Optical Characterization of Silicon Nitride-based Bilayer Thin Films Using Spectroscopic Ellipsometry and the Maxwell-Garnett Model

Abstract

This study investigates the optical and geometrical properties of silicon oxynitride (SiO_xN_y) thin films, which were prepared using a low-pressure chemical vapor deposition (LPCVD) process at 850 °C from SiH₂Cl₂, N₂O, and NH₃ as precursors. The opto-geometrical parameters of these were determined through spectroscopic ellipsometry measurements, employing the Maxwell-Garnett model for data analysis. This model posits silicon oxynitride as a heterogeneous medium composed of silicon oxide (SiO₂) and silicon nitride (Si₃N₄). The refractive index of SiO_xN_y was computed as a function of both wavelength and gas flow ratio NH₃/N₂O, and was further evaluated based on the volume fraction of its constituents and film thickness. Subsequently, the effect of deposition time on film thickness and refractive index was examined. It was observed that the refractive index decreases with increasing wavelength, yet increases with both the gas flow ratio and film thickness, ranging from 1.457 to 1.618. The results also indicate that the SiO₂ volume fraction decreases as the NH₃/N₂O ratio increases, while the Si₃N₄ volume fraction concurrently increases. They equally indicate that prolonged deposition time leads to a significant increase in film thickness and a higher refractive index. This study provides a fundamental basis for selecting deposition parameters that are tailored to specific applications.