ISO 14701:2018

Surface chemical analysis - X-ray photoelectron spectroscopy - Measurement of silicon oxide thickness

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.

Analyse chimique des surfaces — Spectroscopie de photoélectrons par rayons X — Mesurage de l'épaisseur d'oxyde de silicium

General Information

• Status: Published
• Publication Date: 30-Oct-2018
• ICS: 71.040.40 - Chemical analysis
• Technical Committee: ISO/TC 201/SC 7 - Electron spectroscopies
• Drafting Committee: ISO/TC 201/SC 7/WG 2 - Quantification and interpretation of data in electron spectroscopy
• Current Stage: 9060 - Close of review
• Completion Date: 04-Jun-2029

Relations

• Consolidated By: ISO 9288:2022 - Thermal insulation - Heat transfer by radiation - Vocabulary
• Effective Date: 06-Jun-2022
• Revises: ISO 14701:2011 - Surface chemical analysis - X-ray photoelectron spectroscopy - Measurement of silicon oxide thickness
• Effective Date: 28-Oct-2017

Overview

ISO 14701:2018 - "Surface chemical analysis - X-ray photoelectron spectroscopy - Measurement of silicon oxide thickness" specifies validated X-ray photoelectron spectroscopy (XPS) methods to measure the equivalent thickness of silicon dioxide on (100) and (111) silicon wafer surfaces. The standard targets thin thermal oxides in the 1 nm to 8 nm range and is intended for flat, polished samples analyzed with XPS instruments meeting defined hardware and geometry requirements. Typical achievable uncertainties (95% confidence) are around 2% for the standard method and can be near 1% under optimal conditions.

Key topics and requirements

• Scope and applicability
  • Measures oxide thickness as an equivalent SiO2 value for (100) and (111) wafers.
  • Applicable only to flat, polished samples and instruments with the specified capabilities.
• Instrument requirements
  • X-ray source: Al or Mg.
  • Sample stage permitting defined photoelectron emission angles.
  • Spectrometer input lens restrictable to < 6° cone semi-angle.
• Measurement workflow
  • Cleaning and sample handling (powder-free gloves, uncoated stainless-steel tweezers, solvent immersion procedure).
  • Mounting, choosing spectrometer settings, recording spectra, and measuring peak intensities.
  • Two calculation paths: a complex method with better uncertainty and a simpler method with slightly higher uncertainty.
• Metrology and uncertainty
  • Procedures include calculating oxide thickness and quantifying the combined uncertainty at a 95% confidence level.
  • Accurate setting of the emission angle and knowledge of spectrometer acceptance geometry are critical to final accuracy.
• Technical considerations
  • Accounts for interfacial sub-oxides and surface adsorbates that affect absolute thickness values.
  • Uses attenuation lengths, peak synthesis, and intensity normalization for the Si 2p region.

Applications and users

• Semiconductor process control and metrology for advanced node manufacturing.
• Surface-chemical analysis laboratories and national metrology institutes validating thin oxide growth.
• Quality assurance and R&D teams monitoring thermal oxide thickness on silicon wafers.
• XPS instrument manufacturers and service providers implementing or validating measurement protocols.

Related standards

• ISO 18115‑1 - Surface chemical analysis - Vocabulary (normative reference used for terms and definitions in ISO 14701:2018).

Keywords: ISO 14701:2018, XPS, X‑ray photoelectron spectroscopy, silicon oxide thickness, silicon wafers, oxide metrology, Al X-ray source, Mg X-ray source, surface chemical analysis, thin film thickness measurement.

Standards Content (Sample)

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
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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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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.org/iso/foreword .html.
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
complete listing of these bodies can be found at www .iso .org/members .html.
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
required. In selecting gloves, care shall be taken to avoid those with talc, silicone compounds or similar
contaminants. “Powder-free” gloves have no talc, and fresh polyethylene gloves, or gloves of a higher
quality, shall be used in sample handling. Do not use moulded gloves, for example vinyl, which will
probably be covered with highly contaminating release agents. Tweezers that are of uncoated stainless
steel shall be used.
6.1.2 To manipulate samples, the gloves are used to hold the tweezers and not the sample. Avoid any
wiping materials, sometimes used to handle samples, as they might result in unwanted contamination of
the sample surface. Unnecessary contact of the sample with the gloves shall be avoided. Sample mounts
and other materials used to hold samples shall be cleaned regularly whenever there is a possibility of
cross-contamination of samples. The use of tapes containing silicones and other mobile species shall be
[3]
avoided .
6.1.3 Samples shall be prepared and mounted with clean tweezers to ensure that the surface is not
altered prior to analysis and that the best possible vacuum conditions are maintained in the analytical
chamber. Use the gloves to handle the tweezers to avoid contaminating them or any cleaning equipment
with finger grease. Clean the tweezers by one of the following two methods:
a) Immerse the tweezers before use for 16 h in electronic, or equivalent, grade (>99,9 %) isopropyl
alcohol (IPA) that leaves no significant residue. If electronic grade IPA is not available, high-
performance liquid chromatography (HPLC) grade (>99,5 %) IPA may be used.
b) Boil the tweezers in ultra-high-purity water for 10 min.
Grip the sample at the edge only, in a region that will not be analysed. Avoid breathing or speaking over
the sample. Keep these tweezers in a clean glass container for future use. Tools shall not unnecessarily
touch the sample surface to be analysed.
6.1.4 Inspect the samples for any scratches, blemishes or marks on the polished surfaces. Finger marks
should not be present but, if they are, may be removed as described in 6.1.6. Note the condition of the
surface. It should be featureless. Identify the side of the sample for analysis. This is usually the polished
side. If the unpolished side is to be analysed, this document is not applicable. If the sample is too large
for insertion into the instrument, a smaller portion will need to be cut from it. To do this, material with
a (100)-orientated surface may be cut to form a suitably sized rectangular portion by cleavage along
(111) planes. In this way, a square of side 10 mm, bounded by <110> directions, may be conveniently
produced. For those samples with a (111)-orientated surface, a similar cleaving along (111) planes forms
equilateral triangles, also bounded by <110> directions. Triangle sides of length 15 mm are convenient.
The scribing for cleaving often leaves very small fragments of Si on the samples. These fragments shall, as
far as possible, be removed. The cleaning procedure described in 6.1.6 is often sufficient for this purpose.
NOTE The <110> directions are usually indicated by flats cut into the sides of (100)- and (111)-orientated
wafers.
6.1.5 Analyses show that wafers and many other materials, such as metals, accumulate organics,
hydrocarbons, silanes and phthalates from the environment. During storage of wafers, the thickness
4 © ISO 2018 – All rights reserved

of these adsorbed layers increases to around 0,35 nm on the polished surface in normal, uncirculated
laboratory air after 100 days, but is kept below 0,2 nm if a wafer container is used that has been kept
[4]
closed and has not been exposed to excessive heat (i.e. has been kept below ∼35 °C). In either case,
the samples should be analysed without cleaning. If, however, there is evidence that they have been
contaminated by organic contaminants (e.g. finger grease) or the samples have been cut to reduce their
size, the contamination can be reduced to a thickness of about 0,14 nm by cleaning as described in 6.1.6.
6.1.6 If the specimens require cleaning, immerse them in a cleaned glass container in electronic (or
equivalent) grade (>99,9 %) isopropyl alcohol (IPA) for 16 h (e.g. overnight). If electronic grade IPA is
not available, high performance liquid chromatography (HPLC) grade (>99,5 %) IPA may be used. The
top of the test tube can be conveniently closed by a piece of clean aluminium foil. Next, remove the liquid,
renew the IPA, agitate ultrasonically for 1 min, rinse in fresh IPA and remove the excess liquid using a
jet of pure (>99,9 % purity), dry (<0,01 % water) argon or an equivalent rare gas. The samples are now
ready for analysis.
NOTE The procedure using 16 h immersion in solvent leaves significantly less carbon than a simple
[4]
ultrasonic rinse .
If HPLC-quality chloroform or dichloromethane is used instead of IPA, the level of carbon remaining
is generally about twice as high. However, the amount left depends on how the samples have been
contaminated in the first place. Hence, chloroform and dichloromethane are not recommended unless
IPA is unavailable. Note that there are relevant safety requirements in using all solvents. Carbon
deposited during any spectroscopic analysis can be crosslinked by the radiation used, forming a tough
adherent layer that cannot be removed without compromising the oxide integrity. Do not use other
cleaning methods, even if they are known to remove contamination, since they might also change the
[4]
oxide thickness .
If pure, dry argon or another rare gas is not available, do not use gas from pressurized cans that include a
propellant or from compressed-air lines, as these might deposit contaminants. Under no circumstances
use any proprietary cleaning agents or liquids containing surfactants.
6.2 Mounting the sample
Mount the sample on the sample holder using fixing screws, or other metallic means, to ensure electrical
contact. Do not use double-sided adhesive tape. The (100)-surface samples shall be mounted such that
the photoelectron angle of emission is set in the azimuth at 22,5° to one edge of the rectangular samples
and the (111)-surface samples shall be mounted such that the angle of emission is in an azimuth of one
edge of the triangular samples. Set these azimuthal angles as accurately as possible and within 2° of
their nominal values. This is shown in Figure 2.
NOTE The reasons for selecting this geometry are described in detail in Reference [5]. This geometry sets
the emission direction as a single direction available to both the (100) and the (111) surfaces that is as far from
any low index directions as possible. In this direction, the Si substrate forward-focused intensity is avoided.
6.3 Choosing spectrometer settings
−9
6.3.1 Achieve a good high vacuum with a pressure of less than 5 × 10 mbar. Operate the instrument
in accordance with the manufacturer's documented instructions. The instrument shall have fully cooled
following any bakeout. Select the X-ray source. If a twin-anode source is available, it is usually best to
use the Mg anode, rather than the Al anode, since the former gives higher intensity and better energy
resolution. If the instrument is not equipped with a twin-anode source or if the monochromated source
delivers more intense spectra than the twin-anode source, then use the monochromated source. Ensure
that the operation is within the manufacturer's recommended ranges for source power, counting rates,
spectrometer scan rate and any other parameter specified by the manufacturer. Ensure that the entrance
solid angle for the spectrometer is set at a cone semi-angle of less than 6°. Setting too small an entrance
angle will limit the signal quality and the ultimate accuracy of the measurement. Record a survey
(widescan) spectrum to ensure that the only significant peaks are those of Si, O and C. The intensity for
peaks for all other elements shall not exceed 5 % of the intensity of the Si 2p peak for the uncertainty
analysis in 6.7 to be valid. Ensure that there are no significant peaks that are characteristic of the sample
holder. Figure 3 shows an example of a widescan spectrum.
The peaks visible should be the C 1s, O 1s and O Auger peaks and the Si 2p and 2s peaks. For good practice,
the height of the C 1s peak should be less than 30 % of the Si 2s or 2p peak heights although, even for
[5]
significantly larger amounts of carbonaceous contamination, the measurement should not be affected,
except to the extent that the important Si 2p signal intensity will be reduced, leading to an increase in
the measurement uncertainty. Figure 3 shows a typical result for a cleaned, but stored, sample.
NOTE The higher signal level for the unmonochromated Mg X-rays, compared with that for the
monochromated Al X-rays available in many instruments, is important in improving the accuracy of the final
result. This choice leads to a reduction in the term U discussed later.
n
6.3.2 Select slit width and pass energy settings to provide peak widths of around 0,6 eV to 0,9 eV for
the Si 2p and 2p elemental peaks.
3/2 1/2
6.3.3 Set the photoelectron angle of emission to 34° for (100) samples and 25,5° for (111) samples.
It has been found that the accuracy
...