Oscillators and filters are necessary components in the super-heterodyne transceivers which are widely used in various modern wireless communication systems. Today, a majority of oscillators and filters used in transceivers are realized by off-chip acoustic resonators, which offers higher quality factor (Q) compared to LC tank circuit realized by complementary metal oxide semiconductor (CMOS) technology. Among different acoustic resonators, quartz crystal resonators are most popular for the application of oscillators due to its high frequency stability against temperature. For filters, thin-film bulk acoustic wave resonators (FBAR) and surface acoustic wave (SAW) resonators are most favored because of their high Qs. However, the fabrication process of quartz crystal resonators and SAW resonators are not compatible with current CMOS process, which makes on-chip solutions impossible. Although FBARs is CMOS-compatible, their resonant frequencies are determined by the film thickness which makes it challenging to realize a multiple-frequency solution on a single die. To address these problems, laterally-vibrating micromechanical resonators realized by micro-electro-mechanical system (MEMS) technologies have been proposed. Compared to above-mentioned acoustic resonators, laterally-vibrating MEMS resonators offer small form factor, reduced barriers to CMOS integration and the capability to realize the multiple-frequency solution on a single die. The most commonly used types of MEMS resonators include capacitive resonators and piezoelectric resonators. This thesis will first discuss issues pertaining to capacitive resonators to set the context of the rest of this thesis that focuses on piezoelectric resonators.
One of the main issues for capacitive resonators is their typically low electromechanical coupling efficiency. In this thesis, a fully differential electrical characterization configuration setup is first proposed which allows a significant increase in the electromechanical coupling efficiency (14dB) by use of piezoresistive readout while also significantly cancel parasitic capacitive feedthrough (53dB). It is found that the electromechanical coupling efficiency using even piezoresistive readout with capacitive drive still cannot compare to piezoelectric resonators.
Then the thesis moves to the studies of laterally-vibrating thin-film piezoelectric-on-silicon (TPoS) resonators which normally offer much higher electromechanical coupling compared to capacitive resonators. As a preliminary step, an analytical model that more accurately captures the electro-mechanical behavior of laterally-vibrating TPoS resonators is proposed. The proposed modeling approach considers the actual vibration mode shape and allows more accurate prediction of the key lumped parameters in both the mechanical and electrical domain as a function of electrode coverage. Compared to an existing 1D approximation model proposed in a seminal work, the lumped parameters derived from the proposed model show notably closer agreement with 3D coupled-domain finite element (FE) simulations and also the measured results from fabricated devices. In addition, it is shown that the actual vibration mode shape of the TPoS resonator has significant out-of-plane bending due to the stiffness mismatch between AlN film and SCS substrate.
The main issue for TPoS resonators lies in that reported values of Q are still much smaller than those achievable in pure silicon capacitive resonators. To improve Q, better understanding of the main sources of damping in TPoS resonators is needed. This thesis approaches this objective by investigating the temperature dependence of Q of TPoS resonators as the devices are cryogenically cooled from room temperature to 78K. Four designs of TPoS resonators with resonant frequencies in the range of 48MHz to 142MHz were considered. The experimental results from the four designs show that the notable difference in Q observed at room temperature due to differences in the support designs corresponds well with differences in the trends of Q as a function of temperature. This demonstrates the usefulness of cryogenic cooling as a practical approach to ascertain the dominance of temperature-independent energy losses (e.g. anchor loss) in TPoS resonators at room temperature. The experimental results also suggest that the anchor loss plays a major role in setting Q for most of the TPoS resonators under test.
To reduce the anchor loss in laterally-vibrating TPoS resonators, this thesis proposes two strategies which enhance Q by using biconvex edges and etch-holes respectively. The proposed biconvex design serves to confine the acoustic energy to the center of the resonators, thus reducing out-of-plane bending on the supporting tethers that contribute to acoustic energy leakage, thereby enhancing Q. The experimental results demonstrate that the biconvex design concept can be scaled and applied across a range of operating frequencies from 70 to 141MHz with the notable effect of boosting Q by 4-10 times relative to conventional flat-edge designs. In comparison, the proposed etch-hole-assisted design serves to reduce the axial movement on the supporting tethers, thus boosting Q. The measured results show threefold increase in Q of 105MHz TPoS resonators by strategically placing small square etch-holes. The presented results suggest that both out-of-plane bending and axial movement on the tethers contribute to anchor loss in TPoS resonators.