ISO 23420:2021

INTERNATIONAL ISO
STANDARD 23420
First edition
2021-04
Microbeam analysis — Analytical
electron microscopy — Method for the
determination of energy resolution
for electron energy loss spectrum
analysis
Analyse par microfaisceaux — Microscopie électronique analytique
— Méthode de détermination de la résolution énergétique pour
l'analyse spectrale de la perte d'énergie des électrons
Reference number
ISO 23420:2021(E)
©
ISO 2021

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ISO 23420:2021(E)

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ISO 23420:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
5 Definition of the energy resolution . 5
6 Reference materials and energy determination . 5
6.1 General . 5
6.2 Materials selection for energy scale calibration . 5
6.3 Binding energy measurement of graphite in the XPS . 6
7 Measurement procedure and energy resolution determination . 6
7.1 General . 6
7.2 Predetermination of binding energy . 8
7.2.1 Obtain graphite and the other reference sample . 8
7.2.2 Measure C1s of graphite by using the XPS . 8
7.3 Setup of the S/TEM and the EELS, and sample setting . 8
7.4 First energy step, δE , calibration . 8
1
7.4.1 EELS acquisition set-up . 8
7.4.2 Determining the EELS first energy step, δE .
1 8
7.4.3 Acquisition of carbon K-edge EEL spectrum . 9
7.4.4 Calculate calibrated energy step δE .
1C 9
7.5 Measurement of peak close to the zero-loss peak, E , for the other reference
CZLP
sample using energy step δE .
1 12
7.5.1 EEL spectrum acquisition of the second reference sample using energy
step δE .
1 12
7.5.2 Obtain the value for CH between the zero-loss peak and the peak E .
2 CZLP 13
7.5.3 Calculate the peak E energy . .13
CZLP
7.6 Second energy step, δE , calibration .14
2
7.6.1 Determining the EELS second energy step, δE .
2 14
7.6.2 Acquire E EEL spectrum .15
CZLP
7.6.3 Obtain the value for CH between the zero-loss peak and peak E .
3 CZLP 15
7.6.4 Calculate calibrated energy step δE .
2C 15
7.7 Determining the calibrated EEL spectrometer energy resolution, ΔE .15
7.7.1 Acquisition of a ZLP EEL spectrum .15
7.7.2 Obtain the value for CH for the zero-loss peak .15
4
7.7.3 Calculate EEL spectrometer energy resolution, ΔE .15
7.8 Record items .16
8 Uncertainty for the measurement result of energy resolution .17
Annex A (informative) Example of measurement procedure for energy resolution
determination .18
Annex B (informative) Correspondence between energy values of XPS C1s and EELS carbon
K edge .26
Bibliography .28
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ISO 23420:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
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different types of ISO documents should be noted. This document was drafted in accordance with the
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This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee
SC 3, Analytical electron microscopy.
A list of all parts in the ISO 23420 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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ISO 23420:2021(E)

Introduction
In order to understand the chemical composition, the atomic bonding and the electronic structure,
electron energy loss analysis is often performed with the scanning transmission electron microscope
or the transmission electron microscope (S/TEM) equipped with the electron energy loss (EEL)
spectrometer.
In the analysis using EEL spectrometer system, the energy loss of incident electrons by the inelastic
interaction via phonon and plasmon excitations, intra- and inter-band transitions and the inner shell
ionization can be measured. The inner shell ionization is particularly useful and important as it gives
the information on chemical composition of materials. For the precise analysis based on the energy loss
peak decomposition and its energy shifts, it is vitally important to understand the energy resolution
of the EEL spectrometer system. However, the determination method of the energy resolution is not
standardized yet.
This document provides the procedures for energy step calibration and energy resolution determination
useful for the electron energy loss spectrum analysis in the S/TEM equipped with the EEL spectrometer.
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INTERNATIONAL STANDARD ISO 23420:2021(E)
Microbeam analysis — Analytical electron microscopy —
Method for the determination of energy resolution for
electron energy loss spectrum analysis
1 Scope
This document specifies a determination procedure of energy resolution in the scanning transmission
electron microscope or the transmission electron microscope equipped with the electron energy loss
(EEL) spectrometer.
This document is applicable to both in-column type EEL spectrometer and post-column type EEL
spectrometer. These EEL signal detecting systems are applicable to a parallel detecting system and a
serial detecting system.
2 Normative references
There are no normative references in this document.
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:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1
beam diameter
full width at half maximum (FWHM) of the electron beam intensity profile for the STEM observation
3.2
Boersch effect
energy spread of electron beam due to Coulomb interaction (3.5) between electrons in the beam
3.3
channel
range of one pixel of the detector in the parallel detection (3.17) EELS
3.4
collection angle
EELS entrance aperture diameter divided by a camera length and a geometric factor (3.13) for the
STEM or the TEM diffraction mode, or EELS entrance aperture diameter divided by the distance from
crossover of the lens in front of the EEL spectrometer to the EELS entrance aperture for imaging mode
of the energy-filtering TEM
3.5
Coulomb interaction
repulsion of electrons by electric charge
3.6
detection plane
plane where energy dispersed electron focus
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ISO 23420:2021(E)

3.7
electron energy loss
energy shift of the electron kinetic energy due to the inelastic scattering in solids
3.8
energy dispersion
degree of change in position of the dispersed electrons at the detection plane (3.6) per unit energy change
3.9
energy resolution
FWHM of the zero-loss (3.21) peak
3.10
energy step
energy selecting window (3.11) per channel (3.3) in the parallel detection (3.17) EELS, or energy range
limited by the width of energy selecting slit in the serial detection (3.20) EELS
3.11
energy selecting window
energy range for selection of a specific energy loss value
3.12
entrance aperture
aperture for limiting the collection angle (3.4) of the EEL spectrometer
3.13
geometric factor
ratio of distance from a projector lens to an EELS entrance aperture to distance from the projector lens
to an image detection device
3.14
in-column type EELS
EELS system with the EEL spectrometer located in the imaging system of the TEM
3.15
irradiation diameter
diameter of the electron beam irradiation region for the TEM observation
3.16
K edge
energy loss related to K shell electron transition to the lowest empty state
3.17
parallel detection
simultaneous EELS signal detection for all energy-dispersed electrons focused on the detection plane (3.6)
3.18
plasmon-loss
energy loss of electron due to excitation of the quantized plasma oscillations of electrons
3.19
post-column type EELS
EELS system with the EEL spectrometer located behind the imaging/detecting system of the TEM
3.20
serial detection
EEL spectrum detection by scanning the dispersed electrons across the energy selecting slit in front of
the detector
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ISO 23420:2021(E)

3.21
zero-loss
unscattered and elastically scattered electrons (with only minimal loss of energy due to phonon
excitation), giving rise to an intensity peak or the position of which defines zero in the electron energy
loss spectrum
[SOURCE: ISO15932: 2013, 2.2.1.1]
4 Symbols and abbreviated terms
B spatial width of energy selecting window in the serial detection of the EELS
CCD charge coupled device
CFE cold field emission
CH sum of Ch (G, P) and the Ch (G, C-K). In the parallel detection system, CH is the number
1 1 1 1
of channels between the zero-loss peak and carbon K edge of graphite. In the serial detec-
tion system, CH is distance between the zero-loss peak and carbon K edge of graphite.
1
CH number of channels between the zero-loss peak [Figure 5, key 1] and the peak E
2 CZLP
[Figure 5, key 2] on the calibrated energy step δE in the parallel detection EELS. In
1C
the serial detection EELS, CH is distance between the zero-loss peak [Figure 5, key 1]
2
and the peak E [Figure 5, key 2] on the calibrated energy step δE .
CZLP 1C
CH number of channels between the zero-loss peak and the peak E on the energy step
3 CZLP
δE in the parallel detection EELS. In the serial detection EELS, CH is distance between
2 3
the zero-loss peak and the peak E on the energy step δE .
CZLP 2
CH number of channels corresponding to FWHM of the zero-loss peak on the calibrated
4
energy step δE in the parallel detection EELS. In the serial detection EELS, CH is dis-
2C 4
tance between the zero-loss peak and the peak E on the calibrated energy step δE .
CZLP 2C
Ch (G, C-K) number of channels of the range from the graphite plasmon-loss (π + σ) peak [Figure 3,
1
key 1] to carbon K edge E [Figure 3, key 2] on the energy step δE in the parallel
C-K 1
detection system. In the serial detection system, Ch (G, C-K) is distance between the
1
graphite plasmon-loss (π + σ) peak [Figure 3, key 1] and carbon K edge E [Figure 3,
C-K
key 2] on the energy step δE .
1
Ch (G, P) number of channels of the range from the zero-loss peak [Figure 2, key 1] to the graphite
1
plasmon-loss (π + σ) peak E [Figure 2, key 2] on the energy step δE in the parallel
CZLP 1
detection system. In the serial detection system, Ch (G, P) is distance between the ze-
1
ro-loss peak [Figure 2, key 1] and the graphite plasmon-loss (π + σ) peak E [Figure 2,
CZLP
key 2] on the energy step δE .
1
CMOS complementary metal oxide semiconductor
CRM certified reference material
C1s carbon K shell binding energy of graphite measured by the XPS
D energy dispersion on the recording device of the EEL spectrometer
d sample thickness of the electron beam irradiated area
E value of electron energy loss such as plasmon-loss and ionization-loss
E measured plasmon-loss (π - π*) peak energy of boron-nitride under the condition of
BN-P
calibrated energy step δE
1C
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ISO 23420:2021(E)

E position of noticed low-loss peak close to the zero-loss peak
CZLP
E carbon K edge energy in the EELS
C-K
EEL electron energy loss
EELS electron energy loss spectroscope/spectroscopy
FWHM full width at half maximum
GUM guide to the expression of uncertainty in measurement
m total number of available channels in the parallel detection of the EELS
n iteration number in acquisition of electron energy loss spectrum
RM reference material
STEM scanning transmission electron microscope/microscopy
S/TEM scanning transmission electron microscope/microscopy or transmission electron mi-
croscope/microscopy
s detector spatial resolution for the parallel detection. For the serial detection, s is slit
width of energy selecting window
TEM transmission electron microscope/microscopy
t acquisition time in acquisition of electron energy loss spectrum
XPS X-ray photoelectron spectroscope/spectroscopy
ZLP zero-loss peak
ΔE energy resolution
ΔE theoretical energy resolution
r
ΔE energy broadening
SO
δE selected energy step in the first energy calibration. In the parallel detection system, δE
1 1
is selected from the preset value. In the serial detection system, δE is derived from the
1
energy width and its spatial width in the energy selection window
δE calibrated value of energy step δE
1C 1
δE selected energy step in the second energy calibration. In the parallel detection system,
2
δE is selected from the preset value. In the serial detection system, δE is derived from
2 2
the energy width and its spatial width in the energy selection window.
δE calibrated energy step of energy step δE by the second energy calibration step
2C 2
δE energy width of the energy-selecting window in the serial detection system of the EELS
S
λ mean free path of electron inelastic scattering
π π-bonding state
π* π-antibonding state
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ISO 23420:2021(E)

σ σ-bonding state
(π - π*) resonant oscillation of the π-bonding state and the π-antibonding state
(π + σ) resonant oscillation of the π-bonding state and the σ-bonding state
5 Definition of the energy resolution
The theoretical energy resolution ΔE is given from a convolution of an electron beam energy spread
r
[2]
and a spectrometer resolution. The theoretical energy resolution is shown as Formula (1) .
2 2 2 2
(ΔE ) ≈ (ΔE ) + (ΔE ) + (s / D) (1)
r 0 SO
where
ΔE is theoretical energy resolution
r
ΔE is energy spread of the primary electron beam
0
 NOTE ΔE is affected by energy width of electron source and the Boersch effect.
O
ΔE is broadening of energy
SO
 NOTE ΔE is affected both the spectrometer focusing and the angular width of inelastic
SO
scattering.
s is a detector spatial resolution for the parallel detection. For the serial detection, s is a slit
width of energy selecting window.
D is an energy dispersion of the spectrometer
In addition, acquisition time t and acquisition iteration number n influence the energy resolution ΔE .
r
Measurement of energy resolution ΔE is not easy because of the complicated formation of the EELS
r
system. It is well known that the full width at half maximum of the zero-loss peak is proportional
to the energy resolution ΔE . Actually, FWHM of the zero-loss peak is very often used as the energy
r
[3]
resolution . The energy resolution ΔE is also defined as FWHM of the zero-loss peak in this document.
6 Reference materials and energy determination
6.1 General
In order to determine the energy resolution of the EELS equipped in the S/TEM, it is indispensable to
calibrate the energy scale in advance. In this section, material selection for the energy scale calibration
and the procedure for determining the energy scale are described.
6.2 Materials selection for energy scale calibration
For the energy resolution determination, calibration of the energy scale is necessary. As an EEL
spectrometer cannot calibrate energy scale by itself, the reference material is necessary for the
calibration. Since the energy calibrated certified reference materials (CRMs) and/or reference materials
(RMs) are not available, it is necessary to select appropriate materials aiming to energy scale calibration,
as (internal) reference materials. The following characteristics are required for the material.
— Easy to obtain
— Easy to handle,
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ISO 23420:2021(E)

— Homogeneous,
— Stable,
— Having loss peaks at a low-loss energy region,
NOTE 1 For measuring the energy resolution, energy scale calibration is needed to perform within loss
energy region such as zero to several hundred electronvolt.
— Non-chargeable.
NOTE 2 In the first step of energy scale calibration on the EELS, loss energy known sample is needed. The
loss energy value is obtained by the XPS analysis of the sample. Non-chargeable material is needed for XPS
measurements.
In this document, graphite is recommended and used as a reference sample for the coarse energy
scale calibration. The other reference sample for the following fine energy scale calibration should be
selected from the materials, which has low-loss EELS peak, such as boron-nitride.
6.3 Binding energy measurement of graphite in the XPS
XPS C1s (carbon K shell binding energy) peak and EELS carbon K edge E are equivalent. The
C-K
correspondence of the energy values between XPS C1s and EELS carbon K-edge is described in Annex B.
XPS measurement of C1s peak shall be done about graphite standard sample with calibrated XPS
spectrometer.
[4]
The XPS shall be calibrated by ISO 15472:2010 .
7 Measurement procedure and energy resolution determination
7.1 General
In this subclause, the energy scale calibration of EELS and the procedure for determining energy
resolution are described. Annex A shows an example of actual measurement using this procedure.
A flowchart of measurement procedure is shown in Figure 1.
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ISO 23420:2021(E)

NOTE Numbers given in Figure 1 indicate corresponding clauses in this document.
Figure 1 — Flow chart of measurement procedure
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ISO 23420:2021(E)

7.2 Predetermination of binding energy
7.2.1 Obtain graphite and the other reference sample
Obtain a graphite and a second reference sample. The graphite reference sample has a Carbon K-edge
and an EEL peak E in an EEL spectrum. The second reference sample should have an EEL peak
CZLP
E appearing close to the zero-loss peak, but the peak spread should not overlap with the zero-loss
CZLP
peak. For example, the boron nitride plasmon-loss (π - π*) peak is suitable for the E as good as the
CZLP
graphite plasmon-loss (π + σ) peak.
7.2.2 Measure C1s of graphite by using the XPS
Measure the carbon K-shell binding energy of the graphite using XPS. The C1s is used as carbon K edge
energy E of graphite of the EELS. The XPS shall be calibrated by ISO 15472:2010.
C-K
7.3 Setup of the S/TEM and the EELS, and sample setting
Samples are recommended to be drop-cast onto a TEM grid, e.g. a holey carbon film support grid, to
ease the measurement. The electron microscope should be adjusted for S/TEM observation in advance.
EELS should also be adjusted in advance so that EEL spectra can be easily obtained. Parameters of the
S/TEM and the EELS are recorded as described in 7.8.
The quality of an EEL spectrum is easily affected by electron beam induced carbon contamination. The
use of an anti-contamination device is strongly recommended for suppression of the contamination.
Additionally, a clean environment in the column is also needed to minimise the contamination.
7.4 First energy step, δE , calibration
1
7.4.1 EELS acquisition set-up
Set the EEL spectrometer acquisition time and integration numbers so as to secure a sufficient
signal-to-noise ratio without any peak saturation for all measurements, including measurements in
subsequent sections. Since the measurement conditions such as irradiation current, entrance aperture
diameter and capturing device characterization like CCD / CMOS camera are different, acquisition time
and integration number cannot be uniformized. Therefore, users of this procedure should optimise
acquisition time and integration number for their own systems and should record these values.
7.4.2 Determining the EELS first energy step, δE
1
Set the energy step δE to cover all range from the zero-loss peak to the carbon K edge E of graphite.
1 C-K
In the parallel detection system, energy step δE is selected from the preset value under the condition
1
as Formula (2).
δE ≥ E / m (2)
1 C-K
where
E is carbon K edge energy in the EELS
C-K
m is total number of available channels in the parallel detection system of the EELS
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ISO 23420:2021(E)

In the serial detection system, set a full measurement range as the zero-loss peak to the carbon K edge
E of graphite. Energy step δE is obtained as Formula (3).
C-K 1
δE = δE / B (3)
1 S
where
δE is energy width of the energy-selecting window in the serial detection system of the EELS
S
B is spatial width of energy selecting window in the serial detection system of the EELS
7.4.3 Acquisition of carbon K-edge EEL spectrum
Irradiate a thin region of graphite with the electron beam and acquire an EEL spectrum including
the ZLP and the carbon K-shell edge. A d/λ (sample thickness / mean free path) ratio check should be
performed to ensure the thickness of the area irradiated is suitable for the procedure.
Since the intensities of the zero-loss peak, the plasmon-loss (π + σ) peak and carbon K edge E are
C-K
considerably different, spectra including all these peaks may need to be acquired separately.
If it is necessary to acquire separate EEL spectra, an EEL spectrum should be acquired to include the
zero- loss peak and the plasmon-loss (π + σ) peak, this region will give the value for Ch (G, P), (Figure 2).
1
nd
Acquire a 2 spectra which includes the plasmon-loss (π + σ) peak and carbon K edge E , this region
C-K
will give the value for Ch (G, C-K), (Figure 3).
1
Furthermore, the sum of Ch (G, P) and the Ch (G, C-K) gives the value for CH (see 7.4.4).
1 1 1
Certain EELS systems are able to acquire Ch (G, P) and Ch (G, C-K) simultaneously if the EELS system
1 1
has a function capable of obtaining two regions of the EEL spectrum.
7.4.4 Calculate calibrated energy step δE
1C
Obtain calibrated energy step δE as Formula (4). δE is the calibrated value of energy step δE .
1C 1C 1
δE = E / CH = E / (Ch (G, P) + Ch (G, C-K)) (4)
1C C-K 1 C-K 1 1
where
Ch (G, P) is the number of channels of the range from the zero-loss peak [Figure 2, key 1] to the
1
graphite plasmon-loss (π + σ) peak E [Figure 2, key 2] on the energy step δE in the
CZLP 1
parallel detection system. In
...