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

Abstract

The focus of this study is a detailed characterization of hybrid Cu/SiO2 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/SiO2 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 μm and pitch 18 μm) enclosed by SiO2 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 as well as 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 300/400 °C (up to 6 h). No significant changes in texture were found after the reliability tests. The <111> direction with its characteristic <115> twin orientation are 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 towards nearby high angle grain boundaries (60 to 45 °).

3D Integration via D2D Bump-Less Cu Bonding with Protruded and Recessed Topographies

“…It has previously been reported that the higher the nonplanarities, the greater the intergrowth and bond strength. [27][28][29] Figures 5f, 5i show a non-planar interface for the PR sample, confirming its good electrical performance. In Figs.…”

“…Similar observations for hybrid bonded samples were reported before. 27,29 Bond strength analysis.-The ultimate shear strength (USS) of the samples is plotted in Fig. 6a.…”

Recent progress on bumpless Cu/SiO2 hybrid bonding for 3D heterogeneous integration

“…Moreover, for the reliability problem, hybrid bonding may replace solder bump and underfill, which also means that without the stress-buffering underfill, the packaging structure faces the reliability problem again. To solve this problem, more simulation (Li et al, 2020;Ji et al, 2020c;Ji et al, 2019;Ji et al, 2020a;Ji et al, 2020b), material organization research (Panchenko et al, 2022;Panchenko et al, 2020;Kim et al, 2019) and new structure development work should be propelled in time. (Liu et al, 2021a); (e) a daisy chain structure bonded with a 10-nm Pt layer (Liu et al, 2021b); (f) quasi-direct homogeneous bonding achieved by the Pt layer (Kuwae et al, 2020)…”

Solder-less Fine Pitch Copper-to-Copper Bonding

“…Next steps to examine the interface quality and reliability include characterizing the grain structure of the copper pads. Grain structure has been known to influence bonding characteristics [7]. Therefore, it is critical to analyze how grain structure of copper affects the bonding process described here without a separate annealing process.…”