Piezoelectric ladder filter with series resonators having a pair of grooves and method of forming same

A ladder-type piezoelectric filter includes series resonators including substantially rectangular piezoelectric resonators adapted to vibrate in a longitudinal vibration mode, and parallel resonators including substantially square piezoelectric resonators adapted to vibrate in a square vibration mode. The series resonators and the parallel resonators are connected to each other via terminals so as to define a ladder circuit. A pair of grooves are respectively provided on one of the major surfaces of each series resonator at symmetrical positions with respect to the center-line which is substantially perpendicular to the longitudinal direction of the series resonator such that the pair of grooves extend in a direction that is substantially parallel to the center-line of the series resonator and divide the electrode on the one major surface to thereby adjust an electric capacitance between the pair of electrodes respectively disposed on the pair of major surfaces of the series resonator.