Abstract:
Polishing heads to polish the surface of a semiconductor wafer and methods of using the same are disclosed. A disclosed polishing head includes at least one rotating head to apply a downward force; and a plurality of vacuum cells to hold the wafer via a vacuum force and to convey a least some of the downward force from the at least one rotating head to the wafer.

Description:
FIELD OF THE DISCLOSURE  
       [0001]     The present disclosure relates generally to semiconductor fabrication and, more particularly, to CMP polishing heads and method of using the same.  
       BACKGROUND  
       [0002]     As the integration of semiconductor devices increases, multi-layer interconnection technology has been put into practical use. Thus, local and global area planarization of an interlayer insulating layer has become important. A widely used CMP (Chemical Mechanical Polishing) method of polishing the surface of a semiconductor wafer employs chemical components contained in a slurry solution, a polishing pad, and a polishing agent.  
         [0003]     A CMP apparatus is most frequently used to polish the front face of a semiconductor wafer in fabricating semiconductor devices on the wafer. Generally, a wafer is planarized or softened at least one time during the fabricating process in order to make the surface of the wafer as flat as possible. In order to polish the wafer, the wafer is placed on a carrier, put into contact with a polishing pad covered with slurry and then pressed. Wafer polishing is then carried out by rotating the polishing pad and the wafer-loaded carrier.  
         [0004]     A prior art CMP apparatus for polishing a wafer includes a polishing platen, a polishing pad located over the polishing platen, a polishing head, and a retainer ring and/or membrane for holding the wafer in the bottom edge of the polishing head. The wafer is held in the retainer ring so that the surface of the wafer to be polished is disposed toward the polishing pad. The retainer ring has multiple grooves to facilitate the flow of polishing slurry to the surface of the wafer. The grooves are extended from the inner to the outer surface of the retainer ring. Each groove has a round shape structure.  
         [0005]     We will now look at the way in which the retainer ring and polishing pad are used in the prior art. To this end,  FIG. 1   a  through  FIG. 1   c  are schematic, cross-sectional, views which illustrate a retainer ring of a prior art polishing head.  FIG. 1   a  is a schematic, cross-sectional, view illustrating the structure of a prior art CMP polishing head  18  for polishing the surface of a wafer.  FIG. 1   b  and  FIG. 1   c  are schematic, cross-sectional, views which illustrate the retainer ring  14 .  FIG. 1   c  is a cross-sectional view of the channels  16  of the retainer ring  14  depicted in  FIG. 1   b.    
         [0006]     Referring to  FIG. 1   a , the prior art CMP apparatus has a polishing pad  10  which is covered with the flow of polishing slurry. It also has a polishing head  18 . The wafer  12  is positioned between the polishing pad  10  and the polishing head  18 . A retainer ring  14  is disposed at the bottom edge of the polishing head  18 . The retainer ring  14  holds the wafer  12  to prevent it from being derailed during the CMP process. The retainer ring  14  of the polishing head  18  has multiple grooves  16  to facilitate the flow of polishing slurry. The grooves  16  are extended from the inner surface of the retainer ring  14  to the outer surface of the retainer ring  14 . The slurry flows uniformly on the surface of the wafer  12 , since the grooves  16  act as passing channels to facilitate the flow of polishing slurry.  
         [0007]      FIG. 1   b  is a cross-sectional bottom view of the retainer ring  14 . As mentioned above, the retainer ring  14  has multiple grooves  16  which extend from its inner surface to its outer surface. As shown in  FIG. 1   b , the grooves  16  are circularly shaped and skewed at a predetermined angle toward the outer rim against the rotating direction of the retainer ring  14 . During a polishing process, the retainer ring  14  rotates with the desired speed and provides the whole area of the wafer  12  with a uniform flow of the polishing slurry through the grooves  16 .  
         [0008]     As shown in  FIG. 1   c , the cross-sections of the channels  16  have a round shape  16   a . Thus, the channels  16  facilitate the smooth flow of polishing slurry over the surface of the wafer in comparison with the rectangular shape of grooves sometimes employed in the prior art. The worn amount of the polishing pad may be reduced by preventing fast sticking of the polishing pad, so that the durability of the pad can be increased.  
         [0009]     Conventional CMP polishing heads have employed consumables such as a retainer ring or membrane, but these consumables cause a huge increase in maintenance costs.  
         [0010]     Volodarsky et al., U.S. Pat. No. 5,803,799, describes a polishing head for polishing a semiconductor wafer. The polishing head includes a housing, a wafer carrier movably mounted to the housing, and a wafer retainer movably mounted to the housing.  
         [0011]     Quek et al., U.S. Pat. No. 6,245,193, describes a substrate carrier head for use in a CMP apparatus.  
         [0012]     Park et al., U.S. Pat. No. 6,336,846, describes a chemical-mechanical polishing (CMP) apparatus having a polishing head onto which a semiconductor wafer is fixed for holding the surface of the semiconductor wafer in contact with the surface of a polishing pad. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1   a  through  FIG. 1   c  are cross-sectional views which schematically illustrate the retainer ring of a prior art polishing head.  
         [0014]      FIG. 2   a  through  FIG. 2   e  are cross-sectional views which schematically illustrate an example CMP polishing head. 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 2   a  schematically illustrates an example polishing head for polishing the surface of a wafer. The example polishing head of  FIG. 2   a  is connected with a rotating axis  20 . The polishing head comprises a first rotating head  21 , a second rotating head  23 , an air bag  22 , and a plurality of wafer holding cells  28 . The first rotating head  21  rotates and applies a downward force  30 . The second rotating head  23  is installed under the first rotating head  21 . The air bag  22  delivers the downward force  30  from the first rotating head  21  to the second rotating head  23 , and is positioned between the first and the second rotating head  21 ,  23 . The plurality of wafer holding cells  28  conveys the downward force  30  delivered from the second rotating head  23  to the wafer  26 . The wafer  26  is held in place by a vacuum method as explained further below. The plurality of wafer holding cells  28  is located under the second rotating head  23 .  
         [0016]     Conventional CMP polishing heads have held and released the wafer by employing either a retainer ring and membrane or a vacuum method employing a hole. However, the CMP polishing processes disclosed herein are carried out while holding the wafer  26  with a plurality of wafer holding cells  28  distributed throughout the whole area of the backside of the wafer  26 . Each wafer holding cell  28  has a vacuum line  24  and applies a vacuum force to the wafer.  
         [0017]      FIG. 2   b  schematically illustrates an example wafer holding cell  28 . Referring to  FIG. 2   b , the wafer holding cell  28  assists in holding the wafer  26  to prevent the wafer  26  from being derailed during a CMP process. The holding end of the wafer holding cell  28  has a wider area than its opposite end. The wafer holding cells  28  have a funnel shape and have a substantially circular cross-section through the funnel shape.  
         [0018]      FIG. 2   c  is a schematic, horizontal view of the example CMP polishing head of  FIG. 2   a . A method for operating a CMP polishing head of  FIG. 2   a  comprises: holding a wafer by vacuum force with a plurality of wafer holding cells, wherein each of the wafer holding cells comprises a vacuum line and is connected with a second rotating head, performing a CMP process by rotating the wafer while applying downward force to the wafer through the plurality of wafer holding cells, and unloading the vacuum force from the plurality of wafer holding cells when the CMP process is completed. In other words, the wafer holding cells  28  engage and hold the backside of the wafer  26  through a vacuum line  24  positioned at the center of the inner portion in order to prevent the wafer  26  from being derailed. The vacuum force from the plurality of wafer holding cells  28  is greater than the downward force  30 . In order to apply the downward force  30  from the second rotating head  23  to the whole area of the wafer  26 , the wafer holding cells  28  are substantially evenly distributed on the backside of the wafer  26 . Thus, we expect to effectively improve uniformity within the wafer.  
         [0019]     Preferably, the material for the plurality of wafer holding cells  28  comprises flexible rubber. The plurality of wafer holding cells  28  can hold and release the wafer  26  through the vacuum line  24  at the center of the inner portion. Also, the plurality of wafer holding cells  28  are uniformly positioned on the backside of the wafer  26  so that, unlike conventional polishing heads, the polishing head illustrated herein does not employ a retainer ring and/or a membrane in a polishing platen.  
         [0020]     As the pressure applied to the wafer  26  is given to the uniformly arrayed plurality of wafer holding cells  28  through the air bag  22  disposed between the first rotating head  21  and the second rotating head  23 , the quickly rotating surface of the wafer  26  is polished. The air bag  22  controls the pressure applied to the wafer  26 . The wafer holding cells  28  can hold and release the wafer  26 .  
         [0021]      FIG. 2   d  schematically shows the reverse of  FIG. 2   c . Referring to  FIG. 2   d , the top view depicts the same pattern of the bottom surface of a wafer holding cell as  FIG. 2   e.    
         [0022]      FIG. 2   e  schematically shows the pattern of the bottom surface of a wafer holding cell  28 . Thus, looking up at the bottom surface of an example wafer holding cell  28  as in  FIG. 2   e , we see several concentric circles representative of a conical funnel.  
         [0023]     The disclosed CMP polishing head and methods of use of the same reduce the substantial maintenance costs of consumables such as the retainer ring and membrane employed in prior art CMP polishing heads. In particular, the apparatus and methods disclosed herein avoid employing consumables such as the retainer ring and the membrane by providing the downward force used in polishing with an air bag while holding the wafer from the backside of the wafer with a vacuum.  
         [0024]     From the foregoing, persons of ordinary skill in the art will appreciate that the above disclosed methods and apparatus reduce the enormous maintenance cost associated with prior art polishing heads by eliminating the retainer ring and membrane used in the prior art. Further, the above disclosed methods and apparatus achieve improved polishing uniformity of the surface of a wafer.  
         [0025]     From the foregoing, persons of ordinary skill in the art will appreciate that the illustrated CMP polishing head holds and rotates a wafer with downward force to polish the surface of the wafer in a CMP process. The illustrated polishing head comprises a first rotating head to apply a downward force. The first rotating head is connected to a rotating axis. The illustrated polishing head also includes a second rotating head installed under and coupled with the first rotating head, an air bag positioned between the first and the second rotating heads to deliver the downward force from the first rotating head to the second rotating head, and a plurality of wafer holding cells to convey the downward force from the second rotating head to the wafer. The plurality of wafer holding cells is connected under the second rotating head and holds the wafer via a vacuum.  
         [0026]     The illustrated CMP polishing head may be used by holding a wafer by vacuum force with a plurality of wafer holding cells, performing a CMP process by providing downward force through the plurality of wafer holding cells while rotating the wafer, and releasing the vacuum force from the plurality of wafer holding cells when the CMP process is completed. Each wafer holding cell is coupled to a vacuum line and is connected with a second rotating head.  
         [0027]     It is noted that this patent claims priority from Korean Patent Application Serial Number 10-2003-0047495, which was filed on Jul. 12, 2003, and is hereby incorporated by reference in its entirety.  
         [0028]     Although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.