Optical Characterization of Thin Films

Importance of the Topic

Thin films and multilayer optical coatings underpin a wide range of applications in optics, photonics, and electronics. Precise knowledge of their thickness and refractive index is critical for device performance, quality control, and process optimization. Multi‐angle spectral photometry enriches conventional normal‐incidence measurements by providing additional angular‐dependent data, enabling more robust reverse‐engineering of complex coatings.

Study Objectives and Overview

This application note demonstrates the use of a Universal Measurement Accessory (UMA) on an Agilent Cary 5000 UV‐Vis‐NIR spectrophotometer to:

• Obtain multi‐angle transmittance and absolute reflectance data at controlled incidence angles and polarization states.
• Perform optical characterization of single‐layer dielectric films (Ta2O5, SiO2) and multilayer quarter‐wave mirrors.
• Evaluate the accuracy and consistency of reverse‐engineering thin‐film stacks, including dense magnetron‐sputtered and inhomogeneous e‐beam evaporated layers.

Methodology and Instrumentation

Instrumentation:

• Agilent Cary 5000 UV‐Vis‐NIR spectrophotometer
• Universal Measurement Accessory (UMA) for variable‐angle specular reflectance and transmission control

The UMA allows full software control of sample, detector, and polarizer positions. Measurements were conducted in the 300–2,500 nm range, with primary data from 330–1,100 nm to avoid substrate absorption artifacts.

Methodology:

• Multi‐angle measurements at 7°, 10°, 20°, 30°, and 40° for s- and p-polarized light.
• Data acquisition of %T and absolute %R at each angle, followed by reverse‐engineering algorithms to extract layer thicknesses and refractive indices.
• Samples included magnetron‐sputtered Ta2O5 and SiO2 single films, a 15‐layer quarter‐wave mirror with intentional thickness errors, and e‐beam evaporated HfO2 and SiO2 layers.

Main Results and Discussion

• Single‐layer films: Ta2O5 and SiO2 thickness and refractive index at 600 nm showed deviations below 0.1% across angles and polarizations, confirming high measurement precision.
• Multilayer mirror: Intentional thickness offsets of +5%, +7%, −5%, and +5% in selected layers were reliably detected, demonstrating the method’s sensitivity (Figure of error bars showed agreement with planned deviations).
• HfO2 films: Reverse‐engineering of e‐beam evaporated HfO2 revealed refractive index variations (<0.5%) consistent with literature references on thickness‐dependent crystallinity.
• SiO2 layers: Index offsets of 1.5–1.7% relative to nominal values were observed, indicating reproducible characterization of evaporated films.

Benefits and Practical Applications

• Enhanced accuracy and self‐verification through multi‐angle, polarization‐resolved data.
• Improved feedback for coating process control, monitoring, and calibration in production environments.
• Capability to characterize challenging films, such as inhomogeneous or electron‐beam deposited layers.
• Rapid and automated measurement workflows with full software control of all optical components.

Future Trends and Possibilities

• Integration of multi‐angle photometry with in-line process monitoring for real‐time feedback in deposition tools.
• Application of machine‐learning algorithms to accelerate reverse‐engineering and detect subtle coating defects.
• Expansion to other spectral ranges (e.g., mid-IR) and advanced materials such as nanocomposites or metamaterials.
• Development of high-throughput and imaging‐capable accessories for spatially resolved thin‐film analysis.

Conclusion

Multi‐angle spectral photometry using the Agilent UMA on a Cary 5000 provides accurate, reproducible characterization of thin films and complex multilayer coatings. The method delivers richer experimental data than standard normal‐incidence measurements and proves effective for reverse‐engineering and process optimization across a variety of film types.

References

1. Tikhonravov A.; et al. Optical Characterization and Reverse‐engineering Based on Multiangle Spectroscopy. Applied Optics 2012, 51(2), 245–254.
2. Tikhonravov A.; et al. Optical Parameters of Oxide Films Typically Used in Optical Coating Production. Applied Optics 2011, 50, C75–C85.
3. Modreanu M.; et al. Solid Phase Crystallisation of HfO2 Thin Films. Materials Science and Engineering B 2005, 118, 127–131.
4. Modreanu M.; et al. Investigation of Thermal Annealing Effects on Microstructural and Optical Properties of HfO2 Thin Films. Applied Surface Science 2006, 253, 328–334.