Fused Silica Precision Shell Integrating (PSI) Navigation-Grade Micro-Gyroscopes

Abstract

This thesis is aimed at the development of low-cost, vacuum-packaged navigation-grade microelectromechanical system (MEMS) Coriolis vibratory gyroscopes (CVG) using 3-dimensional fused-silica (FS) shell resonators, called Precision Shell Integrating (PSI) gyroscope. PSI gyroscopes consist of 3D shells (dia=10mm, height=5mm) that resemble a wine glass. CVGs are used for measuring rotation angle and rate to aid navigation in GPS-denied environments. While MEMS gyroscopes dramatically reduce cost and size, they struggle to achieve performance comparable to their macro-counterparts. This thesis focuses on making small size/cost and high-performance gyroscopes by understanding bottlenecks in performance and mitigating them through innovative design and fabrication technologies. To improve performance, previous research focused on obtaining high quality factor (Q) from the resonator. In this thesis, navigation-grade performance is achieved by simultaneous improvement in resonator parameters like Q, ring-down time, effective mass, frequency, frequency mismatch, and transduction parameters like signal-to-noise ratio, and capacitance, all of which combine to enhance resolution and accuracy. Improvements in resonant parameters is achieved by making resonators by blowtorching a FS substrate with etched features. This allows fabrication of 3D shells with distributed stiffness/mass (by tailoring thickness in range of 10-450 microns at different locations) exhibiting high-Q (>12.5 Million) and long ring-down time (>500s) with small frequency mismatch (<2Hz) and large modal mass (>12mg) at frequency (5-10kHz). High-Q, long ring-down time and small frequency mismatch is obtained by making symmetric structures with superior surface quality enabled through efficient torching, cleaning and annealing. Modal mass is increased by increasing rim thickness. Similarly, large thickness at anchor transition regions (shoulder) increases shock survivability. Increasing the thickness of both the rim and shoulder from 150microns to 300microns leads to 2x increase in modal mass and 13x decrease in shock-induced displacement. Shells with discrete and mm-scale open windows are also designed and fabricated for the first time. This is done by two-step etching of the substrate before molding and shells after molding. Open windows on shells can increase Q by reducing thermoelastic and anchor loss. Besides, this technology can be used to define discrete electrodes on a curved surface of a molded shell which is otherwise not possible through conventional lithography. It can also be used to fabricate 3D shadow mask for selective metal coating on resonators. A low-cost and fast approach to singulate 3D resonators from their molded substrate using selective hydrofluoric acid etching of the flat substrate. This technology increases throughput and reduces cost by >25x as compared to other conventional processes, without compromising performance. PSI gyroscopes are fabricated by assembling resonators on electrode substrates. Two architectures, one where electrodes are placed beneath the rim (surface electrodes) and other where electrodes are placed at the side of rim (side electrode) is explored. Unprecedented angle random walk of 160 micro-deg/rt-hr and bias instability of <1 mdeg/hr is measured from PSI gyroscope with side electrodes without any temperature compensation. This is not only the best reported performance ever from a MEMS gyroscope, it compares favorably with some of the commercial state-of-the art macro-scale gyroscopes. Furthermore, a novel gyroscope architecture which involves integrating PSI resonators with a custom-designed curved electrode substrate is developed. Curved electrodes which nearly follow the resonator's profile are fabricated. This architecture could potentially increase capacitance by >5x as compared to side electrodes due to its conformal overlap. Curved electrodes improve resolution, frequency tuning, and temperature sensitivity.