ISO 21466:2019

INTERNATIONAL ISO
STANDARD 21466
First edition
2019-12
Microbeam analysis — Scanning
electron microscopy — Method for
evaluating critical dimensions by CD-
SEM
Analyse par microfaisceaux — Méthode d’évaluation des dimensions
critiques par CD-SEM
Reference number
ISO 21466:2019(E)
©
ISO 2019

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ISO 21466:2019(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 21466:2019(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 8
5 Generation of Model-based Library (MBL) . 8
5.1 Basic components of a MBL simulator . 8
5.1.1 Electron probe model . 8
5.1.2 SE signal generation model .10
5.1.3 SE signal detection model .11
5.2 Model of specimen .12
5.2.1 Specimen structure and parameters .12
5.2.2 Specimen specification .15
5.2.3 Generation methods of specimen geometry .15
5.3 Monte Carlo simulation .15
5.3.1 Input parameters .15
5.3.2 Beam-specimen interaction .16
5.4 MBL file structure .16
5.4.1 Variable type and value .16
5.4.2 Model description file .20
5.4.3 Parameter specification file .21
5.4.4 Preparation of library data.21
5.4.5 MBL data structure .22
5.4.6 MBL data file format .22
6 Acquisition of a CD-SEM image.23
6.1 Acceptable image .23
6.2 Specimen tilt .23
6.3 Image quality .23
6.4 Selection of the field of view .23
6.5 CD-SEM image data file .23
7 CD determination .23
7.1 Determination of pixel size.24
7.2 Selection of the field of interest .24
7.3 Coordination and normalization .24
7.4 Matching procedure.25
7.4.1 Interpolation .25
7.4.2 Convolution .25
7.4.3 Matching .26
7.4.4 Averaging .30
8 Module functions and relationship .31
9 Uncertainty of CD measurement .33
Annex A (normative) Flow charts of procedures .35
Annex B (informative) Example of model description file .39
Annex C (informative) Example of parameter specification file .40
Annex D (informative) Example of CD evaluation .41
Bibliography .44
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ISO 21466:2019(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
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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
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee
SC 4, Scanning electron microscopy (SEM).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 21466:2019(E)

Introduction
Nanostructures need strict dimensional control to meet the demands of the semiconductor industry.
Critical dimension (CD) is the minimum size of a feature on an integrated circuit that impacts the
electrical properties of the device, whose value represents the level of complexity of manufacturing.
At nanometer scale, measurement uncertainty control becomes more difficult with much smaller
dimensions. A determination method with algorithm for accurate measurement is a key for CD
valuation. CD-SEMs (critical dimension scanning electron microscopes) are one of the main tools for
CD measurement in semiconductor manufacturing processes, where secondary electrons (SEs) are
the signal source for CD-SEM imaging of surface structure. The CD-SEM image displays the structure
geometry, but the image contrast is not a perfect representation of the structure morphology. The
detected intensity linescan profile of SE signals carries the information about the sample shape and
composition, beam size and shape and the information volume generated by the electron beam-solid
interaction. Restricted by the physical mechanism in the processes of SE signal generation and emission,
the SE signal profiles show an edge effect which leads to difficulty for accurate CD value determination
with image contrast. A reliable CD determination method which bases on physical principle of SE signal
emission is necessary.
Many factors, for example the specimen chemical composition, structural geometric parameters, beam
conditions and other specimen/instrument factors (charging, vibration and drift), can affect CD-SEM
image contrast and hence the CD measurement result. Topographic contrast in the SE mode is resulted
from the enhanced SE emission from an edge as well as tilted local surface in relative to the incident
beam. The quantitative description of contrast or SE intensity profile is crucial in CD metrology.
The physical mechanisms that dominate quantitative measurements by CD-SEM have been well
understood. The CD determination algorithm is based on physical modelling of SE generation and
emission and gives adequate consideration of the influence of various experimental factors during
electron beam-specimen interaction. This document employs the model-based library (MBL) method
for accurate CD determination by CD-SEM. MBL is superior to simpler, unsophisticated, arbitrary
methods that disregard the physics of signal generation, and report only a meagre number, potentially
with unacceptably high bias. MBL uses the whole waveform of the signal, so it can provide results
with less bias and better size and shape accuracy. Once the library is set up, there is essentially no
time penalty for using MBL. Construction of MBL is done with a Monte Carlo (MC) simulator which is
considered as an excellent approach to take into account of every possible physical factor that may affect
signal intensity and shape of linescan profiles. The library generation can be sped up tremendously by
suitable multicore computing environment and MC software that is optimized for a specific measurand.
Such obtained MBL relates the measured signal linescan profiles to both specimen parameters and
instrumental parameters. The library database is consisted of the simulated SE linescan profiles,
having a one-to-one correspondence to a specified value of parameter set. By matching the shape of SE
linescan profile taking from a measured CD-SEM image with those simulated beforehand and stored in
a MBL database, the best fitted CD values used in MC modelling are selected.
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INTERNATIONAL STANDARD ISO 21466:2019(E)
Microbeam analysis — Scanning electron microscopy —
Method for evaluating critical dimensions by CD-SEM
1 Scope
This document specifies the structure model with related parameters, file format and fitting procedure
for characterizing critical dimension (CD) values for wafer and photomask by imaging with a critical
dimension scanning electron microscope (CD-SEM) by the model-based library (MBL) method. The
method is applicable to linewidth determination for specimen, such as, gate on wafer, photomask, single
isolated or dense line feature pattern down to size of 10 nm.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
critical dimension
CD
minimum geometrical feature size limited by the photolithography technology used for the
fabrication process
3.2
CD metrology
measurement of the width of line and space for a trapezoidal line structure model
Note 1 to entry: Extended CD metrology includes the measurement of top CD, middle CD, bottom CD, height,
sidewall angle, top rounding and foot rounding. Figure 1 shows schematically the definition of CDs.
Note 2 to entry: The term “top rounding” indicates a circular arc at the top corner, which is tangent to the top
surface and side surface of a trapezoidal line, and whose value is represented by the circular radius.
Note 3 to entry: The term “foot rounding” indicates a circular arc at the bottom corner, which is tangent to the
bottom surface and side surface of a trapezoidal line, and whose value is represented by the circular radius.
Note 4 to entry: More frequently CD represents the size of a feature on an integrated circuit or transistor that
impacts the electrical properties of the device.
Note 5 to entry: Top rounding and foot rounding are not designed parameters.
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ISO 21466:2019(E)

3.3
critical-dimension scanning electron microscope
CD-SEM
special instrument for measuring CDs (3.1) of the fine patterns formed on a semiconductor wafer
by producing magnified images (3.4) of a specimen (3.19) by scanning its surface with a focused
electron beam
Note 1 to entry: It is mainly used in the manufacturing lines of electronic devices of semiconductors and
optimized for dimensional metrology task, and differs from a general-purpose laboratory SEM in several
aspects: 1. primary electron beam irradiates the sample at normal or nearly normal incidence condition; 2. the
measurement repeatability around 1 % 3σ of the measurement width is guaranteed by improving magnification
calibration to the maximum extent; 3. fine pattern measurements on the wafer are automated.
Figure 1 — Definition of CDs: top CD, middle CD, bottom CD, height, sidewall angle, top rounding
and foot rounding
3.4
image
two-dimensional representation of the specimen (3.19) surface generated by SEM
Note 1 to entry: A photograph of a specimen taken using an SEM is a good example of an image.
[SOURCE: ISO 16700:2016, 3.2]
3.5
SEM imaging
action of forming an image (3.4) by a mapping operation that collects electron signals emitted from the
specimen (3.19) surface and passes the digital signal intensity information into the storage devices
3.6
SE image
scanning (3.9) of electron beam images (3.4) in which the signal is derived from a detector that
selectively measures secondary electrons (3.20) (electrons having energies less than 50 eV) and is not
directly sensitive to backscattered electrons
Note 1 to entry: Intensity of digital CD-SEM image is adjusted to 8 bit (or other) depth of grayscale and does not
equal to the detected physical number of secondary electron signals.
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ISO 21466:2019(E)

[SOURCE: ISO 23833:2013, 4.4.11, modified — Note 1 to entry added.]
3.7
electron probe
electron beam focused by the electron optical system onto the specimen (3.19)
[SOURCE: ISO 22493:2014, 7.1]
3.8
electron probe size
beam size
beam width
diameter of a circle that contains 50 % of the total electron probe (3.7) current
Note 1 to entry: For an ideal Gaussian probe shape in the radial direction:
2
 
1 r

Gr|eσ =−xp (1)
()
 
b
 
2 2
22πσ σ
 
bb
the electron probe size is determined by the standard deviation (σ ) as d =22ln2σ , which is equal to the full
b
pb
width at half maximum (FWHM) of the Gaussian peak.
3.9
scanning
action of obtaining time-controlled movement of the electron probe (3.7) on the specimen (3.19) surface
3.10
linescan profile
signal intensity as function of coordinate along a straight line across an image (3.4)
3.11
focusing
aiming the electrons onto a particular point using an electron lens
[SOURCE: ISO 22493:2014, 3.1.4]
3.12
convergence angle
half-angle of the cone of the beam electrons converging onto the specimen (3.19)
[SOURCE: ISO 22493:2014, 7.1.1]
3.13
working distance
distance between the lower surface of the pole piece of the objective lens and the specimen (3.19) surface
Note 1 to entry: In the past, this distance was defined as the distance between the principal plane of the objective
lens and the plane containing the specimen surface.
[SOURCE: ISO 22493:2014, 4.5.2]
3.14
charging effect
distortion of signal intensity in SEM imaging (3.5) of non-conductive specimens (3.19) due to
accumulation of spatial charges in homogeneously distributed and hence the establishment of surface
electric potential, which alters primary electron incidence (including landing energy and position) and
all emitted electron signal properties
Note 1 to entry: The effect is a time dependent phenomenon and mainly related to current density and beam energy.
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ISO 21466:2019(E)

3.15
pixel
smallest discrete image (3.4) data element that constitutes an SEM image
[SOURCE: ISO 22493:2014, 5.2.4]
3.16
pixel size
length of the pixel (3.15), measured at a specimen (3.19) surface
Note 1 to entry: For a square or circular pixel, the horizontal and vertical pixel sizes should be the same.
[SOURCE: ISO 22493:2014, 5.2.5]
3.17
contrast
difference in signal intensities between two arbitrarily chosen points of interest in the image (3.4) field
3.18
graphics file format
archival digital format for storing the contents of the frame store
Note 1 to entry: The most popular image file formats are: bitmap (BMP), graphics interchange format (GIF), tagged
image format (TIF) and joint photographic experts group (JPG). The TIF format can preserve all data and keeps
the size of each pixel in its header. Consequently, this format is preferred to maintain the integrity of the images.
[SOURCE: ISO 22493:2014, 5.6.4, modified — “TIF format is the scientific format that preserves” is
changed to “TIF format can preserve”, admitted term "image file format" removed.]
3.19
specimen
sampled material designated to be examined or analysed
[SOURCE: ISO 22493:2014, 4.5]
3.20
secondary electron
SE
electron emitted from the specimen (3.19) by the excitation of loosely bound valence electrons of the
specimen in electron inelastic scattering (3.31) events and in a cascade production process as a result of
bombardment by excitation beams, e.g. electrons, ions and photons
Note 1 to entry: By convention, an emitted electron with energy lower than 50 eV is considered as a secondary
electron when primary energy is above 50 eV.
3.21
SE yield
total number of secondary electrons (3.20) per incident electron
[SOURCE: ISO 22493:2014, 3.4.1]
3.22
SE angular distribution
distribution of secondary electrons (3.20) as a function of their emitting angles relative to the
surface normal
[SOURCE: ISO 22493:2014, 3.4.2]
3.23
SE energy distribution
distribution of secondary electrons (3.20) as a function of their emitting energies above the vacuum level
[SOURCE: ISO 22493:2014, 3.4.3, modified — added “above the vacuum level”]
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ISO 21466:2019(E)

3.24
SE tilt dependence
effect on secondary electrons (3.20) of the specimen (3.19) tilt which accompanies a change in incident
beam angle
[SOURCE: ISO 22493:2014, 3.4.5]
3.25
Monte Carlo simulation
MC simulation
broad class of computational algorithms that uses statistical sampling techniques to obtain numerical
results of a math model (Eckhardt 1987)
Note 1 to entry: The calculation models stochastic physical processes in the electron beam-specimen interaction
and SEM image formation (Shimizu 1992; Joy 1995). The incident electron beam strikes the surface of the
specimen, then a series of elastic and inelastic scattering (3.31) process takes place inside the specimen for the
incident electrons and generated SEs. Connection of the spatial location of scattering event forms an electron
trajectory. Tracking of electron trajectory is terminated when an electron is absorbed by losing its kinetic energy
to below surface barrier (3.39) or leave from the specimen surface. Calculation includes the determination of free
path as a function of energy, the outcome of a scattering event, i.e. the new direction, position and energy of a
primary electron and a SE if it is generated.
Note 2 to entry: The simulation for electron beam-specimen interaction is made of the following process (Ding
1996). A primary electron enters into the specimen at an angle α of incidence, which may not be normal to the
local surface even for a normal incident beam onto the substrate plane, shall suffer a scattering after flying
over a distance of free path. This electron step length obeys an exponential probability distribution where the
mean free path is determined by the sum of inverse electron total elastic scattering cross section and electron
inelastic mean free path. By MC simulation technique a particular value of variable shall be randomly sampled
from a given probability density distribution for a continuous variable or from a probability for a discrete
variable with a random number uniformly distributed in the interval of 0-1. In discrete scattering model the
property of scattering event being either elastic or inelastic is determined by another random number based on
the proportion of elastic scattering or inelastic scattering in the total cross section. If it is elastic the scattering
angle is sampled from the differential elastic scattering cross section by a random number. If it is inelastic,
the associated energy loss and scattering angle are sampled from the corresponding differential or double-
differential inelastic scattering cross sections. The new moving direction after scattering can then be determined
to derive the updated coordinates of electron after passing by a new step length (Figure 2). Accompanied with
electron energy loss in an inelastic event, one SE will be generated and its information on the energy, position
and direction will be stored in a stack so that they could be read out after finishing tracing of an incident electron
trajectory. All the simulated electrons, either primary or secondary, shall generate further cascade SEs along
their trajectories in the solid target. If the energy of an electron reaching the surface is high enough to overcome
the surface barrier it is then emitted from the local surface.
Note 3 to entry: MC simulation of SE image and SE linescan profile is performed by counting number of emitted
SEs by calculating a certain number of primary electron trajectories, which are incident onto a location at
specimen surface corresponding to an image pixel, and the generated cascade SE trajectories inside the specimen
for a primary beam scanning the specimen surface.
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ISO 21466:2019(E)

Key
E electron kinetic energy
S sampled electron flight length
ΔE sampled electron energy loss in an inelastic scattering event
θ sampled electron scattering angle (polar angle) in an elastic or inelastic scattering event
φ sampled electron azimuthal angle in an elastic or inelastic scattering event
Figure 2 — Schematic diagram of Monte Carlo simulation of electron trajectory
3.26
model-based library
MBL
database of calculated SE linescan profiles (3.10) with a MC simulation method based on physical
modelling of electron beam interaction with a specimen (3.19) in the CD-SEM (3.3) imaging process,
having one-to-one correspondence between the simulated SE linescan profile and a parameter set for
geometric modelling of specimen topography and beam condition
3.27
MBL simulator
specific MC simulation model and simulation software for producing a MBL database
3.28
scattering cross section
total scattering cross section
effective area that quantifies the essential likelihood of a scattering event when an incident beam strikes
a target object, mathematical description of the probability of a scattering event (elastic or inelastic)
Note 1 to entry: Scattering cross-section is usually measured in units of area.
3.29
differential scattering cross section
cross section which is specified as a function of some final-state variable, such as particle angle and/
or energy
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ISO 21466:2019(E)

3.30
elastic scattering
deflection of an electron in the Coulomb potential of an atomic nucleus where electron energy transfer
is negligible and large-angle
...