Microstructural and morphological characterization of SnAgCu micro-bumps for integration in 3D interconnects

3D interconnects (IC's) studied in this work are ultra fine 50μm pitch structures realized by eutectic bonding of a tin based solder joint. It is well-known that these IC's contain intermetallic compounds (IMC) that grow at the copper/SnAgCu (SAC) alloy interface. Even if this IMC layer evidences a good adhesion between copper and solder alloy, their brittle nature and their resistivity, which is one order of magnitude higher than that of copper, is detrimental to the IC's. Moreover standard manufacturing process often requires up to five reflow steps, during which morphological and microstructural evolutions occur within micro-bumps as well as at the Cu/alloy interface. This study is mainly focused on the microstructural and morphological evolution of copper/SnAgCu (SAC) system during different stages of manufacturing process. Moreover, since Ni barrier is nowadays evaluated as a potential solution for limited IMC growth, it is extended to the case where an additional Ni layer is added between Cu and SAC alloy. The interfacial system is finely characterized after each reflow step or isothermal treatment. In particular, the volume of reaction product formed at the interface is evaluated. In the same way, microstructural evolution of the solder alloy from its electrochemical deposition to its successive reflows is analysed and discussed.