Chemical and Biological Engineering Publications Chemical and Biological Engineering

1996

Development of Surface Impurity Segregationduring Dissolution of AluminumXiaolin WuIowa State University

Kurt R. HebertIowa State University, krhebert@iastate.edu

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J. Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc. 83

5. K. Shimizu, G. E. Thompson, G. C. Wood, and Y Xu,Thin Solid Films, 88, 255 (1982).

6. G. E. Thompson, Y. Xu, P. Skeldon, K. Shimizu, S. H.Han, and G. C. Wood, Philos. Mag. B, 55, 851 (1987).

7. F. Brown and W. D. Mackintosh, This Journal, 120,1096 (1973).

8. J. J. Randall and W. J. Bernard, Elect rochim. Acta, 20,653 (1975).

9. P. Skeldon, M. Skeldon, G. E. Thompson, and G. C.Wood, Philos. Mag. B, 60, 513 (1989).

10. D. L. Cocke, C. A. Polansky, D. E. Halverson, S. M.Kormali, C. V. Barros-Leite, 0. J. Murphy, E. A.Schweikert, and P. Filpus-Luyck, This Journal, 132,3065 (1985).

11. G. E. Thompson, P. Skeldon, K. Shimizu, and G. C.Wood, Phil. Trans. R. Soc. London, A, 350, 143(1995).

12. J. P. S. Pringle, This Journal, 120, 398 (1973).13. P. Skeldon, K. Shimizu, G. E. Thompson, and G. C.

Wood, Thin Solid Films, 123, 127 (1985).14. P. Skeldon, K. Shimizu, G. E. Thompson, and G. C.

Wood, Surf. Interface Anal., 5, 247 (1983).15. G. Amsel, C. Cherki, G. Feuillade, and J. P. Nadai, J.

Phys. Chem. Solids, 30, 2117 (1961).16. K. Shimizu, P. Skeldon, G. E. Thompson, and G. C.

Wood, Surf. Interface Anal., 4, 208 (1981).17. J. R. MacDonald, J. A. Davies, T. E. Jackman, and L.

C. Feldman, J. Appi. Phys., 54, 1800 (1983).18. W. A. Schier, B. K. Barnes, G. P. Couchell, J. J. Egan, P.

Harihar, S. C. Mathur, A. Mittler, and E. Sheldon,Nucl. Phys. A, 254, 80 (1975),

19. J. A. Leavitt, P. Stoss, D. B. Cooper, J. L. Seerveld, L.C. Melntytre, Jr., R. E. Davis, S. Gutierrez, and T. M.Reith, Nucl. Inst rum. Meth. B, 15, 296 (1986).

20. T. A. Belote, E. Kashy, and J. R. Risser, Phys. Rev., 122,920 (1961).

21. R. W. Harris, G. C. Phillips, and C. Miller-Jones, Nucl.Phys., 38, 259 (1962).

22. J. P. Aidridge, G. E. Crawford, and R. H. Davis, Phys.Rev., 167, 1053 (1968).

23. J. R. Cameron, ibid., 90, 839 (1953).24. R. C. Furneaux, G. E. Thompson, and G. C. Wood,

Corros. Sci., 18, 853 (1978).

25. M. Skeldon, P. Skeldon, G. E. Thompson, G. C. Wood,and K. Shimizu, Philos. Mag. B, 68, 787 (1993).

26. P. Skeldon, K. Shimizu, G. E. Thompson, and G. C.Wood, Surf. Interface Anal., 5, 252 (1983).

27. H. Takahashi, K. Fujimoto, and M. J. Nagayama, ThisJournal, 135, 1349 (1988).

28. W. A. Lanford, R. S. Alwitt, and C. K. Dyer, ibid., 127,405 (1980).

29. H. Konno, S. Kobayashi, K. Fujimoto, H. Takahashi,and M. Nagayama, Electrochim. Acta, 25, 1667(1980).

30. K. Shimizu, G. E. Thompson, and G. C. Wood, ThinSolid Films, 77, 313 (1981).

31. G. E. Thompson, G. C. Wood, and K. Shimizu,Elect rochim. Acta, 26, 951 (1981).

32. 5. Chung, J. Robinson, G. E. Thompson, G. C. Wood,and H. S. Isaacs, Philos. Mag. B, 63, 557 (1991).

33. M. Pourbaix, Atlas of Electrochemical Equilibria inAqueous Solutions, National Association ofCorrosion Engineers, Houston, TX (1974).

34. J. S. L. Leach and B. R. Pearson, Electrochim. Acta,29, 1263 (1984).

35. R. Duran-Romero, K. Shimizu, P. Skeldon, G. E.Thompson, and G. C. Wood, Philos. Mag. B, 70, 163(1994).

36. H. Takahashi and M. Nagayama, Electrochim. Acta,23, 279 (1978).

37. P Skeldon, G. E. Thompson, and G. C. Wood, ThinSolid Films, 148, 333 (1987).

38. V. P. Parkhutik, Yu. E. Makushok, and V. I.Shershulskij, in Aluminium Surface TreatmentTechnology, R. S. Alwitt and J. E. Thompson,Editors, PV 86-11, p. 406, The ElectrochemicalSociety Proceedings Series, Pennington, NJ (1986).

39. M. F. Abd Rabbo, J. A. Richardson, and G. C. Wood,Corros. Sci., 16, 689 (1976).

40. M. Skeldon, K. Shimizu, P Skeldon, G. E. Thompson,and G. C. Wood, Corros. Sci., In press.

41. J. J. Randall, Elect rochim. Acta, 20, 663 (1975).42. J. J. Randall, W. J. Bernard, and R. R. Wilkinson, ibid.,

10, 183 (1965).

Development of Surface Impurity Segregation duringDissolution of Aluminum

Xiaolin Wu and Kurt Hebert

Department of Chemical Engineering, Iowa State University, Ames, Iowa 50011, USA

ABSTRACT

Caustic dissolution, when used as a pretreatment for etching of aluminum in chloride solutions, is observed toincrease the rate of pit nucleation. Rutherford backscattering spectrometry (EBS) and Auger electronspectroscopy wereused to measure the composition in the near-surface region of 99.98% purity aluminum after dissolution in1 N NaOH atroom temperature. During dissolution, concentrations of impurities such as Fe, Cu, and Ga were found to accumulatecon-tinuously within a layer less than about 10 nm thick adjacent to the surface, because they dissolved more slowly than didaluminum. Impurity concentrations on the order of 1 atom percent (a/o) in this layer, much higher than equilbrium val-ues, were found after 40 mm dissolution. It is argued that the large impurity concentrations are consistent with a high-ly defective region in the metal near the metal/oxide interface, which has been detected using positron annihilationmeas-urements. Dissolution produced a scalloped surface topography with typically 30 nm high ridgesseparated by 130 nm. Asimulation of RBS measurements based on scattering from spherical particles was developed to test for the possibility ofpreferential impurity segregation to ridges. No such segregation was detected, suggesting that this is not the reason forthe strong tendency for pit nucleation to occur on ridges, as has been observedpreviously.

InfroductjonCaustic dissolution of aluminum, when used as a pre-

treatment for anodic etching in aqueous chloride solu-tions, is found to produce an overall increase in the pitnumber density.' For this reason it is used industrially inthe manufacture of aluminum capacitor electrodes, forwhich enhancement of the surface area is desirable.* Electrochemical Society Active Member.

Several surface changes caused by dissolution may con-tribute to this increased susceptibility to pitting. Bondet al.2 showed that pits may initiate near microsegregatediron, copper, and silicon impurities in high purity alu-minum. It is possible that elevated near-surface concen-trations of these impurities are present after dissolution inNaOH, since concentrations of elements less reactive thanthe metal are typically enhanced by dissolution.35 Asidefrom composition effects, recent positron annihilation

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84 J Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc.

Fig. 1. SEM image of the aluminum surface after 10 mm dissolu-Hon in 1 N NaOH, and then etching in 1 N HCI at 65°C. Appliedpolarization during etching was 0.1 s at —2.0 V vs. the Ag/AgCIreference electrode, followed by 0.1 s at —0.4 V (the pitting poten-tial was —0.74 V).

measurements on aluminum dissolved in NaOH revealed ahighly defective layer within about 10 to 20 nm from theoxide/metal interface, within which the concentration ofvacancy-type defects was estimated to be on the order of1%. It is possible that these defects serve as pit nucleationsites, perhaps by disrupting the structure of the overlyingprotective oxide film.

Sodium hydroxide dissolution also alters the surfacetopography of alumimum: after exposure to NaOH, amicroscopic mosaic of ridges is found on the surface whichsurrounds scalloped depressions. These "scallop cells"(depressed areas bounded by ridges) are 0.1 to 1 p.m inwidth and the ridges are 10 to 100 nm high. Similartopographies are formed by chemical polishing and elec-tropolishing.78 Also, when the metal is anodically oxidizedin acidic solutions, a scalloped surface texture is found onthe metal surface underlying the oxide film.9 Cuff andGrant8 found that, for a constant time in a chemical pol-ishing solution, the scallop cell width increased withincreasing metal purity, and suggested that impurity con-centrations were strongly elevated on ridges compared tothe cell interior. They argued that the cellular pattern wasthe surface manifestation of a three-dimensional networkof impurities in the bulk metal. Thompson et al.1° and Linand Hebert1 subjected aluminum surfaces which had beenpretreated by acid anodizing and caustic dissolution,respectively, to anodic polarization in chloride ion con-taining solutions. They both noted a strong tendencytoward pit nucleation on ridges as opposed to within thecells (Fig. 1). Understanding the preferential attack ofthese very small ridges would be of significant interest,since nanometer-scale compositional or structural inho-mogeneities in the oxide or the metal may be involved.

In the present work, the concentrations of impuritiesnear the surface of high-purity (99.98%) aluminum weremeasured after dissolution in NaOH, in order to determinewhether the increased susceptibility to pitting could becorrelated with surface composition changes. Impuritieswere measured using Rutherford backscattering spec-trometry (RBS) and Auger electron spectroscopy (AES).Using the former technique, the surface composition wasdetermined as a function of dissolution time. This alloweda mole balance on impurities to be made which gave inf or-mation about the mechanism of their retention during dis-solution. In order to determine whether impurities werelaterally segregated to ridges, the backscattering spectrawere modeled using two simulations: one which assumed

that the impurities were present in a uniform flat layeradjoining the surface, and another which took them to belocalized in spheres tens of nanometers in diameter. Thesphere model approximated impurity segregation inridges. To the authors' knowledge the detection of lateralconcentration variations associated with protrusions is anextension of the capabilities of RBS.

ExperimentalAluminum foils were 100 p.m thick and annealed. The

typical grain size was 100 p.m. A composition analysiswith spark-source mass spectrometry revealed concentra-tions on the order of 10 wt-ppm of Cr, Cu, Fe, Ga, Mg, Si,and Zn metallic impurities. Dissolution was carried out atroom temperature (25°C) aqueous 1 N NaOH solution. Thesolution was not circulated during dissolution, but copiousgas evolution occurred from the aluminum surface.

Surface composition was measured with RBS, which isnondestructive and has a sensitivity of about 0.001 atomicfraction for some of the common heavier impurities (Cu,Fe, Ga, Pb) present in high purity aluminum. The RBS sys-tem consisted of a RBS 400 endstation with a 1 MV tan-dem accelerator (Charles Evans and Associates). Spectrawere acquired using beams of alpha particles (4He nuclei)directed normal to the sample surface, with an energy ofeither 1.0 or 2.275 MeV. For each beam energy, spectrawere measured by detectors positioned at both a near-nor-mal angle (160°, measured from the direction of the inci-dent beam) and a grazing angle (102° for the 1.0 MeV beamand 110° for the 2.275 MeV beam). 200 p.C of 4He2 ionswere used to accumulate the spectra. The detector resolu-tion was 20 keV full width at half maximum (FWHM).Some additional measurements to confirm peak assign-ments in RBS were carried out with time-of-flight sec-ondary ion mass spectrometry (TOF-SIMS) (Charles Evansand Associates).

AES, which has greater depth resolution but poorer sen-sitivity than RBS for the metallic impurities (Fe, Cu, Ga)being investigated, was used to help confirm the impurityconcentration distributions. The base pressure in theAuger system (PHI 4300) was 2 x iO-'° Torr. The 100 nmdiam electron beam (energy 10 keV, current 200 nA) wasrastered over an area of several hundred p.m2, and report-ed spectra were averaged over this area. MATLAB soft-ware was used to fit the spectra over 25 channels of datanear each transition of interest. Concentrations were

S.C4,a4,0-C,

E0C

100

80

60

40

20

00 50 100 150 200 250

Sputter Time (s)

Fig. 2. AES depth profiles of Al, 0, and C contaminant. The alu-minum foil was dissolved in NaOH for 40 mi Sputtering rate wasapproximately 5.0 nm/mm.

Ie -0Al

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J. Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc. 85

Fig. 3. AES depth profile of metallic impurities, for the same con-ditions as in Fig. 1.

determined by optimizing the fit to the spectra of the cor-responding pure components; in all cases the residual ofthe fit was smaller than 0.1%. For impurities, tabulatedsensitivity factors were used for conversion of spectra toatomic percent (a/o).'1 The two overlapping Al transitions,metallic Al at 1400 eV and oxidized Al at 1392 eV, wereseparated with principal component analysis, using differ-ent sensitivity factors for metallic and oxidized Al. Depthprofiling was accomplished by Ar ion beam at an angle of36°, and employed a Zalar rotation stage to minimizeshadowing by surface roughness. The energy and currentof the Ar beam were 4 keV and 3 A.

Results and DiscussionAuger electron spectroscopy measurements of surface

composition.—Auger spectra were acquired during sput-tering through the oxide film and the region beneath the

film, on a foil which was treated in NaOH for 40 mm. Thedepth profiles of Al and 0 are shown in Fig. 2, and thoseof the impurities in Fig. 3. Significant concentrations ofCu, Fe, Mg, and Zn impurities near the surface werefound. Appreciable noise is present in the impurity spec-tra in Fig. 3; however, as mentioned in the Experimentalsection, each concentration does not represent a measure-ment at a single energy, but a fit over 25 energy channelsto the spectra of the pure components. Since the residualsof these fits in all cases were smaller than 0.1%, the meas-ured concentrations were considered to be significantlylarger than the detection limits. The oxide/metal interfacewas located at the point where the Al (oxide) and Al sig-nals had decayed to 50% of their maximum values, at asputtering time of 60 s. Two estimates of the sputteringrate were obtained from standard samples: 5.2 nm/mmbased on Si02, and 4.8 nm/mm from anodic aluminumoxide films. Thus, the oxide film thickness was apparent-ly 5 nm. All impurities may have been present in both theoxide and the metal. Concentrations of Zn and Mg wereapparently highest at the film surface and decreasedtoward the metal interface, while the Cu concentrationwas highest at the metal/film interface and decayedtoward the bulk metal. Oxygen was detected surprisinglydeep in the metal, at concentrations which were largerthan the sum of all the metallic impurities and aluminumions. Possibly this persistence of oxygen can be attributedto readsorption during sputtering. On the other hand, arecent Auger and SIMS study of aluminum which did notemploy sputtering also found deep oxygen, which theauthors attributed to the presence of subsurface defectscontaining oxygen.'2

Quantitative interpretation of the Auger profilesrequires knowledge of the sputtering depth resolution.This was estimated from the Al and Al (oxide) signals,since it is assumed that the transition from oxide to metalis actually sharp. The resolution was taken to be 9 nm, thedistance over which the Al and Al (oxide) signals decayedfrom 84 to 16% of their maximum values.13 Since all theimpurity profiles decay over distances which are compa-rable to this value, it is apparent that the true thicknessesof the impurity segregation layers cannot be estimatedfrom the Auger profiles. The measured mean concentra-tions were about 1.0 a/o for Fe and Cu, and 0.3 a/o for Znand Mg. These concentrations are much larger than the

Fig. 4. Part of backscatteringspectra with normal detectorangle (160°), after dissolutionfor various times in NaOH.Edge energies for impurity ele-ments: Fe, 0.7569 MeV; Cu,0.7829 MeV; Ga, 0.8001MeV.'6

CC)1.)I-4)a.aE0

2.5

2

1.5

1

0.5

00 50 100 150 200 250

Sputter Time (s)

Cl)

00

0.8Energy (MeV)

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Fig. 5. Part of backscatteringspectra with grazing detectorangIe (102°), after dissolution forvarious times in NaOH. Edgeenergies for impurity elements:Fe, 0.8247 MeV; Cu, 0.8442MeV;Ga,0.8570MeV. 16

solubilities of the respective elements in aluminummetal,'4 suggesting that the impurities were found in sec-ond-phase particles, or were accompanied by locally highdefect concentrations.

RBS measurements of surface composition.—RBS wasused to characterize surface impurity concentrations inas-received foils, and after immersion in 1 N NaOH atroom temperature for 1, 5, 10, 15, 20, 30, and 40 mm.Representative spectra measured with a near-normaldetector angle (1600)are shown in Fig. 4, while Fig. 5 givesones taken with a grazing angle (102°). The greatest ele-mental resolution was obtained with the detector posi-tioned at the normal angle of 160°. As shown in Fig. 4, twopeaks for metallic impurities could be distinguished. Thepeak on the left was due to iron, while that at lower ener-gy was attributed mainly to copper, with gallium con-tributing a shoulder at low energy. The presence of bothcopper and gallium was confirmed with TOF-SIMS, andall three impurities were found in the bulk metal.Additionally, lead was detected in the 2.275 MeV, 160°spectra (not shown), at a concentration about 100 timeslower than those of the other metallic impurities. Iron wasthe only metallic impurity found in the foil in the as-received condition; the others were detected only after dis-solution. Figures 4 and 5 show that yields of all impuritiesincreased steadily with dissolution time. Arai et al." alsofound strongly enhanced concentrations of iron, bismuth,boron, and magnesium impurities within a distance ofabout 20 nm from the surface of 99.99% Al foils. Lightimpurities such as zinc and magnesium which were foundby AES would not have been detected with BBS.

Quantitative information about the spatial distributionof impurities was obtained by simulation of BBS measure-ments. Two models, which are referred to as the "layer"and "sphere" models, were used to simulate different lat-eral distributions of impurities along the surface.Simulations based on the layer model, which assumed thatall impurities were present in a layer of uniform thicknessadjacent to the surface, are described presently. For sim-plicity, the layer model approximated the impurity con-centration profile by a uniform concentration of impuri-ties within a layer of prescribed thickness. This approxi-mation was valid because the impurity layers wereextremely thin (less than 20 nm); accordingly, the shape ofthe simulated spectra were found to depend more strongly

on the detector resolution than on the shape of the con-centration profile. As an additional simplifying approxi-mation, oxide layers were not explicitly included in thesimulations. The predicted spectra were found to be unaf-fected by the inclusion of an oxide layer, since the densi-ties in atom/cm' of the oxide layer (7 >< 10") and the metal(6 >< 10") are comparable. Simulations using the layermodel were carried out with BUMP software (ComputerGraphic Service).

Figures 6 and 7 show experimental spectra along withthe simulation based on the layer model, in the region ofthe iron, copper, and gallium peaks, for normal (160°,1.0 MeV) and grazing (110°, 1.0 MeV) detector angles.Background levels were subtracted from the experimentalspectra in the figure. In all simulations, the detector reso-lution was taken to be 20 key (FWHM), in agreement withthe experimental system. The aluminum edges of theexperimental 1.0 MeV spectra, showed close agreement

0).4-C0C)

Fig. 6. Portion of backscattering spectra with normal detectorangle (160°) for aluminum after 40 mm dissolution in NaOH. Thesolid line is the experimental spectrum and the dashed line the sim-ulation based on an impurity se9regation layer 10 nm thick.

86 J Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc.

U)4-.C00

Energy (MeV)0.95

Energy (MeV)

250

200

150

100

390 400Channel

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.1 Electrochem. Soc., Vol. 143, No. 1, January 1996 The Eleotrochemical Society, Inc. 87

r. 7. Portion of bockscattering spectra with grazing detector angle(102°) for aluminum after 40 mm dissolution in NaOH. The solid lineis the experimental spectrum and the dashed line the simulation,which was based on an impurity segregation layer 10 nm thkk.

with simulations, thus demonstrating the absence of peakbroadening due to the foil's surface roughness.

When the detector energy resolution assumed in themodel spectra was set to o key, the predicted yield forgiven impurities was independent of energy This obser-vation demonstrates the validity of the "surface energyapproximation"6 for the small layer thicknesses underconsideration; that is, the scattering cross section (prob-ability) of given impurities was independent of depth. Inthis case, the integrated peak area for an impurity is pro-portional to the total number of impurity atoms per sam-ple surface area, and is not affected by their spatial dis-tribution (either laterally or in depth). Accordingly areaconcentrations of metallic impurities were determinedfrom the peaks in the spectra employing normal detectorangles (1600, 2.275 MeV and 160°, 1.0 MeV), for which theelemental resolution was greater than for grazing detec-tor angles. The concentrations were adjusted until theshapes of the experimental and simulated spectra

00 10 20 30

Time (mm)Fig. 8. Total near-surface concentrations of iron, copper and gal-

lium measured with RBS after dissolution for various times in I NNaOH solution.

matched one another, with the integrals of the peaks inagreement to within 10%. Area concentrations of iron,copper, and gallium are shown in Fig. 8 for the variousdissolution times. The nearly linear relationship betweenconcentration and time, which is evident for all threeimpurities, is discussed below.

The thicknesses of impurity layers were determined byadjusting the simulation layer thicknesses for best agree-ment with experimental 1.0 MeV, 102° spectra. Grazingrather than normal angle spectra were used because oftheir greater sensitivity to layer thickness. While varyingthe layer thickness, the impurity concentrations per unitarea (i.e., the product of volume concentration and layerthickness) were kept constant at the values in Fig. 8. Forall dissolution times of 5 mm or greater, simulated peakheights were found to be within 10% of experimental val-ues when layer thicknesses were chosen between 5 to20 nm. The peaks became appreciably shorter and broad-er than those in experimental spectra when the thicknesswas larger than 20 nm. Also, for thicknesses larger thanabout 15 nm, the peak position was shifted to lower ener-gy than the experimental peak by at least 10 key. Fromthese observations, it was inferred than the layer thicknesswas no greater than 15 nm; it was not possible to establisha lower bound on the thickness. The layer includes the 5nm thick oxide film, in which Fe and Cu impurities maynot have been present. No variation of thickness with dis-solution time could be detected, when the foil was dis-solved longer than 5 mm. For shorter dissolution times,the impurity peaks were too small to determine the layerthickness. The average impurity concentrations in thelayer at 40 mm were 1.0 a/o Fe and 0.90 a/o Cu, closelycomparable to the AES profile.

As mentioned above, surfaces after dissolution were notflat, but instead exhibited nanometer-scale roughness.They were covered by a mosaic of scallop cells which con-sisted of shallow, roughly circular depressions bordered byridges (Fig. 1). After 10 mm dissolution, the atomic forcemicroscope (AFM) revealed that the characteristic cellwidth was about 600 nm, while typical heights and widthsof ridges were 50 and 200 nm, respectively (Fig. 9). Twopossible effects of such roughness on backscattering spec-tra were considered. First, roughness might artificiallybroaden peaks for impurity layers. However, this is unlike-ly, since, as stated above, no such broadening of the Aledge occurred. The other roughness effect would be pre-sent if, as proposed by Cuff and Grant,' impurities werefound preferentially at ridges. In this case, when a grazingdetector angle was used, the path length of the beam after

______ scattering from impurity atoms in ridges would be limitedby the thickness of the ridge; thus, significantly narrowerpeaks would have been expected relative to those for flatsurface, or for surfaces with no lateral segregation.

A simple model for backscattering spectra was devisedto test for the possibility of impurity segregation to ridges.This model, which is described in detail in the Appendix,

A treated the ridges as spherical particles having uniformcomposition. The spherical geometry was chosen becauseit incorporated the essential feature of finite width, andyet, owing to its rotational symmetry it was mathemati-cally much simpler than a ridge geometry. Chu et al.1' alsoused a spherical model geometry to simulate the effect ofimpurity-containing protrusions on backscattering spec-

• tra; however, their simulation considered only normaldetector angles, while the present one treated the detectorangle as a parameter. The model sphere contained iron andcopper at the same concentration, and no gallium. Theinclusion of gallium was not necessary for the presentpurposes, since the impurity peak was compared to exper-

40 imental spectra on the basis of its height, and the peakheight was insensitive to the presence of gallium becauseof its low concentration relative to iron and copper(Fig. 8). Figure 10 shows the simulated spectral impuritypeak according to the sphere model for the two detectorangles of 102° and 160°. The peak shapes were determinedby the parameter fRIE0 and the standard deviation of the

400

Energy (MeV)

300

0)-I-.C00

200

100

430Channol

20

15

10

5

CM

E

0E

001'C

C04-ICuI-4-CC)C0C)

• Fe£ Cu• Ga

t•A

.s A

• • U •S •

A •

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88 J Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc.

CC

C

CC

nfl

Fig. 9. AFM line scan along arow of scallop cells on an alu-minum foil dissolved 10 mm inNaOH. The top plot is a topo-

I graphk scan along the line in thebottom image. The friangularmarkers in the plot and imagecorrespond to the same points.

detector energy resolution. The resolution was taken to bethat of the experimental system, 20 key (FWHM), as wasdone in the layer model, and the energy loss rate f wastaken to be that of pure aluminum, 250 eV/nm.Simulations were carried out for sphere radii R of 10 and20 nm, the latter value representative of the ridge heightsmeasured with AFM.

For grazing detector angles, the spectral peaks simulat-ed by the sphere model were found to be higher than thosecalculated by the layer model. This result was to beexpected due to the limited beam path length through thesphere discussed above. Hence, when the detector energyresolution was set to zero, sphere model peaks were nar-rower and higher than the layer model peaks, for the sameimpurity concentration per unit area (i.e., spectral peakarea). When the effect of detector resolution was included,differences in peak width between the two models were nolonger discernable, but the peak heights were still appre-ciably different. The ratio of the peak height for the graz-ing angle to that for the normal angle was 2.7 for both 10and 20 nm radius spheres, significantly larger than theexperimental value of 1.4, and the ratio of 1.7 from thelayer model. Evidently, the layer model represented exper-imental spectra more closely than did the sphere model.The clear differences in predicted peak height ratio for thelayer and sphere models demonstrates the applicability of

this technique for the detection of lateral concentrationvariations associated with topography.

While the impurity concentration may be somewhathigher on ridges than elsewhere, the present results showthat they are not exclusively confined to ridges, as pro-posed by Cuff and Grant. However, even a moderate vari-ation in impurity concentration along the surface may besufficient to produce the variation in dissolution ratewhich gave rise to the scallop topography. Hence, theRBS results do not exclude that the scallops might berelated to impurities. On the other hand, when the dis-solved foil is etched, the preferred nucleation of pits onridges is dramatic, very few pits being found away fromthe ridges. Explaining the location of pit sites solely onthe basis of impurities would then require that nearly allimpurities are associated with ridges, which contradictsthe present backscattering results. Thus, alternativehypotheses for the enhancement of pitting at ridges mustbe sought. Possible explanations may be related to thelocal surface curvature at ridges, which may producestrain energy in the oxide film, or to the presence ofdefects in the metal near the metal/film interface, as dis-cussed below.

Impurity retention during dissolution—The surfaceconcentration measurements provided by RBS allowed amole balance on impurities to be made. The variation of

0 2.00 4.00PH

6.00 8.00

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..L Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc. 89

Energy (MeV)

Fig. 10. Theoretical backscattering spectra at two detector anglesfor a spherical particle with radius 20 nm. The particle containeduniform equal concentrations of iron and copper in aluminum.

impurity surface concentration with dissolution time isexpected to follow

C, = C1 + XVCt [1]

1. is the fraction of surface impurity atoms which areretained on the surface as the surrounding Al atoms dis-solve; that is, 1 — A is the ratio of the dissolution rate of animpurity to that of aluminum. The dissolution rate v wasapproximately constant at 0.22 jim/mm, as determined bythe weight loss measurements shown in Fig. 11. Thus thelinearity of the data in Fig. 8 suggests that A was constantfor each impurity. To assess the tendency of impurities tobe retained during aluminum dissolution, A was set tounity in Eq. 1, and the bulk concentrations Cb were calcu-lated from the surface concentration measurements inFig. 8. The bulk concentrations obtained for iron, copper,and gallium were 39, 34, and 22 wt-ppm, respectively. Thesum of these concentrations is 95 wt-ppm, about half of

Fig. 11. Dissolution of aluminum foils in 1 N NaOH at room tem-perature.

the nominal impurity content of 200 wt-ppm. The differ-ence between the true impurity content and the estimatefrom Fig. 8 can be partly accounted for by light impuritiessuch as Mg and Zn not detected by RBS. Hence, it appearsthat at least half, and perhaps all, of the Fe, Cu, and Gaimpurities exposed at the surface by aluminum dissolutionwere not dissolved, but were retained in the metal.

The retention of at least Fe and Cu is reasonable withregard to dissolution thermodynamics. The thermodynam-ic protection potentials of pure iron and copper againstcorrosion in pH 14 solution are, respectively, —1.0 and—0.4 V vs. the normal hydrogen electrode (NHE).'8 Sincethe potential of aluminum in these experiments wasbetween —1.4 and —1.6 V vs. NHE, iron and copper wouldnot be expected to dissolve. On the other hand, the ther-modynamic protection potential of gallium, which wasdetected by RBS but not AlES, is —1.5 V vs. NHE,16 com-parable to the potential of aluminum during dissolution.Thus gallium retention, as well as that of the reactive Mgand Zn impurities found by Auger spectroscopy, would notbe expected from thermodynamics alone. However, thekinetics of dissolution of these elements may be slowerthan those of aluminum, allowing them to accumulate onthe surface.

The concentrations of Cu and Fe after 40 mm dissolu-tion, which were about 1 a/o, are much larger than the sol-ubilities of these elements at room temperature (roughly0.1 weight percent (w/o) for Cu and 0.001 w/o for Fe 14)Such high concentrations, if present in the metal, implythe existence of either second-phase particles or highdefect concentrations near the metal surface. Recentpositron annihilation measurements on aluminum foilsdissolved in NaOH have detected large increases caused bydissolution in the concentration of vacancy-type defectsnear the metal/oxide film interface.6 The defect layerthicknesses found after 10 mm dissolution were about 10to 20 nm, comparable to the upper bound on the segrega-tion layer thickness determined in this paper, and thevacancy concentration in the defective layer was estimat-ed to be the order of 1%. Hence, it appears that the metalis defective enough near the surface to hold very highimpurity concentrations, without requiring the presenceof second-phase particles.

As dissolved impurity concentrations in the metal nearthe surface built up during dissolution, a concentrationgradient would have appeared favoring their diffusiontoward the bulk. The segregation layer thickness would bedetermined by the depth to which the impurities diffuse.This diffusion length is given by Langer'9 as roughly 2D/v(Eq. 3.7 in his paper). Hence the impurity diffusion coeffi-cient could be estimated from measurements of dissolutionrate and segregation layer thickness. Unfortunately, theuncertainty regarding the layer thickness measurementsdo not allow a reliable estimate of the diffusion coefficientto be made. One would expect, though, that diffusionwould be significantly enhanced by the defective nature ofthe surface layer.

An alternative concept for impurity retention involveserosion and redeposition of impurities from solution. Here,as the nonreactive Cu and Fe were exposed on the surfaceby dissolution, rather than diffusing back into the metal,they would have been undermined by dissolution of thesurrounding aluminum atoms, and would erode into solu-tion. These elements would dissolve in the NaOH solution,and some would then redeposit on the metal surface. Inorder to explain the observed retention of Cu and Fe, atleast half of the exposed impurities would redeposit.However, such an efficient redeposition process was con-sidered unlikely, and the Auger spectra suggest the pres-ence of Cu and Fe in the metal below the oxide, as opposedto on the surface. Thus the most probable retention mech-anism is that the nonreactive impurities simply neverleave the metal, and diffuse toward the bulk when theirconcentration near the surface builds up. After dissolu-tion, the impurities are present within a relatively homo-geneous but highly defective surface layer in which their

tC)

VC)NCe

E0z

3

2.5

2

1.5

1

0.5

00.7 0.75 0.8 0.85 0.9

E

C0

0U,U,

CCe

432

20

15

10

5

00 20 40 60

Time (mm)

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90 J Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc.

concentrations can be quite high. The increased suscepti-bility of the metal toward pitting may be related to thepresence of these defects. It is not known whether thedefects are associated with ridges between scallop cells,where pits preferentially form. Macdonald2° has discusseda mechanism for generation of voids at the aluminummetal/oxide interface near surface protrusions duringanodic oxidation.

ConclusionsDissolution of aluminum in sodium hydroxide solution

is found to increase the rate of corrosion pit initiationduring subsequent anodic etching in chloride solution.The work reported here used Rutherford backscatteringspectrometry and Auger electron spectroscopy to mea-sure near-surface concentrations of metallic impuritiesfollowing dissolution in sodium hydroxide solution. Itwas revealed that nonreactive impurities (iron, copper,gallium) build up continuously during dissolution of alu-minum in sodium hydroxide. The thickness of the impu-rity segregation layer in the metal was no greater than 10nm. Within this layer, the concentrations of Fe and Cuafter 40 mm dissolution were about 1.0 a/o, much higherthan their equilibrium concentrations in the metal atroom temperature. They were most likely not present insecond-phase particles, but were dissolved in the metal,their high concentrations being accomodated by largedefect concentrations near the interface. Large subsur-face defect concentrations after NaOH dissolution haverecently been detected by the authors using positronannihilation measurements.

Dissolution of high purity aluminum leaves a micro-scopic surface topography consisting of a mosaic of 10 to100 nm high ridges. These ridges serve as effective sites forthe initiation of small corrosion pits, and it has been sug-gested that elevated impurity concentrations can be foundon them. To evaluate the lateral distribution of impuritieswith respect to the ridges, a simulation of backscatteringfrom protusions containing elevated impurity concentra-tions was developed. To the authors' knowledge, this sim-ulation represented an extension of the capabilities of RBSfor the detection of such lateral concentration variations.It was concluded that the impurities were not primarilyconfined to ridges, but were spread fairly uniformly alongthe surface. Hence lateral segregation of impurities isprobably not the reason for preferential pitting on micro-scopic ridges. Alternatively, the enhancement of pitting bydissolution might be related to the defective nature of themetal near the interface, or to geometric features of thesurface topography.

AcknowledgmentsThe authors are grateful for financial support for this

work provided by National Science Foundation GrantDMR-9006895. KDK Corporation (Takahagi, Japan) pro-vided aluminum foil.

Manuscript submitted Oct. 3, 1994; revised manuscriptreceived May 8, 1995.

Iowa State University assisted in meeting the publica-tion costs of this article.

APPENDIXRutherfordBackscoitering SpectraofRough Surfaces

Themodel described here parallels that of Chu et al.17 inthat in both models the surface roughness features areassumed to be adequately represented as spheres. Themodel of Chu et at. further assumed that the detector waspositioned along the direction of the incident particlebeam (detector angle of 180c), and consequently a simpleanalytic expression for the scattering yield was obtained.The present model extends their treatment to arbitrarydetector angles. It is now shown that the variation of spec-tral peak height with detector angle is appreciably differ-ent for the sphere and uniform layer models.

Figure A-i shows the model geometry. The incidentbeam is directed along the surface normal, and the detec-tor is oriented at an angle e with respect to the surfacenormal (the "detector angle" is the supplement of 0). The

elastic collision of the beam with a target atom occurs atpoint P1, and P2, and P3 are the points where the beamenters and exits the sphere. The z axis is oriented along thesurface normal. P1, P2, and P3 lie on a plane perpendicularto the y axis. The scattering yield is given by16

H(E3) =cr(E)1l4S(Ei)N[-JAEi

[A-i]

The locus of scattering points P1 from which the beamexits the sphere with the same energy E3 defines a surfaceinside the sphere. S(E1) is the area of this surface project-ed onto the plane of the incident beam, i.e., the xy plane.The energy dependence of a is cr(E) = a(Ej(E0/E)2; E is theparticle energy just before collision. Thus a normalizedyield F is defined according to

F Hf-R21$NAE1(Z7Z2e3 / E3f

— S(E1)(E, d; (E )(zlZle '— R2 E) dEja ° 4E0 ) [A-2]

The energy just after collision is KE, where the kine-matic factor K is smaller than unity because some of theparticle's kinetic energy is transferred to the target atom.As the beam traverses the sphere it loses energy at a con-stant rate f = dE/dx. The distance through the sphere trav-elled by the beam before collision is L1 = PP2 = (E0 — E)/f,while the distance through the sphere after collision isL3 = P1P3 = (KE — E1)/f. Eliminating E between these twoexpressions, the detected energy is found to be

E1 = KE, — (KL1 + L2)f [A-3]

Thus, scattering events which contribute to the yield at agiven energy E1 occur at points within the sphere havingthe same value of L = KL1 + L3. These points lie on a sur-face whose defining equation is derived by calculating L6and L2 in terms of the coordinates P1 = (x1, Pi, z1). Thecoordinates of P3 are [x1, y, (R2 — 4 — y112

— zd, andhence L1 = (B2 — 4 —

y)112— ;. L is found from the equa-

tion of the sphere, 4 + y + 4 = B2, into which is substi-tuted the relations x3 = x1 + L3 sin9, and z = z1 + L2 cosO.Solving the resulting quadratic equation for L3 and com-bining the solutions for L1 and L2, an expression for L is

L = —x1 sin0—z1cos9+KY —Kz1

[A-4]

TOP VIEW SIDE VIEW

From SourceTo Detector

To Detector

P2

x

y z

Fig. A-i. Geometry of the model sphere used in theoretical calcu-lation of bcickscattering spectra.

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J. Electrochem. Soc., Vol. 143, No. 1, January 1996 The Electrochemical Society, Inc. 91

For a given point (x1, y1) on the xy plane, this equationimplicitly gives the coordinate z of a point on the constantL surface.

The normalized yield F was found by integrating (EjE)2over the projection onto the xy plane of the surfacedefined by Eq. A-4. For a point on the surface, E = (E0 —

fL1)/K. To carry out the integration, a grid of points with-in the circle x + y = R2 was constructed, and for eachpoint the solution for z1 was found from Eq. A-4. The inte-grand at that point was set to zero if no solution for z1existed, or if the solution lay outside the sphere. Otherwisethe integrand was set to (E0/E)2 as calculated from x1, Yi,and z1. Integration was according to Simpson's rule. Gridsconsisting of 400 points in both the x and y directions werefound to be sufficient to achieve smaller than 0.1% rela-tive error with respect to the analytic solution of Chu et at.for the case of 0 = 00.

To account for the broadening of the spectral peak bythe limited energy resolution of the spectrometer, the pre-dicted yield was transformed according to

G(E) [l /(21r2)h/2]j F(E') exp [(E_E)2] dE' [A-5]

where i. is the standard deviation of the system energy res-olution, which in the experimental system was 20 keV, theFWHM resolution, divided by 2.35. The predicted spec-trum for a sphere with uniform equal concentrations ofiron and copper impurities is shown in Fig. 7. The ordinatein the figure is 2[(ZG)0 + (ZG)Fe]/[(Z))o + (Z2);e]. Thenormalized yield is a function of the detector resolutionand the parameter fR/E0.

LIST OF SYMBOLSCb impurity concentration in the bulk metal, mol/cm3C, moles of near-surface impurity per unit surface

area, mol/cm2C,, value of C. prior to dissolution, mol/cm2D impurity diffusion coefficient, cm2/sE energy of particle just prior to collision with target

atom, MeVe electronic charge, 1.60210 X 10' CE0 beam energy, MeVE1 particle energy at detector, MeVF normalized scattering yield, dimensionlessf rate of energy loss at beam traverses sphere,

Me V/cmG normalized scattering yield as broadened by

detector energy resolution, dimensionlessH backscattering yield, number of particlesK scattering kinematic factor: ratio of particle energyjust after collision to that just before collision

L KL1 + L2, cmL1 beam path length in sphere before collision, cmL2 beam path length in sphere after collision, cmN volume density of atoms in the target material,

atoms/cm3v dissolution velocity, cm/sR radius of sphere, cmS projected area onto incident beam of surface

comprising points in sphere where collisionsproduce the same particle energy; E1t time, s

x coordinate of point where particle-target atomcollision occurs, cm

y1 coordinate of point where particle-target atom col-ision occurs, cm

Z1, Z2 atomic number of projectile and target atomsz1 coordinate of point where particle-target atom

collision occurs, cmitE1 energy width of one channel of detector, MeV? 1 — (rate of impurity dissoiution)/(rate of aluminum

dissolution)standard deviation of detector energy resolution,key

6 angle between unit normal to surface and beamfrom sample to detector, 0

a differential scattering cross section, cm2!atom-steradian

4) incident beam particle flux, particles/cm2Q solid angle of detector, steradian

REFERENCES1. C.-F. Lin and K. R. Hebert, This Journal, 137, 3723

(1990).2. A. P Bond, G. F. Boiling, H. A. Domian, and H. Biloni,

ibid., 113, 773 (1966).3. K. F9rsvoll and D. Foss, Philos. Mag., 15, 329 (1967).4. H. P Leighly, Jr. and A. Alam, J. Phys. F: Met. Phys.,

14, 1573 (1984).5. C. Towler, R. A. Collins, and G. Dearnaley, J. Vac. Sci.

Technol., 12, 520 (1975).6. X. Wu, P Asoka-Kumar, K. G. Lynn, and K. R. Hebert,

This Journal, 141, 3361 (1994).7. E. A. Culpan and D. J. Arrowsmith, Trans. Inst. Met.

Finish., 51, 17 (1973).8. F. B. Cuff and N. J. Grant, J. Inst. Met., 87, 248 (1958-

59).9. K. Shimizu, K. Kobayashi, G. E. Thompson, and G. C.

Wood, Philos. Mag. A, 66, 643 (1992).10. G. E. Thompson, P. E. Doherty, and G. C. Wood, This

Journal, 129, 1515 (1982).11. L. E. Davis, N. C. MacDonald, P W. Paimberg, G. E.

Riach, and R. E. Weber, Handbook of Auger ElectronSpectroscopy, 2nd ed., Physical Electronics, EdenPrairie, MN (1976).

12. S. A. Larson and L. L. Lauderback, Surf. Sci., 254, 161(1991).

13. S. Hoffmann, in Practical Surface Analysis by Augerand X-Ray Photoelectron Spectroscopy, D. Briggsand M. P. Seah, Editors, p. 141, Wiley, New York(1983).

14. Metals Handbook, Vol. 8, 8th ed., L. Taylor, Editor,American Society for Metals, Metals Park, OH(1973).

15. K. Arai, T. Suzuki, and T. Atsumi, This Journal, 132,1667 (1985).

16. W.-K. Chu, J. W. Mayer, and M.-A. Nicolet,Backscattering Spectrometry, Academic Press, Inc.,New York (1978).

17. Ibid., p. 334.18. M. Pourbaix, Atlas of Electrochemical Equilibria in

Aqueous Solutions, Pergamon Press, Oxford (1966).19. J. Langer, Rev. Mod. Phys., 52, 1 (1980).20. D. D. Macdonald, This Journal, 140, L27 (1993).

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