Interactions of Cu films with Si substrates separated by thin layers of molybdenum and molybdenum nitride were investigated in the viewpoint of diffusion barrier to copper. the diffusion barrier behavior of the layers was studied as functions of deposition and annealing conditions by cross-sectional transmission electron microscopy and Nomarski microscopy. the layers deposited at $N_2$ gas ratios of 0.4 and 0.5 exhibited good diffusion barrier behaviors up to $700^{circ}C$, mainly due to the phase transformation of molybdenum to $gamma$-Mo$_2$N phase. The increase in the N gas ratio in deposition elevates the lower limit of barrier failure temperature. Futhermore, amorphous molybdenum nitride films deposited at 20$0^{circ}C$ and 30$0^{circ}C$ did not fail, while the crystalline $gamma$-Mo$_2$N films deposited at 40$0^{circ}C$ and 50$0^{circ}C$ showed signs of interlayer interactions between Cu and Si after annealing at 75$0^{circ}C$ for 30 minutes. Therefore, the amorphous nature of the molybdenum nitride layer enhanced its ability to reduce Cu diffusion and its stability as a diffusion barrier at elevated temperatures.