**3. Fluxgate sensors**

Fluxgate sensors are inductively working sensors composed of excitation and sensing coils around a ferromagnetic core for detecting static and low frequency fields.

Fluxgate sensors can detect magnetic fields up to approximately 1 mT with a maximum resolution of 100 pT. Classical fluxgate sensors are expensive and have a big size. However in recent years a great effort was devoted to manufacturing micro fluxgate sensors using microfabrication technologies. Beside the small size, the advantages of micro fluxgate sensors are small weight, low power consumption, low cost in mass production and the possibility of on-chip electronics integration. The principal disadvantage is related to the fluxgate sensor parameters dramatically degrading when reducing the core size, which leads to low sensitivity and high noise level.

The fluxgate operation principle can be illustrated with the simple layout in figure 6 ( Ripka, 2001). A ferromagnetic core immersed in an external magnetic field Bx is surrounded by an excitation coil which provides an ac excitation current I. This current periodically saturates the soft magnetic material of the sensor core at a frequency twice the excitation frequency.

Fig. 6. The basic fluxgate principle.

A voltage in the pick-up coil (Vi) is generated due to the changing magnetic flux (Φ). From the Faraday's law:

$$V\_i = \frac{d\Phi}{dt} = \frac{d(NA\mu\_0\mu\_r(t)H(t))}{dt} \tag{3}$$

where µr is the relative permeability, 0 is the vacuum permeability, N is the number of turns and A is the cross sectional area, that we consider here as a constant.

By expanding, the equation (3) becomes:

$$V\_i = \frac{NA\mu\_0\mu\_r dH(t)}{dt} + \frac{NA\mu\_0 Hd\mu\_r(t)}{dt} \tag{4}$$

Fluxgate sensors are inductively working sensors composed of excitation and sensing coils

Fluxgate sensors can detect magnetic fields up to approximately 1 mT with a maximum resolution of 100 pT. Classical fluxgate sensors are expensive and have a big size. However in recent years a great effort was devoted to manufacturing micro fluxgate sensors using microfabrication technologies. Beside the small size, the advantages of micro fluxgate sensors are small weight, low power consumption, low cost in mass production and the possibility of on-chip electronics integration. The principal disadvantage is related to the fluxgate sensor parameters dramatically degrading when reducing the core size, which

The fluxgate operation principle can be illustrated with the simple layout in figure 6 ( Ripka, 2001). A ferromagnetic core immersed in an external magnetic field Bx is surrounded by an excitation coil which provides an ac excitation current I. This current periodically saturates the soft magnetic material of the sensor core at a frequency twice the excitation frequency.

A voltage in the pick-up coil (Vi) is generated due to the changing magnetic flux (Φ). From

<sup>0</sup> ( ( ) ( )) *<sup>r</sup> <sup>i</sup> <sup>d</sup> d NA t H t <sup>V</sup> dt dt*

where µr is the relative permeability, 0 is the vacuum permeability, N is the number of

0 0 ( ) ( ) *r r <sup>i</sup> NA dH t NA Hd t <sup>V</sup> dt dt*

 (4)

 

(3)

turns and A is the cross sectional area, that we consider here as a constant.

around a ferromagnetic core for detecting static and low frequency fields.

**3. Fluxgate sensors** 

leads to low sensitivity and high noise level.

Fig. 6. The basic fluxgate principle.

By expanding, the equation (3) becomes:

the Faraday's law:

where H is the field in the core material and is lower than the measured field Hex in the open air due to demagnetization (Bozorth & Chapin (1942)):

$$H = H\_{ex} \text{ - } DM \tag{5}$$

where D is the demagnetising factor, Hex = Bx/µ0 and M is the magnetization.

The first term in the equation (4) is the basic induction effect, and causes interference. Fluxgate operation is based on the second term, due to the variation of the core permeability with the excitation field. By considering the effect of demagnetization, the basic fluxgate equation becomes (Primdahl, 1979):

$$V\_i = \text{NA}\,\mu\_0 H\_{ex} \frac{1 \text{ - D}}{\text{(1} + D(\mu\_r(t) \text{-1}))^2} \frac{d\mu\_r(t)}{dt} \tag{6}$$

The output voltage is on the second harmonics of the excitation frequency, as permeability reaches its minimum and maximum twice in each excitation cycle.

In accordance with the shape of the magnetic core, parallel-type fluxgate sensors fall into the categories of single core, dual core, ring-type core, racetrack type core (Ripka, 2001). The configuration of figure 6 is single core type. In order to eliminate the induction effect, a dual core configuration has been proposed, as showed in figure 7.

Fig. 7. Dual core (left) and ring type (right) configurations of a fluxgate sensor.

The driving coil is wound in opposite direction around the two cores, thus the induced magnetization fields are opposite in sign. If no external field is applied, the voltage induced in the sensing coil is zero in the ideal case. When an external field is present, a voltage is induced due to the differential change of the permeability (Primdahl, 1979).

High sensitivity can be achieved by increasing the number of turns N (if N is very high coil parasitic capacitance limits the sensitivity), by decreasing the demagnetization factor D or by increasing the excitation frequency, because (dHex/dt) ~ f up to frequency values that make eddy currents negligible.

Such devices can be also classified in parallel type and orthogonal type fluxgate sensors depending on the excitation field is parallel or perpendicular to the sensitive axis of the sensor.

Magnetic Field Sensors Based on Microelectromechanical Systems (MEMS) Technology 113

Fig. 8. Main steps of solenoid fabrication process reported by Woytasik et al. (2006).

Fig. 9. Schematic view of the MEMS based fluxgate sensor proposed by Wu & Ahn, (2008).

Most of the microfluxgate exploiting MEMS technologies are parallel sensors.

(2009) and Kirchhoff & Büttgenbach (2010).

Other examples of MEMS-based fluxgate devices have been reported by Chong Lei et al.

A miniature orthogonal fluxgate realized by exploiting MEMS technologies with a planar structure formed by a permalloy layer electrodeposited on a rectangular copper conductor

Typically these devices contain solenoid systems wiring magnetic cores consisting of a permalloy or an amorphous material.

Figure 7 shows also the ring type configuration. The closed geometry of the ring core has a lower sensitivity but better noise performance, due to the absence of open ends.

A closed core made with oval geometry (race-track fluxgate sensor) lead to a lower demagnetization factor, then to higher sensitivity and less sensitivity to perpendicular fields.

In the orthogonal fluxgate the excitation coil is absent, and the sensor is excited directly by the current flowing through the core.

Three basic types of miniature fluxgate can be distinguished (Ripka & Janosĕk, 2010): plane type sensors with flat coils, PCB-based devices with solenoids made by tracks and vias and 3D type sensors with micro solenoids.

While plane type sensors are typically fabricated by standard CMOS processes, MEMS microfluxgate sensors exploit advanced microfabrication technologies to realize threedimensional coils or three-dimensional cores.

One of the first work which proposed MEMS technology for the development of fluxgate sensors has been reported by Liakopoulos & Ahn, (1999). In this work the authors presented a micro-fluxgate magnetic sensor based on micromachined toroidal type planar coils. In this fluxgate sensor a rectangular-ring shaped magnetic core has been chosen. The operation principle is based on the second harmonic. Excitation and sensing coils as well as permalloy magnetic cores were fabricated by a UV-LIGA thick photoresist process and electroplating techniques to realize a planar three-dimensional magnetic fluxgate sensor on silicon wafers. Excellent linear response over the range of -500 mT to +500 mT, with a sensitivity of 8360 VT-1 and a resolution of 60 nT was achieved with this device. The total response range of the sensor is -1.3 to +1.3 mT. The power consumption is around 100 mW for an operational frequency range of 1-100 kHz.

Woytasik et al. (2006) proposed an alternative fabrication process based on copper micromoulding to realize planar microcoils and microsolenoids for MEMS based magnetic sensors on flexible substrate. Figure 8 shows main steps for solenoid fabrication process.

The main steps for solenoids fabrication consist of the realization of the bottom conductor lines and of the air bridge by copper electrodeposition overflow. The second exposure process uses a gray-tone mask to vary spatially the exposure dose deposited into the photoresist and then to modulate the remaining photoresist thickness after development. The process ends with mould removal.

This technology has been employed for the realization of a micromachined fluxgate sensor (Wu & Ahn, 2008).

The sensor, schematically shown in figure 9, consists of a 30 µm thick electroplated permalloy core, with 56 excitation turns giving a total resistance of 2 Ω and 11 sensing turns. A sensitivity of 650 VT-1 was achieved for a 5.5-mm-long sensor with 14 mW power consumption. The noise is 32 nT/√Hz @1Hz , and the practical resolution is 1 µT.

Typically these devices contain solenoid systems wiring magnetic cores consisting of a

Figure 7 shows also the ring type configuration. The closed geometry of the ring core has a

A closed core made with oval geometry (race-track fluxgate sensor) lead to a lower demagnetization factor, then to higher sensitivity and less sensitivity to perpendicular

In the orthogonal fluxgate the excitation coil is absent, and the sensor is excited directly by

Three basic types of miniature fluxgate can be distinguished (Ripka & Janosĕk, 2010): plane type sensors with flat coils, PCB-based devices with solenoids made by tracks and vias and

While plane type sensors are typically fabricated by standard CMOS processes, MEMS microfluxgate sensors exploit advanced microfabrication technologies to realize three-

One of the first work which proposed MEMS technology for the development of fluxgate sensors has been reported by Liakopoulos & Ahn, (1999). In this work the authors presented a micro-fluxgate magnetic sensor based on micromachined toroidal type planar coils. In this fluxgate sensor a rectangular-ring shaped magnetic core has been chosen. The operation principle is based on the second harmonic. Excitation and sensing coils as well as permalloy magnetic cores were fabricated by a UV-LIGA thick photoresist process and electroplating techniques to realize a planar three-dimensional magnetic fluxgate sensor on silicon wafers. Excellent linear response over the range of -500 mT to +500 mT, with a sensitivity of 8360 VT-1 and a resolution of 60 nT was achieved with this device. The total response range of the sensor is -1.3 to +1.3 mT. The power consumption is around 100 mW for an operational

Woytasik et al. (2006) proposed an alternative fabrication process based on copper micromoulding to realize planar microcoils and microsolenoids for MEMS based magnetic sensors on flexible substrate. Figure 8 shows main steps for solenoid

The main steps for solenoids fabrication consist of the realization of the bottom conductor lines and of the air bridge by copper electrodeposition overflow. The second exposure process uses a gray-tone mask to vary spatially the exposure dose deposited into the photoresist and then to modulate the remaining photoresist thickness after development.

This technology has been employed for the realization of a micromachined fluxgate sensor

The sensor, schematically shown in figure 9, consists of a 30 µm thick electroplated permalloy core, with 56 excitation turns giving a total resistance of 2 Ω and 11 sensing turns. A sensitivity of 650 VT-1 was achieved for a 5.5-mm-long sensor with 14 mW power

consumption. The noise is 32 nT/√Hz @1Hz , and the practical resolution is 1 µT.

lower sensitivity but better noise performance, due to the absence of open ends.

permalloy or an amorphous material.

the current flowing through the core.

3D type sensors with micro solenoids.

frequency range of 1-100 kHz.

The process ends with mould removal.

fabrication process.

(Wu & Ahn, 2008).

dimensional coils or three-dimensional cores.

fields.

Fig. 8. Main steps of solenoid fabrication process reported by Woytasik et al. (2006).

Other examples of MEMS-based fluxgate devices have been reported by Chong Lei et al. (2009) and Kirchhoff & Büttgenbach (2010).

Most of the microfluxgate exploiting MEMS technologies are parallel sensors.

A miniature orthogonal fluxgate realized by exploiting MEMS technologies with a planar structure formed by a permalloy layer electrodeposited on a rectangular copper conductor

Magnetic Field Sensors Based on Microelectromechanical Systems (MEMS) Technology 115

*<sup>z</sup> H OFF*

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

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

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

*<sup>V</sup> <sup>S</sup> I B*

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

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

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

*I*

include temperature sensor and operational amplifiers (op-amps).

1 *<sup>H</sup>*

*z*

thickness of the slab t:

Hall magnetic sensors with respect to metals.

frequencies above 1 kHz at room temperature.

*B I V V*

2

*en*

*D*

(7)

*n nt* <sup>2</sup>*<sup>D</sup>* (8)

(9)

has been reported by Zorlu et al. (2007). The sensor structure is reported in figure 10. The sensor core is only 1 mm in length and the sensor has two flat 60 turn pickup coils.

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 consumption of 8.1 mW.
