Abstract:
An apparatus and method for polishing a workpiece including a polishing pad, and at least one polishing arm for holding a workpiece to be polished on the polishing pad. The polishing arm includes at least one plastic bearing. The plastic bearing prevents lock up of a vacuum chuck coupled to the polishing arm because it is impervious to slurry which often becomes lodged in conventional ball bearings.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fabrication of semiconductor devices, and in particular, to a method and apparatus for polishing semiconductor wafers. 
     DESCRIPTION OF THE RELATED ART 
     Chemical mechanical planarization (“CMP”) processes remove material from the surface of a semiconductor wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals (chemical removal) in the slurry. 
     FIGS. 1 and 2 show a conventional polishing apparatus  10 . The apparatus  10  includes a polishing pad  20  and three polishing stations  30 ,  40 ,  50  for polishing semiconductor wafers  60 . Preferably, the polishing pad  20  spins counter-clockwise to accomplish the polishing of the wafers  60 , as shown by directional arrow A in FIG.  2 . Each station  30 ,  40 ,  50  includes a polishing arm  31 ,  41 ,  51  which holds the wafers  60  during the polishing process. Each polishing arm  31 ,  41 ,  51  includes a U-shaped member for holding vacuum chucks  32 ,  42 ,  52 . Each polishing arm  31 ,  41 ,  51  includes one such vacuum chuck  32 ,  42 ,  52 . The vacuum chucks  32 ,  42 ,  52  are coupled to U-shaped members through metal bearings  35 ,  45 ,  55 . The bearings  35 ,  45 ,  55  allow the vacuum chucks  32 , 42 ,  52  to rotate with respect to the respective polishing arms  31 ,  41 ,  51 . The vacuum chucks  32 ,  42 ,  52  operate to hold the wafers  60  during the polishing process. Each of the vacuum chucks  32 ,  42 ,  52  includes an upper end  33 ,  43 ,  53  which may be coupled to a vacuum device (not shown) to provide a vacuum to the vacuum chucks. The polishing apparatus  10  also includes a slurry dispenser  70  which produces slurry which is dispensed onto a top surface  21  of the polishing pad  20 . 
     A problem associated with the conventional apparatus  10  discussed above is that the slurry dispensed from slurry dispenser  70  often becomes disposed in the metal bearings  35 ,  45 ,  55  which couple the vacuum chucks  32 ,  42 ,  52  to the polishing arms  31 ,  41 ,  51 . The slurry often causes the metal bearings  35 ,  45 ,  55  to “lock up”, thereby preventing the free rotation of the chucks  32 ,  42 ,  52  about the polishing arms  31 ,  41 ,  51 . The locking up of the metal bearings  35 ,  45 ,  55  results in uneven polishing of the wafers  60  attached to the polishing arms  31 ,  41 ,  51 . 
     Therefore, there is currently a need for an improved polishing apparatus which substantially reduces the possibility of lock up of the bearings of the polishing apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus and method for polishing a workpiece including a polishing pad, and at least one polishing arm for holding a workpiece to be polished on the polishing pad, the polishing arm including at least one plastic bearing. 
     The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a conventional polishing apparatus. 
     FIG. 2 is a side elevation view of the conventional polishing apparatus shown in FIG.  1 . 
     FIG.  3 ( a ) is a top plan view of a plastic bearing according to the exemplary embodiment of the present invention. 
     FIG.  3 ( b ) is a side elevation view of the plastic bearing shown in FIG.  3 ( a ). 
     FIG. 4 is an isometric view of a polishing arm including the bearing shown in FIGS.  3 ( a ) and  3 ( b ). 
     FIG. 5 is a top plan view of a polishing apparatus according to an exemplary embodiment of the present invention utilizing the plastic bearing shown in FIGS.  3 ( a ) and  3 ( b ). 
     FIG. 6 is a side elevation view of the polishing apparatus shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS.  3 ( a ) and  3 ( b ), there is shown a plastic bearing  100  according to an exemplary embodiment of the present invention. The bearing includes a substantially cylindrical member  101  with no external openings except for a substantially cylindrical longitudinal opening  105  formed therein which extends from a top side  102  of the bearing to a bottom side  103  of the bearing. The bearing may be made of any suitable plastic material, however, polytetrafluoroethylene (sold under the trademark TEFLON®) is preferred. 
     Because the bearing  100  has no external openings (other than the longitudinal opening  105 ), there are no openings into which foreign materials (e.g. slurry) can penetrate the bearing. Thus, the reliability of the bearing  100  is increased relative to metal bearings, such as ball bearings. 
     FIG. 4 is an isometric view of a polishing arm  131 ,  141 ,  151  including the plastic bearing  100  according to the exemplary embodiment of the present invention. The polishing arm  131 ,  141 ,  151  includes an extension member  135 ,  145 ,  155  with an opening  136 ,  146 ,  156  disposed therein for coupling the polishing arm to a polishing apparatus  110  (see FIGS.  5  and  6 ). The polishing arm  131 ,  141 ,  151  also includes a U-shaped member  137 ,  147 ,  157  with openings  138 ,  148 ,  158  disposed therein for receiving bearings, such as bearing  100  described above. The openings  138 ,  148 ,  158  and the bearings (e.g. bearing  100 ) serve to couple the polishing arm  131 ,  141 ,  151  to at least one vacuum chuck  132 ,  142 ,  152  (see FIG.  6 ). Preferably, each polishing arm  131 ,  141 ,  151  is coupled to two vacuum chucks  132 ,  142 ,  152 . 
     FIGS. 5 and 6 show a polishing apparatus  110  according to an exemplary embodiment of the present invention. The apparatus  110  includes a polishing pad  120  and three polishing stations  130 ,  140 ,  150  (each including bearings  100  as described above with reference to FIGS.  3 ( a ) and  3 ( b )) for polishing semiconductor wafers  160 . Preferably, the polishing pad  120  spins counter-clockwise to accomplish the polishing of the wafers  160 , as shown by directional arrow A in FIG.  6 . Each station  130 ,  140 ,  150  includes a polishing arm  131 ,  141 ,  151  which holds the wafers  160  during the polishing process. Each polishing arm  131 ,  141 ,  151  includes a U-shaped member for holding vacuum chucks  132 ,  142 ,  152 . Each polishing arm  131 ,  141 ,  151  includes one such vacuum chuck  132 ,  142 ,  152 . The vacuum chucks  132 ,  142 ,  152  are coupled to U-shaped members through plastic bearings  100  described above. The bearings  100  allow the vacuum chucks  132 ,  142 ,  152  to rotate with respect to the respective polishing arms  131 ,  141 ,  151 . The vacuum chucks  132 ,  142 ,  512  operate to hold the wafers  160  during the polishing process. Each of the vacuum chucks  132 ,  142 ,  152  includes an upper end  133 ,  143 ,  153  which may be coupled to a vacuum device (not shown) to provide a vacuum to the vacuum chucks. The polishing apparatus  110  also includes a slurry dispenser  170  which produces slurry which is dispensed onto a top surface  121  of the polishing pad  120 . 
     The polishing apparatus  110  described above substantially limits the “lock up” problems experienced by conventional apparatus (e.g. apparatus  10  shown in FIG.  1 ). For example, the metal bearings of conventional apparatus (e.g. apparatus  10 ) can “lock up” (i.e. prohibit rotation) due to slurry which becomes lodged in the bearings during the polishing process. Since the above apparatus  110  utilizes plastic bearings  100  (rather than metal bearings used in the conventional apparatus  10 ) in the polishing arms  131 ,  141 ,  151 , the slurry dispensed by slurry dispenser  170  does not cause “lock up” of the bearings should it become lodged therein. The plastic bearings  100  are unaffected by the slurry, and therefore rotates normally even in the presence of the slurry. Accordingly, the polishing apparatus  110  of the exemplary embodiment of the present invention lasts longer and requires less maintenance than conventional apparatus. 
     Although the polishing apparatus  110  described above has been described as having three polishing stations  130 ,  140 ,  150 , it should be noted that the plastic bearing  100  and polishing arm of the exemplary embodiment of the present invention can be used in a polishing apparatus including only one or any plurality of polishing stations. Further, although the polishing arms (e.g. polishing arm  131 ) described above include an extension member (e.g. extension member  135 ) and a U-shaped member (e.g. U-shaped member  137 ), the polishing arm of the exemplary embodiment of the present invention may be of any shape known to those skilled in the art, as long as the polishing arm includes bearings. Additionally, although the polishing pad  120  of the polishing apparatus  110  is described above as preferably rotating counter-clockwise, the polishing pad may also rotate clockwise without departing from the scope of the invention. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.