Effect of Undercut due to Isotropic Etch while Releasing on the Performance of TPoS Resonators

This paper reports and validates the effect of additional loss due to the isotropic etch used to release the extensional mode thin piezoelectric on silicon (TPoS) resonators. Devices fabricated to operate at a resonant frequency around 1 GHz are kept at varying times in vapor HF to remove the buried oxide, thereby changing the additional area etched. When devices designed with different geometries are present in the same wafer, the etching times differ, causing more undercuts than required in some of the devices. The Quality factor (Q) calculated from the measured transmission characteristics is used to derive a semi-analytical model for the additional loss due to undercut. The total quality factor of these devices is estimated after adding other loss mechanisms with the help of simulations and is validated with measured experimental data from our group as well as those found in the literature. The Q predicted using this model matches the measured data for state-of-the-art devices with different frequencies, dimensions, and materials.