ISO 18554:2016

Surface chemical analysis — Electron spectroscopies — Procedures for identifying, estimating and correcting for unintended degradation by X-rays in a material undergoing analysis by X-ray photoelectron spectroscopy

ISO 18554:2016 provides a simple procedure for identifying, estimating and correcting for unintended degradation in the elemental composition or chemical state of a material which occurs as a result of X-radiation during the time that a specimen material is exposed to the X-rays used in X-ray photoelectron spectroscopy (XPS). ISO 18554:2016 does not address comparisons between different types of material nor does it address the mechanisms, depth, or chemical nature of the degradation that occurs. The correction procedure proposed is only valid if the changes are caused by the X-rays and result in less than a 30 % reduction or increase in intensity of a chosen photoelectron peak from the sample material.

Analyse chimique des surfaces — Spectroscopie d'électrons — Procédures pour l'identification, l'estimation et la correction de la dégradation involontaire par rayons X pendant une analyse de matériau par spectroscopie de photoélectrons par rayons X

General Information

• Status: Published
• Publication Date: 17-Mar-2016
• ICS: 71.040.40 - Chemical analysis
• Technical Committee: ISO/TC 201/SC 7 - Electron spectroscopies
• Drafting Committee: ISO/TC 201/SC 7 - Electron spectroscopies
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 17-Jun-2021

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18554
First edition
2016-03-15
Surface chemical analysis — Electron
spectroscopies — Procedures for
identifying, estimating and correcting
for unintended degradation by X-rays
in a material undergoing analysis by
X-ray photoelectron spectroscopy
Analyse chimique des surfaces — Spectroscopie d’électrons —
Procédures pour l’identification, l’estimation et la correction de
la dégradation involontaire par rayons X pendant une analyse de
matériau par spectroscopie de photoélectrons par rayons X
Reference number
ISO 18554:2016(E)
©
ISO 2016

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ISO 18554:2016(E)

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ISO 18554:2016(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Terms and definitions . 1
3 Symbols and abbreviated terms . 1
4 Sample degradation . 2
4.1 Causes of degradation . 2
4.2 Sample degradation . 3
4.3 Measurements for identifying, and correcting for, degradation . 3
4.3.1 Recognition of degradation . 3
4.3.2 The first survey scan. 3
4.3.3 The detail scans . 4
4.3.4 The final survey scan . 4
4.3.5 Inverting the order of acquisition for unstable compounds . 4
4.3.6 Check for degradation . 4
4.3.7 Deduce the undegraded intensity . 4
4.4 Assessing the likelihood of degradation . 6
4.4.1 Determining the value of A . 6
Z
4.5 Reporting degradation . 6
4.6 Suggested procedures for minimising degradation . 6
4.7 Influence of contamination . 7
4.7.1 Contamination formation during spectrum acquisition. 7
4.7.2 Reporting contamination . 7
Annex A (informative) Materials reported to degrade during analysis . 8
Annex B (informative) Examples of degradation . 9
Annex C (informative) Compensation for formation of a contamination layer.14
Bibliography .16
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ISO 18554:2016(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
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committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information.
The committee responsible for this document is ISO/TC 201, Surface chemical analysis, Subcommittee
SC 7, Electron spectroscopies.
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ISO 18554:2016(E)

Introduction
The basis of X-ray photoelectron spectroscopy is irradiation of a sample surface by soft X-rays and
examination of the excited emission in the form of photo-electrons and Auger electrons. In its most
widely used mode, the X-ray flux is of low intensity and spread over a large area. Thus, the technique
is generally regarded as one of the least destructive of the available “beam” techniques used for the
surface chemical analysis of materials. However, since the time of its inception as a technique for surface
[1]-
analysis, there have been reports of changes in composition arising during the course of analysis.
[4]
These reports indicated that, for some materials, a form of degradation during analysis needs to
be taken into account and, where possible, a correction made. This International Standard addresses
these issues and describes a method by which the extent of degradation can be estimated and a suitable
correction obtained.
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INTERNATIONAL STANDARD ISO 18554:2016(E)
Surface chemical analysis — Electron spectroscopies —
Procedures for identifying, estimating and correcting for
unintended degradation by X-rays in a material undergoing
analysis by X-ray photoelectron spectroscopy
1 Scope
This International Standard provides a simple procedure for identifying, estimating and correcting for
unintended degradation in the elemental composition or chemical state of a material which occurs as
a result of X-radiation during the time that a specimen material is exposed to the X-rays used in X-ray
photoelectron spectroscopy (XPS).
This International Standard does not address comparisons between different types of material nor does
it address the mechanisms, depth, or chemical nature of the degradation that occurs. The correction
procedure proposed is only valid if the changes are caused by the X-rays and result in less than a 30 %
reduction or increase in intensity of a chosen photoelectron peak from the sample material.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
region
part of the photo-excited spectrum chosen for detailed acquisition and analysis
Note 1 to entry: The region may be chosen because it contains a major or minor peak of a given element or to
represent the shape or slope of a background within that energy range, e.g. a detail scan.
Note 2 to entry: This usage of region is not to be confused with the area of analysis.
2.2
time zero
time at which the X-rays start to irradiate the sample
3 Symbols and abbreviated terms
A deduced linear rate of change of I as a result of degradation for a given element or state
Z t,Z
C atomic fraction of contamination carbon from the quantification computation
d thickness of a contamination layer on the surface of the sample
contamina-
tion
DI degradation index
E kinetic energy, in eV, of the detected electrons
FWHM full width at half maximum (intensity)
I intensity of a given photoelectron peak after correction for the formation of a layer of con-
Z,corrected
tamination
I measured intensity of a given photoelectron peak that is influenced by the presence of a layer of
Z,measured
contamination
I undegraded photoelectron intensity of a given element or state, Z
0,Z
I final photoelectron intensity of a given element in the survey spectrum
f,S
I initial photoelectron intensity of a given element in the survey spectrum
i,S
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ISO 18554:2016(E)

I intensity of a given element or state measured after a period of X-ray exposure, t
t,Z m,Z
L attenuation length of detected electrons in the contamination layer
c
PTI photo threshold index
PVC poly(vinyl chloride)
θ angle of emission of the detected electrons from the surface normal
t elapsed time of exposure to X-rays at the start of data collection for a particular element, Z
i,Z
t elapsed time of exposure to X-rays at the termination of data collection for a given element, Z
f,Z
t mean time of X-ray exposure for a given element or state, Z
m,Z
t time at which the sample was first exposed to X-rays
0
t mean time at which the initial survey scan was acquired
i,S
t mean time at which the final survey scan was acquired
f,S
XPS X-ray photoelectron spectroscopy
NOTE t may be the same for all elements if, for example, the scans for individual elements are acquired in
m,Z
a pseudo-parallel mode, but can be very different for each element if element regions are acquired serially, i.e. in
turn, after the previous one has been completed.
4 Sample degradation
Reports of sample degradation during acquisition of a photoelectron spectrum are widespread and
affect most, if not all, classes of materials under certain circumstances. A list of materials reported to
degrade under XPS analysis is provided for information in Annex A.
4.1 Causes of degradation
Sample degradation in the course of analysis by XPS occurs, mainly, because of bonding changes in the
sample caused by the X-ray beam through the direct interaction with the X-rays (characteristic X-rays
or bremsstrahlung) or the electrons emitted from un-monochromated sources or the photoelectrons.
It will occur when the sample is exposed to the X-ray beam before analysis and in the period between
survey or detail scans, as well as in the scans themselves; it does not occur solely during data acquisition.
Degradation can occur also through heating, especially from twin anode sources which are close to
the sample and emit heat. These anodes operate close to 100 °C and are often as close as 5 mm to the
sample stage, covering a large solid angle.
Minor damage can occur from exposure of samples to the vacuum of the instrument and in other cases
from exposure to the low-energy secondary electron flux within the spectrometer chamber. The former
is outside the scope of this International Standard while damage from secondary electrons is likely to
be a concomitant factor of the X-ray flux and does not need to be treated as an independent factor.
Degradation also arises from electron flood guns which may have to be run at high current to neutralize
focused monochromated X-ray sources or may be set at an unnecessarily high value by default. Low-
energy ions used for charge neutralization, also, can have a deleterious effect. Such devices may be on
for some time before analysis starts. Control of some or all of these factors can be helpful in reducing
degradation; however, the procedure recommended in this International Standard considers only
changes that occur once the sample is exposed to the X-rays (defined as time zero in the procedure
[3][4][5][6]
described below).
Material will also be removed from the surface during ion-etching; this is intentional but unintended
changes in chemical state may result. Ion-beam sputtering is outside the scope of this International
Standard but some concepts relating to chemical degradation may be helpful in understanding the
phenomenon.
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ISO 18554:2016(E)

4.2 Sample degradation
Sample degradation is said to have occurred whenever there is a measurable change in the spectrum of
a sample after a period of exposure to the X-ray flux. This change typically arises from a change in the
atomic fraction or chemical state of one or more elements, giving rise to a relative shift in peak positions
or to a change in intensity of one of more peaks and thus a change in the measured constitution of the
sample. A good description of the overall effects of a photon beam has been given by Reference [5].
The change in chemical state can be due to photo-excited reduction of an ion, such as occurs when
Cu(ll) is chemically reduced to Cu(l). Sometimes, but not always, this can be accompanied by
oxidation of another element in the near-surface region. An example is given in Annex B. The change
in composition can arise, particularly in organic compounds, through the breaking of chemical bonds
and the subsequent diffusion and escape of low-molecular weight species generated as a result. Some
relative rates of degradation are given by Reference [6], e.g. using their degradation index, DI, which is
proportional to the parameter, A, defined in this International Standard. Polypropylene has a DI value
of 5 and polyethylene has a DI of 10; these are examples of various forms of degradation, including
bond breaking, radical formation and cross-linking. Poly(vinyl chloride), which has been used to assess
degradation rates has a DI of 25 on this scale. An example of the loss of Cl from a poly(vinyl chloride)
blend is described in Annex B.
Although the degradation noticed in the X-ray photoelectron spectrum concerns only the outermost
10 nm, the X-rays penetrate to much greater depths and damage is not confined to the escape depth
of photoelectrons. Thus, material lost from the surface layers may be compensated by material
diffusing from deeper within the sample. This behaviour can give a dynamic plateau in the degradation
of observed composition or chemical state. An illustration is given in Annex B. In the period prior to
establishment of the plateau, an approximately linear rate of degradation is normally observed and
the procedure recommended in this International Standard applies a correction by means of a linear
extrapolation. Near-surface degradation may be particularly important in analysis of monolayer films
[7][8]
because of disruption to the bond required for attachment to the substrate.
4.3 Measurements for identifying, and correcting for, degradation
4.3.1 Recognition of degradation
Recognition of degradation is based primarily on a comparison of the first and final scan in the
acquisition sequence. For the majority of samples, this is most easily done by comparison of the survey
scans taken at the commencement and closure of analysis. Examples of methods by which the scans
can be compared are given in Annex B. Since degradation is dependent on the total dose of X-rays, it
is necessary to record the time of exposure throughout data acquisition. The following procedure is
recommended for a simple identification and correction of the effects of X-ray induced degradation
with a minimum of effort rather than a detailed study of that degradation.
4.3.2 The first survey scan
Set up the spectrometer for XPS analysis using your usual method and note the time at which the
specimen is first exposed to X-rays. Record this as “time zero”, t . Record a survey spectrum (a 0,4 eV
0
[9]
step interval is recommended ), noting the time at which the acquisition is started and finished. It is
recommended that any exposure of the sample to direct X-ray flux, electron irradiation from electron
flood guns, low energy ions used for charge neutralization, or heat from X-ray anodes is kept as short
as possible prior to acquisition of this scan. The mean exposure time for the first survey scan, t , is the
i,S
difference between the average of the start and finish times of the initial survey scan and time zero, t .
0
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ISO 18554:2016(E)

4.3.3 The detail scans
Record relevant detail scan spectra using your usual operating conditions, noting the times at which
the spectra are started and finished.
a) Procedure for acquisitions recorded individually in a serial mode.
If each detail scan region is recorded individually (serially), use the mean time derived from the
start, t , and finish times t , relative to time zero, for the acquisition of each individual element or
i,Z f,Z
component peak. The mean exposure time for each detail scan, t , is the difference between the
m,Z
average of the start and finish times of the given acquisition and time zero, t .
0
b) Procedure for acquisitions recorded in an interlaced mode (pseudo parallel).
If the detail scans are derived in an interlaced mode (pseudo-parallel), use the mean value of start
time of the series, relative to t , and the finish of the series, relative to t , as t , the mean exposure
0 0 m,Z
time for the measurement. Pseudo-parallel acquisition is the recommended mode of use since the
acquisition of each chosen region of the spectrum will have been made for the same exposure time.
Record this set of intensities and associated mean exposure time to X-rays.
In the event that a signal-to-noise criterion is used to terminate acquisition of individual elements or
peaks in a parallel scan mode, then note shall be taken of the time at which the scan is terminated and
the exposure time calculated from the start and finish times for that region of the spectrum.
4.3.4 The final survey scan
Repeat the survey spectrum at the end of the acquisition sequence using the same instrumental settings
as used for the first survey scan at 4.3.2. Note the time for the start and finish of this acquisition. The
mean exposure time for the final survey scan, t , is the difference between the average of the start and
f,S
finish times of the final survey scan and time zero.
4.3.5 Inverting the order of acquisition for unstable compounds
For organic and other unstable compounds (see Annex A), it may be advantageous to substitute a detail
scan through the carbon 1s region, or other detail region as appropriate, for the first survey scan at
4.3.2 in the above procedure. If a survey scan is required, record this detail scan first. Use a detail scan
through the same region, instead of a survey scan, to terminate the acquisition as 4.3.4.
4.3.6 Check for degradation
To check for degradation, compare the first and final survey scans or, for organic samples the first and
last carbon 1s detail scans, and observe any changes. For insulators, small changes in the charging
shifts can complicate this comparison and realignment against a peak common to both spectra can
be necessary. The spectra may be compared using quantification, subtraction of spectra, division of
spectra channel by channel, or by visual observation. Examples of the results of such procedures are
given in Annex B.
4.3.7 Deduce the undegraded intensity
To deduce the undegraded intensity I for a peak that is degrading and exhibits intensity I at time
0,Z t,Z
of exposure t , the following linear relationship given in Formula (1) is assumed for the rate of
m,Z
degradation:
II=− At (1)
t,ZZ0,mZZ,
where A is a constant for a given element and peak position that depends on the instrument, its settings
Z
and the sample.
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ISO 18554:2016(E)

This relationship is shown in Figure 1. Be aware of the fact that during the necessary set-up time for the
survey scan, the sample might already have degraded: thus, all times are those measured in real time
from the onset of X-ray exposure (t ) and are not the experimental acquisition times for a particular
0
scan. This degradation is assumed to be linear with exposure time and thus, the mean exposure time at
which the peak was acquired, t , may be used in Formula (1).
m,Z
Y
l
0,Z
t ,
i,Z l
i,Z
t , l
f,Z ,Z
Key
X exposure time, t
Y intensity
Figure 1 — Schematic diagram illustrating a linear relationship between initial and final
intensities, I and I , for the corresponding average exposure times, t and t , and
i,Z f,Z i,Z f,Z
extrapolation to time zero to give I
0,Z
The undegraded intensity, I for a given element, Z, is thus given in Formula (2):
0,Z,
II=−tI tt −t (2)
() ()
0,Zi,fZZ,,fZ iZ,,fZ iZ,
where t and t are the initial and final mean exposure times at which the initial and final intensities,
i,Z f,Z
I and I were recorded.
i,Z f,Z
These values may be based on individual regions of a survey scan (or on values from individual detail
scans, obtained under identical conditions and at recorded times, for the elements concerned). This
relation is valid for small changes in intensity resulting from degradation. Ensure that I /I > 0,7 or
f,Z i,Z
I /I < 1,3 for any intensity that is to be corrected.
f,Z i,Z
Each peak intensity that is to be used in determining a composition should be corrected using a value of
I determined for that peak. The corrected composition is then calculated in the usual manner using
0,Z
the individual values for each element. Note that in a multi-element compound or mixture, degradation
can affect different elements in different ways. Thus, A may take positive or negative values. It may be
Z
more convenient to use the compositions, expressed in atomic% instead of intensities in order to check
for degradation. In this case, the procedure above may be followed, correcting the values contributing
to the composition and finally renormalizing to 100 atomic%.
Detail scan spectra that contain two or more chemical states may be examined for degradation using
spectra obtained at differing, known, times. The peak ratio or the peak subtraction method may be
used. If there is evidence of change, then degradation involving a change in chemical state will have
taken place. This can occur even if the overall composition of the sample, determined from comparisons
of the survey scans in 4.3.2 and 4.3.4, show no change. To correct for a change in the chemical state, the
intensities of the individual component peaks should be determined by peak fitting for each time of
acquisition and the zero-time intensities for each chemical state calculated using Formula (2).
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ISO 18554:2016(E)

Organic polymers may give evidence of degradation by changes of parameters characteristic of the
polymer and measurable by XPS. The parameters used by Reference [7] are atomic ratios, the area ratio
of C 1s shake-up peaks to the main peak for unsaturated hydrocarbons or the FWHM of the C 1s peak
for saturated hydrocarbons. These parameters were used in establishing degradation measures for
their Scienta ESCA300 instrument and their degradation index, DI, is proportional to the parameter
A defined here. In a VAMAS inter-laboratory comparison (Project A5) on the degradation of polymers,
Z
the relationship between the degradation rate and X-ray source flux in XPS was investigated by
Reference [10]. An example of the use of the Beamson and Briggs Degradation Index can be found in
Reference [11] together with a suggested method for recording the X-ray flux.
NOTE Measured over longer periods, it will be found that the degradation is not linear with time and might
follow, for instance, an exponential decay. Examples are given in Annex B. The data at longer times can provide
useful information about the degradation process but do not improve the accuracy of the evaluation of I
0,Z
compared with Formula (2).
A values for the same material will differ with the instrument settings and might differ from laboratory to
Z
laboratory. However, the relative values of A for different materials should be approximately constant. Data
Z
from Reference [6] should be proportional to A values.
Z
4.4 Assessing the likelihood of degradation
4.4.1 Determining the value of A
Z
The parameter A in Formula (1) can be used to compare instruments for a given sample, or to compare
Z
samples for a given instrument, or to modify settings to minimize degradation, as given in Formula (3):
AI=− It −t (3)
() ()
Zf ,,Zi Zf ,,Zi Z
For example, a cast layer deposited from PVC dissolved in tetrahydrofuran may be used for comparison
purposes in setting up an instrument to minimize degradation but it should be noted that all PVC
products behave differently. A comparison of data from two laboratories is given in Annex B.
4.5 Reporting degradation
When degradation is observed, it shall be reported. The following formats are recommended.
— The sample showed evidence of degradation during analysis in the form of a decrease in the intensity
of element P and an increase of intensity for element Q by x% and y%, respectively, over the period
of analysis. The reported compositions are those calculated using intensities corrected using linear
extrapolations to the start of X-ray exposure as described in Annex B. The maximum values for x%
and y%, suitable for linear extrapolation, should not exceed 30 %.
— The sample showed evidence of degradation during analysis in the form of a decrease in the relative
abundance of chemical state (Xn) by x% over the period of analysis. The reported compositions are
those corrected using linear extrapolations to the start of X-ray exposure as described in Annex B.
4.6 Suggested procedures for minimising degradation
When degradation is suspected, an attempt should be made to use an adjacent region of the sample
surface to set up the acquisition parameters, such as X-ray intensity, detector settings, and sample
angle or height. Once these parameters are established, the sample can be moved to a new, unexposed
part of the sample and the acquisition started; use the time at which the sample was moved to this new
position as time zero. Note that with unmonochromated sources, regions of the sample holder within
15 mm can be exposed to heat, electrons, and X-rays from the source.
If acquisition needs to be suspended for any period of time, it can be advantageous to switch off
...