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

Abstract

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 x 1095 µm 2 , 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 42Hz 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.

100 &#x00B5;A, 320 nT/vHz, 3-AXIS Lorentz force MEMS magnetometer

“…Note that the sensitivities of the two sensors are almost matched (about 1.5 zF/(nT•mA), in reasonable agreement with the theoretical predictions), and that the cross-axis rejection (> 35dB) is low and likely limited by manual alignment. Sensitivity values are almost 5 times larger than those presented in [3,8] using the same process. The linearity error is lower than 0.3% (the value being limited by system noise) within the ±3 mT full-scale-range, confirming the almost unlimited full-scale of Lorentz-force devices.…”

“…As shown in Fig. 7, the white noise contribution for the two different devices is similar (240-320 nT/•Hz, for only 100 μA rms of driving current), and more than 4-fold better with respect to previous devices fabricated using the same process [3,8].…”

“…The implemented solution (see Fig.1) consists in making the driving current re-circulate inside the device for a given number of loops (N loop ). This solution allows to have a direct increment on the capacitive sensitivity S CB , ideally by a factor N loop [8]:…”

“…by lowering the package pressure without worrying about the system bandwidth). Further, as the working frequency differs from resonance, an improvement on scalefactor stability is observed [8]: the sensitivity to small variations of the frequency mismatch is minimized with respect to resonant operation, thus solving the second and third issues above for resonance operation. However, this solution carries two new drawbacks: (i) an inherently lower scale-factor and (ii) the need for a new drive reference, suitably shifted from the magnetometer resonance.…”

Recent Advances of MEMS Resonators for Lorentz Force Based Magnetic Field Sensors: Design, Applications and Challenges

“…Other IMUs applications will include the navigation systems of ships, trains, military and civil aviation and unmanned operated vehicles [ 43 , 44 , 45 ]. Laghi et al [ 46 ] fabricated a torsional MEMS sensor through surface micromachining for monitoring in-plane magnetic field, as shown in Figure 26 . This sensor operates with the Lorentz force and capacitive sensing.…”