MEMS magnetic field sensor
Encyclopedia
Introduction
Magnetic field sensing can be categorized into four general types depending on the magnitude of the measured field. If the targeted B-field larger than the earth magnetic fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
(maximum value around 60
T), the requirement on the sensitivity of the sensor is not aggressive. To measure the earth field larger than the geomagnetic noise(around 0.1 nT), better sensors is required. For the application of magnetic anomaly detectionMagnetic anomaly detector
A magnetic anomaly detector is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used by military forces to detect submarines ; the military MAD gear is a descendent of geomagnetic survey instruments used to search for...
, sensors at different spots 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, molecule tagging and etc. Figure to the right shows the magnetic sensing category in terms of the magnitude of the field being measured.
There are tons of approaches for magnetic sensing, including Hall effect sensor
Hall effect sensor
A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications....
, magneto-diode, magneto-transistor, AMR
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
, GMR magnetometer, magnetic tunnel junction magnetometer, magneto-optical sensor, Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
based MEMS
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
sensor, Electron Tunneling based MEMS sensor, MEMS compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
, Nuclear precession magnetic field sensor, optically pumped magnetic field sensor, fluxgate magnetometer, search coil
Search coil
A search coil is a device for measuring eye movement using coils that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the eye. In animal studies, the search coil may be surgically implanted into the sclera of the eye. Alternating magnetic fields are generated by...
magnetic field sensor and SQUID magnetometer. MEMS-based magnetic field sensor can offer small-size solution for magnetic field sensing. Smaller device can be placed closer to the measurement spots and thereby achieving higher spatial resolution. Additionally, MEMS magnetic field sensor does not involve the microfabrication
Microfabrication
Microfabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
of magnetic material. Therefore, the cost of the sensor can be largely reduced. Integration of MEMS sensor and microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
can further reduce the size of the entire magnetic field sensing system.
Lorentz-force-based MEMS sensor
This type 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 resonanceResonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
in order to obtain the maximum output signal. Piezoresistive and electrostatic transduction method 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.
Voltage sensing
Beroulle et al. have fabricated a U-shape cantilever beam on silicon substrate. Two piezo-resistors are laid on the support ends. There are 80-turns Al coil passing current along the U-shape beam. Wheatstone bridge is formed by connecting two "active" resistor with another two "passive" resistor free of strain. When there is an external magnetic field applying to the current carry conductor, motion of the U-shape beam will induce strain in two "active" piezo-resistors and thereby generating an output voltage across the Wheatstone bridgeWheatstone bridge
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...
which is proportional to the magnetic field flux density. The reported sensitivity for this sensor is 530 m Vrms/T with a resolution 2 µT. Note that the frequency of the exciting current is set to be equal to the resonant frequency of the U-shape beam in order to maximize the sensitivity.
Herrera-May et al. fabricate a sensor with similar piezoresitive read-out approach but with different mechanical motion. Their sensor relies on the torsional motion of a micro-plate fabricated from silicon substrate. The exciting current loop
Current loop
A current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...
contains 8 turns of aluminum coil. The location of the current loop enables a more uniform Lorentz force distribution compared with the aforementioned U-shape cantilever beam. The reported sensitivity is 403 mVrms/T with a resolution 143 nT.
Kádár et al. also chose the micro-torsional beam as the mechanical structure. Their read-out approach is different. Instead of using piezoresitive transduction, their sensor relies on electrostatic transduction. They patterned several electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s on the surface of the micro-plate and another external glass wafer. The glass wafer is then boned with the silicon substrate to form a variable capacitor
Variable capacitor
A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio , or as a variable reactance, e.g...
array. Lorentz force generated by the external magnetic field will results in the change of capacitor array. The reported sensitivity is 500 Vrms/T with a resolution of a few mT. The resolution can reach 1 nT with vacuum operation.
Emmerich et al. fabricated the variable capacitor array on a single silicon substrate with comb-figure structure. The reported sensitivity is 820 Vrms/T with a resolution 200 nT at the pressure level of 1mbar.
Frequency shift sensing
Another type of Lorentz force based MEMS magnetic field sensor utilize the shift of mechanical resonanceMechanical resonance
Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations matches the system's natural frequency of vibration than it does at other frequencies...
due to the Lorentz force applying to certain mechanical structures.
Sunier et al. change the structure of aforementioned U-shape cantilever beam by adding a curved-in support. The piezoresistive sensing bridge is laid between two heating actuation resistors. Frequency response of the output voltage of the sensing bridge is measured to determine the resonant frequency of the structure. Note that in this sensor, the current flowing through the Aluminum coil is dc current. The mechanical structure is actually driven by the heating resistor at its resonance. Lorentz force applying at the U-shape beam will change the resonant frequency of the beam and thereby change the frequency response of the output voltage. The reported sensitivity is 60 kHz/T with a resolution of 1 µT.
Bahreyni et al. fabricated a comb figure structure on top of the silicon substrate. The center shuttle are connected to two clamped-clamped conductors used to change the internal stress of the moving structure when external magnetic field is applied. This will induce the change of the resonant frequency of the comb finger structure. This sensor use electrostatic transduction to measure the output signal. The reported sensitivity is improved to 69.6 Hz/T thanks to the high mechanical quality factor (Q = 15000 @ 2 Pa) structure in the vacuum environment. The reported resolution is 217 nT.
Optical sensing
The optical sensing is to directly measure the mechanical displacement of the MEMS structure to find the external magnetic field.Zanetti et al. fabricated a Xylophone beam. Current is flowing through the center conductor and the Xylophone beam will be deflected as the Lorentz force is induced. Direct mechanical displacement is measured by an external laser source and a detector. The resolution of 1 nT can be reached. Wickenden had tried to shrink the footprint of this type of device by 100 times. But a much lower resolution of 150 µT was reported.
Keplinger et al. were trying to use an LED source for optical sensing instead of using an external laser source. Optical fibers were aligned on the silicon substrate with different arrangements for the displacement sensing. A resolution 10 mT is reported.