Alloy films of Tal,W,, were prepared by co-sputtering and their diffusion-barrier properties in Cu/Si contacts were examined. The alloy films over 30 at. % of the W composition were in a state of substitutional solid solution, and the resistivity of 100-nm-thick film was similar to45 muOmega cm, at most. In the Cu/Ta1-xWx/Si contacts, the silicidation temperature of 680 degreesC was higher than that reported for the Cu/Ta/Si or Cu/W/Si contact. We confirmed for the Cu/Ta0.5W0.5/Si contact that Cu penetration into the Si substrate was completely suppressed by a thin amorphous interfacial layer upon annealing at temperatures below that of silicidation reaction. It was revealed that at more elevated temperatures, an extremely small amount of Cu atoms was incorporated into the silicide layer at the Ta0.5W0.5/Si interface during reaction and penetrated into the Si substrate through the silicide layer. (C) 2005 American Vacuum Society.