Investigation of copper-tin transient liquid phase bonding reliability for 3D integration

Copper tin transient liquid phase bonding reliability was investigated with different setups including CuSn to Cu and CuSn to CuSn bonding. Additionally, a thermal treatment just after CuSn electrodeposition (ECD) was compared to the classical configuration. Thermal cycling test (TCT) was achieved with electrical and mechanical tests carried out before and after TCT to discriminate the setups. Before TCT, each configuration has very good electrical properties in terms of yield (>90%) and Kelvin resistance. Shear tests show good mechanical properties in all tested cases as well. However, SEM images reveal different kinds and densities of voids. Kirkendall voids are localized at the Cu/Cu₃Sn interface. Smaller voids are visible at the initial bonding interface in the case of CuSn to Cu bonding. After TCT, configurations are well discriminated. CuSn to Cu configuration exhibits the worst properties: electrical yield drops at nearly 30% and shear strength loses 80% of its initial value. On the other hand, CuSn to CuSn configuration and configuration including the post ECD thermal treatment keep correct electrical yield above 80%, and has shear strength loss in the range of 30 to 50%. SEM images after TCT reveals crack localization mainly through Kirkendall voids planes either on top side or on bottom side. Hypotheses regarding these results are discussed.