High-Q MEMS Capacitive-Gap Resonators for RF Channel Selection

Author(s): Wu, Lingqi | Advisor(s): Nguyen, Clark T. -C. | Abstract: On chip capacitive-gap transduced micromechanical resonators constructed via MEMS technology have achieved very high Q’s at both VHF and UHF range, making them very attractive as on-chip frequency selecting elements for filters in wireless communication applications. Still, there are applications, such as software-defined cognitive radio, that demand even higher Q’s at RF to enable low-loss selection of single channels (rather than bands of them) to reduce the power consumption of succeeding electronic stages down to levels more appropriate for battery-powered handhelds.This dissertation focuses on improving the performance of MEMS capacitive-gap resonators to the degree which can be used to build the aforementioned RF channel-select filters. It first aims to enhance quality factor of MEMS capacitive-gap resonators by suppressing vibration energy loss via device substrate, which will lead to low insertion loss in RF channel selection. Then, in order to reduce an RF front-end filter’s bandwidth and termination resistance, it explores the method of building micromechanical resonator array composites that include large number of mechanically coupled resonators. Finally, the dissertation presents an experimentally demonstrated RF narrowband filter built upon mechanically coupled high-Q resonator array composites.Pursuant to further increasing Q at UHF for low insertion loss RF channel select application, the thesis develops an equivalent circuit model of a radial contour mode disk resonator that can analytically predict anchor loss dominated Q. Indicated by this improved equivalent circuit model, this work “hollows” the stems supporting all-polysilicon micromechanical disk resonators to effectively squeeze the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q. By using the same fabrication process flow from the conventional all-polysilicon devices, the use of hollow stem support enhances Q with minimal increase in fabrication complexity. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. Measured Q’s as high as 56,061 at 329 MHz and 93,231 at 178 MHz for whispering gallery modes further attest to the efficacy of this approach.This dissertation also employs mechanically coupled disk array composites to increase resonator stiffness and lower motional resistance, which are both highly desired for RF front-end channel-select filters. By using half-wavelength coupling beams and proper electrode phasing design, measurements confirm that a 215-MHz 50-resonator disk array achieves 46.5× Q-normalized Rx reduction, with no observation of other undesired vibration modes. Notably, as indicated by the newly developed negative-capacitance equivalent circuit model, such array composite also shows enhanced frequency stability against dc-bias voltage fluctuations because of its large electrode-to-resonator overlap capacitance.Finally, the thesis demonstrates a 75MHz 3rd order 210 kHz bandwidth (0.3%) filter with a sharp roll-off of 20dB shape factor of 1.46. This filter employs three high-Q disk array composites connected by quarter-wavelength rotational coupling beams to achieve a weak coupling for narrowband selection. Each array composite itself includes seven flexural disk resonators coupled by strong quasi-zero length beams to enforce desired response. By using electromechanical analogies, the equivalent electrical circuit model of this filter can accurately capture the device’s response and provide insights for filter designers.Most importantly, the accuracy of the described equivalent circuit model in predicting quality factor, frequency stability, and filter response encourages the design of even more complex micromechanical circuits to come, for example, as would be needed in an all-mechanical RF front-end.