Low-temperature and low pressure copper-to-copper direct bonding enabled by creep on highly (111)-oriented Cu surfaces

Cu-to-Cu microbumps have many advantages, such as better electrical and thermal conductivities. The Cu-to-Cu microbumps may be a potential solution to the heat dissipation problem in 3D IC integration. In addition, for packaging of CMOS image sensors, the bonding temperature is preferred to below 200 °C. Therefore, low temperature and low pressure Cu-to-Cu direct is of great interests for packaging industry. However, it is a challenging issue because Cu atoms diffuse slowly below 200 °C. In this study, we achieved low temperature and low pressure Cu bonding using highly (111)-orientated Cu films. Because the diffusivity of Cu atoms on the (111) surface diffusivity is the fastest among all the crystallographic planes of Cu. The bonding temperature can be lowered to 150°C at a compressive stress of 114 psi held for 60 min at 10^-3 torr. Since the melting points of popular Pb-free solders such as SnAg and SnAgCu are over 230°C. Therefore the present technique can be applied to packaging of CMOS image sensors and 3D IC integration.