U.S. patent application number 12/874496 was filed with the patent office on 2012-03-08 for electrodeposition methods of gallium and gallium alloy films and related photovoltaic structures.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Shafaat Ahmed, Hariklia Deligianni, Qiang Huang, Kathleen B. Reuter, Lubomyr T. Romankiw, Raman Vaidyanathan.
Application Number | 20120055612 12/874496 |
Document ID | / |
Family ID | 44534372 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120055612 |
Kind Code |
A1 |
Ahmed; Shafaat ; et
al. |
March 8, 2012 |
ELECTRODEPOSITION METHODS OF GALLIUM AND GALLIUM ALLOY FILMS AND
RELATED PHOTOVOLTAIC STRUCTURES
Abstract
Photovoltaic devices and methods for preparing a p-type
semiconductor layer for the photovoltaic devices generally include
electroplating a layer of gallium or a gallium alloy onto a
conductive layer by contacting the conductive layer with a plating
bath free of complexing agents including a gallium salt, methane
sulfonic acid or sodium sulfate and an organic additive comprising
at least one nitrogen atom and/or at least one sulfur atom, and a
solvent; adjusting a pH of the solution to be less than 2.6 or
greater than 12.6. The photovoltaic device includes an impurity in
the p-type semiconductor layer selected from the group consisting
of arsenic, antimony, bismuth, and mixtures thereof. Various
photovoltaic precursor layers for forming CIS, CGS and CIGS p-type
semiconductor structures can be formed by electroplating the
gallium or gallium alloys in this manner. Also disclosed are
processes for forming a thermal interface of gallium or a gallium
alloy with the electroplating process.
Inventors: |
Ahmed; Shafaat; (Yorktown
Heights, NY) ; Deligianni; Hariklia; (Yorktown
Heights, NY) ; Huang; Qiang; (Yorktown Heights,
NY) ; Reuter; Kathleen B.; (Yorktown Heights, NY)
; Romankiw; Lubomyr T.; (Yorktown Heights, NY) ;
Vaidyanathan; Raman; (Yorktown Heights, NY) |
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
44534372 |
Appl. No.: |
12/874496 |
Filed: |
September 2, 2010 |
Current U.S.
Class: |
156/151 ;
205/170; 205/182; 205/50 |
Current CPC
Class: |
H01L 21/02628 20130101;
Y02E 10/541 20130101; H01L 21/02568 20130101; C25D 7/126 20130101;
H01L 31/0749 20130101; C25D 5/10 20130101; C25D 5/56 20130101; H01L
31/0322 20130101; C25D 3/58 20130101; H01L 2224/73253 20130101;
C25D 3/38 20130101; H01L 23/3738 20130101; C25D 3/54 20130101 |
Class at
Publication: |
156/151 ;
205/170; 205/182; 205/50 |
International
Class: |
C25D 7/12 20060101
C25D007/12; B32B 38/00 20060101 B32B038/00; C25D 5/10 20060101
C25D005/10 |
Claims
1. A method of forming a p-type semiconductor layer for a
photovoltaic device, comprising: electroplating a first layer onto
a conductive surface of a substrate, wherein said first layer is
selected from the group consisting of a copper layer and a
copper-gallium layer; electroplating a second layer onto said first
layer, wherein said second layer is selected from the group
consisting of an indium layer, a gallium layer, an indium-gallium
layer, a copper-indium diselenide layer, and a
copper-gallium-diselenide layer; and optionally electroplating a
third layer onto said second layer, wherein said third layer is
selected from the group consisting of a gallium layer and an indium
layer; and optionally electroplating a fourth layer onto said third
layer, wherein said fourth is selected from the group consisting of
selenium and sulfur; wherein said electroplating is carried out by
a method comprising: contacting: (i) a substrate and (ii) a
solution comprising: a precursor comprising an element selected
from the group consisting of copper, gallium, indium, selenium,
sulfur and a combination thereof; optionally a metalloid compound
additive; further optionally an organic additive having at least a
sulfur atom; and a solvent to dissolve said precursors; wherein the
solution is free of complexing agents; adjusting a pH of said
solution to a range selected from the group consisting of a pH of
about zero to less than about 2.6 and a pH of about 12.6 to about
14, and applying a current to electroplate said substrate to
produce said first, second, third or fourth layers; and annealing
said first, said second and said third layers in the presence of a
selenium source and/or sulfur source to form the p-type
semiconductor layer.
2. The method of claim 1, wherein the conductive surface is
selected from the group consisting of molybdenum, tantalum,
tungsten, titanium, and corresponding nitrides thereof.
3. The method of claim 1, wherein the plating bath comprises sodium
sulfate at a concentration of 0.01 M to 2 M.
4. The method of claim 1, wherein the plating bath further
comprises an oxide of a metalloid.
5. The method of claim 4, wherein the oxide of the metalloid is in
an amount of 1 part per million to 10,000 parts per million.
6. The method of claim 1, wherein the organic additive is selected
from the group consisting of thioureas, thiazines, sulfonic acids,
sulfonic acids, allyl phenyl sulfone, sulfamides,
dithioxo-bishydroxylaminomolybdenum complex, and derivatives
thereof.
7. The method of claim 1, wherein the solution comprises an alkane
sulfonic acid selected from the group consisting of methane
sulfonic acid, ethane sulfonic acid, propane sulfonic acid, and
butane sulfonic acid, and wherein the alkane sulfonic acid is at a
concentration of 0.1 M to 2 M.
8. The method of claim 1, wherein the p-type semiconductor has a
ratio of Cu/(In+Ga) at 0.8 to 0.9 and a ratio of Ga/(Ga+In) at 0.3
to 0.33.
9. A photovoltaic device comprising: at least one layer comprising
gallium or indium or alloys comprising gallium and indium, wherein
the at least one layer is formed by electrodeposition; and an
impurity in the at least one layer selected from the group
consisting of arsenic, antimony, bismuth, selenium, sulfur and
mixtures thereof.
10. The photovoltaic device of claim 9, wherein the at least one
layer forms a copper-gallium-indium-selenium layer.
11. The photovoltaic device of claim 9, wherein the at least one
layer forms a copper-indium-selenium layer.
12. The photovoltaic device of claim 9, wherein the at least one
layer forms a copper-gallium-selenium layer.
13. The photovoltaic device of claim 10, wherein a ratio of
Cu/(In+Ga) is at 0.8 to 0.9 and a ratio of Ga/(Ga+In) is at 0.3 to
0.33.
14. A method for forming a thermal interface, the method
comprising: electroplating a layer of gallium or a gallium alloy
onto a heat emitting surface coupled to a microprocessor, wherein
electroplating the gallium or gallium alloy comprises contacting
the heat emitting surface with a plating bath free of complexing
agents comprising a gallium salt and an optional organic additive
comprising at least one sulfur atom, and a solvent; adjusting a pH
of the plating bath to a range selected from the group of a pH of
greater than about zero to less than 2.6 and a pH greater than
about 12.6 to about 14, and applying a current to electroplate the
heat emitting surface to produce a layer of the gallium or gallium
alloy; and coupling a heat sink or a heat spreader to the layer of
gallium or the gallium alloy to form the thermal interface.
15. The method of claim 14, wherein the plating bath comprises an
alkane sulfonic acid selected from the group consisting of methane
sulfonic acid, ethane sulfonic acid, propane sulfonic acid, and
butane sulfonic acid, and wherein the alkane sulfonic acid is at a
concentration of 0.1 M to 2 M.
16. The method of claim 14, wherein the solution comprises sodium
sulfate at a concentration of 0.01 M to 2 M.
17. The method of claim 14, wherein the organic additive is
selected from the group consisting of thioureas, thiazines,
sulfonic acids, sulfonic acids, allyl phenyl sulfone, sulfamides,
dithioxo-bishydroxylaminomolybdenum complex, and derivatives
thereof.
Description
BACKGROUND
[0001] This invention generally relates to electrodeposition
processes of gallium and gallium alloy films for fabrication of
thin film photovoltaic devices such as those containing copper,
indium, gallium, and/or selenium and as thermal interface
materials.
[0002] For photovoltaic applications, two layers of semiconductor
material having different characteristics are generally used in
order to create an electrical field and a resultant electrical
current. The first layer is typically an n-type semiconductor
material and is generally thin so as to let light pass through to
an underlying p-type semiconductor layer material layer, which is
often referred to as the absorbing layer. The absorbing layer in
combination with the n-type semiconductor material layer provides a
suitable band gap to absorb photons from the light source and
generate a high current and an improved voltage. For the p-type
layer, thin films of a copper-indium-gallium-diselenide
semiconductor material (i.e., CuInGaSe.sub.2 and variations thereof
also referred to as CIGS) or copper indium diselenide (i.e.,
CuInSe.sub.2, also referred to as CIS) or copper gallium diselenide
(i.e., CuGaSe.sub.2, also referred to as CGS) have generated
significant interest over the years for their use in photovoltaic
devices.
[0003] By way of example, the p-type CIGS layer is typically
combined with an n-type CdS layer to form a p-n heterojunction
CdS/CIGS device. Zinc oxide and doped zinc oxide may be added to
improve transparency. The direct energy gap of CIGS results in a
large optical absorption coefficient, which in turn permits the use
of thin layers on the order of 1-2 .mu.m. By way of example, it has
been reported that the absorbed layer band gap was increased from
1.02 electron-volts (eV) for a CuInSe.sub.2 (CIS) semiconductor
material to 1.1-1.2 eV by partial substitution of the indium with
gallium, leading to a substantial increase in efficiency.
[0004] Formation of the CIGS structure is typically by vacuum
deposition, chemical deposition or electrodeposition. The most
common vacuum-based process co-evaporates or co-sputters copper,
gallium, and indium, then anneals the resulting film with a
selenium or sulfur containing vapor to form the final CIGS
structure. An alternative is to directly co-evaporate copper,
gallium, indium and selenium onto a heated substrate. A
non-vacuum-based alternative process deposits nanoparticles of the
precursor materials on the substrate and then sinters them in situ.
Electrodeposition is another low cost alternative to apply the CIGS
layer. Although electrodeposition is an attractive option for
formation of gallium thin films, especially for photovoltaic
applications such as CIGS, current processes are generally not
commercially practical. Gallium is generally considered a difficult
metal to deposit without excessive hydrogen generation on the
cathode because the gallium equilibrium potential is relatively
high. Hydrogen generation on the cathode causes the deposition
efficiency to be less than 100% because some of the deposition
current gets used to form hydrogen gas rather than to form the
gallium film on the substrate or cathode. Low cathodic deposition
efficiency due to excessive hydrogen generation results in poor
process repeatability, partly due to the poor cathodic efficiency,
and most importantly to poor deposit film quality with high surface
roughness and poor deposit morphology.
[0005] Accordingly, there is a need in the art for improved
electrodeposition processes for depositing gallium and gallium
alloys as well as novel photovoltaic devices containing the same
with increased band gap to provide increased photovoltaic
current.
SUMMARY
[0006] The present invention is generally directed to methods of
forming a p-type semiconductor layer for a photovoltaic device. In
one aspect, the method comprises electroplating a first layer onto
a conductive surface of a substrate, wherein said first layer is
selected from the group consisting of a copper layer and a
copper-gallium layer; electroplating a second layer onto said first
layer, wherein said second layer is selected from the group
consisting of an indium layer, a gallium layer, an indium-gallium
layer, a copper-indium diselenide layer, and a
copper-gallium-diselenide layer; and optionally electroplating a
third layer onto said second layer, wherein said third layer is
selected from the group consisting of a gallium layer and an indium
layer; and optionally electroplating a fourth layer onto said third
layer, wherein said fourth is selected from the group consisting of
selenium and sulfur; wherein said electroplating is carried out by
a method comprising: contacting: (i) a substrate and (ii) a
solution comprising: a precursor comprising an element selected
from the group consisting of copper, gallium, indium, selenium,
sulfur and a combination thereof; optionally a metalloid compound
additive; further optionally an organic additive having at least a
sulfur atom; and a solvent to dissolve said precursors; wherein the
solution is free of complexing agents; adjusting a pH of said
solution to a range selected from the group consisting of a pH of
about zero to less than about 2.6 and a pH of about 12.6 to about
14, and applying a current to electroplate said substrate to
produce said first, second, third or fourth layers; and annealing
said first, said second and said third layers in the presence of a
selenium source and/or sulfur source to form the p-type
semiconductor layer.
[0007] In a method for forming a thermal interface, the method
comprises electroplating a layer of gallium or a gallium alloy onto
a heat emitting surface coupled to a microprocessor, wherein
electroplating the gallium or gallium alloy comprises contacting
the heat emitting surface with a plating bath free of complexing
agents comprising a gallium salt and an optional organic additive
comprising at least one sulfur atom, and a solvent; adjusting a pH
of the plating bath to a range selected from the group of a pH of
greater than about zero to less than 2.6 and a pH greater than
about 12.6 to about 14, and applying a current to electroplate the
heat emitting surface to produce a layer of the gallium or gallium
alloy; and coupling a heat sink or a heat spreader to the layer of
gallium or the gallium alloy to form the thermal interface
[0008] A photovoltaic device comprises at least one layer
comprising gallium or indium or alloys comprising gallium and
indium, wherein the at least one layer is formed by
electrodeposition; and an impurity in the at least one layer
selected from the group consisting of arsenic, antimony, bismuth,
selenium, sulfur and mixtures thereof.
[0009] Additional features and advantages are realized through the
techniques of the present invention. Other embodiments and aspects
of the invention are described in detail herein and are considered
a part of the claimed invention. For a better understanding of the
invention with advantages and features, refer to the description
and to the drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0010] The subject matter that is regarded as the invention is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The foregoing and other
objects, features, and advantages of the invention are apparent
from the following detailed description taken in conjunction with
the accompanying drawings in which:
[0011] FIG. 1 depicts a cross sectional view of a CIGS precursor
structure in accordance with the present invention;
[0012] FIG. 2 depicts a cross sectional view of a CIGS precursor
structure in accordance with the present invention;
[0013] FIG. 3 depicts a cross sectional view of a CIGS precursor
structure in accordance with the present invention;
[0014] FIG. 4 depicts a cross sectional view of a CIGS precursor
structure in accordance with the present invention;
[0015] FIG. 5 depicts a cross sectional view of a gallium thermal
interface;
[0016] FIG. 6 schematically illustrates an exemplary
electrodeposition apparatus for deposition of a gallium layer onto
a substrate;
[0017] FIG. 7 shows a scanning electron micrograph of a
cross-sectional view of a film stack wherein gallium was
electrodeposited onto an indium layer and subsequently annealed to
form an indium rich gallium eutectic layer;
[0018] FIGS. 8 and 9 show scanning electron micrographs of top down
views of gallium galvanostatically deposited from an acidic methane
sulfonic acid solution with added thiourea at 20 and 30
mAcm-.sup.2, respectively; and
[0019] FIG. 10 graphically illustrates cyclic voltammetry plots for
acidic gallium plating baths with no additives, with arsenic
trioxide additive, and with arsenic trioxide and thiourea as
additives.
[0020] The detailed description explains the preferred embodiments
of the invention, together with advantages and features, by way of
example with reference to the drawings.
DETAILED DESCRIPTION
[0021] The present invention provides low cost electrodeposition
processes for forming thin layers of gallium and/or gallium alloys
such as may be desired for forming various photovoltaic devices
(e.g., CIGS, CIS, CGS, and the like) and as a thermal
interface.
[0022] The electrodeposition processes utilize electroplating
solutions to deposit compositionally pure, uniform, substantially
defect free, and smooth thin films with high plating efficiency and
repeatability. The electroplating solutions are free of complexing
agents and can be practiced at both high and low pH ranges. Thin
films of alloys may also be electroplated. Examples of desirable
gallium alloys generally depend on the intended application and can
include, without limitation, binary, ternary or higher order alloys
of silver, copper, indium, zinc, tin, lead, silver, bismuth, gold,
selenium, sulfur, and the like. Optionally, the alloy can be formed
by annealing a film stack including an electrodeposited gallium
layer and one or more alloying element metal layers. In this
manner, low cost fabrication of gallium or gallium alloy thin films
is achieved wherein the gallium layer or the gallium alloy layer is
of uniform thickness, excellent morphology, and substantially
defect free.
[0023] In one embodiment, gallium is electrodeposited to form a
CIGS precursor structure configured to control gallium
inter-diffusion within the film stack. In the exemplary embodiment
shown in FIG. 1, a conductive layer 14 is first deposited onto the
substrate 12, which serves as a metal back contact. The conductive
layer may include, without limitation, molybdenum, tantalum,
tungsten, titanium, the corresponding nitrides thereof, and the
like. The conductive layer is generally deposited by any means at a
thickness of about 300 nm to about 600 nanometers (nm). A copper
layer 16 is then disposed onto the conductive layer 14 at a
thickness of about 10 nm to about 500 nm; in other embodiments, the
copper layer is at a thickness of 220 nm to 260 nm; and in still
other embodiments, the copper layer is at a thickness of about 240
nm. An indium layer 18 is then deposited onto the copper layer 16
at a thickness of 50 nm to 500 nm; in other embodiments, the indium
layer is at a thickness of 375 nm to 425 nm; and in still other
embodiments, the indium layer is at a thickness of about 420 nm.
The gallium layer 20 is then deposited onto the indium layer 18 at
a thickness of 20 nm to 200 nm; in other embodiments, the gallium
layer is at a thickness of 100 nm to 150 nm; and in still other
embodiments, the gallium layer is at a thickness of about 140 nm.
The gallium layer is deposited using the electrodeposition process
in accordance with the present disclosure. The other layers may be
deposited by any deposition technique, e.g. vacuum deposition, but
it is generally preferred that these layers be deposited by
electrodeposition.
[0024] FIG. 2 illustrates an exemplary film stack 30 suitable as a
CIGS precursor structure in accordance with another embodiment of
the present disclosure. In this exemplary embodiment, a conductive
layer 34 is first deposited onto the substrate 32 at a thickness of
about 300 nm to about 600 nm. A copper-gallium alloy layer 36 is
then electrodeposited onto the conductive layer 34 at a thickness
of 275 to 330 nm, e.g., 310 nm. An indium-gallium layer 38 is then
electrodeposited onto the copper-gallium layer 36 at a thickness of
420 to 500 nm, e.g., 490 nm. The ratio of Cu/(In+Ga) can be
maintained at 0.8 to 0.9 e.g., 0.88 and the ratio of Ga/(Ga+In) is
can be maintained at 0.3 to 0.33, e.g., 0.31. It should be apparent
in view of this disclosure that the conductive layer may be
deposited by any deposition technique but it is generally preferred
that these layers be deposited by electrodeposition. Gallium is a
very low melting point element. It is liquid at about 35.degree. C.
and as a result is very mobile and inter-diffuses readily. Alloying
gallium with higher melting point metals such as copper and indium
not only reduces the number of electrodeposition process steps but
also stabilizes the microstructure and allows better inter-mixing
of the precursor CIGS material. This ultimately results in better
compositional control of the CIGS p-absorber material.
[0025] FIG. 3 illustrates an exemplary film stack 50 suitable as a
CIGS precursor structure in accordance with another embodiment of
the present disclosure. In this exemplary embodiment, a conductive
layer 54 is first deposited onto the substrate 54. A copper layer
56 is then disposed onto the molybdenum layer 56. The thicknesses
of the copper and molybdenum layers are as previously described. An
indium-gallium layer 58 is then electrodeposited onto the copper
layer 56 at a thickness of 400 nm to 500 nm and most precisely 490
nm. The ratio of Cu/(In+Ga) can be maintained at 0.8 to 0.9, e.g.,
0.88 and the ratio of Ga/(Ga+In) can be maintained at 0.3 to 0.33,
e.g., 0.31. The conductive and copper layers may be deposited by
any deposition technique but it is generally preferred that these
layers be deposited by electrodeposition. Deposition of the InGa
alloy provides better control of CIGS precursor intermixing and
final control of the CIGS composition.
[0026] FIG. 4 illustrates an exemplary film stack 60 suitable as a
CIGS precursor structure in accordance with another embodiment of
the present disclosure. In this exemplary embodiment, a conductive
layer 64 is first deposited onto the substrate 62. A
copper-indium-selenium layer 66 is then electrodeposited onto the
molybdenum layer 64 at a thickness of 1 micron to 2.5 microns. A
gallium alloy layer 68 is then electrodeposited onto the
copper-selenium-indium layers 66. The thicknesses of the molybdenum
and gallium layers are as previously described. It should be
apparent in view of this disclosure that the conductive layer may
be deposited by any deposition technique but it is generally
preferred that these layers be deposited by electrodeposition. With
this method, some of the indium in the CuInSe2 material is
substituted by gallium forming CuInGaSe.sub.2 upon annealing.
[0027] The films stacks including the copper, gallium, and indium
layers as described above in relation to FIGS. 1-4 are then reacted
with selenium and/or sulfur to form a CuInGaSe.sub.2 or
CuInGaSe.sub.2S or CuInGaS structure. For example, a selenium layer
may be deposited onto the film stack and subsequently annealed to
form the selenide. Alternatively, the film stack can be exposed to
hydrogen selenide and/or hydrogen sulfide, for example, and
subsequently annealed. Annealing in sulfur and/or selenium
atmosphere may occur at a temperature of about 400.degree. C. to
about 700.degree. C. and preferably 550.degree. C. It should be
apparent to those skilled in the art that subsequent to CIGS
formation, deposition of an n-type junction layer (not shown) is
then disposed onto the CIGS layer. As noted above, this layer will
interact with the CIGS layer to form a p-n junction. The next layer
to be deposited is typically a ZnO and doped ZnO transparent oxide
layer (not shown). Moreover, it should be apparent that the
electroplating process can be utilized to form precursor layers for
other types of photovoltaic devices, e.g., copper-indium-selenium
(CIS), copper-gallium-selenium (CGS), copper-indium-sulfur (CISu),
copper gallium sulfur (CGSu) and the like.
[0028] In the various embodiments described above, the resulting
CIGS structures generally have a Cu/(in+Ga) ratio of 0.8 to about
0.9 and a Ga/(Ga+In) ratio of 0.3 to about 0.33.
[0029] In another embodiment, a gallium layer or a gallium alloy
layer is electrodeposited to form a thermal interface. The layer of
gallium or gallium alloy can be electroplated as a stack on the
underlayer of Zn, Sn, In, Au, Cu, mixtures thereof, of the like.
Gallium provides low tensile strength as well as high bulk thermal
conductivity. As an alloy, self diffusion provides a low melting
alloy suitable for its application as a thermal interface material.
As such, gallium can be alloyed with other elements to lower the
melting. FIG. 5 illustrates an exemplary device including a
microprocessor chip coupled to a heat sink to prevent overheating
by adsorbing its heat and dissipating the heat into the air. The
device 100 includes a substrate 102 upon which the microprocessor
104 is formed and mounted. A gallium or gallium alloy layer 106 is
electrodeposited onto a surface of the microprocessor. A heat sink
108 is then coupled to the gallium layer. Table 1 provides
exemplary gallium alloys suitable for use as a thermal interface
and the corresponding liquidus and solidus temperatures.
TABLE-US-00001 TABLE 1 Composition Liquidus (.degree. C.) Solidus
(.degree. C.) 61.0Ga/0.25In/13.0Sn/1.0Zn 7.6 6.5
62.5Ga/21.5In/16.0Sn 10.7 10.7 75.5Ga/24.5In 15.7 15.7 95Ga/5In
25.0 15.7 100Ga 29.8 29.8
[0030] The electrodeposition processes for forming the gallium or
gallium alloy layers generally include electroplating a substrate
surface (e.g., a working electrode) disposed in an aqueous plating
bath comprising a gallium salt, a methane sulfonic acid (MSA)
electrolyte, and a solvent. The pH of the bath can be controlled
using an acid or a base. The concentration of gallium ions in the
electrolyte may range from about 0.000005 Molar (M) M up to molar
concentrations close to the saturation limit in the electrolyte and
pH used. Useful gallium sources for the plating bath include
gallium salts soluble within the plating bath including, without
limitation, gallium chloride (GaCl.sub.3), gallium bromide
(GaBr.sub.3), gallium iodide (Gal.sub.3), gallium nitrate
Ga(NO.sub.3).sub.3, gallium sulfate Ga(SO.sub.4).sub.3, mixtures
thereof, and the like. Other suitable gallium salts include salts
of sulfuric acid, sulfamic acid, alkane sulfonic acid, aromatic
sulfonic acid, fluoroborate, and strong bases such as sodium
hydroxide, potassium hydroxide, lithium hydroxide, calcium
hydroxide, magnesium hydroxide, and the like.
[0031] The concentration of acid such as MSA as the electrolyte may
range from about 0.1 M to about 2 M; in other embodiments, the acid
is in a range of about 0.1 M to 1 M; and in still other
embodiments, the acid is in a range of 0.5 M to 1 M. As described,
the electrolyte bath is free from any kind of organic or inorganic
complexing agents. That is, the gallium salt is soluble within the
electrolyte bath.
[0032] The pH of the electrolyte bath is generally less than 2.6 or
greater than 12.6. Applicants have discovered that the plating bath
becomes cloudy, i.e., milky like in appearance, when the solution
pH is in the range of 2.6.ltoreq.pH.ltoreq.12.6. While not wanting
to be bound by theory, it is believed that oxides and/or hydroxides
of gallium are formed within this pH range, e.g. gallium oxides and
hydroxides in aqueous solutions. Suitable acids or bases to provide
and maintain the pH of the electrolyte bath are exclusive of
complexing agents and may include, without limitation, mineral
acids such as sulfuric acid, organic acids such as methane sulfonic
acid, ethane sulfonic acid, propane sulfonic acid, butane sulfonic
acid or other alkane sulfonic acid and aromatic sulfonic acid such
as benzene sulfonic acid, and toluene sulfonic acid.
Advantageously, it has been discovered that the electrodeposition
processes at these pH ranges provide a uniform, thin conformal
gallium layer, thereby preventing individual island formation.
[0033] The alloying elements may be added directly to the bath. For
example, copper in the electrolyte may be provided by a copper
source such as dissolved copper metal or a copper salt such as
copper sulfate, copper chloride, copper acetate, cupper nitrate,
and the like. Likewise, indium may be provided in the electrolyte
by an indium source such as indium chloride, indium sulfate, indium
sulfamate, indium acetate, indium carbonate, indium sulfate, indium
phosphate, indium oxide, indium perchlorate, indium hydroxide, and
the like.
[0034] The gallium electroplating bath may further include an
optional organic additive comprising at least one nitrogen atom or
at least one sulfur atom. The organic additive is added to the
plating bath to effectively increase hydrogen evolution
over-potential and prevent or effectively limit the
co-deposition/evolution of hydrogen during plating of gallium and
to control microstructure of the deposit by controlling nucleation
and growth. Advantageously, the additive also functions as a
brightener and grain refiner while concomitantly assisting with
gallium nucleation. Thinnest layers are formed by instantaneous
nucleation where the same size islands form simultaneously on a
surface. Also, thin layers can be formed by progressive nucleation
where formation of islands is a function of time. In doing so, the
resulting gallium layer is uniform and conformal, thereby
preventing large three dimensional island formations during
deposition. Exemplary organic additives include, without
limitation, aliphatic and/or heterocyclic compounds such as
thioureas, thiazines, sulfonic acids, sulfonic acids, allyl phenyl
sulfone, sulfamides, imidazoles, amines, isonitriles,
dithioxo-bishydroxylaminomolybdenum complex, and derivatives
thereof.
[0035] The organic additive comprising the at least one nitrogen
atom and/or at least one sulfur atom additive has been found to
unexpectedly accelerate gallium plating while suppressing hydrogen
evolution. In this manner, it has been discovered that the organic
additive provides a synergistic effect when employed in combination
with MSA as the electrolyte. The concentration of the organic
additive comprising the at least one nitrogen atom and/or at least
one sulfur atom may range from about 1 parts per million (ppm) to
about 10000 ppm, in other embodiments, the organic additive is in a
range of about 10 ppm to 5000 ppm, and in still other embodiments,
the organic additive is in a range of 100 ppm to 1000 ppm.
[0036] In other embodiments, a metal oxide is added in combination
with the organic additive to poison the cathode, thereby increasing
the onset over-potential of hydrogen evolution (i.e., inhibit
hydrogen generation) and accelerating gallium deposition. The
inorganic metal oxide includes, without limitation, oxides of
metalloids such as arsenic oxides (e.g., As.sub.2O.sub.3;
As.sub.2O.sub.5, KH.sub.2AsO.sub.4, K.sub.2HAsO.sub.4,
K.sub.3AsO.sub.4, K.sub.3AsO.sub.3, KAsO.sub.2, NaH.sub.2AsO.sub.4,
Na.sub.2HAsO.sub.4, Na.sub.3ASO.sub.4, Na.sub.3ASO.sub.3,
NaAsO.sub.2, Na.sub.4AS.sub.2O.sub.7, and the like); antimony
oxides, (e.g., Sb.sub.2O.sub.3, Sb.sub.2O.sub.5, KH.sub.2SbO.sub.4,
K.sub.2HSbO.sub.4, K.sub.3SbO.sub.4, K.sub.3SbO.sub.3, KSbO.sub.2,
NaH.sub.2SbO.sub.4, Na.sub.2HSbO.sub.4, Na.sub.3SbO.sub.4,
Na.sub.3SbO.sub.3, NaSbO.sub.2, Na.sub.4Sb.sub.2O.sub.7, and the
like); and bismuth oxides (e.g., Bi.sub.2O.sub.3, K.sub.3BiO.sub.3,
KBiO.sub.2, Na.sub.3BiO.sub.3, NaBiO.sub.2 and the like).
[0037] Gallium deposition and hydrogen evolution are known to occur
simultaneously, and thus, prior art plating processes generally
exhibit low plating efficiencies in order to prevent hydrogen
evolution, which contributes to porosity within the deposited film
structure The metal oxides described above are effective cathodic
poisons and advantageously increase the onset of over-potential of
hydrogen evolution and unexpectedly accelerate gallium deposition.
Plating efficiencies greater than 90 to 95% have been observed with
gallium plating solutions including the combination of the metal
oxide and the organic additive comprising at least one nitrogen
atom and at least one sulfur atom. The concentration of metal oxide
in the electrolyte may range from about 1 parts per million (ppm)
to about 10,000 ppm, in other embodiments, the metal oxide is in a
range of about 100 ppm to 5,000 ppm, and in still other
embodiments, the metal oxide is in a range of 1,000 ppm to 3,000
ppm. By introducing the metal oxide and/or sulfur in the plating
bath, the resulting layer will include the corresponding metal
(e.g., arsenic, antimony, bismuth or mixtures thereof), and/or
sulfur as an impurity on the order of a few parts per million up to
a few atomic percent in the deposit, which can be detected using an
Auger or SIMS analytical method.
[0038] In another embodiment, the plating bath includes a gallium
salt, a sodium sulfate (Na.sub.2SO.sub.4) electrolyte, an organic
additive comprising the at least one nitrogen atom and/or at least
one sulfur atom, and a solvent. The concentrations of the gallium
salt and the organic additive are as previously described. The
concentration of sodium sulfate as the electrolyte may range from
about 0.01 M to about 2 M; in other embodiments, the sodium sulfate
is in a range of about 0.1 M to 1 M; and in still other
embodiments, the sodium sulfate is in a range of 0.2 M to 60 M.
Optionally, the metal oxide as described above may be included in
plating bath. The pH is less than 2.6 or greater than 12.6 as
previously described.
[0039] In the various embodiments described above, the
electroplating chemistry can be used on conductive and
non-conductive substrates. Suitable conductive substrates include,
without limitation, gold, molybdenum, indium copper, selenium,
zinc, and the like. Suitable non-conductive substrates generally
are those having a metal seed layer thereon and include, without
limitation, glass, quartz, plastic, polymers, and the like. For
example, the non-conductive substrate may include a seed layer,
e.g., a copper seed layer. The particular method for depositing the
seed layer is not limited and is well within the skill of those in
the art. For example, the seed layer may be formed by chemical
vapor deposition, plasma vapor deposition, or electroless
deposition.
[0040] The electroplating baths may also comprise additional
ingredients. These include, but are not limited to, grain refiners,
surfactants, dopants, other metallic or non-metallic elements etc.
For example, other types of organic additives such as surfactants,
suppressors, levelers, accelerators and the like 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, polyalkane sulfonic acids,
coumarin, saccharin, furfural, acryonitrile, magenta dye, glue,
starch, dextrose, and the like.
[0041] Although water is the preferred solvent in the formulation
of the plating baths, 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, and the like.
[0042] Although DC voltage/current can be utilized during the
electrodeposition processes, it should be noted that pulsed or
other variable voltage/current sources may also be used to obtain
high plating efficiencies and high quality deposits. The
temperature of the electroplating baths may be in the range of 5 to
90.degree. C. depending upon the nature of the solvent. The
preferred bath temperature for water based formulations is in the
range of 10 to 30.degree. C.
[0043] Referring now to FIG. 6, in practice, a backside electrical
contact 5 is made to a conductive substrate 4, which functions as
the working electrode, upon which gallium or gallium alloy is to be
electrodeposited. Alternatively, if the substrate is
non-conducting, a conductive layer and/or a seed layer (not shown)
can first be deposited and electrical contact can be made directly
to the seed layer via ohmic contact or to the underlying conductive
layer. An electrolyte solution 1 in accordance with the present
disclosure is placed in contact with the substrate surface 4. A
conductive counter electrode 6, i.e., anode or conductor, is
positioned in the electrolyte solution and spaced apart from the
substrate (working electrode). While the substrate 4 is shown as
having a planar surface, it is understood that substrate 4 can also
have some topography and/or conformal conductive layers thereon.
For electrochemical processing, an electrical current or voltage is
applied to the substrate (electrode) 4 and the counter electrode 6
via a power supply 7 and electrical leads 8. If desired, the
electrochemical potential of the structure/electrolyte can be
controlled more accurately by the introduction of a third
electrode, that is, a reference electrode (not shown), which has
constant electrochemical potential. Examples of reference
electrodes include a saturated calomel electrode (SCE) and
silver-silver chloride (Ag/AgCl) reference electrodes or other
metal reference electrodes such as Cu or Pt. The electrolyte
solution can be agitated during electrodeposition.
[0044] The following examples are presented for illustrative
purposes only, and are not intended to limit the scope of the
invention.
Example 1
[0045] In this example, gallium was electroplated onto a film stack
and subsequently self-annealed to form an indium rich
indium-gallium alloy. The plating chemistry included 0.2M Ga.sup.3+
in 0.5M MSA quenched with 0.5 M NaOH and then adjusted to a pH of
1.21 using additional amounts of MSA. Gallium was electroplated
onto a 360 nm indium layer and a 250 nm copper layer. The gallium
layer with a thickness of 150 nm was subsequently self-annealed at
room temperature 18-22.degree. C. for a period of 3 days. Upon
plating gallium on indium, interdiffusion has onset immediately and
progressively formed In--Ga eutectic alloy.
[0046] FIG. 7 shows a scanning electron micrograph of a
cross-sectional view of a film stack wherein gallium was
electrodeposited onto the indium layer and subsequently annealed to
form an indium-rich gallium eutectic layer. Interestingly, the Ga
interdiffusion did not stop at the indium layer and continued into
the copper forming an alloy of CuInGa.
Example 2
[0047] In this example, various gallium plating baths with and
without the organic additive were used to electro deposit gallium
onto glass substrates having thereon a molybdenum layer that had
previously been seeded with copper. The plating solution included
0.25 M gallium sulfate in 0.5 M MSA with 0 and 500 ppm of thiourea.
The electrolyte bath was at 18-20.degree. C. and agitated at 0 and
550 rpm. The pH was maintained at 1.14 using H.sub.2SO.sub.4.
[0048] The results show that the presence of the organic additive
clearly accelerated gallium plating relative to plating baths that
did not contain the organic additive. Moreover, continuous
agitation of the electrolyte provided significantly higher current
densities than without. FIGS. 8 and 9 pictorially illustrate
surface topographic views of the galvanostatically deposited
gallium film at 20 mA/cm.sup.2 and 30 mA/cm.sup.2, respectively. An
increase in grain size was observed with the increased current
density. No porosity was observed and the films were uniform and of
excellent morphology.
Example 3
[0049] In this example, the plating bath included 0.2MGa.sup.3+ in
0.5M MSA quenched with 0.5M NaOH and then adjusted by adding more
MSA to obtain a pH of 1.18. Varying amounts of As.sub.2O.sub.3 were
included in the plating bath, where indicated. For the plating bath
that included no As.sub.2O.sub.3 or thiourea, the plating bath
included Ga.sup.3+ in 0.5 M MSA quenched with 0.5 M NaOH with the
pH adjusted to 1.18 using additional MSA. The plating bath that
included a combination of As.sub.2O.sub.3 and thiourea contained
As.sub.2O.sub.3 was at 500-6000 ppm and the thiourea was at
100-1000 ppm.
[0050] FIG. 10 provides an overlay of the various voltammetry plots
and includes data for the combination of As.sub.2O.sub.3 and
thiourea. As shown, the increasing amounts of As.sub.2O.sub.3
provided a negative potential shift for onset of hydrogen evolution
over-potential, thereby effectively inhibiting hydrogen generation.
In addition, the combination of thiourea and As.sub.2O.sub.3
accelerated gallium deposition.
Example 4
[0051] In this example, the plating bath included 0.25M Ga.sup.3+
in 0.5 M MSA quenched with 0.5 M NaOH and adjusted to a pH of 1.18
using additional amounts of MSA. Plating was carried out without
any additional additives, with 6000 ppm As.sub.2O.sub.3, and with
6000 ppm As.sub.2O.sub.3 and 500 ppm thiourea. Cyclic voltammetry
plots of these plating chemistries are provided in FIG. 10.
Inhibition of hydrogen evolution and acceleration of gallium
deposition was observed upon addition of As.sub.2O.sub.3 and
further increases in acceleration with the combination of
As.sub.2O.sub.3 and thiourea. It has also been shown that the
combination of As.sub.2O.sub.3 and As.sub.2O.sub.5 is also
effective for inhibiting the hydrogen evolution (results not shown
here). When both of these oxides are combined together then the
effect is much effective even at lower concentrations.
[0052] All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are combinable with each other.
[0053] All cited patents, patent applications, and other references
are incorporated herein by reference in their entirety.
[0054] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the invention (especially in
the context of the following claims) are to be construed to cover
both the singular and the plural, unless otherwise indicated herein
or clearly contradicted by context. Further, it should further be
noted that the terms "first," "second," and the like herein do not
denote any order, quantity, or importance, but rather are used to
distinguish one element from another.
[0055] While the preferred embodiment to the invention has been
described, it will be understood that those skilled in the art,
both now and in the future, may make various improvements and
enhancements which fall within the scope of the claims which
follow. These claims should be construed to maintain the proper
protection for the invention first described.
* * * * *