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Development of 3D High-Q Fused Quartz Micro Structures for Precision Coriolis Vibratory Gyroscopes

  • Author(s): Asadian Ardakani, Mohammad Hossein
  • Advisor(s): Shkel, Andrei M.
  • et al.

The contribution of this Ph.D. thesis is in development of miniaturized 3-dimensional structures with high stiffness symmetry, high damping symmetry, and high Q-factor, for potential implementation as precision rate and rate-integrating Coriolis Vibratory Gyroscopes (CVGs). The focus of this research is toward sensor design and process development, based on Microelectromechanical System (MEMS) techniques, for realization of 3D "Fused Quartz" micro-shell gyroscopes. The challenges in fabrication of low-frequency and 3D high-Q microstructures are addressed. A precision micro-assembly, as a crucial technological step, is developed to complete the development cycle of micro-shell gyroscope devices. "Dual-shell" architectures are introduced as an innovative approach for implementation of 3D micro-resonators and gyroscopes. A test-bed for 2D silicon MEMS gyroscopes is designed and implemented to study the limiting dissipation mechanisms of resonant structures. An ultra-high vacuum sealing process with proven long-term vacuum stability is developed, eliminating the primary damping mechanism for MEMS resonators and gyroscopes.

Several new fabrication processes for "non-flat" 3D microstructures are developed based on the high-temperature micro-glassblowing technique. Limitations of the previously developed baseline process in fabrication of low-frequency shell resonators are addressed by introducing and demonstrating two alternative approaches, leading to realization of micro-shell resonators with a broad range of operational frequencies, from a few to hundreds of kilohertz. The Q-factor up to 1.7 million was demonstrated by improving the surface quality and reducing residual thermal stresses in the micro-shell resonators.

Temporal evolution of 3D micro-shells during the glassblowing process was simulated using a time-dependent fluid flow model. The hybrid fluidic-structural simulation framework enabled a prediction of final geometry, modal characteristics, and dynamic behavior of micro-shells. The simulations were used toward design optimization to (1) reduce the energy dissipation and (2) enhance the shock resilience in micro-shells. The models were used in optimization of structures for operation through environmentally challenging conditions.

A vacuum-compatible micro-assembly process was developed for integration of micro-shell resonators with reduced capacitive gaps (< 5 µm), which is a critical technological step leading to improvements in sensitivity of shell-type devices. The electrostatic actuation, detection, and frequency-mismatch tuning, as well as the rate gyro operation, are demonstrated on fabricated 3D micro-shell resonators.

This thesis developed a practical realization of the fused quartz "dual-shell" structures for 3D MEMS resonators and gyroscopes. A full-cycle of development, including design, modeling, fabrication, as well as instrumentation of the structure as a resonator and a gyroscope, is presented. The high-Q dual-shell resonators with an amplitude ring-down time of over 120 seconds and with a Q-factor of 3.75 million were experimentally demonstrated. The obtained results are a glimpse of design opportunities for high performance and compact form factor MEMS gyro implementations using 3D shell geometries.

Finally, the energy dissipation mechanisms in MEMS vibratory gyroscopes were studied. An ultra-high vacuum sealing process was developed to eliminate viscous damping, one of the major contributing factor to energy dissipation. A 2D "flat" silicon MEMS gyro was designed as a test structure for the energy dissipation study. The Q-factors above 1.1 million were measured on the device, approaching its fundamental thermoelastic damping (TED). The low-temperature Q-factor measurements revealed the Q-factor of 9.29 million, associated with the anchor loss, which, to the best of our knowledge, is the highest Q-factor reported on silicon MEMS resonators. More importantly, the developed approach for cryogenic characterization of micro-resonators offers a methodology for study high precision gyroscopes and resonators, which is expected to be broadly adopted by future research efforts on the topic.

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