ISO/PRF TS 21383

Microbeam analysis -- Scanning electron microscopy -- Qualification of the scanning electron microscope for quantitative measurements

Titre manque

Standards Content (sample)

TECHNICAL ISO/TS
SPECIFICATION 21383
First edition
Microbeam analysis — Scanning
electron microscopy — Qualification
of the scanning electron microscope
for quantitative measurements
PROOF/ÉPREUVE
Reference number
ISO/TS 21383:2021(E)
ISO 2021
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ISO/TS 21383:2021(E)
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© ISO 2021

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Published in Switzerland
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ISO/TS 21383:2021(E)
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 principles ............................................................................................................................................................................................... 5

5.1 Condition setting ................................................................................................................................................................................... 5

5.2 Contrast / brightness setting ...................................................................................................................................................... 5

5.3 Sample preparation ............................................................................................................................................................................ 6

6 Measurement of image sharpness ..................................................................................................................................................... 7

7 Measurement of drift and drift-related distortions (imaging repeatability) ........................................8

7.1 Measurement of image drifts within specified time intervals. ...................................................................... 9

7.1.1 One-minute drift measurement .......................................................................................................................10

7.1.2 Ten-minute drift measurement ........................................................................................................................10

7.1.3 One-hour drift measurement .............................................................................................................................10

7.1.4 Long-term larger than one-hour drift measurement ....................................................................10

7.2 Evaluation of the drift and the drift-related distortions by using image overlay .......................11

7.3 Evaluation of the drift and drift-related distortions by using cross-correlation

function (CCF).......................................................................................................................................................................................13

7.3.1 Measurement of the drifts by using the CCF. ........................................................................................13

7.3.2 Measurement of the distortions by using the CCF. ..........................................................................15

8 Measurement of electron-beam-induced contamination .......................................................................................15

8.1 Cleaning of the sample surface ..............................................................................................................................................16

8.2 Cleaning of the inner surfaces of the sample chamber. .....................................................................................16

8.3 Measurement method of the contamination. ............................................................................................................17

8.3.1 Measurement of the height of the contamination growth. .......................................................17

8.3.2 Measurement of relative carbon concentration of the contamination by

the X-ray analysis..........................................................................................................................................................18

8.3.3 Measurement of the surface contamination by the change of SEM signal

intensities. ..........................................................................................................................................................................18

9 Measurement of the image magnification and linearity ..........................................................................................19

9.1 Measurement of the image magnification ....................................................................................................................20

9.2 Measurement of the image linearity .................................................................................................................................21

10 Measurement of background noise ...............................................................................................................................................22

10.1 Evaluation methods by using noise profiles and processed images ......................................................22

10.2 Evaluation methods by calculating numerical image properties .............................................................28

11 Measurement of the primary electron beam current .................................................................................................30

11.1 Ten-minute primary electron beam current measurement ..........................................................................30

11.2 Long-term primary electron beam current measurement ............................................................................30

12 Reporting Form ...................................................................................................................................................................................................32

Annex A (informative) Measurement of image sharpness ..........................................................................................................34

Annex B (informative) Measurement of image drift and distortions caused by unintended

motions ........................................................................................................................................................................................................................36

Annex C (informative) Measurement of electron beam-induced contamination ...............................................48

Annex D (informative) Measurement of the image magnification and linearity .................................................54

Annex E (informative) Measurement of primary electron beam current ...................................................................57

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Bibliography .............................................................................................................................................................................................................................59

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ISO/TS 21383:2021(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

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 202, Microbeam analysis, Subcommittee

SC 4, Scanning electron microscopy.

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/TS 21383:2021(E)
Introduction

The scanning electron microscope (SEM) is a very versatile instrument, which is widely used in

production, development and scientific research across the world. While they are easy to operate and

provide results quickly, there are a number of notorious problems, which hinder operating them at

their best performance. These are the reasons for lack of excellent repeatability in SEM imaging and

measurements. The most bothersome ones among these are unintended motions of the sample stage

and the primary electron beam, geometry distortions, wrong scale, image blur (lack of sharp focus),

noise and electron beam-induced contamination. Quantification of these essential performance

parameters is very useful to ensure that all SEMs perform at manufacturers specifications and at

users’ own purpose. Quantified knowledge helps in the evaluation of measurement uncertainties, and

necessary repairs.

This document pertains to measurement methods for the following SEM performance parameters:

— Image sharpness (spatial resolution, primary electron beam focusing ability)
— Drifts (the sample stage, the electron beam and the electron-optical column)
— Cleanliness (lack of beam-induced contamination)
— Image magnification and linearity (both in X and Y directions)
— Background noise
— Primary electron beam current

These parameters will also be influenced by the SEM conditions such as the lifetime of source (emitter

conditions), lifetime of liner tube and apertures (contamination of the electron optical parts), time and

intensity of last cleaning of vacuum chamber by the plasma cleaning or Ultra Violet irradiation, the

sample preparation and final surface cleaning.
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TECHNICAL SPECIFICATION ISO/TS 21383:2021(E)
Microbeam analysis — Scanning electron microscopy
— Qualification of the scanning electron microscope for
quantitative measurements
1 Scope

This document describes methods to qualify the scanning electron microscope with the digital imaging

system for quantitative and qualitative SEM measurements by evaluating essential scanning electron

microscope performance parameters to maintain the performance after installation of the instruments.

The items and evaluating methods of the performance parameters are selected by users for their own

purposes.
2 Normative references

The following referenced documents are indispensable for the application 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 16700:2016, Microbeam analysis — Scanning electron microscopy — Guidelines for calibrating image

magnification

ISO/TS 24597:2011, Microbeam analysis — Scanning electron microscopy — Methods of evaluating image

sharpness
ISO 22493, Microbeam analysis — Scanning electron microscopy — Vocabulary

ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 22493 and the following 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
scanning electron microscope
SEM

instrument that produces magnified images of a specimen by scanning its surface with an electron beam

[SOURCE: ISO 16700, 3.1]
3.2
image
two-dimensional representation of the specimen surface generated by SEM
[SOURCE: ISO 16700, 3.2]
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3.3
image magnification

ratio of the linear dimension of the scan display to the corresponding linear dimension of the specimen

scan field
[SOURCE: ISO 16700, 3.3]
3.4
scale marker

line / generated line (intervals) on the image (3.2) representing a designated actual length in the

specimen
[SOURCE: ISO 16700, 3.4]
3.5
reference material

material, sufficiently homogeneous and stable with respect to one or more specified properties, which

has been established to be fit for its intended use in a measurement process

[SOURCE: ISO Guide 30:2015, 2.1.1, modified — Note 1 to entry to Note 4 to entry are omitted.]

3.6
certified reference material
CRM

reference material (RM) (3.5) characterized by a metrologically valid procedure for one or more

specified properties, accompanied by an RM certificate that provides the value of the specified property,

its associated uncertainty, and a statement of metrological traceability

[SOURCE: ISO Guide 30:2015, 2.1.2, modified — Note 1 to entry to Note 4 to entry are omitted.]

3.7
calibration

set of operations which establish, under specified conditions, the relationship between the magnification

indicated by the SEM and the corresponding magnification determined by examination of an RM (3.5)

or a CRM
[SOURCE: ISO 16700, 3.7]
3.8
accelerating voltage
absolute acceleration potential V [V] of the final anode to the electron emitter

Note 1 to entry: For the electron charge q [C], the accelerated electron will obtain the energy qV [J] =

e ea

V [eV] ≡ E and enter the sample with this energy provided that the initial energy E from the emitter

a L I

is negligible. The “landing energy” to the specimen means this energy E , typically expressed in the unit

eV or keV.
Refer to Clause 4 concerning eV.
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4 Symbols and abbreviated terms
A , A
areas of the binarized pictures FI and FI , I respectively, typically ex-
() ()
ACF,1 CCF,n
BAC1 BCC 1 n
pressed in pixel
ACF auto-correlation function
CCF cross-correlation function

D , D Drift quantities of the n-th image I n=12,,  for the horizontal (H) and vertical

Hn Vn n
(V) directions respectively, typically expressed in pixel or in nm
D , D
displacements for X and Y directions respectively from the origin, typically ex-
X Y
pressed in nm

max ||D , max ||D mean the largest absolute values of displacements from the origin

X Y
in X and Y directions respectively
12/
distance DD=+D from the initial position XY, which is regarded as
OX X OO
the origin, typically expressed in nm
max D - the largest value of the distance D
O O
pitch length of the RM or the CRM, typically expressed in nm
d measured mean pitch length, typically expressed in nm
d averaged value of measured d , typically expressed in nm
SA S
CG contrast to gradient method for evaluating image sharpness
DR derivative method for evaluating image sharpness
FT Fourie transform method for evaluating image sharpness

eV electronvolt, a unit of energy equal to approximately 1.6×10−19 joules (J). By definition,

it is the amount of energy gained (or lost) by the charge of a single electron moving

across an electric potential difference of 1 volt.
auto-correlation function of the image I
AC 1
cross-correlation function for the initial image I and n-th image I n=23,,  in
FI , I ()
1 n
CC 1 n
the measurements.

FI binarized picture of the auto-correlation function FI of the initial image I by

() ()
BAC1 AC 1 1
using a thresholding level T

FI , I binarized picture of the cross-correlation function FI , I for the initial image I

() ()
BCC 1 n CC 1 n 1
and the n-th image I n=23,,  by using a thresholding level T
n B
HFW horizontal field width
HV,

horizontal (H) and vertical (V) peak positions of the auto-correlation function FI

PP11
AC 1
respectively

HV, horizontal (H) and vertical (V) peak positions of the cross-correlation function

PPnn
FI , I respectively
CC 1 n
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I background noise image

I flat image whose signal intensity of the element ij, is the mean value S of the

FB MEAN
background noise image I .

I processed images obtained from the background noise images I , for example, by a

PB BG
method of contrast enhancement.
I original secondary electron (SE) or backscattered electron (BE) scanning image

Ii , j signal intensity of the element ij, of the image I , where i and j mean horizontal and

() ()

vertical numbers of the element respectively measured from the initial element ()11,

Ii(), j signal intensity of the element ()ij, of the reference image I which is set to the flat

ref ref
image I when calculating the peak signal-to-noise ratio S
FB PSNR
max Ii, j means the maximum value of the given n -bit imaging mode. If
 ()
 
ref IM
n = 8-bit imaging mode, then max Ii, j =255 .
 ()
IM  ref 

Ii(), j signal intensity of the element ()ij, of the test image I which is set to the back-

test test
ground noise image I when calculating the peak signal-to-noise ratio S
BG PSNR

Ii(), j signal intensity of the element ()ij, of the reference image I which is be obtained

refS refS
from the test image Ii(), j by applying a suitable image filter to reduce the
testS
image noise

Ii, j signal intensity of the element ij, of the test image I which is usually not the

() ()
testS testS
background noise image I but actual SE or BE scanning image
I primary electron beam current (probe current)
kV kilovolt
k ratio of the area A to the area A (/kA= A )
A,n CCF,n ACF,1 A,,nnCCF ACF,1

image size (total pixels of the image area), typically expressed in pixel such as LL×

L , L horizontal (H) and vertical (V) image sizes (lengths) respectively, typically expressed

H V
in pixel
L pixel size, typically expressed in nm

L horizontal (H) line profile which is obtained vertically averaged for specified band

pHA
areas from the background noise image

L vertical (V) line profile which is obtained horizontally averaged for specified band

pVA
areas from the background noise image

l the total measured length by a scaler on the screen or the photograph ( ln= ∙d )

S SP S
image magnification for setting
N number of measurements for beam current
N number of measurements for image drift
number of measurements for image magnification
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N number of measurements for image sharpness
total number of pitches for measurement
R Image sharpness, typically expressed in nm
R image sharpness, typically expressed in pixel
S maximum value of intensities of pixels in an image.
MAX
S mean value of intensities of pixels in an image
MEAN
S minimum value of intensities of pixels in an image
MIN
S peak signal-to-noise ratio
PSNR
S standard deviation of intensities of pixels in an image
STD
V accelerating voltage
WD working distance
5 General principles

The best performance of any SEM is at some optimized set of instrument settings; therefore, throughout

this document for the various assessments those imaging parameters and instrument settings should

be used if those are specified by the SEM’s manufacturer for achieving the best performance. These

basic principles are useful for users' own purpose in many cases.
5.1 Condition setting

Some SEMs have only one accelerating voltage in these specifications; others may have more (e.g.

15 kV and 1 kV). All the assessments should be performed at all specified accelerating voltages and

magnifications if those parameters and settings are applicable to user’s purposes and evaluations. If

optimization of SEM-based measurements requires different parameters (accelerating voltage, beam

current, etc.) for users' own purpose, then for these all the assessments should be performed and the

results recorded in the report.

Beyond setting the magnification, accelerating voltage, beam current, all pertinent parameters to the

values specified by the instrument manufacturer for proving the best resolution performance, it is

important to set the focus (astigmatism), contrast, and brightness to their SEM-specific, best settings

for taking images.
5.2 Contrast / brightness setting

Only images with properly set contrast and brightness should be used for the various measurements

and quantitative SEM assessments. The contrast and the brightness must be set so that all pixels have

grey-scale levels that never reach the lowest (dark, under-saturation) or the highest (bright, over-

saturation) level. This is important to make sure that no information is lost by setting contrast and

brightness values incorrectly.

If the system has the signal monitor or can generate the histogram for the relative signal range [0, 1] in

the acquisition of SEM image, verify that the signals are within the range [0.2, 0.8] approximately.

In 8-bit imaging mode, in properly set images the intensities of the image pixels vary in between 0 and

255 grey levels. In 16-bit imaging mode the intensities of the image pixels vary in between 0 and 65535

grey levels.
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Figure 1 shows examples of secondary electron (SE) images taken at accelerating voltage 5 keV, 19,5 μm

HFW. Contrast and brightness are properly set a) and wrong b), c), d) and e). Figure 2 shows their

corresponding histograms.
5.3 Sample preparation

Concerning the samples, there will not be the perfect or the almighty sample which are applicable to many

evaluation items. In general, the best samples are different for the measurements of image sharpness,

drift and drift-related distortions, electron beam induced contamination, image magnification and

linearity. Select the ideal sample which is suitable for the required evaluation accordingly.

a) Proper contrast and bright- b) Too little contrast c) Too much contrast
ness
d) Too much brightness e) Too little brightness
Figure 1 — Examples of SE images for various contrast and brightness setting.
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a) Proper contrast and bright- b) Too little contrast c) Too much contrast
ness
d) Too much brightness e) Too little brightness
Key
I signal intensity for 8-bit imaging mode
N number of counts
Figure 2 — Examples of histograms corresponding to the images in Figure 1
6 Measurement of image sharpness

The definition and the explanation of the term “image sharpness” are described in ISO TS 24597 for

the SEM. Image sharpness is strongly related to the focusing ability of the SEM in forming the primary

electron beam. It is one of most widely used, but certainly not sufficient performance parameter.

However, it is very useful to know the image sharpness because the algorithms do not depend on the

human sense and give quantitative results by using the procedures described below.

On the other hand, the term “Lateral resolution” or “Spatial resolution” are not defined strictly in SEM

even though these terms are popular in the surface chemical analysis.
[1]
NOTE Refer to ISO 18115-1:2013 for terms used in spectroscopy .

Even if various SEM manufacturers use these terms, the notion of resolution is not established

scientifically, it is sample-and method-dependent, and there is no accurate way of measuring it today.

The focusing ability is related to the size and shape of the primary electron beam at the surface of the

sample. Its measurement is also very difficult, especially for sub-nanometre focuses, i.e., beam sizes.

Furthermore, these values are not related to beam focusing only, but to interaction volume, stability of

probe scanning and external disturbances as well.

To acquire the images for the evaluation of the image sharpness, refer to the Clause “4 Steps for

acquisition of an SEM image” of ISO TS 24597. “4.1 General”, “4.2 Specimen”, “4.4 Selection of the field

of view” and “4.7 Contrast-to-noise ratio of the image” will be useful information. The structure of the

sample should not be “periodic mesh” or “line and space” because some specific features or frequencies

are emphasized in the signal analysis. The samples as shown in Figure 3 a) is typical and appropriate

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because the particles with different diameters are randomly distributed, and their surface are flat and

edges are sharp.

Set the focus and astigmatism to their best, the magnification, accelerating voltage, beam current,

working distance to the values specified by the instrument manufacturer for proving the best image

sharpness performance, and take several images.

To evaluate the SEM image sharpness performance, follow the procedure in ISO/TS 24597:2011. Select

the evaluation method from the 3-methods DR (Derivative) method, FT (Fourier transform) method

and CG (Contrast-to-gradient) method. For one image, plural methods can be used if necessary. Valuate

that the obtained results are allowable or not for the quantitative measurements. Report evaluated

results with the evaluation methods and the valuation.

Figure 3 show the examples of the selected SEM images with the image size L=×512 512 for the

evaluation of the image sharpness R [pixel] or R [nm]. The obtained values of image sharpness R

PX L L

are 1,9 nm for Figure 3 a) and 3,3 – 4,2 nm for Figure 3 b). The example of the evaluation process for

these images is shown in Table A.1.

a) Accelerating Voltage V = 15 kV, beam cur- b) Accelerating Voltage V = 1 kV, beam current

a a
rent I = 43 pA, 265 nm HFW. I = 43 pA, 657 nm HFW.
P P
Figure 3 — Selected SEM images for the evaluation of image sharpness.
Sample: Evaporated gold on carbon.
See 12 Reporting Form and Annex A for further pertinent information.
7 Measurement of drift and drift-related distortions (imaging repeatability)

Unintended motions make the primary electron beam land on wrong, unintended locations on

the sample, which results in poor repeatability and/or distorted, blurry images, especially at high

magnifications. These typically arise from mechanical and acoustical impacts, temperature variation

of the room and the cooling water, hysteresis, adverse external electromagnetic fields, and from the

noise in various circuits of
...