JP2003167038A - Active magnetic shield device and active magnetic shield method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active magnetic shield device for effectively damping external magnetic variation by disposing a sensor group having an object for measuring a magnetic gradient and a sensor group having an object for measuring magnetic variation of the magnetic shield space interior in a magnetic shield area to calculate the proper current value of current flowing through an electromagnetic coil from the mutual measured values. <P>SOLUTION: This active magnetic shield device is provided with magnetic sensor groups 121 to 123 for measuring the magnetic variation amount, electromagnetic coil groups 131 to 133 disposed corresponding to the respective magnetic sensors to thereby independently apply magnetic field to the magnetic sensor disposition portions, and controller groups 141 to 143 for controlling the respective magnetic sensors and the electromagnetic coils disposed corresponding thereto in relation to each other. According to the magnetic variation amount obtained from the measurement of the respective magnetic sensors, magnetic field is applied in the direction of canceling the magnetic variation to the respective magnetic sensor disposition portions independently, thereby forming the space surrounded with the electromagnetic coil groups as a magnetic shield space. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active magnetic shield device and an active magnetic shield method for attenuating a magnetic fluctuation from the outside to a predetermined region where a magnetic shield is required and actively performing magnetic shield. More particularly, the present invention relates to a technique for actively reducing the influence of magnetic fluctuations in the external environment with respect to the environment in which a microscope using an electron beam and an electron beam exposure apparatus that are easily affected by magnetic fluctuations in the external environment are installed.

[0002]

2. Description of the Related Art Electron beam control of a microscope or an electron beam exposure apparatus using an electron beam is easily affected by magnetic fluctuations in the external environment. Particularly, in the case of an electron beam exposure apparatus for making a photomask, recent semiconductor devices have been used. It is known that the influence on the beam position control is a serious problem, while the miniaturization of the pattern in the photomask is further advanced with the increase in the density of the photomask. Recently, in an electron beam drawing apparatus that requires strict position control, the magnetic fluctuation amount is reduced to 0.
It is required to be suppressed to 3 mG to 1 mG or less,
It was necessary to give due consideration to the environment in which the equipment was installed. As a setting room for installing such a device, in general, a so-called shield room is used in which the wall surface is made of a ferromagnetic material typified by Permalloy, and the magnetic fluctuation due to the influence from the outside is attenuated. There is. By this method,
The method of attenuating the magnetic fluctuation due to the influence from the outside world is usually called the passive magnetic shield method. However, in the passive magnetic shield method, it is necessary to increase the number of layers of Permalloy or to increase the thickness in order to meet the recent demands for the device, and the price is accelerating so far, and the installation cost of the device is reduced. The current situation is that it is a shackle. In addition, it has been confirmed that the shielding performance deteriorates in the low frequency range near 0.1 to 1.0 Hz,
It is known that it is difficult to efficiently suppress magnetic fluctuations caused by the passage of vehicles and the like.

Under these circumstances, in recent years, as a new form of a shielded room, an electromagnetic coil is placed in a form surrounding a space to be magnetically shielded (usually a six-sided rectangular parallelepiped), and the magnetic fluctuation due to the influence from the outside is applied to the inside or the outside. A technique has been developed in which a magnetic sensor detects the magnetic field and controls the current flowing in the electromagnetic coil so as to cancel the magnetic field. Such a method of actively attenuating the magnetic fluctuation due to the influence from the outside is called an active magnetic shield method. As a form of the active magnetic shield system, a magnetic sensor for detecting magnetic fluctuations is arranged at one place in the shield space, and the so-called one sensor system that adjusts the amount of current flowing to the electromagnetic coil according to the measured value, all six sides There is known a so-called 6-sensor system in which a sensor is arranged in the X-axis, the magnetic field gradients in the X, Y, and Z axes are measured, and a current is supplied to an electromagnetic coil in a direction to cancel the gradient. The one-sensor system can surely suppress the magnetic fluctuation of the part, but no information can be obtained for other parts. For this reason, when adopting the 1-sensor method, the data of the influence of the magnetic field and magnetic field fluctuation due to the devices placed in the surroundings in advance and the influence of the vehicles and railways running in the surroundings are collected in advance, and the inside of the shield is shielded using the simulation technique. It is general to control so as to suppress the magnetic fluctuation at any point. Therefore, when the environment changes due to the layout change of the peripheral device, it is necessary to perform the simulation again and cannot be called active in the long-term meaning. On the other hand, the 6-sensor method can measure the magnetic gradient in the shielded space, so it can respond quickly to changes in the surrounding environment, but control should be performed by simply approximating the magnetic gradient as a linear gradient. Therefore, it is difficult to suppress magnetic fluctuations in the vicinity and magnetic fluctuations in a frequency band exceeding 5 Hz. Therefore, in actuality, it is possible to suppress the influence of vehicles and railways running far away, but it is difficult to effectively suppress the influence of movements of trolleys and chairs that pass nearby and opening and closing of doors. No matter which method is adopted as described above, none of them has the performance to replace the existing shielded room, and it is particularly effective in the long term and efficiently for the magnetic fluctuations coming from the vicinity in a wide frequency band. It is a difficult situation to control.

[0004]

As described above, the conventional active magnetic shield system, that is, the one-sensor system and the six-sensor system, have their respective drawbacks, and their corresponding measures have been demanded. The present invention addresses this and aims to provide a magnetic shield device having the advantages of both the one-sensor system and the six-sensor system. Specifically, a sensor group having the purpose of measuring the amount of magnetic fluctuation in the magnetic shield area and a sensor group having the purpose of measuring the magnetic fluctuation inside the magnetic shield space are arranged, and the electromagnetic values are measured from each other. The present invention intends to provide an active magnetic shield device that calculates an appropriate current value to be passed through a coil and effectively attenuates external magnetic fluctuations.

[0005]

SUMMARY OF THE INVENTION An active magnetic shield device according to the present invention is a magnetic shield device for attenuating a magnetic fluctuation from the outside to a predetermined region where a magnetic shield is required and for actively performing magnetic shield. And
A magnetic sensor group for measuring the amount of magnetic fluctuation, and an electromagnetic coil group for respectively applying a magnetic field to the magnetic sensor installation location, which is provided corresponding to each magnetic sensor, and each magnetic sensor, respectively. A controller group for associating and controlling each of the electromagnetic coils provided corresponding to this is provided, and based on the amount of magnetic fluctuation obtained by the measurement by each of the magnetic sensors, each independently, A magnetic field is applied to a location where a magnetic sensor is disposed in a direction that cancels magnetic fluctuations, a space surrounded by a group of electromagnetic coils is formed as a magnetic shield space, and the predetermined region is arranged in the magnetic shield space. At least two magnetic fields that sandwich the predetermined region in the predetermined direction of the magnetic shield space and a position between the two positions in the magnetic shield space are arranged along the predetermined direction. An electromagnetic coil that is provided with a magnetic sensor for measuring an amount and applies a magnetic field along each of the predetermined directions at each magnetic sensor position so as to cancel the amount of magnetic fluctuation along each of the predetermined directions measured at each magnetic sensor position. It is characterized by arranging. In the above, the predetermined direction is the X-axis direction, Y
It is characterized in that there are three directions, an axial direction and a Z-axis direction. Further, in the above, the controller includes an A / D conversion unit for A / D converting the signal from the magnetic sensor, an arithmetic processing unit for performing a predetermined arithmetic operation based on the data value from the A / D conversion unit, The data value obtained by the arithmetic processing unit is D / A-converted and further current-amplified, and a D / A conversion unit and a current amplifier are provided to cancel the magnetic fluctuation amount at each magnetic sensor installation position. It is characterized in that a predetermined current is passed through each electromagnetic coil.

The active magnetic shield method of the present invention is
An active magnetic shield method for attenuating magnetic fluctuations from the outside to a predetermined region where a magnetic shield is required, and for actively performing magnetic shielding, at least in a space in which the predetermined region is arranged. A magnetic sensor is provided at each position between the opposite ends in the predetermined direction and the both ends in the magnetic shield space so as to measure the amount of magnetic fluctuation along the predetermined direction, and each magnetic sensor position. In order to offset the amount of magnetic fluctuation along the predetermined direction measured in, arranged an electromagnetic coil for applying a magnetic field along the predetermined direction for each magnetic sensor position,
Each of them independently controls the amount of current flowing through the corresponding electromagnetic coil so that the magnetic fluctuation amount at each magnetic sensor position becomes zero. Then, in the above, the predetermined direction is the X-axis direction, the Y-axis direction,
It is characterized in that there are three directions in the Z-axis direction.

[0007]

The active magnetic shield device of the present invention has such a structure that it is possible to provide a magnetic shield device having both the advantages of the one-sensor system and the six-sensor system. That is, the magnetic shield device of the present invention is
It is supposed to be able to cope with changes in the surrounding environment such as layout changes, and also to cope with magnetic fluctuations in the vicinity in a wide frequency band. Specifically, a group of magnetic sensors for measuring the amount of magnetic fluctuation, and a group of electromagnetic coils provided corresponding to each of the magnetic sensors, respectively, for independently applying a magnetic field to the magnetic sensor installation location, , Each magnetic sensor and a controller group for associating and controlling each magnetic coil provided corresponding to each magnetic sensor, based on the amount of magnetic fluctuation obtained by the measurement by each magnetic sensor, respectively. Independently, a magnetic field is applied to each magnetic sensor installation location in a direction that cancels magnetic fluctuations, and a space surrounded by a group of electromagnetic coils is formed as a magnetic shield space, and the predetermined area is formed in the magnetic shield space. At least,
Magnetic sensors are provided at two positions sandwiching the predetermined region in the predetermined direction of the magnetic shield space and at positions between the two positions in the magnetic shield space so as to measure the amount of magnetic fluctuation along the predetermined direction. By providing an electromagnetic coil for applying a magnetic field along the predetermined direction at each magnetic sensor position so as to cancel the magnetic fluctuation amount along the predetermined direction measured at each magnetic sensor position, Has been achieved. By setting the predetermined direction of the predetermined region to three directions of the X-axis direction, the Y-axis direction, and the Z-axis direction, it is possible to control the magnetic field with higher accuracy than in the case of only one direction.

The active magnetic shield method of the present invention comprises:
With this configuration, one sensor system and six
It is possible to provide a magnetic shield method that has the advantages of the sensor system, and it is possible to deal with changes in the surrounding environment such as layout changes, and also to deal with magnetic fluctuations in the vicinity in a wide frequency band.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment example of an active magnetic shield device of the present invention will be described with reference to the drawings. Figure 1
1A is a schematic diagram of a first example of an embodiment of an active magnetic shield device of the present invention, FIG. 1B is a diagram showing a configuration of each controller, FIG. 2 is an X-axis, a Y-axis, It is the schematic of the 2nd example of embodiment of the active magnetic shield apparatus of this invention which has arrange | positioned the magnetic sensor with respect to each direction of Z-axis. 1 and 2, 110 is a magnetic shield space, 121
~ 123 magnetic sensor, 121a ~ 123a wiring,
131 to 133 are electromagnetic coils, 131a to 133a are wirings, 141 to 143 are controllers, 210 is a magnetic shield space, 221 to 223 are electromagnetic coils, 225 to 22
8 is a magnetic sensor. Incidentally, in FIG. 1, A1, B1, A
c, Bc, A2, and B2 mean respective positions on the wiring.

A first example of the embodiment of the active magnetic shield apparatus of this example will be described with reference to FIG. The active magnetic shield device of the first example is a magnetic shield device for attenuating a magnetic fluctuation from the outside to a predetermined region where a magnetic shield is required and actively magnetically shielding the magnetic field in the X-axis direction. Magnetic sensors 121, 122, 123 are provided at two positions sandwiching a predetermined region where a shield is required and at positions between the two positions so as to measure the amount of magnetic fluctuation along the predetermined direction, and An electromagnetic coil 13 that applies a magnetic field along the predetermined direction at each magnetic sensor position so as to cancel the amount of magnetic fluctuation along the predetermined direction measured at each magnetic sensor position.
1, 132, 133 are arranged, these magnetic sensors,
A controller 141 for controlling the electromagnetic coil in association with the electromagnetic coil,
142 and 143. Then, the amount of current flowing through the corresponding electromagnetic coil is independently controlled so that the amount of magnetic fluctuation at each magnetic sensor position becomes zero. The active magnetic shield device of the first example is X
This device is particularly effective when it has a gradient of the magnetic fluctuation amount only in the axial direction. Here, it is assumed that the device is used in such an environment.

As the magnetic sensors 121 to 123, a semiconductor magnetic sensor, an optical fiber magnetic sensor, and a SQUID (Superconduc) for detecting a magnetic fluctuation amount.
toning Quantum Interferenc
e Device: superconducting quantum interference device) A magnetometer (also referred to as a magnetometer) is used, and it is used according to environmental characteristics and purposes. In this example, a first-order differential type SQUID magnetometer or the like is used. As a broadband magnetic sensor for detecting the amount of magnetic fluctuation that can follow a change in magnetic field at a level of several Hz to 1 KHz or more, for example, one manufactured by Magban Co. is known. The electromagnetic coils 131 to 133 have the magnetic shield space 1
The wiring 10 is surrounded by the wirings 131a, 132a and 133a in a box shape.

The controllers 141 to 143 are connected to the magnetic sensors 121 to 123 and the electromagnetic coils 131 to 133 and control them by associating them with each other, and independently, the magnetic fluctuation amount at each magnetic sensor position. The amount of current flowing through the corresponding electromagnetic coil is controlled so that each of the values becomes zero. In this example, according to the amount of magnetic fluctuation measured by the magnetic sensors 121, 122, 123, the magnetic sensors 131, 132, 133 are independently controlled so that the magnetic fluctuation becomes zero. As shown in FIG. 1B, each controller includes a power supply, an arithmetic processing unit (processor), an A / D converter, a D / A converter, and a current amplifier, and each input from a magnetic sensor. A
A / D conversion is performed by the A / D converter, and a predetermined calculation is performed by the calculation processing unit based on the obtained data value from the A / D conversion unit, and the data value obtained by the calculation processing unit is D / A. D / A conversion is performed by the conversion unit and analogized to obtain current,
Further, this is amplified by a current amplifier to obtain a predetermined current, which is passed through a corresponding electromagnetic coil to cancel the magnetic fluctuation amount.

An example of the operation of attenuating the magnetic fluctuation of the active magnetic shield device of this example will be described with reference to FIG. It should be noted that, instead of this, the description of the embodiment of the active magnetic shield method of the present invention will be omitted. In FIG. 1, for example, the magnetic fluctuation transmitted from the left side (P0 side) to the right side (P1 side) is attenuated by distance, and is transmitted via B1 and B2.
It is assumed that they are respectively transmitted to the controllers 141 and 143. First, the controllers 141 and 143
The measured values of the magnetic sensors 121 and 123 are
Independently, the electromagnetic coils 131 and 133 of the electromagnetic coils 131 and 133, which are respectively taken into the A / D converter 1 and the A / D converter 3, are arranged at the respective magnetic sensor positions in a direction of canceling the magnetic fluctuations at the respective magnetic sensor positions. Control the amount of current. That is, the amounts of currents flowing through the corresponding electromagnetic coils 131 and 133 are controlled in real time so that the magnetic fluctuations at the positions of the magnetic sensors 121 and 123 are always zero. Next, in this state, the magnetic shield space 11
A magnetic sensor 122 arranged in a predetermined area where a magnetic shield of 0 is required monitors the amount of magnetic fluctuation in a predetermined area where a magnetic shield is required. The controller 142 determines the value of the current flowing through the electromagnetic coil 132 arranged at the position of the magnetic sensor 122 by the controller 142.
By controlling independently of 31, 31, 133, the electromagnetic gradient shape in the predetermined region in the magnetic shield space 110 where the magnetic shield is required can be made arbitrary. Normal,
The electromagnetic coil 1 arranged at the position of the magnetic sensor 122
The value of the current flowing through 32 is controlled by the controller 142.
It is controlled independently of the electromagnetic coils 131 and 133, and is independently controlled so that the measured value (value of the magnetic fluctuation amount) of the magnetic sensor at the position of the magnetic sensor 122 becomes zero. In this way
Substantially, the gradient of the magnetic fluctuation amount between the magnetic sensors 121, 122 and 123 can be made zero.

If the magnetic sensor 122 cannot be physically arranged at a place where magnetic fluctuation is desired to be suppressed, the magnetic sensor 122 and the electromagnetic coil 132 are installed at an alternative place,
Based on the measured value of the magnetic sensor at that position (the value of the amount of magnetic fluctuation), the value of the current flowing through the electromagnetic coil at the alternative location may be controlled so that the magnetic fluctuation at the desired location becomes zero. It is necessary to predict the gradient shape of the amount of magnetic fluctuation in the magnetic shield space to some extent and simulate it. In this case, the magnetic shield space extends along the predetermined direction at two locations sandwiching the predetermined region in the predetermined direction. Since the simulation is performed after the magnetic fluctuation amount is controlled to be 0, it can be reduced to an extremely simple function system as compared with that in the conventional active magnetic shield of the 3-axis 1-sensor system. In this case, at least assuming that the magnetic fluctuation amount inside the shield can be approximated by a first-order gradient or a second-order gradient, the amount of current flowing through the electromagnetic coil at this place should be controlled so that the magnetic fluctuation amount at the alternative place becomes zero. If
It can be conceived that the magnetic fluctuation amount approaches 0 even at a portion where the magnetic fluctuation is actually desired to be suppressed.

In the case of the present example, in particular, the magnetic fluctuations at two positions sandwiching the predetermined region in the predetermined direction of the magnetic shield space, that is, at the positions of the magnetic sensors 122 and 123 are controlled to be always zero, and then the magnetic field is controlled. A magnetic sensor 122 is provided at a position between the two locations inside the shielded space 110 so as to measure the amount of magnetic fluctuation along the predetermined direction, and the measured value of this magnetic sensor 122 (the value of the amount of magnetic fluctuation) is set. Originally, since the magnetic fluctuation at the position of the magnetic sensor 122 is monitored and controlled, even if the magnetic fluctuation amount in the X-axis direction changes due to the change of the surrounding environment of the magnetic shield space 110, there is no influence. , Can be used as a true active magnetic shield device. Furthermore, since each control system is independent and simple, the calculation speed of the control system can be made as high as possible, so that the performance can be maintained in a wide frequency band.

In this example, the magnetic sensor (corresponding to 122) and the electromagnetic coil (corresponding to 132) inside the magnetic shield space are arranged at only one place, but the present invention is not limited to this, and is arranged at a plurality of places. It doesn't matter. By arranging at a plurality of positions, it becomes possible to approximate a more complicated magnetic gradient curve, and it is possible to provide a magnetic shield device having an extremely high degree of freedom.

Next, a second example of the embodiment of the active magnetic shield apparatus of the present invention will be briefly described with reference to FIG. In the second example, as shown in FIG. 2, an outer surface (hereinafter, also referred to as a shield surface) of a rectangular (room-shaped) magnetic shield space is arranged so as to be orthogonal to X, Y, and Z directions. Then, arrange each magnetic sensor and each electromagnetic coil,
Regarding each direction, the X-axis direction, the Y-axis direction, and the Z-axis direction, each magnetic sensor, as in the embodiment shown in FIG.
Each electromagnetic coil is associated with and controlled by a corresponding controller. The active magnetic shield device of the second example is a device that can effectively cope with magnetic fluctuations in almost all directions such as the X-axis direction, the Y-axis direction, and the Z-axis direction. Used below. In the present example, in each axial direction, at the shield surface positions facing each other of the electromagnetic shield space 110 orthogonal to each axis,
A magnetic sensor that measures magnetic fluctuations in the axial direction is arranged, and a magnetic sensor that measures magnetic fluctuations in the axial direction is also provided between a pair of magnetic sensors arranged on their opposing shield surfaces. It is to be placed. Incidentally, FIG.
Then, in order to make it easy to see, for convenience, the magnetic sensor 225,
A magnetic sensor paired with 226 and 227 is not shown. The magnetic sensor 225 and a pair of magnetic sensors (not shown) are magnetic sensors for measuring magnetic fluctuations in the Z-axis direction, and the magnetic sensor 226 and a pair of magnetic sensors (not shown) are Y. A magnetic sensor for measuring magnetic fluctuations in the axial direction. A magnetic sensor 227 and a magnetic sensor (not shown) paired with the magnetic sensor 227 are magnetic sensors for measuring magnetic fluctuations in the X-axis direction.
Is a magnetic sensor that measures magnetic fluctuations in the three axial directions of the X-axis direction, the Y-axis direction, and the Z-axis direction. In the first example, each magnetic sensor and each electromagnetic coil are arranged along only one direction of the X-axis direction, but in the second example, the X-axis direction and the Y-axis direction are arranged.
Each magnetic sensor and each electromagnetic coil are arranged along the axial direction and the Z-axis direction, and in the case of the second example, generally, an active magnetic shield device having higher accuracy than in the case of the first example is obtained. be able to. Also in the case of the second example, the number of arrangement axes is simply increased, and the control thereof is basically the same as that of the control method of the first example. In the case of the second example as well, since the control system for each magnetic sensor and each electromagnetic coil in each axial direction is independent and simple, the calculation speed of the control system can be made as high as possible, so that a wide range of frequencies can be used. Performance can be maintained in the band.

[0018]

As described above, the present invention makes it possible to provide a magnetic shield device which has the advantages of both the one-sensor type active shield and the six-sensor type active shield (active type). This makes it possible to efficiently suppress magnetic fluctuations at arbitrary points in the shield region.

[Brief description of drawings]

FIG. 1A is a schematic diagram of a first example of an embodiment of an active magnetic shield device of the present invention, and FIG. 1B is a diagram showing a configuration of each controller.

FIG. 2 is a schematic view of a second example of the embodiment of the active magnetic shield device of the invention.

[Explanation of symbols]

110 Magnetically shielded space 121-123 Magnetic sensor 121a to 123a wiring 131-133 electromagnetic coil 131a to 133a wiring 141-143 controller 210 Magnetically shielded space 221 to 223 electromagnetic coil 225-228 Magnetic sensor

Claims (5)

[Claims] 1. A magnetic shield device for attenuating a magnetic fluctuation from the outside to a predetermined region where a magnetic shield is required and for actively performing magnetic shield, the magnetic shield device for measuring a magnetic fluctuation amount. It is provided corresponding to each sensor group and each magnetic sensor, each independently,
Each of the magnetic sensors includes an electromagnetic coil group for applying a magnetic field to a location where the magnetic sensor is provided, and a controller group for associating and controlling each magnetic sensor and each electromagnetic coil provided corresponding thereto. Based on the amount of magnetic fluctuation obtained by the measurement, the magnetic field is applied to each magnetic sensor installation location in a direction that cancels the magnetic fluctuation.
A space surrounded by a group of electromagnetic coils is formed as a magnetic shield space, and the predetermined region is arranged in the magnetic shield space, and at least two places sandwiching the predetermined region in a predetermined direction of the magnetic shield space. , A magnetic sensor is provided at a position between the two locations in the magnetic shield space so as to measure the amount of magnetic fluctuation along the predetermined direction, and the magnetic sensor is provided at the predetermined direction measured at each magnetic sensor position. An active magnetic shield device characterized in that an electromagnetic coil for applying a magnetic field along the predetermined direction is arranged at each magnetic sensor position so as to cancel the magnetic variation along the magnetic sensor. 2. The active magnetic shield device according to claim 1, wherein the predetermined directions are three directions of an X axis direction, a Y axis direction, and a Z axis direction. 3. The A / D according to claim 1, wherein the controller A / D converts a signal from the magnetic sensor.
A conversion unit, an arithmetic processing unit that performs a predetermined arithmetic operation based on the data value from the A / D conversion unit, and a data value obtained by the arithmetic processing unit is D / A converted and further current-amplified. An active magnetic shield device comprising an A converter and a current amplifier, wherein a predetermined current is passed through each electromagnetic coil so as to cancel out the amount of magnetic fluctuation at each magnetic sensor installation position. 4. An active magnetic shield method for actively magnetically attenuating a magnetic fluctuation from the outside to a predetermined region where a magnetic shield is required, wherein at least the predetermined region is Magnetic sensors are provided at both ends of the space to be arranged in the predetermined direction facing each other and at positions between the both ends in the magnetic shield space so as to measure the amount of magnetic fluctuation along the predetermined direction, and , An electromagnetic coil for applying a magnetic field along the predetermined direction is arranged for each magnetic sensor position so as to offset the magnetic fluctuation amount measured along the predetermined direction at each magnetic sensor position. An active magnetic shield characterized by controlling the amount of current flowing through the corresponding electromagnetic coil so that the magnetic fluctuation amount at the sensor position becomes zero. Method. 5. The active magnetic shield method according to claim 4, wherein the predetermined directions are three directions of an X axis direction, a Y axis direction, and a Z axis direction.