ISO/TR 18392:2005
(Main)Surface chemical analysis — X-ray photoelectron spectroscopy — Procedures for determining backgrounds
Surface chemical analysis — X-ray photoelectron spectroscopy — Procedures for determining backgrounds
ISO/TR 18392:2005 gives guidance for determining backgrounds in X-ray photoelectron spectra. The methods of background determination described are applicable for evaluation of spectra of photoelectrons and Auger electrons excited by X-rays from solid surfaces.
Analyse chimique des surfaces — Spectroscopie de photoélectrons X — Protocoles pour déterminer les fonds continus
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 18392
First edition
2005-12-01
Surface chemical analysis — X-ray
photoelectron spectroscopy —
Procedures for determining backgrounds
Analyse chimique des surfaces — Spectroscopie de
photoélectrons X — Protocoles pour déterminer les fonds
continus
Reference number
ISO/TR 18392:2005(E)
©
ISO 2005
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ISO/TR 18392:2005(E)
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ISO/TR 18392:2005(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Terms and definitions. 1
3 Symbols and abbreviated terms . 1
4 Types of background in XPS . 1
5 Removal of X-ray satellites from electron spectra. 2
6 Estimation and removal of inelastic electron scattering from electron spectra. 2
6.1 General Information. 2
6.2 Procedures to account for inelastic electron scattering . 2
6.2.1 Introduction . 2
6.2.2 Estimation of the linear background and its removal.3
6.2.3 Integral background removal . 3
6.2.4 Removal based on the electron inelastic-scattering cross-section . 4
6.3 Procedures accounting for both inelastic and elastic scattering. 5
6.4 Less commonly used procedures. 5
6.5 Role of surface and core-hole effects in background determination. 6
6.6 Determining the background for inhomogeneous materials . 6
7 Comparisons of procedures for removing effects of inelastic electron scattering from
electron spectra . 7
Bibliography . 8
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ISO/TR 18392:2005(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
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International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 18392 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 5, Auger electron spectroscopy.
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ISO/TR 18392:2005(E)
Introduction
This Technical Report gives guidance for determining backgrounds in X-ray photoelectron spectra. The
methods of background determination described in this report are applicable for quantitative evaluation of
spectra of photoelectrons and Auger electrons excited by X-rays from solid surfaces and surface
nanostructures.
The use of background determination in X-ray photoelectron spectroscopy (XPS) has developed from the
need (i) for accurate quantitative information on chemical composition (including in-depth composition) of
surface/interface layers and nanostructures, (ii) for unambiguous identification of chemical states of surface
species and (iii) for extracting electronic-structure information from photoelectron spectra excited from solids.
It is therefore necessary to separate the intrinsic part of a spectrum, which is associated with the
photoionization or photoexcitation process by the X-radiation of interest in XPS or the Auger-electron decay
process and which is needed for further analysis, from other parts of the spectrum (the background) appearing
due to other processes. There are widely used procedures available for background subtraction in XPS that
are reviewed in detail in References [1] to [4]. Here, the most common procedures and their use are
summarized, including methods (i) commonly available in commercial software systems, (ii) available and
used in more advanced laboratories and (iii) used in individual laboratories to develop understanding of the
processes reflected in the XPS spectra.
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TECHNICAL REPORT ISO/TR 18392:2005(E)
Surface chemical analysis — X-ray photoelectron
spectroscopy — Procedures for determining backgrounds
1 Scope
This Technical Report gives guidance for determining backgrounds in X-ray photoelectron spectra. The
methods of background determination described in this report are applicable for evaluation of spectra of
photoelectrons and Auger electrons excited by X-rays from solid surfaces.
2 Terms and definitions
[5]
For the purposes of this document, the terms and definitions given in ISO 18115 apply.
3 Symbols and abbreviated terms
AES Auger electron spectroscopy
PIA Partial intensity analysis
TM
QUASES Quantitative analysis of surfaces by electron spectroscopy
REELS Reflection electron energy loss spectroscopy
XPS X-ray photoelectron spectroscopy
4 Types of background in XPS
The electrons produced by X-ray irradiation of surfaces are either photoelectrons (as a result of the primary
photoionization process) or Auger electrons (as a result of the secondary, core-hole decay process).
Contributions to the measured spectra (i.e., electron energy distributions) from electrons scattered inelastically
in the sample, from the secondary electron cascade, and — in the case of excitation by non-monochromatic
X-ray irradiation — from photoelectrons induced by X-ray satellites and by bremsstrahlung radiation constitute
the background. It is usually not necessary in practical XPS to determine the secondary-electron cascade
background at low energies.
In this Technical Report, a description of methods for removing X-ray satellites is given in Clause 5 and for
removing inelastic electron scattering in Clause 6. A brief comparison is given in Clause 7 of the effectiveness
of procedures for removing the effects of electron inelastic scattering from electron spectra.
NOTE 1 In some cases, the intensity of the intrinsic part of a spectrum is distributed among features attributable to the
“no-loss” main peak and to various electronic excitations associated with the creation of the core hole. The latter intrinsic
contributions are sometimes denoted as the “intrinsic background”. The identification of the intrinsic loss features and
measurement of their intensities can be important for quantitative applications of XPS.
[5]
NOTE 2 Time-varying fluctuations of the analytical signal due to sources of noise will lead to uncertainty in the signal
intensity. Intensity contributions due to noise are not included in the types of background discussed in this Technical
Report.
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ISO/TR 18392:2005(E)
5 Removal of X-ray satellites from electron spectra
For XPS with non-monochromated X-ray sources, a fixed satellite structure is associated with the exciting
main X-ray line (often Al or Mg Kα radiation). These X-ray satellites lead to corresponding features in the XPS
spectra.
For selected photoelectron peaks measured with Al or Mg X-ray sources, intensity is removed from
higher-kinetic-energy channels corresponding to the energy differences between the Kα , Kβ, etc., X-ray
3,4
[6]
satellite positions and the Kα main peak and the corresponding intensity ratios to remove the satellite
1,2
contributions in the given spectral region. In such a way, scaled photoelectron peaks corresponding to the
peaks excited by the X-ray satellites are subtracted. This subtraction process can be applied in turn to remove
satellite peaks associated with other photoelectron peaks. The subtraction process may also erroneously
remove an equivalent intensity from Auger peaks present in the spectrum if these are mistakenly identified as
photoelectron peaks.
6 Estimation and removal of inelastic electron scattering from electron spectra
6.1 General Information
Various procedures have been developed for separating the part of intrinsic origin in the measured
photo-excited electron spectra from the contributions due to electrons that are inelastically scattered in the
[1-4]
sample . These procedures (including those described in Clause 5) are usually applied to XPS data
following data acquisition and require digital-data acquisition and handling capability.
Prior to application of a procedure for removing the inelastic electron scattering, a measured spectrum
[7, 8]
normally should be corrected for the spectrometer response function in cases where the distortion of the
spectral shape due to instrumental effects is not negligible. To remove the effect of inelastic electron
scattering in the spectrum, two different strategies can be followed: either to remove (subtract) the contribution
attributable to electron inelastic scattering from the spectrum, or to include a background component in the
model function being used to fit the spectrum. The electron-scattering contribution can be considered either as
[9, 10]
a background for the whole spectrum or as a sum of tail contributions from individual photoelectron
peaks. In the case of background removal/subtraction, the parameters of the background components are
fixed and, after creating such a background, can be subtracted from the measured spectrum. In the case of
background fitting, some or all of the parameters of the background components are allowed to vary in the
fitting process.
NOTE The methods described here for removing the contributions of electron inelastic scattering from a spectrum
may not be used for some specific applications of XPS (i.e., total reflection XPS or Auger-photoelectron coincidence
spectroscopy) without further consideration.
6.2 Procedures to account for inelastic electron scattering
6.2.1 Introduction
Even in the case of very thin samples, a considerable fraction of the electrons in a spectrum have been
inelastically scattered; therefore, the estimation of the background for inelastic electron scattering is very
important for quantitative applications. Common procedures for removing the effects of inelastic scattering are
briefly described.
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ISO/TR 18392:2005(E)
6.2.2 Estimation of the linear background and its removal
In this widely used method, two arbitrarily chosen points in the spectrum are selected and joined by a straight
[2]
line as an approximation of the true background. These points are generally chosen such that the peak is
positioned between them. The intensity values at the chosen points may be the values at the corresponding
energies or the average value over a small energy interval around the chosen points. Figure 1 illustrates a
[11]
linear background for a Cu 2p XPS spectrum . This is the most popular method for insulators, where the
3/2
straight line is horizontal. This approach is used for polymers with great success for peak fitting. However, the
use of the linear background in the case of peaks of transition metals (e.g., Fe 2p) leads to significant
systematic errors in estimating the peak areas.
Key
X binding energy (eV)
Y intensity (arbitrary units)
Figure 1 — Example of linear background and its subtraction
(The XPS spectrum used here is copper 2p . The upper curve shows the measured spectrum and the linear
3/2
background. The lower curve is the spectrum after subtraction of the background.)
6.2.3 Integral background removal
[12, 13]
This (widely used) method, proposed by Shirley , 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 energies. This
[14]
method has been modified to optimize the required iterations , to provide for a sloping inelastic
[15]
background , to provide for a background based upon the shape of the loss spectrum from an elastically
[16] [2]
backscattered electron , and to include a band gap for insulators . Figure 2 shows the Shirley or integral
[11]
background for the Cu 2p XPS spectrum given in Figure 1 and the spectrum after this background has
3/2
been subtracted. It should be emphasized that the correct use of this method requires the application of the
[12] [14]
valid algorithm and the proper iteration limit .
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ISO/TR 18392:2005(E)
Key
X binding energy (eV)
Y intensity (arbitrary units)
Figure 2 — Example of integral background and its subtraction
(The XPS spectrum used is copper 2p . The upper curve shows the measured spectrum and the integral
3/2
background. The lower curve is the spectrum after subtraction of the background.)
6.2.4 Removal based on the electron inelastic-scattering cross-section
[4]
This (often used) method, proposed by Tougaard , uses an algorithm that is based on a description of the
inelastic-scattering processes taking place in the sample. The algorithm requires knowledge of the
inelastic-scattering cross-section for the signal electrons in the sample material. This cross-section can be
[4, 17]
obtained from analysis of experimental reflected electron energy loss spectra or from a simple
[4]
approximate formula, the so-called universal cross-section . While the universal cross-section can be
[18]
applied successfully in the case of many solids, more accurate simple formulas are available as well for
particular classes of material (e.g. polymers, Si, Al and other solids). Alternatively, the parameters used in the
[19]
universal cross-section formula may be varied in an algorithm for estimating the inelastic background .
Figure 3 provides an example of the Tougaard background for Au 4p, 4d spectra and of these spectra after
[20]
this background was subtracted .
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ISO/TR 18392:2005(E)
Key
X binding energy (eV)
Y intensity (arbitrary units)
Figure 3 — Example of the Tougaard background and its subtraction
(The XPS spectrum used shows the gold 4p and 4d peaks. The upper curve shows the measured spectrum
and the background obtained from use of the inelastic scattering cross-section. The lower curve is the
spectrum after subtraction of the background.)
6.3 Procedures accounting for both inelastic and elastic scattering
It is the inelastic scattering of photoelectrons that produces the background. However, elastic scattering of the
signal electrons affects their trajectories and therefore has an indirect effect on the background, which can be
[21]
significant. For the case of homogeneous solids, this effect has been described analytically .
A useful method for modelling scattering effects in photo-induced electron spectra is to express the spectral
shape in terms of the partial intensities of electrons that have participated in a given number of inelastic
[22]
collisions . This approach, known as partial intensity analysis (PIA), is based on the assumption that elastic
deflections and energy losses of the scattered electrons are independent and can be considered separately. It
is not necessary for the transmission function of the analyser to be known.
A further, although less frequently applied (because of its higher computational needs and limited practical
use), method to account for scattering effects is the Monte Carlo simulation of the photo-induced electron
spectra using various models and data sets for the material-dependent energy-loss functions, inelastic mea
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