Patent Publication Number: US-2020286764-A1

Title: High resolution stage positioner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/565,951, filed Sep. 29, 2017, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to motion control and positioning of stages used in semiconductor fabrication. 
     INTRODUCTION 
     Controlling the accuracy and precision of the positioning of a substrate relative to a mechanism that acts upon that substrate is a difficult and frankly expensive endeavor. This is particularly so when the positioning must take place over a large region and long travel. 
     In the field of semiconductor manufacturing it is customary to use interferometers, laser or broadband, to accurately position a substrate and the stage or carrier on which it is supported. However interferometers used to position a substrate generally have a measurement leg that extends through open air. Where extreme accuracy or large travel distances are desired, the variability of the index of refraction of air through which the measurement leg of an interferometer passes can negatively affect the position of the substrate. Accordingly, there is a need for a less variable means of determining and/or controlling the position of a substrate. 
     What is more, even if an interferometer can be arranged such that there is little or no variation in the index of refraction of air through which the measurement leg of the interferometer passes, the optical components necessary to create and install an interferometer are expensive. This expense is even greater where the travel of the substrate or a stage on which the substrate is supported is large. For example, mirrors used at the terminus of the measurement leg tend to extend along an axis of travel of a stage so that the position of the stage may be determined continuously. Where the stage is quite large, an optically flat mirror of suitable size is both large and very expensive. Accordingly, there is a need for a mechanism that can be used to measure and/or control large translations of a stage, which also uses relatively small and inexpensive components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic elevation of a lithography machine that may incorporate the present invention. 
         FIG. 2  is a schematic plan view of a pair of traveler mechanisms employed to locate a stage relative to a platen on which the stage travels. 
         FIG. 3A  is a schematic elevation of a traveler mechanism in which a rail is positioned between a stage and a platen. 
         FIG. 3B  is a schematic elevation of a traveler mechanism in which a rail is positioned above a stage. 
         FIG. 3C  is a schematic elevation of another traveler mechanism in which a rail is positioned above a stage. 
         FIGS. 4A and 4B  show the travel and size of a stage relative to one and two projection cameras or other apparatus, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
       FIG. 1  is a diagram of a lithography machine  100  in which the principles of the present invention may be employed. The lithography machine  100  includes a base  102 , which is typically a large block of finished granite that sits on isolation supports (not shown). The combination of the large mass of the base  102  and the design of the isolation supports provides isolation of the lithography machine  100  from floor vibrations. The isolation supports also prevent machine forces from getting into the factory floor and disturbing nearby machinery. The base  102  and isolation supports may be constructed from common commercial parts and materials. 
     On top of the base  102  is a large grid motor platen  104 , such as one disclosed in U.S. Pat. No. 5,828,142, hereby incorporated by reference. The large grid motor platen  104  may include a matrix of soft iron teeth of 1 mm square, separated in X and Y directions by a 1 mm gap. The gaps between all teeth are filled with non-magnetic material, usually epoxy. This surface is ground very flat, to tolerances of a few microns, to provide an air bearing quality surface. Flatness is also useful to control tip and tilt of a main X, Y, Θ stage  106  (hereafter referred to as the main stage  106 ), a possible source of Abbe offset errors in a stage interferometer system. 
     The area covered by the grid motor platen  104  is large enough to allow the main stage  106  to move to all required positions. The travel area allows movement to a substrate exchange position (at the machine front), to substrate alignment location(s), to all calibration locations, and throughout an exposure area. The travel area for the embodiment described herein correlates to the size of a substrate carried on the stage  106 . 
     As in  FIG. 4A , where a single projection camera or other process apparatus is provided, the lateral extents of the base  104  will be on the order of twice the lateral extents of the substrate S so that the projection camera/apparatus can address the entire substrate. In another embodiment such as the one in  FIG. 4B  wherein two projection cameras or other process apparatus are provided, the base  104  will have a lateral extent of approximately 1.5× the corresponding lateral extents of the substrate S. This allows two projection cameras/apparatuses to address the entire substrate S, each camera/apparatus covering approximately one-half of the substrate S. 
     In embodiments where substrates such as disc shaped silicon wafers are carried by the stage  106 , the travel of the stage is, at a minimum, comparable to the lateral extents of the substrate (25 mm to 450 mm), plus a bit more to allow for substrates to be placed onto and removed from the stage  106 . Large panels of glass, silicon, or composite materials used in the packaging of semiconductor devices and in the formation of large screens may also be carried by suitably sized stages  106 . Table 1 below identifies common panel sizes. Additional panel sizes and aspect ratios are contemplated. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Length [mm] 
                 Width [mm] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 GEN 1 
                 300 
                 400 
               
               
                   
                 GEN 2 
                 370 
                 470 
               
               
                   
                 GEN 3 
                 550 
                 650 
               
               
                   
                 GEN 3.5 
                 600 
                 720 
               
               
                   
                 GEN 4 
                 680 
                 880 
               
               
                   
                 GEN 4.5 
                 730 
                 920 
               
               
                   
                 GEN 5 
                 1100 
                 1250-1300 
               
               
                   
                 GEN 6 
                 1500 
                 1800-1850 
               
               
                   
                 GEN 7 
                 1870 
                 2200 
               
               
                   
                 GEN 7.5 
                 1950 
                 2250 
               
               
                   
                 GEN 8 
                 2160 
                 2460 
               
               
                   
                 GEN 10 
                 2880 
                 3130 
               
               
                   
                 GEN 10.5 
                 2940 
                 3370 
               
               
                   
                   
               
            
           
         
       
     
     The stage  106 , in one embodiment, has within its body four forcer motors (not shown). These motors are arranged to drive the stage across the grid motor platen  104 . Two motors are oriented to drive the main stage  106  in an X-axis (“X”) direction. Two additional motors are oriented at 90° to drive the main stage  106  in a Y-axis (“Y”) direction. Either or both pairs of motors may be driven differentially to provide small rotation motion (Θ). In this manner, the main stage  106  may be controlled to move in a very straight line even though the tooth pattern in the grid motor platen  104  may not be straight. Note the present invention is not limited to planar or “Sawyer” type motors. Ball screws, linear actuators of various types, piezo electric actuators or any other suitable motor or actuator that can move the stage  106  relative to a mechanism that acts upon a substrate S may be used. 
     In  FIG. 1  the stage  106  is shown as having a chuck  120  mounted thereon. The illustrated chuck  120  has a form factor adapted to support a substrate S that is a panel of the basic size/shape described in Table 1. As described in U.S. Pat. No. 7,385,671, the chuck  120  may have substituted therefore different numbers or types of chucks adapted to hold different substrates such as silicon wafers. U.S. Pat. No. 7,385,671 is hereby incorporated by reference. 
     A stiff bridge structure  108  supports a projection camera  110  above the main stage  106 . The projection camera  15  has a projection lens  112 , of approximately 2× (i.e., two times) reduction, mounted in a lens housing  114 . The lens housing  114  is mounted on two Z-axis (vertical) air bearings, not shown. These air bearings may be commercially purchased and are preferably a box journal style, which are very stiff. This Z-axis motion is used to move the lens housing  114  and projection lens  112  up and down over small distances needed for focus. The projection lens  112  is telecentric at its image side, so that small changes in focus do not cause image size or image placement errors. Note that other optical arrangements and magnifications are contemplated. 
     The projection lens housing  114  has an individual, real-time, auto-focus sensor (not shown) attached to its bottom. These sensors use simple optics to transform a laser diode light source into a focused slit of light at a substrate S. Some of the light from this slit reflects off the substrate S and is captured by a receiving side of the real-time auto-focus sensor. The reflected slit light is imaged by the receiving optics onto a linear CCD array (not shown). Image processing software is used to locate the image of the reflected slit on the CCD array. Any shift in the position of the image of the reflected slit is then used to control Z-axis drive  116  for projection camera  110 , until the position of the image on the CCD array is restored. In this manner, the “focus” of projection camera  110  is maintained at a constant gap. During machine construction, the motion of the Z-axis in micrometers is used to determine the motion of the image on the CCD array in pixel units. This calibration permits conversion of subsequent focus offsets to be implemented as pixel offsets in the Z-axis focus control system. 
     Attached to the top of the lens housing  114  is a fold mirror  130 . This mirror  21  puts the remainder of the projection camera  110  off to the right side. In this embodiment, the projection lens  112  is designed to have a long working distance at its object side to permit use of the fold mirror  130 . Note that by the omission of fold mirrors from the projection camera  110 , a straight optical path may be achieved. Fold mirrors having different orientations may also be used to further form the optical path of the projection camera  110  to meet whatever space requirements that exist. 
     Projection camera  110  has its own 6-axis reticle chuck  132 , which holds a reticle  134  that includes the pattern or mask being imaged onto the respective substrate. The reticle  134  may be referred to as an image source. It should be understood that other devices may also be used as image sources, such as a multi-mirror light valve or an LCD light valve that dynamically generates a mask (i.e, a maskless image source). 
     Illumination for the lithography exposure is provided by a lamp house  140  that encloses a mercury lamp that in one embodiment outputs about 3500 watts power. The light within the lamp house  140  is collected, focused, and filtered, and then exits the lamp house  140  near a shutter  142 . Note that as shown, the lamp house  140  includes a fold mirror  131  that allows the optical path of the projection camera  110  to be made more compact. The folded arrangement of the projection camera  110  illustrated in  FIG. 1  is only one configuration of many that can be or are commonly used. 
     When the shutter  142  is opened, light from the lamp house  140  passes through a condenser lens assembly  144 , through the reticle  134 , through projection lens  112 , and exposes the substrate S with the image imposed by the reticle  134 . As is well understood, the substrate S is coated with a photo-sensitive resistive coating. A dose sensor (not shown) may be part of the shutter  142 . 
     The foregoing description is of a stepper type configuration for a lithography system. Other configurations such as scanners, imprint, and direct write lithography systems are well known and may benefit from the application of the present invention. What is more, while the present invention is particularly useful in lithography applications, it is not so limited. 
       FIG. 2  illustrates one embodiment of the present invention, which is a traveler mechanism  200  for accurately positioning a substrate S on a stage  106  relative to a projection camera  110  or other mechanism that exposes, inspects, views, measures, or otherwise acts upon a substrate S. Note that in  FIG. 2 , the projection camera  110  and supporting structure have been omitted for the sake of clarity. 
     A traveler mechanism according to the present invention allows for a large format lithography and other operations to be performed on panels and other substrates. The large travel dimensions in scan and travel directions may exceed 2 meters. This is accomplished by providing one or more trucks  210  that travel with a planar stage over a stationary platen in the stage&#39;s scan and travel directions. The truck includes a mechanism that allows travel of the truck relative to the stage along an axis other than the one in which the truck provides position information. The truck  210  runs upon a rail  212  that includes at least one linear encoder  214  to provide a position of the truck relative to the platen  104 . The position of the truck is read by a read head  216  secured to the truck  210  itself. The truck also includes a mechanism for determining a distance between the stage  106  and the truck. 
     As can be seen in  FIG. 2 , a platen  104  and stage  106  may include more than one traveler mechanism  200 . Each traveler mechanism  200  provides a position along a defined axis relative to the platen  104 . Note that cameras and other apparatus that may be addressed to a substrate are generally localized within the same coordinate system as the platen  104 . In this way the position of a substrate may be determined relative to the projection cameras/apparatuses used to modify the substrate. The sensitive axis of each traveler mechanism  200  is defined by the rail or track  212  upon which the truck or block  210  rides. The truck  210  and rail  212  combination may be formed using a linear bearing or an air bearing of a suitable sort. The length of the rail  212  defines the travel distance of the traveler mechanism  200 . One or more scales for measuring position such as linear encoders  214  are included with the rail  212 . The linear encoders  214  may be of an optical or electromagnetic type. Generally, the positioning resolution of the system will be relatively precise, being in the range of ±200 nm over the entire range of travel, which may be more than two meters. One or more read heads  216  are secured to the truck  210  and ride along the linear encoders  214  to determine the position of the truck  210  relative to the platen  104 . The rails  212  may be separate structures that are secured to the platen  104  between the platen  104  and the stage  106 . The rails  212  may also be formed as part of the platen  104  itself by insetting some or all of the structure of the rail  212  into the platen  104 . In another embodiment, the rails  212  are positioned and fixed above the stage  106 . In this embodiment, the rails  212  are coupled to the bridge support  108 . The supporting structure is omitted for clarity. The position of the rails  212  relative to the platen  104  is shown more clearly in  FIGS. 3A and 3B . 
     At the edge of stage  106 , generally perpendicular to the axes of the traveler mechanisms  200  are found reference surfaces  220 . Surfaces  220  are positioned so that distance sensors  222  that are secured to truck  210  will be able to sense a distance between the truck  210  and the stage  106 . Surfaces  220  may be solid, however as it is desired for these surfaces  220  to be as flat as possible so that measurements are as precise as possible, it must be acknowledged that solid surfaces  220  are quite expensive when their length may exceed 2 meters. In a preferred embodiment, the surfaces  220  are made of individual segments  221 . Segments  221  are more easily flat and while there may be variation in the placement of segments  221  to form surfaces  220 , such variation may be minimized and/or calibrated such that error in localization is minimized. The distance sensors  222  may be capacitive, interferometric, chromatic confocal, laser triangulation or similar distance sensors. Note that since the distance to be measured by sensors  222  is relatively short, interferometric sensors may be successfully employed in this setting. In situations where the surfaces  220  are essentially planar, only a single sensor  222  is required to measure the variation in distance between the stage  106  and the truck  210 . Where one is interested in measuring any angular variation between the stage  106  and the truck  210 , one will need to include two sensors  222 , the difference between the measurements reported by the two sensors  222  being used to determined angle between the truck  210  and the stage  106 . 
     Since the truck  210  moves only along the rails  212 , to sense the position of the stage  106  along the sensitive axis of the traveler mechanism it is necessary to couple the truck  210  to the stage  106 . To permit relative translation between a truck  210  and the stage along a non-sensitive or ancillary axis that is often perpendicular to the rails  212 , a linear coupler  226  is used to secure the truck  210  to the stage  106 . When the stage  106  moves along an axis described by arrow  230 , the truck  210  of the left traveler mechanism  200  will move in close conformity with the stage along its rails  212 . The linear encoders  214  and read heads  216  will give the position of the truck  210  relative to the platen  104 . The distance sensors  222  on the truck of the left traveler mechanism  220  provide the position of the stage  106  relative to the truck  210 . Combining the measurements obtained from the read heads  216  and the distance sensors  222  provides the information necessary to accurately localize the stage  104  relative to the platen  106  along the axis of the left traveler mechanism  200 . While the stage  106  moves along the axis  230 , truck  210  of the right traveler mechanism  200  does not move relative to its rails  212 . Rather, the truck  210  remains stationary while the coupler  226  allows the stage  106  to move relative to the truck  210 . The inverse applies when the stage  106  moves along an axis defined by arrow  232 . In this manner, the stage  106  may be easily localized in the plane defined by the platen  104 . 
     In some embodiments, such as where multiple segments  221  form the surfaces  220 , two or more sensors  222  may be desirable. For example, two sensors  222  are emplaced on a truck  210  as seen in  FIG. 2 . This allows for the measurement of an angle between the stage  106  and the truck  210 , providing that each of the segments  221  are coplanar. Two sensors  222  may also be used to measure the distance between the stage  106  and the truck  210  for each individual segment  221 . In this embodiment, the respective sensors  222  measure variation in the distance between the truck  210  and the stage  106  from what a read head  216  reports. This distance is nominally the same at all times. However, backlash may allow this distance to vary. Where the stage  106  moves relative to a truck  210  (e.g. relative motion along axis  230 ) such that different segments  221  are successively presented to sensors  222 , variation in the placement of segments  221  can lead to uncertainty in the readings. In this arrangement, one sensor  222  will precede or lead the remaining, trailing sensor  222  relative to the stage  106 . A single sensor  222  translating between variously positioned segments  221  will report a change in the stage/truck distance that is in truth a variation in position between the segments  221 . Spacing multiple sensors  222  so that they address successive segments  221  allows variation between the respective sensors  222  to be measured and used as a base line or calibration. Further, even without a baseline, a trailing sensor  222  may retain the position of a given segment  221  even as a leading sensor  222  addresses an adjacent segment  221 . Changes in the position reported by the trailing sensor  222  will represent changes in the stage/truck position whereas differences in position reported by the leading and trailing sensors can be attributed to misalignment of the segments. In one embodiment, the spacing between leading and trailing sensors  222  is slightly less than the pitch at which the segments  221  are spaced. This helps ensure that the trailing sensor remains on the current segment  221  for a period sufficient for the position of the segment addressed by the leading sensor  222  to be recorded and accommodated. The sensors  222  may be spaced to skip segments  221  and may be spaced at any suitable pitch, including at a pitch that is slightly less than the pitch of the segments  221  multiplied by any desired number of segments n from one to the total number of segments in a surface  220 . 
     Additional sensors  222  may be used to measure the angle of each segment  221  in a surface  220 . In this embodiment, pairs of sensors  222  are addressed to a given segment  221  to measure its angle as well as the stage/truck distance. As will be appreciated, two pairs of sensors  222  are employed as described as with the two sensors  222  shown mounted on trucks  210  in  FIG. 2 . This permits multiple segments  221  of surface  220  to be localized in their XYO orientation relative to the truck  210 . As will be appreciated, using any suitable number of sensors  220 , one may accurately position the segments  221  for use as a calibration reference. This will reduce the need for multiple sensors  222  and will also make the positioning system much more fault tolerant should there by a discrepancy with a reading from the read heads  216 . 
     While the embodiments shown here have the rails  212  of the traveler mechanisms  200  arranged essentially perpendicularly to one another, this is not required. Any complementary relationship or angle between the traveler mechanisms  200  that permit the stage  106  to be localized relative to the platen  104  may be used. Note that the axes defined by the rails  212  will still define a plane that is parallel to the plane in which stage  106  travels. 
     The couplings  226  are in some embodiments unpowered linear bearings, though air bearings such as pre-stressed air bearings may be used. In order to move the stage  106  a bit faster relative to the platen  104 , it may be desirable to include a linear actuator (not shown) of some sort between the truck  210  and the rails  212  of each traveler mechanism  200 . In this embodiment, the linear couplings  226  would be omitted as the linear actuator would be driven to maintain a nominal distance between the truck  210  and the stage  106 . As expected, the read heads  216  and linear encoders  214  will still indicate the position of the truck  210  relative to the platen  104 . Driving the trucks  210  along a nominal path that the stage  106  will follow allows the distance sensors  222  to measure the distance between the truck  210  and the stage  106 , providing the information needed to accurately localize the stage  106  relative to the platen  104 . The addition of the linear actuator and omission of the linear coupling  226  reduces the inertia of the stage  106  and permits the stage  106  to accelerate at higher rates. 
       FIG. 3A  is a side elevation of how a traveler mechanism  200  with its rails  212  positioned between the stage  106  and platen  104  might be arranged. Note that the traveler mechanism  200  in the figure includes a linear coupler  226 , though as indicated above, this may be omitted in favor of a linear actuator that moves truck  210  along track  212  to follow the stage  106 .  FIG. 3B  is another side elevation of a traveler mechanism  200  but with the rails  212  positioned above the stage  106 . Again, the optional linear coupler  226  is shown in this embodiment. The linear coupler  226  in  FIG. 3B  would be positioned to allow the distance sensor  222  to address the reference surface  220 . 
       FIG. 3C  shows a variation on the embodiment illustrated in  FIG. 3B . In this embodiment, at least one additional linear encoder  214 ′ is provided. Linear encoder  214 ′ is vertically separated from the pictured linear encoder  214 . In normal operation the read heads  214 ,  214 ′ should output substantially identical positions as they linear encoders are nominally parallel to one another. But, where the truck  210  is pitched upward or downward as it moves along rail  212 , the read heads  214 ,  214 ′ will output slightly different values. This differential measurement, along with the known distance between the rails linear encoders  214 ,  214 ′ allow one to compute the amount of pitch the truck  210  is experiencing. Any measurable pitch at the truck  210  is considered to be an aberrant condition that is to be minimized or omitted, if possible. Pitch may come from a number of sources, including aparallel rails  212  or linear encoders  214 ,  214 ′. Pitch may also be derived from the motion of the stage  106  over an non-flat base  104 . This latter source of pitch may be transmitted through the coupler  226  to the truck  210 . The measured pitch values are, in some embodiments, used to modify the position of a reticle  134  by adjusting the reticle chuck  132 . In this way aberration due to pitch may be minimized. 
       FIG. 4A  is a plan schematic view showing an example of the extent of the travel of a stage  106  relative to a platen  104  for a system in which only a single projection camera  110  is employed. It is to be understood that projection camera  110  may be replaced by an inspection, metrology, or other process tool or mechanism that acts upon a substrate secured to stage  106 . In  FIG. 4B , there is shown an example of the extent of the travel of a stage  106  relative to a platen  104  where two projection cameras  110  are employed. Generally, the amount of travel required to address a substrate S to a single projection camera  110  is greater than it would be to address a substrate S to two projection cameras  110 . 
     In building a machine that incorporates a traveler mechanisms  200 , it is desirable to calibrate the traveler mechanism  200  before use. In one instance, a distance sensor such as interferometer (not shown) may be placed on or near the base  102  of the system  100 . It is a good idea to not place the interferometer on the platen  104  as the stage  106  may come into contact with the interferometer. A mirror (also not shown) is placed on the truck  210  to measure the position of the truck relative to the base  102  and platen  104  that is fixed thereto. Each truck  210  is moved through its entire range of motion and its position relative to the base  102  is recorded. Note that in each traveler mechanism  200 , there are illustrated two linear encoders and associated read heads. This is to provide a means to measure an angular position of the stage  106  in addition to its position in a Cartesian coordinate plane. This angular position may be corrected using the forcer motors (not shown) or may be induced in order to angularly align a substrate S to a projection camera  110 . 
     Having calibrated the position of each truck  210  relative to the base/platen, the system  100  may be ready for use. Additional calibration may be helpful however. Using the interferometer or another useful distance sensor, an operator will preferably calibrate the output of the distance sensors  222  relative to the reference surface  220 . Using a calibration target formed directly on the stage  106  or on the chuck  120 , the position of the projection camera  110  relative to the stage  106  may be calibrated. Of course, one will also want to calibrate the position of the stage  106  to the position of the chuck  120 . Using the aforementioned calibrations, one is able to generate a transform that is used with the output of the read heads  216  and the position sensors  222  to accurately position a substrate S relative to the projection camera  110 . When calibrations are complete the distance sensor, such as an interferometer, used in the calibration is removed from the system  100 . 
     One or more embodiments are described as follows: 
     1. A traveler mechanism for localizing a stage comprising:
         a rail, the rail having a linear encoder;   a truck that travels on the rail, the truck having a read head that indicates a nominal position of the truck along the rail;   a linear coupler that connects the traveler to the stage such that when the stage moves in a direction parallel to the rail, the linear coupler causes the truck to move along the rail and when the stage moves in a direction transverse to the rail, the stage will be able to move relative to the truck; and,   a reference surface positioned on the stage and opposing at least one distance sensor positioned on the truck, the position reported by the read head and the nominal distance reported by the distance sensors are combined to localize the stage.
 
2. The traveler mechanism for localizing a stage of clause  1  further comprising at least two distance sensors on the truck, a difference between the distances reported by the distance sensors being used to compute an angle of the stage relative to the truck.
 
3. The traveler mechanism for localizing a stage of clause  1  wherein the reference surface comprises a plurality of individual segments positioned to form a nominally planar reference surface.
 
4. The traveler mechanism for localizing a stage of clause  3  further comprising at least two distance sensors on the truck, the at least two distance sensors on the truck having a spacing therebetween that addresses the at least two distances sensors to spaced apart segments of the reference surface.
 
5. The traveler mechanism for localizing a stage of clause  3  further comprising at least two pairs of distance sensors on the truck, each of the at least two pairs of distance sensors on the truck having a spacing between the individual distance sensors that is less than the width of a segment of the planar reference surface, the at least two pairs of distance sensors having a spacing therebetween that addresses the at least two pairs of distance sensors to spaced apart segments of the reference surface.
 
6. The traveler mechanism for localizing a stage of clause  1  wherein the rail is positioned above the stage.
 
7. The traveler mechanism for localizing a stage of clause  1  wherein the rail is positioned below the stage.
 
8. The traveler mechanism for localizing a stage of clause  1  wherein the stage moves relative to a base, the stage being supported above the base by one of an air bearing mechanism, a mechanical bearing mechanism, and an electromagnetic bearing mechanism.
 
9. A positioning mechanism for localizing a stage comprising:
   a plurality of tracks, each of the plurality of tracks having a scale;   a plurality of blocks, each of the plurality of blocks being arranged to travel on one of the tracks, each of the plurality of blocks having a sensor that indicates a nominal position of a block along its respective track by reading a position from the scale;   a plurality of connectors, each connector coupling one of the plurality of blocks to the stage such that when the stage moves in a direction parallel to one of the plurality of tracks, the connector causes the block associated with the track in question to move along the track and when the stage moves in a direction transverse to the track in question, the stage will be able to move relative to the block;   a reference surface positioned on the stage and opposing at least one distance sensor positioned on each of the plurality of blocks, the position reported by the sensors directed to scales of each track and the nominal distance reported by each of the distance sensors being combined to localize the stage.
 
10. The positioning mechanism for localizing a stage of clause  9  wherein at least two of the plurality of tracks are set perpendicular to one another.
 
11. The positioning mechanism for localizing a stage of clause  9  wherein the track is positioned above the stage.
 
12. The positioning mechanism for localizing a stage of clause  9  wherein the track is positioned below the stage.
 
13. A mechanism for localizing a stage comprising:
   a rail, the rail having a linear encoder;   a truck that travels on the rail, the truck having a read head that indicates a nominal position of the truck along the rail;   a drive coupled between the truck and the rail, the drive being operated to maintain a nominal distance between the stage and the truck as the stage moves along an axis defined by the rail, the   a reference surface positioned on the stage and opposing at least one distance sensor positioned on the truck, the position reported by the read head and the nominal distance reported by the distance sensors are combined to localize the stage.
 
14. The mechanism for localizing a stage of clause  13  further comprising at least two distance sensors on the truck, a difference between the distances reported by the distance sensors being used to compute an angle of the stage relative to the truck.
 
15. The mechanism for localizing a stage of clause  13  wherein the reference surface comprises a plurality of individual segments positioned to form a nominally planar reference surface.
 
16. The mechanism for localizing a stage of clause  15  further comprising at least two distance sensors on the truck, the at least two distance sensors on the truck having a spacing therebetween that addresses the at least two distances sensors to spaced apart segments of the reference surface.
 
17. The mechanism for localizing a stage of clause  15  further comprising at least two pairs of distance sensors on the truck, each of the at least two pairs of distance sensors on the truck having a spacing between the individual distance sensors that is less than the width of a segment of the planar reference surface, the at least two pairs of distance sensors having a spacing therebetween that addresses the at least two pairs of distance sensors to spaced apart segments of the reference surface.
 
18. The mechanism for localizing a stage of clause  13  wherein the rail is positioned above the stage.
 
19. The mechanism for localizing a stage of clause  13  wherein the rail is positioned below the stage.
 
20. The mechanism for localizing a stage of clause  13  wherein the stage moves relative to a base, the stage being supported above the base by one of an air bearing mechanism, a mechanical bearing mechanism, and an electromagnetic bearing mechanism.
 
21. A apparatus for localizing a stage wherein the stage moves in a plane and is localized along at least two axes that are substantially parallel to the plane in which the stage moves, the apparatus comprising:
       

     a mechanism for determining a location along an axis comprising:
         a rail, the rail having a linear encoder;   a truck that travels on the rail, the truck having a read head that indicates a nominal position of the truck along the rail;   a linear coupler that connects the traveler to the stage such that when the stage moves in a direction parallel to the rail, the linear coupler causes the truck to move along the rail and when the stage moves in a direction transverse to the rail, the stage will be able to move relative to the truck;   a reference surface positioned on the stage and opposing at least one distance sensor positioned on the truck, the position reported by the read head and the nominal distance reported by the distance sensors are combined to localize the stage; and,       

     wherein the apparatus includes at least one mechanism for each of the at least two axes. 
     CONCLUSION 
     Although specific embodiments of the present invention have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.