SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 153È156 (1999)

InÑuence of Surface Roughness on the DepthResolution of GDOES Depth ProÐling Analysis

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 Faculty of Science and Technology, Department of Chemistry, 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

Radio-frequency-powered glow discharge optical emission spectroscopy (GDOES) is an extremely powerful andreliable technique for depth proÐling analysis of thin, insulating barrier anodic Ðlms formed on aluminium. Itallows ready and rapid analysis of the Ðlms, with depth resolution and sensitivity comparable with, or better than,those of secondary ion mass spectrometry depth proÐling. However, for successful application of the technique,surfaces of specimens should be microscopically Ñat ; surface roughness of dimensions similar to the thickness ofthe Ðlms can lead to almost total degradation of the depth proÐles. Copyright John Wiley & Sons, Ltd.( 1999

KEYWORDS: glow discharge optical emission spectroscopy ; GDOES; anodic Ðlms ; depth resolution

INTRODUCTION

In the study of impurity distributions in thin, insulatingbarrier anodic Ðlms \200 nm thick formed on alu-minium in a wide variety of electrolytes, the authorshave shown recently that radio-frequency-powered glowdischarge optical emission spectroscopy (GDOES) is anextremely powerful and reliable technique for depthproÐling analyses of such Ðlms.1h3 The Ðlms are amorp-hous and thus possible complications of di†ering etchrates with di†erent crystal orientations that may arise inanalysis of polycrystalline material are avoided. Duringanalysis, surface charging is insigniÐcant and the sput-tering rate of the Ðlm is kept constant, giving rise toexcellent depth resolution that is comparable with, orbetter than, that of secondary ion mass spectrometry(SIMS) depth proÐling. Further, sensitivity is also high,given the relatively low amounts of impurity speciesdetected successfully. The purpose of the present com-munication is to show that surface roughness is a criti-cal factor in achieving depth resolution ; thus, roughnessof dimensions similar to that of the Ðlm thickness canlead to almost total degradation of the depth proÐles.Consequently, for successful applications of GDOESdepth proÐling, specimen surfaces should be micro-scopically Ñat.

EXPERIMENTAL

Aluminium sheets (99.99% pure, dimensions15 ] 50 ] 0.2 mm) were surface pretreated in two di†er-

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

ent ways to obtain a mirror or relatively rough Ðnish :electropolishing was undertaken at a constant currentdensity of 100 mA cm~2 for 5 min in a perchloricÈethanol bath at temperatures below 10 ¡C; and etchingwas performed in 5 wt% NaOH solution at 25 ¡C for 5min. The electropolished, or etched, specimens wereanodized subsequently at a constant current density of5 mA cm~2 to 300 V in 0.1 M ammonium pentaboratesolution at 25 ¡C. In order to assess the surface rough-ness of the variously treated specimens, they were exam-ined, before and after anodizing, by atomic forcemicroscopy (AFM) and by transmission electronmicroscopy (TEM) of ultramicrotomed sections follow-ing the procedures described elsewhere.4

The distributions of electrolyte-derived impurityspecies in the anodic Ðlms (in the present case, boronfrom the pentaborate electrolyte) were determined usinga Jobin-Yvon 5000 RF GDOES instrument. BrieÑy, theanodized specimens were placed in a holder, which wasmade the cathode ; the anodic oxide Ðlms were sput-tered in an argon atmosphere of 3È5 Torr by applyingan r.f. of 13.56 MHz and 40 W power. The Ðlms weresputtered rapidly at D1.1 lm min~1. Light emission ofcharacteristic wavelengths, associated with the sputteredspecies excited mainly by collision of electrons, weremonitored throughout the analysis with a samplingtime interval of 0.01 s to obtain depth proÐles. Thewavelengths of the spectral lines employed were 396.15and 249.68 nm of aluminium and boron, respectively.

RESULTS AND INTERPRETATION

Surface roughness of the specimens

Electropolishing of high-purity aluminium under theconditions employed results in a mirror-Ðnished surface.

CCC 0142È2421/99/030153È04 $17.50 Received 12 October 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 8 December 1998

154 K. SHIMIZU ET AL.

Examination of such a surface by AFM, however,revealed that the surface is not microscopically Ñat, butexhibits a characteristic cellular or scalloped texture asshown in Fig. 1. At the lateral magniÐcation of Fig. 1,with a relatively large area of 10] 10 lm scanned, thecellular or scalloped textures are not revealed clearly.Scrutiny of a reduced area, at increased lateral magniÐ-cation, reveals that the dimensions of the cells areD 100 nm and each cell is separated from adjacent cellsby a network of ridges, up to D 20 nm in height, mea-sured from the base of the cells.4

Figure 2 shows an AFM image of the electropolishedspecimen that had been anodized to 300 V to form abarrier anodic Ðlm D360 nm thick. Due to smoothingof the surface during barrier Ðlm growth,5 which resultsfrom a high-Ðeld-assisted ionic transport process, theinitial roughness of the surface, associated with thecharacteristic cellular texture, is totally eliminated afteranodizing to 300 V. The anodized surface is Ñat towithin a few nanometres over the relatively largescanned area of 5] 5 lm.

The excellent Ñatness of the metal/Ðlm interface hasbeen conÐrmed further by TEM of ultramicrotomedsections (Fig. 3). The barrier Ðlm, of uniform thickness358 nm, is observed over the microscopically Ñat alu-minium substrate.

Figure 1. The AFM image of the electropolished aluminiumsurface.

Figure 2. As Fig. 1, but after anodizing to 300 V at a constantcurrent density of 5 mA cmÉ2 in 0.1 M ammonium pentaboratesolution at 25 ¡C, revealing a relatively smooth film surface.

Figure 3. Transmission electron micrograph of an ultramicro-tomed section of the electropolished aluminium after anodizingto 300 V at a constant current density of 5 mA cmÉ2 in 0.1 M

ammonium pentaborate solution at 25 ¡C.

Unlike the electropolished specimen, the etched alu-minium surface is considerably rough, as shown in Fig.4. Networks of ridges several microns in size and severalhundreds of nanometres in height are observed,although the networks appear rather discontinuous.Further, at local regions surrounded by such ridges,surface roughness of Ðner dimensions, both in size andheight, is observed, along with the occasional presenceof relatively wide and deep cavities with diameters up to1 lm.

The AFM image of the etched specimen that hadbeen anodized to 300 V is shown in Fig. 5, withsmoothing of the surface evident. However, the large-featured surface roughness, associated with the net-works of ridges, remains. Further examination of theetched and anodized specimen by TEM of ultramicro-tomed sections reveals the presence of a barrier Ðlm ofthickness D 360 nm, measured perpendicularly to themicroscopic metal/oxide interface, i.e. the rough andundulating surface (Fig. 6).

Depth proÐles by GDOES

Figure 7 shows a GDOES depth proÐle of the 300 VÐlm formed on the electropolished aluminium specimen.The distribution of boron species is revealed clearly,with sharply deÐned interfaces between the outer layerdoped with boron species and the inner pure Al2O3layer. Further, the metal/oxide interface is also deÐnedsharply. With a depth proÐle of such precision, it is pos-

Figure 4. The AFM image of an etched, relatively rough alu-minium surface.

Surf. Interface Anal. 27, 153È156 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

SURFACE ROUGHNESS AND GDOES DEPTH PROFILING 155

Figure 5. As Fig. 4, but after anodizing to 300 V at a constantcurrent density of 5 mA cmÉ2 in 0.1 M ammonium pentaboratesolution at 25 ¡C, revealing a much rougher film surface thandeveloped on the electropolished aluminium surface.

sible to deÐne the locations of interfaces separating dif-ferent Ðlm layers to an accuracy of a few nanometres.

An interesting feature is associated with the boronproÐle, i.e. the proÐle exhibits waviness. Such wavinessis not due to variation of the amount of boron speciesincorporated into the Ðlm during the Ðlm growth, but isan artifact arising from interference of light emissionthat entered directly into the monochromator from thedischarge lamp and that which entered the monochro-

Figure 6. Transmission electron micrograph of an ultramicro-tomed section of the etched aluminium after anodizing to 300 V ata constant current density of 5 mA cmÉ2 in 0.1 M ammonium pen-taborate solution at 25 ¡C.

Figure 7. The GDOES depth profile of electropolished aluminiumafter anodizing to 300 V at a constant current density of 5 mAcmÉ2 in 0.1 M ammonium pentaborate solution at 25 ¡C.

mator after having been reÑected at the oxide/metalinterface (Y. Shimidz, Attago Bussan Co. Ltd, Japan,1998, personal communication). In reality, the boronspecies are incorporated uniformly into the outer layerof the Ðlm, as indicated by the solid line. A similar, butless pronounced, waviness is also observed in the alu-minium proÐle ; again, this results from the interferencee†ects. Thus, it is evident that the aluminium intensity ishighly stable, as indicated by the solid line, and is con-stant within 10% throughout the analysis of the Ðlm,suggesting uniform sputtering.

From the positions of the boundaries between theouter boron-doped and inner pure layers and theAl2O3metal/oxide interface, the proportion of the outerboron-doped layer to the total Ðlm thickness is 0.44,which agrees precisely with that expected from thecationic transport number of 0.44 during Ðlm growth.6Here, the positions of the boundary between the outerboron-doped and inner pure layers and theAl2O3metal/oxide interface were determined from the half-height positions of the trailing or leading edges of theboron or aluminium proÐles, respectively, as indicatedby the solid lines.

The GDOES depth proÐle for the 300 V Ðlm formedon the etched, relatively rough aluminium specimen isdisplayed in Fig. 8. The intensity of the Al signal isapproximately constant up to 10 s, when it increasesgradually with time until a steady value of intensityabout three times that of the initial stage is reachedafter 25 s. Concerning the boron proÐle, the signalintensity is relatively constant up to 7 s, when itdecreases gradually to zero over a further 6 s. With thisproÐle, it is difficult to determine the positions of theouter boron-doped and inner pure layers and theAl2O3metal/oxide interface with precision, although theproÐle indicates that the Ðlm consists of outer boron-doped and inner pure layers. It is demonstratedAl2O3clearly here that surface roughness leads to almost totaldegradation of the depth proÐles.

Degradation of the depth proÐles may be associatedwith : local enhancement of the electric Ðeld at ridged

Figure 8. The GDOES depth profile of etched aluminium afteranodizing to 300 V at a constant current density of 5 mA cmÉ2 in0.1 M ammonium pentaborate solution at 25 ¡C. The depthresolution is severely degraded by the roughness of the filmedsurface.

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 153È156 (1999)

156 K. SHIMIZU ET AL.

regions, leading to di†erential sputtering ; non-uniformsputtering due to di†erences in impact angles of ions onsurfaces with di†erent orientations ; and di†erences inÐlm thickness between ridged and valley regions overthe area of analysis. Whichever is the main cause of thedegradation of the depth proÐles, the Al signal intensityis constant at the initial stages of analysis when themacroscopic aluminium surface supports an oxide Ðlm.However, with time, the aluminium substrate is exposedat the local regions of the original surface, and the Alsignal intensity increases gradually. The increase con-tinues as the aluminium substrate is exposed over anincreased area of the sample surface, reaching a steadyvalue after the aluminium substrate is exposed com-pletely.

Discrimination between the possible mechanisms ofdegradation of the proÐles may be assisted by high-resolution microscopy. Thus, further detailed studies arenow under way on the sputtering process duringanalysis of thin surface Ðlms on a rough surface byAFM and cross-sectional TEM using a novel surfacepreparation technique that gives, unlike etchingemployed in the present work, a highly controlled, well-deÐned surface roughness suitable for studies of thiskind. Such surfaces of controlled roughness can be pre-pared readily by a sequence of processes involving Ðrstanodization of electropolished aluminium specimens ina phosphoric acid solution to form a relatively large fea-tured porous-type Ðlm with a scalloped metal/Ðlm inter-face, then removal of the Ðlm by dissolution in a hot

solution without changing the scallopedCrO3ÈH3PO4surface texture and Ðnally anodization in an ammoniumpentaborate solution to form a barrier-type Ðlm of 180

nm. The results of these studies will be published in aseparate paper.

CONCLUSIONS

It has been demonstrated clearly that the depthresolution of GDOES depth proÐling depends criticallyon the surface roughness of the specimens. Surfaceroughness of dimensions similar to the Ðlm thickness, inparticular, leads to almost total degradation of depthresolution. Conversely, when thin Ðlms of uniformthicknesses are present over microscopically Ñat sub-strates, GDOES represents an extremely powerful andreliable technique for depth proÐling, allowing readyand rapid analysis of the Ðlm with depth resolution andsensitivity comparable with, or better than, those ofSIMS.1h3 Further, given the relatively low costs ofGDOES, with no ultrahigh vacuum requirements, it isexpected to have an increasing role in depth proÐlinganalysis of various Ðlms of thicknesses up to severalmicrons, particularly in industry.7 However, successfulapplication requires samples to be microscopically Ñat.

Acknowledgements

Thanks are due to Mr Y. Uchida of Attgo Bussan Co. Ltd for provi-sion of time on a Jobin-Yvon 5000 r.f. GDOES instrument. One of theauthors (G.M.B.) wishes to thank the Japan Society for the Pro-motion of Science (JSPS) for the provision of a post-doctoral fellow-ship.

REFERENCES

1. K. Shimizu, G. M. Brown, H. Habazaki, K. Kobayashi, P.Skeldon, G. E. Thompson and G. C. Wood, Corros . Sci ., inpress.

2. K. Shimizu, G. M. Brown, H. Habazaki, K. Kobayashi, P.Skeldon, G. E. Thompson and G. C. Wood, Surf . InterfaceAnal ., 27, 24 (1999).

3. K. Shimizu, G. M. Brown, H. Habazaki, K. Kobayashi, P.Skeldon, G. E. Thompson and G. C. Wood, Electrochim Acta , inpress.

4. K. Shimizu, K. Kobayashi, P. Skeldon, G. E. Thompson andG. C. Wood,Corros . Sci . 39, 701 (1997).

5. J. P. S. Pringle, Electrochim Acta 25, 1403 (1980).6. K. Shimizu, K. Kobayashi, G. E. Thompson, G. C. Wood and P.

Skeldon, Philos . Trans. R. Soc. London A 354, 213 (1996).7. R. Payling, Spectroscopy 13, 36 (1998).

Surf. Interface Anal. 27, 153È156 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.