ISO 17862:2013
(Main)Surface chemical analysis — Secondary ion mass spectrometry — Linearity of intensity scale in single ion counting time-of-flight mass analysers
Surface chemical analysis — Secondary ion mass spectrometry — Linearity of intensity scale in single ion counting time-of-flight mass analysers
ISO 17862:2014 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 time-to-digital converter (TDC) detection system caused by secondary ions arriving during its dead-time. The correction can increase the intensity range for 95 % linearity by a factor of up to more than 50 so that a higher maximum count rate can be employed for those spectrometers for which the relevant correction formulae have been shown to be valid. ISO 17862:2014 can also be used to confirm the validity of instruments in which the dead-time correction is already made but in which further increases can or cannot be possible.
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
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 17862
First edition
2013-12-15
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 17862:2013(E)
©
ISO 2013
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ISO 17862:2013(E)
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ISO 17862:2013(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Symbols and abbreviations . 1
2.1 Abbreviated terms . 1
2.2 Symbols . 1
3 Outline of method . 2
4 Procedure for evaluating the intensity linearity . 5
4.1 Obtaining the reference sample . 5
4.2 Preparation for mounting the sample . 5
4.3 Mounting the sample . 5
4.4 Operating the instrument . 6
4.5 Acquiring the data . 8
4.6 Checking the linearity .12
5 Interval for repeat measurements .17
Annex A (normative) Computation of raster size, ion beam current, number of frames for analysis,
and counts per pulse .18
Annex B (normative) Charge compensation setting .20
Annex C (normative) Ion detector setting .21
Annex D (informative) Instrumental factors affecting linearity .23
Bibliography .25
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ISO 17862:2013(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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The committee responsible for this document is ISO/TC 201, Surface chemical analysis, Subcommittee
SC 6, Secondary ion mass spectrometry.
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ISO 17862:2013(E)
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
[1]
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 can or cannot already be available in the instrument’s data capture
or processing computer. In this International Standard, 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 International Standard 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
exacerbated by unwanted background in the spectra.
This International Standard 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.
M
This upper limit 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 [1]. The results from applying this International Standard 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 International Standard 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 six
monthly intervals.
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INTERNATIONAL STANDARD ISO 17862:2013(E)
Surface chemical analysis — Secondary ion mass
spectrometry — Linearity of intensity scale in single ion
counting time-of-flight mass analysers
1 Scope
This International Standard 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
time-to-digital converter (TDC) detection system caused by secondary ions arriving during its dead-
time. The correction can increase the intensity range for 95 % linearity by a factor of up to more than 50
so that a higher maximum count rate can be employed for those spectrometers for which the relevant
correction formulae have been shown to be valid. This International Standard can also be used to
confirm the validity of instruments in which the dead-time correction is already made but in which
further increases can or cannot be possible.
2 Symbols and abbreviations
2.1 Abbreviated terms
For the purposes of this International Standard, the following abbreviations are used:
MCP microchannel plate
PTFE poly(tetrafluoroethylene)
SIMS secondary ion mass spectrometry
TDC time-to-digital converter
TOF time-of-flight
2.2 Symbols
The term intensity is used below and elsewhere. This refers to a measurement of peak area in the
spectrum.
c
measured counts per pulse intensity
M
c corrected counts per pulse intensity
P
F shorthand for F (i,j)
M M
12 + 13 12 +
F (i,j) ratio of measured intensities for the ith C F and C C F secondary ions in Table 1
M x y x-1 y
F shorthand for F (i,j)
P P
12 + 13 12 +
F (i,j) ratio of corrected intensities for the ith C F and C C F secondary ions in Table 1
P x y x-1 y
i index number for ion pair listed in Table 1
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ISO 17862:2013(E)
i highest primary ion current used for the saturation analysis
max
I integrated measured secondary ion intensity of a specified SIMS peak
M
I (X) integrated measured secondary ion intensity of the SIMS fragment X
M
I integrated corrected secondary ion intensity of a specified SIMS peak
P
I (X) integrated corrected secondary ion intensity of the SIMS fragment X
P
j index number for spectrum in the measurement series
k index number for setting the different primary ion currents
L shorthand for L (i,j)
P P
L (i,j) ratio of F (i,j) to the product α(i) and β(i)
P P
L shorthand for L (i,j)
M M
L (i,j) ratio of F (i,j) to the product α(i) and β(i)
M M
T
theoretical ratio of measured and corrected intensities per pulse
L
M
n number of raster frames used to generate each SIMS intensity
N total number of primary pulses used to generate the SIMS spectrum
R length of the raster side used to generate each SIMS intensity
V mass analyser desired energy acceptance, in eV
E
V mass analyser reflector voltage referred to the sample potential
R
V mass analyser reflector voltage referred to the sample potential for a secondary ion intensity
T
to fall to half the maximum intensity
12 + 13 12 +
α(i) expected isotope ratio of the ith C F and C C F secondary ions in Table 1
x y x-1 y
β(i) scaling factor to correct α(i) for the measured data, found by fitting
τ detection system dead-time
13 12
x number of C or C atoms in the characteristic PTFE secondary ion
y number of F atoms in the characteristic PTFE secondary ion
3 Outline of method
The method is outlined by the flow chart shown in Figure 1. In this method, secondary ion spectra
are measured for PTFE tape analysed in the “as received” condition with no in-house cleaning and no
further sample preparation as described from 4.1 to 4.3. The analytical conditions are chosen by the
analyst as described in 4.4 to provide secondary ion intensities per pulse in the linear and nonlinear
ranges of detector ion counting. This is established using 16 test spectra for a test sample to define
the correct range of primary ion beam current settings and 16 data spectra are then measured for the
analysis sample to provide data for the linearity establishment. In order to ensure that the instrument is
operating in the best condition for linearity, considerations for setting the ion beam, the mass analyser,
the charge compensation, and the ion detection system are described from 4.5.2 to 4.5.5. PTFE is a bulk
insulator and requires charge neutralization.
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ISO 17862:2013(E)
The spectrometer should be operated under conditions that give the most stable performance. It is
[2]
recommended that analysts use ISO 23830 to confirm the repeatability of their instrument. The
protocol described in this International Standard is closely aligned with that in ISO 23830 and those
using that International Standard are already familiar with much of the procedure given here.
The acquisition of data is described in 4.5 and details of the peaks to be measured are given in Table 1.
The behaviour expected is described in 4.6 with relevant formulae. If the linearity is adequate, either
for the data directly or for the data after correction using the instrument’s data capture computer,
the work is complete until, through changes to the instrument or the passage of time, a repeat of this
International Standard is required. If the linearity is inadequate, and if the instrument follows the
predicted behaviour, a correction can be made as described in 4.6.5 which can extend the linear range
by a factor of more than 50. The work is now complete until, through changes to the instrument or the
passage of time, a repeat of this International Standard is required. Finally, if the linearity is inadequate
and if the instrument does not follow the predicted behaviour, annexes are provided to indicate how to
improve the matters. These can improve the linearity range to the extent that this is possible with the
equipment being used. This can or cannot be the full range expected for ideal equipment but should lead
to some improvement.
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ISO 17862:2013(E)
start
4.1 Obtaining the reference sample
4.2 Preparation for mounting the sample
4.3 Mounting the sample
4.4 Operating the instrument
4.4.2 Setting the ion beam
4.4.3 Setting the mass analyser
4.4.4 Setting the charge compensation
4.4.5 Setting the ion detector
4.5 Acquiring the data
4.6.4 Assessing the linear region without and with intensity
Maximum c adequate?
M
Yes
No
4.6.5 Correcting the intensity and checking
Annex D Factors
Linearity adequate?
affecting linearity
No
Yes
4.6.5.1 Correct the intensity
5 Interval for repeat measurements
NOTE The numbers refer to the relevant subclauses.
Figure 1 — Flow chart of the work
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ISO 17862:2013(E)
4 Procedure for evaluating the intensity linearity
4.1 Obtaining the reference sample
For the calibration of static SIMS, spectrometers obtain a new reel of PTFE tape of the type used for
domestic plumbing. Label and keep this reel with your reference samples.
NOTE The PTFE is usually in the form of a reel of tape of length 12 m, width 12 mm, and approximately
0,075 mm thick and is often sold for domestic plumbing.
4.2 Preparation for mounting the sample
Samples shall only be handled with clean, uncoated stainless steel tweezers held using powderless
polyethylene gloves. Vinyl gloves, often used in clean rooms, are coated with a release agent from the
moulding process and shall not be used. The release agent is very mobile and quickly contaminates the
samples. This leads to poor measurement repeatability and poor quality data.
12 13
NOTE This International Standard uses the intensity ratio of natural C and C isotopes to determine the
linearity. For the isotope ratio method to be successful, it is important that the isotope peaks can be measured
without any significant background from peak interferences. Since most TOF-SIMS instruments do not have
13 12
sufficient resolution to completely separate between a fragment with C and the peak interference with CH,
it is important to have no hydrogen in the reference material and a low surface energy so that hydrocarbon
contamination is minimized. PTFE has both of these crucial attributes and, importantly, is very easy to obtain
and use.
In selecting gloves, care should be taken to avoid those with talc, silicone compounds, or similar
contaminants. “Powder-free” gloves have no talc. Coated stainless steel or other tweezers can cause
unwanted contamination.
4.3 Mounting the sample
4.3.1 To manipulate the samples, the gloves are used to hold the tweezers and not the sample. Avoid
any wiping materials, sometimes used to handle samples, as they can result in unwanted contamination
of the sample surface. Unnecessary contact of the sample with the gloves shall be avoided. Sample mounts
and other materials used to hold samples shall be cleaned regularly whenever there is a possibility of
cross-contamination of samples. The use of tapes containing silicones and other mobile species shall be
[3]
avoided.
4.3.2 Remove and discard the first 20 cm of the material from the reel obtained in 4.1 and then cut
appropriately sized samples from the subsequent material with clean scissors. As the reel is unwound,
a fresh surface of PTFE is exposed and it is this surface that is analysed. Do not clean the sample. Mount
samples on the sample holder to produce a flat, even surface using a mechanical clamping or fixing
method. Do not use adhesive tape. Ensure that the reverse side of the sample is against a conducting
surface, electrically connected to the sample holder. The PTFE shall not be placed over a hole.
NOTE 1 Common mounting systems include metal plates with holes of various sizes and metal grids. The grid
often helps if severe charging is experienced.
NOTE 2 The presence of a hole under the sample leads to poor mass resolution and repeatability in systems
that use high extraction fields such as time-of-flight and magnetic sector systems.
A repeat of this International Standard is required later in Clause 5. For this, a fresh sample is required
and for consistency, the sample should be from the same reel.
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ISO 17862:2013(E)
4.4 Operating the instrument
4.4.1 General
Operate the instrument in accordance with the manufacturer’s or local documented instructions.
The instrument shall have fully cooled following any bakeout. Ensure that the operation is within the
manufacturer’s recommended ranges for ion beam current, counting rates, spectrometer scan rate,
and any other parameter specified by the manufacturer. Check that the detector multiplier settings
are correctly adjusted (see 4.4.5). In some instruments, the issue of count rate nonlinearity is dealt
with by providing the user with a warning that a certain count rate should not be exceeded. This can
limit the count rate to below, say 0,1 counts per pulse. It can be possible to correct the nonlinearity to
significantly higher count rates using the present procedure such that a much higher dynamic range
is possible, enabling the work of a higher quality to be achieved in a shorter analytical time. Doing so
can require this warning to be ignored. If the warning is ignored, ensure by checking in the instrument
manual or by contact with the manufacturer that it is safe to do this.
4.4.2 Setting the ion beam
4.4.2.1 In this International Standard, the primary ion current is to be varied to provide secondary
ion intensities per primary ion pulse that range from the linear regime to highly nonlinear regime. A
12 13
measurement of the ion beam current is not required since C and C isotope ratios are used. If many
12 +
ion sources are available, use the one generating the highest intensities for the CF peak from PTFE. It
3
does not matter if it is an atomic primary ion or a cluster primary ion, so long as the beam current can be
adjusted to give a large range of secondary ion intensities and detector saturation as discussed in 4.5.4.
4.4.2.2 In setting the primary ion beam to provide a range of currents on the target sample, it is
important that the following conditions are satisfied.
(a) The pulse width and peak shape do not change drastically at different current settings.
(b) The peak width is much smaller than the dead-time of the detector (determined following 4.5.10 but
is usually –50 ns).
(c) The selected C F peaks (Table 1) do not suffer from mass interference.
x y
The primary ion current can be easily adjusted by altering the alignment of the ion beam through the
internal apertures of the ion beam column using deflection voltages. Other methods can cause changes
to the pulse timing or mass calibration.
4.4.2.3 The ion beam should be centred in the acceptance area of the mass spectrometer as well as
possible. To do this, first, centre all the alignments for the mass analyser and then increase the ion beam
raster area to image the entire mass analyser acceptance area. Using the ion beam controls, centre the
mass analyser acceptance area in the imaged area. In some cases, the maximum field of view cannot
be large enough to observe the acceptance area. If the software restricts it, it can be possible to change
the calibration scaling or “sensitivity” to access a larger raster area and then return to the calibrated
conditions after centring.
16 2
4.4.2.4 A maximum ion fluence of 1 × 10 ions/m is recommended for each measurement. A typical
ion beam raster area for this work is 200 μm × 200 μm but can be increased to 400 μm × 400 μm to satisfy
the fluence requirements given generically by Formula (A.2) with illustrative numbers and where R is
the length of the raster side. For example, a 0,5 pA pulsed beam and a 200 μm square raster requires
128 s acquisition time. This beam would need to be defocused to a diameter greater than 3,1 μm for a
128 × 128 pixel display. If it can only be defocused to 1 μm, a 256 × 256 pixel display shall be used or the
local fluence maximum on a pixel will be exceeded by more than a factor of 2. During a digital raster scan,
an over-focused ion beam will drill a matrix of small holes. For this reason, a large diameter defocused
beam is required. The precise minimum beam size depends on the instrument used but can be evaluated
using Formula (A.1).
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ISO 17862:2013(E)
4.4.2.5 The number of frames acquired, n, should be kept above 20, as described in Formula (A.4), so
that the final frame, which most likely will not be a full frame, represents only a small fraction of the data.
It is recommended that a random raster pattern is used to minimize local sample charging, if available.
4.4.2.6 For good statistics, the integrated spectrum over the whole analysis area is required.
4.4.3 Setting the mass analyser
4.4.3.1 Choose the spectrometer operating settings which provide high intensities and for which the
linearity is to be determined. Select settings to measure positive secondary ions.
NOTE The consistency of the intensities varies with the combination of settings used. In general, the
repeatability is best when using an energy acceptance of the mass analyser of 50 eV or more.
4.4.3.2 In general, the mass analyser should be operated in the condition that gives the most stable
and repeatable performance with good, high mass sensitivity. A suggested method for analyser deflector
alignment, if required, is as follows. With the primary ion beam centred in the analysis area on the PTFE
sample, reduce the raster area to 50 μm × 50 μm. The analyser deflectors should now be systematically
adjusted, one set at a time, to maximize the mass resolution. This alignment process will need to be
iterated by the number of analyser deflector pairs. If the analysis area moves over the sample surface
during this process, re-centre the image using the primary ion beam controls.
4.4.3.3 For reflection instruments, a systematic method for determining the surface potential, in order
+
to set the reflector voltage, is required. The effect of reflector voltage on the peak intensity of the CF ion
2
from PTFE is shown in Figure 2. As the reflector voltage is made more positive, the peaks move down the
apparent mass scale, because the ions are reflected earlier and their flight time is reduced. The reflector
potential at which the ion signal begins to increase rapidly is approximately equal to the sample potential.
In the example shown in Figure 2, the sample potential is approximately −79 V and depends on the sample
thickness and dielectric constant. As the reflector voltage is increased positively, more of the secondary
ion energy distribution is reflected to the detector and the signal rises to a plateau. The reflector voltage
that gives an ion signal of half the maximum peak intensity, V , is accurately and quickly measured. The
T
operating reflector voltage, V , is then set in a reproducible way by adding a further V volts, where V is
R E E
the desired energ
...
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