Investigation of interfacial reaction between Sn-Ag eutectic solder and Au/Ni/Cu/Ti thin film metallization

This paper reports the formation of intermetallic compounds in Au/Ni/Cu/Ti under-bump-metallization (UBM) structure reacted with Ag-Sn eutectic solder. In this study, UBM is prepared by evaporating Au(500 Å)/Ni(1000 Å)/Cu(7500 Å)/Ti(700 Å) thin films on top of Si substrates. It is then reacted with Ag-Sn eutectic solder at 260°C for various times to induce different stages of the interfacial reaction. Microstructural examination of the interface, using both chemical and crystallographic analysis, indicates that two types of intermetallic compounds are formed during the interfacial reaction. The first phase, formed at the initial stage of the reaction, is predominantly Ni₃Sn₄. At longer times the Ni₃Sn₄ phase transforms into (Cu, Ni)₆Sn₅, probably induced by interdiffusion of Cu and Ni. At this stage, the underlying Cu layer also reacts with Sn and forms the same phase, (Cu, Ni)₆Sn₅. As a result, the fully reacted interface is found to consist of two intermetallic layers with the same phase but different morphologies.