Abstract:
A carrier head for supporting a wafer during a chemical mechanical polishing process is provided. In one exemplary implementation, the carrier head may comprise a wear ring, and a slidably movable pressure plate disposed within the wear ring. The pressure plate may be arranged to provide slidable movement relative to the wear ring. This degree of freedom can accommodate height dimensional changes that may occur in the wear ring.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor processing equipment, and, more particularly, to a carrier head for holding a semiconductor wafer during chemical-mechanical polishing. 
     BACKGROUND OF THE INVENTION 
     One commonly used technique for planarizing the surface of a wafer is chemical mechanical polishing (CMP). In a CMP process a wafer, as may be held by a wafer carrier head, is pressed against a polishing pad in the presence of a polishing slurry, and relative motion (rotational, orbital, linear, or a combination of these) between the wafer and the polishing pad is initiated. The mechanical abrasion of the wafer surface combined with the chemical interaction of the slurry with the material on the wafer surface ideally produces a planar surface. 
     As shown in  FIG. 1 , some known carrier heads, such as a carrier head  8 , may include a flexible membrane  10  that contacts the back or unpolished surface of a wafer  12 . A pressure chamber  14  may be defined in a frame  16  of the carrier head and such a pressure chamber  14  may be disposed behind the flexible membrane  10 . Pressure chamber  14  is pressurized by a suitable externally connected pressurizing equipment (not shown) by way of a pressurizing port  15  so that a desired amount of compression force can be applied through membrane  10  to the back surface of the wafer to force the wafer  12  into contact with the polishing pad with a controlled pressure. 
     The carrier head also generally includes a wear ring  18  (sometimes referred to in the art as a “retaining ring” or “edge ring” but hereinafter referred to without limitation as a “wear ring”) that may be attached to an outer periphery of frame  16  and to a corresponding portion of membrane  10  (e.g., by way of glue or mechanical attachments  21 ). The wear ring  18  includes a surface  19  in frictional engagement with a polishing pad  20  and is configured to support the carrier head and wafer relative to the polishing pad  20 . Surface  19  of wear ring  18  can eventually experience some erosion as a function of usage. That is, the height dimension of wear ring  18  identified by line  22  will be reduced and this height reduction of wear ring  18  tends to deform the flexible membrane  10  and create a discontinuity in the pressure applied to the wafer (and to the polishing pad) at or near a gap  24  between the wafer edge and the edge of the wear ring. This may adversely affect the uniformity of the surface of the wafer being polished, particularly near the wafer edge, since the non-uniform compression near the wafer edge may cause uneven polishing at that region of the wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of one known carrier head, as may be used for planarizing the surface of a wafer in a chemical mechanical polishing (CMP) process. 
         FIG. 2  is a cross sectional view of one exemplary embodiment of a carrier head embodying aspects of the present invention. 
         FIG. 3  is a cross sectional view of another exemplary embodiment of a carrier head embodying aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a cross sectional view of one exemplary embodiment of a carrier head  30  embodying aspects of the present invention. For purposes of simplicity of illustration,  FIG. 2  takes advantage of the symmetrical characteristics of a circular structure and just illustrates one half of the cross-section of circular carrier head  30  since the other half would comprise a duplicate of the illustrated section. The inventors of the present invention have innovatively recognized low-cost and mechanically straightforward structure for avoiding or reducing effects that, as described in the context of  FIG. 1 , could develop as the height of a wear ring  32  becomes reduced. Thus, aspects of the present invention are directed to alleviating non-uniform compression effects that could be produced near the wafer edge as the height of wear ring  32  becomes reduced and could cause uneven polishing at that region of the wafer. 
     In one exemplary embodiment, carrier head  30  comprises a pressure plate  34  configured to provide slidable movement relative to wear ring  32 . That is, as wear ring surface  36  becomes eroded due to frictional engagement with the polishing pad (not shown), the pressure plate  34  provides a degree of freedom that allows such a plate to slide relative to wear ring  32 , thereby maintaining a substantially uniform compressive load relative to the back surface of a wafer  37 , and in turn relative to the polishing pad. The pressure plate  34  may be analogized to a piston slidably moveable in a cylinder defined by inner wall  39  of the wear ring  32 . 
     In one exemplary embodiment, wear ring  32  includes a recess  38  that in part defines a shoulder  40  positioned to engage a stop  42  constructed in the pressure plate  34 . For example, engagement of stop  42  against shoulder  40  would provide a limit to downward travel of pressure plate  34 , as may occur in response to pressurization buildup in a pressure chamber  44 . In one exemplary embodiment, pressure chamber  44  is defined by a bottom surface  46  of a frame  48  of the carrier head. Pressure chamber  52  is further defined by a top surface  50  of pressure plate  34  and the surfaces that define recess  38 . When pressure chamber  52  is pressurized by a suitable externally connected pressurizing equipment (not shown) by way of a pressurizing port  52 , a compressive force is applied by pressure plate  34  against the corresponding surface of the wafer  37 . In order to maintain an airtight seal in the pressure chamber  52 , a seal  54  (e.g., an O-ring or other suitable sealing structure) may be provided around the pressure plate  34 . 
     In operation, pressure plate  34  acts as a piston that, regardless of dimensional changes in the height of wear ring  32 , is able to apply a substantially uniform compression force to the wafer  37 . Pressure plate  34  may be made up of a sufficiently rigid material, such as steel, Teflon polymer, ceramic, polymide, relatively hard plastic or similar materials. 
       FIG. 3  is a cross sectional view of another exemplary embodiment of a carrier head  60  embodying aspects of the present invention. In this embodiment a flexible membrane  62  contacts the back or unpolished surface of a wafer  64 . Flexible membrane  62  may be attached to an outer periphery of a frame  66  and to a wear ring  67  (e.g., by way of attachments  68 , such as glue, epoxy, mechanical fasteners, etc.). More particularly, the flexible membrane  62  is configured to provide a preformed segment  70  (e.g., an L-shaped segment or any other shape that may provide a sufficient degree of bending relative to wear ring  67 ). That is, as the height of wear ring  67  becomes reduced, in lieu of creating a sharp mechanical discontinuity that tends to deform the flexible membrane  62  and create a discontinuity in the pressure applied to the wafer (and to the polishing pad), preformed segment  70  allows a gradual mechanical transition in the flexible membrane  62  at or near a gap  71  between the wafer edge and the edge of the wear ring, thereby reducing discontinuity in the pressure applied to the wafer. 
     A pressure chamber  72  when pressurized may function to provide a substantially constant compression force against the entire back surface of the wafer. In yet another aspect of the present invention, if optionally desired, the pressure chamber  72  may receive a plurality of inflatable bladders  74  connected by a manifold  76  to pressurizing equipment (not shown) that would allow applying a distinct (or equal) amount of pressure to the inflatable bladders. Each inflatable bladder may be analogized to a pneumatic tube in a tire. This option would provide the ability to create regions in flexible membrane  62  comprising distinct levels of compression force. The inflatable bladders may take any desired shape, such as toroidal, annular rings, concentric circles, etc. Thus, this embodiment, in addition to alleviating undesirable effects that could arise due to dimensional changes in the height of wear ring  67 , may be optionally used to simultaneously apply selectably distinct levels of compression force to the wafer. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.