Low-Resistance and high-Strength Copper Direct Bonding in no-Vacuum Ambient Using Highly (111)-Oriented Nano-Twinned Copper

In this study, we fabricated (111)-oriented nt-Cu microbumps with 30 µm in diameter, and bonded them together using chip-to-chip bonding scheme in N2 ambient, without vacuum. A well bonded interface in the Cu-to-Cu joint was identified by the microstructure observation. Scanning electron microscope (SEM) images showed a void-less bonding interface within the bonded Cu joint. In addition, a die shear test was conducted. The test results revealed that the shear strength is 124 MPa, which is nearly two times higher than the SnAg solder joint (64 MPa). It indicates that the Cu joint is more robust than the SnAg joint. In addition, fracture analysis showed that 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 vehicles. The resistance is 4.12 mΩ for a single joint and its contact resistivity is 4.26 × 10-8 Ω·cm2. More than 30% resistance reduction has been confirmed as compared to the SnAg solder joint (6.32 mΩ). Moreover, we can further reduce the joint resistance by the second annealing process. The resistance can be brought down to 3.27 mΩ with a resistivity of 3.14 × 10^-8 Ω·cm^2. There is a nearly 50% resistance reduction The resistance for second annealed Cu joint is close the ideal bulk Cu. In summary, the chip-to-chip copper direct bonding has been successfully achieved and low resistance Cu-to-Cu joints has been realized by using (111) oriented nt-Cu in no-vacuum ambient.