Production device for semiconductor device

Abstract

A production device (10) is equipped with a base (12), a table (14) on which a semiconductor device or a material therefor is placed, and a moving mechanism (16) which moves the table (14) with respect to the base (12). The moving mechanism (16) comprises: a movable unit (30) which moves together with the table (14) in a predetermined slide direction; a vibration-damping unit (20) connected with the base (12) so as to be movable in a direction for diminishing a reaction force received from the movable unit (30); a conductor plate (52) which is mounted on the base (12) and is non-magnetic and electroconductive; and a magnetic-flux formation mechanism (70) affixed to the vibration-damping unit (20) and forming a magnetic flux passing through the conductor plate (52). The conductor plate (52) has a shape such that the amount of the magnetic flux capable of passing through the material of the conductor plate (52) increases as the movable unit (30) approaches an end of a stroke.

Description

Semiconductor device manufacturing equipment

[0001] The present specification discloses a semiconductor device manufacturing apparatus including a base, a table, and a moving mechanism for moving the table relative to the base.

[0002] Conventionally, semiconductor device manufacturing apparatuses (hereinafter referred to as "manufacturing apparatuses") such as wire bonding apparatuses and die bonding apparatuses have been used. Such manufacturing apparatuses typically have a table on which a semiconductor device or a substrate is placed, and a movement mechanism that moves the table relative to a base.

[0003] The movement mechanism includes, for example, a movable unit and a vibration-damping unit that moves the movable unit by electromagnetic action. The movable unit is connected to a table, and the vibration-damping unit is connected to a base. If the vibration-damping unit is completely fixed to the base, a reaction force generated when the movable unit moves is transmitted to the base via the vibration-damping unit. This can cause unintended vibrations in the base and, ultimately, the table.

[0004] Patent Document 1 discloses an XY table for semiconductor manufacturing equipment. This XY table has a driver (corresponding to a movable unit) that moves with the table and a motor (corresponding to a vibration control unit) that moves the movable unit by electromagnetic action. Patent Document 1 also describes a mechanism that allows the motor to move in the opposite direction to the driver when the driver is driven. This configuration allows the movement of the motor to counteract the reaction force generated by the movement of the driver. As a result, unintended vibrations of the base and, ultimately, the table can be suppressed, further improving the quality of the manufactured semiconductor devices.

[0005] Patent No. 4021158

[0006] To suppress vibrations of the table, it is better for the damping unit to have a small damping rate of motion. However, if the damping rate at the end of the vibration damping unit's stroke is small, the vibration damping unit may collide with other components, causing a large impact. As a result, unintended vibrations of the table may not be sufficiently suppressed.

[0007] Therefore, this specification discloses a manufacturing apparatus that can more effectively suppress unintended vibrations of the table.

[0008] The manufacturing apparatus disclosed in this specification comprises a base, a table on which a semiconductor device or its material is placed, and a moving mechanism for moving the table relative to the base, wherein the moving mechanism comprises a movable unit connected to the table and moving in a predetermined sliding direction together with the table, a vibration control unit connected to the base so as to be movable in a direction to eliminate the reaction force received from the movable unit, a non-magnetic and conductive conductor plate attached to either the vibration control unit or the base, and a damper magnet attached to the other of the vibration control unit or the base, moving relative to the conductor plate within a predetermined stroke as the vibration control unit moves, and generating magnetic flux that penetrates the conductor plate, wherein the conductor plate is shaped such that when the generated magnetic flux density is uniform, the magnetic flux that penetrates the material of the conductor plate is greater at its ends than at its center.

[0009] In this case, the end of the conductive plate may have a width that continuously increases as it approaches the end point of the stroke.

[0010] The conductive plate may have a plurality of adjustment holes formed therein, and the area of ​​the adjustment holes may be smaller at the end portions of the conductive plate than at the central portion.

[0011] The conductive plate may be fixed to the base, and the damper magnet may be fixed to the vibration control unit.

[0012] Furthermore, a magnetic damper including the conductor plate and the damper magnet may be disposed on both sides of the center of gravity of the vibration control unit.

[0013] In addition, a pair of the damper magnets may be arranged facing each other across the conductor plate, and an adjustment mechanism may be provided for changing the relative positions of the pair of damper magnets in order to change the facing area of ​​the pair of damper magnets.

[0014] The semiconductor device manufacturing apparatus disclosed in this specification can increase the damping force of the vibration damping unit's motion at the ends of the stroke compared to the center of the stroke. As a result, the damping force of the vibration damping unit near the center of the stroke can be reduced, while effectively preventing the vibration damping unit from colliding with the stroke end. This can more effectively suppress unintended vibrations of the table.

[0015] It is a schematic plan view of a manufacturing apparatus. It is a schematic side view of a moving mechanism. It is a cross-sectional view taken along the line AA in FIG. 2. It is a schematic view explaining the action of a magnetic damper. It is a view showing an example of an adjustment mechanism. It is a view showing another example of a conductor plate 52.

[0016] The configuration of the manufacturing apparatus 10 will be described below with reference to the drawings. FIG. 1 is a schematic plan view of the manufacturing apparatus 10. The manufacturing apparatus 10 includes a table 14 on which a semiconductor device or a material therefor (hereinafter, both will be collectively referred to as "object") is placed, and a tool head that performs a predetermined process on the object. The tool head is not shown in FIG. 1. The material for the semiconductor device includes, for example, a substrate or a wafer. The tool head also performs processes such as transfer, wire bonding, and die bonding on the object.

[0017] The table 14 is movable in a first axis direction Af and a second axis direction As perpendicular to the first axis direction Af. To move the table 14, the manufacturing apparatus 10 further includes a first moving mechanism 16f and a second moving mechanism 16s.

[0018] The second moving mechanism 16s moves the table 14 in the second axial direction As. The first moving mechanism 16f moves the table 14 together with the movable unit 30s of the second moving mechanism 16s in the first axial direction Af.

[0019] The first moving mechanism 16f is broadly divided into a movable unit 30f and a vibration suppression unit 20f. The movable unit 30f is further broadly divided into a sub-table 31f and a mover 32f. The sub-table 31f is disposed below the table 14 and holds the table 14 via a movement guide 40s. The table 14 is movable in the first axis direction Af together with the sub-table 31f and in the second axis direction As along the movement guide 40s.

[0020] The mover 32f is fixed to the sub-table 31f. A coil 34f is wound around the mover 32f. When a voltage is supplied to the coil 34f, the movable unit 30f moves in the first axis direction Af according to the direction of the current caused by the voltage.

[0021] The vibration damping unit 20f includes a frame 23f and a permanent magnet 24f. The frame 23f is attached to the base 12 via an absorption guide 42 (not visible in FIG. 1 ), which will be described later. A plurality of permanent magnets 24f, which form magnetic poles, are attached to the frame 23f. The movable unit 30f moves in the first axial direction Af due to electromagnetic interaction between the permanent magnets 24f and the coils 34f. The weight of the vibration damping unit 20f is significantly greater than the weight of the movable unit 30f. The reason for this configuration will be described later. In this example, the coils 34f are provided on the mover 32f, and the permanent magnets 24f are provided on the stator 22f. However, the configuration of the mover 32f and the stator 22f may be modified as appropriate, as long as the movable unit 30f can be moved linearly. For example, the permanent magnets 24f may be provided on the mover 32f, and the coils 34f may be provided on the stator 22f. Moreover, instead of the permanent magnet 24f, an electromagnet that generates a magnetic flux by passing a current through a coil may be used.

[0022] The second moving mechanism 16s has a configuration similar to that of the first moving mechanism 16f. That is, the second moving mechanism 16s is also roughly divided into a vibration control unit 20s and a movable unit 30s. The mover 32s of the movable unit 30s is connected to the table 14 and moves the table 14 in the second axial direction As. As described above, the table 14 is connected to the movable unit 30f of the first moving mechanism 16f via the movement guide 40s, and therefore both the table 14 and the movable unit 30s move in the first axial direction Af.

[0023] Like the vibration damping unit 20f, the vibration damping unit 20s has a frame 23s and a stator 22s. The frame 23f is attached to the base 12 via an absorption guide 42 (not visible in FIG. 1). The stator 22s also has a permanent magnet 24s that generates electromagnetic interaction with the coil 34s. When a voltage is supplied to the coil 34s, the movable unit 30s moves in the second axial direction As depending on the direction of the current caused by the voltage. Here, in the second moving mechanism 16s, the weight of the vibration damping unit 20s is also sufficiently greater than the weight of the movable unit 30s. The configurations of the movable unit 32s and the stator 22s may also be modified as appropriate.

[0024] Here, the reason why the weight of the vibration damping units 20f, 20s is made greater than the weight of the movable units 30f, 30s will be explained. Note that the basic principle is the same for the first moving mechanism 16f and the second moving mechanism 16s, so in the following, the subscripts f and s will be omitted and they will be described as "moving mechanism 16." The same applies to other members.

[0025] Fig. 2 is a schematic side view of the movement mechanism 16, and Fig. 3 is a cross-sectional view taken along line A-A in Fig. 2. As described above, the frame 23 of the vibration damping unit 20 is attached to the base 12 via the absorption guides 42. The absorption guides 42 are, for example, slide rails, and hold the vibration damping unit 20 so that it can slide in a predetermined guide direction. This guide direction (and therefore the sliding direction of the vibration damping unit 20) is parallel to the movement direction of the movable unit 30.

[0026] As described above, in this example, the vibration damping unit 20 is movable in a direction parallel to the direction of movement of the movable unit 30. This configuration is used so that the reaction force generated when the movable unit 30 moves can be absorbed by the sliding movement of the vibration damping unit 20. In other words, when the movable unit 30 moves in a predetermined sliding direction to move the table 14, the vibration damping unit 20 receives an equal but opposite force as a reaction to that movement. At this time, if the vibration damping unit 20 is fixed to the base 12, this opposite force is transmitted to the base 12. When the base 12 vibrates due to this force, the vibration is also transmitted to the table 14, which is indirectly connected to the base 12, resulting in a deterioration in the quality of the semiconductor device.

[0027] In this example, in order to suppress vibration of the base 12, and therefore the table 14, the vibration suppression unit 20 is made slidable relative to the base 12. With this configuration, when the movable unit 30 slides, the vibration suppression unit 20 slides in the opposite direction to the movable unit 30. For example, in Figure 2, when the movable unit 30 moves to the right on the page, the vibration suppression unit 20 moves to the left on the page. This suppresses the transmission of force to the base 12, and suppresses vibration of the table 14.

[0028] Here, the acceleration of the vibration damping unit 20 when it slides is inversely proportional to the weight of the movable unit 30 and the weight of the vibration damping unit 20. Therefore, the greater the weight of the vibration damping unit 20 compared to the weight of the movable unit 30, the smaller the acceleration of the vibration damping unit 20, and therefore the movement distance. In this example, the weight of the vibration damping unit 20 is made sufficiently greater than the weight of the movable unit 30 in order to keep short the movement distance of the vibration damping unit 20 required to suppress recoil.

[0029] However, there is a limit to how heavy the vibration damping unit 20 can be. As a result, it is often not possible to sufficiently reduce the travel distance of the vibration damping unit 20 required to suppress recoil. On the other hand, excessively increasing the stroke of the vibration damping unit 20 leads to another problem: the overall size of the device increases. For this reason, in many cases, the stroke of the vibration damping unit 20 is limited within a certain range. However, in this case, there is a risk that the vibration damping unit 20 will collide with the end of its stroke, i.e., the stroke end, causing an unnecessary impact.

[0030] Therefore, some proposals have been made to place a mechanical damper at the stroke end to mitigate the impact of a collision of the vibration control unit 20. However, it is difficult to absorb impact using a mechanical damper alone. Furthermore, in many cases, a large mechanical damper is required, which leads to problems such as an increase in the size and cost of the device. Another proposal has been made to impart sliding resistance to the absorption guide 42, thereby damping the movement of the vibration control unit 20. However, in this case, it is difficult to change the characteristics of the sliding resistance once installed, making it difficult to adjust the damping force in accordance with the position and speed of the vibration control unit 20, aging deterioration of the absorption guide 42, etc.

[0031] In this example, the vibration control unit 20 is provided with magnetic dampers 50 in order to effectively prevent collisions with the stroke end of the vibration control unit 20 and effectively suppress unintended vibrations of the table 14. The magnetic dampers 50 use the Lorentz force as a damping force. As shown in FIG. 3 , the magnetic dampers 50 are provided on both sides of the vibration control unit 20, one on each side. The magnetic dampers 50 are also located at approximately the same height as the center of gravity of the vibration control unit 20. In other words, the magnetic dampers 50 are provided symmetrically on either side of the center of gravity of the vibration control unit 20.

[0032] The magnetic damper 50 includes a conductor plate 52 and a magnetic flux generating mechanism 70. The conductor plate 52 is a flat plate made of a non-magnetic, electrically conductive metal, such as copper or aluminum. The conductor plate 52 is fixed to the base 12 with its longitudinal direction parallel to the sliding direction. In this example, multiple mounting posts 44 extend from the top surface of the base 12. Both longitudinal ends of the conductor plate 52 are fixed to the mounting posts 44.

[0033] The conductor plate 52 in this example has a shape in which the area per unit distance in the sliding direction increases as it approaches the stroke end. Specifically, the conductor plate 52 has a central portion 56 located in the center, a transition portion 58 that gradually widens as it approaches the end from the central portion 56, and an end portion 54 that extends from the transition portion 58 toward the end. Therefore, the width of the end portion 54 is sufficiently larger than the width of the central portion 56. The reason for this shape will be described later. Note that the width of the conductor plate 52 refers to the dimension in a direction perpendicular to both the direction of the magnetic flux generated by the pair of damper magnets 74 and the direction of movement of the vibration control unit 20.

[0034] The magnetic flux forming mechanism 70 forms a magnetic flux that penetrates the conductor plate 52. As shown in Figures 2 and 3, the magnetic flux forming mechanism 70 is fixed to the frame 23 of the vibration damping unit 20. Here, since the vibration damping unit 20 is slidable with respect to the base 12, the magnetic flux forming mechanism 70 fixed to the vibration damping unit 20 is movable relative to the conductor plate 52 fixed to the base 12.

[0035] The magnetic flux generating mechanism 70 includes a bracket 72 and a pair of damper magnets 74. The bracket 72 is made of a nonmagnetic, insulating material such as resin and is fixed to the frame 23. The bracket 72 is generally U-shaped and has a groove that opens upward. A portion of the conductor plate 52 is inserted into this groove. The bracket 72 also holds a pair of damper magnets 74. The damper magnets 74 are arranged on both sides of the groove, and therefore the conductor plate 52. The two damper magnets 74 face each other across the conductor plate 52. The two damper magnets 74 are magnetized in the same direction. The vertical range of the damper magnet 74 completely encompasses the vertical range of the conductor plate 52. Therefore, a magnetic flux that penetrates the conductor plate 52 is generated between the two damper magnets 74. In this example, the damper magnet 74 is a permanent magnet, but the damper magnet 74 may be an electromagnet that generates a magnetic flux by passing a current through a coil.

[0036] Next, the operation of the magnetic damper 50 will be described with reference to Fig. 4. Fig. 4 is a schematic diagram illustrating the operation of the magnetic damper 50. As mentioned previously and as shown in Fig. 4, a magnetic flux Fm is generated between the two damper magnets 74, penetrating the conductor plate 52. In Fig. 4, the magnetic flux Fm of the magnets is indicated by a dashed arrow.

[0037] Consider the case where the conductor plate 52 moves in the direction of arrow B in Figure 4 as the vibration damping unit 20 slides. In this case, the conductor plate 52 moves through the magnetic flux Fm of the magnet. As a result, an eddy current is generated in the conductor plate 52, the magnitude of which corresponds to the moving speed of the vibration damping unit 20 and the amount of magnetic flux Fm that penetrates the conductor plate 52. Due to this eddy current, a magnetic flux Fp1 in the opposite direction to the magnetic flux Fm of the magnet is generated in the portion of the conductor plate 52 that approaches the damper magnet 74, and a magnetic flux Fp2 in the same direction as the magnetic flux Fm of the magnet is generated in the portion of the conductor plate 52 that moves away from the damper magnet 74. These magnetic fluxes Fp1 and Fp2 generate a repulsive force between the portion of the conductor plate 52 that approaches the damper magnet 74 and the damper magnet 74, and an attractive force between the portion of the conductor plate 52 that moves away from the damper magnet 74 and the damper magnet 74. This repulsive force and attractive force act as a damping force that damps the sliding movement of the magnetic flux generating mechanism 70 and, ultimately, the vibration suppression unit 20 .

[0038] The damping force generated by the magnetic damper 50 increases as the relative speed of the conductor plate 52 relative to the magnetic flux generating mechanism 70 increases, and also as the amount of magnetic flux penetrating the conductor plate 52 increases. In this example, the end portion 54 of the conductor plate 52 is wider than the central portion 56. Therefore, the amount of magnetic flux Fm penetrating the conductor plate 52 increases when the vibration damping unit 20 approaches the stroke end compared to when the vibration damping unit 20 is located at the stroke center. As a result, this example reduces the damping force when the vibration damping unit 20 is located at the stroke center and increases the damping force only when the vibration damping unit 20 approaches the stroke end. Furthermore, this example generates a larger damping force the faster the vibration damping unit 20 moves. As a result, this example effectively prevents the recoil caused by the movement of the movable unit 30 from being transmitted to the base 12 while effectively preventing the vibration damping unit 20 from colliding with the stroke end. As a result, this example effectively prevents unintended vibrations from occurring in the table 14, improving the manufacturing quality of semiconductor devices.

[0039] Furthermore, the magnetic damper 50 has as its main components a conductor plate 52 and a pair of damper magnets 74 sandwiching the conductor plate 52. In this case, the magnetic damper 50 can be made significantly smaller than a mechanical damper. As a result, the overall device can be made smaller. Furthermore, the conductor plate 52 and the damper magnets 74, which are the main components of the magnetic damper 50, are not in contact with each other. Therefore, deterioration due to wear and tear is less likely to occur compared to a mechanical damper or an absorption guide 42 with sliding resistance. As a result, the configuration of this example allows the vibration damping performance of the table 14 to be stably maintained over the long term.

[0040] Furthermore, in this example, the magnetic dampers 50 are arranged on both sides of the center of gravity of the vibration control unit 20. This configuration effectively prevents the vibration control unit 20 from pivoting due to the damping force. That is, if the magnetic dampers 50 are arranged on only one side, in other words, if the damping force is applied to only one point, the vibration control unit 20 may pivot around that point as a fulcrum and tilt. On the other hand, by arranging the magnetic dampers 50 on both sides of the vibration control unit 20 as in this example, it is possible to effectively prevent the vibration control unit 20 from pivoting.

[0041] At least one of the conductor plate 52 and the damper magnet 74 may be replaceable in case of deterioration of the conductor plate 52 or the damper magnet 74. For example, the conductor plate 52 may be fixed to the base 12 using a detachable fastening member such as a screw or a clip. Furthermore, the damper magnet 74 may be detachable from the bracket 72, or the magnetic flux generating mechanism 70 itself may be detachable from the vibration control unit 20.

[0042] Furthermore, an adjustment mechanism may be provided to change the position of at least one of the two damper magnets 74 in order to change the opposing area between the two damper magnets 74. The damper magnets 74 are permanent magnets, but their magnetic force may decrease due to aging or other factors. In this case, the desired damping force may not be obtained. Furthermore, the required damping force may change depending on the situation. Therefore, it may be desirable to appropriately change the amount of magnetic flux between the two damper magnets 74. In such cases, it may be possible to replace the damper magnets 74 themselves, but this would not only increase running costs but also pose the problem of the difficulty of preparing a magnet with the appropriate magnetic force to obtain the desired damping force.

[0043] Therefore, an adjustment mechanism may be provided to change the position of the damper magnets 74 so as to change the opposing area between the two damper magnets 74. The adjustment mechanism is not particularly limited as long as it can change the relative positional relationship between the two damper magnets 74. FIG. 5 shows an example of the adjustment mechanism. As shown in FIG. 5, one of the two damper magnets 74 is housed in a guide hole 78. The guide hole 78 is a hole formed in the bracket 72 and holds the damper magnet 74 movably in a predetermined adjustment direction (left and right directions on the page in the illustrated example). The adjustment mechanism includes a positioning spring 82 that biases the damper magnet 74 in one adjustment direction and a positioning screw 80 that presses the damper magnet 74 in the opposite adjustment direction. The adjustment direction position of the damper magnet 74 can be changed by adjusting the tightening amount of the positioning screw 80.

[0044] Here, as shown in the upper part of Figure 4, if the centers of the two damper magnets 74 are misaligned, the amount of magnetic flux Fm generated between them will be reduced accordingly, and the damping force of the vibration control unit 20 will also be reduced. On the other hand, as shown in the lower part of Figure 4, consider a case in which the relative positions of the two damper magnets 74 are changed using an adjustment mechanism to increase the opposing area. In this case, the amount of magnetic flux Fm generated between them will increase by the amount of the increased opposing area, and the damping force of the vibration control unit 20 will also increase. In other words, by providing an adjustment mechanism that changes the relative positions of the two damper magnets 74, fine adjustment of the damping force is possible. As a result, unintended vibrations of the table 14 can be more effectively prevented. Naturally, the configuration of the adjustment mechanism is not limited to the above, and other configurations may be used.

[0045] Furthermore, the configurations described above are merely examples, and other configurations may be modified as appropriate as long as the configuration described in claim 1 is included. For example, the conductor plate 52 may have any other shape as long as the magnetic flux penetrating the material of the conductor plate 52 is greater at its ends than at its center when the generated magnetic flux density is uniform. Therefore, as shown in FIG. 6 , multiple adjustment holes 60 may be formed in the conductor plate 52, and the density of the adjustment holes 60 may decrease toward the ends of the stroke. Furthermore, a barrier member (e.g., a plate made of a non-magnetic material) that prevents the magnetic flux Fm from penetrating the center portion 56 of the conductor plate 52 may be attached.

[0046] The arrangement and number of magnetic dampers 50 may also be changed as appropriate. For example, only one magnetic damper 50 may be provided, or three or more magnetic dampers 50 may be provided. Furthermore, the magnetic damper 50 is not limited to being attached to the side surface of the frame 23, but may also be attached to the top surface or bottom surface. Furthermore, in the above description, the conductor plate 52 is fixed to the base 12, but the conductor plate 52 may be fixed to the vibration control unit 20, and the magnetic flux generating mechanism 70 may be fixed to the base 12.

[0047] 10 Manufacturing apparatus, 12 Base, 14 Table, 16 Moving mechanism, 16f First moving mechanism, 16s Second moving mechanism, 20 Vibration control unit, 23 Frame, 30 Movable unit, 31f Sub-table, 32 Movable element, 34 Coil, 40 Moving guide, 42 Absorption guide, 44 Mounting support, 50 Magnetic damper, 52 Conductor plate, 54 End portion, 56 Central portion, 58 Transition portion, 60 Adjustment hole, 70 Magnetic flux forming mechanism, 72 Bracket, 74 Damper magnet, 78 Guide hole, 80 Positioning screw, 82 Positioning spring, Fm Magnet magnetic flux.

Claims

1. An apparatus for manufacturing semiconductor devices comprising: a base; a table on which a semiconductor device or material thereof is placed; and a moving mechanism for moving the table relative to the base, wherein the moving mechanism has: a movable unit connected to the table and moving together with the table in a predetermined sliding direction; a vibration control unit connected to the base so as to be movable in a direction to cancel a reaction force received from the movable unit; a non-magnetic and conductive conductor plate attached to either the vibration control unit or the base; and a damper magnet attached to the other of the vibration control unit or the base, moving relatively to the conductor plate within a predetermined stroke as the vibration control unit moves, and generating a magnetic flux that penetrates the conductor plate, wherein the conductor plate is shaped such that when the generated magnetic flux density is uniform, the magnetic flux that penetrates the material of the conductor plate is greater at its ends than at its central portion.

2. An apparatus for manufacturing a semiconductor device according to claim 1, characterized in that the end of the conductive plate has a width dimension which increases continuously as it approaches the end point of the stroke.

3. An apparatus for manufacturing a semiconductor device as described in claim 1, wherein the conductor plate has a plurality of adjustment holes formed therein, and the area of ​​the adjustment holes is smaller at the ends of the conductor plate than at the central portion.

4. An apparatus for manufacturing a semiconductor device according to claim 1, characterized in that the conductor plate is fixed to the base, and the damper magnet is fixed to the vibration control unit.

5. An apparatus for manufacturing a semiconductor device according to claim 1, characterized in that a magnetic damper including the conductor plate and the damper magnet is arranged on either side of the center of gravity of the vibration control unit.

6. An apparatus for manufacturing a semiconductor device as described in claim 1, wherein a pair of the damper magnets are arranged facing each other with the conductor plate in between, and further comprising an adjustment mechanism for changing the relative position of the pair of damper magnets in order to change the facing area of ​​the pair of damper magnets.

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