Diffusion barrier property of sputtered molybdenum nitride films for DRAM copper metallization was investigated as a function of annealing temperature. Molybdenum nitride thin films on silicon remained stable upon annealing 650°C-30min, but h-MoSi2 and t-MoSi2 were formed after the heat treatment at 700°C and Mo5Si3 phase was formed at 850°C. Increasing the annealing temperature decreased the stress of the γ-Mo2N/Si film down to about 0.8×1010dyne/cm2 at 800°C due to the reduction of the intrinsic stress component. Copper films on silicon substrates separated by thin layers of molybdenum nitride remained stable during the heat treatment at 600°C, but they began to fail as a diffusion barrier after the heat treatment at 650°C, when molybdenum silicides and copper silicide were thought to be formed. On heating, Cu/γ-Mo2N/Si films were affected by thermal stress as due to the thermal expansion coefficient between copper and molybdenum nitride thin films. Furthermore, interlayer interactions between copper and silicon increased with increasing the annealing temperature. The interlayer reactions were investigated by Rutherford backscattering spectrometry, X-ray photoelectron spectrscopy and Nomarski microscopy.