VUV/O₃ activated direct heterogeneous bonding towards high-performance LiNbO₃-based optical devices

The combination of lithium niobate (LiNbO₃) and glass without an intermediate layer is an essential component of micro/nanofluidics and optical waveguides. Direct bonding is a popular method for joining homo/heterogeneous materials into a single composite. However, direct bonding of LiNbO₃ and glass is extremely difficult due to the chemical inertness of LiNbO₃ and the large mismatch of the coefficients of thermal expansion of these materials. In this work, we realized direct bonding of LiNbO₃ and glass via VUV/O₃ activation at 150 °C. Focused ion beam microfabrication tests and observation by scanning electron microscopy and transmission electron microscopy confirmed the strong LiNbO₃/glass bonding interfaces. The excellent transmittance and piezoelectric properties of the direct bonded pairs and the single-crystal orientation of the LiNbO₃ close to the glass demonstrated that the VUV/O₃ activated direct bonding method was suitable for the fabrication of high-performance surface acoustic wave (SAW)-actuated LiNbO₃-based devices. Additionally, we investigated the bonding mechanism through water contact angles, atomic force microscopy, Raman spectroscopy, FTIR spectroscopy, and adhesion measurements.