Patent Publication Number: US-2006005770-A1

Title: Independently moving substrate supports

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates generally to method and apparatus for placing a substrate on a substrate support in a processing chamber.  
      2. Description of the Related Art  
      Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors. Generally, flat panels comprise two plates having a layer of liquid crystal material sandwiched therebetween. At least one of the plates includes at least one conductive film disposed thereon that is coupled to a power source. Power, supplied to the conductive film from the power supply, changes the orientation of the crystal material, creating a patterned display.  
      In order to manufacture these displays, a substrate, such as a glass or polymer workpiece, is typically subjected to a plurality of sequential processes to create devices, conductors and insulators on the substrate. Each of these processes is generally performed in a process chamber configured to perform a single step of the production process. In order to efficiently complete the entire sequence of processing steps, a number of process chambers are typically coupled to a central transfer chamber that houses a robot to facilitate transfer of the substrate between the process chambers. A processing platform having this configuration is generally known as a cluster tool, examples of which are the families of AKT PECVD processing platforms available from AKT, a wholly-owned subsidiary of Applied Materials, Inc. of Santa Clara, Calif.  
      Generally, within the cluster tool, the transfer chamber robot transfers the substrates to each chamber on an end effector structure. Within each of the chambers are substrate supports upon which the robot places the substrates during transferring. Once the substrate is on the substrate support, the robot retracts from the chamber. Typically, the substrate support includes a transfer mechanism, such as a plurality of vertically moveable lift pins that are moved upwardly to engage a substrate to facilitate exchange of the substrate between the robot and the substrate support.  
      As demand for flat panels has increased, so has the demand for larger sized substrates. For example, large area substrates utilized for flat panel fabrication have increased in area from 550 mm by 650 mm to over 1,500 mm by 1,800 mm in just a few years and are envisioned to exceed four square meters in the near future. This growth in the size of the large area substrates has presented new challenges in handling and production. For example, to accommodate larger substrates, the lift pins in the substrate support have greater spacing between individual lift pins. This results in greater deflection, or sag, of the unsupported regions of the substrate surrounding the individual lift pins. As the lift pins are retracted to place the substrate upon the substrate support, the sagging regions come into contact with the substrate support prior to the regions beneath the lift pins, resulting in gas becoming trapped between the substrate and the substrate support in one or more locations.  
      The trapped gas, in turn, may cause the substrate to “float” or move on the surface of the substrate support, leading to misalignment of the substrate. Misaligned substrates may result in costly substrate damage or poor processing performance. In addition, the trapped gas pockets between the substrate and the substrate support may result in non-uniform support to substrate heat transfer, further leading to processing non-uniformities and potentially defective structures formed on the substrate.  
      Therefore, there is a need for an improved method and apparatus for transferring a substrate to a substrate support.  
     SUMMARY OF THE INVENTION  
      The present invention generally provides embodiments of a method and apparatus for transferring a large area substrate onto a substrate support. The method and apparatus are utilized to transfer a large area substrate onto a substrate support in a center-to-edge manner which forces substantially all of the gas out from between the substrate and the substrate support.  
      In one embodiment, a support assembly for supporting a substrate in a processing chamber includes a support assembly having a support surface and a bottom surface. A first set of lift pins are movably disposed through the support assembly and have first ends for supporting the substrate disposed proximate the support surface and second ends extending beyond the bottom surface. The first ends of the first set of lift pins are extendable to a first distance above the support surface. A second set of lift pins are movably disposed through the support assembly at a position inward of the first set of lift pins. The second set of lift pins have first ends for supporting the substrate disposed proximate the support surface and second ends extending beyond the bottom surface. The first ends of the second set of lift pins are extendable independently of the first ends of the first set of lift pins to a second distance above the support surface. The second distance is less than the first distance.  
      In another embodiment, a support assembly for supporting a substrate in a processing chamber includes a support assembly moveable between a raised position and a lowered position and having a support surface and a bottom surface. A first set of lift pins are movably disposed through the support assembly and have first ends for supporting the substrate disposed proximate the support surface and second ends extending beyond the bottom surface. The first set of lift pins are passively actuated. The second ends of the first set of lift pins contact a bottom of the chamber at least when the support assembly is in the lowered position. A second set of lift pins are movably disposed through the support assembly and have first ends for supporting the substrate disposed proximate the support surface and second ends extending beyond the bottom surface. An actuator is disposed below the support assembly and is adapted to independently position at least one of the second set of lift pins with respect to the first set of lift pins.  
      In another embodiment, a method for transferring a substrate comprises the steps of simultaneously supporting a substrate above an upper surface of a substrate support on a first set and a second set of lift pins movably disposed through the substrate support. The first set of lift pins are extended to a first height and the second set of lift pins are extended to a second height lower than the first height. The second set of lift pins are disposed inward of the first set of lift pins. A relative distance between both the first set and the second set of lift pins and the upper surface is reduced to cause the substrate to contact the upper surface in a substantially continuous center-to-edge manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  is a sectional side view of one embodiment of a processing chamber having a substrate support of the present invention;  
       FIGS. 2-5  are sectional side views of one embodiment of the substrate support of  FIG. 1  illustrating a substrate being transferred thereto;  
       FIG. 6  is a sectional side view of another embodiment of a processing chamber having a substrate support;  
       FIG. 7  is a sectional side view of another embodiment of a processing chamber having a substrate support;  
       FIG. 8  is a sectional side view of another embodiment of a processing chamber having a substrate support; and  
       FIG. 9  is a flow chart depicting a method of transferring a substrate to a substrate support. 
    
    
     DETAILED DESCRIPTION  
      Embodiments of the invention generally provide a substrate support and method for transferring a large area substrate that are advantageous for placing a large area substrate on a substrate support. The invention is illustratively described below in reference to a plasma enhanced chemical vapor deposition system, such as a plasma enhanced chemical vapor deposition (PECVD) system, such as is available from AKT, a division of Applied Materials, Inc., Santa Clara, Calif. However, it should be understood that the invention has utility in other system configurations such as physical vapor deposition systems, ion implant systems, etch systems, chemical vapor deposition systems and other systems in which transferring a substrate to a substrate support is desired.  
       FIG. 1  is a sectional side view of one embodiment of a plasma enhanced chemical vapor deposition (PECVD) system  100 . The system  100  generally includes a chamber  102  coupled to a gas source  104 . The chamber  102  has walls  106 , a bottom  108  and a lid assembly  110  that define a process volume  112 . The process volume  112  is typically accessed through a port (not shown) in the walls  106  that facilitates movement of the substrate  140  into and out of the chamber  102 . The walls  106  and bottom  108  are typically fabricated from a unitary block of aluminum. The lid assembly  110  contains a pumping plenum  114  that couples the process volume  112  to an exhaust port (that includes various pumping components, not shown).  
      The lid assembly  110  is supported by the walls  106  and can be removed to service the chamber  102 . The lid assembly  110  is generally comprised of aluminum and may additionally contain heat transfer fluid channels for regulating the temperature of the lid assembly  110  by flowing heat transfer fluid therethrough.  
      A distribution plate  118  is coupled to an interior side  120  of the lid assembly  110 . The distribution plate  118  is typically fabricated from aluminum. The distribution plate generally includes a perimeter mounting ring that surrounds a “dish-shaped” center section. The mounting ring includes a plurality of mounting holes passing therethrough, each accepting a vented mounting screw that threads into a mating hole in the lid assembly  110 . The center section includes a perforated area through which process and other gases supplied from the gas source  104  are delivered to the process volume  112 . The perforated area of the distribution plate  118  is configured to provide uniform distribution of gases passing through the distribution plate  118  into the chamber  102 .  
      A heated support assembly  138  is centrally disposed within the chamber  102 . The support assembly  138  supports a substrate  140  during processing. In one embodiment the substrate  140  comprises a large area (e.g., greater than 0.25 square meters) glass or polymer workpiece. The support assembly  138  is generally fabricated from aluminum, ceramic, or a combination of aluminum and ceramic. The support assembly  138  typically includes at least one embedded heating element  132 . A vacuum port (not shown) is used to apply a vacuum between the substrate  140  and support assembly  138 , securing the substrate to the substrate support assembly  138  during processing. The heating element  132 , such as an electrode or resistive element disposed in the support assembly  138 , is coupled to a power source  130 , heating the support assembly  138  and substrate  140  positioned thereon to a predetermined temperature. In one embodiment, the heating element  132  maintains the substrate  140  at a uniform temperature of about 150 to 400 degrees. Alternatively, heating lamps or other heat sources may be utilized to heat the substrate.  
      Generally, the support assembly  138  is coupled to a stem  142 . The stem  142  provides a conduit for electrical leads, vacuum and gas supply lines between the support assembly  138  and other components of the system  100 . The stem  142  couples the support assembly  138  to a lift system (not shown) that moves the support assembly  138  between an elevated position (as shown) and a lowered position. Bellows  146  provides a vacuum seal between the chamber volume  112  and the atmosphere outside the chamber  102  while facilitating the movement of the support assembly  138 .  
      The support assembly  138  generally is grounded such that RF power supplied by a power source  122  to the distribution plate  118  (or other electrode positioned within or near the lid assembly of the chamber) may excite the gases disposed in the process volume  112  between the support assembly  138  and the distribution plate  118 . The RF power, generally having a frequency of between a few Hz to 13 MHz or higher is provided in a wattage suitable for the substrate surface area. In one embodiment, the power source  122  comprises a dual frequency source that provides a low frequency power at less than about 2 MHz (preferably about 200 to 500 kHz) and a high frequency power at greater than 13 MHz (preferably about 13.56 kHz). The frequencies may be fixed or variable. Illustratively, for a 550 mm×650 mm substrate, the low frequency power is about 0.3 to about 2 kW while the high frequency power is about 1 to 5 kW. Generally, the power requirements decrease or increase with a corresponding decrease or increase in substrate size.  
      The support assembly  138  additionally supports a circumscribing shadow frame  148 . The shadow frame  148  is configured to cover the edge of the substrate  140  and is typically comprised of ceramic. Generally, the shadow frame  148  prevents deposition at the edge of the substrate  140  and support assembly  138  so that the substrate does not stick to the support assembly  138 . Optionally, a purge gas is supplied between the shadow frame  148  and the support assembly  138  to assist in preventing deposition at the substrate&#39;s edge.  
      The support assembly  138  has a plurality of holes  128  disposed therethrough to accept a plurality of lift pins  150  comprising a first set  180  and one or more other lift pins  152  that comprises a second set  182 . The second set  182  of lift pins  152  are positioned inward of the first set  180  of lift pins  150 . The lift pins  150  and  152  are typically comprised of ceramic or anodized aluminum. Generally, the lift pins  150  and  152  have respective first ends  160  and  162  that are substantially flush with, or slightly recessed from, a support surface  134  of the support assembly  138  when the lift pins  150  and  152  are in a normal position (i.e., retracted relative to the support assembly  138 ). The first ends  160 ,  162  are generally flared to prevent the lift pins  150 ,  152  from falling through the holes  128 . Additionally, the lift pins  150  and  152  have respective second ends  164  and  166  that extend beyond an underside  126  of the support assembly  138 .  
      The lift pins  150 ,  152  move to a position when actuated where the pins project from the support surface  134 . In the actuated position, the lift pins  150  may project farther from the support surface  134  than the lift pins  152 . Alternatively, the lift pins  150  and the lift pins  152  may project the same distance from the support surface  134 . In one embodiment, the first set  180  of lift pins  150  includes at least eight lift pins that are positioned outwards of the one or more lift pins  152 . In one embodiment, the first set  180  of lift pins  150  include eight pins grouped in pairs wherein a respective pair is positioned proximate each side of a four-sided substrate. In one embodiment, the second set  182  of lift pins  152  includes four lift pins positioned about a center of the support assembly  138 . It is contemplated that any number of lift pins may be utilized in any geometric or random pattern. For example, the substrate  140  may be a mother substrate having many features being formed thereon and intended for subsequent separation into smaller units. The second set  182  of lift pins  152  may be arranged to be situated between the features to prevent inadvertent damage during substrate handling.  
      The lift pins  150 ,  152  may be displaced relative to the support surface  134  of the support assembly  138  to facilitate transfer of the substrate  140  to the support assembly  138 . One or more actuators  170  are disposed below the support assembly  138  and are adapted to control the displacement of at least one of the first or second sets  180 ,  182  of lift pins  150 ,  152  relative to the support surface  134  of the support assembly  138 . The one or more actuators  170  may be a pneumatic cylinder, hydraulic cylinder, lead screw, solenoid, stepper motor, or other device suitable for controlling the displacement of the lift pins  150 ,  152 . Controlling the displacement of at least one of the first and second sets  180 ,  182  of lift pins  150 ,  152  allows for control of the profile of the substrate  140  supported by the lift pins  150 ,  152  as it is brought into contact with the support surface  134  of the support assembly  138 . By controlling the profile of the substrate  140 , the substrate  140  may be brought into contact with the support surface  134  in a substantially continuous center-to-edge manner, thereby enabling transfer of the substrate  140  without substantially trapping air between the substrate  140  and the support surface  134 . Continuous, as used herein, refers to physical continuity, and not temporal continuity. The substrate  140  may be raised, lowered, or held stationary at various moments during the transfer to or from the support assembly  138 .  
      In one embodiment, the actuators  170  are adapted to displace only the second set  182  of lift pins  152  and the first set  180  of lift pins  150  are passively actuated. Passive actuation, as used herein, means that the first set  180  of lift pins  150  are moved relative to the support assembly  138  by contact with a stationary object, such as the bottom  108  of the chamber  102 , when the support assembly  138  is lowered. Alternatively, the actuators  170  may displace both the first and the second sets  180 ,  182  of lift pins  150 ,  152 .  
      In one embodiment, the actuators  170  may be coupled to the bottom  108  of the chamber  102  in general alignment with the second set  182  of lift pins  152 . A plurality of holes  116  formed in the bottom  108  of the chamber  102  allow each actuator  170  to move a strike plate  172  up and down relative to the bottom  108  of the chamber  102 . The strike plate  172  is typically comprised of ceramic or anodized aluminum. The stroke of the actuators  170  controls the amount of displacement of the lift pins  152  and will generally depend on the configuration of the support assembly  138 . For example, in one embodiment, the lift pins  150 ,  152  are of equal length and are actuated as the support assembly  138  lowers and the second ends  164 ,  166  of the lift pins  150 ,  152  contact the chamber bottom  108  and the strike plate  172 , respectively. The actuators  170  may have a stroke that enables the strike plate  172  to be positioned in a range of from at least substantially co-planar with the chamber bottom  108  to a position lower than the chamber bottom  108  sufficient to control the shape of the substrate  140 , as discussed more fully below.  
      Alternatively, in another embodiment where the lift pins  150  are longer than the lift pins  152 , the actuator  170  may have a longer stroke, or be positioned higher, in order to actuate the lift pins  152  as desired. It is contemplated that any combination of relative lengths of the sets  180 ,  182  of lift pins  150 ,  152  may be compensated for by the position and/or stroke length of the actuators  170 . Furthermore, it is contemplated that the actuators  170  may be coupled directly to the lift pins  152  rather than utilizing the strike plate  172 . Optionally, one or more actuators, not shown, may additionally be disposed below the support assembly  138  and adapted to actuate the first set of lift pins  180 .  
       FIGS. 2-5  depict partial sectional side views of the substrate support of the present invention illustrating one mode of operation which advantageously overcomes the deficiencies in the prior art. Specifically,  FIG. 2  is a partial sectional view of a substrate support  138  supporting a substrate  140  thereon in a raised position suitable, for example, for transferring the substrate  140  into or out of the processing chamber  102 . Due to the size of the substrate  140 , areas  204  which are not supported by the lift pins  150 ,  152  will sag. The high temperature of the processing chamber  102  may exacerbate this effect causing even greater sag. Although the first and the second sets  180 ,  182  of lift pins  150 ,  152  are shown extended to a substantially equivalent height above the support assembly  138 , it is contemplated that the second set  182  of lift pins  152  may be at a different height than the first set  180  of lift pins  150  when the substrate  140  is first placed upon the lift pins  150 ,  152 .  
      As shown in  FIG. 3 , the profile of the substrate  140  may be controlled to form a more arcuate shape. In one embodiment, this may be accomplished by lowering the inner, second set  182  of lift pins  152  by the actuators  170 . The inner, second set  182  of lift pins  152  move independently of the outer, first set  180  of lift pins  150 . As shown in  FIG. 4 , a center portion  402  of the substrate  140  is brought into contact with the support surface  134  of the support assembly  138 . This may be accomplished by raising the support assembly  138  to come into contact with the substrate  140 . Alternatively, the first and second sets  180 ,  182  of lift pins  150 ,  152  may be lowered or some other combination of movements of the support assembly  138  and the first and second sets  180 ,  182  of lift pins  150 ,  152  to bring the center portion  402  of the substrate  140  into contact with the support surface  134 .  
      As shown in  FIG. 5 , as the relative distance between the support assembly  138  and the substrate  140  continues to lessen, the substrate  140  smoothly comes into contact with the support assembly  138  along a rolling contact point  502 . The rolling contact point  502  progressively moves outward to squeeze out the air from between the substrate  140  and the support surface  134  of the support assembly  138 . Thus, when the transfer of the substrate  140  to the support assembly  138  is completed, the substrate  140  is uniformly positioned on the support assembly  138  with substantially no gas entrained between the substrate  140  and the support surface  134  of the support assembly  138 .  
      Although the description and drawings depict a method of transferring the substrate  140  in a center to edge manner, it is also contemplated that the substrate  140  could be transferred from one side to the other, e.g., left-to-right, right-to-left, and the like. Specifically, one skilled in the art looking at the placement sequence depicted in  FIGS. 2-5 , could readily arrange and control the actuators  170  and lift pins  150 ,  152  to place the substrate  140  onto the support assembly  138  in a side-to-side manner that squeezes out the air from between the substrate  140  and the support surface  134  of the support assembly  138  as discussed above.  
       FIG. 6  depicts another embodiment of a PECVD system  600  having a support assembly  638  suitable for processing large area substrates. The system  600  is substantially similar to the system  100  described with respect to  FIG. 1 , with the exception of the details described below.  
      A lift plate  612  is disposed proximate the underside  126  of the support assembly  638 . The lift plate  612  is disposed below the second ends  166  of the second set  182  of lift pins  152  such that the lift plate  612  may contact the lift pins  152  and cause them to extend from the support surface  634  of the support assembly  638 . The lift plate  612  is coupled to an actuator such as a pneumatic cylinder, hydraulic cylinder, lead screw, solenoid, stepper motor, or other motion device (not shown) that is typically positioned outside of the process volume  112 . The lift plate  612  is connected to the actuator by a collar  606  that circumscribes a portion of the stem  142 . A bellows  646 , similar to bellows  146  in  FIG. 1 , includes an upper portion  604  and a lower portion  602  that allow the stem  142  and collar  606  to move independently while maintaining the isolation of the process volume  112  from the environment exterior to the chamber  102 . Alternatively, the motions of the lift plate  612  and support assembly  638  may be controlled via a single actuator utilizing a spring and a motion stop that controls the relative motion between the lift plate  612  and support assembly  638 .  
      Generally, the lift plate  612  is actuated to control the position of the lift pins  152  relative to the support surface  634  of the support assembly  638  and the lift pins  150 . By controlling the amount of extension of the lift pins  152  above the support surface  634  relative to the amount of extension of the lift pins  150  from the support surface  634 , the shape of the substrate  140  may be controlled to ensure proper placement of the substrate  140  on the support assembly  638  in a progressive center-to-edge manner, as discussed above. The support assembly  638  may move relative to lift plate  612  either by moving the support assembly  638 , moving the lift plate  612 , or a combination thereof.  
      In the embodiments depicted in  FIGS. 7 and 8 , the desired relative position of the first set  180  of lift pins  150  and the second set  182  of lift pins  152  is predetermined based upon the calculated deflection, or sag, of the substrate  140 . The calculated deflection is generally based upon the physical characteristics of the substrate  140  and processing conditions within the processing chamber  102 , for example, temperature.  
      In the embodiment depicted in  FIG. 7 , a PECVD system  700  is shown having a support assembly  738  disposed in a chamber  102 . A plurality of offsets  702  are disposed on the bottom  108  of the chamber  102  beneath each of the first set  180  of lift pins  150 . Each of the offsets  702  has a height D. As the support assembly  738  is lowered, the second ends  164  of the lift pins  150  contact the offset  702  while the second ends  166  of the lift pins  152  contact the bottom  108  of the chamber  102 . The contact of the lift pins  150 ,  152  respectively with the offset  702  and bottom  108  causes the lift pins  150 ,  152  to stop moving as the support assembly  738  continues to descend, thereby causing the first ends  160 ,  162  of the lift pins  150 ,  152  to extend above the support surface  734  of the support assembly  738 .  
      Due to the presence of the offset  702  on the chamber bottom  108 , the first set  180  of lift pins  150  will be higher than the second set  182  of lift pins  152  by the height D of the offset  702 . The height D is calculated to maintain a desired profile of a substrate  140  placed upon the extended lift pins  150 ,  152  such that, upon raising the support assembly  738 , the substrate  140  comes into contact with the support surface  734  of the support assembly  738  smoothly and continuously from the center of the substrate  140  towards the outer edges of the substrate  140 . As discussed above, this advantageously forces the gas out from between the substrate  140  and the support surface  734  of the support assembly  738 . Alternatively, the lift pins  150  may be longer than the lift pins  152  by the calculated height D without the need for the offset  702  to be disposed in the bottom  108  of the chamber  102 .  
      In the embodiment depicted in  FIG. 8 , a PECVD system  800  is shown having a support assembly  838  disposed in a processing chamber  102 . The system  800  is substantially similar to the system  600  described with respect to  FIG. 6 , with the exception of the following details.  
      A lift plate  812  is disposed proximate the underside  126  of the support assembly  838 . The lift plate  812  is coupled to an actuator (not shown) as described with reference to  FIG. 6 , above. The lift plate  812  may move relative to the support assembly  838  either by moving the support assembly  838 , moving the lift plate  812 , or a combination thereof.  
      The lift plate  812  is disposed below the second ends  164 ,  166  of the lift pins  150 ,  152  such that the lift plate  812  may contact the lift pins  150 ,  152  and cause them to extend from the support surface  834  of the support assembly  838 . The lift plate  812  has an inner surface  814  and a raised outer surface  816 . The outer surface  816  is disposed at a height D above the inner surface  814 .  
      Generally, the lift plate  812  is actuated to control the position of the lift pins  150 ,  152  relative to the support surface  834  of the support assembly  838 . The difference in height D between the inner and outer surfaces  814 ,  816  of the lift plate  812  is calculated to maintain a desired profile of a substrate  140  placed upon the extended lift pins  150 ,  152  such that, upon lowering the lift pins  150 ,  152 , the substrate  140  comes into contact with the support surface  834  of the support assembly  838  smoothly, continuously, and progressively from the center of the substrate  140  towards the outer edges of the substrate  140 . As discussed above, this advantageously forces the air out from between the substrate  140  and the support surface  834  of the support assembly  838 . Alternatively, the lift pins  150  may be longer than the lift pins  152  by the calculated height D without the need for the difference in height between the inner surface  814  and the outer surface  816  of the lift plate  812 .  
       FIG. 9  depicts a flow chart of a method  900  for placing a substrate upon a substrate support. The method  900  begins at step  902  where a substrate  140  is placed, typically by a robot, on the lift pins  150 ,  152  above the support assembly  138  as shown in  FIG. 2 . Alternatively, the second set of lift pins  182  may already be lower than the first set of lift pins  180 , as shown in  FIG. 3 . At optional step  904 , the shape of the substrate is manipulated to the predefined profile by adjusting the relative extensions of the inner and outer lift pins  150 ,  152 . In one embodiment, the inner lift pins  152  are lowered relative to the support surface  134  to control the profile of the substrate  140  to be substantially arcuate, as shown in  FIG. 3 .  
      At step  906 , the support assembly  138  is raised to place the substrate  140  in position on the support surface  134  of the support assembly  138 . As seen in  FIGS. 4 and 5 , a center portion  402  of the substrate  140  comes into contact with the support surface  134  of the support assembly  138  prior to any other portion of the substrate  140 . As shown in  FIG. 5 , as the substrate support continues to rise, the substrate  140  comes into contact with the support surface  134  of the support assembly  138  along rolling contact points  502  which act to squeeze out any gas from between the substrate  140  and the support surface  134  of the support assembly  138  until the substrate  140  rests completely flat on the support surface  134  of the support assembly  138 , as shown in  FIG. 1 . Alternatively, the support assembly  138  may be stationary and the first and the second set of lift pins  180 ,  182  may be actuated to lower the substrate  140  onto the support surface  134 , or the support assembly  138  and the first and the second set of lift pins  180 ,  182  may both move.  
      While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.