Patent Publication Number: US-2009229977-A1

Title: Magnet Structure and Cathode Electrode Unit for Magnetron Sputtering System, and Magnetron Sputtering System

Description:
TECHNICAL FIELD 
     The present invention relates to a magnet structure and a cathode electrode unit for a magnetron sputtering system, and a magnetron sputtering system (hereinafter referred to as “magnet structure and the like”). More specifically, the invention relates to a technique for improving the magnet structure and the like for magnetron sputtering for the purpose of increasing target utilization efficiency. 
     BACKGROUND ART 
     A film formation method based on sputtering phenomenon in which ion (for example Ar ion) is caused to collide with a target material in a vacuum to cause atoms of the target to pop up from the target material and deposit on a substrate disposed opposite to the target material, is conventionally well-known. 
     According to a magnetron sputtering film formation method, which is one of methods based on the above sputtering phenomenon, a tunnel-shaped leakage magnetic field with a specified magnetic flux density or higher can be formed over a target surface (obverse surface opposite to a substrate) to capture secondary electrons generated during a process of the sputtering phenomenon by Lorentz force and cause the secondary electrons to conduct their cycloid motion, thereby increasing a frequency of ionization collision of the secondary electrons with Ar gas. Thereby, high-density plasma is produced in a space lying in the vicinity of the target surface, thereby making a film formation rate higher. 
     Such a magnetron sputtering film formation method, however, has a drawback that, because a target material in a stronger magnetic field is locally eroded faster by the sputtering, a sputtering amount in a plane of the target material becomes non-uniform, decreasing target utilization efficiency. For this reason, various techniques have thus far been developed to overcome this drawback. 
     For example, there has been proposed a drive mechanism for a magnet structure, which moves in a plane direction of a target surface an entire magnetic device (magnet structure) including a plurality of magnets for producing the aforesaid leakage magnetic field, yokes, and connecting members (see patent document 1). 
     Since the drive mechanism enables the magnet to move in the plane direction along a reverse surface of the target, it can change a magnetic force line distribution on the target surface in association with the movement of the magnet. As a result, an erosion promoting region on the target surface changes periodically with time, achieving uniform erosion on the target surface in sputtering. 
     Patent document 1: Japanese Laid-Open Patent Application Publication No. Hei. 4-329874 (FIG. 3) 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the drive mechanism for the magnet structure disclosed in the patent document 1 is required to drive the entire magnet structure, causing complexity and an increase in the size of the drive mechanism. 
     The present invention has been made under the circumstances, and an object of the present invention is to provide a magnet structure and the like which are capable of changing a magnetic force line distribution over a target surface with time to achieve wide erosion of the target with a simple drive mechanism, without moving the entire magnet structure. 
     Means for Solving the Problem 
     To achieve the above described object, a magnet structure for a magnetron sputtering system of the present invention, comprises a main magnet disposed at a reverse surface side of a target to produce a main magnetic force line reaching an obverse surface of the target; an adjustment magnet disposed at the reverse surface side of the target to produce an adjustment magnetic force line for changing a magnetic flux density distribution of the main magnetic force line; a magnetic path of the adjustment magnetic force line which is disposed at the reverse surface side of the target; and a magnetic field adjustment means configured to be able to change strength of the adjustment magnetic force line passing through inside of the magnetic path. 
     In such a configuration, without moving the entire magnet structure and by using a simple drive mechanism, it becomes possible to change the strength of the adjustment magnetic force line passing through the inside of the magnetic path with time, and hence to change the main magnetic force line distribution (magnetic flux density distribution) over the obverse surface of the target, for example, in each specified cycle. As a result, wide erosion of the target can be achieved. 
     One example of the magnetic field adjustment means includes a movable element which is made of a magnetic material and is provided in a predetermined position with respect to the adjustment magnet; and a drive unit for driving the movable element to change the predetermined position. 
     In such a configuration, it becomes possible to achieve a simple drive mechanism which is able to periodically move only the movable element which is a part of the magnet structure. 
     It should be noted that the magnet structure may further comprise a plate-shaped base for holding the main magnet, the base being made of a magnetic material, and the magnetic path may be configured to include the base. Thereby, the base is effectively utilized as the magnetic path for guiding the adjustment magnetic force line. 
     The base may include a pair of an inner base and an outer base. The adjustment magnet may be sandwiched between the inner base and the outer base such that an orientation of a magnetic moment of the adjustment magnet conforms to a plane direction of the inner base and the outer base. 
     One example of the movable element is a plate-shaped member disposed to be opposed to a reverse surface side of the adjustment magnet. 
     By doing so, since there is an appropriate space on the reverse surface side of the adjustment magnet, it is expected that a distance between the movable element and the adjustment magnet can be easily changed. 
     The magnetic path may include a convex portion which is formed of a magnetic member and protrudes toward the reverse surface of the target. The convex portion may be disposed over the adjustment magnet in such a manner that both end surfaces of the convex portion are connected to the inner base and the outer base, respectively. One example of the convex portion is a member curved in an arch shape. 
     A cathode electrode unit for a magnetron sputtering system of the present invention, comprise a target made of a non-magnetic metal; and the aforesaid magnet structure which is disposed on a reverse surface side of the target; and a power supply source for supplying a specified electric power to the target. 
     Also, a magnetron sputtering system of the present invention comprises a vacuum chamber which accommodates the aforementioned cathode electrode unit and a substrate disposed opposite to the target of the cathode electrode unit, the vacuum chamber being configured to be able to reduce its internal pressure. 
     The above and further objects, features and advantages of the invention will more filly be apparent from the following detailed description with accompanying drawings. 
     Effects of the Invention 
     According to the present invention, it is possible to provide a magnet structure and the like which are capable of changing a magnetic force line distribution over a surface of a target with time to thereby achieve wide erosion of the target, using a simple drive mechanism and without moving the entire magnet structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view showing a cathode electrode unit including a magnet structure according to an embodiment 1 of the present invention; 
         FIG. 2  is a perspective view of the cathode electrode unit taken along line II-II of  FIG. 1 ; 
         FIG. 3  is a view showing one exemplary result of analysis on the magnet structure according to the embodiment by a static magnetic field simulation technique; 
         FIG. 4  is a view showing one exemplary result of analysis on a magnet structure according to the embodiment by the static magnetic field simulation technique; and 
         FIG. 5  is a view showing one exemplary result of analysis on the magnet structure according to the embodiment by the static magnetic field simulation technique. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       10  . . . central permanent magnet 
       11  . . . intermediate permanent magnet 
       12  . . . movable element 
       13  . . . outermost permanent magnet 
       14  . . . actuator 
       20  . . . target 
       21 A . . . outer base 
       21 B . . . inner base 
       22  . . . first base piece 
       23  . . . second base piece 
       24  . . . curved magnetic member 
       24 A . . . curved inner surface 
       24 B . . . curved outer surface 
       25  . . . upper magnetic force line 
       26  . . . lower magnetic force line 
       26 A . . . first lower magnetic force line 
       26 B . . . second lower magnetic force line 
       27  . . . inner intermediate magnetic force line 
       28  . . . outer intermediate magnetic force line 
       29  . . . zero point 
       100  . . . cathode electrode unit 
       110  . . . magnet structure 
     V 1  . . . electric power source 
     BM 1 , BM 2  . . . zero cross position 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is a plan view showing a cathode electrode unit including a magnet structure (i.e., magnetic field producing device) according to the embodiment of the present invention. 
       FIG. 2  is a perspective view of the cathode electrode unit taken along line II-II of  FIG. 1 . 
     For simplification of the drawing,  FIG. 1  shows a cross-section of a magnet portion of a magnet structure  110 . 
     For convenience&#39;s sake, a width direction and a thickness direction of a target  20  are represented as X-direction and Y-direction, respectively, in  FIGS. 1 and 2  (as well as in  FIG. 3 ) for description of components of the cathode electrode unit  100 . 
     Further, although the components of the magnet structure  110  are shown as being cut to a predetermined thickness in a depth direction (perpendicular to both the X-direction and the Y-direction) in  FIG. 2 , these components actually extend in the depth direction to have the same sectional shapes. This can be easily understood from  FIG. 1 . 
     According to the present embodiment, as shown in  FIG. 2 , the cathode electrode unit  100  comprises as major components thereof, the rectangular target  20  made of a non-magnetic metal such as aluminum (Al), and the magnet structure  10  having plural magnets and positioned at a reverse surface  20 B side of the target  20 . 
     The target  20  is a parent material of a thin film to be coated over a substrate (not shown) positioned opposite to the target  20 , and is supplied with an electric power from an electric power source VI so as to serve as a cathode for the purpose of attracting Ar ion (positive ion) present in plasma. 
     In this embodiment, a vacuum chamber (not shown) for magnetron sputtering, which accommodates the cathode electrode unit  100  and the substrate therein and which is capable of reducing an internal pressure thereof, is grounded as an anode. 
     During a sputtering phenomenon, high-density plasma containing Ar ion is generated in the vicinity of the obverse surface of the target  20  by a tunnel-shaped leakage magnetic field for plasma confinement, while atoms forming the target  20  (i.e., aluminum atoms in the present embodiment) are beaten out of the obverse surface of the target by collision energy of Ar ion and deposited on the substrate. Since such a technique is well known in the art, detailed description thereof is herein omitted. 
     As shown in  FIG. 2 , the magnet structure  10  includes a pair of an outer base  21 A and an inner base  21 B which are made of ferromagnetic stainless or iron. 
     The outer base  21 A is, in a plan view, defined by an outer peripheral surface having a dimension substantially equal to a outside dimension of the target  20  and an inner peripheral surface having a dimension substantially equal to an outside dimension of the intermediate permanent magnet  11  (to be described later) of a plate-shape and an annularly elongated-circle shape to allow the intermediate permanent magnet  11  to be fitted to the inner peripheral surface of the outer base  21 A. As a result, the outer base  21 A has an annularly plate-shape having an elongate hole. 
     The inner base  21 B has an elongate-circle plate shape to be fitted to the elongate-circle hole of the intermediate permanent magnet  11  in a plan view. 
     Magnets and magnetic members (to be described in detail later) are disposed on each of the inner base  21 B and the outer base  21 A for producing a tunnel-shaped leakage magnetic field for plasma confinement in a space extending above and in the vicinity of the obverse surface  20 A of the target  20 . 
     Actually, the components (including the inner base  21 B and the outer base  21 A, the magnets and the magnetic member) of the magnet structure  110  and the target  20  are fixed to each other integrally by appropriate fixing device. But, illustration and description of such fixing device are herein omitted. 
     As shown in  FIGS. 1 and 2 , the magnet structure  110  includes, as a first main magnet, a central permanent magnet  10  (i.e., internal magnet) of a substantially rectangular shape positioned at the reverse surface  20 B side of the target  20  in a center position in the width direction (i.e., X-direction) of the target  20 . The central permanent magnet  10  is shaped like a rod with its longitudinal center line coinciding with the longitudinal center line of the target  20  in a plan view and is placed on an upper surface of a first base piece  22  resting on the inner base  21 B shown in  FIG. 2 . 
     The central permanent magnet  10  has north and south poles producing a magnetic moment oriented in the direction opposite to the Y-direction (i.e., the direction from the obverse surface  20 A toward the reverse surface  20 B of the target  20 ), as shown in  FIG. 2 . The central permanent magnet  10  is opposed to a center portion of the inner base  21  B with the first base piece  22  interposed therebetween in a state where north-pole of the central permanent magnet  10  is in contact with a center portion of the reverse surface  20 B of the target  20 , while the south-pole of the central permanent magnet  10  is in contact with an upper surface of the first base piece  22  made of a magnetic material (e.g., ferromagnetic stainless or iron). 
     As shown in  FIGS. 1 and 2 , the magnet structure  110  includes, as a second main magnet, an outermost permanent magnet  13  (i.e., outer magnet) of a substantially elongate cylindrical shape positioned at the reverse surface  20 B side of the target  20  inwardly of and in the vicinity of an end portion of the target  20  lying in the width direction (i.e., X-direction) of the target  20 . The outermost permanent magnet  13  has an annular shape extending along the periphery of the end portion of the target  20  in a plan view and is placed on an upper surface of a second base piece  23  resting on the outer base  21 A shown in  FIG. 2 . 
     The outermost permanent magnet  13  has north and south poles producing a magnetic moment oriented in the Y-direction within the outermost permanent magnet  13  (i.e., the direction from the reverse surface  20 B of the target  20  toward the obverse surface  20 A of the target  20 ), as shown in  FIG. 2 . The outermost permanent magnet  13  is opposed to a peripheral portion of the outer base  21 A with the second base piece  23  interposed therebetween in a state where the south-pole of the outermost permanent magnet  13  is in contact with a peripheral portion of the reverse surface  20 B of the target  20 , and the north-pole of the outermost permanent magnet  13  is in contact with an upper surface of the second base piece  23  made of a magnetic material (e.g., ferromagnetic stainless or iron). 
     As shown in  FIGS. 1 and 2 , the magnet structure  110  includes, as an adjustment magnet, an intermediate permanent magnet  11  of an annularly elongate-circle plate shape in a plan view positioned at the reverse surface  20 B side of the target  20 . The intermediate permanent magnet  11  is located between the central permanent magnet  10  and the outermost permanent magnet  13  and is positioned at a substantially center portion of the target  20  in the X-direction of the target  20 . The intermediate permanent magnet  11  is sandwiched between the inner base  21 B and the outer base  21 A. 
     Since the central permanent magnet  11  has a thickness substantially equal to those of the inner base  21 B and the outer base  21 A as shown in  FIG. 2 , there is no step between the central permanent magnet  11 , and the inner base  21 B and the outer base  21 A. Therefore, the members  21 A,  21 B, and  11  are integral with each other to form a single rectangular member in a surface configuration. 
     The intermediate permanent magnet  11  has north and south poles producing a magnetic moment oriented in the direction opposite to the X-direction within the intermediate permanent magnet  11  (i.e., the direction from the end portion of the target  20  to the center portion of the target  20 ), as shown in  FIG. 2 . The south-pole of the intermediate permanent magnet  11  is in contact with the outer peripheral surface of the inner base  21 B, while the north-pole of the intermediate permanent magnet  11  is in contact with the inner peripheral surface of the outer base  21 A. That is, the inner base  21 B and the outer base  21 A are configured to sandwich the intermediate permanent magnet  11  therebetween such that the orientation of the magnetic moment within the intermediate permanent magnet  11  conforms to the plane direction of the inner base  21 B and the outer base  21 A. 
     The permanent magnets  10 ,  11 , and  13  described above can be constructed using various known magnet materials. In cases where these permanent magnets  10 ,  11 , and  13  are used as being immersed in cooling water for cooling the reverse surface  20 B of the target  20 , it is desirable to provide the surfaces of the magnets with a rust-proof treatment or select a rust-proof material (e.g., ferrite magnet). 
     The curved magnetic member  24  (convex-shaped ferromagnetic member), which is made of a magnetic material such as ferromagnetic stainless or iron, is curved like a bow (or arch). As shown in  FIG. 2 , the curved magnetic member  24  has end surfaces extending in parallel with the reverse surface  20 B of the target  20  in such a manner that one end surface thereof contacts the surface of the inner base  21 B and the other end surface thereof contacts the surface of the outer base  21 A. Thus, the curved magnetic member  24  bridges the inner base  21 B and the outer base  21 A over the intermediate permanent magnet  11 . 
     The curved magnetic member  24  is of a substantially annularly elongate-circle shape in a plan view. More specifically, the curved magnetic member  24  has curved inner and outer surfaces  24 A and  24 B which are curved at equal curvature convexly toward the reverse surface  20 B of the target  20  in the thickness direction (i.e., Y-direction) of the target  20 . The curved magnetic member  24  is in the form of a half obtained by cutting an imaginary annular cylindrical body having a substantially elongate-circle section into halves along the end surfaces so that a dimension between the curved inner and outer surfaces  24 A and  24 B corresponds to a wall thickness of the annular cylindrical body. 
     Furthermore, one of both ends in the X-direction of the curved outer surface  24 B of the curved magnetic member  24  is in contact with the side surface of the first base pipe  22 , while the other end is in contact with the side surface of the second base piece  23 . 
     In this manner, the curved magnetic member  24  and a part of the inner base  21 B and the outer base  21 A (to be precise, portions of the inner base  21 B and the outer base  21 A which are respectively located between the magnetic poles of the central permanent magnet  11  and the end surfaces of the curved magnetic member  24 ) form a magnetic path of adjustment magnetic force lines produced by the intermediate permanent magnet  11 . This makes it possible to make use of the inner base  21 B and the outer base  21 A as the magnetic path for guiding the second lower magnetic force line  26 B. Thus, effective use of the magnetic member is achieved. 
     The magnetic path is configured to confine and guide the adjustment magnetic force line so that the magnetic force line emanating from the north pole of the intermediate permanent magnet  11  returns to the south pole of the intermediate permanent magnet  11 . A specific structure of the adjustment magnetic force line will be described in detail later. 
     The top of the curved outer surface  24 B in the Y-direction may be in contact with the reverse surface  20 B of the target  20  or may be spaced apart from the reverse surface  20 B by an appropriate clearance (not shown). 
     When the above two are in contact with each other, the distance between the target  20  and the intermediate permanent magnet  11  is shortened so that magnetic energy contributing to the production of a magnetic field for plasma confinement, which is generated by the magnet  11  works effectively. 
     When the appropriate clearance is provided between the two, there is sometimes an advantage that reverse surface  20 B of the target  20  is cooled with cooling water. For example, in cases where such a cooling structure is employed in which the magnet structure  110  is entirely immersed in cooling water stored in a cooling water vessel (not shown), the cooling water is flowed in the clearance, so that efficient heat exchange between the cooling water and the reverse surface  20 B of the target  20  is suitably carried out. In cases where such a cooling structure is employed in which a rectangular hollow backing plate (not shown) for passage of cooling water is in contact with the reverse surface  20 B of the target  20 , such a clearance is indispensable as a space into which the backing plate is inserted. 
     As components for a magnetic field adjustment device which is capable of changing strength of adjustment magnetic force line produced by the intermediate permanent magnet  11 , there are a movable element  12  which is made of a magnetic material (e.g., ferromagnetic stainless or iron) and is provided in a predetermined position with respect to the intermediate permanent magnet  11 , and an actuator (drive unit)  14  for driving the movable element  12  to change the predetermined position. 
     The movable element  12  has a shape having an outer dimension equal to that of the intermediate permanent magnet  11  in a plan view. As shown in  FIG. 2 , the movable element  12  extending along the intermediate permanent magnet  11  in a plan view is disposed opposite to and in contact with the reverse surface of the intermediate permanent magnet  11 . 
     By positioning the movable element  11  opposite to the reverse surface side of the intermediate permanent magnet  11 , there is formed an appropriate space on the reverse surface side of the intermediate permanent magnet  11 . This can easily change a distance between the movable element  12  and the intermediate permanent magnet  11 . 
     In addition, based on control of suitable control device (microprocessor or the like: not shown), the actuator  14  is able to move the movable element  12  in each predetermined cycle from a state where the movable element  12  and the intermediate permanent magnet  11  are in contact with each other to a state where they are apart a predetermined distance in, for example, downward direction (direction opposite to Y-direction) from each other. As a matter of course, the movable element  11  may be shifted periodically in X-direction by the actuator  14 , or a movement start timing or a movement speed of the movable element  11  may be determined based on a sputtering state of the target  20 . 
     Description will be made of a result of analysis of a magnetic flux density distribution over the above-described target  20  magnetized, by making use of a static magnetic field simulation technique. 
     An analytical model having substantially the same shape as shown in  FIG. 2  is mesh-divided into unit analytical areas for numerical calculation, and is created on a computer. Appropriate physical property data on respective materials and boundary condition data have been input to mesh areas corresponding to the respective components of the magnet structure  110 , mesh area corresponding to the target  20  and boundary mesh areas. As an analytical solver, general-purpose magnetic field analysis software (“MagNet” manufactured by INFOLYTICA CORPORATION) has been used. 
       FIGS. 3 ,  4  and  5  are views each showing one exemplary result of analysis on the magnet structure according to the present embodiment by the static magnetic field simulation technique. 
       FIG. 3  is a view showing a magnetic flux density distribution (constant-height surfaces) and magnetic flux density vectors (indicated by arrows) in the analytical model, and an analysis result view of a two dimensional cross-section taken along II-II in  FIG. 1 . 
     Note that illustration of magnetic flux density vectors within the magnets  10 ,  11 , and  13  is omitted here. 
     In  FIG. 4(   b ), a horizontal axis represents a X-direction position on the target surface and a vertical axis represents a Y-direction component of magnetic flux density on the target surface, and the relationship between the two is plotted using numerical data obtained from the result of analysis.  FIG. 4(   b ) shows an analysis result obtained when the movable element  12  and the central permanent magnet  11  are made in contact with each other as shown in  FIG. 4(   a ). 
     In  FIG. 5(   b ), a horizontal axis represents the X-direction position on the target surface and the vertical axis represents the Y-direction component of magnetic flux density on the target surface, and the relationship between the two is plotted using numerical data obtained from the result of analysis.  FIG. 5(   b ) shows an analysis result obtained when the movable element  12  and the central permanent magnet  11  are apart a predetermined distance from each other as shown in  FIG. 5(   a ). 
     A gray-scale magnetic flux density contour figure (constant-height view) shown in  FIG. 3  represents a high-low distribution of a total (absolute value) of vector components of the magnetic flux density (i.e., magnetic flux density distribution). The contour view shows that the magnetic flux density rises with transition from a lighter gray region to a deeper gray region (It should be noted that the upper limit of magnetic flux density is set to 500 G). 
     As can be seen from such magnetic flux density contour figure and vector view, a magnetic force line as a curve on which the tangential direction at each point coincides with the magnetic field direction at that point is understood. 
       FIG. 3  shows the magnetic flux density contour figure and vector view in accordance with those output from the analytical computer. For easy understanding of these views, however,  FIG. 3  simplifies the magnetic flux density distribution output from the computer while adding bold apparent two-dotted lines representing each of an upper magnetic force line  25 , a lower magnetic force line  26  (first lower magnetic force line  26 A and second lower magnetic force line  26 B), an inner intermediate magnetic force line  27  and an outer intermediate magnetic force line  28 . 
     According to  FIG. 3 , the upper magnetic force line  25  and the lower magnetic force line  26  (first lower magnetic force line  26 A and second lower magnetic force line  26 B) are produced so as to cancel each other&#39;s X-direction vector component of magnetic flux density (component in the width direction of the target  20 ) within the target  20 , while the inner intermediate magnetic force line  27  and the outer intermediate magnetic force line  28  are produced so as to cancel each other&#39;s Y-direction vector component of magnetic flux density (component in the thickness direction of the target  20 ) within the target  20 . 
     Specifically, the upper magnetic force line (main magnetic force line)  25  emanates from the north pole of the central permanent magnet  10 , reaches the obverse surface  20 A of the target  20 , extends substantially in parallel with the X-direction in an inside portion of the target  20  that lies just above a zero point  29  at which the Y-direction vector component of magnetic flux density and the X-direction vector component of magnetic flux density become substantially zero, and enters the south pole of the outermost permanent magnet  13  while curving like an arch within the target  20 . 
     The lower magnetic force line  26  includes the first lower magnetic force line  26 A and the second lower magnetic force line  26 B (adjustment magnetic force line) to be described below. 
     The first lower magnetic force line  26 A emanates from the north pole of the outermost permanent magnet  13 , passes through the second base piece  23 , extends substantially in parallel in the direction opposite to the X-direction while being confined inside the curved magnetic member  24  located just below the zero point  29 , passes through the first base piece  22  while bending like an arch to conform to the shape of the curved magnetic member  24 , and enters the south pole of the central permanent magnet  10 . 
     The second lower magnetic force line  26 B is a magnetic force line whose strength is changed according to the movement of the movable element  12  to be described later. The second lower magnetic force line  26 B emanates from the north pole of the intermediate permanent magnet  11 , passes through the outer base  21 A, extends substantially in parallel in the direction opposite to the X-direction while being confined inside the curved magnetic member  24  located just below the zero point  29 , passes through the inner base  21 B while bending like an arch to conform to the shape of the curved magnetic member  24 , and returns to the south pole of the intermediate permanent magnet  10 . 
     The second lower magnetic force line  26 B, and the magnetic force line passing through inside of the intermediate permanent magnet  11  form an annular magnetic circuit. 
     The inner intermediate magnetic force line  27  emanates from the north pole of the central permanent magnet  10 , reaches an intermediate point in the thickness direction of the target  20 , extends to bend like an arch within the target  20 , passes through a portion of the target  20  that lies beside the zero point  29  (location on the minus side in the X-direction with respect to the zero point  29 ) substantially in parallel with the direction opposite to the Y-direction, and enters the curved magnetic member  24 . 
     The outer intermediate magnetic force line  28  emanates from the curved magnetic member  24 , passes through the portion of the target  20  that lies beside the zero point  29  (location on the plus side in the X-direction with respect to the zero point  29 ) substantially in parallel with the Y-direction, reaches an intermediate point in the thickness direction of the target  20 , extends to bend like an arch within the target  20 , and enters the south pole of the outermost permanent magnet  13 . 
     As shown in  FIG. 3 , the zero point  29  is formed in a region surrounded by the magnetic force lines  25 ,  26 ,  27 , and  28 . Herein, the zero point  29  is present in the vicinity of a contact point between the reverse surface  20 B of the target  20  and the top of the curved magnetic member  24 . 
     By thus positioning the zero point  29  at a location optimum to the constitution of the target  20  (thickness, material and the like), it is possible to correctly adjust the leakage magnetic field for plasma confinement produced over the obverse surface  20 A of the target  20 . 
     To be more specific, the magnetic flux density component (hereinafter referred to as “parallel magnetic flux density”) parallel (X-direction) to the obverse surface  20 A, in the leakage magnetic field in the vicinity of the obverse surface  20 A which leaks to the obverse surface  20 A of the target  20 , serves as the leakage magnetic field for plasma confinement. 
     The magnet structure  110  described above is capable of maintaining the parallel magnetic flux density to a value not less than a predetermined magnetic flux density BS (e.g., 200 to 300 G) over substantially the entire X-direction region of the obverse surface  20 A of the target  20 . As a result, efficient and wide erosion of the target  20  can be achieved advantageously. 
     Further, it is empirically known that a target portion maintaining the magnetic flux density component (hereinafter referred to as “perpendicular magnetic flux density”) perpendicular (Y-direction) to the obverse surface  20 A to near zero in the leakage magnetic field is eroded faster by sputtering. 
     The magnet structure  110  is able to correctly form the zero cross position (e.g., positions of BM 1  In  FIG. 4(   b ) and BM 2  in  FIG. 5)  at which the perpendicular magnetic flux density becomes approximately zero in the X-direction of the obverse surface  20 A of the target  20 . As a result, the magnet structure  110  enables effective utilization of magnetic energy and wide erosion of the target  20  advantageously. 
     Thus, the magnet structure  110  according to the present embodiment is capable of producing a quadridirectional magnetic field comprising the upper magnetic force line  25 , the lower magnetic force line  26  (first lower magnetic force line  26 A and second lower magnetic force line  26 B), the inner intermediate magnetic force line  27  and the outer intermediate magnetic force line  28  which surround the zero point  29  existing at a suitable location in the target  20  (for example, a location in the vicinity of the reverse surface  20 B). Therefore, the magnet structure  110  makes it possible to realize wide erosion with localized sputtering of the target  20  suppressed, thereby to increase the target utilization efficiency, hence, prolong the change period of the target  20 . Thus, the magnet structure  110  can contribute to an improvement in the operating efficiency of a planar-type magnetron sputtering system. 
     As shown in  FIGS. 4 and 5 , by changing the distance between the movable element  12  and the intermediate permanent magnet  11  periodically, it becomes possible to periodically change the X-direction position (hereinafter referred to as “zero cross position”) on the obverse surface  20 A of the target  20  at which the perpendicular magnetic flux density becomes approximately zero. 
     To be specific, by making the movable element  12  contact (as shown in  FIG. 4 ) the intermediate permanent magnet  11  apart from the movable element  12  (as shown in  FIG. 5 ), a part of the magnetic force line emanating from the intermediate permanent magnet  11  is shortened, thereby resulting in the change of a strength of the second lower magnetic force line  26 B (adjustment magnetic force line) passing through the inside of the inner base  21 B and the outer base  21 A as the magnetic path. 
     Thereby, the magnetic flux density distribution of the upper magnetic force line  25  (main magnetic force line) over the obverse surface  20 A of the target  20  changes. Therefore, it can be estimated that the zero cross positions BM 2  at two points at which the perpendicular magnetic flux density becomes approximately zero in the state of  FIG. 5  (state where the movable element  12  and the intermediate permanent magnet  11  are apart from each other) are moved periodically along the obverse surface  20  of the target  20  in X-direction (direction from the center portion of the target  20  to the end portion of the target  20 ), as shown by the zero cross positions BM 1  at two points at which the perpendicular magnetic flux density becomes approximately zero in  FIG. 4  (state where the movable element  12  and the intermediate permanent magnet  11  are in contact with each other). 
     As should be appreciated, according to the magnet structure  110  of this embodiment, using the simple drive mechanism which moves only the movable element  12  which is a part of the magnet structure  110 , relative to the intermediate permanent magnet  11  with time and without moving the entire magnet structure  110 , it becomes possible to change with time the strength of the second lower magnetic force line  26 B (adjustment magnetic force line) passing through the inside of the curved magnetic member  24 , and hence to change the magnetic force line distribution (magnetic flux density distribution) over the obverse surface  20 A of the target  20 , for example, in each specified cycle. As a result, wide erosion of the target  20  can be achieved. 
     It will be apparent from the foregoing description that many improvements and other embodiments of the present invention may occur to those skilled in the art. Therefore, the foregoing description should be construed as an illustration only and is provided for the purpose of teaching the best mode for carrying out the present invention to those skilled in the art. The details of the structure and/or the function of the present invention can be modified substantially without departing from the spirit of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The magnet structure according to the present invention is useful as, for example, magnetic field producing device for use in a magnetron sputtering system.