It is proposed that the plasma enhanced chemical vapor deposited tungsten nitride (PECVD-W-N) thin film is used as a diffusion barrier to prevent the interdiffusion between Al and Si during postannealing process. The atomic concentration of N in W-N film deposited with NH3/WF6 partial pressure ratio of 0.5 is about 33 at. % and its resistivity is 90-110 muOMEGA cm. The Rutherford backscattering spectrometry, Auger electron depth profiles, x-ray diffraction, and scanning electron micrographs show that 900 angstrom PECVD-W67N33 film interposed between Al and Si is more impermeable than PECVD-W film due to N atoms and it also keeps its chemical integrity during the postfurnace annealing at 600-degrees-C for 30 min in Ar ambient.