New Optical Modeling Method Advances Thin Film Analysis Using Spectroscopic Ellipsometry

A recent study investigated a new method that improves the performance capabilities of spectroscopic ellipsometry when analyzing the physicochemical properties of thin films. Published in Optical Materials and led by Yong Liu and Sainan Ma of Zhejiang University, the study showed how advanced optical modeling can improve the ability of spectroscopic ellipsometry in material science applications (1). The new method serves as another piece of evidence that technological innovations are leading to improvements in spectroscopic methods and developing even more versatile, accurate, and non-destructive techniques.

Spectroscopic ellipsometry is a technique routinely used in material science. It is designed to be very sensitive to film thickness, which makes it useful for ultra-thin film applications (2). Spectroscopic ellipsometry not only measures film thickness but also provides highly precise, repeatable measurements of a sample's dielectric properties and optical constants, with a refractive index resolution as fine as 2 × 10⁻³ for certain samples (2). However, despite its usage, its application in analyzing non-stoichiometric and complex materials like amorphous silicon oxide (SiO_x, where 0 ≤ x ≤ 2) has remained limited because of challenges in modeling heterogeneous structures (1).

In their study, the research team attempted to fix this problem by developing a new modeling framework to enhance the ability of spectroscopic ellipsometry. To address this, Liu and Ma’s team proposed a new modeling framework that integrates the Tauc-Lorentz and Bruggeman effective medium approximation (BEMA) models to significantly expand the utility of spectroscopic ellipsometry (1).

The researchers began their study by depositing a series of amorphous silicon (a-Si), silicon dioxide (SiO₂), and SiO_x thin films using mid-frequency magnetron sputtering. Mid-frequency magnetron sputtering is a widely used technique in thin film fabrication in optoelectronics (3). They varied the oxygen partial pressures and sputtering powers to produce films with different compositions, which allowed them to investigate the influence of deposition conditions on the optical and structural properties of the films (1).

Both the Tauc-Lorentz model and the BEMA model were used to help determine key parameters such as refractive index, extinction coefficient, and film thickness. The Tauc-Lorentz model was first applied to extract the optical constants (refractive index n and extinction coefficient k) of the pure a-Si and SiO₂ films, which helped interpret the spectroscopic ellipsometry data accurately (1). These constants served as the foundation for the subsequent application of the BEMA model, which treated the SiO_x films as mixtures of the two reference materials (1). This modeling approach enabled the researchers to precisely characterize the composition, thickness, and optical behavior of the non-stoichiometric SiO_x films. More importantly, it provides insights into the structural composition of SiO_x films, which has traditionally been difficult to obtain using spectroscopic ellipsometry alone (1).

To ensure the reliability of their results, the research team relied on several spectroscopic techniques, including profilometry, UV–vis–NIR spectrophotometry, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). These independent measurements confirmed the accuracy of the optical constants and film compositions derived from the spectroscopic ellipsometry models (1).

In addition to demonstrating the feasibility of the method, the study provided critical insights into how deposition parameters influence film properties. The researchers found that high sputtering power and low oxygen partial pressure favored the formation of hypoxic, sub-stoichiometric SiO_x films (1). These findings are especially valuable for tailoring thin film characteristics in applications ranging from electronics to photovoltaics.

Beyond its implications for materials characterization, the study underscores the growing potential of spectroscopic ellipsometry as a non-invasive, in situ monitoring tool during thin film fabrication (1). By enabling real-time analysis of film growth, the technique could help researchers and engineers optimize materials in semiconductor manufacturing, optical coatings, and flexible electronics (1).

This advancement represents a major step forward in thin film analysis, expanding the utility of spectroscopic ellipsometry from a tool for simple parameter measurement to a comprehensive method for understanding complex material systems.

References

1. Ge, F.; Wang, L.; Han, G.; et al. Optical Modeling of SiO_x Thin Films for Physicochemical Property Measurement by Spectroscopic Ellipsometry. Opt. Mater. 2025, 164, 117044. DOI: 10.1016/j.optmat.2025.117044
2. Bruker, Spectroscopic Ellipsometry. Bruker. Available at: https://www.bruker.com/en/products-and-solutions/test-and-measurement/ellipsometers-and-reflectometers/spectroscopic-ellipsometers.html (accessed 2025-04-29).
3. Yin, M. L.; Zou, C. W.; Li, M.; Liu, C. S.; Guo, L. P.; Fu, D. J. Middle-frequency Magnetron Sputtering for GaN Growth. Phys. Res. B: Beam Inter. Mater. Atoms 2007, 262 (2), 189–193. DOI: 10.1016/j.nimb.2007.05.034