To improve the dielectric properties of sputter-deposited hafnium oxide (Hf O2) films, the supercritical C O2 (SCC O2) fluid technology is introduced as a low temperature treatment. The ultrathin Hf O2 films were deposited on p -type (100) silicon wafer by dc sputtering at room temperature and subsequently treated with SCC O2 fluids at 150 °C to diminish the traps in the Hf O2 films. After SCC O2 treatment, the interfacial parasitic oxide between the Si substrate and Hf O2 layer is only about 5 Å, and the oxygen content of the Hf O2 films apparently increased. From current-voltage (I-V) and capacitance-voltage (C-V) measurements, the leakage current density of the SCC O2 -treated Hf O2 films is repressed from 10-2 to 10-7 A cm2 at electric field=3 MVcm due to the reduction of traps in the Hf O2 films. The equivalent oxide thickness also obviously decreased. Besides, the efficiency of terminating traps is relative to the pressure of the SCC O2 fluids.