Enhanced thermal expansion with nanocrystalline Cu in SiO2 vias for hybrid bonding

The thermal expansion of copper pads within dielectric vias plays a crucial role during hybrid bonding. In this paper, we tailored the thermal expansion of Cu pads by modifying the grain size of Cu, and adopted in-situ heating atomic force microscopy (AFM) to measure their surface profiles at room temperature, 100, 150, and 200 degrees C. Results showed that by reducing the grain size from 2.5 mu m to 0.15 mu m, the expansion (8.4 nm) of nanocrystalline Cu pads at 200 degree celsius increased to more than twice that of coarse-grained Cu counterparts (3.7 nm). This significant enhancement in Cu expansion is critical for enlarging the process window of hybrid bonding in 3D IC fabrication. Microstructural differences and significant plastic deformation in the nanocrystalline Cu pads further indicate the dominance of Coble creep, resulting in the discrepancy in total expansion. Additionally, the thermal expansion of nanocrystalline Cu pads with (111)-preferred orientation was investigated. Their higher- than-expected expansion offers the potential for high-quality hybrid bonding.