Tuning of Copper Grain Sizes for Integration in Hybrid Bonding Applications

The microstructure of Cu plays a crucial role in the formation of Cu-to-Cu metal bonds during the hybrid bonding process. This study demonstrates that controlling the grain size of copper films through varying concentrations of trisodium citrate, a grain size growth inhibitor agent during electrodeposition offers a significant advancement in tailoring material properties for specific applications. Higher citrate concentrations result in smaller grain sizes, which enhance mechanical properties such as hardness and yield stress, making these films ideal for low-temperature bonding and applications requiring robust mechanical strength. For instance, a copper film with a 9 nm grain size exhibited superior hardness (3.59 GPa) and yield stress (1.42 GPa) in the as-deposited state. However, the increased grain boundary area in finer grains also leads to higher electrical resistance, which is a disadvantage for applications requiring high electrical conductivity. Conversely, lower citrate concentrations produce larger grain sizes, which reduce electrical resistance due to fewer grain boundaries, thus improving electrical performance but at the cost of inferior mechanical properties, as observed in a standard copper film deposited without any citrate content with a 273 nm grain size having lower hardness (2.42 GPa) and yield stress (1.06 GPa). This tunable grain size approach via citrate-based chemistry provides a versatile solution to balance and optimize the mechanical and electrical properties of copper films, allowing adaptability based on the specific requirements of various technological applications.