Cobalt-Tin Intermetallic Compounds as Alternative Surface Finish for Low Temperature Die-to-Wafer Solder Stacking

Alternative surface structure on bottom pad formed by cobalt-tin intermetallic compounds has been investigated towards low temperature high through put die to wafer stacking. In this study, the plating conditions for cobalt and tin, which can maintain good within wafer and within die uniformity with ideal intermetallic compounds shape after reflowing, were optimized. The sharp needle Co-Sn intermetallic compounds are utilized for low temperature thermocompression bonding stacking, where high electrical yield was obtained in 20 μm interconnect pitch.