ISO 14706:2014

INTERNATIONAL ISO
STANDARD 14706
Second edition
2014-08-01
Surface chemical analysis —
Determination of surface elemental
contamination on silicon wafers by
total-reflection X-ray fluorescence
(TXRF) spectroscopy
Analyse chimique des surfaces — Détermination de la contamination
en éléments à la surface des tranches de silicium par spectroscopie de
fluorescence X à réflexion totale
Reference number
©
ISO 2014
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2014 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative reference . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 Principle . 2
6 Apparatus . 3
7 Environment for specimen preparation and measurement . 3
8 Calibration reference materials . 3
9 Safety . 4
10 Measurement procedure . 4
10.1 Preparation for measurement . 4
10.2 Preparing a calibration curve . 4
10.3 Measurement of a test specimen . 5
11 Expression of results . 5
11.1 Method of calculation . 5
11.2 Blank correction . 6
12 Precision . 6
13 Test report . 6
Annex A (informative) Reference materials . 8
Annex B (informative) Relative sensitivity factor . 9
[6]
Annex C (informative) Preparation of reference materials .13
Annex D (informative) VPD-TXRF method .16
Annex E (informative) Glancing-angle settings .18
Annex F (informative) International inter-laboratory test results .22
Bibliography .25
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 201, Surface chemical analysis.
This second edition cancels and replaces the first edition (ISO 14706:2000), which has been technically
revised.
iv © ISO 2014 – All rights reserved

Introduction
This International Standard was prepared for the measurement of surface elemental contamination
on silicon wafers on the basis of three existing standards: ASTM F 1526, SEMI Standard M33, and a
UCS (Ultra-Clean Society) standard published by the Institute of Basic Semiconductor Technology
Development.
TXRF needs reference materials to perform quantitative analyses. Certified reference materials are not
10 2
available at low densities of 10 atoms/cm . Even if they were available, the possibility of contamination
from the environment reduces the shelf life of such reference materials.
Therefore, the TXRF reference materials are to be prepared and analysed for calibration by each relevant
analytical laboratory. Thus, two standards, one for the TXRF measurement procedure and the other for
the preparation of reference materials, are necessary. This International Standard concerns the former
part.
INTERNATIONAL STANDARD ISO 14706:2014(E)
Surface chemical analysis — Determination of surface
elemental contamination on silicon wafers by total-
reflection X-ray fluorescence (TXRF) spectroscopy
1 Scope
This International Standard specifies a TXRF method for the measurement of the atomic surface density
of elemental contamination on chemomechanically polished or epitaxial silicon wafer surfaces.
The method is applicable to the following:
— elements of atomic number from 16 (S) to 92 (U);
10 2 14
— contamination elements with atomic surface densities from 1 × 10 atoms/cm to 1 × 10
atoms/cm ;
8 2 12 2
— contamination elements with atomic surface densities from 5 × 10 atoms/cm to 5 × 10 atoms/cm
using a VPD (vapour-phase decomposition) specimen preparation method (see 3.4).
2 Normative reference
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 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
total reflection
complete reflection of glancing X-ray radiation into a medium having larger X-ray optical density value
at a boundary between two media
Note 1 to entry: The refractive index of silicon for X-rays is less than 1. X-rays which are incident on the surface
at a small glancing angle are therefore, totally reflected from the surface at an angle equal to the glancing angle.
3.2
glancing angle
angle between the specimen surface plane and the virtual plane containing incident X-rays which
impinge on the sample surface
3.3
critical angle
glancing angle corresponding to the first point of inflection in the plot of the sample matrix X-ray
fluorescence against the glancing angle
3.4
VPD-TXRF method
method in which impurities on the surface are collected by the so-called VPD procedure, i.e. the non-
volatile products formed by acid decomposition of the oxide at the wafer surface are collected by a
droplet of collecting agent, usually ultra-pure hydrofluoric acid, and dried in a manner which gives the
least environmental contamination, the residue from the droplet, subsequently being analysed by TXRF
3.5
spurious peaks
detected peaks that do not originate from impurities on the silicon wafer
Note 1 to entry: Spurious peaks are due to fluorescent X-rays originating from elements in the detector or the X-ray
path. The fluorescent X-rays are excited by direct scattering or reflection of incident X-rays. This phenomenon
leads to an increase in the measurement error. Spurious peaks seriously affect analytical measurements in the
10 2 11 2
contamination range from ca. 10 atoms/cm to ca. 10 atoms/cm .
4 Abbreviated terms
FWHM full width at half maximum
RM reference material
SSD solid-state detector
SDD silicon drift detector
TXRF total-reflection X-ray fluorescence
VPD vapour-phase decomposition
5 Principle
When a specimen is irradiated with X-rays, fluorescence X-rays at characteristic energies of the elements
that constitute the specimen are generated. The intensities of the fluorescence X-rays are proportional
to the amounts of each element in the specimen.
Total reflection of the incident X-rays on the specimen reduces a penetration depth of the incident X-rays
in the specimen. This allows more selective excitation of the fluorescence X-rays from the surface region,
including atoms deposited on the surface of the silicon wafer. Consequently, a spectrum of fluorescence
X-rays with a large ratio of signal to background (S/B) and signal to noise (S/N) can be obtained.
The detection limit depends upon the atomic number, the excitation energy, the photon flux, the
detector resolution and energy-dependent detection efficiency, the energy bandwidth of the excitation
X-rays, the specimen-related shape and statistics of TXRF spectral background, the instrument-related
noise magnitude, the integration time, and the accuracy of calibration of the RM and the blank value.
For constant instrumental parameters, the interference-free detection limits vary over two orders of
magnitude as a function of the atomic number of the analyte element.
NOTE The depth of measurement can vary with the glancing angle, but in the case of film-type contamination
it is usually less than 5 nm. The area of measurement consists of a circle of ca. 10 mm in diameter, though it
varies depending on the relative position of the X-ray detector and the specimen. In the case of particulate-type
contamination on a clean surface, the yield of fluorescence X-rays varies depending on the sizes, d
...