Thermal stabilities of Cu-contacted n+–p junctions with tungsten nitride (WNx) diffusion barriers deposited at various nitrogen flow ratios are investigated. N2O plasma treatment is applied to improve thermal stability and barrier performance of WNx film. Sheet resistance of Cu/N2O plasma-treated WNx/Si is fairly stable even after annealing at 750 °C for 30 min. Moreover, N2O plasma treatment enables the Cu/WNx/n+–p junction diodes to sustain thermal annealing at 600 °C without electrical degradation. Auger electron spectroscopy depth profiles show that Cu diffusion through the N2O plasma-treated WNx barrier is extremely limited, even after annealing at 675 °C. Analyses of transmission electron microscopy and x-ray photoemission spectroscopy show that nitridation and oxidation on the WNx barrier occur and an amorphous layer is formed after N2O plasma treatment. © 2004 American Vacuum Society.