Patent Document

[0001]     This application claims the benefit of Korean Application No. 10-2005-0039779, filed on May 12, 2005, which is incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus for chemical mechanical polishing of a semiconductor wafer. More specifically, the present invention relates to a wafer carrier assembly for use in a chemical mechanical polishing apparatus.  
         [0004]     2. Description of the Related Art  
         [0005]     Chemical-mechanical polishing (CMP) process plays an important role in high-integration and ultraminiaturization of a semiconductor device. Typically, a semiconductor device including high density circuits requires closely spaced multilevel metallization lines. Thus, a poor surface planarity of a semiconductor wafer exerts a bad influence upon interconnection between metallization lines. CMP technology has been developed as demand for a global planarization of a semiconductor wafer to miniaturize a circuit dimension.  
         [0006]     CMP removes material from the surface of the wafer by chemical and mechanical action(s), in the production of ultra-high density integrated circuits. In a typical CMP process, the top layer of the wafer is exposed to an abrasive medium under controlled chemical, pressure, velocity, and temperature conditions. Conventional abrasive media include slurries and polishing pads.  
         [0007]      FIG. 1  shows a perspective view of a conventional CMP apparatus. Referring to  FIG. 1 , CMP apparatus  10  utilizes a mechanical working of polishing pad  12  and a chemical working of slurry  15  to polish a surface of a wafer. Polishing pad  12  is attached on polishing table or turntable  11 , and slurry  15  is supplied on polishing pad  12  by slurry providing unit  14 . A wafer is contained or arranged in wafer carrier head  13 . Here, turntable  11  is simply rotated, but wafer carrier head  13  is rotated and a downward force is applied by wafer carrier support frame  13   a . In general, a wafer is fastened to wafer carrier head  13  by a ceramic material of carrier ring  20 . A wafer contacts polishing pad  12  by a load of wafer carrier head  13  and a pressure of wafer carrier support frame  13   a . As slurry  15  moves between a wafer and polishing pad  12 , material is removed from the top layer of the wafer mechanically by the abrasive particles in slurry solution  15  and/or surface protrusions of polishing pad  12 , and chemically by the chemicals in slurry solution  15 .  
         [0008]     Meanwhile, in such CMP process, the surface of the wafer to be polished closely contacts with polishing pad  12 . Accordingly, the distribution of pressure between the wafer and polishing pad  12  affects the resultant surface planarity of the wafer. Especially, when carrier ring  20  for fastening the wafer weighs down polishing pad  12  by a pressure of wafer carrier support frame  13   a , unwanted ripples may be generated on polishing pad  12 , owing to the pad bounding effect.  
         [0009]     Specifically, referring to  FIG. 2 , as carrier ring  20  presses polishing pad  12 , a portion of polishing pad  12  protrudes in the vicinity of the edges of carrier ring  20 . In general, carrier ring  20  is formed of ceramic material, but polishing pad  12  is formed of elastic material. Therefore, when carrier ring  20  presses polishing pad  12 , polishing pad  12  is partially deformed so that a ripple  12   a  occurs in the vicinity of carrier ring  20 . The pressure applied to the edge of wafer, near the ripples  12   a , is much larger than that applied to the center of wafer. In other words, a polishing rate of the edge of wafer is relatively increased, comparing with that of the center of wafer. As a result, the polished surface of wafer becomes uneven or non-uniform, which may decrease the yield of semiconductor devices from the wafer.  
       SUMMARY OF THE INVENTION  
       [0010]     It is, therefore, an object of the present invention to provide a wafer carrier assembly and a chemical mechanical polishing apparatus including the same, which can prevent a periphery of a semiconductor wafer from being overpolished during a chemical mechanical polishing process, owing to the deformation of polishing pad.  
         [0011]     To achieve the above objects, an embodiment of a wafer carrier for use in a chemical mechanical polishing apparatus, according to the present invention, comprises: a wafer carrier head rotatably mounted on the apparatus, having a lower surface adapted for contacting a wafer; a retaining ring attached to and/or extending from the lower surface of the wafer carrier head, for retaining the wafer on the wafer carrier head during rotational motion thereof; and a guard ring attached to and/or extending from the lower surface of the wafer carrier head, a distance outside of the retaining ring.  
         [0012]     Especially, in the wafer carrier according to the present invention, an inner wall of the retaining ring is in contact with a periphery of the wafer. In addition, a protrusion height of the guard ring is substantially equal to or larger than that of the retaining ring, relative to a surface of or thickness of the wafer. Moreover, a bottom surface of the retaining ring is arranged on substantially the same plane as a polished surface of the wafer. Preferably, the wafer carrier further includes a resilient member positioned between the lower surface of the wafer carrier head and the wafer.  
         [0013]     In addition, a chemical mechanical polishing apparatus according to the present invention comprises: a turntable rotated by a driving means; a polishing pad positioned on an upper surface of the turntable; a wafer carrier support frame; a wafer carrier rotatably mounted on the wafer carrier support frame, for containing a wafer to be arranged opposite the polishing pad; and a slurry providing unit configured to provide a slurry on an upper surface of the polishing pad. Here, the wafer carrier includes: a wafer carrier head having a lower surface adapted to contact a wafer; a retaining ring attached on and/or extending from the lower surface of the wafer carrier head, for retaining the wafer during rotational motion; and a guard ring attached to and/or extending from the lower surface of the wafer carrier head a distance outside of the retaining ring.  
         [0014]     These and other aspects of the invention will become evident by reference to the following description of the invention, often referring to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a conventional CMP apparatus.  
         [0016]      FIG. 2  is a cross-sectional view of a conventional CMP apparatus, especially illustrating a wafer carrier assembly and a turntable on which a polishing pad is arranged.  
         [0017]      FIG. 3  is a cross-sectional view of an embodiment of a wafer carrier assembly according to the present invention, including a retaining ring and guard ring.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 3  illustrates a cross-sectional view of an embodiment of a wafer carrier assembly including a carrier ring comprising a retaining ring  22  and guard ring  24 , according to the present invention. Wafer carrier head  13  is rotated and forced downwardly by wafer carrier support frame  13   a  during a polishing process. A wafer, referred to as W, is arranged on a lower surface of wafer carrier head  13 . Additionally, a resilient member (e.g., a backing film, not shown) can be positioned between the lower surface of wafer carrier head  13  and the wafer W. Especially, wafer carrier head  13  can be equipped with means for applying vacuum to the wafer, for the purpose of firmly holding the wafer.  
         [0019]     The present invention includes retaining ring  22  and guard ring  24  as a carrier ring. Here, retaining ring  22  may function to fasten the wafer to wafer carrier head  13 , and it is attached on the lower surface of wafer carrier head  13 . Retaining ring  22  retains the wafer W, or prevents the wafer from moving or departing outwardly, during the rotational motion of the wafer carrier head  13 . Preferably, retaining ring  22  has a substantially annular form that can be adjustable to the periphery of the wafer. More preferably, an inner wall  22   a  of retaining ring  22  may be arranged to be in contact with the periphery of the wafer, thus effectively preventing the departure of the wafer from wafer carrier head  13 . In addition, a bottom surface of retaining ring  22  is preferably in substantially the same plane as a polished surface of the wafer. In other words, it is preferable to minimize the clearance and/or the compression between the bottom surface of retaining ring  22  and polishing pad  12 . When the clearance is too large, the wafer may depart from wafer carrier head  13  through a gap between retaining ring  22  and polishing pad  12 . When the compression is too great, a ripple may form inside of the retaining ring  22  and overpolish en edge region or periphery of the wafer. Thus, retaining ring  22  is preferably configured to have a thickness substantially equal to the thickness of the wafer and (if present) any resilient member or backing film on the wafer carrier head  13 .  
         [0020]     Guard ring  24  is attached on the lower surface of wafer carrier head  13 , a predetermined distance from retaining ring  22 . In other words, the inside diameter of guard ring  24  is larger than the outside diameter of retaining ring  22 . The present invention includes guard ring  24  as another carrier ring, as well as retaining ring  22  for fastening the wafer. Thus, two carrier rings (i.e., retaining ring  22  and guard ring  24 ) generally work together to prevent the periphery of the wafer from being overly polished due to the deformation of polishing pad  12 . The deformation of polishing pad  12  mostly occurs in the vicinity of guard ring  24 , particularly when guard ring  24  has a thickness such that its lower surface extends slightly beyond the bottom surface of retaining ring  22 . Therefore, polishing pad  12  rarely deforms in the vicinity of retaining ring  22  and the wafer.  
         [0021]     More specifically, as shown in  FIG. 3 , when wafer carrier head  13  is forced down by wafer carrier support frame  13   a , guard ring  24  primarily presses down polishing pad  12 . As a result, the protrusion portion  12   a  of polishing pad  12 , owing to the pad bounding effect or deformation of the pad, occurs intensively in the vicinity of guard ring  24 . In this case, the pad bounding phenomenon rarely occurs in the vicinity of retaining ring  22  and the wafer, thus the pressure working on the wafer becomes uniform over the periphery and the center of the wafer. Therefore, the polishing rates according to the chemical and mechanical polishing actions can be maintained constantly over the entire surface of the wafer.  
         [0022]     Meanwhile, a gap (referred to as G) between retaining ring  22  and guard ring  24  can be determined or controlled, based on the elasticity of polishing pad  12 , to minimize the pad bounding effect on the wafer. In the case where the polishing pad material has a large elasticity, a region where the pad bounding phenomenon occurs may be wide. In this case, the gap G is preferably relatively large for the pad bounding region not to reach the wafer. In addition, forming a width W 1  of guard ring  24  larger than a width W 2  of retaining ring  22  can minimize the extent of polishing pad&#39;s deformation in the inner region of guard ring  24 .  
         [0023]     It is preferable to form a protrusion height H of guard ring  24  larger than or equal to a protrusion height h of retaining ring  22 , in order to concentrate the pad bounding phenomenon in the vicinity of guard ring  24 . In the case where the protrusion height h of retaining ring  22  is larger than the protrusion height H of guard ring  22 , the pad bounding phenomenon may occur in the vicinity of retaining ring  22 . On the other hand, if the protrusion height H of guard ring  24  is unacceptably larger than the protrusion height h of retaining ring  22 , the wafer may become out of close contact with polishing pad  12 . Accordingly, it is preferable to appropriately control the protrusion heights of retaining ring  22  and guard ring  24 . Here, the protrusion height H or h is defined as a height based on a top surface of the wafer or the lower surface of wafer carrier head  13  in contact with the top surface of the wafer, not a real or actual height of retaining ring  22  or guard ring  24 . In one embodiment, the difference in the protrusion heights of (H−h) retaining ring  22  and guard ring  24  is less than the height of pas protrusion  12   a  under one or more given sets of polishing conditions (e.g., the downward force applied by wafer carrier head  13 , the elasticity and/or compressibility of polishing pad  12 , the width of gap G, the width[s] of retaining ring  22  and/or guard ring  24 , and/or the amount of and/or formulation of the slurry, etc.).  
         [0024]     The wafer carrier assembly, according to the present invention, includes a retaining ring for retaining a semiconductor wafer, and a guard ring as an additional carrier ring. The pad bounding phenomenon is generally concentrated in the vicinity of the guard ring. In other words, when the pad bounding phenomenon is prevented in the vicinity of the wafer, a pressure applied to the wafer can be maintained constantly and uniformly over an entire surface of the wafer to be polished. Therefore, a CMP apparatus including a wafer carrier assembly according to the present invention can prevent a periphery or edge region of a wafer from being overly polished, because a polishing rate is uniform over an entire surface of a wafer. As a result, the yield and reliability of semiconductor devices can be improved (in some cases, significantly so).  
         [0025]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Technology Category: b