Carrier head for workpiece planarization/polishing

An edge control system for deployment on a CMP carrier head comprising a bladder and a carrier head housing having a passage extending therethrough. The bladder includes a flexible diaphragm and is coupled to the carrier head housing. The edge control system comprises first and second annular ribs, each of which comprises a first end portion sealingly coupled to the carrier head housing, a second end portion coupled to the diaphragm, and a strain relief member substantially intermediate the first end portion and the second end portion. A plenum is substantially defined by the first and second annular ribs and the carrier head housing. The passage is fluidly coupled to the plenum to permit the pressurization of the plenum, and the strain relief member promotes the extension of the first and second annular ribs away from the carrier head housing when the plenum is pressurized.

FIELD OF THE INVENTION

The present invention generally relates to workpiece processing and, more particularly, to a carrier head for use in the chemical mechanical polishing or planarizing of a workpiece, such as a semiconductor wafer.

BACKGROUND OF THE INVENTION

For a variety of workpieces (e.g., semiconductor wafers, optical blanks, memory disks, etc.), manufacture requires the substantial planarization of at least one major workpiece surface. For ease of description and understanding, the following description will concentrate on exemplary embodiments of the present invention pertinent to semiconductor wafers. It should be understood, however, that the inventive carrier head may be utilized to planarize a wide variety of workpieces in addition to semiconductor wafers. Furthermore, as appearing herein, the term “planarization” is used in its broadest sense and includes any chemical and/or mechanical process that may be utilized to smooth (e.g., remove irregular topographical features from, change the thickness of, etc.) or polish the surface of a workpiece.

The technique of chemical mechanical polishing, also known as chemical mechanical planarization (referred to herein collectively as “CMP”), has been widely adopted for the planarization of semiconductor wafers. CMP processes produce a substantially smooth, planar face along a major surface of the wafer (referred to herein as the wafer's front surface) to prepare the workpiece surface for subsequent fabrication processes (e.g., photoresist coating, pattern definition, etc.). During CMP, an unprocessed wafer is transferred to a carrier head, which then presses the wafer against a polishing surface (e.g., a polish pad) supported by a platen. Polishing slurry is introduced between the wafer's front surface and the polish pad (e.g., via conduits provided through the polish pad), and relative motion (e.g., rotational, orbital, and/or linear) is initiated between the polish pad and the wafer carrier. The mechanical abrasion of the polish pad and the chemical interaction of the slurry produce a substantially planar topography along the wafer's front surface.

One known type of carrier head generally includes a flexible membrane or bladder that contacts the back (i.e., the unpolished) surface of the work piece during the CMP process. The bladder may be secured to the carrier head by way of a plurality of clamp rings threadably coupled to bolts extending through the carrier head housing. Multiple pressure chambers or plenums are provided behind the bladder to form a number of annular pressure zones across the bladder's working face. The pressure within each zone is independently adjusted to vary the force applied to the wafer's back surface at different locations. The CMP apparatus may be provided with an induction system (e.g., a closed-loop eddy current system) to monitor the topographical features of the wafer's front surface during polishing/planarization. For example, the induction system may identify thicker wafer surface areas requiring a higher rate of removal, and the pressure within the zone or zones corresponding to the thicker surface areas may be increased accordingly. After a major surface of the wafer has been satisfactorily planarized, the carrier head ejects the wafer by, for example, expanding a central portion of the bladder to physically force the wafer away therefrom (commonly referred to as “bullfrogging”).

Despite extensive engineering, conventional carrier heads are still limited in certain respects. For example, the utilization of multiple clamp rings and bolts to attach the bladder to the carrier head housing increases the overall complexity and weight of the carrier head and further complicates the task of refurbishing the carrier head (e.g., replacing exhausted bladders). Moreover, the tightening of each bolt may produce a relatively high and localized clamping force. Consequently, large portions of the carrier head (e.g., the carrier head housing) must typically made of a metal capable of withstanding high axial forces without deformation. The manufacture of the carrier head housing and other carrier head components from metal not only increases the weight of the carrier head, but may also lead to carrier head interference (e.g., signal attenuation) with the induction system utilized to monitor wafer topography during the CMP process.

The limitations associated with conventional carrier head designs are not solely attributable to the bladder attachment means; e.g., known wafer ejectment systems have certain drawbacks as well. By ejecting a wafer in the manner described above, the bladder may place undue stress on inner portions of the wafer. Furthermore, expanding a central portion of the bladder to eject a supported wafer may create suction between the bladder and the wafer, which may ultimately prevent wafer ejection. As a still further limitation, conventional carrier head designs do not provide a large degree of bladder control proximate the outer peripheral edge of the bladder. Consequently, it is difficult to precisely control the planarization of the outer edge of the wafer (e.g., the outer 4-5 mm of a 300 mm wafer), which may result in lower die yields.

In view of the above, it should be appreciated that it would be desirable to provide a CMP carrier head suitable for planarizing a workpiece (e.g., a semiconductor wafer) that overcomes the limitations associated with conventional carrier head designs. In particular, it would be desirable if such a carrier head employed an improved bladder attachment design that utilizes less components, that facilitates refurbishing, and that permits components of the carrier head (e.g., the carrier housing) to be made of materials having lower compressive strengths (e.g., a polymer, such as plastic). In addition, it should be appreciated that it would be advantageous if such a carrier utilized an improved ejection system that did not unduly stress the wafer or create suction between the wafer and the bladder during ejectment. Finally, it should be appreciated that it would be desirable if such a carrier head included a system for providing improved bladder control proximate the outer edge of the wafer during planarization/polishing. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1is a top functional view of a CMP apparatus20comprising a plurality of CMP systems22, which are arranged in two rows and separated by a service access corridor58. Electrical cabinets60may be disposed on either side of corridor58to provide storage space for electrical boards, controllers, and the like. CMP systems22each comprise a carrier head and a polish pad, which are described in detail below in conjunction withFIG. 2. The polish pad may be conditioned by a pad conditioner78comprising an abrasive element attached to an arm configured to pivot from an off-pad location (illustrated) to a conditioning position whereat the abrasive element sweeps across the polish pad. A front end module24resides adjacent CMP systems22opposite cabinets60. Front end module24includes (1) a cleaning module76having a plurality of cleaning stations26thereon, and (2) a wafer cache station28capable of accommodating a plurality of wafer caches30. During the CMP process, unprocessed wafers are retrieved from wafer caches30, cleaned at cleaning stations26, and then planarized/polished by CMP systems22. After planarization/polishing, the wafers may again be transferred to cleaning stations26for post-planarization cleaning and, finally, returned to caches30for transport.

First and second transfer robots32and36may be mounted on front end module24and utilized to transport wafers amongst the various stations of CMP apparatus20. Front end transfer robot32may comprises an extensible arm72having an end effector70attached thereto. Similarly, transfer robot36may comprise an extensible arm75having an end effector74attached thereto. Transfer robots32and36are configured to grasp wafers such that end effectors70and74contact only the outer periphery of the wafer's back surface or the wafer's outer edge. During operation of CMP apparatus20, first transfer robot32transfers selected wafers from caches30to a wafer hand off station34disposed on cleaning module76. As shown inFIG. 1, wafer hand off station34resides at a location accessible to both front end robot32and transfer robot36(e.g., underneath cleaning stations26). Second transfer robot36then retrieves the transferred wafer from hand off station34, inverts the wafer so that its front surface (i.e., the surface to be polished/planarized) is facing downward, and delivers the wafer to a load cup associated with one of CMP systems22.

FIG. 2is an isometric view of two neighboring CMP systems38and40that may be employed by CMP apparatus20. CMP systems38and40are substantially identical and operate in a similar manner; thus, only CMP system40will be discussed herein below. CMP system40comprises a wafer carrier head48and a polish pad50deployed on a polish platen51. CMP system40may also include a load cup52that is configured to transfer wafers to and from carrier head48. Load cup52is configured to pivot about an axis from an off-load position (illustrated) to a load position underneath and aligned with wafer carrier head48. When in the off-load position, load cup52may receive an unprocessed wafer from transfer robot36. After receiving an unprocessed wafer, load cup52pivots about its axis to the load position in which load cup52is raised to contact wafer carrier head48so as to enable wafer transfer to carrier head48. Load cup52then lowers to a plane below wafer carrier head48and pivots back to the off-load position.

After load cup52has returned to the off-load position, wafer carrier head48is lowered to place the surface of the wafer in contact with polish pad50mounted on polish platen51. Polish slurry is supplied to the surface of polish pad50, and relative motion (e.g., rotational, orbital, and/or linear) is initiated between pad50and the wafer carrier head48and, therefore, between pad50and the wafer supported by carrier head48. The front surface of the wafer is polished by the mechanical abrasive action and by the chemical reaction of the slurry with the constituents of the wafer surface. The CMP process terminates when the planarization is complete or when the process has reached a predetermined intermediate point, and carrier head48is raised to a position out of contact with polish pad50. Load cup52again pivots about its axis to the load position, and the processed wafer is transferred from wafer carrier head48to load cup52. If desired, load cup52may spray the planarized surface of the processed wafer with a fluid (e.g., a surfactant) that helps maintain the hydrophilic state of the planarized surface. Load cup52then pivots about its axis to the off-load position in which transfer robot36(FIG. 1) removes the processed wafer. The back or unprocessed side of the wafer may be sprayed with a fluid to help remove residue. Transfer robot36may then transfer the processed wafer to another CMP system22for further processing or a cleaning station26for post-processing cleaning. After the processed wafer has been sufficiently planarized and cleaned, transfer robot32may return the processed wafer to one of caches30.

FIG. 3is an isometric view of a carrier head80in accordance with a first exemplary embodiment of the present invention and suitable for use in conjunction with CMP apparatus20described above in conjunction withFIGS. 1 and 2. Carrier head80may be generally disc-like in shape and comprises an upper surface82, a lower surface84, and an annular rim portion85. The outer annular portion of lower surface84is defined by a retaining ring86(also referred to as a wear ring). Retaining ring86encircles a flexible bladder (hidden from view inFIG. 3) and pre-stresses or pre-compresses the polish pad to protect the leading edge of the workpiece during polishing. As will be described below, the housing of carrier head80cooperates with the bladder to form a plurality (e.g., six) of pressure chambers or plenums. The pressure within each of these plenums may be independently manipulated to vary the pressure applied by the bladder to the wafer's back surface. A plurality of pneumatic fittings88permit the plenums to be fluidly coupled to an external source of pressure (e.g., via flexible connector tubing). Each fitting88is associated with a different pressure plenum. In the illustrated embodiment, carrier head80comprises six fittings88corresponding to six bladder plenums, and a seventh pneumatic fitting92corresponding to a retaining ring plenum. In addition to pneumatic fittings88and92, carrier head80is provided with a plurality of pneumatic fittings90, which allow a plurality (e.g., three) of ejectment mechanisms to be fluidly coupled to an external source of pressure as described below in conjunction withFIG. 11.

Carrier head80is also provided with an induction sensor94, which may be disposed through a central portion of carrier head80as shown inFIG. 3. Induction sensor94may be coupled to an induction system (not shown), such as a closed-loop eddy current system, capable of determining the wafer topography during the CMP process. By utilizing such an induction system, a CMP apparatus (e.g., CMP apparatus20) employing carrier head80may monitor the wafer's topographical features to determine which, if any, plenums require adjustments in pressurization. For example, the induction system may identify thicker wafer surface areas requiring a higher rate of removal, and the CMP apparatus may increase the pressure within the zone or zones corresponding to the thicker surface areas.

FIG. 4is a partially exploded view of carrier head80. The housing of carrier head80comprises: (1) retaining ring86, (2) a mount plate96, (3) an inner clamp ring98, and (4) an intermediate clamp ring100. Clamp rings98and100each comprise an annular base (hidden from view inFIG. 4) having a plurality of arcuate projections extending therefrom. Apertures provided through mount plate96receive the arcuate projections such that only the projections of retaining clamp rings98and100may be seen inFIG. 4. The outer circumferential walls of these projections are preferably threaded such that a first annular fastener may be threadably coupled to clamp ring98and a second annular fastener may be threadably coupled to clamp ring100. For example, a first spanner nut102may be threadably coupled to the exposed arcuate projections of intermediate clamp ring100, and a second spanner nut104may be threadably coupled to the exposed arcuate projections of inner clamp ring98. Spanner nuts102and104may each include a plurality of radially transverse notches or slots106therein to permit tightening with a pronged tool (not shown). This design permits a single annular fastener to be utilized to secure a clamp ring to mount plate96. Spanner nuts102and104may be easily removed to permit clamp rings98and100, respectively, (and the carrier head bladder) to be decoupled from mount plate96. In this manner, the spanner nut/clamp ring assemblies permit carrier head80to be quickly and easily disassembled to facilitate routine maintenance and refurbishing (e.g., replacement of exhausted bladders).

In contrast to conventional bolt/clamp ring assemblies described in the background above, annular fasteners102and104each produce a relatively low and more evenly distributed axial clamping force. As a result, mount plate96, inner clamp ring98, and intermediate clamp ring100may be produced from materials having lower compressive strengths. Preferably, the chosen material is lightweight so as to permit easier manipulation of carrier head80and non-conductive so as to minimize interference (e.g., signal attenuation) with the induction system coupled to induction sensor94; e.g., certain polymers may be employed including various plastics. If the clamp rings are made from a relatively pliable material, it may be desirable to provide supports for the accurate projections of clamp ring98and of clamp ring100. This may be accomplished by, for example, disposing (e.g., press-fitting) first and second stiffening rings107and108along the inner circumference of clamp rings98and100, respectively.

FIG. 5is a partially exploded view of carrier head80absent retaining ring86, annular fasteners102and104, and stiffening rings107and108. In this view, bladder110(described in detail below in conjunction withFIG. 7) and a first outer clamp ring112may be seen. Like clamp rings98and100, outer clamp ring112has a generally annular shape, although the outer diameter of outer clamp ring112is substantially larger than that of clamp ring98and slightly larger than that of intermediate clamp ring100. Unlike clamp rings98and100, however, outer clamp ring112does not include a plurality of arcuate projections that may be threadably coupled to an annular fastener. Instead, outer clamp ring112is coupled to mount plate96by way of a plurality of fasteners118(e.g., bolts). When outer clamp ring112is properly aligned with mount plate96, fasteners118extend through apertures116provided through mount plate and apertures114provided through clamp ring112. In the embodiment shown inFIG. 5, the diameters of apertures116are enlarged (relative to the diameters of apertures114) to accommodate the heads of fasteners118.

A second plurality of apertures120is circumferentially interspersed with apertures114along the upper axial face of clamp ring112. Apertures120extend through clamp ring112and permit ring112to be coupled to a second outer clamp ring122shown inFIG. 6(an exploded view of the lower portion of carrier head80). As can be seen inFIG. 6, outer clamp ring122has a generally annular shape similar to clamp ring112and includes a plurality of apertures124therein. When outer clamp ring122is properly aligned with outer clamp ring112, each of apertures120co-axially align with one of apertures114or apertures124. A fastener126(e.g., a bolt) is disposed through each pair of apertures to couple outer clamp ring122to outer clamp ring112. As was the case previously, the diameters of apertures120may be enlarged (relative to the diameters of apertures124) to accommodate the heads of fasteners126.

FIG. 7is a cross-sectional, isometric view of bladder110. Bladder110comprises a flexible base diaphragm128having a first working surface130, which contacts a workpiece (e.g., a semiconductor wafer) during planarization/polishing, and a second surface131opposite surface130. A plurality of annular ribs (e.g., five) extends from surface131to partially define a plurality (e.g., five) of concentric pressure chambers or plenums. Working outward from the center of bladder110, the ribs are numbered132,134,136,138, and140. Similarly, the plenums are numbered142,144,146,148, and150. Plenum142is laterally defined by rib132, plenum144by ribs132and134, plenum146by ribs134and136, plenum148by ribs136and138, and plenum150by ribs138and140.

It will be noted that bladder110includes an additional rib152disposed along the outer circumference of diaphragm128. Rib152extends from the upper peripheral edge of bladder110to the lower peripheral edge of bladder110. An inner surface of rib152is coupled (e.g., integrally) to an outer annular surface of rib140, and an end portion of rib142is coupled to the outer peripheral edge of diaphragm128. To help distinguish rib152inFIG. 7, dotted lines151separate rib152from rib140and from diaphragm128. Rib152cooperates with an upper portion of rib140to partially define an additional plenum154. As suggested by its peripheral disposition of rib152, plenum154and rib152are utilized to control the outer edge of diaphragm128during planarization/polishing as described below in conjunction withFIG. 10. As will be seen, plenum154may be selectively pressurized to control the vertical displacement of rib152and, therefore, the planarization/polishing characteristics (e.g., the rate of removal) along the outer edge of a wafer during CMP processing.

The annular ribs may be integrally formed with diaphragm128and may each comprise a vertical column having first and second substantially opposite end portions. The annular ribs are preferably oriented substantially orthogonally to the plane of diaphragm128. In preferred embodiments, each annular rib comprises a strain relief member (e.g., an annular brim having a generally J-shaped cross-section) disposed intermediate the first and second end portions. The strain relief members permit greater vertical displacement of the annular ribs and, consequently, permit a greater range of motion substantially orthogonally to working surface130(referred to as a “longer throw”). However, it will be appreciated by one skilled in the art that any or all of the provision of strain relief members is optional and, similarly, that each of the annular ribs may assume a variety of other shapes (e.g., an annular lip having a generally L-shaped cross-section) suitable for attachment to the housing of carrier head80(e.g., to clamp rings98,100,112, and/or122).

FIG. 8is a cross-sectional, isometric view of carrier head80. Retaining ring86is coupled to mount plate96via a plurality of fasteners (e.g., bolts)192(only one of which may be seen inFIG. 8). The inner diameter of retaining ring86is chosen to be slightly larger than the outer diameter of the workpieces to be processed (e.g., 300 mm semiconductor wafers). As stated previously, retaining ring functions to pre-stress or pre-compress the polish pad so as to protect the leading edge of the workpiece during polishing. As such retaining rings are well known in the art, no further discussion is deemed necessary at this time.

FIG. 8also illustrates the manner in which the annular ribs of bladder110are sealingly secured between mount plate96and clamp rings98and100. Since clamp rings98and100, spanner nuts102and104, and the annular ribs secured thereby are similar in structure, only the manner in which spanner nut104couples clamp ring98to mount plate96to sealingly secure annular ribs132and134will be described below.

FIG. 9is a detailed cross-sectional view of a portion of carrier head80including clamp ring98, spanner nut104, and annular ribs132and134. Clamp ring98comprises an annular base156and an arcuate projection158extending therefrom. Mount plate96includes a first surface160, a second surface162substantially opposite surface160, and an aperture164extending from surface160to surface162. Projection158is inserted through aperture164, and spanner nut104is coupled (e.g., threadably) to the outer portion of projection158that protrudes through aperture164. Base156has a larger outer diameter than does aperture164; thus, base156abuts mount plate96when projection158is inserted through aperture164. In particular, base156includes an inner circumferential step166and an outer circumferential step168that abut mount plate96proximate aperture164as shown inFIG. 9.

Base156further comprises a foot portion170having first and second annular recesses172and174therein. Ribs132and134each include a region180of increased thickness proximate an end portion thereof; e.g., proximate the annular rib's inner circumference as shown inFIG. 9. When clamp ring98is attached to bladder110, regions180are received within recesses172and174, which serve to seat and secure regions180. Spanner nut104engages first surface160of mount plate96to sealingly deform regions180between base170and mount plate96in the manner described below. To provide support to bladder110during operation (e.g., to prevent bladder110from caving inward when a partial vacuum is created in plenum144), lower surface181of foot portion170may comprise a flat surface that is substantially parallel the diaphragm of bladder110as shown inFIG. 9.

Mount plate96includes an annular depression182in surface162that receives base156when projection158is inserted through aperture164. Depression182also affords ribs132and134, including respective strain relief members176and178, with space in which to flex. Mount plate96further includes first and second ridges184proximate aperture164. Within depression182, ridges184extend from mount plate96to (1) abut steps166and168, and (2) to contact regions180of ribs132and134. During assembly, as spanner nut104is tightened, base156moves toward mount plate96, and regions180are compressed between base170of clamp ring98and ridges184of mount plate96. Regions180thus deform to contact the inner walls of recesses172and174, and a seal is formed between bladder110and clamp ring98. Steps166and168abut ridge184to prevent over-tightening and extrusion of regions180. To preclude spanner nut104from exerting too high an axial force during tightening, a soft stop may be provided. For example, an annular recess186may be provided in surface160(e.g., where spanner nut104contacts with mount plate96), and a resilient member (e.g., an elastomer washer)189may be disposed within recess186.

It should be appreciated from the forgoing description that ribs132and134of bladder110are sealingly secured between base156of clamp ring98and surface162of mount plate96when spanner nut104is threadably coupled to projection158of clamp ring98. Thus, by sealing securing ribs132and134in this manner, two plenums are fully sealed (i.e., plenums142and144), and one plenum is partially sealed (i.e., plenum146). As may be seen inFIG. 8, ribs136and138are secured between the foot portion of clamp ring100and surface162of mount plate96in a similar fashion. By further sealingly securing ribs136and138, plenums146and148(labeled inFIG. 7) are also sealed. Plenum154is sealed is a different manner described below in conjunction withFIG. 10.

To permit plenum144to be fluidly coupled to an external source of pressure, a passage188(e.g., a pneumatic passage) extends through base156and projection158of clamp ring98. If desired, a fitting88(e.g., a standardized quick connect fitting) may be coupled to projection158; e.g., a threaded insert190may be bonded to an inner portion of projection158, and fitting88may be threadably coupled to insert190. Fitting92may receive an end of a flexible tube coupled to an external source of pressure as described above in conjunction withFIG. 3. Similar pneumatic passages are also provided through clamp rings100and112to permit fluid communication with plenums148and154, respectively; and a plurality of pneumatic passages is provided through mount plate96to provide fluid communication with plenums142,146, and150(FIG. 7).

FIG. 10is a cross-sectional view of an outer portion of carrier head80illustrating an exemplary embodiment of the inventive edge control system. Two plenums are shown inFIG. 10: i.e., plenum150, which is defined by outer clamp ring122and annular ribs138and140; and plenum154, which is defined by outer clamp ring112and annular ribs140and152. Annular ribs140and152include strain relief members194and196, respectively, which are similar to strain relief members176and178described above in conjunction withFIG. 9. In addition, annular ribs140and152each further include a region of increased thickness198formed at an end portion thereof. Region198is received by an annular recess provided in a circumferential shelf200disposed around clamp ring122. When clamp ring122is secured to clamp ring112via fasteners118(FIG. 5) and 126(FIG. 6), region198is compressed between outer clamp ring122and a ledge202extending downward from outer clamp ring112. In this manner, region198forms a seal between shelf200and ledge202. As a result, the outer peripheral wall of plenum150is sealed between bladder110and outer clamp ring122.

Region204of annular rib152is also received by an annular recess provided in a circumferential shelf206disposed around an outer periphery of clamp ring112. When clamp ring112is secured to mount plate96via fasteners118(FIG. 5), region204is compressed between the walls of circumferential shelf206and a ledge208extending from mount plate96thereby forming a seal between mount plate96and clamp ring112, and sealing plenum154. In the illustrated embodiment, plenums150and154are sealed as ribs140and152are secured between stacked clamp rings (i.e., clamp rings112and122) and mount plate96; however, it should be appreciated that plenums150and154may be sealed and that ribs140and152may be secured utilizing other structural configurations, including, but not limited to, an annular fastener/clamp ring assembly similar to that described above in conjunction withFIG. 9.

To permit plenum154to be fluidly coupled to an external source of pressure, a passage211(e.g., a pneumatic passage) is provided through clamp ring112and mount plate96. For example, a first fitting210may be disposed in an aperture provided through mount plate96. A threaded insert212is bonded to an inner portion of fitting210, and a second fitting92(e.g., a standardized quick connect fitting) is threadably coupled to insert212. Passage211may extend through clamp ring112, fitting210, insert212, and fitting92to fluidly engage plenum154.

Plenum154is selectively pressurized to control the vertical displacement of rib152and, to some extent, of rib140, which are each coupled to bladder110proximate an outer peripheral edge thereof. Consequently, selective pressurization of plenum154permits adjustment of an outer peripheral zone of bladder110(labeled X inFIG. 10). By providing increased bladder control proximate the outer peripheral edge of the bladder, carrier head80allows the planarization of the outer edge of the wafer (e.g., the outer 4-5 mm of a 300 mm wafer) to be more precisely managed. To enhance the vertical displacement of rib140and/or rib152, the inventive edge control system may include a guide member disposed proximate ribs140and152. For example, and as shown inFIG. 10, an annular stiffening band218may be coupled (e.g., adhesively) to the outer edge of bladder110. Stiffening band218is preferably configured to have a relatively slim cross-sectional profile to minimize obstruction of retaining ring86. At the same time, stiffening band218is preferably configured to maximize overhang (i.e., the distance between region198and the outer peripheral edge or rib152). To this end, stiffening band218may comprise an upper annular portion that is substantially contiguous with an upper portion of rib152, and a lower annular portion that is substantially contiguous with a lower portion of rib152. Stated differently, stiffening band218may be disposed such that an upper portion of rib152resides between an upper annular portion of band218and plenum154, and a lower portion or rib152resides between a lower annular portion of band218and rib140. Preferably, the outer diameter of the first annular portion is chosen to be slightly larger than the outer diameter of the second annular portion as shown inFIG. 10. To prevent damage to wafers, the lower end of stiffening band218may not extend through to the working surface of bladder110, but instead may abut an outer annular ledge220provided around bladder110.

FIG. 11is a detailed view, partially in cross-section, of an ejectment mechanism222disposed through an aperture224provided through mount plate96. Ejectment mechanism222comprises a cylindrical casing226that abuts mount plate96proximate aperture224(indicated inFIG. 9at228). A piston230is coupled to casing226and configured to translate with respect thereto. Piston230is coupled at its distal end to a plunger head232. Piston230and plunger head232reside within plenum146, which is laterally defined by ribs134and136. To preserve the hermetic integrity of plenum146, ejectment mechanism222preferably forms a seal with mount plate96. As shown inFIG. 11, such a seal may be formed by disposing an elastomer o-ring234around a portion of casing226that is encompassed by mount plate96.

Though ejectment mechanism222may comprise a wide variety of actuators (e.g., an electric or hydraulic actuator), mechanism222is preferably a pneumatic linear actuator. A pneumatic fitting90is coupled to casing226to permit ejectment mechanism222to be fluidly coupled to an external source of pressure. Piston230is biased (e.g., by a spring internal to casing226) toward a retracted position (illustrated inFIGS. 8 and 11) in which head232resides adjacent bladder110. When ejectment mechanism222is sufficiently pressurized, piston230extends away from casing226and plunger head232presses against bladder110. This creates a localized protuberance or bulge along the working surface of the bladder110proximate mechanism222. By simultaneously actuating each of a plurality of ejectment mechanisms222, a wafer supported carrier head80may be ejected (i.e., forced away from bladder110). Ejectment mechanisms222are preferably disposed in an array that evenly distributes the force of ejection over the wafer's back surface. For example, as indicated byFIG. 8, a plurality (e.g., three) of ejectment mechanisms222may be arranged around mount plate96. In contrast to ejectment techniques involving the pressurization of the central plenum, utilizing a plurality of ejectment mechanisms222in this manner does not create suction between bladder110and a supported wafer.

Though described above as having a particular number of ejectment mechanisms and a particular number of spanner nut/clamp ring assemblies, it should be understood that alternative embodiments of the inventive carrier head may employ more or less of these components. As an example,FIG. 12is an isometric view of a carrier head236in accordance with a second embodiment of the present invention. In many respects, carrier head236is similar to carrier head80; carrier head236comprises a mount plate238, a retaining ring240, a flexible bladder (hidden from view), an induction sensor242, and a plurality of pneumatic fittings244. Like carrier head80, carrier head236also comprises first and second spanner nut/clamp ring assemblies246and248that sealingly secure the annular ribs of the bladder to mount plate238as described above. However, unlike carrier head80, carrier head236further comprises a third spanner nut/clamp ring assembly250to sealingly secure the outer peripheral portion of the bladder to mount plate238. Spanner nut/clamp ring assembly250thus replaces fasteners118(FIG. 5) and fasteners126(FIG. 6) employed by carrier head80. By utilizing a plurality of spanner nut/clamp ring assemblies in this manner, carrier head236simplifies assembly/disassembly and streamlines carrier head refurbishment (e.g., replacement of exhausted bladders).

In view of the foregoing description, it should be appreciated that a CMP carrier head has been provided that overcomes many of the limitations associated with conventional carrier head designs. In particular, it should be appreciated that the inventive carrier head employs an improved bladder attachment design that utilizes less components, that facilitates refurbishing, and that permits components of the carrier head (e.g., the carrier housing) to be made of materials having lower compressive strengths (e.g., a polymer, such as plastic). In addition, it should be appreciated that the inventive carrier head employs an improved ejection system that does not unduly stress the wafer or create suction between the wafer and the bladder during ejectment. Finally, it should be appreciated that the inventive carrier head employs an edge control system capable of providing improved bladder control proximate the outer edge of the wafer during planarization/polishing.