Surface chemical analysis — Depth profiling — Measurement of sputtered depth

This document provides guidelines for measuring the sputtered depth in sputtered depth profiling. The methods of sputtered depth measurement described in this document are applicable to techniques of surface chemical analysis when used in combination with ion bombardment for the removal of a part of a solid sample to a typical sputtered depth of up to several micrometres. The depth typically determined by this approach is between 1 nm to 500 µm.

Analyse chimique des surfaces — Profilage d'épaisseur — Mesurage de l'épaisseur bombardée

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Publication Date
16-Mar-2021
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6060 - International Standard published
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17-Mar-2021
Completion Date
17-Mar-2021
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TECHNICAL ISO/TR
REPORT 15969
Second edition
2021-03
Surface chemical analysis — Depth
profiling — Measurement of
sputtered depth
Analyse chimique des surfaces — Profilage d'épaisseur — Mesurage
de l'épaisseur bombardée
Reference number
ISO/TR 15969:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 15969:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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 or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 15969:2021(E)
Contents Page

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

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

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

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

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

4 Methods of determination of the sputtered depth ............................................................................................................ 2

4.1 Crater depth measurement after sputter profiling .................................................................................................. 2

4.1.1 General description ....................................................................................................................................................... 2

4.1.2 Mechanical stylus crater depth measurement ....................................................................................... 2

4.1.3 Optical interferometry crater depth measurement ........................................................................... 3

4.2 Comparison with sputter profiled samples having interfaces as depth markers ......................... 5

4.2.1 General description ....................................................................................................................................................... 5

4.2.2 Reference materials ....................................................................................................................................................... 5

4.2.3 Interface depth determination for layered structures by independent

measurements.................................................................................................................................................................... 6

4.3 Typical applications and uncertainties of the different methods ............................................................10

Annex A Survey of typical applications and uncertainties of the different methods....................................11

Bibliography .............................................................................................................................................................................................................................12

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TR 15969:2021(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.

This second edition cancels and replaces the first edition (ISO/TR 15969:2001), which has been

technically revised.
The main changes compared to the previous edition are as follows:
— in the Scope, the applicable range of depth has been specified more clearly;

— Clause 3 has been revised according to the latest edition of the ISO 18115 series;

— in 4.2.2, the information on reference materials has been updated;
— Table A.1 bas been updated.

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 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 15969:2021(E)
Introduction
This document is intended to be used as follows:

a) for the determination of the depth scale in sputter depth profiling where signal intensity is obtained

as a function of sputtering time (or ion dose density). The sputtered depth per sputtering time is

the sputtering rate (typically reported in nm/s);

b) to enhance the comparability of depth profiling data obtained with different instruments and to

increase the reliability and use of depth profiling in industrial applications;

c) to serve as the basis for the development of International Standards on the measurement of

sputtered depth.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 15969:2021(E)
Surface chemical analysis — Depth profiling —
Measurement of sputtered depth
1 Scope

This document provides guidelines for measuring the sputtered depth in sputtered depth profiling.

The methods of sputtered depth measurement described in this document are applicable to techniques

of surface chemical analysis when used in combination with ion bombardment for the removal of a

part of a solid sample to a typical sputtered depth of up to several micrometres. The depth typically

determined by this approach is between 1 nm to 500 µm.
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

ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy

ISO 22493, Microbeam analysis — Scanning electron microscopy — Vocabulary
ISO 15932, Microbeam analysis — Analytical electron microscopy — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 18115-1, ISO 18115-2,

ISO 22493 and ISO 15932 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/
3.1
sputtered depth

distance z (in m) (perpendicular to the surface) between the original surface and the analysed sample

surface after removal of a measurable amount of matter as a result of sputter profiling, which is given

by Formula (1):
z = (1)
A⋅ρ
where
m is the removed sample mass (kg);
A is the sputtered area (m );
ρ is the density of the sample (kg/m )
© ISO 2021 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/TR 15969:2021(E)
4 Methods of determination of the sputtered depth
4.1 Crater depth measurement after sputter profiling
4.1.1 General description

Usually, the result of sputter profiling is a signal intensity as a function of the sputtering time. The

total sputtering time corresponds to the crater depth and the average sputtering rate is obtained by

dividing the crater depth by the sputtering time. Crater depth measurements are usually performed

[3]

by mechanical stylus profilometry or, less commonly in use, by optical interferometry. Optical

instruments and scanned-probe microscopes give a two-dimensional view of the crater and its non-

uniformities.
4.1.2 Mechanical stylus crater depth measurement

Mechanical stylus profilometers convert the deflection of a stylus in mechanical contact with the

surface into a voltage that is amplified and then displayed directly on a strip chart, or digitized and

processed in a computer. In some instruments, the stylus is scanned across the sample containing

the crater, and in others the sample is scanned under the stylus. Profilometers typically produce one-

dimensional line scans, though some modern instruments and scanned probe microscopes can produce

two-dimensional scans by making an automated series of closely spaced one-dimensional scans.

Stylus profilometry is appropriate for measuring the depths of craters in which the roughness of the

original surface and that of the crater bottom are small compared to the crater depth. It is commonly

used for craters made in semiconductors during SIMS depth profiling. The minimum depth that can

be measured successfully depends on the acoustic and electronic noise of the profilometer as well as

the surface roughness. In modern instruments, the minimum depth can be as small as 10 nm, and the

maximum can be as great as 100 μm.

To perform a crater depth measurement with a one-dimensional profilometer, a scan is made through

the centre of the crater and over a sufficient distance of the unsputtered top surface on either side

to establish an accurate baseline, as shown in Figure 1. Multiple scans are made over different traces

through the crater centre to determine the repeatability of the crater depth measurement. The depth

is measured on a computerized profilometer by determining the average height difference between a

region in the centre of the crater at A and two regions of the reference surface on opposite sides at B

and C. Figure 1 shows an example of a computerized profilometer trace of a sputtered crater in single

crystal silicon approximately 0,5 μm in depth. The three pairs of vertical cursor lines indicate the

regions over which the depth is averaged.
2 © ISO 2021 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TR 15969:2021(E)
Key
X length (µm)
Y depth (µm)

Figure 1 — Example of stylus profilometry trace of a 0,5 μm deep crater in silicon

The depth scale of the stylus profilometer is calibrated with standard step-heights or grooves that are

traceable to fundamental length standards (wavelength of light). A typical calibration uncertainty is

1 % for a 1 μm standard gauge. The uncertainty of a crater depth measurement is a combination of

calibration uncertainty and profilometer noise. In a recent round-robin experiment on craters in silicon,

[3]

uncertainties ranged from ±1,3 % for a 2 μm crater to ±4,7 % for a 0,1 μm crater .

NOTE For the purposes of this document, typical uncertainties are given as one-standard-deviation

uncertainties.
Advantages of stylus profilometry for crater depth measurements are that:
— it is rapid;
— requires no sample preparation; and

— it reveals the size, shape, and flatness of the crater bottom which are measures of the ion beam

current density.

A disadvantage is that corrections can be necessary to convert crater depth to sputtered depth in the

case of non-negligible swelling or oxidation. In the case of layered structures with different sputtering

rates, separate craters are necessary for each interface so that the individual sputtering rates can be

determined. Otherwise, only an average sputtering rate is obtained.
4.1.3 Optical interferometry crater depth measurement

Optical interferometry is a simple and convenient non-contact method of crater depth measurement for

which the equipment is relatively cheap to buy and easy to use.

This method utilizes a metallurgical microscope equipped with an interference attachment (Mireau

or Michelson objective, sample tilting stage and monochromatic light source/interference filter) and

© ISO 2021 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/TR 15969:2021(E)

is only applicable to smooth flat samples, for example flat glass, coatings on glass and semiconductor

wafers. Generally, metal samples are too rough for this method to be suitable.

The crater to be measured is placed on the microscope sample stage, which usually can produce a

controlled tilting movement of the sample as well as the usual x-y translation. Using the interference

objective or a normal objective, the crater of interest is located and placed at the centre of the field of

view. This operation can be done with white light illumination. If a normal objective has been used, the

interference objective is then put in place and the sample height adjusted to give white light interference

fringes across the crater. The interference filter is put in place and the sample illuminated with

monochromatic light. Using the tilting adjustment of the sample stage, the sample is tilted to spread

the fringes to a suitable separation and/or to rotate them so that they produce a suitable contour map

of the crater. Take care to ensure that there are no other craters on the sample near to the crater of

interest that cause displacements of the fringes on either side of the crater that are to be used for the

measurement. Produce a hard copy of the image.

Figure 2 shows an example: Using a straight-edged ruler draw two lines (A and B) through the centres

of two adjacent fringes and measure the separation between them. Preferably, one of these lines (A)

crosses the crater. Draw a third line through the centre of a fringe running through the centre of the

crater (C). Count the number of fringes intersected by the line (A) crossing the crater and estimate the

fraction of a fringe spacing between that line and the line through the fringe in the crater (C). In the case

of Figure 2, this fraction is equal to the ratio of separation of lines B and C to that of A and B. Multiply

this result by the half-wavelength of the light used for illumination to determine the crater depth.

This method is generally applicable to crater depths in the range 0,01 μm to 5 μm although, at the

greater depths, surface roughening during profiling can cause problems. The errors associated with

the measurement are:

a) the ability to count the fringes: getting this wrong usually produces an obvious error;

b) the uncertainty in estimating the fraction of a fringe: this should be less than 1/20 of the wavelength

of the light used; and
c) the uncertainty in the wavelength of light used.

NOTE The greatest uncertainty comes from the estimation of the fractional fringe. This is an absolute

amount, not a percentage. Consequently, the percentage uncertainty is greatest for shallow craters and decreases

with increasing depth. A total of 13 measurements by an experienced user on the crater shown in Figure 2 gave a

crater depth of 325 nm and a standard deviation of 9 nm.

The optical image is also useful for showing the uniformity and any defects of the crater. Another

optical method is confocal laser depth determination.
4 © ISO 2021 – All rights reserved
---------------------- Page: 9 ----------------------
ISO/TR 15969:2021(E)
Figure 2 — Example photograph of optical interferometry crater depth measurement
4.2 Comparison with sputter profiled samples having interfaces as depth markers
4.2.1 General description

A known depth of an interface or the depths of several interfaces can be used to determine the sputtered

depth by comparison with the location of the 50 % drop of the plateau value on the sputtering time

scale in the sputter profile. Errors involved are:

a) the initial change of the sputtering rate (generally an initially slower sputtering rate is expected,

caused by primary-ion implantation and the usual surface contamination layer, leading to typical

errors of the order of 1 nm to 2 nm); and

b) a systematic shift of the 50 % plateau intensity (sputter profile interface location) to apparently

[4]

lower depth as compared to the correct interface location . This error is of the order of the signal

escape depth [electron: Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy

(XPS); or ion escape depth: secondary-ion mass spectrometry (SIMS)] or the atomic mixing length,

depending on the larger value. Under typical profiling conditions, the shift is of the order of 1 nm

to 2 nm. Under favourable conditions, a) and b) can compensate and a linear relation between

sputtering time and depth without a zero*point shift is obtained. In multilayer profiling, both

effects are similar at every interface and, therefore, always cancel in a first order approximation.

4.2.2 Reference materials

Any sample with one or several layers of known thickness can be used to determine the time needed to

proceed from one interface to the other during a sputter profiling experiment with preset conditions

for ion beam species, energy, incidence angle and ion formation chamber parameters determining the

© ISO 2021 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO/TR 15969:2021(E)
[5]

ion beam current density . The latter can be given directly if the sputter yield for the sample material

for the respective ion energy and incidence angle is known. For example, the certified reference material

[6][7]

Ta O /Ta (BCR No. 261T) , with certified oxide thickness z(Ta O ) of 30 nm and of 100 nm, yields

2 5 2 5
immediately an “equivalent” thickness for the ana
...

TECHNICAL ISO/TR
REPORT 15969
Second edition
Surface chemical analysis — Depth
profiling — Measurement of
sputtered depth
Analyse chimique des surfaces — Profilage d'épaisseur — Mesurage
de l'épaisseur bombardée
PROOF/ÉPREUVE
Reference number
ISO/TR 15969:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 15969:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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 or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 15969:2021(E)
Contents Page

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

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

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

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

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

4 Methods of determination of the sputtered depth ............................................................................................................ 2

4.1 Crater depth measurement after sputter profiling .................................................................................................. 2

4.1.1 General description ....................................................................................................................................................... 2

4.1.2 Mechanical stylus crater depth measurement ....................................................................................... 2

4.1.3 Optical interferometry crater depth measurement ........................................................................... 3

4.2 Comparison with sputter profiled samples having interfaces as depth markers ......................... 5

4.2.1 General description ....................................................................................................................................................... 5

4.2.2 Reference materials ....................................................................................................................................................... 5

4.2.3 Interface depth determination for layered structures by independent

measurements.................................................................................................................................................................... 6

4.3 Typical applications and uncertainties of the different methods ............................................................10

Annex A Survey of typical applications and uncertainties of the different methods....................................11

Bibliography .............................................................................................................................................................................................................................12

© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/TR 15969:2021(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.

This second edition cancels and replaces the first edition (ISO/TR 15969:2001), which has been

technically revised.
The main changes compared to the previous edition are as follows:
— in the Scope, the applicable range of depth has been specified more clearly;

— Clause 3 has been revised according to the latest edition of the ISO 18115 series;

— in 4.2.2, the information on reference materials has been updated;
— Table A.1 bas been updated.

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 PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 15969:2021(E)
Introduction
This document is intended to be used as follows:

a) for the determination of the depth scale in sputter depth profiling where signal intensity is obtained

as a function of sputtering time (or ion dose density). The sputtered depth per sputtering time is

the sputtering rate (typically reported in nm/s);

b) to enhance the comparability of depth profiling data obtained with different instruments and to

increase the reliability and use of depth profiling in industrial applications;

c) to serve as the basis for the development of International Standards on the measurement of

sputtered depth.
© ISO 2021 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 15969:2021(E)
Surface chemical analysis — Depth profiling —
Measurement of sputtered depth
1 Scope

This document provides guidelines for measuring the sputtered depth in sputtered depth profiling.

The methods of sputtered depth measurement described in this document are applicable to techniques

of surface chemical analysis when used in combination with ion bombardment for the removal of a

part of a solid sample to a typical sputtered depth of up to several micrometres. The depth typically

determined by this approach is between 1 nm to 500 µm.
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.

[1] [2]
See also and

ISO 18115-1, Surface chemical analysis — Vocabulary — Part 1: General terms and terms used in

spectroscopy

ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy

ISO 22493, Microbeam analysis — Scanning electron microscopy — Vocabulary
ISO 15932, Microbeam analysis — Analytical electron microscopy — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 18115-1, ISO 18115-2,

ISO 22493 and ISO 15932 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/
3.1
sputtered depth

distance z (in m) (perpendicular to the surface) between the original surface and the analysed sample

surface after removal of a measurable amount of matter as a result of sputter profiling, which is given

by Formula (1):
z = (1)
A⋅ρ
where
© ISO 2021 – All rights reserved PROOF/ÉPREUVE 1
---------------------- Page: 6 ----------------------
ISO/TR 15969:2021(E)
m is the removed sample mass (kg);
A is the sputtered area (m );
r is the density of the sample (kg/m )
4 Methods of determination of the sputtered depth
4.1 Crater depth measurement after sputter profiling
4.1.1 General description

Usually, the result of sputter profiling is a signal intensity as a function of the sputtering time. The

total sputtering time corresponds to the crater depth and the average sputtering rate is obtained by

dividing the crater depth by the sputtering time. Crater depth measurements are usually performed

[3]

by mechanical stylus profilometry or, less commonly in use, by optical interferometry. Optical

instruments and scanned-probe microscopes give a two-dimensional view of the crater and its non-

uniformities.
4.1.2 Mechanical stylus crater depth measurement

Mechanical stylus profilometers convert the deflection of a stylus in mechanical contact with the

surface into a voltage that is amplified and then displayed directly on a strip chart, or digitized and

processed in a computer. In some instruments, the stylus is scanned across the sample containing

the crater, and in others the sample is scanned under the stylus. Profilometers typically produce one-

dimensional line scans, though some modern instruments and scanned probe microscopes can produce

two-dimensional scans by making an automated series of closely spaced one-dimensional scans.

Stylus profilometry is appropriate for measuring the depths of craters in which the roughness of the

original surface and that of the crater bottom are small compared to the crater depth. It is commonly

used for craters made in semiconductors during SIMS depth profiling. The minimum depth that can

be measured successfully depends on the acoustic and electronic noise of the profilometer as well as

the surface roughness. In modern instruments, the minimum depth can be as small as 10 nm, and the

maximum can be as great as 100 μm.

To perform a crater depth measurement with a one-dimensional profilometer, a scan is made through

the centre of the crater and over a sufficient distance of the unsputtered top surface on either side

to establish an accurate baseline, as shown in Figure 1. Multiple scans are made over different traces

through the crater centre to determine the repeatability of the crater depth measurement. The depth

is measured on a computerized profilometer by determining the average height difference between a

region in the centre of the crater at A and two regions of the reference surface on opposite sides at B

and C. Figure 1 shows an example of a computerized profilometer trace of a sputtered crater in single

crystal silicon approximately 0,5 μm in depth. The three pairs of vertical cursor lines indicate the

regions over which the depth is averaged.
2 PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TR 15969:2021(E)
Key
X length (µm)
Y depth (µm)

Figure 1 — Example of stylus profilometry trace of a 0,5 μm deep crater in silicon

The depth scale of the stylus profilometer is calibrated with standard step-heights or grooves that are

traceable to fundamental length standards (wavelength of light). A typical calibration uncertainty is

1 % for a 1 μm standard gauge. The uncertainty of a crater depth measurement is a combination of

calibration uncertainty and profilometer noise. In a recent round-robin experiment on craters in silicon,

[3]

uncertainties ranged from ±1,3 % for a 2 μm crater to ±4,7 % for a 0,1 μm crater .

NOTE For the purposes of this document, typical uncertainties are given as one-standard-deviation

uncertainties.
Advantages of stylus profilometry for crater depth measurements are that:
— it is rapid;
— requires no sample preparation; and

— it reveals the size, shape, and flatness of the crater bottom which are measures of the ion beam

current density.

A disadvantage is that corrections can be necessary to convert crater depth to sputtered depth in the

case of non-negligible swelling or oxidation. In the case of layered structures with different sputtering

rates, separate craters are necessary for each interface so that the individual sputtering rates can be

determined. Otherwise, only an average sputtering rate is obtained.
4.1.3 Optical interferometry crater depth measurement

Optical interferometry is a simple and convenient non-contact method of crater depth measurement for

which the equipment is relatively cheap to buy and easy to use.

This method utilizes a metallurgical microscope equipped with an interference attachment (Mireau

or Michelson objective, sample tilting stage and monochromatic light source/interference filter) and

© ISO 2021 – All rights reserved PROOF/ÉPREUVE 3
---------------------- Page: 8 ----------------------
ISO/TR 15969:2021(E)

is only applicable to smooth flat samples, for example flat glass, coatings on glass and semiconductor

wafers. Generally, metal samples are too rough for this method to be suitable.

The crater to be measured is placed on the microscope sample stage, which usually can produce a

controlled tilting movement of the sample as well as the usual x-y translation. Using the interference

objective or a normal objective, the crater of interest is located and placed at the centre of the field of

view. This operation can be done with white light illumination. If a normal objective has been used, the

interference objective is then put in place and the sample height adjusted to give white light interference

fringes across the crater. The interference filter is put in place and the sample illuminated with

monochromatic light. Using the tilting adjustment of the sample stage, the sample is tilted to spread

the fringes to a suitable separation and/or to rotate them so that they produce a suitable contour map

of the crater. Take care to ensure that there are no other craters on the sample near to the crater of

interest that cause displacements of the fringes on either side of the crater that are to be used for the

measurement. Produce a hard copy of the image.

Figure 2 shows an example: Using a straight-edged ruler draw two lines (A and B) through the centres

of two adjacent fringes and measure the separation between them. Preferably, one of these lines (A)

crosses the crater. Draw a third line through the centre of a fringe running through the centre of the

crater (C). Count the number of fringes intersected by the line (A) crossing the crater and estimate the

fraction of a fringe spacing between that line and the line through the fringe in the crater (C). In the case

of Figure 2, this fraction is equal to the ratio of separation of lines B and C to that of A and B. Multiply

this result by the half-wavelength of the light used for illumination to determine the crater depth.

This method is generally applicable to crater depths in the range 0,01 μm to 5 μm although, at the

greater depths, surface roughening during profiling can cause problems. The errors associated with

the measurement are:

a) the ability to count the fringes: getting this wrong usually produces an obvious error;

b) the uncertainty in estimating the fraction of a fringe: this should be less than 1/20 of the wavelength

of the light used; and
c) the uncertainty in the wavelength of light used.

NOTE The greatest uncertainty comes from the estimation of the fractional fringe. This is an absolute

amount, not a percentage. Consequently, the percentage uncertainty is greatest for shallow craters and decreases

with increasing depth. A total of 13 measurements by an experienced user on the crater shown in Figure 2 gave a

crater depth of 325 nm and a standard deviation of 9 nm.

The optical image is also useful for showing the uniformity and any defects of the crater. Another

optical method is confocal laser depth determination.
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ISO/TR 15969:2021(E)
Figure 2 — Example photograph of optical interferometry crater depth measurement
4.2 Comparison with sputter profiled samples having interfaces as depth markers
4.2.1 General description

A known depth of an interface or the depths of several interfaces can be used to determine the sputtered

depth by comparison with the location of the 50 % drop of the plateau value on the sputtering time

scale in the sputter profile. Errors involved are:

a) the initial change of the sputtering rate (generally an initially slower sputtering rate is expected,

caused by primary-ion implantation and the usual surface contamination layer, leading to typical

errors of the order of 1 nm to 2 nm); and

b) a systematic shift of the 50 % plateau intensity (sputter profile interface location) to apparently

[4]

lower depth as compared to the correct interface location . This error is of the order of the signal

escape depth [electron: Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy

(XPS); or ion escape depth: secondary-ion mass spectrometry (SIMS)] or the atomic mixing length,

depending on the larger value. Under typical profiling conditions, the shift is of the order of 1 nm

to 2 nm. Under favourable conditions, a) and b) can compensate and a linear relation between

sputtering time and depth without a zero*point shift is obtained. In multilayer profiling, both

effects are similar at every interface and, therefore, always cancel in a first order approximation.

4.2.2 Reference materials

Any sample with one or several layers of known thickness can be used to determine the time needed to

proceed from one interface to the other during a sputter profiling experiment with preset conditions

for ion beam species, energy, incidence angle and ion formation chamber parameters determining the

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ISO/TR 15969:2021(E)
[5]

ion beam current density . The latter can be given directly if the sputter yield for the sample material

for the respective ion energy and incidence angle is known. For example, the certified reference material

[6][7]

Ta O /Ta (BCR No. 261T) , with certified oxide thickness z(Ta O ) of 30 nm and of 100 nm, yields

2 5 2 5
immediately an “equivalent” thickness
...

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