1、Designation: E995 11Standard Guide forBackground Subtraction Techniques in Auger ElectronSpectroscopy and X-Ray Photoelectron Spectroscopy1This standard is issued under the fixed designation E995; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、 of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 subtrac
3、tion 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 The
4、values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This 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
5、 safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E673 Terminology Relating to Surface Analysis3. Terminology3.1 DefinitionsFor definitions of terms used in this guide,refer to Terminology E673.4. Summary o
6、f Guide4.1 Relevance to AES and XPS:4.1.1 AESThe production ofAuger 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 t
7、he complete energyspectrum and has a maximum (near 10 eV for true secondar-ies), and a second maximum for elastically backscatteredelectrons at the energy of the incident electron beam. Anadditional source of background is associated with Augerelectrons, which are inelastically scattered while trave
8、lingthrough the specimen. Auger electron excitation may alsooccur by X-ray and ion bombardment 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 r
9、esultant core hole states. 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
10、 to diminish or remove 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
11、sub-traction techniques that are described in this guide may dependon available instrumentation and software as well as themethod of acquisition of the original signal. These subtractionmethods fall into two general categories: (1) real-time back-ground subtraction; and (2) post-acquisition backgrou
12、nd sub-traction.5. Significance and 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 inf
13、ormation from the Auger peakline shape, 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 mai
14、nly from the interest in the determina-tion of chemical states (from the binding-energy values forcomponent peaks that may often overlap), greater quantitative1This guide is under the jurisdiction of ASTM Committee E42 on SurfaceAnalysis and is the direct responsibility of Subcommittee E42.03 on Aug
15、er ElectronSpectroscopy and X-Ray Photoelectron Spectroscopy.Current edition approved Oct. 15, 2011. Published October 2011. Originallyapproved in 1984. Last previous edition approved in 2004 as E995 04. DOI:10.1520/E0995-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcon
16、tact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume 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.accuracy from the XPS
17、 spectra, and improvements in dataacquisition. Post-acquisition background subtraction is nor-mally applied to XPS data.5.3 The procedures outlined in Section 7 are popular in XPSand AES; less popular procedures and rarely used proceduresare described in Sections 8 and 9, respectively. General revie
18、wsof background subtraction methods and curve-fitting tech-niques have been published (1-5).35.4 Background subtraction is usually done before peakfitting. Some commercial systems require background removal.Nevertheless, a measured spectral region consisting of one ormore peaks and background intens
19、ities due to inelastic scatter-ing, Bremsstrahlung (for XPS with unmonochromated X-raysources), and scattered primary electrons (for AES) can oftenbe satisfactorily represented by choosing functions for eachintensity component with parameters for each componentdetermined in a single least-squares fi
20、t. The choice of thebackground to be removed if required or desired before peakfitting is suggested by the experience of the analysts and thepeak complexity as noted above.6. Apparatus6.1 Most AES and XPS instruments either already use, ormay be modified to use, one or more of the techniques that ar
21、edescribed.6.2 Background subtraction techniques typically 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 this
22、method, two arbitrarily chosen points in the spectrum areselected and joined by a straight line (1 and 2). This straightline is used to approximate the true background and issubtracted from the original spectrum. For Auger spectra, thetwo points may be chosen either on the high-energy side of theAug
23、er peak to result in an extrapolated linear background orsuch that the peak is positioned between the two points. ForXPS spectra, the two points are generally chosen such that thepeak is positioned between the two points. The intensity valuesat the chosen points may be the values at those energies o
24、r theaverage over a defined number of data points or energyinterval.7.2 Integral (or Shirley) Background Subtraction (AES andXPS)This method, proposed by Shirley (6), employs amathematical algorithm to approximate the inelastic scatteringof electrons as they escape from the solid. The algorithm isba
25、sed on the assumption that the background is proportional tothe area of the peak above the background at higher kineticenergy. This basic method has been modified to optimize therequired iterations (7), to provide for a sloping inelasticbackground (8), to provide for a background based upon theshape
26、 of the loss spectrum from an elastically backscatteredelectron (9), and to include a band gap for insulators (1).7.3 Inelastic Electron Scattering Correction (AES andXPS)This method, proposed by Tougaard (10), uses analgorithm which is based on a description of the inelasticscattering processes as
27、the electrons travel within the specimenbefore leaving it. The scattering cross section which enters inthe algorithm is taken either from a simple universal formulawhich is approximately valid for some solids, similar functionsthat have been optimized for particular materials or materialclasses (11)
28、, or is determined from the energy spectrum of abackscattered primary electron beam by another algorithm (1and 12). Alternatively, the parameters used in the universalformula may also be permitted to vary in an algorithm so as toproduce an estimate of the background (1 and 13). Thisbackground subtra
29、ction method also gives direct informationon the in-depth concentration profile (14 and 15). Tougaard hasassessed the accuracy of structural parameters and the amountof substance derived from the analysis (16).Asimpler but moreapproximate form of the Tougaard algorithm (17). can be usedfor automatic
30、 processing of XPS spectra (for example, spectraacquired for individual pixels of an XPS image).7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES)(18 and 19)Signal differentiation is among the earliestmethods employed to remove the background from an Augerspectrum and to enhance the Auger featu
31、res. 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 theanalyzer used to obtain the Auger spectrum. The output signalis then processed by a lock-in amplifier and d
32、isplayed 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 differentiated bydigital or other methods. The digital method commonly used isthat of the cubic/quadratic derivative a
33、s proposed by Savitzkyand Golay (20).7.5 X-Ray Satellite Subtraction (for Non-MonochromatedX-Ray Sources) (XPS) (21) In this method, photoelectronintensity from the satellite X-rays associated with the K X-rayspectrum from an aluminum or magnesium X-ray source issubtracted. Intensity is removed from
34、 higher kinetic energychannels at the spacing of the Ka3,4,Kb, etc. satellite positionsfrom the Ka1,2main peak and with the corresponding intensityratios (21) to remove their contributions to the XPS spectrum.This subtraction can proceed through the spectrum but not ifthere is anAuger peak in the re
35、gion of interest because it woulderroneously remove an equivalent intensity from any Augerpeaks present in the spectrum.7.6 Implementation of the Linear, Integral, and TougaardBackground Subtraction Methods (XPS)A key choice inimplementation of the linear (7.1), integral (7.2), and Tougaard(7.3) bac
36、kground subtraction methods is the selection of thetwo end points or spectral region for background subtraction.For consistent determination of area, the region over whichbackground subtraction needs to be applied will vary with thepeak width/structure and the background subtraction applied.The cons
37、istent application of a background subtraction processcan produce precise determination of peak areas. In manycircumstances, electrons appropriately associated with the3The boldface numbers in parentheses refer to the references at the end of thisstandard.E995 112photoelectron peaks can occur outsid
38、e of the integration limits;therefore the accuracy of any resulting quantification willdepend on the method by which the sensitivity factors weredetermined. Analytical errors can also occur if there arechanges in AES or XPS lineshapes or shakeup fractions withchanges of chemical state. Uncertainties
39、 in X-ray photoelec-tron spectroscopy intensities associated with different methodsand procedures for background subtraction have been evalu-ated for both monochromatic aluminum X-rays (22) and forunmonochromated aluminum and magnesium X-rays (23).8. Less Common Procedures8.1 Deconvolution (AES and
40、XPS) (24-27)Deconvolu-tion may be used to reduce the effects due to inelastic scatteringof electrons traveling through the specimen. This backgroundis removed by deconvoluting the spectrum with elasticallybackscattered electrons (set at the energy of the main peak) andits associated loss spectrum. T
41、he intensity of the loss spectrum,relative to that of the backscattered primary, is sometimesadjusted to optimize the background subtraction. Deconvolu-tion is usually accomplished using Fourier transforms oriterative techniques.8.2 Linearized Secondary Electron Cascades (AES)In thismethod, proposed
42、 by Sickafus (28 and 29) the logarithm of theelectron energy distribution is plotted as a function of thelogarithm of the electron energy. Such plots consist of linearsegments corresponding to either surface or subsurface sourcesof Auger electrons and are appropriate for removing thebackground forme
43、d by the low energy cascade electrons.9. Rarely Used Procedures9.1 Secondary Electron Analog (AES) (30 and 31)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 pa
44、rticu-larly useful for retarding field analyzers in which low-energysecondary emission is prominent.9.2 Dynamic Background Subtraction (DBS) (AES) (32and 33)Dynamic background subtraction may be used eitherin real time or post acquisition. It involves multiple differen-tiation of an Auger spectrum t
45、o effect background removal,followed by an appropriate number of integrations to reestab-lish a background-free Auger spectrum. The amount of back-ground removal depends on the number of derivatives taken,although two are usually sufficient. In real-time analysis, a firstderivative of the Auger elec
46、tron energy distribution obtainedusing a phase-sensitive detector is fed into an analog integrator,thereby obtaining the Auger electron energy distribution withthe background removed.9.3 Tailored Modulation Techniques (TMT) (AES) (34 and35)This is a real-time method of background subtraction thatuse
47、s special modulation 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) (36)In
48、this method,a structureless background is calculated from a measuredspectrum using a smoothing spline algorithm. This backgroundis then subtracted from the original spectrum.9.5 Digital Filtration (AES) (37 and 38)In a methodborrowed from energy-dispersive X-ray spectroscopy, a “top-hat” digital fre
49、quency filter is applied 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. COMPARISONS AVAILABLE IN THE LITERATUREX1.1 At the present time, the most popular backgroundsubtraction method for AES is digital differentiation (see 7.4).Common methods for XPS include the straight line (see 7.1),Shirley-type (see 7.2), or variations of t
