Patent Publication Number: US-7897007-B2

Title: Substrate holding apparatus and substrate polishing apparatus

Description:
This is a Divisional Application of U.S. patent application Ser. No. 11/907,590, filed Oct. 15, 2007, which is a Divisional Application of U.S. patent application Ser. No. 10/972,579, filed Oct. 26, 2004, now abandoned which is a Divisional Application of U.S. patent application Ser. No. 09/917,732, filed Jul. 31, 2001 now U.S. Pat. No. 6,890,402. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a substrate holding apparatus in which a substrate is held when polished for flattening a surface thereof. The present invention also relates to a polishing apparatus comprising the above-mentioned substrate holding apparatus. 
     A semiconductor device fabricating process comprises forming a thin film layer on a wafer and forming minute patterns and holes in the layer. This process is repeated until a desired number of circuit layers are formed on the wafer. Therefore, raised and recessed portions are created on or added to the surface of the wafer after formation of each circuit layer. In recent years, semiconductor devices have become increasingly minute and element structures of semiconductor devices have become complicated. Further, there is a tendency to increase the number of circuit layers for logic type devices. As a result, raised and recessed portions on the surface of a semiconductor device increase in number and a difference in height between these portions also increases. This leads to a problem such that during formation of a film on the wafer, an extremely thin film is formed over an undulating area containing the raised and recessed portions on the wafer and breaks in a circuit and an electrical insulation defect between circuit layers are likely to occur, leading to a lowering of product quality and a lowering of yield. Although semiconductor devices can operate normally during an initial period of operation, they are not reliable when used over a long period of time. 
     Raised and recessed portions on the wafer are also problematic in a lithography process. That is, when an exposure surface of the wafer contains raised and recessed portions, the lenses of an exposure system partially become out of focus, so that formation of minute patterns becomes difficult. 
     For these reasons, the techniques for surface flattening in fabricating semiconductor devices have been increasingly becoming important. Of various surface flattening techniques, the most important technique is CMP (chemical mechanical polishing), which comprises polishing by using a polishing apparatus, in which while an abrasive liquid containing abrasive particles of silica (SiO2) or the like is supplied onto a polishing surface of a polishing pad, a semiconductor wafer is slidably engaged with the polishing surface. 
     Conventionally, the polishing apparatus of the above-mentioned type comprises a polishing table including a polishing pad having a polishing surface and a wafer holder for holding a semiconductor wafer. The wafer holder is adapted to hold a semiconductor wafer and press the wafer against the polishing table under a predetermined pressure. The wafer holder and the polishing table are moved relative to each other so that the semiconductor wafer is slidably engaged with the polishing surface, to thereby polish the wafer to a flat and mirror-finished surface. 
     In the above-mentioned polishing apparatus, when a relative pressure generated between the semiconductor wafer and the polishing surface of the polishing pad is not uniform over an entire surface of the wafer, insufficient or excessive polishing is likely to occur, depending on the pressure acting on each part of the wafer. Therefore, in order to apply a uniform pressure to an entire surface of the wafer, an elastic membrane made of rubber is provided on the wafer holder on a surface thereof for holding a wafer, and a fluid pressure such as air pressure is applied to a back surface of the elastic membrane. In this case, a circumferential edge of the wafer surface corresponds to a boundary between a contact portion and a non-contact portion of the wafer relative to the polishing surface. Since the polishing pad is elastic, the pressure applied to a portion around the circumferential edge of the wafer surface becomes non-uniform, so that only the circumferential edge of the wafer is polished in an excessive amount, and the wafer is caused to have a “dull” edge. 
     As a countermeasure, it has been proposed to use a wafer holder in which a guide ring or retainer ring for holding an outer circumferential edge of the wafer presses the polishing surface at a position outside the wafer. In this wafer holder, the retainer ring is pressed against the polishing surface under fluid pressure such as air pressure. 
       FIG. 14  is a schematic illustration of a wafer holder of the above-mentioned type, in which a fluid pressure is applied to a wafer so as to press the wafer against a polishing surface, and the fluid pressure is also applied to a retainer ring so as to press the retainer ring against the polishing surface. 
     As shown in  FIG. 14 , a wafer holder  50  comprises: a wafer holder body  51  defining an inner space; a wafer pressurizing mechanism  52  contained in the inner space of the wafer holder body  51  and adapted to press a semiconductor wafer W against a polishing surface  61  of a polishing table  60 ; a retainer ring  53  provided so that it is vertically movable relative to the wafer holder body  51  and adapted to hold an outer circumferential edge of the wafer W; and a retainer ring pressurizing mechanism  54  for pressing the retainer ring  53  against the polishing surface  61 . 
     The wafer pressurizing mechanism  52 , although not shown in detail, comprises an elastic membrane member which is made of an elastic material such as rubber and is connected to the wafer holder body  51 . A pressurized fluid such as pressurized air is supplied to the inside of the elastic membrane member so that the wafer W is pressed against the polishing surface  61  under fluid pressure. The retainer ring pressurizing mechanism  54 , although not shown in detail, also comprises an elastic membrane member which is made of an elastic material such as rubber and is connected to the wafer holder body  51 . A pressurized fluid such as pressurized air is supplied to the inside of the elastic membrane member so that the retainer ring  53  is pressed against the polishing surface  61  under fluid pressure. The wafer holder body  51  is connected to a drive shaft  55  and the drive shaft  55  is adapted to be vertically moved by a lifting mechanism such as an air cylinder. 
     The lifting mechanism such as an air cylinder connected to the drive shaft  55  is operated so as to move the wafer holder body  51  as a whole to a position close to the polishing table  60 . While the wafer W is held in proximity to the polishing surface  61 , the pressurized fluid is supplied under a predetermined pressure to the wafer pressurizing mechanism  52 , to thereby press the wafer W against the polishing surface  61  of the polishing table  60 . The pressure applied to the wafer W during polishing is adjusted to a desired value by adjusting the pressure of the pressurized fluid supplied to the wafer pressurizing mechanism  52 . On the other hand, the pressurized fluid is supplied under a predetermined pressure to the retainer ring pressurizing mechanism  54 , to thereby press the retainer ring  53  against the polishing surface  61  of the polishing table  60 . 
     Since the wafer W is pressed against the polishing surface  61  by using a fluid pressure, it is possible to obtain a uniform pressure distribution across an entire surface of the wafer W from the center to the circumferential edge thereof. This enables uniform polishing of the entire surface of the wafer W. Further, during polishing, a pressure substantially equal to that applied to the wafer W is applied to the retainer ring  53  through the retainer ring pressurizing mechanism  54 , so that the polishing surface of the polishing pad outside the wafer W is pressed under a pressure substantially equal to that of the wafer W. Therefore, a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring  53  outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented. 
     In the above-mentioned conventional wafer holder in which both the wafer and the retainer ring are pressed under fluid pressure, the retainer ring is capable of moving in either a vertical (or perpendicular) direction or a lateral (or radial) direction relative to the wafer holder body. That is, the retainer ring is capable of moving independently of the wafer holder body. Movement of the retainer ring affects uniformity in the polishing of an outer circumferential portion of the wafer surface. Although vertical movement of the retainer ring is necessary for polishing, lateral movement of the retainer ring is unnecessary. Rather, lateral movement of the retainer ring is undesirable because it varies the distance between the retainer ring and the circumferential edge of the wafer surface and impairs uniformity and stability in the polishing of the outer circumferential portion of the wafer surface. 
     Further, in the conventional wafer holder, since the surface of the wafer holder for holding a wafer is covered with the elastic membrane, it is required to form, for example, a suction cup-like configuration in the elastic membrane so as to hold a wafer during transfer thereof. When a wafer is held by the elastic membrane having a suction cup-like configuration, warpage or deformation of the wafer occurs. Due to warpage of the wafer, the wafer can be broken during transfer thereof or a device structure formed on the wafer can be damaged. Further, since the wafer is held by indirect contact with the wafer holder through the elastic membrane, defects in holding of the wafer are likely to occur during transfer of the wafer, leading to a lowering of operating rate of the wafer holder and a lowering of yield of wafers. 
     Further, in chemical mechanical polishing (CMP) utilizing an elastic polishing pad and an abrasive liquid (slurry), the following problem arises. That is, when a wafer surface having raised and recessed portions is polished, the raised portions are polished in preference to the recessed portions during an initial period of polishing, but after the raised portions are polished by a certain amount, the recessed portions are also gradually subjected to polishing (as well as the raised portions). Therefore, the difference in height between the raised portions and the recessed portions cannot be easily reduced. That is, because polishing is conducted by using a relatively soft, elastic polishing pad and a slurry type abrasive liquid containing a large amount of free abrasive particles, chemical mechanical polishing is effected on not only the raised portions, but also the recessed portions of the wafer surface. Further, the effect of polishing varies, depending on the density of raised and recessed portions. 
     Therefore, an attempt has been made with respect to polishing by using a polishing surface comprising fixed abrasive particles such as cerium oxide (CeO2), which are bound by using a binder such as a phenol resin. In this polishing, the polishing surface is hard as compared to the polishing pad conventionally used in chemical mechanical polishing, so that the raised portions are polished in preference to the recessed portions and the recessed portions are unlikely to be polished. Therefore, absolute flatness of the wafer can be easily obtained. 
     However, a wafer holder suitable for a hard polishing surface comprising fixed abrasive particles has not been developed. Generally, a conventional wafer holder for the hard polishing surface comprises a rigid wafer holder body and an elastic backing pad provided on the rigid wafer holder body adapted to be engaged with a wafer to be held by the wafer holder. Although the elastic backing pad can absorb shocks on the wafer, it is difficult for the elastic backing pad to take care of undulations on the hard polishing surface, whereby the undulations are transferred to and affects the wafer surface to be polished. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention has been made. 
     In accordance with the present invention, there is provided a substrate holding apparatus for holding a substrate and bringing it into contact with a polishing surface so that the substrate is subjected to polishing by causing relative movement between the substrate and the polishing surface, the apparatus comprising a substrate holder body having a substrate holding side facing the polishing surface and holding a substrate on the substrate holding side and a retainer ring integrally formed with or fixedly secured to the substrate holder body on the substrate holding side, the retainer ring being arranged to surround an outer periphery of the substrate held by the substrate holder body so that the retainer ring engages with the polishing surface radially outside the substrate as the polishing of the substrate is effected. The substrate holder body is provided on the substrate holding side with a membrane having opposite surfaces including inside and outside surfaces, the inside surface cooperating with a surface of the substrate holder body to define a fluid pressure chamber to which a fluid pressure is applied, the outer surface engaging with the substrate held by the substrate holder body. 
     In accordance with another aspect of the present invention, there is provided a substrate holding apparatus in which, instead of the membrane which covers the entire surface of the substrate, the apparatus comprises a substrate support ring provided in the inner space and arranged to be sealingly engaged with the substrate to be held by the substrate holding apparatus, and a flexible seal member sealingly connected between the substrate support ring and the substrate holder body so that a fluid pressure chamber is defined by the substrate holder body, the flexible seal member and the substrate engaged with the substrate support ring. The fluid pressure chamber is arranged to be selectively connected to a pressurized fluid source or a vacuum source. 
     These substrate holding apparatuses eliminate a relative movement between the retainer ring and the wafer holder body whereby the behavior of the retainer ring can be stabilized during polishing. A substrate is held on the fluid pressure chamber so that the substrate can follow undulations on a polishing surface. 
     In accordance with a further aspect of the present invention, there is provided a polishing apparatus including a substrate holding apparatus as stated above. 
     Further, in accordance with another aspect of the present invention, there is provided a substrate polishing apparatus comprising a first polishing table having a hard polishing surface and a substrate holding apparatus for holding a substrate and bringing it into contact with the hard polishing surface. The substrate holding apparatus comprises a substrate holder body having a substrate holding side facing the polishing surface and holding a substrate on the substrate holding side and a membrane provided on the substrate holding side of the substrate holder body, the membrane having opposite surfaces including inside and outside surfaces, the inside surface cooperating with a surface of the substrate holder body to define a fluid pressure chamber to which a fluid pressure is applied, the outer surface engaging with the substrate held by the substrate holder body. The hard polishing surface has, for example, a modulus of compression of 19.6 MPa (200 kg/cm2) or more. In this apparatus, a substrate is held on the fluid pressure chamber which is supplied with a fluid pressure to press the substrate against the polishing surface so that the substrate can follow undulations on a polishing surface during its polishing operation. 
     The substrate polishing apparatus may further include a second polishing table having a soft polishing surface which is softer (or of smaller elastic module) than the hard polishing of the first polishing table. The substrate holder body is arranged such that the substrate holder body holds a substrate and, then, bring the substrate into contact with the hard polishing surface to effect a first polishing of the substrate and, thereafter, bring the substrate into contact with the soft polishing surface to effect a second polishing of the substrate. By this apparatus, a highly flattened wafer surface having fewer scratch marks can be obtained. 
     This polishing apparatus may be modified as follows. In stead of the membrane which covers the entire surface of the substrate, the apparatus comprises a substrate support ring provided in the inner space and arranged to be sealingly engaged with the substrate to be held by the substrate holding apparatus, and a flexible seal member sealingly connected between the substrate support ring and the substrate holder body so that a fluid pressure chamber is defined by the substrate holder body, the flexible seal member and the substrate engaged with the substrate support ring. The fluid pressure chamber is arranged to be selectively connected to a pressurized fluid source or a vacuum source. 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a substrate holding apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a longitudinal sectional view showing how the substrate holding apparatus of  FIG. 1  is operated. 
         FIG. 3  is a longitudinal sectional view of a substrate holding apparatus according to a second embodiment of the present invention. 
         FIG. 4A  is a bottom view of an example of a retainer ring having grooves formed on a lower surface thereof. 
         FIG. 4B  is a cross-sectional view, taken along line A-A in  FIG. 4A . 
         FIG. 5A  is a bottom view of another example of a retainer ring having grooves formed on a lower surface thereof. 
         FIG. 5B  is a cross-sectional view, taken along line A-A in  FIG. 5A . 
         FIG. 6A  is a bottom view of a further example of a retainer ring having grooves formed on a lower surface thereof. 
         FIG. 6B  is a cross-sectional view, taken along line A-A in  FIG. 6A . 
         FIG. 7  is a longitudinal sectional view of a substrate holding apparatus according to third embodiment of the present invention. 
         FIG. 8  is a schematic view showing an entire structure of a polishing apparatus including the substrate holding apparatus of  FIGS. 1 to 3 . 
         FIG. 9  is a longitudinal sectional view of a substrate holding apparatus according to a fourth embodiment of the present invention. 
         FIG. 10  is a bottom view of the substrate holding apparatus of  FIG. 9 . 
         FIG. 11  is a longitudinal sectional view showing how the substrate holding apparatus of  FIG. 9  is operated. 
         FIG. 12  is a schematic view showing an entire structure of a polishing apparatus including the substrate holding apparatus of  FIGS. 9 to 11 . 
         FIG. 13  is a plan view of a polishing apparatus which is suitably used for two-stage polishing by using the substrate holding apparatus of the present invention. 
         FIG. 14  is a schematic illustration of a conventional substrate holding apparatus in which a fluid pressure is applied to a wafer so as to press the wafer against a polishing surface, and the fluid pressure is also applied to a retainer ring so as to press the retainer ring against the polishing surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, description is made with regard to embodiments of the present invention, with reference to  FIGS. 1 to 13 . 
       FIG. 1  is a longitudinal sectional view of a substrate holding apparatus  1  according to a first embodiment of the present invention.  FIG. 2  is a longitudinal sectional view showing how the substrate holding apparatus of  FIG. 1  is operated. 
     The substrate holding apparatus  1  is adapted to hold a substrate or, in this embodiment, a semiconductor wafer W to be polished and press the wafer against a polishing surface of a polishing table. As shown in  FIG. 1 , the substrate holding apparatus comprises a dish-like wafer holder body  2  defining an inner space and a retainer ring  3  fixed to the wafer holder body  2 . The wafer holder body  2  is made of a material having high strength and high rigidity, such as a metal and a ceramic, and comprises a circular upper plate  2 A and a circumferential wall portion  2 B extending downward from the upper plate  2 A. The retainer ring  3  is fixed to a lower end of the circumferential wall portion  2 B. The retainer ring  3  is made of a resin material having high rigidity. It should be noted that the retainer ring  3  may be formed integrally with the wafer holder body  2 . 
     The wafer holder body  2  and the retainer ring  3  define an inner space for containing an elastic membrane  4  and an elastic membrane supporting member  5  in a generally cylindrical form. The elastic membrane supporting member  5  holds an outer circumferential portion of the elastic membrane  4 . A flexible sheet  6  made of an elastic membrane extends between the elastic membrane supporting member  5  and the wafer holder body  2 . A fluid chamber  8  having a sealable structure is formed by the wafer holder body  2 , the elastic membrane  4 , the flexible sheet  6  and an inner surface of the wafer holder body. Each of the elastic membrane  4  and the flexible sheet  6  is formed from a rubber material which is excellent in strength and durability, such as an ethylene propylene rubber (EPDM), a polyurethane rubber or a silicone rubber. A pressurized fluid such as pressurized air is supplied to the fluid chamber  8  through a fluid passage  10  comprising a tube and a connector. The pressure of pressurized fluid supplied to the fluid chamber  8  can be varied by means of a regulator. A slight gap is formed between an outer circumferential surface of the elastic membrane  4 , and the wafer holder body  2  and the retainer ring  3 . The elastic membrane  4  and the elastic membrane supporting member  5  are vertically movable relative to the wafer holder body  2  and the retainer ring  3 . 
     For insuring high polishing performance, it is preferred to form the fluid chamber  8  by using an elastic membrane as in this embodiment. However, the elastic membrane  4  may not be used and the wafer may be pressed by direct contact with the fluid. When the elastic membrane  4  is not used, the fluid chamber is formed by the wafer holder body  2  and the rear surface of the wafer to be polished. 
     An annular stopper plate  13  is fixed through a support member  12  to the upper plate  2 A of the wafer holder body  2 . An upper end surface  13   a  of the stopper plate  13  is positioned at a predetermined height and the stopper plate  13  provides a restricting member. When the pressurized fluid is supplied to the fluid chamber  8 , the elastic membrane  4  and the elastic membrane supporting member  5  move as a unit downward relative to the wafer holder body  2 . In this instance, an upper end portion  5   a  of the elastic membrane supporting member  5  engages the upper end surface  13   a  of the stopper plate  13 , thus limiting the downward movement of the elastic membrane  4  and the elastic membrane supporting member  5  to a predetermined range. 
     A chucking plate  14  including a plurality of through-holes  14   h  is provided inside the elastic membrane supporting member  5 . In this embodiment, the chucking plate  14  is fixed to an inner side of the elastic membrane supporting member  5 . However, the chucking plate  14  may be formed integrally with the elastic membrane supporting member  5 . A number of spherical recesses  14   a  are formed on a lower surface of the chucking plate  14 . As shown in  FIG. 2 , when a negative pressure is applied to the fluid chamber  8  through the fluid passage  10  from a vacuum source, the elastic membrane  4  is deformed along the spherical recesses  14   a  of the chucking plate  14 . That is, the portions of the elastic membrane  4  corresponding to the spherical recesses  14   a  of the chucking plate  14  form suction cups and hold the wafer W on a lower surface of the elastic membrane  4 . 
     A plurality of stoppers  17  operated by actuators  16  such as air cylinders is provided in the upper plate  2 A of the wafer holder body  2 . By operating the actuators  16 , the stoppers  17  are protruded downward by a predetermined length as shown in  FIG. 2 . When a negative pressure is applied to the fluid chamber  8 , the elastic membrane supporting member  5  moves upward together with the elastic membrane  4  and the upper end portion  5   a  of the elastic membrane supporting member  5  abuts against the stoppers  17 , thus limiting the upward movement of the elastic membrane supporting member  5  and the elastic membrane  4  to a predetermined range. That is, the stoppers  17  provide restricting members having an adjustable heightwise position. When the actuator  16  is arranged so as to have a mechanism such as an air cylinder capable of generating a variable pressure and the stoppers  17  are protruded during polishing so as to press the elastic membrane supporting member  5  in a downward direction, an outer circumferential portion of the wafer W can be mechanically pressed against the polishing surface. A wafer holder drive shaft  18  is provided above the upper plate  2 A of the wafer holder body  2 . The drive shaft  18  and the wafer holder body  2  are connected through a universal joint  19 . 
     The universal joint  19  transmits pressure and torque of the drive shaft  18  to the wafer holder body  2  while permitting inclination of the drive shaft  18  and the wafer holder body  2  relative to each other. The universal joint  19  comprises a spherical bearing mechanism which permits inclination of the wafer holder body  2  and the drive shaft  18  relative to each other and a torque transmitting mechanism which transmits rotation of the drive shaft  18  to the wafer holder body  2 . The spherical bearing mechanism comprises a spherical recess  18   a  formed at a central portion of a lower surface of the drive shaft  18 , a spherical recess  2   a  formed at a central portion of an upper surface of the upper plate  2 A and a bearing ball  21  made of a material having high hardness, such as a ceramic, provided between the spherical recess  18   a  and the spherical recess  2   a.    
     The torque transmitting mechanism comprises a drive pin (not shown) fixed to the drive shaft  18  and a driven pin (not shown) fixed to the upper plate  2 A. The two pins are capable of moving vertically relative to each other and engaging at different contact positions. Therefore, a torque of the drive shaft  18  is surely transmitted to the wafer holder body  2  even when the wafer holder body  2  is inclined. 
     Next, explanation is made with regard to operation of the wafer holder  1  arranged as mentioned above. 
     The wafer holder  1  as a whole is moved to a position for transferring a wafer and the fluid chamber  8  is connected to the vacuum source through the fluid passage  10 . Consequently, as shown in  FIG. 2 , the elastic membrane  4  is deformed and holds the wafer W on the lower surface thereof due to the effect of suction cups formed along the recesses  14   a  of the chucking plate  14 . While holding the wafer W on the elastic membrane  4 , the wafer holder  1  as a whole is moved to a position above a polishing table (designated by reference numeral  30  in  FIG. 8 ) having a polishing surface (such as a polishing pad). The wafer W and the retainer ring  3  are then pressed against the polishing surface to thereby start polishing. An outer circumferential edge of the wafer W is held by the retainer ring  3  so that the wafer W is not separated from the wafer holder  1 . 
     For polishing the wafer W, an air cylinder (designated by reference numeral  33  in  FIG. 8 ) connected to the drive shaft  18  is operated to thereby press the retainer ring  3  fixed to the wafer holder body  2  against the polishing surface of the polishing table under a predetermined pressure. In this state, the pressurized fluid is supplied under a predetermined pressure to the fluid chamber  8  to thereby press the wafer W against the polishing surface of the polishing table. The pressure applied to the wafer W for polishing is adjusted to a desired level by controlling the pressure of pressurized fluid supplied to the fluid chamber  8 . Thus, the pressure of fluid in the fluid chamber  8  is applied to the wafer W, so that it is possible to obtain a uniform pressure distribution for polishing across an entire surface of the wafer W from the center to the circumferential edge thereof, regardless of the thickness of the wafer W. This enables uniform polishing of the entire surface of the wafer W. 
     During polishing, pressure substantially equal to or slightly higher than that applied to the wafer W is applied to the retainer ring  3  through the air cylinder, so that the polishing surface outside the wafer W is pressed under a pressure substantially equal to that of the wafer W. Therefore, a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring  3  outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented. 
       FIG. 3  is a vertical cross-sectional view of a substrate holding apparatus according to a second embodiment of the present invention. In this embodiment, the chucking plate  14  is not provided and a space inside the elastic membrane supporting member  5  is empty. Instead of providing the chucking plate  14 , a plurality of through-holes  4   h  is formed in the elastic membrane  4  in an area between the center and an outer circumferential portion thereof. Therefore, when a negative pressure is applied from the vacuum source through the fluid passage  10  to the fluid chamber  8  for holding the wafer W on the lower surface of the elastic membrane  4 , the wafer W is held due to the effect of vacuum force directly applied through the through-holes  4   h.    
     In the wafer holder  1  in this embodiment, during polishing, as is in the first embodiment, a pressurized fluid is supplied to the fluid chamber  8  so that a wafer W is pressed against a polishing surface by the elastic membrane  4  with the through-holes  4   h  in the membrane  4  being closed by the wafer W. 
     In the embodiments shown in  FIGS. 1 to 3 , the retainer ring  3  is fixedly connected to the wafer holder body  2  having a rigid construction and the retainer ring  3  is vertically moved by vertically moving the wafer holder body  2 . By this arrangement, the pressure applied to the wafer holder body  2  can be utilized as a pressure for pressing the retainer ring  3 . Further, because the retainer ring  3  is fixed to the wafer holder body  2 , undesirable lateral (or radial) movement of the retainer ring  3  can be prevented. Therefore, the distance between the retainer ring  3  and the circumferential edge of the wafer can be constantly minimized, and uniformity and stability in the polishing of the outer circumferential portion of the wafer W can be ensured. 
     Since the retainer ring  3  is fixedly connected to the wafer holder body  2 , the retainer ring can be imparted with high rigidity and the behavior of the retainer ring during polishing can be stabilized. The wafer holding pressurizing mechanism of a floating type structure follows undulations in the polishing surface inside the retainer ring which is stable and has high rigidity. Consequently, the behavior of the retainer ring can be stabilized, even on a hard polishing surface, to thereby achieve excellent stability of the polishing of the wafer. 
     By adjustably positioning the stoppers  17 , upward movement of the elastic membrane supporting member  5  is restricted at a predetermined height, thus limiting upward movement of the chucking plate  14  to a predetermined range. This prevents warpage of the wafer held on the elastic membrane  4  and a lowering of product quality such as breakage of the wafer. Further, by protruding the stoppers  17  by using the cylinder mechanism and pressing the elastic membrane supporting member  5  downward during polishing, the pressure for pressing the wafer W against the polishing surface can be varied on a part of the wafer surface, thus making it possible to obtain desired polishing properties in relation to the profile of the surface to be polished. 
     In the wafer holder shown in  FIG. 3 , in which the through-holes  4   h  are formed in the elastic membrane  4 , the elastic membrane  4  directly holds the wafer W due to the effect of vacuum force applied through the through-holes  4   h.  Therefore, there is no problem of a change in properties of the elastic membrane  4  due to contact with the chucking plate  14  shown in  FIG. 1 . This enhances stability of uniform polishing of the wafer. Further, for holding the wafer W, it is unnecessary to utilize the effect of a suction cup formed by using the chucking plate  14 . Therefore, there is no need to provide the chucking plate  14  and only the elastic membrane supporting member  5  in an annular form is necessary. 
       FIGS. 4A to 6B  show examples of retainer rings having grooves formed on lower surfaces thereof.  FIG. 4A ,  FIG. 5A  and  FIG. 6A  are bottom views of the retainer rings.  FIG. 4B ,  FIG. 5B  and  FIG. 6B  are cross-sectional views, taken along lines A-A in  FIG. 4A ,  FIG. 5A  and  FIG. 6A , respectively. 
     In the example of  FIGS. 4A and 4B , a plurality of radial grooves  3   g - 1  (each extending in a radial direction indicated by an arrow r) is formed on the lower surface of the retainer ring  3 . 
     In the example of  FIGS. 5A and 5B , a plurality of grooves  3   g - 2  inclined at a predetermined angle □ relative to the radial direction r is formed on the lower surface of the retainer ring  3 . 
     In the example of  FIGS. 6A and 6B , a plurality of radial grooves  3   g - 3  (each extending in the radial direction r) is formed on the lower surface of the retainer ring  3 . The radial grooves  3   g - 3  extend from an outer circumferential edge of the retainer ring  3  to an intermediate position at a slight distance from an inner circumferential edge of the retainer ring. 
     Because the retainer ring presses the polishing surface (such as a polishing pad) outside the wafer, when the entire lower surface of the retainer ring is flat, the abrasive liquid (slurry) might not smoothly flow into an area inside the retainer ring. That is, the amount of abrasive liquid supplied to the wafer becomes insufficient, leading to a lowering of uniformity in the polishing of the wafer and a lowering of a rate of polishing. Further, the wafer and the polishing surface are subject to high friction, leading to a problem, namely a high power load on the polishing apparatus. 
     As a countermeasure, it is considered to reduce the width of the lower surface of the retainer ring so as to minimize the effect of the retainer ring of disturbing the inflow of abrasive liquid. In this case, however, the flat portion of the lower surface of the retainer ring is reduced in area due to non-uniform wear of the lower surface of the retainer ring, making it difficult for the retainer ring to press the polishing surface in a stable manner. Further, the amount of wear of the retainer ring becomes large, thereby reducing the life of the retainer ring. 
     In the present invention, as shown in  FIGS. 4A and 4B , the grooves  3   g - 1  may be formed on the lower surface of the retainer ring  3  which is brought into contact with the polishing surface. By this arrangement, the abrasive liquid smoothly flows into an area inside the retainer ring  3  to thereby secure the supply of abrasive liquid to the wafer, thus preventing a lowering of uniformity in the polishing of the wafer and a lowering of a rate of polishing. As shown in  FIGS. 5A and 5B , the grooves  3   g - 2  inclined relative to the radial direction may be formed on the lower surface of the retainer ring. The direction of inclination of the grooves  3   g - 2  corresponds to a rotation direction R of the retainer ring  3 . This enhances smooth flow of the abrasive liquid to the wafer. However, when there is a high possibility of accelerating polishing on a part of the wafer due to oversupply of the abrasive liquid, the grooves  3   g - 3  in  FIGS. 6A and 6B  may be formed on the lower surface of the retainer ring. The grooves  3   g - 3  do not extend to the inner circumferential edge of the retainer ring  3 , so as to leave a wall portion for preventing oversupply of the abrasive liquid, thereby preventing excessive polishing of a part of the wafer while securing the supply of abrasive liquid to the wafer inside the retainer ring. 
     Another advantage of the grooves  3   g - 3  is explained below. When the grooves extend to the inner circumferential edge of the retainer ring, the following problems arise. That is, when relative rotation between the wafer holder and the wafer occurs, an angular portion of the outer circumferential surface of the wafer, which is formed by forming an orientation flat or a notch in the wafer, makes contact with the groove of the retainer ring. Consequently, a portion around the groove at the inner circumferential edge of the retainer ring is likely to become worn due to impact. The orientation flat is especially liable to cause such a wear. Further, pronounced noise is even generated due to impact when the orientation flat makes contact with the groove. The wear at the groove of the retainer ring can be prevented by leaving a wall portion at a terminal end of the groove of the retainer ring as shown in  FIGS. 6A and 6B . 
     The formation of grooves on the lower surface of the retainer ring can be applied to not only wafer holders such as those shown in  FIGS. 1 to 3 , but also various wafer holders as long as they are capable of pressing the retainer ring against the polishing surface.  FIG. 7  shows an illustrative example of a wafer holder other than that shown in  FIGS. 1 to 3 . 
     In the example of  FIG. 7 , a wafer holder  101  comprises a wafer holder body  102  and a holding plate  103  for holding an upper surface of a substrate to be polished, such as a semiconductor wafer W. The holding plate  103  is made of a material having high rigidity, such as a ceramic, and has a wafer holding surface  103   a  which is adapted so as not to be deformed. An elastic mat  106  is adhered to a lower surface of the holding plate  103 . 
     In order to hold the wafer W on the lower surface of the holding plate  103 , a retainer ring (or guide ring)  107  for holding an outer circumferential surface of the wafer W is provided on an outer circumferential surface of the wafer holder  101 . A chamber C is formed between the holding plate  103  and the wafer holder body  102 . The chamber C is used for applying a fluid pressure through communication holes  103   m  formed in the holding plate  103  to a back side of the wafer W. By evacuating the chamber C by means of a vacuum pump, the wafer W can be held on the wafer holding surface  103   a  due to the effect of vacuum force. It should be noted that for separating the wafer W from the wafer holding surface  103   a  of the holding plate  103 , a liquid such as pure water is supplied to the chamber C. 
     In the example of  FIG. 7 , a wafer held on the lower surface of the wafer holder is pressed against the polishing surface by an air cylinder for moving the wafer holder drive shaft  18  in a vertical direction. The retainer ring  107  having grooves  103   g - 3  formed on a lower surface thereof is disposed so that it surrounds the wafer. The retainer ring  107  is independently pressed against the polishing surface due to the effect of pressure of a pressurized fluid supplied to a space  143 . The space  143  is defined by a lower seal ring  140 A and an upper seal ring  140 B. The upper seal ring  140 B comprises a ring  141   b  fixed to a mounting flange portion  102   a  of the wafer holder body  102  and lip seals  142   b  for sealing spaces between the ring  141   b  and a mounting flange portion  102   a  of the wafer holder body  102 . The lower seal ring  140 A comprises a ring  141   a  for pressing the retainer ring  107  and lip seals  142   a  provided radially inside and outside the retainer ring  141   a  for sealing spaces between the ring  141   a  and the mounting flange portion  102   a  of the wafer holder body  102 . The retainer ring  107  comprises a first retainer ring member  107   a  which is vertically movable and a second retainer ring member  107   b  fixed to the wafer holder body  102 . In the embodiment of  FIG. 7 , even when the retainer ring  107  is worn, the retainer ring can be pressed under a desired pressure. The grooves  103   g - 3  formed in the retainer ring  107  are of the same type as the grooves  3   g - 3  in  FIGS. 6A and 6B . That is, the grooves  103   g - 3  extend from the radially outer peripheral edge of the retainer ring and short of the radially inner peripheral edge of the same. The effects of the grooves  103   g - 3  are the same as those described above in connection with the grooves  3   g - 3 . The grooves  103   g - 3  may be inclined at a predetermined angle (□) relative to the radial direction r, as shown in  FIG. 5A . 
       FIG. 8  is a cross-sectional view showing an entire structure of a polishing apparatus including the substrate holding apparatus of  FIGS. 1 to 3 . As shown in  FIG. 8 , the polishing table  30  has a polishing pad  31  attached to an upper surface thereof and is provided below the wafer holder  1 . 
     The wafer holder  1  is connected to the drive shaft  18  through the universal joint  19 . The drive shaft  18  is connected to the air cylinder  33  fixed to a wafer holder head  32 . The drive shaft  18  is vertically moved by means of the air cylinder  33 , thereby moving the wafer holder  1  as a whole in a vertical direction and pressing the retainer ring  3  fixed to the wafer holder body  2  against the polishing table  30 . 
     The drive shaft  18  is connected to a rotary cylinder  34  through a key (not shown). The rotary cylinder  34  has a timing pulley  35  on an outer circumferential surface thereof. The timing pulley  35  is connected through a timing belt  36  to a timing pulley  38  which is connected to a wafer holder motor  37  fixed to the wafer holder head  32 . Therefore, the rotary cylinder  34  and the drive shaft  18  are rotated as a unit by the wafer holder motor  37  through the timing pulley  38 , the timing belt  36  and the timing pulley  35  to thereby rotate the wafer holder  1 . The wafer holder head  32  is supported by a wafer holder head shaft  39  fixedly supported by a frame (not shown). 
     The air cylinder  33  and the fluid chamber  8  are, respectively, connected through a regulator R 1  and a regulator R 2  to a pressurized air source  24 . The pressure of pressurized air supplied to the air cylinder  33  is controlled by the regulator R 1 , to thereby adjust the pressure for pressing the retainer ring  3  against the polishing pad  31 . The pressure of pressurized air supplied to the fluid chamber  8  is controlled by the regulator R 2 , to thereby adjust the pressure for pressing the wafer W against the polishing pad  31 . 
     An abrasive liquid supply nozzle  40  is provided above the polishing table  30 . An abrasive liquid Q is supplied onto the polishing pad  31  on the polishing table  30  through the abrasive liquid supply nozzle  40 . 
     In this polishing apparatus, for polishing, while holding the wafer W on the lower surface of the elastic membrane  4  of the wafer holder  1 , the air cylinder  33  is operated to thereby press the retainer ring  3  fixed to the wafer holder body  2  toward the polishing table  30 , and pressurized air is supplied to the fluid chamber  8  to thereby press the wafer W against the polishing pad  31  on the polishing table  30 , which is rotating. On the other hand, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle  40  so as to retain the abrasive liquid Q on the polishing pad  31 . Thus, polishing is conducted while retaining the abrasive liquid Q between the wafer surface to be polished (a lower surface of the wafer W) and the polishing pad  31 . 
     For polishing, the pressure for pressing the retainer ring  3  against the polishing pad  31 , which is applied through the air cylinder  33 , and the pressure for pressing the wafer W against the polishing pad  31 , which is applied by means of pressurized air supplied to the fluid chamber  8 , are adjusted to a desired level. During polishing, the pressure for pressing the retainer ring  3  against the polishing pad  31  can be varied by means of the regulator R 1 , and the pressure for pressing the wafer W against the polishing pad  31  can be varied by means of the regulator R 2 . Consequently, during polishing, by controlling the pressure for pressing the retainer ring  3  against the polishing pad  31  and the pressure for pressing the wafer W against the polishing pad  31 , a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring  3  outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented. 
     In the present invention, the polishing surface formed on the polishing table may be prepared by a polishing pad such as that described above or an abrasive plate comprising fixed abrasive particles. As the polishing pad, various commercially available polishing pads, for example, SUBA800, IC-1000 and IC-1000/SUBA400 (a two-layered cloth) manufactured and sold by Rodel, Inc., and Surfin xxx-5 and Surfin 000 manufactured and sold by FUJIMI INCORPORATED can be used. The SUBA800, Surfin xxx-5 and Surfin 000 are non-woven cloths which comprise fibers bound by using a urethane resin. The IC-1000 comprises a single layer of hard, foamed polyurethane, which has a porous structure and includes a number of fine recesses or holes formed on a surface thereof. 
     The abrasive plate comprises fixed abrasive particles which are bound by using a binder and formed into a plate. Polishing is conducted by utilizing the abrasive particles freed from the abrasive plate. The abrasive plate comprises the abrasive particles, the binder and pores. Examples of abrasive particles include particles of cerium oxide (CeO2) having an average particle diameter of 0.5 □m or less. As the binder, for example, an epoxy resin is used. The abrasive plate provides a hard polishing surface. The abrasive plate may have a two-layered structure comprising a thin layer of fixed abrasive particles and an elastic polishing pad adhered to a lower side of the fixed abrasive particles. The above-mentioned IC-1000 also provides a hard polishing surface. 
     The wafer holder of the present invention is suitable for use with a polishing member having a hard polishing surface, especially suitable for a polishing surface having a modulus of elasticity of compression of 19.6 MPa (200 kg/cm2) or more. 
     In a conventional wafer holder, a wafer is held on a backing pad provided on a rigid wafer holder body. Because the polishing pad is elastic, shocks on the wafer are absorbed by the polishing pad. However, when a hard polishing surface is used, undulation on the polishing surface is transferred to and affects the wafer surface to be polished. Further, a mark corresponding to a vacuum opening of the backing pad is formed on a rear surface of the wafer. 
     On the other hand, in the wafer holder of the present invention in which a wafer is held on an elastic membrane by utilizing fluid pressure, shocks on the wafer due to a hard, undulating polishing surface can be absorbed by the fluid pressure acting on the rear surface of the wafer. Thus, even when the polishing surface is hard, high polishing performance can be maintained and no mark corresponding to the vacuum opening is formed on the wafer. 
     Further, in the present invention, since the retainer ring is fixedly connected to the wafer holder body, the retainer ring can be imparted with high rigidity and unstable movement of the retainer ring can be suppressed, thereby stabilizing polishing performance. 
       FIG. 9  is a longitudinal sectional view of a substrate holding apparatus  1  according to another embodiment of the present invention.  FIG. 10  is a bottom view of the substrate holding apparatus of  FIG. 9 .  FIG. 11  is a sectional view showing how the substrate holding apparatus of  FIG. 9  is operated. 
     The substrate holding apparatus  1  is adapted to hold a substrate to be polished, such as a semiconductor wafer, and press the wafer against a polishing surface of a polishing table. As shown in  FIG. 9 , the substrate holding apparatus of this embodiment comprises a dish-like wafer holder body  2  defining an inner space and a retainer ring  3  fixed to a lower end of the wafer holder body  2 . The wafer holder body  2  is made of a material having high strength and high rigidity, such as a metal and a ceramic, and comprises a circular upper plate  2 A and a circumferential wall portion  2 B extending downward from the upper plate  2 A. The retainer ring  3  is fixed to a lower end of the circumferential wall portion  2 B. The retainer ring  3  is made of a resin material having high rigidity. It should be noted that the retainer ring  3  may be formed integrally with the wafer holder body  2 . 
     The wafer holder body  2  and the retainer ring  3  define an inner space for containing an elastic membrane  4  and an elastic membrane supporting member  5  in a generally disk-like form. The elastic membrane supporting member  5  holds an outer circumferential portion of the elastic membrane  4 . A flexible sheet  6  made of an elastic membrane extends between the elastic membrane supporting member  5  and the wafer holder body  2 . A fluid chamber  8  is formed by the wafer holder body  2 , the elastic membrane  4 , the flexible sheet  6  and an inner surface of the wafer holder body  2 . Each of the elastic membrane  4  and the flexible sheet  6  is formed from a rubber material which is excellent in strength and durability, such as an ethylene propylene rubber (EPDM), a polyurethane rubber or a silicone rubber. A pressurized fluid such as pressurized air is supplied to the fluid chamber  8  through a fluid passage  10  comprising a tube and a connector. The pressurized fluid supplied to the fluid chamber  8  flows through through-holes  5   h  formed in the elastic membrane supporting member  5  to a rear surface of the elastic membrane  4 , thus applying the pressure of pressurized fluid to the rear surface of the elastic membrane  4 . The pressure of pressurized fluid supplied to the fluid chamber  8  can be varied by means of a regulator. A slight gap is formed between an outer circumferential surface of the elastic membrane  4  and the wafer holder body  2  and the retainer ring  3 . The elastic membrane  4  and the elastic membrane supporting member  5  are vertically movable relative to the wafer holder body  2  and the retainer ring  3 . 
     For insuring high polishing performance, it is preferred to form the fluid chamber  8  by using an elastic membrane as in this embodiment. However, the elastic membrane  4  may not be used so that the wafer may be pressed by direct contact with the fluid. When the elastic membrane  4  is not used, the fluid chamber is formed by the wafer holder body  2  and the rear surface of the wafer to be polished. 
     An annular stopper plate  13  is fixed through a support member  12  to the upper plate  2 A of the wafer holder body  2 . An upper end surface  13   a  of the stopper plate  13  is positioned at a predetermined height and the stopper plate  13  provides a restricting member. When the pressurized fluid is supplied to the fluid chamber  8 , the elastic membrane  4  and the elastic membrane supporting member  5  move as a unit downward relative to the wafer holder body  2 . In this instance, an upper end portion  5   a  of the elastic membrane supporting member  5  engages the upper end surface  13   a  of the stopper plate  13 , thus limiting the downward movement of the elastic membrane  4  and the elastic membrane supporting member  5  to a predetermined range. The elastic membrane  4  includes a plurality of openings  4   a  formed therein. Vacuum portions  14  each having a communication hole  14   h  are exposed from the respective openings  4   a.  The vacuum portions  14  are formed at a central portion of the elastic membrane supporting member  5 . In this embodiment, the vacuum portions  14  are formed integrally with the elastic membrane supporting member  5 . However, the elastic membrane supporting member  5  may be formed into an annular form and a disk-like chucking plate including a plurality of vacuum portions  14  may be employed so that the chucking plate is fixed to an inner side of the elastic membrane supporting member  5 . 
     As shown in  FIG. 10 , five openings  4   a  are formed at a central portion of the elastic membrane  4 , and the vacuum portions  14  are exposed from the respective openings  4   a.  As shown in  FIG. 9 , a lower end of the communication hole  14   h  of each vacuum portion  14  is open. All the communication holes  14   h  join inside the elastic membrane supporting member  5  and are connected through a tube  11  in the fluid chamber  8  to a vacuum source. When a negative pressure is applied to the open ends of the communication holes  14   h  through the vacuum source, a semiconductor wafer W is held on the vacuum portions  14  under vacuum force. As shown in  FIG. 9 , during polishing, the vacuum portions  14  are located inward of a lower end surface of the elastic membrane  4  and do not protrude from the lower end surface of the elastic membrane  4 . When the wafer W is held under vacuum force, as shown in  FIG. 11 , lower end surfaces of the vacuum portions  14  become substantially flush with the lower end surface of the elastic membrane  4 . An elastic sheet  15  such as a thin rubber sheet is attached to the lower end surface of each vacuum portion  14  so that the vacuum force is applied to the wafer through the thin rubber sheet. 
     A wafer holder drive shaft  18  is provided above the upper plate  2 A of the wafer holder body  2 . The drive shaft  18  and the wafer holder body  2  are connected through a universal joint  19 . The universal joint  19  transmits pressure and torque of the drive shaft  18  to the wafer holder body  2  while permitting inclination of the drive shaft  18  and the wafer holder body  2  relative to each other. The universal joint  19  comprises a spherical bearing mechanism which permits inclination of the wafer holder body  2  and the drive shaft  18  relative to each other and a torque transmitting mechanism which transmits rotation of the drive shaft  18  to the wafer holder body  2 . The spherical bearing mechanism comprises a spherical recess  18   a  formed at a central portion of a lower surface of the drive shaft  18 , a spherical recess  2   a  formed at a central portion of an upper surface of the upper plate  2 A and a bearing ball  21  made of a material having high hardness, such as a ceramic, provided between the spherical recess  18   a  and the spherical recess  2   a.    
     The torque transmitting mechanism comprises a drive pin (not shown) fixed to the drive shaft  18  and a driven pin (not shown) fixed to the upper plate  2 A. The two pins are capable of moving vertically relative to each other and engaging at different contact positions. Therefore, a torque of the drive shaft  18  is surely transmitted to the wafer holder body  2  even when the wafer holder body  2  is inclined. 
     Next, explanation is made with regard to operation of the wafer holder  1  explained with reference to  FIGS. 9-11 . 
     The wafer holder  1  as a whole is moved to a position for transferring a wafer and the communication holes  14   h  of the vacuum portions  14  are connected to the vacuum source through the tube  11 . Consequently, as shown in  FIG. 11 , the wafer W is held on the lower end surfaces of the vacuum portions  14  due to the effect of vacuum force applied through the communication holes  14   h.  In this instance, a slight positive pressure is applied to the fluid chamber  8  so as to prevent upward movement of the elastic membrane supporting member  5  and the vacuum portions  14 , and the upper end portion  5   a  of the elastic membrane supporting member  5  engages the upper end surface  13   a  of the stopper plate  13  to thereby hold the elastic membrane supporting member  5  and the vacuum portions  14  at a predetermined position. While holding the wafer W under vacuum force, the wafer holder  1  is moved to a position above a polishing table (designated by reference numeral  30  in  FIG. 12 ) having a polishing surface (such as a polishing pad). The wafer W and the retainer ring  3  are then pressed against the polishing surface to thereby start polishing. An outer circumferential edge of the wafer W is held by the retainer ring  3  so that the wafer W is not separated from the wafer holder  1 . 
     For polishing the wafer W, an air cylinder (designated by reference numeral  33  in  FIG. 12 ) connected to the drive shaft  18  is operated, to thereby press the retainer ring  3  fixed to the wafer holder body  2  against the polishing surface of the polishing table under a predetermined pressure. In this state, the pressurized fluid is supplied under a predetermined pressure to the fluid chamber  8 , to thereby press the wafer W against the polishing surface of the polishing table. The pressure applied to the wafer W for polishing is adjusted to a desired level by controlling the pressure of pressurized fluid supplied to the fluid chamber  8 . Thus, the fluid pressure is directly applied to the wafer W at its portion corresponding to the opening  4   a,  while the fluid pressure is indirectly applied to the remaining portion of the wafer W through the elastic membrane  4 . However, the pressures applied to these two portions of the wafer W are equal. That is, since the pressure of fluid in the fluid chamber  8  is applied to an entire surface of the wafer W, it is possible to obtain a uniform pressure distribution for polishing across an entire surface of the wafer W from the center to the circumferential edge thereof, regardless of the thickness of the wafer W. This enables uniform polishing of the entire surface of the wafer W. During polishing, the elastic membrane  4  is in intimate contact with the rear surface of the wafer W around the openings  4   a,  so that there is substantially no leakage of the pressurized fluid from the fluid chamber  8  to the outside. 
     During polishing, pressure substantially equal to or slightly higher than that applied to the wafer W is applied to the retainer ring  3  through the air cylinder, so that the polishing surface outside the wafer W is pressed under a pressure substantially equal to that of the wafer W. Therefore, a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring  3  outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented. 
     In the wafer holder shown in  FIGS. 9 to 11 , the retainer ring  3  is fixedly connected to the wafer holder body  2  having a rigid construction and the retainer ring  3  is vertically moved by vertically moving the wafer holder body  2 . By this arrangement, the pressure applied to the wafer holder body  2  can be utilized as a pressure for pressing the retainer ring  3 . Further, because the retainer ring  3  is fixed to the wafer holder body  2 , undesirable lateral (or radial) movement of the retainer ring  3  can be prevented. Therefore, the distance between the retainer ring  3  and the circumferential edge of the wafer surface can be constantly minimized, and uniformity and stability in the polishing of the outer circumferential portion of the wafer W can be ensured. 
     Since the retainer ring  3  is fixedly connected to the wafer holder body  2 , the retainer ring can be imparted with high rigidity and the behavior of the retainer ring during polishing can be stabilized. The wafer holding pressurizing mechanism of a floating type structure follows undulation of the polishing surface inside the retainer ring which is stable and has high rigidity. Consequently, the behavior of the retainer ring can be stabilized even on a hard polishing surface to thereby achieve excellent stability of the polishing of the wafer. 
     The openings  4   a  are formed in the elastic membrane  4  and the vacuum portions  14  having the communication holes  14   h  are provided in the openings  4   a.  The wafer W is held due to the effect of vacuum force applied through the communication holes  14   h  connected to the vacuum source. That is, the vacuum portions  14  directly hold the wafer W due to the effect of vacuum force. Therefore, there is no need to impart the elastic membrane  4  with a suction cup-like configuration. Therefore, a change in properties of the elastic membrane  4  is unlikely to occur so that uniformity in the polishing of wafers can be stably maintained. 
       FIG. 12  is a cross-sectional view showing an entire structure of a polishing apparatus including the substrate holding apparatus of  FIGS. 9 to 10 . As shown in  FIG. 12 , the polishing table  30  has a polishing pad  31  attached to an upper surface thereof and is provided below the wafer holder  1 . 
     The wafer holder  1  is connected to the drive shaft  18  through the universal joint  19 . The drive shaft  18  is connected to the air cylinder  33  fixed to a wafer holder head  32 . The drive shaft  18  is vertically moved by means of the air cylinder  33 , thereby moving the wafer holder  1  as a whole in a vertical direction and pressing the retainer ring  3  fixed to the wafer holder body  2  against the polishing table  30 . 
     The drive shaft  18  is connected to a rotary cylinder  34  through a key (not shown). The rotary cylinder  34  has a timing pulley  35  on an outer circumferential surface thereof. The timing pulley  35  is connected through a timing belt  36  to a timing pulley  38  which is connected to a wafer holder motor  37  fixed to the wafer holder head  32 . Therefore, the rotary cylinder  34  and the drive shaft  18  are rotated as a unit by the wafer holder motor  37  through the timing pulley  38 , the timing belt  36  and the timing pulley  35  to thereby rotate the wafer holder  1 . The wafer holder head  32  is supported by a wafer holder head shaft  39  fixedly supported by a frame (not shown). 
     The air cylinder  33  and the fluid chamber  8  are, respectively, connected through a regulator R 1  and a regulator R 2  to a pressurized air source  24 . The pressure of pressurized air supplied to the air cylinder  33  is controlled by the regulator R 1  to thereby adjust the pressure for pressing the retainer ring  3  against the polishing pad  31 . The pressure of pressurized air supplied to the fluid chamber  8  is controlled by the regulator R 2  to thereby adjust the pressure for pressing the wafer W against the polishing pad  31 . The communication holes  14   h  of the vacuum portions  14  are connected through a valve V to a vacuum source  25  such as a vacuum pump. 
     An abrasive liquid supply nozzle  40  is provided above the polishing table  30 . An abrasive liquid Q is supplied onto the polishing pad  31  on the polishing table  30  through the abrasive liquid supply nozzle  40 . 
     In this polishing apparatus, for transferring the wafer W, the communication holes  14   h  of the vacuum portions  14  are communicated with the vacuum source  25  to thereby apply a vacuum force to the wafer W for holding the wafer W on the vacuum portions  14 . For polishing, the vacuum force applied to the wafer W through the vacuum portions  14  is released and, while holding the wafer W on the lower end surface of the elastic membrane  4  of the wafer holder  1 , the air cylinder  33  is operated to thereby press the retainer ring  3  fixed to the wafer holder body  2  toward the polishing table  30 , and pressurized air is supplied to the fluid chamber  8 , to thereby press the wafer W against the polishing pad  31  on the polishing table  30 , which is rotating. On the other hand, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle  40  so as to retain the abrasive liquid Q on the polishing pad  31 . Thus, polishing is conducted while retaining the abrasive liquid Q between the wafer surface to be polished (a lower surface of the wafer W) and the polishing pad  31 . 
     For polishing, the pressure for pressing the retainer ring  3  against the polishing pad  31 , which is applied through the air cylinder  33 , and the pressure for pressing the wafer W against the polishing pad  31 , which is applied by means of pressurized air supplied to the fluid chamber  8 , are adjusted to a desired level. During polishing, the pressure for pressing the retainer ring  3  against the polishing pad  31  can be varied by means of the regulator R 1 , and the pressure for pressing the wafer W against the polishing pad  31  can be varied by means of the regulator R 2 . Consequently, during polishing, by controlling the pressure for pressing the retainer ring  3  against the polishing pad  31  and the pressure for pressing the wafer W against the polishing pad  31 , a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring  3  outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented. 
     In this embodiment, the polishing surface formed on the polishing table may be prepared by a polishing pad such as that described above or fixed abrasives. As the polishing pad, various commercially available polishing pads, for example, SUBA800, IC-1000 and IC-1000/SUBA400 (a two-layered cloth) manufactured and sold by Rodel, Inc., and Surfin xxx-5 and Surfin 000 manufactured and sold by FUJIMI INCORPORATED can be used. The SUBA800, Surfin xxx-5 and Surfin 000 are non-woven cloths which comprise fibers bound by using a urethane resin. The IC-1000 comprises a single layer of hard, foamed polyurethane, which has a porous structure and includes a number of fine recesses or holes formed on a surface thereof. 
     The fixed abrasives comprise particles which are bound by using a binder and formed into a plate. Polishing is conducted by utilizing abrasive particles freed from the abrasive plate. The abrasive plate comprises the abrasive particles, the binder and pores. Examples of abrasive particles include particles of cerium oxide (CeO2) having an average particle diameter of 0.5 □m or less. As the binder, for example, an epoxy resin is used. The fixed abrasives provide a hard polishing surface. The fixed abrasives are generally formed into a disk-like plate and may have a two-layered structure comprising a thin layer of fixed abrasive particles and an elastic polishing pad adhered to a lower side of the fixed abrasive particles. The above-mentioned IC-1000 also provides a hard polishing surface. 
     The wafer holder of this embodiment is suitable for use with a polishing member having a hard polishing surface, and especially suitable for a polishing surface having a modulus of elasticity of compression of 19.6 MPa (200 kg/cm2) or more. 
     In a conventional wafer holder, a wafer is held on a backing pad provided on a rigid wafer holder body. Because the polishing pad is elastic, shocks on the wafer are absorbed mainly by the polishing pad. However, when a hard polishing surface is used, undulation on the polishing surface is transferred to and affects the wafer surface to be polished. Further, a mark corresponding to a vacuum opening of the backing pad is formed on a rear surface of the wafer. 
     On the other hand, in the wafer holder of this embodiment in which a wafer is held on an elastic membrane by utilizing fluid pressure, shocks on the wafer due to a hard, undulating polishing surface can be absorbed by the fluid pressure acting on the rear surface of the wafer. Thus, even when the polishing surface is hard, a high polishing performance can be maintained and no mark corresponding to the vacuum opening is formed on the wafer. 
     Further, in this embodiment, since the retainer ring is fixedly connected to the wafer holder body, the retainer ring can be imparted with high rigidity and unstable movement of the retainer ring can be suppressed, thereby stabilizing polishing performance. 
     A highly flattened wafer surface having less scratch marks can be obtained by conducting two-stage polishing, that is, first conducting polishing of the wafer on a hard abrasive plate while the wafer is held by the wafer holder of the present invention (i.e., the wafer holder which holds a wafer under fluid pressure) and then conducting final polishing of the wafer on a polishing pad which is soft as compared to the abrasive plate while the wafer is held by the wafer holder of the present invention. It should be noted that “soft” means having a low modulus of elasticity. 
       FIG. 13  is a plan view of a polishing apparatus which is suitably used for the above-mentioned two-stage polishing by using the wafer holder of the present invention. The polishing apparatus of  FIG. 13  comprises two polishing tables  30 . An abrasive plate or fixed abrasive polishing tool  29  is attached to one polishing table  30 , to thereby provide a first polishing unit  41   a.  A polishing pad  31  is attached to the other polishing table  30 , to thereby provide a second polishing unit  41   b.  The second polishing unit  41   b  can be used for final polishing. The polishing pad  31  of the second polishing unit  41   b  has a lower elastic modulus than that of the polishing tool fixed abrasive polishing tool of the first polishing unit  41   a.  A wafer holder  1  has the same structure as that shown in  FIG. 1  to  FIG. 3  or  FIG. 9  to  FIG. 11 . The single wafer holder  1  is common to the first polishing unit  41   a  and the second polishing unit  41   b.  That is, as in the case of  FIG. 8  or  FIG. 12 , the wafer holder  1  is supported by a wafer holder head  32 . The wafer holder head is adapted to be pivotally moved by a wafer holder head shaft, and the wafer holder  1  is capable of moving between the fixed abrasive polishing tool  29  and the polishing pad  31 . In this embodiment, the wafer W is picked up by a wafer holder  1  from a wafer supply lift  42 , then moved to the first polishing unit  41   a  to conduct a first polishing of the wafer by the fixed abrasive polishing tool  29 , thereafter to the second polishing unit  41   b  to conduct a second or final polishing of the same by the abrasive pad  31 , and returned to the lift  42  to transfer the polished wafer to the lift. By this arrangement, a highly flattened wafer surface having fewer scratch marks can be obtained. 
     It should be noted that the present invention is not necessarily limited to the foregoing embodiments but can be modified in a variety of ways without departing from the gist of the present invention.