ASTM E995-97

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 995 – 97
Standard Guide for
Background Subtraction Techniques in Auger Electron and
X-ray Photoelectron Spectroscopy
This standard is issued under the fixed designation E 995; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.1.2 XPS—The production of electrons from X-ray excita-
tion of surfaces may be grouped into two categories—
1.1 The purpose of this guide is to familiarize the analyst
photoemission of electrons and the production of Auger
with the principal background subtraction techniques presently
electrons from the decay of the resultant core hole states. The
in use together with the nature of their application to data
source of the background signal observed in the XPS spectrum
acquisition and manipulation.
includes a contribution from inelastic scattering processes, and
1.2 This guide is intended to apply to background subtrac-
for non-monochromatic X-ray sources, Bremsstrahlung radia-
tion in electron, X-ray, and ion-excited Auger electron spec-
tion.
troscopy (AES), and X-ray photoelectron spectroscopy (XPS).
4.2 Various background subtraction techniques have been
1.3 This standard does not purport to address all of the
employed to diminish or remove the influence of these back-
safety concerns, if any, associated with its use. It is the
ground electrons from the shape and intensity of Auger
responsibility of the user of this standard to establish appro-
electron and photoelectron features. Relevance to a particular
priate safety and health practices and determine the applica-
analytical technique (AES or XPS) will be indicated in the title
bility of regulatory limitations prior to use.
of the procedure.
2. Referenced Documents
4.3 Implementation of any of the various background tech-
niques that are described in this guide may depend on available
2.1 ASTM Standards:
instrumentation as well as the method of acquisition of the
E 673 Terminology Relating to Surface Analysis
original signal. These subtraction methods fall into two general
E 996 Practice for Reporting Data in Auger Electron Spec-
categories: (1) real-time background subtraction; and (2) post-
troscopy and X-ray Photoelectron Spectroscopy
acquisition background subtraction.
3. Terminology
5. Significance and Use
3.1 Definitions—For definitions of terms used in this guide,
5.1 Background subtraction techniques in AES were origi-
refer to Terminology E 673.
nally employed as a method of enhancement of weak Auger
4. Summary of Guide
signals to distinguish them from the slowly varying back-
ground of secondary and backscattered electrons. Interest in
4.1 Relevance to AES and XPS:
obtaining useful information from the Auger peak line shape,
4.1.1 AES—The production of Auger electron excitation by
concern for greater quantitative accuracy from Auger spectra,
bombardment of surfaces with electron beams is also accom-
and improvements in data gathering techniques, have led to the
panied by emission of secondary and backscattered electrons.
development of various background subtraction techniques.
These electrons range in energy from a maximum (near 10 eV
5.2 Similarly, the use of background subtraction techniques
for true secondaries), through the Auger spectrum, to a second
in XPS has evolved mainly from the interest in the determina-
maximum for backscattered electrons at the energy of the
tion of chemical states (binding energy values), greater quan-
incident electron beam. An additional source of background is
titative accuracy from the XPS spectra, and improvements in
associated with Auger electrons, which are inelastically scat-
data acquisition. Post-acquisition background subtraction is
tered while traveling through the specimen. Auger electron
normally applied to XPS data.
excitation may also occur by ion bombardment of surfaces.
5.3 The procedures outlined are popular in XPS and AES.
General reviews of background subtraction techniques have
This guide is under the jurisdiction of ASTM Committee E-42 on Surface 3
been published (1 and 2 ).
Analysis and is the direct responsibility of Subcommittee E42.03 on Auger Electron
Spectroscopy and XPS.
Current edition approved Feb. 10, 1997. Published April 1997. Originally
published as E 995 – 84. Last previous edition E 995 – 95. The boldface numbers in parentheses refer to the references at the end of this
Annual Book of ASTM Standards, Vol 03.06. standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 995
6. Apparatus commonly used is that of the cubic/quadratic differential as
proposed by Savitzky and Golay (14).
6.1 Most AES and XPS instruments either already use, or
7.5 X-ray Satellite Subtraction: (15) (XPS)—In this method
may be modified to use, one or more of the techniques that are
a fixed satellite structure associated with any given channel
described.
intensity such as a K X-ray line so that, starting at low kinetic
6.2 Background subtraction techniques may require a digital
energies, intensity is removed from higher kinetic energy
acquisition and digital data handling capability or the attach-
channels at the spacing of the Ka ,Kb, etc. satellite positions
3,4
ment of analog instrumentation to existing equipment.
from the Ka main peak to remove their contribution to the
1,2
spectrum. This subtraction proceeds through the spectrum and
7. Common Procedures
removes the satellite peaks associated with the photoelectron
7.1 Linear Background Subtraction (AES and XPS)—In this
peaks. It may also erroneously remove an equivalent intensity
method, two arbitrarily chosen points in the spectrum are
from any Auger peaks present in the spectrum.
selected and joined by a straight line (1). This straight line is
used to approximate the true background and is subtracted
8. Less Common Procedures
from the original spectrum. For Auger spectra, the two points
8.1 Deconvolution (AES and XPS) (16-19)—Deconvolution
may be chosen either on the high-energy side of the Auger peak
may be used to reduce the effects due to inelastic scattering of
to result in an extrapolated linear background or such that the
electrons traveling through the specimen. This background is
peak is positioned between the two points. For XPS spectra, the
removed by deconvoluting the spectrum with elastically back-
two points are generally chosen such that the peak is positioned
scattered electrons (set at the energy of the main peak) and its
between the two points. The intensity values at the chosen
associated loss spectrum. The intensity of the loss spectrum,
points may be the values at those energies or the average over
relative to that of the backscattered primary, is sometimes
a defined number of channels or energy interval.
adjusted to optimize the background subtraction. Deconvolu-
7.2 Integral Background Subtraction (AES and XPS)—This
tion is usually accomplished using Fourier transforms or
method, proposed by Shirley (3), employs a mathematical
iterative techniques.
algorithm to approximate the inelastic scattering of electrons as
8.2 Linearized Secondary Electron Cascades (AES)—In this
they escape from the solid. The algorithm is based on the
method, proposed by Sickafus (20 and 21) the logarithm of the
assumption that the background is proportional to the area of
electron energy distribution is plotted as a function of the
the peak above the background at higher kinetic energy. This
logarithm of the electron energy. Such plots consist of linear
basic method has been modified to optimize the required
segments corresponding to either surface or subsurface sources
iterations (4), to provide for a sloping inelastic background (5),
of Auger electrons and are appropriate for removing the
to provide for a background based upon the shape of the loss
background formed by the low energy cascade electrons.
spectrum from an elastically backscattered electron (6), and to
include a band gap for insulators (1).
9. Rarely Used Procedures
7.3 Inelastic Electron Scattering Correction (AES and
XPS)—This method, proposed by Tougaard (7), uses an 9.1 Secondary Electron Analog (AES) (22 and 23)—In this
method, a signal that is an electronic analog of the secondary
algorithm which is based on a description of the inelastic
scattering processes as the electrons leave the specimen. The electron cascade is combined with the analyzer signal output so
scattering cross section which enters in the algorithm is taken as to neutralize the secondary emission function. It is particu-
either from a simple universal formula which is approximately larly useful in retarding field systems in which low-energy
valid for some solids, or is determined from the energy secondary emission is prominent.
spectrum of a backscattered primary electron beam by another 9.2 Dynamic Background Subtraction (DBS) (AES) (24 and
algorithm (8). Alternatively, the parameters used in the univer-
25)—Dynamic background subtraction may be used either in
sal formula may also be permitted to vary in an algorithm so as real time or post acquisition. It involves multiple differentiation
to produce an estimate of the background (9). This background of an Auger spectrum to effect background removal, followed
subtraction method also gives direct information on the in- by an appropriate number of iterations to reestablish a
depth concentration profile (10 and 11). background-free Auger spectrum. The amount of background
7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES) removal depends on the number of derivatives taken, although
two are usually sufficient. In real-time analysis, a first deriva-
(12 and 13)—Signal differentiation is among the earliest
methods employed to remove the background from an Auger tive of the Auger electron energy distribution obtained using a
phase-sensitive detector is fed into an analog integrator,
spectrum and to enhance the Auger features. It may be
employed in real time or in post acquisition. In real time, thereby obtaining the Auger electron energy distribution with
the background removed.
differentiation is usu
...