Direct Wafer Bonding of 128° Y-Cut LiNbO3: A Pathway to High Frequency (50 GHz) Filter Applications

Abstract

In this work, room temperature direct wafer bonding of complementarily oriented 128° Y-cut LiNbO3 substrates is achieved with an interfacial layer less than 1 nm thick. This compares well to commercially available structures with thickness of 3 nm and this interfacial width is particularly important for eventual multilayer bonding sequences with each layer on the order of only tens of nm. The (0114) indices describe the surface plane of 128° Y-cut LiNbO3 wafers; and the wafers were bonded such that their in-plane [055.12] directions were antiparallel. Due to its trigonal crystal structure, this zone axis has 1-fold symmetry and therefore is crystallographically distinguishable from either side of the bonded interface. The wafers were cleaned with standard cleaning protocols and subsequently brought in contact face-to-face and bonded. The bonding was performed in ambient atmosphere and the applied pressure was on the order of ~kPa. The bonded pair was then annealed at 200 °C for 10 h in order to strengthen the bond, which is fully compatible with exfoliation of He-implanted LiNbO3 layers. Limited literature has been reported for direct bonding of LiNbO3 to LiNbO3 and even fewer investigate the atomic structure of the bonded interface.1-3 Tomita et al., observe structurally disordered bonded interfaces after annealing at high temperatures (up to 1000 °C). High resolution transmission electron microscopy of the interface presented here reveal an abrupt interface (≤ ~1 nm) and a φ X-ray scan has confirmed a relative twist of ~180° between substrates. This is a follow-up work for what has previously been achieved by our group demonstrating direct wafer bonding and layer transfer of LiNbO3 to substrates such as sapphire and oxidized silicon.4 Combining these latest results will enable the fabrication of a multilayer structure of complementarily oriented LiNbO3 thin films on a sapphire substrate, which has applications for high frequency (50 GHz) filters. Similar structures using a different LiNbO3 orientation (X-cut) have been demonstrated to maintain high quality factor (Q) at 15.8 GHz.5 The authors would like to acknowledge the support from DARPA through the Compact Front-end Filters at the ElEment-level (COFFEE) program.[1] Tulli, D., Janner, D. and Pruneri, V., “Room temperature direct bonding of LiNbO3 crystal layers and its application to high-voltage optical sensing.” Journal of Micromechanics and Microengineering, 21(8), p.085025 (2011).[2] Tomita, Y., Sugimoto, M., & Eda, K., “Direct bonding of LiNbO3 single crystals for optical waveguides.” Applied physics letters, 66(12), 1484-1485 (1995).[3] Berini, P., Mattiussi, G., Lahoud, N. and Charbonneau, R., “Wafer-bonded surface plasmon waveguides.” Applied physics letters, 90(6), p.061108 (2007).[4] Liao, M. E., Matto, L., Huynh, K., and Goorsky, M. S., “Direct Wafer Bonding of 128° Y-Cut LiNbO3 to Sapphire”. WaferBond ’22, Presentation (2022).[5] Lu, R., and Gong, S., 2021 Joint Conf. EFTF/IFCS, 1 (2021). Figure 1