Surface Energy Engineering For Litao3 And Alpha-Quartz Sio2 For Low Temperature (< 220 Degrees 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 alpha-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 18.3x10(-6)/K. This is 1 order of magnitude larger than the CTE for Si, 2.6-2.77x10(-6)/K and 25-30 times larger than the CTE for fused SiO2 and quartz (which ranges 0.54-0.76x10(-6)/K). Thus, even at 200 degrees C, a 4in. LiTaO3/Si bonded pair would delaminate with LiTaO3 expanding 300 mu m in length while Si would expand only by 40 mu m. Therefore, direct wafer bonding of LiTaO3/Si and LiTaO3/SiO2 is investigated with low temperature (T<500K) Nano-Bonding (TM), 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 degrees C due to Li out-diffusion as much as by LiTaO3 fractures due to thermal mismatch.