**4. Hall sensors**

Hall sensors exploits Hall effect as trasduction principle to detect magnetic field. They are commonly fabricated by standard Complementary Metal-Oxide Semiconductor (CMOS) technology. In general, they are applicable in a range from 1 µT to 1 T and have a die size less than one millimeter.

These sensors can measure either constant or varying magnetic field. The frequency limit is around 1 MHz and operate well in a wide temperature range (Ripka & Tipek, 2007).

The Hall effect is based on the Lorentz force felt by charge carriers moving in a magnetic field. Figure 11 shows the schematic of the classical configuration where a thin slab of a conductor is placed in a magnetic field Bz. When a current flows in the x direction, the Lorenz force acts in the y direction, determining a charge distribution that counterbalances the force. Therefore a (Hall) voltage VH builds up, as shown in figure 11.

In the case of a constant current drive, the Hall voltage is given by:

has been reported by Zorlu et al. (2007). The sensor structure is reported in figure 10. The

The overall dimension of the sensor chip is 1.8 0.8 mm, the sensitivity is 0.51 mVmT-1 in a linear operating range of 200 T. The noise was 95nT/Hz@1Hz with an excitation power

sensor core is only 1 mm in length and the sensor has two flat 60 turn pickup coils.

Fig. 10. Sensor structure the orthogonal microfluxgate reported by Zorlu at al. (2007).

Hall sensors exploits Hall effect as trasduction principle to detect magnetic field. They are commonly fabricated by standard Complementary Metal-Oxide Semiconductor (CMOS) technology. In general, they are applicable in a range from 1 µT to 1 T and have a die size

These sensors can measure either constant or varying magnetic field. The frequency limit is

The Hall effect is based on the Lorentz force felt by charge carriers moving in a magnetic field. Figure 11 shows the schematic of the classical configuration where a thin slab of a conductor is placed in a magnetic field Bz. When a current flows in the x direction, the Lorenz force acts in the y direction, determining a charge distribution that counterbalances

around 1 MHz and operate well in a wide temperature range (Ripka & Tipek, 2007).

the force. Therefore a (Hall) voltage VH builds up, as shown in figure 11.

In the case of a constant current drive, the Hall voltage is given by:

consumption of 8.1 mW.

**4. Hall sensors** 

less than one millimeter.

Fig. 11. Hall effect principle.

$$V\_H = V\_{OFF} + \frac{B\_z I}{en\_{2D}}\tag{7}$$

where VOFF is the Hall voltage at zero magnetic field (offset voltage) and n2D is the sheet charge concentration given by the product of the bulk charge concentration n and the thickness of the slab t:

$$m\_{2D} = nt\tag{8}$$

If the current is constant, lower charge carriers concentration involves higher carriers speed resulting in an higher Lorentz force. Therefore low charge carrier concentration leads to high Hall voltage, thus justifying the extensive employment of semiconductor materials for Hall magnetic sensors with respect to metals.

The current related sensitivity is a key figure of merit of a Hall sensor and can be defined as:

$$S\_I = \left| \frac{1}{I} \frac{\partial V\_H}{\partial B\_z} \right| \tag{9}$$

As the Hall voltage is related only to the z-axis magnetic field component, Hall magnetic sensors are basically uniaxial devices. Silicon based Hall sensors are widely employed, due to the suitability of integration with electronics. However, magnetic sensors based on silicon may have intrinsic limits to their sensitivity and resolution, which may limit future performance gains. In addition, they need temperature compensation circuits that can include temperature sensor and operational amplifiers (op-amps).

MEMS technology has been employed in this class of devices in order to solve some of their limitations and to find alternatives to silicon as structural material. For example polymerbased devices are interesting alternatives to silicon, particularly when the polymer materials can be functionalized for enhanced specific material properties (e.g, optical, electrical, and mechanical). Mouaziz et al. (2006) proposed the realization of SU-8 cantilevers with an integrated Hall-probe for advanced scanning probe sensing applications. To this purpose an innovative release method of polymer cantilevers with embedded integrated metal electrodes has been employed. Figure 12 shows the device fabrication process. On the silicon wafer with 0.5 µm of thermal oxide a 2 µm-thick polysilicon sacrificial layer is deposited (figure 12a). The electrodes of the Hall probe (figure 12b) and the metallic thin film electrical connections are obtained by lithographic patterning, metal deposition and liftoff (figure 12c). The device structure is obtained by lithographic patterning of two layers of SU-8 polymer: a 10µm-thick photo-structured layer for the cantilever, and a 200µm-thick layer for the chip body (figure 12d and figure 12e). The releasing method is based on dry etching of a 2µm-thick sacrificial polysilicon layer (figure 12 f).A device sensitivity of 0.05 V/AT was achieved together with a minimum detectable magnetic flux density of 9 µT/√Hz at frequencies above 1 kHz at room temperature.

Sunier et al. (2004) reported on a vertical Hall sensor with precisely defined active area fabricated by a process combining deep-RIE silicon trench etching and anisotropic TMAH silicon wet etching. The fabrication process is showed in figure 13. The main process steps are the thermal oxidation of the p-type silicon wafer (figure 13a), the definition of the trench

Magnetic Field Sensors Based on Microelectromechanical Systems (MEMS) Technology 117

Estrada (2011) proposed a three axis Hall sensor based on MEMS micromachining of SOIwafers. Three Hall sensors embedded in a flexible polyimide carrier was obtained so that appropriate folding of the structure resulted in three Hall-probes positioned to form an orthogonally-oriented array on three faces of a millimeter-sized cube. The key fabrication process steps are as follow: (a) SOI-wafer; (b) micromachining of the active layer using either wet (TMAH) or dry (DRIE) etching; (e) patterning the Al-film needed for ohmic contacts; (f) deposition and curing of the polyimide film; g) etching of the handle wafer to reach the buried oxide film; (h) final metallization of the contact pads for possible soldering

Fig. 14. Key steps for the fabrication of the three-dimensional axis Hall sensor on a flexible

This 3D-sensor configuration allows vector magnetic field measurements where the advantage is that all its elements have the same magnetic sensitivity of about 100 V/AT.

An integrated three axis Hall sensor based on III-V technology was fabricated by employing a micromachining technique for realizing self positioned structures (Todaro et al., 2010). The MEMS technique was applied to a GaAs-based heterostructure containing a sensing layer and a strained layer. The selective removal of a third sacrificial layer allows for the relaxation of the strained layer and the self positioning of the sensing part, which has been

The main fabrication steps are as follows: a) Epitaxial growth of a multilayer with sacrificial layer, strained layer, and sensor multilayer; (b) Photolithography and wet etching to define the mesa active region; (c) Photolithography and wet etching to define the hinge region; (d) metallization by lift off (GeAu/Ni/Au); (e) Photolithography and chemical etching to expose the edge of the sacrificial layer; (f) selective etching of the sacrificial layer and self-

Current related sensitivity of more than 1000 V/AT both for in-plane and for out-of-plane Hall sensors, demonstrates the effectiveness of this method for realizing fully integrated

substrate (polyimide film) proposed by Estrada (2011).

already processed to realize a Hall sensor element (figure 15).

miniaturized high sensitivity three axis magnetic sensors.

or wire bonding (figure 14).

positioning of the structure.

by dry etching, the sidewall implantation and oxidation (figure 13b), another dry etching process to deepen the trenches (figure 13c), TMAH wet etching to release the bottom of the active area (figure 13d), trench oxidation, polysilicon refill and blanket etch, contact formation and metallization (figure 13e) and finally XeF2 etching of the polysilicon in the tranches (figure 13f).

Fig. 12. Schematic illustration of the process for SU-8 cantilever with integrated electrodes reported by Mouaziz et al. (2006).

Beside a very high current related sensitivity of up to 1000 V/AT, the improved insulation from the substrate resulted in a more efficient offset compensation and then in a reduced residual offset.

Fig. 13. Simplified fabrication process for trench Hall devices reported by Sunier et al. (2004).

by dry etching, the sidewall implantation and oxidation (figure 13b), another dry etching process to deepen the trenches (figure 13c), TMAH wet etching to release the bottom of the active area (figure 13d), trench oxidation, polysilicon refill and blanket etch, contact formation and metallization (figure 13e) and finally XeF2 etching of the polysilicon in the

Fig. 12. Schematic illustration of the process for SU-8 cantilever with integrated electrodes

Beside a very high current related sensitivity of up to 1000 V/AT, the improved insulation from the substrate resulted in a more efficient offset compensation and then in a reduced

Fig. 13. Simplified fabrication process for trench Hall devices reported by Sunier et al. (2004).

tranches (figure 13f).

reported by Mouaziz et al. (2006).

residual offset.

Estrada (2011) proposed a three axis Hall sensor based on MEMS micromachining of SOIwafers. Three Hall sensors embedded in a flexible polyimide carrier was obtained so that appropriate folding of the structure resulted in three Hall-probes positioned to form an orthogonally-oriented array on three faces of a millimeter-sized cube. The key fabrication process steps are as follow: (a) SOI-wafer; (b) micromachining of the active layer using either wet (TMAH) or dry (DRIE) etching; (e) patterning the Al-film needed for ohmic contacts; (f) deposition and curing of the polyimide film; g) etching of the handle wafer to reach the buried oxide film; (h) final metallization of the contact pads for possible soldering or wire bonding (figure 14).

Fig. 14. Key steps for the fabrication of the three-dimensional axis Hall sensor on a flexible substrate (polyimide film) proposed by Estrada (2011).

This 3D-sensor configuration allows vector magnetic field measurements where the advantage is that all its elements have the same magnetic sensitivity of about 100 V/AT.

An integrated three axis Hall sensor based on III-V technology was fabricated by employing a micromachining technique for realizing self positioned structures (Todaro et al., 2010). The MEMS technique was applied to a GaAs-based heterostructure containing a sensing layer and a strained layer. The selective removal of a third sacrificial layer allows for the relaxation of the strained layer and the self positioning of the sensing part, which has been already processed to realize a Hall sensor element (figure 15).

The main fabrication steps are as follows: a) Epitaxial growth of a multilayer with sacrificial layer, strained layer, and sensor multilayer; (b) Photolithography and wet etching to define the mesa active region; (c) Photolithography and wet etching to define the hinge region; (d) metallization by lift off (GeAu/Ni/Au); (e) Photolithography and chemical etching to expose the edge of the sacrificial layer; (f) selective etching of the sacrificial layer and selfpositioning of the structure.

Current related sensitivity of more than 1000 V/AT both for in-plane and for out-of-plane Hall sensors, demonstrates the effectiveness of this method for realizing fully integrated miniaturized high sensitivity three axis magnetic sensors.

Magnetic Field Sensors Based on Microelectromechanical Systems (MEMS) Technology 119

Conventional magnetic sensor classes presented in this paper have different application

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

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

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

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,

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

fields depending on their sensitivity range and minimum detectable magnetic field.

into numerous applications for measuring magnetic fields.

solutions for devices in applications requiring low sensitivity.

surfaces.

low cost and low power consumption.

the size and the depth of the buried objects.

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

Figure 16 shows images of the realized three dimensional magnetic Hall sensor highlighting the out-of-plane sensor.

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.
