differential charging in xps. part iii. a comparison of charging in thin polymer overlayers on...

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 904È912 (1997)

Diþ erential Charging in XPS. Part III. AComparison of Charging in Thin PolymerOverlayers on Conducting and Non-ConductingSubstrates

Brian J. Tielsch and Julia E. Fulghum*Chemistry Department, Kent State University, Kent, OH 44242, USA

Previous work has used small-area and imaging XPS to show that di†erential charging in bulk insulators developsas a result of non-uniform x-ray Ñux across the surface, causing lateral di†erential charging. Charging in heter-ogeneous samples can be further a†ected by the local sample environment and sample mounting. The current workextends these studies through an analysis of di†erential charging e†ects in thin overlayers on conducting andinsulating substrates. The charging observed in PnBMA overlayers on indium tin oxide, glass, Ag and Al is dis-cussed as a function of substrate conductivity and photoelectron cross-sections. Substrate conductivity is the mostsigniÐcant factor in determining the magnitude of the overlayer charging observed when no charge compensation isutilized. Di†erential charging in the PnBMA overlayer was used to image a patterned substrate containing insulat-ing and conducting areas. 1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 904È912 (1997)No. of Figures : 17 No. of Tables : 1 No. of References : 11

KEYWORDS: XPS; charging ; polymer ; overlayers

INTRODUCTION

Di†erential charging develops when photoelectron ener-gies are measured with respect to more than one samplepotential, causing unpredictable peak shifts and dis-torted peak shapes. In general it is a problem for theXPS analyst because such phenomena could easily leadto erroneous data interpretation. Di†erential charginge†ects have also been used to good advantage.1,2 Thisrequires, however, knowledge of the sample composi-tion, careful sample preparation and mounting, as wellas adequate control of experimental parameters. Recentadvancements in imaging XPS and the ability toperform e†ective charge compensation have allowed fora better understanding of the nature of di†erentialcharging in bulk insulators as well as heterogeneoussamples. For example, di†erential charging in a bulkinsulator was shown to result from a non-uniform x-rayÑux across the sample.3 Imaging and small-area XPShave been useful in identifying areas of di†ering samplepotential. Factors that a†ect photoelectron peak posi-tions on heterogeneous samples were evaluated usingpatterned conductive layers on glass.4 Local sampleenvironment, sample mounting and spatial variation inx-ray Ñux have been shown to contribute to di†erentialcharging e†ects.

* Correspondence to : J. E. Fulghum, Chemistry Department, KentState University, Kent, OH 44242, USA.

Contract grant sponsor : NSF Science and Technology Center forAdvanced Liquid Crystalline Optical Materials ; grant nos DMR 89-20147 and NSF CHE-9631702.

This work extends the application of imaging XPSand di†erential charging e†ects to the analysis ofsamples containing thin polymer overlayers. The e†ectof substrate conductivity on the photoelectron imagesobtained from thin insulating overlayers is discussed inthe following sections. These results provide a furtherunderstanding of di†erential charging e†ects in heter-ogeneous samples, and demonstrate another method forsample evaluation using XPS.

EXPERIMENTAL

X-ray photoelectron spectroscopy data were acquiredusing a Kratos AXIS HS instrument with the mono-chromatic Al Ka x-ray source operated at 168 W (12mA, 14 kV). Data were acquired using either hybridmode, which uses both electrostatic and magneticlenses, or magnetic mode for imaging and small-areaanalysis. Survey spectra were acquired with an 80 eVpass energy using either hybrid (large-area) or magnetic(100 lm spot size) modes as described. Survey spectra inFigs 6, 11 and 15 were acquired without the use of acharge compensation system. High-resolution spectrawere acquired with a 40 eV pass energy using eitherhybrid or magnetic (100 lm spot size) modes. Spectra inFigs 2, 12 and 16 were acquired without the use ofcharge compensation. Images were acquired using mag-netic mode with a 100 lm spot size and 80 eV passenergy over an area of D1.1 mm2. All images consist of45 ] 43 pixels. Images shown in Figs 13 and 14 wereacquired without the use of charge compensation. Thecharge compensation parameters were : [2.8 V bias

CCC 0142È2421/97/011904È09 $17.50 Received 13 February 1997( 1997 by John Wiley & Sons, Ltd. Accepted 11 June 1997

DIFFERENTIAL CHARGING IN XPS 905

voltage, [1.0 V electron Ðlament voltage and 2.1 A Ðla-ment current.

Samples included a bulk piece of glass, a bulk glasscoated with a 27 nm layer of indium tin oxide (ITO),silver or aluminum foil mounted on bulk glass usingdouble-sided sticky tape and a patterned ITO overlayer(as shown in Fig. 7) on bulk glass. The glass sampleswere obtained from the Donelly Corporation. The silverfoil (99.99% purity) was obtained from Goodfellow. Thealuminium foil was household Reynolds foil. Eachsample was electrically grounded through the sampleholder using conductive tape in contact with both thesample holder and sample surface, as shown in Fig. 7.Prior to XPS analysis, each sample was spun-cast withthree drops of a 5% (w/w) solution of PnBMA in chlo-roform. The glass, Ag and Al substrates were the samesize and were spun-cast at the same time to ensure com-parable overlayer thicknesses. The PnBMA (mol. wt.320 000) was obtained from Aldrich. The chloroformwas obtained from Aldrich and has a purity of 99.8%.The sample with the patterned ITO layer was spun-castwith one drop of a 0.5% (w/w) solution of PnBMA inchloroform. Samples were spun at 6000 rpm for 30 sduring solution casting. Curve Ðtting was performedusing VISION software supplied by the manufacturer.

RESULTS AND DISCUSSION

Lateral di†erential charging in thick PnBMA Ðlms

We have previously demonstrated that spatial varia-tions in x-ray Ñux cross the analysis area contribute sig-niÐcantly to di†erential charging in bulk insulators.3Without proper charge compensation, lateral di†eren-tial charging dominates the observed spectra andimages. Similar e†ects are observed for samples contain-ing thick Ðlms of PnBMA on a variety of substrates.

The data presented here were obtained from samplesconsisting of PnBMA Ðlms on soda-lime glass andsoda-lime glass with a 27 nm overlayer of ITO. Bothsamples were grounded to the sample stage as describedin the experimental section. Large-area XPS surveyspectra from both samples (Fig. 1) show only C and Ofrom the PnBMA, and there is no observable photoelec-tron signal from the glass or ITO substrates. Carbon 1sspectra, acquired from both samples without any chargecompensation, are shown in Fig. 2. Each spectrum wasacquired after D35 min of constant x-ray exposure inorder to reduce dynamic sample charging. The C 1sspectra (Fig. 2) are shifted toward higher binding ener-gies and have an asymmetric tail on the lower bindingenergy side. The overall shift toward high binding ener-gies results from the development of a positive surfacepotential, because no charge compensation was used.The asymmetric tail on the lower binding energy side ofthe C 1s peak occurs as a result of lateral di†erentialcharging, as previously described for a Si 2p signal froma bulk piece of glass analyzed without charge com-pensation.3 One interesting di†erence between the twoC 1s spectra in Fig. 2 is the di†erence in magnitude ofthe shift toward a higher binding energy. The C 1sphotopeak for PnBMA on the glass is shifted by D35.5

Figure 1. Survey spectra from samples consisting of PnBMAfilms spun-cast on: (a) soda-lime glass ; (b) soda-lime glass con-taining a 27 nm ITO overlayer.

Figure 2. Carbon 1s spectra obtained without charge com-pensation from samples containing PnBMA films spun-cast on:(a) soda-lime glass ; (b) soda-lime glass containing a 27 nm ITOoverlayer.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 904È912 (1997)

906 BRIAN J. TIELSCH AND JULIA E. FULGHUM

eV as compared to D14 eV for the PnBMA on ITO,with respect to a C 1s binding energy of 285 eV. Thisresult is examined more closely in the next section.

Carbon 1s and O 1s spectra were also acquired fromeach sample with the charge compensation systemturned on. Spectra acquired at the beginning and theend of the experiments are overlaid in Figs 3 and 4. Thespectra have not been binding energy corrected. Thecomparable peak shapes and intensities show that x-rayÑux was relatively constant throughout the experimentand that an insigniÐcant amount of sample degradationoccurred. The peak shapes also indicate that e†ectivecharge compensation exists under these conditions.Parameters obtained after curve Ðtting the spectra agreewell with those obtained by Beamson and Briggs5 forsamples consisting of thin Ðlms on bare Si substrates, asshown in Table 1. This agreement supports the observ-ation that no signal was detected from the substrates,both of which contain oxygen species that would other-wise complicate the O 1s spectra.

Binding energy shifts of PnBMA on di†erent substrates

The C 1s spectra in Fig. 2 di†er in binding energy by[20 eV as a result of the development of a positivesurface potential in the PnBMA Ðlms. There are severalpossible explanations for this di†erence.

Because no charge compensation was utilized, varia-tion in x-ray Ñux would cause a di†erence in thebinding energy shifts observed. This must be consideredbecause the overall surface potential can be described asthe di†erence between electrons leaving the sample andthose impinging the sample surface, whether by internalor external sample processes.6 The x-ray Ñux has a

Figure 3. Carbon 1s (a) and O 1s (b) spectra from the PnBMAoverlayer on soda-lime glass. The overlaid spectra were collectedbefore and after the experiments, demonstrating minimal x-raydegradation to the PnBMA.

Table 1. Curve-Ðtting comparison for PnBMAa

C 1s O 1s

Curve-fit results 1 2 3 4 1 2

PnBMA reference 5

BE (eV) 285.00 285.71 286.60 288.89 532.07 533.50

FWHM 1.13 0.90 1.19 0.96 1.20 1.36

Area % 63 13 13 12 49 51

% Gaussian 86 98 100 83 80 100

PnBMA on SiO2

BE (eV) 285.00 285.65 286.63 288.87 531.98 533.46

FWHM 1.06 0.94 1.14 0.91 1.13 1.34

Area % 59.39 14.50 14.17 11.94 45.69 54.31

% Gaussian 90 90 90 90 100 80

PnBMA on ITO

BE (eV) 285.00 285.68 286.64 288.87 531.97 533.44

FWHM 1.08 0.96 1.17 0.94 1.14 1.35

Area % 59.13 14.76 13.95 12.15 45.40 54.60

% Gaussian 90 90 90 90 100 80

PnBMA on patterned sample

BE (eV) 285.00 285.66 286.63 288.84 – –

FWHM 1.13 1.00 1.24 1.09 – –

Area % 58.23 15.57 15.12 11.08 – –

% Gaussian 90 90 90 90 – –

a All binding energies are listed with respect to 285 eV, defined for the aliphaticportion of PnBMA.

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 904È912 (1997) ( 1997 by John Wiley & Sons, Ltd.


Figure 4. Carbon 1s (a) and O 1s (b) spectra from the PnBMAoverlayer on the ITO-coated soda-lime glass. The overlaid spectrawere collected at the beginning and end of the experiments,demonstrating minimal x-ray degradation to the PnBMA.

direct e†ect on the number of electrons leaving thesample and must be carefully controlled to make anyreasonable conclusions.

Variation in PnBMA Ðlm thickness will also a†ectthe results. For example, experiments performed byClark et al.7 demonstrate a relationship between Ðlmthickness and the overall extent of charging in Ðlmscovering electrically grounded gold substrates. ClarkÏswork showed an increase in the surface potential as Ðlmthickness increased for polyparaxylene Ðlms on goldsubstrates that were mounted grounded to the spectro-meter. Beamson et al.8 also demonstrated the care thatmust be taken when analyzing thin Ðlms ofpoly(methylmethacrylate) (PMMA) on conducting sub-strates. Optimum sample thickness conditions, thepossibility of overlayerÈsubstrate interactions and theire†ect on the spectra were described. For example,Beamson et al. observed a shift of the CxO componentof PMMA relative to the main peak of the C 1senvelope for thin Ðlms due to Si/PMMA interactions.Careful sample preparation is thus required in order tomake conclusions about the di†erences in sample charg-ing shown in Fig. 2.

The third possible explanation of the binding energyshifts is the di†erence in the photoelectron cross-sections of the substrates. Substrates with larger photo-electron cross-sections may be expected to moree†ectively charge-neutralize an insulating overlayer.6Because of the di†erences in the photoelectron cross-

sections of the components in the substrates used above,this is more carefully examined below.

The di†erence in the relative conductivities of thesubstrates must also be considered. The conductivity ofthe substrate could a†ect the charging observed in anoverlayer. This possibility has been previously suggestedby Barr.9

Three substrates were chosen for a qualitative com-parison of the e†ects of photoelectron cross-sectionsand conductivities. Films of PnBMA on Ag foil, Al foiland glass were prepared as described in the experimen-tal section. Substrate size and spin-coating parameterswere carefully controlled during sample preparation,and the resulting polymer Ðlms are assumed to be ofsimilar thickness. The Ag and Al foils are both conduc-tors but have photoelectron cross-sections that di†er bymore than an order of magnitude. Relative total photo-electron cross-sections reported by ScoÐeld for Ag andAl are 37.33 and 1.35, respectively.10 Although thephotoelectron cross-sections for the Al foil and glasssubstrate (1.87 for Si) are similar, their conductivitiesare quite di†erent because the glass is an insulator. Theresulting sample set contains PnBMA layers of compa-rable thickness on substrates with di†erent conductivi-ties and photoemission cross-sections.

Survey spectra acquired from each sample, with thecharge compensation system on, are shown overlaid inFig. 5. Each spectrum resembles that of bulk PnBMA.There is no apparent substrate signal from the threesamples and the background levels are nearly identical.The similar spectral intensities demonstrate that thex-ray Ñux upon each sample was the same during theexperiments and should have no e†ect on observed dif-ferences in sample charging.

Survey spectra acquired from each sample withoutany charge compensation are shown in Fig. 6. Eachspectrum was acquired after approximately 30 min ofcontinuous x-ray exposure in order to achieve near-static sample charging conditions. The dynamic sample

Figure 5. Overlay of three survey spectra from samples contain-ing a PnBMA overlayer spun-cast on Ag foil, Al foil and bulksoda-lime glass.



Figure 6. Overlay of three survey spectra obtained withoutcharge compensation from samples consisting of a PnBMA over-layer spun-cast on: (a) Ag foil ; (b) Al foil ; (c) bulk soda-limeglass.

charging at this time was \0.3 eV min~1. The overallcharging observed in the PnBMA overlayers on Ag andAl foils is similar. The C 1s peak from the overlayer onAl is D1.6 eV higher than that on Ag. Additionalexperiments are required to determine if this di†erencein charging is reproducible. The C 1s peak shift forPnBMA on glass, however, is considerably higher andshows a di†erence of D23 eV relative to that from thePnBMA on the Al substrate. These results o†er somepreliminary evidence that, as would be expected, sampleconductivity is a signiÐcant factor in the chargingobserved in overlayers. It is interesting to note that theC 1s peak shifts in the PnBMA on the conducting Agand Al substrates are very similar to that from thePnBMA on the ITO substrate, which has a shift that isonly D0.2 eV larger than that from the Ag sample. Itshould also be noted that there was insigniÐcant sampledamage during the course of these experiments basedon a comparison of spectra before and after the contin-uous exposure to x-rays.

Obtaining substrate information from overlayer charging

The above results can be utilized in the analysis of het-erogeneous samples coated with insulating layers usinga combination of small-area and imaging XPS. Thepattern and composition of the sample used for thisexperiment are illustrated in Fig. 7. The sample consistsof four sets of boxes, which were etched through theITO overlayer to the underlying insulating glass. Thesample was spun cast with a 0.5% w/w solution ofPnBMA in chloroform, providing a thinner overlayerthan used in the previous section. The sample wasmounted for XPS analysis with the ITO overlayer inelectrical contact with the sample holder, as shown inFig. 7. This provides a sample with both a conductive

Figure 7. An illustration showing the top view (a) and the sideview (b) of a sample consisting of a patterned ITO layer on a glasssubstrate. The top view shows four boxes etched through the ITOto the glass, as well as the approximate analysis area. The sideview shows the sample after spin casting with a PnBMA overlayer.

ITO pattern and areas of insulating glass underneaththe PnBMA overlayer.

This sample was initially analyzed with the chargecompensation on. A survey spectrum from the sample isshown in Fig. 8. As with the other samples, only photo-electron peaks from the PnBMA overlayer areobserved. The di†erence in the background signal, ascompared to Figs 1 and 5, results from inelasticallyscattered substrate photoelectrons, indicating that thisis a thinner Ðlm, as expected. A C 1s spectrum is shownin Fig. 9. Curve-Ðtting results are shown in Table 1 andare comparable to results from the thicker overlayers.The C 1s image obtained with charge compensation isshown in Fig. 10. The image Ðeld (1.1 mm2) is largerthan the monochromatic x-ray spot. Under propercharge compensation conditions only the PnBMA over-layer is observed, and the image reÑects the x-ray dis-tribution from the monochromatic x-ray source.

Survey spectra obtained without charge com-pensation are shown in Fig. 11. These spectra wereacquired in consecutive 1 min periods and demonstratethe development of a static sample charge within thePnBMA overlayer. The positive surface potential isrelatively small, as reÑected by the small peak shift(D1È2 eV) toward a higher binding energy, comparedto that shown in Fig. 6 (D14È16 eV for Ag and Al, and35.5 eV for the glass substrate). Because x-ray Ñux wasrelatively constant during the experiments, this di†er-ence is ascribed to the di†erence in overlayer thickness.



Figure 8. Survey spectrum from the sample illustrated in Fig. 7,showing C 1s and O 1s photoelectron peaks from the PnBMA.

As mentioned earlier, additional experiments arerequired to further evaluate Ðlm thickness e†ects.

A C 1s spectrum acquired without charge com-pensation is shown in Fig. 12. The spectrum is distortedcompared to the charge-compensated PnBMA C 1sregion shown in Fig. 9. It is possible to discern two setsof C 1s peaks, beginning at 286.2 and 290.1 eV, whicheach resemble the charge-compensated PnBMA peakshape. The two sets of peaks result from the di†erent

Figure 9. A C 1s spectrum from the sample shown in Fig. 7.Curve-fitting results, provided in Table 1, are similar to thoseobtained for bulk PnBMA.

Figure 10. A C 1s photoelectron image from the sample in Fig. 7,showing a uniform PnBMA distribution. Image contrast along theouter regions is indicative of the x-ray flux non-uniformity withinthe image field of view.

conductivities of the substrate areas that lie within theXPS analysis area. To check this assumption, XPSimages were collected using binding energies of 286.2and 290.1 eV, labeled in Fig. 12. Images acquired ateach binding energy are shown in Figs 13 and 14. Theimage in Fig. 13 shows a pattern corresponding to thatof the underlying conductive ITO (see Fig. 7(a)). Theimage acquired at a binding energy of 290.1 eV showsintensity arising from areas over the glass substrate. Thesubstrate pattern is thus apparent from the overlayerimage if di†erential charging e†ects are utilized.

It is also possible to obtain small-area spectra frompoints within the image Ðeld of view. Two points from

Figure 11. Overlay of two consecutively acquired survey spectrafrom the sample shown in Fig. 7. No charge compensation wasused during data acquisition.



Figure 12. A C 1s spectrum obtained from the sample shown inFig. 7. No charge compensation was used during data acquisition.

which spectral data were collected are shown in Fig. 13.Portions of survey spectra collected without chargecompensation are shown overlaid in Fig. 15. There aretwo important features in these data. First, there is nonoticeable signal from the di†erent substrates thatwould give rise to the image contrast shown in Figs 13and 14. Secondly, it is possible to distinguish a peakshift between the C and O signals from the two points.As expected, signal from the polymer over the glass sub-

Figure 13. A C 1s photoelectron map obtained using a bindingenergy of 286. 2 eV from the sample shown in Fig. 7. The intensitydistribution, obtained without charge compensation, is indicativeof the underlying ITO pattern.

Figure 14. A C 1s photoelectron map obtained using a bindingenergy of 290.1 eV from the sample shown in Fig. 7. The intensitydistribution, obtained without charge compensation, is indicativeof the underlying glass substrate.

strate is at a slightly higher binding energy than fromthe polymer over the ITO. It is interesting to note thatthe overall shift between peaks over the two substratesis not as large as that seen in Fig. 2. The proximity ofthe peaks from the patterned sample can be explainedby the thinner overlayer and, potentially, by macro-scopic electrostatic considerations as discussed byCazaux.11 Cazaux has described the potential at anyone point in the sample as being deÐned by the overall

Figure 15. Overlay of small-area (100 mm spot size) surveyspectra acquired from points labeled 1 and 2 in Fig. 13. No chargecompensation was used.



Figure 16. Overlay of small-area (100 lm spot size) C 1s spectrafrom points labeled 1 and 2 in Fig. 13. No charge compensationwas used.

macroscopic potentials within the sample. Using thisdescription, it is likely that the grounded ITO has ane†ect upon the potential of the glass and hence that ofthe polymer above the glass substrate. We have pre-

Figure 17. Small-area (100 lm spot size) C 1s spectra frompoints labeled 1(a) and 2(b) in Fig. 13. The spectra were obtainedusing charge compensation, and resemble the spectra of bulkPnBMA.

viously demonstrated the e†ect of grounded ITO on thesurface potential of neighboring glass.4 During analysiswithout charge compensation, photoelectron peak posi-tions from glass adjacent to ITO were at lower bindingenergies compared to glass further from the ITO. Anadditional possibility is that photoelectrons from theITO could be scattered into areas above the glass.Either case would lower the potential at any pointabove the glass and decrease the separation between thepeaks over the di†erent substrates, as observed. Carbon1s spectra acquired at points 1 and 2 are shown over-laid in Fig. 16. The photopeak from point 1, over theITO, is located at lower binding energy than the photo-peak from point 2. While the peak envelopes are some-what distorted by the di†erential charging, the PnBMApeak shapes are discernible. The peak component frompoint 2 located at a binding energy of 286.2 eV can beexplained by photoelectrons elastically scattered intothe collection angle from areas above the ITO. Thisintensity should decrease as the analysis area or collec-tion angle is decreased.

Small-area spectra were also acquired from points 1and 2 in Fig. 13 using charge compensation. Spot size(100 lm) C 1s spectra from each point are shown in Fig.17. The spectra are representative of bulk PnBMA,demonstrating negligible sample degradation during theexperiment.

CONCLUSIONS

Non-uniform x-ray Ñux across the sample surface is asigniÐcant factor in the development of di†erentialcharging in relatively thick insulating layers. The mag-nitude of the positive surface potential in thin overlay-ers is, however, also a†ected by the properties of theunderlying substrate. Analysis of Ag, Al and glass sub-strates coated with PnBMA shows less charging in thePnBMA overlay on the conducting substrates than onthe insulating substrate. The binding energy shifts in thePnBMA coatings on Ag and Al were similar, while theshift in the overlayer on insulating glass was signiÐ-cantly greater (D23 eV). The overlayer thickness, sub-strate conductivity and x-ray Ñux will all a†ect thecharging that is observed without charge compensation.The substrate conductivity e†ects could be observed inan insulating PnBMA overlayer by using a combinationof small-area spectroscopy and imaging XPS. Thisapproach should be useful in the characterization ofheterogeneous samples. We are currently carrying outadditional studies to both quantify these e†ects andcharacterize the applicability to other sample types andinstruments.

Acknowledgements

The authors would like to acknowledge David Surman and ChuckBryson for helpful discussions. This work is partially supported by theNSF Science and Technology Center for Advanced Liquid CrystallineOptical Materials (ALCOM) under DMR89-20147 and NSF CHE-9631702, and by the W. M. Keck Foundation.



REFERENCES

1. J. D. Miller, W. C. Harris and G. W. Zajac, Surf . Interface Anal .20, 977 (1993).

2. S. J. Simko, D. P. Griffis, R. W. Murray and R. W. Linton,Anal . Chem. 57, 137 (1985).

3. B. J. Tielsch and J. E. Fulghum, Surf . Interface Anal . 24, 422(1996).

4. B. J. Tielsch, J. E. Fulghum and D. J. Surman, Surf . InterfaceAnal . 24, 459 (1996).

5. G. Beamson and D. Briggs, High Resolution XPS of OrganicPolymers . Wiley, New York (1992).

6. R. T. Lewis and M. A. Kelly, J . Electron Spectrosc . Relat .Phenom. 20, 105 (1980).

7. D. T. Clark, A. Dilks , H. R. Thomas and D. Shuttleworth, J.Polym.Sci . Chem. Ed. 17, 627 (1979).

8. G. Beamson, A. Bunn and D. Briggs, Surf . Interface Anal . 17,105 (1991).

9. T. L. Barr, J . Vac. Sci . Technol . A7, 1677 (1989).10. J. H. Scofield, J. Electron Spectrosc . 8, 129 (1976).11. J. Cazaux and P. Leheude, J. Electron Spectrosc. Relat .

Phenom. 59, 49 (1992).


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