Fabrication and Formation Principle of Porous Cu–Sn Bumps for Metal Bonding in Chiplet Integration

Abstract

We have developed porous Cu-Sn bumps by treating conventional Cu-Sn bumps with a gas mixture of oxygen and formic acid for fine-pitch Cu-Sn bonding and low-temperature Cu-Cu bonding. However, the formation mechanisms of the porous Cu-Sn bumps are still unclear, and the fabrication processes are not optimized. This article presents a chemical model for the formation of the porous Cu-Sn bumps. Controlled experiments are performed to investigate the influences of some key factors, such as the temperature and the duration of redox treatment, the gas composition, and the bump sizes. It is found that the formation of porous Cu-Sn bumps is associated with the sequential oxidation and reduction of the Cu-Sn intermetallic compounds (IMCs) as well as the thermal decomposition of their reaction products. Based on these results, a chemical model is proposed for the formation of porous Cu-Sn bumps, which provides deep insights to the reactions and the guidelines for optimizing the porous Cu-Sn bumps.