SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 24È28 (1999)

Glow Discharge Optical Emission Spectrometry(GDOES) Depth ProÐling Analysis of AnodicAlumina FilmsÈa Depth Resolution Study

K. Shimizu,1 G. M. Brown,2 H. Habazaki,3 K. Kobayashi,2 P. Skeldon,4* G. E. Thompson4 andG. C. Wood41 University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223, Japan2 Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223, Japan3 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan4 Corrosion and Protection Centre, UMIST, PO Box 88, Manchester M60 1QD, UK

Anodic alumina Ðlms with precisely known distributions of incorporated species have been used as standards forglow discharge optical emission spectrometry (GDOES) depth proÐling analysis to quantify depth resolution. It isevident that the depth resolution of GDOES is excellent and is comparable with, or better than, secondary ionmass spectrometry depth proÐling of similar Ðlms. Further, the sensitivity for detection of elements is also high,given the amounts of impurity species detected successfully. Thus, GDOES, with its further ability of routine andrapid analysis of Ðlms (organic, inorganic or metallic) of thicknesses up to several hundreds of microns, hassigniÐcant potential in studies of the corrosion and Ðlming behaviour of materials. 1999 John Wiley & Sons,(

Ltd.

KEYWORDS: GDOES; aluminium; anodic oxidation ; alumina

INTRODUCTION

Anodic oxidation of aluminium and tantalum in appro-priate electrolytes leads to the formation of thin (\1lm thick) barrier anodic oxide Ðlms.1,2 The resultantÐlms are of importance as the dielectric layers of elec-trolytic capacitors or in the fabrication of thin-Ðlm elec-tronic devices, such as thin-Ðlm transistors in liquidcrystal display devices.3 The Ðlms are not pure oxides ofthe respective metals but usually contain small amountsof electrolyte-derived species that signiÐcantly inÑuencethe chemical, physical and electronic properties of thethin Ðlms.4h8 Further, the distributions of such speciesin the Ðlms are varied and complex ; the distributionsare not determined simply by the transport numbers ofthe metal ions during Ðlm growth, but also by the rela-tive migrations of the incorporated species.9h12

In previous studies,12 the distributions of species inanodic Ðlms have been determined mostly by secondaryion mass spectrometry (SIMS) depth proÐling. In con-trast, glow discharge optical emission spectrometry(GDOES) has received little, or almost no, attention.This is due mainly to the widely accepted view that thedepth resolution of GDOES is poor, thus rendering thetechnique ill-suited for depth proÐling analysis of thinanodic oxide Ðlms. However, such a view is inapprop-riate, as shown in the present work where selectedanodic alumina Ðlms with precisely known distributionsof incorporated species have been investigated. Theresults reveal that the depth resolution of the technique

* Correspondence to : P. Skeldon, Corrosion and ProtectionCentre, UMIST, PO Box 88, Manchester M60 1QD, UK

is excellent and is comparable with, or better than,SIMS depth proÐling of similar Ðlms.

EXPERIMENTAL

Two standard specimens were prepared for GDOESdepth proÐling analysis by anodic oxidation of electro-polished, mirror-Ðnished, high-purity aluminium speci-mens. BrieÑy, 99.99% purity aluminium sheets ofdimensions 15 ] 50 ] 0.2 mm were electropolished at aconstant current density of 100 mA cm~2 in a per-chloric acidÈethanol bath at temperatures below 10 ¡C,rinsed thoroughly in absolute ethanol and dried in awarm air stream. The electropolished specimens wereanodized at a constant current density of 5 mA cm~2 at20 ¡C in two ways : (1) sequential anodizing, Ðrstly in 0.1M solution to 100 V, followed by anodizing toNa2WO4300 V in 0.1 M ammonium pentaborate solution ; (2)anodizing in 0.1 M solution to 120 V. TheNa2CrO4anodized specimens were rinsed thoroughly in distilledwater and dried in a warm air stream.

The distributions of impurity species derived fromelectrolyte anions were determined by depth proÐlingusing a Johin Yvon 5000 RF plasma solid emissionspectrometer. The anodized specimens were placed in aspecimen holder, which was made the cathode ; then theanodic oxide Ðlms were sputtered in an argon atmo-sphere of 3È5 Torr by applying an r.f. of 13.56 MHz anda power of 40 W. These conditions were selected to givegood sensitivity and depth resolution, as evident in laterdepth proÐles. Light emission of characteristic wave-lengths associated with the sputtered species was moni-tored throughout the analysis with a sampling time

CCC 0142È2421/99/010024È05 $17.50 Received 10 July 1998( 1999 John Wiley & Sons, Ltd. Accepted 14 September 1998

GDOES DEPTH PROFILING OF ANODIC ALUMINA FILMS 25

interval of 0.01 s to obtain depth proÐles. The relevantwavelengths were as follows : Al, 396.15 nm; W, 429.46nm; H, 121.57 nm; B, 249.68 nm; Cr, 425.43 nm; P,178.29 nm.

RESULTS AND INTERPRETATION

Before describing the resultant depth proÐles, the basisfor the selection of the standard Ðlms for determinationof GDOES depth resolution is considered. Firstly, theÐlms are amorphous and highly uniform in thickness,with sharply deÐned, microscopically Ñat metal/oxideinterfaces. Secondly, and most importantly, the distribu-tions of impurity species in these Ðlms, through whichdepth resolutions during GDOES analysis are assessed,have been determined directly and precisely throughtransmission electron microscopy of ultramicrotomedsections. Finally, the Ðlms are prepared readily and maybe used, if required, as standards for depth resolutionstudies of thin Ðlms using other techniques, includingSIMS, Auger electron spectroscopy and Rutherfordbackscattering spectroscopy (RBS), etc.

Figure 1 shows a transmission electron micrograph ofan ultramicrotomed section of the aluminium substrateand the Ðlm formed under the sequential anodizing

Figure 1. Transmission electron micrograph of the anodic filmformed on aluminium by sequential anodizing. Anodizing was per-formed initially at a constant current density of 5 mA cmÉ2 to 100V in 0.1 M sodium tungstate solution at 20 ¡C; anodizing was thencontinued to 300 V in 0.1 M ammonium pentaborate solution.

process (1). The aluminium substrate is observed at thebottom of the micrograph. The anodic Ðlm is evidentabove the aluminium substrate, with a sharply deÐnedand microscopically Ñat metal/oxide interface. The Ðlmis 358 nm thick and appears featureless except for thepresence of a dark band, 34 nm thick, which runs paral-lel to the metal/oxide interface. The band represents aregion that contains a small amount of tungsten speciesincorporated into the Ðlm during the Ðrst stage of anod-izing in sodium tungstate solution ; using RBS,13 theW/Al ratio in the layer has been determined to be8.2] 10~3. Further, the dark appearance of the band isdue to the presence of tungsten, which has an increasedelectron scattering cross-section over that of aluminiumand oxygen constituting the bulk oxide. From themicrograph, the upper edge of the band is located at adepth of 70 nm below the oxide surface.

Figure 2 reveals the GDOES depth proÐle of the Ðlmof Fig. 1. The distributions of tungsten species(introduced into the oxide during the second stage ofanodizing) and boron species are revealed clearly, withsharply deÐned interfaces between the di†erent layers.Concerning the aluminium proÐle, the signal intensity issteady and constant throughout the analysis of theoxide, although the intensities in the outer layers dopedwith boron or boron and tungsten species are slightlyless, by D15%, than in the inner pure layer.Al2O3Thus, assuming that the sputtering rate of the oxide isconstant throughout the analysis, the thickness of thetungsten-doped layer is estimated at 35 nm and theupper edge of the tungsten-doped band is located 73 nmbelow the oxide surface ; here, the locations of the upperand lower edges of the tungsten-doped layer are deter-mined from the half-height positions of the leading andtrailing edges of the tungsten distribution proÐle,because the sharp edges of the tungsten layer are convo-luted by the Gaussian function due to the statisticalnature of sputtering. These values are in excellent agree-ment with those determined directly by transmissionelectron microscopy of ultramicrotomed sections (Fig.1). However, a small di†erence of only 3 nm is evidentin the location of the upper edge of the tungsten-doped

Figure 2. The GDOES profile of the anodic film formed under theconditions of Fig. 1.

( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 24È28 (1999)

26 K. SHIMIZU ET AL .

layer, which most likely arises from a reduced sputter-ing rate of the oxide doped with boron species.

The depth resolution of GDOES has been assessedfurther by analysis of a Ðlm formed in sodium chromatesolution where an extremely Ðne band, only 2È3 nmthick, doped with a relatively high concentration ofchromium species is present in the Ðlm. Figure 3 showsa transmission electron micrograph of an ultramicro-tomed section of the aluminium substrate and Ðlmformed in sodium chromate solution under condition(2). Above the aluminium substrate, which is observedat the bottom of the micrograph, the 142 nm thickanodic Ðlm is observed clearly. Within the anodic Ðlm,a dark and extremely Ðne band 2È3 nm thick is present,passing parallel to the Ñat metal/oxide interface ; theband is located 19 nm below the oxide surface. Theband represents a narrow region that contains a signiÐ-cant amount of Cr3` species, presumed to be present asunits of dispersed in the alumina structure. TheCr2O3band separates an inner pure layer next to theAl2O3metal and an outer layer doped with low concentrationsof and with the proportion of the latterCrO42~ Cr2O3 ,increasing with depth from the oxide surface as deter-mined by analysis of the Ðlm using x-ray photoelectronspectroscopy.14

Figure 4(a) shows the GDOES depth proÐle of theÐlm of Fig. 3. The distribution of chromium species inthe Ðlm is revealed clearly, as well as a sharply deÐnedmetal/oxide interface. Concerning the aluminiumproÐle, the signal intensity is again relatively steady andconstant throughout the analysis of the oxide Ðlm,although its intensity in the outer layer doped withchromium species is slightly less than that in the innerpure layer. The 2È3 nm thick band doped with aAl2O3relatively high concentration of is readilyCr2O3evident, as well as the outer layer doped with smallamounts of and Assuming that the rateCrO42~ Cr2O3 .of sputtering of the oxide is constant throughout theanalysis, the centre of the band is located 19 nm belowthe oxide surface, in agreement with that determineddirectly from transmission electron microscopy of theultramicrotomed section (Fig. 3). However, broadeningis evident in the width of the band ; the width, deter-mined from the half-height positions of the leading andtrailing edges of the chromium distribution proÐle is

Figure 3. Transmission electron micrograph of the anodic filmformed on aluminium at a constant current density of 5 mA cmÉ2

to 120 V in sodium chromate solution at 20 ¡C.

Figure 4. Depth profiles of the anodic film formed under the con-ditions of Fig. 3 : (a) GDOES depth profile ; (b) SIMS depthprofile.

D7 nm, which is more than twice that determineddirectly by transmission electron microscopy. Broaden-ing of this kind is well known for SIMS and AES depthproÐling of thin Ðlms, where the Ðlms are sputtered byhigh-energy primary ion beams such as Ar` or Cs`,and is ascribed generally to the knock-on e†ects by col-lisions of energetic primary ion beams during theanalysis, with inÑuences of sputtering increasing withdepth into the oxide and leading to reduced depthresolution at the oxide/metal interface. The extent ofbroadening in GDOES depth proÐling is now com-pared further with that of SIMS depth proÐling.

Figure 4(b) is a SIMS depth proÐle obtained from theÐlm of Fig. 3. Only the chromium distribution proÐle, ofthickness up to 70 nm from the oxide surface, is shownhere to assist comparison. The proÐle was obtainedusing a Perkin-Elmer Atomica 6500 ion microprobe. ACs` primary ion beam at 3 kV and at a beam current of4 nA was rastered across the surface area of 300 ] 300lm; the secondary ions were collected from a centralarea of D30 ] 30 lm. The incident angle of theprimary ion beam was set to 45¡. In order to ensureuniform sputtering of the oxide and to avoid chargingduring the analysis, a primary Cs` ion beam of rela-tively low energy and current was selected here. Indeed,appropriate instrumental conditions, e.g. energy, currentand incident angle of primary Cs` ion beam, were selec-ted to give the sharpest revelation of the Cr2O3-band. Except for the initial high yield of Cr~enrichedsecondary ions of mass 52, associated with chromium

Surf. Interface Anal. 27, 24È28 (1999) ( 1999 John Wiley & Sons, Ltd.

GDOES DEPTH PROFILING OF ANODIC ALUMINA FILMS 27

species adsorbed at the oxide surface, excellent agree-ment is observed between the chromium distributionproÐles obtained by SIMS [Fig. 4(b)] and GDOES[Fig. 4(a)]. Further, as observed with GDOES depthproÐling, broadening is evident in the narrow bandenriched with From the half-height positions ofCr2O3 .the leading and trailing edges of the chromium distribu-tion proÐle associated with the band, its width is D7nm, i.e. comparable with that determined from GDOESdepth proÐling.

From the previous analyses of standard samples, it isevident that GDOES is extremely powerful and sensi-tive for depth proÐling of thin anodic alumina Ðlms. Anexample is now given where the inÑuence of depth fromthe oxide surface on degradation of depth resolution isdemonstrated clearly. For this purpose, Ðlms arerequired with impurity species located in well-deÐnedlayers of equal thicknesses but at di†erent depths fromthe oxide surface. Such Ðlms are prepared readily byanodic oxidation of electropolished aluminium afterimmersion in a 20 g l~1 and 35 ml l~1CrO3 H3PO4solution at 90 ¡C for 5 min.

As shown schematically in Fig. 5(a), the electro-polished and subsequently treated aluminium surfacesare covered with a thin, hydrated alumina layer of D4nm thick, which contains a signiÐcant concentration ofunits of and species.15 Subsequent anodicCr2O3 PO43~oxidation at a constant current density of 5 mA cm~2to 300 V in 0.1 M ammonium pentaborate at 20 ¡C leadsto the growth of a 358 nm thick barrier Ðlm of appear-ance represented schematically in Fig. 5(b). BrieÑy,growth occurs at the oxide/electrolyte interface by Al3`ion migration outwards, and at the metal/oxide inter-face by O2~ ion migration inwards.16h18 During Ðlmgrowth, small amounts of boron species are incorpor-ated into the Ðlm at the oxide/electrolyte interface, witha pure alumina layer developing at the metal/oxideinterface. Importantly, the Cr3` and speciesPO43~incorporated into the barrier oxide from the initial, 4nm thick surface Ðlm are not immobile, but migrate inopposite directions to give two well-separated distribu-

Figure 5. Schematic diagrams of the aluminium substrate : (a)after electropolishing and immersion in 20 g lÉ1 ml lÉ1CrO

3/35

solution at 90 ¡C for 5 min; (b) after anodizing (a) to 300H3PO

4V at a constant current density of 5 mA cmÉ2 in 0.1 M ammoniumpentaborate solution at 20 ¡C.

tions of comparable thicknesses (D4 nm) at di†erentdepths from the oxide surface. Because the Cr3` ionsgenerated by high Ðeld-assisted dissociation of theincorporated units of migrate outward at a con-Cr2O3stant rate of 0.74 relative to that of Al3` ions,19,20 whilethe incorporated ions migrate inward at a con-PO43~stant rate of 0.50 relative to that of oxygen ions,20 thethin layers doped with Cr3` or species arePO43~expected to be located in the Ðnal 358 nm thick barrierÐlm at depths of 47 and 260 nm from the oxide surface,respectively, as shown in Fig. 5(b). Such doped layersare not revealed directly by transmission electronmicroscopy of ultramicrotomed sections of the Ðlm dueto the relatively low concentrations of the incorporatedCr3` and species. The Ðlm appears uniform uponPO43~examination in the electron microscope, with no layersof di†ering contrast evident.

Figure 6 shows the GDOES depth proÐle of the Ðlmof Fig. 5. The distributions of Cr3`, and boronPO43~species are revealed clearly, with a sharply deÐnedmetal/Ðlm interface. From the boron distribution proÐleit is evident that the boron species are incorporated inan outer part of the Ðlm of thickness relative to the totalÐlm thickness of 0.43, as expected from the transportnumber of Al3` ions of 0.44 during oxide growth.18From the peak positions of the Cr3` and dis-PO43~tribution proÐles, the narrow, doped layers are esti-mated to be located at depths of 47 and 260 nm fromthe oxide surface respectively, as expected. Further, thethicknesses of the layers containing Cr3` and PO43~species, determined from the widths of the half-heightsof the leading and trailing edges of the correspondingproÐles, are D6 and 15 nm respectively. Because theCr3` and species were present initially in a ÐlmPO43~of D4 nm thick, broadening of their distributionsthrough sputtering processes is evident ; the broadeningis increased as the distance from the oxide surface, (47nm for Cr3` species and 260 nm for increases.PO43~)Thus, degradation of depth resolution with distancefrom the oxide surface is demonstrated clearly. Impor-tantly, however, the extents of broadening observedhere, particularly that of the distribution atPO43~increased depth in the oxide, are comparable with, orless than, those observed during SIMS depth proÐling

Figure 6. The GDOES depth profile of the anodic film formedunder the conditions of Fig. 5(b).

( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 24È28 (1999)

28 K. SHIMIZU ET AL .

of a similar Ðlm. Unfortunately, direct comparisonbetween the GDOES depth proÐle of Fig. 6 and theSIMS depth proÐle of the corresponding Ðlm was notpossible, mainly because the Ðlm concerned is too thickto avoid charging e†ects during SIMS depth proÐling.During SIMS depth proÐling analysis of anodicalumina Ðlms of thicknesses greater than D100 nm,Al~ secondary ion yields increased in an acceleratedmanner with sputtering time, due to charging. However,a 100 V Ðlm formed on an electropolished aluminiumspecimen was analysed by SIMS depth proÐling using aCs` primary ion beam of 10 kV and 10 nA with anincident angle of 45¡. In the Ðlm, a thin layer of D4 nmthick containing Cl~ ions from the electropolishingsolution is located at a depth of 91 nm from the oxidesurface. The thickness of the layer containing Cl~ ionsdetermined from the SIMS depth proÐle was 12 nm,comparable with that of the distribution revealedPO43~here. However, the distribution is located at aPO43~depth of 260 nm from the oxide surface, which isroughly three times greater than that of the Cl~ iondistribution. Again, the excellent depth resolutionassociated with GDOES depth proÐling is demon-strated.

SUMMARY AND CONCLUSIONS

Through the examples presented, it is evident thatGDOES is an extremely powerful and reliable tech-nique for depth proÐling analysis of thin anodic

alumina Ðlms. It allows ready and rapid analysis of theÐlms, with excellent sensitivity and depth resolution,comparable with those of SIMS depth proÐling wherehighly controlled primary ion beams are used for Ðlmsputtering. In GDOES depth proÐling, the Ðlm materialis sputtered very rapidly in an argon atmosphere usingr.f. In the case of the 142 nm thick barrier Ðlm formedin sodium chromate solution, for example, sputteringfor 9 s is sufficient to reach the metal/oxide interface,giving a sputtering rate of 15.7 nm s~1. With SIMS,under the conditions described previously, sputteringfor 60 min is required to reach the metal/oxide interface,giving a sputtering rate of 0.04 nm s~1. Thus, the rate ofoxide sputtering during GDOES depth proÐling isD400 times that of SIMS depth proÐling. Additionally,and importantly, specimen charging e†ects are insigniÐ-cant in GDOES analysis, as evident from the steadyand constant aluminium signal intensity throughout theanalysis. This, together with the high sputtering rate ofthe Ðlm material, allows the thicknesses of the Ðlms tobe extended well beyond several tens of microns. Conse-quently, GDOES o†ers signiÐcant potential for precise,yet routine, proÐling analysis of thin or thick Ðlms.

Acknowledgement

Thanks are due to Mr Y. Uchida of Attago Bussan Co. Ltd for theprovision of time on a Jovin Yvon 5000 RF PSS instrument. One ofthe authors (G.M.B.) wishes to thank the Japan Society for the Pro-motion of Science (JSPC) for the provision of a post-doctoral fellow-ship.

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Surf. Interface Anal. 27, 24È28 (1999) ( 1999 John Wiley & Sons, Ltd.