Shama, Yasser2025-04-282025-04-282025-04-282025-04-25https://hdl.handle.net/10012/21652This thesis presents a methodical investigation into the fundamental sensing mechanism of electrostatic MEMS sensors in gas and liquid media. It provides new insights into electrostatic MEMS sensing mechanisms that can improve the sensor design process by combining mass sorption and permittivity change to enhance the sensitivity of gas and liquid sensors. First, it compares among the responsivities of a set of MEMS isopropanol sensors. I found that functionalized static-mode sensors do not exhibit a measurable change in response due to added mass, whereas bare sensors showed a clear change in response to isopropanol vapor. Functionalized dynamic-mode sensors showed a measurable frequency shift due to the added mass of isopropanol vapor. The frequency shift increased by threefold in the presence of strong electrostatic fields. These results show that the sensing mechanism is a combination of a weaker added mass effect and a stronger permittivity effect and that electrostatic MEMS gas sensors are independent of the direction of the gravitational field and are, thus, robust to changes in alignment. It is erroneous to refer to them as `gravimetric' sensors. I investigated the repeatability of electrostatic MEMS sensors over prolonged excitations. The sensors were subjected to two test conditions: continuous frequency sweeps and long-term residence on a resonant branch beyond the cyclic-fold bifurcation. I found that prolonged high-amplitude oscillations undermine repeatability and cause significant shifts in the bifurcation location toward lower frequencies by building up plastic deformations that reduce the capacitive gap. Biased excitation waveforms were also found to lead to charge buildup within dielectrics, exacerbating the drift in frequency of the bifurcation point. In comparison, stiffer in-plane sensors with no metallization operating under unbiased waveforms showed dramatic improvement in repeatability. With a view to deployment of electrostatic MEMS sensors in liquid media, I studied the use of motion-induced current to detect their high frequency vibrations. While current and ground truth (optical) measurements aligned well at lower frequency resonances, current measurements showed valleys rather than peaks at high frequency resonances. The root cause was found to be current behavior switching from capacitive to inductive as the frequency crossed a resonance in the measurement circuit. It was also found that output current diminishes with increasing mode number. Finally, I found a measurable change beyond 10 MHz in the output current of a bare chip carrier when the analyte (mercury acetate) was introduced at the concentration of 100 ppm into deionized water, suggesting a potential for interference with inertial sensing. In the final phase of this work, the fundamental vibration mode of electrostatic MEMS sensors was used to detect 100 ppm of mercury acetate in deionized water. The sensors measured a consistent shift in the frequency and amplitude of the resonant peak. This demonstrates the viability of electrostatic MEMS sensors for underwater applications and the need for further work to improve their detection mechanisms.enelectrostatic MEMSinertial sensorsgas sensorssensing mechanismagingrepeatabilitymotion-induced currentaqueous media sensorsmercury sensorsDoctoral Thesis