Diffusion Enhanced Drive Sub 100 °C Wafer Level Fine-Pitch Cu-Cu Thermocompression Bonding for 3D IC Integration

One of the primary and critical requirements for high quality wafer level thermocompression Copper-Copper (Cu-Cu) bonding is the fast diffusion of Cu atoms across the boundary between two bonding layers. In this paper, we demonstrate low temperature, low pressure and fine pitch Cu-Cu thermocompression bonding by enhancing intrinsic diffusivity at the bonding interface by stress gradient. Stress in the Cu surface was fixed up by simply varying the Ar pressure during physical deposition using the sputtering tool and observed clearly that one of the other deposited wafers had opposite stress and highest differential stress not only led to fine-pitch bonding at sub 100°C with very low external pressure of 0.25 MPa. Retention of stress even in the smaller patterns was clearly observed using conventional non-destructive wafer bow technique and further corroborated using solid mechanic module of COMSOL Multiphysics simulator. The quality of stress engineered fine-pitch bonded sample was examined using the pull test and Idonus Wafer Bonder IR Inspection (WBI) tool. Furthermore, the absence of voids and defect free bonding interface clearly opens up realistic chances for three dimensional (3D) IC integration. Moreover, this novel stress tailoring Cu surface modification prior to bonding is the primary contestant for future heterogeneous integration.