Internal electrostatic transduction of micromechanical resonators [electronic resource]
Keywords: Electrical Engineering

Micromechanical signal processors have a great potential to revolutionize the wireless system architectures by offering a pathway toward full system integration, miniaturization, and lower power consumption. Laterally driven electrostatic micromechanical resonators with high quality factors and frequencies beyond 1GHz have been previously demonstrated. One of the major challenges in the integration of electrostatic MEMS resonators with integrated circuits has been their high motional resistance. Internal electrostatic transduction is introduced as a solution to achieve micromechanical resonators in the radio and microwave frequencies, with low motional resistances, and high quality factors in air. A manufacturable double nanogap process introduced in this thesis allows for fabrication of many high aspect-ratio nano-scale dielectric "gaps" within a single resonator using a combination of lithography and dielectric deposition. Quartz is introduced as a desirable substrate for MEMS resonators, allowing for direct two-port measurements at RF and microwave frequencies. Higher-order mode internally transduced ring resonators are designed, fabricated, and tested in this work. A fifth order dielectrically transduced ring resonator at 1.95GHz is measured on a quartz substrate using direct two-port transmission measurement. The proper placement of dielectric within the resonating structure allowed for a Q of greater than 9000 in air. The concept of serpentine internal electrostatic transduction is introduced, which allows for many capacitances to be transduced in parallel, resulting in a lower motional resistance. The 41st radial bulk mode of a two-port serpentine ring resonator was demonstrated at 2.74 GHz, with a quality factor of 7000 in air. This resonator has a high f x Q product of 1.93x1013, with a low motional resistance of less than 500W. This is the highest f x Q product demonstrated for a polysilicon MEMS resonator in air. This resonator also exhibited the lowest motional resistance demonstrated for an electrostatic polysilicon MEMS resonator in the GHz frequency range. These results demonstrate that internal electrostatic transduction is suitable for the efficient, selective excitation of higher-order lateral bulk modes in resonators. The concept of parallel internal electrostatic transduction is introduced, where a Lamé-mode resonator with integrated electrodes was designed, fabricated, and tested successfully. The integrated electrodes were placed within the resonating plate to efficiently couple to the Lamé mode of resonance, which allows for a fully differential drive/sense scheme. This measurement method allows for lower ohmic losses and helps to suppress the feedthrough significantly. This resonator demonstrated a high quality factor of > 12000 in air at 128.15MHz. Such a high quality factor makes this resonator attractive for different applications. These results validate internal electrostatic transduction of bulk-mode resonators as a viable method for efficient transduction of MEMS resonators in RF and microwave regime. The resonators in this thesis demonstrate the capacity of internal electrostatic transduction to achieve high quality factor, high frequency microresonators with low motional resistance that can be integrated with CMOS technology to achieve fully integrated communication systems.