Chip-to-chip copper direct bonding in no-vacuum ambient using (111) oriented nano-twinned copper

In this study, we demonstrate the chip-to-chip copper direct bonding by using (111) oriented nano-twinned copper in N₂ ambient, without vacuum. A well bonded interface in the Cu-to-Cu joint was identified by the focus ion beam (FIB) microstructure observation. The secondary electron images (SEI) showed a void-less bonding interface within the bonded Cu joint. In addition, a die shear test was conducted. The test result revealed that the Cu joint has a robust bonded Cu structure due to its high value. The shear strength is more than 100 MPa. There is nearly twice strength value higher than the traditional SnAg solder joint (64 MPa). In addition, the scanning electron microscope (SEM) image showed the joint fractured in a ductile manner. Besides, we also performed the resistance measurement by using Kelvin probes on the bonded chip-to-chip test vehicle. The result showed 4.12 mΩ in single joint resistance and 4.26 × 10^-8 Ω·cm² in contact resistivity. More than 30% resistance reduction has been confirmed as compared to the traditional SnAg solder joint (6.32 mΩ). Moreover, for further post annealing process on the bonded test vehicle, we can further reduce the joint resistance by the value of 2.9 mΩ. It approaches the value of bulk Cu. The evidences revealed that the Cu-to-Cu joint is superior to the traditional SnAg solder. In last, the chip-to-chip copper direct bonding by using (111) oriented nano-twinned copper in N₂ ambient was achieved.