ISO/TS 25138:2010

TECHNICAL ISO/TS
SPECIFICATION 25138
First edition
2010-12-01

Surface chemical analysis — Analysis of
metal oxide films by glow-discharge
optical-emission spectrometry
Analyse chimique des surfaces — Analyse de films d'oxyde de métal
par spectrométrie d'émission optique à décharge luminescente




Reference number
ISO/TS 25138:2010(E)
©
ISO 2010

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ISO/TS 25138:2010(E)
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ISO/TS 25138:2010(E)
Contents Page
Foreword .iv
1 Scope.1
2 Normative references.1
3 Principle .1
4 Apparatus.2
4.1 Glow-discharge optical-emission spectrometer .2
5 Adjusting the glow-discharge spectrometer system settings.3
5.1 General .3
5.2 Setting the parameters of a DC source.4
5.3 Setting the discharge parameters of an RF source .5
5.4 Minimum performance requirements.6
6 Sampling .8
7 Calibration.8
7.1 General .8
7.2 Calibration specimens .8
7.3 Validation specimens.10
7.4 Determination of the sputtering rate of calibration and validation specimens .11
7.5 Emission intensity measurements of calibration specimens.12
7.6 Calculation of calibration equations .12
7.7 Validation of the calibration .12
7.8 Verification and drift correction.14
8 Analysis of test specimens .14
8.1 Adjusting discharge parameters .14
8.2 Setting of measuring time and data acquisition rate.14
8.3 Quantifying depth profiles of test specimens .15
9 Expression of results.15
9.1 Expression of quantitative depth profile.15
9.2 Determination of metal oxide mass per unit area .15
9.3 Determination of the average mass fractions of the elements in the oxide.16
10 Precision .16
11 Test report.17
Annex A (normative) Calculation of calibration constants and quantitative evaluation of depth
profiles.18
Annex B (informative) Suggested spectral lines for determination of given elements.29
Annex C (informative) Examples of oxide density and the corresponding quantity ρ .30
O
Annex D (informative) Report on interlaboratory testing of metal oxide films.31
Bibliography.36

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ISO/TS 25138:2010(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.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of document:
⎯ an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
⎯ an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
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/TS 25138 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 8, Glow discharge spectroscopy.


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TECHNICAL SPECIFICATION ISO/TS 25138:2010(E)

Surface chemical analysis — Analysis of metal oxide films by
glow-discharge optical-emission spectrometry
1 Scope
This Technical Specification describes a glow-discharge optical-emission spectrometric method for the
determination of the thickness, mass per unit area and chemical composition of metal oxide films.
This method is applicable to oxide films 1 nm to 10 000 nm thick on metals. The metallic elements of the oxide
can include one or more from Fe, Cr, Ni, Cu, Ti, Si, Mo, Zn, Mg, Mn and Al. Other elements that can be
determined by the method are O, C, N, H, P and S.
2 Normative references
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 14284, Steel and iron — Sampling and preparation of samples for the determination of chemical
composition
ISO 14707, Surface chemical analysis — Glow discharge optical emission spectrometry (GD-OES) —
Introduction to use
ISO 16962:2005, Surface chemical analysis — Analysis of zinc- and/or aluminium-based metallic coatings by
glow-discharge optical-emission spectrometry
3 Principle
The analytical method described here involves the following processes:
a) Cathodic sputtering of the surface metal oxide in a direct-current or radio-frequency glow-discharge
device.
b) Excitation of the analyte atoms in the plasma formed in the glow-discharge device.
c) Spectrometric measurement of the intensities of characteristic spectral-emission lines of the analyte
atoms as a function of sputtering time (depth profile).
d) Conversion of the depth profile in units of intensity versus time to mass fraction versus depth by means of
calibration functions (quantification). Calibration of the system is achieved by measurements on
calibration specimens of known chemical composition and measured sputtering rate.
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ISO/TS 25138:2010(E)
4 Apparatus
4.1 Glow-discharge optical-emission spectrometer
4.1.1 General
[1]
The required instrumentation includes an optical-emission spectrometer system consisting of a Grimm type
or similar glow-discharge source (direct-current or radio-frequency powered) and a simultaneous optical
spectrometer as described in ISO 14707, capable of providing suitable spectral lines for the analyte elements.
The inner diameter of the hollow anode of the glow-discharge source shall be in the range 2 mm to 8 mm. A
cooling device for thin specimens, such as a metal block with circulating cooling liquid, is also recommended,
but not strictly necessary for implementation of the method.
Since the principle of determination is based on continuous sputtering of the surface metal oxide, the
spectrometer shall be equipped with a digital readout system for time-resolved measurement of the emission
intensities. A system capable of a data acquisition speed of at least 500 measurements/second per spectral
channel is recommended, but, for a large number of applications, speeds of > 50 measurements/second per
spectral channel are acceptable.
4.1.2 Selection of spectral lines
For each analyte to be determined, there exist a number of spectral lines which can be used. Suitable lines
shall be selected on the basis of several factors, including the spectral range of the spectrometer used, the
analyte mass fraction range, the sensitivity of the spectral lines and any spectral interference from other
elements present in the test specimens. For applications where several of the analytes of interest are major
elements in the specimens, special attention shall be paid to the occurrence of self-absorption of certain highly
sensitive spectral lines (so-called resonance lines). Self-absorption causes non-linear calibration curves at
high analyte mass fraction levels, and strongly self-absorbed lines should therefore be avoided for the
determination of major elements. Suggestions concerning suitable spectral lines are given in Annex B.
Spectral lines other than those listed may be used, so long as they have favourable characteristics.
4.1.3 Selection of glow-discharge source type
4.1.3.1 Anode size
Most GD-OES instruments on the market are delivered with options to use various anode diameters, 2 mm,
4 mm and 8 mm being the most common. Some older instruments have one anode only, usually 8 mm, while
the most commonly used anode in modern instruments is 4 mm. A larger anode requires larger specimens
and higher power during analysis; therefore the specimen is heated to a greater extent. On the other hand, a
larger anode gives rise to a plasma of larger volume that emits more light, resulting in lower detection limits
(i.e. higher analytical sensitivity). In a large number of applications, the 4 mm anode is a good compromise.
However, in surface analysis applications it is rather common to encounter problems of overheating of the
specimens due to e.g. surface layers of poor heat conductivity and/or very thin specimens. In such cases, the
smaller 2 mm anode is preferable, even if there is some loss of analytical sensitivity.
4.1.3.2 Type of power supply
The glow-discharge source can be either a type powered by a direct-current (DC) power supply or a radio-
frequency (RF) type. The most important difference is that the RF type can sputter both conductive and non-
conductive specimens; hence this is the only type that can be used for e.g. polymer coatings and insulating
oxide layers. On the other hand, it is technically simpler to measure and control the electrical source
parameters (voltage, current, power) of a DC type. Several commercially available GD-OES systems can be
delivered with the option to switch between DC and RF operation, but RF-only systems are becoming
increasingly common. In short, there are a very large number of applications where DC or RF sources can be
used and several where only an RF source can be used.
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ISO/TS 25138:2010(E)
4.1.3.3 Mode of operation
Both DC and RF sources can be operated in several different modes with respect to the control of the
electrical parameters (current, voltage, power) and the pressure. There are several reasons for this:
⎯ “historical” reasons (older instruments have simpler but functional power supplies, while the technology
has evolved so newer models have more precise and easier-to-operate source control);
⎯ different manufacturers have chosen different solutions for source control;
⎯ there are some application-related issues where a particular mode of operation is to be preferred.
This Technical Specification gives instructions for optimizing the source parameters based on several
available modes of operation. The most important reason for this is to make these instructions comprehensive
so as to include several types of instrument. In most applications, there is no major difference between these
modes in terms of analytical performance, but there are other differences in terms of practicality and ease of
operation. For instance, a system equipped with active pressure regulation will automatically be adjusted to
the same electrical source parameters every time a particular analytical method is used. Without this
technology, some manual adjustment of the pressure to achieve the desired electrical source parameters is
normally required.
[2][3]
NOTE It should be noted in this context that what is known as the emission yield forms the basis for calibration
and quantification as described in this Technical Specification. The emission yield has been found to vary with the current,
[8]
the voltage and, to a lesser extent, the pressure . It is impossible in practice to maintain all three parameters constant for
all test specimens, due to variations in the electrical characteristics of different materials. In several instrument types, the
electrical source parameters (the plasma impedance) can therefore be maintained constant by means of automatic
systems that vary the pressure during analysis. Alternatively, there exist methods to correct for impedance variations by
[8]
means of empirically derived functions , and this type of correction is implemented in the software of commercially
available GD-OES systems.
5 Adjusting the glow-discharge spectrometer system settings
5.1 General
Follow the manufacturer's instructions or locally documented procedures for preparing the instrument for use.
For the optical system, the most important preparation step is to check that the entrance slit to the
spectrometer is correctly adjusted, following the procedure given by the instrument manufacturer. This
ensures that the emission intensities are measured on the peaks of the spectral lines for optimum signal-to-
background ratio. For further information, see ISO 14707.
The most important step in developing a method for a particular application is to optimize the parameters of
the glow-discharge source. The source parameters shall be chosen to achieve three aims:
a) adequate sputtering of the test specimen, to reduce the analysis time without overheating the specimen;
b) good crater shape, for good depth resolution;
c) constant excitation conditions in calibration and analysis, for optimum accuracy.
Trade-offs are often necessary among the three specified aims. More detailed instructions on how to adjust
the source parameters are given in the following subclauses.
The settings of the high voltage for the detectors depend on the source parameters, but the procedure is the
same for all modes of operation of the source. This procedure is therefore only described for the first mode of
operation.
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ISO/TS 25138:2010(E)
Similarly, the steps to adjust and optimize the source settings in terms of signal stability and sputter crater
shape are also similar in principle for all modes of operation. Therefore, these procedures are only described
in detail for the first mode of operation.
5.2 Setting the parameters of a DC source
5.2.1 Constant applied current and voltage
5.2.1.1 General
The two control parameters are the applied current and the applied voltage. Set the power supply for the glow-
discharge source to constant-current/constant-voltage operation. Then set the current and voltage to the
typical values recommended by the manufacturer. If no recommended values are available, set the voltage to
700 V and the current to a value in the range 5 mA to 10 mA for a 2 mm or 2,5 mm anode, 15 mA to 30 mA for
a 4 mm anode or 40 mA to 100 mA for a 7 mm or 8 mm anode. If no previous knowledge of the optimum
current is available, it is recommended to start with a value somewhere in the middle of the recommended
range.
5.2.1.2 Setting the high voltage of the detectors
Select test specimens with surface layers of all types to be determined. For all test specimens, run the source
while observing the output signals from the detectors for the analyte atoms. Adjust the high voltage of the
detectors in such a way that sufficient sensitivity is ensured at the lowest analyte mass fraction without
saturation of the detector system at the highest analyte mass fraction.
5.2.1.3 Adjusting the source parameters
For each type of test specimen, carry out a full depth profile measurement, sputtering it in the glow discharge
for a sufficiently long time to remove the metal oxide completely and continue well into the base material. By
observing the emission intensities as a function of sputtering time (often referred to as the qualitative depth
profile), verify that the selected source settings give stable emission signals throughout the depth profile and
into the substrate. If this is found not to be the case, reduce one of the control parameters by a small amount
and sputter through the metal oxide again. If the stability is still unsatisfactory, reduce the other control
parameter by a small amount and repeat the measurements. If found necessary, repeat this procedure for a
number of control parameter combinations until stable emission conditions are obtained.
NOTE Unstable emission signals could indicate thermal instability in the specimen surface layers; specimen cooling
is beneficial in this regard.
5.2.1.4 Optimizing the crater shape
If a suitable profilometer device is available, adopt the following procedure. Sputter a specimen with a metal
oxide typical of the test specimens to be analysed to a depth of about 10 µm to 20 µm, but still inside the
metal oxide. If no such specimen is available, use a brass specimen. Measure the crater shape by means of
the profilometer device. Repeat this procedure a few times using slightly different values of one of the control
parameters. Select the conditions that give an optimally flat-bottomed crater. These conditions are then used
during calibration and analysis.
5.2.2 Constant applied current and pressure
The two control parameters are the applied current and the pressure. Set the power supply for the glow-
discharge source to constant-current operation. Then set the current to a typical value recommended by the
manufacturer. If no recommended values are available, set the current to a value in the range 5 mA to 10 mA
for a 2 mm or 2,5 mm anode, 15 mA to 30 mA for a 4 mm anode or 40 mA to 100 mA for a 7 mm or 8 mm
anode. If no previous knowledge of the optimum current is available, it is recommended to start with a value
somewhere in the middle of the recommended range. Sputter a typical coated test specimen, and adjust the
pressure until a voltage of approximately 700 V is attained in the metal oxide.
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ISO/TS 25138:2010(E)
Set the high voltage of the detectors as described in 5.2.1.2.
Adjust the discharge parameters as described in 5.2.1.3, adjusting first the current and, if necessary, the
pressure.
Optimize the crater shape as described in 5.2.1.4 by adjusting the pressure. These conditions are then used
during calibration and analysis.
NOTE Before sputtering a new specimen type, make a test run in order to ensure that the voltage has not changed
by more than 5 % from the previously selected value. If this is the case, readjust the pressure until the correct value is
attained.
5.3 Setting the discharge parameters of an RF source
5.3.1 Constant applied power and pressure
The two control parameters are the applied power and the pressure. First set the applied power and adjust the
source pressure to the values suggested by the manufacturer. If recommended values are not available, set
the applied power and pressure to somewhere in the middle of the ranges commonly used for depth profiling
of metal specimens. Measure the penetration rate (i.e. depth per unit time) on an iron or steel specimen,
adjusting the power to give a penetration rate of about 2 µm/min to 3 µm/min.
Set the high voltage of the detectors as described in 5.2.1.2.
Adjust the discharge parameters as described in 5.2.1.3, adjusting first the applied power and, if necessary,
the pressure.
Optimize the crater shape as described in 5.2.1.4 by adjusting the pressure.
Remeasure the penetration rate on the iron or steel specimen and adjust the applied power, if necessary, to
return to about 2 µm/min to 3 µm/min. Repeat the cycle of power and pressure adjustment until no significant
change is noted in the penetration rate or crater shape. Note the power and pressure used in units provided
for the instrument type. These conditions are then used during calibration and analysis.
5.3.2 Constant applied power and DC bias voltage
The two control parameters are the applied power and the DC bias voltage. First set the applied power and
adjust the source pressure to attain a DC bias typical of the values suggested by the manufacturer. If
recommended values are not available, set the applied power and DC bias voltage to somewhere in the
middle of the range commonly used for depth profiling of metal specimens. On instruments equipped with
active pressure control, this can be achieved automatically. Measure the penetration rate (i.e. depth per unit
time) on an iron or steel specimen, adjusting the power to give a penetration rate of about 2 µm/min to
3 µm/min.
Set the high voltage of the detectors as described in 5.2.1.2.
Adjust the discharge parameters as described in 5.2.1.3, adjusting first the applied power and, if necessary,
the DC bias voltage.
Optimize the crater shape as described in 5.2.1.4 by adjusting the DC bias voltage.
Remeasure the penetration rate on the iron or steel specimen and adjust the applied power, if necessary, to
return to about 2 µm/min to 3 µm/min. Repeat the cycle of power and DC bias voltage adjustment until no
significant change is noted in the penetration rate or in the crater shape. If this is not the case, readjust the DC
bias voltage until the correct value is attained. Note the power and DC bias voltage used in units provided for
the instrument. These conditions are then used during calibration and analysis.
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ISO/TS 25138:2010(E)
5.3.3 Constant effective power and RF voltage
The two control parameters are the effective power and the RF voltage. Constant effective power is defined
here as the applied power minus the reflected power and the “blind power” measured with the specimen in
place but without plasma (vacuum conditions). The RF voltage is defined here as the RMS voltage at the
coupling electrode.
Set the power supply for the glow-discharge source to constant effective power/constant RF voltage operation.
First set the power to a typical value recommended by the manufacturer. If no recommended values are
available, set the RF voltage to 700 V and the power to a value in the range 10 W to 15 W for a 4 mm anode,
to give an example. If no previous knowledge of the optimum power is available, it is recommended to start
with a value somewhere in the middle of the recommended range.
Set the high voltage of the detectors as described in 5.2.1.2.
Adjust the discharge parameters as described in 5.2.1.3, adjusting first the effective power and, if necessary,
the RF voltage.
Optimize the crater shape as described in 5.2.1.4 by adjusting the RF voltage. Select the conditions that give
an optimally flat-bottomed crater. These conditions are then used during calibration and analysis.
5.4 Minimum performance requirements
5.4.1 General
It is desirable for the instrument to conform to the performance specifications given in 5.4.2 and 5.4.3 below.
NOTE Setting up for analysis commonly requires an iterative approach to the adjustment of the various instrumental
parameters described in this Technical Specification.
5.4.2 Minimum repeatability
The following test shall be performed in order to check that the instrument is functioning properly in terms of
repeatability.
Perform 10 measurements of the emission intensity on a homogeneous bulk specimen with a content of the
analyte exceeding a mass fraction of 1 %. The glow-discharge conditions shall be those selected for analysis.
These measurements shall be performed using a discharge stabilization time (often referred to as “preburn”)
of at least 60 s and a data acquisition time in the range 5 s to 20 s. Each measurement shall be located on a
newly prepared surface of the specimen. Calculate the relative standard deviation of the 10 measurements.
The relative standard deviation shall conform to any requirements and/or specifications relevant to the
intended use.
NOTE Typical relative standard deviations determined in this way a
...