WO2007001334A2 - Activation of aluminum for electrodeposition or electroless deposition - Google Patents

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.

Description

ACTIVATION OF ALUMINUM FOR ELECTRODEPOSITTON OR ELECTROLESS DEPOSITION

This application claims benefits and priority of U.S. provisional application Serial No. 60/601,917 filed August 16, 2004. FIELD OF THE INVENTION

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

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.

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.

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.

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.

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.

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

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.

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 °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.

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.

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).

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.

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.

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

Figure 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.

Figure 2 is a Tafel plot for gold electrodeposition on a processed wafer.

Figure 3 is a Bode plot (magnitude and phase) after a total of 20 minutes of galvanostatic gold electrodeposition at -0.018 mA/cm² performed for 30 or 60 second intervals. The inset represents an equivalent circuit used to model EIS data.

Figure 4 is a plot of resistance and capacitance of the first parallel combination (R₁) and CPEi ) as a function of time during galvanostatic electrodeposition at -0.018 mA/cm² of gold on a processed wafer.

Figure 5 is a plot of mass of gold deposited per unit area and electrodeposition potential as a function of electrodeposition time.

Figure 6 is an EDS spectrum obtained after 5 minutes of gold electrodeposition at -0.018 mA/cm² .

Figures 7a, 7b are SEM micrographs with corresponding histogram insets after gold electrodeposition at -0.54 mA/cm² for 10 seconds (Fig. 7a) and 20 seconds (Fig. 7b).

Figures 8a, 8b are SEM micrographs with corresponding histogram insets after gold electrodeposition at -1.1 mA/cm² for 10 seconds (Fig. 8a) and 20 seconds (Fig. 8b).

Figure 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 °C and subsequent etching a mixture of 0.4 M phosphoric and 0.2 M chromic acids at 60 ⁰C for 110 minutes. The equivalent circuit is shown as an insert in Figure 9.

Figure 10 is a plot of capacitance and thickness of the layer of barrier aluminum oxide during etching.

Figure 11 shows EIS data (magnitude of impedance and phase) collected at OCP after electroless deposition of silver for 3 hours.

Figure 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 (Figure 12) is a continuation of the time axis (Figure 10) with some overlap between 45 and 100 minutes.

Figures 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).

Figure 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

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. 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.

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 ⁰C 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.

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.

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

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.

In particular, aluminum-copper alloy covered wafers used in this Example were fabricated as follows: First, a 600-nm layer of SiO₂ was thermally grown by steam oxidation of each silicon wafer. Second, a 3 -μm thick layer Al-Cu alloy (99.5 weight % aluminum and 0.5 weight % copper) was deposited on the layer of SiO₂ 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 ⁰C. 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 °C for approximately 2 hours. Fifth, gold electrodeposition on the treated Al-Cu alloy layer was carried out at room temperature (22 °C) in 1.0 M Na₂SO₃ (pH 8) with the Oromerse Part B gold plating solution avialable commercially from Technic Inc., Anaheim, CA. The final concentration of Na₃Au(SO₃)₂ of the plating solution was 4.3 mM.

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 cm². 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.

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. Figure 1 shows the current-potential curves collected under anaerobic conditions for three substrates: two with Na₃Au(SO₃)₂, (a) processed wafer and (c) unprocessed wafer, and one without Na₃Au(SO₃)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 Na₃Au(SO₃)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 Na₃ Au(SO₃)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.

To further characterize the electrodeposition of gold, a Tafel plot (Figure 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

where E is the potential, R is the gas constant, T is the absolute temperature, a 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 |/|) 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 a 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

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 Figure 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_u). The CPE is frequently used instead of a pure capacitance to describe interfacial dielectric properties. One of two parallel (R₁ CPEi) combinations is attributed to the layer of barrier aluminum oxide. In this case, CPEi describes the dielectric properties of barrier aluminum oxide, and R₁ describes the resistance to ion migration through the barrier aluminum oxide. The second ( R₂ CPE₂) 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.

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. Figure 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/cm²). 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, R₁₁ is 39.9 ± 0.6 Ω, Ri is 8.7 ± 0.9 kΩ cm², CPEi is 13.0 ± 0.5 μF s^α-1/cm², on is 0.939 ± 0.001, R₂ is 12.2 ± 0.8 Ω cm², CPE₂ is 7.8 ± 0.7μF s^α-1/cm², and α₂ 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/cm²) 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.

Figure 4 illustrates the magnitude of CPE, and R, as a function of deposition time. Over the investigated period of time, CPEi monotonically increases from values typical for electrodes with thin oxide layers (5-6 μF/cm²) to values approaching those typical for metal electrodes (20 μF/cm²). In contrast, the resistance of barrier aluminum oxide (Ri) drops rapidly in the few first minutes of electrodeposition from approximately 1 MΩ cm² to 300 kΩ cm². Thereafter this resistance gradually decreases to values approaching 10 kΩ cm². 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/cm² and less than 0.1 μA/cm², respectively. Thus, a direct comparison of two data sets related to the resistance of barrier aluminum oxide (Ri) is not possible.

In addition to changes in the interfacial electrical properties, it is worthwhile to note a systematic increase in the deposition potential during galvanostatic deposition (Figure 5). After approximately 23 min of electrodeposition, the potential needed to maintain the same current (-0.018 mA/cm²) 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 Figure 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)

where m is the mass of gold deposited (μg), A is the geometric electrode area (cm²), t is the electrodeposition time (s), and M is the atomic weight of gold (g/mol). Analysis of Figure 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. Figure 6 shows an EDS spectrum collected after 5 min of gold electrodeposition. The spectrum confirms the presence of both gold and aluminum. 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/cm² 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. Figures 7 and 8 present micrographs of the aluminum-copper alloy film electrodes with deposited gold particles. Histograms shown as insets in Figures 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 Figures 7a, 7b and 8a, 8b. Whereas Figure 7a,7b demonstrate the state of the samples obtained after electrodeposition at -0.54 mA/cm² for 10 and 20 s, Figure 8a,8b show the results of electrodeposition at -1.1 mA/cm" for 10 and 20 s. Comparison of Figures 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.

The EIS data (Figure 4) indicate that gold particles are electrically connected to the underlying aluminum-copper alloy film.

The effect of electrodeposition time on the particle density and particle diameter is determined from examination of Figures 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/cm² are 2 x 10⁶ and 5 x 10⁶ particles/cm2, respectively. Lower particle densities of (1-5) x 10⁵ particles/cm² were obtained with cathodic current densities of 0.07-0.18 mA/cm². 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.

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 1 : Mean Particle Diameters (μm) for Electrodeposition Times and Current Densitites Shown in Figures 7 and 8 current density (mA/cm²) time(s) 0.54 1.1

10 1.1 ± 0.3 0.75 + 0.19

20 1.5 + 0.3 1.1 ± 0.3

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 10⁵-10⁷ particles cm^"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:

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. In particular, aluminum- copper alloy covered wafers used in this Example were fabricated as follows: First, a 600 nm thick layer of SiO₂ 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 OfSiO₂ 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 ⁰C. 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 ⁰C for approximately 2 hours. Fifth, AgNO₃ 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 ⁰C 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 AgNO₃ 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.

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 . 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.

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. Figure 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 (R₂ CPE₂) 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 (Ri CPEi) combination represents the outer layer of barrier aluminum oxide with the dielectric constant of 8.6. The second (R₂ CPE₂) 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 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_bl) is capacitance of the barrier aluminum oxide, (d) is its thickness, A is the geometric surface area, 1.4 cm², (εø) is the permittivity of vacuum, 8.85x 10^"12 F/m and (ε) is the dielectric constant of aluminum oxide, 8.6.

The capacitance of the barrier aluminum oxide layer was assumed to be equal to the magnitude of CPE) because the frequency dissipation factor (αi) 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 Figure 10 shows that the magnitude of CPEi 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 CPEi 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.

Establishment of the utility of EIS provides a method to monitor the electroless deposition of silver. Figure 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 Figures 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 CPEj (by one order of magnitude) and a decreased value of Ri (by two orders of magnitude). It can be concluded that the electroless silver deposition transforms CPEi from being dominated by the thin (1.4 nm) layer of barrier aluminum oxide (5-6 μF/cm²) to being dominated by the barrier aluminum oxide with silver particles on the top / electrolyte interface (30-40 μF/cm²). Concurrently with increasing capacitance, the resistance of the layer of barrier aluminum oxide decreases from 100 kΩxcm² to 1-2 kΩxcm² (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. Figure 12 demonstrates that after addition of AgNO₃ (the cell concentration of 1.1 mM) and a short incubation period, CPEi monotonically increases over the investigated period of time (3 hours). In contrast, the resistance of barrier aluminum oxide (Ri) 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 (R₂ CPE₂) combination do not appreciably change during electroless deposition. As a result, the (R₂ CPE₂) combination is not influenced by electroless deposition, which takes place on the barrier aluminum oxide / electrolyte interface. Given the observed changes in both Ri and CPEi, EIS is shown to have great practical utility for in-situ monitoring of the silver electroless deposition.

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 (Figure 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. 13 a, 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 ran 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. Figure 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.

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 OfAgNO₃ 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 A1³⁺/A1 is about 2.0 V more negative than that of Cu²⁺/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 AgNO₃, a mixture of chromic and phosphoric acids, pH 1.8, 60 ⁰C). 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.

EIS measurements at OCP indicate that magnitudes of CPEi 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.

Upon completion of etching (0.4 M phosphoric and 0.2 M chromic acids, pH 1.8, 60 °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 wishingto 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.

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 ⁰C increased the magnitude of CPEi to only 7-8 μF/cm². In contrast, the same increase was achieved after electroless deposition for only 4-5 minutes at 60 °C.

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.

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.

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.

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

Claims

CLAIMSWE CLAIM

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.