Design of surface topography for fabricating nanotwinned copper/polyimide hybrid joints using in-situ heating atomic force microscopy

In this study, we fabricated nanotwinned copper (NT-Cu)/polyimide (PI) hybrid joints and examined the surface topography and thermal expansion at different temperatures by in-situ heating atomic force microscope (AFM). Results showed that the surrounding PI films expanded 230 nm more than the theoretical value due to the lateral confinement by the NT-Cu joints. Considering the thermal expansion, deformation capability (approach T_g), post-molding shrinkage, bonding yield, process costs, and proper chemical–mechanical planarization (CMP) process, the low-thermal-budget (220 ℃) and low contact resistance (2.5 × 10^-8 Ω·cm²) NT-Cu/PI hybrid bonding was achieved. To obtain high electrical performance, the optimal protrusion height of NT-Cu is recommended around half of the height difference (at the bonding temperature) between PI and NT-Cu. The scanning acoustic tomography (SAT) result showed large bonding areas (86%). The I-V curve and the resistance measured at different temperatures (25 ℃ to 200 ℃) were linear stable. High bonding strength (> 31.2 MPa) could be achieved without post-bond annealing processes. This study provides the design guideline for surface topography of fabricating Cu/PI hybrid joints.