MEMS sensors for measuring the earth magnetic field: mechanical aspects

The goal of this project was to study, design and optimize novel micromachined MEMS sensors for Earth magnetic field sensing which has found various applications in electronic devices. This work is part of a collaboration between the Department of Civil and Environmental Engineering of Politecnico di Milano, Milano, Italy and STMicroelectronics. After an introduction to the background of the integrated magnetic field sensors, a multi physics modeling of MEMS magnetometers is presented to provide a framework for new designs. Based on the STMicroelectronics technology and the provided framework, novel ideas for MEMS uniaxial magnetometers out of the acoustic bandwidth are proposed, designed and fabricated: a frame-like structure and a Half-Double Ended Tuning Fork (HDETF). Experimental results of the HDETF device shows that the balanced key performance indexes (i.e. mechanical sensitivity, Brownian noise resolution, power consumption and bandwidth) required for market applications can be matched, with a very area efficient device fabricated by standard industrial processes. The design parameters of the tested device were chosen by trial-and- error, considering the trade-offs between all the performance indexes. Hence, geometry optimality of the tested device is discussed by making use of the multi-physics, multi-constrained optimization approach. Multi-axis sensing and sensor integration is the trend of MEMS inertial sensors as they allow chip size and fabrication cost to be reduced, while maintaining the same performance and reliability. Hence, at the end of this work, a novel idea of single-structures for three axis magnetometers and their basic working principles are presented.