Review of Cu-Cu direct bonding technology in advanced packaging

Traditional Sn-based solder interconnects face reliability challenges due to their poor performance at narrow spacing. Driven by the increasing demands for higher performance, greater reliability, and enhanced integration capabilities in modern electronics, Cu-Cu direct bonding has emerged, which offers significant advantages, including narrower spacing, superior electrical and thermal conductivity, and enhanced reliability. However, achieving low-temperature Cu-Cu bonding remains challenging due to copper's high melting point and low self-diffusion rate. This study reviews the recent progress of Cu-Cu direct bonding technology on four parts including the enhancement on Cu microstructure, surface treatments, bonding processes and the assessment methods on performance and reliability. Promising Cu microstructures, such as nanotwinned Cu and nanocrystalline Cu were highlighted in facilitating low-temperature bonding. The performance of surface treatments on promoting bonding were also summarized, including chemical treatment, plasma activation and inert metal passivation. Further, some significant innovations on the bonding process and technology were indicated, and the evaluation methods for bonding quality were discussed. The current research progress provide significant guidance for the development of Cu-Cu bonding technology.