Patent Publication Number: US-6217419-B1

Title: Chemical-mechanical polisher

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of chemical-mechanical polishing, and more particularly, this invention relates to the field of chemical-mechanical polishing using a polishing table that provides a counter force against downward pressure exerted by a semiconductor wafer carrier. 
     BACKGROUND OF THE INVENTION 
     Different types of prior art rotary polishers are used in chemical-mechanical polishing of semiconductor wafers, where a thin layer of semiconductor material is planarized during the overall semiconductor manufacturing operation. The chemical-mechanical polishers typically have a heavy, circular polishing table made from a metal material or rigid ceramic material. The top surface is usually machined flat or formed smooth, and a polishing pad is glued onto the top surface. The polishing table is a predetermined diameter that is larger than the diameter of any semiconductor wafer that will be planarized in the chemical-mechanical polishing step. An abrasive slurry is fed onto the top surface of the polishing table. Typically, a semiconductor wafer carrier is positioned over the polishing table at a position for engaging the top polishing surface. The semiconductor wafer carrier holds the semiconductor wafer through an appropriate mechanism and places the semiconductor wafer against a polishing pad that is positioned on the top surface of the polishing table. 
     Standard prior art polishing pads in some rotary polishers usually consist of a stack with a hard layer on top of a soft under-pad. The hard pad produces a local planarization of topographical features, while the soft layer allows the stack to conform globally to the wafer shape. The use of a soft under-layer degrades the planarization ability of the top pad. However, a single layer hard pad cannot be used on a standard rotary polisher because high spots in the wafer will polish preferentially, resulting in very poor within-wafer uniformity. In order to improve global uniformity, the downward biasing force can be increased to a very high value, about 10 pounds per square inch, which flattens the semiconductor wafer and allows the pad to conform better to the semiconductor wafer. However, within-die uniformity is degraded as the downward directed force increases. 
     One recently developed chemical-mechanical polishing tool uses a “belt sander” approach. A single hard pad is mounted on a thin metal belt. The linear motion of the belt is used to polish the wafer. Good global uniformity can be achieved by tailoring the pressure behind the metal belt under the wafer. However, it is not always desirable to use a reciprocating movement because the technology of most chemical-mechanical polishing tools are directed to the rotary type of polishing table. An example of a reciprocating polishing table is the rectangular configured table shown and disclosed in U.S. Pat. No. 5,908,530 to Hoshizaki et al. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a chemical-mechanical polishing apparatus using a rotary polishing table that can be used in conjunction with a low down-force (less than 5 psi) and a high rotary table speed to maintain a high removal rate without sacrificing the thin wafer uniformity. 
     In accordance with the present invention, a chemical-mechanical polishing apparatus includes a polishing table having a top surface and an annular trench formed in the top surface and defining an annular configured polishing area in the polishing table. A drive mechanism rotates the polishing table. An annular diaphragm positioned within the annular configured polishing area. The annular diaphragm has a top surface and a bottom surface. A fluid actuated pressure mechanism is associated with the annular configured polishing area and exerts pressure upward onto the bottom surface of the annular diaphragm as the polishing table rotates for exerting an upward biasing pressure onto a polishing pad and imparting a desired counter force against any downward pressure exerted against by semiconductor wafer carrier during chemical-mechanical polishing. 
     The fluid actuated pressure mechanism includes a circular fluid head positioned under the polishing table in a predetermined area of the annular configured polishing area in which a semiconductor wafer will be positioned. It includes a plurality of fluid directing orifices through which fluid is directed through the fluid head upward against the bottom surface of the annular diaphragm. 
     In still another aspect of the present invention, an annular trench is formed within the polishing table and defined as the annular configured polishing area in the polishing table. A plurality of supports extend across the annular trench for supporting the annular diaphragm. In still another aspect of the present invention, a semiconductor wafer carrier is positioned over the polishing table at a position for engaging the top polishing surface of the annular diaphragm and holding a semiconductor wafer and placing the semiconductor wafer against a polishing pad positioned on the top surface of the polishing table. The top polishing surface of the annular diaphragm is preferably substantially coplanar with the top surface of the polishing table. A source of fluid can be connected to the fluid actuated pressure mechanism for supplying fluid under pressure. The source of fluid can comprise an air pump. The fluid actuated pressure mechanism can also include a plurality of concentric fluid carrying tubes for exerting pressure against the bottom of the annular diaphragm. 
     In still another aspect of the present invention, the chemical-mechanical polishing apparatus includes a polishing table having a top surface and an annular trench formed in the top surface that is dimensioned wider across the trench than the diameter of a semiconductor wafer to be polished. A drive mechanism rotates the polishing table and a biasing mechanism is positioned within the annular trench and comprises a plurality of concentric fluid carrying tubes positioned within the trench for carrying fluid under pressure. 
     An annular diaphragm is placed within the annular trench and engages the biasing mechanism. The annular diaphragm has a top surface for polishing a semiconductor wafer. A predetermined fluid pressure within the fluid carrying tubes exerts a desired upward biasing pressure through the biasing mechanism onto the annular diaphragm for imparting a desired counter force via a polishing pad against downward pressure exerted against a semiconductor wafer during chemical-mechanical polishing. 
     In still another aspect of the present invention, the chemical-mechanical polishing apparatus includes a polishing table having a top surface and a plurality of openings extending through the polishing table in an annular configuration to form an annular configured polishing area in the polishing table. Each opening is dimensioned wider than the diameter of a semiconductor wafer to be polished. A drive mechanism rotates the polishing table and an annular diaphragm is formed from a substantially rigid material positioned within the annular configured polishing area. The annular diaphragm has a top polishing surface and bottom surface. 
     A fluid directing mechanism is positioned under the polishing table at a predetermined area of the annular configured polishing area in which a semiconductor wafer is to be positioned for directing fluid under pressure and upward through the openings as the polishing table rotates, and onto the bottom surface of the annular diaphragm and onto the polishing pad. It thus exerts an upward biasing pressure onto the annular diaphragm and imparts a desired counter force against any downward pressure exerted against a semiconductor wafer carrier during chemical-mechanical polishing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent from the detailed description of the invention which follows, when considered in light of the accompanying drawings in which: 
     FIG. 1 is a top plan view of a chemical-mechanical polishing apparatus of the present invention. 
     FIG. 2 is a cross-section of one aspect of the chemical-mechanical apparatus of the present invention taken along line  2 — 2  of FIG.  1  and showing a biasing mechanism that carries a plurality of concentric fluid carrying tubes. 
     FIG. 3 is another schematic cross-section drawing of one aspect of the present invention showing the use of a fluid directing mechanism positioned under the polishing table at a predetermined area of an annular configured polishing area. 
     FIG. 4 is a top plan view of a polishing table without the diaphragm, and the polishing pad to show the fluid directing mechanism formed as a circular fluid head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is advantageous because it now provides a chemical-mechanical polishing apparatus and associated method using a novel and unobvious chemical-mechanical polishing set-up where a single layer hard pad can be used in conjunction with a low downward directed force, such as less than 5 psi, and a high rotating table speed to maintain a high removal rate without sacrificing the thin wafer uniformity. 
     Referring now to FIGS. 1 and 2, there is illustrated a first aspect of the present invention with a plan view of the chemical-mechanical polishing apparatus  10  of the present invention and a schematic cross-section view shown in FIG.  2 . The chemical-mechanical polishing apparatus  10  includes a polishing table  12 , such as formed from a metal or ceramic material, as known to those skilled in the art. The polishing table  12  includes a vertical spindle  14  connected to the bottom surface of the polishing table  10 . The spindle is connected to a drive motor  16  that provides the drive force necessary for rotating the polishing table  12  at high operating speeds with increased revolutions per minute. The drive motor  16  could be directly connected to the spindle as illustrated, or connected to a drive mechanism, such as a pulley (not illustrated). The spindle  14  could be secured to the bottom surface of the polishing table by appropriate fasteners, such as bolts or by welding, if there is metal-to-metal contact. 
     The polishing table  12  also has a top surface  19  that is a larger diameter than the diameter of a semiconductor wafer W to be polished via the chemical-mechanical polishing apparatus  10 . As is well known to those skilled in the art, an abrasive slurry is pumped via slurry delivery system  18  onto the top surface  19  of the polishing table to provide the necessary chemical and abrasive action used in chemical-mechanical polishing to planarize the surface of a semiconductor wafer. 
     In accordance with the present invention, the polishing table includes an annular trench  20  formed in the top surface  19  and defines an annular configured polishing area, indicated generally at  22 , in the polishing table  12 . In the embodiment shown in the FIG. 2 cross-section, the annular trench  20  is formed as a milled surface, such as could be formed in a ceramic or metal polishing table  12 . In that aspect of the invention shown in FIG. 2, the trench  20  does not extend completely through the polishing table, but forms a channel with a bottom surface, as illustrated. A fluid biasing mechanism, indicated generally at  24 , is positioned within the annular trench  20 . An annular diaphragm  26 , such as formed from a substantially thin but rigid material, such as a thin metal, is positioned over and preferably on top of the fluid biasing mechanism  24 . Other rigid materials could be used for the diaphragm as suggested by those skilled in the art. 
     The annular trench  20  is formed wider than the wafer diameter that will be chemically-mechanically polished on the polishing table. The annular trench  20  shown in FIG. 2 actually forms an annular channel having a depth to allow a fluid biasing mechanism in the form of concentric fluid carrying tubes  28  contained within a rubber matrix  29  to engage the annular diaphragm, as will be explained below. An annular configured polishing pad  30  is positioned on the annular diaphragm on which the chemical-mechanical polishing occurs. The polishing pad is illustrated a annular configured, but could be other shapes as suggested by those skilled in the art, as long as the annular diaphragm exerts the necessary pressure. The annular diaphragm  26  could be formed substantially coplanar with the top surface  19  of the polishing table. 
     The fluid carrying tubes  28  are positioned in the annular trench  20  and connect to a source of fluid pressure  31 , such as an air source  32  or fluid source  34 . The fluid carrying tubes  28  are preferably flexible to allow expansion within the rubber matrix  29 . Thus, a controller  36  can be connected to the source of fluid pressure  31  to change the pressure exerted within the fluid carrying tubes  28  and expand the rubber matrix  29  onto the bottom surface annular diaphragm to which it is engaged and against the polishing pad  30  positioned on the annular diaphragm  26  to exert a desired upward biasing pressure through the biasing mechanism formed by the rubber matrix and fluid carrying tubes onto the annular diaphragm and thus the polishing pad. As a result, a desired counter force is imparted against downward pressure exerted against a semiconductor wafer during chemical-mechanical polishing. It is possible to place a valve  37  into the fluid carrying tubes. The valve could be adjusted to adjust pressure. 
     As illustrated, a semiconductor wafer carrier  40  is positioned over the polishing table  12  at a position for engaging the top polishing surface of the polishing pad  30  positioned on the annular diaphragm. The wafer carrier  40  holds and places the semiconductor wafer against the top polishing surface of the polishing pad  30 . The semiconductor wafer carrier  40  can hold the semiconductor wafer by vacuum or other techniques, as well known to those skilled in the art. The wafer carrier  40  can be moved downward to apply a predetermined pressure, which is then counteracted by the fluid biasing mechanism  24  against the annular diaphragm  26  positioned within the annular trench. 
     In still another aspect of the present invention shown in FIGS. 3 and 4, the polishing table  50  has a top surface  52  and a plurality of openings  54  extending through the polishing table in an annular configuration to form the annular configured polishing area  56  in the polishing table  50  (FIG.  4 ). Each opening  54  is dimensioned wider than the diameter of the semiconductor wafer to be polished. A plurality of supports  58 , such as wheel spokes, extend across the annular trench (or channel) that extends through the polishing table and defines supports for supporting an annular diaphragm  60 . A support pad  62  can be positioned under the annular diaphragm  60 . The support pad  62  could be formed from plastic, rubber or other materials known to those skilled in the art to provide sufficient support. 
     As shown in FIG. 4, the polishing table  50  has the configuration of a wheel with openings  54 . The outer circumference of the polishing table  50  forms the rim of the wheel. The openings  54  form an annular trench  64  in the polishing area  56 . Thus, the term “trench” as used herein includes both milled channels and milled trenches that extend through the polishing table  12 . The polishing pads  62  is supported by the annular diaphragm  60 . 
     A fluid directing mechanism  66  is positioned under the polishing table  50  at a predetermined area of the annular configured polishing area  56  in which a semiconductor wafer is to be positioned for directing fluid under pressure upward through the openings  54  as the polishing table  50  rotates and onto the bottom surface of the annular diaphragm via the support pad  62 , and exerts an upward biasing pressure onto the annular diaphragm. Thus, a desired counter force is imparted against any downward pressure exerted against a semiconductor wafer during chemical-mechanical polishing. 
     In one aspect of the present invention, the fluid directing mechanism is formed as a circular fluid head  68  similar in function to a shower head having a plurality of fluid directing orifices  70  through which fluid is directed through the fluid head  68  upward against the support pad. The annular diaphragm  60  has a top surface that is substantially coplanar with the top surface of the polishing table. The source of fluid  72  that can be connected to the fluid directing mechanism can be a source of air  74  or source of water  76 . A controller  80  can be connected to the source of air or source of fluid for providing the necessary pressure control to change the pressure exerted against the diaphragm. 
     Thus, the annular diaphragm provides the necessary control over the structure and polishing operation. Naturally, the term “annular diaphragm” corresponds to having separate pie-shaped pieces that are supported by the “wheel spokes” forming the plurality of supports. Thus, the overall structure of the separate pieces would form an annular diaphragm and function purposes. 
     The present invention is advantageous because it now allows very desirable chemical-mechanical polishing operation where a single layer hard pad can be used on conjunction with the low down force of less than 5 pounds psi and a high table speed. The use of the metal diaphragm with the formed trench or channel is advantageous as noted above. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that the modifications and embodiments are intended to be included within the scope of the dependent claims.