Patent Publication Number: US-2007097340-A1

Title: Active damper with counter mass to compensate for structural vibrations of a lithographic system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates generally to damping vibrations associated with lithographic systems. More particularly, the present invention relates to an active damper with a counter mass which provides damping to a lithographic system.  
      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 such a wafer scanning stage or a reticle scanning stage is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly.  
      A photolithography machine which is subjected to vibrations may cause an image to be inaccurately projected by the photolithography machine, and, as a result, be incorrectly aligned on a projection surface such as a semiconductor wafer surface. Many components of a photolithography machine typically have vibrations modes which are relatively lightly damped and difficult to suppress. By way of example, a lens mount system, which includes a lens base and sensor mount, of a photolithography apparatus generally has vibrations modes which, if not damped, may compromise the quality of an image projected onto a wafer surface. When a wafer is exposed during a scanning mode, any vibration or structural oscillation may affect the quality of an image formed on the wafer. Damping the vibrations present in a lens mount system generally enables a relatively high accuracy to be achieved for a reference position associated with the lens mount system.  
      Passive mass dampers are often used to damp vibrations in bodies which vibrate or have vibration modes. While the use of a passive mass damper is often effective to damp vibrations in a vibrating piece, when the vibrations change, as for example when the frequency or the magnitude of the vibrations change, a passive mass damper may no longer significantly damp the vibrations. That is, a passive mass damper is generally arranged to damp a particular range of vibrations in a body, so when the range changes, the passive mass damper may no longer be effective in damping the vibrations to which a body is subjected. As such, even when a passive mass damper is applied to a body which vibrates, e.g., a lens base or a sensor mount, the passive mass damper may not be effective in damping all vibrations to which the body is subjected. By way of example, the application of a passive mass damper to a lens mount system within a photolithographic system may not be effective to damp enough of the vibrational modes to which the lens mount system is subjected to ensure the accuracy of a photolithographic process.  
      Therefore, what is needed is a method and an apparatus that damps vibrations associated with a photolithography apparatus.  
     SUMMARY OF THE INVENTION  
      The present invention relates to an active vibration damping arrangement for a photolithographic system. According to one aspect of the present invention, an assembly that provides damping to a structure, as for example a structure of a photolithographic apparatus, that is subject to structural oscillations includes a counter mass, an active mechanism, an a controller. The active mechanism is coupled to the structure, supports the counter mass, and applies a force to the structure to counteract structural oscillations in the structure. The controller controls the force applied by the active mechanism on the structure, and utilizes information associated with movement of the structure to control the force. In one embodiment, the active mechanism is a voice coil motor (VCM) arrangement. In another embodiment, the active mechanism is a piezoelectric actuator.  
      An active damping arrangement which actively damps vibrations in a lens system of a photolithography apparatus is effective in creating a force to damp the vibrations responsive to the frequency of the vibrations. Typically, such an active damping arrangement includes an active mechanism and a counter mass. Since the active damping arrangement is adjustable, i.e., the force generated by the active damping arrangement may be varied, a wide range of vibrational frequencies may be damped. By way of example, an active damping arrangement controlled by a notch controller may be effective in absorbing energy when the lens system is vibrating at approximately its natural frequency, as well as at other frequencies. As a result, the position of an optical central axis of a lens of the lens system may be relatively stable, and issues associated with aligning a reticle and a wafer with reference to the optical central axis may be significantly reduced.  
      According to another aspect of the present invention, a photolithographic apparatus includes an optical or lens assembly, a sensor arrangement, a controller arrangement, and an active damper. The lens assembly includes an optical element such as a lens, as well as a frame that supports the optical element or lens. The sensor arrangement detects movement of the lens, and generates a first signal indicative of the movement of the lens. The controller arrangement generates a control signal based on the first signal. The active damper includes an active mechanism and a counter mass, and is coupled to the lens assembly. The active mechanism is arranged to be commanded using the control signal to apply a force to the lens assembly to counteract the structural vibrations of the lens assembly.  
      In one embodiment, the sensor arrangement includes either an accelerometer or a velocity sensor, and detects movement of the lens system. In another embodiment, the controller arrangement is a feedforward controller arrangement. In such an embodiment, the controller arrangement may include a notch filter and an amplifier.  
      According to still another aspect of the present invention, a method for counteracting structural oscillations in a body of a system using an active damping arrangement that includes a controller arrangement, a sensor, and an active damper coupled to the system involves obtaining information associated with the structural oscillations using the sensor, and providing the information from the sensor to the controller arrangement. A control signal responsive to the information provided by the sensor is generated using the controller arrangement. The method also includes commanding the active mechanism using the control signal, wherein commanding the active mechanism using the control signal includes creating a force using the active mechanism. Such a force may be applied to the system to counteract the structural oscillations in the body responsive to the control signal.  
      In one embodiment, the sensor is mounted to the body, and the information associated with the structural oscillations is information pertaining to the movement of a central axis of the body. In such an embodiment, the force may be arranged to substantially reduce the movement of the central axis of the body.  
      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  is a block diagram representation of a photolithography apparatus in accordance with an embodiment of the present invention.  
       FIG. 2  is a diagrammatic representation of an active damper which includes a counter mass and a transducer with a piezoelectric arrangement in accordance with an embodiment of the present invention.  
       FIG. 3  is a diagrammatic representation of an active damper which includes a counter mass and a VCM in accordance with an embodiment of the present invention.  
       FIG. 4  is a diagrammatic cross-sectional representation of a lens assembly which includes an active damper in accordance with an embodiment of the present invention.  
       FIG. 5   a  is a block diagram representation of a control system which is suitable for utilizing information associated with vibrations of a lens assembly to control an amount of force applied by an active damper to compensate for the vibrations in accordance with an embodiment of the present invention.  
       FIG. 5   b  is a control diagram representation of a control system, i.e., control system  500  of  FIG. 5   a , in accordance with an embodiment of the present invention.  
       FIG. 5   c  is a control diagram representation of a control system, i.e., control system  500  of  FIG. 5   a , which shows the parameters of a notch filter that is suitable for use as a controller, i.e., controller  538  of  FIG. 5   a , 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 EMBODIMENTS  
      When an apparatus such a photolithography apparatus has components which vibrate, the accuracy of photolithographic processes performed using the apparatus may be compromised. For example, the stability of a lens base and a sensor mount of a lens system within a photolithographic apparatus is crucial, as an optical center of a lens is generally a reference position used to align a wafer and a reticle. In the event that the lens base and sensor mount vibrate or undergo structural oscillation, the reference position may move, and the alignment of the wafer and the reticle may need to be adjusted such that the wafer and the reticle remain aligned with the reference position. If the wafer and the reticle are not aligned with the reference position, the integrity of a photolithographic process which uses the wafer and the reticle may suffer.  
      The utilization an active damper to damp vibrations of a lens system, e.g., a lens base and a sensor mount, allows structural oscillation or vibrations of the lens system to be effectively damped over a range of vibrational frequencies. That is, an active damper may generate suitable amounts of force in response to various vibrational frequencies. The positional stability of a central axis of the lens system may be improved if vibrations of the lens system are reduced. An active damper, which generally includes an active mechanism and a counter mass, may be adjusted such that the force produced by the active damper may vary. More force may be provided by the active damper to counteract vibratory motion of higher frequencies, while less force may be provided to counteract vibratory motion of lower frequencies. That is, the active damper may be commanded as appropriate based upon the vibrational frequency. Hence, the active damper may damp or otherwise counteract substantially any vibrations in the lens system or assembly.  
      An active damper is typically small enough not to greatly affect the dynamics associated with a structure to which the active damper is coupled and arranged to damp. However, the active damper is suitable for generating a substantially direct force to compensate for structural movements or vibrations of the object to which the active damper is coupled.  
      With reference to  FIG. 1 , the use of an active damper to provide damping to a lens assembly of a photolithographic apparatus will be described in accordance with an embodiment of the present invention. A photolithographic apparatus  200  generally includes a reticle stage  204  which supports a reticle  206  and enables reticle  206  to scan, a wafer stage  210  which supports a wafer  211  and enables wafer  211  to scan, and an optical assembly (lens assembly)  208 . As optical assembly (lens assembly)  208  is generally a relatively large structure, and may include a lens base (not shown) and a sensor mount (not shown), lens assembly  208  typically has vibrations modes which are lightly damped and relatively difficult to suppress.  
      To suppress vibrations modes of lens assembly  208 , an active damper  212  may be coupled to lens assembly  208  to provide active damping. Active damper  212  may be substantially directly mounted to lens assembly  208 . Active damper  212  may be arranged, in one embodiment, to include a counter mass that is attached to an active mechanism. Such an active mechanism may be a piezoelectric actuator, e.g., an actuator that includes a piezoelectric component, or a voice coil motor (VCM) with a spring. It should be appreciated, however, that the active mechanism in active damper  212  may be substantially any suitable active mechanism which is capable of generating force.  
      Active damper  212  is arranged to generate a substantially direct force which acts upon lens assembly  208 . Although the type of force generated by active damper  212  may vary, the force is typically an axial force. Typically, structural movement of lens assembly  208  may effectively be cancelled, i.e., vibrations in lens assembly  208  may be damped, when the force produced by the active mechanism in active damper  212  is approximately in phase with the vibrational modes associated with lens assembly. The force produced by the active mechanism may be varied as appropriate such that the force is substantially always of the correct magnitude and in the correct direction to compensate for the structural movement of lens assembly  208 .  
       FIG. 2  is a diagrammatic representation of an active damper which includes a counter mass and a transducer with a piezoelectric arrangement in accordance with an embodiment of the present invention. An active damper  312 , which is mounted on a body  308  with vibrations to be damped, includes a frame  324   b  and a piezoelectric component  324   a  which effectively form a piezoelectric arrangement  324 , and a counter mass  332  that is supported by frame  324   b . Piezoelectric arrangement  324  may be a piezoelectric actuator. Counter mass  332  is generally substantially directly mounted on frame  324   b , while frame  324   b  may be substantially directly mounted to body  308 . In one embodiment, body  308  is a lens structure of a lithographic system, although body  308  may be substantially any structure which has vibrations modes, e.g., vibration modes that are lightly damped and relatively difficult to suppress.  
      Frame  324   b  is arranged such that piezoelectric component  324   a , e.g., a piezo stack, may move within frame  324   b . As will be appreciated by those skilled in the art, a piezoelectric arrangement  324  may be a piezoelectric actuator with a piezo stack. Active damper  312  acts as a damper on body  308  by substantially producing a force that acts on body  308  using piezoelectric arrangement  324  to counteract oscillations or vibrations associated with body  308 . Counter mass  332  is generally arranged to absorb reaction forces created when piezoelectric arrangement  324  acts on body  308 .  
      Active damper  312  is such that the amount of reaction force produced by active damper  312  may effectively be controlled by controlling the amount of voltage associated with piezoelectric arrangement  324 . That is, the amount of force applied by counter mass  332  on body  308  may be controlled by piezoelectric arrangement  324 . Piezoelectric arrangement  324  or, more specifically, piezoelectric component  324   a  in cooperation with frame  324   b , is effectively the active mechanism of active damper  312 . As previously mentioned, the active mechanism of an active damper may be any suitable active mechanism. By way of example, rather than including a piezoelectric arrangement, the active mechanism may instead include a VCM.  FIG. 3  is a diagrammatic representation of an active damper which includes a counter mass and a VCM in accordance with an embodiment of the present invention. An active damper  362  includes a VCM  366  to which a counter mass  372  is coupled. VCM  366  may be substantially directly mounted to a body  358  which is subject to vibrations. In one embodiment, VCM  366  includes a spring and a magnet associated with VCM  366  may include a mechanical damper. Counter mass  372  is arranged to cooperate with VCM  336  to apply force as appropriate in order to reduce the vibrations associated with body  358 .  
      The use of an active damper on a lens assembly of a photolithography apparatus reduces vibrations in the lens assembly and, hence, enables a wafer and a reticle to more effectively track a lens of the lens assembly. In other words, by actively damping vibrations in a lens assembly, the ability of a wafer and a reticle to remain aligned with the lens of the lens assembly is enhanced. For example, when the vibrations of a lens may be dampened such that an optical center of the lens is less likely to move, the alignment of a wafer and a reticle relative to the optical center is facilitated.  
      With reference to  FIG. 4 , an optical assembly (lens assembly) that is acted upon by an active damper will be described in accordance with an embodiment of the present invention.  FIG. 4  is a diagrammatic cross-sectional representation of a lens mount assembly. A lens mount assembly  400  includes a base  404  that is arranged to support a frame  412  which supports a lens  416 . Frame  412  and base  404  may be coupled by an isolation mount  408 . An active damper  428  is positioned on frame  412  in a position near a load point  432  of lens assembly  400 . Active damper  428  typically includes an active mechanism such as a PZT actuator or a VCM, as well as a counter mass. It should be appreciated, however, that active damper  428  may take on substantially any suitable configuration.  
      Active damper  428  uses information pertaining to the vibrational movement of lens  416  to determine an amount of damping, e.g., an amount of force, to apply to lens  416  to reduce the vibrational movement. As shown, active damper  428  damps vibrations in lens  416  by damping vibrations in frame  412  which is coupled to lens  416 . A sensor  424  is mounted on lens  416  is arranged to monitor the movement of lens  416 , and may be positioned along an optical center  420  of lens  416 . Sensor  424  is arranged to obtain information that is then provided via circuitry or a computer system, or both, to active damper, as will be discussed below with respect to  FIG. 5   a . In one embodiment, sensor  424  may be an accelerometer which measures the acceleration of lens  416  when lens  416  vibrates. Sensor  424  may also be a velocity sensor which measures the velocity of lens  416 .  
      It should be appreciated that a sensor such as sensor  424  may be mounted in a variety of different locations. By way of example, a sensor may be located at or near load point  432 , or in proximity to active damper  428 . In one embodiment, multiple sensors such as sensor  424  may be located at or near load point  432  and in proximity to active damper  428 .  
      By reducing vibratory motion in lens  416 , both a wafer  436  onto which an image is to be formed and a reticle  440  which provides a pattern to be formed on wafer  436  may be more accurately positioned. As both wafer  436  and reticle  440  are typically aligned using optical center  420  as a reference point. Hence, when vibrations in lens  416  are reduced using active damper  428 , optical center  420  typically moves less and, as a result, the alignment of wafer  436  and reticle  440  relative to optical center  420  is facilitated. The need to adjust the positioning of wafer  436  and reticle  440  to substantially track optical center  420  is decreased, as reducing vibrations in lens  416  reduces the movement of optical center  420  and facilitates the accurate positioning of optical center  420 . Further, the accuracy of images projected onto wafer  436  is improved, as the position of optical center  420  is less likely to fluctuate.  
      Active damper  428  generally uses information obtained or otherwise received from sensor  424  in the generation of force. By way of example, the amount of force applied to frame  412  by an active mechanism of active damper  428  may be adjusted based upon information obtained from sensor  424 . Referring next to  FIG. 5   a , a control system which is suitable for utilizing information associated with the movement or vibrations of a lens to control an amount of force applied by an active damper to compensate for the movement or the vibration will be described in accordance with an embodiment of the present invention. A control system  500  includes an object  510  that is to be controlled. Specifically, object  510  is a vibrating body, e.g., a lens, in which vibrations or structural oscillations are to be damped. An active damper  528 , which includes an active mechanism  528   a  and a counter mass  528   b , is arranged to provide force which counteracts the vibrations of object  510 .  
      A sensor  530 , which is arranged to effectively sense motion of object  510 , is coupled to object  510 . As previously mentioned, sensor  530  may be an accelerometer which senses any acceleration of object  510 , a velocity sensor which senses the velocity of object  510 , or any other suitable sensor which effectively detects motion of object  510 . Sensor  530  generates a signal  534  which is then provided to a controller  538  which is arranged to utilize signal  534 , as well as feedforward information  542  in one embodiment, to create a control signal  546 . Feedforward information  542  may include information pertaining to the dynamics of disturbances which affect or cause the vibration of object  510 , as well as dynamics associated with sensor  530 .  
      Controller  538  may be any suitable controller, as for example a feedforward controller, which cooperates with an amplifier  550  to generate an amplified control signal  554  that commands active damper  528  to generate a force to counteract vibrations in object  510 . In one embodiment, controller  538  may include a notch filter controller. By properly adjusting the filter parameters in controller  538 , control system  500  provides effective vibration suppression for object  510 . Further, controller  538  is arranged such that control system  500  substantially reacts to vibrations only when necessary, as for example when vibrations are of a frequency that is at a natural frequency of object  510  or higher. In general, controller  538 , in concert with other components of control system  500 , essentially creates a notch effect, and enables energy at a natural frequency of object  510  to effectively be absorbed by active damper  528  to damp the vibrations of object  510 . Typically, the parameters of the notch filter may be selected to provide a high degree of vibration suppression particularly for vibration modes at approximately the natural frequency associated with object  510 .  
      Amplifier  550  is arranged to amplify control signal  546 , and to provide amplified control signal  554  to active damper  528  or, more specifically, to active mechanism  528   a . Active mechanism  528   a  then responds to amplified control signal  554 , and creates a force which is applied to object  510  to counteract vibrations of object  510 . In other words, amplified control signal  554  drives active mechanism  528   a.    
       FIG. 5   b  is a control diagram representation of a control system, i.e., control system  500  of  FIG. 5   a , in accordance with an embodiment of the present invention. A control system  500 ′ is arranged such that dynamics  528 ′ of an active mechanism, e.g., active mechanism  528   a  of  FIG. 5   a , and dynamics associated with disturbances acting on an object, e.g., object  510  of  FIG. 5   a , are used to control the force applied by the active mechanism to the object. Typically, dynamics  528 ′ of the active mechanism are associated with a piezoelectric actuator or a VCM. Dynamics  528 ′ of the active mechanism and disturbance dynamics  562  affect sensor measurement  534 , which is processed using sensor dynamics  530 ′.  
      Sensor measurement  534 , after being processed using sensor dynamics  530 ′, is provided to a filter  538 ′ which, in the described embodiment, is a notch filter. Control signal  546 , which is an output of filter  538 ′, is then provided to amplifier  550 . An output of amplifier  550 , which is amplified control signal  554 , is then processed in accordance with dynamics  528 ′ of the active mechanism to create a force that acts on the object. It should be appreciated that when filter  538 ′ is a notch filter, amplified control signal  554  may have a profile that is indicative of a notch filter.  
      With reference to  FIG. 5   c , the parameters of a notch filter that is suitable for use as a controller, i.e., controller  538  of  FIG. 5   a , will be described in accordance with an embodiment of the present invention. Within a control system  500 ″, disturbance dynamics  562 ′ may be represented as a transfer function GD(s). Similarly, dynamics  528 ″ of an active mechanism of an active damper may be represented by a transfer function GZ(s). As will be appreciated by those skilled in the art, GD(s) and GZ(s) may vary depending upon the configuration of an object and an active damper, respectively.  
      Sensor dynamics  530 ″ may be represented, in one embodiment, as an integrator 1/s. It should be appreciated, however, that sensor dynamics  530 ″ may vary widely depending upon the type of sensor used, or the physical structure of the sensor that is used. A transfer function  538 ″ associated with a controller may be represented as  
           ω   c   2         s   2     +     2   ⁢     ξ   c     ⁢     ω   c     ⁢   s     +     ω   c   2         ,       
 
 where ω c  is a frequency parameter and ξ c  is a damping parameter that are each chosen to enable control signal  546 , as well as amplified control signal  554 , to have the profile of a notch filter. Amplifier  550 ′, which has a gain K, is such that gain K is sufficient to create an amplified control signal  554  that is suitable for driving an active mechanism to suppress vibrations in an object. 
 
      Referring next to  FIG. 6 , a photolithography apparatus which may utilize an active damper with a counter mass 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 . In one embodiment, wafer positioning stage  52  may include a plurality of actuators which are coupled to a common magnet track. 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 VCMs (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 via isolators  54 . Frame  72  may be part of a lens mount system of illumination system  42 , and may be coupled to an active damper (not shown) which damps vibrations in frame  72  and, hence, illumination system  42 . 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 . In one embodiment, projection optics frame  50  is coupled to an active damper (not shown) that is arranged to apply a variable force through a load point of projection optics frame in order to compensate for vibrational modes associated with projection optics frame  50 . 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.  
      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 optics frame  50 , and detects the position of reticle stage  44  which supports a reticle  68 . Interferometer  58  also outputs position information to system controller  62 .  
      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 positioning 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, 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 .  
      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 F 2 -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 (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.  
      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.  
      The present invention may be utilized, in one embodiment, in an immersion type exposure apparatus if suitable measures are taken to accommodate a fluid. For example, PCT patent application WO 99/49504, which is incorporated herein by reference in its entirety, describes an exposure apparatus in which a liquid is supplied to a space between a substrate (wafer) and a projection lens system during an exposure process. Aspects of PCT patent application WO 99/49504 may be used to accommodate fluid relative to the present invention.  
      Further, the present invention may be utilized in an exposure apparatus that comprises two or more substrate and/or reticle stages. In such an apparatus, e.g., an apparatus with two substrate stages, one substrate stage may be used in parallel or preparatory steps while the other substrate stage is utilizes for exposing. Such a multiple stage exposure apparatus is described, for example, in Japan patent Application Disclosure No. 10-163099, as well as in Japan patent Application Disclosure No. 10-214783 and its U.S counterparts, namely U.S. Pat. No. 6,341,007, U.S. Pat. No. 6,400,441, U.S. Pat. No. 6,549,269, U.S. Pat. No. 6,590,634. Each of these Japan patent Application Disclosures and U.S. patents are incorporated herein by reference in their entireties. A multiple stage exposure apparatus is also described in Japan patent Application Disclosure No. 20000-505958 and its counterparts U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,208,407, each of which are incorporated herein by reference in their entireties.  
      The present invention may be utilized in an exposure apparatus that has a movable stage that retains a substrate (wafer) for exposure, as well as a stage having various sensors or measurement tools, as described in Japan patent Application Disclosure No. 11-135400, which is incorporated herein by reference in its entirety. In addition, the present invention may be utilized in an exposure apparatus that is operated in a vacuum environment such as an EB type exposure apparatus and an EUVL type exposure apparatus when suitable measures are incorporated to accommodate the vacuum environment for air (fluid) bearing arrangements.  
      Further, in photolithography systems, when linear motors (see U.S. Pat. No. 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, 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, e.g., a photolithography apparatus which may include an active damper which damps vibrations associated with a lens mount system of the photolithography apparatus, 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. 7 . 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. 7  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 damp 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, an active damper which includes an active mechanism and a damper may be used to suppress vibrations in structures other than a lens assembly associated with a photolithography apparatus. Other structures which may benefit from the vibration suppression afforded by an active damper of the present invention include, but are not limited to, a wafer stage assembly and a reticle stage assembly of a photolithography apparatus. For instance, the present invention may be useful in a loading system that loads a wafer or a reticle onto a wafer positioning stage (wafer table) or reticle stage, respectively, or an unloading system that unloads a wafer or a reticle from a wafer positioning stage (wafer table) or reticle stage, respectively. Further, the present invention may be utilized as a component of a precision machine such as a measuring device.  
      A sensor which is used to monitor the movement of an optical center of a lens may generally be any sensor, or any mechanism, that is able to detect vibrational movement of the lens. That is, while a sensor which monitors the movement of a lens has been described as being an accelerometer or a velocity sensor, an accelerometer and a velocity sensor are merely examples of suitable sensors. For example, a position sensor that detects the position of a lens relative to a reference position may be utilized as a sensor.  
      A controller associated with an active damper system has been described as being a notch filter. A notch filter is particularly suitable, in one embodiment, for effectively absorbing energy at or near the natural frequency of a vibrating object while suppressing vibrations at other frequencies when parameters of the notch filter are selected appropriately. However, a notch filter is just one example of a suitable controller for use in controlling or otherwise commanding an active damper. For example, one of a low pass filter, a high pass filter, and a band-pass filter, or a combination of these filters, may be utilized in a controller in lieu of a notch filter. Many other types of controller may be used in lieu of a notch filter. Additionally, the transfer function which represents a controller within a control system may vary widely 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.