The barrier properties of W, W2N and WN layers to prevent intermixing of Cu wiring with the Si substrate were investigated. W, W2N and WN barrier layers 25 nm thick were found to prevent intermixing after annealing at 650, 790 and 500 degrees C for 30 min, respectively. W2N, which showed the best barrier property, had a polycrystalline structure with disordered gain boundaries, and Cu diffusion in W2N was concluded to be controlled by grain boundaries of the W2N layer based on the diffusional analysis of Cu in W2N.