Surface chemical analysis — Depth profiling — Non-destructive depth profiling of nanoscale heavy metal oxide thin films on Si substrates with medium energy ion scattering

This document specifies a method for the quantitative depth profiling of amorphous heavy metal oxide ultrathin films on Si substrates using medium energy ion scattering (MEIS).

Analyse chimique des surfaces — Profilage d'épaisseur — Profilage d'épaisseur non destructif de films minces d'oxydes de métaux lourds à l'échelle nanométrique sur des substrats de Si par diffusion d'ions de moyenne énergie

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Status
Published
Publication Date
14-Jun-2022
Current Stage
6060 - International Standard published
Due Date
29-Jun-2022
Completion Date
15-Jun-2022
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INTERNATIONAL ISO
STANDARD 23170
First edition
2022-06
Surface chemical analysis — Depth
profiling — Non-destructive depth
profiling of nanoscale heavy metal
oxide thin films on Si substrates with
medium energy ion scattering
Analyse chimique des surfaces — Profilage d'épaisseur — Profilage
d'épaisseur non destructif de films minces d'oxydes de métaux lourds
à l'échelle nanométrique sur des substrats de Si par diffusion d'ions de
moyenne énergie
Reference number
ISO 23170:2022(E)
© ISO 2022
---------------------- Page: 1 ----------------------
ISO 23170:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022

All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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

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Published in Switzerland
© ISO 2022 – All rights reserved
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ISO 23170:2022(E)
Contents Page

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

Introduction .................................................................................................................................................................................................................................v

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

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

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

4 Principle and recommendations of MEIS analysis ........................................................................................................... 1

5 MEIS analysis ...........................................................................................................................................................................................................2

6 MEIS spectra simulation ..............................................................................................................................................................................2

7 Reporting MEIS analysis results .........................................................................................................................................................5

Annex A (informative) Interlaboratory test report ............................................................................................................................. 6

Annex B (informative) List of MEIS spectra simulation program sources and a procedure of

MEIS spectra simulation using PowerMeis ...........................................................................................................................19

Annex C (informative) Reliability of the IAEA electronic stopping power data .................................................22

Annex D (informative) Fitting parameters A, B, C, D from the IAEA database ....................................................24

Bibliography .............................................................................................................................................................................................................................29

iii
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ISO 23170:2022(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 201, Surface chemical analysis,

Subcommittee SC 4, Depth profiling.

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 23170:2022(E)
Introduction

Medium energy ion scattering (MEIS) has been considered to be a quantitative surface and interface

composition analysis method with single atomic depth resolution since its invention in the early of

1980s. MEIS has been widely used for ultrathin films, especially nm gate oxides analysis to determine

its composition, thickness, and the interface. Recently, MEIS has been used for nanoparticle analysis

to determine the size and the composition with the core and shell structure information. In addition

to the toroidal electrostatic energy analyser used in the early stage, different types of energy analyser

such as magnetic energy analyser and time-of-flight (TOF) energy analyser have been used. With the

continued scaling down of electronic devices, demands on accurate and reliable depth profiling have

reached beyond the limit of sputter depth profiling which provides deteriorated depth profiles due

to the sputter damage. Needs have been risen to investigate the consistency between the three types

of energy analyser, ion species, and the different energy range of incident ions used for MEIS analysis

and to set up a procedure for quantitative MEIS analysis. Two international interlaboratory tests were

performed to develop this document which is reported in Annex A.
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INTERNATIONAL STANDARD ISO 23170:2022(E)
Surface chemical analysis — Depth profiling — Non-
destructive depth profiling of nanoscale heavy metal
oxide thin films on Si substrates with medium energy ion
scattering
1 Scope

This document specifies a method for the quantitative depth profiling of amorphous heavy metal oxide

ultrathin films on Si substrates using medium energy ion scattering (MEIS).
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, Surface chemical analysis — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 18115 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
electronic stopping power

retarding force acting on charged particles, typically alpha and proton particles, due to interaction with

electrons, resulting in loss of particle energy
4 Principle and recommendations of MEIS analysis

4.1 Ultrathin films of thickness less than 10 nm can be analysed with MEIS. (100 to 500) keV H or

He ions are used for MEIS analysis. Scattered ion energy and angle are measured precisely so that

measured MEIS spectra be compared with simulated MEIS spectra. MEIS spectra can be simulated with

1 2

various programs from free codes such as PowerMeis and SIMNRA in public websites, MEIS expert

laboratories, and MEIS manufacturers. Simulation programs calculate scattering cross-sections and

electronic stopping powers. Quite often, calculated electronic stopping powers are subject to significant

errors so that tabulated electronic stopping power values in the IAEA website are recommended to

use.

4.2 If not tabulated, it is recommended to measure electronic stopping power by users for more

reliable results. Various types of energy analyzers can be used such as toroidal electrostatic analyser

(TEA), magnetic energy analyser (magnetic), and TOF energy analyser. With MEIS analysis procedures

specified in this document, less than 10 % uncertainty can be expected for ultrathin films under the

guidelines describe in this document. This document is written for amorphous or polycrystalline thin

films but not for crystalline thin films. To reduce the uncertainty of MEIS analysis, additional standards

© ISO 2022 – All rights reserved
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ISO 23170:2022(E)

for calibration of scattering geometry, ion energy, energy resolution, detector efficiency, sample

alignment, are required.
5 MEIS analysis

5.1 Set the ion scattering conditions such as ion energy, scattering angle, incidence angle from the

surface normal, ion species, and ion dose for MEIS analysis. The ion dose is recommended to be in the

15 2

static condition (<10 /cm ) but the static requirement for MEIS analysis is not strict compared to

surface analysis such as XPS and static SIMS.

5.2 For MEIS analysis, specimen should be flat. Slight contamination due to air oxidation and ambient

water and hydrocarbon adsorption does not disturb the MEIS analysis badly. However, if the surface

contamination layer is thicker than 1 nm it shall be cleaned by appropriate methods such as solvent

washing or ion milling.

5.3 Measure a MEIS spectrum of a specimen and generate a MEIS spectrum file with intensity

(counts) vs energy (keV) at a specific angle. Specify all the ion scattering conditions such as ion beam

energy, ion species, incidence angle from the surface normal, scattering angle, ion dose or ion current

with analysis time, beam radius, and the type of energy analyser.

5.4 For MEIS analysis of ultrathin films of approximately 1 nm thickness, a clear plateau does not

appear so that it may have poor reproducibility. It is recommended to express MEIS analysis result in

surface areal density rather than in thickness or concentration for MEIS analysis of ultrathin films of

approximately 1 nm thickness.

5.5 If the primary ion beam current is too high, it can cause multi-hit problems of detector. Under

each MEIS analysis condition, it shall be checked whether the data is affected by the multi-hit problem

by comparing MEIS results from high ion beam current and low ion beam current available from each

MEIS system.
6 MEIS spectra simulation

6.1 MEIS spectra can be simulated with various programs from free codes such as PowerMeis and

SIMNRA in public websites and simulation programs from MEIS laboratories and manufacturers as

listed in Annex B. Detailed procedures for PowerMeis is also given in Annex B as an example. In this

document, general procedures for MEIS spectra simulation are given as a guidance.

Most of MEIS spectra simulation programs are based on Monte Carlo simulation or analytical

calculation of binary scatterings and electronic stopping between binary scatterings. Multiple

scattering is suggested to be included in all MEIS simulations. Thin films thicker than 5 nm are strongly

recommended to be analysed including multiple scattering for the MEIS data obtained by He ions of ion

energy lower than 500 keV. SIMNRA provides multiple scattering simulation so that users can choose

whether including it or not.

Generally, the integration step, and the slab thickness or atom density in MEIS simulations are 0,1 nm,

15 2
and 0,1 nm or 1 x10 atoms/cm , respectively.

In simulation, line shape, cross-section, electronic stopping power, charge neutralization, and energy

straggling shall be selected by users as described below. Electronic stopping power is described in

detail in 6.2
1) http:// tars .if .ufrgs .br
2) https:// home .mpcdf .mpg .de/ ~mam/
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ISO 23170:2022(E)

Generally, simple Gaussian can be used as basic line shape for the first approximation. For MEIS systems

with high resolution of δE/E approximately 8x10 with proton as a projectile, exponentially modified

Gaussian is recommended for ultrathin films. However, other options can be chosen, if needed. Line

shape parameters for each element, σ (additional parameter), is calculated by Casp version 5.2

program for each projectile, projectile energy, and target element. Follow the instructions in Casp

version 5.2.

To calculate cross-sections, interatomic potentials can be chosen from Anderson, L’Ecuyer, Moliere

[1]

potential, or ZBL potential . For MEIS analysis, Moliere potential is widely used. For energy straggling,

[2]

Chu or Yang can be usually chosen. For charge neutralization, Marion and Young’s equation is used for

+ +

30 keV to 150 keV H and for 30 keV to 200 keV He and Armstrong’s equation for 400 keV to 500 keV

+ 3

He . Casp charge-state-fractions algorithm can be used for He with better reliability and for other

heavier ions, if available. PowerMeis provides Marion and Young’s equation and Casp neutralization.

SIMNRA does not include neutralization in simulation.

6.2 Electronic stopping power is automatically calculated by the SRIM 95 or more recent SRIM2013

code, which can be used for non-critical MEIS analysis with inconsistency higher than 10 %. However,

electronic stopping power from the SRIM code can have significant errors in the medium energy range.

For accurate MEIS analysis with consistency lower than 10 %, it is recommended to use the measured

and tabulated electronic stopping power data of elements and compounds from IAEA , as described

in Formula (1). If the IAEA database is insufficient or missing, newly measured electronic stopping

power data by appropriate methods can be added to the IAEA database to improve the accuracy of the

calculated electronic stopping power. The accuracy of electronic stopping power values in the IAEA

database is estimated to be less than 8 % as discussed in Annex C.

The electronic stopping power from IAEA database can be obtained by fitting a set of electronic

stopping power data in which can be used in Formula (1). The equation that can be used to fit the IAEA

database are given in Formula (1). The E is the ion energy, Z is the atomic number of the ion, m is the

2 15

mass of the ion. The unit of S , S , and S is eV·cm /10 atoms, that of E is keV, and that of m is u. For

t l h

fitting to determine A, B, C, and D, Origin or other appropriate fitting programs can be used.

11 1
D D D
SE() SE() SE()
t l h
SE()=A E (1)
zmB CE
 
SE =+ln 1
 
E  m 
where
S (E) is the total electronic stopping power;
S (E) is the electronic stopping power of low energy electrons;
S (E) is the electronic stopping power of high energy electrons;
A, B, C, D are the fitting parameters;
E is the ion energy;
z is the atomic number of the ion;
m is the mass of the ion.
3) Available from: http:// www/ casp -program .org/
4) Available from: https:// www -nds .iaea .org/ stopping/
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ISO 23170:2022(E)

From the IAEA data , available fitting parameters A, B, C, and D are calculated and tabulated in Annex D

for the convenience of the users of this document.

For straggling correction factor, just use 1,0, if the correction factor is not available, which is generally

the case.

6.3 In output, energy channel width and angular channel width can be adjusted according to the

energy and angular resolution for each MEIS analysis. Energy channel width is generally used as 0,1 keV

and angular channel width should be same to the detector angular width when obtaining 1D spectrum.

In beam, atomic number, atomic mass, beam energy, and incidence angle from the surface normal are

given. Projectile number can be adjusted to fit the experimentally measured MEIS spectrum of well-

defined internal reference.

Analyser type is chosen among electrostatic, TOF, and magnetic. For electrostatic and magnetic analyser

type, energy resolution full width half maximum (FWHM) (δE), scattering angle, and angular width

(Δθ) specified from each MEIS measurements are given. For TOF analyser type, TOF length (L), time

resolution (δt), are additionally given according to specific TOF MEIS scattering conditions. Out of plane

angle and angular width are used when MEIS scattering data are obtained from out of plane region or

large detector. Energy resolution FWHM (δE) or time resolution (δt) can be measured by fitting the

leading edge of a MEIS peak from a clean surface peak of heavy metal specimen.

6.4 After all the input parameters for simulation and experimental parameters are given, the structure

of a specimen is to be built. Firstly, list up elements used in simulation. Then, construct layered thin film

structures with the thickness and composition for each layer. Generally, a substrate layer is included

in simulations. If necessary, a surface contamination layer or an interface layer between ultrathin film

layers can be introduced to improve the fitting.

6.5 After all the specified and appropriate values are given, start a simulation. If all the input

parameters are appropriate, then simulated spectra are given. Adjust the parameter of the projectile

number in beam to fit the overall intensity or the intensity of a reference layer for example between

measured and simulated MEIS spectra when an internal reference layer is used. Then adjust the

thickness and composition of each layer manually and iteratively to get the best agreement between

measured and simulated MEIS spectra. A chi-square test can be used to find the best agreement with

a properly chosen energy range of spectrum. The low limit of an energy range in χ calculation is

recommended to be not lower than 10 keV from the substrate surface peak. In case that a reference

layer of known stoichiometry is included in the sample, the height of reference layer can be chosen to

generate minimum χ calculation to determine the projectile number and then the composition and the

thickness of unknown layers can be determined by changing fitting energy range to the unknown layer

2 2

peak position to calculate and minimize χ . In case of no reference layer, overall χ calculation with an

appropriate fitting range can be used to get the best agreement between measured and simulated MEIS

spectra.
15 2

6.6 MEIS simulation programs generate composition and surface areal density (10 atoms/cm ) for

each layer. Therefore, to convert the areal density to the thickness of each layer, the density of each

layer is required. If the density of thin films used in the MEIS analysis is known, use the number. But,

generally the number is not known so that the bulk density can be used. In the case of non-stoichiometric

compounds or mixtures, the sum rule can be used. According to the sum rule, the density of a non-

stoichiometric compound ρ(A B ) can be estimated by the Formula (2).
x+δx y+δy
ρ(A B ) = [(x+δx)m(A) +(y+δy)m(B)[/{[x m(A) + y m(B)]/ρ(A B)} (2)
x+δx y+δy x y
where

ρ(A B ) is the density of a non-stoichiometric compound with A constituent of x+δx composition

x+δx y+δy
and B constituent of y+δy composition;
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ISO 23170:2022(E)
m(A) is the mass of A constituent;
m(B) is the mass of B constituent;

ρ(A B ) is the density of a stoichiometric compound with A constituent of x composition and

x y
B constituent of y composition.

For mixtures of A and B, the density of the mixture A(x%) and B(y%) can be estimated by the

Formula (3).
ρ(x%A+y%B) = [x m(A) + y m(B)]/[x m(A)/ρ(A) + y m(B)/ρ(B)] (3)
In Formulae (2) and (3), the m is the mass of each element or a compound in u.

By dividing the simulated areal atomic density of each layer with the estimated density of each layer,

the thickness of each layer can be obtained. The use of bulk density values or the density estimated by

the sum rule can generate errors in the depth from MEIS analysis.
7 Reporting MEIS analysis results

7.1 Report all the MEIS analysis conditions such as ion beam energy, ion species, ion current,

incidence angle from the surface normal, scattering angle, ion dose, analysis time, beam radius, sample

descriptions including surface contamination and surface flatness, sample preparation, MEIS analysis

chamber pressure, energy analyser type and specifications, detector type and specifications.

7.2 Details of MEIS spectra simulations shall be specified including simulation program name and all

of simulation parameters.

7.3 It is recommended to report MEIS analysis results in the quantity with unit of surface areal

15 2

density (10 atoms/cm ), or concentration (atomic fraction) as a function of depth in the unit of areal

15 2

density (10 atoms/cm ). Using the bulk density, it can be reported as concentration (atomic fraction)

as a function of depth in the unit of thickness (nm), but it shall be clearly stated that the bulk densities

are assumed for ultrathin films.
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ISO 23170:2022(E)
Annex A
(informative)
Interlaboratory test report
A.1 Overview

This annex gives a report of an interlaboratory test on non-destructive depth profiling of nanoscale

thin films with medium energy ion scattering.
A.2 Principle

MEIS spectra of HfO thin films with nominal thickness of 1 nm, 3 nm, 5 nm, and 7 nm on a substrate

12 nm SiO on a Si substrate were measured precisely by 12 participants. Three different types of MEIS

detector were used such as electrostatic, magnetic, and TOF. Measured MEIS spectra were simulated

by each participant laboratories, resulting in the poor consistency of approximately 15 %. They were

simulated again by one key laboratory, K-MAC, Korea and sources of poor consistency such as electronic

stopping power and neutralization correction were investigated, which improved the consistency of

MEIS analysis results less than 8 %.
A.3 Sample description

Samples were prepared at National Nano Fab Center in Daejeon, Korea. Nominally, 1, 3, 5, and 7 nm HfO

were deposited by atomic layer deposition method on a 12 nm thermal SiO layer on a 6 inch Si(001)

wafer. A 12 nm thermal SiO layer that assists the uniform and flat growth of HfO layer is used as an

2 2

internal reference in the MEIS analysis. The uniformity of the thickness over a 6 inch wafer was tested

by measuring 7 points of the wafer 5 times with ellipsometer. As a result, the standard deviations of

thickness were estimated to approximately 0,5 %. The thickness of HfO /SiO /Si samples was also

2 2

measured by transmission electron microscopy (TEM). TEM images of HfO /SiO /Si thin films are

2 2

shown in Figure A.1 and the average thickness of samples are summarized in Table A.1.

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ISO 23170:2022(E)

a) 1 nm HfO on a 12 nm thermal SiO /Si sub- b) 3 nm HfO on a 12 nm thermal SiO /Si sub-

2 2 2 2
strate strate

c) 5 nm HfO on a 12 nm thermal SiO /Si sub- d) 7 nm HfO on a 12 nm thermal SiO /Si sub-

2 2 2 2
strate strate
NOTE The scale bar at the bottom left of the image is 2 nm.
Figure A.1 — TEM images of HfO on a 12 nm thermal SiO /Si substrate
2 2
Table A.1 — Thickness of 1 nm, 3 nm, 5 nm and 7 nm HfO measured by TEM
Thickness (nm)
Samples
Average Standard deviation
1 nm HfO 1,72 0,12
3 nm HfO 3,29 0,10
5 nm HfO 4,86 0,18
7 nm HfO 6,39 0,06
A.4 Measurement procedure
A.4.1 Measurement

For MEIS analysis, the energy and the angle of scattered ions are measured to analyse depth profiles

of ultrathin films. A MEIS instrument consists of an ion source, an accelerator, an energy analyser,

and a detector. A schematic diagram is shown in Figure A.2. Ions generated from an ion source are

accelerated to impinge on a sample surface. The energy of projectile ions scattered by target atom

nuclei is measured by an energy analyser. The scattering angle is determined by the geometry of an ion

source, a sample, and a detector. Electrostatic analyser, and magnetic, TOF are widely used as energy

analysers.
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ISO 23170:2022(E)
Key
+ +
1 ion source (H or He )
2 scattered particles
3 particles to detector
4 accelerator
5 analyser (electrostatic, magnetic or TOF)
6 sample
α incident angle
θ scattering angle
Figure A.2 — Schematic diagram of a MEIS system

Twelve (12) participants in this RRT are UFRGS (Brazil), IBM Watson (US), Western Ontario University

(Canada), Kyoto University (Japan), Global Foundry (US), Samsung Advanced Institute of Technology

(R. O. Korea), Samsung Electronics (R. O. Korea), SK hynix (R. O. Korea), K-MAC, (R. O. Korea), DGIST (R.

O. Korea), KIST (R. O. Korea), and Huddersfield (UK). PowerMeis simulations were performed by W. Min

at K-MAC. The experimental conditions of 12 participants are summarized in Table A.2.

Table A.2 — MEIS analysis conditions for each participant
Incident energy incident angle scattering angle
Participants Incident ion analyser
o o
(keV) ( ) ( )
A H 100 0 120 Electrostatic
B H 100 54,7 110 Electrostatic
C H 94 45 135 Electrostatic
D He 400 31,5 74,1 Magnetic
E He 400 44,5 83,6 Magnetic
F He 500 44,2 78,2 Magnetic
G He 450 54,6 70,5 Magnetic
H He 400 45 68 Magnetic
I He 100 45 130 TOF
J He 80 45 90 TOF
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ISO 23170:2022(E)
Table A.2 (continued)
Incident energy incident angle scattering angle
Participants Incident ion analyser
o o
(keV) ( ) ( )
K He 100 2 120 Electrostatic
L He 100 54,7 70,5 Electrostatic
A.4.2 MEIS Spectrum simulation
[3]

The PowerMeis program was used as a simulation tool for analysis . It was developed mainly by

Mauricio A. Sortica and Gabriel M. Marmitt at the Pedro Grande group of the Universidade Federal

do Rio Grande do Sul (UFRGS), Brazil. PowerMeis uses the Monte-Carlo method to simulate a MEIS

spectrum by summing individual scattering spectrum from randomly selected scattering points of a

sample. The scattered ion energy is calculated by the kinematic factor for each scattering event and

by the electronic stopping energy loss according to the ion beam path in the medium. The peak shape

of each event is calculated by the straggling according to the beam path for each location and a system

resolution.

In this report, the scattering cross-section was calculated by Moliere potential. For the electronic

stopping power, both SRIM95 electronic stopping power and the new refitted electronic stopping power

were used for comparison, and the Chu model was used for the straggling. Neutralization correction

should be applied to the data from electrostatic analyser and magnetic sector. Neutralization of Marion

+ +[2] +[4]

is applied to 100 keV H and He and that of Armstrong is applied to 400 keV to 500 keV He . Casp

[5]

program can be used for both cases with better reliability . The density of HfO used in the simulations

3 3
was 9,68 g/cm , and that of SiO was 2,2 g/cm for the thickness calculation.

The stoichiometry of a thermal SiO layer, (Si:O = 1:2) was used as a reference layer. The number of

incident particles was determined by the Si height of the thermal SiO layer below the HfO layer in

2 2

the sample. When determining the number of incident particles by fitting the Si peak height of the SiO

layer, background signals between Hf and Si peaks should be subtracted. Background signal is caused

by the multiple scattering or the roughness. Given th
...

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