**5. Applications**

Magnetic field sensors based on MEMS technology have potential advantages with respect to conventional magnetic field sensors such as small size, light weight, compactness, lower power consumption. Additionally the MEMS technology achieves low-cost sensors by means of batch fabrication techniques and their potential integration with integrated circuits (IC) on a same substrate.

Figure 16 shows images of the realized three dimensional magnetic Hall sensor highlighting

Fig. 16. Fully integrated three-axis Hall sensor reported by Todaro et al. (2010). a) a schematic of the three-axis Hall sensor; b) Image of a fabricated three axis Hall sensor; c) highlight of an out-of-plane Hall sensor acquired by scanning electron microscopy.

Magnetic field sensors based on MEMS technology have potential advantages with respect to conventional magnetic field sensors such as small size, light weight, compactness, lower power consumption. Additionally the MEMS technology achieves low-cost sensors by means of batch fabrication techniques and their potential integration with integrated circuits

Fig. 15. Three-axis Hall sensor fabrication steps.

the out-of-plane sensor.

**5. Applications** 

(IC) on a same substrate.

Conventional magnetic sensor classes presented in this paper have different application fields depending on their sensitivity range and minimum detectable magnetic field.

Resonant sensors have a magnetic range up to 1 T with a maximum resolution of 1nT, fluxgate sensors range spans from 100 pT to 1 mT, while Hall sensors have a sensitivity ranging from 1 µT to 1 T. Resonant sensors have lower resolution compared to fluxgate, however they present a wide sensitivity range and they could compete with fluxgate sensors into numerous applications for measuring magnetic fields.

Among the sensors presented in this paper, Hall sensors are the less sensitive devices. Their robustness and simple fabrication process justify their use in hundreds of applications.

Improvements in the microfabrication technologies combined with the employment of new and more performing materials as well as novel design solutions for devices on the microscale could enhance further the resolution, making them suitable for applications requiring very high sensitivity, such as in biomedical field and for the realization of new class of hand-held equipments. On the other hand this technology could help in new solutions for devices in applications requiring low sensitivity.

Magnetic field sensors on the microscale with moderate sensitivity, could be used for vehicle detection and recognition (Herrera-May et al. 2009). In fact vehicles moving over ground can generate a succession of impacts on the earth's magnetic field, that can be detected by means of magnetic perturbation using a magnetic sensor, and automatically recognize them by advanced signal processing and recognition method. In this context such sensors could be used for the measure of the speed and size of vehicles for traffic surveillance. Additionally magnetic field sensors can be used in systems containing accelerometers, gyroscopes and pressure devices for vehicle control applications (Niarchos, 2003). For example they can be employed in electronic stability program (ESP) systems to help vehicles to be dynamically stable in critical situations like hard braking and slippery surfaces.

Such microsensors can be employed in electronic compasses for sensing earth's magnetic field for GPS systems in order to provide more precise and instantaneous headings to aid navigation for air, ground and underwater systems. Additionally such devices can be used for global positioning systems (GPS) in cell phones due to the requirements of reduced size, low cost and low power consumption.

These magnetic field sensors find employment for the detection of compact ferrous objects (McFee et al. 1990). Such objects are of major concern in a number of applications. In environment science there is the need for portable sensors for mineral prospecting like measurements of magnetic properties of rocks, as well as detection of pipeline corrosion where geological ore inclusions generate typical peak magnetic induction in the range 1- 1000 nT. In the military field such devices can be used in systems for the detection and mapping of hidden or unexplosed ordnance (mines, bombs, and artillery shells which have a peak magnetic induction in 10-1000 nT range) as well as for detection of armored vehicles (10000 nT) or submarines ( 1-10 nT). The performance of these systems can be enhanced by using two or three dimensional array of sensors. This could give additional informations on the size and the depth of the buried objects.

Magnetic Field Sensors Based on Microelectromechanical Systems (MEMS) Technology 121

In this paper the authors described current research status in magnetic field sensors focusing on devices fabricated by exploiting MEMS technologies. The paper presents advances in some classes of devices such as resonant sensors, fluxgate sensors and Hall sensors that take advantages from these technologies. The authors focused on the description of such microsensors including operation principle, example of realized devices, highlighting the involved fabrication technologies. Possible applications of this new class of compact devices

This work was partially supported by FIRB - Hub di ricerca italo-giapponese sulle

Bahreyni, B. (2008). Fabrication and Design of Resonant Microdevices; William Andrew,

Beeby, S.; Ensell, G.; Kraft, M.; White, N. (2004). MEMS Mechanical Sensors, Artech House,

Bending, S.J. & Khotkevych, V.V. (2009). Scanning Hall Probe Imaging of Nanoscale Magnetic Structures. Sensor Letters, Vol. 7, No. 3, (June 2009), pp. 503-506. Beroulle, V.; Bertrand, Y.; Latorre, L. & Nouet, P. (2003). Monolithic piezoresistive CMOS

Bozorth, R.M. & Chapin, D.M. (1942). Demagnetising factors of rods, J. Appl. Phys., Vol. 13,

Brugger, S. & Paul, O. (2009). Field-Concentrator-Based Resonant Magnetic Sensor With

Chang, H.; Xue, L.; Qin, W.; Yuan, G.; Yuan, W. (2008). An integrated MEMS gyroscope

Elwenspoek, M. & Wiegerink, R. (2001). Mechanical Microsensors, Springer-Verlag: Berling,

Emmerich, H. & Schöfthaler, M. (2000). Magnetic field measurements with a novel surface

Estrada, H.V. (2011). A MEMS-SOI 3D-magnetic field sensor, 24th International Conference on Micro Electro Mechanical Systems (MEMS). IEEE 2011, pp. 664-667. Herrera-May, A. L.; García-Ramírez, P.J.; Aguilera-Cortés, L.A.; Martínez-Castillo, J.M.;

array higher accuracy output, Sensors 2008, Vol. 8, pp. 2886-2899.

DECEMBER (2009), pp. 1432-1443, ISSN 1057-7157.

(May 2000), pp. 972-977, ISSN 0018-9383.

magnetic field sensors, Sens. Actuators A, Vol. 103, No. 1-2, (January 2003), pp. 23-

Integrated Planar Coils, Journal of Microelectromechanical Systems,, Vol. 18, No.6,

micromachined magnetic-field sensor, IEEE Trans. Electron Dev., Vol. 47, No. 5,

Sauceda-Carvajal A.; García-González, L. & Figueras-Costa, E. (2009). A resonant magnetic field microsensor with high quality factor at atmospheric pressure, J. Micromech. Microeng. , Vol. 19, No. 1, (January 2009),pp. 015016 -015026, ISSN

**6. Conclusion** 

has also been reported.

**7. Acknowledgment** 

Norwich, NY, USA.

Norwood, MA, USA.

32, ISSN 0924-4247.

Heidelberg, Germany.

pp. 320.

1057-7157.

nanotecnologie.

**8. References** 

Another application of these sensors is in non-distructive testing for a variety of evaluations including medical implants and aircraft structures, the detection of cracks and corrosion in metals. Archeology is another field requiring systems including magnetic field sensors to resolve non-invasively details, the wide range of artifacts (1-1000 nT magnetic induction range ) and cultural objects. This field requires also new means of mapping prehistoric and historic sites in three dimensions rather than traditional twodimensional methods.

High sensitivity and high resolution magnetic sensors are needed in systems for medical diagnostics. Microfluxgate sensors based on MEMS technology can be employed to build cheap and portable systems for locating metallic foreign objects in the human body (Jing et al. 2009).

Ripka, (2004) showed that fluxgate sensors can be used for mapping the distribution of ferromagnetic particles in the lungs after they are magnetized by strong DC field. Medical applications requiring precise miniaturized magnetic sensors include tracking devices and systems for monitoring magnetic markers such as magnetic "biscuits" and microbeads. Magnetic biscuits can be used for functional tests of digestive tract, while microbeads are used as markers in biotechnology. New types of fluxgate microsensors are being developed for these applications (Vopalensky et al., 2003). Also Hall magnetic sensors have been employed to visualize a magnetically market diagnostic capsule in real time inside human body (Mahfuzul-Aziz, 2008). Tracking devices using fluxgate sensors can be used for monitoring the 3-D position and also orientation of a small permanent magnet which can be attached to body or medical instrument (such as catheter). Another configuration is being used for tracking the motion of the body at further distances: signals from sensors attached to the body are collected and processed.

Typically Hall magnetic devices, due to their low sensitivity are employed for position sensing, current sensing, speed detection, electronic compasses. (Lenz & Edelstein, 2006). Silicon-based Hall sensors are widely employed, due to the suitability of integration with electronics (Popovic, 1997). However, higher sensitivity sensors can be obtained with III-V technology, allowing for applications such as biomolecular function detection (Manandhar et al., 2009). Also recently, the Scanning Hall probe microscopy (SHPM) has been developed based on III-V Hall sensors, allowing for quantitative mapping of nanoscale superconducting and ferromagnetic materials (Bending et al., 2009).

Others nowadays applications such as geomagnetic measurements, environmental disturbance measurements as well as navigation systems demand for Hall miniaturized devices capable of measuring the vector magnetic field. Beside the cumbersome solution of mounting three Hall sensors with their sensitive axis orthogonal to each other, integrated three axis devices have been developed by employing silicon technology and the so called vertical Hall effect (Schott & Popovic, 1999). However these are often characterized by either different sensitivity for each component of the magnetic field (Schott et al., 2000), or by a cross-sensitivity among the direction-components (Popovic, 1999). Furthermore, offset compensation of vertical Hall element is more difficult than in the case of lateral (planar) Hall elements. In this context new materials and device configurations could open the way to realize reliable vector magnetic field sensors to be applied in different fields.
