Abstract:
A polishing apparatus can stabilize polishing speed at the peripheral end portion of a polishing object, such as a wafer for fabricating a semiconductor device. The polishing apparatus performs polishing by arranging the polishing object in opposition to an abrasive cloth on a rotary polishing table, applying a load on a polishing block and with supplying a polishing fluid. A ring-shaped retainer is provided for embracing a given thickness of a elastic buffering plate provided between the polishing object fixing block and the polishing object for making contact pressure between the polishing object and the abrasive plate uniform and whereby for making a polishing speed of the polishing object constant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a polishing apparatus. More specifically, the invention relates to a polishing apparatus for planarizing roughness or unevenness on a surface of a semiconductor substrate or the like. 
     2. Description of the Related Art 
     In the recent years, in a fabrication process of a semiconductor, a polishing process has been used for planarization of roughness or unevenness of a semiconductor substrate or of step of element and wiring on the surface of the semiconductor substrate. The polishing process typically employs a system to selectively polish protruding portion for removal and whereby to achieve complete planarization. 
     Constructions of the conventional polishing apparatus typically employed have been disclosed in Japanese Unexamined Patent Publication No. Showa 59(1984)-187456, Japanese Unexamined Patent Publication No. Heisei 7(1995)-221053 and Japanese Unexamined Patent Publication No. Showa 58(1983)-22657. For example, as shown in FIG.  4 ( a ), the typical polishing apparatus is constructed with a polishing object holding means (hereinafter referred to as “fixing block”)  302  for holding a polishing object  101  via a buffering plate  303 , such as a semiconductor wafer or the like and rotating carrying the polishing object, and a rotary polishing table  106 . The buffering plate  303  is necessary for absorbing a load locally exerted on the polishing object  101  and whereby for improving uniformity of a polished surface. As the buffering plate, DF200 manufactured by Rodel, U.S.A. has been generally used. DF200 has a laminated structure of a polyurethane foamed layer and polyester sheet base material and has a compressibility factor (in accordance with a standard of JIS L-1096) of 33%. For instance, DF200 adapted for 6 inch wafer has an external diameter of 150 mm and a thickness of 0.6 mm. On the other hand, in order to prevent the polishing object  101  from loosing off the fixing block  302 , a retainer ring  304  having an internal diameter of 151 mm and a thickness of 7 mm is engaged on the outer periphery of the fixing block  304 . 
     On an upper surface of the rotary polishing table  106  having a rotary drive shaft  105 A, an abrasive cloth is bonded. The abrasive cloth is typically used with lapping over an abrasive cloth  107  (upper layer) and an abrasive cloth  108  (lower layer). As the abrasive cloth  107 , IC1000 manufactured by Rodel, is frequently used. The abrasive cloth  107  is formed by foaming and hardening urethane resin and is hard having a hardness factor of 95 in an Asker C hardness meter. On the other hand, as the abrasive cloth  108 , Suba400 manufactured by Rodel Nitta Company is frequently used, which is formed by impregnating polyurethane resin into a polyester fiber non-woven fabric and soft having a hardness factor of 61 in the Asker C hardness meter. Functions of the abrasive cloths  107  and  108  is that the abrasive cloth  107  serves for increasing flatness and the abrasive cloth  108  serves for absorbing camber or undulation of the surface of the polishing object  101  to improve uniformity of the polishing surface. On the other hand, a polishing fluid supply nozzle  109  is provided at the center portion above the abrasive cloth  107 . 
     Next, operation of the foregoing polishing apparatus under typical polishing condition will be discussed. When a polishing fluid  110  containing solid component, such as silicon oxide or the like is supplied in a rate of 200 cc/min on the abrasive cloth  107  rotating at a rotation speed of 30 r.p.m., the polishing fluid  110  propagates along the upper surface of the abrasive cloth  107  from the center portion to the end portion. Under this condition, the polishing object  101  held on the fixing block  302  is fitted on the abrasive cloth  107  under pressure. Then, polishing is performed with applying a 7 psi (not shown) of load on the fixing block  302 . At this time, in order to certainly maintain uniformity of in-plane polishing speed of the polishing object  101 , the fixing block  302  is also rotated on own axis at a speed of 30 r.p.m. 
     FIG. 4C is a chart showing a polishing speed in a direction including a diameter extending cross the center of the polishing object  101  polished by an apparatus shown in FIG.  4 A and extending perpendicular to a polishing surface. The shown profile is the same in the profile of any diametrical direction taking the center of the polishing object  101  as an origin. Since FIG. 4C is a bilateral symmetric profile with respect to the origin, discussion will be given for the polishing speed of the polishing object  101  from an edge in the plus direction to the origin. 
     At the edge portion (a region of +73 to +75 mm) of the polishing object  101 , the polishing speed is quite high. Conversely, in a region of +60 to +73 mm, the polishing speed is low, and has a minimum point in a region of +71 to +73 mm. In a region of 0 to +60 mm, the polishing speed is uniform. A tendency of the polishing speed at the end portion of the polishing object  101  is caused due to difference of pressure exerted on the polishing object  101  from the abrasive cloth  107 . Since the abrasive cloth  107  in the upper layer is hard but the abrasive cloth  108  in the lower layer is soft, the abrasive cloths  107  and  108  mating with the edge portion of the polishing object  101  is locally deformed. FIG. 5B shows a result of measurement of a pressure exerted on the edge portion of the polishing object  101  by means of a pressure sensor. In the region of +73 to +75 mm, due to large deformation of the abrasive cloths  107  and  108 , an excessively high pressure is exerted locally. Conversely, in the region of +60 to +73 mm, the pressure is gradually released and lowered. Then, in the region of 0 to +60 mm, the pressure is uniformly exerted. The pressure distribution in respective of these regions correspond to distribution of the polishing speed. 
     In order to improve non-uniformity of the polishing speed, there is a method to use Suba800 manufactured by Rodel Nitta Company, which is formed of the same material as Suba400 but having higher hardness. Suba800 has a hardness factor of 82 in Asker C hardness meter. FIG. 6A is an enlarged section of the edge portion in the case where Suba800 is used as the abrasive cloth  108  in the apparatus of FIG.  4 A. While it is similar even in the case using SuBa400, since the buffer plate  303  is used, displacement is caused between the buffering plate bonding surface of the fixing block  302  and the buffering plate  303  is caused to form a gap portion  316  between the end portion of the buffering plate  303  and the retainer ring  304 . The enlarged section shown in FIG. 6A shows an example causing plus side displacement of the buffering plate  303 , in which the plus side edge of the polishing object  101  is located completely above the buffering plate  303 . 
     Discussion for the pressure distribution and the polishing speed at the plus side edge portion will be given with reference to FIG.  6 B. In this case, Suba800 as the abrasive cloth  108  of the lower layer is hard, local deformation amount of the abrasive cloths  107  and  108  at the edge portion of the polishing object  101  is small even when the polishing object  101  is depressed onto the abrasive cloths  107  and  108 . Therefore, in the region of +73 to +75 mm of FIG. 6B, excessive pressure may be exerted by shifting of the polishing object  101  during polishing. Thus, the pressure exerted from the abrasive cloth  107  is equivalent to the case where Suba400 is used. In the region of +60 to +73 mm, since local deformation amount is small, the pressure to be exerted on the polishing object  101  becomes uniform. Accordingly, as shown in FIG. 7, in a region of +60 to 73 mm, the polishing speed becomes uniform to make the uniform region wider than that in the case where Suba400 is used. 
     It should be noted that displacement between the buffering plate bonding surface of the fixing block  302  and the buffering plate  303  is caused by the structure of the fixing block  302 . Therefore, assembling method of the buffering plate will be discussed with reference to FIGS. 4A and 4B. For example, in the fixing block  302  for 6 inch wafer, on the buffering plate bonding surface having an internal diameter of 151 mm, the buffering plate  303  having an external diameter 150 mm which is smaller than the bonding surface in the extent of 1 mm, is bonded. Next, the retainer ring  304  is engaged to the fixing block  302  and fixed by means of screws. The reason while the buffering plate  303  is made smaller than the buffering plate bonding surface in the extent of 1 mm, is that if displacement is caused in the buffering plate  303  upon bonding, the retainer ring  304  cannot be engaged unless the buffering plate is formed smaller than the buffering plate bounding surface. It may be possible to bond the buffering plate  303  having greater external diameter than the internal diameter of the buffering plate bonding surface and subsequently cut the portion of the buffer plate extending from the bonding surface. However, in such case, cut edge tends to be irregular to make the polishing speed at the edge portion of the polishing object  101  non-uniform. Accordingly, in the shown construction of the fixing block  302 , it becomes necessary to provide a dimensional difference in the extent of 1 mm between the buffering plate  303  and the buffering plate bonding surface. When the external diameter of the buffering plate  302  is smaller than the internal diameter of the buffering plate bonding surface of the fixing block in the extent of 1 mm, a gap portion of 1 mm at the maximum is formed. As a result, as shown by the pressure profile in the region of −75 to −71 mm of FIG. 6B, the pressure to be exerted on the polishing object  101  located above the gap portion, from the abrasive cloth, can be lowered not only in the region of 1 mm from the edge but also in the region of 1 to 4 mm from the edge due to deformation of the buffering plate  303  toward the gap portion  316 . As a result, as shown in the region of −75 to −71 mm of FIG. 7, the polishing speed of the polishing object  101  in the region of 4 mm from the edge becomes lower than that of other portion and thus becomes non-uniform. Even in the method using Suba400 as the abrasive clot  108  in the lower layer, the polishing speed becomes low in the region of −75 to −71 mm due to displacement of the buffering plate  303 . However, since the polishing speed in the region where the edge portion of the polishing object  101  is located above the buffering plate is also low, influence of displacement for non-uniformity of the polishing speed is lesser than that of the case where Suba800 is used as the abrasive cloth  108  of the lower layer. 
     Also, as a solution for non-uniformity of the polishing speed in the edge portion of the polishing object, there is a method to contact a wider width retainer ring to the abrasive cloth to exert a pressure, as proposed on a material of VMIC conference, Jun. 27 to 29, 1995, pp 525 to 527. This is the method to control the polishing speed by applying pressure on the base plate  415  and the retainer ring  404 , as shown in FIG.  8 A. The polishing method under typical polishing condition will be discussed with reference to FIG.  8 A. On the abrasive cloth  107  rotated at 30 r.p.m., the polishing fluid is supplied from the polishing fluid supply nozzle (not shown) in a rate of 200 cc/min. Under this condition, the polishing object  101  held on the base plate  415  via the buffering plate  403  and the retainer ring  404  and the abrasive cloth  107  are fitted under pressure. By supplying air into an air bag  412  for the retainer ring and an air bag  413  for the base plate to apply a load of 7 psi on the retainer ring  404  and the base plate  415 . The fixing block  402  is driven to rotate on the own axis. Under such condition, polishing is performed. 
     In this method, since local deformation of the abrasive cloth  107  and  108  at the edge portion of the polishing object  101  is received by the outer peripheral portion of the lower surface of the retainer ring  404 , the pressure becomes uniform in the region of +60 to +75 mm. As a result, the polishing speed of the edge portion on the plus side becomes the same as in-plane region as shown by the profile. 
     However, since the buffering plate  403  is used even in such retainer ring contact method, similarly to the method using Suba800, displacement may be caused between the buffering plate bonding surface of the base plate and the upper surface of the polishing object  101  to form a gap portion  416  in the extent of 1 mm as shown in the enlarged section of the edge portion on the minus side in FIG.  8 A. As a result, as shown by the pressure profile in FIG. 8B, the pressure on the edge and the area in the vicinity thereof is lowered due to presence of the gap portion  416 . However, since the retainer ring  404  restricts deformation of the abrasive cloths  107  and  108 , the deformation amount of the abrasive cloth  107  located over the edge portion is smaller than that in the case where Suba800 is used. Therefore, the region where the pressure is lowered is in a range of 3 mm from the edge, which is smaller than the range of 4 mm in the case where Suba800 is used. Accordingly, as shown in the region of −75 to −72 mm of FIG. 9, the polishing speed of the polishing object within the range of 3 mm from the edge becomes lower than that in other region and thus to be non-uniform. 
     In the construction of the fixing block in the conventional method, since the external diameter of the buffering plate  303  is smaller than the diameter of the buffering plate bonding portion of the fixing block  302  in the extent of 1 mm as shown in FIG. 6A maximum 1 mm of gap portion  316  is formed at certain bonding position of the buffering plate  303 . By influence of this gap, in the method using Suba800 as the abrasive cloth  108 , the polishing speed of the polishing object  101  in the region 4 mm from the edge becomes lower than that in other region to make polishing speed non-uniform. On the other hand, even in the method, in which the retainer ring  404  is contacted with the abrasive cloth  107  as shown in FIG. 8, the polishing speed of the polishing object  101  in the region 3 mm from the edge becomes lower than that in other region to make polishing speed non-uniform. 
     Accordingly, in the conventional polishing process, a region inappropriate for fabrication of the semiconductor device created in the polishing object is a region of 4 mm from the edge in the method using Suba800 and 3 mm from the edge in the method where the retainer ring  404  is contacted with the abrasive cloth. However, a currently available region where the semiconductor device can be fabricated in the fabrication process other than polishing process, is 2 mm from the edge. Therefore, the region of 2 to 4 mm from the edge lacks flatness to be a cause of defects in the product. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a polishing apparatus which can obtain uniform polishing speed over an entire polishing surface and whereby improve yield of material of a polishing object, upon performing polishing of the polishing object by holding the polishing object by a holding means on a rotary polishing table having an abrasive cloth on the upper surface. 
     In order to accomplish the above-mentioned and other objects, according to one aspect of the present invention, a polishing apparatus comprises: 
     a rotary polishing table carrying an abrasive cloth on the upper surface thereof; 
     a device for supplying a polishing fluid on the rotary polishing table; 
     a polishing object holding device for holding a polishing object opposing the abrasive cloth via a disc-shaped buffering plate having a given elasticity, and for driving the polishing object to rotate with maintaining contact with the abrasive cloth under pressure; 
     a ring-shaped retainer incorporated in the polishing object holding means and detachably fitted on an outer peripheral edge of the polishing object, the ring-shaped retainer extending over the peripheral end portion of the buffering plate for embracing. 
     According to another aspect of the present invention, a polishing apparatus comprises: 
     a rotary polishing table carrying an abrasive cloth on the upper surface thereof; 
     a device for supplying a polishing fluid on the rotary polishing table; 
     a polishing object holding device for holding a polishing object opposing the abrasive cloth via a disc-shaped buffering plate having a given elasticity, and for driving the polishing object to rotate with maintaining contact with the abrasive cloth under pressure; 
     a ring-shaped retainer incorporated in the polishing object holding device and detachably fitted on an outer peripheral edge of the polishing object, the ring-shaped retainer extending over the peripheral end portion of the buffering plate for embracing; and 
     a pressurizing device for depressing the polishing object and the ring-shaped retainer onto the abrasive cloth with a predetermined pressure. 
     In the either construction, it is preferred that the ring-shaped retainer has a first component having an inner periphery mating with the peripheral edge of the buffering plate and a second component formed integrally with the first component has an upper surface mating with the lower surface of the buffering plate. In such case, the second component may have an inner periphery mating with the peripheral edge of the polishing object. 
     In the preferred construction, the polishing object holding device may include a fixing block for holding the polishing object via the buffering plate at fixed position thereon, and the first component of the retainer may extend vertically and detachably engage with the peripheral edge portion of the fixing block. The second component of the retainer may extend horizontally and have an internal diameter slightly greater than an external diameter of the polishing object. 
     In the later-mentioned aspect of the present invention, the pressurizing device may comprise a first pressurizing device for applying a first predetermined pressure for depressing the polishing object onto the abrasive cloth, and second pressurizing device for applying a second predetermined pressure for depressing the retainer onto the abrasive cloth. Preferably, the first and second pressurizing devices may be adjustable of the first and second predetermined pressures independently of the other. The pressurizing devices may be pneumatic devices developing the predetermined pressure by a pneumatic pressure. 
     As set forth above, in the polishing apparatus according to the present invention, the buffering plate having greater external diameter than that of the polishing object. Therefore, no gap portion where the polishing object does not contact with the buffering plate, may be formed. Thus, irregularity of the pressure to be exerted from the abrasive cloth onto the polishing object may not be caused to reduce fluctuation of the polishing speed. 
     In addition, in the later-mentioned embodiment, by depressing the polishing object and the ring-shaped retainer onto the abrasive cloth. The polishing speed can be uniform over the entire polishing surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only. 
     In the drawings: 
     FIG. 1A is a section showing a major construction of the first embodiment of a polishing apparatus according to the present invention; 
     FIG. 1B is a plan view of a polishing object fixing block in the first embodiment of the polishing apparatus of FIG. 1A; 
     FIG. 2A is a partial enlarged section of the fixing block in the first embodiment of the polishing apparatus of FIG. 1A; 
     FIG. 2B is a chart showing a polishing speed in a wafer section corresponding to a distance from a center of the wafer; 
     FIG. 3A is a partial enlarged section of the fixing block in the second embodiment of the polishing apparatus; 
     FIG. 3B is a chart showing a polishing speed in a wafer section corresponding to a distance from a center of the wafer; 
     FIG. 4A is a partial section showing a major construction of the conventional polishing apparatus; 
     FIG. 4B is a cross-section taken along line B—B of FIG. 4A; 
     FIG. 4C is a chart showing a polishing speed distribution in the diametric direction of the wafer; 
     FIG. 5A is a partial enlarged section of the polishing apparatus of FIG. 4A; 
     FIG. 5B is a chart of a pressure applied from the abrasive cloth corresponding to a distance from the center of the wafer; 
     FIG. 6A is a partial enlarged section of another conventional polishing apparatus; 
     FIG. 6B is a chart of a pressure applied from the abrasive cloth corresponding to a distance from the center of the wafer; 
     FIG. 7 is a chart showing a polishing speed in the polishing apparatus of FIG. 6A; 
     FIG. 8A is a partial enlarged section of a further conventional polishing apparatus; 
     FIG. 8B is a chart of a pressure applied from the abrasive cloth corresponding to a distance from the center of the wafer; and 
     FIG. 9 is a chart showing a polishing speed in the polishing apparatus of FIG.  8 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be discussed hereinafter in detail in terms of the preferred embodiment of the present invention with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to avoid unnecessarily obscuring the present invention. 
     FIG. 1A is a section showing a major construction of the first embodiment of a polishing apparatus according to the present invention, FIG. 1B is a plan view of a polishing object fixing block in the first embodiment of the polishing apparatus of FIG. 1A, FIG. 2A is a partial enlarged section of the fixing block in the first embodiment of the polishing apparatus of FIG. 1A, and FIG. 2B is a chart showing a polishing speed in a wafer section corresponding to a distance from a center of the wafer. 
     In FIG. 1A, a polishing object fixing block  102  holding a thin disc shaped polishing object  101  is formed into a disc shaped configuration and has a rotary shaft  105 B for rotation thereabout. On the lower surface, the polishing object fixing block  102  is formed with a flat buffering plate bounding surface  102   a  having a diameter d 3  for holding a buffering plate  103  having an external diameter d 2  (d 2 &lt;d 3 ) in tightly fitted position. A cross-sectionally L-shaped annular retainer ring  104  is provided coaxially with the rotary shaft  105 B on the lower side of the outer peripheral portion of the fixing block  102 . The vertically extending portion of the cross-sectionally L-shaped annular retainer ring, having an internal diameter d 3  is adapted to be detachably engaged with a cylindrical portion of the fixing block  102  having a diameter d 3  for further fixing the polishing object  101 . The inner end surface of a horizontal portion of the cross-sectionally L-shaped retainer ring is in cylindrical shape having an internal diameter d 1  (d 1 &lt;d 2 ) and extends inwardly to clamp the peripheral portion of the buffering plate  103  having the external diameter d 2 . The polishing object  101  having external diameter d 0  is received within the cylindrical portion of the retainer ring  104 . FIG. 1B is a plan view of the lower surface of the fixing block  102 , in which a portion  111  where the retainer ring  104  covers the buffering plate, is shown. 
     A rotary polishing table  106  is a horizontal disc shape having a rotary shaft  105 A at the center thereof. On the upper surface of the rotary polishing table  106 , two layers of abrasive cloths are bonded. The lower surface of the polishing object  101  is polished on the abrasive cloth. It is preferred that, as an upper layer abrasive cloth  107 , IC1000 which is prepared by foaming and hardening polyurethane resin as set forth in the discussion of the background art, and as the lower layer, Suba800 (hard), which is prepared by impregnating polyurethane resin into polyester fiber non-woven fabric. 
     A polishing fluid supply nozzle  109  is provided for supplying a polishing fluid on the abrasive cloth. 
     In the polishing apparatus for the semiconductor wafer of 6 inch diameter (d 0 =150 mm), respective of the foregoing dimensions are internal diameter d 1  of the retainer ring  104  is 151 mm, external diameter d 2  of the buffering plate  103  is 156 mm, and the external diameter d 3  of the buffering plate bonding surface  102   a  is 160 mm. 
     FIG. 2A is a partial enlarged section of the polishing object fixing block  102  of the polishing apparatus of FIG. 1A, and FIG. 2B is a chart showing the polishing speed in the wafer section corresponding to distance from the center of the wafer. In FIG. 2A, the edge portion of the polishing object  101  (wafer) is located at completely inside of the end edge of the buffering plate  103 , the pressure to be exerted on the polishing object  101  becomes uniform. As shown in FIG. 2B, except for the region of 2 mm from the edge of the polishing object  101 , the polishing speed becomes uniform. 
     FIG. 3A is a partial enlarged section of the major components of another embodiment of the polishing apparatus according to the present invention, and FIG. 3 is a chart showing a polishing speed in the wafer section corresponding to a distance from the center of the wafer. In FIG. 3A, a polishing object fixing block  202  is in a form of a rotary disc having a not shown rotary shaft at the center thereof. On the lower surface of the polishing object fixing block  202 , air bags  213  and  212  for applying a load on the polishing object and a retainer ring  204  by a pneumatic pressure. On the lower surface of the air bag  213 , a disc-shaped base plate  215  having an external diameter of d 3  is provided. On the lower surface of the base plate  215 , a bonding surface for a buffering plate  203  having an external diameter d 2  (d 2 &lt;d 3 ) is formed. The retainer ring  204  is annular shaped configuration with L-shaped cross section. The retainer ring  204  is designed to depress a horizontally extending bottom portion in the L-shaped cross-section onto the two layers of abrasive cloths  107  and  108  bonded on the upper surface of the rotary polishing table  106  which is identical to that of the former embodiment of FIG. 1A, by a downward load applied by the air bag  212  for the retainer ring. The polishing object  101  having an external diameter d 0  is arranged at the central portion on the lower surface of the buffering plate  203  having the external diameter d 2  (d 2 &gt;d 0 ). The buffering plate  203  is exerted a load by the pneumatic pressure in the air bag  213  via the base plate  215  to depress the polishing object  101  onto the upper surface of the abrasive cloth  107 . Also, the polishing object  101  is centered by the inner end surface (internal diameter d 1 ) of the horizontally extending portion of the cross-sectionally L-shaped retainer ring  204  in coaxial fashion with the rotary shaft. In addition, the horizontally extending portion of the cross-sectionally L-shaped retainer ring  204  is extended inwardly to clamp the peripheral portion of the buffering plate  203  having the external diameter d 2  (d 1 &lt;d 2 ). Since the internal diameter of the vertically extending portion of the cross-sectionally L-shaped retainer ring  204  is slightly greater than the external diameter d 3  of the base plate  215 , the retainer ring  204  is loosely fitted to the base plate  215 . 
     In the polishing apparatus for 6 inch diameter semiconductor wafer (d 0 =150 mm) constructed as shown in FIG. 3, it is preferred that the internal diameter d 1  of the retainer ring  204  is 151 mm, the external diameter d 2  of the buffering plate is 156 mm, the external diameter d 3  of the base plate is 160 mm, and an external diameter d 4  of the retainer ring is 180 mm. 
     In the polishing apparatus of FIG. 3A, constructed as set forth above, predetermined pressures are supplied to the air bags  212  and  213  with adjusting the pneumatic pressure to depress the polishing object  101  and the retainer ring  204  onto the abrasive cloths  107  and  108  on the rotary polishing table  106  for performing polishing. As a result, the chart of the polishing speed in the section corresponding to the distance from the center of the wafer as shown in FIG.  3 B. 
     In the condition set forth above, the entire edge portion of the polishing object  101  is located on the buffering plate  203 . Also, local deformation of the abrasive clothes  107  and  108  in the edge portion of the polishing object  101  is received by the outer peripheral portion on the lower surface of the retainer ring  204  instead of the edge portion of the polishing object  101 . Thus, the pressure to be exerted on the lower surface of the polishing object  101  becomes uniform. As a result, polishing speed of the polishing object  101  becomes uniform over the entire portion. 
     As set forth above, according to the present invention, since the end portion of the buffering plate is completely covered with the horizontally extending portion of the cross-sectionally L-shaped retainer ring, the pressure to be exerted on the end portion of the polishing object during polishing becomes uniform. Therefore, the polishing speed at the end portion of the polishing object becomes uniform. As a result, the width of the outside region, in which the semiconductor device cannot be fabricated, to be creased through the polishing process, can be reduced from 4 mm in the method using hard abrasive cloth, such as Suba800 in the lower layer to 2 mm. Therefore, number of chip yieldable from the wafer for semiconductor can be increased. On the other hand, even in the system, in which the retainer ring is contacted with the abrasive cloth under pressure, the polishing speed becomes uniform over the entire surface to further increase number of chips yieldable from the wafer. 
     Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.