Enhancing Surface Anisotropy of LiTaO3 (110) and LiNbO3 Piezoelectrics via Surface Energy Engineering (SEE) for Nano-Bonding™ to Si and SiO2 at T < 453 K.*

LiTaO3 and LiNbO3 are strong candidates for Voice Activated Chips (VAC). Li-based metallic oxides (LiXO3) are the strongest candidates for VAC due to their large piezoelectric coefficient (d33).  LiTaO3 has a d33 of 5.7 and LiNbO3 has a d33 of 6.0 pC/N. This work investigates the bonding of LiTaO3 and LiNbO3 to Si. 

LiTaO3 and LiNbO3  can be bonded to Si via heteroepitaxy or Direct Wafer Bonding at T > 473 K. The Coefficient of Thermal Expansion (CTE) of LiTaO3 is about 8 times greater than Si's CTE. Hence, there are large mechanical strains and crystal defects at the interface due to lattice and thermal expansion mismatch. Hence bonding at T > 473 K fractures brittle LiTaO3, and decomposes into Li and TaO3.

In this work, Surface Energy Engineering (SEE) is used to modify surface energies (γT) of crystal wafers into far-from-equilibrium states prior to Nano-Bonding™ at T < 473K. Three Liquid Contact Angle Analysis (3LCAA) and the van Oss-Chaudhury-Good theory yield γT. For example, anisotropic LiTaO3 has water contact angles varying around 45 ± 5.4° in a range of 17 ± 1°. Hence, γT varies along crystal directions by ~ ± 6%, due to anisotropy. The Nano-Bonding™ of LiTaO3 to Si is thus investigated after SEE.