Fluxless Chip to Wafer Bonding with Ultra Fine Pitch (Below 20um) Micro-Bump

The increasing demand for higher density chip interconnects has led to the scaling down of solder bumps. As solder bump approach ultra-fine pitch of 20 μm and smaller. Tradition flux-based bonding technique posed challenges for ultra-fine pitch micro-bumps as flux residues can compromise the reliability and performance of solder interconnects and underfills encapsulation. In this work, the fluxless bonding fine pitch micro-bumps (15 μm) for chip to wafer (C2W) bonding was developed through the implementation of anti-oxidation coating on the Cu bond pads on the bottom wafer, and plasma treatment on the top chip solder bumps. It was found that implementing a thicker seed layer can significantly improve the electroplating uniformity of the micro-bumps. As for the C2W bonding process, the anti-oxidation coating was found to be effective in protecting the Cu bond pads from oxidation and provide a wettable surface for solder joint formation with the top chips during flip-chip thermo-compression bonding (TCB) process. An evaluation was conducted on the top chips with and without plasma treatment prior to the C2W TCB process. The results show that plasma treatment was crucial in removing the solder oxide and improve bonding yield from 24% to 72%. Bonding yield loss was likely attributed to solder oxide formation that occur after plasma treatment and during the duration of the flip chip process as the flip chip bonder used in this work does not has an inert chamber environment to prevent solder oxidation. A room temperature dispensable capillary underfill was evaluated and process parameter was optimized to fill the flip-chip gap of 8 μm.