ISO 21270:2004
(Main)Surface chemical analysis — X-ray photoelectron and Auger electron spectrometers — Linearity of intensity scale
Surface chemical analysis — X-ray photoelectron and Auger electron spectrometers — Linearity of intensity scale
ISO 21270:2004 specifies two methods for determining the maximum count rate for an acceptable limit of divergence from linearity of the intensity scale of Auger and X-ray photoelectron spectrometers. It also includes methods to correct for intensity non-linearities so that a higher maximum count rate can be employed for those spectrometers for which the relevant correction equations have been shown to be valid.
Analyse chimique des surfaces — Spectromètres de photoélectrons X et d'électrons Auger — Linéarité de l'échelle d'intensité
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 21270
First edition
2004-06-01
Surface chemical analysis — X-ray
photoelectron and Auger electron
spectrometers — Linearity of intensity
scale
Analyse chimique des surfaces — Spectromètres de photoélectrons X
et d'électrons Auger — Linéarité de l'échelle d'intensité
Reference number
ISO 21270:2004(E)
©
ISO 2004
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ISO 21270:2004(E)
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ISO 21270:2004(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative reference . 1
3 Symbols . 1
4 Outline of the methods. 2
5 When to use this International Standard. 2
6 Procedure for evaluating the intensity linearity .2
6.1 The samples . 2
6.2 Preparing the copper sample . 3
6.3 Preparing the stainless-steel sample or sample holder . 3
6.4 Choosing the spectrometer settings for which the intensity linearity measurement is
required. 3
6.5 Operating the instrument . 3
6.6 Measurement of the intensity scale linearity by varying the source flux . 4
6.7 Determination of the intensity scale linearity by varying the source flux . 4
6.8 Measurement of the intensity scale linearity in XPS using the spectrum ratio method for
systems with two or more but less than 30 X-ray source emission current settings . 6
6.9 Determination of the intensity scale linearity in XPS using the spectrum ratio method for
systems with two or more but less than 30 X-ray source emission current settings . 7
6.10 Completing the analysis. 9
Annex A (informative) Example results of linearity measurements using the spectrum ratio
method (the second method). 10
Bibliography . 13
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ISO 21270:2004(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 21270 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 7, X-ray photoelectron spectroscopy.
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ISO 21270:2004(E)
Introduction
Quantitative analysis of materials at surfaces by Auger electron spectroscopy (AES) or X-ray photoelectron
spectroscopy (XPS), requires measurements of the spectral intensities. Non-linearities in the respective
instrument intensity scales, unless corrected, lead directly to errors in the amounts of material determined. In
general, intensity scales are linear at very low count rates but become progressively non-linear as the count
rates rise. Measurements of intensity rely on the measurement system delivering an intensity signal which is
fixed in proportion to the intensity being measured. In counting systems, this proportionality is expected to be
unity. If this proportionality varies with the signal level or counting rate, the measurement system is said to be
non-linear. It is rare for non-linearities below 1 % to be treated as significant. The intensity scale non-linearity
[1,2]
may exceed 1 % for count rates which exceed 5 % of the maximum permissible count rate . For many
instruments, the non-linearity behaviour will not vary significantly from month to month, provided the detection
system is correctly set. For these instruments, the count rate may be corrected, using the relevant relationship,
so that the corrected intensity is then linear for a greatly extended fraction of the maximum obtainable count
rate. In this International Standard, two simple relationships are described, involving a parameter known as
the detector system dead time, to make this correction. For some instruments, the non-linearity may not be
predictable or described by any simple relationship. For these instruments, this International Standard allows
the extent of the non-linearity to be measured and a maximum count rate for an acceptable limit of divergence
from linearity to be defined. This limit of divergence from linearity is set by the user appropriately for the
analyses to be conducted.
In this International Standard, two methods for measuring the linearity are provided. The first is based on the
principle that the spectrometer output is proportional to the electron beam current in AES or the X-ray beam
[1]
flux in XPS . This is the simplest method and may be conducted in instruments where the beam current or
flux may be set at 30 or more approximately evenly spaced intervals up to the level required to generate the
maximum count rate for which this International Standard is to be used. In some XPS instruments, this is not
possible and the X-ray flux may only be set at one of two or more (but less than 30) pre-defined levels. For
[2]
these instruments, the first method cannot be realized and a second method is given .
This International Standard should be used when characterising a new spectrometer so that it may be
operated in an appropriate count rate range. It is repeated after any substantive modification to the detection
circuits, or after the multiplier voltage has been increased (since the previous test with this International
Standard) by one-third of the range of increase provided by the manufacturer, or after replacement of the
electron multiplier(s) or at intervals of approximately 12 months.
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INTERNATIONAL STANDARD ISO 21270:2004(E)
Surface chemical analysis — X-ray photoelectron and Auger
electron spectrometers — Linearity of intensity scale
1 Scope
This International Standard specifies two methods for determining the maximum count rate for an acceptable
limit of divergence from linearity of the intensity scale of Auger and X-ray photoelectron spectrometers. It also
includes methods to correct for intensity non-linearities so that a higher maximum count rate can be employed
for those spectrometers for which the relevant correction equations have been shown to be valid.
2 Normative reference
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 18115, Surface chemical analysis — Vocabulary
3 Symbols
E measured energy value for the Cu L VV peak
Cu 3
E energy value for the jth energy channel
j
I measure of the ith flux value of beam current in AES or X-ray anode emission current in XPS
i
k a constant
M (E ) corrected count rate for the high-intensity X-ray spectrum at energy E
H j j
M corrected count rate for the ith flux value
i
M (E ) corrected count rate for the low-intensity X-ray spectrum at energy E
L j j
N (E ) measured count rate for the high-intensity X-ray spectrum at energy E
H j j
N measured count rate for the ith flux value
i
N (E ) measured count rate for the low-intensity X-ray spectrum at energy E
L j j
N maximum count rate for which the system is to be used and for which the system remains within the
max
acceptable limits of divergence from linearity given by k(1 ± δ)
± δ fractional limits to the linearity
τ extended dead time
e
τ non-extended dead time
n
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ISO 21270:2004(E)
4 Outline of the methods
Two methods are available to evaluate the instrument intensity linearity. For AES instruments, and for XPS
instruments in which 30 or more approximately evenly spaced and known increments in the X-ray flux are
available, a general method is described. This is called the method of varying the source flux. For XPS
instruments with two or more but less than 30 settings available for the X-ray flux, this is not possible and a
second method is given. This second method is called the spectrum ratio method.
In the first method, the spectrometer must be fitted with an inert gas ion sputtering gun for cleaning samples.
The intensity scale linearity measurements are then conducted using a sputter cleaned pure copper sample.
In the second method, this sample or, if there is no ion gun, a stainless steel sample or sample holder is used.
The selection of these samples is described in 6.1 and their preparation in 6.2 and 6.3. Next, the spectrometer
settings are selected in 6.4 and the instrument operated as described in 6.5.
In the first method, described in 6.6, the spectrometer is set to detect the count rate at the Cu L VV Auger
3
electron peak. That count rate is then determined as a function of the electron beam current or as a function of
the X-ray flux for 30 or more approximately evenly spaced increments in the X-ray flux. From these data, as
described in 6.7, a plot of the quotient of the measured count rate and the beam current in AES, or of the
quotient of the measured count rate and the X-ray flux in XPS, versus the measured count rate allows the
linearity range and any relevant correction to be determined.
In the second method, described in 6.8 and to be used for those XPS instruments in which 30 settable values
of X-ray flux are not available, widescan spectra are recorded for a high and a low X-ray source emission
current. From these data, as described in 6.9, a plot of the quotient of the count rates of these two spectra, for
each energy channel, versus the count rate for that channel in the high emission current spectrum, allows the
linearity range and any relevant correction to the count rates to be determined.
Finally, 6.10 summarizes the data to be recorded.
5 When to use this International Standard
This International Standard shall be used when characterising a new spectrometer so that it may be operated
in an appropriate count rate range. It shall then be repeated after any substantive modification to the detection
circuits, after the multiplier voltage has been increased by one third of the range of increase (since the
previous test with this standard) provided by the manufacturer, after replacement of the electron multiplier(s)
or at intervals of approximately 12 months.
6 Procedure for evaluating the intensity linearity
6.1 The samples
For the method of varying the source flux, AES or XPS instruments may be used if they incorporate an inert
gas ion gun for cleaning samples. For this method, use a polycrystalline Cu sample of at least 99,8 % purity
and proceed to 6.2. The second method, the spectrum ratio method, is only applicable to XPS instruments.
For this second method, either use a polycrystalline Cu sample of at least 99,8 % purity or a stainless-steel
sample or sample holder. If the instrument does not incorporate an inert gas ion gun, the stainless-steel
sample or sample holder shall be used. Proceed to 6.3.
NOTE 1 For convenience, copper in the form of a foil typically of an area 10 mm by 10 mm, and 0,1 mm to 0,2 mm thick,
is used.
NOTE 2 If stainless steel is to be used, either a foil 10 mm by 10 mm, and 0,1 mm to 0,2 mm thick, or a sample holder,
or some other form may be chosen, as convenient.
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ISO 21270:2004(E)
6.2 Preparing the copper sample
6.2.1 If the sample appears to need cleaning, a short dip in 1 % nitric acid may be used followed by a rapid
rinse in distilled water.
6.2.2 Mount the sample on a sample holder using fixing screws or other metallic means to ensure electrical
contact. Do not use adhesive tape.
6.2.3 Achieve ultra-high vacuum and clean the sample by ion sputtering to reduce the oxygen and carbon
contamination until the heights of the respective signals are each less than 2 % of the height of the most
intense metal peak in a survey spectrum. Record a survey (widescan) spectrum to ensure that the only
significant peaks are those of Cu. The quality of vacuum necessary here is such that the oxygen and carbon
peak heights shall not exceed 3 % of the heights of the most intense metal peaks by the time you have
reached completion of the data acquisition in 6.6 or 6.8 or at the end of the working day (whichever is the
earlier).
NOTE 1 Inert gas ion sputtering conditions that have been found suitable for cleaning are 1 min of a 30 µA beam of
2
5 keV argon ions covering 1 cm of the sample.
NOTE 2 Example AES and XPS spectra for Cu may be found in bibliography references [3] to [6].
6.2.1 Try to conduct all relevant parts of this International Standard in one working day. If more than one
day is required, confirm the cleanness of the samples at the start of each day’s work. Now proceed to 6.4.
6.3 Preparing the stainless-steel sample or sample holder
6.3.1 Wash the stainless-steel sample or sample holder in distilled water and then in ethanol to remove
handling contaminants. If a sample is to be used, it is mounted on a sample holder using fixing screws or
other metallic means to ensure electrical contact. Adhesive tape shall not be used.
NOTE Example XPS spectra for air-contaminated stainless steel may be found in bibliography references [2], [7]
and [8].
6.3.2 Achieve the system working pressure and, if possible, leave the sample in the vacuum overnight to
allow the desorption of contaminants to stabilize.
6.4 Choosing the spectrometer settings for which the intensity linearity measurement is
required
Choose the spectrometer operating settings for which the intensity linearity measurement is required. The
procedure from 6.4 to 6.10 shall be repeated for each combination of spectrometer settings of pass energy,
retardation ratio, slits, lens settings etc for which a linearity measurement is required. Record the values of
these settings in the spectrometer log.
NOTE The designs of spectrometers and their circuits vary and a spectrometer linearity measurement for one
combination of lens settings, slits and pass energy will not necessarily be valid for any other setting of the lens, slits and
pass energy. Many spectroscopists make accurate measurements under one optimum set of conditions and then only that
set of analyser conditions needs a linearity assessment. Any result is only valid for the combination of settings used,
although the designs of some of the simpler instruments do lead to linearity results that are consistent for all settings.
6.5 Operating the instrument
6.5.1 Operate the instrument in accordance with the manufacturer’s or locally defined documented
instructions. The instrument shall have fully cooled following any bakeout. Ensure that the operation is within
the manufacturer’s recommended ranges for source power (for XPS), primary beam current (for AES),
counting rates, spectrometer scan rate and any other parameter specified by the manufacturer. Check that the
detector multiplier settings are correctly adjusted. For multidetector systems, ensure that any necessary
optimizations or checks described by the manufacturer are conducted prior to this assessment.
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ISO 21270:2004(E)
NOTE Incorrect detector settings may lead to an excessive non-linearity of the intensity scale, the behaviour of which
may vary from day to day and from sample to sample. It is particularly important that the multiplier voltage is correctly
[9] [10]
set as this voltage generally needs to be increased during the multiplier life . It is also important that the counting
electronics discriminator is correctly set.
6.5.2 For AES instruments, and for XPS instruments in which 30 or more approximately evenly spaced and
known increments in the X-ray flux are available, use the method of varying the source flux, described in 6.6.
For XPS instruments with two or more but less than 30 settings available for the X-ray flux, proceed directly to
the spectrum ratio method, described in 6.8.
6.6 Measurement of the intensity scale linearity by varying the source flux
6.6.1 Using the sputter-cleaned Cu sample, identify the Cu L VV Auger electron peak. Use a low beam
3
current or X-ray flux and measure and record the energy of the peak maximum, E , to a precision of 0,1 eV.
Cu
NOTE The kinetic energy of the Cu L VV peak for spectrometers with energy resolutions better than 1 eV is
3
[11,12]
918,69 eV when referenced to the Fermi level or approximately 914,2 eV when referenced to the vacuum level. The
binding energy equivalent values for XPS are 334,91 eV
...
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