Research on MEMS technology of Micro fluxgate …Research on MEMS technology of Micro fluxgate...

Research on MEMS technology of Micro fluxgate sensor

1 Hui Lv, *2 Shibin Liu 1,2School of Electronics and information, Northwestern Polytechnical University, Xi’an,

China 1Electrical Engineering and Automation institute, Henan Polytechnic University,

Jiaozuo, China, [email protected]

Abstract Micro fluxgate sensors are widely applied in detecting the weak magnetic field. Compared with

traditional fluxgate, Micro fluxgate sensor has some advanced features. In this paper, the general development and present status of micro fluxgate processing technologies are summarized, and the main developing tendency and directions are analyzed. The best method that solves sensitivity and power consumption problem efficiently is proposed and discussed.

Keywords: Micro Fluxgate Sensor, Mems, Fabrication Process

1. Introduction

Fluxgate magnetic sensors are widely applied since it appeared. They are vector devices which are used to measure the components of magnetic fluxes density vector at one certain point of the dc or low-frequency ac magnetic fields up to 1mT [1]. The theory exploited is based on the saturation of ferromagnetic materials. They have the resolution of 0.1-10nT [2, 3], even up to 10pT, and the absolute precision of 1nT to 100nT [3-5]. Compared with other solid-state magnetic sensors, they have higher temperature stability and better long-term stability as the most primary reasons for their extensive application in space vehicle.

Micro fluxgate sensors are based on MEMS (Micro-Electro-Mechanical Systems) technology. By compared with traditional fluxgate magnetic sensor, Micro fluxgate sensors has some advanced features, such as less mass, smaller size, less power consumption, better performance and batch production. The original goal is to apply of them to portable equipments. In addition, there are broad applications in many fields, such as to mineral prospecting, geomagnetic detection, automation, scientific research, and process control of industry, particularly nano satellites and pico satellites.

Figure 1. The principle of micro fluxgate sensor

In this paper, the general development and present status of micro fluxgate processing technologies

are summarized, and the main developing tendency and directions are analyzed. 2. The technology problem of Micro fluxgate sensor

The principle of micro fluxgate sensor is show in Figure 1.A exciting current Iex generate two equal

and opposite magnetic field Hex in the two same cores. When external magnetic fields Hx are present,

Research on MEMS technology of Micro fluxgate sensor Hui Lv, Shibin Liu

International Journal of Digital Content Technology and its Applications(JDCTA) Volume7,Number6,March 2013 doi:10.4156/jdcta.vol7.issue6.132

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the two cores is subjected to Hex+Hx and Hex-Hx, respectively. The flux density is shown as formula (1) and (2).

0 1'= ( - )B B H H (1)

0 1"= ( + )B B H H (2)

The induced electromotive force of measuring coil Usc is shown as formula (3).

=- ( '+ ")se

dU SW B B

dt (3)

S - cross sectional area of core. W - turns of measuring coil. The magnetic property is shown as formula (4).

3= +B aH bH (4)

The constants a,b are determined by the shape and the material of core. Then plugged formula (4) into formulas (1) and (2):

3 2 2 30 1 0 0 1 0 1 1'= - + -3 +3 -B aH aH bH bH H bH H bH (5)

3 2 2 30 1 0 0 1 0 1 1"= + + +3 +3 +B aH aH bH bH H bH H bH (6)

3 20 0 0 1'+ "=2 +2 +6B B aH bH bH H (7)

Substituting formula (7) into formulas (3)) yields:

20 1=6 [ ( )]se

dU bSWH H t

dt (8)

1 1 2( )= sinω + cosωm mH t H t H t (9)

Hm1, Hm2 -amplitude of alternating magnetic field. ω1, ω2 - angular frequency of alternating magnetic field. When ω2=0, the formula (9) is transformed to (10).

1 2( )= sinω +mH t H t H (10)

H2 - auxiliary fixed field. The induced electromotive force of measuring coil Use is

20 2=6ω (2 cosω + sinω )se m mU bSWH H H t H t (11)

The principle of a fluxgate is based on the external magnetic field dependent periodical saturation of the ferromagnetic material.The combined action of the external field to be measured and excitation coils, driving the ferromagnetic core in saturation, leads to the generation of higher order harmonics of the fundamental excitation frequency in a detection coil surrounding the magnetic core. The second harmonic of the output can be measured using a lock-in technique, and is proportional to the external field for a certain field range (typically 0.01–100 μT) [4, 6-8].

Figure 2 shows a design diagram for a fluxgate sensor with rectangular shaped magnetic core. Two excitation coils of five windings each are symmetrically positioned around the inner detection coil. Air gap lengths Lg have been varied between 0 and 1 mm.

Figure 2. Schematic diagram of a fluxgate sensor with rectangular shaped magnetic core


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Figure 3 represents the second harmonic voltage V2f of the detection coil as a function of the

external field Bext for various gap lengths Lg, and at a frequency of 10 kHz for an excitation current Ie of 300 mAp-p. Bext is applied in-plane perpendicular to the detection coil, as shown schematically by the arrow in the insert of Figure 2. The three curves are characterized by a linear slope at small Bext. The slopes of the linear parts of the V2f -Bext curves decrease with increasing gap length Lg. This graph clearly shows that, by choosing a certain air gap Lg, one can select the fluxgate sensor’s sensitivity and its linear magnetic field range.

Figure 4 shows the sensitivity SB=(dV2f)/(dBext) and the maximum of the linear range Blin,max, determined from the curves of Figure 2. There is a decrease of the sensitivity SB and an increase of the maximum of the linear range Blin, max with increasing gap length Lg. By choosing a certain air gap Lg, it hence is possible to make a compromise between the sensitivity and the magnetic field detection range.

Figure 3. Second harmonic voltage V2f of the detection coil as a function of Bext

Figure 4. The sensitivity and maximum of the linear range

Figure 5. Magnetic offset Boffset of the fluxgate sensor as a function of excitation current

To evaluate the minimum detection limit of our fluxgate sensing devices, the magnetic offset of the fluxgate sensor Boffset should be measured. When cycling Bext from zero to saturation and back to zero, based the value of V2f at this point, one can calculate the offset by using the fluxgate’s measured sensitivity curves. The results are shown in Figure 5 as a function of excitation current Ie at 20 kHz. There is a decreasing Boffset and an improved minimum field detection limit with increasing Ie. The


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reason for this behavior has to be sought in the better defined magnetic saturation state at high applied Ie. The observed values of Boffset clearly are well below the value of the earth’s magnetic field, enabling application of magnetic compass. 3. General development and present status

The one-ax magnetic sensors with unenclosed magnetic core on top of planar spiral coils were researched by Almazan and Choi [9-11]. The sensors with closed core were also developed by Ripka. The two-axis planar ones with unenclosed core were designed by Kejik. Gottfried-Gottfried tried to invent one-ax sensors with toroidal coils and unenclosed core, while Park and Liakopoulos researched the micro magnetometers based on enclosed core with toroidal coils and with solenoid coils, respectively.

Now different types of micro fluxgate sensors have been researched according to the publications. According to the literature, the manufacturing process of micro fluxgate sensors is mainly divided into two types.

3.1. Printed circuit board technology (PCB)

O.Dezuari adapted a new printed circuit board (PCB) technology to the fabrication of ultraflat and

sensitive fluxgate magnetic field sensors. The two outer layers of the PCB stack compose the electrical windings of fluxgates, while the inner layer is made of a micro-patterned amorphous magnetic ribbon with extremely high relative magnetic permeability.

Figure 6. Schematic diagram of a toroidal fluxgate sensor

Two basic configurations were considered: one based on a toroidal magnetic core and the other on a

rectangular core with and without an air gap. The field response and sensitivity of the fluxgate devices have been studied as a function of the gap length, the excitation current, and excitation frequency. Compared to fluxgate sensors of similar size, a relatively high sensitivity of 60 V/T was found at 30 kHz for a five-winding detection coil wound around a rectangular E-shaped magnetic core. This high performance is primarily attributable to the high-permeability magnetic core.

O.Dezuari designed and fabricated two different types of fluxgate sensors: one type has the inner magnetic foil patterned into a toroidal shape and the other type has a magnetic core of rectangular shape. Diagrams of structure are displayed in Figure 2 and Figure 6.

As shown in Figure 6, the two excitation coils are positioned symmetrically at both sides of the detection coil. Each excitation coil has 18 windings while only four were used for the detection coil. Figure 2 shows a schematic diagram of a rectangular fluxgate sensor whose excitation coils have been grouped into four clusters of five winding and are symmetrically positioned around the inner five detection coil windings. Photographs of the two types of fluxgate sensors are displayed in Figure 7[5, 12, 13].

A.Tipek describes the basic parameters of a PCB technology fluxgate sensor. The main objective of his work is to develop a cheap fluxgate sensor technology based on embedding magnetic materials


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inside multilayer PCBs. The construction of sensor is derived from the current sensor in PCB technology [14-16]. The magnetic circuit has the form of a 7/10 mm ring made from two 22-μm thick layers of electrodeposited Permalloy. This core is sandwiched between the layers of the PCB as shown in Figure 8(a). The sensor excitation winding is also integrated into the PCB design; individual conductor sections in the lower and upper copper layers are connected by vias, thus forming 40 turns with a total resistance of 700mΩ. For an excitation current of 1A, a magnetic field intensity of 1.45 kA/m is created in the core.

(a) (b)

Figure 7. Photographs of fluxgate sensors: (a) a toroidal fluxgate (b) a rectangular fluxgate The pick-up coil of the fluxgate sensor was implemented using an external winding with 0.2-mm-

diameter copper wire. The 60-turns coil was wound around the frame with the PCB-integrated core. Figure 8 (b) shows the schematic design.

Figure 8. (a) Electrodeposited core sandwiched in the PCB, (b) one-axis PCB fluxgate schematic

design (pick-up coils: layers 1 and 9; excitation coils: layers 3 and 7; magnetic core: layer 5; dielectric: layers 2, 4, 6 and 8)

3.2. Micromachining processing on silicon wafer

T.Seitz firstly proposed a planar fluxgate element integrating a magnetic thin film and a spiral pick-

up coil. But the excitation coil is not integrated in the silicon chip [15-18]. Furthermore other authors developed a fully integrated micro-fluxgate sensing element, which is composed of micro-solenoids for the pick-up and the excitation coils and a NiFe (Permalloy) film formed by electrodeposition.

S.Kawahito pointed out that the performance of the micro-fluxgate sensor can be remarkably enhanced at high-frequency excitation, and the integration of the excitation coil is essential for such a high-frequency excitation.

3.2.1. 2D Planar Micro fluxgate

Andrea Baschirotto realized a fluxgate sensor in a planar structure. As shown in Figure 9. In this

case, the excitation and sensing coils are implemented as spirals, realized with two different metal layers, while the magnetic core is usually obtained with a post-processing of the silicon wafer. In Figure 9 both magnetic axes are shown but, for simplicity, only a pair of sensing coils is indicated.


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This structure is able to detect a magnetic field coplanar with the structure itself, the output signal being proportional to the projections of the field along the directions of the two cross arms of the magnetic core[7, 19].

The photograph of integrated micro-fluxgate is shown in Figure 10, which was realized in a 0.5-μm CMOS integrated circuit process. The planar excitation coil is characterized by 5.5-μm thickness, 71 turns, and 12-μm pitch (8-μm metal width and 4 μm of spacing between two metals). The total area for the excitation coil is 1760 × 1760 μm2, and its resistance is about of 123.4 Ω. This allows the excitation circuitry to be supplied with a single 5-V source, which is standard in portable systems.

The geometrical parameters of the sensing coil, which was realized in aluminum, are given as follows: 1-μm thickness,1.4-μm metal width, and 1.6 μm of spacing between two metals. The total area for the sensing coils, which consists of 66 turns, is 650 × 650 μm2, and their resistance is about of 1.84 kΩ. The ferromagnetic cross-shaped core was realized with the Vitrovac 6025 X., has been obtained with the dc-magnetron sputtering technique, with a very small thickness (about 1μm) [8].

Figure 9. Planar double axis fluxgate magnetic sensor

Figure 10. Fluxgate sensor microphotograph

3.2.2. 3D Microfluxgate

The 3D fluxgate magnetic sensor has been reported. This micro-fluxgate design concept differs

from recently developed planar fluxgate sensors realizes the implementation of conventional wire-winded fluxgate sensors on a silicon wafer and features miniature dimensions and simple post-CMOS process. The novel device has low noise decreasing with the increasing excitation frequency. It not


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only has enhanced field-to-voltage transfer coefficient and excellent linear B-V characterization, but also is volume-miniature and CMOS-compatible when compared with conventional fluxgate and planar fluxgate sensors.

Wen-Sheng Huang fabricated a 3D fluxgate in a 2.5 mm × 2.5 mm chip and comprises two excitation solenoids enclosing the ferromagnetic cores by using Al wire-bonding technique, which enables a sufficient excitation field to saturate the magnetic cores. The bottom part of the excitation coils are patterned by CMOS standard process and made of two levels of metal 3 and metal 4, which are 180 μm wide and duplicated with 23 turns, as illustrated in Figure 11. Two pairs of planar pick-up coils are allocated beneath each single ferromagnetic core to sense the alteration of magnetic flux cause by the external (measured) field within the core, and thus induce voltage output between the coils. The line width of the pick-up coil is 5μm and 168 turns are winded. To reduce the excitation magnetic signal in the pick-up coils, the excitation coils are winded reversely so that the magnetic fields along the cores are opposite and thus revoked. The dual-core design can provide induced output voltage that is purely provoked from the ferromagnetic cores. The cross-sectional diagram of the 3D CMOS fluxgate magnetometer is shown in Figure 12. Figure 13 demonstrates the micrograph of the 3D fluxgate [9].

Figure 11. Configuration of the 3D fluxgate magnetometer

Figure 12. The cros-sectional illustration of the 3D fluxgate chip

Figure 13. A micrograph of the fabricated 3D fluxgate


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4. Developing tendency and main directions

The major trends of micro fluxgate were decreasing of the sensor size, power consumption and

price, and, on the other hand, increasing of the precision in the large range of the measured fields. And many micro fluxgate sensors using MEMS technologies have been developed and have the attractive features of small size, light weight, high sensitivity and integration of signal processing circuits, etc. But power consumption higher than dozens mW of the micro fluxgate sensors is a serious problem, which does not decrease along the reduction of the sensor dimension due to saturation magnetizing principle and high coil resistance. The contradiction between power consumption, sensitivity and size weakens the competitiveness of the micro fluxgate sensors. With the further development of micro fluxgate, low-power technology will become important factor which affects the practicability of micro fluxgate technology.

The core was chosen as a compromise between the sensitivity and power consumption. The thicker core required higher current to bring it into saturation. High permeability and low coercivity, but non-rectangular shape of the magnetization curve is preferred for the core. The material should have low number of structural imperfections, no random internal stresses, uniform cross-section, smooth surface and high homogeneity of the parameters. Low saturation magnetization (for low-power) and high electrical resistivity (for low eddy current losses) are advantageous. Materials suitable for fluxgate cores are permalloys (with 78–81% of nickel) and amorphous alloys. The minimum noise is achieved for alloys possessing very low magnetostriction. 5. Conclusions

Further work to realize the more excellent integrated fluxgate sensor will be executed. If we can

obtain the higher quality core, a new micro integrated fluxgate sensor with extremely lower power consumption and higher sensitivity will be realized to meet the requirement of portable electronic equipments and small satellite. Acknowledgement

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (No.20126102110031). References

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Research on MEMS technology of Micro fluxgate …Research on MEMS technology of Micro fluxgate...

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