Low-Temperature Cu-Cu Bonding Using <111>-oriented and Nanocrystalline Hybrid Surface Grains

Cu-to-Cu direct bonding has been considered a promising approach to overcome the issues that conventional solder bumps face. However, reducing the bonding temperature to minimize the thermal budget was still a challenging task. Highly <111>-oriented nanotwinned copper (NT-Cu) has drawn lots of attention on account of its highest surface diffusivity and lowest oxidation rate among all crystal directions that helped <111> NT-Cu achieve low-temperature bonding. Recent studies also focused on nanocrystalline Cu (NC-Cu) since the fine grains contain a large area of grain boundaries that can not only serve as diffusion paths for atomic movement during bonding but also store high grain boundary energy which can function as a driving force for interface elimination. Thus, NC-Cu was regarded as a candidate for complete Cu bonding under low temperatures.In this work, we successfully fabricated <111>-oriented and nanocrystalline hybrid Cu surfaces that can be employed in Cu-Cu bonding. Taking the advantages of extremely high surface diffusivity on <111> Cu surfaces and the high energy state of nanocrystalline grains, we achieved low-thermal-budget Cu-Cu bonding at 200°C for 1 h. A nearly seamless bonding interface along with its high bonding shear strength both demonstrate that the synergy of <111>-orientation and nanocrystalline grain is a feasible approach to be adopted as copper interconnects in advanced packaging technology.