Grain morphology effect on interfacial void closure in Cu–Cu bonding for advanced semiconductor packaging

Driven by the demands of increased integration density and heterogeneous integration in the post-Moore era, Cu–Cu direct bonding has become a critical technology for enabling fine-pitch interconnects in advanced packaging. However, achieving reliable bonds remains challenging due to the existence of interfacial voids and the complex grain morphology that develops during the bonding process. In this work, a phase field model is developed and employed to investigate the effect of grain morphology on the kinetics of interfacial void closure during the Cu–Cu bonding process. Representative configurations are constructed by varying grain size and morphology in both bonded Cu pads to simulate realistic microstructural scenario. The results show that configurations with a larger number of smaller grains on both sides of the bonding structures promote faster void closure and achieve more uniform interfacial densification. This behavior is attributed to the increased presence of grain boundaries, which serve as enhanced diffusion pathways across the interface. Furthermore, the evolution of stress fields during bonding indicates that stress becomes more evenly distributed as voids close. The stress triaxiality near void regions remains below −1 throughout the closure process, demonstrating a favorable hydrostatic compressive state for void elimination. These findings provide further fundamental understanding of the mechanisms of void closure and may offer valuable guidance for improving the reliability of Cu–Cu bonding in advanced packaging applications.