Anal. Chem. 1996, 68, 309R-331R
Surface Analysis: X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Noel H. Turner*
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5342 John A. Schreifels
Department of Chemistry, George Mason University, Fairfax, Virginia 22030 Review Contents X-ray Photoelectron Spectroscopy Reviews Instrument Calibration Data Handling Background Subtraction Line Shape Analysis Binding Energies Surface Core-Level Shifts Shake Structure Valence Band Spectra X-ray Excited Auger and Auger Parameter Polymers Films Instrumentation Auger Electron Spectroscopy (AES) Line Shapes Beam Interactions Background Backscattering Quantitative Analysis Depth Profiling Instrumentation Scanning Auger Microscopy Combined XPS-AES Topics Analysis Instrumentation Analysis Depth Depth Profiles Angular Diffraction Electron Spectroscopy Coincidence XPS and AES Literature Cited
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This fundamental review is on the subject of X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), and Auger electron spectroscopy (AES) and will cover publications published in Chemical Abstracts between October 1993 and October 1995. The review is written in three separate parts for the convenience of the reader: section A, XPS; section B, AES; and section C, combined XPS-AES topics. However, for those who use only one of these techniques, there may be items of interest in the other sections. XPS and AES are used widely for the analysis of surfaces. From about 1970 to the present time, these techniques have grown in acceptance by the scientific community. Much of this activity has been documented in earlier Fundamental and Application Reviews in Analytical Chemistry (1-22). While this review is lengthy, it is not an all-inclusive bibliography of XPS and AES during the review period. The articles S0003-2700(96)00014-5 CCC: $25.00
© 1996 American Chemical Society
have been selected with the idea of improvement in the “state of the art” of these techniques and new trends. The goal of this review is to help analysts solve the problems that are encountered in using XPS and AES in a regular laboratory with commercially available equipment. In addition, reviews on related topics, e.g., ion-induced Auger transitions, are cited. A section on inelastic mean free paths (IMFP) and related topics will be in the combined XPS-AES part of this review. Finally, the names of the authors of the papers cited could not be included in the text due to space limitations. X-RAY PHOTOELECTRON SPECTROSCOPY Reviews. A historical review of photoemission and how XPS has become a major investigative technique in solid-state studies was given (A1). The theoretical and experimental aspects of XPS (A2-A5) angular resolved XPS (ARXPS) using synchrotron radiation (A6) and the opportunities available with these source generators for XPS (A7) were discussed. Reviews dealt with corehole and surface coupling for atomic and molecular systems (A8), core-level XPS spectra of transition metal oxides using the Anderson-type impurity model (A9), theoretical methods of interpreting XPS and ARXPS results for metals and alloys (A10), core and valence band (VB) XPS spectra to investigate corrosion and electrochemical processes (A11, A12), XPS studies of electrochemically formed surface oxide films (A13), and corrosion and adhesion at metal oxide surfaces (A14, A15), and thin films, especially oxides (A16, A17). XPS contributions were discussed for determining oxidation states (especially mixed oxidation states) in supported catalysts (A18-A20) and pure and supported metal clusters (A21). XPS and nuclear magnetic resonance (NMR) analysis of high surface area zeolites (A22) and studies of organic crystals with the H bond coupled to the charge-transfer state were reviewed (A23). Reviews were published also on the analysis of soil surfaces (A24), glass (A25), minerals (A26), ultrafine ceramic powders (A27), and single-particle structure of strongly correlated electron materials (A28). Applications to semiconductors especially using a synchrotron radiation source (A29) and the determination of electronic structure of semiconductor surfaces (A30) and of small-area XPS for the determination of microscopic contamination in semiconductor materials and processes (A31) were reviewed. Cu 2p spectral studies from copper oxide-based high-temperature superconductors (A32) and data from core-level orbitals of alkaline earth high-temperature superconductors were reviewed (A33). Reviews discussed the preparation and characterization of carbon fibers (A34) and composites (A35-A37), the characterization of polymer surfaces (A38-A40), including electroactive Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 309R
polymers (A41), molecular bonding and adhesion at polymermetal interfaces (A42, A43), metal-polyimide interfaces (A44), chemical derivatization procedures (A45), and other data analysis methods (A46). Recent results of core level spectroscopic and X-ray-excited Auger spectra (XAES) studies of physisorbed O2 and N2 on graphite (A47) and XPS contributions to chemisorption and chemical reactions (A48) were outlined. Methods of analysis of minerals and coal (A49) and biomaterial surfaces (A50, A51) by XPS were assessed. Instrument Calibration. Reported binding energy (BE) discrepancies of some oxides were discussed in terms of the possible electronic structure differences as a result of the preparation method (A52). Upward shifts of the Fermi edge due to the electron extraction from the elemental metal into an evolving oxide would cause the observed BE to differ from one that is connected to the Fermi edge. This prompted a review on the use of adventitious carbon (AC) as a BE calibrant (A53). AC, which occurs at a BE of 284.7 ( 0.2 eV, is polymeric and not graphitic; some of the potential problems with its application along with other referencing systems were discussed. Inhomogeneous differential charging on natural minerals of more than 1 eV was found (A54). Satisfactory results were obtained only when two internal reference sources were used. Shifts in XPS peaks as large as 188 eV due to surface charging were observed with an alumina single crystal using a monochromatic X-ray source, and varying differences between the BEs of the O 1s and Al 2p peaks were ascribed to the differing orbital cross sections (A55). Spectral acquisition at various flood gun currents made it possible to determine the optimal conditions for data collection. Spatially localized BE shifts were observed in the XPS study of insulators, even though an electron flood gun was used (A56). Although the exact cause of the shifts was not determined, it was attributed to local variations in the bulk and surface resistivities. Surface charging and its relationship to the presence of carbon or water contamination on cleaved mica surfaces was studied (A57). A contaminant overlayer reduced the charging and was suggested to affect changes in the dipole layer. With the failure of a photocured resin on Al2O3, a difference in C 1s structure was observed between spectra acquired on a conventional X-ray source and data from a monochromatic source with charge compensation (A58). Vertical differential charging from the sample during analysis with the conventional source was the attributed cause of the variations. An interlaboratory comparison for calibration procedures of XPS spectrometers assumed that the BE scale was linear and could be calibrated at two points (Au 4f7/2 line at low BE and Cu 2p3/2 line at high BE) (A59). Checking the linearity with lines at intermediate BEs demonstrated the scale was satisfactorily linear; significant deviations from reference values were related to differences in the average energy of nonmonochromatic vs monochromatic X-rays. A combined standard deviation for the mean elemental BE values of 0.06 eV was obtained when spectra from four laboratories were corrected relative to the National Physical Laboratory; the results suggested that the reported BEs could be used as a reference set for relative chemical shifts in XPS (A60). A thin film of 99mTc as an internal conversion electron source was proposed as an absolute method of calibrating a spectrometer (A61). Measurement of the emission rate prior to XPS analysis gave a known flux of electrons that was used with the XPS 310R
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intensities to determine the absolute instrument efficiency; line widths from these spectra provided a reference for determining the instrumental contribution to the line shape. An external reference from 20 ( 10 nm Pd particles provided a reliable way of static charge correction on copper-zinc-aluminum oxide catalyst precursors than some normal methods (A62). Colloidal Pd particles were deposited from a metal dispersion in water. The gold decoration technique was used to calibrate the energy scale of a series of oxidic model catalysts (A63). The width of the Au 4f7/2 signal could distinguish between physically and chemically induced line broadening. When a bias voltage was applied to a composite, the C 1s peaks shifted relative to each other; these shifts, due to the differing electrical properties of the fiber and the matrix, made it possible to separate the signals from the two components in the mixture (A64). This procedure was called Voltage Contrast XPS. Extra high-intensity peaks were observed in the XPS spectra of both Au and Si when analyzed by using both Al KR and Mg KR X-rays (A65). Analysis of the surface and near-surface of the anodes revealed the presence of Ag; all extra peaks could be explained in terms of emission after excitation from Ag LR, Ag Lβ, and O KR photons. Data Handling. The compositions of the top, bottom, and sides of submicrometer features in polycrystalline Si were determined by geometric shadowing, electrostatic charging, various takeoff angles, and modeling of the surface signal (A66). The dimensions of the etched structures were found by scanning electron microscopy (SEM); it was suggested that the method could be extended to other systems. Analysis was performed on Mo-Ni alloys using the tail heights after each major peak (A67). Since the height ratios were not on a straight line for the alloy series, it was concluded that variations in crystal structures have to be considered. The presence of hydroxide on AlN powders was confirmed using Brehmstrahlung-excited Auger spectra and XPS (A68). The relative amounts of hydroxide were determined even though it cannot be detected in the normal XPS spectra. Changes in the O 1s and O KLL spectra from mixtures of Al2O3/TiO2 were correlated with the particle size and mode of mixing (A69). Charging could be controlled using either a flood gun or Ar+ sputtering. A modification of the conventional method (by accounting for matrix and instrumental effects) for using instrumental sensitivity factors to determine relative atomic amounts was described (A70). Quantitative analysis of high-Tc superconductors and deviations of less than (2% were obtained for binary metal alloys. An iterative algorithm that determined the density and elemental composition of all the elements including hydrogen in leached optical glasses was described (A71). The method considered the attenuation caused by hydrocarbon contamination layers and used calculated densities to correct for matrix dependence of the attenuation length (AL). Background Subtraction. The importance of background subtraction has received much attention in the past several years, due to its role in quantitative XPS determinations. The Shirley procedure to correct for inelastic scattering was modified to produce a sloping background (A72). Using the universal constants from the Tougaard model of background subtraction, the background tail and height could be estimated. Far from the XPS peak the background was very similar to the Tougaard tail, but near the photopeak the background was too low. A Monte
Carlo algorithm that simulates the inelastic background in XPS and possibly spectra also was described (A73). Input parameters of the IMFP and the loss function gave reasonable agreement between experimental and simulated Si 2p spectra, but there were some discrepancies. A theoretical expression describing the energy and angular yield of photoelectrons after multiple elastic and inelastic scattering was tested by a Monte Carlo simulation of partial escape distributions (A74). Corrections for elastic and inelastic collisions of observable spectra were shown and compared with the Tougaard method. Better background correction was obtained when this theory was used. The peak shape parameter, defined as the ratio of the area of XPS peak to the height of the background 30 eV below the peak, was studied for homogenous samples of Au, Ag, and Cu (A75). It was shown that the ratio of backgrounds from the two peaks should be approximately equal to the ratio of the peak areas. Experimentally determined ratios of backgrounds from the two peaks were compared with predictions from the peak areas. A database of correction parameters, βeff and Qx, for elastic scattering effects in XPS was published (A76). The parameters were determined by direct simulation with the Monte Carlo method for XPS intensities taking into account the elastic scattering; the algorithm required knowledge of IMFP and elastic scattering cross sections for the 27 elements. Eight laboratories participated in a comparison of the consistency and accuracy of the Shirley, straight line, and Tougaard methods for determining peak areas from XPS spectra of Au and Ni (A77). All techniques were consistent to within 2-6%, but the Tougaard method was the only approach to produce peak area ratios that agreed with the expected theoretical ratio within the presumed accuracy. The error during alloy analysis using a firstprinciples approach and a narrow energy range was greater than when larger energy ranges were used (A78). Smaller errors were obtained when the Tougaard background was used, since it was possible to account for the presence of shake-up intensities. These methods of background subtraction also were compared in terms of validity and consistency for several polycrystalline elemental solids and alloys (A79). The error was ∼3% for the Tougaard method compared to theory, while it ranged from 33 to 60% for the Shirley and straight line methods. The performance of a recently developed software package (QUASES) for quantitative surface analysis (based upon the formal treatment of electron transport processes) was studied using thin films of Au or Cu/ Au over polycrystalline Cu (A80). When the results from this analysis were compared with those from other methods, a 10fold improvement in the consistency of results was obtained. The inelastic differential cross section and XPS background were determined from the IMFP, and the assumptions that the peak intensity is proportional to excitation probability and only intensity from the peak should be observed after background removal (A81). The background in a survey scan of Au and Al metals was completely removed with these assumptions only; the procedure was based on the Tougaard algorithm and used a minimization procedure. Line Shape Analysis. Methods using the nonlinear leastsquares curve fitting (NLLSCF) procedure for the quantification of the oxidation state distributions on reduced catalyst surfaces were reviewed (A82, A83). Correlations of catalytic activity with oxidation state were made, and limitations of this technique also were discussed. Often there is uncertainty associated with the
determination of the number and positions of the components contributing to the overall spectrum. Factor analysis (FA) reduced the uncertainty in the number, positions, and shapes of the principle components in Mo 3d spectra without making any assumptions. The method was tested further on Cr/Al2O3 catalysts to determine the distribution of chromium oxidation states (A84). Cr3+ and Cr6+ were detected. A combination of deconvolution, curve fitting, and FA proved to be effective in determining the reduction behavior of W/TiO2 catalysts (A85). Since the W 4f and Ti 3p spectra overlapped severely, spectral broadening was first removed using a deconvolution procedure to give preliminary estimates of peak positions; NLLSCF provided more exact peak positions and widths. The results were compared with FA and indicated the number of W components in the reduction series. FA of XPS data from depth profiling identified components attributed to Cu, Cu2O, and CuO on oxidized Cu foils (A86). Rutherford backscattering spectrometry (RBS) analysis confirmed the presence of Cu2O. Improvements were proposed to fitting procedures for overlapping peaks to determine peak areas, positions, and widths, and the difficulties with the present least-squares methods were discussed (A87). Some fits are arbitrary and it was proposed that, with the addition of certain statistical diagnostics, a better judgement of fit quality could be obtained. Inference testing of regression parameters was introduced as a method of error analysis (A88). This approach accounts for correlated uncertainties and can help determine the number of peaks that should be used to fit the overall spectrum. The problem of ill-conditioning in terms of multicollinearity was considered (A89). Only synthetic data were presented for each concept discussed in refs A87-A89 and attempts were not made to demonstrate the concepts on real XPS spectra. A very flexible NLLSCF PC-based program that permitted up to six variables to describe an individual peak was developed and tested with experimental data and simulated spectra (A90). Much of the emphasis with using the program rested on having the fits make reasonable chemical sense. The statistical estimation of XPS data using residuals analysis to infer goodness of fit was criticized (A91). The originality of this approach, the use of residual analysis, and the choice of line shape were questioned. In a reply, it was pointed out that the method of using residual analysis had not been used with XPS data (A92). Residuals of high-resolution C 1s spectra of CH4 were presented to support the line shape employed. Line shape changes of K 2p spectra were observed with increasing amounts of coadsorbed water on K/H2O/Ni(111) (A93). When interpreting peak area results, line shape changes with coverage must be considered. A procedure to routinely deconvolute XPS spectra produced Si 2p peaks with full-width at half-maxima (fwhm) of 0.3 eV that were acquired using nonmonochromatic X-rays (A94). The technique involved smoothing the data, experimental measurement of the instrument response function (for both the electron analyzer and the X-ray source), and deconvolution using the iterative Jansson algorithm. An accurate analytical approximation to the Voigt function (used for nonmetallic solids), and a method of implementation was described (A95). Using a NLLSCF method, satisfactory fits of spectra were obtained using a personal computer. Kalman filteroptimized cubic spline functions for digital smoothing of noisy data produced better results with Pb 4f doublets than the Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Savitzgy-Golay method (A96). Since aliased frequencies were greatly reduced, the signal-to-noise ratio was improved. Binding Energies. Semiempirical SCC-MO, AM1, and PM3 calculations of BEs were performed to determine which procedure would best reproduce chemical shifts in the N 1s region (A97) and the F 1s, Cl 2s, Cl 2p3/2, Br 3d5/2, and I 3d5/2 peaks (A98). The results were poor with the PM3 and AM1 methods for N 1s; good agreement was obtained for the fluorine and chlorine peaks, but less successful outcomes were obtained for bromine and iodine. XPS spectra could be used for the paramaterization of these semiempirical methods. XPS BE shifts of group IV atoms (C 1s, Si 2p, Ge 3p, Ge 3d, Sn 3d5/2, Sn 4d5/2) were reproduced best by the SCC-MO method instead of the MNDO and AM1 procedures (A99). Reorganization energies were useful tests of the semiempirical wavefunctions. Experimental and theoretical studies of the C 1s and N 1s levels of gas-phase acetonitrile, acrylonitrile, and proprionitrile were performed to model the spectra of polyacrylonitrile (A100). This made it possible to assign the individual peaks to particular structures. The ∆SCF and ∆MP2 calculations gave reasonable predictions of BEs for the polymer, while ∆MCSCF computations did not. A saturated monomer of 2-methyl-2-propenenitrile bound to Ni clusters was proposed as a model for the reaction products of cathodic eletropolymerization (A101). From Hartree-Fock calculations, a C 1s BE via Koopmans’ theorem of 283.8 eV was close to an experimental value of 283.6 eV. BEs obtained from high-resolution XPS studies on biodegradable poly(ortho esters) prepared from N-methyl and N-phenyldiethanolamine (MDE and PDE, respectively) were correlated with BEs calculated with ab initio MO methods (A102). Good agreement between calculated and measured BEs was obtained by assuming the nitrogen was planar in PDE and was pyramidal in MDE. Nonlocal screening effects were investigated using a large-cluster model for the Cu 2p region of a CuO2 plane (A103). Since the matrix size increases exponentially with cluster size and hole number, an approximation was made to perform calculations on larger clusters and to describe the spectra using multiple sites for Cu in the model. Shifts in the C 1s spectra of a series of alkanes were compared with the calculated charge obtained from the Mulliken population and Karlstro¨m methods (A104). The Karlstro¨m procedure was better, since it considers multipole moments. Hydrogen bonding in a series of substituted anilinium chloride and trichlorostannate(II) derivatives was studied by infrared spectroscopy and XPS (A105). Variations in the N 1s BEs correlated with force constant changes; structural data changes in the N-H bond length caused by variations in the crystal structure were suggested for most of the observed shifts. Differences in XPS core spectral features between coadsorbed CO/ H/Ni and N2 adsorbed on a Ni-Ti alloy surface, CO/Ni, and N2/ Ni were studied using the cluster configuration interaction (CI) approach (A106). Changes in the spectra were related to differences in the local metal configuration energy ordering from the ground state to the core ionized state. Reversible temperatureinduced changes in the XPS spectra of NiO were observed even with UHV cleaved NiO single crystals (A107). These differences were not the result of oxidation to Ni3+; a simple model that involved vibronic mixing of different final states was proposed to explain these findings. The cause for the unusual stability of the ordered stacking with misfit layer compounds having the general formula (MX)1-δTX2 (M ) Sn, Pb, Bi, or a rare earth metal; X ) 312R
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S, Se; and T ) Ti, V, Cr, Nb, and Ta) was investigated (A108). The stability was ascribed to covalent interlayer bonds and not to charge transfer between the layers. Shifts in BEs of Si and Ge for several Si- and Ge-based compounds were correlated with the local electronegativity (Sanderson’s scale) (A109). New values for Si and Ge of 2.6 and 3.1, respectively, were proposed. The ionicity of UHV grown films of Mg2Si/Si(100) was calculated by the Madelung potential model, from XPS shifts, and by Pauling’s electronegativity model (A110). A shift of +0.25 and -1.02 eV with respect to the pure Mg and Si levels, respectively, indicated an ionicity of ∼8%, and this agreed with the Madelung potential approach. Brønsted and Lewis acid sites were detected on large-pore zeolites using the N 1s spectrum after vapor-phase adsorption of pyridine (A111). Brønsted acid sites produced high BE peaks. Bulk and surface acidity of commercial protonated Y zeolites was studied by adsorption of NH3 and pyridine followed by XPS and temperature-programmed desorption (TPD) (A112). Desorption temperatures and the N 1s BEs demonstrated the proportions of Brønsted and Lewis sites. Surface acid-base interactions between poly(methyl methacrylate) (PMMA) and glass were investigated using ARXPS by measuring the relative intensities of O 1s peaks from the underside of pealed PMMA films (A113). The surface acidity prior to deposition of PMMA was determined from shifts in the O 1s and Si 2p BEs, and a preferential orientation of the carbonyl group was suggested. Uptake of Na+ from a glass slide to the polymer was observed after removing the polymer from the glass (A114). The extent of the uptake was directly correlated with the basicity, via the amount of Na+ on the polymer. An improvement to account for the possibility of complexation for adsorption on MO- sites by determining the isoelectronic point of solid surfaces with XPS was suggested (A115). The method involved measurement of the formation constant for this process prior to the determination of pKa1 and pKa2. BEs in the Si 2p spectra for a series of pentacoordinate Si compounds were compared to SiO2 data (A116). Shifts were explained in terms of coordination number, kind, concentration of cations, and presence of organic species. Physisorbed O2 and N2 on graphite produced XPS spectra that were very similar to gas-phase data, but shifted to lower BEs by ∼1.4 eV (A117). Between the first and second layers, BEs also decreased. Inconsistencies in published BEs of high-Tc materials were explained in terms of vacuum-temperature induced losses of O (A118). These losses were reversible and were reduced when air-exposed materials formed a stable surface byproduct. A technique called surface charge spectroscopy was described that uses XPS with an electron flood gun to measure the Fermi level and the surface potential of a dielectric (A119). Examples with SiO2/Si and SiNx/InP samples were given. Experimental requirements for surface charge spectroscopy were discussed, and important sources of imprecision and incorrect results were established (A120). Poor electrical contact between the sample and spectrometer and difficulties with BE referencing were the most important sources of error. The energy distribution of surface states in the semiconductor bandgap of a sample consisting of Pt/oxide/p-InP(100) were determined by acquiring XPS spectra under bias (A121). Differences in BEs between In 3d and Pt 4f peaks changed when an applied bias was used for these measurements. Results of characterization of S species by Raman spectroscopy were compared with those of XPS experiments and
were found in general agreement (A122). The applicability of XPS for characterizing surface species on electrodes previously had been questioned. The 3s peak of metallic iron was investigated to clarify its spindependent nature of this level (A123). A previously uncharacterized v-spin component was identified; its relative intensity increased at higher photon energies and was assigned to a remnant of the atomic 1G term for the 4s23d6 valence configuration of Fe. Spin-resolved XPS (SRXPS) experiments of core-level metallic Ni satellites were consistent with a model where extraatomic screening in Ni involves pure 3d states and 3d-4sp states (A124). Final-state occupation of these levels led to a 50% 3d9 and 50% 3d10 configuration with the 3d9 state being the origin of satellite structure. On the other hand, SRXPS studies of Fe 2p3/2, 2p1/2, 2s, and 3p levels demonstrated the importance of intraatomic exchange (A125). The electronic response of the metal to corehole creation depended upon its the spin. High-resolution XPS and SRXPS studies of Co, CoF2, and CoF3 found a linear variation of the multiplet splitting for the Co 3s level with (2S + 1) (A126). This and the absence of core-hole satellites in Co metal were attributable to localized final-state 3d charge fluctuations and suggested that intraatomic exchange dominates the Co 3s spectrum. The influence of electron spin on the Co 2s, 2p1/2, 2p3/2, and 3p core-levels was also studied using the same techniques (A127). A comparison of Co metal data with spectra from the amorphous ferromagnetic glass Co66Fe4NiB14Si15 further revealed the sensitivity of Co core-level spectra to variations in the valence spin polarization by intraatomic exchange. Surface Core-Level Shifts. Core-level 4f7/2 spectra from a bifacial crystal having flat W(110) and stepped W(110) [W(320)] surfaces reduced uncertainties in the results of the W(320) surface and showed that the average surface core-level shift (SCS) was less for W(320) than for W(110) (A128). The absence of a large SCS for the step-edge atoms contradicted earlier results and theoretical models. Linear muffin-tin-orbital (LMTO) calculations of the bulk band structure and VB spectra revealed highly hybridized electron states between the Pt d and Ti d valence levels in Pt3Ti(111) (A129). Only a small amount of charge (0.37 electron/Ti atom) was transferred from Ti to Pt, and the large surface core-level shifts reflected the stability of the alloy rather than a charge transfer. With high-resolution spectra surface corelevel shifts in BEs were observed with CO, CCH3, and O on Pt(111) (A130). Using data from low-energy electron diffraction to indicate adsorbate orientations, it was possible to determine the nature and distribution of adsorbate sites and the local character of the surface core-level shift. A decrease in the core-level BE of Ni in a Ni/Al2O3/Al structure was attributed to the charge transfer from the cation to Ni (A131). A Ni-Al alloy formed via interdiffusion through oxide defects. BE shifts, relative to the bulk metal, of well-characterized small gold clusters were correlated with cluster size (A132). Very small clusters (6 keV and the Bethe ionization cross sections were used. Poorer results were obtained when the film was Cu or the Gryzinski ionization cross sections were used. Monte Carlo calculations of the electron backscattering from redistributed primary and secondary electrons agreed well with earlier models and standardized experimental results (B80). However, studies are still required to define better the region below ∼200 eV and a variety of electron beam energies. A technique to correct for backscattering and topographic effects that relies on measured backscattered electrons was developed (B81). In the procedure, a ratio of the Auger peak heights to the background signal at 2.5 keV is used for quantification purposes and examples with SAM images were reported. Two formulas that considered backscattering for the calculation of the Auger yield were compared over 320R
Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
normal to oblique incident electron beam angles (B82). Only at high incidence angles was divergence noted. Quantitative Analysis. Research has continued to make AES a technique that is capable of quantitative determinations in the surface region of solid materials. A review was published of the factors involved in quantitative AES analysis (B83). AES relative sensitivity factors were modified to take into account matrix effects, backscattering, and IMFP (B84). Calculations were made for many elements, and comparisons with some experimental results were made. Of the components needed for the quantitative analysis of binary alloys, correction factors for elemental sensitivity, atomic density, and preferential sputtering were found to be the most important (B85). Of lesser importance were sputter yield and roughness. RBS was employed to find AES sensitivity factors for FeSix (1.9 < x < 2.3) during sputter profiling (B86). Measured sensitivity factors changed with Ar+ energy for Fe and x, but not for Si. The differences between using a continuum and a discrete model for layer quantification were found to be small up to one layer for composition determinations (B87). Beyond a monolayer the differences were nonexistent, and overall, the variations between the approaches were less than originally thought. It was pointed out, however, that the approach used to find the small differences between the discrete and continuum models would not handle the case where the composition varied in a linear fashion (B88). The ability to determine submonolayer coverages was described by using low and high kinetic energy transitions and noting the signal loss at these levels falls linearly rather than exponentially (B89). Two examples from previous studies of S on Fe and Ni(001) were given. Corrections for the electron flux emitted from a layered structure were made from a theoretical model that reduces to a general expression using only the IMFP of electrons and the backscattering factors from pure elements (B90). This allows modification for differential scattering, even for complex stratified samples. Several methods to determine relative atomic amounts of CdS, CdSe, and CdSxSe(1-x) (0.47 < x < 0.87) were examined (B91). The results that agreed best with chemical analysis were based on first-principle calculations. The effect on the determination of relative amounts with Cu-Al alloys was made using several approaches for computing IMFPs, backscattering coefficients, and matrix factors (B92). It was found that the backscattering models had little effect, and the choice of IMFP procedure depended on the relative atomic amount of the lower Z component in the determinations. It also was concluded that matrix effects can vary with composition. Several AES quantitative methods were tested using SrTiO3 with the best results found with a first-principle semiempirical approach (B93). The use of a standard compound with matrix factor corrections yielded almost as good results, while some simpler methods gave much poorer values. Several methods for quantitative AES of SixC1-x films were evaluated, i.e., negative peak height and negative peak times the peak width squared with derivative data, and integrated data (B94). Little difference was found between the procedures when matrix and backscattering effects were considered. A method to measure the amount of Na2SO4 electrodeposited on Au was developed based on pph measurements that considered backscattering, atomic density, and IMFP (B95). The results agreed with radiochemical determinations, and electron beam degradation was minimal under the experimental conditions used. The composition of TiNx (0 < x < 1.07) could be determined from
the peak height of a shoulder on the integral mode Ti L3M23M45 peak; a background correction was required (B96, B97). With Ti-B-N films, the ratio of the negative excursion of the Ti L3M23M23 to the L3M23M45 (that overlaps with the N KLL peak line) with derivative spectra was found to give unreliable results. Bonding information about Ti can also be obtained. An analytical procedure that used the negative excursion of derivative spectra for TiN was extended to Ti-Al-N films; results agreed well with wavelength-dispersive spectrum analysis (B98). Due to some Al-N bonding, the method needed to be modified with the use of an AlN standard. Ternary compounds, InxGa(1-x)As with 0 < x < 1, were analyzed with pph measurements based on a relative determination with the As signal (B99). Close agreement for the relative atomic amounts with X-ray double-diffraction and electron microprobe analysis determinations was obtained when matrix and sputter yields were considered. In the quantitative analysis of PtSi and Pd2Si it was found that both matrix and sputter effects needed to be used because of peak shape changes (B100). Also, it was not possible to separate these effects. Matrix factors affecting three binary alloy systems (Ag/Cu, Ni/Cr, Pb/Sb) were due to contributions from elemental sensitivity, atomic density, and preferential sputtering (B101). Empirically derived matrix factors did not fully describe the data. FA was employed with both the MVV and LMM regions of Cu and Zn to analyze scratched brass samples (B102). From the more surface sensitive MVV spectra, an enrichment of Zn was observed that was not noted with the LMM data. A “top-hat” filter was capable of reducing the effects of backscattered and inelastic electrons for Auger spectra (B103). Then, with NLLSCF the composition of ternary compounds was found by assuming that two of the components could be considered a single component. A weighting function method was proposed for use with a linear leastsquares analysis of alloy spectra to calculate compositions (B104). Zero intensity, small, and overlapping elemental points are not employed in fitting, and this can reduce computation time; examples were given. The use of a set of standard multicomponent spectra for determining composition was proposed; the set has to have all of the elements in the specimen being analyzed (B105). Each standard spectrum has to have one of the elements contained in the sample along with an element common to another standard spectrum. The standards are based in a spectral library. An extension was made to a model for quantitative AES of single crystals to polycrystalline binary materials that uses intensity ratios and ALs (B106). The procedure gave results constant with the bulk composition of Ce3Rh2 films. Depth Profiling. One of the biggest uses of AES is depthprofiling materials, especially to depths of up to several hundred nanometers. Target factor analysis was used for the analysis of an oxidized stainless steel that included Cl, Mo, Cr, O, and Fe (B107). The proper choice of the sputter gas and base spectra permitted analysis where overlapping peaks or chemical differences occurred during the profile. A new procedure to find compositions in a depth profile was utilized to study the interaction of rubber on brass (B108). The method is called maximum likelihood common factor analysis and multiple region transformation and does not require reference spectra or target vectors. An iterative method to analyze sputter depth profiles was proposed (B109). The approach involves using transitions that have sufficiently large IMFP differences. The differences in plasmon loss features for depth profiling with elastically reflected spectra
at different voltages was utilized to study a Mo/Si multilayer (B110). The advantage of this method is that by varying the electron beam energy the IMFP of the detected features is altered. A procedure to evaluate AES profiles of thin-film sandwiches that employs standards, relative sputtering rates, and a conservative estimate of the depth resolution was proposed (B111). The method was able to infer diffusion regions with a Nb-Al2O3-Nb structure. A rotation speed of 0.1 rpm, or lower, depending on the material being studied, was sufficient for improved depth profile resolution with Ar+ or Xe+ (B112). However, with 1-keV Xe+, differences in depth resolution were negligible. Profiles of Ni, Cr, and Si multilayers with sample rotation displayed poorer depth resolution with increasing number of layers (B113). With heating, the profiles showed that diffusion of the individual elements occurred, and this was confirmed by other techniques. O+ had a better depth resolution in sputter profiles compared to Ar+ under a variety of conditions (e.g., beam voltage and angle, sample rotation, or nonrotation) for Ni-Cr multilayers (B114). However, with glancing angle ion beams and sample rotation, both O+ and Ar+ gave the best overall resolution. Improved Ar+ sputter profile depth resolution with sample rotation was found with Zr/Nd and InP/InGaAs multilayers (B115). However, sample rotation with AlAs/GaAs multilayers did not improve depth resolution, and this was ascribed to the latter material being in a single crystalline state. A comparison of depth profiles using Al2O3 on Al samples (produced in several ways) was made with and without specimen rotation and varying Ar+ beam angles (B116). Sample rotation improved depth resolution, but the sputter rates remained about the same. A near-grazing ion beam angle significantly decreased the sputter rate. Better depth resolution with a Cr/Ni multilayer was obtained with Xe+ vs Ar+ with incident angles up to 65°; at 80° there were no differences (B117). Two sputtering effects were suggested for these observations: (1) different channeling effects of the gases and (2) different sputter yields within the metallic grains and oxides at grain boundaries. The effect of ion beam energy and incidence angle on the interfacial depth resolution was studied for a superlattice of GaAs/Ga0.81In0.19 (B118). The best results were found with low beam energies (500 eV) and a neargrazing incidence angle. Instrumentation. Better understanding of the operational characteristics of laboratory instruments is being gained. Also, new, specialized spectrometers are being constructed and tested. A procedure was given so that comparisons of S/N between different analyzers can be made by using the Cu L3M45M45 peak under varying resolution, incident electron beam angle, and electron emission angle (B119). The incident electron beam angle had a strong effect on the S/N with a CHA, but not with a CMA. A CMA for obtaining standardized spectra was built with cylinders that were coated with a carbonaceous coating to reduce secondary electron emission and that allowed the distance between the inner and outer cylinders to be varied (B120, B121). It was designed also to reduce fringe effects and employed Faraday cups for measuring both the beam and Auger currents. Reasonable agreement of Cu, Ag, and Au was obtained with standardized spectra from a CHA system. Numerical calculations for the transmission factor of a CHA were made for a point source (B122). It was found that the angular magnification is the most important operational parameter in describing analyzer response, but the energy retardation ratio and aberrations in the CHA input lens Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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also must be considered. Micromachined Bessel boxes (2.6 mm thick) were made that measured Auger spectra for Cu and graphite samples (B123). For electrons of 100-eV kinetic energy, 1.2 eV was the computed fwhm. Energy calibration using the Ni M3VV and L3VV transitions was found to be better if the spectra were referenced to the Fermi level rather than the vacuum level (B124). This approach was used in a round-robin study and was observed to reduce the scatter by ∼75%. It was noted also that instruments with better energy resolution had lower standard errors. A procedure to obtain detailed features was described when spectra are recorded with different and known analyzer functions (B125). Spectra with good S/N are needed; an example with Ag was given. A modification for older CMAs to use the beam brightness modulation method and the associated computerization was described (B126). This allowed N(E) spectra to be obtained along with some background removal. A real-time Auger analysis system (based on a commercial analyzer) that can record spectra in a molecular beam epitaxy chamber was constructed with an electron source used for reflected high-energy electron diffraction (B127). Results with some deposition material were given along with some difficulties employing the equipment. A commercial CHA was modified with the addition of a Mott detector for spinpolarized electrons (B128). Further improvements in performance may be possible with the addition of a microchannel detector and better scattered electron reduction. A modified sample holder was described that permitted rotational depth profiles at multiple locations on 50-µm sample features (B129). Repositioning the sample in situ was accomplished by bumping a movable portion of the sample holder. A procedure to correct distortions with retarding field analyzers (RFAs) on the low-energy side of elastic peaks and Auger lines employs putting a bias on the specimen (B130). This yields somewhat sharper peaks, and a method to find the best potential was given. By making thin films of polymers (
