Abstract:
A system for polishing a substrate has a controller, pressure source, a platen, and a carrier for handling the substrate. The carrier must be able to detect if a substrate is present. In either the case of a false detection of substrate presence or the failure to detect substrate presence, the likely result is damaged substrates, wasted polishing consumables, and down time of the manufacturing facility. Detection is achieved by the substrate causing movement of a plunger and by such movement resulting in a pressure differential that is detected. The reliability of this detection is improved by one or more of a precise relationship of the plunger to a plate that applies pressure to the substrate, a controlled seal that is ensured of being broken when the plunger is moved by the presence of a substrate, and proper spring pressure applied to the plunger to prevent spurious plunger movement.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to the field of semiconductor manufacturing, and more specifically to a polishing system having a carrier head with substrate presence sensing. 
     2. Related Art 
     A wafer carrier is a critical component of a polisher. The wafer carrier serves two main purposes. A first purpose is to transport a wafer to/from a load station and between each polishing process area. A second purpose is to press the wafer downward against a polishing pad using a backside pressure while the polish pad and the wafer carrier rotate at high speeds. The type of carrier determines how pressure is applied to the backside of the wafer. One type of carrier includes an internal wafer presence sensor to verify that a wafer is loaded onto the carrier. 
       FIG. 1  is a cross sectional view of a carrier head having a substrate sensing mechanism according to the prior art. Carrier head  10  includes a perforated plate  12 , and a gimbal plate  14  disposed within retaining ring  16 . An edge control ring  20  holds a membrane  22  across a bottom surface of perforated plate  12 . The substrate sensing mechanism of carrier head  10  includes a plunger  24  disposed within a sensor venting port  50  of gimbal plate  14 . Plunger  24  is resiliently held within the venting port by a weak spring  26  disposed between a top portion of plunger  24  and an encapsulated region defined by reference numeral  28 . An oversized non-captured O-ring  30  is disposed between a flange portion of the plunger  24  and a top surface of gimbal plate  14 , around the venting port  50 . Pressure sensor  32  monitors a pressure within encapsulated region  28 . Under normal operating conditions, encapsulated region  28  is either pressurized or vented. 
     Plunger  24  can move vertically within sensor venting port  50  between a lower most travel position and an upper most travel position. The lower most travel position is defined by a combination of the plunger flange, the oversized non-captured O-ring  30 , and the top surface of the gimbal plate  14 . When in the lower most travel position, a bottom portion of plunger  24  extends below a lower most surface of perforated plate  12  by a distance indicated by reference numeral  36 . The upper most travel position is defined by a top surface of the plunger flange and a surface above the flange within encapsulated region  28 . When in the upper most travel position, a top portion of the plunger  24  is moved a distance as indicated by reference numeral  34 . 
       FIG. 2  is a top view of a substrate sensor venting port and an oversized non-captured O-ring according to the prior art. For example, a portion of gimbal plate  14  containing the substrate sensor venting port  50  is shown. The diameter of venting port  50  is slightly larger than a diameter of the plunger  24  to allow the plunger  24  to move within port  50 . To provide for venting, venting arteries or channels  52  are disposed along an inner sidewall of port  50 , extending from a top surface of gimbal plate  14  to a bottom surface of gimbal plate  14 . The use of the oversized non-captured O-ring  30  increases a possibility for impeding the venting of the encapsulated region, resulting in an erroneous sensing performance. That is, O-ring  30  is subject to various placements about the venting port  50 , for example, off-center from the venting port  50 . It is also possible for the placement of O-ring  30  to preclude passage of vacuum or pressure through one or more arteries  52 . 
     Carrier head  10  suffers from reliability issues of the wafer sensing mechanism. Such reliability issues lead to various handling problems that include one or more of dechuck errors, false wafer loss alarms, and failure to detect wafer loss. A dechuck error generally refers to a situation wherein a wafer slips off the carrier onto the underlying polishing pad as the carrier attempts to lift off the polishing pad after processing, typically resulting in breakage of the wafer. A false wafer loss alarm generally refers to a situation wherein the carrier incorrectly senses no wafer presence although a wafer is physically loaded, typically resulting in various handling errors. A failure to detect wafer loss generally refers to a situation wherein the carrier incorrectly senses a wafer when a wafer is not physically present, typically resulting in wafer breakage of a wafer that gets left behind. Such problems cause product scrap, tool downtime/reduced availability, and increased wafer polishing and carrier consumable cost. 
     Accordingly, it would be desirable to provide a carrier head with improved wafer sensing to overcome the problems in the art. 
     SUMMARY 
     According to one embodiment, a system for polishing a substrate includes a controller, a platen, and a carrier head. The carrier head is coupled to the controller. The carrier head is for carrying the substrate and holding the substrate against the platen during polishing. The carrier head includes a retaining ring for laterally supporting the substrate, a holding mechanism for applying positive pressure to the substrate during polishing and negative pressure when carrying the substrate, a gimbal plate coupled to the holding mechanism, and substrate detection means, coupled to the gimbal plate for detecting if the substrate is secured by the holding mechanism when the holding mechanism is applying negative pressure. The substrate detection means includes a plunger passing through a hole in the gimbal plate. The plunger has a maximum travel distance in the hole, has a bottom surface that extends below the gimbal plate and is coupled to the substrate during detecting. When the holding mechanism is pressed to the gimbal plate, the plunger extends past the holding mechanism by an amount substantially equal to the maximum travel distance of the plunger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present disclosure are illustrated by way of example and not limited by the accompanying figures, in which like references indicate similar elements, and in which: 
         FIG. 1  is a cross sectional view of a carrier head having a substrate sensing mechanism according to the prior art; 
         FIG. 2  is a top view of a substrate sensor venting port and an oversized non-captured O-ring according to the prior art; 
         FIG. 3  is a cross sectional view of a carrier head with a substrate presence sensing mechanism according to an embodiment of the present disclosure; 
         FIG. 4  is a top view of a substrate sensor venting port and a captured compliant sealing ring according to an embodiment of the present disclosure; 
         FIG. 5  is a section view of a substrate sensing plunger according to an embodiment of the present disclosure; 
         FIG. 6  is a section view of a substrate sensing plunger with a captured sealing ring according to an embodiment of the present disclosure; and 
         FIG. 7  is a block diagram view of a polishing system having a carrier head with substrate presence sensing according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a cross sectional view of a carrier head with a substrate presence sensing mechanism according to an embodiment of the present disclosure. Carrier head  38  includes a perforated plate  40 , and a gimbal plate  14  disposed within retaining ring  16 . An edge control ring  20  holds a membrane  22  across a bottom surface of perforated plate  40 . In one embodiment, the perforated plate  40  has a thickness on the order of 0.100+/−0.005 in. Such a thickness enables an optimal wafer sense plunger extension, allowing the wafer sensor to vent the membrane  22  when a wafer is physically present. 
     The substrate sensing mechanism of carrier head  38  includes a plunger  46  disposed within a sensor venting port  50  of gimbal plate  14 . Plunger  46  is resiliently held within the venting port by spring  42  disposed between a top portion of plunger  46  and an encapsulated region defined by reference numeral  28 . 
     A captured resilient sealing ring  44  is disposed between a flange portion of the plunger  46  and a top surface of gimbal plate  14 , around the venting port  50 . Sealing ring  44  includes any suitable resilient material capable of withstanding polishing process conditions, as appropriate. Pressure sensor  32  monitors a pressure within encapsulated region  28 . Under normal operating conditions, encapsulated region  28  is either pressurized or vented. 
     Spring  42  is a spring of sufficient strength for sealing with the captured resilient sealing ring  44 , the chamber defined by the encapsulated region  28 , and the region between the bottom portion of the gimbal plate  14  and the membrane  22 . Spring  42  is selected to also provide a sufficient force such that in response to pulling a vacuum on the membrane  22 , in the absence of a substrate, the membrane  22  does not overcome the force provided by spring  42 , and accordingly, does not breach the seal provided by the sealing ring  44  and the top of plate  14 . Still further, spring  42  must allow the sensor to be depressed in the event of pulling vacuum on a substrate, wherein the substrate acts upon the plunger  46 , breaking the seal otherwise provided by the sealing ring  44  and the top of plate  14 . Spring  42  must also not prevent a bottom portion of plunger  46  from aligning flush with a bottom side of perforated plate  40 . In one embodiment, spring  42  has a stiffness rating on the order of 19+/−5 lb/in, which allows wafer sensor actuation even under the highest possible membrane vacuum setting, while reliably actuating during physical wafer presence. 
     Plunger  46  can move vertically within sensor venting port  50  between a lower most travel position and an upper most travel position. The lower most travel position is defined by a combination of the plunger flange, the captured resilient sealing ring  44 , and the top surface of the gimbal plate  14 . When in the lower most travel position, a bottom portion of plunger  46  extends below a lower most surface of perforated plate  40  by a distance indicated by reference numeral  48 . Note that the distance  48  is greater than the distance  36 , shown in FIG.  1 . The upper most travel position is defined by a top surface of the plunger flange and a surface above the flange within encapsulated region  28 . When in the upper most travel position, a top portion of the plunger  46  is moved a distance as indicated by reference numeral  34 . To ensure that a bottom portion of plunger  46  does not extend below the lower surface of perforated plate  40  when the plunger is in an uppermost position, distance  48  must be less than or equal to distance  34 . 
       FIG. 4  is a top view of a substrate sensor venting port and a captured compliant sealing ring according to an embodiment of the present disclosure. For example, a portion of gimbal plate  14  containing the substrate sensor venting port  50  is shown. The diameter of venting port  50  is slightly larger than a diameter of the plunger  46  to allow the plunger  46  to move within port  50 . To provide for venting, venting arteries or channels  53  are disposed along an inner sidewall of port  50 , extending from a top surface of gimbal plate  14  to a bottom surface of gimbal plate  14 . The use of the captured resilient sealing ring  44  increases a possibility for assuring the venting of the encapsulated region, as well as sealing of the encapsulated region, resulting in an improved sensing performance. That is, plunger  46  captures resilient sealing ring  44  in a manner which makes the captured resilient sealing ring  44  subject to repeatable placement about and on-center with the venting port  50 . Accordingly, the placement of captured resilient sealing ring  44  assures both the passing and the blocking of vacuum or pressure, as needed, through arteries  53 . 
     In one embodiment, arteries  53  are constructed to have equal or greater area than the orifice  55  between encapsulated region  28  and pressure sensor  32 . For example, orifice  55  may have an orifice size within encapsulated region  28  on the order of approximately 0.050″ in diameter. The size of the three arteries  53  can each be on the order of an approximately 0.025″ radius half circle. 
     A benefit of the increased volume provided by arteries  53  can be understood from the following illustration. During a wafer dechuck or removal of a wafer from the polishing pad, the encapsulated region  28  is under positive pressure. The wafer presses against the wafer sensor. In addition, the vacuum within the membrane area must overcome the positive pressure and cause a delta-pressure on sensor  32 . With the embodiments of the present disclosure, a threshold on the order of approximately 0.8 to 1.0 Vdc on sensor  32  can be obtained, in contrast to a threshold on the order of approximately 0.3 to 0.5 Vdc with known wafer sensor embodiments. As a result of increased threshold, a tool constant value on the order of approximately 0.5 Vdc can be used, in comparison to a tool constant value on the order of 0.2 Vdc of a known wafer sensor embodiments. Accordingly, the embodiments of the present disclosure provide more reliable sensing and greater confidence that a wafer is actually pressed against the sensor and removed from the pad, rather than in a transition of moving from the pad and against the sensor. 
       FIG. 5  is a section view of a substrate sensing plunger according to an embodiment of the present disclosure. More particularly, plunger  46  includes a top portion and a bottom portion, separated by a flange portion. Between the flange portion and the bottom portion, plunger  46  includes a recessed region  54 . The recessed region is adapted for receiving and capturing the resilient sealing ring  44  therein. Once captured, movement of the resilient sealing ring with respect to the venting port  50  is more precisely controlled by plunger  46 . Accordingly, a reliability of sensing the presence or absence of a semiconductor substrate is greatly enhanced. 
       FIG. 6  is a section view of a substrate sensing plunger with a captured sealing ring according to an embodiment of the present disclosure. As shown, resilient sealing ring  44  is captured within recess  54 . A bottom portion of plunger  46  has a first diameter, as indicated by reference numeral  56 . Recess  54  has a second diameter, as indicated by reference numeral  58 . The second diameter  58  is on the order of less than the first diameter  56 . In one embodiment, diameter  58  is on the order of slightly larger than an inner diameter of resilient sealing ring  44 . In addition, the inner diameter of resilient sealing ring  44  is less than the diameter  56  of the bottom portion of plunger  46 . 
       FIG. 7  is a block diagram view of a polishing system having a carrier head with substrate presence sensing according to an embodiment of the present disclosure. Polishing system  60  includes a carrier head  38 , a platen  62 , polishing pad  64 , motor  66 , one or more pressure sources ( 68 , 70 , 72 ), and controller  74 . Carrier head  38  includes the substrate carrier head discussed herein above with respect to  FIGS. 3 ,  4 ,  5  and  6 . Carrier head  38  retains a substrate  76  within the retaining ring  16  during a polishing operation. 
     A polishing operation generally includes a substrate attach/detach step and a substrate transport step, in addition to the substrate polishing. During a substrate transport portion of a polishing operating, the carrier head transports the substrate between a substrate loading and unloading position, as well as, transports the substrate from a non-contact polishing position (i.e., substrate not in contact with the polishing pad) to a contact polishing position (i.e., substrate in contact with the polishing pad), or vice versa. Substrate attachment and/or detachment prior to transport is accomplished with the carrier head  38 , one or more pressure sources ( 68 , 70 , 72 )and membrane  22 . In particular, for carrying out attachment of a substrate to the carrier head, a vacuum is drawn behind membrane  22  and within the openings of perforated plate  40 . The vacuum causes a suctioning effect between the membrane  22  and the substrate to be transported. For detachment, the vacuum behind membrane  22  is vented, thereby releasing the suctioning effect between the membrane  22  and the substrate. 
     Platen  62  and pad  64  can include any suitable platen/pad for a particular polishing operation. For example, in one embodiment, platen  62  and polishing pad  64  may include a single platen/pad unit. Motor  66  provides rotation of carrier head  38 , as indicated by reference numeral  67 . Pressure sources ( 68 , 70 , 72 ) provide either vacuum or pressure to carrier head  38 , as appropriate, for use in a given portion of a polishing operation. Additional pressure sources may also be used. Controller  74  provides control of one or more portions of polishing operations via pressure sources ( 68 , 70 , 72 ) and motor  66 . In addition, controller  74  can provide additional controls as may be needed for the requirements of a particular polishing operation. 
     During an initial loading for a polishing operation, the carrier head  38  is positioned over a loading mechanism (not shown) for picking up a substrate, for example, as indicated by reference numeral  76 . Membrane  22  is vented, i.e., pressure is relieved from the region between the lower surface of plate  14 , perforated plate  40 , and an upper surface of membrane  22 . A dechuck bladder (not shown), such as is well known in the art, allows pressurizing of the encapsulated region  28 . The pressurized region  28  is sensed by pressure sensor  32 . The substrate is raised to a loading position by the loading mechanism, wherein the substrate acts upon plunger  46  in an upward fashion. Vacuum is applied to membrane  22 , in a region between an underside of plate  14 , the perforated plate  40 , and above membrane  22 . Subsequent venting of the region  28  occurs due to the upward displacement of plunger  46  by the underlying substrate, moving the captured resilient sealing ring  44  in a controlled manner to enable an assured venting of region  28 . Accordingly, a change in pressure sensed by pressure sensor  32  indicates the presence of the substrate. 
     During a polishing operation, membrane  22  and retaining ring  16  are pressurized to provide polishing pressures to polish the substrate. During the polishing operation, the perforated plate extends downward beyond the end of plunger  46 , rendering the substrate sensor inactive. 
     Upon a completion of the polishing operation, a dechuck operation is performed to remove the substrate from a surface of the platen/pad surface of the polisher. The retaining ring pressure is maintained according to requirements of a given dechuck operation. The membrane  22  is vented. The perforated plate  40  is extended, until contacting the substrate. Extending of the perforated plate  40  also causes encapsulated region  28  to be pressurized due to the spring action of spring  42  acting upon plunger  46  and causing the captured resilient sealing ring  44  to seal off sensor venting ports  53 . The pressurized region  28  is sensed by pressure sensor  32 . Vacuum is pulled on membrane  22 , wherein vacuum is drawn behind membrane  22  and within the openings of perforated plate  40 , causing a suctioning effect between the membrane  22  and the substrate. In response to suctioning of the by membrane  22 , the substrate acts upon plunger  46  in an upward fashion, causing plunger  46  to be displaced. Displacement of plunger  46  moves the captured resilient sealing ring  44  in a controlled manner to break the seal, thereby allowing region  28  to vent. The venting of region  28  causes a change in pressure of the encapsulated region. Accordingly, pressure sensor  32  senses the change in pressure, thus indicating the presence of the substrate. 
     According to one embodiment, a system for polishing a substrate includes a controller, a platen, and a carrier head. The carrier head is coupled to the controller. The carrier head is for carrying the substrate and holding the substrate against the platen during polishing. The carrier head includes a retaining ring for laterally supporting the substrate, a holding mechanism for applying positive pressure to the substrate during polishing and negative pressure when carrying the substrate, a gimbal plate coupled to the holding mechanism, and substrate detection means, coupled to the gimbal plate for detecting if the substrate is secured by the holding mechanism when the holding mechanism is applying negative pressure. 
     The substrate detection means includes a plunger passing through a hole in the gimbal plate. The plunger has a maximum travel distance in the hole, has a bottom surface that extends below the gimbal plate and is coupled to the substrate during detecting. When the holding mechanism is pressed to the gimbal plate, the plunger extends past the holding mechanism by an amount substantially equal to the maximum travel distance of the plunger. 
     In one embodiment, the plunger has a reduced thickness in an area above the gimbal plate, wherein the substrate detection means further comprises a compliant sealing ring around the area of the plunger having the reduced thickness. In addition, the substrate detection means has a spring applied to a top portion of the plunger above the gimbal plate, wherein the spring has a spring rate greater than 12 pounds per inch and less than 50 pounds per inch. Still further, the compliant sealing ring is captured by the plunger in the area of reduced thickness. The compliant sealing ring is also snugly against the plunger in the area of reduced thickness. The holding mechanism comprises a rigid perforated plate having a uniform thickness of less than 0.12 inch. 
     In another embodiment, the carrier head includes a retaining ring for laterally supporting the substrate, a holding mechanism for applying positive pressure to the substrate during polishing and negative pressure when carrying the substrate, a gimbal plate coupled to the holding mechanism, and substrate detection means, coupled to the gimbal plate for detecting if the substrate is secured by the holding mechanism when the holding mechanism is applying negative pressure. The substrate detection means includes a plunger passing through a hole in the gimbal plate. The plunger has a reduced thickness in an area above the gimbal plate. In addition, the substrate detection means also includes a compliant sealing ring around the area of the plunger having the reduced thickness. 
     Accordingly, the embodiments of the present disclosure provide improvements to wafer sensing reliability in a carrier head. Such improvements reduce the occurrence of wafer breakage, provide increased equipment availability, and decrease a cost of ownership of the carrier head and the polishing system. 
     In the foregoing specification, the disclosure has been described with reference to various embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present embodiments as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present embodiments. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the term “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements by may include other elements not expressly listed or inherent to such process, method, article, or apparatus.