ISO 11952:2019

INTERNATIONAL ISO
STANDARD 11952
Second edition
2019-05
Surface chemical analysis — Scanning-
probe microscopy — Determination
of geometric quantities using SPM:
Calibration of measuring systems
Analyse chimique des surfaces — Microscopie à sonde à balayage
— Détermination des quantités géométriques en utilisant des
microscopes à sonde à balayage: Étalonnage des systèmes de mesure
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .v
Introduction .ix
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Characteristics of SPMs . 4
5.1 Components of an SPM . 4
5.2 Metrological categories of SPMs . 6
5.3 Block diagram of an SPM . 6
5.3.1 For category C: . 6
5.3.2 Additionally, for category B: . 6
5.3.3 Additionally, for category A: . 7
5.4 Calibration interval . 7
6 Preliminary characterization of the measuring system . 8
6.1 Overview of the instrument characteristics and influencing factors to be investigated . 8
6.2 Waiting times after interventions in the measuring system (e.g. instrument
installation, intrinsic effects, carrying out operation, warm-up, tip/specimen change) .10
6.2.1 Adjustment of the instrument to ambient conditions .10
6.2.2 Potential causes of drift .10
6.2.3 Procedure .10
6.3 External influences .11
6.3.1 Sources of external influences .11
6.3.2 Consequences of external influences and countermeasures .11
6.4 Summary .11
7 Calibration of scan axes .12
7.1 General .12
7.2 Measurement standards .12
7.2.1 Requirements for measurement standards .12
7.2.2 Handling of measurement standards .13
7.3 Xy-scanner guidance deviations of the x- and y-axes (xtz, ytz) .13
7.3.1 Definition of xy-scanner guidance deviations in vertical direction (z-plane) .13
7.3.2 Measurement strategy .13
7.3.3 Flatness measurement standards .14
7.3.4 Measurements .15
7.3.5 Evaluation of results .15
7.3.6 Summary .16
7.3.7 Extended calibration measurements .16
7.4 Calibration of x- and y-axis (Cx, Cy) and of rectangularity (ϕxy) and determination
of deviations (xtx, yty, ywx) .17
7.4.1 General.17
7.4.2 Definition of pitch p and p and rectangularity (ϕ ) in the x-y-plane .17
x y xy
7.4.3 Measurement strategy .17
7.4.4 Selection of lateral measurement standards .18
7.4.5 Basic calibration — Adjustments and measurements .19
7.4.6 Extended calibrations (scan speed, angle, and eccentric measurements) .20
7.4.7 Evaluation .21
7.4.8 Extended evaluations: nonlinearity of the x-y-axis .24
7.4.9 Summary .25
7.5 Calibration of the z-axis C , ϕ , and ϕ , and determination of the deviations ztz,
z xz yz
zwx, and zwy .26
7.5.1 General.26
7.5.2 Definitions of the step height .26
7.5.3 Measurement strategy .27
7.5.4 Selection of step height measurement standards .27
7.5.5 Basic calibration — Adjustments and measurements .28
7.5.6 Extended calibrations .29
7.5.7 Evaluations . . .30
7.5.8 Summary .34
7.6 3D measurement standards for alternative and extended calibration .34
7.6.1 General.34
7.6.2 Requirements for 3D measurement standards .34
7.6.3 Selection of the 3D measurement standards .35
7.6.4 Carrying-out of the basic calibration .36
7.6.5 Evaluation of the measurements .36
7.6.6 Extended calibrations .37
7.6.7 Advantages and disadvantages of the 3D measurement standard .37
8 Report of calibration results .38
8.1 General .38
8.2 Equipment used .38
8.3 Statements on ambient conditions .38
8.4 From Clause 5: Preliminary investigations .39
8.5 From Clause 5: Calibration — Details of measurement standards, scan range and
scan speed .39
8.6 Further statements .39
9 Uncertainties of measurement .39
9.1 General .39
9.2 Vertical measurand (height and depth).40
10 Report of results (form) .41
Annex A (informative) Example of superposition of disturbing influences in the topography
image .42
Annex B (informative) Sound investigations: effects of a sound-proofing hood .44
Annex C (informative) Thermal isolation effect of a sound-proofing hood/measuring cabin .46
Annex D (informative) Handling of contaminations in recorded topography images .48
Annex E (informative) Step height determination: comparison between histogram and
ISO 5436-1 method .49
Annex F (normative) Uncertainty of measurement for lateral measurands (pitch, position,
diameter) .51
Bibliography .56
iv © ISO 2019 – All rights reserved

Foreword
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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 9, Scanning probe microscopy.
This second edition cancels and replaces the first edition (ISO 11952:2014), of which it constitutes a
minor revision. The changes to the previous edition are as follows:
Previous edition Revised edition
Figure 1 “interferometery” “interferometry”
Figure 1 Note “The calibration of a user’s SPM by means “The calibration of a user’s SPM by
of traceably calibrated measurement means of traceable calibrated meas-
standards is the object of this Internation- urement standards is the object of this
al Standard (done by the user).” document (done by the user).”
Clause 4 N “N ” “N ”
i i ij
Clause 4 r “tip” “tip radius”
Clause 4 α “thermal expansion coefficient” “thermal expansion coefficient of the
m
specimen”
Clause 4 ytx “positional deviation Δx measured along a “straightness deviation Δx measured
y-coordinate line” along a y-coordinate line”
Clause 4 yty “straightness deviation Δy measured “positional deviation Δy measured
along a y-coordinate line” along a y-coordinate line”
Clause 4 ztz “straightness deviation Δz measured “positional deviation Δz measured
along a z-coordinate line” along a z-coordinate line”
6.3.1 Title “Kinds of external influence” “Sources of external influences”
6.3.1 First sentence “As SPMs are most sensitive to interfer- “As SPMs are very sensitive to interfer-
ence from the environment, the following ence from the environment, the influ-
quantities are to be accounted for”: ences of the following quantities need
to be determined”:
Previous edition Revised edition
6.3.1 Fourth bullet “mechanical vibrations (e.g. structural “mechanical vibrations (e.g. structural
vibrations, foot fall sounds/human traffic, vibrations, human traffic, pumps)”;
pumps)”;
6.4 a) and b) a) For measurement a) and b) switched
b) For installation or familiarization of
the staff
7.3.4 Second bullet “Adjust the z-position of the scanner in “Adjust the z-position of the scanner in
such a way that the z-scanner operates such a way that the z-scanner oper-
symmetrically around the central position ates symmetrically around the central
in the z-deflection range (see also Fig- position in the z-deflection range as
ure 18).” illustrated in Figure 17 around its
medium (central) deflection, i.e. 50 % of
its range”
7.3.7 Fourth bullet “adjust the z-position of the scanner in “adjust the z-position of the scanner in
such a way that the z-scanner operates, such a way that the z-scanner operates
e.g. by 20% (see also Figure 20) above or above or below the central position in
below the central position in the z-deflec- the z-deflection range, i.e. symmetrical-
tion range (see also Figure 18 and 20).” ly around 10 %, 30 %, 70 % and 90 %
(as illustrated in Figure 17), in addition
to the basic z calibration performed
around 50 % deflection”
Figure 9 Title “Flow diagram of calibration of the lateral “Calibration of the lateral axes: materi-
[35] [35]
axes “ als, steps and methods ”
7.4.4 Seventh par- “needs to” “should”
agraph
“great” “large”
7.4.4 Eighth para- “(relatively feeble)” “(low)”
graph
7.4.5 2) “(see also Figure 18)” “as illustrated in Figure 15 and shown
for the medium (central) deflection case
in Figure 17.”
7.4.7 Figure 10
7.4.7 4) and Note 1 “Appurtenant” “relevant”
7.4.8 Second bullet “In good gratings, the mean values of the “In good gratings, the fit lines g to g
0 n
pitches of all the straight lines are a good are nearly parallel so that the mean
approximation and should be used for value of the gradients of all these
further evaluation. If this is not the case, straight lines is a good approximation
the parallelism of the straight lines is to and should be used for further evalua-
be forced by fitting as above.” tion. If this is not the case, the parallel-
ism of the straight lines is to be forced
by fitting as above.”
7.4.8 Eighth bullet “(example in Figure 15)” “(the example in Figure 12 shows a pol-
ynomial fit of the third degree)”
vi © ISO 2019 – All rights reserved

Previous edition Revised edition
7.4.8 Note 2 “In the case of clear deviations of the “The certified pitch values of a transfer
specimen temperature (e.g. in deep-tem- standard are valid for a certain refer-
perature applications) from the reference ence temperature, typically 20 °C. In
temperature 20 °C in particular, for which case of significant deviations of the
the calibration of the measurement stand- sample temperature from the reference
ard is valid, the thermal expansion is to be temperature (e.g. in low-temperature
accounted for.” chambers or if the sample is heated in
the particular setup), the material-de-
pendent thermal expansion is to be
taken into account.”
7.5.7.2.1 Second para- “For the one straight line — besides the “For the one straight line — besides
graph parallelism requirement — the determi- the parallelism requirement — the
nation uses only an area C in the middle determination uses only the section C in
of the indention or elevation whose width the middle of the indention or elevation
can be selected by the user; it is usual to whose width w can be selected by the
m
select one (according to ISO 5436 1) to user; it is usual to select one (according
two-thirds (Figure 18) of the total width to ISO 5436-1) to two-thirds (like in the
w of the indention/elevation.” example shown on the left of Figure 18)
of the total width w (defined as full-
width at half maximum) of the inden-
tion/elevation.”
7.5.7.2.1 Third para- “Taking account of the parallelism re- “Taking account of the parallelism
graph quirement, the second straight is selected requirement, the second straight is
through two areas A and B which lie sym- selected through two sections A and
metrically about the indention/elevation B which lie symmetrically about the
and usually show the same width as C. indention/elevation. The lengths w in
s
The distance of A” sections A and B are identical, but might
be different from w . The sections A
m
and B should not start/end with the
beginning/end of the profile (scanline),
as irregularities in height measurement
are to be expected especially at the
beginning/end of scanlines. A spacing
w to the left of section A and w to the
l r
right of section B should be allowed for.
As a general rule, the total length of the
measured profile should be at least 3w.
The distance we of A”
7.5.7.2.1 Fourth para- “As to the mathematics, the determina- “As to the mathematics, the determina-
graph tion of the step height, h, is reduced to the tion of the step height, h, is reduced to
calculation of only one regression line by the calculation of only one regression
appropriately shifting the points area by line by least squares approximation
area by +h/2 and –h/2, respectively.” with h being the fit variable. This
reduction to only one regression line is
achieved by introducing a vertical shift
of the data points in the sections A, B
and C according to the following rules”:
7.5.7 Figure 18
title
Previous edition Revised edition
“Step height determination according to “Step height determination according
ISO 5436-1 (left: step height measurement to ISO 5436-1 (left: example of a step
standard 6 nm)” height measurement standard 6 nm,
right: profile section C of length w
m
and profile sections A and B each of
length w taken into account for step
s
height analysis, spacings of lengths w
e
from the edge and of lengths w to the
l
left of section A and w to the right of
r
section B)”
Table C.1 Sum “+1,49” “+1,50”
Annex E Fifth para- “The bars in Figure E.1 give the uncertain- “The bars in Figure E.1 give the stand-
graph ty” ard uncertainty”
Bibliogra- [16] DZIOMBA T., KOENDERS L., WILKENING KLAPETEK P., Quantitative data pro-
phy G., FLEMMING M., DUPARRÉ A. Entwick- cessing in scanning probe microscopy.
lung einer Kalibrierrichtlinie für Raster- Elsevier, Amsterdam, The Netherlands,
sondenmikroskope; tm — Technisches ISBN: 978–0–12–813347–7, 2018
Messen 72 (2005) 5, S. 295–307; siehe
auch http: //www .tm -messen .de/
Bibliogra- [26] Koenders, L.; Dziomba, T.; Thom- Yacoot A., Koenders L. Recent develop-
phy sen-Schmidt, P.; Senoner M.: Normale ments in dimensional metrology using
für die dimensionelle und analytische AFMs. Meas. Sci. Technol. 2011, 22, p.
Nanometrologie. PTB-Mitteilungen 114 12201
(2004) 1, S. 16–24
Bibliogra- [39] ZHAO X. PTB-report F-20. Wirtschaftsver- Zhao X. “Scanning Probe Microscope
phy lag NW, Bremerhaven, Germany, 1995 with high resolution capacitive trans-
ducers”, PTB-report F-32, Wirtschafts-
verlag NW — Verlag für neue Wissen-
schaft GmbH, Bremerhaven, www-nw-
verlag.de ISBN 3-89701-207–3, 1998,
158 pages
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.
viii © ISO 2019 – All rights reserved

Introduction
The progress of miniaturization in semiconductor structuring, together with the rapid advance of
many diverse applications of nanotechnology in industrial processes, calls for reliable and comparable
[9]
quantitative dimensional measurements in the micro- and submicrometre range . Currently, a
measurement resolution, in or below the nanometre region, is frequently required. Conventional
optical or stylus measurement methods or coordinate measuring systems are not able to offer this level
of resolution.
For this reason, scanning-probe microscopes (SPMs) are increasingly employed as quantitative
measuring instruments. Their use is no longer confined only to research and development, but has been
extended to include industrial production and inspection.
For this category of measuring instrument, standardized calibration procedures need to be developed,
as have already been established, for example, for contact stylus instruments (see ISO 12179). For
efficient and reliable calibration of SPMs to be carried out, the properties of the measurement standards
used need to be documented and accounted for in the calibration (see Figure 1). At the same time, the
procedure for the calibration should be clearly defined.
Only if this prerequisite is satisfied will it be possible to perform traceable measurements of geometrical
quantities.
NOTE The calibration of a user’s SPM by means of traceable calibrated measurement standards is the object
of this document (done by the user).
Figure 1 — Traceability chain for SPMs
An SPM is a serially operating measuring device which uses a probe with a tip of adequate fineness to
trace the surface of the object to be measured by exploitation of a local physical interaction (such as the
quantum-mechanical tunnel effect, interatomic or intermolecular forces, or evanescent modes of the
electromagnetic field). The probe and the object to be measured are being displaced in relation to one
[10]
another in a plane (hereinafter referred to as the x-y-plane) according to a defined pattern , while
the signal of the interaction is recorded and can be used to control the distance between probe and
object. In this document, signals are considered which are used for the determination of the topography
(hereinafter called the “z-signal”).
This document covers the verification of the device characteristics necessary for the measurement of
[11]
geometrical measurands and the calibration of the axes of motion (x, y, z) , i.e. the traceability to the
unit of length via measurement on traceable lateral, step height and 3D measurement standards (see
Figure 2).
While this document aims at axis calibrations at the highest level and is thereby intended primarily for
high-stability SPMs, a lower level of calibration might be required for general industry use.
Key
1 measurement standards for verification purposes
1a flatness
1b probe shape
2 measurement standards for calibration purposes
2a 1D and 2D lateral
2b step height
3 calibration of the measurement standards by reference instruments (certified calibration, measurement value
including uncertainty)
Figure 2 — Verification and calibration of SPMs with test specimens and measurement
standards
This document is mainly based on the guideline VDI/VDE 2656, Part 1, drafted by a guideline committee
of the VDI (Verein Deutscher Ingenieure/Association of German Engineers) from 2004 to 2008, with the
final whiteprint of that guideline being released in June 2008.
x © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 11952:2019(E)
Surface chemical analysis — Scanning-probe microscopy
— Determination of geometric quantities using SPM:
Calibration of measuring systems
1 Scope
This document specifies methods for characterizing and calibrating the scan axes of scanning-probe
microscopes (SPMs) for measuring geometric quantities at the highest level. It is applicable to those
providing further calibrations and is not intended for general industry use, where a lower level of
calibration might be required.
This document has the following objectives:
— to increase the comparability of measurements of geometrical quantities made using SPMs by
traceability to the unit of length;
— to define the minimum requirements for the calibration process and the conditions of acceptance;
— to ascertain the instrument's ability to be calibrated (assignment of a “calibrate-ability” category to
the instrument);
— to define the scope of the calibration (conditions of measurement and environments, ranges of
measurement, temporal stability, transferability);
— to provide a model, in accordance with ISO/IEC Guide 98-3, to calculate the uncertainty for simple
geometrical quantities in measurements using an SPM;
— to define the requirements for reporting results.
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 11039, Surface chemical analysis — Scanning-probe microscopy — Measurement of drift rate
ISO 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
IEC/TS 62622, Artificial gratings used in nanotechnology — Description and measurement of dimensional
quality parameters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-2 and IEC/TS 62622
and the following 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
scanner bow
additional deflection in the z-direction when the scanner is displaced in the x-y-direction
Note 1 to entry: Scanner bow is also known as out-of-plane motion (see also xtz, ytz in Clause 4).
3.2
look-up table
table in which a set of correction factors for the scanner are filed for different modes of operation (e.g.
scan ranges, scan speeds, deflections)
3.3
step height
height of an elevation (bar) or depth of a groove (see ISO 5436-1); on atomic surfaces, the distance
between neighbouring crystalline planes
3.4
levelling
correction of the inclination between the ideal x-y-specimen plane and the x-y-scanning plane
4 Symbols
x, y, z position value related to the respective axis
C , C , C calibration factors for the x-, y-, and z-axes
x y z
h step height
w width of a structure of the specimen
N ith pitch value in a profile used for the determination of the pitch/period (number of pitch
ij
values i over all lines j = 1,., Nj)
p pitch or period in the x-direction
x
p pitch or period in the y-direction
y
a vector in the x-direction of a grating (not to be confused with p )
x x
a vector in the y-direction of a grating (not to be confused with p )
y y
γ non-orthogonality of 2D gratings
xy
P-V peak-to-valley value
r tip radius
Rq (Sq) root mean square deviation of the assessed roughness profile (Rq) or of the assessed
area (Sq)
T temperature
α thermal expansion coefficient of the specimen
m
T temperature of the air
L
T temperature of the specimen during measurement
m
j angle of rotation about the x-axis
x
2 © ISO 2019 – All rights reserved

j angle of rotation about the y-axis
y
j angle of rotation about the z-axis
z
θ levelling angle
x value of the measurement standard for shift in the x-direction
L
x shift in the x-direction measured with the x-displacement transducer
m
xtx positional deviation Δx measured along an x-coordinate line
xty straightness deviation Δy measured along an x-coordinate line
xtz straightness deviation Δz measured along an x-coordinate line
xrx rotational deviation j measured along an x-coordinate line
x
xry rotational deviation j measured along an x-coordinate line
y
xrz rotational deviation j measured along an x-coordinate line
z
xwy measured rectangularity deviation in the coordinate plane x-y
xwz measured rectangularity deviation in the coordinate plane x-z
y value of the measurement standard for displacement in the y-direction
L
y displacement measured with the y-displacement transducer in the y-direction
m
ytx straightness deviation Δx measured along a y-coordinate line
yty positional deviation Δy measured along a y-coordinate line
ytz straightness deviation Δz measured along a y-coordinate line
yrx rotational deviation j measured along a y-coordinate line
x
yry rotational deviation j measured along a y-coordinate line
y
yrz rotational deviation j measured along a y-coordinate line
z
ywz rectangularity deviation measured in the coordinate plane y-z
z value of the measurement standard for displacement in the z-direction
L
z displacement in the z-direction measured with z-displacement transducer
m
ztx straightness deviation Δx measured along a z-coordinate line
zty straightness deviation Δy measured along a z-coordinate line
ztz positional deviation Δz measured along a z-coordinate line
zrx rotational deviation j measured along a z-coordinate line
x
zry rotational deviation j measured along a z-coordinate line
y
zrz rotational deviation j measured along a z-coordinate line
z
cos(φ ) rotational correction, for example in pitch measurement
i
cos(θ ) tilt-related correction, for example in pitch measurement
i
λ short-wavelength filter (see ISO 4287 for details)
s
λ long-wavelength filter (see ISO 4287 for details)
c
Λ correlation length
ϕ angle between the x- and y-direction, counterclockwise
xy
ϕ angle between the x- and z-direction, counterclockwise
xz
ϕ angle between the y- and z-direction, counterclockwise
yz
Rqx noise in the x-direction
Rqy noise in the y-direction
Rqz (Sqz) noise in the z-direction in a measured profile (or within a measured area)
v scan speed (i.e. distance travelled by the probe tip per unit time, not to be confused with
the scan rate, i.e. the number of scanlines recorded per unit time)
5 Characteristics of SPMs
5.1 Components of an SPM
4 © ISO 2019 – All rights reserved

Key
1 x-y-scanner
2 z-scanner
3 position detector
4 probe
5 specimen
6 coarse z-approach, i.e. move the probe or the specimen in the vertical direction to bring it close enough to the
specimen or probe, respectively (afterwards, start automatic approach techniques)
7 coarse x-y-positioning, i.e. move the specimen or probe laterally close to or into the region of interest on the
specimen
Figure 3 — Schematic sketch of an SPM
Several components shown in Figure 3 are defined in ISO 18115-2. In this document, they fulfil the
following functions:
— Probe: equipped with a tip at its apex. This probes the specimen surface, exploiting a local physical
interaction whose changes can be detected, for example cantilever bending in the case of an atomic
force microscope.
— Position detector: transformation of the probe’s interaction response (e.g. bending or oscillation of
the cantilever) into an electrical signal.
— z-scanner: element for the realization of the vertical tracking of the specimen/probe distance during
x-y-scanning to a constant value of the physical interaction used for distance control (e.g. of the
action of force on the probe in the case of an atomic force microscope), to ensure an approximately
constant distance between specimen and probe.
— x-y-scanner: element for realization of the lateral displacement of the probe (or of the specimen) in
the x-y-plane (the plane parallel to the seating face of the specimen), which is used, among other
things, to record a location-dependent interaction signal that contains information about a local
property of the specimen (above all, the local height).
— Specimen holder: where appropriate, with coarse positioning and coarse approach mechanics.
— Casing/mounting: structure for mounting the scanner and specimen.
5.2 Metrological categories of SPMs
SPMs can generally be subdivided into the three following categories, depending on their metrological
equipment:
— category A: reference instruments with integrated laser interferometers, allowing direct traceability,
1)
via the wavelength of the laser used, to the SI unit of length .
— category B: SPMs with position measurement using displacement transducers, for example
capacitive/inductive sensors, strain gauges or encoders calibrated by temporarily connecting laser
interferometers to the instrument or by making measurements on high-quality measurement
standards. A distinction is made between the following two types:
— those with active position control: tracking to a scheduled position by means of a closed loop
(so-called closed-loop configuration);
— those with position measurement but without a closed loop for position control (so-called open-
loop configuration).
— category C: SPMs in which the position is determined from the electrical voltage applied to the
adjustment elements and, if need be, corrected using the look-up table. Calibration is against
measurement standards.
These definitions of metrological categories imply that it is not possible for certain instruments to
be assigned to a single category, but that, with respect to their scan axes, they need to be considered
separately.
5.3 Block diagram of an SPM
The block diagram shown in Figure 4 has been obtained from the schematic diagram of an SPM in
Figure 3. The characteristics of the essential components are given below and need to be investigated
individually in the course of verification and calibration.
5.3.1 For category C:
— casing/mounting (mechanical, acoustic, electromagnetic and thermal characteristics);
— specimen holder, where appropriate with coarse positioning and coarse approach mechanics
(acoustic, mechanical and thermal characteristics);
— z-scanner;
— x-y-scanner;
— detector loop, for example using the beam deflection method, with a beam on the rear side of the
cantilever in the case of an atomic force microscope and detection of the reflected beam from the
rear side of the cantilever with a position-sensitive photodiode. The signal of the position-dependent
photodiode serves as input to the feedback loop of the z-scanner in order to keep the set-point
constant;
— probe.
5.3.2 Additionally, for category B:
— category B2: x-, y- and/or z-displacement transducer, for example encoder, capacitive or inductive
displacement transducer or strain gauge;
— category B1: where appropriate, active (closed-loop) position control.
1) Instruments of this category are often referred to as “metrological SPMs”, although the definition of a
“metrological SPM” in ISO 18115-2:2013 does not necessarily imply laser-interferometric position control.
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5.3.3 Additionally, for category A:
Traceability by integrated laser interferometers, i.e. systems as for category B, but equipped with:
— integrated laser interferometers for position measurement/control;
— where appropriate, additional angle sensors.
Figure 4 — Block diagram of an SPM
The classification above is a first rough estimation of the effort necessary to achieve the desired
accuracy of calibration. It is not necessary, for example, to purchase a set of measurement standards
with minimum uncertainties of measurement for the calibration of category C instruments. Less
sophisticated measurement standards are usually sufficient here.
5.4 Calibration interval
The interval at which the instrument will need to be calibrated depends on the type of instrument
(i.e. the metrological category), its stability, especially with respect to time, the intended purpose of
the measurements and the constancy of the ambient conditions. As most calibrations are of a complex
nature, and thus, are labour- and time-intensive, a compromise needs to be found between the cost of
calibration and the measurement uncertainty which can be tolerated.
Generally, the following repetition patterns for calibrations (K) and measurements (M) are suitable.
KMM …, KMM . for instruments of high stability in the medium term: calibration is necessary only at
defined intervals of time, for example once weekly/monthly/yearly.
KM, KM, KM … for instruments with acceptable short-term but bad long-term stability: calibration
is necessary before each measurement.
KMK, KMK . when the maximu
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