Abstract:
An XY stage for precision movement for use in aligning a wafer in a microlithography system. A main stage supporting the wafer straddles a movable beam that is magnetically driven in a first linear direction in the XY plane. A follower stage, mechanically independent of the main stage, also moves in the first linear (X) direction and its motion is electronically synchronized by a control system with the main stage motion in the X direction. Electromagnetic drive motors include magnetic tracks mounted on the follower stage which cooperate with motor coils mounted on the edges of the main stage to move the main stage in a second linear (Y) direction normal to the X direction. Thus the main stage is isolated from mechanical disturbances in the XY plane since there is no mechanical connections and is lightened by removing the weight of the magnetic tracks from the beam. A cable follower stage moves in the Y direction on the follower stage and supports the cables connecting to the main stage, thereby reducing cable drag. An air circulation system is provided in the magnetic tracks on the follower stage to remove heat from operation of the electromagnetic motors. Air is removed from a central region of each track by a vacuum duct enhanced by air plugs fitting at the two ends of the motor coil assembly on the main stage to contain the air therein.

Description:
CLAIM OF PRIORITY 
     This application is a divisional Ser. No. 09/454,691 filed Dec. 3, 1999 now U.S. Pat. No. 6,134,981 which is a division of application Ser. No. 08/799,674, filed Feb. 11, 1997, now U.S. Pat. No. 5,996,437 which is a continuation of application Ser. No. 08/325,740, filed Oct. 19, 1994 now U.S. Pat. No. 5,623,853. Priority is hereby claimed to the foregoing applications which are also hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to electromechanical alignment and isolation, and more particularly to a method and apparatus for supporting and aligning a wafer in a microlithographic system having extreme precision. 
     2. Description of the Prior Art 
     Various support and positioning structures are well known for use in microlithographic instruments. Typically in the prior art, XY guides including a separate X guide assembly and a Y guide assembly are utilized with one guide assembly mounted on and movable with the other guide assembly. Often a separate wafer stage is mounted on top of the guide assemblies. These structures require high precision in manufacturing and many components. These structures are typically used in a wafer stepper apparatus where the alignment of an exposure field to the reticle being imaged affects the success of the circuit i.e., the yield. In a scanning exposure system, the reticle and wafer move simultaneously and scan across one another during the exposure sequence. 
     A related system is disclosed in copending and commonly owned U.S. Pat. application Ser. No. 08/221,375 filed Apr. 1, 1994, titled “Guideless Stage Isolated Reaction Stage” invented by Martin Lee, now U.S. Pat. No. 5,528,118 issued Jun. 18, 1996, and copending and commonly owned U.S. Pat. application Ser. No. 08/266,999, filed Jun. 27, 1994, titled “Electromagnetic Alignment and Scanning Apparatus”, invented by Akimitsu Ebihara. See also U.S. Pat. No. 5,040,431 issued Aug. 20, 1991 to Sakino et al. and U.S. Pat. No. 4,667,139 issued May 19, 1987 to Hirai et al. All of the above patent disclosures are incorporated herein by reference. Many other examples of such stage structures, often called “XY stages”, are known in the art. 
     Prior art stages typically suffer from a significant drawback in that the sensitivity of measurement accuracy of the stage position is adversely affected by temperature. The electromagnetic motors which drive the elements of the stage relative to one another are a significant heat source adversely affecting the performance of the laser interferometry typically used to determine the actual stage position. 
     Another disadvantage of prior art systems is that the numerous cables including electrical cables, fiber optic cables, coolant tubes, vacuum tubes and air hoses connecting to the stage from external devices impose a significant amount of drag and mechanical forces, both steady and impulsive, on the actual stage, thus degrading performance. Thus, cable drag occurs as the stage moves about pulling the cables with it, causing thereby mechanical friction and disturbances. 
     Additionally, prior art stages suffer from reduced performance due to the relatively high mass of the stage which typically carries the heavy electromagnetic drive motor magnets for positioning the stage in at least one axis direction. Higher mass may reduce the stage mechanical resonance frequency and thereby lower the stage performance. If the stage is made stiffer to compensate, this may add even more mass. Higher mass requires more motor power, thus undesirably more potential for heating. 
     Therefore, there is a significant problem in the prior art of impeded stage performance in terms of accuracy and speed caused by the relatively high weight of the stage support, the cable drag, and the heat generated by the stage movement impeding sensing accuracy in terms of position. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an XY stage apparatus is capable of high accuracy positioning and motion control in three dimensions. The apparatus uses linear commutated motors to drive the main stage in a two-dimensional plane and controls out-of-plane linear motion (along the Z axis) as well as rolling and pitching rotations through the use of air bearings. 
     The main stage in one embodiment straddles a beam (guide bar) that is mechanically driven on a base by linear motors in a first linear direction. A follower stage (follower frame), not mechanically connected to the main stage, also moves independently in the first linear (X) direction between fixed guides mounted on the base, and its motion is synchronized to the main stage motion in the X direction. As the main stage and follower stage move independently but simultaneously in the X direction, linear electromagnetic motors, the magnetic tracks of which are mounted on the follower stage and the coil portions of which are mounted on the main stage, move the main stage in a second linear Y direction normal to the X direction. 
     Thereby the main stage is isolated from mechanical disturbances in the XY plane and control of accuracy of movement of the main stage is improved by removing the weight of the magnetic tracks from the main stage itself. 
     Additionally, the sensitivity of measurement accuracy of stage location to temperature effects is improved by minimizing the number and size of heat sources in close proximity to the main stage and the measuring laser beam paths which are part of the interferometry measurement system. 
     Air circulation is provided through the slots in the magnetic tracks in which the motor-coils ride. These slots are partially sealed at either end of the main stage so as to cause air flow through the tracks which is ducted away from the main stage, and thus away from the interferometry laser beam paths. 
     Additionally, a cable follower stage is mounted on the follower stage. The cable bundle is connected from the main stage to the cable follower stage. The cable follower stage moves in one dimension along the follower stage in synchronization with the movement in that direction of the main stage, and thus supports the bulk of the weight of the cable bundle (including electrical and optical cables, air and vacuum tubes) connected external to the apparatus. 
     Thus, high accuracy and movement is achieved by obtaining optimum control, minimizing thermal effects, and substantially eliminating cable drag. 
     The apparatus in accordance with the present invention is suited for use as a wafer stepper and scanner in a scanning exposure system by providing smooth and precise stepping and scanning in two-dimensions. Additionally, the present apparatus is adapted to a scanner system wherein the Y direction is the scan direction and the X direction is the cross scan direction. 
     Advantageously, precision, accuracy, acceleration, velocity and settling time are improved over the prior art. The main stage and its supporting beam (movable guide bar) are advantageously reduced in weight because the relatively heavy magnetic tracks for driving the stage are mounted on the independent follower stage instead of on the beam. The forces applied to accelerate and decelerate the main stage are effectively applied at or near the center of gravity of the stage. This advantageously reduces torque moment on the stage and thereby reduces a tendency for rolling and pitching of the stage. Thus the control in the X and Y directions is optimized. 
     Additionally, use of the beam for driving the main stage reduces the number of heat sources located on the stage, thereby reducing thermal effects on the interferometry system which determines stage location. 
     Further use of the follower stage to drive the main stage in the Y direction means that the linear motor coil, often located in the prior art at the center of the main stage, is replaced by two motor coils at two edges of the main stage, each motor coil requiring only one half of the power compared to the use of a single prior art drive motor to achieve similar movement. This not only physically locates the heat source away from the wafer (which is located at the center of the main stage), but also reduces the concentration of heat generated, thereby facilitating a limitation of thermal effects on the stage and on interferometry positioning. 
     The cable follower stage serves as an intermediate resting place for the cables between the main stage and the external cable connections, the cable follower stage thereby being a mechanical buffer between the main stage and mechanical disturbances due to cable drag. Thus precise movement of the stage is enhanced. 
     Advantageously, an apparatus in accordance with the invention uses commercially available electromagnetic drive motor components rather than the special commutatorless electromechanical drive elements used in some prior art stages. Thus, the apparatus is relatively easily manufactured and cost is reduced. Further, the heat generated is substantially reduced over that of the prior art commutatorless motors, reducing thermal effects. 
     The stage need not surround or even straddle the beam, so long as the beam can mechanically drive the stage. In one embodiment, a linear motor coil is located at each end of the beam and drives the beam along magnetic tracks located on the guides elongated in the X direction and fixedly mounted on the base. Thus, the motion of the beam in the X direction (cross scan direction) is transmitted to the main stage because the main stage is coupled to the beam. The beam also serves as a guide for the main stage in the Y direction (scan direction) as the main stage slides freely along the beam via air bearings disposed on surfaces of the main stage facing the beam. The driving motion of the main stage in the Y direction is provided by two linear motor coil assemblies, each mounted on one of two parallel edges of the main stage. The two coil assemblies move inside the magnetic tracks mounted on the follower stage which is located surrounding but mechanically independent of the stage. 
     The follower stage is a rigid rectangular structure including two parallel members (called herein bridges) that connect two other parallel members at right angles. Two linear motors move the follower stage along the guides in synchronization with the main stage and the beam motion in the X direction. 
     To ensure smooth sliding motion of the moving members (main stage, beam, follower stage) with respect to the principal surface of the base plate, air (or other fluid) bearings are located underneath the main stage, the beam, and the follower stage. Additional air bearings are also disposed between the follower stage and the fixed guides for smooth follower stage motion in the X direction. Air bearings disposed between the main stage and the beam ensure smooth relative motion of the stage in the Y direction. In one embodiment only one fixed guide is provided, and so the beam and follower frame are guided only at one end. 
     The follower stage is guided in the X direction and restrained in the Y direction by two opposed pairs of air bearings which clamp to and glide along one of the fixed guides. The beam is guided in the X direction by a combination vacuum and air bearing which moves along the other fixed guide. 
     Alternatively in another embodiment no air bearings are disposed between the beam and the fixed guides. Instead, air bearings are disposed between the beam and the bridge portions of the follower stage for restraining the beam in the Y direction. Because the beam motion and the follower stage motion are synchronized in their respective X direction movements by an electronic control system, relative motion (only a few millimeters) is limited between the beam and the follower stage and only arises from the lack of perfect synchronicity in the control system between these two elements. Thus bearing mechanical noise at this location is minimized. 
     “Air plug” structures and corresponding air ducts improve heat removal from the linear motors that drive the stage in the Y direction along the beam. The slots which accommodate the magnetic tracks on the follower stage are closed at each corner of the main stage by air plug structures so as to contain the warm air inside. These air plugs are formed in the shape of the interior portion of the slot in cross section and have a similar clearance to that of the motor coil relative to the magnetic track. Thus there is a small gap between the air plug structures and the magnetic track interior. Thus the air plug structures do not completely seal the air movement in the slots, but form sufficient restriction between the air inside the slots and outside to allow a slight negative pressure to develop. This ensures, in conjunction with a ducted vacuum outlet located in a center portion of the magnetic track, the controlled flow of air from the outside moving into the stage motor coil slots. This air in turn is removed by the ducts. Duct tubing is attached to the outlet in the magnetic track assembly to circulate air therethrough to cool the coil assemblies at its center portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows a perspective view of an XY stage structure in accordance with the present invention. 
     FIG. 1 b  shows a view of the structure of FIG. 1 a  rotated 90°. 
     FIG. 2 a  shows a view of the wafer stage and beam of FIG. 1 a.    
     FIG. 2 b  shows the underside of the wafer stage and beam of FIG. 2 a.    
     FIG. 3 a  shows the wafer stage and base of FIG. 1 a , without the follower stage and associated structures. 
     FIG. 3 b  shows the follower stage and cable follower stage and base of FIG. 1 a , without the beam or main stage. 
     FIG. 3 c  shows detail of a cross section of the cable follower stage. 
     FIG. 4 shows a cross section of the stage, beam and heat, dissipation structures of FIG. 1 a.    
     FIG. 5 shows a block diagram of a control system for the XY stage of FIG. 1 a.    
     FIG. 6 shows a cross section of another embodiment of the beam, stage, and follower stage. 
     FIG. 7 shows a plan view of yet another embodiment, with one fixed guide. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a  shows a perspective view of an XY stage structure in accordance with one embodiment of the invention. (Axes X, Y, Z are shown for purposes of illustration but are not structural elements.) Granite base  12  having flat and smooth principal surface  12   a  is a conventional base for stability. Two fixed granite guides  18   a  and  18   b  which are spaced apart in the Y direction and parallel are mounted on the surface  12   a  of base  12 . 
     Each of the guides  18   a ,  18   b  is almost orthogonal to the surface  12   a  (X-Y surface) and has guiding surfaces  180   a ,  180   b  in the X direction. The guiding surface  180   a  of the guide  18   a  guides the air bearing/vacuum structure  100  that is fixed to one end of the beam  20  so as to move the beam (movable guide bar)  20  in the X direction. A guiding surface in the Y direction is formed on the side surface of the beam  20 , and the main stage (upper and lower stages  22   a ,  22   b ) is movable in the Y direction along that guiding surface. Magnetic tracks  16   a ,  16   b  are mounted on the base  12  via spacers  14   a ,  14   b  respectively, and the guides  18   a ,  18   b  are disposed above magnetic tracks  16   a ,  16   b  respectively. The spacers  14   a ,  14   b  are formed into a square/rectangle pillar shape in the X direction respectively and also provide internal cooling ducts to extract warm air from the vicinity of the magnetic tracks  16   a ,  16   b . In the slot in the X direction of the magnetic tracks  16   a , a coil unit that is fixed at the lower part of the structure  100  on one end of the beam  20 , is inserted. Similarly, a coil unit that is fixed on the other end of the beam  20 , is inserted in the slot in the X direction of the magnetic tracks  16   b . Only one of these coil units could be mounted (preferably on the structure  100  side) in one embodiment. 
     These coil units and the slots for each of magnetic tracks  16   a ,  16   b  are disposed in the Y and Z direction so as not to contact each other, and the beam  20  moves on the surface  12   a  of base  12  in the X direction by providing an electric current to the coil units. As described later in detail, air bearings are installed at the lower part of the both ends of beam  20 , and the weight of the beam  20 , structure  100 , and coil unit is borne by the surface  12   a . The lower stage  22   a  surrounds the beam  20  and is supported on the surface  12   a  via air bearings. Therefore, the weight of the main stage does not fall on the beam  20  but entirely on the base  12 . 
     On the other hand, guide  18   b  guides the movement of the follower stage assembly ( 72 ,  74 ,  76 ,  78 ) which has a rectangle frame structure in the X direction. Inside the frame of this follower stage assembly, there is space to accommodate the Y direction stroke of the main stage movement. As described later in detail, the follower stage assembly is supported on the base  12  via air bearings and has a coil unit that is magnetically connected with the magnetic track  16   b . This coil unit is installed on the bridge structure  72  side in FIG. 1 a  and moves the follower stage assembly in the X direction. Furthermore, the bridge structure  78  in FIG. 1 a  is disposed leaving a gap of several mm with the air bearing/vacuum structure  100  so as to avoid contact. Furthermore, in the present embodiment, the follower stage assembly is guided only by the guide  18   b , and there is no restraint by the guide  17   a  in the Y direction. 
     Magnetic tracks  16   a  and  16   b  are of the “ 300  series” available from Trilogy Corporation, Webster, TX, and are a first part of the conventional electromagnetic drive motors, as described below. The inner motor coil assemblies which are in one embodiment also the second part of the drive motor are commercially available parts. These are commutated linear mptors, part number LM310-3 from Trilogy Corporation. (This particular motor is apporximately three times as powerful as is needed; this overdesign is intentional so that the motor coils operate well under their capacity, to minimize heat production.) The wires which comprise the coil windings are very fine and embeded in a material such as epoxy. Each coil includes three sets of windings, each winding space one-third of the magnetic pitch as defined by the magnetic tracks from one another. The magnetic tracks are a linear assembly of magnets, for instance pairs of individual magnets mounted to an iron frame with an air gap between. The pairs are installed adjacent with their poles reversed for each adjacent pair of magnets. This set of magnets is magnetically attached to the iron or steel backing structure and held in place by adhesive. The magnetic tracks may be of various configurations. In one configuration, the coil moves between two opposed sets of magnets. In another embodiment, a coil having two spaced-apart portions is located above and below a magnetic track. 
     The beam  20  is a rigid srtucture rectangular in cross section of the X-Z plane and formed for instance of ceramic (e.g. alumina) or aluminum. In another embodiment, the beam  20  is an I bar shape in cross-section. The beam  20  defines internal-voids (see FIG. 4) so as to minimize its weight, and includes damping features. 
     Lower stage  22   a  straddles and surrounds guide bar  20 . The lower stage  22   a  is for instance of aluminum or ceramic material (silicon carbide). The upper stage  22   b  made of ceramic is mounted on lower stage  22   a . Also a conventional wafer chuck  24  made of ceramic and two mirrors  28 ,  30  for laser interferometer are mounted on upper stage  22   b . The mirror  28  reflects the Y interferometer beam that measures the stage  22   b  position in the Y direction, and the mirror  30  reflects the X interferometer beam that measures the stage  22   b  position in the X direction. It is to be understood that typically in use, the stage  22   b  is covered by a rectangular cover with openings for the conventional chuck, mirrors, sensors, and other fixtures (not shown) as appropriate. 
     The XY stage depicted herein is for lithography on semiconductor wafers. However, the invention is not limited to this application. To give an idea of the relative dimensions involved, in this case wafer chuck  24  is of a size to accommodate a  12  inch diameter semiconductor wafer. Thus the overall dimensions of base  12  are approximately 4½′×4½′. In one embodiment, leveling devices (also not shown) are provided for leveling the upper stage  22   b  relative to the lower stage  22   a . Also conventional structures (not shown) are mounted on upper stage  22   b  for adjusting the rotation of wafer chuck  24 . Fiducial marks are mounted on upper stage  22   b , as are conventional interferometry mirrors  28  and  30  for determining the stage position as described hereinafter. Lower stage  22   a  rides on the principal surface  12   a  of the base  12 . 
     The follower stage is a rigid rectangular structure including four e.g. aluminum members. These are opposing and parallel members  74  and  76  located on either side of and lying parallel to the beam  20  and connected by opposing and parallel “bridge” members  72  and  78 . 
     The cable bundle  50   a  with cable connector  52  is supported on a cable follower stage,  140  mounted on follower frame member  78  (shown better in other views). Magnetic tracks  84   a  and  84   b  are fixed to respectively follower stage members  76  and  74  and form a part of the follower stage. In each of the slots of the magnetic tracks  84   a ,  84   b , coil units that are fixed to both sides of the lower stage  22   a  are inserted. These coil units energize the main stage to be moved in the Y direction. The follower stage also includes air bearing members  64   a  and  64   b  which are located on either side guide surface  180   b  of upper guide  18   b  and are connected by connecting member  64   c . Similarly, air bearing members  66   a  and  66   b  are located on either side of upper guide  18   b  and are connected by connecting member  66   c . Thus, members  64   a ,  64   b ,  64   c  and  66   a ,  66   b , and  66   c  guide the follower stage along upper guide  18   b . An air bearing  134   a  (and others, not shown here) also guide the follower stage on principal surface  12   a  as described in further detail below. 
     As evident from the above described structure, the counter force generated by acceleration and deceleration of the main stage in the Y direction is entirely transmitted to the guide  18   b  via bearing members  64   a ,  64   b ,  66   a ,  66   b . In other words, this counter force is not transmitted to the other guide  18   a  at all via the beam  20  that guides the main stage and the air bearing/vacuum. 
     The air bearings are of a type available from Devett Machinery Co., Aston, Pa. The air bearings are of three general types each providing a typical riding height of 5 microns. One type is a combination vacuum/air bearing. The air bearing is formed of a porous carbon body with a varnish sealant along its non-bearing sides. The carbon body defines a central bore which is connected to a source of vacuum. The outer perimeter of the carbon body is the air bearing. The upper portion of the carbon body is connected to a source of compressed air. Thus, the compressed air is pushed out through the carbon body at its concentric outer portion and the vacuum sucks in the air at the center portion, thus providing a proper supporting effect whereby the vacuum “loads” the air bearing. 
     In a second air bearing/vacuum structure used herein, the vacuum portion is physically separated from and adjacent to the air bearing portion. It is to be understood that the vacuum and compressed air are provided externally via tubing in the cable bundle and an internal tubing distribution system (not shown in the figures for simplicity). 
     The air bearing clamping structures  64   a ,  64   b ,  64   c  and a similar structure  66   a ,  66   b  and  66   c  are of a third type and provide the desired low friction relationship between the upper guide  18   b  and the follower stage member  72 . These are conventional air bearing clamping members with opposed air bearings  64   a  and  64   b  connected by a connecting member  64   c . This type of clamping air bearing structure is relatively bulky and hence is not used in other portions of the structure, where instead the air bearing/vacuum combinations are used. However, it is to be understood that where space permits this type of clamping air bearing structure and other types of air bearing structures may be substituted for the air bearing/vacuum combination bearing structures illustrated herein. In general, it is to be understood that air bearings must be loaded, either by a weight, by a physical clamping structure, or by vacuum, to perform properly. 
     FIGS. 1 b ,  2   a ,  2   b ,  3   a , and  3   b  are other views of either the entire structure of FIG. 1 a  or portions thereof. 
     FIG. 1 b  shows the identical structure of FIG. 1 a  rotated 90° clockwise so as to better illustrate the portion which is to the rear of FIG. 1 a . Thus, additionally in FIG. 1 b  are seen the other portions of the cable bundle assembly including the additional cable bundle loops  50   b ,  50   c , and  50   d  and the terminus  52  of the cable bundle loop  50   c  which connects to the upper stage  22   b . A preload type air bearing system (not visible) supports the cable follower stage  140  with respect to the surface  12   a  of the base  12  and outer side face of the member  76  of the follower assembly. It is to be appreciated that the cable bundle includes electrical wires, optical fibers, vacuum tubes, fluid coolant tubes, and pneumatic tubes for carrying compressed air for the air bearings. The cable bundle is attached to the cable follower stage  140  by stage connector  90   d  and is moved by a steel band  48  which is driven by a rotary drive motor  90   a  via gear mechanism  90   b . Plate  46  is located over steel band  48  for purposes of supporting cable  50   d  as it rolls out onto the plate  46 . Thus, the cable follower stage  140  is mounted on and moves relative to member  76  of the follower stage in the X-direction. The cable follower stage moves the cable bundle attached to connector  90   d  to the left and right directions (Y direction) in FIG. 1 b  in synchronization with the movement of the main stage. Thus, the cable dragging effect on the main stage is eliminated. 
     With regard to the cable bundle, its operation is such that as shown in FIG. 1 b , the loop  50   a  widens and narrows with the movement of the follower stage elements ( 72 ,  74 ,  76 ,  78 , and  140 ) along its axis in the X direction. The loop  50   d  also rolls to extend and shorten as the cable follower stage  140  moves in the orthogonal direction i.e., the Y direction with the main stage by actuating the motor  90   a  and gear mechanism  90   b  which are fixedly attached to the member  76  without contact to the base surface  12   a . Thus all of the cable movement is in the main loops  50   a  and  50   d , the other cable loop  50   c  being essentially stationary with only minimal movement, i.e., approximately one or two millimeters, which is the maximum movement between the cable follower stage and the main stage. 
     As further seen in FIG. 1 b , the magnetic track  84   b  is identical to magnetic track  84   a  including an internal slot for accommodating the motor coil attached to lower stage  22   a  as described further below. The other elements shown in FIG. 1 b  are identical to those in FIG. 1 a . (Not every element shown in FIG. 1 a  is also referenced in FIG. 1 b , for simplicity.) 
     Although not shown in FIG. 1 a , a coil unit  92  to be inserted in the slot for the fixed magnetic track  16   b  is shown in FIG. 1 b  at the end part on the guide  18   b  side of the follower stage assembly. It is preferable to install this coil unit  92  not only on one end side of the member  75  in the Y direction (clamping assembly  66   a ,  66   b , and  66   c ) that constitutes the follower stage assembly but also on one end of the follower stage assembly member  74  (claiming assembly  64   a ,  64   b ,  64   c  side). 
     FIG. 2 a  shows portions of the structure of FIG. 1 a , from an identical perspective. However, the base  12  and the associated fixed guides and the follower stage are not illustrated in FIG. 2 a  so as to better show the beam structure  20  and the lower stage  22   a.    
     As shown in FIG. 2 a , a vacuum/air bearing structure  100  is attached to the right end of beam  20 . Air bearings  114   a ,  114   b , and  114   c  and  114   d  as well as vacuum areas  116   a ,  116   b , and  116   c  are mounted on structure  100 . (The air bearing structure  100  is also partly shown in FIG. 1 a  but only its top portion is visible in FIG. 1 a .) As shown in FIG. 2 a , air bearing structure  100  is mounted so that each of the air bearings (pressure gas exhaust pads) and vacuum areas faces the side guide surface  180   a  of the fixed guide  18   a  on an upper surface of beam  20 . The X direction motor coil  110   a  is mounted on the end portion of beam  20  and the X direction length of the structure  100  is almost equivalent to the dimension of the main stage in the X direction. As shown, motor coil  110   a  is an I shaped structure in cross section. A similar motor coil  110   b  (also for movement in the X direction) is mounted on the left hand portion of beam  20 . The motor coils  110   a ,  110   b  are attached by brackets as shown to the main portion of the beam  20 . 
     FIG. 2 a  shows at the left hand portion of lower stage  22   a  the Y direction motor coil  102   a  which is mounted on lower stage  22   a . The structure of motor coil  102   a  is similar to that of motor coils  110   a  and  110   b . At the right hand side of lower stage  22   a , a small portion of the other stage motor coil  102   b  is visible. It is to be understood that motor coils  102   a  and  102   b  fit into the corresponding magnetic tracks  84   a  and  84   b  respectively on the follower stage shown in FIGS. 1 a  and  1   b . Hence, the “I” shape of motor coils  102   a  and  102   b  in cross section fits into the corresponding slots of magnetic tracks  84   a  and  84   b . The underside portion  22   d  of lower stage  22   a  is an air is bearing pad assembly described in further detail below. Also shown in FIG. 2 a  is guide surface  106   a  at the left hand side of beam  20 . This guide surface  106   a  is in contact with an air bearing pad assembly fixedly mounted on the corresponding inner surface of lower stage  22   a . A similar guide surface on the opposite side of beam  20  is not visible in FIG. 2 a.    
     FIG. 2 a  also shows “air plug” structures  104   a ,  104   b  located at either end of the side slot portion in lower stage  22   a  accommodating motor coil  102   a . Such air plugs are not provided for motor coil  102   b  in this embodiment, but may be used for motor coil  102   b , as described below. 
     FIG. 2 b  shows the underside of stage  22   a  including an (aluminum) plate  22   d  on which four pairs of air bearing/vacuum structures are mounted at each corner portion of the plate  22   d . The first pair of pads  120   a ,  120   b  each includes a circular central portion which is a vacuum inlet and an outer concentric portion which is the air bearing portion, as described in detail below. Similar structures are shown at the other corners of the plate  22   d , including air bearings/vacuum structures  126   a ,  124   a , and  122   a . (The other air bearings are not designated here for simplicity.) Also visible in FIG. 2 b  is the underside of beam  20 , cable bundle loop  50   a  and the air bearing  142  which supports the cable follower stage  140 . One end of the cable bundle  50   a  is connected to a terminus connector  52  (fixed, for example, on the base  12  side), and the other end is connected to the connector  53  that is fixed to the side end of the frame member  76  of the follower stage assembly. 
     It is to be understood that in FIG. 2 b  only the base  12  is removed; all other structures are shown, including the guides and magnetic tracks  16   a ,  18   a , and  16   b  and  8   b . Also, the under portion of the follower stage including members  72  and  76  is illustrated. Also shown are the air bearing and vacuum structures on the underside of beam  20 . These are, at the left hand side of beam  20 , vacuum area  136   b  and air bearings pads  132   a  and  132   b  which are arranged adjacent to both sides of the vacuum area  136   b  with respect to the X direction. At the right hand side of beam  20  are shown vacuum area  134   b  and the corresponding air bearings  130   a  and  130   b.    
     Also shown are the air bearing/vacuum structures for supporting the follower frame members  72  and  76  on surface  12   a  and mounted on the underside of the members  72  and  76 . At the left hand portion of member  76 , the rectangular vacuum area at the center portion of structure  136   c  is surrounded by the concentric air bearing (pressurized gas exhaust) portion. Corresponding air bearing vacuum structure  134   c  is located at the right hand side of follower stage member  76 . Similar pad structures  134   a ,  136   a  are mounted on member  72 . Also illustrated are the magnetic tracks  84   a  and  84   b  mounted on the follower stage members respectively  76  and  72 . A cover (heat baffle)  22   c  attached to one side of lower stage  22   a  encloses a portion of the magnetic track  84   b  and the follower stage, as described below. The underside of the heat baffle  22   c  (partly shown in FIG. 1 b ) is shown, as is heat extraction duct  201  (described further below). Also shown is the interferometry mirror  30  and portions  90   a  and  90   b  of the cable follower stage drive mechanism, an air bearing  142  on the bottom surface of cable follower stage  140 . Round holes  137   a ,  137   b  respectively in spacers  14   a ,  14   b  are ducts to extract heat from the magnetic tracks  16   a ,  16   b . Air hoses (not shown) are attached to holes  137   a ,  137   b . Duct  86  is a hole into the interior of magnetic track  84   b ; an exhaust (vacuum) air hose is attached thereto (but not shown) to remove warm air from inside track  84   b.    
     The inside of the spacers  14   a ,  14   b  is formed to be a cavity, and on the steel cover of the magnetic tracks  16   a ,  16   b  holes that lead to the cavity of the spacers are formed spaced apart in the longitudinal direction. Therefore, if the air extracting hoses are connected to the each of the holes  137   a ,  137   b  in the spacers  14   a ,  14   b , warm air floating in the slots of each of the magnetic tracks  16   a ,  16   b  is forcibly exhausted. Such a structure makes it possible to prevent the warm air in the slot, that has been warmed up by the heat generated by coils  110   a ,  110   b  that are magnetically connected to each of the magnetic tracks  16   a ,  16   b  from flowing out to the surrounding portions of the main stage. 
     FIG. 3 a  illustrates the base  12 , the beam  20 , the lower stage  22   a  and upper stage  22   b  and associated structures with the follower stage shown removed. FIG. 3 a  shows how the wafer stage of FIG. 2 a  is located on the principal surface  12   a  of the base  12 . FIG. 3 b  shows the base  12  and the follower stage assembly ( 72 ,  74 ,  76 ,  78 ,  64   a - 64   c ,  66   a - 66   c ,  136   a ,  136   c , etc.) and cable follower stage ( 140 ,  148 , etc.) without the main stage and the beam. Thus, all the structures illustrated in FIG. 3 b  are also illustrated in the foregoing figures. 
     As evident from the main stage structure shown in FIG. 3 a , because linear motor coils  110   a  and  110   b  are installed at both ends of the beam  20 , the beam  20  can be finely rotated in the X-Y plane by controlling the current (e.g., three-phase current) that drives the coils  110   a  and  110   b . This enables active adjustment of the yawing of the main stage ( 22   a ,  22   b ). Similarly, it is also possible to finely rotate the main stage together with the beam  20  in the X-Y plane by controlling the driving current to the two coils  102   a  and  102   b  installed on the both sides of the lower stage  22   a.    
     If an interferometer that measures the position of the mirror  28  in the Y direction has measuring laser beams BY 1 , BY 2  of two axes or more, for instance, then the main stage yawing can be determined by obtaining the difference between the measurement values taken by the beams BY 1  and BY 2 . Similarly, the X interferometer for the mirror  30  can be a multi-axis (laser beam BX 1 , BX 2 ) type. 
     Incidentally, the counter force (acceleration and deceleration) generated accompanying the Y direction movement of the main stage is not transmitted to the beam  20  at all. Because of this, the stiffness of the static pressure gas bearing that is formed in the gap (a few micrometers) between the air bearing/vacuum structure  100  that restrains the beam  20  in the Y direction and the guide surface  180   a  of the guide  18   a  can be small. Thus the regulation for the air pressure or vacuum pressure that is supplied to the air bearing/vacuum structure  100  can be relaxed. 
     The relaxation of this stiffness constraint also implies that the positioning and the movement accuracy (stability at the time of acceleration and deceleration in the Y direction) of the main stage will not deteriorate as a result. On the other hand, the principal function of the follower stage assembly (follower frame) shown in FIG. 3 b  is to have the magnetic tracks  84   a ,  84   b  of the linear motor that drives the main stage ( 22   a ,  22   b ) follow the main stage and to move them in the X direction. Furthermore, another principal function is to transmit the counter force generated at the time of the main stage movement in the Y direction to the fixed guide  18   b  via the clamping structures  64   a - 64   c ,  66   a - 66   c . Because of this, the clamping structures  64   a - 64   c ,  66   a - 66   c  are connected the guide  18   b  via a supporting structure where high stiffness can be obtained so as to firmly restrain the entire follower stage assembly in the Y direction. 
     In one embodiment of the invention, small elastic bumpers (not shown) are affixed to the principal surface  12   a  of granite base  12  along its edges (other than the edges where the guides are located). These bumper cushion the movement of the stage in case of a failure of the control system causing the stage to move to the very edge of the surface  12   a.    
     As seen in FIG. 1 b , a motor coil  92  is mounted on the right hand portion of follower frame member  76 . A corresponding motor coil (not shown) is mounted on the left hand portion of follower frame motor  76 . This coil is inserted in the slot of the fixed magnetic track  16   a  and drives the follower stage assembly in the X direction in cooperation with the coil  92  shown in FIG. 1 b . These motor coils provide the driving force for the follower stage and are similar to but shorter in length and hence lower in power than the motor coils  102   a ,  102   b  of the main stage, due to the lower mass of the follower stage. In the embodiment, the opposing follower stage member  74  does not include any drive motor coils. 
     Comparison of FIGS. 3 a  and  3   b  shows how both beam  20  and the follower stage  76  member are driven on the identical magnetic tracks  16   a  and  16   b . Also, comparison of these figures shows how the follower stage rides against the guide  18   b  while the beam  20  rides against the guide  18   a . A comparison of FIGS. 3 a ,  3   b  also illustrates the purpose of the bridge structure of follower stage members  72 ,  78 . 
     Thus when the coils  102   a ,  102   b  on the stage  22   b  are energized, the stage  22   a  generates force (in the Y axis direction) in proportion to the magnetic strength of the follower stage magnetic tracks. Thus the reaction force (in the Y axis direction) accompanying the main stage movement is all imparted to the follower stage. Therefore, since no reaction force is provided to the beam  20  which guides the main stage, no such force that would deflect the guiding surface affects the fixed guide  18   a . The other guide  18   b  bears the reaction force of the follower stage, and the guide  18   b  may be deflected thereby. However, the deflection would only affect the follower stage guiding accuracy, and thus desirably the main stage guiding accuracy is not adversely affected. 
     It is also to be understood that the particular configuration of the air bearings shown herein is only illustrative of one embodiment of the invention. For instance, the circular air bearing/vacuum structures  120   a ,  120   b , etc. shown in FIG. 2 b  are used, but the rectangular type vacuum and air bearing structures also shown in FIG. 2 b  could be used. Also, the concentric combination air bearing/vacuum structures such as  126   a  and  136   c  for example, are used in places but the separated vacuum structure/air bearing structure such as  132   b  and  136   b  could be used. The selection of a particular air bearing configuration depends on the geometry of the available surface area. 
     FIG. 3 c  shows a cross section along line  3 — 3  in FIG. 1 b , thereby illustrating details of the cable follower stage and associated structures. Elements shown in FIG. 3 c  which are identical to those in FIG. 1B are the base  12 , the cable follower stage  140 , the outer side portion of the follower stage member  76 , one surface of which is slanted at about 45° and bears up against the cable follower stage  140  and thereby serves as a guide therefore, and the air bearing  142  (shown in FIG. 2 b  also) which bears on base  12 . Additional structure shown in FIG. 3 c  includes a magnet  144   a  mounted on cable follower stage  140  which is attracted to iron plate  144   b  installed in the (aluminum) follower member  76 . The iron plate  144   b  is elongated in the X direction over the stroke of the cable follower stage  140 . Magnet  144   a  serves to load air bearings  142   d ,  142  located on two of the surfaces of cable follower stage  140 , due to the attraction between magnet  144   a  and iron plate  144   b  mounted on the corresponding surface of member  76 . Therefore, the cable follower stage  140  is guided by the  450  slanted surface (formed in the X direction) of the member  76  and moves in the X direction (the direction perpendicular to the paper on which FIG. 3 c  is drawn). 
     Other elements shown in FIG. 3 c  include photodetector  148  which detects the cable follower stage movement limit. Also included are steel plate  46  upon which a loop of the cable bundle  50   b  is supported, and cable clamp members  146   b ,  146   d  between which the end portion of the cable bundle  50   b , that is the bundle  50   d , is located, and structural member  146   c  for supporting cable clamp members  146   b  and  146   d . (It is to be understood that the cable bundle as shown in FIG. 1 b  is looped around and folded several times on structures  146   b ,  146   c ,  146   d .) 
     The steel plate  46  shown in FIG. 3 c  (and FIG. 1 b ) is fixed in the X direction on the member  76  side so as to cover the upper part of the cable follower stage  140 . The structures  146   b ,  146   c ,  146   d  move in the space above the steel plate  46  in the Y direction (the direction perpendicular to the paper on which FIG. 3 c  is drawn) altogether with the stage  140 . 
     The end portion of the bundle  50  on the supply source side is connected to the terminus connector  53  (shown in FIG. 2 b ) that is fixed on the member  76  side. The bundle  50   b  ( 50   d ) loops in the Y-Z plane because the other end of the bundle  50   b  (i.e., the bundle  50   d ) is fixed by the clamp structures  140   b  and  140   d . The structural member  146   c  includes several joints and electric terminals as necessary to connect the bundle  50   d  to the upper bundle  50   c . The bundle  50   c  loops in the X-Z plane and is connected to the terminus  51  (see FIG. 1 b ) of the main stage. 
     Furthermore, in FIG. 1 b , the terminus  51  is installed on the upper stage  22   b , but it is also acceptable to install it on the lower stage  22   a  side. As evident from FIGS. 1 b ,  2   b , and  3   c , only the cable bundle  50   a  that loops on the X-Y plane deforms when the main stage and the follower stage assembly synchronize and move in the X direction. Because the cable follower stage  140  and the structures  146   b ,  146   c , and  146   d  move in the Y direction altogether, only the cable bundle  50   b  (or  50   d ) deforms when the main stage moves in the Y direction. Because of this, the drag force of the cable bundle  50   a  is absorbed by the follower frame assembly, and the drag force of the cable bundle  50   b ,  50   d  is absorbed by the cable follower stage  140  (and structures  146   b - 146   d ) and the follower frame assembly. What could possibly give the drag force to the main stage in the end is the cable bundle  50   c . However, the shape of the bundle  50   c  rarely changes no matter how the main stage moves because of the operation of the cable follower stage  140 . Therefore, only a force that is smaller than the cable bundle  50   c &#39;s weight will be added in the Z direction to the terminus  51  on the main stage side. 
     FIG. 4 illustrates in cross-section the beam  20 , lower stage  22   a , and associated structures. This cross-section is along line  4 — 4  shown in FIG. 3 a.    
     The lower stage  22   a  in this embodiment surrounds the beam  20 . (The beam depicted here is of the I-beam type in cross-section.) Beam  20  defines three internal void,  20   a ,  20   b  and  20   c  to minimize its weight. For guiding the main stage along the beam  20 , air bearing/vacuum pads  142   a ,  142   b  (not seen in the other figures) are provided on both inner side portions of the lower stage  22   a  to produce static pressure gas bearings between the pads  142   a ,  142   b  and both side surfaces  106   a  of the beam  20 . Pads  142   a ,  142   b  include rectangular air bearings located near each outside edge of lower stage  22   a  so as to minimize the friction between beam  20  and lower stage  22   a . Furthermore, in FIG. 4, both the top portion and the bottom portion of the beam  20  are arranged leaving spaces of several mm or less from both the inner top wall and the inner bottom wall of the lower stage  22   a . The air bearing/vacuum pad  22   d  for bearing on base  12  is shown on the lower portion of lower stage  22   a . The linear motor coils  102   a ,  102   b  are attached to the sides of stage  22   a  and are shown in cross-section. The associated magnetic tracks  84   a  and  84   b  respectively surround motor coils  102   a  and  102   b.    
     Magnetic tracks  84   a ,  84   b  are not part of the main stage, but instead are attached to the follower stage members  74 ,  76  respectively, and shown here for purpose of illustration. Also shown are backing plates  84   c ,  84   d  for magnetic tracks respectively  84   b ,  84   a . A portion of follower frame member  74  is shown, to which is attached the magnetic track backing plate  84   d . (The corresponding structure for track  84   b  is not shown). 
     The thermal stability of the stage mechanism is important to ensure accurate interferometry measurements and to eliminate change of thermal expansion due to thermal differentials. The interferometry problem is that warm air has a different index of refraction than does cooler air. The objective is maintain air temperature in the location of the interferometry laser beams within a few thousandths of a degree centigrade. In accordance with the embodiment of the invention, a general air flow supplied from an air temperature and humidity conditioner and HEPA filter (not shown) is maintained (in FIG. 1 a ) from the bottom left hand portion of the figure to the upper right hand portion. As an air conditioner, one of a type for an environment chamber that encloses an exposure apparatus, for instance, can be used. If the chamber is the side flow type (where the air flows laterally by installing an opening for ventilation with a HEPA filter on one side of the inner side wall, and installing a return duct on the other side of the inner side wall), the general air flow can be supplied by arranging the stage apparatus so as to agree with the X direction in FIG. 1 a . Thus, it is important to keep warm air away from the interferometry mirrors  28  and  30 . This accomplished by three cooling subsystems as described hereinafter. 
     First, the motor coils  102   a  and  102   b  on the stage are each liquid cooled by conventionally circulating liquid coolant within the base of the coil structure. The liquid coolant is water (or some other material such as Fluorinert) which is supplied by conventional tubing (part of the cable bundle not shown) and circulates through channels formed in the base of each motor coil  102   a ,  102   b . (These channels are part of the above-described commercially available motor coils.) 
     Second, for further cooling, liquid coolant is circulated on the outer surfaces of the motor coils  102   a ,  102   b.    
     Third, an air cooling system is provided for the is side slot portions of the lower stage  22   a  surrounding the motor coils  102   a ,  102   b  in FIG. 2 a . Heat baffle  22   c  (see FIG.  4  and FIG. 2 b ) surrounds the frame member  74  with magnetic track  84   a  and is attached to lower stage  22   a  without contacting the frame member  74 . Heat exhaust tube (duct)  201  is a linear structure extending the length of the exterior of track  84   a  and located immediately below the track for elimination of warm air from within the “chamber”formed by the stage  22   a  and baffle  22   c . Tube  201  is approximately 3 mm high and approximately 67 mm wide and defines at least one opening to admit warm air from within this “chamber”. Tube  201  thus is a thin rectangular tube connected externally to an exhaust tube subject to vacuum (not shown). Tube  201  thereby is disposed inside heat baffle  22   c  to exhaust warm air from inside heat baffle  22   c.    
     As evident from the structure shown in FIG. 4, the air in the slot of the magnetic track  84  is warmed by the coil  102   a . If either the heat buffle  22   c  or the tube  21  is omitted, that warm air would flow out of the slot of the track  84 , and a large fluctuation of the index of refraction caused in the space in front of the mirror  30 , i.e., in the laser beam path of the X direction interferometer system. Therefore, it is advisable to install several spaced-apart through holes in the Y direction below the track  84   a  and construct track  84   a  so that the warm air can be rapidly exhausted to the inner space of the tube  201  from the slot space of the track  84   a.    
     To enhance the circulation of air through the ducting, the “air plug” structures  104   a ,  104   b  shown in FIG. 2 a  are located at either end of the slot portion in the lower stage  22   a  in which the motor coil  102   a  is located. Each air plug  104   a ,  104   b  is of a size to closely fit around the corresponding magnetic track, but leaving a small gap between the air plug and the magnetic track so as to admit a flow of air into the slot of magnetic track  84   b  and the slot portion of lower stage  22   a . This provides a positive flow of air which then flows into the “chamber” and then into the duct  201  adjacent the magnetic track and out through the connected vacuum tube. The air plugs  104   a ,  104   b  are thin pieces of e.g. plastic. A gap of approximately 1 mm between the edge of each air plug and the corresponding portion of the magnetic track allows air to enter past the air plug from outside. Thus the air plugs have the same cross sectional shape as do the coil  102   a  and are affixed to the end of the slot portion locating the coil  102   a  in the lower stage. 
     In this embodiment the heat baffle  22   c  and exhaust =tube  201  are only located on one side of stage  22   a , because the object is to divert heat away from the inferometery mirrors, and the overall air flow direction is from left to right in FIG. 1 a . Thus in another embodiment, the heat baffle, duct and air plugs are also provided for motor coil  102   b  on the other side of stage  22   a.    
     The follower stage and the beam are driven synchronously as described above. The typical maximum amount of motion between them is approximately plus or minus 1.0 mm. Thus the locational relationship between these two parts is essentially static. In one embodiment, the relative precision of movement of the main stage  22   a  is approximately plus or minus 10 nanometers. The follower stage accuracy of movement is approximately plus or minus 1 mm. Thus the main stage and the beam are driven in the X-axis direction so as to maintain a predetermined minimum gap (about 1 mm) between the motor coils on the main stage and the corresponding magnetic tracks on the follower stage. The cable follower stage-accuracy is approximately 1 to 2 mm in the Y direction. 
     A system for controlling movement of the beam, the main stage, the follower frame stage, and the cable follower stage to meet these tolerances is shown in FIG. 5 in a block diagram. The control system synchronizes the movement in one dimension (X-axis) of the beam and the follower frame stage so that they move closely together with a minimum spacing between the main stage and the follower frame stage, by controlling the current to their respective motor coils  92 ,  110   a ,  110   b . Also, the cable follower stage, which rides on the follower frame stage, is moved in synchronism with the main stage in the orthogonal direction (Y axis) so as to minimize cable drag by controlling the actuation of the motor  90   a . Thus the control system synchronizes movement of the three stages and the beam. 
     The controlling portion  161  includes three axis controllers or CPUs. These are typically microprocessors or microcontrollers and may all be resident in one microprocessor. 
     Thus, the main stage axis controller  162   a  issues control signals to digital to analog (D/A) converter  162   d , the output of which is then amplified by amplifier  162   e  to drive the motor coils  102   a ,  102   b  of the main stage (here designated  162   f ) and beam coils  110   a ,  110   b . Follower frame stage axis controller  164   a  issues signals to the digital to analog converter  164   d  which are amplified by amplifier  164   e  to control the motor coils  92  of the follower frame stage, here designated  164   f . Similarly, cable follower stage axis controller  168   a  via digital to analog converter  168   d  and amplifier  168   e  controls the motor  90   a  for the cable follower stage, here designated  168   f.    
     It is to be understood that also a host computer  160  conventionally issues commands to axis controllers  162   a ,  164   a , and  168   a  for movements in both X and Y directions. 
     Three position measuring systems  162   g ,  164   g  and  168   g  provide feedback. Measuring system  162   g  for the main stage includes the interferometry mirrors  28  and  30  mounted on the upper stage  22   b . By means of laser beams reflected by mirrors  28 ,  30  the exact location on the base  12  in both the X and Y direction (and also the yaw component) of the upper stage  22   b  is measured. This is then provided for feedback purpose via lines  162   h  and  162   k  respectively to the axis controllers  162   a  and  168   a.    
     A second measuring system  164   g  measures the position of the follower frame stage relative to the main stage. Measuring system  164   g  in one embodiment is an inductive-type proximity measurement sensor. The sensor element is disposed on one of the members of the follower frame stage, for instance, member  78  and senses the proximity of member  100 . Sensor  164   g  thus measures the position of the follower frame stage relative to the main stage. The output of measuring system  164   g  is fed back via feedback line  164   h  to axis controller  164   a  for controlling coils  92  of the follower frame stage. 
     The third measuring system  168   g  for the cable follower stage is a conventional mechanical encoder which is a portion of cable follower stage rotary motor  90   a . This encoder indicates the position of the cable follower stage. This measurement is fed back via line  168   h  to the cable follower stage axis controller  168   a . This controller  168   a  mainly inputs the measuring signal in Y direction of the main stage via the line  162   k  and drives the motor  90   a  so that the measuring signal from the encoder system  168   g  would be within the range of 1 to 2 mm. 
     Signal path  162   j  is optional and provides a “feed forward” feature whereby the follower stage control elements, instead of merely following the beam movement along the X-axis, receive the movement commands from host computer  160  at the same time as do beam  20  control elements, thereby providing faster performance. 
     Thus the overall desired synchronism in the movements of the various elements of the entire stage assembly is maintained to within a desired tolerance. The movements of the main stage are most closely controlled by the most accurate measuring system, while the follower stage is somewhat less accurately controlled by a less accurate measuring system. Finally, the least accurate measuring system (mechanical encoding) is used for the portion of the system where motion control is least precise, which is the cable follower stage. 
     Additionally the linear motors  110   a  and  110   b  at either end of beam  20  are differentially driven by axis controller  162  to prevent and overcome any tendency of the beam to yaw. Thus control unit  162   a  provides different levels of current to each motor coil  110   a  and  110   b . The amount of current provided to each motor coil is determined by a computer program in controller  162  in accordance with the measured location of the stage  22   a  on beam  20  as indicated on feedback line  162   h . It is to be understood that the above-described air bearing structure associated with beam  20  also provides an anti-yaw effect; however, the differential drive provides higher anti-yaw performance than does the air bearings. Thus any tendency of beam  20  to yaw due to the location of stage  22   a  is overcome by this differential drive control. The goal is to prevent the beam  20  from pivoting, i.e. moving faster on one side versus the other, due to the reaction force exerted on beam  20  by the stage  22   a.    
     An alternate embodiment of the stage is shown in cross section in FIG. 6 showing only the beam, main stage, and follower frame stage. The follower frame stage  274 , which is formed as a single bar, carries a single magnetic track  284  which is positioned at the center of the main (lower) stage  222   a , near its center of gravity. The follower frame stage is constructed as in the previous embodiments with air bearings at the outside ends for support in the Z and Y directions and with linear motor coils at each end. A motor coil  202  is attached to the main stage  222   a . The beam  220  is a “U” shaped structure with smooth side surfaces which guide air bearings  242   a ,  242   b  fixed on the main stage  222   a  to move the main stage  222   a  in the Y direction. The stage  222   a  is supported on the base surface  12   a  (not shown) through the air bearing system  222   d  provided to the underside of the stage  222   a . The beam  220  is supported on air bearings  232 . This embodiment is similar to the above-described embodiment but differs in that a single magnetic track  284  (single follower bar  274 ) drives the main stage  222   a  in the Y direction. It is also characterized by the stage  222   a  not completely surrounding the beam  220 . That is, the main stage  222   a  is supported on the base  12  so that it straddles not only the beam  220  but also the follower bar  274  (with magnetic track  284 ). Furthermore, the beam is a “U” shaped structure with air bearings  232  directly under the beam  220  for support along its length instead of only at the ends of the beam. In the case shown in FIG. 6, a structure is necessary to support the follower bar stage  274  to the base  12  because the beam  220  will be arranged right underneath it. One example would be to have a structure where both ends in the Y direction of the follower bar stage  274  are supported in the Z direction by two fixed guides. 
     FIG. 7 shows still another embodiment of the invention, in a plan view. This embodiment is the same as that of FIGS. 1 a  to  3   c  with similar elements identically labelled, except that guide  18   a  has been eliminated, and the air bearing structure  100  thus bears against the remaining guide  18   b.    
     As another embodiment, it is also acceptable to form the pads  114   a - 114   d  of the air bearing/vacuum structure  100  that are shown in detail in FIG. 2 a  and the vacuum areas  116   a - 116   c  at the back of the structure  100 , and the restraint of the entire beam  20  in Y direction can be provided by the bridge member  78  (FIGS. 1 a ,  1   b ). Guide  18   a  is not essential, but guide  18   a  helps prevent the entire beam  20  from significantly displacing itself in the guide  18   a  direction when the suction force in the Y direction between the structure  100  and the bridge  78  is lost under an unexpected condition. Furthermore, the relative position of the magnetic tracks and coils can be reversed. 
     The above description is illustrative and not limiting. Further modifications will be apparent to one skilled in the art in the light of this disclosure, and are intended to fall within the scope of the appended claims.