Investigation of bonding front propagation for wafer direct bonding

Bonding front propagation driven by the work of adhesion is restricted by the air drag and mechanical deformation of wafers. Energy variation during room temperature bonding can influence the bonding initiation and front propagation dramatically. In order to evaluate the dissipated energy and elastic strain energy resulting from the air cushion and the wafer bow, both theoretical and finite element models are established in this paper. The effects of bonding propagation velocity, wafer materials, and the ambient pressure on the bonding front propagation are investigated respectively. A large bonding propagation velocity can be obtained with a high surface energy thanks to the surface treatment prior to bonding (e.g. plasma activation). Compared to the silicon wafers, quartz wafers need less propagating energy to overcome the elastic deformation. Ambient vacuum can help to improve the bonding quality. Our results indicate that an external pressure needs to be applied on the wafers with hydrophobic surfaces to achieve a good pre-bonding interface in wafer scale.