2D Silicon Magnetometer - Science Direct

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ScienceDirect Procedia Engineering 168 (2016) 666 – 669

30th Eurosensors Conference, EUROSENSORS 2016

2D Silicon Magnetometer S. V. Lozanova, S. A. Noykov, Ch. S. Roumenin* Institute of Systems Engineering and Robotics at Bulgarian Academy of Sciences, Block 2, Acad. G.Bonchev St. 1113 Sofia, BULGARIA

Abstract A novel single-chip sensing device for measurement of two orthogonal magnetic-field components using one and the same transducer zone is presented. On a rectangular n-type silicon substrate, four n+- ohmic planar contacts are implemented – two of them are elongated and serves as power supply, and other two terminals positioned in the middle of the region between elongated ones, function as outputs only. A proper coupling arrangement is used for obtaining the information about vector components Bx (parallel to the supply contacts) and Bz (perpendicular to the substrate). The sensor operation is determined by the direction of individual parts of the curvilinear current trajectory and the Lorentz force action on them. The 2D magnetometer interface circuitry comprises three instrumentation amplifiers and a differential amplifier. Simple fabrication technology is applied. The effective spatial resolution is high consisting 90 x 60 x 40 ȝm3. The respective channel-magnetosensitivities without amplification reaches Sx § 17 V/AT and Sz § 23.3 V/AT, the channel cross-talk at induction B 1.0 T is no more than 3 % and lowest detected induction Bmin for the two-axis device at a supply 3 mA over frequency range f 100 Hz is about 11 µT. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

© 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

Keywords: 2D magnetometer; silicon Hall sensor; magnetic-field measurement

1. Introduction The most advanced 2-D and 3-D vector magnetometers are those using the Hall effect principle, since their action involves only one well defined and investigated physical phenomenon [1-4]. These multidimensional devices,

* Corresponding author. Tel.: +359 2 870 33 61 E-mail address: [email protected]

1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.243

S.V. Lozanova et al. / Procedia Engineering 168 (2016) 666 – 669

irrespective of the pronounced progress in characteristics, for example the spatial resolution, feature some essential drawbacks. They contain quite many contacts and numerous electrical connections between them; each of the presented in [4-7] widespread advanced vector microsensor solutions require at least 8 electrodes. This seriously complicates technology fabrication, impedes high spatial resolution and obstructs the achievement of the required miniaturization degree. One of the promising means overcoming these significant problems is the functional multisensor approach [1]. A silicon magnetic-field vector device was proposed measuring subsequently the Bx, By, and Bz components [1, 8, 9]. However, its imperfection is reduced accuracy due to the large initial offset in the individual channel outputs, the enlarged dimensions of the magnetometers due to the specific construction, requiring relatively many contacts. Thus, the resolution is limited. Also, the presented in [8, 9] magnetometers require complicated signal-extracting circuit, containing pulse generators, counters, multi-channel analog multiplexers, sample & hold circuits, etc. In this paper, a novel single-chip sensing device for measurement of two orthogonal magnetic-field components using one and the same transducer zone, simple construction, high resolution and flexible interface electronics is presented. 2. Sensor design and operation principle The novel 2-D magnetometer consists of a rectangular n-type silicon substrate with four n+ ohmic planar contacts C1, C2, C3 and C4. Two of them - C1 and C2, are elongated and serves as power supply. Other two contacts - C3 and C4, are square and function as outputs only. They are positioned in the middle of the region between elongated ones and near to the edges, Fig. 1(a,b). A deep surrounding p-ring around the n+-contacts for surface current restriction is formed too. The vector multisensor operates in the following way. The device through the elongated contacts C1 and C2 is fed with constant supply current, Is Ł IC1,2, due to the load resistor R, Fig. 1(a,b). A current trajectory IC1,2 in the bulk of the n-Si substrate, between the planar contacts C1 and C2, is curvilinear and penetrate in a depth 30 - 40 µm, Fig. 1(a). a)

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Fig. 1. (a) Sensor design and operation principle; (b) Sensor interface circuitry.

In a magnetic field Bz, perpendicular to the upper surface, the well-known Lorentz force FL(Bz) appeared, where ± FL(Bz) = ± q vy x Bz, Fig. 1(a) [1-3]. The force FL(Bz) deflects the current lines to the front or back side of the structure, depend on the directions of the supply current IC1,2 and field Bz. (Both front and back side of the structure are perpendicular to the short edges, Fig. 1(a)). This way, the concentration of the current charges in the vicinity of the contact C3 increases or decreases, and respectively the concentration of the charges around the contact C4 decreases or increases. Thus, between the terminals C3 and C4, a Hall voltage VH(Bz) appear. The Lorentz force FL(Bx) appeared too, when the field Bx is parallel to the substrate plane, where ± FL(Bx) = ± q vy x Bx, Fig. 1. The

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force FL(Bx) deflect the current lines to the upper or bottom surface of the structure, depend on the directions of the current IC1,2 and magnetic field Bx. This way, non-compensated current charges appear simultaneously in the vicinity of both terminals C3 and C4. Thus, between the middle point of the potentiometer P and either of the contacts C3 or C4, output signals arise, containing a Hall voltage VH(Bx), Fig. 1(a). Two output voltage, containing VH(Bx), may be measured - between the middle point P and C3: VH(Bx) + V0, and between the point P and contact C4 respectively: VH(Bx) - V0. The additional voltage V0 is proportional to VH(Bz) as follows: V0 = ± GHVH(Bz), where GH is the geometrical correction factor for Hall voltage. The Hall geometrical factor is in the range 0 < GH < 1, [1-3]. 3. Interface circuitry The 2D magnetometer interface circuitry, shown in Fig. 1(b), is very simple. It comprises three instrumentation amplifiers and a differential amplifier too. The channel for obtaining the voltage VH(Bx), proportional to the induction Bx, includes two instrumentation amplifiers- A1 and A2 with gains G1 = G2 = 1. The inputs of in-amp A1 are connected between the middle point of the potentiometer P and terminal C4 in such a way that the output of inamp A1 gives a voltage VH(Bx) +V0. The inputs of in-amp A2 are connected between the point P and contact C3 in such a way that the output of in-amp A2 gives a voltage -VH(Bx) +V0. The differential amplifier A3 has a gain G3 > 1. It subtracts the outputs voltages of in-amp A1 and in-amp A2, and gives output signal Vout(Bx) = 2G3VH(Bx), proportional to VH(Bx) as follows: G3[VH(Bx) +V0 - (-VH(Bx) + V0)] = 2G3VH(Bx). This way, the parasitic signal V0 is completely compensated. The channel for obtaining the voltage VH(Bz) contains only a instrumentation amplifier inamp A4 with a gain G4 > 1. It gives an output voltage Vout(Bz) = G4VH(Bz), proportional to VH(Bz). 4. Realization The experimental prototype has been implemented using part of the processing steps applied in bipolar IC technology using four masks. The n-Si plates are 300 ȝm thick, with resistivity ȡ § 7.5 ȍ.cm. The current carrier’s concentration is about n ~ 4.3 ɯ 1015 cm-3. Similar to [10], four masks are used. The mask 1 determine n+ implanted zones for ohmic contacts C1…C4 as depth of the ohmic n+-n junctions is about 1 ȝm. The mask 2 form deep p-ring with rectangular shape, the mask 3 is intended for metalized busses and zones for bonding. The mask 4 is intended for the contact holes on the surface layer SiO2 for the electrical contacts between the busses and n+ areas. The dopant donor concentration of the n+-n junctions is n § 1020 cm-3. The deep surrounding p-ring width at the surface is about 20 ȝm (on the mask). The sizes of ohmic contacts are 10 x 60 µm2 for C1 and C2, and 10 x 10 µm2 for C3 and C4. The microsystem is achieved by hybrid realization, using three instrumentation amplifiers and a differential amplifier. A precise instrumentation amplifier like AD623, AD8553, AD8230 is used for our application. 5. Experimental results Some of the 2D magnetometer characteristics are shown. The respective channel-magnetosensitivities consist Sx § 17 V/AT and Sz § 23.3 V/AT, without amplification, Fig. 2(a). The offsets are nullified in advance; the op-amp gains are 1. The non-linearity in the range – 1.0 B 1.0 T is no more than NL 1.3 %. The channel cross-talk, mainly due to the geometrical magnetoresistance reaches no more than 3.0 % at induction B 1 T. The internal noise in the range 10 Hz f 1 kHz is of the 1/f type, Fig. 2(b). With the increase of bias current Is, the noise grows too. The mean lowest detected magnetic induction B for the two-axis magnetometer at a supply current 3 mA over the frequency range f 100 Hz at a signal to noise ratio equal to unity, is Bmin § 11 µT for the channels. The effective operational volume is about 90 x 60 x 40 ȝm3, which provides the high spatial resolution of this 2D magnetometer.

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6. Conclusion The novel silicon sensing device for simultaneously measureing of two orthogonal magnetic-field components using one and the same transducer zone is very promising. The interface electronics is simple and reliable. The obtained results and performance are appropriate for many contactless applications, like: robotics, mechatronics and industrial controls – tactile systems, space orientation, measuring angular and linear displacements, speed sensors, end-of-travel sensors, encoders, magnetic compass and robotized unmanned flight vehicles; automobiles - ignition timing, antilock braking (ABS) systems; various electronic equipment - commutation for brushless fans, disk drive index sensors etc. The future objective is to develop a three-axis magnetometer for both 3D magnetic field sensing and contactless in-plane 360º absolute angle encoding based on the presented in this paper simple as design device.

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