Plate wave resonator using rotated Y-cut single crystal LiTaO3 thin film made by ion implant technology

Realization of a high frequency resonator for 3.6 GHz-band filter in the International Mobile Telecommunication Advanced (IMT-Advanced) system is desired. It is difficult to accomplish such a high frequency resonator using conventional surface acoustic wave (SAW) technology. The strip width of the inter-digital transducers (IDT) becomes too narrow in the range of 0.2-0.3 μm, because it has a low SAW velocity of approximately 4000 m/s. Therefore, the formation of IDT is challenging. The authors attempted to realize A1 mode Lamb type plate wave air-gap type membrane resonator by using rotated Y-cut single crystal LiTaO₃ thin film. The calculation results of various propagation characteristics in plate wave are presented. Rotated Y-cut single crystal LiTaO₃ thin film is difficult to obtain by conventional thin film methods. We focus on utilization of the Smart-cut^® process. Smart-cut^® process is usually used for manufacturing of silicon on insulator (SOI) wafers. We show that such technique can be applied to form the membrane structure of a single crystal LiTaO₃ thin film. The plate wave resonator based on rotated Y-cut single crystal LiTaO₃ thin film has a high resonant frequency of 3.38 GHz, a large k² of about 8 %, and the impedance ratio of 63 dB.