Hafnium oxide (HfO2) gate dielectric film was prepared by Hf sputtering in oxygen, and the thermal instability of HfO2 was investigated by rapid thermal annealing (RTA) in nitrogen. X-ray photoelectron spectroscopy study reveals that the HfO2 film is thermally unstable at postmetallization annealing temperatures (>500 degreesC). The HfO2 film decomposes and some oxygen atoms are released upon the RTA in nitrogen. In addition, the current-voltage characteristics of the Al/HfO2/Si capacitor are also highly unstable at temperatures higher than 300 K. These observations suggest that although HfO2 has a much higher dielectric constant, it may not be suitable for the gate dielectric application because the postdeposition thermal treatment deteriorates both the physical and the electrical properties of the HfO2 film. (C) 2003 The Electrochemical Society.