1、Designation: E995 11E995 16Standard 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 t
2、he case 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 analyst with the principal background
3、 subtraction techniques presently in usetogether with the nature of their application to data acquisition and manipulation.1.2 This guide is intended to apply to background subtraction in electron, X-ray, and ion-excited Auger electron spectroscopy(AES), and X-ray photoelectron spectroscopy (XPS).1.
4、3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish app
5、ropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E673 Terminology Relating to Surface Analysis (Withdrawn 2012)32.2 ISO Standard:4ISO 181151 Surface chemical analysisVocabularyPart 1: General terms a
6、nd terms used in spectroscopy3. Terminology3.1 DefinitionsFor Since Terminology E673 was withdrawn in 2012, for definitions of terms used in this guide, refer toTerminologyISO E673. 18115-1.54. Summary of Guide4.1 Relevance to AES and XPS:4.1.1 AESThe production of Auger electrons by bombardment of
7、surfaces with electron beams is also accompanied byemission of secondary and backscattered electrons. These secondary and backscattered electrons create a background signal. Thisbackground signal covers the complete energy spectrum and has a maximum (near 10 eV for true secondaries), and a secondmax
8、imum for elastically backscattered electrons at the energy of the incident electron beam. An additional source of backgroundis associated withAuger electrons, which are inelastically scattered while traveling through the specimen.Auger electron excitationmay also occur by X-ray and ion bombardment o
9、f surfaces.4.1.2 XPSThe production of electrons from X-ray excitation of surfaces may be grouped into two categoriesphotoemissionof electrons and the production of Auger electrons from the decay of the resultant core hole states. The source of the backgroundsignal observed in the XPS spectrum includ
10、es a contribution from inelastic scattering processes, and for non-monochromatic X-raysources, electrons produced by Bremsstrahlung radiation.1 This guide is under the jurisdiction of ASTM Committee E42 on Surface Analysis and is the direct responsibility of Subcommittee E42.03 on Auger ElectronSpec
11、troscopy and X-Ray Photoelectron Spectroscopy.Current edition approved Oct. 15, 2011Nov. 1, 2016. Published October 2011December 2016. Originally approved in 1984. Last previous edition approved in 20042011as E995 04.E995-11. DOI: 10.1520/E0995-11.10.1520/E0995-16.2 For referencedASTM standards, vis
12、it theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 The last approved version of this historical standard is referenced on www.astm.org.4 Availabl
13、e from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http:/www.ansi.org.5 https:/www.iso.org/obp/ui/#iso:std:iso:18115:-1:ed-2:v1:en.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what
14、 changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the
15、official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.2 Various background subtraction techniques have been employed to diminish or remove the influence of these backgroundelectrons from the shape and intensity ofAuger e
16、lectron and photoelectron features. Relevance to a particular analytical technique(AES or XPS) will be indicated in the title of the procedure.4.3 Implementation of any of the various background subtraction techniques that are described in this guide may depend onavailable instrumentation and softwa
17、re as well as the method of acquisition of the original signal. These subtraction methods fallinto two general categories: (1) real-time background subtraction; and (2) post-acquisition background subtraction.5. Significance and Use5.1 Background subtraction techniques in AES were originally employe
18、d as a method of enhancement of the relatively weakAuger signals to distinguish them from the slowly varying background of secondary and backscattered electrons. Interest inobtaining useful information from the Auger peak line shape, concern for greater quantitative accuracy from Auger spectra, andi
19、mprovements in data gathering techniques, have led to the development of various background subtraction techniques.5.2 Similarly, the use of background subtraction techniques in XPS has evolved mainly from the interest in the determinationof chemical states (from the binding-energy values for compon
20、ent peaks that may often overlap), greater quantitative accuracyfrom the XPS spectra, and improvements in data acquisition. Post-acquisition background subtraction is normally applied to XPSdata.5.3 The procedures outlined in Section 7 are popular in XPS and AES; less popular procedures and rarely u
21、sed procedures aredescribed in Sections 88 and 9 and 9, respectively. General reviews of background subtraction methods and curve-fittingtechniques have been published elsewhere (1-5).65.4 Background subtraction is usually done before peak fitting. commonly performed prior to peak fitting, although
22、it can beassessed (fitted) during peak fitting (active approach (6, 7). Some commercial systems data analysis packages require backgroundremoval. removal before peak fitting. Nevertheless, a measured spectral region consisting of one or more peaks and backgroundintensities due to inelastic scatterin
23、g, Bremsstrahlung (for XPS with unmonochromated X-ray sources), and scattered primaryelectrons (forAES) can often be satisfactorily represented by choosing applying peak functions for each intensity component withparameters for each componentone determined in a single least-squares fit.The choice of
24、 the background to be removed, if requiredor desired, before or during peak fitting is suggested by the experience of the analysts analysts, the capabilities of the peak fittingsoftware, and the peak complexity as noted above.6. Apparatus6.1 Most AES and XPS instruments either already use, or may be
25、 modified to use, one or more of the techniques that aredescribed.6.2 Background subtraction techniques typically require a digital acquisition and digital data handling capability. In earlieryears, the attachment of analog instrumentation to existing equipment was usually required.7. Common Procedu
26、res7.1 The following background subtraction methods are widely employed. It is common for an analyst to choose one amongthem depending on the shape of the spectrum. As shown in a Round Robin study, different groups chose different backgroundmethods for analyzing the same spectrum (8).Although the pu
27、rpose of this guide is to describe the common procedures employedfor background subtraction, 7.3.2 provides a short guide of how to choose one or more background types depending on the shapeof the spectrum.7.2 Linear Background Subtraction (AES and XPS)Commonly Employed Background Types: In this met
28、hod, two arbitrarilychosen points in the spectrum are selected and joined by a straight line (1 and 2). This straight line is used to approximate the truebackground and is subtracted from the original spectrum. For Auger spectra, the two points may be chosen either on thehigh-energy side of the Auge
29、r peak to result in an extrapolated linear background or such that the peak is positioned between thetwo points. For XPS spectra, the two points are generally chosen such that the peak is positioned between the two points. Theintensity values at the chosen points may be the values at those energies
30、or the average over a defined number of data points orenergy interval.7.2.1 Linear Background (AES and XPS)In this method, two arbitrarily chosen points in the spectrum are selected and joinedby a straight line (1 and 2). This straight line is used to approximate the true background and is subtracte
31、d from the originalspectrum. For Auger spectra, the two points may be chosen either on the high-energy side of the Auger peak to result in anextrapolated linear background or such that the peak is positioned between the two points. For XPS spectra, the two points aregenerally chosen such that the pe
32、ak is positioned between the two points. The intensity values at the chosen points may be thevalues at those energies or the average over a defined number of data points or energy interval. The linear method can be extendedto a polynomial version when the peaks are small and riding on top of a more
33、complex (than linear) background (7).6 The boldface numbers in parentheses refer to the references at the end of this standard.E995 1627.2.2 Shirley (or Integral) Background (AES and XPS)This method, proposed by Shirley (9), employs a mathematicalalgorithm to approximate the step in the background c
34、ommonly found at the position of the peak. The algorithm is based on theassumption that the background is proportional to the area of the peak above the background at higher kinetic energy. This impliesan iterative procedure, which was described in detail by Proctor and Sherwood (10), that should be
35、 employed to guaranteeself-consistency (11). With another variant proposed by Vegh (12) and fully discussed by Salvi and Castle (13), it is possible toemploy a self-consistent Shirley-type background (SVSC-background) without the need of an iterative process; it is especiallypractical for complex sp
36、ectra (7).7.2.2.1 The original Shirley method was modified by Bishop to include a sloping component to reproduce the decay of thebackground intensity (14). Another modification provides for a background based upon the shape of the loss spectrum from anelastically backscattered electron (15), and to
37、include a band gap for insulators (1).7.2.3 2-Parameter and 3-Parameter Tougaard Backgrounds (XPS)This corresponds to a practical version of the approachdescribed in 8.1. Under this method, the K function, which enters in the algorithm, is taken from a simple universal formula whichis approximately
38、valid for some solids. Similar functions have been optimized for particular materials or material classes (16). Theapplication of this background might require the acquisition of background data in a 50 to 100 eV range below (in the lowerkinetic-energy side) the main peaks. Alternatively, the parame
39、ters used in the universal formula may also be permitted to vary inan optimizing algorithm so as to produce an estimate of the background (1 and 17). Tougaard has assessed the accuracy ofstructural parameters and the amount of substance derived from the analysis (18). A more approximate form of the
40、Tougaardalgorithm (19) can be used for automatic processing of XPS spectra (for example, spectra acquired for individual pixels of an XPSimage). A simpler form of the Tougaard background, the slope-background (20), can be employed for spectra with a reduced (5to 15 eV) background acquisition range b
41、elow the main peaks. It is designed to reproduce the onset of the background growth dueto extrinsic inelastic electron scattering, which correspond to the near-peak part of theTougaard background (it cannot be employedto reproduce the background signal farther than 15 eV from the main peaks).7.3 Int
42、egral (or Shirley) Implementation of the Various Background Subtraction (AES and XPS)Methods (XPS): This method,proposed by Shirley (6), employs a mathematical algorithm to approximate the inelastic scattering of electrons as they escape fromthe solid. The algorithm is based on the assumption that t
43、he background is proportional to the area of the peak above thebackground at higher kinetic energy. This basic method has been modified to optimize the required iterations (7), to provide fora sloping inelastic background (8), to provide for a background based upon the shape of the loss spectrum fro
44、m an elasticallybackscattered electron (9), and to include a band gap for insulators (1).7.3.1 Background End-Points (XPS)A key choice in implementation of the methods described in 7.2 is the selection of thetwo end points or spectral region for background subtraction. These points are selected far
45、enough from the peaks to assure thatthe intensity at those energies is only due to the background.7.3.1.1 However, in some cases, one peak might still contribute to the signal at the chosen points, so the total intensity is notpurely due to the background. This is common for spectra containing peaks
46、 with large kurtosis (large Lorentzian width) since thepeak contribution at energies as far as five times the Lorentzian width from the peak center is still 1 %. In these cases it is possibleto employ an active approach during peak fitting in which the intensity of the background is not tied to the
47、intensity of the signalat the chosen points but calculated during peak-fitting (6, 7). The advantages of an active approach are discussed in various reports(12, 13); an early example can be found in Figure A3.7 of Ref (21).7.3.2 Choosing the Background Type Based on the Shape of the Spectrum (XPS)Th
48、e linear background is recommended whenthe background at both sides of the peaks is a straight line, one side the continuation of the other. The polynomial background isrecommended for small peaks riding on top of the background of a larger peak or on wide Auger structures. A step-shapedincrement on
49、 the background intensity from the low to the high binding energy side of the main features could be treated with the(iterative) Shirley or with the SVSC method. Besides the plasmon features, the Tougaard-type backgrounds also reproduce anincrement on the slope of the background signal near the peak on the high binding energy side.7.3.2.1 The high binding-energy side of a photoelectron peak commonly shows both a step-shaped increment and an incrementon the slope of the background signal. In these and other cases, the total background might consist of the sum
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