SURFACE AND INTERFACE ANALYSIS, VOL. 24, 28-33 (1996)

sample swface I

Differential Charging in XPS. Part I: Demonstration of Lateral Charging in a Bulk Insulator Using Imaging XPS

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

Recent literature has drawn a distinction between charging across the surface (lateral differential charging) and charging with depth (vertical differential charging) for insulating samples. In this study, differential charging in a bulk insulator was investigated using imaging and small-area spectra acquired with a monochromatic x-ray source. With no charge neutralization, reproducible photoelectron images and spectra could be acquired from a glass sample after -90 min of x-ray exposure. The charge shifts and photoelectron peak shapes vary as a function of x-ray flux on the sample, indicating that lateral charging is the dominant mechanism on a bulk insulator. Uniform binding energies, photoelectron peak shapes and images are acquired with the charge neutralizer on.

INTRODUCTION

As the development of x-ray photoelectron spectros- copy (XPS) has progressed, significant effort has been expended to evaluate the causes and manifestations of differential charging. In general, differential charging occurs as a result of more than one potential developing within or across the surface of a sample. The photoelec- tron spectra acquired from areas of differing potential contain severely distorted and broadened peak shapes. This can make chemical state identification and quanti- fication difficult or impossible.

Differential charging has been described by Bryson as resulting from the establishment of a potential gradient across the sample surface.' In this case, a positive charge density distribution is assumed to develop on a non-conducting uniform sample upon exposure to x-rays, as shown in Fig. 1, causing lateral differential charging. The most extreme potential gradient would

monochromaled x-ray spot c-3 + + + +++Y+++ +++ + + +

+ + + + + + + + + + + + + + + + + +

+ + +

therefore result from exposure to a focused, monochro- matic x-ray source that has a spot size smaller than the field of view of the lens. The resulting photoelectron spectra are shifted to a higher binding energy than expected, and contain a long tail on the low-binding- energy side (see, for example, Fig. 7). If the charging phenomena is caused primarily by potential variations across the surface of the sample, the highest binding energy component will come from the center of the x-ray spot, because the sample charging should be most significant at the point of highest x-ray flux. The tailing structure is then a result of the decreasing charge gra- dient as one moves away from the center of the x-ray spot. To obtain useful information, a charge neutral- izing system that completely eliminates this surface potential distribution, and hence the differential charg- ing effect, is required.

Potential variations as a function of depth within the sample have been suggested for samples containing thin insulating over layer^.'.^ However, Cazaux and Lehuede4 have calculated that an electric field of lo5 V cm-' results in a change of only 10 mV in 10 k This implies that differential charging in homogenous insula- tors should not have a significant component due to potential variations with depth.

In this study, a monochromatic x-ray source has been used in the analysis of a bulk, homogenous insulator in order to investigate the effect of x-ray flux on differen- tial charging. The use of small-area spectra and photo- electron imaging makes it possible to demonstrate differential charging across the sample surface.

EXPERIMENTAL

Figure 1. Potential gradient that develops on an insulating sample with x-ray exposure.

* Author to whom correspondence should be addressed

CCC 0 142-242 1 /96/0 10028-06 0 1996 by John Wiley & Sons, Ltd

X-ray photoelectron spectroscopy data were acquired using a Kratos AXIS HS with the monochromatic A1 Ka x-ray source operated at 300 W. Survey spectra were acquired in hybrid mode, which uses both electro-

Received 15 May 1995 Accepted 9 August 1995

DIFFERENTIAL CHARGING IN XPS. PART I 29

static and magnetic lenses, at a pass energy of 80 eV. Acquisition times of 1 min and a 40 eV pass energy were used for both the standard and small-area Si 2p and 0 1s spectra. Images were acquired using the mag- netic lens with a 100 pm diameter analysis area, a pass energy of 80 eV and 1 min acquisition times. The image field of view was 1.6 x 1.6 mm and contained 50 x 50 pixels. Images were acquired point by point using elec- trostatic deflection to change the area on the sample from which photoelectrons are collected. Small-area spectra were collected with the same lateral resolution as the images. The charge neutralization system was used with the standard operating conditions: -2.8 V bias voltage, - 1.0 V electron filament voltage and + 2.1 A filament current.

Samples were mounted on standard holders using electrically conductive tape. The only electrical contact between the samples and the spectrometer was through the tape to the back of the sample. The 0.125 mm thick Ag foil was obtained from Goodfellow and had a purity of >99.99%. The foil was sputter cleaned for -7 min using an Ar ion beam rastering an area of 6 x 6 mm, in order to remove carbon contamination. The silica glass was sputter cleaned for 2 min to remove carbon con- taminants and contaminants from cutting the glass. Small-area spectra were processed using a nine-point smooth. Quantitative results were obtained using VISION software provided by the manufacturer. Regions of peak area integration were defined using

linear backgrounds, with endpoints averaged over 7-9 data points.

RESULTS A N D DISCUSSION

Variation in x-ray flux

The most severe differential charging should result from a monochromatic x-ray source that is focused within the field of view of the lens. The x-ray spot size with respect to the imaging field of view is shown in Fig. 2, which is an image of the Ag 3d intensity from a piece of sputter-cleaned Ag foil. There is significant image con- trast within the 1.6 x 1.6 mmz image field. For the pur- poses of this study, it is important to confirm that this image contrast is due to a non-uniform x-ray flux across the image field, and not to differences in sample com- position or variations in x-ray flux during the image acquisition. Survey spectra acquired from 100 pm areas show no variation in carbon or oxygen contamination across the imaged area, as shown in Fig. 3. The loca- tions of these small-area spectra are noted in Fig. 2. High-resolution Ag 3d and C 1s spectra were also col- lected and show no major hydrocarbon contamination to be present. The possibility of temporal variations in x-ray flux during the 2 min acquisition of the Ag image was also evaluated. Three Ag 3d spectra, each acquired

Figure 2. The Ag 3d image of sputter-cleaned Ag foil showing x-ray flux variation within the imaged area of 1.6 x 1.6 mm. The image was acquired in 2 min using the 100 pm aperture and an 80 eV pass energy.

30 B. J. TIELSCH AND J. E. FULGHUM

(a)

I I

I200 1WI 800 60 400 200 0

Binding Energy (eV)

Figure 3. Small-area survey spectra acquired from points 1 (3a), 2 (3b) and 3 (3c) on Fig. 2. No significant C Is or 0 I s peaks are observed.

in 1 min, are shown overlaid in Fig. 4. No significant x-ray flux variation is evident. These experiments thus confirm that the intensity variation in Fig. 2 is due to the spatial variation of x-ray flux from the monochro- matic source. The imaged field is thus larger than the area of uniform x-ray flux, as required for significant lateral differential charging to be observed.

55 .I

380 378 316 374 312 370 368 366 364 362 360

Binding Energy (eV) Figure 4. Overlay of three large-area Ag 3d spectra acquired in consecutive 1 min intervals. No variation in x-ray flux is apparent.

Q 5 8

106 104 IM 100 98 96 94

Binding Energy (eV)

Figure 5. Comparison of (a) 0 I s and (b) Si 2p large-area spectra acquired with the charge neutralizer (CN) on (solid line) and off (dotted line). The spectra acquired with the charge neu- tralizer off have been shifted down in binding energy by -100 eV.

Analysis of an insulating sample with no charge compensation

As a point of reference, 0 1s and Si 2p spectra acquired with the charge neutralizer on are shown in Fig. 5. The spectra are large-area spectra from approximately the entire x-ray irradiated region. The small shoulder on the low-binding-energy side of the 0 Is spectrum, due to the presence of metal oxides, makes it difficult to observe the differential charging, which will cause tailing on the low-binding-energy side of the photoelec- tron peak. For this reason, the differential charging phe- nomenon was exafnined using the less intense but more symmetric Si 2p peak originating from the glass. The 0 1s and Si 2p spectra acquired without the charge neu- tralizer are also included in Fig. 5. Note that the spectra acquired with no charge neutralization have been shifted down in binding energy by -100 eV in Fig. 5. Both spectra show the expected tailing on the low- binding-energy side and a loss of intensity. It is easier, however, to evaluate the differential charging effects on the Si 2p peak.

One additional consideration in the investigation of differential charging effects is the existence of time- dependent charging, resulting in peaks shifting toward higher binding energies during the analysis. As has been discussed previously,’ a steady-state static charge may not be obtained on an insulating surface if no charge compensation is used. Figure 6 shows Si 2p spectra from the glass recorded at 5 min intervals over a time span of -90 min. Although the shift in peak position is initially quite dramatic, after 90 min of x-ray exposure the time-dependent peak shifts become small enough to be manageable, as will be demonstrated below.

Plate 1. The Si 2p images acquired with 1 min acquisition times from points A (a), B (b) and C ( c ) on Fig. 7.

Plate 2. Color overlay of images shown in Plate 1.

DIFFERENTIAL CHARGING IN XPS. PART I 31

1 Moo:

2800:

2600

200 19S 190 185 180 175 170 165

Binding Energy (eV)

2400.:

2200:

2000 7

Figure 6. Large-area Si 2p spectra showing time-dependent charging. Spectra were acquired for 1 min at 5 min intervals: after -90 min, time-dependent charge shifts are reduced to tenths of an electron-volt.

After the time-dependent charge shifts were reduced, data were acquired to evaluate the differential charging with respect to the x-ray flux variation across the sample surface. Figure 7 shows the Si 2p peak acquired after >90 min of exposure to the x-ray source. This large-area spectrum continues to show a broadened peak with a low-binding-energy tail, as in Fig. 5.

4100

3200 A (198.8 ev) I

Y 9 8 v

0 ’P f

J

400:

200 - 2172 ux) 198 1% 194 192 190 188

Binding Energy (eV)

Figure 7. Large-area Si 2p spectrum acquired after -90 min of x-ray exposure. Images were acquired at the binding energies labeled A-C.

Mapping of differential charging effects

The Si 2p images were acquired at the three binding energies indicated in Fig. 7 using the 100 pm aperture. Image acquisition times of 1 min were used to minimize the effects of time-dependent peak shifts. The resulting images are shown in Plate 1. If lateral charging is the predominant mechanism in this sample, the most sig- nificant charging should correspond to the analysis area containing the highest surface potential, which will be the center of the monochromatic, focused x-ray spot as shown in Fig. 2. Plate l(a) was acquired using the binding energy at the peak maxima (‘A’ in Fig. 7), and shows a good correlation with the area of highest x-ray flux. The image acquired using the photoelectron inten- sity from a point on the lower binding energy tail of the Si 2p peak (‘B’ in Fig. 7) should correspond to an area of intermediate potential distribution. The region of highest intensity thus moves out from the center of the x-ray spot, as shown in Plate l(b). The image in Plate l(c), acquired using the photoelectron intensity from point ‘C’, is most intense at the outer edges of the imaging field of view, as would be expected if lateral differential charging is dominant. A color overlay of Plate l(a)-(c) is presented in Plate 2, clearly showing the variation in intensity in the Si images acquired at differ- ent binding energies. A comparison of Si 2p spectra acquired before and immediately after the image acqui- sition shows an overall peak shift of -0.3 eV higher in binding energy. This minimal shift during the analysis will not have resulted in the differing intensity distribu- tions. It is important to note that the overall time- dependent peak shift is consistently moving toward higher binding energies, while the images were acquired in order of decreasing binding energy.

Small-area spectra

Small-area (100 pm) Si 2p spectra were acquired from the points marked 1-3 in Plate 2, beginning with point 1 and ending with point 3. Acquisition times of 1 min were used. This should result in peaks occurring at pro- gressively lower binding energies if the lateral differen- tial charging model is further supported. The order of acquisition is important because the relative peak posi- tions should occur in the opposite direction of the overall time-dependent peak shifts shown in Fig. 6. The small-area Si 2p spectra from points 1-3 are overlaid in Fig. 8. As expected, the peaks appear at progressively lower binding energies due to lower surface potentials in moving from the center of the image in Plate 2 (point 1) to the edge (point 3). The extremely broad peak struc- ture from point 3 is reasonable, considering the lateral resolution (100 pm) used to obtain these spectra. The broadening probably results from a large variation in surface potential at the edges of the x-ray spot. The spectra also show that, despite the differential charging induced during these analysis conditions, it may be pos- sible to obtain useful data without charge neutralization if photoelectrons are acquired from an area of equal surface potential. This is demonstrated in Fig. 11, which compares the Si 2p spectra acquired using the charge neutralization system (Fig. 5 ) and the spectrum

32 B. J. TIELSCH AND J. E. FULGHUM

Binding Energy (eV)

Figure 8. Small-area Si 2p spectra from points 1-3 on Plate 2.

obtained from multipoint 1, located in the center of the x-ray spot, with no charge neutralization. The spectrum from point 1 has been shifted by almost 100 eV in Fig. 9. The normalized spectra are similar in peak width and shape. Thus, if the x-ray flux is made uniform across the entire field of view of the lens or if photoelectrons can be efficiently collected from within a small enough area of the x-ray spot so as to avoid differential charging, it

204 202 200 198 1% 194 192 1W 188

Binding Energy (eV)

Figure 9. Comparison of large-area Si 2p spectrum acquired with the charge neutralizer (CN) on (dotted line) and small-area Si 2p spectrum acquired from point 1 (Plate 2) with the charge neutral- izer off (dotted line). The small-area spectrum has been shifted down in binding energy by -- 100 eV. Spectra are normalized at the binding energy marked by the arrows.

may be possible in certain cases to successfully acquire and interpret data from insulating samples, with no charge compensation. This is, however, difficult to accomplish, as a further evaluation of Fig. 8 shows. A comparison of points 1 and 3 shows an expected decrease in the signal intensity due to the decrease in x-ray flux. However, points 1 and 2 show comparable intensities but clear evidence of differential charging. The small-area spectra provide further evidence that, for this bulk insulator, lateral charging is a more significant problem than charging with depth (vertical charging). If significant vertical charging were contributing to the photoelectron peak, peak broadening and tailing should be observed for all three spectra shown in Fig. 8.

The experiments summarized above were repeated in order to evaluate further the effect of time-dependent binding energy shifts. Spectra were shifted by a few tenths of an electron-volt, but the image appearances and spectral trends are reproducible. This also confirms that the x-ray spot was not drifting during the analysis.

These results are in good agreement with SIMS studies of insulators by Cazaux and L e h e ~ d e . ~ The authors showed that narrower energy distributions were obtained from a glass sample using a defocused ion beam and a small image field than when the ion beam and image field were of comparable size. However, the SIMS results were acquired with the use of an electron flood gun, whereas the XPS results presented here use no charge neutralization.

Insulator analysis with charge compensation

The Si 2p images and small-area spectra were also acquired with the charge neutralizer on. Figure 10 shows large-area Si 2p spectra from three 1 min acquisi- tions. The excellent reproducibility in the peak inten-

106 104 102 100 98 96 94 92 90

Bindmg Energy (eV)

Figure 10. Overlay of large-area Si 2p spectra from three con- secutive 1 min acquisitions.

DIFFERENTIAL CHARGING IN XPS. PART I 33

sities and shapes is important for two reasons. First, it confirms proper charge neutralization of the glass, based on the peak shape and the consistency of the peak position. Additionally, reproducibility of the peak intensities assures that the x-ray flux was constant during the time when the images and the multipoint spectra were acquired.

A Si 2p map was acquired with the charge neutral- ization system on, using the binding energy of the peak maxima in Fig. 10. The resulting image (Fig. 11) is rea- sonably uniform in intensity, with a slight contrast apparent at the outer edges of the image field, as was apparent in the Ag image in Fig. 2. Small-area analyses from the same locations as in Plate 2 were acquired for comparison with Fig. 8. Spectra from points 1-3 are shown overlaid in Fig. 12. The similar peak positions and shape indicate uniform charge neutralization across the image field. The similarity in peak intensities obtained from multipoints 1 and 2 was also observed in the small-area spectra acquired without charge neutral- ization. The decrease in the intensity at point 3 confirms that the image contrast in Fig. 11 is due to non-uniform x-ray flux. Quantitative analysis showed no difference in composition in the three areas.

450

P' '

Figure 11. The Si 2p image acquired in 1 min with the charge neutralizer on. The imaged area is the same as in Plates 1 and 2.

Binding Energy (eV j

Figure 12. Small-area Si 2p spectra acquired from points 1-3 in Plate 2, with the charge neutralizer on.

CONCLUSIONS

These results support a view of differential charging as a result of non-uniform surface potentials due to varia- tions in x-ray flux in the analysis area. Lateral charging thus dominates the observed spectra and images. The images in Plate 1 show that significant image intensity variation can result if effective charge neutralization is not used in the analysis of insulating samples. Images acquired under conditions of insufficient charge neutral- ization may appear to show changes in chemical state or composition, which are in fact due to variations in surface potential.

Acknowledgements

The authors gratefully ackdowledge Chuck Bryson for helpful dis- cussions. This work is partially supported by the NSF Science and Technology Center for Advanced Liquid Crystalline Optical Materials (ALCOM), under DMR89-20147, and the W. M. Keck Foundation.

REFERENCES ~~

1 . C. E. Bryson, I I I , Surf. Sci. 189/190, 50 (1 987). 2. T. L. Barr, J. Vac. Sci. Techno/. A7, 1677 (1 989). 3. X. Yu and H. Hantsche, Surf. lnterface Anal. 20, 555 (1993). 4. J. Cazaux and P. Leheude, J. Electron Specrrosc. Relat.

5. P. Swift, D. Shuttleworth and M. P. Seah, in Practical Surface Analysis. ed. by D. Briggs and M. P. Seah, Appendix 2. Wiley, Chichester (1 990).

Phenom. 59,49 (1 992).