Atomistic behavior of Cu–Cu solid-state bonding in polycrystalline Cu with high-density boundaries

Low-temperature Cu–Cu solid-state bonding is key for interconnect miniaturization and higher current densities in advanced semiconductor devices. Achieving reliable joints requires effective void closure at the interface, driven by diffusion. We employed molecular dynamics simulations to elucidate the dominant atomic transport mechanism enhancing void closure. We modeled two polycrystalline Cu structures with high-density boundaries—a randomly oriented nanocrystalline (NC) structure with high-density grain boundaries (GBs) and a unidirectional [111]-oriented columnar nanotwinned (NT) structure with high-density twin boundaries (TBs). Thermocompression bonding was simulated for both structures, with the bonding surfaces designed to replicate surface roughness. The NC structure exhibited significant atomic movement at the GBs with simultaneous grain coalescence, leading to early void closure. In contrast, the NT structure showed limited atomic movement and void closure, despite active surface diffusion. Potential energy analysis revealed that the NC structure's energy decreased significantly over time, promoting void closure, while the NT structure's quasi-stable energy state hindered this process. This was attributed to its high potential energy state, low activation energy for GB diffusion, and complex GB migration during grain coalescence at randomly oriented GBs. This study provides a deeper understanding of the GB-driven atomic transport mechanism that promotes void closure.