Patent Publication Number: US-2009224444-A1

Title: Vibration suppression apparatus, exposure apparatus, and method of manufacturing device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vibration suppression apparatus, an exposure apparatus, and a method of manufacturing a device. 
     2. Description of the Related Art 
     Along with improvements in the precision of exposure apparatuses, a demand has arisen for techniques of vibration suppression with a higher performance in order to prevent vibration which adversely affects exposure from occurring in, for example, a structure which forms the main body of the exposure apparatus and a projection lens. To achieve this, it is being demanded to insulate as much as possible the main body of the exposure apparatus from external vibration transmitted from, for example, the base on which the exposure apparatus is installed. It is also being demanded to quickly reduce vibration which occurs upon operation of a device including a driving mechanism such as a stage device mounted in the main body of the exposure apparatus. 
     To meet these demands, an active vibration suppression apparatus is widely applied to exposure apparatuses. The active vibration suppression apparatus detects by sensors the position and vibration of a surface plate which mounts the apparatus main body, and drives an actuator, that applies a control force to the surface plate, based on the detection results. Moreover, a technique of more effectively suppressing vibration by compensating the signal from a device including a driving mechanism such as a stage device mounted on the surface plate, and feed-forwarding the compensation signal to the actuator is applied to an apparatus of this type. 
     Japanese Patent Laid-Open No. 11-294520 discloses an active vibration suppression apparatus configured to reduce or suppress vibration of a surface plate using an air spring which supports the surface plate as an air pressure actuator, and, simultaneously, using an electromagnetic linear motor arranged dynamically parallel to the air spring. The active vibration suppression apparatus detects, for example, the position or acceleration of the surface plate using a sensor, and controls each actuator based on the signal obtained by compensation calculation for the detection result. In addition, the active vibration suppression apparatus more effectively controls vibration by controlling each actuator using the signal obtained by compensating the signal from a stage device mounted on the surface plate. 
     Assume that the vibration suppression apparatus supports the surface plate or devices vulnerable to vibration, which are mounted on the surface plate. In this case, the vibration suppression apparatus often supports the surface plate by four or more support mechanisms having a vibration suppression function, depending on the design limitations of the device arrangement and the support structure of the surface plate. Conventionally, such a vibration suppression apparatus including four or more support mechanisms having a vibration suppression function controls control signals for the position or vibration of six degrees of freedom of a rigid body using four or more actuators for each of the vertical and horizontal directions. 
       FIG. 7  shows a prior art of a vibration suppression apparatus including four support mechanisms having a vibration suppression function of supporting a surface plate  1  in the vertical direction while suppressing its vibration. The apparatus shown in  FIG. 7  supports the surface plate  1  by four support mechanisms  2   a  to  2   d  having a vibration suppression function. Although not shown, each of the support mechanisms  2   a  to  2   d  having a vibration suppression function includes a spring element and damper element. If an air spring is used as the support mechanism having a vibration suppression function, a control valve which controls its internal pressure is often used as an actuator. The apparatus shown in  FIG. 7  uses an air spring as the support mechanism having a vibration suppression function and as an air pressure actuator. 
     The vibration suppression apparatus includes position detectors  3   a  to  3   c  which detect the position of the surface plate  1  with respect to a reference position, and vibration sensors  4   a  to  4   c  which detect vibration of the surface plate  1 . The detection signals from these detectors and sensors are sent to a compensation calculator  41  of a controller  40 . The compensation calculator  41  performs appropriate compensation calculation for difference signals between the position detection signals and their target values, and the vibration detection signals. The signals obtained as a result of the compensation calculation are sent to driving circuits  6   a  to  6   d  to drive the actuators of the support mechanisms  2   a  to  2   d  having a vibration suppression function. 
     In such a vibration suppression apparatus using four or more support mechanisms  2   a  to  2   d  having a vibration suppression function, and four or more actuators, the actuators for use in control may be redundant for the degrees of freedom of rigid-body motion of the apparatus, resulting in deformation of a structure such as the surface plate. To avoid this situation, the following approaches have been proposed in order to suppress deformation of the structure. 
     The first approach is to detect deformation of the structure and control to suppress it, or to control each actuator with a force balance good enough to prevent deformation of the structure. This approach is disclosed in, for example, Japanese Patent Laid-Open Nos. 7-83276 and 7-139582. An apparatus of this type extracts signals corresponding to rigid-body motion modes and some deformation modes by coordinate transformation by taking account of deformation modes which can occur depending on the support balance of four support mechanisms having a vibration suppression function, and configures a control system for each mode. By taking account of the geometrical arrangement and characteristics of the actuators which apply control forces to the surface plate, the control commands for respective modes obtained as a result of control calculation are distributed to the actuators so as to act on the surface plate. As a matter of course, it is also effective to distribute thrusts to the actuators so as not to deform the structure in calculation for distributing control forces without extracting any signals representing the deformation modes. 
     However, depending on the above-mentioned first approach, the air pressure control system cannot sufficiently prevent deformation of the structure. In many cases, a servo valve which is widely used for the air pressure control system generally exhibits input/output characteristics with hysteresis. For this reason, when the servo valve is operated with a large range, it often cannot produce a control force according to the input signal with high precision. Because the support balance of the surface plate is excessively constrained by redundant actuators, a deviation in the control force of the actuator often fluctuates the support balance, resulting in dynamic deformation of the structure. 
     The second approach is to improve the control precision of action forces by configuring a control system which feeds back the control force of each actuator to the signal input to it. More specifically, a widely known approach feeds back, for example, the control force detected by a force sensor, the control pressure of an air pressure actuator, or the driving current or current value of an electromagnetic actuator. 
     Japanese Patent Laid-Open Nos. 10-256141 and 11-141599 and the like disclose techniques in which the second approach is applied to a control suppression apparatus. The second approach reduces a deviation between a control force commanded by the input signal and a force actually produced by the actuator, thus greatly improving the followability of the control force with respect to the input signal. Hence, appropriately distributing the signals input to the actuators makes it possible to reduce any forces which trigger deformation of the structure, thus suppressing deformation of the structure. Even when the operation range of the servo valve in the air pressure control system is relatively large, deformation of the apparatus main body is expected to be suppressed. 
     The third approach is to operate a plurality of support mechanisms having a vibration suppression function by the same control force. As disclosed in Japanese Patent No. 3337906, this approach is implemented by connecting two air springs or tanks which communicate with them through a pipe in a vibration suppression apparatus including, for example, four support mechanisms having a vibration suppression function. In such an apparatus, two out of four air springs have an equal internal pressure, and the four air springs have three pressure values accordingly, leading to a constant support balance which is determined by the load of the structure and its center of gravity. This approach determines one combination of forces that achieves support balance of the load. This makes it possible to prevent the deformation state of the structure from changing free from any fluctuation in the support balance of the structure. 
     As described above, the conventional vibration suppression apparatus suppresses deformation of the structure by various approaches even when it is inevitable that the support balance of the structure is excessively constrained by redundant actuators due to, for example, the limitations of the apparatus structure. 
     However, along with the recent improvements in the precision of devices mounted on the vibration suppression apparatus, it is being demanded to further suppress deformation of the structure. A fluctuation in the support balance of the vibration suppression apparatus triggers deformation of a mounted surface plate. As a consequence, the deformation adversely affects the devices mounted on the surface plate as well, which, in turn, adversely affects the measurement performance and exposure performance, which are required to improve. 
     Under the circumstances, suppression of deformation of the structure is becoming a more serious challenge than ever before, and therefore a fluctuation in the support balance at a conventionally negligible level is becoming problematic. 
     Vibration suppression is indispensable to attaining an improvement in the precision of an exposure apparatus. This makes it necessary to suppress local vibration harmful to the exposure apparatus by increasing the rigidity of a structure which forms the exposure apparatus. However, the higher the rigidity of the structure, the heavier the structure. Because of this conflict, a design which prioritizes weight reduction of the structure is often required. This may make it impossible to sufficiently ensure the rigidity of the structure. In this manner, it becomes harder to suppress deformation of the structure when four or more support mechanisms having a vibration suppression function are used under the condition in which a sufficient rigidity of the structure cannot be ensured. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a vibration suppression apparatus which includes four or more actuators that apply forces to a structure and, at the same time, suppresses vibration of the structure while suppressing its deformation. 
     According to the first aspect of the present invention, there is provided a vibration suppression apparatus including three first actuators and at least one second actuator which are configured to support a structure by applying forces to the structure in a vertical direction and do not align themselves on an identical straight line, a detector which detects at least one of vibration and a position of the structure with respect to a reference position, and a controller which controls the forces, applied to the structure by the three first actuators, based on the output from the detector, wherein the second actuator is controlled so that a force applied to the structure by the second actuator is maintained constant. 
     According to the second aspect of the present invention, there is provided a vibration suppression apparatus including three first actuators and at least one second actuator which are configured to support a structure by applying forces to the structure in a horizontal direction and do not align themselves on an identical straight line, a detector which detects at least one of vibration and a position of the structure with respect to a reference position, and a controller which controls the forces, applied to the structure by the three first actuators, based on the output from the detector, wherein the second actuator is controlled so that a force applied to the structure by the second actuator is maintained constant. 
     According to the third aspect of the present invention, there is provided an exposure apparatus which supports, by three active vibration suppression mechanisms, a surface plate which mounts at least one of an illumination system, a projection optical system, a substrate stage, and a reticle stage, the exposure apparatus comprising a support mechanism which supports the surface plate and includes an actuator, in addition to the three active vibration suppression mechanisms, wherein the actuator is controlled so that a force applied to the surface plate by the actuator is maintained constant. 
     According to the present invention, it is possible to provide a vibration suppression apparatus which includes, for example, four or more actuators that apply forces to a structure and, at the same time, suppresses vibration of the structure while suppressing its deformation. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a view showing a vibration suppression apparatus according to the first embodiment; 
         FIG. 2  is a view showing an actuator according to the first embodiment; 
         FIG. 3  is a view showing a vibration suppression apparatus according to the second embodiment; 
         FIG. 4  is a view showing an actuator according to the third embodiment; 
         FIG. 5  is a view showing a vibration suppression apparatus according to the third embodiment; 
         FIG. 6  is a view showing a vibration suppression apparatus according to the fourth embodiment; 
         FIG. 7  is a view showing a conventional vibration apparatus; and 
         FIG. 8  is a view for explaining an exposure apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of a vibration suppression apparatus according to the present invention will be described below with reference to the accompanying drawings. The vibration suppression apparatus according to the present invention can be used for exposure apparatuses, electron microscopes, machine tools, and the like. However, a vibration suppression apparatus for use in exposure apparatuses will be exemplified in the following embodiments. 
     A first embodiment of the present invention will be explained below referring to the drawings. A vibration suppression apparatus includes support mechanisms  2  having a vibration suppression function (to be simply referred to as support mechanisms hereinafter). The support mechanisms  2  support a surface plate  1  serving as a structure which mounts devices vulnerable to vibration in the main body of an exposure apparatus, such as an illumination system, a projection optical system, various types of measuring devices, and a substrate stage (wafer stage), while suppressing vibration of the structure.  FIG. 1  does not show the illumination system, projection optical system, various types of measuring devices, substrate stage, and the like which are mounted on the surface plate  1 . 
     The support mechanism  2  includes, for example, a spring element and damper element. In a technical field which requires precise vibration suppression, an air spring is widely used for the support mechanism  2 . Especially in the field of active vibration suppression, the following arrangement is often adopted. That is, the arrangement forms an air pressure actuator by connecting a control valve such as a servo valve, which controls the internal pressure of a gas chamber (air chamber) storing air, to an air spring, and controls the position and vibration of a structure to support, using the actuator. In this case, an air pressure actuator can often be easily designed to have a spring rigidity by connecting a tank with an appropriate capacity to an air spring, and therefore often functions as the spring element of the support mechanism  2  as well. Note that the gas for use in the actuator is not particularly limited to air, and the air spring and air pressure actuator used herein are more generally interpreted as a gas spring and gas pressure actuator. A case in which air is used as an example of the gas will be explained hereinafter. 
     The support mechanism  2  according to this embodiment includes an air pressure actuator  20 . Each of support mechanisms  2   a  to  2   d  includes an air pressure actuator. As a matter of course, an air pressure actuator using, for example, an air cylinder can be used in place of an air spring. Alternatively, a spring mechanism having no function as an actuator and an air pressure actuator, electromagnetic actuator, or the like may be used together. 
     Four support mechanisms  2   a  to  2   d  which constitute actuators  20  are assumed to be provided herein, as shown in  FIG. 1 . Needless to say, the present invention incorporates an apparatus including five or more support mechanisms  2  in accordance with its design limitations. 
     The support mechanism  2  can be preferably the one as shown in  FIG. 2 .  FIG. 2  shows details of the internal arrangement of the support mechanism  2 . The support mechanism  2  includes the air pressure actuator  20 . The air pressure actuator  20  includes an air spring  21 , an air tank  22  connected to it, a control valve  24  which controls air supply/exhaust to/from the air spring  21  and air tank  22  which constitute an air chamber, and an air pipe  23 . The air pipe  23  connects the air spring  21  and air tank  22  to the control valve  24 . The control valve  24  is connected to a supply system Ps, which supplies compressed air, and an exhaust system Pr. The control valve  24  controls the internal pressures of the air spring  21  and air tank  22  by adjusting air supply/exhaust to/from them based on electrical command signals. 
     The air spring  21  also functions as a spring mechanism which constitutes the support mechanism  2 . The spring rigidity of the air spring  21  is determined by, for example, the pressure-receiving area of the air spring  21 , and the capacities and internal pressures of the air spring  21  and air tank  22 . These design values are determined in accordance with a required specification. 
     The apparatus shown in  FIG. 2  includes a pressure detector  9  which measures the internal pressures of the air spring  21  and air tank  22 . This arrangement can be employed in the embodiments to be described later as well, and is effective to improve the control precision and response characteristics of a control force produced by the air pressure actuator. 
     For the sake of descriptive convenience, the air pressure actuator  20  is assumed to act in the vertical direction in this embodiment. As a matter of course, the same effect as described hereinafter can be produced even by using an air pressure actuator  20  which acts in the horizontal direction. 
     The plurality of support mechanisms  2  and actuators  20  are mounted and fixed on a base  7  such that three sets of them do not align themselves on the same straight line. 
     Conventionally, a reticle stage which mounts a reticle serving as a circuit original and moves, and a substrate stage which mounts and precisely aligns a substrate (wafer) in an exposure apparatus are mounted on a vibration suppression apparatus. The vibration suppression apparatus disclosed in the present invention may mount these stages as well. However, along with the recent increases in the performance of stages, the stages are often directly mounted on the base  7  without using the support mechanisms  2 . 
     The vibration suppression apparatus according to this embodiment also includes a plurality of position detectors  3   a  to  3   c  which detect a displacement of the surface plate  1  with respect to a reference position, a plurality of vibration sensors  4   a  to  4   c  which detect vibration such as acceleration, and a compensation calculator  5  which performs compensation calculation for these detection signals. 
     The plurality of position detectors  3   a  to  3   c  are arranged such that their detection axes do not match each other. Likewise, the plurality of vibration sensors  4   a  to  4   c  are arranged such that their detection axes do not match each other. 
     The compensation calculator  5  is provided in a controller  50  and performs appropriate compensation calculation for a difference signal between the target position signal and position detection signal of the surface plate  1 , or its vibration detection signal. The compensation signals output from the compensation calculator  5  are sent to driving circuits  6   a  to  6   c  to drive the actuators  20  of the support mechanisms  2   a  to  2   c  in accordance with this signal. 
     First actuators driven in accordance with the compensation signals output from the compensation calculator  5  are only actuators  20  of the three support mechanisms  2   a  to  2   c.  A second actuator  20  of the support mechanism  2   d  is not driven in accordance with any signals obtained by the compensation calculator  5 . 
     The controller  50  includes a signal generator  8  which generates a constant signal representing a constant value. The signal provided by the signal generator  8  is sent to the second actuator  20  of the support mechanism  2   d  via a driving circuit  6   d . The constant value can be set to the value of the signal input to the second actuator  20  of the support mechanism  2   d  when an apparatus (for example, an exposure apparatus) which mounts a control device is provided and aligns the surface plate  1  to a desired state. 
     The operation of the vibration suppression apparatus according to this embodiment will be explained next. First, the position detectors  3   a  to  3   c  detect the position of the surface plate  1  with respect to a reference position. In addition, the vibration sensors  4   a  to  4   c  detect vibration such as acceleration of the surface plate  1 . These detection signals are sent to the compensation calculator  5 . 
     The compensation calculator  5  performs appropriate compensation calculation for these detection signals. As for the position detection signal, in general, the difference (deviation) between the position detection signal and a target value signal associated with the position of the surface plate  1  is calculated, and compensation calculation is performed for the calculated difference. To align the position of the surface plate  1  with the target position free from any deviation, compensation calculation including integral compensation, such as PI compensation or PID compensation, is typically adopted. 
     As for the vibration detection signal, compensation calculation according to the detection characteristics of the vibration sensor  4 , and the response characteristics of the actuators  20  of the three support mechanisms  2   a  to  2   c  which apply control forces to the surface plate  1  are performed. For example, if an accelerometer and air pressure actuators are applied to the vibration sensor  4  and air pressure actuators  20  of the three support mechanisms  2   a  to  2   c , respectively, gain compensation, integral compensation, or compensation as a combination thereof can be preferably used. When the actuators  20  of the three support mechanisms  2   a  to  2   c  have response frequencies sufficiently lower than the natural frequency of a mechanism system including the support mechanisms  2  and surface plate  1 , the characteristics of the actuators  20  of the three support mechanisms  2   a  to  2   c  can be approximated by integral systems. For this reason, if, for example, gain compensation is performed, it is possible to produce a control force proportional to the velocity of the surface plate  1 , thus controlling its damping characteristics. If integration compensation is adopted, it is possible to produce a control force proportional to a displacement of the surface plate  1 , thus adjusting its support rigidity. 
     The compensation calculator  5  desirably adopts the above-mentioned compensation calculation after coordinate-transforming the position or vibration detection signal into the translational and rotational modes of the surface plate  1 . The compensation signals obtained for the respective motion modes undergo calculation for distributing thrusts formulated based on the geometrical arrangement of the actuators  20  of the three support mechanisms  2   a  to  2   c,  and the resultant signals are distributed to the actuators  20  of the three support mechanisms  2   a  to  2   c.    
     The compensation signals output from the compensation calculator  5  are sent to the actuators  20  of the support mechanisms  2   a  to  2   c  via the driving circuits  6   a  to  6   c  to drive the actuators  20  of the three support mechanisms  2   a  to  2   c.    
     With the above-mentioned arrangement, the position and vibration of the surface plate  1  are controlled using the actuators  20  of the three support mechanisms  2   a  to  2   c.    
     The second actuator  20  of the support mechanism  2   d,  which is not used for the above-mentioned position and vibration control, is driven in accordance with a signal provided by the signal generator  8  which generates a constant signal. The signal generator  8  generates a signal representing a constant value in accordance with the operation conditions of the vibration suppression apparatus. This signal may be always generated or turned on/off in accordance with the operation conditions of the vibration suppression apparatus. 
     The signal output from the signal generator  8  is sent to the driving circuit  6   d  to drive the second actuator  20  of the support mechanism  2   d.  As a consequence, the control force of the second actuator  20  of the support mechanism  2   d  is maintained as a constant force according to the signal output from the signal generator  8 . 
     The rigid-body motion of the structure is based on a kinetic system defined by six degrees of freedom of motion, that is, three translational degrees of freedom and three rotational degrees of freedom. For example, the air pressure actuator  20  which acts in the vertical direction has degrees of freedom of motion defined by translation Z in the vertical direction and rotations θx and θy about the respective axes of translations X and Y in the horizontal direction perpendicular to the translation Z. In other words, because the air pressure actuator  20  has three degrees of freedom of motion, three air pressure actuators  20  can theoretically control the position and vibration of the rigid body. If three air pressure actuators  20  are adopted, there is only one combination of forces that achieves support balance of the motions Z, θx, and θy. 
     However, if there are four or more actuators  20 , there are an infinite number of combinations of the control forces of the air pressure actuators  20  for maintaining an orientation defined by the same motions Z, θx, and θy. This is because the air pressure actuators  20  are redundant. 
     Even in this case, as long as a force produced by the actuator  20  of the support mechanism  2   d  other than three actuators of the three support mechanisms  2   a  to  2   c  is maintained constant, as in the vibration suppression apparatus disclosed in the present invention, there is only one combination of the control forces of the actuators for maintaining an orientation defined by the motions Z, θx, and θy. In other words, it is possible to keep the support balance of the surface plate  1  constant. 
     With the above-mentioned arrangement of the vibration suppression apparatus, one steady combination of forces that achieves support balance of the actuators  20  is obtained. Unless the load or its center of gravity changes, there is only one combination of forces that achieves support balance of the structure. For this reason, the support balance of the structure theoretically does not fluctuate. When a force produced by the second actuator  20  of the support mechanism  2   d  which maintains the produced force constant is set appropriately, it is possible to suppress deformation of the structure to support. 
     Although control of the position and vibration of the structure has been explained assuming that the number of actuators is four, the same applies to a case in which five or more air pressure actuators  20  are provided. In other words, it is only necessary that a minimum number of actuators required to control the rigid-body motion, that is, three actuators for the vertical direction and three actuators for the horizontal direction control the position and vibration of the structure, and actuators other than them produce constant forces. 
     Also, control of the position and vibration of the surface plate  1  is not particularly limited to that described herein. For example, Japanese Patent Laid-Open No. 11-294520 exemplifies a case in which the position control is performed by the same air pressure actuator as in this embodiment, and the vibration control based on an acceleration signal is performed by an electromagnetic linear motor arranged parallel to it. Needless to say, even a vibration suppression apparatus having such a control system is expected to produce the same effect by operating actuators other than three actuators to produce constant control forces as in this embodiment. The present invention naturally incorporates such an arrangement. 
     A second embodiment of the vibration suppression apparatus according to the present invention will be explained next. The second embodiment is different from the first embodiment in a method of controlling an actuator which produces a constant force. 
       FIG. 3  is a view showing an arrangement according to the second embodiment. A vibration suppression apparatus according to the second embodiment shown in  FIG. 3  includes a pressure detector  9   d  in a second actuator  20  of a support mechanism  2   d , and controls a force produced by the second actuator  20  of the support mechanism  2   d,  using the detection signal from the pressure detector  9   d.  The pressure detector  9   d  detects the internal pressures of an air spring and air tank of the second air pressure actuator  20  of the support mechanism  2   d  in real time. 
     Also, the vibration suppression apparatus in this embodiment is different from that in the first embodiment in a method of generating a control command signal issued to the second actuator  20  of the support mechanism  2   d  which produces a constant force. Their detailed difference lies in that a pressure compensation calculator  10  which performs compensation calculation for a difference signal between the signal from a signal generator  8   b  which generates a constant signal and the detection signal from the pressure detector  9   d  is provided, and a driving circuit  6   d  receives the output from the pressure compensation calculator  10 . That is, in this embodiment, a controller  50   b  controls a control valve of the second actuator  20  of the support mechanism  2   d  based on the difference between a reference pressure and the output from the pressure detector  9   d.    
     The signal generator  8   b  plays a role of generating target values set for the internal pressures of the air spring and air tank of the second air pressure actuator  20  of the support mechanism  2   d.  To control the internal pressures of the air spring and air tank of the second actuator  20  of the support mechanism  2   d  to follow the target values set by the signal generator  8   b  free from any deviations, the pressure compensation calculator  10  can preferably use a compensation calculation mechanism which exploits compensation calculation including integral compensation, such as PI compensation or PID compensation. 
     The controller in this embodiment is different from that in the first embodiment in the above-mentioned points, so it is denoted by reference numeral  50   b  in  FIG. 3 , differently from the controller  50  in  FIG. 1 . The compensation signal output from the pressure compensation calculator  10  is sent to the driving circuit  6   d  to drive the second actuator  20  of the support mechanism  2   d.    
     With the above-mentioned arrangement, the same effect as in the first embodiment can be produced by maintaining a force produced by the second actuator  20  of the support mechanism  2   d  constant. Moreover, the vibration suppression apparatus disclosed in this embodiment is configured to detect by the pressure detector  9   d  a force produced by the second actuator  20  of the support mechanism  2   d,  and to feed back the detection signal. Hence, the vibration suppression apparatus according to this embodiment additionally has a merit of maintaining a force produced by the second actuator  20  of the support mechanism  2   d  constant even when any disturbance such as a fluctuation in the supply air pressure of the second actuator  20  of the support mechanism  2   d  occurs. 
     A third embodiment of the present invention will be explained next. A vibration suppression apparatus disclosed in this embodiment is different from those in the above-mentioned embodiments in the arrangement of an actuator for maintaining a produced force constant.  FIG. 4  shows the arrangement of a second actuator  25  disclosed in this embodiment. 
     The second actuator  25  includes the same elements, that is, an air spring  26 , air tank  27 , and air pipe  28 , as in the second actuator  20  of the support mechanism  2   d,  but does not include a control valve  24 . The second actuator  25  includes a precision air pressure regulator  11  as a mechanism which adjusts and sets the internal pressures of the air spring  26  and air tank  27 . 
     The precision air pressure regulator  11  is also called a precision pressure reducing valve and inserted between an air pressure source and the second actuator  25 . The precision air pressure regulator  11  functions to adjust the internal pressures of the air spring  26  and air tank  27  of the second actuator  25  to constant pressures set in advance. The precision air pressure regulator  11  is also simply called a pressure reducing valve, pressure reducing regulator, or pressure (air pressure) regulator. The precision air pressure regulator  11  used herein may be the one which can adjust its set pressure in accordance with an electrical command or the one the set pressure of which is adjusted by manual operation. The purpose of use of the precision air pressure regulator  11  is to maintain the internal pressures of the air spring  26  and air tank  27  constant, so even a precision air pressure regulator  11  of a manual type suffices. 
       FIG. 5  is a view showing the schematic arrangement of an apparatus disclosed in this embodiment. This embodiment is different from the first and second embodiments in that a support mechanism  2   d  includes the second actuator  25  in place of the second actuator  20  of the support mechanism  2   d , and the internal pressure of the second actuator  25  is adjusted by the precision air pressure regulator  11  connected to it. 
       FIG. 5  assumes an apparatus including a precision air pressure regulator  11  of a manual type. On this assumption, an electrical mechanism for adjusting the internal pressure of the second actuator  25  is unnecessary, that is, a pressure detector  9 , a driving circuit  6   d,  and a signal generator  8  or  8   b  for generating a constant signal are unnecessary. A controller  50   c  in this embodiment includes only a compensation calculator  5  necessary for a control operation using first actuators  20  of three support mechanisms  2   a  to  2   c  other than the support mechanism  2   d.    
     With the above-mentioned arrangement, the same effect as in the first and second embodiments can be produced by maintaining a force produced by the second actuator  25 . Moreover, the vibration suppression apparatus disclosed in this embodiment requires no electrical mechanism for adjusting the internal pressure of the second actuator  25 . This makes it possible to further simplify the apparatus arrangement. 
     A fourth embodiment of the present invention will be explained next. The first to third embodiments have been explained assuming that there are four support mechanisms  2 . 
       FIG. 6  is a view showing an example in which the vibration suppression apparatus according to the third embodiment is improved to an apparatus including five support mechanisms  2 . A vibration suppression apparatus according to the fourth embodiment includes five support mechanisms  2   a  to  2   e , and operates second actuators of the support mechanisms  2   d  and  2   e  to produce constant control forces. The apparatus shown in  FIG. 6  uses the precision air pressure regulator, which is disclosed in the third embodiment, as a mechanism which maintains forces produced by the second actuators of the support mechanisms  2   d  and  2   e  constant. 
     The apparatus in this embodiment is different from that in the third embodiment shown in  FIG. 5  in that a support mechanism  2   e  of a second actuator which produces a constant force is added. Other arrangements and operations of the apparatus in this embodiment are the same as in the apparatus shown in  FIG. 5 . 
     In this manner, even a vibration suppression apparatus including a larger number of actuators can produce the same effect as in the vibration suppression apparatuses according to the first to third embodiments. 
       FIG. 6  shows an arrangement in which a vibration sensor  4  and a control system based on the detection signal from it are omitted. As long as the vibration suppression apparatus can ensure damping characteristics good enough to stably operate, a vibration sensor  4  and a control system based on the detection signal from it may be omitted, as shown in  FIG. 6 . 
     [Exposure Apparatus] 
     An exemplary exposure apparatus to which the vibration suppression apparatus according to the present invention is applied will be explained below. The exposure apparatus includes exposure units such as an illumination system  101 , a reticle stage  102  which mounts a reticle, a projection optical system  103 , and a wafer stage  104  which mounts a wafer, as shown in  FIG. 8 . The exposure apparatus also includes a surface plate which supports the exposure units, and the vibration suppression apparatus shown in any of the first to fourth embodiments, which supports the surface plate while suppressing its vibration. The exposure apparatus projects and transfers a circuit pattern formed on a reticle onto a wafer by exposure, and may be of the step &amp; repeat projection exposure scheme or the step &amp; scan projection exposure scheme. 
     The illumination system  101  illuminates a reticle on which a circuit pattern is formed, and includes a light source unit and illumination optical system. The light source unit uses, for example, a laser as the light source. The laser can be, for example, an ArF excimer laser having a wavelength of about 193 nm, a KrF excimer laser having a wavelength of about 248 nm, or an F 2  excimer laser having a wavelength of about 153 nm. However, the type of laser is not particularly limited to an excimer laser and may be, for example, a YAG laser, and the number of lasers is also not particularly limited. When a laser is used as the light source, an optical system for shaping a collimated light beam from the laser beam source into a desired beam shape, and an optical system for converting a coherent laser beam into an incoherent laser beam are preferably used. Also, the light source which can be used for the light source unit is not particularly limited to a laser, and one or a plurality of mercury lamps or xenon lamps can be used. 
     The illumination optical system illuminates a mask and includes, for example, a lens, mirror, light integrator, and stop. 
     The projection optical system  103  can be, for example, an optical system including a plurality of lens elements alone, an optical system including a plurality of lens elements and at least one concave mirror, an optical system including a plurality of lens elements and at least one diffractive optical element, or an optical system including only mirrors. 
     The reticle stage  102  and wafer stage  104  can move by, for example, a linear motor. In the step &amp; scan projection exposure scheme, the stages  102  and  104  move synchronously. An actuator is separately provided to at least one of the wafer stage  104  and the reticle stage  102  to align the reticle pattern onto the wafer. 
     The above-described exposure apparatus can be used to manufacture micropatterned devices, for example, a semiconductor device such as a semiconductor integrated circuit, a micromachine, and a thin-film magnetic head. 
     [Method of Manufacturing Device] 
     An embodiment of a method of manufacturing a device using the above-mentioned exposure apparatus will be explained next. Devices (for example, a semiconductor integrated circuit device and liquid crystal display device) are manufactured by a step of exposing a substrate (for example, a wafer or glass plate) coated with a photosensitive agent, using the exposure apparatus according to any of the above-mentioned embodiments, a step of developing the substrate exposed in the exposing step, and other known steps (for example, etching, resist removing, dicing, bonding, and packaging steps). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2008-054067, filed Mar. 4, 2008, which is hereby incorporated by reference herein in its entirety.