CMOS magnetic-field sensor system

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 29, NO. 8. AUGUST 1994

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CMOS Magnetic-Field Sensor System Andreas Sprotte, Rolf Buckhorst, Werner Brockherde, Bedrich J. Hosticka, and Dieter Bosch

Abstract-A magnetic-field sensor system integrated in CMOS technology with additional processing steps necessary for sensor fabrication is presented. The system contains a magnetoresistive permalloy microbridge acting as a sensor, temperature compensation circuitry, programmable readout electronics, reference voltage bias, and clock generation. It features maximum magnetic flux sensitivity of 70 mV/pT (corresponds to the magnetic-field sensitivity of 88.2 mV/(A/m) @ p r = 1) and its temperature gain is below 260 ppm/"C in the range hetween -50°C and +lOO°C.

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I. INTRODUCTION

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AGNETORESISTIVE permalloy sensors exhibit two significant features: they are highly sensitive to magnetic fields and their fabrication sequence is basically compatible with standard CMOS processes. Therefore, they appear to be very attractive for integration of devices measuring lowlevel magnetic fields. However, their temperature dependence is also high and must be compensated for (typically, in the range between -50°C and +lOO"C, the sensitivity changes by about 50%). The solution chosen in the presented work was to place the magnetoresistive sensor on a silicon microchip and co-integrate an electronic temperature gain cancellation. This approach also enables the realization of sensor readout electronics and reference voltage necessary for sensor biasing on the same chip and yields a single-chip magnetoresistive sensor system.

Fig. 1, Magnetoresistive permalloy full bridge. Permalloy

Fig. 2. Process cross-section of the permalloy bridge.

11. SENSOR PRINCIPLE AND FABRICATION The sensor principle used in this work is based on the fact that the resistance of anisotropic permalloy depends on its magnetization [ 11-[3]. While the initial magnetization state is consistent with the anisotropy axis, it can be changed by applying an external magnetic field. This causes a change in permalloy resistance and can be used for detection and measurement of magnetic fields. The resistance variation depends on the angle between the direction of the applied field and the anisotropy axis. The actual sensor developed in this work consists of a thin permalloy layer structured in such way that it forms four magnetoresistors. These are arranged to form a full magnetoresistive bridge (Fig. 1). As the magnetization characteristic is nonlinear, the sensor has been made linear by applying an additional magnetic bias field. The anisotropy axis of the sensor has been chosen to be identical with the measurement direction. The magnetic bias field is then perpendicular to the anisotropy axis. The resistive permalloy strips have been aligned at an angle of f 1 0 " with Manuscript received September 10, 1993; revised April 3, 1994. A. Sprotte, W. Brockherde, and B. J. Hosticka are with Fraunhofer Institute of Microelectronic Circuits and Systems, D-47057 Duisburg, Germany. R. Buckhorst is with Hanning GmbH, Oerlinghausen, Germany. D. Bosch is with Deutsche Aerospace AG, Ottobrunn, Germany. IEEE Log Number 9402143.

respect to the anisotropy axis (see Fig. 1). This arrangement offers two significant advantages: it yields high magnetic-field sensitivity and reduces sensitivity to variations of the magnetic bias field. The magnetoresistive strips change their resistance according to the inclination of their current paths measured with respect to the anisotropy axis: if the magnetic field applied in measurement direction increases, the resistance of strips arranged at the angle of +lo" increases, while it decreases for these lying at -10". The sensor fabrication sequence can be easily included in a standard CMOS process. The only requirement is that the standard CMOS sequence must be interrupted after aluminum patterning. Now an additional oxide layer is deposited instead of the passivation oxide. After planarization contact cuts are etched to establish contact between the aluminum and the permalloy that will be deposited in the next step. This is followed by permalloy patterning and etching contact holes over bonding pad locations in the additional oxide layer. The process cross-section of the permalloy sensor is illustrated in Fig. 2 . In order to achieve the desired anisotropy the permalloy layer was deposited in presence of an external magnetic field. Measurements at fabricated sensors have yielded a typical bridge sensitivity of 5.25 x 10-6(A/m)-1 at 20°C and lin-

0018-9200/94$04.00 0 1994 IEEE

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SPROTTE et al.: CMOS MAGNETIC-FIELD SENSOR SYSTEM

Integrated Magnetic Field Sensor System

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Fig. 4. Block diagram of the single-chip magnetic-field sensor system

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earity better than 0.1% in the range *240 A/m. The bridge resistance at 20°C was 2.47 kR for layer thickness of 70 nm. The temperature dependence of the bridge sensitivity S and the bridge resistance R B are ~ shown in Figs. 3(a) and (b), respectively. The total bridge sensitivity Stot depends on the bridge voltage V,, or the bridge current I,,, subject to biasing mode. The sensitivity of the permalloy bridge compares favorably with sensitivity of other semiconductor magnetic sensors: integrated Hall devices show an absolute sensitivity of 8.75 x lo-' ( A h - ' , SD MAGFET 1.5 x (A/m)-' and magnetotransistors (LMT) 1.88 x lop6 ( A h - ' [41. 111. SENSORSYSTEM In order to ensure wide sensor marketability, sensor systems have not only to deliver excellent performance data but must also require only a minimum of additional extemal components and adjustments. In addition, they have to deliver a buffered sensor signal in a convenient format at the output. The monolithic cointegration approach employed in this work adheres exactly to this concept. Above considerations have led to the development of a magnetic-field sensor system on a CMOS chip. The block diagram of the system is depicted in Fig. 4. Its centerpiece is the magnetoresistive permalloy full bridge, as described in the previous section. Programmable readout electronics with offset compensation and temperature compensation circuitry

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provide exactly those functions that are necessary for reliable and successful operation without extra off-chip components. Also on the chip are located additional peripheral circuits which support the sensor operation: a reference voltage bias for the magnetoresistive bridge and a clock generator.

IV. TEMPERATURE COMPENSATION Since the permalloy is deposited on the top of several isolating oxide layers (see Fig. 2), it is thermally well-insulated. In order to ensure good temperature sensing that is necessary for temperature compensation of the magnetoresistive bridge, we have decided to use the magnetoresistive sensor as a temperature sensor as well. The sensor is then operated in a time-multiplexed mode: during one period the sensor operates as a temperature sensor and during the other period as a magnetoresistive sensor while the temperature dependence is being compensated using results of the temperature measurement (i.e., the second period is the actual readout period). This method makes it possible to reduce significantly the temperature dependence while ensuring that the temperature sensing does not suffer from any temperature transients or gradients. The principle of the realized temperature compensation electronics is shown in Fig. 5. The actual bridge signal is sensed only during the readout period defined by the clock

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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 29, NO. 8, AUGUST 1994

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Fig. 7. Block diagram of the readout electronics.

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summing amplifier yields at its output an output voltage u l . V ~ ~+, u2 p . V ~ 2 which , contains a term proportional to R B ~ , as desired. This output voltage is then held constant during the clock period @I when the feedback loop is opened and it During the temperature measurement period (clock phase remains frozen. The readout electronics are now connected to @z, see Fig. 6(b)), the longitudinal bridge voltage V B ~ is, ~ the bridge biased by I B and ~ ,read ~ out the differential voltage measured in order to determine the vertical bridge resistance across the bridge Vsig,l. R B ~ ( Twhile ) a constant quiescent bias current I B ~is ,applied ~ to the bridge. Assuming no temperature changes between both measurements, we can use VBr,2(T) for generation of I B r , l ( T ) V. READOUTELECTRONICS so that The readout electronics contain a chain of three cascaded where

switched-capacitor precision amplifiers with programmable voltage gains. This is necessary for adjustment of absolute It can be shown that due to the different slope signs of the sensitivity of the sensor system because the sensor sensitivity temperature dependencies of S ( T ) and RB,(T) even simple can change due to processing variations, e.g., of permalto )RB,(T), loy thickness, and because the sensitivity requirements for linear current adjustment proportional of I B ~ , I ( T i.e., I B ~ , I ( 0; T )RB,(T),can lead to a substantial reduction of different applications may vary, too. The readout chain is the temperature coefficient of Stot. Simulations have shown followed by a smoothing RC-active third order Sallen-Key that square-law current adjustment would further reduce the lowpass filter to provide a continuous-time output voltage. The temperature error by a factor of 3.5 when compared to the first two switched-capacitor amplifiers have fully differential linear adjustment. signal paths, while the third one yields a single-ended output. The temperature compensation has been implemented on All amplifiers features capacitive resetting in order to obtain the chip using an analog switched-capacitor feedback loop open-loop gain enhancement and track & hold behavior [7]. containing a summing amplifier and a voltage/current con- Furthermore, they employ the principle of correlated double verter (see Fig. 5). During the clock phase @ 2 , a fixed voltage sampling for low frequency noise and offset reduction. The VEI is applied to the converter which generates I B ~ ,The ~ . time-multiplex operation betweeen the temperature measure-

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TABLE I Power supply voltages Oscillator frequency System clock frequency System bandwidth Bridge resistance @2OoC Sensor sensitivity @2OoC Measuring field range Programmable system gain Sensor system sensitivity at gain of 25 Meas. temperature range TC of system sensitivity witWwithout on-chip temp compensation Integral system nonlinearity Chip area (3 u m CMOS)

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ment and the readout measurement is fully synchronized with the clock of the switched-capacitor circuits. The first switchedcapacitor amplifier features an additional input for cancellation of the bridge offset (see VOC, and V O Cin~ Fig. 7) which is typically in the order of magnitude of the maximum bridge signal swing. The temperature dependence of the bridge offset, however, has not been compensated because the sensor is intended only for use in dynamic measurements. For static measurements of magnetic fields the result of the temperature measurement period would have to be added as an additional signal at the input for the bridge offset cancellation.

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VI. MEASUREMENTS The single-chip sensor system has been fabricated in a standard 3 pm n-well silicon-gate CMOS technology with included permalloy processing steps. The chip microphotograph of the magnetoresistive sensor system is shown in Fig. 8. Besides the sensor, readout electronics, and temperature compensation, the chip contains all circuits necessary for its operation: clock generator including an RC oscillator matched to the time constants of the RC smoothing filter and stabilized voltage reference source based on the bandgap principle. As we needed at least 1-phase clock during both periods (i.e., readout and temperature) the clock generation is a bit more complicated. Actually, a 5-phase clock is used on the chip. In Fig. 9, the temperature sensitivity of the sensor system with and without temperature compensation is plotted. The longitudinal bridge voltage VB, for the measurement with and without compensation was 2.5 V and 1.5 V (@ 20°C), respectively. The linearity measurement is shown in Fig. 10 and the measured data are summarized in Table I. VII. SUMMARY We have presented a magnetic-field sensor system based on magnetoresistive principle and fabricated in CMOS technology. The magnetoresistive permalloy microbridge has been

integrated on the same chip as the system circuitry which contains temperature compensation, programmable readout electronics, reference voltage bias, and clock generation. The maximum magnetic-field sensitivity of the system is 88.2 mV/(A/m) and integral full-scale linearity is better then 0.1%).

REFERENCES G. R. Hoffmann, J. K. Birtwistle and E. W. Hill, “The performance of magnetoresistive vector magnetometer with optimised conductor and anisotropy axis angles,” IEEE Trans. Magnetics, vol. 19, no. 5, pp. 2139-2141, Sept. 1983. G. R. Hoffmann and J. K Birtwistle, “Factors affecting the performance of a thin film magnetoresistive vector magnetometer,” J. Appl. Physics, vol. 53, no. 11, pp. 8266-8268, Nov. 1982. U. Dibbern, “Magnetoresistive sensors,” Sensors -A Comprehensive Survey, W. Goppel, J. Hesse and J. N. Zemel, Eds., vol. 5, Magnetic Sensors, R. Boll and K. J. Overshott, Eds., Weinheim, 1989, pp. 341-380 H. P. Bakes and R. S. Popovic, “Integrated semiconductor magnetic field sensors,” in IEEE Proc., vol. 74, no. 8, Aug. 1986, pp. 1107-1 132 Y. Kanda, et al., “Silicon Hall-effect power IC’s for brushless motors,’’ IEEE Trans. Electron Devices, vol. 29, no. 1, pp. 151-154, Jan. 1982. Van Zieren, et al., “Magnetic-field sensitive multi-collector npn transistor,” IEEE Trans. Electron Devices, vol. 29, no. I , pp. 83-90, Jan. 1982. K. Martin, L. Ozcolak, Y. S. Lee and G. C. Temes, “A differential switched capacitor amplifier,” IEEE J. Solid-Srare Circuits, vol. 22, no. 1, pp. 104-106, Feb. 1987.