Development of Directly Bonded LiNbO3 Plates and Microfabricated Quartz Crystal Structures

Surface acoustic wave (SAW) devices are widely used in frequency filters in mobile communication due to their wide frequency band, small dimensions and productivity. However, conventional SAW device structures consisting of piezoelectric plates and interdigitated electrodes have a high-frequency operation limit of approximately 4 GHz because of electrode miniaturization limits and power resistances. SAW devices fabricated using heterostructures or thinning piezoelectric plates have been actively studied to achieve higher frequency operations. Recently, Lamb mode operation in a novel SAW structure with a thin LiNbO3 plate on microfabricated quartz crystal as a support substrate has been proposed in theoretical calculations. Such a SAW structure is promising for future mobile communication applications due to the improvement of temperature coefficient of frequency (TCF) and mechanical strength compared with general self-standing Lamb mode device structures. This study focused on the direct bonding of single-crystal LiNbO3 plates onto microfabricated quartz crystals via a unique anisotropic etching technique with HF gas and catalytic materials. Utilizing this anisotropic etching process, extremely low roughness (Ra = 0.14 nm) of the microfabricated quartz surface comparable to that on the polished surfaces was obtained without using hard mask materials. Furthermore, the direct bonding of the microfabricated quartz to a LiNbO3 plate was demonstrated without requiring additional polishing. This study paves the way for the efficient mass production of SAW devices utilizing microfabricated quartz crystal substrates.