There is disclosed acoustic resonators and filter devices. An acoustic resonator includes a substrate and a piezoelectric diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) has interleaved fingers on the diaphragm. A thickness of the interleaved fingers is greater than or equal to 0.85 times a thickness of the diaphragm and less than or equal to 2.5 times the thickness of the diaphragm.