Single-crystalline LiNbO₃ integrated onto Si-based substrates via Ar plasma-activated low-temperature direct bonding

Lithium niobate (LiNbO3) crystals are multifunctional materials with excellent performance and are widely used in integrated optical devices. In this study, 4-inch LiNbO3/Si and LiNbO3/SiO2/Si bonded pairs were obtained by optimizing Ar plasma activation. After pre-bonding was completed, a slicer was used to cut the LiNbO3/Si and LiNbO3/SiO2/Si pairs into 10 x 10 mm(2) squares, respectively. The optimal annealing temperature was determined through multiple annealing experiments. Scanning acoustic microscopy was used to confirm the high bonding rates of the two bonding pairs. Based on hydrophilic experiments and atomic force microscopy, the changes in the hydrophilicity and roughness of the LiNbO3, Si, and SiO2 surfaces before and after activation can be compared. X-ray photoelectron spectroscopy was used to characterize the chemical structure composition of LiNbO3, Si, and SiO2 surfaces. The dense interface without defects was observed by transmission electron microscopy. In addition, we explained the bonding mechanism between LiNbO3 and Si-based materials. The reasons for the different bonding strengths of LiNbO3 with Si and SiO2 were also analyzed. Finally, the high bonding quality of LiNbO3 and Si-based materials can meet the stringent material requirements of Si-based LiNbO3 devices.