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

A cognitive radio system using a frequency from 400 MHz to 6 GHz requires tunable filters with a wide variable frequency range. The cognitive radio also requires a tunable filter in high frequency such as 5 to 6 GHz among them. Ultra wide band resonators are effective to realize their tunable filters. A Lamb wave on a LiNbO₃ this plate having a high velocity and a large electro-mechanical coupling factor is suitable for their resonators. Ultra wide band resonator in high frequency requires an optimum large reflection coefficient and a substrate with a large electro-mechanical coupling factor and a high velocity. This time, authors attempt to fabricate a higher frequency Lamb wave resonator on a LiNbO₃ single crystal thin plate instead of the LiNbO₃ thin film, though it has been considered that it is difficult to make a very thin LiNbO₃ plate such as thinner than 1 μm. A new one-port Lamb wave resonator was fabricated on an Al electrode/LiNbO₃ crystal thin plate/air gap/Si substrate with the plate thickness of 0.15A (actual thickness = 395 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 large impedance ratio of 62 dB at the fr and the fa. Moreover, the authors attempted to construct theoretically the ladder type of tunable filter using this Lamb wave resonators. The filter with the tunable range of 9% from 5.58 to 6.06 GHz have been obtained.