Without doping,plasma enhanced chemical vapor deposition (PECVD) of silica films on Si substrates with gas mixtures of SiH_4 and N_2O is considered. Various factors affecting the refractive index and the deposition rate of the deposited film are studied in order to optimize the growth process of the films. The microstructures and optical properties of the films are examined by a prism coupler,a Fourier transform infrared spectroscopy (FTIR) and an atom force microscopy (AFM). The results show that a thick film with a uniform surface and small loss for infrared light can be rapidly deposited by the PECVD technology and refractive index of the film can be controlled accurately.