x-ray photoelectron spectroscopy, Auger electron ... - ACS Publications

Anal. Chem. 1984, 56,373R-416R

Surface Analysis: X-ray Photoelectron Spectroscopy, Auger Electron Spectroscopy, and Secondary Ion Mass Spectrometry Noel H. Turner, Brett I. Dunlap, and Richard J. Colton* Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375

This fundamental review is on the subject of surface analysis and includes the fields of X-ray photoelectron spectroscopy (XPS),Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) for the period of 1981-1983. This review will cover the literature abstracted in Chemical Abstracts between November 2,1981 and October 31,1983 (plus several important articles that have appeared in the latter part of 1983). The review is written in four separate sections for the reader’s convenience: section A -XPS, section B-AES, section C-Combined XPS-AES Experiment and Theory, apd section D-SIMS. XPS, AES, and SIMS are the most widely used techniques in surface analysis and, in fact, they are often used in combination. Since the early 1970s, each technique has grown rapidly. Much of this growth is documented in earlier Fundamental and Application Reviews in Analytical Chemistry (1-10). Recent developments in molecular SIMS are also documented in the Fundamental Reviews on Mass Spectrometry (11-13). This review, although lengthy, is not all-inclusive (2 year) biblionraDhv of XPS. AES. or SIMS. We selected the most imporvan’t papers that (in our opinion) will advance the “state-of-the-art” of XPS, AES, and SIMS. We also decided to omit some topics such as the use of synchrotron radiation in XPS, for example, and to limit or ignore the large number of papers on applications in order to keep this review to a manageable size. We have, in addition, paid more attention to theoretical developments in XPS and AES. The most difficult task in writing this review (other than the task of handling such a large number of papers) is trying to write coherently while limiting the scope of the review to only those papers published during the last 2 years. We apologize, therefore, when the topics seem disjointed.

X-RAY RHOTOELECTRON SPECTROSCOPY Introduction

X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA), continues to be a widely used technique for the elemental analysis of surfaces and the neap surface region. In addition the technique also provides information about the chemical environment of the observed atoms. While these are some difficulties in the complete understanding of the measured binding energies and intensities, much useful information can be extracted in most cases from XPS. During the reporting period there have been several general reviews of XPS along with numerous reviews on specific topics (e.g., electronics) in which XPS has been used. The Nobel Prize Award address by Siegbahn ( A I ) has been widely published. Other reviews, in which much of the basics of XPS are covered, have been prepared by Roberts (A2,A3) Windawl and Wagner (A4),Verdrine (A5), and Hercules and Klein (A@. A book by Ghosh ( A n has been published on photoelectron spectroscopy with a portion of the text devoted to solids and surf@ce@. In this review studies that have used synchrotron radiation have not been reviewed. While much new and basic information has been obtained from such radiation sources, the availability of synchrotron radiation to most researchers is severely limited. Thus much of the information derived from such studies cannot be directly put to use by most researchers. With new synchrotron sources being built or planned, this situation will change in the future. Those interested in the use of synchrotron radiation for surface studies are directed to a review by Smith and Woodruff (A8). This review will not cover gas or liquid phase XPS. Jolly (A91 recently has reviewed gas photoelectron spectroscopy. Papers concerned with liquid phase XPS have been published

by Sie bahn et al. (A10). A discussion of inelastic mean free paths FIMFP) will be in the combined AES-XPS section of this review. Factors Affecting XPS Analysis

Binding Energies. Nefedov ( A l l ) has conducted a round-robin survey of several spectrometers, almost all of which were of different designs using inorganic compounds as the basis of the test. Variations of up to several tenths of an eV between instruments were noted for both binding and peak separation energies. The standard deviations were usually much smaller. In an analysis of the errors of reported binding energies from an earlier round-robin study, Lee (A12) has concluded the referencing to the “Fermi level” is the major cause of the observed discrepencies. A method to overcome the problems associated with the determination of binding energies has been developed by Anderson et al. (A13). Fermi-level referenced ESCA, or FRESCA, is based upon varying the potential of a field emitter until the peak corresponds to that of the photoelectron of interest. Errors in binding energies of about 0.17 eV were noted. Swift (A14)has reviewed the use of the adventitious C 1s line for energy referencing. While discrepancies have been observed in many instances, it was concluded that the adventitious C is line can be of use, but the limitations must be considered. Kohiki et al. (A15) have proposed the use of a low dose of Arf to correct the charging. The 2p3/, level of Ar in metals is used as a standard, and this line along with all of the other XPS lines of interest in the insulator are shifted to match the standard. In their studies reduction of the various insulators has not been observed. This method is based upon the assumption that the interactions of the Ar in the metals is the same as that in the insulators. The possibility of selective charging has been presented in the data of Ayame et al. (A16) for Ag-NaC1 catalmt. In this case several peaks were split by a nearly equal amount. The history and or preparation of a sample can affect the observed XPS bin ing energy. Egelhoff ( A 1 3 has shown that with expitiaxl growth of Ag, Au, or Cu on Al(100) the binding energies of the overlayers do not approach the bulk levels until there are 10-20 monolayers. The core level binding energies decreased by 0.8-1.8 eV with increasing thickness. Grunthaner et al. (A18) have made similar measurements with Ni/Si or Pd/Si systems as a function of coverage or temperature. The binding energies of Si and Ni fall as coverage increases, but the effect is much less with Pd. Changes in binding energies as function of time and heating of compounds originally containing water have been found by Hirokawa and Danzaki (A19). In some cases more than a simple dehydration may be taking place. Shock loaded minerals have been observed by Jakubith and Hornemann (A20)to have new peaks compared to the original state. The measurement of the energy differences between a core level and the valence band maximum to within h0.04 eV has been reported for GaAs and Ga by Kraut et al. (A21). The coye and valence level spectra were obtained simultaneously, and a number of experimental and theoretical factors must be taken into account for such accurate measurements. The changes in the binding energies in the oxide-GaP interface, produced under different conditions, have been assigned primarily to differences in the Fermi level position by Mizokawa et al. (A22). Other effects such as charging, dipole movements, and extra-atomicrelaxation were considered much less important. Smart (A23) has considered the shifts in binding energies of semiconductors with different surface treatments to be due to band bending and sample charging. From a simple model the changes in sample charging can

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This article not subject to US. Copyright. Published 1984 by the American Chemical Society

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indicate the amount of band bending. Citrin et al. (A241 have used monochromatic A1 Ka excitation and angular resolution to distinguish between surface and bulk energies of Au, Ag and Cu. Differences of -0.08 to -0.40 eV were found, via very accurate line-shape fits. Shifts of the same magnitude (but not necessarily the same sign) for Au, Pd, Rh, Sc, and Ca bulk vs. surface binding energies have been found by Erbudak et al. (A25). The difference in the value for Au between these two studies was within the quoted experimental errors. Cross Sections and Sensitivity Factors. In the roundrobin study mentioned previously ( A l l ) ,it has been determined that the relative intensities could be measured to about 10% between different instruments with the proper corrections. Empirically determined XPS sensitivity factors have been determined for two commercial XPS spectrometers by Wagner et al. (A26). Areas and peak heights have been'given for major and strong secondary peaks and comparisons have been made to previous studies. Elliott et al. (A27) have used theoretical cross sections, asymmetry parameters, and instrument throughput to compute sensitivity factors for a commercial XPS spectrometer with a monochromatic X-ray source. An IMFP correction has to be used in combination with the report sensitivity factors for quantitative XPS analysis. Relative sensitivity factors for several elements have been determined to within -10% by Yabe and Yamashina (A28).Matrix and overlayer contamination effects contributed to the error. A much higher attenuation factor for the IMFP of a contamination overlayer compared to most other studies was found. Intensity ratios for Ni and Fe and some of their oxides have been measured by Olefjord and Marcus (A29). Nonuniform Surfaces. The measurement of island diameter and number from a statistical analysis of XPS intensities over different parts of a sample of gold dots on Si has been compared to a simple theory by Ebel et al. (A30). A contamination layer that was on both the islands and substrates had to be considered also. The effects of roughness and electron takeoff angle have been studied for a nonretaring type XPS spectrometer by Ebel and Wernisch (A31). Experiments with groved samples were compared to model calculations (a set of cubes a t various angles) that included the effect of overlayer, and the experiment and calculations agreed remarkably well. Changes in XPS intensities have correlated moderately well with BET gas adsorption and light scattering data, as studied by Wu et al. (A32). However, Au (- 100-300 nm) was deposited on the substrates before the XPS measurements, but after the surface area measurements, which might have some effect on the measurements. Huizinga and Prins (A33) have observed that Pt particles on Ti02y d A1203 show different binding energies as a function of dispersion (and with Ti02,a phase change). These were in the region of about 0.3 eV when the A1 2p and Ti 2p3 positions were fixed to a constant level via the C 1s line. kina1 state effects were suggested to be the cause of this observation. X-ray Excited Auger Transitions. Many XPS spectra contain X-ray induced Auger transitions that can provide additional information about the system under investigation. The use of such information, mostly in terms of the Auger parameter, i.e., the addition of the binding energy of a major photoelectron line and the kinetic energy of a major Auger transition, has been reviewed by Wagner (A34). The Auger parameters for a number of Si-0 and A1-0 containing compounds have been reported and correlated with structure by Wagner et al. (A35). The location of the points on a plot of Auger vs. photoelectron energies has been explained by the polarizability and the final state energy of the ion in the Auger process. West and Castle (A36) have determined the Auger parameter for 30 silicates (with the use of a Zr La X-ray source) and have found reasonable agreement with the refractive index of the commands, as is expected from theory. Also the bulk polarizability, predominated by 0, has been correlated with the Auger parameter for tetrahedral structures, but not for octahedral sites. A correlation of Pauling's electronegativity with extra-atomic relaxation energy (Le., Auger parameter) for Si and some Si compounds has been made by Fellenberg (A37). Bechstedt et al. (A38) have attempted to calculate relaxation energies from Auger parameters, dielectric constants, and the plasmon energy of solid Si compounds. This effort was successful only when the 2p level of Si level was used. The Auger parameters for Cd in CdSe, CdTe, and 374R

ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

CdSeo.,Te0., have been compared by Polak (A39)and a range of 0.9 eV has been noted amon the compounds. The Auger parameter was larger than the knding energy shifts and this gives an indication of the relaxation effect of the species studied. From previous data it was found that the Auger parameter for gases was less than or equal to that for all solids with the same atom. With gas phase w. solid phase data with the same compound, this trend has been confirmed by Wagner and Taylor (A40). This suggests that atoms in the solid not directly bound to the atom of interest still contribute to the screening energy. The Auger parameter has been measured by Henry et al. (A411 for Xe in the gas phase and adsorbed on to clean and oxidized Mo(ll0). A contraction of the conduction band at the surface has been suggested as a reason for the change in the final state polarizability, and hence the Auger parameter. Wagner et al. (A42) have outlined the procedures that are needed in order to obtain reproducible data for the determination of Auger parameters. The calibration of the spectrometer against natural lines and taking the data so that any instrumental drift can be corrected were suggested. XPS and Auger parameter data from Lampert (A43) for Ba and some of its oxygen containing compounds have indicated that extra-atomic relaxation accounts for the lower binding energies of the oxides. Pederson (A44) has observed larger shifts in the Auger parameter for numerous Pd compunds compared to XPS binding energy shifts. A practical example of the use of the Auger parameter has been in the study of Na vapor deposited onto vitreous silica by Lau et al. (A45). The Auger parameter of Na indicated that the Na was present as an ion. While the use of the Auger parameter has been one of the major areas of work with X-ray induced Auger transitions, other studies have not been centered on this particular topic. X-ray excited Auger L2,SMM and L2,3MVtransition of Ca have been reported by Vayryner et al. (A46). The extrinsic losses of the L2,3MMtransition were very similar to those of the Ca 2p lines, and the spectra were in reasonable agreement with spin-density function calculations. Yin et al. (A47) have used X-ray excited LMM spectra and the 2p lines of completely oxidized V and Cr compounds to study the reductive effects of ion bombardment and the bonding of these materials. Intraatomic AES transitions have been proposed by these investigators for the observed L3M2,3M4,5 and L3M4,6M4,6 transitions. The measurement of the Cu LMV LVV ratio from X-ray radiation for analysis purposes has een tested by Klein et al. (A48) for several different compounds. Differences could be observed in many cases if the removal of the background due to inelastic peaks with a crude model was done. X-ray excitation has been employed by Koel (A49) to observed the C KVV Auger spectrum of CO on Ni. These results were similar to those of Fe(CO), and suggested that the hole-hole repulsion energy was very small due charge transfer. Radiation and Ion Beam Effects. The X-ray radiation used in XPS usually has little or no effect on the material being studied. There are cases, however, where the X-ray radiation has been found to alter the sample. Deaton and Walton (A50)have found that Mo mixed carbonyl-isocyanide complexes decompose rapidly when XPS spectra were made at room temperature and relatively high X-ray power. However, when the temperature was lowered to about 190 K and the X-ray power was reduced, degradation was not observed. Cu(I1) intercalated into a and /3 Zr(HP04)2has been observed to decompose under X-ray radiation by Marletta et al. (A51). Variations in rates of decomposition were ascribed to differences in the crystal structures of the phosphates. Sharma et al. (A52)have noted that AgCl will decompose under X-ray radiation, but at a much slower rate than under UV irradiation. An overall loss of both Ag and C1 were observed. XPS is well suited to study the possibility of chemical alteration of a material as a result of ion bombardment. Christie et al. (A53) have studied the results of ion bombardment of the carbonates, nitrates, and sulfates of Ca, Sr, Ba, and Pb and have noted changes in the oxidization state and stoichiometry of the anions with little or no changes in the cation. They also have observed that the extent of the changes was related to the free energies of formation, with those compounds having the lower values being altered the most. Oxides of Ti, Ta, Zr, and Hf, but not Al, have been

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SURFACE ANALYSIS

r a t w and worked in me area 01 ps-soim amorption. HIS cunent research intere~ta are In me area 01 Auger electron specbasCOPY and x-ray Photoelecbm specaoscopy. h Tuner Is a membet 01 the American

1976. His dissBRBWn research was directed by Uoyd Armstroog. Jr. Hs has held poptdataal posnbns with Jdln W. D. CanmW and Jdln R. SaMn In OUBntUm lhe. ay project 01 the Univsrsny 01 F-. J. W. oadrlk Sutate in Science me E W Di*lSbn m physlc4 d me Nmional0vSection and

aks can provide additional information about the sample King investigated. Wagner (A34) has reviewed briefly this topic. Examples of this effect with organic compounds have been given by Distefano et al. (A.58) for para-substituted benzonitrites and benzaldehydes. Shake-up features were observed in both the N and 0 1s regions, and reas6 :able agreement with CNDO-CI calculationswas found. Both shake up in the 0 region and shake down in the N region satellites have been observed in some nitro Schiff bases by Fritzpatrick and Katrih (A.59). The shake-down process in these systems has been ascribed to a charge transfer from the non-nitro N atoms in the HOMO to N atoms in the LUMO; the shake-up peak has been assigned to a T T * transition. Shake-up features for Mo mixed carbonyl-isocyanide complexes in the C, N, and 0 1s regions have been noted hy Daton and Walton (A.50). Hedge et al. (A60) have studied the satellite region near the core level for transition metal oxides and some transition metal oxyanions. While they were able to suggest the source of the satellites, no trend could be obtained for the satellite shift vs. the number of d electrons for a given element. These workers have studied a number of transition metals and rare earth compounds with regards to the core and lignand core hole spectra and associated satellite structure (A61). A simple theory has been proposed that explains the experimental trends. Satellite structure, i.e., distance from the main core line and intensity, could not be predicted for NiPd alloys from a thermochemical approach by Steiner and Hiifner (A62). This approach has been successful for the shifts in core level positions, and the failure for satellite structure has been ascribed to the neglecting of d-band contributions to cohesive energies. Scrocco (A63) has observed that by studying the satellite structure of the core levels of Zn and Cd, the positions and intensities of unoccupied bands can be observed. XPS core level spectra and satellite structure of several a-Zr (PO,),M, compouna (M is an alkali metal) have been reported by Pignataro et al. (A64). They have found that the satellite structure depends upon M and sputtering. The sputtering was suggested to have changed the relative composition of the material and to have created damage that increased the satellite relative intensity. This group also has investigated a- and 7-Zr acid phosphates and have found that the satellite structure was dependent on the phase examined and electronic defects in the material (A65). A low energy satellite from the 3ds line of SmAI, has been ascribed to a divalent surface layer inslead of a shake-down peak from the bulk by Raaen and Parks (A66). The contraction of 4f level would be too small to expect such a relatively large eak Plasmon peaks for Au and Ag from XPS spectra have fee, observed hy Leiro et al. (A67). While the positions of the peaks have been found to agree with those noted by other techniques, the intensities do not. These peaks are about 100 times less intense than the main photoelectron lines. The various oxides of U have been investigated by Allen et al. (A@) by XPS. I t was found that the satellite structure was very useful in understanding the complete surface chemistry of this system. Data Handling. The use of computers can greatly improve the data collection and/or data reduction in XPS. In some eases new information may come to light as a result of refined data handling. Brunix and vanEenbergen (A69) have developed a program that can be used for peak identification in survey scans with a resolution of 1 eV. Peak mea and width and background can be determined, but some broad peaks cannot he readily identified. The determination of the area of XPS peaks is important if quantitative analysis is desired. However, the problem often occurs as to a what the base line should be. This has been illustrated by Bishop (A70)who used a simulated spectra similar to those for mixtures of Fe and Fe,O in the LMM and 2p region. Then four workers were asked to measure the area or areas of the 2p peak(s). Errors of up to 37% were obtained, but this could be reduced to 17% if the area of both peaks was determined. Verma (A711 has demonstrated with a simple series expansion of the experimental data a narrowing of an observed spectrum can he obtained. The procedure is much simpler than either a Fourier transform or vanCittert calculation;an example with data from a Zircaloy-Z sample was given. Viljoen (A72) has used the vancittert method to improve the line widths from XPS spectra of Ge S and Ge Se. A combined Gaussian-Lorentizan function was used. The Gaussian function was used for the instrument response, and the actual peak was quite close to

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r e m 01 Standards. and wHh DavM E. Rameksr in me Chemktry mprtment of ' Oeage Wsshlngton Unlversny betwe lolnhg the staff at M I L In me ThBMetlcal chemlsw Section 1" 1980. Hts Vlmary merests are actuate moiecuiar X a c~lcuisllMlsand me rob 01 symmetry In physlcBl phenomena. He is a memba 01 Slgma Xi and me Amerlcan phvsicai Soclsty.

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Rkhard J. Conon Is a s w a v l w n , research Chemist and Head 01 me s t i o n on Advanced Swlace S~BCQ'OSCOPYat me Naval Research Lamalay. He earned hls 0.5. and RI.D. degrees horn me U " h S n y Of Pmsbum hl 1972 and 1976. reswcnvsly. He pertDrmed his gadasto work under the dhsction 01 Dr. J. W. RabalaIs In me areas 01 ukavkiet and X+ay photoelectron specboscopy. In 1976. he was awarded a NRC Resk$3nt Research ASSOClateShip at me Naval Research Labaatoly la a research pe POsBl MISllCq vim secondary ion m a s specamby (SIMS). h.Conon jolned the staff a1 NRL in 1977 and m M u m bask and applied research in me area 01 amlaw c h m s v . m r-rsn momslnclule swtaw and materials analyses by s w m SD~C~~OSCOPY and SBO ondary b n mass specvometry. the development of new surface analytical tools. and me study 01 me mechanism 01 mie%uIBr and polyatornk ion eml&n. h.Callon Is a meof Its ACS. ASMS. ASTM. and AVS. He Is also drakman of ASTM E42 wbcommmW On SIMS. chairman of me ASMS uxnmmse on soIkIs and ourlac~s.and a member of me edit.x!al board la me InlamaIh?a Journal 01 Mass Specbanetry and Ion processes.

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found to be reduced from the action of Ar+ by Hofmann and Sanz (A.54). The layer of reduction rcducts was estimated to be about 2 nm. In the case of inert gas ion hombardment has not caused observable r uction, hut the XPS lines have been broadened. Hofmann and Thomas (ASS)have suggested the broadening is due to disorder in the Si0 bonds. Coyle et al. (A.56) have investigated the effect of Art on a number of fluorosilicates. For the alkali and alkali eartb compounds damage was not observed. For the transition metals decomposition depends upon upon the reactivity of the cation. The more reactive cations yield metal fluorides while the less reactive cations suffer reduction and then logs of Si and F. These workers also have studied the effect of Art bombardment on transition metal sulfates (A.57). For the metals that tend to form stable oxides, metal oxides have been observed, and the metals that are more inert to oxidization undergo reduction to the metallic state and formation of elemental S. Satellite Effects. XPS satellite structure is often easily observed near a major photoelectron peak. These satellite

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ANALYTICAL CHEMISTRY. VOC. 56. NO. 5, APRIL 1984

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SURFACE ANALYSIS

a Lorentizan. A generalized program to analyze overlapping peaks has been described by Dunn and Dunn (A73). It also has a combined Gaussian-Lorentizan line shape and can account for electron energy loss. The program has been written so that certain parameters may be held constant during the fitting process. Examples with several polymer systems were given. The factor analysis approach to resolving overlapping XPS peaks has been studied by Gilbert et al. (A74). In this method of calculation there is no need to assume any given line shape; the procedure was used on data from the X-ray induced transformation of Pt(en)z(OH)zClzto Pt(en),Cl,. A small laboratory computer can be used for the factor analysis approach. Proctor and Sherwood (A75)have taken another approach to fiiding multiple peaks in an XPS spectrum. They have given a procedure to account for the inelastic background subtraction. In addition they have shown that a derivative XPS spectra can indicate the number of peaks in an envelope, but that intensities cannot be found. Also they have suggested that small peaks can be detected by difference spectra, but alignment and normalization are very important in this analysis, and there must be model spectra. Examples of carbon fibers that have undergone various treatments have been given (A76). Quantification. The ability of XPS for quantitative analysis has been reviewed by Carlson (A77). The factors that are involved in such analysis have been considered along with the use of photo induced Auger lines. In making relative measurements many of these variables cancel and most investigators are able to use fairly simple expressions. Many of the tabulations of relative intensitivities in theory can be used directly if a similar XPS spectrometer to the one used in compiling the intensities is being used (e.g., Warner et al. (A26)). The results from the round-robin test noted above ( A l l )should serve as a warning to the use of such intensity ratios without checking. Ebel et al. (A78) have proposed a method for quantitative analysis by XPS without standards by the use of a complete expression for the determination of XPS lines. However, the energy dependence of IMFPs and variable angle and/or different elemental subshell information has to be supplied in order to employ this approach. Several examples of quantitative XPS analysis in comparison to AES are given in the section on combined techniques. Other checks on the quantitative capability of XPS have been made by Houalla et al. (A79). These investigators have studied the dispersion of NiO on B modified silicas by XPS and transmission electron Microscopy (TEM). The intensity ratio of the Ni/Si XPS lines agreed well with the TEM data. Sulfate groups on polystyrene latexes have been analyzed by Stone and Stone-Masui (A80) with XPS. Qualitative and quantitative determinations have been made using standard procedures and sensitivities. Via variable angle analysis, it was observed that the Sod2groups were on the surface and that the amount a reed with a conductometric titration. Hydrolysis of the SOP- groups could be monitored also. Instrumentation

The past 2 ears has seen the ability to analyze areas of about 150 pm become commercially available in XPS spectrometers, The possibility of obtaining resolution of 100 pm in a nonretarding XPS spectrometer has been investigated by Gurker et al. ( A 8 l ) . In order to accomplish this goal a scanning sample positioning device along with a position sensitive detector would be required. Scanning X-ray radiography has been developed by Cazaux et al. (A82). In this procedure the sample becomes an anode with electrons striking the back of the sample. Resolution of about 20 ym has been achieved, and a factor of 10 improvement may be possible. Also, it is possible to X-ray induce Auger electrons with a similar resolution (A83). A simple collimator for a commerical dual anode X-ray source has been developed by Hawn (A84). The signal to background ratio has been improved without much overall loss in signal intensity. In addition, cross-talk has been reduced by a factor of 6. A dual anode (Al and Mg) X-ray source that can replace earlier, commercial designs that have only one anode has been demonstrated by Ganschow and Steffens (A85). Count rates 10 times those obtained from the earlier source have been measured; the power supply for this X-ray

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ANALYTICAL CHEMISTRY, VOL. 56,

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source is described also. Cazaux and Duc (A86) have shown the ability of a W anode to produce Bremsstrahlung radiation useful for X-ray excited Auger electrons. Good improvements in the signal to background ratio for the A1 and Si KLL transitions compared to the use of an A1 anode under the same conditions have been reported. The spectrometer function, dependence on the sample area, spectrometer transmission factor, and the energy dependence of the detector has been measured by putting a bias on the sample by Ebel et al. (A87). The results of this analysis have shown that the presumed energy dependence is only followed approximately. Cazaux (A88) has considered the intensity of the background in XPS and its influence on the elemental is the mindetection limit. It has been concluded that imum detection limit with conventional XPS spectrometers. An optical position sensitive electron detector for a nonretarding XPS spectrometer has been credited by Bertrand et al. (A89)with a 250 times improvement in data collection rate. The associated computer program has been described also. The use of a small microprocessor to control a commercial XPS system has been demonstrated by Evans and Elliott (A90). Some data reduction capability was available also. Depth Profiles

XPS does not find as wide use for depth profiles studies as do other techniques, Le., AES and SIMS. This is due to larger area that must be sputtered in most instruments for XPS analysis, the relatively slow XPS data collection rate, and in many instruments, the inability to sputter and take XPS spectra simultaneously. There are some cases where XPS depth profiles can be useful, e.g., thin layers that may not require sputtering and samples that may be altered drastically by an electron beam. Tougaard and Iguatiev ( A 9 l ) have proposed a method of obtaining quantitative depth information that involves the measurement of the peak areas of an XPS line vs. the background (Le., loss features) for pure elements. Then, the same quantity is measured for the sample of interest. Initial results were presented for A1 on Ag. A stop flow chemical etching for depth profiling Si,N4 on SiO, by XPS has been developed by Wurzbach and Grunthaner (A92). A resolution of 0.5-1 nm was achieved, but consideration of residual contamination has to be made. Depth profile analysis from variable angle XPS data has been claimed to give a resolution of -X/3 (A is IMFP) by Pijolat and Hollinger (A93). Examples have been iven for Ag-A1203 and Si-SiO, interfaces and a Cu-Ni alloy. 8i/SiO, Si02interfaces of different thickness have been studied y Finster and Schulze (A94). They have made use of variable angle intensity measurements and binding energy changes; equations have been derived for the analysis of a three layer system. Bhide et al. (A95)have used both XPS intensity and X-ray induced Auger electron ratios to study CdS:Cu2Ssolar cells. Information about the chemical state of Cu as a function of pretreatment was obtained. The determination of the thickness of Si02layers on Si with the use of a CMA has been demonstrated by Ito et al. (A96). The angle between the sample and the X-ray source was varied and the ratio of oxide vs. substrate Si was measured. A value of the IMFP of Si had to be assumed in order to determine the interface depths.

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Polymers

XPS is used quite widely for the analysis of polymers and biological surfaces. Clark (A97) has reviewed this topic with a special emphasis on IMFP determinations, plasma polymerization of thin films, and polymer photodegradation. A review of fiber and polymer surfaces has been prepared by Millard (A98). Dilks (A99) has reviewed XPS studies of polymers with respect to binding energles, escape depths, angular effects, reactions of polymer surfaces, and stoichiometry. Pireaux et al. (Al00) have covered the use of the X-ray induced VB spectra of polymers. Briggs (AlOl)has reviewed XPS with an emphasis on adhesion. XPS has been employed by Thomas and O’Malley (A102) to study several classes of multicomponent polymers, Le., regular block copolymers, random block copolymers, and physical mixtures. Intensity ratios, variable angle electron take-off angle, and shake-up satellites have been used to compare the surface to bulk

SURFACE ANALYSIS

compositions of the individual pol mer8 and the various classes. The conditions that are nee ed to use reactions; Le., derivatizing specific, surface functional groups have been analyzed by Batich and Wendt (A103). They have found that thallium ethoxide or sodium hydroxide can be used to tag surface carboxylic acids. Methods to determine surface hydroxide groups on methyl methacrylate and hydroxypropyl methacrylate copolymers by derivatizationwith trifluoroacetic anhydride have been developed by Dickie et al. (A104). Via angular resolution studies it has been determined that the F (and hence the hydroxy groups) were away from the surface. The adsorption from solution of polyethyleneimide and colloidal silica on sulfated polyethylene has been studied by XPS for a variety of conditions by Larsson et al. (A105). The structure and extend of the adsorption process could be inferred by angular resolution studies. The capability to distinguish carboxylic, carbonyl and alcohol groups on plasma activated polyethylene terephthalate has been developed by Ohmichi (A106). The procedure involves reacting the surface with reagents that are fairly specific to the groups noted above. Pennings (A107) has demonstrated that the reaction of heptafluorobutanic acid chloride with vinyl chloride, vinyl acetate, and vinyl alcohol copolymers will selectively cause fluorine to be attached to the latter two compounds. The extent of the reaction can be monitored by XPS.

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AUGER ELECTRON SPECTROSCOPY (AES) Introductlon

Auger electron spectroscopy continues to be very widely used in surface analysis, with new applications appearing during the reporting period. Several reviews of a more or less general nature have been published on AES by Grant ( B I ) , Braun et al. (B2) (along with other topics), and Allen and Wild (B3). Somewhat more specific reviews with an emphasis on quantification have been prepared by Venables and Fathers (B4)and Lea (B5). The use of ion-induced AES will not be covered in this review; those interested in this topic should read an extended article by Baragiola (B6). Other reviews concerned with specific topics AES (e.g., depth profiling) will be given in that specific headin . Powell et al. (B7)have reviewel the results of a round-robin study of AES by the ASTM committee E-42. In this investigation energy and intensity measurements for Cu and Ag were obtained from 28 different spectrometers made by four manufacturers. Energy spreads of 7 and 32 eV for nominal peaks at 60 and 2025 eV, respectively, were reported. The range of intensity ratios varied by more than a factor -38 for the 60 and 920 eV peaks of Cu and -120 for the 70 and 2025 eV peaks of Au. Reasons for the large variations were advanced. A discussion of inelastic mean free paths (IMFP) of electrons will be in the AES-XPS combined portion of this review. Auger Line Shapes

Elements. The use of AES as a valence band spectroscopy has been reviewed by Madden (B8)with an emphasis on data processing and atomic and band structure for elements and compounds. Two methods of obtaining the local density of states (DOS) from AES C W (C = Core, V = Valence) spectra have been investigated by BartoEi and MBca (B9). A point by point method tends to increase the effect of random noise and gives oscillations. A Fourier transformation procedure minimizes individual point contributions, but requires more computations. Maguire (B10) has suggested that the high energy features of the Be AES spectra are that due to plasmon gain (Le., an energy absorption of a bulk plasmon before emission). The main peak or peaks are dependent upon the method of preparation or treatment. An interpretation of the AES KVV spectrum of oxidized Be has been given by Rogers and Knotek (BIO)that is based upon similarities to SOz. They have suggested that the hole-hole repulsion energy is -0 eV. The KVV spectrum of F2(and the XPS 2a and 2a, spectra) have been measured and analyzed by Weighman et al. (BI2). Several models were tried, and the best results were obtained when screening and both initial and final state effects were included. Miura (B13) has investigated the AES LVV spectra of S on several substrates. The high energy peak has been ascribed to substrate interactions, i.e., with band structure and that the hole-hole interactions were very small.

The variation in energy and line shape of the AES Cu MMV spectra on epitaxial Cu(ll1) with new layer formation has been shown by Namba et al. (B14).Slight changes in the valence band in the edges vs. bulk were noted. The AES spectra of T c , the M4,5NS,7N6,7 and M4,5N6,702region of Yb (as a vapor), of the high energy region for U, and of the low energy region for 241Amhave been reported (B15-418) and compared to theoretical predictions of the respective elements. Compounds, Alloys, and Complexes. Houston and Rye (B19) have reviewed AES line shapes in molecules and surfaces; they also have included some theoretical background. The use of AES (and other techniques) to study hydride surfaces has been reviewed by Malinowski (B20). Obviously, AES observations have to be based upon changes in line shape. A number of Li compounds have been studied by Ferguson et al. (B21),and there were differences in the number of peaks and their position between many of the compounds. Many interesting effects in the line shape of C have been noted for a number of different systems. Kelber et al. (B22) have compared the loss corrected AES K W spectra of polyethylene and poly(ethy1ene oxide) to their respective gas phase analogues. The polyethylene C KVV spectrum was similar to n-alkanes as was the 0 KVV in poly(ethy1ene oxide) to dimethyl ether. However, the C KVV spectrum for the latter polymer was similar only to ethane. Variations in both the C and Cs AES signals have been found for intercalation of Cs into highly ordered pyrolytic graphite under different temperature and pressure conditions by Lagues et al. (B23), Craig et al. (B24) have proposed that the amount of C-C vs. C-M bonding in metal-carbon films can be analyzed from derivative AES line shapes. aC:H and Fe3C served as standards in combination to which the unknown is compared on the basis of peak heights. Good results were obtained. Derry and Sellschop (B25)have compared the AES line shapes of C ion implanted into diamond to standards for damage studies. Many ionic fluorides have a single AES F KLL peak, but several have a doublet structure. This latter feature has been correlated except for KF, to the K ( Y ~ satellite , ~ structure in X-ray emission spectra by Deconnlnek and Van den Brook (B26). L3L2,3Vand L2,3Wspectra of Mg and L2,3VV spectra of Si in Mg2Sihave been recorded and compared to valence band calculations by Bevolo and Shanks (B27). However, base line and loss corrections were not made (and are important in the energy region where the measurements were done), and the conclusions are open to question. The effect of H in SiH on the AES L23VV spectra has been compared to Si via line shapes by Madden (B28). H was suggested to “tie-ofP dangling surface bonds and thus change the VB DOS. The AES Si L2,3VVline shapes of plasma-deposited silicon nitride and chemical vapor deposited silicon oxynitride could be constructed from curves of Si-N, Si-Si, Si-0, and Si-H. Madden and Nelson 0329) have observed for this procedure to be successful background subtraction and the removal of loss features was required. Plasma formed nitride films on Si have been investigated by Koyama and Kashiwaki (B30). Ar+ was found to destroy the film, where as N2+both removes and helps form the fii. The Si-N vs. pure Si ratio could be found from variations in the L W spectra (no background correction) and the N-H/Si-H ratio was proportional to the Si-N Si AES peak heights. Hezel and Lieske (B31)also have ma e use of the Si LVV spectra line shapes of plasma deposited films for chemical state identification. A practical example of the use of AES Si line shapes has been made by Zironi and Poppa (B32) who have studied W-SiC/Ti fiber matrix with quantification made by peak heights. High resolution AES spectra of HC1 (g) have been reported and compared to LCAO MO SCF calculations with CI considered by Akesla et al. (B33). The theoretical results were reasonable, The AES spectra of pure Ti, V, and Mn and their oxides have shown that the is proportional to the number of valence electrons. Rao and Sharma (B34)also have found shape changes with oxidization and that interatomic transitions occur as the 3d level is depleted vs. intraatomic transitions. Landot and Mauri (B35) have noted spin polarized electrons from magnetically ordered Fe6 B for the major Fe transitions. This effect can be only about 20% of the total intensity and was not observed in the B KLL spectra of this compound. Changes in the AES line shape and energy of Be in Fe-B-Be glasses have been determined when the Be composition goes from 4 to 5% by Bevolo et al. (B36).

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The change has been ascribed to an atomic like state going to a valence band transition. In addition, a small change in the B line shape has has been found, which may be due to the Fe-B bond and might be involved with the Be line-shape changes. Vishneuetskii and Fasmar (B37) have investigated various NiAl containing catalyst with different surface composition. Chemical information was derived from the AES A1 line shapes. However, the quantitative analysis in this study were based upon standards from an analyzer of a much different design. The AES line shapes of Cu-Pb alloy have been studied by Sundaram et al. (B39). Surface effects were noted in the Cu L3M2,3M45 v s L3M4,6M4,5 transitions. As expected from theory, the Pd line shape changed, but the Cu line shape did not, as a function of composition. Sen et al. (B39) have noted shifts of 3 f 1and 7 f 1eV in the AES spectra of SnO and Sn02compared to pure Sn. This effect was observed for MNN and MNV peaks. AES has been used to study Ba cathodes by Lamartine et al. (B40). These investigators found changes in the Ba line shape and composition as a function of temperature treatment. The high resolution spectra of Cs(v) and CsI(v) have been recorded by and the results compared reasonably Aksela and Aksela (B41), well with theory. Ellis and Powell (B42)have determined the AES spectrum of V2Ti and have suggested that Ti is an electron donor in this command due to the line-shape changes. Reactions. Differences in the C KLL lineshape with various carbon moieties have been used in several investigations to determine the type of surface carbon. Christie (B43) has suggested that CO2 adsorbed on Li gave a carbonate type structure. AES has been used to follow the hydrogenation reaction of graphite in the presence of Ni and W by Bliznakov (B44);the line shape changed to a carbide type structure with an increase in temperature. Houston et al. (B45) have been able to correlate the reactivity of Rh and Ni single crystals for methanation with the AES loss deconvoluted line shapes of C on these metals. In addition, suggestions were made as to the C coordination sites from the line shape. Differences in AES C KVV line shape as a function of temperature for the decomposition of C2H4on Fe(ll0) have been followed by Channing and Chesters (B46). This group has also studied the adsorption of C2H4on Cu(ll1) and Pt(ll1) and have suggested that a ?r bonding adsorbed ethylene could be identified (B47). One of the species was thought to be ECH. Lower temperature of preparation formation of a (W, Mo) C electrocatalyst favored carbide type C vs. a graphitic C at higher temperatures has been found from AES C KVV line shapes by Kudo et al. (B48). The adsorption of several C containing gases and NH3 on transition metals has been shown by Kamath et al. (B49) to result in the “finger printing” of the resultant AES spectra. AES LVV line shapes of A1 implanted with N have been found to be similar to those for AI-N bonds by Lieske and Hezel (B50). In this case the chemical composition was near A1N. Kelemen et al. (B51)have studied the oxidization of a-SiH by AES line shapes. With changes in experimental conditions differences in the Si LVV line shapes were found and correlated with the oxide structure being formed. Progressive oxidization or nitration of Si has been demonstrated by differences in the background corrected and deconvoluted Si L W spectra by Knotek and Houston (B52). Low coordination intermediates were postulated also on the basis of the observed line shapes. The reaction of excited 02 with Si has been followed by noting changes in the Si LZn3VVand L1L2,3V spectra with different exposures by Brockman and Russell (B53). These spectra were correlated with the removal of “danglingbonds”. Kobayashi et al. (B54)have used differences in the L,L2,3Vspectra of Si with overlayers of A1 to identify the interface electron structure. Fukuda and Igatiev (B55) have assigned the low energy peaks at 31 and 44 eV of oxidized Cr to cross transitions M2,,V[cr)V(o) and MILl(o)V(c,.The oxidization of Ni has been investigated by Holloway and Outlaw (B56)by the observation of the Ni M2,3W and AES Ni L 3 W line shapes. The changes occurred only upon the formation of NiO. AES has been used to study the effects of a laser annealed single crystal of GaAs by Zehner et al. (B57). Changes in the line shape of the Ga M2,3M45M4,5 transition were suggested to be due to nonstoichiometric surface regions as a result of the surface interaction. The reaction products of CO, C02, H20, and O2 on Ba films have been identified by AES line shapes. Shih et al. (B58) 378 R

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have suggested that intraatomic transitions occur for BaO, which indicates an electron transfer to 0 (Le., Ba 4d, 5p, 0 2p vs. Ba 4d, 5p, 6s). Factors Involving AES Analysls

There are a number of factors that can affect the final observed AES spectrum. In the following several paragraphs these effects will be discussed in detail, although in many cases there are interrelationships. Finally, quantification of AES will be covered. Sputter Effects. Sputtering of practical (i.e., rough) surfaces with AES analysis using several different ion beam, electron beam, and analyzer configurations has been considered by Keenlyside et al. (B59). Without the ability to adjust each of these variables for the best possible efficiency for an individual experiment, they have concluded that normal ion beam incidence is the simplest approach for a uniform etch. The angle of the ion beam has been found by Skinner et al. (B60) to be one variable (along with the smoothness of the surface) with the determination of the interface of A1 on I or P. An iterative calculation that included preferential sputtering corrections has been forwarded by Sekine et al. (B61). This approach was tested with a Ni-Pt sputter deposited alloy and the results compared to electron microprobe analysis (EMPA);the AES results were somewhat larger, but only peak heights (without corrections) were used. Ritz and Bermudez (B62)have found that Ar+ bombardment causes reduction on the surface of LiNb03 and LiTaO, as observed by line-shape changes. Carbide formation during ion beam bombardment has been noted by Ingrey et al. (B63) on several metals. The source of the C was thought to be from CHI in the vacuum system; this effect cannot be easily predicted. Changes in the high energy side of the AES Si L,,,VV line shape with Ar+ bombardment have been suggested by Madden and Hjalmarson (B64) to have more p character in the DOS or new states were created in the band gap. Bandyopadhyay et al. (B65)have observed that in the AES analysis of a basalt glass an increase in intensity of Ca, Si, and 0 with a decrease in C with Ar+ bombardment. Fe, Al, and Mg remained fairly constant. Difficulties in AES analysis of TiSi, due to sputter effects have been noted by Blom et al. (B66) when compared to RBS results. More consistent values were obtained when the KLL transition was used. Preferential sputtering of Cu in a Pd-Cu alloy has been determined by Sundararaman et al. (B67) over the range of Ar+ energies of 0.5-5 keV. At 0.5 keV the effect was greatest with little differences noted above 1keV. Heron et al. (B68)have investigated the effect of ion beams of different gases and bombardment energies on Cu20(111). Relatively small differences in the Cu to 0 ratio were found. The effect of heating and ion bombardment on the surface composition and depth profile analysis by AES has been studied by Rivaud et al. (B69). Preferential sputtering of the In and the depth of mixing increased with higher Ar+ energies. Frankenthal and Siconolfi (B70)have used AES to determine the surface composition of In-Sn and In-Pb alloys after Ar+ bombardment. The element with the highest heat of atomization was observed to be enriched on the surface; the temperature and alloy composition were found not to have a surface composition effect. Small particles (-20 Mm), inbedded on In, will be covered by the In with Ar+ bombardment during AES analysis, Hock et al. (B71)have concluded that the effect could not be eliminated and seemed to be more pronounced when the oxide layer on the In was removed. Tu and Schlier (B72) have found that islands of In were formed with Ar+ bombardment of an InP single crystal. This effect was found even at low ion doses and low energy. Preferential sputtering of Si or Ag in PtSi, Nisi2and 0.55 Ag-0.45 AU with Ar+ or Xe+ has been noted by Holloway and Bhattacharya (B73). The effect was greater for Ar+ than Xe+, and ions of lower energy (i.e., 0.5 vs. 2 keV). Ar+ bombardment has been found to reduce SnO but not Sn02 (B39). Electron Beam Effects. The interactions of electron beams and various damage mechansims with solids with regard to AES and high spatial resolution has been reviewed by Levenson (B74). CO decomposition in an electron beam to form a carbide type C on LiNb03 and LiTaO, has been found (B62). A similar type of observation has been noted for several metals with C from CH, (B63). Both C and 0 from residual gases have been observed to be adsorbed onto a Ni

SURFACE ANALYSIS

surface with electron beam bombardment by Li et al. (B75). Fontaine et al. (B76) have studied the electron beam effect on 0 chemisorbed on two A1 surfaces (single crystal and polycrystalline). Little adsorption was found on the single crystal (100); on the polycrystalline surface it was suggested that the electrons caused the 0 to become an oxide. 0 outside the beam was thought to diffuse by interactions with low energy electrons to the irradiated area. In the analysis of A1 on InP, the energy of the electron beam also had an effect on observed interface (B60). The various stages of electron induced oxidization of Si by O2has been investigated by Carriere et al. (B77). Miotello and Mazzoldi (B78) have analyzed the decay of the Na in glasses by electron beams, by both regular and electric field effects on the diffusion process. A reasonable agreement with experiment has been obtained, but many complicating factors were not considered. A similar type of study has been made by Whitkop (B79) on changes in the metal ion concentration during AES analysis in glasses with a kinetic model. Changes in the oxidization states of the ion have to be considered. The decay of Na in AES spectra of lasses has been explained by Ohuchi and Holloway (B80)with 80th electron stimulated desorption and field enhanced ionic diffusion being important effects. Good agreement between experiment and theory was achieved. Bouquet et al. (B81) have studied the migration of C1 by electron beam interaction on a Ti-A1 alloy. The C1 was found to diffuse from outside the beam area, and that the damage decreased as the beam voltage was increased, The diffusion of K and C1 through films of Au or Au Ag/KC1 has been investigated by Farias et al. (B82).The iffusion has been found to be caused by electrons rather than heating. F has been shown to disappear by electron beam effects for CaFZwith the Ca becomin metallic (XPS observation). For a 10% change, a current ensity of 0.2 C/cm2 has been found by Strecker et al. (B83). The AES analysis of Fe-Ni alloys containing B,P, and Mo have been observed to change with electron beam interactions by Burstein (B84). In this case the electron beam was used as a depth profile tool. No beam effect has been found on Cu20 (B68). With CuInSz an effect of the electron beam has been noted by Polak (B85). Cu was found to increase while S decreased, and C and 0 were present due to CO adsorption and interaction with the electron beam. The effect of an electron beam on 20 pm insulating particles on In foil has been explored by Hock et al. (B86). Charging and nonreproducible analysis were found to be reduced or eliminated by higher electron beam currents, but thermal effects using this approach may alter the surface being analyzed. Electron beam interaction has been found to reduce SnO, but not SnOz (B39). Backscattering. Jablonski (B87) has computed backscattering coefficients via a Monte Carlo approach for Mg, Al, and Si (KLL) with different electron beam voltages and angles of incidence. The biggest changes were found when the angle of incidence was varied between 45 and 90'. For adsorbed layers the increase is monotonic up to 90'. The Monte Carlo technique to compute backscattering coefficients for 25 materials under different experimental conditions has been used also by Ichimura et al. (B88, B89). The analysis of Ni-Pt and Cu-Au alloys by AES gave good agreement with theory, provided that there was not any preferential sputtering. An empirical equation for computing the backscattering coefficient has been proposed by Ichimura (B90). The equation follows reasonably points computed earlier by a Monte Carlo approach. Ferrdn et al. (B91) have concluded that forward scattering of electrons in AES analysis has an important contribution to the backscattering factor at intermediate electron beam energies. At higher energies this particular contribution becomes less important and other factors; Le., Auger transition rate and IMFP cause more uncertainity in quantitative analysis. An iterative approach that included backscattering has been tested against the analysis of a Ni-Pt layer with the results previously noted (B61). Experimentally determined backscattering coefficients have been determined by Gergely (B92)for a number of materials with a wide range of Z values. Geller et al. (B93) have determined that the backscattered electrons have the largest contribution to the change in signal current when oxygen is allowed to react with Si from 30 eV to the primary beam energy. Angular Effects. A model has been proposed by Gerlach and Cargill (B94) for the AES signal as a function of sample

k

t

and detector angles, multiple scattering, and IMFP. Experiments with Ti spheres on Sn agreed well with the theory except where edge effects are important; correction could be made for this regime also. Angular dependent AES spectra of Ni(ll0) and S over Ni (110) have been reported by Baudoing et al. (B95) and compared favorably to a previously developed model. Angular effects in the AES analysis of thin film growth (along with other considerations) has been analyzed by Rhead et al. (B96). Secondary Electrons. Various computational procedures for determining the secondary electron contribution have been reviewed by Smith and McGuire (B97). In the region of the primary beam voltages used in AES, the secondary electron yield has been determined by Gergely (B98). These values can then be used in quantitative AES. Secondary electron yields have been measured by Ruzic et al. (B99)for a number of treated stainless steels with different primary beam energies and angles of incidence. A procedure to determine AES peak heights in the low energy region using previously developed expressions has been tested by Andersson (B100). However, caution must be used because valence effects are often in this energy region. Duc et al. (BIOI)have presented a very simple method to remove the secondary electron contribution to AES spectra. It is based upon using an experimental point on the low and high energy side of an AES peak of interest and fitting to a simple exponent expression. Examples were given with alloys where standard curves from the individual elements are combined so that a smooth background resulted. Unfortunately comparisons to other more detailed procedures were not given. Another approach to correct for the sloping base line due to secondary electrons has been forwarded by Labohm (B102). In this method the peak separation distance and the angle that the base line makes relative to the normal in the region of the peak are employed. An example of C and Ni(100) has been used to illustrate the ease of application of the method; however, it has been assumed that the background does not change from spectrum to spectrum. Quantitative Analysis. In the preceding sections many of the factors that can affect quantitative anlysis by AES have been discussed. In this section a somewhat more general picture will be presented. In many cases several of the effects mentioned above have been considered with regard to quantitative analysis, and several other schemes will be presented. Seah (B103)has presented a review of quantitative analysis by AES. Methods for quantitative AES have been compared by Minni (B104),i.e., elemental standards, similar compounds as standards, and "first principle calculations" with several mixtures. Firm conclusions were not reached from this study except that sputtering can cause large errors. A similar approach involving calculations where ionization cross sections, Auger transition coefficients, atomic density, IMFP, backscattering, and sputter yield has been tried by Mroczkowski and Lichtman (B105).The computed results were compared to elemental standards and found to be better in two of the three alloy systems tested. Griffis and Linton (B106)have shown integral area AES (after corrections for base line, backscattering, and IMF'P) has given comparable quantitative results compared to derivative mode spectra for several metal sulfides. XPS was used to correct for surface vs. bulk stoichiometries. Sekine et al. (B107) have considered a method that is based upon the ZAF theory in EPMA for matrix corrections. It is made by an iterative calculation with a correction factor that includes atomic density, backscattering, and IMFP. This procedure gave improved results with the analysis of two alloy systems. Another approach that is based upon EPMA has been tested by Jablonski (B108)for quantitative AES. Backscattering, IMFP, and atomic density were considered and the model was tested with Au-Pd alloys. Better results were obtained when the Au peak at 2024 eV was employed in place of the 64 eV peak. Another method for the analysis of real surfaces and materials that considers matrix effects and roughness has been proposed by Keenlyside et al. (B109)that is based upon standard spectra. However, preferential sputtering and changes in the peak to peak height due to different chemical species may be buried in the corrections needed for the method. Surface roughness with two types of structure has been considered by Wu and Butler (B110).Fair correlation with AES signal intensity were observed and the results did show an independence of the peak energy up to 1000 eV. ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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The effect on modulation voltage on derivative mode AES spectra has received the attention of Anthony and Seah (B111).Shifts in the observed energy and intensity as a function of modulation voltage of derivative mode spectra have been studied. Unless this factor is carefully noted and controlled, intensity ratios for peaks at different energies, and the energies of the transitions themselves, may not have good transferability between different instruments. A procedure has been developed by these authors (B112)that makes use of the shape (i.e., the ratio of the peak to base-line height) of the Ag M4 5 NN doublet. The effect of analyzer resolution and modulation voltage on the ability to resolve the M4,5NN peaks of Pd and Ag in a Ag-Pd alloy has been considered by Mathieu and Landot (B113). Only at higher resolution and low modulation voltage can a cylindrical mirror analyzer (CMA) give good resolution. Methods that make use of features of AES spectra have been tested for quantitative AES analysis. Dolizy and Groliere (B114) have claimed that the high energy sides of derivate AES spectra can be used for qualitative as well as quantitative analysis with examples given with the Na LVV (-30 eV), K LMM (-250 eV), and Sb MNN (-450 eV) transitions. The low energy side can display chemical effects; no consideration of background corrections was given. The use of the R value (the ratio of the overall peak-to-peak height vs. the small doublet peak-to-peak height in Cu or Ag) has been used by Vook and Namba (B115) to study deposition overlayers. Various stages of layer growth, i.e., island formation and the “smoothness”of a surface, could be determined. Siuda (B116) has employed the R value concept to study thin films of Au on Ag. The calibration of AES spectra by measuring P on Fe single crystals and comparing the results with low energy electron diffraction (LEED) results has been done by Viefhaus et al. (B117). It was noted also that the relative peak heights changed with the angle of incidence of the electron beam, and the results were used in studies of fracture surfaces. Auger electron emission coefficients have been measured for several transition metal L level initial states as a function of beam energy by Zaporozhehenko et al. (B118).Fair agreement with theoretical values was observed. Okada et al. (B119)have used a diamond scribe to create clean surfaces on brass. AES spectra of the scratch showed a bulk composition when compared to pure elemental standards. The quantitative analysis of Ag-Au alloys by AES with both scribed and sputtered surfaces has been studied by Holloway and Hofmeister (B120). Corrections for atomic density, IMFP, and backscattering had to be made for comparison to elemental standards. In some instances surface segregation and recoil implantation occurs. Fe-Cr and Fe-Ni-Cr alloys have been analyzed by AES with comparison with peak-to-peak heights for bulk standards by Matthews et al. (B1.21). Careful choice of the peaks used resulted in good agreement in most cases except for one peak in which the base to negative exertion was employed. A linear combination of the AES Co and Fe M23VV spectra of pure materials has been tested for a Fe-Co ahoy (50-50 mixture) by Allie and Lauroz (B122).Good results were obtaned even without background subtraction. Another example of the addition of standard spectra has been provided by Dawson and Burke (B123) for a 50-50 alloy of W and Mo. In this system there are overlapping peaks also, and differences in IMFP and backscattering had to be taken into account for good analytical results. The analysis of Ag,Al,-,As by AES, ion microprobe mass analysis, and photoluminescence has been investigated by Stewart et al. (B124). With the standard peak-to-peak heights good agreement between the techniques was noted in most of the samples analyzed. Quantitative AES of coevaporated layers of Au and Cu has been attempted by Li et al. (B125), who compared their results to atomic absorption spectroscopy. Good agreement could be obtained only if electron beam effects, residual 0, and backscattering were taken into account. Strausser et al. (B126)have use N(E) AES spectra of pure Co, Ni, and Cu to study the surface composition of Ni-Co alloys and Cu/Ni-Co layer structures. By starting with the highest kinetic energy peak and subtracting out its contribution, the experimental curve is separated into its individual components. An analysis can be made from the scale factors needed to eliminate a given elemental contribution. 380R

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A procedure to reduce topographic effects for AES analysis has been suggested by Prutton et al. (B127). An expression of the form N1 - N2/(N1 + N,) was found to give the best normalized results for Au or Ag deposited on etched Si. (N, and N2 are proportional to the electron count at the peak and just above the peak, respectively). Instrumentation and Calculation. In this section advances in instrumentation and computation for AES are described. In most of the papers cited one of these areas has been the main thrust of the report. The design factors for electron and ion beam guns have been reviewed Drummond (B128)and some examples of their use have been given. Roberts (B129)has described a small, relatively inexpensive CMA that had a tested resolution of less than 1%.With high spatial resolution becoming more imortant, Bishop (B130) has determined that 10 kV was the est overall beam voltage to use for AES analysis from a signal-to-noiseviewpoint. However, other effects, i.e., spatial resolution, sample degradation, etc., must be considered also. A Scanning Auger Microscope (SAM) has been developed by Topokoro et al. (B131)with a field emission gun that results in a maximum resolution of 20 nm at 10 kV. The electron gun is at a right angle with the CMA; an example with A1 on Si has been given. A SAM with 50-nm spatial resolution that uses a concentric hemispherical analyzer with an energy resolution of up to 630 or a constant energy of 1eV has been described by Prutton et al. (B132). AES spectra, micrographs, and line scans have been presented. These workers also have given details about the use of a desktop computer to operate the system (B133). A modification to a retarding field analyzer (RFA) by the addition of two Helmholtz coils has been made by Bauer et al. (B134). This allows scanning capabilities with this type of analyzer to a resolution of -100 pm. Smith and Southworth (B135)have described a fracture stage device for use with a RFA. The ability to make N(E) AES measurements with a conventional, single pass CMA by the use of an isolation amplifier has been demonstrated by Burrell et al. (B136). Examples with several mixtures have been given and background subtraction has been considered. The modification of a commercial electron beam power supply to allow for more stable beam currents in the nA region for N(E) AES spectra has been described by Woodward et al. (B137). Nishimori et al. (B138) have designed and tested a circuit to approximate low energy secondary electrons for a RFA used in AES analysis. Digital recording of N(E) AES and secondary electron spectra with a high resolution voltage to frequency digitizer has been reported by Roberts and O’Neill (B139). Higher currents than are normally used in pulse counting can be employed with this system. Noise reduction of actual AES data by nine-point smoothing, spline, and a piecewise polynominal method has been tested by Yu et al. (B240). Overall the spline procedure has been found to be the best method. The use of microcomputers to collect and analyze (to a certain degree) AES spectra has been given by Frank and Vasina (B141) and Chornik (B142). Software for SAM that includes collection, processing, line scans, images, differentiation, and background subtraction has been described by Prutton and Peacock (B143). Also procedures for improvement of S/N ratios, electron beam current fluctuation, and surface topography have been included. Forman and Lesny (B144) have given details of the modification of a commercial SAM and the development of computerized data collection. Examples have been shown of rough cathodes studied with the system. A microcomputer system to be used with a RFA has been developed Nakanishi et al. (B145). stabilized beam currents, improved S / N ratios, and faster data collection have been achieved. Scanning Auger Microscopy. SAM has many practical applications in the area of electronics, fracture and lubrication surfaces,catalyst, grain boundaries, etc. In this section some of the progress in a basic understanding of SAM will be reviewed. In the previous section several articles that were concerned with SAM hardware and software were considered. Prutton (B146) has reviewed the factors involved in quantitative SAM and has concluded that that current design limits resolution to around 100 nm. Several possible improvements have been suggested. Factors such as electron beam effects due to the impact parameter, backscattering,

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X-rays, and Bremsstrahlung induced Auger electrons have been considered by Cazaux (B147) for their effect on SAM resolution. It has been concluded that the material being studied, beam operating conditions, and resolution criteria can all affect,the observed resolution. SIMS and AES have been compared for microanalysis by Welkie and Gerlach ( B I B ) . SIMS was considered to be better than AES for larger areas and thicker samples, while AES is capable of better analysis for smaller areas and the surface region. ElGomati et al. (B149) have shown the effects of scan direction with sample orientation on SAM resolution. In addition, they have found that high energy secondary and backscattered electrons can reduce resolution. Depth Profile Analysis. AES is one of the primary methods that is used to gain information about the elemental composition vs. depth of materials. The knowledge that can be gained from such studies has been used to answer many fundamental and practical questions. The advanta es of AES for surface and depth profiles has been reviewef by Holm (B150)with several industrial applications serving as examples. There are several experimental and physical factors that influence AES depth resolution experiments. Malherbe et al. (B151)have investigated the effect of Gaussian ion beam and electron beam profiles in AES depth profile studies. In most cases misalignment of the beams is not important. Duncan et al. (B152) also have considered depth resolution as a function of ion and electron beam angles with respect to the sample normal, beam diameters, and misalignments. Depth resolution becomes poorer with large misalignments or when the electron beam to ion beam diameter ratio increases or the electron beam is far from the sample normal. Instrumental and sample factors for AES depth profile resolution have been considered by Seah and Hunt (B153). Under ideal conditions the type of material, Le., polycrystalline metal, amorphous elements or compounds, single crystals, or epitaxial layers, will have different depth characteristics. Also, the depth at which the desired information is desired has been shown to be important. At shallow depths the IMFP of the Auger electron is the most important factor in depth resolution, while at intermediate depths statistical terms and cascade mixing are important. At large depths electron stimulated desorption, crater uniformity and developing roughness are the predominate effect on depth resolution. With the observation of both the Auger electrons and X-rays produced by an electron beam during a depth profile, Kirschner and Etzkorn (B154) have been able to make an estimate of the depth of the Ge/Si interface. Broadening due to sputtering was estimated to be at least 2.5 nm. By plotting the log of the AES intensity vs. depth (or sputter time) a more precise determination of the interface was obtained (Le., -1 nm). A number of studies have been made to improve the data analysis of depth profiles. Garrenstroom (B155)has proposed the use of “target transformation” along with principle component analysis to identify and quantify species present in AES depth profiles. The procedure involves using the AES spectra over a limited energy region during sputtering compared to standard spectra. The results were obtained more quickly than by other methods. Total line shapes have been used by Palmer and Roll (B156)to investigate the composition of Cu-Ni alloys during sputtering. This approach has been found to be lengthy; peak-to-peak heights can be used where there is no overlap. A summation method is possible, but only over limited compositional ranges. A Monte Carlo calculation that considered cascade mixing for alternate Ni-Cr layers has been compared to experimental results by Davarya et al. (B157). The experimental interfaces became broader than the predicted results, and this was attributed to topographic effects. Palacio and Martinez-Duart (B158)have tested three computation methods to improve depth resolution, i.e., a power series, Fourier transform, and the vanCittert method. The results were comparable, but the power series approach involved less computer time. The correction for differential sputtering with Cu-Ni alloys has allowed Furman (B159)to use a simple summation intensity model to be used for AES analysis. The change in surface to bulk composition in AES depth profiles of Ni-Pt alloys on a layer basis has been calculated from the peak-to-peak heights of standards by Sedlacek et al. (B160). Reasonable agreement was found, but chemical effects must be considered in each case. Mitchell (B161)has modeled sputter profiles when the layer thickness

is about the same as the IMFP. The model also includes the statistical nature of sputtering, and good agreement with several experiments was found. An equation has been developed by Wildman and Schwartz (B162)to investigate the A1/A1,O3/A1Cu interface by AES depth profiling (NB the sputter rate for A1 is several times that of Al,03). For films 3-4.5 nm thick, the estimates were -1 nm too small. Smith and Southworth (B163) have proposed to determine an overlayer thickness by the use of AES transitions with two largely different kinetic energies. The approach has been tested with two enriched grain boundaries (i.e., Sn and S on Fe), and good agreement between experiment and previous predictions was found. The width of a thin SiO, overlayer on Si analyzed by AES has been shown by Taubenblatt and Helms (B164)to be affected by knock-on effects. A theoretical model was developed to test this system, but it was not successful for all of the data. While most depth profiles are done with accelerated gas ions, ball cratering and tapering of surfaces are used also. The factors involved with depth profiles greater than 1 pm from ball cratering for AES analysis has been investigated by Brown et al. (B165)from the point of view of crater geometry, beam diameter, and the mechanical roughness of the track. Resolution in certain cases is better than 0.3 pm. This group (B166) has examined the experimental conditions that need to be used to employ this procedure. Bisaro et al. (B167)have employed a chemical beveling technique to determine the interfacing width of InP/GaInAs/InP interface. From the AES results a thickness of 7 nm was obtained, and it was suggested the ultimate resolution with this procedure is 1.5 nm. An enamel-metal interface has been examined by Baumgartl et al. (B168)by tapering. Depth resolution of -0.35 pm was obtained in this stud . Larson et al. (B169)have used the crater edge from the Ar Y bombardment to study the results of oxidization of a NiCrAl-Zr alloy. An interfacial layer -0.3 pm under an oxide scale of -2 pm was observed. The comparison of AES depth profiles with results from other techniques has been the subject of several studies. Hammer (B170)has investigated the depth profiles of rubber cured in contact with brass-plated steel. Spectrum subtraction of the raw AES data is used for analyzing low concentration peaks that have overlapping peaks. The AES results were comparable to those obtained from simultaneous SIMS profiles. The comparison of AES, SIMS, and IIR for depth profiles of dielectric coatings on glass has been investigated by Hauser et al. (B171). The results from the various techniques were similar, but for a particular sample one method might give a better profile than the others. Al/Ag Al/C films have been studied by AES depth profile and utherford backscattering by Oliver and Santibanez (B172). The results from the techniques were in agreement. Kempf (B173) has measured the phase difference and reflectance with a He-Ne laser focused on a spot being sputtered. A correlation with an AES depth profile sputter rate was observed. The implantation depth of various elements into an Inconel alloy has been investigated by Pessall et al. (B174) with AES depth profiles. Some of the implants had non-Gaussian distributions, but in many cases good agreement between calculated and experimental depths was obtained after sensitivity corrections were made.

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COMBINED XPS-AES TOPICS Introduction

Since XPS and AES are related in several ways, i.e., combined instruments and the similarity of many of the basic principles, it is not unexpected that many investigations will use both techniques. In many cases the combination of techniques will yield more information that will just one procedure alone. Also, many problems are common to both techniques. Thus, there should be items of interest in this section for those who are interested mainly in just XPS or AES. The activities of the ASTM E-42 committee on surface analysis have been reviewed by Holloway ( C I ) . There have been many reviews of surface analysis that have included XPS and AES, and they contain many of the basic principles of the techniques and can serve as introductions for those interested. These reviews have been written by Powell (C2), Katz (C3),Buono et al. (C4),Brundle (C5), Helms (C6), LaANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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gally (0, and Lee (C8). A lood into the future of surface analysis has been made by Duke (C9). There also have been numerous reviews of these techniques that often have included basic information along with an emphasis on specific, applied research areas. These reviews include analysis in the following areas of interest: high spatial resolution by Venables (CIO); electronic structure of point defects on insulator surfaces by Bermudez ((211);glass surfaces by Hench and Clark (C12); electronicsby Shaffner (C13)and by Ryan and McGuire (C14); solar energy materials (C15);by Czanderna; corrosion by Baer and Thomas (C16);the nuclear industry by McIntyre (C17); modified electrodes by Kareik et al. (C18);and environmental applications by Linton et al. (1‘219). Inelastic Mean Free Paths. The low values of the inelastic mean free path (IMFP) of Auger or photoelectrons provide the surface sensitivity of AES and XPS. IMFPs also are important for any attempts at quantification of the techniques. There continues to be a wide range of reported values for IMFPs. An IMFP of 0.07 nm for a 61-eV electron from Ni through a S layer has been reported by Bouquet et al. (C20);no explanation for this value was offered. With an overlayer of C from the reaction of CFC1, on Fe(100)the IMFP for an electron of 270 eV has been determined to be -0.3 nm by Dowben and Grunze (C21). IMFPs that are more in agreement with many previous investigationshave been found for Xe adsorbed on to various metals (electron energies of 16-60 eV) by Baker and Klauber (C22). Burke and Schreurs (C23)have investigated thin layers of Cu on Ni and vice versa. They have found that the IMFP in Cu was greater than in Ni for electrons of the same energy. However, the values were in general agreement with previous results. With thin layers of Pb on Cu (grown on a layer by layer basis) DeCooman et al. (C24) have found IMFP’s of 0.57 and 0.69 nm for electrons with energies of 60 and 329 eV, respectively. Kirschner and Etzkorn (C25) have used SAM with energy-dispersive X-ray detection for sputter profile determinations of IMFP with Ge/Si layers. They plot semilog AES intensities vs. depth to determine the interface, and then IMFPs may be determined from self-consistent calculations. The IMFPs in Si were somewhat greater than those in Ge for the same electron energy. The values range from 1.9 nm for a 1147 eV electron in Ge to 2.5 nm for a 1619 eV electron in Si. In a study of angle resolved XPS from noble metals with monochromatic X-ray radiation, Citrin et al. (C26)have found IMFPs of 1.4 f 0.3 and 1.9 f 0.3 nm for electron energies of 550 and 1400 eV, respectively. Clark et al. (C27) have determined IMFPs for overlayers of poly-p-xylene on Au for electron kinetic energies of 590-4430 eV (Cu and Ti X-ray sources were employed). The IMFPs ranged from 1.7 to 3.7 nm over the above stated energy span; a rough Ell2 energy dependence was found. Values much larger than might be anticipated have been observed by Fowler et al. (C28) for BeO/Be interfaces of different thickness. For 100 eV electrons they have determined the IMFP to be -6 equivalent monolayers, twice that expected from the “universal curve”. RBS was used as an independent check of these results. Hupfer et al. (C29) have determined IMFPs for Cd salts of poly(diacetylene) in mono- and multilayers of 8.3 and 12.1 nm for electrons of 788 and 1480 eV, respectively. Experimental factors may play a role in observed IMFPs. Various roughness models have been tested for their effect on IMFPs by Wagner and Brummer (C30). In some instances the IMFPs ranged from 40-170% of the mean values, depending on the electron take off angles. Ferron et al. (C31) have given consideration to the effect of elastic scattering by surface layers upon the observed IMFP with angular dependence. They have concluded also that the simple exponential attenuation law does not hold with electrons that are nearly parallel to the surface. Several theoretical calculationshave been made for IMFPs. Approximate expressions, derived from constants of optical data, have been used by Ashley (C32) to compute IMFPs for organic compounds. Overall, these results have not been good. A curve based upon electron impact cross sections from Lotz has been used for IMFPs and has been compared favorably to earlier, empirical determinations by Ballard (C34). From a Monte Carlo calculation of the buildup of layers, Londry and Seavin ((734)have shown that if 85% or more of the atoms go down as a single layer, then reasonable values of IMFPs could be expected to within 10%. From earlier models Ashley 382R

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and Tung ((235) have calculated, for several materials, the exponent P in the equation X = kEP,and it was in the range of 0.65-0.8 (A is the IMFP and E is the kinetic energy of the ejected photo or Auger electron). This is in agreement with much, but not all, of the earlier work in this area. New Comparison Studies. If both XPS and AES were well developed, then quantitative analysis with each method would be expected to agree with the other. However, this does not appear to be the case in two studies. Baer et al. (C36) have analyzed an amorphous compound containing Fe, Ni, Cr, P, and B by both techniques and found disagreements, if standard sensitivity factors were used. An iterative approach was applied to both sets of data, along with depth analysis, and much better agreement was obtained. Several mixed metal oxide powders have been studied by Garbass (C37) for comparison purposes also. Again, with the use of standard sensitivity factors the O/Me ratios usually were within 30% of the expected values, but there was poor correlation between the two techniques. Electron beam and matrix effects may have been the cause of these poor results. Hirokawa et al. (C38) have found resonable agreement between AES and XPS studies of thin overlayers for several different materials. With XPS the use of electron mean free paths and cross sections gave adequate agreement, and for AES the use of standards also worked well. The AES and XPS results were in good agreement also. Haynes alloy, which contains Cr, Co, Ni, and W, has been analyzed by Graham and Hercules (C39). After a small amount of sputtering, they have found that the surface and bulk concentrations were nearly the same with the use of published standards for both AES and XPS analysis. AEX-XPS Depth Profiles. Some investigations have used both XPS and AES in depth profile investigations. The determination of depth profiles from 1nm to 1 mm by AES and XPS has been reviewed by Walls (C40). It was concluded that in most cases AES is better, or the only way, (i.e., angle lapping or ball cratering) to obtain the desired information. XPS does have an advantage, if different photon energies can be used in the near surface region. The analysis of fracture surfaces and depth profiles with regards to sample preparation has been reviewed briefly by Lea (C41). The area sputtered in depth profiling vs. the area of analysis has been shown by Mathieu and Landot (C42) to be important in AES and XPS analysis. They have studied depth profiles of Ta206with AES and XPS and have shown that the best results were obtained when the analyzed area was about l/lo the sputter crater area. AES gave better resolutions, due to the different, and usually lower, IMFPs with that technique. They also have suggested that a raster ion gun is better, and that the alignment of the ion gun is important. A few examples of the use of AES-XPS profiles in areas of practical interest are solar cell thin films of Cr-Si02-Si (C43), catalyst (C39, C44), passive oxide films on Ni (C45), the adhesive properties of metal films on Si3N4(C46), and leached glass surfaces (C47). Instrumentation. A combined AES and XPS spectrometer for surface microscopy in the pm2 region has been described by Cazaux, et al. (C48). Some examples with the system were given. Knapp et al. (C49) have devised a modification to the cylindrical mirror analyzer (CMA) so that angle-resolved electron spectroscopy can be performed. A simple procedure to determine the Fermi edge, and hence the energy calibration has been given by Oelhafen and Freeouf (C50). The method is based upon the conduction band of states of graphite (Le., the “a peak). Electrons, UV radiation, or X-rays can be used to excite the secondary electrons that are the basis of the determination of the ‘‘0 peak”. Several investigators have described sample inlet chambers of varing capabilities and cost to commercial analysis systems (C51C53).

Practical AES-XPS Combined Analysis. In this section a few studies where both XPS and AES have been used will serve to show the wide range of applications where combined studies can be of use. Livshits and Polak (C54)have used XPS and AES to study Na migrations in NaB-Alumina in an electron beam. XPS showed Na in several states and the beam damage was assessed over a range of conditions and residual gases. The use of AES and XPS data of an anodized aluminum has been employed by Davis et al., (C55) to construct “surface behavior diagrams” which can help to provide information on surface reactions. Other areas of study have

SURFACE ANALYSIS

included brazing (C56),biological analysis of intra- and extracellular tissues (C57), ceramic cleaning processes (C58), corrosion (C59),sliding friction (C60), catalyst (C61), plasma grown products (C62),electrodes (C63),and minerals (C64). These are just examples, and the listing is not meant to be complete for an individual area. Other uses have been and will continue to be explored. Theory of Electron Spectroscoples

Here we combine theoretical work on XPS and AES as much of this work pertains to both electron spectroscopies. We use 13 other categories to annotate the various theoretical works covered by this review; several papers, nevertheless, could easily have been described in more than one section. For these subsections, the appearence of a reference number will generally mark the end of our discussion of each paper. Studles of Theoretical Approxlrnations

Gofman et al. extended to the case of multiple scattering the method of Gelius and Siegbahn for using molecular orbital populations and atomic cross sections to obtain molecular photoelectron intensities. They use the muffin-tin approximation of MS Xa and assume absorption within each atomic sphere leads to out-going electron flux that is coherently multiply scattering to yield the final intensity (C65). The electron density of the incident beam as a function of depth and position within the unit cell for a 1-2 keV electron beam incident on a Mo(ll0) surface was calculated, and the relevance of these large density variations to electron spectroscopies discussed (C66). Higashi et al. calculated nonsperical contributions to the C KVV transition rates of CH4 using continuum wave functions of definite angular momentum but two-hole final states of tetrahedral symmetry. The nonspherical contributions are large for the (lt2J2 ‘Tz transition (C67). Morales et al. computed relative XPS intensities using two different sudden approximations and Gaussian type orbital Hartree-Fock (HF) calculations on CO (C68). Chorkendorff computed all the angular factors necessary to express the Auger matrix elements as a sum of radial integrals where the initial state is described in jj coupling and the final state in LS coupling (C69). Ridder described a computer program to determine whether an Auger transition is allowed or its degree of forbiddenness in LS coupling, given the initial and final electronic configurations (C70). Von Barth and Grossman defended their final-state rule for XPS and X-ray emission spectroscopy (XES), which is that, apart from the edge singularity, the line shape in a solid derived from a single band is better approximated by the final DOS than the initial DOS. Thus, in a homogeneous solid the XES line shape approximates the DOS of the pure solid, whereas the X-ray absorption line shape approximates the unoccupied local DOS (LDOS) of an impurity having 2 1 nuclear charge. The work is thorough, comparing results from both finite-electron determinantal methods and infiniteelectron Green’s function methods on model systems (C71). They have used the final-state rule and impurity (an atom with a 2s or 2p core hole) LDOS calculation to obtain distinct Na KLIV and KL2,3VAuger line shapes which are in agreement with experiment (C72). Ramaker proposed the finalstate rule for AES (C73). Schulman and Dow analyzed the Li KVV Auger line shape in the free-electron model and at various other levels of approximation, including shake u due to the nonorthogonality of the initial and final-state orgitals (C74). Mehreteab and Dow have shown that shake-off side bands due to shape resonances can drastically affect the XPS and X-ray emission line shapes rendering the final-state rule quantitatively and qualitatively inappropriate (1275, C76). Hansch and Minnhagen studied the XPS problem for simple metals. They avoid the approximation of a se arable potential and the “bozonization” approximation of 8unnarsson and Schonhammer yet obtain the MND edge singularity (C77).

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A thorough review of Green’s function calculations of ionization energies and intensities in the valence regions of molecules was authored by von Niessen (C78). Muller and Agren have reviewed anlyses of high resolution (vibrationally

resolved) photoemission spectra of first-row molecules (C79). In a novel approach, Braicovich suggested the use of the Cooper minimum, the minimum in the photoelectron cross section as a function of frequency, to suppress the number of photoelectrons from higher angular momentum initial states, which usually dominate the XPS spectrum. The method was used to determine s and p DOS of Pd and Ag films on Si (C80). Meyers and Feuchtwang reformulated the problem of photoemission from a free-electron metal. Their central quantity is the photon flux, which can vanish deep into the solid, rather than the gradient of the potential at the surface. Thus, they can predict processes involving crystal momentum perpendicular to the surface (C81). Bose et al. computed as a function of depth (into the surface) the intensities of surface and bulk plasmon satellites (C82, C83) and the intensity and asymmetry parameters of the main XPS peak for simple metals (C84). The metal glasses V,Zr,, Cf30zr70,and Mn&r7, have a high density of states at the Fermi level, as determined by ultraviolet photoelectron spectroscopy (UPS) and ASW calculations on the ordered MZr3 (M = V, Cr,and Mn) compounds. The XPS M(2p3/,) and Zr(3d6/,) chemical shifts and the change in asymmetry index relative to the pure compounds were also given (C85). The VB and Pd (3d) XPS spectra of pure and H saturated Pd (PdH,,.J were interpreted as showing that the Pd 3d levels were completely occupied in palladium hydride on the basis of the behavior of satellites and asymmetrical line shapes (C86). Empirical tight-binding LDOS were compared with UPS and vibrational spectra to address the surface geometry during various stages of oxidation of the Si(ll1) and Si(lO0) surfaces (C87). To model nitrogen-containing activated charcoals, CNDO/2 calculations on perinaphthene and perinaphthene with an N or NH impurity replacin various C atoms were performed. As would be expectet due to the different number of electrons in N and NH, differences in the XPS spectra were predicted (C88). Ab initio calculations were preformed and XPS chemical shifts for compounds containing the nontransition elements N, Sb, S, Se, Te, C1, Br, and I in different states of oxidation were obtained in order to determine the degree of d-orbital occupation. The calculations and the linear relationship between chemical shift and oxidation state indicated little d-orbital occupancy (C89). AES

There have been a number of Auger reviews during this time period. The most general is by Weissmann and Muller (C90). Weightman’sreview has the most complete discussion of ab initio theoretical approaches (C91). Ramaker advocated a semiempirical approach (C92),Kleiman stressed localization in solids (C93),Jennison emphasized localization in segments of molecules (C94),and Larkins stressed complex and highly resolved atomic and atomic-like condensed-phase spectra ((295). Friedel gave an elementary but broad in scope review of the Hubbard model that includes a brief discussion of Auger lifetimes and line shapes (C96). In an elegant note, Kondratenko viewed the entire Auger process, including excitation, within the sudden approximation. The resultant expression for the cross section includes overlap between the core-hole and final (N-1)-electron wave functions, which he related to the standard expression through perturbation theory (C97). Fitchek et al. computed using a renormalized Green’s function technique the L2,sVV A1 Auger lineshape that is in excellent agreement with experiment including the plasmon-loss intensity (C98). Using APW symmetry-projected DOS for Al, Bloch sums of HF atomic A1 wave functions for valence orbitals, and atomic Si wave functions for final-state core holes. Kucharenko and Aleshin computed the A1 KLIV and KL2,3V Auger line shapes that agree with experiment ((299).

Liegener has extended the molecular Green’s function method to Auger line-shape analysis. The method allows one the option of using experimental one-electronspectra and was successfully applied to the F KVV line shape in HF (ClOO) and Fz (C101, C102). Luken and Leonard calculated the atomic B K W Auger energies using Hartree-Fock. By adding correlation corrections of > B, direct emission of an AB pair is possible when the heavier species (A) is knocked on during a collision cascade that results in sputtering (e.g., the formation of MO molecules sputtered from heavy metal oxides); and (2) for weak (metallic and covalent) bonds between atomic constituents of a specimen with comparable masses, sputtered molecules are formed by recombination during the ejection of not necessarily next nearest neighbors but individually sputtered surface atoms from a single collision cascade. Koppel et al. (0236) detected metal cluster ions M,+ (n = 1-6) and oxide ions M,O,+ (n = 1-6, m = 1-5) for oxidized Fe, Co, Ni, Rb, Pd, In, and Pt surfaces bombarded with Ar+ ions. The yield of the various ions belonging to a given cluster size n varied as a Gaussian function of the “fragment valence” K = (q 2m)/n. Gijbels and co-workers (0272) compared the emission of Si,+ and Simon+cluster ions obtained from LAMMA with those from SIMS and spark source mass spectrometry. The positive ion yields can be described by a Gaussian distribution, when plotted as a function of the “fragment valence”. Richter and Trapp (0273) studied the emission of atomic and cluster ions of doping elements (e.g., B, Al, P, Ga, As) in silicon. Marien and DePauw (0274) studied ion beam induced effects which occur during the bombardment of polyoxyanion salts such as sulfites, sulfates, nitrites, and nitrates. The dependence of the ion-induced artifacts upon beam energy, ion dose, and the partial pressure of oxygen is analyzed. Cluster ions are also formed by sputtering rare-gas and molecular solids (0275-0279). Michl(D275) summarized the experimental observations from the study of solid Ar, Kr, Xe, CO, C02, N2, 02, N20, NO, Nz03, and Nz04 with respect to the effect of primary ion mass and energy, secondary ion energy distribution, and cluster ion composition. He also describes a mechanism which accounts for the results. The mechanism has three principal features: primary damage formation during the collision cascade regime (e.g., fragmentation of molecules, ejection of fragments, ionization and

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excitation of molecules and their fragments, and local charging in the impact regime), secondary damage formation during the thermal spike regime (e.g., chemical reactions in the damage track), and ejection (e.g., explosive expansion of high-pressure gas into vacuum). Orth et al. (0276) studied secondary ion emission from neat solid NzO, NO, Nz03, and Nz04as a function of the nature and energy of the primary ions (He+,N+, Ar+,Kr+,Xe’, 0.5-5 keV). The clustering unit generally did not agree with the molecular formula of the solid, suggesting that extensive ion-neutral and neutral-neutral chemical reactions occurred before the cluster ions reached the mass analyzer. In other work, Stulik et al. (0278) studied secondary ion emission from neat solid Cot, COS, and CS2 as a function of the nature and energy of the primary ions. Friedman and Beuhler (0279) found evidence for the existence of magic numbers for cluster ions of argon atoms and nitrogen molecules with high molecular weight mass spectra of their cluster ions. The magic numbers coincided with the number of particles required for complete shell icosahedra. Marien and DePauw (0280) observed negative cluster ions such as [OH(H,O),]- ( n = 0-5) in the SIMS of H 0 and positive cluster ions containing 1-17 molecules of Me6H in the SIMS of MeOH. Tantsyrev (0281) recorded the SIMS spectra of normal and deuterated benzene and of their equimolar mixture as frozen films on a Ni substrate using an ArO beams. Gas-phase mass spectra are also reported for normal and deuterated benzene using an Aro beam. The measured gasphase and SIMS data are used to calculate the relative yields of ions sputtered from the equimolar mixture. The mechanism of sputtered ion formation involving ion/molecule reactions is discussed. Cluster ions are also formed by sputtering ionic solids such as the alkali halides. SIMS spectra of alkali halides, MX, show intense ions of the type M+, M2+,[MX]’, [M(MX),]+,X-, and [X(MX),]- (0282-0286). Researchers at the Naval Research Laboratory observed cluster ions from CsI extending to the [CS(CSI)~~]+ ion at m / z 25 854 and the [ I ( C S I ) ~ion ~ ] at m / z 22 730 (0286). They present evidence supporting the formation of cluster ions with specific geometric structures (0282-0284). In addition, they find that the stability of the cluster ions is dependent on the surface energy of the cluster (0283),on the size of the constituent atoms (0285), and on the effective lifetimes of the ions (0286). The stability of doubly charged alkali halide clusters was studied theoretically by Martin (0287) and Gay and Berne (0288). In other work, Standing and co-workers (0289) show that the yield of [Cs(CsI),]+ cluster ions decreased smoothly with n when observed in a time-of-flight mass spectrometer at effective times -0.2 ps after emission. Cluster ions with n > 7 are found to be metastable with lifetimes electron ionization. The higher efficiency of the direct emission process lowers the detection limits for organic salts in SIMS such that picogram quantities can be detected. Cooks and Delgass and co-workers (0293) using molecular SIMS studied the cationization of organic molecules adsorbed on metal surfaces. Comparing the cationization of thiophene on Ag, Cu, or Pt showed a direct dependence of adduct ion yield on secondary ion sputtering efficiency; A and Cu’ yields are greater than 10 times higher than Pt yields. A similar result is observed for benzene adducts on Ag vs. Mo. Kloppel and co-workers (0294) compared the secondary ion mass spectra of selected organic compounds with electron impact (EI), chemical ionization (CI), field ionization (FI),and field desorption (FD) measurements. At low primary ion currents the SIMS data are similar to CI spectra. Kralj and co-workers (0295) ionized volatile organic molecules in the gas phase by fast atom bombardment. Busch and Cooks (045) in an excellent review article on the mass spectrometry of large, fragile,and involatile molecules collectively refer to FD, PD, SIMS, EHMS (electrohydynamic ionization mass spectrometry), LD, thermal desorption, and FAB as the techniques of desorption ionization (DI). The new DI methods are credited with opening up exciting new areas of biomolecular analysis such that the mass spectrum of vitamin B12can be readily obtained even though just a few years ago it would have seemed virtually impossible. A qualitative model of desorption ionization is summarized (045, 0296); the chief features of this model are (1) isomerization (loss of identity) of the input energy, (2) desorption of preformed ions or intact molecules, (3) ion/molecule reactions such as cationization occurring in the selvedge region, and (4) dissociation of energetic (metastable) ions well-removed from the surface. Cooks and Busch (0296, 0297) also describe coupling DI with mass spectrometry/mass spectrometry (MS/MS) and discuss ways in which they can increase signal and signal-to-noise ratios. DI increases sensitivity while MS MS increases specificity, and together they offer improved ana ytical performance. Hunt (046) also gives a general overview of the ionization techniques for nonvolatile organic molecules including a discussion of the mechanisms for desorption/ionization and the parameters influencing the performance of each of the methods. Vestal (D97)also published an extensive review of the ionization techniques for nonvolatile molecules including a detail account of the mechanism of ion emission from liquids. Vestal presented a unified model in which he speculates that the intermediate between energy deposition and molecular ion formation is a cluster or droplet which is separated from the bulk sample by a rapid, nonequilibrium process. The important steps in the overall process are summarized as

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energy deposition, nucleation, formation of clusters and droplets, and production of molecular ions from clusters and droplets. The recent use of liquid matrices in SIMS has completely revolutionized the way mass spectrometrists can analyze biomolecules. The technique known as fast atom bombardment (FAB)-since it uses a beam of fast atoms instead of ions-or liquid SIMS (0298) was developed by Barber and co-workers at the University of Manchester and introduced in 1981. In the FAB/liquid SIMS technique, the analyte is dissolved in a liquid matrix, such as glycerol, at some optimum concentration in order to provide a surface which, during particle bombardment, is constantly replenished with sample molecules that diffuse from the bulk to the surface (0299). The mobility of the sample molecules in the liquid matrix is the most important property of the technique. The use of fast atoms versus fast ions is inconsequential. Developments in FAB have been reviewed recently by Barber and co-workers (040,O B ) ,Rinehart (049),Taylor (050),and Vickerman and co-workers (061). Fenselau (0300) outlined the analysis of large molecules by FAB giving information about the experiment setup, target and matrix conditions, performance characteristics, characteristics of the spectra, the role of the liquid matrix, mechanistic models, and analytical potential of FAB. In other work, Fenselau and co-workers (0301) investigate the information which can be deduced from the molecular ion envelope of an unknown middle mass or large molecule, evaluate the need for unit resolution above 5000 amu, and illustrate the ability of particle induced desorption to provide stable isotope analysis, absolute and relative quantitation in mixtures, and molecular weights and structural information. Biemann and co-workers (0302) conducted a systematic investigation of the experimental variables in FAB in order to optimize the experimehtal techniques. Among the parameters studied are the shape and material of the target, the nature of the ionization gas, and sample preparation, concentration, and matrix. The use of liquid matrices has also been exploited by several other mass spectrometric ionization techniques including direct liquid introduction for chemical ionization (DLI-CIMS) (0303, 0304), thermospray (097), liquid ion evaporation (0305), and electrohydrodynamic ionization (EHMS) (097). Some controversy associated with the development and application of FAB has been reported in the recent literature. Dievenne and Roustan (030) published a paper entit1ed'"Fast Atom Bombardment'-A Rediscovered Method for Mass Spectrometry" citing that, in 1966, Dievenne and co-workers studied (and published work on) the sputtering of various target materials (organic, inorganic, biological, geological, and metallic) by a high energy molecular beam obtained by charge exchange. The method was called molecular beam solid analysis (MBSA). Numerous examples of MBSA's sensitivity and application are given. In a recent letter, Campana (0306) directs attention to recent publications on FAB studies which apparently neglect previous well-established experimental and theoretical results of research on SIMS and sputtering. Magee (0307) described in a recent paper the sputtering of organic molecules in terms of the nature of the momentum-transfer process and the amount of radiation damage incured by the uppermost monolayer of the sample from which the molecules are emitted. He concluded that (1) the charge on the primary bombarding particle does not affect the sputtering process, (2) particle bombardment of a liquid is identical to that of a solid on the time scale of a collision cascade, (3) liquid martices are useful for rapidly exchanging the sample surface exposed to the radiation damage caused by the sputtering beam, (4)the degree of fragmentation observed in a mass spectrum can be affected by changing the angle of incidence that the bombarding beam makes with the surface normal (i.e., a large angle of incidence results in a higher overall yield of sputtered particles with a lower degree of fragmentation than observed at normal incidence), and (5) energetic particle bombardment is not really a "soft ionization" technique, but rather a "graded fragmentation" technique. Van der Greef and Ten Noever de Brauw (0308) compared FAB and FD results. They showed that the FAB data are easier to obtain and that FAB produces more fragment ions which is a favorable aspect for structure elucidation. Rollgen and Barofsky and coworkers (0309) introduced mercury into a conventional FAB source in order to estimate 396 R

ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

the difference between Hg and Ar atom bombardment in the yield of secondary ions. They report a yield for Hg-FAI3 which is 10 times higher than Ar-FAB. Rbllgen and co-workers (0310) bombarded large organic molecules with fast molecular ions and obtained a higher yield with fast molecular ions than with fast Hg atoms. By selecting primary molecular ions of appropriate size and mass, the ionization of larger organic molecules may be considerably increased. Kambara (031I) compared the relative secondary ion intensities of some bioorganic compounds bombarded by Xe+ and Ar+ ions. The ion intensities are higher for Xe+ bombardment than for Ar+ bombardment. In other work Kambara (0312) describes the effects of primary ion current, the amount of matrix substance (glycerol),and the presence of salts on the secondary ion and metastable ion mass spectral patterns. Aberth et al. (0313, 0314) compared the sputtering efficiency between fast Cs+ and XeO primary beams on the yield of molecular ions from bioorganic compounds. Their results indicate that an improvement in sensitivity of at least a factor of 3 can be achieved with the Cs ion beam. In addition, no charging of the liquid matrix is observed when a primary ion beam is used. Morris and co-workers (0315)compared the SIMS spectra obtained using a variety of noble gases. As can be expected, Xeo gave more intense spectra than Aro while Neo gave still weaker spectra. Standing and co-workers (0316) using aTOF mass spectrometer measured the relative yields for the prominent ions in the secondary ion mass spectrum of alanine for primary alkali ions (Cs+,K+, Na+,and Li+) at energies from 1 to 16 keV. Yields increase greatly with increasing energy and with the mass of the bombarding particle, suggesting that in this energy region the nuclear stopping is mainly responsible for the secondary ion production. Barofsky and co-workers (0317) used Ga, In, SiAu, and Bi liquid metal ion (LMI) emitters as primary sources in matrix-assistedmolecular SIMS studies. The principal observations are a very high molecular ion abundance from the glycerol solution of every compound tested (i.e,, 10-50 times that attainable with Aro fast atoms), a strong increasing dependence of molecular ion signal on primary beam current density up to a saturation level at 10" A/cm2, a strong increasing dependence of molecular ion abundance on atomic number of the bombarding element, a secondary ion energy spread of - 3 eV, and a statistical fluctuation of -3 - 10% in the secondary ion signals. In other work, Barofsky and co-workers (0318)observed two effects associated with ion emission from liquid samples. First, a period of -1 - 30 s. after the onset of sample bombardment is needed to establish a nearly constant secondary ion emission. Second, after prolonged bombardment the intensities of the molecular ions of certain compounds increase significantly shortly before the glycerol is depleted. In a most unusual experiment, Krueger and Knabe (0319, 0320), using a van de Graaf accelerator to produce fast Fe dust particles -0.2 - 15 pm in diameter of velocity 1 - 60 km/s, obtained TOF mass spectra of several organics and salts. Almost no radical ions are found. Also, there is almost no dependence of relative ion intensities on projectile velocity. Strong sudden perturbation, caused by a shockwave, leading to a far from equilibrium surface-gas phase transition where kinetic time scales are more important than enthalpy energy scales, is believed responsible for these impact phenomena. Benninghoven (0321)discusses the relationship between organic SIMS and FAEL He points out that the unique feature of FAB which allows it to be used in low transmission instruments (e.g., double focusing magnetic sector field mass spectrometers) is not fast atom bombardment, but the regenerating liquid glycerol matrix (i.e., liquid matrix SIMS). In this and other work, Benninghoven (053,0321-0323) gives the main points of a precursor model for secondary ion formation and emission: precursor formation, fast transfer of kinetic energy, charge sign conversion, and fragmentation. Prior to any bombardment a precursor of the finally emitted secondary ion exists at the target surface; this concerns the atomic composition as well as the average charge state of the finally emitted secondary ion. The low-energy tail of the impact cascade, that is generated near the surface as a result of the ion impact, is responsible for the fast evaporation (lo-'' s) of the unfragmented precursor. A tendency exists to maintain the average charge sign of the precursor during its transformation into the correspondin secondary ion. The high energy region of the surface casca e produces fragments

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SURFACE ANALYSIS

of the surface precursors, including atomic species. Additional fragmentation may occur during the decomposition of larger excited molecular ions after their separation from the surface. Benninghoven also reports the results of a static SIMS investigations of the [M + HI+, [M - HI-, and [M - HI+ ion emission from metal (Cu, Au, Ag, and Pt) supported amino acids and of different organic compounds in a glycerol matrix. The formation of a closed monolayer of the sample molecules on the surface of glycerol is indicated. The performance of a new TOF instrument for SIMS is also discussed. In related work, Benninghoven and co-workers (0324-0326) investigated the secondary ion emission from metal supported amino acid layers that are prepared under UHV conditions by a molecular beam technique. The influence of the substrate metal, the relative coverage, the sample temperature, and the ion and electron prebombardment on the secondary ion emission suggests a preformation of the emitted parent-like ions on the sample surface before ion bombardment. In the submonolayer and monolayer range, the seconary ion emission from amino acids depends strongly on the chemical nature of the metal. For the formation of protonated molecular ions, [M HI+,proton exchange between adjacent molecules is an important mechanism. Sichtermann (0327) studied the temperature dependence of secondary ion emission from phenylalanine. Laxhuber et al. (0328)used monolayer fatty acids of various chemical composition, structure, and thickness to construct a model system to study biological processes. The chemical composition of the organic system can be determined by SIMS with a depth resolution of