Performance optimization of lateral-mode thin-film piezoelectric-on-substrate resonant systems

The main focus of this dissertation is to characterize and improve the performance of thin-film piezoelectric-on-substrate (TPoS) lateral-mode resonators and filters. TPoS is a class of piezoelectric MEMS devices which benefits from the high coupling coefficient of the piezoelectric transduction mechanism while taking advantage of superior acoustic properties of a substrate. The use of lateral-mode TPoS designs allows for fabrication of dispersed-frequency filters on a single substrate, thus significantly reducing the size and manufacturing cost of devices. TPoS filters also offer a lower temperature coefficient of frequency, and better power handling capability compared to rival technologies all in a very small footprint. Design and fabrication process of the TPoS devices is discussed. Both silicon and diamond substrates are utilized for fabrication of TPoS devices and results are compared. Specifically, the superior acoustic properties of nanocrystalline diamond in scaling the frequency and energy density of the resonators is highlighted in comparison with silicon. The performance of TPoS devices in a variety of applications is reported. These applications include lateral-mode TPoS filters with record low IL values (as low as 2dB) and fractional bandwidth up to 1%, impedance transformers, very low phase noise oscillators, and passive wireless temperature sensors

Temperature Coefficient Of Frequency In Silicon-Based Cross-Sectional Quasi Lam E; Mode Resonators

Temperature coefficient of frequency (TCF) is studied in silicon-based cross-sectional quasi Lamé modes (CQLMs). Such modes are demonstrated in thin-film piezoelectric-on-silicon (TPoS) resonators and the TCF curves are modeled using eigenfrequency analysis in COMSOL for highly n-type doped silicon. It is shown that the ratio between the finger-pitch and the resonator thickness affects the turnover temperature of these resonators which could be predicted using this model. The CQLM-TPoS resonators fabricated on a 40\mu\mathbf{m} thick SOI substrate, are characterized and the measured TCF values are confirmed to be in close agreement with the prediction. A relatively high turnover temperature of \u3e100°C is reported for a third-order CQLM-TPoS resonator aligned to \u3c100\u3e silicon plane while a turnover temperature of \u3c20°C is recorded for the \u3c110\u3e counterpart

Investigation Of Phonon-Carrier Interactions In Silicon-Based Mems Resonators

In this work, a technique is introduced for isolating the energy loss associated with the interaction of charge carriers with acoustic phonons in thin film piezoelectric-on-silicon (TPoS) MEMS resonators. This method facilitates the investigation of acoustoelectric loss mechanism. The variation in quality factor (Q) and insertion loss of high frequency TPoS resonators is reported while the surface carrier concentration of the silicon layer is varied through application of a voltage to the metal-dielectric-silicon capacitor that is intrinsically formed during the conventional fabrication of TPoS resonators. A maximum of 3% improvement in the insertion loss (IL~9 dB) of a ~926 MHz resonance mode is recorded when a 4 V bias is applied to the said capacitance which is believed to stem from a reduction in interaction of acoustic phonons with carriers

Cross-Sectional Quasi- Lamé Modes In Thin-Film Piezoelectric-On-Silicon Resonators

In this work, it is demonstrated for the first time that cross-sectional quasi-Lamé modes (CQLM) could be efficiently excited in silicon with reasonably high quality factor (Q). Third-harmonic Lamé modes of a silicon block are piezoelectrically exited in thin-film-piezoelectric on silicon (TPoS) resonators in which the thickness is chosen to be in proximity of half acoustic wavelength. Finite element analysis is used to show that the support loss in these resonators could be reduced by an order of magnitude through usage of acoustic isolation techniques. A quality factor of 14,500 is measured in partial vacuum for a third-harmonic 67 MHz CQLM-TPoS resonator designed within a circular acoustic isolation frame and fabricated on a 40μm thick silicon-on-insulator (SOI) substrate