High-frequency devices operating at 3 GHz or higher are required, for instance, for future 4th generation mobile phone systems in Japan. Using a substrate with a high acoustic velocity is one method to realize a high-frequency acoustic or elastic device. A Lamb wave has a high velocity when the substrate thickness is thin. To realize a high-frequency device operating at 3 GHz or higher using a Lamb wave, a very thin (less than 0.5 μm thick) single-crystal plate must be used. It is difficult to fabricate such a very thin single crystal plate. The authors have attempted to use a c-axis orientated epitaxial LiNbO(3) thin film deposited by a chemical vapor deposition system (CVD) instead of using a thin LiNbO(3) single crystal plate. Lamb wave resonators composed of a interdigital transducer (IDT)/the LiNbO(3) film/air gap/base substrate structure like micro-electromechanical system (MEMS) transducers were fabricated. These resonators have shown a high frequency of 4.5 and 6.3 GHz, which correspond to very high acoustic velocities of 14,000 and 12,500 m/s, respectively, have excellent characteristics such as a ratio of resonant and antiresonant impedance of 52 and 38 dB and a wide band of 7.2% and 3.7%, respectively, and do not have spurious responses caused by the 0th modes of shear horizontal (SH(0)) and symmetric (S(0)) modes.

Currently, an ultra-wideband resonator is greatly needed to realize a tunable filter with a wide tunable range, because mobile phones with multiple bands and cognitive radio systems require such tunable filters to simplify their circuits. Although tunable filters have been studied using SAW resonators, their tunable range was insufficient for the filters even when wideband SAW resonators with a bandwidth of 17% were used. Therefore, the fabrication of wider-bandwidth resonators has been attempted with the goal of realizing tunable filters with wide tunable ranges. In this study, an SH0- mode plate wave resonator in a 27.5°YX-LiNbO3 plate with an ultra-wide bandwidth of 29.1%, a high impedance ratio of 98 dB, and a high Q (Q(r) = 700 and Q(a) = 720) was realized. Two types of tunable filters were constructed using such SH0-mode resonators and capacitors. As a result, tunable ranges (bands) of 13% to 19% were obtained. The possibility of applying the SH0-mode resonator in the high-frequency gigahertz range is discussed.

Lamb wave modes can be classified as a fundamental anti-symmetric mode (A 0 ), a symmetric mode (S 0 ), and their higher order modes. Membrane-type A 1 mode Lamb wave resonators have been reported by the authors for future mobile communication systems. Although the reports show promising results, it is difficult to fabricate the resonators because they have a large dispersion of phase velocity and electromechanical coupling factor, and also are fragile. In this study, the authors suggest a solidly mounted resonator (SMR)-type S 0 mode Lamb wave resonator, which can solve the above-mentioned difficulty. An SMR-type S 0 mode Lamb wave resonator was successfully realized for the first time. The resonator exhibits a 1.5 times faster phase velocity and a 1/3 smaller temperature coefficient of frequency than the commonly used surface acoustic wave (SAW) devices composed of an Al electrode and a 36-48 YX-LiTaO 3 substrate.

It has been considered that it is difficult to realize a high-frequency device of 3 GHz or more, for instance, for a fourth generation mobile phone system in Japan, using a conventional surface acoustic wave (SAW) substrate. In this study, we attempted to fabricate a highfrequency resonator using Lamb waves, which has a high velocity and consists of a thin LiNbO 3 film deposited by chemical vapor deposition (CVD). As a result, a 1-port Lamb wave resonator composed of an electrode/thin epitaxial LiNbO 3 film/air-gap/base substrate was fabricated. The measured resonator has a high resonant frequency of 4.5 GHz, which corresponds to a very high velocity of 14,000 m/s, a large impedance ratio of 52 dB, and a relatively wide bandwidth of 7.2%.

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