A Lorentz-Force MEMS Magnetometer With an Annular Resonance Beam for Z-Axis Sensing

Abstract

Restricted by the shapes of utilized resonance structures, conventional Lorentz-force micro-electro-mechanical systems (MEMS) magnetometers are rarely designed with the multiturn coil layout for the performance enhancement of Z-axis magnetic sensing by increasing the effective coil length. We put forward a novel Lorentz-force MEMS magnetometer with a low-frequency annular resonance beam for Z-axis sensing. Wherein, four resonance elements are interconnected and coupled into the annular resonance beam with the contraction-expansion (CE) mode (~37.4 kHz), which contributes to generating together Lorentz forces on four sides of the MEMS magnetometer structure. Meanwhile, a three-turn Lorentz force coil with only two coil pads, which is utilized for injecting the alternating current (ac), is located on and electrically insulated with the annular Si resonance beam. Under the Z-axis magnetic field, capacitance comb structures around the annular beam are employed to detect the resonance displacement induced by the Lorentz force. In this article, the proposed MEMS magnetometer is illustrated with device designs, theory analyses, device fabrications, interface circuit realizations, and performance characterizations. Specifically, the designed MEMS magnetometer is fabricated using bulk Si micro-machining processes. In addition, a board-level interface circuit is realized by amplification, phase shift, demodulation, and filtering. By experimental tests, the fabricated MEMS magnetometer has a sensitivity of 214.2 mV/mT with a nonlinearity of 0.42% and a resolution of $\sim 3~\mu $ T estimated from noise measurements. Also, it exhibits fine performances on the cross-axis rejection ratios (37 dB), hysteresis (0.34%), and repeatability (~0.9%).