Studying Energy Loss Mechanisms in MEMS Based Contour-Mode Resonators Using Laser Doppler Vibrometery
The work in this thesis aims to uncover energy loss mechanisms in MEMS-based piezoelectric contour-mode resonators. Quality factors for these devices has been constrained to under 10,000, limiting their commercial applications. The first part of this thesis studies Q degredation related to anchor losses. During fabrication, the outer edge of the anchors are released from the substrate due to an isotropic etch step used in releasing the device from the substrate. This allows both ends of the anchors and the surrounding region of the device layers to undergo strain resulting in energy loss during operation. This study finds that a variation in Q of up to 31% can occur as a result of this released area. A novel method for minimizing this loss through modified boundary conditions is also analyzed and experimentally tested.\par
The second part of this thesis develops a new method of measuring ring-downs at ultra high frequencies using laser Doppler vibrometry. Due to an inherently low signal to noise ratio and a 4ns timing error in the vibrometer measurement triggering, traditional ring-down methods are not possible at ultra high frequencies. The method outlined here overcomes this and produces results with an error of less than 6% when compared to electrically determined values.
Next, a method is outlined for measuring the magnitudes of the individual material wavelengths on the surface of the resonator. The devices are composed of multiple material layers, each with a unique acoustic velocity. As a result, at resonance, each layer has a unique wavelength that can be measured using laser Doppler vibrometery.
The final section combines the previous two methods to measure the ring-downs of the individual material layers used in the construction of the resonators. Due to their unique material properties, the rate at which the vibrations of each material decays can be quantified to determine future fabrication choices.