An array CMOS magnetic-field sensor

209

Sensors and Actuators A, 35 (1993) 209-212

An array CMOS magnetic-field sensor Xmyu Zheng,

Dongsheng

Zhang,

Lltlan Lm and ZhlJlan Ll

Inrtltuie of Mcroelectronrcs, Tsmghua Umversrty, Beyrng (Chma) (Received May 8, 1992, UI remed form June 30, 1992, accepted July 21, 1992)

Abstract An array integrated magnetlofield sensor has been fabncated by CMOS processlugtechnology It works in a new mode, and has current-mode output wth an on-&p reference current source The array form of the sensitive units Increases the signal-to-noise ratlo and the sensltlvlty The sensor has good hnear charactenstlcs

Introduction

Design and fabrication

A sphtdram MOSFET (MAGFET) is a magnetic-field sensor using the Lorentz deflection of the channel current m a magnetic field Because of its good magnetic-field-sensltlve performance [l] and its compatlblllty to CMOS technology, It 1s considered to be a promlsmg Integrated magnetlc-field-sensitive component [2] It seems posable to use an integrated magnetic-field sensor wrth a MAGFET as the sensltlve unit instead of the tradltlonal Hall device m order to make an integrated magnetic sensor urlth very small volume and with the display and measurement devices integrated together Such an integrated device can be used for measurement of direct and alternating currents and the magnetic field strength and dlstrtbutlon Nevertheless, the output of the present MAGFET magnetic-field sensor 1s non-linear and 1s difficult to digitize The array CMOS magnetic-field sensor presented m this paper works m a new mode which dvercomes these dlfficultles The new mode 1s charactetlzed by the followmg features (1) the MAGFET 1s m the optmuzed equal-dram-voltage saturated state, (2) the current mode output with on-chip reference current source provides a convement digital display and self-compensation, and (3) the array form of the sensltlve units increases the slgnal-tonoise ratio and the sensltlvlty Expenments show that the array sensor has good linear charactenstics, as expected

A photo of the chip of the array CMOS magnetic-field sensor 1s shown m Fig 1 Figure 2 shows the scheme of the chip (a) and the structure of the magnetic-field-sensltlve MAGFET (b) The circuit consists of bias voltage sources (M,, Ms), a MAGFET (24 devices, M 1, connected m parallel), current nurrors (a total of 24 groups of MZ, M3 and Mq, connected m parallel) and reference current sources (M,, M6, output posltlve and negatlve reference current, respectively), where M1, M6, M8 are NMOS transistors and the others are PMOS transistors The design values of the widthto-length ratio confirm that all the MAGFETs are blased m the saturated state The central dram

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l?g 1 Photograph of the chip of the array CMOS magnetic-field sensor

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Measurement and the method of digitiziug the output

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output & IS taken as the reference level The nght and left dram-current output terminals AI+ and .U are kept at P’,.eflevel by the outer circuits The reference current output termmals Z+ and I_ are also maintained at the V,, level The ratio of the output current to the total current flowing through the MAGFET 1s determmed by the layout design, thus (AZ+ - AZ_)/(Z+ -I_) 1s effectively the ratto of the MAGFET magnetic-field-senatlve current to the total current This ratio 1s finally transferred mto a d@al output According to the analysis of ref 1, the equal dram-voltage current output, saturated state and the measurement of relative sensltlvlty are the condltlons for an optlmlzed MAGFET which possesses good linearity and stability of its magnetic-field-sensltlve characteristics These condltlons are well satisfied by the chip shown m Fig 1 Furthermore, the bias voltage V, of the sensor can be adjusted outside the chip, and since it 1s easier to obtam higher sensltlvlty with lower V,, this capability also performs as one of the features of the given array sensor The MAGFETs are fabncated on ( lOO)-onented p-type substrates with 10 R cm reslstlvlty using the standard polyslhcon gate MOS technology The oxide thckness and the threshold voltage are approximately 50 nm and 0 5 V, respectively The aspect ratio of the MAGFETs 1s one with a channel length of 90 w and a notch spacmg between the two drams of 5 pm

Figure 3 shows the relationship between the differential current output AZ (AZ+ - AI_) of the array CMOS magnetic-field sensor and the magnetic field under different bias voltages V, For all bias voltages, the hneanty 1s very good When V, = 2 V and the magnetic field 1s 1 T, the dIfferenteal current output can reach 50 PA Figure 4 shows the external cn-cult which converts the output of the CMOS array magnetic-field sensor to digital form Four buffer operational amplifiers (ICL7650) have not been integrated on the chip When the development of the CMOS operational amplifier with self-zero-adJustment circult IS finished m this laboratory, all the clrcults concerned, except the ADC, will be integrated m a single chip A commercially available three and a half digit double lptegrator circuit ICL7107 or ICL7106 1s adopted for the ADC AZ+ and AI_, I, and I_ provide dlfferentlal slgnal input and dlfferentlal reference input, respectively VRf offers the reference level The indicated dlgt should be D = A(AZ+ - AZ_)/(Z+ -I_)

= K&B

where A, K are constants determined by the chip and external clrcult, respectively, and S, IS the relative sensltlvlty of the MAGFET In practice, the digital output 1s dlvlded mto two scales, with full scale of (1999) 0 2 T and 2 T, respectively On the 0 2 T scale, a change of 3 to 4 Gs 1s dlstmgulshable The sensor m this paper has been used practltally m the measurement of a magnetic field Further improvement (increasing the number of array ,v,=-a

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units, reducing the umt area, commonly mtegratmg the buffer operational amphfiers, etc ) will be made, which ~11 enable the magnetic-field-sensltlve device to be much smaller, smarter and more sensltlve

sensor to dlg&d form

Electromc Engmeermg, Tsmghua Umverslty, m 1966 He received M S and Ph D degrees from Tsmghua Umverslty m 1983 and 1986, respectively He IS presently an associate professor m the Institute of Mlcroelectromcs, Tsmghua University His research area 1s integrated sensors and OEIC

References

Biographies

Dongsheng Zhung was born m Shandong Provmce, China, m 1967 He graduated from the Department of Electronic Engmeermg, Tsmghua Umverslty, m 1990 He 1s presently a research assistant m the Institute of Mlcroelectromcs, Tsmghua University His research area IS mtegrated sensors

Xmyu Zheng was born m Shanghai, China, m 1944 He graduated from the Department of

Lltran Liu was born m Nanchang, China, m 1947 He graduated from the Department of Elec-

I X Zheug and S Wu, General charactenstlcs and current output mode of a MOS magnetic-field sensor, Sensors and Actualors A, 28 (1991) 1-5 2 D Mlsra andT R Kswanathan and E L Heasell, A novel high-gam MOS magnetic-field sensor, Sensors and Actuators, 9( 1986) 213-221

212

tromc Engmeenng, Tsmghua University, BeiJmg, China, m 1970 Since then he has been working on the research and development of semiconductor devices and KS Currently, he 1s an associate professor m the Institute of Mlcroelectromcs, Tsmghua Umverslty His present research interests focus on the development of solid-state Integrated sensors

Zhgzan LI was born m ZheJiang Province, China, m 1928 He received a Ph D degree from the Department of Physics and Mathematics, Umverslty of Leningrad, USSR Since 1958, he has worked in the Department of Electronic Engmeermg, Tsmghua University, BelJmg, China Currently, he 1s a professor of Mxroelectromcs Science and the director of the Institute of Mlcroelectromcs, Tsmghua University