Dilute Cu Alloying for Sn-Cu Bumping by Annealing Electroplated Cu/Sn Stacks on Ti/Ni/Pd UBM

Sn-Cu bumping has been demonstrated to employ sequential electroplating of Cu and Sn, followed by alloying the Cu/Sn stacks during reflow. Alloying behavior of the Cu/Sn stacks has been investigated with varying the underlying Cu thickness. The Cu₆Sn₅ based compounds were observed at the interface between a sputter deposited Ti/Ni/Pd under bump metallization and the Cu/Sn plated stack The underlying Cu was consumed by forming the intermetallic compounds as well as alloying with Sn, limiting the alloying Cu content in the Sn-Cu bump. With elevating reflow temperature, the alloying Cu content in the bump was slightly decreased. The result cannot be predicted by the solubility limit of Cu into Sn at a given reflow temperature based on the equilibrium phase diagram. The basic process design for the Cu/Sn stack has been provided. The intermetallic phase has been confirmed to work as a good diffusion barrier to Sn, leading to integrity of the Ni after solid state aging. The Cu/Sn stack plating process allows us to realize dilute Cu alloying with Sn for the eutectic composition and less Cu contents in the Sn-Cu bumps. In the bumping process, the thickness ratio of the Cu/Sn stack as plated does not need to be strictly controlled and a thick electrodeposited Ni layer is not necessary as the under bump metallization. This study confirms that electroplating provides a robust and cost-effective process for mass production of lead free bumping