Thin SiO2 films were deposited by plasma enhanced chemical vapor deposition (PECVD) in N2O/SiH4 mixture at different process conditions. The influences of chamber pressure, substrate temperature, and RF power on growth rate, refractive index, and etch rate of SiO2 films were investigated. Our results indicate that the refractive index and the etch rate vary significantly with the SiO2 thickness for values below 30 nm. This effect is attributed to the formation of non-stochiometric low dense SiO2 films at the onset of deposition. A chemical reaction mechanism model for PECVD is proposed and discussed.