Abstract:
A carrier head has a base, a flexible membrane, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that contacts an upper surface of the flexible membrane so that if a substrate is attached to the lower surface of the flexible membrane when the first chamber is evacuated, the valve is actuated to generate a signal to the substrate detection system.

Description:
BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to the detection of a substrate in a carrier head. 
     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. 
     Chemical mechanical polishing (CMP) is one accepted method of planarizing a substrate surface. 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 or moving polishing belt. The polishing pad may be a “standard” pad with a durable roughened surface, or a fixed-abrasive pad with abrasive particles embedded in a binder. The carrier provides a controllable load 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 at least one chemically-reactive agent, and an abrasive if a standard pad is being used, may be distributed over the polishing pad. 
     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 to 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 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, an 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 carrier head may include a rigid base having 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 base. 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 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 carrier head, then the 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 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 
     In one aspect, the invention is directed to a carrier head that has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that contacts an upper surface of the flexible membrane so that if a substrate is attached to the lower surface of the flexible membrane when the first chamber is evacuated, the valve is actuated to generate a signal to the substrate detection system. 
     Implementations of the invention may include the following features. The valve may be positioned in a passage that fluidly couples the first chamber to a second chamber. The valve may be biased in an open or closed position, and actuation of the valve may close or open the valve. The valve stem may extend through an aperture in a support structure, and may project slightly beyond a lower surface of the support structure. The support structure may be movable relative to the base. The valve may be biased by a spring, and the spring constant of the spring may be selected so that the force from the spring is sufficient to counteract a force from a flexible membrane when the substrate is not attached, but is insufficient to counteract a force from a flexible membrane when the substrate is attached. The valve stem may contacts the upper surface of the flexible membrane if the first chamber is evacuated. The flexible membrane may wrap around a lower portion of the valve if the substrate is not present. 
     In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that projects past a support surface, so that if the first chamber is evacuated and a substrate is attached to the lower surface of the flexible membrane, the substrate abuts the support surface and actuates the valve. 
     In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a plurality of valves in the carrier head that form part of a wafer detection system. If a substrate is attached to the flexible membrane when the first chamber is evacuated, either of the valves may be actuated to generate a signal to the wafer detection system. 
     In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a plurality of valves in the carrier head that form part of a wafer detection system. If a substrate is attached to the flexible membrane when the first chamber is evacuated, both of the valves must be actuated to generate a signal to the wafer detection system. 
     In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, a second chamber, a passage through the base between the first and second chambers, a first valve that is biased open and actuates to close the passage if the first chamber is evacuated a substrate is attached to the flexible membrane when the first chamber, and a second valve connected in series with the first valve, the second valve biased closed and actuatable to open the passage if the second chamber is evacuated. 
     Advantages of the invention include the following. The CMP apparatus includes a sensor to detect whether the substrate is properly attached to the carrier head. The sensor is less prone to false alarms. 
     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 partially a schematic cross-sectional view of a carrier head with a flexible membrane and a chamber, and partially a schematic diagram of a pneumatic control system for the carrier head. 
     FIG. 3A is an expanded view of the valve from the carrier head of FIG.  2 . 
     FIG. 3B is a view of the carrier head of FIG. 3A with an attached substrate. 
     FIG. 4 is a graph showing pressure as a function of time in a CMP apparatus using the carrier head of FIG.  2 . 
     FIG. 5 is a schematic cross-sectional view of a carrier head that includes multiple valves connected in parallel. 
     FIG. 6 is a schematic cross-sectional view of a carrier head that includes multiple valves connected in series. 
     FIG. 7 is a schematic cross-sectional view of a carrier head in which valves are separated by a diaphragm. 
     FIG. 8 is a schematic cross-sectional view of a carrier head in which valves are biased in opposite directions. 
    
    
     Like reference numbers are intended in the various drawings to indicate like elements, although some elements in different implementations may have different structures, operations or functions. 
     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 a CMP apparatus can be found in pending U.S. Pat. No. 5,738,574, the entire disclosure of which is hereby incorporated by reference. The CMP apparatus  20  includes a series of polishing stations  25  and a transfer station  27 . 
     Each polishing station  25  includes a rotatable platen  30  on which is placed a polishing pad  32 . Each polishing station may further include an associated pad conditioner apparatus  34  to periodically recondition the polishing pad surface. Each polishing station can also include a combined slurry/rinse arm  36  to supply a slurry  38  containing an active 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) to the surface of polishing pad  32 . 
     The CMP apparatus  20  also includes a rotatable multi-head carousel  40  that supports four carrier heads  100 . Three of the carrier heads receive and hold substrates and polish them by pressing them against the polishing pad  32  on platen  30  of polishing stations  25 . One of the carrier heads receives a substrate from and delivers the substrate to transfer station  27 . The carousel can rotate to orbit the carrier heads, and the substrates attached thereto, between the polishing stations and the transfer station. Each carrier can be independently rotated about its own axis, and independently laterally oscillated by a drive shaft  42 . 
     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. 2, carrier head  100  includes a housing hub  102 , a base  104 , a loading chamber  108 , a retaining ring  110 , and a substrate backing assembly  112 . Descriptions of similar carrier heads may be found in U.S. Pat. No. 5,957,751, and in pending U.S. application Ser. No. 09/169,500, filed Oct. 9, 1998, each of which is incorporated herein by reference in its entirety. 
     The housing hub  102  is connected to drive shaft  42  to rotate therewith about an axis of rotation which is substantially perpendicular to the surface of the polishing pad. Three passages  130 ,  132  and  134  are formed through housing hub  104  for pneumatic control of the carrier head. 
     Base  104  includes a gimbal mechanism  106  and an outer clamp ring  144 . The vertical position of base  104  relative to housing hub  102  is controlled by loading chamber  108 . Chamber  108  also controls the downward pressure on base  104  and retaining ring  110 . Loading chamber  108  is sealed by a diaphragm  140  that is clamped to housing hub  102  by an inner clamp ring  142  and clamped to base  104  between outer clamp ring  144  and flexure ring  152 . Outer clamp ring  144  includes an inwardly projecting flange  146  which extends over a lip of housing hub  102  to prevent over-extension of the carrier head and to prevent slurry from contaminating diaphragm  140 . 
     A first pump or pressure source  52   a  may be connected to loading chamber  108  via passage  130  in housing hub  102 . If pump  52   a  pumps fluid into loading chamber  108 , the volume of the chamber will increase and base  104  will be pushed downwardly. On the other hand, if pump  52   a  pumps fluid out of loading chamber  108 , the volume of chamber  108  will decrease and base  104  will be pulled upwardly. 
     Gimbal mechanism  106  permits base  104  to move with respect to housing hub  102  so that the retaining ring may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism  106  includes a gimbal rod  150  and a flexure ring  152 . Gimbal rod  150  may slide vertically in passage  132  in housing  102  so that base  104  can move vertically with respect to housing  102 . However, gimbal rod  150  prevents any lateral motion of base  104  with respect to housing  102 . A first passage  154  can be formed through gimbal rod  150 , and a second passage  156  can be formed through gimbal rod  150 , flexure ring  152  and outer clamp ring  144  for pneumatic control of the carrier head. 
     Retaining ring  110  may be secured at the outer edge of base  104 . Retaining ring  110  can have a flat bottom surface  126 , or the bottom surface can include channels to permit slurry flow. When fluid is pumped into chamber  108  and base  104  is pushed downwardly, retaining ring  110  is also pushed downwardly to apply a load to polishing pad  32 . An inner surface of 124 retaining ring  110  restrains the substrate from lateral motion. 
     A membrane  162  may be clamped to a lower surface of base  104  by a clamp ring  164  to form an annular bladder  160 . A passage  166  extends through clamp ring  164  and is aligned with passage  156  in base  104 . A second pump or pressure source  52   b  can connected to bladder  160  via passage  134  in housing hub  102 , passage  156  in base  104 , and passage  166  in clamp ring  164 . If pump  52   b  forces a fluid into bladder  160 , then bladder  160  will expand downwardly. On the other hand, if pump  52   b  evacuates fluid, 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 . 
     The substrate backing assembly  112  includes a flexible membrane  118 , a support ring  116 , a support structure  114 , and a spacer ring  128 . Each of these elements will be explained in greater detail below. 
     Flexible membrane  118  is a generally circular sheet formed of a flexible and elastic material with a central portion  170  and a peripheral portion  172  that extends between spacer ring  128  and support plate  114 . The central portion  170  of flexible membrane  118  extends below support structure  114  to provide a mounting surface for the substrate. An inner edge of the peripheral portion  172  is folded back over the perimeter of the central portion  170  to form an expandable lip  174 , as discussed in pending U.S. application Ser. No. 09/296,935, filed Apr. 22, 1999, the entirety of which is incorporated herein by reference. An outer edge of membrane  118  is clamped between retaining ring  110  and outer clamp ring  144  to define a pressurizable chamber  120 . 
     A third pump or pressure source  52   c  can be connected to chamber  120  via passage  154  in gimbal rod  150 . If pump  52   c  forces a fluid into chamber  120 , then the volume of the chamber will increase and flexible membrane  118  will be forced downwardly. On the other hand, if pump  52   c  evacuates air from chamber  120 , then the volume of the chamber will decrease and the membrane will be drawn upwardly. 
     Spacer ring  128  is an annular body positioned between support structure  114  and retaining ring  110  to maintain the proper shape of flexible membrane  118 . Spacer ring  128  can rest on the lip portion of flexible membrane  118 . 
     Support ring  116  is an annular piece with a C-shaped cross-section that rests inside chamber  120  on flexible membrane  118 . The central portion  170  of flexible membrane  118  can include an inwardly extending flap  176  that engages support ring  116  to maintain the proper shape of flexible membrane  118 . 
     Support structure  114  also rests inside chamber  120  on flexible membrane  118 . The support structure  114  includes a disk-shaped plate portion  180  with a plurality of unillustrated apertures, an outwardly extending flange portion  182  that extends over support ring  116 , and a downwardly extending flange portion  184  that extends between support ring  116  and peripheral portion  172  of flexible membrane to rest on the central portion  170  of the flexible membrane. 
     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. 
     Three pressure sensors or gauges  56   a,    56   b  and  56   c  may be connected to the fluid lines between pumps  52   a,    52   b  and  52   c,  and chambers  108 ,  160 , and  120 , respectively. Controllable valves  58   a,    58   b  and  58   c  may be connected across the fluid lines between pressure gauges  56   a,    56   b  and  56   c,  and pumps  52   a,    52   b  and  52   c,  respectively. Pumps  52   a - 52   c,  pressure gauges  56   a - 56   c  and valves  58   a - 58   c  may be appropriately connected to a general-purpose digital computer  60 . Computer  60  may operate pumps  52   a - 52   c,  as described above, to pneumatically power carrier head  100  and to vacuum-chuck a substrate to the bottom of the carrier head. In addition, computer  60  may operate valves  58   a - 58   c  and monitor pressure gauges  56   a - 56   c,  as described in more detail below, to sense the presence of the substrate in the carrier head. 
     Referring to FIGS. 3A and 3B, the carrier head  100  includes a mechanically actuated valve  200  to provide the carrier head with a wafer detection capability. In one implementation, passage  156  is connected to a chamber  220  in flexure ring  152 , and valve  200  is positioned near the center of the carrier and extends between chamber  220  and chamber  120 . In this implementation, valve  200  includes a valve stem  202 , an annular flange  204  that extends radially outwardly from the valve stem  202 , an O-ring  206 , and a spring  214 . Valve stem  202  extends through an aperture  208  in flexure ring  152  between valve chamber  220  and lower chamber  120 , with valve flange  204  positioned in valve chamber  220 . The portion of valve stem  202  that extends into lower chamber  120  passes through an aperture  210  in support structure  114 . When lower chamber  120  is evacuated and support structure  114  is retracted against base  104 , valve stem  202  can extend slightly below a bottom surface  186  of support structure  114 . Channels  212  may be formed in flexure ring  152  surrounding aperture  208  and valve stem  202  to connect chamber  120  to valve chamber  220 . However, O-ring  206  is positioned around valve stem  202  in valve chamber  220  between annular flange  204  and flexure ring  152 . In addition, spring  214  is positioned between annular flange  204  and a ceiling  222  of valve chamber  220 . Spring  214  biases the valve  200  into a closed position. (as shown in FIG.  3 A). More specifically, O-ring  206  is compressed between annular flange  204  and flexure ring  152  to seal channels  212  from valve chamber  220 , thereby isolating valve chamber  220  from lower chamber  120 . However, if valve stem  202  is forced upwardly (as shown in FIG.  3 B), then O-ring  206  will no longer be compressed and fluid may leak through a gap  218  around the O-ring. As such, valve  200  will be open and valve chamber  220  and lower chamber  120  will be in fluid communication via channels  212 . 
     A CMP apparatus including carrier head  100  senses whether the substrate has been successfully vacuum-chucked to the carrier head as follows. The substrate is positioned against the flexible membrane  118 . Pump  52   b  inflates bladder  160  to a predetermined pressure, and then valve  58   b  is closed to isolate bladder  160  from pump  52   b.  A first measurement of the pressure in bladder  160  is made by means of pressure gauge  56   b.  Then pump  52   c  evacuates lower chamber  120  to create a low-pressure pocket between the flexible membrane and the substrate in order to vacuum chuck the substrate to the carrier head. Then a second measurement of the pressure in bladder  160  is made by means of pressure gauge  56   b.  The first and second pressure measurements may be compared to determine whether the substrate was successfully vacuum-chucked to the carrier head. 
     Carrier head  100  is configured so that valve  200  will actuate if the substrate is present, and will not actuate if the substrate is absent. As shown in FIG. 3A, if the substrate is not present, then when chamber  120  is evacuated, flexible membrane  118  will move upwardly and contact the valve stem. However, since flexible membrane  118  is flexible and is partly supported against support structure  114  when chamber  120  is evacuate, the flexible membrane will tend to wrap around the valve stem, and the upward tension force on valve stem  202  from flexible membrane  118  will be insufficient to overcome the downward spring force from spring  204 , and the valve  200  will remain closed. On the other hand, as shown in FIG. 3B, if the substrate is vacuum-chucked to the flexible membrane, the relatively rigid substrate will press on valve stem  202 . In this case, the upward tension force from flexible membrane  118  and substrate  10  will overcome the downward spring force from spring  204 , and the valve  200  will open, thereby fluidly connecting lower chamber  120  to valve chamber  220 . This permits fluid to be drawn out of bladder  160  through valve chamber  220  and lower chamber  120 , and evacuated by pump  52   c.    
     It should be noted that spring  204  is selected to provide a downward force that is sufficient to counteract the upward force applied by the membrane alone, but insufficient to counteract the upward force applied when a substrate is attached to the membrane. In general, the larger the aperture  210  in support structure  114 , the stiffer the membrane  118 , and the farther the valve stem  202  extends past lower surface  176 , the more force flexible membrane  118  will apply to the valve stem  202 , and the larger the spring constant of spring  204  will need to be. However, a lower spring constant results in less stress on the substrate as the valve is actuated. 
     Referring to FIG. 4, bladder  160  may initially be at a pressure P 1 . The first pressure measurement is made at time T 1  before pump  52   c  begins to evacuate lower chamber  120 . When chamber  120  is evacuated at time T 2 , flexible membrane  118  is drawn upwardly. If the substrate is present, valve  200  remains closed, and the pressure in bladder  160  will remain constant at pressure P 1 , or even rise to a pressure P 2  if support structure  114  applies an upward force to compress the bladder  160 . Thus, the pressure in bladder  160  measured by gauge  56   b  will remain at or above pressure P 1 . On the other hand, if the substrate is present, then valve  200  is opened and fluid is evacuated from volume  160  so that the pressure measured by gauge  56   b  falls to pressure P 3 . Therefore, if the second measured pressure is less than the first measured pressure, the substrate is attached to the carrier head. However, if the second measured pressure is equal to or larger than the first measured pressure, the substrate is not attached to the carrier head. 
     Computer  60  may be programmed to store the two pressure measurements, compare the pressure measurements, and thereby determine whether the substrate was successfully vacuum-chucked to the carrier head. This can provide an extremely reliable substrate detector that is not subject to “false” signals, e.g., indications that the substrate is absent when it is, in fact, present. In addition, the sensor is contained within the carrier head behind the flexible membrane, so that the sensor does not provide an opportunity for slurry to contaminate the interior of the carrier head. 
     Referring to FIG. 5, in another implementation, carrier head  100   a  includes two or more valves  300 ,  310  connected in parallel between lower chamber  120  and bladder  160 . For example, the first valve can extend between lower chamber  120  and a first chamber  302 , whereas the second valve can extend between lower chamber  120  and a second chamber  312 . A passage  320  in flexure ring  154  can connect first chamber  302  to second chamber  312 . Thus, chamber  120  will be connected to bladder  160  if either or both valves  300  is triggered. This implementation increases the sensitivity of the carrier head to the presence of the wafer, and provides redundancy in case one valve becomes stuck. In addition, if the carrier head includes three or more valves spaced at equal angular intervals around the carrier base, the substrate will not be tilted as it is lifted. 
     Referring to FIG. 6, in another implementation, carrier head  100   b  includes two or more valves  400 ,  410  connected in series between chamber  120  and bladder  160 . For example, the first valve can extend between lower chamber  120  and a first chamber  402 , and a passage  420  through flexure ring  152  can connect first chamber  402  to a passage  414  that is sealed from a second chamber  412  by the O-rings of second valve  410 . The second chamber  412  is connected to bladder  160  by passage  156 . In short, the input of first valve  400  is connected to chamber  120 , the output of the first valve  400  is connected to the input of second valve  402  by passage  420 , and the output of second valve  410  is connected to bladder  160 . Thus, chamber  120  will be connected to bladder  160  only if both valves  400  and  410  are triggered. This implementation increases the sensitivity of the carrier head to the absence of the substrate and to situations in which the substrate is not sufficiently firmly secured to the flexible membrane, e.g., if the substrate is attached to the flexible membrane by surface tension alone, and not by vacuum-chucking, and tilts rather than actuating both sensors. The input passage  414  of second valve  410  can be separated from chamber  120 , while allowing the valve stem of second valve  410  to extend into the chamber  120 , by O-rings  416 . 
     As shown in FIG. 7, a flexible diaphragm  430  can be used instead of O-rings to separate passage  414  of second valve  410 ′ from the chamber  120 . Valve stem  202 ′ of valve  410 ′ can rest on diaphragm  430 , and a bumper  432  can be affixed to the underside of diaphragm  430 . Flexible diaphragm  430  is sufficiently elastic that when bumper  432  is pressed upwardly by flexible membrane  118 , bumper  432  can be forced up into aperture  210 ′ in support structure  114 , thus forcing valve stem  202 ′ upwardly to actuate second valve  410 ′. 
     Referring to FIG. 8, in another implementation, carrier head  100   c  includes two valves  500  and  510  connected in series between chambers  120  and  108 . This implementation would be appropriate for the carrier head discussed in pending U.S. application Ser. No. 60/114,182, filed Dec. 30, 1998. In this implementation, the valves  500  and  510  can be formed between flexure ring  152 ′ and an annular gimbal clamp  158 , and multiple fluid passages through the gimbal rod and the flexure ring are not required. First valve  500  fluidly connects chamber  120  to a first valve chamber  502  via channels  508  in flexure ring  152 ′ surrounding valve stem  506 , second valve  510  fluidly connects chamber  108  to a second valve chamber  512  via channels  518  in gimbal clamp  158  surrounding valve stem  516 , and first valve chamber  502  is connected to second valve chamber  512  by an unillustrated passage. The first valve  500  is biased open by spring  504 , and second valve  510  is biased closed by spring  514 . If the lower chamber  120  is evacuated and a substrate is vacuum-chucked to the carrier head, then valve stem  506  of first valve  500  will be actuated to press O-ring  506  against gimbal clamp  158  to close the first valve, and the pressure in chamber  108  will remain constant. On the other hand, if the lower chamber  120  is evacuated but no substrate is present, then first valve  500  will remain open. If loading chamber  108  is also evacuated when chamber  120  is evacuated, e.g., in order to lift the entire substrate backing assembly and the retaining ring away from the polishing pad, then valve stem  516  will be pressed against housing hub  102 . This generates a downward force on the valve stem which can overcome an upward force from spring  514  that presses O-ring against gimbal clamp  158 , causing the second valve  510  to open and thus connecting loading chamber  108  to lower chamber  120 . Fluid will then flow out of loading chamber  108  via lower chamber  120 . On the other hand, if the loading chamber  108  is pressurized when chamber  120  is evacuated, e.g., to control the contact area and pressure on the substrate during polishing, then valve  510  will remain closed. In sum, the valve assembly will be actuated to connect loading chamber  108  to lower chamber  120  only if a substrate is not present and chamber  108  is evacuated. The drop in pressure in lower chamber  120  can be detected by pressure gauge  52   c  to indicate that the substrate is not present. 
     Alternatively, carrier head  100   c  could include a single valve that opens when chamber  120  is evacuated if a substrate is present. In this case, the valve that separates chamber  108  from a pump or pressure source can remain open so that chamber  120  does not entirely evacuate, thus preventing the membrane  118  from pull so far into chamber  120  that the substrate becomes overstressed and damaged. 
     Although in several implementations the valves are described as connecting lower chamber  120  to bladder  160 , the valve could be used to connect any two chambers in the carrier head, or the valve can connect a chamber in the carrier head to the ambient atmosphere. Moreover, the valve can be biased opened or closed, so that the presence of the substrate can either close or open the valve, respectively, when the valve is actuated. The valve can be positioned in parts of the carrier head other than the flexure ring. For example, the valve can be offset from the center of the carrier and attached to a base ring with the valve chamber formed between the flexure ring and the base ring. In addition, the passages formed through the carrier head to provide the fluid connections are exemplary. For example, fluid communication can be provided by a flexible hose that is coupled to fixtures on the housing hub and base ring, a first passage can connect the fixture on the base ring to the valve chamber, and a second passage can connect the valve chamber to the bladder. 
     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. The scope of the invention is defined by the appended claims.