Sensors and Actuators A 110 (2004) 236–241

Single core fully integrated CMOS micro-fluxgate magnetometer

Predrag M. Drljacaa,∗, Pavel Kejika, Franck Vincenta, Dominique Piguetb,François Gueissazb, Radivoje S. Popovica

a Swiss Federal Institute of Technology, Institute of Microelectronics and Microsystems, CH-1015 Lausanne, Switzerlandb Asulab S.A., Rue des Sors 3, CH-2074 Marin, Switzerland

Received 23 July 2003; received in revised form 23 July 2003; accepted 15 September 2003

Abstract

A new fully integrated 2D micro-fluxgate magnetometer is presented. This magnetometer is integrated in a standard CMOS process anduses a ferromagnetic core integrated on the chip by a photolithographic post-process compatible with the integrated circuit technology. Thecross-shaped ferromagnetic core is placed diagonally above four excitation coils, two for each measurement axis. A novel electronic signalextraction technique is presented. The integrated 2D magnetometer exhibits a sensitivity of 160 V/T and a linear range of±50�T. The mag-netic equivalent noise spectral density is 70 nT/

√Hz at 1 Hz, and the total power consumption is as low as 17 mW for the 5 V power supply.

© 2003 Published by Elsevier B.V.

Keywords: Magnetic sensors; Fluxgate; Planar structure; CMOS

1. Introduction

The low noise and low temperature sensitivity of thesensors working on a fluxgate principle make them suitablefor low magnetic field measurements. They can achieveas low as pico-Tesla resolution and nano-Tesla accuracy[1–3]. The main disadvantages of conventional fluxgatesensors are their large size, high power consumption andcost [4]. Manufacturing miniature fluxgate magnetometersusing micro-technology is a complex task. Moreover, theirperformances dramatically degrade with size reduction[5,6].

The new micro-fluxgate sensor presented in this paper hasbeen developed on the well-established and cost-effectiveCMOS integrated circuit process, providing planar coilmetallizations and all the necessary electronic circuitry,with the addition of a compatible post-process to in-tegrate a ferromagnetic core. The combination of highquality amorphous ferromagnetic material for the coreand completely integrated high-speed electronics allowsthe sensor to maintain high performance in spite of sizereduction.

∗ Corresponding author. Tel.:+41-21-693-6424; fax:+41-21-693-6670.E-mail address: predrag.drljaca@epfl.ch (P.M. Drljaca).

2. Sensor structure

The sensor shown here is a two axes micro-fluxgate mag-netometer (Fig. 1). It consists of two excitation coils, with27 turns, for each measurement axis. The coils are madeof two superimposed metal layers of the CMOS process,which gives us the opportunity to decrease the overall coilresistance. The excitation coils are connected in series, withopposite windings direction, to produce an excitation fieldparallel to the surface of the chip (Fig. 2). In contrary tothe previously presented sensor[7], the core is not dividedby the excitation magnetic field into two parts, to createa parallel configuration. In the actual sensor, the core ismagnetized along the whole length, which made us call it:single core fluxgate magnetometer. This working principleleads to a better core saturation and to further miniaturiza-tion prospective for the sensor. The main advantages arethe core’s length reduction by a factor of 1.7 and a decreaseof the coils resistance, from 280 to 190�, which result ina current increase. Finally, the core is positioned along thediagonal of the excitation coils to maximize the magneticfield as much as possible.

For each axis, we use two pickup coils to detect the signalinduced from the ferromagnetic core. Each coil is made with27 turns of the Metal-1 layer and is positioned at one tip ofthe ferromagnetic core cross. This way, we double the outputsignal, which enables us to reduce the number of turns in

0924-4247/$ – see front matter © 2003 Published by Elsevier B.V.doi:10.1016/j.sna.2003.09.014

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Fig. 1. Exploded view of the two-axis single core magnetometer. Elec-tronics as well as the driving and pickup coils are made by a CMOSprocess. Ferromagnetic core is added by a compatible post process.

the coil. This leads to a smaller resistance (from 6 k� to450�), a smaller noise, and above all a reduced capacitivecoupling to the excitation.

3. Fabrication process

Prototypes of the sensor were manufactured using a stan-dard 1�m CMOS (2M/2P/HR) integrated circuit process. Abatch post-process was used for the first time to integrate the

Fig. 3. Scanning electron microscope (SEM) image of the sensor with the integrated ferromagnetic core.

Fig. 2. Integrated planar fluxgate configuration. The excitation coils areconnected in series, with windings in opposite direction, to produce anexcitation field parallel to the surface of the chip and, therefore, saturatethe whole ferromagnetic core.

ferromagnetic cores on the CMOS wafers. We use a mag-netically soft amorphous alloy (Metglas® 2714 A), which iscommercially available as ribbon (26 mm×0.018 mm) fromHoneywell Inc., USA. It is characterized by its high perme-ability (µr > 100000), a low coercive force (Hc = 0.5 A/m)and a saturation magnetic flux densityBr of 0.55 T.

The post-processing begins with the preparation of theribbons. They are first thinned down to 10�m using amechanical–chemical process, to improve the sensitivityof the sensor and the saturation of the ferromagnetic core.Simultaneously, the roughness of the surface is drasticallyreduced. These treated ribbons are then glued to the waferusing a 3�m thick epoxy layer. After that, we structure thecore in a shape of the cross by a classical photolithographyprocess. The cores are chemically etched and the remain-ing apparent glue is removed using a dry plasma etching(Fig. 3). They are, finally, encapsulated with a SU-8 barrier

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Fig. 4. Micrograph of the 2D CMOS micro-fluxgate chip. Most of thesurface of the chip is occupied by the four excitation coils with thepost-integrated ferromagnetic core on top. The rest of the chip is dedicatedto the electronics and contact pads.

(negative photosensitive resin) liberating only the pads. Thecore’s final dimensions are 1.2 mm of length, 20�m ofwidth and 8�m of thickness. After dicing, the total chipsize is 3.0 mm× 3.2 mm, where a substantial portion of thesurface is occupied by numerous testing pads (Fig. 4).

4. Operation principle

The block diagram of the microsystem is presented inFig. 5. The microsystem can be divided, from the pointof view of the operation, into three basic parts. A digital

Fig. 5. Block diagram of the fluxgate microsystem. The electronics consist of a digital part, a driving part and a rectifying electronics that will producean output signal proportional to the measured magnetic field.

block creates the different clocks necessary for the properoperation of the chip. A driver that generates the current inthe excitation coils to produce the excitation magnetic field.At the end, a rectifier electronics that samples the signalinduced in the pickup coils and demodulates it to a dc outputvoltage, proportional to the measured magnetic field.

To generate the excitation magnetic field along the wholeferromagnetic core, we alternatively connect both excitationcoils to the supply voltage[7,8] with a H-bridge made of fourpower MOS transistor switches. The high current slew rateinduces large pulses in the pickup coils, and the excitationduty cycle of 1/16 results in a low power consumption fromthe excitation driver.

A novel signal extraction technique is proposed for thisnew single core micro-fluxgate sensor, which differs fromthe usual parallel fluxgate structure. For each axis, two sep-arate pickup coils detect the signal induced during the risingand falling edges of the core’s magnetization field (Fig. 6).All four induced peaks, for each pickup coil, are sampledand their values are stored. If no external field is present,all induced voltage peaks have the same height. When thesensor is exposed to a constant external magnetic fieldBext,the excitation pulses are shifted over this value. Due to thenon-linearity of theB(H) curve, a difference between thepulses appears. If we now introduce the rectification for-mula:

Vout = (V2 + V4) − (V1 + V3) − (V6 + V8) + (V5 + V7),

(1)

whereV1–V8 are stored values of voltage peak, we have atthe output signalVout proportional to the external magneticfield. As we can see, sampling and processing the voltage, inthe proposed way, reproduces the parallel fluxgate structure.We can conclude that this configuration is a quasi-parallelfluxgate sensor in the time domain with the respect to thetrue parallel fluxgate structure.

The most critical point, in the proposed extraction of thesignal, is to sample the peaks induced in the pickup coil.

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Fig. 6. Operation principle of the single core fluxgate magnetometer. Ifthe external field is present, the output peaks induced in the pickup coilswill be different. Using the proposed rectification formula we obtain asignal proportional to the output field.

Due to a short rise time of the excitation pulse, the widthof the peaks induced in the pickup coils is very narrow. Wemeasured them to be about 60 ns. It is almost impossibleto detect their maximum value by using a standard sampleand hold circuit. To solve this problem we used the box-cartechnique[9]. The principle of operation is described inFig. 7. It consists of an analog switch, created from a MOScomplementary pair, a capacitor and a buffer. The internalresistance of the switch creates a low pass filter with the ca-pacitor. We presume that the input signal is either repetitiveor very slowly varying. This is true for the fluxgate sensor,since the excitation frequency is in the order of hundredsof kilohertz and the measured magnetic field is assumed tobe at a low frequency or constant. If we assume that thetime constant of the low pass filter is much larger than thesampling time we can write that, aftern sampling times,the voltage at the output of the buffer will be:

Vn1 = 1

RC

n∑k=1

exp

(−1

RC(kε + (n − k)T)

) ∫ (n−k)T+ε

(n−k)T

× exp

(t′

RC

)(Vpickup + Vnoise)dt′, (2)

whereε is the sampling time,Vpickup the voltage at the inputof the circuit,T the time between two samples andVnoisea noise voltage coming from the pickup coil and the ferro-magnetic core. It can be shown that, although the box-car is

Fig. 7. Schema and operating principle of the box-car averaging. Samplingthe peak, for a short time, we will slowly charge the capacitor to a valueequal to the input voltage and in the same time reduce the noise comingfrom input.

a non-linear averaging circuit, its noise improvement reduc-tion follows an1/2 law. If the number of samplesn satisfies:

n RC

ε, (3)

the input and output voltages will be equal.Using eight of this previously described circuit and as

many different clock signals we sample every induced peaksin both pickup coils. To realize the rectification described inEq. (1), the induced voltage is fed to a summing amplifier.The output voltage is finally buffered for output stability.For both axes we use the same amplifier and buffer, to savechip’s surface and avoid matching problems. The axis canbe externally chosen by a digital input.

5. Sensor performance

The characterization of the microsystem is performed witha 5 V power supply and the excitation frequency is 250 kHz.The power consumption of the chip is 17 mW, includingbiasing, driving and readout electronics.

To measure the sensitivity and the linear working rangewe place the chip in Helmholtz coils. The applied magneticfield is swept in the range of±400�T, using a low noisecurrent source to supply the coils. The output voltage ofthe sensor is plotted inFig. 8 against the external magneticfield for one measurement axis. The measured response ofthe micro-fluxgate sensor is linear in the range of±50�Twith a sensitivity of 160 V/T and a linearity error of 1.4%full scale. At higher magnetic fields, the sensor enters intoa saturation region and the sensitivity decreases drastically.Since the readout electronics has a voltage gain close to

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Fig. 8. Response of the micro-fluxgate sensor to the external magneticfield measured on one axis. The measured response is linear in the rangeof ±50�T with a linearity error of 1.4% full scale.

Fig. 9. Equivalent spectral noise density characteristics of the single corefluxgate. The characteristics is dominated by the 1/f noise of the frontend amplifiers.

unity, the measured sensitivity is approximately equal to theintrinsic sensitivity of the sensor part itself. The hysteresischaracteristic of this sensor, measured by sweeping the ap-plied magnetic field, was found to lie below the noise level.

Noise measurements are performed using a HP 35670Adynamic signal analyzer. To protect the magnetometer fromthe external magnetic field disturbances, we use a five-layermumetal shielding box. A battery is used as the sensorpower supply to avoid noise induced from an external volt-age source. The measured magnetic equivalent noise spectraldensity is 70 nT/

√Hz at 1 Hz (Fig. 9). The rms noise value

is about 150 nT in the 0.2–10 Hz domain. The overall noiseof this micro-fluxgate sensor is actually dominated by the 1/fnoise of the sampling and output amplifiers. This problemwill be one of the main concerns for future improvement ofthe fluxgate magnetometer.

6. Conclusion

A new design of a 2D CMOS fully integrated single coremicro-fluxgate magnetometer has been presented. Accord-

ing to this new sensor configuration, a novel signal extrac-tion technique is proposed. High quality ferromagnetic coreswere structured for the first time directly on the entire wafer,using a photolithographic and wet etching process.

This presented micro-fluxgate sensor meets the high res-olution, small dimensions and low power characteristics re-quired by compasses integrated in portable applications suchas wristwatches, pocket navigation instruments or portablecellular phones.

References

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[2] J. Piil-Henriksen, J.M.G. Merayo, O.V. Nielsen, H. Petersen, J. Raa-gaard, F. Primdahl, Digital detection and feedback fluxgate magne-tometer, Meas. Sci. Technol. 7 (1996) 897–903.

[3] C. Hinnrichs, C. Pels, M. Schilling, Noise and linearity of a fluxgatemagnetometer in racetrack geometry, J. Appl. Phys. 87 (2000) 7085–7087.

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[6] P. Ripka, S.O. Choi, A. Tipek, S. Kawahito, M. Ishida, Symmetricalcore improves micro-fluxgate sensors, Sens. Actuators A 92 (2001)30–36.

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Biographies

Predrag M. Drljaca was born in Belgrade, Yugoslavia (Serbia), in 1972.He obtained his BSc and MSc degree in electrical engineering from theUniversity of Belgrade, Yugoslavia, in 1996 and 1999, respectively. From1997 to 1999, he has been working as research and teaching assistant at theDepartment of Microelectronics and Engineering Physics, the Universityof Belgrade. Since 2000, he has joined the Institute of Microelectronics andMicrosystems at the Swiss Federal Institute of Technology in Lausanne(EPFL) as a research assistant and PhD student. His research interestsinclude noise and non-linear effects in magnetic sensor microsystems.

Pavel Kejik was born in Prostejov, in the Czech Republic in 1971. Hereceived the Diploma degree in 1994 and the PhD degree in 1999 at theCzech Technical University of Prague. In 1999, he joined the Institute ofMicroelectronics and Microsystems at the EPFL to work on Institute’scircuit design and testing. His research interests include fluxgate mag-netometry and contactless current measurement, mixed-signal IC designand low noise circuit design.

Franck Vincent was born in Epinal, France, in 1972. He obtained a MSc atthe University Louis Pasteur- Strasbourg, in 1998. Currently, he is workingas an engineer at the Institute for Microelectronics and Microsystems of the

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Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland.His work includes developing microfabrication technologies in the cleanroom facility at EPFL, in collaboration with industrial partners.

Dominique Piguet was born in Switzerland, in 1973. He received hisDiploma degree in microengineering at the Swiss Institute of Technologyof Lausanne (EPFL) in 1996, where he worked on medical applicationsof optical probes, in the Institute of Applied Optics. Since 1997, he hasbeen working at Asulab SA, the Central Research Laboratories of theSwatch Group, developing electronic compass systems, based on variousmagnetic sensors, mainly for wristwatches.

François Gueissaz was born in Switzerland in 1959. He obtained hisDipl Ing degree in electrical engineering from the Swiss Institute ofTechnology of Zürich (ETHZ) in 1988, and the Dr Sc degree from theSwiss Institute of Technology of Lausanne (EPFL) in 1992. His Dipl Ingthesis related to hall sensors based on gallium–arsenide layers, and hisDr Sc thesis to high speed field effect transistors on III–V compoundsemiconductor heterostructures. From 1992 to 1994, he worked for theAdvanced Devices and Technology Laboratory of the Nippon Telegraphand Telephone (NTT) Corporation in Atsugi, Japan, developing ultra-highspeed III–V compound semiconductor heterostructure based electronicdevices. From 1994 to 1996, he operated as manager of the Swiss Priority

Research Program of Optical Sciences, Applications and Technologies.In 1996, he joined Asulab SA, the Central Research Laboratories of theSwatch Group, and became Head of the Sensor Group, in charge ofdeveloping various sensors and transducers oriented toward low powerportable applications.

Radivoje S. Popovic was born in Yugoslavia (Serbia) in 1945. He obtainedthe Dipl Ing degree in applied physics from the University of Beograd,Yugoslavia, in 1969, and the MSc and Dr Sc degrees in electronics fromthe University of Nis, Yugoslavia, in 1974 and 1978, respectively. From1969 to 1981, he was with Elektronska Industrija Corp., Nis, Yugoslavia,where he worked on research and development of semiconductor devicesand later became Head of the company’s CMOS department. From 1982to 1993, he worked for Landis & Gyr Corp., Central R & D, Zug,Switzerland, in the field of semiconductor sensors, interface electronic,and microsystems. There he was responsible for research in semiconductordevice physics (1983–85), for microtechnology R & D (1986–90) and wasappointed Vice President (Central R & D) in 1991. In 1994, he joined theSwiss Federal Institute of Technology at Lausanne (EPFL) as Professorfor microtechnology systems. He teaches conceptual products and systemdesign and microelectronics at the Department of Microengineering ofthe EPFL. His current research interests include sensors for magnetic,optical, and mechanical signals, the corresponding microsystems, physicof submicron devices, and noise phenomena.