Microstructure Development of Cu/SiO₂ Hybrid Bond Interconnects After Reliability Tests

The focus of this study is a detailed characterization of hybrid Cu/SiO
₂
wafer-to-wafer bonding interconnects after reliability testing. Hybrid bonding (or direct bond interconnect) is a technology of choice for fine pitch bonding without microbumps. The main challenge of the hybrid bonding technology is the preparation of a clean Cu/SiO
₂
surface with controlled Cu dishing. The Cu/Cu interface after hybrid bonding and after reliability testing was investigated by electron backscatter diffraction (EBSD) in this study. The Cu interconnects (diameter
$4 ~\mu \text{m}$
and pitch
$18 ~\mu \text{m}$
) enclosed by SiO
₂
were formed by wafer-to-wafer bonding. The small diced stacks were used for subsequent reliability tests. Three types of tests were carried out: temperature shock test (TST) at −40 °C/125 °C up to 1000 cycles, isothermal storage at 150 °C, 300 °C, and 400 °C, and multiple bonding cycles. The results include a comprehensive estimation of the changes in grain diameter, grain boundaries, and texture of the interconnects. All samples showed good reliability and stayed intact after all tests. Grain refinement was observed after TST, storage at 150 °C, and multiple bonding cycles compared to the state after bonding. Grain growth was found for the storage at 300 °C/400 °C (up to 6 h). No significant changes in texture was found after the reliability tests. The
$\langle 111\rangle $
direction with its characteristic
$\langle 115\rangle $
twin orientation is the dominating orientations perpendicular to the bonding interface. The migration of 60° twin boundaries has been observed after reliability testing. The results indicate a rotation toward nearby high-angle grain boundaries (HAGBs, 60°–45°).