Abstract:
A chemical mechanical polishing apparatus includes a plate on which a substrate is received, and a movable polishing pad support and coupled polishing pad which move across the substrate and orbit a local region of the substrate during polishing operation. The load of the pad against the substrate, the revolution rate of the pad, and the size, shape, and composition of the pad, may be varied to control the rate of material removed by the pad.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the architecture of a chemical mechanical polishing (CMP) system. 
       BACKGROUND 
       [0002]    An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the metallic layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non-planar surface. In addition, planarization of the substrate surface is usually required for photolithography. 
         [0003]    Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad. 
       SUMMARY 
       [0004]    The present disclosure provides systems and apparatus for polishing of substrates in which the contact area of the polishing pad against the substrate is substantially smaller than the radius of the substrate. During polishing, the polishing pad can undergo an orbital motion with a fixed angular orientation. 
         [0005]    In one aspect, a chemical mechanical polishing system includes a substrate support, a movable pad support and a drive system. The substrate support is configured to hold a substrate in a substantially fixed angular orientation during a polishing operation. The movable pad support is configured to hold a polishing pad having a diameter no greater than a radius of the substrate. The drive system is configured to move the pad support and polishing pad in an orbital motion while the polishing pad is in contact with an upper surface of the substrate. The orbital motion has a radius of orbit no greater than a diameter of the polishing pad and maintains the polishing pad in a fixed angular orientation relative to the substrate. 
         [0006]    In another aspect, a chemical mechanical polishing system includes a substrate support, a polishing pad, a movable pad support and a drive system. The substrate support is configured to hold the substrate in a substantially fixed angular orientation during a polishing operation. The polishing pad has a contact area for contacting the substrate, the contact area having a diameter no greater than a radius of the substrate. The movable pad support is configured to hold the polishing pad. The drive system is configured to move the pad support and polishing pad in an orbital motion while the contact area of the polishing pad is in contact with an upper surface of the substrate. The orbital motion has a radius of orbit no greater than a diameter of the polishing pad and maintains the polishing pad in a fixed angular orientation relative to the substrate. 
         [0007]    In another aspect, a method of chemical mechanical polishing includes bringing a polishing pad into contact with a substrate in a contact area having a diameter no greater than a radius of the substrate, and generating relative motion between the polishing pad and the substrate while the contact area of the polishing pad is in contact with an upper surface of the substrate. The relative motion includes an orbital motion having a radius of orbit no greater than a diameter of the polishing pad. The polishing pad is maintained in a substantially fixed angular orientation relative to the substrate during the orbital motion. 
         [0008]    Advantages of the invention may include one or more of the following. A small pad that undergoes an orbiting motion can be used to compensate for non-concentric polishing uniformity. The orbital motion can provide an acceptable polishing rate while avoiding overlap of the pad with regions that are not desired to be polished, thus improving substrate uniformity. In addition, in contrast with rotation, an orbital motion that maintains a fixed orientation of the polishing pad relative to the substrate provide a more uniform polishing rate across the region being polished. A polishing pad with a bottom protrusion that makes contact with the substrate during a polishing operation and a larger radius top portion that is coupled to a polishing pad support with a pressure sensitive adhesive can be less susceptible to delamination during polishing operation. Non-uniform polishing of the substrate is reduced, and the resulting flatness and finish of the substrate are improved. 
         [0009]    Other aspects, features, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a schematic cross-sectional side view of a polishing system; 
           [0011]      FIG. 2  is a schematic cross-sectional side view of an implementation of a polishing system that includes a vacuum chuck to hold the substrate; 
           [0012]      FIG. 3  is a schematic cross-sectional side view of an implementation of a polishing system with a polishing pad that does not include a downward projection; 
           [0013]      FIG. 4  is a schematic cross-sectional side view of an implementation of a polishing system with a polishing pad that has an upper layer that has a larger diameter than the substrate, and a downward projection with a smaller diameter than the substrate; 
           [0014]      FIG. 5  is a schematic cross sectional top view illustrating a polishing pad that moves in an orbit while maintaining a fixed angular orientation; 
           [0015]      FIG. 6  is a schematic cross-sectional top view of the polishing pad support and drive train system of a polishing system; 
           [0016]      FIG. 6A  is a schematic cross-sectional top view of the system of  FIG. 6  with relation to a substrate; 
           [0017]      FIG. 6B  is a schematic cross-sectional top view of the system of  FIG. 6 , with a quarter revolution turn with respect to  FIG. 6A ; 
           [0018]      FIG. 7A  is a schematic cross-sectional side view of a movable polishing pad support connected to the polishing pad with a plurality of clamps; 
           [0019]      FIG. 7B  is a schematic cross-sectional view of an implementation of a movable polishing pad support that includes an interior pressurized space enclosed by an internal membrane; 
           [0020]      FIG. 8A  is a schematic cross-sectional side view of the movable polishing pad support of  FIG. 7B  in a state of low pressure; 
           [0021]      FIG. 8B  is a schematic cross-sectional side view of the movable polishing pad support of  FIG. 7B  in a state of high pressure; 
           [0022]      FIG. 9  is a schematic bottom view of a contact area of a polishing pad; 
           [0023]      FIGS. 10A and 10B  are schematic cross-sectional side views of implementations of a polishing pad; 
           [0024]      FIG. 11  is a schematic cross-sectional side view of another implementation of a movable polishing pad support; 
           [0025]      FIG. 12  is a schematic top view of an implementation of a polishing system with a polishing pad that has an arc-shaped projection layer which forms a corresponding arc-shaped loading area; and 
           [0026]      FIG. 13  is a schematic cross-sectional side view of an implementation of a polishing system with an arc-shaped polishing surface that undergoes orbital motion. 
       
    
    
       [0027]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0028]    1. Introduction 
         [0029]    Some polishing processes result in thickness non-uniformity across the surface of the substrate. For example, a bulk polishing process can result in under-polished regions on the substrate. To address this problem, after the bulk polishing it is possible to perform a “touch-up” polishing process that focuses on portions of the substrate that were underpolished. 
         [0030]    Some bulk polishing processes result in localized non-concentric and non-uniform spots that are underpolished. A polishing pad that rotates about a center of the substrate may be able to compensate for concentric rings of non-uniformity, but may not be able to address localized non-concentric and non-uniform spots. However, a small pad that undergoes an orbiting motion can be used to compensate for non-concentric polishing uniformity. 
         [0031]    Referring to  FIG. 1 , a polishing apparatus  100  for polishing localized regions of the substrate includes a substrate support  105  to hold a substrate  10 , and a movable polishing pad support  300  to hold a polishing pad  200 . The polishing pad  200  includes a polishing surface  250  that has a smaller diameter than the radius of the substrate  10  being polished. 
         [0032]    The polishing pad support  300  is suspended from a polishing drive system  500  which will provide motion of the polishing pad support  300  relative to the substrate  10  during a polishing operation. The polishing drive system  500  can be suspended from a support structure  550 . 
         [0033]    In some implementations, a positioning drive system  560  is connected to the substrate support  105  and/or the polishing pad support  300 . For example, the polishing drive system  500  can provide the connection between the positioning drive system  560  and the polishing pad support  300 . The positioning drive system  560  is operable to position the pad support  300  at a desired lateral position above the substrate support  105 . For example, the support structure  550  can include two linear actuators  562  and  564 , which are oriented perpendicular relative to one another over the substrate support  105 , to provide the positioning drive system  560 . Alternatively, the substrate support  105  could be supported by two linear actuators. Alternatively, the substrate support  105  can be rotatable, and the polishing pad support  300  can be suspended from a single linear actuator that provides motion along a radial direction. Alternatively, the polishing pad support can be suspended from a rotary actuator  508  and the substrate support  105  can be rotatable with a rotary actuator  506 . 
         [0034]    Optionally, a vertical actuator  506  and/or  508  can be connected to the substrate support  105  and/or the polishing pad support  300 . For example, the substrate support  105  can be connected to a vertically drivable piston  506  that can lift or lower the substrate support  105 . 
         [0035]    The polishing apparatus  100  includes a port  60  to dispense polishing liquid  65 , such as abrasive slurry, onto the surface  12  of the substrate  10  to be polished. The polishing apparatus  100  can also include a polishing pad conditioner to abrade the polishing pad  200  to maintain the polishing pad  200  in a consistent abrasive state. 
         [0036]    In operation, the substrate  10  is loaded onto the substrate support  105 , e.g., by a robot. The positioning drive system  500  positions the polishing pad support  300  and polishing pad  200  at a desired position on the substrate  10 , and the vertical actuator  506  moves the substrate  10  into contact with the polishing pad  200  (or vice versa). The polishing drive system  500  generates the relative motion between the polishing pad support  300  and the substrate support  105  to cause polishing of the substrate  10 . 
         [0037]    During the polishing operation, the positioning drive system  560  can hold the polishing drive system  500  and substrate  10  substantially fixed relative to each other. For example, the positioning system can hold the polishing drive system  500  stationary relative to the substrate  10 , or can sweep the polishing drive system  500  slowly (compared to the motion provided to the substrate  10  by the polishing drive system  500 ) across the region to be polished. For example, the instantaneous velocity provided to the substrate by the positioning drive system  500  can be less than 5%, e.g., less than 2%, of the instantaneous velocity provided to the substrate by the polishing drive system  500 . 
         [0038]    The polishing system also includes a controller  90 , e.g., a programmable computer. The controller can include a central processing unit  91 , memory  92 , and support circuits  93 . The controller&#39;s  90  central processing unit  91  executes instructions loaded from memory  92  via the support circuits  93  to allow the controller to receive input based on the environment and desired polishing parameters and to control the various actuators and drive systems. 
         [0039]    2. The Polishing System 
         [0040]    A. The Substrate Support 
         [0041]    Referring to  FIG. 1 , the substrate support  105  is plate-shaped body situated beneath the polishing pad support. The upper surface of the body provides a loading area large enough to accommodate a substrate to be processed. For example, the substrate can be a 200 to 450 mm diameter substrate. The upper surface of the substrate support  105  contacts the back surface of the substrate  10  (i.e., the surface that is not being polished) and maintains its position. 
         [0042]    The substrate support  105  is about the same radius as the substrate  10 , or larger. In some implementations, the substrate support  105  is slightly narrower (e.g., see  FIG. 2 ) than the substrate, e.g., by 1-2% of the substrate diameter. When placed on the support  105 , the edge of the substrate  10  slightly overhangs the edge of the support  105 . This can provide clearance for an edge grip robot to place the substrate on the support. In some implementations, the substrate support  105  is wider than the substrate. In either case, the substrate support  105  can make contact with a majority of the surface the backside of the substrate. 
         [0043]    In some implementations, as shown in  FIG. 1 , the substrate support  105  maintains the substrate  10  position during polishing operation with a clamp assembly  111 . In some implementations, the clamp assembly  111  can be a single annular clamp ring  112  that contacts the rim of the top surface of the substrate  10 . Alternatively, the clamp assembly  111  can include two arc-shaped clamps  112  that contact the rim of the top surface on opposite sides of the substrate  10 . The clamps  112  of the clamp assembly  111  can be lowered into contact with the rim of the substrate by one or more actuators  113 . The downward force of the clamp restrains the substrate from moving laterally during polishing operation. In some implementations, the clamp(s) include downwardly a projecting flange  114  that surrounds the outer edge of the substrate. 
         [0044]    In some implementations, as shown in  FIG. 2 , the substrate support  105  is a vacuum chuck  106 . The vacuum chuck  106  includes a chamber  122  and a plurality of ports  124  connecting the chamber  122  to the surface  127  that supports the substrate  10 . In operation, air can be evacuated from of the chamber  122 , e.g., by a pump  129 , thus applying suction through the ports  124  to hold the substrate in position on the substrate support  106 . 
         [0045]    In some implementations, as shown in  FIG. 3 , the substrate support  105  includes a retainer  131 . The retainer  131  can be attached to and project above the surface  116  that supports the substrate  10 . Typically the retainer is at least as thick (measured perpendicular to the surface  12 ) as the substrate  10 . In operation, the retainer  131  surrounds the substrate  10 . For example, the retainer  131  can be an annular body with a diameter slightly larger than the diameter of the substrate  10 . During polishing, friction from the polishing pad  200  can generate a lateral force on the substrate  10 . However, the retainer  131  constrains the lateral motion of the substrate  10 . 
         [0046]    The various substrates support features described above can be optionally be combined with each other. For example, the substrate support can include both a vacuum chuck and a retainer. 
         [0047]    In addition, although substrate support configurations are shown in conjunction with the pressure sensitive adhesive movable pad support configurations for ease of illustration, they can be used with any of the embodiments of the pad support head and/or drive system described below. 
         [0048]    B. The Polishing Pad 
         [0049]    Referring to  FIG. 1 , the polishing pad  200  has a polishing surface  250  that is brought into contact with the substrate  10  in a contact area, also called a loading area, during polishing. The polishing surface  250  can be of a smaller diameter than the radius of the substrate  10 . For example, for the diameter of the polishing pad can be about can be about 5-10% of the diameter of the substrate. For example, for wafer that ranges from 200 mm to 300 mm in diameter, the polishing pad can be between 10 and 30 mm in diameter. Smaller pads provide more precision but are slower to use. 
         [0050]    In the example in  FIG. 1 , the polishing pad  200  is located above the upper surface of the substrate  10 , and includes an upper portion  270  which is coupled to the bottom of the movable pad support  300 , and a lower portion  260  which has a bottom surface  250  that makes contact with the substrate  10  during polishing operation. In some instances, as shown in  FIG. 1 , the bottom portion  260  of the polishing pad  200  is provided by a protrusion from a wider upper portion  270 . The bottom surface  250  of the protrusion  260  comes into contact with the substrate during polishing operation and provides the polishing surface. 
         [0051]    In the example in  FIG. 1 , the movable pad support  300  is coupled to the top portion  270  of the polishing pad  200  using a pressure sensitive adhesive  231 . The pressure sensitive adhesive  231 , applied between the bottom surface of the polishing pad support  300  and the top surface  270  of the polishing pad, maintains the polishing pad  200  on the pad support  300  coupling during the polishing operation. 
         [0052]    By making the upper portion  270  of the polishing pad  200  wider than the lower portion  260 , the available surface area for the adhesive  231  is increased. Increasing the surface area of the adhesive  231  can improve the bond strength between the pad  200  and pad support, and reduce the risk of delamination of the polishing pad during polishing. 
         [0053]    Referring to  FIG. 3 , the polishing pad  203  can have the same radius in its bottom portion  260  as in its top portion  273 . However, when a pressure sensitive adhesive  231  provides the coupling between the pad and the movable pad support  300 , it is preferable for the bottom portion  263  to be narrower than the top portion  273 . 
         [0054]    Referring to  FIG. 5 , the contact area  5  of the polishing pad can be a disk-shaped geometry  5  formed by a disk-shaped bottom protrusion of the polishing pad. 
         [0055]    Referring to  FIGS. 9A and 9B , the contact area  901  of the polishing pad  110  which makes contact with the substrate  10  can be an arc-shaped contact area  901  formed by an arc-shaped protrusion  290  of the polishing pad. 
         [0056]    Referring to  FIG. 1 , in some implementations the diameter of the upper portion  270  of the polishing pad  200  can be smaller than the diameter of the substrate  10 . 
         [0057]    Referring to  FIG. 4 , in some implementations the diameter of the upper portion  274  of the polishing pad  204  can be larger than the diameter of the substrate  10 . 
         [0058]    Referring to  FIG. 1 , the polishing pad  200  can consist of a single layer of uniform composition. In this case, the material composition of the upper portion  270  and of the lower portion  260 , also called the protrusion  260 , are the same. 
         [0059]    Referring to  FIG. 10B , in some implementations, the polishing pad  200  can include two or more layers of different composition, e.g., a polishing layer  1062  and a more compressible backing layer  1052 . Optionally, an intermediate pressure sensitive adhesive layer  1032  can be used to secure the polishing layer  1061  to the backing layer  1061 . In this case, the upper portion  1221  can correspond to the backing layer  102  and the lower portion  1222  can correspond to the polishing layer  1062 . The polishing pad can be coupled to a polishing pad support via the pressure sensitive adhesive layer  231 . 
         [0060]    Referring to  FIG. 10A , in some implementations, the polishing pad can include two or more layers of different composition, and the upper portion  1221  of the polishing pad  200  can include both the backing layer  1052  and an upper section  1064  of the polishing layer  1062 . Thus, the polishing layer  1062  includes both a lower section  1066  that provides the protrusion  1222  and the upper section  1062 , with the supper section  1064  wider than the lower section  1066 . In either implementation shown in  FIG. 10A  or  FIG. 10B , the portion of the pad that contacts the substrate can be of a conventional material, e.g., a microporous polymer such as polyurethane. The polishing pad can be coupled to a polishing pad support via the pressure sensitive adhesive layer  321 . 
         [0061]    Referring to  FIG. 10A , the backing layer  1052  can be relatively soft to allow for better polishing pad flexibility when polishing an uneven substrate surface spot. The polishing layer  1064  can be a hard polyurethane. 
         [0062]    Referring to  FIG. 10B , the backing layer  1052  can be relatively soft, but also can be a flexible incompressible layer made of material, such as Mylar™. For example, such a pad configuration can be used in implementation in which the polishing pad of  FIG. 10B  is coupled to the pressurized chamber polishing pad support of  FIG. 11 . The polishing layer  1062  can be a hard polyurethane. 
         [0063]    Referring to  FIG. 11 , in some implementations, the polishing pad  205  can include an upper portion  275  and a lower portion  265 . The polishing pad  205  has a thicker lateral section  267  which includes the combined lower portion  265  and upper portion  275 . The upper portion  275  extends laterally  285  on either side of the thicker section  267 . The lateral side sections  285  flex in response to pressure on the thicker section  267 . The thicker section  267  can have a pad thickness of about 2 mm in the polishing area, which is similar to a large sized pad. The pad thickness in the flexing lateral sections  285  can be about 0.5 mm. 
         [0064]    In some implementations, the bottom surface of the lower portion of the polishing pad  200  can include grooves to permit transport of slurry during a polishing operation. The grooves  299  can be shallower than the depth of the lower portion  265  (e.g., see  FIG. 11 ). However, in some implementations the lower portion does not include grooves. 
         [0065]    Referring to  FIG. 9 , the bottom surface  1900  of the polishing pad  200  can be an arc-shaped area. If such a polishing pad includes grooves, the grooves  299  can extend entirely through the width of the arc-shaped area. The grooves  299  can be spaced at uniform pitch along the length of the arc-shaped area. Each grooves  299  can extend along a radius that passes through the groove and the center  1903  of the arc-shaped area, or be positioned at an angle, e.g., 45°, relative to the radius. 
         [0066]    C. The Drive System and Orbital Motion of the Pad 
         [0067]    Referring to  FIGS. 1 and 5 , the polishing drive system  500  can be configured to move the coupled polishing pad support  300  and polishing pad  200  in an orbital motion above the substrate  10  during the polishing operation. In particular, as shown in  FIG. 5 , the polishing drive system  500  can be configured to maintain the polishing pad in a fixed angular orientation relative to the substrate during the polishing operation. 
         [0068]    Referring to  FIG. 5 , the radius of orbit  20  of the polishing pad in contact with the substrate is preferably smaller than the diameter  22  of the contact area. For example, the radius of orbital can be about 5-50%, e.g., 5-20%, of the diameter of the contact area. For a 20 to 30 mm diameter contact area, the radius of orbit can be 1-6 mm. This achieves a more uniform velocity profile in the loading area  5 . The orbit of the polishing pad should preferably revolve at a rate of 1,000 to 5,000 revolutions per minute (“rpm”). 
         [0069]    Referring to  FIG. 6 , the drive train can include a mechanical system base  910  which achieves orbital motion with a single actuator  915 . A motor output shaft  924  is connectively coupled to a cam  922 . The cam  922  extends into a recess  928  in the polishing pad holder  920 . During the polishing operation, the motor output shaft  924  rotates around a rotational axis  990 , causing the cam  922  to revolve the polishing pad holder  920 . A plurality of anti-rotation links  912  extend from the mechanical system base  910  to the upper portion of the polishing pad holder  920  to prevent rotation of the pad holder  920 . The anti-rotation links  912 , in conjunction with motion of cam  922 , achieve orbital motion of the polishing pad support, in which the angular orientation of the polishing pad holder  920  does not change during polishing operation. 
         [0070]    Orbital motion, as depicted in  FIGS. 6A and 6B , can maintain a fixed angular orientation of the polishing pad relative to the substrate during polishing operation. As the central motor output shaft  620  rotates, the cam  625 , in combination with anti-rotational links  630  connecting the mechanical system base above to the polishing pad support, translates the rotational motion into orbital motion for the polishing pad  610 . This achieves a more uniform velocity profile than simple rotation. 
         [0071]    In some implementations, the polishing drive system and the positioning drive system are provided by the same components. For example, a single drive system can include two linear actuators configured to move the pad support head in two perpendicular directions. For positioning, the controller can cause the actuators to move the pad support to the desired position on the substrate. For polishing, the controller can cause the actuators to the actuators to move the pad support in the orbital motion, e.g., by applying phase offset sinusoidal signals to the two actuators. 
         [0072]    Referring to  FIG. 1 , in some implementations, the polishing drive system  500  can include two rotary actuators. For example, the polishing pad support can be suspended from a rotary actuator  508 , which in turn is suspended from a second rotary actuator  509 . During the polishing operation, the second rotary actuator  509  rotates an arm  510  that sweeps the polishing pad support  300  in the orbital motion. The first rotary actuator  508  rotates, e.g., in the opposite direction but at the same rotation rate as the second rotary actuator  509 , to cancel out the rotational motion such that the polishing pad assembly orbits while remaining in a substantially fixed angular position relative to the substrate. 
         [0073]    D. Pad Support 
         [0074]    The movable pad support  300  holds the polishing pad, and is coupled to the polishing drive system  500 . 
         [0075]    In some implementations, e.g., as shown in  FIGS. 1-4 , the pad support  300  is a simple rigid plate. The lower surface  311  of the plate is sufficiently large to accommodate the upper portion  270  of the polishing pad  200 . 
         [0076]    However, the pad support  300  can also include an actuator  508  to control a downward pressure of the polishing pad  200  on the substrate  10 . 
         [0077]    In the example in  FIG. 7A , a pad support  300  that can apply an adjustable pressure on the polishing pad  200  is shown. The pad support  300  includes a base  317  that is coupled to the polishing drive system  500 . A bottom of the base  317  includes a recess  327 . The pad support  300  includes a clamp  410  that hold the rim of the polishing pad  200  on the base  317 . The polishing pad  200  can cover the recess  327  to define a pressurizable chamber  426 . By pumping a fluid into or out of the chamber  426 , downward pressure of the polishing pad  200  on the substrate  10  can be adjusted. 
         [0078]    In the some implementations, as in  FIGS. 7B ,  8 A, and  8 B the pad support  300  can have an interior membrane  405  defining a first pressurizable chamber  406  between the membrane  405  and the base  317 . The membrane is positioned to contact the side  275  of the polishing pad  200  farther from the polishing surface  258 . The membrane  405  and the chamber  406  are configured such that when the pad support  300  holds the polishing pad  200  during a polishing operation, the pressure in the chamber  406  controls the size of the loading area  809  of the polishing pad  200  on the substrate  10 . When the pressure inside the chamber increases, the membrane expands its radius, applying pressure to a larger portion of the bottom protrusion layer of the pad and thus increasing the area of the loading area  810 . When pressure decreases, the result is a smaller-sized loading area  809 . 
         [0079]    Referring to  FIG. 11 , in some implementations, the polishing pad support  315  can include an internal pressurizable chamber  325  formed by walls  320  of the polishing pad support  315 . The chamber  325  can have a substrate-facing opening  327 . The opening  327  can be sealed by securing the polishing pad  200  to the polishing pad support  315 , e.g., by a clamp  410 . The pressure in the pressure chamber  425  can be dynamically controlled, e.g., by a controller and hydrostatic pump, during a polishing operation to adjust to the non-uniform spot being polished. 
         [0080]    Referring to  FIG. 12 , in some implementations, the contact area  1301  of the polishing pad  20  can be arc-shaped area. For example, the protrusion can be arc-shaped. The drive system  500  can rotate the arc around a center  1302  of the substrate  10 . 
         [0081]    Referring to  FIG. 13 , in some embodiments, the polishing pad  200  contact area  901  can be an arc-shaped area that undergoes orbital motion relative to the substrate  10 . 
         [0082]    3. CONCLUSION 
         [0083]    The size of a spot of non-uniformity on the substrate will dictate the ideal size of the loading area during polishing of that spot. If the loading area is too large, correction of underpolishing of some areas on the substrate can result in overpolishing of other areas. On the other hand, if the loading area is too small, the pad will need to be moved across the substrate to cover the underpolished area, thus decreasing throughput. Thus, this implementation permits the loading area to be matched to the size of the spot. 
         [0084]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the substrate support could, in some embodiments, include its own actuators capable of moving the substrate into position relative to the polishing pad. As another example, although the system described above includes a drive system that moves the polishing pad in the orbital path while the substrate is held in a substantially fixed position, instead the polishing pad could be held in a substantially fixed position and the substrate moved in the orbital path. In this situation, the polishing drive system could be similar, but coupled to the substrate support rather than the polishing pad support. Although generally circular substrate is assumed, this is not required and the support and/or polishing pad could be other shapes such as rectangular (in this case, discussion of “radius” or “diameter” would generally apply to a lateral dimension along a major axis). 
         [0085]    Accordingly, other embodiments are within the scope of the following claims.