The barrier properties of sputter-deposited tantalum carbide (TaCx) and tungsten carbide (WCx) for Cu metallization were investigated and compared. The incorporation of the C atom is shown to be effective in decorating local defects of the barrier layer and improving the thermal stability of the Cu/barrier/Si contact system. Owing to a higher melting point and better chemical inertness, with respect to Si substrate, the TaCx (600 Angstrom) film shows a better thermal stability (up to 650 degreesC) than that of WCx (similar to 600 degreesC) film based on material analyses, while the highest stable temperature as evaluated by the n(+)p-diode leakage measurement is found to be approximately 550 degreesC for the TaCx and 500 degreesC for the WCx barrier layer. The failure of the TaCx and WCx barriers is found to be mainly due to Cu diffusion through the localized defects of the barrier layers into the Si substrate. The interfacial reaction between the WCx layer and the Si substrate is found to play an additional role in the failure of the WCx films