Low-temperature direct Ru-Ru bonding enabled by ternary synergistic plasma surface activation

Ruthenium (Ru) is a leading candidate for next-generation ultra-high-density interconnects due to its low resistivity at nanoscale dimensions, superior electromigration resistance, and excellent compatibility with advanced back-end-of-line (BEOL) integration. However, achieving low-temperature direct Ru-Ru bonding remains a fundamental challenge because of Ru's high melting point and surface oxidation, which typically necessitate high-temperature, high-pressure thermal compression processes incompatible with advanced heterogeneous integration. In this study, we introduce a low-temperature, pressure-assisted direct Ru-Ru bonding strategy enabled by a synergistic Ar/N₂/H₂ ternary plasma surface activation. The plasma treatment simultaneously removes native Ru oxides, suppresses re-oxidation, and generates highly reactive surface states through combined physical sputtering (Ar), chemical reduction (H₂), and nitrogen-assisted surface passivation and defect engineering (N₂). This engineered surface chemistry enables spontaneous atomic-scale interfacial reconstruction and metallurgical bonding at temperatures below conventional thermal compression thresholds. Robust, void-free Ru-Ru interfaces are achieved with a measured bonding strength of ~8 MPa at 250℃, without the need for intermediate layers or high-temperature annealing. The approach establishes a generalizable surface-activation paradigm for refractory metal bonding and opens new pathways for advanced packaging, chiplet integration, and monolithic 3D system architectures.