Thermosonic direct Cu pillar bonding for 3D die stacking

The emerging 3D IC stacking technology as one of the main platforms for 3D integration gains significant performance advantages by using copper (Cu) pillar interconnections. Although thermocompression Cu bonding offers reliable interconnections with demonstrated bond stability at high temperatures, it requires high pressures, high temperatures and long process time. As an alternative to thermocompression bonding, thermosonic flip-chip bonding of Cu-pillared silicon chips is studied here. By exploiting ultrasonic energy and inducing mechanical vibrations, which promotes the interfacial atomic diffusion, Cu-to-Cu bonding was facilitated in significantly lower bonding forces (3-6 MPa), shorter process windows (300-500 ms) and lower thermal budgets (140-240 degrees C). In this study, thermosonic bonding parameters (i.e., ultrasonic energy, bonding temperature and pressure) were optimized for Cu-pillared chips. The bonded samples were further characterized by die shear strength analysis and interfacial microscopy. It was found that thermosonic bonding can be regarded as a substitute for thermocompression bonding with enhanced bonding strength and invisible Cu interfaces. It was also proposed that thermosonic flip-chip bonding of Cu pillars can be utilized for die-to-wafer integration either as a single process or as a tacking process combined with a global post-processing.