Design and Implementation of Solid/Solid Phononic Crystal Structures in Lateral Extensional Thin-film Piezoelectric on Silicon Micromechanical Resonators

Achieving high quality factor in MEMS resonator devices is a critical demand for today’s wireless communication and sensing technologies. In order to reach this goal, several dedicated prior works have been conducted based on published literature at different frequency ranges. Particularly, piezoelectrically transduced resonators, which are widely deployed in commercial wireless communication systems, could benefit from greatly improved qualify factor. So far, their development has evolved from thin film bulk acoustic resonators (FBAR’s) using surface attached piezoelectric thin-film transducers with moderate Q factors to high Q resonators equipped with a side-supporting tether (anchor) attached vibrating resonators that allow the devices to operate at very high frequency (VHF) and ultra-high frequency (UHF) ranges. This dissertation presents a newly developed fabrication methodology to replace existing expensive SOI technologies with much cheaper single crystalline wafers using a modified Single-Crystalline Reactive Etched and Metallization (SCREAM) process. Piezoelectrically transduced MEMS resonators have been fabricated at USF cleanroom facility, which have been designed and tested successfully in air with a quality factor of 1,528 and an insertion loss of -32.1 dB for a disk shaped resonators. A quality factor of 1,013 along with an insertion loss of -19 dB have been achieved for a rectangular plate resonator. In these devices, varied silicon layer thickness ranging from submicrons to tens of microns from a single layer were achieved as opposed to an uniform thickness of the device layer across the silicon-on-insulator (SOI) wafers, allowing device batch fabrication while maintaining the same number of photolithography steps. Resonators with varied Si resonator structure layer thickness have been implemented and studied in terms of motional resistance (Rm), quality factor (Q) and resonance frequency. To our best knowledge, this work has pioneered the implementation of soild/soild phononic crystals (PnCs) in fully suspended, lateral extensional and contour mode bulk acoustic wave (BAW) resonators. The in-house fabrication of the PnCs was performed on silicon-on-insulator (SOI) substrate. Silicon and tungsten were chosen as alternated layers for PnCs with a 4.5 ratio of acoustic impedance mismatch between the two chosen solid materials. The analysis of solid/solid PnCs bandgap is also conducted for determining the frequency regime, where no phonons exist. PnCs are strategically designed with piezoelectric transduction mechanism to operate within the phononic bandgap regime. Finite Element method (FEM) is also performed to investigate PnCs behavior in acoustic wave rejection, in which it was evaluated to be ~11 dB rejection per crystal. Lastly, the fully released thin-piezo on silicon (TPoS) resonators in this work have been fabricated, characterized and modeled. The work of fabricating fully released BAW resonators with embedded PnCs one of the pioneering work of solid/solid PnCs in the MEMS resonator field. The electrical equivalent circuit parameters of the devices were extracted and the quality factors for these devices have shown 7-10 times enhancement as compared to counterparts without PnCs