SOI circuit exhibits excellent performance and rehabilitee but with the component miniaturization trend and the clock frequency increase, the self-heating phenomena that arise from the SOI structure itself must not be underestimated. In order to minimize this problem, several candidates have been identified to replace the buried silicon oxide (SiO2) by high thermal conductive dielectric layers such as HfO2, Si3N4, diamond or Al2O3. In order to elaborate a SOI structure using this kind of innovative buried dielectric, first of all, their direct bonding with silicon has to be studied. In this work, we investigate the bonding thermal behaviour of Si/Al2O3 and Al2O3/Al2O3 direct bonded structures: bondings are submitted to room temperature up to 900 A degrees C annealing. Amorphous or crystallized Al2O3 thin films were used in this study. Bonding energies are measured in an anhydrous atmosphere and bonding defectivity is analysed using scanning acoustic microscope (SAM). With amorphous a-Al2O3 layer, for T > 200 A degrees C, high bonding energy are obtained even if high defect density appeared when annealing temperature exceeded 400-500 A degrees C. Spontaneous debonding phenomena even occurred for a-Al2O3/a-Al2O3 direct bonding. This defectivity, unobservable using infrared camera, may be explained by chemical or structural Al2O3 modification such as gases desorption, internal stress or crystallisation state. Bonding with crystallized Al2O3 film has been also characterized by infrared spectroscopy and complementary analysis. No high defect density is observed with crystallized Al2O3 layer. Based on these results, an Al2O3 bonding mechanism is proposed.