein the surface of the conductor
comprises copper.
13. The method of claim 11 wherein the surface of the conductor
comprises gallium.
14. The method of claim 11 further comprising the step of
controlling the temperature of the solution within the range of
10-60.degree. C. before the step of electrodeposition and wherein
the In source comprises at least one of In-chloride, In-sulfate,
In-acetate, In-carbonate, In-nitrate, In-perchlorate, In-phosphate,
In-oxide and In-hydroxide.
15. The method of claim 14 wherein the In source comprises at least
one of In-chloride, In-sulfate, In-acetate, In-carbonate,
In-nitrate, In-perchlorate, In-phosphate, In-oxide and
In-hydroxide.
16. The method of claim 15 wherein the conjugate pair salt of
citric acid is at least one of sodium citrate, lithium citrate,
potassium citrate, and an organically modified citrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field
[0002] This invention relates to indium (In) electroplating methods
and chemistries to deposit uniform, defect free and smooth In thin
films with high plating efficiency and repeatability. These In thin
films may be used in fabrication of electronic and semiconductor
devices such as thin film solar cells.
[0003] 2. Description of the Related Art
[0004] Indium (In) is an important metal used in semiconductor and
electronics industries. Indium is generally recovered as a
by-product from zinc and lead-zinc production. Electrodeposition is
a common method to recover bulk In. A number of In electroplating
baths have also been formulated to deposit layers of In on various
conductive substrates. For example, In plating baths containing
sulfamate (U.S. Pat. No. 2,458,839), cyanide (U.S. Pat. No.
2,497,988), alkali hydroxides (U.S. Pat. No. 2,287,948), tartaric
acid (U.S. Pat. No. 2,423,624), and fluoborate (U.S. Pat. No.
3,812,020, U.S. Pat. No. 2,409,983) have been developed and
commercialized. Some details on such chemistries may be found in
the review paper of Walsh and Gabe (Surface Technology, vol: 8,
page: 87-99, 1979). In addition to the common aqueous In solutions,
In can also be plated from organic solvent based solutions such as
formamide (U.S. Pat. No. 2,452,361). Some of the In alloys that can
be electroplated include indium-silver (U.S. Pat. No. 1,935,630),
indium-tin (U.S. Pat. No. 6,331,240) and indium-nickel (U.S. Pat.
No. 4,626,324, U.S. Pat. No. 4,686,015). Although it is possible to
deposit In and In alloy layers using various prior-art plating
chemistries, such layers may not be suitable for preparation of
smooth, uniform, defect-free In films with sub-micron thickness
that are needed for thin film solar cell applications. Common
defects in electroplated In layers include holes, or pinholes as
will be described later.
[0005] One recent application of electroplated In films is in
formation of Cu(In,Ga)(Se,S).sub.2 or CIGS(S) films which are the
most advanced compound absorbers for polycrystalline thin film
solar cells. A thin In layer, for example, may be electroplated on
a Cu layer. The Cu/In precursor stack thus obtained may then be
reacted with Se to form a CuInSe.sub.2 or CIS absorber. Reaction
with S would form a CIS(S) layer. For CIGS or CIGS(S) formation Ga
is also included in the precursor stack. The CIGS(S) absorber may
be used in fabrication of thin film solar cells with a structure of
"contact/CIGS(S)/buffer layer/TCO", where contact is a metallic
layer such as a Mo layer, the buffer layer is a thin transparent
film such as a CdS film and TCO is a transparent conductive oxide
layer such as a ZnO and/or ITO layer.
[0006] In a thin film solar cell employing a Group IBIIIAVIA
compound absorber such as CIS or CIGS, the cell efficiency is a
strong function of the molar ratio of IB/IIIA. If there are more
than one Group IIIA materials in the composition, the relative
amounts or molar ratios of these IIIA elements also affect the
properties. For a Cu(In,Ga)(S,Se).sub.2 or CIGS(S) absorber layer,
for example, the efficiency of the device is a function of the
molar ratio of Cu/(In+Ga). Furthermore, some of the important
parameters of the cell, such as its open circuit voltage, short
circuit current and fill factor vary with the molar ratio of the
IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for
good device performance Cu/(In+Ga) molar ratio is kept at or below
1.0. For ratios close to or higher than 1.0, a low resistance
copper selenide phase may form, which may introduce electrical
shorts within the solar cells. As the Ga/(Ga+In) molar ratio
increases, on the other hand, the optical bandgap of the absorber
layer increases and therefore the open circuit voltage of the solar
cell increases while the short circuit current typically may
decrease. It is important for a thin film deposition process to
have the capability of controlling both the molar ratio of IB/IIIA,
and the molar ratios of the Group IIIA components in the
composition. Therefore, if electrodeposition is used to introduce
the In into the film composition, it is essential that the
electroplated In films have smooth morphology and be free of
defects such as pinholes. It should be appreciated that any
protrusions in the In film will cause an In-rich region in the
CIGS(S) absorber obtained after reaction with Se and/or S.
Alternately, any pinholes or regions with thinner In would yield
Cu-rich or In-poor regions which, after reaction with Se and/or S
would turn into copper selenide-rich regions with low resistance,
which would introduce electrical shunt between the contact layer
and TCO layer of the solar cell, reducing the conversion efficiency
of the device. Such non-uniform layers cannot be used for high
efficiency solar cell manufacturing.
[0007] One other factor that is also important for manufacturing
devices such as solar cells utilizing electroplating technique is
the necessity for high electroplating efficiency and for stability
of this efficiency as a function of time. Since electroplated In
layer thickness depends directly on the plating efficiency and
since the In layer thickness determines the Cu/(In+Ga) and
Ga/(In+Ga) ratios which are crucial for CIGS(S) type solar cell
operation, controlling and increasing the plating efficiency
improves manufacturability and it also increases throughput of
CIGS(S) absorber formation processes. The maximum cathode
efficiency in the prior-art fluoborate In plating bath is only 75%.
The In cyanide bath contains very toxic potassium cyanide and its
cathode efficiency drops down with aging of the bath and thus In
thickness changes with time. The alkali hydroxide In plating baths
may result in corrosion on the plated In surfaces and they usually
need some additives to increase the stability and the deposition or
plating efficiencies. Control of additives is difficult and costly
in the manufacturing environment. As the most popular In
electroplating bath, the In sulfamate plating solution generates In
thin films with a cathode efficiency of about 90%. However, this
bath requires some additives to improve the quality and morphology
of the deposited layers. Organic additives may gradually decompose
and start affecting deposit quality in a negative manner. They need
to be monitored and replenished periodically and the bath needs to
be totally replaced when a large concentration of decomposed
additives accumulate in the solution. These are costly approaches
and sulfamic acid and In-sulfamate salts are considerably expensive
chemicals to be used in a low cost processing approach.
[0008] In most metal plating baths, the low plating efficiencies or
cathode efficiencies are due to hydrogen generation. In high
efficiency electrolytes hydrogen generation is reduced or
eliminated. Hydrogen generation may not be very critical to most
electroplating applications where low plating efficiency, high In
film thickness control tolerance (+/-10% or more) and high defect
density in the deposited layers may be tolerated. However, in
semiconductor and electronic industries and especially in the above
mentioned application of CIGS(S) solar cell manufacturing, low
plating efficiency and associated generation of hydrogen bubbles
become major problems. Hydrogen generated during a low efficiency
electroplating process typically results in formation of pinholes
on the plated thin films, mostly due to the formation of sub-micron
or micron size hydrogen bubbles on the cathodic surface. These
small bubbles typically stick to the cathode surface and restrict
material deposition at those sites. As explained before, these
sites without In or with less In compared to the other parts of the
cathode surface later turn into Cu-rich regions in a finished CIGS
absorber. Copper-rich regions, in turn, introduce low shunt
resistance to solar cells and reduce their conversion efficiency.
Furthermore, hydrogen generation is sensitive to the substrate or
cathode surface conditions, which may change as a function of time
and reduce the repeatability of producing pin-hole free In thin
films. It should be noted that the typical thickness of In layers
to be electroplated for CIGS(S) absorber formation is in the range
of 100-500 nm.
[0009] Because In.sup.3+ cations precipitate as In(OH).sub.3
between pH values of about 4-12, complexing agents need to be used
to get a clear In electroplating solution within this pH range.
Some complexing agents, such as EDTA, cyanides, tartrates, have
been employed for this purpose. However, the In electroplating
baths containing these complexing agents yielded low cathodic
current efficiencies and required some organic additives to
stabilize the solutions. It may be for this reason that complexing
agents are not popular in commercial In plating baths.
[0010] Fouda et al (Bull. Soc. Chim. Fr., vol: 2, page: 270-272,
1987) compared the process results of some In plating baths
containing acetate, thiocyanate, chloride, iodide, sulphate,
oxalate, acetamide and citrate. Unfortunately, some of the data in
this publication appears to be inconsistent. For example, the
authors refer to the cathode current efficiencies listed in Table 1
and state that "The effect of complexing agent is shown in table 1,
all current efficiencies were quite low". In Table 1 all the
current efficiencies are low (in the range of 18-64%), except one
using citrate that is listed as 100%. Obviously this is a
typographic error in Table 1, especially since in the same table,
the rate of deposition is listed as 0.17 mg/min for the 100%
efficient bath at 2.5 mA/cm.sup.2 current density whereas the
acetamide bath with 64% plating efficiency at the same current
density is shown to yield a rate of deposition of 0.33 mg/min,
which is higher than 0.17 mg/min. It should be appreciated that
this is impossible. If we adjust for this discrepancy, the citrate
bath efficiency would be about 50%, which would be consistent with
the statement of the authors that all efficiencies in table 1 are
low. One other possible explanation for the discrepancy may be the
fact that the solution with citrate yielded non-uniform and gray
deposits. According to the authors, gray In deposits can be
attributed to the deposition of double salts. In other words, the
deposits may not be pure metallic In but may comprise In salts. In
either case, it is clear from this publication that its authors
could not obtain pure and uniform In films at high plating
efficiency using the plating bath chemistries they listed
[0011] As described by Fouda et al., the low cathodic current
efficiencies might be attributed to the dilute In.sup.3+
concentration (0.02 M). It is possible that the authors kept this
concentration low to avoid precipitation of In.sup.3+ species
within the relatively high pH ranges they selected for
formulations. Some of the baths in this study generated poor
quality In deposits. For example, a bath with 0.02 M InCl.sub.3 and
60 g/L of tri-sodium citrate (pH=5) yielded impure In deposits that
were thick, non-uniform and gray, suggesting a powdery and grainy
deposit with rough morphology which possibly comprised salts as
stated by the authors.
[0012] As can be seen from the foregoing discussion it is necessary
to develop a new In electroplating bath with high cathodic
efficiency that can provide high quality, smooth, substantially
defect-free and pure electrodeposited In layers at low cost, which
may be used in electronic and semiconductor applications such as in
processing thin film solar cells.
SUMMARY OF THE INVENTION
[0013] The present invention relates to an In electroplating bath
to deposit silvery white, uniform, substantially defect-free,
smooth and pure metallic In films with high cathodic current
efficiency and repeatability. Such layers may be used in
fabrication of electronic devices such as thin film solar
cells.
[0014] In certain embodiments, the present invention provides a
plating solution or bath for application of an In layer on a
conductive surface. The solution includes an In source, a weak acid
and its conjugate pair salt, and a solvent, wherein the solution
provides a sub-micron thick chemically pure In film on the
conductor with a cathodic plating efficiency of about 95-100%,
preferably an efficiency of 98-100%. The pH value of such solution
is below about 4.0. In addition, the In plating bath of certain
embodiments of the present invention features non-corrosive,
environmentally green chemistry which is low cost and highly stable
so that In layers with repeatable thickness and morphology may be
electroplated employing simple maintenance of the bath in a
manufacturing environment.
[0015] In another embodiment of the invention, the In plating
solution has been applied to roll-to-roll electroplating to obtain
an In containing film possessing a sub-micron thickness on the
surface of a conductor in a large scale manufacturing line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic illustration of an In layer
electroplated on a conductive bottom layer.
DETAILED DESCRIPTION OF THE INVENTION
[0017] An embodiment of the invention provides an electroplating
solution for application of a substantially pure indium (In) film
onto a conductive surface at high plating efficiency, comprising a
solvent, an In source providing In ions to the solvent with an In
ion concentration of at least about 0.05 M, and a weak acid and a
conjugate pair salt of the weak acid, wherein the pH value of the
electroplating solution is below about 4.0. The solvent may
optionally comprise water, and is preferably aqueous. The In source
may be any suitable source of In ions and is preferably one or more
of In-chloride, In-sulfate, In-acetate, In-carbonate, In-nitrate,
In-perchlorate, In-phosphate, In-oxide and In-hydroxide. Exemplary
weak acids which are suitable for this embodiment are citric acid,
acetic acid, formic acid, thioacetic acid, glycolic acid, lactic
acid, ascorbic acid, malic acid, butanoic acid, and pentanoic acid.
In certain preferred embodiments the weak acid is citric acid. The
conjugate pair salt of the weak acid may be any suitable salt and
in preferred embodiments is a sodium, lithium, potassium, ammonium
or alkyl (e.g., C.sub.1-C.sub.6 alkyl)ammonium salts of the weak
acid. In certain preferred embodiments where the weak acid is
citric acid, the conjugate pair salt is one or more of sodium
citrate, lithium citrate, potassium citrate, and a salt of an
organically modified citrate, wherein the organically modified
citrate comprises a citrate moiety such that one or more organic
groups replaces (a) one or more hydrogens that are directly
connected to a carbon or oxygen bonded to carbon 2 of the citrate
moiety, or (b) the hydroxyl group bonded to carbon 2 of the citrate
moiety.
[0018] Another embodiment of the invention provides a method of
obtaining a substantially pure and substantially defect-free In
film on a surface of a conductor comprising the steps of (i)
providing a solution with a pH value below 4 wherein that solution
comprises a solvent, an In source providing In ions to the solvent
with an In ion concentration of at least 0.05 M, a weak acid and a
conjugate pair salt of such weak acid; (ii) applying the solution
onto an anode and the surface of the conductor, (iii) establishing
a potential difference between the anode and the conductor, and
(iv) electrodepositing the substantially pure In film on the
surface of the conductor. In certain preferred embodiments the weak
acid is citric acid and the conjugate pair salt is a citrate salt,
preferably at least one of sodium citrate, lithium citrate,
potassium citrate, and an organically modified citrate. In certain
embodiments the In source includes one or more of In-chloride,
In-sulfate, In-acetate, In-carbonate, In-nitrate, In-perchlorate,
In-phosphate, In-oxide and In-hydroxide. In certain embodiments the
temperature of the solution is within the range of 10-60.degree. C.
during the step of electrodeposition and preferably is controlled
within such range prior to the step of electrodeposition.
[0019] The present invention provides a method to electroplate In
films onto conductive surfaces at close to 100% deposition
efficiency and high repeatability. In one embodiment, the present
invention may be used to manufacture Group IB/IIIA/VIA compound
solar cell absorbers including Group IB (such as Cu), Group IIIA
(such as Ga and In), and Group VIA (such as Se and S) elements. To
manufacture a solar cell absorber layer initially an absorber
precursor layer must be formed over a base which may include a
substrate and a contact layer formed on a surface of the substrate.
FIG. 1 shows an absorber precursor structure 10 having a conductive
bottom layer 12 and an In layer 14 electroplated on the conductive
bottom layer 12 using the present invention. The conductive bottom
layer 12 is formed on the base 16 which may comprise the substrate
and the contact layer (not shown). The typical conductive bottom
layers used in this invention may be copper (Cu) and gallium (Ga)
layers. After electroplating the In film in accordance with the
present invention, another element, e.g., selenium (Se), or Ga can
be directly plated onto the resultant In layer to form multiple
metal stacks. By electroplating an In film in an efficient manner
on a Cu surface, for instance, the present invention may be used to
manufacture Cu/In/Se, Cu/In/Ga/Se and other metallic stacks, which
in turn may be employed in processing CIS or CIGS type solar cell
absorbers.
[0020] The electrochemical methods have recently received more
attention for CIGS film formation due to their potential low cost.
For example, the CIGS films have been prepared with electrochemical
co-deposition method from acidic solutions containing CuCl.sub.2,
InCl.sub.3, GaCl.sub.3 and SeO.sub.2 (see for example, U.S. Pat.
No. 6,872,295). The electrochemical co-deposition of CIS films was
also performed from a solution containing Cu.sup.2+, In.sup.3+,
Se.sup.4+ and citrate salts, as reported in the literature
(Oliveira et al., Thin Solid Films, vol: 405, p: 129-134, 2000).
However, all these electrochemical co-plating or co-deposition
methods have problems such as long plating times (about one hour),
non-repeatable Cu/Group IIIA molar ratios, extremely low cathodic
plating efficiencies, and defective films arising from hydrogen
generation. Therefore, they have not been suitable for
manufacturing and have only been conducted for academic
purpose.
[0021] At the present time, the most practical industrial approach
for growth of the CIS or CIGS thin films using electroplating is a
"two-stage" process. In this process, controlled amounts of Cu, In,
Ga and sometimes Se are electrodeposited in the form of Cu, In, Ga
and Se containing thin film precursor stacks such as Cu/In/Ga/Se,
Cu/Ga/In/Se, In/Cu/Ga/Se, Ga/Cu/In/Se, In/Ga/Cu/Ga/Se,
In/Ga/Cu/In/Se, Ga/In/Cu/Ga/Se, Ga/In/Cu/In/Se, Cu/Ga/Cu/In/Se,
Cu/In/Cu/Ga/Se etc., on a base such as a substrate coated with a
conductive contact layer. These stacks may then be annealed, or
reacted, optionally with more Se, sulfur (S) or sodium (Na), to
form a uniform thin film of the CIGS(S) alloy or compound on the
contact layer. By controlling the thickness and morphology of the
Cu, In, and Ga layers within the precursor stacks, the process
yield in terms of compositional control may be improved compared to
the above mentioned alloy plating approaches where two or more of
the Cu, In, Ga, Se species are co-plated on the substrate.
[0022] As described before the control of the thickness and
morphology of the electrodeposited layers, such as the In layer, is
extremely important. High yield and repeatability of a solar cell
manufacturing process utilizing two-stage processing and
electrodeposition of at least one of a Cu layer, an In layer and a
Ga layer critically depend on the repeatability of the deposited
thickness of the electroplated layers, from run to run.
Furthermore, micro-scale compositional uniformity requires these
electroplated films with sub-micron thickness to have smooth
morphology with a surface roughness of typically less than 10% of
the film thickness, and with desirable and controllable
microstructure, which is typically a small-grain microstructure
with sub-micron size grains. Stacks utilizing In films with rough
surface morphology, for example, would result in the In content to
be changing locally, in micro-scale throughout the film although on
the average the In content may be in the acceptable range. It
should be noted that the typical acceptable CIGS(S) film
composition has a Cu/(In+Ga) molar ratio in the 0.8-1.0 range
whereas the Ga/(In+Ga) molar ratio may be in the range of
0.3-0.5.
[0023] With these thin film stacks, copper layers (or Ga layers)
may be electroplated or sputter deposited on a base comprising a
substrate which, on its surface, may have a conductive contact film
such as a Mo layer and/or a ruthenium (Ru) containing layer. The
substrate may be a metallic foil, glass or polymeric sheet or web.
The Ru containing layer on the substrate surface may be a Ru layer,
a Ru alloy layer, a Ru compound layer or a stack containing Ru such
as a Mo/Ru stack or in general a M/Ru stack, where M is a conductor
or semiconductor. Indium electroplating on the Cu surface (or the
Ga surface) can be carried out at various current densities, such
as at 5, 10, 20 and 30 mA/cm.sup.2, using the electrolytes of the
present invention. Both DC and/or variable (such as pulse or
ramped) voltage/current waveforms may be used for electroplating
the In layers.
[0024] In particular, this invention provides an efficient In
plating bath employing weak acids and conjugates salts such as
citrates. Films obtained using this solution are substantially free
from defects such as pinholes since hydrogen bubble formation on
the cathode during plating is drastically reduced by the high
plating efficiency. Electronic applications such as solar cells may
tolerate a total pinhole area to be about 0.0001% of the total film
area. Therefore, substantially pinhole-free means that on a 1
cm.sup.2 size In-plated surface the total are of the pinholes
(number of pinholes times the average size of pinholes) is less
than about 10.sup.-6 cm.sup.2.
[0025] Although citrates have been used in the In electrodeposition
before, as described above, it did not provide pure and uniform
layers. The optimized pH for the bath of the present invention is
less than about 4, preferably less than about 3.5, more preferably
less than 2.5, and most preferably the pH is about 2. Although
there may be other explanations for the superior performance of the
formulated bath, and without limiting the invention with respect to
any particular theory, the following non-limiting points are
observed with respect to the embodiment of the invention using
citric acid and citrate. Present inventors realized that the citric
acid (H.sub.3Cit) has three pK.sub.a values. These are:
pK.sub.a1=3.06, pK.sub.a2=4.74, and pK.sub.a3=5.40, at 25.degree.
C. As a result of this, at a pH of 2, about 90% of the citric acid
may not be dissociated in the solution. The other 10% of the
citrates may stay in the solution in the form of
H.sub.2Cit.sup.-Na.sup.+ that may, in turn, complex the In.sup.3+
species in the form of
[(H.sub.2Cit.sup.-).sub.nIn.sup.3+].sup.m+/-, where n may be in the
range of 1-6 and m may be in the range of 0-3. As a result, most of
In.sup.3+ cations in the solution of the present invention are not
complexed with the citrates. This is an important difference of the
present invention from the literature (see, e.g. Fouda et al.). In
Fouda a pH value of 5 was utilized. In the solution of the present
invention In precipitates as In(OH).sub.3 at such pH values. Use of
lower pH values allows the present solution to accommodate more In
without precipitation and this improves plating efficiency and
allows high current densities, which in turn reduces hydrogen
evolution, improves the process throughput and therefore yields
lower manufacturing cost. The solubility product of In(OH).sub.3 is
1.times.10.sup.-33. The citrate seems to be a weak complexing agent
for In. Therefore, the main role the citrates play in the solution
of this embodiment of the present invention may not be complexing.
Instead, there may be two aspects operating in our bath. On one
hand, the sodium citrate may form a kind of buffer solution with
the citric acid to stabilize the solution pH value. On the other
hand, citrate may consume some excess protons to reduce hydrogen
generation during the In plating process.
[0026] As mentioned before, a pH value of 5 was utilized in the
work described by Fouda et al. Use of low pH values in the present
invention provides several benefits including: i) adjustment of pH
to low values can be achieved using citric acid instead of other
acids. Citric acid is the acid of the citrate anions, which provide
preferable plating results as explained before. ii) Using citric
acid allows control of the cation concentration in the bath. If,
for example, Na-citrate was used solely as a source of citrate, the
Na concentration and citrate concentration in the bath would be
tied together. By using citric acid in addition to its conjugated
salt such as Na-citrate, we have independent means of controlling
pH as well as the citrate concentration and Na concentration in the
plating bath. This flexibility allows adjustment of the bath so
that plating efficiencies close to 100% can be achieved for the
first time.
[0027] The invention will now be further described with reference
to certain examples, however the invention is not limited to the
examples set forth herein. The electroplating experiments in these
examples were carried out using a potentiostat/galvanostat
(EG&G Model 263 A). The solutions were stirred during plating.
De-oxygenation was not found to be necessary during the In plating
process although this may be helpful in reducing the In anode
oxidation during plating. The substrates for the plating tests
included stainless steel and soda-lime glass, both coated with a
contact layer comprising a Cu film on its surface. Indium was
electroplated on the Cu surface and the results were evaluated. The
surface areas for the substrates were varied from several cm.sup.2
to several hundreds of cm.sup.2 to understand the suitability of
the method for large scale manufacturing. After the In deposition,
the uniformity and the plating efficiency were evaluated by
dissolving various portions of the films and using Inductively
Coupled Plasma (ICP) method to measure the In amounts in the
dissolved samples.
EXAMPLE 1
In Plating Bath Containing Citrates
[0028] A set of exemplary aqueous plating solutions were prepared
containing 0.1-0.3 M InCl.sub.3, 0.2-0.5 M sodium citrate
(Na.sub.3C.sub.6H.sub.5O.sub.7), and 0.1-0.3 M citric acid
(H.sub.3C.sub.6H.sub.5O.sub.7). The pH was adjusted to a range
between 1.5 and 3.5. Indium was electrodeposited on the Cu surfaces
at current densities of 5-30 mA/cm.sup.2. Highly adherent In films
with surface roughness less than 10 nm were obtained for a
thickness of 200-400 nm. The plating efficiency was measured and
found to be in the 95-100% range. The typical anode used in the
plating was an In plate. The resultant In films were shiny, silvery
white, smooth and substantially defect free, as examined with SEM
and optical microscopes. No hydrogen evolution could be observed
during plating. Indium was also plated on other metal surfaces
using the citrate In plating baths with high plating efficiency. An
accelerated test that continuously lasted 80 hours demonstrated
that the bath chemistry was stable without any oxide or hydroxide
precipitation and the deposition efficiencies were repeatable.
Another plating bath containing 50 liters of the In plating
solution was used to plate In onto the 6''.times.8'' substrates for
nine months with repeatable results if the pH was adjusted about
once in two weeks. The In.sup.3+ concentration was stable if the In
plate was used for the anode and did not require any adjustment.
The In thickness was very repeatable and the non-uniformity over
the whole 6''.times.8'' substrates could be controlled at levels
below 2% of relative standard deviation for different locations on
the substrate.
[0029] The indium citrate solution containing 0.2 M In Cl.sub.3,
0.15 M citric acid, and 0.35 M sodium citrate (pH=2.0) was applied
to a roll-to-roll plating line. The web substrate used in this
roll-to-roll plating line was 13'' wide and moved at a speed of
about 2 ft/min. The current density used in the plating was about
10 mA/cm.sup.2. This plating process produced uniform, smooth and
defect free In films demonstrating the suitability of the solution
to large scale manufacturing.
EXAMPLE 2
In Plating Bath Containing Glycine as the Complexing Agent
[0030] An aqueous plating bath was formulated with 0.2 M InCl.sub.3
and 0.5 M Glycine to compare the bath of the present invention with
a bath comprising a complexing agent. The pH was adjusted to the
range of 2.0-2.5 using HCl. The plating tests were carried out on
Cu surfaces at current densities of 10-30 mA/cm.sup.2. All the In
films looked shiny and smooth but the cathodic plating efficiencies
were only about 60-85%. Extensive hydrogen bubbling was observed on
the cathode surface during the plating, which resulted in defects
and pinholes on the In films that were visible to naked eye.
[0031] Example 1 above demonstrated the good performance of citric
acid and its conjugate pair citrate in the In plating bath. It
should be appreciated that other acids and their conjugate pairs
may play the same roles in other In plating baths with a pH less
than about 4.0, preferably less than about 3.5. These acids include
but are not limited to acetic acid, tartaric acid, phosphoric acid,
oxalic acid, carbonic acid, ascorbic acid, boric acid, butanoic
acid, thioacetic acid, glycolic acid, malic acid, formic acid,
heptanoic acid, hexanoic acid, hydrocyanic acid, hydrofluoric acid,
lactic acid, nitrous acid, octanoic acid, pentanoic acid, uric
acid, sulfurous acid, sulfuric acid HSO.sub.4.sup.-, nonanoic acid,
decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic
acid, octadecanoic acid, eicosanoic acid, tetracosanoic acid, etc.
All of these acids may be combined with Li.sup.+, Na.sup.+,
K.sup.+, NH.sub.4.sup.+ or (C.sub.nH.sub.(2n+1)).sub.4N.sup.+(e.g.,
where n may be 1 to 6) salts of their conjugate pairs and In.sup.3+
salts to form In plating baths with high efficiency.
[0032] Although water is the preferred solvent in the formulation
of the In plating baths of the present invention, it should be
appreciated that organic solvents may also be added in the
formulation, partially or wholly replacing the water. Such organic
solvents include but are not limited to alcohols, acetonitrile,
propylene carbonate, formamide, dimethyl sulfoxide, glycerin
etc.
[0033] Although the DC voltage/current was utilized during the In
electrodeposition processes in the present invention, it should be
noted that pulsed or other variable voltage/current sources may
also be used to obtain high plating efficiencies and high quality
In deposits employing the In plating baths of the present
invention. The temperature of the In electroplating baths may be in
the range of 5-120.degree. C. depending upon the nature of the
solvent. It is preferable to keep this temperature below the
boiling point of the solvent. The preferred bath temperature for
water based formulation is in the range of 10-60.degree. C. The
most preferred range is 15-30.degree. C.
[0034] Although not preferable, the electroplating baths of the
present invention may comprise additional ingredients. These
include, but are not limited to, grain refiners, surfactants,
dopants, other metallic or non-metallic elements etc. For example,
organic additives such as surfactants, suppressors, levelers,
accelerators etc. may be included in the formulation to refine its
grain structure and surface roughness. Organic additives include
but are not limited to polyalkylene glycol type polymers, propane
sulfonic acids, coumarin, saccharin, furfural, acrylonitrile,
magenta dye, glue, SPS, starch, dextrose, etc. In fact, dextrose
and triethanolamine were used in the In citrate baths of the
present invention, but the difference is insignificant because the
plated In films have already shown good qualities without any
additives.
[0035] It should be noted that the present invention is directed to
the electrodeposition of substantially pure In layers (more than
about 99.5% In, preferably more than 99.9% In) since the
electronics application and specifically CIGS(S) solar cell
application of such layers require good thickness and compositional
control. However, trace amounts of other materials may be included
in the bath formulation of the present invention without changing
its fundamentals. For example, small amounts (typically less than
0.01M) Ga, Cu, S and/or Se may be present in the formulation,
provided that they do not interfere with the high plating
efficiency. The In layers produced using the bath compositions of
the present invention were successfully employed to fabricate some
all-electroplated metallic stacks on bases comprising stainless
steel substrates coated with contact layers comprising Mo and/or
Ru. These stacks had various deposition sequences yielding
base/Cu/Ga/In/Se, base/Cu/Ga/Cu/In/Se, base/Cu/In/Cu/Ga/Se,
base/Cu/In/Ga/Se, base/Cu/Ga/Cu/In/Ga/Se and base/Ga/Cu/In/Se
multiple layers. A Ga citrate based electroplating bath developed
by the present inventors (US Pat Appl Pub. 20070272558) was
utilized for Ga depositions. The stacks were then reacted in a tube
furnace at 500.degree. C. for 50 minutes under inlet gas to form
Cu(In,Ga)Se.sub.2 absorbers. The Cu/(In+Ga) molar ratio was kept in
the 0.8-0.9 range while the Ga/(In+Ga) molar ratio was nominally
50% in these samples. After the reaction step, a 100 nm thick CdS
layer was chemically deposited onto the absorber surfaces yielding
a base/Cu(In,Ga)Se.sub.2/CdS structure. A ZnO containing
transparent oxide layer was then deposited over the CdS films by
the sputtering technique. Solar cell was completed by printing Ni
or Ag finger contacts over the transparent oxide layer. Solar cell
efficiencies over 15% were recorded from these devices
demonstrating the quality of the electrodeposited stacks comprising
the In layers of the present invention. This efficiency value is
the highest that has been achieved by a two stage process employing
In electrodeposition for the precursor preparation.
* * * * *