Ultra-fine pitch Cu-Cu bonding of 6μm bump pitch for 2.5D application

Ultra-fine pitch (6μm) interconnects are essential for high-end application-products that demands high speed and high bandwidth inter-chip communication. Achieving Cu-Cu bonding with such a fine pitch is challenging since bond time is too long and bond interface gets easily oxidized. Throughput issue associated with long-bonding time is solved by using 2-step bonding procedure where first step is a temporary bonding and second step is a permanent bonding using a gang bonder. Gaseous formic acid is used to remove surface oxide on Cu surface and wafer level pre applied underfill is used as a tacking material in temporary bonding. Bonding is carried out with dies having 500,000 interconnects and the results show excellent bonding and electrical properties.