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

Abstract

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 Omega and 4.14 m Omega, respectively. The contact resistivity is 3.98 x 10(-8) Omega.cm(2). More than 30% resistance reduction in the single Cu joint has been confirmed as compared to the SnAg solder joint (6.32 m Omega). 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.