Research On Magnetic Field Sensors Based On Piezoelectric MEMS Resonators

Magnetic field sensing is indispensable in everyday life.With the continuous development of science and technology,how to realize a integrated high performance magnetometer has become one of the important problems.In recent decades,continued advances in micromachining processes have led to a boom in Micro-Electro-Mechanical Systems(MEMS)technology,resulting in the emergence and widespread commercialization of various MEMS devices.MEMS sensors offer the benefits of small size,low power consumption,excellent performance and easy integration on a chip.They usually have characteristic feature sizes typically in micron range,enabling functions not possible in conventional sensors.Among various MEMS devices,MEMS resonators based on Thin-film Piezoelectric-on-Silicon(TPo S)structures exhibit advantages of high electromechanical transduction efficiency,high quality factor and mechanical stability,thus recently attracting widespread interest in the field of on-chip resonant sensors.This thesis designs a resonant magnetic field sensor based on TPo S resonator.Based on the measured results,a generic equivalent circuit model is developed to characterize the magnetic field sensor.The organization of this thesis is as follows.Firstly,this thesis provides an in-depth analysis for the electromechanical conversion principle and circuit model of TPo S resonators.The main structure of the resonator is a square flat plate,which is designed to vibrate in corner-flapping vibration mode upon external magnetic field.The input excitation AC current,passing through a metal track on the top of the plate,is responsible for generating Lorentz force,which drives the resonator to vibrate and thus yields output current due to piezoelectric effect.With the help of the theoretical and simulation model,four resonant magnetic field sensors were designed and optimized.Measurement and data analysis on the fabricated devices were subsequently carried out.Next,the interface circuits of four resonators were implemented with different wire bonding schemes.The magnetic field sensors were electrically characterized using vector network analyzer and lock-in amplifier,focusing on investigating the effect of bias current.The measured results show that the fabricated magnetic field sensor exhibit sensitivity of0.281 ppm/m T.Finally,this thesis analyzed the causes of bias current and improved the equivalent circuit model in order to include the effect of bias current.The proposed equivalent circuit model is verified by comparing the simulation and measured results.The proposed equivalent circuit model can also be used for analyzing other type of resonant MEMS magnetic field sensors,providing a new idea for relevant research.