Employment of a Bi-Layer of NiP/Cu as a Diffusion Barrier Layer for Cu bump/Sn Bonding Structures for the 3D Integration Applications

Cu bump/Sn layer/Cu bump is a widely-adopted bonding structure for 3D integration; nevertheless, a strong material interaction between Cu and Sn that occurs during manufacturing processes and under actual usage conditions is potentially a serious problem for both thermal and mechanical reliabilities. Here, we propose the introduction of a bi-layer of plated Ni(P) and Cu as a diffusion barrier. The Ni(P) layer with a thickness of 1um serves to prevent Cu diffusion whereas the Cu layer with a thickness of 0.5um is expected to react with Sn and form Cu6Sn5, which would subsequently suppress a reaction between Ni and Sn. We deposited the bi-layer barrier using a wet process, electroless-plating for Ni(P) and electroplating for Cu. We examined the effects of the barrier on the thermal stability and bonding morphology using SEM and TEM. Then we carried out lap-shear tests to characterize the mechanical properties.