Patent Publication Number: US-2010120335-A1

Title: Partial Contact Wafer Retaining Ring Apparatus

Description:
BACKGROUND 
     Wafer polishing is often performed during semiconductor fabrication operations to planarize the surface of the wafer between process steps. Common components of wafer polishing systems include a polishing surface, a clamping system for holding the wafer against the polishing surface, and a wafer retaining ring to restrict lateral motion of the wafer while it is being polished. 
     When polishing a wafer, it is often desired that the material removal rate be uniform across the diameter of the wafer, and that the profile of material removal be consistent between wafers. However, in light of the extremely small scale of modern integrated circuits, and the corresponding accuracy and precision required in fabrication steps such as polishing, desired levels of wafer-to-wafer reproducibility in polishing profiles may be difficult to achieve. 
     SUMMARY 
     Accordingly, various embodiments herein related to wafer polishing are disclosed. For example, one disclosed embodiment provides a wafer retaining ring comprising a ring for retaining the wafer, the ring having an inner diameter surface configured to restrict lateral wafer motion, and at least one interface surface configured to interface with a polishing surface. The interface surface comprises a recessed section adjacent to the ring inner diameter, configured to preclude contact between the recessed section and the polishing surface. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of an embodiment of a system for polishing wafers. 
         FIG. 2  shows a partial axisymmetric section view of an embodiment of a system for polishing wafers, and illustrates a deformation of a polishing pad under a wafer edge during a polishing process. 
         FIG. 3  shows a partial axisymmetric section view of another embodiment of a system for polishing wafers, and illustrates a deformation of a polishing pad at a location spaced from a wafer edge during a polishing process. 
         FIG. 4  illustrates a separation between areas of the polishing surface that have contacted the wafer, and areas of the polishing surface that have contacted the wafer retaining ring during a polishing process, of the embodiment of  FIG. 3 . 
         FIG. 5  shows a cutaway view of an embodiment of a wafer retaining ring. 
         FIG. 6  shows an alternate embodiment of the cutaway section of FIG. 
         FIG. 7  shows an alternate embodiment of the cutaway section of  FIG. 1 . 
         FIG. 8  shows an alternate embodiment of the cutaway section of  FIG. 1 . 
         FIG. 9  shows a plan view of an embodiment of a wafer retaining ring. 
         FIG. 10  shows a flow diagram depicting an embodiment of a method for polishing wafers. 
     
    
    
     DETAILED DESCRIPTION 
     Prior to discussing the partial contact wafer retaining ring apparatus, an example use environment is described.  FIG. 1  shows a block diagram of an embodiment of a wafer polishing system  100 . Wafer polishing system  100  comprises a retaining ring  102 , a polishing surface  104 , a wafer  106 , and a clamping device  108 . Wafer  106  is disposed within wafer retaining ring  102 . Retaining ring  102  is configured to move laterally with clamping device  108 . Retaining ring  102  and wafer  106  are held against polishing surface  104  by clamping device  108 . 
     Retaining ring  102  comprises any ring shaped device suitable for retaining a wafer during polishing. In one embodiment, retaining ring  102  comprises a partial contact wafer retaining ring. The term “partial contact” as used herein refers to a configuration of a retaining ring  102  in which the interface surface that contacts the polishing surface  104  comprises a recessed section located adjacent an inner diameter of the retaining ring  102 , configured to preclude contact between the recessed section and the polishing surface. The partial contact retaining ring configuration is described in more detail below. 
     Polishing surface  104  comprises any flat surface suitable for polishing a wafer. In one embodiment, polishing surface  104  comprises a polishing pad which is affixed to a wafer polishing head. 
     Wafer  106  comprises any flat disc shaped object suitable for polishing. In one embodiment, wafer  106  comprises a semiconductor wafer with one or more layers of thin film deposition product on its surface. In other embodiments, wafer  106  may comprise a similarly shaped piece of an alternate material such as glass, plastic, or metal, with or without a coating of thin film deposition product. 
     Clamping device  108  comprises any device suitable for urging a wafer and a wafer retaining ring against a polishing surface. In one embodiment, clamping device  108  comprises a wafer carrier head with a pressurized bladder. In other embodiments, clamping device  108  may comprise a mechanically driven rigid wafer carrier head. 
     During polishing, a relative lateral motion is established between polishing surface  104 , and clamping device  108 . Retaining ring  102 , which is configured to move laterally with clamping device  108 , restricts the lateral motion of wafer  106 . This results in a relative lateral motion also being established between wafer  106  and polishing surface  104 . In one embodiment, the relative lateral motion is an orbital motion. In some embodiments, the relative lateral motion may also include additional in-plane rotations or translations which are superimposed over the orbital motion. 
     When polishing a wafer, it is often desired that the material removal rate be uniform across the diameter of the wafer, and that the profile of material removal be consistent between wafers. Material removal rates near the edge of the wafer may be influenced when the wafer comes into contact with areas of the polishing surface that have also contacted the wafer retaining ring, or localized surface deformations on the polishing surface which are caused by interaction between the wafer retaining ring and the polishing surface. When either or both of these conditions occur, the uniformity of material removal across the diameter of the wafer, and the consistency of the material removal profile between wafers may be adversely affected. Additionally, areas of polishing surface  104  that contact both wafer  106  and retaining ring  102  may experience different wear characteristics than areas of polishing surface  104  that contact only wafer  106 . This may lead to further difficulties in achieving uniform removal profiles. 
       FIG. 2  shows an axisymmetric view of an embodiment of a wafer polishing system  100  that illustrates the above concerns. In this embodiment, wafer retaining ring  102  comprises a full contact retaining ring, and clamping device  108  comprises a pressurized bladder. The term “full contact” as used herein refers to a configuration of a retaining ring  102  in which the interface that contacts the polishing surface  104  is directly adjacent to the inner diameter of the retaining ring  102 . This is in contrast to the “partial contact” retaining ring mentioned above, in which the interface that contacts the polishing surface  104  comprises a recessed section located adjacent an inner diameter of the retaining ring  102 , configured to preclude contact between the recessed section and the polishing surface. 
     Continuing with  FIG. 2 , the outer edge of wafer  106  is directly adjacent to the interface between wafer retaining ring  102  and polishing surface  104 . Because of this proximity, a localized surface deformation  110 , which is present near the interface of retaining ring  102  and polishing surface  104 , may be introduced beneath the edge of wafer  106 . Further, a region of the polishing surface  104  adjacent to the inner diameter of retaining ring  102  contacts both wafer  106  and retaining ring  102  during polishing. This may cause this region of the polishing surface  104  to exhibit different wear characteristics, and therefore different polishing characteristics, than portions of the polishing surface that contact only the wafer  106 . As discussed above, these factors may cause a variation in the material removal rate across the diameter of wafer  106 , and may thereby compromise overall polishing uniformity. 
     The use of a partial contact wafer retaining ring may reduce the presence of such localized surface deformations near the edge of the wafer.  FIGS. 3 and 4  show an axisymmetric view of an alternate embodiment of a wafer polishing system  100 . In this embodiment, retaining ring  102  comprises a partial contact wafer retaining ring with a recessed section  302 . Recessed section  302  provides a geometric buffer between the edge of wafer  106 , and the area of interface between retaining ring  102  and wafer polishing surface  104 . First referring to  FIG. 3 , recessed section  302  is configured such that localized surface deformation  110  does not come into contact with wafer  106  during polishing. As discussed above, contact between the wafer and localized surface deformations on the polishing surface which are caused by interaction between the wafer retaining ring and the polishing surface may have an adverse effect on polishing uniformity. The partial contact wafer retaining ring reduces this adverse effect by precluding contact between the edge of the wafer and the localized surface deformations. 
     Next referring to  FIG. 4 , a relative lateral polishing motion between polishing surface  104  and the interfacing surfaces of wafer retaining ring  102  and wafer  106  is illustrated. Because the surfaces move relative to each other, contact between wafer  106  and areas of polishing surface  104  that have contacted wafer retaining ring  102  may occur if a full contact wafer retaining ring is used during polishing. In contrast, in this embodiment, recessed section  302  is configured such that wafer  106  does not come into contact with any areas of polishing surface  104  that have also contacted wafer retaining ring  102 . This may reduce any adverse polishing effects caused by overlapping contact between the two areas. 
     Moving now to the discussion of the partial contact wafer retaining ring apparatus,  FIG. 5  shows a cutaway view of one embodiment of a wafer retaining ring  102 , and illustrates a cross section  502  of wafer retaining ring  102 . Wafer retaining ring  102  comprises an inner diameter surface  504 , an interface surface  506 , and a recessed section  302 . Inner diameter surface  504  is located on the inner diameter of wafer retaining ring  102 . Interface surface  506  is located on the underside of wafer retaining ring  102 . Recessed section  302  is recessed from interface surface  506 , and is immediately adjacent to inner diameter surface  504 . 
     Wafer retaining ring  102  may be constructed of any material or combinations of materials suitable for retaining a wafer. In one example, wafer retaining ring  102  is constructed entirely of a polymer such as polyethylene terapthalate, polyetheretherketone, polyacetyl, or polyamideimide. In other embodiments, wafer retaining ring  102  may be formed from a ceramic material such as silicon carbide, or aluminum oxide. In still other embodiments, wafer retaining ring  102  may be constructed of a combination of ceramic and polymer materials. 
     Inner diameter surface  504  serves to restrict lateral motion of a wafer, and may be configured in any manner suitable for this purpose. For example, in one embodiment, inner diameter surface  504  comprises a continuous cylindrical surface which is perpendicular to interface surface  506 . In this embodiment, the vertical distance from surface  506  to the top edge of inner diameter surface  504  is equal to or larger than the thickness of a wafer. 
     In another embodiment, inner diameter surface  504  is configured to restrict the lateral motion of a wafer with one or more discrete surfaces of equal or non-equal size.  FIG. 6  shows an example of such an embodiment. In this example, inner diameter surface  504  comprises a cylindrical surface which is interrupted at regular intervals by a series of vertically oriented channels that span the entire vertical distance of the surface. It will be understood that the above specified geometric descriptions of inner diameter surface  104  are disclosed for the purpose of example, and that inner diameter surface  104  may have other suitable geometric configurations. 
     In yet another embodiment, inner diameter surface  504  comprises a removable surface  510 , which is configured to restrict lateral wafer motion.  FIG. 7  shows one example of this embodiment. In this example, removable surface  510  is sized to cover the entire inner diameter surface of retaining ring  102 . Removable surface  510  is joined to the inner diameter of retaining ring  102  through any suitable means, including but not limited to gluing, bonding, fastening, or press fitting. Removable surface  510  may be made from any material suitable for restraining a wafer during polishing. In one embodiment removable surface  510  is constructed of a soft material, such as polyethylene teraphthalate, or other suitable polymer. Such a material may help to reduce a possibility of wafer edge chipping or other such damage due to contact between the wafer edge and the inner diameter of the retaining ring. Likewise, the body of wafer retaining ring  102  is constructed of a material which is more resistant to wear, such as aluminum oxide. Such a material may help to reduce a frequency of retaining ring replacement, and therefore help to reduce system downtime. 
     The use of the removable surface  510  may allow the surface  510  to be removed and replaced separately from the body  102  of the retaining ring. Further, the use of a softer material for the removable surface  510  to contact the wafer and a harder material for the body  102  of the retaining ring may allow the removable surface  510  and the body  102  of the retaining ring to be serviced or replaced at different intervals, and therefore may help to save costs compared to the use of a soft material for the entire retaining ring. 
     Interface surface  506  provides an interface between wafer retaining ring  102  and a wafer polishing surface, and may be configured in any manner suitable for this purpose. For example, in one embodiment, interface surface  506  comprises an outermost planar surface on one side of wafer retaining ring  102 . In this embodiment, interface surface  506  forms a continuous planar surface, which is perpendicular to the cylindrical axis of wafer retaining ring  102 , and which spans the radial distance between recessed surface  302  and the outer diameter of wafer retaining ring  102 . It will be understood that this embodiment is described for the purpose of example, and that the interface surface may have any other suitable configuration. 
     Recessed section  302  provides a geometric buffer between interface surface  506 , and inner diameter surface  504 . During wafer polishing, this buffer creates a separation between the outer edge of the wafer being polished, and the inner diameter of interface surface  506 . By maintaining this separation, it may be possible to control the interaction between the edge of the wafer, and interface surface  506  during wafer polishing. For example, the separation distance may be configured to preclude contact between the wafer and any portion of the polishing surface that has come into contact with interface surface  506 . In another example the separation distance may be configured to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface. 
     Recessed section  302  may be configured in any manner suitable for the above described purpose. For example, in one embodiment, recessed section  302  is defined by a cylindrical recess which is concentric to the inner diameter of wafer retaining ring  102 . In this embodiment the recessed surface is approximately planar, and is parallel to interface surface  506 . The walls of the cylinder defining the outer diameter of recessed section  302  are concentric with the axis of wafer retaining ring  102 . The depth of recessed section  302  is fixed with respect to interface surface  506 , and may have any suitable dimension. For example, in embodiments configured for use with semiconductor wafers, recessed section  302  may have a maximum depth of one half the thickness of a wafer due to the location of the bevel on such wafers. In other embodiments, the recessed section  302  may have depths outside of this range, depending upon the specific configuration of the wafers for which retaining ring  102  will be used. The radial width of recessed section  302  is configured to preclude contact between the wafer and any portion of the polishing surface that has come into contact with the retaining ring, and to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface. 
     In another embodiment, recessed section  302  comprises one or more discrete surfaces of equal or non equal size.  FIG. 8  shows an example of this embodiment. In this example, recessed section  302  comprises a cylindrical recess which is concentric to the inner diameter or wafer retaining ring  102  and which is interrupted at regular intervals by a series of radially oriented channels which span the entire radial width of the surface. 
     In yet another embodiment, recessed section  302  comprises a varied diametric width.  FIG. 9  shows one example of this embodiment. In this example, the outer wall of recessed section  302  is defined by an elliptically shaped boundary. It will be understood that the above specified descriptions of recessed section  302  are disclosed for the purpose of example, and that recessed section  302  may have other suitable configurations. 
     As described above, the embodiments of partial contact wafer retaining rings detailed herein may be used during wafer polishing to preclude contact between the wafer and any portion of the polishing surface that has come into contact with the wafer retaining ring, and to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface.  FIG. 10  shows an embodiment of a method  1000  for polishing a wafer. Method  1000  comprises at  1002  placing a wafer on a polishing surface, then at  1004  restricting the lateral motion of the wafer with a retaining device, and then at  1006  clamping the wafer and retaining device against the polishing surface. 
     Continuing with  FIG. 10 , method  1000  next comprises, at  1008 , polishing the wafer such that the wafer does not contact any area of the polishing surface that has also contacted the retaining device. Any suitable method for polishing the wafer in this manner may be used. For example in one embodiment, the wafer is restrained during polishing with a partial contact wafer retaining ring that has been configured to preclude contact between the wafer and any area of the polishing surface that has also contacted the retaining device. 
     Polishing the wafer at  1008  may additionally be performed by polishing such the wafer such that the wafer does not come into contact with any localized surface deformations on the polishing surface that are caused by the interaction between the wafer retaining device and the polishing surface. Any suitable method for polishing the wafer in this manner may be used. For example in one embodiment, the wafer is restrained during polishing with a partial contact wafer retaining ring that has been configured to preclude contact between the wafer and any area of the polishing surface that has also contacted the retaining device. 
     It will be understood that the configurations and/or approaches described herein are exemplary in nature and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein as well as an and all equivalents thereof.