This work investigated the thermal stability, of tungsten carbide (WCx) films deposited by a sputtering process with a WC target as diffusion barrier layer between Cu and Si. The as-deposited WCx film has a nanocrystalline structure and a low electrical resistivity of around 227 mu Omega cm. Film characterization reveals that the WCx film was able to preserve the integrity of the Cu (2000 Angstrom)/WCx (500 Angstrom)/n-Si structure without formation of Cu3Si, up to a 600 degreesC annealing for 30 min. In addition, diode leakage current measurements on the same contact structure, but with a p(+)n-Si substrate did not show deterioration of electrical characteristics up to a 550 degreesC annealing. As. the thickness of the WCx barrier was reduced to 150 Angstrom, the WCx film retained the integrity of diodes up to 500 degreesC without increasing the diode leakage current. The failure of WCx film after high temperature annealing is attributed to the Cu diffusion into the Si substrate through grain boundaries or local defects of the WCx barrier layer, in which some local defects may arise from the formation of W5Si3.