Overview
ISO 23216:2021 specifies a standardized spectroscopic ellipsometry method to determine the optical properties of amorphous carbon films - specifically the refractive index (n) and extinction coefficient (k) - and to classify film types within the n‑k plane. The procedure is applicable to amorphous carbon films deposited by sputtering, ionized evaporation, arc deposition, plasma‑assisted CVD, hot filament techniques and similar processes. It is focused on non‑modified, homogeneous films (thickness 0.02 µm to 5 µm) on optically isotropic substrates (recommended: Si wafer).
Key topics and technical requirements
- Measurement method: Spectroscopic ellipsometry with light source and detector spectral range >450 nm to 950 nm.
- Recommended measurement conditions: wavelength 450–950 nm, angle of incidence 70° (or multi‑angle), spot size >500 µm × 500 µm, measurement duration ~5 s.
- Data point for classification: n and k values at or close to 550 nm are used for optical classification.
- Optical modelling:
- First layer: Tauc‑Lorentz dispersion model for the amorphous carbon layer.
- Surface roughness: Modelled as a mixture of carbon and voids using Bruggeman effective medium approximation (EMA).
- Two optical model variants are specified depending on film transparency (whether light reaches the substrate).
- Specimen preparation: Cleaning by ultrasonic bath in high‑purity solvent, oven dry (120 °C) and storage in the measurement atmosphere.
- Repeatability: Minimum of three test repeats under identical conditions.
- Classification: Defined n‑k ranges identify film types (e.g., ta‑C, ta‑C:H, a‑C, a‑C:H, polymer‑like, graphite, and DLC subtypes) per Annex A.
- Exclusions: Films modified with metals or silicon, films with composition/property gradients through thickness, paints and varnishes.
Applications and who uses it
- Coating manufacturers and process engineers - process fingerprinting and selection of the appropriate amorphous carbon type (DLC, ta‑C, a‑C:H) for wear, corrosion or optical performance.
- R&D and thin‑film developers - material characterization and optical modelling during development.
- Quality control and production metrology labs - non‑destructive, fast verification of optical constants and film classification on silicon wafers.
- Semiconductor, automotive, biomedical and optics sectors - where amorphous carbon coatings are used for protection, tribology or optical interfaces.
Practical value
- Provides a repeatable, international procedure for deriving optical constants (n, k) and a standardized n‑k classification that acts as a process or material fingerprint.
- Enables consistent quality control, inter‑lab comparability and easier selection of carbon film types for application‑specific requirements.
Related standards
- ISO 20523 - Carbon based films - Classification and designations
- ISO 23131 - Ellipsometry - Principles
- ISO 17328 - Optical materials - Refractive index (infrared)
- VDI 2840 - Carbon films - Basic knowledge, film types and properties
Keywords: spectroscopic ellipsometry, amorphous carbon films, refractive index, extinction coefficient, n‑k plane, DLC, Tauc‑Lorentz, Bruggeman EMA, silicon wafer, optical classification, thin film metrology.
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