Patent Publication Number: US-2016243670-A1

Title: Retainer ring, substrate holding apparatus, and polishing apparatus, and retainer ring maintenance method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Priority Patent Application JP 2015-33668 filed on Feb. 24, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present technology relates to substrate holding apparatuses that hold substrates, such as wafers, and in particular, relates to a substrate holding apparatus used for polishing a surface of a substrate by pressing the substrate onto a polishing tool, such as a polishing pad. The present technology further relates to a polishing apparatus that uses such a substrate holding apparatus. The present technology still further relates to a retainer ring used in the above described substrate holding apparatus. 
     BACKGROUND AND SUMMARY 
     A polishing apparatus for polishing a surface of a wafer is widely used in a process for manufacturing semiconductor devices. Such a kind of the polishing apparatus includes a polishing table supporting a polishing pad having a polishing face, a substrate holding apparatus holding a wafer referred to as a top ring or a polishing head, and a polishing liquid supply nozzle supplying a polishing liquid onto the polishing face. 
     The polishing apparatus polishes a wafer as described below. While allowing the polishing table to rotate together with the polishing pad, the apparatus supplies the polishing liquid onto the polishing face from the polishing liquid supply nozzle. The substrate holding apparatus holds and rotates the wafer at an axial center of the wafer. In this state, the substrate holding apparatus presses a surface of the wafer onto the polishing face of the polishing pad to allow the surface of the wafer to come into sliding contact with the polishing face while the polishing liquid is being supplied. The surface of the wafer is evenly polished through a mechanical action of abrasive grain contained in the polishing liquid and a chemical action of the polishing liquid. The described polishing apparatus is also referred to as a chemical mechanical polishing (CMP) apparatus. 
     While polishing the wafer is underway, the surface of the wafer comes into sliding contact with the polishing pad, causing a friction force on the wafer. To prevent the wafer from coming off the substrate holding apparatus due to the friction force occurred while the wafer is polished, the substrate holding apparatus includes a retainer ring. The retainer ring is arranged to enclose the wafer to press the polishing pad outside the wafer. 
     A wafer polishing rate (or, removal rate) could change depending on polishing conditions such as a load of the wafer against the polishing pad, a load of the retainer ring, a rotating speed of the polishing table and the wafer, and a type of the polishing liquid. To obtain the same level of polishing finish when polishing a plurality of wafers successively, such polishing conditions are normally maintained at a constant level. However, a polishing profile at an edge of each of the plurality of wafers could change gradually as polishing of the plurality of wafers proceeds even though the polishing conditions are kept unchanged. Specifically, the polishing rate at the edge increases as many wafers are polished. 
     A reason of such increase of the polishing rate is assumed to be that the retainer ring changes in shape.  FIG. 19  is a schematic view illustrating a retainer ring while a wafer is polished. As shown in  FIG. 19 , a retainer ring  200  wears while polishing the wafer W is underway, because the retainer ring  200  is pressed against a polishing face  201   a  of a polishing pad  201 . Inner and outer peripheral surfaces of the retainer ring  200  particularly wear, resulting in the rounded inner and outer peripheral surfaces. When the inner peripheral surface of the retainer ring  200  wears as shown in  FIG. 19 , a force of the retainer ring  200  to press a pad area near an edge of the wafer W lowers, increasing the polishing rate at the edge. 
     Therefore, there is a need for substrate holding apparatuses that are able to stabilize a substrate polishing rate even when successively polishing a plurality of substrates (wafers, for example). There is a further need for polishing apparatuses that use such substrate holding apparatuses. There is a still further need for retainer rings used in such substrate holding apparatuses, as well as a retainer ring maintenance method. 
     According to one embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; and an outside ring arranged outside the inside ring, abrasion-resistant of the outside ring being higher than abrasion-resistant of the inside ring, wherein the inside ring having a thickness in a radial direction in a range from a minimum of 0.05 mm to a maximum of 5 mm. 
     According to another embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; and an outside ring arranged outside the inside ring, the outside ring comprising a lower ring and an upper ring, wherein abrasion-resistant of the lower ring is higher than abrasion-resistant of the inside ring, and toughness of the upper ring is higher than toughness the lower ring. 
     According to another embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; an outside ring arranged outside the inside ring, abrasion-resistant of the outside ring being higher than abrasion-resistant of the inside ring and the outside ring being harder than the inside ring; wherein the outside ring and the inside ring are individually, vertically movable each other, and the outside ring is either contact-less with the polishing pad, or is pressed onto the polishing pad with a force smaller than another force pressing the inside ring onto the polishing pad. 
     According to another embodiment, a substrate holding apparatus includes: a top ring body for holding a substrate; and the above retainer ring, the retainer ring being arranged to enclose the substrate held by the top ring body. 
     According to another embodiment, a polishing apparatus includes: a polishing pad; and the substrate holding apparatus according to claim  10 , the substrate holding apparatus pressing the substrate onto the polishing pad. 
     According to another embodiment, a method for maintaining the above retainer ring, includes: removing the inside ring from the outside ring; and fixing a new inside ring onto the outside ring. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view illustrating a polishing apparatus including a substrate holding apparatus according to an embodiment. 
         FIG. 2  is a view illustrating a detailed configuration of the polishing apparatus. 
         FIG. 3A  is a cross-sectional view illustrating the top ring  1 . 
         FIGS. 3B and 3C  are enlarged cross-sectional views illustrating different phases of an outer periphery of a drive ring  81 . 
         FIG. 4  is a plain view illustrating the drive ring  81  and the coupling member  75 . 
         FIG. 5  is a view illustrating the spherical bearing  85 . 
         FIG. 6A  is a drawing illustrating vertical movement of the coupling member  75  relative to the spherical bearing  85 . 
         FIGS. 6B and 6C  are drawings illustrating tilt movement of the coupling member  75  together with the inner ring  101 . 
         FIG. 7  is an enlarged cross-sectional view illustrating another configuration example of a spherical bearing  85 . 
         FIG. 8A  is a drawing illustrating vertical movement of the coupling member  75  relative to the spherical bearing  85 . 
         FIGS. 8B and 8C  are drawings illustrating tilt movement of the coupling member  75  together with the intermediate ring  91 . 
         FIG. 9  is an enlarged partial cross-sectional view of the retainer ring  40 . 
         FIG. 10  is a view schematically illustrating a change in a surface shape of the retainer ring  40  shown in  FIG. 9  after used for a long period of time. 
         FIGS. 11A to 11D  are graphs showing measurement results of cross sectional shapes of a typical retainer ring after used for a long period of time. 
         FIGS. 12 to 14  are graphs showing measurement results of shapes of lower surfaces of typical retainer rings after used for a long period of time. 
         FIGS. 15A to 15F  are enlarged views of various outside rings  40   b.    
         FIG. 16  is a cross-sectional view illustrating an example method for fixing an inside ring  40   a  and an outside ring  40   b.    
         FIG. 17  is an enlarged partial cross-sectional view of a retainer ring  40 ′, a modified example of the retainer ring  40  shown in  FIG. 9 . 
         FIG. 18  is a bottom view of a retainer ring  40 . 
         FIG. 19  is a schematic view illustrating a retainer ring while a wafer is polished. 
     
    
    
     DETAILED DESCRIPTION OF NON-LIMITING EXAMPLE EMBODIMENTS 
     A detailed description will hereinafter be given with reference to the drawings. 
     According to one embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; and an outside ring arranged outside the inside ring, abrasion-resistant of the outside ring being higher than abrasion-resistant of the inside ring, wherein the inside ring having a thickness in a radial direction in a range from a minimum of 0.05 mm to a maximum of 5 mm. 
     Since the outside ring is highly abrasion-resistant, and an appropriate distance is set between an inner peripheral surface of the inside ring and a convex portion, it is possible to maintain a load acting point of the retainer ring near an edge of the substrate, stabilizing a polishing rate. 
     According to another embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; and an outside ring arranged outside the inside ring, the outside ring comprising a lower ring and an upper ring, wherein abrasion-resistant of the lower ring is higher than abrasion-resistant of the inside ring, and toughness of the upper ring is higher than toughness of the lower ring. 
     Since the outside ring is highly abrasion-resistant, it is possible to maintain a load acting point of the retainer ring near an edge of the substrate, stabilizing a polishing rate. Also, the tough upper ring allows the retainer ring to firmly be attached with another component. 
     It is preferable that a convex portion toward the polishing pad is formed on a lower surface of the outside ring. 
     By such a manner, the load acting point is prevented from being moved away from a substrate in a radial direction. 
     It is more preferable that a distance between a tip of the convex portion and an end of the outside ring with no convex portion is below 30 μm. 
     Therefore, a substrate polishing rate can be stabilized. 
     It is preferable that the inside ring is detachably attached to the outside ring. 
     Therefore, only the inside ring can be replaced when the inside ring is worn, while continuously using a highly abrasion-resistant outside ring. 
     According to another embodiment, a retainer ring for retaining a substrate to be polished, includes: an inside ring arranged to enclose the substrate; an outside ring arranged outside the inside ring, abrasion-resistant of the outside ring being higher than abrasion-resistant of the inside ring and the outside ring being harder than the inside ring; wherein the outside ring and the inside ring are individually, vertically movable each other, and the outside ring is either contact-less with the polishing pad, or is pressed onto the polishing pad with a force smaller than another force pressing the inside ring onto the polishing pad. 
     Since the outside ring is highly abrasion-resistant, it is possible to maintain a load acting point of the retainer ring near an edge of the substrate, stabilizing a polishing rate. 
     A material of the inside ring may include one or more of PPS, PEEK, PTFE, PP, PET, polycarbonate, and polyacetal. A material of the outside ring may include one or more of ceramic materials and metallic materials. 
     Preferably, surface roughness of a lower surface of the outside ring is a maximum of Ra 1.6 μm, and Rockwell M hardness of the inside ring is in a range from a minimum of 70 to a maximum of 120. 
     Preferably, on a lower surface of the inside ring and a lower surface of the outside ring, a plurality of grooves extending in radial directions and communicating the inside ring and the outside ring are formed. 
     According to another embodiment, a substrate holding apparatus includes: a top ring body holding a substrate; and the above retainer ring, the retainer ring being arranged to enclose the substrate held by the top ring body. 
     According to another embodiment, a polishing apparatus includes: a polishing pad; and the substrate holding apparatus according to claim  31 , the substrate holding apparatus pressing the substrate onto the polishing pad. 
     According to another embodiment, a method for maintaining the above retainer ring, includes: removing the inside ring from the outside ring; and fixing a new inside ring onto the outside ring. 
       FIG. 1  is a schematic view illustrating a polishing apparatus including a substrate holding apparatus according to an embodiment. As shown in  FIG. 1 , the polishing apparatus includes a top ring (substrate holding apparatus)  1  holding and rotating a wafer (substrate) W, a polishing table  3  supporting a polishing pad  2 , a polishing liquid supply nozzle  5  supplying polishing liquid (slurry) onto the polishing pad  2 , and a film thickness sensor  7  retrieving film thickness signals that changes depending a film thickness of the wafer W. The film thickness sensor  7  is arranged inside the polishing table  3 , retrieving the film thickness signals at a plurality of areas including a center of the wafer W each time the polishing table  3  rotates once. The film thickness sensor  7  may be, for example, an optical sensor or an eddy current sensor. 
     The top ring  1  is configured to hold the wafer W by vacuum contact onto the lower surface of the wafer W. The top ring  1  and the polishing table  3  rotate in the same direction indicated by an arrow, during which the top ring  1  presses the wafer W against a polishing face  2   a  of the polishing pad  2 . The polishing liquid supply nozzle  5  supplies the polishing liquid onto the polishing pad  2 , during which the wafer W comes into sliding contact with the polishing pad  2  for polishing. While the wafer W is polished, the film thickness sensor  7  rotates together with the polishing table  3  across a surface of the wafer W, as indicated by symbol A, to retrieve the film thickness signals. The film thickness signals are index values directly or indirectly indicate film thicknesses, and the film thickness signals change as the film thickness of the wafer W decreases. The film thickness sensor  7  is connected to a polishing controller  9 , to which the film thickness signals is sent. The polishing controller  9  stops polishing the wafer W upon the film thickness signals indicate that the film thickness of the wafer W reaches a predetermined target value. 
       FIG. 2  is a view illustrating a detailed configuration of the polishing apparatus. The polishing table  3  is coupled, via a table shaft  3   a,  to a motor  13  arranged under the polishing table  3  so that the polishing table  3  can rotate around the table shaft  3   a.  The polishing pad  2  is pasted on an upper surface of the polishing table  3  so that an upper surface of the polishing pad  2  configures the polishing face  2   a  polishing the wafer W. The motor  13  rotates the polishing table  3 , resulting in that the polishing face  2   a  moves relative to the top ring  1 . The motor  13  therefore configures a polishing face moving mechanism that horizontally moves the polishing face  2   a.    
     The top ring  1  is connected to a top ring shaft  11  that vertically moves by a vertical movement mechanism  27  relative to a top ring head  16 . By vertical movement of the top ring shaft  11 , the top ring  1  raises or lowers entirely relative to the top ring head  16  for positioning. An upper end of the top ring shaft  11  is attached with a rotary joint  25 . 
     The vertical movement mechanism  27 , which vertically moves the top ring shaft  11  and the top ring  1 , includes a bridge  28  rotatably supporting the top ring shaft  11  via a bearing  26 , a ball screw  32  attached to the bridge  28 , a support table  29  supported by a support column  30 , and a servomotor  38  arranged on the support table  29 . The support table  29  supporting the servomotor  38  is fixed to the top ring head  16  via the support column  30 . 
     The ball screw  32  includes a screw shaft  32   a  coupled to the servomotor  38  and a nut  32   b  into which the screw shaft  32   a  is screwed. The top ring shaft  11  vertically moves together with the bridge  28 . When the servomotor  38  is driven, accordingly the bridge  28  is vertically moved via the ball screw  32 , and thus the top ring shaft  11  and the top ring  1  are vertically moved. 
     The top ring shaft  11  is coupled to a rotating cylinder  12  via a key (not shown). An outer periphery of the rotating cylinder  12  is equipped with a timing pulley  14 . A top ring motor  18  is fixed to the top ring head  16 , and the above described timing pulley  14  is connected, via a timing belt  19 , to a timing pulley  20  attached to the top ring motor  18 . Therefore, rotary-driving the top ring motor  18  allows the rotating cylinder  12  and the top ring shaft  11  to rotate together, via the timing pulley  20 , the timing belt  19 , and the timing pulley  14 , and thus allows the top ring  1  to rotate at an axial center. The top ring motor  18 , the timing pulley  20 , the timing belt  19 , and the timing pulley  14  configure a rotation mechanism rotating the top ring  1  at the axial center. The top ring head  16  is supported by a top ring head shaft  21  rotatably supported by a frame (not shown). 
     The top ring  1  can hold the substrate, such as the wafer W, at the lower surface. The top ring head  16  is configured in a rotatable manner around the top ring head shaft  21 , moving the top ring  1  holding the wafer W at the lower surface, through rotation of the top ring head  16 , from a wafer W—receiving position to above the polishing table  3 . The top ring  1  is then lowered to press the wafer W against the polishing face  2   a  of the polishing pad  2 . At this time, the top ring  1  and the polishing table  3  are rotated separately, and the polishing liquid is supplied from the polishing liquid supply nozzle  5  arranged above the polishing table  3  onto the polishing pad  2 . In this manner, the wafer W is come into sliding contact with the polishing face  2   a  of the polishing pad  2  to polish the surface of the wafer W. 
     Next, the top ring  1  configuring the substrate holding apparatus will be described.  FIG. 3A  is a cross-sectional view illustrating the top ring  1 . As shown in  FIG. 3A , the top ring  1  includes a top ring body  10  holding and pressing the wafer W against the polishing face  2   a,  and a retainer ring  40  arranged to enclose the wafer W. The top ring body  10  and the retainer ring  40  are configured to rotate together through rotation of the top ring shaft  11 . The retainer ring  40  is configured in a vertically movable manner relative to the top ring body  10 . The present embodiment has an object to stabilize a polishing rate by appropriately configuring the retainer ring  40 , the retainer ring  40  being described in detail in and after  FIG. 9 . 
     The top ring body  10  includes a circular flange  41 , a spacer  42  attached at a lower surface of the flange  41 , and a carrier  43  attached at a lower surface of the spacer  42 . The flange  41  is coupled to the top ring shaft  11 . The carrier  43  is coupled, via the spacer  42 , to the flange  41  so that the flange  41 , the spacer  42 , and the carrier  43  rotate together and vertically move. The top ring body  10  configured with the flange  41 , the spacer  42 , and the carrier  43  is molded of a resin such as engineer plastic (PEEK, for example). The flange  41  may be molded of metal such as SUS or aluminum. 
     Under a lower surface of the carrier  43 , an elastic film  45  that comes into contact with a back of the wafer W is attached. A lower surface of the elastic film  45  configures a substrate holding face  45   a.  The elastic film  45  has a plurality of annular partition walls  45   b  forming, between the elastic film  45  and the top ring body  10 , four pressure chambers, i.e. a center chamber  50 , a ripple chamber  51 , an outer chamber  52 , and an edge chamber  53 . The four pressure chambers  50  to  53  are connected, via the rotary joint  25 , to a pressure regulator  65  that supplies a pressurized fluid. The pressure regulator  65  can separately adjust pressure in the four pressure chambers  50  to  53 . The pressure regulator  65  can further form negative pressure in the four pressure chambers  50  to  53 . 
     The elastic film  45  has a through hole (not shown) at a position corresponding to the ripple chamber  51  or the outer chamber  52  so that, by forming negative pressure in the through hole, the top ring  1  can hold the wafer W at the substrate holding face  45   a.  The elastic film  45  is molded of a highly strong and durable rubber material, such as ethylene propylene rubber (EPDM), polyurethane rubber, or silicone rubber. The center chamber  50 , the ripple chamber  51 , the outer chamber  52 , and the edge chamber  53  are also connected to an atmosphere open mechanism (not shown) so that the center chamber  50 , the ripple chamber  51 , the outer chamber  52 , and the edge chamber  53  are atmospherically open. 
     The retainer ring  40  is arranged to enclose the carrier  43  and the elastic film  45  of the top ring body  10 . The retainer ring  40  is a ring-shaped member for coming into contact with the polishing face  2   a  of the polishing pad  2 . The retainer ring  40  is arranged to enclose an outer periphery edge of the wafer W to hold the wafer W so that the wafer W does not come off the top ring  1  while the wafer W is polished. 
       FIGS. 3B and 3C  are enlarged cross-sectional views illustrating different phases of an outer periphery of a drive ring  81 . As shown in  FIGS. 3B and 3C , an upper surface of the retainer ring  40  is positioned to the drive ring  81  by a plurality of reinforcing pins  82 , and is fixed with a plurality of screws  83 . The plurality of reinforcing pins  82  and the plurality of screws  83  are respectively arranged circumferentially and evenly. A top of the drive ring  81  is coupled to an annular retainer ring pressing mechanism  60  applying a load downwardly, evenly, and entirely to the upper surface of the retainer ring  40  via the drive ring  81  to press a lower surface of the retainer ring  40  against the polishing face  2   a  of the polishing pad  2 . 
     The retainer ring pressing mechanism  60  includes an annular piston  61  fixed to the top of the drive ring  81  and an annular rolling diaphragm  62  connected to an upper surface of the piston  61 . A retainer ring pressure chamber  63  is formed inside the rolling diaphragm  62 . The retainer ring pressure chamber  63  is connected to the pressure regulator  65  via the rotary joint  25 . 
     When the pressurized fluid (pressurized air, for example) is supplied from the pressure regulator  65  to the retainer ring pressure chamber  63 , the rolling diaphragm  62  presses down the piston  61 , and then the piston  61  presses down, via the drive ring  81 , the retainer ring  40  entirely. The retainer ring pressing mechanism  60  presses in this manner the lower surface of the retainer ring  40  against the polishing face  2   a  of the polishing pad  2 . In addition to this, the pressure regulator  65  can form negative pressure inside the retainer ring pressure chamber  63  to raise the retainer ring  40  entirely. The retainer ring pressure chamber  63  is also connected to the atmosphere open mechanism (not shown) so as to be atmospherically open. 
     The drive ring  81  is detachably coupled to the retainer ring pressing mechanism  60 . More specifically, the piston  61  is molded of a magnetic material, such as metal, and the top of the drive ring  81  is arranged with a plurality of magnets  70 . The plurality of magnets  70  attracts the piston  61  to secure the drive ring  81  to the piston  61 . The magnetic material used to mold the piston  61  may be, for example, corrosion-resistant magnetic stainless steel. In contrast, the drive ring  81  may be molded of a magnetic material, and the piston  61  may be arranged with a plurality of magnets. 
     The retainer ring  40  is coupled to a spherical bearing  85  via the drive ring  81  and a coupling member  75 . The spherical bearing  85  is arranged inside the retainer ring  40  inwardly in a radial direction.  FIG. 4  is a plain view illustrating the drive ring  81  and the coupling member  75 . As shown in  FIG. 4 , the coupling member  75  includes a shaft  76  arranged at a center of the top ring body  10 , a hub  77  fixed to the shaft  76 , and a plurality of spokes  78  radially extending from the hub  77 . 
     One end of the spokes  78  is fixed to the hub  77 , and the other end of the spokes  78  is fixed to the drive ring  81 . The hub  77  and the plurality of spokes  78  are integrally formed together with the drive ring  81 . The carrier  43  is fixed with a plurality of pairs of drive pins  80 ,  80 . Each pair of the drive pins  80 ,  80  is arranged on both sides of each of the spokes  78  to transmit rotation of the carrier  43  to the drive ring  81  and the retainer ring  40  via the drive pins  80 ,  80  so that the top ring body  10  and the retainer ring  40  can rotate together. 
     As shown in  FIG. 3A , the shaft  76  extends longitudinally inside the spherical bearing  85 . As shown in  FIG. 4 , in the carrier  43  is formed a plurality of radial grooves  43   a  accommodating the plurality of spokes  78  so that each of the spokes  78  are longitudinally movable in each of the grooves  43   a.  The shaft  76  of the coupling member  75  is longitudinally and movably supported by the spherical bearing  85  arranged at the center of the top ring body  10 . According to the above described configuration, the coupling member  75  is longitudinally movable together with the drive ring  81  and the retainer ring  40  coupled to the coupling member  75  relative to the top ring body  10 . The drive ring  81  and the retainer ring  40  are further tiltably and movably supported by the spherical bearing  85 . 
       FIG. 5  is a view illustrating the spherical bearing  85 . The shaft  76  is fixed to the hub  77  with a plurality of screws  79 . The shaft  76  is formed with a longitudinally extending through hole  88 . The through hole  88  acts as an air vent hole when the shaft  76  longitudinally moves relative to the spherical bearing  85 , allowing the retainer ring  40  to longitudinally and smoothly move relative to the top ring body  10 . 
     The spherical bearing  85  includes an annular inner ring  101  and an outer ring  102  slidably supporting an outer peripheral surface of the inner ring  101 . The inner ring  101  is coupled, via the coupling member  75 , the drive ring  81  and the retainer ring  40 . The outer ring  102  is fixed to a supporting member  103  fixed to the carrier  43 . The supporting member  103  is arranged inside a recessed portion  43   b  of the carrier  43 . 
     The outer peripheral surface of the inner ring  101  has a spherical shape where a top and a bottom are cut into flat surfaces, and a center (fulcrum) O of the spherical shape positions at a center of the inner ring  101 . An inner peripheral surface of the outer ring  102  is configured with a recessed surface along with the outer peripheral surface of the inner ring  101  so that the outer ring  102  slidably supports the inner ring  101 . The inner ring  101  is therefore tiltably and omni-directionally) (360°) movable relative to the outer ring  102 . 
     An inner peripheral surface of the inner ring  101  is configured with a through hole  101   a  into which the shaft  76  is inserted. The shaft  76  is only longitudinally movable relative to the inner ring  101 . The retainer ring  40  coupled to the shaft  76  therefore cannot move laterally, and thus a lateral (horizontal) position of the retainer ring  40  is secured by the spherical bearing  85 . The spherical bearing  85  functions as a supporting mechanism to restrict lateral movement of the retainer ring  40  (i.e. securing the horizontal position of the retainer ring  40 ), while receiving a lateral force (a force in an outward radial direction of the wafer) applied from the wafer W against the retainer ring  40  due to friction between the wafer W and the polishing pad  2  while the wafer W is polished. 
       FIG. 6A  illustrates vertical movement of the coupling member  75  relative to the spherical bearing  85 , while  FIGS. 6B and 6C  illustrate tilt movement of the coupling member  75  together with the inner ring  101 . The retainer ring  40  coupled to the coupling member  75  is tiltably movable together with the inner ring  101  at the fulcrum O, and is vertically movable relative to the inner ring  101 . 
       FIG. 7  is an enlarged cross-sectional view illustrating another configuration example of a spherical bearing  85 . As shown in  FIG. 7 , the spherical bearing  85  includes an intermediate ring  91  coupled to a retainer ring  40  via a coupling member  75 , an outer ring  92  slidably supporting the intermediate ring  91  from above, and an inner ring  93  slidably supporting the intermediate ring  91  from beneath. The intermediate ring  91  has a partial spherical shell shape where a lower half thereof is smaller than an upper half, the intermediate ring  91  being interposed between the outer ring  92  and the inner ring  93 . 
     The outer ring  92  is arranged in a recessed portion  43   b.  The outer ring  92  has a brim  92   a  at an outer periphery of the outer ring  92 , through which brim  92   a,  by securing the brim  92   a  onto a step of the recessed portion  43   b  with a bolt (not shown), the outer ring  92  is fixed to the carrier  43 , as well as pressure can be applied onto the intermediate ring  91  and the inner ring  93 . The inner ring  93  is arranged on a bottom of the recessed portion  43   b  to support from beneath the intermediate ring  91  so as to form a gap between a lower surface of the intermediate ring  91  and the bottom of the recessed portion  43   b.    
     An inner surface  92   b  of the outer ring  92 , an outer surface  91   a  and an inner surface  91   b  of the intermediate ring  91 , and an outer surface  93   a  of the inner ring  93  are each configured in an approximate hemispherical shape formed around a fulcrum O. The outer surface  91   a  of the intermediate ring  91  slidably contacts with the inner surface  92   b  of the outer ring  92 , while the inner surface  91   b  of the intermediate ring  91  slidably contacts with the outer surface  93   a  of the inner ring  93 . The inner surface  92   b  (sliding contact surface) of the outer ring  92 , the outer surface  91   a  and the inner surface  91   b  (sliding contact surfaces) of the intermediate ring  91 , and the outer surface  93   a  (sliding contact surface) of the inner ring  93  each has a partial spherical shape where a lower half thereof is smaller than an upper half. According to the above described configuration, the intermediate ring  91  is tiltably and omni-directionally) (360°) movable relative to the outer ring  92  and the inner ring  93 , and the fulcrum O, i.e. a center of tilt movement, positions below the spherical bearing  85 . 
     The outer ring  92 , the intermediate ring  91 , and the inner ring  93  are respectively formed with through holes  92   c,    91   c,  and  93   b  into which a shaft  76  is inserted. A gap is formed between the through hole  92   c  of the outer ring  92  and the shaft  76 , as well as another gap is formed between the through hole  93   b  of the inner ring  93  and the shaft  76 . The through hole  91   c  of the intermediate ring  91  has a smaller diameter than diameters of the respective through holes  92   c  and  93   b  of the outer ring  92  and the inner ring  93  so that the shaft  76  is only longitudinally movable relative to the intermediate ring  91 . The retainer ring  40  coupled to the shaft  76  therefore cannot substantially move laterally, and a lateral (horizontal) position of the retainer ring  40  is secured by the spherical bearing  85 . 
       FIG. 8A  illustrates vertical movement of the coupling member  75  relative to the spherical bearing  85 , while  FIGS. 8B and 8C  illustrate tilt movement of the coupling member  75  together with the intermediate ring  91 . As shown in  FIGS. 8A to 8C , the retainer ring  40  coupled to the coupling member  75  is tiltably movable together with the intermediate ring  91  at the fulcrum O, as well as is vertically movable relative to the intermediate ring  91 . The spherical bearing  85  shown in  FIG. 7  is the same with the spherical bearing  85  shown in  FIG. 5  in terms of the fulcrum O, i.e. a center of tilt movement, positioned on a central axis line of the retainer ring  40 . However, the spherical bearing  85  shown in  FIG. 7  is different from the spherical bearing  85  shown in  FIG. 5  because the fulcrum O shown in  FIG. 7  is positioned lower than the fulcrum O shown in  FIG. 5 . For the spherical bearing  85  shown in  FIG. 7 , a height of the fulcrum O may be the same or below a surface of the polishing pad  2 . 
     Next, the retainer ring  40 , which is one of features of the present embodiment, will now be described herein. 
       FIG. 9  is an enlarged partial cross-sectional view of the retainer ring  40 . The retainer ring  40  includes an inside ring  40   a  and an outside ring  40   b.  The inside ring  40   a  has an annular shape, enclosing the wafer W. The outside ring  40   b  also has an annular shape, and is arranged outside the inside ring  40   a.    
     When polishing the wafer W, an inner peripheral surface of the inside ring  40   a  comes into contact with the edge of the wafer W. To prevent the wafer W from being chipped or damaged, the inside ring  40   a  is molded of a softer material than the wafer W, preferably a material having a Rockwell M hardness of in a range from a minimum of 70 to a maximum of 120. In addition, the inner peripheral surface of the inside ring  40   a  receives a force from the edge of the wafer W. To bear such a force, the inside ring  40   a  is molded of a highly abrasion-resistant material. Highly abrasion-resistant materials having low hardness property may be, for example, PPS (Poly Phenylene Sulfide), PEEK (Poly Ether Ether Ketone), PTFE (Poly Tetra Fluoro Etylene), PP (Poly Propylene), PET (Polyethylene Terephthalate), polycarbonate, polyacetal, and other resin materials that contain one or more of the described materials. 
     A lower surface of the outside ring  40   b  is pressed by the polishing face  2   a  to be rotated. Therefore, the outside ring  40   b  is molded of a material that is more abrasion-resistant than a material of the inside ring  40   a,  preferably a material with a wear amount, due to contact with the polishing face  2   a,  of a maximum of one fifth of a wear amount of the inside ring  40   a.  Such a material may be, for example, a ceramic material, such as SiC, SiN, alumina, or zirconia, or a metallic material, such as stainless steel or titanium. The material of the outside ring  40   b  may be harder than a material of the inside ring  40   a.    
     The retainer ring  40  normally requires breaking-in polishing in an initial stage of use to stabilize surface roughness of a lower surface that comes into contact with the polishing face  2   a,  because, if the retainer ring  40  having a rough surface is used when polishing the wafer W, an action of grinding a surface of the polishing pad  2  occurs, leading to an unstable polishing rate of the wafer W. Since the outside ring  40   b  is highly abrasion-resistant, and will be rarely worn in a polishing process, it is preferable that surface roughness be a maximum of Ra 1.6 μm (JIS B 0601:2001) in an initial state. 
       FIG. 10  is a view schematically illustrating a change in a surface shape of the retainer ring  40  shown in  FIG. 9  after used for a long period of time. The view shows that the lower surface of the retainer ring  40  wears through contact with the polishing face  2   a,  where wear on the inner peripheral surface of the inside ring  40   a  due to contact with the edge of the wafer W is ignored. Wear on a lower surface of the inside ring  40   a  advances through polishing of many wafers W in a long period of time due to contact with the polishing face  2   a.  In particular, the more close to the inner peripheral surface, the more the wear amount, and the more close to an outer periphery (i.e. more close toward the outside ring  40   b ), the less the wear amount. On the other hand, since the outside ring  40   b  is molded of a highly abrasion-resistant material, the lower surface is not changed significantly, almost keeping the initial shape. 
     With the retainer ring  40  having such a shape, an innermost point A on the lower surface of the outside ring  40   b  becomes a load acting point against the wafer W. Accordingly, the load acting point can be prevented from being moved away from the edge of the wafer W. As a result, even when the retainer ring  40  is used for a long period of time, a polishing rate at the edge of the wafer W can be prevented from being increased, achieving the stabilized polishing rate. 
     An appropriate value of thickness d of the inside ring  40   a  in a radial direction will now be described herein. 
     Since the inner peripheral surface of the inside ring  40   a  wears due to contact with the wafer W, a thickness d must be thicker than a wear amount. 
       FIGS. 11A to 11D  are graphs showing measurement results of cross sectional shapes of a typical retainer ring after used for a long period of time. The typical retainer ring is molded of a single material (PPS), different from the retainer ring  40  including the inside ring  40   a  and the outside ring  40   b  shown in  FIG. 9 .  FIGS. 11A to 11D  respectively show the results of measuring the annular retainer ring at four points shifted by 90 degrees (0 deg., 90 deg., 180 deg., and 270 deg., respectively). 
     Horizontal axes of  FIGS. 11A to 11D  each represents position R of the retainer ring in a radial direction, indicating that an inner peripheral surface of the unworn retainer ring is positioned at a value of 90 μm. Vertical axes each represents height Z of the retainer ring, indicating that a lower surface of the retainer ring is positioned at a value of 800 μm, and an edge of a wafer W is coming into contact with a value of approximately 400 μm. 
     As can be seen in the measurement results, the retainer ring wears approximately 0.04 mm after used for a long period of time. When using the retainer ring  40  configured with the inside ring  40   a  and the outside ring  40   b,  a preferable thickness d of the inside ring  40   a  be a minimum of 0.05 mm by taking into account some additional thickness because the inside ring  40   a  could be worn 0.04 mm. The thickness d of the inside ring  40   a  may be increased further, for example, a minimum of 0.1 mm, because a too thin thickness might be disadvantageous in terms of rigidity, as well as might lead to difficult processing. 
     On the other hand, although the load acting point of the retainer ring  40  is the innermost point A on the lower surface of the outside ring  40   b,  a too high thickness d causes the load acting point of the retainer ring  40  to move away from the edge of the wafer W, leading to an unstable polishing rate. Therefore, the thickness d is set so that the load acting point of the retainer ring  40  comes close to the edge of the wafer W. 
       FIGS. 12 to 14  are graphs showing measurement results of shapes of lower surfaces of typical retainer rings after used for a long period of time. The retainer rings used for the measurements were also molded of a single material (PPS), where the lower surfaces were flat initially. As can be seen in the graphs, widths of the retainer rings (distances in a radial direction between an inner peripheral surface and an outer peripheral surface) were 5 mm ( FIG. 12 ), 7.5 mm ( FIG. 13 ), and 15 mm ( FIG. 14 ). A horizontal axis of each of the graphs represents positions in radial direction relative to the inner peripheral surface of the retainer ring as an origin. A vertical axis represents a relative distance from the polishing face  2   a,  where a scale represents 10 μm. As can be seen, although the lower surfaces of the retainer rings were flat initially, the lower surfaces were worn after use for a long period of time, forming a downwardly raised portion (convex portion) P′ on the lower surface. 
     Regarding the retainer rings with the initial widths of 5 mm ( FIGS. 12 ) and 7.5 mm ( FIG. 13 ), respectively, polishing rates were stable, and wafers W were polished successfully. On the other hand, regarding the retainer ring with the initial width of 15 mm ( FIG. 14 ), a polishing rate was unstable, resulting in unsuccessful polishing of a wafer W. 
     According to  FIGS. 12 and 13  where proper polishing was realized, distances s′ between the inner peripheral surfaces and the downwardly raised portions P′ of the retainer rings were approximately 3 mm and 5 mm, respectively. The results mean, for the retainer ring  40  including the inside ring  40   a  and the outside ring  40   b  shown in  FIG. 10 , that a maximum thickness d of 5 mm can achieve a stabilized polishing rate. 
     On the other hand, according to  FIG. 14  where proper polishing was not realized, the distance s′ between the inner peripheral surface and the downwardly raised portion P′ of the retainer ring was approximately 7 mm. The result means, for the retainer ring  40  shown in  FIG. 10 , that a minimum thickness d of 7 mm could lead to an unstable polishing rate. 
     Therefore, it is preferable that a thickness d should not be too high, but be below 7 mm, and more preferably a maximum of 5 mm. 
     As shown in  FIG. 9 , although the lower surface of the outside ring  40   b  may be flat, it is preferable that a downwardly raised portion be formed toward the polishing face  2   a  (lower side). 
       FIGS. 15A to 15F  are enlarged views of various outside rings  40   b.  In the views, inside rings  40   a  are illustrated with two-dot chain lines. A downwardly raised portion P may be formed at an innermost side of an outside ring  40   b  ( FIGS. 15A and 15D ), or may be formed at a position slightly away from the innermost side ( FIGS. 15B and 15E ). In addition, lines from a downwardly raised portion P to both ends of a lower surface of an outside ring  40   b  may be straight ( FIGS. 15A and 15B ) or curved ( FIGS. 15D and 15E ). 
     By providing a downwardly raised portion P at a position near an inside ring  40   a,  the downwardly raised portion P becomes a load acting point against a wafer W. Accordingly, the load acting point is prevented from being moved away from a wafer W in a radial direction. 
     As shown in  FIGS. 15C and 15F , downwardly raised portions P may be at outermost positions of outside rings  40   b  (or near outside). In each case of  FIGS. 15C and 15F , an innermost point on an outside ring  40   b  becomes a load acting point against a wafer W, maintaining the load acting point near the wafer W as well. 
     When a distance h between an end (where no downwardly raised portion P is formed) of an outside ring  40   b  and a downwardly raised portion P is too large, a load could concentrate into the downwardly raised portion P, leading to an unstable polishing rate, the worn downwardly raised portion P, and the damaged polishing face  2   a.    
     According to  FIGS. 12 and 13  where proper polishing was realized, distances h′ between tips of downwardly raised portions P′ and ends of retainer rings, the distances h′ being occurred due to wear, were approximately 5 μm and 20 μm, respectively. The results mean, for the retainer ring  40   s,  shown in  FIGS. 15A to 15F  where downwardly raised portions P are formed, that an acceptable distance h is a maximum of 20 μm. 
     On the other hand, according to  FIG. 14  where proper polishing was not realized, the distance h′ was approximately 30 μm. The result means, for the retainer rings  40  shown in  FIGS. 15A to 15F , that a polishing rate becomes unstable if the distance h is a minimum of 30 μm. In other words, a maximum distance h of 30 μm can prevent a load from being excessively concentrated at a downwardly raised portion P. 
     Therefore, it is preferable, for the retainer rings  40 , that a distance h be below 30 μm, and more preferably, a maximum of 20 μm. 
     Based on the above information, it is preferable that a thickness d of the inside ring  40   a  in a radial direction shown in  FIG. 10  be in a range from a minimum of 0.05 mm to a maximum of 5 mm. When arranging a downwardly raised portion P on a lower surface of the outside ring  40   b,  it is preferable that a distance h between an end of the outside ring  40   b  and a tip of a downwardly raised portion P be below 30 μm. 
       FIG. 16  is a cross-sectional view illustrating an example method for fixing an inside ring  40   a  and an outside ring  40   b.  The inside ring  40   a  and the outside ring  40   b  may be fixed using an adhesive agent. However, it is preferable that a screw  40   c  be used so that the inside ring  40   a  is detachable from the outside ring  40   b.  As shown in  FIG. 10 , the inside ring  40   a  could wear due to contact with the polishing face  2   a,  while the outside ring  40   b  rarely wears. By making the inside ring  40   a  detachable, only the inside ring  40   a  can be replaced, if worn, in a cost effective manner. 
     The inside ring  40   a  and the outside ring  40   b  are fixed each other at least when polishing a wafer W so as to vertically move together. 
     A specific configuration example of  FIG. 16  shows that the cross sections of the inside ring  40   a  and the outside ring  40   b  are L-shaped which can be engaged each other. Screw holes are formed in portions where the inside ring  40   a  and the outside ring  40   b  abut in a vertical direction. Through the screw holes, a screw  40   c  is tightened to secure the inside ring  40   a  and the outside ring  40   b.  A screw hole  40   d  is formed on the outside ring  40   b  to fix the retainer ring  40  onto the drive ring  81  by inserting one of the plurality of screws  83  shown in  FIG. 3C . 
     Although this configuration only represents an example, it is obvious that those skilled in the art will understand that there are various methods of detachably fixing the inside ring  40   a  and the outside ring  40   b.    
     When replacing the inside ring  40   a  as a maintenance, firstly the screw  40   c  is removed to remove the inside ring  40   a  from the outside ring  40   b.  Next, a new inside ring  40   a  is fit onto the outside ring  40   b,  and then again the screw  40   c  is tightened to fix the inside ring  40   a  and the outside ring  40   b.    
     It is unnecessary to always fix the inside ring  40   a  and the outside ring  40   b  each other, but the inside ring  40   a  and the outside ring  40   b  may be vertically moved individually. In this case, a force pressing the inside ring  40   a  onto the polishing face  2   a,  and another force pressing the outside ring  40   b  onto the polishing face  2   a  may be the same, or may be different. For example, the outside ring  40   b  may be more strongly pressed onto the polishing face  2   a.  In contrast, the outside ring  40   b  may be more gently pressed onto the polishing face  2   a,  or the outside ring  40   b  may be contact-less with the polishing face  2   a,  if required. 
     When the outside ring  40   b  is pressed onto the polishing face  2   a  separately from the inside ring  40   a,  an amount of wear of the inside ring  40   a  in a vertical direction can be refrained, comparing with a case where no outside ring  40   b  is provided, because, although a wear amount of the inside ring  40   a  increases since an outer periphery of the inside ring  40   a  strongly comes into contact with the polishing pad  2  when no outside ring  40   b  is provided, the outer periphery of the inside ring  40   a  can more gently come into contact with the polishing pad  2  when an outside ring  40   b  is provided. 
       FIG. 17  is an enlarged partial cross-sectional view of a retainer ring  40 ′, a modified example of the retainer ring  40  shown in  FIG. 9 . An outside ring  40   b  of the retainer ring  40 ′ includes a lower ring  40   b   1  and an upper ring  40   b   2 . 
     Since the lower ring  40   b   1  comes into contact with the polishing face  2   a,  the lower ring  40   b   1  is molded of a highly abrasion-resistant, chemically stable material, such as a ceramic material, than a material of an inside ring  40   a.  Since the upper surface of the upper ring  40   b   2  is mechanically fixed to the drive ring  81  with a reinforcing pin  82 , a screw  83  and the like ( FIGS. 3B and 3C ), the upper ring  40   b   2  is molded of a tougher (damage-resistant, or, rigid) material, such as a stainless steel metallic material or a resin material, than a material of the lower ring  40   b   1 . The lower ring  40   b   1  and the upper ring  40   b   2  may be fixed each other with screws or an adhesive agent. 
       FIG. 18  is a bottom view of a retainer ring  40 . As shown in  FIG. 18 , a plurality of grooves  40   d  communicating the inside ring  40   a  and the outside ring  40   b  and extending in radial directions are formed at an approximately even interval so that a polishing liquid supplied onto the polishing face  2   a  can flow, through the plurality of grooves  40   d,  from inside to outside, and vice versa, of the retainer ring  40  for efficient polishing of a wafer W. 
     As described above, configuring the retainer ring  40  in two pieces, i.e. the inside ring  40   a  and the outside ring  40   b , while using a highly abrasion-resistant material for the outside ring  40   b,  allow a load acting point of the retainer ring  40  to be kept close to an edge of the wafer W, stabilizing a rate of polishing the wafer W (the edge, particularly). 
     While the embodiment has been described above, the present invention is not limited to the embodiment, and various other changes and modifications can be made within the spirit of the technology.