MEMS cantilever based magnetic field gradient sensor

Abstract

This paper describes major contributions to a MEMS magnetic field gradient sensor. An H-shaped structure supported by four arms with two circuit paths on the surface is designed for measuring two components of the magnetic flux density and one component of the gradient. The structure is produced from silicon wafers by a dry etching process. The gold leads on the surface carry the alternating current which interacts with the magnetic field component perpendicular to the direction of the current. If the excitation frequency is near to a mechanical resonance, vibrations with an amplitude within the range of 1–103 nm are expected. Both theoretical (simulations and analytic calculations) and experimental analysis have been carried out to optimize the structures for different strength of the magnetic gradient. In the same way the impact of the coupling structure on the resonance frequency and of different operating modes to simultaneously measure two components of the flux density were tested. For measuring the local gradient of the flux density the structure was operated at the first symmetrical and the first anti-symmetrical mode. Depending on the design, flux densities of approximately 2.5 µT and gradients starting from 1 µT mm−1 can be measured.

100 pT/cm single-point MEMS magnetic gradiometer from a commercial accelerometer

“…We have characterized the performance of a new MEMS magnetic gradiometer with a resolution of 100 pT cm −1 in a shielded vacuum and a range spanning over 3 decades under ambient conditions (1.1 nT cm −1 to 4.6 µT cm −1 ). Compared to existing MEMS magnetic sensor designs, our resolution presents a 30-fold improvement compared with that of MEMS magnetometer technology 17 and a 1000-fold improvement compared with that of MEMS gradiometer technology 24 . The sensor has a small spatial resolution based on a 0.25 mm magnetic sensing element and a total sensorplus-package footprint under 10 mm 2 .…”

“…Finally, an MEMS cantilever design with deflections measured by laser Doppler vibrometry was suggested to be capable of measuring gradient magnetic fields as low as 100 nT cm −1 (ref. 24 In this work, we show that the marriage of a permanent micromagnet and a commercial accelerometer can realize a single-point MEMS gradiometer with a high gradient field resolution (100 pT cm −1 in vacuum and 1 nT cm −1 in air at resonance) and high spatial resolution (250 µm magnetic sensing element). The calculated noise floor is 110 fT cm −1 Hz −1/2 , well within the range of biomagnetic field sensing 25 .…”

“…More recently, MEMS gradiometers have been investigated for the same benefits of traditional MEMS technology and the added benefit of reducing interference from the Earth's field by measuring the gradient [20][21][22][23][24] . Here, both the gradient field resolution and spatial resolution (footprint) are important when measuring the positional change of a magnetic field.…”

“…20 ). Other works have developed socalled single-point gradiometers that do not require the subtraction of two uniform field sensors, circumventing the issues of asymmetry and drift [21][22][23][24] . These works typically integrate permanent magnets and have achieved resolutions in the high mT cm −1 range with large footprints (>1 mm 2 ).…”

Responsitivity Measurement of a Lorentz Force Transducer for Homogeneous and Inhomogeneous Magnetic Fields

“…This combines the advantages of magnitude amplification and large dynamic measurement range. MEMS magnetic field sensors have their potential application in telecommunications, navigation, non-destructive testing [1], dipole characterisation (e.g., CERN [2]) and magnetic resonance tomography [3].…”