An Insertion-Based Mechanical Interlock Cu-Sn Bonding Structure for Three-Dimensional Integration

In three-dimensional integration, multi-chip bonding requires substantial processing time. This paper proposes a novel mechanical interlock Cu-Sn bonding structure, which can significantly reduce the occupation time of bonding equipment. The columnar bumps are inserted into the annular bumps with pressure in the bonding machine to make the plastically deformed Sn fill the annular bumps and form a mechanical interlock structure. Subsequently, Cu-Sn solid-liquid interdiffusion (SLID) bonding or solid-state diffusion (SSD) bonding is performed by thermal treatment. This structure can be applied in chiplet assembly, where multiple chips can be gang-assembled onto one certain substrate then annealed simultaneously, avoiding reliability issues caused by repeated heating of the chiplets and substrate. We conducted five sets of experiments and determined the following two process conditions. For Cu-Sn SLID bonding, the insertion parameters include 25 degrees C temperature, 80 MPa pressure, 120 s load hold time, and the thermal treatment parameters include 260 degrees C temperature, 20 MPa pressure, 60 s dwell time. For Cu-Sn SSD bonding, the insertion parameters include 25 degrees C temperature, 300 MPa pressure, 120 s load hold time, and the thermal treatment parameters include 150 degrees C temperature, 4 h dwell time. The formation of intermetallic compounds (IMCs) has been successfully observed. The shear strength is 75.62 MPa for Cu-Sn SLID bonding and 85.37 MPa for Cu-Sn SSD bonding.