CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of and claims priority to U.S. application Ser. No. 09/658,417, filed on Sep. 8, 2000, now U.S. Pat. No. 6,676,497 the entirety of which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to chemical mechanical polishing systems and processes. 
     BACKGROUND 
     Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After a layer is deposited, a photoresist coating is applied on top of the layer. A photolithographic apparatus, which operates by focusing a light image on the coating, is used to remove predetermined portions of the coating, leaving the photoresist coating on areas where circuitry features are to be formed. The substrate is then etched to remove the uncoated portions of the layer, leaving the desired circuitry features. 
     As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Specifically, the photolithographic apparatus may not be able to focus the light image on the photoresist layer if the maximum height difference between the peaks and valleys of the non-planar surface exceeds the depth of focus of the apparatus. Therefore, there is a need to periodically planarize the substrate surface. 
     Chemical mechanical polishing (CMP) is one accepted method of planarization. Chemical mechanical polishing typically requires mechanically abrading the substrate in a slurry that contains a chemically reactive agent. During polishing, the substrate is typically held against a rotating polishing pad by a carrier head. The carrier head may also rotate and move the substrate relative to the polishing pad. As a result of the motion between the carrier head and the polishing pad, abrasives, which may either be embedded in the polishing pad or contained in the polishing slurry, planarize the non-planar substrate surface by abrading the surface. 
     The polishing process generates vibrations that may reduce the quality of the planarization or damage the polishing apparatus. 
     SUMMARY 
     In a first aspect, the invention is directed to a carrier head for chemical mechanical polishing that has a base having at least a portion formed of a polymer, a mounting assembly connected to the base having a surface for contacting a substrate, and a retainer secured to the portion of the base to prevent the substrate from moving along the surface. 
     Implementations of the invention may include one or more of the following features. The portion of the base may be a ring-shaped body extended around a perimeter of the base. A damping material may be secured between the retainer and the portion of the base. At least one screw may extend through apertures in the base, the ring-shaped body and the damping material and into a receiving recess in the retaining ring to secure the retaining ring to the base. The ring-shaped body may include at least one boss extending to contact the retaining ring, and the boss may surround the screw. The polymer may include polyphenylenesulfide, carbon fibers and polytetrafluoroethylene, e.g., about 50-55%, 30-35%, and 10-15% respectively. The damping material may Includes a polyvinylchoride thermoplastic. The entire base may be formed from the polymer. A bottom portion of the retainer may include at least one of carbon, fluoropolymer, and polyester. 
     In another aspect, the invention is directed to a carrier head for chemical mechanical polishing that has a base, a mounting assembly attached to the base having a surface for contacting a substrate, a retainer secured to the portion of the base to prevent the substrate from moving along the surface, and a damping material secured between the retainer and the base. 
     Implementations of the invention may include one or more of the following features. The damping material may include at least one of polyurethane and polyvinylchoride thermopolastic. At least a portion of the base may be formed of a polymer and the retainer may be secured to the portion of the base. The portion of the base may be a ring-shaped body extended around a perimeter of the base. At least one screw may extend through apertures in the base, the ring-shaped body and the damping material and into a receiving recess in the retainer to secure the retainer ring to the base. The ring-shaped body may include at least one boss surrounding the screw and extending to contact the retainer. A bottom portion of the retainer may include at least one of carbon, fluoropolymer, and polyester. 
     In another aspect, the invention is directed to a carrier head for chemical mechanical polishing that has a base, a mounting assembly attached to the base having a surface for contacting a substrate, and a retainer secured to the portion of the base to prevent the substrate from moving along the surface. At least a bottom portion of the retainer including a material selected from the group consisting of polytetrafluoroethylene, perfluoroalkoxy, polyethylene terephthalate, polyetheretherketone, polyetherketoneketone, polybenzimidazole, an imidized thermoset polyimide, a semi-crystalline thermoplastic polyester, and a long molecular chain molecule produced from poly-paraphenylene terephthalamide. 
     Implementations of the invention may include one or more of the following features. The bottom portion of the retaining ring may further include carbon, e.g., graphite or carbon fibers. 
     In another aspect, the invention is directed to an article for attachment to a carrier head that has a ring-shaped body configured to be detachably secured at an outer perimeter of a carrier head. The ring-shaped body is formed of a polymer and has a plurality of apertures therethrough and plurality of bosses surrounding the apertures. 
     In an implementation of the invention, the polymer may include polyphenylenesulfide, carbon fibers and polytetrafluoroethylene. 
     In another aspect, the invention is directed to an article for attachment to a carrier head that has a generally flat annular body configured to be detachably secured at an outer perimeter of a carrier head. The annular body is formed of a damping material and has a plurality of apertures therethrough. 
     In an implementation of the invention, the damping material may include at least one of polyurethane and polyvinylchoride thermopolastic. 
     In another aspect, the invention is directed to a retaining ring for a chemical mechanical polishing head. The retaining ring has an upper portion configured to be secured to a base, and a bottom portion that includes a material selected from the group consisting of polytetrafluoroethylene, perfluoroalkoxy, polyethylene terephthalate, polyether-etherketone, polyetherketoneketone, polybenzimidazole, an imidized thermoset polyimide, a semi-crystalline thermoplastic polyester, and a long molecular chain molecule produced from poly-paraphenylene terephthalamide. 
     In an implementation of the invention, the bottom portion of the retaining ring may further include at least one of graphite and carbon fibers. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a polishing machine having three polishing stations and four carrier heads; 
         FIG. 2  is a cross-sectional view of a carrier head of  FIG. 1 , which includes a retaining ring; 
         FIG. 3  is a more detailed cross-sectional view of the retaining ring of  FIG. 2  during polishing; and 
         FIG. 4  is a cross-sectional view of the polishing station of  FIG. 1 . 
         FIG. 5  is a cross-sectional view of another implementation of a carrier head. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a chemical mechanical polishing (CMP) apparatus  1  for polishing a substrate  10 . A description of a similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is hereby incorporated by reference. 
     The CMP apparatus  1  includes a lower machine base  22  and a multi-head carousel  60 . The lower machine base  22  has three polishing stations  25   a ,  25   b , and  25   c  on a tabletop  23 . Each polishing station  25   a - 25   c  includes a circular polishing pad  32 , which is secured to a circular platen  30  of about the same diameter as the polishing pad  32 , e.g., using a pressure sensitive adhesive (PSA). Platen  30  is driven by a platen drive motor located inside machine base  22 . The polishing pad  32  can be a fixed-abrasive polishing pad, manufactured by 3M Superabrasives and Microfinishing Systems Division, or a standard polyurethane pad, such as IC-1010, manufactured by Rodel, Inc. Assuming the apparatus  1  is used for polishing “eight-inch” or “twelve-inch” substrates, the diameter of the polishing pad  32  and the platen  30  is between twenty and thirty inches. 
     A slurry arm  52  provides an abrasive or non-abrasive slurry to the polishing pad  32  through several spray nozzles (not shown). The slurry contains a reactive agent and a chemically reactive catalyzer. To polish an oxide substrate, deionized water is used as the reactive agent and potassium hydroxide is used as the catalyzer. The slurry arm  52  also provides fluid for rinsing the substrate. 
     The carousel  60  is positioned above the lower machine base  22 . Carousel  60  includes four carrier head systems  70   a - 70   d  that are spaced at equal angular intervals about an axis  64  of symmetry of the carousel. Each carrier head system  70   a - 70   d  has a circular carrier head  100  for holding a substrate  10 . The carrier head  100  is mounted on a drive shaft  74 , which extends through a slot  72  to connect the carrier head to a carrier head rotation motor  76 . The carrier head rotation motor  76  is supported on a slider (not shown). 
     During polishing, a pneumatic system (described below) lowers the carrier head  100  onto a polishing pad  32  to press the substrate  10  against the polishing pad  32  with a pre-determined loading force. The platen drive motor rotates the platen, thereby causing the polishing pad  32  to rotate. At the same time, the rotation motor  76  rotates the substrate  10  by rotating the carrier head  100 , while the slider (not shown) linearly drives the rotation motor  76  back and forth along the slot  72  to oscillate the carrier head  100  and the substrate  10  laterally on the surface of the polishing pad. Thus the apparatus moves the substrate  10  relative to the polishing pad  32 , thereby abrading the surface of the substrate against abrasives contained within the polishing pad. The slurry arm  52  provides slurry  50 , which contains a reactive agent (as previously described), to facilitate the polishing of the substrate. The loading and motion of the carrier head against the polishing pad, and the rotation speed of the polishing pad are carefully controlled to maintain a desired rate and quality of polishing. 
     One problem that can occur during chemical mechanical polishing is excessive vibration of the one or more structures in the polishing apparatus. For example, in some metal polishing processes, particularly in some copper polishing processes, friction between the substrate and the polishing pad causes vibration in the carrier head. This vibration can be transmitted through the drive shaft to other parts of the polishing apparatus, such as the carousel. In general, the vibration is dissipated as noise or shaking in the polishing apparatus. 
     We will describe several implementations of the polishing apparatus  10  according to the invention. The implementations use a vibration damping material at different locations to significantly reduce the transfer of vibrational energy from one part of the polishing apparatus adjacent to the damping material to another adjacent part of the polishing system and thereby reducing or preventing vibration during polishing. Generally, the damping material has significantly better vibration damping characteristics than both adjacent parts of the polishing apparatus, which are typically made from stiff materials, e.g., metals. The damping material can be a viscoelastomer with little or no memory so as to provide good vibration damping characteristics. In general, the damping material can be a material that absorbs vibrational energy and dissipates it as heat. The damping material can be a soft polymeric material, such as a polyvinylchloride (PVC). A suitable damping material is Isodamp C-1002, which is manufactured by EAR Specialty Composites of 7911 Zionesville Road, Indianapolis, Ind. 46268. Alternatively, the damping material can be a hard polymer, such as a mixture of polyphenylenesulfide (PPS), carbon fibers and polytetrafluoroethylene (PTFE, e.g., Teflon®, available from E.I. Dupont), e.g., with 55%/35%/10% by weight. 
     Referring to  FIG. 2 , a first implementation that has the vibration damping material in the carrier head  100  will be described. Carrier head  100  typically includes a housing  102 , a base  104 , a gimbal mechanism  106 , a retaining ring  110 , and a substrate backing assembly  112 . The housing  102  is substantially cylindrical and can be connected to a drive shaft  74  to rotate about an axis  107 . A passage  126  extends through the housing for pneumatic control of the carrier head, as will be described below. The housing  102  can have a cylindrical bushing  122  fitted into a vertical bore  124  which runs vertically through the housing. 
     Gimbal mechanism  106  has a gimbal rod  150 , which is fitted into the bushing  122  so that the rod  150  is free to move vertically within the bore. The bushing  122  prevents lateral motion of the gimbal rod  150 . A gimbal ring  220  is attached to the gimbal rod  150 . A flexure ring  152  is attached to the gimbal ring  220  through a damping material  230 , to prevent or reduce the transmission of vibration energy from the flexure ring  152  to the housing  102 , through the gimbal ring  220 . The damping material  230  can be about 0.06 inches thick. Pressure sensitive adhesive (not shown) adheres the damping material  230  to both the housing  102  and the flexure ring  152 . 
     The flexure ring  152 , which is a generally planar annular ring, is attached to the generally ring-shaped base  104 . The flexure ring  152  flexes in a direction perpendicular to the plane of the flexure ring  152 , thereby gimballing the base  104  to the gimbal rod  150  and the housing  102 . The gimbal mechanism also allows the base  104  to move up and down by allowing the gimbal rod  150  to move vertically within the bore  122 , while preventing any lateral motion of the base. The damping material  230  reduces or prevents the transmission of vibrational energy from the base  104  into the housing  102  through the gimbal mechanism  106 . 
     An outer clamp ring  164  clamps a rolling diaphragm  160  to the base  104 , and an inner clamp ring  162  lamps the rolling diaphragm  160  onto the housing  102 . Thus, the rolling diaphragm  160  seals the loading chamber  108  formed by the housing  102 , the gimbal rod  106 , the gimbal ring  220 , the damping material  230 , the flexure ring  152 , and the base  104 , leaving an opening  126  into the chamber  108 . The opening  126  is connected to a pump (not shown), which lowers or raises the base by pumping fluid, e.g., air, into or out of the chamber  108 , respectively. By controlling the pressure of the fluid pumped into the loading chamber  108 , the pump can press down the base towards the polishing surface with a desired loading force. 
     The retaining ring  110  is a generally annular ring bolted onto the base  104 , e.g., by bolts  194  (only one is shown in the cross-sectional view of  FIG. 2 ). During polishing, fluid is pumped into the loading chamber  108 , thereby generating pressure in the chamber  108 . The generated pressure exerts a downward force on the base  104 , which in turn exerts a downward force on the retaining ring  110 . The downward force presses the retaining ring  110  against the polishing pad  32 . 
     Substrate backing assembly  112  includes a flexure diaphragm  116 , which is clamped between the retaining ring  110  and the base  104 . An inner edge of the flexure diaphragm  116  is clamped between an annular lower clamp  172  and an annular upper clamp  174  of a support structure  114 , and an outer edge of the flexure diaphragm is clamped between the base  102  and the retaining ring  110 . A support plate or support ring  170  of the support structure  114  is attached to the lower clamp  172 . The flexure diaphragm allows some vertical motion of the support plate  170  relative to the base  104 . The support plate  170  is a generally disk-shaped rigid member with a plurality of apertures  176  through it (only one is labeled in  FIG. 2 ). The support plate  170  has a downwardly projecting lip  178  at its outer edge. A flexible membrane  118  extends around the lip  178  of the support plate  170  and is clamped between the support plate  170  and the lower clamp  172 , to form a generally disk shaped lower surface  120 . The flexible membrane is formed from a flexible and elastic material. Alternatively, the flexure diaphragm and the flexible membrane can be combined in a single-piece membrane. 
     The sealed volume between the flexible membrane  118 , support structure  114 , flexure diaphragm  116 , base  104 , and flexure ring  152  defines a chamber  190  with an opening  250  that runs through the gimbal rod  150 . A pump (not shown) is connected to the opening  250  to control the pressure in the chamber  190  by pumping fluid, into the chamber through the opening  250 , thereby controlling the downward pressure of the membrane lower surface  120  on the substrate  10 . 
     An inner surface  188  of the retaining ring  110  in conjunction with the lower surface  120  of the flexible membrane  188  define a cavity  192  for receiving a substrate. The retaining ring keeps the substrate from slipping laterally out of the cavity  192 , while the lower surface  120  of the flexible membrane  188  pushes the substrate, contained within the cavity  192 , against the polishing pad  32  ( FIG. 1 ). 
     A second implementation includes the damping material in the retaining ring itself. Referring to  FIG. 3 , the annular retaining ring  110  includes four portions, which are stacked one on top of another. An upper portion  203  and a middle portion  184  of the retaining ring  110  are a rigid rings. For example, the upper portion  203  can be a stainless steel ring with a thickness of about 0.1 inches, and the middle portion  184  can be a stainless steel ring with a thickness of about 0.25 inches. The upper portion  203  is attached to the middle portion  184  through a damping material  200 , which is similar in thickness and is made from the same material as the damping material  230  of  FIG. 2 . The damping material  200  reduces or prevents the transmission of vibration energy from the middle portion  184  to the upper portion  203 . Pressure sensitive adhesive  202  adheres the damping material  200  to the upper portion  203 , while pressure sensitive adhesive  201  adheres the damping material  200  to the middle portion  184 . The lower portion  180  is a relatively softer material that is chemically inert in the polishing process, such as polyphenylene sulfide (PPS), available from DSM Engineering Plastics of Evansville, Ind. The lower portion  180  can be durable but gradually wears away with use. The lower portion  180  has a bottom surface  182 , which contacts the polishing pad  32  during polishing. The bottom surface can have substantially radial grooves (not shown) for transporting slurry from the outside of the retaining ring to the surface of the substrate  10 . The middle portion  184  can adds rigidity to the lower portion  180 , thereby reducing the deformation of the retaining ring during polishing. The middle portion  184  can be secured to the lower portion  180  by a layer of epoxy adhesive  186 , such as Magnobond-6375™, available from Magnolia Plastics of Chamblee, Ga. 
     The thickness of the lower portion  180  should be larger than the thickness TS of the substrate  10 . Specifically, the lower portion  180  should be thick enough that the substrate  10  does not contact The adhesive layer  186 . On the other hand, if the lower portion  180  is too thick, The bottom surface  182  of the retaining ring  110  may be subject to deformation due to the flexible nature of the lower portion  180 . The initial thickness of the lower portion is typically between 200 to 400 mils. The lower portion  180  is replaced when the remaining thickness of the retaining ring is about the same as the thickness of the substrate. 
     Referring to  FIG. 4 , a third implementation has a damping material  211  located between the polishing pad  240  and the platen  210  to reduce or prevent the transmission of vibration energy from the polishing pad  240  to the platen  210 . The damping material  211  is similar in thickness and is made from the same material as the damping material  230  of  FIG. 2 . A pressure sensitive adhesive layer  213  adheres the damping material  211  to the polishing platen  210 . 
     The damping material  211  is attached to the polishing pad  240  through a protective layer  215 . The protective layer  215  is a 0.01-inch thick Teflon sheet that makes it easier to detach the polishing pad  240  from the damping material  211 . A layer of pressure sensitive adhesive  212  adheres the protective layer  215  to the damping material  213 , while a second layer of pressure sensitive adhesive (not shown) adheres the protective layer  215  to the polishing pad  240 . 
     Referring to  FIG. 5 , in a fourth implementation the retaining ring  302  includes an annular upper portion  316  that is more rigid than the lower portion  310 . For example, the upper portion  316  of the retaining ring  302  can be stainless steel and the lower portion  310  of the retaining ring  302  can be PPS. Optionally, a more rigid sleeve may be inserted into the inner diameter of the retaining ring to reduce wear caused by the substrate. Optionally, the entire retaining ring  302  may be formed of the same material. 
     A layer or gasket of a damping material  304  is positioned between the retaining ring and the base  306  of the carrier head  300  to absorb and dissipate vibrational energy. The damping material can be a polyurethane foam or a polymeric material Composites. Depending on the polishing conditions, a minimum thickness may be required for the gasket  304 . The damping material can be a polyvinylchoride thermopolastic, such as Isodamp C-1002, available from EAR Specialty. In this case, the damping material should be precompressed by about 5-15% in thickness. 
     In addition, a portion  308  of the base to which the retaining ring is attached is formed from a polymer material. For example, a ring-shaped insert  308  may be placed between the base  306  and the damping material  304 . The retaining ring  302  can be secured to the base  306  by inserting screws or bolts through the holes  318  in the insert  308  and gasket  304  into the upper layer  316  of the retaining ring. The ring-shaped insert  308  can have bosses around each screw. The tops of the bosses can contact the top surface of the upper portion  316  of the retaining ring. The bosses can control the amount of compression of the damping material and can secure the screws to ensure a tight connection between the base  306  and the retaining ring  300 . The polymer material can be a mixture of polyphenylenesulfide (PPS), carbon fibers and polytetrafluoroethylene, e.g., 50-55%, 30-35%, 10-15% by weight, respectively. 
     Alternatively, the entire base  306  can be formed of a polymer material. In addition, the retaining ring  302  could be secured to the base  306  by an adhesive, such as an epoxy, by a clamp, or by some other mechanism. 
     An edge of a flexible membrane  314  can be clamped directly between the upper surface of the retaining ring  302  and the base  306  as illustrated in  FIG. 5 . Alternatively the flexible membrane can be clamped between the damping material  304  and the base  306 , or the flexible membrane  314  can be clamped between the retaining ring  302  and the damping material  304 , or the flexible membrane could be attached in another fashion to the retaining ring, the base, or to another section of the carrier head. 
     Separately or in combination with one or more of the above implementations, it may also be possible to reduce vibrations by proper selection of the materials in the lower portion of the retaining ring. Possible materials for the lower portion include polytetrafluoroethylene (PTFE, e.g., Teflon®, available from E.I. Dupont), perfluoroalkoxy PTFE (PFA), polyethylene terephthalate (PET), polyetheretherketone (PEEK, e.g., Arlon®-1000, available from Green, Tweed &amp; Co.), polyetherketoneketone (PEKK), polybenzimidazole (PBA, e.g., Celazole®, available from Celanese AG), an imidized thermoset polyimide (such as Duratron® XP, available from DSM Engineering Plastics Products, Inc.), a semi-crystalline thermoplastic polyester (such as Ertalyte®, available from DSM Engineering Plastics), a long molecular chain molecule produced from poly-paraphenylene terephthalamide (such as Kelvar®, available from E.I. DuPont), or a blend of one or more of the above materials, possibly including other materials, such as graphite or carbon fibers. For example, the retaining ring can include Zymaxx® (a composite material available from E.I. DuPont with about 80% Teflon® and 20% carbon fibers), Zymaxx® 6400 (a composite material with about 80% Teflon® and 20% Kelvar®), bearing grade Ryton® (a composite material with about 75% PPS, 15% carbon fiber and 10% Teflon®, available from Chevron Phillips Chemical Company LP), Avalon®-69 (a composite material with about 80% Teflon®, 17% PPS and 3% graphite, available from Green, Tweed &amp; Co), Arlon®-1286 (a composite material with about 60% PEEK and 40% carbon fiber), Arlon®-1330 (a composite material with about 85% PEEK and 15% Teflon®), Arlon®-1555 (a composite material with about 70% PEEK, 10% Teflon®, 10% carbon fibers and 10% graphite), and Ertalyte(® TX (a composite material with Ertalyte® and Teflon®). 
     The lower portion should be chosen to be chemically inert in the polishing process. The lower portion should be sufficiently pliant that the force of the substrate edge against the inner surface of the retaining ring does not chip or otherwise damage the substrate, without excessive wear or particle generation. The specific optimal material may depend on other polishing parameters, such as slurry composition, platen and head rotation rates and applied pressure to the retaining ring and substrate. 
     For a working example, a carrier head according to  FIG. 5  was constructed using a gasket  308  composed of Isodamp C-1002 having a thickness of 60 mils, a ring-shaped insert  308  about 280 mils thick (including bosses which were about 56 mils tall) composed of a composite material with about 50-55% PPS, 30-35% carbon fiber, and 10-15% Teflon®, a stainless steel upper portion  316 , and a PPS lower portion  310 . The construction demonstrated reduced noise during copper polishing, using an applied pressure of 6 psi on the polishing pad from the substrate membrane, an applied pressure of 2.2 to 5.8 psi on the polishing pad from the retaining ring, and simultaneous conditioning. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the damping material may be used with other kinds of polishing apparatus known to persons skilled in the art. For instance, the retaining ring in the apparatus need not contact the polishing pad, as described in the specification. One of the polishing pad and the retaining ring of the polishing system may not rotate at all. The damping material may be used in a polishing apparatus that uses an abrasive or a non-abrasive polishing pad, and the polishing liquid provided to the polishing pad can be a slurry that contains abrasives, such as silicon dioxide particles, in a chemically reactive agent, such as deionized water or potassium hydroxide, or an abrasiveless liquid. 
     The vibration damping material may also be used in any pair of the locations described in the specification, or even in all of the locations described. Other materials with suitable damping properties may be used to damp vibrations, so long as they significantly reduce or prevent the transmission of vibrational energy from one end of the material to another. Any material that does not rebound to its original shape when deformed may be used as a damping material. Specifically, when subjected to a deformation, the damping material should rebound by less then ten percent of the deformation, although a rebound of less than six percent of the deformation is preferred. For instance, the damping material may be any isodamp C-1000 series isolation damping material, manufactured by EAR Specialty Composites, a visco-elastomer, a soft-plastic, or any other material that has better vibration damping properties than materials immediately adjacent to the damping material. 
     The thickness of the damping material may be varied to provide optimum results in operating conditions that have different loading, carrier head rotation speed, polishing pad rotation speed, damping material, and so on. A thicker damping material may be used to improve the vibration damping, although poor control of the relative motion of the substrate and the polishing pad may result from a damping material that is too thick. A thinner damping material may also be used, although if the damping material is too thin, it may not sufficiently reduce or prevent the transmission of vibrational energy. 
     The middle portion  184  and the upper portion  203  ( FIG. 3 ) of the retaining ring may be manufactured from aluminum or any other material that provides a suitable amount of stiffness to the retaining ring. The thickness of the middle portion  184  and the upper portion  203  may be varied, although if the middle and upper portions are too thin, the retaining ring may deform and reduce the quality of polishing. Alternatively, the middle portion  184  and the lower portion  180  of the retaining ring  110  may be one integrated piece formed from the same kind of material, e.g., PPS or stainless steel. Other adhesive or attachment methods known to persons of skill may be used to affix the damping material. 
     Accordingly, other embodiments are within the scope of the following claims.
A carrier head for chemical mechanical polishing that has a base having at least a portion formed of a polymer, a mounting assembly connected to the base having a surface for contacting a substrate, a retainer secured to the portion of the base to prevent the substrate from moving along the surface, and a dampening material secured between the retainer and the base.