Surface energy engineering for LiTaO₃ and α-quartz SiO₂ for low temperature (<220 °c) wafer bonding

Wafer bonding can be substituted for heteroepitaxy when manufacturing specific heterojunction-based devices. Devices manufactured using wafer bonding include multijunction solar cells, integrated sensors, heterogeneously integrated photonic devices on Si (such as high-performance laser diodes), Mach-Zehnder modulators, photodetectors, optical filters, and surface acoustic wave devices. In these devices, creating heterointerfaces between different semiconductors with heavily mismatched lattice constants and/or significant thermal expansion mismatch presents significant challenges for heteroepitaxial growth. High costs and poor yields in heavily mismatched heteroepitaxy can be addressed by wafer bonding in these optoelectronic devices and sensors, including the LiTaO₃/Si and LiTaO₃/SiO₂ heterostructures. In the present work, heterostructure formation between piezoelectric LiTaO₃ (100) and Si (100) and α-quartz SiO₂ (100) is investigated via wafer bonding. Direct bonding is selected instead of heteroepitaxy due to a significant thermal expansion mismatch between LiTaO₃ and Si-based materials. The coefficient of thermal expansion (CTE) of LiTaO₃ is 18.3 × 10^-6/K. This is 1 order of magnitude larger than the CTE for Si, 2.6-2.77 × 10^-6/K and 25-30 times larger than the CTE for fused SiO₂ and quartz (which ranges 0.54-0.76 × 10^-6/K). Thus, even at 200 °C, a 4 in. LiTaO₃/Si bonded pair would delaminate with LiTaO₃ expanding 300 μm in length while Si would expand only by 40 μm. Therefore, direct wafer bonding of LiTaO₃/Si and LiTaO₃/SiO₂ is investigated with low temperature (T < 500 K) Nano-Bonding™, which uses surface energy engineering (SEE). SEE is guided by fast, high statistics surface energy measurements using three liquid contact angle analysis, the van Oss/van Oss-Chaudhury-Good theory, and a new, fast Drop Reflection Operative Program analysis algorithm. Bonding hydrophobic LiTaO₃ to hydrophilic Si or SiO₂ is found to be more effective than hydrophilic LiTaO₃ to hydrophobic Si or SiO₂ temperatures for processing LiTaO₃ are limited by thermal decomposition LiTaO₃ into Ta₂O₅ at T ≥ 180 °C due to Li out-diffusion as much as by LiTaO₃ fractures due to thermal mismatch.