In the last decade, the implementation of three-dimensional integration circuit (3D IC) technology has received much attention in the semiconductor industry. 3D Through Silicon Via technology garnered much attention from the semiconductor industry as it can potentially integrate two or more active dies with diverse technologies with passive components in a single package by an intermediate metal bonding process. In this work, the utilization of ultrathin palladium (Pd) as an effective passivation layer to achieve low pressure and thermal budget Copper-Copper (Cu-Cu) bonding is reported. The effect of Pd thickness on bond quality was systematically studied with a specific focus on inter-diffusion behavior, surface roughness, and controlling surface oxidation of the Cu surface. As a result of this study, upon various surface and interface investigations, it was observed an ultra-thin 3nm palladium layer on the Cu surface not only inhibited surface oxidation but also reduced the surface roughness from about 2.1 nm (pure Cu surface) to about 0.89 nm. This resulted in Cu-Cu bonding at temperatures as low as 140 °C and operating pressures as low as about 3 bar, with very short bonding time. This simple passivation mechanism with enhanced diffusion of Cu across the bonding interface is attributed to the high diffusion constant of Pd in Cu, which resulted in grain growth across the entire interface bonding at low thermal budgets. The proposed approach could be a potential candidate for the future vertical interconnection of dies at lower thermal budgets. © 2022 IEEE.