ISO 17862:2022

INTERNATIONAL ISO
STANDARD 17862
Second edition
2022-09
Surface chemical analysis —
Secondary ion mass spectrometry —
Linearity of intensity scale in single
ion counting time-of-flight mass
analysers
Analyse chimique des surfaces — Spectrométrie de masse des ions
secondaires — Linéarité de l'échelle d'intensité des analyseurs de
masse à temps de vol pour comptage des ions individuels
Reference number
© ISO 2022
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Symbols . 1
3.3 Abbreviated terms . 3
4 Outline of method . 3
5 Procedure for evaluating the intensity linearity . 5
5.1 Obtaining the reference sample . 5
5.2 Preparation for mounting the sample . 5
5.3 Mounting the sample . 5
5.4 Operating the instrument . 5
5.4.1 General . 5
5.4.2 Setting the ion beam . 6
5.4.3 Setting the mass analyser . 6
5.5 Acquiring the data . 7
5.6 Checking the linearity . 10
5.6.1 The relation of corrected and measured counts . 10
5.6.2 The measured ratios for isotopes. 11
5.6.3 Fitting the data .12
5.6.4 Assessing the linear region without and with any instrumental intensity
correction .12
5.6.5 Correcting the intensity and checking the validity of any instrumental
correction . 13
6 Interval for repeat measurements .15
Annex A (normative) Computation of raster size, ion beam current, number of frames for
analysis and counts per pulse .16
Bibliography .18
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 6, Secondary ion mass spectrometry.
This second edition cancels and replaces the first edition (ISO 17862:2013), which has been technically
revised.
The main changes are as follows:
— the procedure has been simplified by removing the informative background (including Annexes B
to D);
— all figures have been fixed to adhere with ISO standards.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
For the quantitative analysis of materials using secondary ion mass spectrometry (SIMS), measurements
are made of the spectral intensities. Nonlinearity in the instrument intensity scale, unless corrected,
leads directly to errors in the relative amounts of material determined at surfaces and in-depth profiles.
In general, intensity scales are linear at very low count rates, or more correctly low counts per pulse,
but become progressively nonlinear as the count rates rise. Measurements of intensity rely on the
measurement system delivering an intensity signal 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 nonlinear. It is rare for nonlinearities
below 1 % to be treated as significant. The intensity scale nonlinearity can exceed 1 % for count rates
[2]
that exceed 5 % of the maximum permissible count rate . For many instruments, the nonlinearity
behaviour will not vary significantly from month-to-month, provided the detection system is correctly
set. For these instruments, the count rate can 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. This correction to the intensity scale might or might not already be available in the instrument's
data capture or processing computer. In this document, a simple test of linearity is provided for the
intensity lost in systems in which secondary ions arrive at a detector based on a microchannel plate or
scintillator and photomultiplier followed by a time-to-digital converter. If this test is shown to be valid,
a correction is provided that, for suitable instruments, can extend the intensity scale by up to a factor
of more than 50. For some instruments, the nonlinearity cannot be predictable nor described by any
simple relationship. For these instruments, this document allows the extent of the nonlinearity to be
measured and a maximum count rate for an acceptable limit of divergence from linearity to be defined.
In some cases, adjustments to the instrumental settings can improve the situation so that the required
correction is then valid. The limit of divergence from linearity is set by the user appropriately for the
analyses to be conducted.
Although there are a number of causes of nonlinearities in TOF-SIMS instrumentation, the most
significant is intensity saturation caused by the effective dead-time of the detector system. This arises
since only one secondary ion count per primary ion pulse can be detected within a dead-time interval
τ, regardless of the actual number of secondary ions arriving at the detector. Nonlinearity can also be
aggravated by unwanted background in the spectra.
This document provides, and can only provide, a correction to the dead-time nonlinearity for a somewhat
ideal situation and not for all cases. Nevertheless, the significantly enhanced dynamic range or rate of
working can be very important. Suggestions are included to optimize the instrument to provide the
best measurement capability and to diagnose simple instrumental defects such as detector faults, e.g.
a low detector efficiency or a detector not providing single ion counting. Then, a dead-time Poissonian
correction is established to correct the measured counts within certain limits set by the analyst. This
establishes an upper value for c , the count per pulse, either before or after correction. This upper limit
M
is generally applicable to peaks where the signal is constant with both time and spatial distribution,
where there is only one peak within the dead-time interval, and where the background intensities
are negligible (these conditions are not always satisfied in practice). This is explored and explained in
detail in Reference [2]. The results from applying this document relate to a “best-case scenario” and
the linearity achievable with Formula (1) can be lower in real cases where it is not practical to use a
wide peak integration limit of ± the dead-time. More advanced dead-time correction routines should be
sought in these cases and their effectiveness can be tested using the methodology here.
This document requires technical skills that may go beyond everyday operation and should be used
when characterizing a new spectrometer so that it can be operated in an appropriate intensity range. It
should then be repeated after any substantive modification to the detection circuits, after replacement
of the microchannel plate (MCP), or at approximately 1 year intervals.
v
INTERNATIONAL STANDARD ISO 17862:2022(E)
Surface chemical analysis — Secondary ion mass
spectrometry — Linearity of intensity scale in single ion
counting time-of-flight mass analysers
1 Scope
This document specifies a method for determining the maximum count rate for an acceptable limit of
divergence from linearity of the intensity scale in single ion counting time-of-flight (TOF) secondary
ion mass spectrometers using a test based on isotopic ratios in spectra from poly(tetrafluoroethylene)
(PTFE). It also includes a method to correct for intensity nonlinearity arising from intensity lost from
a microchannel plate (MCP) or scintillator and photomultiplier followed by a ti
...