This work investigates the thermal stability of chemically vapor deposited amorphous WSix layers used as a diffusion barrier between Cu and Si substrate, in which the WSix layers were deposited to a thickness of about 50 nm using the SiH4 reduction of WF6 at various SiH4/WF6 how ratios. For 30 min annealing in nitrogen ambient, the effectiveness of the WSix layers as barriers between a copper overlayer and a p(+) -n junction diode decreases as the Si/W atomic ratio, x, increasing from 1 to 1.3. This composition change is obtained by raising the SiH4/WF6 flow ratio from 3 to 50. As deposited, all films are x-ray amorphous. Their resistivity increases roughly linearly with x. The barrier capability of WSix layers can be significantly improved by an in situ N-2 plasma treatment. The N-2 plasma treatment produces a very thin layer of WSiN (about 5 nm) on the surfaces of WSix layers. In particular, the Cu/WSiN/WSix/ p(+) -n junction diodes with the WSix layers deposited with a SiH4/WF6 flow ratio of 3 were able to remain intact up to at least 600 degrees C.