The effect of fluorine incorporation on the electrical properties of HfO2 gate oxide were investigated, especially on the frequency dispersion, hysteresis and the density of interface states. By treating HfO2 films using octafluorocyclobutane (C4F8) 60 MHz/2 MHz dual-frequency capacitively coupled plasmas, fluorine atoms were incorporated into the HfO2 films, but thinner C:F films also deposited on the surface of the HfO2 films. After a following thermal annealing, the C:F films were removed, accompanied the formation of the C-C group and Hf-F bonds. By optimizing the low frequency (LF) power, the appropriate fluorine incorporation significantly improved the quality of the gate oxide, resulting in excellent electrical properties. At the LF power of 30 W, the smallest Delta V-fb, hysteresis and the lowest interface state density were obtained. These improvements were attributed to the passivation of oxygen vacancies and the reduction of defects states density in the gap by forming Hf-F bonds. (C) 2014 Published by Elsevier B.V.