ISO 21466:2019

Overview

ISO 21466:2019 defines a standardized, physics‑based method for evaluating critical dimensions (CDs) using a critical-dimension scanning electron microscope (CD‑SEM). The standard specifies the model structure, parameter sets, file format and fitting procedures for the model‑based library (MBL) approach to CD metrology for wafers and photomasks. It is applicable to linewidth determination of features - including gate structures, single isolated and dense line patterns - down to sizes of 10 nm.

Key technical topics and requirements

• Model‑based Library (MBL) generation
  • Basic simulator components: electron probe model, secondary‑electron (SE) signal generation model, and SE detection model.
  • Specimen modelling: geometric parameters for trapezoidal line structures (top/middle/bottom CDs, height, sidewall angle, top/foot rounding).
  • Use of Monte Carlo (MC) simulation to generate SE linescan profiles that relate instrument and specimen parameters to measured signals.
• MBL file structure and format
  • Specification of variable types, model description files, parameter specification files and preparation of library data for matching.
• CD‑SEM image acquisition
  • Requirements for acceptable images, field‑of‑view selection, tilt control, and image quality considerations.
• CD determination and matching
  • Procedures for pixel‑size determination, field‑of‑interest selection, normalization/coordination of experimental data.
  • Matching workflow including interpolation, convolution, profile matching and averaging to select best‑fit library entries.
• Measurement uncertainty
  • Methods for assessing and reporting uncertainty of CD measurements.
• Supporting material
  • Normative flow charts and informative examples of model/parameter files and CD evaluation (Annexes A–D).

Practical applications and users

ISO 21466:2019 is directly useful for:

• Semiconductor fabs and process engineers performing CD metrology and process control.
• Photomask manufacturers verifying feature linewidths.
• Metrology laboratories seeking traceable, physics‑based CD determination.
• CD‑SEM instrument and software vendors implementing MBL workflows and file formats.
• QA teams and calibration labs evaluating measurement uncertainty and method comparability.

Benefits include improved accuracy and reduced bias by using full SE waveform matching, reproducible library formats for automated, high‑throughput CD measurements, and clear procedures for uncertainty assessment.

Related standards

• ISO 16700:2016 (referenced in the standard for SEM image terminology and related definitions)

Keywords: ISO 21466:2019, CD‑SEM, model‑based library, MBL, critical dimension, CD metrology, Monte Carlo simulation, wafer, photomask, linewidth measurement, microbeam analysis.