High frequency lamb wave resonator using LiNbO₃ crystal thinplate and application to tunable filter

A cognitive radio system using a frequency from 400 MHz to 6 GHz requirestunable filters with a wide variable frequency range. The cognitive radio alsorequires a tunable filter in high frequency such as 5 to 6 GHz among them. Ultrawide band resonators are effective to realize their tunable filters. A Lambwave on a LiNbO₃ this plate having a high velocity and a largeelectro-mechanical coupling factor is suitable for their resonators. Ultra wideband resonator in high frequency requires an optimum large reflectioncoefficient and a substrate with a large electro-mechanical coupling factor anda high velocity. This time, authors attempt to fabricate a higher frequency Lambwave resonator on a LiNbO₃ single crystal thin plate instead of theLiNbO₃ thin film, though it has been considered that it is difficultto make a very thin LiNbO₃ plate such as thinner than 1 m. A newone-port Lamb wave resonator was fabricated on an Al-electrode/ LiNbO₃crystal thin plate/air gap/Si substrate with the plate thickness of 0.15(actual thickness395 nm). It showed a high resonant frequency (fr) of 5.44 GHz,a high anti-resonant one (fa) of 6.09 GHz, a wide band-width of 12%, and a largeimpedance ratio of 62 dB at the fr and the fa. Moreover, the authors attemptedto construct theoretically the ladder type of tunable filter using this Lambwave resonators. The filter with the tunable range of 9% from 5.58 to 6.06 GHzhave been obtained.