Effects of the citrate-coated nanosized Ag pastes on joining reliable Cu-Cu joints for Current 3D ICs

The emergence of third-generation semiconductor materials (represented by SiC and GaN) has raised the operating temperature and higher requirements of devices. According to the Restriction of Hazardous Substances (RoHS) directive, the use of lead-containing solder is prohibited as it is a hazard to the ecological environment and to human health. Currently, lead-free, soft solders, such as Sn-Ag, Sn-Zn, and Sn-Cu, are the most commonly used multistage packaging and interconnection materials in the electronics industry. However, the electronics industry has an urgent need for new interconnect materials for high power devices and reliability. In this study, reliable Cu-Cu joints were successfully bonded using citrate-coated nanosized Ag paste in ambient air. With the increase of washing times, the joint strength tended to first increase and then decrease. The joint strength first increased and then decreased as the joining temperature rose. Based on the practical application considerations, three washing times, a joining temperature of 260 °C, a holding time of 30 min, and a joining pressure of 1 MPa were selected as the optimal process conditions for the bare Cu-Cu joints. The thermal stability of the joints made by the optimal process at 150 °C and 250 °C was investigated. The results showed that the joint has excellent thermal stability at 150 °C. After long-term aging at 250 °C, however, its strength was significantly reduced. This is because the aging temperature of 250 °C caused significant oxidation of the interface between the Ag nanoparticle paste and the Cu substrate, resulting in a significantly lowered joint strength. The present work aids in the understanding of the bare Cu-Cu joining for three-dimensional (3D) ICs fabricated under atmospheric conditions.