Room-temperature transfer bonding of lithium niobate thin film on micromachined silicon substrate with Au microbumps

This paper introduces a room-temperature transfer bonding method for a future integration of LiNbO₃ thin-film device with a micromachined Si platform. A single-crystal LiNbO₃ thin film (5 μm thickness) prepared by mechanical polishing was successfully transfer-bonded onto a micromachined Si substrate with Au microbumps in ambient air using surface activated bonding. Tensile testing showed the strong bond strength between the Au thin film and the Au microbumps, which was sufficient for device applications. An air/LiNbO₃ thin film/air structure was demonstrated on a Si substrate using the proposed method. In addition, our simulation and experimental results showed that room-temperature bonding was essential to overcome the large coefficient of thermal expansion mismatch between LiNbO₃ and Si. We expect that this technology can be utilized to realize new configurations of highly-functional integrated microelectromechanical systems, including Si-based high-density integrated photonic devices.