Patent Publication Number: US-2007115451-A1

Title: Lithographic System with Separated Isolation Structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application is a divisional of related co-pending U.S. patent application Ser. No. 10/919,771, filed Aug. 17, 2004, which is incorporated herein by reference in its entirety. The present application is also related to co-pending U.S. patent application Ser. No. 09/721,733 and to co-pending U.S. patent application Ser. No. 09/721,734, which are each incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a lithographic device which uses an isolation system such as an active vibration isolation system to vibrationally isolate a reticle stage from a lens arrangement.  
      2. Description of the Related Art  
      For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument is adversely affected, as for example by disturbance forces or vibrations, products formed using the precision instrument may be improperly formed and, hence, defective. For instance, a lithography device such as a photolithography machine which is subjected to vibrations may cause an image projected by the photolithography machine to move, and, as a result, be aligned incorrectly on a projection surface such as a semiconductor wafer surface.  
      Scanning stages such as wafer scanning stages and reticle scanning stages are often used in semiconductor fabrication processes, and may be included in various photolithography and exposure apparatuses. Wafer scanning stages are generally used to position a semiconductor wafer such that portions of the wafer may be exposed as appropriate for masking or etching. Reticle scanning stages are generally used to accurately position a reticle or reticles for exposure over the semiconductor wafer. Patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. When a reticle is positioned over a wafer as desired, a beam of light or a relatively broad beam of electrons may be collimated through a reduction lens, and provided to the reticle on which a thin metal pattern is placed. Portions of a light beam, for example, may be absorbed by the reticle while other portions pass through the reticle and are focused onto the wafer.  
      Many photolithographic systems use an active vibration isolation system (AVIS) to reduce the amount of vibrations which may be transmitted through a lens frame to a lens assembly within the photolithographic system.  FIG. 1   a  is a diagrammatic representation of a photolithographic system which includes an AVIS. A system  100  includes a wafer stage  104  which is supported on a wafer stage base  108  and supports a wafer table  112  which holds a wafer (not shown). A counter mass  116  is also supported on wafer stage base  108 . Wafer stage base  108  is positioned substantially atop a frame caster  120  onto which a trim motor  124 , which cooperates with counter mass  116  to substantially compensate for reaction forces caused by the scanning of wafer stage  104  and wafer table  112 , and for some external vibratory motion, is mounted. In some instances, wafer stage base  108  may be mounted on an AVIS, e.g., AVIS  180  as shown in  FIG. 1   b , in order to reduce the transmissibility of wafer stage vibrations to frame caster  120  and, hence, to lens frame  132 .  
      Returning to  FIG. 1   a , a lens assembly  128  is supported on a lens frame  132  which, as shown, is isolated from frame caster  120  through AVIS  136  to reduce vibrations that are transmitted through frame caster  120  to lens assembly  128 . Lens frame  132  also supports a reticle stage base  140  on which a reticle fine stage  144  and a reticle coarse stage  148  may move to position a reticle (not shown) positioned on reticle fine stage  144 . A trim motor  156 , which cooperates with a counter mass  152  to compensate for reaction forces created by scanning reticle fine stage  144  and reticle coarse stage  148 , and to reduce the transmission of vibrations to reticle fine stage  144  and reticle coarse stage  148 , is supported on lens frame  132 . Various sensors  160 , e.g., interferometers which measure lateral motion of wafer table  112  and interferometers which measure lateral motion of reticle fine stage  144 , are also mounted on lens frame  132 .  
      Often, vibrations associated with the movement of a reticle (not shown) positioned on reticle file stage  144  may be transmitted through reticle stage base  140  to lens frame  132 . Such vibrations may adversely affect lens assembly  128  by causing lens assembly  128  to vibrate or otherwise move, thereby causing an image projected through lens  128  onto a wafer (not shown) on wafer table  112  to be inaccurately projected. In other words, any images formed on a surface of a wafer (not shown) on wafer table  112  may not be accurately formed, i.e., the images may not be precise. As a result, the integrity of the wafer (not shown) positioned on wafer table  112  may be compromised.  
      Therefore, what is needed is a method and an apparatus for reducing vibrations which are transmitted through a lens frame to a lens assembly. More specifically, what is desired is a system which effectively isolates a reticle stage assembly from a lens assembly in a photolithographic system such that vibrations associated with the reticle stage assembly may be substantially prevented from adversely affecting the operation of the lens assembly and, hence, the processing of a wafer positioned beneath the lens assembly.  
      BRIEF SUMMARY OF THE INVENTION  
      The present invention relates to separated isolation structures which enable a reticle stage arrangement to be vibrationally isolated from a lens assembly. According to one aspect of the present invention, an apparatus includes a reticle stage assembly, a lens assembly, and an isolator assembly. The isolator assembly is arranged to substantially prevent vibrations from being transmitted from the reticle stage assembly to the lens assembly. In one embodiment, the apparatus also includes a frame structure that supports the lens assembly and the reticle stage assembly. In such an embodiment, the isolator assembly is mounted on the frame structure.  
      An isolator which substantially prevents vibrations from being transmitted through a lens frame to a lens assembly allows the accuracy with which images may be formed on the surface of a wafer to be improved. When a lens of a lens assembly is substantially prevented from vibrating or oscillating, the position of the lens relative to a reticle and a wafer may be more accurately determined and, as a result, the reticle and the wafer may be positioned more accurately relative to the lens. Isolating a reticle stage structure from a lens assembly typically reduces the transmission of vibrations which are generated when a reticle stage moves to a lens assembly. As such, an overall imaging process which uses the lens assembly is less likely to be compromised due to a vibrating lens assembly.  
      According to another aspect of the present invention, a lithographic apparatus includes a wafer stage assembly, a reticle stage assembly, a lens assembly, and a first isolation system. The wafer stage assembly includes a wafer table that supports a wafer and serves to scan the wafer. The reticle stage includes a reticle table that supports a reticle and serves to scan the reticle. The lens assembly, which is disposed substantially between the wafer stage assembly and the reticle stage assembly, is isolated from the reticle stage assembly to substantially prevent vibrations associated with the reticle stage assembly from being transmitted to the lens assembly.  
      In one embodiment, the first isolation system is further arranged to substantially compensate for a shift in a center of gravity associated with the reticle stage assembly. In another embodiment, the first isolation system is one of an active vibration isolation system and a piezoelectric actuator.  
      These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
       FIG. 1   a  is a diagrammatic representation of a photolithographic system which includes one active vibration isolation system (AVIS).  
       FIG. 1   b  is a diagrammatic representation of a photolithographic system which includes an AVIS which separates a lens frame from a frame caster and an AVIS which separates a wafer stage base from the frame caster.  
       FIG. 2   a  is a diagrammatic representation of a first lithographic system which includes an AVIS that substantially isolates a reticle stage structure from a lens assembly in accordance with an embodiment of the present invention.  
       FIG. 2   b  is a diagrammatic representation of a second lithographic system which includes an AVIS that substantially isolates a reticle stage structure from a lens assembly will be described in accordance with an embodiment of the present invention.  
       FIG. 3  is a diagrammatic representation of a lithographic system which includes a piezoelectric actuator assembly that substantially prevents vibrations from being transmitted between a reticle stage and a lens assembly in accordance with an embodiment of the present invention.  
       FIG. 4  is a control block diagram which illustrates the control logic associated with enabling the movement of a reticle to substantially track the movement of a wafer in accordance with an embodiment of the present invention.  
       FIG. 5  is a diagrammatic representation of a lens assembly and an interferometer system in accordance with an embodiment of the present invention.  
       FIG. 6  is a diagrammatic representation of a photolithography apparatus in accordance with an embodiment of the present invention.  
       FIG. 7  is a process flow diagram which illustrates the steps associated with fabricating a semiconductor device in accordance with an embodiment of the present invention.  
       FIG. 8  is a process flow diagram which illustrates the steps associated with processing a wafer, i.e., step  1304  of  FIG. 7 , in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Preventing a lens assembly of a photolithography apparatus from being subjected to significant vibrations is crucial to ensure the accuracy with which an image may be transmitted through the lens assembly to the surface of a wafer during a semiconductor fabrication process. Such vibrations may stem from the movement of a wafer stage, or from the movement of a reticle stage, for example. In many photolithographic systems, a reticle stage assembly and a lens assembly may be supported by a common frame, e.g., an overall lens frame. As a result, any vibrations associated with the reticle stage assembly may be transmitted through the lens frame to the lens assembly.  
      By preventing vibrations from being transmitted through a lens frame to a lens assembly, the accuracy with which images may be formed on the surface of a wafer through the use of the lens assembly may be improved. Isolating a reticle stage structure from a lens assembly typically reduces the transmission of vibrations which are generated when a reticle stage moves to a lens assembly. In one embodiment, a reticle stage structure may be isolated from a lens frame which supports a lens assembly through the use of an active vibration isolation system (AVIS). Alternatively, a reticle stage structure may be isolated from a lens frame through the use of a system which includes piezoelectric actuators.  
      With reference to  FIG. 2   a , one lithographic system which includes an AVIS that substantially isolates a reticle stage structure from a lens assembly will be described in accordance with an embodiment of the present invention. A lithographic system  200  includes a wafer stage  204  which is supported on a wafer stage base  208  and supports a wafer table  212  which holds a wafer (not shown). Typically, a wafer (not shown) may be held on wafer table  212  by a wafer chuck (not shown). In one embodiment, wafer stage  204  may be a coarse stage which enables a wafer (not shown) supported on wafer table  212  to undergo coarse movements and wafer table  212  may be a fine stage which enables the wafer to undergo fine movements. A counter mass  216  is positioned on wafer stage base  208  and is arranged to absorb some reaction forces generated when wafer stage  204  or wafer table  212  moves. A trim motor  224 , which is mounted to a frame caster  220 , may prevent external vibrations or oscillations from being transmitted from frame caster  220 , or a grounding surface, to counter mass  216  such that the movement of wafer stage  204  or wafer table  212  is not significantly affected by external vibrations.  
      In the embodiment as shown, wafer stage base  208  is isolated from a frame caster  220 , e.g., a grounded surface, through the use of an AVIS  280  positioned substantially atop frame caster  280 . AVIS  280  serves to prevent a significant amount of wafer stage vibrations from adversely affecting a lens assembly  228 , and to prevent external vibrations from affecting wafer table  212 . AVIS  280  may generally include either a “passive isolator” such as an air mount or an “active isolator” such as a voice coil motor. It should be appreciated that AVIS  280  is optional and is not included in system  200  in some embodiments. By way of example, for an embodiment in which counter mass  216  is effective in balancing reaction forces associated with wafer stage  204  such that there is substantially no center of gravity shift associated with wafer stage  204 , then AVIS  280  may be eliminated.  
      Wafer stage  204  and wafer table  212  are each typically arranged to move in multiple degrees of freedom, e.g., between three to six degrees of freedom, such that a wafer (not shown) may be positioned relative to a lens assembly  228 , e.g., a projection optical system. As will be appreciated by those skilled in the art, movement in three degrees of freedom is typically translational or lateral movement along an X-axis  298   a , lateral movement along a Y-axis  298   b , and rotational movement about a Z-axis  298   c , while movement in six degrees of freedom includes lateral movement along each axis  298  as well as rotational movement about each axis  298 . The choice of the number of degrees of freedom for wafer table  212  is generally dependent upon the requirements of system  200 . For example, when AVIS  280  is not included in system  200 , then wafer table  212  may move in six degrees of freedom such that a low transmissibility and a high bandwidth may be achieved. When wafer table  212  may move in six degrees of freedom, then image distortion associated with images projected through lens assembly  228  onto a wafer (not shown) supported on wafer table  212  may be reduced. Often, when wafer table  212  is arranged to move in three degrees of freedom, AVIS  280  is included in system  200  to reduce the amount of external vibrations transmitted to wafer table  212 .  
      Lens assembly  228  is supported on a lens frame  232  which is effectively vibrationally isolated from frame caster  220  by an AVIS  236  such that vibrations transmitted between frame caster  220  and lens assembly  228  may be reduced. Like AVIS  280 , AVIS  236  may be either a passive isolator or an active isolator. Lens assembly  228  supports sensors  260 , which are generally position or motion measurement sensors such as interferometers, which are arranged to determine positions of components of system  200 . By way of example, sensor  260   a  may be arranged to effectively measure a position of a wafer (not shown) mounted on wafer table  212 , while sensor  260   b  may be arranged to measure a position of a lens assembly  228 . Sensor  260   c  may be used to measure a position, e.g., a lateral position, of a reticle (not shown) supported on a reticle fine stage  244 . It should be understood that system  200  includes various other sensors which have not been shown for ease of illustration. Such sensors include, but are not limited to, sensors which measure a position of wafer table  212  along Z-axis  298   c , sensors which measure a position of reticle stage base  240  along Z-axis  298   c , and sensors which measure a position of the top of lens assembly  228  along X-axis  298   a.    
      A reticle support frame  286  is arranged to support a reticle stage base  240  on which a reticle fine stage  244  and a reticle coarse stage  248  may move to position a reticle (not shown) held in a reticle fine stage  244 . In general, reticle support frame  286 , lens frame  232 , and frame caster  220  may form an overall support frame. It should be appreciated that although both a reticle fine stage  244  and a reticle coarse stage  248  are included in system  200 , some systems may include only a single reticle stage. A counter mass  252  which is positioned on reticle stage base  240  and a trim motor  256 , which is mounted on reticle support frame  286  such that trim motor  256  is substantially isolated from reticle stage base  240 , serve to position counter mass  252  when a reticle (not shown) is scanned and to reduce the transmission of external vibrations to reticle fine stage  244  and reticle coarse stage  248 , respectively.  
      An AVIS  290  is arranged to isolate reticle fine stage  244  and reticle coarse stage  248  from lens assembly  228  by preventing significant vibrations from being transmitted from either or both reticle fine stage  244  and reticle coarse stage  248  through reticle stage base  240  to lens assembly  228 . As shown, AVIS  290  is also arranged to substantially isolate reticle fine stage  244  and reticle coarse stage  248  from sensors  260   a ,  260   b ,  260   c  thereby reducing the effect of external vibrations on the operation of sensors  260   a ,  260   b ,  260   c . AVIS  290  is effectively mounted on frame caster  220 , as for example through reticle support frame  286 . In one embodiment, AVIS  290  may be mounted substantially directly to frame caster  220 . AVIS  290 , in addition to being used to reduce the amount of vibrations transmitted from reticle fine stage  244  and reticle coarse stage  248 , may generally serve to compensate for a shift in the center of gravity associated with a reticle stage assembly which generally includes reticle fine stage  244  and reticle coarse stage  248 . When counter mass  252  is used, then AVIS  290  is not necessarily used for center of gravity shift compensation associated with reticle fine stage  244  and reticle coarse stage  248 , and is instead used to reduce the transmissibility of vibrations generated by the movement of reticle fine stage  244  or reticle coarse stage  248 .  
      By isolating reticle stage base  240  from lens assembly  228  using AVIS  290 , lens assembly  228  is effectively not subjected to vibrations generated when a reticle (not shown) supported on reticle fine stage  244  is scanned. Hence, the accuracy associated with system  200  may be improved, as lens assembly  228  is less likely to move and, further, sensors  260  are also less likely to move. AVIS  290  may be substantially any suitable isolation system which is effective in preventing reticle stage vibrations from being transmitted to lens assembly  228 . Suitable isolation system typically include, but are not limited to, various air mounts and voice coil motors.  
      A lithographic system which includes an AVIS that prevents significant reticle stage vibrations from affecting a lens assembly, e.g., AVIS  290  of  FIG. 2   a , may generally vary widely. By way of example, as discussed above, such a system may include both reticle fine stage  244  and reticle coarse stage  248 . Alternatively, such a system may include only a single reticle stage. In addition, a system which includes an AVIS that isolates an overall reticle stage assembly from a lens assembly may or may not include an AVIS that isolates a wafer stage assembly from a frame caster.  
       FIG. 2   b  is a diagrammatic representation of a second lithographic system which includes an AVIS that substantially isolates a reticle stage structure from a lens assembly will be described in accordance with an embodiment of the present invention. A lithographic system  300  is similar to lithographic system  200  of  FIG. 2   a , and includes wafer stage  204 , wafer stage base  208 , and wafer table  212 . System  300  also includes reticle fine stage  244 , reticle coarse stage  248 , and reticle stage base  240  which are substantially isolated from lens assembly  228  by AVIS  290 .  
      In some situations, the use of a counter mass and a trim motor with a wafer stage assembly, e.g., counter mass  216  and trim motor  224  of  FIG. 2   a , may not be desirable, as for example when the mass of system  300  is to be reduced. When a counter mass and a trim motor are not substantially used with a wafer stage assembly, a reaction frame  294  may instead be used to effectively “absorb” reaction forces associated with the movement of wafer stage  204  and wafer table  212 . Specifically, reaction frame  294  may transmit reaction forces and vibrations to frame caster  220 .  
      When reaction frame  294  is used, avis  280  is used to reduce the transmissibility of vibrations between wafer stage base  208  and frame caster  220 . In other words, when reaction frame  294  is used in lieu of a counter mass and a trim motor, avis  280  is typically included in system  300 , i.e., avis  280  is effectively no longer optional. As previously mentioned, the inclusion of AVIS  280  generally entails the use of a three degree of freedom wafer table  212  in system  300 , although it should be appreciated that a six degree of freedom wafer table  212  may instead be used.  
      While the use of AVIS  290  is effective in reducing the transmissibility of vibrations resulting from the movement of reticle fine stage  244  or reticle coarse stage  148  to lens assembly  128 , aligning AVIS  290  within system  300  may be difficult. For example, difficulties may be the result a relatively low stiffness in air mounts and voice coil motors associated with AVIS  290 . In one embodiment, a piezoelectric actuator assembly may be used instead of an AVIS to prevent vibrations from being transmitted between a reticle stage and a lens assembly. With reference to  FIG. 3 , a lithographic system which includes a piezoelectric actuator assembly that substantially prevents vibrations from being transmitted between a reticle stage and a lens assembly will be described in accordance with an embodiment of the present invention. A lithographic system  400  includes lens assembly  228 , which is supported on lens frame  232 . Reticle stage base  240  supports a reticle stage  446  which is arranged to move a reticle (not shown) that is positioned atop reticle stage  446 .  
      A piezoelectric actuator assembly  490  is arranged to isolate reticle stage base  240 , reticle stage  446 , and counter mass  252  from lens assembly  228  such that vibrations associated with the movement of reticle stage  446  may be substantially prevented from being transmitted to lens assembly  228 . In general, when piezoelectric actuator assembly  490  is used instead of an AVIS, i.e., instead of AVIS  290  of  FIGS. 2   a  and  2   b , trim motor  256  as shown in  FIGS. 2   a  and  2   b  is not needed within system  400 . Piezoelectric actuator assembly  490  may include actuators with a relatively fast response time that effectively maintain a desired position along Z-axis  298   c , and about X-axis  298   a  and Y-axis  298   b . It should be understood that in order to control a position along Z-axis  298   c , and about X-axis  298   a  and Y-axis  298   b , feedback signals may be measured between lens assembly  228  and reticle stage base  240 . In one embodiment, piezoelectric actuator assembly  490  may include voice coil motors instead of piezoelectric actuators to control position relative to X-axis  298   a  and Y-axis  298   b , and about Z-axis  298   c , since the accuracy requirements generally associated with such position is relatively low.  
      Although the stiffness associated with piezoelectric actuator assembly  490  typically enables piezoelectric actuator assembly  490  to be aligned more readily than an AVIS, e.g., AVIS  290  of  FIGS. 2   a  and  2   b , when the stiffness of piezoelectric actuators included in piezoelectric actuator assembly  490  is too high, there may be disturbance effects associated with piezoelectric actuator assembly  490 . In general, the amount of vibration transmitted from caster  220  to reticle stage base  240  is dependent upon the stiffness of piezoelectric actuator  490 . Adding a component made of rubber or a material with characteristics similar to rubber, in one embodiment, to piezoelectric actuator assembly  490  may serve to reduce vibrations from caster  220 .  
      Typically, a reticle is arranged to track the movement of a wafer during a lithography process. As such, when the actual trajectories of the wafer and the reticle differ, the trajectory of the reticle is generally corrected or adjusted such that the trajectory of the reticle substantially matches the trajectory of the wafer.  FIG. 4  is a control block diagram which illustrates the control logic associated with enabling the movement of a reticle to substantially track the movement of a wafer in accordance with an embodiment of the present invention. A desired trajectory  500  is provided, e.g., through a controller arrangement, to a reticle stage assembly  504  and a wafer stage assembly  508 . In the described embodiment, desired trajectory  500  is specified using at least lateral positions along an X-axis and a Y-axis, as well as rotational positions about a Z-axis.  
      Reticle stage assembly  504  and wafer stage assembly  508  may then move a reticle and a wafer, respectively. A reticle output position  512  which is associated with the position to which reticle stage assembly  504  has moved and a wafer output position  516  which is associated with the position to which wafer stage assembly  508  has move may be fed back to reticle stage assembly  504  and wafer stage assembly  508 , respectively. When wafer stage assembly  508  includes a six degree of freedom wafer table, wafer output position  516  may include up to six coordinates, e.g., translational and rotational coordinates associated with an X-axis, a Y-axis, and a Z-axis. In other words, information relating to every degree of freedom associated with wafer stage assembly  508  may be fed back to wafer stage assembly  508 . While the position along the X-axis and the Y-axis, as well as the position about the Z-axis, of a wafer table included in wafer stage assembly  508  may be adjusted to enable the wafer table to track a desired trajectory using information that is fed back, the position of the wafer table may also be adjusted or repositioned based on the information that is fed back to reduce image distortion, e.g., by altering a rotational position about the X-axis and the Y-axis and a translational position along the Z-axis.  
      In general, reticle output position  512  is measured laterally along an X-axis and a Y-axis, and rotationally about a Z-axis. Wafer output position  516  may generally include lateral and rotational measurements about an X-axis, a Y-axis, and a Z-axis. A wafer stage controller (not shown) uses wafer output position  516  and desired trajectory  500  to correct errors in the stage position. A reticle stage controller (not shown) takes reticle output position  512 , desired trajectory  500 , and filter output  528  to generate a force command to move the stage.  
      Reticle output position  512  and wafer output position  516 , which typically represent the current positions of a reticle and a wafer, respectively, may be processed to create an error signal  520 . That is, the difference between the trajectories, e.g., as measured along an X-axis and a Y-axis, and about a Z-axis, of the reticle and the wafer may effectively be determined by determining the difference between the current position of the reticle and the current position of the wafer. When the difference between the current positions is substantially negligible, then the indication may be that the actual trajectory followed by reticle stage assembly  504  is currently successfully tracking the actual trajectory of wafer stage assembly  508 .  
      When there is a difference between the current or actual positions of a reticle and a wafer, then error signal  520  is passed through a filter  524  which is arranged to filter out any lens vibrations associated with a lens assembly of a lithography apparatus, e.g., lens assembly  228  of  FIGS. 2   a ,  2   b , and  3 . That is, filter  524  may be used to effectively separate out lens body vibrations from stage motion in error signal  520 . Filter  524  typically has parameters which may be determined using an interferometer system associated with the lens assembly, as will be discussed below with respect to  FIG. 5 . In general, filter  524  is added to the interferometer system associated with the lens assembly, and may be substantially any suitable filter which is effective to filter out vibrational components, e.g., vibrational components in lens body vibrations, that have an effect on either or both reticle output position  512  and wafer output position  516 . Suitable filters may include, but are not limited to, low pass filters and notch filters. As will be appreciated by those skilled in the art, a suitable filter may be selected based upon the characteristics of the vibrational components.  
      Once error signal  520  is filtered, the resultant filtered error signal  528  is provided as input to reticle stage assembly  504 . As a result, filtered error signal  528 , reticle output position  512 , and desired trajectory  500  may be used to substantially dictate the movement of reticle stage assembly  504  such that reticle stage assembly  504  allows a reticle supported thereon to follow the trajectory of a wafer supported on wafer stage assembly  508 .  
      Filter  524 , as previously mentioned, includes parameters which may be selected depending upon readings generated from an interferometer system.  FIG. 5  is a diagrammatic representation of a lens assembly and an interferometer system in accordance with an embodiment of the present invention. A lens assembly  550  is generally positioned between a reticle stage assembly  554  and a wafer stage assembly  558 . Specifically, lens assembly  550  is positioned between a reticle stage base and a wafer table which supports a wafer  
      Reference beams  562  and a measurement beam  570   a  which are associated with an interferometer system  566  are used to determine suitable parameters, e.g., parameters F 1  and F 2 , for filter  524  of  FIG. 4 . In general, vibrations of lens assembly  550  are effectively not compensated for. Rather, vibrations of wafer stage assembly  558  are controlled using parameters F 1 , F 2 . In one embodiment, reference beam  562   a  and measurement beam  570   b  may be used such that parameters F 1 , F 2  may be chosen to effectively control vibrations of wafer stage assembly  558  and reticle stage assembly  554 . Parameters F 1 , F 2  may be changed when the characteristics of vibrations changes, e.g., when oscillations increase or decrease in either frequency or magnitude.  
      With reference to  FIG. 6 , a general photolithography apparatus which may include an AVIS which reduces vibrations transmitted from a reticle stage to a lens assembly will be described in accordance with an embodiment of the present invention. A photolithography apparatus (exposure apparatus)  40  includes a wafer positioning stage  52  that may be driven by a planar motor (not shown), as well as a wafer table  51  that is magnetically coupled to wafer positioning stage  52  by utilizing an EI-core actuator. The planar motor which drives wafer positioning stage  52  generally uses an electromagnetic force generated by magnets and corresponding armature coils arranged in two dimensions. A wafer  64  is held in place on a wafer holder or chuck  74  which is coupled to wafer table  51 . Wafer positioning stage  52  is arranged to move in multiple degrees of freedom, e.g., between three to six degrees of freedom, under the control of a control unit  60  and a system controller  62 . The movement of wafer positioning stage  52  allows wafer  64  to be positioned at a desired position and orientation relative to a projection optical system  46 .  
      Wafer table  51  may be levitated in a z-direction  10   b  by any number of voice coil motors (not shown), e.g., three voice coil motors. In the described embodiment, at least three magnetic bearings (not shown) couple and move wafer table  51  along a y-axis  10   a . The motor array of wafer positioning stage  52  is typically supported by a base  70 . Base  70  is supported to a ground via isolators  54 . Reaction forces generated by motion of wafer stage  52  may be mechanically released to a ground surface through a frame  66 . One suitable frame  66  is described in JP Hei 8-166475 and U.S. Pat. No. 5,528,118, which are each herein incorporated by reference in their entireties.  
      An illumination system  42  is supported by a frame  72 . Frame  72  is supported to the ground or a frame caster (not shown) via isolators  54 . Illumination system  42  includes an illumination source, and is arranged to project a radiant energy, e.g., light, through a mask pattern on a reticle  68  that is supported by and scanned using a reticle stage which includes a coarse stage and a fine stage. The radiant energy is focused through projection optical system  46 , which is supported on a projection optics frame  50  and may be supported to the ground or a frame caster (not shown) through isolators  54 . Suitable isolators  54  include those described in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, which are each incorporated herein by reference in their entireties. In one embodiment, at least one of isolators  54  may be an AVIS.  
      A first interferometer  56  is supported on projection optics frame  50 , and functions to detect the position of wafer table  51 . Interferometer  56  outputs information on the position of wafer table  51  to system controller  62 . In one embodiment, wafer table  51  has a force damper which reduces vibrations associated with wafer table  51  such that interferometer  56  may accurately detect the position of wafer table  51 . A second interferometer  58  is supported on projection optical system  46 , and detects the position of reticle stage  44  which supports a reticle  68 . Interferometer  58  also outputs position information to system controller  62 . Reticle stage  44  is supported on a reticle stage frame  48  which may include at least one AVIS which prevents vibrations associated with reticle stage  44  from being transmitted to projection optical system  46 .  
      It should be appreciated that there are a number of different types of photolithographic apparatuses or devices. For example, photolithography apparatus  40 , or an exposure apparatus, may be used as a scanning type photolithography system which exposes the pattern from reticle  68  onto wafer  64  with reticle  68  and wafer  64  moving substantially synchronously. In a scanning type lithographic device, reticle  68  is moved perpendicularly with respect to an optical axis of a lens assembly (projection optical system  46 ) or illumination system  42  by reticle stage  44 . Wafer  64  is moved perpendicularly to the optical axis of projection optical system  46  by a wafer stage  52 . Scanning of reticle  68  and wafer  64  generally occurs while reticle  68  and wafer  64  are moving substantially synchronously.  
      Alternatively, photolithography apparatus or exposure apparatus  40  may be a step-and-repeat type photolithography system that exposes reticle  68  while reticle  68  and wafer  64  are stationary. In one step and repeat process, wafer  64  is in a substantially constant position relative to reticle  68  and projection optical system  46  during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer  64  is consecutively moved by wafer positioning stage  52  perpendicularly to the optical axis of projection optical system  46  and reticle  68  for exposure. Following this process, the images on reticle  68  may be sequentially exposed onto the fields of wafer  64  so that the next field of semiconductor wafer  64  is brought into position relative to illumination system  42 , reticle  68 , and projection optical system  46 .  
      It should be understood that the use of photolithography apparatus or exposure apparatus  40 , as described above, is not limited to being used in a photolithography system for semiconductor manufacturing. For example, photolithography apparatus  40  may be used as a part of a liquid crystal display (LCD) photolithography system that exposes an LCD device pattern onto a rectangular glass plate or a photolithography system for manufacturing a thin film magnetic head.  
      The illumination source of illumination system  42  may be g-line (436 nanometers (nm)), i-line (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193 nm), and an F2-type laser (157 nm). Alternatively, illumination system  42  may also use charged particle beams such as x-ray and electron beams. For instance, in the case where an electron beam is used, thermionic emission type lanthanum hexaboride (LaB6) or tantalum (Ta) may be used as an electron gun. Furthermore, in the case where an electron beam is used, the structure may be such that either a mask is used or a pattern may be directly formed on a substrate without the use of a mask.  
      With respect to projection optical system  46 , when far ultra-violet rays such as an excimer laser is used, glass materials such as quartz and fluorite that transmit far ultra-violet rays is preferably used. When either an F 2 -type laser or an x-ray is used, projection optical system  46  may be either catadioptric or refractive (a reticle may be of a corresponding reflective type), and when an electron beam is used, electron optics may comprise electron lenses and deflectors. As will be appreciated by those skilled in the art, the optical path for the electron beams is generally in a vacuum.  
      In addition, with an exposure device that employs vacuum ultra-violet (VUV) radiation of a wavelength that is approximately 200 nm or lower, use of a catadioptric type optical system may be considered. Examples of a catadioptric type of optical system include, but are not limited to, those described in Japan Patent Application Disclosure No. 8-171054 published in the Official gazette for Laid-Open Patent Applications and its counterpart U.S. Pat. No. 5,668,672, as well as in Japan Patent Application Disclosure No. 10-20195 and its counterpart U.S. Pat. No. 5,835,275, which are all incorporated herein by reference in their entireties. In these examples, the reflecting optical device may be a catadioptric optical system incorporating a beam splitter and a concave mirror. Japan Patent Application Disclosure (Hei) No. 8-334695 published in the Official gazette for Laid-Open Patent Applications and its counterpart U.S. Pat. No. 5,689,377, as well as Japan Patent Application Disclosure No. 10-3039 and its counterpart U.S. Pat. No. 5,892,117, which are all incorporated herein by reference in their entireties. These examples describe a reflecting-refracting type of optical system that incorporates a concave mirror, but without a beam splitter, and may also be suitable for use with the present invention.  
      Further, in photolithography systems, when linear motors (see U.S. Pat. Nos. 5,623,853 or 5,528,118, which are each incorporated herein by reference in their entireties) are used in a wafer stage or a reticle stage, the linear motors may be either an air levitation type that employs air bearings or a magnetic levitation type that uses Lorentz forces or reactance forces. Additionally, the stage may also move along a guide, or may be a guideless type stage which uses no guide.  
      Alternatively, a wafer stage or a reticle stage may be driven by a planar motor which drives a stage through the use of electromagnetic forces generated by a magnet unit that has magnets arranged in two dimensions and an armature coil unit that has coil in facing positions in two dimensions. With this type of drive system, one of the magnet unit or the armature coil unit is connected to the stage, while the other is mounted on the moving plane side of the stage.  
      Movement of the stages as described above generates reaction forces which may affect performance of an overall photolithography system. Reaction forces generated by the wafer (substrate) stage motion may be mechanically released to the floor or ground by use of a frame member as described above, as well as in U.S. Pat. No. 5,528,118 and published Japanese Patent Application Disclosure No. 8-166475. Additionally, reaction forces generated by the reticle (mask) stage motion may be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and published Japanese Patent Application Disclosure No. 8-330224, which are each incorporated herein by reference in their entireties.  
      Isolaters such as isolators  54  may generally be associated with an active vibration isolation system (AVIS). An AVIS generally controls vibrations associated with forces  112 , i.e., vibrational forces, which are experienced by a stage assembly or, more generally, by a photolithography machine such as photolithography apparatus  40  which includes a stage assembly.  
      A photolithography system according to the above-described embodiments may be built by assembling various subsystems in such a manner that prescribed mechanical accuracy, electrical accuracy, and optical accuracy are maintained. In order to maintain the various accuracies, prior to and following assembly, substantially every optical system may be adjusted to achieve its optical accuracy. Similarly, substantially every mechanical system and substantially every electrical system may be adjusted to achieve their respective desired mechanical and electrical accuracies. The process of assembling each subsystem into a photolithography system includes, but is not limited to, developing mechanical interfaces, electrical circuit wiring connections, and air pressure plumbing connections between each subsystem. There is also a process where each subsystem is assembled prior to assembling a photolithography system from the various subsystems. Once a photolithography system is assembled using the various subsystems, an overall adjustment is generally performed to ensure that substantially every desired accuracy is maintained within the overall photolithography system. Additionally, it may be desirable to manufacture an exposure system in a clean room where the temperature and humidity are controlled.  
      Further, semiconductor devices may be fabricated using systems described above, as will be discussed with reference to  FIG. 8 . The process begins at step  1301  in which the function and performance characteristics of a semiconductor device are designed or otherwise determined. Next, in step  1302 , a reticle (mask) in which has a pattern is designed based upon the design of the semiconductor device. It should be appreciated that in a parallel step  1303 , a wafer is made from a silicon material. The mask pattern designed in step  1302  is exposed onto the wafer fabricated in step  1303  in step  1304  by a photolithography system. One process of exposing a mask pattern onto a wafer will be described below with respect to  FIG. 8 . In step  1305 , the semiconductor device is assembled. The assembly of the semiconductor device generally includes, but is not limited to, wafer dicing processes, bonding processes, and packaging processes. Finally, the completed device is inspected in step  1306 .  
       FIG. 8  is a process flow diagram which illustrates the steps associated with wafer processing in the case of fabricating semiconductor devices in accordance with an embodiment of the present invention. In step  1311 , the surface of a wafer is oxidized. Then, in step  1312  which is a chemical vapor deposition (CVD) step, an insulation film may be formed on the wafer surface. Once the insulation film is formed, in step  1313 , electrodes are formed on the wafer by vapor deposition. Then, ions may be implanted in the wafer using substantially any suitable method in step  1314 . As will be appreciated by those skilled in the art, steps  1311 - 1314  are generally considered to be preprocessing steps for wafers during wafer processing. Further, it should be understood that selections made in each step, e.g., the concentration of various chemicals to use in forming an insulation film in step  1312 , may be made based upon processing requirements.  
      At each stage of wafer processing, when preprocessing steps have been completed, post-processing steps may be implemented. During post-processing, initially, in step  1315 , photoresist is applied to a wafer. Then, in step  1316 , an exposure device may be used to transfer the circuit pattern of a reticle to a wafer. Transferring the circuit pattern of the reticle of the wafer generally includes scanning a reticle scanning stage which may, in one embodiment, include a force damper to dampen vibrations.  
      After the circuit pattern on a reticle is transferred to a wafer, the exposed wafer is developed in step  1317 . Once the exposed wafer is developed, parts other than residual photoresist, e.g., the exposed material surface, may be removed by etching. Finally, in step  1319 , any unnecessary photoresist that remains after etching may be removed. As will be appreciated by those skilled in the art, multiple circuit patterns may be formed through the repetition of the preprocessing and post-processing steps.  
      Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, a lithographic system which includes a piezoelectric actuator which serves as a vibration isolator has been described as including only a single reticle stage. In some embodiments, a piezoelectric actuator may be implemented in a system which includes a plurality of reticle stages, e.g., a fine stage and a coarse stage. Generally, lithographic systems which include either an AVIS or a piezoelectric actuator to isolate a reticle stage assembly from a lens assembly may be widely varied. For instance, a lithographic system may include a reaction frame instead of a counter mass arrangement to absorb reaction forces, as discussed above.  
      An AVIS has generally been described as being either passive or active. A passive AVIS has been described as including an air mount, while an active AVIS has been described as including a voice coil motor. It should be appreciated that substantially any suitable device may be used as a passive AVIS or an active AVIS. That is, the configuration of an AVIS may vary widely.  
      Each AVIS or piezoelectric actuator assembly has generally been described as being mounted substantially directly to a frame caster, e.g., through a frame such as a reticle frame to substantially isolate a lens assembly from vibrations associated with the movement of various stages. In one embodiment, an AVIS may instead be mounted substantially on the lens assembly in order to isolate the lens assembly from the vibrations, e.g., an AVIS which isolates a reticle stage assembly from a lens assembly may be substantially mounted on the lens assembly without departing from the spirit or the scope of the present invention.  
      The trajectory of a reticle has been described above as being altered such that the reticle effectively follows or tracks the trajectory of a wafer. It should be appreciated that instead of altering the actual trajectory of a reticle to track the trajectory of a wafer, the actual trajectory of the wafer may instead be altered to track the trajectory of the reticle. Typically, the trajectory of the reticle is altered due to the fact that there are fewer mechanism associated with a reticle stage assembly than there are associated with a wafer stage assembly, i.e., it may be less complicated to alter the trajectory of the reticle. In addition, the bandwidth associated with adjusting the trajectory of the reticle is higher than the corresponding bandwidth of the wafer.  
      The control logic or flow used to enable the trajectory of a reticle to track the trajectory of a wafer may vary widely. By way of example, position output signals associated with a reticle stage assembly and a wafer stage assembly may each be filtered before an error signal is determined without departing from the spirit or the scope of the present invention. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.