ISO 16531:2020

INTERNATIONAL ISO
STANDARD 16531
Second edition
2020-10
Surface chemical analysis — Depth
profiling — Methods for ion beam
alignment and the associated
measurement of current or current
density for depth profiling in AES and
XPS
Analyse chimique des surfaces — Profilage d'épaisseur — Méthodes
d'alignement du faisceau d'ions et la mesure associée de densité de
courant ou de courant pour le profilage d'épaisseur en AES et XPS
Reference number
ISO 16531:2020(E)
©
ISO 2020

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ISO 16531:2020(E)

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© ISO 2020
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ISO 16531:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
4 System requirements . 2
4.1 General . 2
4.2 Limitations . 3
5 Ion beam alignment methods . 3
5.1 General . 3
5.2 Important issues to be considered prior to ion beam alignment . 3
5.3 Alignment using circular-aperture Faraday cup . 6
5.4 Alignment using elliptical-aperture Faraday cup .10
5.5 Alignment using images from ion-induced secondary electrons during ion beam
rastering .10
5.6 Alignment in X-ray photoelectron microscope/photoelectron imaging system .12
5.7 Alignment by observing direct ion beam spot or crater image during and after ion
sputtering .13
5.8 Alignment by observing phosphor sample .14
6 When to align and check ion beam alignment .14
Annex A (informative) Comparison of AES depth profiles with good/poor ion beam alignment .15
Annex B (informative) Alignment using cup with co-axial electrodes .17
Bibliography .19
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ISO 16531:2020(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.
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
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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 4, Depth profiling.
This second edition cancels and replaces the first edition (ISO 16531:2013), which has been technically
revised.
The main changes to the previous edition are as follows:
— Table 1, in reference to 5.4: a comment has been added to mention the use of automated alignment
routine.
— 5.3.2, 5.3.3 and 5.5.4: some descriptions in notes have been changed to body text.
— minor editorial changes have been introduced for clarity.
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.
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ISO 16531:2020(E)

Introduction
In surface chemical analysis with Auger electron spectroscopy (AES) and X-ray photoelectron
spectroscopy (XPS), ion sputtering has been extensively incorporated for surface cleaning and for
the in-depth characterization of layered structures in many devices and materials. Currently, ultra-
thin films of < 10 nm thickness are increasingly used in modern devices and so lower energy ions are
becoming more important for depth profiling. For reproducible sputtering rates and for good depth
resolution, it is important to align the ion beam at the optimal position. This optimization becomes
increasingly critical as better and better depth resolutions are required. It is not necessary to conduct a
beam alignment routinely but it is necessary to align the beam when instrument parameters change as
a result of, for example, replacement of ion-gun filaments or an instrument bake-out. During the beam
alignment, care must be taken not to sputter or otherwise affect samples for analysis on the sample
holder. Instruments have different facilities to conduct alignment and six methods are described
to ensure that most analysts can conduct at least one method. Two of these methods are also useful
for measuring the ion beam current or the current density – important when measuring sputtering
yields and for measuring sputtering rate consistency. With commercial instruments, the manufacturer
may provide a method and equipment to conduct the beam alignment. If this is adequate, the methods
described here might not be necessary but could help to validate that method.
ISO 14606 describes how the depth resolution may be measured from a layered sample and used to
monitor whether the depth profiling is adequate, properly optimized or behaving as intended. That
method, from the instrumental setup to the depth resolution evaluation via in-depth measurement,
is, however, time-consuming and so the present, quicker procedure is provided to ensure that the ion
beam is properly aligned as the first step to using ISO 14606 or for more routine checking.
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INTERNATIONAL STANDARD ISO 16531:2020(E)
Surface chemical analysis — Depth profiling — Methods
for ion beam alignment and the associated measurement
of current or current density for depth profiling in AES and
XPS
1 Scope
This document specifies methods for the alignment of the ion beam to ensure good depth resolution
in sputter depth profiling and optimal cleaning of surfaces when using inert gas ions in Auger electron
spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). These methods are of two types: one
involves a Faraday cup to measure the ion current; the other involves imaging methods. The Faraday
cup method also specifies the measurements of current density and current distributions in ion beams.
The methods are applicable for ion guns with beams with a spot size less than or equal to 1 mm in
diameter. The methods do not include depth resolution optimization.
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 18115-1, Surface chemical analysis — Vocabulary — Part 1: General terms and terms used in
spectroscopy
3 Terms, definitions, symbols and abbreviated terms
For the purposes of this document, the terms and definitions given in ISO 18115-1 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/
For the purposes of this document, the following symbols and abbreviated terms apply.
A Area of Faraday cup aperture
A Raster area at a known orientation to the ion beam
R
A Area of ion beam raster in sample plane
0
AES Auger electron spectroscopy
B Ion beam broadening parameter equal to ratio I /I
outer inner
C Current
CD Current density
D′ Ion dose rate at the sample
F′ Ion fluence rate delivered by ion gun
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ISO 16531:2020(E)

FC Faraday cup
FWHM Full width at the half maximum
I Rastered ion beam current measured in aperture of Faraday cup
I Ion current measured at inner electrode of co-axial cup
inner
I Ion current measured at outer electrode of co-axial cup
outer
I Beam current as measured into dark region in the method specified in 5.5
S
I Stationary, small diameter ion beam current measured in aperture of Faraday cup
0
J Current density in ion beam measured per unit area of sample surface
OMI Optical microscope image
SEI Secondary electron image
SEM Secondary electron microscope
X Position of ion beam on x-axis set by ion gun controller
X Aligned position on x-axis of ion beam set by ion gun controller
0
XPS X-ray photoelectron spectroscopy
Y Position of ion beam on y-axis set by ion gun controller
Y Aligned position on y-axis of ion beam set by ion gun controller
0
θ Angle of incidence of ion beam with respect to sample surface normal
θ Angle of incidence of ion beam with respect to Faraday cup surface normal in usual position
a
θ Minimized angle of incidence of ion beam with respect to Faraday cup surface normal
b
4 System requirements
4.1 General
This document is applicable to the focusable ion gun for sputtering with inert gases that is usually
supplied with most AES and XPS instruments or available from after-market suppliers. The beam size
or raster area of the ion beam shall be larger than and uniform over the analysis area. Six alternative
methods of ion beam alignment are described that require the equipment to have provision for the
measurement of the ion current or the detection of excited secondary signals or to have an optical
microscope aligned at the analytical point. Depending on the equipment available, measurements
of increasing sophistication may be made. The methods for measuring the ion beam current involve
measurement by a circular-aperture Faraday cup, an elliptical-aperture Faraday cup or a co-axial
electrode cup. The methods involving the excited secondary signals are categorized by ion/electron-
induced secondary electrons or emitted photons that are detected with a secondary electron detector,
an optical microscope or a phosphor screen.
To conduct the relevant surface analysis, the electron energy analyser, the analysis probe beam and the
ion beam need to be focused and aligned correctly on the same analysis point or area to be analysed. To
apply this document, the electron energy analyser and the analysis probe beam shall already be aligned
to the optimum position using the manufacturer's or in-house documented procedure.
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4.2 Limitations
This document is an important part of the setting up of depth profiling generally; nevertheless,
depending on the material of the sample and its structure, there are several depth profiling procedures
that may be applied to achieve the best depth resolution, not all of which are aided by this document.
Some of the most popular procedures are:
a) ion bombardment of fixed position samples at angles of incidence in the range 0° to 60° with respect
to the surface normal;
b) ion bombardment at grazing angles of incidence;
c) sample rotation during ion bombardment;
d) simultaneous ion bombardment applying two ion guns;
e) sample rotation and grazing angle of incidence for ion bombardment.
This document will assist in the use of procedure a). Some aspects could relate to the other procedures
but further considerations might be required that are not necessarily included in this document.
5 Ion beam alignment methods
5.1 General
This document describes six simple methods for ion beam alignment, all easily applied. These methods
and a summary of their advantages are set out in Table 1. Also indicated are which methods are best for
ion beam current or current density measurement.
Each method has different advantages and requires different instrumental capabilities. The analyst
needs to select the method based on requirements and equipment capabilities. Some issues depend
on the raster size of the ion beam. A small raster is good, since little material is consumed or sputter
deposited in the spectrometer. Additionally, for industrial samples, the material to be profiled may only
occupy a small area. A very small raster is possible in AES where the electron beam is small and some
users may deliberately use higher ion beam energies where ion beams tend to be better focused to
obtain small sputtered areas with a faster sputtering rate. In these cases, and for systems with small-
area XPS analysis, particular care needs to be taken with alignment. For broader ion beams, such as
for some XPS instruments, the alignment accuracy may be more relaxed. If more than one method is
suitable, tests with each will show which is most convenient for the sputtering conditions intended.
The effects of good and poor ion beam alignment in sputter depth profiling are illustrated in Annex A.
General precautions are given in 5.2. If analysts wish to align the beam and measure the ion beam
current or current density, or change the ion beam energy, they can choose one of the two methods that
use a Faraday cup. The alignment methods specified in 5.3 and 5.4 are those using Faraday cups with a
circular aperture and an elliptical aperture, respectively. Annex B introduces a method using co-axial
electrodes giving measurements proportional to the ion current or current density. If analysts wish
to align the beam and not measure the ion current or current density, they can align the beam using
images from secondary electrons or ions excited by ions or primary electrons, an optical image or by
ion-induced luminescence, using the methods specified in 5.5, 5.6, 5.7 and 5.8, respectively. The method
chosen depends on the capability and facility of the instrument used.
Clause 6 describes when to conduct the ion beam alignment.
5.2 Important issues to be considered prior to ion beam alignment
5.2.1 For consistent, high-quality analysis, the analytical probe beam, whether stationary or rastered
over an area, and the electron energy analyser axis shall be aligned at the analysis position. The
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ISO 16531:2020(E)

intersection of these two axes with the sample surface shall also define the centre for the sputtered area
for sputter depth profiling.
5.2.2 It is important that the analysis area be located in the central, uniform region of the ion beam
[4]
irradiation area. This is shown in Figure 1 . It is useful to know the sputtering rate for the ion gun and
sample as a function of sputtering parameters such as the ion beam energy, beam current, raster size
and so on or their equivalent instrumental control settings in order to choose the best settings for the
alignment. The two most important aspects for the analyst are to ensure that, through alignment of the
ion beam, the analysis area coincides with the central uniform region of the ion beam irradiation area
and also that an appropriate ion beam current density and raster size can be set. Ion beam currents
and current densities may be measured using a Faraday cup using the methods specified in 5.3 and 5.4,
as summarized in Table 1. Some design details and the accurate measurement of both electron and ion
beam currents using Faraday cups are given in References [5] and [6].
Key
1 specimen
2 analysis area
3 flat area
4 sputtered area
a) top view
b) cross-section view along line A–A′
SOURCE Urushihara N., Sanada N., Paul D., Suzuki M. Ion Beam Alignment Procedures using a Faraday Cup or a
[4]
Silicon Dioxide Film on Silicon Substrate with Auger Electron Microscope. J. Surf. Anal. 2007, 14 (2) pp. 124–130 ,
reproduced with the permission of the Surface Analysis Society of Japan.
Figure 1 — Configuration of sputtered flat and analysis areas
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ISO 16531:2020(E)

Table 1 — Detected signals when aligning ion beam — Summary of methods
Minimum
Measurement
Detected Subclause: a Equipment
ion energy
Feature of current and
signal method required
current density
eV
Good for alignment. Gives the best
measure of C and CD for quantitative FC may be
sputtering rates but, for this, may orientated
5.3: FC with require a FC that can be set normal to towards the
C: good
circular the ion beam at the analytical posi- ~50 ion gun or in
CD: good
aperture tion. If the FC is in the sample plane, the
CD measurement may be poor at sample
incidence angles greater than that for plane
which the FC is designed, often ~45°.
Ion current
FC with
Good for alignment. This modification
elliptical ap-
can allow greater angles to be used
erture may
than those given by 5.3.
5.4: FC with be orientat-
C: good
Some manufactures have such auto-
elliptical ~50 ed towards
mated routines based on this standard CD: good
aperture the ion gun
incorporated as default. When using
or in the
the automated routines, follow the
sample
manufacturer’s instructions.
plane
Allows rastered ion beam to be
5.5: Ion- aligned to within a fraction of the
C: poor
induced beam size and the raster size to be Raster for
~50
secondary determined, but quantitative C and ion beam
CD: poor
electrons CD measurements are poor or shall be
conducted separately.
5.6: Ion- Allows unrastered ion beam to be
Imaging for
induced aligned to within a fraction of the
C: poor
secondary
secondary beam size, but quantitative C and CD ~50
electrons or
CD: poor
emission measurements are poor or shall be
ions
imaging conducted separately.
Allows an unscanned ion beam to be Raster for
focused and aligned in a system with ~2 000 electron
Excited
i) an electron beam raster either dur- [i), d)] beam or
secondary
5.7: Ion spot
C: no
ing d) or after a) sputtering or ii) an optical
signal
image in SEI ~50 [i), a)]
optical microscope after a) sputtering. microscope
CD: poor
or OMI
C and CD measurements shall be con- ~1 000 aligned at
ducted separately. After sputtering, [ii), a)] analytical
methods are very slow. point
Allows an unscanned ion beam to be
focused and aligned in a system, but
the ion beam energy range available is
Phosphor
5.8: Ion- limited. C and CD measurements shall
C: no
screen for
induced be conducted separately. Most, if not ~2 000
ion
CD: no
luminescence all, phosphorescent materials are also
detection
electrical insulators and not stable
under irradiation from either ions or
electrons.
Key
C current
CD current density
FC Faraday cup
a
The minimum energy is the energy for which the beam size is below ~1 mm and which is rarely below 50 eV.
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5.2.3 In general, the components on the sample stage used in the methods given in this document do
not all lie in a single plane, for essential mechanical reasons, and this may cause errors in alignment
unless understood. Figure 2 shows an example of the essential problem of using a screw head on the
sample stage in an AES instrument when aligning the ion beam. If the region to be analysed is at a
different height from the region used for setting the analytical position, an alignment error may occur.
For both AES and XPS, the analyst needs a method to compensate for the different heights of any different
components involved. Methods that have been used include optical or electron optical components with
restricted depths of focus so that the item is only in focus to the monitoring microscope or the electron
spectrometer, if at the correct position, or by stage adjustments for the known dimensions of these
components.
5.2.4 When applying this document there shall be a manufacturer's or in-house documented procedure
to set the sample at the correct analysis position and this shall be used whenever such setting is required.
a)  Set-up b)  Analysis
Key
1 electron/X-ray beam
2 ion beam aligned on screw head centre or Faraday cup hole
3 screw head or Faraday cup set at analysis position
4 sample set at analytical height
5 ion beam actually misaligned on sample being analysed
6 screw head or Faraday cup
NOTE In a) the ion beam has been aligned to the screw head on the axis but is above the sample plane.
Therefore, in b), when moving the sample to sputter an appropriate region without changing the height, the ion
beam is misaligned.
Figure 2 — Diagram illustrating importance of aligning ion beam at component set at
correct height
5.2.5 While aligning the ion beam, ensure that the samples to be analysed are not in a position where
they can be sputtered or contaminated from ion-sputtered material while conducting this work.
5.3 Alignment using circular-aperture Faraday cup
5.3.1 The Faraday cup method is generally applicable in the ion energy range from below 50 to
above 100 000 eV. A Faraday cup is shown in Figure 3. Operate the Faraday cup according to the
manufacturer's instructions. Turn off, or blank, the electron or X-ray beam. Turn off or blank any other
items recommended by the manufacturer before using the ion beam. Set the centre of the Faraday cup
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ISO 16531:2020(E)

at the analysis position. Turn on the ion beam with settings as close to those intended as is possible,
without the ion beam raster.
NOTE 1 In many cases the defining aperture in the outer electrode of the Faraday cup is designed to be at the
same height as the sample holder or a typical mounted wafer.
NOTE 2 If there is no instruction concerning the bias voltage to use on the inside of the Faraday cup, usually
+15 V is sufficient. A lower voltage cannot stop most electrons being emitted and a higher voltage can enlarge the
apparent aperture size.
5.3.2 If the Faraday cup aperture size is able to be selected, a size smaller than the ion beam allows
the best alignment and current density measurement, but it should still be sufficiently large to accept
sufficient current for adequate measurement by the current meter available. Apertures larger than
at least twice the FWHM of the ion beam profile may be used to measure the total beam current. For
non-normally incident ions, care is required in considerations of the aperture edge and the depth and
structure of the Faraday cup if measurement errors are to be avoided.
NOTE Parameters of the Faraday cup design and applied voltages on both the outer shield and inner cup,
important for accurate measurements, are described by References [5] and [6]. Accuracies better than 1 % are
achievable.
5.3.3 Adjust the ion beam X and Y position offsets and tune the voltages of the ion gun objective (final)
lens until attaining a maximum current in the Faraday cup as shown in Figure 4. If a condenser lens is also
available, increasing its voltage usually allows a smaller spot size to be obtained but with a lower beam
current. After changing the condenser lens strength, the objective lens may need refocusing. Depending
on the equipment available, this sequence may need to be repeated iteratively until final settings for
beam size, position and current are attained. This provides the X and Y settings for the X and Y offsets
0 0
for correct alignment for the given setting of the beam energy, lenses and so on.
If the ion beam is not at normal incidence to the sample or Faraday cup, the X and Y deflections may
either give equal angular deflections for equal settings or one deflection may be scaled against the other
to give equal deflections on the sample surface. It is useful to check this to understand the equipment
behaviour. The apparent width of the Faraday cup aperture in the X and Y directions will be affected by
these considerations.
5.3.4 If the beam width observed is greater than required, it may be possible to reduce it by reducing
the beam current and checking the focus. Ion beams may exhibit an astigmatic focus. This can be checked
by scanning over the Faraday cup in both X and Y directions, optimizing the focus each time, with the
average focal setting being used. Ensure that the X and Y offsets are tuned last for the final operating
condition to set X and Y .
0 0
Ion beam bombardment increases the emission of secondary electrons, and it may damage the
secondary electron detector or electron energy analyser. Therefore, before switching on the ion beam,
check if it is necessary to turn off or decrease the voltages supplied to the electron multiplier detectors,
any other radiation-sensitive detector or both.
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Key
1 outer electrode
2 electrical insulator
3 inner electrode
SOURCE Urushihara N., Sanada N., Paul D., Suzuki M. Ion Beam Alignment Procedures using a Faraday Cup or a
[4]
Silicon Dioxide Film on Silicon Substrate with Auger Electron Microscope. J. Surf. Anal. 2007, 14 (2) pp. 124–130 ,
reproduced with the permission of the Surface Analysis Society of Japan.
Figure 3 — Schematic of Faraday cup with design accepting ions up t
...