U.S. patent application number 11/384208 was filed with the patent office on 2006-11-16 for method of depositing films on aluminum alloys and films made by the method.
This patent application is currently assigned to Science & Technology Corporation @ UNM. Invention is credited to Dmitri A. Brevnov, Tim S. Olson.
Application Number | 20060254922 11/384208 |
Document ID | / |
Family ID | 37418072 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060254922 |
Kind Code |
A1 |
Brevnov; Dmitri A. ; et
al. |
November 16, 2006 |
Method of depositing films on aluminum alloys and films made by the
method
Abstract
Method for depositing a metallic material on an aluminum alloy
surface for galvanic displacement type deposition,
electrodeposition or electroless deposition of a metallic film on
the surface wherein the alloy surface is oxidized (e.g. anodized)
to form aluminum oxide and the oxidized surface is etched to leave
a partial thickness of a barier aluminum oxide on the alloy
surface. The partial thickness of the barrier oxide is controlled
by etching to form a porous, metallic particulate film for a thin
barrier oxide, or a continuous metallic film for thicker barrier
oxide. The metallic film then is electrodeposited or electroless
deposited on the barrier film.
Inventors: |
Brevnov; Dmitri A.;
(Albuquerque, NM) ; Olson; Tim S.; (Albuquerque,
NM) |
Correspondence
Address: |
Mr. Edward J. Timmer
P.O. Box 770
Richland
MI
49083-0770
US
|
Assignee: |
Science & Technology
Corporation @ UNM
|
Family ID: |
37418072 |
Appl. No.: |
11/384208 |
Filed: |
March 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11201766 |
Aug 11, 2005 |
|
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|
11384208 |
Mar 17, 2006 |
|
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60663659 |
Mar 21, 2005 |
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Current U.S.
Class: |
205/205 |
Current CPC
Class: |
C25D 11/18 20130101;
C23C 18/42 20130101; C25D 15/00 20130101; C23C 18/1844 20130101;
C23C 18/54 20130101 |
Class at
Publication: |
205/205 |
International
Class: |
C25D 5/34 20060101
C25D005/34 |
Claims
1. Method of depositing a metallic material on a substrate,
comprising the steps of providing a substrate comprising an alloy
of aluminum and an alloying element, oxidizing a surface of the
substrate to form aluminum oxide thereon, etching the oxidized
surface to leave a partial thickness of a barrier aluminum oxide of
said aluminum oxide on the surface, and depositing by galvanic
displacement type deposition, electroless deposition or
electrodeposition discrete metallic nanoparticles on the barrier
oxide having a particle density of about 10.sup.4 to about
10.sup.12 particle/cm.sup.2.
2. The method of claim 1 wherein said etching is conducted to leave
a barrier oxide portion having a partial thickness to increase
nucleation of the discrete nanoparticles thereon.
3. The method of claim 1 wherein the substrate comprises a film or
layer of the alloy.
4. The method of claim 1 wherein the alloying element includes one
or more of Au, Cu, Cr, Mn, Mo, Ni, Si, Ta, Ti, or Zn.
5. The method of claim 1 wherein the surface is oxidized by
anodizing, polishing, alkaline etching, acid pickling,
electropolishing, or heating in an oxygen bearing atmosphere.
6. The method of claim 1 wherein the surface is acid etched by
contact with a mixture of phosphoric acid and an inhibitor for
aluminum dissolution.
7. The method of claim 6 wherein the inhibitor comprises chromic
acid.
8. The method of claim 1 wherein the metallic material comprises
one of Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe, W, Mo, or Co.
9. The method of claim 1 wherein the substrate is disposed on a
silicon wafer.
10. A substrate comprising an alloy of aluminum and an alloying
element, said substrate having a barrier oxide on a substrate
surface and a porous, three dimensional structure of electrically
interconnected, metallic nanoparticles deposited by electroless
deposition or electrodeposition on the barrier oxide.
11. The substrate of claim 10 wherein the structure includes
randomly packed, generally spherical metallic nanoparticles having
a distribution of particle sizes.
12. The substrate of claim 10 wherein the nanoparticles have a
particle diameter in the range of about 20 nm to about 1000 nm.
13. The substrate of claim 10 wherein the nanoparticles have an
interparticle spacing sufficient to provide electrolyte access
among the nanoparticles, providing a high surface area
material.
14. The substrate of claim 10 wherein the structure has a ratio
between electrolyte accessible area and geometric surface area of
about 100 and above.
15. Electrode comprising a substrate comprising an alloy of
aluminum and an alloying element, said substrate having a barrier
oxide on a substrate surface and a porous, electrolyte accessible,
three dimensional structure of electrically interconnected,
generally spherical, randomly packed metallic nanoparticles
deposited by galvanic displacement type deposition, electroless
deposition or electrodeposition on the barrier aluminum oxide.
16. The electrode of claim 15 wherein the nanoparticles have a
particle diameter in the range of about 20 nm to about 1000 nm.
17. The electrode of claim 15 wherein the structure has a ratio
between electrolyte accessible area and geometric surface area of
about 100 and above.
18. The electrode of claim 15 wherein the barrier oxide film has
been chemically etched.
19. The electrode of claim 15 wherein the substrate is disposed on
a silicon wafer.
20. Capacitor having an electrode in accordance with claim 15.
21. Method of depositing a metallic material on a substrate,
comprising the steps of providing a substrate comprising an alloy
of aluminum and an alloying element, oxidizing a surface of the
substrate to form aluminum oxide thereon, etching the oxidized
surface to leave a partial thickness of a barrier aluminum oxide of
said aluminum oxide on the surface, and depositing by galvanic
displacement type deposition, electroless deposition or
electrodeposition a continuous metallic film on the barrier
oxide.
22. The method of claim 21 wherein said etching is conducted to
leave a barrier oxide portion having a partial thickness to deposit
a continuous metallic film.
23. The method of claim 21 wherein the substrate comprises a film
or layer of the alloy.
24. The method of claim 21 wherein the alloying element includes
one or more of Au, Cu, Cr, Mn, Mo, Ni, Si, Ta, Ti, or Zn.
25. The method of claim 21 wherein the surface is oxidized by
anodizing, polishing, alkaline etching, acid pickling,
electropolishing, or heating in an oxygen bearing atmosphere.
26. The method of claim 21 wherein the surface is acid etched by
contact with a mixture of phosphoric acid and an inhibitor for
aluminum dissolution.
27. The method of claim 26 wherein the inhibitor comprises chromic
acid.
28. The method of claim 21 wherein the metallic material comprises
one of Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe, W, Mo, or Co.
29. The method of claim 21 wherein the substrate is disposed on a
silicon wafer.
Description
[0001] This application is a continuation-in-part of U.S. Ser.
No.11/201,766 filed Aug. 11, 2005, and claims benefits and priority
of provisional application Serial No. 60/663,659 filed Mar. 21,
2005.
FIELD OF THE INVENTION
[0002] The invention relates to galvanic displacement type
deposition, electroless deposition or electrodeposition of metallic
films on treated aluminum alloys as well as to the deposited
metallic films and components, such as capacitor electrodes,
embodying the metallic films.
BACKGROUND OF THE INVENTION
[0003] The surface of aluminum metal is spontaneously oxidized in
the ambient atmosphere. This oxidation creates a dielectric film of
native aluminum oxide, which has an adverse effect on
electrodeposition or electroless deposition of metals or alloys
such as Ni, Ag, Au, and Cu and their alloys.
[0004] With respect to overcoming the problem of electrodeposition,
the zincate process has been employed in industry for the
deposition of adhesive metallic films on aluminum. The process
consists of immersing the aluminum substrate in a strong alkaline
zincate solution. The native aluminum oxide is dissolved, and zinc
is deposited on the surface via galvanic displacement of aluminum.
As a result, the zinc-coated aluminum surface becomes amenable for
electrodeposition of adhesive layers of metals, including nickel
and copper. Zincate surface activation of aluminum has proven to be
a cost-effective process for nickel bumping of wafers prior to
flip-chip assembly.
[0005] Since the zincate method is sensitive to many variables,
there are incentives for developing alternative methods for the
deposition of metals on aluminum. One alternative method has
involved direct electrodeposition of copper on aluminum using
several copper complexes. An electroplating procedure for nickel
displacement of aluminum followed by electroless nickel deposition
has also discussed. In addition, an organic solvent has been used
to lay a seed layer of copper or palladium on aluminum substrates.
Then, electroless deposition with a reducing agent was utilized to
deposit substantially more copper.
[0006] In general, the galvanic displacement type deposition
process proceeds via two concurrent electrochemical reactions,
which involve the reduction of ions of metals and the oxidation of
the substrate surface. The driving force for this process is
determined by a difference in half-cell potentials (e.g. redox
potentials for corresponding metal/metal ion and oxidized
substrate/substrate pairs). The half-cell potential of the reduced
species has to be more positive than that of the oxidized
substrate. Chemical etching, which effectively removes the surface
layer of oxide, precedes and/or takes place simultaneously with the
deposition of a film of metal.
[0007] The present invention involves deposition of metallic films
(either porous or continuous) on an aluminum alloy surface to offer
the opportunity for fabrication of heat dissipation systems, energy
conversion and storage devices. For example, double layer
capacitors are energy storage devices that store electrical energy
by sustaining an electrical charge in a thin double layer at the
interface between an ionically conducting electrolyte and an
electronically conducting electrode. Potential applications for
double layer capacitors include memory protection in electronic
circuitry, portable electronic, and communication devices. Double
layer capacitors can be built as either a self-standing device or a
part of integrated electronic system. Mesoporous carbon, carbon
nanotubes and other carbonaceous materials have been extensively
investigated for use in double layer capacitors because of their
very high specific surface areas. In contrast to porous metallic
films, limitations of double layer capacitors based upon
carbonaceous materials are two-fold. First, capacitance of these
materials typically degrades at frequencies higher than 10 Hz. The
second limitation arises from problematic incorporation of these
capacitors into technologically relevant materials such as
silicon.
[0008] In addition, the present invention involves deposition of
metallic films on an aluminum alloy surface to offer the
opportunity for fabrication of optical devices for surface enhanced
FT-IR spectroscopy, surface enhanced Raman scattering and
metal-enhanced fluorescence. In addition, composite materials with
noble metal particles may have useful photo-catalytic,
anti-microbial properties and tunable surface plasmon resonances.
In addition, the deposition of continuous metallic films on
aluminum alloys may be used for metallization of aluminum, for
providing a soldering surface on aluminum and, consequently, for
packaging of electronic devices (zincate-free nickel bumping of
wafers prior to flip-chip assembly).
SUMMARY OF THE INVENTION
[0009] One embodiment of the present invention provides a method of
depositing a metallic material on a substrate wherein the method
includes the steps of providing a substrate comprising an alloy of
aluminum and an alloying element, oxidizing a surface of the alloy
substrate to form aluminum oxide thereon, etching the oxidized
surface to leave a partial thickness of a barrier aluminum oxide on
the alloy surface, and depositing by galvanic displacement type
deposition, electroless deposition or electrodeposition discrete
metallic nanoparticles having a particle density of about 10.sup.4
to about 10.sup.12 particles/cm.sup.2 on the barrier oxide.
[0010] In an illustrative embodiment of the invention, the etching
step is conducted to leave a partial thickness of the barrier oxide
to increase nucleation of discrete metallic nanoparticles thereon.
This embodiment of the present invention provides an aluminum alloy
substrate having a partial thickness of the barrier oxide on the
surface and a porous, three dimensional film structure of
electrically interconnected, discrete metallic metal nanoparticles
deposited by galvanic or electroless deposition or
electrodeposition on the barrier oxide. In this case, the
nucleation density is high enough so that the neighboring metallic
particles form the electrical connections to each other. The film
structure includes randomly packed, generally spherical, metallic
nanoparticles having a distribution of particle sizes. An
electrode, such as a double layer capacitor electrode, can comprise
the alloy substrate having the barrier oxide and porous and
metallic film structure thereon.
[0011] Still another embodiment of the present invention provides a
method of depositing a metallic material on a substrate wherein the
method includes the steps of providing a substrate comprising an
alloy of aluminum and an alloying element, oxidizing a surface of
the substrate to form aluminum oxide thereon, etching the oxidized
surface to leave a partial thickness of the barrier oxide on the
alloy surface, and depositing by electroless deposition or
electrodeposition a continuous metallic film on the barrier oxide.
As an illustration, the metallic film is composed of Ni. In an
illustrative embodiment, the etching step is conducted to almost
completely remove the barrier oxide in order to deposit a
continuous metallic film.
[0012] Features and advantages of the invention will become more
readily apparent from the following detailed description taken with
the following drawings.
DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a SEM (scanning electron micrograph) collected
after the electroless deposition of silver particles on anodized
and chemically etched aluminum-copper alloy film for 24 hours. FIG.
1B is a high magnification SEM of the deposit cross-section.
[0014] FIG. 2A are cyclic voltammograms obtained for silver
particles/aluminum-copper alloy film electrode, and FIG. 2B shows
charging current dependence upon the scan rate. While the solid
line shows the fit over the whole range of scan rates, two dotted
lines show the fit over two regions of slow and fast scan
rates.
[0015] FIG. 3A is a Bode plot (data as symbols, modeling as lines)
of EIS results, FIG. 3B is a Nyquist plot. FIG. 3C shows the
equivalent circuit.
[0016] FIG. 4 shows polarization curves of the anodized and etched
Al-Cu alloy film electrode where curve (a) is before and curve (b)
is after addition of AgNO.sub.3 to the final concentration of 3.0
mM.
[0017] FIG. 5 is a SEM micrograph collected after galvanic
displacement type deposition of silver for 40 min.
[0018] FIG. 6 is a SEM micrograph collected after electrodeposition
of silver at--1.3 V for 40 min.
[0019] FIG. 7 is a Bode plot of EIS data collected at OCP after
galvanic displacement type deposition of silver by galvanic
displacement for 40 min. Experimental data are symbols and results
of modeling are solid lines. FIG. 7A shows the equivalent circuit
is shown as an inset.
[0020] FIG. 8 is a Bode plot of EIS data collected at OCP after
electrodeposition of silver at--1.3 V for 40 min. Experimental data
are symbols and results of modeling are solid lines.
[0021] FIG. 9 shown the equivalent circuit used for modeling of the
electrodeposited porous film of silver (FIG. 8).
[0022] FIGS. 10A-10F are diagrams of electrodeposition on patterned
Al-Cu alloy film substrates where FIG. 10A shows photolithographic
patterning; FIG. 10B shows anodization at 90 V for 2-3 min to form
a dense layer of Al.sub.2O.sub.3; FIG. 10C shows photoresist
removal to expose the underlying Al-Cu alloy layer or film; FIG.
10D shows anodization at 50 V for 20 min to form porous
Al.sub.2O.sub.3; FIG. 10E shows chemical etching of porous
A1.sub.20.sub.3; and FIG. 10F shows electrodeposition of
silver.
[0023] FIG. 11A is a SEM micrograph of patterned features, showing
four circular regions of electrodeposited silver and FIG. 11B is a
SEM micrograph of a patterned feature with electrodeposited
silver.
DETAILED DESCRIPTION THE INVENTON
[0024] The invention provides a method for the galvanic
displacement type deposition, electroless deposition or
electrodeposition of a metallic film on a treated surface of an
alloy of aluminum and an alloying element. The aluminum alloy can
comprise an alloy of aluminum and one or more alloying elements to
provide a binary, ternary, quaternary, etc. aluminum alloy. For
purposes illustration and not limitation, the alloying element can
include, but is not limited to, one or more of Au, Cu, Cr, Mn, Mo,
Ni, Si, Ta, Ti, or Zn, or combinations thereof.
[0025] The invention can be practiced to deposit a variety of
metallic layers or films on the aluminum alloy surface. For
purposes of illustrating and not limiting the invention, the
invention is useful to deposit a metallic layer or film comprising
Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe, W, Mo, or Co wherein the term
metallic film includes a layer or film comprising a metal or an
alloy of these metals one with another or with another different
metal, or mixture of two or more metals, deposited concurrently or
sequentially to provide a metallic film on the surface whereby the
deposited metallic film comprises a binary alloy deposit, ternary
alloy deposit, quaternary alloy deposit and so on. The metallic
film can have a thickness in the range of 1 nm to 10 microns for
purposes of illustration and not limitation; however, practice of
the invention is not limited to any particular thickness of the
metallic film since any suitable metallic film thickness can be
deposited.
[0026] The method envisions providing an aluminum alloy surface
that is treated in a manner effective to render the alloy surface
amenable to galvanic displacement type deposition,
electrodeposition or electroless deposition of a metallic film
thereon. Galvanic deposition and electroless deposition both can
occur with no external electrical power requirement such that
galvanic deposition is considered by some to be a form of
electroless deposition. Galvanic deposition generally is a
deposition process in which the substrate comprises a less noble
element that acts as a reducing agent for a metal cation dissolved
in the deposition solution to effect deposition of the metal.
Electroless deposition involves providing a reducing agent in the
deposition solution containing the metal cation to be deposited to
effect its deposition on the substrate, which may not be less noble
than the metal to be deposited. The alloy surface to be treated
pursuant to the invention can include, but is not limited to, any
type of aluminum alloy substrate, layer, film, or other surface on
which the metallic film comprising a metal or alloy is to be
deposited by electrodeposition or electroless deposition.
[0027] The method of the invention involves treating the aluminum
alloy surface by oxidizing the surface to form aluminum oxide
thereon wherein the aluminum oxide comprises an outer porous
aluminum oxide and an inner barrier aluminum oxide adjacent the
alloy surface. The anodization results in enrichment of the
alloying element present in the aluminum alloy, such as for example
Cu, under and/or in the barrier aluminum oxide.
[0028] The invention can be practiced using anodizing to oxidize
the surface to form aluminum oxide thereon. However, practice of
the invention is not limited to any particular anodizing process.
For example, the anodizing process can vary with particular type of
surface to be treated. Any conventional anodizing process can be
used with the type of electrolyte and parameters of anodizing, such
as anodization voltage, electrical current density, temperature and
electrolyte acidity being selected as desired. For example, the
anodizing process can be conducted in any conventional aqueous
electrolyte that includes, but is not limited to, solutions of
oxalic acid, sulfuric acid, phosphoric acid, chromic acid, and
mixtures of two or more of these acids. The invention also can be
practiced using other oxidizing processes to form aluminum oxide on
the surface. For purposes of illustration and not limitation,
alternative oxidizing treatments to anodization include polishing,
alkaline etching, acid pickling, electropolishing, heating up to
700.degree. C. in an oxygen bearing atmosphere such as air, and any
other treatment, which results in oxidation of the aluminum alloy
surface and formation of aluminum oxide on the surface.
[0029] The aluminum oxide then is etched in a manner to remove the
porous outer aluminum oxide and partially remove the barrier
aluminum oxide to leave a portion of its original thickness on the
alloy surface. The etching step is conducted for a time in a
selected etchant to leave a controlled partial thickness of the
barrier aluminum oxide remaining on the alloy surface in dependence
upon the type of metallic film to be subsequently deposited.
[0030] For example, in an illustrative embodiment of the invention,
the etching step is conducted to leave a partial thickness of the
barrier aluminum oxide that is effective to enhance nucleation of
discrete metallic nanoparticles on the barrier oxide during
subsequent electroless or electrodeposition so as to form a porous,
metallic particulate film. For purposes of illustration and not
limitation, in this embodiment, the thickness of the partial
thickness of the barrier oxide remaining on the alloy surface can
be about 50 nm or less depending on deposition parameters, such as
overpotential for electrodeposition, and presence of additives in
the solution.
[0031] The resulting porous, metallic particulate film deposited on
the Al alloy substrate comprises the partial thickness of the
barrier oxide on the alloy surface and a porous, three dimensional
film structure of electrically interconnected metallic metal
nanoparticles deposited by galvanic or electroless deposition or
electrodeposition on the barrier oxide. The film structure includes
randomly packed, generally spherical metallic nanoparticles having
a distribution of particle sizes and high particle density in the
range of about 104 to about 10.sup.12 particles/cm.sup.2. For
example, the metallic nanoparticles can have a particle diameter in
the range of about 20 nm to about 1000 nm. An electrode, such as a
double layer capacitor electrode, can embody such alloy substrate
having the partial thickness of the barrier oxide and this film
structure thereon.
[0032] In another illustrative embodiment of the invention, the
etching step is conducted to leave a partial thickness of the
barrier aluminum oxide that is thin enough to yield a continuous
(non-porous) metallic film on the barrier oxide during subsequent
electroless or electrodeposition. For purposes of illustration and
not limitation, in this embodiment, the thickness of the partial
thickness of the barrier oxide remaining on the alloy surface can
be about 5 nm or less depending on deposition parameters, such as
overpotential for electrodeposition, and presence of additives in
the solution.
[0033] Practice of the invention is not limited to any particular
etching process. For example, the etching process can vary with the
particular type of aluminum alloy surface to be treated. Any
conventional etching process can be used with the type of etchant
and time of etching being selected empirically to achieve a desired
etched barrier aluminum oxide of controlled partial thickness
described above depending upon the metallic film to be deposited.
For example, the etching process can be conducted in any
conventional acid etchant that includes, but is not limited to, an
acidic aqueous solution (phosphoric acid, oxalic acid, sulfuric
acid) or a mixture of an acid and an inhibitor of aluminum
oxidation, such as chromic acid. Other inhibitors can be used as an
alternative to chromic acid. Etching also can be performed in an
alkaline solution of sodium hydroxide, or any other hydroxide.
[0034] Although the Examples set forth below involve anodizing
using an aqueous oxalic acid solution and certain anodizing
parameters and acid etching using an aqueous solution of phosphoric
acid and chromic acid, these are offered merely for purposes of
illustrating and not limiting the invention. Similarly, although
the Examples are described with respect to a surface of a thin film
or layer of an alloy of Al and Cu where Al and Cu are present in
respective amounts of 99.5 weight % and 0.5 weight % of the alloy,
the Examples are offered merely for purposes of illustrating and
not limiting the invention.
[0035] Example 1 describes a method pursuant to an illustrative
embodiment of the invention wherein an aluminum-copper alloy is
anodized and then chemically etched followed by electroless
deposition of silver on the treated alloy surface.
EXAMPLE 1
Galvanic Displacement Type Deposition
[0036] In particular, aluminum-copper alloy film covered wafers
used in this Example were fabricated as follows: First, a 600-nm
layer of SiO.sub.2 was thermally grown by steam oxidation of each
silicon wafer. Second, a 3 -.mu. m thick layer Al-Cu alloy (99.5
weight % aluminum and 0.5 weight % copper) was deposited on the
layer of Sio.sub.2 by physical vapor deposition (PVD). Third, each
wafer having the Al-Cu alloy layer was anodized in an
electrochemical cell at 50 V dc for 20 min in 3% by weight oxalic
acid aqueous solution at 0.degree. C. The electrical contact was
made to the top metallic layer outside the electrochemical cell.
The steady state current density, established after 5 minutes of
anodization, was approximately 1.4 mA/cm . Preliminary experiments
revealed that the entire 3 .mu. m thick metallic layer was anodized
in approximately 80-85 min. Thus, anodization for 20 min consumed
about 0.75 .mu. m of the metallic layer. Following anodization, the
porous and barrier layers of aluminum oxide were etched in a
mixture of 0.4 M H.sub.3PO.sub.4 and 0.2 M H.sub.2CrO.sub.4 acids
at 60.degree. C. and for approximately 1 hour to remove the outer
porous aluminum oxide and partially remove the thickness of the
barrier aluminum oxide. Chromic acid is known to be an inhibitor
for corrosion of aluminum and was used to decelerate the
dissolution of the remaining metallic layer. Galvanic displacement
of Al-Cu by silver (1.5 mM AgNO.sub.3) was performed in a mixture
of 0.4 M H.sub.3PO.sub.4 and 0.2 M H.sub.2CrO.sub.4 acids, at
60.degree. C. with no stirring and for approximately 48 hours.
Anodization of Al-Cu alloy films was carried out with a Pt mesh
counter electrode. All electrochemical measurements were carried
out using a three-electrode cell with the Pt mesh counter electrode
and a reference electrode (either a Pt wire or a Ag/AgCl
electrode). Cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS) were performed with an IM6-e impedance
measurement unit (Zahner), in the same mixture of 0.4 M
H.sub.3PO.sub.4 and 0.2 M H.sub.2CrO.sub.4 acids, at 22.degree. C.
EIS data were acquired at open circuit potential (OCP) over a
frequency range between 0.1 Hz and 100 kHz and with a potential
amplitude of 5 mV. The CV and EIS data were normalized to the
geometric electrode area (1.4 cm 2). During the course of
electroless deposition, the depletion of silver and accumulation of
copper in the deposition solution were monitored with an
inductively coupled plasma (ICP) atomic absorption Perkin-Elmer
spectrophotometer. The surface morphology of deposited silver
particles was evaluated by a Hitachi (S-5200) scanning electron
microscope operated at 2-3 kV.
[0037] Galvanic displacement proceeds via two concurrent
electrochemical reactions: the reduction of metal ions and
oxidation of the substrate surface. If a substrate has a layer of
surface oxide, electroless deposition follows and/or coincides with
chemical etching of the oxide layer. Little is known about the
displacement deposition of silver on aluminum films containing
copper in acidic media. Therefore, it is important to address the
issue of the substrate pretreatment for the electroless deposition
of silver on aluminum/copper films.
[0038] Three main observations can be made from work related to
this Example. First, under the conditions of this Example, no
electroless deposition of silver was observed with the pure
aluminum substrate (99.997 % pure aluminum foil), which was
processed in the same way as alloy films containing 99.5 wt % Al
and 0.5 wt % Cu. Second, electroless deposition of silver did not
take place on either of these two substrates if anodization and
etching steps were omitted. Third, the Al-Cu alloy films are made
amenable for electroless deposition of silver by anodization
followed by complete chemical etching of porous aluminum oxide and
partial chemical etching of barrier aluminum oxide. Upon completion
of chemical etching, the thickness of barrier aluminum oxide
remaining on the treated alloy surface is approximately 1.5 nm. A
combination of anodization and chemical etching results in the
copper enrichment in and underneath the thin layer of barrier
aluminum oxide. This enrichment enables the charge transfer between
silver cations and metallic substrate. In contrast to pure aluminum
substrates, the reduction of silver cations on anodized and etched
Al-Cu alloy substrates becomes possible. Thus, silver particles are
deposited by galvanic displacement.
[0039] With respect to the species that are oxidized during the
electroless deposition, after approximately 48 hours of electroless
deposition, the depletion of silver in the electrochemical cell
corresponded to the deposition of 4.0 mg (37 .mu. mole) of silver.
In contrast, only 0.45 mg (7 .mu. mole) of copper was determined to
accumulate in the deposition solution. If copper were the only
reducing species, the molar ratio between amounts of depleted
silver and accumulated copper would be 2 to 1. The high molar ratio
of approximately 5 suggests that both copper and aluminum are
oxidized during the electroless deposition. One can expect that
aluminum is oxidized because aluminum alloys containing copper are
known to have lower corrosion resistance and are more susceptible
for an attack by an oxidizing agent (silver cations in our case)
than pure aluminum.
[0040] Additional evidence that aluminum along with copper is
oxidized during the electroless deposition of silver comes from
examining the steady state mixed potential, E.sub.mp, established
during the electroless deposition of silver. Based upon the mixed
potential theory, E.sub.mp is located between the formal potentials
of the reduced and oxidized species. Under our experimental
conditions, E.sub.mp (-0.7/-0.5 V vs. Ag/AgCl) was more negative
than the formal potential of Cu.sup.2+/Cu. Such a negative value
can be rationalized if the partial reaction of oxidation involves a
species with a sufficiently negative formal potential (in this case
aluminum).
[0041] FIG. 1A shows that galvanic displacement deposition for 48
hours proceeds via the formation of a network of spherical and
randomly packed particles of silver with a broad distribution of
particle diameters. The particle diameter varies from 50 to 600 nm
and the mean particle diameter is about 200 nm. Particles overlap
with the neighboring particles and are electrically interconnected
to each other. The interconnection of particles enables the charge
transfer between the aluminum-copper alloy substrate and silver
cations in the solution. As a result, the growth of particles does
not stop upon the deposition of the first layer of silver particles
adjacent to the aluminum-copper alloy surface. Rather, galvanic
displacement deposition proceeds with the formation of a 1-2 .mu. m
thick multi-layer structure, where the average particle diameter
(the thickness of a single layer) is smaller than the overall
thickness of the porous layer of silver particles. FIG. 1A shows
that the particle density in a single layer is about 10.sup.9
particles cm.sup.-2. The inter-particle space allows for the
electrolyte access among silver particles. Thus, by analyzing FIG.
1A, one can conclude that the multi-layer and porous structure
composed of interconnected silver particles is expected to have a
high ratio between electrolyte accessible and geometric surface
areas.
[0042] Careful examination of individual silver particles (FIG. 1B)
reveals that silver particles are slightly rough and interconnected
to each other by other particles. Shown as white spots on the
surface of 200 nm particles are silver nano-particles, which tend
to grow into 200 nm particles over the time of galvanic
displacement deposition. Analysis of FIG. 1A, 1B confirms that
nucleation of new silver particles is a progressive process, which
coincides in time with growth of existing silver particles.
[0043] In order to characterize the dielectric properties of the
silver particles/electrolyte interface, a series of cyclic
voltammograms was collected at scan rates of 25, 50, 100 and 200
mV/sec (FIG. 2A). While the negative potential limit is determined
by water electrolysis, the positive potential limit is defined by
copper oxidation. Applied potentials (FIG. 2A) are more negative
than potentials, which result in significant oxidation of silver.
FIG. 2A demonstrates that the current magnitude does not
significantly change during either cathodic or anodic scan in the
potential window of about 0.6 V between -0.5 and 0.1 V vs. Ag/AgCl.
Thus, at the reported range of scan rates the faradaic current due
to any possible oxygen and/or proton reduction is small. Moreover,
the observed current is linearly proportional to the scan rate
(FIG. 2B), which indicates its capacitive origin. This capacitance,
(C.sub.area), normalized to the electrode geometric area and
determined over the shown range of scan rates, reaches a value of
1.7.+-.0.2 mF/cm.sup.2. We note that the capacitance is larger at
slow scan rates (25 and 50 mV/s) than at fast scan rates (100 and
200 mV/s) as indicated by the slopes of dotted lines (FIG. 2B).
This observation is consistent with previously reported
observations that the double-layer capacitance of a porous
electrode depends upon the time scale of measurements. Measurements
over a long time scale result in the deep penetration of potential
perturbation into the porous structure and a large sampled surface
area. Thus, it is not surprising that the linear regression
analysis of cyclic voltammograms with slow scan rates (25 and 50
mV/s) produces larger values of the double layer capacitance (2.0
mF/cm.sup.2) than that obtained by analyzing the whole range of
scan rates.
[0044] Electroless deposition of silver and nickel has been
previously shown to increase the double layer capacitance. A high
capacitance of the metal/electrolyte interface has been explained
by an increased surface roughness (a ratio between real and
geometric surface areas). However, the capacitances reported in
literature for electroless deposition of metal particles have been
only one order of magnitude higher than ones typical for the metal
/ electrolyte interface (20-40 .mu. F/cm.sup.2). In contrast,
C.sub.area of the electroless deposited silver particles pursuant
to this Example of the invention exceeds these values almost by two
orders of magnitude.
[0045] It would be worthwhile to estimate the electrolyte
accessible surface area of silver particles per gram of silver
(S.sub.a, m.sup.2/g) and compare this value with the specific
surface area of spherical particles of silver per gram of silver
(S.sub.g, m.sup.2/g). Equation (1) connects S.sub.a with the
gravimetric capacitance of silver particles/electrolyte interface
per gram of deposited silver (C.sub.mass; F/g) and specific
capacitance of smooth silver/electrolyte interface
(C.sub.spec=20.times.10.sup.6 F/cm.sup.2).
S.sub.a=C.sub.mass/C.sub.spec=(C.sub.area.times.A)/(m.times.C.sub.spec)=3-
.5m.sup.2/g (1)
[0046] In Equation (1), A is the geometric electrode area (1.4
cm.sup.2) and m is the mass of deposited silver (4.0 mg). Thus, one
can calculate that C.sub.mass is equal to 0.70 F/g and S.sub.a is
equal to 3.5.times.10.sup.4 cm.sup.2/g (3.5 m.sup.2/g). For spheres
of silver, S.sub.g can be calculated according to Eq. (2), where
.rho.is silver density (11.times.10.sup.6 g/m.sup.3) and D is the
mean diameter of spheres (200 nm), as determined from FIGS. 1A, 1B.
S.sub.g.times.6/(.rho..times.D).times.2.7 m.sup.2/g (2) Comparison
of these two values indicates that S.sub.a is larger than S.sub.g
most likely due to slight roughness of silver particles (FIG. 1B).
Thus, the surface area of silver particles is completely (except
interconnected areas) utilized to increase the double layer
capacitance.
[0047] S.sub.g of interconnected silver particles is lower than
S.sub.g reported for activated carbonaceous materials (.about.1000
m2/g). The difference between these specific surface areas is
compensated for by one order of magnitude, when one considers that
the atomic weights of carbon and silver are, respectively, 12 and
109 g/mol. Moreover, S.sub.g of carbonaceous materials is usually
determined by gas adsorption methods. Thus, the electrolyte
accessible area, which is available for charged species, may be
appreciably smaller than the specific surface area due to possible
hydrophobicity. As shown in the previous paragraph, the opposite
trend is observed for interconnected silver particles. To alleviate
the difference in S.sub.g for two materials, S.sub.g of silver
particles/electrolyte interface may be increased with the
deposition of smaller particles, which have a higher inner surface
to volume ratio than one obtainable with 200 nm particles.
[0048] In addition to the electrolyte accessible area and
gravimetric capacitance, another point to evaluate technological
utility of electroless deposition of silver particles for the
fabrication of double layer capacitors is the frequency response.
This variable is obtained using EIS, electrochemical impedance
spectroscopy. Although EIS results are frequently presented in
literature dealing with super-capacitors, the experimental data are
rarely analyzed with an equivalent circuit. The difficulty in
modeling of porous electrodes results from the distributed nature
of the double layer capacitance along the pore length in the
direction perpendicular to the electrode surface. The charge
transfer resistance and Warburg impedance have to be considered in
the presence of a faradaic reaction. Due to the distributed
character of the interfacial impedance, the impedance of the porous
electrode (including the multi-layer and porous structure composed
of interconnected silver particles) is properly described by the
transmission line model described in R. de Levie, Electrochem Acta
8, page 751 (1963). The model can be applied to either straight or
tortuous pores.
[0049] The EIS results obtained for the silver
particles/aluminum-copper electrode are summarized in FIG. 3A, 3B.
While FIG. 3A shows the Bode plot (both the magnitude and phase),
FIG. 3B shows the Nyquist plot. The equivalent circuit used for
modeling of EIS data is shown in FIG. 3C. According to the
previously developed transmission line model (www.zehner.de
Application Note 01 (1997), the impedance of the porous layer
consists of three elements: R.sub.pore, the ionic resistance of
pores filled with the electrolyte, R.sub.silver, the electronic
resistance of the solid layer (interconnected silver particles) and
Z.sub.q, the impedance of the interior interface between silver and
electrolyte. Z.sub.q is modeled as a serial connection of two
constant phase element/resistor combinations. The constant phase
element (CPE) is often used instead of a pure capacitance to
describe interfacial dielectric properties. EIS were performed at
OCP while the electroless deposition of silver was taking place.
Therefore, resistors (R.sub.1 and R.sub.2) model the charge
transfer across the silver particles/electrolyte interface due to
reduction of silver cations and concurrent oxidation of the
aluminum/copper substrate. It is worthwhile to note that the EIS
data between 0.1 and 100 Hz can be adequately modeled with
R.sub.pore, R.sub.silver and Z.sub.q containing only a single
(CPE.sub.1 R.sub.1) combination. However, satisfactory description
of the frequency response between 100 Hz and 100 kHz requires that
the second (CPE.sub.2 R.sub.2) combination be introduced. The exact
origin of the second (CPE.sub.2 R.sub.2) combination is undefined.
In addition to the impedance of the porous layer, R.sub.s is used
in the equivalent circuit to model the bulk electrolyte resistance.
The underlying silicon substrate does not appear in the analysis of
EIS data because the electrical connection was made to the top
metallic layer.
[0050] The magnitude of CPE.sub.1, which describes the silver
particles/electrolyte capacitance, is determined to be 1.4.+-.0.1
mF.times.s.sup..alpha.-1/cm.sup.2, a value that is slightly smaller
than one obtained by CV (assuming that .alpha..sub.1 1). The
smaller magnitude of CPE.sub.1, results from the fact that EIS is
performed over a shorter time scale than CV. Other parameters of
the equivalent circuit, which is used to fit the EIS data to the
transmission line model, are calculated as follows: R.sub.s
=27.+-.1.OMEGA..times.cm.sup.2,
R.sub.pore=370.+-.60.OMEGA..times.cm.sup.2,
R.sub.silver=31.+-.2.OMEGA.=cm.sup.2, .alpha..sub.1=0.98.+-.0.01,
R.sub.1=3.9.+-.1.6 k.OMEGA..times.cm.sup.2, CPE.sub.232 200.+-.25
.mu. F.times.s.sup..alpha.-/cm.sup.2, .alpha..sub.2=0.87.+-.0.02,
R.sub.2=1.9.+-.0.2 .OMEGA..times.cm.sup.2. These values agree with
the qualitative analysis of EIS data. For example, the total cell
impedance at frequencies higher than 10 kHz is indicative of a pure
resistor of 58 .OMEGA..times.cm.sup.2 (FIG. 3B). Due to negligible
impedance of capacitors, this value is a sum of the bulk
electrolyte resistance, R.sub.s, and the electronic resistance of
interconnected silver particles, R.sub.silver. At frequencies
between 200 Hz and 10 kHz, a combination of the ionic resistance of
pores, R.sub.pore, and increased impedance of CPE.sub.1 produces a
depressed semi-circle (FIG. 3B). At frequencies between 10 and 200
Hz, the impedance of the interior interface between silver
particles and electrolyte, Z.sub.q, becomes dominant, which results
in a straight line on the Nyquist plot (FIG. 3B).
[0051] The Nyquist plot for porous electrodes is typically divided
into two regions by the "knee" frequency. As discussed in the
previous paragraph, the impedance in the high frequency region
(.gtoreq.200 Hz) is due the porous electrode structure, the
impedance in the low frequency region (.ltoreq.200 Hz) is dominated
by the whole interior interface between silver particles and
electrolyte. Examination of FIG. 3A reveals that the "knee"
frequency is located at about 200 Hz. This value suggests that the
electrical energy can be stored in the double-layer capacitor at
frequencies up to 200 Hz. In contrast, the majority of carbon-based
super-capacitors, with a few exceptions, show the "knee" frequency
around a few Hz. Therefore, their capacitive behavior deteriorates
between 10 and 100 Hz. The superior performance (the high "knee"
frequency) achievable with the capacitor described here is believed
to result from the suitable porosity (the inter-particle distance
of about 20-40 nm) and easy access of the electrolyte among the
deposited silver particles. The high "knee" frequency of 200 Hz is
an important advantage of the capacitors reported in this
paper.
[0052] A critical issue in the design of high power density
super-capacitors is the low electronic resistivity of the porous
electrode structure. For example, it ahs been reported that the
contact resistance between the elements (particles or fibers) in
the electrode must be very small. Among metals, silver has the
lowest electrical resistivity (1.6 .mu..OMEGA..times. cm). However,
the electronic resistance of interconnected silver particles,
R.sub.silver, is comparatively high. This value is determined by
tiny interconnected areas among silver particles as well as the
contact resistance of silver particles to the aluminum substrate.
One can speculate that the interconnected areas can be increased
and, accordingly, R.sub.silver can be decreased with the deposition
of smaller particles. Future study will aim at the deposition of
30-60 nm particles of silver, which are expected to have a higher
inner surface to volume ratio and larger interconnected areas than
those reported in this paper.
[0053] To summarize this Example, the utilization of electroless
deposition of silver by the galvanic displacement mechanism on
aluminum-copper alloy films has been demonstrated in order to
fabricate a multi-layer and porous network composed of electrically
interconnected silver particles. This structure has a high ratio
between electrolyte accessible and geometric surface areas ( 100).
Two electrochemical techniques (CV and EIS) independently suggest
that the capacitance of the silver particles/electrolyte interface
normalized to the electrode geometric area (1.7 mF/cm.sup.2)
exceeds those typical for the smooth silver/electrolyte interface
(20 .mu.F/cm.sup.2) by two orders of magnitude. Evaluation of
electrochemical data and SEM micrographs suggests that the surface
area of silver particles is completely accessible to the
electrolyte (except interconnected areas). The gravimetric
capacitance of silver particles/electrolyte interface per gram of
silver is 0.70 F/g and the useful DC potential range is
approximately 0.6 V. Analysis of the Nyquist plot shows that the
network of silver particles is capable of storing electrical energy
at frequencies up to 200 Hz. This value is higher than those
typically reported for carbon based double layer capacitors. In
addition, use of silicon wafers with aluminum/copper alloy films is
attractive because these wafers are frequently employed in standard
micro-fabrication lines. Given these two advantages, the described
capacitor could find applications for special electronic circuits
where a high frequency response is required.
[0054] Example 2 describes a method pursuant to another
illustrative embodiment of the invention wherein an aluminum-copper
alloy surface is anodized and then chemically etched followed by
galvanic deposition or electrodepostion of silver on the treated
alloy surface.
EXAMPLE 2
[0055] In particular, aluminum-copper alloy covered wafers used in
this Example were fabricated as follows: First, silicon wafers with
a 600 nm thick layer of SiO.sub.2 overlayed with a 3 .mu. m thick
layer of 99.5 wt % Al and 0.5 wt % Cu (deposited by physical vapor
deposition) were used in all experiments. The Al-Cu alloy layer was
anodized with a Pt mesh counter electrode at 50 V DC for 20 min in
3 wt % H.sub.2C.sub.2O.sub.4 acid and at 0.degree. C. The
electrical contact was made to the top metallic layer outside the
electrochemical cell. After anodization, the porous and barrier
layers of aluminum oxide were etched in a mixture of 0.4 M
H.sub.3PO.sub.4 and 0.2 M H.sub.2CrO.sub.4 acids at 50 .degree. C.
for approximately 90 minutes to remove the outer porous aluminum
oxide and partially remove the thickness of the barrier aluminum
oxide. Upon completion of etching the specific capacitance of
barrier aluminum oxide was determined to be 5.8 .mu. F/cm.sup.2.
Assuming that the dielectric constant was 8.6, the layer of barrier
aluminum oxide remaining on the treated alloy surface was estimated
to be 1.3 nm thick. H.sub.2CrO.sub.4 was used to inhibit the
dissolution of the remaining Al-Cu alloy film. Following
anodization and chemical etching, either galvanic displacement type
or potentiostatic electrodeposition of silver (3.0 mM AgNO.sub.3)
was performed in a mixture of 0.4 M H.sub.3PO.sub.4 and 0.2 M
H.sub.2CrO.sub.4 acids at 50.degree. C., with no stirring and for
40 min. Potentiodynamic polarization measurements and
electrochemical impedance spectroscopy (EIS) were performed with an
IM6-e impedance measurement unit (Zahner). All electrochemical
measurements were carried out using a three-electrode cell with a
Pt mesh counter electrode and a reference electrode (either a Pt
wire or a Hg/HgSO.sub.4 electrode) in the mixture of 0.4 M
H.sub.3PO.sub.4 and 0.2 M H.sub.2CrO.sub.4 acids at 50 .degree. C.
For polarization measurements, the potential step was 2 mV and the
time delay to sample the steady state current was 1 s. EIS data
were acquired at open circuit potential (OCP) over a frequency
range between 0.5 Hz and 100 kHz, with a potential amplitude of 5
mV and were normalized to the geometric electrode area (1.4
cm.sup.2). The surface morphology of deposited silver particles was
evaluated by a Hitachi (S-5200) scanning electron microscope
operated at 2-3 kV.
[0056] According to the mixed-potential theory for galvanic
displacement type deposition, the steady state mixed potential,
E.sub.mp, is determined by the partial currents of reduction and
oxidation reactions, which are equal to each other in the magnitude
and opposite in the sign. When the rate of partial cathodic
reaction is increased by adding a suitable reducible species,
E.sub.mp shifts in the anodic direction. In this Example, the
effect of addition of AgNO.sub.3 on both the exchange current
density and E.sub.mp is further investigated with potentiodynamic
polarization experiments (FIG. 4) performed prior to either
galvanic displacement type or electrodeposition. Three features are
notable by comparing the representative E-log (j) curves of
stationary Al-Cu alloy electrodes, (a) after anodization and
etching and (b) upon addition of AgNO.sub.3. First, before addition
of AgNO.sub.3, OCP of -0.88 V is determined by partial reactions
involving oxidation of the Al-Cu substrate and reduction of protons
and/or residual oxygen. Upon addition of AgNO.sub.3, E.sub.mp
shifts to 230 mV more positive indicating that reduction of silver
cations becomes the dominant cathodic reaction. Second, the induced
anodic shift of E.sub.mp results in a larger overpotential for the
oxidation reaction, which increases the partial anodic current
density and accelerates oxidation and dissolution of the Al-Cu
substrate. Third, as a result of higher rates of cathodic and
anodic reactions, the exchange current density increases from (a)
2.times.10.sup.-7A/cm2 to (b) 7.times.10.sup.-6A/cm.sup.2. FIG. 4
illustrates that both the exchange current density and E.sub.mp are
correlated with the rates of partial electrode reactions.
Therefore, polarization experiments are informative to predict
whether galvanic displacement type deposition proceeds with a
reasonable deposition rate.
[0057] FIG. 5 shows the state of the surface of anodized and etched
Al-Cu alloy substrate after deposition of silver by galvanic
displacement for 40 min. The black scallops with white edges can be
observed behind silver particles. These shallow scallops are formed
on the surface as a result of anodization and subsequent chemical
etching. The coverage of randomly distributed particles of silver
is about 4.times.10.sup.9 particles cm.sup.-2. A relatively large
D.sub.mean of 180.+-.110 nm and S.sub.g of 3 m.sup.2/g indicate
that new particles do not nucleate as fast as existing particles
grow.
[0058] FIG. 4 allows one to determine how the cathodic electrode
polarization increases the rate of silver reduction. While the
exchange current density at E.sub.mp (-0.65 V) is 7.times.10.sup.-6
A/cm.sup.2, the cathodic current density at -1.3 V (0.65 V more
negative than E.sub.mp) is 6.times.10.sup.-4 A/cm .sup.2. One can
hypothesize that the cathodic electrode polarization would increase
the rate of nucleation of new particles rather than the growth of
existing particles. In order to investigate this hypothesis, the
Al-Cu alloy electrode was polarized to -1.3 V vs. Hg/HgSO.sub.4 for
a time interval of 40 min, during which the cathodic current
density gradually increased from 0.6 to 2 mA/cm.sup.2. The faradaic
efficiency for silver reduction at -1.3 V is about 40% due to
reduction of protons and/or residual oxygen. This assumption is
based upon the comparison of curves (a) and (b) (FIG. 4) and
observing an inflexion point in the curve (b) around -1.1 V. The
inflexion point indicates that the faradaic current due to residual
faradaic reactions starts to exceed the faradaic current due the
electrodeposition of silver. Comparison of curves (a) and (b)
suggests that the current density due to residual faradaic
reactions is higher in the latter case. This observation can be
explained by a high catalytic activity and a high surface area of
silver particles electrodeposited during a potentiodynamic scan in
comparison with those of the anodized and etched Al-Cu film. Taking
into account the faradaic efficiency for silver reduction, a mass
of deposited silver (1.0 mg) is determined by integrating the
current density over time because electrodeposition dominates over
galvanic displacement at -1.3 V.
[0059] FIG. 6 shows a cauliflower type film composed of densely
packed nanoparticles, which are produced by electrodeposition of
silver on the anodized and etched Al-Cu alloy surface. The coverage
of nanoparticles is about 10.sup.11 particles cm.sup.-2 and
D.sub.mean is 30.+-.7 nm. Comparison of FIG. 5 and 6 indicates a
dramatic difference in the surface morphology between two methods
of deposition. Electrodeposition results in the formation of
particulate silver films with D.sub.mean one order of magnitude
smaller and the particle coverage two orders of magnitude higher
than those obtained with galvanic displacement type deposition. For
spherical particles, S.sub.g can be calculated according to
Equation (1'), where p is the density of silver (11.times.10.sup.6
g/m.sup.3). Table 1 summarizes S.sub.g obtained with both methods
(galvanic displacement and electrodeposition).
S.sub.g=6/(.rho..times.D.sub.mean ) (1')
[0060] Due to the small D.sub.mean , the fabricated 3-dimensional
network of electrodeposited silver nanoparticles has a higher
S.sub.g than that composed of silver particles deposited by
galvanic displacement.
[0061] Microscopic examination of deposited silver particles was
supplemented with macroscopic electrochemical measurements. In
order to confirm that electrodeposition results in the formation of
silver nanoparticles with a high inner-to-geometric surface area
ratio, the electrolyte accessible surface area, S.sub.a, was
estimated by EIS. FIGS. 7 and 8 show the Bode representation of two
EIS data sets collected after galvanic displacement type deposition
and electrodeposition, respectively. Qualitative analysis of EIS
spectra (FIGS. 7 and 8) around 100 Hz and 0.5 Hz results in two
observations. The magnitude of the total cell impedance is lower
and the phase of the total cell impedance is less negative for the
electrode with electrodeposited particles of silver than those for
the electrode with particles deposited by galvanic displacement.
Both observations can be explained by the fact that a larger
capacitance makes a smaller contribution to the total cell
impedance in the former case in comparison with the latter
case.
[0062] For quantitative analysis, EIS spectra are modeled with the
equivalent circuits shown in FIG. 7A and FIG. 9. The equivalent
circuit describing particles deposited by galvanic displacement for
40 min (FIG. 7A) does not require considering the 3-dimensional
structure of deposited particles. On the contrary, the equivalent
circuit used for modeling of electrodeposited particles (FIG. 9)
includes the transmission line model developed for a porous
electrode. This model has to be employed because the capacitance
normalized to the geometric electrode area, C.sub.area, obtained
for the electrode with electrodeposited particles of silver exceeds
the specific capacitance of a smooth silver/electrolyte interface,
C.sub.spec(20.times.10.sup.-6 F/cm.sup.2), by two orders of
magnitude (Table 1). Both models were described in details in
previous publications. C.sub.area, and gravimetric capacitances
normalized to the mass of deposited silver, C.sub.mass, are shown
in Table 1. The electrolyte accessible surface area of
electrodeposited silver particles per gram of silver (S.sub.a,
m.sup.2/g) can be determined as the ratio of C.sub.mass and
C.sub.specaccording to Equation (2').
S.sub.a=C.sub.mass/C.sub.spec=(C.sub.area.times.A)/(m
.times.C.sub.spec) (2') In Equation (2'), A is the geometric
electrode area (1.4 cm.sup.2) and m is the mass of deposited
silver. As expected, S.sub.a is approximately the same as S.sub.g
(Table 1). Thus, the surface area of electrodeposited particles of
silver is completely accessible to the electrolyte. Based upon
these observations, one can conclude that electrodeposition results
in the deposition of porous silver films composed from electrically
interconnected nanoparticles of silver. S.sub.g of electrodeposited
particles is one order of magnitude higher than that obtained with
particles deposited by galvanic displacement.
[0063] It is important to note why the cathodic polarization of the
Al-Cu electrode favors the deposition of silver particles with a
higher nucleation density than that obtained with galvanic
displacement. The nucleation density is known to exponentially
increase with the applied overpotential. Therefore, it is not
surprising that a high nucleation density is obtained when the
Al-Cu alloy electrode is polarized to 0.6-0.7 V more negative than
E.sub.m (-0.65 V) established during galvanic displacement. The
cathodic polarization increases the applied overpotential for
reduction of silver cations, which translates in a high nucleation
density.
[0064] Particles of silver remain adhesive to the Al-Cu alloy
substrate during electrodeposition, electrochemical and microscopic
examination. Mechanical strength of the fabricated porous structure
can be explained by the fact that the D.sub.mean of 30 nm allows
for a plenty of interconnection points among silver particles per
unit volume. The corrosion resistance of electrodeposited particles
of silver can be further improved with co-deposition of refractory
metals such as tungsten.
[0065] Electrodeposition of silver on anodized and etched Al-Cu
alloy substrates can be compared with galvanic displacement. In
both cases, the combination of anodization and chemical etching
results in the copper enrichment in and underneath the thin layer
of barrier aluminum oxide. The enrichment in copper during
anodization enables subsequent electrodeposition of silver in this
Example.
[0066] In the case of electrodeposition, cathodic polarization
allows one to control the nucleation and growth of silver particles
and, consequently, properties of deposited silver films. It is
worthwhile to point out that the technologically important
zincation of aluminum substrates is galvanic displacement type
deposition.
[0067] In order to increase the technological utility, the
procedure to activate the Al-Cu alloy film for electrodeposition
must be compatible with standard photolithographic methodology.
Previous results obtained in our laboratory indicated that
photoresist did not exhibit acceptable adhesion to the Al-Cu alloy
film during porous type anodization. This problem was overcome with
the negative pattern transfer technique. The procedure for
electrodeposition on patterned Al-Cu alloy films is summarized at
FIGS. 10A-10F. Silicon wafers with Al-Cu films were patterned with
photoresist, producing two circular patterns with diameters of 10
and 30 um (FIG. 10A-step 1). The photoresist mask was transferred
to an approximately 100 nm thick layer of barrier aluminum oxide
(FIG. 10B-step 2). This layer was formed in the regions not covered
with the photoresist under the following conditions: voltage of 90
V, anodization time of 2-3 min, temperature of 3.degree. C. and in
3 % w/v Na.sub.2C.sub.20.sub.4 (pH 4.5). After removal of
photoresist (FIG. 10C-step 3), subsequent 20 min anodization (FIG.
10D-step 4) at voltage of 50 V, temperature of 3.degree. C. and in
3 % w/v H.sub.2C.sub.20.sub.4 (pH 1.5) resulted in the formation of
porous aluminum oxide only in those regions, which were not covered
with the layer of barrier aluminum oxide and were initially covered
with the photoresist. In order to enable electrodeposition, the
whole layer of porous aluminum oxide and almost all of underlying
barrier aluminum oxide (1.3 nm left) were chemically etched (FIG.
10E-step 5) in a mixture of 0.4 M H.sub.3PO.sub.4 and 0.2 M
H.sub.2CrO.sub.4 acids at 50.degree. C. for approximately 90 min as
described in the experimental section for blanket Al-Cu films.
Electrodeposition of silver (3.0 mM AgNO.sub.3) was performed at
-1.3 V vs. Hg/HgSO.sub.4 (FIG. 10F-step 6).
[0068] FIGS. 11A, 11B demonstrate the state of the alloy surface
after completion of all steps in the fabrication procedure (FIGS.
10A-10F). Silver is electrodeposited (FIG. 10F-step 6) only in the
circular regions, which underwent porous type anodization (FIG.
10D-step 4). The space-selective deposition of silver provides
indirect evidence that the copper enrichment, which occurs during
porous typelanodization of Al-Cu alloy films, enables the
electrodeposition of silver on anodized and chemically etched
regions of Al-Cu alloy films. The silver deposits are porous and
similar to those shown in the high magnifications micrograph (FIG.
6). FIGS. 11A, 11B confirm that the pretreatment method developed
for activation of Al-Cu alloy films (anodization and chemical
etching) is compatible with photolithographic techniques.
[0069] To summarize this Example, activation of technologically
relevant Al-Cu alloy substrates for electrodeposition is achieved
by anodization followed by chemical etching of aluminum oxide.
Electrodeposition of silver on anodized and etched Al-Cu alloy
substrates results in the fabrication of a porous film built from
electrically interconnected nanoparticles of silver with D.sub.mean
of 30 nm. The coverage of electrodeposited particles of silver is
4.times.10.sup.11 particles cm.sup.-2. Microscopic examination by
SEM is supplemented with macroscopic electrochemical measurements
(EIS). EIS is shown to be a useful in-situ method to monitor the
surface area of deposited particulate films. The frequency response
of the porous network of electrodeposited silver nanoparticles is
evaluated using the transmission line model. The capacitance
normalized to the geometric electrode area is 2.9.+-.0.1
mF/cm.sup.2 and the capacitance normalized to the mass of deposited
silver is 3.9.+-.0.1 mF/g. The electrolyte accessible area of
electrodeposited silver nanoparticles is 20 m.sup.2/g. The method
developed for electrodeposition on anodized and etched Al-Cu alloy
films is compatible with photolithographic techniques.
Electrodeposition on patterned Al-Cu alloy films is accomplished by
transferring the photoresist mask to a layer of barrier aluminum
oxide. This layer acts as a mask for porous type anodization.
Following anodization and chemical etching, electrodeposition of
silver takes place only on anodized and etched areas. Table 1. The
mean particle diameter, D.sub.mean ; specific surface area,
S.sub.g; mass of deposited silver, m; capacitance normalized to the
geometric electrode area; C.sub.area, gravimetric capacitances;
C.sub.mass, and electrolyte accessible surface area; S.sub.a, for
silver particles deposited by either galvanic displacement type
deposition or electrodeposition. TABLE-US-00001 D.sub.mean (nm)
S.sub.g (m.sup.2/g) m C.sub.area C.sub.mass S.sub.a Process (SEM)
(SEM) (mg) (mF/cm.sup.2) (F/g) (m.sup.2/g) Galvanic 180 .+-. 110
3.0 .+-. 1.8 N/A 0.058 .+-. 0.002 N/A N/A displacement
ElectroDeposition 30 .+-. 7 18 .+-. 4 1.0 2.9 .+-. 0.1 3.9 .+-. 0.2
19 .+-. 1
Although the invention has been described in connection with
certain embodiments thereof, those skilled in the art will
appreciate that the invention is not limited to these illustrative
embodiments and that changes and modifications can be made thereto
within the scope of the invention as set forth in the following
claims.
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