Large Hybrid High Precision MEMS Mirrors

This thesis presents the design and microfabrication technology of a tip-tilt, 2 degree of freedom "mirror system" made out of Silicon On Insulator (SOI) wafers with two movable mirrors that have diameter of 10 mm. The system was intended for precise beam steering applications and tracking for inter-satellite telecommunication. Implementation of two mirrors allowed one mirror to have large static mechanical scan angles (±3.5°) and the other to have fast fine pointing capabilities within ±0.2° static mechanical scan angle. The first mirror was magnetically actuated and has a resonance frequency of 200 Hz and the large rotation angle. The second mirror was designed to use either electrostatic actuation or electromagnetic actuation. The resonance frequency is 1 KHz. The main objective of this thesis was the microfabrication of large mirrors (10 mm) and actuators. The study of upper limits of achievable resonant frequencies in combination with static deflection was another major aim of the project, as well as optimization of the flatness of the mirrors. The undertaken research presents a study of different non-contact actuation schemes for use in MOEMS devices, namely electrostatic and magnetic actuation schemes. Determination of the best magnetic actuation scheme for use in this thesis was carried out according to the design constraints and size limitations. It was concluded that the best magnetic actuation scheme consists of a moving magnet and a microfabricated stationary coil. This scheme created high force and the stationary microfabricated coils could easily dissipate heat through conduction into a heat sink. This configuration was of great advantage in space where heat convection does not exist. The considered electrostatic schemes included parallel plate actuators and vertical comb actuators. The vertical comb actuator was chosen to be fabricated due to its scalability that allows the microfabrication process to be easily adapted to smaller or larger devices. The microfabrication and implementation of the magnetically actuated mirrors was proven to be very straightforward resulting in high yield and uniformity of both the high and low frequency devices. The microfabrication of the electrostatic actuator was also achieved successfully. The operation performance of the electrostatically actuated mirror was not as satisfactory as the magnetically actuated mirrors. It was concluded that electrostatic actuation might be more suited to smaller devices, rather than the devices designed in the present study. The fabrication and actuation performance of the components of a "mirror system" that combines fast and large angle actuation was successfully demonstrated in this thesis. Very good mirror flatnesses were also obtained with RMS flatness of λ/10 for near infrared wavelengths. This project shows the general advantage of magnetic actuation for large MOEMS devices with large actuation ranges and proves the feasibility of their fabrication. Potential applications of these devices are robotic 3D vision, imaging LIDARs (Light Detection And Ranging), docking sensors and inter-satellite laser communications.

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