Reactively sputtered WNx thin films have been studied as a diffusion barrier between Cu and Si. By changing the ratio of N-2 to Ar + N-2 gas flow rate at a total pressure of 5 mTorr with RF power of 200 W, WNx films with seven different compositions were deposited. Amorphous WNx structure was obtained for the films below 32% nitrogen content, and polycrystalline W2N structure above that. After annealing of Si/WNx/Cu samples in 10% H-2/Ar forming gas ambient for 1 h, X-ray diffraction, scanning electron microscopy, Anger electron spectroscopy and sheet resistance measurements were employed to characterize barrier performance. Amorphous WNx with thickness of 100 nm maintained its barrier property up to 800 degrees C, while poly-W2N failed at temperature lower than 800 degrees C because of the release of excess nitrogen. Even though the thickness of WNx barrier was reduced to 5 nm, amorphous WNx films maintained the Sin/WNx/Cu structure up to 600 degrees C for 1 h. Among the amorphous WNx films, W68N32 was the most stable composition for various thicknesses.