In this work, HfO2 was deposited by r.f. sputtering at room temperature and then annealed for different times at 200 degrees C in a forming gas atmosphere. After annealing for 2 hours the HfO2 layers present a reduction on the flat band voltage of about 1 V, relatively to the as deposited film, decreasing from -2.23V down to -1.28 V. This means an improvement of the interface properties and a reduction on the oxide charge density from 1.33x10(12) cm(-2) to 7.62x10(12) cm(-2). The dielectric constant reaches a maximum of 18.3 after 5h annealing due to film's densification. When annealing for longer times such as 10h a small degradation of the electrical properties is observed. After 10h annealing the dielectric constant, flat band voltage and fixed charge density are respectively, 14.9, -2.96 V and 1.64x10(12) cm(-2) and the leakage current also increases due to film's crystallization.