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New magnetoresistive sensors: Engineering and applications
763
Sensors and Actuators A, 25-27
(1991) 763-766
New Magnetoresistive
Sensors: Engineering and Applications
F. ROT-I-MANN HL-Planartechnik
GmbH. Dortmund (F.R.G.)
F. DE-I-I-MANN Terhnikum
ftir Srhichttechnologie
und Sunderbuuelementr,
Sektion Physik der Uniwrsitiit Jean, Jenu (G.D.R.)
Abstract The magnetoresistive effect within thin anisotropic layers of ferromagnetic materials has been known for a long time. A new sensor layout and an appropriate selection of materials allows the hysteresis to be drastically reduced and the operation temperature to be increased up to 200 “C. Due to the short reaction times in the nanosecond field and their extreme sensitivity, magnetoresistive sensors are predestined to locate fast dynamic fields. But static magnetic fields can also be reliably measured with this type of sensor, since the resistance change is carried out in proportion to the field strength and not in proportion to the temporal change of the magnetic induction, as with inductively operated sensors. The typical applications of magnetoresistive sensors are revolution counting at gear wheels, digital or analog contactless distance measurement or remotely operating joysticks.
1. Introduction The dependence of the resistance of ferromagnetic materials on the magnetic field has been known for a long time [ 11. Practical experience has shown that the possibilities of modern thin-film and structuring technology can be used in order to produce components based on this effect which are suitable for the measurement of magnetic fields or, more specificially, for reading out information stored in magnetic materials [2]. Advantages in the 0924-4247/91/$3.50
use of such components result from their sensitivity, which is higher by a factor of 50 than that of modern Hall effect sensors [3], and from the fast reaction of the resistance of thin magnetoresistive films to field changes [4], which should permit working frequencies of the order of several 100 MHz. The problems involved in the design of magnetoresistive barberpole sensors and their various applications are described in the relevant literature [5,6]. The basic calculations for such sensors are summarized in ref. 7. This paper presents a special barberpole sensor which, as a result of its structure and the manufacturing technique employed, has substantially improved characteristics as regards its parameters and area of application. The discussion of a number of examples of use demonstrates the wide range of potential applications.
2. Structure and Manufacture of the Sensor The magnetoresistive sensor consists of a Wheatstone bridge circuit with four meander stripes. Figure 1 is a chip photograph illustrating the structure. Details of the barberpole structure can be seen more clearly in Fig. 2, which is an enlarged view of one balancing area. The chip area is 1.5 mm x 1 mm. The layout of the sensor element shows a high degree of symmetry. It is also important to ensure that only parallel strips of magnetoresistive material of identical width are used throughout, including the balancing areas. These strips are made of 0 Elsevier Sequoia/Printed
in The Netherlands
Fig. 1. Photograph of a magnetoresistlve sensor element with four resistance meanders in a bridge arrangement.
Fig. 2. Enlarged view of a balancing area of the sensor bridge.
permalloy (81% Ni, 19% Fe) and have a width of 10 pm. Identical strip dimensions and parallel alignment of all the strips are a prerequisite for low hysteresis in the sensor characteristic curve. This can, however, only be attained if, during the manufacture of the magnetoresistive film, a magnetic field of several 100 A/m, coinciding in direction with the longitudinal axis of the strips, ensures that a uniform anisotropic field strength is impressed on the entire area of the chip. The symmetry of the arrangement with respect. to the x-axis theoretically running through the centre of the component has the advantage that parasitic fields dispersed inductively or capacitively in the bridge are self-cancelling within the sensor bridge, and
so make no contributioh to the output signal. In normal sensor operation, the heat generated on the chip is not insignificant and the temperature coefficient of resistance of the magnetoresistive film strip is also not inconsiderable. This symmetry is therefore also necessary to prevent the occurrence of components of the output voltage which may vary as a function of the operating voltage or current. Results have shown that the slightly offset position of the balancing area located in the bottom left has no influence. It is, however, required in order to increase the permitted tolerance of the approach point for laser compensation. The structure of the balancing areas in magnetoresistive strips with the same thickness and width as in the resistance meanders is also necessary to ensure the same dependence of the resistance on the magnetic field at this point and thus also to exclude non-linearities in the characteristic curves, particularly in the case of very low measuring magnetic fields. The sensor elements are produced with a number of film planes on oxidized silicon wafers and structured by photolithography. The necessary degree of precision can only be obtained by means of electron beam guided masks, particularly for the barberpole structures with a width of only 2 pm. An exact line width must be guaranteed and an angle of +45” or -45” in relation to the longitudinal axis of the magnetoresistive strips respected over the entire chip area in order to ensure a symmetrical characteristic curve. The various films are produced by means of the following work stages: (i) Deposition of the film for the contact surfaces (on the right in Fig. 1) and for the aligning crosses (on the left in Fig. 1) with a rough surface structure in order to permit further automatic processing of the chip. (ii) Deposition of the magnetoresistive film with a protective film, the barberpole and connecting conductor layer and finally a passivation film which leaves only the contact surfaces exposed. The purpose of the protective film is to ensure good contact between the magnetoresistive film and the barberpole
765
6
a
HY [kA Im]
Fig. 3. Sensor characteristics of the sensor element shown in Fig. 1. Operating voltage 12 V. Values of the stabilization fields H,: I, 0 kA/m; 2, 2 kA/m; 3, 4 kA/ m; 4, 7 kA/m; 5, 10 kA/m.
layer, while at the same time preventing diffusion processes between the latter. The marks shown at the top and bottom of Fig. 1 are used to adjust the various masks and permit precise determination of any dimensional disparity which may have occurred. Figure 3 shows the sensor characteristic curves of an element. The magnetic field Hv is vertical in relation to the sensor structure shown in Fig. 1. Different characteristic curve rise patterns are obtained in the known manner for magnetic fields H, of different intensities (horizontal in relation to Fig. 1). Magnetic fields H, vertical to the chip surface have no influence. The dimensions of the sensor structure were chosen in such a way that with a bridge resistance of 3 kR and a stabilization field H, of 3 kA/m, a sensor sensitivity of typically 3 (mV/V) (kA/m) is obtained. The temperature coefficients are as follows: bridge resistance: 2.5 x 1O-3/K; sensor sensitivity with constant bridge voltage: -3.5 x 10P3/K;
sensor sensitivity with constant bridge current: - 1.l x 10v3/K. The hysteresis of the characteristic curves is less than 3 pV/V; disturbances of the magnetic field in the normal laboratory environment give much greater deviations. In order to determine the temperature strength of the magnetoresistive sensors, they were loaded at operating currents of 5 mA for an extended period at 200 “C in the atmosphere. In an initial phase lasting for about 40 h, the bridge resistance fell by 2 to 4% and the sensor sensitivity increased by about the same value. No further significant changes were observed during operation for a total of 800 h. The standardized offset voltage of the sensor bridge (output voltage without application of the measuring field) as a criterion for the uniformity of the four bridge resistances, changes, in the case of some components, by up to 600 ,uV/V before the first 40 h have elapsed. Subsequent changes occurring during the test period always remain below 100 /Jv/v.
3. Applications The quick reaction combined with a negligibly small hysteresis and a permissible high working temperature presents numerous forms of applications within the most varied areas for magnetoresitive sensors (MRS) : (i) Revolution counting at gear wheels. With MRS, ABS systems can be produced for the automobile industry which enable a controlled braking to occur until the vehicle comes to an absolute standstill. (ii) Two MRS could help to create an electronic compass, e.g., for intelligent traffic guiding systems in future cars. (iii) High resolution contactless distance measurement in the range from 1 pm up to more than 10 mm. (iv) Contactless angle measurement up to 360”, e.g., to detect the throttle valve angle in electric fuel injection systems. (v) Galvanically separated broadband current measurement to control and measure
766
harmonic currents, e.g., in static frequency transformers. Furthermore, this enables the wire break in, motor vehicles (control lamp) to be controlled at a favourable price. (vi) Touch-less operating joystick for personal computers or control levers for industrial machines.
References 1 W. Thomson, On the electro-dynamic qualitities of metals: effects of magnetization on the electric conductivity of nickel and iron, Proc. R. Sot. London Ser. A, 8 (1857) 546-550.
2 R. L. Hebbert and L. J. Schwee, Thin film magnetoresistance magnetometer, Rev. Sci. Instrum., 37 (1966) 1321-1323. 3 Semiconductive magnetic sensor, Catalog MURA TA, No. SG OIE-6, 1989, p. IO. 4 Packer, Diinne magnet&he Schichten und ihre Anwendung in der Speichertechnik, Fernmelde-Ing., 18 (1964) l-32. 5 K. Dibbern, Magnetic field sensors using the magnetoresistive effect, Sensors and Acruufors, 10 (1986) 127-140. 6 U. Loreit, P. Pertsch, H. Prowol and 0. Gcbgardt,
Magnetorestistive Sensoren in der Mess- und Speichertechnik, Radio Fernsehen Elektron., 34 (1985) 316-319. 7 F. Dettmann,
U. Loreit, S. Linke and P. Pertsch, Magnetoresistive Sensorelemente, Mess., Steuern,
Regeln, 31 (1988) 34X-350.
Sensors and Actuators A, 25-27
(1991) 763-766
New Magnetoresistive
Sensors: Engineering and Applications
F. ROT-I-MANN HL-Planartechnik
GmbH. Dortmund (F.R.G.)
F. DE-I-I-MANN Terhnikum
ftir Srhichttechnologie
und Sunderbuuelementr,
Sektion Physik der Uniwrsitiit Jean, Jenu (G.D.R.)
Abstract The magnetoresistive effect within thin anisotropic layers of ferromagnetic materials has been known for a long time. A new sensor layout and an appropriate selection of materials allows the hysteresis to be drastically reduced and the operation temperature to be increased up to 200 “C. Due to the short reaction times in the nanosecond field and their extreme sensitivity, magnetoresistive sensors are predestined to locate fast dynamic fields. But static magnetic fields can also be reliably measured with this type of sensor, since the resistance change is carried out in proportion to the field strength and not in proportion to the temporal change of the magnetic induction, as with inductively operated sensors. The typical applications of magnetoresistive sensors are revolution counting at gear wheels, digital or analog contactless distance measurement or remotely operating joysticks.
1. Introduction The dependence of the resistance of ferromagnetic materials on the magnetic field has been known for a long time [ 11. Practical experience has shown that the possibilities of modern thin-film and structuring technology can be used in order to produce components based on this effect which are suitable for the measurement of magnetic fields or, more specificially, for reading out information stored in magnetic materials [2]. Advantages in the 0924-4247/91/$3.50
use of such components result from their sensitivity, which is higher by a factor of 50 than that of modern Hall effect sensors [3], and from the fast reaction of the resistance of thin magnetoresistive films to field changes [4], which should permit working frequencies of the order of several 100 MHz. The problems involved in the design of magnetoresistive barberpole sensors and their various applications are described in the relevant literature [5,6]. The basic calculations for such sensors are summarized in ref. 7. This paper presents a special barberpole sensor which, as a result of its structure and the manufacturing technique employed, has substantially improved characteristics as regards its parameters and area of application. The discussion of a number of examples of use demonstrates the wide range of potential applications.
2. Structure and Manufacture of the Sensor The magnetoresistive sensor consists of a Wheatstone bridge circuit with four meander stripes. Figure 1 is a chip photograph illustrating the structure. Details of the barberpole structure can be seen more clearly in Fig. 2, which is an enlarged view of one balancing area. The chip area is 1.5 mm x 1 mm. The layout of the sensor element shows a high degree of symmetry. It is also important to ensure that only parallel strips of magnetoresistive material of identical width are used throughout, including the balancing areas. These strips are made of 0 Elsevier Sequoia/Printed
in The Netherlands
Fig. 1. Photograph of a magnetoresistlve sensor element with four resistance meanders in a bridge arrangement.
Fig. 2. Enlarged view of a balancing area of the sensor bridge.
permalloy (81% Ni, 19% Fe) and have a width of 10 pm. Identical strip dimensions and parallel alignment of all the strips are a prerequisite for low hysteresis in the sensor characteristic curve. This can, however, only be attained if, during the manufacture of the magnetoresistive film, a magnetic field of several 100 A/m, coinciding in direction with the longitudinal axis of the strips, ensures that a uniform anisotropic field strength is impressed on the entire area of the chip. The symmetry of the arrangement with respect. to the x-axis theoretically running through the centre of the component has the advantage that parasitic fields dispersed inductively or capacitively in the bridge are self-cancelling within the sensor bridge, and
so make no contributioh to the output signal. In normal sensor operation, the heat generated on the chip is not insignificant and the temperature coefficient of resistance of the magnetoresistive film strip is also not inconsiderable. This symmetry is therefore also necessary to prevent the occurrence of components of the output voltage which may vary as a function of the operating voltage or current. Results have shown that the slightly offset position of the balancing area located in the bottom left has no influence. It is, however, required in order to increase the permitted tolerance of the approach point for laser compensation. The structure of the balancing areas in magnetoresistive strips with the same thickness and width as in the resistance meanders is also necessary to ensure the same dependence of the resistance on the magnetic field at this point and thus also to exclude non-linearities in the characteristic curves, particularly in the case of very low measuring magnetic fields. The sensor elements are produced with a number of film planes on oxidized silicon wafers and structured by photolithography. The necessary degree of precision can only be obtained by means of electron beam guided masks, particularly for the barberpole structures with a width of only 2 pm. An exact line width must be guaranteed and an angle of +45” or -45” in relation to the longitudinal axis of the magnetoresistive strips respected over the entire chip area in order to ensure a symmetrical characteristic curve. The various films are produced by means of the following work stages: (i) Deposition of the film for the contact surfaces (on the right in Fig. 1) and for the aligning crosses (on the left in Fig. 1) with a rough surface structure in order to permit further automatic processing of the chip. (ii) Deposition of the magnetoresistive film with a protective film, the barberpole and connecting conductor layer and finally a passivation film which leaves only the contact surfaces exposed. The purpose of the protective film is to ensure good contact between the magnetoresistive film and the barberpole
765
6
a
HY [kA Im]
Fig. 3. Sensor characteristics of the sensor element shown in Fig. 1. Operating voltage 12 V. Values of the stabilization fields H,: I, 0 kA/m; 2, 2 kA/m; 3, 4 kA/ m; 4, 7 kA/m; 5, 10 kA/m.
layer, while at the same time preventing diffusion processes between the latter. The marks shown at the top and bottom of Fig. 1 are used to adjust the various masks and permit precise determination of any dimensional disparity which may have occurred. Figure 3 shows the sensor characteristic curves of an element. The magnetic field Hv is vertical in relation to the sensor structure shown in Fig. 1. Different characteristic curve rise patterns are obtained in the known manner for magnetic fields H, of different intensities (horizontal in relation to Fig. 1). Magnetic fields H, vertical to the chip surface have no influence. The dimensions of the sensor structure were chosen in such a way that with a bridge resistance of 3 kR and a stabilization field H, of 3 kA/m, a sensor sensitivity of typically 3 (mV/V) (kA/m) is obtained. The temperature coefficients are as follows: bridge resistance: 2.5 x 1O-3/K; sensor sensitivity with constant bridge voltage: -3.5 x 10P3/K;
sensor sensitivity with constant bridge current: - 1.l x 10v3/K. The hysteresis of the characteristic curves is less than 3 pV/V; disturbances of the magnetic field in the normal laboratory environment give much greater deviations. In order to determine the temperature strength of the magnetoresistive sensors, they were loaded at operating currents of 5 mA for an extended period at 200 “C in the atmosphere. In an initial phase lasting for about 40 h, the bridge resistance fell by 2 to 4% and the sensor sensitivity increased by about the same value. No further significant changes were observed during operation for a total of 800 h. The standardized offset voltage of the sensor bridge (output voltage without application of the measuring field) as a criterion for the uniformity of the four bridge resistances, changes, in the case of some components, by up to 600 ,uV/V before the first 40 h have elapsed. Subsequent changes occurring during the test period always remain below 100 /Jv/v.
3. Applications The quick reaction combined with a negligibly small hysteresis and a permissible high working temperature presents numerous forms of applications within the most varied areas for magnetoresitive sensors (MRS) : (i) Revolution counting at gear wheels. With MRS, ABS systems can be produced for the automobile industry which enable a controlled braking to occur until the vehicle comes to an absolute standstill. (ii) Two MRS could help to create an electronic compass, e.g., for intelligent traffic guiding systems in future cars. (iii) High resolution contactless distance measurement in the range from 1 pm up to more than 10 mm. (iv) Contactless angle measurement up to 360”, e.g., to detect the throttle valve angle in electric fuel injection systems. (v) Galvanically separated broadband current measurement to control and measure
766
harmonic currents, e.g., in static frequency transformers. Furthermore, this enables the wire break in, motor vehicles (control lamp) to be controlled at a favourable price. (vi) Touch-less operating joystick for personal computers or control levers for industrial machines.
References 1 W. Thomson, On the electro-dynamic qualitities of metals: effects of magnetization on the electric conductivity of nickel and iron, Proc. R. Sot. London Ser. A, 8 (1857) 546-550.
2 R. L. Hebbert and L. J. Schwee, Thin film magnetoresistance magnetometer, Rev. Sci. Instrum., 37 (1966) 1321-1323. 3 Semiconductive magnetic sensor, Catalog MURA TA, No. SG OIE-6, 1989, p. IO. 4 Packer, Diinne magnet&he Schichten und ihre Anwendung in der Speichertechnik, Fernmelde-Ing., 18 (1964) l-32. 5 K. Dibbern, Magnetic field sensors using the magnetoresistive effect, Sensors and Acruufors, 10 (1986) 127-140. 6 U. Loreit, P. Pertsch, H. Prowol and 0. Gcbgardt,
Magnetorestistive Sensoren in der Mess- und Speichertechnik, Radio Fernsehen Elektron., 34 (1985) 316-319. 7 F. Dettmann,
U. Loreit, S. Linke and P. Pertsch, Magnetoresistive Sensorelemente, Mess., Steuern,
Regeln, 31 (1988) 34X-350.