Employment of a bi-layer of Ni(P)/Cu as a diffusion barrier in a Cu/Sn/Cu bonding structure for three-dimensional interconnects

This study explored the possibility of employing a bi-layer barrier of electroless-plated Ni(P)/thin Cu layers in a Cu/Sn/Cu bonding structure for three-dimensional interconnects. Our materials analysis revealed that the bi-layer barrier served effectively as a diffusion barrier and prevented full-scale materials interaction for temperatures higher than 300 °C. Such suppression of an intermetallic compound reaction and limiting Cu diffusion led to the formation of a rod-shaped Cu6Sn5 compound, rendering a unique microstructure of ductile Sn embedded with strong Cu6Sn5 rods. Our mechanical characterization using lap-shear testing and fracture analysis revealed that the sample with such a microstructure displayed a high bonding strength with some ductility, a desirable combination for high mechanical reliability.