Low-temperature Cu–Cu bonding enabled by two-step plasma-induced Cu nitride formation and post-bond annealing

The effect of post-bond annealing on the quantitative interfacial adhesion energy of low-temperature Cu–Cu bonding interfaces was systematically investigated using a double cantilever beam test. A two-step Ar/N₂ plasma treatment was applied to achieve low-temperature bonding. This treatment protected the Cu surface from oxidation by forming Cu nitrides such as Cu₃N and Cu₄N, which were confirmed via electron backscatter diffraction and X-ray photoelectron spectroscopy analyses. Cu₃N and Cu₄N completely decomposed at 200 °C, enabling pure Cu–Cu low-temperature bonding. The interfacial adhesion energy at post-bond annealing temperatures of 250 °C, 300 °C, and 350 °C was 0.65 ± 0.05 J/m², 3.81 ± 0.61 J/m², and 4.12 ± 1.12 J/m², respectively. As the post-bond annealing temperature increased, Cu atomic diffusion was enhanced, leading to the elimination of voids and seams, grain growth, and the disappearance of grain boundaries. Consequently, the improved Cu‒Cu bonding quality resulted in an increase in interfacial adhesion energy.