Abstract:
An alignment device is provided that includes (1) a first pusher adapted to contact an edge of a substrate supported on a stage and to laterally translate along a first path; (2) a second pusher adapted to contact the substrate edge and to laterally translate along a second path that is at an angle to and intersects the first path; (3) a frame, to which the first and second pushers are movably coupled, adapted to maintain the first and second pushers at an elevation of the substrate edge; (4) a first biasing element coupled between the first pusher and the frame and adapted to bias the first pusher against the substrate edge; and (5) a second biasing element coupled between the second pusher and the frame and adapted to bias the second pusher against the substrate edge independent of the biasing of the first pusher. Other aspects are provided.

Description:
The present application claims priority from U.S. Provisional Patent Application No. 60/448,855, filed Feb. 20, 2003, which is hereby incorporated by reference herein in its entirety. 
   CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to the following co-pending U.S. Provisional Patent Applications:
         U.S. Provisional Patent Application Ser. No. 60/448,821, filed Feb. 20, 2003, and titled “METHODS AND APPARATUS FOR DETERMINING A POSITION OF A SUBSTRATE RELATIVE TO A SUPPORT STAGE”; and   U.S. Provisional Patent Application Ser. No. 60/448,972, filed Feb. 20, 2003, and titled “METHODS AND APPARATUS FOR POSITIONING A SUBSTRATE RELATIVE TO A SUPPORT STAGE”.       

   Each of these provisional patent applications is hereby incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates generally to substrate processing, and more particularly to positioning a substrate relative to a support stage. 
   BACKGROUND OF THE INVENTION 
   During the manufacture of flat panel displays, a glass substrate may be placed on a support stage for processing and/or testing purposes. Typical substrate processing may include lithography, deposition, etching, annealing, etc., and typical substrate testing may include verifying the operation of thin film transistors formed on the substrate, e-beam inspection, defect detection, etc. 
   To accurately identify device and/or substrate locations for processing and/or testing, and/or to reduce device/location search times, a position of a substrate relative to a support stage should be determined. Accordingly, improved methods and apparatus for quickly and accurately positioning a substrate relative to a support stage would be desirable. 
   SUMMARY OF THE INVENTION 
   In a first aspect of the invention, an alignment device is provided that is adapted to laterally urge a substrate supported on a support stage so as to cause the substrate to slide relative to the support stage. The alignment device includes (1) a first pusher adapted to contact an edge of a substrate supported on the support stage and to laterally translate along a first path of translation; (2) a second pusher adapted to contact the edge of the substrate and to laterally translate along a second path of translation, the second path of translation being at an angle to the first path of translation and intersecting the first path at a path intersection; (3) a frame to which the first and second pushers are movably coupled, the frame adapted to maintain the first and second pushers at an elevation of the edge of the substrate supported on the support stage; (4) a first biasing element coupled between the first pusher and the frame and adapted to bias the first pusher against the edge of the substrate; and (5) a second biasing element coupled between the second pusher and the frame and adapted to bias the second pusher against the edge of the substrate independent of the biasing of the first pusher. 
   In a second aspect of the invention, a substrate calibration system is provided that includes a plurality of alignment devices arranged in a spaced relation around a support stage. Each of the plurality of alignment devices comprises (1) a first pusher adapted to contact an edge of a substrate supported on the support stage and to laterally translate along a first path of translation; (2) a second pusher adapted to contact the edge of the substrate and to laterally translate along a second path of translation, the second path of translation being at an angle to the first path of translation and intersecting the first path at a path intersection; (3) a frame to which the first and second pushers are movably coupled, the frame adapted to maintain the first and second pushers at an elevation of the edge of the substrate supported on the support stage; (4) a first biasing element coupled between the first pusher and the frame and adapted to bias the first pusher against the edge of the substrate; and (5) a second biasing element coupled between the second pusher and the frame and adapted to bias the second pusher against the edge of the substrate independent of the biasing of the first pusher. Numerous other aspects are provided, as are methods and apparatus in accordance with these and other aspects of the invention. 
   Other features and aspects of the present invention will become more fully apparent from the following detailed description of exemplary embodiments, the appended claims and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic top view of an exemplary embodiment of a substrate calibration system in accordance with the present invention. 
       FIG. 2A  is a schematic top view of one of the pushing devices of  FIG. 1  wherein pushers of the pushing device are retracted away from a hub portion of the frame of the pushing device. 
       FIG. 2B  is a schematic top view of one of the pushing devices of  FIG. 1  wherein the pushers of the pushing device are in contact with the edge of a substrate prior to calibration of the substrate relative to a support stage. 
       FIG. 2C  is a schematic top view of one of the pushing devices of  FIG. 1  wherein the substrate has been calibrated relative to the support stage. 
       FIGS. 3 and 4  are, respectively, a perspective exploded assembly view, and a cross-sectional side view of a pushing device that comprises an exemplary embodiment of the pushing devices of FIGS.  1  and  2 A– 2 C. 
   

   DETAILED DESCRIPTION 
   Conventional manufacturing methods for the production of glass substrates, e.g. for use in flat panel displays and other applications, may produce substrates that vary in size. For example, a glass substrate having nominal width and length dimensions of 1 meter×1.2 meter may have a variation of up to +/−0.5 millimeters per side or more. 
   Such variation in glass substrate dimensions can cause problems during device processing and/or testing. For example, a lithographic system may rely on one or more electron beams (e-beams) to search for registration marks on the surface of a substrate during mask alignment. An e-beam will function best when the surface area of the substrate within which the e-beam is expected to scan, e.g. in order to find a particular registration mark therein, is minimized. However, variations in substrate size, as described above, tend to increase the area in which an e-beam must scan in order to locate registration marks. Any increase in e-beam scanning area may increase the time required to locate registration marks. Since the printing of a mask on the surface of a substrate must be delayed until proper alignment is established, longer scan times directly affect the efficiency of the lithographic process. Moreover, long e-beam scanning times may damage devices formed on a substrate due to excessive charge that may build up during the e-beam scanning process. 
   In a first aspect of the present invention, the methods and apparatus disclosed herein are adapted to subject a substrate placed on a support stage, which can be a test stage or other type of substrate processing stage, to positional and/or orientational adjustment relative to the support stage. Preferably, positioning and/or orientation of the substrate in accordance with the first aspect of the invention will result in at least rough alignment of the substrate relative to a known position and orientation of the support stage. In a particular embodiment, such rough alignment may be performed despite variations in substrate size, thereby reducing and/or minimizing e-beam scan areas and/or scan times during lithographic processing. As described further below, such substrate alignment may be performed quickly and inexpensively, and may be employed with other substrate processing steps and/or during device testing. 
     FIG. 1  is a schematic top view of an exemplary embodiment of a substrate calibration system  101  in accordance with the present invention. The substrate calibration system  101  is shown positioned adjacent a support stage  103  that supports a substrate  105  having an edge  107 . 
   In the embodiment of  FIG. 1 , the substrate calibration system  101  comprises four alignment devices (e.g., pushing devices  109 ) arranged in a spaced relation around the support stage  103 . Few or more pushing devices may be employed. Each pushing device  109  is positioned adjacent a corner of the support stage  103  (alternative positions are also acceptable, such as adjacent a side of the support stage  103 ). Each pushing device  109  includes a first and a second pusher support  113   a ,  113   b , the first pusher support  113   a  being adapted to move relative to the frame  111  of the pushing device  109  along a first translation path  115   a  (and to contact the substrate  105  via a first pusher  117   a ), and the second pusher support  113   b  being adapted to move relative to the frame  111  of the pushing device  109  along a second translation path  115   b  (and to contact the substrate  105  via a second pusher  117   b ). The frame  111  of each pushing device  109  may include a hub portion  119  (e.g., generally located at a path intersection point  121  where the first translation path  115   a  and the second translation path  115   b  intersect). Other configurations for the frame  111  of the pushing device  109  may also be employed, e.g. wherein no portion of the frame  111  is located at the path intersection point  121 . 
   Also as shown in  FIG. 1 , the hub portion  119  of the frame  111  of the pushing device  109  may be positioned and configured so as to be wholly or partially beneath the substrate  105 . Furthermore the hub portion  119  may be positioned and configured so as to be wholly or partially beneath the support stage  103 . It will be apparent therefore that if the presence of the substrate calibration system  101  increases the footprint of the overall system (e.g. as might be primarily established by the breadth and length of the support stage  103  in a given processing or inspection tool), such an increase may be minimized and/or relatively small compared to the footprint of the support stage  103 . 
   Movement of each pusher support  113  relative to the frame  111  of the pushing device  109  can be facilitated by a guide shaft  123 . Each guide shaft  123  may be used to define a translation path  115 , and in the case when a translation path  115  is linear, the guide shaft  123  may also be straight. In other embodiments, the guide shaft  123  may deviate from straight, depending on the desired shape of the translation path  115 . In the example of  FIG. 1  each pushing device  109  comprises two guide shafts  123 , and an end of each guide shaft  123  is coupled to the hub portion  119  of the frame  111  of the pushing device  109 . Other arrangements may also be employed, e.g. wherein the frame  111  is coupled to both ends of one or both guide shafts  123 . 
   Each pushing device  109  may further be adapted to confine the movement of each pusher support  113  along its translation path  115  to within a predefined range (e.g., a first and second predefined range, respectively). For example, as shown in  FIG. 1 , the pushing device  109  may further comprise a stop plate  125  (shown in profile) coupled to the frame  111  of the pushing device  109  adapted to perform such a function. In the example of  FIG. 1 , each pushing device  109  comprises two stop plates  125 , and each stop plate  125  is coupled to the frame  111  of the pushing device  109  via a projection portion  127  of the frame  111  that extends outward from the hub portion  119  of the frame  111 . While two projection portions  127  are shown in  FIG. 1 , it will be understood that more or fewer than two projections may be employed. 
   In operation, the substrate calibration system  101  of  FIG. 1  is adapted to allow the substrate  105  to be loaded onto the support stage  103  by moving the pushers  117  of the pushing devices  109  generally away from the support stage  103  (e.g., via translation of the pusher supports  113 ). Thereafter, the substrate  105  may be loaded onto the support stage  103  and the pushers  117  may be moved toward the edge  107  of the substrate  105  so as to calibrate the substrate  105  relative to the support stage  103 . Further details regarding modes of operating the pushing devices  109  of the substrate calibration system  101  are discussed below with reference to  FIGS. 2A–2C . 
     FIG. 2A  is a schematic top view of one of the pushing devices  109  of  FIG. 1  wherein the pushers  117  of the pushing device  109  are retracted away from the hub portion  119  of the frame  111  of the pushing device  109 . When the pushers  117  are so positioned, the substrate  105  may be placed on or unloaded from the support stage  103 . In a particular embodiment, the substrate  105  of  FIG. 2A  is in contact with the support stage  103  only. In such an embodiment the pushing device  109  may be further configured such that the remainder of the pushing device  109  (other than the pushers  117 ) is spaced apart from and below the substrate  105  so as to facilitate loading and unloading of the substrate  105  with respect to the support stage  103 . Other embodiments provided for the substrate  105  of  FIG. 2A  to contact both the support stage  103  and a top surface  131  of one or both of the pusher supports  113  of the pushing device  109  so as to provide additional support to the substrate  105 . (Note that in  FIG. 2A , the substrate  105  still requires calibration to the support stage  103 , e.g. such that the edge  107  of the substrate  105  will thereafter be aligned with an x-y coordinate system  133  of the support stage  103 .) 
     FIG. 2B  is a schematic top view of one of the pushing devices  109  of  FIG. 1  wherein the pushers  117  of the pushing device  109  are in contact with the edge  107  of the substrate  105  prior to calibration of the substrate  105  relative to the support stage  103 . For example, each pusher  117  has moved along its respective translation path  115  from a retracted position as shown in  FIG. 2A  and toward the support stage  103  at least as far as is necessary to achieve contact with the edge  107  of the substrate  105  as the substrate  105  rests on the support stage  103 . As mentioned above, the substrate  105 , as it appears in  FIG. 2B , may also be in contact with the top surface  131  of one or both of the pusher supports  113 . Alternatively, contact between the substrate  105  and the pushing device  109  may be confined to the pushers  117 . Surfaces of the pushing device  109  that contact the substrate  105  (e.g. vertical surfaces the pushers  117  and/or the top surface  131  of the pusher supports  113 ), and/or the surface of the support stage  103  that supports the substrate  105 , may be treated so as to promote smooth sliding therebetween and to protect against scratching a surface of the substrate  105 . For example, such surfaces may be coated with Teflon® (e.g., polytetrafluoroethylene), or with a similar low-friction coating material. Such a low-friction coating may be advantageous when a substrate must slide against a pusher  117  during positioning/calibration of the substrate to the support stage  103 . 
     FIG. 2C  is a schematic top view of one of the pushing devices  109  of  FIG. 1  wherein the substrate  105  has been calibrated relative to the support stage  103  (e.g., via the pushers  117  contacting and pushing the edge  107  of the substrate  105 ). That is, the edge  107  of the substrate  105  is aligned with an x-y coordinate system  133  of the support stage  103 . In one or more embodiments, the substrate calibration system  101  ( FIG. 1 ) is adapted to calibrate the substrate  105  to the support stage  103  by confining and/or enclosing the substrate  105  within a perimeter (not separately shown) defined by the pushers  117 . In some such embodiments, it is not necessary for any of the pushers  117  to continue to push against the edge  107  of the substrate  105  once the substrate  105  is within the perimeter. In yet another embodiment, the substrate calibration system  101  and the support stage  103  can be moved in tandem during processing or inspection, with the substrate  105  continuing to be confined within the perimeter defined by the pushers  117 . In other such embodiments, one or more of the pushers  117  may continue to push against the edge  107  of the substrate  105 , providing a further measure of confinement for applications that require the same (whether or not the support stage  103  may be moved in tandem with the substrate calibration system  101 ). 
     FIGS. 3 and 4  are, respectively, a perspective exploded assembly view, and a cross-sectional side view of a pushing device  109   a  that comprises an exemplary embodiment of the pushing devices  109  of FIGS.  1  and  2 A– 2 C. 
   With reference to  FIGS. 3 and 4 , the pushing device  109   a  comprises a frame  111  and two pusher supports  113  movably coupled to the frame  111 . Each pusher support  113  is adapted to move relative to the frame  111  along a linear translation path  115 . The pushing device  109   a  further includes two pushers  117 , one each pusher  117  coupled to the top surface  131  of one of the pusher supports  113  for movement relative to the frame  111  along the translation path  115 . The frame  111  includes a hub portion  119  generally located at a path intersection point  121  where the translation paths  115  of the pusher supports  113  and the pushers  117  intersect. The pushing device  109   a  further includes two guide shafts  123  coupled to the hub portion  119  of the frame  111  of the pushing device  109   a . For example, each guide shaft  123  may extend from the hub portion  119  in a cantilevered arrangement. Each guide shaft  123  is adapted to guide one of the pusher supports  113  along the translation path  115  of the pusher support  113 . For example, each pusher support  113  may comprises a first and a second downward-extending portion  135 ,  137 , each including a hole within which may be disposed a shaft bearing  139  for facilitating smooth movement of the pusher support  113  along the guide shaft  123 . 
   The frame  111  of the pushing device  109   a  further includes two projection portions  127 , each projection portion  127  enclosing one of the guide shafts  123  within an open enclosure  141 . The pushing device  109   a  also includes two stop plates  125 , one stop plate  125  coupled to a different one of the projection portions  127  and forming an end boundary for its respective open enclosure  141 . 
   The pushing device  109   a  further includes two biasing elements  143 , one biasing element  143  being extensibly coupled within each open enclosure  141  between the stop plate  125  and the second downward-extending portion  137  of the pusher support  113  that is located within the open enclosure  141 . It will be apparent that each biasing element  143  of the pushing device  109  is adapted to (1) push against a stop plate  125 , which is preferably fixedly coupled to the frame  111 ; (2) extend within an open enclosure  141  toward the hub portion  119 ; and (3) move the pusher support  113  and the pusher  117  along a guide shaft  123  (and along the translation path  115  defined by the guide shaft  123 ) toward the hub portion  119  as the biasing element  143  so extends. In this manner each pusher support  113  and pusher  117  may be moved from a retracted position ( FIG. 2A ) to a contact position relative to the edge  107  of the substrate  105  ( FIG. 2B ) and finally to an extended position ( FIG. 2C ). Referring specifically to  FIG. 4 , one of the pusher supports  113  and one of the pushers  117  of the pushing device  109   a  are shown in a fully-extended position. As shown in  FIG. 4 , the pushing device  109   a  may comprise one or more adjustable spacing mechanisms  145 . Each spacing mechanism  145  is adapted to establish a limit to which a pusher support  113  may be moved along a guide shaft  123  toward the hub portion  119  under the force of a biasing element  143  (e.g., to establish a fully-extended position of the pusher support  113  and its pusher  117 ). In the specific example of  FIG. 4 , the spacing mechanism  145  comprises a setscrew  147  occupying a threaded through-hole that perforates the first downward-extending portion  135  of the pusher support  113 , and a nut  149  coupled to the setscrew  147  for adjustably setting the length of the setscrew  147  that extends beyond the first downward-extending portion  135  and toward the stop plate  125 . As shown in  FIG. 4 , an end  151  of the setscrew  147  is adapted to contact the stop plate  125 , preventing any further motion of the pusher support  113  toward the hub portion  119 . Any other configuration for limiting motion of the pusher support  113  and/or pusher  117  similarly may be employed. 
   Referring to  FIGS. 3 and 4 , the pushing device  109   a  may be further adapted to establish a limit to which a pusher support  113  may be moved along a guide shaft  123  away from the hub portion  119  against the force of a biasing element  143  (e.g., to establish a fully-retracted position of the pusher support  113  and its pusher  117 ). For example, each pusher support  113  of the pushing device  109   a  may comprise a lower portion  153 , adapted to contact the stop plate  125 , and to prevent further motion of the pusher support  113  away from the hub portion  119 . As illustrated in  FIG. 4 , when the pusher support  113  is in a fully-extended position, the lower portion  153  of the pusher support  113  is spaced apart from the stop plate  125  by a first distance  155  (e.g., the maximum range of motion (not separately shown) of the pusher support  113  along the guide shaft  123 ). 
   The pushing device  109   a  may also be adapted to substantially prevent rotation of the pusher support  113  relative to the frame  111 , e.g. as the pusher support  113  moves relative to the frame  111  along the translation path  115 . For example, each pusher support  113  may comprise a guidepin  157  coupled to the lower portion  153  of the pusher support  113  and extending downward from the lower portion  153 . Each projection portion  127  may be shaped so as to define a guide channel  159  for retaining the guidepin  157  as the pusher support  113  moves along the translation path  115 . 
   The pushing device  109   a  may be adapted to retract each pusher support  113  away from the hub portion  119  of the frame  111  of the pushing device  109   a  against the force of the biasing element  143  associated with each pusher support  113  so as to cause each pusher support  113  to assume the fully-retracted position described above. For example, each pusher support  113  may include an open area  161  ( FIG. 4 ) between the second downward-extending portion  137  and the lower portion  153  of the pusher support  113  and defined in part by a pair of wall portions  163  that span the distance between and connect the second downward-extending portion  137  and the lower portion  153 . An axle  165  may be coupled to each wall portion  163  and a rotary member  167  such as a roller wheel may be rotatably mounted on the axle  165  within the open area  161 . 
   Furthermore the pushing device  109   a  may comprise a plunger  169  or similar retraction device adapted to urge each pusher support  113  away from the hub portion  119  by interacting with the rotary member  167  of each pusher support  113 . For example, the plunger  169  may comprise a plunger head  171  having an inclined surface  173 , and the plunger  169  may be pushed upward so as to cause the rotary member  167  of each pusher support  113  to roll along the inclined surface  173  as the plunger head  171  moves upward. In this manner, each pusher support  113  will be displaced (along with each rotary member  167 ) outward from the hub portion  119  of the frame  111 . For this purpose the pushing device  109   a  may further comprise an actuator  175 , e.g. a linear actuator such as a motor or pneumatically-driven actuator, adapted to retractably extend a pusher  177  upward, so as to push a shaft  179  of the plunger  169  upward, causing the above-described outward displacement of each rotary member  167 . 
   As stated above, the pushing device  109   a  may be adapted to utilize the plunger  169  and the actuator  175  to retract both pusher supports  113  simultaneously. For example, the shaft  179  of the plunger  169  may be vertically aligned beneath the path intersection point  121  where the translation paths  115  of the pusher supports  113  and the pushers  117  intersect. A downward-extending portion  181  ( FIG. 4 ) of the hub portion  119  of the frame  111  may at least partially define a plunger head insertion chamber  183  of the pushing device  109   a  for accommodation of the plunger head  171  of the plunger  169  as the plunger  169  is retractably extended upward. As best seen in  FIG. 4 , the plunger head insertion chamber  183  may be further defined by each projection portion  127  of the frame  111 ; and the rotary member  167  of each pusher support  113  may be adapted to project, at least partially, into the plunger head insertion chamber  183  such that rolling communication between the rotary member  167  and the inclined surface  173  of the plunger head  171  may be achieved. 
   For precise positioning and alignment of the pushing device  109   a  relative to the support stage  103  and for secure mounting of the pushing device  109   a  to a mounting member such as the chamber bottom  185  shown in  FIG. 4 , the pushing device  109   a  may include a shaft alignment member  187 , a chamber interface  189 , and an actuator mounting frame  191 . The shaft alignment member  187  may include shaft bearings  193  for guiding the vertical movement of the shaft  179  of the plunger  169 . 
   The shaft alignment member  187  may include an upward-extending portion  195  for mating with the downward-extending portion  181  of the hub portion  119  of the frame  111 , and for establishing a proper elevation of the pushers  117  of the pushing device  109   a  relative to the edge  107  of the substrate  105  (and/or of the hub portion  119  of the frame  111  relative to the support stage  103 , as shown in  FIG. 4 ). In one or more embodiments, the shaft alignment member  187  may also include a machined cylindrical surface  197  so that the shaft alignment member  187  may be inserted into a machined through-hole  199  ( FIG. 4 ) that penetrates the chamber bottom  185  and be precisely located therein. The chamber bottom  185  may include multiple such holes (not separately shown) arranged in a pattern (not separately shown) around the support stage  103  ( FIG. 1 ) so as to facilitate precise calibration of the substrate  105  relative to the support stage  103 . 
   The chamber interface  189  may couple to the shaft alignment member  187 , as well as attach to a surface of the chamber bottom  185  such that a sealing ring (not shown) residing in a channel  201  formed within the chamber interface  189  seals the interface between the chamber bottom  185  and the chamber interface  189 . As well, the actuator mounting frame  191  may comprise a surface  203  to which the actuator  175  may be mounted and through which the pusher  177  of the actuator  175  may be inserted, and a coupling portion  207  so the actuator mounting frame  191  may be coupled to the chamber interface  189  and/or fixed in place relative to the machined through-hole  199  of the chamber bottom  185 . 
   In operation, the pushing device  109   a  of  FIGS. 3 and 4  may be operated in accordance with a substrate pushing mode wherein the pushing device  109   a  applies forces to the edge  107  of the substrate  105  along two different directions, each such direction coinciding with a direction of a translation path  115  through which the pushing device  109   a  moves a pusher support  113  and a pusher  117 . In one embodiment, the pushing device  109   a  applies a first force to the edge  107  of the substrate  105  along a first direction, applies a second force to the edge  107  along a second direction that is substantially perpendicular to the first direction, and moves the substrate  105  relative to the support stage  103 , e.g. based on a combination of the first and second forces. The first and second forces may be generated by the biasing element  143  of each pusher support  113  ( FIGS. 1A–3 ) and may be transmitted to the edge  107  of the substrate  105  via the pushers  117 . 
   As described above, the pushing device  109   a  moves each pusher support  113  along a translation path  115  toward the hub portion  119  of the frame  111  under the force of a biasing element  143 . The motion of each pusher support  113  toward the hub portion  119  may continue until an equal and opposite force prevents further such motion. This force may be supplied in a number of different ways. For example, the substrate  105  may stop at a position relative to the support stage  103  at which multiple pushers  117  are pushing the edge  107  of the substrate  105  in different directions (e.g., such that all of the different direction lateral forces generated by and/or through the pushers  117  are balanced). Alternatively the substrate  105  may stop at a position relative to the support stage  103  at which all of the pusher supports  113  (and/or pushers  117 ) have reached a fully extended position (as shown in  FIG. 4 ). In the latter circumstance, the substrate  105  may be adequately calibrated to the support stage  103  even though positive location of the edge  107  of the substrate  105  to a physical barrier or stop may not have been achieved. For example, the edge  107  of the substrate  105  may then be within a perimeter established by fully-extended pushers  117 . 
   In at least one embodiment of the invention, the substrate calibration system  101  ( FIG. 1 ) may comprise four pushing devices  109  that contact a substrate near its four corners (e.g., at a total of eight locations). In one aspect, such a system may position the substrate accurately to within about 0.3 mm. Probes or other measuring devices may be coupled to one or more pushing devices  109  so as to be “pre-aligned” with a substrate positioned by the pushing devices  109 . Note that each pusher support  113  and/or pusher  117  may extend independently, and by an amount that is independent, of other pusher supports  113  (whether the other pusher supports are part of the same pushing device  109  or another pushing device  109 ). 
   The pushing device  109  may also be operated in a pusher retraction mode wherein the actuator  175  of the pushing device  109   a  pushes the shaft  179  of the plunger  169  upward. The plunger head  171 , being moved upward within the plunger head insertion chamber  183  of the hub portion  119 , contacts the rotary member  167  of each pusher support  113  and displaces the rotary member  167  outward from the hub portion  119  while the rotary member  167  rolls along the inclined surface  173  of the plunger head  171 . The upward motion of the plunger head  171  may continue and thereby move each pusher support  113  away from the hub portion  119  along its translation path  115 , e.g. until the lower portion  153  of the pusher support  113  contacts the stop plate  125 . The pushers  117  will then be fully retracted and a substrate may be loaded onto or removed from the stage  103  by any number of means, e.g. one or more human operators, a robot equipped with an end effector or the like. 
   The forgoing description discloses only exemplary embodiments of the invention; modifications of the above-disclosed apparatus and methods which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For example, while the present invention has been described primarily with regard to adjusting the position of glass plates relative to a support stage, it will be understood that the present invention may be employed to adjust the position of other types of substrates. A controller (not shown), such as an appropriately programmed microprocessor or microcontroller, may be coupled to the actuator  175  of each pushing device  109  and adapted to control retraction and extension of the pushers  117  as described above. 
   Accordingly, while the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.