y using an alkoxy silane in which R is an aryl group in the general formula described above. [0053] Further, while N2O was used as the oxidative gas to form the first insulative diffusion barrier layer in this example, there is no particular restriction to this gas but a similar effect can be obtained also by the use of a gas containing oxygen atom such as O2, NO2, NO2, CO, CO2 and H2O. However, u