U.S. patent application number 11/201766 was filed with the patent office on 2006-02-16 for activation of aluminum for electrodeposition or electroless deposition.
This patent application is currently assigned to SCIENCE & TECHNOLOGY CORPORATION @ UNM. Invention is credited to Plamen B. Atanassov, Dmitri A. Brevnov, Gabriel P. Lopez, Tim S. Olson.
Application Number | 20060032757 11/201766 |
Document ID | / |
Family ID | 37595556 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060032757 |
Kind Code |
A1 |
Brevnov; Dmitri A. ; et
al. |
February 16, 2006 |
Activation of aluminum for electrodeposition or electroless
deposition
Abstract
Method for treating an aluminum alloy surface for
electrodeposition or electroless deposition of a metal or alloy on
the surface, the surface is oxidized (e.g. anodized) to form
aluminum oxide, and then the oxidized surface is chemically etched
to render the surface amenable for electrodeposition or electroless
deposition of the metal or alloy thereon. A metallic coating can be
electrodeposited or electroless deposited on the treated
surface.
Inventors: |
Brevnov; Dmitri A.;
(Albuquerque, NM) ; Olson; Tim S.; (Albuquerque,
NM) ; Lopez; Gabriel P.; (Albuquerque, NM) ;
Atanassov; Plamen B.; (Albuquerque, NM) |
Correspondence
Address: |
Edward J. Timmer
P.O. Box 770
Richland
MI
49083-0770
US
|
Assignee: |
SCIENCE & TECHNOLOGY
CORPORATION @ UNM
|
Family ID: |
37595556 |
Appl. No.: |
11/201766 |
Filed: |
August 11, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60601917 |
Aug 16, 2004 |
|
|
|
Current U.S.
Class: |
205/205 ;
205/213 |
Current CPC
Class: |
C23C 18/1844 20130101;
C23C 18/54 20130101; C25D 11/18 20130101; C25D 5/44 20130101; C23F
1/20 20130101 |
Class at
Publication: |
205/205 ;
205/213 |
International
Class: |
C25D 5/34 20060101
C25D005/34 |
Claims
1. Method of treating a surface comprising aluminum for
electrodeposition or electroless deposition of a metal or alloy on
the surface, comprising the steps of: providing a surface
comprising an alloy of aluminum and an alloying element, oxidizing
the surface on the alloy to form aluminum oxide thereon, and
chemically etching the oxidized surface to render the surface
amenable for electrodeposition or electroless deposition.
2. The method of claim 1 wherein the surface is provided as an
alloy of aluminum and an element selected from the group consisting
of copper, silicon, magnesium, zinc, silver, gold, tungsten,
chromium, lead, nickel, titanium or combination thereof.
3. The method of claim 1 wherein the surface is provided on a film
or layer of the alloy.
4. The method of claim 3 wherein the film or layer is deposited on
a substrate by physical vapor deposition.
5. The method of claim 3 wherein the alloy includes about 0.5
weight % copper and balance aluminum.
6. 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.
7. The method of claim 1 wherein the anodized surface is acid
etched for a time to render the surface amenable for deposition of
the metal or alloy thereon.
8. The method of claim 7 wherein the surface is acid etched by
contact with a mixture of phosphoric acid and chromic acid.
9. The method of claim 1 wherein the metal or alloy comprises one
or more noble metals.
10. The method of claim 1 wherein the metal or alloy comprises a
non-noble metal including Cu, Ni, Cr, Cd, Pb, Sn, or a combination
thereof.
11. The method of claim 9 wherein the metal or alloy further
comprises another metal including Ni, Co, Fe, Cr, Mo, W, or a
combination thereof.
12. The method of claim 1 including the additional step of
electrodepositing the metal or alloy on the surface.
13. The method of claim 1 wherein the metal or alloy is
electrodeposited on the surface as a metallic coating.
14. The method of claim 13 wherein the coating comprises a
particle-type noble metal coating with both controlled particle
density and controlled narrow particle size distribution.
15. The method of claim 1 including the additional step of
electroless depositing the metal or alloy on the surface.
16. The method of claim 15 wherein the metal or alloy comprises one
or more noble metals.
17. The method of claim 15 wherein the metal or alloy comprises a
non-noble metal including Cu, Ni, Cr, Cd, Pb, Sn, or a combination
thereof.
18. The method of claim 16 wherein the metal or alloy further
comprises another metal including Ni, Co, Fe, Cr, Mo, W, or a
combination thereof.
19. The method of claim 15 wherein the metal or alloy is
electroless deposited as a metallic coating on the surface.
20. The method of claim 15 wherein the electroless depositing
occurs in the presence of an external agent or by galvanic
displacement where a reducing agent resides in and/or underneath
the aluminum oxide.
Description
[0001] This application claims benefits and priority of U.S.
provisional application Ser. No. 60/601,917 filed Aug. 16,
2004.
FIELD OF THE INVENTION
[0002] The invention relates to treatment of a surface comprising
an aluminum alloy in a manner to render the surface amenable to
electrodeposition or electroless deposition of a metal or alloy,
such as a noble metal or alloy, on the surface.
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] Regardless of the acceptance of the zincate process in
commercial applications, there are incentives for developing
alternative methods for the electrodeposition of metals on aluminum
and its alloys since the zincate method is sensitive to many
variables. For example, direct electrodeposition of copper on
aluminum has been reported for several copper complexes. A plating
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 addition to being useful for metallization of the
aluminum surface, electrodeposition of noble metals on aluminum and
its alloys has a variety of potential applications. For example, a
porous network of electrodeposited metalic particles
electrodeposited on the aluminum surface can be utilized for
fabrication of heat dissipation systems, energy conversion and
storage devices. In addtion, gold nanoparticles deposited on
aluminum alloys may exhibit useful catalytic and electrocatalytic
properties.
[0007] The electroless deposition of metals (e.g. Au, Ag, Cu) by
galvanic displacement on semiconductor or metal surfaces is a
well-known process. This 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.
Galvanic displacement has been reported for deposition of Au on Si,
Au on Ge, Pt on Ge, Cu on TaN, Cu on Si, Cu on Al, Zn on Al, Ni on
Al and other combinations.
[0008] Electroless deposited films of silver on aluminum and
aluminum alloys can be utilized in a number of diverse
applications, including, for example, miniature silver-zinc
batteries. The electroless deposition of silver can also be used to
fabricate optical devices for surface enhanced FT-IR spectroscopy,
surface enhanced Raman scattering and metal-enhanced fluorescence.
In addition, composite materials with silver particles are shown to
have useful photo-catalytic, anti-microbial properties and tunable
surface plasmon resonances.
SUMMARY OF THE INVENTION
[0009] The present invention provides a method for treating a
surface comprising an alloy of aluminum for electrodeposition or
electroless deposition of a metal or alloy on the treated
surface.
[0010] In an illustrative embodiment of the invention, the surface
to be treated is comprised of an alloy of aluminum and a second
element (e.g. Cu, Si, and/or others), the surface is oxidized by
anodizing to form aluminum oxide, and then the anodized surface is
chemically etched to remove aluminum oxide for a time to render the
surface amenable for deposition of the metal or alloy thereon. The
deposited coating can be either a particle type or continuous.
Although anodizing is described as the oxidizing process for the
illustrative embodiment, the invention is not so limited since
alternative oxidizing processes to anodizing can be used in
practice of the invention such as including, but not limited to,
polishing, alkaline etching, acid pickling, electropolishing and
any other treatment (e.g. thermal treatment by heating up to
700.degree. C. in an oxygen bearing atmosphere such as air), which
results in oxidation of aluminum alloy and formation of aluminum
oxide on the surface where the coating is to be deposited.
[0011] In other embodiments of the invention, a particle-type
coating comprising a metal or alloy of one or more noble metals can
be deposited on the treated surface by electrodeposition with both
controlled particle density and controlled particle size
distribution of the deposited material. A coating comprising a
metal or alloy of one or more noble metals also can be deposited on
the treated surface by electroless deposition.
[0012] In other embodiments of the invention, a porous and
multi-layer network of interconnected metalic particles is
deposited on the oxidized (e.g. anodized) and etched surface by
electroless deposition (galvanic displacement).
[0013] In other embodiments of the invention, electrodeposition of
a metal or metal alloy on oxidized (e.g. anodized) and etched
aluminum/copper films is used to fabricate a porous electrode built
from electrically interconnected and spherical nanoparticles with
the mean particle diameter ranging from 10 to 1000 nm.
[0014] In other embodiments of the invention, electrodeposition of
a metal or metal alloy on oxidized (e.g. anodized) and etched
aluminum/copper films is used to deposit a continious film.
[0015] 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
[0016] FIG. 1 shows cyclic voltammograms (100 mV/s) for gold
electrodeposition on processed wafer (curve "a") and on unprocessed
wafer (curve "c"). Background scan curve "b" involves a processed
wafer without gold (I) sodium thiosulfate as described in Example
1.
[0017] FIG. 2 is a Tafel plot for gold electrodeposition on a
processed wafer.
[0018] FIG. 3 is a Bode plot (magnitude and phase) after a total of
20 minutes of galvanostatic gold electrodeposition at -0.018
mA/cm.sup.2 performed for 30 or 60 second intervals. The inset
represents an equivalent circuit used to model EIS data.
[0019] FIG. 4 is a plot of resistance and capacitance of the first
parallel combination (R.sub.1) and CPE.sub.1) as a function of time
during galvanostatic electrodeposition at -0.018 mA/cm.sup.2 of
gold on a processed wafer.
[0020] FIG. 5 is a plot of mass of gold deposited per unit area and
electrodeposition potential as a function of electrodeposition
time.
[0021] FIG. 6 is an EDS spectrum obtained after 5 minutes of gold
electrodeposition at -0.018 mA/cm.sup.2.
[0022] FIGS. 7a, 7b are SEM micrographs with corresponding
histogram insets after gold electrodeposition at -0.54 mA/cm.sup.2
for 10 seconds (FIG. 7a) and 20 seconds (FIG. 7b).
[0023] FIGS. 8a, 8b are SEM micrographs with corresponding
histogram insets after gold electrodeposition at -1.1 mA/cm.sup.2
for 10 seconds (FIG. 8a) and 20 seconds (FIG. 8b).
[0024] FIG. 9 shows EIS data (magnitude of impedance and phase)
collected at OCP after anodization at 50 V, for 20 minutes in 3%
w/v oxalic acid, at 0.degree. C. and subsequent etching a mixture
of 0.4 M phosphoric and 0.2 M chromic acids at 60.degree. C. for
110 minutes. The equivalent circuit is shown as an insert in FIG.
9.
[0025] FIG. 10 is a plot of capacitance and thickness of the layer
of barrier aluminum oxide during etching.
[0026] FIG. 11 shows EIS data (magnitude of impedance and phase)
collected at OCP after electroless deposition of silver for 3
hours.
[0027] FIG. 12 is a plot of capacitance and resistance of the layer
of barrier aluminum oxide during electroless deposition of silver
on aluminum-copper alloy film substrates. The time axis (FIG. 12)
is a continuation of the time axis (FIG. 10) with some overlap
between 45 and 100 minutes.
[0028] FIGS. 13a through 13d are SEM micrographs collected after
electroless deposition of silver for 9 minutes (FIG. 13a), 1 hour
(FIG. 13b), 2 hours (FIG. 13c), and 3 hours (FIG. 13d).
[0029] FIG. 14 is an EDS spectrum collected after electroless
deposition of silver for 3 hours on 99.5% aluminum and 0.5% copper
films.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The invention provides a method for treating a surface
comprised of an alloy of aluminum to render the surface amenable to
electrodeposition or electroless deposition of a metal on the
surface. The surface to be treated 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 Cu, Si, Mg, Zn and/or other alloying
elements. Although the invention is especially useful as a surface
treatment prior to deposition of one or more noble metals, the
invention is not so limited since the invention can be practiced as
a surface treatment prior to deposition of any metal or alloy on
the surface wherein the term "metal or alloy" includes, but is not
limited to, a metal or an alloy or mixture of two or more metals
deposited concurrently or sequentially to provide a metallic
deposit on the surface. For purposes of illustrating and not
limiting the invention, the metal or alloy to be deposited can
comprise Au, Ag, Pt, Pd, Cu, Ni, Cr, Cd, Pb, Sn, or W, or alloy
thereof with one another, or with one or more other alloying
elements such as including but not limited to one or more of Ni,
Co, Fe, Cr, Mo, and W, whereby the deposited material comprises a
binary alloy deposit (e.g. Ag--W, Ag--Co, etc.), ternary alloy
deposit, quaternary alloy deposit and so on.
[0031] The method envisions providing a surface that is comprised
of an alloy of aluminum and one or more alloying elements where the
alloying element(s) is/are present in an amount effective to render
the treated surface amenable to electrodeposition or electroless
deposition of a metal or alloy thereon. The surface to be treated
pursuant to the invention can include, but is not limited to, any
type of substrate, layer, film, or other surface on which the metal
or alloy is to be deposited by electrodeposition or electroless
deposition.
[0032] The method of the invention involves oxidizing the surface
to form aluminum oxide thereon and then chemically etching the
oxidized surface (i.e. etching the aluminum oxide layer formed on
the surface) in a manner to render the surface amenable for
electrodeposition or electroless deposition of the metal or alloy
thereon. The invention can be practiced using anodizing to oxidize
the surface to form aluminum oxide thereon. Practice of the
invention is not limtied 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, phosporic 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 nto 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.
[0033] Practice of the invention is not limited to any particular
etching process. For example, the etching process can vary with
particular type of 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 surface that
amenable to electrodeposition or electroless deposition. For
example, the etching process can be conducted in any conventional
acid etchant that includes, but is not limited to, a pure acidic
solution (phosphoric acid, oxalic acid, sulfuric acid, phosphoric
acid) and a mixture of an acid and an inhibitor of aluminum
oxidation such as a chromic acid. Other inhibitors can be used as
an alternative to chromate. 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 using 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.
EXAMPLE 1
Electrodeposition
[0035] This Example describes an illustrative method pursuant to an
embodiment of the invention for the pretreatment of an aluminum
surface that makes it amenable for the electrodeposition of gold.
This illustrative method is achieved by alloying aluminum with
copper, anodizing the surface, and then chemically etching the
anodized surface (i.e. etching the aluminum oxide layer on the
surface) prior to electrodepostion.
[0036] In particular, aluminum-copper alloy 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% weight by volume oxalic acid
aqueous solution at 0.degree. C. Fourth, the porous and barrier
aluminum oxide layers formed by the anodization were chemically
etched in an aqueous solution of 0.4 M phosphoric acid and 0.2 M
chromic acid at 60.degree. C. for approximately 2 hours. Fifth,
gold electrodeposition on the treated Al--Cu alloy layer was
carried out at room temperature (22.degree. C.) in 1.0 M
Na.sub.2SO.sub.3 (pH 8) with the Oromerse Part B gold plating
solution avialable commercially from Technic Inc., Anaheim, Calif.
The final concentration of Na.sub.3Au(SO.sub.3).sub.2 of the
plating solution was 4.3 mM.
[0037] Anodization of the Al--Cu alloy layer was carried out with a
platinum mesh counter electrode and a Hewlett-Packard 4140B pA
meter/dc voltage source. Electrodeposition, cyclic voltammetry
(CV), and electrochemical impedance spectroscopy (EIS) were
performed in a three-electrode cell with the same platinum counter
electrode and either a platinum wire (quasireference) or Ag/AgCl
reference electrode. All experiments were performed with an IM6-e
impedance measurement unit (BAS-Zahner). EIS data were acquired at
open-circuit potential (OCP) over a frequency range between 1 Hz
and 100 kHz with an AC potential amplitude of 5 mV and were
normalized to the electrode geometric area of 1.4 cm.sup.2. The
surface morphology of the deposited gold was evaluated with a
Hitachi (S-5200) scanning electron microscope equipped with a PGT
spectrometer for energy-dispersive spectroscopy (EDS). The
microscope was operated at 10 kV for imaging and at 25 kV for EDS.
As used in this example, the term "processed" wafer refers to a
silicon wafer with an aluminum-copper alloy film or layer that has
been anodized and etched, whereas the phrase "unprocessed" wafer
refers to a silicon wafer with an aluminum-copper alloy film or
layer that has not been anodized and etched.
[0038] The films or layers comprised of 99.5% aluminum and 0.5%
copper were anodized and chemically etched to activate the film or
layer surface for subsequent electrodeposition of gold. Whereas
anodization forms both barrier and porous aluminum oxide layers,
etching results in complete dissolution of the porous aluminum
oxide and partial dissolution of the barrier aluminum oxide. FIG. 1
shows the current-potential curves collected under anaerobic
conditions for three substrates: two with
Na.sub.3Au(SO.sub.3).sub.2, (a) processed wafer and (c) unprocessed
wafer, and one without Na.sub.3Au(SO.sub.3).sub.2, (b) processed
wafer. Evaluation of collected currentpotential curves results in
the following conclusions. First, a comparison of curves "a" and
"b" suggests that an excess of cathodic current for curve "a" at
sufficiently negative potentials corresponds to the reduction of
Na.sub.3Au(SO.sub.3).sub.2 and the concurrent deposition of gold
metal. Second, evaluation of curves "a" and "c" indicates that the
cathodic current that corresponds to electrodeposition of gold is
significantly more pronounced for the processed wafer. Third,
current-potential curve "a" shows no peak on the reverse scan;
thus, the reduction of Na.sub.3Au(SO.sub.3).sub.2 is irreversible
process. Fourth, the cathodic current for curve "a" increases in
exponential fashion over the investigated range of overpotentials
and shows neither peak nor plateau, which could be attributed to
mass-transport limitations. This observation could be explained by
an onset of water electrolysis catalyzed by deposited gold.
[0039] To further characterize the electrodeposition of gold, a
Tafel plot (FIG. 2) was obtained for potentials up to 60 mV more
negative than OCP of the aluminum-copper alloy electrode in the
investigated electrolyte (-0.76 V vs Ag/AgCl). This relatively
small potential range was chosen to minimize the amount of gold
deposited on the substrate during the experiments. The form of the
Tafel equation used here is given by E = const - RT .alpha. .times.
.times. n .times. .times. F .times. .times. ln .times. .times. j (
1 ) ##EQU1## where E is the potential, R is the gas constant, T is
the absolute temperature, .alpha. is the cathodic charge-transfer
coefficient, n is the number of electrons, F is the Faraday
constant, and j is the current density. The Tafel slope of the
shown data (E vs In |j|) is found to be -0.022 V. This value
indicates that the reduction of the gold complex in solution is a
one-electron process, assuming that .alpha. is equal to 1. A slope
of -0.026 V is expected from eq 1. The one-electron process
corresponds to the following electrochemical reaction
Au(SO.sub.3).sub.2.sup.3-+e.sup.-.fwdarw.Au(s)+2(SO.sub.3).sup.- 2-
(2)
[0040] EIS was employed as method for in situ monitoring of the
thickness of the layer of barrier aluminum oxide after anodization
and during chemical etching. An equivalent circuit (inset in FIG.
3) used for modeling of the total cell impedance includes two
parallel combinations of a constant phase element (CPE) and a
resistor (R) connected in series with each other and the cell
uncompensated resistance (R.sub.u). The CPE is frequently used
instead of a pure capacitance to describe interfacial dielectric
properties. One of two parallel (R.sub.1 CPE.sub.1) combinations is
attributed to the layer of barrier aluminum oxide. In this case,
CPE.sub.1 describes the dielectric properties of barrier aluminum
oxide, and R.sub.1 describes the resistance to ion migration
through the barrier aluminum oxide. The second (R.sub.2 CPE.sub.2)
combination possibly represents the inner layer of aluminum oxide
with different dielectric properties, which is located between the
aluminum phase and outer layer of barrier aluminum oxide. The
analysis of EIS data allows establishment of the necessary duration
of the etching process (typically 90-120 min). At the end of the
etching process, the layer of barrier aluminum oxide reaches its
minimal thickness, which facilitates the subsequent
electrodeposition process.
[0041] In addition to monitoring of the etching rate of barrier
aluminum oxide, EIS can be used as a convenient quality-control
method to observe changes in the interfacial electrical properties
induced by the electrodeposition of gold particles on the
aluminum-copper alloy film substrate. FIG. 3 depicts a Bode plot
for the aluminum-copper alloy film with deposited gold (deposited
for approximately 20 min at a current density of -0.018
mA/cm.sup.2). Fitting of the experimental EIS data to the same
equivalent circuit allowed extraction values of the components of
the equivalent circuit. For this EIS data set, R.sub.u is
39.9.+-.0.6 .OMEGA., R.sub.1 is 8.7.+-.0.9 k.OMEGA. cm.sup.2,
CPE.sub.1 is 13.0.+-.0.5 .mu.F s.sup..alpha.-1/cm.sup.2,
.alpha..sub.1 is 0.939.+-.0.001, R.sub.2 is 12.2.+-.0.8
.OMEGA.cm.sup.2, CPE.sub.2 is 7.8.+-.0.7 .mu.F
s.sup..alpha.-1/cm.sup.2, and .alpha..sub.2 is 0.812.+-.0.008. To
better understand the time dependence of gold electrodeposition on
the aluminum-copper alloy films, EIS experiments were carried out
according to the following protocol. Galvanostatic
electrodeposition (-0.018 mA/cm.sup.2) was preformed over
relatively short time intervals (30 s-1 min) and was followed by
EIS at the OCP. Interruption of galvanostatic electrodeposition was
necessary to satisfy one of the requirements for the validity of
EIS measurements. The system under investigation is required not to
change over the time (about 3 min) necessary to collect an EIS
spectrum.
[0042] FIG. 4 illustrates the magnitude of CPE, and R, as a
function of deposition time. Over the investigated period of time,
CPE.sub.1 monotonically increases from values typical for
electrodes with thin oxide layers (5-6 .mu.F/cm.sup.2) to values
approaching those typical for metal electrodes (20 .mu.F/cm.sup.2).
In contrast, the resistance of barrier aluminum oxide (R.sub.1)
drops rapidly in the few first minutes of electrodeposition from
approximately 1 M.OMEGA. cm.sup.2 to 300 k.OMEGA. cm.sup.2.
Thereafter this resistance gradually decreases to values
approaching 10 k.OMEGA. cm.sup.2. This observation most likely
results from the incorporation of gold in the layer of barrier
aluminum oxide and, as a result, an increase in the electronic
conductivity in this layer, although applicants do not wish to be
bound by any theory in this regard. On the basis of these
observations, it is concluded that electrodeposition of gold
results in the formation of gold particles that directly affect
both the interfacial capacitance and resistance. Thus, these gold
particles have an intimate electrical contact to the conductive
aluminum-copper alloy film substrate. Low resistances are important
for the aluminum-copper alloy film with gold particles to be used
in electrocatalysis and electro-analytical applications. Note that
EIS and Tafel data are collected at different time scales. Whereas
the time scale for EIS is less than 1 second, the time scale for
the Tafel experiment is longer. In addition, the EIS and Tafel data
are acquired at significantly different cathodic current densities,
0.018 mA/cm.sup.2 and less than 0.1 .mu.A/cm.sup.2, respectively.
Thus, a direct comparison of two data sets related to the
resistance of barrier aluminum oxide (R.sub.1) is not possible.
[0043] In addition to changes in the interfacial electrical
properties, it is worthwhile to note a systematic increase in the
deposition potential during galvanostatic deposition (FIG. 5).
After approximately 23 min of electrodeposition, the potential
needed to maintain the same current (-0.018 mA/cm.sup.2) was over
0.4 V more positive than the initial value of -1.1. V. This
observation indicates that the overall overpotential diminishes for
sequential electrodeposition intervals. Also shown in FIG. 5 is the
amount of gold deposited per unit area, which was calculated
according to eq 3, assuming ideal conditions for electrodeposition
(100% faradaic efficiency) m / A = ( j .times. .times. t n .times.
.times. F ) .times. .times. M .times. 10 6 ( 3 ) ##EQU2## where m
is the mass of gold deposited (.mu.g), A is the geometric electrode
area (cm.sup.2), t is the electrodeposition time (s), and M is the
atomic weight of gold (g/mol). Analysis of FIG. 5 suggests that a
monotonic increase in the deposition potential correlates with the
amount of deposited gold. The deposited particles were also
characterized with EDS. FIG. 6 shows an EDS spectrum collected
after 5 min of gold electrodeposition. The spectrum confirms the
presence of both gold and aluminum.
[0044] The mechanism of electrodeposition, i.e., nucleation and
growth of gold particles, was investigated by generating four
samples at two different current densities of -0.54 and -1.1
mA/cm.sup.2 and two electrodeposition times of 10 and 20 s. During
these experiments, the electrodeposition potential at which the
aluminum/copper electrode was polarized at the end of
electrodeposition became only slightly more positive than its
initial value. FIGS. 7 and 8 present micrographs of the
aluminum-copper alloy film electrodes with deposited gold
particles. Histograms shown as insets in FIGS. 7a, 7b and 8a, 8b
were collected by analyzing low-magnification micrographs. As a
result, the particle counts are higher than the number of particles
shown in FIGS. 7a, 7b and 8a, 8b. Whereas FIG. 7a, 7b demonstrate
the state of the samples obtained after electrodeposition at -0.54
mA/cm.sup.2 for 10 and 20 s, FIG. 8a, 8b show the results of
electrodeposition at -1.1 mA/cm.sup.2 for 10 and 20 s. Comparison
of FIGS. 7a, 7b and 8a, 8b at corresponding electrodeposition times
indicates that the particle density increases with current density
(or overpotential). This result is consistent with previous
observations that the nucleation density exponentially increases
with overpotential. The exponential dependence is due to a
distribution of activation energies associated with nucleation
sites.
[0045] The EIS data (FIG. 4) indicate that gold particles are
electrically connected to the underlying aluminum-copper alloy
film.
[0046] The effect of electrodeposition time on the particle density
and particle diameter is determined from examination of FIGS. 7a,b
and 8a,b. Table I shows that mean particle diameters increase with
the electrodeposition time. The particle densities for samples
prepared at cathodic current densities of 0.54 and 1.1 mA/cm.sup.2
are 2.times.10.sup.6 and 5.times.10.sup.6 particles/cm2,
respectively. Lower particle densities of (1-5).times.10.sup.5
particles/cm.sup.2 were obtained with cathodic current densities of
0.07-0.18 mA/cm.sup.2. Analysis of micrographs and histograms leads
to conclusions as follows. First, at a given current density, the
particle density remains almost constant as the electrodeposition
proceeds over the investigated period of time. Second, for a given
sample the distribution of particle diameters is comparatively
narrow, with the relative standard deviation being approximately
25%. These two observations indicate that nucleation occurs only at
the onset of the deposition process. Thus, electrodeposition
proceeds by the instantaneous nucleation mechanism, although the
invnetors do not wish to be bound by any theory.
[0047] Gold electrodeposition can be compared with electroless
deposition of silver on the aluminum-copper alloy film substrate
described in Example 2. In a dramatic contrast to electroless
deposition, the electrodeposition of gold allows control of both
the particle density and particle diameter. Whereas electroless
deposition is determined by the overpotential and the
elecrodepositon is controlled by the electrodeposition time.
Therefore, electrodeposition is the method of choice for
fabrication of particle-type films with a controlled particle
density and a narrow distribution of particle diameters. For
example, electrodeposition of a metal or metal alloy on oxidized
etched Al/Cu films can be used to make a porous electrode built
from electrically interconnected and spherical nanoparticles with
mean particle diameter of from 10 to 1000 nm. TABLE-US-00001 TABLE
1 Mean Particle Diameters (.mu.m) for Electrodeposition Times and
Current Densitites Shown in FIGS. 7 and 8 current density
(mA/cm.sup.2) time(s) 0.54 1.1 10 1.1 .+-. 0.3 0.75 .+-. 0.19 20
1.5 .+-. 0.3 1.1 .+-. 0.3
[0048] Example 1 described above demonstrates that aluminum-copper
alloy films are made amenable for subsequent electrodeposition by
anodization followed by chemical etching of aluminum oxide on the
anodized surface. Scanning electron microscopy examination of
aluminum-copper alloy films following gold electrodeposition shows
the presence of gold particles with densities of 10.sup.5-10.sup.7
particles cm.sup.-2. The relative standard deviation of mean
particle diameters is approximately 25%. Whereas the gold particle
density was determined by the overpotential, the gold particle
diameter was controlled by the electrodeposition time. Therefore,
electrodeposition is the method of choice for the fabrication of
particle-type noble metal films with a controlled particle density
and a narrow particle size distribution. The fabricated films of
gold particles with a controlled particle density and particle
diameter distribution can be utilized in a number of applications,
including catalysis, electrocatalysis, and optical and electronic
devices. The method of the invention thus can be used as an
alternative to the traditionally used zincate process for
electrodeposition on aluminum.
EXAMPLE 2
Electroless Deposition
[0049] This Example describes an illustrative method pursuant to
another embodiment of the invention for the pretreatment of an
aluminum surface that makes it amenable for the electroless
deposition of silver (Ag). This illustrative method is achieved by
alloying aluminum with copper, anodizing the surface, and then
chemically etching the anodized surface (i.e. etching the aluminum
oxide layer on the surface) prior to electroless deposition.
[0050] In particular, aluminum-copper alloy covered wafers used in
this Example were fabricated as follows: First, a 600 nm thick
layer of SiO.sub.2 was thermally grown by steam oxidation of a Si
wafer. Second, a 3 micron thick layer (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 was anodized in
an electrochemical cell, described in detail elsewhere, at 50 V DC
for 20 minutes in 3% weight by volume oxalic acid aqueous solution
at 0.degree. C. Fourth, the porous and barrier aluminum oxides were
etched in a mixture of 0.4 M phosphoric and 0.2 M chromic acids at
60.degree. C. for approximately 2 hours. Fifth, AgNO.sub.3 was
added to the etching solution to obtain the 1.1 mM concentration of
Ag+ to effect electroless deposition of silver. Electroless
deposition was carried out in the etching solution at 60.degree. C.
and with no stirring. That is, silver (Ag) was deposited on the
treated surface of the Al--Cu alloy films or layers by the galvanic
displacement mechanism (electroless deposition) during the etching
step by adding AgNO.sub.3 to the etching solution. Copper in and/or
underneath the film or layer as a result of anodizing appears to
act as a reducing agent, although applicants do not intend to be
bound by this. The invention also envisions using an external
reducing agent during electroless deposition.
[0051] Anodization of aluminum-copper alloy films prior to etching
was carried out with a platinum mesh counter electrode and a
Hewlett-Packard 4140B pA meter/DC voltage source. EIS experiments
were performed in a three-electrode cell with the same working and
counter electrodes and a platinum wire as a quasi-reference
electrode. EIS was carried out with an IM6-e impedance measurement
unit (BAS-Zahner) and the acquired EIS data were analyzed with
impedance modeling software (BAS-Zahner). EIS data were acquired at
open circuit potential (OCP) over a frequency range between 1 Hz
and 100 kHz and with an AC potential amplitude of 5 mV. A low
amplitude of AC potential is customarily employed in EIS in order
to satisfy the condition of linearity. The impedance data were
normalized to the geometric electrode area, 1.4 cm.sup.2. The
surface morphology of deposited silver films was evaluated by a
Hitachi (S-5200) scanning electron microscope equipped with a PGT
spectrometer for energy dispersive spectroscopy (EDS). The
microscope was operated at 5-6 kV for imaging and at 25 kV for
EDS.
[0052] The anodization of aluminum-copper alloy films and
subsequent etching were carried out in order to generate a clean
surface with a controlled thickness of a layer of barrier aluminum
oxide. While anodization forms barrier and porous aluminum oxide
layers, etching results in complete dissolution of porous aluminum
oxide and partial dissolution of barrier aluminum oxide.
[0053] FIG. 9 demonstrates the Bode representation of an EIS
spectrum collected after anodization for 20 minutes and etching for
110 minutes. While the exact physical origin of the second (R.sub.2
CPE.sub.2) combination is uncertain its introduction to the
equivalent circuit is necessary in order to obtain a more accurate
estimate of CPE associated with barrier aluminum oxide as shown in
Table 1' (left column). The presence of two (R CPE) combinations
may be attributed to a two-layer structure of the aluminum oxide
film 26 In this case, the first (R.sub.1 CPE.sub.1) combination
represents the outer layer of barrier aluminum oxide with the
dielectric constant of 8.6. The second (R.sub.2 CPE.sub.2)
combination possibly represents the inner layer of aluminum oxide
with different dielectric properties, which is located between the
aluminum phase and outer layer of barrier aluminum oxide.
[0054] The etching of the layer of barrier aluminum oxide was
followed by EIS measurements. The thickness of the barrier oxide
layer was calculated according to Equation [1'], where (C.sub.bl)
is capacitance of the barrier aluminum oxide, (d) is its thickness,
A is the geometric surface area, 1.4 cm.sup.2, (.epsilon..sub.0) is
the permittivity of vacuum, 8.85.times.10.sup.-12 F/m and
(.epsilon.) is the dielectric constant of aluminum oxide, 8.6.
C.sub.bl=.epsilon..epsilon..sub.0A/d (1')
[0055] The capacitance of the barrier aluminum oxide layer was
assumed to be equal to the magnitude of CPE.sub.1 because the
frequency dissipation factor (.alpha..sub.1) was almost equal to 1
(0.96.+-.0.01). Due to a slow rate of dissolution, the layer of
barrier aluminum oxide was considered to be quasi-stable over the
time period of EIS measurements (about 3 minutes). The EIS scan was
repeated every 10 minutes. The left part of FIG. 10 shows that the
magnitude of CPE.sub.1 increases and the thickness of the barrier
aluminum oxide layer almost linearly decreases with time at a
constant temperature. A constant dissolution rate of barrier
aluminum oxide was a consequence of the constant electrode area
exposed to the etching electrolyte. The etching was carried out for
approximately 50 minutes after establishing that both the magnitude
of CPE.sub.1 and, as a result, thickness of barrier aluminum oxide
did not vary with time. At this moment the layer of barrier
aluminum oxide was assumed to be thinnest, which favored the
electroless deposition of silver.
[0056] Establishment of the utility of EIS provides a method to
monitor the electroless deposition of silver. FIG. 11 shows the
Bode representation of an EIS spectrum collected after 180 minutes
of electroless deposition of silver. Table 1' (right column) lists
the results of modeling by using the same equivalent circuit as
discussed above. Careful comparison of FIGS. 9 and 11 shows that
the magnitude of total cell impedance significantly decreases and
the phase becomes less negative in the low frequency region between
1 Hz and 500 Hz. As shown in Table 1' (right column), both of these
observations result from an increased value of CPE.sub.1 (by one
order of magnitude) and a decreased value of R.sub.1 (by two orders
of magnitude). It can be concluded that the electroless silver
deposition transforms CPE.sub.1 from being dominated by the thin
(1.4 nm) layer of barrier aluminum oxide (5-6 .mu.F/cm.sup.2) to
being dominated by the barrier aluminum oxide with silver particles
on the top/electrolyte interface (30-40 .mu.F/cm.sup.2).
Concurrently with increasing capacitance, the resistance of the
layer of barrier aluminum oxide decreases from 100
k.OMEGA..times.cm.sup.2 to 1-2 k.OMEGA..times.cm.sup.2 (Table 1).
This observation most likely results from the incorporation of
silver in the layer of barrier aluminum oxide and, as a result, an
increase in the electronic conductivity in this layer. FIG. 12
demonstrates that after addition of AgNO.sub.3 (the cell
concentration of 1.1 mM) and a short incubation period, CPE.sub.1
monotonically increases over the investigated period of time (3
hours). In contrast, the resistance of barrier aluminum oxide
(R.sub.1) suddenly drops in the few first minutes of electroless
deposition and slightly decreases afterward over 3 hours. It is
noted that both elements of the second (R.sub.2 CPE.sub.2)
combination do not appreciably change during electroless
deposition. As a result, the (R.sub.2 CPE.sub.2) combination is not
influenced by electroless deposition, which takes place on the
barrier aluminum oxide/electrolyte interface. Given the observed
changes in both R.sub.1 and CPE.sub.1, EIS is shown to have great
practical utility for in-situ monitoring of the silver electroless
deposition.
[0057] In order to investigate the electroless deposition of
silver, the galvanic displacement was interrupted after 9, 60, 120
and 180 minutes of continuous deposition. The silver deposits were
examined by SEM (FIGS. 13a, 13b, 13c and 13d), respectively. The
black pseudo-hexagonal spots with white edges shown at FIG. 13a
represent the scallops of barrier aluminum oxide left on the
surface after anodization and etching. As observed from FIG. 13a,
the silver phase formation preferably starts in the centers of
scallops. The thickness of oxide layer is known to play a
significant role in determining the location of nucleation sites
during electroless and electrodeposition. Importantly, it is noted
that no electroless deposition of silver was observed if the
anodization and etching steps were omitted. The analysis of the
micrographs revels that the electroless deposition proceeds via the
formation of spherical particles of silver randomly distributed on
the surface. Both the particle density and average particle
diameter increase with the deposition time. The average diameter of
particles increases from 50 nm after 9 minutes of deposition to 180
nm after 120 minutes. The fact that the electroless deposition
results in a distribution of particle diameters indicates the
silver phase formation is a continuous process (e.g. new
nano-particles are formed while old particles increase in
diameter). By varying the duration and temperature of silver
electroless deposition, it is possible to fabricate coatings
containing silver particles with a variety of diameters. The
chemical composition of the deposited particles was confirmed by
EDS. FIG. 14 shows an EDS spectrum collected after 180 minutes of
electroless plating. The spectrum shows the presence of aluminum,
silicon, silver, and a trace amount of copper.
[0058] In order to further study silver electroless deposition on
the aluminum-copper alloy films or layers, control experiments were
performed under the same conditions, but with the pure aluminum
substrate (99.997% aluminum foil). These experiments revealed no
increase in the interfacial capacitance over of a period of 2 hours
after addition of the same amount of AgNO.sub.3 during the etching
step. In addition, SEM examination of the samples revealed no
particles of silver. Aluminum is known to be a stronger reducing
agent than copper (the redox potential of Al.sup.3+/Al is about 2.0
V more negative than that of Cu.sup.2+/Cu. Therefore, the driving
force for galvanic displacement of aluminum by silver is
significantly larger than that of copper by silver (0.46 V).
However, aluminum is also a very inert metal due to the presence of
the surface oxide layer. Thus, it was not surprising that galvanic
displacement of aluminum by silver was not observed under our
experimental conditions (1.1 mM AgNO.sub.3, a mixture of chromic
and phosphoric acids, pH 1.8, 60.degree. C.). These results confirm
that the electroless deposition of silver on pure aluminum
substrates is a kinetically prohibited process due the presence of
the layer of barrier aluminum oxide, which prevents the electron
transfer from the aluminum substrate to cations of silver.
[0059] EIS measurements at OCP indicate that magnitudes of
CPE.sub.1 and, as a result, the thickness of barrier aluminum oxide
are approximately the same for the treated alumium-copper alloy
films or layers treated pursuant to the invention and the 99.997%
pure aluminum foil were anodized and chemically etched. However,
the samples show the aforementioned striking difference toward the
electroless silver deposition.
[0060] Upon completion of etching (0.4 M phosphoric and 0.2 M
chromic acids, pH 1.8, 60.degree. C.) OCP of the aluminum-copper
alloy film or layer is determined to be sufficiently negative
(-0.70 V vs. a Ag/AgCl reference electrode). Although not wishing
to be bound by any theory or explanation, applicants note that this
measurement suggests that the oxidation state of copper
incorporated in the layer of barrier aluminum oxide is zero such
that silver may deposit by galvanic displacement of such
copper.
[0061] Additional experiments also demonstrated that the elevated
temperatures were necessary in order to accelerate the electroless
deposition of silver. At a room temperature, the electroless
deposition of silver was achievable, but occurred at a
significantly slower rate as determined by EIS and SEM. For
example, the electroless deposition of silver for 2 hours at
22.degree. C. increased the magnitude of CPE.sub.1 to only 7-8
.mu.F/cm.sup.2. In contrast, the same increase was achieved after
electroless deposition for only 4-5 minutes at 60.degree. C.
[0062] Scanning electron micrographs show that electroless
deposition results in the formation of films composed of silver
particles on the aluminum-copper alloy films. By varying the
conditions for silver electroless deposition (e.g. duration and
temperature), it is possible to fabricate silver particles with a
range of diameters (10-200 nm). These films are of interest for
fabrication of miniature silver-zinc batteries, optical devices for
surface enhanced Raman scattering and FT-IR spectroscopy, composite
materials with photocatalytic properties and surfaces with
anti-microbial properties.
[0063] Moreover, traditionally, zincating or stannating processes
are used as the initial treatment of aluminum surfaces for
sequential electroless or electrodeposition of metals (e.g. Ni).
The method of the invention described in this Example 2 to achieve
electroless deposition of silver particles by galvanic displacement
can be used as an alternative method to zincating or stannating. As
a result of the activation, the aluminum-copper alloy surface can
be further coated with a metal (e.g. Ni, Ag, Au, etc.) by means of
electroless deposition.
[0064] In addition, methods have been develpoed for patterning and
anodization of the aluminum films only in those areas, which do not
have a protective mask. A combination of these methods and
electroless deposition of silver pursunt ot the invention is
attractive for selective metallization of aluminum surfaces.
Therefore, structures with particles of silver deposited only in
the selective areas can be fabricated by combining of electroless
deposition pursuant to the invention and photolithographic
methods.
[0065] 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 TABLE-US-00002 TABLE 1' Results of modeling of EIS data
Anodization and Silver deposition Elements etching (FIG. 9) (FIG.
11) R.sub.u/.OMEGA. 27.6 .+-. 0.6 14.6 .+-. 0.6 R.sub.t/k.OMEGA.
.times. cm.sup.2 102 .+-. 4 1.36 .+-. 0.14 CPE.sub.1/.mu.F .times.
5.74 .+-. 0.06 41.6 .+-. 1.7 s.sup..alpha.-t/cm.sup.2 .alpha..sub.1
0.968 .+-. 0.001 0.968 .+-. 0.005 R.sub.2/.OMEGA. .times. cm.sup.2
15.1 .+-. 0.8 13.8 .+-. 0.8 CPE.sub.2/.mu.F .times. 10.2 .+-. 0.6
5.2 .+-. 0.5 s.sup..alpha.-t/cm.sup.2 .alpha..sub.2 0.712 .+-.
0.008 0.777 .+-. 0.008
* * * * *