Spectroscopic ellipsometry is gaining attention as a non-destructive and rapid method for evaluating multilayer semiconductor thin film structures. Compared to traditional reflectance methods and broadband ellipsometry, spectroscopic ellipsometry offers several advantages, including the ability to evaluate multilayer structures (complex dielectric functions and film thickness), interface roughness, and film density. The technique involves measuring the polarization state changes of light reflected from the sample surface at multiple wavelengths. The measurement range is typically 0.25 to 0.85 μm, with about 100 measurement points. The measured quantities are the amplitude reflection ratio (tan ψ) and phase difference (cos Δ), which are functions of the complex dielectric function and film thickness. For amorphous silicon (a-Si) films, the complex dielectric function can be determined using the Bruggeman approximation, which relates the effective dielectric function of a heterogeneous layer to the individual components' dielectric functions and volume fractions. The measured values of tan ψ and cos Δ are fitted to these theoretical values using least squares regression, with the Marquardt method improving the accuracy of the fitting. The evaluation of a-Si film structures reveals that the complex dielectric function decreases at lower energies as the film thickness decreases. This is attributed to the presence of an interface layer (5-8 nm thick) and an a-Si layer (50 nm thick). As the film thickness increases, additional surface roughness (about 10 nm) is observed. Regression analysis of a 16.8 nm thick sample showed that a three-layer model with a 9.0% void in the first a-Si layer and no void in the second layer provided the best fit. For thicker films (300 nm and beyond), periodic oscillations in cos Δ at lower energies were observed, indicating the presence of a two-layer structure. The three-layer model with a 10.6% void at the interface between the a-Si layer and the c-Si substrate provided the best fit. Spectroscopic ellipsometry is expected to become widely used in both research and quality control due to its non-destructive nature and the development of new regression algorithms and software. It is particularly effective for analyzing multilayer structures in amorphous materials and III-V compound semiconductors. Future research directions include in-situ measurements of a-Si film growth and the extension of the technique to the infrared domain.Spectroscopic ellipsometry is gaining attention as a non-destructive and rapid method for evaluating multilayer semiconductor thin film structures. Compared to traditional reflectance methods and broadband ellipsometry, spectroscopic ellipsometry offers several advantages, including the ability to evaluate multilayer structures (complex dielectric functions and film thickness), interface roughness, and film density. The technique involves measuring the polarization state changes of light reflected from the sample surface at multiple wavelengths. The measurement range is typically 0.25 to 0.85 μm, with about 100 measurement points. The measured quantities are the amplitude reflection ratio (tan ψ) and phase difference (cos Δ), which are functions of the complex dielectric function and film thickness. For amorphous silicon (a-Si) films, the complex dielectric function can be determined using the Bruggeman approximation, which relates the effective dielectric function of a heterogeneous layer to the individual components' dielectric functions and volume fractions. The measured values of tan ψ and cos Δ are fitted to these theoretical values using least squares regression, with the Marquardt method improving the accuracy of the fitting. The evaluation of a-Si film structures reveals that the complex dielectric function decreases at lower energies as the film thickness decreases. This is attributed to the presence of an interface layer (5-8 nm thick) and an a-Si layer (50 nm thick). As the film thickness increases, additional surface roughness (about 10 nm) is observed. Regression analysis of a 16.8 nm thick sample showed that a three-layer model with a 9.0% void in the first a-Si layer and no void in the second layer provided the best fit. For thicker films (300 nm and beyond), periodic oscillations in cos Δ at lower energies were observed, indicating the presence of a two-layer structure. The three-layer model with a 10.6% void at the interface between the a-Si layer and the c-Si substrate provided the best fit. Spectroscopic ellipsometry is expected to become widely used in both research and quality control due to its non-destructive nature and the development of new regression algorithms and software. It is particularly effective for analyzing multilayer structures in amorphous materials and III-V compound semiconductors. Future research directions include in-situ measurements of a-Si film growth and the extension of the technique to the infrared domain.