Observation of Thermal Expansion Behavior of Nanotwinned-Cu/SiO₂ & Regular-Cu/SiO₂ Hybrid Structure via In-Situ Heating AFM

One critical challenge in downscaling hybrid bonding is the decrease in the expansion of Cu pads in dielectric vias with decreasing dimensions. A deeper understanding on the mechanism of hybrid bonding is required. In this study, we investigated the thermal expansion characteristics of both nanotwinned-Cu (NT-Cu) and regular-Cu in SiO₂ hybrid structure using in-situ heating atomic force microscopy (AFM). The scanning microscopy was conducted from room temperature to 100, 150 and 200 under an argon atmosphere. It was found that the NT-Cu/SiO₂ via with 8 μm in diameter exhibited a 6-nm recess at room temperature, began to protrude at 150 , and eventually transitioned to a 4-nm protrusion at 200 . This result provides direct evidence of the hybrid bonding mechanism. The NT-Cu/SiO₂ samples could also be bonded at 150 for 2 h. The regular-Cu via exhibited a similar thermal expansion during the measurement. In contrast, the expansion of the regular-Cu via was discovered to be larger than that of the NT-Cu via, which could be attributed to the difference in Young's modulus. The plastic deformation was also observed to contribute to the total expansion in Cu/SiO₂ vias over 150 . This work provides deeper insight into the mechanism of hybrid bonding and design of Cu-Cu joints.