This work studied the barrier capability of sputter-deposited Mo and reactively sputter-deposited Mo-N layers against Cu diffusion in a device structure of Cu/barrier/p(+)n junction diodes. With a 500 Angstrom thick Mo film as barrier layer, the Cu/Mo/p(+)n junction diodes were capable of withstanding 30 min of thermal annealing at temperatures up to 500 degrees C without causing degradation to the devices electrical characteristics. The incorporation of nitrogen in the Mo layer improved the barrier capability of the metal layer. In particular, the Mo-N films sputter-deposited in a gas mixture of Ar and N-2 with Ar/N-2 flow rates of 24/8 to 24/12 seem were found to possess the best barrier capability. With a 500 Angstrom thick Mo-N barrier layer, the thermal stability of Cu/Mo-N/p(+)n junction diodes were found to reach 600 degrees C. At temperatures exceeding the thermal stability limit, we presumed that the failure of Cu/barrier/p(+)n junction diodes arose from two mechanisms: the grain boundary diffusion for the Mo film and the Mo-N films deposited with small N-2 flow rates, and the defects diffusion for the Mo-N barriers deposited with large N-2 flow rates. For the Mo-N films deposited in a gas mixture of medium nitrogen content (e.g. N-2 flow rate of 8 to 12 sccm), both of the failure mechanisms might exist simultaneously.