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 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

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
Foreword
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iv © ISO 2019 – All rights reserved

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 result
...