Patent Publication Number: US-6705924-B2

Title: Carrier head with a substrate detection mechanism for a chemical mechanical polishing system

Description:
This application is a continuation of U.S. application Ser. No. 09/989,498, filed Nov. 19, 2001, now U.S. Pat. No. 6,517,415 which is a continuation of U.S. application Ser. No. 09/595,500, filed Jun. 16, 2000, now U.S. Pat. No. 6,343,973, which is a continuation of U.S. application Ser. No. 09/314,462, filed May 18, 1999, now U.S. Pat. No. 6,093,082, which is a divisional of U.S. application Ser. No. 08/862,350, filed May 23, 1997, now U.S. Pat. No. 5,957,751. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting the presence of a substrate in a carrier head of 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. Therefore, the substrate surface is periodically planarized 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 affect the relative velocity distribution over the surface of the substrate. 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. 
     Typically, the carrier head is used to remove the substrate from the polishing pad after the polishing process has been completed. The substrate is vacuum-chucked to the underside of the carrier head. When the carrier head is retracted, the substrate is lifted off the polishing pad. 
     One problem that has been encountered in CMP is that the substrate may not be lifted by the carrier head. For example, if the surface tension binding the substrate to the polishing pad is greater than the force binding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. Also, if a defective substrate fractures during polishing, then the carrier head may be unable to remove the fractured substrate from the polishing pad. 
     A related problem that has been encountered in CMP is that the attachment of the substrate to the carrier head may fail, and the substrate may detach from the carrier head. This may occur if, for example, the substrate was attached to the carrier head by surface tension alone, rather than in combination with vacuum-chucking. 
     As such, the operator may not know that the carrier head no longer carries the substrate. The CMP apparatus will continue to operate even though the substrate is no longer present in the carrier head. This may decrease throughput. In addition, a loose substrate, i.e., one not attached to a carrier head, may be knocked about by the moving components of the CMP apparatus, potentially damaging the substrate or the polishing pad, or leaving debris which may damage other substrates. 
     Another problem encountered in CMP is the difficulty of determining whether the substrate is present in the carrier head. Because the substrate is located beneath the carrier head, it is difficult to determine by visual inspection whether the substrate is present in and properly attached to the carrier head. In addition, optical detection techniques are impeded by the presence of slurry. 
     A conventional carrier head may include a rigid base. The base has a bottom surface which serves as a substrate receiving surface. Multiple channels extend through the base to the substrate receiving surface. A pump or vacuum source can apply a vacuum to the channels. When air is pumped out of the channels, the substrate will be vacuum-chucked to the bottom surface of the carrier head. A pressure sensor may be connected to a pressure line between the vacuum source and the channels in the carrier head. If the substrate was not successfully vacuum-chucked to the underside of the carrier head, then the channels will be open and air or other fluid will leak into the channels. On the other hand, if the substrate was successfully vacuum-chucked to the underside of the carrier head, then channels will be sealed and air will not leak into the channels. Consequently, the pressure sensor will measure a higher vacuum or lower pressure when the substrate is successfully vacuum-chucked to the underside of the carrier head as compared to when the substrate is not properly attached to the carrier head. 
     Unfortunately, there are several problems with this method of detecting the presence of a substrate in the carrier head. Corrosive slurry may be suctioned into the channels and contaminate the carrier head. In addition, the threshold pressure for determining whether the substrate has been lifted from the polishing pad must be determined experimentally. 
     Accordingly, it would be useful to provide a CMP system capable of reliably sensing the presence of a substrate in a carrier head. It would also be useful if such a system could operate without exposing the interior of the carrier head to contamination by a slurry. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention is directed to a carrier head for a chemical mechanical polishing system. The carrier head includes a base and a flexible member connected to the base to define a chamber. A lower surface of the flexible member provides a substrate receiving surface. There is an aperture in the flexible member between the substrate receiving surface and the chamber. 
     Implementation of the invention may include the following. The aperture may be configured such that if a substrate is attached to the substrate receiving surface, the substrate blocks the aperture. If fluid is forced into or evacuated from the chamber and a substrate is attached to the substrate receiving surface, a pressure in the chamber may reach a first pressure which is different than a second pressure that would result if the substrate were not attached to the substrate receiving surface. The carrier head may be part of an assembly including a vacuum source connected to the chamber, a sensor to measure a pressure in the chamber and generate an output signal representative thereof, and a processor configured to indicate whether the substrate is attached to the substrate receiving surface in response to the output signal. The processor may be configured to indicate that the substrate is attached to the substrate receiving surface if the pressure in the chamber is greater than a threshold pressure. 
     In another aspect, the carrier head includes a base, a flexible member connected to the base to define a chamber, a first passage in the base connecting the chamber to the ambient atmosphere and a second passage in the base connecting the chamber to a passage in a drive shaft. A lower surface of the flexible member provides a substrate receiving surface. 
     Implementations of the invention may include the following. The second passage may be positioned such that, if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to block the second passage so that a pressure in the second passage drops to a first pressure which is less than a second pressure that would result if the substrate were attached to the substrate receiving surface. The carrier head may include a check valve in the first passage to prevent fluid from exiting the chamber through the first passage. The carrier head may include a mechanically actuatable valve across the first passage, the valve configured such that if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to actuate the valve. 
     In another aspect, the carrier head includes a base, a first flexible member connected to the base to define a first chamber, a second chamber in the base, and a valve across a passage between the first chamber and the second chamber. A lower surface of the first flexible member provides a substrate receiving surface. 
     Implementations of the invention include the following The valve may be configured such that if fluid is evacuated from the first chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects to actuate the valve so that a pressure in the second chamber reaches a first pressure which is different from, e.g., less than, a second pressure that would result if the substrate were attached to the substrate receiving surface. A second flexible member may define the second chamber. The second flexible member may be positioned above the first flexible member, and an upward motion of the first flexible member may exert a force on the second flexible member. A pressure source may be connected to the second chamber to pressurize the second chamber. A pressure sensor may measure the pressure in the second chamber at a first time and a second time and generate output signals representative thereof, and a processor may be configured to indicate whether the substrate is attached to the carrier head in response to the output signals. A second valve may isolate the pressure source from the second chamber. 
     In another aspect, the invention is directed to a carrier head including a base, a first flexible member connected to the base to define a first chamber, a second flexible member connected to the base to define a second chamber, and a passage in the base connecting the chamber to a passage in a drive shaft. The first flexible member exerts a force on the second flexible member. The passage in the base is positioned such that if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to block the second passage so that a first force on the second flexible member is different than a second force that would result if the substrate were attached to the substrate receiving surface. 
     Advantages of the invention include the following. The CMP apparatus includes a sensor to detect whether the substrate is present or properly attached to the carrier head. The interior of the carrier head is not exposed to slurry. The sensor is able to detect whether a substrate is held on the carrier head by surface tension rather than by vacuum. 
     Other advantages and features of the invention become apparent from the following description, including the drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a chemical mechanical polishing apparatus. 
     FIG. 2 is a schematic top view of a carousel, with the upper housing removed. 
     FIG. 3 is partially a cross-sectional view of the carousel of FIG. 2 along line  3 — 3 , and partially a schematic diagram of the pressure regulators used by the CMP apparatus. 
     FIG. 4 is a schematic cross-sectional view of a carrier head with a flexible membrane and a chamber in accordance with the present invention. 
     FIG. 5A is a schematic cross-sectional view of a carrier head with a vented chamber in accordance with the present invention. 
     FIG. 5B is a view of the carrier head of FIG. 5A without an attached substrate. 
     FIG. 6A is a schematic cross-sectional view of a carrier head with a valve connecting the chamber to a bladder in accordance with the present invention. 
     FIG. 6B is a view of the carrier head of FIG. 6A without an attached substrate. 
     FIG. 7 is a schematic cross-sectional view of a carrier head with a valve connecting the chamber to ambient atmosphere in accordance with the present invention. 
     FIGS. 8A and 8G is are graphs showing pressure as a function of time in a CMP apparatus using the carrier head of FIG.  4 . 
     FIGS. 8B and 8C are graphs showing pressure as a function of time in a CMP apparatus using the carrier head of FIG.  5 A. 
     FIGS. 8D and 8E are graphs showing pressure as a function of time in a CMP apparatus using the carrier head of FIG.  6 A. 
     FIG. 8F is a graph showing pressure as a function of time in a CMP apparatus using the carrier head of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     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 pending U.S. patent application Ser. No. 08/549,336, by Perlov, et al., filed Oct. 27, 1995, 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. 
     CMP apparatus  20  includes a lower machine base  22  with a table top  23  mounted thereon and a removable upper outer cover (not shown) Table top  23  supports a series of polishing stations  25   a ,  25   b  and  25   c , and a transfer station  27 . Transfer station  27  may form a generally square arrangement with the three polishing stations  25   a ,  25   b  and  25   c . Transfer station  27  serves multiple functions of receiving 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 substrate  10  is an eight-inch (200 mm) diameter disk, then platen  30  and polishing pad  32  will be about twenty inches in diameter. Platen  30  may be a rotatable plate connected by a platen drive shaft (not shown) to a platen drive motor (also not shown). For most polishing processes, the drive motor rotates platen  30  at about thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. 
     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 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 polishing pad  32  by a combined slurry/rinse arm  52 . Sufficient slurry is provided to cover and wet the entire polishing pad  32 . Slurry/rinse arm  52  includes several spray nozzles (not shown) which provide a high pressure rinse of polishing pad  32  at the end of each polishing and conditioning cycle. 
     Two or more intermediate washing stations  55   a  and  55   b  may be positioned between neighboring polishing stations  25   a ,  25   b  and  25   c . The washing stations rinse the substrates as they pass from one polishing station to another. 
     A rotatable multi-head carousel  60  is positioned above lower machine base  22 . Carousel  60  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly located within base  22 . Center post  62  supports a carousel support plate  66  and a cover  68 . Multi-head carousel  60  includes four carrier head systems  70   a ,  70   b ,  70   c , and  70   d . Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pad  32  on platen  30  of polishing stations  25   a - 25   c . One of the carrier head systems receives a substrate from and delivers the substrate to transfer station  27 . 
     The four carrier head systems  70   a - 70   d  are mounted on carousel support plate  66  at equal angular intervals about carousel axis  64 . 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 carousel axis  64 . 
     Each carrier head system  70   a - 70   d  includes a polishing or carrier head  100 . Each carrier head  100  independently rotates about its own axis, and independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A carrier drive shaft  74  connects a carrier head rotation motor  76  to carrier head  100  (shown by the removal of one-quarter of cover  68 ). There is one carrier drive shaft and motor for each head. 
     Referring to FIG. 2, in which cover  68  of carousel  60  has been removed, carousel support plate  66  supports the four carrier head systems  70   a - 70   d . Carousel support plate includes four radial slots  72 , generally extending radially and oriented 90° apart. Radial slots  72  may either be close-ended (as shown) or open-ended. The top of support plate supports four slotted carrier head support slides  80 . Each slide  80  aligns along one of the radial slots  72  and moves freely along a radial path with respect to carousel support plate  66 . Two linear bearing assemblies bracket each radial slot  72  to support each slide  80 . 
     As shown in FIGS. 2 and 3, each linear bearing assembly includes a rail  82  fixed to carousel support plate  66 , and two hands  83  (only one of which is illustrated in FIG. 3) fixed to slide  80  to grasp the rail. Two bearings  84  separate each hand  83  from rail  82  to provide free and smooth movement therebetween. Thus, the linear bearing assemblies permit slides  80  to move freely along radial slots  72 . 
     A bearing stop  85  anchored to the outer end of one of the rails  82  prevents slide  80  from accidentally coming off the end of the rails. One of the arms of 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  87  mounted on carousel support plate  66 . When motor  87  turns lead screw  86 , slide  80  moves radially. The four motors  87  are independently operable to independently move the four slides along the radial slots  72  in carousel support plate  66 . 
     A carrier head assembly or system, each including a carrier head  100 , a carrier drive shaft  74 , a carrier motor  76 , and a surrounding non-rotating shaft housing  78 , is fixed to each of the four slides. Drive shaft housing  78  holds drive shaft  74  by paired sets of lower ring bearings  88  and a set of upper ring bearings  89 . 
     A rotary coupling  90  at the top of drive motor  76  couples three or more fluid lines  92   a ,  92   b  and  92   c  to three or more channels  94   a ,  94   b  and  94   c , respectively, in drive shaft  74 . Three vacuum or pressure sources, such as pumps, venturis or pressure regulators (hereinafter collectively referred to simply as “pumps”)  93   a ,  93   b  and  93   c  may be connected to fluid lines  92   a ,  92   b  and  92   c , respectively. Three pressure sensors or gauges  96   a ,  96   b  and  96   c  may be connected to fluid lines  92   a ,  92   b  and  92   c , respectively. Controllable valves  98   a ,  98   b  and  98   c  may be connected across the fluid lines between pressure gauges  96   a ,  96   b  and  96   c  and pumps  93   a ,  93   b  and  93   c , respectively. Pumps  93   a - 93   c , pressure gauges  96   a - 96   c  and valves  98   a - 98   c  may be appropriately connected to a general-purpose digital computer  99 . Computer  99  may operate pumps  93   a - 93   c , as described in more detail below, to pneumatically power carrier head  100  and to vacuum-chuck a substrate to the bottom of the carrier head. In addition, computer  99  may operate valves  98   a - 98   c  and monitor pressure gauges  96   a - 96   c , as described in more detail below, to sense the presence of the substrate in the carrier head. In the various embodiments of the carrier head described below, the pumps remain coupled to the same fluid lines, although the function or purpose of the pumps may change. 
     During actual polishing, three of the carrier heads, e.g., those of carrier head systems  70   a - 70   c , are positioned at and above respective polishing stations  25   a - 25   c . Carrier head  100  lowers a substrate into contact with polishing pad  32 , and slurry  50  acts as the media for chemical mechanical polishing of the substrate or wafer. 
     Generally, carrier head  100  holds the substrate against the polishing pad and evenly distributes a force across the back surface of the substrate. The carrier head also transfers torque from the drive shaft to the substrate and ensures that the substrate does not slip from beneath the carrier head during polishing. 
     Referring to FIG. 4, carrier head  100  includes a housing  102 , a base  104 , a gimbal mechanism  106 , a loading mechanism  108 , a retaining ring  110 , and a substrate backing assembly  112 . A more detailed description of a similar carrier head may be found in pending U.S. patent application Ser. No. 08/745,670 by Zuniga, et al., filed Nov. 8, 1996, entitled A CARRIER HEAD WITH A FLEXIBLE MEMBRANE FOR A CHEMICAL MECHANICAL POLISHING SYSTEM, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference. 
     The housing  102  is connected to drive shaft  74  to rotate therewith about an axis of rotation  107  which is substantially perpendicular to the surface of the polishing pad. The loading mechanism  108  is positioned between housing  102  and base  104  to apply a load, i.e., a downward pressure, to base  104 . The vertical position of base  104  relative to polishing pad  32  is also controlled by loading mechanism  108 . Pressurization of a chamber  290  positioned between base  104  and substrate backing assembly  112  generates an upward force on the base and a downward force on the substrate backing assembly. The downward force on the substrate backing assembly presses the substrate against the polishing pad. 
     The substrate backing assembly  112  includes a support structure  114 , a flexure  116  connected between support structure  114  and base  104 , and a flexible membrane  118  connected to support structure  114 . The flexible membrane  118  extends below support structure  114  to provide a mounting surface  274  for the substrate. Each of these elements will be explained in greater detail below. 
     Housing  102  is generally circular in shape to correspond to the circular configuration of the substrate to be polished. The housing includes an annular housing plate  120  and a generally cylindrical housing hub  122 . Housing hub  122  may include an upper hub portion  124  and a lower hub portion  126 . The lower hub portion may have a smaller diameter than the upper hub portion. The housing plate  120  may surround lower hub portion  126  and be affixed to upper hub portion  124  by bolts  128 . 
     An annular cushion  121  may be attached, for example, by an adhesive, to an upper surface  123  of housing plate  120 . As discussed below, the cushion acts as a soft stop to limit the downward travel of base  104 . 
     Base  104  is a generally ring-shaped body located beneath housing  102 . A lower surface  150  of base  104  includes an annular recess  154 . A passage  156  may connect a top surface  152  of base  104  to annular recess  154 . A fixture  174  may be inserted into passage  152 , and a flexible tube (not shown) may connect fixture  133  to fixture  174 . The base  104  may be formed of a rigid material such as aluminum, stainless steel or fiber-reinforced plastic. 
     A bladder  160  may be attached to lower surface  150  of base  104 . Bladder  160  may include a membrane  162  and a clamp ring  166 . Membrane  162  may be a thin annular sheet of a flexible material, such as a silicone rubber, having protruding edges  164 . The clamp ring  166  may be an annular body having a T-shaped cross-section and including wings  167 . A plurality of tapped holes, spaced at equal angular intervals, are located in the upper surface of the clamp ring. The holes may hold bolts or screws to secure the clamp ring to the base. To assemble bladder  160 , protruding edges  164  of membrane  162  are fit above wings  167  of clamp ring  166 . The entire assembly is placed in annular recess  154 . Clamp ring  166  may be secured to base  104  by screws  168  (not shown in FIG. 4, but one screw is shown on the left hand side of the cross-sectional view of FIG.  6 A). Clamp ring  166  seals membrane  162  to base  104  to define a volume  170 . A vertical passage  172  extends through clamp ring  166  and is aligned with passage  152  in base  104 . An O-ring  178  may be used to seal the connection between passage  156  and passage  172 . 
     Pump  93   b  (see FIG. 3) may be connected to bladder  160  via fluid line  92   b , rotary coupling  90 , channel  94   b  in drive shaft  74 , passage  132  in housing  102 , the flexible tube (not shown), passage  152  in base  104 , and passage  172  in clamp ring  166 . If pump  93   b  forces a fluid, for example a gas, such as air, into volume  170 , then bladder  160  will expand downwardly. On the other hand, if pump  93   b  evacuates fluid from volume  170 , then bladder  160  will contract. As discussed below, bladder  160  may be used to apply a downward pressure to support structure  114  and flexible membrane  118 . 
     Gimbal mechanism  106  permits base  104  to move with respect to housing  102  so that the base may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism  106  includes a gimbal rod  180  and a flexure ring  182 . The upper end of gimbal rod  180  fits into a passage  188  through cylindrical bushing  142 . The lower end of gimbal rod  180  includes an annular flange  184  which is secured to an inner portion of flexure ring  182  by, for example, screws  187 . The outer portion of flexure ring  182  is secured to base  104  by, for example, screws  185  (not shown in FIG. 4, but one screw is shown in the left hand side of the cross-sectional view of FIG.  6 A). Gimbal rod  180  may slide vertically along passage  188  so that base  104  may move vertically with respect to housing  102 . However, gimbal rod  180  prevents any lateral motion of base  104  with respect to housing  102 . 
     Gimbal mechanism  106  may also include a vertical passage  196  formed along the central axis of gimbal rod  180 . Passage  196  connects upper surface  134  of housing hub  122  to chamber  290 . O-rings  198  may be set into recesses in bushing  142  to provide a seal between gimbal rod  180  and bushing  142 . 
     The vertical position of base  104  relative to housing  102  is controlled by loading mechanism  108 . The loading mechanism includes a chamber  200  located between housing  102  and base  104 . Chamber  200  is formed by sealing base  104  to housing  102 . The seal includes a diaphragm  202 , an inner clamp ring  204 , and an outer clamp ring  206 . Diaphragm  202 , which may be formed of a sixty mil thick silicone sheet, is generally ring-shaped, with a flat middle section and protruding edges. 
     Inner clamp ring  204  is used to seal diaphragm  202  to housing  102 . Inner clamp ring  204  is secured to base  104 , for example, by bolts  218 , to firmly hold the inner edge of diaphragm  202  against housing  102 . 
     Outer clamp ring  206  is used to seal diaphragm  202  to base  104 . Outer clamp ring  206  is secured to base  104 , for example, by bolts (not shown), to hold the outer edge of diaphragm  202  against the top surface of base  104 . Thus, the space between housing  102  and base  104  is sealed to form chamber  200 . 
     Pump  93   a  (see FIG. 3) may be connected to chamber  200  via fluid line  92   a , rotary coupling  90 , channel  94   a  in drive shaft  74 , and passage  130  in housing  102 . Fluid, for example a gas, such as air, is pumped into and out of chamber  200  to control the load applied to base  104 . If pump  93   a  pumps fluid into chamber  200 , the volume of the chamber will increase and base  104  will be pushed downwardly. On the other hand, if pump  93   a  pumps fluid out of chamber  200 , the volume of chamber  200  will decrease and base  104  will be pulled upwardly. 
     Outer clamp ring  206  also includes an inwardly projecting flange  216  which extends over housing  102 . When chamber  200  is pressured and base  104  moves downwardly, inwardly projecting flange  216  of outer clamp ring  206  abuts cushion  121  to prevent over-extension of the carrier head. Inwardly projecting flange  216  also acts as a shield to prevent slurry from contaminating components, such as diaphragm  202 , in the carrier head. 
     Retaining ring  110  may be secured at the outer edge of base  104 . Retaining ring  110  is a generally annular ring having a substantially flat bottom surface  230 . When fluid is pumped into chamber  200  and base  104  is pushed downwardly, retaining ring  110  is also pushed downwardly to apply a load to polishing pad  32 . An inner surface  232  of retaining ring  110  defines, in conjunction with mounting surface  274  of flexible membrane  118 , a substrate receiving recess  234 . The retaining ring  110  prevents the substrate from escaping the receiving recess and transfers the lateral load from the substrate to the base. 
     Retaining ring  110  may be made of a hard plastic or a ceramic material. Retaining ring  110  may be secured to base  104  by, for example, bolts  240  (only one is shown in this cross-sectional view). 
     The substrate backing assembly  112  is located below base  104 . Substrate backing assembly  112  includes support structure  114 , flexure.  116  and flexible membrane  118 . The flexible membrane  118  connects to and extends beneath support structure  114 . 
     Support structure  114  includes a support plate  250 , an annular lower clamp  280 , and an annular upper clamp  282 . Support plate  250  may be a generally disk-shaped rigid member. Support plate  250  may have a generally planar lower surface  256  with a downwardly-projecting lip  258  at its outer edge. A plurality of apertures  260  may extend vertically through support plate  250  connecting lower surface  256  to an upper surface  254 . An annular groove  262  may be formed in upper surface  254  near the edge of the support plate. Support plate  250  may be formed of aluminum or stainless steel. 
     Flexible membrane  118  is a circular sheet formed of a flexible and elastic material, such as a high-strength silicone rubber. Membrane  118  may have a protruding outer edge  270 . A portion  272  of membrane  118  extends around a lower corner of support plate  250  at lip  258 , upwardly around an outer cylindrical surface  268  of the support plate, and inwardly along upper surface  254 . Protruding edge  270  of membrane  118  may fit into groove  262 . The edge of flexible membrane  118  is clamped between lower clamp  280  and support plate  250 . A small aperture or plurality of apertures may be formed at the approximate center of membrane  118 . The apertures may be about one to ten millimeters across, and are used, as discussed below, to sense the presence of the substrate. 
     The flexure  116  is a generally planar annular ring. Flexure  116  is flexible in the vertical direction, and may be flexible or rigid in the radial and tangential directions. The material of flexure  116  is selected to have a durometer measurement between 30 on the Shore A scale and 70 on the Shore D scale. The material of flexure  116  may be a rubber such as neoprene, an elastomeric-coated fabric such as NYLON™ or NOMEX™, a plastic, or a composite material such as fiberglass. 
     The space between flexible membrane  118 , support structure  114 , flexure  116 , base  104 , and gimbal mechanism  106  defines chamber  290 . Passage  196  through gimbal rod  180  connects chamber  290  to the upper surface of housing  102 . Pump  93   c  (see FIG. 3) may be connected to chamber  290  via fluid line  92   c , rotary coupling  90 , channel  94   c  in drive shaft  74  and passage  196  in gimbal rod  180 . If pump  93   c  forces a fluid, for example a gas, such as air, into chamber  290 , then the volume of the chamber will increase and flexible membrane  118  will be forced downwardly. On the other hand, if pump  93   c  evacuates air from chamber  290 , then the volume of the chamber will decrease and the membrane will be forced upwardly. It is advantageous to use a gas rather than a liquid because a gas is more compressible. 
     The lower surface of flexible membrane  118  provides a mounting surface  274 . During polishing, substrate  10  is positioned in substrate receiving recess  234  with the backside of the substrate positioned against the mounting surface. The edge of the substrate may contact the raised lip  258  of support ring  114  through flexible membrane  118 . 
     By pumping fluid out of chamber  290 , the center of flexible membrane  118  may be bowed inwardly and pulled above lip  258 . If the backside of the substrate is placed against mounting surface  274 , then the extension of the flexible membrane above lip  258  creates a low-pressure pocket  278  between the substrate and the flexible membrane (see FIGS.  5 A and  6 A). This low-pressure pocket vacuum-chucks the substrate to the carrier head. 
     A CMP apparatus utilizing carrier head  100  may operate as follows. Substrate  10  is loaded into substrate receiving recess  234  with the backside of the substrate abutting mounting surface  274  of flexible membrane  118 . Pump  93   b  pumps fluid into bladder  160 . This causes bladder  160  to expand and force support structure  114  downwardly. The downward motion of support structure  114  causes lip  258  to press the edge of flexible membrane  118  against the edge of substrate  10 , creating a fluid-tight seal at the edge of the substrate. Then pump  93   c  evacuates chamber  290  to create a low-pressure pocket between flexible membrane  118  and the backside of substrate  10  as previously described. Finally, pump  93   a  pumps fluid out of chamber  200  to lift base  104 , substrate backing assembly  112 , and substrate  10  off a polishing pad or out of the transfer station. Carousel  60  then, for example, rotates the carrier head to a polishing station. Pump  93   a  then forces a fluid into chamber  200  to lower the substrate  10  onto the polishing pad. Pump  93   b .evacuates volume  170  so that bladder  160  no longer applies a downward pressure to support structure  114  and flexible membrane  118 . Finally, pump  93   c  may pump a gas into chamber  290  to apply a downward load to substrate  10  for the polishing step. 
     The CMP apparatus of the present invention is capable of detecting whether a substrate is properly attached to carrier head  100 . If the CMP apparatus detects that the substrate is missing or is improperly attached to the carrier head, the operator may be alerted and polishing operations may be automatically halted. 
     The CMP apparatus may sense whether carrier head  100  successfully chucked the substrate as follows. After pump  93   c  evacuates chamber  290  to create low pressure pocket  278  between flexible membrane  118  and the backside of substrate  10 , pressure gauge  96   c  is used to measure the pressure in chamber  290 . 
     Referring to FIG. 8A, chamber  290  is initially at a pressure P a1 . Then pump  93   c  begins to evacuate chamber  290  at a time T a0 . On the one hand, if the substrate is properly attached to the carrier head, substrate  10  will block aperture  276  and pump  93   c  will successfully evacuate chamber  290 . Consequently, the pressure in chamber  290  will fall to a pressure P a2 . If the substrate is not present or is not properly attached to the carrier head, then aperture  276  will not be blocked, and air from the ambient atmosphere will leak into chamber  290 . Consequently, pump  93   c  will not be able to completely evacuate chamber  290 , and the pressure in chamber  290  will only fall to a pressure P a3  which is greater than pressure P a2 . The exact values of pressures P a1 , P a2  and P a3  depend upon the efficiency of pump  93   c  and the size of aperture  276  and chamber  290 , and may be experimentally determined. Pressure gauge  96   c  measures the pressure in line  92   c , and thus in chamber  290 , at time T a1  after the pump is activated. Computer  99  may be programmed to compare the pressure measured by pressure gauge  96   c  to a threshold pressure P aT  which is between pressures P a2  and P a3 . An appropriate threshold pressure P aT  may be determined experimentally. If the pressure measured by gauge  96   c  is below threshold pressure P aT  then it is assumed that the substrate is chucked to the carrier head and the polishing process may proceed. On the other hand, if the pressure measured by gauge  96   c  is above threshold pressure P aT , this provides an indication that the substrate is not present or is not properly attached to the carrier head. 
     In the alternate embodiments of the carrier head of the present invention discussed below, elements with modified functions or operations will be referred to with single or double primed reference numbers. In addition, in the embodiments discussed below, although pressure sensors  96   a - 96   c  remain coupled to fluid lines  92   a - 92   c , respectively, the purpose or function of the pressure sensors may change. 
     Referring to FIG. 5A, flexible membrane  118 ′ of carrier head  100 ′ does not include an aperture. Rather, carrier head  100 ′ includes a vent  300  between chamber  290  and the ambient atmosphere. 
     Vent  300  includes a passageway  302  formed in flexure ring  182 ′, a passageway  304  formed in base  104 ′, and a passageway  306  formed in outer clamp ring  206 ′. Vent  300  may also include a check valve  308  to prevent fluid from exiting chamber  290 . Check valve  308  may be located between base  104 ′ and outer clamp ring  206 ′. During polishing, when pump  93   c  pressurizes chamber  290 , the air pressure in passageway  304  will close check valve  308 . This ensures that the pressure in chamber  290  remains constant. 
     Support plate  250 ′ may include a large central aperture  320  located beneath an entry port  322  of passage  196 . As discussed below, flexible membrane  118 ′ may deflect upwardly through aperture  320  to close entry port  322 . In addition, a spacer (not shown) may be attached to the bottom surface of flexure ring  182 . The spacer prevents direct contact between support plate  250  and flexure ring  182  and provides a gap for fluid to flow from passageway  302  to entry port  322 . 
     A CMP apparatus using carrier head  100 ′ senses whether the substrate has been successfully chucked to the carrier head as follows. The substrate is loaded into substrate receiving recess  234  so that the backside of the substrate contacts mounting surface  274 . Pump  93   c  evacuates chamber  290  to create low-pressure pocket  278  between flexible membrane  118 ′ and substrate  10 . Pressure gauge  96   c  measures the pressure in chamber  290  to determine whether the substrate was successfully vacuum-chucked to the carrier head. 
     As shown in FIG. 5A, if the substrate was successfully vacuum-chucked, flexible membrane  118 ′ is maintained in close proximity to substrate  10  by low-pressure pocket  278 . Consequently, air may flow into chamber  290  through vent  300  as pump  93   c  attempts to evacuate chamber  290 . As shown in FIG. 5B, if the substrate is not present or is not properly attached to the carrier head, then membrane  118 ′ will deflect through aperture  320  and be pulled against a lower surface  324  of gimbal rod  180  to close entry port  322  of passage  196 . 
     Referring to FIG. 8B, chamber  290  is initially at a pressure P b1 . Pump  93   c  begins to evacuate chamber  290  at time T b0 . If the substrate is properly attached to the carrier head, then the pressure measured by gauge  96   c  will fall from pressure P b1  to a pressure P b2 . If the substrate is not present or is improperly attached to the carrier head, then the pressure measured by gauge  96   c  will fall from pressure P b1  to a pressure P b3 . Since air may leak into chamber  290  through vent  300  if the substrate is present, pressure P b2  is greater than pressure P b3 . 
     Computer  99  may be programmed to compare the pressure measured by gauge  96   c  at time T b1  after activation of pump  93   c  to a threshold pressure P bT . If the pressure measured by gauge  96   c  is greater than the threshold pressure P bT , it is assumed that the substrate is chucked to the carrier head and the polishing process may continue normally. On the other hand, if the pressure measured by gauge  96   c  is less than the threshold pressure P bT , the computer this is an indication that the substrate is not present or is not properly attached to the carrier head. Pressures P b1 , P b2 , P b3  and P bT  depend upon the efficiency of pump  93   c , the size and shape of chamber  290 , and the size and shape of vent  300 , and may be determined experimentally. 
     In order for carrier head  100 ′ to function properly, membrane  118 ′ must deflect sufficiently to block entry port  322 . The deflection of membrane  118 ′ depends upon the diameter of aperture  320 , the vertical distance that membrane  118  needs to deflect, the elastic modulus and thickness of membrane  118 ′, and the vacuum level in chamber  290 . Aperture  320  may be about 1.25 inches in diameter, the distance between bottom surface  256  of support plate  250  and the bottom surface of flexure ring  182  may be about 120 to 140 mils, membrane  118 ′ may have a thickness of {fraction (1/32)} inch and a durometer measurement of about forty to forty-five on the Shore A scale, and the vacuum level in chamber  290  may be about twenty-two to twenty-four inches of mercury (inHg) when aperture  274  is blocked and about ten to fifteen inHg when the aperture is not blocked. 
     Referring to FIG. 8C, in an alternate method of operating a CMP apparatus including carrier head  100 ′, the pressure in volume  170  may be measured to determine whether the substrate was successfully chucked to the carrier head. If this alternate method is used, carrier head  100 ′ need not have a vent  300 . Volume  170  may initially be at a pressure P c1 , and valve  98   b  is closed to seal volume  170  from pressure regulator  93   b . After pump  93   c  evacuates chamber  290  to create low pressure pocket  278  between flexible membrane  118  and the backside of substrate  10 , pressure gauge  96   b  is used to measure the pressure in volume  170 . As pump  93   c  evacuates chamber  290 , support structure  114  is drawn upwardly. This causes annular upper ring  282  to press upwardly on membrane  162  and reduces the volume of bladder  160 . 
     If substrate  10  is properly attached to carrier head  100 ′, the pressure in volume  170  will rise to a pressure P c2 . On the other hand, if the substrate is not present or is improperly attached to the carrier head, membrane  118 ′ will deflect through aperture  320  to close entry port  322  of passage  196 . Consequently, some fluid will be trapped in chamber  290 , and chamber  290  will not reach as low a pressure. Since support structure  114  will not be drawn as far upwardly and bladder  160  will not be as compressed, the pressure measured by gauge  96   b  will rise only to a pressure P c3  which is less than pressure P c2 . If the pressure measured by gauge  96   b  is greater than a threshold pressure P cT , it is assumed that the substrate is chucked to the carrier head and the polishing process may continue normally. On the other hand, if the pressure measured by gauge  96   b  is less than the threshold pressure P cT , the computer this is an indication that the substrate is not present or is not properly attached to the carrier head. 
     Referring to FIG. 6A, in another embodiment a mechanically actuated valve  350  is located between chamber  290  and volume  170 . Valve  350  may be at least partially located in a chamber  366  formed across passage  156 ″ between fixture  174  and bladder  160 . Valve  350  includes a valve stem  352  and a valve press plate  356 . Valve stem  352  may extend through an aperture  354  between chamber  366  and chamber  290  in flexure ring  182 ″. Valve press plate  356  is connected to the lower end of valve stem  352  and fits in a shallow depression  358  in a lower surface  360  of flexure ring  182 ″. Three channels  362  (only one channel is shown in the cross-sectional view of FIG. 6A) may be formed in flexure ring  182 ″ surrounding aperture  354  and valve stem  352  to connect chamber  290  to chamber  366 . Valve  350  may also include an annular flange  364  positioned above flexure rings  182 ″ in chamber  366 . An O-ring  368  may be positioned around valve stem  352  between annular flange  364  and flexure ring  182 ″. In addition, a spring  370  may be positioned between annular flange  364  and a ceiling  372  of chamber  366 . Spring  370  biases valve stem  352  downwardly so valve  350  is closed. More specifically, O-ring  368  is compressed between annular flange  364  and flexure ring  182 ″ to seal channels  362  from chamber  366 , thereby isolating chamber  366  from chamber  290 . However, if valve stem  352  is forced upwardly (as shown in FIG.  6 B), then O-ring  368  will no longer be compressed and fluid may leak around the O-ring. As such, valve  350  will be open and chamber  366  and chamber  290  will be in fluid communication via channels  362 . 
     Support plate  250 ″ may include a generally circular aperture  374  located beneath valve press plate  356 . As discussed below, flexible membrane  118 ″ may deflect upwardly through aperture  374  to open valve  350 . 
     A CMP apparatus including carrier head  100 ″ sense whether the substrate has been successfully vacuum-chucked to the carrier head as follows. The substrate is positioned in the substrate receiving recess  234  so that the backside of the substrate contacts mounting surface  274 . Pump  93   b  inflates bladder  160  to form a seal between flexible membrane  118 ″ and substrate  10 . Then valve  98   b  is closed to isolate bladder  160  from pump  93   b . A first measurement of the pressure in volume  170  is made by means of pressure gauge  96   b . Pump  93   c  evacuates chamber  290  to create low-pressure pocket  278  between the flexible membrane and the substrate. Then a second measurement of the pressure in volume  170  is made by means of pressure gauge  96   b . The first and second pressure measurements may be compared to determine whether the substrate was successfully vacuum-chucked to the carrier head. 
     As shown in FIG. 6A, if the substrate was successfully vacuum-chucked, flexible membrane  118 ″ is maintained in close proximity to substrate  10  by low pressure pocket  278 , and valve  350  will remain in its closed position. On the other hand, as shown in FIG. 6B, if the substrate is not present or is improperly attached to the carrier head, then when chamber  290  is evacuated, flexible membrane  118 ″ will deflect upwardly. The flexible membrane will thus contact valve press plate  356  and open valve  350 , thereby fluidly connecting chamber  290  to chamber  366 . This permits fluid to be drawn out of volume  170  through chamber  290  and evacuated by pump  93   c.    
     Referring to FIG. 8D, volume  170  may initially be at a pressure P d1 . The first pressure measurement is made at time T d1  before pump  93   c  begins to evacuate chamber  290 . When chamber  290  is evacuated at time T d1 , support structure  114  is drawn upwardly. This causes annular upper ring  282  to press upwardly on membrane  162 . This will reduce the volume of bladder  160 . The second pressure measurement is made at time T d2  after chamber  290  has been evacuated. 
     If the substrate is present, valve  350  remains closed, and the reduction of the volume of bladder  160  will thereby increase the pressure in volume  170  measured by gauge  96   b  as pressure P d1 . On the other hand, if the substrate is not present, then valve  350  is opened and fluid is evacuated from volume  170  so that the pressure measured by gauge  96   b  falls to pressure P d3 . Therefore, if the second measured pressure is larger than the first measured pressure, the substrate has been successfully chucked by the carrier head. However, if the second measured pressure is smaller than the first measured pressure, the substrate has not been successfully chucked by the carrier head. 
     Computer  99  may be programed to store the two pressure measurements, compare the pressure measurements, and thereby determine whether the substrate was successfully vacuum-chucked to the carrier head. 
     For carrier head  100 ″ to function properly, membrane  118 ″ must deflect sufficiently to actuate valve  350 . In addition to the factors discussed with reference to carrier head  100 ′, the ability of membrane  118 ″ to actuate valve  350  depends upon the diameter of valve press plate  356  and the downward load of spring  370  on valve stem  352 . Aperture  374  may be about 1.0 to 1.5 inches in diameter, spring  370  may apply a downward load of about two to three pounds, valve press plate  376  may be about the distance between bottom surface  256  of support plate  250  and the bottom surface of flexure ring  182  may be about 80 to 100 mils, and the vacuum level in chamber  290  may be about ten to fifteen inHg. 
     Referring to FIG. 8E, in an alternate method of operating a CMP apparatus including carrier head  100 ″, valve  98   b  may remain open when pump  93   c  evacuates chamber  290 . Volume  170  may initially be at a pressure P e1 . The first pressure measurement is made at time T e1  before pump  93   c  begins to evacuate chamber  290 . The second pressure measurement is made at time T e2  after pump  93   c  begins to evacuate chamber  290 . If the substrate is present, valve  350  remains closed, and pressure regulator  93   b  will maintain the pressure in volume  170  at pressure P e1 . On the other hand, if the substrate is not present, valve  350  is opened. Pressure regulator  93   b  will be unable to maintain the pressure in volume in  170  as fluid is evacuated, and the pressure in volume  170  will fall to pressure P e2 . Therefore, if the second measured pressure is smaller than the first measured pressure, the substrate was not successfully chucked by the carrier head. However, if the second measured pressure is equal to the first measured pressure, the substrate is properly attached to the carrier head. 
     Carrier head  100 ″ provides several benefits. First, carrier head  100 ″ is a sealed system in which there are no leaks or apertures to the atmosphere. Therefore, it is difficult for slurry to contaminate the interior of the carrier head. In addition, carrier head  100 ″ provides an absolute method of determining whether the substrate has been vacuum-chucked to the carrier head: if the pressure in volume  170  increases, the substrate is properly attached to the carrier head, whereas if the pressure in volume  170  decreases, the substrate is not present or is not properly attached to the carrier head. Experimentation is not required to determine a threshold pressure. In addition, because valve  350  is biased closed by spring  370 , the valve only opens if chamber  290  is under vacuum and a substrate is not present or is improperly attached to the carrier head. Consequently, the wafer sensor mechanism is not sensitive to the sequence of pressure or vacuum states in chamber  290  and volume  170 . 
     Referring to FIG. 7, in another embodiment mechanically actuated valve  350  is connected across a passage  380  between chamber  290  and the ambient atmosphere. Valve  350  may be at least partially located in a chamber  366 ′ formed across passage  380 , and includes valve stem  352 , valve press plate  356 , and annular flange  364 . In its closed position, valve  350 ′ isolates chamber  366 ′ from chamber  290 . However, if valve stem  352  is forced upwardly (as shown in FIG.  6 B), then O-ring  368  will no longer be compressed and fluid may leak around the O-ring. As such, valve  350  will be open and chamber  290  will be in fluid communication with the ambient atmosphere via passage  380 . 
     A CMP apparatus including carrier head  100 ′″ senses whether the substrate has been successfully vacuum-chucked to the carrier head as follows. Referring to FIG. 8F, chamber  290  is initially at a pressure P f1 . Then pump  93   c  begins to evacuate chamber  290  at a time T f0 . If the substrate is present, valve  350  remains closed, and the pressure in chamber  290  as measured by gauge  96   c  will fall to a pressure P f2 . On the other hand, if the substrate is not present, then valve  350  is opened. Consequently, pump  93   c  will not be able to completely evacuate chamber  290 , and the pressure in chamber  290  will only fall to a pressure P f3  which is greater than pressure P f2 . Computer  99  may be programmed to compare the pressure measured by pressure gauge  96   c  to a threshold pressure P fT  which is between pressures P f2  and P f3  to determine whether the substrate is present and properly attached to the carrier head. 
     As discussed above, the CMP apparatus may detect whether the carrier head has successfully chucked the substrate. In addition, in any of the embodiments, the pressure gauges may also be used to continuously monitor the presence of a substrate in the carrier head. If pressure gauges  96   c  or  96   b  detect a change in the pressure of chamber  290  or volume  170 , for example, while transporting the substrate between polishing stations or between a polishing station and a transfer station, then this is an indication that the substrate has detached from the carrier head. In this circumstance, operations may be halted and the problem corrected. 
     Another problem that has been encountered in CMP is that the substrate may escape from the carrier head during polishing. For example, if the retaining ring accidentally lifts off the polishing pad, the frictional force from the polishing pad will slide the substrate out from beneath the carrier head. 
     A CMP apparatus using carrier head  100  may sense whether the substrate is properly positioned beneath the carrier head during polishing. If carrier head  100  is to be used in this fashion, it is advantageous to have several apertures  278  located near the periphery of the flexible membrane  118 . When pump  93   c  pressurizes chamber  290  to apply a load to the substrate  10 , pressure gauge  96   c  is used to measure the pressure in chamber  290 . Referring to FIG. 8G, chamber  290  is initially at a pressure P g1 . If the substrate is properly positioned beneath the carrier head, substrate  10  will block apertures  278  and the pressure in chamber  290  will remain constant. However, if the substrate escapes, then apertures  278  will not be blocked, and fluid from chamber  290  will leak through the apertures into the ambient atmosphere. Consequently, the pressure in chamber  290  will fall to a pressure P g2 . 
     The present invention has been described in terms of a number of preferred embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims.