Abstract:
A method of detecting a substrate in a carrier head for a chemical mechanical polishing system includes connecting a chamber in a carrier head to a pressure source, measuring the pressure in the chamber as a function of time, calculating the derivative of the pressure in the chamber, and determining whether the substrate is adjacent a substrate receiving surface in the carrier head from the derivative.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 10/121,143, filed on Apr. 10, 2002, which is a divisional application of U.S. application Ser. No. 09/470,820, filed Dec. 23, 1999, now U.S. Pat. No. 6,422,927, which claims the benefit of priority under 35 USC 119(e) to Provisional Application Serial No. 60/114,182, filed Dec. 30, 1998. The disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.  
           [0003]    Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.  
           [0004]    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 placed against a rotating polishing pad. The polishing pad may be either a “standard” or a fixed-abrasive pad. A standard polishing pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.  
           [0005]    The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.  
           [0006]    A reoccurring problem in CMP is the so-called “edge-effect”, i.e., the tendency of the substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in non-uniform polishing at the substrate perimeter, e.g., the outermost three to fifteen millimeters of a 200 millimeter (mm) wafer. A related problem is the so-called “center slow effect”, i.e., the tendency of the center of the substrate to be underpolished.  
         SUMMARY  
         [0007]    In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a first pressurizable chamber at least partially bounded by a first flexible membrane, and a second pressurizable chamber positioned to apply a downward force to the first chamber. A lower surface of the first flexible membrane provides a first surface to apply a pressure to a substrate in a loading area having a controllable size, and the first and second chambers are configured such that a first pressure in the first chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the second chamber controls the size of the loading area.  
           [0008]    Implementations of the invention may include one or more of the following features. A vertically movable base may form at least part of an upper boundary of the second pressurizable chamber. A housing may be connectable to a drive shaft and a third chamber may be disposed between the housing and the base. A retaining ring may be connected to the base to maintain the substrate beneath the carrier head. A boundary between the first and second chambers may be formed by a rigid member or a flexible member, and the second chamber may form a generally annular volume or a generally solid volume. The lower surface of the first flexible membrane may provide a mounting surface for the substrate, or a second flexible membrane may extend beneath the first flexible membrane to provide a mounting surface for the substrate. The volume between the first flexible membrane and the second flexible membrane may define a third pressurizable chamber. The first flexible membrane may be movable into contact with an upper surface of the second flexible membrane in the loading area to apply pressure to the substrate. The lower surface of the first flexible membrane may be textured to provide fluid flow between the first and second flexible membranes when they are in contact.  
           [0009]    A first support structure may positioned inside the first chamber, and the first flexible membrane may extends around an outer surface of the first support structure. A first spacer ring may be positioned outside the first chamber, and the first flexible membrane may extend in a serpentine path between the first structure and the first spacer ring, around an inner surface of the first spacer ring, and outwardly around an upper surface of the first spacer ring. A second support structure may be located in the third chamber between the first and second flexible membranes and positioned to surround the first supports structure. A second spacer ring may be located outside the third chamber above the second support ring, and the second flexible membrane may extend in a serpentine path between the second support structure and the second spacer ring, around an inner surface of the second spacer ring, and outwardly around an upper surface of the second spacer ring.  
           [0010]    In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a base, a first flexible membrane portion, and a second flexible membrane portion. The first flexible membrane portion extends beneath the base and defines a first pressurizable chamber, and a lower surface of the first flexible membrane portion provides a mounting surface to apply a pressure to a substrate in a loading area having a controllable size. The second flexible membrane portion couples the first flexible membrane portion to the base and defines a second pressurizable chamber so that a first pressure in the first pressurizable chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the second chamber controls the size of the loading area.  
           [0011]    In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a base, a first flexible membrane portion, a second flexible membrane portion, and a third flexible membrane portion. The first flexible membrane portion extends beneath the base to define a first pressurizable chamber, and a lower surface of the first flexible membrane provides a mounting surface for a substrate. The second flexible membrane portion extends beneath the base and defines a second pressurizable chamber, and a lower surface of the second flexible membrane contacts a top surface of the first flexible membrane in a loading area having a controllable size. The third flexible membrane portion couples the second flexible membrane portion to the base and defines a third pressurizable chamber so that a first pressure in the second pressurizable chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the third chamber controls the size of the loading area.  
           [0012]    In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a first biasing member and a second biasing member. The first biasing member includes a first pressure chamber, and a lower surface of the first pressure chamber is bounded by a flexible membrane that provides a first surface to apply a load to a substrate in a loading area having a controllable size. The second biasing member is connected to the first biasing member, and the second biasing member controls the vertical position of the first biasing member so that the second biasing member controls the size of the loading area and the first biasing member controls the pressure applied to the substrate in the loading area.  
           [0013]    In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a flexible membrane that provides a mounting surface for a substrate, means for controlling a size of a loading area in which a load is applied to the substrate, and means for controlling a pressure applied to the substrate in the loading area.  
           [0014]    In another aspect, the invention is directed to a method for chemical mechanical polishing a substrate. In the method, a substrate is held against a polishing pad with a carrier head, a load is applied to the substrate in a loading area with a first chamber in the carrier head, the size of the loading area is controlled with a second chamber in the carrier head, and relative motion is created between the substrate and the polishing pad.  
           [0015]    In another aspect, the invention is directed to a method of detecting a substrate in a carrier head for a chemical mechanical polishing system. In the method, a chamber in a carrier head is connected to a pressure source. The pressure in the chamber is measured as a function of time, and the derivative of the pressure in the chamber is calculated. Whether the substrate is adjacent a substrate receiving surface in the carrier head is determined from the derivative.  
           [0016]    Implementations of the invention may include the following features. The substrate may be indicated as present if the derivative exceeds a critical value, or absent if if the derivative does not exceed a critical value.  
           [0017]    Advantages of the invention may include the following. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. Non-uniform polishing of the substrate is reduced, and the resulting flatness and finish of the substrate are improved.  
           [0018]    Other advantages and features of the invention will be apparent from the following description, including the drawings and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is an exploded perspective view of a chemical mechanical polishing apparatus.  
         [0020]    [0020]FIG. 2 is a schematic cross-sectional view of a carrier head according to the present invention.  
         [0021]    [0021]FIG. 3 is an enlarged view of a substrate backing assembly from the carrier head of FIG. 2.  
         [0022]    [0022]FIGS. 4A and 4B are schematic cross-sectional views illustrating the pressure and force distribution on a hypothetical flexible membrane.  
         [0023]    [0023]FIGS. 5A and 5B are schematic cross-sectional views illustrating the variable loading area of an internal flexible membrane from the carrier head of FIG. 2 against the substrate.  
         [0024]    [0024]FIG. 6 is a graph illustrating the relationship between the diameter of the contact area and the pressure in the upper floating chamber.  
         [0025]    [0025]FIGS. 7A and 7B are a graph illustrating the pressure and derivative of the pressure (dP/dt) in the lower floating chamber as a function of time during a substrate detection procedure.  
         [0026]    [0026]FIG. 8 is a schematic cross-sectional view of a carrier head having an internal support plate.  
         [0027]    [0027]FIG. 9 is a schematic cross-sectional view of a carrier head having a flexible membrane with a lip.  
         [0028]    [0028]FIG. 10 is a schematic cross-sectional view of a carrier head having a flexible membrane that directly contacts the substrate in a variable loading area.  
         [0029]    [0029]FIG. 11 is a schematic cross-sectional view of carrier head having a valve for sensing the presence of a substrate. 
     
    
       [0030]    Like reference numbers are designated in the various drawings to indicate like elements. A reference number with a letter suffix indicates that an element has a modified function, operation or structure.  
       DETAILED DESCRIPTION  
       [0031]    Referring to FIG. 1, one or more substrates  10  will be polished by a chemical mechanical polishing (CMP) apparatus  20 . A description of a similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference.  
         [0032]    The CMP apparatus  20  includes a series of polishing stations  25  and a transfer station  27  for loading and unloading the substrates. Each polishing station  25  includes a rotatable platen  30  on which is placed a polishing pad  32 . If substrate  10  is a six-inch (150 millimeter) or eight-inch (200 millimeter) diameter disk, then platen  30  and polishing pad  32  may be about twenty inches in diameter. If substrate  10  is a twelve-inch (300 millimeter) diameter disk, then platen  30  and polishing pad  32  may be about thirty inches in diameter. For most polishing processes, a platen drive motor (not shown) rotates platen  30  at thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station  25  may further include an associated pad conditioner apparatus  40  to maintain the abrasive condition of the polishing pad.  
         [0033]    A slurry  50  containing a reactive agent (e.g., deionized water for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) may be supplied to the surface of polishing pad  32  by a combined slurry/rinse arm  52 . If polishing pad  32  is a standard pad, slurry  50  may also include abrasive particles (e.g., silicon dioxide for oxide polishing). Typically, sufficient slurry is provided to cover and wet the entire polishing pad  32 . Slurry/rinse arm  52  includes several spray nozzles (not shown) which provide a high pressure rinse of polishing pad  32  at the end of each polishing and conditioning cycle.  
         [0034]    A rotatable multi-head carousel  60  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly (not shown). Multi-head carousel  60  includes four carrier head systems  70  mounted on a carousel support plate  66  at equal angular intervals about carousel axis  64 . Three of the carrier head systems position substrates over the polishing stations, and one of the carrier head systems receives a substrate from and delivers the substrate to the transfer station. The carousel motor may orbit the carrier head systems, and the substrates attached thereto, about the carousel axis between the polishing stations and the transfer station.  
         [0035]    Each carrier head system  70  includes a polishing or carrier head  100 . Each carrier head  100  independently rotates about its own axis, and independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A carrier drive shaft  74  extends through slot  72  to connect a carrier head rotation motor  76  (shown by the removal of one-quarter of a carousel cover  68 ) to carrier head  100 . There is one carrier drive shaft and motor for each head. Each motor and drive shaft may be supported on a slider (not shown) which can be linearly driven along the slot by a radial drive motor to laterally oscillate the carrier head.  
         [0036]    During actual polishing, three of the carrier heads are positioned at and above the three polishing stations. Each carrier head  100  lowers a substrate into contact with polishing pad  32 . The carrier head holds the substrate in position against the polishing pad and distributes a force across the back surface of the substrate. The carrier head also transfers torque from the drive shaft to the substrate.  
         [0037]    Referring to FIG. 2, carrier head  100  includes a housing  102 , a base assembly  104 , a gimbal mechanism  106  (which may be considered part of the base assembly), a loading chamber  108 , a retaining ring  110 , and a substrate backing assembly  112  which includes three pressurizable chambers, such as a floating upper chamber  236 , a floating lower chamber  234 , and an outer chamber  238 . A description of a similar carrier head may be found in U.S. Pat. No. 6,183,354, the entire disclosure of which is incorporated herein by reference.  
         [0038]    The housing  102  can be connected to drive shaft  74  to rotate therewith during polishing about an axis of rotation  107  which is substantially perpendicular to the surface of the polishing pad during polishing. Housing  102  may be generally circular in shape to correspond to the circular configuration of the substrate to be polished. A vertical bore  130  may be formed through the housing, and three additional passages (only two passages  132 ,  134  are illustrated in FIG. 2) may extend through the housing for pneumatic control of the carrier head. O-rings  138  may be used to form fluid-tight seals between the passages through the housing and passages through the drive shaft.  
         [0039]    The base assembly  104  is a vertically movable assembly located beneath housing  102 . The base assembly  104  includes a generally rigid annular body  140 , an outer clamp ring  164 , gimbal mechanism  106 , and a lower clamp ring  144 . A passage  146  may extend through the body of the gimbal mechanism, the annular body, and the clamp ring, and two fixtures  148  may provide attachment points to connect a flexible tube between housing  102  and base assembly  104  to fluidly couple passage  134  to one of the chambers in substrate backing assembly  112 , e.g., chamber  238 . A second passage (not shown) may extend through annular body  140 , and two fixtures (also not shown) may provide attachment points to connect a flexible tube between housing  102  and base assembly  104  to fluidly couple the unillustrated passage in the housing to a second chamber in substrate backing assembly  112 , e.g., chamber  236 .  
         [0040]    The gimbal mechanism  106  permits the base assembly to pivot with respect to housing  102  so that the retaining ring may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism  106  includes a gimbal rod  150  which fits into vertical bore  130  and a flexure ring  152  which is secured to annular body  140 . Gimbal rod  150  may slide vertically along bore  130  to provide vertical motion of base assembly  104 , but it prevents any lateral motion of base assembly  104  with respect to housing  102  and reduces momement generated by the lateral force of the substrate against the retaining ring. Gimbal rod  150  may include a passage  154  that extends the length of the gimbal rod to fluidly couple bore  130  to a third chamber in substrate backing assembly  112 , e.g., chamber  234 .  
         [0041]    The loading chamber  108  is located between housing  102  and base assembly  104  to apply a load, i.e., a downward pressure or weight, to base assembly  104 . The vertical position of base assembly  104  relative to polishing pad  32  is also controlled by loading chamber  108 . An inner edge of a generally ring-shaped rolling diaphragm  160  may be clamped to housing  102  by an inner clamp ring  162 . An outer edge of rolling diaphragm  160  may be clamped to base assembly  104  by outer clamp ring  164 . Thus, rolling diaphragm  160  seals the space between housing  102  and base assembly  104  to define loading chamber  108 . A first pump (not shown) may be fluidly connected to loading chamber  108  by passage  132  to control the pressure in the loading chamber and the vertical position of base assembly  104 .  
         [0042]    The retaining ring  110  may be a generally annular ring secured at the outer edge of base assembly  104 , e.g., by bolts  128 . When fluid is pumped into loading chamber  108  and base assembly  104  is pushed downwardly, retaining ring  110  is also pushed downwardly to apply a load to polishing pad  32 . A bottom surface  124  of retaining ring  110  may be substantially flat, or it may have a plurality of channels to facilitate transport of slurry from outside the retaining ring to the substrate. An inner surface  126  of retaining ring  110  engages the substrate to prevent it from escaping from beneath the carrier head.  
         [0043]    Referring to FIGS. 2 and 3, substrate backing assembly  112  includes a flexible internal membrane  116 , a flexible external membrane  118 , an internal support structure  120 , an external support structure  230 , an internal spacer ring  122 , and an external spacer ring  232 . Support structures  120  and  230  and spacer rings  122  and  232  may be “free-floating”, i.e., not secured to the rest of the carrier head, and may be held in place by the internal and external flexible membranes.  
         [0044]    The flexible internal membrane  116  includes a central portion  200  which will apply pressure to the substrate in a controllable area, a relatively thick annular portion  202  with an “L-shaped” cross-section, an annular inner flap  204  that extends from the corner of L-shaped portion  202 , an annular outer flap  206  that extends from the outer rim of L-shaped portion  202 , and a perimeter portion  208  that extends around internal support structure  120  to connect L-shaped portion  202  and central portion  200 . The rim of inner flap  204  is clamped between flexure ring  152  and annular body  140 , whereas the rim of outer flap  206  is clamped between outer clamp ring  164  and lower clamp ring  144 . The volume between base assembly  104  and internal membrane  116  that is sealed by inner flap  204  provides a pressurizable floating lower chamber  234 . The annular volume between base assembly  104  and internal membrane  116  that is sealed by inner flap  204  and outer flap  206  defines a pressurizable floating upper chamber  236 . A second pump (not shown) may be connected to the unillustrated passage to direct fluid, e.g., a gas, such as air, into or out of the floating upper chamber  236 . A third pump (not shown) may be connected to bore  130  to direct a fluid, e.g., a gas, such as air, into or out of floating lower chamber  234 . The second pump controls the pressure in the upper chamber and the vertical position of the lower chamber, and the third pump controls the pressure in the lower chamber. As explained in greater detail below, the pressure in floating upper chamber  236  will control a contact area of internal membrane  116  against a top surface of external membrane  118 . Thus, the second pump controls the area of the substrate against which pressure is applied, i.e., the loading area, whereas the third pump controls the downward force on the substrate in the loading area.  
         [0045]    The external membrane  118  includes a central portion  210  that extends below external support structure  230  to provide a mounting surface to engage the substrate, and a perimeter portion  212  that extends in a serpentine path between external support structure  230  and external spacer ring  232  to be secured to the base assembly. For example, an edge of the external membrane may be clamped between lower clamp ring  144  and retaining ring  110 . The sealed volume between internal membrane  116  and external membrane  118  defines a pressurizable outer chamber  238 . Thus, outer chamber  238  can actually extend below the lower chamber  234 . A fourth pump (not shown) may be connected to passage  134  to direct fluid, e.g., a gas, such as air, into or out of outer chamber  238 . The fourth pump controls the pressure in outer chamber  238 .  
         [0046]    The internal support structure  120  may be a generally rigid annular washer-shaped body located inside floating lower chamber  234  to maintain the desired shape of internal membrane  116 . Alternatively, the internal support structure may be a disk-shaped body with a plurality of apertures therethrough. The disk-shaped support structure would provide a backing surface to prevent the substrate from being damaged due to warping.  
         [0047]    The internal spacer ring  122  is a generally rigid annular body which may have a “C-shaped” cross-section. The internal spacer ring may include a cylindrical portion  190 , an annular upper flange  192 , and an annular lower flange  194 . The internal spacer ring  122  may be located in outer chamber  238  above internal support structure  120 . The annular lower flange  194  can be supported by the internal support structure, whereas annular upper flange  192  can extend over external support structure  230  and external spacer ring  232 .  
         [0048]    The internal membrane  116  is formed of a flexible and elastic material, such as an elastomer, an elastomer coated fabric, or a thermal plastic elastomer (TPE), e.g., HYTREL™ available from DuPont of Newark, Del., or a combination of these materials. Preferably, internal membrane  116  is somewhat less flexible than external membrane  118 . As discussed above, a controllable region of central portion  200  of internal membrane  116  can contact and apply a downward load to an upper surface of external membrane  118 . The load is transferred through the external membrane to the substrate in the loading area. The bottom surface of central portion  200  of internal membrane  116  may be textured, e.g., with small grooves, to ensure that fluid can flow between the internal and external membranes when they are in contact. The perimeter portion  208  of the internal membrane extends upwardly around an outer surface  180  of internal support structure  120 , and inwardly between lower flange  194  of internal spacer ring  122  and an upper surface  182  of the internal support structure to connect to the lower edge of L-shaped portion  202 . The L-shaped portion  202  of the internal membrane extends inside cylindrical portion  190  and over annular upper flange  192  of the internal spacer ring  122 .  
         [0049]    The external support structure  230  is located inside outer chamber  238  between internal membrane  116  and external membrane  118  to maintain the desired shape of external membrane  118  and to seal the external membrane against the substrate during vacuum-chucking. Specifically, external support structure  230  may have a generally rigid ring-shaped portion  170  with an annular projection  172  that extends downwardly from the rim of the ring-shaped portion. Alternatively, projection  172  may be positioned to contact a top surface of the external membrane to preferentially apply pressure to selected areas of the substrate, as discussed in U.S. Pat. No. 6,146,259, the entire disclosure of which is incorporated herein by reference. The projection  172  may be formed by adhesively attaching a layer of compressible material to a lower surface of ring-shaped portion  170 .  
         [0050]    The external spacer ring  232  is a generally annular member positioned between retaining ring  110  and external membrane  118 . Specifically, external spacer ring  232  may be located above external support structure  230 . External spacer ring  232  includes a cylindrical portion  184  and a flange portion  186  which extends outwardly toward inner surface  126  of retaining ring  110  to maintain the lateral position of the external spacer ring.  
         [0051]    External membrane  118  is a generally circular sheet formed of a flexible and elastic material, such as chloroprene or ethylene propylene rubber, or silicone. As noted, central portion  210  of the external membrane defines a mounting surface for the substrate, whereas perimeter portion  212  extends in a serpentine fashion between external support structure  230  and external spacer ring  232  to be clamped between base assembly  104  and retaining ring  110 . Specifically, perimeter portion  212  extends upwardly around an outer surface  174  of external support structure  230 , inwardly between flange portion of external spacer ring  232  and an upper surface  176  of external support structure  230 , upwardly around cylindrical portion  184  of external spacer ring  232 , and then outwardly to a rim portion  214  which is clamped between lower clamp ring  144  and retaining ring  110  to form a fluid-tight seal. A “free span” portion  216  of the external membrane extends between rim portion  214  and the outer diameter of the upper surface of external spacer ring  232 . The external membrane  118  may also include a thick portion  218  that extends upwardly between internal spacer ring  122  and external spacer ring  232 . The external membrane may be pre-molded into a serpentine shape.  
         [0052]    In operation, fluid is pumped into or out of floating lower chamber  234  to control the downward pressure of internal membrane  116  against external membrane  118  and thus against the substrate, and fluid is pumped into or out of floating upper chamber  236  to control the contact area of internal membrane  116  against external membrane  118 . The ability of carrier head  100  to control both the loading area and the pressure applied to the substrate will be explained with reference to the schematic diagrams of FIGS. 4A and 4B. Referring to FIG. 4A, a hypothetical and highly schematic polisher  300  includes a “free-floating” flexible membrane  302  that defines a pressurizable chamber  306 . Assuming that no external pressures are applied to flexible membrane  302 , it will be generally spherical and have an interior pressure P 1 . However, if the membrane is compressed, e.g., between a rigid plate  304  and substrate  10 , the flexible membrane will deform into an oblate shape which contacts the substrate in a generally circular contact region  308 . Assuming that rigid plate  304  applies a downward force F to flexible membrane  302 , force balancing requires that F=ΔP*A c , where ΔP is the difference between the internal pressure P 1  in the chamber  306  and the external pressure P 2  surrounding the flexible membrane, and A c  is the surface area of contact region  308 . Thus, the diameter D c  of contact region  308  will be given by:  
         D   C     =       4        F     π                 Δ                 P                                 
 
         [0053]    Consequently, any circular contact profile and pressure can be obtained by a two step process where the pressure P 1  is selected, and the applied force F is adjusted to determine the diameter of the loading area. Although FIGS. 4A and 4B illustrate the concept in a highly schematic fashion, the invention may be generally implemented by applying a downward force to a free-floating membrane chamber.  
         [0054]    Referring to FIGS. 5A and 5B, the contact area of internal membrane  116  against external membrane  118 , and thus the loading area in which pressure is applied to substrate  10 , may be controlled by varying the pressure in floating upper chamber  236 . By pumping fluid out of floating upper chamber  236 , L-shaped portion  202  of internal membrane  116  is drawn upwardly, thereby pulling the outer edge of central portion  200  away from external membrane  118  and decreasing the diameter of the loading area. Conversely, by pumping fluid into floating upper chamber  236 , L-shaped portion  202  of internal membrane  116  is forced downwardly, thereby pushing central portion  200  of the internal membrane into contact with external membrane  118  and increasing the diameter of the loading area. In addition, if fluid is forced into outer chamber  238 , L-shaped portion  202  of internal membrane  116  is forced upwardly, thereby decreasing the diameter of the loading area. Thus, in carrier head  100 , the diameter of the loading area will depend on the pressures in both the upper chamber and the outer chamber.  
         [0055]    An exemplary graph  400  of diameter of the contact area as a function of the pressures in upper chamber  235 , lower chamber  234  and outer chamber  238  is shown in FIG. 6. Such a graph can be determined by experimentation or calculated by finite element analysis. In the graph in FIG. 6, the x-axis represents the pressure in the upper chamber  234  and the y-axis represents the contact area. The sets of graph lines  402 - 418  represent the relationship of the upper chamber pressure to contact area for various pressures in the lower chamber  236  and the outer chamber  238 , as summarized by the following chart:  
                                                                                     Pressure P1   Pressure P2                   in Outer   in Lower           Graph Line   chamber 238   Chamber 234   P2 − P1                                        402   1.0   1.5   0.5           404   1.0   2.0   1.0           406   3.0   3.5   0.5           408   3.0   4.0   1.0           410   3.0   4.5   1.5           412   5.0   5.5   0.5           414   5.0   6.0   1.0           416   5.0   6.5   1.5           418   5.0   7.0   2.0                      
 
         [0056]    Carrier head  100  may also be operated in a “standard” operating mode, in which floating chambers  234  and  236  are vented or depressurized to lift away from the substrate, and outer chamber  238  is pressurized to apply a uniform pressure to the entire backside of the substrate.  
         [0057]    As previously discussed, one reoccurring problem in CMP is non-uniform polishing of the substrate center. However, the controllable loading area can be used to compensate for polishing profiles in which the center of the substrate is underpolished by applying a sequence of polishing steps with different diameters of the loading area. For example, the carrier head may be used to polish a region of the substrate having radius r 1  for a first duration T 1 , then polish a larger region having a radius r 2  for a second duration T 2 , and then polish a still larger region having a radius r 3  for a third duration T 3 . This ensures that the different regions of the substrate are polished with a total time and pressure required to reduce polishing non-uniformities.  
         [0058]    As previously discussed, another reoccurring problem in CMP is non-uniform polishing near the edge of the substrate. However, external spacer ring  232  may be used to control the pressure distribution applied by external membrane  118  near the substrate edge. Specifically, as discussed in U.S. Pat. No. 6,277,014, the entire disclosure of which is incorporated herein by reference, the surface area of an upper surface of the external spacer ring can be selected to adjust the relative pressure applied at the corner of the external membrane to the substrate perimeter.  
         [0059]    In order to remove the substrate from the polishing pad, floating upper chamber  236  is pressurized to force projection  172  of external support structure  230  downwardly against the upper surface of external membrane  118 . This forces the external membrane into contact with the substrate to form a seal. The floating lower chamber  234  is vented, e.g., connected to the external atmosphere, and outer chamber  238  is depressurized. This causes the external membrane  118  to be drawn inwardly to vacuum-chuck the substrate to the carrier head. Then the floating upper chamber  236  is depressurized to draw the internal and external membranes upwardly and lift the substrate off the polishing pad. Finally, loading chamber  108  is evacuated to lift base assembly  104  and substrate backing assembly away from the polishing pad.  
         [0060]    The operation of carrier head  100  to load a substrate into the carrier head at transfer station  27 , dechuck the substrate from a polishing pad at polishing station  25 , and unload the substrate from the carrier head at the transfer station  27 , is summarized by the following tables.  
                                                         Load Operation                    Retract       Push sub-               Initial   lower   Inflate   strate into       Step   State   assembly   Membrane   Membrane   Grip Wafer               Outer   vent   vent   pressure   vent   vacuum       Lower   vent   vent   vent   vent   vent       Upper   vent   vacuum   vacuum   vacuum   vacuum       Ring   vacuum   vacuum   vacuum   vacuum   vacuum                  
 
         [0061]    Time delays may be taken after the inflation, pushing and griping steps, respectively.  
                                                         Dechuck Operation                Initial   Apply Seal   Grip   Lift Substrate   Lift Ring       Step   State   Force   Substrate   from Pad   from Pad               Outer   vent   vent   vacuum   vacuum   vacuum       Lower   vent   vent   vent   vent   vent       Upper   vent   pressure   pressure   vacuum   vacuum       Ring   pressure   pressure   pressure   pressure   vacuum                  
 
         [0062]    Time delays may be taken after the sealing, gripping and lifting steps, respectively.  
                                                         Unload Operation                    Extend                       Initial   Lower   Release   Eject   Deflate       Step   State   Assembly   Substrate   Substrate   Membrane               Outer   vacuum   vacuum   vent   vent   vent       Lower   vent   vent   vent   pressure   vent       Upper   vacuum   pressure   vent   vent   vent       Ring   vacuum   vacuum   vacuum   vacuum   vacuum                  
 
         [0063]    Time delays may be taken after the lowering and ejection steps, respectively.  
         [0064]    In order to determine whether the substrate was successfully attached to the carrier head after the loading or dechucking operations, the CMP apparatus may perform a substrate detection procedure. This procedure starts with outer chamber  238 , upper floating chamber  236  and loading chamber  108  under vacuum, and lower floating chamber  234  vented. The lower floating chamber  234  is connected to a pressure source at a fixed pressure. Referring to FIG. 7A, the pressure in the lower floating chamber is measured as a function of time. Referring to FIG. 7B, the first derivative (dP/dt) of the pressure in the lower floating chamber is calculated as the chamber is pressurized. If the substrate is not present, the lower chamber will bow outwardly and have room to expand. In contrast, if the substrate is present and chucked to the carrier head, the volume in the lower chamber will be limited, and consequently the pressure in the lower chamber will rise more quickly. Therefore, if the substrate may be detected by determining whether the derivative dP/dt is exceeds a critical value C 1 . This critical value C 1  may be determined experimentally. If the derivative dP/dt exceeds the critical value C 1 , then the substrate is present. On the other hand, If the derivative dP/dt does not exceed the critical value C 1 , then the substrate is absent. Lower floating chamber  234  may be returned to a vacuum after the substrate detection procedure is complete.  
         [0065]    Referring to FIG. 8, in another embodiment, carrier head  100   a  includes a generally disk-shaped internal support plate  120   a  that provides a barrier between floating upper chamber  236   a  and floating lower chamber  234   a . The internal membrane  116   a  is a generally circular sheet, with a central portion  200   a , an edge portion  240  secured to base assembly  104   a , and an annular interior region or flap  242  secured to an outer edge  244  of internal support plate  120   a . The central portion  200   a  of the interior membrane extends beneath internal support plate  120   a  to define floating lower chamber  234   a , whereas the volume between the backing plate and the base assembly that is sealed by edge portion  240  of internal membrane  116   a  defines floating upper chamber  236   a . The disk-shaped internal support plate  120   a  increases the contact area between floating upper chamber  236   a  and floating lower chamber  234   a.    
         [0066]    The external support structure  230   a  may include a ring-shaped portion  170   a , an annular flange portion  178   a  that projects upwardly from an inner edge of ring-shaped portion  170   a , and a projection  172   a  that extends downwardly from the outer edge of ring-shaped portion  170   a  to contact an upper surface of external membrane  118   a . The flange portion  178   a  of external support structure  230   a  may be secured to internal support plate  120   a  or to internal membrane  116   a . Alternatively, external support structure  230   a  may be free-floating in outer chamber  238 .  
         [0067]    Carrier head  100   a  functions in a fashion similar to carrier head  100 . Specifically, the pressure in floating upper chamber  236   a  controls the contact area of the internal membrane against the upper surface of the external membrane, and the pressure in floating lower chamber  234   a  controls the pressure applied to the substrate in the loading area. To remove a substrate from the polishing pad, floating upper chamber  236   a  is pressurized to force projection  172   a  on external support structure  230   a  against the upper surface of external membrane  118   a . This presses the external membrane against the substrate to form a fluid-tight seal therebetween. Then the floating lower chamber is vented, and outer chamber  238   a  is depressurized to pull the external membrane against the internal membrane. Finally, the floating upper chamber is depressurized to pull the substrate off the polishing pad.  
         [0068]    Referring to FIG. 9, in another embodiment, carrier head  100   b  may include an external membrane  118   b  having an annular lip  250 . When outer chamber  238   c  is evacuated, lip  250  may be pulled against substrate  10  to form a seal and improve the vacuum-chucking of the substrate, as described in U.S. Pat. No. 6,159,079, the entire disclosure of which is incorporated herein by reference.  
         [0069]    Referring to FIG. 10, in another embodiment, carrier head  100   c  includes a single flexible membrane  118   c  and a disk-shaped backing structure  122   c . A center portion  260  of flexible membrane  118   c  extends below backing structure  122   c  to provide a mounting surface to engage the substrate. A perimeter portion  262  of the flexible membrane extends upwardly and inwardly around a cylindrical rim  264  of the backing structure. The perimeter portion  262  includes an inner flap  266  which is clamped between a clamp ring  268  and an upper surface  270  of backing structure  122   c , and an outer flap  272  which wraps around spacer ring  120   c  to be clamped between retaining ring  110   c  and annular body  140   c . Thus, the volume between backing structure  122   c  and flexible membrane  118  defines a pressurizable floating lower chamber  234   c , and the volume between base assembly  104  and backing structure  122   c  that is sealed by inner and outer flaps  266  and  272  defines a pressurizable floating upper chamber  236   c . One pump may be connected to floating upper chamber  236   c  by passage  154  in gimbal rod  150 , and another pump may be connected to floating lower chamber  234   c  by passage  134  in housing  102 , passage  280  in base assembly  104   c , and a passage  282  through backing structure  122   c . Fixtures  284  and  286  provide attachment points for flexible tubing to fluidly couple the passages the passages through the base assembly and the backing structure to connect passage  134  to floating lower chamber  234   c.    
         [0070]    The bottom surface  274  of the backing structure may have a projection  276  that extends downwardly from an outer edge of the structure. A plurality of grooves  278  may also be formed in bottom surface  274  of backing structure  122   c  to ensure that fluid can be evacuated from between the backing structure and the flexible membrane.  
         [0071]    By controlling the pressure in the upper and floating lower chambers, both the contact pressure and loading area of flexible membrane  118   c  against the substrate can be controlled. To remove the substrate from the polishing pad, floating upper chamber  236   c  is pressurized to force projection  276  downwardly and create a seal between the substrate and flexible membrane, and then floating lower chamber  234   c  is evacuated to vacuum-chuck the substrate to the carrier head.  
         [0072]    Referring to FIG. 11, in another embodiment, carrier head  100   d , which is similar in construction to carrier head  100   c , may include a valve  300  in backing structure  122   d  to fluidly couple upper chamber  236   d  to lower chamber  234   d . Valve  300  includes a disk-shaped valve body  302  and an annular valve flange  304 . Valve body  302  may fit in an aperture  306  in backing structure  122   d , and valve flange  304  may be adhesively secured to a top surface  312  of backing structure  122   d . An annular seal  308  fits in a shallow depression  310  in top surface  312  surrounding aperture  306 . A plurality of vertical channels  314  may be formed through disk-shaped valve body  302  above seal  308  to fluidly couple lower chamber  234   d  and upper chamber  236   d . Valve flange  304  acts as a flexure spring to biases valve body  302  downwardly so that vertical channels  314  abut annular seal  308  to close the valve. However, if valve body  302  is forced upwardly, then the seal will no longer be contact the valve body and fluid may leak through channels  314 . As such, valve  300  will be open and lower chamber  234   d  and upper chamber  236   d  will be in fluid communication via channels  314 .  
         [0073]    Valve  300  may be used to sense whether a substrate has been chucked to flexible membrane  118   d . Specifically, a first measurement of the pressure in upper chamber  234   d  can be made with a pressure gauge (not shown) after the upper chamber is pressurized but before the lower chamber is evacuated. The upper chamber  234   d  should be isolated from the pump that pressurizes or evacuates that chamber. Then, after the lower chamber is evacuated, a second measurement of the pressure in the upper chamber is made by means of the pressure gauge. The first and second pressure measurements may be compared to determine whether the substrate was successfully vacuum-chucked to the carrier head.  
         [0074]    If the substrate was successfully vacuum-chucked, flexible membrane  118   d  will be maintained in close proximity to the substrate by a low pressure pocket between the substrate and the flexible membrane. Consequently, valve  300  will remain biased in its closed position, and the pressure in the upper chamber will remain constant or may increase. On the other hand, if the substrate is not present or is not vacuum-chucked to the carrier head, then when lower chamber  234   d  is evacuated, flexible membrane  118   d  will deflect upwardly. The flexible membrane will thus apply an upward force to valve body  302  and will open valve  300 , thereby fluidly connecting upper chamber  234   d  to upper chamber  236   d . This permits fluid to be drawn out of upper chamber  236   d  through lower chamber  234   d . Consequently, the resulting pressure in the upper chamber will be lower if the substrate is not present or is not vacuum-chucked to the flexible membrane than if the substrate is properly attached. This difference may be detected to determine whether the substrate is chucked to the carrier head. Similar apparatus and methods for sensing the presence of a substrate in a carrier head are described in pending U.S. Pat. No. 5,957,751, the entire disclosure of which is incorporated herein by reference.  
         [0075]    A variety of configurations are possible for a carrier head that implements the invention. For example, the floating upper chamber can be either an annular or a solid volume. The upper and lower chambers may be separated either by a flexible membrane, or by a relatively rigid backing or support structure. The substrate can be contacted directly by a flexible membrane in a variable loading area, or an internal membrane can contact the interior surface of an external membrane in a variable contact area. The support structures could be either ring-shaped or disk-shaped with apertures therethrough.  
         [0076]    The present invention has been described in terms of a number of embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims.