Abstract:
Actuator devices, as well as related systems and methods, are disclosed. In some embodiments, the devices, systems and methods are within the field of microlithography.

Description:
FIELD 
       [0001]    The disclosure relates to actuator devices, as well as related systems and methods. In some embodiments, the devices, systems and methods are within the field of microlithography. 
       BACKGROUND 
       [0002]    Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element modules including optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units. 
         [0003]    With such optical systems, typically, such optical element units are often built from a stack of optical element modules holding one or more—typically but not necessarily rotationally symmetric—optical elements. These optical element modules usually include an external generally ring shaped support structure supporting one or more optical element holders each, in turn, holding one or more optical elements. 
       SUMMARY 
       [0004]    In one aspect, the disclosure generally features an actuator device that includes an actuation unit that defines an actuator axis and a direction of actuation along the actuator axis. The actuation unit is adapted to receive an actuation energy defining an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section which, in a neutral state of the actuation unit, has a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation. The actuation unit also includes a second section which, in the neutral state of the actuation unit, has a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation. The actuation unit further includes a linking section that links the first and second sections. The linking section, in the neutral state of the actuation unit, has a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation. The third axial rigidity is smaller than at least one of the first and second axial rigidities and/or the third transverse rigidity is smaller than at least one of the first and second transverse rigidities. 
         [0005]    In another aspect, the disclosure generally features an optical element unit that includes an optical element and a support structure supporting the optical element. The support structure includes at least one actuator device connected to the optical element and configured to exert an actuation force onto the optical element along a direction of actuation. The at least one actuator device includes an actuation unit defining an actuator axis and a direction of actuation along the actuator axis. The actuation unit is adapted to receive an actuation energy defining an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section which, in a neutral state of the actuation unit, has a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation. The actuation unit also includes a second section which, in the neutral state, has a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation. The actuation unit further includes a linking section that links the first and second sections. The linking section, in the neutral state, has a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation. The third axial rigidity is smaller than at least one of the first axial rigidity and the second axial rigidity, and/or the third transverse rigidity is smaller than at least one of the first transverse rigidity and the second transverse rigidity. 
         [0006]    In a further aspect, the disclosure generally features an actuator device that includes an actuation chamber defining an actuator axis and a direction of actuation along the actuator axis. The actuation chamber is adapted to receive an actuation fluid having an actuation pressure. The actuation pressure defines an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element. The at least one wall element includes a first wall section and a second wall section. At least one of the first and second wall sections is at least partially shaped in the manner of a bellows. The second wall section, at least in a neutral state of the actuator device, is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber. 
         [0007]    In an additional aspect, the disclosure generally features an optical element unit that includes an optical element and a support structure supporting the optical element. The support structure includes at least one actuator device connected to the optical element and configured to exert an actuation force onto the optical element along a direction of actuation. The at least one actuator device includes an actuation chamber defining an actuator axis and the direction of actuation along the actuator axis. The actuation chamber is configured to receive an actuation fluid having an actuation pressure. The actuation pressure defines an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element. The at least one wall element includes a first wall section and a second wall section. At least one of the first wall section and the second wall section is at least in part shaped in the manner of a bellows. The second wall section, at least in a neutral state of the actuator device, is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber. 
         [0008]    In one aspect, the disclosure generally features a method that includes providing a component and a support structure supporting the component. The support structure includes at least one actuator device connected to the component. The at least one actuator device includes an actuation unit. The actuation unit defines an actuator axis and a direction of actuation along the actuator axis. The actuation unit includes a first section, a linking section and a second section. The linking section links the first section and the second section. The first section, in a neutral state of the actuation unit, has a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation. The second section, in the neutral state, has a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation. The linking section, in the neutral state, has a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation. The third axial rigidity is smaller than at least one of the first axial rigidity and the second axial rigidity, and/or the third transverse rigidity is smaller than at least one of the first transverse rigidity and the second transverse rigidity. The method also includes providing an actuation energy to the actuation unit, the actuation energy defining an actuation force exerted by the actuator device along a direction of actuation along the actuator axis onto the component. 
         [0009]    In another aspect, the disclosure generally features a method that includes providing a component and a support structure supporting the component. The support structure includes at least one actuator device connected to the component. The at least one actuator device includes an actuation chamber. The actuation chamber defines an actuator axis and being at least partially confined by at least one wall element. The at least one wall element includes a first wall section and a second wall section. At least one of the first wall section and the second wall section is at least in part shaped in the manner of a bellows. The second wall section, at least in a neutral state of the actuator device, is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber. The method also includes providing an actuation fluid to the actuation chamber. The actuation fluid has an actuation pressure. The actuation pressure defines an actuation force exerted by the actuator device along a direction of actuation along the actuator axis onto the component. 
         [0010]    In a further aspect, the disclosure features a method that includes providing and connecting a first wall section of a wall element and a second wall section of the wall element. At least one of the first and second wall sections is at least in part shaped in the manner of a bellows. The first wall section and the second wall section are connected such that the second wall section is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of an actuation chamber at least partially confined by the wall element. 
         [0011]    In an additional aspect, the disclosure generally features an actuator device that includes an actuation unit defining an actuator axis and a direction of actuation along the actuator axis. In a neutral state of the actuator device, it has an actuation unit length along the actuator axis. The actuation unit is adapted to receive an actuation energy. The actuation energy defines an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section, a second section and a linking section that links the first and second section. The linking section, at least in the neutral state, is folded back along the actuator axis such that the actuation unit has an effective length along the actuator axis that corresponds at least to 140% of the actuation unit length. 
         [0012]    In one aspect, the disclosure generally features an actuator device that includes an actuation chamber defining an actuator axis and a direction of actuation along the actuator axis and, in a neutral state of the actuator device, has a chamber length along the actuator axis. The actuation chamber is adapted to receive an actuation fluid having an actuation pressure. The actuation pressure defines an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element. The at least one wall element includes a first wall section and a second wall section. The second wall section, at least in the neutral state, is arranged in a nested manner within the first wall section such that the at least one wall element has an effective length along the actuator axis that corresponds at least to 120% of the chamber length. 
         [0013]    The present disclosure can provide an actuator device with relatively high flexibility in the adjustment of the axial and/or transverse rigidity of the actuator device at given geometric boundary conditions. 
         [0014]    The present disclosure can provide an actuator device having a relatively high range of displacement even at narrow geometric boundary conditions in its fully retracted state while at the same time keeping the axial and/or transverse rigidity of the actuator device low over a the entire range of displacement. 
         [0015]    The present disclosure can provide a relatively simple to use and/or design optical device used in an exposure process for at least maintaining the imaging accuracy during operation of the optical device. 
         [0016]    In some embodiments, a higher flexibility in the adjustment of the axial and/or transverse rigidity the actuator device at given geometric boundary conditions may be achieved by increasing the effective length of the actuation unit along the actuator axis. It is possible to adjust the axial and/or transverse rigidity of the actuator device by simply modifying the effective length of the actuation unit. An increase in the effective length may be achieved by folding back a linking section of the actuation unit linking a first and second section of the actuation unit along the actuator axis such that the first and second section of the actuation unit overlap along the actuator axis. 
         [0017]    If a fluidic actuation principle can be used, a higher flexibility in the adjustment of the axial and/or transverse rigidity the actuator device at given geometric boundary conditions may be achieved by increasing the effective length along the actuator axis of the at least one wall element confining the actuation chamber. It is possible to adjust the axial and/or transverse rigidity of the actuator device by simply modifying the effective length of the at least one wall element confining the actuation chamber. Furthermore, an increase in this effective length provides an increased range of displacement at a lower variation of the axial and/or transverse rigidity of the actuator device over the range of displacement since, due to the increased effective length, at a given range of displacement, the corrugations of the at least one wall element can be less straightened than in the known actuator devices leading to (if any) a lower increase in the axial and/or transverse rigidity of the actuator device. 
         [0018]    At given geometric boundary conditions in the fully retracted state of the actuator device, it is possible to achieve a higher range of displacement while at the same time keeping the axial and/or transverse rigidity of the actuator device low. Thus, due to this low axial and/or transverse rigidity lower parasitic loads can be introduced into the optical system leading to a lower amount of imaging errors resulting from such parasitic loads introduced into the optical system. 
         [0019]    An increase in the effective length of the actuation unit can be achieved by folding back at least one part of the actuator into itself such that at least one section of the actuator is arranged in a nested manner within another section of the actuator. 
         [0020]    If a fluidic actuation principle is used, the increase in the effective length of the at least one wall element confining the actuation chamber can be achieved by folding back the at least one wall element into itself such that at least one wall section of the at least one wall element is arranged in a nested manner within another wall section of the at least one wall element, both sections then defining a ring shaped section of the actuation chamber. 
         [0021]    In some embodiments, an actuator device includes an actuation unit, the actuation unit defining an actuator axis and a direction of actuation along the actuator axis. The actuation unit is adapted to receive an actuation energy defining an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section, a linking section and a second section, the linking section linking the first section and the second section. The first section, in a neutral state of the actuation unit, has a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation. The second section, in the neutral state, has a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation. The linking section, in the neutral state, has a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation. At least one of the third axial rigidity is smaller than at least one of the first axial rigidity and the second axial rigidity, and the third transverse rigidity is smaller than at least one of the first transverse rigidity and the second transverse rigidity. 
         [0022]    In certain embodiments, an optical element unit includes an optical element and a support structure supporting the optical element, the support structure including at least one actuator device connected to the optical element and exerting an actuation force onto the optical element along a direction of actuation. The at least one actuator device includes an actuation unit defining an actuator axis and a direction of actuation along the actuator axis, the actuation unit being adapted to receive an actuation energy defining an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section, a linking section and a second section, the linking section linking the first section and the second section. The first section, in a neutral state of the actuation unit, has a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation. The second section, in the neutral state, has a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation. The linking section, in the neutral state, has a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation. At least one of the third axial rigidity is smaller than at least one of the first axial rigidity and the second axial rigidity, and the third transverse rigidity is smaller than at least one of the first transverse rigidity and the second transverse rigidity. 
         [0023]    In some embodiments, an actuator device includes an actuation chamber. The actuation chamber defines an actuator axis and a direction of actuation along the actuator axis. The actuation chamber is adapted to receive an actuation fluid having an actuation pressure, the actuation pressure defining an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element, the at least one wall element including a first wall section and a second wall section, at least one of the first wall section and the second wall section being at least partially shaped in the manner of a bellows. The second wall section, at least in a neutral state of the actuator device, is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber. 
         [0024]    In certain embodiments, an optical element unit includes an optical element and a support structure supporting the optical element. The support structure includes at least one actuator device connected to the optical element and exerting an actuation force onto the optical element along a direction of actuation. The at least one actuator of device includes an actuation chamber, the actuation chamber defining an actuator axis and the direction of actuation along the actuator axis. The actuation chamber receives an actuation fluid having an actuation pressure, the actuation pressure defining an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element, the at least one wall element including a first wall section and a second wall section, at least one of the first wall section and the second wall section at least in part being shaped in the manner of a bellows. The second wall section, at least in a neutral state of the actuator device, is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber. 
         [0025]    In some embodiments, a method of exerting a force onto a component of an optical device includes, in a first step, providing the component and a support structure supporting the component, the support structure including at least one actuator device connected to the component, the at least one actuator device including an actuation unit, the actuation unit defining an actuator axis and a direction of actuation along the actuator axis, the actuation unit including a first section, a linking section and a second section, the linking section linking the first section and the second section; the first section, in a neutral state of the actuation unit, having a first axial rigidity along the direction of actuation and a first transverse rigidity transverse to the direction of actuation; the second section, in the neutral state, having a second axial rigidity along the direction of actuation and a second transverse rigidity transverse to the direction of actuation; and the linking section, in the neutral state, having a third axial rigidity along the direction of actuation and a third transverse rigidity transverse to the direction of actuation; at least one of the third axial rigidity being smaller than at least one of the first axial rigidity and the second axial rigidity, and the third transverse rigidity being smaller than at least one of the first transverse rigidity and the second transverse rigidity. The method further includes, in a second step, providing an actuation energy to the actuation unit, the actuation energy defining an actuation force exerted by the actuator device along a direction of actuation along the actuator axis onto the component. 
         [0026]    In certain embodiments, a method of exerting a force onto a component of an optical device includes, in a first step, providing the component and a support structure supporting the component, the support structure including at least one actuator device connected to the component, the at least one actuator device including an actuation chamber, the actuation chamber defining an actuator axis and being at least partially confined by at least one wall element, the at least one wall element including a first wall section and a second wall section, at least one of the first wall section and the second wall section at least in part being shaped in the manner of a bellows, and the second wall section, at least in a neutral state of the actuator device, being arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of the actuation chamber, and, in a second step, providing an actuation fluid to the actuation chamber, the actuation fluid having an actuation pressure, the actuation pressure defining an actuation force exerted by the actuator device along a direction of actuation along the actuator axis onto the component. 
         [0027]    In some embodiments, a method of manufacturing a wall element of an actuator device includes providing and connecting a first wall section of the wall element and a second wall section of the wall element, at least one of the first wall section and the second wall section at least in part being shaped in the manner of a bellows, the first wall section and the second wall section being connected such that the second wall section is arranged in a nested manner within the first wall section so as to define a ring shaped chamber section of an actuation chamber at least partially confined by the wall element. 
         [0028]    In certain embodiments, an actuator device includes an actuation unit, the actuation unit defining an actuator axis and a direction of actuation along the actuator axis and, in a neutral state of the actuator device, having an actuation unit length along the actuator axis. The actuation unit is adapted to receive an actuation energy, the actuation energy defining an actuation force exerted by the actuator device along the direction of actuation. The actuation unit includes a first section, a second section and a linking section linking the first and second section, the linking section, at least in the neutral state, being folded back along the actuator axis such that the actuation unit has an effective length along the actuator axis that corresponds at least to 140% of the actuation unit length. 
         [0029]    In some embodiments, an actuator device includes an actuation chamber, the actuation chamber defining an actuator axis and a direction of actuation along the actuator axis and, in a neutral state of the actuator device, having a chamber length along the actuator axis. The actuation chamber is adapted to receive an actuation fluid having an actuation pressure, the actuation pressure defining an actuation force exerted by the actuator device along the direction of actuation. The actuation chamber is at least partially confined by at least one wall element, the at least one wall element including a first wall section and a second wall section. The second wall section, at least in the neutral state, is arranged in a nested manner within the first wall section such that the at least one wall element has an effective length along the actuator axis that is at least 120% of the chamber length. 
         [0030]    Further aspects and embodiments of the disclosure will become apparent from the figures, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a schematic representation of an optical imaging arrangement which includes an optical element unit. 
           [0032]      FIG. 2  is a schematic sectional representation of an optical element unit being a part of the optical imaging arrangement of  FIG. 1 . 
           [0033]      FIG. 3  is a schematic sectional view of the detail III of the optical element unit of  FIG. 2  in a neutral state. 
           [0034]      FIG. 4  is a block diagram of a method of exerting a force on a component of the optical element unit of  FIG. 2 . 
           [0035]      FIG. 5  is a block diagram of a method of manufacturing the actuator device of  FIG. 3 . 
           [0036]      FIG. 6  is a schematic sectional view of an actuator device in a neutral state. 
           [0037]      FIG. 7  is a schematic sectional view of an actuator device in a neutral state. 
           [0038]      FIG. 8  is a schematic sectional view of an actuator device in a neutral state. 
           [0039]      FIG. 9  is a schematic sectional view of an actuator device in a neutral state. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    In the following, an optical imaging arrangement  101  according to the disclosure will be described with reference to  FIGS. 1 to 5 . 
         [0041]      FIG. 1  is a schematic and not-to-scale representation of the optical imaging arrangement in the form of an optical exposure apparatus  101  used in a microlithography process during manufacture of semiconductor devices. The optical exposure apparatus  101  includes a first optical device in the form of an illumination unit  102  and a second optical device in the form of an optical projection unit  103  adapted to transfer, in an exposure process, an image of a pattern formed on a mask  104 . 1  of a mask unit  104  onto a substrate  105 . 1  of a substrate unit  105 . To this end, the illumination unit  102  illuminates the mask  104 . 1  with exposure light at a wavelength of 193 nm. The optical projection unit  103  receives the exposure light coming from the mask  104 . 1  and projects the image of the pattern formed on the mask  104 . 1  onto the substrate  105 . 1 , e.g. a wafer or the like. 
         [0042]    The illumination unit  102  includes a light source (not shown) and an optical element system  106  including a plurality of optical element units such as optical element unit  106 . 1 . The optical projection unit  103  includes a further optical element system  107  including a plurality of optical element units  107 . 1 . The optical element units of the optical element systems  106  and  107  are aligned along an (eventually folded) optical axis  101 . 1  of the optical exposure apparatus  101  and may include any type of optical element, such as lenses, mirrors, gratings or the like. 
         [0043]    The optical element system  107  is held by a stack of optical element units in the form of optical element modules including an optical element module  107 . 1 . As may be seen from  FIG. 2 , the optical element unit  107 . 1  includes a first optical element in the form of a lens  108  held by a support structure  109 . The support structure  109  includes a ring shaped lens holder  109 . 1  contacting the lens  108 . The lens holder  109 . 1  in turn is supported by a ring shaped support device  109 . 2  which in turn is connected to the housing  103 . 1  of the optical projection unit  103 . In particular, the lens holder  109 . 1  is suspended via four holding elements  109 . 3  to an upper ring shaped support element  109 . 4  of the support device  109 . 2 . The holding elements  109 . 3 , in the state shown in  FIG. 2 , take the gravitational force G acting on the assembly formed by the lens  108  and the lens holder of  109 . 1 . 
         [0044]    The holding elements  109 . 3  are evenly distributed at the circumference of the lens holder  109 . 1 . and define the position of the assembly formed by the lens  108  and the lens holder of  109 . 1 . To this end, the holding elements  109 . 3  may be simple passive elements. However, it is also possible that the holding elements  109 . 3  are active control elements actively adjusting the position of the assembly formed by the lens  108  and the lens holder of  109 . 1  under the control of a suitable control device connected thereto. 
         [0045]    Furthermore, four identical fluidic actuator devices in the form of actuators  110  are evenly distributed at the circumference of the lens holder  109 . 1 . However, it will be appreciated that, any other desired and suitable number of actuators may be chosen. The holding elements  109 . 3  and the actuators  110  are alternately and evenly distributed at the circumference of the lens holder  109 . 1  such that each holding element  109 . 3  is rotated by an angle of 45° (about the optical axis  101 . 1 ) with respect to an adjacent actuator  110 . 
         [0046]    As can be seen best from  FIG. 3 , the actuator  110 , at a first end, is supported on a lower ring shaped support element  109 . 5  of the support device  109 . 2 . Furthermore, the actuator  110 , at a second end, contacts the lower surface of the lens holder  109 . 1  in order to be able to exert an actuation force F onto the lens holder  109 . 1  along an actuator axis  110 . 1  defining a direction of actuation  110 . 2 . 
         [0047]    The actuator  110  includes a monolithic thin walled wall element  110 . 3  confining an actuation chamber  110 . 4  in a gas tight manner. The wall element  110 . 3 , among others, includes an outer first wall section  110 . 5  and an inner second wall section  110 . 6 . 
         [0048]    The actuation chamber  110 . 4  may be filled with a pressurized actuation fluid such that a certain actuation pressure prevails within the actuation chamber  110 . 4 . In  FIG. 3 , the actuator  110  is shown in a neutral state where the actuation pressure p c  within the actuation chamber  110 . 4  substantially corresponds to the pressure p a  in the atmosphere surrounding the actuator  110  such that the gage pressure p g , i.e. the pressure difference between the surrounding atmosphere and the actuation pressure within the actuation chamber  110 . 4 , is substantially zero, whereby the gage pressure p g  calculates as: 
         [0000]        p   g   =p   c   −p   a.   (1) 
         [0049]    The first wall section  110 . 5  is of generally cylindrical design defining a cylinder axis that is collinear with the actuator axis  110 . 1 . Thus, the first wall section  110 . 5 , in a plane perpendicular to the actuator axis  110 . 1 , has a generally circular cross-section. The first wall section  110 . 5 , along the actuator axis  110 . 1 , has a first length L 1  substantially corresponding to the length LC of the actuation chamber of  110 . 4  along the actuator axis  110 . 1 . 
         [0050]    Over its entire length L 1 , the first wall section  110 . 5  has a corrugated design, i.e. a plurality of corrugations  110 . 7 , such that, in other words, the first wall section  110 . 5 , over its entire length L 1 , is shaped in the manner of a bellows. However, it will be appreciated that the corrugated design may also be limited to a certain fraction of the length L 1  of the first wall section while the remaining part of the first wall section may be of straight design. 
         [0051]    The second wall section  110 . 6  is also of generally cylindrical design defining a cylinder axis that is collinear with the actuator axis  110 . 1  and, consequently, collinear with the axis of the first wall section  110 . 5 . Thus, the second wall section  110 . 6 , in a plane perpendicular to the actuator axis  110 . 1 , also has a generally circular cross-section. The second wall section  110 . 6 , along the actuator axis  110 . 1 , in the neutral state shown, has a second length L 2  along the actuator axis  110 . 1  corresponding to more than 50%, namely roughly 66%, of the first length L 1  of the first wall section  110 . 5 . 
         [0052]    The second wall section  110 . 6 , along the actuator axis  110 . 1 , also has a corrugated design, i.e. a plurality of corrugations  110 . 8 , such that, in other words, the second wall section  110 . 6  as well is shaped in the manner of a bellows. However, it will be appreciated that the corrugated design may also be limited to a certain fraction of the length L 2  of the second wall section while the remaining part of the second wall section may be of straight design. 
         [0053]    It will be appreciated that the number and/or the geometry on the corrugations  110 . 7  and  110 . 8  may be selected as a function of a desired axial rigidity of the actuator  110  along the actuator axis  110 . 1  and/or of a desired transverse rigidity of the actuator  110  transverse to the actuator axis  110 . 1 . While in the embodiment shown continuously curved corrugations are a used it will be appreciated that, other types of corrugated designs may be used. In particular, designs having a zigzagging shape (in a sectional plane including the actuator axis) may also be used. 
         [0054]    It will be further appreciated that, the first and/or second wall section do not necessarily have to be of generally cylindrical shape. It is also possible that either one of the first and second wall section is of generally prismatic shape, i.e. has a polygonal cross-section in a plane perpendicular to the actuator axis. Furthermore, it will be appreciated that the axis defined by the first wall section and the axis defined by the second wall section do not necessarily have to be collinear. In particular, if (apart from the actuation force F along the actuator axis) a further actuation force or moment in a different direction is desired, such as non-collinear design may be chosen. 
         [0055]    As can be seen from  FIG. 3 , the second wall section  110 . 6  is folded back into the first wall section  110 . 5 . Thus, the second wall section  110 . 6  is arranged in a nested manner within the first wall section  110 . 5  such that a ring shaped chamber section  110 . 9  of the actuation chamber  110 . 4  is confined by the first wall section  110 . 5  and the second wall section  110 . 6 . The ring shaped chamber section  110 . 9  also defines an axis that is collinear with the actuator axis  110 . 1 . 
         [0056]    At its inner (here lower) end located within the first wall section  110 . 5  the second wall section  110 . 6  is monolithically connected to a third wall section  110 . 10  of the wall element  110 . The third wall section  110 . 10  substantially extends in a plane perpendicular to the actuator axis  110 . 1 . The third wall section  110 . 10  forms a mounting base for a first end of a force transmitting device in the form of a push-rod  110 . 11 . The rod  110 . 11  extends along the actuator axis  110 . 1  and, at its second end, is connected to the lens holder in  109 . 1 . 
         [0057]    It will be appreciated that, the actuation chamber and may be confined by a plurality of separate wall elements connected in a suitable fluid tight manner. In particular, the separate wall elements may be of different design, different wall thickness distribution, different material etc. 
         [0058]    In its neutral state, the actuator  110  is in a retracted state. If the actuation pressure within the actuation chamber  110 . 4  is raised above the pressure within the atmosphere surrounding the actuator of  110 , i.e. if the gage pressure p g  is raised to a positive value, the corrugations  110 . 7  of the first wall section  110 . 5  are straightened while the corrugations  110 . 8  of the second wall section  110 . 6  are further compressed due to the now existing positive pressure difference between the actuation pressure within the actuation chamber  110 . 4  and the pressure within the atmosphere surrounding the actuator of  110 . Thus, the actuator  110  extends and, via the rod  110 . 11 , exerts a positive force F onto the lens holder  109 . 1 . Furthermore, if the actuation pressure within the actuation chamber  110 . 4  is lowered below the pressure within the atmosphere surrounding the actuator of  110 , i.e. if the gage pressure p g  is lowered to a negative value, the corrugations  110 . 7  of the first wall section  110 . 5  are further compressed while the corrugations  110 . 8  of the second wall section  110 . 6  are straightened due to the now existing negative pressure difference between the actuation pressure within the actuation chamber  110 . 4  and the pressure within the atmosphere surrounding the actuator of  110 . Thus, the actuator  110  further retracts and, via the rod  110 . 11 , exerts a negative force F onto the lens holder  109 . 1 . 
         [0059]    This actuation force F causes a corresponding (positive or negative) shift of the part of the lens holder  109 . 1  immediately adjacent to the actuator  110  and, consequently, of the part of the lens  108  immediately adjacent to the actuator  110  along the optical axis  101 . 1 . On the contrary, the holding elements  109 . 3  may maintain their length along the optical axis  101 . 1  such that the respective part of the lens holder  109 . 1  immediately adjacent to the respective holding element  109 . 3  and, consequently, of the part of the lens  108  immediately adjacent to the respective holding element  109 . 3  maintains its position along the optical axis  101 . 1 . 
         [0060]    Thus, via the respective actuation pressure within the four actuators  110 , a certain deformation of the lens  108  (i.e. an alteration within the geometry of the lens  108 ), may be achieved while the position of the lens  108  (more precisely, the position of e.g. the optical center or another suitable reference point of the lens  108 ) in space remains unchanged. 
         [0061]    A so called two-wave deformation may be introduced into the lens  108  by operating two diagonally opposite actuators  110  at a suitable positive gage pressure and the other two diagonally opposite actuators  110  at a suitable negative gage pressure. Furthermore, a so called four-wave deformation may be introduced into the lens  108  by operating all four actuators  110  at a suitable positive (or negative) gage pressure. It will be appreciated that depending on the number of actuators and holding elements used, any other desired n-wave deformation may be introduced into the lens  108 . 
         [0062]    It will be further appreciated that the actuation force F provided by the actuator  110  (typically acting in combination with further actuators and acting in a more direct manner on the lens) may also be used for altering the position of the lens  108  in the z-direction as well as the tilting angle of the lens  108  about the x- and y-axis may be adjusted. The maximum displacement of the lens  108  in the z-direction is defined by the range of displacement of the actuator  110 , i.e. the achievable maximum extension of the actuator  110  along the actuator axis  110 . 1 . 
         [0063]    The folded back or nested design of the wall element  110 . 3  has the advantage that, at given geometric boundary conditions for the actuator  110  in the neutral state, an increase in the effective length LE of the wall element  110 . 3  along the actuator axis  110 . 1  may be achieved with respect to known non-nested actuator designs where the effective length of the wall element confining the actuation chamber typically corresponds to the length of the actuation chamber. 
         [0064]    In particular, the effective length LE calculates as the sum of the first length L 1  of the first wall section  110 . 5  and the second length L 2  of the second wall section  110 . 6 , i.e. as: 
         [0000]        LE=L 1+ L 2.  (2) 
         [0065]    Since, the first length L 1  substantially corresponds to the length LC of the actuation chamber  110 . 4  and the second length L 2  corresponds to about 66% of the first length L 1 , the effective length LE of the wall element  110 . 3  corresponds to about 166% of the length LC of the actuation chamber  110 . 4 . 
         [0066]    This increase in the effective length LE of the wall element  110 . 3  has the advantage that, on the one hand, it is possible to adjust the axial and/or transverse rigidity of the actuator  110  by simply modifying the second length L 2  of the second wall section  110 . 6 . In particular, while the transverse rigidity is predominantly influenced by the additional effective length provided via the nested second wall section  110 . 6 , the axial rigidity is mainly influenced by the additional corrugations available via the nested second wall section  110 . 6 . It will be appreciated that the modification of the second length L 2  does not affect the length LC of the actuation chamber  110 . 4  such that it is feasible within given geometric boundary conditions. 
         [0067]    It will be further appreciated that the axial and transverse rigidity of the actuator  110  may also be adjusted by the material and/or the geometry and/or the thickness distribution of the first wall section  110 . 5  and in the second wall section  110 . 6 . Thus, a broad range of desired axial rigidities and transverse rigidities of the actuator device  110  may be achieved. 
         [0068]    Furthermore, an increase in this effective length LE provides an increased range of displacement at a lower variation of the axial and/or transverse rigidity of the actuator  110  over the range of displacement since, thanks to the increased effective length LE, at a given range of displacement, the corrugations of the first wall section  110 . 5  are less straightened than in the known actuator devices leading to (if any) a lower increase in the axial and/or transverse rigidity of the actuator  110 . 
         [0069]    Furthermore, at given geometric boundary conditions in the fully retracted state of the actuator  110 , it is possible to achieve a higher range of displacement while at the same time keeping the axial and/or transverse rigidity of the actuator  110  low. Thus, thanks to this low axial and/or transverse rigidity lower parasitic loads are introduced into the lens holder  109 . 1  and, consequently, into the lens  108  leading to a lower amount of imaging errors resulting from such parasitic loads introduced into the optical system. 
         [0070]    The axial rigidity and the transverse rigidity of the actuator  110 , in the neutral state shown, correspond to less than 75% of the axial rigidity and transverse rigidity, respectively, of a reference actuator in a reference neutral state corresponding to this neutral state. The reference actuator has a reference actuation chamber that is confined by a reference wall element. The reference wall element is made of the same material as the first wall section  110 . 5 , has a reference length along the actuator axis  110 . 1  that corresponds to the length LC of the actuation chamber  110 . 4  and has a reference geometry that corresponds to the geometry of the first wall section  110 . 5 . In other words, the reference wall element corresponds to the wall element  110 . 3  without the nested second and third wall sections  110 . 3  and  110 . 10 . 
         [0071]    It will be appreciated that, the effective length LE may have any other desired value. The effective length LE of the wall element  110 . 3  can correspond to at least 120% of the length LC of the actuation chamber. In particular, with a single-nested design as shown in  FIGS. 2 and 3 , where a negative gage pressure is used, the effective length LE may be selected to be between 120% and 150% of the length LC of the actuation chamber in order to provide a sufficient range of displacement. Typically displacements up to 1 mm can be readily achieved. 
         [0072]    Typically, with a single-nested design as shown in  FIGS. 2 and 3 , where negative and positive gage pressures are used (to provide an alternating deformation of the lens  108 ), the effective length LE may be selected to range from 130% to 170% (e.g., from 145% to 155%) of the length LC of the actuation chamber in order to provide a sufficient range of displacement at advantageously low axial and transverse rigidity. In case only positive gage pressure is to be employed for actuation in one direction only, the effective length may be increased to more than 170%, e.g. even up to 190%. 
         [0073]    It will be further appreciated that the effective length LE may even be pushed beyond 200% of the length LC of the actuation chamber by choosing a multiple-nested design where, for example, a further wall section can be arranged in a nested manner within the second wall section in an analogous manner (i.e. the second wall section and the further wall section defining a further ring shaped chamber section of the actuation chamber). 
         [0074]    Consequently, the axial rigidity and the transverse rigidity of the actuator, in the neutral state, may even correspond to less than 50% and even down to 25% of the axial rigidity and transverse rigidity, respectively, of a reference actuator as outlined above. 
         [0075]    During exposure of the pattern formed on the mask  104 . 1  onto the substrate  105 . 1  the deformation (i.e. the geometry) of the lens  108  is actively controlled, among others, via the actuators  110  under the control of a control device  111 . The active deformation control of the deformation of the lens  108  is performed as a function of certain operating factors of the exposure apparatus  101 . Furthermore, the position (i.e. location and orientation) of the lens  108  may be actively controlled, among others, via the holding elements  109 . 3  under the control of the control device  111  (in this case also connected to the holding elements  109 . 3 ). The active position control of the position of the lens  108  may also be performed as a function of certain operating factors of the exposure apparatus  101 . 
         [0076]    The actual value of these operating factors is captured by a capturing device  111 . 1  of the control device  111  and provided to a control unit  111 . 2  of the control device  111 . The control unit  111 . 2 , in response to the actual value of the respective operating factor provided by the capturing device  111 . 1 , generates corresponding control signals. The control unit  111 . 2  provides these control signals to a pressure source  110 . 12  of the respective actuator  110 . 
         [0077]    The pressure source  110 . 12  is connected to the actuation chamber  110 . 4  and provides an actuation fluid, here an actuation gas, to the actuation chamber of  110 . 4  at a pressure that has been adjusted as a function of the control signals provided by the control unit  111 . 2 . It will be appreciated that, instead of a gaseous medium, a liquid medium may also be used as the actuation fluid. 
         [0078]    The operating factors used for the active position control of the lens  108  may be any operating factor that may be influenced by the active position control of the lens  108 . Typically, this operating factor is an imaging error of the optical system that may be counteracted by the active deformation and/or position control of the lens  108 . 
         [0079]    With the optical exposure apparatus  101  of  FIG. 1  a method of exerting a force on a component of an optical device may be executed as it will be described in the following with reference to  FIG. 1 to 4 . 
         [0080]    In a step  112 . 1 , the components of the optical exposure apparatus  101 , in particular and the lens  108  and the support structure  109 , as they have been described above are provided and put into a spatial relation to provide the configuration as it has been described in the context of  FIGS. 1 to 3 . 
         [0081]    In a step  112 . 2 , the pattern formed on the mask  104 . 1  is projected (eventually several steps and/or several times) onto the substrate  105 . 1 . Concurrently with this exposure process, in a step  112 . 3 , the actual value of at least one operating factor of the optical exposure apparatus  101  is captured as it has been described above. 
         [0082]    In a step  112 . 4 , the geometry of the lens  108  is actively controlled as it has been described above. Furthermore, as it had been outlined above, in this step  112 . 4  of the position of the lens  108  may be actively controlled. 
         [0083]    In a step  112 . 5  it is determined if the processes to be stopped. If this is not the case, e.g. if a further substrate  105 . 1  is to be exposed, the method jumps back to step  112 . 2 . Otherwise the process stops in a step  112 . 6 . 
         [0084]    The wall element  110 . 3  of the actuator  110  of the optical exposure apparatus  101  of  FIG. 1  may be manufactured using a method of manufacturing such a wall element as it will be described in the following with reference to  FIG. 1 to 5 . 
         [0085]    In a step  113 . 1 , a core (also referred to as a mandrel) made of aluminum is provided. The core has the geometry of the actuation chamber  110 . 4  in the neutral state shown in  FIG. 3 . Further components of the later actuator  110  may already be mounted to the core. For example, the rod  110 . 11  may be mounted to the core as well as a suitable connector for the actuation fluid line from the pressure source  110 . 12 . 
         [0086]    In a step  113 . 2 , one or several layers of nickel (Ni) are galvanically deposited (e.g. electroplated) on the surface of the core to form the wall element  110 . 3  while at the same time firmly connecting the further components (rod  110 . 11 , fluid line connector etc.) in a gas tight manner to the wall element  110 . 3 . It will be appreciated that other materials or material combinations may be deposited in one or several layers on the core. For example, beryllium copper (BeCu) or titanium (Ti) may also be used as a material for the wall element  110 . 3 . 
         [0087]    The wall element  110 . 3  is given a uniform thickness. However, it will be appreciated that any desired thickness distribution over the wall element may be provided. For example, the first wall section and the nested second wall section may be given a different thickness. Furthermore, the thickness may be altered over the respective wall section etc. 
         [0088]    In a step of  113 . 3 , the core is removed by dissolving it such that the thin walled wall element  110 . 3  with the embedded further components is obtained. Finally, in a step  113 . 4 , the actuator  110  is assembled to provide the configuration as it has been described above. 
         [0089]    As it had already been mentioned above, the actuation chamber and may be confined by a plurality of separate wall elements connected in a suitable fluid tight manner. In particular, the separate wall elements may be of different design, different wall thickness distribution, different material etc. Furthermore, in this case, the separate wall elements do not necessarily have to be formed in a galvanic deposition process. For example, one or several of these separate wall elements may in this case also be formed by a suitably deformed stainless steel material. 
         [0090]    A actuator  210  will be described with reference to  FIG. 6 . The actuator  210  in its basic design and functionality largely corresponds to the actuator  110  and may replace the actuator  110  in the optical imaging device  101  of  FIG. 1 . Thus, it is here mainly referred to the explanations given above and only the differences with respect to the actuator  110  will be explained in further detail. In particular, similar parts are given the same reference numeral raised by the amount 100 and (unless explicitly described in the following) in respect to these parts reference is made to the explanations given above. 
         [0091]    The second wall section  210 . 6  nested within the first wall section  210 . 5  is a substantially straight cylindrical wall section without any corrugations. As a consequence, the increase in the effective length LE achieved by this nested arrangement mainly has an influence on the transverse rigidity of the actuator  210  while the axial rigidity along the actuator axis  210 . 1  substantially remains unchanged with respect to a corresponding reference actuator. Consequently, it is possible to separately influence the transverse rigidity and the axial rigidity of the actuator. 
         [0092]    It will be appreciated that it may also be provided that the first wall section is a substantially straight wall section (without corrugations) while the second wall section is a corrugated wall section as it has been described above. 
         [0093]    A fluidic actuator  310  will be described with reference to  FIG. 7 . The actuator  310  in its basic design and functionality largely corresponds to the actuator  110  and may replace the actuator  110  in the optical imaging device  101  of  FIG. 1 . Thus, it is here mainly referred to the explanations given above and only the differences with respect to the actuator  110  will be explained in further detail. In particular, similar parts are given the same reference numeral raised by the amount 200 and (unless explicitly described in the following) in respect to these parts reference is made to the explanations given above. 
         [0094]    Both the second wall section  310 . 6  nested within the first wall section  310 . 5  and the first wall section  310 . 5  are substantially straight cylindrical wall sections without any corrugations. The extension of the actuator  310  is achieved by a rolling unfolding process in the curved transition section  310 . 13  between the first wall section  310 . 5  and the second wall section  310 . 6 . 
         [0095]    As a consequence, a comparatively high axial rigidity is achieved while the increase in the effective length LE achieved by this nested arrangement mainly has an influence on the transverse rigidity of the actuator  310 . Consequently, it is possible to achieve a low transverse rigidity while at the same time providing a high axial rigidity of the actuator. 
         [0096]    An actuator  410  will be described with reference to  FIG. 8 . The actuator  410  in its basic design and functionality corresponds to the actuator  110  and may replace the respective actuator  110  in the optical imaging device  101  of  FIG. 1 . Thus, similar parts are given the same reference numeral raised by the amount 300 and (unless explicitly described in the following) in respect to these parts reference is made to the explanations given above. 
         [0097]    As can be seen from  FIG. 8  (showing the actuator  410  in a highly schematic manner), the actuator  410 , at a first end, is supported on the lower ring shaped support element  109 . 5  of the support device  109 . 2 . Furthermore, the actuator  410 , at a second end, contacts the lower surface of the lens holder  109 . 1  in order to be able to exert an actuation force F onto the lens holder  109 . 1  along an actuator axis  410 . 1  defining a direction of actuation  410 . 2 . 
         [0098]    The actuator  410  includes an actuation unit  410 . 3  with a first section  410 . 4 , a second section  410 . 11  and a linking section  410 . 13  arranged kinematically in series between the support element  109 . 5  and the lens holder  109 . 1 . The linking section  410 . 13  is folded back along the actuator axis  410 . 1  such that the first section  410 . 4  and the second section  410 . 11  overlap along the actuator axis  410 . 1 . More precisely, the linking section  410 . 13  links the first section  410 . 4  and the second section  410 . 11  in such a manner that the second section  410 . 11  is arranged in a nested manner within the first section  410 . 4 . By this means the effective length LE of the actuation unit  410 . 3  along the actuator axis  410 . 1  is increased with respect to the length LA of the actuation unit  410 . 3  along the actuator axis  410 . 1  as will be explained in further detail below. 
         [0099]    Either one of the first section  410 . 4  and the second section  410 . 11  may be an actuation section including an actuation element (not shown in further detail) providing an actuation force F along the actuator axis  410 . 1  when an actuation energy is provided to the respective actuation element by an energy source  410 . 12 . Only the first section  410 . 4  is connected to the energy source  410 . 12  and provides a first actuation force F 1  along the actuator axis  410 . 1 , while the second section  410 . 11  is a simple passive component, for example, a simple push rod. However, it will be appreciated that, in addition or as an alternative, the second section  410 . 11  may also be connected to the energy source  410 . 12  (as it is indicated in  FIG. 8  by the dashed contour) or to a further energy source in order to provide a second actuation force F 2  along the actuator axis  410 . 1 . In this case, the first section may be a simple passive component. 
         [0100]    The respective actuation element may be any suitable actuation element providing an actuation force. For example, the actuation element may be a fluidic actuator provided with an actuation energy in the form of a correspondingly pressurized actuation fluid. Furthermore, the respective actuation element may be any type of electric or electromechanical actuation element, such as a piezoelectric actuator, a so-called Lorentz actuator etc. Such an electric actuation element is provided with actuation energy in the form of electric energy. Of course, any desired combination of such actuation elements may be used within either one of the first section  410 . 4  and the second section  410 . 11  as well as within the actuation unit  410 . 3 . 
         [0101]    In  FIG. 8 , the actuator  410  is shown in a neutral state. If a fluidic actuation element is used, this neutral state is the state where the actuation pressure p c  within the fluidic actuation element substantially corresponds to the pressure p a  in the atmosphere surrounding the actuation element  410  such that the gage pressure p g  (i.e. the pressure difference between the surrounding atmosphere and the actuation pressure within the actuation element) is substantially zero, whereby the gage pressure p g  calculates according to equation (1) as: 
         [0000]        p   g   =p   c   −p   a . 
         [0102]    If an electric actuation element is used, the neutral state may be the state where no electric energy is provided to the actuation element. 
         [0103]    The first section  410 . 4  is of generally cylindrical design defining a cylinder axis that is collinear with the actuator axis  410 . 1 . Thus, the first section  410 . 4 , in a plane perpendicular to the actuator axis  410 . 1 , has a generally circular cross-section. The first section  410 . 4 , along the actuator axis  410 . 1 , has a first length L 1 . 
         [0104]    The linking section  410 . 13  is also of generally cylindrical design defining a cylinder axis that is collinear with the actuator axis  410 . 1  and, consequently, collinear with the axis of the first section  410 . 4 . Thus, the linking second section  410 . 13 , in a plane perpendicular to the actuator axis  410 . 1 , also has a generally circular cross-section. 
         [0105]    The linking section  410 . 13  includes a thin walled wall element  410 . 6  of generally cylindrical shape. The wall element  410 . 6 , at a first end, is connected to a flange element  410 . 14  of the linking section  410 . 13 , the flange element  410 . 14  being connected to the first section  410 . 4 . At a second end, the wall element  410 . 6  is connected to a circular base element  410 . 10  which in turn is connected to the second section  410 . 11 . 
         [0106]    The linking section  410 . 13 , along the actuator axis  410 . 1 , in the neutral state shown, has a second length L 2  along the actuator axis  410 . 1  corresponding to more than 50%, namely roughly 66%, of the first length L 1  of the first wall section  410 . 5 . 
         [0107]    The first section  410 . 4 , in the neutral state, has a certain first axial rigidity along the actuator axis  410 . 1  and a certain first transverse rigidity transverse to the actuator axis  410 . 1 . Furthermore, the second section  410 . 11 , in the neutral state, has a certain second axial rigidity along the actuator axis  410 . 1  and a certain second transverse rigidity transverse to the actuator axis  410 . 1 . The first and second axial rigidity may have any desired relation. The same applies to the first and second axial rigidity. 
         [0108]    The wall element  410 . 6  of the linking section  410 . 13 , along the actuator axis  410 . 1 , has a straight design. Thus, the third axial rigidity of the linking section  410 . 13  along the actuator axis  410 . 1  and the third transverse rigidity transverse to the actuator axis  410 . 1  is mainly defined by the material and the cross-section of the wall and into  410 . 6  (in a plane transverse to the actuator axis  410 . 1 ). Since the wall element  410 . 6  is thin walled, the third axial rigidity is considerably smaller than the first and second axial rigidity. The same applies to the third transverse rigidity which is considerably smaller than the first and second transverse rigidity. 
         [0109]    It will be appreciated that, both, the third axial rigidity and the third transverse rigidity may be easily adjusted to a desired value by simply adjusting the material properties and/or the geometry of the wall element  410 . 6 , in particular, by adjusting the length, the diameter and/or the wall thickness of the wall element  410 . 6 . Thus, configurations may be easily achieved wherein the third axial rigidity is less than 50%, even less than 25%, of one of the first and second axial rigidity (e.g., of the smaller one of the first and second axial rigidity). The same applies to the third transverse rigidity where configurations may be easily achieved where the third transverse rigidity is less than 50%, even less than 25%, of one of the first and second transverse rigidity (e.g., of the smaller one of the first and second transverse rigidity). 
         [0110]    It will be appreciated that, to further reduce the third axial rigidity and/or the third transverse rigidity, the wall element of the linking section may have a corrugated design, i.e. a plurality of corrugations, such that, in other words, the wall element  410 . 6  is shaped in the manner of a bellows as it has been described above. However, it will be appreciated that the corrugated design may also be limited to a certain fraction of the length L 2  of the linking section while the remaining part of the linking section may be of straight design. 
         [0111]    It will be appreciated that the number and/or the geometry on the corrugations may be selected as a function of a desired axial rigidity of the actuator  410  along the actuator axis  410 . 1  and/or of a desired transverse rigidity of the actuator  410  transverse to the actuator axis  410 . 1 . It will be appreciated that continuously curved corrugations may be used as well as other types of corrugated designs. In particular, designs having a zigzagging shape (in a sectional plane including the actuator axis) may also be used. 
         [0112]    It will be further appreciated that the first and/or second section do not necessarily have to be of generally cylindrical shape. It is also possible that either one of the first and second section is of generally prismatic shape, i.e. has a polygonal cross-section in a plane perpendicular to the actuator axis. Furthermore, it will be appreciated that the axis defined by the first wall section and the axis defined by the second wall section do not necessarily have to be collinear. In particular, if (apart from the actuation force F along the actuator axis) a further actuation force or moment in a different direction is desired, such as non-collinear design may be chosen. 
         [0113]    As can be seen from  FIG. 8 , the wall element  410 . 6  is folded back into the first section  410 . 4 . Thus, the wall element  410 . 6  is arranged in a nested manner within the first wall section  410 . 5 . At its inner (here lower) end located within the first section  410 . 6  the wall element  410 . 6  is connected to the base element  410 . 10  of the wall element  410 . The base element  410 . 10  substantially extends in a plane perpendicular to the actuator axis  410 . 1 . The base element  410 . 10  forms a mounting base for a first end of the second section  410 . 11 . The second section  410 . 11  extends along the actuator axis  410 . 1  and, at its second end, is connected to the lens holder in  109 . 1 . 
         [0114]    In its neutral state, the actuator  410  is in a retracted state. If actuation energy of a first polarity is provided to the actuation element of the first section  410 . 4  (e.g. if the gage pressure p g  within a fluidic actuation element is raised to a positive value or if a positive electric tension is applied to an electric actuation element), the actuation element of the first section  410 . 4  extends and, via the linking section  410 . 13  and the second section  410 . 11 , exerts a positive force F onto the lens holder  109 . 1 . Furthermore, if actuation energy of a second polarity is provided to the actuation element of the first section  410 . 4  (e.g. if the gage pressure p g  within a fluidic actuation element is lowered to a negative value or if a negative electric tension is applied to an electric actuation element), the actuation element of the first section  410 . 4  further retracts and, via the linking section  410 . 13  and the second section  410 . 11 , exerts a negative force F onto the lens holder  109 . 1 . 
         [0115]    This actuation force F causes a corresponding (positive or negative) shift of the part of the lens holder  109 . 1  immediately adjacent to the actuator  410  and, consequently, of the part of the lens  108  immediately adjacent to the actuator  410  along the optical axis  101 . 1 . On the contrary, the holding elements  109 . 3  may maintain their length along the optical axis  101 . 1  such that the respective part of the lens holder  109 . 1  immediately adjacent to the respective holding element  109 . 3  and, consequently, of the part of the lens  108  immediately adjacent to the respective holding element  109 . 3  maintains its position along the optical axis  101 . 1 . 
         [0116]    Thus, via the respective actuation energy provided to the four actuators  410 , a certain deformation of the lens  108  (i.e. an alteration within the geometry of the lens  108 ), may be achieved while the position of the lens  108  (more precisely, the position of e.g. the optical center or another suitable reference point of the lens  108 ) in space remains unchanged. 
         [0117]    A so called two-wave deformation may be introduced into the lens  108  by providing two diagonally opposite actuators  410  with a suitable actuation energy of the first polarity and the other two diagonally opposite actuators  410  with a suitable actuation energy of a second polarity. Furthermore, a so called four-wave deformation may be introduced into the lens  108  by providing to all four actuators  410  and actuation energy at a suitable first (or second) polarity. It will be appreciated that, depending on the number of actuators and holding elements used, any other desired n-wave deformation may be introduced into the lens  108 . 
         [0118]    It will be further appreciated that the actuation force F provided by the actuator  410  (typically acting in combination with further actuators and acting in a more direct manner on the lens) may also be used for altering the position of the lens  108  in the z-direction as well as the tilting angle of the lens  108  about the x- and y-axis may be adjusted. The maximum displacement of the lens  108  in the z-direction is defined by the range of displacement of the actuator  410 , i.e. the achievable maximum extension of the actuator  410  along the actuator axis  410 . 1 . 
         [0119]    The folded back or nested design of the actuation unit  410 . 3  has the advantage that, at given geometric boundary conditions for the actuator  410  in the neutral state, an increase in the effective length LE of the actuation unit  410 . 3  along the actuator axis  410 . 1  may be achieved with respect to known non-nested actuator designs where the effective length of the actuation unit typically corresponds to the length of the actuator. 
         [0120]    In particular, the effective length LE calculates as the sum of the first length L 1  of the first section  410 . 4 , the second length L 2  of the linking section  410 . 13  and the third length L 3  of the second section  410 . 11 , i.e. as: 
         [0000]        LE=L 1 +L 2 +L 3.  (3) 
         [0121]    Since, the first length L 1  corresponds to about 95% of the length LA of the actuation chamber  410 . 4  and the second length L 2  corresponds to about 66% of the first length L 1  while the third length L 3  corresponds to about 70% of the first length L 1 , the effective length LE of the wall element  410 . 3  corresponds to about 224% of the length LA of the actuator  410 . 
         [0122]    This increase in the effective length LE of the actuation unit  410 . 3  has the advantage that, on the one hand, it is possible to adjust the axial and/or transverse rigidity of the actuator  410  by simply modifying the second length L 2  of the linking section  410 . 13  and the third length L 3  of the second section  410 . 11 . It will be appreciated that the modification of the second length L 2  and the third length L 3  does not affect the length LA of the actuator  410  such that it is feasible within given geometric boundary conditions. 
         [0123]    It will be further appreciated that the axial and transverse rigidity of the actuator  410  may also be adjusted by the material and/or the geometry and/or the thickness distribution of the wall element  410 . 6 . Thus, a broad range of desired axial rigidities and transverse rigidities of the actuator device  410  may be achieved. 
         [0124]    Furthermore, an increase in the effective length LE of the actuation unit  410 . 3 , at a given actuator length LA, provides an increased range of displacement if the first section  410 . 4  and the second section  410 . 11  are active sections, i.e. include at least one actuation element, while at the same time keeping the axial and/or transverse rigidity of the actuator  410  low. Thus, thanks to this low axial and/or transverse rigidity lower parasitic loads are introduced into the lens holder  109 . 1  and, consequently, into the lens  108  leading to a lower amount of imaging errors resulting from such parasitic loads introduced into the optical system. 
         [0125]    The axial rigidity and the transverse rigidity of the actuator  410 , in the neutral state shown, correspond to less than 75% of the axial rigidity and transverse rigidity, respectively, of a reference actuator in a reference neutral state corresponding to this neutral state. The reference actuator includes the first section  410 . 4  and the second section  410 . 11  connected by a reference linking section that is substantially rigid, i.e. has a fourth axial rigidity that is higher than the first and second axial rigidity and a fourth transverse rigidity that is higher than the first and second transverse rigidity. 
         [0126]    It will be appreciated that the effective length LE may have any other desired value. The effective length LE of the actuation unit  410 . 3  can correspond to at least 140% of the length LA of the actuator. In particular, with a single-nested design as shown in  FIG. 8 , where negative force is generated by further retracting the actuator, the effective length LE may be selected to be between 150% and 200% of the length LA of the actuator in order to provide a sufficient range of displacement. Typically displacements up to 1 mm can be readily achieved. 
         [0127]    Typically, with a single-nested design as shown in  FIG. 8 , where negative and positive forces F are provided (to provide an alternating deformation of the lens  108 ), the effective length LE may be selected to range from 160% to 240% (e.g., from 190% to 224%) of the length LA of the actuator in order to provide a sufficient range of displacement at advantageously low axial and transverse rigidity. In case only positive forces are to be generated for actuation in one direction only, the effective length may be increased to more than 240%, e.g. even up to 280% of the length LA of the actuator. 
         [0128]    It will be further appreciated that the effective length LE may even be pushed beyond 300% of the length LA of the actuator by choosing a multiple-nested design where, for example, a further linking section is arranged in a nested manner within the second section in an analogous manner (i.e. the second wall section and the further linking section defining a further ring shaped section of the actuation unit). 
         [0129]    Consequently, the axial rigidity and the transverse rigidity of the actuator, in the neutral state, may even correspond to less than 50% and even down to 25% of the axial rigidity and transverse rigidity, respectively, of a reference actuator as outlined above. 
         [0130]    A fluidic actuator  510  will be described with reference to  FIG. 9 . The actuator  510  in its basic design and functionality largely corresponds to the actuator  410  and may replace the respective actuator  110  in the optical imaging device  101  of  FIG. 1 . Thus, it is here mainly referred to the explanations given above and only the differences with respect to the actuator  410  will be explained in further detail. In particular, similar parts are given the same reference numeral raised by the amount 100 and (unless explicitly described in the following) in respect to these parts reference is made to the explanations given above. 
         [0131]    The second section  410 . 11  is not nested within the first section  510 . 4 . Rather the linking section  510 . 13  is only folded back along the actuator axis  510 . 1  in such a manner that the first section  510 . 4  and the second section  510 . 11  simply overlap along the actuator axis  510 . 1 . To this end, the linking section  510 . 13  includes a guiding section  510 . 15  arranged kinematically parallel to the thin walled wall element  510 . 6  and connected to the flange  510 . 14  and to the base element  510 . 10  in order to provide a suitable guide function along and transverse to the actuator axis  510 . 1 . 
         [0132]    As a consequence, a comparatively high axial rigidity is achieved while the increase in the effective length LE achieved by this folded back arrangement mainly has an influence on the transverse rigidity of the actuator  510 . Consequently, it is possible to achieve a low transverse rigidity while at the same time providing a high axial rigidity of the actuator. However, it will be appreciated that it is also possible to achieve a low axial rigidity while at the same time providing a high transverse rigidity simply by replacing the guide mechanism between the guiding section  510 . 15  and the base element  510 . 10  by a guide mechanism as it is indicated by the dashed contour  510 . 16 . 
         [0133]    In the foregoing, the disclosure has been described above where optical element units including an optical element and a holder holding the optical element have been used. However, it will be appreciated that the disclosure may also be applied to embodiments where the actuator is directly connected to the optical element. 
         [0134]    Furthermore, the disclosure has predominantly been described in the context of embodiments where the fluidic actuator is used for altering the position (i.e. location and/or orientation in space) of an optical element. However, as already indicated above, it will be appreciated that the actuation force provided by the actuator according to the disclosure may also be used for altering the geometry of such an optical element or any other component of an optical device. Furthermore, the actuation force provided by the actuator according to the disclosure may be used for any other task in such an optical device. 
         [0135]    In the foregoing, the disclosure has been described solely in the context of microlithography systems working with exposure light at a wavelength of 193 nm. However, it will be appreciated that the disclosure may also be used in the context of any other optical device working at any other wavelength, in particular, any other optical device using deformation sensitive components. In particular, the disclosure may also be used in the context of so called EUV systems working at a wavelength below 20 nm, typically at about 13 nm. 
         [0136]    Finally, it will be appreciated that the disclosure may be used in the context of any type of optical element at any location within an optical device, in particular, in the context of refractive, reflective and diffractive optical elements or any combination thereof. 
         [0137]    U.S. Pat. No. 5,822,133 is incorporated herein by reference. 
         [0138]    Other embodiments are in the claims.