Research on the Crystallinity of Metal Passivation Layer for Improving Low-Temperature Copper Bonding Performance

The study of copper bonding for hybrid bonding is crucial for high-performance devices; however, copper, which is easily oxidized, faces challenges in low-temperature bonding. Metal passivation of copper bonding has emerged as an appropriate solution to address these issues. This research compares the properties of metal passivation layers (titanium, platinum, tantalum, chromium) that protect the copper layer from native oxidation, and investigates their diffusion behavior and the underlying causes. The effects of copper diffusion on grain size and surface roughness are compared, along with the identification of additional factors. It was found that the crystallinity of the metal passivation layer aligns more closely with the trends in diffusion behavior than grain size and surface roughness. The interfaces after the actual bonding process were examined for metal passivation specimens with different crystallinities. Differences in diffusion behavior based on crystallinity were consistent with XPS results across various conditions, including bonding temperature, ramp-up, and pressure. Further experiments were conducted to investigate the influence of crystallinity on diffusion behavior in greater detail. The deposition methods were varied to alter the crystallinity of the passivation metals, and factors including grain size and surface roughness were compared. Additionally, bonding and diffusion experiments were performed to observe the behavior. This research aims to enhance the understanding of copper bonding and is expected to positively influence advanced packaging applications such as hybrid bonding.