Effects of Surface Roughness and Bonding Pressure on Cu Direct Bonding: a Molecular Dynamics Study

Low-temperature, low-pressure Cu-to-Cu direct bonding technology is a potential replacement for solder microbump interconnects. Here, the effects of the surface roughness of microbumps and bonding pressure on the bonding behavior and mechanical properties of (111)-oriented nanotwinned Cu (NT-Cu) are investigated by using molecular dynamics simulations. The simulation results show that for direct bonding with nonflat microbumps, a modest increase in bonding pressure can decrease the size of residual gaps and increase the bonding area, leading to an improvement in mechanical properties. During the bonding process, the number of partial dislocations around the bonding interface increases with increasing radius (r) of an asperity on the bottom of the upper microbump of a microbump pair, leading to an increase in residual compressive stress for a bonded microbump. The mechanical properties of bonded microbumps strongly depend on the size of the residual gaps. For a given bonding pressure, the mechanical properties of bonded microbumps generally decrease with increasing r value.