Refractive index and etched structure of silicon nitride waveguides fabricated by PECVD

Silicon nitride (SiN_X) thin films were deposited by means of an RF plasma enhanced chemical vapor deposition (PECVD) reactor using SiH₄ and N₂ gases. The refractive index of the SiN thin films increased from 1.5652 to 2.7621 as the SiH₄/N₂ flow ratio was increased from 0.16 to 1.66, since the amount of Si in the film increased, while that of N decreased, as the SiH₄/N₂ flow ratio was increased. The core shape became circular after annealing at 1200 °C. This change is related to a decrease in the viscosity with increasing annealing temperature. This decreased viscosity causes condensation of the core layer due to surface tension, which leads to the change in shape from rectangular to circular. The thickness, refractive index and shape of the films were characterized by ellipsometry, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).