- Email: [email protected]
Reference data for Auger electron spectroscopy and X-ray photoelectron spectroscopy combined
Applied Surface Science 144–145 Ž1999. 161–167
Reference data for Auger electron spectroscopy and X-ray photoelectron spectroscopy combined M.P. Seah
)
Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK
Abstract Recent reference work at NPL, in which measurements for Auger electron spectroscopy ŽAES. and X-ray photoelectron spectroscopy ŽXPS. are combined, is reviewed. For the energy calibration of both AES and XPS instruments new tables have been derived. These tables extend the existing work and homogenise it with new data for the X-ray energies and calculations of the X-ray lineshapes. For the intensity calibration of these instruments, new software has been developed incorporating many spectra as a reference base. In order to develop our understanding of the theory for the emitted intensities, elemental AES and XPS databases have been compiled. These allow a number of improvements in existing theory to produce a good convergence between theory and experiment. In all aspects, the combination of the two techniques is necessary to realise the full potential of the data and correlations required. Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 82.80.Pv; 79.20.Fv; 79.20.Hx; 79.60.Cn Keywords: AES; XPS; Reference data; Sensitivity factors; Intensities; Quantification
1. Introduction Since the initial developments of Auger electron spectroscopy ŽAES. and X-ray photoelectron spectroscopy ŽXPS. as surface analytical tools in the late 1960s, the amount of experimental work common to both techniques has not been extensive. Theoretical work in understanding chemical shifts w1,2x and general properties of electron transport w3,4x, on the other hand, have been carefully considered in relation to both forms of electron spectroscopy. However, there is much in common between the two
)
Tel.: q44-181-943-6634; Fax: q44-181-943-6453; E-mail: [email protected]
methods that may be used to advantage to provide data not available from one method in isolation. It is fortunate that many modern instruments may be used for both techniques and so, in the present work we shall describe a set of experiments, using one instrument, to generate reference data for both AES and XPS. We shall first describe recent measurements of reference energies for calibrating the energy scales of AES and XPS instruments. In this study the reference zero point for the AES energy scale could only be obtained by using the XPS data first. For relating spectral intensities to theoretical cross-sections it is important to calibrate the intensityrenergy response functions of instruments. Here absolute intensity measurements are only possible using AES
0169-4332r99r$ - see front matter Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 8 8 - 0
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M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
since there is no general and accurate method of measuring the X-ray flux densities at the sample in XPS. Thus, again, certain comparisons are only fully possible in AES and not XPS, although they are important for XPS. As we shall show, on the other hand, some of the parameters needed are best established from XPS rather than AES. These problems have now been resolved by developing extensive data banks of both AES and XPS spectra using a fully calibrated spectrometer.
Table 1 Recommended binding energies, eV, for peaks for a spectrometer of 0.2 eV to 0.4 eV resolution and an angle of emission of 458 Peak
Al mono
Mg
Al
Au 4f 7r 2 Ag 3d 5r 2 Cu 2p 3r 2
83.96"0.01 368.21"0.01 932.62"0.02
83.95"0.01 368.22"0.01 932.62"0.02
83.95"0.01 368.22"0.01 932.63"0.02
At 08 angle of emission add 7 meV and at 568 subtract 7 meV. The uncertainties represent the one standard deviation value of the measurements excluding the uncertainty of establishing the Fermi energy, after Seah et al. w7x.
2. Energy scales The positions of peaks on the energy scales for AES and XPS are illustrated in Fig. 1. The binding energies of peaks for reference core levels are given by EB , referred to a determination of the energy, XFL, of X-ray excited electrons from the Fermi level. The kinetic energies of Auger electrons are given by EA referred to the energy, FL, of the Fermi level. In practice, FL is not directly accessible and its position may only be deduced from the energy of XFL minus the photon energy hn . Thus, the AES reference energies may only be deduced as a result of prior XPS reference work. Realisation of the simple scheme described above is, unfortunately, more complex than it may seem at
Fig. 1. XPS spectrum for Cu showing the photoelectron peaks, X, the Auger electron peaks, A, the Fermi level, FL, and the photoemission from the Fermi level, XFL, after Seah and Smith w5x.
first sight. The position of XFL is best determined using a monochromated XPS system to record the position of the silver Fermi edge w6,7x. However, for monochromators, we do not know hn exactly since it can be within "0.2 eV of the nominal setting w8x. Instead, we measure the binding energy of the Au 4f 7r2 peak with respect to the Ag Fermi edge using the monochromated X-rays and then deduce the relative positions of the relevant XPS reference peaks of Cu, Ag and Au, for calibrating XPS instruments, by convolution of the lineshapes measured using a monochromator, with theoretical lineshapes for unmonochromated X-rays. This analysis allows us to relate measurements with unmonochromated sources, where the X-ray energies have been accurately determined w9x, to data calculated for an ideal monochromator aligned at the K a 1 , X-ray energy. The result of this study provides Table 1 w7x for calibrating the binding energy scales of XPS instruments, with all three common X-ray sources, when operated at resolutions in the range from 0.2 eV to 0.4 eV. The electron angle of emission is defined at 458 since small energy shifts are observed with the angle of emission, arising from the intensity of the surface core level shifts. Table 1 is consistent with earlier work w10x but involves a reduction in the binding energies, cited earlier, of approximately 0.04 eV arising from the re-establishment of the Fermi level position using silver. Having defined XFL accurately for the K a 1 component of the X-ray energies, FL may be defined as indicated in Fig. 1 and the Auger electron calibration energies determined. These are given in Table 2 w11x and are approximately 0.07 eV greater than the values published earlier w12x. The AES energies are
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
163
Table 2 Recommended calibration kinetic energies for Auger electron peaks referred to the Fermi level in the direct mode at high resolution for nŽ E . and EnŽ E . spectra using 5 keV or 10 keV electron beam energies Transition
Kinetic energy, eV a
Cu M 2,3VV Au N6,7VV a Ag M 4 NN Cu L 3VV Al KL 2,3 L 2,3 Au M 5 N6,7 N6,7 5 keV, nŽ E . 5 keV, EnŽ E . 10 keV, nŽ E . 10 keV, EnŽ E .
62.37"0.02"0.03 71.23"0.02"0.03 357.88"0.01"0.02 918.69"0.01"0.02 1393.09"0.01"0.04 2015.80"0.03"0.04 2015.81"0.03"0.04 2015.79"0.03"0.04 2015.79"0.03"0.04
The uncertainties represent the one standard deviation values for the measurement repeatability and the traceability, respectively. A further "0.03 eV arises from the uncertainty of the X-ray establishment of FL, after Seah w11x. a For the copper and gold low energy doublets the single value represents the average of the two peak positions found by the tangent intercept method w11,12x.
defined, as for XPS, by a parabola fit to the top of the appropriate peak except for the low energy Cu and Au doublets. Here a tangent is used to the peaks in the doublet and the average of the tangent contact points is determined. Note that the Au M 5 N6,7 N6,7 peak is weak and its peak energy depends on the underlying background slope. Four archetypal conditions are, therefore, given to define this energy. Use of these new values will change the energies given in tables where high accuracy is appropriate but will, of course, not alter the relative energies of peaks or values of Auger parameters.
Fig. 2. Intensityrenergy response function for a VG Scientific ESCALAB II with a 5 CEM 210 analyser at 50 eV pass energy with 6 mm input and output slits, after Seah w13x.
spectra to remove their instrument intensityrenergy response function and derive spectra in the correct units of sry1 eVy1 . If we integrate the resulting spectra over energy, as we would in evaluating peak areas, the result is in units of sry1 , i.e., the number of emitted electrons per unit solid angle for each incident electron. An example of a calibrated intensityrenergy response function is given in Fig. 2. Examples of the reference spectra and of typical
3. Intensity scales Here, we start with AES rather than XPS since the flux of the incident radiation is easily measured in AES and the emitted spectrum is dominated by a smooth background rather than intense peaks. By a series of studies, standard AES spectra were established for Cu, Ag and Au using a 5 keV electron beam at 308 to the average surface normal w13x. These have been built into a software calibration system w14x that allows users to re-calibrate their
Fig. 3. Reference XPS spectra for Cu at 54.78 between the X-ray direction and the photoelectron direction, Ža. unmonochromated Al X-rays, Žb. monochromated Al X-rays, Žc. spectrum Ža. with satellites removed and intensity consolidated into the K a 1 line, minus spectrum Žb..
164
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
response functions are given in Refs. w13,15x, respectively. With the above-calibrated instrument we have generated reference XPS spectra for unmonochromated Al and Mg X-rays and also for monochromated Al X-rays to calibrate the intensityrenergy response function of XPS instruments w13x. Examples of the unmonochromated and monochromated Al X-ray reference spectra for copper are shown in Fig. 3. Unlike AES, the experimental spectra cannot simply be ratioed to these reference spectra to obtain the intensityrenergy response functions, since the XPS process is more complex than that for AES. Thus, in the software calibration system for XPS w16x there are many more spectra and algorithms to produce the intensityrenergy response function than for AES. An interesting aspect, also shown in Fig. 3, is that the Bremsstrahlung contribution to the unmonochromated spectrum, given by the lowest spectrum, spectrum Žc., adds a total electron flux of only 15% to that generated by the characteristic K X-rays. In the past, Briggs cites w17x, that it was often believed that monochromators degraded samples less because there was no Bremsstrahlung contribution. However, it is clear from Fig. 3 that the main source of sample degradation will either be the flux density of characteristic X-rays, mainly through low energy electrons generated in the sample, or heat or electrons from the X-ray source. In addition to the reference spectra for calibration, spectra have been recorded for the elements shown in Fig. 4 for AES and somewhat fewer for XPS. These form digital databases. The digital Auger data base is described in some detail elsewhere w18,19x.
Fig. 4. Elements whose spectra are available in elemental or compound form in the high resolution digital Auger database. The symbols circumscribed with an O are available in both elemental and oxide form, after Seah and Gilmore w18x.
All of these spectra are recorded with good signal levels for widescans at a resolution of 1 eV with 1 eV step intervals and narrowscans, from 10 eV higher kinetic energy than any peaks to 50 eV lower energy, at a resolution of 0.25 eV with 0.1 eV step intervals.
4. Reference spectra The start of our programme of reference data was for AES spectra recorded at 5 keV and 10 keV beam energies w18x. From these spectra, to deduce the peak areas, we needed to subtract the inelastically scattered primary electrons, the inelastically scattered Auger electrons and the cascade secondary electrons. The inelastically scattered primary electrons were modelled according to Jousset and Langeron’s w20x analysis which gives a spectrum shape of the form, expŽ E r E1 ., where E is the kinetic energy and E1 is a constant for that element. Analysis of many elements gave E1 s 1988.4 " 148.2 eV for a 5 keV beam but a more complex function for the 10 keV beam w21x. This allowed removal of the inelastically backscattered primary electrons. Next, the Tougaard background w22x was removed and finally the secondary electron cascade w23x. In our original study w18x, the standard values of Tougaard’s parameters B and C, of 2866 eV 2 and 1643 eV 2 , were used. However, a study of 30 metals by XPS shows that if C is taken as 1643 eV 2 , B must be increased to 3006.1 " 38.5 eV 2 w19x. Fig. 5 shows the XPS spectrum for Sm using unmonochromated Al K a X-rays with C s 1643 eV 2 and B increasing in 5% intervals from 2866 to 3006 and 3156 eV 2 . The 3006 eV 2 value gives an excellent fit until the presence of the cascade electrons is seen at kinetic energies below 400 eV. It is clear from Fig. 5 that B may be determined with an accuracy of 1%, consistent with the scatter noted above for the 30 metals studied. The effect of the change in B value is generally small and, for AES where a further background is removed, reduces the measured Ag and Cu intensities by only 3%. However, for Au, the Auger electron peaks of which cover 1200 eV, the reduction in peak area in AES is 32% w24x. Thus, again, the XPS data are needed to interpret the AES data.
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
165
well as any electrons in the 4f shell w19x. The results in Fig. 6 look excellent with an average ratio of experiment to theory of 1.04, indicating that the total net bias of all of the parameters used in both theory and experiment is surprisingly small. This ratio is based on the Auger electrons with peak energies above 150 eV shown in Fig. 6Ža.. At lower energies it is difficult to define the peak area accurately and the measurement precision deteriorates. As seen in Fig. 5, the definition of peak areas in XPS is much easier than it is for AES, although one could use the AES peaks obtained in XPS in many
Fig. 5. Reference XPS spectrum for Sm using unmonochromated Al X-rays, curve Ža. spectrum corrected for instrument intensityrenergy response function, Žb. after removal of a Tougaard background with B s 2866 eV 2 and C s1643 eV 2 , Žc. as Žb. but B s 3006 eV 2 , Žd. as Žb. but B s 3156 eV 2 .
When all backgrounds have been removed, the final AES peak areas may be deduced in units of sry1 , with values in the range from 10y5 to 10y2 . Integrated over the full solid angle the range of intensities is from 10y4 to 10y2 . This final result is the total probability of Auger electron emission from any given shell per incident electron. We have calculated these intensities from first principles, incorporating such terms as the loss of intensity arising from X-ray production, the crosssection for ionisation described by Casnati et al. w25x, the backscattering factor of Shimizu w26x, the inelastic mean free path formula, TPP-2M of Tanuma et al. w27x and the elastic scattering correction of Jablonski w28x. In both theory and experiment it is not easy to determine the intensities of individual peaks and so, in both cases, areas are determined for the total intensity from each shell of a given principal quantum number. Experimental and theoretical results for 10 keV electrons are shown in Fig. 6. Correlations of the data w29x show that Casnati et al.’s w25x cross-section describes the results much more accurately than that of Gryzinski w30x. Further analysis shows that a cut-off should be applied to the number of valence electrons used in the TPP-2M formula and that cut-off is usefully set to exclude both electrons of more than 14 eV binding energy as
Fig. 6. Auger electron intensities summed over all contributions for a given shell, for a 10 keV electron beam, Žq. K shell, ŽU . L shell, Ž`. M shell and Ž=. N shell, Ža. experimental data with peaks below 150 eV separated and Žb. theoretical calculations, after Seah and Gilmore w19x.
166
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
cases to do this by scaling and matching via, say, the differential spectra. As for AES, we may calculate XPS intensities from first principles, incorporating the cross-section for ionisation described by Scofield w31x, the anisotropy parameters of Yeh and Lindau w32x, the inelastic mean free path formula, TPP-2M, of Tanuma et al. w27x and the elastic scattering correction of Jablonski w28x. In Fig. 7, we show the experimental data for the peak areas, after correction of the instrument intensityrenergy response function and removal of the Tougaard background with parameters as discussed above, together with the theoretical calculations. Note that, unlike Fig. 6, the ordinate axis in Fig. 7Ža. has
been scaled by a factor of 6.64 = 10y7 since we cannot measure the X-ray flux at the sample. Thus, there is one degree of freedom in comparing the data. Both parts of Fig. 7 are for Mg K a X-rays. The data are recorded using unmonochromated Xrays and the satellite structure is then removed by deconvolution before removing the Tougaard background. Similar data sets are recorded for both monochromated and unmonochromated Al X-rays. There is an excellent fit between experiment and theory apart from one remaining scaling factor, typically in the range from 0.8 to 1.2 which affects all peaks for a given element. This remaining systematic error is thought to arise mainly from the use of the Universal Tougaard loss function to describe all elements w34,35x. Analyses, as above, for the experimental data using the popular Shirley background w36x lead to very much larger scatters between theory and experiment, as seen earlier by Jansson et al. w37x.
5. Conclusions Reference spectra have been derived for AES and XPS by considering both spectroscopies simultaneously. For energy calibration, new tables of peak energies are derived which are consistent with current measurements of X-ray energies and X-ray line structures whilst maintaining traceability to earlier tables. For the intensity calibration new data are available, integrated with systems for determining the instrument characteristics. Calibration of an instrument at NPL has enabled major AES and XPS elemental databases to be established. These provide reference data for quantitative analysis as well as effective tests of basic theory in order to provide an improved understanding of the calculation of intensities and an improved correlation between theory and experiment.
Acknowledgements Fig. 7. X-ray photoelectron intensities for Mg K a X-rays, ŽU . p levels, Ž`. d levels and Ž=. f levels, Ža. experimental data divided by 1.506=10 6 and Žb. theoretical calculations, after Seah and Gilmore w33x.
The author would like to thank H.E. Bishop, P.J. Cumpson, I.S. Gilmore, G. Lorang, S.J. Spencer and M. Tosa for assistance with work included in this review. This work is partly supported by the UK
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
Department of Trade and Industry’s National Measurement System Valid Analytical Measurement Programme and partly by the Standards Measurement and Testing Programme of the European Commission. References w1x P. Weightman, Rep. Prog. Phys. 45 Ž1982. 753. w2x J.C. Riviere, ` J.A.A. Crossley, G. Moretti, Surf. Interface Anal. 14 Ž1989. 257. w3x A. Jablonksi, S. Tougaard, Surf. Interface Anal. 26 Ž1998. 17. w4x W.S.M. Werner, Surf. Interface Anal. 26 Ž1998. 455. w5x M.P. Seah, G.C. Smith, in: D. Briggs, M.P. Seah ŽEds.., Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1990, p. 531. w6x A.C. Miller, C.J. Powell, U. Gelius, C.R. Anderson, Surf. Interface Anal. 26 Ž1998. 606. w7x M.P. Seah, I.S. Gilmore, G. Beamson, Surf. Interface Anal. 26 Ž1998. 642. w8x M.P. Seah, I.S. Gilmore, S.J. Spencer, Surf. Interface Anal. 26 Ž1998. 617. w9x J. Schweppe, R.D. Deslattes, T. Mooney, C.J. Powell, J. Elec. Spectrosc. 67 Ž1994. 463. w10x M.P. Seah, Surf. Interface Anal. 14 Ž1989. 488. w11x M.P. Seah, J. Elec. Spectrosc., to be published. w12x M.P. Seah, I.S. Gilmore, J. Elec. Spectrosc. 83 Ž1997. 197. w13x M.P. Seah, J. Elec. Spectrosc. 71 Ž1995. 191. w14x NPL System for the Intensity Calibration of Auger Electron Spectrometers—Version A.1, NPL, Teddington, Middlesex TW11 0LW, UK or internet site: http:www.npl.co.ukr nplrcmmtrsisrindex.html.
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w15x M.P. Seah, G.C. Smith, Surf. Interface Anal. 17 Ž1991. 855. w16x NPL System for the Intensity Calibration of X-ray Photoelectron Spectroscopy—Version X.I, to be available from NPL, Teddington, Middlesex TW11 0LW, UK or internet site: http:www.npl.co.ukrnplrcmmtrsisrindex.html. w17x D. Briggs, in: D. Briggs, M.P. Seah ŽEds.., Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1990, p. 437. w18x M.P. Seah, I.S. Gilmore, J. Vac. Sci. Technol. A 14 Ž1996. 1401. w19x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 908. w20x D. Jousset, J.P. Langeron, J. Vac. Sci. Technol. A 5 Ž1997. 989. w21x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 723. w22x S. Tougaard, Surf. Interface Anal. 11 Ž1988. 453. w23x E.N. Sickafus, Phys. Rev. B 16 Ž1977. 1436. w24x M.P. Seah, Surf. Interface Anal. 24 Ž1996. 830. w25x E. Casnati, A. Tartari, C. Baraldi, J. Phys. B 15 Ž1982. 155. w26x R. Shimizu, Jpn. J. Appl. 22 Ž1983. 1631. w27x S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 Ž1994. 165. w28x A. Jablonski, Surf. Interface Anal. 23 Ž1995. 29. w29x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 815. w30x M. Gryzinski, Phys. Rev. A 138 Ž1965. 336. w31x J.H. Scofield, J. Elec. Spectrosc. 8 Ž1976. 129. w32x J.J. Yeh, I. Lindau, Atomic Data and Nucl. Data Tables 32 Ž1985. 1. w33x M.P. Seah, I.S. Gilmore, to be published. w34x M.P. Seah, Surf. Sci., to be published. w35x M.P. Seah, I.S. Gilmore, to be published. w36x D.A. Shirley, Phys. Rev. B 5 Ž1972. 4709. w37x C. Jansson, S. Tougaard, G. Beamson, D. Briggs, S.F. Davies, A. Rossi, R. Hauert, G. Hobi, N.M.D. Brown, B.J. Meenan, C.A. Anderson, M. Repoux, C. Malitesta, L. Sabbatini, Surf. Interface Anal. 23 Ž1995. 484.
Reference data for Auger electron spectroscopy and X-ray photoelectron spectroscopy combined M.P. Seah
)
Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK
Abstract Recent reference work at NPL, in which measurements for Auger electron spectroscopy ŽAES. and X-ray photoelectron spectroscopy ŽXPS. are combined, is reviewed. For the energy calibration of both AES and XPS instruments new tables have been derived. These tables extend the existing work and homogenise it with new data for the X-ray energies and calculations of the X-ray lineshapes. For the intensity calibration of these instruments, new software has been developed incorporating many spectra as a reference base. In order to develop our understanding of the theory for the emitted intensities, elemental AES and XPS databases have been compiled. These allow a number of improvements in existing theory to produce a good convergence between theory and experiment. In all aspects, the combination of the two techniques is necessary to realise the full potential of the data and correlations required. Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 82.80.Pv; 79.20.Fv; 79.20.Hx; 79.60.Cn Keywords: AES; XPS; Reference data; Sensitivity factors; Intensities; Quantification
1. Introduction Since the initial developments of Auger electron spectroscopy ŽAES. and X-ray photoelectron spectroscopy ŽXPS. as surface analytical tools in the late 1960s, the amount of experimental work common to both techniques has not been extensive. Theoretical work in understanding chemical shifts w1,2x and general properties of electron transport w3,4x, on the other hand, have been carefully considered in relation to both forms of electron spectroscopy. However, there is much in common between the two
)
Tel.: q44-181-943-6634; Fax: q44-181-943-6453; E-mail: [email protected]
methods that may be used to advantage to provide data not available from one method in isolation. It is fortunate that many modern instruments may be used for both techniques and so, in the present work we shall describe a set of experiments, using one instrument, to generate reference data for both AES and XPS. We shall first describe recent measurements of reference energies for calibrating the energy scales of AES and XPS instruments. In this study the reference zero point for the AES energy scale could only be obtained by using the XPS data first. For relating spectral intensities to theoretical cross-sections it is important to calibrate the intensityrenergy response functions of instruments. Here absolute intensity measurements are only possible using AES
0169-4332r99r$ - see front matter Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 8 8 - 0
162
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
since there is no general and accurate method of measuring the X-ray flux densities at the sample in XPS. Thus, again, certain comparisons are only fully possible in AES and not XPS, although they are important for XPS. As we shall show, on the other hand, some of the parameters needed are best established from XPS rather than AES. These problems have now been resolved by developing extensive data banks of both AES and XPS spectra using a fully calibrated spectrometer.
Table 1 Recommended binding energies, eV, for peaks for a spectrometer of 0.2 eV to 0.4 eV resolution and an angle of emission of 458 Peak
Al mono
Mg
Al
Au 4f 7r 2 Ag 3d 5r 2 Cu 2p 3r 2
83.96"0.01 368.21"0.01 932.62"0.02
83.95"0.01 368.22"0.01 932.62"0.02
83.95"0.01 368.22"0.01 932.63"0.02
At 08 angle of emission add 7 meV and at 568 subtract 7 meV. The uncertainties represent the one standard deviation value of the measurements excluding the uncertainty of establishing the Fermi energy, after Seah et al. w7x.
2. Energy scales The positions of peaks on the energy scales for AES and XPS are illustrated in Fig. 1. The binding energies of peaks for reference core levels are given by EB , referred to a determination of the energy, XFL, of X-ray excited electrons from the Fermi level. The kinetic energies of Auger electrons are given by EA referred to the energy, FL, of the Fermi level. In practice, FL is not directly accessible and its position may only be deduced from the energy of XFL minus the photon energy hn . Thus, the AES reference energies may only be deduced as a result of prior XPS reference work. Realisation of the simple scheme described above is, unfortunately, more complex than it may seem at
Fig. 1. XPS spectrum for Cu showing the photoelectron peaks, X, the Auger electron peaks, A, the Fermi level, FL, and the photoemission from the Fermi level, XFL, after Seah and Smith w5x.
first sight. The position of XFL is best determined using a monochromated XPS system to record the position of the silver Fermi edge w6,7x. However, for monochromators, we do not know hn exactly since it can be within "0.2 eV of the nominal setting w8x. Instead, we measure the binding energy of the Au 4f 7r2 peak with respect to the Ag Fermi edge using the monochromated X-rays and then deduce the relative positions of the relevant XPS reference peaks of Cu, Ag and Au, for calibrating XPS instruments, by convolution of the lineshapes measured using a monochromator, with theoretical lineshapes for unmonochromated X-rays. This analysis allows us to relate measurements with unmonochromated sources, where the X-ray energies have been accurately determined w9x, to data calculated for an ideal monochromator aligned at the K a 1 , X-ray energy. The result of this study provides Table 1 w7x for calibrating the binding energy scales of XPS instruments, with all three common X-ray sources, when operated at resolutions in the range from 0.2 eV to 0.4 eV. The electron angle of emission is defined at 458 since small energy shifts are observed with the angle of emission, arising from the intensity of the surface core level shifts. Table 1 is consistent with earlier work w10x but involves a reduction in the binding energies, cited earlier, of approximately 0.04 eV arising from the re-establishment of the Fermi level position using silver. Having defined XFL accurately for the K a 1 component of the X-ray energies, FL may be defined as indicated in Fig. 1 and the Auger electron calibration energies determined. These are given in Table 2 w11x and are approximately 0.07 eV greater than the values published earlier w12x. The AES energies are
M.P. Seah r Applied Surface Science 144–145 (1999) 161–167
163
Table 2 Recommended calibration kinetic energies for Auger electron peaks referred to the Fermi level in the direct mode at high resolution for nŽ E . and EnŽ E . spectra using 5 keV or 10 keV electron beam energies Transition
Kinetic energy, eV a
Cu M 2,3VV Au N6,7VV a Ag M 4 NN Cu L 3VV Al KL 2,3 L 2,3 Au M 5 N6,7 N6,7 5 keV, nŽ E . 5 keV, EnŽ E . 10 keV, nŽ E . 10 keV, EnŽ E .
62.37"0.02"0.03 71.23"0.02"0.03 357.88"0.01"0.02 918.69"0.01"0.02 1393.09"0.01"0.04 2015.80"0.03"0.04 2015.81"0.03"0.04 2015.79"0.03"0.04 2015.79"0.03"0.04
The uncertainties represent the one standard deviation values for the measurement repeatability and the traceability, respectively. A further "0.03 eV arises from the uncertainty of the X-ray establishment of FL, after Seah w11x. a For the copper and gold low energy doublets the single value represents the average of the two peak positions found by the tangent intercept method w11,12x.
defined, as for XPS, by a parabola fit to the top of the appropriate peak except for the low energy Cu and Au doublets. Here a tangent is used to the peaks in the doublet and the average of the tangent contact points is determined. Note that the Au M 5 N6,7 N6,7 peak is weak and its peak energy depends on the underlying background slope. Four archetypal conditions are, therefore, given to define this energy. Use of these new values will change the energies given in tables where high accuracy is appropriate but will, of course, not alter the relative energies of peaks or values of Auger parameters.
Fig. 2. Intensityrenergy response function for a VG Scientific ESCALAB II with a 5 CEM 210 analyser at 50 eV pass energy with 6 mm input and output slits, after Seah w13x.
spectra to remove their instrument intensityrenergy response function and derive spectra in the correct units of sry1 eVy1 . If we integrate the resulting spectra over energy, as we would in evaluating peak areas, the result is in units of sry1 , i.e., the number of emitted electrons per unit solid angle for each incident electron. An example of a calibrated intensityrenergy response function is given in Fig. 2. Examples of the reference spectra and of typical
3. Intensity scales Here, we start with AES rather than XPS since the flux of the incident radiation is easily measured in AES and the emitted spectrum is dominated by a smooth background rather than intense peaks. By a series of studies, standard AES spectra were established for Cu, Ag and Au using a 5 keV electron beam at 308 to the average surface normal w13x. These have been built into a software calibration system w14x that allows users to re-calibrate their
Fig. 3. Reference XPS spectra for Cu at 54.78 between the X-ray direction and the photoelectron direction, Ža. unmonochromated Al X-rays, Žb. monochromated Al X-rays, Žc. spectrum Ža. with satellites removed and intensity consolidated into the K a 1 line, minus spectrum Žb..
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response functions are given in Refs. w13,15x, respectively. With the above-calibrated instrument we have generated reference XPS spectra for unmonochromated Al and Mg X-rays and also for monochromated Al X-rays to calibrate the intensityrenergy response function of XPS instruments w13x. Examples of the unmonochromated and monochromated Al X-ray reference spectra for copper are shown in Fig. 3. Unlike AES, the experimental spectra cannot simply be ratioed to these reference spectra to obtain the intensityrenergy response functions, since the XPS process is more complex than that for AES. Thus, in the software calibration system for XPS w16x there are many more spectra and algorithms to produce the intensityrenergy response function than for AES. An interesting aspect, also shown in Fig. 3, is that the Bremsstrahlung contribution to the unmonochromated spectrum, given by the lowest spectrum, spectrum Žc., adds a total electron flux of only 15% to that generated by the characteristic K X-rays. In the past, Briggs cites w17x, that it was often believed that monochromators degraded samples less because there was no Bremsstrahlung contribution. However, it is clear from Fig. 3 that the main source of sample degradation will either be the flux density of characteristic X-rays, mainly through low energy electrons generated in the sample, or heat or electrons from the X-ray source. In addition to the reference spectra for calibration, spectra have been recorded for the elements shown in Fig. 4 for AES and somewhat fewer for XPS. These form digital databases. The digital Auger data base is described in some detail elsewhere w18,19x.
Fig. 4. Elements whose spectra are available in elemental or compound form in the high resolution digital Auger database. The symbols circumscribed with an O are available in both elemental and oxide form, after Seah and Gilmore w18x.
All of these spectra are recorded with good signal levels for widescans at a resolution of 1 eV with 1 eV step intervals and narrowscans, from 10 eV higher kinetic energy than any peaks to 50 eV lower energy, at a resolution of 0.25 eV with 0.1 eV step intervals.
4. Reference spectra The start of our programme of reference data was for AES spectra recorded at 5 keV and 10 keV beam energies w18x. From these spectra, to deduce the peak areas, we needed to subtract the inelastically scattered primary electrons, the inelastically scattered Auger electrons and the cascade secondary electrons. The inelastically scattered primary electrons were modelled according to Jousset and Langeron’s w20x analysis which gives a spectrum shape of the form, expŽ E r E1 ., where E is the kinetic energy and E1 is a constant for that element. Analysis of many elements gave E1 s 1988.4 " 148.2 eV for a 5 keV beam but a more complex function for the 10 keV beam w21x. This allowed removal of the inelastically backscattered primary electrons. Next, the Tougaard background w22x was removed and finally the secondary electron cascade w23x. In our original study w18x, the standard values of Tougaard’s parameters B and C, of 2866 eV 2 and 1643 eV 2 , were used. However, a study of 30 metals by XPS shows that if C is taken as 1643 eV 2 , B must be increased to 3006.1 " 38.5 eV 2 w19x. Fig. 5 shows the XPS spectrum for Sm using unmonochromated Al K a X-rays with C s 1643 eV 2 and B increasing in 5% intervals from 2866 to 3006 and 3156 eV 2 . The 3006 eV 2 value gives an excellent fit until the presence of the cascade electrons is seen at kinetic energies below 400 eV. It is clear from Fig. 5 that B may be determined with an accuracy of 1%, consistent with the scatter noted above for the 30 metals studied. The effect of the change in B value is generally small and, for AES where a further background is removed, reduces the measured Ag and Cu intensities by only 3%. However, for Au, the Auger electron peaks of which cover 1200 eV, the reduction in peak area in AES is 32% w24x. Thus, again, the XPS data are needed to interpret the AES data.
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well as any electrons in the 4f shell w19x. The results in Fig. 6 look excellent with an average ratio of experiment to theory of 1.04, indicating that the total net bias of all of the parameters used in both theory and experiment is surprisingly small. This ratio is based on the Auger electrons with peak energies above 150 eV shown in Fig. 6Ža.. At lower energies it is difficult to define the peak area accurately and the measurement precision deteriorates. As seen in Fig. 5, the definition of peak areas in XPS is much easier than it is for AES, although one could use the AES peaks obtained in XPS in many
Fig. 5. Reference XPS spectrum for Sm using unmonochromated Al X-rays, curve Ža. spectrum corrected for instrument intensityrenergy response function, Žb. after removal of a Tougaard background with B s 2866 eV 2 and C s1643 eV 2 , Žc. as Žb. but B s 3006 eV 2 , Žd. as Žb. but B s 3156 eV 2 .
When all backgrounds have been removed, the final AES peak areas may be deduced in units of sry1 , with values in the range from 10y5 to 10y2 . Integrated over the full solid angle the range of intensities is from 10y4 to 10y2 . This final result is the total probability of Auger electron emission from any given shell per incident electron. We have calculated these intensities from first principles, incorporating such terms as the loss of intensity arising from X-ray production, the crosssection for ionisation described by Casnati et al. w25x, the backscattering factor of Shimizu w26x, the inelastic mean free path formula, TPP-2M of Tanuma et al. w27x and the elastic scattering correction of Jablonski w28x. In both theory and experiment it is not easy to determine the intensities of individual peaks and so, in both cases, areas are determined for the total intensity from each shell of a given principal quantum number. Experimental and theoretical results for 10 keV electrons are shown in Fig. 6. Correlations of the data w29x show that Casnati et al.’s w25x cross-section describes the results much more accurately than that of Gryzinski w30x. Further analysis shows that a cut-off should be applied to the number of valence electrons used in the TPP-2M formula and that cut-off is usefully set to exclude both electrons of more than 14 eV binding energy as
Fig. 6. Auger electron intensities summed over all contributions for a given shell, for a 10 keV electron beam, Žq. K shell, ŽU . L shell, Ž`. M shell and Ž=. N shell, Ža. experimental data with peaks below 150 eV separated and Žb. theoretical calculations, after Seah and Gilmore w19x.
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cases to do this by scaling and matching via, say, the differential spectra. As for AES, we may calculate XPS intensities from first principles, incorporating the cross-section for ionisation described by Scofield w31x, the anisotropy parameters of Yeh and Lindau w32x, the inelastic mean free path formula, TPP-2M, of Tanuma et al. w27x and the elastic scattering correction of Jablonski w28x. In Fig. 7, we show the experimental data for the peak areas, after correction of the instrument intensityrenergy response function and removal of the Tougaard background with parameters as discussed above, together with the theoretical calculations. Note that, unlike Fig. 6, the ordinate axis in Fig. 7Ža. has
been scaled by a factor of 6.64 = 10y7 since we cannot measure the X-ray flux at the sample. Thus, there is one degree of freedom in comparing the data. Both parts of Fig. 7 are for Mg K a X-rays. The data are recorded using unmonochromated Xrays and the satellite structure is then removed by deconvolution before removing the Tougaard background. Similar data sets are recorded for both monochromated and unmonochromated Al X-rays. There is an excellent fit between experiment and theory apart from one remaining scaling factor, typically in the range from 0.8 to 1.2 which affects all peaks for a given element. This remaining systematic error is thought to arise mainly from the use of the Universal Tougaard loss function to describe all elements w34,35x. Analyses, as above, for the experimental data using the popular Shirley background w36x lead to very much larger scatters between theory and experiment, as seen earlier by Jansson et al. w37x.
5. Conclusions Reference spectra have been derived for AES and XPS by considering both spectroscopies simultaneously. For energy calibration, new tables of peak energies are derived which are consistent with current measurements of X-ray energies and X-ray line structures whilst maintaining traceability to earlier tables. For the intensity calibration new data are available, integrated with systems for determining the instrument characteristics. Calibration of an instrument at NPL has enabled major AES and XPS elemental databases to be established. These provide reference data for quantitative analysis as well as effective tests of basic theory in order to provide an improved understanding of the calculation of intensities and an improved correlation between theory and experiment.
Acknowledgements Fig. 7. X-ray photoelectron intensities for Mg K a X-rays, ŽU . p levels, Ž`. d levels and Ž=. f levels, Ža. experimental data divided by 1.506=10 6 and Žb. theoretical calculations, after Seah and Gilmore w33x.
The author would like to thank H.E. Bishop, P.J. Cumpson, I.S. Gilmore, G. Lorang, S.J. Spencer and M. Tosa for assistance with work included in this review. This work is partly supported by the UK
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Department of Trade and Industry’s National Measurement System Valid Analytical Measurement Programme and partly by the Standards Measurement and Testing Programme of the European Commission. References w1x P. Weightman, Rep. Prog. Phys. 45 Ž1982. 753. w2x J.C. Riviere, ` J.A.A. Crossley, G. Moretti, Surf. Interface Anal. 14 Ž1989. 257. w3x A. Jablonksi, S. Tougaard, Surf. Interface Anal. 26 Ž1998. 17. w4x W.S.M. Werner, Surf. Interface Anal. 26 Ž1998. 455. w5x M.P. Seah, G.C. Smith, in: D. Briggs, M.P. Seah ŽEds.., Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1990, p. 531. w6x A.C. Miller, C.J. Powell, U. Gelius, C.R. Anderson, Surf. Interface Anal. 26 Ž1998. 606. w7x M.P. Seah, I.S. Gilmore, G. Beamson, Surf. Interface Anal. 26 Ž1998. 642. w8x M.P. Seah, I.S. Gilmore, S.J. Spencer, Surf. Interface Anal. 26 Ž1998. 617. w9x J. Schweppe, R.D. Deslattes, T. Mooney, C.J. Powell, J. Elec. Spectrosc. 67 Ž1994. 463. w10x M.P. Seah, Surf. Interface Anal. 14 Ž1989. 488. w11x M.P. Seah, J. Elec. Spectrosc., to be published. w12x M.P. Seah, I.S. Gilmore, J. Elec. Spectrosc. 83 Ž1997. 197. w13x M.P. Seah, J. Elec. Spectrosc. 71 Ž1995. 191. w14x NPL System for the Intensity Calibration of Auger Electron Spectrometers—Version A.1, NPL, Teddington, Middlesex TW11 0LW, UK or internet site: http:www.npl.co.ukr nplrcmmtrsisrindex.html.
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w15x M.P. Seah, G.C. Smith, Surf. Interface Anal. 17 Ž1991. 855. w16x NPL System for the Intensity Calibration of X-ray Photoelectron Spectroscopy—Version X.I, to be available from NPL, Teddington, Middlesex TW11 0LW, UK or internet site: http:www.npl.co.ukrnplrcmmtrsisrindex.html. w17x D. Briggs, in: D. Briggs, M.P. Seah ŽEds.., Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1990, p. 437. w18x M.P. Seah, I.S. Gilmore, J. Vac. Sci. Technol. A 14 Ž1996. 1401. w19x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 908. w20x D. Jousset, J.P. Langeron, J. Vac. Sci. Technol. A 5 Ž1997. 989. w21x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 723. w22x S. Tougaard, Surf. Interface Anal. 11 Ž1988. 453. w23x E.N. Sickafus, Phys. Rev. B 16 Ž1977. 1436. w24x M.P. Seah, Surf. Interface Anal. 24 Ž1996. 830. w25x E. Casnati, A. Tartari, C. Baraldi, J. Phys. B 15 Ž1982. 155. w26x R. Shimizu, Jpn. J. Appl. 22 Ž1983. 1631. w27x S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 Ž1994. 165. w28x A. Jablonski, Surf. Interface Anal. 23 Ž1995. 29. w29x M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 26 Ž1998. 815. w30x M. Gryzinski, Phys. Rev. A 138 Ž1965. 336. w31x J.H. Scofield, J. Elec. Spectrosc. 8 Ž1976. 129. w32x J.J. Yeh, I. Lindau, Atomic Data and Nucl. Data Tables 32 Ž1985. 1. w33x M.P. Seah, I.S. Gilmore, to be published. w34x M.P. Seah, Surf. Sci., to be published. w35x M.P. Seah, I.S. Gilmore, to be published. w36x D.A. Shirley, Phys. Rev. B 5 Ž1972. 4709. w37x C. Jansson, S. Tougaard, G. Beamson, D. Briggs, S.F. Davies, A. Rossi, R. Hauert, G. Hobi, N.M.D. Brown, B.J. Meenan, C.A. Anderson, M. Repoux, C. Malitesta, L. Sabbatini, Surf. Interface Anal. 23 Ž1995. 484.