Seed Layer Etching, Thermal Reflow and Bonding of Cu-Sn Micro Bumps with $5\ \mu \mathrm{m}$ Diameters

Transient-liquid-phase (TLP) Cu-Sn bonding is an attractive method for fabrication of intra-chip connects in 3-D and chiplet applications due to the ease of manufacturing and low cost. With the continuous increase in the interconnect densities, it is highly desired to shrink the Cu-Sn bump diameters to $\mathbf{5}\ \boldsymbol{\mu} \mathbf{m}$ or less from the current mainstream $\mathbf{20}\sim \mathbf{30}\ \boldsymbol{\mu} \mathbf{m}$ . However, small Cu-Sn bumps shows significant difference from large ones because some surface related properties that are insignificant for large bumps become dominant for small ones. This paper reports the differences between bonding of large and small Cu-Sn bumps, such as surface oxidation and bump undercut, and proposes the methods to address these problems. By optimizing the bonding processes, TLP bonding of Cu-Sn bumps with a $\mathbf{5}\ \boldsymbol{\mu} \mathbf{m}$ diameter has been successfully achieved.