This work studies the thermal stability of Cu/WSx/p(+)-n and Cu/WSiN/WSix/p(+)-n diodes in which the WSix barrier layers were deposited by chemical vapor deposition to a thickness of about 50 nm using SiH4/WF6 chemistry with the SiH4/WF6 flow rates of 6/2 seem, while the WSiN layers were formed by in situ N-2 plasma treatment on the chemically vapor deposited WSix (CVD-WSix) surfaces. Without N-2 plasma treatment, the thermal stability of Cu/WSix (50 nm)/p(+)-n junction diodes was found to reach 500 degrees C; with N-2 plasma treatment, the resultant Cu/WSiN/WSix (50 nm)/p(+)-n junction diodes were able to retain integrity of electrical characteristics up to at least 600 degrees C. Failure mechanism of the WSiN/WSix bilayer in the Cu/WSiN/WSix/Si structure was closely related to the WSix/Si interface reaction and the tungsten silicide formation of the WSix layer. Thus, barrier capability of the WSiN/WSix bilayer can be further improved by suppressing the WSix/Si interface reaction and the silicidation of the WSix layer. The thermal stability of Cu/barrier/p(+)-n diodes was further raised to 650 degrees C by using a multilayer barrier structure of WSiN/Si-x (50 nm)/WSiN/WSix (10 nm) or a WSiN/WSiy (y > 1) barrier We conclude that the post-CVD-WSix treatment with in situ N-2 plasma is a simple, practical, and efficient method of improving the WSix-based barrier capability for Cu metallization.