Glass-on-LiNbO_3 heterostructure formed via a two-step plasma activated low-temperature direct bonding method

摘要

Lithium niobate (LiNbO3) crystal has been widely used in integrating optical and acoustic devices. However, direct bonding of LiNbO3 to Si-based materials is still a challenge due to the large coefficient of thermal expansion mismatches. In this paper, strong bonding strength of the LiNbO3/glass pair was obtained via the optimized O-2 plasma activation followed by N-2 plasma treatment. After the pre-bonding was accomplished at room temperature, the bonded pair was annealed at 150 degrees C for 12 h. A defect-free and tight bonding interface was confirmed by the scanning electron microscopy and transmission electron microscopy. Based on atomic force microscopy, water contact angle and X-ray photoelectron spectroscopy characterizations, both LiNbO3 and glass surfaces became smoother and more hydrophilic due to the densities of -OH and N-related groups increased. Additionally, combined with the bonding strength obtained via different plasma activated processes, we demonstrated that the mechanism of the enhanced bonding strength could be attributed to N-related covalent bonds formed across the bonding interface. Furthermore, the excellent optical transmittance of LiNbO3/glass bonded pairs indicated this facile bonding process can meet the demand of high-performance glass-on-LiNbO3 devices.