Passivation process is a key feature to improve the efficiency of silicon solar cells. So far, a 20nm-thick layer of Al2O3 grown by Plasma Enhanced Atomic Layer Deposition (PE-ALD) followed by a 450°C anneal during 15 to 30 min gives the best results in surface carrier recombination velocities on p-type silicon. In this study we used chemical (XPS profiling, SIMS), and electrical (C-V, QSS-PCD) characterisation to obtain a better understanding of the passivation activation process. A 20nm-thick Al2O3 film is deposited on 200µm Cz p-type c-Si (100) wafers with a resistivity of 5Ω.cm by PE-ALD. Starting from as-deposited state to anneal time up to 1 hour, the evolution of chemicals components at the interface Al2O3/Si is recorded in conjunction with macroscopic parameters (Density of effectives charges (Qeff), Density of interface defects (Dit) and carrier lifetime). First XPS analyses show that the SiO2 interface layer is not the driving parameter of a good passivation since it is not affected by annealing. However chemical bonds, such as Al-O-Si appear as the sample is annealed. At the same time, SIMS measurements confirm an outgassing of hydrogen, contained in the Al2O3 layer, toward the silicon during the thermal treatment with a maximum penetration depth into the silicon at 30 min of annealing. This evolution is in good agreement with the electrical values, deduced from the C(V) measurements, where the Dit at 30min is low (109 cm-3) and the Qeff is high (-1.1013 cm-3).