This disclosure provides implementations of electromechanical systems resonator structures, devices, apparatus, systems, and related processes. In one aspect, a resonator structure includes a first conductive layer of electrodes and a second conductive layer of electrodes. A piezoelectric layer including a piezoelectric material is disposed between the first conductive layer and the second conductive layer. One or more trenches can be formed in the piezoelectric layer on one or both sides in space regions between the electrodes. In some implementations, a process for forming the resonator structure includes removing an exposed portion of the piezoelectric layer to define a trench, for instance, by partial etching or performing an isotropic release etch using a XeF2 gas or SF6 plasma. In some other implementations, a portion of a sacrificial layer is removed to define a trench in the piezoelectric layer.