Low-Temperature and Pressureless Cu-to-Cu Bonding by Electroless Pd Plating using Microfluidic System

Recently, 3D-IC technology has been implemented for finer pitch and higher interconnect density over the past decades. Among all Cu-to-Cu bonding technologies, the microfluidic electroless interconnection (MELI) process can directly fabricate full-metal Cu pillar joints at temperatures below 100°C and without applying any pressure on the chips. The vertical interconnections are formed by a forced flow of electroless plating solution through a microchannel so that the reduced atoms self-assemble between the gaps of two facing Cu pillars. Our previous research shows that the MELI process using electroless Ni(P), Cu, and Au could form vertical interconnection uniformly under controlled flow. This study extends the MELI process's application range to 5G generation usage with electroless Pd. Using electroless Pd allows a more significant skin depth than other plating materials, causing less signal loss and delay during high-frequency transmission. In this study, highly uniform electroless-Pd-bonded Cu pillar joints were fabricated in a patterned microfluidic system of around 70°C. The bonded pillars were analyzed by scanning electron microscope. In addition, the cross-section images were further investigated by a focused ion beam to confirm the bonding interface. A TEM analysis examined the bonding characteristics. Epitaxial growth of deposited Pd along the Cu pillar's surface orientation was demonstrated, indicating enhanced electrical properties for future high-frequency applications. Furthermore, the die shear test verified a shear strength of around 55 MPa of the electroless-Pd-bond. This method might be a potential candidate for future low-stress and low-thermal-budget bonding.