ISO 16129:2012

INTERNATIONAL ISO
STANDARD 16129
First edition
2012-04-15
Surface chemical analysis — X-ray
photoelectron spectroscopy — Procedures
for assessing the day-to-day performance
of an X-ray photoelectron spectrometer
Analyse chimique des surfaces — Spectroscopie de photoélectrons
X — Modes opératoires d’évaluation de la performance au jour le jour
d’un spectromètre de photoélectrons X
Reference number
ISO 16129:2012(E)
©
ISO 2012

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ISO 16129:2012(E)
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Published in Switzerland
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ISO 16129:2012(E)
Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Terms and definitions . 1
3 Initial approach . 1
4 Initial instrument calibration, alignment and assessment . 1
5 Test specimen selection . 2
5.1 General information . 2
5.2 The conductive specimen . 2
5.3 The non-conductive specimen . 2
5.4 Specimen for assessing the X-ray source . 4
6 Collection of reference data . 4
6.1 General information . 4
6.2 Rapid test of the instrument using a conductive specimen . 4
6.3 Rapid test of the instrument using a non-conductive specimen . 8
6.4 Rapid test of the X-ray source using a phosphor specimen . 8
6.5 Rapid test of the X-ray source using a uniform conductive specimen . 8
7 Collection of subsequent performance data . 9
8 Analysis of the performance data . 9
8.1 General information . 9
8.2 Survey spectrum . 9
8.3 High-resolution spectrum from the conductive specimen .10
8.4 High-resolution spectrum from the non-conductive specimen .10
8.5 Images from the phosphor specimen .10
8.6 Images from the uniform conductive specimen .10
8.7 Spectrum ratios .10
9 Control charts .15
Bibliography .18
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ISO 16129:2012(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
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.
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.
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 16129 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 7, Electron spectroscopies.
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ISO 16129:2012(E)
Introduction
XPS instruments are complex, and unsatisfactory performance is not always obvious to an operator. It is
therefore necessary to provide a test for the correct operation of the system that can be performed regularly
and frequently without interfering excessively with the normal work of the laboratory.
A full diagnostic test can require many hours or even days; such a test can be appropriate only when the
instrument is known to have a fault that needs to be remedied or following a major maintenance procedure.
Data acquired before a problem is uncovered become suspect if the spectrometer has not been routinely
tested, leading to a loss of confidence in those data. If a regular check of the instrument is made, changes in
performance can be monitored and corrective action taken in good time to ensure that the data supplied are
fit-for-purpose. In the event that a serious fault is uncovered, then only the data since the last check can be in
doubt and need to be repeated.
The purpose of this document is to provide a user with a procedure which is not excessively time-consuming so
that it is capable of being completed on a regular and frequent basis, daily if required. The user will then gain an
awareness of the current characteristics of the instrument so that a decision can be made as to whether or not a
more complete and time-consuming action is required to return the instrument to a satisfactory level of performance.
This procedure is intended to be applied to an XPS instrument that has been correctly calibrated and aligned
in accordance with ISO standards or manufacturer’s instructions. It is designed to highlight aspects of the
instrument’s characteristics that differ significantly from those that were measured immediately following the
calibration procedure. The procedure does not show how the instrument can be returned to its original state.
Instead, it guides the user to possible areas of concern. The procedure provides data that can be used in control
charts, allowing trends to be observed and acted upon before data quality deteriorates to an unacceptable level
for the needs of the analyst.
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INTERNATIONAL STANDARD ISO 16129:2012(E)
Surface chemical analysis — X-ray photoelectron
spectroscopy — Procedures for assessing the day-to-day
performance of an X-ray photoelectron spectrometer
1 Scope
This International Standard is designed to allow the user to assess, on a regular basis, several key parameters
of an X-ray photoelectron spectrometer. It is not intended to provide an exhaustive performance check, but
instead provides a rapid set of tests that can be conducted frequently. Aspects of instrument behaviour covered
by this International Standard include the vacuum, measurements of spectra of conductive or non-conductive
test specimens and the current state of the X-ray source. Other important aspects of the instrument performance
(e.g. lateral resolution) fall outside the scope of this International Standard. The standard is intended for use
with commercial X-ray photoelectron spectrometers equipped with a monochromated Al Kα X-ray source or
with an unmonochromated Al or Mg Kα X-ray source.
2 Terms and definitions
For definitions of the spectroscopy terms used in this International Standard, the reader is referred to
[10]
ISO 18115-1 .
The following abbreviations are used:
FWHM full width at half maximum
PET poly(ethylene terephthalate)
PTFE polytetrafluoroethylene
XPS X-ray photoelectron spectroscopy
3 Initial approach
Most instruments are fitted with a vacuum gauge or gauges. These shall be read frequently and the reasons for
large variations understood. A large increase in the pressure can be due to the properties of a test specimen inside
the instrument, a fault in the pumping system, an increase in the temperature of the vacuum system or a leak.
Similarly, most instruments have status indicators, either for the system as a whole or for sub-systems or
modules. Examples of such indicators include water flow rate, data system communications status and electrical
power. These can be visible as part of the instrument hardware itself or on screen through an instrument
control (data acquisition) system. Such indicators again shall be carefully monitored, along with any measured
values that are reported.
4 Initial instrument calibration, alignment and assessment
Before undertaking the procedure described in the following clauses, it is essential that the instrument be
calibrated and aligned to an optimum performance level. This is achieved by following the relevant International
Standards (References [5] to [9]) or the manufacturer’s instructions. Choose the two settings of the instrument
operating conditions that are needed to obtain survey spectra and high-resolution spectra. These should be
settings that you regularly use and will be described in Clause 6. Since this is a rapid check, only these two
settings are chosen, but these can show faults that are common to all settings. These settings shall always
be used in future checks unless they are later found to be less effective than other settings. If the designated
settings are changed, data at both the new and old settings shall be recorded at the time of change.
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ISO 16129:2012(E)
5 Test specimen selection
5.1 General information
Three types of test specimen are required for the full procedure described in this International Standard: a
conductive specimen, a non-conductive specimen and a specimen suitable for assessing the quality of the
X-ray beam (e.g. X-ray spot size, shape and uniformity). The conductive specimen provides information that
the basic energy and intensity calibrations of the instrument are consistent from day to day. When using a non-
monochromatic X-ray source having more than one anode material, the survey spectrum shall be examined to
check for peaks arising from specimen irradiation from the anode that is not currently being used (this is due
to “cross-talk” within the X-ray source). Similarly, when anode coatings wear, X-rays can be emitted from the
substrate material, which is often copper. The survey spectrum shall be examined for peaks arising from X-rays
produced from the substrate material. When using a magnesium anode, there can be peaks due to radiation
from O Kα arising from oxidation of the anode coating. These peaks are often called ghost peaks. Table 1
shows the approximate positions of commonly encountered ghost peaks when acquiring an XPS spectrum
from silver using Mg Kα radiation.
Table 1 — Examples of approximate positions, on the binding-energy scale, of frequently
encountered ghost peaks in a silver spectrum collected using a magnesium anode
(The photon energy of Mg Kα radiation is 1 253,6 eV)
Peak position on the
Radiation giving rise to Photon energy Possible origin of
binding-energy scale
ghost peaks eV radiation
eV
Al Kα 1 486,6 From second anode in a 135
twin anode source
O Kα 524,9 From oxide on the surface 1 097
of the magnesium anode.
Cu Lα 929,7 From anode substrate 692
The non-conductive specimen is required to confirm that the charge compensation system is operating
satisfactorily when non-conducting specimens are being analysed. The nature of the specimen that is required
for assessing the X-ray beam depends upon the type of instrument being used. If the analysis position is visible
during the normal operation of the instrument, the quality of the focus and the alignment of the X-ray beam from
a monochromated source can be assessed using a phosphor specimen. If the analysis position is not visible in
normal operation and the instrument is capable of imaging, a uniform (there should be no features visible in an
image of the specimen when the instrument is in its optimum condition), conductive specimen can be used; this
can be the conductive specimen mentioned above. All specimens used shall be large enough to completely fill
the defined analysis region of the spectrometer.
5.2 The conductive specimen
A suitable conductive specimen shall be selected. This should be a material that produces several peaks in the
photoelectron spectrum. Preferably, these peaks should be widely spaced in binding energy. The specimen
shall be one whose surface can be cleaned easily by sputtering with noble-gas ions.
A pure (≥ 99,8 %) specimen of silver foil is suitable for this measurement and is recommended. If, however, a
different material is commonly analysed in the user’s laboratory and conforms to the above criteria, this may
be used instead. For convenience, it is assumed here that silver has been selected as the conductive reference
material. The same conductive specimen shall be used for all measurements.
5.3 The non-conductive specimen
Non-conductive specimens, and conductive specimens with a non-conductive surface layer, charge under the
X-ray flux, resulting in shifts in the peak binding energies relative to the uncharged state.
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ISO 16129:2012(E)
Select a non-conductive material often studied in your laboratory, for which you have a good stock and which
is capable of being maintained with a surface in a reproducible state. If you are unsure of a material to use,
examples that have been found to be useful for tests of instrumental performance are shown in Table 2.
Examples are given with different forms. You might wish to select a specimen with a form similar to that of the
specimens most commonly analysed with the instrument. Care shall be taken to select a material which does
not exhibit degradation under the X-ray beam during the analysis.
Table 2 — Examples of non-conductive specimens that may be used for this procedure
Material Form Cleaning
PET Sheet or fibre Not required
Laboratory filter
Sheet Not required
paper
Poly(ethylene terephthalate) (PET) has long been used to evaluate both the energy resolution and the
effectiveness of charge control in XPS. It shows a structure of three C 1s peaks together with shake-up
intensity. The minimum between the peak at the lowest binding energy and the adjacent peak at a separation
of ~1,5 eV is highly sensitive to the combination of the instrumental resolution and the uniformity of the charge
correction. The ease of achieving suitable and consistent energy resolution will depend on both the operator
and the instrumental capability.
An example of the C 1s spectrum from PET is shown in Figure 1.
Y
9 000
8 000
7 000
6 000
5 000
4 000
3 000
2 000
1 000
0
X 294 292 290 288 286 284 282 280
Key
X binding energy (eV)
Y number of counts
Figure 1 — Example of a C 1s spectrum from PET
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ISO 16129:2012(E)
More recently, careful studies on cellulose-based materials (paper) have indicated that these are suitable
1)
materials for use in this test (see References [1] and [2]). It has been shown that laboratory filter paper
provides reproducible XPS spectra. If using paper, avoid releasing loose fibres into the instrument as they can
cause a deterioration in performance.
Alternatively, the specimen can be one with which the user is familiar. It shall be a material that provides a
reliably reproducible spectrum with little or no specimen preparation.
5.4 Specimen for assessing the X-ray source
A phosphor specimen is suitable for those instruments that are fitted with a monochromatic X-ray source and
in which the analysis position is visible, preferably with a microscope and camera. The phosphor specimen
should be as flat and uniform as possible and produce easily visible light under the X-ray bombardment.
Not all phosphor specimens are vacuum-compatible and not all phosphor materials are designed for optimum
excitation by X-rays. Ensure that a phosphor specimen of suitable quality is obtained from the supplier of the
2)
instrument or from another reputable supplier.
The uniform conductive specimen shall be used if an optical image of the analysis position is not available during
the normal operation of the instrument and the instrument is capable of producing X-ray-induced images. The
specimen shall be flat and of uniform composition over an area greater than the maximum area that is imaged
in the instrument. The silver specimen described in 5.2 would be suitable for this purpose.
6 Collection of reference data
6.1 General information
Once fully calibrated and functioning correctly at the designated settings described in Clause 4, the instrument
shall be used to collect a set of reference data. The specimens and the way in which data shall be collected are
described in this clause. If the spectrometer is routinely used for conductive specimens, follow the procedure
described in 6.2 regularly and that in 6.3 occasionally, whereas for instruments used mainly for insulating
specimens follow the procedure described in 6.3 regularly and that in 6.2 occasionally. The interpretation of
“regular” and “occasional” depends on the use of the instrument and its behaviour. If data are critical, “regular”
might need to be interpreted as daily. For modern instruments used daily in normal use, “regular” may be
interpreted as “an interval of one week” and “occasionally” as “a period of one month”, but it must be stressed
that the appropriate interval does depend on the instrument and its intended use. Document your reasons for
your choice of the intervals. The control charts of Clause 9 will also be useful in determining the frequency at
which checks should be made.
All electron optics have an optimum focal point. The specimens shall be at the common focal point of the
analyser, the ion gun (if used), the neutralizer (if used) and the X-ray source. If the spectrometer is equipped
with an X-ray source whose height above the specimen is adjustable, the X-ray source shall be in the same
position for each test.
6.2 Rapid test of the instrument using a conductive specimen
6.2.1 Specimen mounting and pre-treatment
Mount the specimen on an appropriate specimen holder. It is important that the specimen be in good electrical
contact with the specimen holder. Place the specimen in the optimum analysis position in the spectrometer.
1) A suitable type of filter paper is S&S 589 Blue Ribbon Ashless, which can be obtained from Whatman plc, Springfield Mill,
James Whatman Way, Maidstone, Kent, ME14 2LE, UK, or one of this company’s international distributors. This information
is given for the convenience of users of this International Standard and does not constitute an endorsement by ISO of this
product.
2) Suitable phosphor specimens can be obtained from a number of suppliers, including TMS Vacuum Components,
Unit 21 Stirling Road, Castleham Industrial Estate, Hastings, East Sussex, TN38 9NP, UK. This information is given for the
convenience of users of this International Standard and does not constitute an endorsement by ISO of this supplier.
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ISO 16129:2012(E)
Take care to follow the manufacturer’s recommendations for positioning the specimen at the focal point, or use
whatever documented procedure is required for analysis. Ensure that all specimen stage parameters (X, Y, Z,
rotation and tilt) are correctly set.
If the instrument is fitted with a noble-gas ion gun, the specimen surface shall be cleaned by ion etching,
using conditions commonly found to be successful for this purpose and which remove a minimum amount of
2
material. A typical sputtering fluence for cleaning silver is 20 A⋅s/m . A typical value for the ion energy is 3 keV.
Excessive sputtering leads to significant roughening of the specimen so that it needs frequent replacement.
6.2.2 Survey spectrum
A survey or wide-scan spectrum shall be collected over the binding-energy range from −10 eV to a value within
50 eV of the value of the photon energy used to irradiate the specimen (e.g. a binding energy of 1 437 eV
for Al Kα X-rays) or to whatever maximum range is available. The analyser shall be operated in the constant
analyser energy (CAE) mode. The data shall be collected in energy steps no wider than 0,5 eV. The dwell time
5
at each energy step shall be sufficient to collect at least 10 counts in at least one energy channel in an XPS
peak. In the case of silver, this will be the Ag 3d peak at ~368 eV binding energy. Bear in mind that it is the
5/2
number of counts that is important here, not the count rate. The acquisition time depends upon the sensitivity of
5
the instrument, so the minimum dwell time per channel is given by 10 counts divided by the signal strength in
counts per second. On modern instruments, such a spectrum will be collected in a few minutes. Care shall be
taken to avoid selecting conditions that would result in count rates that are high enough to induce nonlinearity
in the detector.
The pass energy of the analyser used for this spectrum should be the same as that typically used for a survey
spectrum. If using monochromatic Al Kα X-rays, it is likely that the pass energy chosen would result in a full
width at half-maximum (FWHM) amplitude of between 1,5 eV and 2,5 eV for the Ag 3d peak from a clean
5/2
silver foil. If using non-monochromatic Al or Mg Kα X-rays, it is likely that the pass energy chosen would result
in an FWHM of between 1,5 eV and 3,0 eV.
The anode power in the X-ray source should be the same as that typically used for analysis.
Once collected, the spectrum shall be examined, first to ensure that there is a tolerably low level of detectable
contamination present on the surface. The presence of a significant quantity of adventitious carbon indicates
that the surface is contaminated. The height of the peak due to C 1s should be less than 1 % of the height of
the peak due to Ag 3d . If the carbon peak is too intense, the cleaning procedure shall be repeated and the
5/2
spectrum collected again. Such a spectrum is shown in Figure 2. In this spectrum, the FWHM of the Ag 3d
5/2
peak is 2,4 eV.
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ISO 16129:2012(E)
Y
140
120
100
80
60
40
20
0
X 1 390 1 190 990 790 590 390 190 -10
Key
X binding energy (eV)
Y kcounts
Figure 2 — An example of a survey spectrum from a clean silver specimen to be used as a reference
(The spectrum was collected using Al Kα radiaton from an X-ray monochromator)
The survey spectrum shall be examined and the following data recorded to provide reference values for
control charts:
a) The number of counts in the channel having the greatest number of counts.
b) The energy position of the channel having the greatest number of counts on the binding-energy scale.
c) The FWHM amplitude of the Ag 3d peak.
5/2
d) The average number of counts per channel in a background region of the spectrum. For example, the
range could be from 450 eV to 460 eV. This range includes the inelastic tail from the Ag 3d peaks and so
the background is relatively high, allowing greater precision in the measurement of the number of counts.
e) The total number of counts in all channels in the binding-energy range from –5 eV to –10 eV.
f) The intensity of any ghost peaks present. This is only required if a non-monochromatic X-ray source
is being used.
It is not necessary to include the intensity of the C 1s peak in a control chart, but the height of the C 1s peak above its
local background shall be noted as it provides a record of the state of the specimen at the time of the measurement.
When measuring the background intensity from a spectrum produced using a non-monochromatic source,
care shall be taken to avoid those areas of the spectrum in which ghost or satellite peaks might be present.
6.2.3 High-resolution spectrum
A high-resolution or narrow-scan spectrum shall then be recorded of the Ag 3d peak. At a minimum, this
5/2
spectrum shall be recorded over the binding-energy range from 365 eV to 372 eV. The spectrum shall be
collected with an energy step size not larger than 0,05 eV and the dwell time shall be such that there are at
4
least 10 counts in the channel containing the greatest number of counts. The acquisition time will depend
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ISO 16129:2012(E)
4
upon the sensitivity of the instrument, so the minimum dwell time per channel is given by 10 counts divided by
the signal strength in counts per second. For this spectrum, the pass energy of the analyser should be set to a
value normally used by the analyst for the collection of high-resolution spectra. It is anticipated that the FWHM
of the Ag 3d peak will be in the range 0,55 eV to 0,65 eV if an X-ray monochromator is used and between
5/2
0,8 eV to 1,0 eV if a monochromator is not used. Figure 3 shows an example of such a spectrum.
Y
160
140
120
100
80
60
40
20
0
X 371 370 369 368 367 366 365
Key
X binding energy (eV)
Y kcounts
Figure 3 — High-resolution Ag 3d spectrum from clean silver using Al Kα radiaton from an X‑ray
5/2
monochromator
Once collected, the spectrum shall be examined and the following parameters recorded:
a) The maximum peak height after subtraction of a suitable background (often a Shirley background).
b) The ratio of the intensity of this peak to the intensity of the same peak in the survey spectrum.
c) The peak energy in the binding-energy scale (peak fitting could be used to determine this parameter).
d) The FWHM of the peak above the background chosen.
e) The signal-to-background ratio, defined as the number of counts in the peak maximum divided by the
average number of counts in a range of channels remote from the peak. In Figure 3, a suitable range would
be those channels between 365,5 eV and 366,5 eV binding energy.
These measurements will be used as reference values in control charts
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ISO 16129:2012(E)
6.3 Rapid test of the instrument using a non-conductive specimen
6.3.1 Specimen mounting and positioning
Mount and position the specimen in the spectrometer as described for the conductive specimen in 6.2.1. If
cleaning with an ion beam is appropriate for the specimen type chosen, sputter the specimen using conditions
that have been proved to be effective.
6.3.2 High-resolution spectrum
A high-resolution spectrum shall be recorded over a representative peak in this spectrum. If the specimen
is organic in nature, this is likely to be the C 1s peak. The scan range for the spectrum shall be
...