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Anal. Chem. 1992,64, 1030-1033
Electrodeposition of Metallic Films on Aluminum Specimen Supports for Characterization by Scanning Electron Microscopy and Energy-Dispersive X-ray Analysis Ngee-Sing Chong Cantrell Research, Znc., P.O. Box 19076, Austin, Texas 78760
Michael L. Norton* Department of Chemistry, Marshall University, Huntington, West Virginia 25755
James L. Anderson* Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602
Methods of alumlnum surface treatment are Investigated so that alloy thln fllms can be easlly electrodeposited onto alu” I stubs dltectty for physlcal characterlzatkn by s c a m electron mkroscopy and elemental determinatkn by energydkperslve X-ray mlcroanalyds. Undedrable mlnor components such as silicon In the cast alumlnum are removed by the treatment steps of acldlc and bask dlswlutlon. The adheslon between the alumlnum substrate electrode and the depodted alloy fllms can be Improved by the process of electrochemical anodization In which a porous layer of alumlnum oxlde Is formed to provide polnts of anchorlng for the deporH wlthln pores of ca. 5-pm dlameter. The reactlvlty of the alumlnum substrate wlth the aqueous plating bath Is effectively subdued by the formation of a spontaneous zlnc hwMwdon coatlng. The standardbm elemental det8fminatbn of alloy fllms directly depodted onto the alumlnum stubs Is expected to be more accurate than for electrodes mounted on lhe stubs because the tltt angle and the takaofl angle can be more rellably measured and used to account for the dHferences In absorptlon pathlength and backscattering coeff Iclent durlng quantttatlve analysls.
INTRODUCTION Electrochemists frequently use scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX) to study the microstructure and chemical composition of electrodeposited thin films on electrode^.'-^ Combined SEM/ EDX affords a nondestructive, quantitative elemental composition map with high spatial resolution, while preserving the sample for subsequent measurements, with lower cost and easier quantitation than alternate electron beam techniques such as scanning Auger microscopy and ion microprobe analysis. Other methods for the analysis of metallic films, including atomic absorption spectrometry,4 inductively coupled plasma-atomic emission spectrometry,5 spectrophotometric determination by colorimetry,6 and stripping voltammet$ are destructive and do not afford spatial composition information despite their impressive detection limits relative to EDX. EDX is well enough established to carry out “standardless” quantitative analysis* and determination of light element content in carbides, oxides, and fluorides using continuum radiation? Standardless analysis is particularly attractive because of the scarcity of suitable reference standards and the tremendous amount of time saved in performing the 0003-2700/92/0364-1030$03.00/0
calibration, although care is required to correct for effects of the morphology of the deposit. Morphology strongly affects the shape of the primary electron excitation volume and the extent of self-absorption in the deposited films, and hence affects the accuracy of the correction analysis. This is especially important in electrodeposited f i i which tend to have unique localized three-dimensional geometric features. Conventional SEM/EDX analysis of electrodeposited thin films involves mounting of either electrodes with electrodeposited films or the stripped f i themselves onto a specimen support/stub. The difficulty of mounting electrodes coplanar with the stub surface interferes with the requirements of standardless quantitative EDX analysis for a flat specimen with a well-characterized tilt angle and take-off angle of the X-ray photons. Uncontrolled tilt or take-off angles or nonplanar geometries cause X-ray excitation volumes and absorption pathlengths to be uncontrolled or to vary across the sample surface. Mounting of stripped films is rarely used because of the adhesion between the substrate and the deposited film and the difficulty of mounting the micrometerthick film perfectly flat on top of the stub. Mounting of electrodes on stubs tends to confound the morphologicalinfluence on the X-ray signal with uncontrolled geometric factors. However, if the deposited f i i is coplanar with the stub surface, the estimate (i.e. by microscopy) of the surface orientation of deposit relative to the stub surface can potentially allow morphologicalcorrection. A similar problem in X-ray fluorescence spectrometry of aluminum alloys exists with regard to the effects of different surface preparation techniques.1° The extent of signal variation caused by structural characteristics of the specimen surface should be significantly greater for EDX than for X-ray excited fluorescence,since the primary exciting electron beam in EDX has a considerably shorter penetration range than X-rays. For films of submicrometer thickness, signal enhancement/suppression may arise from absorptive or fluorescent interactions or insufficiently deconvolved spectral overlap between the electrode material and the film constituents at the substrate-deposit interface (e.g. a Ag electrode gives rise to a background peak that overlaps with the Cd peak in the analysis of CdSe films). Both signal distortion and spectral overlap can be ameliorated by tilting the specimen toward the detector to minimize excitation of the electrode substrate and to reduce the absorption pathlength of the emitted X-rays simultaneously. However, a more fundamental solution to this problem lies in the careful selection of electrode substrate material and deposition time which ultimately determines the film thickness. 0 1992 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 9, MAY 1, 1992
We outline here a chemical and electrochemical procedure for polishing, etching, anodizing, and spontaneously coating conventional aluminum SEM stubs with a zinc film to enable direct elecgodepition of a suitable thickness of metallic alloy films of interest for subsequent analysis by SEM/EDX. The procedure alleviates most of the difficulties outlined above, while avoiding problems such as slow rates of depoeition, poor adhesion of coating, and relatively high expense due to the requirements of vacuum associated with alternative stub metallization techniques based on vacuum evaporation and sputtering of metals. The difficulty of electroplating onto the a l e u m substrate due to aluminum oxide passivation is mitigated by pretreatment procedures of chemical etching of the surfam oxides and impurities, electrochemicalanodization for the formation of surface pores to improve adhesion, and the application of a more inert zinc immersion coating."J2 The SEM examination of the electrodepait directly plated onto the stub allows minute and fragile morphological features to be studied in an intact state without any structural deformation or introduction of artifacts during the electrode mounting step. An additional advantage is that the substrate metals of choice (i.e. Au, Ag, and Pt) can be electrodeposited from solution just prior to the electrodepositionof the desired thin f i i . This approach should be able to offer a very clean surface for thin-film formation and economy in forming just the desired thickneaa of substrate metal as electrode material.
EXPERIMENTAL SECTION Chemicals and Equipment. Reagent-gradechemicals were used as received. Milli-Q deionized and distilled water of 18 MQ-cmresistivity was used for the preparation of plating baths and treatment solutions for the aluminum stub surface. A locally constructed electrochemical cell using the aluminum stubs as rotating working electrodes was employed for the electrochemical procedures of aluminum surface treatment and the subsequent deposition of alloy thin f i i . The aluminum stubs (EMSL, Inc.) had a circular cross section of 1.23 cm2with a coaxial peg 8 mm in length extending from the back of the stub. The groove on the side wall of the disk of the stub was wound with a 0.25-mmdiameter niobium wire which was in turn fastened with Teflon tape. The stub was press-fitted snugly into a Teflon holder such that only the disk surface was exposed to the solution. A rotating disk electrode was fabricated by press-fitting the holder into a glass tube, with the niobium wire passing through the tube for electrical contact. The rotating electrode was used so that the rate of convective transport of electrolyte was maximized and so that any gas bubbles evolved could be dislodged by rotation at 750 rpm. The cell had divided compartments so that the counterelectrode of Pt tubing was separated from the working electrode by a fritted disk to avoid contamination or undesirable dissolution of deposit by anodic products from the counter electrode. The plating solution was purged with nitrogen gas before and during electrodeposition. The reference electrode was a Ag/AgCl wire dipped in a 3 M KCl fiing solution isolated from the solution by a porous Vycor separator. The deposition of alloy thin films was achieved using a Bioanalytical Systems SP-2 potentiostat. A Philips 505 scanning electron micrawow was used to evaluate the surface treatment of the stubs and to examine the morphology of the electrodeposited fiis. An accelerating voltage of 20-30 kV was used for both imaging and elemental analysis with an electron beam spot of 50 nm in diameter. A lithium-driftedsilicon detector and a data acquisition time of 120 s were used for EDX analysis. A Tracor Northern 5500 data acquisition system was used for spectral collection, and the quantitative X-ray determination was carried out by using the StandardlessFundamental Parameters program. Procedures. For direct electrodeposition onto the SEM stubs, the following procedures were followed. The aluminum stubs were rinsed with acetone and then mounted on the flat surface of a cylindrical block and immobilized with epoxy glue. The disk surface of the stubs was polished by manually pressing the block
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containing the potted stubs against emergy paper mounted on a motor-driven rotating plate. The polishing was carried out with succeasively fiier gradea of emery paper from 240-grit to 600-grit before the final polishing with 0.25-pm diamond grit immersed in lapping oil to give a mirror-smooth finish. To remove organic smudges, the polished stubs were cleaned in a proprietary desmutting or degreasing solution (Patchlin Chemical Co. 103-B) which contained ethanolamine. The stubs were then etched in 5% NaOH followed by 20% concentrated HzS04 1% concentrated HF at 70 OC for 10 s each time to provide an oxide-free surface with most of the silicon impurities stripped out by the HF. Dissolution of other metal impurities was done subsequently by immersing the etched stubs in 1:l (volume) concentrated HN03-H20 for 20 s followed by the immediate formation of a zinc immersion coating by dipping the stubs in an alkaline &-containing solutionll with the composition of 502 g/L NaOH, 98 g/L ZnO, 2 g/L FeC13.6H20,and 10 g/L NaKC4H406-4H20 for 60 s. Finally, the electrodeposition was carried out on the zinc-coated stub. Another variation of the pretreatment procedures for aluminum stubs involved mechanical polishing of stubs with up to 400-grit emery paper followed by the steps of degreasing, alkaline etch, acidic etch, and acidic dissolution of impurities a described above. Next, the disk surfaces of the stubs were electrochemically anodized at a current density of 1A/dm2 in 20 w t % concentrated H2S04or at an applied cell potential of 50 V in 20 vol % concentrated H3P04for 10-40 min, both at room temperature. The anodization process resulted in the production of surface pores that help improve adhesion of the deposited fii. Finally, the porous stubs were subjected to the zinc immersion coating process which produced a stable substrate surface for electrodeposition. The deposition of Fe-Cr-Ni alloy thin film from a recently proposed plating bath13was used to evaluate the characteristics of the pretreated aluminum stub surface as a substrate for electrodeposition. The aluminum stubs were rinsed in deionized and distilled water before being mounted in the electrochemical cell with 40 mL of the plating bath consisting of 0.16 mol of CrCl3-6H20,0.4 mol of NiC12.6H20,0.06 mol of FeC12.4H20,1.0 mol of NaC1, 1.0 mol of NH4C1,and 0.3 mol of H3B03in 1.00 L of dimethylformamide plus 1.00 L of H20.13 The controlled potential electrodeposition was carried out for 20 min at -1.25 V to give a coating of at least 10-pm thickness, enough to avoid the electron excitation of either the underlying zinc or aluminum substrate. The electrodeposited stub was finally washed in deionized and distilled water before being analyzed by EDXRF microanalysis in a scanning electron microscope in order to evaluate the reproducibility of the quantitative determination as a function of tilt angle under various instrumental conditions. Co and Ni-Co films were also deposited to confirm the efficacy of aluminum surface treatment to produce films comparable to those deposited on other substrates.
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RESULTS AND DISCUSSION Ideally, the surface treatment of aluminum stubs for successful direct electrodepositionof alloy thin f i i should solve or mitigate the following problems of deposition onto an aluminum substrate. First, the presence of aluminum oxide on the surface of stubs must be eliminated or masked so that its amphoteric nature does not interfere with the reduction of metal ions. Second, the spontaneous formation of certain metallic immersion deposita can alter the desired composition of the alloy deposited. Third, the difference in potential between the aluminum matrix and impurity phases containing Mg or Si in the stub can cause the deposition of alloy thin films with various intermetallic phases or heterogeneous compo~ition.~'Fourth, the difference in the coefficients of thermal expansion between aluminum and the deposited alloys may cause the thin fiim to peel off after the residual solvent evaporates and removes the heat of vaporization. Fifth, the mismatch in the atomic diameter and crystal lattice structure between aluminum and the deposited alloys may result in poor adhesion and cracks in the thin film. Figure 1 shows an SEM micrograph of the surface of an unpolished aluminum stub that has been treated sequentially
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 9, MAY 1, 1992
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. 1
Figure 1. SEM micrograph of the surface of an unpolished aluminum stub that has been treated sequentially in solutions of NaOH, H,SO, with 1% HF, and HNO,. The scale bar corresponds to 1 mm.
Figure 2. SEM micrograph of aluminum stub surface that has been deposited with a spontaneous zinc immersion coating. The scale bar corresponds to 1 mm.
in solutions of NaOH, H2S04/HF,and HN03. The etching by the solution reveals the concentric circular pattern due to the cutting of the disk along with some tiny holes arising from the selective dissolution of impurities included in the cast aluminum matrix. Mechanical polishing to a mirror-smooth finish prior to the etching treatment can eliminate these undesirable surface features that may interfere with either the uniformity and homogeneity of the alloy f i i deposited or the accuracy of X-ray fluorescence determination. Accuracy is adversely affected by the difficulty of applying appropriate absorption correction due to geometric effects. For instance, the analysis of the same alloy deposit on the ridges and the troughs of the concentric pattern gives rise to rather different results. The presence of any holes or surface irregularities after etching and the reactive sites due to aluminum oxide formation are masked by the spontaneous immersion coating of zinc as shown in Figure 2 for a specimen stub without mechanical polishing. The zinc coating also improves the adhesion and the homogeneity of deposited films by surmounting the difficulties mentioned earlier. In particular, the lattice parameter of zinc is better matched to that of most commonly plated transition metals, leadmg to better adhesion. The adhesion between zinc and aluminum is remarkably good because of the epitaxial relationship provided by the 2:3 ratio unit cell parameters of Zinc is an ideal choice aluminum (4.04 A) and zinc (2.66 for coating aluminum because of the stability of the couple
i
Figure 3. SD.4 micrograph Of aluminum stub surface that has been anodized in 20 wt % sulfuric acid at 1 A/dm2 for 10 minutes. The scale bar corresponds to 10 pm.
as opposed to other couples which have a large difference in reduction potentials and hence are susceptible to bimetallic corrosion. The use of anodization to produce pores on the aluminum surface to provide anchoring points for thin-film deposits depends on several variables. As anodization time increases, the thickness of the overall oxide layer also increases. The action of oxide dissolution by the electrolyte results in the formation of a porous oxide layer above a compact barrier layer oxide. The nature of the electrolyte greatly affects the rate of dissolution of the aluminum oxide layer and hence the pore size and pore density. Phosphoric acid produces larger pores than sulfuric acid and chromic acid." Sulfuric acid gives higher pore densities than both phosphoric and chromic acid.12 Furthermore, the increase of applied voltage tends to increase the thickness of both the barrier and porous layer but decrease the pore density. Figure 3 shows the surface of an aluminum stub that has been anodized in 20 w t % concentrated sulfuric acid at 1A/dm2 for 10 min at room temperature. The uniform distribution of pores with sizes of 3-5-pm diameter without pore agglomeration represents an ideal situation for improving the adhesion of electrodeposition. By using the same anodization conditions but lowering the electrolyte concentration to 10 wt % of concentrated sulfuric acid, a rather different aluminum surface with smaller pores and a heterogeneous distribution of pore size and depth is obtained (not shown), which seem to contribute to the poor adhesion and nonuniformity of the alloy film produced in subsequent electrodeposition. The deleteriouseffect of unduly large pores and pore agglomeration produced in 20 vol % concentrated phosphoric acid at room temperature for 40 min is shown in Figure 4. The X-ray elemental map of the corresponding SEM image of the Fe-Cr-Ni deposit reveals that only nickel is deposited inside the pores whereas the Fe-Cr-Ni alloy is deposited along the ridges or perimeter of the pores. A possible explanation for this phenomenon is that the effective potential inside the pore is shielded due to iR drop such that only the most readily reducible element, nickel, is deposited. In order to avoid heterogeneity of the alloy deposit, the anodization time or the concentration of phosphoric acid has to be reduced. It should be noted that aluminum stubs from different suppliers do respond slightly differently to the treatment procedures recommended above, and hence adjustments in terms of conncentrations of solutions, treatment times, and anodization voltage/current may be required.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 9, MAY 1, 1992
Figure 4. SEM micrograph of aluminum stub surface that has been anodized in 20 vol YO concentrated phosphoric acid at room temperature for 40 min before undergoing electrochemical deposition in Fe-Cr-Ni plating bath. Nickel deposits are found inside the pores whereas the Fe-Cr-Ni alloy is found along the ridges around the pores.
It is important to control the thickness of the barrier layer oxide because its electrical resistance contributes to the voltage drop across the interfacial region and thus makes the deposition of elements with rather negative reduction potentials difficult to achieve in aqueous plating baths. The commonly observed relationship of 10-12 A of barrier layer oxide per volt applied12for most anodizing baths may be used as a guide in ensuring a barrier layer thickness less than 10 nm. Besides masking the reactive amphoteric aluminum oxide and providing better deposit adhesion, the application of a zinc immersion coating on the anodized aluminum surface f& the pores and makes the electric field gradient more uniform and consequently yields a more uniform deposit. The successful electrodeposition of films that are uniform, crack-free, smooth, and homogeneous in composition onto the zinc-coated anodized aluminum surface is demonstrated by Figure 5, which shows a Fe-Cr-Ni alloy frlm deposited onto the exposed a l u " stub surface. The barely discernible pattern of swirl caused by the rotating electrode can be minimized by using a rotation speed of about 750 rpm or slower. Films of Ni-Co and Co (not shown) also exhibit similar morphology. The Fe-Cr-Ni, Ni-Co, and Co films are free from the usual problems associated with direct electrodeposition on aluminum such as poor adhesion, spotty deposits, and intermetallic phases containing aluminum due to spontaneous dissolution. It is worth noting that even though the two alloy plating baths consist of chloride solutions, there is no adverse effect of pitting on the substrate which usually occurs with direct deposition onto aluminum using a chloride plating bath. This pitting immunity is attributed to the application of the zinc coating. The analytical precision of EDX analysis of directly electrodeposited Fe-CFNi thin films is evaluated by the elemental determination of Ni and Cr as a function of tilt angle from -7' to 37.5". The overall improvement of the anaysis in terms of precision compared to the case of a similar deposit on a mounted electrode is quite pronounced and will be treated elsewhere.16 The mean and standard deviation for the major level of Ni at various tilt angles under various analytical conditions are 77.6 f 1.1% and those of the minor level of Cr are 2.54 f 0.40%. These values are equivalentto a relative standard deviation of 1.4% for Ni and 16% for Cr in com-
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Flgure 5. SEM micrographs of electrodeposited thin film of Fe-Cr-NI on treated aluminum stubs. The width of the micrograph corresponds to 8 mm of the specimen.
parison to the corresponding values of 4.3% and 39% for EDX analysis of a similar deposit on a mounted niobium disc electrode of 0.79 mm2. The difference between deposits onto stubs and deposits on mounted electrodes arises mainly from the fact that a film with well-characterizedtilt and take-off angles was analyzed with minimal substrate interference in the present approach. Despite the variety of analytical conditions imposed which includes differences in deposit morphology, electron beam spot, and accelerating voltage, the relatively small deviation implies that standardless analysis can be used reliably for various tilt angles for directly electrodeposited thin films.
ACKNOWLEDGMENT We gratefully acknowledge the financial support of NSF Grant CHE-8600224 and the use of electron microscopy facilities a t the Ultrastructural Center at the University of Georgia. REFERENCES Mishra, K.; Rajeshwar, K.; Weiss. A.; Murley, M.; Engelken, R. D.; Slayton, M.; McCloud. H. E. J . Electrochem. SOC. 1989, 136, 1915-1923. Chong, N. S.; Anderson, J. L.; Norton, M. L. J . Electrochem. SOC. 1989, 136, 1245-1246. Bhattacharya, R. N.; Rajeshwar. K. J . Electrochem. SOC. 1984, 131, 2032-2037. Abd El-Rehim, S. S.; Abd ECHalim. A. M.; Osman. M. M. J . Appl. Electrochem. 1985, 15, 107-112. Felloni, L.; Fratesi, R.; Quadrini, E.; Roventi, G. J . Appl. Electrochem. 1987, 17,574-582. Krohn, A.; Brown, T. M. J . Electrochem. SOC. 1961, 108, 60-64. Jovic, V. D.;Zejnilovic. R. M.; Despic, A. R.; Stevanovic. J. S. J . Appl. Electrochem. 1988, 18, 511-520. Zhang, R. J.; Wu, Z. Q. Acta Phys. Sinica 1982, 31. 1395-1400. Wu, Z. Q. J . Electron Microsc. 1987, 7 ,323-329. Feret, F. R.; Sokolowski, J. Spectroscopy 1989, 4 (7). 36-41. Murphy, M.. Ed. Metal Finishing Guidebook; Metals and Plastics Publications, Inc.: Hackensack, NJ. 1989; pp 439-449. Wernick. S.; Pinner. S.; Sheasby, P. G. The Surface Treatment and Finishing of Aluminum and its Albys (Volume 2 ) ;Finishing Publications Ltd.: Middlesex, England, 1987; pp 989-1053. Vertes. A.; Watson, A; Chisholm, C. U.; Czako-Nagy. I.; Kuzmann. E.: El-Sharif, M. R. Electrochim. Acta 1987, 32, 1761-1767. Binger, W. W.; Hollingsworth, E. H.; Sprawls, D. 0. Aluminum; Van Horn, K. R., Ed.; ASM: Metals Park, OH, 1967; Vol. 1, pp 209-276. Bullough. W.; Graham, G. E. J . Electrodepositors' Tech. SOC. 1947, 22, 169. Chong, N. S.; Anderson, J. L.; Norton, M. L. I n preparation.
RECEIVED for review October 10,1991. Accepted February 13, 1992.
