Patent Publication Number: US-2010112905-A1

Title: Wafer head template for chemical mechanical polishing and a method for its use

Description:
FIELD OF THE INVENTION  
     The present invention relates to a wafer template for chemical mechanical polishing and a method for its use. 
     BACKGROUND OF THE INVENTION  
     The present invention is a planar template for a CMP polishing tool possessing a means of securing a wafer in CMP polishing wherein the back surface of the said template is held to the polishing head by retaining means. 
     Chemical Mechanical Polishing (CMP) polishing tools, slurry, polishing heads, polishing pads and diamond conditioner disks form the key components of the equipment used to carry out CMP processes in recent years. These various pieces of equipment have been produced and marketed by several vendors to standards of reliable quality and effectiveness. The function of the slurry is to deliver continuously the mechanical abrasive particles and chemical components to the surface of the wafer and to provide a means of removing reaction products and wafer debris from the polishing surface. 
     The function of the polishing pad is to abrade the surface of the wafer during polishing in concert with the slurry. The polishing pad is typically made of polyurethane or a substance of similar properties and the surface is roughened to enhance its effectiveness as a polishing surface for the wafer. It is positioned face up both to hold the slurry and to facilitate and is rotated from below. The surface of the polishing pad is usually provided with long grooves that serve both to assist in the dispersion of slurry under the wafer head during polishing and to provide effective removal of spent slurry and wafer detritus which are typically swept off of the pad by centripetal force as the pad rotates. The pad rotates at a rate of typically about 20 to 100 RPM 
     The diamond conditioner discs serve the purpose of continual and consistent roughening of the polishing pad and are suspended from an arm or a bridge, platform or similar structure of the polishing tool so that they are pressed diamond-bearing face down into the polishing pad. Diamonds on the conditioner disc surface cut and roughen the pad during CMP operation. 
     This is necessary because the action of the slurry and the wafer on the pad quickly smooth the pad otherwise greatly diminishing its effectiveness and the rate of removal of the wafer surface during polishing. The diamond conditioner disk sits under a load on the polishing pad and is both rotated and moved back and for the between the center of the pad to ensure an even dispersal of roughening over the polishing pad. 
     The polishing head consists an apparatus for holding, supporting and rotating the wafer and is hung from a supporting arm, bridge, platform or similar structure of the polishing tool and holds the wafer face down on the face of the polishing pad while applying pressure and rotating it. 
     Depending upon the type of wafer to be polished and the specific objectives of the operator, a variety of loads, respective rotation rates and, in the case of the diamond conditioner disc, motions across the polishing pad surface may be employed. Likewise, the type, quantity and concentration of slurry may be varied to obtain different results. 
     The polishing head itself is a structure attached to the shaft that descending from the apparatus, resting on a bridge or platform typically part of the framework of the CMP polishing tool. The polishing head is suspended above the polishing pad except during polishing when it is rotated at between typically 20 and 100 RPM and lowered onto the pad under a load that may be controlled by the operator. 
     The face of the polishing head is typically a disc or membrane with a flat downward facing surface that holds the wafer by some means. The surface may be composed of a number of materials including ceramic or rubber plates or sheets. Where ceramic plates have been used it has been possible to equip the plate with a large number of small holes passing through it and to install a vacuum device behind the plate. Fujikoshi Machinery Corporation has been able to develop this kind of ceramic plate faced vacuum-backed polishing head for the polishing of silicon wafers. However, for the polishing of silicon dioxide and metal layers, this method of polishing has demonstrated problems with the ability to hold the wafer firmly to the wafer head surface. 
     To overcome this problem, Fujikoshi Machinery Corporation developed a template in the shape of a disc made of a single piece of PVC and machined so that there was a raised ring around the upward facing circumference of the template of a half centimetre to a centimetre or so in width and a height a half centimetre to a centimetre or so. This allowed them to firmly fit the plastic PVC template on the wafer head and it was effectively held with the vacuum delivered through pores in the ceramic plate. The silicon dioxide or other non-silicon type CMP wafers were affixed to the front of the template using a wet, porous polymeric backing film with a retaining ring. The raised ring serves the double purpose of holding the template in place and preventing slippage and keeping slurry from seeping back behind the template and into the vacuum system. This template with retaining ring design has the further quality that is easily removable and allows quick and efficient removal of and attachment of different wafers to the polishing tool resulting in a very considerable time savings and increase in process cost effectiveness. 
     However, these templates have the problem of being unable to delivery uniformity of polishing primarily due to the physical properties of the PVC material used. The PVC is machined into the desired shape using a lathe which tends to result in slightly thicker center that in turn distorts the wafer surface as it is pressed against the polishing pad leading to unacceptable levels of radial non-uniformity in polishing and a significantly flawed final product. 
     HOW THE PRESENT INVENTION OVERCOMES THE PROBLEMS OF THE PRIOR ART  
     For CMP polishers using ceramic plates of this type which are designed to hold the wafer in place by vacuum, the present invention avoids the problems of the prior art by using a template prepared from a stable and initially very flat material such as polycarbonate sheet which maintains a very smooth surface and precise thickness even under the conditions of CMP polishing and maintains the position of the wafer on the lower downward facing surface of the template of the present invention by means of a commercially available polymeric wafer backing film. The surface of the template can be further lapped or polished to achieve an even greater degree of flatness and it is easily possible to achieve the desired degree of flatness without machining. Additionally the dimensionally stable material used in the present invention is less likely to deform under stress than PVC which leads to even more stable support for the wafer on the wafer head. The critical part of the design is that the plate that holds the wafer backing film is made from a material that is manufactured with precisely controlled thickness uniformity. Thickness uniformity of the plate is the most critical element for obtaining polish rate uniformity on suitable polishing tools. 
     The template of the present invention is held in place not only by vacuum which holds it tightly against the said ceramic plate but also by a retaining ring attached to the said template which both serves to keep the template and by extension the wafer from slipping on the ceramic plate and also prevents slurry from coming behind the template and entering the vacuum holes. The retaining ring can be prepared from the same material as the template itself and if suitably sealed and attached to the template with means such as pins and adhesive can as effectively maintain the template position and prevent infiltration of the vacuum system with excess slurry as the templates of the prior art. Thus the templates of the present invention possess all of the advantages of the templates of the prior art but provide substantial improvement over the prior art from the standpoint of uniformity of polishing of the wafer surface. 
     SUMMARY OF THE PRESENT INVENTION  
     The present invention relates to a planar template for a CMP tool polishing head possessing a means of securing a wafer in CMP polishing wherein the back surface of the said template is held to the polishing head by retaining means. The present invention more particularly relates to such a planar template wherein the size of the template is the same size or larger than the size of the wafer backing film, the means of holding the wafer comprises a wafer backing film, the material from which the template is made is polycarbonate sheet, the retaining means is a vacuum, and a retaining ring is also affixed around the edge of the said template on the face toward the polishing head to secure the template in place. 
     The present invention further relates to a method for CMP polishing using a planar template for a CMP tool polishing head possessing a means of securing a wafer in CMP polishing wherein the back surface of the said template is held to the polishing head by retaining means. using a planar template with a means of securing a wafer in CMP polishing wherein the back surface of the said template is held to the polishing head by retaining means and more particularly to a method for using such a planar template wherein the size and shape of the template are the same as the size and shape of the wafer backing film, the means of securing the wafer comprises a wafer backing film, the material from which the template is made is polycarbonate sheet, the retaining means is vacuum and a retaining ring is affixed around the edge of the said template on the face toward the polishing head to hold the template in place. 
     Finally, the present invention refers to a such a method for using a planar template wherein the diameter of the wafer is half or less than that of the template and two or more wafers may be attached to the same time to the same template and polished. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a top view and cross sectional view of the template of the present invention. 
         FIG. 2  is a cross sectional view of the template on the polishing head with wafer attached. 
     
    
    
     Unless otherwise indicated, in the Figures like numbers denote the same elements throughout the Figures 
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
     The template and related methods of use thereof of the present invention have been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available wafer supporting removable templates for CMP polishing in CMP polishers with vacuum supplied polishing heads. Thus, it is an overall objective of the present invention to provide highly dimensionally stable and flat vacuum supported CMP polishing head templates for CMP that are easily removable and still effectively prevent slurry from entering the vacuum system and related methods that remedy the shortcomings of the prior art. 
     The purpose of this device and method are to allow more effective CMP of wafers on CMP polishers equipped with vacuum in the wafer head and in particularly to substantially improve and render practical in an industrial or research setting the level of polishing uniformity of the wafer surface possible with the use of such a template. 
     Furthermore, this improved template may be easily and cost-effectively applied to facilitate and enhance the use of CMP polishing devices and the polishing heads used in them. These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     All dimensions for parts in the present invention follow are based on a wafer size of about 200 mm to 300 mm in diameter and may be altered as needed in proportion to changes in the size of the wafers used. The specific dimensions given herein are in no way limiting but are by way of example to demonstrate an effective embodiment of the invention. 
     As shown in  FIG. 1 , the template ( 8 ) of the present invention comprises a hard flat sheet of material ( 10 ). The material that may be used is not particularly limited but it should be hard, dimensionally stable under the conditions of operation of CMP and resistant to the chemical composition of the slurries used in CMP. Any hard material may be used as the material for the sheet ( 10 ) of the present invention but hard plastic is preferred. Any hard plastic sheet such as acrylic, polyurethane or polycarbonate sheet may be used but polycarbonate sheet is preferred for its availability, cost effectiveness, toughness, flatness and dimensional stability. 
     The thickness of this material is not particularly limited but it is preferably between 1/16 th  inch and ¼ inch and more preferably between 0.093 inch and 0.17 inch. The outer shape and dimensions of the sheet ( 10 ) are not particularly limited and any sheet dimensions and shape that do not interfere with the normal operation of the polishing head may be used but a circular shape is preferred and the diameter thereof should be sufficiently large to encompass the outer dimensions of the ceramic plate at the bottom of the polishing head by at least about ¼ inch and more preferably by ½ inch or more. It is preferred that the diameter of the sheet ( 10 ) not exceed the diameter of the template by more than 2 inches. 
     Around the circumference ( 12 ) of the sheet ( 10 ) on the upward facing face of the sheet ( 10 ) is attached the retaining means. The retaining means is not particularly limited and may comprise a series of architectural supports around the circumference of the sheet placed so as to prevent the template from slipping during CMP polishing or a complete retaining ring with an inner diameter equal to or no more than ⅛ inch larger than the diameter of the ceramic plate on the lower face of the polishing head but a retaining ring ( 14 ) is preferred. The retaining ring ( 14 ) may be made of any hard material but hard plastic is preferred. Any hard plastic sheet such as acrylic, polyurethane or polycarbonate sheet may be used but polycarbonate sheet is preferred. The sheet ( 10 ) and the retaining ring ( 14 ) may be of the same or different material and the use of the same material is preferred. The degree of flatness of the sheet ( 10 ) is not particularly limited. 
     The dimensions of the retaining ring ( 14 ) are not particularly limited but it is preferred that the outer edge ( 16 ) of the retaining ring be concentric with the sheet ( 10 ) where the sheet ( 10 ) is circular in shape and it is also preferred that the said outer edge ( 16 ) be the same diameter as the outer edge ( 18 ) of the sheet. The inner edge ( 20 ) of the retaining ring ( 14 ) should be concentric with the center of the sheet ( 10 ) and should be the same diameter as the polishing head or up to about ⅛ inch in diameter larger and a diameter as close in size as possible to the size of the plate on the lower face of the polishing head is preferred. The thickness of the retaining ring of the present invention is not particularly limited but must be sufficient to allow the retaining ring to hold against the sides of the polishing head securely and is preferably no thicker than the sheet ( 10 ), more preferably approximately half the thickness of the sheet ( 10 ) and is further preferably between 0.093 inches and 0.17 inches in thickness. 
     The means of attaching the retaining ring ( 14 ) of the present invention to the sheet ( 10 ) is not particularly limited and any suitable means including bolting, pinning, lamination with adhesive or mechanical attachment may be used but secure attachment with adhesive and by a series of pins ( 34 ) in recessed openings ( 36 ) in both the retaining ring ( 14 ) and the sheet ( 10 ) supplied for this purpose is preferred, combinations of methods of attachment may be used, and the use of adhesive between the retaining ring ( 14 ) and the sheet ( 10 ) in addition to bolting or pinning is more preferred. If pins are used any number of pins may be used but between 10 and 20 stainless steel, aluminium or brass pins is preferred. 
     The wafer is secured to the sheet by means of a backing film ( 22 ) with a ring ( 24 ) for securing the lateral edges of the wafer. Any suitable backing film commercially available for CMP polishing may be used for this purpose. 
     Drawing  2  shows the ceramic plate ( 26 ) on the lower face of the polishing head ( 28 ) and the affixation of the template ( 8 ) thereto. The wafer ( 30 ) is attached to the backing film ( 22 ) by liquid capillary forces within the ring ( 24 ) for securing it on the side opposite the affixation of the retaining ring and the side of the template possessing the retaining means is placed on the polishing head ( 28 ) so that the sheet ( 10 ) face contacts the face of the ceramic plate ( 26 ). Vacuum is introduced through the multiple small openings (not shown) in the surface of the ceramic plate ( 26 ) equipped for that purpose and the template is affixed and ready to commence CMP polishing on the CMP polishing pad ( 32 ). To remove the template, maintenance of the vacuum to the ceramic plate ( 26 ) is discontinued after the completion of polishing operations and the template ( 8 ) with the wafer ( 30 ) attached by means of the backing film ( 22 ) and the ring ( 24 ) for securing the wafer is removed from the CMP polisher. 
     The CMP polisher of the present invention with a polishing head that is equipped with vacuum to hold a wafer directly or a disc during polishing (not shown) is not particularly limited and CMP polishing tools such as the APD 500 or APD 800 may be used. 
     PRACTICE EXAMPLES  
     Example 1  
     A circular polycarbonate sheet of thickness 0.17 inches was cut from GE Lexan® to a diameter of 8 and 59/64 inches. To this sheet was affixed a ring made of the same material having the same external diameter, an internal diameter of 8 and 9/64 inches for a width of 25/64 inches and a thickness of 3/32 nd  of an inch. This was affixed to the sheet with 16 brass pins ⅛ inch in diameter and 0.2 inches long set in both holes drilled in the sheet and in the retaining ring at equidistant points around the ring halfway between the inner and outer diameter thereof and set in the holes with Loktite super glue (cyanoacrylate glue). The pins fully penetrate the ring but do not fully penetrate the sheet. The ring was further affixed to the sheet by lamination with Loktite super glue (cyanoacrylate glue). 
     A backing film was attached centered to the side of the template that did not possess the retaining rings and a wafer secured therein. The wafer was then affixed with the retaining ring side facing to the ceramic plate head of an Araca APD 500 polisher, the vacuum of the polishing head was engaged and the wafer and template were secured to the polishing head. 
     The unexpected and superior results achieved by means of the present invention were a substantial improvement in uniformity of the polishing of the surface of the wafer during the CMP processes using the aforementioned CMP polishing tool. 
     DETAILED DESCRIPTION OF THE DRAWINGS  
     
       FIG. 1 
     
       FIG. 1  is a top view and cross sectional view of the template of the present invention. 
     ( 8 ) is the entire template 
     ( 10 ) is a sheet 
     ( 12 ) is the circumference of the sheet 
     ( 14 ) is the retaining ring. 
     ( 16 ) is the outer edge of the retaining ring 
     ( 18 ) is the outer edge of the sheet 
     ( 20 ) is the inner edge of the retaining ring 
     ( 22 ) is the backing film 
     ( 24 ) is the ring for securing the wafer 
       FIG. 2  is a cross sectional view of the template on the polishing head with wafer attached. 
     ( 26 ) is the ceramic plate on the lower face of the polishing head 
     ( 28 ) is the polishing head 
     ( 30 ) is the wafer 
     ( 32 ) is the CMP polishing pad 
     ( 34 ) are the pins 
     ( 36 ) is the recessed opening for the pins. 
     EFFECTS OF THE INVENTION  
     The present invention has the effect of unexpectedly improving the uniformity of the polishing of the wafer surface by providing a much flatter and more stable surface. Since material with this degree of planarity is readily and cheaply available it provides a cheap and effective way to move various wafers on and off of the CMP polishing tool quickly and efficiently.