The electrical properties of a Ta layer prepared with and without RuO2 addition were investigated. The Ta + RuO2TiSi2/poly-Si/SiO2/Si contact system exhibited lower total resistance and ohmic characteristics up to 800 degrees C. Meanwhile, the Ta/TiSi2/poly-Si/SiO2/Si contact system showed higher total resistance and nonohmic behavior after annealing at 650 degrees C, attributed to the oxidation of both Ta and TiSi2 layers, In the former case, a Ta + RuO2 diffusion barrier showed an amorphous Ta microstructure and embedded RuOx nanocrystals in the as-deposited state. The conductive RuO2 crystalline phase in the Ta + RuO2 film was formed by reaction between the nanocrystalline RuOx and oxygen indiffused from air during annealing, When the Ta layer was deposited with RuO2 addition, therefore, both the electrical properties and the oxidation resistance of the Ta + RuO2 diffusion barrier were better than those of TiN, TaN, and Ta-Si-N barriers.