Designation: E995 − 11

Standard Guide for

Background Subtraction Techniques in Auger Electron
Spectroscopy and X-Ray Photoelectron Spectroscopy1
This standard is issued under the fixed designation E995; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope emission of secondary and backscattered electrons. These

1.1 The purpose of this guide is to familiarize the analyst secondary and backscattered electrons create a background
with the principal background subtraction techniques presently signal. This background signal covers the complete energy
in use together with the nature of their application to data spectrum and has a maximum (near 10 eV for true
acquisition and manipulation. secondaries), and a second maximum for elastically backscat-
tered electrons at the energy of the incident electron beam. An
1.2 This guide is intended to apply to background subtrac- additional source of background is associated with Auger
tion in electron, X-ray, and ion-excited Auger electron spec- electrons, which are inelastically scattered while traveling
troscopy (AES), and X-ray photoelectron spectroscopy (XPS). through the specimen. Auger electron excitation may also
1.3 The values stated in SI units are to be regarded as occur by X-ray and ion bombardment of surfaces.
standard. No other units of measurement are included in this 4.1.2 XPS—The production of electrons from X-ray excita-
standard. tion of surfaces may be grouped into two categories—
1.4 This standard does not purport to address all of the photoemission of electrons and the production of Auger
safety concerns, if any, associated with its use. It is the electrons from the decay of the resultant core hole states. The
responsibility of the user of this standard to establish appro- source of the background signal observed in the XPS spectrum
priate safety and health practices and determine the applica- includes a contribution from inelastic scattering processes, and
bility of regulatory limitations prior to use. for non-monochromatic X-ray sources, electrons produced by
Bremsstrahlung radiation.
2. Referenced Documents 4.2 Various background subtraction techniques have been
2.1 ASTM Standards:2 employed to diminish or remove the influence of these back-
E673 Terminology Relating to Surface Analysis (Withdrawn ground electrons from the shape and intensity of Auger
2012)3 electron and photoelectron features. Relevance to a particular
analytical technique (AES or XPS) will be indicated in the title
3. Terminology of the procedure.
3.1 Definitions—For definitions of terms used in this guide, 4.3 Implementation of any of the various background sub-
refer to Terminology E673. traction techniques that are described in this guide may depend
on available instrumentation and software as well as the
4. Summary of Guide method of acquisition of the original signal. These subtraction
4.1 Relevance to AES and XPS: methods fall into two general categories: (1) real-time back-
4.1.1 AES—The production of Auger electrons by bombard- ground subtraction; and (2) post-acquisition background sub-
ment of surfaces with electron beams is also accompanied by traction.

5. Significance and Use

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 Electron 5.1 Background subtraction techniques in AES were origi-
Spectroscopy and X-Ray Photoelectron Spectroscopy. nally employed as a method of enhancement of the relatively
Current edition approved Oct. 15, 2011. Published October 2011. Originally weak Auger signals to distinguish them from the slowly
approved in 1984. Last previous edition approved in 2004 as E995 – 04. DOI:
10.1520/E0995-11. varying background of secondary and backscattered electrons.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Interest in obtaining useful information from the Auger peak
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM line shape, concern for greater quantitative accuracy from
Standards volume information, refer to the standard’s Document Summary page on
Auger spectra, and improvements in data gathering techniques,
the ASTM website.
3
The last approved version of this historical standard is referenced on have led to the development of various background subtraction
www.astm.org. techniques.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

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 E995 − 11
5.2 Similarly, the use of background subtraction techniques energy. This basic method has been modified to optimize the
in XPS has evolved mainly from the interest in the determina- required iterations (7), to provide for a sloping inelastic
tion of chemical states (from the binding-energy values for background (8), to provide for a background based upon the
component peaks that may often overlap), greater quantitative shape of the loss spectrum from an elastically backscattered
accuracy from the XPS spectra, and improvements in data electron (9), and to include a band gap for insulators (1).
acquisition. Post-acquisition background subtraction is nor-
7.3 Inelastic Electron Scattering Correction (AES and
mally applied to XPS data.
XPS)—This method, proposed by Tougaard (10), uses an
5.3 The procedures outlined in Section 7 are popular in XPS algorithm which is based on a description of the inelastic
and AES; less popular procedures and rarely used procedures scattering processes as the electrons travel within the specimen
are described in Sections 8 and 9, respectively. General reviews before leaving it. The scattering cross section which enters in
of background subtraction methods and curve-fitting tech- the algorithm is taken either from a simple universal formula
niques have been published (1-5).4 which is approximately valid for some solids, similar functions
5.4 Background subtraction is usually done before peak that have been optimized for particular materials or material
fitting. Some commercial systems require background removal. classes (11), or is determined from the energy spectrum of a
Nevertheless, a measured spectral region consisting of one or backscattered primary electron beam by another algorithm (1
more peaks and background intensities due to inelastic and 12). Alternatively, the parameters used in the universal
scattering, Bremsstrahlung (for XPS with unmonochromated formula may also be permitted to vary in an algorithm so as to
X-ray sources), and scattered primary electrons (for AES) can produce an estimate of the background (1 and 13). This
often be satisfactorily represented by choosing functions for background subtraction method also gives direct information
each intensity component with parameters for each component on the in-depth concentration profile (14 and 15). Tougaard has
determined in a single least-squares fit. The choice of the assessed the accuracy of structural parameters and the amount
background to be removed if required or desired before peak of substance derived from the analysis (16). A simpler but more
fitting is suggested by the experience of the analysts and the approximate form of the Tougaard algorithm (17). can be used
peak complexity as noted above. for automatic processing of XPS spectra (for example, spectra
acquired for individual pixels of an XPS image).
6. Apparatus
7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES)
6.1 Most AES and XPS instruments either already use, or (18 and 19)—Signal differentiation is among the earliest
may be modified to use, one or more of the techniques that are methods employed to remove the background from an Auger
described. spectrum and to enhance the Auger features. It may be
6.2 Background subtraction techniques typically require a employed in real time or in post acquisition. In real time,
digital acquisition and digital data handling capability. In differentiation is usually accomplished by superposition of a
earlier years, the attachment of analog instrumentation to small (1 to 6 eV peak-to-peak) sinusoidal modulation on the
existing equipment was usually required. analyzer used to obtain the Auger spectrum. The output signal
is then processed by a lock-in amplifier and displayed as the
7. Common Procedures derivative of the original energy distribution N(E) or EN(E). In
7.1 Linear Background Subtraction (AES and XPS)—In this post-acquisition background subtraction, the already acquired
method, two arbitrarily chosen points in the spectrum are N(E) or EN(E) signal may be mathematically differentiated by
selected and joined by a straight line (1 and 2). This straight digital or other methods. The digital method commonly used is
line is used to approximate the true background and is that of the cubic/quadratic derivative as proposed by Savitzky
subtracted from the original spectrum. For Auger spectra, the and Golay (20).
two points may be chosen either on the high-energy side of the 7.5 X-Ray Satellite Subtraction (for Non-Monochromated
Auger peak to result in an extrapolated linear background or X-Ray Sources) (XPS) (21) —In this method, photoelectron
such that the peak is positioned between the two points. For intensity from the satellite X-rays associated with the K X-ray
XPS spectra, the two points are generally chosen such that the spectrum from an aluminum or magnesium X-ray source is
peak is positioned between the two points. The intensity values subtracted. Intensity is removed from higher kinetic energy
at the chosen points may be the values at those energies or the channels at the spacing of the Ka3,4, Kb, etc. satellite positions
average over a defined number of data points or energy from the Ka1,2 main peak and with the corresponding intensity
interval. ratios (21) to remove their contributions to the XPS spectrum.
7.2 Integral (or Shirley) Background Subtraction (AES and This subtraction can proceed through the spectrum but not if
XPS)—This method, proposed by Shirley (6), employs a there is an Auger peak in the region of interest because it would
mathematical algorithm to approximate the inelastic scattering erroneously remove an equivalent intensity from any Auger
of electrons as they escape from the solid. The algorithm is peaks present in the spectrum.
based on the assumption that the background is proportional to 7.6 Implementation of the Linear, Integral, and Tougaard
the area of the peak above the background at higher kinetic Background Subtraction Methods (XPS)—A key choice in
implementation of the linear (7.1), integral (7.2), and Tougaard
4
The boldface numbers in parentheses refer to the references at the end of this (7.3) background subtraction methods is the selection of the
standard. two end points or spectral region for background subtraction.

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 E995 − 11
For consistent determination of area, the region over which electron cascade is combined with the analyzer signal output so
background subtraction needs to be applied will vary with the as to counteract the secondary emission function. It is particu-
peak width/structure and the background subtraction applied. larly useful for retarding field analyzers in which low-energy
The consistent application of a background subtraction process secondary emission is prominent.
can produce precise determination of peak areas. In many
9.2 Dynamic Background Subtraction (DBS) (AES) (32 and
circumstances, electrons appropriately associated with the
photoelectron peaks can occur outside of the integration limits; 33)—Dynamic background subtraction may be used either in
therefore the accuracy of any resulting quantification will real time or post acquisition. It involves multiple differentiation
depend on the method by which the sensitivity factors were of an Auger spectrum to effect background removal, followed
determined. Analytical errors can also occur if there are by an appropriate number of integrations to reestablish a
changes in AES or XPS lineshapes or shakeup fractions with background-free Auger spectrum. The amount of background
changes of chemical state. Uncertainties in X-ray photoelec- removal depends on the number of derivatives taken, although
tron spectroscopy intensities associated with different methods two are usually sufficient. In real-time analysis, a first deriva-
and procedures for background subtraction have been evalu- tive of the Auger electron energy distribution obtained using a
ated for both monochromatic aluminum X-rays (22) and for phase-sensitive detector is fed into an analog integrator,
unmonochromated aluminum and magnesium X-rays (23). thereby obtaining the Auger electron energy distribution with
the background removed.
8. Less Common Procedures
9.3 Tailored Modulation Techniques (TMT) (AES) (34 and
8.1 Deconvolution (AES and XPS) (24-27)—Deconvolution
35)—This is a real-time method of background subtraction that
may be used to reduce the effects due to inelastic scattering of
uses special modulation waveforms tailored to the analyzer and
electrons traveling through the specimen. This background is
removed by deconvoluting the spectrum with elastically back- phase sensitive detection to measure the Auger signal. The
scattered electrons (set at the energy of the main peak) and its N(E) distribution, EN(E) distribution, or areas under Auger
associated loss spectrum. The intensity of the loss spectrum, peaks over specified energy ranges may be obtained directly
relative to that of the backscattered primary, is sometimes using these techniques.
adjusted to optimize the background subtraction. Deconvolu- 9.4 Spline Technique (AES and XPS) (36)—In this method,
tion is usually accomplished using Fourier transforms or a structureless background is calculated from a measured
iterative techniques. spectrum using a smoothing spline algorithm. This background
8.2 Linearized Secondary Electron Cascades (AES)—In this is then subtracted from the original spectrum.
method, proposed by Sickafus (28 and 29) the logarithm of the 9.5 Digital Filtration (AES) (37 and 38)—In a method
electron energy distribution is plotted as a function of the borrowed from energy-dispersive X-ray spectroscopy, a “top-
logarithm of the electron energy. Such plots consist of linear hat” digital frequency filter is applied to an Auger spectrum to
segments corresponding to either surface or subsurface sources suppress the slowly varying background continuum, while the
of Auger electrons and are appropriate for removing the
more rapidly varying Auger peaks remain unaffected.
background formed by the low energy cascade electrons.
9. Rarely Used Procedures 10. Keywords
9.1 Secondary Electron Analog (AES) (30 and 31)—In this 10.1 Auger electron spectroscopy; background subtraction;
method, a signal that is an electronic analog of the secondary surface analysis; X-ray photoelectron spectroscopy

APPENDIX

(Nonmandatory Information)

X1. COMPARISONS AVAILABLE IN THE LITERATURE

X1.1 At the present time, the most popular background mentioned here have been published in the literature. In the
subtraction method for AES is digital differentiation (see 7.4). case of 7.1 and 7.2, the effect on the peak area calculated in
Common methods for XPS include the straight line (see 7.1), terms of the choice of end points is examined (7 and 8, 22 and
Shirley-type (see 7.2), or variations of the Tougaard method 23). Further comparisons of these procedures and that in 7.2 on
(see 7.3). Comparisons of background subtraction methods a number of materials are also offered (22 and 23, 39-49).

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 E995 − 11
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