ISO/TR 15969:2021

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

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ISO/TR 15969:2021(E)

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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
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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.
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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.
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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):
m
z = (1)
A⋅ρ
where
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m is the removed sample mass (kg);
2
A is the sputtered area (m );
3
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.
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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
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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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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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[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
...