MAGNETIC SENSOR

Disclosed herein is a magnetic sensor that includes a sensor chip having a first magnetic layer, a second magnetic layer, and a magnetosensitive element; a first magnetism collecting member covering the first magnetic layer; and a second magnetism collecting member having a body part covering a back surface of the sensor chip, a first protruding part connected to the body part and covering a side surface of the sensor chip, and a second protruding part connected to the first protruding part and covering the second magnetic layer. The second protruding part has a first surface facing the second magnetic layer. The first surface is higher in flatness than at least one of the other surfaces of the second magnetism collecting member.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a magnetic sensor and, more particularly, to a magnetic sensor having a magnetism collecting member for collecting magnetic flux in a sensor chip.

Description of Related Art

Magnetic sensors are widely used in ammeters, magnetic encoders, and the like. Some magnetic sensors are provided with a magnetism collecting member for collecting magnetic flux in a sensor chip for the purpose of enhancing detection sensitivity. For example, JP 2019-158508A discloses a magnetic sensor having a first magnetism collecting member (top splitter) that covers a magnetic layer disposed at the center portion of an element formation surface and a second magnetism collecting member (back splitter) that covers the back and side surfaces of a sensor chip and covers a magnetic layer disposed at both end portions of the element formation surface.

The magnetic sensor described in JP 2019-158508A uses both the top splitter and back splitter and thus can detect a magnetic field in a direction perpendicular to the element formation surface with higher sensitivity than when using only the top splitter.

However, a method of fabricating the back splitter simply by cutting a block made of ferrite involves difficulty in detecting an extremely weak magnetic field with high sensitivity.

SUMMARY

It is therefore an object of the present invention to enhance detection sensitivity with respect to a magnetic field in a magnetic sensor having a magnetism collecting member for collecting magnetic flux in a sensor chip.

A magnetic sensor according to the present invention includes: a sensor chip having a first magnetic layer, a second magnetic layer, and a magnetosensitive element which are formed on an element formation surface, the magnetosensitive element being positioned on a magnetic path formed by a magnetic gap between the first and second magnetic layers; a first magnetism collecting member covering the first magnetic layer; and a second magnetism collecting member having a body part covering a back surface of the sensor chip positioned on the side opposite to the element formation surface thereof, a first protruding part connected to the body part and covering a side surface of the sensor chip perpendicular to the element formation surface and back surface thereof, and a second protruding part connected to the first protruding part and covering the second magnetic layer. The second protruding part has a first surface facing the second magnetic layer, and the first surface is higher in flatness than at least one of the other surfaces of the second magnetism collecting member.

According to the present invention, the first surface of the second magnetism collecting member has a high degree of flatness, so that a gap between the first surface of the second magnetism collecting member and the sensor chip can be reduced, whereby detection sensitivity with respect to a magnetic field can be enhanced.

In the present invention, a distance between the first surface and the sensor chip may be 50 μm or less. This can further enhance detection sensitivity with respect to a magnetic field.

In the present invention, the arithmetic mean of waviness Wa of the first surface may be 50 μm or less. This can reduce a gap between the first surface and the sensor chip in a state where the first surface is pressed against the sensor chip.

The magnetic sensor according to the present invention may further include a substrate for mounting the sensor chip, the first magnetism collecting member, and the second magnetism collecting member thereon. The second magnetism collecting member may further have a second surface facing the substrate, and the second surface may be higher in flatness than at least one of the other surfaces of the second magnetism collecting member. Thus, the second surface of the second magnetism collecting member and the substrate are in close contact with substantially no gap left therebetween. This reduces friction between the second magnetism collecting member and substrate, facilitating assembly work of bringing the second magnetism collecting member into abutment against the sensor chip while sliding it on the substrate.

In the present invention, the second magnetism collecting member may be made of a ferrite material. The ferrite material remaining in a cut state is low in flatness, but the first surface can be selectively flattened through grinding or polishing.

As described above, according to the present invention, detection sensitivity with respect to a magnetic field can be enhanced in a magnetic sensor having a magnetism collecting member for collecting magnetic flux in a sensor chip.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be explained in detail with reference to the drawings.

FIG. 1is a schematic perspective view illustrating the outer appearance of a magnetic sensor10according to a preferred embodiment of the present invention.

As illustrated inFIG. 1, the magnetic sensor10according to the present embodiment includes a substrate2with a surface4constituting the xz plane, a sensor chip20, a first magnetism collecting member30, and a second magnetism collecting member40each of which is placed on the surface4of the substrate2. The sensor chip20has an element formation surface21. The element formation surface21constitutes the xy plane and faces one end of the magnetism collecting member30in the z-direction. The magnetism collecting member40is provided on the back surface side of the sensor chip20. The magnetism collecting members30and40are each a block made of a soft magnetic material having a high permeability, such as ferrite.

As illustrated inFIG. 1, in the present embodiment, the sensor chip20is mounted such that the element formation surface21thereof is perpendicular to the surface4of the substrate2. That is, the sensor chip20is laid at 90° with respect to the substrate2. Thus, even when the magnetism collecting members30and40are large in length in the z-direction, they can be stably fixed on the substrate2.

FIG. 2is a schematic perspective view for explaining the structure of the sensor chip20.

As illustrated inFIG. 2, the sensor chip20has a substantially rectangular parallelepiped shape and has four magnetosensitive elements R1to R4on the element formation surface21constituting the xy plane. The opposite side of the element formation surface21is a back surface22constituting the xy plane. Besides, the sensor chip20has side surfaces23and24constituting the yz plane and positioned on the mutually opposite sides and lower and upper surfaces25and26constituting the xz plane and positioned on the mutually opposite sides. The sensor chip20is placed on the substrate2such that the lower surface25faces the surface4of the substrate2.

The magnetosensitive elements R1to R4are not particularly limited in type as long as characteristics thereof change according to the direction or strength of a magnetic field and may be, for example, magnetoresistive elements. In the following description, the magnetosensitive elements R1to R4are assumed to be magnetoresistive elements and to have the same fixed magnetization direction. The magnetosensitive elements R1and R3are at the same x-direction position, and the magnetosensitive elements R2and R4are at the same x-direction position. Further, the magnetosensitive elements R1and R4are at the same y-direction position, and the magnetosensitive elements R2and R3are at the same y-direction position.

Magnetic layers M1to M3are formed on the element formation surface21of the sensor chip20. The magnetic layer M1is positioned at substantially the center on the element formation surface21in a plan view, and the magnetic layers M2and M3are positioned on opposite sides of the magnetic layer M1in the x-direction. Although not particularly limited, the magnetic layers M1to M3each may be a film made of a composite magnetic material obtained by dispersing magnetic filler in a resin material or may be a thin film or a foil made of a soft magnetic material, such as nickel or permalloy, or may be a thin film or a bulk sheet made of ferrite or the like. The magnetosensitive elements R1and R3are disposed around a magnetic gap formed by the magnetic layers M1and M2, and the magnetosensitive elements R2and R4are disposed around a magnetic gap formed by the magnetic layers M1and M3. The magnetosensitive elements R1to R4each need not necessarily be positioned within the magnetic gap and only need to be disposed on a magnetic path formed by the magnetic gap, i.e., at a position where a magnetic field to be detected passing through the magnetic gap can be detected.

FIG. 3is a schematic perspective view for explaining the positional relation between the sensor chip20and the magnetism collecting members30,40.

As illustrated inFIG. 3, the magnetism collecting member30has a substantially rectangular parallelepiped shape elongated in the z-direction and overlaps the magnetic layer M1in a plan view (as viewed in the z-direction). The magnetism collecting member30functions as a top splitter that applies magnetic flux in the z-direction to the magnetic layer M1and distributes the magnetic flux to the magnetic layers M2and M3positioned on the opposite sides of the magnetic layer M1in the x-direction. The height of the magnetism collecting member30in the z-direction is not particularly limited; however, when the height thereof is increased, selectivity with respect to magnetic flux in the z-direction can be improved. In the present embodiment, the width of the magnetism collecting member30in the y-direction substantially coincides with the width of the sensor chip20in the y-direction, but not limited to this.

The back surface22and side surfaces23,24of the sensor chip20are covered with the magnetism collecting member40. The magnetism collecting member40has a body part A covering the back surface22of the sensor chip20, first protruding parts B1and B2connected to the body part A and covering respectively the side surfaces23and24of the sensor chip20, and second protruding parts C1and C2connected respectively to the first protruding parts B1and B2and covering the element formation surface21of the sensor chip20. The protruding parts C1and C2cover the magnetic layers M2and M3, respectively.

Thus, as viewed in the z-direction, the magnetosensitive elements R1and R3are positioned between the magnetism collecting member30and the protruding part C1of the magnetism collecting member40, and the magnetosensitive elements R2and R4are positioned between the magnetism collecting member30and the protruding part C2of the magnetism collecting member40. It follows that magnetic flux collected by the magnetism collecting member at the magnetic layer M1is substantially evenly distributed to the magnetic layers M2and M3and then absorbed into the body part A of the magnetism collecting member40through the protruding parts C1and C2. At this time, a part of the magnetic flux passes through the magnetosensitive elements R1to R4. That is, magnetic fluxes in mutually opposite directions are applied to the magnetosensitive elements R1, R3and magnetosensitive elements R2, R4. Thus, the magnetism collecting member40functions as a back splitter that collects the magnetic flux split by the magnetism collecting member30.

FIG. 4is a circuit diagram for explaining the connection relation between the magnetosensitive elements R1to R4.

As illustrated inFIG. 4, the magnetosensitive element R1is connected between the terminal electrodes53and56, the magnetosensitive element R2is connected between the terminal electrodes53and55, the magnetosensitive element R3is connected between the terminal electrodes54and55, and the magnetosensitive element R4is connected between the terminal electrodes55and56. The terminal electrode56is applied with a power supply potential Vcc, and the terminal electrode54is applied with a ground potential GND. The magnetosensitive elements R1to R4have the same fixed magnetization direction, and a difference occurs between the resistance variation of the magnetosensitive elements R1, R3positioned on one side as viewed from the magnetism collecting member30and the resistance variation of the magnetosensitive elements R2, R4positioned on the other side as viewed from the magnetism collecting member30. As a result, the magnetosensitive elements R1to R4constitute a differential bridge circuit, and thus a change in electrical resistance of the magnetosensitive elements R1to R4according to a magnetic flux density appears in the terminal electrodes53and55.

Differential signals output from the terminal electrodes53and55are input to a differential amplifier provided on or outside the substrate2. An output signal from the differential amplifier61is fed back to the terminal electrode52. As illustrated inFIG. 4, a compensation coil63is connected between the terminal electrodes51and52and thus generates a magnetic field according to the output signal from the differential amplifier61. The compensation coil63can be integrated in the sensor chip20. Thus, when a change in electric resistance of the magnetosensitive elements R1to R4according to a magnetic flux density appears in the terminal electrodes53and55, a current corresponding to the magnetic flux density flows in the compensation coil63to generate magnetic flux in the opposite direction, whereby the external magnetic flux is canceled. Then, by converting the current output from the differential amplifier61into voltage using a detection circuit62, the direction and strength of the external magnetic flux can be detected.

FIG. 5is a schematic perspective view for explaining the structure of the magnetism collecting member30.

As illustrated inFIG. 5, the magnetism collecting member30has a substantially rectangular parallelepiped shape with six surfaces31to36. The surface31constitutes the xy plane and faces the element formation surface21of the sensor chip20in a mounted state. The surface33constitutes the xz plane and faces the surface4of the substrate2in a mounted state. The surface32constitutes the xy plane positioned on the side opposite to the surface31. The surface34constitutes the xz plane positioned on the side opposite to the surface33. The surfaces35and36constitute the yz plane and opposite to each other.

FIG. 6is a schematic perspective view for explaining the structure of the magnetism collecting member40.

As illustrated inFIGS. 1, 3, and 6, the magnetism collecting member40has the body part A with a substantially rectangular parallelepiped shape, the protruding parts B1and B2connected to the body part A and protruding in the z-direction, and the protruding parts C1and C2connected respectively to the protruding parts B1and B2and protruding in the x-direction. As described above, the protruding part C1overlaps the magnetic layer M2in the z-direction, and the protruding part C2overlaps the magnetic layer M3in the z-direction.

The magnetism collecting member40has upper and lower surfaces41and42constituting the xz plane, side surfaces43and44constituting the yz plane, an inner surface45constituting the xy plane, an inner surface46constituting the yz plane, an inner surface47constituting the xy plane, an end surface48constituting the yz plane, and an end surface49constituting the xy plane. In a mounted state of the magnetism collecting member40, the lower surface42faces the surface4of the substrate2, the inner surface45faces the back surface22of the sensor chip20, the inner surface46faces the side surfaces23and24of the sensor chip20, and the inner surface47faces the element formation surface21of the sensor chip20so as to overlap the magnetic layers M2and M3. In particular, the inner surface47preferably contacts a protective film covering the magnetic layers M2and M3, i.e., the surface of the sensor chip20.

While the inner surface45of the magnetism collecting member40may contact the back surface22of the sensor chip20, a gap may be provided therebetween so as to press the protruding parts C1and C2of the magnetism collecting member40against the element formation surface21of the sensor chip20. Further, the inner surface46of the magnetism collecting member40may be slightly separated from the side surfaces23and24of the sensor chip20so as to allow adjustment of the relative position between the sensor chip20and the magnetism collecting member40in the z-direction.

In the present embodiment, at least the inner surface of the magnetism collecting member40is flattened. This results from grinding or polishing applied to the inner surface47of the magnetism collecting member40. Thus, when the inner surface47of the magnetism collecting member40is pressed against the sensor chip20, they are in close contact with substantially no gap left therebetween, which can suppress a reduction in detection sensitivity due to the gap therebetween and reduce a variation in detection sensitivity among products.

Specifically, grinding or polishing is preferably performed so as to make the arithmetic mean of waviness Wa (defined in JIS B 0601: 2013) of the inner surface47equal to or less than 50 μm and more preferably equal to or less than 20 μm.

As illustrated inFIG. 7, when the arithmetic mean of waviness Wa of the inner surface47is equal to or less than 50 μm, a gap between the inner surface47of the magnetism collecting member40and the sensor chip20can be reduced to 50 μm or less in a state where the protruding parts C1and C2of the magnetism collecting member40are pressed against the surface of the sensor chip20. Further, when the arithmetic mean of waviness Wa of the inner surface47is equal to or less than 20 μm, a gap between the inner surface47of the magnetism collecting member40and the sensor chip20can be reduced to 20 μm or less in a state where the protruding parts C1and C2of the magnetism collecting member40are pressed against the surface of the sensor chip20.

FIG. 8is a graph illustrating the relation between sensor sensitivity and a gap formed between the inner surface47of the magnetism collecting member40and the sensor chip20. When ferrite is used as the material of the magnetism collecting member40, the arithmetic mean of waviness Wa of the inner surface47is about 300 μm unless the flattening is performed and, in this case, sensor sensitivity is about 45,000 μV/nT. When a gap between the inner surface47of the magnetism collecting member40and the sensor chip20is about 50 μm, sensor sensitivity increases to about 46,000 μV/nT. Further, when a gap between the inner surface47of the magnetism collecting member40and the sensor chip20is about 20 μm, sensor sensitivity increases to about 48,000 μV/nT.

Not only the inner surface47of the magnetism collecting member40, but also other surfaces of the magnetism collecting member40may be flattened. However, the upper surface41and side surfaces43,44need not be flattered because the surface properties thereof have no influence on sensor sensitivity. Rather, they are preferably not to be flattened (preferably made to remain in a state where the magnetism collecting member40is cut out from a block made of a magnetic material such as ferrite) for reduction in machining cost. On the other hand, the lower surface42is preferably flattened like the inner surface47. When the lower surface42is flattened, the lower surface42and the surface4of the substrate2are in close contact with substantially no gap left therebetween in a state where the magnetism collecting member40is mounted on the substrate2. This reduces friction between the lower surface42of the magnetism collecting member40and the surface4of the substrate2, thus facilitating assembly work of bringing the magnetism collecting member40into abutment against the sensor chip20while sliding it on the surface4of the substrate2. The arithmetic mean of waviness Wa of the lower surface42may be the same or larger than that of the inner surface47.

Further, not only the magnetism collecting member40, but also the surface of the magnetism collecting member30is preferably flattened. In particular, when the surface31of the magnetism collecting member30is flattened, it is possible to significantly suppress a reduction in detection sensitivity due to a gap between the element formation surface21and the magnetism collecting member30and to significantly reduce a variation in detection sensitivity among products. Further, when the surface33of the magnetism collecting member30is flattened, the surface33and the surface4of the substrate2are in close contact with substantially no gap left therebetween, and an angle formed by the surfaces31and33becomes close to 90° , so that a variation in gap size between the element formation surface21of the sensor chip20and the magnetism collecting member30can be reduced. In addition, friction between the surface33of the magnetism collecting member30and the substrate2is reduced, thus facilitating assembly work of bringing the magnetism collecting member30into abutment against the sensor chip20while sliding it on the substrate2.

As described above, in the magnetic sensor10according to the present embodiment, the magnetism collecting member30that functions as a top splitter and the magnetism collecting member40that functions as a back splitter are used to collect magnetic flux in the sensor chip20, and the inner surfaces47of the protruding parts C1and C2of the magnetism collecting member40are selectively flattened, so that it is possible to enhance detection sensitivity while suppressing an increase in machining cost.

While the preferred embodiment of the present disclosure has been described, the present disclosure is not limited to the above embodiment, and various modifications may be made within the scope of the present disclosure, and all such modifications are included in the present disclosure.