Abstract:
An exposure apparatus includes an exposure device disposed between a mask and an object. The exposure device exposes a pattern of the mask onto the object. The apparatus also includes a movable mask stage to hold the mask and a movable object stage to hold the object. The apparatus also includes a reaction frame that is dynamically isolated from the exposure device. A reaction force caused by movement of the mask stage and the object stage when the mask stage and the object stage are moved by a drive is transferred substantially to the reaction frame.

Description:
This is a division of application Ser. No. 09/192,153 filed Nov. 12, 1998, which in turn is a continuation of application Ser. No. 08/416,558 filed Apr. 4. 1995. now U.S. Pat. No. 5,874,820, The entire disclosure of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     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 a 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 present 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 the present 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 present 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 present window frame guided stage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a top view of a stage mechanism in accordance with the present 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, original docket no. NPI0500 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 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 TX. 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,  1 B,  1 C. 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 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 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 labelled 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 respectively 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 respectively 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 labelled 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 histeresis (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° 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.