Patent ID: 7535154

Claim:
A piezoelectric resonator comprising: a substrate; and a thin film unit including a portion supported by the substrate and a portion acoustically isolated from the substrate, the portion acoustically isolated from the substrate including a vibration unit and an additional film, the vibration unit including a piezoelectric film sandwiched between a pair of electrodes and having a Poisson ratio less than or equal to about 1/3, the piezoelectric film being overlapped with the pair of electrodes in plan view, the additional film being disposed on one of the piezoelectric film and the electrode so as to extend along at least a portion of a periphery of the vibration unit in plan view; wherein when x (MN·second/m 3 ) denotes an acoustic impedance of the additional film defined by the square root of the product of the density and Young's modulus, A denotes the product of the density and the thickness of the additional film, B denotes the product of the densities and the thicknesses of the electrodes, and y=A/B, the A/B ratio y satisfies one of the following conditional expressions (1a) and (1b): In the range of 9.0 ≦x <44.0, 0.0092 ·x +0.88 ≦y <0.067 ·x +0.60 (1a) In the range of 44.0 ≦x <79.0, −0.0035 ·x +1.45 ≦y <0.015 ·x +2.9 (1b).