Patent Publication Number: US-2004057817-A1

Title: Switchable damping mechanism for use in a stage apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of Invention  
       [0002] The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a damping mechanism that reduces vibrations associated with a wafer table when the wafer table is accelerating, but does not adversely affect the wafer table during a wafer exposure process.  
       [0003] 2. Description of the Related Art  
       [0004] 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 vibrations, products formed using the precision instrument may be improperly formed and, hence, function improperly. For instance, 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. Further, vibrations may cause control problems during an acceleration portion of a scanning process or a scan.  
       [0005] 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.  
       [0006] Within a photolithography apparatus, vibrations may be particularly problematic, especially when the vibrations are excited to uncontrollable levels during the acceleration and the deceleration of components of the apparatus, such as a wafer table. While such vibrations often cause accuracy issues during an acceleration or deceleration portion of an overall wafer scanning process, the same vibrations often do not significantly affect the accuracy of a constant velocity, or exposure portion, of a scan. In other words, although vibrations may cause significant stage control problems during an acceleration or deceleration portion of an overall scanning process, vibrations generally do not significantly affect the ability to control the stage during an exposure portion of the scanning process. However, the transient acceleration or deceleration effects which may remain from the acceleration portion of a scanning process may cause accuracy issues to arise during the exposure portion of the scanning process.  
       [0007] Transient effects which result from acceleration or deceleration of a wafer table may result in measurement inconsistencies. For example, inertia may be exerted on an interferometer mirror, which is used to enable an interferometer to effectively determine a location of the wafer table, when a wafer table accelerates. As will be appreciated by those skilled in the art, a conventional interferometer such as a laser interferometer generally uses mirrors to facilitate a determination of the position of the wafer table. The inertia exerted on an interferometer mirror may cause the interferometer mirror to vibrate and, hence, deflect. Deflection of the interferometer mirror may cause the interferometer mirror to bend significantly, e.g., the interferometer mirror may bend by up to approximately 600 nanometers (nm). If the interferometer mirror bends when the wafer table accelerates or decelerates, once the wafer table has ceased to accelerate or to decelerate, transient effects may be such that the interferometer mirror has not returned to an unbent state before a wafer exposure portion of an overall scan begins. When the interferometer mirror is bent or flexed during wafer exposure, measurement inconsistencies may occur due to the interferometer mirror effectively being moved from an anticipated position while a wafer situated on the wafer table has not moved. Inconsistent or inaccurate position measurements may result in an inaccurate wafer exposure.  
       [0008] Since vibrations during an acceleration or deceleration portion of a wafer scan may compromise the accuracy of the scan, e.g., by causing an interferometer mirror to bend, mechanisms are often incorporated into a photolithography apparatus or, more generally, a precision instrument which includes a wafer table or a stage. The mechanisms are generally arranged to effectively reduce the effect of the vibrations. Such mechanisms typically include passive dampers which apply damping within the precision instrument. Typically, as will be understood by those skilled in the art, dampers often include hysteretic materials which dissipate the energy associated with vibrations. Although hysteretic materials are effective in absorbing vibrational energy and, hence, reducing the effect of vibrations, hysteric materials often introduce hysteresis with respect to the precision instrument. By way of example, hysteretic materials may introduce hysteretic effects which result in the distortion of a wafer table. When the wafer table is distorted during the exposure portion of a scan, the accuracy with which a wafer may be exposed may be adversely affected and, hence, a semiconductor formed using the wafer may not be reliable. That is, more generally, the integrity of an exposure portion of a scan may be affected.  
       [0009] Therefore, what is desired is a method and an apparatus which damps vibrations during an acceleration or deceleration portion of a scan substantially without adversely affecting a wafer exposure portion of the scan. That is, what is needed is a system which damps vibrations when a wafer table is accelerating or decelerating, but does not cause a distortion of the wafer table during an exposure process.  
       SUMMARY OF THE INVENTION  
       [0010] The present invention relates to damping vibrations within a stage device using controllable dampers. According to one aspect of the present invention, a method for adjusting an amount of resistance associated with a stage device which has a table, a first surface, and a coupler positioned substantially between the table and the first surface includes accelerating the table and applying a resistance between the table and the first surface when the table is accelerating. Applying the resistance includes providing a first adjustment to the coupler. The method also includes substantially removing the resistance from between the table and the first surface when the table is not accelerating, wherein the resistance is substantially removed by providing a second adjustment to the coupler. In one embodiment, the stage device further includes a first damper that is arranged to substantially damp an elastic body vibration of the wafer table. In such an embodiment, the method may also include applying a damping to the wafer table to substantially counteract the elastic body vibration of the wafer table, wherein applying the damping includes substantially adjusting the first damper.  
       [0011] In another embodiment, the coupler is a controllable damper arrangement, and applying the resistance between the table and the first surface includes increasing a signal provided to the controllable damper arrangement. In such an embodiment, substantially removing the resistance from between the table and the first surface includes decreasing the signal provided to the controllable damper arrangement.  
       [0012] Damping elastic body vibrations on a wafer table of a stage device, as well as vibrations between the wafer table and a long mirror, or a mirror associated with an interferometer, during an acceleration portion of a scan reduces the effect of vibrations on the stage device. Using a controllable damper or dashpot to provide damping, e.g., during an acceleration portion of a scan, enables the amount of damping to be varied as needed. In addition, the controllable damper enables no damping to be applied when appropriate, e.g., during an exposure or constant velocity portion of a scan. As a result, distortion of the of a wafer table may be reduced during exposure, along with the effect of vibrations during acceleration.  
       [0013] According to another aspect of the present invention, a method for performing a scan using a stage device which has a table and a damping device includes initiating a scan using the table. Initiating the scan using the table may excite at least one elastic body vibration mode associated with the table. The elastic body vibration mode may cause a deflection of at least a section of the table. The method also includes adjusting a parameter associated with the damping device to substantially counteract the elastic body vibration mode to substantially reduce the deflection. In one embodiment, the damping device is a magneto-rheological damper arrangement, and the method further includes determining when the elastic body vibration mode is not excited, and adjusting the magneto-rheological damper arrangement to provide substantially no damping when it is determined that the elastic body vibration mode is not excited.  
       [0014] In accordance with another aspect of the present invention, a stage apparatus includes a first stage, a mirror, at least one actuator, and a first arrangement. The actuator enables the first stage to scan, and may excite at least one vibrational mode associated with the stage apparatus. The first arrangement is positioned between the first stage and the mirror, and receives a variable signal that controls an amount of resistance associated with the first arrangement. A first amount of resistance is arranged to cause the vibrational mode to be substantially damped. In one embodiment, the first stage is arranged to have an associated elastic body vibration mode, and the stage apparatus further includes a second arrangement that is positioned on the first stage. Such a second arrangement receives a controllable signal that controls an amount of resistance associated with the second arrangement. A second amount of resistance is arranged to cause the elastic body vibration mode to be substantially damped.  
       [0015] According to still another aspect of the present invention, a damping device includes a damper and a controller. The damper is connected to a movable member, and is arranged to change its damping characteristic in accordance with a variable signal that is provided to the damper. The controller, which is connected to the damper, controls the variable signal based on information of the movement of the movable member. In one embodiment, the information of the movement of the movable member includes a first mode and a second mode. In such an embodiment, the movable member may move with one of an acceleration and a deceleration in the first mode, and move at a substantially constant velocity in the second mode. 
     
    
    
     [0016] 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  
     [0017] The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
     [0018]FIG. 1 is a diagrammatic representation of a portion of a stage apparatus which includes magneto-rheological dampers positioned substantially between a wafer table and long mirrors in accordance with an embodiment of the present invention.  
     [0019]FIG. 2 a  is a diagrammatic top-view representation of a stage apparatus, e.g., stage apparatus  100  of FIG. 1, in accordance with an embodiment of the present invention.  
     [0020]FIG. 2 b  is a diagrammatic side-view cross-sectional representation of a stage apparatus, e.g., stage apparatus  100  of FIG. 1, in accordance with an embodiment of the present invention.  
     [0021]FIG. 3 is a diagrammatic representation of a magneto-rheological damper in accordance with an embodiment of the present invention.  
     [0022]FIG. 4 is a diagrammatic block diagram representation of the functionality of a magneto-rheological damper in accordance with an embodiment of the present invention.  
     [0023]FIG. 5 is a process flow diagram which illustrates the steps associated with operating a stage device which includes a magneto-rheological damper in accordance with an embodiment of the present invention.  
     [0024]FIG. 6 is a diagrammatic side-view representation of the deflection of a wafer table due to a double cantilever vibration mode.  
     [0025]FIG. 7 is a diagrammatic representation of the deflection of a wafer table due to a common table vibration mode.  
     [0026]FIG. 8 a  is a diagrammatic representation of a wafer table which includes magneto-rheological mirror dampers and a magneto-rheological table damper in a first position in accordance with an embodiment of the present invention.  
     [0027]FIG. 8 b  is a diagrammatic representation of a wafer table, e.g., wafer table  810  of FIG. 8 a , which includes magneto-rheological mirror dampers and a magneto-rheological table damper in a second position in accordance with an embodiment of the present invention.  
     [0028]FIG. 8 c  is a diagrammatic representation of a wafer table, e.g., wafer table  810  of FIG. 8 a , which includes magneto-rheological mirror dampers and multiple magneto-rheological table dampers in accordance with an embodiment of the present invention.  
     [0029]FIG. 9 is a diagrammatic representation of a photolithography apparatus in accordance with an embodiment of the present invention.  
     [0030]FIG. 10 is a process flow diagram which illustrates the steps associated with fabricating a semiconductor device in accordance with an embodiment of the present invention.  
     [0031]FIG. 11 is a process flow diagram which illustrates the steps associated with processing a wafer, i.e., step  1304  of FIG. 10, in accordance with an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
     [0032] Reducing the effect of vibrations on a precision instrument during an acceleration or deceleration portion of a wafer scan process is generally necessary in order to enable a wafer table of the precision instrument to be accurately scanned. While passive damping solutions are effective in dissipating vibrational energy, passive damping solutions may cause the wafer table to distort during an exposure portion of the scan.  
     [0033] In order to enable vibrations to be damped during an acceleration or deceleration portion of a wafer scan substantially without causing significant distortion or deceleration, dampers which may be controlled or adjusted in real-time may be implemented within a precision instrument. Such dampers, e.g., magneto-rheological dampers, may be activated during acceleration or deceleration in order to damp vibrations such as mirror vibrations, and deactivated during exposure to substantially prevent a wafer table from experiencing significant distortion. In other words, the use of dynamically adjustable or switchable dampers such as magneto-rheological dampers enables vibrations due to a moving stage, for example, to be damped when necessary or desired, while allowing the dampers to apply effectively no damping when damping is either not necessary or not desired.  
     [0034] With reference to FIG. 1, a portion of an overall precision instrument such as a stage apparatus which includes dampers that may be dynamically adjusted will be described in accordance with an embodiment of the present invention. An overall stage apparatus  100  includes a wafer table  110 , e.g., a fine stage, which supports a wafer chuck  124 . Stage apparatus  100  may be moved in three degrees of freedom (x, y, θ z ) by a mover assembly, e.g., a linear motor or a planar motor that utilizes a Lorentz Force. Interferometer mirrors  114 , which are part of an overall interferometer, are also included in overall stage apparatus  100 . Typically, wafer table  110  is situated over a coarse stage (not shown) which imparts coarse movements while wafer table  110  imparts finer movements. When wafer table  110  is scanned, a wafer (not shown) that may be supported by wafer chuck  124  may be moved as appropriate. By way of example, a wafer may be scanned such that a particular portion of the wafer is positioned under a particular portion of a reticle.  
     [0035] Magneto-rheological dampers  120 , or mirror deflection dampers, are positioned between wafer table  110  and interferometer mirrors  114 . In general, wafer table  110  and interferometer mirrors  114  may also be coupled using substantially any suitable fastening device. Specifically, wafer table  110  and interferometer mirror  114   a  may be coupled through fastening devices at areas of contact, while wafer table  110  and interferometer mirror  114   b  may also be coupled through fastening devices at areas of contact. Suitable fastening devices include, but are not limited to, bolts and various adhesive materials.  
     [0036] As shown, magneto-rheological damper  120   a  is positioned between wafer table  110  and interferometer mirror  114   a , while magneto-rheological damper  120   b  is positioned between wafer table  110  and interferometer mirror  114   b . Interferometer mirror  114   a  is utilized to detect the position of wafer table  110  in a y-direction, and interferometer mirror  114   b  is utilized to detect the position of wafer table  110  in an x-direction. In general, one end of a magneto-rheological damper  120  may be coupled to wafer table  110  while a substantially opposite end of magneto-rheological damper  120  may be coupled to an interferometer mirror  114 . Although the components of magneto-rheological dampers  120  may be widely varied, magneto-rheological dampers  120  generally include a fluid-filled cavity that contains fluid which has a relatively low viscosity when there is substantially no magnetic field, and a higher viscosity when there is a magnetic field. By applying magnetic fields around magneto-rheological dampers  120  using electromagnetic coils  128 , the higher viscosity may be achieved and, hence, damping may be provided by magneto-rheological dampers  120 .  
     [0037]FIG. 2 a  is a diagrammatic top-view representation of a stage apparatus, e.g., stage apparatus  100  of FIG. 1, and FIG. 2 b  is a diagrammatic cross-sectional side-view representation of a stage apparatus, e.g., stage apparatus  100  of FIG. 1, in accordance with an embodiment of the present invention. As shown, magneto-rheological dampers  120  are effectively coupled to wafer table  110  and to interferometer mirrors  114 . Actuators  206 , e.g., voice coil motors, are positioned under wafer table  110  to enable wafer table  110  to translate linearly. Actuators may be driven for leveling motion (at least one of z, θ x , θ y  directions) of wafer table  110 . While wafer table  110  enables a wafer (not shown) that is positioned on wafer chuck  124  to effectively undergo fine movements, a coarse stage  204  which is positioned substantially under wafer table  110  enables the wafer to effectively undergo coarse movements.  
     [0038] In general, magneto-rheological dampers  120  may take substantially any suitable configuration. Referring next to FIG. 3, one embodiment of a magneto-rheological damper will be described in accordance with the present invention. A magneto-rheological damper  320  may be coupled to an interferometer mirror  314  and to a wafer table  310 . In the described embodiment, magneto-rheological damper  320  is a dashpot which includes a cylinder  332  and a piston  336 . Cylinder  332 , which may be formed from a material such as aluminum, is typically coupled to wafer table  310 , while piston  336  is typically coupled to mirror  314 .  
     [0039] Cylinder  332  is arranged to contain a magneto-rheological fluid  340 , or a fluid which is relatively viscous in the presence of a magnetic field and less viscous when a magnetic field is not present. In general, a magneto-rheological fluid is a controllable fluid which may be controlled by a magnetic field. Magneto-rheological fluids are non-colloidal suspensions of polarizable or magnetically soft particles. The particles in a magneto-rheological fluid generally have a size on the order of a few microns. One example of a magneto-rheological fluid is a hydrocarbon base fluid which holds iron particles in suspension.  
     [0040] When magneto-rheological fluid  340  is in a non-active state, i.e., when magneto-rheological fluid  340  is not subjected to an applied magnetic field or is not magnetized, the particles in magneto-rheological fluid  340  are effectively randomly positioned. As such, when piston  336  is pushed against magneto-rheological fluid  340 , piston  336  may meet relatively little resistance since magneto-rheological fluid  340  has a relatively low viscosity and, hence, magneto-rheological damper  320  provides substantially no appreciable damping. Alternatively, when magneto-rheological fluid  340  is in an active state, i.e., when magneto-rheological fluid  340  is subjected to an applied magnetic field or is in a substantially magnetized state, then the particles in magneto-rheological fluid  340  become aligned and magneto-rheological fluid  340  is characterized by a higher viscosity. Specifically, the particles in magneto-rheological fluid  340  may become aligned into structures which change the rheology of magneto-rheological fluid  340 , e.g., into a substantially plastic state by altering the shear strength of magneto-rheological fluid  340 . As a result, when piston  336  is pushed against magneto-rheological fluid  340 , piston  336  may meet significant resistance, thereby causing magneto-rheological damper  320  to provide an appreciable amount of damping. The amount of resistance provided by magneto-rheological fluid  340  and, hence, the amount of damping provided by magneto-rheological damper  320  typically varies with the strength of the magnetic field that is effectively applied to magneto-rheological fluid  340 . As the strength of the magnetic field increases, the amount of damping provided by magneto-rheological damper  320  increases.  
     [0041] As will be appreciated by those skilled in the art, a piston included in a dashpot typically either includes holes, or a thin gap may exist around the perimeter of the piston. When the piston is compressed, fluid flows either through the holes or around the gap with varying viscosity causing varying damping.  
     [0042] An electromagnetic coil  328  is positioned with respect to cylinder  332  such that when coil  328  active, as for example when current is provided to coil  328 , coil  328  causes a magnetic field to be applied to magneto-rheological fluid  340 . In one embodiment, by controlling the amount of current that is provided to coil  328 , the strength or the magnitude of the magnetic field that is created by coil  328  may be varied. As a result, adjusting the amount of current provided to coil  328  essentially enables the amount of resistance provided by magneto-rheological fluid  340  to be adjusted in real time such that the amount of damping provided by magneto-rheological damper  320  is effectively continuously variable in real time. It should be appreciated that an off-the-shelf magneto-rheological damper, such as a magneto-rheological damper available from the Lord Corporation in Cary, N.C., may be used as an alternative to magneto-rheological damper  320 .  
     [0043] The current that is sent to coil  328  may be controlled using substantially any suitable process. Current may be controlled by a controller which is coupled to a current amplifier and is external to the system. By way of example, a controller (not shown) which adjusts the current provided to coil  328  may receive a signal from a sensor that is arranged to sense when wafer table  310  is being scanned at a constant velocity or when wafer table is being accelerated or decelerated. The controller may then use the signal to adjust the amount of current provided to coil  328  as appropriate.  
     [0044] Magneto-rheological damper  320  is generally arranged to enable vibrations, e.g., elastic body vibrations, to be damped between mirror  314  and wafer table  310  when magneto-rheological fluid  340  has a relatively high viscosity. When magneto-rheological fluid  340  has a relatively low viscosity, magneto-rheological damper  320  is arranged to provide little or no damping between mirror  314  and wafer table  310 . As previously mentioned, damping vibrations during an acceleration or deceleration component of a scan may prevent a subsequent exposure process from being compromised. However, applying damping during an exposure process may distort wafer table  310  and, hence, cause the exposure process to be performed inaccurately. Since magneto-rheological damper  320  is controllable, the amount of damping applied during a scan may be adjusted.  
     [0045] In general, magneto-rheological damper  320  is arranged to control vibrations by providing a measure of damping when necessary, and to provide substantially no damping when it is not necessary to control vibrations. FIG. 4 is a diagrammatic block diagram representation of the functionality of a magneto-rheological damper in accordance with an embodiment of the present invention. A magneto-rheological damper  420  is positioned between a wafer table  410  and a mirror  414 , e.g., a long mirror or an interferometer mirror. When wafer table accelerates or decelerates, magneto-rheological damper  420  provides non-zero damping such that vibrations that are excited to uncontrollable levels may be damped. An actuator arrangement  450 , which causes wafer table  410  to accelerate or decelerate, may provide a signal to a controller  460  which controls the current provided to a coil (not shown) associated with magneto-rheological damper  420 . Increasing the amount of current provided to the coil increases the strength of the magnetic field generated by the coil and, hence, the amount of damping applied by magneto-rheological damper  420 . Actuator arrangement  450  may include an actuator and, in one embodiment, a control mechanism which uses interferometer signals to determine how wafer table  410  is to be moved.  
     [0046] When wafer table  410  is moved at a constant velocity, e.g., during a wafer exposure process, magneto-rheological damper  420  may be controlled by controller  460  such that magneto-rheological damper  420  provides substantially no damping. Controller  460  may reduce the current provided to a coil (not shown) that is associated with magneto-rheological damper  420  such that a significant magnetic field is not generated by the coil. In one embodiment, controller  460  may be in communication with a computing device  470 .  
     [0047]FIG. 5 is a process flow diagram which illustrates the steps associated with a scanning process performed using a stage device which includes a magneto-rheological damper in accordance with an embodiment of the present invention. A scanning process  500  begins at step  504  in which the acceleration of a wafer table is initiated. It should be appreciated that although the wafer table is described as accelerating, the acceleration of the wafer table may include the deceleration of the wafer table, as deceleration is generally a negative acceleration.  
     [0048] Upon initiation of the acceleration of the wafer table, a magneto-rheological damper which is coupled to the wafer table and to an interferometer mirror is activated in step  508  to provide damping. In other words, during the acceleration portion of a scan, the magneto-rheological damper is arranged to provide damping. The magneto-rheological damper may be activated by providing electric current to an electromagnetic coil associated with the magneto-rheological damper. The amount of electric current provided to the electromagnetic coil may be controlled by a control device which is in communication with a computing system. Providing damping between the wafer table and the interferometer mirror allows vibrational energy associated with the movement of the wafer table to be substantially damped. Specifically, the magneto-rheological damper, e.g., magneto-rheological damper  120   a  of FIG. 1, may allow elastic body vibrations on the interferometer mirror to be damped between the wafer table and the interferometer mirror, e.g., wafer table  110  and interferometer mirror  114   a  of FIG. 1.  
     [0049] Once the magneto-rheological damper provides damping, the wafer table may accelerate as necessary in step  512 . Typically, the wafer table accelerates until a wafer carried by the wafer table is in a desired position. In step,  516 , the acceleration of the wafer table ends. When the wafer table is no longer accelerating, and any transient effects of acceleration have been exhausted, the magneto-rheological damper no longer needs to provide damping, as vibrations generally do not significantly affect the wafer table during a constant velocity portion of a scan. Since the magneto-rheological damper no longer needs to provide damping, the magneto-rheological damper is effectively adjusted to provide substantially no appreciable damping in step  520 . In one embodiment, adjusting the magneto-rheological damper to provide substantially no appreciable damping may include effectively deactivating the electromagnetic coil associated with the magneto-rheological damper, as for example by cutting the supply of current to the electromagnetic coil.  
     [0050] After the magneto-rheological damper is adjusted to provide substantially no appreciable damping, a wafer exposure process is initiated in step  524 . Once the wafer exposure process is initiated, process flow moves to step  528  in which the wafer table moves at a constant velocity. Since the magneto-rheological damper effectively provides no damping while the wafer table moves at a constant velocity, there is substantially no distortion of the wafer table while the wafer table moves at the constant velocity.  
     [0051] In step  532 , the wafer exposure process is completed. A determination is then made in step  536  regarding whether the scan is completed. If it is determined that the scan is not completed, then process flow returns to step  504  in which an acceleration of the wafer table is initiated. Alternatively, if it is determined in step  536  that the scan is completed, then process  500  ends.  
     [0052] As discussed above, magneto-rheological dampers, e.g., mirror deflection dampers, may be used to damp out vibrations which may cause mirrors to deflect or bend. Magneto-rheological dampers may also be suitable for damping vibrational modes, as for example double cantilever vibration modes or common table vibration modes, of a wafer table that are relatively common to lithography systems. A double cantilever vibration mode is often excited when a wafer table accelerates. FIG. 6 is a diagrammatic side-view representation of the deflection of a wafer table due to a double cantilever vibration mode. A wafer table  610  which is situated over a coarse stage  604  may be deflected to a deflected position  610 ′ as a result of a double cantilever vibration mode. Typically, the largest displacement or deflection in deflected position  610 ′ is near a center portion of wafer table  610 .  
     [0053] A common table vibration mode is generally an elastic body mode. FIG. 7 is a diagrammatic representation of the deflection of a wafer table due to a common table vibration mode. A wafer table  710 , when subjected to a common table vibration mode, deflects to a deflected position  710 ′ such that opposite corners  722 ,  726  deflect substantially together. That is, corners  722  deflect upwards to deflected positions  722 ′, while corners  726  deflect downward to deflected positions  726 ′.  
     [0054] The various deflections of a wafer table that are caused by vibrations may be compensated for by placing magneto-rheological dampers on the wafer table. For example, a magneto-rheological damper which may function as a table vibration damper may be positioned on a wafer table in a spot where the relative deflection of the wafer table between the two ends of the damper is the greatest. Such magneto-rheological dampers may be used to provide damping whenever it is anticipated that the wafer table is likely to deflect. Specifically, magneto-rheological table dampers may be controlled in real-time to adjust the amount of damping provided to a wafer table in order to provide resistance to vibrations and, hence, deflections.  
     [0055]FIG. 8 a  is a diagrammatic representation of a wafer table which includes magneto-rheological mirror dampers and a magneto-rheological table damper in accordance with an embodiment of the present invention. A wafer table  810  may include magneto-rheological mirror dampers  820  which damp vibrations between wafer table  810  and interferometer, or long, mirrors  814 . A magneto-rheological table damper  860  may be positioned on either a top surface or a bottom surface of wafer table  810 , as appropriate, to damp vibration modes that are likely to be experienced by wafer table  810 . In other words, the placement of table damper  860  is generally determined by the vibration modes associated with wafer table  810 . By way of example, if wafer table  810  is likely to be subjected to a double cantilever vibration mode which is generally characterized by a relatively large displacement of a center of wafer table  810 , then table damper  860  may be positioned substantially such that the ends of table damper  860  are placed at a location on wafer table  810  where the relative deflection between the ends is substantially greatest, as shown in FIG. 8 b . Alternatively, if wafer table  810  may be subjected to both a double cantilever vibration mode or a common table vibration mode, multiple table dampers  860  may be positioned on wafer table  810 , as shown in FIG. 8 c.    
     [0056] In general, although table damper  860  of FIG. 8 a  may be positioned substantially anywhere on a surface of wafer table  810 , table damper  860  is typically positioned such that the two ends of table damper  860  are placed at a location of wafer table  810  where the relative deflection between the two ends is most likely to be relatively large. By positioning table damper  860  near or at a location which is most likely to be subjected to deflection, table damper  860  may substantially counteract the deflection by damping out vibrations which effectively give rise to the deflection. Similarly, table dampers  860  of FIG. 8 c  may each be positioned in locations on wafer table  810  that may be expected to have the largest relative displacements.  
     [0057] With reference to FIG. 9, a photolithography apparatus which may include magneto-rheological mirror dampers, as well as at least one magneto-rheological table damper, 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 El-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 . Heat generated during the movement of wafer positioning stage  52  may be stored by a detachable heat sink (not shown) that is coupled to wafer positioning stage  52 .  
     [0058] 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.  
     [0059] An illumination system  42  is supported by a frame  72 . Frame  72  is supported to the ground 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  44  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 the ground through isolators  54 . Reticle stage  44  is supported on a reticle stage frame  48  and may be supported on the ground 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.  
     [0060] 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 (magneto-rheological damper or table damper described in the previous embodiments of the present invention) 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 . This invention may be embodied in reticle stage  44  in addition to the wafer table of wafer positioning stage  52 . In this case, the magneto-rheological damper may be positioned between the reticle fine stage and the interferometer mirror for second interferometer  58 . Also, the magneto-rheological table damper may be positioned on the fine stage of reticle stage  44 .  
     [0061] 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.  
     [0062] 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, i.e., at a substantially constant velocity of approximately zero meters per second. 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 .  
     [0063] 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. Further, an adjustable force damper may also be applied to a proximity photolithography system that exposes a mask pattern by closely locating a mask and a substrate without the use of a lens assembly. Additionally, an adjustable force damper may be used in other devices including, but not limited to, other semiconductor processing equipment, machine tools, metal cutting machines, and inspection machines.  
     [0064] 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 F 2 -type laser (157 rn). 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 (LaB 6 ) 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.  
     [0065] 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.  
     [0066] 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.  
     [0067] 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.  
     [0068] 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.  
     [0069] 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.  
     [0070] 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.  
     [0071] A photolithography system according to the above-described embodiments, e.g., a photolithography apparatus which may include one or more detachable heat sinks, 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.  
     [0072] Further, semiconductor devices may be fabricated using systems described above, as will be discussed with reference to FIG. 10. 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. 11. 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 .  
     [0073]FIG. 11 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  313 , 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.  
     [0074] 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.  
     [0075] 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.  
     [0076] 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, magneto-rheological dampers have been described as being suitable for use in providing variable damping properties between a wafer table and an interferometer, or long, mirror of an overall stage device. It should be appreciated, however, that substantially any suitable damper which may be arranged to provide at least some damping during an acceleration portion of a scan and to provide negligible or no damping during an exposure portion of the scan may be implemented in lieu of a magneto-rheological damper. That is, substantially any sort of controllable or non-passive damper, or a damper which includes a controllable fluid, may be used. Other controllable fluids which may be used in a damper include, but are not limited to, electro-rheological fluids or ferrofluids. Ferrofluids, as will be understood to those skilled in the art, are generally suspensions of iron particles which are smaller in size than the particles which are typically included in magneto-rheological fluids.  
     [0077] Controllable or variable dampers other than magneto-rheological dampers may also be used to compensate for displacements of a wafer table that may occur as a result of a vibrational mode, e.g., a double cantilever vibration mode or a common table vibration mode. In one embodiment, different types of controllable dampers may be used within a single precision instrument.  
     [0078] While an electromagnetic coil has been described as being suitable for use in creating a magnetic field around a magneto-rheological fluid, it should be appreciated that substantially any mechanism which is capable of enabling a variable magnetic field to be created in real time may be used in lieu of an electromagnetic coil. In addition, although an electromagnetic coil has been described as being positioned around a cylinder of a damper, the electromagnetic coil may be positioned substantially anywhere without departing from the spirit or the scope of the present invention. For example, a coil may be positioned within a cylinder or within a piston.  
     [0079] A magneto-rheological mirror damper may be activated substantially any time a wafer table is accelerating or decelerating. As such, in the event that the acceleration or the deceleration of the wafer table does not excite vibrations, the magneto-rheological mirror damper may still be activated, i.e., arranged to provide damping. In one embodiment, however, a magneto-rheological mirror damper may be arranged to be activated when a wafer table is accelerating or decelerating substantially only when vibrations are sensed. That is, a magneto-rheological mirror damper may be controlled such that the magneto-rheological mirror damper is substantially not activated unless an accelerating or decelerating wafer table causes vibrations to be excited.  
     [0080] In general, the steps associated with the methods of the present invention may vary widely. Steps may be added, removed, altered, and reordered. By way of example, a magneto-rheological damper may be arranged to provide damping before an acceleration of a wafer table is initiated. 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.