sims with sample rotation: an experimental novelty or a practical necessity?

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 28, 49–55 (1999)

SIMS with Sample Rotation: an ExperimentalNovelty or a Practical Necessity?

D. E. Sykes*Loughborough Surface Analysis Ltd, PO Box 5016, Loughborough LE11 3WS, UK

The application of sample rotation during SIMS analysis of practical samples is described. It is shown thatsample rotation gives improved depth resolution in profiles of metal layers on flat semiconductor substrates,allowing thin interfacial layers to be identified. For thick metal layers on non-ideal substrates, the benefitsof sample rotation are less clear. Sample rotation is also shown to be useful as a simplein situ samplepreparation tool for the removal of metal overlayers on semiconductor substrates where the objective of theanalysis is the semiconductor material rather than the metal itself. Copyright 1999 John Wiley & Sons,Ltd.

KEYWORDS: SIMS; depth profiling

INTRODUCTION

The development of surface topography and the con-sequential loss of depth resolution during sputter depthprofiling is a problem that has limited practical analysesand has attracted academic interest for many years. Oneapproach, which attempts to minimize the effects of sur-face topography development, is the use of sample rotationduring sputtering. This has been shown to be effectiveboth in Auger1 and SIMS2 – 4 depth profiling. These firstapplications of sample rotation in SIMS were directed atsuppression of the characteristic ripple topography5 – 8 thatdevelops on semiconductor surfaces. Subsequently, stud-ies of metal films on smooth substrates, i.e. aluminiumon silicon, demonstrated the potential of sample rotationfor improving the depth resolution at buried interfacesin ideal samples.9,10 The application of sample rotationduring SIMS analysis to a practical problem, quantify-ing the relative amounts of contaminant left at interfacesfollowing physical and chemical etching steps, has beendescribed elsewhere.11 This paper illustrates further appli-cations of sample rotation during SIMS depth profilingand discusses some of the limitations in the analysis ofpractical samples as well as demonstrating the advantagesof the technique.

EXPERIMENTAL

The SIMS analyses were carried out in a Cameca IMS 4ffitted with a sample rotation stage manufactured by KoreTechnology Ltd.12 Primary ion beams of either CsC or O2

C

were used, the potential of the ion sources being 10.0 and12.5 kV, respectively. The primary ion current densitywas adjusted to allow the profiles to be carried out in a

* Correspondence to: D. E. Sykes, Loughborough Surface AnalysisLtd, PO Box 5016, Loughborough LE11 3WS, UK.E-mail: [email protected]

reasonable time, of the order of 20 min for the thinnerstructures and up to 3 h for the 14µm thick metal layers.Positive secondary ions were detected using a 150µmtransfer lens. For the profiles with sample rotation, therotation rate was¾3 rpm.

The scanning electron microscope images were obtai-ned in a JEOL JAMP 10s SEM/Auger instrument usingan electron beam energy of 10 keV.

RESULTS AND DISCUSSION

Rotation as an in situ sample preparation tool

Determination of the dopant distribution within a layeredsemiconductor sample is a relatively routine analyticaltask for SIMS. However, when the semiconductor struc-ture is buried beneath a metal contact layer, the taskbecomes more difficult if the metal overlayers cannot beremoved easily by chemical means. In this example, it wasnecessary to determine the zinc dopant profile through anInP-based laser structure that had been metallized and heattreated. Chemical etching had failed to remove the met-allization successfully and a conventional SIMS profilethrough the full structure gave an unacceptably broad-ened profile in the interfaces of interest. Figure 1 showsthe profiles for the matrix elements arsenic and phospho-rus, and for the zinc dopant. (The profile was obtainedusing CsC primary ion bombardment and detecting theelements of interest as the CsMC species, which are rel-atively standard conditions for zinc analyses.) No signalsfor the metal species are shown because these were not ofinterest in this application. Figure 2 shows the part of thesample on which the analysis was performed. The sam-ples were in the form of metallized bars of semiconductor250µm wide and several millimetres in length. A 250µmsquare raster was used for the analysis and this can beseen in the low magnification image. At higher magnifi-cation the surface roughness of the metallization can beseen clearly at the edge of the crater, propagating downinto the semiconductor substrate. The central region of the

CCC 0142–2421/99/130049–07 $17.50 Received 30 November 1998Copyright 1999 John Wiley & Sons, Ltd. Accepted 22 January 1999

50 D. E. SYKES

Figure 1. SIMS depth profile, without sample rotation, though an InGaAs/InP/InGaAsP/InP laser structure buried beneath metal contactlayers.

Figure 2. The SEM images (ð200 and ð3000) of the crater resulting from the profile shown in Fig. 1.

crater appears quite rough. It is interesting to observe thatthe depth resolution of the profile shown in Fig. 1 appearsto improve with increasing depth into the semiconductorstructure, the interfaces of the quaternary active regionbeing more abrupt than those of the ternary p-contact.

The most effective way to perform routine SIMS anal-yses on this type of sample has proved to be simplyto use rotational SIMS as a sample preparation stage toremove the metal layers and then to carry out a nor-mal SIMS profile as described above. This procedure hasthe advantage that the results are immediately compa-rable with those obtained from conventional profiles onnon-metallized samples but the loss of depth resolutionincurred by performing a non-rotated profile through the

metal layers is not present. Figure 3 shows the profilethrough the same structure but with rotation (using a500 µm raster) up until the point at which the CsAsC sig-nal began to rise, indicating that the metal/semiconductorinterface had been reached. The rotation was then stopped,the primary ion beam raster was reduced to 250µm andthe profile was continued using exactly the same condi-tions as those for Fig. 1. The crater produced in this wayis shown in Fig. 4. As in Fig. 2, the surface roughnessof the starting surface is clear but the interfaces in themetallic layers are sharper and no roughness is immedi-ately obvious in the crater base. The SEM images fromthe two craters, which are adjacent to each other on thesample, were recorded using identical imaging conditions.

Surf. Interface Anal. 28, 49–55 (1999) Copyright 1999 John Wiley & Sons, Ltd.

SIMS WITH SAMPLE ROTATION 51

Figure 3. The SIMS depth profile, with sample rotation, through an InGaAs/InP/InGaAsP/InP laser structure buried beneath metalcontact layers.

Figure 4. The SEM images (ð200 and ð 3000) of the crater resulting from the profile shown in Fig. 2.

The depth profile with rotation (Fig. 3) shows increaseddetail in the signals through the metal layers when com-pared with the profile without rotation. It is not immedi-ately obvious which species these signals represent but,as the composition of the metallization system was out-side the scope of this investigation, these signals werenot investigated further. The profile through the semicon-ductor structure of interest now shows acceptable depthresolution, comparable to that expected from virgin semi-conductor surfaces using these experimental conditions,and allowed meaningful comparisons to be made betweensamples exposed to a range of treatments.

Analysis of a thick metal bi-layer system

Figure 5 shows a conventional SIMS depth profile throughtwo relatively thick metal layers on a metallic substrate

taken from an engineered component. The outermost layeris of the order of 8µm in thickness and the second layer is6 µm. As in the previous example, the primary ion beamwas CsC and CsMC species were detected. (This samplewas also analysed using O2

C primary ion bombardmentbut no significant differences were found in the shapes ofthe profiles, only in the relative intensities of the signalsfrom the elements of interest.)

As may be expected, oxygen shows peaks in the inter-faces between metal 1 and metal 2 and between metal 2and the substrate. There are also secondary oxygen fea-tures about one-third of the way into each layer (in thegrowth direction). Carbon also shows features, but not atthe same depths as the oxygen features, and other impuritysignals also show structure through the layers.

The sample had a rough surface texture and examinationof the crater base showed considerable sputter-induced

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 49–55 (1999)

52 D. E. SYKES

Figure 5. The SIMS depth profile, without sample rotation, of a thick metal bi-layer on a non-flat substrate.

Figure 6. The SIMS depth profile, with sample rotation, of a thick metal bi-layer on a non-flat substrate.

topography. It seemed appropriate to use sample rotationin order to suppress the sputter-induced topography andthus to improve the resolution in the profile. The result of aprofile carried out with sample rotation is shown in Fig. 6.At first sight there is little difference to be seen betweenthe two profiles. Closer inspection, however, shows thatthe features in the profile recorded without rotation aresharper in the profile recorded with rotation, althoughmetal 1 appears to have a tail extending into metal 2 thatis not present in the non-rotated case. Also, the signalfrom metal 1 shows periodic fluctuations characteristic ofa tilted sample. The improvement in depth resolution isbest illustrated by the broad feature in the carbon profilein metal 1 just before the interface with metal 2, whichis resolved into two peaks in the profile recorded withsample rotation.

Low-magnification SEM images of the two craters areshown in Fig. 7. These images clearly show the presenceof surface roughness on the as-received surface and inthe base of both craters. The crater from the non-rotatedprofile shows the presence of cones and columnar features,pointing in the direction of the incident ion beam, that aretypical of the topography generated in multiphase systems.The crater produced with sample rotation also showstopography but, in this case, the features are rounded and

appear no worse than those on the as-received surface.Closer inspection of the interface between metal 2 andthe substrate reveals that the initial surface on which thelayers were deposited was not itself flat, as perhaps wouldbe expected for an industrial component.

In this example, the improvement in depth resolutionobtained by using sample rotation is small. This inabilityto improve significantly on the results obtained withoutsample rotation is a direct result of the structure of thesample itself and not a failure of the experimental method.Sample rotation can be used to suppress sputter-inducedtopography and hence give better depth resolution in caseswhere that information is there to be obtained. However,in cases where the depth resolution is limited by thestructure of the sample itself, sample rotation can donothing to improve on the quality of the results obtainedand only leads to unnecessary experimental complication.

Identification of a high resistance layer in ametallization on silicon

The problem that was presented in this case was to identifythe cause of a high series resistance in a titanium/titaniumnitride/tungsten metallization system on silicon. Threenominally identical layers had been produced in cluster



Figure 7. The SEM (ð200) images of the craters resulting from: (a) the profile shown in Fig. 5; (b) the profile shown in Fig. 6.

Figure 8. The SIMS profiles of tungsten/tinitride metallizations (on silicon dioxide on silicon): (a) tool 1, without sample rotation;(b) tool 1, with sample rotation; (c) tool 2, chamber A, with sample rotation; (d) tool 2, chamber B, with sample rotation.


54 D. E. SYKES

Figure 8. (continued).

tools, two of which were on the same system. Tool 1and Tool 2, chamber A, produced well-behaved layers;the high series resistance was only present in the layersproduced in Tool 2, chamber B.

The thicknesses of the metal layers in this case wereconsiderably less than in the case described above: 200 nmof tungsten on 10 nm of titanium nitride on 10 nm of tita-nium. The analysis of these samples was performed usingO2C primary ion bombardment and positive secondary ion

detection.Figure 8(a) shows the profile from the Tool 1 sam-

ple recorded without rotation; although the tungsten andtitanium signals show something of the layer structure,the interfaces are not well resolved and the outer tungstenlayer appears to meet the underlying silicon dioxide layer.Figure 8(b), on the other hand, shows that with rotation

the layer structure is better resolved when sample rotationis used. Comparison of the fluorine profiles in the twoplots illustrates that loss of depth resolution has occurredvery rapidly in the profile recorded without rotation; thesharp peak that occurs some two-thirds of the way throughthe tungsten layer in the profile with rotation appears as abroad feature that is first seen about half-way through thetungsten layer in the profile recorded without rotation.

Figures 8(c) and 8(d) show profiles recorded withrotation from the samples prepared in chambers A andB of Tool 2. The major features of both profiles resemblethose of the sample prepared in Tool 1. Although the risein the oxygen signal follows that of the silicon signal inthe Tool 1 profile—and is generally similar in the Tool 2,chamber A, profile, both showing a small shoulder to theoxygen profile in the titanium-based layers—in the Tool 2,



chamber B, profile the oxygen signal rises well beforethe silicon signal, indicating the presence of an oxidizedlayer. Thus, the cause of the higher series resistance canbe attributed to the presence of an oxidized titanium layerresulting from a leak in chamber B. Only through theuse of sample rotation could the subtle differences inthe profiles be revealed and the cause of the problemidentified.

CONCLUSIONS

The use of sample rotation during SIMS analysis has beenshown to be effective in the practical analysis of thinmetal layers on silicon, enabling a process problem to beidentified. In the case of thick metal layers on machinedcomponents, sample rotation provides little gain in depth

resolution because the intrinsic variations in sample com-position can be of a similar scale to those introduced bysputter-induced roughening. When the objective of theanalysis is the underlying semiconductor material, samplerotation can be used as anin situ sample preparation stageto remove metallic layers before carrying out a conven-tional profile without rotation. Sample rotation is a usefulexperimental tool but only in those cases where there issome benefit to be gained; for many analytical tasks it willnot be necessary. In circumstances where sputter-inducedroughening can otherwise obscure the information that issought, sample rotation is essential.

Acknowledgements

The author would like to thank the three clients who have allowed theresults of the analyses of their samples to be used in this paper, all beit anonymously, and his colleague, Dr Alison Chew, for her part in thework described above.

REFERENCES

1. A. Zalar, Thin Solid Films 124, 223 (1985).2. E.-H. Cirlin and J. J. Vajo, in SIMS VIII, p. 347. Wiley,

Chichester (1992).3. M. Hatada, A. Karen, Y. Nakagawa, M. Saeda, M. Uchida,

K. Okuno, F. Soeda and A. Ishitani, in SIMS VIII, p. 351.Wiley, Chichester (1992).

4. M. R. Houlton, O. D. Dosser, M. T. Emeny, A. Chew andD. E. Sykes, in SIMS VIII, p. 343. Wiley, Chichester (1992).

5. F. A. Stevie, P. M. Kahora, D. S. Simons and P. Chi, J. Vac.Sci Technol. A 6, 76 (1988).

6. A. Karen, K. Okumo, F. Soeda and A. Ishitani, in SIMS VII,p. 139. Wiley, Chichester (1990).

7. S. P. Smith, in SIMS VII, p. 107. Wiley, Chichester (1990).8. M. R. Houlton, G. W. Blackmore, M. T. Emeny, C. R. White-

house, A. Chew and D. E. Sykes, Surf. Interface Anal. 20, 69(1993).

9. F. A. Stevie and J. L. Moore, Surf. Interface Anal. 18, 147(1992).

10. D. E. Sykes and A. Chew, Surf. Interface Anal. 21, 231 (1994).11. D. E. Sykes, A. Chew, J. Hems and K. Stribley, Appl. Surf.

Sci. 100/101, 77 (1996).12. A. Chew, D. E. Sykes, S. J. Mullock and C. A. Coreltt, in SIMS

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sims with sample rotation: an experimental novelty or a practical necessity?

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