Low temperature and pressureless Cu-to-Cu direct bonding by green synthesized Cu nanoparticles

Background: Low temperature bonding is the key for developing advanced electronic packaging technique. While Cu is regarded as a promising material for interconnects with high conductivity, the oxidation issue during processing needs to be solved. Methods: In this study, Cu-to-Cu direct bonding was achieved at low temperature without applying pressure by a green chemical method. Synthesized Cu nanoparticles (NPs) without capping agents were mixed with polyethylene glycol (PEG). Significant findings: An iterating mechanism is proposed as Cu oxide catalyzing the decomposition of PEG, and the decomposed products further reduce the Cu oxide. Particle size and size distribution of Cu NPs were simply controlled by pH value of the reducing solution. A random close packing model proves that packing density of the pre-sintered Cu NPs directly correlates to bonding strength. Ductile fracture surface confirms the realization of well-sintered joints by a green process that can be applied to devices.