Date of Award

4-2010

Degree Name

MS in Engineering - Materials Engineering

Department

Materials Engineering

Advisor

Richard Savage

Abstract

Micrometer-scale mirrors are used in many electronic devices today such as digital light projection. One common type of mirror is a thin plate structure supported by torsional hinges which rotates when actuated. These devices are popular because the design allows for high stability and reliability. Parallel plate electrostatic actuation is commonly used to create the force which rotates the plate structure. The device consists of one deformable parallel plate electrode and one immovable electrode. In order for these devices to rotate to a specific angle when actuated, both the mechanical and electrostatic forces must be characterized. This project analyzes both of these forces through modeling equations, and compares theoretical performance to experimentally measured values. The rotational measurements involve reflecting a laser spot off the surface of the actuator face and recording any displacement of the laser spot with a position sensitive device. The electrostatic device created consists of a rotating hinged structure, a fixed aluminum electrode, and a polymer spacer to create an air gap between the electrodes. These components are created using standard semiconductor fabrication techniques. The hinged structures are created from a 500μm thick, double-sided polished, single crystal (100) silicon wafer. The wafers are etched using both wet etching, and reactive ion etching techniques, which produce approximately 8μm thick plate structures. Physical vapor deposition is used to deposit a thin aluminum film onto the silicon in order to form a conductive layer. Rigid aluminum counter-electrodes and SU-8 polymer spacers are fabricated on a glass slide. The silicon actuator chip is aligned and mounted onto the glass slide. Once fabricated, the micro-mirror actuator was tested for angular rotation as a function of applied voltage. The applied voltage ranged from 0 to 100V and produced an angular rotation up to 0.3 degrees. During testing it was observed that the stability of the angular tilt was poor enough to merit further examination. Angular stability over time can be a serious issue for micro mirrors, and can cause complete failure of the device. Short and long term angular drift as well as a rotational settling phenomenon were investigated. Angular drift was found to be most likely caused by electrical or environmental factors. The rotational settling had an electrical root cause, which caused charge to migrate on the glass substrate beneath the actuator. The charge formed by the migration created a counteracting force on the actuator causing the rotation angle to reduce over time. The migrating charge was eliminated by creating a second neutrally charged counter-electrode which prevented charge from building on the glass surface.

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