Abstract:
This invention relates to an apparatus and method for positioning dual stages during semiconductor wafer processing. More particularly, the invention facilitates the use of interferometers to determine the positions of both wafer stages at all times during processing. While the movement of a typical twin stage apparatus causes one of the stages to eclipse the other and requires the addition of a significant number of additional interferometers, this invention minimizes the number of interferometers necessary through dimensioning the stages so that one stage never totally eclipses the other.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to lithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with an apparatus and method for using interferometers to determine the position of substrate stages during the simultaneous processing of the substrates affixed to these stages.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lithography processes require positioning a reticle between an electron beam and the substrate chip or wafer. System throughput is dependent upon the speeds of many separate processes that are performed in series. Throughput is therefore dependent on the duration of each process.  
           [0003]    In a typical modern lithography process an individual wafer undergoes a number of sub-processes. These can include: loading, field image alignment, global alignment, and exposure. The production of an acceptable final product requires the complex interaction of the systems necessary to implement each sub-process. For example, in the sub-process for exposing patterns on wafers and other substrates, the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. Both reticle and wafer must be held without slippage and in a way that does not cause distortion of the reticle pattern. The system is further complicated by the fact that lithography processes typically occur in a clean room/vacuum environment; this is also an indication of the sensitivity of the processes.  
           [0004]    A typical exposure apparatus  10  employing a single wafer stage is shown in FIG. 1 and FIG. 2. Exposure apparatus  10  transfers a pattern of an integrated circuit from reticle  12  onto semiconductor wafer  14 . Apparatus frame  16  preferably is rigid and supports the components of exposure apparatus  10 . These components include: reticle stage  18 , wafer stage  20 , lens assembly  22 , and illumination system  24 . Alternatively, separate, individual structures (not shown) can be used to support wafer stage  20 , reticle stage  18 , illumination system  24 , and lens assembly  22 .  
           [0005]    Illumination system  24  includes an illumination source  26  and an illumination optical assembly  28 . Illumination source  26  emits an exposing beam of energy such as light or electron energy. Optical assembly  28  guides the beam from illumination source  26  to lens assembly  22 . The beam illuminates selectively different portions of reticle  12  and exposes wafer  14 . In FIG. 1, illumination source  26  is illustrated as being supported above reticle stage  18 . Typically, however, illumination source  26  is secured to one of the sides of apparatus frame  16  and the energy beam from illumination source  26  is directed to above reticle stage  18  with illumination optical assembly  28 . Where illumination source  26  is an electron beam, the optical path for the electron beam should be in a vacuum.  
           [0006]    Lens assembly  22  projects and/or focuses the light passing through reticle  12  to wafer  14 . Depending upon the design of apparatus  10 , lens assembly  22  can magnify or reduce the image illuminated on reticle  12 .  
           [0007]    Reticle stage  18  holds and precisely positions reticle  12  relative to lens assembly  22  and wafer  14 . Similarly, wafer stage  20  holds and positions wafer  14  with respect to the projected image of the illuminated portions of reticle  12 . In the embodiment illustrated in FIG. 1 and FIG. 2, wafer stage  20  and reticle stage  18  are positioned by shaft-type linear motors  30 . Depending upon the design, apparatus  10  may include additional servo drive units, linear motors and planar motors to move wafer stage  20  and reticle stage  18 , but other drive and control mechanisms may be employed.  
           [0008]    The basic device as described may be used in different types of lithography processes. For example, exposure apparatus  10  can be used in a scanning type lithography system, which exposes the pattern from reticle  12  onto wafer  14  with reticle  12  and wafer  14  moving synchronously. In a scanning type lithography process, reticle  12  is moved perpendicular to an optical axis of lens assembly  22  by reticle stage  18 , and wafer  14  is moved perpendicular to an optical axis of lens assembly  22  by wafer stage  20 . Scanning of reticle  12  and wafer  14  occurs while reticle  12  and wafer  14  are moving synchronously.  
           [0009]    Alternatively, exposure apparatus  10  may be employed in a step-and-repeat type lithography system that exposes reticle  12  while reticle  12  and wafer  14  are stationary. In the step-and-repeat process, wafer  14  is in a constant position relative to reticle  12  and lens assembly  22  during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer  14  is consecutively moved by wafer stage  20  perpendicular to the optical axis of lens assembly  22  so that the next field of semiconductor wafer  14  is brought into position relative to lens assembly  22  and reticle  12  for exposure. Following this process, the images on reticle  12  are sequentially exposed onto the fields of wafer  14 .  
           [0010]    This complexity and sensitivity of the exposure apparatus and the processes involved result in a significant time expenditure for each sub-process. When a single wafer is undergoing one of these sub-processes, the mechanisms for the others are normally idle. Consumer demand for the end product has created a need for increased throughput and, thus, the development of methods to decrease the idle time. One current method uses two stages that run simultaneously, but with each stage at different steps in the process. This method relies upon a combination of encoders and interferometers to determine the position of each stage at any given point throughout processing.  
           [0011]    Encoders, however, are less than ideal devices for this use for a number of reasons. The encoder must be placed in an area that does not interfere with the requirements of other sub-processes, such as substrate exposure. This leads to apparatus design problems in harmonizing the requirements of the encoder, interferometers, and the process. Also, encoders are less precise than interferometers. Precision in planar placement of the stage is necessary, since errors in reticle or wafer position result in similar errors in the final product and, therefore, reduced functionality of that final product.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a dual stage assembly and method where stage position may be determined using interferometers. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions. The two stages move between multiple positions on the base and have mirrors affixed to them that cooperate with the other interferometer components to provide position data. At times, the two stages are positioned so that the first stage eclipses the second stage with respect to said at least one of the interferometers. Whenever such an eclipse occurs, the mirror on the second (eclipsed) stage is configured to cooperate with the non-eclipsed interferometers so that the position of said second stage is continuously determinable. This is achieved by appropriately dispersing the interferometers about one side of the base and by causing the mirror on the second stage to extend from behind the eclipsing shadow of the first stage. In a preferred embodiment, the second stage is the same size as the first and merely supports the larger mirror. In another preferred embodiment, the second stage is approximately the same size as the mirror in the relevant dimensions. In both the previously mentioned preferred embodiments the stages are the same size in the direction parallel to the axes of the interferometers. But the invention could also be practiced in two dimensions resulting in the need for interferometers on only two orthogonal sides of the base.  
           [0013]    A method incorporating the invention comprises: sizing the stages based on wafer and exposure apparatus parameters; dispersing interferometers about the sides of the base at appropriate positions based on the stage sizes; configuring the mirror on the second stage to continue to cooperate with enough other interferometer elements to provide position data even if the first stage is positioned between the second stage and some of the interferometers; moving the stages as desired during the course of using the exposure apparatus; and determining the positions of both stages at all times during the process. A preferred embodiment of the invention practices the method with respect to one dimension of the apparatus; resulting in interferometers on three sides of the base. Another embodiment of the invention practices the method with respect to both dimensions of the plane of movement; resulting in interferometers on two orthogonal sides of the base. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing and other aspects and advantages will be better understood from the following detailed description of the preferred embodiment of the invention with reference to the drawings, in which:  
         [0015]    [0015]FIG. 1 is a side view of a lithography exposure apparatus of the prior art;  
         [0016]    [0016]FIG. 2 is a different side view of a lithography exposure apparatus of the prior art;  
         [0017]    [0017]FIG. 3 is a top view of a dual wafer stage assembly incorporating a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 3 a  is a schematic plan view of an alternative embodiment of a stage according to the invention;  
         [0019]    [0019]FIG. 4 a  is a top view of a first wafer stage;  
         [0020]    [0020]FIG. 4 b  is an enlarged detail view of portion B of the first wafer stage shown in FIG. 4 a.    
         [0021]    [0021]FIG. 5 is a top plan view of a first wafer stage eclipsing a second wafer stage in a preferred embodiment of the present invention;  
         [0022]    [0022]FIG. 6 is a top view of a second wafer stage in a preferred embodiment of the present invention;  
         [0023]    [0023]FIG. 7 is a top view of first and second wafer stages showing the Y swept area of dual wafer stage assembly  40  in example 1;  
         [0024]    [0024]FIG. 8 illustrates a first position involved in determining the X swept area of dual wafer stage assembly  40  in example 1;  
         [0025]    [0025]FIG. 9 illustrates a second position involved in determining the X swept area of dual wafer stage assembly  40  in example 1;  
         [0026]    [0026]FIG. 10 illustrates the mechanical margin as described in example 1;  
         [0027]    [0027]FIG. 11 illustrates an arrangement of wafer stage cover as described in example 1;  
         [0028]    FIGS.  12 ( a )-( d ) illustrate interferometer axes configuration in example 1;  
         [0029]    FIGS.  13 ( a )-( c ) is a flowchart illustrating a method according to the present invention; and  
         [0030]    FIGS.  14 ( a )-( v ) illustrate steps  1 - 22  of the method shown in FIGS.  13 ( a )-( c ). 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Referring now to FIG. 3, dual wafer stage assembly  40  is illustrated from above according to a preferred embodiment of the invention. This invention minimizes the number of interferometers necessary for positioning by configuring the stages according to the invention so that one stage never totally eclipses the other. Dual wafer stage assembly  40  provides the capability for the system to rely on interferometers  46 ,  48 ,  50 ,  52 ,  54 ,  56  to determine the position of wafer stages  42 ,  44  during processing. Dual wafer stage assembly  40  generally comprises a first wafer stage  42 , a second wafer stage  44 , a base  60 , and the interferometers mentioned above. Each interferometer cooperates with a reflective surface (mirror)  58 ,  62 ,  64 ,  66 ,  68 ,  70  mounted on stages  42 ,  44  so that they reflect back to the intended interferometer.  
         [0032]    As shown in FIG. 3, focal point  72  refers to the point where the image from reticle  12  is focused by the projection apparatus onto stage  40 . Area  74  identifies an area that an operator views from above (typically with a microscope) to determine if the wafer is properly aligned. This area is referred to as the field image alignment area or point. Interferometer  50  is positioned approximately midway between projection lens  72  and field image alignment point  74  in the X-direction and is used for control when stages  42 ,  44  are moving in the Y-direction. Interferometer  54  is positioned to determine stage location during loading (stage safety position) as illustrated.  
         [0033]    Mirrors  58 ,  62 ,  64 ,  66 ,  68 ,  70  are known as “moving mirrors” because they move with wafer stages  64 ,  66 . As is understood by persons skilled in the art, other non-moving mirrors known as “reference mirrors” are located within the interferometers  46 ,  48 ,  56 ,  52 ,  50  and  54 , but in order to clarify the schematic representation of the invention standard reference beams, reference mirrors, and sensors for each interferometer are not shown.  
         [0034]    Elements of base  60  necessary for the support, positioning, and movement of wafer stages  42 ,  44  are also not illustrated in FIG. 3 for purposes of clarity, but as one of skill in the art would recognize, these functions may be accomplished by the shaft-type linear motors  30  or other known actuators and additional static support elements of the prior art (see FIGS. 1 &amp; 2). Also, the individual axes (beams) of interferometers  46 ,  48 ,  56 ,  52 ,  50  and  54  are configured and utilized either singularly or in combination to make possible the measurements necessary at any particular moment. For example, although the single beam of interferometer  50  is primarily used to determine Y position of wafer stage A when switching in the Y direction, it is also used at times in conjunction with interferometer  52  to determine the stage A yaw. In a preferred embodiment, the present invention employs one, two, and three axes interferometers that are incident on dual wafer stages  42 ,  44  from three sides to determine wafer stage position at all times. As shown in FIGS.  12 ( a )-( d ), the axes of the interferometers are directed in parallel towards the moveable mirrors on stages  42 ,  44 , but are arranged to provide position data from different points on the moveable mirrors. Position data from one axis may therefore be combined with position data from one or more other axes and manipulated to yield stage yaw and pitch. As is well known to one of skill in the art, the spacing between axes necessary for them to combine to yield yaw and pitch data is infinitely variable in three dimensions so long as the beam is incident upon the desired mirror.  
         [0035]    Continuing with FIG. 3, and viewing the interferometers in more detail in FIG. 12, interferometers  46 ,  48 ,  56  each have 3 axes  46   a,    46   b,    46   c,    48   a,    48   b,    48   c,    56   a,    56   b,    56   c  and is capable of determining X position, yaw, and pitch. Interferometers  52 ,  54  each have 2 axes  52   a,    52   b,    54   a,    54   b  and is capable of determining Y position and pitch. Interferometer  50  has one axis  50   a  and is capable of determining Y position, but if roll and pitch control are also required in addition to the X position, Y position, and yaw control, then interferometer  50  preferably has 2 axes and is capable of determining Y position and pitch, similar to interferometer  52 . If roll and pitch control are also required in addition to the X position, Y position, and yaw control, then interferometer  54  preferably has 3 axes and is capable of determining Y position, yaw, and pitch. In this preferred embodiment the minimum number of axes necessary to implement the invention is 14, as shown in FIG. 14.  
         [0036]    In a preferred embodiment shown in FIG. 3 interferometers  46 ,  48 ,  52 , and  56  are directed so their axes determine the positions of wafer stages  42 ,  44  closest to the most significant areas. Interferometers  46 ,  48  intersect at focal point  72  where the projection lens focuses the image from the reticle. Interferometers  56  and  52  similarly intersect at field image alignment point  74 . In order to prevent all of interferometers  48 ,  50 ,  52 ,  54  from being eclipsed by first stage  42  whenever it is positioned between second wafer stage  44  and said interferometers, second stage  44  is preferably larger in the X-direction than first stage  42 . Thus, moving mirrors positioned on second stage  44  that cooperate with interferometers  48 ,  50 ,  52 ,  54  are at least in part disposed at peripheral edge portions of the second stage so as to be accessible to the interferometers around the first stage. In a preferred embodiment, these moving mirrors comprise a single, continuous mirror that extends along the entire side of the stage to permit interferometer readings at any point. Alternatively, rather than increasing the size of the stage as a whole, it will be appreciated that extension structures  43  may be provided on second stage  44 ′ to carry the moving mirror(s) as schematically illustrated in FIG. 3 a.    
         [0037]    By properly placing the interferometers and configuring the stages or the moveable mirrors mounted on the stages, this invention provides a dual wafer stage assembly that positions the stages with the precision of the interferometer, while only requiring the interferometers be mounted on three sides of the stages. The principals of the present invention are further illustrated by the following example. This example describes one possible preferred embodiment for illustrative purposes only, the example does not limit the scope of the invention as set forth in the appended claims.  
       EXAMPLE 1  
       [0038]    The following example describes the sizing of the stages in an exemplary embodiment according to the present invention using equations (1)-(17). In this example, as illustrated in FIG. 3, the X direction dimensions of the first and second stages are different, but the Y direction dimension of each is the same. One of skill in the art will understand that the present invention can be practiced with stage dimensions of infinite variety. This is due to the different potential ranges of values possible for each of the variables and the changes possible with the configurations of the stages, mirrors, and other elements. In particular, depending on a preferred orientation of interferometers, the Y direction dimension of either the first or second stage may be larger than that of the other stage in accordance with the teachings set forth herein.  
         [0039]    The dimensions of first wafer stage  42  are now described with reference to FIGS. 4 a  and  4   b.  In FIG. 4 a,  wafer  6  is illustrated positioned on stage  42  and a projection on the stage of the exposure slit of the exposure apparatus is represented by outline  8 . As is known in the art, the X dimension of mirror  64  may be determined based on the wafer size and slit width, as well as certain other parameters as explained below. In this example, it is assumed that the stage  42  size is the same as the size of mirror  64  in the X direction. Also, in FIGS.  4 - 6 ,  10 , and  11  the actual interferometer axes are depicted as thick lines, such as beam diameter  90  in FIG. 4 a,  while the virtual path of the interferometer is a thin dotted or solid line used in the remainder of the figures.  
         [0040]    In an exemplary embodiment, as shown in FIGS.  4 - 7 , the predetermined parameters are:  
                                           300   mm   Wafer size (WS) 82       25   mm   Slit width (SW) 84       26.2   mm   Yaw separation (YS) 86       13.1   mm   Double path separation (DPS) 88       6   mm   Beam diameter (BD) 90       5   mm   Polishing margin (PM) 92       4       n value chosen for New bow correction (NBC)                  
 
         [0041]    The X direction mirror size may be determined by solving the following five equations:  
         Exposure area ( EA ) 94 =WS+SW   (1)  
           NBC=YS/n   (2)  
         Integered exposure area ( IEA ) 95 =NBC* ( Int ( EA/NBC )+1)  (2)  
         Additional length ( AL ) 98=( YS/ 2 +DPS/ 2 +BD/ 2 +PM )  (4)  
           X  Mirror size ( XMS ) 100 =IEA+ 2* AL   (5)  
         [0042]    As shown in FIG. 4 b,  exposure area  94  is the area that is swept under projection lens focal point  72 . The X dimension of this area is determined by wafer size  82  and slit width  84  as Eq. (1) shows. Yaw separation  86  is the distance interferometer axes must be separated to accurately determine wafer stage  42  and  44  yaw. Integered exposure area  95  is based on yaw separation  86 , exposure area  94 , and NBC as shown in Eq. (3). For a discussion of the new bow correction, see U.S. Pat. No. 5,790,253, which describes the conventional method of correcting for curving deviations associated with mirror imperfections, and is incorporated by reference. In this conventional method, when measuring linearity errors of a moving mirror by means of a laser interferometer having two measuring length axes, the bow correction is made by shifting the moving mirror by less than yaw separation  86  and, therefore, the measurement points of the axes overlap. The decree of overlap is indicated by the denominator n. The linearity errors which indicate the curving of the reflecting surface of the moving mirror are then based on the relationship of adjacent measurement values. New bow correction indicates the length of mirror necessary to perform the conventional method of correcting for mirror linearity errors. It is a function of YS 86 and an arbitrary denominator n, chosen by one of skill in the art based on the resolution of the deviation correction needed and interferometer accuracy. A higher value for n is chosen where higher resolution is needed. and, therefore, n is greater than or equal to one.  
         [0043]    Additional length  98  is based on yaw separation  86 , double path separation  88 , beam diameter  90 , and polishing margin  92  as shown in Eq. (4). The X dimension of first wafer stage  42  is equal to its X mirror dimension and shown by Eq. (5) to be the sum of integered exposure area  95  and additional length  98  needed on each side of stage  42 . In this example, given the wafer size, slit width, and other fixed parameters, solving equations (1)-(5) produces a X dimension for mirror  42  of 382.8 mm.  
         [0044]    Now referring to FIG. 5, this X dimension then provides the basis for calculating the X direction separation of field image alignment area  74  and interferometer  48  from focal point  72  and interferometer  52  necessary to allow stages  42  and  44  to move in the Y direction. This is baseline separation  80  and it drives the X distance between interferometers  48  and  52 . Baseline separation  80  is 393 mm and is calculated:  
         Baseline Separation ( BS ) 80=Stage 42  XMS−DPS + 2*( YS− ( DPS/ 2 +BD/ 2)− PM )  (6)  
         [0045]    Given a specific first wafer stage size, the size of second wafer stage  44  is also then calculated according to the invention. More specifically, the parameter of beam clearance  108 , the distance that an interferometer beam is designed to clear the wafer stage, is needed in addition to the predetermined parameters of FIG. 4 b.  Wafer stage  44  X mirror size may be determined by solving the following two equations:  
         Beam clearance ( BC )= YS−PM− 2*( DPS/ 2 +BD/ 2)  (7)  
         Stage 44  XMS= Stage 42  XMS+ 2*( DPS+BD+BC+PM )  (8)  
         [0046]    Second wafer stage  44  is thus made greater than first wafer stage  42  with additions to its X dimension. Each of these additions is the sum of double path separation  88 , beam diameter  90 , beam clearance  108 , and polishing margin  92 . Again it is assumed that the stage  44  size is the same as the size of mirror  70  in the X direction. Thus, for the given first stage size and predetermined parameter values, solving equations (7) and (8) produces a second wafer stage  44  X dimension of 435.2 mm.  
         [0047]    Wafer stages  42  and  44  are the same dimension in the Y direction. In the Y direction, stage size is not equated to mirror size, unlike the X direction in this example. Computation of the Y dimension requires the additional predetermined parameters of: maximum velocity, acceleration, settling time, X mirror thickness and slit length.  
                                               For:   375   mm/sec   Maximum velocity           0.75   g   Acceleration           50   msec   Settling time           25   mm   X mirror thickness (MT) 87           9   mm   Slit length (SL) 85                  
 
         [0048]    Y table size is computed using Equations (9)-(13). These equations that follow can best be understood by referring to FIG. 6. Acceleration and deceleration area  104  is determined by the maximum acceleration and velocity of wafer stages  42 ,  44  and the settling time required once the correct position has been reached, per Eq. (11). Interferometer length  96  is a function of yaw separation  86 , double path separation  88 , and beam diameter  90 ; parameters previously discussed with reference to FIGS. 4 a,    4   b,  and  5 . In this example, solving equations (9)-(13) gives a Y dimension of 435.9 mm.  
         Exposure area  Y  ( EAY ) 102 =WS+SL   (9)  
         Acceleration and deceleration area (“ Acc ”) 104 (“ Acc ”)=[(Maximum velocity**2)/(2*Acceleration)+(Maximum velocity*Settling time)]  (10)  
         Interferometer length (“ Inf ”) 96 (“ Inf ”)=[ YS/ 2 +DPS/ 2 +BD/ 2]  (11)  
           Y  mirror size ( YMS )= EA+ 2*( Acc+Inf )  (12)  
           Y  stage size= YMS+MT   (13)  
         [0049]    [0049]FIG. 7 illustrates the Y dimension needed for stages  42 ,  44  to move without contacting each other within dual wafer stage assembly  40 . Interferometers and supporting structure are not shown in this diagram as this is a representation of the space necessary for the stages to move freely. The necessary distance is Y swept area  106 . Mechanical margin  112  is the parameter defining the clearance between stages as they pass each other and is 24 mm, as discussed within. Y swept area  106  is determined by equation (14) to be 895.8 mm, with the Y separation of interferometers  46  and  56 , or Y baseline dimension  89 , determined by equation (15) to be 150.9 mm.  
           Y  swept area=2* Y  stage dimension+mechanical margin  (14)  
           Y  Baseline Dimension ( YBD ) 89=2*( Acc+Inf )+ MT   (15)  
         [0050]    [0050]FIGS. 8 and 9 illustrate the X dimension needed for stages  42 ,  44  to move without hindrance within dual wafer stage assembly  40 . X swept area  114  is the distance covered by second wafer stage  44  during the course of its movement. X swept area includes the space necessary for stage  44  to undergo field image alignment while stage  42  is being exposed, plus the baseline separation  80  of interferometers  48  and  52  (Eq. (6)), plus the area necessary for stage  44  to undergo exposure while stage  42  undergoes field image alignment. X swept area  114  is the sum of the X dimension of wafer stages  42 , twice the X dimension of second wafer stage  44  and mechanical margin  112  minus additional length  98  and baseline dimension  80  as shown in Eq. (16).  
           X  swept area 114=3*wafer stage 42  XMS/ 2 −AL+ 2* MM+ 2*wafer stage 44  XMS−BS   (16)  
         [0051]    [0051]FIG. 10 illustrates the reasons driving mechanical margin  112  to 24 mm which are described by Eq. (17). Interferometer  48   a  is a dual beam interferometer. For it to function both beams must be incident on the mirror, avoiding polishing margin  92  to ensure beam coherency. If mechanical margin  112  is greater than 24 mm then interferometer  48   a  cannot be used for measuring first wafer stage  42  position in the safety area as shown in FIG. 14( o ). Another interferometer axis would be required. A mechanical margin  112  of greater than 24 mm would result in one of the dual beams of interferometer  48  a hitting wafer stage  42  in the polishing margin  92 .  
           MM  112 =WS/ 2+Stage 44  XMS/ 2 −DPS/ 2 +BD/ 2 −BS+PM   (17)  
         [0052]    [0052]FIG. 11 illustrates the reason why interferometer  52  cannot cover first wafer stage  42  when it is in the loading position and interferometer  54  must be added. Wafer stage  42  must be covered by interferometer  48  when in the exposure position and nearest to wafer stage  44  while stage  44  is in the field image alignment position. Wafer stage  42  must also be covered by an interferometer while in the loading position. With interferometers  48  and  52  set at the baseline dimension  80  apart, and given mechanical margin  112 , the center of wafer stage  44  during loading is fixed along the axis of interferometer  54  as shown. The center of wafer stage  42  must therefore also be on this axis, as the loading positions for the wafers are the same. But given the smaller size of wafer stage  42 , when wafer stage  42  is in line for loading, interferometer  52  is too far from the center of wafer stage  42  to be functionally incident upon wafer stage  42 , thus making interferometer  54  necessary. The left beam of double-path interferometer  52  would not be reflected by the mirror on wafer stage  42  since it is shown by Eq. (18) to be 212.05 mm from the center of the loading position. This is further than half of the X dimension of wafer stage  42 , which is only 191.4 mm.  
                                       Eq. (18)   Distance of interferometer 52 left beam from Wafer stage 44           center 91 = Wafer stage 44 XMS/2 −           [BS − WS/2 − SW/2 − Wafer stage 42 XMS/2 − MM −           DPS/2 − BD/2] = 212.05 mm                  
 
         [0053]    FIGS.  13 ( a )-( d ) is a flowchart of a preferred method for utilizing the dual wafer stage of the present invention. This flowchart illustrates a preferred method of utilizing dual wafer stage assembly  40  of the present invention. The flowchart further illustrates interferometer use during the movement of the stages. Individual steps in the flowchart are discussed in more detail in the referenced sub-illustrations of FIG. 3 and FIGS.  14 ( a )-( v ).  
         [0054]    Referring again to FIG. 13( a ), in step  200 , further illustrated by FIG. 14( a ), first wafer stage  42  is loading and its X position is controlled by interferometer  56   a  changing to interferometer  56 , its Y position by interferometer  54 , and Yaw is controlled by interferometer  54 . Second wafer stage  44  is stopped to begin the exposure sequence if loaded with a wafer and its X position is controlled by interferometer  46 , its Y position by  48 , and its Yaw by Interferometer  48 .  
         [0055]    In step  202 , further illustrated by FIG. 14( b ), first wafer stage  42  is starting enhanced global alignment and its X position is controlled by interferometer  56 , its Y position by interferometer  54  changing to interferometer  52 , and Yaw is controlled by interferometer  54  changing to interferometer  56 . At times stage  42  is halted to avoid disturbing the movement of stage  44 . Second wafer stage  44  is in the exposure sequence if loaded with a wafer and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0056]    In step  204 , further illustrated by FIG. 14( c ), first wafer stage  42  is in enhanced global alignment and its X position is controlled by interferometer  56 , its Y position by interferometer  52  changing to  50 , and Yaw is controlled by interferometer  56 . Second wafer stage  44  X is stopped at the end of the exposure sequence if loaded with a wafer and its position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0057]    In step  206 , further illustrated by FIG. 14( d ), first wafer stage  42  has ended enhanced global alignment and is switching in the Y direction. Its X position is controlled by interferometer  56 , its Y position by  50 , and Yaw is controlled by interferometer  56  changing to interferometer  52  and  50 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0058]    In step  208 , further illustrated by FIG. 14( e ), first wafer stage  42  is switching in the Y direction and its X position is controlled by interferometer  56   a,  its Y position by  50 , and Yaw is controlled by interferometer  52  and  50 . Second wafer stage  44  is waiting for interferometer  56   c  to become available to control its X position and its Y position is controlled by interferometer  48 , and its Yaw by interferometer  48 .  
         [0059]    In step  210 , further illustrated by FIG. 14( f ), first wafer stage  42  is waiting for interferometer  46  to become available to control its X position. Its Y position is controlled by interferometer  50 , and Yaw is controlled by interferometer  52 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  56   c,  its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0060]    In step  212 , further illustrated by FIG. 14( g ), first wafer stage  42  is switching in the Y direction and its X position is controlled by interferometer  46 , its Y position by  50 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  56   c  changing to interferometer  56 , its Y position by interferometer  48 , and its Yaw by interferometer  48  changing to interferometer  56 .  
         [0061]    Now referring to FIG. 13( b ), in step  214 , further illustrated by FIG. 14( h ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by interferometer  52  changing to interferometer  48   c,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is waiting for stage  42  to finish moving. Its X position is controlled by interferometer  56 , its Y position by interferometer  48  changing to interferometer  48   a,  and its Yaw by interferometer  56 .  
         [0062]    In step  216 , further illustrated by FIG. 14( i ), first wafer stage  42  is now stopped and waiting for stage  44  to finish moving. Its X position is controlled by interferometer  46 , its Y position by interferometer  48   c,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the X direction and its X position is controlled by interferometer  56 , its Y position by interferometer  48   a  changing to interferometer  52 , and its Yaw by interferometer  56 .  
         [0063]    In step  218 , further illustrated by FIG. 14( j ), first wafer stage  42  is starting the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48   c  changing to interferometer  48 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the X direction and its X position is controlled by interferometer  56 , its Y position by interferometer  52  changing to interferometer  54 , and its Yaw by interferometer  56 .  
         [0064]    In step  220 , further illustrated by FIG. 14( k ) (also known as FIG. 3), first wafer stage  42  is starting the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is loading or unloading and its X position is controlled by interferometer  56 , its Y position by interferometer  54 , and its Yaw by interferometer  56 .  
         [0065]    In step  222 , further illustrated by FIG. 14( l ), first wafer stage  42  is in the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  46 . Second wafer stage  44  is still stopped and loading or unloading. Its X position is controlled by interferometer  56 , its Y position by interferometer  54  changing to interferometer  52 , and Yaw is controlled by interferometer  56 .  
         [0066]    In step  224 , further illustrated by FIG. 14( m ), first wafer stage  42  is in the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is in enhanced global alignment and its X position is controlled by interferometer  56 , its Y position by interferometer  52 , and its Yaw by interferometer  56 .  
         [0067]    In step  226 , further illustrated by FIG. 14( n ), first wafer stage  42  is ending the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is in enhanced global alignment and its X position is controlled by interferometer  56 , its Y position by interferometer  52 , and its Yaw by interferometer  56 .  
         [0068]    In step  228 , further illustrated by FIG. 14( o ), first wafer stage  42  is stopped in the safety position waiting for stage  44  to finish EGA and its X position is controlled by interferometer  46 , its Y position by interferometer  48   a,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is ending enhanced global alignment and its X position is controlled by interferometer  56 , its Y position by interferometer  52 , and its Yaw by interferometer  56 .  
         [0069]    Now referring to FIG. 13( c ), in step  230 , further illustrated by FIG. 14( p ), first wafer stage  42  is stopped in the safety position and its X position is controlled by interferometer  46 , its Y position by interferometer  48   b,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the X direction and its X position is controlled by interferometer  56 , its Y position by interferometer  52 , and its Yaw by interferometer  56 .  
         [0070]    In step  232 , further illustrated by FIG. 14( q ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by interferometer  48   b  changing to  50 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is waiting for interferometer  48   a  to become available and its X position is controlled by interferometer  56 , its Y position by interferometer  52 , and its Yaw by interferometer  56 .  
         [0071]    In step  234 , further illustrated by FIG. 14( r ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by  50 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is waiting in the switch X/wait position and its X position is controlled by interferometer  56 , its Y position by interferometer  52  changing to interferometer  48   a,  and its Yaw by interferometer  56 .  
         [0072]    In step  236 , further illustrated by FIG. 14( s ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by interferometer  50  changing to interferometer  52 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is in the waiting in the X direction and its X position is controlled by interferometer  56 , its Y position by interferometer  48   b  changing to interferometer  48 , and its Yaw by interferometer  56  changing to interferometer  48 .  
         [0073]    In step  238 , further illustrated by FIG. 14( t ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by interferometer  52  changing to interferometer  54 , and Yaw is controlled by interferometer  46  changing to interferometer  54 . Second wafer stage  44  is waiting and its X position is controlled by interferometer  56 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0074]    In step  240 , further illustrated by FIG. 14( u ), first wafer stage  42  is switching in the Y direction and its X position is controlled by interferometer  46  changing to interferometer  56   a,  its Y position by interferometer  54 , and Yaw is controlled by interferometer  54 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  56  changing to interferometer  56   c,  its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0075]    Now referring to FIG. 13( d ), in step  242 , further illustrated by FIG. 14( v ), first wafer stage  42  is waiting in the switch Y/wait position before moving to the loading position and its X position is controlled by interferometer  56   a,  its Y position by interferometer  54 , and Yaw is controlled by interferometer  54 . Second wafer stage  44  is in position to begin the exposure sequence switching in the Y direction and its X position is controlled by interferometers  56   c  changing to interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0076]    Step  243  is the decision block for whether to continue to process more wafers. Should the answer be “yes” the method returns to step  200  and continues in sequence, otherwise the method continues to step  244  which is also illustrated by FIG. 14( a ), except the actions taken account for the fact that wafer stage  42  is now empty.  
         [0077]    In step  244 , further illustrated by FIG. 14( a ), first wafer stage  42  is unloaded but not reloaded. Its X position is controlled by interferometer  56   a  changing to interferometer  56 , its Y position by interferometer  54 , and Yaw is controlled by interferometer  54 . Second wafer stage  44  is stopped to begin the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0078]    In step  246 , further illustrated by FIG. 14( b ), first wafer stage  42  X position is controlled by interferometer  56 , its Y position by interferometer  54  changing to interferometer  52 , and Yaw is controlled by interferometer  54  changing to interferometer  56 . At times stage  42  is halted to avoid disturbing the movement of stage  44 . Second wafer stage  44  is in the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0079]    In step  248 , further illustrated by FIG. 14( c ), first wafer stage  42  X position is controlled by interferometer  56 , its Y position by interferometer  52  changing to  50 , and Yaw is controlled by interferometer  56 . Second wafer stage  44  is stopped at the end of the exposure sequence and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0080]    In step  250 , further illustrated by FIG. 14( d ), first wafer stage  42  is switching in the direction. Its X position is controlled by interferometer  56 , its Y position by  50 , and Yaw is controlled by interferometer  56  changing to interferometer  52  and  50 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0081]    In step  252 , further illustrated by FIG. 14( e ), first wafer stage  42  is switching in the direction and its X position is controlled by interferometer  56   a,  its Y position by  50 , and Yaw is controlled by interferometer  52  and  50 . Second wafer stage  44  is waiting for interferometer  56   c  to become available to control its X position and its Y position is controlled by interferometer  48 , and its Yaw by interferometer  48 .  
         [0082]    In step  254 , further illustrated by FIG. 14( f ), first wafer stage  42  is waiting for interferometer  46  to become available to control its X position. Its Y position is controlled by  50 , and Yaw is controlled by interferometer  52 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  56   c,  its Y position by interferometer  48 , and its Yaw by interferometer  48 .  
         [0083]    In step  256 , further illustrated by FIG. 14( g ), first wafer stage  42  is switching in the Y direction and its X position is controlled by interferometer  46 , its Y position by  50 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the Y direction and its X position is controlled by interferometer  56   c  changing to interferometer  56 , its Y position by interferometer  48 , and its Yaw by interferometer  48  changing to interferometer  56 .  
         [0084]    In step  258 , further illustrated by FIG. 14( h ), first wafer stage  42  is switching in the X direction and its X position is controlled by interferometer  46 , its Y position by interferometer  52  changing to interferometer  48   c,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is waiting for stage  42  to finish moving. Its X position is controlled by interferometer  56 , its Y position by interferometer  48  changing to interferometer  48   b,  and its Yaw by interferometer  56 .  
         [0085]    In step  260 , further illustrated by FIG. 14( i ), first wafer stage  42  is now stopped and waiting for stage  44  to finish moving. Its X position is controlled by interferometer  46 , its Y position by interferometer  48   c,  and Yaw is controlled by interferometer  46 . Second wafer stage  44  is switching in the X direction and its X position is controlled by interferometer  56 , its Y position by interferometer  48   a  changing to interferometer  52 , and its Yaw by interferometer  56 .  
         [0086]    In step  262 , further illustrated by FIG. 14( j ), first wafer stage  42  is not beginning the exposure sequence since it was not loaded. Its X position is controlled by interferometer  46 , its Y position by interferometer  48   c  changing to interferometer  48 , and Yaw is controlled by interferometer  46 . Second wafer stage  44  is unloading X position is controlled by interferometer  46 , its Y position by interferometer  48 , and its Yaw by interferometer  48 . The unloading of second wafer stage  44  ends the method.  
         [0087]    The use of the exposure apparatus and dual wafer stage assembly described herein is not limited to a lithography system for semiconductor manufacturing. This arrangement may be employed advantageously in other assemblies wherein objects other than wafers must be precisely positioned while they are processed simultaneously. The apparatus, for example, can be used as an LCD lithography system that exposes a liquid crystal display device pattern onto a rectangular glass plate or a lithography system for manufacturing a thin film magnetic head. Furthermore, the exposure apparatus and dual wafer stage assembly can also be applied to a proximity lithography system that exposes a reticle pattern by closely locating a reticle and a substrate without the use of a lens assembly. Additionally, an exposure apparatus utilizing a dual wafer stage assembly according to the invention can be used in other devices, including other semiconductor processing equipment, machine tools, metal cutting machines, and inspection machines.  
         [0088]    It is to be understood that while illustrative embodiments of the invention have been shown and described herein, various changes and adaptions in accordance with the teachings of the invention will be apparent to those of skill in the art. Such changes and adaptions nevertheless are included within the spirit and scope of the invention as defined in the following claims.