Characterization and optimization of electrostatic actuators for MEMS micropumps

This thesis explores the comprehensive characterization and optimization of electrostatic actuators for Microelectromechanical Systems (MEMS) micropumps. With the rapid proliferation of MEMS in various industrial and medical applications, optimizing the performance of the crucial compo nents, such as electrostatic actuators, is of paramount importance. These actuators are fabricated using micro-fabrication technologies and can achieve a total displacement of 5 µm. With the aim to optimize their performance and efficiency, the investigation relies on the analytical and numerical models along with experimental studies. The study delves into understanding the electromechanical response of actuators and determining performance parameters such as pull-in and release voltage, displacement, capacitance, charge stored, current, stroke volume etc, with a focus on finding their dependency on applied voltages and driving frequencies. The study also provides a comprehensive exploration of both quasi-static and dynamic characterization of the actuators, aiming to enhance their functionality and reliability. The quasi-static characterization conducted using a slow-varying voltage to analyse the actuator membrane deflection at both wafer and chip levels, finds the pull-in voltage to be roughly 70 V and release 50 V. It calculates the stroke volume as approximately 47.3 nL using line scans and numerical integration. A pressure-dependent deflection measurement revealed pull-in at 40 kPa pressure. Also, the actuator capacitance and impedance were analyzed through quasi-static C-V characterization, showing an initial capacitance of 300 pF, which sharply increased to 384.75 pF during the pull-in. The dynamic response of electrostatic actuators to different voltage waveforms, including sine, ramp, square and arbitrary waveforms across various frequencies are investigated. The pull-in occurs at around 68.7 V and release at approximately 52 V, with pull-in and release durations averaging 0.924 s and 1.342 s respectively. This indicates that the actuator discharges for a longer period than it charges. The membrane deflection tends to decrease with increasing frequency from 100 mHz to 500 mHz, when the frequency is so high that the actuator cannot respond quickly enough. Both the quasi-static and dynamic characterization measurements were conducted under negative pressure of -85 kPa. Observations indicated a notably accelerated response speed from the actuator under these negative pressure conditions as opposed to its performance under normal atmospheric v pressure. Moreover, the charging and discharging process of the actuator are evaluated. The actuator behaves like a capacitor, whereby the current only flows during the charging or discharging, but does not continue indefinitely. Rather, the current ceases once the actuator becomes fully charged or completely discharged. Measurements indicate that the current passing through the actuator is approximately 40.56 mA during the charging process and -43.69 mA throughout the discharging. The measurement results are compared with the analytical results, quasi-static simulation results of ANSYS FEA model and dynamic simulation results of nonlinear Simulink system model. These comparisons are critically important for improving actuator reliability and lifetime. Also, the optimizations done for the fabrication of electrostatic actuators to maximize actuation performance are explained.