Low resistance and high reliable Cu-to-Cu joints using highly (111)-oriented nano-twinned copper

In this study, we present a chip-to-chip copper direct bonding technique in N
₂
ambient, without vacuum by using (111) oriented nano-twinned copper. A well bonded copper joint was identified by the cross-sectioning process and microstructure observation. The cross-sectional images showed a void-less bonding interface in the bonded copper joint. In addition, we conducted resistance measurement using Kelvin probes on the bonded chip-to-chip test vehicle. The resistance for a daisy chain loop (400 joints) and the Kelvin structure (single joint) are 5.28 Ω and 4.14 mΩ, respectively. The contact resistivity is 3.98 × 10
^-8
Ω cm
²
. More than 30% resistance reduction in the single Cu joint has been confirmed as compared to the SnAg solder joint (6.32 mΩ). Besides, temperature cycling test shows that the daisy chain loop with 400 Cu joints can stand the 1,000 cycles without failure. In summary, the chip-to-chip copper direct bonding in no-vacuum ambient has been successfully achieved by using (111) oriented nt-Cu. The low resistance as well as a reliable Cu-to-Cu joint can be achieved.