Abstract:
A polishing pad includes at least one conductive polishing element supported by a compressible under layer having conductive patterning therein, the conductive patterning adapted to permit coupling of a potential to the conductive polishing element; a guide plate above the compressible under layer, the guide plate having a hole through which the polishing element passes and further having a cathodic element connected thereto; and a slurry distribution layer adhered to the guide plate opposite the compressible under layer. The polishing pad may further include a proton exchange membrane placed over the cathodic element. A semiconductor wafer having a metal film thereon may be polished using the polishing pad by placing the wafer in contact with the polishing element, applying anodic current to the polishing element and cathodic current to the cathodic element, and polishing with an anodic solution. For copper films, a sulfuric acid-copper sulfate solution may be used.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is a non-provisional of, claims priority to and incorporates by reference U.S. provisional patent application No. 60/739,252, filed 23 Nov. 2005; and U.S. provisional patent application No. 60/758,006, filed 10 Jan. 2006. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of electro-chemical mechanical planarization (ECMP) and relates specifically to methods of using engineered polishing pads in ECMP processes.  
       BACKGROUND  
       [0003]     In modern integrated circuit (IC) fabrication, layers of material are applied to embedded structures previously formed on semiconductor wafers. Chemical mechanical planarization (CMP) is an abrasive process used to remove these layers and polish the surface of a wafer flat to achieve a desired structure. CMP may be performed on both oxides and metals and generally involves the use of chemical slurries applied in conjunction with a polishing pad in motion relative to the wafer (e.g., pad rotation relative to the wafer). The resulting smooth flat surface is necessary to maintain the photolithographic depth of focus for subsequent steps, and to ensure that the metal interconnects are not deformed over contour steps. Damascene processing requires metal, such as tungsten or copper, to be removed from the top surface of a dielectric to define interconnect structures, using CMP.  
         [0004]     As CMP is a chemical-mechanical process, planarization/polishing performance is impacted by the mechanical properties and the slurry distribution ability of the polishing pad. Polishing slurries are formulated to cause passivation layers on the wafer surface, which layers are removed by the mechanical action of the pad. Higher points on the wafer are subject to higher pressure while lower points are protected by passivation and the inability of the pad to reach them. As advanced technology requirements are to be met, low K dielectric materials have to be used. These materials have low mechanical strength and cannot be polished with traditional down force processes.  
         [0005]      FIG. 1  illustrates the surface of post-CMP copper wafer  100  polished with a traditional polishing pad alone. The low K dielectric material  102  is capped with a protective dielectric  104  such as silicon dioxide, silicon nitride or silicon carbide. Copper features  106  are etched into the dielectric stack. After polishing with a standard process besides dishing and erosion, damage to dielectric stack is seen in the form of material damage  108  or delamination  110 . It is desirable to have a pad that can enable good planarization performance with low or near-zero down force.  
       SUMMARY OF THE INVENTION  
       [0006]     A polishing pad configured in accordance with an embodiment of the present invention includes at least one conductive polishing element supported by a compressible under layer having conductive patterning therein, the conductive patterning adapted to permit coupling of a potential to the conductive polishing element; a guide plate above the compressible under layer, the guide plate having a hole through which the polishing element passes and further having a cathodic element connected thereto; and a slurry distribution layer adhered to the guide plate opposite the compressible under layer. The polishing pad may further include a proton exchange membrane placed over the cathodic element.  
         [0007]     The conductive polishing element may be made of a conductive polymer, graphite or a combination of graphite and a conducting or non-conducting polymer. Alternatively, or in addition, the conductive polishing element may be coated with a non-conducting material other than at its top and bottom.  
         [0008]     In further embodiments, the conductive polishing element generally has a top portion and a base, and the diameter of the base is preferably larger than the diameter of the top portion and the hole in the guide plate. The conductive polishing element may be adapted to make rolling contact with a metal film on an article undergoing polishing (e.g., by having a rounded tip).  
         [0009]     In still other embodiments, the polishing pad may include a combination of electrically conductive and electrically non-conductive polishing elements. The guide plate may therefore have a number of thru holes arranged in a pre-determined pattern for accommodating the polishing elements. The conductive polishing elements may be made of one or more of a conductive polymer, graphite or combination thereof, while the non-conductive polishing elements may be made of a thermoplastic polymer such as polyurethane, Delrin, nylon, etc. These different polishing elements may have the same shape or different shapes.  
         [0010]     The conductive patterning in the polishing pad may be made of copper, while the guide plate may be made of polycarbonate. The slurry distribution layer may be a polyethylene open cell foam having a pore size of approximately 10-50 microns. The compressible under layer may be one of: a polyurethane foam, a rubber foam, a solid polyurethane, or a solid rubber.  
         [0011]     A semiconductor wafer having a metal film thereon may be polished with a polishing pad having at least one conductive polishing element supported by a compressible under layer with conductive patterning therein, the conductive patterning adapted to permit coupling of a potential to the conductive polishing element; a guide plate above the compressible under layer, the guide plate having a hole through which the polishing element passes and further having a cathodic element connected thereto; and a slurry distribution layer adhered to the guide plate opposite the compressible under layer. Such polishing may include placing the wafer in contact with the polishing element, applying anodic current to the polishing element and cathodic current to the cathodic element, and polishing with an anodic solution.  
         [0012]     Further, a semiconductor wafer having a copper film thereon may be polished with a polishing pad having at least one conductive polishing element supported by a compressible under layer with conductive patterning therein, the conductive patterning adapted to permit coupling of a potential to the conductive polishing element; a guide plate above the compressible under layer, the guide plate having a hole through which the polishing element passes and further having a cathodic element connected thereto; and a slurry distribution layer adhered to the guide plate opposite the compressible under layer. Such polishing may include placing the wafer in contact with the polishing element and supplying a sulfuric acid-copper sulfate solution while applying anodic current to the polishing element and cathodic current to cathodic element.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, in which:  
         [0014]      FIG. 1  illustrates examples of the types of damage which may occur to a wafer surface using conventional polishing pads;  
         [0015]      FIG. 2A  illustrates a cut-away side profile view of a circular engineered ECMP pad configured according to one embodiment of the present invention;  
         [0016]      FIG. 2B  illustrates the compressible foam under layer of the ECMP pad shown in  FIG. 2A  with conductive patterning laid out in a concentric manner to enable the potential of the polishing elements to be controlled in the radial direction;  
         [0017]      FIG. 2C  illustrates the guide plate of the ECMP pad shown in  FIG. 2A  with holes for the polishing elements and cathodic elements arranged between the holes;  
         [0018]      FIG. 3  illustrates a top down view of the ECMP pad configured according to one embodiment of the present invention; and  
         [0019]      FIG. 4  illustrates a polishing element for the ECMP pad shown in  FIG. 2  configured according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]     The present invention relates to electrochemical-mechanical polishing (ECMP) pads adapted to permit the use of low down force CMP. Although discussed with reference to certain illustrated embodiments, however, the scope of the invention should be measured only in terms of the claims following this description.  
         [0021]     CMP processes require a balance of chemical and mechanical force to achieve removal of material from high points and thereby planarize an article. With the advent of damascene processing, copper CMP is one of the most critical processes in IC fabrication. Typical copper CMP processes have operated at down forces in the range of 3-5 Psi. Moderate operating pressures (3-5 Psi) are adequate when using silicon dioxide as a dielectric to inlay copper interconnects. As more advanced technologies for 90 nm, 65 nm and 45 nm are under consideration and development, lower K dielectrics need to be utilized to achieve desired circuit performance. These low K materials may be carbon-doped silicon dioxide or other materials made porous for even lower K performance. Typical Young&#39;s modulus values of silicon dioxides is on the order of 70 GPa and it drops to 10-15 Gpa for carbon-doped films. Porous materials are expected to have even lower strength. It is therefore necessary to mitigate or completely replace the need for down force to achieve desired planarization and removal performance for copper interconnects.  
         [0022]     The engineered CMP pad described herein may be used in a variety of steps associated with CMP processing through selection of appropriate polishing elements and electrolytic chemistries. Such polishing elements are preferably made of conducting material.  
         [0023]      FIG. 2A  illustrates a cut-away side profile view of a circular engineered ECMP pad  200 , configured according to one embodiment of the present invention. Pad  200  is placed on polish table  212 . Polishing elements  202  are placed through a guide plate  204  and supported by compressible foam layer  206 . A fluid distribution layer  208  is adhered on top of the guide plate  204  and a cathodic conductive element  216  is placed between the guide plate and the fluid distribution layer. An optional proton exchange membrane  210  may be placed over the cathodic conductive element. During polishing, the ECMP pad  200  is placed on the polishing table  212 , which rotates relative to wafer  218  as the wafer contacts the polishing elements  202 .  
         [0024]     At the core of the engineered polishing pad is the guide plate  204  and compressible foam layer  206 . The guide plate  204  provides lateral support for the polishing elements  202  and may be made of a conducting or non-conducting material or combination of the two. Conductive polishing elements  202  are made such that they have a conductive core and a non-conducting coating except the top surface, which is in contact with the wafer. In one embodiment of the present invention, the guide plate  204  includes holes fabricated into or drilled out of the guide plate  204  to accommodate each of the polishing elements  202 .  
         [0025]     The polishing elements  202  may protrude above surface of the guide plate  204 , as illustrated. The polishing elements may be of varying geometric shapes (e.g., circular and/or triangular cross sections).  
         [0026]     The compressible under layer  206  provides, among others features, a positive pressure directed toward the polishing surface when compressed. Typically, the compression may vary around 10% at 5 psi (pounds per square inch), however, it will be appreciated that the compression may be varied dependent upon the materials used in constructing the engineered polishing pad  200  and the type of ECMP process. For example, the compressible under layer  206  may be BONDTEX™ made by RBX Industries, Inc.  
         [0027]      FIG. 2B  shows compressible foam layer  206  with conductive patterning  214 . The pattern is laid out in a concentric manner to enable the potential of the polishing elements to be controlled in the radial direction. Potential for each ring of the various concentric patterns can be independently adjusted to V 1 , V 2 , V 3 , etc., respectively (although only three voltage rings are illustrated in the figure, it should be appreciated that any number of such rings could be used). It should be understood that the conductive patterning can be achieved in any manner required for the process.  
         [0028]      FIG. 2C  shows guide plate  204  with holes  220  for the polishing elements and cathodic elements  216  arranged between the holes. The cathodic elements  216  are laid out in a concentric arrangement providing control across the pad in a radial manner. The potential for each concentric ring corresponds to an appropriate anodic ring at V 1 , V 2 , V 3 , etc., respectively. It is understood that the conductive patterning can be achieved in any manner required for the process  
         [0029]      FIG. 3  illustrates a top down view of an engineered polishing pad  200 , configured according to one embodiment of the present invention. The conductive polishing elements  202  are interdigitated throughout the pad  300  and the distribution of the polishing elements may vary according to specific requirements. In varying embodiments, the polishing elements  202  may have a density of between 30 and 80 percent of the total pad surface area, as determined by the diameter (D) of each polishing element and the diameter of the polishing pad. In one embodiment, the diameter (D) is at least 50 micrometers. In other embodiments, the diameter (D) is between 50 micrometers and 25 millimeters.  
         [0030]      FIG. 4  illustrates a polishing element  202 , configured according to one embodiment of the invention. The element has a conductive core  404  and a non conducting shell  402  which occupies the top portion of the element. The element has a base  406  which has a size greater than the diameter of the top portion of the element and also greater than the size of the opening in the guide plate. It should be understood that a non-conducting element may be constructed from same shape.  
         [0031]     Thus, an ECMP pad adapted to permit the use of low down force CMP has been described. The present ECMP pad has the capability to provide uniform pressure across a wafer undergoing polishing. During such polishing operations, the ECMP pad is placed on a polish table, a wafer is pressed against the pad to ensure good contact with the conductive polish elements and an electrolytic solution is supplied to the pad surface (i.e., the fluid distribution layer  208 ). An anodic potential is applied to the elements in contact with the wafer (i.e., the conductive polishing elements) while a cathodic potential is applied to a counter surface. For example, the anodic potential may be applied through conductive patterning  214  in the compressible under layer, which the cathodic potential may be applied to the guide plate through cathodic elements  216 . As discussed above, the cathodic element may be placed between the fluid distribution layer and the guide plate. The polishing elements may be made (at least in part) of an electrically conductive material, and protrude through holes in the guide plate. The polishing elements are supported at their base by the compressible under layer. An optional proton exchange membrane may be applied on top of the cathodic element and below the fluid distribution layer, enabling separation of anodic and cathodic halves of the electrolytic cell.  
         [0032]     During polishing, a wafer is placed on top of the conductive polishing elements in the presence of an electrolytic solution while a potential is applied to the cathodic element. This causes the electrochemical circuit to be completed, causing the metal film on the wafer to dissolve into the anodic solution while a reduction reaction is observed on the cathode. In the case of a copper film, for example, the electrolytic solution may be copper sulfate or sulfuric acid. The electrolytic solution may or may not contain an inhibitor to limit the removal of copper. The copper removal rate may be regulated by the number of anodic contact elements, the strength of the electrolytic solution, and the distance between the anodes.  
         [0033]     One advantage of the present system is that it allows for multiple contact points ensuring a uniform potential across the wafer (which is specially useful for thin film removal). The present ECMP pad design also enables multiple potentials to be applied across the wafer, through anodic or cathodic channels.  
         [0034]     The polishing elements apply local pressure to the wafer to achieve electrical contact and initiate removal. Since removal is electro-chemically driven, pressure or down force is used only to ensure good contact between the pad and the wafer and not for removal; hence, very low down force is required to achieve desired material removal results. The polish elements are able to move independently of one another in a vertical axis (i.e., normal to the wafer surface) to apply pressure on the wafer. This enables the elements to apply uniform pressure while complying with pressure tolerances at a global level. This eliminates “hot spots” on the wafer which might cause local pressure variations or, in case of low K materials, initiate material or interface failure damage. As will be evident to those of ordinary skill in the art, this structure also ensures good WIWNU at low down force for CMP pads and uniform electrical contact for ECMP pads enabling multiple contacts to work repeatably.  
         [0035]     In varying embodiments of the present invention, the polishing elements may be made of any suitable conducting material such as carbon-filled polymer, metal, graphite or combination thereof and are capable of movement in the vertical axis. Further, polishing elements may be of different sizes and/or geometries may used with varying density across the pad surface and a combination of conducting and non-conducting elements may be used in the pad.  
         [0036]     In still other embodiments, the polishing pad may include a combination of electrically conductive and electrically non-conductive polishing elements. The guide plate may therefore have a number of thru holes arranged in a pre-determined pattern for accommodating the polishing elements. The conductive polishing elements may be made of one or more of a conductive polymer, graphite or combination thereof, while the non-conductive polishing elements may be made of a thermoplastic polymer such as polyurethane, Delrin, nylon, etc. These different polishing elements may have the same shape or different shapes.  
         [0037]     The conductive patterning in the polishing pad may be made of copper, while the guide plate may be made of polycarbonate. The slurry distribution layer may be a polyethylene open cell foam having a pore size of approximately 10-50 microns. The compressible under layer may be one of: a polyurethane foam, a rubber foam, a solid polyurethane, or a solid rubber.  
         [0038]     In some cases, a copper pad may be placed on one polishing platen and barrier pad placed on another platen to remove copper and barrier material sequentially, utilizing separate copper and barrier solutions. The copper pad (whether used separately or in combination with the barrier material removal pad) may be used in combination with a copper sulfate/sulfuric acid solution on the anodic side and silver nitrate on the cathodic side. In yet another embodiment the electrolytic solution may contain an inhibitor such as benzotriazole to inhibit the removal of copper.