This work investigates the barrier capability of W layers as well as WSiN/WSix/W stacked layers against Cu diffusion. The VV layers were selectively chemical vapor deposited (CVD) in contact holes to a thickness of about 450 nm using SiH4 reduction of WF6. We found that the CVD-W layers functioned as effective barriers against Cu diffusion, and the Cu/W(450 nm)/p(+)-n junction diodes were able to sustain a 30 min furnace annealing up to 650 degrees C without causing degradation in electrical characteristics. The use of WSiN/WSix/W stacked layers as diffusion layers further improved the thermal stability of Cu/WSiN/WSix/W(450 nm)p(+)-n junction diodes to at least 700 degrees C. The WSix layers were deposited by CVD to a thickness of 75 nm using SiH4/WF6 chemistry, and the subsequent in situ N-2 plasma treatment produced a very thin layer of WSiN on the WSix surface. This thin WSiN layer was very thermally stable and effective in suppressing Cu diffusion, failure of barrier capability for the W films was presumably due to interdiffusion of Cu and Si along grain boundaries of the W films, and the interdiffusion was probably enhanced by the formation of WSi2, The formation of WS(i)2 consumed the W layer and Si substrate, resulting in a volume change in barrier layer, which, in turn, developed local defects, such as microcracks and stress-induced weak points, and thus provided fast paths for Cu diffusion.