Fabrication of Fine-Pitch Cu-Sn Microbumps Using Electroplating and Wet Seed Layer Etching

Cu-Cu thermocompression bonding and Cu-Sn-Cu transient-liquid-phase (TLP) bonding are the mainstream metal bonding technologies to electrically connect the vertically stacked chips/wafers in 3D/2.5D integration. Compared with Cu-Cu thermocompression bonding which needs chemical mechanical polishing (CMP), Cu-Sn-Cu TLP bonding is CMP-free and can be realized at low temperature and low pressure. Shrinkage of the Cu-Sn bumps significantly increases the surface to volume ratios, and some related characteristics that are insignificant for 50 μm bumps become dominant in 5 μm bumps, such as remarkable Sn volatilization, fast Sn consumption during heating, and serious bump undercut during seed layer etching. In this paper, we report the fabrication of small Cu-Sn bumps with a diameter of 5 μm as well as the bonding results.