SiN thin films having excellent surface morphology for the optical device application were synthesized using a plasma enhanced chemical vapor deposition (PECVD) method at low temperature (350∘C) using silane (SiH4) and nitrogen (N2). The effects of the SiH4/N2 flow ratio, rf power and annealing on the SiN films were investigated. The optical and structural properties of SiN films were characterized using an ellipsometry, a fourier-transform infrared spectroscopy (FT-IR), and an atomic force microscope (AFM). The refractive index increased from 1.6 to 2.3 as the SiH4/N2 ratio was increased from 0.17 to 1.67. The rms surface roughness decreased from 14.1 to 3.6Å after post-deposition annealing process performed at 800∘C for 1hr in an air ambient. We could fabricate straight waveguides based on a three layer structure and have no problems with step coverage.