ISO/TR 19319:2003
(Main)Surface chemical analysis — Auger electron spectroscopy and X-ray photoelectron spectroscopy — Determination of lateral resolution, analysis area, and sample area viewed by the analyser
Surface chemical analysis — Auger electron spectroscopy and X-ray photoelectron spectroscopy — Determination of lateral resolution, analysis area, and sample area viewed by the analyser
ISO/TR 19319:2003 provides information for measuring (1) the lateral resolution, (2) the analysis area, and (3) the sample area viewed by the analyser in Auger electron spectroscopy and X-ray photoelectron spectroscopy.
Analyse chimique des surfaces — Spectroscopie des électrons Auger et spectroscopie de photoélectrons — Détermination de la résolution latérale, de l'aire de la surface d'analyse et de l'aire de la surface de l'échantillon contribuant au signal détecté
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TECHNICAL ISO/TR
REPORT 19319
First edition
2003-12-01
Surface chemical analysis — Auger
electron spectroscopy and X-ray
photoelectron spectroscopy —
Determination of lateral resolution,
analysis area, and sample area viewed by
the analyser
Analyse chimique des surfaces — Spectroscopie des électrons Auger
et spectroscopie de photo-électrons — Détermination de la résolution
latérale, de l'aire de la surface d'analyse et de l'aire de la surface de
l'échantillon contribuant au signal détecté
Reference number
ISO/TR 19319:2003(E)
©
ISO 2003
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ISO/TR 19319:2003(E)
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ISO/TR 19319:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Terms and definitions. 1
3 Symbols and abbreviated terms. 2
4 Background information on lateral resolution, analysis area, and sample area viewed by
the analyser . 2
4.1 General information . 2
4.2 Lateral resolution . 3
4.3 Analysis area . 10
4.4 Sample area viewed by the analyser. 12
5 Measurements of lateral resolution, analysis area, and sample area viewed by the
analyser. 12
5.1 General information . 12
5.2 Lateral resolution . 13
5.3 Analysis area . 14
5.4 Sample area viewed by the analyser. 14
Bibliography . 16
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ISO/TR 19319:2003(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
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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 exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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/TR 19319 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 5, Auger electron spectroscopy.
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ISO/TR 19319:2003(E)
Introduction
This Technical Report is intended to be used as follows:
a) To provide guidance on the determination of lateral resolution in Auger electron spectroscopy and X-ray
photoelectron spectroscopy where measurements are made of Auger electron or X-ray photoelectron
peak intensities as a function of position on a sample surface.
b) To provide guidance on the determination of analysis area in similar applications of Auger electron
spectroscopy and X-ray photoelectron spectroscopy.
c) To provide guidance on the determination of sample area viewed by the analyser in applications of Auger
electron spectroscopy and X-ray photoelectron spectroscopy.
d) To serve as a basis for the development of International Standards for measurements of lateral resolution,
analysis area, and sample area viewed by the analyser in Auger electron spectroscopy and X-ray
photoelectron spectroscopy.
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TECHNICAL REPORT ISO/TR 19319:2003(E)
Surface chemical analysis — Auger electron spectroscopy and
X-ray photoelectron spectroscopy — Determination of lateral
resolution, analysis area, and sample area viewed by the
analyser
1 Scope
This Technical Report provides information for measuring (1) the lateral resolution, (2) the analysis area, and
(3) the sample area viewed by the analyser in Auger electron spectroscopy and X-ray photoelectron
spectroscopy.
2 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115 [1] apply. The definitions of
“analysis area ” and “lateral resolution” from ISO 18115 are given for convenience here. A definition
of “sample area viewed by the analyser” is proposed. This definition is similar to the definition of “analysis area
” in ISO 18115. The term “sample area viewed by the analyser” is preferred in this Technical
Report to distinguish this area from the corresponding area when the sample is set in a plane at right angles
to the spectrometer axis.
2.1
analysis area
two-dimensional region of a sample surface measured in the plane of that surface from which the
entire analytical signal or a specified percentage of that signal is detected
2.2
resolution, lateral
distance measured either in the plane of the sample surface or in a plane at right angles to the axis of the
image-forming optics over which changes in composition can be separately established with confidence
NOTE 1 The choice of plane should be stated.
NOTE 2 In practice, the lateral resolution may be realised as either (i) the FWHM of the intensity distribution from a
very small emitting point on the sample or (ii) the distance between the 12% and 88% intensity points in a line scan across
a part of the sample containing a well-defined step function for the signal relating to the property being resolved. These
two values are equivalent for a Gaussian intensity distribution. For other distributions, other parameters may be more
appropriately chosen. Often, for a step function, the distance between the 20% and 80% intensity points or the 16% and
84 % intensity points in the line scan are used. The latter pair gives the two sigma width for a Gaussian resolution function.
2.3
sample area viewed by the analyser
two-dimensional region of a sample surface measured in the plane of that surface from which the analyser
can collect an analytical signal from the sample or a specified percentage of that signal
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ISO/TR 19319:2003(E)
3 Symbols and abbreviated terms
AES Auger electron spectroscopy
erf error function
FWHM full width at half maximum
I Auger electron intensity
I incident beam current (in AES)
i
I maximum Auger electron intensity
max
J (r) intensity distribution of detected Auger electrons as a function of the radius r
A
J (r) intensity distribution of detected Auger electrons that were created by backscattered electrons as a
Ab
function of the radius r
J (r) intensity distribution of detected Auger electrons that were created by the incident beam as a function
Ai
of the radius r
R backscattering factor (in AES)
r radius from the centre of the incident electron beam on the sample surface (in AES)
r upper limit of integration in equation (5)
max
XPS X-ray photoelectron spectroscopy
δr lateral resolution
δr(50) lateral resolution determined from a 25% to 75% intensity change in a line profile
σ Gaussian parameter describing the radial distribution of backscattered electrons (in AES)
b
σ Gaussian parameter describing the radial distribution of the incident electron beam (in AES)
i
4 Background information on lateral resolution, analysis area, and sample area
viewed by the analyser
4.1 General information
A common need in AES and XPS is the measurement of composition as a function of position on the sample
surface. Typically, an analyst wishes to determine the local surface composition of some identified region of
interest. This region of interest could be a feature on a semiconductor wafer (such as an unwanted defect
particle or contamination stain), a corrosion pit, a fibre, or an exposed surface of a composite material. With
growing industrial fabrication of devices with dimensions on the micrometer and nanometer scales, particularly
in the semiconductor industry [2] and for emerging nanotechnology applications, there is an increasing need
to characterize materials using tools with lateral resolutions and dimensions of analysis areas that are smaller
than those of the features of interest. It is generally necessary in these applications to be able to determine
that devices have been fabricated as intended (quality control), to evaluate new or current fabrication methods
(process development and process control), and to identify failure mechanisms (failure analysis) of a device
during its service life or after exposure to different ambient conditions. The lateral resolution and the analysis
area are important and related parameters in the application of characterization techniques such as AES and
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ISO/TR 19319:2003(E)
XPS for the surface characterization of materials containing features with micrometer and nanometer
dimensions. Another parameter that is important in some measurements is the sample area viewed by the
electron energy analyser. The needs for measurements of lateral resolution, analysis area, and sample area
viewed by the analyser are described in the following sections.
As in optical [3-6] and various forms of electron microscopy [7-9], the achievable lateral resolution is related to
the contrast found in a measured image. A discussion of contrast mechanisms, various definitions of lateral
resolution, and image quality is beyond the scope of this report, and the reader is referred to detailed analyses
available elsewhere [3-9]. It is pointed out, however, that the contrast transfer function is a useful means for
describing the contrast in an image as a function of spatial frequency [3-9]. At the highest detectable spatial
frequency, the contrast approaches zero. The achievable lateral resolution in a particular AES or XPS
measurement will therefore depend not only on the instrumental characteristics but on the available contrast
(e.g., from the signals associated with two neighbouring chemical phases for a particular measurement time).
An overview is given in this report of certain instrumental and measured properties that are described in terms
of Gaussian functions. This approach is believed to be a useful guide but it should be emphasized that the
properties of real instruments and of real measurements can depart from the Gaussian model considered here.
In addition, the detectability of a feature in AES and XPS measurements depends in part on the measure of
lateral resolution of the instrument and in part on the difference in signal intensities for measurements made
on and off the possible feature and the observation time (through the statistical variations in the signal
intensities). The detectability of a feature thus depends on the contrast transfer function for the measurement
and the measurement time. The specific results will thus be a function of both instrumental and sample
properties. Reliable detection of a feature will also depend on instrumental stability (particularly the stability of
the incident electron beam current in AES and the X-ray flux in XPS, and the positional stability of the sample
stage with respect to the electron or X-ray beam) and the chemical stability of the sample during the time
needed for acquisition of AES or XPS data.
4.2 Lateral resolution
4.2.1 Introduction
It is clearly desirable that the lateral resolution of the technique be smaller than the lateral dimensions of the
feature of interest in order that the feature can be reliably analysed. The feature of interest in an AES
instrument might typically be initially detected in a scanning electron micrograph. The primary electron beam
could then be positioned on the feature, and an Auger electron spectrum recorded. In XPS instruments, the
feature of interest must generally be detected from an image or a line scan in which a particular signal (often
the intensity of a selected photoelectron peak) is displayed as a function of position on the sample surface.
Many authors have described and discussed the lateral resolution (often referred to as spatial resolution) of
AES and XPS instruments. Useful information can be found in a review by Cazaux [9] for AES and in a review
by Drummond [10] for XPS.
Figures 1 to 3 show schematic diagrams of typical experimental configurations for AES and XPS. These
Figures show the exciting radiation incident on the sample surface. For AES (Figure 1), an electron beam with
an energy between 3 keV and 25 keV is focused to a “spot” on the sample surface. With a field-emission
electron source, the full width at half maximum (FWHM) intensity of the focused spot may be between 5 nm
(or even lower) and 50 nm depending on the beam energy and the beam current. The beam is scanned
across a region of interest on the sample surface, and various signals collected (such as secondary-electron
and Auger electron signals). The Auger electron signal arises from inner-shell ionisations caused in part by
the incident beam and in part by backscattered electrons [9]. The lateral resolution in AES is mainly
determined by the FWHM of the focused spot [9]; further details are given in 4.2.2.
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ISO/TR 19319:2003(E)
Key
1 focused electron beam for AES
focused X-ray beam for XPS
2 to analyser
3 sample
Figure 1 — Schematic outlines of experimental configurations for AES and XPS
with a focused incident beam
Figure 1 also indicates an XPS configuration in which the incident X-ray beam is focused to a spot on the
sample surface. With a conventional X-ray source and a bent-crystal focusing X-ray monochromator, the
FWHM of the focused spot can be less than 10 µm. With a synchrotron source of X-rays and a zone-plate, the
FWHM of the focused spot can be less than 100 nm [11]. The lateral resolution is determined by the FWHM of
the focused spot. The experimental configurations for AES and XPS in Figure 1 are thus similar in that an
incident beam is focused to a small area on the sample surface. Lateral variations of surface composition can
thus be detected as the beam is positioned on different regions of interest, is linearly scanned across a
selected region, or is rastered to obtain information from a selected area. If the incident beam in Figure 1 is
not normally incident on the sample surface, the beam profile will be elliptical instead of circular. In such cases,
the lateral resolution will be given by the FHWM of the beam profile in two orthogonal directions (parallel and
perpendicular to the plane of incidence).
Figure 2a) illustrates an XPS configuration in which the electron energy analyser is part of an electron-optical
configuration that views a selected single small area on the sample surface. The lateral resolution for this
configuration depends on the electron-optical design and can be less than 10 µm. Figure 2b) shows an XPS
configuration in which the electron-optical system produces an image of a selected region of the surface. In
this mode, different pixels of the image correspond to particular regions of the surface; information from
multiple points on the surface can be recorded in parallel. Figures 2a) and 2b) are similar in that the regions of
interest are selected by the electron-optical system. Lateral variations of surface composition can be detected,
in principle, by mechanically moving the sample with respect to the analyser or, usually, by adjustment of the
electron-optical system to select the particular regions of interest on the sample surface from which
photoelectrons are detected. As for Figure 1, photoelectron signals can be obtained from a selected region,
from multiple regions along a line, or from multiple regions within a selected area.
Figure 3 shows a simpler XPS configuration in which the sample is irradiated by X-rays from a nearby X-ray
source and photoelectrons are detected as in Figure 2 from an area defined by the electron-optical properties
of the analyser. Unlike the configurations of Figure 2, however, the instruments represented by Figure 3 were
not designed to detect lateral variations of surface composition except by movement of the sample with
respect to the analyser. In this way, a lateral resolution of about 0,1 mm to 1 mm could be achieved.
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ISO/TR 19319:2003(E)
a) XPS with single-point analysis b) XPS with multi-point analysis
Key
1 X-ray beam
2 to analyser
3 sample
Figure 2 — Schematic outlines of XPS configurations in which a) the analyser accepts photoelectrons
from a selected area on the sample surface (single-point analysis)
or b) the analyser accepts photoelectrons from multiple regions on the sample surface
to create an image of the surface (multi-point analysis)
Key
1 X-ray beam
2 to analyser
3 sample
Figure 3 — Schematic XPS configuration in which the sample is irradiated by a broad X-ray beam
and in which photoelectrons are accepted by the analyser from a larger area
of the sample surface than for Figure 2
4.2.2 Lateral resolution for AES
For simplicity in the following discussion, it will be assumed that the sample has a plane surface and that the
primary electron beam is normally incident on the sample. It is also assumed that the analysis area is smaller
than the sample area viewed by the analyser and that the detection efficiency of the analyser is uniform within
the analysis area.
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ISO/TR 19319:2003(E)
Although the incident electron beam in AES can be focused to a spot with FWHM less than 50 nm, detected
Auger electrons originate from ionisations caused by the incident beam and by backscattered electrons [9,12].
Due to multiple elastic- and inelastic-electron scattering, the backscattered electrons can cause inner-shell
ionisations that lead to detected Auger electrons from sample regions of up to about 1 µm from the incident-
beam spot. The intensity distribution J ()r of detected Auger electrons as a function of radius r can be
A
described by the sum of two Gaussian functions [9,12,13]:
22 2 2 2 2
Jr( )=−(I / 2πσ )exp(r / 2σ )+ [(R−1)I / 2πσ ]exp(−r / 2σ ) (1)
Ai i i i b b
or
J ()rJ=+()r J (r) (2)
AAi Ab
where I is the incident beam current, σ is the Gaussian parameter describing the radial distribution of the
i i
incident electron beam, σ is the Gaussian parameter describing the radial distribution of backscattered
b
electrons, R is the backscattering factor, and J (r)and (J r) are the radial intensity distributions for Auger
Ai Ab
electrons created by the incident beam and by backscattered electrons, respectively. The FWHM values for
these two Gaussian functions are 2,35σ and 2,35σ , respectively.
i b
Seah [14] has shown that, with 20 keV incident electrons, the FWHM values for backscattered electrons vary
between about 0,2 µm and 3,0 µm for different elements; values of σ thus range from about 0,085 µm to
b
about 1,3 µm. As an illustrative example, Figure 4 shows a plot of J (r) with σ = 10 nm, σ = 200 nm, and
A i b
R = 1,5; for simplicity, J (r) has been normalised to unity at r = 0. Because σ >>σ, (Jr=0) is about
A b i Ai
three orders of magnitude greater than Jr( = 0) in this example. It is thus possible for the lateral resolution
Ab
to be determined mainly by the value of σ although, as will be shown, the magnitude of J (r) in the vicinity
i Ab
of r= 0 also affects the lateral resolution.
Figure 4 — Plot of the total Auger electron intensity distribution J (r) (normalised to unity at r = 0)
A
from equation (1) as a function of r with σ = 10 nm, σ = 200 nm, and R = 1,5
i b
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ISO/TR 19319:2003(E)
The lateral resolution in AES has often been determined by scanning the primary electron beam across a
sufficiently sharp chemical gradient (a chemical edge) in the plane of the sample; the scan direction is normal
to the chemical edge in such measurements. The Auger electron intensity, I, for one of the materials is then
measured as a function of beam position on the sample. The lateral resolution, δr, has been variously defined
as the distance over which the intensity I changes from 25% to 75% of its maximum value, I , the distance
max
over which I changes from 20% to 80% of I , the distance over which I changes from 16% to 84% of I ,
max max
the distance over which I changes from 12% to 88% of I , and the distance over which I changes from 10%
max
to 90% of I [1, 12].
max
The intensity distribution of detected Auger electrons shown in equation (1) can also be written in Cartesian
coordinates [12,13]. It is then possible to calculate the change in detected Auger intensity as the primary
beam is scanned across an abrupt chemical interface as in the experiments. Figure 5 shows a plot of I/I as
max
a function of scan distance for the same parameters used in Figure 4. While there is a steep increase in the
value of I/I in the vicinity of the origin in Figure 5 (corresponding to the primary-beam component J ()r in
max Ai
Figure 4), there are significant tails in the plotted I/I due to the backscattered-electron component
max
J ()r in Figure 4.
Ab
In the example of Figure 5, the measures of the lateral resolution are about 15 nm, 22 nm, 32 nm, 102 nm,
and 150 nm for the 25 % to 75 %, 20 % to 80 %, 16 % to 84 %, 12 % to 88 %, and 10 % to 90 % Auger
intensity changes, respectively. It is clear that the measure of lateral resolution is mainly determined by the
FWHM of the primary beam [that is, the parameter σ in equation (1)] if, in this example, the measure of
i
lateral resolution is found from the distances corresponding to the 25 % to 75 %, 20 % to 80 %, and 16 % to
84 % Auger electron intensity changes (although, as will be shown shortly, these measures of lateral
resolution also depend weakly on σ and R). In contrast, the measure of lateral resolution is a strong function
b
of all three parameters in equation (1) (σ , σ , and R) if the measure of lateral resolution is found from the
i b
distances corresponding to the 12% to 88% and 10% to 90% Auger electron intensity changes. Since the
values of σ and R depend on the sample and the primary electron energy [7],[15], it is desirable for the
b
measure of lateral resolution in AES to be determined in a way that is least dependent on the sample
properties. It is therefore recommended that the lateral resolution be obtained from the distances
corresponding to the 25% to 75% Auger electron intensity changes in a scan such as that shown in Figure 5.
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ISO/TR 19319:2003(E)
Key
X scan distance (nm)
Y I/I
max
Figure 5 — Plot of the ratio of the calculated Auger electron intensity, I,
to the maximum Auger electron intensity, I , as a function of scan distance as the primary beam
max
is scanned across a sharp chemical boundary located at the origin
(with the beam and backscattering parameters used in Figure 4)
(In this example, I/I is plotted for the material on the right-hand side of the edge. The horizontal lines show
max
I/I = 0,25 and I/I = 0,75. The lateral resolution can be determined from the difference between the scan
max max
distances for these values of I/I ; in this example, the lateral resolution is about 15 nm.)
max
Seah [13] has shown that the measure of the lateral resolution, δr(50), corresponding to the 25 % to 75 %
change in Auger electron intensity across an abrupt chemical edge can be determined from the relation:
0,5Rz=+erf[ (σσ)] (R−1)erf[z( )] (3)
ib
where erf(z) is the error function defined by:
z
2
erf(z)=−(2/ π ) exp(td)t (4)
∫
0
and where tr=δ (50)/ 2σ and tr=δ (50)/ 2σ for the first and second terms in equation (3), respectively.
i b
Figure 6 shows plots of δr(50)/σ versus σ /σ for four values of the backscattering factor R. These plots show
i b i
that the value of δr(50)/σ does not vary appreciably with σ /σ when the latter ratio is greater than about 20.
i b i
The value of δr(50)/σ does, however, depend on R although δr(50)/σ is between 1,35 (when R = 1) and about
i i
3,1 (when R= 1, 8). If, however, the measure of the lateral resolution was determined from the 10% to 90%
changes in Auger electron intensity across an edge, Cazaux has shown that this measure changes almost
linearly with σ /σ and with a slope that depends on the value of R [12].
b i
The results shown in Figures 4 to 6 were for normal incidence of the primary electron beam. Cazaux [12] has
made similar analyses for primary beams at non-normal incidence. The results of these model calculations
agree well with experimental measurements and with Monte Carlo simulations of Auger electron production by
backscattered electrons [12,16]. Cazaux [9] has also considered the detectability of features in the form of
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ISO/TR 19319:2003(E)
stripes in the plane of the surface and has examined Auger electron intensity profiles for chemically
non-abrupt edges. El Gomati et al. [17] have shown the importance of edge effects in Auger electron line
profiles when the pri
...
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