In this paper, we investigate the impact of thermal budget and HfO2 thickness on the chemical and electronic properties of the HfO2/SiO2/Si stack. High temperature anneal at 750 degrees C induces both the regrowth and reoxidation of the SiO2 interfacial layer. A bias drop of 1.1 eV is observed along the whole stack via the C 1s core-level shift and is ascribed both to the HfO2/SiO2 interfacial dipole and to fixed charges in HfO2. Electrical measurements suggest a dipole strength of 0.2 eV. Ellipsometry and UV photoelectron spectroscopy are combined to deduce the HfO2 electron affinity (1.8 +/- 0.2 eV). This value does not change with increasing thermal budget or dielectric thickness. The HfO2/Si barrier height is estimated to be 2.1 +/- 0.2 eV in agreement with previous internal photoemission results.