a structure to enable damascene copper semiconductor fabrication is disclosed . there is a silicon nitride film for providing a diffusion barrier for cu as well as an etch stop for the duel damascene process . directly above the silicon nitride film is a silicon oxynitride film . the silicon oxynitride film is graded , to form a gradual change in composition of nitrogen and oxygen within the film . directly above the silicon oxynitride film is silicon oxide . the silicon oxide serves as an insulator for metal lines . preferably , the film stack of silicon nitride , silicon oxynitride and silicon oxide is all formed in sequence , within the same plasma - processing chamber , by modifying the composition of film - forming gases for forming each film .