This work examines the thermal stability of Ta barrier layer for Cu metallization with the effect of oxygen. The Cu/Ta/SiO2/Si films were annealed at temperatures ranging from 400 to 600 degreesC under various vacuum conditions. Transmission electron microscopy has been performed to characterize the microstructure of the films after annealing. The results show that an amorphous interlayer of oxide between Cu and Ta can be formed at 400 degreesC in a vacuum of 10(-2) mbar. X-Ray energy dispersive spectroscopy and electron energy loss spectroscopy confirm that this interlayer is tantalum oxide. This interlayer transformed into a crystalline phase of Ta-Cu oxide at 600 degreesC. In addition, formation of tantalum oxide interlayer is more thermodynamically favorable than that of copper oxide layer at the Cu/Ta interface. Growth of the amorphous interlayer is atmosphere-dependent, as evidenced by the thickness of tantalum oxide being decreased with better vacuum or argon gas. This observation suggests that the oxidation source may arise from the annealing atmosphere rather than from interior SiO2. Furthermore, it has been observed that oxygen diffuses along grain boundaries in copper films to cause tantalum oxidation.