Abstract:
A method and apparatus for slurry distribution is provided. The apparatus for the distribution of slurry over a polishing pad surface used in chemical mechanical polishing includes a roller positioned over a polishing pad surface. The roller is connected with a gimbaling attachment to a positioning arm and is configured to apply a force against the polishing pad surface while maintaining a surface of the roller substantially parallel to the polishing pad surface. The gimbaled roller drives the slurry into and over the porous texture of the polishing pad surface and ensures a substantially even distribution of slurry. In another example, a double roller apparatus is also provided and is configured to combine slurry distribution and pad conditioning.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to the distribution of micro-abrasive suspension, or slurry, underneath the wafer in CMP operations. 
     2. Description of the Related Art 
     In the fabrication of semiconductor devices, there is a need to perform CMP operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from each other by dielectric materials, such as silicon dioxide, for example. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metal line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization, e.g., such as copper. 
     In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface. 
     FIG. 1A illustrates an exemplary prior art CMP system  100 . The CMP system  100  in FIG. 1A is a belt-type system, so designated because the preparation surface is an endless belt  108  mounted on two drums  114  which drive the belt  108  in a rotational motion as indicated by belt rotation directional arrows  116 . As used herein, the belt  108  should be understood to include a polishing pad or other preparation surface material in addition to any supporting material, such as aluminum, stainless steel or any suitable supporting structural material for holding the pad or other preparation surface. A wafer  102  is mounted on a carrier  104 . The carrier  104  is rotated in direction  106 . The rotating wafer  102  is then applied against the rotating belt  108  with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen  112  is provided to stabilize the belt  108  and to provide a solid surface onto which to apply the wafer  102 . 
     Slurry  118  is introduced upstream of the wafer  102 . In a belt-type CMP system  100 , slurry  118  is commonly introduced in a region that is upstream and off-center from the wafer  102  as illustrated in FIG.  1 A. The movement of the belt  108  carries the slurry  118  to the wafer  102  which is mounted on the carrier  104  and being applied against the belt  108  with a force F as it is being rotated  106 . The rotation  106  of the wafer  102  and the friction of the wafer  102  against the belt  108  further distributes the slurry  118  across and into the polishing pad or other preparation surface of the belt  108  and over the surface of the wafer  102 . In FIG. 1C, the effect of the moving belt  108  and the rotating wafer  102  is illustrated. As the slurry  118  approaches the wafer  102  from upstream and off-center, it is distributed across the belt  108 , facilitating the CMP operation and moving beyond the wafer  102  having been distributed across a larger region of the belt  108 . Slurry  118   a  is shown after passing the wafer  102  and having been distributed across the belt  108  during the CMP operation on wafer  102 . 
     Slurry  118 , as is known, is a water-based suspension consisting of dispersed micro-abrasives, dissolved chemicals and in some cases, lubricants. The fluid properties of the suspension allow for the even distribution of the abrasive material across a surface and enhance the effectiveness of the CMP operation. Both solid abrasives and fluid chemicals, including water, modify the surface properties of interacting objects, thus promoting smooth removal. A section of a typical CMP belt  108  and the porous texture of the polishing pad or other preparation surface is illustrated in FIG.  1 B. As stated above, the belt provides the supporting structure for the polishing pad or other preparation surface. In FIG. 1B, the polishing pad surface of the belt  108  is shown as uneven or rough and contributing an abrasiveness. Slurry  118  is distributed over the pad surface  108 , but due to the fluid properties, the micro-abrasives, surface tension, capillary openings in the belt  108  surface blocked by air, and other such factors, the slurry does not penetrate into the surface cavities through the usual distribution method described above. FIG. 1B shows capillary openings blocked by air pockets  119  that form in the surface cavities and result in uneven and unstable slurry distribution. Non-uniform slurry distribution can result in less efficient and non-uniform planarization of wafers being processed. Further, if slurry  118  loses the fluid properties due to build-up and drying, then the micro-abrasives collect and “cake” forming chunks of abrasive debris. This abrasive debris, in the extreme, can damage the quality of the semiconductor wafer being processed in a CMP operation. Typically, such debris contributes to non-uniform planarization and wafer defects. In addition to the air pockets  119  shown in FIG. 1B, the polishing pad surface  108  can accumulate build-up of dried slurry  118 . As the slurry build-up dries, chunks of abrasive debris form. 
     One method of removing and preventing build-up on the pad surface  108  is illustrated in FIG. 1A. A belt conditioner assembly  110  is mounted down-stream from the wafer  102 . The belt conditioner assembly  110  consists of an abrasive head that is applied against the polishing pad surface  108  to dislodge any abrasive debris that may be on the polishing pad  108 . Further, the belt conditioner assembly  110  renews the surface cavities in the polishing pad  108  to ensure the pad  108  retains its abrasive properties, and the ability to hold and transport slurry into the CMP operation. As the belt continues to rotate, the conditioner assembly  110  provides constant conditioning of the pad  108  during CMP operations. Or, the conditioning assembly  110  can be programmed to condition the pad  108  at intervals according to operator requirements. 
     The increased complexity of multi-layered semiconductor chips requires more precise and more uniform planarization techniques. CMP is and will remain an integral part of the semiconductor wafer manufacturing process, but must be made more effective and more controllable to meet the increasing demands for more complex fabrication. In view of the foregoing, there is a need for slurry distribution methods and apparatus in CMP operations that are more controllable, that more evenly and uniformly distribute slurry across a preparation surface, and that minimize the risks of damage due to dried slurry and abrasive debris. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by providing systems and methods for the uniform and even distribution of slurry in a CMP system. The gimbaled roller system and method provide a controllable distribution of slurry to create a more efficient and effective CMP operation with fewer substrate defects. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below. 
     In one embodiment, an apparatus for even slurry distribution in CMP operations is disclosed. The apparatus is a gimbaled roller consisting of a roller, a gimbaling roller attachment attached to the roller, and a positioning arm attached to the gimbaling roller attachment to position the roller for the even distribution of slurry. 
     In another embodiment, an apparatus for distributing slurry over a polishing pad surface used in chemical mechanical polishing of a substrate is disclosed. The apparatus includes a belt assembly having the polishing pad surface that rotates in a loop. A roller is positioned over the polishing pad surface and has a gimbaling attachment configured to ensure that a surface of the roller is maintained substantially parallel to the polishing pad surface. 
     In still a further embodiment, a method for distributing slurry in a chemical mechanical polishing system is disclosed. The method includes introducing slurry onto a chemical mechanical polishing surface and moving the chemical mechanical polishing surface. The method further provides applying a roller against the chemical mechanical polishing surface as the slurry is moved on the moving chemical mechanical polishing surface toward the roller, and distributing the slurry as an even film over the chemical mechanical polishing surface. 
     In yet another embodiment, an apparatus for combined pad conditioning and slurry distribution is disclosed. The apparatus includes a polishing pad with a polishing pad surface, and a first and a second roller positioned before a wafer polishing application location. The first roller has an abrasive surface that is applied to the polishing pad surface, and the second roller is defined below the polishing pad surface to support the polishing pad at a location where the first roller is applied. Thus configured, the first roller conditions the polishing pad surface and distributes slurry over the polishing pad surface. 
     The advantages of the present invention include the providing of more control over the chemical mechanical polishing operation. The present invention allows for the setting and maintaining of a designated thickness of slurry on a polishing pad or other preparation surface. By applying pressure with a gimbaled roller of the invention, air pockets are displaced in the porous surface of the polishing pad, and slurry is distributed in an even and uniform thickness across the pad. The slurry is also distributed across the width of the polishing pad surface of the belt resulting in a uniform and controllable amount of slurry at the substrate for CMP processing. More control over the slurry in CMP operations yields more precise processing with fewer defects. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements. 
     FIG. 1A illustrates an exemplary prior art CMP system. 
     FIG. 1B illustrates section of a typical CMP belt and the porous texture of the polishing pad or other preparation surface. 
     FIG. 1C shows the effect of the moving belt and the rotating wafer on slurry distribution. 
     FIG. 2A is a three dimensional view of a CMP system in accordance with one embodiment of the present invention. 
     FIG. 2B is a three dimensional view of a CMP system in accordance with another embodiment of the present invention. 
     FIG. 2C shows the distribution of slurry on a polishing pad or other preparation surface by the roller in accordance with one embodiment of the invention. 
     FIG. 2D shows the distribution of slurry on a polishing pad or other preparation surface by the roller in accordance with another embodiment of the invention. 
     FIG. 3A shows the action of the roller on the slurry in accordance with one embodiment of the invention. 
     FIG. 3B shows a detail view of the gimbaling roller attachment in accordance with one embodiment of the invention. 
     FIGS. 4A and 4B show two different embodiments of surface textures of a polishing pad or other preparation surface. 
     FIG. 5A shows three rollers connected to three roller arms positioned in parallel across a belt in accordance with one embodiment of the present invention. 
     FIG. 5B shows multiple rollers positioned perpendicular to the direction of movement of belt in accordance with an embodiment of the invention. 
     FIG. 6 shows the position of a gimbaled roller on a CMP system in accordance with yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An invention for CMP wafer operations, namely, a gimbaling roller (e.g., made of elastomeric material), for the distribution of slurry on a CMP pad, belt, or other preparation surface is disclosed. In preferred embodiments, methods for the even distribution of slurry in a CMP system include using a gimbaled roller to ensure even distribution across the preparation surface as well as constant and even pressure to infuse slurry into the porous texture of a polishing pad or other preparation surface as appropriate. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIG. 2A is a three dimensional view of a CMP system  200 , in accordance with one embodiment of the present invention. The CMP system  200  shown is an exemplary belt-type system including a belt  108  preparation surface mounted around two drums  114 . The drums  114  rotate and impart the rotation on the belt in the direction as shown by arrows  116 . A wafer  102  is mounted on a carrier  104  which has a rotation  106 . The rotating carrier  104  and wafer  102  are applied against the belt  108  as it rotates  116  with a force F. A platen  112  is located under the belt  108  and provides a stable and secure support for CMP operations. 
     Slurry  118  is introduced onto the pad surface  108  upstream of the wafer  102 . In one embodiment, slurry is dispensed through a manifold (see FIG. 2D) that is configured to span the width of the belt  108 . In a slurry distribution manifold embodiment, slurry  118  is not deposited in a single region that is off-center from the wafer  102 , but rather is dispensed across the entire pad surface  108 . Since the belt  108  rotates  116 , the slurry  118  is transported to the wafer  102  by the movement of the belt  108 . 
     Before reaching the wafer  102 , the slurry  118  travels under a gimbaled roller apparatus  202  in accordance with one embodiment of the present invention. A roller  204  is attached by a roller arm  205  to a gimbaling roller attachment  206 . As described in greater detail below, the gimbaling roller attachment  206  ensures the roller  204  is maintained at a constant, even force against the surface of the moving belt  108 . In one embodiment, the roller  204  is made of an elastomeric material, like polyurethene to provide better conformity with the polishing pad surface. The gimbaled roller apparatus  202  is positioned over the belt  108  by a control arm  208  which is controlled by a position controller  220 . 
     A pad conditioning assembly  110  is configured downstream from the wafer  102 . It could be also installed upstream from the wafer. As described above, the pad conditioning assembly  110  provides an abrasive head for the conditioning and maintenance of the polishing pad or other preparation surface of the belt  108 . As can be seen in FIG. 1B, one embodiment of the polishing pad or other preparation surface  108  is porous and includes multiple surface cavities. Process by-products can collect in these cavities as build-up, or slurry  118  can dry out and be trapped in the cavities, blocking them. Acting as a key feature providing slurry transport underneath the wafer, the pores and the surface of the polishing pad or other preparation surface  108  need to be kept free of build-up and other debris. It is the function of the pad conditioning assembly  110  to clean, condition, and maintain the polishing pad or other preparation surface  108  for optimum CMP operations. 
     In one embodiment, the belt conditioner assembly  10  includes an abrasive head with a narrower surface area than the width of the belt  108 . In this embodiment, the belt conditioner assembly  110  is configured to sweep across the belt  108  while being applied against the belt  108  with a down force. Due to the sweeping action across the rotating belt  108 , the entire polishing pad or other preparation surface  108  is conditioned during sustained CMP operations. 
     In another embodiment, the belt conditioner assembly includes an abrasive head spanning the width of the belt  108 . In this embodiment, the belt conditioner assembly  110  is configured to be applied against the moving belt  108  across the entire width of the belt  108 . In one embodiment, the conditioning is a constant process of the CMP operation. In another embodiment, the conditioning is programmed to occur intermittently in accordance with the needs of the specific operation. 
     FIG. 2B is a three dimensional view of a CMP system  200 , in accordance with another embodiment of the present invention. FIG. 2B depicts the same CMP system  200  as described in FIG. 2A, but the CMP system  200  in FIG. 2B incorporates a support roller  210 . The support roller  210  functions in a similar manner as the platen  112 . Just as the platen  112  provides support and a stable surface for the rotating carrier  104  to be applied against the belt  108  with force F, the support roller  210  provides an opposing roller to the gimbaled roller apparatus  202 . The support roller  210  is designed as a roller to minimize generated friction between the moving belt  108 , the roller  204  and the support roller  210  while providing support for the roller  204  to be applied against the belt  108  with force. In another embodiment, the support roller  210  is designed with a platen-like structure. Because the gimbaled roller apparatus  202  is a gimbaled structure, the support roller  210  need not be gimbaled, and is configured to provide a support surface with a minimum of generated friction. 
     In another embodiment, a double roller device (e.g., a first top roller and a second bottom roller) can be used to provide a combined conditioning plus slurry distribution action. In this case the upper roller  204  is made of a rigid material, covered with a diamond grid to provide an abrasive action. The lower roller  210  is made of elastic material like polyurethene to provide system compliance. 
     FIG. 2C shows the distribution of slurry  118   b  on a polishing pad or other preparation surface  108  by the roller  204  in accordance with one embodiment of the invention. The roller  204 , attached to the roller arm  205  is positioned at an angle θ across the belt  108 . The position controller  220  (see FIG. 2A) moves the control arm  208  (see FIG. 2A) to position the roller  204  at an optimum position across the belt  104  according to the configuration of a particular system. The control arm  205  moves in movement direction  212  to achieve the desired position of the roller  204  across the belt  108 . In FIG. 2C, slurry  118  is introduced at a point upstream and off-center from the wafer  102 . The roller  204  is positioned at angle θ across the belt  104  to provide maximum distribution of slurry  118  across the belt  108 . 
     The slurry  118  travels along the length of the roller  204 . As is described in greater detail below, the gimbaling feature of the roller  204  provides a constant, flat point of contact between the roller  204  and the polishing pad or other preparation surface  108  (or the slurry that is on the polishing pad or other preparation surface  108 ). As the slurry  118  travels under the roller  204 , it is pressed into the porous surface of the polishing pad  108  and is evenly and uniformly distributed across the polishing pad surface of the belt  108 . The slurry  118   b  then travels with the belt  108  to the wafer  102  where the CMP process is accomplished with more precision and control. 
     FIG. 2D shows the distribution of slurry  118   b  on a polishing pad or other preparation surface  108  by the roller  204  in accordance with another embodiment of the invention. As described in reference to FIG. 2C, the control arm  205  moves in movement direction  212  to achieve the desired position of the roller  204  across the belt  108 . In FIG. 2D, the optimum position is with the roller  204  perpendicular to the belt  108 . A slurry distribution manifold  220  is shown positioned over and across the belt  108 . Slurry  118  is dispensed by the slurry distribution manifold  220  through slurry distribution ports  220   a . Slurry  118  is dispensed across the width of the belt  108 . As the slurry  118  travels under the roller  204 , it is pressed into the porous texture of the polishing pad surface  108  and distributed evenly and uniformly across the polishing pad surface  108 . The slurry  118   b  then travels to the wafer  102  for more precise and controllable CMP processing. 
     FIG. 3A shows the action of the roller  204  on the slurry  118  in accordance with one embodiment of the invention. Slurry  118 , as is known, consists of micro-abrasives and dissolved chemicals in suspension. Typically, slurry  118  is introduced into the CMP operation in droplets from a slurry dispensing system. One example of a slurry dispensing system is the slurry distribution manifold  220  as described in reference to FIG.  2 D. In FIG. 3A, a droplet of slurry  118  reaches the roller  204 . The roller  204  has a roller surface  204   a  designed to distribute the slurry  118  across the surface of the belt  108  and into the porous texture of the polishing pad surface  108 . The roller  204  is attached to a roller arm  205  that positions the roller  204  across the polishing pad surface  108  as described above in reference to FIG.  2 C. The roller arm  205  also positions the roller  204  at a determined distance from or with a determined pressure against the belt  108 . Applying the roller  204  against the belt  108  with force F presses the slurry  118  into the porous polishing pad surface  108  for more even, uniform, and controllable distribution. The roller  204  is configured to distribute the slurry  118  in a uniform thickness  117  across the belt  108  to deliver a uniform distribution of a controlled amount of slurry  118   b  to the wafer  102  (not shown) as required for specific CMP operations. A polishing pad surface  108  that is constantly conditioned as described above will deliver a constant and controllable amount of slurry  118   b  at a desired thickness  117  for specific CMP operations. 
     FIG. 3B shows a detail view of the gimbaling roller attachment  206  in accordance with one embodiment of the invention. The gimbaling roller attachment  206  is attached to a position controller  220  (see FIG. 2A) with a control arm  208 . The control arm  208  rotates the gimbaling roller attachment  206  in the horizontal plane. This controls the positioning of the roller  204  across the belt  108  (not shown). Rotation of the control arm positions the roller  204  from exactly perpendicular to the direction of motion of the belt  108 , through any angle θ as described above in reference to FIG.  2 C. Movement of the roller in the horizontal plane is represented by directional arrows  212 . Further, the position controller  220  (not shown) raises and lowers the control arm  208  to position the roller  204  in the vertical plane from a set distance above the belt  108  through a desired pressure against the belt  108 . Movement of the roller  204  in the vertical plane is represented by directional arrows  213 . 
     The roller  204  is attached to the gimbaling roller attachment  206  by the roller arm  205 . The roller arm  205  connects to the gimbal connector  206   b  which is attached to the gimbal support  206   a . The gimbal connector freely spins in its mounting in the gimbal support  206   a , being mounted by known gimbaling techniques. Thus mounted, the gimbaling roller attachment  206  provides that contact between the roller  204  and the polishing pad or other preparation surface  108  (or layer of slurry on the polishing pad or other preparation surface  108 ) is constantly maintained. The gimbaling action controls movement of the roller in the vertical plane, but such movement is equal and opposite on opposite ends of the roller  204 . If one end of the roller  204  is moved in an upward component of direction  213 , then the opposite end of the roller  204  must move an equal distance in the downward component of direction  213 . This provides both uniform distribution of slurry across the pad  108  as well as forcing the slurry into the surface cavities of the polishing pad or other preparation surface  108 . Under a constant, even pressure, the air is forced out of the capillary openings in the porous pad surface  108  and displaced by slurry  118 . This provides not only the uniform distribution of slurry  118  across the belt  108 , but the ability to set and control a uniform thickness of slurry  118  on and into the polishing pad or other preparation surface  108 . 
     Further still, the control arm  208 , responding to directional signals from the position controller  220  (not shown) positions the gimbaling roller attachment  206  and roller  204  along the belt  108  at a desired distance from the carrier  104  (see FIG.  2 A). In a preferred embodiment, the roller  204  is positioned as close to the carrier  104  (not shown) as possible. The control arm positions the roller along the belt  108  in accordance with the requirements of the specific CMP operation. The control arm  208  and gimbaling roller attachment  206  thus provide positioning and movement of the roller  204  through the various planes of movement in the “x,” “y” and “z” axes. 
     FIGS. 4A and 4B show two different embodiments of surface textures of a polishing pad or other preparation surface  108 . As described in detail above, the gimbaled roller apparatus  202  (see FIG. 2A) distributes slurry  118  (not shown) across the belt  108 , as well as into the porous surface of the pad  108 . In one embodiment of the present invention, the uniform distribution of slurry across the polishing pad or other preparation surface  108  is enhanced by the surface texture of the pad  108 . In FIG. 4A, the porous surface texture of the pad  108   a  contains diagonal ridges and/or troughs in order to facilitate the distribution of slurry across the surface of the pad  108   a . In FIG. 4B, the ridges and or troughs are configured to provide a spiral or swirl surface texture to the pad  108   b.    
     As described above in reference to FIG. 3A, roller  204  has a roller surface  204   a . In one embodiment of the present invention, the exemplary polishing pad or other preparation surface textures  108   a ,  108   b  illustrated in FIGS. 4A and 4B are also the textures configured to roller surface  204   a . In this manner, the distribution of slurry  118  is enhanced by both the roller  204  and the belt  108  for an effective uniform distribution of slurry  118  across the belt  108 . 
     FIGS. 5A and 5B illustrate multiple roller  304  configurations of a gimbaled roller apparatus in accordance with another embodiment of the present invention. In FIG. 5A, three rollers  304  connected to three roller arms  305  are positioned in parallel across belt  108 . In this embodiment, the rollers  304  are positioned in parallel at an angle θ across belt  108  such that slurry  118  traveling along the belt  108  must come in contact with some surface of at least one of the three rollers  304  before reaching wafer  102 . As described in detail above in reference to FIG. 2C, slurry  118  is distributed across and into the porous polishing pad or other preparation surface  108  to form a controllable and uniform slurry  118   b  at the wafer  102  for precise CMP processing. 
     FIG. 5B shows multiple rollers  304  positioned perpendicular to the direction of movement of belt  108  in accordance with an embodiment of the invention. In FIG. 5B, three rollers  304  are connected to three roller arms  305  and positioned across belt  108  and perpendicular to the movement of belt  104 . Slurry  118  traveling on belt  108  must pass through some surface of at least one of the rollers  304 . In this manner, slurry  118  is distributed along and into the porous polishing pad or other preparation surface  108  to form evenly distributed and controllable slurry at wafer  102  enhancing the precision of CMP operations. 
     FIG. 6 shows the position of a gimbaled roller  404  on a CMP system  400  in accordance with yet another embodiment of the present invention. The CMP system  400  represented in FIG. 6 is an orbital CMP system  400 . A wafer  202  with rotation  231  is applied to pad  408  which has rotation  230 . As described in detail above, in a preferred embodiment of the invention, the roller  404  is positioned as close to the wafer  202  as possible. In the embodiment shown in FIG. 6, the roller  404  is positioned in the quadrant of the pad  408  adjacent to the wafer  202 . The roller  404  is attached to roller arm  205  for positioning and application of force as described above. The roller  404  in the embodiment shown in FIG. 6 is cone-shaped to accomplish the uniform and controllable distribution of slurry  118  over the circular-shaped pad  408 . Slurry distribution can be further enhanced by using textured pads  408  and textured roller surface  404   a  (not shown) as described above in reference to FIGS. 4A and 4B. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.