Piezoelectric MEMS: Microsystems Based on Bulk PZT Lateral Bimorphs for Low-Power Applications and Toward Bulk Diffraction Wave Gyroscopes

Abstract

Piezoelectric microelectromechanical systems (MEMS) technology, compared to conventional discrete manufacturing techniques, allows the development of sensors and wireless transmitters in smaller sizes, lowered cost, and lower power consumption. This dissertation presents two research areas of piezoelectric MEMS: microsystems based on bulk lead zirconate titanate (PZT) lateral bimorphs, and high-overtone bulk diffraction wave gyroscopes. First, the microsystems for low power applications are developed based on piezoelectric lateral bimorphs fabricated by a 150-µm-resolution laser-micromachining process on 500-µm-thick bulk PZT. Compared to piezoelectric thin-film bimorphs, bulk PZT provides higher electromechanical coupling (k33 = 0.72) and high dielectric coefficient (εr = 1275). The high k33 enables efficient transduction from electrical to mechanical domain for actuators, and high mechanical to electrical signal for sensors. The high εr enables greater charge to be generated for energy harvesters. This dissertation presents PZT-bimorph-based microsystems designed for the following applications: 1) near-zero power consumption event detection devices, 2) in-situ MEMS gyroscope calibration, and 3) ultralow frequency (ULF) communication by mechanical motion of magnets. The first application is the near-zero power event detection, where zero-power PZT-bimorph-based sensors are used to measure acceleration, rotation, magnetic field, and sound. NEMS switches or CMOS comparators detect desired signal patterns and generate wakeup triggers. Prototypes are evaluated in the laboratory and field tests consisting of detections of electrical generators, cars, and trucks. The power consumption of the system with a low-power CMOS comparator is 2-6 nW, which potentially enables the development of long-lifetime battery-powered IoT devices. The second application is the calibration of MEMS gyroscopes, where PZT-bimorph-based dither stages are used to calibrate and reduce gyroscope scale factor errors down to 50 ppm from 5.5%. Gyroscopes integrated with the calibration dither stages potentially allows for low-cost gyroscopes to be used for navigation, significantly reducing the cost, weight, and size of navigation grade gyroscopes. The third application is the ULF communication, which permanent magnets are actuated by the PZT-bimorph-based dither stages to generate a time-varying magnetic field in the ULF range (300 Hz - 3 kHz). A transmitter prototype consumes lower power (1.8 µW) and is capable of wirelessly communicating up to 20 m at a frequency of 893 Hz and modulation bandwidth of 2.4 Hz. The transmitter can potentially communicate underwater, underground, and in the air due to low attenuation, reflection, and refraction of the ULF magnetic induction. High-overtone bulk diffraction wave gyroscopes are developed to operate in high-shock environments such as autonomous vehicles during accidents. Unlike most commercial MEMS gyroscopes, this gyroscope eliminates the need for proof masses, which can impact parts of the package when exposed to high shocks. The gyroscope uses interdigitated electrodes that excite thickness mode resonances of the longitudinal waves in a lithium niobate substrate. The gyroscope measures rotation from the effects of the Coriolis force on bulk acoustic shear waves generated by the diffraction and reflection of the longitudinal wave.