SURFACE AND INTERFACE ANALYSIS, VOL. 25, 788È789 (1997)

Topographic Correction of Images3D SIMS

M. L. Wagter, A. H. Clarke, K. F. Taylor, P. A. W. van der Heide and N. S. McIntyre*Surface Science Western, Room G-1, Western Science Centre, The University of Western Ontario, London, Ontario N6A 5B7,Canada

In order to provide an accurate rendition of a three-dimensional (3D) volume obtained by SIMS, it is necessary totake account of the topography of the original surface and the relative sputter rates of the di†erent structureswithin the volume. We describe a method that corrects both distortions to 3D SIMS images. An atomic forcemicroscope is used to produce a topographic images of the area analysed by SIMS, both before and after the depthproÐle. This information is convoluted with the 3D SIMS image to produce a correct 3D image of the changes incomposition within the volume of the material. 1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 788È789 (1997)No. of Figures : 5 No. of Tables : 0 No. of References : 7

KEYWORDS: SIMS; topography ; AFM; imaging

INTRODUCTION

Depth proÐling of surfaces by ion bombardment is aprocedure that makes a number of assumptions aboutthe response of the surface to the ion beam. Surfaces arenormally considered to be single-phase entities with asputtering rate peculiar to that phase ; in practice, thesituation is usually more complex. Most solids consistof multiple phases, each of which will exhibit contrast-ing sputtering behaviour resulting from di†erent surfaceenergies. Even the exposure of di†erent crystallographicorientations of the same phase frequently results in sig-niÐcant alteration in the sputter rate. Di†erential sput-tering can also be caused by the original surfacetopography. Highly-accented asperities and depressionscan give rise to accentuated structures.1

The result of this is that most one-dimensional (1D)depth proÐles are distorted by topography and by thepresence of multiple phases. Thus, regions of obvioussurface roughness should be avoided when choosing alocation in which to proÐle. Recently, samplerotation2,3 has been demonstrated as a remedy tocounteract some types of di†erential sputtering thatresult from beam shadowing or crystallographic etchinge†ects.

When one wishes to expand the proÐle to two orthree dimensions of information, topographic e†ectsbecome a very critical factor because the increaseddetail available is even more susceptible to distortion bydi†erential sputtering. Secondary ion mass spectrometry(SIMS) is particularly useful for generating such two-dimensional (2D) and three-dimensional (3D) proÐlesbecause modern SIMS instruments have the capacity togenerate the prodigious amount of data for intensityand spatial coordinates in the time of a typical proÐle.

* Correspondence to : N. S. McIntyre, Surface Science Western,Room G-1, Western Science Centre, The University of WesternOntario, London, Ontario N6A 5B7, Canada

In the past, this laboratory has demonstrated severalexamples where localized structural and chemicaldetails of corrosion Ðlms have become apparentthrough SIMS 3D proÐling.4h7 This paper reports somee†orts to correct 2D and 3D proÐles for surface topo-graphic e†ects present prior to proÐling, as well as thosegenerated during the proÐle itself. The proceduresdescribed make use of the detailed 3D topographicinformation that can be obtained using atomic forcemicroscopy (AFM) and combine this with data fromSIMS to give a partial volumetric presentation.

EXPERIMENTAL METHOD

The method of collection of 3D SIMS proÐles has beenreported previously.4h7 A Cameca IMS-3f SIMS instru-ment using a caesium (Cs`) primary beam with an inci-dence angle of 20¡ (reduced from 30¡ by 4.5 keV) and anincidence energy of 14.5 keV was used to obtain the 3Dimages.

The AFM instrument used in collecting topographicdata was a Topometrix Explorer with a tripod scanhead and a typical silicon nitride pyramidal-shapedprobe.

Prior to SIMS imaging, several (usually three) iden-tifying marks were made in the surface of interest usinga micro-indenter. The purpose of these marks was toallow easy overlaying of SIMS and AFM images duringsubsequent analysis ; accordingly, these were spaced insuch a way as to all fall within the region imaged by theSIMS instrument (typically a diameter of 150 lm). Thisarea was then scanned by AFM, depth proÐled bySIMS and then re-scanned by AFM. The AFM dataÐles were then adjusted to Ðt the area covered by the3D SIMS proÐle.

The Ðrst topographic scan was overlayed on a screenwith the Ðrst SIMS image in a proÐle sequence. ThemagniÐcation and position of the images were adjustedso that the three indenter marks coincided and then the

CCC 0142È2421/97/010788È02 $17.50 Received 23 December 1996( 1997 by John Wiley & Sons, Ltd. Accepted 27 March 1997

TOPOGRAPHIC CORRECTION OF 3D SIMS IMAGES 789

SIMS intensity data was overlayed on the Ðeld of theAFM image and was stored in a new Ðle. A similar pro-cedure was used to Ðt the last SIMS image in thesequence with the AFM data obtained after the depthproÐle had been completed. Then, interpolated AFMdata were constructed from the “topÏ and “bottomÏ AFMimages for each intermediate location in the SIMSimage sequence. Such intermediate images were thenÐtted to the SIMS images in sequence. Such imagemanipulation was readily completed on a 386 PC. Purevolumetric rendering of such data is not possible on aPC.

The SIMS depth proÐles presented here wereobtained for a sample of ZrÈ2.5%Nb alloy whosesurface had been polished to a 1 lm diamond Ðnish.Depth proÐles were obtained using a 1000 nA Cs`primary beam rastered over a 250 lm square area.Most of the proÐle occurred within the metallic phase.

RESULTS AND DISCUSSION

Figure 1(a) shows 2D SIMS images of Zr~ and ZrO~through the oxide surface Ðlm. The numbers 1, 2 and 3on both AFM and SIMS images point to three distinctgrains on the sample surface used as secondary refer-ence points to exactly match up the topographicalimages with the SIMS images. Figure 1(b) shows apseudo-3D image stack of this structure. TheZrO2/Zrterm pseudo is used to indicate that no topographicalcorrection has been made (the thickness of the layermeasured is to be 2 lm). Figure 2 shows atomic forceimages of the surface before and after depth proÐling.The presputtered surface contains some pits andgrooves. The grain structure is barely visible, but it ispossible to identify the positions of the three grainsobserved in Fig. 1. One diamond-shaped micro-indentermark is found within the area. For this sample, only one

micro-indentation mark was used, because many of thegrain shapes could be identiÐed in both SIMS andAFM images throughout the proÐle. This is not alwaysthe case ; typically multiple micro-indenter marks arerequired for overlaying. Figure 2(b) shows the AFMimage of the surface after the depth proÐle. Preferentialsputtering of certain grain orientations has resulted in arange of heights for di†erent grains. Sputter-inducedroughening is also visible on some grain surfaces.

In Fig. 3, a 2D image of the topographically-corrected Nb~ intensity distribution is shown for a 50lm section cut through the 3D volume. It can be seenthat the topography on the top and bottom di†ers ; thisis the result of di†erential sputtering and shows theimportance of making both “topÏ and “bottomÏ correc-tions. Figure 4 shows topographically-corrected 3Dimages of Nb~ and H~. The hydrogen and niobiumintensities are displayed with a thermal linear colourscale and show the presence of areas where hydrogenappears more prominently. There is, however, one area(see arrow), where the hydrogen secondary ion inten-sities are strong, that coincides exactly with anobliquely-angled facet. The enhanced hydrogen, in thiscase, could be the result of : more adsorption on metallicregions that are partly obscured from the primary ionbeam; a hydrogen rich area ; or a matrix e†ect. Bothimages show a slotted region along the side of thevolume, which is due to mechanical depressions thatexceed the depth of the proÐle. The ultimate value of adisplay of this type is the perspective that it can bring tothe relationship between mechanical structure andchemical composition. In Fig. 5, four separate depths inthe Nb~ intensity proÐle are shown.

We believe that such correlations between SIMSimages and topography will provide valuable insightinto the relations of microstructure and composition inmany metal and oxide systems and their relationship tothe substrate metal structure.

REFERENCES

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International Conference on Secondary Ion Mass Spectrometry ,Amsterdam, 1991, ed. by A. Benninghoven, K. Janssen, J.Tumpner and H. W. Werner pp. 347–350. Wiley, Chichester(1992).

3. M. Hatada, A. Karen, Y. Nakagawa, M. Saeda, M. Uchida, K.Okuno, F. Soeda and A. Ishitani, in SIMS VIII : Proceedings ofthe International Conference on Secondary Ion Mass Spec-trometry , Amsterdam, 1991, ed. by A. Benninghoven, K.Janssen, J. Tumpner and H. W. Werner, pp. 351–354. Wiley,Chichester (1992).

4. S. F. Lu, G. R. Mount, N. S. McIntyre and A. Fenster, Surf .Interface Anal . 21, 177 (1994).

5. N. S. McIntyre, K. F. Taylor, G. R. Mount and C. G. Weisener, inSIMS VIII : Proceedings of the International Conference onSecondary Ion Mass Spectrometry , Amsterdam, 1991, ed. by A.Benninghoven, K. Janssen, J. Tumpner and H. W. Werner, pp.513–516. Wiley, Chichester (1992).

6. N. S. McIntyre, R. D. Davidson, K. F. Taylor, C. G. Weisener, F.C. Gonzalez, E. M. Rasile and A. M. Brennenstu� hl, Surf . Inter-face Anal . 18, 601 (1992).

7. N. S. McIntyre, P. A. W. van der Heide, D. M. Kingston, G. R.Mount, A. H. Clarke, K. F. Taylor and B. D. Warr, Proceedingsof the European Conference on Applied Surface and InterfacialAnalysis (ECASIA), Montreux, 1995, ed. by H. J. Mathieu, pp.151–155. Wiley, Chichester (1996).

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 788È789 (1997)