Thermal instability of nanocrystalline Cu enables Cu-Cu direct bonding in interconnects at low temperature

Cu-Cu direct bonding has provided an alternative packaging method to circumvent various issues that arise in conventional Cu/Sn/Cu interconnects, and has potential applications in three-dimensional integrated circuits (3D IC). However, achieving a low-temperature bonding with high integrity remains a challenge. In this paper, we demonstrate a method of low-temperature Cu to Cu direct bonding with a perfectly indistinguishable bonding interface achieved using a randomly oriented nanocrystalline Cu interlayer at 250 °C. No orientation control is needed to be performed to assist bonding, neither any post-bonding annealing step is required. The elimination of the bonding interface was enabled by the low thermal stability of nanocrystalline Cu at low temperatures. Micro-scale tensile testing of the interfacial region has shown a ductile fracture behaviour which proves excellent mechanical integrity.