First Demonstration of Superior Characteristics of Co-Co Bonding with Passivation Structure at Low Thermal Budget for Advanced Packaging and Ultrafine Pitch Applications

This study investigates Co-Co direct bonding with and without Ag passivation, with a focus on bonding quality and electrical performance. Bonding quality was confirmed through SAT and SEM analyses, revealing excellent bonding results. The electrical measurements showed that the Ag-passivated structure achieved a specific contact resistance of approximately $10^{-7} \Omega \cdot ~\text{cm}^{2}$, surpassing the performance of direct Co bonding. Therefore, the results confirmed that Ag passivation structure significantly reduced the bonding temperature compared to direct Co bonding. XPS depth profile analysis further demonstrated that Ag passivation can effectively prevent surface oxidation while allowing cobalt diffusion through the passivation layer. These findings underscore the potential of Ag passivation for highquality, low-temperature Co-Co bonding for next-generation three-dimensional integration technologies, highlighting the potential of novel fine-pitch applications for advanced packaging.