ISO 16531:2013
(Main)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
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
ISO 16531:2013 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 and X-ray photoelectron spectroscopy. 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 below ~1 mm in diameter. The methods do not include depth resolution optimization.
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
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 16531
First edition
2013-06-01
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 16531:2013(E)
©
ISO 2013
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ISO 16531:2013(E)
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ISO 16531:2013(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 . 2
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 . 9
5.5 Alignment using images from ion-induced secondary electrons during ion
beam rastering . 9
5.6 Alignment in X-ray photoelectron microscope/photoelectron imaging system .11
5.7 Alignment by observing direct ion beam spot or crater image during and/or after
ion sputtering .12
5.8 Alignment by observing phosphor sample .13
6 When to align and check ion beam alignment .13
Annex A (informative) Comparison of AES depth profiles with good/poor ion beam alignment .14
Annex B (informative) Alignment using cup with co-axial electrodes .16
Bibliography .18
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ISO 16531:2013(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. 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. 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.
The committee responsible for this document is ISO/TC 201, Surface chemical analysis, Subcommittee
SC 4, Depth profiling.
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ISO 16531:2013(E)
Introduction
In surface chemical analysis with AES (Auger electron spectroscopy) and XPS (X-ray photoelectron
spectroscopy), 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, for
example, from replacement of ion-gun filaments or from an instrument bake-out. During the beam
alignment, care must be taken not to sputter or otherwise affect specimens for analysis on the sample
holder. Instruments have different facilities to conduct alignment and seven 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 may not be necessary but may help to validate that method.
[1]
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 16531:2013(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 International Standard 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 and X-ray photoelectron spectroscopy. 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 below ~1 mm in diameter. The methods
do not include depth resolution optimization.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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 and the following
symbols and abbreviated terms apply.
A Area of Faraday cup aperture
A Area of ion beam raster in sample plane
0
A Raster area at a known orientation to the ion beam
R
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 Stationary, small diameter ion beam current measured in aperture of Faraday cup
0
I Ion current measured at inner electrode of co-axial cup
inner
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ISO 16531:2013(E)
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
J Current density in ion beam measured per unit area of sample surface
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
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 posi-
a
tion
θ Minimized angle of incidence of ion beam with respect to Faraday cup surface normal
b
AES Auger electron spectroscopy
OMI Optical microscope image
SEI Secondary electron image
SEM Secondary electron microscope
XPS X-ray photoelectron spectroscopy
4 System requirements
4.1 General
This International Standard is applicable to the focusable ion gun for sputtering with inert gases that
is usually supplied with most of 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.
Seven alternative methods of ion beam alignment are described that require the equipment to have
provision for the measurement of the ion current, or for detecting excited secondary signals, or 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, 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 International Standard, 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.
4.2 Limitations
This International Standard 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
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ISO 16531:2013(E)
procedures that may be applied to achieve the best depth resolution, not all of which are aided by this
International Standard. Some of the most popular procedures are
a) ion bombardment of fixed position samples at angles of incidence in the range of 0°–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, and
e) sample rotation and grazing angle of incidence for ion bombardment.
This International Standard will assist in the use of procedure a). Some aspects may relate to the
other procedures but further considerations may be required that are not necessarily included in this
International Standard.
5 Ion beam alignment methods
5.1 General
This International Standard describes not all but seven simple methods for ion beam alignment, 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; whereas 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, or 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 intersection
of these two axes with the specimen surface shall also define the centre for the sputtered area for sputter
depth profiling.
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ISO 16531:2013(E)
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].
Specimen
(a) Analysis area
Flat area
A A’
Sputtered area
(b)
AA’
Flat area
Sputtered area
Figure 1 — Configuration of sputtered flat and analysis areas — (a) Top view (b) cross-section
[4]
view along line A–A′
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ISO 16531:2013(E)
Table 1 — Detected signals when aligning ion beam — Summary of methods
Measurement
Minimum
Detected Subclause: of current and Equipment
a
ion energy
Feature
signal Method current den- required
eV
sity
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
C: good
towards the
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 sample
CD measurement may be poor at
plane.
incidence angles greater than that for
which the FC is designed, often ~45°.
Ion current
FC with
elliptical
aperture
5.4: FC with Good for alignment. This modification may be
C: good
elliptical can allow greater angles to be used ~50 orientated
CD: good
aperture than those given by 5.3. towards the
ion gun or in
the sample
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 CD ion beam
CD: poor
electrons measurements are poor or must 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 must 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) sputter- microscope
CD: poor
or OMI
ing. C and CD measurements must be ~1 000 aligned at
conducted 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
5.8: Ion- Phosphor
limited. C and CD measurements must
C: no
induced screen for
be conducted separately. Most, if not ~2 000
lumines- ion detec-
CD: no
all, phosphorescent materials are also
cence tion
electrical insulators and not stable
under irradiation from either ions or
electrons.
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.
5.2.3 In general, the components on the sample stage used in the methods given in this International
Standard do not all lie in a single plane, for essential mechanical reasons, and this may cause errors in
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ISO 16531:2013(E)
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 International Standard, 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.
Electron/X-ray beam Electron/X-ray beam
Ion beam aligned on screw
head center or Faraday
Ion beam actually
cup hole
misaligned on sample
being analyzed
Screw head
or Faraday
cup
Screw head or Faraday cup
set at analysis position
Sample set at analytical height
Sample set at analytical height
a) Set-up b) Analysis
In a) the ion beam has been aligned to the screw head on the axis but is above the sample plane, and so,
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 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 but this may or may not be the case in the
instrument being used.
NOTE 2 If there is no instruction concerning the bias voltage to use on the inside of the Faraday cup, usually
+15 V is found sufficient. A lower voltage may not stop most electrons being emitted and a higher voltage may
enlarge the apparent aperture size.
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ISO 16531:2013(E)
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.
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 % may be
achievable. 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.
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 for
0 0
correct alignment for the given setting of the beam energy, lenses, etc.
NOTE 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 both the voltages supplied to the electron multiplier detectors and or
any other radiation sensitive detector.
++
ArAr elelectrectrical inical insulatosulatorr
II
ininnenerr
II
ououteterr
AAAA
Figure 3 — Schematic of Faraday cup with design accepting ions up to 45° from its surface
normal (modified from Reference[4])
5.3.5 This provides the X and Y settings for the X and Y offsets for correct alignment for the given
0 0
setting of the beam energy, lens settings, etc. The values of X and Y are valid only for those settings and
0 0
are likely to change if the settings are changed. Record these settings.
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ISO 16531:2013(E)
5.3.6 To measure the ion beam current or current density, it is necessary to check that the Faraday cup
specification is valid for a beam entering at the angle of incidence from the surface normal, θ , relevant
a
for the analysis position. If so, this position may be retained and 5.3.7 and 5.3.8 proceeded to. However, if
this is not the case, the Faraday cup shall be orientated at an angle from the surface normal, θ , as small
b
as possible before proceeding.
I /nA
inner
10
Faraday cup - 300V
↓ X
0
8
6
4
2
0
-2 -1 01 2
X offset (mm)
The Y-direction has been optimized at Y and the position of X indicated.
0 0
The Faraday cup aperture diameter is 200 μm.
Figure 4 — Example plot of ion current vs. X-direction offset for 300 eV ions at about 47° to
circular aperture
5.3.7 If the measured beam profile achieved by scanning the beam across the Faraday cup is as shown
in Figure 4, it is too broad to measure the beam current accurately. In this case, only the current density in
the rastered mode is measurable. Set the raster running with all other ion beam settings kept constant and
measure the current recorded into the Faraday cup. This requires current measurement with a sufficient
integration time that the current is averaged over many raster frames. For the Faraday cup retained in the
original analysis position, the ion beam current density per unit area of the surface, J, is given
...
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