Abstract
This study focuses on the formation mechanism of impinged-grain of Cu6Sn5 and interfolded-grain of Cu6(Sn,Zn)5 in Cu/Sn-3.5Ag/Cu and Cu/Sn-3.5Ag/Cu-15Zn (wt %) transient liquid-phase (TLP) bonding after reflow, respectively. Due to the application of TLP process in 3D-IC technology, the Cu-Sn intermetallic compounds (IMCs) rapidly form from the Cu pads and occupy the entire joint. These Cu-Sn IMCs with large volume fraction weaken the bond reliability resulting from its homogeneous grain structure and brittleness. It is demonstrated that doping Zn into one of the Cu substrates effectively modify the homogeneous grain structure into interfolded structure. The interfolded grain structure of Cu6(Sn,Zn)5 acts as a barrier for crack propagation and therefore enhances the bond reliability. The mechanisms involved are explained in details using thermodynamic related theories, First-principle simulation, FE-SEM observation and electron backscatter diffraction (EBSD) analyses. Owing to the effective modification of grain structure, Cu/Sn-3.5Ag/Cu-15Zn TLP bonding is potentially useful in strengthening the interconnections for novel 3D-IC technologies.
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