UC Irvine

This Ph.D. dissertation focuses on developing microfabrication processes to realize planar Fused Silica (FS) Micro-Electro-Mechanical System (MEMS) technology for the implementation of Micro-Rate-Integrating Gyroscopes (MRIG). Additionally, it includes a study on identifying error sources in Coriolis Vibratory Gyroscopes (CVG) with Whole-Angle (WA) mode of operation. The FS planar MRIG is envisioned to provide navigational-grade noise performance, unprecedented dynamic range, and thermal stability at the Size, Weight, Power, and Cost (SWaP-C) factor of today's commercially-available silicon micro-scale gyroscopes.

As part of this work, we developed the Fused silica-On-Silicon (FOS) process for batch fabrication of resonators from low-loss amorphous FS material based on conventional microfabrication techniques. As the core step of the fabrication process for plasma etching of FS, we identified optimal parameters to achieve a 7:1 aspect ratio of etching with near-vertical sidewalls. For demonstration, a Toroidal Ring Gyroscope (TRG) architecture was fabricated through the developed process. With the metal-coated FS-TRG devices, we demonstrated resonator functionality and measured a quality factor on the order of 539,000 and a frequency split in the operational modes as low as 7~Hz. We also introduced a digital manufacturing process, which utilizes Femtosecond Laser-Induced Chemical Etching (FLICE) to fabricate stand-alone FS MEMS vibratory structures for the first time. Through process optimization, we demonstrated that FLICE is an enabling technology for patterning micro-channels with an aspect ratio of 55:1 and higher, ideal for fabricating MEMS resonators with ultrahigh capacitive transduction. By employing FLICE as part of a 3-step process, we fabricated Disk Resonator Gyroscopes (DRG) from a single-layer FS material. We demonstrated resonator functionality and measured the frequency split as low as 54.7~Hz and the quality factor as high as 614,000. To the best of our knowledge, the quality factors demonstrated as part of this dissertation are the highest quality factors reported for a planar FS resonator in the kilohertz frequency range.

We developed a mathematical electro-mechanical model of WA operation, through which we simulated and characterized the effect of mechanical imperfections of the structure and imperfections in the WA control electronics on precession of oscillations. We demonstrated that mechanical imperfections in MRIG, including anisodamping and anisoelasticity, limit the resolution of angle measurements by introducing angle-dependent bias errors and angular drift. Imperfections in the control electronics, including phase errors, asymmetries in motion actuation and detection gains, were shown to adversely affect the outcome of the WA control loops and cause interference on the precession. We verified the simulation results experimentally by implementing the WA control with an FPGA/DSP-based platform and applying it to a planar silicon MRIG as the testbed.

Finally, we identified a mismatch in MRIG's Temperature Coefficients of Frequency (TCF) as the primary mechanism causing Angular Gain Temperature Sensitivity (AGTS) and angular drift in the WA mode of operation. We provided mitigation strategies to significantly reduce AGTS, despite the TCF-mismatch in the mechanical sensing element. We demonstrated angular gain stability better than 223~ppb (equivalent to a rate-bias-stability better than 0.2~deg/hr), which would be critical for high accuracy angle measurements in prolonged operations.