Patent Publication Number: US-2005142995-A1

Title: Method of controlling carrier head with multiple chambers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation and claims the benefit of priority under 35 U.S.C. Section 120 of application Ser. No. 10/666,003, filed Sep. 17, 2003, which is a continuation of application Ser. No. 10/251,302, filed Sep. 19, 2002, now U.S. Pat. No. 6,648,740, which is a continuation of U.S. application Ser. No. 09/908,868, filed Jul. 18, 2001, now U.S. Pat. No. 6,506,104, which is a continuation of U.S. application Ser. No. 09/611,246, filed Jul. 7, 2000, now U.S. Pat. No. 6,277,010, which is a divisional of U.S. application Ser. No. 09/368,396, filed Aug. 4, 1999, now U.S. Pat. No. 6,106,378, which is a divisional of U.S. application Ser. No. 08/891,548, filed Jul. 11, 1997, now U.S. Pat. No. 5,964,653. The disclosures of the prior applications are considered part of and are incorporated by reference in the disclosure of this application. 
    
    
     BACKGROUND  
      The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing system.  
      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, the layer 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 non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus, and it will be impossible to properly focus the light image onto the outer substrate surface.  
      It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in a further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a substantially planar layer surface.  
      Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.  
      A CMP process is fairly complex, and differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and the abrasive particles with the reactive sites results in polishing.  
      An effective CMP process should have a high polishing rate and generate a substrate surface that is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing rate sets the maximum throughput of the polishing apparatus.  
      The polishing rate depends upon the force with which the substrate is pressed against the pad. Specifically, the greater this force, the higher the polishing rate. If the carrier head applies a non-uniform load, i.e., if the carrier head applies more force to one region of the substrate than to another, then the high pressure regions will be polished faster than the low pressure regions. Therefore, a non-uniform load may result in non-uniform polishing of the substrate.  
      One problem that has been encountered in CMP is that the edge of the substrate is often polished at a different rate (usually faster, but occationally slower) than the center of the substrate. This problem, termed the “edge effect”, may occur even if the load is uniformly applied to the substrate. The edge effect typically occurs in the perimeter portion, e.g., the outermost five to ten millimeters, of the substrate. The edge effect reduces the overall flatness of the substrate, makes the perimeter portion of the substrate unsuitable for use in integrated circuits, and decreases yied.  
      Therefore, there is a need for a CMP apparatus that optimizes polishing throughput while providing the desired flatness and finish. Specifically, the CMP apparatus should have a carrier head which provides substantially uniform polishing of a substrate.  
     SUMMARY  
      In one aspect, the invention is directed to a carrier head for use in a chemical mechanical polishing system. The carrier head comprises a base and a flexible member connected to the base to define a first chamber, a second chamber and a third chamber. A lower surface of the flexible member provides a substrate receiving surface with an inner portion associated with the first chamber, a substantially annular middle portion surrounding the inner portion and associated with the second chamber, and a substantially annular outer portion surrounding the middle portion and associated with the third chamber. Pressures on the inner, middle and outer portions of the flexible member are independently controllable.  
      Implementations of the invention may include the following. The width of the outer portion may be significantly less than the width of the middle portion. The outer portion may have an outer radius approximately equal to or greater than  100  mm, such as  150  mm, and the width of the outer portion may be between about  4  and  20  mm, such as  10  mm. The flexible member may include an inner annular flap, a middle annular flap, and an outer annular flap, each flap being secured to a lower surface of the base to define the first, second and third chambers.  
      In another aspect, the carrier head comprises a flange attachable to a drive shaft, a base, a gimbal pivotally connecting the flange to the base, and a flexible member connected to the base and defining a chamber. A lower surface of the flexible member provides a substrate receiving surface. The gimbal includes an inner race connected to the base, an outer race connected to the flange to define a gap therebetween, and a plurality of bearings located in the gap.  
      Implementations of the invention may include the following. A spring may urge the inner race and outer race into contact with the bearings, and an annular retainer may hold the bearings. A plurality of pins may extends vertically through a passage in the flange portion such that an upper end of each pin is received in a recess in the drive shaft and a lower end of each pin is received in a recess in the base portion to transfer torque from the drive shaft to the base. A retaining ring may be connected to the base to define, in conjunction with the substrate receiving surface, a substrate receiving recess.  
      In another aspect, the invention is directed to an assembly for use in a chemical mechanical polishing system. The assembly comprises drive shaft, a coupling slidably connected to the drive shaft, a carrier head secured to a lower end of the drive shaft to rotate with the drive shaft, a vertical actuator coupled to an upper end of the drive shaft to control the vertical position of the drive shaft and the carrier head, and a motor coupled to the coupling to rotate the coupling to transfer torque to the drive shaft.  
      Implementations of the invention may include the following. The drive shaft may extend through a drive shaft housing, and the vertical actuator and the motor may be secured to the drive shaft housing. The coupling may include an upper rotary ring surrounding the upper end of the drive shaft and a lower rotary ring surrounding the lower end of the drive shaft, a first bearing rotatably connecting the upper rotary ring to the drive shaft housing and a second bearing rotatably connecting the lower rotary ring to the drive shaft housing. The upper and lower rotary rings may be spline nuts and the drive shaft may be a spline shaft.  
      In another aspect, the invention is directed to a carrier head assembly for use in a chemical mechanical polishing system, comprising a drive shaft a first ball bearing assembly laterally securing an upper end of the drive shaft, a second ball bearing assembly laterally securing a lower end of the drive shaft, and a carrier head connected to the lower end of the drive shaft by a gimbal. The gimbal permits the carrier head to pivot with respect to the drive shaft. The distance between the first ball bearing assembly and the second ball bearing assembly is sufficient to substantially prevent lateral forces transferred through the gimbal from pivoting the drive shaft.  
      In another aspect, the carrier head assembly comprises a drive shaft and a carrier head connected to a lower end of the drive shaft. The drive shaft includes a bore and at least one cylindrical tube positioned in the bore to define a central passageway and at least one annular passageway surrounding the central passageway. The carrier head includes a plurality of chambers, each chamber connected to one of the passageways.  
      Implementations of the invention may include the following. The draft shaft may include two concentric tubes positioned in the bore to define three concentric passageways, each of the passageways connected to one of the chambers. A rotary union may couple a plurality of pressure sources to respective ones of the plurality passageways.  
      In another aspect, the invention is directed to a carrier head comprising first, second and third independently pressurizable chambers, a flexible inner member associated with the first chamber to apply a first pressure to a central portion of a substrate, a substantially annular flexible middle member associated with the second chamber and surrounding the inner member to apply a second pressure to a middle portion of the substrate, and a substantially annular flexible outer member associated with the third chamber and surrounding the middle member to apply a third pressure to an outer portion of the substrate. The outer member is substantially narrower than the middle member.  
      Advantages of the invention include the following. The carrier head applies a controllable load to different portions of the substrate to improve polishing uniformly. The carrier head is able to vacuum-chuck the substrate to lift it off the polishing pad. The carrier head contains few moving parts, and it is small and easy to service.  
      Other advantages and features of the present invention will become apparent from the following description, including the drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic exploded perspective view of a chemical mechanical polishing apparatus.  
       FIG. 2A  is a schematic top view of a carousel of  FIG. 1 , with the upper housing removed.  
       FIG. 2B  is a schematic exploded perspective view of a portion of the carrier head assembly located above the carousel support plate.  
       FIG. 3  is partially a cross-sectional view of a carrier head assembly along line  3 - 3  of  FIG. 2A , and a schematical illustration of the pumps used by the CMP apparatus.  
       FIG. 4  is a schematic cross-sectional view along line  4 - 4  of  FIG. 3 .  
       FIG. 5  is an enlarged view of the carrier head of the present invention.  
       FIG. 6  is a schematic bottom view of the carrier head of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Referring to  FIG. 1 , one or more substrates  10  will be polished by a chemical mechanical polishing (CMP) apparatus  20 . A complete description of CMP apparatus  20  may be found in U.S. patent application Ser. No. 08/549,336, by Perlov, et al., filed Oct. 27, 1996, entitled CONTINUOUS PROCESSING SYSTEM FOR CHEMICAL MECHANICAL POLISHING, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference.  
      The CMP apparatus  20  includes a lower machine base  22  with a table top  23  mounted thereon and a removable upper outer cover (not shown). The table top  23  supports a series of polishing stations  25   a ,  25   b  and  25   c , and a transfer station  27 . The transfer station  27  forms a generally square arrangement with the three polishing stations  25   a ,  25   b  and  25   c . The transfer station  27  serves multiple functions of receiving the individual substrates  10  from a loading apparatus (not shown), washing the substrates, loading the substrates into carrier heads (to be described below), receiving the substrates from the carrier heads, washing the substrates again, and finally transferring the substrates back to the loading apparatus.  
      Each polishing station  25   a - 25   c  includes a rotatable platen  30  on which is placed a polishing pad  32 . If the substrate  10  is an eight-inch (200 mm) diameter disk, then the platen  30  and the polishing pad  32  will be about twenty inches in diameter. The platen  30  may be a rotatable aluminum or stainless steel plate connected by a stainless steel platen drive shaft (not shown) to a platen drive motor (also not shown). For most polishing processes, the drive motor rotates the platen  30  at about thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used.  
      The polishing pad  32  may be a composite material with a roughened polishing surface. The polishing pad  32  may be attached to the platen  30  by a pressure-sensitive adhesive layer. The polishing pad  32  may have a fifty mil thick hard upper layer and a fifty mil thick softer lower layer. The upper layer may be a polyurethane mixed with fillers. The lower layer may be composed of compressed felt fibers leached with urethane. A common two-layer polishing pad, with the upper layer composed of IC-1000 and the lower layer composed of SUBA-4, is available from Rodel, Inc., located in Newark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.).  
      Each polishing station  25   a - 25   c  may further include an associated pad conditioner apparatus  40 . Each pad conditioner apparatus  40  has a rotatable arm  42 , holding an independently rotating conditioner head  44  and an associated washing basin  46 . The conditioner apparatus  40  maintains the condition of the polishing pad so that it will effectively polish any substrate pressed against it while it is rotating.  
      A slurry  50 , containing a reactive agent (e.g., deionized water for oxide polishing), abrasive particles (e.g., silicon dioxide for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing), is supplied to the surface of the polishing pad  32  by a slurry supply port  52  in the center of the platen  30 . Sufficient slurry is provided to cover and wet the entire polishing pad  32 . Optional intermediate washing stations  55   a ,  55   b  and  55   c  may be positioned between the neighboring polishing stations  25   a ,  25   b  and  25   c  and the transfer station  27 . The washing stations are provided to rinse the substrates as they pass from one polishing station to another.  
      A rotatable multi-head carousel  60  is positioned above the lower machine base  22 . The carousel  60  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly located within the base  22 . The center post  62  supports a carousel support plate  66  and a cover  68 . The carousel  60  includes four carrier head assemblies  70   a ,  70   b ,  70   c , and  70   d . Three of the carrier head assemblies receive and hold substrates, and polish them by pressing them against the polishing pad  32  on the platen  30  of the polishing stations  25   a - 25   c . One of the carrier head assemblies receives a substrate from and delivers the substrate to the transfer station  27 .  
      The four carrier head assemblies  70   a - 70   d  are mounted on the carousel support plate  66  at equal angular intervals about the carousel axis  64 . The center post  62  allows the carousel motor to rotate the carousel support plate  66  and to orbit the carrier head systems  70   a - 70   d , and the substrates attached thereto, about the carousel axis  64 .  
      Each carrier head system  70   a - 70   d  includes a carrier head  200 , three pneumatic actuators  74  (see  FIGS. 2A and 2B ), and a carrier motor  76  (shown by the removal of one-quarter of the cover  68  and the pneumatic actuators  74 ). Each carrier head  200  independently rotates about its own axis, and independently laterally oscillates in a radial slot  72 . There are four radial slots  72  in the carousel support plate  66 , generally extending radially and oriented 90□ apart. Each carrier drive motor  76  is connected to a carrier drive shaft assembly  78  which extends through the radial slot  72  to the carrier head  200 . There is one carrier drive shaft assembly and motor for each head.  
      During actual polishing, three of the carrier heads, e.g., those of carrier head assemblies  70   a - 70   c , are positioned at and above the respective polishing stations  25   a - 25   c . The pneumatic actuators lower the carrier head  200  and the substrate attached thereto into contact with the polishing pad  32 . A slurry  50  acts as the media for chemical mechanical polishing of the substrate wafer. Generally, the carrier head  200  holds the substrate against the polishing pad and evenly distributes a downward pressure across the back surface of the substrate. The carrier head also transfers torque from the drive shaft assembly  78  to the substrate and ensures that the substrate does not slip from beneath the carrier head during polishing.  
      Referring to  FIG. 2A , in which the cover  68  of the carousel  60  has been removed, the carousel support plate  66  supports four support slides  80 . Two rails  82  fixed to the carousel support plate  66  bracket each slot  72 . Each slide  80  rides on two of the rails  82  to permit the slide  80  to move freely along the associated radial slot  72 .  
      A bearing stop  84  anchored to the outer end of one of the rails  82  prevents the slide  80  from accidentally coming off the end of the rails. Each slide  80  contains an unillustrated threaded receiving cavity or nut fixed to the slide near its distal end. The threaded cavity or nut receives a worm-gear lead screw  86  driven by a slide radial oscillator motor  88  mounted on the carousel support plate  66 . When the motor  88  turns the lead screw  86 , the slide  80  moves radially. The four motors  88  are independently operable to independently move the four slides  80  along the radial slots  72 .  
      Referring to  FIGS. 2A and 2B , three pneumatic actuators  74  are mounted on each slide  80 . The three pneumatic actuators  74  are connected by an arm  130  (shown in phantom in  FIG. 2A ) to the carrier drive shaft assembly  78 . Each pneumatic actuator  74  controls the vertical position of a corner of the arm  130 . The pneumatic actuators  74  are connected to a common control system and undergo identical vertical motion so that the arm  130  is maintained in a substantially horizontal position.  
      Referring to  FIG. 3 , each carrier head assembly  70   a - 70   d  includes the previously mentioned carrier head  200 , pneumatic actuators  74  (only one is shown due to the cross-sectional view), carrier motor  76  and drive shaft assembly  78 . The drive shaft assembly  78  includes a spline shaft  92 , an upper spline nut  94 , a lower spline nut  96 , and an adaptor flange  150 . Each carrier head assembly  70   a - 70   d  further includes a drive shaft housing  90 . The carrier motor  76  may be secured to the drive shaft housing  90 , and the pneumatic actuators  74  and the drive shaft housing  90  may be secured to the slide  80 . Alternately, the carrier motor  76 , the pneumatic actuators  74 , and the drive shaft housing  90  may be secured to a carrier support plate (not shown), and the carrier support plate may be attached to the slide  80 . The drive shaft housing  90  holds the upper spline nut  94  by means of a pair of upper ball bearings  100 ,  102 . Similarly, the lower spline nut  96  is held by a pair of lower ball bearings  104 ,  106 . The ball bearings permit the spline shaft  92 , and the spline nuts  94  and  96  to rotate with respect to the drive shaft housing  90 , while holding the spline nuts  96  and  94  in a vertically fixed position. A cylindrical tube  108  may be located between the ball bearings  102  and  104  to connect the upper spline nut  94  to the lower spline nut  96 . The spline shaft  92  passes through the spline nuts  94  and  96  to support the carrier head  200 . The spline nuts  94  and  96  hold the spline shaft  92  in a laterally fixed position, but allow the spline shaft  92  to slide vertically. The adaptor flange  150  is secured to the lower end of the spline shaft  92 . The distance between the upper ball bearings  100 ,  102  and the lower ball bearings  104 ,  106  is sufficient to substantially prevent the spline shaft from pivoting under an applied side load from the carrier head. In addition, the ball bearings provide a low-friction rotary coupling. In combination, the ball bearings and the spline shaft help prevent the spline nuts from frictionally “sticking” to the drive shaft housing as a result of the side load.  
      Referring to  FIG. 4 , an outer cylindrical surface  110  of the spline shaft  92  includes three or more projections or tabs  112  which fit into corresponding recesses  116  in an inner cylindrical surface  114  of the spline nut  96 . Thus, the spline shaft  92  is rotationally fixed but is free to move vertically relative to the spline nut  96 . A suitable spline shaft assembly is available from THK Company, Limited, of Tokyo, Japan.  
      Returning to  FIG. 3 , a first gear  120  is connected to a portion of the upper spline nut  94  which projects above the drive shaft housing  90 . A second gear  122  is driven by the carrier motor  76  and meshes with the first gear  120 . Thus, the carrier motor  76  may drive the second gear  122 , which drives the first gear  120 , which drives the upper spline nut  94 , which in turn drives the spline shaft  92  and the carrier head  200 . The gears  120  and  122  may be enclosed by a housing  124  to protect them from slurry or other contaminants from the chemical mechanical polishing apparatus.  
      The carrier motor  76  may be affixed to the drive shaft housing  90  or to the carrier support plate. The carrier motor  76  may extend through an aperture in the carousel support plate  66  (see  FIG. 2B ). Advantageously, in order to maximize usage of available space and reduce the size of the polishing apparatus, the carrier motor  76  is positioned adjacent to the drive shaft assembly  78  in the radial slot  72 . A splash guard  126  may be connected to the underside of the carousel support plate  66  to prevent slurry from contaminating the carrier motor  76 .  
      The arm  130  is connected to the spline shaft  92 . The arm  130  includes a circular aperture  136 , and the spline shaft  92  projects above the upper spline nut  94  and through the aperture  136  in the arm  130 . The arm  130  holds the spline shaft  92  with an upper ring bearing  132  and a lower ring bearing  134 . The inner races of the ring bearings  132  and  134  are secured to the spline shaft  92  and the outer races of the ring bearings are secured to the arm  130 . Thus, when the pneumatic actuators  74  lift or lower the arm  130 , the spline shaft  92  and the carrier head  200  undergo a similar motion. To load the substrate  10  against the surface of the polishing pad  32 , the pneumatic actuators  74  lower the carrier head  200  until the substrate is pressed against the polishing pad. The pneumatic actuators  74  also control the vertical position of the carrier head  200  so that it may be lifted away from the polishing pad  32  during the transfer of the substrate between the polishing stations  25   a - 25   c  and the transfer station  27 .  
      The substrate is typically subjected to multiple polishing steps, including a main polishing step following a final polishing step. For the main polishing step, usually performed at station  25   a , the polishing apparatus may apply a force of approximately four to ten pounds per square inch (psi) to the substrate. At subsequent stations, the polishing apparatus may apply more or less force. For example, for a final polishing step, usually performed at station  25   c , the carrier head  200  may apply a force of about three psi. The carrier motor  76  rotates the carrier head  200  at about 30 to 200 revolutions per minute. The platen  30  and the carrier head  200  may rotate at substantially the same rate.  
      Referring to  FIGS. 3 and 4 , a bore  142  is formed through the length of the spline shaft  92 . Two cylindrical tubes  144   a  and  144   b  are positioned in the bore  142  to create, for example, three concentric cylindrical channels. As such, the spline shaft  92  may include, for example, an outer channel  140   a , a middle channel  140   b , and an inner channel  140   c . Various struts or cross-pieces (not shown) may be used to hold the tubes  144   a  and  144   b  in place inside the bore  142 . A rotary coupling  146  at the top of the spline shaft  92  couples three fluid lines  148   a ,  148   b  and  148   c  to the three channels  140   a ,  140   b  and  140   c , respectively. Three pumps  149   a ,  149   b  and  149   c  may be connected to the fluid lines  140   a ,  140   b  and  140   c , respectively. Channels  140   a - 140   c  and pumps  149   a - 149   c  are used, as described in more detail below, to pneumatically power the carrier head  200  and to vacuum chuck the substrate to the bottom of the carrier head  200 .  
      Referring to  FIG. 5 , the adaptor flange  150  is detachably connected to the bottom of the spline shaft  92 . The adaptor flange  150  is a generally bowl-shaped body having a base  152  and a circular wall  154 . Three passages  156   a - 156   c  (passage  156   a  is shown in phantom in this cross-sectional view) extend from an upper surface  158  to a lower surface  160  of the base  152  of the adaptor flange  150 . The upper surface  158  of the base  152  may include a circular depression  162  and its lower surface  160  may include a lower hub portion  164 . The lowermost end of the spline shaft  92  fits into the circular depression  162 .  
      A generally annular connector flange  170  may be joined to the lower portion of the spline shaft  92 . The connector flange  170  includes two passages  172   a  and  172   b  (passage  172   b  is shown in phantom in this cross-sectional view). Two horizontal passages  174   a  and  174   b  extend through the spline shaft  92  to connect the channels  140   a  and  140   b  to the passages  172   a  and  172   b.    
      To connect the adaptor flange  150  to the spline shaft  92 , three dowel pins  180  (only one is shown due to the cross-sectional view) are placed into matching recesses  182  in the upper surface  158  of the adaptor flange  150 . Then the adaptor flange  150  is lifted so that the dowel pins  180  fit into matching receiving recesses  184  in the connector flange  170 . This circumferentially aligns passages  172   a  and  172   b  with passages  156   a  and  156   b , respectively, and aligns channel  140   c  with passage  156   c . The adaptor flange  150  may then be secured to the connector flange  170  with screws (not shown).  
      The circular wall  154  of adaptor flange  150  prevents slurry from contacting the spline shaft  92 . A flange  190  may be connected to the drive shaft housing  90  and the circular wall  154  may project into a gap  192  between the flange  190  and the drive shaft housing  90 .  
      The carrier head  200  includes a housing flange  202 , a carrier base  204 , a gimbal mechanism  206 , a retaining ring  208 , and a flexible membrane  210 . The housing flange  202  is connected to the adaptor flange  150  at the bottom of the drive shaft assembly  72 . The carrier base  204  is pivotally connected to the housing flange  202  by the gimbal mechanism  206 . The carrier base  204  is also connected to the adaptor flange  150  to rotate therewith about an axis of rotation which is substantially perpendicular to the surface of the polishing pad  32 . The flexible membrane  210  is connected to the carrier base  204  and defines three chambers, including a circular central chamber  212 , an annular middle chamber  214  surrounding the central chamber  212 , and an annular outer chamber  216  surrounding the annular middle chamber  214 . Pressurization of the chambers  212 ,  214  and  216  controls the downward pressure of the substrate against the polishing pad  32 . Each of these elements will be explained in greater detail below.  
      The housing flange  202  is generally annular in shape and may have approximately the same diameter as the adaptor flange  150 . The housing flange  202  includes three vertical passages  220  (only one of which is shown due to the cross-sectional view) formed at equal angular intervals around the axis of rotation of the carrier head  200 . The housing flange  202  may have a threaded cylindrical neck  260 .  
      The carrier base  204  is a generally disc-shaped body located beneath the housing flange  202 . The diameter of the carrier base  204  is somewhat larger than the diameter of the substrate to be polished. A top surface  222  of the carrier base  204  includes an annular rim  224 , an annular recess  226 , and a turret  228  located in the center on the recess  226 . A bottom surface  230  of the carrier base  204  includes an annular outer depression  232  which will define the edges of the middle chamber  214 . The bottom surface  230  of the carrier base  204  also includes a shallower, annular inner depression  234  which will define a cieling of the inner chamber  212 .  
      The carrier base  204  also includes three passageways  236   a - 236   c  (passage  236   a  is shown in phantom in this cross-sectional view) which extend from an upper surface  238  of the turret  228  to the lower surface  230 . O-rings  239  are placed into recesses in the upper surface  238  and surround the three passageways  236   a - 236   c  to seal the passageways when the carrier head  200  is connected to the adaptor flange  150 .  
      As previously mentioned, the carrier base  204  is connected to the housing flange  202  by the gimbal mechanism  206 . The gimbal mechanism  206  permits the carrier base  204  to pivot with respect to the housing flange  202  so that the carrier base  204  can remain substantially parallel to the surface of the polishing pad. Specifically, the gimbal mechanism permits the carrier base  204  to rotate about a point on the interface between the polishing pad  32  and the substrate  10 . However, the gimbal mechanism  206  holds the carrier base  204  beneath the spline shaft  92  to prevent the carrier base  204  from moving laterally, i.e., parallel to the surface of the polishing pad  32 . The gimbal mechanism  206  also transfers the downward pressure from the spline shaft  92  to the carrier base  204 . Furthermore, the-gimbal mechanism  206  can transfer any side load, such as the sheer force created by the friction between the substrate and the polishing pad  32 , to the housing flange  202  and drive shaft assembly  78 .  
      An annular biasing flange  240  with an inwardly projecting lip  242  is fixed to the carrier base  204 . The biasing flange  240  may be bolted to the carrier base  204  in the annular recess  226 .  
      The gimbal mechanism  206  includes an inner race  250 , an outer race  252 , a retainer  254 , and multiple ball bearings  256 . There may be twelve ball bearings  256 , although only two are shown in this cross-sectional view. The inner race  250  is secured to or formed as part of the carrier base  204  and is located in the recess  226  adjacent the turret  228 . The outer race  252  is secured to or formed as part of the housing flange  202  and includes an outwardly-projecting lip  258  which extends beneath the inwardly-projecting lip  242  of the biasing flange  240 . An annular spring washer  244  fits in the gap between the inwardly projecting lip  242  and the outwardly projecting lip  258 . The washer  244  biases the inner race  250  and outer race  252  into contact with the ball bearings  256 . The retainer  254  is a generally annular-shaped body having a plurality of circular apertures. The ball bearings  256  fit into the apertures in the retainer  254  to be held in place in the gap between the inner race  250  and the outer race  252 .  
      To connect the carrier head  200  to the adaptor flange  150 , three vertical torque transfer pins  262  (only one of which is shown in this cross-sectional view) are inserted through the passages  220  in the housing flange  202  and into three receiving recesses  264  in the carrier base  204  or the biasing flange  240 . Then the carrier head  200  is lifted so that the. vertical torque transfer pins  262  are fitted into three receiving recesses  266  in the adaptor flange  150 . This aligns the passages  156   a - 156   c  in the adaptor flange  150  with the passageways  236   a - 236   c , respectively, in the carrier base  204 . A lower hub  178  of the adaptor flange  150  contacts the upper surface  239  of the turret  228 . Finally, a threaded perimeter nut  268  can fit over an edge  269  of the adaptor flange  150  and be screwed onto the threaded neck  260  of the housing flange  202  to firmly secure the carrier head  200  to the adaptor flange  150  and thus to the drive shaft assembly  78 . The rim  224  of the carrier base  204  may fit into an annular recess  259  in the lower surface of the perimeter nut  268 . This creates a restricted pathway that prevents slurry from contaminating the gimbal mechanism  206  or the spring washer  244 .  
      The retaining ring  208  may be secured at the outer edge of the carrier base  204 . The retaining ring  208  is a generally annular ring having a substantially flat bottom surface  270 . When the pneumatic actuators  74  lower the carrier head  200 , the retaining ring  208  contacts the polishing pad  32 . An inner surface  272  of the retaining ring  208  defines, in conjunction with the bottom surface of the flexible membrane  210 , a substrate receiving recess  274 . The retaining ring  208  prevents the substrate from escaping the substrate receiving recess  274  and transfers the lateral load from the substrate to the carrier base  204 .  
      The retaining ring  208  may be made of a hard plastic or ceramic material. The retaining ring  208  may be secured to the carrier base  204  by, for example, a retaining piece  276  which is secured, for example, to the carrier base  204  by bolts  278 .  
      The flexible membrane  210  is connected to and extends beneath the carrier base  204 . The bottom surface of the flexible membrane  210  provides a substrate receiving surface  280 . In conjunction with the base  204 , the flexible membrane  210  defines the central chamber  212 , the annular middle chamber  214 , and the annular outer chamber  216 . The flexible membrane  210  is a generally circular sheet formed of a flexible and elastic material, such as a high strength silicone rubber. The substrate backing membrane  210  includes an inner annular flap  282   a , a middle annular flap  282   b , and an outer annular flap  282   c . The flaps  282   a - 282   c  are generally concentric. The flaps  282   a - 282   c  may be formed by stacking three separate flexible membranes and bonding the central portions of the membranes so as to leave the outer annular portions of each membrane free. Alternatively, the entire flexible membrane  210  may be extruded as a single part.  
      An annular lower flange  284  may be secured in a depression  232  on the bottom surface  230  of the carrier base  204 . The lower flange  284  includes an inner annular groove  286  and an outer annular groove  287  on its upper surface. A passage  288  may extend through the lower flange  284  and connect to passageway  236 b. The lower flange  284  may also include an annular indentation  289  on its lower surface. The inner flap  282   a , the middle flap  282   b , and the outer flap  282   c  may each include a protruding outer edge  290   a ,  290   b  and  290   c , respectively. To secure the flexible membrane  210  to the carrier base  204 , the inner flap  282   a  is wrapped around the inner edge of the lower flange  284  so that its protruding edge  290   a  fits into the inner groove  286 , and the middle flap  282   b  is wrapped around the outer edge of the lower flange  284  so that its protruding edge  290   b  fits into the outer groove  287 . Then the lower flange  284  is secured in depression  232  by screws (not shown) which may extend from the top surface  222  of the carrier base  204 . The inner and middle flaps  282   a  and  282   b  are thus clamped between the lower flange  284  and the carrier base  204  to seal the inner and middle chambers  212  and  214 . Finally, the outer edge of  290   c  of outer flap  282   c  is clamped between the retaining ring  208  and the carrier base  204  to seal the outer chamber  216 .  
      Pump  149   a  (see  FIG. 3 ) may be connected to the inner chamber  212  by the fluid line  148   a , the rotary coupling  146 , the inner channel  140   a  in the spline shaft  92 , the passage (not shown) in the adaptor flange  150 , and the passageway  236   c  (not shown) through the carrier base  204 . Pump  149   b  may be connected to the middle chamber  214  by the fluid line  148   b , the rotary coupling  146 , the middle channel  140   b , the passage (not shown) in the adaptor flange  150 , the passageway  236   b  in the carrier base  204 , and the passage  288  in the lower flange  284 . Pump  149   c  may be connected to the outer chamber  216  by the fluid line  148   c , the rotary coupling  146 , the outer channel  140   c , the passage  156   c  in the adaptor flange  150 , and the passageway  236   c  in the carrier base  204 . If a pump forces a fluid, preferably a gas such as air, into one of the chambers, then the volume of that chamber will increase and a portion of the flexible membrane  210  will be forced downwardly or outwardly. On the other hand, if the pump evacuates a fluid from the chamber, then the volume of the chamber will decrease and a portion of the flexible membrane will be drawn upwardly or inwardly.  
      The flexible membrane  210  may include a circular inner portion  292 , an annular middle portion  294 , and an annular outer portion  296  located beneath the inner chamber  212 , middle chamber  214 , and outer chamber  216 , respectively (see also  FIG. 6 ). As such, the pressures in chambers  212 ,  214  and  216  can control the downward pressure applied by the respective flexible membrane portions  292 ,  294  and  296 .  
      The flexible membrane portions may have different dimensions. The majority of the edge effect occurs at the outer-most six to eight millimeters of the substrate. Therefore, the annular outer membrane portion  296  may be fairly narrow in the radial direction in comparison to the annular middle membrane portion  294  in order to provide pressure control of a narrow edge region at the edge of the substrate which is independent of the pressures applied to the center and middle portions of the substrate.  
      Referring to  FIG. 6 , the inner membrane portion  292  may have a radius R 1 , the middle membrane portion  294  may have an outer radius R 2 , and the outer membrane portion  296  may have an outer radius R 3 . The width W 1  of the middle membrane portion  294  may be equal to R 2 -R 1 , and width W 2  of the outer membrane portion  296  may be equal to R 3 -R 2 . The radius R 3  may be equal to or greater than about 100 mm (for a 200 mm diameter substrate), and the width W 2  may be between five and thirty millimeters. If the radius R 3  is 5.875 inches (for a 300 mm diameter substrate), the widths W 1  and W 2  may be 2.375 inches and 0.625 inches, respectively. In this configuration, the radii R 1  and R 2  are 2.875 and 5.25 inches, respectively.  
      The pressures in chambers  212 ,  214  and  216  may be independently controlled by pumps  149   a ,  149   b  and  149   c  to maximize the uniformity of polishing of the substrate  10 . The average pressure in outer chamber  216  may be lower than the average pressure in the other two chambers so that the pressure on the outer annular membrane portion  296  is lower than the pressure on the inner membrane portion  292  or the middle membrane portion  294  during polishing so as to compensate for the over-polishing created by the edge effect.  
      The flexible membrane  210  deforms to match the backside of the substrate  10 . For example, if the substrate is warped, the flexible membrane  210 , will in effect, conform to the contours of the warped substrate. Thus, the load on the substrate should remain uniform even if there are surface irregularities on the back side of the substrate.  
      Rather than applying a different pressure to each chamber, the time during which a positive pressure is applied to each chamber may be varied. In this fashion, uniform polishing may be achieved. For example, rather than apply a pressure of 8.0 psi to the inner chamber  212  and the middle chamber  214  and a pressure of 6.0 psi to the outer chamber  216 , a pressure of 8.0 psi may be applied to the inner chamber  212  and the middle chamber  214  for one minute while the same pressure is applied to the outer chamber  216  for forty-five seconds. This technique permits pressure sensors and pressure regulators to be replaced by simple software timing controls. In addition, the technique may allow for a more accurate process characterization and consequently better uniformity in polishing the substrate.  
      The carrier head  200  can vacuum-chuck the substrate  10  to the underside of the flexible membrane  210 . As such, the pressure in the middle chamber  214  is reduced as compared to the pressure in the other chambers and this causes the middle membrane portion  294  of the flexible membrane  210  to bow inwardly. The upward deflection of the middle membrane portion  294  creates a low pressure pocket between the flexible membrane  210  and the substrate  10 . This low pressure pocket will vacuum-chuck the substrate  10  the carrier head. It is advantageous to use the middle membrane portion  294  as opposed to the inner membrane portion  292  in order to avoid bowing the center of the substrate, which can create a low pressure pocket between the substrate and the polishing pad. Such a low pressure pocket would tend to vacuum-chuck the substrate to the polishing pad. In addition, the pressure in the outer chamber  2116  may be increased while the pressure in the middle chamber  214  is reduced. An increased pressure in the outer chamber  216  forces the outer membrane portion  296  against the substrate  10  to effectively form a fluid-tight seal. This seal can prevent ambient air from entering the vacuum between the middle membrane portion  294  and the substrate. The outer chamber  216  may be pressurized for only a short period of time, for example, less than a second, while the vacuum pocket is being created, as this appears to provide the most reliable vacuum-chucking procedure.  
      The polishing apparatus  20  may operate as follows. The substrate  10  is loaded into the substrate receiving recess  274  with the backside of the substrate abutting the flexible membrane  210 . The pump  149   a  pumps fluid into the outer chamber  216 . This causes the outer membrane portion  296  to form a fluid-tight seal at the edge of the substrate  10 . Simultaneously, pump  149   b  pumps fluid out of the middle chamber  214  to create a low pressure pocket between the flexible membrane  210  and the backside of the substrate  10 . The outer chamber  216  is then quickly returned to normal atmospheric pressure. Finally, the pneumatic actuators  74  lift the carrier head  200  off of the polishing pad  32  or out of the transfer station  27 . The carousel  60  rotates the carrier head  200  to a new polishing station. The pneumatic actuators  74  then lower the carrier head  200  until the substrate  10  contacts the polishing pad  32 . Finally, the pumps  149   a - 149   c  force fluid into the chambers  212 ,  214  and  216  to apply a downward load to the substrate  10  for polishing.  
      The present invention is described in terms of the preferred embodiment. The invention, however, is not limited to the embodiments depicted and described herein. Rather, the scope of the invention is defined by the appended claims.