JP2022142942A - magnetic sensor - Google Patents

Abstract

To reduce the 1/f noise of a magnetic sensor.SOLUTION: A magnetic sensor 1 comprises: a sensor chip 20 having a magneto-sensitive element; external magnetic substances 31, 32 for collecting magnetic fluxes to be detected in the magneto-sensitive element; and an MEMS chip 40 that overlaps a magnetic gap Gz provided in the external magnetic substances 31, 32. The MEMS chip 40 includes piezoelectric structures 43a, 43b that cause the distance between a magnetic substance layer 44 and the magnetic gap Gz to change in accordance with a drive voltage. Thus, as the MEMS chip 40 is arranged at a position overlapping the magnetic gap Gz provided in the external magnetic substances 31, 32, stresses are in no case applied to the sensor chip 20. Furthermore, as the external magnetic substances 31, 32 do not themselves need to be displaced, a magnetic path can be mechanically displaced with a high frequency.SELECTED DRAWING: Figure 1

Description

The present invention relates to a magnetic sensor, and more particularly to a magnetic sensor that reduces 1/f noise by mechanically displacing a magnetic path.

At present, magnetic sensors using magneto-sensitive elements are used in various fields, but a magnetic sensor with a high S/N ratio is required in order to detect extremely weak magnetic fields. Here, 1/f noise can be cited as a factor that lowers the S/N ratio of the magnetic sensor. 1/f noise becomes more pronounced as the frequency component of the magnetic field to be measured is lower. Therefore, in order to detect magnetic fields in the low-frequency range, such as 1 kHz or less, with high sensitivity, it is necessary to reduce the 1/f noise. is important.

As a method for reducing 1/f noise in a magnetic sensor, as described in Patent Documents 1 and 2, a magnetic field to be measured is reduced by mechanically displacing a magnetic path using a piezoelectric layer. A method of amplitude modulation has been proposed.

For example, the magnetic sensor described in Patent Literature 1 has a structure in which a mechanical driving section is adhered onto the element forming surface of the sensor chip. The mechanical drive section has a structure in which a magnetic layer and a piezoelectric layer are laminated, and a magnetic path made up of the magnetic layers is displaced by applying a drive voltage to the piezoelectric layer. Further, the magnetic sensor described in Patent Document 2 has a structure in which a piezoelectric layer is added to an external magnetic body that collects magnetic flux, and the external magnetic body itself is deformed by a drive voltage. According to the magnetic sensors described in Patent Documents 1 and 2, the magnetic field to be measured is amplitude-modulated by applying a high-frequency drive voltage to the piezoelectric layer, thereby reducing 1/f noise. becomes possible.

JP 2020-067331 A JP 2020-106309 A

However, in the magnetic sensor described in Patent Document 1, since the mechanical drive unit is adhered to the element formation surface of the sensor chip, there is a risk that stress will be applied to the sensor chip due to the operation of the mechanical drive unit. . Moreover, since the magnetic sensor described in Patent Document 2 displaces the external magnetic body itself, it is not easy to increase the driving frequency.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a magnetic sensor capable of reducing 1/f noise by mechanically displacing the magnetic path at a high frequency without applying stress to the sensor chip.

The magnetic sensor according to the present invention comprises a sensor chip having a magneto-sensitive element, an external magnetic body that collects the magnetic flux to be detected on the magneto-sensitive element, a magnetic layer that overlaps a magnetic gap provided in the external magnetic body, and a driving voltage. It is characterized by comprising a variable magnetic path structure including a piezoelectric structure that changes the distance between the magnetic layer and the magnetic gap accordingly.

According to the present invention, since the variable magnetic path structure is arranged at the position overlapping the magnetic gap provided in the external magnetic body, no stress is applied to the sensor chip. Moreover, since it is not necessary to displace the external magnetic body itself, it is possible to mechanically displace the magnetic path at a high frequency.

In the present invention, the external magnetic body has a first external magnetic body with one end facing the sensor chip and a second external magnetic body with one end facing the other end of the first external magnetic body via a magnetic gap. It doesn't matter if you do. According to this, the magnetic flux flowing from the second external magnetic body to the first external magnetic body can be amplitude-modulated by the variable magnetic path structure.

In the present invention, the variable magnetic path structure may be integrated on the MEMS chip. According to this, it becomes possible to reduce the number of parts. In this case, the MEMS chip may have a structure in which the MEMS substrate is removed or the thickness of the MEMS substrate is selectively reduced at positions overlapping the magnetic layer and the piezoelectric structure. According to this, it becomes possible to increase the amount of displacement of the magnetic layer.

The magnetic sensor according to the present invention further includes a circuit board on which the sensor chip, the external magnetic material, and the MEMS chip are mounted, the MEMS chip further includes terminal electrodes supplied with a driving voltage, and the terminal electrodes of the external magnetic material are exposed. A notch may be provided at a position overlapping with the terminal electrode so that the terminal electrode is overlapped. According to this, it is possible to supply the drive voltage to the MEMS chip without being blocked by the external magnetic material.

In the present invention, the length of the magnetic layer in the direction in which the magnetic gap extends may be longer than the length in the direction in which the magnetic gap extends, so that the entire magnetic gap may be covered with the magnetic layer. According to this, it is possible to reduce the magnetic resistance in a state in which the magnetic layer is brought close to the magnetic gap.

Thus, according to the present invention, it is possible to provide a magnetic sensor capable of reducing 1/f noise by mechanically displacing the magnetic path at a high frequency without applying stress to the sensor chip. It becomes possible.

FIG. 1 is a schematic perspective view showing the appearance of a magnetic sensor 1 according to one embodiment of the present invention. FIG. 2 is a schematic exploded perspective view of the magnetic sensor 1. FIG. FIG. 3 is a schematic plan view of the sensor chip 20. FIG. 4 is a schematic cross-sectional view along line AA of FIG. 3. FIG. FIG. 5 is a schematic perspective view showing the structure of the MEMS chip 40 viewed from the main surface side. FIG. 6 is a schematic perspective view showing the structure of the MEMS chip 40 viewed from the back side. FIG. 7 is a schematic perspective view showing a state in which the insulating layer 42 included in the MEMS chip 40 is removed. FIG. 8 is a schematic perspective view showing the positional relationship between the MEMS chip 40 and the external magnetic bodies 31 and 32. As shown in FIG. FIG. 9 is a schematic cross-sectional view for explaining a state in which no drive voltage is applied to the MEMS chip 40. As shown in FIG. FIG. 10 is a schematic cross-sectional view for explaining a state in which a drive voltage is applied to the MEMS chip 40. FIG.

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

FIG. 1 is a schematic perspective view showing the appearance of a magnetic sensor 1 according to one embodiment of the present invention. 2 is a schematic exploded perspective view of the magnetic sensor 1. FIG.

As shown in FIGS. 1 and 2, the magnetic sensor 1 according to the present embodiment includes a circuit board 10, a sensor chip 20 mounted on a surface 11 constituting the xz plane of the circuit board 10, external magnetic bodies 31 to 33, and A MEMS chip 40 is provided. The sensor chip 20 has an element forming surface 21 and a back surface 22 forming the xy plane, side surfaces 23 and 24 forming the yz plane, and side surfaces 25 and 26 forming the xz plane. It is mounted on the circuit board 10 so as to face the surface 11 of the board 10 . On the element forming surface 21 of the sensor chip 20, magneto-sensitive elements and magnetic layers M1 to M3, which will be described later, are formed. Thus, in this embodiment, the surface 11 of the circuit board 10 and the element formation surface 21 of the sensor chip 20 are perpendicular. However, in the present invention, it is not essential that both are completely vertical, and they may have a predetermined inclination with respect to the vertical.

The external magnetic bodies 31 to 33 play the role of collecting magnetic flux to the sensor chip 20, and are all made of a high magnetic permeability material such as ferrite. Among them, the external magnetic bodies 31 and 32 are rod-shaped bodies whose longitudinal direction is the z direction, and are arranged in a straight line with the magnetic gap Gz interposed therebetween. The width direction of the magnetic gap Gz is the z direction, and the extending direction is the y direction. The magnetic gap Gz is covered with the MEMS chip 40 from the width direction and the x direction perpendicular to the extension direction. The structure of the MEMS chip 40 will be described later. The magnetic gap Gz may be a space, or may be filled with a non-magnetic material such as resin. In other words, a structure in which the tip of the external magnetic body 31 and the tip of the external magnetic body 32 are connected by a non-magnetic resin may be used. According to this, the number of parts can be reduced, and the gap width of the magnetic gap Gz can be fixed.

One end of the external magnetic body 31 in the z direction is positioned substantially at the center of the element formation surface 21 in the x direction so as to cover a portion of the magnetic layer M1. The other end of the external magnetic body 31 in the z-direction faces one end of the external magnetic body 32 in the z-direction via a magnetic gap Gz. The other end of the external magnetic body 32 in the z-direction is open, and this portion is used as a magnetic field detection head. The external magnetic body 33 has a rod-like shape whose longitudinal direction is the z-direction, covers part of the magnetic layers M2 and M3, and also covers the rear surface 22 and the side surfaces 23 and 24 of the sensor chip 20 . With such a configuration, the magnetic field in the z direction is selectively collected, and the collected magnetic field is applied to the sensor chip 20 .

3 is a schematic plan view of the sensor chip 20, and FIG. 4 is a schematic cross-sectional view taken along line AA of FIG.

As shown in FIGS. 3 and 4, the element forming surface 21 of the sensor chip 20 is formed with four magneto-sensitive elements R1 to R4. The magneto-sensitive elements R1 to R4 are not particularly limited as long as they are elements whose electric resistance changes depending on the direction of the magnetic flux, and for example, MR elements can be used. The fixed magnetization directions of the magneto-sensitive elements R1 to R4 are aligned in the same direction (for example, the plus side in the x direction). The magneto-sensitive elements R1 to R4 are covered with an insulating layer 27, and magnetic layers M1 to M3 made of permalloy or the like are formed on the surface of the insulating layer 27. As shown in FIG. The magnetic layers M1 to M3 are covered with an insulating layer . Of the magnetic layers M1 to M3, portions positioned on one side in the y direction (upper side in FIG. 3) are defined as magnetic layers M11, M21, and M31, and the other side in the y direction (lower side in FIG. 3) ) are defined as the magnetic layers M12, M22, and M32, the magneto-sensitive element R1 is positioned between the magnetic layers M11 and M21 in plan view (viewed from the z direction), The magneto-sensitive element R2 is positioned between the magnetic layers M12 and M22, the magneto-sensitive element R3 is positioned between the magnetic layers M11 and M31, and the magneto-sensitive element R4 is positioned between the magnetic layers M12 and M12. It is positioned between the magnetic layers M32. As a result, a magnetic field passing through the magnetic gaps G1-G4 is applied to the magneto-sensitive elements R1-R4. Here, since the direction of the magnetic field applied to the magneto-sensitive elements R1 and R2 and the direction of the magnetic field applied to the magneto-sensitive elements R3 and R4 are different from each other by 180°, the magneto-sensitive elements R1 to R4 are bridge-connected. Thus, the direction and strength of the magnetic flux applied through the external magnetic body 31 can be detected.

However, in the present invention, it is not essential to arrange the magneto-sensitive elements R1 to R4 between the two magnetic layers. It is sufficient that the magneto-sensitive elements R1 to R4 are arranged on the magnetic path formed by. Further, the width of the magnetic gaps G1-G4 need not be wider than the width of the magneto-sensitive elements R1-R4, and the width of the magnetic gaps G1-G4 may be narrower than the width of the magneto-sensitive elements R1-R4.

3 and 4, the area indicated by reference numeral 31a indicates the area covered by the external magnetic material 31, and the areas indicated by reference characters 33a and 33b indicate the areas covered by the external magnetic material 33. As shown in FIG. As shown in FIGS. 3 and 4, the external magnetic body 31 covers the magnetic layer M1, and the external magnetic body 33 covers the magnetic layers M2 and M3.

5 is a schematic perspective view showing the structure of the MEMS chip 40 viewed from the main surface side, and FIG. 6 is a schematic perspective view showing the structure of the MEMS chip 40 viewed from the back surface side. Also, FIG. 7 is a schematic perspective view showing a state in which the insulating layer 42 included in the MEMS chip 40 is removed.

As shown in FIGS. 5 to 7, the MEMS chip 40 has a MEMS substrate 41 made of silicon or the like. The MEMS substrate 41 has a main surface 41a forming the yz plane, a back surface 41b located opposite to the main surface 41a, and a through hole 41c penetrating from the back surface 41b to the main surface 41a. Instead of providing the through holes 41c, a membrane structure may be employed in which the thickness of the MEMS substrate 41 is selectively reduced at positions corresponding to the through holes 41c.

A main surface 41a of the MEMS substrate 41 is provided with two piezoelectric structures 43a and 43b. Although some of the piezoelectric structures 43a and 43b are supported by the MEMS substrate 41, the rest of the piezoelectric structures 43a and 43b are positioned to overlap the through holes 41c. The piezoelectric structures 43a and 43b are composed of piezoelectric layers made of a piezoelectric material such as PZT and electrode layers formed on both sides thereof. One electrode layer is connected to a terminal electrode 48 via a wiring pattern 46 provided on the main surface 41a of the substrate 41, and the other electrode layer is connected via a wiring pattern 47 provided on the main surface 41a of the substrate 41. is connected to the terminal electrode 49. Accordingly, when a driving voltage is applied between the terminal electrodes 48 and 49, the piezoelectric material such as PZT included in the piezoelectric structures 43a and 43b is deformed. As described above, part of the piezoelectric structures 43a and 43b are arranged at positions overlapping the through holes 41c (or the membrane portion), so that the piezoelectric structures 43a and 43b are displaced in the x direction by the drive voltage.

A main surface 41 a of the MEMS substrate 41 is covered with an insulating layer 42 . The insulating layer 42 is provided with a plurality of slits 42a-42d. The slits 42a, 42b extend in the z-direction and the slits 42c, 42d extend in the y-direction. A magnetic layer 44 is arranged in a region surrounded by these slits 42a to 42d and sandwiched between the piezoelectric structures 43a and 43b when viewed from the x direction. The magnetic layer 44 is a metal foil made of a high magnetic permeability material such as permalloy, and its surface is covered with a protective film 45 . The magnetic layer 44 is elastically supported by an insulating layer 42 provided with a plurality of slits 42a-42d. With such a configuration, when the piezoelectric structures 43a and 43b are deformed by applying a driving voltage between the terminal electrodes 48 and 49, the position of the magnetic layer 44 in the x direction is displaced.

After forming the piezoelectric structures 43a and 43b, the wiring patterns 46 and 47, the insulating layer 42, the magnetic layer 44, etc. on the main surface 41a of the MEMS substrate 41 using a semiconductor process, the MEMS chip 40 is mounted on the substrate from the back surface 41b side. It can be produced by etching 41 to form a through hole 41c (or a membrane portion). In the semiconductor process, the MEMS chip 40 may be exposed to high temperatures, but since the MEMS chip 40 is a separate chip from the sensor chip 20, the magneto-sensitive elements R1 to R4 are not exposed to high temperatures.

FIG. 8 is a schematic perspective view showing the positional relationship between the MEMS chip 40 and the external magnetic bodies 31 and 32. As shown in FIG.

As shown in FIG. 8, the MEMS chip 40 is mounted on the surface 11 of the circuit board 10 such that the magnetic layer 44 overlaps the magnetic gap Gz in the x-direction. Here, when the width of the magnetic gap Gz in the y direction is L1 and the width of the magnetic layer 44 in the y direction is L2, L1<L2 and the entire magnetic gap Gz is covered with the magnetic layer 44. It is preferable that According to this, the magnetic resistance of the magnetic gap Gz whose magnetic path is the magnetic layer 44 is reduced.

In addition, the external magnetic bodies 31 and 32 have a reduced thickness in the x direction and a reduced width in the y direction at the portion where the magnetic gap Gz is formed. This reduces the cross-sectional area of the magnetic gap Gz, thereby increasing the magnetic flux density of the magnetic flux passing through the magnetic gap Gz. In particular, if notches 31b and 32b for narrowing the width in the y direction are provided on the circuit board 10 side to expose the terminal electrodes 48 and 49 and the solder 50 connected thereto, the external magnetic bodies 31, It becomes possible to supply the driving voltage to the MEMS chip 40 without being interrupted by 32 .

9 is a schematic cross-sectional view for explaining a state in which no drive voltage is applied to the MEMS chip 40, and FIG. 10 is a schematic cross-sectional view for explaining a state in which a drive voltage is applied to the MEMS chip 40. It is a cross-sectional view.

As shown in FIGS. 9 and 10, the MEMS chip 40 is mounted on the circuit board 10 such that the magnetic gap Gz formed by the external magnetic bodies 31 and 32 and the magnetic layer 44 overlap in the x direction. Specifically, the MEMS chip 40 is mounted on the circuit board 10 so that the yz planes at the tip portions of the external magnetic bodies 31 and 32 and the magnetic layer 44 are in contact or very close to each other via the protective film 45 . When no drive voltage is applied to the MEMS chip 40, the initial state shown in FIG. 9 is maintained. Join. As a result, the magnetic resistance of the magnetic gap Gz is reduced, so that the magnetic field to be detected is efficiently applied to the magneto-sensitive elements R1 to R4 provided on the sensor chip 20. FIG.

On the other hand, when a drive voltage is applied to the MEMS chip 40, the magnetic layer 44 is displaced in the +x direction as shown in FIG. As a result, a magnetic gap Gx in the x direction is formed between the magnetic gap Gz and the magnetic layer 44, and the degree of magnetic coupling between the external magnetic bodies 31 and 32 is greatly reduced. That is, the magnetic gap Gz functions as a large magnetic resistance. As a result, the sensitivity of the sensor chip 20 to the magnetic field to be detected is greatly reduced. Thus, the MEMS chip 40 constitutes a variable magnetic path structure that changes the distance Gx between the magnetic layer 44 and the magnetic gap Gz according to the driving voltage.

Therefore, when a drive voltage having a predetermined frequency is applied to the piezoelectric structures 43a and 43b through the terminal electrodes 48 and 49, the output signals of the magneto-sensitive elements R1 to R4 are amplitude-modulated with the frequency of the drive voltage as the sampling frequency. be done. By demodulating the amplitude-modulated output signal, it is possible to obtain measurement results with reduced 1/f noise. As an example, if the frequency component of the magnetic field to be measured is in a low frequency band of 0.1 Hz to 1 kHz, 1/f noise will reduce the S/N ratio, so if the magnetic field to be measured is particularly weak. difficult to measure. However, if the magnetic sensor according to this embodiment is used, the output signal can be amplitude-modulated at an arbitrary sampling frequency. Therefore, by setting the sampling frequency to several kHz, for example, the influence of 1/f noise can be almost eliminated. It becomes possible.

As described above, the magnetic sensor according to this embodiment includes the MEMS chip 40 that amplitude-modulates the magnetic flux flowing through the external magnetic bodies 31 and 32, so that 1/f noise can be reduced. Moreover, since the MEMS chip 40 is a separate chip from the sensor chip 20, it is possible not only to prevent an increase in manufacturing costs and to prevent deterioration of the characteristics of the magneto-sensitive elements R1 to R4 due to a high-temperature process, but also to prevent the operation of the MEMS chip 40. Therefore, the sensor chip 20 is not stressed. Further, since there is no need to displace the external magnetic bodies 31 and 32 themselves, the drive frequency can be easily increased.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. Needless to say, it is included within the scope.

For example, in the above embodiment, the four magneto-sensitive elements R1 to R4 are bridge-connected, but the number of magneto-sensitive elements formed on the sensor chip 20 and the connection method are not particularly limited.

Further, in the above embodiment, the sensor chip 20 is provided with the magnetic layers M1 to M3, but the use of the magnetic layers is not essential in the present invention.

Furthermore, although the magnetic gap Gz extends in the y direction in the above embodiment, the magnetic gap Gz may extend in the x direction.

In the above embodiment, the MEMS chip 40 constitutes the variable magnetic path structure. The configuration of the structure does not matter.

1 magnetic sensor 10 circuit board 11 circuit board front surface 20 sensor chip 21 element forming surface 22 sensor chip rear surface 23 to 26 sensor chip side surfaces 27, 28 insulating layers 31 to 33 external magnetic bodies 31a, 33a, 33b external magnetic bodies and Overlapping regions 31b, 32b Notch 40 MEMS chip 41 MEMS substrate 41a Main surface 41b of MEMS substrate Rear surface 41c of MEMS substrate Through hole 42 Insulating layers 42a to 42d Slits 43a, 43b Piezoelectric structure 44 Magnetic layer 45 Protective films 46, 47 Wiring patterns 48, 49 Terminal electrodes 50 Solders G1 to G4, Gx, Gz Magnetic gaps M1 to M3, M11, M21, M31, M12, M22, M32 Magnetic layers R1 to R4 Magnetosensitive elements

Claims (6)

a sensor chip having a magneto-sensitive element;
an external magnetic body that collects the magnetic flux to be detected on the magneto-sensitive element;
a variable magnetic path structure including a magnetic layer that overlaps a magnetic gap provided in the external magnetic body; and a piezoelectric structure that changes a distance between the magnetic layer and the magnetic gap according to a driving voltage. A magnetic sensor characterized by: The external magnetic body has a first external magnetic body with one end facing the sensor chip and a second external magnetic body with one end facing the other end of the first external magnetic body via the magnetic gap. The magnetic sensor according to claim 1, characterized in that: 3. The magnetic sensor according to claim 1, wherein the variable magnetic path structure is integrated on a MEMS chip. 4. The structure according to claim 3, wherein the MEMS chip has a structure in which the MEMS substrate is removed or the thickness of the MEMS substrate is selectively reduced at positions overlapping with the magnetic layer and the piezoelectric structure. A magnetic sensor as described. further comprising a circuit board on which the sensor chip, the external magnetic body and the MEMS chip are mounted;
the MEMS chip further includes a terminal electrode to which the driving voltage is supplied;
5. The magnetic sensor according to claim 3, wherein the external magnetic body has a notch at a position overlapping with the terminal electrode so that the terminal electrode is exposed. The length of the magnetic layer in the extending direction of the magnetic gap is longer than the length of the magnetic gap in the extending direction, whereby the entire magnetic gap is covered with the magnetic layer. 6. A magnetic sensor as claimed in any one of claims 1 to 5.

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