Thin films of silicon oxide were prepared on copper substrates using several techniques, such as DC sputtering, sol-gel and microwave decomposition of gases (SiH4 and O-2). The sol-gel films were made using a conventional mixture of tetraethoxysilane (TEOS), water and ethanol, with various TEOS/water ratios. The oxygen diffusion coefficient is found from the thickness of the copper oxide layer formed at the copper-SiO2 interface, which was estimated from reflectivity and Auger spectroscopy measurements. Thermal treatments at various temperatures allowed us to determine the temperature dependence of the diffusion coefficient. It is found that the diffusion coefficient depends on the method of preparation of the SiO2 layer, which could be related to differences in the structure of the oxide layer. This conclusion is confirmed by the dielectric breakdown measurements. It follows from the results obtained that the oxygen diffusion constants and the dielectric breakdown field could be the indicators of the structural quality of silica.