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 (μbump) connections using copper (Cu) or nickel (Ni) Under-Bump-Metallurgy (UBM) with interconnect pitches of 40 μm 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 μbumps. Furthermore, a novel Ni/Cu/Sn μbump scheme is developed to replace the conventional Cu/Ni/Sn or Cu/Sn schemes in order to improve the μbump 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)₃Sn or Cu₃Sn and significantly reduce void formation.