The effects of CeO2 addition on the barrier properties of a Ta diffusion barrier were investigated by using four point probe, optical microscopy, Auger electron microscopy, x-ray diffraction spectroscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy. The amorphous microstructure of the Ta+CeO2 diffusion barrier was up to high temperatures and the thermal stability of the Ta+CeO2/Si interface was higher than that of Ta/Si interface. The Cu/Ta+CeO2/Si system retained its structure up to 800 degrees C without any increase in resistivity, whereas the Cu/Ta/Si structure degraded after annealing at 550 degrees C. The cerium dioxide (CeO2) in the Ta layer plays a role in amorphizing the microstructure of the Ta+CeO2 barrier film and strongly binds the Ta-Ce-O system during deposition of the Ta layer. It appeared that the thermal stability of Cu/Ta+CeO2 interface as well as that of the Ta+CeO2/Si interface was much higher than that of both the Cu/Ta and the Ta/Si interfaces. Therefore, the Ta film prepared with the addition of CeO2 effectively prevented the interdiffusion of Cu and Si through the diffusion barrier up to 800 degrees C.