Metallurgical aspects and joint properties of Cu-Ni-In-Cu fine-pitch interconnects for 3D integration

Low-temperature bonding technologies for advanced packaging gained substantial interest in both research and industry within recent years. The replacement of Sn in fine-pitch interconnects with In presents a viable approach in the formation of micron-scale metallic interconnections using liquid interlayers. The employment of Cu-In bumps is associated with the rapid formation of intermetallic compounds (IMCs) in the solid state thus reducing the available liquid interlayer volume for prolonged shelf times. To overcome this drawback, the use of a barrier layer between the Cu bump and the deposited In solder presents a viable option. In this work we investigated the impact of a Ni barrier in Cu-In bumps on the formation of IMCs at both room temperature and above the melting point of In. The results show an effective suppression of the IMC growth in the solid state, a slow growth at the solid-liquid interface below 180 °C which significantly accelerates at further increasing temperatures. The fabricated Cu-Ni-In-Cu fine-pitch interconnects exhibit die shear strengths up to 30 MPa. On a microscopic level the formed interconnect exhibit a compact microstructure in the bonding zone and an excellent wetting behavior at the Cu-In interface but also delaminated areas at the Ni-In interface.