Abstract:
Methods and apparatus for planarizing the surface of a semiconductor wafer by applying non-uniform pressure distributions across the back side of the wafer are disclosed. According to one aspect of the present invention, a chemical mechanical polishing apparatus for polishing a first surface of a semiconductor wafer includes a polishing pad which polishes the first surface of the semiconductor wafer. The apparatus also includes a first mechanism which is used to hold, or otherwise support, the wafer during polishing, and a second mechanism that is used to apply a non-uniform pressure distribution through the first mechanism, directly onto a second surface of the wafer. The second mechanism is further used to facilitate polishing the first surface of the semiconductor wafer such that the first surface of the semiconductor wafer is evenly polished. In one embodiment, the second mechanism is arranged to apply both positive pressure and negative pressure substantially simultaneously across the second surface of the semiconductor wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to methods and apparatus for polishing the surface of a semiconductor wafer using a chemical mechanical polishing process. More particularly, the present invention relates to methods and apparatus for applying pressure differentials on the back side of a semiconductor wafer to improve the performance of chemical mechanical polishing processes. 
     2. Description of Relevant Art 
     Chemical mechanical polishing, which is often referred to as “CMP,” typically involves mounting a wafer, faced down, on a holder and rotating the wafer face against a polishing pad mounted on a platen. The platen is generally either rotating or in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer surface layer and an abrasive that physically removes portions of the surface layer is flowed between the wafer and the polishing pad, or on the pad in the vicinity of the wafer. 
     In semiconductor wafer fabrication, CMP is often utilized in an effort to planarize various wafer layers which may include layers such as dielectric layers and metallization layers. The planarity of the wafer layers is crucial for many reasons. For example, during wafer fabrication, planar layers reduce the likelihood of the accidental coupling of active conductive traces between different metallization layers, e.g., layers of active conductive traces, on integrated circuits housed on the wafer. Planar layers further provide a surface with a constant height for any subsequent lithography processes. 
     Polishing pressure, or the pressure applied to a wafer by a polishing pad, is generally maintained at a constant, e.g., uniform, level across the wafer. A uniform polishing pressure is maintained in an effort to ensure that the same amount of material, or film, is removed from all sections on the surface of a wafer. The amount of material removed from the surface of a wafer is governed by Preston&#39;s Equation, which states that the amount of material removed from the surface of a wafer is proportional to the product of the polishing pressure and the relative velocity of the wafer. The relative velocity of the wafer is generally a function of the rotation of the wafer. Using Preston&#39;s Equation, if the relative velocity of the wafer is maintained at a constant level, and the polishing pressure is at a uniform level across the wafer, then the amount of material removed from the wafer is constant. 
     During CMP, a wafer is held against a polishing pad with a uniform downforce such that the surface of the wafer may be evenly polished by the polishing pad. FIG. 1 is a diagrammatic cross-sectional representation of a wafer carrier assembly which may be used with a CMP apparatus such as an Avantgaard 676, available commercially from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz. A wafer carrier assembly  104  is generally used to transport a wafer  112  in order to position wafer  112  over a polishing pad  124 , which is mounted on a platen  125 . Wafer carrier assembly  104  typically includes a wafer carrier  106 , or carrier plate, a wafer carrier film  108 , and a retaining ring  110 . Wafer  112  is supported by wafer carrier assembly  104  such that when a negative pressure, i.e., a vacuum, is applied through vacuum inlet  116  when wafer  112  is to be moved over a polishing pad  124 , the negative pressure “permeates” openings  120  in wafer carrier  106  and wafer carrier film  108  to force wafer  112  against carrier film  108 . That is, the vacuum created through openings  120  essentially suctions wafer  112  against carrier film  108  for transport. 
     When wafer  112  comes into contact with polishing pad  124  for polishing purposes, the vacuum applied through vacuum inlet  116  is released, and wafer  112  may be held against polishing pad  134  with a uniform back pressure applied by a pneumatic cylinder mechanism (not shown). In general, wafer carrier assembly  104  includes a shaft  126  which is coupled to a pneumatic cylinder mechanism (not shown) that is arranged to apply a downforce on wafer  112  in order to polish a front side  128  of wafer  112  using polishing pad  124 . The downforce on wafer  112  is applied when the pneumatic cylinder mechanism presses down on wafer carrier assembly  104 . 
     Once a polishing pad has been repeatedly used, e.g., is near the end of its pad life, the effectiveness of the polishing pad decreases. Since replacing polishing pads is time-consuming and expensive, a polishing pad is typically repeatedly used until non-uniformity on the surfaces of wafers polished using the polishing pad is at a level which is considered to be unacceptable. Generally, after a polishing pad has been repeatedly used to polish wafers over a period of time, the polishing pad has a tendency to become “glazed.” As is well known in the art, pad glazing occurs when the particles eroded from wafer surfaces, in addition to particles from abrasives in the slurry, glaze or otherwise accumulate over the polishing pad. 
     Pad glazing is generally most evident during CMP performed on an oxide layer such as a silicon dioxide layer. Herein and after, CMP performed on an oxide layer will be referred to as “oxide CMP.” During oxide CMP, eroded silicon dioxide particulate residue, along with abrasives in the slurry, have the tendency to glaze the polishing pad. When pad glazing occurs, the polishing rate of the wafer surface is reduced, and a non-uniformly polished wafer surface is produced due to uneven removal of the glaze. 
     In general, during CMP, as the number of wafers processed using a particular polishing pad increases, the material, or film, removal rate near the axial center of the wafer typically becomes increasingly slower due to pad glazing. Pad conditioning generally helps to prevent the glazing effect. However, as the polishing pad degrades, film removal non-uniformity increases. The film removal non-uniformity typically results in faster film removal at the wafer edge than near the center of the wafer. The increasingly slower material removal rate near the center of the wafer is generally known as “center-slow” polishing. In order to compensate for center-slow polishing, pad conditioning may also be used to shape the profile of a polishing pad such that contact between the polishing pad and the center of a wafer is increased. In general, a polishing pad is fabricated from a material such as a compressible poromeric polyurethane. As will be appreciated by those skilled in the art, conditioning of a compressible poromeric polyurethane becomes less effective after repeated conditioning. 
     Increasing the contact between a polishing pad and the center of the wafer results in an increased polish rate at the center of the wafer. However, conditioning the polishing pad has the tendency to become less effective as the polishing pad ages. Further, replacing polishing pads is both time-consuming and expensive. Hence, prolonging the life of a polishing pad while reducing film removal non-uniformity is desirable. As such, what is desired is a method and apparatus for reducing wafer surface non-uniformity that occurs during CMP after a polishing pad has been used repeatedly. In other words, what is desired is a method and apparatus slows down the film removal non-uniformity degradation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, non-uniform pressure distributions are provided across the back side of a semiconductor wafer to enable polishing pressure to be varied across the wafer and, hence, the polishing pad which is used to polish the wafer during a chemical mechanical polishing (CMP) process. Varying the polishing pressure across the polishing pad enables problems which may arise when a polishing pad has been used repeatedly, e.g., center slow polishing, to be alleviated. By way of example, to compensate for center slow polishing, the pressure applied around the axial center of the wafer may be higher than pressures applied away from the center of the wafer. 
     According to one aspect of the present invention, a chemical mechanical polishing apparatus for polishing a first surface of a semiconductor wafer includes a polishing pad which polishes the first surface of the semiconductor wafer. The apparatus also includes a first mechanism which is used to hold, or otherwise support, the wafer during polishing, and a second mechanism that is used to apply a non-uniform pressure distribution through the first mechanism, directly onto a second surface of the wafer. The second mechanism is further used to facilitate polishing the first surface of the semiconductor wafer such that the first surface of the semiconductor wafer is evenly polished. In one embodiment, the second mechanism is arranged to apply both positive pressure and negative pressure substantially simultaneously across the second surface of the semiconductor wafer. 
     According to another aspect of the present invention, a chemical mechanical polishing apparatus for polishing a first surface of a semiconductor wafer includes a polishing pad which polishes the first surface of the wafer and a mechanism which applies a non-uniform pressure distribution directly across portions of a second surface of the wafer. The mechanism also supports the wafer while the first surface of the wafer is being polished. In one embodiment, the mechanism for applying the non-uniform pressure distribution includes a retaining ring, a carrier, and a carrier film which cooperate to support the wafer. In such an embodiment, the mechanism may also include an air supply which provides the non-uniform pressure distribution along the second surface of the wafer. 
     In another embodiment, a carrier and a carrier film are used to facilitate the application of the non-uniform pressure distribution along the second surface of the wafer. In such an embodiment, a plurality of openings, coupled to an air supply, are defined through both the carrier and the carrier film to provide the non-uniform pressure distribution along the second surface of the semiconductor wafer. 
     According to yet another aspect of the present invention, a method for planarizing a first surface of a semiconductor wafer using chemical mechanical polishing includes holding the wafer over a chemical mechanical polishing pad. A non-uniform pressure distribution is then applied directly over a second surface of the wafer, and the first surface of the wafer is polished with the chemical mechanical polishing pad. In one embodiment, applying the non-uniform pressure distribution over the second surface of the wafer involves simultaneously applying both a positive pressure and a negative pressure. In another embodiment, pressurized air is applied directly over the second surface of the semiconductor wafer. 
     These and other features and advantages of the present invention will be presented in more detail in the following detailed description of the invention and in the associated figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagrammatic cross-sectional representation of a wafer carrier assembly which is a part of a chemical mechanical polishing apparatus in accordance with prior art. 
     FIG. 2 a  is a diagrammatic cross-sectional representation of a wafer carrier assembly which is arranged to apply a non-uniform pressure distribution to a back side of a wafer in accordance with a first embodiment of the present invention. 
     FIG. 2 b  is a diagrammatic top-view representation of a wafer carrier in accordance with the first embodiment of the present invention. 
     FIG. 3 a  is a diagrammatic cross-sectional representation of a wafer carrier assembly which is arranged to apply a non-uniform pressure distribution to a back side of a wafer in accordance with a second embodiment of the present invention. 
     FIG. 3 b  is a diagrammatic top-view representation of a wafer carrier in accordance with the second embodiment of the present invention. 
     FIG. 4 is a diagrammatic representation of a top view of a wafer carrier in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The planarity, or uniformity, of the surface of a semiconductor wafer layer is important for a number of different reasons. For example, ensuring the planarity of the surface of a semiconductor wafer reduces the likelihood of accidentally coupling metallization lines in different metallization layers of the semiconductor wafer. One process which is used to form planar surfaces on a wafer is chemical mechanical polishing (CMP). While CMP is generally effective in forming planar surfaces on wafers, when polishing pads used in CMP become glazed, the polishing rate of wafer surfaces may be reduced. As a polishing pad degrades, the film removal non-uniformity also degrades. As a result, non-uniformly polished wafer surfaces may be produced due to uneven removal of the glaze. Specifically, in many cases, center-slow polishing occurs which causes the portion of a wafer around the axial center of the wafer to be polished to a lesser extent than other portions of the wafer. 
     By applying a pressure differential across the back of a semiconductor wafer, the wafer may be bowed to promote contact between particular portions of the wafer and a polishing pad. Therefore, the polishing pad may consistently and uniformly polish the wafer surface, even after the polishing pad has been used extensively, or is otherwise approaching the end of its pad life. Specifically, applying pressure differentials across the back side of a wafer allows polishing pressures exerted between the polishing pad and the surface of a wafer to be varied. As such, by varying pressures applied across the back side of a wafer as necessary, polishing pressures across a wafer may then be effectively varied to enable a CMP process to produce a planar surface on the wafer. In other words, the film removal rate may be varied by varying the polishing pressure. For example, less material is removed from the surface of a wafer as the polishing rate of the wafer decreases. Hence, by increasing the polishing pressure, the amount of material removed from the wafer may be increased. In general, polishing pressures may be varied between both positive pressures and negative pressures, e.g., vacuums. Further, both a positive pressure and a negative presssure may be simultaneously applied across different sections of the wafer to achieve differential polishing pressures across the wafer. 
     A pressure differential, or a non-uniform pressure distribution, may be created across the back side of a wafer by including a plurality of air sources, coupled to a plurality of air lines. The air sources and the air lines may provide air pressurized to different pressures directly to the wafer to thereby create a non-uniform pressure distribution across the wafer. 
     Referring next to FIG. 2 a , a wafer carrier assembly  204  which is arranged to apply a non-uniform pressure distribution directly to a back side  217  of a wafer  212  will be described in accordance with a first embodiment of the present invention. As shown, the features and dimensions of wafer carrier assembly  204  have been exaggerated for purposes of discussion. Wafer carrier assembly  204  includes a wafer carrier  206 , a wafer carrier film  208 , and a retaining ring  210 . Wafer  212  is supported by wafer carrier assembly  204 , which further includes a shaft  216  that is coupled, in one embodiment, to a pneumatic cylinder mechanism (not shown). In general, a pneumatic cylinder mechanism, or an equivalent mechanism, is arranged to apply a downforce on back side  217  of wafer  212  while a front side  218  of wafer  212  is polished against a polishing pad  220  which is mounted on a platen  221 . While polishing pad  220  has been shown as having a smaller diameter than wafer  212 , it should be appreciated that in some embodiments, polishing pad  220  has a larger diameter than wafer  212 . By way of example, wafer  212  may have a diameter of approximately six inches to approximately eight inches, while polishing pad  220  may have a diameter of approximately ten inches. 
     Air sources  234 , e.g., sources of nitrogen, provide air through air lines  236  which pass through a sealing space  240 , in one embodiment. Air is generally passed through air lines  236  such that air flows through openings  238  in wafer carrier  206  and carrier film  208  to substantially directly contact back side  217  of wafer  212 . One embodiment of a pattern of openings  238  in wafer carrier  206  and, hence, carrier film  208  will be described in more detail below with respect to FIG. 2 b . As will be appreciated by those skilled in the art, carrier film  208  is typically a thin, polymeric film which is intended to cushion wafer  212 . In some embodiments, carrier film  208  may not be included as part of wafer carrier assembly  204 . 
     In order to provide a non-uniform pressure distribution on back side  217  of wafer  212  to facilitate the even polishing of front side  218  of wafer  212 , the air which passes through air lines  236  may be at different pressures. By way of example, as shown, air which passes through air line  236   a  is at a first pressure PI, while air which passes through air lines  236   b  is at a second pressure P 2 . Similarly, air which passes through air lines  236   c  is at a third pressure. By including a plurality of air lines  236 , the pressures on the back side  217  of wafer  212  may be finely controlled, as different air lines  236  may be used to essentially control the polishing pressure on different sections of wafer  212 . 
     In general, air pressures P 1 , P 2 , and P 3  may be widely varied. For example, air pressure P 1  may be a negative pressure, i.e., a vacuum, while air pressures P 2  and P 3  may be positive pressures. The magnitudes of air pressures P 1 , P 2 , and P 3  may also be widely varied, and are generally chosen based upon the desired uniformity front side  216  of wafer  212 . In general, the magnitudes of air pressures P 1 , P 2 , and P 3  will not exceed the downforce applied on wafer  212  by a pneumatic cylinder mechanism (not shown). In one embodiment, the magnitudes of air pressures P 1 , P 2 , and P 3  will not exceed a value which is greater than approximately seventy percent of the magnitude of the downforce, which may be, but is not limited to being, in the range of approximately five to approximately ten pounds-per-square inch (psi). By way of example, when the -downforce is approximately 7 psi, the magnitudes of air pressures P 1 , P 2 , and P 3  may be in the range of approximately 0.5 psi to approximately 3 psi. 
     As polishing pad  220  reaches the end of its life, when wafer  212  is polished using polishing pad  220 , center-slow polishing tends to occur. In other words, the area of wafer  212  near the axial center of wafer  212  may be polished to a lesser extent than areas of wafer  212  which are further from the axial center. In order to compensate for center-slow polishing, air pressure P 1  may be greater than air pressure P 2  which, in turn, may be greater than air pressure P 3 . Increasing the air pressure on back side  217  of wafer  212  near the axial center of wafer  212  with respect to the air pressure on back side  217  of wafer  212  away from the axial center of wafer  212  allows the area of front side  218  near the axial center of wafer  212  to be polished at a faster rate. That is, the portion of front side  218  of wafer  212  may be slightly bowed out with respect to other portions of wafer  212 . 
     The distribution of pressure on back side  217  of wafer  212  may be varied depending upon the pattern of cylindrical openings  238  in wafer carrier  206  and carrier film  208 . FIG. 2 b  is a diagrammatic top-view representation of one pattern of openings  238  in wafer carrier  206  in accordance with the first embodiment of the present invention. Openings  238  are arranged as substantially concentric circles with respect to the axial center of wafer carrier  206 . Arranging openings  238  in substantially concentric circles enables pressure to be distributed across back side  217  of wafer  212  in a concentric, circular pattern as will be appreciated by those skilled in the art. As shown, opening  238   a  is located approximately at the axial center of wafer carrier  206 , while openings  238   b  and openings  238   c  are patterned on concentric circles which are substantially centered around opening  238   a.    
     Although the air pressure which passes through all openings  238  may be different, e.g., separate air lines  236  may be associated with each opening  238 , in the described embodiment, opening  238   a  is associated with air pressure P 1 , openings  238   b  are associated with air pressure P 2 , and openings  238   c  are associated with air pressure P 3 . Accordingly, as described above, to compensate for center-slow polishing, air pressure P 1  may be higher than air pressure P 2 , which may be higher than air pressure P 3 . Alternatively, to compensate for center-fast polishing, i.e., to compensate for a higher polishing rate near the edges of wafer  212 , air pressure P 3  may be higher than air pressure P 2 , which may be higher than air pressure P 1 . 
     By providing openings  238  in wafer carrier  206  such that air pressure may be applied directly to back side  217  of wafer  206 , wafer carrier  206  may remain substantially rigid during a CMP process. Minimizing any flexing in wafer carrier  206  during CMP protects the integrity of wafer carrier assembly  204 , e.g., may reduce the wear of wafer carrier  206 , and, hence, any wafer  212  polished using wafer carrier assembly  204 . 
     As described above, to generate a non-uniform pressure distribution across the back side of a wafer, a plurality of air lines may be implemented in a wafer carrier system to substantially simultaneously apply different pressures to the back side of the wafer. Alternatively, a single air line, coupled to a single air source, may also be used to create a non-uniform pressure distribution across the back side of a wafer, as will be described with respect to FIG. 3 a . When a single air line is used, modifications may be made to a wafer carrier, or a carrier plate, to create the non-uniform pressure distribution. 
     FIG. 3 a  is a diagrammatic cross-sectional representation of a wafer carrier assembly with a wafer in accordance with a second embodiment of the present invention. A wafer carrier assembly  304  includes a wafer carrier  306 , a wafer carrier film  308 , and a retaining ring  310 . A wafer  312  is supported by wafer carrier assembly  304 , as will be appreciated by those skilled in the art. Wafer carrier assembly  304  further includes a shaft  316  which is generally coupled to a pneumatic cylinder mechanism (not shown), or any other suitable mechanism that is arranged to apply a downforce on wafer  312  while a front side  318  of wafer  312  is polished against a polishing pad  320  which is supported on a platen  321 . 
     An air source  334 , e.g., a source of nitrogen, provides air through an air line  336  to sealing space  340 . It should be appreciated that the air provided by air source  334  may be at any suitable pressure P. In order to provide a non-uniform pressure distribution on a back side  322  of wafer  312  such that the even polishing of front side  318  of wafer  312  is facilitated, openings  330  of varying diameters are provided in wafer carrier  306  and wafer carrier film  308 . The range of suitable diameters for openings  330  may be widely varied. By way of example, in one embodiment, diameters may be in the range of approximately 0.03 millimeters to approximately 1 millimeter. One embodiment of wafer carrier  306 , with openings  330  of varying diameters, will be described below with reference to FIG. 3 b.    
     In the described embodiment, the pressure distribution on back side  322  of wafer  312  is dependent upon both the pattern of openings  330  in wafer carrier  306  and carrier film  308  and the relative size of openings  330 . Although the pattern of openings  330  may be widely varied, one particularly suitable pattern of openings  330  is an essentially concentric pattern of openings. Although air which flows through air line  336  is typically at a single pressure, when the air is dispersed within sealing space  340  and passed through openings  330 , due to the fact that openings  330  are of different diameters, e.g., opening  330   c  has a larger diameter than opening  330   a , the pressure of air passing through opening  330   c  will be different from the pressure of air passing through opening  330   a.    
     Referring next to FIG. 3 b , one pattern of openings  330  of different sizes in wafer carrier  306  will be described in accordance with a second embodiment of the present invention. Openings  330  are arranged as substantially concentric circles with respect to the axial center of wafer carrier  306 . As shown, opening  330   b  is located approximately at the axial center of wafer carrier  306 , while openings  330   a  and openings  330   c  are located along concentric circles which are substantially centered around opening  330   b.    
     As previously mentioned with respect to FIG. 3 a , in the described embodiment, openings  330   c  have a larger diameter than openings  330   a . Opening  330   b , as shown, has approximately the same diameter as openings  330   c . Varying the sizes of openings  330  on wafer carrier  306  enables a single source of air pressure, e.g., air source  334 , to create a non-uniform pressure distribution on back side  322  of wafer  312 . By way of example, when air pressure provided through air line  336  is a negative air pressure, or a vacuum, then a higher vacuum may be produced through openings  330   a  than through openings  330   b ,  330   c . As such, wafer  312 , when subjected to pressurized air through openings  330 , may be bowed such that the portion of wafer which is “suctioned” through openings  330   b ,  330   c  may be polished to minimize the effects of center-slow polishing. 
     Openings in a wafer carrier may generally be arranged in any suitable configuration, and are not limited to being organized in a pattern of concentric circles. Specifically, openings may be situated on a wafer carrier at specific locations determined by an acceptable level of uniformity for a polished surface of a wafer. That is, openings are arranged to provide a pattern of pressure distribution across the back side of a wafer which allows the front side of the wafer to be polished to a desired level of uniformity. FIG. 4 is a diagrammatic representation of a top view of a wafer carrier in accordance with a third embodiment of the present invention. A wafer carrier  404  includes a plurality of openings  412 . As shown, openings  412   a  have diameters which are larger than those of openings  412   b . Openings  412  are arranged such that the central portion of wafer carrier  404  includes smaller openings  412   a , while the peripheral portion of wafer carrier  404  includes larger openings. Accordingly, a wafer which is held in a wafer carrier assembly which uses wafer carrier  404  may have one pressure applied across the central portion of the wafer, and another pressure applied across the peripheral portion of the wafer. Such an arrangement of openings  412  in wafer carrier  404  may be suitable to promote contact between a central portion of a wafer and a polishing pad. 
     Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, although openings in a wafer carrier and, hence, a carrier film have been described as being cylindrical, it should be appreciated that the openings may take on a variety of different shapes, as well as sizes. In one embodiment, openings may be conically shaped to produce a nozzle effect in terms of directing pressurized air at the back side of a wafer. 
     While openings of different diameters in a wafer carrier have been described as being associated with a wafer carrier system which has a single air line, in general, openings of different diameters may be implemented for use with a wafer carrier system which includes a plurality of air lines and, hence, a plurality of air sources. By way of example, a single, large opening which is coupled to a first air source may be located at the axial center of a wafer carrier, while multiple smaller openings which are coupled to a second air source may be located nearer to the periphery of the wafer carrier, without departing from the spirit or the scope of the present invention. 
     In addition to providing a non-uniform back pressure on the back side of a wafer during CMP, the density associated with the non-uniform back pressure may also be modified. For example, rather than arranging openings in a wafer carrier in concentric circles, openings of the same size and shape may be arranged such that one portion of the wafer carrier may have more concentrated openings than another portion. By varying the density of openings in a wafer carrier, the density of the back pressure applied to a wafer held by the wafer carrier may be varied. 
     The application of a non-uniform back pressure on the back side of a wafer has been described as enabling the bowing of the wafer to be controlled in order to control the uniformity of a polishing process by affecting the contact between a polishing pad and the wafer. However, it should be appreciated that the application of a non-uniform back pressure on the back side of a wafer during polishing also serves to secure the wafer and, therefore, prevent the wafer from rotating during polishing. For example, a non-uniform vacuum may be applied to the back side of a wafer during polishing. 
     Further, during CMP, polishing inconsistency may occur near a wafer carrier contact point, as for example the contact point between a wear ring and a wafer. Generally, the edge of a wafer is polished faster than the center of the wafer, due to the compression of the polishing pad. By varying the back pressure applied to the wafer such that a vacuum is applied near the edge of the wafer while higher pressures are applied to other portions of the wafer, the edge exclusion problem may be solved without departing from the spirit or the scope of the present invention. Therefore, the present examples 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 of the appended claims.