Patent Publication Number: US-9403255-B2

Title: Polishing apparatus and polishing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This document claims priorities to Japanese Patent Application No. 2012-124663 filed May 31, 2012 and Japanese Patent Application No. 2012-279751 filed Dec. 21, 2012, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polishing apparatus and a polishing method for polishing a substrate, such as a wafer. 
     2. Description of the Related Art 
     With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin firm formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus. 
     Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer with use of a polishing apparatus by placing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO 2 ), onto the polishing surface. 
     The polishing apparatus of this type has a polishing table that supports the polishing pad thereon, and a substrate holder for holding the wafer. The substrate holder is often called a top ring or a polishing head. This polishing apparatus polishes the wafer as follows. The substrate holder holds the wafer and presses it against the polishing surface of the polishing pad at predetermined pressure. The polishing table and the substrate holder are moved relative to each other to bring the wafer into sliding contact with the polishing surface to thereby polish a surface of the wafer. 
     When polishing the wafer, if a relative pressing force applied between the wafer and the polishing pad is not uniform over the surface of the wafer in its entirety, lack of polishing or excessive polishing would occur depending on the pressing force applied to each portion of the water. Thus, in order to even the pressing force exerted on the wafer, the substrate holder has at its lower part a pressure chamber formed by a flexible membrane. This pressure chamber is supplied with fluid, such as air, to press the wafer through the flexible membrane under the fluid pressure. 
     However, because the above-described polishing pad has elasticity, the pressing force becomes non-uniform in an edge portion (a peripheral portion) of the wafer during polishing of the water. Such non-uniform pressing force would result in so-called “rounded edge” which is excessive polishing that occurs only in the edge portion of the wafer. In order to prevent such rounded edge, the substrate holder has a retaining ring for retaining the edge portion of the wafer. This retaining ring is configured to be vertically movable relative to a top ring body (or carrier head body) and press a region in the polishing surface of the polishing pad around the wafer. 
     As the types of semiconductor devices have been increasing dramatically in recent years, there is an increasing demand for controlling a polishing profile in the wafer edge portion for each device or each C process (e.g., an oxide film polishing process and a metal film polishing process). One of the reasons is that each wafer has a different initial film-thickness distribution because a film-forming process, which is performed prior to the CMP process, varies depending on the type of film. Typically, a wafer is required to have a uniform film-thickness distribution over its entire surface after the CMP process. Therefore, different initial film-thickness distributions necessitate different polishing profiles. 
     Other reason is that types of polishing pads and polishing liquids, both of which are consumables in the polishing apparatus, are increasing greatly from a viewpoint of costs. Use of different polishing pads or different polishing liquids results in different polishing profiles particularly in the wafer edge portion. Because the polishing profile in the wafer edge portion can greatly affect a product yield, it is very important to precisely control the polishing profile in the wafer edge portion. 
     As described above, for the purpose of preventing the rounded edge of the wafer, the conventional substrate holder has the retaining ring configured to press the polishing surface of the polishing pad around the wafer. It is possible to control a polishing rate in the wafer edge portion by regulating pressure of the retaining ring. However, changing the pressure of the retaining ring could result in a change in the polishing rate not only in the wafer edge portion, but also in other regions with a relatively large area. Therefore, this approach is not suitable in the case of precisely controlling the polishing profile in the wafer edge portion. 
     SUMMARY OF THE INVENTION 
     The polishing profile, particularly the polishing profile in the wafer edge portion, can be controlled precisely by exerting a local force on the retaining ring for holding the edge portion of the wafer. In embodiments, a polishing apparatus and a polishing method capable of precisely controlling a polishing profile, particularly a polishing profile in an edge portion, of a substrate, such as a wafer. 
     In embodiments, a polishing apparatus for polishing a substrate by bringing the substrate into contact with a polishing surface, comprises: a substrate holder having a substrate holding surface configured to press the substrate against the polishing surface, a retaining ring coupled to the substrate holding surface and configured to surround the substrate, wherein the retaining ring is brought into contact with the polishing surface during operation of the polishing apparatus, the retaining ring being configured to be tiltable independently of the substrate holding surface; a rotating mechanism configured to rotate the substrate holder about its own axis; and at least one local load exerting mechanism configured to exert a local load on a part of the retaining ring in a direction perpendicular to the polishing surface, the at least one local load exerting mechanism being arranged so as not to move in accordance with the substrate holder. 
     In embodiments, the polishing apparatus further comprises a retaining ring pressing mechanism configured to press the retaining ring against the polishing surface. 
     In embodiments, the substrate holding surface and the retaining ring are vertically movable relative to each other. 
     In embodiments, the polishing apparatus further comprises a supporting mechanism configured to receive a lateral force applied from the substrate to the retaining ring during polishing of the substrate. 
     In embodiments, the at least one local load exerting mechanism comprises an air cylinder configured to exert the local load on a part of the retaining ring. 
     In embodiments, the at least one local load exerting mechanism comprises a magnet configured to exert the local load on a part of the retaining ring. 
     In embodiments, the magnet is an electromagnet configured to exert a downward local load and an upward local load selectively on a part of the retaining ring. 
     In embodiments, the polishing apparatus further comprises a load cell configured to measure a force that varies in accordance with the local load. 
     In embodiments, the polishing apparatus further comprises structure configured to allow an installation position of the at least one local load exerting mechanism to be changed. 
     In embodiments, the polishing apparatus further comprises a polishing surface moving mechanism configured to move the polishing surface horizontally relative to the substrate holder, the at least one local load exerting mechanism being located downstream of the substrate with respect to a moving direction of the polishing surface. 
     In embodiments, the at least one local load exerting mechanism comprises a plurality of local load exerting mechanisms. 
     In another embodiment, a polishing apparatus for polishing a substrate by bringing the substrate into contact with a polishing surface, comprises: a retaining ring arranged so as to surround the substrate; a first load exerting mechanism configured to bring the retaining ring into contact with the polishing surface during operation of the polishing apparatus; and at least one second mechanism configured to exert a second load on a part of the retaining ring in a direction perpendicular to the polishing surface. 
     In embodiments, the polishing apparatus further comprises structure configured to allow an installation position of the at least one second exerting mechanism to be changed. 
     In embodiments, the polishing apparatus further comprises a retaining ring height sensor configured to measure a height of the retaining ring. 
     In embodiments, the polishing apparatus is configured to change either one or both of a magnitude and a position of the second load based on a measurement result of the height of the retaining ring. 
     In embodiments, the polishing apparatus further comprises a film thickness sensor configured to obtain a film thickness signal indicating a film thickness of the substrate, the polishing apparatus being configured to change either one or both of a magnitude and a position of the second load based on the film thickness signal obtained. 
     In embodiments, the at least one second mechanism is configured to exert a second load on a part of the retaining ring downstream of a rotation of the polishing surface. 
     In another embodiment, a polishing method of polishing a substrate by bringing the substrate into contact with a polishing surface, comprises: pressing the substrate against the polishing surface while rotating the substrate; bringing a retaining ring, arranged so as to surround the substrate, into contact with the polishing surface while rotating the retaining ring; and when pressing the substrate against the polishing surface, exerting a local load on a part of the retaining ring in a direction perpendicular to the polishing surface, the position of the local load not changing during the rotation of the retaining ring. 
     In embodiments, the polishing method further comprises changing the position of the local load based on polishing result of the substrate. 
     In embodiments, the polishing method further comprises: measuring a height of the retaining ring; and changing either one or both of a magnitude and the position of the local load based on a measurement result of the height of the retaining ring. 
     In embodiments, the polishing method further comprises: obtaining a film thickness signal indicating a film thickness of the substrate; and changing either one or both of a magnitude and the position of the local load based on the film thickness signal. 
     In another embodiment, a polishing method, comprises: pressing a first substrate against a polishing surface while rotating the first substrate; bringing a retaining ring, arranged so as to surround the first substrate, into contact with the polishing surface while rotating the retaining ring; when pressing the first substrate against the polishing surface, exerting a local load on a part of the retaining ring in a direction perpendicular to the polishing surface, with use of a local load exerting mechanism which is stationary at a first position; after polishing of the first substrate, pressing a second substrate against the polishing surface while rotating the second substrate; bringing the retaining ring into contact with the polishing surface while rotating the retaining ring; when pressing the second substrate against the polishing surface, exerting a local load on a part of the retaining ring in the direction perpendicular to the polishing surface, with use of the local load exerting mechanism which is stationary at a second position differing from the first position; obtaining polishing results of the first substrate and the second substrate; and determining a position of the local load exerting mechanism based on the polishing results. 
     In embodiments, the local load when polishing the second substrate is different from the local load when polishing the first substrate. 
     In another embodiment, a polishing apparatus for polishing a substrate by bringing the substrate into sliding contact with a polishing surface, comprises: a substrate holder having a substrate holding surface for pressing the substrate against the polishing surface and further having a retaining ring arranged so as to surround the substrate, the retaining ring being configured to be tiltable independently of the substrate holding surface, wherein the retaining ring is brought into contact with the polishing surface during operation of the polishing apparatus; a rotating mechanism configured to rotate the substrate holder about its own axis; a local load exerting mechanism configured to generate a load; and a pressure ring disposed between the local load exerting mechanism and the retaining ring, the local load exerting mechanism being configured to exert the load on a part of the pressure ring in a direction perpendicular to the polishing surface, the pressure ring having a load transmitting element configured to transmit the load, received from the local load exerting mechanism, to a part of the retaining ring, the local load exerting mechanism and the pressure ring being substantially fixed in position relative to rotation of the substrate holder during operation of the polishing apparatus. 
     In embodiments, the polishing apparatus further comprises structure configured to allow a position of the load transmitting element to be changed along a circumferential direction of the retaining ring. 
     In embodiments, the load transmitting element comprises a rolling member. 
     In embodiments, the substrate holder farther has a retaining ring pressing mechanism configured to press the retaining ring against the polishing surface. 
     In embodiments, the local load exerting mechanism comprises load generators, a bridge configured to receive loads generated by the load generators, and a connector configured to transmit the loads, received by the bridge, to the pressure ring. 
     In embodiments, of the load generators, one which is closer to the connector generates a relatively large load, and one which is away from the connector generates a relatively small load. 
     In embodiments, the load generators are operable to generate the loads such that a center of gravity of the loads coincides with a position of the connector. 
     In embodiments, the polishing apparatus further comprises a load cell configured to measure a force that varies in accordance with the load exerted by the local load exerting mechanism. 
     In embodiments, the polishing apparatus further comprises a suction line coupling the pressure ring to a vacuum source. 
     In another embodiment, a polishing apparatus for polishing a substrate by bringing the substrate into contact with a polishing surface, comprises: a substrate holder having a substrate holding surface for pressing the substrate against the polishing surface and further having a retaining ring arranged so as to surround the substrate, the retaining ring being configured to be tiltable independently of the substrate holding surface, the retaining ring configured to contact the polishing surface during operation of the polishing apparatus; a rotating mechanism configured to rotate the substrate holder about its own axis; local load exerting mechanisms configured to generate local loads; and a pressure ring disposed between the local load exerting mechanisms and the retaining ring, each of the local load exerting mechanisms being configured to exert a local load on a part of the pressure ring in a direction perpendicular to the polishing surface, the pressure ring having load transmitting elements configured to transmit the local loads, received from the local load exerting mechanisms, to the retaining ring, the local load exerting mechanisms and the pressure ring being arranged so as not to rotate together with the substrate holder. 
     In embodiments, the load transmitting elements comprise rolling members. 
     In embodiments, the substrate holder further has a retaining ring pressing mechanism configured to press the retaining ring against the polishing surface. 
     In embodiments, the local load exerting mechanisms are configured to be able to change a center of the gravity of the local loads exerted on the pressure ring. 
     In embodiments, the polishing apparatus further comprises load cells configured to measure forces that vary in accordance with the local loads. 
     In another embodiment, a polishing apparatus for polishing a substrate by bringing the substrate into sliding contact with a polishing surface, comprises: a substrate holder having a substrate holding surface for pressing the substrate against the polishing surface, a retaining ring arranged so as to surround the substrate and brought into contact with the polishing surface, a pressing member configured to exert a local load on a part of the retaining ring in a direction perpendicular to the polishing surface, and a load generator configured to generate the local load, the retaining ring being configured to be tiltable independently of the substrate holding surface; a rotating mechanism configured to rotate the substrate holder about its own axis; and a position retaining mechanism arranged so as not to rotate together with the substrate holder and configured to retain a position of the pressing member so as not to allow the pressing member to rotate together with the substrate holder. 
     In embodiments, the position retaining mechanism is configured to retain the position of the pressing member via a magnetic force. 
     In another embodiment, a polishing method of polishing a substrate by bringing the substrate into sliding contact with a polishing surface, comprises: pressing the substrate against the polishing surface while rotating the substrate; bringing a retaining ring, arranged so as to surround the substrate, into contact with the polishing surface while rotating the retaining ring; and when pressing the substrate against the polishing surface, exerting local loads each on a part of the retaining ring in a direction perpendicular to the polishing surface, with positions of the local loads fixed relative to the rotation of the retaining ring. 
     In embodiments, the polishing method further comprises changing a center of gravity of the local loads by changing magnitude of the local loads. 
     In another embodiment, a polishing apparatus for polishing a substrate, comprises: a rotatable polishing table for supporting a polishing pad; a rotatable top ring having a substrate holding device and a retainer ring, wherein the substrate holding device is configured to hold the substrate against the polishing pad and the retainer ring is configured to surround the substrate and to press the polishing pad and is capable of tilting independently from the substrate holding device relative to an axis of the rotation of the top ring; and a first load exerting device for exerting a pressure to the retainer ring to press the polishing pad wherein the load exerting device is stationary relative to the rotation of the top ring. 
     In embodiments, the top ring further comprises a second load exerting device for pressing the retaining ring against the polishing pad. 
     In embodiments, the retaining ring is vertically movable independent from the substrate holding device. 
     In embodiments, the first load exerting device presses the retaining ring at more than two locations of the retaining ring. 
     In embodiments, the first load exerting device has an air cylinder for pressing the retaining ring. 
     In embodiments, the first load exerting device has a magnetic device for pressing the retaining ring. 
     In embodiments, the magnetic device comprises an electromagnet. 
     In embodiments, the first load exerting device is able to change the location to be pressed on the retaining ring. 
     In embodiments, the location on the retaining ring to be pressed by the first load exerting device is located downstream of the substrate relative to the rotation of the polishing table. 
     In embodiments, the substrate holding device is configured to hold the substrate against the polishing pad at different pressures at a central area and a peripheral area. 
     In embodiments, the polishing apparatus is a chemical mechanical polishing apparatus. 
     In embodiments, the substrate is a semiconductor wafer. 
     In another embodiment, a method for polishing a substrate., comprises: providing a rotatable top ring having a substrate holding device and a retaining ring; holding a substrate in the substrate holding device against a polishing pad; rotating the polishing pad; rotating the top ring and the substrate held in the substrate holding device; and exerting a pressure to the retaining ring to press the polishing pad by a load exerting device, wherein a position of the exerted pressure is stationary relative to the rotation of the top ring. 
     In embodiments, the substrate is a semiconductor wafer. 
     According to various embodiments described above, it is possible to actively control a surface pressure distribution of the retaining ring, a deformed state of the polishing surface, a deformed state of the retaining ring, and the like by exerting the local load on a part of the retaining ring. As a result, the polishing rate (i.e., the polishing profile) in the wafer edge portion, which is adjacent to the retaining ring, can be controlled precisely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a polishing apparatus according to an embodiment; 
         FIG. 2  is a view showing a detailed structure of the polishing apparatus; 
         FIG. 3  is a cross-sectional view of a top ring; 
         FIG. 4  is a plan view showing a retaining ring and a coupling member; 
         FIG. 5  is an enlarged cross-sectional view of a spherical bearing and a part of the coupling member; 
         FIG. 6A  is a view showing a manner in which the coupling member is vertically moved relative to the spherical bearing; 
         FIG. 6B  is a view showing a manner in which the coupling member tilts in unison with an intermediate bearing ring in one direction; 
         FIG. 6C  is a view showing a manner in which the coupling member tilts in unison with the intermediate bearing ring in the other direction; 
         FIG. 7  is a view showing an enlarged cross-sectional view of another example of the spherical bearing; 
         FIG. 8A  is a view showing a manner in which the coupling member is vertically moved relative to the spherical bearing; 
         FIG. 8B  is a view showing a manner in which the coupling member tilts in unison with an inner bearing ring in one direction; 
         FIG. 8C  is a view showing a manner in which the coupling member tilts in unison with the inner bearing ring in the other direction; 
         FIG. 9  is a view showing an example in which a rotary cover is provided on the retaining ring and a stationary cover is provided so as to surround the rotary cover; 
         FIG. 10  is a view showing another embodiment of a local load exerting mechanism; 
         FIG. 11  is a view showing still another embodiment of the local load exerting mechanism; 
         FIG. 12  is a view showing still another embodiment of the local load exerting mechanism; 
         FIG. 13  is a view showing another embodiment of the top ring; 
         FIG. 14  is a diagram showing a positional relationship as viewed from above a polishing surface; 
         FIG. 15  is a view showing an example in which a moving mechanism for moving the local load exerting mechanism is provided; 
         FIG. 16  is a view showing an example in which a retaining ring height sensor is provided; 
         FIG. 17  is a flowchart for illustrating a method of determining a pressing position of the local load exerting mechanism; 
         FIG. 18  is a diagram showing another example of a method of determining the pressing position of the local load exerting mechanism; 
         FIG. 19  is a diagram showing still another example of a method of determining the pressing position of the local load exerting mechanism; 
         FIG. 20  is a view showing still another embodiment of the local load exerting mechanism; 
         FIG. 21  is a plan view showing the coupling member which couples the retaining ring to the spherical bearing; 
         FIG. 22  is a perspective view of the top ring, a pressure ring, and the local load exerting mechanism; 
         FIG. 23A  is a view showing a load cell for measuring a local load applied from the local load exerting mechanism to the retaining ring; 
         FIG. 23B  is a view showing the pressure ring taken along line B-B in  FIG. 23A ; 
         FIG. 24  is a plan view showing a bridge; 
         FIG. 25  is a plan view showing a positional relationship between air cylinders and a local load point; 
         FIG. 26A  is an enlarged view showing a junction between a first suction line and the pressure ring; 
         FIG. 26B  is an enlarged view showing a junction between a second suction line and the pressure ring; 
         FIG. 27  is an enlarged view showing a magnetic member and the pressure ring; 
         FIG. 28  is a plan view of a mount ring; 
         FIG. 29  is a cross-sectional view of local load exerting mechanisms according to another embodiment; 
         FIG. 30A  shows an example in which two air cylinders exert local loads on the pressure ring; 
         FIG. 30B  shows an example in which three air cylinders exert local loads on the pressure ring; 
         FIG. 30C  shows an example in which four air cylinders exert local loads on the pressure ring; 
         FIG. 31  is a cross-sectional view of the top ring according to another embodiment; 
         FIG. 32A  is a plan view showing a pressing member and a position retaining mechanism; 
         FIG. 32B  is a side view of the pressing member; 
         FIG. 33  is a cross-sectional view showing another embodiment of a shaft portion supported by the spherical bearing; 
         FIG. 34  is a cross-sectional view of the top ring according to still another embodiment; and 
         FIG. 35  is a cross-sectional view showing a part of the top ring according to still another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described in detail below with reference to the drawings. Identical or corresponding parts are denoted by identical reference numerals throughout the views and their repetitive explanations will be omitted. 
       FIG. 1  is a schematic view of a polishing apparatus according to an embodiment. As shown in  FIG. 1 , the polishing apparatus includes a top ring (a substrate holder)  1  for holding and rotating a wafer (i.e., a substrate) W, a polishing table  3  for supporting a polishing pad  2  thereon, a polishing liquid supply mechanism  5  for supplying a polishing liquid (slurry) onto the polishing pad  2 , and a film thickness sensor  7  for obtaining a film thickness signal that varies according to a film thickness of the wafer W. The film thickness sensor  7  is disposed in the polishing table  3  and obtains the film thickness signal in a plurality of regions, including a central region, of the wafer W every time the polishing table  3  makes one revolution. Examples of the film thickness sensor  7  include an optical sensor and an eddy current sensor. 
     The top ring  1  is configured to hold the wafer W on its lower surface by vacuum suction. The top ring  1  and the polishing table  3  rotate in the same direction as indicated by arrows in  FIG. 1 . In this state, the top ring  1  presses the wafer W against a polishing surface  2   a  of the polishing pad  2 . The polishing liquid is supplied from the polishing liquid supply mechanism  5  onto the polishing pad  2 , so that the wafer W is polished by sliding contact with the polishing pad  2  in the presence of the polishing liquid. During polishing of the wafer W, the film thickness sensor  7  rotates together with the polishing table  3  and obtains the film thickness signal while passing across a surface of the wafer W as shown by a symbol A. This film thickness signal is an index value representing the film thickness directly or indirectly, and varies in accordance with a decrease in the film thickness of the wafer W. The film thickness sensor  7  is coupled to a polishing controller  9  so that the film thickness signal is transmitted to the polishing controller  9 . This polishing controller  9  is configured to terminate polishing of the wafer W when the film thickness of the wafer W, which is indicated by the film thickness signal, has reached a predetermined target value. 
       FIG. 2  is a view showing a detailed structure of the polishing apparatus. The polishing table  3  is coupled to a motor  13  through a table shaft  3   a  and is rotated about the table shaft  3   a  by the motor  13  which is disposed below the polishing table  3 . The polishing pad  2  is attached to an upper surface of the polishing table  3 . An upper surface of the polishing pad  2  provides the polishing surface  2   a  for polishing the wafer W. When the polishing table  3  is rotated by the motor  13 , the polishing surface  2   a  moves relative to the top ring  1 . Therefore, the motor  13  serves as a polishing surface moving mechanism for moving the polishing surface  2   a  horizontally. 
     The top ring  1  is coupled to a top ring shaft  11 , which is movable vertically relative to a top ring head  16  by a vertically moving mechanism  27 . A vertical movement and positioning of the top ring  1  in its entirety relative to the top ring head  16  are achieved by the vertical movement of the top ring shaft  11 . A rotary joint  25  is mounted to an upper end of the top ring shaft  11 . 
     The vertically moving mechanism  27  for elevating and lowering the top ring shaft  11  and the top ring  1  includes a bridge  28  for rotatably supporting the top ring shaft  11  through a bearing  26 , a ball screw  32  mounted to the bridge  28 , a support base  29  supported by pillars  30 , and a servomotor  38  mounted to the support base  29 . The support base  29  for supporting the servomotor  38  is secured to the top ring head  16  through the pillars  30 . 
     The ball screw  32  has a screw shaft  32   a  coupled to the servomotor  38  and a nut  32   h  which is in engagement with the screw shaft  32   a . The top ring shaft  11  is configured to move vertically together with the bridge  28 . Therefore, when the servomotor  38  is set in motion, the bridge  28  moves vertically through the ball screw  32  to cause the top ring shaft  11  and the top ring  1  to move vertically. A top ring height sensor  39  is mounted to the top ring head  16  so as to face the bridge  28 . This top ring height sensor  39  is configured to measure a height of the top ring  1  based on a position of the bridge  28  which is vertically movable in unison with the top ring  1 . 
     The top ring shaft  11  is further coupled to a rotary cylinder  12  through a key (not shown). This rotary cylinder  12  has a timing pulley  14  on its outer circumferential surface. A top ring motor  18  is secured to the top ring head  16 , and a timing pulley  20  is mounted to the top ring motor  18 . The timing pulley  14  is coupled to the timing pulley  20  through a timing belt  19 . With these configurations, rotation of the top ring motor  18  is transmitted to the rotary cylinder  12  and the top ring shaft  11  through the timing pulley  20 , the timing belt  19 , and the timing pulley  14  to rotate the rotary cylinder  12  and the top ring shaft  11  in unison, thus rotating the top ring  1  about its own axis. The top ring motor  18 , the timing pulley  20 , the timing belt  19 , and the timing pulley  14  constitute a rotating mechanism for rotating the top ring  1  about its own axis. The top ring head  16  is supported by a top ring head shaft  21  which is rotatably supported by a frame (not shown). 
     The top ring  1  is configured to hold a substrate, such as the wafer W, on its lower surface. The top ring head  16  is configured to be able to pivot on the top ring shaft  21 , so that the top ring  1 , holding the wafer W on its lower surface, is moved from a wafer transfer position to a position above the polishing table  3  by the pivotal movement of the top ring head  16 . The top ring  1  is then lowered and presses the wafer W against the polishing surface  2   a  of the polishing pad  2 , while the top ring  1  and the polishing table  3  are rotated and the polishing liquid is supplied onto the polishing pad  2  from the polishing liquid supply mechanism  5  disposed above the polishing table  3 . The wafer W is placed in sliding contact with the polishing surface  2   a  of the polishing pad  2 , whereby the surface of the wafer W is polished. 
     The top ring  1 , which serves as the substrate holder, will be described in detail below.  FIG. 3  is a cross-sectional view of the top ring  1 . As shown in  FIG. 3 , the top ring  1  includes a top ring body  10  for pressing the wafer W against the polishing surface  2   a , and a retaining ring  40  arranged so as to surround the wafer W. The top ring body  10  and the retaining ring  40  are rotatable in unison by the rotation of the top ring shaft  11 . The retaining ring  40  is configured to be vertically movable independently of the top ring body  10 . 
     The top ring body  10  has a circular flange  41 , a spacer  42  mounted to a lower surface of the flange  41 , and a carrier  43  mounted to a lower surface of the spacer  42 . The flange  41  is coupled to the top ring shaft  11 . The carrier  43  is coupled to the flange  41  through the spacer  42 , so that the flange  41 , the spacer  42 , and the carrier  43  rotate and vertically move in unison. The top ring body  10 , which is constructed by the flange  41 , the spacer  42 , and the carrier  43 , is made of resin, such as engineering plastic (e.g., PEEK). The flange  41  may be made of metal, such as SUS, aluminum, or the like. 
     A flexible membrane  45 , which is brought into contact with a back surface of the wafer W, is attached to a lower surface of the carrier  43 . This flexible membrane  45  has a lower surface which serves as a substrate holding surface  45   a . The flexible membrane  45  further has annular partition walls  45   b  which define four pressure chambers: a central chamber  50 ; a ripple chamber  51 ; an outer chamber  52 ; and an edge chamber  53 , which are located between the flexible membrane  45  and the top ring body  10 . These pressure chambers  50  to  53  are in fluid communication with a pressure regulator  65  via the rotary joint  25 , so that pressurized fluid is supplied into these pressure chambers  50  to  53  from the pressure regulator  65 . This pressure regulator  65  is configured to be able to regulate pressures in the respective four pressure chambers  50  to  53  independently. Further, the pressure regulator  65  is configured to be able to produce negative pressure in the pressure chambers  50  to  53 . The flexible membrane  45  has a through-hole (not shown) in a position corresponding to the ripple chamber  51  or the outer chamber  52 , so that the top ring  1  can hold the substrate on its substrate holding surface  45   a  by producing the negative pressure in the through-hole. The flexible membrane  45  is made of a highly strong and durable rubber material, such as ethylene propylene rubber (EPDM), polyurethane rubber, silicone rubber, or the like. The central chamber  50 , the ripple chamber  51 , the outer chamber  52 , and the edge chamber  53  are further coupled to a pressure relief mechanism (not shown), which can establish a fluid communication between the atmosphere and these four pressure chambers  50  to  53 . 
     The retaining ring  40  is disposed so as to surround the carrier  43  and the flexible membrane  45 . This retaining ring  40  has a ring member  40   a  that contacts the polishing surface  2   a  of the polishing pad  2 , and a drive ring  40   b  fixed to an upper portion of the ring member  40   a . The ring member  40   a  is secured to the drive ring  40   b  by a plurality of bolts (not shown). The ring member  40   a  is arranged so as to surround a peripheral edge of the wafer W and retains the wafer W therein so as to prevent the wafer W from being separated from the top ring  1  when the wafer W is being polished. 
     The retaining ring  40  has an upper portion coupled to an annular retaining ring pressing mechanism  60 , which is configured to exert a uniform downward load on an upper surface of the retaining ring  40  (more specifically, an upper surface of the drive ring  40   b ) in its entirety to thereby press a lower surface of the retaining ring  40  (i.e., a lower surface of the ring member  40   a ) against the polishing surface  2   a  of the polishing pad  2 . 
     The retaining ring pressing mechanism  60  includes an annular piston  61  fixed to an upper portion of the drive ring  40 , and an annular rolling diaphragm  62  connected to an upper surface of the piston  61 . The rolling diaphragm  62  defines a retaining ring pressure chamber  63  therein. This retaining ring pressure chamber  63  is in fluid communication with the pressure regulator  65  through the rotary joint  25 . When the pressure regulator  65  supplies a pressurized fluid (e.g., pressurized air) into the retaining ring pressure chamber  63 , the rolling diaphragm  62  pushes down the piston  61 , which in turn pushes down the retaining ring  40  in its entirety. In this manner, the retaining ring pressing mechanism  60  presses the lower surface of the retaining ring  40  against the polishing surface  2   a  of the polishing pad  2 . Further, when the pressure regulator  65  develops the negative pressure in the retaining ring pressure chamber  63 , the retaining ring  40  in its entirety is elevated. The retaining ring pressure chamber  63  is further coupled to a pressure relief mechanism (not shown), which can establish a fluid communication between the atmosphere and the retaining ring pressure chamber  63 . 
     The retaining ring  40  is removably coupled to the retaining ring pressing mechanism  60 . More specifically the piston  61  is made of a magnetic material, such as metal., and a plurality of magnets  70  are disposed on the upper portion of the drive ring  40   b . These magnets  70  magnetically attract the piston  61 , so that the retaining ring  40  is secured to the piston  61  via a magnetic force. The magnetic material of the piston  61  may be corrosion resisting magnetic stainless steel. The drive ring  40   b  may be made of a magnetic material, and magnets may be disposed on the piston  61 . 
     The retaining ring  40  is coupled to a spherical bearing  85  through a coupling member  75 . The spherical bearing  85  is disposed radially inwardly of the retaining ring  40 .  FIG. 4  is a plan view showing the retaining ring  40  and the coupling member  75 . As shown in  FIG. 4 , the coupling member  75  includes a vertically extending shaft portion  76  disposed centrally in the top ring body  10 , a hub  77  secured to the shaft portion  76 , and a plurality of spokes  78  extending radially from the hub  77 . The spokes  78  have one ends fixed to the shaft portion  76  and have the other ends fixed to the drive ring  40   b  of the retaining ring  40 . In this embodiment, the hub  77 , the spokes  78 , and the drive ring  40   b  are formed integrally. Plural pairs of drive pins  80  and  80  are secured to the carrier  43 . The drive pins  80  and  80  of each pair are arranged on both sides of each spoke  78 . The rotation of the carrier  43  is transmitted to the retaining ring  40  through the drive pins  80  and  80  to thereby rotate the top ring body  10  and the retaining ring  40  in unison. 
     As shown in  FIG. 3 , the shaft portion  76  extends vertically in the spherical bearing  85 . As shown in  FIG. 4 , the carrier  43  has a plurality of radial grooves  43   a  in which the spokes  78  are disposed, respectively. Each spoke  78  is movable freely in the vertical direction in each groove  43   a . The shaft portion  76  of the coupling member  75  is supported by the spherical bearing  85  such that the shaft portion  76  can move in the vertical direction. The spherical bearing  85  is located at the center of the top ring body  10 . The coupling member  75  and the retaining ring  40  that is secured to the coupling member  75  are thus vertically movable relative to the top ring body  10 . Further, the retaining ring  40  is tiltably supported by the spherical hearing  85 . 
     The spherical bearing  85  will now be described in more detail.  FIG. 5  is an enlarged cross-sectional view of the spherical bearing  85  and a part of the coupling member  75 . As shown in  FIG. 5 , the shaft portion  76  is secured to the hub  77  by a plurality of screws  79 . The shaft portion  76  has a vertically extending through-hole  88  formed therein. This through-hole  88  acts as an air vent hole when the shaft portion  76  moves vertically relative to the spherical bearing  85 . Therefore, the retaining ring  40  can move smoothly in the vertical direction relative to the top ring body  10 . 
     The spherical bearing  85  includes an intermediate bearing ring  91  coupled to the retaining ring  40  through the coupling member  75 , an outer bearing ring  92  slidably supporting the intermediate bearing ring  91  from above, and an inner bearing ring  93  slidably supporting the intermediate bearing ring  91  from below. The intermediate bearing ring  91  is in the form of a partial spherical shell smaller than an upper half of a spherical shell. The intermediate bearing ring  91  is sandwiched between the outer bearing ring  92  and the inner bearing ring  93 . 
     The carrier  43  has a recess  43   b  formed at the central portion thereof, and the outer bearing ring  92  is disposed in this recess  43   b . The outer bearing ring  92  has a flange portion  92   a  on its outer circumferential surface. The flange portion  92   a  is secured to a step of the recess  43   b  by bolts (not shown), thereby securing the outer bearing ring  92  to the carrier  43  and applying pressure to the intermediate bearing ring  91  and the inner bearing ring  93 . The inner bearing ring  93  is disposed on a bottom surface of the recess  43   b . This inner bearing ring  93  supports the intermediate bearing ring  91  from below so as to form a gap between a lower surface of the intermediate bearing ring  91  and the bottom surface of the recess  43   b.    
     The outer bearing ring  92  has an inner surface  92   b , the intermediate bearing ring  91  has an outer surface  91   a  and an inner surface  91   b , and the inner bearing ring  93  has an outer surface  93   a . Each of these surfaces  92   b ,  91   a ,  91   b , and  93   a  is a substantially hemispheric surface whose center is represented by a fulcrum O. The outer surface  91   a  of the intermediate bearing ring  91  slidably contacts the inner surface  92   h  of the outer bearing ring  92 . The inner surface  91   b  of the intermediate bearing ring  91  slidably contacts the outer surface  93   a  of the inner bearing ring  93 . The inner surface  92   b  (sliding contact surface) of the outer bearing ring  92 , the outer surface  91   a  and the inner surface  91   b  (sliding contact surfaces) of the intermediate bearing ring  91 , and the outer surface  93   a  (sliding contact surface) of the inner bearing ring  93  have a partial spherical shape smaller than an upper half of a spherical surface. With these configurations, the intermediate bearing ring  91  is tiltable in all directions through 360° with respect to the outer bearing ring  92  and the inner bearing ring  93 . The fulcrum O, which is the center of the tilting movement of the intermediate bearing ring  91 , is located below the spherical bearing  85 . 
     The outer bearing ring  92 , the intermediate bearing ring  91 , and the inner bearing ring  93  have respective through-holes  92   c ,  91   c , and  93   b  formed therein in which the shaft portion  76  is inserted. There is a gap between the through-hole  92   c  of the outer bearing ring  92  and the shaft portion  76 . Similarly, there is a gap between the through-hole  93   b  of the inner bearing ring  93  and the shaft portion  76 . The through-hole  91   c  of the intermediate bearing ring  91  has a diameter smaller than those of the through-holes  92   c  and  93   b  of the outer bearing ring  92  and the inner bearing ring  93  such that the shaft portion  76  is movable relative to the intermediate bearing ring  91  only in the vertical direction. Therefore, the retaining ring  40 , which is coupled to the shaft portion  76 , is substantially not allowed to move laterally, i.e., horizontally. That is, the retaining ring  40  is fixed in its lateral position (i.e., its horizontal position) by the spherical bearing  85 . 
       FIG. 6A  shows the manner in which the coupling member  75  is vertically moved relative to the spherical bearing  85 , and  FIGS. 6B and 6C  show the manner in which the coupling member  75  tilts in unison with the intermediate bearing ring  91 . As shown in  FIGS. 6A through 6C , the retaining ring  40 , which is coupled to the coupling member  75 , is tiltable around the fulcrum O in unison with the intermediate bearing ring  91  and is vertically movable relative to the intermediate bearing ring  91 . The fulcrum O, which is the center of the tilting movement, lies on a central axis of the retaining ring  40 . 
     The spherical bearing  85  allows the retaining ring  40  to move vertically and tilt, while restricting the lateral movement (i.e., the horizontal movement) of the retaining ring  40 . During polishing of the wafer, the retaining ring  40  receives a lateral force from the wafer (i.e., a force in a radially outward direction of the wafer). This lateral force is generated due to friction between the wafer and the polishing pad  2 . The lateral force is received by the spherical bearing  85 . Therefore, the spherical bearing  85  serves as a supporting mechanism capable of supporting the lateral force (i.e., the force in the radially outward direction of the wafer) applied to the retaining ring  40  from the wafer due to the friction between the wafer and the polishing pad  2  and capable of restricting the lateral movement of the retaining ring  40  (i.e., capable of fixing the horizontal position of the retaining ring  40 ). 
     Since the spherical bearing  85  is arranged in the top ring body  10  and housed in the recess  43   h  of the carrier  43 , wear debris produced from the sliding contact surfaces of the spherical bearing  85  is confined in the top ring body  10  and does not fall onto the polishing surface  2   a.    
       FIG. 7  shows an enlarged cross-sectional view of another example of the spherical bearing. Components shown in  FIG. 7  identical to those shown in  FIG. 5  are denoted by the same reference numerals. A spherical bearing  100  shown in  FIG. 7  includes an annular inner bearing ring  101 , and an annular outer bearing ring  102  which slidably supports an outer circumferential surface of the inner bearing ring  101 . The inner bearing ring  101  is coupled to the retaining ring  40  through the coupling member  75 . The outer bearing ring  102  is secured to a support member  103 , which is secured to the carrier  43 . The support member  103  is disposed in the recess  43   b  which is formed at the central portion of the carrier  43 . 
     The outer circumferential surface of the inner bearing ring  101  has a spherical shape whose upper and lower portions are cut off. A central point (fulcrum) O′ of this spherical shape is located at the center of the inner bearing ring  101 . The outer bearing ring  102  has an inner circumferential surface which is a concave surface shaped so as to fit the outer circumferential surface of the inner bearing ring  101 , so that the outer bearing ring  102  slidably supports the inner bearing ring  101 . The inner bearing ring  101  is tiltable in all directions through 360° with respect to the outer bearing ring  102 . 
     The inner bearing ring  101  has an inner circumferential surface which forms a through-hole  101   a  in which the shaft portion  76  is inserted. The shaft portion  76  is movable relative to the inner bearing ring  101  only in the vertical direction. Therefore, the retaining ring  40 , which is coupled to the shaft portion  76 , is substantially not allowed to move laterally, i.e., horizontally. That is, the retaining ring  40  is fixed in its lateral position (i.e., its horizontal position) by the spherical bearing  100 . As well as the spherical bearing  85 , the spherical bearing  100  serves as a supporting mechanism capable of supporting the lateral force (i.e., the force in the radially outward direction of the wafer) applied to the retaining ring  40  from the wafer due to the friction between the wafer and the polishing pad  2  and capable of restricting the lateral movement of the retaining ring  40  (i.e., capable of fixing the horizontal position of the retaining ring  40 ). 
       FIG. 8A  shows the manner in which the coupling member  75  is vertically moved relative to the spherical bearing  100 .  FIGS. 8B and 8C  show the manner in which the coupling member  75  tilts in unison with the inner bearing ring  101 . The coupling member  75  and the retaining ring  40  (not shown in  FIGS. 8A through 8C ) coupled thereto are tiltable around the fulcrum O′ in unison with the inner bearing ring  101  and are vertically movable relative to the inner bearing ring  101 . 
     While the spherical bearing  100  shown in  FIG. 7  has the same functions as those of the spherical bearing  85  shown in  FIG. 5 , the fulcrum O′ as the center of the tilting movement of the spherical bearing  100  is higher in position than the fulcrum O of the spherical bearing  85 . More specifically, the fulcrum O′ is located within the spherical bearing  100 . The spherical bearing  100  thus constructed still allows the retaining ring  40  to tilt smoothly and positively when the frictional force produced between the wafer and the polishing pad  2  is applied to the retaining ring  40 . Since the spherical bearing  100  has the higher fulcrum than that of the spherical bearing  85 , the frictional force between the wafer and the polishing pad  2  generates a larger moment that causes the retaining ring  40  to tilt around the fulcrum. Therefore, the retaining ring  40  can tilt more greatly during polishing of the wafer. Use of the spherical bearing  100  may offer a wider controllable range of the tilting movement of the retaining ring  40 , and may enable the top ring  1  to control a variety of polishing profiles. 
     As shown in  FIG. 3 , the retaining ring  40  has an upper surface extending radially outwardly from the top ring body  10 . A local load exerting mechanism  110 , which serves to exert a local load on a part of the retaining ring  40 , is disposed above the retaining ring  40 .  FIG. 3  shows an embodiment of the local load exerting mechanism  110 . The local load exerting mechanism  110  is fixed to the top ring head  16 . While the retaining ring  40  is configured to rotate about its own axis during the polishing process, the local load exerting mechanism  110  does not rotate together with the retaining ring  40  and is fixed in position. 
     A load transmitting member  111  is fixed to a peripheral upper surface of the retaining ring  40 . A guide ring  112  is secured to an upper surface of the load transmitting member  111 . The local load exerting mechanism  110  exerts a downward load on a part of the retaining ring  40  through the guide ring  112  and the load transmitting member  111 . The load transmitting member  111  may be of a ring shape or may comprise a plurality of columns arranged along a circumferential direction of the drive ring  40   b . The downward load of the local load exerting mechanism  110  is transmitted from the guide ring  112  to the retaining ring  40  through the load transmitting member  111 . Operations of the local load exerting mechanism  110  are controlled by the polishing controller  9  shown in  FIG. 1 . The local load exerting mechanism  110  may directly exert a downward local load on the retaining ring  40  without providing the load transmitting member  111  and the guide ring  112 . 
     While the top ring  1  rotates about its own axis, the local load exerting mechanism  110  does not rotate with the top ring  1  because the local load exerting mechanism  110  is secured to the top ring head  16 . Specifically, during polishing of the wafer W, the top ring  1  and the wafer W rotate about their own axes, while the local load exerting mechanism  110  is stationary at a predetermined position. In  FIG. 3 , the single local load exerting mechanism  110  is shown. However, a plurality of local load exerting mechanisms  110  may be installed along a circumferential direction of the top ring  1 . These local load exerting mechanisms  110  may be radially spaced from the central axis of the top ring  1  by the same distance or different distances. Installation positions of the local load exerting mechanisms  110  may be variable along the circumferential direction of the top ring  1 . 
     In the embodiment shown in  FIG. 3 , the local load exerting mechanism  110  includes an air cylinder  114  having a piston  114   a , and a wheel  115  coupled to the piston  114   a . The air cylinder  114  is fixed to the top ring head  16 . The wheel  115  is mounted to a distal end of the piston  114   a . When the wheel  115  is lowered by the air cylinder  114 , the wheel  115  exerts the load on the guide ring  112 . 
     The wheel  115  is configured to be vertically moved by the air cylinder  114 . When the top ring  1  is lowered to bring the retaining ring  40  into contact with the polishing surface  2   a , the air cylinder  114  lowers the wheel  115  to exert the local downward local force to a part of the retaining ring  40 . Before the top ring  1  moves upward, the air cylinder  114  elevates the wheel  115  away from the guide ring  112 . It is possible to change the load of the wheel  115  on the retaining ring  40  by changing pressure of a gas supplied to the air cylinder  114 . The wheel  115  is rotatably supported by a non-rotatable wheel shaft  116  disposed centrally so that the wheel  115  can rotate about the wheel shaft  116 . The wheel  115  is made of a material having a low coefficient of friction. A bearing, such as a ball bearing, may be interposed between the wheel shaft  116  and the wheel  115 . 
     When the wafer W is being polished, the guide ring  112  rotates about the rotational axis of the top ring  1 , but the local load exerting mechanism  110  does not rotate. Therefore, the guide ring  112  moves horizontally relative to the local load exerting mechanism  110 . When the wheel  115  is pressed against the guide ring  112 , the retaining ring  40  exerts a downward local load (force) to the polishing surface  2   a  in a direction perpendicular to the polishing surface  2   a . Since the wheel  115  is made of a material having a low coefficient of friction, the frictional force that acts between the wheel  115  and the guide ring  112  can be minimized. As a result, an effect on a change in an attitude of the retaining ring  40  due to the frictional force can be minimized. The material having a low coefficient of friction may be a resin material, such as nylon, PTFE (polytetrafluoroethylene), PEEK (polyetheretherketone), PPS (polyphenylene sulfide), or the like, or may be a resin material with fiber, such as carbon fiber, and a solid lubricant added. 
     As shown in  FIG. 9 , it is preferable to provide a cylindrical rotary cover  120  on the retaining ring  40  and to provide a cylindrical stationary cover  121  so as to surround the rotary cover  120 . The rotary cover  120  is fixed to an outer circumferential surface of the retaining ring  40 , while the stationary cover  121  is fixed to the top ring head  16 . A seal member  122  is interposed between the rotary cover  120  and the stationary cover  121 , isolating a space above the top ring  1  from a polishing space in which the polishing surface  2   a  exists. The covers  120  and  121  and the seal member  122  can prevent droplets of the polishing liquid or other liquid from adhering to sliding elements, such as the wheel  115 , and can further prevent particles, which may be produced from the wheel  115 , from falling onto the polishing surface  2   a . Instead of the seal member  122 , a labyrinth structure may be used to prevent the particles from falling onto the polishing surface  2   a  and prevent the droplets from entering the polishing space. Such a labyrinth structure may be constructed by the covers  120  and  121 , one of which is radially superposed on the other. 
     The retaining ring  40  is supported by the spherical bearing  85  (or  100 ) arranged on the rotational axis of the top ring  1 . This spherical bearing  85  (or  100 ) allows the retaining ring  40  to tilt smoothly and move in the vertical direction independently of the top ring body  10 . During polishing of the wafer W, the retaining ring  40  on the polishing surface  2   a  tilts around the fulcrum as a result of receiving the frictional force of the wafer W. More specifically, the retaining ring  40  on the polishing pad  2  tilts such that it sinks into the polishing pad  2  at an upstream side of the wafer W (i.e., at an incoming polishing-surface side) while it rises from the polishing pad  2  at a downstream side of the wafer W (i.e., at an outgoing polishing-surface side). The local load exerting mechanism  110  exerts the local load on the tilted retaining ring  40  to make it possible to control the tilt of the retaining ring  40 . For example, when the local load is exerted on a part of the retaining ring  40  downstream of the wafer W, the tilt of the retaining ring  40  can be reduced while the wafer W is being polished. 
     By controlling the tilt of the retaining ring  40  in this manner, it is possible to change the polishing profile of the wafer edge portion as a result of changes in a surface pressure distribution of the retaining ring  40 , a deformed state of the polishing pad  2 , a deformed state of the wafer W that is pressed against the retaining ring  40  by the frictional force, and a deformed state of the retaining ring  40  subjected to the frictional force. The edge portion of the wafer W is an annular region having a width ranging from 3 mm to 10 mm at the outermost circumferential edge of the wafer W. 
     Controlling the tilt of the retaining ring  40  can further change a distribution of the polishing liquid that is supplied to the lower surface of the wafer W through a gap between the retaining ring  40  and the polishing surface  2   a . As a result, the polishing rate and the polishing profile of the surface of the wafer W in its entirety can be controlled. Only the local load exerting mechanism  110  may be used to press the retaining ring  40 . Alternatively, the retaining ring pressing mechanism  60  may be used to press the retaining ring  40  uniformly, while the local load exerting mechanism  110  is additionally exerting the local load on the retaining ring  40 . 
     An example of the polishing profile control using the local load exerting mechanism  110  will be described below. In this example, the wafer W is polished under a first polishing condition and a second polishing condition described below. The first polishing condition is that the retaining ring  40  is pressed against the polishing pad  2  at a surface pressure P 1  only by the retaining ring pressing mechanism  60  without using the local load exerting mechanism  110 . The second polishing condition is that the retaining ring  40  is pressed against the polishing pad  2  at the surface pressure P 1  by the retaining ring pressing mechanism  60  and further pressed against the polishing pad  2  at a surface pressure P 2  by the local load exerting mechanism  110 . 
     Under the second polishing condition, since the local load is applied to the retaining ring  40 , the attitude of the retaining ring  40  is changed and particularly the polishing profile of the edge portion of the wafer W is changed, as compared with the case of polishing the wafer W under the first polishing condition. Under the second polishing condition, the total surface pressure applied from the retaining ring  40  to the polishing surface  2   a  is P 1 +P 2 , which is greater than the surface pressure P 1  applied under the first polishing condition. If the surface pressure applied by the retaining ring pressing mechanism  60  under the second polishing condition is changed from P 1  to P 1 −P 2 , then the total surface pressure applied from the retaining ring  40  to the polishing surface  2   a  is P 1 , which is the same as the surface pressure P 1  applied from the retaining ring  40  under the first polishing condition. In such a case, while the same surface pressure of the retaining ring  40  as the surface pressure applied under the first polishing condition is maintained, the polishing profile can be obtained under the different condition in terms of the attitude of the retaining ring  40 . For controlling the polishing profile of the edge portion of the wafer W, the surface pressure applied by the retaining ring pressing mechanism  60  under the second polishing condition is preferably in the range of P 1 −2.5×P 2  to P 1 −0.5×P 2 , and more preferably in the range of P 1 −1.8×P 2  to P 1 −1.2×P 2 . 
     The retaining ring  40  is tiltable and vertically movable relative to the substrate holding surface  45   a  and the wafer W held thereon, and is capable of pressing the polishing pad  2  independently of the wafer W. Therefore, when the local load exerting mechanism  110  presses a part of the retaining ring  40  downwardly, the wafer W does not tilt and the pressure applied from the substrate holding surface  45   a  to the wafer W remains unchanged. Therefore, the local load exerting mechanism  110  can control the attitude of the retaining ring  40  independently of the attitude of the wafer W and the pressure acting on the wafer W. In other words, it is possible to control the polishing profile by controlling the attitude of the retaining ring  40  while keeping the pressure applied from the substrate holding surface  45   a  to the wafer W constant. 
     The retaining ring  40  is pressed against the polishing pad  2  while rotating in unison with the wafer W held by the top ring  1 . Since the retaining ring  40  and the wafer W are rotating during the polishing process, the region of the polishing surface  2   a  contacting the retaining ring  40  and the wafer W changes gradually as the polishing table  3  rotates. Therefore, a particular region of the polishing pad  2  is prevented from remaining in contact only with a particular area of the wafer W. As a result, the surface of the wafer W can be uniformly polished. For the same reason, the retaining ring  40  is prevented from being locally worn away. 
     An inside diameter of the retaining ring  40  is larger than a diameter of the wafer W. Therefore, as the top ring  1  rotates, a relative position between the retaining ring  40  and the wafer W in the circumferential direction changes gradually in a sun-and-planet motion. As a result, the effect of the flatness of the retaining ring  40  can be averaged so that the flatness of the retaining ring  40  does not affect any certain circumferential position of the wafer W. As a consequence, the surface of the wafer W can be polished uniformly, especially over the circumferential direction thereof. 
       FIG. 10  shows the local load exerting mechanism  110  according to another embodiment. Structures that are not described particularly in this embodiment are identical to those of the previously-discussed embodiment shown in  FIG. 3  and their repetitive descriptions are omitted. In this embodiment, the local load exerting mechanism  110  exerts an upward local load (force) on a part of the retaining ring  40  in a direction perpendicular to the polishing surface  2   a . As shown in  FIG. 10 , guide ring  112  supported by load transmitting member  111  has an annular ledge  112   a  projecting inwardly in the radial direction of the top ring  1 . The local load exerting mechanism  110  has wheel  115  that is held in rolling contact with a lower surface of the annular ledge  112   a  and air cylinder  114  for elevating the wheel  115 . 
     The wheel  115  is rotatably supported by wheel shaft  116 , which is mounted to piston  114   a  of the air cylinder  114 . The wheel  115  is coupled to the air cylinder  114  via the wheel shaft  116 . The air cylinder  114  elevates the wheel  115  to push the annular ledge  112   a  upwardly, exerting an upward local load on a part of the retaining ring  40  perpendicularly to the polishing surface  2   a . The local load exerting mechanism  110  according to the embodiment shown in  FIG. 10  offers the same effects as the local load exerting mechanism  110  according to the embodiment shown in  FIG. 3 . 
     An example of the polishing profile control using the local load exerting mechanism  110  shown in  FIG. 10  will be described below. In this example, the wafer W is polished under a first polishing condition and a second polishing condition described below. The first polishing condition is that the retaining ring  40  is pressed against the polishing pad  2  at a surface pressure P 1  only by the retaining ring pressing mechanism  60 , without using the local load exerting mechanism  110 . The second polishing condition is that the retaining ring  40  is pressed against the polishing pad  2  at the surface pressure P 1  by the retaining ring pressing mechanism  60  and further an upward local load is exerted on the retaining ring  40  by the local load exerting mechanism  110  such that the surface pressure applied to the polishing pad  2  is reduced by a surface pressure P 2 . 
     Under the second polishing condition, since the local load is applied to the retaining ring  40 , the attitude of the retaining ring  40  is changed and particularly the polishing profile of the edge portion of the wafer W is changed, as compared with the case of polishing the wafer W under the first polishing condition. Under the second polishing condition, the total surface pressure applied from the retaining ring  40  to the polishing surface  2   a  is P 1 −P 2 , which is smaller than the surface pressure P 1  applied under the first polishing condition. If the surface pressure applied by the retaining ring pressing mechanism  60  under the second polishing condition is changed from P 1  to P 1 +P 2 , then the total surface pressure applied from the retaining ring  40  to the polishing surface  2   a  is P 1 , which is the same as the surface pressure P 1  applied from the retaining ring  40  under the first polishing condition. In such a case, while the same surface pressure of the retaining ring  40  as the surface pressure applied under the first polishing condition is maintained, the polishing profile can be obtained under the different condition in terms of the attitude of the retaining ring  40 . For controlling the polishing profile of the edge portion of the wafer W, the surface pressure applied by the retaining ring pressing mechanism  60  under the second polishing condition is preferably in the range of P 1 +0.5×P 2  to P 1 +2.5×P 2 , and more preferably in the range of P 1 +1.2×P 2  to P 1 +1.8×P 2 . 
       FIG. 11  shows the local load exerting mechanism  110  according to still another embodiment. Top ring  1  shown in  FIG. 11  is identical in structure to the top ring  1  shown in  FIG. 3 , and its repetitive descriptions are omitted. In this embodiment, the local load exerting mechanism  110  includes a permanent magnet  130  instead of the guide ring  112  shown in  FIG. 3 , and an electromagnet  131  instead of the wheel  115  shown in  FIG. 3 . The permanent magnet  130  is supported by load transmitting member  111  fixed to the retaining ring  40 . The electromagnet  131  is arranged so as to face the permanent magnet  130  arranged above the retaining ring  40 . The local load exerting mechanism  110  is capable of exerting a vertical local load on a part of the retaining ring  40  via a magnetic force acting between the electromagnet  131  and the permanent magnet  130 . 
     When the electromagnet  131  generates a magnetic force with the same magnetic pole as the permanent magnet  130 , the local load exerting mechanism  110  can exert a downward local load (repulsive force) on the retaining ring  40 . When the electromagnet  131  generates a magnetic force with a magnetic pole opposite to the permanent magnet  130 , the local load exerting mechanism  110  can exert an upward local load (attractive force) on the retaining ring  40 . In this manner, the electromagnet  131  can selectively exert a downward load or an upward load on a part of the retaining ring  40 . Controlling the load on the retaining ring  40  makes it possible to change the attitude of the retaining ring  40 . As a result, the polishing profile of the wafer W, especially the polishing profile of the wafer edge portion, can be controlled. While the permanent magnet  130  is arranged above the retaining ring  40  in the present embodiment, the permanent magnet  130  may be replaced with a magnetic member, and the electromagnet  131  may attract the magnetic member to exert an upward local load (force) on the retaining ring  40 . 
     The electromagnet  131  is supported by an elevating mechanism  133  which is coupled to the top ring head  16 . The elevating mechanism  133  may be constituted by an air cylinder or a mechanism including a ball screw and a servomotor. If the lowered position of the electromagnet  131  is to be controlled precisely as described below, it is preferable to use the mechanism including the ball screw and the servomotor as the elevating mechanism  133 . 
     With the retaining ring  40  in contact with the polishing surface  2   a , the elevating mechanism  133  lowers the electromagnet  131  to a position near the permanent magnet  130 , and then the electromagnet  131  is energized to exert a local force on a part of the retaining ring  40 . The force applied to the retaining ring  40  can be changed by varying the magnetic force generated by the electromagnet  131  and/or a distance between the permanent magnet  130  and the electromagnet  131 . If a relationship between the distance of the permanent magnet  130  from the electromagnet  131 , electric current supplied to the electromagnet  131 , and magnetic force corresponding to the supplied electric current and the distance is obtained in advance, it is possible to determine the magnetic force generated by the electromagnet  131  from the electric current supplied to the electromagnet  131  and the distance between the permanent magnet  130  and the electromagnet  131 . 
     In the present embodiment, the local load exerting mechanism  110  is configured to exert the local load via the magnetic force. In order to prevent a magnetic field from adversely affecting other parts, magnetic shielding materials may be used to constitute components of the top ring  1 , such as the top ring body  10 , or may be used to cover those components. The adverse effects of the magnetic field include destruction of semiconductor devices of the wafer W caused by a large electric current induced in fine electric interconnects of the semiconductor devices, and a decrease in an accuracy of the eddy current sensor that obtains the film-thickness signal of the metal film from the polishing-surface side. 
     Typically, every time a wafer W is polished, the polishing surface  2   a  of the polishing pad  2  is conditioned by a dresser that uses diamond particles or the like. Therefore, each time a wafer W is polished, the polishing pad  2  is worn and the distance between the electromagnet  131  and the permanent magnet  130  increases gradually. Since the retaining ring  40  is placed in sliding contact with the polishing pad  2  while the wafer W is being polished, a pad contact surface of the retaining ring  40  is also worn gradually, and as a result the distance between the electromagnet  131  and the permanent magnet  130  increases gradually. Because the local load applied to the retaining ring  40  greatly depends on the gap between the electromagnet  131  and the permanent magnet  130 , it is highly important to keep the gap between the electromagnet  131  and the permanent magnet  130  constant regardless of a change in the height of the polishing pad  2  and the retaining ring  40 , both of which are consumables. For example, when the height of the polishing pad  2  is reduced by ΔH, the position of the electromagnet  131  is lowered by ΔH by the elevating mechanism  133 . The change in the gap between the electromagnet  131  and the permanent magnet  130  may be determined by a process of measuring amounts of wear of a polishing pad and a retaining ring as disclosed in Japanese laid-open patent publication No. 2006-128582 and Japanese laid-open patent publication No. 2006-255851. 
     As the polishing pad  2  is worn, it is necessary to change the height of the top ring  1  when polishing the wafer W (which will hereinafter be referred to as “polishing position”). This polishing position of the top ring  1  can be adjusted using the vertically moving mechanism  27 . The lowered position of the electromagnet  131  is adjusted in accordance with the change in the polishing position of the top ring  1 . 
     In the present embodiment, a load cell  135  is disposed between the top ring head  16  and the elevating mechanism  133 . The load cell  135  is capable of measuring a force that varies according to the local load applied to the retaining ring  40 . The force that varies according to the local load is a force determined by subtracting weights of suspended components including the elevating mechanism  133  and the electromagnet  131 , which are suspended from the load cell  135 , from the repulsive force acting between the magnets  130  and  131 . The weights of the suspended elements are measured beforehand. The force measured by the load cell  135  changes at the same rate as a rate of change in the local load applied to the retaining ring  40 . It is possible to determine the load that is actually exerted from the local load exerting mechanism  110  on the retaining ring  40  during polishing of the wafer by adding the weights of the suspended components to the force measured by the load cell  135 . 
     If the measurement result is different from a desired load, the electric current supplied to the electromagnet  131  is controlled so as to adjust the magnetic force. The magnetic force generated by the permanent magnet  130  may vary with time. In order to correct such a change in the magnetic force, a correction process may be performed which includes the steps of energizing the electromagnet  131  while the wafer W is not polished, calculating the load on the retaining ring  40  at that time, and correcting the electric current supplied to the electromagnet  131  such that a difference between a preset initial load and the present load is eliminated. The corrected electric current can be used in polishing of subsequent wafers W. Although no load cell is shown in  FIG. 3 , a load cell may be disposed between the top ring head  16  and the air cylinder  114  shown in  FIG. 3  as well. In this case also, the actual load can be calculated, and the pressing force of the air cylinder  114  can be corrected based on the difference between the calculated load and the initial value of the load. 
     According to the embodiment shown in  FIG. 11 , the local load exerting mechanism  110  does not include sliding parts, such as the guide ring  112  and the wheel  115  shown in  FIG. 3 , and can thus exert a force on the retaining ring  40  without physical contact. Therefore, the local load exerting mechanism  110  according to this embodiment does not need the covers  120  and  121  for covering the top ring  1  as shown in  FIG. 3 . This embodiment with no covers  120  and  121  enables easy maintenance, such as replacement of the consumables of the top ring  1 . The permanent magnet  130  may be a neodymium magnet. The permanent magnet  130  may be an integrally-formed ring-shaped magnet, a ring-shaped assembly of a plurality of separate permanent magnets, or a plurality of separate permanent magnets embedded in a ring-shaped member. The permanent magnet  130  may have its surface protected against corrosion by coating or plating. Instead, the permanent magnet  130  may be covered with a corrosion-resistant magnetic material, such as magnetic stainless steel. 
       FIG. 12  shows the local load exerting mechanism  110  according to yet another embodiment. In this embodiment, a permanent magnet  140  is provided instead of the electromagnet  131  shown in  FIG. 11 . The permanent magnet  140  is disposed in facing relation to the permanent magnet  130  which is supported by the load transmitting member  111  so that a repulsive force is generated between the permanent magnets  130  and  140 . In the present embodiment, an air cylinder is used as the elevating mechanism  133 . The air cylinder  133  lowers the permanent magnet  140  with a force that is smaller than the maximum repulsive force acting between the permanent magnets  130  and  140  when a gap between the permanent magnets  130  and  140  is minimum. This is for the purpose of preventing the upper permanent magnet  140  from contacting the lower permanent magnet  130 . 
     When the gap between the permanent magnets  130  and  140  is large, the upper permanent magnet  140  is lowered, because the downward force of the air cylinder  133  is larger than the repulsive force between the permanent magnets  130  and  140 . When the gap between the permanent magnets  130  and  140  is reduced until the repulsive force between the permanent magnets  130  and  140  counterbalances the downward force of the air cylinder  133 , the downward movement of the permanent magnet  140  is stopped. At this time, a load corresponding to the downward force generated by the air cylinder  133  is exerted on the retaining ring  40 . Consequently, it is possible to regulate the load on the retaining ring  40  by changing the pressure of the gas supplied to the air cylinder  133 . 
     The local load exerting mechanism  110  shown in  FIGS. 11 and 12  uses the magnetic force for its operations. However, the local load exerting mechanism  110  is not limited to the above embodiments. For example, an electromagnet may be mounted to the upper surface of the retaining ring  40 , or a combination of an electromagnet and an air cylinder may be used to constitute the local load exerting mechanism  110 . 
       FIG. 13  shows the top ring  1  according to another embodiment. The local load exerting mechanism  110  shown in  FIG. 13  includes the air cylinder  114  and the wheel  115  shown in  FIG. 3 . Alternatively, the local load exerting mechanism  110  having the magnetic components shown in  FIG. 11 or 12  may be used. Structures of this embodiment which will not be described particularly are identical to those shown in  FIG. 3 . 
     The top ring body  10  includes a top ring base  150  and carrier  43  holding flexible membrane  45 . The flexible membrane  45  has a lower surface serving as substrate holding surface  45   a  for holding a wafer W. A spherical bearing  155  is disposed between the top ring base  150  and a drive flange  158  to allow the top ring base  150  to tilt freely with respect to the drive flange  158 . The spherical bearing  155  includes a hard ball made of ceramic or the like. 
     The retaining ring  40  is fixed to the top ring base  150  and is configured to be tiltable in unison with the top ring base  150 . The drive flange  158  is fixed to the lower end of the top ring shaft  11  and rotates in unison with the top ring shaft  11 . Rotation of the drive flange  158  is transmitted to the top ring base  150  through a plurality of torque transmitting pins (not shown) that are fixed to the top ring base  150 . 
     The carrier  43 , which is separated from the top ring base  150 , is coupled to the top ring base  150  by an elastic membrane  160 . The carrier  43  is movable vertically and tiltable relative to the top ring base  150 . The carrier  43 , the top ring base  150 , and the elastic membrane  160  jointly form a pressure chamber  161 . When the pressure chamber  161  is supplied with a pressurized fluid from the pressure regulator  65  (see  FIG. 3 ), the carrier  43 , the flexible membrane  45 , and the wafer W are lowered. When negative pressure is developed in the pressure chamber  161  by the pressure regulator  65 , the carrier  43 , the flexible membrane  45 , and the wafer W are elevated. 
     The top ring base  150  and the retaining ring  40  that are supported by the spherical bearing  155  are tiltable in all directions (360°) relative to the drive flange  158 . The center of the tilting movement of the spherical bearing  155  lies on the central axis of the retaining ring  40 . The drive flange  158  and the top ring base  150  should desirably be made of a relatively rigid material, such as metal (e.g., stainless steel or aluminum) or ceramic. 
     The downward load and the torque of the top ring shaft  11  are transmitted to the top ring base  150  through the drive flange  158 . Specifically, the downward load of the top ring shaft  11  is transmitted to the top ring base  150  through the drive flange  158  and the spherical bearing  155 , and the torque of the top ring shaft  11  is transmitted to the top ring base  150  through the drive flange  158  and the torque transmitting pins (not shown). In this embodiment, the top ring  1  does not have the retaining ring pressing mechanism  60  shown in  FIG. 3 . Therefore, the retaining ring  40  is tiltable, rotatable, and vertically movable in unison with the top ring base  150 . 
     The local load exerting mechanism  110  is disposed above the retaining ring  40  and the top ring base  150 . The local load exerting mechanism  110  exerts a downward local load on a part of the retaining ring  40  through the top ring base  150 . The retaining ring  40  is rotatable in unison with the wafer W that is held on the substrate holding surface  45   a  of the flexible membrane  45 , while the retaining ring  40  is tiltable independently of the substrate holding surface  45   a . Therefore, when the local load exerting mechanism  110  presses a part of the retaining ring  40  downwardly, the wafer W held on the substrate holding surface  45   a  W remains unchanged in its attitude. 
     While the wafer W is being polished, the pressure chambers formed between the flexible membrane  45  and the carrier  43 , the central chamber  50 , the ripple chamber  51 , the outer chamber  52 , and the edge chamber  53  are supplied with the pressurized fluid. Therefore, the top ring body  10  receives an upward reaction force from these pressure chambers  50  through  53 . The load of the retaining ring  40  on the polishing pad  2  is determined by subtracting the upward reaction force from the downward load applied through the drive flange  158  to the top ring base  150 . It is possible to change the load applied from the retaining ring  40  to the polishing pad  2  by changing the downward load applied from the vertically moving mechanism  27  (see  FIG. 2 ) to the top ring shaft  11 . In this embodiment, the vertically moving mechanism  27  may use an air cylinder instead of the combination of the ball screw and the servomotor. 
     The top ring  1  shown in  FIG. 13  may be combined with the magnetic local load exerting mechanism  110  shown in  FIG. 11  or  FIG. 12 . In this case, the change in the gap between the permanent magnet  130  and the electromagnet  131  due to wear of the polishing pad  2  and/or the retaining ring  40  can be determined from a measured value of the height of the top ring  1 . Specifically, the change in the gap between the permanent magnet  130  and the electromagnet  131  can be determined from the difference between an initial measured value and a current measured value of the height of the top ring  1 . As shown in  FIG. 1 , the top ring height sensor  39  is mounted to the top ring head  16 . This top ring height sensor  39  is configured to measure the height of the top ring  1  based on the position of the bridge  28  which vertically moves in unison with the top ring  1 . 
     As with the above embodiment, the lateral force (i.e., the frictional force acting between the wafer W and the polishing pad  2 ) that is applied to the retaining ring  40  during polishing of the wafer W is received by the spherical bearing  155 . According to the present embodiment, even if a perpendicularity of the top ring shaft  11  to the polishing surface  2   a  may be slightly shifted, the spherical bearing  155  allows the top ring base  150  to tilt so as to follow the polishing surface  2   a . Further, during polishing of the wafer W, the top ring base  150  and the retaining ring  40  can tilt smoothly under the frictional force generated between the wafer W and the polishing pad  2 . Since the top ring base  150  is made of a relatively rigid material, such as metal or ceramic, an effect of the deformation of the top ring base  150  can be minimized, and the top ring base  150  is allowed to tilt smoothly by the spherical bearing  155 . 
     The carrier  43  is separated from the top ring base  150  and coupled to the top ring base  150  through the elastic membrane  160 . That is, the carrier  43  is in a so-called floating state. The top ring  1  thus constructed allows the retaining ring  40  to tilt independently of the substrate holding surface  45   a  and the wafer W held thereon, and allows the retaining ring  40  to press the polishing pad  2  independently. Therefore, when the local load exerting mechanism  110  presses a part of the retaining ring  40  downwardly, the wafer W does not tilt, and the pressure applied from the substrate holding surface  45   a  of the flexible membrane  45  to the wafer W remains unchanged. Therefore, the local load exerting mechanism  110  can control the attitude of the retaining ring  40  without affecting the attitude of the wafer W and the pressure on the wafer W. In other words, it is possible to control the polishing profile by controlling the attitude of the retaining ring  40  while keeping the pressure applied from the substrate holding surface  45   a  to the wafer W constant. 
     The retaining ring  40  is pressed against the polishing pad  2  while rotating in unison with the wafer W held by the top ring  1 . Since the retaining ring  40  and the wafer W are rotating during the polishing process, the region of the polishing surface  2   a  contacting the retaining ring  40  and the wafer W changes gradually as the polishing table  3  rotates. Therefore, a particular region of the polishing pad  2  is prevented from remaining in contact only with a particular area of the wafer W. As a result, the surface of the wafer W can be uniformly polished. For the same reason, the retaining ring  40  is prevented from being locally worn away. 
     The inside diameter of the retaining ring  40  is larger than the diameter of the wafer W. Therefore, as the top ring  1  rotates, the relative position between the retaining ring  40  and the wafer W in the circumferential direction changes gradually due to sun-and-planet motion. As a result, the effect of the flatness of the retaining ring  40  can be averaged so that the flatness of the retaining ring  40  does not affect any certain circumferential position of the wafer W. As a consequence, the surface of the wafer W can be polished uniformly, especially over the circumferential direction thereof. 
       FIG. 14  is a diagram showing a positional relationship as viewed from above the polishing surface  2   a . In  FIG. 14 , a line interconnecting the center of the wafer W and the center of the polishing surface  2   a  is defined as a virtual line VL. The polishing surface  2   a  can be divided into an upstream side of the virtual line VL and a downstream side of the virtual line VL with respect to the rotating direction of the polishing surface  2   a . Stated otherwise, the upstream side of the virtual line VL and the downstream side of the virtual line VL are an upstream side of the wafer W and a downstream side of the wafer W with respect to the moving direction of the polishing surface  2   a . In  FIG. 14 , the polishing surface  2   a  rotates as illustrated. In a polishing apparatus in which the polishing surface is provided by a belt and moves at a constant speed over the surface of the wafer W, it is possible to more easily define the upstream side and the downstream side of the polishing surface. 
     In  FIG. 14 , a circle S represents a path of a point on the polishing surface  2   a  passing through the center of the wafer W as the polishing surface  2   a  rotates about its central axis. A line T which is tangential to the circle S at the center of the wafer W intersects with a wafer circle at two points. One of the two points that lies at the upstream side is defined as an angle 0°, and the other at the downstream side is defined as an angle 180°. The virtual line VL intersects with the wafer circle at two points. One of the two points which is near the center of the polishing surface  2   a  is defined as an angle 270°, and the other which is near the peripheral edge of the polishing surface  2   a  is defined as an angle 90°. The wafer circle represents a peripheral edge of the wafer W. 
     When the wafer W is being polished, the retaining ring  40 , which receives the frictional force from the wafer W, rises from the polishing pad  2  at the downstream side, i.e., in a region around the angle 180°. Conversely, the retaining ring  40  sinks into the polishing pad  2  at the upstream side, i.e., in a region around the angle 0°. When the local load exerting mechanism  110  exerts a downward local load on a downstream part of the retaining ring  40 , this load can effectively change the attitude of the retaining ring  40  for controlling the polishing profile. Since the wafer W receives the frictional force from the polishing surface  2   a  during the polishing process, the wafer W is pressed against an inner circumferential surface of the retaining ring  40 . Therefore, the smallest gap between the retaining ring  40  and the wafer W lies in the downstream side. When the local load exerting mechanism  110  increases the downward local load on the retaining ring  40  in the downstream side, the local load exerting mechanism  110  can impart a significant pad rebounding effect on the wafer W. In contrast, when the local load exerting mechanism  110  exerts an upward local load on an upstream part of the retaining ring  40 , such an upward local load can effectively change the attitude of the retaining ring  40  for controlling the polishing profile. 
     Under processing conditions that increase the frictional force between the wafer W and the polishing surface  2   a , the retaining ring  40  may rise greatly at the downstream side until the wafer W eventually slips out of the top ring  1 . This problem can be solved by exerting a local downward load on the downstream part of the retaining ring  40  to reduce the degree to which the retaining ring  40  rises at the downstream side. As a result, the wafer W can be polished safely. In addition, polishing of the wafer W can be performed safely even under conditions that have been difficult to be applied in the conventional polishing process. For example, even when a high load is exerted on the wafer W or a high frictional force is produced as a result of a combination of the polishing pad  2 , the polishing liquid, the wafer W and the like, the wafer W can be safely polished. 
     The polishing liquid flows from the upstream side into the top ring  1  as the polishing surface  2   a  rotates. When a local downward load is exerted on the downstream part of the retaining ring  40  so as to reduce the degree to which the retaining ring  40  sinks into the polishing pad  2  at the upstream side, the polishing liquid can easily flow into the top ring  1 . Hence the polishing liquid can effectively be used in polishing of the wafer W. In addition, the polishing rate can increase with a less amount of polishing liquid used, while an increase in polishing temperature is suppressed. Moreover, since the wafer W is polished in the presence of the abundant polishing liquid, it is possible to reduce defects, such as scratch, of the polished wafer W and reduce surface steps of the polished wafer W. In most cases, the local load exerting mechanism  110  configured to exert the downward local load is installed preferably at the downstream side, more preferably in an angular range of 180°±60°. 
     As described above, types of semiconductor devices and types of consumables have been dramatically increasing in recent years. In this trend, there is a need to establish controllability of a variety of polishing profiles for improvement of a product yield. When the position at which the local load is applied to the retaining ring  40  is changed, the manner of change in the polishing profile is changed. Therefore, the polishing profile can be changed in various ways depending on the installation position of the local load exerting mechanism  110 . In some processes, the local load exerting mechanism  110  may preferably be installed in an angular range of 270°±60°, 90°±60°, or 0°±60°. 
     In order to meet the demands for realizing various polishing profiles, the local load exerting mechanism  110  may be movably installed. According to an embodiment shown in  FIG. 15 , the local load exerting mechanism  110  is movable through 360° around the center of the top ring  1  by an annular moving mechanism  170  that is mounted to the top ring head  16 . This moving mechanism  170  can move the local load exerting mechanism  110  to a desired position. The moving mechanism  170  may include, for example, a motion guide mechanism for enabling a curvilinear motion of the local load exerting mechanism  110  and a servomotor for moving the local load exerting mechanism  110 . The position of the local load exerting mechanism  110  may be set and changed according to a polishing recipe for a wafer. 
     Instead of using the moving mechanism  170  shown in  FIG. 15 , the local load exerting mechanism  110  may be mounted to a desired one of a plurality of predetermined sites on the top ring head  16  by fasteners, such as screws. In this embodiment, the local load exerting mechanism  110  is removably mounted to the top ring head  16  by the fasteners. The moving mechanism  170  and the fasteners that allow the installation position of the local load exerting mechanism  110  to change are also applicable to the embodiments shown in  FIGS. 11 through 13 . 
     It is also possible to install a plurality of local load exerting mechanisms  110  along the circumferential direction of the top ring  1 . For example, two local load exerting mechanisms  110  may be installed at the downstream side (i.e., the angle 180°) and the polishing-surface center side (i.e., the angle 270°). With this arrangement, the local load exerting mechanisms  110  can control the attitude of the retaining ring  40  about two axes that extend parallel to the polishing surface  2   a  and perpendicularly to each other. The local load exerting mechanisms  110  thus arranged can control the polishing profile in a wider range, compared with the case of the single local load exerting mechanism  110 . 
     The local load exerting mechanism  110  using the magnetic repulsive force and the magnetic attractive force is advantageous in that the polishing profile control is applicable to a wider range. In the case of using the downward load of the air cylinder or only the magnetic repulsive force, it is preferable to install at least three local load exerting mechanisms  110 . If two local load exerting mechanisms  110  each having an air cylinder are installed in two positions, e.g., at the downstream side (i.e., the angle 180°) and the polishing-surface center side (i.e., the angle 270°), the local load exerting mechanism  110  at the polishing-surface center side is capable of changing the attitude of the retaining ring  40  so as to cause the retaining ring  40  to sink into the central region of the polishing pad  2 , but is unable to exert a local load on the retaining ring  40  so as to cause the retaining ring  40  to sink into the peripheral region of the polishing pad  2 . Therefore, in addition to the two local load exerting mechanisms  110  installed at the downstream side (i.e., the angle 180°) and the polishing-surface center side (i.e., the angle) 270°, a local load exerting mechanism  110  is preferably installed at a polishing-suffice peripheral side (the angle 90°). Three local load exerting mechanisms  110  may be installed at 120° intervals. The installation positions of plural local load exerting mechanisms  110  may be changed by the moving mechanism  170  shown in  FIG. 15 . 
     The structure that allows the change in the installation position of local load exerting mechanisms  110  and the change in the local load on the retaining ring  40  makes it possible to realize a variety of polishing profiles, thus improving a yield of a wide variety of semiconductor devices. 
     In an embodiment shown in  FIG. 16 , in addition to the local load exerting mechanism  110 , a retaining ring height sensor  175  is fixed to the top ring head  16 . The retaining ring height sensor  175  is capable of measuring a vertical displacement of the retaining ring  40 , i.e., the height of the retaining ring  40 . Although not shown in  FIG. 16 , the local load exerting mechanism  110  may include the air cylinder, a combination of the electromagnet and the permanent magnet, or a combination of the permanent magnets as described above, or may be of any other type. 
     The guide ring  112  has a sensor target surface, which may be in the same location where the local load exerting mechanism  110  exerts a local load, or may be in a different location. Typically, the sensor target surface is a flat surface. While a contact-type displacement sensor may be used as the retaining ring height sensor  175 , a non-contact-type displacement sensor capable of measuring the height of the retaining ring  40  in a non-contact manner, such as a laser displacement sensor, is preferably used. If a contact-type displacement sensor is used as the retaining ring height sensor  175 , particles may be produced from contact surfaces of a sensor probe and the guide ring  112  and may fall onto the polishing surface  2   a , possibly causing defects of the polished wafer. Moreover, the sensor probe which contacts the guide ring  112  may exert a downward load on the retaining ring  40 , possibly changing the attitude of the retaining ring  40 . 
     In the embodiments shown in  FIGS. 11 and 12 , the amount of wear of the polishing pad  2  and/or the amount of wear of the retaining ring  40  can be measured using a process of measuring amounts of wear of a polishing pad and a retaining ring as disclosed in Japanese laid-open patent publication No. 2006-128582 and Japanese laid-open patent publication No. 2006-255851. A change in the height of the retaining ring  40  during the polishing process is calculated from the measured amount of wear of the polishing pad  2  and the measured amount of wear of the retaining ring  40  by the polishing controller  9 . The polishing controller  9  feeds the calculated amount of change in the height of the retaining ring  40  back to the elevating mechanism  133  of the local load exerting mechanism  110  to control the elevating mechanism  133  so as to keep the gap between the magnets constant. In the embodiment shown in  FIG. 16 , the polishing controller  9  may feed the amount of change in the height of the retaining ring  40 , which is determined from the measured values sent from the retaining ring height sensor  175 , back to the elevating mechanism  133 . Based on the measurement result of the height of the retaining ring  40 , the magnitude of the local load and/or the position of the local load on the retaining ring  40  (i.e., the position of the local load exerting mechanism  110 ) may be changed for polishing of the current wafer W or polishing of a next wafer. It is preferable to change the position of the local load on the retaining ring  40  in advance before the next wafer is polished. If a plurality of local load exerting mechanisms  110  are installed, it is possible to change the position of the local load by switching the local load exerting mechanism  110  to be operated from one to another. Alternatively, a plurality of local load exerting mechanisms  110  may be operated simultaneously so as to change a ratio of the magnitudes of the local loads generated by the respective local load exerting mechanisms  110 . In this case also, the same effect can be achieved as if the position of the local load exerting mechanism  110  is changed. 
     In the case where only one retaining ring height sensor  175  is installed, the local load exerting mechanism  110  may be operated such that the measured value of the retaining ring height sensor  175  (i.e., the measured height of the retaining ring  40 ) is equal to or smaller than a predetermined threshold value. Depending on the installation position of the retaining ring height sensor  175 , the local load exerting mechanism  110  may be operated such that the measured value of the height of the retaining ring  40  is equal to or larger than a predetermined threshold value or falls in a predetermined range. 
     A plurality of retaining ring height sensors  175  may be arranged along the circumferential direction of the top ring  1 . In a case of installing two retaining ring height sensors  175 , they are disposed in respective positions that are symmetric with respect to the central axis of the top ring  1 . Preferably, the two retaining ring height sensors  175  are disposed at an upstream side and a downstream side of the central axis of the top ring  1 . A difference in height of the retaining ring  40  between two positions on the retaining ring  40  can be calculated from two measured values obtained from the two retaining ring height sensors  175 . The polishing controller  9  may control operation of the local load exerting mechanism  110  such that the calculated difference in the height of the retaining ring  40  lies within a desired range. Preferably, at least three retaining ring height sensors  175  are arranged along the circumferential direction of the top ring  1  in order for the polishing controller  9  to be able to calculate a slope plane of the retaining ring  40 . The polishing controller  9  controls operation of the local load exerting mechanism  110  so as to realize a desired slope plane of the retaining ring  40 . If the calculated slope plane is different from the desired slope plane, then the load on the retaining ring  40  exerted by the local load exerting mechanism  110  is adjusted. 
     Instead of controlling the slope plane so as to coincide with the desired plane, the polishing controller  9  may control the local load exerting mechanism  110  using an index calculated from the slope plane. For example, a maximum slope quantity and a maximum slope position may be adjusted to be kept within desired ranges. The maximum slope quantity represents a difference between a maximum height and a minimum height of the retaining ring  40  in the slope plane, and the maximum slope position represents a position of the highest part of the retaining ring  40 . For example, if the polishing controller  9  determines that the maximum slope quantity is 0.05 mm and the maximum slope position is 180°, then the polishing controller  9  controls one or more local load exerting mechanisms  110  so that the maximum slope quantity is within a range of 0.03 mm±0.02 mm and the maximum slope position is within a range of 270°±30°. The local load exerting mechanism  110  may have a retaining ring height measuring function, or more specifically, may have a retaining ring height sensor. 
     A method of determining a desired installation position (pressing position) of the local load exerting mechanism  110  will be described below  FIG. 17  is a flowchart of a method of determining the pressing position of the local load exerting mechanism  110 . In step  1 , a wafer is polished. While the local load exerting mechanism  110  is pressing the retaining ring  40 . In step  2 , the local load exerting mechanism  110  is moved, and then another wafer is polished while the local load exerting mechanism  110  is pressing the retaining ring  40  at a position different from the position in step  1 . Step  1  and step  2  are repeated as many times as required. In step  3 , polishing results are obtained at the respective pressing positions. In step  4 , a desirable local pressing position is determined from the polishing results. 
     Typical examples of the “polishing results” include in-plane uniformity of a polishing rate distribution, in-plane uniformity of a remaining film thickness distribution, a polishing profile of a wafer edge portion, a polishing rate, a polishing temperature, defects of a polished wafer (e.g., scratch and foreign matter), flatness characteristics (typically dishing and erosion) of a polished wafer, a frictional force between a wafer and the polishing surface  2   a , and a drive current supplied to the polishing table rotating motor  13  which varies due to the frictional force. The “polishing results” thus include items that can be measured and calculated by a sensor and a film thickness measuring device disposed in the polishing apparatus and items that can be acquired by an instrument disposed outside of the polishing apparatus, such as a defect inspection device. 
       FIG. 18  is a diagram showing another example of a method of determining the pressing position of the local load exerting mechanism  110 . In this reference example, the local load exerting mechanism  110  is operated in each of the pressing positions 90°, 180°, and 270° (corresponding to step  3  of the flowchart). The number of pressing positions may be more than or less than three. The “polishing result” shown in  FIG. 18  represents the in-plane uniformity of the remaining film thickness distribution. As shown in  FIG. 18 , the in-plane uniformity is minimized at the pressing position 180°. Since it is generally desirable that the in-plane uniformity be small, the pressing position 180° is determined to be a desirable pressing position (corresponding to step  4  of the flowchart). The polishing results may be interpolated by linear interpolation or curve approximation for use in determination of the desirable pressing position.  FIG. 18  further shows a curve representing the polishing results approximated by a quadratic expression. It can be seen from the curve that the pressing position where the polishing result is minimized is an angle slightly greater than the angle 180°. In this manner, the desirable pressing position can be determined. 
       FIG. 19  is a diagram showing still another example of a method of determining the pressing position of the local load exerting mechanism  110 . In this example shown in  FIG. 19 , a polishing condition is the local load applied by the local load exerting mechanism  110  to the retaining ring  40 . Conditions  1 ,  2 ,  3  in  FIG. 19  represent different local loads. The polishing result represents the in-plane uniformity of the remaining film thickness distribution. 
     As shown in  FIG. 19 , the polishing result obtained under the different polishing conditions (i.e., the conditions  1 ,  2 ,  3 ) changes greatly when the pressing position is 180°. In contrast, the change in the polishing result under the different polishing conditions is small when the pressing position is 90° and 270°. More specifically, the polishing result at the pressing position 90° is large in its entirety, and the polishing result at the pressing position 270° is small in its entirety. Since it is generally desirable that the in-plane uniformity of the remaining film thickness distribution be small, the pressing position 270° where the polishing result is small and stable may be selected. 
     In the pressing position 180°, the in-plane uniformity varies greatly depending on the polishing condition. This means that the polishing profile varies greatly depending on the polishing condition. In a case where a wide controllable range of the polishing profile is required, such as when various types of semiconductor devices are polished, the angle 180° may be selected as the desirable pressing position. With use of the above-described interpolation, a pressing position which is different from the position where the polishing result was actually obtained may be selected as the desirable pressing position. 
     A desirable pressing position and a desirable load of the local load exerting mechanism  110  may be determined based on a period of time for which consumables, such as the polishing pad  2 , have been used. For example, the desirable pressing position and load may be determined in advance for an initial stage, a middle stage, and a final stage of a service life of the polishing pad  2 , and the pressing position and the load may be changed in accordance with the period of time for which the polishing pad  2  has been used. The other consumables of the polishing apparatus include the retaining ring  40 , the flexible membrane  45 , and the dresser. 
     A wafer may be polished in two stages in one polishing apparatus. For example, in a first polishing stage, a relatively high pressure is applied to the wafer to remove surface steps of the wafer. In a second polishing stage, a reduced pressure is applied to the wafer during polishing thereof so as to reduce surface steps (e.g., dishing) that have been produced as a result of locally excessive polishing. The pressing position and the load of the local load exerting mechanism  110  may be different between the first polishing stage and the second polishing stage. For example, in the first polishing stage, the local load exerting mechanism  110  may exert a first local load on the retaining ring  40  at a first pressing position, and in the second polishing stage, the local load exerting mechanism  110  may exert a second local load, which is different from the first local load, on the retaining ring  40  at a second pressing position which is different from the first pressing position. 
     Typically, after the retaining ring  40  contacts the polishing surface  2   a  at the start of polishing of the wafer, the operation of the local load exerting mechanism  110  is started to press the retaining ring  40 , and is stopped after polishing of the wafer is terminated and before the top ring  1  is elevated. If the local load exerting mechanism  110  presses the retaining ring  40  when the top ring  1  is in its elevated position, the retaining ring  40  is forced to tilt, causing a trouble in transporting of the wafer. In order to avoid such a trouble, the local load exerting mechanism  110  does not press the retaining ring  40  when the top ring  1  is in its elevated position with the retaining ring  40  out of contact with the polishing surface  2   a.    
     Based on the measurement results of the film-thickness sensor  7  that are obtained when a wafer W is polished, the magnitude of the local load and/or the position of the local load on the retaining ring  40  (i.e., the position of the local load exerting mechanism  110 ) may be changed for polishing of the wafer W or polishing of a next wafer, it is preferable to change the position of the local load on the retaining ring  40  in advance before the next wafer is polished. If a plurality of local load exerting mechanisms  110  are installed, it is possible to change the position of the local load by switching the local load exerting mechanism  110  to be operated from one to another. Alternatively, a plurality of local load exerting mechanisms  110  may be operated simultaneously so as to change a ratio of the magnitudes of the local loads generated by the respective local load exerting mechanisms  110 . In this case also, the same effect can be achieved as if the position of the local load exerting mechanism  110  is changed. Furthermore, the polishing controller  9  may produce a polishing profile (i.e., a film thickness distribution along the radial direction of the wafer) from the measurement results, and the local load and the local pressing position may be changed based on the polishing profile, particularly the polishing profile of the edge portion of the wafer. Alternatively, the local load and the local pressing position may be changed based on the measurement results obtained from another film-thickness measuring device disposed in the polishing apparatus. 
       FIG. 20  shows the local load exerting mechanism  110  according to still another embodiment. Structures and operations of the top ring  1  which will not be described below are identical to those of the top ring  1  shown in  FIGS. 3 and 4 , and repetitive descriptions thereof are omitted. The top ring  1  shown in  FIG. 20  includes the spherical bearing  100  shown in  FIGS. 7 and 8A through 8C . The top ring  1  may include the spherical bearing  85  shown in  FIGS. 5 and 6A through 6C , instead of the spherical bearing  100 . 
     The retaining ring  40  has the upper portion that contacts the annular retaining ring pressing mechanism  60 . This retaining ring pressing mechanism  60  exerts a uniform downward load on the upper surface of the retaining ring  40  (more specifically the upper surface of the drive ring  40   h ) to thereby press the lower surface of the retaining ring  40  (i.e., the lower surface of the ring member  40   a ) against the polishing surface  2   a  of the polishing pad  2 . 
     The retaining ring  40  is not fixed to the retaining ring pressing mechanism  60 , and simply receives the load from the retaining ring pressing mechanism  60 . Instead of this configuration, the retaining ring  40  may be fixed to the retaining ring pressing mechanism  60 . In the case where the retaining ring  40  is fixed to the retaining ring pressing mechanism  60 , the piston  61  of the retaining ring pressing mechanism  60  is formed by a magnetic material, such as metal, and a plurality of magnets are disposed on the upper portion of the drive ring  40   b  as shown in  FIG. 3 . 
       FIG. 21  is a plan view showing coupling member  75  which couples the retaining ring  40  to the spherical bearing  100 . The basic structure shown in  FIG. 21  is identical to the structure shown in  FIG. 4  except that the coupling member  75  shown in  FIG. 21  has six spokes  78 . The other structural details shown in  FIG. 21  are identical to those shown in  FIG. 4 , and their repetitive descriptions are omitted. 
     As shown in  FIG. 20 , the retaining ring  40  has an upper surface extending radially outwardly from the top ring body  10 . A pressure ring  200 , which is configured to exert a local load on a part of the retaining ring  40 , is disposed above the retaining ring  40 . The local load exerting mechanism  110  is disposed above the pressure ring  200 . This local load exerting mechanism  110  is configured to exert a local load on a part of the pressure ring  200  in the direction perpendicular to the polishing surface  2   a . The pressure ring  200  has a support ring  201  disposed above the retaining ring  40  and a load transmitting element  202  fixed to a lower portion of the support ring  201 . The local load applied to the pressure ring  200  is transmitted to the retaining ring  40  through the load transmitting element  202 . In this manner, the local load exerting mechanism  110  exerts a local load on the retaining ring  40  through the pressure ring  200  so as to change the tilt of the retaining ring  40  with respect to the substrate holding surface  45   a.    
     The support ring  201  of the pressure ring  200  is made of a resin material, such as engineering plastic (e.g., PEEK, PPS), or metal, such as stainless steel or aluminum. A cylindrical lower cover  211  is fixed to an outer circumferential surface of the pressure ring  200 . A cylindrical upper cover  212  is fixed to an outer circumferential surface of the lower cover  211 . The lower cover  211  and the upper cover  212  extend upwardly from the pressure ring  200 , and serve to prevent a liquid, such as the polishing liquid, from entering the top ring  1 . 
     In this embodiment shown in  FIG. 20 , the load transmitting element  202  is constructed by a roller (rolling member) which is rotatably supported by the support ring  201 . The load transmitting element will hereinafter be referred to as “roller”. The roller  202  is rotatably supported by a roller shaft  203  which is secured to the support ring  201 . The roller  202  has a bearing (not shown) therein, so that the load transmitting element  202  can freely rotate around the roller shaft  203 . 
     The roller  202  is placed in rolling contact with an annular plate  215  fixed to the upper surface of the retaining ring  40  (more specially, the upper surface of the drive ring  40   b ). The pressure ring  200  receives the local load from the local load exerting mechanism  110  and exerts the local load on a part of the retaining ring  40  through the roller  202 . The annular plate  215  is integrally connected to the piston  61  of the retaining ring pressing mechanism  60 . Therefore, the annular plate  215  receives both the uniform pressing force from the retaining ring pressing mechanism  60  and the local load from the local load exerting mechanism  110 . The annular plate  215  and the piston  61  may be provided as separate members. Alternatively, the annular plate  215  may be omitted, and the roller  202  may be placed in direct rolling contact with the upper surface of the retaining ring  40 . 
     The annular plate  215  is made of a resin material, such as engineering plastic (e.g., PEEK, PPS), or metal, such as stainless steel or aluminum. If a plurality of magnets are disposed in the drive ring  40   b  as described above, the annular plate  215  may be made of magnetic metal or magnetic corrosion-resistant stainless steel. The roller  202  is placed in rolling contact with an upper surface of the annular plate  215 . This upper surface of the annular plate  215  may be plated with a hard material, such as electroless nickel or hard chromium, or may be coated with a hard material, such as DLC (diamond-like carbon), for increasing a wear resistance. 
     The local load exerting mechanism  110  is secured to the top ring head  16  and fixed in position. The pressure ring  200  is held by the local load exerting mechanism  110  so as not to rotate in unison with the top ring  1 . Specifically, during polishing of the wafer W, the retaining ring  40  rotates about its own axis, while the local load exerting mechanism  110  and the pressure ring  200  do not rotate with the retaining ring  40  and are stationary. 
       FIG. 22  shows a perspective view of the top ring  1 , the pressure ring  200 , and the local load exerting mechanism  110 . For illustrative purpose, certain components are omitted from  FIG. 22 . The pressure ring  200  is disposed on the top ring  1  and is concentric with the retaining ring  40 . The pressure ring  200  and the retaining ring  40  have substantially the same diameter. The local load exerting mechanism  110  is coupled to the pressure ring  200  and configured to exert a local load on a part of the pressure ring  200 . 
     The local load exerting mechanism  110  includes two push rods  221 , a bridge  222 , a plurality of (three in  FIG. 22 ) air cylinders (load generators)  231 ,  232 , and  233 , a plurality of (four in  FIG. 22 ) linear guides  224 , and a unit base  225 . The unit base  225  is secured to the top ring head  16 , and the air cylinders  231 ,  232 , and  233  and the linear guides  224  are mounted to the unit base  225 . The air cylinders  231 ,  232 , and  233  have piston rods  231   a ,  232   a , and  233   a , respectively. The piston rods  231   a ,  232   a , and  233   a  and a plurality of guide rods  226  are coupled to the common bridge  222 . The guide rods  226  are vertically movably supported by the respective linear guides  224  with low friction. Therefore, the linear guides  224  allow the bridge  222  to move smoothly in the vertical direction without being inclined. 
     The air cylinders  231 ,  232 , and  233  generate loads that are transmitted to the common bridge  222 . The bridge  222  is coupled to the pressure ring  200  through the push rods (connector)  221 , which transmit the loads, applied from the air cylinders  231 ,  232 , and  233  to the bridge  222 , to the pressure ring  200 . The roller  202  as the load transmitting element is located below a position where the pressure ring  200  is coupled to the push rods  221 . The local load that is exerted from the push rods  221  on the pressure ring  200  is transmitted to the retaining ring  40  through the roller  202 . In  FIG. 22 , the roller shaft  203  is disposed below a middle point between the two push rods  221 . The air cylinders  231 ,  232 , and  233  are coupled respectively to pressure regulators, vertical movement controllers, and air vent mechanisms (not shown), so that they can generate loads independently of each other. 
       FIG. 23A  is a view showing a load cell  135  for measuring the local load applied from the local load exerting mechanism  110  to the retaining ring  40 .  FIG. 23B  is a view showing the pressure ring  200  taken along line B-B in  FIG. 23A . The pressure ring  200  includes the load cell  135  therein. More specifically, the support ring  201  of the pressure ring  200  has a recess  201   a  formed in an upper surface thereof, and the load cell  135  is disposed in the recess  201   a . The load cell  135  is located above the roller  202 . A load plate  204  is mounted to the load cell  135 . The two push rods  221  have respective lower ends coupled to the load plate  204 . The local load generated by the local load exerting mechanism  110  is transmitted through the two push rods  221  and the load plate  204  to the load cell  135 , which measures the local load. 
     The pressure ring  200  may have at least three rollers for stabilizing its own attitude. In the present embodiment, the pressure ring  200  has, in addition to the roller  202  (which may hereinafter be referred to as “first roller  202 ”), a second roller, and a third roller (not shown), which are disposed in positions that are angularly spaced from the first roller  202  by angles of ±150°, respectively. Since the first roller  202  is located under the push rods  202 , the local load from the local load exerting mechanism  110  is transmitted to the retaining ring  40  through the first roller  202 , and the second roller and the third roller do not substantially transmit the local load from the local load exerting mechanism  110  to the retaining ring  40 . The push rods  221  are arranged radially inwardly of the first roller  202  to prevent the pressure ring  200  from being inclined when the push rods  221  press the pressure ring  200 . 
       FIG. 24  is a plan view of the bridge  222 . As shown in  FIG. 24 , the bridge  222  is of a substantially arcuate shape and has a plurality of through-holes  222   a  which are arranged along a circumferential direction of the bridge  222 . The through-holes  222   a  are also arranged along a circumferential direction of the pressure ring  200 . The two push rods  221  are removably inserted into either two of the through-holes  222   a . Instead of the two push rods  221 , only one push rod  221  may be used. According to the bridge  222  thus constructed, a connecting position between the bridge  222  and the push rods  221  is allowed to change, while a connecting position between the push rods  221  and the pressure ring  200  is maintained as it is. By changing the connecting position between the bridge  222  and the push rods  221 , the position of the local load on the retaining ring  40  can be changed along the circumferential direction of the retaining ring  40 . 
     In order to prevent a liquid, such as the polishing liquid, from entering the top ring  1 , it is desirable to vertically move the top ring  1  when the pressure ring  200  and the retaining ring  40  are not away from each other. Specifically, when the top ring  1  is elevated, the air cylinders  231 ,  232 , and  233  are supplied with atmospheric pressure or low pressure. When the top ring  1  is lowered (e.g., at the time of starting the wafer polishing process), the pressurized fluid is supplied to the three air cylinders  231 ,  232 , and  233  to lower the three piston rods  231   a ,  232   a , and  233   a  at the same time the downward movement of the top ring  1  is started. In order to lower the three piston rods  231   a ,  232   a , and  233   a  at the same speed, the three air cylinders  231 ,  232 , and  233  are typically supplied with the fluid having the same pressure. When the top ring  1  reaches its lowered position, the pressure of the supply fluid is changed, and the loads of the air cylinders  231 ,  232 , and  233  are controlled according to a control process described below. When the top ring  1  is elevated (e.g., after the wafer W has been polished), the fluid in the air cylinders  231 ,  232 , and  233  is vented to the atmosphere or a low-pressure fluid is supplied into the air cylinders  231 ,  232 , and  233 , so that the pressure ring  200  is elevated by the ascending top ring  1  in unison. 
     A method of controlling the loads generated by the air cylinders  231 ,  232 , and  233  will be described below with reference to  FIG. 25 .  FIG. 25  is a plan view showing a positional relationship between the air cylinders  231 ,  232 , and  233  and a local load point (i.e., the roller  202 ). As described above, a position of a local load point Q on the retaining ring  40  can be changed by changing the attachment position of the push rods  221  to the bridge  222 . The loads generated by the air cylinders  231 ,  232 , and  233  are controlled based on the attachment position of the push rods  221 . Specifically, a load balance between the air cylinders  231 ,  232 , and  233  is determined such that moments about the local load point Q are balanced. If the air cylinders  231 ,  232 , and  233  press the bridge  222  with the moments about the local load point Q unbalanced, then the bridge  222  is inclined, thus increasing sliding resistance of the piston rods  231   a ,  232   a , and  233   a  and the linear guides  224 . As a result, the local load on the retaining ring  40  will become unstable. 
     As shown in  FIG. 25 , assuming that distances in a X direction from the air cylinders  231 ,  232 , and  233  to the local load point Q are x1, x2, x3, distances in a Y direction, which is perpendicular to the X direction, from the air cylinders  231 ,  232 , and  233  to the local load point Q are y1, y2, y3, the loads generated by the air cylinders  231 ,  232 , and  233  are F 1 , F 2 , F 3 , and a target local load is F, the following equations hold:
 
 F 1 *x 1 +F 2 *x 2 −F 3 *x 3=0
 
 F 1 *y 1 −F 2 *y 2 −F 3 *y 3=0
 
 F 1 +F 2 +F 3 =F  
 
     Since values of x1, x2, x3, y1, y2, y3 are determined from the position of the local load point Q, ratios of F 1 , F 2 , F 3  to F are calculated according to the above equations. Of the air cylinders  231 ,  232 , and  233 , one which is closer to the push rods (connector)  221  generates a relatively large load, and one which is away from the push rods  221  generates a relatively small load. It is preferable that the air cylinders  231 ,  232 , and  233  generate loads such that the center of gravity of the loads generated coincides with the position of the push rods  221 , i.e., the position of the local load point Q. The loads generated by the air cylinders  231 ,  232 , and  233  are controlled based on the load balance thus calculated. 
     If the local load measured by the load cell  135  is different from the desired local load F, a warning may be issued or the loads F 1 , F 2 , F 3  generated by the air cylinders  231 ,  232 , and  233  may be changed. If the measured local load is smaller than the desired local load F, the loads F 1 , F 2 , F 3  are increased, and if the measured local load is greater than the desired local load F, the loads F 1 , F 2 , F 3  are reduced. In these cases, the loads F 1 , F 2 , F 3  may be changed while the calculated ratio F 1 :F 2 :F 3  is maintained. 
     The position of the local load point Q, i.e., the position of the roller  202 , can be selected as desired. For better stability of the load, the roller  202  is preferably located within a triangle (illustrated by bold lines) that interconnects the three air cylinders  231 ,  232 , and  233 . The roller  202  located within the triangle interconnecting the three air cylinders  231 ,  232 , and  233  can locally press the retaining ring  40 , while the moments about the local load point Q are in equilibrium. 
     Next, a suction mechanism  240  for sucking a liquid and particles from the top ring  1  will be described. As shown in  FIG. 22 , the suction mechanism  240  has a first suction line  241  and a second suction line  242  which are coupled to a vacuum source  239 , e.g., a vacuum pump, and a suction line holder  244  which holds the first suction line  241  and the second suction line  242 . The first suction line  241  and the second suction line  242  have respective distal ends coupled to the pressure ring  200 . 
       FIG. 26A  is an enlarged view showing a junction between the first suction line  241  and the pressure ring  200  and  FIG. 26B  is an enlarged view showing a junction between the second suction line  242  and the pressure ring  200 . The first suction line  241  is in fluid communication with an annular trench  248  formed on an upper surface of the pressure ring  200 . A downwardly extending annular protrusion  249  that is fixed to a circumferential surface of the top ring body  10  is loosely inserted in the annular trench  248 . The annular protrusion  249  and the annular trench  248  jointly construct a labyrinth structure that prevents a liquid from entering the gap between the pressure ring  200  and the top ring  1 . The first suction line  241  serves to suck the liquid collected in the annular trench  248 . 
     As shown in  FIG. 26B , the distal end of the second suction line  242  is connected to a vertical hole  247  formed in the pressure ring  200 . The vertical hole  247  extends through the pressure ring  200  and is in fluid communication with the gap between the pressure ring  200  and the retaining ring  40 . The second suction line  242  draws in dust particles (e.g., particles worn of from the roller  202 ) which may be produced as a result of the rolling contact of the roller  202  serving as the load transmitting element. 
     As shown in  FIG. 22 , a magnetic member  250  which extends downwardly to the upper surface of the pressure ring  200  is fixed to the suction line holder  244 .  FIG. 27  is an enlarged view showing the magnetic member  250  and the pressure ring  200 . As shown in  FIG. 27 , an upper permanent magnet  251  and a plurality of lower permanent magnets  252  are embedded in the pressure ring  200 . The upper permanent magnet  251  is located at a position corresponding to a position of the magnetic member  250 , so that an attractive magnetic force is produced between a lower end of the magnetic member  250  and the upper permanent magnet  251 . The two suction lines  241  and  242  are removably secured to the pressure ring  200  via this magnetic force. With such a construction, the magnetic member  250  and the suction lines  241  and  242  can be easily removed from the pressure ring  200  for maintenance. The lower permanent magnets  252  are arranged along the circumferential direction of the pressure ring  200 . An attractive magnetic force is produced between the lower permanent magnets  252  and the annular plate  215 , which is made of a magnetic material, to thereby stabilize the position of the pressure ring  200 . 
     As shown in  FIG. 22 , the suction mechanism  240  is removably mounted to a mount ring  255 , which is secured to the unit base  225 .  FIG. 28  is a plan view of the mount ring  255 . The mount ring  255  is of an arcuate shape and has a plurality of attachment holes  255   a  arranged along a circumferential direction of the mount ring  255 . The suction line holder  244  is removably secured to the mount ring  255  by at least one screw (not shown) which is inserted into at least one of the attachment holes  255   a . Therefore, the position of the suction mechanism  240  mounted to the mount ring  255  can be changed. 
     As described previously, changing of the position of the local load point is achieved by changing the attachment position of the push rods  221  relative to the bridge  222 . When changing the attachment position of the push rods  221 , it is necessary to move the suction lines  241  and  242  together with the pressure ring  200  along the circumferential direction of the pressure ring  200 . Therefore, when the attachment position of the push rods  221  is changed, the position of the suction mechanism  240  is also changed. 
       FIG. 29  is a cross-sectional view of the local load exerting mechanisms  110  according to another embodiment. Structures and operations of the local load exerting mechanisms  110  which will not be described below are identical to those of the local load exerting mechanisms  110  shown in  FIG. 20 , and repetitive descriptions are omitted. In this embodiment, the plural local load exerting mechanisms  110  are secured to the top ring head  16 . Air cylinders are used as the local load exerting mechanisms  110 , respectively. These local load exerting mechanisms  110  are coupled to the pressure ring  200 . Each local load exerting mechanism  110  is configured to exert a local load on a part of the pressure ring  200  in the direction perpendicular to the polishing surface  2   a.    
     The pressure ring  200  has a plurality of rollers (rolling members)  202  as load transmitting elements for receiving the local loads from the local load exerting mechanisms  110  and transmitting the local loads to the retaining ring  40 . The rollers  202  are disposed just below the local load exerting mechanisms  110 , respectively. The load transmitting elements may be convex sliding members in sliding contact with the retaining ring  40 , instead of the rollers  202 . 
     Although not shown, a plurality of load cells are disposed in the pressure ring  200 . The load cells are disposed between the rollers  202  and the local load exerting mechanisms  110 . The load cells are arranged in the same manner as the load cell  135  shown in  FIGS. 23A and 23B . In the embodiment shown in  FIG. 29 , the load cells are disposed above the respective rollers  202 . If the loads measured by these load cells are different from desired values, then a warning may be issued or the loads generated by the local load exerting mechanisms  110  may be changed. 
     A load balance between the local load exerting mechanisms (e.g., the air cylinders in the present embodiment)  110  can control the center of the gravity of the local loads applied to the retaining ring  40 , i.e., a pressure distribution along the circumferential direction of the retaining ring  40 . In the embodiment shown in  FIG. 22 , the change in the relative position of the push rods  221  with respect to the bridge  222  results in the change in the position of the local load point. According to the embodiment shown in  FIG. 29 , it is possible to change the position of the local load point on the retaining ring  40  with greater ease by changing the load balance between the local load exerting mechanisms  110 . In the top ring  1  shown in  FIG. 20 , the retaining ring pressing mechanism  60  exerts a uniform load on the retaining ring  40  in its entirety, and the pressure ring  200  exerts a local load on the retaining ring  40 . In the present embodiment, the retaining ring pressing mechanism  60  may be omitted, and the local load exerting mechanisms  110  may exert both a uniform load and a local load on the retaining ring  40 . 
     A method of controlling the local loads of the local load exerting mechanisms  110  shown in  FIG. 29  will be described below with reference to  FIGS. 30A through 30G .  FIGS. 30A through 30C  are plan views of the air cylinders and the pressure ring  200 . The angle θ shown in  FIGS. 30A through 30C  corresponds to the angle illustrated in  FIG. 14 .  FIG. 30A  shows an example in which two air cylinders exert local loads on the pressure ring  200 .  FIG. 30B  shows an example in which three air cylinders exert local loads on the pressure ring  200 .  FIG. 30C  shows an example in which four air cylinders exert local loads on the pressure ring  200 . In any one of these examples, the center of gravity of the local loads generated by the air cylinders is deviated from the center of the retaining ring  40 . Although not shown, the principle of the present embodiment is also applicable to a case of using five or more air cylinders. 
     In  FIG. 30A , two air cylinders  110 A and  110 B are axisymmetrical with respect to a line of the angle 180° (the angle is represented by θ). When the two air cylinders  110 A and  110 B exert respective local loads on the retaining ring  40 , the center of gravity of the two local loads can be located within a line segment interconnecting the two air cylinders  110 A and  110 B. For example, if a local load is to be applied to a position on the line of the angle 180°, the loads generated by the two air cylinders  110 A and  110 B are set to be equal to each other. 
     In  FIG. 30B , three air cylinders  110 A,  110 B, and  110 C are arranged at equal intervals around the center of the retaining ring  40 , i.e., the center of the pressure ring  200 . In this example, the center of gravity of the three local loads generated by the three air cylinders  110 A,  110 B, and  110 C can be located within an equilateral triangle interconnecting the three air cylinders  110 A,  110 B, and  110 C. For example, if a local load is to be applied to a position on the line of the angle 180°, the load generated by the air cylinder  110 C at the downstream side is set to be the highest one, while the loads generated by the two air cylinders  110 A and  110 B at the upstream side are set to be equal to each other. Further, the total load generated by the two air cylinders  110 A and  110 B at the upstream side is set to be smaller than the load generated by the air cylinder  110 C at the downstream side. When the loads of the two air cylinders  110 A and  110 B at the upstream side are zero, the air cylinder  110 C can tilt the retaining ring  40  most effectively. 
     In  FIG. 30C , four air cylinders  110 A,  110 B,  110 C, and  110 D are arranged at equal intervals around the center of the retaining ring  40 , i.e., the center of the pressure ring  200 . The center of gravity of the four local loads generated by the four air cylinders  110 A,  110 B,  110 C, and  110 D can be located within a regular square interconnecting the four air cylinders  110 A,  110 B,  110 C, and  110 D. For example, if a local load is to be applied to a position on the line of the angle 180°, the loads generated by the two air cylinder  110 C and  110 D at the downstream side are set to be equal to each other, and the loads generated by the two air cylinders  110 A and  110 B at the upstream side are also set to be equal to each other. Further, the total load generated by the two air cylinders  110 A and  110 B is set to be smaller than the total load generated by the two air cylinders  110 C and  110 D. In this example also, when the loads generated by the two air cylinders  110 A and  110 B at the upstream side are zero, the air cylinders  110 C and  110 D can tilt the retaining ring  40  most effectively. 
     The four-load-point arrangement shown in  FIG. 30C  is capable of bringing the center of gravity of the loads close to the retaining ring  40  in a wider area than the three-load-point arrangement shown in  FIG. 30B . Therefore, the four-load-point arrangement shown in  FIG. 30C  is capable of tilting the retaining ring  40  in a wider area than the three-load-point arrangement shown in  FIG. 30B . For example, when the three-load-point arrangement is used to press a position on a line of an angle 135°, the air cylinder  110 A and the air cylinder  1100  press the retaining ring  40  at the same load L, while the air cylinder  110 B exerts a zero load. Where a distance from the center of the retaining ring  40  to each of the air cylinders is R, a moment of changing the tilt of the retaining ring  40  is L*R. With the four-load-point arrangement, only the air cylinder  110 C presses the retaining ring  40  at a load 2L, which means twice the load L. In this case, a moment of changing the tilt of the retaining ring  40  is 2L*R. While the total local load acting on the retaining ring  40  is 2L in both cases using the three-load-point arrangement and the four-load-point arrangement, the four-load-point arrangement can produce a greater moment for changing the tilt of the retaining ring  40  than the three-load-point arrangement. 
       FIG. 31  is a cross-sectional view of the top ring  1  according to still another embodiment. In this embodiment, the top ring  1  includes a pressing member  260  for pressing a part of the retaining ring  40  in the direction perpendicular to the polishing surface  2   a , and a load generator  261  for exerting, on the pressing member  260 , a pressing force for pressing the retaining ring  40  against the polishing surface  2   a . The load generator  261  and the pressing member  260  jointly constitute a local load exerting mechanism for exerting a local load on a part of the retaining ring  40  perpendicularly to the polishing surface  2   a . The pressing member  260  is secured to a support ring  262  disposed between the load generator  261  and the retaining ring  40 . The polishing apparatus has a position retaining mechanism  270  configured to retain a position of the pressing member  260  so as not to allow the pressing member  260  to rotate together with the top ring  1 . 
     The load generator  261  is disposed in the flange  41  of the top ring body  10 . This load generator  261  has a piston  263  and a rolling diaphragm  264 , as with the retaining ring pressing mechanism  60 . The load generator  261  can change the pressing force applied to the pressing member  260  by changing the pressure of the pressurized fluid supplied into a pressure chamber  265  which is defined by the rolling diaphragm  264 . 
       FIG. 32A  is a plan view showing the pressing member  260  and the position retaining mechanism  270 .  FIG. 32B  is a side view of the pressing member  260 . The pressing member  260  has upper rollers  260   a  and  260   b  for receiving a load from the load generator  261 , lower rollers  260   c  and  260   d  for transmitting the load to a part of the retaining ring  40 , and a roller holder  260   e  holding the rollers  260   a ,  260   b ,  260   c , and  260   d  thereon. The roller holder  260   e  is fixed to the support ring  262 . 
     The gap between the top ring body  10  and the retaining ring  40  is sealed by a seal ring  272  and a seal sheet  273 , which prevent droplets of the polishing liquid from entering the top ring  1  and also prevent particles, produced in the top ring  1 , from falling onto the polishing surface  2   a . The seal ring  272  has an L-shaped cross section and has a lower surface fixed to the retaining ring  40 . The seal ring  272  is rotatable together with the retaining ring  40 , and is tiltable in unison with the retaining ring  40 . The lower rollers  260   c  and  260   d  are held in rolling contact with an upper surface of the seal ring  272 . Therefore, the lower rollers  260   c  and  260   d  transmit the load, generated by the load generator  261 , to a part of the retaining ring  40  through the seal ring  272 , thus tilting the retaining ring  40  with respect to the substrate holding surface  45   a . The lower rollers  260   c  and  260   d  may be placed in direct contact with the upper surface of the retaining ring  40  so as to directly transmit the load to a part of the retaining ring  40 . 
     The position retaining mechanism  270  has a retaining target  275  mounted to the support ring  262  and a target retainer  276  for retaining the retaining target  275 . The support ring  262  has a plurality of attachment holes  262   a  which are arranged along the entire circumference of the support ring  262 . The retaining target  275  is removably attached to either one of the attachment holes  262   a . By changing the attachment position of the retaining target  275 , the position of the pressing member  260  can be changed along the circumferential direction of the retaining ring  40 , the position of the local load applied to the retaining ring  40  can be changed. 
     The target retainer  276  is located closely to the retaining target  275 . The target retainer  276  is fixed in position. The target retainer  276  retains the position of the retaining target  275  via a magnetic force in a non-contact manner. More specifically, one of the target retainer  276  and the retaining target  275  is constituted by a permanent magnet, and the other is constituted by a magnetic material. The permanent magnet may be replaced with an electromagnet. An attractive magnetic force is produced between the target retainer  276  and the retaining target  275 , so that the target retainer  276  can retain the retaining target  275  via the magnetic force without physical contact with the retaining target  275 . As a result, the support ring  262  to which the retaining target  275  is fixed is magnetically fixed in position and is not allowed to rotate in unison with the top ring  1 . 
     Since the target retainer  276  and the retaining target  275  are spaced from each other, it is possible to provide the seal ring  272  and/or the seal sheet  273  between the target retainer  276  and the retaining target  275 , as shown in  FIG. 31 . Alternatively, the target retainer  276  may be directly connected to the retaining target  275  so as to retain the retaining target  275  in a contact manner. 
     As shown in  FIG. 32A , a sensor target  281  may be provided on the support ring  262 , and a proximity sensor  282  for sensing the sensor target  281  may be provided. The proximity sensor  282  is disposed outside of the top ring  1  and does not rotate together with the top ring  1 . The proximity sensor  282  thus arranged is capable of detecting whether or not the position of the support ring  262  is retained by the position retaining mechanism  270 , i.e., whether or not the support ring  262  and the pressing member  260  are not rotating in unison with the top ring  1 . The sensor target  281  is removably attached to one of the attachment holes  262   a  of the support ring  262 , so that an installation position of the sensor target  281  can be changed. In  FIG. 32A , the sensor target  281  is disposed adjacent to the retaining target  275 . The sensor target  281  may be located away from the retaining target  275 . The sensor target  281  may be made of nonmagnetic metal and the proximity sensor  282  may be an eddy-current sensor. 
     In order to retain the position of the support ring  262  via the magnetic force, the target retainer  276  may be vertically movable in synchronism with the top ring  1  or may have a vertical dimension larger than a vertically movable distance of the top ring  1 . In  FIGS. 31 and 32A , the target retainer  276  and the proximity sensor  282  are coupled to a vertically moving mechanism  285 , which moves the target retainer  276  and the proximity sensor  282  in the vertical direction in synchronism with the vertical movement of the top ring  1 . The vertically moving mechanism  285  may be constituted by a combination of a servomotor and a ball screw. 
     While the target retainer  276  is fixed in position, the position at which the retaining target  275  is mounted to the support ring  262  can be changed. Therefore, by changing the relative position of the pressing member  260  and the retaining target  275  on the support ring  262 , it is possible to change the local load point on the retaining ring  40 . For example, when the pressing member  260  is located in a position spaced apart from the retaining target  275  by an angle of 180°, the pressing member  260  can exert a local load on the position spaced apart from the retaining target  275  by an angle of 180°. When the pressing member  260  is located in a position spaced apart from the retaining target  275  by an angle of 90°, the pressing member  260  can exert a local load on the position spaced apart from the retaining target  275  by an angle of 90°. Instead of this structure in which the attachment position of the retaining target  275  is variable, the attachment position of the pressing member  260  may be variable. 
       FIG. 33  is a cross-sectional view showing another embodiment of the shaft portion  76  supported by the spherical bearing  100 . As shown in  FIG. 33 , the shaft portion  76  includes a flange element  76 A and a shaft element  7613 , which are separated components. The flange element  76 A and the shaft element  7613  are bonded to each other by an adhesive layer  76 C. Preferably, the flange element  76 A is made of metal, such as aluminum or stainless steel, and the shaft element  7613  is made of highly rigid and highly wear-resistant ceramic, such as alumina, SiC, or zirconia. The shaft portion  76  thus constructed is suitable in the case where an eddy-current sensor is used as the film thickness sensor  7  embedded in the polishing table  3 . If metal exists near the substrate holding surface  45   a  of the top ring  1 , a measurement value of the eddy-current sensor may adversely be affected. In order to avoid such an adverse effect, the shaft element  7613  is preferably made of ceramic. The modification shown in  FIG. 33  is also applicable to the shaft portion  76  supported by the spherical bearing  85  shown in  FIG. 5 . 
       FIG. 34  is a cross-sectional view of the top ring  1  according to still another embodiment. Structures and operations of the top ring  1  which will not be described below are identical to those of the top ring  1  shown in  FIG. 20 , and repetitive descriptions thereof are omitted. As shown in  FIG. 34 , the flange  41  is connected to the top ring shaft  11 . The top ring shaft  11  and the top ring body  10  have a plurality of fluid passages  290  formed therein which are connected to the pressure chambers  50 ,  51 ,  52 , and  53  and the retaining ring pressure chamber  63 , respectively. In  FIG. 34 , only some of the fluid passages  290  are depicted. These fluid passages  290  are not constituted by pipes, such as tubes, but are formed by drilling holes in the top ring shaft  11  and the top ring body  10 . 
       FIG. 35  is a cross-sectional view showing a part of the top ring  1  according to still another embodiment. As shown in  FIG. 35 , a plurality of radially inwardly extending stopper pins  295  are mounted to the drive ring  40   b . These stopper pins  295  are inserted in respective holes  43   c  formed on a circumferential surface of the carrier  43 . When the carrier  43 , the drive ring  40   b , and the retaining ring  40  are removed for maintenance, the carrier  43  and the drive ring  40   b  are not separated from each other because of the stopper pins  295  inserted in the holes  43   c.    
     A reinforcement ring  297  is embedded in the retaining ring  40 . This reinforcement ring  297  is disposed between the drive ring  40   b  and the ring member  40   a  and arranged concentrically with the retaining ring  40 . The reinforcement ring  297  serves to prevent the deformation of the retaining ring  40  when the retaining ring  40  is subjected to the frictional force produced between the wafer and the polishing pad  2  during polishing of the wafer. A cover ring  298  is disposed to an outer circumferential surface of the drive ring  40   b . An O-ring  301  is interposed between the cover ring  298  and the drive ring  40   b , and an O-ring  302  is interposed between the cover ring  298  and the ring member  40   a . The O-rings  301  and  302  serve to prevent a liquid, such as the polishing liquid, from entering the top ring  1 . 
     The polishing apparatus and the polishing method according to the embodiments described above may be appropriately combined. 
     Although certain embodiments have been shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the technical concept.