Abstract:
An apparatus and method are disclosed for planarizing a wafer in a carrier with adjustable pressure zones and adjustable barriers between zones. The carrier has an independently controlled central zone and concentric surrounding zones for distributing the pressure on the backside of a wafer while the wafer is being pressed against an abrasive surface in a chemical-mechanical polishing tool. The pressure zones may be created by mounting an elastic web diaphragm to a carrier housing that has a plurality of recesses. A corresponding plurality of elastic ring shaped ribs may extend from the web diaphragm opposite the recesses. The plurality of ring shaped ribs thereby defines a central zone surrounded by one or more concentric surrounding zones. The zones and barriers may be individually pressurized by utilizing corresponding fluid communication paths during the planarization process. 
     A method for practicing the present invention starts by selecting a carrier with adjustable pressure zones that correspond to the number and locations of the bulges and troughs on the wafer. Zones that correspond to high regions receive greater pressure than zones that correspond to low regions on the wafer. The pressure on the barriers between zones may be optimized to prevent leakage between zones or to smooth the pressure distribution between neighboring zones on the back surface of the wafer.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the art of planarizing a workpiece against an abrasive surface. For example, the present invention may be used to planarizing a wafer, or thin films deposited thereon, in an improved wafer carrier with adjustable pressure zones and adjustable pressure barriers against a polishing pad in a chemical-mechanical planarization (CMP) tool. 
     BACKGROUND OF THE INVENTION 
     A flat disk or “wafer” of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. In addition, applicant has noticed other semiconductor wafer processing steps, e.g. shallow trench isolation (STI) and copper deposition, produce predictable concentric bulges of excess material on the wafer. For example, applicant has noticed that conventional STI processes usually produce a wide peripheral ring shaped bulge and a small central disk shaped bulge with a narrow trough between bulges. Applicant has also noticed that conventional copper deposition processes usually produce a narrow peripheral ring shaped bulge and a small central disk shaped bulge with a wide trough between bulges. 
     The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness. Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create integrated circuitry or interconnects on the wafer. To this end, machines have been developed to provide controlled planarization of both structured and unstructured wafers. 
     A conventional method of planarizing a wafer will now be discussed. The wafer is secured in a carrier that is connected to a shaft in a CMP tool. The shaft transports the carrier, and thus the wafer, to and from a load or unload station and a position adjacent a polishing pad mounted to a platen. A pressure is exerted on the back surface of the wafer by the carrier in order to press the wafer against the polishing pad, usually in the presence of slurry. The wafer and/or polishing pad may be rotated, orbited, linearly oscillated or moved in a variety of geometric or random patterns via motors connected to the shaft and/or platen. 
     Numerous carrier designs are known in the art for holding and distributing a pressure on the back surface of the wafer during the planarization process. Conventional carriers commonly have a hard flat pressure plate that is used to press against the back surface of the wafer that does not conform to the back surface of the wafer. As a consequence, the pressure plate is not capable of applying a uniform polish pressure across the entire area of the wafer, especially at the edge of the wafer. In an attempt to overcome this problem, the pressure plate is often covered be a soft carrier film. The purpose of the film is to transmit uniform pressure to the back surface of the wafer to aid in uniform polishing. In addition to compensating for surface irregularities between the carrier plate and the back surface of the wafer, the film deforms around and smoothes over minor contamination on the wafer surface. Such contamination could produce high pressure points in the absence of such a carrier film. Unfortunately, the films are only partially effective with limited flexibility and no capability for globally adjusting once they have been applied to the pressure plate. 
     A common problem for conventional carriers having a hard flat plate is that they cannot compensate for incoming wafers that have one or more bulges. The hard flat plate is limited by the fact that it cannot adjust the pressure applied to different zones on the back surface of the wafer. It is common for some wafer processing steps to leave bulges on the wafer. Conventional carriers typically remove approximately the same amount of material across the entire front face of the wafer, thereby leaving the bulges on the wafer. Only sufficiently smooth, flat portions of the wafer surface may be effectively used for circuit deposition. Thus, the depressions limit the useful area of the semiconductor wafer. 
     Other conventional carriers implement means for applying more than one pressure region across the back surface of the wafer. Specifically, some conventional carriers provide a carrier housing with a plurality of concentric internal chambers that may be independently pressurized separated by barriers. By pressurizing the individual chambers in the top plate to different magnitudes, a different pressure distribution can be established across the back surface of the wafer. 
     However, Applicants have discovered that the pressure distribution across the back surface of the wafer for conventional carriers is not sufficiently controllable. This is due to the lack of control of the pressure caused by the barriers on the back surface of the wafer. The barriers are important in controlling the pressure on the back surface of the wafer between internal chambers. Therefore, the ability to control the applied pressure across the entire back surface of the wafer is limited, thereby restricting the ability to compensate for anticipated removal problems. 
     What is needed is a system for controlling the application of multiple pressure zones and the pressure from the barriers between zones across the entire back surface of a wafer during planarization. 
     SUMMARY OF THE INVENTION 
     Thus, it is an object of the present invention to provide an apparatus and method for controlling the pressure distribution on the back surface of a wafer through independently controllable concentric zones and barriers while planarizing the wafer. 
     In one embodiment of the present invention, a carrier is disclosed for planarizing a surface of a wafer. The carrier includes a central disk shaped plenum, a plurality of concentric ring shaped plenums surrounding the central plenum and a plurality of concentric barriers between neighboring plenums. The pressure distribution on the back surface of the wafer may thus be controlled by adjusting the pressure in the plenums and the pressure exerted on the barriers. 
     In another embodiment, a carrier is disclosed that includes a carrier-housing that advantageously comprises a rigid non-corrosive material. The carrier housing is preferably cylindrically shaped with a first major surface being used to couple the carrier to a CMP tool and a second major surface with a plurality of concentric ring-shaped plenums. 
     An elastic web diaphragm is placed over the second major surface thereby covering the carrier plenums. A plurality of elastic ring shaped ribs extends orthogonally from the web diaphragm opposite the ring shaped carrier plenums. The web diaphragm and ribs may be made from a single mold, but are preferably separate pieces. The plurality of ring shaped ribs extending from the web diaphragm thereby defines a central disk shaped web plenum surrounded by one or more concentric ring shaped web plenums. The web diaphragm and ribs may be held in place by clamping rings that are tightened against the carrier housing thereby trapping the web diaphragm and ribs placed between the clamping rings and carrier housing. 
     The carrier plenums may be pressurized by corresponding carrier fluid communication paths in fluid communication with each of the carrier plenums. The carrier plenums are used to control an urging force on the ribs to assist the ribs in sealing against the wafer or to assist in the distribution of force on the back surface of the wafer between neighboring web plenums. 
     The web plenums may be pressurized by corresponding web fluid communication paths in fluid communication with the central web plenum and each of the plurality of ring shaped web plenums. The web plenums are used to control an urging force on concentric zones to assist in controlling the distribution of pressure on the back surface of the wafer. The wafer may then be supported by the ribs and the central and ring shaped web plenums during the planarization process. 
     The ribs are supported by the web diaphragm on one end while the other end (rib foot) supports the wafer. The rib foot may be flat, round or have other shapes that improve the pivoting of the foot on the wafer or the sealing of the foot against the wafer. A vacuum path may be routed through the rib to further assist in sealing the rib to the wafer. While using ribs as the barrier between neighboring web plenums is the preferred method, other barriers such as o-rings, bellows or shields may be used to prevent fluid exchange between neighboring web plenums. 
     The carrier preferably has a floating retaining ring connected to the carrier housing. The retaining ring surrounds the wafer during the planarization process to prevent the wafer from escaping laterally beneath the carrier when relative motion is generated between the wafer and the abrasive surface. The floating retaining ring may be attached to the carrier housing with a retaining ring diaphragm held taut over a ring shaped recess in the periphery of the carrier housing. A retaining ring plenum is thus created between the ring shaped recess in the carrier housing and the retaining ring diaphragm. A retaining ring fluid communication path may be placed in either the carrier housing and/or retaining ring to communicate a desired pressure onto the retaining ring. The retaining ring preloads and shapes a portion of the polishing pad prior to the wafer moving over that portion of the polishing pad. The pressure on the retaining ring may thus be used to enhance, particularly near the wafer&#39;s edge, the planarization process for the wafer. 
     In another embodiment, a disk shaped wafer diaphragm is placed adjacent the feet of the ribs, thereby enclosing the web plenums. The wafer diaphragm is placed over, and is supported partially by, the ribs. To prevent leakage between the web plenums, the rib feet may be bonded to the wafer diaphragm or they may be made from a single mold. Alternatively, the rib feet may be sealed to the wafer diaphragm using the same methods as described above for sealing the rib feet to the wafer. A wafer may then be placed against the wafer diaphragm during the planarization process while the carrier plenums and/or web plenums are adjusted to control the distribution of force on the back surface of the wafer. As a further alternative, the outermost rib may be a bellows molded as a single piece with the wafer diaphragm or may be bonded to the wafer diaphragm. As a further alternative, a spring ring may be placed inside the outermost web plenum against the juncture of the outermost rib and the wafer diaphragm. The compressed spring ring will try to uniformly expand radially outward and assist in maintaining a taut wafer diaphragm. 
     The present invention may be practiced by analyzing incoming wafers for repeating geometric patterns. Some semiconductor wafer processing steps leave predictable concentric bulges on the wafer. The number, position, width and height of the bulges from these processing steps are often substantially the same from wafer to wafer. By using a carrier with adjustable concentric pressure zones and adjustable barrier pressures between zones, the carrier may optimize a pressure distribution across the entire back surface of the wafer. The pressure distribution on the back surface of the wafer is optimized by pressing harder on zones with larger bulges during the planarization process to produce a wafer with a substantially uniform thickness. 
     These and other aspects of the present invention are described in full detail in the following description, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the appended drawing Figures, wherein like numerals denote like elements, and: 
     FIG. 1 is a cross section view of a simplified carrier having adjustable concentric ribs defining adjustable pressure zones there between; 
     FIG. 2 is a bottom view of a web diaphragm with orthogonally attached concentric ribs defining a central disk shaped web plenum surrounded by concentric ring shaped web plenums; 
     FIG. 3 is a cross section view of a simplified carrier having adjustable concentric ribs defining adjustable pressure zones there between wherein the zones are enclosed by a wafer diaphragm; 
     FIG. 4 is a graph relating pressure to corresponding zones on the back surface of a wafer; 
     FIG. 5 is a cross section view of a rib with a square foot; 
     FIG. 6 is a cross section view of a rib with a round foot; 
     FIG. 7 is a cross section view of a rib with an “elephant” or self-sealing foot; 
     FIG. 8 is a cross section view of a rib with a self-sealing foot with a vacuum assist system; 
     FIG. 9 is a cross section view of another embodiment of the invention; 
     FIG. 10 is a flow chart of an exemplary process to practice the invention; 
     FIG. 11 is a more detailed drawing of a carrier similar to the carrier in FIG. 1; and 
     FIG. 12 is a cross section view of a carrier having adjustable concentric ribs defining adjustable pressure zones wherein the zones are enclosed by a wafer diaphragm and the outermost rib is configured as a bellows. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The preferred embodiment of the present invention is as an improved wafer carrier for planarizing a wafer in a CMP tool. The present invention may be used with a variety of CMP tools, such as the AvantGaard 676, 776 or 876 or Auriga C or CE made commercially available by SpeedFam-IPEC headquartered in Chandler, Ariz. CMP tools that may be used to practice the present invention are well known in the art and will not be discussed in detail to avoid obscuring the nature of the present invention. 
     A wafer carrier in a CMP tool must retain the wafer and assist in the distribution of a pressing force on the back of the wafer while the front of the wafer is planarized against an abrasive surface. The abrasive surface typically comprises a polishing pad wetted by chemically active slurry with suspended abrasive particles. The preferred polishing pad and slurry are highly dependant on the particular process and workpiece being used. Conventional CMP polishing pads and slurries are made commercially available by Rodel Inc. from Newark, Delaware for typical applications. 
     Referring to FIG.  1  and FIG. 11, an exemplary embodiment of the present invention will be discussed in detail. The carrier  156  has a rigid cylindrical carrier housing  154  providing a rigid superstructure. The carrier housing  154  may comprise, for example, stainless steal to give the carrier housing  154  the necessary rigidity and resistance to corrosion needed in a CMP environment. The top major surface of the cylindrical carrier housing  154  may be adapted to be connected to almost any conventional CMP tool. Most conventional CMP tools have a movable shaft used for transporting the carrier  156  and wafer  150 . The movable shaft typically allows the carrier  156  to move between a wafer loading and/or unloading station and a position in proximity and parallel to an abrasive surface in a CMP tool. 
     The bottom major surface of the carrier housing  154  has a plurality of concentric ring shaped recesses (hereinafter called carrier plenums)  131 - 134 . For maximum control of the pressure distribution on the back surface of a wafer, at least one carrier fluid communication path  141 - 144  is in fluid communication with each carrier plenum  131 - 134 . The carrier fluid communication paths  141 - 144  are routed through the carrier housing  154  to an apparatus for delivering an independently pressurized fluid to each carrier plenum  131 - 134 , the purpose for which will be explained below. 
     A web diaphragm  100  is coupled to the carrier housing  154  across the carrier housing&#39;s bottom major surface thereby sealing the carrier plenums  131 - 134 . The web diaphragm  100  may be coupled to the carrier housing  154  with adhesives, screws or other known techniques. However, the web diaphragm  100  is preferably kept in place by tightening a plurality of bolts  158  that pull clamp rings  157  against the carrier housing  154  thereby trapping the web diaphragm  100  placed between the carrier housing  154  and the clamp rings  157 . 
     A plurality of concentric barriers  101 - 104  extends orthogonally from a major surface of the web diaphragm  100  opposite the carrier plenums  131 - 134 . The barriers  101 - 104  may take the form of o-rings, bellows or other known configurations capable of separating neighboring pressure zones having a pressure differential. However, the preferred barrier is a short elastic piece of material hereafter referred to as a “rib”. The head of each rib  101 - 104  is connected to the web diaphragm  100  while the foot of each rib  101 - 104  is used to support either a wafer  150  or a wafer diaphragm  300  (the wafer diaphragm  300  is discussed below with reference to FIG.  3  and FIG.  12 ). The ribs  101 - 104  are made as short as possible, preferably less than 15 mm and about 2.5 mm wide, to maximize the load capabilities and minimize deflections during the planarization process. While the web diaphragm  100  and ribs  101 - 104  may be manufactured as a single piece of elastic material, they are preferably separate pieces held together against the carrier housing  154  by clamping rings  157 . The web diaphragm  100  and ribs  101 - 104  may comprise an elastic material such as EPDM. 
     The number of concentric barriers or ribs the web  155  has will directly correspond to the number of independently controllable pressure zones that may be created. Using FIG. 2 as an example (which is a bottom view of the web  155  in FIG.  1  and FIG.  11 ), four concentric ribs  101 - 104  are used to create a central disk shaped web plenum  111  surrounded by three concentric ring shaped web plenums  112 - 114 . The central disk shaped web plenum  111  is defined by the inner diameter of the innermost rib  101 , while the surrounding web plenums  112 - 114  are defined by the outer diameter and inner diameter of the ribs  101 - 104 . The spacing between the ribs  101 - 104  (and carrier plenums  131 - 134 ) may be adjusted to control the width of the web plenums  111 - 114 . The position of the ribs  101 - 104  (in combination with the carrier plenums  131 - 134 ) may be adjusted to alter the position of the web plenums  111 - 114 . For optimum control of the pressure distribution on the back surface of the wafer, at least one independently controllable web fluid communication path  121 - 124  is in fluid communication with each web plenum  111 - 114 . The web fluid communication paths  121 - 124  may be routed through the carrier housing and out the center of the carrier. 
     With reference to FIG. 1, an example of one possible method for routing a pressurized fluid to the carrier plenums  131 - 134 , web plenums  111 - 114  and retaining ring plenum  115  will now be given for a typical CMP tool design. A compressor may be used to generate a pressurized fluid that may be fed through a manifold to one or more regulators. The pressure generated by the compressor should be higher than the pressure actually needed by any of the plenums. One independently controllable regulator is preferably used for each carrier plenum  131 - 134 , web plenum  111 - 114  and retaining ring plenum  115  on the carrier  156 . The regulators are in fluid communication with their corresponding carrier fluid communication paths  141 - 144 , web fluid communication paths  121 - 124  and retaining ring fluid communication path  125 . The fluid communication paths may be routed through a rotary union on a hollow shaft, commonly found in CMP tools, connected to the carrier  156 . The fluid communication paths may then be routed through the hollow shaft and carrier  156  to their respective plenums. The present invention may be practiced using a variety of compressors, manifolds, regulators, fluid communication paths, rotary unions and hollow shafts that are well known in the art. 
     The central disk shaped web plenum  111  and surrounding ring shaped web plenums  112 - 114  may be individually pressurized to produce a plurality of concentric constant pressure zones on the back surface of a wafer  150 . The web plenums  111 - 114  may be made smaller, and are thus easier and quicker to pressurize, by increasing the size of the clamp rings  157 . The particular pressure chosen for each pressure zone depends on the surface geometry and materials comprising the incoming wafers in combination with the other process parameters of the CMP tool. For STI or copper deposition semiconductor wafers, pressures from 1 to 10 psi, and preferably 3 to 7 psi, on conventional CMP tools may be used. 
     Carriers  156  with additional controllable pressure zones have zones with a smaller average width, thereby giving the carrier  156  finer control of the pressure distribution on the backside of the wafer  150 . However, additional zones increase the cost of manufacturing, the cost of additional plumbing and the complexity of the carrier  156 . The preferred carrier  156  therefore uses the minimum number of web plenums  111 - 114  necessary for a given wafer surface geometry. 
     Additional structural support may be used to increase the ribs&#39; hoop strength and minimize the deflection of the ribs  101 - 104 . Additional structural support for the ribs  101 - 104  may be added with external or internal hoops being attached on the side of the ribs  101 - 104 , external or internal structural threads attached to the ribs  101 - 104  or by using materials for the ribs  101 - 104  having a higher modulus of elasticity. 
     An individually controllable pressing force may be placed on the head of each rib  101 - 104  by pressurizing the rib&#39;s corresponding carrier plenum  131 - 134 . The down forces generated by the carrier plenums  131 - 134  may be transmitted through the ribs  101 - 104  to the rib feet. The force on each rib  101 - 104  presses the rib&#39;s feet against either a wafer  150  or a wafer diaphragm  300  (discussed below with reference to FIG.  3  and FIG. 12) to create a superior seal for each web plenum  111 - 114 . The pressure on each rib  101 - 104  is advantageously made equal to or greater than the pressure in the neighboring web plenums  111 - 114  to help prevent fluid from leaking between the neighboring web plenums  111 - 114 . The pressurized fluid for the carrier plenums  131 - 134 , web plenums  111 - 114  and retaining ring plenum  115  may be a liquid or gas and is preferably filtered air. 
     The rib feet may be enhanced to prevent pressurized fluid from leaking between neighboring web plenums  111 - 114 . The shape of the rib feet will affect how well the feet seal, the pressure transmission through the rib  101 - 104  to the wafer  150  and how well the feet “gimbal” on the wafer  150 . 
     Referring to FIG. 5, a cross section of a square foot  101   a  is shown connected to a web diaphragm  100   a  prior to being sealed to surface  501 . The square foot  100   a  is easy to manufacture and provides a medium size contact area with the surface  501  to be sealed against, but has limited gimballing characteristics. 
     Referring to FIG. 6, a cross section of a rounded foot  101   b  is shown connected to a web diaphragm  100   b  to be sealed to surface  601 . The rounded foot  101   b  is harder to manufacture than the square foot, has minimal contact area with the surface  601  to be sealed against, but has excellent gimballing characteristics. 
     Referring to FIG. 7, a cross section of an “elephant” foot  101   c  is shown connected to a web diaphragm  100   c  prior to being sealed to surface to surface  701 . The elephant foot  101   c  is the most difficult to manufacture and has poor gimballing characteristics, but provides a large contact area with the surface  701  to be sealed against. In addition, pressure in the neighboring web plenums  702  and  703  may be used to press on the “elephant” foot  101   c  as graphically illustrated by arrows A 702  and A 703  to assist the “elephant” foot  101   c  in sealing against surface  701 . 
     Referring to FIG. 8, a cross section of an “elephant” foot  101   d  is shown connected to a web diaphragm  100   d  prior to being sealed to a surface  801 . For this rib foot  101   d  configuration, a vacuum line  802  is passed through to the rib foot  101   d  to assist in the rib foot  101   d  sealing against a surface  801 . While the vacuum line  802  is shown in combination with the “elephant” foot design, it may also be used with other rib foot designs to improve their sealing capability. 
     Referring to FIG.  1  and FIG. 11, a floating retaining ring  151  is suspended from the carrier housing  154  by a retaining ring membrane  153 . The retaining ring membrane  153  preferably comprises an elastic material such as fairprene. The upper portion of the retaining ring  151  is enclosed in a retaining ring plenum  115  defined by the carrier housing  154  and retaining ring membrane  153 . The lower portion of the retaining ring  151  extends below the retaining ring membrane  153  and makes contact with a polishing pad. A pressurized fluid may be introduced to the retaining ring plenum  115  through a retaining ring fluid communication path  125  to control the pressure the retaining ring  151  exerts on the polishing pad. The optimum pressure of the retaining ring  151  on the polishing pad will vary depending on the particular application, but for most conventional wafer process applications will typically be less than 10 psi and usually between 4 and 8 psi. The optimum pressure for the retaining ring  151  will usually be about the same pressure as that for the wafer  150  against the polishing pad. 
     Adjusting the pressure of the retaining ring  151  in relation to the pressure of the wafer  150  against a polishing pad may be used to control the rate of removal of material, particularly at the periphery, of the wafer  150 . Specifically, a higher retaining ring  151  pressure will usually slow the rate of material removal, while a lower retaining ring  151  pressure will usually increase the rate of material removal, at the periphery of the wafer  150 . 
     The retaining ring  151  surrounds the wafer  150  during the planarization process and prevents the wafer  150  from laterally escaping from beneath the carrier  156 . The retaining ring membrane  153  allows the retaining ring  151  to adjust to variations in the polishing pad&#39;s thickness, without undesirably tilting the carrier housing  154 . Because the retaining ring  151  rubs against the abrasive polishing pad, it preferably comprises a wear resistant material such as a ceramic. However, the inner diameter of the retaining ring  151  makes repeated contact with the wafer  150  and may undesirably chip the wafer  150 . To prevent the wafer  150  from being chipped, a material softer than the wafer, such as delrin, may be used to create a barrier  152  between the wafer  150  and the retaining ring  151 . 
     With reference to FIG. 3, another embodiment of the present invention will be discussed. The illustrated carrier  305  has a similar carrier housing  154 , carrier plenums  131 - 134 , carrier fluid communication paths  141 - 144 , web diaphragm  100 , ribs  101 - 104 , rib plenums  111 - 114 , web fluid communication paths  121 - 124  and floating retaining ring  151  as previously discussed. However, a wafer diaphragm  300  is positioned between the wafer  150  and the ribs  101 - 104  and is supported on the feet of the ribs  101 - 104 . The ribs  101 - 104  may be sealed against the wafer diaphragm  300  in a manner similar to the ribs&#39; feet sealing against the wafer  150  in the previous embodiment of the carrier  158 . However, the ribs  101 - 104  are preferably bonded to the wafer diaphragm  300  to assist in preventing leakage between neighboring web plenums  111 - 114 . 
     A compressed spring ring  301  may be inserted in the outermost web plenum  114  near the junction between the outermost rib  114  and the wafer diaphragm  300 . The spring ring  301  is advantageously designed to expand uniformly in a radial direction to assist in maintaining a taut wafer diaphragm  300 . 
     With reference to FIG. 12, another embodiment of a carrier  156  is shown. This embodiment has ribs  101 - 103 , web plenums  111 - 114 , carrier plenums  131 - 133 , carrier fluid communication paths  141 - 143  and web plenum fluid communication paths  121 - 124  as shown in the prior embodiments. However, the outermost rib  104  shown in FIG. 3 is replaced with a bellows  304 . The bellows  304  does not need a carrier plenum  134  or carrier fluid communication path  144  (both shown in FIG.  3 ), thereby simplifying the design and construction of the carrier  1200 . 
     FIG. 9 illustrates another embodiment where the wafer diaphragm  300   a  is actually attached to the rib  901  thereby sealing web plenum  904 . Web plenum  904  may be pressurized by web fluid communication path  903  in a manner similar to the other embodiments already discussed. This embodiment has the additional feature of a vacuum or discharge path  900  for either assisting in picking-up the wafer  150  with a vacuum or removing the wafer  150  from the carrier with a rapid discharge of fluids at point  905   a.    
     The carriers in FIG.  3  and FIG. 12 have the advantage of the wafer diaphragm  300  preventing the backside of the wafer  150  from being exposed to a fluid, such as air, that might dry or adhere the slurry onto the back surface of the wafer. Once slurry has dried or adhered to the wafer  150 , it is extremely difficult to remove, thereby introducing contaminates that may be harmful to the wafer  150 . 
     The carrier  156  in FIG.  1  and FIG. 11, the carrier  305  in FIG.  3  and the carrier  1200  in FIG. 12 may be used to pick-up a wafer  150  by creating one or more vacuum zones on the back surface of the wafer  150 . A vacuum zone may be created by one or more of the web fluid communication paths  121 - 124  communicating a vacuum to one of the web plenums  111 - 114 . The vacuum for carrier  156  in FIG.  1  and FIG. 11 is communicated directly to the back surface of the wafer  150 . The vacuum for the carrier  305  in FIG. 3 or the carrier  1200  in FIG. 12 lifts the wafer diaphragm  300  from the backside of the wafer  150  creating a vacuum between the wafer diaphragm  300  and the wafer  150 . 
     The carrier  156  in FIG.  1  and FIG. 11, the carrier  305  in FIG.  3  and the carrier  1200  in FIG. 12 may be used to discharge a wafer  150  from the carrier. A rapid discharge of fluids through one or more of the web fluid communication paths for the carrier  156  in FIG.  1  and FIG. 11 will directly impact the wafer  150  and blow the wafer  150  out of the carrier  156 . A wafer  150  in carrier  305  in FIG. 3 or carrier  1200  in FIG. 12 may be removed from the carrier by pressurizing the web plenums  111 - 114  which will cause the wafer diaphragm  300  to extend outwards thereby dislodging the wafer  150  from the carrier  305 . 
     An exemplary process for using the present invention will now be discussed with reference to FIG.  4  and FIG.  10 . The first step is to determine the number, location, height and/or width of concentric bulges on incoming wafers (step  1000 ). This may be done by reviewing incoming wafers prior to planrazation with various known metrology instruments, such as a UV1050 manufactured by KLA-Tencor located in San Jose, Calf. 
     A carrier with adjustable concentric pressure zones that correspond to the surface geometry of the incoming wafers may be advantageously selected for use (step  1001 ). The carrier should have adjustable pressure zones that correspond to the ridges and adjustable pressure zones that correspond to the troughs between ridges on the wafer. 
     A wafer may then be loaded into the selected carrier and the carrier and wafer moved so that the wafer is parallel to and adjacent (near or just touching) an abrasive surface such as a polishing pad (step  1002 ). The wafer may then be pressed against the abrasive surface by pressurizing the independently controlled pressure zones (web plenums). The pressure in each zone may be independently controlled by adjusting the pressure communicated through the zone&#39;s corresponding web fluid communication path to provide an optimum planarization process for the surface geometry of that wafer (step  1003 ). 
     FIG. 4 illustrates one possible pressure distribution on the back surface of a wafer with a central zone  1  and three surrounding zones  2 - 4 . The central zone  1  (web plenum  111  in FIG. 3) is pressurized to 4 psi, zones  2  and  3  (web plenums  112  and  113  respectively in FIG. 3) are pressurized to 5 psi and zone  4  (web plenum  114  in FIG. 3) is pressurized to 6 psi. This distribution of pressure on the back surface of a wafer may be used for wafers with a thin bulge around the periphery and a small depression near the center of the wafer. The variation of pressures allows the carrier to press harder on zones with bulges and softer on zones with troughs or depressions during the planarization process to produce a wafer with a substantially uniform thickness. Additional zones, smaller zones or zones of varying sizes may be used to give finer control over the distribution of pressure on the back surface of the wafer, but increase the complexity and manufacturing cost of the carrier. 
     Applicant has noticed certain semiconductor wafer processing steps leave predictable concentric bulges on the wafer. The bulges from these processing steps are substantially the same from wafer to wafer in that the wafers typically have the same surface geometry. For example, applicant has noticed current copper deposition processes typically have a narrow bulge near the periphery and another bulge in the shape of a small disk near the center of the wafer. Additionally, applicant has noticed current STI processes typically have a wide bulge near the periphery and another bulge in the shape of a small disk near the center of the wafer. A single carrier design with four roughly equal zones, as illustrated in FIG.  1  and FIG. 3, may be advantageously used for both copper deposition and STI wafers in this situation. For a specific example, zones  1  and  4  that correspond to bulges on a copper deposition wafer may have a higher pressure, e.g. 6 psi, while the zones  2  and  3  that correspond to the trough may have a lower pressure, e.g. 5 psi. Likewise, zones  1 ,  3  and  4  that correspond to bulges on an STI wafer may have a higher pressure, e.g. 6 psi, while zone  2  that corresponds to a trough may have a lower pressure, e.g. 5 psi. This strategy allows one carrier design to be used to planarize wafers after two different processes. 
     The carrier preferably also has carrier plenums that may be individually pressurized by corresponding carrier fluid communication paths. Each pressurized carrier plenum exerts a force against the head of each rib that is transmitted through the rib to assist in pressing the feet of the rib against the back surface of the wafer (or wafer diaphragm if one is used). This pressing force assists the feet of the ribs in making a good seal with the back surface of the wafer. The pressure in the carrier plenums may be made equal to or slightly greater (about 0.1 to 0.3 psi) than the pressure in the neighboring web plenums to assist in preventing leakage between neighboring web plenums (step  1004 ). Alternatively, the pressure in each carrier plenum may be set between the pressure in its neighboring web plenums to create a smoother distribution of pressure on the back surface of the wafer. 
     Relative motion is necessary between the wafer and the abrasive surface to remove material from the front face of the wafer thereby planarizing the front face of the wafer. The abrasive surface and/or carrier of the present invention may be rotated, orbited, linearly oscillated, moved in particular geometric patterns, dithered, moved randomly or moved in any other motion that removes material from the front face of the wafer. In addition, the abrasive surface and/or carrier may be moving relative to each other prior to, or after, the front face of the wafer contacts the abrasive surface (step  1005 ). However, the preferred relative motion is generated by the carrier rotating and the polishing pad orbiting. The carrier and polishing pad motion may be ramped up to their desired speeds simultaneously with the pressure on the back surface of the wafer being ramped to its desired level. 
     Although the foregoing description sets forth preferred exemplary embodiments and methods of operation of the invention, the scope of the invention is not limited to these specific embodiments or described methods of operation. Many details have been disclosed that are not necessary to practice the invention, but have been included to sufficiently disclose the best mode of operation and manner and process of making and using the invention. Modification may be made to the specific form and design of the invention without departing from its spirit and scope as expressed in the following claims.