Microbeam analysis -- Electron probe microanalysis -- Guidelines for the determination of experimental parameters for wavelength dispersive spectroscopy

ISO 14594:2014 gives the general guidelines for the determination of experimental parameters relating to the primary beam, the wavelength spectrometer, and the sample that need to be taken into account when carrying out electron probe microanalysis. It also defines procedures for the determination of beam current, current density, dead time, wavelength resolution, background, analysis area, analysis depth, and analysis volume. It is intended for the analysis of a well-polished sample using normal beam incidence, and the parameters obtained can only be indicative for other experimental conditions. It is not designed to be used for energy dispersive X-ray spectroscopy.

Analyse par microfaisceaux -- Analyse par microsonde électronique (Microsonde de Castaing) -- Lignes directrices pour la détermination des paramètres expérimentaux pour la spectrométrie à dispersion de longueur d'onde

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Status
Published
Publication Date
20-Oct-2014
Current Stage
9092 - International Standard to be revised
Start Date
10-Aug-2020
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ISO 14594:2014 - Microbeam analysis -- Electron probe microanalysis -- Guidelines for the determination of experimental parameters for wavelength dispersive spectroscopy
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INTERNATIONAL ISO
STANDARD 14594
Second edition
2014-10-15
Microbeam analysis — Electron
probe microanalysis — Guidelines for
the determination of experimental
parameters for wavelength dispersive
spectroscopy
Analyse par microfaisceaux — Analyse par microsonde électronique
(Microsonde de Castaing) — Lignes directrices pour la détermination
des paramètres expérimentaux pour la spectrométrie à dispersion de
longueur d’onde
Reference number
ISO 14594:2014(E)
ISO 2014
---------------------- Page: 1 ----------------------
ISO 14594:2014(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2014

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

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Published in Switzerland
ii © ISO 2014 – All rights reserved
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ISO 14594:2014(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Abbreviated terms .............................................................................................................................................................................................. 2

5 Experimental parameters ........................................................................................................................................................................... 2

5.1 General ........................................................................................................................................................................................................... 2

5.2 Parameters related to the primary beam ......................................................................................................................... 2

5.3 Parameters related to wavelength dispersive X-ray spectrometers ......................................................... 3

5.4 Parameters related to the specimen ..................................................................................................................................... 4

6 Procedures and measurements ............................................................................................................................................................ 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Beam current ............................................................................................................................................................................................ 5

6.3 Parameters related to measured peaks ............................................................................................................................. 6

6.4 Parameters related to the specimen ..................................................................................................................................... 8

7 Test report ................................................................................................................................................................................................................... 9

Annex A (informative) Methods of estimating analysis area ....................................................................................................10

Annex B (informative) Methods of estimating analysis depth ................................................................................................12

Annex C (informative) Method of estimating X-ray analysis volume by applying the Monte Carlo

(MC) simulation ..................................................................................................................................................................................................14

Bibliography .............................................................................................................................................................................................................................18

© ISO 2014 – All rights reserved iii
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ISO 14594:2014(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 on the meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers

to Trade (TBT) see the following URL: Foreword - Supplementary information

The committee responsible for this document is ISO/TC 202, Microbeam analysis, Subcommittee SC 2,

Electron probe microanalysis.

This second edition cancels and replaces the first edition (ISO 14594:2003), of which it constitutes a

minor revision. It also incorporates the Technical Corrigendum ISO 14594:2003/Cor 1:2009.

iv © ISO 2014 – All rights reserved
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INTERNATIONAL STANDARD ISO 14594:2014(E)
Microbeam analysis — Electron probe microanalysis
— Guidelines for the determination of experimental
parameters for wavelength dispersive spectroscopy
1 Scope

This International Standard gives the general guidelines for the determination of experimental

parameters relating to the primary beam, the wavelength spectrometer, and the sample that need to be

taken into account when carrying out electron probe microanalysis. It also defines procedures for the

determination of beam current, current density, dead time, wavelength resolution, background, analysis

area, analysis depth, and analysis volume.

This International Standard is intended for the analysis of a well-polished sample using normal beam

incidence, and the parameters obtained can only be indicative for other experimental conditions.

This International Standard is not designed to be used for energy dispersive X-ray spectroscopy.

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/IEC 17025:2005, General requirements for the competence of testing and testing laboratories

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
analysis area

two-dimensional region of sample surface from which the full signal or a specified percentage of that

signal is detected
3.2
analysis depth

distance from the sample surface to the bottom normal to the surface from which the full signal or a

specified percentage of that signal is detected
3.3
analysis volume

three-dimensional region of a sample from which the full signal or a specified percentage of that signal

is detected
3.4
background

non-characteristic component of an X-ray spectrum arising from the X-ray continuum

3.5
beam current
electron current contained within the focused beam
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ISO 14594:2014(E)
3.6
beam current density
beam current incident on the sample per unit area
3.7
dead time

time associated with the measurement of a signal photon in a detector and/or counting system,

representing the time that the system is unavailable to process the next photon
3.8
wavelength resolution
full peak width at half maximum of a characteristic X-ray peak
4 Abbreviated terms
EPMA Electron Probe Microanalysis
FWHM Full Width at Half Maximum
WDX Wavelength Dispersive X-ray
5 Experimental parameters
5.1 General

The parameters given in 5.2.1, 5.2.3, and 5.2.4 should be known and recorded. Checking the calibration

of beam energy, beam current, and magnification together with counter dead time should be included in

the maintenance schedule of the instrument.
5.2 Parameters related to the primary beam
5.2.1 Beam energy

The beam energy typically ranges from 2 keV to 30 keV. In most cases, the calibration of the beam energy

is not critical for qualitative analysis.

NOTE Calibration is very critical in the case of use of low overvoltage ratio or during measurements relating

to layer thickness or elemental depth distributions.
5.2.2 Beam current

Because X-ray peak intensity is directly proportional to beam current, the precision of the measurement

of the beam current should be better than the precision required for quantitative analysis.

The beam current stability over long periods of time is absolutely essential for consistent quantitative

analysis. The beam current stability should be tested periodically, especially prior to quantitative

calibration and analysis. It is possible to compensate for small changes in beam current if this is recorded

prior to and following each measurement. Then all X-ray peak and background measurements should be

scaled appropriately by I /I where I is the initial beam current and I is the beam current at the time

i m, i m
of the measurement.
5.2.3 Beam current density

Beam current density is especially important when analyzing beam sensitive materials. The current

4 -2

density in a focused probe can exceed 10 A m . The effective current density can be reduced for a

measurement by lowering the incident electron beam current or, where lateral resolution is not critical,

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ISO 14594:2014(E)

by either defocusing or rastering the probe. If a rastered probe is used, a similar scan should be used

for comparative measurements on standards and other specimens because the effective spectrometer

efficiency for the selected wavelength decreases as a function of the beam deflection. See 5.3.5.

5.2.4 Magnification

To properly define the dimensional scale for line-scans and images acquired by deflecting the primary

electron beam, it is essential to calibrate the magnification scale while operating in the scanning electron

mode.
5.3 Parameters related to wavelength dispersive X-ray spectrometers
5.3.1 General

An instrument may be fitted with one or more WDX spectrometers, each with a number of diffracting

crystals that may be selected to cover a particular range of X-ray wavelengths depending on the line

of the analysed element. The following parameters are important for the proper operation of WDX

spectrometers.
5.3.2 Take-off angle

The take-off angle affects quantitative analysis. Any comparison of measurements from instruments

with different take-off angles should be taken into account and the procedures used be noted in the

analysis report.

NOTE The value of this angle, which is normally fixed, is provided by the instrument manufacturer.

5.3.3 Wavelength resolution
The spectral resolution depends on the following parameters:
— crystal material (and Miller indices of the crystal planes);

— the radius of curvature of the diffracting crystal (fully focusing vs. semi-focusing crystal);

— the presence of a crystal mask (if semi-focusing crystal);

— the size and position of the counter entrance window or of the entrance slit if present.

All these settings determine the wavelength resolution of the measured X-ray spectrum and the observed

line-width (FWHM) of the characteristic X-ray peaks.

Resolution can also influence the ability of the system to discriminate against overlapping peaks,

background signals, and the sensitivity of measurements to specimen height and beam position on the

specimen.
5.3.4 X-ray detector and counting chain

Many spectrometers use a gas-filled proportional counter to detect X-rays. The magnitude of the output

pulses from these detectors is determined by the incident X-ray energy and/or the voltage applied to

the counters. Two discriminators are used to select the pulse of interest. A low discriminator setting is

used to eliminate pulses due to noise, while a high discriminator setting excludes pulses from high order

reflections of more energetic X-rays. Optimum settings depend on the X-ray lines of interest.

It is important to set the discriminator to ensure that any unintended shift in pulse amplitude, for

example, due to high count rates or changes in atmospheric temperature and pressure (flow counter),

has no significant effect on the measured count rate.

Because X-ray counting efficiency decreases with increasing count rate, it is important to correct the

measured count rate for the effect of the dead time. In an automated system, the discriminator settings

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ISO 14594:2014(E)

can be set automatically. These settings should be routinely checked to ensure proper automatic

operation.
5.3.5 Peak location (wavelength)

Under normal circumstances, the wavelength which has the maximum peak intensity is used to define

the location of an X-ray peak. It is necessary, using suitable reference materials, to periodically check

and correct for the difference in a peak’s theoretical position and its actual measured position on a

given spectrometer and diffraction crystal. The time between checks will depend on the stability of the

instrument spectrometers.

The measured maximum intensities of peaks which have narrow FWHM values are strongly affected by

the errors in peak location. The peak intensity can be changed due to the chemical state and polarization

effects.

NOTE 1 If the element in the sample of interest is in a different chemical state than that of the reference material,

then the shape of the characteristic X-ray peak might be different for specimen and standard. In this case, the

peak maximum might not provide a reliable measure of the total peak intensity and an alternative approach,

such as peak area measurements, might be required to obtain reliable results. These chemical state effects are

particularly important for X-ray peaks with low energy values.

NOTE 2 If a crystalline sample causes the polarization effects in relation to the position between the sample

and the analysis crystal, the peak shape and location can be changed. This can be checked by rotating the sample

around an axis perpendicular to the electron beam and observing the effect on peak shape and location. The

problem might occur in systems with symmetry lower than cubic and higher than triclinic and is worst when the

[1] [2]

Bragg angle is close to 45°. The phenomenon has been found in graphite and certain borides. The effect can

be much reduced by using peak area measurements.

The position of the peak maximum varies with deviation of the probe from the focal point of the

spectrometer on the sample. Calibration measurements and quantitative analysis on the sample should

normally be made with the probe in the same position relative to this focal point, and using the same

beam defocus or raster setting, if applied. For all quantitative and qualitative analyses carried out using

a defocused and scanned beam, the area of the sample surface irradiated should not be so large as to

cause a significant fall in X-ray counts from that obtained with the static focused electron beam.

5.3.6 Background

The characteristic X-ray peaks are superimposed on a background of continuum X-rays.

To properly calculate the intensities of characteristic X-rays, the magnitude of this background needs to

be determined and corrected if it is statistically significant.
5.4 Parameters related to the specimen
5.4.1 Specimen stage

High precision X, Y, and Z stages allow the sample and standards to be accurately positioned under the

electron beam by using an attached optical microscope; the user can set the height of the sample so

the axis of the WDX spectrometer and the primary beam position coincide at the surface of the sample.

Orthogonality between the electron beam (the optical axis) and the specimen stage is essential in order

to perform a proper quantitative analysis. A check on the adjustment of the optical microscope should

be included in the routine instrument maintenance schedule.

In an automatic mode of operation, where the measurements are to be made at preset points on the

standards and the specimen, it is important to know the reproducibility with which the stage retrieves

preset points and to adopt appropriate strategies to overcome any obvious limitations.

5.4.2 Surface roughness
For best results, the surface roughness of the specimen should be minimized.
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ISO 14594:2014(E)
5.4.3 Analysis volume
Analysis volume is determined by the incident beam area, the depth
...

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