ISO 18516:2019

INTERNATIONAL ISO
STANDARD 18516
Second edition
2019-01
Surface chemical analysis —
Determination of lateral resolution
and sharpness in beam based methods
with a range from nanometres to
micrometres
Analyse chimique des surfaces — Détermination de la résolution
latérale et de la netteté par des méthodes à base de faisceau utilisant
une gamme allant des nanomètres aux micromètres
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
5 General information . 4
5.1 Background . 4
5.2 Survey on principal methods to characterize lateral resolution in imaging surface
chemical analysis . 5
5.3 Measurement of lateral resolution in imaging surface chemical analysis . 6
5.4 Dependence of lateral resolution on scan direction . 6
5.5 Reporting results . 7
6 Measurement of lateral resolution using the straight edge method .8
6.1 Introduction . 8
6.2 Model functions and sharpness parameters . 8
6.3 Requirements for a test sample . 9
6.4 Cleaning the straight-edge specimen .10
6.5 Mounting the straight-edge specimen .10
6.6 Operating the instrument .10
6.7 Data acquisition requirements .10
6.8 Determination of D .11
12−88
6.9 Determination of line spread function .11
6.10 Reporting .11
7 Measurement of lateral resolution using the narrow line method .13
7.1 Introduction .13
7.2 Requirements for a test sample .13
7.3 Cleaning the narrow stripe specimen .13
7.4 Mounting the narrow stripe specimen .14
7.5 Operating the instrument .14
7.6 Data acquisition requirements .14
7.7 Determination of w .14
LSF
7.8 Reporting .15
8 Measurement of lateral resolution using the grating method .17
8.1 Introduction .17
8.2 Requirements for a test sample .17
8.3 Cleaning the grating specimen .19
8.4 Mounting the grating specimen.19
8.5 Operating the instrument .20
8.6 Data acquisition .20
8.7 Estimation of effective lateral resolution r by visual inspection of an image or a
e
line scan .20
8.8 Determination of effective lateral resolution r by numerical analysis of a line profile .23
e
8.8.1 General.23
8.8.2 Consideration of noise and determination of reduced noise σ .23
Nr
8.8.3 Determination of dip D .24
8.8.4 Resolution criterion .25
8.9 Determination of the effective lateral resolution r using graded gratings .26
e
8.9.1 Estimation of effective lateral resolution r by using the resolution
e
criterion in Formula (17).26
8.9.2 Determination of effective lateral resolution r by using Formula (17) and
e
an interpolation–extrapolation method .27
8.10 Reporting .29
Annex A (informative) The relation between sharpness parameters and effective lateral
resolution .31
Annex B (informative) Straight edge method: systematic underestimation of D caused
12−88
by insufficient plateau length L .33
pl
Annex C (informative) Straight edge method: required length ranges for L .35
ESF
Annex D (informative) Straight edge method: the uncertainty of D .36
12−88
Annex E (informative) Narrow stripe method: systematic overestimation of w caused by
LSF
inappropriately large widths w of the imaged stripe .37
s
Annex F (informative) Narrow line method: the uncertainty of w .39
LSF
Annex G (informative) Imaging of square gratings: reduction of image period for three-
stripe gratings .40
Annex H (informative) Imaging of square gratings: relation between signal-to-noise ratio
and effective lateral resolution .43
Annex I (informative) Imaging of square gratings: minimum number of sampling points per
period .46
Annex J (informative) Imaging of square gratings: uncertainty of r determined by visual
e
inspection of an image or line scan over a series of gratings .47
Annex K (informative) Imaging of square gratings: Uncertainty of r determined by
e
interpolation–extrapolation .48
Annex L (informative) Determination of lateral resolution by imaging of square wave
gratings — practical example for SIMS .50
Bibliography .53
iv © ISO 2019 – All rights reserved

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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 2, General procedures.
This second edition cancels and replaces the first edition, ISO 18516:2006, which has been technically
revised as follows:
1. content related to straight edge method expanded;
2. new content addressing the narrow stripe method added;
3. new content addressing the use of gratings in the determination of lateral resolution added;
4. implementation of concepts developed in ISO/TR 19319:2013(E);
5. title and scope changed to address nanotechnology following the recommendations of TC 201/SG 1.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
Introduction
Secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES) and X-ray photoelectron
spectroscopy (XPS) are surface-analytical techniques that are used to generate chemical maps of
surfaces and line scans across surfaces. These techniques can have lateral resolutions for instance as
good as 10 nm for AES, 50 nm for SIMS and 5 µm for laboratory XPS and can cover areas as large as
many square millimetres by using stitching techniques. Different instruments generate images with
different lateral resolutions. Moreover, an analyst needs to have a suitable method to measure the
lateral resolution of an instrument for any given settings. In this way, analysts can obtain the optimum
lateral resolution from a given instrument, appropriate to the analytical requirements, in a consistent
and clear way. The ability of the analyst to realize these resolutions in an effective way will, of course,
also depend on the quality of the signal levels obtained and the level of noise. Resolution is a quality
parameter of images and line scans and describes the performance of imaging instruments used to
deliver them. This document is based on ISO/TR 19319:2013, which explains theoretical backgrounds
of a determination of resolution and sharpness parameters used to express the performance of imaging
[1]
instruments .
This document describes different methods for the determination of lateral resolution in beam-based
methods as AES, SIMS and XPS. These are (a) the straight edge method, (b) the narrow line method and
(c) the grating method. The method to be chosen for use depends on the expected value of the lateral
resolution and the specific needs to be addressed. The standard is targeted at the needs of different
communities: the manufacturers for specifying or benchmarking an instrument, the analysts in a
laboratory for their day-to-day running of instruments to match the needs of good laboratory practice
(GLP) and the analysts in testing laboratories operating under a formal accreditation scheme, for
example ISO 17025, who must prepare and run standard operation protocols (SOP) for regular function
control of instruments.
The annexes provide forthcoming information on how to find appropriate measurement parameters,
considerations of the uncertainty of measurement and one practical example, the determination of
effective lateral resolution by evaluation of a secondary ion image of a grating.
vi © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 18516:2019(E)
Surface chemical analysis — Determination of lateral
resolution and sharpness in beam based methods with a
range from nanometres to micrometres
1 Scope
This document describes methods for measuring lateral resolution and sharpness in imaging surface
chemical analysis. It applies to all methods of surface analysis which use a beam to analyse the chemical
composition of surfaces under defined settings of an instrument. It applies to scanning instruments,
where a finely focused beam is scanned over the sample in a preselected field of view, as well as to
full field imaging instruments, where the field of view is simultaneously imaged by a broad beam,
an imaging lens system and a pixelated detector. The methods for measuring lateral resolution and
sharpness are
— the straight edge method;
— the narrow line method;
— the grating method.
This document applies to instruments and methods that provide information on layers with nanometre
thicknesses and to surfaces with nanometre-sized structures and individual nano-objects.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 16242:2011, Surface chemical analysis — Recording and reporting data in Auger electron
spectroscopy (AES)
ISO 16243:2011, Surface chemical analysis — Recording and reporting data in X-ray photoelectron
spectroscopy (XPS)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
edge spread function
ESF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely sharp edge
[SOURCE: ISO 12231:2012, 3.43, modified — note removed]
3.2
effective lateral resolution
r
e
minimum spacing at which two features of the image can be recognised as distinct and separate
Note 1 to entry: For the analytical methods in the scope of this document the image is formed from chemical
composition data.
Note 2 to entry: “effective” has been added because the lateral resolution characteristic of an image is not only
defined by the instrument used for taking it but also by the noise in the image data (see Reference [2] and ISO/
TR 19319:2013). Effective lateral resolution r as introduced in this document takes account of that noise.
e
Note 3 to entry: The said minimum spacing can be determined as that distance between two stripes of a square-
wave grating at which the dip of signal intensity between two maxima of the measured grating profile is at least
four times the reduced noise.
Note 4 to entry: In microbeam analysis [ISO 22493:2008, 7.2] the term “image resolution” is defined in a similar way.
3.3
line spread function
LSF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely thin line
[SOURCE: ISO 12231:2012, 3.94]
3.4
noise
time-varying disturbances superimposed on the analytical signal with fluctuations leading to
uncertainty in the signal intensity
Note 1 to entry: An accurate measure of noise can be determined from the standard deviation of the fluctuations.
[SOURCE: ISO 18115-1:2013, 4.315, modified — Note 2 to entry removed.]
3.5
reduced noise
1/2
standard deviation of noise σ multiplied by (5/S ) , where S is the number of sampling points per
N pp pp
period in the measured grating profile
3.6
sharpness
property of an image or line scan to show a sharp line of demarcation between two adjacent areas of
different signal intensities
Note 1 to entry: For the analytical methods in the scope of this document the signal intensities in images and line
scans relate to chemical composition.
Note 2 to entry: In practical laterally resolved surface analysis objective measures of sharpness are realized
as the distance D between the 12 % and 88 % intensity points in a line scan across a part of the sample
12−88
containing a well-defined step function for the signal relating to the property being resolved.
[SOURCE: ISO 6196-5:1987, 05.04, modified]
3.7
signal-to-noise ratio
R
S/N
ratio of the signal intensity to a measure of the total noise in determining that signal
[SOURCE: ISO 18115-1:2013, 4.427, modified — Notes to entry removed.]
2 © ISO 2019 – All rights reserved

4 Symbols and abbreviated terms
AES Auger electron spectroscopy
CRM certified reference material
d distance between two narrow stripes
D dip between two maxima of signal intensity in the line profile over a grating
d distance between two consecutive gratings
gr
D ESF steepness parameter giving the distance between points of well-defined intensities x
x−(100−x)
and 100-x (e.g. 20 % to 80 %) of the profile over a straight edge
ESF edge spread function
F fit range
r
FWHM full width at half maximum
I mean value of signal intensity in the image of a three-stripe-grating (A-B-A)
m
I signal intensity of the left maximum in the image of a three-stripe-grating (A-B-A)
max l
I signal intensity of the right maximum in the image of a three-stripe-grating (A-B-A)
max r
I signal intensity of the minimum in the image of a three-stripe-grating (A-B-A)
min
L length
L length range of measured values of the ESF
ESF
L length of a plateau of constant concentration
pl
LSF line spread function
P grating period
P period of the largest non-resolved grating
P period of the first (finest) resolved grating
P period of the second resolved grating
P period of the image of a grating
im
P period at R = 4 determined by interpolation between P and P
int 0 1
P period at R = 4 determined by extrapolation with P and P
ext 1 2
PSF point spread function
q grading factor of consecutive grating periods q = P / P
n+1 n
R R = D/σ – dip-to-reduced-noise ratio
Nr
r effective lateral resolution
e
R signal-to-noise ratio
S/N
S sampling step width
w
S sampling points per period as a variable
pp
SIMS secondary ion mass spectrometry
U expanded uncertainty of a quantity
w full width at half maximum of the line spread function
LSF
w width of a stripe in the object pattern
s
XPS X-ray photoelectron spectroscopy
σ standard deviation of noise
N
σ standard deviation of reduced noise
Nr
5 General information
5.1 Background
A common need in imaging surface chemical analysis by methods such as SIMS, 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 micrometre and nanometre scales, particularly in the
semiconductor industry and for emerging nanotechnology applications, there is an increasing need to
characterize materials using tools with lateral resolutions 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 is an
important parameter in the application of characterization techniques such as SIMS, AES and XPS for the
surface characterization of materials containing features with micrometre and nanometre dimensions.
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 readily imaged. The feature of interest shall
generally be detected from an image or a line scan in which a particular signal (the intensity of a
selected secondary ion, a photoelectron peak or an Auger electron transition) is displayed as a function
of position on the sample surface. In practice, the detectability of a feature in SIMS, AES and XPS
measurements depends not only on the lateral resolution but also the difference in signal intensities
for measurements made on and off the possible feature (materials contrast) and the observation time
(through the statistical variations in the signal intensities, i.e. noise). The detectability of a feature thus
depends on an instrumental setting enabling the required lateral resolution, the particular constituents
of the sample, and the measurement time needed to reach a required (low) noise level. Reliable detection
of a feature will also depend on instrumental stability (particularly the stability of the incident beam
current in SIMS and AES or the X-ray flux in XPS, and the positional stability of the sample stage and the
chemical stability of the sample during the time needed for acquisition of data.
Many authors have described and discussed the lateral resolution (often referred to as spatial
resolution) of SIMS, AES and XPS instruments. ISO/TR 19319:2013 provides guidance and theoretical
background on the determination of lateral resolution and related parameters in SIMS, AES and XPS.
4 © ISO 2019 – All rights reserved

5.2 Survey on principal methods to characterize lateral resolution in imaging surface
chemical analysis
All methods presented in this document are designed for a determination of parameters that
characterize the lateral resolution of an imaging instrument at specific settings. They use images or
line scans obtained from specific test samples. These methods are:
— the straight edge method (Clause 6);
— the narrow line method (Clause 7);
— the grating method (Clause 8).
Principally all methods are suitable for any resolution from micrometre down to nanometre scales and
work for all beam-based instruments used for imaging surface chemical analysis. They can be used either
— in a quick and easy manner as it is required by an analyst for daily performance tests. Data evaluation
is made by visual real-time inspection of an image on a screen or simple graphical analysis of a line
scan by only using pencil and ruler; or
— in a sophisticated manner which delivers accurate and objective values for resolution parameters
by using numerical data evaluation. These sophisticated methods shall be used for instrument
specification including benchmarking of instruments and documented validation of instrument
performance in an accredited testing laboratory.
NOTE 1 The lateral resolution might or might not depend either on the measured electron energy, for example
in imaging XPS or AES, or the selected secondary ion in SIMS.
The well-known straight edge method is a simple method suitable for the characterization of lateral
resolution. A line scan across or an image of a straight edge measured with high signal-to-noise ratio
and appropriate sampling step width shall be acquired using a test sample which displays a sharp
straight chemical edge between material A and material B and appropriately long plateau lengths left
and right from the edge. Different test samples for the micro and nano scale are on the market (see
References [3-5]). The method delivers lateral resolution expressed as the sharpness parameter D
12−88
which characterizes the (sigmoidal) edge spread function (ESF). The method is well suited for daily in-
house performance tests.
NOTE 2 As a single parameter, D cannot fully characterize the complex sigmoidal ESF (see Reference [1]
12−88
and references therein). Basically, the ESF is determined by the shape of the profile of the probe beam (e.g.
Gaussian, Lorentzian, mixed shape) used for imaging. Therefore, the character of D is ambiguous and
12−88
a minimum spacing at which two features of an image can be recognized as distinct and separate cannot be
deduced from that parameter in a simple manner. This ambiguity of D is illustrated by Figure A.1 in Annex A.
12−88
NOTE 3 For laboratory XPS instruments where the best lateral resolution reached nowadays is around 5 µm,
knife edges or mesh bars are principally suitable as test samples. In this case the vacuum becomes material
B. However, when using knife edges or mesh bars the individual geometry of the straight edge has, via edge
effects, impact on the resulting value of D and comparability of those results will be limited. There is no
12−88
comprehensive investigation of the impact of edge geometry on D published so far.
12−88
The use of gold islands (on graphite) to run the straight edge method is not recommended because of
imperfections of the straight edge due to arbitrary topography (leading to arbitrary D data) and
12−88
the risk of too small plateau lengths for material A or B.
The narrow line method is a simple method suitable for the characterization of lateral resolution. A
line scan across or an image of a narrow stripe measured with high signal-to-noise ratio and appropriate
sampling step width shall be acquired using a test sample which displays a narrow line of material A in
B where the line width shall be sufficiently low in comparison with the expected lateral resolution. As
a rough estimate the resolution shall be three to five times the width of the narrow line (see Annex E).
The method delivers lateral resolution expressed as the parameter w the full width at half maximum
LSF,
(FWHM) of the line spread function (LSF). Different test samples for the micro and nano scale are on
the market (see References [3-5]). The method is well suited for daily in-house performance tests.
NOTE 4 As a single parameter, w cannot fully characterize the complex line spread function (LSF). Basically,
LSF
the LSF is determined by the shape of the profile (e.g. Gaussian, Lorentzian, mixed shape) of the probe beam used
for imaging. Therefore, the character of w is ambiguous and a minimum spacing at which two features of an
LSF
image can be recognized as distinct and separate cannot be deduced from that parameter in a simple manner.
The grating method enables the determination of the lateral resolution in terms of the minimum
spacing at which two features of an image can be recognized as distinct and separate. To run the grating
method a line scan across or image of a series of A-B-A gratings of material A in B measured with high
signal-to-noise ratio and appropriate sampling step width (see Annex I) shall be acquired using a test
sample which displays a series of gratings narrower and broader than the expected resolution. The
method delivers the effective lateral resolution r , estimated by using the period P of the finest resolved
e
grating. Hence, r is the minimum spacing at which two features of the image can be recognized.
e
Different test samples for the micro and nano scale are on the market (see References [3-5]). Using
its visual inspection mode (see 8.7) the method is well suited for (daily) in-house performance tests.
Using its more sophisticated modes (see 8.9) it enables instrument specification and formal validation
of instrument performance.
NOTE 5 The advantage of the grating method is that it is linked to a widely accepted and overarching definition
of lateral resolution in microscopy. Results obtained by using the grating method for different instruments
(settings) can be compared with each other.
Considerations of the uncertainty of measurement related to all three methods used for a determination
of parameters that characterize the lateral resolution are given in Annexes D, F, J and K.
Test files containing data of real or simulated line profiles representing results obtained by each of
the three methods are available at: http: //isotc .iso .org/livelink/livelink ?func = ll & objId = 1864863 &
objAction = browse & viewType = 1 .
5.3 Measurement of lateral resolution in imaging surface chemical analysis
The lateral resolution in imaging surface chemical analysis typically depends on either the
characteristics of the incident radiation or the characteristics of the lens-analyser-detector system used
in the spectrometer. In the former case, the lateral resolution will depend mainly on the cross-sectional
dimensions (e.g. the beam diameter) of the incident radiation (e.g. electron beam in AES, X-ray beam
in XPS or primary ion beam in SIMS) at the sample surface, and will improve as the beam diameter
decreases.
All methods described in Clauses 6 to 8 involve measurements of the intensity of a selected spectral
feature while a sufficiently sharp chemical gradient on the sample is translated through the analysis
position defined by the incident beam or the analysis position is translated across the chemical gradient.
The measured lateral resolution will depend on the instrumental design (i.e. the beam diameter or the
electron/ion-optical design of the spectrometer) and the intrinsic sharpness of the chemical gradients
used for the measurements.
NOTE For AES, the magnitude and width of the Auger signal excited by back-scattered electrons have an
impact on lateral resolution.
5.4 Dependence of lateral resolution on scan direction
The measured lateral resolution can depend upon the direction in which the translation of the sample
with respect to the incident beam or the spectrometer is made. This variation can arise in any of the
following three situations:
a) if an X-ray, electron or primary ion beam of circular cross-section (i.e. the beam has axial symmetry)
is incident on the sample at a non-zero angle Θ relative to the surface normal; the beam-intensity
profile on the sample will then be an ellipse, as shown in Figure 1;
6 © ISO 2019 – All rights reserved

b) if the lateral resolution is defined by the analyser or lens, and the sample normal is not parallel
with the entrance axis of the analyser;
c) if the incident beam is astigmatic.
Lateral resolution should therefore be measured in at least two directions. In the case of a circular beam
incident on a sample at some angle with respect to the surface normal, the measurements should be
made along the directions of the short and long axes of the ellipse shown in the plan view of Figure 1. In
the case of an astigmatic beam, the measurements should be made in at least two directions; normally,
these directions should be orthogonal to each other. If possible, these directions should be chosen to
show the smallest and the largest values of the lateral resolution.
Key
1 direction of linescan
2 surface normal
3 axis of primary beam
Figure 1 — Circular beam incident on a sample impinging at an angle Θ with respect to the
surface normal and direction of line scan used for determination of the lateral resolution
5.5 Reporting results
Basic experimental conditions and analysis parameters shall be reported for XPS and AES instruments
following the standards ISO 16242:2011 and ISO 16243:2011, respectively. Clauses 4.6 “Maps and line
scans” of those standards should be considered carefully.
For SIMS such a standard is not available. Basic experimental conditions and analysis parameters to be
reported for SIMS are:
— nature of analytical ion gun and mode of its use;
— primary ion species, energy of primary ions, primary ion density, nominal beam diameter and beam
shape (if known), impact geometry;
— kind of mass analyser, mode and parameters of use;
— calibrations (intensity scale, mass scale, length scale for images and line scans);
— validation of linearity of intensity scale.
There are additional parameters to be reported for the straight edge method, the narrow line method
and the grating method. These method-specific parameters are summarized at the ends of Clauses 6 to 8.
6 Measurement of lateral resolution using the straight edge method
6.1 Introduction
[6]
Sharpness is a quality parameter of images and related to the performance of imaging instruments .
The term is in use in photography, microscopy and other imaging techniques. In all cases, sharpness
refers to the ability to show a sharp line of demarcation between two adjacent areas of different signal
intensities. This ability can be characterized by the ESF, which is the response of an imaging system to
a sharp edge between material A and material B.
6.2 Model functions and sharpness parameters
A line profile over a straight edge, i.e. a step transition, yields the ESF, which is the integral over the
LSF (see ISO/TR 19319:2013, Figure 2 and 4.3). The shape of the measured ESF characterizes the
sharpness of images and line scans. Usually, the ESF is approximated by a specific sigmoidal function
characteristic of the used imaging instrument and its settings (see ISO/TR 19319:2013, 4.3.1). For
practical applications, a comparison of those specific sigmoid functions is not straightforward and a
single parameter for characterizing sharpness is needed. The solution here is to use parameters D
x−
which characterize the steepness of the ESF by distances between points of well-defined relative
(1−x)
intensities x.
A variety of such parameters with different values of x is in use. Among them, only the parameter D
12−88
is recommended. The reason is that for the limiting case of a Gaussian LSF the parameter D is
12−88
equal to the FWHM of the LSF, expressed as w , and is close to the value of effective lateral resolution
LSF
r (see ISO/TR 19319:2013, 4.3.4, Clause 7 and Annex A).
e
8 © ISO 2019 – All rights reserved

Key
A material A
B material B
X length
Y signal intensity
1 ESF profile
Figure 2 — Schematic profile over a straight edge and definition of the sharpness parameter
D
12−88
6.3 Requirements for a test sample
Test samples shall have a sharp transition between two different materials, A and B, each with a
homogeneous concentration of the detected analyte. Sharp transition means that the transition
region between the materials is narrow compared with the expected rise of the ESF. The straight-
edge specimen shall have a straight sharp edge whose length is at least 10 times larger than the lateral
resolution to be measured. It is an advantage if the material has a large cross-section for photoelectron
or Auger-electron emission or a high secondary ion yield because this minimizes the time needed to
produce a signal of sufficient intensity. The specimen shall also be as smooth as possible so that signal
variations due to the changing topography of the specimen are minimized.
The length of the two plateaus of constant concentration of the analytes on both sides of the chemical
edge shall be large enough to enable an accurate determination of the 0 % and 100 % levels of signal
intensity leading to the respective ESF parameter D . Insufficient plateau length regularly results
x−(1−x)
in underestimations of D . For recommended plateau lengths for the limiting cases of Gaussian and
x−(1−x)
Lorentzian beam profiles see Annex B.
In practice, common test samples are specimens with a slot, copper or gold grids on carbon, respectively.
However, slots and grids are characterized by a 3D topography and edge effects, i.e. emission from non-
horizontal parts of the analysed surface, will distort the intensity distribution within images and line
scans. To avoid this, appropriate test samples shall have the surface topography minimized, for nano-
scaled lateral resolution down to the low nanometre scale.
NOTE A suitable specimen for the straight edge method is the BAM L200 CRM where a straight edge between
Al Ga As and GaAs with a plateau length of 691 nm on each side is established. This CRM is available
(0,70) (0,30)
from https: //www .webshop .bam .de. From the ETHZ, Zürich, Switzerland, a straight edge is available with a low
1)
2 [5]
topography (~1 nm) 200 × 500 µm Ti bar as part of a Ti (material A) in Au (material B) pattern .
6.4 Cleaning the straight-edge specimen
If the straight-edge specimen has appreciable surface contamination, the required data-acquisition
time will become very long. Sample cleaning using the following procedure is recommended for this
situation. The straight-edge sample should be washed in research-grade alcohol and dried by passing
dry argon over the surface. The region of the straight-edge specimen where the measurements are to
be made should be cleaned with caution by ion etchi
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