Abstract:
The disclosure relates to a support structure for an optical element and an optical element module including such a support structure. The disclosure also relates to a method of supporting an optical element. The disclosure may be used in the context of photolithography processes for fabricating microelectronic devices, such as semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of, and claims priority under 35 U.S.C. §120 to, international application PCT/EP2007/057689, filed Jul. 25, 2007, which claims benefit of U.S. Provisional Application Ser. No. 60/923,000, filed Apr. 12, 2007, and Ser. No. 60/822,227, filed Jul. 25, 2006, and European patent application 06 117 813.3, filed Jul. 25, 2006. International application PCT/EP2007/0576 is incorporated by reference herein in its entirety. 
     
    
     FIELD 
       [0002]    The disclosure relates to a support structure for an optical element and an optical element module including such a support structure. The disclosure also relates to a method of supporting an optical element. The disclosure may be used in the context of photolithography processes for fabricating microelectronic devices, such as semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes. 
       BACKGROUND 
       [0003]    Semiconductor devices are undergoing miniaturization. Accordingly, it is desirable for the good performance of the optical system used in the exposure process during semiconductor fabrication. The same can hold for auxiliary systems participating in the fabrication process, such as the support structure supporting the semiconductor device, e.g. a wafer, to be manufactured. 
       SUMMARY 
       [0004]    In some embodiments, the disclosure provides an optical element module and a support to an optical element, respectively, that may be used for highly dynamic positioning applications with larger positioning ranges, such as positioning ranges of 10 mm and more. 
         [0005]    In certain embodiments, the available positioning range of an actuating support structure for an optical element is increased by a relatively simple approach while maintaining the influence of the actuation on the imaging accuracy of the optical element as low as possible. 
         [0006]    The disclosure is based, in part at least, on the understanding that a highly dynamic introduction of forces into an optical element, e.g. for a gravity compensation allowing increased positioning ranges without deteriorating the imaging accuracy or for actuating and/or deforming an optical element, may be achieved by using a negative pressure for generating a force that acts on the optical element. The force generated using the negative pressure and acting on the optical element may be used for any desired purpose. For example, such a force may be used for counteracting the gravitational force acting on the optical element to be supported or for generating a force actuating the optical element, such as, positioning and/or deforming the optical element. For example, when using the disclosure for a pressure based gravity compensation or any other purpose, due to the simple pressure control that may be achieved, the force generated using the negative pressure may be easily kept at least close to its optimum value over a virtually unlimited range of motion, e.g. over a virtually unlimited positioning range of the optical element. Since virtually no energy has to be supplied to the system in proximity of the optical element the problem of heat generation and introduction into the optical system under static load conditions is largely avoided. 
         [0007]    Furthermore, apart from the simple pressure control that may be achieved, the use of a negative pressure has the advantage that a lower mass of working medium can be conveyed when shifting part of the optical element or even the entire the optical element (e.g. during positioning the optical element). Thus, a lower inertia and lower internal friction can be dealt with leading to improved dynamic properties of the system. Furthermore, the use of the negative pressure can simply eliminate the contamination problem since there is no material transport through any eventual sealing gap of the force exerting device used towards the surroundings of the optical element. This can be particularly valid if a gaseous working medium is used. However, a liquid medium may also be used. 
         [0008]    Furthermore, the force exertion may be achieved in a simple and space saving manner by implementing a simple bellows or a simple cylinder and piston arrangement forming a negative pressure chamber wherein the negative pressure is provided by a suitable negative pressure source. The control keeping, for example, the gravity compensation force substantially equal to the gravitational force acting on the optical element during the positioning process may be a simple pressure control. It may be provided, for example, via a pressure sensor providing the actual level of negative pressure to a suitable control device adjusting the negative pressure to a given setpoint value. 
         [0009]    It will be appreciated that, positioning ranges—i.e. a travel of the optical element from one extreme position to its other extreme position—of more than 10 mm to 30 mm, even more than 50 mm may be achieved at substantially optimized gravity compensation force. This may be done within a very short interval of less than two seconds, even less than one second. 
         [0010]    In some embodiments, the disclosure provides an optical element module including an optical element and a support structure supporting the optical element. The support structure includes a force exerting device that is mechanically connected to the optical element and adapted to exert a force on the optical element when a negative pressure is acting within the force exerting device. 
         [0011]    In certain embodiments, the disclosure provides an optical element module including an optical element and a support structure supporting the optical element. The support structure includes an actuator device and a gravity compensation device. The actuator device is mechanically connected to the optical element and adapted to exert an actuation force on the optical element. The actuation force accelerates the optical element. The gravity compensation device includes a gravity compensator. The gravity compensator is mechanically connected to the optical element and adapted to exert a gravity compensation force on the optical element when a negative pressure is acting within the gravity compensator. The gravity compensation force counteracts at least a part of the gravitational force acting on the optical element. It will be appreciated here that more than one gravity compensator may be used to fully compensate the gravitational force acting on the optical element. 
         [0012]    In some embodiments, the disclosure provides an optical exposure apparatus for transferring an image of a pattern formed on a mask onto a substrate. The apparatus includes an illumination system adapted to provide light of a light path, and a mask unit located within the light path and adapted to receive the mask. The apparatus also includes a substrate unit located at an end of the light path and adapted to receive the substrate. The apparatus further includes an optical projection system located within the light path between the mask location and the substrate location and adapted to transfer an image of the pattern onto the substrate. The illumination system and/or the optical projection system includes an optical element module. The optical element module includes an optical element and a support structure supporting the optical element. The support structure includes a force exerting device that is mechanically connected to the optical element and adapted to exert a force on the optical element when a negative pressure is acting within the force exerting device. 
         [0013]    In certain embodiments, the disclosure provides an optical exposure apparatus for transferring an image of a pattern formed on a mask onto a substrate. The apparatus includes an illumination system adapted to provide light of a light path, and a mask unit located within the light path and adapted to receive the mask. The apparatus also includes a substrate unit located at an end of the light path and adapted to receive the substrate. The apparatus further includes an optical projection system located within the light path between the mask location and the substrate location and adapted to transfer an image of the pattern onto the substrate. The illumination system and/or the optical projection system includes an optical element module. 
         [0014]    In some embodiments, the disclosure provides a support structure for supporting an optical element. The support structure includes a force exerting device adapted to be mechanically connected to the optical element and to exert a force on the optical element when a negative pressure is acting within the force exerting device. 
         [0015]    In certain embodiments, the disclosure provides support structure for supporting an optical element including an actuator device and a gravity compensation device. The actuator device is adapted to be mechanically connected to the optical element and to exert an actuation force on the optical element. The actuation force accelerates the optical element. The gravity compensation device includes a gravity compensator adapted to be mechanically connected to the optical element and to exert a gravity compensation force on the optical element when a negative pressure is acting within the gravity compensator. The gravity compensation force counteracts at least a part of the gravitational force acting on the optical element. 
         [0016]    In certain embodiments, the disclosure provides a method of supporting an optical element. The method includes providing an optical element and a force exerting device and supporting the optical element. Supporting the optical element includes exerting a force on the optical element via the force exerting device, where the force is generated using a negative pressure. 
         [0017]    In some embodiments, the disclosure provides a method of supporting an optical element including providing an optical element and a gravity compensation device, exerting a gravity compensation force on the optical element via the gravity compensation device, the gravity compensation force counteracting at least a part of the gravitational force acting on the optical element. The exerting the gravity compensation force includes generating the gravity compensation force using a negative pressure. 
         [0018]    It will be appreciated in this context that more than one gravity compensator and gravity compensation force, respectively, may be used to fully compensate the gravitational force acting on the optical element. However, is also possible that the full gravity compensation of the optical element is provided by one single gravity compensator and gravity compensation force, respectively. 
         [0019]    Optionally, the above aspects of the disclosure are used in the context of microlithography applications. However, it will be appreciated that the disclosure may also be used in any other type of optical exposure process or any other type of supporting an element being either an optical or not. 
         [0020]    Further embodiments of the disclosure will become apparent from the dependent claims and the following description with reference to the appended drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic representation of an optical exposure apparatus including an optical element module with a support structure; 
           [0022]      FIG. 2  is a schematic view of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 ; 
           [0023]      FIG. 3  is a schematic view of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 ; 
           [0024]      FIG. 4  is a schematic view of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 ; 
           [0025]      FIG. 5  is a schematic view of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 ; 
           [0026]      FIG. 6  is a schematic view of a part of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 ; and 
           [0027]      FIG. 7  is a schematic view of a part of an optical element module that may be used in the optical exposure apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    An optical exposure apparatus  101  includes an illumination system  102 , a mask unit  103  holding a mask  104 , an optical projection system  105  and a substrate unit  106  holding a substrate  107  will be described with reference to  FIGS. 1 and 2 . 
         [0029]    The optical exposure apparatus is a microlithography apparatus  101  that is adapted to transfer an image of a pattern formed on the mask  104  onto the substrate  107 . To this end, the illumination system  102  illuminates the mask  104  with exposure light. The optical projection system  105  projects the image of the pattern formed on the mask  104  onto the substrate  107 , e.g. a wafer or the like. 
         [0030]    The illumination system  102  includes a light source  102 . 1  and a first optical element group  108  with a plurality of optical elements cooperating to define the beam of exposure light—schematically indicated by the double-dot-dashed contour  109  in FIG.  1 —by which the mask  104  is illuminated. The optical projection system  104  includes a second optical element group  110  with a plurality of optical elements cooperating to transfer an image of the pattern formed on the mask  104  onto the substrate  107 . 
         [0031]    The light source  102 . 1  provides light at a wavelength of 193 nm. Thus, the optical elements of the first optical element group  108  and the second optical element group  110  are refractive and or reflective optical elements, i.e. lenses, mirrors or the like. However, it will be appreciated that, in embodiments operating at different wavelengths, such as in the so called EUV range (i.e. at a wavelength between 5 nm and 20 nm, typically about 13 nm), any types of optical elements, e.g. lenses, mirrors, gratings etc. may be used alone or in an arbitrary combination. 
         [0032]    During the exposure process, the wafer  107  is temporarily supported on a wafer table  106 . 1  forming part of the substrate unit  106 . Depending on the working principle of the of the microlithography apparatus  101  (wafer stepper, wafer scanner or step-and-scan apparatus) the wafer  107  is moved at certain points in time relative to the optical projection system  105  to form a plurality of dies on the wafer  107 . Once the entire wafer has been exposed, the wafer  107  is removed from the exposure area and the next wafer is placed in the exposure area. 
         [0033]    Depending on the working principle of the microlithography apparatus  101 , when switching from one die to the next die and/or from one wafer to the next wafer, the illumination setting of the illumination system  102  has to be rapidly changed frequently. To this end, the position of an optical element in the form of a lens  108 . 1  of the first optical element group  108  has to be rapidly changed in order to achieve a high throughput of the microlithography apparatus  101 . 
         [0034]    As can be seen from  FIG. 2 , the lens  108 . 1 —shown in a highly schematic manner—forms part of an optical element module  111 . The optical element module  111  includes a support structure  112  supporting the lens  108 . 1 . The support structure  112 , in turn, includes a base structure  112 . 1 , an actuator device  113  and a force exerting device in the form of a gravity compensation device  114 . 
         [0035]    The actuator device  113  includes three actuator pairs  113 . 1  (only one of them being shown in  FIG. 1  for reasons of clarity). The actuator pairs  113 . 1  are evenly distributed at the perimeter of the lens  108 . 1 . 
         [0036]    Each actuator pair  113 . 1  includes two contactless actuators  113 . 2 , such as voice coil motors (Lorentz actuators) or the like, each mechanically connected to the base structure  112 . 1  and the lens  108 . 1 . The actuator device  113  serves to accelerate and, thus, to position the lens  108 . 1 . To this end, it exerts a corresponding actuation force on the lens  108 . 1  as will be explained in greater detail below. 
         [0037]    The gravity compensation device  114  includes three gravity compensators  114 . 1  each of them being associated to one of the actuator pairs  113 . 1 . Thus, the gravity compensators  114 . 1  as well are evenly distributed at the perimeter of the lens  108 . 1 . Each gravity compensator  114 . 1  is mechanically connected to the base structure  112 . 1  and the lens  108 . 1 . 
         [0038]    The gravity compensation device  114 , in sum, exerts a total gravity compensation force F G ct which counteracts and fully compensates the gravitational force F G  acting in the center of gravity (COG)  108 . 2  of the lens  108 . 1 . Depending on the mass distribution of the lens  108 . 1  the individual gravity compensation forces F G &amp; exerted by the respective gravity compensator on the lens  108 . 1  are chosen such that, together, they fully compensate and balance the static forces and moments acting on the lens  108 . 1 . It will be appreciated that, depending on the design of the actuators  113 . 2 , eventually, this may also include forces and/or moments resulting from the weight of certain components of the actuator device  113  mechanically connected to the lens  108 . 1 . 
         [0039]    In other words, under static load conditions, the individual gravity compensation forces F G &amp; exerted by the individual gravity compensators  114 . 1  are selected such that the sum ΣF COG  of all forces acting in the centre of gravity  108 . 2  and the sum ΣM COG  of all moments acting in the centre of gravity  108 . 2  is zero, i.e.: 
         [0000]      ΣF COG =0,  (1) 
         [0000]      ΣM COG =0.  (2) 
         [0040]    To this end, each gravity compensator  114 . 1  includes a cylinder  114 . 2  and a piston  114 . 3  slidably mounted within the cylinder  114 . 2 . A piston rod  114 . 4  guided in a suitable bush of the cylinder  114 . 2  mechanically connects the piston  114 . 3  to the lens  108 . 1 . The cylinder  114 . 2  and the piston  114 . 3  define a negative pressure chamber  114 . 5 . A negative pressure source  114 . 6  provides a suitable negative pressure NP within the negative pressure chamber  114 . 5 . 
         [0041]    This negative pressure provided by the negative pressure source  114 . 6  corresponds to a negative pressure setpoint value NP 5  which is selected such that, under static load conditions, the above equations (1) and (2) or are fulfilled, i.e. the desired individual gravity compensation force F G &amp; as outlined above is exerted via the piston rod  114 . 4  on the lens  108 . 1 . 
         [0042]    The negative pressure source  114 . 6  includes a simple pressure control which controls the negative pressure NP using the negative pressure setpoint value NP 5 . In other words, the pressure control tries to maintain the negative pressure NP within the negative pressure chamber  114 . 5  as close as possible to the negative pressure setpoint value NP S  at any time. 
         [0043]    The pressure control may be fully integrated within the negative pressure source. However, it is also possible, for example, that a suitable pressure sensor of the pressure control is provided within or close to the cylinder  114 . 2  in order to reduce the reaction time of the control. 
         [0044]    The actuator device  113  is optionally arranged to position the lens  108 . 1  in more than one degree of freedom (DOF), optionally in up to all six degrees of freedom (DOF). Depending on the positioning movement provided by the actuator device the location and/or orientation of the lens  108 . 1  may change such that the negative pressure setpoint value NP 5  has to be adjusted accordingly in order to achieve fulfillment of the above equations (1) and (2) under static load conditions for this location and/or orientation of the lens  108 . 1 . Thus, a corresponding control of the negative pressure setpoint value NP S  may be superimposed to the negative pressure control as outlined above. 
         [0045]    It will be appreciated that, in certain embodiments, the control of the negative pressure setpoint value NP 5  may be performed as a function of an operational parameter of the actuator device  113  optionally being representative of the power taken up by the actuator device  113 . This may be done in order to reduce the power consumed and, thus, the heat generated by the actuator device  113 . For example, it is possible to adjust the negative pressure setpoint value NP 5  as a function of the electrical current taken by the actuator device  113 . 
         [0046]    The control of the negative pressure setpoint value NP S  and, thus, of the negative pressure within the negative pressure chamber  114 . 5  can be provided at a low bandwidth, optionally at less than 5 Hz, such that the control does substantially not interfere with the dynamic position control of the lens  108 . 1  provided via the actuator device  113 . Thus, the current taken and, consequently, the power consumed by the actuator device  113  may be reduced, both, under static load conditions as well as even under dynamic load conditions. This leads to an overall reduction of the heat generated within the actuator device  113  and, thus, within the optical system reducing thermally induced problems such as thermally induced degradation of imaging quality. 
         [0047]    Thanks to the use of a negative pressure on the gravity compensation device  114  has very short reaction times and thus very good dynamic properties. This is due to the fact that, as already outlined above, only a rather low mass of working medium is to be conveyed within the negative pressure chamber  114 . 5 , within the negative pressure lines connecting the negative pressure chamber  114 . 5  and the negative pressure source  114 . 6  and within the components of the negative pressure source  114 . 6  when positioning the optical element  108 . 1 . Thus, a low inertia and a low internal friction on the working medium is to be dealt with leading to improved dynamic properties of the system. 
         [0048]    It will be appreciated that the negative pressure NP is provided to be negative in relation to the pressure prevailing in the atmosphere  115  outside the negative pressure chamber  114 . 5  and surrounding the lens  108 . 1 . 
         [0049]    Thus, furthermore, the use of the negative pressure NP simply eliminates the contamination problem since there is no material transport through any sealing gap, such as the gap  114 . 7  formed between the cylinder  114 . 2  and the piston  114 . 3  and the gap  114 . 8  formed between the cylinder  114 . 2  and the piston rod  114 . 4 , towards the atmosphere  115  surrounding the lens  108 . 1 . On the contrary, if any, there is only material transport from the atmosphere  115  towards the negative pressure chamber  114 . 5 . 
         [0050]    However, it will be appreciated that, in some embodiments, it may be provided that there is no material flow between the negative pressure chamber and the atmosphere surrounding it, e.g. by providing suitable seals such as highly compliant membrane seals or the like. In this case the negative pressure within the negative pressure chamber may also be only negative in relation to an atmosphere prevailing within a further pressure chamber within the cylinder and lying on the opposite side of the piston. This further pressure chamber is then also sealed from the atmosphere surrounding the lens. 
         [0051]    The lens  108 . 1  may be positioned over a range of more than 50 mm within less than 1 s. Furthermore, accelerations up to 100 m/s 2  may be achieved with lenses (or other optical elements) weighing 5 kg and more. 
         [0052]    As can be seen from  FIG. 2 , the gravity compensator  114 . 1  and the actuators  113 . 2  of the associated actuator pair  113 . 1  contact the lens  108 . 1  in a single interface  116  in such a manner that the gravity compensation force line of the individual gravity compensation force F GCI  and the actuation force line of the respective actuation force F A  intersect at the interface  116 . Thus, an advantageous three-point support is provided to the lens  108 . 1 . 
         [0053]    As can be also seen from  FIG. 2 , an end stop device  117  is associated to the respective gravity compensator  114 . 1 . The end stop device  117  is formed by a tube  117 . 1  the upper end of which faces the piston  114 . 3  while its lower end is mechanically connected to the base of structure  112 . 1  via two membrane elements  117 . 2 . In case of a failure of the negative pressure supply to the negative pressure chamber the piston  114 . 3  will move towards the upper end of the tube  117 . 1  due to the weight of the lens  108 . 1 . 
         [0054]    Once the lower face of the piston  114 . 3  engages the upper end of the tube  117 . 1  the membrane elements  117 . 2  gradually build up forces acting in the vertical direction in order to slow down and stop the movement of the lens  108 . 1 . The tube  117 . 1  and the membrane elements may also build up such forces in a horizontal plane such that movement of the lens having a horizontal component may also be slowed down and stopped. Thus, in other words, the end stop device  117  may damp the forces acting on the lens  108 . 1  in case of a failure of its support and avoid damage to the lens  108 . 1  in this case. 
         [0055]    It will be appreciated that the end stop device may be of any other suitable design in order to fulfill this task. In particular, any other resilient and/or damping support may be selected for the part engaging the piston  114 . 3 . Furthermore, it will be appreciated that the piston and/or the end stop device may have any suitable design which guarantees a proper force transmitting engagement in case of their contact upon a failure. 
         [0056]    Finally, as can be seen from  FIG. 2 , the base structure  112 . 1  also forms support for a metrology arrangement  118  capturing the relative position of the lens  108 . 1  in relation to the base structure  112 . 1 . This relative position of the lens  108 . 1  is then used to control the active positioning of the lens  108 . 1  via the actuator device  113 . 
         [0057]    It will be appreciated that the base structure  112 . 1  may be supported on a ground structure or a further base structure—not shown in FIG.  2 —in a vibration isolated manner in order to avoid introduction of vibrations into the optical system. 
         [0058]    It will be further appreciated that, in case the optical element  108 . 1  is a mirror or another optical element that is not optically used in its central area, instead of the distribution with three gravity compensation devices  114  and three actuator pairs  113 . 1  as described above, there may also be provided a single, centrally located gravity compensation device  114  and a plurality of actuators  113 . 2  associated thereto. 
         [0059]    The gravity compensator  114 . 1  is then located such that the gravity compensation force line of its gravity compensation force F G &amp; extends through the center of gravity  108 . 2  of the optical element  108 . 1 . The gravity compensation force F G &amp; then in itself fully compensates the gravitational force F G  acting on the optical element  108 . 1 . The interface  116  then it is a rigid interface that is capable of transmitting forces and moments of the optical element  108 . 1  in up to six degrees of freedom (DOF). 
         [0060]    An optical element module  21   1  which may replace the optical element module  111  in the exposure apparatus  101  of  FIG. 1  will be described with reference to  FIGS. 1 and 3 . 
         [0061]    The basic design and functionality largely correspond to  FIG. 2  such that it is here at mainly referred to the differences only. As a consequence, like or identical parts have been given the same reference number raised by 100. 
         [0062]    As can be seen from  FIG. 3 , the lens  208 . 1  is supported by a support structure  212  including a base structure  212 . 1 , an actuator device  213  and a force exerting device in the form of a gravity compensation device  214  and an interface device in the form of a support ring  216 . The lens  208 . 1  is connected to the support ring  216  via three or more leaf springs  219  evenly distributed at the perimeter of the lens  208 . 1 . 
         [0063]    The actuator device  213  includes two contactless actuators  213 . 2  similar to the ones described above. Each actuator  213 . 2  is mechanically connected to the base structure  212 . 1  and the support ring  216 . The actuator devices  213  serve to accelerate and, thus, to position the lens  208 . 1  in one degree of freedom (DOF) while suitable guide mechanisms—not shown in FIG.  3 —restrict the movement of the lens  208 . 1  in the five other degrees of freedom (DOF). The gravity compensation device  214  includes two gravity compensators  214 . 1 . Each gravity compensator  214 . 1  is mechanically connected to the base structure  212 . 1  and the lens  208 . 1 . 
         [0064]    The actuators  213 . 2  and the gravity compensators  214 . 1  are evenly distributed at the perimeter of the lens  208 . 1 . The distribution is such that the gravity compensation force lines of the individual gravity compensation forces F G &amp; exerted by the respective gravity compensator on the lens  208 . 1  lie in a common plane with the center of gravity (COG)  208 . 2  of the lens  208 . 1 . Furthermore, the distribution is such that the actuator force lines of the individual actuator forces F A  exerted by the respective actuator on the lens  208 . 1  lie in a common plane with the center of gravity (COG)  208 . 2  as well. 
         [0065]    Furthermore, the gravity compensation force lines and the actuator force lines are substantially parallel to each other and to the force line of the gravitational force F G  acting on the lens  208 . 1 . 
         [0066]    The gravity compensation device  214 , in sum, exerts a total gravity compensation force F G ct which counteracts and fully compensates the gravitational force F G  acting in the center of gravity (COG)  208 . 2  of the lens  208 . 1 . Depending on the mass distribution of the lens  208 . 1  the individual gravity compensation forces F GCI  exerted by the respective gravity compensator on the lens  208 . 1  are chosen such that, together, they fully compensate and balance the static forces and moments acting on the lens  208 . 1  and the support ring  216 , i.e. such that the equations (1) and (2) are fulfilled. It will be appreciated that, depending on the design of the actuators  213 . 2 , eventually, this may also include forces and/or moments resulting from the weight of certain components of the actuator device  213  mechanically connected to the lens  208 . 1 . 
         [0067]    Each gravity compensator  214 . 1  again includes a cylinder  214 . 2  and a piston  214 . 3  slidably mounted within the cylinder  214 . 2 . A piston rod  214 . 4  guided in a suitable bush of the cylinder  214 . 2  mechanically connects the piston  214 . 3  to the lens  208 . 1 . The cylinder  214 . 2  and the piston  214 . 3  define a negative pressure chamber  214 . 5 . Again a negative pressure source  214 . 6  provides a suitable negative pressure NP within the negative pressure chamber  214 . 5 . This negative pressure is controlled and has been explained above. 
         [0068]    Again, as can be also seen from  FIG. 3 , an end stop device  217  identical to the end stop device  117  of  FIG. 2  is associated to the respective gravity compensator  214 . 1 . 
         [0069]    It will be appreciated that the base structure  212 . 1  may be supported on a ground structure or a further base structure—not shown in FIG.  3 —in a vibration isolated manner in order to avoid introduction of vibrations into the optical system. 
         [0070]    An optical element module  311  which may replace the optical element module  111  in the exposure apparatus  101  of  FIG. 1  will be described with reference to  FIG. 4 . 
         [0071]    The basic design and functionality largely correspond to  FIG. 2  such that it is here at mainly referred to the differences only. As a consequence, like or identical parts have been given the same reference number raised by 200. 
         [0072]    As can be seen from—highly schematic— FIG. 4 , the lens  308 . 1  is supported by a support structure  312  including a base structure  312 . 1 , an actuator device  313  and a force exerting device in the form of a gravity compensation device  314 . 
         [0073]    The base structure  312 . 1  includes a first base structure part  312 . 2  on which a second base structure part  312 . 3  and a third base structure part  312 . 4  are each supported in a vibration isolated manner. While the second base structure part  312 . 3  supports the actuator device  313 , the third base structure part  312 . 4  supports the gravity compensating device  314  and the metrology arrangement  318 . This has the advantage that the gravity compensating device  314  and the metrology arrangement  318  are dynamically decoupled from actuator device  313  reducing the overall vibration disturbances introduced into the system. 
         [0074]    It will be appreciated that the gravity compensating device and the actuator device may be of any suitable design. In particular, they may be of the design as it has been described above. 
         [0075]    An optical element module  411  which may replace the optical element module  111  in the exposure apparatus  101  of  FIG. 1  will be described with reference to  FIG. 5 . 
         [0076]    The basic design and functionality largely correspond to  FIG. 2  such that it is here at mainly referred to the differences only. As a consequence, like or identical parts have been given the same reference number raised by 300. 
         [0077]    As can be seen from  FIG. 5 , the lens  408 . 1  is supported by a support structure  412  including a base structure  412 . 1 , an actuator device  413  and a force exerting device in the form of a gravity compensation device  414 . 
         [0078]    The actuator device  413  includes a plurality of contactless actuators  413 . 2  similar to the ones described above. Each actuator  413 . 2  is mechanically connected to the base structure  412 . 1  and the lens  408 . 1 . The actuator device  413  serves to accelerate and, thus, to position the lens  408 . 1 . The gravity compensation device  414  includes a plurality of gravity compensators  414 . 1 . Each gravity compensator  414 . 1  is associated to an actuator  413 . 2  and mechanically connected to the base structure  412 . 1  and the lens  408 . 1 . 
         [0079]    Each actuator  413 . 2  and its associated gravity compensator  414 . 1  form a support unit. Furthermore, the actuator  413 . 2  and its associated gravity compensator  414 . 1  are arranged such that the gravity compensation force lines and the actuator force lines are substantially collinear to each other and parallel to the force line of the gravitational force F G  acting on the lens  408 . 1 . To this end, the piston rod  414 . 4  of the gravity compensator  414 . 1  extends through a tube shaped actuator rod of the actuator  413 . 2 . By this approach, a very compact arrangement may be achieved. 
         [0080]    The actuator  413 . 2  and the associated gravity compensator  414 . 11  connected to the lens  408 . 1  and a common interface  416  located close to the neutral plane of deformation  408 . 3  of the lens  408 . 1 . Herewith an advantageous introduction of loads into the lens  408 . 1  is achieved. 
         [0081]    A suitable number of the support units formed by an actuator  413 . 2  and its associated gravity compensator  414 . 1  are evenly distributed at the perimeter of the lens  408 . 1 . The gravity compensation device  414 , in sum, exerts a total gravity compensation force F G ct which counteracts and fully compensates the gravitational force F G  acting in the center of gravity (COG)  408 . 2  of the lens  408 . 1 . Depending on the mass distribution of the lens  408 . 1  the individual gravity compensation forces F G &amp; exerted by the respective gravity compensator on the lens  408 . 1  are chosen such that, together, they fully compensate and balance the static forces and moments acting on the lens  408 . 1 , i.e. such that the equations (1) and (2) are fulfilled. It will be appreciated that, depending on the design of the actuators  413 . 2 , eventually, this may also include forces and/or moments resulting from the weight of certain components of the actuator device  413  mechanically connected to the lens  408 . 1 . 
         [0082]    Each gravity compensator  414 . 1  again includes a cylinder  414 . 2  and a piston  414 . 3  slidably mounted within the cylinder  414 . 2 . A piston rod  414 . 4  guided in a suitable bush of the cylinder  414 . 2  mechanically connects the piston  414 . 3  to the lens  408 . 1 . The cylinder  414 . 2  and the piston  414 . 3  define a negative pressure chamber  414 . 5 . Again a negative pressure source  414 . 6  provides a suitable negative pressure NP within the negative pressure chamber  414 . 5 . This negative pressure is controlled and has been explained above. 
         [0083]    Again, as can be also seen from  FIG. 3 , an end stop device  417  identical to the end stop device  117  of  FIG. 2  is associated to the respective gravity compensator  414 . 1 . 
         [0084]    An optical element module  511  which may replace the optical element module  111  in the exposure apparatus  101  of  FIG. 1  will be described with reference to  FIGS. 1 and 6 . 
         [0085]    As can be seen from  FIG. 6 , a lens  508 . 1 —shown in a highly schematic manner—forms part of the optical element module  511 . The optical element module  511  includes a support structure  512  supporting the lens  508 . 1 . The support structure  512 , in turn, includes a base structure  512 . 1 , a support device  520  and a force exerting device  514 . 
         [0086]    The support device  520  includes four passive support elements  520 . 1  (only one of them being shown in  FIG. 6 ). However, it will be appreciated that, in some embodiments, any other number of support elements may be chosen as long as, together with other components of the support structure, a defined and stable support to the optical element is achieved. 
         [0087]    Each of the support elements  520 . 1  is mechanically connected to the base structure  512 . 1  and to the outer perimeter of the lens  508 . 1 . The support elements  520 . 1  may be connected to the lens  508 . 1  by any suitable mechanism. For example, the support elements  520 . 1  may be clamped to the outer perimeter of the lens  508 . 1 . However, it will be appreciated that, in some embodiments, the connection between the support elements and the lens may be of any other suitable type, e.g. a frictional connection, a positive connection, an adhesive connection or any combination thereof. 
         [0088]    Furthermore, the respective support elements  520 . 1  may provide mechanical decoupling in the radial direction of the lens  508 . 1  in order to allow compensation of thermally induced position alterations between the lens  508 . 1  and the base structure  512 . 1 . Suitable mechanism(s) for providing such mechanical decoupling in the radial direction are all well-known in the art, e.g. from U.S. Pat. No. 4,733,945 (Bacich), the entire disclosure of which is incorporated herein by reference, such that this will not be explained here in further detail. 
         [0089]    The support elements  520 . 1  are evenly distributed at the perimeter of the lens  508 . 1 , i.e. mutually rotated by 90° about the optical axis  508 . 3  (not shown at its real location in  FIG. 6 ) of the lens  508 . 1 , in order to provide even support to the lens  508 . 1 . 
         [0090]    The force exerting device  514  includes four force exerting units  514 . 1  mechanically connected to the base structure  512 . 1  and the lens  508 . 1  (only one of them being shown in  FIG. 6 ). However, it will be appreciated that, in some embodiments, any other number of force exerting units may be chosen as long as, eventually together with one or more support elements as they have been described above, a defined and stable support to the optical element is achieved. 
         [0091]    The force exerting units  514 . 1  are evenly distributed at the outer perimeter of the lens  508 . 1  (i.e. mutually rotated by 90° about the optical axis  508 . 3  of the lens  508 . 1 ). Furthermore, the first locations where each force exerting unit  514 . 1  contacts in the lens  508 . 1 , in the peripheral direction of the lens  508 . 1 , is located substantially halfway between the two second locations where two neighboring support elements  520 . 1  contact the lens  508 . 1 . Thus, an even distribution of the components of the support structure  512  contacting the lens  508 . 1  is achieved. 
         [0092]    Each force exerting unit  514 . 1  includes a force exerting element in the form of a bellows  514 . 9  and a lever  514 . 10 . The lever  514 . 10 , at a first end  514 . 11 , is connected by suitable connection mechanism  522  (shown in highly schematic way in  FIG. 6 ) to a first location at the outer perimeter of the lens  508 . 1 . For example, the lever  514 . 10  may be clamped via the connection mechanism  522  to the outer perimeter of the lens  508 . 1 . However, it will be appreciated that, in certain embodiments, the connection mechanism may provide any other suitable connection between the lens and the lever, e.g. a frictional connection, a positive connection, an adhesive connection or any combination thereof. 
         [0093]    At its second end  514 . 12 , the lever  514 . 10  is mechanically connected to a first end  514 . 13  of the bellows  514 . 9 . The second end  514 . 14  of the bellows  514 . 9 , in turn, is mechanically connected to the base structure  512 . 1 . 
         [0094]    Between its first end  514 . 11  and its second end  514 . 12  the lever  514 . 10  is articulated via a hinge  514 . 15 , e.g. via a flexure, to the base structure  512 . 1 . The articulation via the hinge  514 . 15  is such that the lever  514 . 10  is pivotable about a pivot axis extending substantially tangential to the peripheral direction of the lens  508 . 1 . Depending on the distance between the flexure  514 . 13  and the location of connection to the lens  508 . 1  and the bellows  514 . 9 , respectively, a desired ratio of motion and/or force transmission may be achieved between the bellows  514 . 9  and the lens  508 . 1 . 
         [0095]    Similar to the support element  520 . 1 , the connection mechanism  522  may provide mechanical decoupling in the radial direction of the lens  508 . 1 . To this end, the connection mechanism  522  may, for example, include a flexure or a leaf spring element or any other spring element providing the radial decoupling function. Furthermore, as an alternative or in addition, the connection mechanism  522  may also provide a radial guide function. 
         [0096]    On the one hand, this allows compensation of thermally induced position alterations between the lens  508 . 1  and the base structure  512 . 1 . On the other hand, this radially flexible configuration allows for a mutual tilt between the lens  508 . 1  and the lever  514 . 10 , thus reducing the introduction of bending moments (about an axis tangential to the peripheral direction of the lens  508 . 1 ) when the lever  514 . 10  is pivoted about the hinge  514 . 15 . Such bending moments otherwise might, for example, promote undesired loads to the connection between the connection mechanism  522  and the lens  508 . 1 . 
         [0097]    The bellows  514 . 9 , along a line of action  514 . 16  (substantially parallel to the optical axis  508 . 3 ), exerts a bellows force F 6 , on the lever  514 . 10 . In turn, via the lever  514 . 10 , each force exerting unit  514 . 1  exerts a desired deformation force F DI  on the lens  508 . 1  which is also directed substantially parallel to the optical axis  508 . 3  of the lens  508 . 1  (or the optical axis  508 . 3  of an optical system including the lens  508 . 1  if the lens  508 . 1  is a plane parallel plate). 
         [0098]    Depending on the shape and, thus, the mass distribution of the lens  508 . 1  and the forces exerted by the support elements  520 . 1 , the individual deformation force F DI  exerted by the respective force exerting unit  514 . 1  on the lens  508 . 1  is chosen such that, together, they provide a desired deformation of the lens  508 . 1 . In other words, via the deformation forces F DI  the first locations where the force exerting units  514 . 1  contact the lens  508 . 1  are displaced parallel to the optical axis  508 . 3  with respect to the second locations where of the support elements  520 . 1  contact the lens  508 . 1  leading to the desired deformation on the lens  508 . 1 . 
         [0099]    Such a deformation of the lens  508 . 1  may for example be used in a generally well-known manner for at least partly compensating imaging errors inherent to and/or introduced into the optical system of the optical exposure apparatus  101 . It will be appreciated that, in some embodiments, depending on the deformation of the optical element to be achieved, any other suitable number and/or distribution of support elements and/or force exerting units may be chosen. 
         [0100]    In particular, passive support elements may be even omitted and the support to the optical element may be provided exclusively via force exerting units. Under these circumstances, the deformation forces introduced into the optical element may also account for a shift in the position of an optical reference of the optical element (e.g. the focal point of the optical element) associated therewith. In other words, it is even possible to achieve a desired position of such an optical reference of the optical element (e.g. keep this position unchanged) while at the same time providing a desired deformation of the optical element. 
         [0101]    To provide the deformation forces F DI , the respective bellows  514 . 9  defines a negative pressure chamber  514 . 5 . A negative pressure source  514 . 6  provides a suitable negative pressure NP within a gaseous working medium provided to the negative pressure chamber  514 . 5 . This negative pressure provided by the negative pressure source  514 . 6  corresponds to a negative pressure setpoint value NP S  which is selected such that, under static load conditions, the desired individual deformation force F DI  as outlined above is exerted via the force exerting unit  514 . 1  on the lens  508 . 1 . 
         [0102]    The negative pressure source  514 . 6  includes a simple pressure control which controls the negative pressure NP using the negative pressure setpoint value NP S . In other words, the pressure control tries to maintain the negative pressure NP within the negative pressure chamber  514 . 5  as close as possible to the negative pressure setpoint value NP S  at any time. 
         [0103]    It will be appreciated that the pressure provided within the pressure chambers  514 . 5  may be the same for all the force exerting units  514 . 1  (e.g. by providing the pressure via a common pressure line). Optionally, the pressure source  514 . 6  is adapted to provide different individual pressure values (e.g. via a separate pressure lines) within selected ones of the pressure chambers  514 . 5 . 
         [0104]    The pressure control may be fully integrated within the pressure source  514 . 6 . However, it is also possible, for example, that a suitable pressure sensor of the pressure control is provided within or close to the bellows  514 . 9  (as it is indicated in  FIG. 6  by the dashed contour  521 ) in order to reduce the reaction time of the control. 
         [0105]    The pressure source  514 . 6  optionally (but not necessarily) is also arranged to act as a positive pressure source providing a positive pressure to pressure chamber  514 . 5  of the bellows  514 . 9 . By this approach it is possible to exert the above deformation force F DI  as a first force on the lens  508 . 1  when a negative pressure NP prevails within the pressure chamber of  514 . 5  and to exert an opposite bellows force—F BI  and, thus, an opposite deformation force—F DI  as a second force on the lens  508 . 1  when a positive pressure PP prevails within the pressure chamber of  514 . 5  (then being a positive pressure chamber). 
         [0106]    By this approach, it is possible to achieve a wide range of deformation of the lens  508 . 1 . In particular, deformation in both directions from a neutral state of the lens  508 . 1  with no deformation forces introduced via the force exerting units  514 . 1  may be achieved using one single bellows  514 . 9  per location of deformation. Furthermore, it is possible to actively reverse the deformation of the lens  508 . 1  using one single bellows  514 . 9  per location of deformation. 
         [0107]    The control of the pressure within the pressure chamber  514 . 5  may be provided by the pressure source  514 . 6  at any desired bandwidth depending on the desired dynamic properties of the deformation of the lens  508 . 1  to be achieved. 
         [0108]    Thanks to the use of a negative pressure the force exerting device  514  has very short reaction times and thus very good dynamic properties. This is due to the fact that, as already outlined above, only a rather low mass of working medium is to be conveyed within the negative pressure chamber  514 . 5 , within the negative pressure lines connecting the pressure chamber  514 . 5  and the pressure source  514 . 6  and within the components of the pressure source  514 . 6  when acting on the lens  508 . 1 . Thus, a low inertia and a low internal friction on the working medium is to be dealt with leading to improved dynamic properties of the system. 
         [0109]    It will be appreciated that the negative pressure NP is provided to be negative in relation to the pressure prevailing in the atmosphere  515  outside the negative pressure chamber  514 . 5  and surrounding the lens  508 . 1 . Optionally a negative pressure of down to −0.8 bar (e.g., down to −0.7 bar) is chosen. If the pressure source is also used as a positive pressure source providing a positive pressure PP (the pressure being positive in relation to the pressure prevailing in the atmosphere  515  outside the pressure chamber  514 . 5  and surrounding the lens  508 . 1 ), a positive pressure of up to +0.5 bar (e.g., up to +0.7 bar) is chosen. 
         [0110]    Furthermore, the use of the negative pressure NP simply eliminates potential contamination problems since there is no material transport through leakage points of the pneumatic system towards the atmosphere  515  surrounding the lens  508 . 1 . On the contrary, if any, there is only material transport from the atmosphere  515  towards the negative pressure chamber  514 . 5 . 
         [0111]    However, it will be appreciated that, in certain embodiments, it may be provided that there is no material flow between the negative pressure chamber and the atmosphere surrounding it, e.g. by providing suitable seals such as highly compliant membrane seals or the like. In this case the negative pressure within the negative pressure chamber may also be only negative in relation to an atmosphere prevailing within a further pressure chamber within the cylinder and lying on the opposite side of the piston. This further pressure chamber is then also sealed from the atmosphere surrounding the lens. 
         [0112]    The geometry of the lens  508 . 1  may be changed within a wide range within a very short time in the range of down to a few milliseconds (e.g., 200 ms, 20 ms, 2 ms). 
         [0113]    Finally, as can be seen from  FIG. 6 , the base structure  512 . 1  also forms support for a metrology arrangement  518  capturing the deformation and relative position of the lens  508 . 1  in relation to the base structure  512 . 1 . The information on the deformation of the lens  508 . 1  is provided to the pressure source  514 . 6  and used for the control of the pressure provided by the pressure source  514 . 6 . The information on the relative position of the lens  508 . 1  may be used to control an eventual active positioning of the lens  508 . 1 . Such a position control may for example be provided by an actuating device positioning the base structure  512 . 1 . 
         [0114]    It will be appreciated that the base structure  512 . 1  may be supported on a ground structure or a further base structure—not shown in FIG.  6 —in a vibration isolated manner in order to avoid introduction of vibrations into the optical system. 
         [0115]    It will be further appreciated that, in case the optical element  508 . 1  is a mirror or another optical element that is not optically used in its central area, instead of the distribution with a plurality of force exerting units  514 . 1  that the outer perimeter of the optical element as described above, there may also be provided a single, centrally located force exerting device  514 . 
         [0116]    Furthermore, it will be appreciated that, in some embodiments, any other orientation in space of the force exerting device and/or of the force exerted by the force exerting device on the optical element may be chosen. For example, the force exerted on the optical element may have at least a force component in a radial and/or tangential direction of the optical element. 
         [0117]    Furthermore, any other suitable design of the force exerting device and force exerting units may be chosen. For example, the force exerting unit may simply consist of the bellows acting directly on the optical element (i.e. without any further transmission mechanism located in between). It will be also appreciated that a cylinder and piston configuration may be chosen instead of the bellows to define the pressure chamber. 
         [0118]    In particular, as it is shown in  FIG. 7 , a cylinder and piston configuration defining two pressure chambers (e.g. on both sides of the piston) may be chosen.  FIG. 7  shows the optical element module  508  of  FIG. 6  in a configuration where the bellows  509  is replaced by such an arrangement with a cylinder  614 . 2  and a piston  614 . 3 . 
         [0119]    The piston  614 . 3  is slidably mounted within the cylinder  614 . 2 . A piston rod  614 . 4  guided in a suitable bush of the cylinder  614 . 2  mechanically connects the piston  614 . 3  to the lever  514 . 10 . The cylinder  614 . 2  and the piston  614 . 3  define two negative pressure chambers, a first negative pressure chamber  614 . 5  and a second negative pressure chamber  614 . 17 . Apart from that, the cylinder  614 . 2  and the piston  614 . 3  largely correspond to the above description. 
         [0120]    The negative pressure source  514 . 6  then provides a suitable first negative pressure NP 1  within the first negative pressure chamber  614 . 5 . and a second negative pressure NP 2  within the second negative pressure chamber  614 . 17 . In other words, in this case, the negative pressure source  514 . 6  is adapted to independently control the negative pressure level within the first negative pressure chamber  614 . 5  and the negative pressure level within the second negative pressure chamber  614 . 17  according to the desired direction and amount of the force F DI  to be exerted on the lens  508 . 1 . 
         [0121]    By this approach it is possible to provide force exertion in opposite directions using exclusively negative pressures in both pressure chambers, i.e. without the need for providing a positive pressure as it has been described above in the context of the bellows  514 . 9 . 
         [0122]    Finally, it may be provided that the force exerting device does not act directly on the optical element but on a deformable holding structure (e.g. a deformable holding ring or the like) to which the optical element is connected. 
         [0123]    In the foregoing, the disclosure has been described in the context of operating at a wavelength of 193 nm mainly with refractive optical elements. However, it will be appreciated that, in some embodiments working at different wavelengths, in particular also in the EUV range, the use of other types of optical elements (e.g. mirrors, gratings) is possible as well. 
         [0124]    Furthermore, the disclosure has been described in the context of contactless actuator devices such as voice coil motors (Lorentz actuators). However, it will be appreciated that, in some embodiments, it is also possible to apply the disclosure in a configuration where any other type of actuator is used for adjusting the position of the respective optical element. 
         [0125]    Furthermore, the disclosure has been described in the context of adjusting the position of an optical element in a rather large positioning range which is achievable under satisfying dynamic conditions thanks to the use of the negative pressure. However, it will be appreciated that, with smaller positioning ranges as they are often desired for the position adjustment of optical elements in the optical projection system, it is also possible to realize the geometric configurations described above with mechanical and/or magnetic gravity compensators as they have been described initially. 
         [0126]    Furthermore, the disclosure has been described in the context of adjusting the position of an optical element of an illumination system. However, it will be appreciated that, in some embodiments, it is also possible to apply the disclosure to an optical element of the optical projection system or any other part of an optical exposure apparatus. 
         [0127]    In the foregoing, the disclosure has been described only in the context of microlithography applications. However, it will be appreciated that the disclosure may be used in the context of any other imaging process.