ASTM E995-16

Standard Guide for Background Subtraction Techniques in Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy

SIGNIFICANCE AND USE
5.1 Background subtraction techniques in AES were originally employed as a method of enhancement of the relatively weak Auger signals to distinguish them from the slowly varying background of secondary and backscattered electrons. Interest in obtaining useful information from the Auger peak line shape, concern for greater quantitative accuracy from Auger spectra, and improvements 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 determination of chemical states (from the binding-energy values for component peaks that may often overlap), greater quantitative accuracy from the XPS spectra, and improvements in data acquisition. Post-acquisition background subtraction is normally applied to XPS data.  
5.3 The procedures outlined in Section 7 are popular in XPS and AES; less popular procedures and rarely used procedures are described in Sections 8 and 9, respectively. General reviews of background subtraction methods and curve-fitting techniques have been published elsewhere (1-5).6  
5.4 Background subtraction is commonly performed prior to peak fitting, although it can be assessed (fitted) during peak fitting (active approach  (6, 7)). Some commercial data analysis packages require background removal before peak fitting. Nevertheless, a measured spectral region consisting of one or more peaks and background intensities due to inelastic scattering, Bremsstrahlung (for XPS with unmonochromated X-ray sources), and scattered primary electrons (for AES) can often be satisfactorily represented by applying peak functions for each component with parameters for each one determined in a single least-squares fit. The choice of the background to be removed, if required or desired, before or during peak fitting is suggested by the experience of the analysts, the capabilities of the peak fitting software, and the p...
SCOPE
1.1 The purpose of this guide is to familiarize the analyst with the principal background subtraction techniques presently in use together 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).  
1.3 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 responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Published
• Publication Date: 31-Oct-2016
• ICS: 17.180.30 - Optical measuring instruments
• Technical Committee: E42 - Surface Analysis
• Drafting Committee: E42.03 - Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy

Relations

• Replaces: ASTM E995-11 - Standard Guide for Background Subtraction Techniques in Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy
• Effective Date: 01-Nov-2016
• Refers: ASTM E673-03 - Standard Terminology Relating to Surface Analysis (Withdrawn 2012)
• Effective Date: 01-Dec-2003
• Refers: ASTM E673-02a - Standard Terminology Relating to Surface Analysis
• Effective Date: 10-Dec-2002
• Refers: ASTM E673-01 - Standard Terminology Relating to Surface Analysis
• Effective Date: 10-Nov-2001
• Refers: ASTM E673-98E1 - Standard Terminology Relating to Surface Analysis
• Effective Date: 10-Nov-2001
• Referred By: ASTM E996-19 - Standard Practice for Reporting Data in Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy
• Effective Date: 01-Nov-2016
• Referred By: ASTM E2735-14(2020) - Standard Guide for Selection of Calibrations Needed for X-ray Photoelectron Spectroscopy (XPS) Experiments
• Effective Date: 01-Nov-2016

Overview

ASTM E995-16 is the internationally recognized standard titled "Standard Guide for Background Subtraction Techniques in Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy." Issued by ASTM International, this guide serves as a comprehensive resource for analysts engaged in surface analysis using Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS). The main aim of this standard is to familiarize users with principal background subtraction techniques, ensuring greater quantitative accuracy and improved data interpretation in surface chemical analysis.

Background subtraction is a crucial step in AES and XPS, as it enables the enhancement and interpretation of weak signals amidst complex backgrounds generated by secondary and backscattered electrons (in AES) or inelastic scattering (in XPS). The standard details both commonly used and less popular background subtraction methods suitable for various instruments and data analysis software.

Key Topics

• Importance of Background Subtraction:
  Enhances weak spectroscopic signals, improves quantification, and allows more accurate interpretation of line shapes and chemical states.

• Techniques in AES and XPS:
  The standard covers an array of background subtraction approaches used in both electron and X-ray-induced surface analysis. This includes:

  • Linear Background Subtraction: Connecting end points in a spectrum with a straight line to approximate and subtract the background.
  • Shirley (Integral) Background: Uses an iterative algorithm assuming background is proportional to peak area above background at higher energies. Well suited for step-shaped backgrounds.
  • Tougaard Background: Models background from inelastic electron scattering, applicable to complex spectra, and useful for quantitative depth profiling.
  • Signal Differentiation: Enhances Auger features by differentiating the spectrum, either in real-time or during post-acquisition.
  • Other Methods: Includes deconvolution, dynamic background subtraction, spline fitting, and digital filtration, tailored for specific analytical challenges.

• Implementation Factors:
  Selection of technique depends on analyst experience, software capabilities, and sample characteristics. Consistent application and careful choice of endpoints or regions for background subtraction are essential for reproducible results.

Applications

ASTM E995-16 background subtraction methods are widely applied in scientific and industrial laboratories for:

• Surface Composition Analysis:
  Accurately distinguishing elemental and chemical state information from complex backgrounds in AES and XPS spectra.

• Materials Science and Nanotechnology:
  Facilitating the quantitative study of thin films, coatings, interfaces, and nanostructures by minimizing artifacts in spectral interpretation.

• Quality Control and Research:
  Supporting data-driven decisions in quality assurance, failure analysis, and R&D in fields ranging from semiconductor fabrication to corrosion studies.

• Software Integration:
  Many commercial and custom data analysis packages incorporate these background subtraction methods, underlining their practical relevance.

Related Standards

• ASTM E673: Terminology Relating to Surface Analysis (historical reference)
• ISO 18115-1: Surface chemical analysis - Vocabulary - Part 1: General terms and terms used in spectroscopy
• Other ASTM and ISO standards concerning surface analysis, spectral data processing, and quantification methods

Keywords: Auger electron spectroscopy, X-ray photoelectron spectroscopy, background subtraction, surface analysis, AES, XPS, spectral data analysis, ASTM E995-16

By following ASTM E995-16, laboratories can ensure reliable, standardized background removal and enable high-confidence surface analytical results in both research and industry.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E995 − 16
Standard Guide for
Background Subtraction Techniques in Auger Electron
Spectroscopy and X-Ray Photoelectron Spectroscopy
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 3. Terminology
1.1 The purpose of this guide is to familiarize the analyst 3.1 Definitions—Since Terminology E673 was withdrawn
with the principal background subtraction techniques presently in 2012, for definitions of terms used in this guide, refer to ISO
in use together with the nature of their application to data 18115-1.
acquisition and manipulation.
4. Summary of Guide
1.2 This guide is intended to apply to background subtrac-
4.1 Relevance to AES and XPS:
tion in electron, X-ray, and ion-excited Auger electron spec-
4.1.1 AES—The production ofAuger electrons by bombard-
troscopy (AES), and X-ray photoelectron spectroscopy (XPS).
ment of surfaces with electron beams is also accompanied by
1.3 The values stated in SI units are to be regarded as
emission of secondary and backscattered electrons. These
standard. No other units of measurement are included in this
secondary and backscattered electrons create a background
standard.
signal. This background signal covers the complete energy
spectrum and has a maximum (near 10 eV for true
1.4 This standard does not purport to address all of the
secondaries), and a second maximum for elastically backscat-
safety concerns, if any, associated with its use. It is the
tered electrons at the energy of the incident electron beam.An
responsibility of the user of this standard to establish appro-
additional source of background is associated with Auger
priate safety and health practices and determine the applica-
electrons, which are inelastically scattered while traveling
bility of regulatory limitations prior to use.
through the specimen. Auger electron excitation may also
2. Referenced Documents occur by X-ray and ion bombardment of surfaces.
2 4.1.2 XPS—The production of electrons from X-ray excita-
2.1 ASTM Standards:
tion of surfaces may be grouped into two categories—
E673 Terminology Relating to SurfaceAnalysis (Withdrawn
3 photoemission of electrons and the production of Auger
2012)
electrons from the decay of the resultant core hole states. The
2.2 ISO Standard:
source of the background signal observed in the XPS spectrum
ISO 18115–1 Surface chemical analysis—Vocabulary—Part
includes a contribution from inelastic scattering processes, and
1: General terms and terms used in spectroscopy
for non-monochromatic X-ray sources, electrons produced by
Bremsstrahlung radiation.
4.2 Various background subtraction techniques have been
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
employed to diminish or remove the influence of these back-
Spectroscopy and X-Ray Photoelectron Spectroscopy.
ground electrons from the shape and intensity of Auger
Current edition approved Nov. 1, 2016. Published December 2016. Originally
electron and photoelectron features. Relevance to a particular
approved in 1984. Last previous edition approved in 2011 as E995-11. DOI:
10.1520/E0995-16. analytical technique (AES or XPS) will be indicated in the title
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
of the procedure.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
4.3 Implementation of any of the various background sub-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
traction techniques that are described in this guide may depend
The last approved version of this historical standard is referenced on
on available instrumentation and software as well as the
www.astm.org.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. https://www.iso.org/obp/ui/#iso:std:iso:18115:-1:ed-2:v1:en.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E995 − 16
method of acquisition of the original signal. These subtraction among them depending on the shape of the spectrum. As
methods fall into two general categories: (1) real-time back- showninaRoundRobinstudy,differentgroupschosedifferent
ground subtraction; and (2) post-acquisition background sub- background methods for analyzing the same spectrum (8).
traction. Although the purpose of this guide is to describe the common
procedures employed for background subtraction, 7.3.2 pro-
5. Significance and Use
vides a short guide of how to choose one or more background
5.1 Background subtraction techniques in AES were origi-
types depending on the shape of the spectrum.
nally employed as a method of enhancement of the relatively
7.2 Commonly Employed Background Types:
weak Auger signals to distinguish them from the slowly
7.2.1 Linear Background (AES and XPS)—In this method,
varying background of secondary and backscattered electrons.
two arbitrarily chosen points in the spectrum are selected and
Interest in obtaining useful information from the Auger peak
joined by a straight line (1 and 2). This straight line is used to
line shape, concern for greater quantitative accuracy from
approximate the true background and is subtracted from the
Auger spectra, and improvements in data gathering techniques,
original spectrum. For Auger spectra, the two points may be
have led to the development of various background subtraction
chosen either on the high-energy side of the Auger peak to
techniques.
result in an extrapolated linear background or such that the
5.2 Similarly, the use of background subtraction techniques
peakispositionedbetweenthetwopoints.ForXPSspectra,the
in XPS has evolved mainly from the interest in the determina-
twopointsaregenerallychosensuchthatthepeakispositioned
tion of chemical states (from the binding-energy values for
between the two points. The intensity values at the chosen
component peaks that may often overlap), greater quantitative
points may be the values at those energies or the average over
accuracy from the XPS spectra, and improvements in data
a defined number of data points or energy interval. The linear
acquisition. Post-acquisition background subtraction is nor-
method can be extended to a polynomial version when the
mally applied to XPS data.
peaks are small and riding on top of a more complex (than
5.3 TheproceduresoutlinedinSection7arepopularinXPS linear) background (7).
and AES; less popular procedures and rarely used procedures
7.2.2 Shirley (or Integral) Background (AES and XPS)—
aredescribedinSections8and9,respectively.Generalreviews
This method, proposed by Shirley (9), employs a mathematical
of background subtraction methods and curve-fitting tech- algorithm to approximate the step in the background com-
niques have been published elsewhere (1-5).
monlyfoundatthepositionofthepeak.Thealgorithmisbased
on the assumption that the background is proportional to the
5.4 Backgroundsubtractioniscommonlyperformedpriorto
areaofthepeakabovethebackgroundathigherkineticenergy.
peak fitting, although it can be assessed (fitted) during peak
This implies an iterative procedure, which was described in
fitting(active approach (6, 7)).Somecommercialdataanalysis
detail by Proctor and Sherwood (10), that should be employed
packages require background removal before peak fitting.
to guarantee self-consistency (11). With another variant pro-
Nevertheless, a measured spectral region consisting of one or
posed by Vegh (12) and fully discussed by Salvi and Castle
more peaks and background intensities due to inelastic
(13), it is possible to employ a self-consistent Shirley-type
scattering, Bremsstrahlung (for XPS with unmonochromated
background (SVSC-background) without the need of an itera-
X-ray sources), and scattered primary electrons (for AES) can
tive process; it is especially practical for complex spectra (7).
often be satisfactorily represented by applying peak functions
7.2.2.1 The original Shirley method was modified by
for each component with parameters for each one determined
Bishop to include a sloping component to reproduce the decay
in a single least-squares fit.The choice of the background to be
of the background intensity (14). Another modification pro-
removed, if required or desired, before or during peak fitting is
vides for a background based upon the shape of the loss
suggested by the experience of the analysts, the capabilities of
spectrumfromanelasticallybackscatteredelectron (15),andto
the peak fitting software, and the peak complexity as noted
include a band gap for insulators (1).
above.
7.2.3 2-Parameter and 3-Parameter Tougaard Backgrounds
6. Apparatus
(XPS)—This corresponds to a practical version of the approach
6.1 Most AES and XPS instruments either already use, or described in 8.1. Under this method, the λ K function, which
may be modified to use, one or more of the techniques that are enters in the algorithm, is taken from a simple universal
described. formula which is approximately valid for some solids. Similar
functions have been optimized for particular materials or
6.2 Background subtraction techniques typically require a
materialclasses (16).Theapplicationofthisbackgroundmight
digital acquisition and digital data handling capability. In
require the acquisition of background data in a 50 to 100 eV
earlier years, the attachment of analog instrumentation to
range below (in the lower kinetic-energy side) the main peaks.
existing equipment was usually required.
Alternatively, the parameters used in the universal formula
7. Common Procedures
may also be permitted to vary in an optimizing algorithm so as
toproduceanestimateofthebackground (1 and 17).Tougaard
7.1 The following background subtraction methods are
has assessed the accuracy of structural parameters and the
widely employed. It is common for an analyst to choose one
amount of substance derived from the analysis (18). A more
approximate form of the Tougaard algorithm (19) can be used
The boldface numbers in parentheses refer to the references at the end of this
standard. for automatic processing of XPS spectra (for example, spectra
E995 − 16
acquired for individual pixels of an XPS image). A simpler derivative of the original energy distribution N(E) or EN(E).In
form of the Tougaard background, the slope-background (20), post-acquisition background subtraction, the already acquired
can be employed for spectra with a reduced (5 to 15 eV) N(E) or EN(E) signal may be mathematically differentiated by
background acquisition range below the main peaks. It is digital or other methods.The digital method commonly used is
designed to reproduce the onset of the background growth due that of the cubic/quadratic derivative as proposed by Savitzky
to extrinsic inelastic electron scattering, which correspond to and Golay (24).
the near-peak part of the Tougaard background (it cannot be
7.5 X-Ray Satellite Subtraction (for Non-Monochromated
employed to reproduce the background signal farther than
X-Ray Sources) (XPS) (25)—In this method, photoelectron
~ 15 eV from the main peaks).
intensity from the satellite X-rays associated with the K X-ray
spectrum from an aluminum or magnesium X-ray source is
7.3 Implementation of the Various Background Subtraction
Methods (XPS): subtracted. Intensity is removed from higher kinetic energy
channels at the spacing of the Kα ,Kβ, etc. satellite positions
7.3.1 Background End-Points (XPS)—A key choice in
3,4
from the Kα main peak and with the corresponding intensity
implementationofthemethodsdescribedin7.2istheselection
1,2
ratios (25) to remove their contributions to the XPS spectrum.
of the two end points or spectral region for background
subtraction. These points are selected far enough from the This subtraction can proceed through the spectrum but not if
thereisanAugerpeakintheregionofinterestbecauseitwould
peaks to assure that the intensity at those energies is only due
to the background. erroneously remove an equivalent intensity from any Auger
peaks present in the spectrum.
7.3.1.1 However, in some cases, one peak might still con-
tribute to the signal at the chosen points, so the total intensity
7.6 Reporting—For consistent determination of a peak area,
is not purely due to the background. This is common for
the region over which background subtraction needs to be
spectra containing peaks with large kurtosis (large Lorentzian
applied will vary with the peak width, peak shape, and the
width) since the peak contribution at energies as far as five
background-subtraction method applied. The consistent appli-
times the Lorentzian width from the peak center is still 1 %. In
cationofabackground-subtractionprocesscanproduceprecise
these cases it is possible to employ an active approach during
determination of peak areas. In many circumstances, electrons
peak fitting in which the intensity of the background is not tied
appropriately associated with the photoelectron peaks can
to the intensity of the signal at the chosen points but calculated
occur outside of the integration limits; therefore the accuracy
duringpeak-fitting (6, 7).Theadvantagesofanactiveapproach
of any resulting quantification will depend on the method by
are discussed in various reports (12, 13); an early example can
whichthesensitivityfactorsweredetermined.Analyticalerrors
be found in Figure A3.7 of Ref (21).
can also occur if there are changes in AES or XPS lineshapes
7.3.2 Choosing the Background Type Based on the Shape of
or shakeup fractions with changes of chemical state. Uncer-
the Spectrum (XPS)—The linear background is recommended
tainties in X-ray photoelectron spectroscopy intensities associ-
when the background at both sides of the peaks is a straight
ated with different methods and procedures for background
line, one side the continuation of the other. The polynomial
subtraction have been evaluated for both monochromatic
background is recommended for small peaks riding on top of
aluminumX-rays (8)andforunmonochromatedaluminumand
the background of a larger peak or on wideAuger structures.A
magnesiumX-rays (26).Sincethepeakareawilldependonthe
step-shaped increment on the background intensity from the
chosen background and how it is applied, the analyst should
low to the high binding energy side of the main features could
specifythebackgroundtypeortypesandthechosenendpoints
be treated with the (iterative) Shirley or with the SVSC
when reporting peak areas and the derived analytical results.
method. Besides the plasmon features, the Tougaard-type
8. Less Common Procedures
backgrounds also reproduce an increment on the slope of the
background signal near the peak on the high binding energy
8.1 Inelastic Electron Scattering Correction (AES and
side.
XPS)—This method, proposed by Tougaard (27), uses an
7.3.2.1 The high binding-energy side of a photoelectron algorithm which is based on a description of the inelastic
peak commonly shows both a step-shaped increment and an
scattering processes as the electrons travel within the specimen
increment on the slope of the background signal. In these and before leaving it. The energy loss function (or scattering cross
other cases, the total background might consist of the sum of
section) multiplied by the inelastic mean free path (the λ K
various types. The simultaneous application of various back- function) is iteratively convolved with the primary signal to
ground types can be done under the active approach (7). Some reproduce the background in a large energy region. This
examples are discussed in References (7 and 20). background subtraction method also gives direct information
on the in-depth concentration profile (28 and 29). The λ K
7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES)
function could be assessed from reflected electron energy loss
(22 and 23)—Signal differentiation is among the earliest
spectroscopy (REELS) measurements by applying a certain
methods employed to remo
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it 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 version
of the standard as published by ASTM is to be considered the official document.
Designation: E995 − 11 E995 − 16
Standard Guide for
Background Subtraction Techniques in Auger Electron
Spectroscopy and X-Ray Photoelectron Spectroscopy
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
1.1 The purpose of this guide is to familiarize the analyst with the principal background subtraction techniques presently in use
together 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).
1.3 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 responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E673 Terminology Relating to Surface Analysis (Withdrawn 2012)
2.2 ISO Standard:
ISO 18115–1 Surface chemical analysis—Vocabulary—Part 1: General terms and terms used in spectroscopy
3. Terminology
3.1 Definitions—For —Since Terminology E673 was withdrawn in 2012, for definitions of terms used in this guide, refer to
TerminologyISO E673. 18115-1.
4. Summary of Guide
4.1 Relevance to AES and XPS:
4.1.1 AES—The production of Auger electrons by bombardment of surfaces with electron beams is also accompanied by
emission of secondary and backscattered electrons. These secondary and backscattered electrons create a background signal. This
background signal covers the complete energy spectrum and has a maximum (near 10 eV for true secondaries), and a second
maximum for elastically backscattered electrons at the energy of the incident electron beam. An additional source of background
is associated with Auger electrons, which are inelastically scattered while traveling through the specimen. Auger electron excitation
may also occur by X-ray and ion bombardment of surfaces.
4.1.2 XPS—The production of electrons from X-ray excitation of surfaces may be grouped into two categories—photoemission
of electrons and the production of Auger electrons from the decay of the resultant core hole states. The source of the background
signal observed in the XPS spectrum includes a contribution from inelastic scattering processes, and for non-monochromatic X-ray
sources, electrons produced by Bremsstrahlung radiation.
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
Spectroscopy 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 20042011
as E995 – 04.E995-11. DOI: 10.1520/E0995-11.10.1520/E0995-16.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
https://www.iso.org/obp/ui/#iso:std:iso:18115:-1:ed-2:v1:en.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E995 − 16
4.2 Various background subtraction techniques have been employed to diminish or remove the influence of these background
electrons from the shape and intensity of Auger electron 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 on
available instrumentation and software as well as the method of acquisition of the original signal. These subtraction methods fall
into two general categories: (1) real-time background subtraction; and (2) post-acquisition background subtraction.
5. Significance and Use
5.1 Background subtraction techniques in AES were originally employed as a method of enhancement of the relatively weak
Auger signals to distinguish them from the slowly varying background of secondary and backscattered electrons. Interest in
obtaining useful information from the Auger peak line shape, concern for greater quantitative accuracy from Auger spectra, and
improvements 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 determination
of chemical states (from the binding-energy values for component peaks that may often overlap), greater quantitative accuracy
from the XPS spectra, and improvements in data acquisition. Post-acquisition background subtraction is normally applied to XPS
data.
5.3 The procedures outlined in Section 7 are popular in XPS and AES; less popular procedures and rarely used procedures are
described in Sections 88 and 9 and 9, respectively. General reviews of background subtraction methods and curve-fitting
techniques have been published elsewhere (1-5).
5.4 Background subtraction is usually done before peak fitting. commonly performed prior to peak fitting, although it can be
assessed (fitted) during peak fitting (active approach (6, 7)). Some commercial systems data analysis packages require background
removal. removal before peak fitting. Nevertheless, a measured spectral region consisting of one or more peaks and background
intensities due to inelastic scattering, Bremsstrahlung (for XPS with unmonochromated X-ray sources), and scattered primary
electrons (for AES) can often be satisfactorily represented by choosing applying peak functions for each intensity component with
parameters for each componentone determined in a single least-squares fit. The choice of the background to be removed, if required
or desired, before or during peak fitting is suggested by the experience of the analysts analysts, the capabilities of the peak fitting
software, and the peak complexity as noted above.
6. Apparatus
6.1 Most AES and XPS instruments either already use, or may be modified to use, one or more of the techniques that are
described.
6.2 Background subtraction techniques typically require a digital acquisition and digital data handling capability. In earlier
years, the attachment of analog instrumentation to existing equipment was usually required.
7. Common Procedures
7.1 The following background subtraction methods are widely employed. It is common for an analyst to choose one among
them depending on the shape of the spectrum. As shown in a Round Robin study, different groups chose different background
methods for analyzing the same spectrum (8). Although the purpose of this guide is to describe the common procedures employed
for background subtraction, 7.3.2 provides a short guide of how to choose one or more background types depending on the shape
of the spectrum.
7.2 Linear Background Subtraction (AES and XPS)—Commonly Employed Background Types: In this method, two arbitrarily
chosen points in the spectrum are selected and joined by a straight line (1 and 2). This straight line is used to approximate the true
background and is subtracted from the original spectrum. For Auger spectra, the two points may be chosen either on the
high-energy side of the Auger peak to result in an extrapolated linear background or such that the peak is positioned between the
two points. For XPS spectra, the two points are generally chosen such that the peak is positioned between the two points. The
intensity values at the chosen points may be the values at those energies or the average over a defined number of data points or
energy interval.
7.2.1 Linear Background (AES and XPS)—In this method, two arbitrarily chosen points in the spectrum are selected and joined
by a straight line (1 and 2). This straight line is used to approximate the true background and is subtracted from the original
spectrum. For Auger spectra, the two points may be chosen either on the high-energy side of the Auger peak to result in an
extrapolated linear background or such that the peak is positioned between the two points. For XPS spectra, the two points are
generally chosen such that the peak is positioned between the two points. The intensity values at the chosen points may be the
values at those energies or the average over a defined number of data points or energy interval. The linear method can be extended
to a polynomial version when the peaks are small and riding on top of a more complex (than linear) background (7).
The boldface numbers in parentheses refer to the references at the end of this standard.
E995 − 16
7.2.2 Shirley (or Integral) Background (AES and XPS)—This method, proposed by Shirley (9), employs a mathematical
algorithm to approximate the step in the background commonly found at the position of the peak. The algorithm is based on the
assumption that the background is proportional to the area of the peak above the background at higher kinetic energy. This implies
an iterative procedure, which was described in detail by Proctor and Sherwood (10), that should be employed to guarantee
self-consistency (11). With another variant proposed by Vegh (12) and fully discussed by Salvi and Castle (13), it is possible to
employ a self-consistent Shirley-type background (SVSC-background) without the need of an iterative process; it is especially
practical for complex spectra (7).
7.2.2.1 The original Shirley method was modified by Bishop to include a sloping component to reproduce the decay of the
background intensity (14). Another modification provides for a background based upon the shape of the loss spectrum from an
elastically backscattered electron (15), and to 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 approach
described in 8.1. Under this method, the λ K function, which enters in the algorithm, is taken from a simple universal formula which
is approximately valid for some solids. Similar functions have been optimized for particular materials or material classes (16). The
application of this background might require the acquisition of background data in a 50 to 100 eV range below (in the lower
kinetic-energy side) the main peaks. Alternatively, the parameters used in the universal formula may also be permitted to vary in
an optimizing algorithm so as to produce an estimate of the background (1 and 17). Tougaard has assessed the accuracy of
structural parameters and the amount of substance derived from the analysis (18). A more approximate form of the Tougaard
algorithm (19) can be used for automatic processing of XPS spectra (for example, spectra acquired for individual pixels of an XPS
image). A simpler form of the Tougaard background, the slope-background (20), can be employed for spectra with a reduced (5
to 15 eV) background acquisition range below the main peaks. It is designed to reproduce the onset of the background growth due
to extrinsic inelastic electron scattering, which correspond to the near-peak part of the Tougaard background (it cannot be employed
to reproduce the background signal farther than ~ 15 eV from the main peaks).
7.3 Integral (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 from
the solid. The algorithm is based on the assumption that the background is proportional to the area of the peak above the
background at higher kinetic energy. This basic method has been modified to optimize the required iterations (7), to provide for
a sloping inelastic background (8), to provide for a background based upon the shape of the loss spectrum from an elastically
backscattered 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 the
two end points or spectral region for background subtraction. These points are selected far enough from the peaks to assure that
the 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 not
purely due to the background. This is common for spectra containing peaks with large kurtosis (large Lorentzian width) since the
peak contribution at energies as far as five times the Lorentzian width from the peak center is still 1 %. In these cases it is possible
to employ an active approach during peak fitting in which the intensity of the background is not tied to the intensity of the signal
at 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)—The linear background is recommended when
the background at both sides of the peaks is a straight line, one side the continuation of the other. The polynomial background is
recommended for small peaks riding on top of the background of a larger peak or on wide Auger structures. A step-shaped
increment on 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 an
increment 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 increment
on the slope of the background signal. In these and other cases, the total background might consist of the sum of various types.
The simultaneous application of various background types can be done under the active approach (7). Some examples are discussed
in References (7 and 20).
7.3 Inelastic Electron Scattering Correction (AES and XPS)—This method, proposed by Tougaard (10), uses an algorithm which
is based on a description of the inelastic scattering processes as the electrons travel within the specimen before leaving it. The
scattering cross section which enters in the algorithm is taken either from a simple universal formula which is approximately valid
for some solids, similar functions that have been optimized for particular materials or material classes (11), or is determined from
the energy spectrum of a backscattered primary electron beam by another algorithm (1 and 12). Alternatively, the parameters used
in the universal formula may also be permitted to vary in an algorithm so as to produce an estimate of the background (1 and 13).
This background subtraction method also gives direct information on the in-depth concentration profile (14 and 15). Tougaard has
assessed the accuracy of structural parameters and the amount of substance derived from the analysis (16). A simpler but more
approximate form of the Tougaard algorithm (17). can be used for automatic processing of XPS spectra (for example, spectra
acquired for individual pixels of an XPS image).
E995 − 16
7.4 Signal Differentiation, dN(E)/dE or dEN(E)/dE (AES) (22 and 23)—(Signal18 and 19)—Signal differentiation is among the
earliest methods employed to remove the background from an Auger spectrum and to enhance the Auger features. It may be
employed in real time or in post acquisition. post-acquisition. In real time, differentiation is usually accomplished by superposition
of a small (1 to 6 eV (1 to 6 eV peak-to-peak) sinusoidal modulation on the analyzer used to obtain the Auger spectrum. The output
signal is then processed by a lock-in amplifier and displayed as the derivative of the original energy distribution N(E) or EN(E).
In post-acquisition background subtraction, the already acquired N(E) or EN(E) signal may be mathematically differentiated by
digital or other methods. The digital method commonly used is that of the cubic/quadratic derivative as proposed by Savitzky and
Golay (2024).
7.5 X-Ray Satellite Subtraction (for Non-Monochromated X-Ray Sources) (XPS) (25)—(In21) —In this method, photoelectron
intensity from the satellite X-rays associated with the K X-ray spectrum from an aluminum or magnesium X-ray source is
subtracted. Intensity is removed from higher kinetic energy channels at the spacing of the Kα , Kβ, etc. satellite positions from
3,4
the Kα main peak and with the corresponding intensity ratios (2125) to remove their contributions to the XPS spectrum. This
1,2
subtraction can proceed through the spectrum but not if there is an Auger peak in the region of interest because it would
erroneously remove an equivalent intensity from any Auger peaks present in the spectrum.
7.6 Implementation of the Linear, Integral, and Tougaard Background Subtraction Methods (XPS)—Reporting—A key choice
in implementation of the linear (7.1), integral (7.2), and Tougaard (7.3) background subtraction methods is the selection of the two
end points or spectral region for background subtraction. For consistent determination of a peak area, the region over which
background subtraction needs to be applied will vary with the peak width/structure width, peak shape, and the background
subtractionbackground-subtraction method applied. The consistent application of a background subtraction background-
subtraction process can produce precise determination of peak areas. In many circumstances, electrons appropriately associated
with the photoelectron peaks can occur outside of the integration limits; therefore the accuracy of any resulting quantifica
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