Anal. Chem. 1900, 60, 377 R-387 R (462) Alden, M.; Wallln, S. Appl. Opt. 1985, 24(21), 3434-3437. (463) England, W. A.; Glass, D. H. W.; Brennan, J.; Greenhalgh, D. A. J . Catel. 1988, 700(1), 103-117. (464) Eckbreth, A. C.; Anderson, T. J. Appl. Opt. 1985, 24(16),2731-2736.
(465) Pubanz, 0. A.; Maronelli, M.; Nibler, J. W. Chem. f h y s . Lett. 1985, 720(3), 313-3 17. (466) Van Hare, D. R.; Carrera, L. A.; Rogers, L. B. Appl. Spectrosc. 1988, 39(2), 347-352.
Surface Analysis: X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Noel H. Turner Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000
This fundamental review is on the subject of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) and will cover the literature abstracted in Chemical Abstracts between November 18, 1985, and November 6,1987. 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 (1-14). 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 the improvement in the "state-of-the-art" of these techniques. The goal of this review is to help analysts solve the problems that are encountered in using XPS and AES in a routine laboratory setting with commerciallyavailable equipment. A section on inelastic mean free paths (IMFP) will be in the combined XPS-AES art of this review. Finally, t t e names of the authors of the papers cited could not be included in text due to space limitations.
X-RAY PHOTOELECTRON SPECTROSCOPY Introduction
XPS or electron spectroscopy for chemical analysis (ESCA) is one of the most widely used techniques for elemental analysis of the near surface region. Also, this method gives information about the chemical environment of the observed atoms. Much useful information can be obtained from XPS, even though a complete understanding of binding energies and intensities has not been achieved. There have been a number of reviews of XPS during the past 2 years, both general ( A I )and specific with an emphasis on topics such as small clusters (A2, A3), metal complexes (A4),binding energy shifts (A5), electronics (A6, A n , the Si02/Si interface (At?), failure analysis (A9), soils (AIO), surface reactions ( A l l ) , ceramics (A12), mineral flotation processes (A13),and polymers (A14,AI5). Investigations that have used synchrotron radiation have not been included in this review; this topic has been discussed elsewhere (A16). Blndlng Energies
A compliation of 13000 binding energies from XPS data was reported to be near completion (A17). This information will be invaluable to XPS users. A review of XPS with an emphasis on energy scale calibration and instrument response functions for various commercial spectrometers was presented ( A B ) . It was suggested that Zn or Ga 2p3I2and 3d peaks be used for XPS binding energy calibration, since the 3d peaks can be resolved (A19). Also, the energy that separates these peaks is known accurately from X-ray emission spectroscopy (XES). The variation in binding energies (f0.2 eV) with different instruments was found to be smaller than in an earlier round-robin study for selected transitions (A20). However, for some other peaks that were not used earlier, the
range of reported binding energies was somewhat lar er. Studies of the effect of charging have continued. i n investigation of charging by the deposition of metal dots on various insulating substrates was undertaken (A21). It was found that the surface charge could change during the determination if a flood gun was not used. With the use of a flood gun it was noted that the reference levels were to the vacuum level and often dependent on the work function of the specimen. Variations in the observed binding energy of Au on different insulators were noted (A22). Also, the shift was larger with materials that have a greater bond ionicity. With insulating samples it was observed that charging decreases when the current and voltage of the X-ray source increase (A23). Some differences were found also when there was an A1 window used with the X-ray source. Pd clusters and SiOzwere shown to have shifts that were related on A1203 to particle size (A24). At low coverages initial state effects were suggested to be important. The polarizability of the substrate and number of available d electrons were considered to become more important as the cluster size increased. Binding energy shifts to higher values for Ag and Pd on C substrates were greater for smaller clusters (A%). With Si02 the shifts were dominated by preferential charging, and A1203 and it was proposed that final state effects could account for these observations. Shifts in the binding energy of Pd on CdTe (a poor conductor) were suggested to be due to initial state effecta at low coverage (i.e. small particles) and changes in valence band energies (A26). At higher coverages final state effects approximately balance changes in the valence band binding energies. The calculation of relaxation energies with an electron gas model for small particles indicated that a fall off starts with sizes about 4 nm (A27). These findings agree with other determinations. Small negative binding energy shifts were noted upon the addition of up to several layers of Au on Ag(ll1) ( A B ) . From data of this type and the use of a Born-Haber cycle, a cohesion energy of the adsorbate can be computed. Surface binding energies were found to be shifted relative to bulk peak positions for Au-Pt alloys (A29). In this case the surface atoms had lower binding energies than the bulk. It was observed that for Ar+ bombardment on plasma-induced C films on glass, binding energies increased with higher doses (A30). These findings were explained by charge transfer effects. It was suggested that H impurities in the surface region can be observed with binding energy changes with different compositions in Ag-Pd alloys (A31). The energy separation between the A1 2s or 2p and the first loss peak was found to be sufficiently different to distinguish A1N from A1203(A32). Data Handling
A number of different approaches to the treatment of XPS data have been investigated during the review period. A relationship for the S/N ratio in terms of the S B (B is the background) for XPS was developed and teste (A33). To use this method, the noise level of the spectrometer must be known. This method allows an estimate of the time required for obtaining separate spectra with a given S/N ratio. A comparison of curve fitting and factor analysis for W03 overlayers on W indicated that the agreement between the methods was within 20% (A34). Where there is a large
This article not sublect to U.S. Copyright. Published 1988 by the American Chemical Society
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number of spectra, factor analysis is easier to employ, while with only a limited number of experiments, curve fitting would be preferable. The use of an asymmetric-combined Gaussian Lorentizan curve for fitting XPS peaks was proposed (A35). This method adds another variable to the calculation and reduces the usefulness of the approach; an example with W and W03 was given. The use of the fast Fourier transform to reduce data collection time was demonstrated (A36). The procedure makes use of the elimination of higher order coefficients that contribute very little to the observed peaks. Both van Cittert and Fourier transform methods to narrow overlapping XPS lines were investigated for block copolymers containing bisphenol-A-polycarbonate/dimethyl siloxane. The resultant curves were then followed by curve fitting procedures (A37). The results indicated that the choice of deconvolution function, and subsequent curve fitting procedure, affected the final computed line shapes and that the expected Lorentizan features were not present. A comparison of deconvolution (van Cittert) of a standard Mg X-ray source vs a monochromatic Al X-ray source on the same instrument showed that the S/N ratio for a given peak was a function of the S/B ratio (A38). To obtain a spectrum with a given S N ratio and full width at half maximum (fwhm) in a given ata collection time, the choice of method will depend upon a comparison to previously obtained standard peaks. A narrowing of about 0.2 eV in the Au 4f7/2peak by the use of the Fourier transform to remove the KaZ X-ray component was noted (A39). The removal of inelastic loss features on XPS spectra has received attention recently. These effects have to be considered as part of any quantitative estimates based upon XPS data. Corrections for inelastic peaks in XPS spectra by a “universal function” were studied (A40). This procedure can be applied with reasonable ease. An approximation method was compared to a more exact procedure to remove the inelastic background in XPS spectra (A41). For homogenous solids the approximation gave similar curves to a previous procedure with less computational effort. Background subtraction methods that included IMFP based upon the employment of a reflected high-energy electron beam were studied (A42). One procedure did not use a fitting parameter and requires less computation than the other methods investigated. However, it does give negative intensities and adjustments to the IMFP used are required. The shapes of the loss features for several metals were measured (A43). All of the intensity from this source of the observed signal is confined to 50 eV below the main peak. The use of a previously developed method based upon the peak area ratio to the increased background signal was extended to the case of a layer below the surface (A44). It was found that an empirical approach resulted in better agreement vs a more rigorous method when compared to experimental results. The effects of energy loss, X-ray source, and analyzer resolution were removed from XPS spectra by a ratio deconvolution procedure (A45). Examples were given for Fe and several of its oxides. XPS is gaining use as a method to study elemental distribution in the near-surface region in a nondestructive manner. A previously developed method to account for energy loss features in XPS spectra was examined with several systems that had thin overlayers (A46). The method improves quantitative estimates and elemental distributions in the near-surface region. Changes in XPS line shapes as a function of depth for inhomogenous transition metals were evaluated theoretically (A47). A single parameter that is a function of the kinetic energy of an X P S peak and the distance an electron goes in the solid characterizes the peak shape. The ability to determine near-surface inhomogeneity by the determination of the intensity of the background in XPS spectra was studied (A#). An empirical relationship for such a measurement was obtained. A method to determine the composition of separate, homogenous layers that have different composition by the use of angular resolved XPS (ARXPS) has been derived (A49). Examples with oxidized Si and Ca stearate layers on Si were given with reasonable results. A procedure was developed that allows intensity data to be correlated with depth of an interface region (A50). The method was used with residual F that closely followed the N profile (after a chemical etch) in SiOzthat had been treated with a nitride. Composition depth profiles analyzed from ARXPS using a Laplace transform approach were shown to be difficult to apply due to both experimental data problems and nonunique solutions ( A 5 1 ) .
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In some cases curve fitting can be used to find the profiles. This method is more applicable with larger IMFPs. In many cases an ion beam is used for depth profiling with XPS, although not as extensively as in AES. Backscattered electron images have been employed to analyze the beam crater for a wide area ion gun used for XPS profiles (A52). It was found that the beam was uniform over an area of about 2 mm, which would be poor from many systems. Often important information can be obtained from measurement of the relative intensities of XPS peaks with minimal corrections to the experimental data. A model to use observed XPS intensities for various geometries of supported catalyst (i.e. flat, spherical, and hemispherical particles) were proposed (A53). This model takes into account the effects of angular and layer thickness averaging. A model to determine the extent of dispersion and surface segregation from XPS intensity ratios was proposed and tested with RuOz on Si (A54). When combined with IR spectral data, the results agreed with SEM findings. A method to analyze the relative amounts in mixtures with a common element that can be resolved by XPS was developed (A55). The results were within 10% in almost all of the mixtures analyzed. Variations in the relative intensity of the Hf 4f7/2vs 4f5j2peaks in HfN, were noted as x increases relative to the growth of the N 2s line (A56). However, the Hf doublet goes to higher binding energies as x increases, but this does not occur with the N h e . Flat fiiters were found best for the analysis of aerosol particles by XPS (A57). A linear dependence of signal intensity vs mass of the particles was noted. X-ray Induced Auger Transitions
Auger transitions that are induced by X-rays often can provide a useful adjunct to conventional XPS analyses. One approach is to study the line shapes of Auger transitions. For example, two different types of AES N line shapes were observed (A58). One type, based upon NH3, was noted for borazine and boron nitride. The other line shape which is similar, was found with pridine and poly(2-viny1)pyridine. Investigations into determination of the Auger parameter have been extended to energy regions beyond those usually accessible to Mg or Al. A number of Si compounds were analyzed with a Zr X-ray source and Bremsstrahlung radiation from an A1 source (A59). Also, the chemical state of Si in various oxidized Fe samples was studied. A monochromatic Ag X-ray source was employed to determine Auger parameters for C1 in a number of alkali metal chlorides (A60). X-rayexcited Auger peak positions and relative intensities with Al, Ag, and Ti anodes for 65 elements were reported (A61). This information extends the range of Auger parameter determinations. No change in the Auger parameter was noted for either Si or SiOzwith various thicknesses of SiOz on Si (A62). A correlation of polarizability of various Si compounds was found (A63). Initial-state and final-state effects were studied with data from Auger parameter determinations (A60, A6365). Angular Resolved XPS
AFtXPS continues to be found useful in many analyses, and improvements in equipment and procedures have occurred. Improved angular resolution (down to just over 1’) by the use of tube arrays or glass microchannel plates was demonstrated (A66). The design considerations and some examples were given. ARXPS of Ni(100) with a resolution of about 1’ revealed structural features not found with lower resolution (A67). Also, it was noted that the source signal appeared to be from a point rather than a plane wave. A single scattering model with spherical waves reasonably fit the experimental data. This same approach was used with studies on Cu(100) (A68). This method was found to be better than a simple Kikuchi band model. High-resolution ARXPS valence band spectra were obtained for W(OO1) and W(0ll) as a function of temperature (A69). These results were compared to various theoretical models with semiquantitative agreement. Reasonable agreement between theoretical angular distribution curves (from a small scattering center approximation) and previous experimental curves was achieved ( A70). However, improvements still can be made. Enhanced intensity of XPS and Auger lines of Cu overlayers on Ni(100) were explained
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n
Nod H. Tvmr is a R-rch
Chemkt at the Naval Research Labwatay. Washing tw. DC. He recslvsd a B.S. in ChemisbY tran ms univnrrsny Of CBIifMIIIB. Balkeley. in 1962 and a Ph.D. in physical chemistry Iran lha Unlverrity of Rochester in 1968. Hk oTaduate wolk was in the area of mspha& kineticS u& tha diredon of l h a k W. 0. Wanerr. In Janualy of 1968 Or. Turner joined the staff of the Naval Re. search Laboratory and worked in the area of gas-iolki adswption. His current research interests are in ms area of Auger &&on spectroscopy and X-ray photoeiecbon spctroscopy. or.Turner is a member ot the American Chemical Society and has been active in the Division of Colloid and Surtace Chemishy. He ais0 is a member of Sigma X I . American Asso~klionfor the Advancement ot Science. and American Vacuum Socie. ty.
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by forward scattering (A7Z). Electron channeling could not account for these observations. A method to analyze ARXPS data for depth distributions with linear functions from a delta function approach was derived (A72). This procedure was compared to an earlier iterative method with both actual and simulated data. Both approaches gave reasonable results, but better profiles were obtained when the newer method was used in conjunction with the older one. The simplex method was used to analyze ARXPS spectra of a heterojunction to determine the interface composition (A73). However, some assumptions about the system under study are required. The use of a Au X-ray source with simultaneous Mg radiation did show differences in the SiO/Si ratio of a native oxide (A74). It was suggested that the Si KLL Auger transitions could be used also, but these lines may have different cross sections for Si vs SiO,. A method to find the variation in composition with depth of 0 diffused into various Nb and Si compounds by ARXPS was developed and tested (A75). Various surface inhomogeneities can he taken into account. Polymers
XPS has been used in the analysis of a number of polymer systems to investigate various aspects of the near-surface region. Surface enrichment of poly(viny1methyl ether) mixed with polystyrene was observed by ARXPS not to follow a simple parallel layer model (A76). Also, a procedure to correct for hydrocarbon overlayer contamination was given. Poly(dimethylsiloxane) was found to be in excess when mixed with bisphenol A polycarbonate (BPAC) compared to the hulk composition (A77). The use of ARXPS was found not to he precise enough for this polymer system. The use of unique C 1s features in the high binding energy region of BPAC was employed for quantitative determinations. The employment of F substitution in segmented poly(ether urethanes) and poly(ether urea urethanes) with poly(tetramethy1ene glycol) (PTMO) permitted the investigation of the surface distrihution of these systems (A78). The presence of F with its relatively large photoemission cross section and ARXPS indicated the PTMO segments are located preferentially a t the surface. ARXPS and the spectra of individual films of poly(N'-trifluoroacetyl-L-lysine) (Kt) and polysarcosine showed that Kt predominates when the K t is in low relative amounts on the surface of the block copolymer (A79). This effect disappears when the amount of Kt is about 50%. Analysis of the C 1s line shapes in polyacrylate copolymers was questioned on the grounds of X-ray damage, sample contamination, and the curve fitting methods used (A80). A reply followed (A8Z). Neither article completely addressed all of the questions raised. Surface enrichment of poly(viny1 chloride) (PVC) blended with poly(methy1 methacrylate) (PMMA) was found from the elemental ratio of the 0 1s to CI 2p peaks and agreed with contact angle measurements except with high relative amounts of PVC (A82). Changes in surface groups of plasma altered polystyrene vs the bulk were noted by ARXPS (A74). Derivatization continues to be a useful procedure to study polymer surfaces. The reaction of glassy carbon with titanium diisopropoxide bis(2,4-pentanedionate) to determine the
number of surface hydroxyl sites was investigated (A83). The method was validated by using the reaction with known phenoxy resin compounds. The relative number of carbonyl groups on glassy carbon and poly(methy1 vinyl ketone) was determined by derivatization with pentduorophenylhydrazine (A84). These findings were in agreement with results by other reactions. The reaction of Pd(OAc), with surface olefins was proposed from a study of irradiated poly(ethy1ene terephthalate) (A85). The derivatization of cellulose fibers by using the reaction of trichloro-s-triazine with the surface hydroxyl groups was reported (A86). By proper choice of substituent groups on the aromatic ring, further coupling reactions are possible. Reaction of Ba ions with oxidized carhon fibers, followed by XPS analysis, was employed to determine the amount of surface acidity (A87, A88). This method gives comparable results to other methods. The number of adjacent acid groups (
