Abstract:
The method for positioning two stages during semiconductor wafer processing facilitates the use of two stages to improve system throughput by decreasing the rest-time of certain system components. While a typical single-stage apparatus requires that each step in the process be performed serially, this invention allows an amount of parallel processing with each stage at different steps of the process, and thus improves system throughput.

Description:
FIELD OF THE INVENTION 
     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 a method for positioning stages during the processing of the substrates affixed to those stages. 
     BACKGROUND OF THE INVENTION 
     In lithography processes the image from a reticle is transferred to a substrate, typically a wafer. System throughput is dependent upon the speeds of many separate steps that are performed in series. Throughput is therefore dependent on the duration of each step, which 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 step. This complexity creates time requirements of its own. For example, when 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 given time to settle to reduce the vibration that can cause distortion of the transferred pattern. Lithography processes typically occur in a clean room/vacuum environment; this is a source of further complexity and also an indication of the sensitivity of the processes. 
     A typical exposure apparatus 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 . 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     Processing a single wafer requires a significant time expenditure because of the complexity and sensitivity of the exposure apparatus and the steps involved. When a single wafer is undergoing one step, the apparatus for the others are normally idle. For example, when a single wafer is being exposed the apparatus for determining the alignment of the wafer relative to the wafer stage is typically 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. A way to decrease idle time is to use two stages and position them so that each stage can undergo different steps of the process at the same time. The present invention is a method that 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. Encoders being rather less accurate than interferometers; the method preferably relies on them during the less position-sensitive steps of the process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a two stage method where stage position may be determined using interferometers and one or more encoders. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions and encoders where interferometers are not feasible. 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. Should such an eclipse occur, and another interferometer not be available for determining the eclipsed stage&#39;s position, an encoder is configured to supply position. The apparatus is designed so that encoders are required during the less position-sensitive steps of the process, such as when switching from one step to another. 
     A method incorporating the invention comprises: sizing the stages based on wafer and exposure apparatus parameters; dispersing interferometers and encoders about the base at appropriate positions based on the stage sizes; moving the stages as desired while using the exposure apparatus; and determining the positions of both stages at all times during the process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a side view of a lithography exposure apparatus of the prior art; 
     FIG. 2 is a different side view of a lithography exposure apparatus of the prior art; 
     FIG. 3 is a plan view of a two wafer stage assembly employed in a preferred embodiment of the present invention; 
     FIGS.  4 ( a )-( c ) illustrate the interferometer axes as seen from the wafer stages; 
     FIGS.  5 ( a )-( e ) is a flowchart illustrating a method according to the present invention; and 
     FIGS.  6 ( a )-( t ) illustrate steps  1 - 20  of the method shown in FIGS.  5 ( a )-( c ). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 3, in which dual wafer stage assembly  40  is illustrated from above according to a preferred embodiment of the invention. Two wafer stage assembly  40  provides the capability for the system to rely on interferometers  46 ,  48 ,  50 ,  52 ,  54 ,  56  and encoders  55  and  57  to determine the position of wafer stages  42  and  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 and encoders mentioned above. Each interferometer cooperates with a reflective surface (mirror)  58 ,  62 ,  64 ,  66 ,  68 , and  70  mounted on stages  42  and  44  so that they reflect back to the intended interferometers. Elements of base  60  necessary for the support, positioning, and movement of wafer stages  42  and  44  are illustrated as support elements  30  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  51  and  53 , depicted schematically, or other known actuators and additional static support elements of the prior art (see FIGS.  1  &amp;  2 ). 
     As shown in FIG. 3, projection lens area  72  refers to the point where the image from reticle  12  (of FIG. 1) is focused by the projection apparatus onto stage assembly  40 . Area  74  identifies an area that an operator views from above (typically with a microscope) to align wafer  14  with, eventually, reticle  12 . Area  74  is referred to as the field image alignment (FIA) area  74 . 
     Overall apparatus alignment is a multi-step process designed to obtain an accurate alignment of the image of reticle  12  on wafer  14 . Alignment is performed by determining the position of wafer  14  relative to wafer stage  42  or  44  in FIA area  74  using a microscope, for example, to determine the positions of selected marks or elements on wafer  14  relative to fiducial mark  43  on stage  42  or  44 . These elements are preferably dispersed about the wafer surface so as to provide data about the entire surface, or “global” data. These elements could be marks on the wafer dedicated for this purpose similar to the fiduciary mark in the wafer stage, or the elements could be images formed by previous exposures, or otherwise formed. Only some of these marks are measured, with that data used in calculations to estimate all chip positions. With the additional “global” data, the chip position relative to the fiducial mark is determined more precisely, or “enhanced”. Thus, with the additional calculations for estimating each chip position, this procedure is known as “enhanced global alignment” (EGA). Global alignment must be performed after each time a new wafer is loaded. 
     To determine the position of the image of reticle  12  with respect to apparatus  10 , called a “reset,” stage  42  or  44  is positioned with fiducial mark  43  at projection lens area  72 . Then interferometer data is taken of stage position and a reticle alignment microscope (not shown) is used to determine the position of fiducial mark  43  with respect to the image of reticle  12 . With this image to fiducial mark data, and fiducial mark to apparatus data, the position of the image of reticle  12  with respect to apparatus  10  is determined. A similar procedure is conducted to determine the position of FIA area  74  using fiducial mark  43 . These procedures are called “resets” with one for FIA area  74  and another for the reticle and projection lens  72 . With both the position of the image of reticle  12 , and the position of FIA area  74  determined with respect to fiducial mark  43 , and with the position of wafer  14  determined with respect to fiducial mark  43 , the position of the image of reticle  12  can be accurately positioned on wafer  14 . 
     The alignment is a multistep process, using both a reset for FIA area  74  and a reset step for projection lens area  72 , because more traditional methods of relating reticle image to wafer position can interfere with processing in this twin stage method. With two stages, it is preferable to process wafer  14  on one stage while performing an EGA reset and EGA on the second stage. Traditional alignment methods, as is known to one of skill in the art, direct projection lens area  72  and FIA area  74  to fiducial marks on the same stage during alignment. 
     A reset must be performed after a reticle is changed. So long as reticle  12  does not change and apparatus  10  is stable enough, a reset need not be performed at every wafer loading unless particularly precise exposures are required. Resets are, however, typically performed after several wafer exposures. One of skill in the art will recognize that many methods may be employed to align the reticle image with the field image alignment microscope. 
     Interferometer  50  is positioned at approximately the center Y-position of projection lens area  72  and is mainly used for determining the stage X-position, yaw, and roll during exposure. Interferometer  52  is positioned at approximately the center Y-position of FIA area  74  microscope and is mainly used for determining the X-position, yaw, and roll during EGA. Interferometer  48  is positioned at approximately the X-position of projection lens area  72  and is mainly used for determining stage Y-position and pitch during exposure. Similarly, interferometer  56  is positioned at approximately the center X-position of FIA area  74  and is mainly used for determining stage Y-position and pitch during EGA. Remaining interferometers  46  and  54  are mainly used during the switching or waiting steps because, since they are offset from the lens assembly and FIA focal points, any error in the data they produce is compounded by the offset. 
     Mirrors  58 ,  62 ,  64 ,  66 ,  68 , and  70  are known as “moving mirrors” because they move with wafer stages  42  and  44 . As is understood by persons skilled in the art, other non-moving mirrors known as “reference mirrors” are located within the interferometers  46 ,  48 ,  50 ,  52 ,  54  and  56 , but in order to clarify the schematic representation of the invention standard reference beams, reference mirrors, and sensors for each interferometer are not shown. 
     Linear motors  51  and  53  position wafer stages  42  and  44  in the X-direction respectively. Encoders  55  and  57  are located beneath linear motors  51  and  53  and are illustrated as gratings. Encoders  55  and  57  provide X-position data when stage movement causes one stage to eclipse the other from interferometers  50  or  52 . In the preferred embodiment shown this occurs when the stages are switching in the Y direction. At that time X position is not as critical and the less precise measurements given by encoders are adequate. 
     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 interferometer  46   b  is most often used to provide yaw data for stage  42 , it also provides Y-position data at the end of the exposure sequence for stage  42 . In a preferred embodiment, the present invention employs two and three axes interferometers that are incident on wafer stages  42  and  44  from three sides as well as encoders  55  and  57  to determine wafer stage position at all times. As shown in FIG.  3  and FIGS.  4 ( a )-( c ), the axes of the interferometers are directed in parallel towards the moveable mirrors on stages  42  and  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. 
     Continuing with FIG. 3, and viewing the interferometers in more detail in FIG. 4, interferometers  50  and  52  each have 3 axes  50   a ,  50   b ,  50   c ,  52   a ,  52   b , and  52   c  and is used to determining X position, yaw, and roll. Interferometers  46 ,  48 ,  54 ,  56  each have 2 axes  46   a ,  46   b ,  48   a ,  48   b ,  54   a ,  54   b ,  56   a , and  56   b  and are used to determine Y position and yaw. Only interferometers  48  and  56  are used to determine pitch. 
     In a preferred embodiment shown in FIG. 3 interferometers  48 ,  50 ,  52 , and  56  are directed so their axes intersect at approximately the center of the most sensitive areas. Interferometers  48  and  50  intersect at projection lens area  72  where the projection lens focuses the image from the reticle. Interferometers  52  and  56  similarly intersect at field image alignment point  74 . Remaining interferometers  46  and  54  and encoders are relied upon during the less critical movements; interferometers  46  and  54  because they are offset from the positions of interest and the encoders because they are less precise. 
     Where the interferometers, encoders, stages, and base are properly configured and dimensioned, the present invention provides a method that increases system throughput by reducing the idle time of the individual system components. Idle time is reduced by providing a second substrate upon a second stage without interfering with the steps being performed on the first stage. 
     As illustrated in FIG. 3, the dimensions of the first and second stages are the same. One of skill in the art, however, 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 involved in stage design and the changes possible with the configurations of the stages, mirrors, and other elements. 
     FIGS.  5 ( a )-( c ) is a flowchart of a preferred method for utilizing the two wafer stage method of the present invention. The flowchart illustrates interferometer and encoder use during the movement of the stages. Individual steps in the flowchart are discussed in more detail in the referenced sub-illustrations of FIGS.  6 ( a )-( t ). 
     Referring again to FIG. 5, in step  200 , further illustrated by FIG.  6 ( a ), first wafer stage  42  is loaded and its X position is monitored by interferometer  52 , Y position by interferometer  46   b  changing to  56 , yaw is monitored by interferometers  46   a  and  46   b  changing to  52 , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is beginning the exposure sequence if loaded with a wafer and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  202 , further illustrated by FIG.  6 ( b ), first wafer stage  42  is reset and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  continues the exposure sequence if loaded with a wafer and its X position is monitored by interferometer  50 , Y position by interferometer  48 , pitch by interferometer  48 , yaw by interferometer  50 , and roll by interferometer  50 . 
     It should be noted that resetting is necessary whenever the reticle has been moved or changed, but thereafter the frequency of resets is determined by the stability of the apparatus. The method illustrated by FIGS.  5 ( a )-( c ) and  6 ( a )-( t ) contains, practically speaking, the most resets possible. Some of these would be deleted when using a more positionally stable apparatus. 
     In step  204 , further illustrated by FIG.  6 ( c ), first wafer stage  42  continues enhanced global alignment and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  continues the exposure sequence if loaded with a wafer and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  206 , further illustrated by FIG.  6 ( d ), first wafer stage  42  ends enhanced global alignment and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  continues the exposure sequence if loaded with a wafer and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  208 , further illustrated by FIG.  6 ( e ), first wafer stage  42  is waiting in position and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  ends the exposure sequence if loaded with a wafer and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  210 , further illustrated by FIG.  6 ( f ), first wafer stage  42  starts to switch in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  56  changing to  48 , yaw by interferometer  52  changing to interferometers  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  starts to switch in the Y direction and its X position is monitored by interferometer  50 , Y position by interferometer  48  changing to interferometers  54   a  and  54   b , yaw by interferometer  50 , pitch is not monitored, and roll is not monitored. 
     In step  212 , further illustrated by FIG.  6 ( g ), first wafer stage  42  is switching in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  56  changing to interferometer  48 , yaw by interferometer  52  changing to interferometers  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  starts to switch in the Y direction and its X position is monitored by interferometer  50 , Y position by interferometer  54   b , yaw by interferometer  50  changing to interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  214 , further illustrated by FIG.  6 ( h ), first wafer stage  42  ends switching in the X direction and its X position is monitored by encoder  55 , Y position by interferometer  48 , yaw by interferometer  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  216 , further illustrated by FIG.  6 ( i ), first wafer stage  42  is X position is reset and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometers  46   a  and  46   b  changing to interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  218 , further illustrated by FIG.  6 ( j ), first wafer stage  42  is waiting in position and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  220 , further illustrated by FIG.  6 ( k ), first wafer stage  42  starts the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  ends switching in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b  changing to interferometer  52 , pitch is not monitored, and roll is not monitored. 
     In step  222 , further illustrated by FIG.  6 ( l ), first wafer stage  42  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is loaded and/or unloaded and its X position is monitored by interferometer  52 , Y position by interferometer  54   b , yaw by interferometer  52 , pitch is not monitored, and roll is not monitored. 
     In step  224 , further illustrated by FIG.  6 ( m ), first wafer stage  42  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is reset and its X position is monitored by interferometer  52 , Y position by interferometer  54   b  changing to  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . 
     In step  226 , further illustrated by FIG.  6 ( n ), first wafer stage  42  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  continues enhanced global alignment and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . 
     In step  228 , further illustrated by FIG.  6 ( o ), first wafer stage  42  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  ends enhanced global alignment and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . 
     Now referring to FIG.  5 ( b ), in step  230 , further illustrated by FIG.  6 ( p ), first wafer stage  42  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  starts to switch in the X direction and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52  changing to interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  232 , further illustrated by FIG.  6 ( q ), first wafer stage  42  ends the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48  changing to interferometer  46   b , yaw by interferometer  50  changing to interferometers  46   a  and  46   b , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is waiting in position and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  234 , further illustrated by FIG.  6 ( r ), first wafer stage  42  starts to switch in the X direction and its X position is monitored by encoder  55 , Y position by interferometer  46   b , yaw by interferometer  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is waiting in position and its X position is monitored by interferometer  52 , Y position by interferometer  56  changing to interferometer  48 , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  236 , further illustrated by FIG.  6 ( s ), first wafer stage  42  is switching in the X direction and its X position is monitored by encoder  55 , Y position by interferometer  46   b , yaw by interferometer  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  ends switching in the X direction and its X position is monitored by encoder  57 , Y position by interferometer  48 , yaw by interferometer  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  238 , further illustrated by FIG.  6 ( t ), first wafer stage  42  is switching in the X direction and its X position is monitored by encoder  55 , Y position by interferometer  46   b , yaw by interferometer  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is reset and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  54   a  and  54   b  changing to interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     Step  240  is the decision block for whether to continue to process more wafers. Should the answer be “yes” the method returns to step  200  of FIG.  5 ( a ) and continues in sequence, otherwise the method continues to step  244  which is also illustrated by FIG.  6 ( a ), except the actions taken differ. 
     In step  242 , further illustrated by FIG.  6 ( a ), first wafer stage  42  is unloaded and its X position is monitored by interferometer  52 , Y position by interferometer  46   b  changing to  56 , yaw by interferometer  46   a  and  46   b  changing to  52 , pitch is not monitored, and roll is not monitored. Second wafer stage  44  begins the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  244 , further illustrated by FIG.  6 ( b ), first wafer stage  42  is in the reset area and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , pitch by interferometer  48 , yaw by interferometer  50 , and roll by interferometer  50 . 
     In step  246 , further illustrated by FIG.  6 ( c ), first wafer stage  42  is in the enhanced global alignment area and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  continues the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     Now referring to FIG.  5 ( c ), in step  248 , further illustrated by FIG.  6 ( e ), first wafer stage  42  is waiting in position and its X position is monitored by interferometer  52 , Y position by interferometer  56 , yaw by interferometer  52 , pitch by interferometer  56 , and roll by interferometer  52 . Second wafer stage  44  ends the exposure sequence and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . 
     In step  250 , further illustrated by FIG.  6 ( f ), first wafer stage  42  starts to switch in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  56  changing to  48 , yaw by interferometer  52  changing to interferometers  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  starts to switch in the Y direction and its X position is monitored by interferometer  50 , Y position by interferometer  48  changing to interferometers  54   a  and  54   b , yaw by interferometer  50 , pitch is not monitored, and roll is not monitored. 
     In step  252 , further illustrated by FIG.  6 ( g ), first wafer stage  42  is switching in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  56  changing to interferometer  48 , yaw by interferometer  52  changing to interferometers  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is switching in the Y direction and its X position is monitored by interferometer  50 , Y position by interferometer  54   b , yaw by interferometer  50  changing to interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  254 , further illustrated by FIG.  6 ( h ), first wafer stage  42  ends switching in the X direction and its X position is monitored by encoder  55 , Y position by interferometer  48 , yaw by interferometer  46   a  and  46   b , pitch is not monitored, and roll is not monitored. Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  256 , further illustrated by FIG.  6 ( i ), first wafer stage  42  is in the reset area and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometers  46   a  and  46   b  changing to interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  258 , further illustrated by FIG.  6 ( j ), first wafer stage  42  is waiting in position and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is switching in the Y direction and its X position is monitored by encoder  57 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b , pitch is not monitored, and roll is not monitored. 
     In step  260 , further illustrated by FIG.  6 ( k ), first wafer stage  42  is in the exposure area and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  ends switching in the Y direction and its X position is monitored by interferometer  52 , Y position by interferometer  54   b , yaw by interferometers  54   a  and  54   b  changing to interferometer  52 , pitch is not monitored, and roll is not monitored. 
     In step  262 , further illustrated by FIG.  6 ( l ), first wafer stage  42  is in the exposure area and its X position is monitored by interferometer  50 , Y position by interferometer  48 , yaw by interferometer  50 , pitch by interferometer  48 , and roll by interferometer  50 . Second wafer stage  44  is unloading and its X position is monitored by interferometer  52 , Y position by interferometer  54   b , yaw by interferometer  52 , pitch is not monitored, and roll is not monitored. The unloading of second wafer stage  44  ends the method. 
     The use of the two stage method 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 two stage method 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, a two stage method according to the invention can be used in other devices, including other semiconductor processing equipment, machine tools, metal cutting machines, and inspection machines. 
     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.