Toward inertial-navigation-on-chip: The physics and performance scaling of multi degree-of-freedom resonant MEMS gyroscopes
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Gyroscopes based on microelectromechanical systems (MEMS) are micro-scale inertial sensors that measure the rotation of an object without relying on external references. Due to their small size and low cost, integrated single-chip MEMS inertial measurement units (IMUs) consisting of 3-axis gyroscopes and 3-axis accelerometers have received great success in personal electronics applications for self-sustained motion tracking. However, IMUs with robust higher performance are required by emerging applications like health informatics, robotics, industrial guidance, and indoor navigation, which are unavailable with current MEMS gyroscope technology. In addition, high-performance MEMS IMUs can be used for exploratory and scientific applications such as direction guidance for field studies in GPS-denied environments, low-cost geophysical measurements, and relativity experiments. Therefore, breakthroughs in multi-axis MEMS gyroscope technology for robust high-performance IMUs are highly desirable. Commercially available MEMS gyroscopes suffer from poor robustness due to their low-frequency design. Contrarily, high-frequency resonant gyroscope technology developed in recent years promises robust high performance with mode-matched resonant operation. While high performance is shown in various resonant yaw gyroscopes, large discrepancies are seen between the promised and delivered performance in resonant pitch and roll gyroscopes, keeping multi-axis resonant gyroscopes from high-end IMU applications and prohibiting a fully integrated single-chip IMU design based on high-frequency gyroscopes. This dissertation focuses on understanding the physical phenomena behind non-idealities in resonant MEMS gyroscopes to identify and provide solutions to the performance scaling limits, especially for resonant pitch and roll gyroscopes, as well as to provide insights into the future path toward resonant-gyroscope-based single-chip inertial navigation systems (INSs). This work has led to novel designs and microfabrication technologies that enabled the implementation of the first high-performance single-chip timing and inertial measurement unit (TIMU) with robust 3-axis resonant gyroscopes. In this thesis, a nano-gap slanted electrode technology was introduced and demonstrated through an advanced high aspect-ratio poly- and single-crystal silicon RIE plus wet-etching (HARPSS+) process to enable, for the first time, electrostatic quadrature error cancellation in pitch and roll gyroscopes with both in-plane (IP) and out-of-plane (OOP) degree-of-freedom (DOF), which has been the bottleneck in multi-axis resonant gyroscope technology. A novel high-frequency resonant framed-annulus pitch and roll gyroscope design with high Coriolis sensitivity was proposed and experimentally verified, incorporating the slanted electrodes and a novel nano-gap comb-drive technology to match the performance of state-of-the-art high-frequency yaw gyroscopes. The framed-annulus gyroscopes were integrated with a novel quasi-solid disk bulk acoustic wave (BAW) yaw gyroscope and MEMS accelerometers to form a wafer-level packaged (WLP) single-chip TIMU. The 3-axis gyroscopes on the TIMU demonstrated high performance with below 1 deg/rthr angle random walk (ARW) and ~10 deg/hr bias instability (BI) while possessing small form-factor and high robustness, meeting the requirements for portable short-term navigational applications. Furthermore, experiments and numerical studies were performed on the presented gyroscopes, which for the first time, established a solid understanding of the limiting phenomena responsible for BI in high-frequency resonant gyroscopes and provided strategies for future improvements toward long-range on-chip inertial navigation.