(Invited) Bonding Strength of Cu-Cu Hybrid Bonding for 3D Integration Process

Introduction Cu-Cu hybrid bonding is a significant technology in the manufacture of 3D stacked semiconductor devices and is indispensable for the integration of 3D heterogeneous stacked packages. 1 A process is used to thin the bonded semiconductor during 3D stacking integration, but delamination can occur when the bonding strength of the bonded interface is insufficient. Therefore, the bonding strength is a key factor in the 3D stacked integration process, and mechanisms for improving the strength have been thoroughly investigated. Cu-Cu hybrid bonding strength During Cu-Cu hybrid bonding, the bonding strength between bonded wafers, which is a crucial factor, is complicated to describe because of the various materials contained in the bonding interface. Because the bonding interface includes not only Cu/Cu and dielectric/dielectric films, but also Cu/dielectrics as a result of the misalignment of Cu pads, the strength of the Cu-Cu hybrid bond needs to comprehensively account for these interfaces. We investigated the relationship between the bonding strength and amount of misalignment using patterned Si substrates that were bonded after intentionally being aligned with an offset and thermally annealed. Although the bonding strength before annealing was confirmed to decrease lineally depending on the amount of misalignment, the wafers showed a different behavior after annealing in the region where the amount of misalignment was low. Simulation calculations elucidated the behavior of the overall bonding strength. When a Cu pad with a large recess contacted the opposing Cu pad, an uneven amount of pressure was generated between their contact surfaces because of the thermal expansion produced by the annealing. An area with very low pressure was also found between the surfaces, and extremely little or no diffusion occurred in this area. 2 This phenomenon was not observed in a case where the amount of recess was small in both the experiments and simulation calculations, which showed linear relationships dependent on the misalignment after annealing. This study determined the misalignment dependence of the bonding strength after annealing considering the recess of the Cu pad, and an integrated model of the comprehensive bonding strength that included three interfaces involved in the Cu-Cu hybrid bonding could be presented. Improvement of bonding strength between dielectric films We are also continuing to study ways to improve the bonding strength of the interface