Surface chemical analysis — Depth profiling — Measurement of sputtered depth

This Technical Report gives guidelines for measuring the sputtered depth in sputtered depth profiling. The methods of sputtered depth measurement described in this Technical Report 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 severalmicrometres.

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

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Status
Withdrawn
Publication Date
30-May-2001
Withdrawal Date
30-May-2001
Current Stage
9599 - Withdrawal of International Standard
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17-Mar-2021
Completion Date
17-Mar-2021
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TECHNICAL ISO/TR
REPORT 15969
First edition
2001-06-01
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 15969:2001(E)
ISO 2001
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ISO/TR 15969:2001(E)
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© ISO 2001

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ii © ISO 2001 – All rights reserved
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ISO/TR 15969:2001(E)
Contents Page

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

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

2 Terms and definitions ...................................................................................................................................1

3 Abbreviated terms .........................................................................................................................................2

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

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

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

4.3 Typical applications and uncertainties of the different methods.............................................................9

Annex A Survey of typical applications and uncertainties of the different methods.....................................10

Bibliography..............................................................................................................................................................11

© ISO 2001 – All rights reserved iii
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ISO/TR 15969:2001(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted

by the technical committees are circulated to the member bodies for voting. Publication as an International

Standard requires approval by at least 75 % of the member bodies casting a vote.

In exceptional circumstances, when a technical committee has collected data of a different kind from that which is

normally published as an International Standard ("state of the art", for example), it may decide by a simple majority

vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature

and does not have to be reviewed until the data it provides are considered to be no longer valid or useful.

Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent

rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO/TR 15969 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee

SC 4, Depth profiling.
iv © ISO 2001 – All rights reserved
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ISO/TR 15969:2001(E)
Introduction
This Technical Report 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 2001 – All rights reserved v
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TECHNICAL REPORT ISO/TR 15969:2001(E)
Surface chemical analysis — Depth profiling — Measurement of
sputtered depth
1 Scope

This Technical Report gives guidelines for measuring the sputtered depth in sputtered depth profiling. The methods

of sputtered depth measurement described in this Technical Report 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

typicalsputtereddepthof uptoseveralmicrometres.
2 Terms and definitions

For the purposes of this Technical Report, the following terms and definitions apply.

[1]

NOTE The terms used in this Technical Report follow basically the definitions in ASTM E 673-95c . These definitions are

to be modified to conform to those being developed by ISO/TC 201/SC 1 on Terminology. See also [2] and [3].

2.1
sputtered depth

distance z (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

z � (1)
A��
where
m is the removed sample mass (kg);
A is thesputteredarea(m );
� is the density of the sample (kg/m )
2.2
crater depth

average distance (perpendicular to the surface) between the original surface and the region of a crater bottom from

which the measured signal is derived

NOTE The crater depth is equal to the sputtered depth if primary-ion implantation and retention, which may cause

[5]

enlargement (“swelling”) of the sample in the direction perpendicular to the surface, is negligible . If the sputtered depth is

measured by crater depth measurement outside the analysis chamber, surface reactions (e.g. oxidation) may add to the

swelling of the crater bottom, i.e. the crater depth is generally measured as being less than the sputtered depth.

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ISO/TR 15969:2001(E)
3 Abbreviated terms
AES Auger electron spectroscopy
AFM Atomic force microscopy
EDS Energy dispersive spectrometry
EPMA Electron probe microanalysis
FIB Focused ion beam
GIXR Grazing incidence X-ray reflectivity
MEIS Medium energy ion scattering
RBS Rutherford backscattering spectrometry
SAM Scanning Auger microscopy
SEM Scanning electron microscopy
SIMS Secondary-ion mass spectrometry
TEM Transmission electron microscopy
XPS X-ray photoelectron spectroscopy
XRF X-ray fluorescence
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

[6]

sputtering time. Crater depth measurements are usually performed 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 are capable of producing 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 may be as small as 10 nm, and the maximum may be as great as 100 �m.

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

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ISO/TR 15969:2001(E)

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 �m indepth. The threepairs of

vertical cursor lines indicate the regions over which the depth is averaged.
Figure 1 — Exampleof stylusprofilometrytraceof a0,5����m 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 �m 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, uncertainties ranged from � 1,3 % for a 2 �mcrater

[6]
to � 4,7 % for a 0,1 �mcrater .

NOTE For the purposes of this Technical Report, 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 reveals the size, shape, and flatness of the crater bottom which are measures of the ion beam

current density. A disadvantage is that corrections may 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 must be made 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 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 is capable of producing 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

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ISO/TR 15969:2001(E)

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) should cross 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 will be 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 0,2 �mto 5 �m 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.
Figure 2 — Example photogragh of optical interferometry crater depth measurement
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ISO/TR 15969:2001(E)
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)

[5]

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

escape depth [electron (AES, XPS) or ion escape depth (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 favorable conditions,

(a) and (b) may 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,

[7]

energy, incidence angle and ion formation chamber parameters determining the 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

[8, 9]

angle is known. For example, the certified reference material Ta O /Ta (BCR No. 261R) , with certified oxide

2 5

thickness z(Ta O ) of 30 nm and of 100 nm, yields immediately an “equivalent” thickness for the analysed layer if

2 5

the sputtering time for the 50 % drop of the plateau intensity is used as a marker. By knowledge of the sputtering

� � �

rate z =dz/dt of the analysed material M, z (M), relative to that of Ta O , z (Ta O ), for the same ion beam

2 5 2 5

conditions, the analysed layer thickness z is determined by the following relation:

tz�MM�
� �
zz��Ta O � (2)
tzTa O � Ta O
����
25 2 5

where t(M) and t(Ta O ) are the sputtering times for the analysed layer and for the reference layer, respectively.

2 5

Owing to the build-up of the steady-state altered layer at the beginning of sputter

...

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