1、Designation: E 995 04Standard Guide forBackground Subtraction Techniques in Auger ElectronSpectroscopy and X-ray Photoelectron Spectroscopy1This standard is issued under the fixed designation E 995; the number immediately following the designation indicates the year oforiginal adoption or, in the ca
2、se of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 The purpose of this guide is to familiarize the analystwith the principal background subt
3、raction techniques presentlyin use together with the nature of their application to dataacquisition and manipulation.1.2 This guide is intended to apply to background subtrac-tion in electron, X-ray, and ion-excited Auger electron spec-troscopy (AES), and X-ray photoelectron spectroscopy (XPS).1.3 T
4、his standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Docume
5、nts2.1 ASTM Standards:2E 673 Terminology Relating to Surface AnalysisE 996 Practice for Reporting Data in Auger Electron Spec-troscopy and X-ray Photoelectron Spectroscopy3. Terminology3.1 DefinitionsFor definitions of terms used in this guide,refer to Terminology E 673.4. Summary of Guide4.1 Releva
6、nce to AES and XPS:4.1.1 AESThe production of Auger electrons by bombard-ment of surfaces with electron beams is also accompanied byemission of secondary and backscattered electrons. Thesesecondary and backscattered electrons create a backgroundsignal. This background signal covers the energy spectr
7、um andhas a maximum (near 10 eV for true secondaries), and a secondmaximum for elastically backscattered electrons at the energyof the incident electron beam. An additional source of back-ground is associated with Auger electrons, which are inelasti-cally scattered while traveling through the specim
8、en. Augerelectron excitation may also occur by X-ray and ion bombard-ment of surfaces.4.1.2 XPSThe production of electrons from X-ray excita-tion of surfaces may be grouped into two categoriesphotoemission of electrons and the production of Augerelectrons from the decay of the resultant core hole st
9、ates. Thesource of the background signal observed in the XPS spectrumincludes a contribution from inelastic scattering processes, andfor non-monochromatic X-ray sources, electrons produced byBremsstrahlung radiation.4.2 Various background subtraction techniques have beenemployed to diminish or remov
10、e the influence of these back-ground electrons from the shape and intensity of Augerelectron and photoelectron features. Relevance to a particularanalytical technique (AES or XPS) will be indicated in the titleof the procedure.4.3 Implementation of any of the various background tech-niques that are
11、described in this guide may depend on availableinstrumentation and software as well as the method of acqui-sition of the original signal. These subtraction methods fall intotwo general categories: (1) real-time background subtraction;and (2) post-acquisition background subtraction.5. Significance an
12、d Use5.1 Background subtraction techniques in AES were origi-nally employed as a method of enhancement of the relativelyweak Auger signals to distinguish them from the slowlyvarying background of secondary and backscattered electrons.Interest in obtaining useful information from the Auger peakline s
13、hape, concern for greater quantitative accuracy fromAuger spectra, and improvements in data gathering techniques,have led to the development of various background subtractiontechniques.5.2 Similarly, the use of background subtraction techniquesin XPS has evolved mainly from the interest in the deter
14、mina-tion of chemical states (binding energy values), greater quan-titative accuracy from the XPS spectra, and improvements in1This guide is under the jurisdiction of ASTM Committee E42 on SurfaceAnalysis and is the direct responsibility of Subcommittee E42.03 on Auger ElectronSpectroscopy and XPS.C
15、urrent edition approved July 1, 2004. Published August 2004. Originallyapproved in 1984. Last previous edition approved in 1997 as E 995 97.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volum
16、e information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.data acquisition. Post-acquisition background subtraction isnormally applied to XPS data.5.3 The procedure
17、s outlined are popular in XPS and AES.General reviews of background subtraction techniques havebeen published (1 and 2 ).36. Apparatus6.1 Most AES and XPS instruments either already use, ormay be modified to use, one or more of the techniques that aredescribed.6.2 Background subtraction techniques t
18、ypically require adigital acquisition and digital data handling capability. Inearlier years, the attachment of analog instrumentation toexisting equipment was usually required.7. Common Procedures7.1 Linear Background Subtraction (AES and XPS)In thismethod, two arbitrarily chosen points in the spect
19、rum areselected and joined by a straight line (1). This straight line isused to approximate the true background and is subtractedfrom the original spectrum. For Auger spectra, the two pointsmay be chosen either on the high-energy side of the Auger peakto result in an extrapolated linear background o
20、r such that thepeak is positioned between the two points. For XPS spectra, thetwo points are generally chosen such that the peak is positionedbetween the two points. The intensity values at the chosenpoints may be the values at those energies or the average overa defined number of channels or energy
21、 interval.7.2 Integral (or Shirley) Background Subtraction (AES andXPS)This method, proposed by Shirley (3), employs amathematical algorithm to approximate the inelastic scatteringof electrons as they escape from the solid. The algorithm isbased on the assumption that the background is proportional
22、tothe area of the peak above the background at higher kineticenergy. This basic method has been modified to optimize therequired iterations (4), to provide for a sloping inelasticbackground (5), to provide for a background based upon theshape of the loss spectrum from an elastically backscatteredele
23、ctron (6), and to include a band gap for insulators (1).7.3 Inelastic Electron Scattering Correction (AES andXPS)This method, proposed by Tougaard (7), uses analgorithm which is based on a description of the inelasticscattering processes as the electrons leave the specimen. Thescattering cross secti
24、on which enters in the algorithm is takeneither from a simple universal formula which is approximatelyvalid for some solids, or is determined from the energyspectrum of a backscattered primary electron beam by anotheralgorithm (8). Alternatively, the parameters used in the univer-sal formula may als
25、o be permitted to vary in an algorithm so asto produce an estimate of the background (9). This backgroundsubtraction method also gives direct information on the in-depth concentration profile (10 and 11).7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES)(12 and 13)Signal differentiation is amon
26、g the earliestmethods employed to remove the background from an Augerspectrum and to enhance the Auger features. It may beemployed in real time or in post acquisition. In real time,differentiation is usually accomplished by superposition of asmall (1 to 6-eV peak-to-peak) sinusoidal modulation on th
27、eanalyzer used to obtain the Auger spectrum. The output signalis then processed by a lock-in amplifier and displayed as thederivative of the original energy distribution N(E) or EN(E).Inpost-acquisition background subtraction, the already acquiredN(E) or EN(E) signal may be mathematically differenti
28、ated bydigital or other methods. The digital method commonly used isthat of the cubic/quadratic differential as proposed by Savitzkyand Golay (14).7.5 X-ray Satellite Subtraction: (15) (XPS)In this methoda fixed satellite structure associated with any given channelintensity such as a K X-ray line so
29、 that, starting at low kineticenergies, intensity is removed from higher kinetic energychannels at the spacing of the Ka3,4,Kb, etc. satellite positionsfrom the Ka1,2main peak to remove their contribution to thespectrum. This subtraction proceeds through the spectrum andremoves the satellite peaks a
30、ssociated with the photoelectronpeaks. It may also erroneously remove an equivalent intensityfrom any Auger peaks present in the spectrum.8. Less Common Procedures8.1 Deconvolution (AES and XPS) (16-19)Deconvolutionmay be used to reduce the effects due to inelastic scattering ofelectrons traveling t
31、hrough the specimen. This background isremoved by deconvoluting the spectrum with elastically back-scattered electrons (set at the energy of the main peak) and itsassociated loss spectrum. The intensity of the loss spectrum,relative to that of the backscattered primary, is sometimesadjusted to optim
32、ize the background subtraction. Deconvolu-tion is usually accomplished using Fourier transforms oriterative techniques.8.2 Linearized Secondary Electron Cascades (AES)In thismethod, proposed by Sickafus (20 and 21) the logarithm of theelectron energy distribution is plotted as a function of thelogar
33、ithm of the electron energy. Such plots consist of linearsegments corresponding to either surface or subsurface sourcesof Auger electrons and are appropriate for removing thebackground formed by the low energy cascade electrons.9. Rarely Used Procedures9.1 Secondary Electron Analog (AES) (22 and 23)
34、In thismethod, a signal that is an electronic analog of the secondaryelectron cascade is combined with the analyzer signal output soas to counteract the secondary emission function. It is particu-larly useful for retarding field analyzers in which low-energysecondary emission is prominent.9.2 Dynami
35、c Background Subtraction (DBS) (AES) (24 and25)Dynamic background subtraction may be used either inreal time or post acquisition. It involves multiple differentiationof an Auger spectrum to effect background removal, followedby an appropriate number of integrations to reestablish abackground-free Au
36、ger spectrum. The amount of backgroundremoval depends on the number of derivatives taken, althoughtwo are usually sufficient. In real-time analysis, a first deriva-tive of the Auger electron energy distribution obtained using aphase-sensitive detector is fed into an analog integrator,3The boldface n
37、umbers in parentheses refer to the references at the end of thisstandard.E995042thereby obtaining the Auger electron energy distribution withthe background removed.9.3 Tailored Modulation Techniques (TMT) (AES) (26 and27)This is a real-time method of background subtraction thatuses special modulatio
38、n waveforms tailored to the analyzer andphase sensitive detection to measure the Auger signal. TheN(E) distribution, EN(E) distribution, or areas under Augerpeaks over specified energy ranges may be obtained directlyusing these techniques.9.4 Spline Technique (AES and XPS) (28)In this method,a struc
39、tureless background is calculated from a measuredspectrum using a smoothing spline algorithm. This backgroundis then subtracted from the original spectrum.9.5 Digital Filtration (AES) (29 and 30)In a methodborrowed from energy-dispersive X-ray spectroscopy, a “top-hat” digital frequency filter is ap
40、plied to an Auger spectrum tosuppress the slowly varying background continuum, while themore rapidly varying Auger peaks remain unaffected.10. Keywords10.1 Auger electron spectroscopy; background subtraction;surface analysis; X-ray photoelectron spectroscopyAPPENDIX(Nonmandatory Information)X1. COMP
41、ARISONS AVAILABLE IN THE LITERATUREX1.1 At the present time, the most popular method for AESis digital differentiation (see 7.4). Popular methods for XPSinclude the straight line (see 7.1), modified Shirley (see 7.2), orvariations of the Tougaard method (see 7.3). Comparisons ofbackground subtractio
42、n methods mentioned here have beenoffered in the literature. In the case of 7.1 and 7.2, the effect onthe peak area calculated in terms of the choice of end points isexamined (4 and 5). Further comparisons of these proceduresand that in 7.2 on a number of materials are also offered(31-38).REFERENCES
43、(1) Briggs, D., and Seah, M. P., Practical Surface Analysis, Vol 1, 1990,Wiley and Sons, New York, NY, pp. 233239 and pp. 555586.(2) Grant, J. T., “Background Subtraction Techniques in Surface Analy-sis,” Journal of Vacuum Science and Technology A, Vol 2, 1984, pp.11351140.(3) Shirley, D. A., “High
44、Resolution X-Ray Photoemission Spectrum ofthe Valence Bands of Au,” Physical Review B, Vol 5, No. 12, 1972, pp.47094714.(4) Proctor, A., and Sherwood, P. M. A., “Data Analysis Techniques inX-ray Photoelectron Spectroscopy,” Analytical Chemistry, Vol 54,1982, pp. 1319.(5) Bishop, H. E.,“ Practical Pe
45、ak Area Measurements in X-Ray Photo-electroin Spectroscopy,” Surface and Interface Analysis, Vol 3, 1981,pp. 272274.(6) Burrell, M. C., and Armstrong, N. R., “A Sequential Method forRemoving the Inelastic Loss Contribution from Auger Electron Spec-troscopic Data,” Applications of Surface Science, Vo
46、l 17, 1983, pp.5369.(7) Tougaard, S., “Quantitative Analysis of the Inelastic Background inSurface Electron Spectroscopy,” Surface and Interface Analysis,Vol11, 1988, pp. 453472.(8) Jansson, C., Hansen, H. S., Yubero, F., and Tougaard, S., “Accuracy ofthe Tougaard Method for Quantitative Surface Ana
47、lysis. Comparisonof the Universal and REELS Inelastic Cross Sections,” Journal ofElectron Spectroscopy and Related Phenomenon, Vol 60, 1992, pp.301319.(9) Tougaard, S., “Practical Algorithm for Background Subtraction,”Surface Science, Vol 216, 1989, pp. 343360.(10) Tougaard, S., “In-Depth Concentrat
48、ion Profile Information ThroughAnalysis of the Entire XPS Peak Shape,” Applied Surface Science,Vol 32, 1988, pp. 332337.(11) Tougaard, S.,“ Formalism for Quantitative Surface Analysis byElectron Spectroscopy,” Journal of Vacuum Science and TechnologyA, Vol 8, 1990, pp. 21972203.(12) Harris, L. A., “
49、Analysis of Materials by Electron Excited AugerElectrons,” Journal of Applied Physics, Vol 39, No. 3, 1968, pp.14191427.(13) Taylor, N. J., “Resolution and Sensitivity Considerations of an AugerElectron Spectrometer Based on LEED Display Optics,” Review ofScientific Instruments, Vol 40, No. 6, 1969, pp. 792804.(14) Savitzky, A., and Golay, M., Analytical Chemistry, Vol 61, 1964, pp.16271639.(15) Klauber, C., “Refinement of Magnesium and Aluminum K X-raySource Functions,” Surface and Interface Analysis, 1993, pp.703715.(16) Mularie, M. C., and Peria, W.
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