On-Chip Compensation of Stress, Temperature, and Nonlinearity in a MEMS Gyroscope

2018-12-12T21:22:32Z (GMT) by Erdinc Tatar
The micro-gyroscope performance has improved throughout the years as a result of extensive research, and advancements in fabrication technologies and readout electronics. Today the performance has reached to a point where the environmental effects play a significant role on the gyroscope drift. This thesis proposes on-chip stress, temperature and nonlinearity compensation techniques for high performance MEMS gyroscopes to further improve the gyroscope stability without sacrificing the noise performance.<br>Long term drift limits the performance of the micro-gyroscopes. Temperature has been found to be one of the important sources of long term drift. But constant temperature testing of one of the state of the art gyroscopes still exhibits long term drift. The main motivation of this thesis is solving the long term gyroscope drift problem by measuring the stress of the mechanical structure with on-chip stress sensors. Towards that end, a methodology that couples finite element analysis and circuit simulation has been developed to understand the stress zero rate output (ZRO) relation. To validate the trends in the simulations, stress tests have been performed on a silicon-on-insulator (SOI) MEMS gyroscope that is in-house fabricated, vacuum packaged, and ovenized. This thesis successfully demonstrates that stress compensation significantly suppresses long-term drift resulting in 9°/hr/√Hz angle random walk (resolution) and 1°/hr bias instability at 10,000 s (around 3 hr) averaging time, which is a seven times improvement over the uncompensated gyroscope output.<br>While stress compensation improves the long term stability, the noise performance of the gyroscope directly depends on the drive displacement. High drive displacement improves the gyroscope signal to noise ratio but also leads to a nonlinear force displacement behavior that isobservable as a hysteresis in the frequency-phase and frequency-amplitude relations. These nonlinearities lead to an amplitude-frequency effect where resonance frequency depends on the displacement. This affects the gyroscope long term stability. This thesis proposes a cubically shaped nonlinearity tuning comb finger design that cancels the inherent softening nonlinearity of the gyroscope by introducing a dc voltage controlled hardening nonlinearity. The functionality of the fingers is demonstrated and, furthermore, cancelling drive nonlinearities at high displacement leads to a better bias instability compared to the same displacement with uncompensated, nonlinear characteristics.<br>