Quantitative Auger electron spectroscopic analysis of carbides,nitrides and carbonitrides

A novel method for the quantitative evaluation of Auger electron spectra based on peak areas is presented. Sample and reference spectra in integral mode are filtered by an area conserving digital filter. This transforms the peak shapes influenced by chemical effects into standard peak shapes. After filtering a linear combination of reference spectra, differentiated spectra accounting for peak shifts and some low order polynomials to account for variations in the background is fitted to the sample spectrum by a least squares method. The need to approximate the spectrum of the secondary electron background explicitly for direct calculation of peak areas is thus eliminated. Filters of different widths are applied to reduce errors by chemical effects. The composition of the sample is computed from the composition of the reference samples and the coefficients obtained from the fit.To demonstrate the validity of this technique it has been applied to both, Gaussian model peaks and spectra of titanium carbonitrides. A further test on an alloy series is under investigation. The results show that the method works as predicted and gives accurate quantification.     

Data Preprocessing in Peak Shape Analysis of Auger Electron Spectra

 Factor analysis is an established method of peak shape analysis in Auger electron spectrometry. The influence of different commonly used data preprocessing tools onto the results of factor analysis is demonstrated on AES depth profiles of multilayers and implantation profiles. For the analysis of Auger electron spectra it has been traditional to differentiate spectra by Savitzky and Golay’s method to remove background and to elucidate changes in peak shape. For phosphorus implanted in titanium it is shown that background removal works not ideal so that inelastic losses of the Ti(LMM) Auger peak can affect the result of factor analysis for the P(LVV) peak located at ca. 250 eV lower in kinetic energy. The contribution of such losses to the background can be corrected by shifting the spectra so that the high energy side above the peak equals zero. Numerical differentiation can introduce correlated error into the data set. To diminish edge effects the reduction of filter width at the edges and cutting off the outermost data points is recommended. The precision of spectrum reproduction is considered as a crucial test for the number of principal components. The reliability factor is investigated as a measure for the goodness of spectrum reproduction.     

Quantitative surface analysis by X-ray photoelectron spectroscopy and X-ray excited auger electron spectroscopy

It is shown that X-ray excited KLL Auger electron spectra allow it to describe measured signal strengths similarly to X-ray photoelectron signals, thus offering valuable information on the quantitative surface composition of a solid sample. The principal equation and corresponding fundamental parameters are discussed. As a result Auger spectra of C, N, O, F, and Na can be easily used in a multiline approach for quantitative analysis. LMM and MNN spectra give rise to more problems, due to their more complicated structure, uncertainties with regard to the background and the influence of Coster-Kronig transitions. These problems are overcome by the use of empirical ratios of the strongest lines of 2p/LMM or 3d/MNN. Since these ratios are independent of sample composition, they allow it to transform the Auger signal into the corresponding photoelectron signal, provided that a standard sample has been measured. Thus a true additional information is obtained and moreover difficulties in cases of photoelectron spectra with overlapping lines from other chemical elements can be overcome.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

A fast and simple method for background removal in Auger electron spectroscopy

A purely formal method for background removal in electron beam induced Auger electron spectroscopy is presented. The method has been developed for practical purposes. It is typically used to remove the background of a complete recorded spectrum, no fit is necessary to remove the backscattered electrons background. An overcompensation of the background, resulting in negative values of the background removed spectrum is not possible, all values of the background removed spectrum are positive or zero. Since the Auger peaks are separated by zeros after background removal, the method is well suited for peak finding and identification.     

A fast and simple method for background removal in Auger electron spectroscopy

A purely formal method for background removal in electron beam induced Auger electron spectroscopy is presented. The method has been developed for practical purposes. It is typically used to remove the background of a complete recorded spectrum, no fit is necessary to remove the backscattered electrons background. An overcompensation of the background, resulting in negative values of the background removed spectrum is not possible, all values of the background removed spectrum are positive or zero. Since the Auger peaks are separated by zeros after background removal, the method is well suited for peak finding and identification.Dedicated to Professor Dr. H. Seiler on the occasion of his 65th birthday     

Quantitative Auger electron spectroscopy of PtSi and Pd2Si

Summary Quantitative AES analysis has been carried out on PtSi and Pd₂Si using dN(E)/dE spectra. The concentration ratios are determined by elemental sensitivity factors after correcting for matrix and sputter effects. In the case of matrix correction an improved correction procedure has been developed which considers effects created by different atomic densities, electron backscattering, electron attenuation and peak shapes. The concentration ratios X_metal/X_Si deduced from measurements of mechanically cleaned sample surfaces agree very well with the nominal bulk values of the silicides. Ion bombardment creates strong intensity and line shape changes of the Auger signals. Therefore, in case of sputtered silicide surfaces, both matrix and sputter effects must be corrected.     

Summary of ISO/TC201 standard: ISO/TR 18394:2006—surface chemical analysis—Auger electron spectroscopy—derivation of chemical information

ISO/TR 18394 provides guidance for the identification of chemical effects on x‐ray or electron‐excited Auger electron spectra as well as for applications of these effects in chemical characterization of surface/interface layers of solids. In addition to elemental composition, information can be obtained on the chemical state and the surrounding local electronic structure of the atom with the initial core hole from the changes of Auger electron spectra upon the alteration of its local environment. The methods of identification and use of chemical effects on Auger electron spectra, as described in this Technical Report, are very important for accurate quantitative applications of Auger electron spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

X-ray and electron induced Auger processes for I2 adsorption on uranium

The accurate determination of the kinetic energy of X-ray induced Auger electrons, which is necessary in XPS experiments, e.g. for calculating the Auger parameter, is sometimes hampered by peak interferences or by the high secondary electron background. The latter is of special importance for low kinetic energy electrons like e.g. the U(OPV) and U(OVV) Auger electrons. These problems can be circumvented by using electron induced Auger transitions (AES). However, since XPS and AES use different reference points for the energy scales, both scales have to be matched. This can be done by measuring the kinetic energy of an appropriate Auger transition in XPS and relating this value to the maximum of the second derivative of the same peak in AES.     

Reminiscences with Martin P. Seah

Starting in the mid-1960s, the detection and display of peaks in Auger electron spectroscopy (AES) were improved by using modulation of the electron energy analyzer coupled with electron detection using a lock-in amplifier. This allowed a derivative of the electron energy distribution, N(E), to be obtained directly at the output of the lock-in amplifier thereby removing most of the effect from the relatively large, slowly varying, electron background signal due to secondary and backscattered electrons. For relatively low modulation amplitudes, the peak-to-peak intensity of the Auger features increased linearly with modulation amplitude (for a deflection-type analyzer), improving the signal-to-noise ratio. However, with relatively large modulations, the Auger peak shapes distorted, and the peak-to-peak heights eventually decreased in size, and this nonlinearity would cause problems in quantitative analysis. A universal curve was developed for singlet Auger peaks to approximate corrections due to this peak distortion, but an approach to exactly correct for such distortions was largely ignored by the AES community. This approach was called Dynamic Background Subtraction and is even relevant today as some Auger instruments using modulation and lock-in amplifiers are still being manufactured. This review paper describes approximate and exact corrections for modulation effects in AES data.     

Physical foundation of quantitative Auger analysis

The possibility of using an Auger peak height in the dN (E) /dE spectrum and an integrated N (E) spectrum as a measure of the Auger current is discussed and necessary relations are presented. The methods of the background determination are reviewed and discussed.

The relation between the Auger current and the atomic cancentration of a corresponding sample component is derived and the state of art in the field of theoretical and experimental determination of factors appearing in this relation (ionization cross-section, Auger transition probability. backscattering factor, and inelastic mean free path of Auger electrons) is presented.

Approaches to the quantitative Auger analysis (QAA) of homogeneous, isotropic samples, including corrections for matrix factors, are presented and discussed. Problems arising when heterogeneous samples are analyzed are discussed and practical approaches to such an analysis are presented.

The role of crystalline effects (the dependence of the Auger signal from crystalline samples on the direction of the primary electron beam and angular distribution of Auger electron emission from such samples) in QAA is discussed and examples of such crystalline effects are presented together with their physical foundation.

Some rules are suggested allowing the quantitative Auger analysis to be performed with the smallest possible error.     

Green's function calculations on the Auger spectra of CO

The Auger spectra of carbon monoxide have been calculated by the Green's function method and the unsolved assignment problems have been studied. Electron correlation plays an important role in all parts of the spectra. The results favour the assignment of the 250 eV peak in the C (KVV) spectrum as mainly 4σ¹5σ¹.     

Radiationless decay in the region of the 2t2g and 4eg resonances in SF6

The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.     

Der trans-Einfluß der Metall-Metall-Bindung in den ESCA-, Röntgenemissions- und Augerspektren von Koordinationsverbindungen

The Trans-Influence of the Metal-Metal Bond in ESCA, X-ray Emission, and Auger Spectra of Coordination Compounds The contribution of different factors to the chemical shift in ESCA, x-ray emission, and Auger spectra is analyzed for the manifestation of the metal-metal bond trans-influence in coordination compounds. In ESCA and x-ray emission spectra the chemical shift is determined in this case by the atomic effective charge. The trans-influence of metal-metal bond is manifested by the decrease in the Kα-line or the core level positive shift of the coordinated ligand atom in comparison with the values in other coordination compounds.     

Site-selected Auger electron spectroscopy of N2O

The N 1s Auger spectra for the two nonequivalent N atoms in N2O have been measured via Auger electron-photoelectron coincidence spectroscopy. The site-selected Auger spectra are compared with the normal Auger spectrum and with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. Such effects are found to be crucial factors in determining the different band shapes in the site-selected spectra.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Radiative auger effect and extended X-ray emission fine structure (EXEFS).

Radiative Auger spectra are weak X-ray emission spectra near the characteristic X-ray lines. Radiative Auger process is an intrinsic energy-loss process in an atom when a characteristic X-ray photon is emitted, due to an atomic many-body effect. The energy loss spectra correspond to the unoccupied conduction band structure of materials. Therefore the radiative Auger effect is an alternative tool to the X-ray absorption spectroscopy such as EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure), and thus it is named EXEFS (Extended X-ray Emission Fine Structure). By the use of a commercially available X-ray fluorescence spectrometer or an electron probe microanalyzer (EPMA), which are frequently used in materials industries, we can obtain an EXEFS spectrum within 20 min. The radiative Auger effect, as an example, demonstrates that the study on atomic many-body effects has become a powerful tool for crystal and electronic structure characterizations. The EXEFS method has already been used in many industries in Japan. Reviews about the applications and basic study results on the radiative Auger effect are reported in this paper.     

High energy resolution Auger spectroscopy on a CMA instrument

An experimental method that increases the analyzer resolution of cylindrical mirror analyzer CMA‐based Auger spectrometers is described. By means of electrically biasing the sample, the effective energy resolution obtainable from the CMA instrument is improved from the native 0.5 to 0.1% or even better for higher kinetic energy Auger transitions. In addition, the maximum kinetic energy Auger transition observable by the CMA Auger instrument is increased from 3200 to 5700 eV, in the current realization. It is also shown that the sensitivity of the energy scale calibration to sample working distance with respect to the analyzer is simultaneously reduced, making the method suitable for chemical surface analysis. The biasing is accomplished using a special sample holder with electronics and software that can be added to an existing instrument. The overall capability of the Auger instrument for chemical analysis is, therefore, increased, while preserving all the analytical functionality and features of the CMA. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of the first nucleation steps of thin films by XPS inelastic peak shape analysis

The initial steps in the formation of thin films have been investigated by analysis of the peak shape (both inelastic background and elastic contributions) of X‐ray photoelectron spectra. Surface coverage and averaged height of the deposited particles have been estimated for several overlayers (nanometre range) after successive deposition cycles. This study has permitted the assessment of the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of an XPS spectrometer. To check the performance of the method, several materials (i.e. cerium oxide, vanadium oxide and cadmium sulfide) have been deposited on different substrates using a variety of preparation procedures (i.e. thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition). It is shown that the first deposited nuclei of the films are usually formed by three‐dimensional particles whose heights and degree of surface coverage depend on the chemical characteristics of the growing thin film and substrate materials, as well as the deposition procedure. It is concluded that XPS peak shape analysis can be satisfactorily used as a general method to characterize morphologically the first nanometric moieties that nucleate a thin film. Copyright © 2006 John Wiley & Sons, Ltd.     

Determination in gas chromatography—mass spectrometry data of mass spectra free of background and neighboring substance contributions

The determination in a g.c.—m.s. data matrix by singular value analysis and least squares of the mass spectra of the substances present and of their corresponding resolved g.c. peak profiles has been supplemented by the determination of a background for each mass, assumed to be constant over 10–12 contiguous scans. The norm for the g.c. peaks has been changed to a maximum of one so that the mass spectral intensities are proportional to the true ion currents at the respective g.c. peak maxima. Complete resolved spectra are computed by using all measured masses. Examples are given of close resolutions (less than one scan separation) and multiple overlap resolution (8 overlapping substances). The method is compared with other published clean-up methods.     

Spectral deconvolution for overlapping GC/MS components

The use of mass chromatogram peak centroids has been investigated as a means of deconvoluting the spectra of overlapping gas chromatography/mass spectrometry components. The peak centroids have been calculated with a precision of 0.04 scans (sd). This proved sufficient to allow deconvolution of the mass spectra belonging to two chemical components which were eluted 0.48 scans apart. For a more complex chromatography peak, it was possible to deconvolute the spectra of six components which were eluted within a 9 scan window. All the spectra produced by using this deconvolution mechanism agreed well with National Institute of Standards and Technology database spectra.