As shown in FIG. 23A, it was found that (W+N) and Si were detected in addition to W in a range of 100 nm of the sample surface formed of a W layer after the heat treatment at 800??? C. for 30 minutes, but diffusion of the impurity As in polysilicon into W was sufficiently suppressed in a portion except the surface portion since a WSix Ny diffusion preventing layer was formed on the interface betwe