Fine-pitch bonding technology with surface-planarized solder micro-bump/polymer hybrid for 3D integration

The scaling of conventional solder-based flip chip bonding is facing its limitations due to thermal compression bonding overlay tolerance when using conventional bumping. To decrease the tolerance, planarization can be used to fabricate two flat surfaces for bonding. However, planarization of these soft and ductile surfaces is challenging by polishing. Here, we assess the creep-feed fly-cutting process, the so-called surface planer process for planarization of fine-pitch Sn bumps and polymer simultaneously. It is revealed that the polishing process causes a lot of scratches on the Sn and polymer surface; however, these surfaces are smooth for the case of the surface planer process. The planarized Sn and polymer surface has only a 50 nm step height which does not have any impact during thermal compression bonding. Using a planarized Sn and polymer surface, stacking of below 10 mu m pitch has been achieved.