X-ray diffraction, x-ray photoelectron spectroscopy, and "in situ" sheet resistance measurements were used to study the thermal stability of TiW films as diffusion barriers between a gold overlay and the Ni2S/SiC ohmic contact. The degradation phenomenon is totally different in the O-2 environment as compared to the vacuum ambient. The sheet resistance shows an anomalous behavior with thermal annealing in vacuum from 300 to 580 degreesC, which is correlated to the Ti diffusion through the Au layer. In particular, Ti diffuses through Au grain boundaries at 315 degreesC forming TiOx on the sample surface, while at 400 degreesC, Ti bulk diffusion occurs. The activation energy for titanium diffusion in gold layers is 1.9 +/- 0.2 eV, a typical value for the volume diffusion in a metal layer. Thermal annealing performed in an oxygen environment prevented the Ti grain boundary diffusion at low temperature (until 450 degreesC) while at higher temperatures (higher than 500 degreesC) a complete degradation of the diffusion barrier occurs. (C) 2004 American Vacuum Society.