Silicon oxynitride films were deposited using ammonia as a nitrogen source via PECVD (plasma enhanced chemical vapor deposition) to study the physical properties of the films. Silane and nitrous oxide were used as silicon and oxygen sources, respectively. The composition of the silicon oxynitride films was well controlled by changing the ratios of the sources and confirmed by XPS. The silicon oxynitride films with high oxygen content showed bigger compressive stress and less refractive index, while the values of surface roughness were around 1 nm, irrespective of the variation of the source ratios.