Patent Publication Number: US-2009219123-A1

Title: Magnet unit for magnetron sputtering system

Description:
CROSS-REFERENCE TO APPLICATION 
     This application is based upon and claims the benefit of priority to Japanese Patent Application No. 2008-50956, filed on Feb. 29, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An aspect of the invention relates to a magnet unit for a magnetron sputtering system. 
     2. Description of the Related Art 
     A magnetron sputtering system has generally been used to form various thin films on a substrate, such as a semiconductor substrate. The magnetron sputtering system performs sputtering using plasma while generating a magnetic field in the vicinity of the surface of a target, which is a sputtering material. A rotary magnet cathode is used to effectively utilize a target and form a uniform thin film by sputtering the target. The rotary magnet cathode is a device that rotates a permanent magnet on the rear surface of the target to rotate a magnetic field having a predetermined pattern in the vicinity of the front surface of the target. A magnet unit formed by arranging a plurality of permanent magnets in a predetermined pattern on a base board (which is also referred to as a yoke) made of a soft magnetic material is used to generate the magnetic field having a predetermined pattern. 
     The plurality of permanent magnets are arranged in a predetermined pattern such that the target is effectively sputtered. In the same arrangement of the magnets, the sputtering speed of the target or the deposition of the target on the substrate depends on the process conditions or the kind of target during sputtering. Therefore, in this case, it is necessary to change the arrangement of the magnets according to the process conditions or the kind of target. In order to change the arrangement of the magnets, magnet units capable of changing the arrangement of some or all of the permanent magnets provided therein have been proposed. 
     For example, a magnet unit has been proposed in which a ring-shaped magnet is provided on a rotatable base board at a position that is eccentric from the center of rotation of the base board, and another central magnet is provided in the ring-shaped magnet, thereby changing the position of one or both of the ring-shaped magnet and the central magnet (for example, see Patent Document 1). 
     In addition, a magnet unit has been proposed in which a plurality of strip-shaped magnets are arranged in a predetermined pattern on a base board, and a plurality of magnet segments are provided in a portion of the strip-shaped magnet, thereby changing the position of each of the magnet segments (for example, see Patent Document 2). 
     Further, a magnet unit of a parallel displacement type, not a rotary type, has been proposed in which a base board is divided into a plurality of plates, and a magnet is provided on each of the divided plates, thereby changing the position of each plate (for example, see Patent Document 3). 
     [Patent Document 1] 
     Japanese Laid-open Patent Publication No. 2004-269952 
     [Patent Document 2] 
     Japanese Laid-open Patent Publication No. 2003-531284 
     [Patent Document 3] 
     Japanese Laid-open Patent Publication No. 2000-212739 
     Patent Document 1 discloses a technique for changing the position of the entire ring-shaped magnet or the entire central magnet. In order to change the position of the magnet, the magnet is detached from the base plate, and then attached to a different position. Since the magnet used for the magnet unit has a very strong attraction force, it is necessary to detach or attach the magnet using a dedicated jig, and it is difficult for persons other than a magnet unit manufacturer to change the position of the magnet. 
     As in Patent Document 2, when the position of a portion of the magnet is changed, in a rotary magnet unit, the center of the magnet unit is also changed, and the rotation balance is broken. Therefore, it is necessary to adjust the rotation balance again, and an operation of adjusting the position of the magnet becomes complicated. The magnet unit disclosed in Patent Document 2 arranges a plurality of thin strip-shaped magnets that overlap each other in a predetermined pattern to obtain a magnetic field pattern, but does not form a magnetic field using leakage flux between a pair of magnets. 
     The magnet unit disclosed in Patent Document 3 is a parallel displacement type, not a rotary type. Therefore, the magnet unit does not consider a rotation balance, and cannot be applied to a rotary magnet unit without any change. 
     SUMMARY 
     According to an aspect of an embodiment, a magnet unit for a magnetron sputtering system includes a base board, an inner magnet fixed to the base board and an outer magnet fixed to the base board. The outer magnet is fixed around the inner magnet, and at least one of a portion of the inner magnet or a portion of the outer magnet is displaceable on the base board. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the overall structure of a magnetron sputtering system; 
         FIG. 2  is a plan view illustrating a magnet unit according to a first embodiment; 
         FIG. 3  is a front view illustrating the magnet unit according to the first embodiment; 
         FIG. 4  is a plan view illustrating the magnet unit when a sliding portion is displaced; 
         FIG. 5  is an enlarged view illustrating the leading end of a pressure screw; 
         FIG. 6  is a cross-sectional view taken along the line V-V of  FIG. 5 ; 
         FIG. 7  is a plan view illustrating the magnet unit when the sliding portion is displaced; 
         FIGS. 8A and 8B  are diagrams illustrating modifications of a sliding groove; 
         FIG. 9  is a plan view illustrating a magnet unit including adjustment weights; 
         FIG. 10  is a front view illustrating the magnet unit shown in  FIG. 9 ; 
         FIG. 11  is an enlarged cross-sectional view taken along the line XI-XI of  FIG. 9 ; 
         FIG. 12  is a plan view illustrating a magnet unit including a mechanism that automatically adjusts a central position such that the central position is not changed when the sliding portion is moved; 
         FIG. 13A  is a front view illustrating a pin sliding jig; 
         FIG. 13B  is a side view illustrating the pin sliding jig; 
         FIG. 14  is a plan view illustrating a magnet unit including two sliding portions fitted into a sliding groove in parallel; 
         FIG. 15  is a cross-sectional view taken along the line XV-XV of  FIG. 14 ; 
         FIG. 16  is a diagram illustrating the movement of a sliding portion by a pin sliding jig; 
         FIG. 17  is a plan view illustrating a magnet unit according to a second embodiment of the invention; 
         FIG. 18  is a front view illustrating the magnet unit according to the second embodiment; 
         FIG. 19  is a plan view illustrating the magnet unit when a rotating portion is rotated; 
         FIG. 20  is an enlarged cross-sectional view taken along the line XX-XX of  FIG. 17 ; and 
         FIG. 21  is a plan view illustrating the magnet unit having a rotating portion provided in an outer magnet. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments of the invention will be described with reference to the accompanying drawings. 
     First, the overall structure of a magnetron sputtering system using a magnet unit according to an embodiment of the invention will be described with reference to  FIG. 1 . 
     A magnetron sputtering system  10  shown in  FIG. 1  sputters a target T, which is a deposition target, in a vacuum chamber  12  to form a film on a substrate W. A substrate holder  14  is provided at an upper part in the vacuum chamber  12 , and the substrate W is mounted on the substrate holder  14  made of an insulating material. A target holder  16  is provided below the substrate holder  14 , and the target T is mounted on the target holder  16 . 
     A magnetron cathode  18  is provided on the rear side of the target T mounted on the target holder  16 . The magnetron cathode  18  includes a magnet unit  20  that generates a magnetic field and a rotating mechanism  22  that rotates the magnet unit  20 . 
     In the above-mentioned structure, the vacuum chamber  12  and the substrate W (substrate holder  14 ) are connected to the ground. A DC power source  24  applies a negative voltage of several hundred volts to the target T (target holder  16 ). In general, in a sputtering method, an inert gas, such as argon (Ar), is used to generate plasma. The inert gas is supplied from a gas supply source  26  to the vacuum chamber  12  through a supply port  12   a.  In addition, the inert gas in the vacuum chamber  12  is discharged by a vacuum pump  28  through an exhaust port  12   b.    
     When a high voltage is applied between the substrate W and the target T, Ar in the vacuum chamber  12  is changed into plasma, and the plasma is confined in the vicinity of the front surface of the target T by the magnetic field generated by the magnet unit  20 . Electrons in the plasma collide with Ar atoms by the voltage applied to the target T to generate Ar ions (Ar+). The Ar ions (Ar+) are accelerated by a sheath electric field generated between the plasma and the target T and collide with the target T. In this way, the target T is sputtered, and the sputtered target material is deposited on the substrate W held by the substrate holder  14 . 
     Next, a magnet unit  20 A according to the first embodiment will be described.  FIG. 2  is a plan view illustrating the magnet unit  20 A according to the first embodiment.  FIG. 3  is a front view illustrating the magnet unit  20 A shown in  FIG. 2 . 
     The magnet unit  20 A includes a base board  30  formed of a soft magnetic material, and an outer magnet  32  and an inner magnet  34  provided on the base board  30 . The outer magnet  32  is a frame-shaped permanent magnet, and the inner magnet  34  is a rectangular permanent magnet. The outer magnet  32  surrounds the inner magnet  34  with a predetermined gap between the outer magnet  32  and the inner magnet  34 . The outer magnet  32  is magnetized such that the upper surface thereof serves as the N-pole and the lower surface thereof serves as the S-pole. In this case, the inner magnet  34  is magnetized such that the upper surface thereof serves as the S-pole and the lower surface thereof serves as the N-pole. Therefore, leakage flux is generated from the upper surface (S-pole) of the inner magnet  34  to the upper surface (N-pole) of the outer magnet. The leakage flux is a magnetic field for confining the plasma. 
     The centers of the outer magnet  32  and the inner magnet  34  are located at a position (eccentric position) that deviates from the center of rotation of the base board  30 . In this state, the center of the magnet unit  20 A is also located at a position (eccentric position) that deviates from the center of rotation of the base board  30 . Therefore, two balance weights  38  are screwed to the base board  30 . The balance weights  38  make it possible to align the center of the magnet unit  20 A with the center of rotation of the base board  30  and smoothly rotate the magnet unit  20 A. 
     In this embodiment, as shown in  FIG. 2 , a portion of the outer magnet  32  and a portion of the inner magnet  34  can slide together with a portion of the base board  30 . A slidable portion (sliding portion  30   a ) of the base board  30  has a strip shape, and is slidably fitted into a sliding groove  30   b  formed in the base board  30 . With the sliding portion  30   a  fitted into the sliding groove  30   b  of the base board  30 , the surface of the sliding portion  30   a  is flush with the surface of the base board  30 . 
     A portion of the outer magnet  32  and a portion of the inner magnet  34  on the sliding portion  30   a  are fixed to the sliding portion  30   a,  and are movable together with the sliding portion  30   a.  That is, a portion  32   a  of the outer magnet  32  and the other portion  32   b  of the outer magnet  32  are formed as individual magnets. The portion  32   a  of the outer magnet  32  and the other portion  32   b  of the outer magnet  32  are combined into the frame-shaped outer magnet  32 . Similarly, a portion  34   a  of the inner magnet  34  and the other portion  34   b  of the inner magnet  34  are formed as individual magnets. The portion  34   a  of the inner magnet  34  and the other portion  34   b  of the inner magnet  34  are combined into the inner magnet  34  as a whole. Since the outer magnet  32  and the inner magnet  34  are formed of materials that are difficult to machine, it is preferable that the magnets be fixed to the base board  30  and the sliding portion  30   a  by an adhesive. In addition, it is preferable that the outer magnet  32  and the inner magnet  34  be symmetric with respect to a line that passes through the center of rotation and is aligned with a direction in which the sliding portion extends. In this way, it is not necessary to balance the rotation of the magnet unit in a direction vertical to the line that passes through the center of rotation and is aligned with the direction in which the sliding portion extends, and it is easy to perform a balance adjusting operation. However, in the structure that balances the rotation of the magnet unit in a direction vertical to the line that passes through the center of rotation and is aligned with the direction in which the sliding portion extends, the arrangement of the outer and inner magnets is not limited to the line symmetry. 
     In the magnet unit  20 A shown in  FIG. 2 , the sliding portion  30   a  is fitted into the sliding groove  30   b  of the base board  30 , and the center of the sliding portion  30   a  in the longitudinal direction is aligned with the center of rotation of the base board  30 . In this state, the outer magnet  32  and the inner magnet  34  serve as one magnet, and a desired magnetic field is formed between the outer magnet  32  and the inner magnet  34 . In this case, it is possible to change or adjust the magnetic field by slightly displacing the sliding portion  30   a  in the sliding groove  30   b.    
     Long holes  30   c  are formed in the vicinities of both ends of the sliding portion  30   a  so as to be elongated in the direction in which the sliding portion  30   a  can move. Screws  31  are tightened to the base board  30  through the long holes  30   c,  thereby fixing the sliding portion  30   a  to the base board  30 . 
       FIG. 4  is a plan view illustrating the magnet unit  20 A when the sliding portion  30   a  is displaced. It is preferable to press the end of the sliding portion  30   a  to displace the sliding portion  30   a.  In this embodiment, in order to press the end of the sliding portion  30   a  to displace the sliding portion  30   a,  a sliding jig  40  is mounted on the side surface of the base board  30 . 
     As shown in  FIG. 4 , the sliding jig  40  includes a supporting portion  40   a  that is screwed to the side surface of the base board  30  and a pressure screw  40   b  that is inserted into a screw hole formed at the center of the supporting portion  40   a.  As shown in  FIG. 5 , the leading end of the pressure screw  40   b  is engaged with an engaging concave portion  30   d  formed at the end of the sliding portion  30   a.    FIG. 5  is an enlarged view illustrating the leading end of the pressure screw  40   b,  and  FIG. 6  is a cross-sectional view taken along the line V-V of  FIG. 5 . 
     It is possible to tighten the pressure screw  40   b  to press the sliding portion  30   a,  and it is possible to loose the pressure screw  40   b  to pull out the sliding portion  30   a.  In this way, it is possible to displace the sliding portion  30   a  at a desired position in the sliding groove  30   b.  As shown in  FIG. 4 , portions of the outer magnet  32  and the inner magnet  34  can be displaced. When portions of the outer magnet  32  and the inner magnet  34  are displaced, the magnetic field also varies. Therefore, it is possible to adjust the magnetic field by adjusting the displacement of the magnets. 
     In the example shown in  FIG. 4 , the pressure screw  40   b  of the sliding jig  40  is engaged with one end of the sliding portion  30   a  to apply pressing force and tensile force to the sliding portion  30   a.  However, as shown  FIG. 7 , sliding jigs  40 A may be provided at both sides of the sliding portion  30   a.  In this case, the sliding jigs  40 A just press the sliding portion  30   a,  and the leading ends of pressure screws  40 Ab of the sliding jigs  40 A just come into contact with the ends of the sliding portion  30   a.  That is, the engaging concave portion  30   d  is not provided in the sliding portion  30   a,  and no engaging portion is formed in the leading end of the pressure screw  40 Ab. 
     In addition, the sliding jigs  40  and  40 A may be removed after a displacement adjusting operation. 
     The shape of the sliding groove  30   b  of the base board  30  into which the sliding portion  30   a  is slidably fitted is not limited to the rectangular shape shown in  FIG. 3 , but the sliding groove  30   b  may have other shapes. For example, as shown in  FIG. 8A , the sliding groove  30   b  may have an inverted trapezoidal shape (so-called dovetail groove) such that the sliding portion  30   a  does not come off from the base board  30 . In this way, it is possible to improve stability during a magnet adjustment operation. Alternatively, as shown in  FIG. 8B , comb-shaped uneven portions may be provided in the bottom of the sliding groove  30   b,  and uneven portions corresponding to the comb-shaped uneven portions may be formed in the bottom of the sliding portion  30   a.  In this case, magnetic resistance between the sliding portion  30   a  and the base board  30  is reduced, and it is possible to form a strong magnetic field. 
     In the above-described embodiment, when the sliding portion  30   a  is moved, the central position of the magnet unit  20 A is changed. When the central position of the magnet unit is changed, the rotation balance is adjusted by changing the positions of the balance weights  38 . In addition, it is possible to easily adjust the rotation balance by attaching detachable adjustment weights  44  to both ends of the sliding portion  30   a,  without changing the positions of the balance weights  38 , as shown in  FIGS. 9 to 11 . 
       FIG. 9  is a plan view illustrating a magnet unit  20 B including the sliding portion  30   a  having the adjustment weights  44  attached thereto.  FIG. 10  is a front view illustrating the magnet unit  20 B shown in  FIG. 9 .  FIG. 11  is an enlarged cross-sectional view taken along the line XI-XI of  FIG. 9 . In  FIGS. 9 to 11 , the same components as those shown in  FIG. 2  are denoted by the same reference numerals, and a description thereof will be omitted. 
     The magnet unit  20 B has the same basic structure as the magnet unit  20 A shown in  FIG. 2  except that the adjustment weights  44  that are moved together with the sliding portion  30   a  are provided and non-magnetic members  46 ,  47 ,  48 , and  49  are provided on the sliding portion  30   a.  As shown in  FIG. 9  and  FIG. 10 , the non-magnetic members  46 ,  47 ,  48 , and  49  are slightly lower than the outer magnet  32 , and are formed of a non-magnetic material having a specific gravity slightly larger than that forming the outer magnet  32  and the inner magnet  34 . In addition, the weight per area of the non-magnetic members is substantially equal to that of the outer magnet  32  and the inner magnet  34 . When the non-magnetic members  46 ,  47 ,  48 , and  49  are attached to the sliding member  30   a,  the portion  32   a  of the outer magnet  32 , the portion  34   a  of the inner magnet  34 , and the non-magnetic members  46 ,  47 ,  48 , and  49  mounted on the sliding portion  30   a  become a strip-shaped member having a substantially uniform weight distribution in the longitudinal direction. 
     The adjustment weights  44  are screwed to the non-magnetic members  46  and  49  that are provided at both ends of the sliding portion  30   a.  A plurality of adjustment weights  44  (three adjustment weights in  FIG. 9 ) are provided at one end of the sliding portion  30   a,  and three adjustment weights  44  are also provided at the other end. 
     In the state shown in  FIG. 9 , assume that the sliding portion  30   a  is moved a distance corresponding to the thickness of one adjustment weight  44  to change the positions of portions of the outer magnet  32  and the inner magnet  34 . Then, one end of the sliding portion  30   a  protrudes a distance corresponding to the thickness of one adjustment weight  44 , and the other end of the sliding portion is recessed a distance corresponding to the thickness of one adjustment weight  44 . In this case, one adjustment weight  44  is detached from the protruding end, and the detached adjustment weight  44  is attached to the adjustment weights  44  at the recessed end. In the example shown in  FIG. 9 , when the sliding portion  30   a  is moved towards the balance weights  38 , the sliding portion protrudes a distance corresponding to the thickness of one adjustment weight  44  on the side of the balance weight  38 , and the protruding adjustment weight  44  is detached such that two adjustment weights  44  remain on the side of the balance weight  38 . Then, the detached adjustment weight  44  is attached to the three adjustment weights  44  on the opposite side. One adjustment weight  44  is added to the three adjustment weights  44  on the opposite side, and four adjustment weights fill up the recessed portion. 
     As described above, the adjustment weight  44  protruding by the movement of the sliding portion  30   a  is detached, and the detached adjustment weight is attached to the opposite side. In this way, even when the slider  30   a  is moved, the central position does not vary, and it is possible to align the central position of the magnet unit  20 B with the center of rotation. 
     In the example shown in  FIG. 9 , the adjustment weights  44  are manually detached and attached to adjust a weight balance. However, a mechanism that automatically adjusts the central position when the sliding portion is moved may be provided.  FIG. 12  is a plan view illustrating a magnet unit  20 C including the mechanism that automatically adjusts the central position such that the central position does not vary when the sliding portion is moved. In  FIG. 12 , the same components as those shown in  FIG. 9  are denoted by the same reference numerals, and a description thereof will be omitted. 
     In the magnet unit  20 C, the sliding portion  30   a  is divided into two portions by the center of rotation. A fixed pin  50  is provided at the center of rotation of the base board  30 . In addition, a movable pin  52  is provided in one of the two divided portions, that is, a sliding portion  30   a - 1 , and another movable pin  52  is provided in the other portion, that is, a sliding portion  30   a - 2 . 
     In the above-mentioned structure, it is possible to use a pin sliding jig  54  shown in  FIG. 13  to move the movable pins  52  at the same distance from the fixed pin  50  in the opposite directions.  FIG. 13A  is a front view illustrating the pin sliding jig  54 , and  FIG. 13B  is a side view illustrating the pin sliding jig  54 . The pin sliding jig  54  includes a pin engaging portion  54   a  and a handle portion  54   b.  The pin engaging portion  54   a  is provided with a pin hole  56  into which the fixed pin  50  is fitted and pin holes  58  into which the two movable pins  52  are fitted. The pin hole  56  into which the fixed pin  50  is fitted is a circular hole having a sufficient size for the fixed pin  50  to be inserted. The pin holes  58  into which the movable pins  52  are fitted are holes that are elongated in the horizontal direction such that the movable pins  52  can be moved in the holes. 
     In this way, it is possible to use the pin sliding jig  54  to displace (move) the sliding portion  30   a - 1  and the sliding portion  30   a - 2  in the opposite directions. That is, the pin sliding jig  54  is arranged such that the fixed pin  50  and the two movable pins  52  are inserted into the pin holes  56  and  58  of the pin sliding jig  54 , respectively, and the pin sliding jig  54  is rotated about the fixed pin  50 . Then, the pin holes  58  are rotated on the fixed pin  50 . However, the movable pins  52  can be moved only in the direction in which the sliding portion  30   a - 1  and the sliding portion  30   a - 2  can move (in the direction in which the sliding groove  30   b  extends). Therefore, the movable pins are moved in a direction corresponding to the rotation of the pin holes  58 , and the sliding portion  30   a - 1  and the sliding portion  30   a - 2  are moved along the sliding groove  30   b  in the direction in which they approach or are separated from each other.  FIG. 12  shows the state in which the sliding portion  30   a - 1  and the sliding portion  30   a - 2  are slightly displaced to be separated from each other. 
     As described above, the sliding portion  30   a - 1  and the sliding portion  30   a - 2  are moved the same distance in the direction in which they are symmetric with respect to the center of rotation of the magnet unit  20 C. Therefore, even when the sliding portion  30   a - 1  and the sliding portion  30   a - 2  are moved, the central position of the magnet unit  20 C does not vary. As a result, after the sliding portion  30   a - 1  and the sliding portion  30   a - 2  are moved to adjust the magnetic field, it is not necessary to perform an operation of adjusting the weights to adjust the central position, and it is possible to simplify an operation of adjusting the magnetic field. 
     Two sliding portions may be fitted into a sliding groove in parallel so as to move in the opposite directions.  FIG. 14  is a plan view illustrating a magnet unit  20 D including two sliding portions fitted into a sliding groove in parallel. In  FIG. 14 , the same components as those shown in  FIG. 2  are donated by the same reference numerals, and a description thereof will be omitted. 
     The magnet unit  20 D shown in  FIG. 14  includes two sliding portions  30   a  in a sliding groove  30   b.  A fixed pin  60  is provided in the vicinities of the sliding portions  30   a  between the sliding portions  30   a  on the bottom (that is, the base board  30 ) of the sliding groove  30   b.  In addition, movable pins  62  are provided in two sliding portions  30   a -A and  30   a -B. The two movable pins  62  are provided at both sides of the fixed pin  60  that is erected from the base board  30  so as to be symmetric with respect to the fixed pin. 
       FIG. 15  is an enlarged cross-sectional view taken along the line XV-XV of  FIG. 14 . The fixed pin  60  is vertically provided in the base board  30 . One of the movable pins  62  is vertically provided in the sliding portion  30   a -A, and the other movable pin  62  is also vertically provided in the sliding portion  30   a -B. 
     In the magnet unit  20 D having the above-mentioned structure, it is possible to use a pin sliding jig  64  shown in  FIG. 16  to move the movable pins  62  in the opposite direction. An operation of moving the sliding portions  30   a -A and  30   a -B using the pin sliding jig  64  is the same as that of moving the sliding portions  30   a - 1  and  30   a - 2  in the magnet unit  20 C shown in  FIG. 12 . 
     That is, the pin sliding jig  64  is arranged such that the fixed pin  60  and the two movable pins  62  are inserted into the pin holes  66  and  68  of the pin sliding jig  64 , respectively, and the pin sliding jig  64  is rotated about the fixed pin  60 . Then, the pin holes  68  are rotated on the fixed pin  60 . However, the movable pins  62  can be moved only in the direction in which the sliding portion  30   a -A and the sliding portion  30   a -B can move (in the direction in which the sliding groove  30   b  extends). Therefore, the movable pins are moved in a direction corresponding to the rotation of the pin holes  68 , and the sliding portion  30   a -A and the sliding portion  30   a -B are moved along the sliding groove  30   b  in the opposite directions. In  FIG. 14 , the sliding portion  30   a -A is slightly moved in the upward direction, and the sliding portion  30   a -B is slightly moved in the downward direction. 
     As described above, the sliding portion  30   a -A and the sliding portion  30   a -B are moved the same distance in the direction in which they are symmetric with respect to the center of rotation of the magnet unit  20 D. Therefore, even when the sliding portion  30   a -A and the sliding portion  30   a -B are moved, the central position of the magnet unit  20 D does not vary. As a result, when the sliding portion  30   a -A and the sliding portion  30   a -B are moved to adjust the magnetic field, it is not necessary to perform an operation of adjusting the weights to adjust the central position, and it is possible to simplify an operation of adjusting the magnetic field. 
     Further, in the above-described embodiment, the shapes of the inner magnet and the outer magnet are not limited to those shown in the drawings. However, for example, the inner magnet may have a circular shape, and the outer magnet may have a circular ring shape that surrounds the inner magnet. Alternatively, the inner magnet and the outer magnet may have any shapes as long as portions of the inner and outer magnets can be deformed. The shape of the inner magnet and the shape of the outer magnet may depend on the pattern of a magnetic field to be formed. In addition, the outer magnet  32  does not need to completely surround the inner magnet  34 , and the outer magnet  32  may have any shape and arrangement as long as it can form a leakage magnetic field between the outer magnet  32  and the inner magnet  34 . 
     It is preferable that the inner magnet and the outer magnet have shapes and arrangement so as to be symmetric with respect to a line passing through the center of rotation, but the invention is not limited thereto. The inner magnet and the outer magnet may have any shapes and arrangement. In this case, it is preferable to adjust weights to align the central position of the magnet unit with the center of rotation. 
     In this embodiment, the sliding portion is slidably mounted on the base board such that the center line (a line passing through the center of rotation) of the base board is aligned with the center line of the sliding portion, as shown in the drawings, but the position of the sliding portion is not limited thereto. The sliding portion may be provided such that the center line of the sliding portion deviates from the center line (a line passing through the center of rotation) of the base board. 
     According to the above-mentioned structure, both the portion  32   a  of the outer magnet  32  and the portion  34   a  of the inner magnet  34  are not necessarily fixed to the upper surface of the sliding portion  30   a,  but any one of them may be fixed to the sliding portion  30   a  such that it can be displaced. 
     Next, a magnet unit according to a second embodiment will be described with reference to  FIGS. 17 to 21 .  FIG. 17  is a plan view illustrating a magnet unit  20 E according to the second embodiment, and  FIG. 18  is a front view illustrating the magnet unit  20 E. In  FIGS. 17 and 18 , the same components as those shown in  FIG. 2  are denoted by the same reference numerals, and a description thereof will be omitted. 
     Similar to the magnet unit according to the first embodiment, the magnet unit  20 E includes a base board  30 , and an outer magnet  32  and an inner magnet  34  fixed to the base board  30 . However, no sliding portion is provided in the magnet unit  20 E, but the magnet unit  20 E includes a rotating portion  70  that can rotate a portion  34   a  of the inner magnet  34 . 
       FIG. 19  is a plan view illustrating the magnet unit  20 E when the rotating portion  70  is rotated. It is possible to fix the inner magnet  34  with the semicircular portion  34   a  thereof being rotated by rotating the rotating portion  70 . In this way, it is possible to displace a portion of the inner magnet  34  to change or adjust the magnetic field formed by the outer magnet  32  and the inner magnet  34 . 
       FIG. 20  is an enlarged cross-sectional view taken along the line XX-XX of  FIG. 17 .  FIG. 20  shows the sectional structure of the rotating portion  70 . The rotating portion  70  includes a movable base board  30   e  and the portion  34   a  of the inner magnet  34 . The movable base board  30   e  is a circular board, and is rotatably accommodated in a circular concave portion  30   f  formed in the base board  30 . The portion  34   a  of the inner magnet  34  is a cylinder having a semicircular shape in a cross-sectional view, and is fixed to the movable base board  30   e  by an adhesive. 
     The movable base board  30   e  is supported by a detachment preventing member  72  from the rear side of the base board  30  while it is accommodated in the circular concave portion  30   f  of the base board  30 . The detachment preventing member  72  is provided at the center of the movable base board  30   e,  and the movable base board  30   e  can be rotated about the center of the detachment preventing member  72  in the circular concave portion  30   f.    
     The movable base board  30   e  is supported by the detachment preventing member  72 , and is fixed by a fixing screw  74 . The fixing screw  74  passes through an arc-shaped long hole formed in the rear surface of the base board and is then tightened to the movable base board  30   e.  When the fixing screw  74  is loosened, the rotating portion  70  including the movable base board  30   e  can rotate. When the fixing screw  74  is tightened, the rotating portion  70  including the movable base board  30   e  is fixed. In this way, it is possible to rotate the portion  34   a  of the inner magnet  34  of the rotating portion  70 , and change or adjust the magnetic field formed by the outer magnet  32  and the inner magnet  34 . 
     When the rotating portion  70  is rotated, the portion  34   a  of the inner magnet  34  is rotated, and the central position of the magnet unit  20 E slightly deviates. However, it is possible to adjust the deviation of the central position by changing the positions of the balance weights  38 . Alternatively, as represented by a dotted-chain line in  FIG. 20 , a non-magnetic member  76  having specific gravity and height that are more than or equal to those of the inner magnet  34  and a weight per area that is substantially equal to that of the inner magnet  34  may be provided in the movable base board  30   e.  In this case, even when the rotating portion  70  is rotated, the central position of the magnet unit does not vary. 
     In this embodiment, the rotating portion  70  is provided to rotate the portion  34   a  of the inner magnet  34 . However, as in a magnet unit  20 F shown in  FIG. 21 , a rotating portion  80  that rotates the portion  32   a  of the outer magnet  32  may be provided. The structure of the rotating portion  80  is the same as that of the rotating portion  70  shown in  FIG. 20 , and thus a description thereof will be omitted. 
     In this embodiment, the size of the magnet is half the size of the rotating portion, but the invention is not limited thereto. The magnet may have a circular shape having any size. In addition, the position of the rotating portion is not limited to that shown in the drawings, but the rotating portion may be disposed at any position around the magnet. The rotating portions may be provided in both the outer magnet  32  and the inner magnet  34 , and a plurality of rotating portions may be provided in the outer magnet  32  and the inner magnet  34 . 
     As described above, according to the second embodiment, it is possible to change or adjust the magnetic field generated by a magnet unit with a simple operation, without detaching or removing a magnet. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.