Electrical Performance of Hybrid Bonding with Sub-Micron Cu-Cu Bonding Contacts: Effects of Scaling, Microstructure, and Surface Morphology

To understand the implications of ultra-fine pitch scaling of the interconnects (≤ 1 µm) enabled by hybrid bonding, we evaluated pre-bonding surface conditions of the dies and the electrical performance with the scaling of Cu pad size and pitch spacing. First, the test dies were coated with a surface protection layer that needed to be removed before hybrid bonding. The pre-bonding surface was then characterized using Fourier transform infrared (FTIR) and Raman spectroscopy techniques. The electrical performance was evaluated through four-point resistance measurements via Kelvin structures, with a focus on the correlation between the performance and the microstructural changes resulting from reduction in the pad size and pitch spacing. Electrical resistance values were measured for initial bonding quality assessments, followed by current stressing at elevated temperatures. The results show an increasing trend in initial resistance values and their spread as pad size and pitch spacing decrease, while the bonding yield decreases. High temperature testing improved bonding quality, leading to lower resistance values and higher yield. Additionally, in-situ current stressing data revealed a resistance decrease during the initial testing hours, likely due to diffusive densification and mechanical deformation induced by thermal compression. Finally, we correlate the Cu interconnect scaling with Cu pad microstructure developments for improved hybrid bonding quality.