Abstract: A method includes forming a first connector and a second connector over a first wafer and a second wafer, respectively, in which each of the first and second connectors are formed by forming an opening in a dielectric layer; depositing a first metal layer in the opening, in which the first metal layer has a nano-twinned structure with (111) orientation; and depositing a second metal layer over the first metal layer, the second metal layer and the first metal layer being made of different materials, in which the second metal layer has a nano-twinned structure with (111) orientation; attaching the first wafer to the second wafer, such that that the second metal layer of the first connector on the first wafer is in contact with the second metal layer of the second connector on the second wafer; and performing a thermo-compression process to bond the first and second wafers.