Abstract:
A polishing apparatus polishes a workpiece to a planar mirror finish stably, and is prevented from being vibrated while polishing is carried out. The polishing apparatus has a rotatable holding member for holding the workpiece, and a bearing supporting an outer circumferential surface of the holding member, for suppressing vibrations transmitted to the holder while the workpiece is being polished.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a planar mirror finish, and more particularly to a polishing apparatus for polishing a workpiece by pressing a polishing pad or a grinding plate and the workpiece against each other while moving them in sliding contact with each other. 
     2. Description of the Related Art 
     Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithography process can form narrower interconnections, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surface of semiconductor wafers has been to polish semiconductor wafers by polishing apparatus. 
     Heretofore, polishing apparatus for polishing semiconductor wafers comprises a turntable with a polishing pad attached thereto and a top ring for holding a semiconductor wafer to be polished. The top ring which holds a semiconductor wafer to be polished presses the semiconductor wafer against the polishing pad on the turntable. While an abrasive liquid is being supplied to the polishing pad, the top ring and the turntable are rotated about their own axes to polish the surface of the semiconductor wafer to a planar mirror finish. 
     FIG. 1 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 1, the conventional polishing apparatus comprises a turntable  5  with a polishing pad  6  attached to an upper surface thereof, a top ring  1  for holding a semiconductor wafer  4  which is a workpiece to be polished while rotating and pressing the semiconductor wafer  4  against the polishing pad  6 , and an abrasive liquid supply nozzle  9  for supplying an abrasive liquid Q to the polishing pad  6 . The upper surface of the polishing pad  6  provides a polishing surface. The top ring  1  is connected to a top ring drive shaft  8 , and supports on its lower surface a resilient mat  2  such as of polyurethane or the like. The semiconductor wafer  4  is held on the top ring  1  in contact with the resilient mat  2 . The top ring  1  also has a cylindrical guide ring  3  mounted on a lower outer circumferential surface thereof for preventing the semiconductor wafer  4  from being disengaged from the lower surface of the top ring  1  while the semiconductor wafer  4  is being polished. The guide ring  3  is fixed to the top ring  1  against relative movement in the circumferential direction. The guide ring  3  has a lower end projecting downwardly beyond the lower supporting surface of the top ring  1 . The guide ring  3  holds the semiconductor wafer  4  on the lower supporting surface of the top ring  1  against dislodgment from the top ring  1  due to frictional forces developed between the semiconductor wafer  4  and the polishing pad  6  while the semiconductor wafer  4  is being polished. 
     In operation, the semiconductor wafer  4  is held against the lower surface of the resilient mat  2  on the top ring  1 , and pressed against the polishing pad  6  by the top ring  1 . The turntable  5  and the top ring  1  are rotated about their own axes to move the polishing pad  6  and the semiconductor wafer  4  relatively to each other in sliding contact for thereby polishing the semiconductor wafer  4 . At this time, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle  9  to the polishing pad  6 . The abrasive liquid Q comprises, for example, an alkaline solution with fine abrasive grain particles suspended therein. Therefore, the semiconductor wafer  4  is polished by both a chemical action of the alkaline solution and a mechanical action of the fine abrasive grain particles. Such a polishing process is referred to as a chemical and mechanical polishing (CMP) process. 
     Another known polishing apparatus employs a grinding plate made of abrasive grain particles bonded by a synthetic resin for polishing a workpiece. The grinding plate is mounted on the turntable, and an upper surface of the grinding plate provides a polishing surface. Since this polishing apparatus does not employ a soft polishing pad and a slurry-like abrasive liquid, it can polish the workpiece to a highly accurate finish. The polishing process by the grinding plate is also advantageous in that it is less harmful to the environment because it discharges no waste abrasive liquid. 
     The conventional polishing apparatus shown in FIG. 1 has a spherical bearing  7  positioned between the top ring  1  and the top ring drive shaft  8 . The spherical bearing  7  allows the top ring  1  to be tilted quickly with respect to the top ring drive shaft  8  even when the top ring  1  encounters a small slant on the upper surface of the turntable  5 . The top ring drive shaft  8  is kept in driving engagement with the top ring  1  by a torque transmission pin  107  on the top ring drive shaft  8  and torque transmission pins  108  on the top ring  1 . The torque transmission pins  107 ,  108  are held in sliding point-to-point contact with each other. When the top ring  1  is tilted with respect to the top ring drive shaft  8 , the torque of the top ring drive shaft  8  is smoothly and reliably transmitted to the top ring  1  because the torque transmission pins  107 ,  108  change their point of contact while transmitting the torque. 
     The above conventional polishing apparatus are problematic in that while polishing a workpiece, the polishing apparatus suffer large vibrations owing to frictional forces developed between the turntable and the top ring with the workpiece interposed therebetween. An analysis suggests that such large vibrations are caused by a combined action of resistant forces by the rotating top ring and the rotating turntable which are rotated independently of each other, such resistant forces being dependent on frictional forces developed between the surface of the workpiece and the surface of the polishing pad or grinding plate, and restoring forces exerted by the top ring drive shaft and a turntable drive shaft. 
     When the vibrations become large, the polished surface of the workpiece develops polish irregularities or scratches or other surface damage, and hence the workpiece cannot be polished stably. The vibrations may become so intensive that the workpiece may be forcibly detached from the top ring and no longer will be polished. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a polishing apparatus which is capable of preventing undue vibrations during a polishing process and of stably polishing a workpiece to a planar mirror finish. 
     According to the present invention, there is provided a polishing apparatus for polishing a workpiece to a planar mirror finish by pressing the workpiece against a polishing surface while keeping the workpiece and the polishing surface in sliding motion, comprising a holding member for holding the workpiece, a mechanism for rotating the holding member, and a bearing supporting an outer circumferential surface of the holding member, for suppressing vibrations transmitted to the holder while the workpiece is being polished. 
     The outer circumferential surface of the holding member which holds the workpiece to be polished is rotatably supported by the bearing for suppressing vibrations of the holding member. Vibrations produced owing to a combined action of frictional forces developed on the surface being polished and restoring forces exerted by drive shafts of the holding member and the abrasive member, are maximized on the holding member supported by the drive shaft which is relatively small in diameter. Therefore, since the holding member is rotatably supported at its outer circumferential surface by the bearing, vibrations of the holding member are suppressed, and hence vibrations of the polishing apparatus in its entirety are also suppressed. Consequently, even when the rotational speeds of the workpiece and the abrasive member increase or the pressure applied therebetween increases to develop a buildup of frictional force, the polishing apparatus is effectively prevented from being unduly vibrated, and can polish the workpiece stably under desired operating conditions. 
     As described above, vibrations of the holding member can be suppressed because the holding member is rotated with its outer circumferential surface being rotatably supported by the bearing. This structure is also applicable to other polishing apparatus than polishing apparatus which have a top ring and a turntable. Specifically, the structural details are applicable to a cup-type polishing apparatus in which the workpiece is arranged with its surface to be polished facing upwardly and the abrasive member rotates and presses against the workpiece, and also to a scrolling-type polishing apparatus in which the grinding plate or polishing pad is arranged with its polishing surface facing upwardly and the workpiece is arranged with its surface to be polished facing downwardly against the grinding plate or polishing pad, which is caused to make a scrolling motion such as circulate orbital motion to polish the workpiece. 
     The bearing may comprise a mechanical bearing or a non-contact-type bearing such as a magnetic bearing. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-sectional view of a conventional polishing apparatus; 
     FIG. 2 is a perspective view of a polishing apparatus according to a first embodiment of the present invention; 
     FIG. 3 is a vertical cross-sectional view of the polishing apparatus shown in FIG. 2; 
     FIG. 4 a perspective view of a modification of the polishing apparatus shown in FIG. 2; 
     FIG. 5 is a vertical cross-sectional view showing in detail one example of a top ring of the polishing apparatus shown in FIGS. 2 and 4; 
     FIG. 6 is a perspective view of a polishing apparatus according to a second embodiment of the present invention; 
     FIG. 7 is a perspective view of a polishing apparatus according to a third embodiment of the present invention; 
     FIG. 8 is a perspective view of a polishing apparatus according to a fourth embodiment of the present invention; 
     FIG. 9 is a fragmentary vertical cross-sectional view of a portion of the polishing apparatus shown in FIG. 8; and 
     FIG. 10 is a fragmentary vertical cross-sectional view of a modification of the polishing apparatus shown in FIGS.  8  and  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Like or corresponding parts are denoted by like or corresponding reference numerals throughout views. 
     As shown in FIGS. 2 and 3, a polishing apparatus according to a first embodiment of the present invention generally comprises a turntable  5  with a polishing pad or grinding plate  6  attached to an upper surface thereof and a top ring  1  for holding a semiconductor wafer  4  which is a workpiece to be polished. The upper surface of the polishing pad or the grinding plate provides a polishing surface. The turntable  5  is fixed to the upper end of a vertical drive shaft  11  which is rotatably supported by bearings (not shown) in a mount base  10  and can be rotated by a motor and a belt-and-pulley mechanism (not shown). The top ring  1  is fixed to the lower end of a vertical drive shaft  8  which can be rotated by a motor and a belt-and-pulley mechanism (not shown) that are housed in a top ring casing  12 . The top ring casing  12  also houses bearings by which the drive shaft  8  is rotatably supported, and a presser mechanism such as an air cylinder or the like for pressing the top ring  1  toward the turntable  5 . Therefore, the top ring  1  can be pressed against the turntable  5  while being rotatably supported by the drive shaft  8 . The top ring casing  12  is supported on the mount base  10  by a vertical support shaft  16 . The top ring  1  comprises a disk-shaped element which holds on its lower surface the semiconductor wafer  4  to be polished. 
     The top ring  1  has its outer circumferential surface supported by a bearing  13  housed in a bearing casing  14 . The bearing casing  14  is detachably fixed to the mount base  10  by a vertical post  15 . 
     The polishing apparatus shown in FIGS. 2 and 3 operates as follows: 
     The semiconductor wafer  4  is attracted to and held in a recess in the lower surface of the top ring  1  by a suction force or the like. The top ring  1  holding the semiconductor wafer  4  is moved to a position over the turntable  5 , and lowered to bring the semiconductor wafer  4  into contact with the polishing pad or grinding plate  6  on the turntable  5 . The bearing casing  14  is fixed to the support post  15  and firmly fixed to the mount base  10 . 
     The turntable  5  and the top ring  1  start being independently rotated about their own axes up to a predetermined speed, and the top ring  1  is lowered to press the lower surface of the wafer  4  to be polished against the polishing pad or grinding plate  6  under a predetermined pressure. If the polishing pad is employed on the turntable  5 , then an abrasive liquid is supplied onto the polishing pad, and the lower surface of the semiconductor wafer  4  is polished by abrasive grain particles contained in the abrasive liquid. If the grinding plate is employed on the turntable  5 , then the lower surface of the semiconductor wafer  4  is polished by abrasive grains produced and contained in the grinding plate. It is preferable that the turntable  5  and the top ring  1  be rotated at the same speed for uniformly polishing the entire lower surface of the semiconductor wafer  4 . 
     As the speed of the turntable  5  and the top ring  1  increases, the rate at which the semiconductor wafer  4  is polished increases. The rate of polishing is also increased when the pressure under which the semiconductor wafer  4  is pressed against the polishing pad or grinding plate  6  by the top ring  1  increases. However, an increase in the rotational speed of the turntable  5  and the top ring  1  or an increase in the pressure applied to the semiconductor wafer  4  by the top ring  1  tends to cause the polishing apparatus to vibrate in its entirety. Such vibrations of the polishing apparatus would be liable to increase because the drive shaft  8  of the top ring  1  is usually relatively small in diameter. 
     According to the embodiment shown in FIGS. 2 and 3, the outer circumferential surface of the top ring  1  is rotatably secured firmly to the mount base  10  by the bearing  13 . Since the top ring  1  is firmly supported on the mount base  10 , i.e., the bearing casing  14  is fixed to the mount base  10 , it is effectively prevented from vibrating. Therefore, the top ring casing  12  and the support shaft  16  are also prevented from vibrating, and hence the turntable  5  and the drive shaft  11  are also prevented from vibrating. 
     FIG. 4 shows a modification of the polishing apparatus according to the first embodiment shown in FIGS. 2 and 3. 
     According to the modification shown in FIG. 4, the support post  15  is swingably supported on the mount base  10 . While the semiconductor wafer is not polished by the polishing apparatus, the support post  15  is swingable to retract the bearing casing  14  away from the position above the turntable  5 . Specifically, the mount base  10  has a swing slot D defined therein, and the support post  15  is movably disposed in the swing slot D. When the support post  15  moves along the swing slot D in either of the directions indicated by the arrow d, the bearing casing  14  is allowed to swing in unison with the top ring  1 . The swing slot D is defined as an arcuate slot extending about the axis of the support shaft  16  and having a radius of curvature equal to the distance from the support shaft  16  to the support post  15 . While a semiconductor wafer is being polished by the polishing apparatus, the support post  15  is firmly fixed to the mount base  10  by a pair of stop bars  25  extending across the swing slot D in sandwiching relation to the support post  15 . When the polishing of the semiconductor wafer is finished, the stop bars  25  are retracted out of the swing slot D, and the support post  15  is moved along the swing slot D to retract the bearing casing  14  and the top ring  1  away from the turntable  5 . In this manner, the semiconductor wafer  4  which has been kept in close contact with the polishing pad or grinding plate  6  on the turntable  5  during the polishing process can easily be lifted off and moved away. 
     FIG. 5 shows in detail an example of the top ring  1  of the polishing apparatus shown in FIGS. 2 and 4. The top ring  1  has a top ring body  1   a  which supports on its lower surface a resilient mat  2  such as of polyurethane or the like. The top ring  1  has a cylindrical guide ring  3  mounted on a lower outer circumferential surface of the top ring body  1   a  for preventing the semiconductor wafer  4  from being disengaged from the lower surface of the top ring body  1   a  while the semiconductor wafer  4  is being polished. The semiconductor wafer  4  is retained in a recess surrounded by the guide ring  3  and the resilient mat  2 . The surface, to be polished, of the semiconductor wafer  4  is held in contact with the polishing pad or grinding plate  6 . 
     The top ring body  1   a  is connected to the drive shaft  8  by a spherical bearing  7 . The drive shaft  8  can be rotated by a rotating mechanism comprising a motor and a belt-and-pulley mechanism (not shown) which are housed in the top ring casing  12 , for thereby rotating the top ring body  1   a  by torque transmission pins such as shown in FIG.  1 . The drive shaft  8  can also be lowered by a presser mechanism such as an air cylinder or the like housed in the top ring casing  12 , for thereby pressing the top ring body  1   a  toward the turntable  5 . The semiconductor wafer  4  held on the lower surface of the top ring body  1   a  is polished while it is being rotated in sliding motion with respect to the polishing pad or grinding plate  6  and also being pressed against the polishing pad or grinding plate  6 . 
     The guide ring  3  is coupled to the top ring body  1   a  by keys  18  such that the guide ring  3  is vertically movable with respect to the top ring body  1   a  and rotatable in unison with the top ring body  1   a.  The guide ring  3  is coupled to a guide ring presser  20  by a guide ring bearing  19  such that the guide ring  3  is rotatable in a horizontal plane with respect to the guide ring presser  20  and vertically movable in unison with the guide ring presser  20 . Specifically, the guide ring bearing  19  has an inner race mounted on an outer circumferential surface of the guide ring  3  and an outer race mounted on an inner circumferential surface of the guide ring presser  20 . The guide ring presser  20  is connected to the air cylinder or the like in the top ring casing  12  by vertical shafts  21 . Therefore, the guide ring presser  20 , when lowered by the air cylinder or the like, can press the guide ring  3  down against the polishing pad or grinding plate  6 . The guide ring presser  20  has its outer circumferential surface supported by a bearing  33  housed in a bearing casing  32  which is firmly secured to the mount base  10  by the support post  15 . A vibration damper  30  such as an O-ring is interposed between the top ring body  1   a  and the guide ring  3  for absorbing vibrations generated while the semiconductor wafer  4  is being polished. 
     Even when vibrations are developed by a combined action of frictional forces developed between the surface of the semiconductor wafer  4  and the surface of the polishing pad or grinding plate  6  and restoring forces exerted by the drive shafts  8 ,  11 , such vibrations are first absorbed by the resilient mat  2  and then by the vibration damper  30  interposed between the top ring  1  and the guide ring  3 . Greater vibrations are transmitted from the surface of the semiconductor wafer  4  via the resilient mat  2 , the top ring  1 , and the keys  18  to the guide ring  3 . However, since the guide ring  3  is rotatably supported by the bearing  33  housed in a bearing case  32  which is firmly secured to the mount base  10  by the support post  15 , such greater vibrations are suppressed by the bearing  33 . 
     In FIG. 5, the bearings  19 ,  33  are illustrated as comprising ball bearings. However the bearings  19 ,  33  may comprise plain bearings. In the illustrated embodiment, the guide ring  3  is supported by the bearing  19 . However, the top ring body  1   a  itself may be supported directly by a bearing. In the illustrated embodiment, the top ring body  1   a  is connected to the drive shaft  8  by the spherical bearing  7 . However, the top ring body  1   a  may be coupled directly to the drive shaft  8 . 
     In the illustrated embodiment, the bearing  33  is fixed to the mount base  10 . However, the bearing  33  may be fixed to a different member, other than the top ring casing  12 , which is less subject to vibrations of the top ring  1 . 
     FIG. 6 shows a polishing apparatus according to the second embodiment of the present invention. The polishing apparatus shown in FIG. 6 is of a cup-type configuration. As shown in FIG. 6, a grinding wheel holder  41  supports a ring-shaped grinding wheel  42  fixed to its lower surface. The grinding wheel holder  41  is actuated by a drive mechanism  43  to press the ring-shaped grinding wheel  42  against a semiconductor wafer  4 . The drive mechanism  43  is reciprocally movable in the directions indicated by the arrow A while the semiconductor wafer  4  is being polished by the ring-shaped grinding plate  42 . While the grinding plate  42  is ring shaped in the illustrated embodiment, it may comprise an annular array of small disk-shaped grinding members. The semiconductor wafer  4  is supported on a wafer holder  44  which is rotatable about its own axis in the direction indicated by the arrow B. The wafer holder  44  is fixedly mounted on a support shaft  45 , which is rotatable by a motor (not shown) housed in a mount base  48  thereby to rotate the semiconductor wafer  4 . The wafer holder  44  has an outer circumferential surface rotatably supported by a bearing (not shown) housed in a bearing case  46 . The bearing case  46  is firmly fixed to the mount base  48  by a support post  47 . Vibrations developed while the semiconductor wafer  4  is being polished are suppressed because the outer circumferential surface of the wafer holder  44  is supported by the bearing. 
     FIG. 7 shows a polishing apparatus according to the third embodiment of the present invention. The polishing apparatus shown in FIG. 7 is of a scrolling-type configuration. As shown in FIG. 7, a holder  52  with a grinding plate or polishing pad  51  attached thereto is supported on a support shaft  53  which can be driven to make a scrolling motion in the direction indicated by the arrow D. The term “scrolling motion” used herein means a circulatory rotating orbital motion having a radius “d” imparted to the grinding plate or polishing pad  51  while the grinding plate or polishing pad  51  is being translated. A wafer holder  54  for holding a semiconductor wafer (not shown) is fixed to a support shaft  55  which can be rotated in the direction indicated by the arrow C by a drive mechanism  57 . The wafer holder  54  has an outer circumferential surface rotatably supported by a bearing (not shown) housed in a bearing case  55   a.  The wafer holder  54  is thus rotatably supported while being prevented from radial movement. The bearing case  55   a  is firmly fixed to a mount base  58  by a support post  56 . The grinding plate or polishing pad  51  held by the holder  52  has its grinding or polishing surface facing upwardly, and the semiconductor wafer is held by the wafer holder  54  with its surface, to be polished, facing downwardly. The semiconductor wafer is rotated by the drive mechanism  57  and pressed against the grinding plate or polishing pad  51 . The semiconductor wafer is polished as the grinding plate or polishing pad  51  makes the scrolling motion and the semiconductor wafer is rotated and pressed against the grinding plate or polishing pad  51 . Even when vibrations are developed by a combined action of frictional forces developed between the surface of the semiconductor wafer and the surface of the grinding plate or polishing pad  51  and restoring forces exerted by the support shaft  55 , such vibrations are suppressed because the wafer holder  54  is rotatably supported by the bearing in the bearing case  55   a.    
     FIGS. 8 and 9 show a polishing apparatus according to the fourth embodiment of the present invention. The polishing apparatus shown in FIGS. 8 and 9 is free of a top ring shaft and top ring support arm. The polishing apparatus has a grinding plate or polishing pad  61  mounted on a turntable rotatably supported by a support shaft  62 . A semiconductor wafer  4  to be polished is held by a top ring  63  which has no support shaft and no top ring support arm, and pressed against the grinding plate or polishing pad  61 . The top ring  63  has its outer circumferential surface rotatably supported by a bearing  64   a  housed in a bearing case  64 . The bearing case  64  is fixed to a mount base  70  by an arm  64   b  and a support post  66 . The top ring  63  is rotatable by a motor  69  disposed in the support post  66  via a belt  65 . The support post  66  also houses a pressing cylinder  67  for lowering the bearing case  64  to impose a vertical load on the surface, to be polished, of the semiconductor wafer  4 . Vibrations developed by the semiconductor wafer  4  being polished and transmitted to the top ring  63  are suppressed because the outer circumferential surface of the top ring  63  is rotatably supported by the bearing  64   a  housed in the bearing case  64 . In this embodiment, because the top ring  63  is rotatably supported by the bearing, which is fixed to the mount base, a top ring support arm  12  such as shown in FIG. 2 is not necessary. Therefore, the size of the polishing apparatus becomes compact in height. 
     FIG. 10 shows a modification of the polishing apparatus according to the fourth embodiment shown in FIGS. 8 and 9. The modified apparatus has a magnetic bearing  71  by which the top ring  63  is rotatably supported. The magnetic bearing  71  comprises an electromagnet whose magnetically attractive forces are controlled to keep the top ring  63  in a levitated position out of physical contact with the magnetic bearing  71 . The top ring  63  is thus magnetically held in a constant position regardless of disturbances applied thereto. Accordingly, the top ring  63  is prevented from being unduly vibrated by disturbances. The top ring  63  has an upper end  63   a  connected to a pressure transmitting beam  65   a  connected to the pressing cylinder  67  in the support post  66 . The magnetic bearing  71  imparts rotating forces to the top ring  63  out of contact therewith. The semiconductor wafer  4  held by the top ring  63  is polished by the rotation of the grinding plate or polishing pad  61 , the rotation of the semiconductor wafer  4 , and the pressure applied from the pressing cylinder  67  to the semiconductor wafer  4 . Vibrations transmitted from the semiconductor wafer  4  to the top ring  63  are suppressed by the magnetic bearing  71 . 
     The conventional polishing apparatus have had a spline shaft, a top ring arm, a swing shaft, and other parts supporting the top ring, and those parts have been designed for large rigidity in order to suppress vibrations developed in the polishing apparatus. Therefore, the conventional polishing apparatus have been very heavy and large in size. According to the present invention, however, since the holder for holding the workpiece to be polished is rotatably supported by the bearing, the holder is prevented from being unduly vibrated. Therefore, various shafts and arms associated with the workpiece holder are not required to be highly rigid, and the polishing apparatus can thus be reduced in weight and made compact. 
     In the above embodiments, the holder for holding the workpiece to be polished is rotatably supported at its outer circumferential surface by the bearing for suppressing vibrations transmitted thereto. In the cup-type or scrolling-type polishing apparatus, the holder which holds the grinding plate or polishing pad may be rotatably supported at its outer circumferential surface by a bearing for suppressing vibrations transmitted thereto. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made thereto without departing from the scope of the appended claims.