Surface Analysis: X-ray Photoelectron Spectroscopy and Auger

Anal. Chem. 2000, 72, 99R-110R

Surface Analysis: X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Noel H. Turner*

Department of Chemistry, George Washington University, Washington, D.C. 20052 John A. Schreifels

Department of Chemistry, George Mason University, Fairfax, Virginia 22030 Review Contents X-ray Photoelectron Spectroscopy Instrument Calibration Data Handling and Line Shape Analysis Background Subtraction and Energy Loss Features Binding Energies Valence Band Spectra Shake Effects and Multiplet Splitting X-ray Excited Auger Electrons and Auger Parameter Semiconductors Polymers Films Instumentation Auger Electron Spectroscopy Line Shapes Quantitative Analysis Depth Profiling Instrumentation and Technique Small Area Analysis Combined XPS-AES Topics Standards and Databases Inelastic Mean Free Paths Instrumentation and Technique Multitechnique Analyses 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 articles published in Chemical Abstracts between October 1997 and October 1999. 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 (1-26). While this review is lengthy, it is not an all-inclusive bibliography of XPS and AES during the review period. The articles have been selected with the idea of improvement in the “state of the 10.1021/a10000110 CCC: $19.00 Published on Web 05/18/2000

© 2000 American Chemical Society

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 XPS is a widely used surface analysis technique since one can determine the surface composition as well as electronic environment nondestructively (in many cases). This is invaluable in many fields of science, such as (but not limited to) catalysis, corrosion, and semiconductors. The principles of XPS (A1) and methods of avoiding the pitfalls of surface analysis (A2) were published. The use of Auger parameter and Wagner plots in the characterization of chemical states in XPS (A3) and the simulation and interpretation of core and valence band (VB) spectra of polymers (A4) were discussed. The effects of photon beam damage and surface charging along with methods used to reduce the effect (A5) and practical techniques of charge compensation (A6) were outlined. A book on the analysis of polymers by XPS and static SIMS (A7) was published. Techniques of polymer and catalyst characterization (A8) and the application of quantitative models for the analysis of supported oxides (A9) were also discussed. Instrument Calibration. The location of the zero point on the binding energy (BE) scale of XP spectrometers was different when determined with Ni than with Ag (A10) and was due to variations in VB density of states. Measurement of the Fermi level from Ag was recommended over Ni. The effects of different parts of a spectrometer on the error in BE measurement were examined (A11). Drift from the instrument’s electronics had the largest effect on the uncertainties in the calibration. The results of the previous two studies led to a reevaluation of the BE standard values for peaks from copper and silver (A12). The new recommended BEs differed from accepted values by less than 0.05 eV and their standard deviations were generally lower than the earlier ones. A procedure was then outlined to calibrate the BE scale using traditional X-ray sources (A13). Uncertainties from peak energy repeatability, scale linearity, and instrumental drift were incorporated into the procedure. Analytical Chemistry, Vol. 72, No. 12, June 15, 2000 99R

The results of a round-robin study on the calibration of the BE scale using the Au 4f7/2 and Cu 2p3/2 lines were reported (A14). It was possible to determine a BE to within (0.2 eV relative to a previously reported value. Energy calibration procedures were outlined and uncertainties from major error sources were discussed (A15). The BE scale could be calibrated as well by use of a Lorentzian, Gaussian, or asymmetric Gaussian function by employing a quadratic equation when the top 10-20% of the peak was fit. The vertical alignment of the sample with some focused X-ray beam instruments was critical for the accurate measurement of relative peak intensities (A16). A misalignment of only 0.1 mm in the sample height can cause a 10% change in the relative intensities over the 0-1000 eV BE range. Inadequate design was one of several causes for nonlinearity between signals at different count rates (A17, A18). Making use of ratio plots from survey scans at different X-ray source emission currents makes it possible to monitor the extent of nonlinearity without additional equipment. Nonlinearities of up to 50% were observed with commercial electron counting systems. Sources of surface charging of insulating polystyrene (PS) and poly(dimethylsiloxane) (PDMS) deposited on NaCl particles were examined (A19). Charging shifts indicated the kinetic energy of the photoelectron is determined by the electrical potential in the phase of origin and not by the potential at the sample surface. Placement of a metal wall around a sample reduced the extent of differential charging occurring on a nonconducting surface during XPS analysis with a nonmonochromatized X-ray source (A20). A correction procedure, which uses the F 1s line as an internal standard, for differential charging from fluorocarbon polymer films was proposed (A21). Mixing nonconducting powders and carbon black in a 50/50 volume ratio greatly reduced charging effects (A22). Examples were given with SiO2 and LiOH. A differentialcharging factor was determined from line shape analysis of the F 1s spectrum and was applied to the C 1s spectrum. Corrections for charging could be made when factor analysis (FA) was used for depth profiles (A23). Either a strong peak that is not removed or an implant peak from the bombarding gas can be employed. Data Handling and Line Shape Analysis. A factorial-based design of standard test data for evaluating computer-based data analysis tools of XPS spectra was presented (A24). Simulated polymer C 1s spectra were evaluated by 20 analysts using a variety of software packages to determine the errors in the binding energies of doublet peaks with varying conditions of peak overlap, intensity, and noise. Three commercially available peak-fitting XPS data analysis systems were evaluated in terms of their ability to accept synthetically generated VAMAS (Versailles Project on Advanced Materials and Standards) data file formats (A25). Not all software packages produced the same accuracy. The fitting of Si 2p spectra was demonstrated using the Levenberg-Marquardt fitting technique and a Voigt line shape (A26). BE values were very consistent, but poor estimates of peak areas were obtained unless the noise level was very low. A method of determining sensitivity factors using peak intensities, photoionization cross sections, and kinetic energies was demonstrated (A27). The technique worked well except when high-level s peaks (e.g., 3s, 4s, 5s, etc.) were used. Core spectra of selected polymers were satisfactorily fit using the least-squares method with a Voigt 100R

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function and a modified Tougaard background (A28). The model loss function involved free parameters that were determined during the fitting process. The true ratio of peak areas of substrate peaks was estimated by varying the level of contamination of the substrate and plotting the areas of these peaks relative to the area of the C 1s peak (A29). The effect of flatness, random roughness, and shadowing was taken into account; the method should improve the estimation of the true spectrometer transmission function. A procedure involving a cubic spline approximation to determine instrumental broadening of a commercially available instrument from measurement of the Ag Fermi edge was proposed (A30). The Ag Fermi edge was approximated by a step function convoluted with broadening functions. Instrument broadening was removed from spectra of thin Ti films on diamond by the maximum entropy method (MEM) using a function that had been obtained from the silver Fermi edge (A31). Reconstructed Ti 2p3/2 spectra revealed temperature-dependent changes that were attributed to a phase transition. MEM was shown also to be a valuable method of resolution enhancement with the smallest error of several other iterative deconvolution methods (A32). The broadening factor was determined using the silver VB spectrum. The C 1s spectrum of polyethylene was narrower than had been reported from an instrument with higher resolution. MEM reduced the width of Al 2p spectra to what is obtained using synchrotron radiation (A33). At least 250 data points were required over a 10 eV scan range; the code for the method was written using the MATLAB program, and the sequential quadratic programming algorithm converged in about 15 min on a fast personal computer. The usefulness of MEM was also demonstrated in the study of thin films containing oxides of chromium and in the detection of individual phases in Au-Al alloys (A34). The full width at half-maximum (fwhm) of each S 2p spectrum from fractured pyrite, marcasite, and arsenopyrite after deconvolution by MEM was 50% of the fwhm of the original spectrum (A35). The enhanced resolution revealed small peaks that were not otherwise observable and were attributed to broken S-S bonds. The theoretical effect of large clusters of quasi-one-dimensional copper compounds on the line shape of the Cu 2p spectrum was examined (A36). Both the Cu-O-Cu bond angle and magnetic coupling between Cu spins were key factors in determining the line shape. The shapes of Cu 2p spectra for a series of cuprates were studied using the Anderson impurity model with hopping matrix elements (A37). The observed leading edge was attributed to the presence of 180° Cu-O-Cu bond angles. A theory of line shape based on the Green’s function calculation of the atomic vacancy structure was developed (A38). Asymmetric photoelectron peaks are always found and significant broadening is expected for satellites. Chemical states of semi-insulating silicon, using a pattern recognition method and the fuzzy k-nearest rule, were analyzed by the line shapes of Si 2p, O 1s, and O KLL spectra (A39). Estimates of Si, SiOx, and SiO2 relative amounts from this analysis were compared to those from conventional methods. Estimates of oxygen content from this procedure were lower compared to the conventional method. Temperaturedependent changes in the Cu 2p line shapes of quasi-onedimensional organic conductors were observed with a salt having

a charge density wave/metal insulator transition at 210 K (A40). The line shape change was related to the transfer of charge from Cu 3d states to the organic ligand. Background Subtraction and Energy Loss Features. A simple iterative procedure to correct for the attenuation of signals from advantitious carbon was developed (A41). Two examples were given, and the lack of sensitivity to hydrogen usually is unimportant. After using a procedure for determining the analyzer transmission function, the beginning and ending points of a Shirley background could be determined by an iterative procedure, via a comparison between the areas of two peaks (A42). This method was applicable to practical quantitative analysis. An asymmetry factor, κ, accounted for intrinsic and final state losses in XPS peaks of metals (A43). This factor, when used with the Shirley background subtraction method, made it possible to use narrower scans than employed by the Tougaard background. The value of κ was independent of the peak kinetic energy and was similar for all peaks from the same element. Also, it did not depend on the spectrometer type and varied uniformly with atomic number. In another study of copper and chromium oxides, κ exhibited a slight dependence upon the chemical state of the element (A44). The presence of layered structures, on the other hand, did not influence the fit. This parameter was determined for a series of aluminum and titanium intermetallic compounds (A45). The formation of the intermetallic compound caused κ to increase compared with the value of the pure metal; this suggests an intermetallic bond formed between the aluminum and titanium. Inelastic backgrounds were subtracted from a set of synthetic angle-dependent XP spectra using principle component analysis (PCA) and a polynomial approximation (A46). Optimal fitting depended upon the signal-to-noise ratio (S/N), but ranged between two- and five-degree polynomials. Energy loss functions for Si 2p electrons were obtained from clean and oxygen-adsorbed Si(111), as well as SiO2 surfaces, by reflection of an electron beam at the kinetic energy of the photoelectron, and by analysis with the extended Landau theory (A47). Features in the processed spectra of the oxygen-adsorbed surface confirmed the applicability of the technique to the study of initial oxidation processes. Energy loss features near the O 1s peak from LiTaO3 and LiNbO3 were measured and analyzed theoretically using the full-potential linearized augmented plane-wave method (A48). The observed peaks were assigned to both interband transitions from the valence to the conduction bands and to a transition from the O 2s level to the lower conduction band. The effect of inelastic loss was studied by considering the differential inverse IMFP for bulk electrons and the differential surface excitation parameter (A49). It was found that the correction for loss resulted in a narrower band than with the Tougaard approach. The calculated angular effects on peak intensities were less than those from Monte Carlo calculations. Binding Energies. Linear correlations between C 1s BEs and atomic charge, calculated using the modified Sanderson (MS) formalism and a MNDO quantum mechanical method, were observed for a series of aromatic compounds (A50). Better results were obtained from the MS method, although a better correlation with MNDO charges in molecules where screening is efficient would be expected, since the MS method does not account for electron correlation. An extension of the MS method was proposed

for polycyclic molecules (A51). Molecules were drawn in linear equivalent structures, which permitted the calculation of the partial charge on a carbon atom in the molecule. A linear correlation with BE was obtained. Experimental core-level chemical shifts of C 1s spectra from Langmuir-Blodgett films of fatty acid salts and amine complexes agreed with those predicted from the MS formalism (A52). Inductive effects on calculated partial charge correctly predicted the presence of spectral components arising from carbons directly coordinated to the carboxylate carbon/ amine group. Calibration of the parameters necessary to determine the oxidation state of iron in biotite was performed by use of constrained least-squares fitting techniques on spectra obtained from natural single crystals (A53). Similar parameters were obtained from target transformation factor analysis. Bulk amounts of Fe(III) in individual crystals of biotite were obtained by XPS from the above calibration and compared with results from Mo¨ssbauer spectra (A54). Relative amounts could be obtained to an accuracy of about 9%, and some differences between the techniques were found. Three-way parallel factor analysis, an improvement on the traditional FA method, was used to study the spectra from the oxidation of aluminum surfaces (A55). A peak found at 72.4 eV in the composite Al 2p spectrum was attributed to the presence of aluminum hydride at the metal/oxide interface. Core-level ab initio calculations of BEs were performed with SPARTAN, using a Pentium PC (A56). Reasonable agreement between these binding energies and those obtained by more powerful computers was obtained. Density functional theory was applied to calculate binding energies for two systems, Co adsorbed on Pt and (γ-aminopropyl)hyroxylsilane adsorbed on Si (A57). BE differences between experiment and theory were less than 0.3 eV if some assumptions were made concerning the orientation of the adsorbed species. N 1s BEs in hard and elastic carbon nitride (CNx) films were investigated using the ∆-self-consistentfield technique and model compounds (A58). On the basis of calculated values for different chemical environments, recently reported BEs were justified; a peak at 402.6 eV was attributed to either nitrogen at graphitic edges or to nitrogen bound to oxygen. Negative BE and kinetic energy Auger shifts were observed for AgF and AgF2 powders after sputtering the surface clean (A59). These shifts, opposite to what was expected, were attributed to factors other than electronegativity differences such as lattice potential, work function changes, and extra-atomic relaxation energies. The effect of relaxation on observed BE shifts was discussed (A60). Initial state effects, such as changes in structure and oxidation state, generally affect the extra-atomic relaxation energy while changes in composition, such as in network oxides, produce little change. Core-level binding energies of small Pd particles supported on Al2O3 decreased upon addition of Na (A61). Both a shift in the Fermi level and donation of an electron to the particles were considered to explain the observations. Since a Fermi level shift was ruled out due to a very small predicted shift and the predicted shift from a negative particle was of the correct magnitude as the observed shift, the decrease was attributed to the development of a negative charge on the Pd particles. Shifts in BE and line shape changes in the VB spectra of Al70Co15Ni15 relative to the pure metals were very similar to those in crystalline Al3Ni3 (A62). Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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The large shifts in core-level Ni spectra were accounted for in terms of a large heat of formation for AlNi. Temperature-dependent changes in the Ti/Sr ratio were observed in the spectra of nanocrystalline SrTiO3 that were not found for a single-crystal thin film (A63). The use of nanocrystals increased the number of grain boundaries so that effects at these sites could be observed. Spectra from S 2p and S 2s core levels revealed three types of sulfur from two nanocrystallites and only one type of sulfur from bulk CdS (A64). Using mean escape depths, crystallite sizes were estimated. A model describing the structure of oxidized nanoparticulates predicted the existence of a large dipole between the metal core and oxide shell (A65). Spectra of Sn 3d core levels revealed large shifts relative to the metal that depended upon the size of the nanoparticles. Results of round-robin studies of the decomposition of nitrocellulose revealed that the intensity of the N 1s spectrum decreased rapidly with time of X-ray exposure, power of the X-ray source, and distance to sample (A66). These results could be explained in terms of a set of first-order rate equations (A67). VB spectra indicated that poly(tetrafluoroethylene) (PTFE) did not decompose significantly during a 120 min X-ray exposure (A68). Decreases in BE were observed with increasing coverage of 3-perfluorohexyl-1,2-epoxypropane chemisorbed on Si(111) (A69). Maximum decreases of -0.56 eV for C 1s and -0.35 eV for F 1s peaks were found. The observation was explained in terms of a relaxation shift caused by the increased molecular density at higher coverages from intermolecular interactions. Evidence of interpolymer hydrogen bonding was observed by XPS for a series of poly(styrene-co-4-vinylphenol) and poly(styreneco-4-vinylpyridene) polymer blends (A70). After a threshold point, BE shifts of 0.6-0.7 eV for the N 1s peak and 0.4-0.5 eV for the O 1s peak were observed. Core-level BEs of unsaturated organic molecules bonded to Si(001) were investigated after referencing them to Si 2p spectra (A71). Using this internal standard, the observed BE shifts for alkenes were about 0.6-0.9 eV higher than for alkane-like carbons. Combined XPS and electron spin resonance studies established that a peak at 230.8 eV in the Mo 3d5/2 spectra of various standards was derived from a Mo(V) species (A72). Peaks at this BE in stainless steel were thus attributed to this species. Valence Band Spectra. Published examples of the combination of an anaerobic electrochemical cell and XPS were reviewed (A73). The complimentary natures of core and VB spectra help in the determination of the chemical states on the electrode surface. Multiple linear regression analyses of the VB spectra of polymer blends and derivatized surfaces were compared with analysis from other techniques (A74). Variations between the VB and spectra from other techniques were explained in terms of surface concentration variations. Simulated VB spectra for 10 polymers were very similar to measured spectra (A75). Calculated spectra were obtained using a quantum chemistry software package and model compounds. A linear relationship in the VB between peak intensities and weight percent of poly(ethylene) (PE) was obtained for a series of PE-poly(propylene) (PP) standards. This demonstrated the potential use of this type of data for quantitative analysis of the relative amounts of PE on PE-PP polymer surfaces (A76). 102R


Increased errors in the measurements occurred when significant amounts of hydrocarbon surface impurities were present. Differences between VB spectra of industrial lubricants on aluminum surfaces were observed, thus making it possible to differentiate between similar lubricants (A77). Core XPS spectra were very similar and therefore were not able to distinguish between the different lubricants. Theoretical, XPS, VB, and X-ray emission spectroscopic studies of the electronic states of VO2 were compared (A78). The correspondence of theoretical with experimental results led to the conclusion that the electronic structure is more band like than correlated. Shake Effects and Multiplet Splitting. The interplay between intra-atomic multiplet coupling and interatomic hybridization in determining core-level spectra of rare-earth oxides was reviewed (A79). Analysis of spectra using the Anderson impurity model, which included the affect of interatomic hybridization, resulted in close agreement between the predicted and observed spectra. The maximum intensities of shake-up satellites from the C 1s spectra of vapor-deposited, electroluminescent thin films of tris(8-hydroxyquinoline)aluminum and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine were at 5.5 and 6.7 eV, respectively (A80). Since optically derived energy differences between the HOMO and LUMO for these were roughly half of these values, it was concluded that the shake-up peaks must not be derived from these orbitals, but instead from the π f π* transition. The O/V peak area ratios could not be used to determine the oxidation state of vanadium in a series of vanadium oxides due to the presence of shake-up satellite peaks in the spectra for both vanadium and oxygen (A81). The satellite peak areas were approximately 20% of the areas of the main peaks. Comparison of XP spectra of a series of Fe-based, binary transition metal systems demonstrated that the shape of 3s spectra depended upon the type of chemical bond between the metals (A82). Multiplet splitting and shake-up effects determined the shape of the spectrum. XP and X-ray emission spectra of manganese and iron oxides were compared to each other and to reference spectra of compounds containing these elements in which the electronic structure does not change due to charge-transfer processes (A83). Differences in line widths of 2p spectra with the oxides, but not with the reference compounds, were attributed to the development of satellite structure as a result of charge-transfer transitions. X-ray Excited Auger Electrons and Auger Parameter. The growth behavior of Pd on single crystalline Al2O3 was monitored using the fwhm as well as the modified Auger parameter (A84). Each variable was sensitive to the coverage as well as the crystalline face and indicated a pseudo Stranski-Krastanov growth mechanism. Application of linear algebra to the equations in the electrostatic model led to the determination of the in-framework coordination number and polarizability of aluminum from the Auger parameter (A85). The model was applied to all published zeolite Auger parameter data. Shifts in BEs and Auger parameters for very thin films of TiO2 and Ti2O3 were not in accord with previous work (A86). These changes were ascribed to chargetransfer processes at the metal oxide/metal interface and different relaxation energies compared to thick systems.

Auger parameters of Mg and V from vapor-deposited alloys with compositions of 0.48, 2.9, 8.9, and 15 atom % V were analyzed using the charge-transfer model of Thomas and Weightman (A87). A charge transfer between 0.09 and 0.11 electrons from Mg to V was calculated. The nonstoichiometry of surface lattice oxygen for a series of LaMn1-λCuxO3+λ compounds was investigated by XPS and the modified Auger parameter (A88). Manganese was a mixture of Mn3+ and Mn4+, and the modified Auger parameter was correlated with the extent of nonstoichiometry. Chemical effects in the electron excited LMM Auger spectra of a series of copper oxides were compared with VB spectra (A89). The amount of broadening relative to the pure metal in the Auger spectra was directly related to the oxidation state of the metal and the width of the main VB peak. Semiconductors. Interface states at ultrathin SiO2/Si(100) interfaces were studied by XPS analysis while samples were electrically biased (A90). Reversible BE shifts under these conditions were attributed to charges accumulated in the interface states. The chemical composition of high aspect ratio contact holes of etched SiO2 on Si was determined by combination of a charge neutralizer beam with XPS (A91). Peaks from the resist were separated from the contact holes so that chemical states from each could be determined. Broadness on the low BE side of the Si 2p peak of hydrogen plasma sputtered SiO2/Si substrates was observed (A92). Peak-fitting analysis indicated some of the broadness could not be accounted by the presence of Si3+, Si2+, or Si+, but was in the correct range for a Si-H species. Accurate film thicknesses from thin SiO2/Si and Si/SiO2 surfaces were obtained using angle-resolved XPS (ARXPS) by considering photoelectron yields which were obtained from plasmon-loss peaks (A93). Close agreement with film thickness measurements from ellipsometry was obtained. Since no adsorbed carbon or oxygen were detected after in situ scraping of the surface of LiNbO3 and GaSb with a ceramic file, it was concluded that this procedure produced a surface whose XP spectra were characteristic of the bulk composition (A94). The extent of surface removal during scraping ensured the removal of surface oxides. Spectra of the Pb 5d level were broader on a Si(111) crystal that was misoriented by 4° than on Si(100) when PbS was deposited by laser ablation (A95). The terraces on the substrate induced Pb-rich clusters. A 3% doping of Sb in SnO2 resulted in significant changes in both the valence and core-level spectra (A96). Shifts in the VB were attributed to conduction-band occupation, while strong screening by the conduction electron gas produced “screened” and “unscreened” final-state peaks. Polymers. A consistently asymmetric peak shape for the C 1s spectrum in aliphatic hydrocarbons was observed under highresolution conditions for a variety of practical samples (A97). Since the peak shape varied little with sampling depth from angleresolved experiments, the asymmetry appeared to be derived from “vibrational” structure. Relative degradation rates of poly(vinyl alcohol) films on silicon, copper, and gold substrates during exposure to monochromatic Al KR X-rays were compared (A98). Rates were consistent with the yield of photoelectrons and secondary electrons from the substrates, and the initial rate varied linearly with X-ray flux. A method of rapidly determining the chlorine atomic percent in poly(vinylidene chloride) polymers was

outlined (A99). It involved relating various parameters (e.g., satellite area) associated with the C 1s spectrum to the chlorine atomic %. Films. Thicknesses and compositions of aluminum oxide overlayers on aluminum substrates were calculated using primary zero-loss intensities of the metallic and oxide Al 2p peaks (A100). Exclusion of the characteristic tail, which extends about 30 eV on the lower kinetic side leads to an overestimation of the film thickness by as much as 20%. A constant C 1s intensity after n ) 6-8 for a series of self-assembled films made from alkanethiols, CnH2n+1SH, was observed (A101). This observation was attributed to an increasing degree of order until n ≈ 8. A linear variation in the inelastic energy loss background for the S 2p peak and in the film thickness with chain length was observed. Preferential sputtering of Si was observed during Ar+ sputtering of zeolite materials (A102). The intensities of the inelastic peaks from Si KLL and Al KLL transitions improved the estimation of the surface composition. The effect of anisotropic photoemission was studied experimentally, analytically, and by Monte Carlo modeling (A103). Elastic scattering of photoelectrons causes the mean escape depth to be several times larger than the value predicted by normal XPS formalism. Intensity ratios for various peaks varied markedly from the nominally expected values (A104). Two phenomena were suggested to explain the results: (1) effects due to errors in IMFP, cross sections, etc., and (2) neglect of elastic collisions for the escaping photoelectrons. The amounts of Au deposited on Ni surfaces were determined by analysis of the Au 4d peak shapes using the software package QUASES and compared with thicknesses determined by Rutherford backscattering measurements (A105). Disagreement of greater than 7% was observed from the results of some of the films but was attributed to a nonuniform Au layer. The nanostructures of Ge films on Si(001) were also studied using line shape analysis routines of the background in the QUASES software and atomic force microscopy (AFM) (A106). The islands predicted by line shape analysis were confirmed by AFM, and the combined use of these techniques provided a more detailed understanding of the nanostructure than either technique by itself. A simulation study, using a singular system approach for the inversion of ARXPS data to obtain the concentration-depth profile used equidistance, gravimetric, and equiangular sampling schemes (A107). Random error and the type of depth profiles were most important in determining the quality of the recovered profile. Using standard equations to express the angular dependence of the XPS signal, the molecular organization of thin films of Langmuir-Blodgett and self-assembled monolayers was examined (A108). An enhancement in the signal at 30° for films from 12alkanethiol and 40° for a bis(4-dithyannodithiobenzil)Ni film suggested, in agreement with earlier work, that the molecules were tilted by that amount. Strategies for obtaining realistic depth profiles from angular resolved XP spectra were outlined (A109). Three regions for the depth distribution of an overlayer were defined and strategies for optimized estimation of the depth distribution in that portion of the layer were included. Angular distributions of photoelectrons were measured for Cu, Ag, and Au and were compared to predicted values from transport theory calculations (A110). Much better agreement between experiment and theory was found when elastic scattering was included. The Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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angular distribution of Ni 2p3/2 photoelectrons using Al KR X-rays also was measured and compared with the commonly used straight-line approximation model calculations (A111). The inadequacy of the theoretical results was improved by modifying the model to include elastic scattering. The complimentary nature of XPS and glancing angle X-ray diffraction in the characterization of complex oxides was reviewed (A112). The combined use of these techniques was particularly useful when films for which structural information was not available were studied. The shortcomings of the techniques also were discussed. The detection limits of surface impurities of Fe and Cu on Si wafers decreased by more than 100-fold when spectra were acquired in a total reflection mode (glancing angle for the X-ray source) (A113). The detection limit of about 1010 atoms/ cm2 was achieved since there was a great reduction in the background of the spectra. However, the detection limit has to be further reduced to meet the needs of ultraclean Si wafer surfaces in the semiconductor industry. Mathematical relationships relating the signal intensities from ion scattering spectroscopy (ISS) and XPS analysis of films during film growth were developed for the layer-by-layer and island growth mechanisms (A114). The success of the relationships was demonstrated by studying the growth characteristics of SnO, SnO2, CoO, and Co3O4 on MgO and SiO2. An analytical approximation for overlayer corrections of spectra from rough or powdered surfaces was proposed (A115). A “magic angle” of ∼40-45° was shown to exist in which roughness effects could be ignored. XPS analysis of porous silicon, after room temperature derivatization using Grignard reagents, confirmed the direct formation of covalently attached organic layers (A116). Surface composition was roughly what was expected. Several models for quantitative XPS and X-ray absorption spectroscopic analysis of nanometric powders, coated by an overlayer, were described (A117). Spherical, cubic, and continuous particle types were considered and the advantages of each technique were outlined. A method for the global analysis of XPS signals from surfaces of arbitrary measurable roughness was presented (A118). A frequency histogram of the “local slopes” was generated from the atomic force microscope and combined with XPS data. Reduction of CeO2 in CeO2/Si films during XPS monitored sputter profiles could not be attributed to X-ray-induced reactions (A119). Comparison of simulations for short- and long-term effects with experimental results suggested that the energy released, as the implanting ion collided with an atom, was the likely cause. Measurement of the intensity of the Cu 2p3/2 spectrum (a line profile) across an etched crater from a Ti film on Cu produced a more accurate estimate of film thickness than sputter depth profiling (A120). It was necessary to scan in the nondispersive direction to avoid inaccuracies associated with variations in intensity. The relative concentrations of Fe2+ and Fe3+ in an oxide film on iron were determined by a reconstruction of the composite oxide spectrum from standard spectra of each pure chemical state (Fe, Fe2+, Fe3+) (A121). The film was assumed to be of constant composition. The reconstruction also yielded film thicknesses that were in excellent agreement with those determined from the intensity of the O 1s spectra and ellipsometric analysis. 104R


Spatially resolved chemical maps of the C 1s region across the surface of atmospheric particles were reported (A122). Although spatial variations were observed for major chemical species, trace species were not observed in the maps due to a low S/N. XPS and ellipsometric measurements of polyperfluoropolyether film thicknesses were calibrated using X-ray reflectivity as an absolute calibrant (A123). Accurate estimates of film thickness were obtained when an escape depth of 2.5 nm was used for XPS and a bulk reflectivity index for ellipsometry. The extent of damage as a result of X-ray irradiation during XPS measurements of self-assembled monolayers was evaluated by XPS and Fourier transform infrared spectroscopy (A124). XPS data alone were not sufficiently sensitive to changes in the surface chemistry or composition. Instrumentation. A small incident angle XP spectrometer was based upon modifications of a conventional XPS instrument with a 300 W monochromatic X-ray source (A125). Although the angle of incidence for the X-rays was greater than the critical angle, noise was greatly reduced and examples of its use to monitor surface cleanliness were provided. Unlike an earlier paper, time-resolved XPS spectra could be obtained during a temperature-programmed desorption of poly(ethylene glycol) films on silica with conventional instrumentation (A126). Each spectrum was acquired in 25 s with a heating rate 0.1 K s-1. Others noted that they had performed kinetic experiments with conventional as well as synchrotron-excited instrumentation. They showed that the synchrotron radiation decreased acquisition time by 2 orders of magnitude (A127). It was reiterated that it was possible to perform kinetic studies using conventional equipment in certain experiments (A128). The use of a laser plasma for an X-ray source (∼255 eV) was demonstrated with a spectrum showing the Si 2p3/2 and 2p1//2 peaks (A129). Spatial resolution of ∼60 µm can be obtained, and the system uses a timeof-flight electron analyzer. An integrated circuit was designed, which can be used with a microchannel plate electron multiplier to produce a 2D electron imaging system that was designed for XPS (A130). The chip is composed of a total of 256 × 256 channels of nMOS chargesensing circuits. Although its functionality was demonstrated, it was not used in an XP spectrometer. Charge neutralization of insulating materials was improved by the combined use of a redesigned electron gun neutralizer and a defocused low-energy ion gun (A131). The neutralizer was made from a rare earth oxide having a work function of about 2 eV and produced a current of 400 nA/mm2. The ion gun neutralized the negative charge outside of the X-ray illumination area. Negligible damage from these was observed in the C 1s spectra of PTFE after a 1-h exposure. Equations to calculate the spectral distortion from distances between detectors were derived (A132). The noise level distortion from the use of two or more detectors could be calculated. AUGER ELECTRON SPECTROSCOPY Auger electron spectroscopy by electron beam interaction with a solid surface is employed widely for both elemental and chemical analysis of the near-surface region. The atomic environment of elements in the analysis region often can be evaluated by analysis

of the observed transitions. AES is one of the most widely used approaches for depth profiles in the near-surface region, i.e., up to 1 µm or so. Also, efforts continue to make the technique more quantitative. The analysis of small features, well under 1 µm can be accomplished by AES. A review of the last thirty years in AES was published (B1). Line Shapes. Investigations of Auger line shapes from both experimental and theoretical perspectives have continued during the review period. Line shapes often can be used to “fingerprint” certain species and provide an understanding of chemical bonding. Small changes in the C KVV line shape were noted with various levels of hydrogenation of single-crystal diamond (B2). Theoretical calculations that accounted for the hydrogen coverage described the observations. A comparison between an experimental target factor analysis (TFA) and a theoretical line shape for the N contribution in TiN was made (B3). There was reasonable agreement between the approaches. The ability to determine conduction band density of states (DOS) using typical AES equipment with variable beam voltages was shown for Al, Be, and Ni (B4). The procedure, called core-level inelastic electronscattering spectroscopy, relies on observing peak shape changes with different compounds. The use of the linear muffin tin orbital method found the Pd atoms in Cu-rich Cu-Pd alloys affected the Cu valence states up to three atoms away (B5). Thus, the experimental Pd M4,5N4,5N4,5 peak shape could be described correctly. Satellite features in the L1M4,5M4,5 transitions for Ag, Rh, and Pd were ascribed to shake processes (B6). This interpretation was based on using Ti KR X-rays and analysis of the L1L2,3M and L2L3M lines. X-ray-induced LMM and LMN transitions for several 4d metals were measured and compared to an initial-state j-j coupling and final-state intermediate coupling model (B7). Good agreement between experiment and theory was found in most cases. Nonrelativistic calculations for the LM4,5M4,5 spectra of Rh and Pd, as well as those that included relativistic and configuration interactions, agreed with experiment (B8). Shake-up was proposed to account for the observed satellite features. The effects of induced extra-atomic charge due to site correlation and atomic potential parameters for calculated energy shifts were evaluated for alloys (B9). The first effect correlated with Fermi level changes, but the latter effect was too small to be observed. Quantitative Analysis. Research has continued to make AES a technique that is capable of quantitative determinations in the surface region of solid materials. The use of a self-organizing map method for quantification was applied to AES spectra of Co-Ni alloys with different compositions as standards (B10, B11). An “unknown” alloy composition was found with an error of less than 1%. Analyses of mixed molybdenum and tungsten carbides, based on the linear least-squares method, were made in the spectral region where significant peak overlap occurs (B12). Simulated spectra were computed using two standard spectra of the individual carbides, and internal agreement usually was within several percent. Monte Carlo calculations of loss features for Cu were made and compared to the Tougaard background subtraction procedure (B13). Small differences were found only in the L2,3M2,3M2,3 region for the LMM spectral envelope. A comparison of compositions determined by peak area, peak-to-peak heights (pph) in derivative mode, and peak height in integral mode for

the O KLL transition showed a wide variation with the transition metal oxides (B14). When the direct peaks were broadened with a 20 eV Gaussian peak and then differentiated, the variations in the pph and negative peak to baseline became only several percent. The use of the L2,3M4,5M4,5 metal peaks produced better results by considering the effect of the number of O electrons in the M4,5 level. An earlier equation described the background in the 2 kV region for many materials in a standard data set with a beam energy of 5 kV (B15). With data obtained using a 10 kV beam, a more complex expression was needed; small corrections for surface roughness could be made. Sets of calculated Auger ionization cross sections were compared to absolute measured values (B16). Only one calculation method agreed well with experimental values; differences with other approaches in some instances were approximately 50%. A wide ranging comparison of theoretically calculated AES spectra that included ionization cross sections, backscattering coefficients, and IMFPs was made to standard data (B17). Overall, the agreement with experimental spectra was very close and equations for matrix factors were included also. Depth Profiling. Reviews of depth profiling were published (B18, B19). Several models for sputter correction were tested using various semiconductor compounds, and a fitting equation was proposed (B20). Mass effects predominated over chemical bonding in this analysis. The TRIM program was employed to simulate ion mixing in depth profiles while taking into account IMFPs and backscattering (B21). The method closely followed experimental results and requires only a single variable. A sputtering model that involves mixing, roughness, and information depth was applied to profiles of GaAs/AlAs (B22, B23). The model agreed well with experimental data taken with sample rotation except in the earliest stages of a profile. Depth profiles of GaAs/ AlAs were determined at various ion beam angles, gases, and ion energies (B24). The best depth resolution was achieved with lowenergy (500 eV) SF6 and at a high grazing angle; the results were modeled also. Surface roughness has the largest contribution to the overall depth resolution compared to atomic mixing and IMFP (using low kinetic energy transitions) (B25). Monte Carlo simulation of depth profiles for GaAs/AlAs multilayers were made that included effects of interstitial atoms and vacancies with annihilation of the latter (B26). The calculations agreed well with experimental data. Atomic force microscope topographical data were employed to find the depth distribution function (DDF) and the angular distribution function of emitted electrons with various levels of sputtering for Al deposited on Si (B27). The depth profile using the derivative Al KVV peak agreed closely with experimental data. While sputtering to a depth of only 5 nm, the composition of fractured GaP and InP changed (B28). However, there was no surface roughening of the GaP. FA revealed added phases and sputter-induced compound formation for ZnN/BN multilayers (B29). These results were observed with transmission electron microscopy and Rutherford backscattering, which confirmed the overall structure of the multilayers. Three-dimensional analyses were made by using a focused Ga ion beam (diameter 0.1 µm) that was parallel with the electron beam perpendicular to the surface (B30, B31). Using scanning Auger microscopy (SAM), the lateral resolution was less than 0.1 µm and the depth resolution was less than 0.01 µm; reduction of Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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these values may be possible. Bevels produced by etching solutions have been demonstrated using computer-controlled times of immersion (B32). The depth profiles with GaAs/GaAlAs heterostructures were somewhat smoother than those produced by conventional low-energy, glancing ion beam profiles. A method to calibrate depth profiles with any material based on using a mesh over a specimen was described (B33). The depth is measured with a profilometer after the profile is completed. Instrumentation and Technique. An electron gun, operating at floating potential of about 100 V, reduced charging greatly when used at a relatively high incident beam angle (B34). The approach was less successful with XPS. One hundred volt Ar+ at relatively high incidence angles also largely reduced charging (B35). A newly designed input lens for a concentric hemispherical analyzer (CHA) could produce an energy resolution of 0.03% (B36). Examples with Si, SiO2, and Si3N4 were given. A scanning tunneling microscope tip was used as an electron source along with a modified commercial energy analyzer to detect Auger electrons and elastic peak features on Si (B37). The tip was operated in the field emission mode. Elastic peak electron spectroscopy (EPES)sthe measurement of the ratio of the elastic current to the primary beam currentswas compared to AES for Au deposited on InSb/InP and employed to measure the relative amount of As in InAsxP1-x (B38). The results were reasonably close between the techniques, and the differences were explained by differing surface sensitivities. Small Area Analysis. A method was developed to analyze the same micrometer-sized particles in instruments by AES and energy-dispersive X-ray analysis by marking the sample planchet (B39). However, some of the particles could not be correlated between the two systems; better results might have been achieved by using SAM. An advanced sample positioning stage for 20 cm Si wafers found locations to within about 2 µm (B40). The assembly introduced low levels of particulate contamination. Energy-analyzed secondary electrons in the peak maximum region could detect stepped structural features on a Si (111) surface (B41). These features could not be observed using either the LVV or KLL Si Auger peaks. Ion beam-induced Auger electrons were produced using a Ga source at 20 kV, and elemental maps with a resolution below 1 µm were produced (B42). Sample loss was relatively low, and better spatial resolution may be possible. With the capability to have a complete spectrum at the individual pixel level, some of the problems of data analysis and visualization were explored (B43). At present there is still much development work needed, since the quantity of raw data is greater than present quantification methods can process easily. COMBINED XPS-AES TOPICS In this section, the review covers publications that contain material of interest to users of both XPS and AES. General reviews on XPS and AES appeared (C1, C2) along with aspects of quantification (C3). The applications of PCA and FA in Auger electron spectroscopy and XPS were presented (C4). The performance of AES, XPS, and secondary ion mass spectrometry (SIMS) for elemental analysis was reviewed (C5). Standards and Databases. Changes in standard values for transition energies using several metals were recommended for XPS and AES (C6). These changes were about 0.07 eV different 106R


from previous values. In addition, the effect of oxidized anodes on observed XPS transition energies was evaluated. This work was furthered with respect to both peak energy and intensities for XPS and AES (C7). This has lead also to the production of standard data for many elements. The use of thin-film reference materials in XPS and AES was reviewed (C8). The composition of TiN, Ti(C,N), and (TiAl)N films was analyzed by wet chemical methods, XPS, and AES (C9). Deviations between the XPS and AES determinations, especially with substoichiometric materials, and the bulk values were ascribed to sputter cleaning effects. The use of “home grown” GaAs/AlAs multilayers for routine depth profile standards was discussed (C10). The ability to share XPS and AES spectra via the Internet was described (C11). Data are submitted, (converted to the VAMAS/ ISO standard if needed) and are checked before inclusion. ISO is the International Standards Organization. A database with 35 000 spectra that can be used with a personal computer was described (C12). The background in XPS and AES spectra continues to receive attention. The contribution of elastic scattering to XPS and AES peak intensities can be approximated by a simple multiplicative factor for most experimental geometries (C13). These results were based on Monte Carlo calculations with several elements, and the effect of large emission angles needs to be determined. Three primary peak parameters, based on IMFP, loss features below about 100 eV for the analyzed peaks of interest, and peak shape and intensity, predominated in quantification (C14). Other factors only improved to a limited extent the quantification results. An equation to remove the effects of secondary and redistributed primary electrons for AES (and extendable to XPS) was tested with Ag, Pt, Ta, and Ni using a cylindrical mirror analyzer (CMA) (C15). The resulting spectra then had segmented straight lines to eliminate extrinsic contributions to give flat baselines. Inelastic Mean Free Paths. A review for both experimental and theoretical determinations of IMFPs was made along with evaluations of consistency for a number of elements and selected compounds (C16). It was concluded also that EPES has produced better IMFP values than other procedures. The various quantities (e.g., IMFPs and attenuation lengths) for the information depth of XPS and AES were critically assessed (C17). Factors such as elastic scattering and electron emission angle on these parameters may be significant depending on the technique employed, and the simple relationships commonly employed may have large errors in actual practice. Values of IMFPs determined by EPES were somewhat lower than those calculated by the TPP-2 equation (B38). A factor that considers the signal intensity derived from the signal due to elastic electron collisions alone was proposed to account for elastic scattering effects (C18). It was found that this calculational approach greatly reduced errors throughout a wide range of experimental conditions and gave reasonable agreement with Monte Carlo computations. Attenuation lengths were determined for photopolymerized Cd 10,12-pentacosadiynoate on the native oxide layer of Si (C19). The data were taken over a wide range of takeoff angles, and the findings were in good agreement with earlier studies. The escape probability of Auger and photoelectrons from thin overlayers on nonuniform surfaces was calculated using kinetic equations over a wide range of

emissions angles (C20). A comparison with experimental data for Al2O3 and Al using photoelectrons was in good agreement. The escape probability of O 1s photoelectrons for a thin CuO on Cu did not follow a simple exponential probability (C21). The intensity results were fitted reasonably well employing Monte Carlo calculations with the depth distribution factor (DDF). The transport mean free path was found to follow the Born approximation over a wide range of energies for low-Z elements, but more complex behavior was noted for higher Z elements (C22). These findings followed results from other methods. With carefully grown FeO on Fe, attenuation lengths reasonably close to empirically determined values were obtained (C23). Standard materials and background corrections for the XPS spectra were required for these analyses. The IMFP values (not attenuation lengths), determined by EPES for GaAs and InP, were in good agreement with prior calculations (C24). However, effects such as ion beam roughening of the surface required consideration. A correction factor for elastic scattering effects on the origin depth of the electron was tested with Monte Carlo calculations for indepth distributions (C25). When the electron emission angle was less than 30°, the correction factor was reasonably valid. IMFPs for Cu, Cu2O, and CuO were determined by EPES over an energy range of 400-1600 eV (C26). The experimental values were slightly greater than those calculated by several theoretical models. Monte Carlo calculations of IMFPs for Cu over an energy range of 50-5000 eV were lower than those computed from optical data (C27). Including surface excitation corrections raised somewhat the values from the Monte Carlo procedure. IMFPs using InSb and GaSb were determined by EPES with different instruments (C28). The experimental values were in good agreement between the various laboratories and fell between various theoretical determinations. Corrections were made to earlier IMFP values obtained from Fe/Ag/Fe(110) multilayers that made the new values greater (C29). The corrected values were larger compared to IMFPs from the TTP-2 model. Instrumentation and Technique. The elastic reflection coefficient was determined for two types of energy analyzers and compared to data from others (C30). This type of information allows comparisons to be made between various systems for quantification. An interconnected analysis and exposure system was constructed (C31). It allowed specimen exposure over a wide temperature and pressure range. A portable sample chamber was constructed for use with different analysis techniques (C32). Specimens could be heated or cooled as needed. A transfer cell that just covers the specimen area on a sample mount was described (C33). With this design, contamination from the assembly is greatly reduced. Ozone in the 10-4 Pa range greatly reduced charging with SiO2 during AES analyses (C34). However, with XPS the results were not as good. Multitechnique Analyses. Simulations for the X-ray-excited valence band and L1L23V spectra of hydrogenated amorphous Si were produced using the MOPAC computational program (C35). Moderate agreement with experimental results was obtained. The positions and peak shapes of the O KVV and O 1s lines in films of Ti-Al-O indicated a wide range in solubility between Al2O3 and TiO2 (C36). Also, the structure of the Al2O3 was deduced. The use of self-organizing maps to determine elemental composition by XPS and AES was explored (C37). The method involves

comparison with materials of known composition against those that are unknown. Coincidence XPS and AES. Auger photoelectron coincidence spectroscopy (APECS) has been used to gain further understanding of the influence of such processes as loss effects and holehole repulsion energy on Auger line shapes. The technique involves measuring both the photoelectron and Auger transition from the same ionization event. A Green’s function approach was employed to calculate coincidence spectral features for the L2,3VV line of Ni and some Cu species and the metals (C38). There were insufficient data for a complete test of this approach. The Cu 2p1/2 photo line in coincidence experiments shifted to a higher binding energy and then returned to the starting value as the kinetic energy for the L3M4,5M4,5 Auger transition being analyzed was reduced (C39). This effect was ascribed to altered screening that the Auger and photoelectrons encountered upon leaving the solid. Coincidence measurements of the Cu 4p and N2,3 VV transitions indicated complex atomic-like behavior for the Auger transition (C40). The 4p photo peaks cannot be considered as simple spin-orbit, but angular momentum coupling between the core hole and the open shell has to be taken into account. A onestep model for coincidence XPS-AES was developed and applications of this approach to understanding decay processes were discussed (C41). The count rate for a coincidence 2p-L2,3 VV spectrum of Cu was improved by a factor of 10 (C42). Some new satellite and shake-up features in this regime were observed. Acknowledgment: The authors wish to thank the Mr. D. Stickel of the Chemical Abstracts Service for providing many of the abstracts that are the basis of this review. Noel H. Turner’s research interests for over twenty-five years have been in the area of surface analysis using X-ray photoelectron spectroscopy and Auger electron spectroscopy. He received a B.S. in chemistry from the University of California, Berkeley, and a Ph.D. in physical chemistry from the University of Rochester. Currently he is associated with George Washington University in Washington, D.C. and the Lectromechanical Design Co. of Sterling, VA. John A. Schreifels is an Associate Professor of Chemistry at George Mason University, Fairfax, VA. He received his B.S. in 1975 and Ph.D. from the University of South Florida. In 1988, he became an Associate Professor at George Mason University. He has worked in the field of surface science for over 20 years. His research interests are in the field of solid surface interactions with gases and liquids using Auger and photoelectron spectroscopies. Dr. Schreifels is a member of the American Chemical Society and the American Vacuum Society.

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A10000110

Surface Analysis: X-ray Photoelectron Spectroscopy and Auger

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