The study of rapid Cu–Cu bonding using thermally stable nanostructured Cu layers

• A small/large-grains mixed nanostructure enables thermal stability in Cu layers. • 10-min Cu–Cu bonding is achieved at 250 °C using thermally stable Cu layers. • A kinetic model verifies grain boundary diffusion mechanism enables rapid Cu bonding. • High-density grain boundaries provide abundant channels for rapid Cu diffusion. Cu–Cu bonding is an important interconnection technology in the advanced packaging industry. However, compared to the 3–5 min solder bonding process, Cu–Cu bonding typically requires a significantly longer bonding time of over 30 min, which leads to low throughput and high costs. To address this issue, we designed thermally stable nanostructured Cu layers that combine small grains (100–150 nm) and large grains (200–300 nm) for rapid Cu–Cu bonding. During the bonding process, the average grain size increases to around 381 nm within the initial 2 min and then stabilizes, maintaining a high-density grain boundary network in the Cu layers. Subsequently, this network serves as the fast pathway for Cu atom diffusion, promoting rapid Cu–Cu bonding. With the designed Cu layers, successful bonding is achieved within 10 min at 250 °C. Furthermore, when the bonding time is extended to 30 min, the bonding interface is eliminated. We proposed a diffusion-based bonding kinetic model to describe the bonding process. In this model, with the grain boundary diffusion as the dominant bonding mechanism, a 23-min bonding time is predicted for void-free bonding, which aligns with the experimental bonding results, further indicating that thermally stable nanostructured Cu layers are viable for rapid Cu–Cu bonding.