Bumpless interconnect of Cu electrodes in millions-pins level

In this study, we demonstrated the bumpless interconnect of Cu electrodes in millions-pins level in the wafer-scale by means of the surface activated bonding (SAB) method. The bumpless interconnect is a highly effective way to compensate the thermal expansion mismatch to obtain the bonding pitch of microns where ultra-low profiled Cu electrodes are bonded directly at low heating temperature to achieve high alignment accuracy and reliability. For the realization of wafer-scale interconnection, 12.5 million electrodes with the dimensions of 10times30mum² and intervals of 10mum were fabricated on 8 and 6-inch wafers by the photolithography followed by the CMP (chemical mechanical polishing) process. In order to clarify appropriate SAB conditions for CMP-Cu to ensure both the cleanness and flatness of the surface to obtain an excellent quality of bonding, we modified the surface cleaning and vacuum conditions. The bonding experiments were performed with the SAB wafer bonder that enables the alignment accuracy of microns all over the wafer in the vacuum condition, and all electrodes were successfully interconnected with the contact load of around 2.0times10^-4N/pin. The misalignment turned out as low as 2mum in the entire wafer, and the shear strength could be larger than 70MPa