JP2009085646A - Magnetic field strength sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic sensor, capable of high-sensitivity and high-accuracy evaluation of the strength of unidirectional magnetic field, even between narrow magnetic poles, using an MR element without having to make its constitution complex. <P>SOLUTION: A magnetic field strength sensor for measuring the strength of a unidirectional magnetic field, by detecting changes in a resistance value of a magneto-resistive membrane is provided with both a magnetoresistive membrane element in which a substantially rectangular electrically conductive pattern having flat upper and lower parts of a magneto-resistive membrane is formed in a substrate and a holding means for holding the magnetoresistive membrane element with its membrane surface rotated by a prescribed angle to a vertical surface in the direction of the magnetic field on a straight line, extending in the vertical directions at right angles with respect to the long side of the substantially rectangular pattern as an axis. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a sensor using a magnetoresistive thin film element (hereinafter referred to as an MR element), and is particularly effective when applied to the evaluation of magnetic field strength above geomagnetism in magnetic measurement, and also suitable for measuring DC current and the like. It relates to technologies that can be applied.

The magnetic field strength is evaluated using a magnetic sensor capable of quantitatively detecting the magnetic field strength, and a magnetoresistive thin film element or a Hall element is widely used as the magnetic sensor. In this case, the MR element is mainly used for evaluation of a small magnetic field intensity of 800 A / m or less, such as geomagnetic field or magnetic field leakage magnetic field evaluation (see Patent Document 1). In addition, Hall elements are mainly used in magnetic field strength evaluation of about 8000 A / m or more such as electromagnetic actuators (see Patent Document 2).
JP-A-9-145375 JP-A-7-22667

  The Hall element is configured by using a compound semiconductor substrate such as GaAs, and generates an electromotive force, that is, a so-called Hall voltage that is substantially proportional to the magnetic field intensity under a constant current supply. The magnetic field strength can be quantitatively detected from the Hall voltage.

  However, a compound semiconductor such as GaAs used for the Hall element is a very fragile material, and it is necessary to form a protective film of several hundreds μm or more on the element surface. Therefore, the Hall element cannot be flexed flexibly. Moreover, since the thickness of the element itself including the protective film is 500 μm or more, it is difficult to measure a magnetic field between very narrow magnetic poles such as a micro electromagnetic actuator.

  Further, since the Hall element is a four-terminal active element and requires current supply, there is a problem that the configuration of the sensor unit and the measurement circuit is likely to be complicated. There is also a problem in temperature characteristics, and the maximum heat-resistant temperature is about 125 ° C. even in a high-temperature type InAs element, so that the use environment is greatly restricted.

  On the other hand, an MR element is formed by forming a conductive pattern of a magnetoresistive thin film on a substrate, and detects the magnetic field intensity from the change in the resistance value of the magnetoresistive thin film. The circuit configuration is simple, thin and flexible. Since it can be formed into a simple substrate shape, the degree of freedom in shape is high. For this reason, there exists an advantage that the structure of a sensor part and a measurement circuit can be simplified compared with a Hall element.

  Furthermore, although an alloy thin film made of a ferromagnetic element such as Ni, Fe, Co or the like can be used for the magnetoresistive thin film, its Curie temperature is high and exhibits a Curie temperature of 400 ° C. or higher. Therefore, it is easy to obtain one having excellent temperature characteristics and heat resistance, and there is an advantage that it can be used even in an environment where the use of the Hall element is difficult.

  However, this MR element has a problem that the linearity of the resistance value change with respect to the magnetic field strength is poor. Therefore, the magnetic field strength measurement requiring high accuracy is greatly inferior to the Hall element. There is also a problem that the sensitivity as a magnetic sensor is low and it is not suitable for evaluation of a large magnetic field strength.

  As a method for increasing the detection sensitivity of the MR element, a method of applying a magnetic bias has been proposed. In this case, the configuration is greatly complicated to apply the magnetic bias, and the advantage of using the MR element is lost. It will be.

  The present invention has been made in view of the background as described above. The object of the present invention is to increase the strength of a unidirectional magnetic field with high sensitivity even between narrow magnetic poles using MR elements without complicating the configuration. The object is to provide a magnetic sensor that can be evaluated with high accuracy.

The present invention for achieving the above object is a magnetic field strength sensor for measuring the strength of a unidirectional magnetic field by detecting a change in resistance value of a magnetoresistive thin film,
A magnetoresistive thin film element formed by forming a substantially rectangular conductive pattern with a flat top and bottom by a magnetoresistive thin film on a substrate, and a straight line extending in the vertical direction perpendicular to the long side of the substantially rectangular pattern Holding means for holding the film surface of the magnetoresistive thin film element in a state rotated by a predetermined angle with respect to the vertical surface in the magnetic field direction;
It is characterized by having.

  Preferably, the holding means is a magnetic field intensity sensor that holds the substrate surface in a state of being rotated at an angle of 5 degrees to 60 degrees with respect to a vertical plane in the magnetic field direction.

  Without complicating the structure, the MR element can be used to evaluate the strength of the unidirectional magnetic field with high sensitivity and high accuracy. The operations / effects other than the above will be clarified in the description of the present specification and the accompanying drawings.

=== Basic structure of magnetic sensor ===
The magnetic field strength sensor according to the present invention is mainly composed of an MR element formed by patterning a magnetoresistive thin film on a substrate surface, and measures the strength of a unidirectional magnetic field by detecting a change in the resistance value of the magnetoresistive thin film. It is. FIG. 1 is a plan view showing an example of the MR element 10 constituting the magnetic field strength sensor in the embodiment of the present invention. In the following, the left-right direction (one-dot chain line arrow) and the vertical direction (two-dot chain line arrow) of the MR element 10 are defined based on FIG. In the MR element 10, a conductive pattern made of a magnetoresistive thin film 12 is formed in a vertically flat rectangular shape, that is, a horizontally long strip shape, on a nonmagnetic, insulating thin element substrate 11. The magnetoresistive thin film 12 is formed by sputtering and patterned into a strip-shaped conductive path by a photolithography technique. As the magnetoresistive thin film 12, an alloy thin film mainly made of a ferromagnetic element such as Ni, Fe, or Co is used. This type of alloy thin film has a very small magnetostriction constant and a high Curie temperature of 400 ° C. or higher. In this embodiment, a polyimide film having a thickness of 125 μm is used as a substrate, and a resist film is formed on the substrate 11 by removing a pattern in a horizontally long strip shape by photolithography. (100 nm thickness) is laminated by sputtering without a magnetic field to form a magnetoresistive thin film. Then, the resist is removed to obtain a strip-shaped magnetoresistive thin film. In this embodiment, the strip-shaped pattern is formed with a size of width w = 2 mm and height h = 20 μm.

  Further, two terminals for connecting the rectangular terminal pads 13 disposed symmetrically below the element substrate 11 and the terminal pad portions 13 from the strip-shaped magnetoresistive thin film 12 by photolithography and sputtering. Terminal wiring 14 is formed. The two terminal wires 14 are slightly spaced apart and extend symmetrically in the vertical direction, with the upper end traversing the strip-shaped magnetoresistive thin film and projecting upward from the thin film, and the lower end in the left-right direction. Each is bent and connected to the inside upper side of each terminal pad. In the present embodiment, the inter-terminal distance L between the upper and lower extensions of the two terminal wirings 14 is 100 μm.

  Then, the wafer is cut into a desired shape and size by dicing, and then heat-treated in a vacuum at 300 ° C. for 1 hour to complete the MR element 10. It is assumed that the top and bottom and left and right sides of the element substrate 11 are parallel to the top and bottom and left and right sides of the strip-shaped magnetoresistive thin film 12.

  In the magnetic field strength sensor of the present invention, the direction of the magnetic field to be measured and the substrate surface on which the pattern 12 of the MR element 10 is formed are not orthogonal to each other. It is characterized in that it is held in a state where it is rotated by a predetermined angle with a direction parallel to the side 15 (vertical extension direction) as an axis. FIG. 2 shows the positional relationship between the magnetic field to be evaluated by the magnetic field intensity sensor and the MR element 10. (A) is a plan view when the MR element is viewed from above, and (B) is a perspective view. A line (two-dot chain line) parallel to the extending direction of the short side 15 of the strip-shaped pattern, that is, a vertical line is an axis z, and the film surface (element substrate surface) P2 of the magnetoresistive thin film 12 is directed to the magnetic field direction Mx. The rotation angle is an angle θ between the magnetic field direction Mx and the normal line N of the element substrate surface P2. That is, the element substrate surface P2 is inclined at an angle θ with respect to the vertical plane P1 orthogonal to the magnetic field direction Mx. In the drawing, a vertical line that divides the element substrate 11 left and right symmetrically and bisects in the thickness direction is shown as an axis z.

  FIG. 3 shows the magnetic sensor of this example. (A) is the top view, (B) is a perspective view from the upper left. The magnetic sensor 1 includes an MR element 10, a lead on which a lead wiring 21 for holding and outputting the resistance value between the terminal pads 13 to an external circuit such as an external measuring device is printed and printed. A substrate 20 and holding means 40 for inclining the element substrate surface P2 of the MR element 10 at a predetermined angle with respect to the measured magnetic field direction Mx are provided.

  In this embodiment, the lead substrate 20 is made of a polyimide film that is a high heat-resistant non-magnetic insulating material, has a long plate shape with substantially the same width as the substrate 11 of the MR element 10, and retains its shape. And has flexibility. That is, it has a long thin leaf spring shape. The MR element 20 is mounted and held on the tip of the plate by bonding or the like.

  Four lead wires 21 are printed on the lead substrate 20. An upper end portion 22 of the lead wiring 21 is connected to the terminal pad portion 13 through a bonding wire 30. A lower base end portion 23 of the lead wiring 21 is connected to an external circuit via a cable or the like. In the present embodiment, two pads are connected from each pad portion to the distal end portion 22 of the lead wiring 21 so as to measure the resistance value by the four-terminal method.

  The holding means is a wedge-shaped spacer member 40 interposed between the element substrate 11 and the lead substrate 21, and the wedge is inclined from one side to the other side of the element substrate 11. Intervened. Therefore, by appropriately setting the height of the spacer member 40, the MR element surface P2 can be held in a state of being rotated at a predetermined inclination angle about the axis z extending in the vertical direction with respect to the magnetic field direction. it can. The magnetic sensor of the present embodiment having the above configuration is installed so that the holding surface of the MR element 10 on the lead substrate 20 coincides with the vertical plane P1 in the measured magnetic field direction Mx.

=== Characteristic evaluation ===
<Evaluation method>
About the magnetic sensor 1 of a present Example, resistance value with respect to magnetic field intensity is measured by using rotation angle (theta) with respect to the magnetic field direction Mx as a parameter. FIG. 4 shows an outline of an apparatus for measuring the magnetoresistance change characteristic of the magnetic sensor 1. The measuring apparatus 100 is provided with a processed cylindrical rod-like probe 101, and the magnetic sensor 1 is attached to the tip of the probe 101. (A) is a block diagram of the measuring apparatus 100, and (B) is an enlarged view of the probe 101 with the magnetic sensor 1 attached thereto. The probe 101 has a cylindrical rod-shaped tip cut out with a flat diameter, and the magnetic sensor 1 is placed and fixed on the flat portion 102. The magnetic sensor 1 for characteristic evaluation used here has a configuration in which a wedge-shaped spacer member 40 is not interposed between the MR element 10 and the lead substrate 20, and the probe 101 is rotatable about the cylindrical axis Zp. It has become. Thereby, the MR element surface of the magnetic sensor can be inclined at an arbitrary angle with respect to the magnetic field direction.

  The measurement apparatus main body 110 includes an angle detection circuit 103 that detects a rotation angle of the probe 101, that is, an inclination angle of the substrate surface P2 of the MR element 10 with respect to the measurement magnetic field direction Mx, and a resistance measurement circuit 104 that measures the magnetic resistance of the MR element 10. The data conversion circuit 105 converts the measured magnetic resistance into magnetic field strength, and the output circuit 106 outputs the magnetic field strength converted from the magnetic resistance to a display or the like.

<Evaluation results>
The magnetic field strength sensor 1 of the present invention detects the change in the resistance value of the MR element 10 to quantitatively detect the strength of the unidirectional magnetic field Mx, and the magnetoresistive thin film surface (element substrate surface) P2 is placed on a vertical surface in the magnetic field direction ( The change in the resistance value can be detected in a state inclined by a predetermined angle θ with respect to P1). The results of evaluating the characteristics of the magnetic sensor 1 of this example using the measuring apparatus 100 are shown in graphs in FIGS. In the evaluation method, the resistance of the MR element 10 was detected by a four-terminal method while reciprocating a unidirectional magnetic field from −1100 kA / m to +1100 kA / m. The graphs shown in FIGS. 5 to 11 show the magnetic properties of the MR element 10 with respect to changes in magnetic field strength when the inclination angle θ is 0 °, 5 °, 10 °, 30 °, 45 °, 60 °, and 75 °, respectively. The resistance change state is shown. When the tilt angle θ is 0 ° (FIG. 5), the magnetoresistive thin film is parallel to the vertical plane P1 in the magnetic field direction Mx. In this case, the applied magnetic field strength is ± 300 kA / m or less, that is, in one direction. When the magnetic field is −300 kA / m or more and +300 kA / m or less, disorder is observed in the change in magnetoresistance. For this reason, the sensitivity of magnetic field strength detection is reduced in the region where the magnetoresistance change is disturbed, and the detection accuracy is deteriorated in the hysteresis region.

  Furthermore, a sharp bending of the resistance change curve occurs when the strength of the applied magnetic field is around ± 500 kA / m. For this reason, when the strength of the applied magnetic field is ± 500 kA / or more, there is a problem that the linearity of the resistance value change with respect to the magnetic field strength is poor and high-precision detection cannot be performed. That is, the range of magnetic field intensity that can be measured with high accuracy is narrow.

  However, when the magnetoresistive thin film surface P2 is tilted with respect to the vertical plane P1 in the magnetic field direction, that is, when the tilt angle θ is set to a significant angle larger than zero, as shown in FIGS. The linearity of the magnetoresistance change in the low strength region is improved, and the sharp bending of the resistance change curve is eliminated or alleviated, and the magnetoresistance change becomes linear in a wide magnetic strength range. This has been found to significantly improve both detection sensitivity and accuracy.

  In the case of 5 ° ≦ θ ≦ 60 ° (FIGS. 5 to 10), the change rate of the resistance value between the two points when the magnetic field strength is 0 kA / m and 1000 kA / m is about 0.5% or more. Yes, for example, in an application for evaluating the magnetic field of a general electromagnetic actuator, the resistance change can be read sufficiently. However, in the case of θ = 75 ° (FIG. 11), the change rate of the resistance value when the absolute value of the magnetic field strength is 0 kA / m and 1000 kA / m is 0.2% or less, and It is not suitable for use in evaluating magnetic fields. Therefore, it is desirable to satisfy 5 ≦ θ ≦ 60 for applications that require more accurate evaluation.

=== Features of the Invention ===
In the MR element, for example, even when a magnetoresistive thin film having a very small magnetostriction constant of 10 ppm or less is used, such as a Ni80Fe20 alloy thin film, anisotropic dispersion occurs due to the change in magnetization of the thin film due to strain. This anisotropic dispersion is considered to cause disturbance in the magnetoresistance change. However, from the above evaluation results, it has been found that the influence of this anisotropic dispersion is reduced by inclining the magnetoresistive thin film surface with respect to the vertical plane in the magnetic field direction. Furthermore, it has been found that in order to eliminate the influence of the anisotropic dispersion, it is more desirable that the tilt angle θ is in a range of 5 ° ≦ θ ≦ 60 °.

  The present invention is a magnetic field intensity sensor using an MR element having high heat resistance, and can solve the problems related to heat resistance in the Hall element by forming the MR element on a substrate having heat resistance. An oxide film silicon wafer or the like can be used as the element substrate, but a polyimide film can be used as in the above embodiment. The polyimide film has a heat-resistant temperature of 350 ° C., and not only has a high heat-resistant temperature like the oxide silicon wafer, but also has a shape retaining property and flexibility, and has a high degree of flexibility in bending and the like. Therefore, by using a polyimide film for the substrate of the MR element, it is combined with the heat resistance of 400 ° C. or more in the metal or alloy constituting the magnetoresistive thin film, and 150 ° C. or more which is the limit of the magnetic field strength sensor using the Hall element. There is an advantage that a magnetic sensor capable of sufficiently achieving heat resistance can be configured.

  Further, in the Hall element using a fragile compound semiconductor, it was essential to firmly maintain and fix the shape of the element, whereas in the magnetic field strength sensor of the present invention, a magnetoresistive thin film that can be bent is used. Therefore, the magnetoresistive thin film can be formed on the element substrate having shape retention and flexibility. Thereby, for example, even when the space part for measuring the unidirectional magnetic field strength is narrow or irregularly shaped, the MR element can be bent and inserted into the measurement space part to perform measurement. It is done.

  In addition, the magnetoresistance change characteristic is linear in a wide magnetic intensity range. Therefore, if the magnetic field strength sensor of the present invention is installed in a specific magnetic field so as to be rotatable with respect to the vertical plane of the magnetic field, the resistance value corresponding to the rotation angle of the magnetic field strength sensor can be accurately detected. For example, it can be used as a highly accurate angle sensor. As mentioned above, although this invention was demonstrated based on the typical Example, this invention can have various aspects other than having mentioned above. For example, the size of the magnetoresistive thin film in the MR element can be appropriately set according to the magnetic field to be measured. For example, when measuring the magnetic field strength in a narrow region, the pattern should be a flatter thin line pattern. Is also possible. Further, the holding means 30 of the MR element 10 can take various forms other than the form using the spacer member 40 described above.

It is a top view of the magnetoresistive thin film element which comprises the magnetic field intensity sensor in the Example of this invention. It is a figure which shows typically the installation state of a magnetoresistive thin film element when measuring a magnetic field intensity, and is a top view when the angle of the film surface of a magnetoresistive thin film element and a measurement magnetic field direction is set to (theta) in an installation state (A And a perspective view (B). It is the top view (A) and perspective view (B) of the said magnetic field intensity sensor. It is a figure which shows schematic structure of a magnetic intensity sensor and a measuring apparatus. It is a graph which shows the change of magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 0 degree. It is a graph which shows the change of the magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 5 degree. It is a graph which shows the change of magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 10 degrees. It is a graph which shows the change of magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 30 degrees. It is a graph which shows the change of the magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 45 degrees. It is a graph which shows the change of magnetic resistance with respect to the magnetic field intensity at the time of (theta) = 60 degrees. It is a graph which shows the change of magnetoresistance with respect to the magnetic field intensity at the time of (theta) = 75 degree.

Explanation of symbols

1 Magnetic field strength sensor 10 Magnetoresistive thin film element (MR element)
11 Element substrate 12 Magnetoresistive thin film 13 Terminal pad portion 20 Lead substrate (holding jig)
21 Lead wiring 30 Bonding wire 40 Spacer member 100 Measuring device Mx Magnetic field θ Inclination angle

Claims (2)

A magnetic field strength sensor that measures the strength of a unidirectional magnetic field by detecting a change in resistance value of a magnetoresistive thin film,
A magnetoresistive thin film element formed by forming a substantially rectangular conductive pattern with a flat top and bottom by a magnetoresistive thin film on a substrate;
Holding means for holding the film surface of the magnetoresistive thin film element rotated by a predetermined angle with respect to a vertical surface in the magnetic field direction, with a straight line extending in the vertical direction perpendicular to the long side of the substantially rectangular pattern as an axis When,
A magnetic field strength sensor comprising:   2. The magnetic field strength sensor according to claim 1, wherein the holding means holds the substrate surface in a state where the substrate surface is rotated at an angle of 5 degrees to 60 degrees with respect to a vertical plane in the magnetic field direction.

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