Abstract:
A mask having a mask pattern is provided on a mask table. A substrate is provided on a substrate table. The mask is irradiated to project an image of at least a portion of the mask pattern onto a radiation-sensitive layer of the substrate using a projection system. At least one of the mask and substrate tables is positioned using a drive unit having a stationary part coupled to a reaction frame. A position of at least one of the mask and substrate tables is measured using a measuring system having a plurality of measurement sensors that have a stationary part and a movable part. The movable part of one of the sensors is coupled to the one of the mask table and the substrate table whose position is measured, and the stationary parts of the sensors are coupled to a support frame mechanically isolated from the reaction frame.

Description:
This is a Division of application Ser. No. 09/449,762 filed Nov. 26, 1999, which in turn is a Continuation of application Ser. No. 09/192,153 filed Nov. 12, 1998 (now U.S. Pat. No. 6,246,202), which is a Continuation of application Ser. No. 08/416,558 filed Apr. 4,1995 (now U.S. Pat. No. 5,874,820). Said application Ser. No. 09/449,762 also is a Continuation-In-Part of application Ser. No. 09/127,288 filed Jul. 31, 1998 (now U.S. Pat. No. 6,049,186), which is a Continuation of application Ser. No. 08/627,824 filed Apr. 2, 1996 (now U.S. Pat. No. 5,942,871), which is a Continuation of application Ser. No. 08/221,375 filed Apr. 1, 1994 (now U.S. Pat. No. 5,528,118). The entire disclosures of the above-identified prior applications are hereby incorporated by reference herein in their entireties. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to precision motion stages and more specifically to a stage suitable for use in a photolithography machine and especially adapted for supporting a reticle. 
   2. Description of the Prior Art 
   Photolithography is a well known field especially as applied to semiconductor fabrication. In photolithography equipment, a stage (an X-Y motion device) supports the reticle (i.e., mask) and a second stage supports the semiconductor wafer, i.e., the work piece being processed. Sometimes only a single stage is provided, for the wafer or the mask. 
   Such stages are essential for precision motion in the X-axis and Y-axis directions and often some slight motion is provided for adjustments in the vertical (Z-axis) direction. A reticle stage is typically used where the reticle is being scanned in a scanning exposure system, to provide smooth and precise scanning motion in one linear direction and insuring accurate, reticle to wafer alignment by controlling small displacement motion perpendicular to the scanning direction and a small amount of “yaw” (rotation) in the X-Y plane. It is desirable that such an X-Y stage be relatively simple and be fabricated from commercially available components in order to reduce cost, while maintaining the desired amount of accuracy. Additionally, many prior art stages include a guide structure located directly under the stage itself. This is not desirable in a reticle stage since it is essential that a light beam be directed through the reticle and through the stage itself to the underlying projection lens. Thus, a stage is needed which does not include any guides directly under the stage itself, since the stage itself must define a fairly large central passage for the light beam. 
   Additionally, many prior art stages do not drive the stage through its center of gravity which undesirably induces a twisting motion in the stage, reducing the frequency response of the stage. Therefore, there is a need for an improved stage and especially one suitable for a reticle stage. 
   SUMMARY 
   A precision motion stage mechanism includes the stage itself which moves in the X-Y plane on a flat base. The stage is laterally surrounded by a “window frame” guide structure which includes four members attached at or near their corners to form a rectangular structure. The attachments are flexures which are a special type of hinge allowing movement to permit slight distortion of the rectangle. In one version, these flexures are thin stainless steel strips attached in an “X” configuration, allowing the desired degree of hinge movement between any two adjacent connected window frame members. 
   The window frame guide structure moves on a base against two spaced-apart and parallel fixed guides in, e.g., the X axis direction, being driven by motor coils mounted on two opposing members of the window frame cooperating with magnetic tracks fixed on the base. 
   The window frame in effect “follows” the movement of the stage and carries the magnetic tracks needed for movement of the stage in the Y axis direction. (It is to be understood that references herein to the X and Y axes directions are merely illustrative and for purposes of orientation relative to the present drawings and are not to be construed as limiting.) 
   The stage movement in the direction perpendicular (the Y axis direction) to the direction of movement of the window frame is accomplished by the stage moving along the other two members of the window frame. The stage is driven relative to the window frame by motor coils mounted on the stage and cooperating with magnetic tracks mounted in the two associated members of the window frame. 
   To minimize friction, the stage is supported on the base by air bearings or other fluid bearings mounted on the underside of the stage. Similarly, fluid bearings support the window frame members on their fixed guides. Additionally, fluid bearings load the window frame members against the fixed guides and load the stage against the window frame. So as to allow slight yaw movement, these loading bearings are spring mounted. The stage itself defines a central passage. The reticle rests on a chuck mounted on the stage. Light from an illuminating source typically located above the reticle passes to the central passage through the reticle and chuck to the underlying projection lens. 
   It is to be understood that the present stage, with suitable modifications, is not restricted to supporting a reticle but also may be used as a wafer stage and is indeed not limited to photolithography applications but is generally suited to precision stages. 
   An additional aspect in accordance with the invention is that the reaction force of the stage and window frame drive motors is not transmitted to the support frame of the photolithography apparatus projection lens but is transmitted independently directly to the earth&#39;s surface by an independent supporting structure. Thus, the reaction forces caused by movement of the stage do not induce undesirable movement in the projection lens or other elements of the photolithography machine. 
   This physically isolating the stage reaction forces from the projection lens and associated structures prevents these reaction forces from vibrating the projection lens and associated structures. These structures include the interferometer system used to determine the exact location of the stage in the X-Y plane and the wafer stage. Thus, the reticle stage mechanism support is spaced apart from and independently supported from the other elements of the photolithography machine and extends to the surface of the earth. 
   Advantageously, the reaction forces from operation of the four motor coils for moving both the stage and its window frame are transmitted through the center of gravity of the stage, thereby desirably reducing unwanted moments of force (i.e., torque). The controller controlling the power to the four drive motor coils takes into consideration the relative position of the stage and the frame and proportions the driving force accordingly by a differential drive technique. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a top view of a window frame guided stage. 
       FIG. 2  shows a side view of the window frame guided stage and associated structures. 
       FIGS. 3A and 3B  show enlarged views of portions of the structure of FIG.  2 . 
       FIG. 4  shows a top view of a photolithography apparatus including the window frame guided stage. 
       FIG. 5  shows a side view of the photolithography apparatus of FIG.  4 . 
       FIGS. 6A and 6B  show a flexure hinge structure as used, e.g., in the window frame guided stage. 
       FIG. 7  is a perspective view of a microlithography system disclosed in U.S. patent application Ser. No. 08/221,375. 
       FIG. 7A  is a view of a portion of the structure shown in  FIG. 7  delineated by line A—A and with the reaction stage which is shown  FIG. 7  removed. 
       FIG. 7B  is an elevational view, partially in section, of the structure shown in FIG.  7 . 
       FIG. 7C  is a schematic elevational view, partially in section, of the object positioning apparatus disclosed in U.S. patent application Ser. No. 08/221,375. 
       FIG. 8  is a plan view of the wafer XY stage position above the reaction stage. 
       FIG. 9  is a side elevational view of a portion of the structure shown in  FIG. 8  taken along line  9 — 9  in the direction of the arrows. 
       FIG. 9A  is an enlarged view of a portion of the structure shown in  FIG. 9  delineated by line B—B. 
       FIG. 10  is a perspective view of the reaction stage showing the XY followers without the means for coupling to the XY stage for positioning of the XY stage. 
       FIG. 10A  is an enlarged perspective view of the XY followers illustrated in FIG.  10 . 
       FIG. 11  is a schematic block diagram of the position sensing and control system for the preferred embodiment disclosed in U.S. patent application Ser. No. 08/221,375. 
       FIGS. 12 and 13  are views similar to  FIGS. 8 and 9  of an alternative embodiment disclosed in U.S. patent application Ser. No. 08/221,375. 
       FIGS. 14 and 15  are views similar to  FIGS. 8 and 9  of still another embodiment disclosed in U.S. patent application Ser. No. 08/221,375. 
       FIG. 16  is an enlarged top view of a portion of the structure shown in FIG.  14 . 
       FIG. 17  is an end view of the structure shown in  FIG. 16  taken along line  17 — 17  in the direction of the arrows. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a top view of a stage mechanism in accordance with the invention. See also copending commonly owned and invented U.S. patent application Ser. No. 08/221,375 entitled “Guideless Stage with Isolated Reaction Stage” filed Apr. 1, 1994, which is incorporated herein by reference and shows a related method of supporting elements of a stage mechanism so as to isolate reaction forces from the projection lens and other parts of a photolithography apparatus. 
   The detailed description from U.S. patent application Ser. No. 08/221,375 is reproduced below. FIGS. 1-11 of that application have been renumbered respectively as  FIGS. 7-17 , and the reference numerals have been increased by 200 in order to avoid the use of duplicate reference numerals for different elements. 
   The Detailed Description from U.S. patent application Ser. No. 08/221,375 
   While it will be appreciated by those skilled in the art that the guideless stage, with or without its isolating reaction frame, has many applications to many different types of instruments for precise positioning of objects, the invention will be described with respect to a preferred embodiment in the form of a microlitholigraphic instrument for aligning wafers in a system where a lens produces an image which is exposed to the photoresist on the wafer surface. In addition, while the guideless stage with or without its isolation stage can be utilized as a guideless object stage movable in just one direction, such as a X or an Y direction, the preferred embodiment is directed to a guideless XY wafer stage as described below. 
   Referring now to the drawings, with particular reference to  FIGS. 7 and 8 , there is shown a photolithographic instrument  210  having an upper optical system  212  and a lower wafer support and positioning system  213 . The optical system  212  includes an illuminator  214  including a lamp LMP, such as a mercury lamp, and an ellipsoid mirror EM surrounding the lamp LPM. The illuminator  214  comprises optical integrator such as a fly&#39;s eye lens FEL producing secondary light source images and a condenser lens CL for illuminating a reticle (mask) R with uniformed light flux. A mask holder RST holding the mask R is mounted above a lens barrel PL of a projection optical system  216 . The lens barrel PL is fixed on a part of a column assembly which is supported on a plurality of rigid arms  218  each mounted on the top portion of an isolation pad or block system  220 . 
   Inertial or seismic blocks  222  are located on the system such as mounted on the arms  218 . These blocks  222  can take the form of a cast box which can be filled with sand at the operation site to avoid shipment of a massive structure. An object or wafer stage base  228  is supported from the arms  218  by depending blocks  222  and depending bars  226  and horizontal bars  227  (see FIG.  7 A).  FIG. 7B  is an elevational view, partially in section, of the structure shown in  FIG. 7  except that in  FIG. 7B  the blocks  222  are shown as being a different configuration than in  FIGS. 7 and 7A . Referring now to  FIGS. 8 and 9 , there are shown plan and elevational views, respectively, of the wafer supporting and positioning apparatus above the object or wafer stage base  228  including the object or wafer or XY stage  230  and the reaction frame assembly  260 . The XY stage  230  includes a support plate  232  on which the wafer  234 , such as a 12 inch wafer, is supported. The plate  232  is supported in space above the object stage base  228  via vacuum pre-load type air bearings  236  which can be controlled to adjust Z, i.e., tilt roll and focus. Alternatively, this support could employ combinations of magnets and coils. 
   The XY stage  230  also includes an appropriate element of a magnetic coupling means such as a linear drive motor for aligning the wafer with the lens of the optical system  216  for precisely positioning an image for exposure of a photoresist on the wafer&#39;s surface. In the embodiment illustrated, the magnetic coupling means takes the form of a pair of drive members such as X drive coils  242 X and  242 X′ for positioning the XY stage  230  in the X direction and a pair of Y drive members such as drive coils  244 Y and  244 Y′ for positioning the XY stage  230  in the Y direction. The associated portion of the magnetic coupling means on the reaction frame assembly  260  will be described in later detail below. 
   The XY stage  230  includes a pair of laser mirrors  238 X operative with respect to a pair of laser beams  240 A/ 240 A′ and  238 Y operative with respect to a pair of laser beams  240 B/ 240 B′ of a laser beam interferometer system  292  for determining and controlling the precise XY location of the XY stage relative to a fixed mirror RMX at the lower part of the lens barrel PL of the projection optical system  216 . 
   Referring to  FIGS. 10 and 10A , the reaction frame assembly  260  has a reaction frame  261  which includes a plurality of support posts  262  which are mounted on the ground or a separate base substantially free from transferring vibrations between itself and the object stage. 
   The reaction frame  261  includes face plates  264 X and  264 X′ extending between support posts  262  in the X direction and  266 Y and  266 Y′ extending between support posts in the Y direction. Inside the face plates  264 - 266  a plurality of reaction frame rails  267 - 269  and  267 ′- 269 ′ are provided for supporting and guiding an X follower  272  and a Y follower  282 . Inside face plate  264 X are an upper follower guide rail  267  and a lower follower guide rail  268  (not shown) and on the inside surface of the opposite face plate  264 X′ are upper and lower follower guide rails  267 ′ and  268 ′. On the inside surfaces of each of the face plates  266 Y and  266 Y′ is a single guide rail  269  and  269 ′, respectively, which is positioned vertically in between the guide rails  267  and  268 . 
   The X follower includes a pair of spaced apart arms  274  and  274 ′ connected at their one end by a cross piece  276 . Drive elements such as drive tracks  278  and  278 ′ (see  FIG. 8 ) are mounted on the arms  274  and  274 ′, respectively, for cooperating with the drive elements  242 X and  242 X′ of the XY stage. Since in the illustrated embodiment the drive elements  242 X and  242 X′ on the XY stage are shown as drive coils, the drive tracks on the X follower  272  take the form of magnets. The coupling elements could be reversed so that the coils would be mounted on the X follower and the magnets mounted on the XY stage. As the XY stage is driven in the X and Y direction, the laser interferometer system  292  detects the new position of the XY stage momentarily and generates a position information (X coordinate value). As described in greater detail below with reference to  FIG. 11 , a servo position control system  294  under control of a host processor (CPU)  296  controls the position of the X follower  272  and the Y follower  282  in response to the position information from the interferometer system  292  to follow the XY stage  230  without any connection between the drive coils  242 X,  242 X′ and the tracks  274 ,  274 ′. 
   For movably mounting the X follower  272  on the reaction frame  261 , the ends of the arms  274  and  274 ′ at the side of the reaction frame  261  ride or are guided on the rail  269 , and the opposite ends of the arms  274  and  274 ′ ride on rail  269 ′ adjacent face plate  266 Y′. For moving the X follower  272  a drive member  277  is provided on the cross piece  276  for cooperating with the reaction frame guide  269  for moving the follower  272  in a direction which is perpendicular to the X direction of the XY stage. Since the precision drive and control takes place in the XY stage  230 , the positioning control of the X follower  272  does not have to be as accurate and provide as close tolerances and air gaps as the XY stage  230 . Accordingly, the drive mechanism  277  can be made of a combination of a screw shaft rotated by a motor and a nut engaged by the X follower  272  or a combination of a coil assembly and a magnet assembly to establish a linear motor and each combination can be further combined with a roller guiding mechanism. 
   Similar to the X follower  272 , the Y follower  282  includes a pair of arms  284  and  284 ′ connected at their one end by a crossbar  286  and including drive tracks  288  and  288 ′ for cooperating with the Y drive members  244 Y and  244 Y′. The arms  284  and  284 ′ of the Y follower  282  are guided on separate guide rails. The ends of arm  284  ride or are guided on the upper rails  267  and  267 ′ and the ends of arm  284 ′ are guided on lower rails  268  and  268 ′. A drive mechanism  287  is provided on the cross piece  286  of the Y follower  282  for moving the Y follower  282  along guides  267 ,  267 ′,  268  and  268 ′ between the face plates  266 Y and  266 Y′ in a direction perpendicular to the Y direction of the XY stage. 
   As best illustrated in  FIG. 10A , the arms  274  and  274 ′ and crossbar  276 ′ of the X follower  272  all lie within and move in the same plane crossing the Z axis. The center of gravity of the XY stage  230  lies within or is immediately adjacent to this plane. In this construction the drive forces from each of the drive coils  242 X and  242 X′ are in a direction along the length of the arms  274  and  274 ′, respectively. However, the arms  284  and  284 ′ of the Y follower  282  lie within and move in different parallel planes spaced apart along the Z axis from one another respectively above and below and parallel to the plane containing the X follower  272 . In the preferred embodiment, the crossbar  286  lies in the lower plane containing the arm  284 ′ and a spacer block  286 ′ is positioned between the overlapping ends of the arm  284  and crossbar  286  to space the arms  284  and  284 ′ in their respective parallel planes. As with X follower  272 , the drive forces from each of the drive coils  244 Y and  244 Y′ are in a direction along the length of the arms  284  and  284 ′. Also, predetermined gaps in X and Z directions are maintained between the drive coils  244 Y ( 244 Y′) and the drive tracks  288  ( 288 ′) to achieve the guideless concept. 
   In operation of the guideless stage and isolated reaction frame of the invention, the XY stage  230  is positioned in an initial position relative to the projection lens as sensed by the interferometer system  292 , and the XY stage  230  is supported in the desired Z direction from the object stage base  228  by the air bearings  236  with the drive coils  242 X,  242 X′,  244 Y and  244 Y′ spaced from the drive elements in the form of drive tracks  278 ,  278 ′,  288  and  288 ′, respectively. There is no direct contact between the XY stage  230  and the reaction frame  261 . That is, there is no path for the vibration of the reaction frame to affect the position of the XY stage and vice versa. There is only indirect contact via the transmission means that deliver the signals to the coils and the laser interferometer position sensing system which then transmits sensed position information to the controller which receives other commands to initiate drive signals which result in movement of the XY stage  230 . 
   With the known position of the XY stage  230  from the interferometer system  292 , drive signals are sent from the position control system  294  to the appropriate drive coils,  242 X,  242 X′,  244 Y and  244 Y′ to drive the XY stage to a new desired position. The motion of the XY stage is sensed by the interferometer system  292  and position sensors  298 X and  298 Y (see FIG.  11 ), and the X follower  272  and Y follower  282  are driven by the drive members  277  and  287 , respectively, to follow the XY stage. As illustrated in  FIG. 11 , the position sensor  298 X detects a variation of the Y direction space between the XY stage  230  and the X follower  272  and generates an electric signal representing the amount of space to the position control system  294 . The position control system  294  generates a proper drive signal for the drive member  277  on the basis of the X position information from the interferometer system  292  and the signal from the position sensor  298 X. 
   Also, the position sensor  298 Y detects a variation of X direction space between the XY stage  230  and the Y follower  282  and generates an electric signal representing the amount of space, and the drive member  287  is energized on the basis of the Y position information from the interferometer system  292  and the signal from the position sensor  298 Y. 
   Yaw correction is accomplished by the pairs of linear motors which can be used to hold or offset yaw, or the pairs of linear motors can change the rotational position of the XY stage. The data from either or both pairs of laser beams  240 A/ 240 A′ and  240 B/ 240 B′ are used to obtain yaw information. Electronic subtraction of digital position data obtained from measurement using the laser beams  240 A and  240 A′ or  240 B and  240 B′ is performed or both differences are added and divided by two. 
   This invention allows the positioning function of the XY stage to be accomplished faster than if XY guides were used. Reaction forces created in moving the XY stage can be coupled away from the image forming optics and reticle handling equipment. 
   This invention needs no precision X or Y guides as compared to a guided stage, and precision assembly and adjustment of the wafer XY stage is reduced due to the lack of precision guides. The servo bandwidth is increased because the linear motor forces in the XY axes act directly on the wafer stage; they do not have to act through a guide system. 
   Forces from the XY linear motors can all be sent substantially through the center of gravity of the XY stage thereby eliminating unwanted moments of force (torque). 
   With the X follower  272  and the Y follower  282  mounted and moved totally independently of one another, any vibration of a follower is not conveyed to the wafer XY stage or to the optical system when using commercially available electromagnetic linear motors for the magnetic coupling between each of the followers  272  and  282  and the XY stage  230  and with clearance between the coils and magnet drive tracks less than about 1 mm. Additionally, with the arms of one of the followers spaced above and below the arms of the other follower, the vector sum of the moments of force at the center of gravity of the XY stage due to the positioning forces of cooperating drive members is substantially equal to zero. 
   No connection exists between the XY stage and the follower stages that would allow vibrations to pass between them in the X, Y or theta degrees of freedom. This allows the follower stages to be mounted to a vibrating reference frame without affecting performance of the wafer stage. For example, if the reaction frame were struck by an object, the XY stage and the projection optical system would be unaffected. 
   It will be appreciated by a person skilled in the art that if the center of gravity is not equidistant between either of the two X drive coils or either of the two Y drive coils, that appropriate signals of differing magnitude would be sent to the respective coils to apply more force to the heavier side of the stage to drive the XY stage to the desired position. 
   For certain applications the drive elements  242 X/ 242 X′ or  242 Y/ 242 Y′ of the actuator or magnetic coupling assembly for supplying electromagnetic force to the movable XY stage may be held stationary (see  FIG. 11 ) in a static position with respect to movement of the stage in either the X or Y direction, respectively. 
   In the last of the explanation of this embodiment, referring to  FIG. 7C  again, the essential structure of the invention will be described. As illustrated in  FIG. 7C , the XY stage  230  is suspended on the flat smooth surface (parallel with the X-Y plane) of the stage base  228  through the air bearings  236  having air discharge ports and vacuum pre-load ports and is movable in X, Y and theta direction on the stage base  228  without any friction. 
   The stage base  228  is supported on the foundation (or ground, base structure)  221  by the isolation blocks  220 , arms  218 , blocks  222 , the vertical bars  226  and the horizontal bars  227 . Each of the isolation blocks  220  is composed of a vibration absorbing assembly to prevent transmission of the vibration from the foundation  221 . 
   Since  FIG. 7C  is a sectional view of the XY stage  230  along a line through the drive coils  242 X,  242 X′ in Y direction, the following description is restricted about the X follower  272 . 
   In  FIG. 7C , the drive coils  242 X are disposed in a magnetic field of drive track (magnet array elongated in X direction)  278  mounted on the follower arm  274  and the drive coils  242 X′ are disposed in a magnetic field of drive track  278 ′ mounted on the follower arm  274 ′. 
   The two arms  274 ,  274 ′ are rigidly assembled to move together in Y direction by the guide rails  269 ,  269 ′ formed inside of the reaction frame  261 . Also, the guide rails  269 ,  269 ′ restrict the movement of the two arms  274 ,  274 ′ in X and Z directions. The reaction frame  261  is directly supported on the foundation  221  by the four support posts  262  independently from the stage base  228 . 
   Therefore, the drive coils  242 X ( 242 X′) and the drive tracks  278  ( 278 ′) are disposed with respect to each other to maintain a predetermined gap (a few millimeters) in Y and Z directions. 
   Accordingly, when the drive coils  242 X,  242 X′ are energized to move the XY stage  230  in X direction, the reaction force generated on the drive tracks  278 ,  278 ′ is transferred to the foundation  221 , not to the XY stage  230 . 
   On the other hand, as the XY stage  230  moves in Y direction, the two arms  274 ,  274 ′ are moved in Y direction by the drive member  277  such that each of the drive tracks  278 ,  278 ′ follows respective coils  242 X,  242 X′ to maintain the gap in Y direction on the basis of the measuring signal of the position sensor  298 X. 
   While the invention has been described with reference to the preferred embodiment having a pair of X drive members or coils  242 X and  242 X′ and a pair of Y drive members or coils  244 Y and  244 Y′, it is possible to construct a guideless stage with an isolated reaction frame in accordance with the invention with just three drive members or linear motors such as shown in  FIGS. 12 and 13 . As illustrated in  FIG. 12 , a pair of Y drive coils  344 Y and  344 Y′ are provided on the stage  330  and a single X drive coil or linear motor  342 X is mounted centered at the center of gravity CG′ of the XY stage. The Y drive coils  344 Y and  344 Y′ are mounted on the arms  384  and  384 ′ of the Y follower  382 , and the X drive coil  344 X is mounted on an arm  374 ″ of a X follower  372 . By applying appropriate drive signals to the drive coils  342 X and  344 Y and  344 Y, the XY stage can be moved to the desired XY positions. 
   Referring now to  FIGS. 14-17 , there is shown an alternative embodiment of the invention which includes links between the XY drive coils  442 X,  442 X′,  444 Y and  444 Y′ and the attachment to the XY stage  230 ′. These connections include a double flexure assembly  500  connecting the drive coil  444 Y to one end of a connecting member  520  and a double flexure assembly  530  connecting the other end of the connecting member  520  to the XY stage  230 ′. The double flexure assembly  500  includes a flange  502  connected to the coil  444 Y. A clamping member  504  is attached via clamping bolts to the flange  502  to clamp therebetween one edge of a horizontal flexible link  506 . The other end of the flexible link  506  is clamped between two horizontal members  508  which are in turn integrally connected with a vertical flange  510  to which are bolted a pair of flange members  512  which clamp one edge of a vertical flexible member  514 . The opposite edge of the vertical flexible member  514  is clamped between a pair of flange members  516  which are in turn bolted to a flange plate  518  on one end of the connecting member  520 . At the other end of the connecting member  520  a plate  548  is connected to two flange members  236  which are bolted together to clamp one end of a vertical flexible member  544 . The opposite edge of the vertical member  544  is clamped by flange members  542  which are in turn connected to a plate  540  connected to a pair of clamping plates  538  clamping one edge of a horizontal flexible member  536 , the opposing edge of which is in turn clamped onto the XY stage  230 ′ with the aid of the plate  534 . Thus, in each of the double flexure assemblies  500  and  530  vibrations are reduced by providing both a horizontal and a vertical flexible member. In each of these assemblies the vertical flexible members reduce X, Y and theta vibrations and the horizontal flexible members reduce Z, tilt and roll vibrations. Thus, there are eight vertical flex joints for X, Y and theta and eight horizontal flex joints for Z, tilt and roll. 
   As illustrated in  FIG. 17 , the coil  444 Y is attached to a coil support  445 Y which has an upper support plate  446  attached thereto which rides above the top of the magnetic track assembly  488 . Vacuum pre-load type air bearings  490  are provided between the coil support  445 Y and upper support plate  446  on the one hand and the magnetic track assembly  488  on the other hand. 
   In an operative example of the embodiment illustrated in  FIGS. 14-17  the flexible members  506 ,  514 ,  544  and  536  are stainless steel 1¼″ wide, ¼″ long and 0.012″ thick with the primary direction of flex being in the direction of the thickness. In the embodiment illustrated members  506  and  514  are mounted in series with their respective primary direction of flex being orthogonal to one another; members  544  and  536  are similarly mounted. 
   The Detailed Description from U.S. patent application Ser. No. 08/416,558 
   The stage  10  is (in plan view) a rectangular structure of a rigid material (e.g., steel, aluminum, or ceramic). Two interferometry mirrors  14 A and  14 B located on stage  10  interact conventionally with respectively laser beams  16 A and  16 B. Conventionally, laser beams  16 A are two pairs of laser beams and laser beams  16 B are one pair of laser beam, for three independent distance measurements. The underside of stage  10  defines a relieved portion  22  (indicated by a dotted line, not being visible in the plane of the drawing). A reticle  24  is located on stage  10  and held by conventional reticle vacuum groove  26  formed in the upper surface of chuck plate  28 . Stage  10  also defines a central aperture  30  (passage) below the location of reticle  24 . Central aperture  30  allows the light (or other) beam which penetrates through reticle  24  to enter the underlying projection lens, as described further below. (It is to be understood that the reticle  24  itself is not a part of the stage mechanism.) Moreover, if the present stage mechanism is to be used for other than a reticle stage, i.e., for supporting a wafer, aperture  30  is not needed. 
   Stage  10  is supported on a conventional rectangular base structure  32  of, e.g., granite, steel, or aluminum, and having a smooth planar upper surface. The left and right edges (in  FIG. 1 ) of base structure  32  are shown as dotted lines, being overlain by other structures (as described below) in this view. In operation, stage  10  is not in direct physical contact with its base structure  32 ; instead, stage  10  is vertically supported by, in this example, conventional bearings such as gas bearings. In one embodiment three air bearings  36 A,  36 B and  36 C are used which may be of a type commercially available. 
   In an alternative air bearing/vacuum structure, 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 a conventional cable bundle and internal tubing distribution system (not shown in the drawings for simplicity). In operation, stage  10  thereby floats on the air bearings  36 A,  36 B,  36 C approximately 1 to 3 micrometers above the flat top surface of base structure  32 . It is to be understood that other types of bearings (e.g., air bearing/magnetic combination type) may be used alternatively. 
   Stage  10  is laterally surrounded by the “window frame guide” which is a four member rectangular structure. The four members as shown in  FIG. 1  are (in the drawing) the top member  40 A, the bottom member  40 B, the lefthand member  40 C, and the righthand member  40 D. The four members  40 A- 40 D are of any material having high specific stiffness (stiffness to density ratio) such as aluminum or a composite material. These four members  40 A- 40 D are attached together by hinge structures which allow non-rigid movement of the four members relative to one another in the X-Y plane and about the Z-axis as shown in the drawing, this movement also referred to as a “yaw” movement. The hinge is described in detail below, each hinge  44 A,  44 B,  44 C and  44 D being, e.g., one or more metal flexures allowing a slight flexing of the window frame guide structure. 
   The window frame guide structure moves in the X axis (to the left and right in  FIG. 1 ) supported on horizontal surfaces of fixed guides  46 A and  46 B, and supported on vertical surfaces of fixed guides  64 A,  64 B. (It is to be understood that each pair of fixed guides  46 A,  64 A and  46 B,  64 B could be, e.g., a single L-shaped fixed guide, or other configurations of fixed guides may be used.) Mounted on window frame guide member  40 A are two air bearings  50 A and  50 B that cause the member  40 A to ride on its supporting fixed guide member  46 A. Similarly, air bearings  52 A and  52 B are mounted on the member  40 B, allowing member  40 B to ride on its supporting fixed guide member  46 B. Air bearings  50 A,  50 B,  52 A,  52 B are similar to air bearings  36 A, etc. 
   The window frame guide is driven along the X axis on fixed guides  46 A and  46 B,  64 A and  64 B by a conventional linear motor, which includes a coil  60 A which is mounted on window frame guide member  40 A. Motor coil  60 A moves in a magnetic track  62 A which is located in (or along) fixed guide  64 A. Similarly, motor coil  60 B which is mounted on window frame guide member  40 B moves in magnetic track  62 B which is located in fixed guide  64 B. The motor coil and track combinations are part no. LM-310 from Trilogy Company of Webster Tex. These motors are also called “linear commutator motors”. The tracks  62 A,  62 B are each a number of permanent magnets fastened together. The electric wires which connect to the motor coils are not shown but are conventional. Other types of linear motors may be substituted. It is to be understood that the locations of the motor coils and magnetic tracks for each motor could be reversed, so that for instance the magnetic tracks are located on stage  10  and the corresponding motor coils on the window frame guide members, at a penalty of reduced performance. 
   Similarly, stage  10  moves along the Y axis in  FIG. 1  by means of motor coils  68 A and  68 B mounted respectively on the left and right edges of stage  10 . Motor coil  68 A moves in magnetic track  70 A mounted in window frame guide member  40 C. Motor coil  68 B moves in magnetic track  70 B mounted in window frame guide member  40 D. 
   Also shown in  FIG. 1  are air bearings  72 A,  72 B and  72 C. Air bearing  72 A is located on window frame guide member  40 A and minimizes friction between window frame guide member  40 A and its fixed guide  64 A. Similarly, two air bearings  72 B and  72 C on window frame guide member  40 B minimize its friction with the fixed guide  64 B. The use of a single air bearing  72 A at one end and two opposing air bearings  72 B and  72 C at the other end allows a certain amount of yaw (rotation in the X-Y plane about the Z-axis) as well as limited motion along the Z-axis. In this case, typically air bearing  72 A is gimbal mounted, or gimbal mounted with the gimbal located on a flexure so as to allow a limited amount of misalignment between the member  40 A and fixed guide  64 A. 
   The use of the air bearing  72 A opposing bearings  72 B and  72 C provides a loading effect to keep the window frame guide in its proper relationship to fixed guides  64 A,  64 B. Similarly, an air bearing  76 A loads opposing air bearings  76 B and  76 C, all mounted on side surfaces of the stage  10 , in maintaining the proper location of stage  10  relative to the opposing window frame guide members  40 B and  40 D. Again, in this case one air bearing such as  76 A is gimbal mounted to provide a limited amount of misalignment, or gimbal mounted with the gimbal on a flexure (spring). Air bearings  72 A,  72 B,  72 C and  76 A,  76 B, and  76 C are conventional air bearings. 
   The outer structure  80  in  FIG. 1  is the base support structure for the fixed guides  46 A,  46 B,  64 A,  64 B and the window frame guide members  40 A, . . . ,  40 D of the stage mechanism, but does not support stage base structure  32 . Thus, the underlying support is partitioned so the reaction force on base support structure  80  does not couple into the stage base structure  32 . Base support structure  80  is supported by its own support pillars or other conventional support elements (not shown in this drawing) to the ground, i.e., the surface of the earth or the floor of a building. An example of a suitable support structure is disclosed in above-referenced U.S. patent application Ser. No. 08/221,375 at FIGS. 1, 1B, 1C ( FIGS. 7 ,  7 B,  7 C of the present application). This independent support structure for this portion of stage mechanism provides the above-described advantage of transmitting the reaction forces of the reticle stage mechanism drive motors away from the frame supporting the other elements of the photolithography apparatus, especially away from the optical elements including the projection lens and from the wafer stage, thereby minimizing vibration forces on the projection lens due to reticle stage movement. This is further described below. 
   The drive forces for the stage mechanism are provided as close as possible through the stage mechanism center of gravity. As can be understood, the center of gravity of the stage mechanism moves with the stage  10 . Thus, the stage  10  and the window frame guide combine to define a joint center of gravity. A first differential drive control (not shown) for motor coils  60 A,  60 B takes into account the location of the window frame guide to control the force exerted by each motor coil  60 A,  60 B to keep the effective force applied at the center of gravity. A second conventional differential drive control (not shown) for motor coils  68 A,  68 B takes into account the location of stage  10  to control the force exerted by each motor coil  68 A,  68 B to keep the effective force applied at the center of gravity. It is to be understood that since stage  10  has a substantial range of movement, that the differential drive for the motor coils  60 A,  60 B has a wide differential swing. In contrast, the window frame guide has no CG change, hence the differential drive for the motor coils  68 A,  68 B has a much lesser differential swing, providing a trim effect. Advantageously, use of the window frame guide maintains the reaction forces generated by movement of the reticle stage mechanism in a single plane, thus making it easier to isolate these forces from other parts of the photolithography apparatus. 
     FIG. 2  shows a cross-sectional view through line  2 — 2  of FIG.  1 . The structures shown in  FIG. 2  which are also in  FIG. 1  have identical reference numbers and are not described herein. Also shown in  FIG. 2  is the illuminator  90  which is a conventional element shown here without detail, and omitted from  FIG. 1  for clarity. Also shown without detail in  FIG. 2  is the upper portion of the projection lens (barrel)  92 . It is to be understood that the lower portion of the projection lens and other elements of the photolithography apparatus are not shown in  FIG. 2 , but are illustrated and described below. 
   The supporting structure  94  for the projection lens  92  is also shown in FIG.  2 . As can be seen, structure  94  is separated at all points by a slight gap  96  from the base support structure  80  for the reticle stage mechanism. This gap  96  isolates vibrations caused by movement of the reticle stage mechanism from the projection lens  92  and its support  94 . As shown in  FIG. 2 , stage  10  is not in this embodiment a flat structure but defines the underside relieved portion  22  to accommodate the upper portion of lens  92 . Magnetic track  70 A is mounted on top of the window frame guide  40 B, and similarly magnetic track  70 B is mounted on top of the opposite window frame guide member  40 D. 
     FIGS. 3A and 3B  are enlarged views of portions of  FIG. 2 , with identical reference numbers;  FIG. 3A  is the left side of FIG.  2  and  FIG. 3B  is the right side of FIG.  2 . Shown in  FIG. 3A  is the spring mounting  78  for air bearing  76 A. Air bearing  78 A being spring mounted to a side surface of stage  10 , this allows a certain amount of yaw (rotation in the X-Y plane about the Z-axis) as well as limited motion along the Z-axis. A gimbal mounting may be used in place of or in addition to the spring  78 . The spring or gimbal mounting thereby allows for a limited amount of misalignment between stage  10  and members  40 C,  40 D (not shown in FIG.  3 A). 
     FIG. 4  is a top view of a photolithography apparatus including the stage mechanism of  FIGS. 1 and 2  and further including, in addition to the elements shown in  FIG. 1 , the supporting base structure  100  which supports the photolithography apparatus including frame  94  except for the reticle stage mechanism. (Not all the structures shown in  FIG. 1  are labeled in  FIG. 4 , for simplicity.) Base structure  100  supports four vertical support pillars  102 A,  102 B,  102 C and  102 D connected to structure  94  by respective bracket structures  106 A,  106 B,  106 C and  106 D. It is to be appreciated that the size of the base structure  100  is fairly large, i.e., approximately 3 meters top to bottom in one embodiment. Each pillar  102 A,  102 B,  102 C,  102 D includes an internal conventional servo mechanism (not shown) for leveling purposes. Also shown in  FIG. 4  are the supports  108  and  110  for respective laser interferometer units (beam splitter etc.)  112 A,  112 B,  112 C.  FIG. 4  will be further understood with reference to  FIG. 5  which shows a view of FIG.  4  through cross-sectional line  5 — 5  of FIG.  4 . 
   In  FIGS. 4 and 5  the full extent of the supporting structure  94  can be seen along with its support pillars  102 A,  102 C which rest on the base structure  100  which is in contact with the ground via a conventional foundation (not shown). The independent support structure for the reticle stage base support structure  80  is shown, in  FIG. 4  only (for clarity) and similarly includes a set of four pillars  114 A,  114 B,  114 C,  114 D with associated bracket structures  116 A,  116 B,  116 C,  116 D, with the pillars thereby extending from the level of base support structure  80  down to the base structure  100 . 
   The lower portion of  FIG. 5  shows the wafer stage  120  and associated support structures  122 ,  124 . The elements of wafer stage  120  conventionally include (not labeled in the drawing) a base, the stage itself, fixed stage guides located on the base, magnetic tracks located on the fixed stage guides, and motor coils fitting in the magnetic tracks and connected to the stage itself. Laser beams from laser  124  mounted on support  126  locate lens  92  and the stage itself by interferometry. 
     FIG. 6A  shows detail of one of the window frame guide hinged flexure structures, e.g.,  44 C, in a top view (corresponding to FIG.  1 ). Each of hinges  44 A,  44 B,  44 C and  44 D is identical. These flexure hinges have the advantage over a mechanical-type hinge of not needing lubrication, not exhibiting hysteresis (as long as the flexure is not bent beyond its mechanical tolerance) and not having any mechanical “slop”, as well as being inexpensive to fabricate. 
   Each individual flexure is, e.g., ¼ hard 302 stainless steel approximately 20 mils (0.02 inch) thick and can sustain a maximum bend of 0.5 degree. The width of each flexure is not critical; a typical width is 0.5 inch. Two, three or four flexures are used at each hinge  44 A,  44 B,  44 C and  44 D in FIG.  1 . The number of flexures used at each hinge is essentially determined by the amount of space available, i.e., the height of the window frame guide members. The four individual flexures  130 A,  130 B,  130 C,  130 D shown in  FIG. 6A  (and also in a 90 degree(s) rotated view in  FIG. 6B ) are each attached by clamps  136 A,  136 B,  136 C,  136 D to adjacent frame members (members  40 B and  40 D in  FIGS. 6A and 6B ) by conventional screws which pass through holes in the individual flexures  130 A,  130 B,  130 C,  130 D and through the clamps and are secured in corresponding threaded holes in frame members  40 B and  40 D. 
   Note that the frame members  40 B,  40 D of  FIGS. 6A and 6B  differ somewhat from those of  FIG. 1  in terms of the angular (triangular) structures at the ends of frame members  40 B,  40 D and to which the metal flexures  130 A,  130 B,  130 C,  130 D are mounted. In the embodiment of  FIG. 1 , these angular structures are dispensed with, although their presence makes screw mounting of the flexures easier. 
   In an alternate embodiment, the window frame guide is not hinged but is a rigid structure. To accommodate this rigidity and prevent binding, one of bearings  72 C or  72 B is eliminated, and the remaining bearing moved to the center of member  40 B, mounted on a gimbal with no spring. The other bearings (except those mounted on stage  10 ) are also gimballed. 
   This disclosure is illustrative and not limiting; further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.