A novel 3-D magnetic-field sensor in standard CMOS technology

Sensors

and Actuator.,

A

67

34 (1992) 67 75

A novel 3-D magnetic-field sensor in standard CMOS technology* Durga Misra, Mingming Zhang and Ziyan Cheng Department of Electrical and Computer Engmrwrmg, New Jersey Institute of Technology, Newark, NJ 07102 (USA) (Received November

26, 1991, accepted February 25, 1992)

Abstract Measurement of the three components of the magnetic-field vector simultaneously, by an integrated magnetic-field sensor, has many practical applications In this paper we present a three-dimensional (3-D) magnetic-field sensor in standard 2 pm CMOS technology The sensor uses two CMOS-compatible one-dimensional vertical magnetotransistors positioned at tight angles to each other to measure two field components, a MAGFET and/or a lateral bipolar transistor is used to measure the third component To minimize the offset due to asymmetry during fabncation, two base contacts are used The MOS device controls the base surface potential and thereby improves the device characteristics and the noise performance in the lateral device The device is designed and fabricated by MOSIS using standard p-well technology Simulations are performed to study the electrical characteristics and to find out the optimized geometry in order to achieve high current gain by using PISCES-ITBa 2-D device simulation-6software The measured device characteristics show relative sensitivities .S .,,. = 7 19 x 10 -7 G', S,, = 1 88 x 10 G - ' and S-.. = 6 80 x 10 -7 G i in the X-, Y- and Z-directions respectively, to the applied magnetic field

Introduction Integrated silicon magnetic-field sensors (MFSs) can now be manufactured using standard integrated circuit-processing technologies without invoking additional processing steps such as micromachining, as in the case of most mechanical sensors The MFS device [1] is an input transducer that is capable of converting the magnetic field H into a useful electronic signal This sensor is also needed whenever a nonmagnetic signal is detected by means of an intermediate conversion into H ('tandem' transduction), e g , the detection of a current through its magnetic field or the mechanical displacement of a magnet Thus we can distinguish two main groups of magnetic-field sensor applications In the case of direct applications, the magnetic-field sensor is part of a magnetometer Examples are (i) Earth's magnetic field measurements, (n) reading magnetic tapes and disks, (iii) recognition of magnetic ink patterns of bank notes and crcdit cards, (iv) control of magnetic appara-

'Paper presented at the 6th international Conference on SolidState Sensors and Actuators (Transducers '91), San Francisco, CA, USA, June 24-28 1991

0924-4247)9215500

Ins In the case of indirect applications, the magnetic-field sensor is used as an intermediary carver for the detection of nonmagnetic signals Examples are (i) contactless switching, (it) linear and angular displacement detection (e g , in automotive systems), (in) potential-free current detection, (iv) integrated wattmeters With respect to highdensity magnetic recording, some of the recently devised integrated silicon magnetic-field sensors are now able to compete with traditional NiFe thin-film magnetoresistor devices [1] On the other hand, there are potential applications of a three-dimensional magnetic-field sensor, as in most real cases the magnetic-field vector is not simply one dimensional The possible 3-D magnetic-field sensor applications are (t) full magnetic vector measurements on magnetic materials and apparatus, (u) Earth's magnetic field measurements for navigational or geological purposes, (iii) proximity switches, (iv) contactless angular position encoders Practical applications require the detection of magnetic fields in the range of pT and mT This can be achieved by the integrated semiconductor sensors Recent reports show that the detection of magnetic fields in the nanotesla range is possible by using vertical dual-collector magnetotransistors with large current gain [2] For

(c) 1992 - Elsevier Sequoia All tights reserved

68

those large-scale applications, inexpensive batchfabricated integrated sensor prototypes are highly desirable Various applications come with a specific sensor requirement, such as a required sensitivity, field resolution or sensor geometry We have several ways to realize a high-performance magnetic-field sensor based on different physical mechanisms and different standard technologies So far the best explored magnetic-field sensors are the split-drain (SD) MAGFET [8, 12], dual-collector magnetotransistor (MT) [3 5], including vertical (VMT) and lateral (LMT) types, and the suppressed sidewall injection magnetotransistor [7] Each type has various advantages and disadvantages In general, high absolute sensitivities can he obtained with SD MAGFETs when devices are operating in the saturation region and drains are loaded by high resistance In this case, the useful signal is the differential voltage between the two drains SD MAGFETs are one-dimensional magnetic-field sensors For the detection of magnetic fields in the nanotesla range [2], a dual-collector magnetotransistor with a large current gain is desirable [2] Also by merging two 1-D VMTs and one LMT, a 3-D magnetic-field sensor can be designed and fabricated by standard bipolar technology [9, 10] However, due to strong cross-sensitivities, the overall sensitivities of this kind of 3-D magneticfield sensor are relatively low The newly reported suppressed sidewall injection magnetotransistor displays a range of sensitivities from 0 5 to 30 T - i This sensitivity exceeds by an order of magnitude or more the previously reported values for comparable magnetotransistors It can be fabricated by standard CMOS technology However, this kind of magnetic-field sensor is still not able to measure simultaneously more than two components of the magnetic-flux-density vector B This paper describes the design of a 3-D magnetic-field sensor which has been fabricated in standard CMOS technology The cross-sensitivities are minimized by taking advantage of the technology The paper presents details about the operation principles, design considerations, computer-aided simulations and experimental results of the sensor device The sensitivities in all three dimensions are measured as a function of the bias currents The effect of a finite voltage at the gate of the split-drain MOS structure on the overall operation of the sensor is outlined

A 3-D magnetic sensor There are some general design criteria that should be taken into account when designing a magnetic-field sensor to satisfy one or two requirements First, we must consider the availability of technology At the present time, standard bipolar, NMOS and CMOS technologies, and the new BiCMOS technology [11] are readily available Secondly, we need to estimate the total manufacturing cost, which should he as low as possible The environment (such as temperature, humidity, chemical stress, mechanical stress and vibrations) to which the sensor is exposed should also be taken into consideration, since these conditions can influence the sensor performance The basic principle of the MFS operation is that it exploits the Lorentz force F = qv x B on electrons in a metal, a semiconductor, or an insulator in one way or another, where q denotes the electronic charge, v the electron velocity and B the magnetic induction [1] In an n-type semiconductor material, for instance, the magnitude of the sensor effect is controlled by the product of the electron mobility and the magnetic induction The operation of integrated devices to measure a one-dimensional field is based on the identical principle A split-drain MAGFET is a MOSFET with two or three adjacent drain regions that share the drain current A magnetic field perpendicular to the chip surface can cause deflection of the current lines in the channel region By operating the MAGFET in the saturation mode, a large and linear output voltage swing may be obtained The first split-drain MAGFET was initially proposed by Fry and Hoey [12] A dual-collector magnetotransistor is a bipolar transistor whose design and operating conditions are optimized with respect to the magnetic-field sensitivity of the collector current I, Because of the geometrical symmetry, the two collector currents should be equal without a magnetic field However, when a magnetic field is applied, the collector currents will be modified, causing more current in one of the collectors and less current in the other The first vertical magnetotransistor showed a relative sensitivity of about 0 5 T - ' A lateral magnetotransistor device was essentially a merged combination of a planar Hall plate and a lateral dual-collector transistor The relative sensitivity was also about 05 T - '



69

During most real applications, the measurement of the 3-D magnetic field is required This can be performed by successively changing the onentation of 1-D sensor and measuring the field components, or by a simultaneous measurement of all three field components by attaching three I-D sensors to the orthogonal faces of a cube The spatial resolution of a measurement would not be satisfactory in this case if highly divergent fields were to be measured, nor is the sensor integrable A recent report [10] shows that a 3-D magnetic sensor has been designed and fabricated by standard bipolar technology The current flow in a 2-D in-plane magnetotransistor does not completely sense the two components of the magnetic field, since there are significant lateral components of the total current in the epitaxial collector region In the case of lateral magnetotransistors, the lateral component of the total collector current can be deflected by the third field component B_ But the lateral device can show some sensitivity due to the vertical component of its total current In general, the output signal of a 3-D sensor can be written as [ 10]

eft ery AL where Ali ( i = x, y or z) is the collector-current difference of the associated channel S should be a 3 x 3 sensitivity diagonal matrix But it is not necessarily a diagonal matrix in the case of the above bipolar device, because the output signal A!, of channel r will not only depend on the field component B,, but also on the other two components The sensitivity matrix elements of the 3-D sensor with one z -collector pair are [10]

r

0

S = 0

0 5,.,

S, r

A 3-D magnetic sensor in CMOS technology CMOS being the most popular and widely used technology, the CMOS-compatible MT groups seem to offer a good opportunity with respect to both scientific research and various low-power applications A widely applicable approach lies in using bipolar devices that are realizable with the existing CMOS technologies One possibility is the vertical transistor using the substrate as the collector and a separate well as its base [8], a lateral bipolar transistor can also be formed in the well region [7] In this Section, we first describe the device structure and then analyze its operation Device structure and operation

The structure of the new 3-D magnetic-field sensor is shown in Fig I The device is situated in a p-well, serving as the base of the transistor The substrate serves as collector for the two vertical npn transistors and another lateral bipolar transistor with a two-collector pair is formed within the base region The two vertical magnetotransistors are sensitive to the two in-plane magnetic-field components and the lateral magnetotransistor can measure the magnetic field perpendicular to the chip surface Two base contacts, BI and B2, are used to allow this application distribution of collector current For example, if the device has ideal geometrical symmetry, the collector currents should be equal However, asymmetry in the device structure arising during the fabrication process will cause cural

Bi

0 0 S'_

Therefore, it is essential to eliminate any cross-sensitivity In CMOS technology 5, can be eliminated or minimized by using a split-drain MAGFET in place of the lateral collector pair while detecting the third component of the magnetic field To increase the current level of the split-drain MAGFET, the drains can simultaneously be used as a pair of lateral collectors, as discussed in the following paragraphs

c E

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0

W c.i M c®

W Top view Fig I

3-D magnetic sensor structure in CMOS technology showing the cross section and the top view

70 rent imbalance even without a magnetic field By applying slightly different voltages to BI and B2, the imbalance can be eliminated Also from Fig I

Cy i

Bi

E

02

E

Ca

B1

Cn

L

1

l

1

eye

we can see the additional Z-collector pair partakes of the X-collector current Adjusting the two base voltages will provide a reasonable distribution of those collector currents Secondly, the device is also similar to a MOS

tie

transistor, as the base region is covered by the thin oxide and polysilicon gate layers This was done for

1

two reasons (i) the split-drain MAGFET device (if

(b)

used) can measure the third component indepen-

1

B2

dent of the vertical magnetotransistors, (u) the gate

Fig 2 Device structures show the directional mode of simulation (a) in Y direction, (b) in X, Z directions

makes it possible to control the base surface potential if the lateral transistor is used This feature is

increased by using minority-carrier deflection in the

important for both sensitivity and noise perfor-

long-base region The deflection efficiency is not

mance of the device For example, when we apply

linked to the device geometry, rather biasing condi-

a magnetic field perpendicular to the device plane

tions are more related to the device sensitivity Evidently, this kind of 3-D magnetic-field sensor

(B,), that is, in a direction such that the electron flux lines are deflected toward the device surface, the lateral collector current will be reduced because

has better performance than those 3-D magnetotransistors discussed earlier [10]

of the additional recombination at the silicon-oxide interface The MOS structure on top of the base

Computer-aided device simulation

region serves to reduce this recombination Appli-

Based on the theory presented in the previous

cation of a negative voltage to the gate enriches the base-region surface and pushes the minority carri-

Sections, a 3-D magnetotransistor has been designed and fabricated by MOSIS using a standard

ers away from the interface, as recently demon-

2 pin p-well technology Computer-aided simula-

strated by Vittoz [6]

tions have been done by using a 2-D device simulation software, PISCES-IIB As the device is a 3-D

For a qualitative device operation, let us assume that the device is biased adequately for forward active operation Electrons are injected into the base region laterally and vertically and are collected

structure, it is simulated in the Y direction and the X, Z directions separately, as shown in Fig 2 As the fabrication process is standard, the device ge-

by the corresponding collectors If we apply a

ometry can only be modified to optimize its opera-

magnetic field B, or B, it will cause a change in the two vertical collector pairs due to the deflection of

tion During this work, four devices with different sizes have been designed to get an optimized perfor-

the electron paths When the field is directed per-

mance Device simulations provided the electrical

pendicular to the plane of the Figure, the Lorentz force on the charge carriers will cause an imbalance

characteristics and the proper geometry in order to

of collector current in the Z-collector pair

showing the base current as a function of baseemitter voltage are presented in Fig 3(a) for the

Now the carrier deflection in the long base region is discussed In this device, since the p-well with a depth of about 5 pin is used as the base region, we must consider the Lorentz force on the minority

achieve high current gain The simulation results

device in the Y direction A simulated device charactenstic (1, as a function of VVE ) is shown in Fig 4(a) for all three directions During the simulation

earners (electrons) as well as on the majonty carriers It is known that the Hall field generated by

the collector pair in each direction was shorted The collector currents show different gains, with a min-

deflected electrons in silicon is in direction opposite

imum gain for the Z-collector pair This is mainly

to that of the Hall field generated by holes Therefore, the majority carriers (holes) cancel the Hall

due to the current sharing problem between the X and Z directions

field that the minority carriers (electrons) would induce, in the same way as they cancel any other

From the simulation results, it is seen that without an applied magnetic field, the current distribu-

space-charge effect Thus the sensitivity can be

tion in the X-, Y- and Z-directional collector pairs



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Fig 3 Simulation results showing the base current as a function of base-emitter voltage (b) Experimental results showing the collector current as a function of base-emitter voltage

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for V, Y and Z collector pairs (b) Experimental measurements of the,

the higher is the gain The transduction area was minimized by using the minimum possible separation rules for the process Again, the size of the collector current influences the current gain, and the enutter area is the principal factor determining the output current level



72

Experimental results and discussion A photomicograph of the device is shown in Fig 5 The experimental measurements of the device characteristics are carried out by using a curve tracer The experimental characteristic showing the collector current as a function of baseemitter voltage is presented in Fig 3(b) The simulation and the experimental results show a base-emitter voltage of the devices of 0 72 V The collector currents Ix , I,, and I, are presented in Fig 4(b) The experimental results are identical to the simulation results A test circuit has been designed (Fig 6) to measure the magnetic-field effects Load resistors of 4 6 kQ were used at each set of collectors and a base voltage was provided by a voltage-divider circuit (R, = 15 kQ and R z = 700 0) The collector currents 1, and I, were set at 728 pA and 728 RA, respectively, whereas the collector current L, was set to 75 pA with a zero gate potential, when a gate potential of 5 V is applied, the current increased to 202 pA All the measurements were carried out keeping the transistors under the linear active mode of operation, with a V,, of 53V The measured response of the 3-D sensor to the magnetic-field vector as a function of the magnetic-field strength at different currents is given in Fig 7 The output current difference is 47,,, where i represents x, y and z, for the collector current difference in the i direction due to a magnetic field B, The relative sensitivity S,,, in Fig 7(a) is almost identical for all current values,

Fig 5 Photomicrograph of a 3-D magnetic sensor device designed in CMOS technology

Vee-53 V

to

(10

0 40

40

L Fig 6 Test circuit used to measure the sensitivity of the magnetic sensor

whereas a decrease in the case of S„ (Fig 7(b)) and an increase for S,= (Fig 7(c)) are noticed with an increase in collector current This difference is attributed to sharing of current between the x and z collectors At higher currents it is believed that some carriers moving under the influence of the magnetic field B, r are lost to the z collectors, whereas during the influence of B, more carvers are collected at the z collectors before reaching the x collectors Figure 8 presents a comparison of the sensitivities of the magnetic sensor for all the three directions and also shows the influence of a finite potential on the gate of the MOS device The device shows relative sensitivities S ir = 7 19 x 10 - i G - i, S,, = 1 88 x 10 6 G - ' and S,_ = 6 80 x 10 -7 G - i in the X, Y and Z directions, respectively, to the applied magnetic field An increase in AI, is seen (Fig 8) when a finite voltage (5 V) is applied at the gate of the MOS device This is because the current in the split drain has increased to a higher value due to the formation of a channel and to reduced recombination in the bulk, thereby increasing AL_ However, the relative sensitivity was not improved due to the rather poor transduction capacity of the MOS device, where the transduction takes place mainly in the pinch-off region at the dram end The cross-sensitivity 5,,, (not shown) is reduced though not eliminated by the gate voltage

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(a)

100 200 300 400 500 600 700 600 900 1000

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2 A Nathan and H P Buttes, Huw to achieve nanutesla resulution with integrated silicon magnetoresistance, IEDM 99 pp 511-514 3 T Smy, Lj Ristic and H Baltes, An analytical model of a new magnetoresistor the SSIMT, /ED.M 87, pp 286 289 4 Lj Ristic, T Smy and H P Buttes A lateral magnetotranststor structure with a linear response to the magnetic field, IEEE Trom Electron Demvcs, ED-36 (1989) 1076 1086 5 R S Popovic and R Widmer, Magnetotranststor in CMOS technology, IEEE Trans Electron Devices . ED-33 (1986) 1334 1340 6 E A Vittoz, MOS transistors operated in the lateral bipolar mode and their application in CMOS technology IEEE J Solid State Cut , SSC-18 (1983) 173 279 7 A W Vmal and N A Masnari Magnetic transistor behavior explained by modulation of emitter injection, not carrier deflection, IEEE Electron Device "it EDL-3 (1982) 203-205 8 D Misra, T R Viswanathan and E L Ileasell, A novel high . gain magnetic field sensor Sensors and Actuators 9 (1986) 213 221 9 S Kordic . Integrated 3-D magnetic sensor based on an n-p-n transistor IEEE Electron Deuc+ Let, ED1:7 (1996) 196198 10 S Kordie and P J A Munier, Three-dimensional magnetic field sensors IEEE Trans Electron Devices ED-35 (1988) 771779 11 K 0 Kenneth, R Reif and Hae-Sung Lee, BiMOS transistors merged bipolar/sidewall MOS transistors IEEE Electron Device Left, EDL-10 (1989) 517-519 12 P W Fry and S J Hoey, A silicon MOS magnetic field transducer of high sensitivity IEEE Trans Electron Devices ED-16 (t969) 35- 39 13 B Wang and D Mists . A novel 3-dimension magnetic field sensor with a merged structure in BiCMOS technology, IEEE Trans Electron Devices submitted for publication

Conclusions It is demonstrated that a 3-D magnetic-field sensor can be effectively implemented in a standard 2 (lm CMOS technology with relative sensitivities S ir =7 19 x 10 -7 G -t , S ' = 1 88 x 10 -6 G'and S__ = 6 80 x 10 - 'G - 'in X, Y and Z directions, respectively, to an applied magnetic field The MOS device controls the base surface potential and thereby improves the device gain by increasing the current, but does not necessarily improve the relative sensitivity of the device due to the current splitting limitations The cross-sensitivity was not eliminated due to the current sharing between the X and Z directions This can be improved by using a BiCMOS device [13], where the MOS transistor will not be able to share current with the bipolar devices

References I H P Balms and R S Popovic, Integrated semiconductor magnetic field sensors . Proe IEEE, 4(8) (1986) 1107-1132

Biographies Durga Misra is an assistant professor of the Department of Electrical and Computer Engineering at the New Jersey Institute of Technology Dr Misra received his M S and PhD degrees, both in electrical engineering, from the University of Waterloo, Waterloo, Canada in 1985 and 1988, respectively His current research interests are integrated sensors and microengineering He has worked extensively on IC fabrication and published a number of papers on integrated magnetic sensors and reactive ion etching He is a member of IEEE, the Electrochemical Society and Sigma Xi M1ngmlng Zhang was born in China in 1966 She received a B S in electrical engineering from Tsinghua University, China in 1989 and an M S degree in electrical engineering in December 1991 from the New Jersey Institute of Technology



75

She is currently with Rockwell International, Newport Beach, CA Ziyan Cheng was born in China in 1963 She received a B S and M S in electrical engineering

from East China Normal University, Shanghai, China in 1988 She received her degree in electrical and computer engineering in May 1992 She is currently working on software development at Easy Soft, Inc, NJ