ISO 14701:2018

INTERNATIONAL ISO
STANDARD 14701
Second edition
2018-11
Surface chemical analysis — X-ray
photoelectron spectroscopy —
Measurement of silicon oxide
thickness
Analyse chimique des surfaces — Spectroscopie de photoélectrons par
rayons X — Mesurage de l'épaisseur d'oxyde de silicium
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms and symbols . 1
4.1 Abbreviated terms . 1
4.2 Symbols . 1
5 Outline of method . 2
6 Method for measuring the oxide thickness . 4
6.1 Cleaning and preparing the sample . 4
6.2 Mounting the sample . 5
6.3 Choosing spectrometer settings . 5
6.4 Recording data . 8
6.5 Measuring intensities . 9
6.6 Calculating the oxide thickness .12
6.7 Calculating the uncertainty of the oxide thickness .14
Bibliography .16
Foreword
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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 7, X-ray photoelectron spectroscopy.
This second edition cancels and replaces the first edition (ISO 14701:2011), which has been technically
revised.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv © ISO 2018 – All rights reserved

Introduction
The measurement of the thickness of silicon oxide at the surface of silicon wafers has been conducted in
the past by many methods. These generally apply to oxide layers thicker than 20 nm. It is often important
to measure thicknesses in the range below 10 nm, and this document addresses the range below 8 nm
using X-ray photoelectron spectroscopy. Problems arise in measuring film thicknesses in this thickness
range since, for a layer to bond well to the substrate, it must form strong inter-atomic bonds at the
interface so that a monolayer or more of layer and substrate interfacial material exists there. This
material would not necessarily be a thermodynamically stable bulk material. Additionally, if the layer
is reactive, its outer surface might have reacted with the environment and so be changed between
fabrication and measurement. For the particular case of silicon dioxide on silicon, at the interface
there is approximately a monolayer of sub-oxides and, at the surface, adsorbed materials containing
carbon, oxygen and probably hydrogen atoms. These effects lead to offsets for the thicknesses deduced
from many methods that, although reliably measuring changes in thickness between one sample and
another, have difficulty in defining an absolute thickness.
The procedures described in this document provide methods to measure the thickness with high
accuracy (optimally 1 %) and also, more rapidly and simply, at lower accuracy (optimally 2 %). It could
also form a basis for the measurement of many film thicknesses on substrates, but without considerable
further work, the uncertainties will be undefined.
INTERNATIONAL STANDARD ISO 14701:2018(E)
Surface chemical analysis — X-ray photoelectron
spectroscopy — Measurement of silicon oxide thickness
1 Scope
This document specifies several methods for measuring the oxide thickness at the surfaces of (100)
and (111) silicon wafers as an equivalent thickness of silicon dioxide when measured using X-ray
photoelectron spectroscopy. It is only applicable to flat, polished samples and for instruments that
incorporate an Al or Mg X-ray source, a sample stage that permits defined photoelectron emission
angles and a spectrometer with an input lens that can be restricted to less than a 6° cone semi-angle. For
thermal oxides in the range 1 nm to 8 nm thickness, using the best method described in this document,
uncertainties, at a 95 % confidence level, could typically be around 2 % and around 1 % at optimum. A
simpler method is also given with slightly poorer, but often adequate, uncertainties.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 18115-1, Surface chemical analysis — Vocabulary — Part 1: General terms and terms used in
spectroscopy
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-1 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/
4 Abbreviated terms and symbols
4.1 Abbreviated terms
HPLC high-performance liquid chromatography
IPA isopropyl alcohol
4.2 Symbols
d total oxide thickness
oxide
thickness contribution to the Si O peak
d 2
Si O
d thickness contribution to the SiO peak
SiO
thickness contribution to the Si O peak
d 2 3
Si O
thickness contribution to the SiO peak
d
SiO
I intensity of the Si contribution to the Si 2p peak
Si
intensity of the Si O contribution to the Si 2p peak
I
Si O
I intensity of the SiO contribution to the Si 2p peak
SiO
intensity of the Si O contribution to the Si 2p peak
2 3
I
Si O
intensity of the SiO contribution to the Si 2p peak
I
SiO
L attenuation length for Si 2p electrons in Si
Si
attenuation length for Si 2p electrons in Si O
L
Si O
L attenuation length for Si 2p electrons in SiO
SiO
attenuation length for Si 2p electrons in Si O
2 3
L
Si O
attenuation length for Si 2p electrons in SiO
L 2
SiO
intensity normalization parameter for the Si O contribution to the Si 2p peak
R
Si O
R intensity normalization parameter for the SiO contribution to the Si 2p peak
SiO
intensity normalization parameter for the Si O contribution to the Si 2p peak
2 3
R
Si O
intensity normalization parameter for the SiO contribution to the Si 2p peak
R
SiO
U uncertainty contribution, at a 95 % confidence level, for the spectrum measurement statistics
n
U uncertainty contribution, at a 95 % confidence level, for θ
θ
U uncertainty contribution, at a 95 % confidence level, for the analyser electron optics defining
A
the solid angle of acceptance
U uncertainty contribution, at a 95 % confidence level, for the validity of the equations for thicknesses
E
U uncertainty contribution, at a 95 % confidence level, for peak synthesis without the interme-
F
diate oxides
U uncertainty contribution, at a 95 % confidence level, for the attenuation length
L
θ angle of emission of electrons measured from the surface normal
5 Outline of method
Here, the method is outlined so that the detailed procedure, given in Clause 6, can be understood in
context. Typical spectra are available in the literature and given later in Figures 3 and 4.
The initial step of cleaning the samples, if necessary, is given in 6.1. In 6.2 and 6.3, the samples are
mounted and suitable spectrometer settings chosen. In 6.4 and 6.5, the procedures for recording
the data and measuring the intensities are given. Finally, in 6.6 and 6.7, the oxide thickness and its
uncertainty at a confidence level of 95 % are calculated. In 6.5 and 6.6, two methods are provided for
calculating the oxide thicknesses from the data: a more complex method with better uncertainties and
a simpler method with poorer uncertainties. The more complex method might achieve uncertainties
as low as 1 %, but the simpler method is restricted to uncertainties that are greater than 2 %. This
2 © ISO 2018 – All rights reserved

greater figure is often adequate for many purposes, however. The sequence of steps is illustrated in the
flowchart in Figure 1. It might be useful to refer to this while using this document.
Key
Y Yes
N No
Figure 1 — Flowchart of the measurement process
Subclause 6.3 requires the angle of emission to be set accurately, and it is often the accuracy of this
setting that limits the final accuracy. Users of this procedure will need to ensure that the accuracies of
these settings are known in order to evaluate the final uncertainty. The settings can be checked to an
adequate level using reflectors mounted on the sample stage, a laser beam and standard geometrical
[1][2]
relationships .
6 Method for measuring the oxide thickness
6.1 Cleaning and preparing the sample
6.1.1 For cleaning and preparing the samples, gloves and uncoated stainless-steel tweezers are
requ
...