Abstract:
A chemical mechanical polishing (CMP) system is provided. A carrier has a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer having a one or more formed layers to be prepared. A preparation head is also included and is designed to be applied to at least a portion of the wafer that is less than an entire portion of the surface of the wafer. Preferably, the preparation head and the carrier are configured to rotate in opposite directions. In addition, the preparation head is further configured to oscillate while linearly moving from one of the direction of a center of the wafer to an edge of the wafer and from the edge of the wafer to the center of the wafer so as to facilitate precision controlled removal of material from the formed layers of the wafer.

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 a wafer carrier utilized in a subaperture CMP system. 
     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 having diffusion regions 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 other conductive layers by dielectric materials, such as silicon dioxide. 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, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. 
     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 one or both sides of 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. 
     In a typical CMP system, a wafer is mounted on a carrier, which rotates in a direction of rotation. The CMP process is achieved when the exposed surface of the rotating wafer is applied with force against a polishing pad, which moves or rotates in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad. 
     Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. However, in some CMP systems, the polishing pads may contain fixed abrasive particles throughout their surfaces. Depending on the form of the polishing pad used, a slurry composed of an aqueous solution such as NH 4 OH or DI water containing dispersed abrasive particles may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the wafer. 
     Several problems may be encountered while using a typical CMP system. One recurring problem is called “edge-effect,” which is caused when the CMP system polishes the edge of the wafer at a different rate than other regions, thereby causing a non-uniform profile on the surface of the wafer. The problems associated with edge-effect can be divided to two distinct categories. The first category relates to the so-called “pad rebound effect” caused as a result of the initial contact of the polishing pad with the edge of the wafer. The second category will be described below. 
     FIG. 1A is an illustration of the pad rebound effect associated with the prior art. A wafer  202  is mounted on a carrier  100 . Subsequently, the wafer  202  is applied against the pad surface  102  with a force F to accomplish a CMP process. At a certain point in time, the pad surface  102  contacts the edge of the wafer  202  at an edge contact zone  104 c, and the pad surface is shown bouncing off the edge of the wafer, thereby creating a non-contact zone  104   a . Thereafter, the pad surface comes into contact with the wafer  202  at a contact zone  104   b . However, the pad surface  102  bounces off the surface of the wafer  202  again, so as to create another non-contact zone  104   a . Then, once more the pad surface comes into contact with the wafer  202  at another contact zone  104   b . However, it bounces off again. Thus, the regions of the wafer  202 , which came into contact with the pad surface  102  like the contact zones  104   b , are polished more than other regions. As a result, the CMP processed wafer will tend to show a non-uniform profile. 
     The “burn-off” effect, which constitutes the second category of problems associated with the edge-effect is shown in FIG.  1 B. As illustrated, the burn-off effect occurs when the sharper edge of a wafer  202  is excessively polished as it makes contact with the pad surface  102  (e.g., at the edge contact zone  104   c ). This happens because a considerable amount of pressure is exerted on the edge of the wafer  202  as a result of the surface pad  102  applying the force F on an infinitely small contact area defined as the edge contact zone  104   c . As a consequence of the burn-off effect, the edge of the resulting polished wafers exhibit a burn ring that renders the edge region unusable, thereby wasting silicon device area. 
     Another shortcoming of conventional CMP systems is their inability to polish the surface of the wafer  202  along a desired finishing layer profile. Ordinarily, the surface of a wafer  202  that has undergone some fabrication tends to be of a different thickness in the center region and varies in thickness out to the edge. As illustrated in FIG. 1C-1, in a typical conventional CMP system, the pad surface  102 , which covers the entire wafer surface, is designed to apply a force on a finishing layer  202   a  surface. As a result, all the regions of the finishing layer  202   a  are polished until the finishing layer  202   a  is substantially flat. Thus, as shown in FIG. 1C-2, the pad surface  102  polishes the finishing layer  202   a , irrespective of its wavy profile, thereby causing the thickness of the finishing layer  202   a  to be non-uniform (i.e., at points  202   a   1 ,  202   a   2 ,  202   a   3 , and  202   a   4 ). As is well known, some circuit fabrication applications require that a certain thickness of material be maintained in order to build a working device. For instance, if the finishing layer  202   a  were a dielectric layer, a certain thickness would be needed in order to define metal lines and conductive vias therein. 
     In view of the foregoing, a need therefore exists in the art for a chemical mechanical polishing system that enables precision and controlled polishing of specifically targeted wafer surface regions, while substantially eliminating damaging edge-effects, pad rebound effects, and edge burn-off effects. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by providing a system which implements precision controlled polishing of layer surfaces of a wafer. In one implementation, the CMP system can be made to follow the topography of the layer surfaces of the wafer so as to create a layer surface, which has a uniform thickness throughout. In a preferred embodiment, the CMP system is designed to implement a rotating carrier in a subaperture polishing configuration, thereby eliminating the above-mentioned drawbacks, edge-effects, pad rebound effects, and edge burn-off effects. 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, a chemical mechanical polishing (CMP) system is disclosed. The CMP system includes a carrier, which has a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer having a one or more formed layers to be prepared. Further included is a preparation head, which is designed to be applied to at least a portion of the wafer that is less than an entire portion of the surface of the wafer. 
     In another embodiment, a chemical mechanical polishing (CMP) system is disclosed. A carrier is designed to hold and rotate a substrate at a fixed location. The carrier includes a surface to be prepared. Also included is a primary head. The primary head is designed to move over the fixed location of the rotating carrier in one of a first direction or a second direction. A first direction begins at about the center of the surface to an edge of the surface, and a second direction begins at about the edge of the surface to about the center of the surface. The primary head is further designed to be applied to at least a portion of the surface, which is less than an entire portion of the surface. 
     In still a further embodiment, a chemical mechanical polishing (CMP) system is disclosed. The CMP system includes a carrier having a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer, which has one or more formed layers to be prepared. Also included is a preparation head designed to be applied to at least a portion of the surface of the wafer that is less than an entire portion of the surface of the wafer. Further included is a conditioning head positioned beside the carrier. The conditioning head is designed to have a conditioning surface that is substantially planer with a top surface of the wafer. The conditioning head is further designed to deliver the preparation head as the preparation head is moved onto the top surface of the wafer. Alternatively, the conditioning head is designed to receive the preparation head as the preparation head is moved onto the conditioning head. 
     The advantages of the present invention are numerous. Primarily, rather than polishing all the regions of the surface of a wafer until the surface of the wafer is substantially flat, the subaperture CMP system, precisely and controllably, polishes specifically targeted wafer surface regions. Thus, in one embodiment, the CMP system can be made to follow the topography of the finishing layer, thereby creating a finishing layer having a uniform thickness throughout. In addition, the subaperture configuration of the CMP system in conjunction with the carrier implemented, substantially eliminate the edge-effects, pad rebound effects and edge burn-off effects associated with the prior art. Further advantages associated with the subaperture CMP, system include, without limitations, substantially lower footprint, machine volume, and cost of ownership. 
     Other aspects and advantages of the 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 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 is an illustration of the pad rebound effect associated with the prior art. 
     FIG. 1B is an illustration of the edge burn-off effect associated with the prior art. 
     FIG. 1C-1 is a cross-sectional view of a wafer revealing the non-uniform topography of a wafer. 
     FIG. 1C-2 is a cross-sectional view of a typical post-CMP wafer. 
     FIG. 2A-1 is a plan-view of a subaperture CMP system, in accordance with one embodiment of the present invention. 
     FIG. 2A-2 is a cross-sectional view of a subaperture CMP system, illustrating the co-planer relationship of the retainer ring and the wafer, in accordance with another embodiment of the present invention. 
     FIG. 2A-3 is a plan-view of a subaperture CMP system, illustrating the movement of the primary head from the center of the wafer to the edge of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 2A-4 is a cross-sectional view of a subaperture CMP system, revealing the fixed position of the secondary head, in accordance with still another embodiment of the present invention. 
     FIG. 2B-1 is a diagram of a contact area at a time when the edge of the primary head is positioned at a center of the wafer, according to one embodiment of the present invention. 
     FIG. 2B-2 is a diagram of a contact area at a time when the primary head has moved away from the center of the wafer, according to another embodiment of the present invention. 
     FIG. 2B-3 is a diagram of a contact area when the primary head is approaching an edge of the wafer, according to yet another embodiment of the present invention. 
     FIG. 3A-1 is a plan-view of a subaperture CMP system, illustrating the respective positions of the primary head and the secondary head relative to the carrier, in accordance with still another embodiment of the present invention. 
     FIG. 3A-2 is a cross-sectional view of a subaperture CMP system, revealing the co-planer relationship of the retainer ring and the carrier as well as the position of the edge of the primary head at the center of the wafer and the head of the secondary head positioned slightly to the left of the center of the wafer, according to yet another embodiment of the present invention. 
     FIG. 3A-3 is a plan-view of a subaperture CMP system, illustrating the linear movements of the primary head and the secondary head in the opposite directions. 
     FIG. 3A-4 is a cross-sectional view of a subaperture CMP system, depicting the non-stationary position of the secondary head, in accordance with yet another embodiment of the present invention. 
     FIG. 4A-1 is a plan-view of a subaperture CMP system, wherein the edge of the primary head is positioned at the edge of the wafer while the edge of the secondary head is positioned at the center of the wafer, according to still another embodiment of the present invention. 
     FIG. 4A-2 is a cross-sectional view of a subaperture CMP system, revealing the stationary position of the secondary head from a linear movement standpoint, according to yet another embodiment of the present invention. 
     FIG. 4A-3 is a plan-view of a subaperture CMP system, wherein the edge of the primary head has linearly moved from the edge of the wafer to the center of the wafer, according to still another embodiment of the present invention. 
     FIG. 4A-4 is a cross-sectional view of a subaperture CMP system, depicting the linear movement of the primary head and the fixed position of the secondary head from a linear movement standpoint, in accordance with yet another embodiment of the present invention. 
     FIG. 5A-1 is a plan-view of a subaperture CMP system revealing the respective positions of the primary head and the secondary head relative to the carrier, in accordance with still another embodiment of the present invention. 
     FIG. 5A-2 is a cross-sectional view of a subaperture CMP system, wherein the primary head and secondary head are depicted to be located on the opposite edges of the wafer, according to yet another embodiment of the present invention. 
     FIG. 5A-3 is a plan-view of a subaperture CMP system, wherein the primary head and the secondary head have linearly moved from the opposite edges of the wafer to the center of the wafer, according to yet another embodiment of the present invention. 
     FIG. 5A-4 is a cross-sectional view of a subaperture CMP system, depicting the linear movements of the primary head and the secondary head, in accordance still another embodiment of the present invention. 
     FIG. 6A-1 is a plan-view of a subaperture CMP system, wherein the edge of the primary head and the edge of the secondary head are positioned at the center of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 6A-2 is a cross-sectional view of a subaperture CMP system depicting the positions of the edges of the primary head and the secondary head at about the center of the wafer, according to yet another embodiment of the present invention. 
     FIG. 6A-3 is a plan-view of a subaperture CMP system, wherein the primary head and the secondary head have linearly moved in the same direction, in accordance with yet another embodiment of the present invention. 
     FIG. 6A-4 is cross-sectional view of a subaperture CMP system, illustrating the linear movements of the primary head and the secondary head in the same direction, according to still another embodiment of the present invention. 
     FIG. 7A-1 is a plan-view of a subaperture CMP system, depicting the respective positions of the edges of the primary head and the secondary head with respect to the carrier, in accordance with yet another embodiment of the present invention. 
     FIG. 7A-2 is a cross-sectional view of a subaperture CMP system, illustrating the direction of the movement of the primary head and the secondary head, according to still another embodiment of the present invention. 
     FIG. 7A-3 is a plan-view of a subaperture CMP system, depicting the linear movements of the edges of the primary head and the secondary head from the edge of the wafer to the center of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 7A-4 is a cross-sectional view of a subaperture CMP system, wherein the primary head and the secondary head have linearly moved in the same direction, according to yet another embodiment of the present invention. 
     FIG. 8A-1 is a plan-view of a subaperture CMP system, wherein a support point has been used to support the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 8A-2 is a cross-sectional view of a subaperture CMP system, revealing the position of the support point on the carrier with respect to the primary head, according to yet another embodiment of the present invention. 
     FIG. 8A-3 is a plan-view of a subaperture CMP system, illustrating the fixed position of the support point as well as the linear movement of the primary edge from the center of the wafer to the edge of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 8A-4 is a cross-sectional view of a subaperture CMP system, wherein the primary head has linearly moved from the center of the wafer to the edge of the wafer, in accordance with still another embodiment of the present invention. 
     FIG. 9A-1 is a plan-view of a subaperture CMP system utilizing a support point, wherein the edge of the primary head is located at the edge of the wafer, according to still another embodiment of the present invention. 
     FIG. 9A-2 is a cross-sectional view of a subaperture CMP system utilizing a support point, revealing the location of the edge of the primary head at the edge of the wafer, according to yet another embodiment of the present invention. 
     FIG. 9A-3 is a plan-view of a subaperture CMP system utilizing a support point, wherein the edge of the primary head has linearly moved from the edge of the wafer to the center of the wafer, in accordance with still another embodiment of the present invention. 
     FIG. 9A-4 is a cross-sectional view of a subaperture CMP system utilizing a support point, depicting the movement of the primary head in the direction of the edge of the wafer to the center of the wafer, according to yet another embodiment of the present invention. 
     FIG. 10A-1 is a plan-view of a subaperture CMP system utilizing a plurality of support points, in accordance with yet another embodiment of the present invention. 
     FIG. 10A-2 is a cross-sectional view of a subaperture CMP system utilizing a plurality of support points, wherein the edge of the primary head is positioned at the center of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 10A-3 is a plan-view of a subaperture CMP system utilizing a plurality of support points, depicting the linear movement of the primary head in the direction of the center of the wafer to the edge of the wafer, according to yet another embodiment of the present invention. 
     FIG. 10A-4 is a cross-sectional view of a subaperture CMP system utilizing a plurality of support points, wherein the edge of the primary head has moved from the center of the wafer to the edge of the wafer, according to still another embodiment of the present invention. 
     FIG. 11A-1 is a plan-view of a subaperture CMP system utilizing multiple support points positioned substantially opposite the location of the edge of the primary head, in accordance with yet another embodiment of the present invention. 
     FIG. 11A-2 is a cross-sectional view of a subaperture CMP system utilizing multiple support points, wherein the edge of the primary head is positioned at the edge of the wafer, in accordance with yet another embodiment of the present invention. 
     FIG. 11A-3 is a plan-view of a subaperture CMP system utilizing multiple support points, wherein the edge of the primary head has moved from the edge of the wafer to the center of the wafer, in accordance with still another embodiment of the present invention. 
     FIG. 11A-4 is a cross-sectional view of a subaperture CMP system utilizing multiple support points, depicting the movement of the primary head in the direction of the center of the wafer to the edge of the wafer, according to yet another embodiment of present invention. 
     FIG. 12 is a cross-sectional view of a non-gimbaled subaperture CMP system, in accordance with still another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An invention for a CMP system, which enables precision controlled polishing of layer surfaces of a wafer, is disclosed. The CMP system substantially eliminates the aforementioned edge-effects, pad rebound effects, and edge burn-off effects, while at the same time effectively controls the degree of polishing over selected regions of the wafer. In preferred embodiments, the CMP system implements a wafer carrier that is implemented in subaperture polishing configurations. The subaperture polishing configuration is herein defined as a configuration, wherein the contact surface of a polishing pad with the wafer can at certain points in time be smaller than the surface of the wafer. 
     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 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-1 is a plan view of a subaperture CMP system  200 , in accordance with one embodiment of the present invention. The embodiment of FIG. 2A-1 includes a primary head  208 , which is configured to polish the surface of a wafer  202 . The primary head  208  is designed so as to polish the wafer  202  utilizing any type of polishing pad including, without limitation, Linear Polisher Technology (LPT) pad materials, rotary CMP pad materials, fixed abrasive pad materials, etc. In general, any pad material that enables the desired polishing levels and precision can be used. 
     The subaperture configuration of the system  200  introduces flexibility into the polishing operation by utilizing different or same removal rates on different regions of the wafer  202 . Unlike the conventional CMP system wherein the entire polishing head is in contact with the entire surface of the wafer, in the subaperture CMP system  200 , at any given time, the size of the contact surface of the primary head  208  with the wafer  202  may vary. More specifically, in the conventional CMP systems the polishing head applies force on the entire surface of the wafer, thereby removing materials from the entire surface of the wafer so as to create a substantially flat wafer. In contrast, in the subaperture CMP system  200 , the primary head  208  applies force only to selected regions of the wafer  202 , thereby removing excess materials from those selected portions, exclusively, at a particular time. 
     Since the wafer contact surface area changes (e.g., where the pad meets the wafer), the removal rate also changes based on the Preston&#39;s Equation. According to the Preston&#39;s Equation, Removal Rate=KpPV, where the removal rate of material is a function of Downforce (P) and Linear Velocity (V), with Kp being the Preston Coefficient, a constant determined by the chemical composition of the slurry (or fixed abrasive material and chemicals), the process temperature, and the pad surface, among others. Thus, the smaller the contact surface of the primary head  208  and the wafer  202 , the greater the removal rate of the surface material. In one embodiment, the ability to control the position of the primary head  208  over the wafer  202  also enables polishing of material along a desired topography of the wafer  202 , rather than polishing the entire wafer  202  until a substantially flat surface is achieved. 
     In this embodiment, a carrier  206  is positioned below the primary head  208  and is configured to engage the wafer  202  using a retainer ring  204 . Preferably, the retainer ring  204  is defined so as to maintain a co-planer relationship with the wafer  202  while the wafer  202  is being polished by the primary head  208 . The retainer ring  204  is made of a hard material, which can withstand repeated polishing, buffing, and cleaning (i.e., SiC). Preferably, the carrier  206  is oriented in a manner that the exposed surface of the wafer  202  faces the primary head  208 . During the polishing, the wafer  202  is configured to rotate in the direction of a wafer rotation  209  while the primary head  208  is designed to rotate in the opposite direction, a direction of preparation  207 . In addition to rotating in the direction of preparation  207 , the primary head  208  is configured to move back and forth, in a small magnitude, in the direction of oscillation  211  so as to create an oscillating movement. Thus, while rotating, the primary head  208  oscillates in the oscillation direction  211 , thereby improving the polishing operation performed on the wafer  202 . 
     In one exemplary implementation, the primary head  208  may be configured to be about the same size as the carrier  206 . However, in a different embodiment, preferably, the primary head  208  may be designed so as to be about the same size as the wafer  202 . Thus, in the embodiments wherein the size of the primary head  208  and the wafer  202  vary (i.e., the primary head  208  is smaller or larger than the wafer  202 ), the angular velocity of the carrier  206  can be changed so that the angular velocity of the carrier  206  matches the angular velocity of the primary head  208 . 
     In another embodiment, the subaperture CMP system  200  can include a secondary head  212  positioned to the left of the primary head  208  and above the carrier  206 . In this example, the secondary head is configured to provide additional support to the wafer  202  while the wafer  202  is being polished by the primary head  208 . In one embodiment, in addition to providing additional support to the wafer, the secondary head  212  can be designed to function as a buffer or a cleaner. The secondary head  212  may be fixed or mobile, and similar to the primary head  208 , rotates at an angular velocity in the direction of preparation  207 . 
     In yet another embodiment, a conditioning head  210  may be positioned to the right of the carrier  206  and below the primary head  208  so as to condition the primary head  208 . Similar to the primary head  208  and secondary head  212 , the conditioning head  210  rotates in the direction of preparation  207 . 
     Initially, in the embodiment of FIG. 2A-1, the edge of the primary head  208  is defined at a position x 0 , which defines the center of the wafer  202 . Thus, the edge of the primary head  208  is positioned at position x 0 , thereby causing the edge of the secondary head  212  to be located slightly to the left of x 0 . By positioning the secondary head  212  slightly to the left of x 0 , it is possible for the primary head  208 , which is oscillating in the direction  211 , to polish the entire surface of the wafer  202 . Accordingly, the primary head  208  will preferably contact the surface of the wafer  202  slightly to the left of x 0  in view of the slight oscillation. 
     FIG. 2A-2 is a cross-sectional view of the embodiment of FIG. 2A-1, which illustrates the co-planer relationship of the retainer ring  204  and the wafer  202 . In a preferred embodiment, a gimbal  216  is positioned underneath the carrier  206  and is defined to align the carrier  206  to the moving primary head  208  or secondary head  212  during the polishing, buffing or cleaning operations. The gimbal  216  is mounted on an extending spindle  224 , which rotates in the direction of wafer rotation  209 . The extending spindle  224  is configured to apply a force F onto the carrier  206 . 
     Further shown in the embodiment of FIG. 2A-2 is the fixed conditioning head  210  mounted on a conditioning spindle  226 . The conditioning spindle  226  is configured to apply the force F onto the conditioning head  210 . The carrier  206  and the conditioning head  210  are configured to be moved up and down, along the spindle  224  and conditioning spindle  226  so as to adjust the amount of force F to be applied to the carrier  206  and conditioning head  210 . 
     As depicted, initially, the edge of the primary head  208  is positioned at the position x 0  and is configured to linearly move at a movement direction  214 . Also shown is the secondary head  212  having its edge positioned slightly to the left of position x 0 . In this embodiment, the secondary head  212  is configured to be stationary (from a linear movement standpoint). However, the secondary head  212  is preferably rotating along the direction of preparation  207 . 
     FIG. 2A-3 is a plan view of the embodiment of FIG. 2A-1, wherein the edge of the primary head  208  has moved from the position x 0  to a position x 1 , which defines the edge of the wafer in the movement direction  214 . FIG. 2A-4 is a cross-sectional view of the subaperture CMP system  200 ′ shown in FIG. 2A-3. As shown, the secondary head  212  has remained stationary. 
     In this embodiment, the polishing operation started by the edge of the primary head  208  polishing the center of the wafer  202  first. Due to the subaperture configuration of the system, at any given time, the size of the contact areas (i.e., the areas of the wafer  202  being polished by the primary head  208 ) were different, and thus the removal rates. At the instances when the contact areas were small (i.e., the areas around the position x 0 ) the removal rates were higher. In contrast, on the occasions when the contact areas were larger (i.e., the areas closer to the position x 1 ) the removal rates were lower. However, it must be noted that the amount of materials to be removed depended on the profile of the wafer  202  in the areas being polished. 
     For example, if the profile of the wafer  202  were initially flat in the areas being polished, the primary head  208  removed excess materials so that a flat polished surface was achieved; however, if the desired profile were not flat, the primary head  208  ensued the profile of the regions being polished while removing the desired amount of material. 
     Once all the desired amount of material (e.g., surface layer material) were removed from the areas around the center of the wafer  202 , the polishing operation was continued by precision controlling the edge of the primary head  208  to a different position, which was farther from the position x 0  and closer to the position x 1 . Accordingly, the primary head  208  left the polishing of the edge of the wafer  202  to the end, thereby allowing the utilization of greater control over the removal of materials from the edge of the wafer  202 . 
     In addition to controlling the removal rate at the edge of the wafer  202 , this embodiment also eliminated the edge-effect, pad rebound effect, and edge burn-off effect because the carrier  206  is designed to engage the wafer  202  by utilizing the retainer ring  204 . The existing co-planer relationship between the wafer  202  and the retainer ring  204  creates an arrangement, wherein the retainer ring  204  supports the wafer  202  while the wafer  202  is being polished by the primary head  208 . Thus, the co-planer arrangement allows the primary head  208  to continue polishing to the very edge of the wafer  202  without falling off the very edge of the wafer  202  or excessively polishing the edge of the wafer  202 , as is common in prior art burn-off effect. 
     It must be noted that the subaperture CMP system  200 ′ performed an improved polishing operation, thereby yielding a better polished wafer  202 . Unlike the polishing operations used in the conventional CMP systems, the primary head  208  performed the polishing operation by rotating in the direction of preparation  207 , and simultaneously oscillating in the oscillation direction  211 . 
     FIGS. 2B-1,  2 B- 2 , and  2 B- 3  are illustrations of the differences in size of the contact areas at different instances of time. In one exemplary embodiment of FIG. 2B-1, initially, the edge of the primary head  208  was located at the position x 0 , the center of the wafer  202 , thereby creating a contact area  230 . As shown, the primary head  208  intersected the wafer  202  at a point  232   a  and a point  232   b , thereby creating the contact area  230  defined as the area between an arc radial  234  and an arc radial  236 . 
     Subsequently, the edge of the primary head  208  linearly moved farther away from the center of the wafer  202  thus creating a smaller contact area  230 ′. The implementation of FIG. 2B-2 depicts the contact area  230 ′, defined between an arc radial  232   a ′ and an arc radial  232   b ′ created as a result of the intersection of the primary head  208  and wafer  202  at a point  232   a ′ and  232   b ′. As depicted in the embodiment of FIG. 2B-3, once the edge of the primary head  208  approximately reached the edge of the wafer  202 , a substantially smaller contact area  230 ″ was created. As illustrated, the contact are  230 ″ is the small area created between an arc radial  236 ″ and an arc radial  234 ″, defined between the intersection of the primary head  208  and the wafer  202  at a point  232   a ″ and a point  232   b ″. Accordingly, as the edge of the primary head  208  moved from the position x 0  to the position x 1 , the contact areas became smaller, thereby enabling the primary head  208  to perform a precision polishing operation at the regions substantially close to the edge of the wafer  202 . Although the movement of the primary head  208  is described moving from position x 0  to x 1 , it should be apparent to those skilled in the art that the movement can also be from the position x 1  to x 0 , and the position x 0  to x −1 . 
     Similarly, in the embodiment of FIGS. 3A-1 and  3 A- 2 , the primary head  208  is defined at the position x 0 , thereby causing the edge of the secondary head  212  to be located slightly to the left of the position x 0 . In this embodiment, while the secondary head  212  rotates in the direction of preparation  207 , it is configured to linearly move in a movement direction  222 . As the secondary head  212  moves, the primary head, which is located at the position x 0 , linearly moves in the opposite direction, the movement direction  214 , while oscillating in the direction of oscillation  211 . In one preferred embodiment, the secondary head  212  and the primary head  208  may be configured to both move at about the same linear velocity. However, in alternative embodiments, the linear velocity of the primary head  208  and the secondary head  212  can be different and will vary depending upon the desired process recipe. 
     FIGS. 3A-3 and  3 A- 4  depict a subaperture CMP system  200   a ′, wherein the edge of the primary head  208  has moved from the position x 0  to the position x 1  while the edge of the secondary head  212  has moved from the position x 0  to a position x −1 , which defines the edge of the wafer  202  in the movement direction  222 . The support provided by the co-planer arrangement of the wafer  202  and the retainer ring  204  permitted the primary head  208  and the secondary head  212  to continue polishing the wafer  202  to its very edge without falling off the edge of the wafer  202 . Accordingly, the subaperture CMP system  200   a ′ permitted precision polishing of the edge of the wafer  202  by polishing the edge of the wafer  202  to the end. 
     The embodiment of FIGS. 4A-1 and  4 A- 2  depict a subaperture CMP system  200   b , wherein the edge of the secondary head  212  is positioned at the position x 0 . In this embodiment, the secondary head  212  is designed to be fixed (from a linear movement standpoint), while it preferably rotates in the direction of preparation  207 . In yet another embodiment, the secondary head  212  can be set to stay in a non-rotational configuration. As shown, the edge of the primary head  208  is positioned at the position x 1 , the edge of the wafer  202  in the movement direction  214 . As depicted in the embodiment of FIGS. 4A-3 and  4 A- 4 , the edge of the primary head  208  has linearly moved from the position x 1  to the position x 0 , while the secondary head  212  has remained fixed. Thus, the subaperture CMP system  200   b ′ interjects a great deal of flexibility by allowing the precision polishing of the edge of the wafer  202  prior to polishing the center of the wafer  202  while eliminating the aforementioned edge burn-off effect and/or the pad rebound effect. 
     In the embodiment of FIGS. 5A-1 and  5 A- 2  the primary head  208  is positioned at the position x 1 , the edge of the wafer  202  in the movement direction  214 , while the edge of the secondary head  212  is located at x −1 , the edge of the wafer  202  in the movement direction  222 . Both the primary head  208  and the secondary head  212  are configured to rotate in the direction of preparation  207 . Besides rotating, the primary head  208  is defined to linearly move in the movement direction  214  at about the same linear velocity as the secondary head  212  while it linearly moves in the opposite direction, the movement direction  222 . In one embodiment, while the secondary head  212  supports the wafer  202  during the time it is being polished by the primary head  208 , the secondary head  212  may additionally function as a buffer or a cleaner. 
     The subaperture CMP system  200   c ′ represents the embodiment of FIG. 5A-1, wherein the primary head  208  and the secondary head  212  have both linearly moved. As depicted, the primary head  208  has moved from the position x 1 , the edge of the wafer  202  in the movement direction  214  to the position x 0 , the center of the wafer  202 . Concurrently, the secondary head  212  has linearly moved from the position x −1  to the position x 0 . As such, the  200   c ′ system controls the polishing of a particularly targeted region of the surface of the wafer  202 , the very edge of the wafer  202 , without having to deal with the shortcomings associated with the prior art (i.e., the edge burn-off effect or pad rebound effect). 
     The primary head  208  of the embodiment of FIGS. 6A-1 and  6 A- 2  is designed to rotate in the direction of preparation  207  while oscillating in the direction of oscillation  211 . The primary head  208  is further configured to linearly move in the direction of movement  214 . As shown, the edge of the primary head  208  is located at the location x 0  thus causing the edge of the secondary head  212  to be located slightly to the left of the position x 0 . In this embodiment, the secondary head  212  is also designed to rotate at the direction of preparation  207  while it linearly moves in the movement direction  214 . In one embodiment, the secondary head  212  may function as a buffer or a cleaner besides supporting the wafer  202  while it is being polished by the primary head  208 . 
     FIGS. 6A-3 and  6 A- 4  represent the embodiment of FIG. 6A-1 following the primary head  208  linear movement from the position x 0  to the position x 1 . Also shown is the concurrent movement of the secondary head  212  from the position x 0  to x 1 . It must be noted that while moving from the position x 0  to the position x 1 , the linear velocity of the primary head  208  may be about the same as the linear velocity of the secondary head  212 . Accordingly, in specific situations, the subaperture CMP system  200   d ′ has the capability of polishing the center of the wafer  202  first, thereby removing surface materials from the center of the wafer  202  prior to removing any materials from the edge region. This capability allows the CMP system to polish the surface of the wafer  202  in order to remove surface materials along a desired topography of the wafer  202 . Furthermore, by leaving the polishing of the edge of the wafer  202  to the end, the subaperture CMP system  200   d ′ can exercise a greater control over the removal of surface materials so as to eliminate the drawbacks associated with the prior art. 
     In the subaperture CMP system of  200   e , as shown in FIGS. 7A-1 and  7 A- 2 , the edge of the primary head  208  is located at the position x 1 , the edge of the wafer  202  in the movement direction  214  thus causing the edge of the secondary head  212  to be positioned slightly to the left of the position x 1 . In this embodiment, the secondary head  212  is designed to linearly move in the movement direction  222  while it rotates. Similarly, the primary head  208  is defined to linearly move at the linear direction  214  while rotating in the preparation direction  207  and oscillating at the direction of oscillation  211 . The primary head  208  and the secondary head  212  are designed to move at a comparable linear velocity. 
     FIGS. 7A-3 and  7 A- 4  depict the embodiment of FIG. 7A-1 after the primary head  208  and secondary head  212  have concurrently moved in the linear movement direction  222 , at a comparable linear velocity. As shown, the edge of the primary head  208  has linearly moved from the position x 1  to the position x 0  while the edge of the secondary head  212  has also moved from the position x 1  to the position x 0 . FIGS. 7A-3 and  7 A- 4  reveal a flexible subaperture CMP system, wherein initially, the edge of the wafer  202  is polished without being subjected to the pad rebound effect and edge burn-off effect associated with the prior art. 
     FIG. 8A-1 is a plan view of a subaperture CMP system  200   f  in accordance with one embodiment of the present invention. The primary head  208  of this embodiment is defined to rotate in the preparation direction  207  while oscillating in the oscillation direction  211 . In one embodiment, the subaperture CMP system  200   f  is designed to preferably include a support point  220  rather than the secondary head  212 . The support point  220  is configured to support the retainer ring  204  while the wafer  202  is being polished by the primary head  208 . In one embodiment, the support point  220  is positioned at a position symmetrically opposite to the primary head  208  so as to create a moment equal to the moment generated by the rotating primary head  208 . In one embodiment, the support point  220  may be designed to be rigid. However, in a different embodiment, the support point  220  may be designed so as to support the retainer ring  204  while the wafer  202  is being polished by utilizing an automatic mechanism configured to create a programmable force comparable to the moment generated by the rotating primary pad  208 . The support point  220  may support the wafer  202  utilizing any variety of force output devices including, without limitation, pneumatic output device, hydraulic output device, electromagnetic output device, or mechanical output device. The support point  220  may further be implemented by the use of an open or closed loop system, wherein the resulting forces and/or the feedback can be monitored. In general, any force output device that can generate a programmable force so as to compensate the moment generated by the primary head  208  can be used. 
     FIG. 8A-2 is a cross-sectional view of the embodiment of FIG. 8A-1, which illustrates the position of the support point  220  with respect to the co-planer relationship of the retainer ring  204  with the wafer  202 . As shown, initially, the edge of the primary head  208  is positioned at the position x 0 , the center of the wafer  202 . 
     The subaperture CMP system  200   f ′ of FIGS. 8A-3 and  8 A- 4  depict the embodiment of the FIG. 8A-1, wherein the edge of the primary head  208  has moved from the position x 0  to the position x 1 . As shown, the primary head  208  started polishing the wafer  202  at the center of the wafer  202 . As the primary head  208  continued polishing the surface of the wafer  202 , the support point  220  supported the wafer  202  by imparting a force on the retainer ring  204  so as to compensate for the moment generated by the primary head  208 . As a result of the support point  220  supporting the wafer  202 , while the wafer  202  was being polished by the primary head  208 , the subaperture CMP system  200   f ′ was able to polish the wafer  202  to its very edge without facing the shortcomings associated with the prior art. 
     In the embodiment of FIGS. 9A-1 and  9 A- 2  the support point  220  applies force on the retainer ring  204  while the retainer ring  204  holds the wafer  202  being polished by the primary head  208 . In this embodiment, the edge of the primary head  208  is positioned at the position x 1  while the support point  220  is positioned at a symmetrically opposite point on the retainer ring  204 . As shown in the embodiment of FIGS. 9A-3 and  9 A- 4 , the primary head  208  has moved in the movement direction  222  from the position x 1 , the edge of the wafer  202 , to the position x 0 , the center of the wafer. Thus enabling the subaperture CMP system  200   g ′ to polish the very edge of the wafer  202  without confronting the pad rebound effect and edge burn-off effect associated with the prior art. 
     In one embodiment of a subaperture CMP system  200   h , a plurality of support points  220  may be implemented. As depicted in the embodiments of FIGS. 10A-1 and  10 A- 2 , two support points  220  have been used to apply force on the retainer ring  204  while the retainer ring  204  holds the wafer  202  as it is being polished by the primary head  208 . In this embodiment, the edge of the primary head  208  is positioned at the position x 0 , while the support points  220  are positioned on opposite positions on the retainer ring  204 . The multiple support points  220  support the retainer ring  204  and, thereby the wafer  202  by generating a moment comparable to that of the primary head  208 . 
     The benefits of utilizing multiple support points  220  become evident particularly when the primary head  208  is polishing the edge of the wafer  202 . The embodiment of FIGS. 10A-3 and  10 A- 4  depict a subaperture CMP system  200   h ′, wherein the edge of the primary head  208  has moved from the position x 0  to the position x 1 . As shown, the support points  220  supported the retainer ring  204  and, thereby the wafer  202  while the primary head  208  moved from the position x 0  to the position x 1 , the edge of the wafer  202 . The co-planer relationship of the retainer ring  204  with the wafer  202  in conjunction with the support provided by the support points  220  have allowed the polishing head  208  to polish the wafer  202  to the very edge without falling off the edge of the wafer  202 . 
     FIGS. 11A-1 and  11 A- 2  depict a subaperture CMP system in accordance with yet another embodiment of the present invention. This embodiment also implements a plurality of support points  220 , which apply force on the retainer ring  204  while it holds the wafer  202  as the wafer  202  is being polished by the primary head  208 . In this embodiment, the edge of the primary head  208  is positioned at the position x 1 , while the multiple support points  220  are located on opposite positions on the retainer ring  204 . As shown in the embodiment of FIGS. 11A-3 and  11 A- 4 , the primary head  208  has moved from the position x 1  to the position x 0 . The multiple support points  220  enabled the primary head  208  to start polishing at the very edge of the wafer  202  without creating any pad rebound effect or edge burn-off effect. 
     FIG. 12 depicts a non-gimbal exemplary embodiment of the present invention. As shown, the carrier  206  is situated on the extending spindle  224  while engaging the retainer ring  204 , which holds the wafer  202 . The support point  220  applies force on the retainer ring  204  while the primary head  208  polishes the wafer  202 . This implementation, preferably, requires the utilization of a primary head  208 , which can adjust to the carrier head  206  as the primary head  208  is being applied on the carrier head  206 . Specifically, the primary head  208  is configured to properly apply appropriate amount of force on the carrier head  206  so as to prevent improper angled contact of the primary head  208  with the carrier  206 . 
     It should be appreciated that the present invention may be implemented in a manner so as to include a system to monitor the finishing layer thickness (e.g., such as any one or more layers/films being prepared/polished), material removal rate, and/or finishing layer profile. However, such monitoring systems have not been shown in the above-mentioned Figures. Furthermore, although in the exemplary above-mentioned embodiments the wafer  202  and the other components were oriented horizontally, the present invention may be implemented in a manner so that the wafer  202  is oriented in any attitude (i.e., pitch or roll). Lastly, the present invention may be implemented so that all axis motions, forces, flows, and process parameters may be fully programmable. 
     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. For instance, the embodiments are applicable to any substrate, such as, storage media disks, semiconductor wafers (e.g., 200 mm wafers, 300 mm wafers, etc.), and any other type of substrate requiring polishing, planarization, buffing, or other suitable preparation operations. 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.