ASTM E1523-97
(Guide)Standard Guide to Charge Control and Charge Referencing Techniques in X-Ray Photoelectron Spectroscopy
Standard Guide to Charge Control and Charge Referencing Techniques in X-Ray Photoelectron Spectroscopy
SCOPE
1.1 This guide covers the acquainting of the XPS user with the various charge control and charge shift referencing techniques that are and have been used in the acquisition and interpretation of X-ray photoelectron spectroscopy (XPS) data from surfaces of insulating specimens.
1.2 This guide is intended to apply to charge control and charge referencing techniques in XPS and is not necessarily applicable to electron-excited systems.
1.3 SI units are standard unless otherwise noted.
1.4 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
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Designation: E 1523 – 97
Standard Guide to
Charge Control and Charge Referencing Techniques in
X-Ray Photoelectron Spectroscopy
This standard is issued under the fixed designation E 1523; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope This binding energy shift may reach a nearly steady-state value
of between 2 and 5 eV for spectrometers equipped with
1.1 This guide acquaints the XPS user with the various
nonmonochromatic X-ray sources. The surface potential
charge control and charge shift referencing techniques that are
charge and the resulting binding energy shift is, generally,
and have been used in the acquisition and interpretation of
larger for spectrometers equipped with monochromatic X-ray
X-ray photoelectron spectroscopy (XPS) data from surfaces of
sources because of the, generally, lower flux of low-energy
insulating specimens.
electrons impinging on the specimen surface. This lower flux
1.2 This guide is intended to apply to charge control and
arises because focused, monochromatic X-ray beams irradiate
charge referencing techniques in XPS and is not necessarily
only a portion of the specimen and not other nearby surfaces
applicable to electron-excited systems.
(for example, the specimen holder) that are sources of low-
1.3 SI units are standard unless otherwise noted.
energy electrons. The absence of an X-ray window in many
1.4 This standard does not purport to address all of the
monochromatic X-ray sources (or a greater distance of the
safety problems, if any, associated with its use. It is the
specimen from the X-ray window) also eliminates another
responsibility of the user of this standard to establish appro-
source of low-energy electrons.
priate safety and health practices and determine the applica-
4.2 The amount of induced surface charge, its distribution
bility of regulatory limitations prior to use.
across the specimen surface, and its dependence on experimen-
2. Referenced Documents tal conditions are determined by several factors including
specimen composition, homogeneity, magnitude of surface
2.1 ASTM Standards:
conductivity, total photoionization cross-section, surface to-
E 673 Terminology Relating to Surface Analysis
pography, and availability of neutralizing electrons. The pres-
E 902 Practice for Checking the Operating Characteristics
ence of particles on or different phases in the specimen surface
of X-Ray Photoelectron Spectrometers
may result in an uneven distribution of charge across the
E 1078 Guide for Specimen Handling in Auger Electron
surface, a phenomenon known as differential charging. Some
Spectroscopy, X-Ray Photoelectron Spectroscopy, and
specimens undergo time-dependent changes in the level of
Secondary Ion Mass Spectrometry
charging because of electron, X-ray, or thermal damage or
E 1829 Guide for Specimen Preparation and Mounting in
because of volatilization. Such specimens may never achieve
Surface Analysis
steady-state potentials.
3. Terminology
4.3 Several techniques have been developed for the purpose
of controlling charge buildup and the subsequent changes in
3.1 Definitions:
surface potential in order to obtain meaningful and reproduc-
3.1.1 See Terminology E 673 for definitions of terms used in
ible data from insulating specimens. These techniques are
X-ray photoelectron spectroscopy.
employed during the data acquisition and are discussed in 7.1.
4. Overview of Charging Effects
4.4 Several techniques have been developed for the purpose
of correcting the binding energy shifts that result from surface
4.1 For insulating specimen surfaces, the emission of pho-
charging. These corrections are performed after the data has
toelectrons following X-ray excitation may result in a buildup
been accumulated and are discussed in 7.2.
of a positive surface charge. This positive surface charge
4.5 The use of the various charge control or charge refer-
changes the surface potential thereby shifting the measured
encing techniques described in this guide may depend on the
energies of the photoelectron peaks to higher binding energy.
available instrument as well as the specimen being analyzed.
This guide is under the jurisdiction of ASTM Committee E-42 on Surface
5. Significance and Use
Analysis and is the direct responsibility of Subcommittee E42.03 on Auger Electron
5.1 The acquisition of chemical information from variations
Spectroscopy and X-ray Photoelectron Spectroscopy.
Current edition approved Sept. 10, 1997. Published July 1998. Originally
in the energy position of peaks in the XPS spectrum is of
published as E 1523 – 93. Last previous edition E 1523 – 93.
primary interest in the use of XPS as a surface analytical tool.
Annual Book of ASTM Standards, Vol 03.06.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1523
Surface charging acts to shift spectral peaks independent of flood gun and must, in general, be determined by the user. Use
their chemical relationship to other elements on the same low-electron energies (usually 10 eV or less) to maximize the
surface. The desire to eliminate the influence of surface neutralization effect and reduce the number of electron
charging on the peak positions and peak shapes has resulted in bombardment-induced reactions. A metal screen placed on or
the development of several empirical methods designed to above the specimen can help (6,7).
assist in the interpretation of the XPS peak positions, determine
7.1.1.2 Ultraviolet Flood Lamp (8)—Ultraviolet radiation
surface chemistry, and allow comparison of spectra of conduct-
can also produce low-energy electrons (for example, from the
ing and nonconducting systems of the same element. It is
specimen holder) that may be useful in neutralizing specimen
assumed that the spectrometer is generally working properly
charging.
for non-insulating specimens (see Practice E 902).
7.1.1.3 Specimen Heating—For a limited number of speci-
5.2 No ideal method has been developed to deal with
mens, heating can increase the electrical conductivity of the
surface charging. For insulators, an appropriate choice of any
specimen, thus decreasing charging.
control or referencing system will depend on the nature of the
7.1.1.4 Specimen Biasing—Applying a low-voltage bias
specimen, the instruments, and the information needed. The
(−10 to + 10 V) to the specimen and observing the changes in
appropriate use of charge control and referencing techniques
the binding energies of various peaks can be used to learn
will result in more consistent, reproducible data. Researchers
about the electrical contact of a specimen (or parts of a
are strongly urged to report both the control and referencing
specimen) with the specimen holder. Peaks in XPS spectrum
techniques that have been used, the specific peaks and binding
that shift when the bias is applied are from conducting regions
energies used as standards (if any), and the criteria applied in
of the specimen. Other peaks from insulating regions may not
determining optimum results so that the appropriate compari-
shift nearly as much or at all and can be interpreted accord-
sons may be made.
ingly. This method can sometimes verify that the peaks being
used for charge referencing (for example, Au 4f or C 1s) are
6. Apparatus
behaving in the same manner as the peaks of interest from the
6.1 One or more of the charge compensation techniques
specimen (1,8,9). For nonuniform or composite (nonconduct-
mentioned in this guide may be employed in virtually any XPS
ing) specimens, a variety of charge shifts may be observed
spectrometer.
upon biasing.
6.2 Some of the techniques outlined require special acces-
7.1.1.5 Low Energy Ion Source—Recent work indicates that
sory apparatus, such as electron flood sources or a source for
portions of an insulator surface can be negatively charged, even
evaporative deposition.
when some areas exposed to X-rays are charged positively
6.3 Certain specimen mounting procedures, such as mount-
(10). Such effects appear to be particularly important for
ing the specimen under a fine metal mesh (1) , can enhance
focused X-ray beam systems, where the X-rays strike only a
electrical contact of the specimen with the specimen holder, or
relatively small portion of the specimen. In these circum-
reduce the amount of surface charge buildup. This and other
stances the use of a low-eneergy positive-ion source, in
methods of specimen mounting to reduce static charge are
addition to an electron source, may help stabilize (and make
described in detail in Guide E 1078 and Guide E 1829 .
more uniform) the surface potential of the specimen.
7.2 Binding Energy Reference Methods:
7. Procedures
7.2.1 Adventitious Carbon Referencing (1,2,8,11-15)—
7.1 The methods described here involve charge control (the
Unless specimens are prepared for analysis under carefully
effort to control the buildup of charge at a surface or to
controlled atmospheres, the surface, generally, is coated by
minimize its effect), charge referencing (the effort to determine
adventitious contaminants. Once introduced into the spectrom-
a reliable binding energy despite buildup of charge), or some
eter, further specimen contamination can occur by the adsorp-
combination of the two. For charge control, peak shape is the
tion of residual gases, especially in instruments with oil
most important parameter to consider. Correcting the peak
diffusion pumps. These contamination layers can be used for
position is accomplished separately using an appropriate
referencing purposes if it is assumed that they truly reflect the
charge referencing technique. In some circumstances the Auger
steady-state static charge exhibited by the specimen surface
parameter can provide chemical information without the need
and that they contain an element with a peak of known binding
to resort to surface potential corrections.
energy. Carbon is most commonly detected in adventitious
7.1.1 Methods to Control Surface Potential:
layers, and photoelectrons from the C 1s transition are those
7.1.1.1 Electron Flood Gun (2-5)—Use low-energy electron
most often adopted as a reference.
flood guns to stabilize the static charging of insulators exam-
7.2.1.1 A binding energy of 284.8 eV is often used for the C
ined by XPS (3), in particular when monochromatized X-rays
1s level of this contamination and the difference between the
are employed. Optimum operating conditions, for example,
measured position in the energy spectrum and the reference
filament position, electron energy, and electron current, depend
value, above, is the amount of surface potential shift caused by
upon the orientation of the electron flood gun with respect to
charging.
the specimen and upon the particular design of the electron
7.2.1.2 The main disadvantage of this method lies in the
uncertainty of the reference values as reported in the literature
(2,12,13) that ranges from 284.6 to 285.2 eV for the C 1s
The boldface numbers given in parentheses refer to a list of references at the
end of the text. electrons. Therefore, it is recommended that if adventitious
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1523
carbon is to be used for referencing, the reference binding such a nature that a portion of it has spectral lines of known
energy should be determined on the user’s own spectrometer. binding energy that can be used as the charge reference (11).
Ideally, this measurement should be carried out on a substrate This method assumes the invariance of the binding energy of
similar in its chemical and physical properties to the material to the chosen chemical group in different molecules. The mea-
be analyzed and covered by only a thin, uniform contamination sured peak energy will include the static charge of the
layer (that is, of the order of a monolayer). specimen. A shift factor, calculated to correct the binding
7.2.1.3 Care must be taken where adventitious hydrocarbon energy of the reference chemical group to the assumed value,
can be chemically transformed, as, for example, by a strongly can be applied to other measured peaks. If carbon is used, the
oxidizing specimen (13). With less than one monolayer cover- technique is called internal carbon referencing.
age of adventitious carbon, the C 1s binding energy sometimes
7.3 Bias Referencing (9)—This method of charge referenc-
decreases (14). The carbon binding energy may also shift as a
ing uses a small amount of a known external calibrant with an
consequence of ion sputtering; evidence has been found for
electron flood gate to reference an insulating surface. Typically,
carbon of lower binding energy, possibly graphite or, more ˚
a small gold dot (1 to 3-mm diameter and 250 A thick) is
likely, carbon in domains approaching atomic dimensions (8).
placed on the specimen surface by vacuum evaporation. XPS
Despite the limitations and uncertainties associated with the
spectra of both the gold dot and a representative area on the
use of adventitious carbon for static-charge referencing, it is
specimen surface are obtained under the influence of a negative
the most convenient and commonly applied technique.
bias (approximately 10 V) that may be produced by electrons
7.2.2 Gold Decoration (2,3,11,16-19)—Traditionally,“
from a standard neutralizer. Resulting spectra can be refer-
gold decoration” refers to the application of a uniform thin
enced to gold by the application of a shift factor, calculated
layer (0.5 to 0.7 nm) of elemental gold to the entire surface of
from the difference between the Au 4f peak under negative
7/2
an insulator in order to provide a metal calibrant on the sample
bias conditions and that same peak when the gold dot is in
surface. This layer is also connected to the spectrometer by
electrical contact with the spectrometer. In practice, Au 4f
7/2
mechanical contact with the sample holder so that both the
spectra are usually examined before and after obtaining data
spectrometer and the layer are at the same electrical potential.
from the specimen in order to monitor system drift. It appears
It is assumed that the contact between the deposited layer and
that this method brings about vacuum level alignment rather
the surface of the sp
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