ISO 13084:2011
(Main)Surface chemical analysis — Secondary-ion mass spectrometry — Calibration of the mass scale for a time-of-flight secondary-ion mass spectrometer
Surface chemical analysis — Secondary-ion mass spectrometry — Calibration of the mass scale for a time-of-flight secondary-ion mass spectrometer
ISO 13084:2011 specifies a method to optimize the mass calibration accuracy in time-of-flight SIMS instruments used for general analytical purposes. It is only applicable to time-of-flight instruments but is not restricted to any particular instrument design. Guidance is provided for some of the instrumental parameters that can be optimized using this procedure and the types of generic peaks suitable to calibrate the mass scale for optimum mass accuracy.
Analyse chimique des surfaces — Spectrométrie de masse des ions secondaires — Étalonnage de l'échelle de masse pour un spectromètre de masse des ions secondaires à temps de vol
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INTERNATIONAL ISO
STANDARD 13084
First edition
2011-05-15
Surface chemical analysis — Secondary-
ion mass spectrometry — Calibration of
the mass scale for a time-of-flight
secondary-ion mass spectrometer
Analyse chimique des surfaces — Spectrométrie de masse des ions
secondaires — Étalonnage de l'échelle de masse pour un spectromètre
de masse des ions secondaires à temps de vol
Reference number
ISO 13084:2011(E)
©
ISO 2011
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ISO 13084:2011(E)
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ii © ISO 2011 – All rights reserved
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ISO 13084:2011(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 13084 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 6, Secondary ion mass spectrometry.
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ISO 13084:2011(E)
Introduction
Secondary-ion mass spectrometry (SIMS) is a powerful technique for the analysis of organic and molecular
surfaces. Over the last decade, instrumentation has improved significantly so that modern instruments now
have very high repeatability and constancy (Reference [2] in the Bibliography). An increasing requirement is
for the identification of the chemical composition of complex molecules from accurate measurements of the
mass of the secondary ions. The relative mass accuracy to do this and to distinguish between molecules that
contain different chemical constituents, but are of the same nominal mass (rounded to the nearest integer
mass), is thus an important parameter. A relative mass accuracy of better than 10 ppm is required to
distinguish between C H (28,031 30 u) and Si (27,976 92 u) in a parent ion with total mass up to 1 000 u, and
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between CH (14,015 65 u) and N (14,003 07 u) in parent ions with total mass up to 300 u. However, in a
2
recent interlaboratory study (Reference [3] in the Bibliography), the average fractional mass accuracy was
found to be 150 ppm. This is significantly worse than is required for unambiguous identification of ions. A
detailed study (Reference [4] in the Bibliography) shows that the key factors degrading the accuracy include
the large kinetic energy distribution of secondary ions, non-optimized instrument parameters and extrapolation
of the mass scale calibration.
This International Standard describes a simple method, using locally sourced material, to optimize the
instrumental parameters, as well as a procedure to ensure that accurate calibration of the mass scale is
achieved within a selectable uncertainty.
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INTERNATIONAL STANDARD ISO 13084:2011(E)
Surface chemical analysis — Secondary-ion mass
spectrometry — Calibration of the mass scale for
a time-of-flight secondary-ion mass spectrometer
1 Scope
This International Standard specifies a method to optimize the mass calibration accuracy in time-of-flight SIMS
instruments used for general analytical purposes. It is only applicable to time-of-flight instruments but is not
restricted to any particular instrument design. Guidance is provided for some of the instrumental parameters
that can be optimized using this procedure and the types of generic peaks suitable to calibrate the mass scale
for optimum mass accuracy.
2 Symbols and abbreviated terms
2.1 Symbols
m mass of interest
m calibration mass 1
1
m calibration mass 2
2
∆M mass accuracy (u)
M measured peak mass (u)
P
M true mass (u)
T
U(m) mass uncertainty for a mass m, arising from calibration
U uncertainty in the accurate mass measurement of m
1 1
U uncertainty in the accurate mass measurement of m
2 2
U average uncertainty in an accurate mass measurement
0
V reflector or acceptance voltage (V)
R
W relative mass accuracy
x number of carbon atoms
y number of hydrogen atoms
σ(∆M) standard deviation of the mass accuracy for a number of peaks
+
σ average of the standard deviations of ∆M for each of the four C H cascades with 4, 6, 7 and 8
M x y
carbon atoms
2.2 Abbreviated terms
MEMS micro-electromechanical system
PC polycarbonate
ppm parts per million
r/min revolutions per minute
SIMS secondary-ion mass spectrometry
THF tetrahydrofuran
ToF time of flight
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ISO 13084:2011(E)
3 Outline of method
Here, the method is outlined so that the detailed procedure, given in Clause 4, may be understood in context.
Firstly, to optimize a time-of-flight mass spectrometer using this procedure, obtain a thin film of PC on a
conducting substrate (silicon). The optimization procedure is achieved by carrying out the procedures in 4.3 to
4.5 iteratively; it uses 19 specific C H peaks in the polycarbonate (PC) positive-ion mass spectrum. In 4.6, a
x y
general calibration procedure is given which provides the rules by which calibrations for inorganics and
organics may be incorporated. This leads to a new generic set of ions for mass calibration that can improve
the mass accuracy from some often used calibrations by a factor of 5. The effects of extrapolation beyond the
calibration range are discussed and a recommended procedure is given to ensure that accurate mass is
achieved, within a selectable uncertainty, for large molecules. Therefore, the procedure has two parts,
optimization and calibration. Subclauses 4.1 to 4.5 are only required as part of the regular maintenance of the
instrument as defined by the testing laboratory. Subclause 4.6 is required for all calibrations of the mass scale.
This is summarized in the flowchart in Figure 1.
START
Optimize
No
instrumental
parameters?
Yes
4.1/4.2 Obtaining/Preparing the
reference sample for optimization
4.3 Obtaining SIMS spectral data
4.4 Calculating mass accuracy
4.5 Optimizing instrumental parameters
4.6 Calibration procedure
for spectra
Figure 1 — Flowchart of sequence of operations of the method
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ISO 13084:2011(E)
4 Method for improving mass accuracy
4.1 Obtaining the reference sample for optimization
A sample of thin (10 to 100 nm) PC on a flat conducting substrate (e.g. silicon wafer) shall either be obtained
or prepared, as described at 4.2.
4.2 Preparation of polycarbonate sample
4.2.1 Instructions for the preparation of a PC reference sample are provided. This method can give
sample-to-sample repeatability in ToF SIMS spectra of better than 1,9 % [2]. To prepare such a sample for
static SIMS analysis requires a clean working environment. To reduce surface contamination, clean glassware,
tweezers and powderless gloves shall be used. The equipment required is a 1 ml glass pipette, a 100 ml
glass-stoppered measuring flask and a device for spin casting. If a device for spin casting is not available,
droplet deposition of the PC solution may be used. However, this will give poor repeatability, which will need
to be carefully taken into account during spectral analysis.
4.2.2 Using poly(bisphenol A carbonate), abbreviated to PC, weigh out 100 mg on a clean piece of
aluminium foil. Introduce the PC into the 100 ml, glass-stoppered measuring flask, add tetrahydrofuran (THF)
of analytical reagent quality, to the 100 ml level line. Shake the flask to mix the PC and allow time to dissolve it
completely. This produces a 1 mg/ml solution of PC in THF. The aluminium foil shall be freshly unrolled and
the shiny surface used. Ensure that the THF is anhydrous. Otherwise, streaks will appear from water when
spin coating as described in 4.2.3. The shelf life of freshly prepared stock solution shall be no more than one
month owing to water take-up.
NOTE 1 It does not matter if the PC contains low levels of additives such as Irgafos.
NOTE 2 It does not matter if the final PC/THF solution concentration varies by ±20 %.
4.2.3 Use a conveniently sized (1 cm by 1 cm) piece of silicon, or another flat or polished conducting
substrate, and clean it overnight by soaking in propan-2-ol (isopropyl alcohol). Ultrasonically clean the
substrate in fresh propan-2-ol and dry. If an ultrasonic bath is not available, just rinse the sample in fresh
propan-2-ol. Mount the substrate on the spin casting device. Pipette approximately 0,2 ml of the PC solution
onto the substrate and spin cast at 4 000 r/min for 25 s. Samples may be prepared by depositing the PC
solution using a 5 ml pipette onto the silicon surface then air drying under ambient conditions. However, this
method will result in an uneven PC film, so care shall be taken when comparing spectra, as peak intensities
will vary.
NOTE 1 It is not essential what substrate is used, as long as it is conducting. Silicon has been found to give
good-quality films.
NOTE 2 Using this procedure, the film thickness will be approximately 10 nm. The absolute thickness is not critical.
4.3 Obtaining the SIMS spectral data
4.3.1 Insert the PC sample inside the chamber of the SIMS instrument.
4.3.2 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 the ion-beam current, counting rates and any other parameter
specified by the manufacturer. Check that the detector multiplier settings are correctly adjusted.
4.3.3 Select the normal analytical settings and acquisition time. For ToF instruments, select a repetition rate
that gives a maximum mass of at least 800 u. If the total counts in the C H O peak are less than 10 000,
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increase the acquisition time to ensure that this peak contains more than 10 000 counts. This may not be
possible if the signal is too weak and it is not possible to achieve 10 000 counts within a reasonable time. To
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ensure that the maximum ion fluence (1×10 ions/m ) is not exceeded, an enlarged raster area may be
required. The acquisition time finally chosen will be a compromise between the data quality and the duration
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ISO 13084:2011(E)
of the work. Record the parameters set. Ensure that the detector is not saturated using the manufacturer's or
local documented instructions. This may be achieved by reducing the number of primary ions per pulse.
[1]
NOTE For details of acquiring high-quality SIMS spectra with good repeatability and constancy, refer to ISO 23830 .
4.4 Calculating mass accuracy
4.4.1 Instrument manufacturers' software may provide the calculation of the peak position automatically; it is
often sufficient to use this to obtain a value of M . A more accurate and reliable method for measurement of
o
the mass of the peak in the spectra, M , can be used. An asymmetric Gaussian function, G , can be used to
o A
fit to the signal intensity versus the mass position, M , and the fitting used to calculate the peak position, M .
P o
Where M is the peak centre, M is the peak mass and G is a scaling term, G , the fit to signal intensity, is
...
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