Epitaxial encapsulation of MEMS inertial sensors

MEMS (microelectromechanical systems) inertial sensors have become ubiquitous in today's society thanks to recent advancement in MEMS technology and their wide scale adoption in consumer electronics. Silicon MEMS inertial sensors are batch fabricated using well-developed CMOS micromachining technology. This means that large numbers of MEMS sensors can be co-fabricated and directly integrated with CMOS technology, providing low manufacturing costs and low noise performance. Additionally, these inertial sensors can be fabricated on the micro-scale, resulting in small footprints on the order of a few millimeters. With the advent of established technologies such as smart phones, and cutting-edge emergent technologies such as the Internet of things (IoT), wearable devices, and augmented/virtual reality, MEMS inertial sensors have begun to see a much wider range of consumer applications. MEMS technology is well suited for such applications due to their small size, low cost, and CMOS integration. In addition to the consumer market, MEMS inertial sensors have received attention for navigation grade applications due to technological advancements in MEMS technology allowing for the fabrication of high performance sensors on the micro-scale. These high performance sensors are critical for the miniaturization of dead reckoning and inertial navigation systems. Since MEMS devices operate at the micro-scale, particles and operational environment must be carefully regulated to ensure device performance and robustness. This is why MEMS packaging is of critical importance. Epi-seal is a MEMS packaging technology developed to hermetically package MEMS devices at the wafer-level, which retains the advantages of batch fabrication and CMOS compatibility as well as offer unprecedented performance of MEMS devices. The standard Epi-seal fabrication process, however, does not lend itself directly to the fabrication of MEMS inertial sensors. The process lacks integration of differential, out-of-plane capacitive electrodes which are essential for the implementation of multi-axis MEMS inertial sensors. Additionally, the narrow transduction gaps required for the process severely limits the design and performance of MEMS inertial sensors in Epi-seal. Previous solutions to this limitation have been shown to result in drastic reduction of process robustness and manufacturability. Like in many MEMS processes, stiction in Epi-seal has also proven to be a difficult challenge for the fabrication of inertial sensors. In this work, we demonstrate new fabrication processes and techniques which address each of these limitations in such a way that maintains the inherent advantages of the Epi-seal process - thereby achieving a robust MEMS process for the fabrication of high performance, batch fabricated, MEMS inertial sensors. Application of the MEMS inertial sensors is also demonstrated. Dynamic interactions of the MEMS structures, namely 1:1 modal coupling of resonant devices, are studied. Dynamic tuning of the quality factor through 1:1 modal coupling is demonstrated for the MEMS resonators. Derivation of an autonomous calibration scheme capable of nullifying quadrature and frequency split of degenerate gyroscope modes is detailed. The calibration method was demonstrated to be intolerant of specific device parameters such as gyroscope geometry, material, and fabrication tolerances. The autonomous calibration may therefore be applied to a wide range of MEMS gyroscope applications towards high performance, mode-matched, operation.

Keywords: Mechanical Engineering.
Advisor: Kenny Thomas William 
Howe Roger Thomas 
Tang Sindy (Sindy K.Y.)