A semiconductor device according to this invention comprises a substrate 100 in which semiconductor elements are formed, a first conductor 301 at least a portion of the peripheral surface of which is made of a material comprising copper as a main ingredient, and a first insulative diffusion barrier layer 203 covering at least a portion of the first conductor 301. The first insulative diffusion barrier layer 203 is formed by using a gas mixture at least containing an alkoxy silane represented by the general formula (RO).sub.nSiH.sub.4-n (n is an integer in a range from 1 to 3, R represents an alkyl group, an aryl group or a derivative thereof), and an oxidative gas by a plasma CVD. Thus, a semiconductor device comprising copper wiring of high reliability and with less wiring delay time can be provided.