A MEMS magnetic field sensor is a small-scale microelectromechanical systems (MEMS) device for detecting and measuring magnetic fields (magnetometer). Many of these operate by detecting effects of the Lorentz force: a change in voltage or resonant frequency may be measured electronically, or a mechanical displacement may be measured optically. Compensation for temperature effects is necessary. Its use as a miniaturized compass may be one such simple example application.
Magnetic field sensing Magnetometers can be categorized into four general types depending on the magnitude of the measured field. If the targeted B-field is larger than the earth magnetic field (maximum value around 60 μT), the sensor does not need to be very sensitive. To measure the earth field larger than the geomagnetic noise(around 0.1 nT), better sensors are required. For the application of magnetic anomaly detection, sensors at different locations have to be used to cancel the spatial-correlated noise in order to achieve a better spatial resolution. To measure the field below the geomagnetic noise, much more sensitive magnetic field sensors have to be employed. These sensors are mainly used in medical and biomedical applications, such as MRI and molecule tagging. There are many approaches for magnetic sensing, including the Hall effect sensor, magneto-diode, magneto-transistor, AMR magnetometer, GMR magnetometer, magnetic tunnel junction magnetometer, magneto-optical sensor, Lorentz force based MEMS sensor, electron tunneling based MEMS sensor, MEMS compass, Nuclear precession magnetic field sensor, optically pumped magnetic field sensor, fluxgate magnetometer, search coil magnetic field sensor and SQUID magnetometer.
Figures of merit of MEMS magnetic sensor MEMS magnetic sensors have several parameters: quality factor (Q), resonance frequency, mode shape, responsivity, and resolution. Quality factor is a measure of how much energy can be maintained during vibration of the resonator. There might be several factors that can damp the resonator, such as mechanical damping of resonator itself or damping from outside pressure and temperature. Resonance frequency is the frequency at which the device vibrates with the highest amplitude (or the longest, as a struck bell or tuning fork). Resonance frequency is governed by geometry of the device. We can calculate resonance frequency when we know dimension of the device, equivalent Young's modulus of the device, and the equivalent density of the device. Mode shape is the pattern of the vibration of resonator. Responsivity (which contributes to resolution) describes the size of the oscillation we can get from devices with same external condition. If we apply the same current and B field to several resonators, devices that show larger vibration amplitudes are said to have a higher responsivity. All other things being equal, a higher responsivity device is more sensitive. The range of magnetometers based on piezoelectric resonators is mV/T (millivolt/tesla), so higher responsivity is generally better. Resolution refers to the smallest magnetic field a device can measure. The smaller the number, the more sensitive the device. The range of magnetometers based on piezoelectric resonator is a few nT (nanotesla).
Advantages of MEMS-based sensors A MEMS-based magnetic field sensor is small, so it can be placed close to the measurement location and thereby achieve higher spatial resolution than other magnetic field sensors. Additionally, constructing a MEMS magnetic field sensor does not require the microfabrication of magnetic material. Therefore, the cost of the sensor can be greatly reduced. Integration of MEMS sensor and microelectronics can further reduce the size of the entire magnetic field sensing system.
Lorentz-force-based MEMS sensor This type of sensor relies on the mechanical motion of the MEMS structure due to the Lorentz force acting on the current-carrying conductor in the magnetic field. The mechanical motion of the micro-structure is sensed either electronically or optically. The mechanical structure is often driven to its resonance in order to obtain the maximum output signal. Piezoresistive and electrostatic transduction methods can be used in the electronic detection. Displacement measurement with laser source or LED source can also be used in the optical detection. Several sensors will be discussed in the following subsections in terms of different output for the sensor.
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