Wafer-level vacuum-encapsulated ultra-low voltage tuning fork MEMS resonator

Abstract

In this thesis, a low-voltage 32 kHz silicon tuning fork MicroElectroMechanical Systems (MEMS)-based resonator design with a high Quality factor of over 73,000 is presented with a Complementary Metal-Oxide Semiconductor (CMOS) sustaining amplifier towards a low power oscillator. The resonator is designed using MEMS Integrated Design for Inertial Sensors (MIDIS) process developed by Teledyne DALSA Semiconductor Inc. (TDSI). MIDIS offers wafer-level vacuum encapsulation with ultra-low leak rate. Ultra-low polarization voltage, as low as 10mV, is needed to excite the resonator by using a transduction gap reduction technique based on electrostatic deflection of movable electrodes and subsequent localized melting of welding pads for permanent position locking. Further, the technique helps to minimize unexpected electrostatic stiffness induced by time-varying capacitance across transduction gaps to just-0.6 N/m. The motional resistance drops down to about 2kΩ as a result of a small gap size and the technique helps to improve the Quality Factor (Q). A sustaining amplifier using a transimpedance operational amplifier configuration is system-integrated with the tuning fork resonator to establish continuous oscillation with low damping losses. An average power consumption of around 600µW is measured on the oscillator, which is suitable for mobile electronic systems. v Dedicated to my parents vi ACKNOWLEDGEMENTS My special thanks to Dr. Vamsy Chodavarapu, my advisor, for giving me guidance all through my research work as well as unlimited access to the software and other resources necessary for completing the design of this work. I would also like to appreciate everyone who has offered me assistance in this work. This includes George Xereas, who spared some of his valuable time to teach me with patience how to use the CoventorWare software for MEMS-based resonator design; Balaadithya Uppalapati, who gave me the basic tutorials on how Silvaco EDA tools should be used for oscillator circuit design; and Dr. Weisong Wang, for imparting knowledge of MEMS fundamentals to me through lectures in class. I also deeply appreciate Dr. Guru Subramanyam, who provided me with several useful points on which parts I should improve regarding the work in the future. vii