U.S. patent application number 12/404690 was filed with the patent office on 2009-07-09 for efficient gallium thin film electroplating methods and chemistries.
This patent application is currently assigned to SoloPower, Inc.. Invention is credited to Serdar Aksu, Bulent M. Basol, Jiaxiong Wang.
Application Number | 20090173634 12/404690 |
Document ID | / |
Family ID | 39230522 |
Filed Date | 2009-07-09 |
United States Patent
Application |
20090173634 |
Kind Code |
A1 |
Aksu; Serdar ; et
al. |
July 9, 2009 |
EFFICIENT GALLIUM THIN FILM ELECTROPLATING METHODS AND
CHEMISTRIES
Abstract
The present invention relates to gallium (Ga) electroplating
methods and chemistries to deposit uniform, defect free and smooth
Ga films with high plating efficiency and repeatability. Such
layers may be used in fabrication of electronic devices such as
thin film solar cells. In one embodiment, the present invention
provides a solution for application on a conductor that includes a
Ga salt, a complexing agent, a solvent, and a Ga-film having
submicron thickness is facilitated upon electrodeposition of the
solution on the conductor. The solution may further include one or
both of a Cu salt and an In salt.
Inventors: |
Aksu; Serdar; (Milpitas,
CA) ; Wang; Jiaxiong; (Castro Valley, CA) ;
Basol; Bulent M.; (Manhattan Beach, CA) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
SoloPower, Inc.
San Jose
CA
|
Family ID: |
39230522 |
Appl. No.: |
12/404690 |
Filed: |
March 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11535927 |
Sep 27, 2006 |
7507321 |
|
|
12404690 |
|
|
|
|
Current U.S.
Class: |
205/264 ;
205/261 |
Current CPC
Class: |
Y02E 10/541 20130101;
C25D 3/54 20130101; C25D 3/56 20130101; H01L 31/0322 20130101; H01L
31/0749 20130101 |
Class at
Publication: |
205/264 ;
205/261 |
International
Class: |
C25D 3/54 20060101
C25D003/54; C25D 3/56 20060101 C25D003/56 |
Claims
1. A solution for electrodeposition of a gallium (Ga) containing
film on a conductor comprising: a gallium (Ga) salt, a complexing
agent with at least one of a carboxylic chelating group, and an
ammine chelating group, and a solvent, and wherein the pH of the
solution is higher than 7.0.
2. The solution of claim 1 wherein the complexing agent is selected
from the group consisting of a citrate, ethylenediaminetetraacetic
acid (EDTA), a tartrate, an oxalate, ethylenediamine,
nitrilotriacetic acid, and an amino acid.
3. The solution of claim 1 wherein the gallium (Ga) salt is
selected from the group consisting of gallium (Ga)-chloride,
gallium (Ga)-sulfate, gallium (Ga)-acetate and gallium
(Ga)-nitrate.
4. The solution of claim 2 wherein the gallium (Ga) salt is
selected from the group consisting of gallium (Ga)-chloride,
gallium (Ga)-sulfate, gallium (Ga)-acetate and gallium
(Ga)-nitrate.
5. The solution of claim 2 further comprising an alkali metal salt
of the complexing agent.
6. The solution of claim 5 wherein the alkali metal salt of the
complexing agent is one of a sodium salt of the complexing agent, a
potassium salt of the complexing agent, and a lithium salt of the
complexing agent.
7. The solution of claim 1 wherein the solvent is water.
8. The solution of claim 1 wherein the solvent is a mixture of
water with an organic solvent.
9. The solution of claim 1 further comprising an organic
additive.
10. The solution of claim 1 further comprising at least one of a
copper (Cu) salt and an indium (In) salt.
11. The solution of claim 1 further comprising a pH adjustment
agent that is one of NaOH and KOH.
12. The solution of claim 1 wherein the pH of the solution is
within a range of 9-14.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
U.S. application Ser. No. 11/535,927 filed Sep. 27, 2006 entitled
"Efficient Gallium Thin Film Electroplating Methods and
Chemistries", the entirety of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to gallium (Ga) electroplating
methods and chemistries to deposit uniform, defect free and smooth
Ga films with high plating efficiency and repeatability. Such
layers may be used in fabrication of electronic devices such as
thin film solar cells.
BACKGROUND
[0003] Gallium is an element that is used in semiconductor and
electronics industries. Gallium is generally recovered as a
by-product from Bayer-process liquors containing sodium aluminate
(see for example, U.S. Pat. No. 2,793,179 and U.S. Pat. No.
2,582,377). Although electrodeposition is a common method to
recover bulk Ga (see for example, U.S. Pat. No. 3,904,497) out of
basic or acidic solutions, or to purify bulk Ga, there have not
been many applications for this material where thin films were
deposited with controlled uniformity, morphology and thickness.
Therefore, only a few electroplating bath chemistries and processes
were developed and reported for the deposition of thin layers of Ga
on substrates for electronic applications. For example, Ga-chloride
solutions with pH values varying between 0 and 5 were evaluated by
S. Sundararajan and T. Bhat (J. Less Common Metals, vol. 11, p.
360, 1966) for electroplating of Ga films. Other researchers
investigated Ga deposition out of high pH solutions comprising
water and/or glycerol. Bockris and Enyo, for example, used an
alkaline electrolyte containing Ga-chloride and NaOH (J.
Electrochemical Society, vol. 109, p. 48, 1962), whereas, P.
Andreoli et al. (Journal of Electroanalytical Chemistry, vol. 385,
page. 265, 1995) studied an electrolyte comprising KOH and
Ga-chloride.
[0004] The above mentioned prior-art methods and plating baths
reportedly all achieved Ga film deposition. There are, however,
some common problems associated with the prior-art electrochemical
deposition processes. These problems include, low cathodic
deposition efficiency due to excessive hydrogen generation, poor
repeatability of the process, partly due to the poor cathodic
efficiency, and the poor quality of the deposited films such as
their high surface roughness and poor morphology. These issues may
not be important for bulk Ga electroplating or for Ga films
deposited for the purpose of investigating scientific topics such
as deposition mechanisms. Poor film morphology or inadequate
thickness control may also not be important for the electrically
inactive applications of Ga layers, such as their use as
lubricating coatings etc. However, properties of the Ga films
become important for certain new electronic applications where Ga
film plays a role in forming an active portion of an electronic
device, such as a solar cell.
[0005] Prior-art Ga electroplating techniques utilizing simple
electrolytes operating under acidic or basic pH values are not
suitable for the above mentioned electronics applications for a
variety of reasons, including that they result in poor plating
efficiencies and films with rough morphology (typically surface
roughness larger than about 20% of the film thickness). Gallium is
a difficult metal to deposit without excessive hydrogen generation
on the cathode because Ga plating potential is high. Hydrogen
generation on the cathode causes the deposition efficiency to be
less than 100% because some of the deposition current gets used on
forming the hydrogen gas, rather than the Ga film on the substrate
or cathode. Hydrogen generation and evolution also causes poor
morphology and micro defects on the depositing films due to the
tiny hydrogen bubbles sticking to the surface of the depositing
film, masking the micro-area under them, and therefore impeding
deposit on that micro-area. This causes micro-regions with less
than optimum amount of Ga in the film stack. Poor plating
efficiencies inherently reduce the repeatability of an
electrodeposition process because hydrogen generation phenomenon
itself is a strong function of many factors including impurities in
the electrolyte, deposition current densities, small changes on the
morphology or chemistry of the substrate surface, temperature, mass
transfer etc. As at least one of these factors may change from run
to run, hydrogen generation rate may also change, changing the
deposition efficiency.
[0006] Electrodeposition of Ga out of low pH aqueous electrolytes
or solutions may suffer from low cathodic efficiencies arising from
the presence of a large concentration of H.sup.+ species in such
electrolytes. Therefore, hydrogen gas generation may be expected to
lessen at higher pH values. However, as the pH is increased in the
solution, Ga forms oxides and hydroxides which may precipitate as
reported in the literature. Only at extremely alkaline pH values
these oxides/hydroxides dissolve as soluble Ga species. Therefore,
it becomes possible to electrodeposit Ga in a bath of pH>14
containing Ga salts as was done in prior-art techniques using high
concentrations of KOH and NaOH in the bath formulation. High
concentrations of alkaline species, however, cause corrosion
problems for the equipment as well as the cathode material itself.
There is also a limit of the Ga amount that can be dissolved in the
form of acidic Ga salts (GaCl.sub.3, Ga(NO.sub.3).sub.3 etc) in
such solutions before Ga starts to precipitate. Therefore, the pH
needs to be adjusted again by further addition of alkaline species
such as NaOH and KOH. As pointed out above, solutions comprising a
large molar amount of caustics are difficult to handle and they
also have high viscosity. High viscosity makes the hydrogen bubbles
formed on the cathode stick more to the cathode making it very
difficult to remove them by stirring or other means of mass
transfer. As explained above, such gas bubbles on the cathode
surface increase defectivity of the deposited Ga layer.
[0007] As can be seen from the foregoing discussion there is a need
to develop new electroplating chemistries and methods that can
provide high quality electrodeposited Ga layers which may be used
in electronic applications such as in processing thin film solar
cells.
SUMMARY OF THE INVENTION
[0008] The present invention relates to gallium (Ga) electroplating
methods and chemistries to deposit uniform, defect free and smooth
Ga films with high plating efficiency and repeatability. Such
layers may be used in fabrication of electronic devices such as
thin film solar cells.
[0009] In one embodiment, the present invention provides a solution
for application on a conductor. The solution includes a Ga salt, a
complexing agent, and a solvent, wherein the solution provides by
electrodeposition a sub-micron thickness Ga containing film on the
conductor.
[0010] In another embodiment of the invention, the solution further
includes one or both of a Cu salt and an In salt, and the pH of the
solution is substantially 7 or higher.
[0011] In another embodiment of the invention there is provided a
method of obtaining a Ga containing film having a sub micron
thickness on a surface of a conductor.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The present invention provides a method to electroplate Ga
films onto conductive surfaces at high deposition efficiency and
repeatability. Two particular conductive surfaces used in this
invention are Cu and In surfaces. By electroplating Ga in an
efficient manner on Cu and In surfaces, the present invention may
be used to manufacture Cu/In/Ga, Cu/Ga/In, In/Ga/Cu and other
metallic stacks, which in turn may be employed in processing
CIGS(S) type solar cell absorbers.
[0013] In this regard, there has been significant progress by the
present assignee, including inventors, in thin film solar cell
technologies. Copper-indium-selenide-(sulfide), or CIGS(S) and
similar materials in the family of Group IBIIIAVIA chalcopyrite
semiconductors have emerged as important compounds for thin film
polycrystalline solar cell applications. In a recently developed
"two-stage" processing method for growth of CIGS(S) thin films, for
example, controlled amounts of Cu, In and Ga are electrodeposited
in the form of Cu, In and Ga containing thin film stacks such as
Cu/In/Ga, Cu/Ga/In, In/Cu/Ga, Ga/In/Cu, Ga/Cu/In, Cu/Ga/Cu/In,
Cu/In/Cu/Ga etc. stacks, on a base such as a substrate coated with
a conductive contact layer. These stacks are then reacted with Se
and/or S to form a thin film of the CIGS(S) compound on the contact
layer. Details of such a processing approach may be found in the
following patent applications, each of which are expressly
incorporated by reference herein: [0014] U.S. Provisional
Application Ser. No. 60/756,750 filed Jan. 6, 2006 entitled
"Precursor Copper Indium and Gallium for Selenide (Sulfide)
Compound Formation"; [0015] U.S. Provisional Application Ser. No.
60/782,373 filed Mar. 14, 2006 entitled: Method and Apparatus for
Converting Precursor Layers Into Photovoltaic Absorbers"; [0016]
U.S. Provisional Application Ser. No. 60/782,373 filed Mar. 14,
2006 entitled "Method and Apparatus for Converting Precursor Layers
Into Photovoltaic Absorbers"; [0017] U.S. Provisional Application
Ser. No. 60/820,479 filed Jul. 26, 2006 entitled "Technique for
Doping Compound Layers Used In Solar Cell Fabrication"; [0018] U.S.
patent application Ser. No. 11/081,308 filed Mar. 15, 2005 entitled
"Technique and Apparatus for Depositing Thin Layers of
Semiconductors For Solar Cell Fabrication"; [0019] U.S. patent
application Ser. No. 11/266,013 filed Nov. 2, 2005 entitled
"Technique and Apparatus For Depositing Layers of Semiconductors
For Solar Cell and Module Fabrication"; and [0020] U.S. patent
application Ser. No. 11/462,685 filed Aug. 4, 2006 entitled
"Technique For Preparing Precursor Films and Compound Layers for
Thin Film Solar Cell Fabrication and Apparatus Corresponding
Thereto.
[0021] It should be appreciated that in this processing technique,
control of thickness for the deposited layers, such as the Cu, In
and Ga layers is extremely important. The thickness of Ga layers in
such stacks is typically sub-micron, more typically in the range of
50-200 nm. The electrical and optical properties of the compound
semiconductors such as CIGS(S) are highly sensitive to the
stoichiometry or composition of the material. Specifically, these
properties strongly depend on the Cu/(In+Ga) and Ga/(Ga+In) molar
ratios throughout the film. Efficiency of solar cells fabricated on
such compound semiconductor layers, in turn, depends on the optical
and electrical properties of the layers. Therefore, 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
layer(s), from run to run. Furthermore, micro-scale compositional
uniformity requires these electrodeposited 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 submicron size grains. Stacks utilizing Ga
films with rough surface morphology, for example, would cause the
Ga content to be changing locally, in micro-scale throughout the
film although on the average the Ga 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/(Ga+In) molar ratio may be in the
range of 0.1-0.3.
[0022] With these thin film stacks, copper layers (or In 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 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 N/Ru stack, where M is a conductor
or semiconductor. Gallium electroplating on the Cu surface (or the
In surface) can be carried out at various current densities, such
as at 5, 10, 20, 30, 40 and 50 mA/cm.sup.2, using the electrolytes
of the present invention. Both DC and/or variable (such as pulsed
or ramped) voltage/current waveforms may be used for electroplating
the Ga layer.
[0023] In particular, this invention provides a class of Ga plating
baths containing complexing agents. Complexing agents complex the
Ga in the bath, forming complexes which may be in general
represented by Ga.sup.k+ (L.sup.m-).sub.n. During plating, a
cathodic reaction of Ga.sup.k+
(L.sup.m-).sub.n+k.sup.e-=Ga+nL.sup.m- may take place causing Ga
deposition on the cathode surface and release of the complexing
species. The value of "k" may be 3. Complexing agents may serve
multiple purposes. Among these are: i) reduction of free Ga ion
concentration in the bath, ii) reduction of Ga salt precipitation,
and, iii) maintenance of a stable pH. With the assistance of
complexing agents, Ga salts may be dissolved in a basic solution at
reasonably large concentrations of 0.1-1.0M, without precipitation,
so that hydrogen generation is reduced and deposition efficiency is
enhanced. Some of the advantages for the bath compositions of the
present invention are: i) since the pH is typically higher than 7,
preferably higher than 9, hydrogen generation is reduced, ii) since
the pH is preferably lower than 14, excessive corrosion problems
are avoided, iii) complexed Ga species form small grained smooth Ga
deposits in a repeatable manner.
[0024] The invention will now be described by presenting several
examples. The electroplating experiments in these examples were
carried out using a potentiostat/galvanostat (EG&G Model 263
A). During plating, the solutions were stirred. The substrates for
the plating tests included stainless steel and soda-lime glass,
both coated with a 500 nm thick Mo layer followed by a Ru layer
which had a thickness in the range of 5-100 nm. First, a 50-200 nm
of Cu layer was electroplated on the Ru surface. Gallium was then
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 cm.sup.2 to understand the suitability of the
method for large scale manufacturing. After the Ga deposition, the
uniformity and the plating efficiency were evaluated by dissolving
various portions of the film and using Atomic Absorption
Spectroscopy to measure the Ga amounts in the dissolved
samples.
Example 1
Citrate as the Complexing Agent
[0025] A set of exemplary aqueous plating baths were prepared
containing 0.2-0.5 M GaCl.sub.3, and 0.5-0.8 M sodium citrate
(Na.sub.3C.sub.6H.sub.5O.sub.7). The pH was adjusted to a range
between 10 and 13. Gallium was electrodeposited on the copper
surface at current densities of 30-50 mA/cm.sup.2. Highly adherent
Ga films with surface roughness of <10 nm were obtained for a
thickness of 100 nm. The plating efficiency was measured and found
to be in the 85-100% range, the higher current density yielding
more efficient deposition process. Gallium was also plated on other
metal surfaces also using the citrate containing complexed baths.
Deposition on Ru surface directly yielded a plating efficiency of
75-90%. On the surface of In, Ga deposition efficiency reached
100%. An accelerated test that lasted 50 hours demonstrated that
the bath chemistry was stable without any oxide/hydroxide
precipitation and the deposition efficiencies were repeatable.
Example 2
EDTA as the Complexing Agent
[0026] An aqueous plating bath was formulated with 0.2 M GaCl.sub.3
and 0.4 Molar EDTA. The pH was adjusted to the range of 12-14 using
NaOH. The plating tests were carried out on electroplated copper
surfaces at current densities of 10-50 mA/cm.sup.2. All Ga films
were shiny with smooth morphology. Surface roughness was <10 nm
for 100 nm thick films. In this case the deposition efficiency was
found to be higher at current densities around 20-30 mA/cm.sup.2
compared to lower and higher current density values. These
efficiency values were in the range of 75-95%.
Example 3
Glycine as the Complexing Agent
[0027] An aqueous plating bath was formulated with 0.2 M GaCl.sub.3
and 0.5 M Glycine. The pH was adjusted to the range of 11-13 using
NaOH. The plating tests were carried out on the surfaces of
electroplated copper at current densities of 10-50 mA/cm.sup.2. All
Ga films were shiny with smooth surfaces. Surface roughness was
<10 nm for 100 nm thick layers. In this case the deposition
efficiency was found to be in the range of 75-90% at 20-30
mA/cm.sup.2. Efficiency went down at lower and higher current
density values.
[0028] Although three specific complexing agents, i.e. citrate,
EDTA and glycine have been used for bath formulation in the above
examples, it is possible to employ other complexing agents with
carboxylic and/or ammine chelating groups in addition to or in
place of those that are cited. Citrates used may be organically
modified such as triethyl citrate and tributyl citrate. Other
complexing agents include but are not limited to tartrates (such as
sodium tartrate, lithium tartrate, potassium tartrate, sodium
potassium tartrate, diethyl tartrate, dimethyl tartrate, dibutyl
tartrate, diisopropyl tartrate, and ammonium tartrate), oxalates
(such as sodium, potassium and lithium oxalates), ammonia and
ammonium salts, ethylenediamine, nitrilotriacetic acid and its
salts, hydroxyethylethylenediaminetriacetic acid and its salts,
aminobutyric acids and their salts, amino acids including alanine,
valine, leucine, isoleucine, praline, phenylalanine, tyrosine,
tryptophan, lysine, arginine, histidine, aspartate, glutamate,
serine, threomine, cysteine, methionine, asparagine, and
glutamine.
[0029] It should be noted that although some of these complexing
agents may perform well even in acidic solutions, the preferred Ga
plating bath compositions of this invention have a pH value of
higher than 7, preferably higher than 9, and most preferably in the
range of 9 to 14. The above examples employed simple aqueous
chemistries with water as the solvent. Although water is the
preferred solvent in the formulation of Ga plating baths of the
preferred 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
glycerin, alcohols, ethylene glycol, ethylene carbonate, propylene
carbonate, acetonitrile, formamide, dimethyl sulfoxide, sulfolane
etc.
[0030] The examples above utilized DC voltage/current during the Ga
electrodeposition process. It should be noted that pulsed or other
variable voltage/current sources may also be utilized to obtain the
high plating efficiencies and high quality Ga deposits employing
the Ga plating baths of the present invention. The temperature of
the Ga electroplating baths may be in the range of 5-150 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 C. The most preferred range is 15-30 C.
[0031] 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. There are many such additives commonly used in
the field. Organic additives include but are not limited to
polyalkylene glycol type polymers, propane sulfonic acids,
coumarin, saccharin, furfural, acryonitrile, magenta dye, glue,
SPS, starch, dextrose, etc. It should be noted that other materials
may be included in the bath formulation to deposit thin layers of
Ga-alloys or mixtures of Ga with the other materials. For example,
addition of Cu species (such as Cu-sulfate, Cu-chloride etc) in the
bath formulation allows deposition of Cu-Ga alloys and/or thin
films comprising a mixture of Cu and Ga. Similarly, In may be added
in the formulation to obtain Ga--In alloys and/or films comprising
mixture of Ga and In. Addition of both In and Cu may yield deposits
comprising Cu, In and Ga in controlled amounts. Dopants may also be
added in quantities that allow deposition of "doped Ga" films. A
doped Ga film comprises a dopant in a quantity that is typically
less than 1 atomic percent, preferably less than 0.1 atomic
percent. Such dopants include but are not limited to Na, Li, K, S,
Se, Sb, P etc.
[0032] The Ga layers produced using the bath compositions of the
present invention were employed to fabricate exemplary
all-electroplated metallic stacks on bases comprising stainless
steel substrates coated with Mo/Ru or only Ru layers. These stacks
had various deposition sequences yielding base/Cu/Ga/In,
base/Cu/Ga/Cu/In, base/Cu/In/Cu/Ga and base/Cu/In/Ga structures. An
indium sulfamate-based plating bath marketed by Indium Corporation
of America was utilized for In film depositions. The stacks were
reacted in a tube furnace at 500 C for 50 minutes with Ar+H.sub.2Se
gas mixture, forming Cu(In,Ga)Se.sub.2 absorbers. The Cu/(In+Ga)
molar ratio was kept in the 0.88-0.94 range while the Ga/(Ga+In)
molar ratio was nominally 30% in these samples. After the reaction
step a 100 nm thick CdS layer was formed on the absorber surface
yielding a base/Cu(In,Ga)Se.sub.2/CdS structure. A well known
chemical dip method was used for CdS deposition out of a solution
comprising Cd-sulfate, Thiorea, triethylenamine (TEA) and ammonia.
A ZnO/ITO transparent conductive layer was then deposited over the
CdS film by the sputtering technique. Solar cell was completed by
evaporating Ni finger contacts over the ITO layer. Solar cell
efficiencies as high as 11% were recorded from these devices
demonstrating the quality of the electrodeposited stacks comprising
the Ga layers of the present invention.
[0033] What we claim:
* * * * *