Ni/Cu/Sn Bumping Scheme for Fine-pitch Micro-bump Connections

3D integration requires a physical stacking of die/wafer onto another die/wafer while forming a permanent electrical and mechanical connection between the input/output pins of the devices. Tin-based micro-bump (mu bump) connections using copper (Cu) or nickel (Ni) Under-Bump-Metallurgy (UBM) with interconnect pitches of 40 mu m and smaller are generally considered to be the leading candidates for these high density Si-to-Si interconnects, particularly because of their tolerance to height variation, similarity to standard flip-chip solder joints and ease of processing. In this paper, a systematic study of the metallurgical interactions in the Ni/Cu/Sn system is presented, using both stacks of blanket films and mu bumps. Furthermore, a novel Ni/Cu/Sn mu bump scheme is developed to replace the conventional Cu/Ni/Sn or Cu/Sn schemes in order to improve the mu bump solder joint quality and reliability. Here a very thin layer of Cu between Ni UBM and Sn solder is used to avoid the formation of (Ni,Cu)(3)Sn or Cu3Sn and significantly reduce void formation.