| This disservation describes an electrostatic charge sensor based on a microelctromechanical systems (MEMS) resonator with dual micro-levers to improve sensitivity, including theory analysis, simulation optimization, fabrication and experiment. The device comprises a double-ended tuning fork resonant element working at out-of-phase mode with a quality factor (Q) of about 4900 under operating pressure of 4 mTorr and room teperature. The DETF resonator is drived and sensed by capacitance transmission. The resonant frequency shifts in proportion to an axial force induced by charge additions and then transformed by dual micro-levers with an amplification factor over 8. The measured sensitivity is 1.3×10-3 Hz / fC2, which matches well with the results from analytical models and finite element method. The resolution of the charge sensor is 21 femto Coulomb under 4 ppm frequency fluctuation. Additionally, eletrostatic spring softening and nonlinear feature of the resonator are also investigated.This disservation also reports a novel resonant electrometer with resolution of sub-1 femto Coulomb, in which scheme of adjustable sensitivity is firstly proposed to realize both high resolution and large dynamic range. The maximum sensitivity of the electrometer was found to be 1.03×10-2Hz/fC2 after polynomial fitting. The adjustable capacitance will increase 9.7 fF when 40V DC voltage is added on the electrostatic actuator. As a result, the sensitivity can be adjusted by 91.6%.This disservation is organized as follows:In Chapter 1, the definition, classification and applications of MEMS resonators are described and the mass-spring model and equivalent circuit model for resonatos are deduced in detail. Then research status of charge sensors and advantages for MEMS resonant charge sensors are reviewed. Finally, the work of this disservation is summarized systematically.In Chapter 2, the novel structure design for resonant charge sensor is proposed. The amplification factor of the micro-levers, resonator characteristic and sensitivity of the charge sensor are analyzed theoretically. Some key parameters of the structure are optimized by Finite Element Method.In Chapter 3, the fabrication process is introduced. The devices are all fabricated based on SOI process. The movable structures of the device are released by two methods, from backside and frontside.In Chapter 4, the measurement of the device is introduced. The devices are tested under vacuum pressure and room temperature. The characteristics of the resonator including spring softening, nonlinearity and sensitivity are tested and analyzed.In Chapter 5, a novel resonant charge sensor with both high resolution and large dynamic range are proposed. A comb-drive actuator used to adjust sensitivity is firstly introduced to solve the trade-off between high resolution and large dynamic range. The device is fabricated and validated.In Chapter 6, the research work is summarized as well as its scientific contributions. Future work is also presented. |
