Direct wafer bonding of amorphous or densified atomic layer deposited alumina thin films

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 (SiO 2 ) by high thermal conductive dielectric layers such as HfO 2 , Si 3 N 4 , diamond or Al 2 O 3 . 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/Al 2 O 3 and Al 2 O 3 /Al 2 O 3 direct bonded structures: bondings are submitted to room temperature up to 900 °C annealing. Amorphous or crystallized Al 2 O 3 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-Al 2 O 3 layer, for T > 200 °C, high bonding energy are obtained even if high defect density appeared when annealing temperature exceeded 400–500 °C. Spontaneous debonding phenomena even occurred for a-Al 2 O 3 /a-Al 2 O 3 direct bonding. This defectivity, unobservable using infrared camera, may be explained by chemical or structural Al 2 O 3 modification such as gases desorption, internal stress or crystallisation state. Bonding with crystallized Al 2 O 3 film has been also characterized by infrared spectroscopy and complementary analysis. No high defect density is observed with crystallized Al 2 O 3 layer. Based on these results, an Al 2 O 3 bonding mechanism is proposed.