ISO 19318:2004
(Main)Surface chemical analysis — X-ray photoelectron spectroscopy — Reporting of methods used for charge control and charge correction
Surface chemical analysis — X-ray photoelectron spectroscopy — Reporting of methods used for charge control and charge correction
ISO 19318:2004 specifies the minimum amount of information describing the methods of charge control and charge correction in measurements of core-level binding energies for insulating specimens by X-ray photoelectron spectroscopy that shall be reported with the analytical results. Information is also provided on methods that have been found useful for charge control and for charge correction in the measurement of binding energies.
Analyse chimique des surfaces — Spectroscopie de photoélectrons — Indication des méthodes mises en oeuvre pour le contrôle et la correction de la charge
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INTERNATIONAL ISO
STANDARD 19318
First edition
2004-05-01
Surface chemical analysis — X-ray
photoelectron spectroscopy — Reporting
of methods used for charge control and
charge correction
Analyse chimique des surfaces — Spectroscopie de photoélectrons —
Indication des méthodes mises en oeuvre pour le contrôle et la
correction de la charge
Reference number
ISO 19318:2004(E)
©
ISO 2004
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ISO 19318:2004(E)
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ISO 19318:2004(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative reference. 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms. 1
5 Apparatus. 2
6 Calibration of binding-energy scale. 2
7 Reporting of information related to charge control . 2
7.1 Methods of charge control. 2
7.2 Information on specimen . 2
7.3 Instrument and operating conditions . 3
7.4 General method for charge control. 3
7.5 Reasons for needing charge control and for choosing the particular method for charge
control . 3
7.6 Values of experimental parameters . 3
7.7 Information on the effectiveness of the method of charge control . 4
8 Reporting of method(s) used for charge correction and the value of that correction. 4
8.1 Methods of charge correction . 4
8.2 Approach. 4
8.3 Value of correction energy. 4
Annex A (informative) Description of methods of charge control and charge correction. 5
A.1 Introduction. 5
A.2 Methods of charge control. 5
A.3 Methods of charge correction . 7
[17]
A.4 Bias referencing . 8
[32-34]
A.5 Auger parameter measurements . 9
Bibliography . 10
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ISO 19318:2004(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 19318 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 5, Auger electron spectroscopy.
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ISO 19318:2004(E)
Introduction
X-ray photoelectron spectroscopy (XPS) is widely used for characterization of surfaces of materials. Elements
in the test specimen (with the exception of hydrogen and helium) are identified from comparisons of the
binding energies of their core levels, determined from measured photoelectron spectra, with tabulations of
these binding energies for the various elements. Information on the chemical state of the detected elements
can frequently be obtained from small variations (typically between 0,1 eV and 10 eV) of the core-level binding
energies from the corresponding values for the pure elements. Reliable determination of chemical shifts often
requires that the binding-energy scale of the XPS instrument be calibrated with an uncertainty that could be as
small as 0,1 eV.
The surface potential of an insulating specimen will generally change during an XPS measurement due to
surface charging, and it is then difficult to determine binding energies with the accuracy needed for elemental
identification or chemical-state determination. There are two steps in dealing with this problem. First,
experimental steps can be taken to minimize the amount of surface charging (charge-control methods).
Second, corrections for the effects of surface charging can be made after acquisition of the XPS data (charge-
correction methods). Although the buildup of surface charge can complicate analysis in some circumstances,
it can be creatively used as a tool to gain information about a specimen.
The amount of induced charge near the surface, its distribution across the specimen surface, and its
dependence on experimental conditions are determined by many factors including those associated with the
[1, 2]
specimen and characteristics of the spectrometer. Charge buildup is a well-studied three-dimensional
phenomenon that occurs along the specimen surface and into the material. Charge buildup may also occur at
phase boundaries or interface regions within the depth of the specimen that is irradiated by X-rays. Some
specimens undergo time-dependent changes in the level of charging because of chemical changes or
volatilization induced by photoelectrons and secondary electrons, X-rays, or heating. Such specimens may
never achieve steady-state potentials.
There is, at present, no universally applicable method or set of methods for charge control or for charge
[3, 4]
correction . This International Standard specifies the information that shall be provided to document the
method of charge control during data acquisition and/or the method of charge correction during data analysis.
Information is given in Annex A on common methods for charge control and charge correction that can be
useful for many applications. The particular charge-control method that may be chosen in practice depends on
the type of specimen (e.g., powder, thin film or thick specimen), the nature of the instrumentation, the size of
the specimen, and the extent to which the specimen surface might be modified by a particular procedure.
This International Standard is expected to have two main areas of application. First, it identifies information on
methods of charge control and/or charge correction to be included in reports of XPS measurements (e.g., from
an analyst to a customer or in publications) in order to evaluate, assess and reproduce data on insulating
materials and to ensure that measurements on similar materials can be meaningfully compared. Second,
adherence to this International Standard will enable published binding energies to be used with confidence by
other analysts and will lead to the inclusion of more reliable data in XPS databases.
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INTERNATIONAL STANDARD ISO 19318:2004(E)
Surface chemical analysis — X-ray photoelectron
spectroscopy — Reporting of methods used for charge control
and charge correction
1 Scope
This International Standard specifies the minimum amount of information describing the methods of charge
control and charge correction in measurements of core-level binding energies for insulating specimens by
X-ray photoelectron spectroscopy that shall be reported with the analytical results. Information is also provided
on methods that have been found useful for charge control and for charge correction in the measurement of
binding energies.
2 Normative reference
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 18115, Surface chemical analysis — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115 apply.
4 Symbols and abbreviated terms
BE Binding energy, in eV
BE Corrected binding energy, in eV
corr
BE Measured binding energy, in eV
meas
BE Reference binding energy, in eV
ref
FWHM Full width at half maximum amplitude of a peak in the photoelectron spectrum above the
background,
in eV
XPS X-ray photoelectron spectroscopy
∆ Correction energy, to be added to measured binding energies for charge correction, in eV
corr
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ISO 19318:2004(E)
5 Apparatus
5.1 One or more of the charge-control techniques mentioned in Clause A.2 may be employed in most XPS
spectrometers. The XPS instrument shall be operated in accordance with the manufacturer’s or other
documented procedures.
5.2 Some of the techniques outlined in Clause A.2 require special apparatus, such as an electron flood gun
or a source for evaporative deposition of gold.
[5]
5.3 Certain specimen-mounting procedures, such as mounting the specimen under a fine metal mesh ,
can enhance electrical contact of the specimen with the specimen holder, or reduce the amount of surface
charge buildup. This and other methods of specimen mounting to reduce static charge are described in detail
[6, 7]
in ASTM E 1078 and ASTM E 1829 .
6 Calibration of binding-energy scale
[8]
The binding-energy scale of the X-ray photoelectron spectrometer shall be calibrated using ISO 15472 or
another documented method before application of this International Standard.
7 Reporting of information related to charge control
7.1 Methods of charge control
Many of the methods commonly used to control the surface potential and to minimize surface charging are
summarized in Clause A.2. Information on the following critical specimen and experimental conditions shall be
reported for individual specimens or collections of similar specimens.
7.2 Information on specimen
7.2.1 Specimen form
The form of the specimen shall be reported. The physical nature, source, preparation method and specimen
[2]
structure can influence charging behaviour .
EXAMPLE 1 Powder
EXAMPLE 2 Thin film spin-cast on silicon
EXAMPLE 3 Macroscopic mineral specimen
7.2.2 Specimen dimensions
The size and shape of a specimen can have a significant effect on the extent of specimen charging. The
shape of the specimen shall be reported together with approximate values of the dimensions of the specimen
or of any relevant specimen features (e.g., particle diameters).
7.2.3 Specimen-mounting methods
[1, 2]
Specimen mounting and contact with the specimen holder can significantly impact charging . The method
by which a specimen is mounted, including information about special methods used to increase conductivity or
isolate a specimen from ground, shall be reported.
EXAMPLE 1 Powder specimen pressed into foil, which was attached to specimen holder by tape
EXAMPLE 2 1 ml of contaminated liquid deposited on a silicon substrate and dried prior to analysis
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ISO 19318:2004(E)
EXAMPLE 3 Specimen held to holder using conductive adhesive tape of a specified type
EXAMPLE 4 Corroded specimen held on specimen holder by metal screw
7.2.4 Specimen treatment prior to or during analysis
The specimen treatment prior to or during analysis includes any physical or chemical treatment that can affect
charging of the specimen during XPS measurements.
NOTE Such treatment of the specimen may modify the surface composition as well as the electrical conductivity, and
hence charging, of the surface region.
7.3 Instrument and operating conditions
The instrument operating conditions include details of the particular XPS instrument, the nature of the X-ray
source, the approximate size of the X-ray beam on the specimen surface, the analyser pass energy, a
measure of energy resolution such as the FWHM of the silver 3d photoelectron line for the selected
5/2
operating conditions, the angle between the specimen normal and the X-ray source, and the use or not of a
magnetic lens.
7.4 General method for charge control
The particular instrumental component(s) used for charge control shall be identified.
EXAMPLE 1 Electron flood gun
EXAMPLE 2 Electron flood gun in combination with an ion gun
EXAMPLE 3 Specimen heating
EXAMPLE 4 Irradiation with ultraviolet light
If the components used are not standard for the XPS instrument, information shall be provided on the
manufacturer or on the relevant design characteristics.
7.5 Reasons for needing charge control and for choosing the particular method for charge
control
The reasons for needing charge control and for choosing a particular method shall be reported.
EXAMPLE 1 As supplied to us, the portion of the specimen of interest was isolated from ground. We supplied flood
gun electrons for charge compensation using the standard flood gun for this instrument.
EXAMPLE 2 Experience with similar specimens indicated that differential charging was likely. To obtain good spectra,
we totally isolated these specimens from ground. The application of the combined fluxes of a low-energy electron flood
gun and a low-energy ion flux produced well resolved peaks.
EXAMPLE 3 Initial spectra without any charge control showed peak shifting and broadening. Placing a grounded fine
grid above the specimen solved these problems without leading to a significant signal due to the grid material. This
method is easy to apply, and is used routinely in our measurements with similar specimens.
7.6 Values of experimental parameters
Values of parameters used to control charge, such as flood gun settings, shall be reported.
EXAMPLE For the flood gun, the cathode voltage was − 5 V (with respect to instrumental ground), the emission
current was 20 mA, and the gun cathode was 5 cm from the specimen.
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ISO 19318:2004(E)
7.7 Information on the effectiveness of the method of charge control
The adequacy of the charge-control methods for the type of analysis being conducted shall be established.
FWHMs and the binding energies (BE ) of peaks in the measured spectra, after charging effects have
meas
been minimized, but before any charge correction has been made, provide one useful method for determining
adequacy of the charge-control method. To document the effectiveness of the procedure(s) used to produce
appropriate BE and FWHM measurements, it may useful to have as a comparison a measurement of the
FWHM of at least one photoelectron peak of similar chemistry in another specimen that is known to be
conductive or for which the method of charge control is believed to be effective.
EXAMPLE The FWHM of the oxidized Si 2p photoline was reduced from 2,4 eV to 1,6 eV by application of flood gun.
The 1,6 eV width is consistent with measurements made on a thin SiO layer on Si.
2
The ability to control charge compensation over a wide energy range might be documented by measuring the
energy separation between different photopeaks from the same element. The adequacy of such a
measurement assumes that there are no complications due to chemical state changes with depth or the
presence of second phases.
NOTE It is recommended that specimens be examined for the presence or absence of specimen damage and that
the results be recorded.
8 Reporting of method(s) used for charge correction and the value of that
correction
8.1 Methods of charge correction
Many of the methods commonly used for charge correction are summarized in Clause A.3. The following
critical specimen and experimental parameters shall be reported:
8.2 Approach
The general method for correcting measured binding energies (peak positions) for charging effects shall be
specified in sufficient detail so that the method can be reproduced and the effectiveness judged.
8.3 Value of correction energy
Information shall be given on the magnitude of the correction energy (∆ ) for each spectrum and how this
corr
correction energy was determined. The corrected binding energies and values of the reference energies shall
be reported.
The correction energy ∆ is determined by taking the difference between the measured binding energy of a
corr
reference line (BE ) and the accepted or reference value for this binding energy (BE ) using the following
meas ref
relation:
∆ = BE − BE (1)
corr ref meas
The corrected binding energy for another photoelectron peak in the same spectrum (BE ) can then be found
corr
from the sum of the measured binding energy for that peak (BE ) and the correction energy:
meas
BE = BE + ∆ (2)
corr meas corr
NOTE Equations (1) and (2) apply only when charge compensation has adequately removed differential charging
effects.
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ISO 19318:2004(E)
Annex A
(informative)
Description of methods of charge control and charge correction
A.1 Introduction
The methods described here involve charge control (the effort to control the buildup of charge at a surface or
to minimize its effect) as described in Clause A.2, charge correction (the effort to determine a reliable binding
energy despite buildup of charge) as described in Clause A.3, or some combination of the two as described in
Clause A.4. For charge control, peak shape is one of the most important parameters to consider in assessing
the effectiveness of a method. Correcting a measured peak-energy position (i.e., binding energy) is
accomplished separately using an appropriate charge-correction technique. When both a photoelectron line
and a major Auger peak from the same element can be observed, the Auger parameter or the modified Auger
parameter, described in Clause A.5, can be used to provide chemical-state information without the need to
resort to charge corrections. Although the buildup of charge during XPS is often an unwanted complication, it
can also be used to obtain important information about a specimen as noted in A.2.5.2 and A.2.5.3.
The amount and distribution of surface and near-surface charge for a specific experimental system are
determined by many factors, including specimen composition, homogeneity, magnitude of bulk and surface
conductivities, photoionization cross-section, surface topography, spatial distribution of the exciting X-rays,
and availability of neutralizing electrons. Charge buildup occurs along the specimen surface and into the
[1, 2]
material . The presence of particles on or different phases in the specimen surface may result in an
uneven distribution of charge across the surface, a phenomenon known as differential charging. Charge
buildup may also occur at phase boundaries or interface regions within the specimen that is irradiated by
X-rays. Some specimens undergo time-dependent changes in the amount of charging because of chemical
and physical changes induced by electrons, X-rays or heat.
[3]
There is no single method to overcome all charging problems in all instruments . Several new methods were
developed in the 1990s, including those that involve electrons, ions and/or magnetic fields. All methods
described in this annex are based on the assumption that charging is not dependent on the kinetic energy of
the signal electrons. This may not be the case for some spectrometers or when differential charging occurs as
a function of depth into the specimen. An inter-laboratory comparison reported in 2000 of static-charge
stabilization methods for a variety of insulating specimens using referencing to both gold and carbon showed
[4]
that the standard deviation of the binding-energy measurements from 27 laboratories was, at best, 0,15 eV .
The report concluded that the reproducibility was unsatisfactory and that considerable additional work was
needed.
A.2 Methods of charge control
A.2.1 Damage caution
A variety of methods are and have been applied to control the extent of charge accumulation at the surface
during analysis. As some of these methods involve charged-particle or photon irradiation or the addition of
materials to the surface, possible specimen damage or specimen change from any such irradiations or
treatments should be considered.
[9-12]
A.2.2 Electron flood gun
Low-energy electron flood guns are frequently used to stabilize the static charging of insulators examined by
[10]
XPS , in particular when monochromatized X-rays are employed. Optimum operating conditions, for
example filament position, electron energy and electron current, depend upon the orientation of the electron
flood gun with respect to the specimen and upon the particular design of the electron flood gun and should, in
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ISO 19318:2004(E)
general, be determined by the user. Low electron energies (usually 10 eV or less) are used to maximize the
neutralization effect and reduce the number of electron-bombardment-induced reactions. Currents need to be
high enough to be effective, but low enough to avoid specimen damage or unwanted heating. A metal screen
[13, 14]
placed on or above the specimen can sometimes help .
[15]
A.2.3 Ultraviolet flood lamp
Ultraviolet radiation can produce low-energy electrons (for example, from the specimen holder) that may be
useful in neutralizing specimen charge.
A.2.4 Specimen heating
For a limited number of specimens, heating can increase the electrical conductivity of the specimen, thus
[2]
decreasing charging . The effects of specimen temperature and possible surface segregation need to be
considered.
A.2.5 Electrical connection
A.2.5.1 Grounding and enhanced conduction path
Surrounding insulating materials with a conducting material has been a common approach to minimizing the
charge buildup on specimens. This can mean masking a solid specimen with a conducting aperture, grid or
[2]
foil, or mounting particles on a conducting foil or tape .
A.2.5.2 Isolation from ground
For some materials, or mixtures of materials with different electrical conductivities, differential charging can
occur. This phenomenon can be used to obtain information about the chemistry or composition of conducting
[16]
or insulating parts of the specimen and can sometimes be minimized (and a more uniform specimen
potential achieved) by isolating the specimen from ground.
A.2.5.3 Biasing
Applying a low-voltage bias (−10 V to +10 V or more) to the specimen and observing the changes in the
binding energies of various peaks can be used to give information about the electrical contact of a specimen
(or parts of a specimen) with the specimen holder. Peaks in an XPS spectrum that shift when the bias is
applied are from conducting regions of the specimen. Other peaks from insulating regions may not shift nearly
as much or at all and can be interpreted accordingly. This method can sometimes verify that the peaks being
used for charge correction (for example, Au 4f or C 1s) are behaving in the same manner as the peaks of
[5, 15, 17]
interest from the specimen . For non-uniform or composite (non-conducting or partially conducting)
specimens, a variety of charge shifts may be observed upon biasing. These measurements may provide
useful information about the specimen and indicate a need to connect the specimen more carefully to ground
or to isolate the specimen from ground. Sometimes, all data for some specimens are collected with a bias
applied (see also Clause A.4).
A.2.6 Low-energy ion source
Portions of an insulator surface can be negatively charged, even when some areas exposed to X-rays are
[18]
charged positively . Such effects appear to be particularly important for focused X-ray beam systems,
where the X-rays strike only a relatively small portion of the specimen. In these circumstances, the use of a
low-energy positive-ion source, in addition to an electron source, may help stabilize (and make more uniform)
the surface potential of the specimen.
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ISO 19318:2004(E)
A.3 Methods of charge correction
A.3.1 Differential charging
A variety of methods are often used to determine the amount of binding energy shift due to surface charging.
Each of these methods is based on the assumption that differential charging (along the surface or within the
specimen) is not present to a significant degree. If significant differential charging is found to occur or thought
to be present, it may be necessary to alter the method of charge control.
[5, 9, 15, 19-23]
A.3.2 Adventitious-hydrocarbon referencing
A.3.2.1 Unless specimens are prepared for analysis under carefully controlled atmospheres, the surface,
generally, is coated by adventitious contaminants. Once introduced into the spectrometer, further specimen
contamination can occur by the adsorption of residual gases, especially in instruments with oil diffusion pumps.
These contamination layers can be used for correction purposes if it is assumed that they truly reflect the
steady-state static charge exhibited by the specimen surface and that they contain an element with a peak of
known binding energy. Carbon is most commonly d
...
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