Electrostatic micro-actuator capacitive sensor providing insight into one- and two-dimensional resonant behavior

Electrostatically-driven microactuators are used in a variety of applications where small size, low power, high speed, and large deflections are priorities. Applications span from mass sensors to microelectromechanical (MEMS) devices used to scan or shutter light such as miniaturized microscopes, LIDAR, and optical switches. Electrostatically-driven microactuators are driven on resonance to maximize the benefits of using microactuators but because phase behavior near resonance can change drastically even with minute frequency drifts due to small environmental changes, such as temperature, the ability to measure the phase of microactuators is essential to high-resolution operation. Previous solutions have been hampered by low bandwidths; parasitic capacitance requiring modifications to the chip or board; and low signal-to-noise output signals only applicable to monitoring amplitude. This Dissertation will describe the design and verification of a capacitive sensing technique developed to measure the capacitance change across interdigitated combs while providing open-loop, high bandwidth, high gain, and high signal-to-noise performance to sense the amplitude and phase motion information through the same leads used to drive the mirror. This Dissertation first introduces a microelectromechanical phase sensor system with a signal-to-noise ratio of 42.55dB and an 11kHz bandwidth. The sensor is modular and capable of simultaneously driving and sensing the movement of an electrostatically-driven microactuator without modifying the chip or its packaging. The operational principle is to electromechanically modulate the amplitude of a high frequency signal with the changing capacitance of the MEMS actuator. The MEMS Phase Monitor (MPM) is characterized by measuring the frequency response and small phase drifts of an unmodified, unpackaged commercial MEMS scanning mirror and comparing the measurement results to simultaneous measurements of the position of a laser beam reflected off the mirror. This novel sensing technique also offers insight into the changing sub-harmonic behavior of the mirror. The MPM sensing technique is then applied to create the four-channel MPM (4CMPM), which is capable of simultaneously measuring the capacitive change across all four sides of a two-dimensional (2D) mirror. The focus of the 4CMPM is to first look at each side of each axis separately to gain insight into 1D operation, which previous MEMS motion sensors have not explored. Then, the 4CMPM is used to simultaneously sense the motion on all four sides of the 2D micromirror to identify and quantify observations of two dominant artifacts of 2D resonant operation using the 2D commercial electrostatic microactuator as a test structure. Simultaneously sensing on all four sides of the micromirror provides a close look at the phase deviations experienced during 2D resonant operation and by applying these phase deviation measurements to a simulated Lissajous scan, this Dissertation is able to present an error metric to describe the impact phase deviations experienced during 2D operation have on imaging applications. Finally, observations of cross-talk effects on both the frequency response behavior and amplitude modulation due to one axis influencing the operation of the other is presented and quantified. This Dissertation demonstrates that the circuitry of the MPM and 4CMPM are insensitive to complex changing overlap behavior and that they are capable of surpassing conventional characterization sensors as a measurement technique for describing the motion using the capacitance to give a real-time readout of the micromirror phase and amplitude without requiring additional leads, modification, or access to the chip. The open-loop, high sensitivity sensor design presented and verified here provides future opportunities to characterize the dynamic behavior of a microactuator in detail, leading to new insights and enabling new types of operation, including non-linear operation, and applications.

Keywords: Electrical Engineering.
Advisor: Solgaard Olav 
Dutton Robert W. 
Fraser-Smith A. C. (Antony C.)