Coupling of interdigital transducer to plate modes in a slotted acoustically-thin membrane

Piezoelectric coupling with an interdigital transducer (IDT) to plate modes in a composite ZnO-SiO2-Si membrane that is thinner than an acoustic wavelength has been examined for possible use in forming on-chip filters or resonators. Tests at 80 MHz on a composite membrane approximately 45 µm thick with IDT´s having a 76 µm period indicates preferential coupling to the lowest quasi-symmetric plate wave having a phase velocity roughly 60% higher than that of the ordinary surface-acoustic-wave (SAW) on the unthinned portion of the same silicon wafer. Results are presented for a resonator made on such a structure in which end reflectors are single slots etched in the membrane. Numerical calculations confirm the observation and reveal that the electromechanical coupling coefficient is 1.4% as compared with 0.1% for ordinary SAW´s on semi-infinite substrate.