Abstract:
A carrier head for a chemical mechanical polishing apparatus includes a flexible membrane with an expandable lip portion to engage a substrate for improved chemical mechanical polishing. The lip portion can form or break a seal with the substrate in response to pressure changes in a chamber in the carrier head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part application of copending U.S. patent application Ser. No. 09/149,806, filed Sep. 8, 1998, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing a substrate. 
     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. 
     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 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. 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. 
     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. 
     One problem encountered in CMP is that a central portion of the substrate is often underpolished. This problem, which may be termed the “center slow effect”, may occur even if pressure is uniformly applied to the backside of the substrate. 
     Another problem is the difficulty in removing the substrate from the polishing pad surface once polishing has been completed. As mentioned, a layer of slurry is supplied to the surface of the polishing pad. When the substrate is placed in contact with the polishing pad, the surface tension of the slurry generates an adhesive force which binds the substrate to the polishing pad. The adhesive force may make it difficult to remove the substrate from the pad. 
     Typically, the substrate is vacuum-chucked to the underside of the carrier head, and the carrier head is used to remove the substrate from the polishing pad. When the carrier head is retracted from the polishing pad, the substrate is lifted off the pad. However, if the surface tension holding the substrate on the polishing pad is greater than the vacuum-chucking force holding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. This may cause the substrate to fracture or chip. In addition, failure to remove the substrate can cause a machine fault requiring manual intervention. This requires shutting down the polishing apparatus, decreasing throughput. To achieve reliable operation from the polishing apparatus, the substrate removal process should be essentially flawless. 
     Several techniques have been employed to reduce the surface tension between the substrate and the polishing pad. Once such technique is to slide the substrate horizontally off the polishing pad to break the surface tension before vertically retracting the carrier head. This technique may, however, scratch or otherwise damage the substrate as it slides off the edge of the polishing pad. The mechanical configuration of the CMP apparatus may also prohibit use of this technique. 
     Another technique is to treat the surface of the polishing pad to reduce the surface tension. However, this technique is not always successful, and such treatment of the pad surface may adversely affect the finish and flatness of the substrate and reduce the polishing rate. 
     Another technique is to apply a downward pressure to the edge of the substrate to create a seal that prevents ambient atmosphere from interfering with the vacuum-chucking process. However, this technique may require complex pneumatic controls for the carrier head. In addition, the structure of the carrier head may prevent the application of pressure to the edge of the substrate. 
     SUMMARY 
     In one aspect, the invention is directed to a carrier head for chemical mechanical polishing of a substrate. The carrier head has a base and a flexible membrane extending beneath the base to define a pressurizable chamber. A lower surface of the flexible membrane provides a mounting surface for a substrate. The flexible membrane includes an inner portion and an outer expandable lip portion. The lip portion is configured to inflate or collapse and arranged to break or form a seal with a substrate positioned against the mounting surface, according to pressure changes in the chamber. 
     Implementations of the invention may include one or more of the following. A portion of the flexible membrane may be folded to define the lip portion. The lip portion may include a pocket in fluid communication with the chamber. The lip portion may include an upper part, a lower part, and a pocket located between the upper and lower parts. The flexible memberane further may include an edge portion joined to the upper part. The flexible memberane may further include an annular wing portion having a first end joined to the edge portion and a second end secured to a retainer ring. A spacer may surround an edge portion of the lip portion to maintain the structural integrity of the lip portion. The lip portion may form the seal with the substrate when the chamber is evacuated. The lip portion may break the seal with the substrate when the chamber is pressurized. 
     In another aspect, the invention is directed to a method of chemical mechanical polishing. A substrate is positioned against a mounting surface of a flexible membrane of a carrier head, the flexible membrane defining a pressurizable chamber within the carrier head and including an expandable lip portion in fluid communication with the chamber. The chamber is evacuated to collapse the expandable lip portion and form a seal with the substrate. The substrate is transfered from a first location to a second location. 
     Advantages of the invention may include the following. The substrate can be reliably loaded to and removed from the polishing pad. A uniform load is applied to the substrate during polishing by allowing removal of air trapped between the substrate and the flexible membrane. 
    
    
     Other advantages and features of the invention will be apparent from the following description, including the drawings and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a chemical mechanical polishing apparatus. 
     FIG. 2 is a schematic cross-sectional view of a carrier head according to the present invention. 
     FIG. 3 is an enlarged view of the carrier head of FIG. 2 showing a flexible lip at the edge of a flexible membrane. 
     FIG. 4A is a view of the carrier head of FIG. 2 illustrating a method of removing a substrate from the polishing pad. 
     FIG. 4B is a view of the carrier head of FIG. 2 illustrating a method of removing a substrate from the carrier head. 
     FIG. 5 is a cross-sectional view of a carrier head in which the edge portion of the flexible membrane extends over the lip portion. 
     FIG. 6 is a schematic cross-sectional view of a carrier head in which the flexible membrane includes an expandable lip portion. 
     FIG. 7A illustrates a method of polishing a substrate on a polishing pad using the carrier head of FIG.  6 . 
     FIGS. 7B and 7C illustrate a method of removing the substrate from the polishing pad using the carrier head of FIG.  6 . 
     FIG. 7D illustrates a method of removing the substrate from the carrier head using the carrier head of FIG.  6 . 
     FIG. 8 is a schematic cross-sectional view of a carrier head which includes a flexible membrane with an expandable lip portion and a separate flexure. 
    
    
     Like reference numbers are designated in the various drawings to indicate like elements. A letter suffix indicates that an element has a modified function, operation or structure. 
     DETAILED DESCRIPTION 
     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. 
     The CMP apparatus  20  includes a lower machine base  22  with a table top  23  mounted thereon and a removable upper outer cover (not shown). Table top  23  supports a series of polishing stations  25 , and a transfer station  27  for loading and unloading substrates. The transfer station may form a generally square arrangement with the three polishing stations. 
     Each polishing station includes a rotatable platen  30  on which is placed a polishing pad  32 . If substrate  10  is an eight-inch (200 millimeter) or twelve-inch (300 millimeter) diameter disk, then platen  30  and polishing pad  32  will be about twenty or thirty inches in diameter, respectively. Platen  30  may be connected to a platen drive motor (not shown) located inside machine base  22 . For most polishing processes, the platen drive motor rotates platen  30  at thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station may further include an associated pad conditioner apparatus  40  to maintain the abrasive condition of the polishing pad. 
     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. 
     A rotatable multi-head carousel  60 , including a carousel support plate  66  and a cover  68 , is positioned above lower machine base  22 . Carousel support plate  66  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly located within machine base  22 . Multi-head carousel  60  includes four carrier head systems  70  mounted on carousel support plate  66  at equal angular intervals about carousel axis  64 . Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pads of the polishing stations. One of the carrier head systems receives a substrate from and delivers the substrate to transfer station  27 . The carousel motor may orbit the carrier head systems, and the substrates attached thereto, about carousel axis  64  between the polishing stations and the transfer station. 
     Each carrier head system 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 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. 
     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 a polishing pad  32 . Generally, carrier head  100  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. 
     Referring to FIGS. 2 and 3, carrier head  100  includes a housing  102 , a base  104 , a gimbal mechanism  106 , a loading chamber  108 , a retaining ring  110 , and a substrate backing assembly  112 . A description of a similar carrier head may be found in U.S. application Ser. No. 08/861,260 by Zuniga, et al., filed May 21, 1997, entitled A CARRIER HEAD WITH A FLEXIBLE MEMBRANE FOR A CHEMICAL MECHANICAL POLISHING SYSTEM, and assigned to the assignee of the present invention, the entire disclosure of which is incorporated herein by reference. 
     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. Loading chamber  108  is located between housing  102  and base  104  to apply a load, i.e., a downward pressure, to base  104 . The vertical position of base  104  relative to polishing pad  32  is also controlled by loading chamber  108 . 
     Substrate backing assembly  112  includes a support structure  114 , a flexure diaphragm  116  connecting support structure  114  to base  104 , and a flexible member or membrane  118  connected to support structure  114 . Flexible membrane  118  extends below support structure  114  to provide a mounting surface  192  for the substrate. The sealed volume between flexible membrane  118 , support structure  114 , flexure diaphragm  116 , base  104 , and gimbal mechanism  106  defines a pressurizable chamber  190 . Pressurization of chamber  190  forces flexible membrane  118  downwardly to press the substrate against the polishing pad. A first pump (not shown) may be fluidly connected to chamber  190  to control the pressure in the chamber and thus the downward force of the flexible membrane on the substrate. 
     Housing  102  may be generally circular in shape to correspond to the circular configuration of the substrate to be polished. A cylindrical bushing  122  may fit into a vertical bore  124  through the housing, and two passages  126  and  128  may extend through the housing for pneumatic control of the carrier head. 
     Base  104  is a generally ring-shaped body formed of a rigid material and is located beneath housing  102 . A passage  130  may extend through the base, and two fixtures  132  and  134  may provide attachment points to connect a flexible tube between housing  102  and base  104  to fluidly couple passage  128  to passage  130 . 
     An elastic and flexible membrane  140  may be attached to the lower surface of base  104  by a clamp ring  142  to define a bladder  144 . Clamp ring  142  may be secured to base  104  by screws or bolts (not shown). A second (not shown) may be connected to bladder  144  to direct a fluid, e.g., a gas, such as air, into or out of the bladder and thereby control a downward pressure on support structure  114 . Specifically, bladder  144  may be used to cause a projection  179  (see FIG. 3) from a support plate  170  of support structure  114  to press a central area of flexible membrane  118  against substrate  10 , thereby applying additional pressure to the central portion of the substrate. 
     Gimbal mechanism  106  permits base  104  to pivot with respect to housing  102  so that the base may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism  106  includes a gimbal rod  150  which fits into a passage  154  through cylindrical bushing  122  and a flexure ring  152  which is secured to base  104 . Gimbal rod  150  may slide vertically along passage  154  to provide vertical motion of base  104 , but it prevents any lateral motion of base  104  with respect to housing  102 . 
     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 clamp ring  164  may clamp an outer edge of rolling diaphragm  160  to base  104 . Thus, rolling diaphragm  160  seals the space between housing  102  and base  104  to define loading chamber  108 . A third pump (not shown) may be fluidly connected to loading chamber  108  to control the pressure in the loading chamber and the load applied to base  104 . 
     Retaining ring  110  may be a generally annular ring secured at the outer edge of base  104 , e.g., by bolts (not shown). When fluid is pumped into loading chamber  108  and base  104  is pushed downwardly, retaining ring  110  is also pushed downwardly to apply a load to polishing pad  32 . A bottom surface  194  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  196  of retaining ring  110  engages the substrate to prevent it from escaping from beneath the carrier head. 
     Support structure  114  of substrate backing assembly  112  includes support plate  170 , an annular lower clamp  172 , and an annular upper clamp  174 . Support plate  170  may be a generally disk-shaped rigid member having a plurality of apertures  176  formed therethrough. The outer surface of support plate  170  may be separated from inner surface  196  of retaining ring  110  by a gap having a width of about 3 mm. An annular recess  178  having a width W 1  of about 2-4 mm, e.g., 3 mm, may be formed in the outer edge of support plate  170 . In addition, projection  179  (see FIG. 3) may extend downwardly from a central region of the bottom surface of the support plate. The projection may be formed by attaching a carrier film to the bottom of the support plate, or it may be formed integrally with the support plate. Support plate  170  may not include apertures through the area above projection  179 . Alternately, the apertures may extend through both the support plate and the projection. 
     Flexure diaphragm  116  of substrate backing assembly  112  is a generally planar annular ring. An inner edge of flexure diaphragm  116  is clamped between base  104  and retaining ring  110 , and an outer edge of flexure diaphragm  116  is clamped between lower clamp  172  and upper clamp  174 . Flexure diaphragm  116  is flexible and elastic, although it could be rigid in the radial and tangential directions. Flexure diaphragm  116  may be formed of rubber, such as neoprene; an elastomeric-coated fabric, such as NYLON™ or NOMEX™; plastic; or a composite material, such as fiberglass. 
     Flexible membrane  118  is a generally circular sheet formed of a flexible and elastic material, such as chloroprene, ethylene propylene rubber or silicone. Flexible membrane  118  includes an inner portion  180 , an annular edge portion  182  which extends around the edges of support plate  170  to be clamped between the support plate and lower clamp  172 , and a flexible lip portion  186  which extends outwardly from a juncture  184  between inner portion  180  and edge portion  182  to contact a perimeter portion of a substrate loaded in the carrier head. The juncture  184  is located generally beneath recess  178  in support plate  170 , and is thicker, e.g., about twice as thick, than inner portion  180  or edge portion  182 . 
     The lip portion  186  may be wedge-shaped and taper from a thickness about equal to that of the juncture to a thickness at its outer rim  188  about equal to that of inner portion  180  of flexible membrane  118 . Outer rim  188  of lip portion  186  may be angled toward the substrate. Specifically, the lip portion should extend sufficiently downwardly so that if chamber  190  is evacuated and flexible membrane  118  is pulled upwardly, rim  188  of lip portion  180  still extends below projection  179  on support plate  170 . This ensures that a seal can be formed between the substrate and flexible membrane  118  even if projection  179  prevents the application of pressure to the edge of the substrate. As discussed in greater detail below, lip portion  186  assists in the removal of the substrate from the polishing pad. 
     In one implementation, the inner and edge portions of flexible membrane  118  may be about 29-33 mils thick, whereas the juncture section may be about 60-66 mils thick and may extend inwardly from the edge portion about 1-5 mm, e.g., 3.5 mm. The lip portion may extend downwardly at an angle of about 0-30°, e.g., 15°, from inner portion  180 , and may extend about 1-5 mm, e.g., 3.5 mm, beyond edge portion  182 . 
     As previously discussed, one reoccurring problem in CMP is underpolishing of the substrate center. Carrier head  100  may be used to reduce or minimize the center slow effect. Specifically, by providing support plate  170  with projection  179  which contacts the upper surface of the flexible membrane in a generally circular contact area near the center of the substrate-receiving surface, additional pressure may be applied by bladder  144  to the potentially underpolished region at the center of the substrate. This additional pressure increases the polishing rate at the center of the substrate, improving polishing uniformity and reducing the center slow effect, as discussed in U.S. patent application Ser. No. 08/907,810, filed Aug. 8, 1997, the entire disclosure of which is incorporated herein by reference. 
     When polishing is completed, fluid is pumped out of chamber  190  to vacuum chuck the substrate to flexible membrane  118 . Then loading chamber  108  is evacuated to lift base  104  and backing structure  112  off the polishing pad. 
     As mentioned above, another problem in CMP is the difficulty in removing the substrate from the polishing pad. However, carrier head  100  substantially eliminates this problem. 
     Referring to FIG. 4A (for simplicity, only the elements involved in attaching and removing the substrate are illustrated in FIGS.  4 A and  4 B), when chamber  190  is evacuated, inner portion  180  of flexible membrane  118  is pulled inwardly. This causes a decrease in pressure in the volume between the backside of the substrate and the mounting surface of the flexible membrane. The decrease in pressure causes lip portion  186  to be drawn against a perimeter portion of the substrate to form a seal therebetween. This provides an effective vacuum-chuck of the substrate to the flexible membrane. Thus, when loading chamber  108  is evacuated, substrate  10  will be securely held to the carrier head. In addition, the seal is sufficiently fluid-tight that it may not be necessary to apply an additional downward force to the portion of the flexible membrane over the perimeter of the substrate to form the seal. Consequently, the seal may be implemented without requiring additional pneumatic controls in the carrier head. 
     Referring to FIG. 4B, to remove the substrate from the carrier head, fluid is pumped into chamber  190 . This causes inner portion  180  to bulge outwardly, causing juncture  184  to pivot downwardly. Consequently, lip portion  186  pivots upwardly so that it lifts away from the substrate. This breaks the seal between the flexible membrane and substrate, and the downward pressure from the inner portion of the flexible membrane removes the substrate from the carrier head. The thickness of juncture  184  should be selected to provide sufficient rigidity to ensure that the lip portion pivots upwardly when the inner portion of flexible membrane  118  is urged downwardly. 
     Referring to FIG. 5, a carrier head  100   a  includes a flexible membrane  118   a  that folds over lip portion  186   a . An advantage of this implementation is that the gap between the outer cylindrical surface of support plate  170  and the inner surface of retaining ring  110  is smaller. The edge portion  182   a  of flexible membrane  118   a  includes a folded portion  198  which extends over lip portion  186   a  to connect to juncture  184   a . The folded portion  198  may fit into recess  178   a  in support plate  170 . Support plate  170  may also include a projection  179  that is formed integrally with the support plate. 
     Referring to FIG. 6, in another embodiment, a carrier head  100   b  includes a flexible membrane  118   b  having an inner portion  180   b , an annular edge portion  200  which extends around the edges of support plate  170 , a wing portion  202  extending radially outward from an upper end  204  of edge portion  200  to retaining ring  110  and base  104  to be secured therebetween, and an expandable peripheral lip portion  206  to contact a perimeter portion of the substrate loaded in the carrier head. Wing portion  202  is integrally joined to the flexible membrane and replaces flexure  116  of FIGS. 3 and 5. Edge portion  200  is located generally between wing portion  202  and expandable lip portion  206 . A spacer ring  208  includes an inwardly-extending flange  210  that extends into a gap between the wing portion and the edge portion. The spacer ring generally surrounds the edge portion to maintain the structural integrity of expandable lip portion  206  when chamber  190  is pressurized, as explained later. 
     Expandable lip portion  206  extends radially outward from edge portion  200  and inner portion  180   b  of flexible membrane  118   b . Lip portion  206  may be formed by folding the portion of the flexible membrane between perimeter portion  200  and inner portion  180   b  into an upper part  216  and a lower part  218 . The space between upper part  216  and lower part  218  defines a pocket  220  that is in fluid communication with chamber  190 . An outer rim  222  of expandable lip portion  206  may be angled toward the substrate. Specifically, expandable lip portion  206  should extend sufficiently downwardly so that rim  222  of expandable lip portion  206  extends below projection  179  on support plate  170 . This ensures that a seal can be formed between the substrate and flexible membrane  118   b  even if projection  179  prevents the application of pressure to the edge of the substrate. 
     In one implementation, inner portion  180   b  of flexible membrane  118   b  may be about 29-33 mils thick, whereas edge portion  200  may be about 150-250 mils thick. Lip portion  206  may extend downwardly at an angle of about 0-30°, e.g., 15°, from inner portion  180   b , and may extend about 1-5 mm, e.g., 3.5 mm, beyond edge portion  200 . 
     Referring to FIG. 7A, carrier head  100   b  is used to apply a uniform load to a substrate during a polishing operation. To perform the polishing operation, substrate  10  is first vacuum-chucked to flexible membrane  118   b  and placed on a polishing pad  32 . The vacuum-chuck procedure is performed by evacuating chamber  190  and forming a seal between substrate  10  and expandable lip portion  206  of flexible membrane  118   b . During this procedure, air may be trapped between the flexible membrane and the substrate. If the trapped air is not removed, it may exert a force on the substrate when a load is applied to the backside of the substrate during a polishing operation via a rigid object, e.g., a projection extending downwardly from the support plate, thereby preventing application of uniform load to the substrate. 
     Expandable lip portion  206  provides a way to remove the trapped air. After placing the substrate on polishing pad  32 , fluid is pumped into chamber  190  to pressurize the chamber and apply a uniform load to the substrate. Pocket  220  of expandable lip portion  206 , which is in fluid communication with chamber  190 , is also pressurized by the inflow of fluid and inflates expandable lip portion  206 . Arrows AA show pressurization of chamber  190  and pocket  220 . Expandable lip portion  206  is inflated as the pocket is pressurized. The expansion of the lip portion forces the rim of the flexible membrane away from the substrate, thereby breaking the seal between the lip portion and the substrate. As a result, any air that may have been trapped between flexible membrane  118   b  and the substrate is forced out when a load is applied to the substrate at the beginning of the polishing operation. 
     Throughout the pressurization precedure, spacer  208  surrounding edge portion  200  allows expandable lip portion  206  to maintain its structural integrity by preventing the pressure in chamber  190  and pocket  220  from pushing edge portion  200  too far out and deforming expandable lip portion  206 . 
     Referring to FIGS. 7B and 7C, expandable lip portion  206  provides a reliable means of removing a substrate from the polishing pad after a polishing operation. Once the polishing procedure has been completed, chamber  190  is evacuated to remove the substrate from the polishing pad. Arrows BB show evacuation of chamber  190 . The evacuation of chamber  190  collapses inflated expandable lip portion  206 , and atmospheric pressure is exerted on expandable lip portion  206 , as shown by arrows CC. Spacer  208  is not shown in order to illustrate the atmospheric pressure being exerted on expandable lip portion  206 . 
     Further evacuation of chamber  190  lifts flexible membrane  118   b  upward, as shown in FIG. 7C. A low pressure region  281  between flexible membrane  118   b  and the substrate is created by the lifting of flexible membrane  118   b . The pressure difference across flexible membrane  118   b  causes lip portion  206  to press firmly against the substrate. The sealing force between lip portion  206  and the substrate is proportional to the force trying to separate flexible membrane  118   b  from the substrate. Therefore, lip portion  206  tightly holds the substrate as carrier head  100   b  is lifted upward to remove the substrate from the polishing pad. 
     FIG. 7D illustrates the removal procedure of the substrate from carrier head  100   b  after the polishing operation. Once the substrate is chucked to the carrier heads, carrier head  100   b  lifts the substrate off polishing pad  32 , and the carousel rotates until the carrier head is positioned above a transfer station (not shown). Chamber  190  and pocket  220  are pressurized, as shown by arrows DD. Spacer  208  surrounding edge portion  200  allows expandable lip portion  206  to maintain its structural integrity by preventing the pressure in chamber  190  and pocket  220  from pushing edge portion  200  too far out and deforming the expandable lip portion. The pressurization of chamber  190  and pocket  220  expands flexible membrane  118   b  and inflates expandable lip portion  206 . The expansion of the edge portion forces the rim of the flexible membrane away from the substrate, thereby breaking the seal between lip portion  206  and the substrate. The substrate then drops onto the transfer station. Expandable lip portion  206  provides carrier head  100   b  with a reliable means of removing the substrate from the carrier head after a polishing operation by pressurizing chamber  190  and pocket  220 . 
     Referring to FIG. 8, in another embodiment, carrier head  100   c  includes a flexure  116  secured to retainer ring  110  and a flexible membrane  118   c  with a rim  224  secured between support plate  170  and lower clamp  172 . The flexible membrane  118   c  includes an expandable lip portion  206   c . A spacer  208   c  surrounds edge portion  200   c  to maintain the structural integrity of expandable lip portion  206   c  when chamber  190  is pressurized. 
     The polishing and substrate removal procedures of carrier head  100   c  of FIG. 8 is substantially similar to the procedures illustrated above with respect to carrier head  100   b.    
     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.