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 un-thinned 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. Copyright © 1983 by The Institute of Electrical and Electronics Engineers, Inc.