Torsional MEMS magnetometer operated off-resonance for in-plane magnetic field detection

• Description and operation of a novel torsional MEMS magnetometer for in-plane field detection.
• The magnetometer is fabricated in the same process used for 3-axis accelerometers, 3-axis gyroscopes, and out-of-plane magnetometers and can be part of a 9-axis Inertial Measurement Unit.
• Operation of the device according to off-resonance operation, which theoretically allows to extend the bandwidth without worsening the minimum measurable field (per unit bandwidth), as opposed to resonant operation where intrinsic trade-offs arise.
• Description of a driving and low-noise readout electronics suitable for the characterization of the proposed device.
• Demonstration of sensitivity, linearity, cross-axis rejection and bandwidth in line with consumer grade specifications.

The work presents a microelectromechanical system (MEMS) based magnetometer, targeting compass applications performance, which measures magnetic fields along an in-plane direction. The magnetometer is fabricated with the surface micromachining process used for consumer gyroscopes, accelerometers, and recently proposed out-of-plane magnetometers. The magnetometer is based on the Lorentz force principle, so to avoid the need for magnetic materials integration. It features an area of 282 μm×1095 μm, and it is wafer–wafer packaged at a nominal pressure (0.35 mbar) similar to the one used for gyroscopes. In agreement with theoretical predictions, operation is demonstrated both at-resonance and off-resonance: in both situations the measured resolution, normalized to unit bandwidth and applied Lorentz current, is about 120 nT mA/Hz, but the maximum sensing bandwidth is extended from 4 Hz (at resonance) to 42 Hz in off-resonance mode, which copes with consumer specifications. Within magnetic fields of ±5 mT, the device shows measured linearity errors <0.5% of the full-scale-range (demonstrating a large linearity) and a cross-axis rejection of ∼50 dB. The bias stability in off-resonance operation mode (80 nT mA at 100 s) improves by a factor 100 with respect to resonance operation.