Optical Characteristics and Thickness of 2-layered Structures

Importance of the Topic

Multilayer optical coatings are central to applications across ultraviolet to infrared regions, serving in telecommunications, sensors, and protective layers. Accurate knowledge of film thickness, refractive index, and extinction coefficient is critical for optimizing performance and reliability.

Objectives and Study Overview

This study aims to determine thickness (d), refractive index (n), and extinction coefficient (k) of two layered thin-film structures using a non-destructive multi-angle spectrophotometric approach on an Agilent Cary 5000 system.

Methodology

The double-angle method was applied by measuring transmittance for transparent quartz samples at 0° and 20°, and reflectance for opaque silicon-based samples at 6° and 20°. Wavelengths corresponding to identical interference peaks at each angle were extracted and processed through established equations to calculate n and d. Optical homogeneity and sample preparation conditions were controlled to minimize measurement uncertainty.

Instrumentation Used

• Agilent Cary 5000 spectrophotometer with Universal Measurement Accessory (UMA)
• Absolute reflectance (5°–85°) and transmittance (0°–85°) capabilities with 0.02° step resolution
• Wavelength range: 190–2800 nm (unpolarized) and 250–2500 nm (s- and p-polarized)

Main Results and Discussion

Nanocomposite Zr-Si-B-(N) films on quartz exhibited refractive indices from 2.64 to 2.17 across 470–955 nm, with an optical thickness corresponding to ~1380 nm (±5 percent). LiNbO3 layers on silicon showed n values from 2.58 to 2.09 over 544–709 nm and physical thickness around 250 nm (±5 percent). Dispersion trends for both structures were mapped, revealing a typical decrease of n with increasing wavelength. Film heterogeneity detected by optical microscopy contributed to an n uncertainty of ±0.01.

Advantages and Practical Applications

• Non-destructive simultaneous measurement of reflectance and transmittance at multiple angles and polarizations
• Automated unattended operation over a wide spectral range
• Elimination of sample repositioning and accessory changes
• Preliminary n and d evaluation to guide detailed ellipsometric modeling

Future Trends and Potential Applications

Integration of multi-angle spectrophotometry with advanced inverse modeling for real-time process control and inline quality assurance.
Extension to absorbing or graded-index films and incorporation into machine-learning frameworks for predictive coating design.

Conclusion

The Cary 5000 with UMA provides a robust, non-invasive approach for rapid determination of thin-film optical parameters. The double-angle spectrophotometric method yields refractive index accuracy of ±0.01 and practical thickness estimates, supporting efficient coating development workflows.

References

1. Arndt D.P. et al. Multiple determination of optical constants of thin-film coating materials. Appl. Opt. 23(20):3571–3596 (1984).
2. Tikhonravov A.V. et al. Optical parameters of oxide films in coating production. Appl. Opt. 50(9):C1–C12 (2011).
3. Tikhonravov A.V. et al. Optical characterization based on multiangle spectroscopy. Appl. Opt. 51(2):245–254 (2012).
4. Shih W.-Ch. et al. Growth of c-axis LiNbO3 films by pulsed laser deposition. Jpn. J. Appl. Phys. 47(5):4056–4059 (2008).
5. Simoes A.Z. et al. Influence of thickness on LiNbO3 thin films. Mater. Charact. 50:239–244 (2003).
6. Ayupov B.M. et al. Inverse problems in ellipsometry and spectrophotometry. J. Opt. Technol. 78(6):350–354 (2011).
7. Kirikhanstev-Korneev F.V. et al. Nanocomposite coatings Zr-Si-B-(N): structure and properties. Proc. VII Int. Conf. 2017, p.104.
8. Kozlova N.S. et al. Spectrophotometric determination of optical parameters of LiNbO3 films. Modern Electron. Materials 3:122–126 (2017).
9. Zhukov R.N. et al. Synthesis and properties of LiNbO3 thin films for nanogradient structures. PIERS Proc:98–101 (2013).