ISO/TS 15338:2020

TECHNICAL ISO/TS
SPECIFICATION 15338
Second edition
2020-02
Surface chemical analysis — Glow
discharge mass spectrometry —
Operating procedures
Analyse chimique des surfaces — Spectrométrie de masse à décharge
luminescente (GD-MS) — Introduction à l'utilisation
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 1
6 Routine operations . 6
6.1 Cleaning the system . 6
6.2 Support gas handling . 6
7 Calibration . 7
7.1 Mass calibration . 7
7.2 Detector calibration . 7
7.3 Routine checks . 8
8 Data acquisition. 9
8.1 Sample preparation . 9
8.2 Procedure setup . 9
8.3 Data acquiring .10
9 Quantification .10
9.1 Element integral calculation .10
9.2 Ion beam ratios .11
9.3 Fully quantitative analysis .11
9.4 Semi quantitative analysis .12
9.5 Combination of semi quantitative and quantitative analysis .12
Bibliography .13
Foreword
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 8, Glow discharge spectroscopy.
This second edition cancels and replaces the first edition (ISO/TS 15338:2009), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— This document is more generic and covers not only the static, cryogenic cooled source, but also the
fast flow high power source.
— This document no longer refers to calibration factors specific to one particular instrument type.
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 © ISO 2020 – All rights reserved

TECHNICAL SPECIFICATION ISO/TS 15338:2020(E)
Surface chemical analysis — Glow discharge mass
spectrometry — Operating procedures
1 Scope
This document gives procedures for the operation and use of glow discharge mass spectrometry (GD-
MS). There are several GD-MS systems from different manufacturers in use and this document describes
the differences in their operating procedures when appropriate.
NOTE This document is intended to be read in conjunction with the instrument manufacturers’ manuals and
recommendations.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle
In a glow discharge source, a potential difference is applied between the cathode (the sample to be
analysed) and the anode, and a plasma is supported by the introduction of an inert gas, normally argon.
This potential difference can be either direct current (DC) or radio frequency (RF), the advantage of
RF being that electrically insulating materials can be analysed directly. Inert gas ions and fast neutrals
formed within the plasma are attracted to the surface of the sample and their impact results in the
production of neutrals by sputtering from surface.
These neutrals diffuse into the plasma where they are subsequently ionised within the equipotential
area of the plasma and can then be extracted to a mass spectrometer for analysis. Both magnetic sector
and time of flight spectrometers are available.
5 Apparatus
5.1 Ion source
There are two fundamental types of ion source used for the GD-MS, a low flow or “static” source, and a
fast flow source. Both types can accept pin samples or samples with a flat surface. A typical pin would
be 20 mm long with a diameter of 3 mm, and a typical flat sample would be 20 mm to 40 mm diameter.
More details of these dimensions can be found later.
In the low flow source the plasma cell is effectively a sealed unit held within a high vacuum chamber,
with a small exit slit or hole to allow the ions to exit the cell and enter the mass spectrometer. The cell
body is at anode potential, the acceleration potential of the mass spectrometer, and the sample is held at
cathode potential, typically 1 kV below anode potential. In this type of source, the argon flow is typically
one sccm (standard atmosphere cubic centimetres per minute) or less, and the gas used, normally
argon, should be of very high purity, six nines five or better. The power of the plasma is relatively low,
typically 2 W or 3 W; the potential difference is typically 1 kV and the current 2 mA or 3 mA.
Key
1 insulator
2 sample (cathode)
3 anode (GD cell)
a
Gas inlet (0,3 sccm to 0,6 sccm)
b
To mass spectrometer
Figure 1 — Low flow source pin geometry
A schematic diagram of the low flow source in pin geometry is shown in Figure 1. The gas is introduced
into the cell through a metal pipe which forms a metal to metal seal with the cell body. On some systems
an alternative of a PEEK tube with a ferrule seal to the cell body is used. The pin sample is held in a
chuck which sits at cathode potential and the cell body is at anode potential, so the two are separated
by an insulating disc. The chuck is actually located against a metal (tantalum) plate which also sits at
cathode potential (not shown in the schematic diagram). The whole assembly forms a good gas seal
while maintaining good electrical insulation. The only escape for the gas and any ions formed in the
plasma is through a small slit or hole at the back of the cell, and this creates a pressure differential
between the cell and the surrounding source vacuum chamber. It is normal to measure the pressure
outside the cell in a low flow source rather than in the cell itself, the presence of a plasma making
the measurement difficult. In this geometry, the potential difference between the anode and cathode
“drops” in a small sheath approximately 1mm around the sample, thus leaving the main gas volume in
the cell at the same potential. So any ions formed in the “plasma cloud” will not be electrically attracted
back to the cathode.
It is standard practice in the low flow source to cool the plasma to near liquid nitrogen temperatures.
This has been shown to reduce significantly the formation of molecular species associated with the
matrix and plasma support gas combined as dimers or trimers, or with gas backgrounds such as
hydrogen, nitrogen and oxygen. Cooling the sample in this way also allows for the measurement of low
melting point materials such as gallium and indium, materials that would melt under normal plasma
conditions.
Heat transfer between the components of the plasma cell needs to be considered. The whole cell
assembly is floated up to the accelerating potential, so the anode will typically be around 6 kV to 8 kV
while the sample (cathode) is at approximately 1 kV lower during operation. The design of the heat
exchanger, or cooling assembly, means that it will be sitting at ground potential, and so it is connected
to the cell insulating disc which is of a larger diameter than the cell body (not shown in Figure 1). Thus,
it is necessary for the insulating disc to have a good coefficient of heat transfer at the same time as
being electrically insulating; the material boron nitride is ideal for this and is used in most systems.
It is important to consider heat transfer through all junctions, particularly from the cell body and
cathode plate through the insulator to the heat exchanger. And in order to make the sample cold, it is
2 © ISO 2020 – All rights reserved

important to use a sample holder, or chuck, that is of a similar diameter to the sample itself. It is possible
to maximise the heat transfer if attention is paid to detail, for example, a piece of Indium foil can be cut
and shaped to fit between the heat exchanger and the insulator, providing that care is taken to ensure
that there can be no electrical leakage.
Key
1 insulator
2 sample (cathode)
3 anode (GD cell)
a
Gas inlet (0,2 sccm to 0,4 sccm)
b
To mass spectrometer
Figure 2 — Low flow source flat geometry
Figure 2 shows a schematic of the low flow source in flat geometry. The sample, which forms the
cathode in the plasma, is pressed against an insulator which in turn is pressed against the cell body
which is at anode potential. Again, a good gas seal is formed and the only escape route for the gas and
the sputtered particles is through the ion exit slit or hole. The area of the sample exposed to the plasma
can be varied by the choice of the insulator used. If a larger area of sample is exposed then a larger ion
beam will be produced, but it is possible to use smaller insulators to allow smaller samples to make a
gas seal. Or to reduce the area exposed to analysis. Commercial systems will generally have insulators
available to allow areas from 2 mm to 20 mm diameter to be exposed, but 10 mm to 15 mm is the
normal. The most important need is that the sample surface shall be flat in order to make a good gas
seal with the insulating disc.
The gap between the sample (cathode) and the anode has to be small (less than a critical distance,
typically 1 mm) to avoid creation of a discharge in the gap. It is often a problem that sputtered material
can be deposited on the inner diameter of the insulator, creating a short circuit between anode and
cathode. This can be avoided by the use of two insulators with different diameter holes.
Figure 3 shows the fast flow source. In this geometry the sample (shown here as a flat sample) is pushed
against the back of the source and is raised to cathode potential. The discharge gas (normally argon)
is injected between a tube at anode potential and a concentric flow tube and is directed towards the
sample surface. The flow rate of the argon is in the region of 300 ml/min to 500 ml/min. The same high
purity support gas as used in the low flow source is not required, a typical purity would be five nines.
Key
1 insulator
2 sample (cathode)
3 anode
4 flow tube
5 cone
Figure 3 — Fast flow source flat geometry
The potential difference between anode and cathode is typically 800 V and the current typically 30 mA
or 40 mA. Sample atoms that are sputtered from the surface and ionised enter the mass spectrometer
through an orifice in the sampling cone as shown in the diagram.
In a fast flow source the signals are largely independent of the sample temperature and Peltier cooling is
used to reduce sample overheating. The use of liquid nitrogen cooling is not necessary as the high power
of the plasma inhibits the formation of molecular ions. A significant reduction from room temperature
is required only for certain high melting point samples.
In all cases described above, the ions are accelerated into the mass analyser by a high potential
difference, typically 6 kV to 8 kV. In most cases the source is held at high potential and the analyser at
ground, but it is possible to achieve the same potential difference by grounding the source and taking
the analyser section to a negative high voltage.
The extracted ions are focussed and steered onto a source defining slit by a series of steering plates and
lenses at different potentials. This source defining slit or sampling cone is typically at ground potential
and forms the object that the analyser will focus onto the detector. Different sizes of source defining slit
are available, in some instruments there is a finite number of alternatives, in others the source defining
slit is infinitely adjustable. For high resolution, where interference peaks need to be separated from
the peak of interest, a small source defining slit shall be used. But for the analysis of impurities where
there is no interference peak, it can be possible to have a large source defining slit to allow more ionised
particles to pass through, hence making it possible to detect lower levels of the impurity.
The potential between anode and cathode can either be a steady DC voltage, a radio frequency (RF)
voltage, or can be a modulated pulsed voltage. In this last case, the cathode potential is switched
rapidly, typically at a frequency of 2 000 Hz with a pulse duration of 50 μs, a duty cycle of 10 %. It
has been shown that in the case of the fast flow source, this mode of operation can be beneficial for
bulk analysis by improving long term stability and precision. It also makes possible depth profiling of
layered samples and secondary cathode analysis of insulators.
In the case of the recently developed Time of Flight (TOF) mass spectrometer, the source is similar to
the fast flow source and the accelerating potential is always pulsed so that very short bursts of ions are
introduced to the analyser section.
4 © ISO 2020 – All rights reserved

5.2 Mass analyser
The purpose of the mass analyser is to separate ions of different species by mass. More precisely the
separation is by mass charge ratio; an Ar ion with a single charge would be seen at 40 u (atomic mass
units), but the same ion with two electrons removed would appear at 20 u. As well as multiple charged
40 +
species, some molecular species are seen in the spectrum, an example being Ar which would appear
at 80 u.
Some GD-MS instruments work at high resolution and are capable of resolving most interferences from
the peak of interest. These systems can also be operated at low resolution if there are no interferences
to be resolved, thereby giving a larger signal and more precise measurement or better detection limits.
All mass spectrometers operate under vacuum to ensure that the mean free path without collision is as
great as possible, resulting in a cleaner signal at the collector. The analyser section of the instrument
should be left under vacuum whenever possible, and for this reason all systems incorporate a valve to
isolate the analyser from the source which needs to be exposed to atmosphere frequently.
In those systems which consist of a magnet and ESA (Electrostatic Analyser) the ions are separated
in mass by the magnetic field, and any small spread in energy is removed by the ESA. Some systems
use a forward style Nier Johnston geometry (ESA then magnet), others use a reverse style (magnet
then ESA). A continuous stream of ions will enter the analyser and the masses of interest are measured
sequentially. In the case of the TOF, a complete spectrum is acquired for each individual pulse.
5.3 Detector system
The magnetic sector GD-MS instruments are designed to measure as large a spread of signal intensities
as possible, and there may be more than one type of detector on an instrument.
−9
Very large signals (around 10 A) can be produced by GD-MS instruments, an ion current equivalent to
10 ions per second. These signals cannot be measured by any ion counting system and so are collected
by a Faraday cup and converted to a voltage by a current amplifier. A typical amplifier will produce
−9 −14
a signal of 1 V for an ion current of 10 A. Currents down to 10 A can be registered by the same
amplifier using a digital voltmeter with good resolution.
−13 6
Below 3 × 10 A, equivalent to 2 × 10 counts per second, ion counting systems are used. On some
instruments this is a Daly system, which consists of a conversion electrode, a scintillator and a
photomultiplier tube outside the vacuum. On others it is an electron multiplier mounted inside the
vacuum system of the mass analyser. For both of these alternatives the associated electronics will
be able to switch the signal between the alternative detectors and will protect the more sensitive
multiplier for the incidence of large signals which could cause damage.
On systems which use more than one detector, there will be software to cross calibrate the detectors.
Signals of suitable strength are measured both detectors to show the effective efficiency of the
multipliers. This can change for many reasons, supply voltage changes or aging being two reasons, and
so it is important to monitor it on a regular basis. More details of this calibration and the monitoring
are given later.
In time of flight systems, Micro Channel Plates (MCPs) are used to measure the signal as a very fast
response is needed. When a pulse of ions is introduced into the analyser, all ions have the same kinetic
energy and so different masses travel at different velocities and reach the detector at different times.
Data for the whole mass spectrum is acquired and stored for each individual pulse, a full spectrum
being acquired in a few tens of microseconds.
6 Routine operations
6.1 Cleaning the system
As with all vacuum systems, cleanliness is extremely important. Any components that fit within
the vacuum system should only be handled when wearing gloves. Powder free gloves are preferred
otherwise a residue of chemical such as silicon can be left behind.
Some systems have the internal source parts made from tantalum as this is effectively mono-isotopic
with an odd number mass that produces less background interferences and can be thoroughly cleaned
with acids. The source parts need to be cleaned regularly to remove sputter deposited coating that can
produce instability, and if different matrices are analysed these depositions can cause memory effects.
In the case of tantalum parts, they should be cleaned mechanically by scrubbing with an abrasive
sponge-like material then soaked in an acid suitable to dissolve the matrix material. For particularly
difficult matrices a strong acid such as aqua regia (1:3, HNO : HCl) with a small percentage (~2 %) HF
can be used. The acid wash should then be followed with several washes with deionised water and
finally with methanol.
Stainless steel components should be treated more carefully than the tantalum items, but as they are
not usually in direct contact with the ion beam a mechanical wash followed by soaking briefly in 50 %
HNO and a rinse in deionised water then methanol is normally sufficient.
Any ceramic components should not be cleaned with liquids as they tend to be absorbent, a simple
mechanical scrub should suffice.
In order to reduce down time as a result of cleaning source components, it is possible to keep a batch of
such parts so that they can be exchanged when necessary and cleaned while the system continues to run.
6.2 Support gas handling
In the low flow versions of GD-MS the purity of the plasma support gas is more critical than in the fast
flow systems, so the handling of the entire gas system is critical. Manufacturers will generally supply
instructions on the best way to install new bottles and handle the system during both use and idle period.
When installing a new bottle, the valve on the top of the bottle should be left closed and the regulator
...