Pressureless wafer bonding by turning hillocks into abnormal grain growths in Ag films

We demonstrate pressureless wafer bonding using silver abnormal grain growth caused by stress migration at 250 °C, which is very low for a direct solid-state bonding temperature. The bonding achieved a die-shear strength of more than 50 MPa, which exceeds the fracture toughness of Si wafer. Various deposition temperatures for the silver films, i.e., initial residual stress, reveal that the bonding process is driven by thermomechanical stress. Abnormal grain growth is induced at the contact interface instead of hillocks growing on the film surface. Pressureless wafer bonding can be applied to advanced devices such as thin-wafer multi-chip integrations.