Fluxless Bonding Via In-Situ Oxide Reduction

This work describes the details of a fluxless thermal compression bonding process using an in-situ oxide reduction method. Current, TCB processes mainly rely on dip fluxing of the chip or/and pre-applied fluxing of the substrate to achieve removal of oxides. In some instances, an inert environment e.g. N 2 is maintained during the TCB process. A post bonding flux cleanup is generally required, however, there are some near zero residue fluxes that my not require cleaning but these fluxes have the tradeoff that they are not very good at removing oxides. Flux cleanup becomes even more challenging as the chip area and interconnect density is increased. We have developed an in-situ oxide reduction method using localized delivery of formic acid vapors just prior to the bonding step. This method avoids fluxing and post-bonding cleanup steps. The resulting bond quality is equally comparable to the conventional flux based TCB bonds. Furthermore, the FA based oxide reduction is further extendable to the direct Cu-Cu TCB processes