Fabrication and properties of high thermal stability nanocrystalline Cu for low temperature Cu–Cu bonding

This study reports the fabrication, microstructure, and properties of nanocrystalline copper (NC-Cu) films for low-temperature Cu-Cu bonding applications. NC-Cu films with grain sizes below 100 nm were successfully synthesized using grain refiner additives, demonstrating high thermal stability after annealing at 150 °C for 2 h. The as-fabricated NC-Cu films exhibited superior hardness (186.2 HV), significantly higher than regular Cu films. Electrical measurements revealed that NC-Cu films have 25% higher resistivity (2.20 × 10 −6 Ω·cm for 12 ASD and 2.15 × 10 −6 Ω·cm for 4 ASD) than regular Cu, primarily due to grain size and impurity effects. The NC-Cu films were then bonded and post-annealed at 280 °C for 6 h to further trigger cross-interface grain growth. Shear tests were also performed showing the high bonding strength of NC-Cu joints (>31.53 MPa). Failure analysis indicated strong bonding of the NC-Cu films. The stable grain size, zig-zag interface formation, and strong shear strength places the NC-Cu as a robust candidate for low-temperature Cu-Cu bonding in advanced packaging.