JP2010145147A - Magnetic sensor element and magnetic sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized magnetic sensor having a small power consumption. <P>SOLUTION: When a magnetic field is applied to a magnetic sensor element 10, a magnetostrictive material plate 2 is elongated, or shrunk when a magnetostriction constant is minus, to thereby generate a strain in a piezoelectric body flat plate 3, and an interval S between comb teeth 41A, 41B in a propagation direction of an elastic surface wave of an SAW (Surface Acoustic Wave) element 1 is changed, and a resonance frequency f of the SAW element 1 is changed. Consequently, a change of the magnetic field is converted into a change of a frequency, and detection of the change of the magnetic field becomes possible. The SAW element 1 is constituted by forming an electrode on the piezoelectric body flat plate 3, and has a small size, and can reduce a power consumption. Further, the magnetostrictive material plate 2 can be miniaturized corresponding to the SAW element 1. Consequently, the small-sized magnetic sensor element 10 having a small power consumption can be obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic sensor element for detecting a magnetic field such as geomagnetism and a magnetic sensor using the same.

  A fluxgate sensor is known as a magnetic sensor. A fluxgate sensor has an excitation coil and a detection coil wound around a core of magnetic material, has high detection sensitivity, a wide range of detectable magnetic fields, and high resolution (see, for example, Patent Document 1). Patent Document 1 relates to a direct current sensor, but describes the basics of a fluxgate sensor.

JP 7-128373 A (page 3, FIG. 1)

  However, since the coil is wound around the core, it is difficult to reduce the size. In addition, since it is necessary to excite with a current enough to saturate the core, power consumption increases.

  The present invention has been made to solve the above-described problems, and can be realized as the following forms or application examples.

[Application Example 1]
A magnetic sensor element comprising: a SAW element including a piezoelectric body; and a magnetostrictive material that imparts strain to the propagation direction of the surface acoustic wave of the SAW element.

According to this application example, when a magnetic field is applied to the magnetic sensor element, the magnetostrictive material expands or contracts when the magnetostriction constant is negative, causing the piezoelectric body to be distorted, and the SAW element surface acoustic wave propagation direction. The distance between the electrodes changes, and the resonance frequency of the SAW element changes. Therefore, the change in the magnetic field is converted into the change in the frequency, and the change in the magnetic field can be detected.
The SAW element is formed by forming electrodes on the surface of the piezoelectric body, and is small and consumes little power. Also, the magnetostrictive material may be small depending on the SAW element. Therefore, a small magnetic sensor element with low power consumption can be obtained.

[Application Example 2]
The magnetic sensor element according to claim 1, wherein the magnetostrictive material is bonded to the piezoelectric body.
In this application example, since the magnetostrictive material is bonded and integrated with the piezoelectric body, a smaller magnetic sensor element can be obtained.

[Application Example 3]
The magnetic sensor element, wherein the magnetostrictive material is disposed between the piezoelectric body and a support member.
In this application example, the magnetostrictive material is disposed between the support member and the piezoelectric body so as to give strain to the propagation direction of the surface acoustic wave. The strain generated in the magnetostrictive material applies a force to the piezoelectric body with the support member as a support, and the piezoelectric body is distorted. Therefore, a magnetic sensor element having the above-described effect can be obtained.

[Application Example 4]
A magnetic sensor comprising: the magnetic sensor element described above; a container for housing the magnetic sensor element; and an oscillation circuit.

  According to this application example, a magnetic sensor having the above-described effects can be obtained.

[Application Example 5]
The magnetic sensor according to claim 1, further comprising a bias magnetic field applying unit that applies a bias magnetic field in the propagation direction of the magnetostrictive material.
In this application example, a bias magnetic field can be applied to the magnetostrictive material by the bias magnetic field application unit. Magnetostrictive materials have a nonlinear magnetostriction constant relative to the strength of the magnetic field when the magnetic field is near zero, and the magnetostriction constant is small and the sensitivity of the magnetic sensor decreases when the magnetic field is near zero. The magnetostriction constant with respect to reaches a linear region, and a magnetic sensor with improved sensitivity can be obtained.

Hereinafter, embodiments will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 shows a schematic diagram of a magnetic sensor element 10 in the present embodiment. (A) is a schematic plan view of the magnetic sensor element 10, (b) is a schematic sectional view, and (c) is a schematic bottom view.

In FIG. 1, a magnetic sensor element 10 includes a SAW (Surface Acoustic Wave) element 1 and a magnetostrictive material plate 2 as a magnetostrictive material.
The SAW element 1 includes a piezoelectric plate 3 as a piezoelectric body, comb electrodes 4A and 4B, take-out electrodes 5A and 5B, and reflecting electrodes 6A and 6B formed on one surface of the piezoelectric plate 3.

The comb electrodes 4A and 4B include comb teeth 41A and 41B and base portions 42A and 42B that connect the comb teeth 41A and 41B. Further, the comb electrodes 4A and 4B are formed such that the comb teeth 41A and the comb teeth 41B alternately enter between the respective teeth. Furthermore, extraction electrodes 5A and 5B are formed on the respective comb electrodes 4A and 4B.
In this embodiment, the extraction electrodes 5A and 5B are formed on the same surface at a position different from the comb electrodes 4A and 4B. However, the base portions 42A and 42B may be used as the extraction electrodes, or the piezoelectric electrodes. A hole may be provided in the body flat plate 3, and an extraction electrode connected to the base portions 42A and 42B through the hole from the back surface may be formed.
The reflection electrodes 6A and 6B are in a lattice shape and are formed on both sides of the comb electrodes 4A and 4B in the direction in which the comb teeth 41A and 41B are arranged.

The comb-tooth electrodes 4A and 4B, the extraction electrodes 5A and 5B, and the reflection electrodes 6A and 6B are vapor-deposited by sputtering a metal conductor such as Al, Au, Ag, and Cn on the surface of the piezoelectric flat plate 3 that has been subjected to mirror polishing. It is obtained by forming a fine pattern using a method such as photolithography after forming by a film forming means such as photolithography.
In the SAW element 1, the distance S between the comb teeth 41A and 41B is substantially the same as the wavelength λ of the generated surface acoustic wave. If the ratio L / S between the width L of the comb teeth 41A and 41 and the interval S between the comb teeth 41A and 41B is approximately 1, surface acoustic waves are likely to occur.

  In the SAW element 1, surface acoustic waves are generated in the direction in which the comb teeth 41A and 41B of the piezoelectric plate 3 are arranged by the comb electrodes 4A and 4B, and the surface acoustic waves reflected by the reflective electrodes 6A and 6B are detected. It can be carried out. Therefore, the direction in which the comb teeth 41A and 41B are arranged is the propagation direction D of the surface acoustic wave. The propagation direction D is indicated by an arrow in the figure.

A surface acoustic wave is a wave in which energy concentrates and propagates on the surface of an elastic body in the same manner as a wave traveling on the water surface, and its amplitude attenuates exponentially with respect to the depth direction.
The frequency f of the surface acoustic wave is given by f = V / λ where V is the propagation velocity of the surface acoustic wave and the wavelength λ of the surface acoustic wave. The propagation speed of the surface acoustic wave has a different value depending on the piezoelectric flat plate 3. For example, in the case of ST cut quartz, it is about 3,100 m / sec. The most suitable frequency band for the SAW element 1 is about several tens of MHz to 3 GHz, but the upper and lower limits thereof are determined by the resolution of the fine electrode pattern of the comb teeth 41A and 41B and the characteristics and size of the piezoelectric plate 3. The

The piezoelectric flat plate 3 is a rectangular parallelepiped plate, and the surface of the surface on which the electrodes are formed is subjected to mirror polishing. The piezoelectric flat plate 3 includes quartz (SiO ₂ ), langasite (La ₃ Ga ₅ SiO ₁₄ ), lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), and barium titanate (BaTiO ₃ ) which are ferroelectric materials. ₃ ), lead titanate (PbTiO ₃ ), lead zirconate titanate, lithium tetraborate (Li ₂ B ₄ O ₇ ), diamond and the like are used.
Quartz is superior in that it has a small temperature coefficient. Diamond has a high propagation speed and is used in a high frequency band. Lithium niobate and the like have a large electromechanical coupling coefficient k ² of ˜5%, and the generation efficiency of surface acoustic waves is good.
Further, if the thickness H1 of the piezoelectric flat plate 3 is 1.5 to 2.0 times the wavelength λ of the surface acoustic wave, the influence on the surface acoustic wave is small.

Hereinafter, the case where the piezoelectric plate 3 is quartz will be mainly described.
In FIG. 1 (b), the electrode film thickness H2 varies depending on the material of the piezoelectric plate 3 and the cut angle orientation. In the case of quartz ST cut and LST cut, if the electrode metal is mainly Al, the electrode film The ratio (H2 / λ) of the thickness H2 to the surface acoustic wave wavelength λ is preferably set to about 0.01 to 0.03.
At this time, the cutting angle of the ST cut and the cutting direction of the LST cut are set so that the first-order temperature coefficient of the frequency temperature characteristic is zero near room temperature. In ST cutting, the crystal optical axis is 30 ° counterclockwise. A Y plate rotated about ˜35 ° + X axis is used, and in LST cutting, a mechanical axis of quartz is a Z plate rotated about 9-15 ° + X axis counterclockwise.

  Table 1 shows the amorphous magnetic materials used for the magnetostrictive material plate 2. Table 1 shows the composition and magnetostriction of the amorphous magnetic material, but the magnetostrictive material that can be used is not limited to these, and may be anisotropic or have a negative magnetostriction constant. May be.

The shape of the magnetostrictive material plate 2 is a rectangular parallelepiped plate shape, and the widest surface of the magnetostrictive material plate 2 is bonded to the back surface of the surface on which the electrodes of the piezoelectric flat plate 1 are formed. The bonding position of the magnetostrictive material plate 2 is preferably a position where the long side of the magnetostrictive material plate 2 is parallel to the propagation direction D of the surface acoustic wave.
When the magnetostrictive material has anisotropy, it is preferable to bond the direction in which the magnetostriction is large and the propagation direction D of the surface acoustic wave together. Even if the direction of large magnetostriction and the propagation direction D of the surface acoustic wave are shifted, the direction of large magnetostriction has a component of the propagation direction D of the surface acoustic wave and is distorted with respect to the propagation direction D of the surface acoustic wave. Should be given.

The thickness H3 of the magnetostrictive material plate 2 only needs to be thick enough to bend the piezoelectric plate 3 compared to the thickness H1 of the piezoelectric plate 3.
Further, the surface of the magnetostrictive material plate 2 is preferably smooth with few irregularities, like the piezoelectric plate 1. The thin adhesive layer can ensure sufficient adhesion, and as a result, sufficient adhesive strength can be obtained.

A method for bonding the magnetostrictive material plate 2 to the piezoelectric flat plate 3 will be described below.
First, when the magnetostrictive material is an amorphous material, the surface roughness depends on the surface state of the rotating roll of the quenching apparatus used for manufacturing. In the case where the surface roughness Ra is 5 μm or more, the surface roughness Ra is finished to about 1 μm by polishing using a polishing paper or the like so as not to give a nonuniform stress as much as possible. In this case, only the surface to which the magnetostrictive material plate 2 is bonded needs to be polished. This is because if the adhesive application thickness is thinner than the surface roughness, the adhesion area cannot be secured.

  Adhesives are organic such as epoxy resin, acrylic resin, olefin resin, urethane resin, vinyl chloride resin, modified silicon, polyamide resin, polyimide, polybenzimidazole, and polymethacrylate resin. An inorganic adhesive such as a silica adhesive or a ceramic adhesive can be used.

As a method of controlling the coating thickness of the adhesive, an apparatus that transfers the adhesive from one roller to another with a plurality of rollers, the adhesive that adheres to the rollers is made thinner and thinner until the desired thickness is reached. Then, there is a method of transferring and applying an adhesive to an object to be bonded. In this method, the number of times of transfer by the roller and the transfer pressure are adjusted according to the viscosity of the adhesive.
There is also a method of thinning by transferring an adhesive using a tape or the like instead of a roller. For example, when transferring from a roller to a tape, the applied thickness of the adhesive that was first attached to the roller is separated to the tape side and the roller side, so that the thickness is about half. Further, the adhesive attached to the tape is halved by transferring it to the next tape. By repeating this, a desired coating thickness can be obtained.

FIG. 2 shows a schematic cross-sectional view of the magnetic sensor 100 in the present embodiment. In the figure, an example of a direction in which a magnetic field is applied is indicated by a thick arrow.
The magnetic sensor 100 includes a magnetic sensor element 10, a container 20, and an oscillation circuit 30. The magnetic sensor element 10 is supported by a support base 21 and stored in a container 20.
The container 20 is made of ceramic or Kovar and includes a main body 22 and a lid 23. The main body 22 includes electrodes 24A and 24B for soldering and mounting the container 20 on the bottom.

The main body 22 has a two-layer structure in order to connect the internal electrodes and the electrodes 24A and 24B.
Specifically, in order to electrically connect the electrodes 25A and 25B electrically connected to the electrodes of the magnetic sensor element 10 by wires or the like and the electrodes 24A and 24B, the main body 22 has holes 26A and 26B. The holes 26A and 26B are filled with conductive substances 27A and 27B.

Further, in order to minimize the change with time of the magnetic sensor element 10, the inside of the container 20 is in a nitrogen gas atmosphere or a vacuum atmosphere, and the lid 23 is hermetically sealed with sealing materials 28A and 28B.
As a sealing method of the main body 22 and the lid 23, seam welding, soldering, glass sealing, Ag or Au—Sn brazing material sealing, epoxy resin sealing, and the like are suitable.

  The size of the obtained container 20 is, for example, about 2.5 × 2.0 × 0.86 mm, 4.8 × 5.2 × 1.5 mm.

FIG. 3 shows an example of a typical Colpitts circuit as a feedback oscillation circuit as the oscillation circuit.
Reference numeral 1 in the drawing indicates the SAW element 1. In the figure, Rb1 and Rb2 are bias setting resistors, Re is an emitter resistor, Rd and Cd are power supply decoupling resistors, capacitors, and Vcc is a power supply. CL1 and CL2 indicate load capacities for satisfying the oscillation condition. CL1 and CL2 are set according to a desired frequency. The output is via a capacitor C _0.
The transistor Tr is used for high frequency (the cutoff frequency fT is several GHz), and the chip and the resistor and the capacitor are used at 100 MHz or more.

According to this embodiment, there are the following effects.
(1) When a magnetic field is applied to the magnetic sensor element 10, the magnetostrictive material plate 2 expands or contracts when the magnetostriction constant is negative, whereby the piezoelectric flat plate 3 is distorted, and the surface acoustic wave of the SAW element 1 The distance S between the comb teeth 41A and 41B in the propagation direction D changes, and the resonance frequency f of the SAW element 1 changes. Therefore, the change in the magnetic field is converted into the change in the frequency, and the change in the magnetic field can be detected. The SAW element 1 is configured by forming electrodes on the piezoelectric plate 3 and is small in size and can reduce power consumption. In addition, the magnetostrictive material plate 2 can be reduced in size according to the SAW element 1. Therefore, it is possible to obtain a magnetic sensor element 10 that is small and consumes less power.

  (2) Since the magnetostrictive material plate 2 is bonded to the piezoelectric flat plate 3 and integrated, the smaller magnetic sensor element 10 and magnetic sensor 100 can be obtained.

(Second Embodiment)
FIG. 4 shows a schematic cross-sectional view of the magnetic sensor 200 in the present embodiment. For the oscillation circuit, electrodes, wiring, and the like, the same ones as in the first embodiment can be used.
In FIG. 4, the container 40 contains the main body 42 of the SAW element 1 and the magnetostrictive material 7, and the lid 43 is hermetically sealed with sealing materials 28A and 28B. As the SAW element 1, the same element as in the first embodiment can be used.
Here, the magnetic sensor element 50 includes the SAW element 1, the magnetostrictive material 7, and the main body 42 as a support member. However, the magnetic sensor element 50 may be configured using a support member that is separate from the main body 42.

The magnetostrictive material 7 is disposed between the piezoelectric flat plate 3 and the main body 42. The main body 42 is formed with a recess 9 as a relief when the piezoelectric flat plate 3 is bent.
The SAW element 1 and the magnetostrictive material 7 are placed adjacent to each other in the container 40. There are variations in the inner dimension L1 of the container 40, the dimension L2 of the SAW element 1, and the dimension L3 of the magnetostrictive material 7, and these are adjusted. Therefore, the gap can be reduced by inserting the spacer 8 (thickness L4). That is, the place where L1> L2 + L3 is set to L1≈L2 + L3 + L4. As a material for the spacer 8, a metal material that can obtain appropriate flexibility and stable dimensions is suitable for insertion into a narrow gap. For example, stainless steel, copper, aluminum, brass, etc.
When the magnetostriction constant is negative, the magnetostrictive material 7, the piezoelectric flat plate 3, and the main body 42 are bonded to each other, whereby a strain can be applied to the propagation direction D of the surface acoustic wave.

The magnetostriction constants of nickel and cobalt are as small as −40 to −60 ppm, whereas the rare earth Laves phase system Tb _0.3 Dy _0.7 Fe _2.0 (trade name Terfenol-D), which is called a giant magnetostrictive material, exceeds 1000 ppm. However, it is difficult to process into a thin plate or a wire, and the actuator is limited to a lot type. Therefore, the magnetostrictive material 7 can be disposed adjacent to the piezoelectric plate 3.
Examples of magnetostrictive materials include a magnetostriction constant exceeding 1000 ppm for the rare earth iron-based intermetallic compound TbFe ₂ system, and a reverse perovskite-based Mn ₃ CuN having a magnetostriction constant of 0.2% at maximum in a polycrystalline sintered body. Heusler type Ni ₂ MnGa alloy is a single crystal and has a magnetostriction constant of about 6% (60000 ppm).

According to this embodiment, there are the following effects.
(3) The magnetostrictive material 7 is disposed between the main body 42 and the piezoelectric plate 3 so as to give a strain to the propagation direction D of the surface acoustic wave. The strain generated in the magnetostrictive material 7 applies a force to the piezoelectric flat plate 3 with the main body 42 as a support, and the piezoelectric flat plate 3 is distorted. Therefore, the magnetic sensor element 50 and the magnetic sensor 200 having the above-described effects can be obtained.
Further, the piezoelectric flat plate 3 can be distorted even if the magnetostrictive material is difficult to process into a thin plate or a wire.

(Third embodiment)
FIG. 5 shows a schematic diagram of the magnetic sensor 300 in the present embodiment.
In the present embodiment, the magnetic sensor 300 is configured by arranging the coil 15 as a bias magnetic field application unit on the outer periphery of the magnetic sensor 200 of the second embodiment.
Note that the magnetic sensor used may be the magnetic sensor 100 of the first embodiment.
The coil 15 may be a solenoid coil obtained by uniformly winding a wire made of a good conductor of copper or aluminum with an insulating coating, or a Helmholtz coil in which two coils are arranged in parallel to obtain a uniform magnetic field. Also good.
In the present embodiment, a current is passed through the coil 15 arranged on the outer periphery of the magnetic sensor 200 to form a uniform bias magnetic field throughout the magnetic sensor 200.

FIG. 6 shows the relationship between the applied magnetic field and the strain amount (ΔL / L) in the magnetostrictive material.
In the magnetostrictive material 7, since the magnetostriction constant does not change linearly with respect to the strength of the magnetic field and has the same tendency as the magnetization curve, the magnetostriction constant is small when the magnetic field is near zero, and the sensitivity of the magnetic sensor 200 is also reduced as a result. End up. Therefore, if a magnetic field corresponding to a region where the magnetostriction constant is increased to some extent is given in advance, the sensitivity of the magnetic sensor 200 is improved. This region is also a linear region.
For example, a magnetic field between the magnetic fields H1 and H2 may be given in the curve shown in the figure.

FIG. 7 shows the relationship between the detected magnetic field Hm and the accompanying frequency change Δf.
If the detected magnetic field Hm is a magnetic field between the magnetic fields H1 and H2 shown in FIG. 6, the relationship between the detected magnetic field Hm and the frequency change Δf is also linear, and the magnetic field can be easily detected.
Further, if a magnetic field in a direction opposite to the magnetic field applied by the coil 15 is applied, the detected oscillation frequency becomes smaller than the reference oscillation frequency, so that the direction of the detected magnetic field can also be known.

According to this embodiment, there are the following effects.
(4) A bias magnetic field can be applied to the magnetostrictive material 7 by the coil 15. The magnetostrictive material 7 has a non-linear magnetostriction constant with respect to the strength of the magnetic field when the magnetic field is near zero, and the magnetostriction constant is small and the sensitivity of the magnetic sensor is small when the magnetic field is near zero. The magnetostriction constant with respect to the strength reaches the linear region, and the magnetic sensor 300 with improved sensitivity can be obtained.

Various modifications other than the above-described embodiment can be made.
For example, in the first embodiment, the piezoelectric body may not be a flat plate, and the magnetostrictive material plate 2 may be bonded to any surface as long as no electrode is formed on the piezoelectric flat plate 3. Good. Further, the shape of the magnetostrictive material is not limited to a rectangular parallelepiped plate, and the shape in plan view may be a rhombus.

In addition, the coil 15 in the third embodiment may be disposed in the container 40.
Furthermore, as a method for applying a magnetic field in the third embodiment, a permanent magnet and a yoke are adjacent to the magnetic sensor 200 as a bias magnetic field application unit in addition to a method of applying a magnetic field by passing a current through the outer coil 15 of the magnetic sensor 200. And a method of applying a magnetic field.
In the latter case, iron-based materials with high saturation magnetic flux density, electromagnetic soft iron, low carbon steel, silicon steel plate, etc. are suitable for the yoke, and plating and coating for heat treatment and rust prevention are performed as necessary. Is done. The shape of the yoke is preferably such that the magnetic resistance becomes small so that the magnetic flux from the permanent magnet passes through the magnetostrictive material 7 inside the magnetic sensor 200 as much as possible. Suitable permanent magnets include ferrite magnets, NdFeB magnets, SmFe magnets, and alnico magnets. The number of permanent magnets may be one or two, and a suitable shape can be selected from permeance of the permanent magnet, and a yoke shape corresponding to the shape can be designed.

(A) in 1st Embodiment is a schematic plan view of a magnetic sensor, (b) is a schematic sectional drawing, (c) is a schematic bottom view. The schematic sectional drawing of a magnetic sensor. The figure which shows the example of the Colpitts circuit as an oscillation circuit. The schematic sectional drawing of the magnetic sensor in 2nd Embodiment. Schematic of the magnetic sensor in 3rd Embodiment. The figure which showed the relationship between the applied magnetic field and the amount of distortion in a magnetostrictive material. The figure which showed the relationship between a detection magnetic field and the frequency change accompanying it.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... SAW element, 2 ... Magnetostrictive material board, 3 ... Piezoelectric flat plate as a piezoelectric body, 7 ... Magnetostrictive material, 10, 50 ... Magnetic sensor element, 15 ... Coil as a bias magnetic field application part, 20, 40 ... Container, DESCRIPTION OF SYMBOLS 30 ... Oscillation circuit, 42 ... Main body as support member, 100, 200, 300 ... Magnetic sensor, D ... Propagation direction of surface acoustic wave.

Claims (5)

A SAW element comprising a piezoelectric body;
A magnetic sensor element comprising: a magnetostrictive material that imparts strain to the propagation direction of the surface acoustic wave of the SAW element. The magnetic sensor element according to claim 1,
The magnetic sensor element, wherein the magnetostrictive material is bonded to the piezoelectric body. The magnetic sensor element according to claim 1,
The magnetostrictive material is disposed between the piezoelectric body and a support member. The magnetic sensor element according to any one of claims 1 to 3,
A container for housing the magnetic sensor element;
A magnetic sensor comprising an oscillation circuit. The magnetic sensor according to claim 4.
A magnetic sensor comprising: a bias magnetic field applying unit that applies a bias magnetic field in the propagation direction of the magnetostrictive material.

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