Bonding properties and interfacial diffusion mechanism of low-temperature Cu–Cu bonded joints using electroless Ag layer

Cu–Cu bonding is a promising technology for next-generation semiconductor packaging due to its superior electrical and thermal properties. In this study, an Ag layer was deposited onto Cu via electroless plating, followed by Cu–Cu bonding at a low temperature of 220 °C. The Ag thickness was controlled by adjusting the time and temperature during the strike and electroless plating processes. These conditions also affected the surface roughness and morphology, which in turn influenced the interface structure and bonding quality. Cross-sectional analysis revealed that Cu diffused to the center of the bonding interface. Because Ag and Cu do not form additional stoichiometric compounds at 220 °C, the Ag was displaced from the center. Following post-bonding annealing at 250 °C, the Cu initially located at the joint center was redistributed away from the center region, while Ag diffused toward the joint center and became repositioned. This redistribution significantly reduced the number of voids, resulting in higher shear strength after annealing. Excellent bonding interface quality was confirmed through cross-sectional TEM analysis, SAT observations, electrical characterization, and mechanical measurements. This study elucidates the interdiffusion behavior and the role of an Ag layer in tailoring interfacial diffusion and vacancy behavior at interfaces, thereby advancing the fundamental understanding of surface and interface phenomena relevant to high-density semiconductor packaging.