Modeling of Cu-Cu Thermal Compression Bonding

A simple bonding model is proposed to correlate the bonding time with some parameters such as surface roughness, temperature, pressure, and grain boundary diffusivity. The theoretical bonding time is defined as the time required for the bonding area to reach 95% of the surface area. Cu-Cu direct bonding is accomplished through the surface creep mechanism, which are divided into four stages, surface contact and plastic deformation, isolated void and grain boundary formation, interfacial void ripening, and interface elimination by grain growth. In this study, we established a surface creep model for the second bonding stage. The driving force is a pressure gradient, which triggers Cu atoms to fill voids at the bonding interface via grain boundary and surface diffusion. This is driven by the release of Gibbs free energy in the system. We took the critical parameters, including surface roughness, bonding temperature, and pressure into account of the model. Using such a kinetic model, we are able to estimate the theoretical bonding time as functions of surface roughness, grain boundary diffusivity, temperature, and pressure. The results indicate that surface roughness and orientation play critical roles on the bonding time. The theoretic bonding time is estimated as 104 s for the Cu films with a surface roughness of 10 nm bonded at 200 °C and 0.5 MPa. As the surface roughness is reduced to 1.0 nm, a bonding time of 10 s is predicted.