A Novel Copper Microporous-Assisted Bonding Method for Fine-Pitch Cu/Sn Microbump 3D Interconnects

This work presents a novel approach incorporating micro/nanoscale porous copper inverse opals (CIO) within Sn-based solder microbumps, compatible with low-temperature CMOS Back-end-of-line (BEOL) processes. The microporous structure enables critical pore sizes as small as 5 μm and down to 200 nm based on the bump size. This porous-assisted bonding technique has great potential to enhance the thermal conductivity and mechanical reliability of the fine-pitch Cu/Sn bonding interface. In this work, we have successfully fabricated and demonstrated CIO-based microporous structure with a 3 μm pore size on a 100 μm diameter Cu bump, achieving a targeted thickness of 3 μm - 5 μm, as confirmed by Focused Ion Beam microscope (FIB) analysis. Microstructure and element mapping of the Cu-CIO and Sn solder bonding interface reveal that molten solder can permeate these copper CIO microporous structures. This allows for the self-alignment of microbumps through capillary forces, forming robust mechanical interdiffusion bonds. Additionally, employing a simplified finite element method (FEM) suggests that the CIO-based micro/nano porous copper matrix structure has the potential to enhance the equivalent thermal conductivity of the Cu/Sn bonding layer by a factor of 2-3.