Lorentz Force MEMS Magnetometer Research Articles

A Test Setup for the Characterization of Lorentz-Force MEMS Magnetometers

Lorentz-force MEMS magnetometers are interesting candidates for the replacement of magnetometers in consumer electronics products. Plenty of works in the literature propose MEMS magnetometers, their readout circuits and modulations. However, during the standalone characterization of such MEMS devices, a great variety of instruments and strategies are used, making it very complex to compare results from different works in the literature. For this reason, this article proposes a test setup to characterize Lorentz-force MEMS magnetometers. The proposed setup is based around the use of an impedance analyzer for the driving of voltage and Lorentz-current of the MEMS in-phase and in quadrature, which allows the device Amplitude Modulation and Frequency Modulation characterization. The proposed solution is validated by using the designed circuit to characterize two CMOS-MEMS magnetometers with very different characteristics.

Stochastic Effects on the Dynamics of the Resonant Structure of a Lorentz Force MEMS Magnetometer

Resonance features of slender mechanical parts of Lorentz force MEMS magnetometers are affected by the (weakly) coupled thermo-electro-magneto-mechanical multi-physics governing their dynamics. We recently showed that reduced-order models for such parts can be written in the form of the Duffing equation, whose nonlinear term stems from the mechanical constraint on the vibrations and is affected by the driving voltage. As some device performance indices vary proportionally to the amplitude of oscillations at resonance, an optimization of the operational conditions may lead to extremely slender, imperfection-sensitive movable structures. In this work, we investigate the effects of imperfections on the mechanical response of a single-axis magnetometer. At the microscopic length-scale, imperfections are given in terms of uncertainties in the values of the over-etch depth and of the Young’s modulus of the vibrating polycrystalline silicon film. Their effects on the nonlinear structural dynamics are investigated through a Monte Carlo analysis, to show how the output of real devices can be scattered around the reference response trend.

Sensitivity Enhancement of a Lorentz Force MEMS Magnetometer With Frequency Modulated Output

This paper presents a frequency output micromachined silicon magnetometer comprised of a low stiffness thermal-piezoresistive electromechanical resonator coupled to a Lorentz force generator. The Lorentz force generator utilizes a leverage mechanism amplifying the force by ~ 55X. Upon application of a magnetic field perpendicular to the device, the leveraged Lorentz force deflects the resonator central beam changing its resonance frequency. Furthermore, different structures for the piezoresistive beam have been explored and the design has been modified to enhance the sensitivity by ~ 950X (from 7.73 to ~7200 ppm/mA.T). Frequency shifts as high as 0.123ppm/ $\mu \text{T}$ have been demonstrated, for the best-case design operating at a Lorentz force current of 17 mA, which is in good agreement with its simulated values. The drift in the resonance frequency has been experimentally measured for various operating bias currents to get an insight into the noise floor for such devices. The noise floor for the best-case design is measured to be 14 $\mu \text{T}/\surd $ Hz, which is in the same order when compared with the prior state-of-the-art frequency modulated Lorentz force MEMS magnetometers. [2016-0111]

Amplitude modulated Lorentz force MEMS magnetometer with picotesla sensitivity

This paper demonstrates ultra-high sensitivities for a Lorentz force resonant MEMS magnetometer enabled by internal-thermal piezoresistive vibration amplification. A detailed model of the magneto-thermo-electro-mechanical internal amplification is described and is in good agreement with the experimental results. Internal amplification factors up to ~1620 times have been demonstrated by artificially boosting the effective quality factor of the resonator from 680 to 1.14 × 106 by tuning the bias current. The increase in the resonator bias current in addition to the improvement in the quality factor of the device led to a sensitivity enhancement by ~2400 times. For a bias current of 7.245 mA, where the effective quality factor of the device and consequently the sensitivity is maximum (2.107 mV nT−1), the noise floor is measured to be as low as 2.8 pT (√Hz)−1. This is by far the most sensitive Lorentz force MEMS magnetometer demonstrated to date.

Off-Resonance Low-Pressure Operation of Lorentz Force MEMS Magnetometers

This paper demonstrates bandwidth extension for Lorentz force microelectromechanical systems magnetometers operated at a frequency slightly lower than the mechanical resonance. This off-resonance mode of operation also minimizes the critical tradeoff between maximum sensing bandwidth and minimum measurable field, enabling low-pressure operation and the associated resolution improvement. The experimental verification is obtained using two identical devices packaged at two different pressures and tested with a suitable low-noise board-level electronics. The first device, packaged at 1 mbar, has a (mechanical) sensing bandwidth of ~50 Hz, and a resolution per unit bandwidth and driving current of 290 nT · mA/√Hz. With the second device, packaged at 0.25 mbar and operated off-resonance, roughly twice better resolution (160 nT · mA/√Hz) and four times larger bandwidth (>200 Hz) are obtained.

Operation of Lorentz-Force MEMS Magnetometers With a Frequency Offset Between Driving Current and Mechanical Resonance

The paper discusses the operation of Lorentz-force-based microelectromechanical magnetometers at a driving-current frequency slightly lower than the device resonance frequency. Among the advantages with respect to operation at resonance, there are a higher achievable signal to noise ratio (thanks to the lower permitted pressure and damping coefficient, which have now no influence on the maximum sensing bandwidth) and the possibility of driving more magnetometers in series through a single current source, enabling the fabrication of low-power 3-axis magnetic field sensors. A partial drawback is represented by a loss in gain-factor. Experimental results obtained on a sample device confirm the trade-off between gain-factor decrease and bandwidth increase. Guidelines for an optimized design of Lorentz force magnetometers are given together with a comparison with other state-of-the-art technologies through the introduction of a figure of merit. In particular, it is shown how Lorentz force devices can reach better performance in terms of minimum detectable magnetic flux density per unit current consumption and bandwidth.