Grain boundary motions of low temperature and low pressure copper to copper direct bonding by electroplating ultra-fine-grain (UFG) Cu

Cu–Cu direct bonding using electroplated ultrafine-grain Cu (107.24 nm) was studied in air at 110–150 °C. Unstable grain boundaries enabled ultrafast grain growth across the bonding interface, analyzed via coincidence site lattice (CSL) boundaries using EBSD. Above 125 °C, the Σ3 boundary length exceeded 40%, while below 120 °C it rapidly declined, transforming into Σ27a, indicating a critical transition dominated by the {115} plane. A temperature–time-dependent grain growth model was developed, incorporating CSL effects. Simulations showed grain evolution and timing of CSL boundary formation, with transition times from 316 to 190 s as temperature increased.