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 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

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
Foreword
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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
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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.
iv © ISO 2020 – All rights reserved

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.
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
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
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
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
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
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 focu
...