Silicon oxide films were deposited by plasma enhanced chemical vapor deposition on crystalline silicon substrates from a mixture of silane and nitrous oxide. Substrate temperature and [N2O]/[SiH4] precursor gas flow ratio were varied between 200–350 °C and 5.5–110, respectively. Rapid thermal annealing at 900 °C to achieve impurity effusion led to significant changes in the dependence of the mechanical stress with the gas flow ratio R. In situ stress measurements during thermal cycles up to 300 °C as well as measurements of transmission infrared spectra for the as-deposited and annealed samples were performed. The changes in the stress behavior were explained considering the layer shrinkage resulting from the effusion of the different impurities present and the network rearrangement taking place during rapid thermal annealing, for different ranges of flow ratios.