(Invited) Bonding Strength of Cu-Cu Hybrid Bonding for 3D Integration Process
Authors: Fujii Nobutoshi , Furuse Shunsuke , Yoshioka Hirotaka , Ogawa Naoki , Yamada Taichi , Hirano Takaaki , Saito Suguru , Hagimoto Yoshiya , Iwamoto Hayato 
Published: February 2024

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 between the dielectrics films that exist in the interfaces of a Cu-Cu hybrid bond. Elucidating the mechanism of strength generation would offer a guideline to improve the bonding strength.
The dielectric films are bonded using a dehydration condensation reaction on the plasma-activated surface by thermal annealing. This bonding process has been widely studied from the perspective of the amount of surface Si-OH by considering the activation method and condition, type of dielectrics, and flatness of the surface. Because few investigations have considered factors other than the bonding interfaces, we focused on the influence of the H2O contained in the dielectric film to be bonded and investigated the relationship between the amount of H2O in the film and the bonding strength.
The deposition conditions of the dielectric films produced using Plasma Enhanced Chemical Vapor Deposition (PE-CVD) were varied to prepare five types of films that contained various amounts of H2O. After CMP treatment of each film, the surface roughness and amount of Si-OH were measured, and it was confirmed that there was no significant difference. When the bonding strength was obtained by bonding using these films and plotted against the amount of H2O, it was found that the bonding strength was dependent on the amount of H2O. In addition, after being bonded and annealed, it was confirmed that the expansion of the dielectric film depended on the amount of H2O, as seen in cross-sectional STEM images of the interface.
An investigation of the mechanism for this phenomenon suggested that the bonding strength was increased by filling the gap in the bonding interface through the expansion of the film. This study suggested that filling up the gap in the interface would be one approach for improving the bonding strength.
These results offer a deep understanding of the mechanism for the bonding strength of Cu-Cu hybrid bonds and will contribute to the development of 3D stacking technology in the future.
References
[1] P. Ramm, et al., “Handbook of Wafer Bonding”, Wiley-VCH Verlag & Co., 2012
[2] S. Furuse, et al., “Behavior of Bonding Strength on Wafer to Wafer Cu-Cu Hybrid Bonding”, ECTC Proc., pp591, 2023