Patent Publication Number: US-2023154721-A1

Title: Ion gun and vacuum processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2021/025743, filed Jul. 8, 2021, which claims the benefit of International Patent Application No. PCT/JP2020/028362, filed Jul. 22, 2020, both of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an ion gun and a vacuum processing apparatus. 
     Description of the Related Art 
     An ion gun is an apparatus for ejecting generated ions as an ion beam, and is used in a vacuum processing apparatus or the like used for manufacturing a semiconductor device or the like. Among them, a type of an ion gun called a closed drift ion source has an ejection port of an ion beam that literally forms a closed loop and is used in various fields by taking advantage of the feature that an area of the ion gun can be easily increased. 
     While the closed drift type ion gun has an advantage that plasma generation and ion acceleration can be performed simultaneously, due to its structure, it is inevitable that some accelerated ions collide with a magnetic pole constituting the ejection port. Therefore, the magnetic pole is eroded over time, the discharge stability gradually decreases, and finally the discharge cannot be maintained. In addition to the erosion of the magnetic pole, this also causes problems such as contamination of a processed object due to the eroded magnetic pole material, heat generation of the magnetic pole, and reduction in etching rate due to beam loss. 
     As a method for solving such a problem, U.S. Patent Application Publication No. 2005/0247885 discloses an ion gun in which a mirror ratio in the vicinity of an ejection port is increased. Further, U.S. Patent Application Publication No. 2004/0016640 discloses an ion gun in which magnetic poles are coated with a member having high sputtering resistance. 
     As a closed drift type ion gun, it is known that there is an ion gun having an annular ejection port in which a linear portion and a curved portion are combined in shape. However, the present inventors find it clear for the first time that, in such an ion gun, if the linear portion and the curved portion are designed in the same manner, the amount of ions colliding with the magnetic pole is small in the linear portion but very large in the curved portion. In U.S. Patent Application Publication No. 2005/0247885 and U.S. Patent Application Publication No. 2004/0016640, no special consideration has been made on the shape of the ejection port. 
     An object of the present invention is to provide an ion gun capable of improving the ejection efficiency and uniformity of an ion beam and stably operating over a long period of time, and a vacuum processing apparatus using it. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present disclosure, there is provided an ion gun including an anode, a cathode opposed to the anode and having a first portion and a second portion, and a magnet configured to form a magnetic field space between the first portion and the second portion. An annular gap including a curved portion is provided between the first portion and the second portion of the cathode. The magnet is configured to form, between the first portion and the second portion of the curved portion, a magnetic field line having a bottom inside a cross-sectional centerline of the gap. 
     According to another aspect of the present disclosure, there is provided an ion gun including an anode, a cathode opposed to the anode and having a first portion and a second portion, and a magnet configured to form a magnetic field space between the first portion and the second portion. An annular gap including a curved portion is provided between the first portion and the second portion of the cathode. The first portion is arranged inside with respect to the gap and the second portion is arranged outside with respect to the gap. The magnet is configured to form, in a space between the first portion and the second portion and the anode, a magnetic field line in a direction from the second portion to the first portion. In the curved portion, a magnetic field vector at a point where the magnetic field line and the cross-sectional centerline of the gap intersect with each other is inclined to a side of the first portion and the second portion, with respect to a plane orthogonal to the cross-sectional centerline, at a first angle smaller than 1.5 degrees and larger than 0 degrees to a side of the anode. 
     According to yet another aspect of the present disclosure, there is provided an ion beam adjusting method adjusting an ion beam ejected from a gap in an ion gun including an anode, a cathode opposed to the anode and having a first portion and a second portion, and a magnet configured to form a magnetic field space between the first portion and the second portion wherein the gap is an annular gap including a curved portion and provided between the first portion and the second portion of the cathode. The method including adjusting a center position of an ion beam ejected from the gap by shifting a position of a bottom of a magnetic field line formed between the first portion and the second portion of the curved portion toward inside of a cross-sectional centerline of the gap. 
     According to yet another aspect of the present disclosure, there is provided an ion beam adjusting method adjusting an ion beam ejected from a gap in an ion gun including an anode, a cathode opposed to the anode and having a first portion and a second portion, and a magnet configured to form a magnetic field space between the first portion and the second portion wherein the gap is an annular gap including a curved portion and provided between the first portion and the second portion of the cathode, wherein the first portion is arranged inside with respect to the gap and the second portion is arranged outside with respect to the gap, and wherein the magnet is configured to form, in a space between the first portion and the second portion and the anode, a magnetic field line in a direction from the second portion to the first portion. The method including adjusting a center position of an ion beam ejected from the gap by inclining in the curved portion, a magnetic field vector at a point where the magnetic field line and the cross-sectional centerline of the gap intersect with each other toward the anode with respect to a plane orthogonal to the cross-sectional centerline. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view illustrating a structure of an ion gun according to a first embodiment of the present invention. 
         FIG.  2    is a plan view illustrating the structure of the ion gun according to the first embodiment of the present invention. 
         FIG.  3 A  is a schematic cross-sectional view illustrating the structure of the ion gun according to the first embodiment of the present invention (Part  1 ). 
         FIG.  3 B  is a schematic cross-sectional view illustrating the structure of the ion gun according to the first embodiment of the present invention (Part  2 ). 
         FIG.  4    is an enlarged schematic cross-sectional view illustrating a structure in the vicinity of an ejection port of the ion gun according to the first embodiment of the present invention. 
         FIG.  5 A  is a diagram illustrating the operation of the ion gun according to the first embodiment of the present invention (Part  1 ). 
         FIG.  5 B  is a diagram illustrating the operation of the ion gun according to the first embodiment of the present invention (Part  2 ). 
         FIG.  6    is a schematic diagram illustrating a magnetic mirror force generated by a mirror magnetic field. 
         FIG.  7 A  is a diagram illustrating the structure and operation of the ion gun according to a reference example (Part  1 ). 
         FIG.  7 B  is a diagram illustrating the structure and operation of the ion gun according to the reference example (Part  2 ). 
         FIG.  8    is a schematic diagram illustrating the magnetic mirror force generated in a curved portion of the ejection port. 
         FIG.  9    is a schematic diagram illustrating an electrical relationship among plasma, an anode, and a magnetic pole plate in the curved portion of the ejection port. 
         FIG.  10    is a schematic diagram illustrating a direction of a magnetic field vector on a cross-sectional center line of an ejection port. 
         FIG.  11    is a schematic diagram illustrating movement of electrons in a space between the magnetic pole plate and the anode. 
         FIG.  12    is a graph illustrating a result of simulation of the amount of an erosion of the magnetic pole plate when the position of the bottom of the magnetic field line is changed. 
         FIG.  13    is a graph illustrating a simulation result of a relationship between a distance under a magnet and a bottom position of a magnetic field line. 
         FIG.  14    is a graph illustrating the relationship between a distance from the anode on the centerline of the cross-section of the ejection port and the slope angle of the magnetic field line acquired by simulation. 
         FIG.  15    is a perspective view illustrating the structure of a second embodiment of the present invention. 
         FIG.  16    is a diagram illustrating the operation of an ion gun according to the second embodiment of the present invention. 
         FIG.  17    is a graph illustrating a result of a simulation performed on an eroded amount of a magnetic pole plate caused by a change in the sectional area of a first yoke. 
         FIG.  18    is a diagram illustrating the operation of an ion gun according to a third embodiment of the present invention. 
         FIG.  19    is an enlarged view around magnetic pole plates covered with first magnetic pole covers. 
         FIG.  20    is an enlarged view around magnetic pole plates covered with second magnetic pole covers. 
         FIG.  21    is an enlarged view around magnetic pole plates covered with third magnetic pole covers. 
         FIG.  22    is a schematic diagram illustrating a vacuum processing apparatus according to a fourth embodiment of the present invention. 
         FIG.  23    is a perspective view illustrating the structure of an ion gun in which an annular opening has a perfect circular shape. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A structure of an ion gun according to a first embodiment of the present invention will be described with reference to  FIGS.  1  to  3 B .  FIG.  1    is a perspective view illustrating a structure of an ion gun according to the present embodiment.  FIG.  2    is a plan view illustrating the structure of the ion gun according to the present embodiment.  FIGS.  3 A and  3 B  are schematic cross-sectional views illustrating the structure of the ion gun according to the present embodiment.  FIG.  3 A  is a cross-sectional view taken along line A-A′ of  FIG.  2   , and  FIG.  3 B  is a cross-sectional view taken along line B-B′ of  FIG.  2   . 
     As illustrated in  FIGS.  1  to  3 B , the ion gun  10  according to the present embodiment includes magnetic pole plates  20 A and  20 B, a magnet  32 , a yoke  34 , and an anode  40 , and has a substantially rectangular parallelepiped appearance. As illustrated in  FIG.  1   , an ejection port  22  for ejecting an ion beam is provided in one main surface of the ion gun  10 . 
     The magnetic pole plate  20 A and the magnetic pole plate  20 B are tabular bodies made of a high permeability magnetic material having conductivity. The magnetic pole plate  20 B is an annular tabular body having an opening corresponding to the outer peripheral shape of the magnetic pole plate  20 A. The magnetic pole plate  20 A is arranged inside the opening of the magnetic pole plate  20 B so as to ensure a predetermined gap between the magnetic pole plate  20 A and the magnetic pole plate  20 B. The magnetic pole plate  20 A and the magnetic pole plate  20 B may function as magnetic poles that form a magnetic field space by arranging the magnetic pole plate  20 A and the magnetic pole plate  20 B with a predetermined gap therebetween. The magnetic pole plate  20 A and the magnetic pole plate  20 B also function as a cathode opposed to the anode  40 . The magnetic pole plate  20 A constitutes a first portion of the cathode, and the magnetic pole plate  20 B constitutes a second portion of the cathode. 
     The gap between the magnetic pole plate  20 A and the magnetic pole plate  20 B forms an annular opening along the outer circumference of the magnetic pole plate  20 A and the inner circumference of the magnetic pole plate  20 B. The annular opening formed in this manner constitutes the ejection port  22  of the ion beam. For example, as illustrated in  FIGS.  1  and  2   , the ejection port  22  may include a linear portion  22   a  and a semicircular curved portion  22   b.  The ejection port  22  is preferably annular in order to maintain discharge, but the shape of the ejection port  22  is not particularly limited. For example, an arbitrary shape such as a part of a perfect circular shape as shown in  FIG.  23    or a part of an elliptical shape can be applied to the curved portion  22   b.  The curvature of the curved shape may be constant or may be inconstant. 
     When viewed from the center of the ejection port  22  in a plan view, as illustrated in  FIG.  2   , the magnetic pole plate  20 A is positioned inside the annular ejection port  22 , and the magnetic pole plate  20 B is positioned outside the annular ejection port  22 . In this specification, the expression “inside” of the ejection port  22  indicates the side of the magnetic pole plate  20 A with respect to the ejection port  22 , and the expression “outside” of the ejection port  22  indicates the side of the magnetic pole plate  20 B with respect to the ejection port  22 . 
     The magnetic pole plate  20 A and the magnetic pole plate  20 B are not particularly limited as long as they are high permeability magnetic materials having conductivity, and may be composed of, for example, ferromagnetic stainless steel such as SUS430, SmCo alloy, NdFe alloy, or the like. 
     As illustrated in  FIGS.  2  and  3 A , the magnet  32  and the yoke  34  constitute a structure  30  having an annular recessed portion  36 . The magnetic pole plate  20 A and the magnetic pole plate  20 B are bonded onto the surface of the structure  30  on which the recessed portion  36  is provided so that the ejection port  22  is positioned thereabove along the recessed portion  36 . The yoke  34  is magnetically coupled to the magnet  32  and the magnetic pole plates  20 A and  20 B, and functions as a magnetic conductor for guiding magnetic flux generated from the magnet  32  to the magnetic pole plates  20 A and  20 B. The magnet  32  and the yoke  34  are arranged such that a recessed portion  36  is positioned inside a magnetic circuit (magnetic path) formed by the magnetic pole plates  20 A and  20 B, the magnet  32  and the yoke  34 . The magnet  32  forms a magnetic field space between the magnetic pole plate  20 A and the magnetic pole plate  20 B via the yoke  34 . The yoke  34  and the magnet  32  may be electrically connected to the magnetic pole plates  20 A and  20 B. 
     The yoke  34  is not particularly limited as long as it is a high permeability magnetic material having conductivity, and may be composed of, for example, ferromagnetic stainless steel such as SUS 430, SmCo alloy, NdFe alloy, or the like. The magnet  32  may be a permanent magnet or an electromagnet. The maximum magnetic flux density in the magnetic field formed between the magnetic pole plate  20 A and the magnetic pole plate  20 B by the magnet  32  is preferably about 1000 [Gauss]. 
     The yoke  34  is provided with a gas inlet port  38  communicating with the recessed portion  36  from the outside of the structure  30 . Although  FIGS.  3 A and  3 B  illustrate a plurality of gas inlet ports  38  provided at the bottom of the structure  30 , the number and arrangement of the gas inlet ports  38  are not particularly limited. 
     As illustrated in  FIG.  3 A , the magnet  32  is positioned on the cross-sectional centerline  24  of the ejection port  22  in the linear portion  22   a  of the ejection port  22 . In other words, the distance between the magnet  32  and the magnetic pole plate  20 B is substantially equal to the distance between the magnet  32  and the magnetic pole plate  20 A. Alternatively, the length of the magnetic path between the magnet  32  and the magnetic pole plate  20 B is substantially equal to the length of the magnetic path between the magnet  32  and the magnetic pole plate  20 A. In this specification, the cross-sectional centerline  24  of the ejection port  22  refers to a straight line parallel to the ejection direction (Z direction) of the ion beam passing through the center of the ejection port  22  in the width direction. 
     As illustrated in  FIG.  3 B , the magnet  32  is positioned outside the cross-sectional centerline  24  of the ejection port  22  in the curved portion  22   b  of the ejection port  22 . In other words, the distance between the magnet  32  and the magnetic pole plate  20 B is shorter than the distance between the magnet  32  and the magnetic pole plate  20 A. Alternatively, the length of the magnetic path between the magnet  32  and the magnetic pole plate  20 B is shorter than the length of the magnetic path between the magnet  32  and the magnetic pole plate  20 A. 
     The anode  40  is an annular structure corresponding to the shape of the recessed portion  36 , and is accommodated in the recessed portion  36  away from the magnetic pole plate  20 A, the magnetic pole plate  20 B, the magnet  32 , and the yoke  34 . The anode  40  is opposed to the magnetic pole plates  20 A and  20 B as cathodes, and functions as an acceleration electrode that accelerates ions in plasma generated in the space between the magnetic pole plates  20 A and  20 B and the anode  40  together with the magnetic pole plates  20 A and  20 B. The anode  40  is not particularly limited, but may be fixed to the structure  30  by, for example, a spacer made of an insulating material (not illustrated). The anode  40  may be electrically conductive and does not need to be magnetically considered, and may be composed of, for example, non-magnetic stainless steel. 
     Next, a specific shape of a portion where the magnetic pole plates  20 A and  20 B face the anode  40  will be described with reference to  FIG.  4   .  FIG.  4    is a cross-sectional view conceptually illustrating a shape of a portion where the magnetic pole plates  20 A and  20 B face the anode  40 . 
     The characteristic of the ion beam ejected from the ejection port  22  greatly changes depending on the shape of the tips of the magnetic pole plates  20 A and  20 B, the positional relationship between the magnetic pole plates  20 A and  20 B and the anode  40 , and the like. Therefore, the shapes of the tips of the magnetic pole plates  20 A and  20 B, the positional relationship between the magnetic pole plates  20 A and  20 B and the anode  40 , and the like are appropriately set in accordance with the required characteristics of the ion beam, but are typically set in a relationship as illustrated in  FIG.  4   , for example. 
     For example, as illustrated in  FIG.  4   , the tips of the magnetic pole plates  20 A and  20 B may be formed in a tapered shape with chamfered corner portions on the side facing the anode  40 . When the width of the ejection port  22  is G, the thickness of the thinnest portion of the magnetic pole plates  20 A and  20 B is T 1 , the thickness of the tapered portion is T 2 , and the gap between the magnetic pole plates  20 A and  20 B and the anode  40  is S, all of them can be set to a size of about several millimeters. The taper angle θ may be set to about 45 degrees. The shapes of the tips of the magnetic pole plates  20 A and  20 B and the sizes of the respective portions are merely examples, and are not particularly limited. Depending on the ion gun, corner portions of the magnetic pole plates  20 A and  20 B on the side opposite to the anode  40  may be chamfered. 
     Next, the operation of the ion gun according to the present embodiment will be described with reference to  FIGS.  5 A and  5 B .  FIGS.  5 A and  5 B  illustrate the operation of the ion gun according to the present embodiment.  FIG.  5 A  corresponds to a cross-sectional view taken along line A-A′ of  FIG.  2   , and  FIG.  5 B  corresponds to a cross-sectional view taken along line B-B′ of  FIG.  2   . It is assumed that the cross-section illustrated in  FIGS.  5 A and  5 B  is a surface which appears when the ion gun  10  is cut in a direction in which the width of the ejection port  22  becomes minimum. Further, the cross section of the ion gun in the present application is a surface appearing when the ion gun  10  is cut in a direction where the width of the ejection port  22  is the smallest. 
     First, a discharge gas such as argon (Ar) is supplied to the recessed portion  36  through the gas inlet port  38  to adjust the pressure inside the ion gun  10  to about  0 . 1  Pa. When the pressure of the environment in use (for example, the pressure in the chamber of the vacuum processing apparatus in which the ion gun  10  is installed) is already about 0.1 Pa and discharge is possible in that state, the gas supply operation may be omitted. 
     Next, the magnetic pole plates  20 A and  20 B as cathodes and the yoke  34  are set to a ground potential (0 V), and a voltage of, for example, about 1000 V to 4000 V is applied to the anode  40  from a power source (not illustrated). Thus, an electric field is generated between the anode  40  and the magnetic pole plates  20 A and  20 B, and the gas introduced into the ion gun  10  is excited, dissociated, and ionized by the electric field, thereby generating plasma  50 . 
     On the other hand, the magnetic flux emitted from the N pole of the magnet  32  (magnetic field line  60 ) passes through the yoke  34  and the magnetic pole plate  20 B, and is emitted from the tip of the magnetic pole plate  20 B. The magnetic field lines  60  emitted from the tips of the magnetic pole plates  20 B spread by repulsive force and are then sucked into the magnetic pole plates  20 A. As a result, in the gap space between the magnetic pole plates  20 A and the magnetic pole plates  20 B, as illustrated in  FIG.  5 A  and  FIG.  5 B , the magnetic field lines  60  are vertically convex. The magnetic field space having such a shape is called a mirror magnetic field. The mirror magnetic field acts to confine charged particles therein. 
       FIG.  6    is a schematic diagram illustrating a magnetic mirror force generated by a mirror magnetic field. As illustrated in  FIG.  6   , when the magnetic field lines  60  have dense portions and sparse portions, the charged particles in the mirror magnetic field receive the magnetic mirror force  64  in a direction from the dense portions to the sparse portions of the magnetic field lines  60 . Thereby, the charged particles are confined in the mirror magnetic field. In addition, when the plasma  50  is generated in the mirror magnetic field, the density of the plasma  50  is higher than that when the magnetic field is not present. 
     Electrons in the plasma  50  are drawn into the anode  40  by an electric field between the magnetic pole plates  20 A and  20 B and the anode  40 . Positive ions in the plasma  50  are accelerated by a potential difference between the magnetic pole plates  20 A and  20 B and the anode  40  to form an ion beam  52 . 
     As described above, the ion gun  10  according to the present embodiment is characterized in that the magnet  32  is arranged outside the cross-sectional centerline  24  of the ejection port  22  in the curved portion  22   b  of the ejection port  22 . The reason why the ion gun  10  of the present embodiment is configured in this manner will be described below while comparing with the ion gun of the reference example. 
       FIGS.  7 A and  7 B  are schematic cross-sectional views illustrating the structure and operation of the ion gun according to the reference example.  FIG.  7 A  corresponds to a cross-sectional view taken along line A-A′ of  FIG.  1   , and  FIG.  7 B  corresponds to a cross-sectional view taken along line B-B′ of  FIG.  1   . The ion gun according to the reference example illustrated in  FIGS.  7 A and  7 B  is similar to the ion gun  10  according to the present embodiment except that the arrangement of the magnets  32  is different. That is, in the ion gun according to the reference example, the magnet  32  is arranged at a position opposed to the ejection port  22  with the anode  40  interposed therebetween. That is, the magnet  32  is positioned on the cross-sectional centerline  24  of the ejection port  22  in both the linear portion  22   a  and the curved portion  22   b  of the ejection port  22 . 
     In a general ion gun, a magnetic field is designed so that the magnetic field is symmetrical with respect to the cross-sectional centerline  24  of the ejection port  22 . By designing the magnetic field in this way, the center of the generated plasma  50  is positioned on the cross-sectional centerline  24  of the ejection port  22 , so that collision of the ion beam  52  with the magnetic pole plates  20 A and  20 B can be minimized, and the ion beam  52  can be ejected efficiently. For this purpose, the magnet  32  is often arranged at a position opposed to the ejection port  22  with the anode  40  interposed therebetween, for example, as in the ion gun according to the reference example illustrated in  FIGS.  7 A and  7 B . 
     However, when the magnetic field is designed in the curved portion  22   b  of the ejection port  22  in the same manner as the linear portion  22   a,  as illustrated in  FIG.  7 B , the center of the generated plasma  50  is shifted outside the cross-sectional centerline  24  of the ejection port  22 , and the ion beam  52  which collides with the magnetic pole plate  20 B increases. Thus, the ion beam  52  cannot be efficiently ejected from the ejection port  22 . 
     Further, the magnetic pole plates  20 A and  20 B are eroded over time by the sputtering action of the ion beam  52  which collides with the magnetic pole plates  20 A and  20 B, and the discharge stability gradually decreases, and finally the discharge cannot be maintained. Therefore, the magnetic pole plates  20 A and  20 B need to be periodically replaced, but when the collision ion beam  52  increases, the amount by which the magnetic pole plates  20 A and  20 B are eroded increases, and the maintenance cycle shortens. Further, since particles generated by sputtering the magnetic pole plates  20 A and  20 B cause contamination of the processing apparatus, it is desirable that the amount of ion beams  52  which collide with the magnetic pole plates  20 A and  20 B be as small as possible. 
     The reason why the center of the plasma  50  is shifted outside from the center of the cross-section of the ejection port  22  in the curved portion  22   b  of the ejection port  22  will be described with reference to  FIGS.  8  and  9   .  FIG.  8    is a schematic diagram illustrating a magnetic mirror force generated in the curved portion  22   b  of the ejection port  22 .  FIG.  9    is a diagram illustrating the electrical relationship between the plasma  50  and the anode  40  and the magnetic pole plates  20 A and  20 B in the curved portion  22   b  of the ejection port  22 . 
     As illustrated in  FIG.  8   , the interval between the magnetic field lines  60  of the curved portion  22   b  in the plane parallel to the magnetic pole plates  20 A and  20 B (XY plane) gradually spreads from the magnetic pole plate  20 A toward the magnetic pole plate  20 B. That is, the magnetic field lines  60  are dense on the magnetic pole plate  20 A side (inside of the ejection port  22 ) and sparse on the magnetic pole plate  20 B side (outside of the ejection port  22 ). As a result, as illustrated in  FIG.  7 B , the plasma  50  receives the magnetic mirror force  64  directed outside by the magnetic mirror effect, and shifts toward the magnetic pole plate  20 B side (outside the cross-sectional centerline  24  of the ejection port  22 ). 
     When the areas where the magnetic pole plates  20 A and  20 B are in contact with the plasma  50  in the curved portion  22   b  are compared, the area where the magnetic pole plate  20 B is in contact with the plasma  50  (electrode area) is larger than the area where the magnetic pole plate  20 A is in contact with the plasma  50  (electrode area). Therefore, when the resistance R A  between the plasma  50  and the magnetic pole plate  20 A and the resistance R B  between the plasma  50  and the magnetic pole plate  20 B are compared, the resistance between the magnetic pole plate  20 B having a larger electrode area than the magnetic pole plate  20 A and the plasma  50  becomes smaller (R A &gt;R B ). That is, the magnetic pole plate  20 B acts more strongly as a cathode than the magnetic pole plate  20 A. As a result, a larger amount of current flows toward the magnetic pole plate  20 B, and the plasma  50  is generated more on the magnetic pole plate  20 B side. 
     Due to these two actions, in the curved portion  22   b  of the ejection port  22 , the center of the plasma  50  is shifted outside from the cross-sectional centerline  24  of the ejection port  22 . 
     From this point of view, in the ion gun  10  according to the present embodiment, the magnetic field is designed such that the bottom  62  of the magnetic field line  60  is positioned closer to the magnetic pole plate  20 A than the cross-sectional centerline  24  of the ejection port  22  in the curved portion  22   b  of the ejection port  22  (see  FIG.  5 B ). That is, the center position of the ion beam  52  ejected from the ejection port  22  is adjusted so that the position of the bottom  62  of the magnetic field line  60  formed between the magnetic pole plate  20 A and the magnetic pole plate  20 B of the curved portion  22   b  is shifted inside from the cross-sectional centerline  24  of the ejection port  22 . In the present specification, the bottom  62  of the magnetic field line  60  means a point on the magnetic field line  60  in the space between the magnetic pole plates  20 A and  20 B and the anode  40 , where the tangential direction to the magnetic field line  60  is parallel to the surface of the anode  40  facing the ejection port  22 . 
     From another point of view, in the space between the magnetic pole plates  20 A,  20 B and the anode  40  of the curved portion  22   b,  the direction of the magnetic field line  60  (magnetic field vector) from the magnetic pole plate  20 B toward the magnetic pole plate  20 A is inclined toward the anode  40  with respect to the plane  66  orthogonal to the cross-sectional centerline  24  at the point where the cross-sectional centerline  24  of the ejection port  22  and the magnetic field line  60  intersect with each other (see  FIG.  10   ). That is, in the curved portion  22   b,  the center position of the ion beam  52  ejected from the ejection port  22  is adjusted such that the magnetic field vector at the point where the magnetic field line  60  and the cross-sectional centerline  24  of the ejection port  22  intersect is inclined toward the anode  40  with respect to the plane orthogonal to the cross-sectional centerline  24 . 
       FIG.  11    is a schematic diagram illustrating movement of electrons in a space between the magnetic pole plates  20 A and  20 B and the anode  40 . In the plasma, electrons and ions are continuously generated by ionization. The generated electrons e are attracted to the anode  40  by the potential difference between the magnetic pole plates  20 A and  20 B and the anode  40 , but are also subjected to the force of the magnetic field (Lorentz force), and thus move along the magnetic field line  60  so as to be entangled with the magnetic field line  60 , and reciprocate around the bottom  62 . The reciprocally moving electrons e gradually lose energy due to collision with the gas and are finally collected by the anode  40 . The kinetic energy of the electrons e is highest during the reciprocating motion when the electrons e are positioned at the bottom  62  of the magnetic field line  60 . Therefore, the frequency of occurrence of ionization, that is, the density of the plasma  50  is the highest in the vicinity of the bottom  62  of the magnetic field line  60 . Therefore, if the magnetic field is designed so that the position of the bottom  62  of the magnetic field line  60  is shifted from the cross-sectional centerline  24  of the ejection port  22 , the centers of the plasma  50  and the ion beam  52  can also be shifted from the cross-sectional centerline  24  of the ejection port  22 . 
     Therefore, by shifting the position of the bottom  62  of the magnetic field line  60  toward the inside of the cross-sectional centerline  24  of the ejection port  22  so as to cancel the movement of the plasma  50  toward the outside due to the magnetic mirror effect, the center of the plasma  50  can be shifted to the vicinity of the cross-sectional centerline  24  of the ejection port  22 . Thus, the collision of the ion beam  52  with the magnetic pole plates  20 A and  20 B can be minimized in the curved portion  22   b  of the ejection port  22  as well as in the linear portion  22   a,  and the ion beam  52  can be efficiently ejected. 
     Since the magnetic field lines  60  exist in an unlimited number, the bottoms  62  of the magnetic field lines  60  also exist in an unlimited number. What is important in the present invention is the position of the bottom  62  of the magnetic field lines  60  at the height at which the plasma  50  is generated, as can be seen from the mechanism. In many cases, plasma  50  is generated near a height of about 1 mm from the surface of anode  40 . Thus, in one example, the position of the bottom  62  of the magnetic field line  60  can be defined as the position of the bottom  62  of the magnetic field line  60  at a height of 1 mm from the surface of the anode  40 . 
     Next, the amount by which the position of the bottom  62  of the magnetic field line  60  is shifted inside from the cross-sectional centerline  24  of the ejection port  22  will be described. The shift amount of the bottom  62  was examined by simulation using ELF/MAGIC, which is general-purpose magnetic field analysis software, and PEGASUS, which is general-purpose plasma analysis software. As the conditions of the simulation, the gas for plasma generation was Ar, the pressure in the chamber was 0.07 Pa, and the voltage applied to the anode  40  was 3000 V. 
       FIG.  12    is a graph illustrating a result of simulation of the amount of erosion of the magnetic pole plates  20 A and  20 B when the position of the bottom  62  of the magnetic field line  60  is changed. The vertical axis represents the ratio of the amount of erosion of the magnetic pole plate  20 A to the amount of erosion of the magnetic pole plate  20 B (inside/outside ratio of erosion). The vertical axis represents a value obtained by dividing the larger value of the erosion amount of the magnetic pole plate  20 A and the erosion amount of the magnetic pole plate  20 B by the smaller value. The vertical axis represents a positive value when the erosion amount of the magnetic pole plate  20 B is larger than the erosion amount of the magnetic pole plate  20 A, and represents a negative value when the erosion amount of the magnetic pole plate  20 A is larger than the erosion amount of the magnetic pole plate  20 B. The horizontal axis represents the distance (the position of the bottom of the magnetic field line) from the center line of the cross-section of the ejection port  22  to the bottom  62  of the magnetic field line  60 . The horizontal axis indicates the shift amount when the position of the bottom  62  is shifted outside with respect to the cross-sectional centerline  24  of the ejection port  22  as a positive value and indicates the shift amount when the position of the bottom  62  is shifted inside with respect to the cross-sectional centerline  24  of the ejection port  22  as a negative value. 
     As illustrated in  FIG.  12   , the inside/outside ratio of the erosion of the magnetic pole plates  20 A and  20 B is approximately proportional to the position of the bottom  62  of the magnetic field line  60 . The inside/outside ratio of the erosion increases in the positive direction as the position of the bottom  62  of the magnetic field line  60  shifts in the outside direction, and increases in the negative direction as the position of the bottom  62  of the magnetic field line  60  shifts in the inside direction. 
     For example, when the bottom  62  of the magnetic field line  60  in the curved portion  22   b  of the ejection port  22  is positioned on the cross-sectional centerline  24  of the ejection port  22  (shift amount=0 mm), the centers of the plasma  50  and the ion beam  52  shift outside from the cross-sectional centerline  24  of the ejection port  22  as described above. In this case, the amount of the erosion of the magnetic pole plate  20 B was about 2.1 times the amount of the erosion of the magnetic pole plate  20 A. 
     In order to evenly shift the amount of the erosion of the magnetic pole plates  20 A and  20 B, that is, in order to shift the centers of the plasma  50  and the ion beam  52  to the vicinity of the cross-sectional centerline  24  of the ejection port  22 , the bottom  62  of the magnetic field line  60  may be shifted toward the inside of the cross-sectional centerline  24  of the ejection port  22 . In the example of  FIG.  12   , it has been found that the position of the bottom  62  of the magnetic field line  60  is preferably shifted toward the inside direction by about 0.1 mm to 0.4 mm from the center of the cross-section of the ejection port  22 , and it is optimal to shift toward the inside direction by 0.25 mm from the center of the cross-section of the ejection port  22 . 
     At positions where the inside/outside ratios of the erosion are even, the absolute amount of the erosion of the magnetic pole plates  20 A and  20 B tends to be small. In the study by the inventors, by setting the amount of shift of the bottom  62  to 0.1 mm to 0.4 mm, the amount of the erosion of the magnetic pole plates  20 A and  20 B can be reduced by about 20% at maximum as compared with the case where the amount of shift of the bottom  62  is set to 0 mm. This means that the heat generation of the magnetic pole plates  20 A and  20 B, the contamination of the processed object, the beam loss, and the like can be reduced by about 20% at maximum. 
     Further, in the study by the present inventors, by setting the shift amount of the bottom  62  to 0.1 mm to 0.4 mm, the peak value of the erosion rate can be reduced from about 1/1.3 to about 1/1.8 of that in the case where the shift amount of the bottom  62  is set to 0 mm. This corresponds to an increase in the life of the component and the maintenance cycle of about 1.3 to 1.8 times. 
     Note that an appropriate shift amount of the position of the bottom  62  of the magnetic field line  60  changes depending on the structure of the ion gun  10 , the discharge condition, and the like. For example, when the size of the ion gun  10  or the curvature of the curved portion  22   b  of the ejection port  22  is increased, the optimal shift amount is considered to be larger than the above-described value. The shift amount of the position of the bottom  62  of the magnetic field line  60  is preferably set as appropriate in accordance with the structure of the ion gun  10 , the discharge condition, and the like. 
     Although the method for shifting the position of the bottom  62  of the magnetic field line  60  in the curved portion  22   b  of the ejection port  22  is not particularly limited, an example is a method of changing the position of the magnet  32  as described in the present embodiment. Shifting the position of the bottom  62  of the magnetic field line  60  is to disrupt the symmetry of the magnetic field with respect to the cross-sectional centerline  24  of the ejection port  22 , and it can be said that moving the location of the magnet  32  is the most direct method. 
       FIG.  13    is a graph illustrating a simulation result of a relationship between a distance under a magnet and a bottom position of a magnetic field line. The “distance under magnet” on the vertical axis represents the distance x (see  FIG.  5 B ) from the lower surface of the structure  30  to the lower surface of the magnet  32 . In the “position of bottom of magnetic field line” on the horizontal axis, when the bottom  62  is positioned on the cross-sectional centerline  24  of the ejection port  22  is 0, a position inside the cross-sectional centerline  24  is represented by a negative sign, and a position outside the cross-sectional centerline  24  is represented by a positive sign. In the simulation, the distance x under the magnet  32  is changed without changing the size of the magnet  32 . 
     As illustrated in  FIG.  13   , the distance x under the magnet and the position of the bottom  62  of the magnetic field line  60  are approximately proportional to each other. By changing the distance x under the magnet, the inside/outside balance of the magnetic field in the vicinity of the magnetic pole changes, and the position of the bottom  62  of the magnetic field line  60  changes. By increasing the distance x under the magnet, the position of the bottom  62  of the magnetic field line  60  can be shifted toward the inside direction of the ejection port  22 . According to the simulation result, by setting the distance x under the magnet to Y, the position of the bottom  62  of the magnetic field line  60  can be shifted inside by 0.25 mm. Y may be set to a size of several tens of mm. 
       FIG.  14    is a graph illustrating the relationship between the distance from the anode  40  on the cross-sectional centerline  24  of the ejection port  22  and the inclination angle of the magnetic field lines  60 .  FIG.  14    illustrates simulation results when the positions of the bottoms  62  of the magnetic field lines  60  are set to −0.1 mm, −0.4 mm, and 0 mm, respectively. The position of the bottom  62  of the magnetic field line  60  is indicated by a negative sign at a position inside the cross-sectional centerline  24  of the ejection port  22  and a positive sign at a position outside the cross-sectional centerline  24  of the ejection port  22 . The inclination angle of the magnetic field lines  60  is 0 degree when the magnetic field lines  60  are parallel to the plane  66  ( FIG.  10   ) orthogonal to the cross-sectional centerline  24  of the ejection port  22 , is represented by a negative sign when the magnetic field lines  60  are inclined toward the anode  40 , and is represented by a positive sign when the magnetic field lines  60  are inclined toward the magnetic pole plates  20 A and  20 B. 
     As illustrated in  FIG.  14   , as the magnetic field line  60  is closer to the anode  40 , the inclination angle of the magnetic field line  60  toward the anode  40  at a point intersecting the cross-sectional centerline  24  of the ejection port  22  becomes larger. Further, as the shift amount of the position of the bottom  62  of the magnetic field line  60  increases in the inside direction, the inclination angle toward the anode  40  at the point intersecting the cross-sectional centerline  24  of the ejection port  22  increases. According to the simulation results, it was found that the inclination angle of the magnetic field lines  60  in the range of the shift amount −0.1 mm to −0.4 mm in which improvement was seen in the absolute amount of the erosion of the magnetic pole plates  20 A and  20 B and the peak value of the erosion rate was in the range of 1.5 degrees to −3.5 degrees. 
     That is, in the curved portion  22   b,  it is desirable that the magnetic field vector at the point where the magnetic field line  60  and the cross-sectional centerline  24  of the ejection port  22  intersect be inclined at a first angle in the range of 0 degrees to 1.5 degrees toward the magnetic pole plates  20 A and  20 B and in the range of 0 degrees to 3.5 degrees toward the anode  40  with respect to the plane  66  orthogonal to the cross-sectional centerline  24 . Further, in the linear portion  22   a,  it is desirable that the magnetic field vector at the point at which the magnetic field line  60  and the cross-sectional centerline  24  of the ejection port  22  intersect each other forms a second angle smaller than the first angle with respect to the plane  66  orthogonal to the cross-sectional centerline  24 . The optimal value of the second angle is 0 degrees where the magnetic field vector is parallel to the plane  66 . 
     The method for shifting the position of the bottom  62  of the magnetic field line  60  by disrupting the symmetry of the magnetic field is not limited to the method of moving the position of the magnet  32 . Other methods include, for example, a method for controlling the applied current when an electromagnet is used as the magnet  32 . 
     As discussed above, according to the present embodiment, it is possible to improve efficiency and uniformity of emission of an ion beam. Further, it is possible to suppress collision of an ion beam with magnetic poles and thus realize excellent effects such as a reduction in a temporal change, improvement on a maintenance cycle, or a reduction in the running cost. 
     Second Embodiment 
     An ion gun according to a second embodiment of the present invention will be described with reference to  FIG.  15    and  FIG.  16   .  FIG.  15    is a perspective view illustrating the structure of the ion gun according to the present embodiment.  FIG.  16    illustrates the operation of the ion gun in a cross section taken along a line C-C′ of  FIG.  15   . The same components as those in the ion gun according to the first embodiment are labeled with the same references, and the description thereof will be omitted or simplified. 
     As illustrated in  FIG.  15    and  FIG.  16   , the ion gun  10  according to the present embodiment includes the magnetic pole plates  20 A,  20 B, the magnet  32 , a first yoke  34 A, a second yoke  34 B, an adjustment yoke  34 C, and the anode  40 . As illustrated in  FIG.  15   , one primary surface of the ion gun  10  is provided with the ejection port  22  from which an ion beam is emitted. 
     As illustrated in  FIG.  16   , the magnetic pole plate  20 A is magnetically coupled to the magnet  32  via the first yoke  34 A. Further, the magnetic pole plate  20 B is magnetically coupled to the magnet  32  via the second yoke  34 B. The adjustment yoke  34 C is provided so as to be in contact with the first yoke  34 A, the second yoke  34 B, and/or the magnet  32 . In the present embodiment, as illustrated in  FIG.  15    and  FIG.  16   , the adjustment yoke  34 C is arranged on a part of the outer circumferential surface of the first yoke  34 A. However, the arrangement of the adjustment yoke  34 C is not limited to the above. For example, the adjustment yoke  34 C may be provided on a part of the outer circumferential surface of the second yoke  34 B, may be provided on parts of the outer circumferential surfaces of the first yoke  34 A and the magnet  32 , may be provided over the entire outer circumferential surface of the second yoke  34 B, or may be provided indirectly to the first yoke  34 A. Alternatively, the adjustment yoke  34 C can also be installed inside the structure  30 , for example, on the inner wall of the recessed portion  36  or inside the first yoke  34  having a hollowed inside. Further, a plurality of adjustment yokes  34 C having different thicknesses or a plurality of adjustment yokes  34 C having the same thickness may be used and thereby configured so that the magnetic resistance of yokes in contact with the adjustment yokes  34 C can be adjusted. 
     In the first embodiment, as illustrated in  FIG.  5 B  and  FIG.  13   , the magnet  32  is arranged outside with respect to the cross-sectional centerline  24  of the ejection port  22 , the under-magnet distance x is adjusted, and thereby the position of the bottom  62  of the lines of magnetic field line  60  is controlled. That is, the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B is adjusted by changing the under-magnet distance x. For some structure of ion guns, however, it may be difficult to change the under-magnet distance x after the ion gun or a vacuum processing apparatus is assembled. 
     Accordingly, in the present embodiment, the adjustment yoke  34 C is used to allow to adjust the position of the bottom  62  of the lines of magnetic field line  60  without changing the under-magnet distance x. The position of the bottom  62  of the lines of magnetic field line  60  may vary in accordance with a ratio of respective magnetic resistances from the tips of the magnetic pole plates  20 A,  20 B to the magnet. Specifically, the bottom of lines of magnetic field tends to be shifted on the larger magnetic resistance side. Herein, a magnetic resistance is inversely proportional to the sectional area and the magnetic permeability of a magnetic pole plate or a yoke, which is a magnetic material, and is proportional to the length thereof. 
     In the present embodiment, the position of the bottom  62  of the lines of magnetic field line  60  is adjusted by using the properties described above. As illustrated in  FIG.  16   , the ion gun  10  of the present embodiment is formed such that the first yoke  34 A and the second yoke  34 B having the same thickness are arranged above and under the magnet  32 . To adjust the ratio of the magnetic resistance from the tips of the magnetic pole plates  20 A,  20 B to the magnet  32 , the adjustment yoke  34 C is arranged to the structure  30  in the present embodiment. The substantial thickness and the substantial sectional area of the first yoke  34 A and the second yoke  34 B include the thickness and the sectional area of the adjustment yoke  34 C in contact. Therefore, use of the adjustment yoke  34 C allows to adjust the ratio of the magnetic resistance and adjust the position of the bottom  62  of the lines of magnetic field line  60  without changing the under-magnet distance x. 
       FIG.  17    is a graph illustrating a result of a simulation performed on a eroded amount of a magnetic pole plate caused by a change in the sectional area of the first yoke. The horizontal axis represents the sectional area change rate of the first yoke, and the vertical axis represents the inside-outside ratio of erosion. Note that, in the present embodiment, the sectional area change rate of the first yoke corresponds to the ratio of the sectional area of the first yoke  34 A and the adjustment yoke  34 C relative to the sectional area of the second yoke  34 B. 
     As illustrated in  FIG.  17   , the inside-outside ratio of erosion is shifted to the plus side as the sectional area change rate of the first yoke  34 A increases. Therefore, by suitably adjusting the sectional area of the first yoke  34 A, it is possible to optimize the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B to be ±1. 
     For example, when the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B has been shifted to the minus side from ±1 after assembly of the ion gun or the vacuum processing apparatus, the bottom position of lines of magnetic field has been shifted inside. Therefore, in such a case, the bottom position of lines of magnetic field is caused to be shifted outward by increasing the thickness of the adjustment yoke  34 C to increase the sectional area change rate of the first yoke  34 A. In contrast, when the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B has been shifted to the plus side from ±1 after assembly of the ion gun or the vacuum processing apparatus, the bottom position of lines of magnetic field has been shifted outward. Therefore, in such a case, the bottom position of lines of magnetic field is caused to be shifted inside by reducing the thickness of the adjustment yoke  34 C to reduce the sectional area change rate of the first yoke  34 A. By suitably increasing or decreasing the thickness of the adjustment yoke  34 C in such a way, it is possible to adjust the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B to a value near ±1 that is the optimal value. 
     Note that the bottom position of lines of magnetic field may be adjusted by changing the sectional area of the first yoke  34 A by using the adjustment yoke  34 C or may be adjusted by changing both the sectional areas of the first yoke  34 A and the second yoke  34 B by using the adjustment yoke  34 C. 
     As described above, in the present embodiment, it is possible to control the position of the bottom  62  of the line of magnetic field line  60  by using the adjustment yoke  34 C. Therefore, in the present embodiment, it is possible to adjust the bottom position of lines of magnetic field even after an ion gun or a vacuum processing apparatus has been assembled. 
     Note that, in the present embodiment, the case where the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B is adjusted after an ion gun or a vacuum processing apparatus has been assembled has been described as an example. However, the adjustment according to the present embodiment is not limited thereto. For example, it is also possible to control the position of the bottom  62  of the lines of magnetic field line  60  in a design phase of an ion gun to adjust the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B. That is, it is possible to adjust the inside-outside ratio of erosion of the magnetic pole plates  20 A,  20 B by separately designing the magnetic pole plates  20 A,  20 B or the first yoke  34 A and the second yoke  34 B with respect to the sectional area, the length, or the magnetic permeability in a design phase of an ion gun. For example, the first yoke  34 A and the second yoke  34 B may be designed to have substantially the same magnetic permeability, or the first yoke  34 A and the second yoke  34 B may be designed to have different magnetic permeability. 
     Third Embodiment 
     An ion gun according to a third embodiment of the present invention will be described with reference to  FIG.  18    to  FIG.  21   .  FIG.  18    illustrates the operation of the ion gun in the curved portion  22   b  of the ejection port  22  (corresponding to the cross section taken along the line B-B′ of  FIG.  2   ).  FIG.  19    is an enlarged view around the magnetic pole plates  20 A,  20 B illustrated in  FIG.  18   . The same components as those in the ion gun according to the first embodiment are labeled with the same references, and the description thereof will be omitted or simplified. 
     The ion gun according to the present embodiment further has first magnetic pole covers  70 A,  70 B. The first magnetic pole covers  70 A,  70 B cover the entire surfaces of the magnetic pole plate  20 A and the magnetic pole plate  20 B. That is, the magnetic pole plates  20 A,  20 B and the first magnetic pole covers  70 A,  70 B integrally form a cathode of the ion gun according to the present embodiment. Since the first magnetic pole covers  70 A,  70 B are provided so as to cover the entire surfaces of the magnetic pole plates  20 A,  20 B, the magnetic pole plates  20 A,  20 B will not directly come into contact with plasma or ions. It is thus possible to reduce erosion of the magnetic pole plates  20 A,  20 B due to spattering and suppress temporal changes of the magnetic pole plates  20 A,  20 B and the ion beam characteristics. Further, it is possible to suppress contamination of the magnetic pole plates  20 A,  20 B due to influence from outside of the ion gun. Further, by optimizing selection of the material forming the first magnetic pole covers  70 A,  70 B, it is possible to avoid deposition of an unwanted material onto a processed object (a processed substrate  132 ). Furthermore, selection of a low spattering rate material for the magnetic pole cover can reduce deposition, onto the anode, of the material of the magnetic pole cover eroded by an ion beam, and this enables improvement in the maintenance cycle of the anode and a reduction in the running cost. 
     The magnetic pole cover used in the present embodiment is not limited to have the shape or the like of the first magnetic pole covers  70 A,  70 B illustrated in FIG.  18  and  FIG.  19   , and various changes as illustrated in  FIG.  20    and  FIG.  21   , for example, are possible.  FIG.  20    is an enlarged view around the magnetic pole plates  20 A,  20 B covered with second magnetic pole covers  71 A,  71 B. The second magnetic pole covers  71 A,  71 B cover the entire surfaces of the magnetic pole plates  20 A,  20 B in the same manner as the first magnetic pole covers  70 A,  70 B. The second magnetic pole covers  71 A,  71 B each have a chamfered corner on the surface side opposite to the anode  40  (in the present embodiment, on the surface side facing the processed object (the processed substrate  132 )) and each have an inclined surface in a portion covering the surface where the magnetic pole plate  20 A and the magnetic pole plate  20 B face each other. An ion beam is emitted with a divergence angle from the ion gun. Thus, if the corners on the processed object side of the facing portions are not chamfered, an ion beam may collide with the magnetic pole covers and be unable to be efficiently emitted. Accordingly, the corners of the outer surface of the ion gun (the surface opposite to the anode  40 ) of the ejection port  22  portion of the magnetic pole covers are chamfered, thereby collision of an ion beam with the magnetic pole covers can be prevented, and the ion beam can be efficiently emitted. 
       FIG.  21    is an enlarged view around magnetic pole plates  20 A,  20 B covered with third magnetic pole covers  72 A,  72 B,  73 A,  73 B. The third magnetic pole cover  72 A,  72 B cover the outer surface on the surface side opposite to the anode  40  (on the surface side facing the processed object (the processed substrate  132 )) of the magnetic pole plates  20 A,  20 B. Further, the third magnetic pole covers  73 A,  73 B cover the surfaces on the surface side where the magnetic pole plate  20 A and the magnetic pole plate  20 B face each other, respectively. Unlike the magnetic pole covers illustrated in  FIG.  18    to  FIG.  20   , the third magnetic pole covers  72 A,  72 B,  73 A,  73 B cover only the outer surfaces on the surface side opposite to the anode  40  (on the surface side facing the processed object (the processed substrate  132 )) and the surfaces on the surface side where the magnetic pole plate  20 A and the magnetic pole plate  20 B face each other out of the surfaces of the magnetic pole plates  20 A,  20 B. The outer surfaces of the magnetic pole plates  20 A,  20 B covered with the magnetic pole covers  72 A,  72 B are portions where the material of the processed object eroded by spattering is likely to be deposited. Further, the surfaces of the magnetic pole plates  20 A,  20 B covered with the magnetic pole covers  73 A,  73 B are portions which are likely to be eroded by an ion beam. Accordingly, by limiting the portions covered by the magnetic pole cover to such portions which are likely to be eroded, it is possible to reduce influence on lines of magnetic field due to provision of the magnetic pole covers. Further, it is no longer necessary to detach the magnetic pole plates  20 A,  20 B during maintenance, the number of components required for installation of the magnetic pole covers is reduced, and operability is thus improved. Furthermore, since the structure of the magnetic pole covers is simplified, the attachment error in installation of the magnetic pole covers can be reduced, and the tolerance in manufacturing the magnetic pole covers can be increased. 
     Note that, in the configuration example illustrated in  FIG.  21   , the third magnetic pole cover  72 A and the third magnetic pole cover  73 A are formed as separate components, and the third magnetic pole cover  72 B and the third magnetic pole cover  73 B are formed as separate components. However, the third magnetic pole covers  72 A,  73 A and the third magnetic pole covers  72 B,  73 B may be formed as an integral component, respectively. Further, although the chamfering is applied to only the third magnetic pole covers  73 A,  73 B in the configuration example illustrated in  FIG.  21   , corners on the ejection port  22  side of the third magnetic pole covers  72 A,  72 B may be chamfered in accordance with the divergence angle of an ion beam to have inclined surfaces. 
     It is desirable to form the magnetic pole covers with a non-magnetic material taking influence on lines of magnetic field into consideration. Furthermore, to suppress the material of the magnetic pole cover eroded by an ion beam from being redeposited onto the anode, it is desirable to form the magnetic pole covers with a low spattering rate material. For example, titanium, tantalum, tungsten, single carbon, or a compound containing these elements can be used to form the magnetic pole cover. Further, a thermal spray deposit or a plate-like bulk can also be used to form the magnetic pole cover. When a plate-like bulk is used to form a magnetic pole cover, a longer maintenance cycle can be set, and the magnetic pole cover can be easily replaced. Furthermore, with use of a plate-like bulk as a magnetic pole cover, it is possible to fix the magnetic pole cover to the magnetic pole plate by using a fastening member, or it is also possible to the magnetic pole cover to the magnetic pole plate by a dovetail tenon and a dovetail groove without using a fastening member. 
     Fourth Embodiment 
     A vacuum processing apparatus according to a fourth embodiment of the present invention will be described with reference to  FIG.  22   .  FIG.  22    is a schematic diagram of a vacuum processing apparatus according to the present embodiment. The same components as those in the ion gun according to the first embodiment are labeled with the same references, and the description thereof will be omitted or simplified. 
     In the present embodiment, as an example of an apparatus to which the ion gun  10  according to the first to third embodiments is applied, an ion beam etching apparatus which is one of vacuum processing apparatuses used for manufacturing a semiconductor device or the like will be described. The application example of the ion gun according to the first to third embodiments is not limited to the ion beam etching apparatus, and may be a film forming apparatus such as an ion beam sputtering apparatus. Further, the application example of the ion gun according to the first to third embodiments is not limited to the vacuum processing apparatus, but may be another apparatus including the ion gun. 
     As illustrated in  FIG.  22   , the vacuum processing apparatus  100  according to the present embodiment may include, as main components, a vacuum chamber  110 , a vacuum pump  120 , a holder  130  that holds a substrate  132  to be processed, and an ion gun  140 . The vacuum pump  120  is connected to the vacuum chamber  110 . The holder  130  and the ion gun  140  are installed in the vacuum chamber  110 . 
     The vacuum chamber  110  is a processing chamber in which the inside can be maintained in a vacuum state, and various processes such as etching, surface modification, and surface cleaning can be performed inside the vacuum chamber  110 . 
     The vacuum pump  120  is an exhausting device for exhausting gas in the vacuum chamber  110  and bringing the inside of the vacuum chamber  110  into a vacuum state. By exhausting the gas in the vacuum chamber  110  by the vacuum pump  120 , the inside of the vacuum chamber can be brought into a high vacuum state of about 10 −3  to 10 −6  Pa. 
     The holder  130  is a member for holding a processed object (substrate  132 ) made of, for example, Si, Ga, carbon, or the like. The holder  130  may include a swing mechanism. Since the holder  130  includes the swing mechanism, the substrate  132  can be subjected to processing with high in-plane uniformity. The holder  130  may further have another function, for example, a heating function for heating the substrate  132 . 
     The ion gun  140  is the ion gun described in the first embodiment, and is arranged at a position opposed to the substrate  132  held by the holder  130 . The ion gun  140  irradiates the positive ion beam  52  toward the substrate  132 . The ion beam  52  emitted from the ion gun  140  collides with the substrate  132  with high kinetic energy. Thus, the surface of the substrate  132  can be subjected to a predetermined process such as etching. 
     By configuring the vacuum processing apparatus  100  using the ion gun  10  according to the first to third embodiments, it is possible to irradiate the substrate  132  with the ion beam  52  with high uniformity, thereby improving the processing quality. In addition, since collision of the ion beam  52  with the magnetic pole plates  20 A and  20 B can be reduced, the maintenance cycle can be extended. Thus, the production cost can be improved, and the processing capability of the substrate  132  can be improved. In addition, contamination of the inside of the vacuum chamber  110  and the substrate  132  due to particles generated by sputtering the magnetic pole plates  20 A and  20 B by the ion beam  52  can be suppressed. 
     Modified Embodiments 
     The present invention is not limited to the embodiments described above, and various modifications are possible. 
     For example, an example in which a configuration of a part of any embodiment is added to another embodiment or an example in which a configuration of a part of another embodiment is substituted is also an embodiment of the present invention. In addition, a known technique or a known technique in the technical field can be appropriately applied to a specific description or a portion not illustrated in the embodiments. 
     Further, in the above embodiment, although the magnet  32  is arranged on the cross-sectional centerline  24  of the ejection port  22  in the linear portion  22   a  of the ejection port  22 , it is not necessarily required to be arranged on the cross-sectional centerline  24  if the magnetic field is symmetrical with respect to the cross-sectional centerline  24  of the ejection port  22 . 
     Although argon gas is exemplified as the discharge gas in the above embodiments, the discharge gas is not limited to a rare gas such as argon, and may be a reactive gas represented by oxygen gas or nitrogen gas. The discharge gas may be appropriately selected depending on the purpose of use of the ion gun  10  or the like. 
     According to the present invention, the ejection efficiency and uniformity of the ion beam can be improved. In addition, it is possible to suppress collision of the ion beam with the magnetic pole, and realize excellent effects such as reduction in temporal change, improvement in maintenance cycle, and reduction in running cost. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.