Anal. Chem. 1990, 62, 113R-125R (E52) Katsanos. A. A.; Aravantinos, A,; Keliithrakas-Kontos, N. X-Ray Spectrom. 1988, 17, 13-16. (E531 Szabo, Qy.; Zolnal, L. Nucl. Instrum. Methods Phys. Res. 1989, 836, 88-92. (E54) Khaiiquuaman, M.; Lam, S. T.; Otsubo, T.; Hussain, A. H.; StephensNewsham, L. G. Nucl. Instrum. Methods Phys. Res. 1989, 836, 259-262. (E551 Ishiwari, R.; Shiomi-Tsuda, N.; Sakamoto, N. Nucl. Instrum. Methods phys. Res. 1988, 831, 503-517. (E561 Campbell. J. L.; DeForge, D. X-Ray Specbpm. 1989, 78, 235-242. (E57) Teesdale, W. J.; Maxwell, J. A.; Perujo, A.; Campbell, J. L. Van der Zwan, L.; Jackman, T. E. Nucl. Instrum. kelhods Phys. Res. 1988, 835. 57-66. (E581 Ishii. K.; Morita, S. Nuci. Instrum. Methods Phys. Res. 1988. 834, 209-2 16. (E59) Cookson, J. A. Nucl. Instrum. Methods Phys. Res. 1988. 830, 324-330. (E60) Watt, F.; Grime, G. W.; Perry, C. C. Nucl. Instrum. Methods Phys. Res. 1988, 830, 331-336. (E61) Myklebust, R. L.; Fiori, C. E. Roc., Annu. Conf.-Microbeam Anal. SOC.. 24th 1989, 219-222. (E621 Myklebust, R. L.; Newbury, D. E. Proc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1988, 261-262. (E63) Murata, K. Proc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1988, 133. .. 137. . .. . (E64) Dei Giorgio, M.; Trincaveili, J.; Riverso, J. A. X-Ray Spectrom. 1989, 18. 229-234. (E89 Czyzewski, 2.; Joy, D. C. Proc., Annu. Conf.-Microbeam Anal. Sot., 24th 1989, 396-398. (E661 Small, J. A.; Myklebust, R. L. Proc. Annu. Conf.-Microbeam Anal. SoC.,24th 1989, 223-2268, (E67) Karduck, P.; Rehbach. W. Proc., Annu. Conf.-Microbeam Anal. SOC., 23rd 1988. 277-283. (E68) Love, G.; Scott, V. D. Proc., Annu. Conf.-Microbeam Anal. SOC., 23rd 1988, 247-250. (E691 Pouchou, J. L.; Pichoir, F. M. A. Pfoc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1988, 315-318. (E70) Li. X.; Guangxiang, J.; Zi-qin, W.; Lee, R. J. Proc., Annu. Conf.Microbeam Anal. SOC.,23rd 1988, 295-296. (E71) Li, X.; Ouang-xiang, J.; Zi-qin, W.; Lee, R. J. Proc., Annu. Conf.Mlwobeam Anal. Soc., 23rd 1988, 300. (E72) Heinrich, K. F. J.; Newbury, D. E.; Mykiebust, R. L. Proc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1988, 273-276. (E731 Packwood, R.; Parker, C.; Moore, V. Roc., Annu. Conf.-Mcrobeam Anal. Soc., 23rd 1988, 258-260. (E74) Labar, J. L. Proc., Annu. Conf.-Mlcrobeam Anal. SOC.,23rd 1988, 253-257. (E79 Armstrong, J. T. Proc., Annu. Conf.-Microbeam Anal. SOC..23rd 1988, 239-248. (E76) Armstrong, J. T. Proc., Annu. Conf.-Microbeam Anal. Soc., 23rd 1988, 469-478. (E77) Abdrlamous, S. A. Roc.,Annu. Conf.-hficrobeamAnai. Soc., 24th 1988, 239-241. (E781 Newbury, D. E.; Marinenko, R. B. Proc.. Annu. Conf.-Microbeam Anal. SOC., 24th 1989, 257-259. (E79) Fialin, M. X-Ray Spectrom. 1988, 77, 103-108. (E80) Armstrong, J. T. Proc.. Annu. Conf.-Microbeam Anal. Soc., 23rd 1988, 301-304. (E81) Cheng, W.; Lee, R. J. Proc., Annu. Conf.-Microbeam Anal. SOC., 23rd 1988, 305-306. (E82) Wllllch, P.; Obertop, D.; Krumme. J. P. Roc., Annu. Conf.Microbeam Anal. Soc., 23rd 1988, 307-309. (E83) Rehbach, W.; Karduck, P. R o c . Annu. Conf .-Microbeam Anal. SOC., 23rd 1988, 285-289. (E84) Bastin, G. F.; Heijilgers, H. J. M.; Pinxter, J. F. M. Proc., Annu. Conf.-Mlcrobeam Anal. Soc., 23rd 1988, 290-294. (E85) Okumura, T. Roc.,Annu. Conf.-Mlwobeam Anal. Soc.. 23rd 1988, 297-299.
(E86) Bastin, 0. F.; Heijligers. H. J. M. Roc., Annu. Conf.-Microbeam Anal. SOC.. 24th 1989, 207-210. (E87) Newbury, D. E.; Myklebust, R. L. Roc., Annu. Conf.-Microbeam Anal. Soc.. 23rd 1088, 139-142. (E88) Hoeft, H.; Schwaab. P. X-Ray Spectrom. 1988, 778, 201-208. (E89) Pouchou, J. L.; Pichoir, F. M. A. Roc., Annu. Coni.-Microbeam Anal. Soc., 23rd 1088, 319-324. (E90) Storms, H. M.; Janssens, K. H.; T&&, Sz. B.; Van Grieken, R. E. X-Ray Spectrom. 1080, 18, 45-52. (E911 Van Borm, W. A.: Adams, F. C.: Maenhaut. W. Atmos. Envkon. 1989. 23, 1139-1151. (E92) Nomizu, T.; Goto. K.; Mizuike, A. Anal. Chem. 1988, 60. 2653-2858. (E93) Zreiba, N. A.; Kelly, T. F. X-Ray Spectrom. 1988, 77, 229-238. (E94) Lifshin, E.; Peluso, L. A.; MogroCompero, A,; Turner, L. G. Roc., Annu. Conf.-Microbeam Anal. Soc.. 24th 1989, 243-246. (E95) Waldo. R. A. Roc., Annu. Gmf.--Mlcrobeam Anal. Soc.,23rd 1988, 310-314. (E96) Brown, J. D. Proc., Annu. Conf.-Microbeam Anal. Soc., 23rd 1988, 271-272. (E97) Packwood, R.; Moore, V.; Thomas, S. Roc., Annu. Conf.Microbeam Anal. SOC.,24th 1989, 211-215. (E98) Goidstein, J. I.; Williams, D. B. Proc., Annu. Conf.-M/crobeam Anal. SOC.,24th 7089, 501-506. (E99) Gauvin, R.; L'Esperance. G. Proc., Annu. Conf.-Microbeam Anal. SOC., 24th 1989, 527-530. (E100) Romig, A. D., Jr.; Headiey, T. J.; Carr, M. J.; Cieslak, M. J. Proc., Annu. Conf.-Microbeam Anal. SOC.,24th 1989, 51 1-514. (E101) Harrowfield, I . R.; MacRae, C. M. Proc., Annu. Conf.-Microbeam Anal. SOC..23rd 1988, 267-270. (E102) Roomans, G. M. Scanning Mlcrosc. 1988, 2 , 311-317. (E103) De Bruijn, W. C.; Van Miert, M. P. C. Scanning Microsc. 1988, 2 , 319-322. (E104) Aarnio, P. A.; Lauranto, H. Nucl. Instrum. Methods P h p . Res. 1989, A276, 608-613. (E105) Bostrom, T. E.; Nockolds, C. E. Proc., Annu. Conf.-Microbeam Anal. SOC..24th 1989. 233-235. (E106) Yang, S. V.; Wagstaff, J.; McKay, G. Proc., Annu. Conf.Microbeam Anal. SOC.,24th 1989, 247-248. (E107) Fresenius' 2.Anal. Chem. 1988. 332, 518-743. (E108) Roelandts, I.Spectrochim. Acta 1980. 448, 281-290. (E109) Roelandts, I.Spectrochim. Acta 1989. 448, 985-988. (E110) Roelandts, I . Spectrochh. Acta 1989, 448, 5-29. ( E l l l ) Roelandts, I.Spectrochim. Acta 1989, 448, 925-934. (E112) Agrawai, R. M.;Kapoor, S. K. X-Ray Spectrom. 1989, 78, 151-155. (E113) Blake, D. F.; Allard, L. F.; Echer, C. J.; Freund, F. Proc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1988, 129-132. (E114) Marinenko, R. B.; Small, J. A.; Blackburn. D. H.; Retorick, D. R.; Shire, N. J. Proc., Annu. Conf.-Microbeam Anal. SOC., 24th 1989, 254-256. (E115) Couture, R. A. Adv. X-Ray Anal. 1989, 32, 233-238. (El 16) Bi-Shia King, Vivit, .D. X-Ray Spectrom. 1988, 77, 85-87. (E117) Bi-Shia King, Vivlt, D. X-Ray Spectrom. 1988, 77, 145-147. (E118) Eddy, 8. T.; Balaes, A. M. E. X-Ray Spectrom. 1988, 77, 17-18. (E119) Simonoff, M.; Hamon, C.; Moretto, P.; Llabador, Y.; Simonoff, G. Nucl. Instrum. Methods Phys. Res. 1988, 83 7 , 442-448. (E120) Kasrai, M.; Fazoonmayeh, L.; Payrovan, H. X-Ray Spectrom. 1988, 17, 219-222. (E121) Eddy, B. T.; Baiaes, A. M. E. X-Ray Spectrom. 1088, 77, 195-199. (E122) Cohen, L. H.; Smith, D. K. Anal. Chem. 1989, 67, 1837-1840. (E123) Watson, W.; Parker, J.; Harding, A. R. A&. X-Ray Anal. 1989, 32, 22 1-226. (E124) Prange, A.; Schwenke, H. Adv. X-Ray Anal. 1989. 32, 211-220. (E125) Marshall, A. T.; Condron, R. J. Proc., Annu. Conf.-Microbeam Anal. SOC.,23rd 1088, 443-444. (E126) Zierokl, K. Proc., Annu. Conf.-Mlcrobeam Anal. SOC.,24th 1989. 109-1 11. (E127) Geiss, S.; Einax, J.; Danzer, K. Fresenius' 2.Anal. Chem. 1989, 333, 97-101 (in German).
Surface Analysis: X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Noel H. Turner Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 This fundamental review is on the subject of X-ray photoelectron s ectrosco y (XPS) and Auger electron spectroscopy (AESY and wi! cover the literature abstracted in Chemical Abstracts between November 1987 and August 1989. The review is written in three separate parts for the convience of the reader: section A, XPS; section B, AES; and section C, combined XPS-AES topics. However, for those who use This article not subject to U.S. Copyright.
only one of these techni ues, there may be items of interest in the other sections. X'liS and AES are used widely for the analysis of surfaces. From about 1970 to the present time, these techniques have grown in acceptance by the scientific community. Much of this activity has been documented in earlier Fundamental and Application Reviews in Analytical Chemistry (1-16).
Published 1990 by the American Chemical Society
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While this review is len h ,it is not an all-inclusive bibliqraphy of XPS and A& Juring the review period. The articles have been selected with the idea of improvement in the “state of the art” of these techniques. The goal of this review is to help anal sts solve the problems that are encountered in using XPgand AES in a regular laboratory with commercially available equipment. A section on inelastic mean free paths (IMFP)will be in the combined XPS-AES part of this review. Finally, the names of the authors of the apers cited could not be included in the text due to space Emitations.
A. X-RAYPHOTOELECTRON SPECTROSCOPY Introduction. XPS or electron spectroscopy for chemical analysis (ESCA) is one of the most widely used techniques for elemental analysis of the near surface region. Also, this method gives information about the chemical environment of the observed atoms. Much useful information can be obtained from XPS,even though a complete understanding of binding energies and intensities has not been achieved. There have been a number of reviews of XPS during the report4 period of a eneral nature (AI,A21 and a book (A3). In addition, reviews Raving much basic information but emphasizing molten salts (A4),Si and Si02 (A5),biomaterials (A6),corrosion ( A n , catalysts (At?),adhesion performance with ion implantation (A9),theory and experimental results of the rare earth compounds (AIO),bonding of minerals ( A l l ) ,and small particles (A1.2)have ap ared. The American Society for Testin and Materials ( A G M ) has issued a standard for reporting 5PS spectra (A13). Investigations that have used s chrotron radiation have not been included in this review; t is to ic has been reviewed elsewhere (A14). Biniing Energies and X-ray Excited Auger Transitions. The National Institute of Standards and Technolo (Former1 National Bureau of Standards) has released NISY StandariReference Database 20 (A15). This XPS database contains over 13000 line positions, chemical shifts, and splittings from data reported through 1985. The database is used with a personal computer. A number of metals, some as oxides, were studied in a round-robin test of bindin energies (A16). In many cases there were differences of a%out 1eV between different laboratories; sample handling may have caused some of the differences. Binding energies for a number of elements in the 1700-3900-eV region were determined with an XPS spectrometer that had four anodes, Le., Mg, Al, Ag, and Ti (A17). The standard deviations for most of the peak positions was less than 1eV. N+ implanted in a Si wafer was shown to form Si3N4as a phase that can serve as a binding energy reference standard (AM). This fmding was based upon the binding energies, Auger line sha s, and Auger parameter data com ared to earlier studies. l?e bindin energy of the 3d612pea&, for Cs which was electrodepositei from Cs2S04 solutions, showed differences of 0.8 eV that depended upon the electrochemical potential (A19). These shifts occurred in the double layer, since no differences were noted in the substrate. Theoretical descriptions of surface versus bulk binding ener shifts and line shapes were reviewed (AZO). Estimates of gnding energies of aromatic compounds based upon charge estimates indicated that a second-order term was required for carbon (A21). The usual linear relationship was found to be adequate for other elements studied ( A B ) . The use of Born-Haber cycles to describe binding energy shifts of adsorbed gases at various coverages on metals was explored (A23). Such an approach can be useful in certain cases. Studies concerned with the roblems of charging samples continued. A review that consigred the various contributions, i.e. sample morpholog , experimental equipment, and the various hysical contrigutions with examples was published (A24). was found that mixing raphite with As2S3-T12S could be used to com ensate for cgharging by usin the Cls line as a binding stanlard (A%). Reasonable correfations of expected versus observed bonding to nonbonding S ratios and binding energy shift trends were obtained. However, with several zeolites the Cls line, from adventitious carbon, was found to vary in a parent binding ener y by a much larger amount than the $a KL13L23A er pea% ( A H ) . Variations in the position and shape*occurrzeven with samples stored under vacuum. This indicates that the Cls line would be a poor choice for use as binding energy reference with these
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compounds. A comparison of a gold dot (with a flood of 10-eV electrons) versus the use of the adventitious Cls peak for charge correction was made with sapphire and Ca- and Cedoped yttrium aluminum garnets (A27). The gold dot procedure gave more consistant results, and it was suggested that the Fermi levels of the metal overlayer and the insulator substrates were the same. Au deposited from a colloidal solution was used with reasonable success as a bindin energy reference; examples were given with Pt-Sn on A1 %3 and a zeolite (A28). Some problems may occur due to t%enature of a specific sample, e.g., pores etc. A controversy concerning the cause (either initial-or final-state effects) of binding energy shifts of small metallic particles on insulating substrates has continued (A29,A30). The shape, size, and cluster-support interactions play a very important role in the interpretation of both the experimental and theoretical understandin of screen to rJuce these systems. The use of a charging with a focused X-ray placed above an insulating sample was investigated (A31). At the point where enough electrons from a flood gun compensated for the exiting photo electrons, it was suggested that a uniform surface potential across the sample resulted. With this approach, sample charging has been employed to determine the surface species of Si02 deposited on an A1 substrate (A32). The effects of various process treatments were then evaluated. Si with a thin oxide film showed char ing of both s ecies with and without the use of a flood gun h 3 3 ) . With g 3 N on InP the signals from the bulk did not shift when the flood gun was employed. Also, small C and 0 peak shifting was greater than that of the Si, thus the contaminants did not appear to be associated with the other species. The use of the Au er parameter for analysis purposes was extensively reviewef (A34). An examination of the Auger parameter concluded that this quantity is useful for “fingerprinting” purposes, but it cannot usually be used for determining extraatomic relaxation energies (A35). This is due to the fact that binding energy shifb from different levels usually are not the same. It was proposed that a different parameter based upon two binding energy levels and an Auger peak energy can be used for both fingerprintin and extraatomic energy determinations. Correlation of &e Auger parameter with Hammett u values for several elements was determined (A36). Also, polarization contributions for the Auger electronic transitions can be separated. A resonant electron-transfer mechanism was em loyed to account for a shoulder peak on the X-ray-excited#peak with alkali metal and alkaline-earth fluorides (A37). This effect in some cases could cause a shift in the peak position and the Auger parameter. Differences in the Auger parameter determined with A1 versus combined Al and Ag anodes for Pb were calculated from an earlier study (A38). It was su gested that the line width of a nonmonochromatic Ag anoje is too broad to be useful to determine peak positions accurately. Changes in the Au er arameter as the thickness of a Cu overlayer differed on ghlgfi00) were observed for both Cu (going from Cu20 to Cu metal) and Mg (A39). The Mg Auger parameter drop ed by about 0.5 eV when the first layer was completed and &en returned to its initial value as more Cu was deposited. It was suggested that this was due to a change in the electronic structure with the completion of a Cu monolayer. MNN Auger spectra of Ta through Au (kinetic energies from about 1700 to 2100 eV) produced by Bremsstrahlungradiation from a Cu anode were found to have reasonable signal strength (A40). Spectral Features. In some cases spectral features can be used for analysis, but care has to be exercised. Bindin shifts for the N i p and Zr3d, peakq in hydrogena T E a l l o y s were note and compare%to those of unreacted materials (A41). However, the samples were sputtered, and some changes to the materials may have occurred. A shift of 0.9 eV to higher binding energy with H2+-implantedZr was found along with a new feature in the valence band spectra (A42). Determination of the relative mixed-valence states in Ti-containing materials by the use of a standard TiOz spectrum was demonstrated when the binding ene s h h were greater than 1 eV (A43). However, some pro%ms such as surface preparation, char ing, etc. have to be considered. Ar and K ion im lanted in showed different 2s and 2p line shapes, i.e., t e Ar was symmetric and the K asymmetric (A44). It was suggested that K interacted strongly with the A1 substrate. An attempt was made to correlate the Fe3s
u ins mission
4
K
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SURFACE ANALYSIS NO.I
n. ~urneris a research chemist at me
Naval Research Laboratory. Washington
He received a B.S. in chemistry from the University of California. Berkeiey. in 1962 and a Ph.0. in physhxi chemishy from the University of Rochester in 1966. His graduate work was in lhe area 01 gas-phase kinetics under me direction of the late W. 0. Waiters. I n January of 1968 Or. Turner ioined the Staff of the Naval Research LabDC.
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splitting and the mapetir moment for a number nf mawrials hased upon previous suggestinns (A4.5, A46I. Rut with high-resdution data there were nnt any nhsrrvahle trends based upon magnetic moments I n the case of NiFeSiH glasses. rhanges in the Nipp, satellite strurture were found t.4 $ti I .
1)ifferenres were noted as a function of temperature and cwtrage u,ith high-resnliitinn 0 1 s spertra of CO adwrhed nn Ni1100) ( A 4 n . I t was suggested that this nhwrvatinn could he explained by vibrational effects rather than srrerning of the ionized final core level states. Shifts were found hetween Mg versus AI X-ray sources in the (;e2p1 binding enerm for several compounds (A4rlr. The cause rnf these shifts was the near threshold energy uithis peak with Mg radiation. The swnewhat Img-lived photoelectron 2p, hule i i affected liy t h r I.,M ,M,,s Auger transit ion. Image chargr calculations were maAe for XPS'hinding energirs and Auger parameters for thin S O 2films on Si b44.9~.The shift in Auger parameter and binding energies Idecreming with thinner films) was found both i n the calculations and experiments. Thus, special ropertie- of these t h i n films ascrihed from the spectra may e questionable. I t was found with t h i n insulating films that the rure lewl lines ran be shifted and broadened due to the creation uf high potrntial grndients ditring samplr irradiattnn (A,jlli. The wnsequences nf this efftct can he minimi7ed by increasing the electron takeoff angle and deconvolution with a known spectrum. Data Handling. Sophisticated procedures for treating XI'S spertral results continue to be investigated. These areas include barkground rorrertions. depth profile analysis, rurve-fitting methods. and the use of standard materials f n r making quantitative estimates. A harkround model for XPS based upon an rlrctron scattering cross-serf ion apprnach was reviewed 1A,5/J. Also diiaussed was the use o t a rnutine procedure for quantitative XPS analvsi, A previously develuped method for hackground correction was tested with Au for different S.ray excitatiun energies (A521. It was found that small differences in the currected spectra could be asrrihed t u the neglect of rimsideratiun of inelastic surface exritatiuns. A prucedure that empluys reflected elertron energy loss spertra tlIKKl,Si tu remwe the inelastic background from XI'S spectra was testrd with AI 1A5JI. The methid involvrs only H single REELS spectrum and does not require a fitting conitant that ,,[her approaches enipluy. Huwe\,er, small negative excersioiis in the surface plasmon region wrur hecause the bulk efiert is overestimated. Anuther simple procedure u'as propgsed to ronsider the rffert o f the inelastic background (A.541. 'l'he rorrertion yieldwl similar rrsiilts to the Shirley method. The difficiilty in arhieving gnod qiiantitative estimates was illuhtratrd i n a rotind.rnbin stuily with Fe-(:r allnya (Alh'i. A precisinn uf 12% was found, hut with these elemenu good area determinations are difficult to make. Some o f t he partiripants used peak heights instead uiarea, whirh would rediicr rhr arriirary of the rrsults. Several studies investigated curve-fitting mrthotls. 'l'hr use oi a damped nonlinear leait-sqiiarrs ( Y I L S Japproach ior fitting XI'S peaks was explored with buth personal and main-framr rwnputrrs tA.551. Setera1 variations of this prrwdure wcrr te*trrl with g d fiu of rxperimrntiil data being obtained. In anorhtr SI.1.S approach. phpiral cnnstratnts, i e., q~in-vrbitriitios and energy
separations, were limited to reasonable values (A56). An exam le of this method was given. A decomposition method baserfupon a Fourier-transform procedure combined with a truncation function was used to investigate the oxidation of GaAs (A57). The method was able to resolve multiple peaks under a broad signal without making assumptions about the system. A previously developed method for quantitative XPS was extended for use with X-ray-excited Auger peaks from several different anode materials (A58). However, in order to apply this procedure the entire series of Auger peaks, e.g., all of the KLL peaks, must be recorded. By curve fitting the 01s spectra of single-component oxides, the relative amounts of oxide mixtures could be determined (A59). In addition, it was found that X-ray-induced 0 Auger spectra were useful in qualitative determinations (A60). Examples with oxides of Cr, AI, Fe, and Si were given, and good agreement wth other studies was found in many cases. Modification to previously used procedures for taking into account Coster-Kronig broadening for the 3dfi? transition of Tc was made for estimates of the core leve lifetime (A61). This broadening will affect any fitting routine; the method uses an assumed spinorbit ratio in the fitting process of the spectrum. It was found that relative sensitivity factors did not agree with theoretical estimates for iodine compounds (A62). However, the data treatment procedures may affect the determinations, and the theoretical values have been used widely with reasonable success. Often it is possible to determine compositional changes in the near surface region by XPS without the use of sputtering. A method to determine a concentration profile from the entire background of an XPS signal was developed (A63). Initial results with pure Au were correct, but more tests with inhomogenous materials will be required to employ this method. Laplacian-transformed angle-resolved XPS (ARXPS) data were shown to give good compositional determinations as a function of depth (A64). Examples were given with a native oxide layer on Si and sputtered GaAs. The determination of nondestructive depth profiles from ARXPS data with the use of staged applications of the Laplace transform was suggested (A65). However, there are still some difficulties in the use of this method. A procedure to compute nondistructive depth profiles of thin overlayers by ARXPS that is based upon a series of linear equations and information about the system was developed (A66). The sources of errors in the method were discussed and results from two materials were given. By the addition of standard spectra of polycarbonate and poly(butylene terephthalate) it was determined that the former was enriched in the surface of blends of these polymers (A67). However, careful alignment of the energy scale was required to use this approach. CuzO and Cu were used as standards for the determination of oxide layer thickness in electrochemical oxidation ( A m ) . The L 3 W X-ray induced lines were modeled as combined Gaussian-Lorentzian peaks; good agreement between the electrochemical and surface analysis determinations was obtained. Angular-Resolved XPS. ARXPS continues to be useful in many different analyses. Many single-crystal systems have been studied since diffraction effects often can be observed. However, many other ARXPS analyses have been with thin films. The effect of orbital angular factors in X-ray photoelectron diffraction in single crystals with the plane wave approximation were studied (A6.9). It was found that these factors were unimportant in the description of these determinations. ARXPS spectra of GaAs(001) for both photoelectrons and Auger electrons were compared hy using a single scattering model with plane wave corrections (A70). Qualitative agreement was obtained, but quantitative estimates from the data were difficult to obtain. ARXPS was employed to study the structure of Au and Ag overlayers on Ni(100) by analysis of the forward-scattered enhanced photoemission peaks (A71). When this approach is combined with other techniques, a better understanding of the structures of these thin films was obtained. The same method was employed to study submonolayer deposits of Ni on GaAs(ll0) (A72). The location of the Ni atoms upon annealing was partially determined. Diffraction of the XPS signal was observed for a 1.2-nm layer of Rh on AIz0,(0001) (A73). This indicated that Rh was an expitaxial layer in this system, and the experimental spectra agreed reasonably with a theoretical calculation. ANALYTICAL CHEMISTRY. VOL. 62, NO. 12. JUNE 15, 1990
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The ability to determine band bending of semiconductors by metal overlayers, ARXPS,and different X-ray source8 was explored (A74). In most instances the combination of metal overlayers and ARXPS was the most practical experimental choice. ARXPS was used to indicate nonbrid and bridging versus suboxide ox gens a t the surface of Si with small amounts of vapor- eposited K (A75). The sukoxide peak intensity increased at higher takeoff angles, which indicated differential charging was not a large problem. Thin oxide layers that have rou h interfaces with the bulk material were analyzed with a NL%S model that handled several ARXPS spectra as part of the calculation procedure (A76). In addition, background effects were taken into account, and several examples were given. ARXPS indicated two 0 1 s signals, Le., oxide and hydroxide, from an electrochemically oxidized stainless steel (A77). However, the sputtering was not over a large enough area to confirm the suggestion. Small Particle Analysis. XPS has been employed in the anal sis of small particles for analysis of air pollution and cadysts. The methods of making quantitative estimates of particle composition by XPS were reviewed (A78). The dispersion of a metal oxide (V20,) on another (TiOz) was estimated by an extension of the simple XPS overlayer model (A79). Several assumptions were made that simplified the determination for this system that might not be true in other cases. A procedure to find the thickness of the oxide layer on small metal articles was developed that includes changes in sputter yiellover the article as a function of ion beam angle, electron takeoff ang e, and the particle size (A80). Good agreement between experimental results and the model was achieved in most instances for thin oxide layers. The binding energy of the Aulf, peak decreases as cluster size increases in deposits on C ( A h ) . Extra-atomic relaxation was proposed to explain the results, but this type of explanation is the subject of controversy (A29, A30). This effect was not observed on alkali halide substrates due to the high mobility of the deposited atoms. Via an analysis of the binding energies of Hg for Hg,-,Cd,Te, a method to determine the mean diameter of Hg clusters was given (A82). A number of assumptions were required along with curve-fittingmethods that must be viewed with caution. Polymers. The use of XPS to study the near surface region of polymers has advanced in several areas. However, in some instances XPS has given ambigious results. The use of XPS and other techniques in this regard was reviewed (A83). Different methods of solvent extraction of Biomer indicated that this is a blended polymer by XPS analysis (A84). Previously it had been considered to be copolymer of urethane and polyether. ARXPS was employed to investi ate surface segregation in three polymer classes, Le., poly(et ylene terephthalate) (PET) with cyclic oli omers, a copolymer of ethylene chlorotrifluoroethylene (&CTFE), and segmented polyuret anes (PU) (A85). Surface composition alterations due to different preparations, e.g., amount of crystallinity or number of surface soft segments, were identifed from the XPS spectra. Surface segregation of poly(viny1 methyl ether) blended with polystyrene was observed by ARXPS under a wide variety of experimental conditions (A86). The findings agreed with mean-field theories of polymer solutions. The investigation of films of benzatriazole on Zn and Zn-Cu alloys required the use of both XPS and X-ray-induced Auger peaks to study preferential interactions (A87). The Auger peaks had much larger shifts between the metal and films compared to base metals. The structure of poly(s0dium p-styrenesulfonate) cast on vinyl alcohol/4-vinylpyridine was suggested to have vertial microdomains by ARXPS and transmission electron microsco y (TEM) (AM). However, the ARXPS results were not concksive due to com arisons with weak signals and the use of C and 0 that couli have several sources. The use of chemical derivatizationfor polymer analysis with XPS was reviewed (A89). The penetration depth of two derivatization reagents, trifluoroaceticanhydride (TFAA) and pentafluorophenylhydrazine (PFPH) often used in XPS, was studied ( A N ) . It was observed that both penetrated rapidly, but TFAA reacted quickly and PFPH did not. The use of gas-phase reactions to derivatize amine, carboxyl, and hydroxy groups with various fluorine-containing compounds was investigated (A91). Some selectivity was achieved on a qualitative basis, but reaction yields were not 10%. Trimeth lsilyl imidazole was used in a vapor-phase reaction with cartoxyl
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
groups in a derivative identification procedure (A92). Water must be excluded in this method, and reactions with other functional groups have to be considered. Derivatization of a plasma-treated PET with TFAA indicated the presence of surface hydroxy groups by ARXPS (A93). The extent of reaction was the same with either liquid or vapor exposure of the TFAA. PolyCTFE with surface hydroxy groups was found by XPS to react quantitatively with (3-isocyanatopropy1)triethoxysilane(A94). This reaction may be useful in similar systems to determine the amount of surface hydroxy groups. Derivatization can be employed with carbon fibers also. The use of several F-containin organic reagents to determine the presence of carbonyl, car oxy, and hydroxy groups was extended to carbon fibers (A95). The reactions a peared to be confined to the outermost surface. With B a d i t is possible to monitor the extent of carboxyl grou s on carbon fibers (A96). This allows a determination of t t e number of these groups that have reacted with an epoxy versus unreacted surfaces. It was shown that the adsorption of CHC13 can be used to determine porosity of carbon fibers (A97). The CHCl:, a pears to be bound to the edge atoms of pores on the surface; tfese results agree with other observations. Sample Degradation. Changes in samples due to X-ray irradiation, electron and ion beams, and heat are observed on occasion. Moat of the problems occur with polymers or oxides. Damage to filter paper was found even with brief exposure to nonmonochromatic X-rays (A98). It was suggested that dehydroxylation was the main decomposition reaction. It was found that poly(tetrafluoroethy1ene)(PTFE) (in the form of a tape) lost more F compared to C with ex osure to both mono- and nonmonochromatic Al X-rays (A99f: Little change under the same conditions was noted for poly(arylnitri1e) and poly(viny1 chloride) and none for PET and poly(methy1 methacrylate). The effect of Mg X-rays on PFFE was found by posttreatment with a commercial etch solution (A100). No differences in the XPS spectra were found under the experimental conditions of this investigation. The COH moiety in poly(viny1 alcohol) was found to be unstable under Mg X-ray irradiation (A101). Several new C-0 functionalities were noted. It was observed that poly(oxymethy1ene) degraded under monochromatic X-rays (A102). The samples had a wide range of mo hologies and the effect was ascribed to electrons leaving theyulk. The result of electron bombardment of poly(viny1diene fluoride) (PVDF) was investigated with XPS using different anodes, i.e., Mg and Ti (A103). Differences in the Fls/F2s ratios were not found with Mg radiation but were noted when the Ti anode was employed. The depth where the changes occurred was in the 10-nm region. Low-energy Ar+ (1keV) ions were observed to complete% remove the CF roups from PVDF with a dose of 10 ions/cm2 (AIM).' 8amage started to be noted with an ion dose of 10l2ions/cm2. Two polyimides, made from pyromellitic dianh dride and oxydianiline or biphenyltetracarboxylic dianhychde and phenylenediamine, were found to undergo changes with Ar bombardment (A105). The changes, which resulted mostly in the removal of carbonyl groups, occurred after the equivalent of one layer of Ar had impinged on the surface. Low-energy ion beams were found to improve adhesion in many polymer-metal interface systems (AIM). XPS analysis indicated that intermixing in the interface region had occurred. With Ar+ sputtering, chan es in the surface composition of glass surfaces (compare to in situ fractured surfaces) were noted by XPS (A107). The alterations, i.e., surface depletion or enhancement of 0, depended upon the glass. Surface enrichment of Si in sputtered alkaline glass surfaces was studied by ARXPS (AIM). Detailed conclusions about the elemental distribution the near surface region could not be made, but somewhat eneralized findings could be concluded. ARXPS showed &at (100) and (110) Ti0 will be reduced to different de ths depending on the Ar+ kombardment ener (A109). strong depth de endence of the reduced Ti anysurface diffusion was notex From a controversary about ion beam induced changes in the XPS s of sin le-crystal Fe203, it appears that experimental con 'tions and treatment affect any conclusions about this system (A110,A111). Another factor involved in this roblem is the large background of the Fe2p si nal. In anotier study only FezO was observed to be alterefby Ar+ bombardment compared to oxides of Si, Al, and Cr (A60). LiNbOs was observed
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SURFACE ANALYSIS
to undergo reduction by Ar+ beams (A40). It appears that the specimens and experimental conditions play a very important role in any observed effects of ion bombardment of oxides. With Ar+ bombardment (4 keV) the central metal atom of K XFp-type compounds (X = Nd, Ta, or Ti) underwent reauction (A112). However, the reduction reached a steady-state value as indicated by the XPS spectra. Instrumentation. Durin the reporting period several manufacturers have i n t r o d u J X P S instrumentation that can analyze small areas. Features the size of 0.01 mm have been observed by XPS with one instrument, and with other spectrometers resolution is approximately 10 times that dimension. With these new instruments elemental ma s similar to those obtained by scanning Auger microscopy (#AM) are produced. A review of some of the advances in XPS instrumentation was made (A113). A shorter review with an emphasis on the use of a focused X-ray beam for small-area analysis was published also (A114). The ability to obtain elemental XPS images with a resolution of 0.15 mm from a field of view of 10 mm was demonstrated (A115). This was achieved with the addition of a prelens of four lates to a commercial concentric hemis herical analyzer &HA) that allowed scanning of the phot0er)eCtrrons. It was suggested that an improvement of the resolution by a factor of 10 would be ossible. XPS analysis of a spot with a diameter of 0.25 mm y modification to the in ut lens of a CHA was described (A116). The area samplezshowed less than a 10% variation with changes in kinetic energy, and the transmission factor for the system was determined. The addition of a positionsensitive electron detector to an XPS spectrometer with a nonretarding CHA was made that gave a spatial resolution of about 0.2 mm (A117). With a step ing motor adjustment to the sample stage, two-dimensiona images were obtained with a width of 0.2 mm. Variations in signal intensity and eak position across a sample were observed in a CHA with 0th electron beam images and XPS spectra (A118). The effect depended upon several factors, including pass energy, slit width, acceptance area, and energy dispersion of the analyzer. A method to anal e by small-area XPS about 20 hair strands or an individuzhair was developed (A119). The measurement of Ag particle size on AlzO was studied as a function of sample preparation (A120). T i e best agreement (compared to scanning electron microscopy (SEM)) was with in situ deposition and a hemispherical shape. The use of small-area analysis and a differentially pum ed ion gun for XPS depth profiles was illustrated for sever3materials that would be difficult to study with AES (A121). Re ions of samples that conduct versus insulators were identifie! by the use of an electron flood gun and a monochromatic X-ray source (A122). The peaks due to the insulating part of the sample shift relative to the peaks from the conducting portion of the specimen.
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B. AUGER ELECTRON SPECTROSCOPY The use of Auger electron spectroscopy by electron beam interactions with a solid surface is employed widely for both elemental and, in many instances, chemical analysis of the near surface region. Also, efforts continue to make the technique more quantitative. The ASTM published a standard for re ort' AES data (BI). Two overview reviews of AES, inclu&n Z M ,were made (B2, B3). A book (B4) and an article (E!on ? the use of AES in analysis of materials were published. A number of methods to eliminate charging with nonconducting samples in AES were reviewed (B6). Line Shapes. AES line shapes can offer detailed information about the chemical environment of many materials in a ver small area. Guidelines for the identification of chemicdand matrix effects were published by ASTM (B7). The use of AES to anal e the electronic structure in metals and alloys was revieweZB8). From the investigation of Cu overlayers on A (111)it was shown that the extended Auger fine structwe fepends upon the local atomic environment (B9). Tbil fsy.i c vas reviewed also (BIO). The short-range structure found in the high-energy region of the MZs3VV spectra of transition metals was questioned (B11).Another explanation based u on long-range structure was proposed as an alternate. At tfis time the reason for the fine structure has not been resolved. In many systems the use of Auger line shapes has made contributions to understanding material properties. Changes
in the 0 KLL AES line sha es were found with different treatment procedures for an AfLi alloy (B12). The differences included energy separation and relative peak intensities. However, background and loss feature corrections were made which may alter the results. A deconvolution process previously employed in XPS was applied to the AES spectra of Pt on Si(ll1) (B13). A number of spectral features were found in the Si b W and Pt N,OO r 'ons that were not observable in the raw spectra. Changes in x e Zr AES spectra were noted at varying degrees of oxidation when high-resolution data were carefully treated for background shape and loss features (B14). Some features and line shape changes were not observable with lower resolution spectra. In the temperature range from 25 to 400 "C, differences in the L W spectra of V were found (B15).Part of these changes were ascribed to differences in the electron loss features. It was found for free atoms that the line shapes were not Lorentzian and the observed fullwidth at half-maxima were somewhat larger than the decay width (B16). It was suggested that these effects came from long-range continuum interactions. A comparison of the A1 L2,VV (about 60 eV) and KVV (about 1560 eV) line shapes (after differences in resolution were considered) showed few differences (B17).This indicated that the C W AES spectra sampled predominately bulk effects even for the b W region. Differences in the 0 KLL spectra at high-energy resolution were noted between ZnO and MgO (B18). However, both materials were sputtered and this may be the cause of the observations. A review of the C K W line sha in a number of materials was made (B19). Chan es in the &?W line shape of CH30H with temperature and jifferent amounts adsorbed on Fe(ll0) were observed (B20). In the physisorbed regime some electron beam effects were found even with the lowest ex osure. The hole-hole repulsion energy in P E was suggestexto be much larger than predicted theoretically by a one-electron density of state (DOS)model (B21). However, the shape of the peak (bandlike) did follow the expected theoretical predictions. Changes in the AES spectra of Fe-doped polyacetylene and polypyrrole (conducting polymers) as a result of various treatments were shown by pattern recognition techniques (B22). The success of this approach is based upon the choice of appropriate classifiers for the pattern reco ition procedure. The use of derivative AES spectra of the LVV region to compute line shapes was explored (B23). It was claimed that new features, after loss effects were removed, could be observed in the calculations compared to integral data. It was found that changes in the positive to negative excersion ratio in derivative mode spectra of Tic, were correlated with chemical composition (B24). However, the information content is the same, or even better, with integral data compared to differentiated data, but features are often easier to see with differential spectra. In several investigations attempts were made to employ changes in derivative mode spectra for identificationpurposes. Most often this uses only two points in a s ectrum. It was s gested that by taking the ratios of the p e d - b p e a k hei hts ( S H ) of the various L 3 W transitions of Cu and Mn a n t t h e K W transitions of 0, identification of the oxidation state of the metals could be determined (B25). However, the deviations from the averages often were such that the identification could be ambiguous. The peak-to-background (P/B) ratio method for N(E) spectra was found to be better in the study of the Ti-Si interface than using the PPH procedure (B26). Lower beam currents could be used and better handlin of chemical effects were noted. With the use of relative Pf" of N and Fe in various steel samples a detection limit of about 1% N was achieved with good linearity (when compared to wet chemical analysis) (B27, B28). An attempt was made to use the ratio of the positive peak height to the full PPH to distinguish various calcium aluminates (B29).Small differences were noted in the various ratios in most cases. However, the error estimates were large enough to make many identifications questionable. Small differencesin the PPH of small amounts of Au in Cu in the 60-70-eV region compared to the puue metals were reported (B30). However, the resolution conditions of these determinations may have contributed to the changes in the spectra. Beam Interactions. It is well-known that both electron and ion beams can affect materials in AES experiments. Electron-excited C KVV spectra for various graphite and ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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graphite oxide aam les showed chemical and structural damage compared to k r a y excited spectra (B31). In some instances the changes could be lessened by reducing the beam current. Changes in the valence band region after electron beam exposure versus X-ray irradiation were noted also. The diffusion of Ca and Na in soda-lime glasses under electron beam irradiation was analyzed with an irreversible thermodynamic model (B32). Good eement over a large range of current density was obtainedYiNbO3 under relatively mild electron beam conditions (1keV and 4 pA/mm2) degraded with the emission of O+ (B33). The results were explained with Knotek-Feibelman mechanism. Thin films of CdC12on Si were found to decompose under electron beam irradiation (B34). The degradation was localized, but the lateral extent of it was not measured. It was noted that if 0 (about lo4 torr) were present during the sputtering of Tic, &e depth of 0 in the solid was much greater than with a static exposure (B35). The O/Ti PPH ratio was found to vary in a linear fashion for different oxide overlayers on Cu that were sputtered (B36). However, the species preeent at the Ti/Cu interfama could not be identified by this ap roach. XPS or AES line shape differences might be able t o f k d l e this roblem. Preferential s uttering of Ni with various Co-Ni &ys was found in the &V re ion but not in the LMM spectra (B37). This was ascribe8 to the differences in the IMFP in the two ener regions. However, corrections for the secondary and rexstributed primary electrons were not made, and the fitting was done by a grid procedure. It was observed that preferential sputtering of metal alloys was dependent upon the relative sputter yield of the pure metals; i.e., the lower sputter yield material predominated at steadystate conditions (E38). In most instances low ion beam energy gave a composition nearer that of the bulk. Sputtering of cleaved GaSe(100) indicated that Se initially was removed referentially (B39). With longer bombardment times 8 a disappeared preferential1 and changes in the Se line shape occurred that suggestedrmore atomic-like surface species were created. It was noted that the referential sputtering of Li in a heated Li-Co alloy depenzed upon the amount of 0 present in the surface region (B40).With more bulk 0 the rate of Li removal was reduced and the steady-state amount of Li was larger. The anal is of Si,Nb films by AES showed N was removed referenti& in an ion beam (B41). With prolonged electron geam exposure increases in the relative amount of N was noted. Cooling H CdTe double layers reduced the effects of both electron ancfion beam changes during a depth profile (B42). The results with etched (to produce a tapered surface) and nonetched samples were very similar. It was found that its formed on Ar+ sputtered thin filmsof Pb on Cu (produced gy different methods) (E43). Thin oxide films had little effect on these observations. Backscattering. Backscatteringis one of the factors that has to be considered when attempting quantitative AES. The effect of elasticall scattered electrons on the observed Auger current was consiiered with Monte Carlo calculations and was found to cause at most a 10% decrease in intensit (B44). However, this results in a decrease in the escape iepth of about 30% and would affect overlayer determinations. By a classical calculational approach it was determined that elastically scattered electrons had about the same backscattering value as was found in a previous study (B45).A range of elements and beam vol es was considered. Monte Carlo calculations, based upon di erent physical models than had been tried previously, were used to estimate backscatterin at both the low- and high-ener background regions in AE8 spectra (B46). Comparisons Etween the calculations and experimental results with layered materials were given. Better agreement for the energy profiles compared to the experimental spectra was obtained than for the backscattering coefficients. Backscattering factor calculations were shown to vary appreciably for low-2 materials as a function of the angle of incidence of the electron beam (B47).Changes in the beam energy were found to have only a minor effect on the computed backscattering coefficients. A model was developed to calculate the energy and angular distribution of backscattered and secondary electrons by Monte Carlo procedures (B48).The results agreed well with experimentalfmdings for the s stems studied, and could be extended to systems where the Jelectric constant is known. Backscattering correction
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factors determined from Boltzmann transport eed well with findings from Monte Carlo calculations ( B 4 r The effects of electron channeling and backscattering on the Auger signal intensity for Al(111) were evaluated as a function of beam voltage (B50).From the calculations experimental conditions that give both good spatial resolution and surface sensitivity were found. Angular Effects. Variations in the C KVV Auger line shape and peak position maxima were found with singlecrystal aphite as a function of electron emission angle (B51). The digerences were not noted with amorphous graphite. This effect was ascribed to anisotropy in the DOS, and agreement with calculations was best at intermediate takeoff angles (B52). The angular variations of AES spectra of Si(OO1) with and without a K overlayer were observed (E531 The data suggest that the position of the K atoms on the Si could be determined. Angular resolved AES spectra of varyin layer thickness of Cu on and overlayered with Ni single crystab were analyzed both experimentally and theoretically for forward A single scattering ap roach was able focused electrons (Ed). to describe many cases, but with deep layers method was inadequate. A multiple scattering approach was required to ex lain the surface structure observed in this instance. Angu ar resolved Auger electron emission was observed from Ni(100) and oxidized Mg(0001), and the results com ared reasonably well with theoretical predictions (E&). In addlition, the osition of the 0 atoms in the Mg was determined. A m d l was develo ed to calculate the angular dependence of the backscatteref Auger and reflected signals (B56) The model was tested with changes in the azimuthal angle for GaAs(ll1). Moderate agreement between the computations and the experimental results for both Ga and As L3M4bM4,5 peaks and the reflected peak was obtained. Measurements of the effects of diffraction and backscattered electrons on the total Auger signal for W(110) and a Si overlayer were made (B57).Variations with the emission angle of up to 10% were noted. Sensitivity Factors. A comparison of semiempirical calculations of ionization cross sections and IMFP was made for AES sensitivity factors (B58).The best match between experimental results and the computations had a standard deviation of 56%. Ab initio Hartree-Fock-Slater calculations for AES cross sections that included backscattering were made for Si, Cu, Ag, and W (B49). The trends were similar to those for other computations,but the values were higher especially near the threshold level. The inclusion of the probability of core hole ionization in the calculation of AES sensitivity factors showed improved agreement with experimentalvalues (B59). However, the factors cannot be used with transitions involving different levels, i.e., KLL and LMM. It was determined that the signal-to-backgroundratio increases and the signal-to-noise ratio decreases for AES peaks when the beam voltage increases from 30 to 100 keV (B60). These results were consistent with theoretical expressions. These findin s are important when experiments involving better spati resolution are being considered. Monte Carlo calculations of the full N(E) spectra of Si, Cu, and Au showed reasonable agreement with experimentaldata with a cylinderical mirror analyzer (CMA) (E61).It was found also that a 0.5-nm Cu layer on Si obliterated the underlyin s i plasmon peak structure. A randium-jellium model was to simulate the Cu LMM spectral region (B62). The s ctra had reasonable agreement with expermental results a n g o r e complete Monte Carlo calculations. It was found b Monte carlo calculations that elastic collisions reduced the and sampling depth of B in various metal matrices ( 8 6 3 ) S g effect was greatest with a 2 of about 30. Background Correction. The ASTM issued a ide for AES background subtraction techniques (B64). T E use of a power law expression, AEm, to describe the background for AES spectra was reviewed from both experimental and theoretical viewpoints (B65). The value of A depends upon the number of valence electrons and m varied in value from about 0.5 to a little more than 1. A global approach for AES background correction using splines was proposed that involves very little computational effort (B66). Reasonable agreement for Si and oxidized Si with this procedure and an earlier method was achieved. An iteriative deconvolution procedure to remove loss and instrumental effects was demonstrated for the CVV transitions of C and Si (B67). The
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method gave results similar to those from other rocedures. A method to compute the background of an m! spectrum based upon a single scattering event convoluted with the experimental spectrum was develo ed (B68). Examples for several elements were given and t e corrected spectra appeared to be reasonable. Quantitative Analysis. Several of the topics covered above (i.e., backscattering, sensitivity factors, beam effects, and background) need to be considered when trying to do uantitative analysis by AES. Some of these difficulties were i ustrated in a round-robin study using Au-Cu alloys (B69). It was found that relative sensitivity factors vary less when the high-ener transitions were used compared to those These variations, u to determined at%wer kinetic ener 40% from the mean value, were rauced by about a half wRen comparisons were made with instruments from the same manufacturer. In another round-robin study with several iron nlloys, the quantitative results varied from about 10% with cleaved binary samples to about 20% if the specimens were polished (B70). With three elements present that required the use of transitions with a wide range of kinetic energies, differences of about 60% were reported. A study of the intensity ratio of the Cu M2,3Wto Cu L 3 W lines depended strongly on the incident electron beam le (B71). This effect was ascribed to changes in the overal3ackground spectra. From these findings it was s ested that an incidence angle of 30" should be used for inte%wratory comparison purposes. The use of "Top Hat" or Savitzky-Golay smoothing of AES integral and derivative spectra before fitting mixture data was shown to improve the psquared values and reduce the number of iterations for a calculated fit (B72). The computations were made with a NLLS approach that would consider ener shifts,chemical effects mght cause problems with this m e t h s An example with an alloy was given that yielded reasonable agreement with the bulk composition. Several studies were made on systems where peaks overlap. The use of low-resolution AES spectra (obtained by smoothing and postdifferentiation)for quantitative analysis of TiN, was suggested (B73). In this a proach a Ti metal standard is used to account for the Ti& 23M4gpeak. Standard curves with varing amounts of N were constructed, and agreement with other approaches was good. The effect of oxygen on the ositive excersion of the overlapping Ti L , M , M4,and Ni ELL eaks in impure TiN, was questi~~ed2(B74!, This procefure was suggested previously as a way to determine N to Ti ratios. Least-squares fitting and factor analysis (FA) were employed in the analysis of the oxidization of Ni and a Ni-Fe-Cr all0 (2375). With FA it was possible to infer the chemisorbed species in the early states presence of a of the reaction. Analyses of Cu-Au alloys were made with a linear addition of inte al AES spectra of the pure metals in the range 0-lo00 eV (E76).A comparison of a ran e of compositions was used with the best fit chosen visualfy with an accuracy of several percent. However, large deviations were noted in the lower energy regions due to differences in secondary electron distribution. Backscattering and matrix effects could contribute also. Even poorer results were found when just PPH measurements were used. It was determined that PPH measurements do not accurately represent Auger intensities if the resolution of the energy analyzer is >0.3 times the natural line width (B77). A procedure to overcome this problem was developed and teated with good results. A method to analyze ternary systems, AxB1,C, by AES that can consider sputter and matrix effects was proposed (B78). The procedure is based upon an internal reference of element C; A1,Ga ,As was used as an example. Sputter and matrix effects with quantitative derivative AES spectra for Ti and Co silicides were explored (B79). The IMFP value assumed had the est effect on the matrix correction for the analysis of several inary mixtures (B80).The influence of beam ener and incidence angle and relative atomic amounts was n o g t o be relatively smaller. It was found that the experimental values for these effects did not agree with
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ployed in studies of the diffusion of 0 and Si in Ti-Si layered material. The relative amounts of Cu and Au in alloys of these metals did not follow a simple linear relationship when the
PPH of various transitions was used with scraped thin film specimens (B82). With sputtered samples a linear relationship was noted except when the Ar+ beam voltage was less than 1keV. Surface versus bulk composition of Au-Cu alloys was found to differ dependent upon the bulk composition by using both low and high kinetic energy AES transitions (B83). These observations were made with sputtered surfaces, which may have an effect on the findings. Effects such as adventitiousmaterials adsorbing on a sample will affect quantitative analyses by AES. Reasonable agreement between quantitative AES PPH analysis and Rutherford backscattering spectrometry (RBS) analysis of a series of silicon oxy-nitride films was reported (B84).In most instances sputter effects could be taken into account, but readsorption of residual oxygen-containing species was noted in some instances. A method to take into account the usual adventitious overlayer was proposed (B85).It was based upon the simple overlayer equation and was employed to eliminate effects from sputtering. The procedure assumed that a Pd-Au alloy would have the same surface and bulk composition (if mechanically olished) and that the sputter yield was about the same for 0th elements. In this instance the assumptions a eared to be roughly applicable. A method to quantify A@ spectra based u on peak heights that takes into account backscattering, I h F P , and atomic density was developed for use with a minicomputer (B86). The ability to include thin overlayer films was included also. Models for the AES analysis of the growth of overlayers on nonideal surfaces, Le., hills and pits, were developed (B87). Small deviations from the ideal flat surface were found to give changes in the ex cted intensities. It was su gested that scratching (with a ramond scribe) a Co-Ni alfoy caused a small relative increase of Ni at the This conclusion was based upon a leasbsquares surface (B). fit with a number of assumptions about the M,W line shape with the pure metals. Depth Profiling. AES is often used to determine the composition of the near-bulk re ion of many materials by depth profilin . The use of the KLlL2 transition (about 369 eV) for a cfepth profile of TiN was proposed (B89). The PPH method with spectra obtained at high resolution yielded a N detection limit of about 5%. Four methods of error analysis to determine the number of components by FA in a profile of Pd on Si were tested (B90).Depending upon the system, target transforms, after principal component analysis (PCA) were able to detect the various species encountered in the profile. FA of Pd films on Si annealed for different times at 200 "C indicated that different Pd-Si species existed at the interface (B91). A computed spectrum attributed to Pd2Si was very similar to one previously reported. PCA of the de th profile of the M,W region of Cu implanted in a Co-Ni J o y was compared to an earlier study where the LMM lines were em loyed (B92). The more surface sensitive M,W findings in8cated that preferential sputtering of Cu, and to a lesser extent Co, occurred and that corrections for redistributed primary and secondary electrons were required. A sequential layer sputter model was extended to passive films of Fe-Cr alloys where a sharp interface was assumed (B93). Separate sensitivity factors and IMFPs for the oxides and metals were used, and the s utter rate was an adjustable parameter. The true interface (8ased upon a sharp interface) was deeper than the calculated interface. The effect of sam le temperature (30-700 "C) on the AES depth profile of a tu-Au alloy was studied (B94). Up to 300 "C surface enhancement of Cu predominated due to sputterin Au preferentially segregated to the surface. Above 300 "C d k i o n was the more important mechanism. For the depth profile anal sis of thin C/W multilayers it was found that Gaussian andrerf functions did not properly describe the results (B95). The basic sputtering process appeared to be the dominant broadening feature, and the use of an asymmetry distribution function gave reement with the experimental observations. At smally e p t h s microroughness was found to dominate while at greater depths the substrate was more important (B96).A procedure to determine the com osition of the outermost layer with a heterogenous distritution of elements in subsequent layers was developed (B97). In a test with a Fe-Si-P alloy reasonable agreement with experimental results was found. A model that takes into account macrostructures (features in the micrometer region) caused by sputtering was tested
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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against experimental results for a standard profiie material, i.e., Ni-Cr (Bsls). Good agreement was noted for a wide range of experimental conditions, i.e., incident ion beam angle and sample rotation. The improved depth resolution of A /Mo and Cu/Ag multilayers with Nz+and Ar+ was ascribe! to a combination of chemical and energy effects (B99). SEM micrographs showed smoother sputtered areas with Nz+and N was implanted in the surface region of Mo. A composition nearer that of the bulk with a Ni-Cr alloy was obtained with sample rotation than with nonrotation (B100).A decrease in the preferential sputtering of Ni was noted. Methods to detect an interface durin an AES sputter profile were investigated with the aim to rduce the time spent in obtaining spectra in the middle of a la er and increase the number of data points at an interface &101). Chan es in SIMS peak intensities, absorbed ion, and electron currents and the decay of an Auger peak were examined as monitors. The choice of which approach or approaches depends upon the material being investigated. A matched filter method that is easy to appl was described that improves the S/Nratio of AES spectra (2102). In many cases this method will suppress the background and the smoothin results are similar to other procedures. An example with a fepth profile was given. The angular distribution of sputtered atoms from a Au-Ag alloy was determined by AES analysis of a catcher foil (B103).The distribution followed a cos* 0 relationship where x de ended upon the gas and the metal being analyzed and range in value from 0.5 to 1.7. Thus, a simple cosine distribution was not found from these observations. Sputter-redeposited material from standard multilayer sam les partly reduces profiie resolution (B104).However, the incizent angle of the ion beam and the smoothness of the sample usually are more important. AES can be used to analyze olymer/metal interfaces by using taper techniques (B105). !'he determination of the taper position was done b combining AES data and energy dlspersive X-ray analysis ( DXA) results. Profile determinations of Pd deposited on Si were made by AES and RBS (B106). The RBS data 'elded interface depth values for the AES rofiles. AErdetected trace amounts that were not founfby RBS. Carrier-gas heat extraction was emplo ed to calibrate ion 'elds for AES depth (B107).The comprofiles of oxide firms up to 1 pm on position of the homogenous layer must be known, and the precision in this work was 13% By simultaneous SIMS and AES analysis, preferential sputtering and mix' in Si02and InP multilayers were investigated with Oz+a 3 A r + (B108). AES showed 0 honding to In and P in the interface region and 02+ ave preferential In s uttering versus P at the interface. combined AES-SI& profiie of a standard Ta205 overlayer on Ta showed the same depth resolution with both techniques (B109). An increase of the ion beam raster frequency gave slightly better AES profile resolution. Instrumentation. Determination of AES spectrometer sensitivity for different analysis modes, i.e., pulse countering, beam blankin , etc., was made on the basis of the S/N ratio of the Cu LJ8 M peak (BIIO).Good agreement with a theoretical ana!&is%as achieved, and this procedure allows comparisons to be made between different instruments. A procedure to convert standard PPH values for use with a specific analyzer was given with examples ( B I I I ) . With a modification to the ower sup ly for a CMA, sequential ISS-AES s ctra can obtained)(Bll2). A small percentage Ar+ used for ISS is required. Differences in the of He+ in outermost layer com osition versus the near surface region for two alloys were ogserved. The ion gun uses a LaBBfilament to increase ionization efficency and a slit is placed between the sample and the CMA (B113). Angular resolved AES was demonstrated in a system with a CMA that had a movable le electron gun (B114).The sample aperture and a glanc' is positioned so t h a x x e t e c t e d electrons come from a low angle off of the surface; examples with two specimens were given. An experimental s tem to obtain angular resolved AES s ectra with a three-gridrsretarding field analyzer (RFA) was gveloped (B115).An example with a Ni(100) surface was demonstrated. An electron s in polarization anal zer that can be added to an AES or SAL spectrometer was &eloped (B116). All three polarization componentscan be determined and the magnetic image can be analyzed independently of the secondary electron image. It was found that modulating a
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
potential on the specimen reduced the background signal when a RFA was employed for AES spectra (B117). The replacement of a lock-in amplifier with a computer to obtain derivative AES spectra was demonstrated with a double-pass CMA (B118).This method allowed spectra to be obtained more rapidly and integral spectra with low noise could be easily computed. There were several reports of the construction of in situ pin-on-disk assemblies in UHV systems (B1194121). Scanning Auger Microscopy. An extended overview of S A M was published during the review period (B122). Reviews on the use of SAM for industrial ap lications (B123),fiber reinforced materials (B124),biologidspecimens (B1251,and small particles (B126) were also made. One of the most important topics in SAM is the wish to obtain more information from experimental micrographs. A brief review was made on background and S A M image anal is (B127). A method that uses eak heights was em loye to uantify SAM images with fackscattering, IMFf, atomic Iensity, sample drift,and the data collection procedure taken into account (B86). Image analysis procedures could be included also. The effects of shadowing and forward scattering and backscattering in using SAM with rough surfaces were studied by Monte Carlo calculations and model samples (BIB). Procedures to reduce these effects were proposed and the agreement between the calculations and the experimental f i n d y displayed the trends correctly. The effect of electron beam scattering on spatial resolution in S A M was studied with a kinematic model of a shar ,flat interface (BI29). It was found that the spatial resorution de ends mostly on the electron beam diameter and decreases s\ghtly with increasing beam voltage. Scatter diagrams to analyze digital SAM micrographs were demonstrated for a series of samples (B130). These diagrams are used to distinguish elemental spatial differences, topological effects, and improve the S/N ratios of aks. A method to compensate for topographic effects in R M images was developed that takes into account the electron beam incidence angle and the Auger transition energy (B131). Some additional measurements are required to use this procedure that would add to the time needed to obtain an image. Several cases were found where the P B approach can give erroneous results for quantitative SA and AES (B132). These instances include a thin coating of one element on another, ed e effects, changes in the background signal, chanelling, a n f isotro ic emission. The use of P/(P+ 2B) or a simple backgrountffunction often was found to give better results. Changes in peak intensity due to different chemical states were not explored. Improvement in spatial resolution was reported. AES spectra of individual &nm Pd particles on Si were obtained with a scanning transmission electron microscope (STEM) operated at 100 keV (B133). With submonolayer amounts of Si buried in GaAs, it was found that the minimum detection limit was similar to that in conventional analysis. The signals observed in both cases came from about 5OOO atoms. Analysis of GaAs-ALAS multilayers and a superconducting film were re orted also (B134). In both of these reports a conventional ChA was used. The use of a modified parallel-plate mirror analyzer in a STEM for detecting Auger electrons was roposed (B135). Higher throughput compared to a C M i or CHA with 0.1% energy resolution was considered achievable. Resolution of 20 nm for multilayer materials with a new commercial SAM system was reported (B136). Other improvements in instrumentation were suggested. Modifications to the Everhart-Thornley detector (normally used in SEM)for SAM were made (B137). This dectector has cost and lifetime advantages compared to the normal detectors used. The useful current range was 1-50 nA, but it could be extended. A computer-controlledstage for SAM that allows mapping for areas of up to 20 mm on a side was developed (B138).Image analysis and computer controls of an ion gun and tensil were incorporated with this system. In situ ben fracture devices for SAM were built and applie to hydrogen-embrittled specimens (B139). A number of other experimental refinements were made also during the report period. A method to measure the onset of secondary electrons with a SAM was described and comparisons with AES profile and depth analysis were given (B140). This approach can give very good spatial and interfacial resolution, since low beam currents can be used. Examples were shown of the y-modulation technique (the
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intensity of an Auger peak in a map is a vertical dis lacement instead of the intensity dis lay usually employed) for several samples (B141).The dispyays by this approach had better contrast than the usual images. Electron beam induced current measurements along with secondary electron images and AES spectra (all determined in a SAM) were employed to study a metal-semiconductor interface (B142). Good correlation between the .techniques was obtained. With a lap ing angle of
