This paper investigates the barrier capability of tantalum nitride (TaNx) layers against Cu diffusion. The TaNx layers were reactively sputtered in contact holes to a thickness of 50 nm by using a different nitrogen flow rate. Results indicate that the TaNx layers fail to be a diffusion barrier due to a relative high resistivity for nitrogen flow ratios exceeding 10%. in addition, we found that the phase of alpha -Ta(-N) functions as an effective barrier against Cu diffusion and that Cu/TaN(3-5%)/n(+)-p junction diodes are able to sustain a 30 min furnace anneal up to 500 degreesC without causing degradation of the electrical characteristics. The high-temperature failure of barrier capability for the TaNx layers is due to interdiffusion of Cu and Si across the TaNx film structure to form Cu3Si. The surface roughness and the film structure of TaNx layers determine the ability of Cu and Si interdiffusion.