Thin-Film properties and barrier effectiveness against copper (Cu) diffusion of a thin amorphous WSix layer were investigated. The amorphous WSix layer was deposited by the chemical vapor deposition (CVD) method using the SiH4/WF6 chemistry with the activation energy determined to be 3.0 kcal/mol. The CVD-WSix film has a low film stress, low electrical resistivity, and excellent step coverage. The resistivity of the amorphous CVD-WSix layer increases with the deposition temperature, but the residual stress of the layer decreases with the deposition temperature. The WSix/Si structure is thermally stable up to at least 600 degrees C, while the copper-contacted Cu/WSix/Si structure with a 50 nm thick WSix barrier is stable only up to 550 degrees C. Moreover, the Cu/WSix/p(4)-n junction diodes can sustain a 30 min thermal annealing up to 500 degrees C without causing degradation in electrical characteristics. Barrier failure of the WSix layer in the Cu/WSix/Si structure at temperatures above 550 degrees C is attributed to Cu atoms diffusion via fast paths in the WSix layer. These fast paths were presumably developed from grain growth of the WSix layer and/or thermal-stress-induced weak points in the WSix layer.