Patent Publication Number: US-6705932-B1

Title: Carrier head for chemical mechanical polishing

Description:
This application is a division of U.S. application Ser. No. 09/236,187, filed Jan. 23, 1999 now U.S. Pat. No. 6,162,116. 
    
    
     BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing. 
     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. 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. The chemical and mechanical interaction between the polishing pad, slurry and substrate results in polishing. 
     One problem, particularly in a carrier head with a flexible membrane, relates to the attachment of the flexible membrane to the carrier head. Typically, the flexible membrane is secured to the carrier head with a clamping ring. Unfortunately, there are a variety of potential problems with this arrangement, such as difficulty in securing the clamping ring or ensuring that the seal between the flexible membrane and carrier head is fluid-tight. 
     SUMMARY 
     In general, in one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a support structure movably connected to the base, and a flexible membrane. The support structure has an outer surface and a recess formed in the outer surface. The flexible membrane extends beneath the base to define a pressurizable chamber, and a lower surface of the flexible membrane provides a mounting surface for a substrate. An edge portion of the flexible membrane extends into the recess and a sealant in the recess secures the flexible membrane to the support structure. 
     Implementations of the invention may include one or more of the following. The edge portion of the flexible membrane may extend along the outer surface of the support structure. The sealant may be injected in a liquid state into the recess. A plurality of ports may be formed between an upper surface of the support structure and the recess. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a support structure movably connected to the base, and a flexible membrane. The support structure has an outer surface and a recess formed in the outer surface. The flexible membrane extends beneath the base to define a pressurizable chamber. A lower surface of the flexible membrane provides a mounting surface for a substrate. The rim portion of the flexible membrane engages the recess to form an O-ring seal between the flexible membrane and the support structure. 
     Implementations of the invention may include the following. The rim portion of the flexible membrane may have a diameter in an unstretched state which is less than a diameter of the recess in the outer surface of the support structure. The flexible membrane may include an edge portion that may extend along the outer surface of the support structure. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a support structure movably connected to the base, and a flexible membrane. The support structure has an outer surface and a recess formed in the outer surface. The flexible membrane extends beneath the base to define a pressurizable chamber. A lower surface of the flexible membrane provides a mounting surface for a substrate. An edge portion of the flexible membrane extends into the recess. The edge portion and recess are configured such that if the chamber is pressurized, the edge portion is pressed against a first surface of the recess to form a seal between the flexible membrane and the support structure. When the chamber is evacuated, the edge portion is pulled against a second surface of the recess to form a seal between the flexible membrane and the support structure. 
     Implementations of the invention may include the following. The recess may be disposed in a generally horizontal arrangement. The first surface may be a top surface of the recess and the second surface may be a bottom surface of the recess. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a support structure movably connected to the base, and a flexible membrane. The support structure has an outer surface and a recess formed in the outer surface. The flexible membrane extends beneath the base to define a pressurizable chamber. The lower surface of the flexible membrane provides a mounting surface for a substrate, and a rim portion of the flexible membrane is adhesively attached to the support structure. 
     Implementations of the invention may include the following. The flexible membrane may have an edge portion that extends around the outer surface of the support structure. The rim portion of the flexible membrane may be adhesively attached to a top surface of the support structure. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a flexible membrane that extends beneath the base to define a pressurizable chamber, and a retaining ring. A lower surface of the flexible membrane provides a mounting surface for a substrate. The retaining ring has an inner surface surrounding the mounting surface and a recess formed in the inner surface. An edge portion of the flexible membrane extends into the recess. The sealant in the recess secures the flexible membrane to the retaining ring. 
     Implementations of the invention may include the following. The sealant may be injected in a liquid state into the recess. A plurality of injection ports may be formed between an upper surface of the retaining ring and the recess. The flexible membrane may extend along the inner surface of the retaining ring. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a flexible membrane extends beneath the base to define a pressurizable chamber a lower surface of the flexible membrane provides a mounting surface for a substrate. The retaining ring surrounds the mounting surface, it includes an upper surface and a recess formed in it. The rim portion of the flexible membrane engages the recess to form an O-ring seal between the flexible membrane and the retaining ring. 
     Implementations of the invention may include the following. The flexible membrane may have an edge portion and may extend along the inner surface of the retaining ring. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a flexible membrane extends beneath the base to define a pressurizable chamber. The lower surface of the flexible membrane provides a mounting surface for a substrate. The retaining ring includes an inner surface surrounding the mounting surface and a recess formed in the inner surface. The edge portion of the flexible membrane extends into the recess. The edge portion and recess are configured such that if the chamber is pressurized, the edge portion is pressed against a first surface of the recess to form a seal between the flexible membrane and the retaining ring. If the chamber is evacuated, the edge portion is pulled against a second surface of the recess to form a seal between the flexible membrane and the retaining ring. 
     Implementations of the invention may include the following. The recess may be horizontal. The first surface may be a top surface, and the second surface may be a bottom surface of the recess. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base, a flexible membrane extends beneath the base to define a pressurizable chamber, a lower surface of the flexible membrane provides a mounting surface for a substrate. The retaining ring surrounds the mounting surface. The edge portion of the flexible membrane extends along an inner surface of the retaining ring and a rim portion of the flexible membrane is adhesively attached to a top surface of the retaining ring. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base which had an outer surface and a recess formed in the outer surface. The flexible membrane extends beneath the base to define a pressurizable chamber. A lower surface of the flexible membrane provides a mounting surface for a substrate. The rim portion of the flexible membrane engages the recess to form an O-ring seal between the flexible membrane and the base. 
     Implementations of the invention may include the following. The retaining ring may surround the mounting surface. The rim portion of the flexible membrane may have a diameter in an unstretched state which may be less than a diameter of the recess in the outer surface of the base. 
     In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus including a base which has a lower surface and a recess formed in the lower surface. The flexible membrane extends beneath the base to define a pressurizable chamber. The lower surface of the flexible membrane provides a mounting surface for a substrate. The edge portion of the flexible membrane extends into the recess, it is configured so that if the chamber is pressurized, the edge portion is pressed against a first surface of the recess to form a seal between the flexible membrane and the base. If the chamber is evacuated, the edge portion is pulled against a second surface of the recess to form a seal between the flexible membrane and the base. 
     Implementations of the invention may include the following. The retaining ring may surround the mounting surface. The recess may be vertical. The first surface may be an outer surface, and the second surface may be an inner surface of the recess. 
     Advantages of the invention may include the following. The membrane is easy to install and remove, with reduced chance of assembly errors and reduced time to change the membrane. The shape of the retaining ring should not distort when the membrane is installed. The membrane assembly accommodates retaining ring wear, i.e., the pressure applied by the membrane should not change as the lower surface of the retaining ring is worn away. The membrane may be removed without removing the retaining ring. A reliable fluid-tight seal is formed between the flexible membrane and the support plate, retaining ring or base. The membrane may “self-align”, i.e., pressurization of the chamber will naturally cause the membrane to move into the proper position for polishing. The membrane assembly has a low manufacturing cost. The membrane and the retaining ring or support structure may form a unitary part that is easy to install. 
    
    
     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. 3A is an enlarged view of the carrier head of FIG. 2 showing an injection molded connection between a flexible membrane and a support structure. 
     FIG. 3B is a cross-sectional view of a carrier head in which the flexible membrane is snap-fit to the support structure. 
     FIG. 3C is a cross-sectional view of a carrier head in which a flap of the flexible membrane fits into a sealing slot in the support structure. 
     FIG. 3D is a cross-sectional view of a carrier head in which the flexible membrane is adhesively attached to the support structure. 
     FIG. 4 is a cross-sectional view of a carrier head according to the present invention in which the flexible membrane is attached to the retaining ring. 
     FIG. 5A is an enlarged view of the carrier head and FIG. 4 showing an injection molded connection between the flexible membrane and the retaining ring. 
     FIG. 5B is a cross-sectional view of a carrier head in which the flexible membrane is snap-fit to the retaining ring. 
     FIG. 5C is a cross-sectional view of a carrier head in which a flap of the flexible membrane fits into a sealing slot in the retaining ring. 
     FIG. 5D is a cross-sectional view of a carrier head in which the flexible membrane is adhesively attached to the retaining ring. 
     FIG. 6 is a cross-sectional view of a carrier head according to the present invention in which a flexible membrane is attached to a carrier base. 
     FIG. 7A is an enlarged view of the carrier head of FIG. 6 showing a snap-fit connection between the flexible membrane and the carrier base. 
     FIG. 7B is a cross-sectional view of a carrier head in which a flap of flexible membrane fits into a sealing slot in the carrier base. 
     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 
     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 series of polishing stations  25  and a transfer station  27  for loading and unloading the substrates. 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) which, for most polishing processes, 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. 
     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 , is supported by a center post  62  and rotated about a carousel axis  64  by a carousel motor assembly (not shown). Multi-head carousel  60  includes four carrier head systems  70  mounted on carousel support plate  66 . Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pads of polishing stations  25 . 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  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 heads. 
     Referring to FIGS. 2 and 3A, 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 hereby incorporated 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. 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. 
     Base  104  is a generally ring-shaped or disk-shaped body located beneath housing  102  and formed of a rigid material. An elastic and flexible membrane  140  may be attached to the lower surface of base  104  to define a bladder  144 . A first pump (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 . 
     An inner edge of a ring-shaped rolling diaphragm  160  is clamped to housing  102  by an inner clamp ring  162 , and an outer edge of rolling diaphragm  160  is clamped to base  104  by an outer clamp ring  164 . Thus, rolling diaphragm  160  seals the space between housing  102  and base  104  to define loading chamber  108 . A second 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 . The vertical position of base  104  relative to polishing pad  32  is also controlled by loading chamber  108 . 
     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 may slide vertically in bushing  122  to provide vertical motion of base  104 , while preventing lateral motion and excessive rotation of base  104  with respect to housing  102 . 
     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  136  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  134  of retaining ring  110  engages the substrate to prevent it from escaping from beneath the carrier head. 
     Substrate backing assembly  112  is positioned below base  104  and 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  132  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  130 . A third pump (not shown) may be fluidly connected to chamber  130  to control the pressure in the chamber and thus the downward force of the flexible membrane on the substrate. 
     Support structure  114  of substrate backing assembly  112  includes a support plate  170  and an annular clamp  172 . Support plate  170  may be a rigid disk-shaped member having a plurality of apertures  176  therethrough. Alternately, support plate  170  could be replaced by a ring-shaped member having a central aperture. A generally horizontal annular recess or slot  182  is formed in an outer surface  180  of the support plate, and a plurality of ports or through-holes  184  are formed between a top surface  186  of support plate  170  and the interior of annular slot  182 . For example, there may be twelve through-holes spaced at equal angular intervals. Support plate  170  may also have a downwardly-projecting lip  178  at its outer edge. 
     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 support plate  170  and clamp  172 . Flexure diaphragm  116  is flexible and elastic, although it could be rigid in the radial and tangential directions. 
     Flexible membrane  118  is a generally circular sheet formed of a flexible and elastic material. An edge portion  174  of flexible membrane  118  extends along inner surface  134  of retaining ring  110 . The edge portion  174  also extends around outer surface  180  of support plate  170  and fits into annular slot  182 . To secure the flexible membrane to the support plate, a liquid sealant is injected into through-holes  184  to fill annular slot  182 . The liquid sealant may be a room temperature vulcanizing (RTV) rubber or another elastomeric material. The sealant may be formed of the same material as the flexible membrane, e.g., silicone. The sealant is heated or otherwise cured to secure the flexible membrane in the annular slot. Advantages of may include low risk that the shape of the retaining ring will distort when the membrane is installed, the ability to remove the membrane without removing the retaining ring, and a reliable fluid-tight seal between the support plate and the flexible membrane. In addition, this embodiment accommodates retaining ring wear, i.e., the pressure applied by the membrane should not change as the lower surface of the retaining ring is worn away. Furthermore, the membrane and the support structure form a unitary part that is easy to install and which requires little maintenance. 
     In operation, fluid is pumped into chamber  130  to control the downward pressure applied to the substrate by flexible membrane  118 . When polishing is completed, fluid is pumped out of chamber  130  to vacuum chuck the substrate to flexible membrane  118 . Then loading chamber  108  is evacuated to lift base  104  and substrate backing assembly  112 . 
     Referring to FIG. 3B, a carrier head  100   a  may includes a flexible membrane  118   a  which is snap-fit to a support plate  170   a . An outer surface  180   a  of support plate  170   a  includes a relatively shallow annular recess  192 . Flexible membrane  118   a  includes a thick rim portion  190 . In an unstretched state, rim portion  190  has a diameter slightly smaller than the diameter of the outer surface of support plate  170   a . However, the flexible membrane can be stretched to slide rim portion  190  around the outer surface of support plate  170   a  until rim portion  190  fits into annular recess  192 . When rim portion  190  is located in and engages recess  192 , it forms an O-ring seal between the support plate and the flexible membrane. The inner surface of the retaining ring and the substrate act to contain the membrane and prevent the O-ring from escaping the recess. Advantages of this embodiment may include ease of installation and removal of the membrane, reduced risk of retaining ring distortion, accommodation of retaining ring wear, a reliable fluid-tight seal between the support plate and the flexible membrane, and a low manufacturing cost. 
     Referring to FIG. 3C, a carrier head  100   b  includes a flexible membrane  118   b  with a flap or edge portion  200  that extends inwardly into a generally annular recess  202  formed in an outer surface  180   b  of a support plate  170   b . The recess  202  includes a lower sealing surface  204  and an upper sealing surface  206 . If chamber  130  is pressurized, flap portion  200  of flexible membrane  118   b  is forced upwardly and into contact with upper sealing surface  206 . On the other hand, if chamber  130  is evacuated, flap portion  200  is pulled downwardly into contact with lower sealing surface  204 . Thus, flexible membrane  118   b  forms a fluid-tight seal with support plate  170   b . Advantages of this embodiment include ease of assembly, reduced risk of retaining ring distortion, accommodation of retaining ring wear, “self-alignment” of the membrane, i.e., that pressurization of the chamber will naturally cause the membrane to move into the proper position for polishing, and a low manufacturing cost. 
     Referring to FIG. 3D, a carrier head  100   c  includes a flexible membrane  118   c  which is secured to a support plate  170 c with an adhesive layer  210 . Specifically, adhesive layer  210  may be placed on an annular outer area  212  of top surface  186  of a support plate  170   c  . The adhesive layer  210  may be an epoxy or a pressure sensitive adhesive. An advantage of the adhesive attachment is that it provides a relatively permanent attachment between the flexible membrane and the support plate so that the membrane and the support structure form a unitary part that is easy to install and which requires little maintenance. Additional advantages of this embodiment may include reduced risk of retaining ring distortion, accommodation of retaining ring wear, and a reliable fluid-tight seal between the support plate and the flexible membrane. 
     Referring to FIGS. 4 and 5A, a carrier head  100   d  includes a flexible membrane  118   d  that is secured to a retaining ring  110   d . A generally horizontal annular slot o recess  220  is formed in an inner cylindrical surface  134   d  of the retaining ring. In addition, a plurality of through-holes or ports  224  are formed between an upper surface  226  of retaining ring  110   d  and an annular slot  220 . Flexible membrane  118   d  includes a flap or edge portion  228  that extends outwardly into slot  220 . To secure the flexible membrane to the retaining ring, a sealant, such as RTV or the membrane material, is injected into through-holes  224  into annular slot  220 . The sealant is cured to secure the flexible membrane to the retaining ring. Although carrier head  100   d  is illustrated without a support plate, flexure, or bladder, these elements could be included in the carrier head. Advantages of this embodiment may include a relatively permanent attachment between the flexible membrane and the retaining ring support plate which provides a unitary part that is easy to install and requires little maintenance. Additional advantages of this embodiment may include a reliable fluid-tight seal between the retaining ring and the flexible membrane. 
     Referring to FIG. 5B, a carrier head  100   e  includes a flexible membrane  118   e  which is snap-fit to a retaining ring  110   e . Retaining ring  110   e  includes an annular recess or groove  230  formed in an upper surface  226   e  of the retaining ring. The edge portion  174  of flexible membrane  118   e  extends along an inner surface  134   e  of retaining ring  110   e , and a flap portion  238  of the flexible membrane extends outwardly across upper surface  226   e  of retaining ring  110   e  and downwardly into annular groove  230 . Flexible membrane  118   e  includes a thick rim portion  232  which fits into a relatively shallow recess  234  in an inner surface  236  of annular groove  230 . In an unstretched state, the diameter of rim portion  232  may be slightly smaller than the diameter of recess  234 . Thus, when flexible membrane  118   e  is stretched over the retaining ring to fit rim portion  232  into recess  234 , the flexible membrane forms an O-ring seal with retaining ring  110   e . Advantages of this embodiment may include ease of assembly, accommodation of retaining ring wear, a reliable fluid-tight seal between the support structure and the flexible membrane, and a low manufacturing cost. 
     Referring to FIG. 5C, a carrier head  100   f  includes a flexible membrane  118   f  which has an edge or flap portion  240  that extends into a generally horizontal annular slot  242  formed in an inner surface  134   f  of a retaining ring  110   f . When chamber  130  of carrier head  100   f  is pressurized, flap  240  of flexible membrane  118   f  is pressed against a lower surface  244  of annular slot  242 . On the other hand, when the chamber  130  of carrier head  100   f  is evacuated, flap  240  of flexible membrane  118   f  is pulled against an upper surface  246  of annular slot  242 . Thus, flexible membrane  118   f  forms a fluid-tight seal with the retaining ring. Advantages of this embodiment may include ease of assembly, “self-alignment” of the membrane, and a low manufacturing cost. 
     Referring to FIG. 5D, a carrier head  100   g  includes a flexible membrane  118   g  which is secured to a retaining ring  110   g  by an adhesive layer  252 . Specifically, an edge portion  250  of flexible membrane  118   g  may be secured to a rim  254  formed in an upper surface  256  of the retaining ring. The adhesive layer  252  may be an epoxy or pressure-sensitive adhesive. Advantages of this embodiment may include a unitary part that is easy to install, and a reliable fluid-tight seal between the retaining ring and the flexible membrane. 
     Referring to FIGS. 6 and 7A, a carrier head  100   h  includes a flexible membrane  118   h  which is snap-fit to a base  104   h . Base  104   h  includes an annular projection  260  which extends downwardly from a main body portions  262 . An annular groove or recess  264  is formed in an outer cylindrical surface  266  of projection  260 . An edge portion  174   h  of flexible membrane  118   h  extends through a gap  269  between an inner surface  134   h  of retaining ring  110   h  and outer surface  266  of projection  260 . Flexible membrane  118   h  includes a protruding rim portion  268  which fits into groove  264  on projection  260 . In an unstretched state, the diameter of rim portion  268  may be slightly less than the diameter of groove  264 . Thus, when flexible membrane  118   h  is stretched and pulled over annular projection  260  so that rim portion  268  fits in groove  264 , the flexible membrane forms an O-ring seal with the base. Advantages of this embodiment may include ease of assembly, reduced risk of retaining ring distortion, a reliable fluid-tight seal between the base and the flexible membrane, and a low manufacturing cost. 
     Referring to FIG. 7B, carrier head  100   i  includes a generally vertical annular slot or recess  270  formed in a lower surface  272  of a base  104   i . A flexible membrane  118   i  includes an edge or flap portion  274  that extends upwardly into annular slot  270 . When chamber  130  is pressurized, flap portion  274  is urged outwardly against an outer sealing surface  276  of annular slot  270 . On the other hand, if chamber  130  is evacuated, flap portion  274  is pulled against inner surface  278  of annular slot  270 . Thus, a fluid-tight seal is formed between the flexible membrane and the base. Advantages of this embodiment may include the ability to remove the retaining ring without removing the membrane, ease of assembly, reduced risk of retaining ring distortion, accommodation of retaining ring wear, “self-alignment” of the membrane, and a low manufacturing cost. 
     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.