Low voltage large stroke bipolar tri-electrode electrostatic actuator for deformable mirrors in adaptive optics

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Allameh, Mohammadmehdi
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Image distortion is one of the most common problems of optical receivers. Adaptive Optics (AO) technology was invented to solve that issue for ground-based telescopes. The deformable mirror (DM) is the corrector which is being used to correct the aberrated received image in an AO system. The arrays of actuators and the flexible mirror face sheet are the two main elements of a DM. In this thesis, a tri-electrode electrostatic actuator was studied which is comprised of one moving MEMS electrode and two fixed controlling electrodes. The actuator is primarily beneficial for usage in an array of actuators for DMs, since only one common high-voltage electrode is employed as the background electrode for all actuators in the array, and the movement of each individual actuator can be controlled by an individual low voltage electrode. The tri-electrode topology, and the additionally presented extended electrode tri-electrode configurations, are designed such that they can be utilized for other applications such as MEMS switches. In this work, all the design parameters of the tri-electrode actuator were studied in simulation using the finite element method and restoring spring force method, as well as experimentally. The design studies were dimensionless. Therefore, the optimizations can be used for an actuator of any size. The studies showed that the controlling voltage of the actuator can be decreased significantly. For one specific design, it was shown that the controlling voltage is almost four times smaller than for a conventional parallel plate electrostatic actuator. The maximum displacement of the actuator can be enhanced significantly compared to that of the conventional configuration. The increased range of motion can be bidirectional, and for one specific case it was 2.5 times larger than for the conventional actuator using the extended electrode design. Two silicon MEMS actuators were designed in order to experimentally verify the simulation studies. The measurements showed a close agreement between the simulations and the demonstrated prototype designs. Secondary research was also performed to study the issues in building a tri-layer flexible mirror that can be utilized as the face sheet for DMs. The conducted stress analysis investigations result in valuable conclusions and a fabrication recipe was developed to enable tensile stress mirrors. The recipe was used to build tensile stress metal-polymer-metal tri-layer structures to enable wrinkle free mirrors, and a high level of smoothness as it is necessary for a flexible mirror face sheet.
MEMS Electrostatic Actuator, Deformable Mirror