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NIST data resources for surface analysis by X-ray photoelectron spectroscopy and Auger electron spectroscopy
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 1097–1102 www.elsevier.nl / locate / elspec
NIST data resources for surface analysis by X-ray photoelectron spectroscopy and Auger electron spectroscopy a, b c d a C.J. Powell *, A. Jablonski , A. Naumkin , A. Kraut-Vass , J.M. Conny , J.R. Rumble Jr.a a
National Institute of Standards and Technology, Gaithersburg, MD 20899 -8370, USA Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44 /52, 01 -224 Warsaw, Poland c A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Science, 28 Vavilov Street, 117813 Moscow, Russia d 10946 Baskerville Road, Reistertown, MD 21136, USA b
Received 8 August 2000; received in revised form 7 September 2000; accepted 9 September 2000
Abstract A description is given of data resources that are available from the National Institute of Standards and Technology (NIST) for X-ray photoelectron spectroscopy (XPS) and Auger-electron spectroscopy. NIST currently has three databases available: an XPS Database, an Electron Elastic-Scattering Cross-Section Database, and an Electron Inelastic-Mean-Free-Path Database. NIST also offers Standard Test Data (STD) for XPS, a set of simulated XPS data designed to evaluate algorithms and procedures for detecting, locating, and measuring the intensities of overlapping peaks in a doublet. The XPS database and the XPS-STD are available over the internet. 2001 Elsevier Science B.V. All rights reserved. Keywords: Auger-electron spectroscopy; Databases; Electron elastic-scattering cross sections; Electron inelastic mean free paths; Standard test data; Surface analysis; X-ray photoelectron spectroscopy
1. Introduction Reliable data are needed for many applications in science and technology. The National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA currently offers three databases for applications in surface analysis and surface science [1] 1 . These databases are intended principally for use with X-ray photoelectron spectroscopy (XPS) and Auger*Corresponding author. Tel.: 11-301-975-2534; fax: 11-301-2161134. E-mail address: [email protected] (C.J. Powell). 1 Further information on the NIST databases can be obtained from the following internet address: http: / / www.nist.gov / srd.
electron spectroscopy (AES) but are also useful for other surface-sensitive spectroscopies in which electron beams are employed, electron microprobe analysis, electron-beam lithography, and radiation physics. The three NIST databases are: • X-Ray Photoelectron Spectroscopy Database (SRD 20) • Electron Elastic-Scattering Cross-Section Database (SRD 64) • Electron Inelastic-Mean-Free-Path Database (SRD 71) NIST has also developed a set of Standard Test
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00252-8
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C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
Data (STD) for XPS that can be used to assess algorithms and procedures for determining the positions and intensities of overlapping peaks in a doublet 2 . The databases and the XPS-STD will be described in turn.
2. X-ray photoelectron spectroscopy database (SRD 20) Version 3.0 of this database, released in 2000, is available for on-line access through the internet and is free 3 . It contains a substantial amount of new data and additional information (for the new data) on the specimen material, the measurement conditions, and the data-analysis procedure for each reported measurement. Table 1 shows the main search and display options of Version 3.0. New features of Version 3.0 are the ability to retrieve data for all compounds containing a selected element, to retrieve data for surface and interface core-level shifts, and to search the database using citation information. As in previous versions, the database can be used to identify unknown spectral lines, retrieve data for selected elements, retrieve data for selected compounds or classes of compounds, and display Wagner plots. Work is ongoing to provide additional evaluated data for the database. Version 3.0 contains over 19 000 line positions, chemical shifts, doublet splittings, and energy separations of photoelectron and Auger-electron spectral lines. It has an expanded set of descriptive codes for the newly added data that are used to describe general features of the specimen material. Information is provided on the specimen temperature (at the time of the XPS measurement) and on methods which may have been used to determine the surface composition and the surface crystallinity. The X-ray source for the XPS measurement is identified and the overall energy resolution is given. Information is given on the type of curve-fitting function that may have been used for peak location, the full-width at 2
Further information on the XPS-STD can be obtained from the following internet address (from which the XPS-STD can be downloaded and individual results uploaded for analysis): http: / / www.acg.nist.gov / std. 3 Version 3.0 of the NIST XPS database can be accessed through the following internet address: http: / / srdata.nist.gov / xps /.
Table 1 Main search and display options for Version 3.0 of the NIST XPS database Identify unknown spectral lines (photoelectron line, Auger-electron line, Auger parameter, doublet separation) Retrieve data for selected elements Binding energy Auger kinetic energy Chemical shift Surface / interface core-level shift Elemental reference Retrieve data for selected compounds Selected groups of elements Selected element in a compound Chemical name Chemical Classes Inorganic Organic Ligand Other Display Wagner plot Retrieve data by scientific citation
half-maximum intensity of the peak, whether a background function had been utilized, and the uncertainty of peak location (if this information was provided in the data source). Finally, comments are included where necessary to describe specific details of the specimen morphology and processing history. The database screens have been redesigned to facilitate user access, user searches, and user convenience.
3. Electron elastic-scattering cross-section database (SRD 64) Version 2.0 of this database was released in 2000 and is now available without charge [1] 1 . This database, to be used on a personal computer (PC), provides differential and total elastic-electron-scattering cross-sections for elements with atomic numbers from 1 to 96 and for electron energies between 50 and 20 000 eV (in steps of 1 eV) [2]. The differential cross-sections were computed from relativistic theory, and can be presented in three different coordinate systems. These cross-sections can be displayed on the screen of the PC, and changes can
C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
be observed as the electron energy is varied. Files can be created with cross-sections for specified elements, energies, and coordinates. Differential cross-sections for different elements or for the same element at different energies can be graphically compared. Fig. 1 is an example of such a comparison. Random number generators can be created for particular cross-sections, and these provide polar scattering angles for use in Monte Carlo simulations of electron transport in solids (e.g., signal-electron transport in XPS and AES). Differential cross-sections from the random number generators can be compared with the differential cross-sections from the database. The database also provides phase shifts and transport cross-sections for elements with atomic numbers from 1 to 96 and for electron energies between 50 and 20 000 eV (in steps of 1 eV). The variation of the transport cross-section with electron energy for a
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selected element can be graphically displayed, and files can be created with transport cross-sections or phase shifts for specified elements and energies. Transport cross-sections are useful in XPS and AES for the calculation of effective attenuation lengths (needed for the determination of overlayer-film thicknesses) [3], for calculation of mean escape depths (measures of surface sensitivity) [4], for calculation of emission depth distributions for the signal of interest [4,5], and for correcting signal intensities for the effects of elastic-electron scattering [6,7].
4. Electron inelastic-mean-free-path database (SRD 71) Version 1.0 of this database was released in 1999 and an enhanced version (Version 1.1) will be released in 2000; it is also available without charge
Fig. 1. Comparison of differential elastic-electron-scattering cross sections from the NIST Electron Elastic-Scattering Cross-Section Database for gold at 50 eV (solid line), 100 eV (dotted line), 200 eV (dashed line), and 400 eV (dot-dashed line).
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[1] 1 . This database, to be used on a PC, contains inelastic mean free paths (IMFPs) calculated from experimental optical data for certain elements and compounds and IMFPs measured by elastic-peak electron spectroscopy for certain elements [8]. If no calculated or measured IMFPs are available for a material of interest, values can be estimated from the predictive formulae of Tanuma et al. [9] or Gries [10]. IMFPs can be displayed graphically or as values for one or more user-specified electron energies. The IMFPs can be presented in different units ˚ nm, or mg / m 2 ) and in linear, semi-logarithmic, (A, or logarithmic displays. Files containing IMFPs for selected materials can be created, and IMFPs for different materials or from different sources can be graphically compared. IMFPs are needed in AES and XPS for quantitative analyses (correction of matrix effects), calcula-
tion of effective attenuation lengths [3], determination of mean escape depths [4], determination of specimen morphology, and film thicknesses (from analysis of spectral shapes) [11,12], and for Monte Carlo simulations of the transport of signal electrons. A comparison of IMFPs for silicon from the database is given in Fig. 2. The solid line shows IMFPs measured by elastic-peak electron spectroscopy [13] and the dotted line shows IMFPs calculated from optical data [14].
5. Standard test data for XPS Standard test data (STD) are simulations of instrument responses that can be used to assess dataanalysis procedures involving different algorithms and different operator choices. NIST has developed a
Fig. 2. Comparison of inelastic mean free paths for Si from the NIST Electron Inelastic-Mean-Free-Path Database: IMFPs measured by elastic-peak electron spectroscopy by Gergely et al. [13] (solid line) and IMFPs calculated from experimental optical data by Tanuma et al. [14] (dotted line).
C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
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Fig. 3. Factorial design for various conditions of modeled doublets based on C 1s spectra for polymers [15,16]. For each spectrum, intensity (counts per channel) is shown as a function of binding energy (increasing to the left). Factor 1 (component peak separation) and factor 2 (relative peak height) have three levels. Factor 3 (fractional Poisson noise) has two levels.
set of STD for XPS that analysts can use for detecting, locating, and measuring the intensities of overlapping peaks in a doublet [15,16]. The XPSSTD simulate varying degrees of peak overlap, varying relative intensities, and varying levels of random noise. These XPS-STD were constructed by adding scaled spectra (derived from carbon 1s spectra for polymers) according to a factorial design. Fig. 3 shows the XPS-STD doublet spectra [15,16]. The XPS-STD have been assessed by 20 analysts who used different software and different equations for fitting the component peaks of each spectrum [15,16]. While most spectra in the XPS-STD were doublets, some were singlets. Analysts therefore had to decide whether a particular spectrum was a doublet or a singlet, but this decision was frequently not made correctly [15]. Analysts were asked to report peak positions and peak intensities. These results have been analyzed to give information on the bias and the random error for particular spectra in the XPS-STD set and for particular curve-fitting equations [15,16]. The XPS-STD can be downloaded and individual results can be uploaded for analysis and comparison with results from the 20 analysts.2
Acknowledgements The authors are grateful to C.D. Wagner, J.W. Allison, D.M. Blakeslee, and A.Y. Lee for their many contributions to the NIST XPS Database.
References [1] C.J. Powell, J.R. Rumble Jr., D.M. Blakeslee, M.E. DalFavero, A. Jablonski, S. Tougaard, Characterization and metrology for ULSI technology, in: D.G. Seiler, A.C. Diebold, W.M. Bullis, T.J. Shaffner, R. McDonald, E.J. Walters (Eds.), American Institute of Physics, New York, 1998, p. 887. [2] A. Jablonski, S. Tougaard, Surf. Interface Anal. 22 (1994) 129. [3] C.J. Powell, A. Jablonski. J. Electron Spectrosc. Relat. Phenom. (this volume). [4] A. Jablonski, C.J. Powell, J. Electron Spectrosc. Relat. Phenom. 100 (1999) 137. [5] A. Jablonski, S. Tougaard, Surf. Interface Anal. 26 (1998) 374. [6] A. Jablonski, I.S. Tilinin, J. Electron Spectrosc. 74 (1995) 207. [7] A. Jablonski, C.J. Powell, J. Vac. Sci. Tech. A 15 (1997) 2095.
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[8] C.J. Powell, A. Jablonski, J. Phys. Chem. Ref. Data 28 (1999) 19. [9] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 (1994) 165. [10] W.H. Gries, Surf. Interface Anal. 24 (1996) 38. [11] S. Tougaard, Surf. Interface Anal. 26 (1998) 249. [12] W.S.M. Werner, Phys. Rev. B 52 (1995) 2964. [13] G. Gergely, A. Konkol, M. Menyhard, B. Lesiak, A. Jablonski, D. Varga, J. Toth, Vacuum 48 (1997) 621.
[14] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 17 (1991) 911. [15] J.M. Conny, C.J. Powell, L.A. Currie, Surf. Interface Anal. 26 (1998) 939. [16] J.M. Conny, C.J. Powell, Surf. Interface Anal. 29 (2000) 444.
NIST data resources for surface analysis by X-ray photoelectron spectroscopy and Auger electron spectroscopy a, b c d a C.J. Powell *, A. Jablonski , A. Naumkin , A. Kraut-Vass , J.M. Conny , J.R. Rumble Jr.a a
National Institute of Standards and Technology, Gaithersburg, MD 20899 -8370, USA Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44 /52, 01 -224 Warsaw, Poland c A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Science, 28 Vavilov Street, 117813 Moscow, Russia d 10946 Baskerville Road, Reistertown, MD 21136, USA b
Received 8 August 2000; received in revised form 7 September 2000; accepted 9 September 2000
Abstract A description is given of data resources that are available from the National Institute of Standards and Technology (NIST) for X-ray photoelectron spectroscopy (XPS) and Auger-electron spectroscopy. NIST currently has three databases available: an XPS Database, an Electron Elastic-Scattering Cross-Section Database, and an Electron Inelastic-Mean-Free-Path Database. NIST also offers Standard Test Data (STD) for XPS, a set of simulated XPS data designed to evaluate algorithms and procedures for detecting, locating, and measuring the intensities of overlapping peaks in a doublet. The XPS database and the XPS-STD are available over the internet. 2001 Elsevier Science B.V. All rights reserved. Keywords: Auger-electron spectroscopy; Databases; Electron elastic-scattering cross sections; Electron inelastic mean free paths; Standard test data; Surface analysis; X-ray photoelectron spectroscopy
1. Introduction Reliable data are needed for many applications in science and technology. The National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA currently offers three databases for applications in surface analysis and surface science [1] 1 . These databases are intended principally for use with X-ray photoelectron spectroscopy (XPS) and Auger*Corresponding author. Tel.: 11-301-975-2534; fax: 11-301-2161134. E-mail address: [email protected] (C.J. Powell). 1 Further information on the NIST databases can be obtained from the following internet address: http: / / www.nist.gov / srd.
electron spectroscopy (AES) but are also useful for other surface-sensitive spectroscopies in which electron beams are employed, electron microprobe analysis, electron-beam lithography, and radiation physics. The three NIST databases are: • X-Ray Photoelectron Spectroscopy Database (SRD 20) • Electron Elastic-Scattering Cross-Section Database (SRD 64) • Electron Inelastic-Mean-Free-Path Database (SRD 71) NIST has also developed a set of Standard Test
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00252-8
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C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
Data (STD) for XPS that can be used to assess algorithms and procedures for determining the positions and intensities of overlapping peaks in a doublet 2 . The databases and the XPS-STD will be described in turn.
2. X-ray photoelectron spectroscopy database (SRD 20) Version 3.0 of this database, released in 2000, is available for on-line access through the internet and is free 3 . It contains a substantial amount of new data and additional information (for the new data) on the specimen material, the measurement conditions, and the data-analysis procedure for each reported measurement. Table 1 shows the main search and display options of Version 3.0. New features of Version 3.0 are the ability to retrieve data for all compounds containing a selected element, to retrieve data for surface and interface core-level shifts, and to search the database using citation information. As in previous versions, the database can be used to identify unknown spectral lines, retrieve data for selected elements, retrieve data for selected compounds or classes of compounds, and display Wagner plots. Work is ongoing to provide additional evaluated data for the database. Version 3.0 contains over 19 000 line positions, chemical shifts, doublet splittings, and energy separations of photoelectron and Auger-electron spectral lines. It has an expanded set of descriptive codes for the newly added data that are used to describe general features of the specimen material. Information is provided on the specimen temperature (at the time of the XPS measurement) and on methods which may have been used to determine the surface composition and the surface crystallinity. The X-ray source for the XPS measurement is identified and the overall energy resolution is given. Information is given on the type of curve-fitting function that may have been used for peak location, the full-width at 2
Further information on the XPS-STD can be obtained from the following internet address (from which the XPS-STD can be downloaded and individual results uploaded for analysis): http: / / www.acg.nist.gov / std. 3 Version 3.0 of the NIST XPS database can be accessed through the following internet address: http: / / srdata.nist.gov / xps /.
Table 1 Main search and display options for Version 3.0 of the NIST XPS database Identify unknown spectral lines (photoelectron line, Auger-electron line, Auger parameter, doublet separation) Retrieve data for selected elements Binding energy Auger kinetic energy Chemical shift Surface / interface core-level shift Elemental reference Retrieve data for selected compounds Selected groups of elements Selected element in a compound Chemical name Chemical Classes Inorganic Organic Ligand Other Display Wagner plot Retrieve data by scientific citation
half-maximum intensity of the peak, whether a background function had been utilized, and the uncertainty of peak location (if this information was provided in the data source). Finally, comments are included where necessary to describe specific details of the specimen morphology and processing history. The database screens have been redesigned to facilitate user access, user searches, and user convenience.
3. Electron elastic-scattering cross-section database (SRD 64) Version 2.0 of this database was released in 2000 and is now available without charge [1] 1 . This database, to be used on a personal computer (PC), provides differential and total elastic-electron-scattering cross-sections for elements with atomic numbers from 1 to 96 and for electron energies between 50 and 20 000 eV (in steps of 1 eV) [2]. The differential cross-sections were computed from relativistic theory, and can be presented in three different coordinate systems. These cross-sections can be displayed on the screen of the PC, and changes can
C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
be observed as the electron energy is varied. Files can be created with cross-sections for specified elements, energies, and coordinates. Differential cross-sections for different elements or for the same element at different energies can be graphically compared. Fig. 1 is an example of such a comparison. Random number generators can be created for particular cross-sections, and these provide polar scattering angles for use in Monte Carlo simulations of electron transport in solids (e.g., signal-electron transport in XPS and AES). Differential cross-sections from the random number generators can be compared with the differential cross-sections from the database. The database also provides phase shifts and transport cross-sections for elements with atomic numbers from 1 to 96 and for electron energies between 50 and 20 000 eV (in steps of 1 eV). The variation of the transport cross-section with electron energy for a
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selected element can be graphically displayed, and files can be created with transport cross-sections or phase shifts for specified elements and energies. Transport cross-sections are useful in XPS and AES for the calculation of effective attenuation lengths (needed for the determination of overlayer-film thicknesses) [3], for calculation of mean escape depths (measures of surface sensitivity) [4], for calculation of emission depth distributions for the signal of interest [4,5], and for correcting signal intensities for the effects of elastic-electron scattering [6,7].
4. Electron inelastic-mean-free-path database (SRD 71) Version 1.0 of this database was released in 1999 and an enhanced version (Version 1.1) will be released in 2000; it is also available without charge
Fig. 1. Comparison of differential elastic-electron-scattering cross sections from the NIST Electron Elastic-Scattering Cross-Section Database for gold at 50 eV (solid line), 100 eV (dotted line), 200 eV (dashed line), and 400 eV (dot-dashed line).
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C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
[1] 1 . This database, to be used on a PC, contains inelastic mean free paths (IMFPs) calculated from experimental optical data for certain elements and compounds and IMFPs measured by elastic-peak electron spectroscopy for certain elements [8]. If no calculated or measured IMFPs are available for a material of interest, values can be estimated from the predictive formulae of Tanuma et al. [9] or Gries [10]. IMFPs can be displayed graphically or as values for one or more user-specified electron energies. The IMFPs can be presented in different units ˚ nm, or mg / m 2 ) and in linear, semi-logarithmic, (A, or logarithmic displays. Files containing IMFPs for selected materials can be created, and IMFPs for different materials or from different sources can be graphically compared. IMFPs are needed in AES and XPS for quantitative analyses (correction of matrix effects), calcula-
tion of effective attenuation lengths [3], determination of mean escape depths [4], determination of specimen morphology, and film thicknesses (from analysis of spectral shapes) [11,12], and for Monte Carlo simulations of the transport of signal electrons. A comparison of IMFPs for silicon from the database is given in Fig. 2. The solid line shows IMFPs measured by elastic-peak electron spectroscopy [13] and the dotted line shows IMFPs calculated from optical data [14].
5. Standard test data for XPS Standard test data (STD) are simulations of instrument responses that can be used to assess dataanalysis procedures involving different algorithms and different operator choices. NIST has developed a
Fig. 2. Comparison of inelastic mean free paths for Si from the NIST Electron Inelastic-Mean-Free-Path Database: IMFPs measured by elastic-peak electron spectroscopy by Gergely et al. [13] (solid line) and IMFPs calculated from experimental optical data by Tanuma et al. [14] (dotted line).
C. J. Powell et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1097 – 1102
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Fig. 3. Factorial design for various conditions of modeled doublets based on C 1s spectra for polymers [15,16]. For each spectrum, intensity (counts per channel) is shown as a function of binding energy (increasing to the left). Factor 1 (component peak separation) and factor 2 (relative peak height) have three levels. Factor 3 (fractional Poisson noise) has two levels.
set of STD for XPS that analysts can use for detecting, locating, and measuring the intensities of overlapping peaks in a doublet [15,16]. The XPSSTD simulate varying degrees of peak overlap, varying relative intensities, and varying levels of random noise. These XPS-STD were constructed by adding scaled spectra (derived from carbon 1s spectra for polymers) according to a factorial design. Fig. 3 shows the XPS-STD doublet spectra [15,16]. The XPS-STD have been assessed by 20 analysts who used different software and different equations for fitting the component peaks of each spectrum [15,16]. While most spectra in the XPS-STD were doublets, some were singlets. Analysts therefore had to decide whether a particular spectrum was a doublet or a singlet, but this decision was frequently not made correctly [15]. Analysts were asked to report peak positions and peak intensities. These results have been analyzed to give information on the bias and the random error for particular spectra in the XPS-STD set and for particular curve-fitting equations [15,16]. The XPS-STD can be downloaded and individual results can be uploaded for analysis and comparison with results from the 20 analysts.2
Acknowledgements The authors are grateful to C.D. Wagner, J.W. Allison, D.M. Blakeslee, and A.Y. Lee for their many contributions to the NIST XPS Database.
References [1] C.J. Powell, J.R. Rumble Jr., D.M. Blakeslee, M.E. DalFavero, A. Jablonski, S. Tougaard, Characterization and metrology for ULSI technology, in: D.G. Seiler, A.C. Diebold, W.M. Bullis, T.J. Shaffner, R. McDonald, E.J. Walters (Eds.), American Institute of Physics, New York, 1998, p. 887. [2] A. Jablonski, S. Tougaard, Surf. Interface Anal. 22 (1994) 129. [3] C.J. Powell, A. Jablonski. J. Electron Spectrosc. Relat. Phenom. (this volume). [4] A. Jablonski, C.J. Powell, J. Electron Spectrosc. Relat. Phenom. 100 (1999) 137. [5] A. Jablonski, S. Tougaard, Surf. Interface Anal. 26 (1998) 374. [6] A. Jablonski, I.S. Tilinin, J. Electron Spectrosc. 74 (1995) 207. [7] A. Jablonski, C.J. Powell, J. Vac. Sci. Tech. A 15 (1997) 2095.
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[8] C.J. Powell, A. Jablonski, J. Phys. Chem. Ref. Data 28 (1999) 19. [9] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 21 (1994) 165. [10] W.H. Gries, Surf. Interface Anal. 24 (1996) 38. [11] S. Tougaard, Surf. Interface Anal. 26 (1998) 249. [12] W.S.M. Werner, Phys. Rev. B 52 (1995) 2964. [13] G. Gergely, A. Konkol, M. Menyhard, B. Lesiak, A. Jablonski, D. Varga, J. Toth, Vacuum 48 (1997) 621.
[14] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 17 (1991) 911. [15] J.M. Conny, C.J. Powell, L.A. Currie, Surf. Interface Anal. 26 (1998) 939. [16] J.M. Conny, C.J. Powell, Surf. Interface Anal. 29 (2000) 444.