Influence of water diffusion in deposited silicon oxides on direct bonding of hydrophilic surfaces

Direct bonding of PECVD SiOx films is of great interest in the field of microtechnologies for applications such as stacked structures, thin film transfers, whose fabrication processes still deserve to be investigated. This work deals with the influence of water diffusion in the SiOx films deposited onto 200 mm Si wafers and its impact on adherence of hydrophilic surfaces. The as-deposited film nature induces various stresses. Stresses evolutions and water penetration are characterized and a correlation is made between the kinetics of both stress variations and water diffusion through the oxide thin films. Impacts of room temperature (RT) storage prior to bonding and thermal treatments applied for strengthening the bonding is shown. As direct bonding operates through surface asperities deformation, specific mechanical properties of such contact asperities can lead to a stronger bonding by increasing bonding area (Fournel et al., ECS J Solid State Sci Technol 4(5):124–130, 2015; Rieutord et al., ECS Trans 3(6):205–215, 2006; Ventosa et al., J Appl Phys 104:123524, 2008, Electrochem Solid State Lett 12(10):H373–H375, 2009). As the mechanical properties of silicon oxide material are greatly influenced by internal water concentration, aging and water diffusion on asperity have a real impact in term of direct bonding energy. Moreover at bonding interfaces, it is observed by X-ray reflectivity (XRR) that aging of deposited SiOx prior to bonding enables shallower bonding gaps, indicating better bonding interface closure which is coherent with the bonding energy enhancement. All these results confirm the link between water diffusion and hydrophilic surface adherence of deposited SiOx films.