Patent Publication Number: US-2011063590-A1

Title: Optical element module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/015,894, filed Jan. 17, 2008, which is a continuation under 35 U.S.C. §120 of international application PCT/EP2006/064427, filed Jul. 19, 2006, which claims the benefit under 35 U.S.C. 119(e)(1) of provisional U.S. Patent Application Ser. No. 60/700,517 filed 19 Jul. 2005. The entire contents of these applications are hereby incorporated herein by reference. 
    
    
     FIELD 
     The disclosure relates to optical element modules used in exposure processes, in particular to optical element modules of microlithography systems. It further relates to optical element units comprising such optical element modules. It also relates to optical exposure apparatuses comprising such optical element units. Furthermore, it relates to a method of holding an optical element. The disclosure may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes. 
     BACKGROUND 
     Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image formed on a mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical element units. Such optical element units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element. 
     Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical element units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels. 
     Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system. 
     Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical elements of such an optical system is desirably supported in a defined manner in order to maintain a predetermined spatial relationship between said optical elements to provide a high quality exposure process. 
     In this context there exist, among others, two general requirements for the support of optical elements of the optical system. One is that the rigidity of the support system of the optical elements has to be as high as possible in certain directions, in particular in the direction of the optical axis, to keep the resonant frequencies of the system as high as possible. Furthermore, deformations of the optical elements of the optical system are to be avoided to the greatest possible extent in order to keep imaging errors resulting from such deformations as low as possible. 
     One such imaging error is for example stress induced birefringence of refractive optical elements. Such stress induced birefringence mainly results from stresses introduced into the optical element via its peripheral support structure and radially propagating through the optically used area of the optical element. Such stresses are often thermally induced, resulting from differences in the coefficient of thermal expansion (CTE) of the optical element and its peripheral support structure. Variations in the temperature situation of the optical element and its peripheral support structure lead to relative movements between the optical element and its peripheral support structure. These relative movements are counteracted by the holding forces acting between the optical element and its peripheral support structure leading to the above undesired stress situations. 
     To avoid thermally induced stresses and deformations within an optical element due to differences in the coefficient of thermal expansion of the optical element and its optical element holder, it is known to connect the optical element and its optical element holder via deformation uncoupling elements. These deformation uncoupling elements generally allow for relative movements between the optical element and its optical element holder. 
     These deformation uncoupling elements may provide a reduction of the stresses and, thus, the deformations introduced into the optical element. However, they have the disadvantage that they also reduce the rigidity of the support system. To deal with this effect, the rigidity of the uncoupling elements might be increased, but this would reduce their deformation uncoupling abilities leading to increased stresses and, thus, the deformations introduced into the optical element. 
     Another approach to deal with this problem is known from US 2001/0039126 A1 (to Ebinuma et al.). Here, it is provided for an adaptation of the coefficients of thermal expansion between an optical element and a support ring contacting the optical element in order to reduce the introduction of thermally induced deformations into the optical element resulting from differences in the coefficients of thermal expansion. However, this solution my have the disadvantage that, for certain optical elements with a certain coefficient of thermal expansion, the adaptation of the coefficient of thermal expansion may only be achieved with comparatively expensive materials for such large parts as the support ring. 
     SUMMARY 
     It is thus an object of the disclosure to, at least to some extent, overcome the above disadvantages and to provide good and long term reliable imaging properties of an optical system used in an exposure process. 
     It is a further object of the disclosure to increase imaging accuracy of an optical system used in an exposure process by reducing thermally induced stresses introduced into an optical element of the optical system. 
     It is a further object of the disclosure to increase imaging accuracy of an optical system used in an exposure process by reducing stress induced birefringence introduced into an optical element of the optical system via its support structure. 
     These objects are achieved which is based on the teaching that a reduction in the deformations introduced into an optical element of the optical system via its support structure and a high rigidity of the support mechanism for the optical element may be achieved when at least one of the module components of an optical element module is adapted to provide, compared to conventional deformation uncoupling elements, at least a reduction of forces introduced into the optical element upon a thermally induced position change in the relative position between the optical element and the optical element holder by maintaining, at the same time, a high rigidity of the support mechanism. This reduction of disturbing forces upon maintained support rigidity may be achieved in several ways. 
     One solution is to provide a contact element that compensates by its thermal expansion properties for the difference in the coefficient of thermal expansion between the optical element and the associated optical element holder such that, at least at a given variation in the temperature situation, no relative shift between the contact points of the module components occurs. With this solution, the thermally induced introduction of disturbing forces into the optical element may even be completely avoided. This solution has the further advantage that, compared to the known adaptation of the coefficients of thermal expansion of the optical element and the optical element holder, with the contact element only a relatively small part has to be adapted to the given coefficient of thermal expansion situation. Furthermore, at a given coefficient of thermal expansion situation, adaptation may be provided easily by simply adapting the effective distance between the contact points of the contact element with the optical element and the optical element holder, respectively. 
     A second solution is to allow for a relative movement between the optical element and the associated optical element holder at a variation in the temperature situation, but to reduce the disturbing forces introduced into the optical element as a result of such a relative movement. Since these disturbing forces predominantly result from frictional forces between the coupled module components, a reduction of these frictional forces at the interface of the coupled module components is provided. This may be achieved by adapting the frictional properties of the module components at the interface location to provide a low friction contact. Furthermore, the relative motion between the module components may be adapted to provide a type of motion with low friction. In some embodiments, due to the low frictional forces transmitted at such a motion type, a rolling motion is provided at the interface location between the module components. 
     A third solution is to overall reduce, under normal operating conditions, the holding forces exerted on the optical element and, thus, also the disturbing forces introduced into the optical element at a thermally induced relative movement between the module components. This solution is based on the concept that the holding forces usually counteract also the thermally induced relative movement between the module components and, thus, have an influence on the frictional forces introduced into the optical element at such a relative movement between the module components. Usually, due to the manufacture and mounting of the optical system at a location different from the location of its later use, the holding forces provided for the optical elements do not only account for the forces occurring under normal operating conditions of the optical system but also have to account for considerably higher abnormal forces occurring during, for example, transport of the optical system. Thus, in conventional systems, holding forces are considerably higher than necessary in normal use. This obviously leads to considerable disturbing forces introduced into the optical element at a thermally induced relative movement between the module components. These disturbing forces can be reduced by providing a securing device which is only activated under abnormal load conditions in order to hold the optical element in place. Thus, under normal operating conditions, holding forces which are considerably lower than in conventional systems may be applied to the optical element leading, in turn, to reduced disturbing forces. 
     It will be appreciated that arbitrary combinations of the above solutions may be selected to combine their beneficial effects and to further reduce the disturbing forces introduced into the optical element at thermally induced relative movements between some of the module components. 
     Thus, according to a first aspect of the disclosure there is provided an optical element module comprising an optical element, an optical element holder and a first contact element. The optical element has a first coefficient of thermal expansion. The optical element holder holds the optical element via the first contact element and has a second coefficient of thermal expansion, the second coefficient of thermal expansion being different from the first coefficient of thermal expansion. A first contact point is formed on a first module component, the first module component being one of the optical element and the optical element holder. The first contact element has a second contact point and a third coefficient of thermal expansion. At a first temperature situation, the first contact point contacts the second contact point at a first location. Furthermore, the first contact element contacts a second module component at a second location, the second location, at the first temperature situation, being located at a first contact location distance from the first location, and the second module component being different from the first module component and being one of the optical element and the optical element holder. At least one of the third coefficient of thermal expansion and the contact location distance is selected such that, at a given second temperature situation different from the first temperature situation, a thermally induced modification in the size of the first contact element with respect to the first temperature situation compensates for the difference between the first coefficient of thermal expansion and the second coefficient of thermal expansion such that, at the second temperature situation, there is substantially no shift between the first contact point and the second contact point. 
     According to a second aspect of the disclosure there is provided an optical element module comprising an optical element, an optical element holder and a first contact element. The optical element has a first coefficient of thermal expansion. The optical element holder holds the optical element via the first contact element and has a second coefficient of thermal expansion, the second coefficient of thermal expansion being different from the first coefficient of thermal expansion. One of the optical element and the optical element holder forms a first module component and one of the optical element and the optical element holder forms a second module component being different from the first module component. A first contact surface is formed on the first module component, the first contact element having a curved second contact surface contacting the first contact surface. The first contact element is adapted such that the second contact surface executes a rolling motion with respect to the first contact surface upon a thermally induced change in the relative position between the optical element and the optical element holder. 
     According to a third aspect of the disclosure there is provided an optical element module comprising a plurality of module components. The module components comprise an optical element, an optical element holder and a contact element. The optical element has a first coefficient of thermal expansion. The optical element holder holds the optical element via the first contact element and has a second coefficient of thermal expansion, the second coefficient of thermal expansion being different from the first coefficient of thermal expansion. At least one of the module components is adapted to provide at least a reduction of forces introduced into the optical element upon a thermally induced position change in the relative position between the optical element and the optical element holder, the position change resulting from a temperature situation variation in a temperature situation of the plurality of module components. 
     According to a fourth aspect of the disclosure there is provided an optical element unit comprising a plurality of optical element modules connected to each other and supporting a plurality of optical elements. The plurality of optical element modules comprises a first optical element module being an optical element module. 
     According to a fifth aspect of the disclosure there is provided an optical exposure apparatus for transferring an image of a pattern formed on a mask onto a substrate comprising a light path, a mask location located within the light path and receiving the mask, a substrate location located at an end of the light path and receiving the substrate and an optical element unit located within the light path between the mask location and the and the substrate location. 
     According to a sixth aspect of the disclosure there is provided a method of holding an optical element comprising, in a first step, providing a plurality of module components, the module components comprising an optical element, an optical element holder and a contact element, and, in a second step, holding the optical element using the optical element holder, the optical element holder holding the optical element via the contact element. The optical element having a first coefficient of thermal expansion. The optical element holder has a second coefficient of thermal expansion, the second coefficient of thermal expansion being different from the first coefficient of thermal expansion. At least one of the module components is adapted to provide at least a reduction of forces introduced into the optical element upon a thermally induced position change in the relative position between the optical element and the optical element holder, the position change resulting from a temperature situation variation in a temperature situation of the plurality of module components. 
     Further aspects and embodiments of the disclosure will become apparent from the dependent claims and the following description of preferred embodiments which refers to the appended figures. All combinations of the features disclosed, whether explicitly recited in the claims or not, are within the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an optical exposure apparatus comprising preferred embodiments of an optical element unit and optical element modules; 
         FIG. 2  is a perspective view of a schematic sectional representation of a part of an optical element unit of the optical exposure apparatus of  FIG. 1 ; 
         FIG. 3  is a perspective view of schematic sectional representation of another part of the optical element unit along line III-Ill of  FIG. 2 ; 
         FIG. 4  is a block diagram of a method of holding an optical element; 
         FIG. 5  is a schematic sectional representation of a part of a further optical element module used in the optical exposure apparatus of  FIG. 1 ; 
         FIG. 6A  is a schematic perspective view of a further contact element that may be used in the optical element module of  FIG. 5 ; 
         FIG. 6B  is another schematic view of the contact element of  FIG. 6A ; 
         FIG. 7A  is a schematic view of a part of a further optical element module used in the optical exposure apparatus of  FIG. 1 ; 
         FIG. 7B  is a schematic view of the detail B of  FIG. 7A ; 
         FIG. 8A  is a schematic view of a part of a further optical element module used in the optical exposure apparatus of  FIG. 1 ; 
         FIG. 8B  is a schematic view of the detail B of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     First Embodiment 
     In the following, a first preferred embodiment of an optical exposure apparatus  1  comprising an optical projection system  2  with an optical element unit  3  will be described with reference to  FIGS. 1 and 2 . 
     The optical exposure apparatus  1  is adapted to transfer an image of a pattern formed on a mask  4  onto a substrate  5 . To this end, the optical exposure apparatus  1  comprises an illumination system  6  illuminating said mask  4  and the optical element unit  3 . The optical element unit  3  projects the image of the pattern formed on the mask  4  onto the substrate  5 , e.g. a wafer or the like. 
     To this end, the optical element unit  3  holds an optical element group  7 . This optical element group  7  is held within a housing  3 . 1  of the optical element unit  3 . The optical element group  7  comprises a number of optical elements  107  and  207  as well as optical elements  407  and  507  such as lenses, mirrors or the like. These optical elements  107 ,  207 ,  407 ,  507  are aligned along a folded optical axis  3 . 2  of the optical element unit  3   
     The optical projection system  2  receives the part of the light path between the mask  4  and the substrate  5 . Its optical elements  107 ,  207 ,  407 ,  507  cooperate to transfer the image of the pattern formed on the mask  4  onto the substrate  5  located at the end of the light path. To increase the numerical aperture NA of the optical projection system  2 , the optical projection system  2  may comprise an immersion zone located between the lower end of the optical element unit  3  and the substrate  5 . 
     The optical element unit  3  is composed of a plurality of optical element modules  3 . 3  and  3 . 4 , optical element modules  3 . 5  and  3 . 6  as well as an optical element module  3 . 7  stacked and tightly connected to form the optical element unit  3 . Each optical element module  3 . 3  to  3 . 6  holds one or more of the optical elements  107 ,  207 ,  407 ,  507 , respectively. The optical element module  3 . 7  is an interface module holding a reflecting optical element  12  used to fold the optical axis  3 . 2 . The optical element module  3 . 7  is an interface module providing an interface for the respective module stack. 
       FIG. 2  shows a schematic perspective view of a sectional representation of a part of the optical element module  3 . 3  of the optical element unit  3  at a first temperature situation T 1  of the optical element unit  3 . The first temperature situation T 1  is characterized by a certain temperature profile within the components of the optical element unit  3 . 
     The optical element  107  of the optical element module  3 . 3  is a rotationally symmetric lens having an optical axis  107 . 1 . The lens  107  is made of Quartz (SiO 2 ) and has a first coefficient of thermal expansion. 
     The lens  107  is usually positioned in space such that the optical axis  107 . 1  of the lens  107  is substantially collinear with the optical axis  3 . 2  of the optical element unit  3 . It should be noted that the position of the optical axis  107 . 1  of the lens  107  shown in  FIG. 2  is not to scale. In reality, the optical axis  107 . 1  is located at a distance from the outer circumference of the lens  107  that by far exceeds the distance shown in  FIG. 2 . 
     The lens  107  is held by an optical element holder in the form of a ring shaped lens holder  108  which, in turn, is held by a ring shaped frame element  109 . The lens holder  108  is made of Invar that has a second coefficient of thermal expansion different from, namely larger than the first coefficient of thermal expansion of the lens  107 . The lens holder  108  holds the lens  107  in place via a plurality of first contact elements  110  and a plurality of second contact elements  111 . In the sectional view of  FIG. 2 , half of a first contact element  110  and half of a second contact element  111  is shown, both contact elements  110  and  111  being symmetric with respect to the sectional plane. 
     In the embodiment shown, three first contact elements  110  and three second contact elements  111  are evenly distributed at the inner circumference of the lens holder  108 . However, it will be appreciated that, with other embodiments of the disclosure, a different number of first and/or second contact elements may be provided. Thus, for example, a large number of narrowly spaced first and/or second contact elements may be formed to provide a configuration similar to the one disclosed in the context of  FIG. 2  of U.S. Pat. No. 6,392,825 B1 (Trunz et al.), the entire disclosure of which is hereby incorporated herein by reference. In such a configuration, the respective first and/or second contact elements may be formed by a radially resilient element similar to the ones disclosed in the context of  FIG. 4  of U.S. Pat. No. 4,733,945 (Bacich), the entire disclosure of which is hereby incorporated herein by reference. Furthermore, these first and/or second contact elements may be formed as separate elements or monolithically connected—in groups or altogether—via a contact element connecting element, e.g. a connecting ring similar to the one shown in  FIG. 2  of U.S. Pat. No. 6,392,825 B1 (Trunz et al.), which is then connected lens holder. Furthermore, only one type of contact elements may be provided. For example, only the lower first contact elements may be provided to support the lens from below. 
     At the first temperature situation T 1 , a first contact nose  110 . 1  formed on one end of the first contact element  110  contacts the lens  107  at a first location  112 . Thereby a first contact point  107 . 2  of a plane first contact surface  107 . 3  of the lens  107  is contacted by a second contact point  110 . 2  on the first contact nose  110 . 1  of the first contact element  110 . Furthermore, the first contact element  110  contacts the lens holder  108  at a second location  113 , where the first contact element  110  is connected to the lens holder  108  by means of screws or any other suitable fastening technique. 
     Similarly a second contact nose  111 . 1  formed on one end of the second contact element contacts the lens  107  at a third location  114 . Thereby a third contact point  107 . 4  of a plane third contact surface  107 . 5  of the lens  107  is contacted by a fourth contact point  111 . 2  on the second contact nose  111 . 1  of the second contact element  111 . Furthermore, the second contact element  111  contacts the lens holder  108  at a fourth location  115 , where the second contact element  111  is connected to the lens holder  108  by means of screws or any other suitable fastening technique. 
     It will be appreciated that, with other embodiments of the disclosure, the first and third contact surface may be curved surfaces as well. Furthermore, the optical element may form contact one or both noses as well, while one or both contact elements form plane contact surfaces. 
     Along the optical axis  107 . 1 , the first location  112  is aligned with the third location  114  and the second location  113  is aligned with the fourth location  115 . Thus, both, the first location  112  and the second location  113  as well as the third location  114  and the fourth location  115  are spaced apart in a radial direction  116  of the optical element module  3 . 3  by a contact location distance D. 
     In general, in the radial direction  116 , as a function of the temperature situation T and the first coefficient of thermal expansion α L  of the lens  107 , the first and third contact point  107 . 2  and  107 . 4  are located at a radius L(T;α L ) from the optical axis  107 . 1 . Furthermore, as a function of the temperature situation T and the second coefficient of thermal expansion α R  of the lens holder  108 , the second location  113  and the third location  115  are located at a radius R(T;α R ) from the optical axis  107 . 1 . Thus, as a function of the temperature situation T, the first coefficient of thermal expansion α L  and the second coefficient of thermal expansion αR, the contact location distance D(T;α L ;α R ) follows the equation: 
         D ( T;α   L ;α R )= R ( T ;α R )− L ( T;α   L )   (1)
 
     The first contact element  110  is designed such that, at the first temperature situation T 1  shown in  FIG. 2 , the distance E in the radial direction  116  between its second contact point  110 . 2  and the second location  113  (where the first contact element  110  is fixedly connected to the lens holder  108 ) is equal to the contact location distance D(T;α L ;α( R ). Furthermore, the second contact element  111  is designed such that, at the first temperature situation T 1  shown in  FIG. 2 , the distance E in the radial direction  116  between its fourth contact point  111 . 2  and the fourth location  115  (where the second contact element  111  is fixedly connected to the lens holder  108 ) is also equal to the contact location distance D(T;α L ;α R ). 
     In general, the distance E again is a function of the temperature situation T and of the coefficient of thermal expansion α E  of the respective contact element  110 ,  111 , i.e. E(T;α E ). Thus, at the first temperature situation T 1  the following equation is valid: 
         D ( T 1;α L ;α R )= E ( T 1;α E )   (2)
 
     It should be noted that, in the embodiment shown in  FIG. 2 , the first and second contact element  110 ,  111  are made of the same material having a third coefficient of thermal expansion. However it will be appreciated that they may also be made of different materials with different coefficients of thermal expansion, which would then have to be accounted or in the above Equation (2). 
     While the first contact element  110  is a substantially rigid element, the second contact element  111  is formed such that its second contact nose  111 . 1  is supported in a manner to be resilient in a direction parallel to the optical axis  107 . 1  of the optical element  107 . To this end, the second contact nose  111 . 1  is connected to a base part  111 . 3  of the second contact element  111  via two leaf spring arms  111 . 4 . 
     The second contact nose  111 . 1  protrudes from the second contact element  111  in such a manner that the arms  111 . 4  are elastically deflected when the base part  111 . 3  is screwed to the lens holder  108 . As a consequence, the second contact nose  111 . 1  exerts a clamping force F 1  onto the second contact surface  107 . 5  of the lens  107 , said clamping force F 1  being substantially perpendicular to the second contact surface  107 . 5 . The amount of the clamping force may be adjusted by a spacer  111 . 5  of suitable thickness placed between the base part  111 . 3  and the lens holder  108 . 
     The first contact nose  110 . 1  protruding from the first contact element  110  exerts a counteracting force F 2  onto the first contact surface  107 . 3  of the lens  107 , said counteracting force F 2  being substantially perpendicular to the first contact surface  107 . 3 . The first contact nose  110 . 1  and the second contact nose  111 . 1  are arranged such that the counteracting force F 2  is collinear with the clamping force F 1  and counteracts the clamping force F 1 . In other words, the lens is clamped between the respective first contact element  110  and the associated second contact element  111 . 
     The first contact surface  107 . 3  and the second contact surface  107 . 5  are substantially perpendicular to the optical axis  107 . 1  of the lens. Thus, at the first temperature situation T 1 , substantially no radial forces directed radially towards the center of the lens  107  are introduced into the lens by its holding mechanism. 
     At a second temperature situation T 2  different from the first temperature situation T 1 , the temperature within the components of the optical element unit  3  is raised by a given amount. As a consequence of the raised temperature, among others, the lens  107  and the lens holder  108  expand in a radial direction  116 . Since the second coefficient of thermal expansion of the lens holder  108  is higher than the first coefficient of thermal expansion of the lens  107 , the raise in the temperature causes a relative movement between the lens  107  and the lens holder  108  in the radial direction  116  such that the lens holder  108  radially moves away from the lens  107 . In other words, the contact location distance D(T;α L ;α( R ) increases according to Equation (1). 
     In conventional systems with a conventional clamping mechanism, this would lead to a relative radial movement and a residual elastic deformation at the interface between the respective contact element and the lens, both leading to the introduction of stresses into the optical element radially propagating through the optically used area of the lens. As previously explained, such radial stresses lead to imaging errors such as stress induced birefringence. 
     However, the first contact element  110  and the second contact element  111  compensate for this difference in the first and second coefficient of thermal expansion such that, at the second temperature situation T 2 , there is substantially no shift between the first contact point  107 . 2  and the second contact point  110 . 2  as well as substantially no shift between the third contact point  107 . 4  and the fourth contact point  111 . 2 . 
     To this end, the first contact elements  110  and the second contact elements  111  are made of a steel material having a third coefficient of thermal expansion different from the first and second coefficient of thermal expansion of the lens  107  and the lens holder  108 , respectively. The third coefficient of thermal expansion is higher than the first and second coefficient of thermal expansion. 
     For the first contact elements  110 , at least one of the second location  113  and the third coefficient of thermal expansion α E  are selected such that 
         E ( T 2;α E )= D ( T 2;α L ;α R )= R ( T 2;α R )− L ( T 2;α L )   (3)
 
     The same applies for the second contact elements  111 , i.e. at least one of the fourth location  115  and the third coefficient of thermal expansion α E  are selected such that Equation (3) is valid. 
     In other words, due to the higher thermal expansion of the first and second contact elements  110  and  111 , the respective contact element  110 ,  111 , at the given temperature situation variation between the first and second temperature situation T 1  and T 2 , spans the gap between the lens  107  and the lens holder  108  that results from the difference in the first and second coefficient of thermal expansion of the lens  107  and the lens holder  108 , respectively. Thus, at the second temperature situation T 2  as well, despite the thermal expansion of the module components, substantially 
     The first and second contact elements do not necessarily have to be fixedly mounted to the optical element holder. It will be appreciated that, with other embodiments of the disclosure, in the manner of a kinematic reversal, at least one of the respective first and second contact element may be fixedly mounted to the optical element and contact the optical element holder in the manner as it has been described above for the contact between the contact elements  110 ,  111  and the lens  107 . 
     It will be appreciated that, with other embodiments of the disclosure, other materials or combinations may be chosen. In any case, the compensation of the difference in the coefficient of thermal expansion between the lens holder and the lens by suitably selecting the material (i.e. the coefficient of thermal expansion), the size, and the location of the respective contact element. 
     It will be further appreciated that the compensation of the difference in the first coefficient of thermal expansion of the lens  107  and the second coefficient of thermal expansion of the lens holder  108  provided by the first and second contact elements  110  and  111  may be effective during the entire temperature situation variation, i.e. the transition between the first temperature situation T 1  and the second temperature situation T 2 . However, depending on the change in the temperature profile in the module components (lens  107 , lens holder  108  and contact elements  110 ,  111 ) during the temperature situation variation, the first and second contact elements  110  and  111  may not provide for a complete compensation during the entire temperature situation variation. 
     Thus, it may be that, during the transition between the first temperature situation T 1  and the second temperature situation T 2 , certain thermally induced radial disturbing forces are introduced into the lens  107 . However, the disclosure also provides for a reduction of these thermally induced radial disturbing by the following means. 
     First of all, as may be seen from  FIG. 3 , a gravity compensation means  117  is provided. This gravity compensation means  117  is mounted to the lens holder  108  and located close to the outer circumference of the lens  107 . The gravity compensation means  117 , in sum, exerts a force onto the lens that balances the gravitational force acting onto the lens  107  due to its mass. 
     To this end, the gravity compensation means  117  comprises force generating means  117 . 1  contacting the first contact surface  107 . 3  of the lens  107  over a certain fraction of the outer circumference of the lens  107 . In the embodiment of  FIG. 3 , three force generating means  117 . 1  are extend over substantially the entire part of the circumference of the lens  107  that is not taken by the first contact elements  110 . 
     Each force generating means  117 . 1  exerts a line force onto the lens  107  that is parallel to the optical axis  107 . 1  of the lens. The force generating means  117 . 1  is provided in the form of a helical spring with elliptical coils that are inclined with respect to the spring axis such as a so called BAL SPRING® manufactured by Bal Seal Engineering Co. Inc., Pauling, Calif., U.S.A. However, it will be appreciated that, with other embodiments of the disclosure, another number and other types of force generating means, e.g. leaf spring elements, magnetic or pneumatic elements etc., may be used for the gravity compensation means. 
     The force generating means  117 . 1  is supported on a support element  117 . 2  fixedly connected to the inner circumference of the lens holder  108 . Here again, similar to the first contact element  110 , the support element  117 . 2  may be made of a material with a coefficient of thermal expansion as well as mounted and designed such that it compensates for the difference in the first coefficient of thermal expansion of the lens  107  and the second coefficient of thermal expansion of the lens holder  108 . In other words, the support element  117 . 2  may be designed such that, upon the above temperature situation variation, there is substantially no shift in the contact points between the force generating means  117 . 1  and the lens  107 . 
     This avoids introduction of radial disturbing forces into the lens via the gravity compensation means  117 . However, it will be appreciated that this complete compensation may also be omitted to some extent. For example a radial relative movement may be admitted between the force generating means  117 . 1  and the lens  107  upon thermal expansion since the force generating means  117 . 1 , due to its design, may execute a rolling movement with respect to the first contact surface  107 . 3  of the lens  107 . Such a rolling movement is associated with a very low rolling friction acting onto the lens  107  and, thus, leads to a considerably reduced introduction of disturbing radial forces into the lens  107 . To further reduce the frictional forces introduced into the lens  107  at least one of the force generating means  117 . 1  and the first contact surface  107 . 3  of the lens  107  may be provided with a low friction coefficient contact surface, e.g. with a friction coefficient coating at the respective contact surface. 
     An advantage of the gravity compensation means  117  lies within the fact that the normal reaction force acting between the first contact elements  110  and the lens  107  perpendicular to the first contact surface  107 . 3  does not have to include a component resulting from the balancing of the gravitational force acting onto the lens  107 . Thus, the first contact elements  110  only exert a reduced normal contact force only balancing the clamping force exerted by the associated second contact elements  111 . This reduced normal contact force has the advantage that upon any thermally induced radial relative movement between the lens  107  and the first contact elements  110  only a reduced frictional disturbing force acts in the radial direction  116  onto the lens  107 , said frictional disturbing force being a function of the normal contact force and the friction coefficient at the contact location. 
     A further reduction of the frictional disturbing force acting onto the lens  107  upon any thermally induced radial relative movement between the lens  107  and the contact elements  110  and  111  may be achieved by providing at least one of the lens  107  and the first and second contact nose  110 . 1  and  111 . 1  with a low friction coefficient contact surface, e.g. with a low friction coefficient coating at the respective contact surface, i.e. the first and second contact surface  107 . 3  and  107 . 5 . and/or the contact surface of the first and second contact nose  110 . 1  and  111 . 1 . By this means as well only a reduced frictional disturbing force acts in the radial direction  116  onto the lens  107  upon such a thermally induced radial relative movement, said frictional disturbing force being a function of the normal contact force and the friction coefficient at the respective contact location. 
     As mentioned above, the lens holder  108  is held by a ring shaped frame element  109 . The frame element  109  itself may form a part of the housing  3 . 1  of the optical element unit  3  or may be connected to a separate part, said separate part then forming a part of the housing  3 . 1 . 
     The lens holder  108  has a first axis of symmetry  108 . 1  substantially coinciding with the optical axis  107 . 1 . The same applies to the frame element  109 , i.e. the frame element  109  has a second axis of symmetry  109 . 1  substantially coinciding with the optical axis  107 . 1  as well. 
     For reasons of reduced weight and good thermal conductivity, the frame element  109  is made of aluminum. Thus, the frame element  109  has a fourth coefficient of thermal expansion different from the second coefficient of thermal expansion of the lens holder  108 . To account for this fact, the lens holder  108  is connected to the frame element  109  via a plurality of radial deformation uncoupling elements  109 . 1  evenly distributed at the inner circumference of the frame element  109 . 
     The lens holder  108  is connected to the frame element  109  via one screw  118  per deformation uncoupling element  109 . 1 . To avoid distortion of the deformation uncoupling elements  109 . 1  when tightening the screws  118 , a protection ring  119  is placed between the heads of the screws  118  and the deformation uncoupling elements  109 . 1 . 
     In the following, a preferred embodiment of a method of holding an optical element according to the present disclosure will be described with reference to  FIGS. 1 to 4 . 
       FIG. 4  shows a block diagram of a preferred embodiment of a method of holding an optical element. 
     In a first step  20 , a plurality of module components  107 ,  108 ,  109 ,  110 ,  111 ,  117  of the optical element module  3 . 3  is provided. At least one of these module components is adapted to provide at least a reduction of forces introduced into the lens  107  upon a thermally induced position change in the relative position between the lens  107  and the lens holder  108 . 
     As mentioned above, the plurality of module components comprises the lens  107  as it has been described above in the context of  FIGS. 2 and 3 . The lens  107  is provided in a step  20 . 1 . 
     The plurality of module components further comprises the lens holder  108  and the frame element  109  as they have been described above in the context of  FIGS. 2 and 3 . The lens holder  108  and the frame element  109  are provided in a step  20 . 2   
     The plurality of module components further comprises the first and second contact elements  110  and  111  as they have been described above in the context of  FIGS. 2 and 3 . The first and second contact elements  110  and  111  are provided in a step  20 . 3 . In this step  20 . 3  the first and second contact elements  110  and  111  are designed such that they may compensate for the difference in the coefficient of thermal expansion between the lens  107  and the lens holder  108  at a temperature situation variation as it has been described above in the context of  FIGS. 2 and 3 . 
     In a step  21 . 1  of a second step  21 , the module components of the optical element module  3 . 3  are mounted together such that the lens  107 , at a first temperature situation, is held by the lens holder  108  via the first and second contact elements  110  and  111  to provide a configuration as it has been described above in the context of  FIGS. 2 and 3 . 
     In a step  21 . 2  a temperature situation variation is provided wherein the temperature situation of the optical element module  3 . 3  changes from the first temperature situation T 1  to the second temperature situation T 2  as it has been described above in the context of  FIGS. 2 and 3 . 
     In a step  21 . 3  the lens  107 , at said second temperature situation T 2 , is held by the lens holder  108  via the first and second contact elements  110  and  111  to provide a configuration as it has been described above in the context of  FIGS. 2 and 3 . As it has been described above in the context of  FIGS. 2 and 3 , the first and second contact elements  110  and  111  are designed and mounted to the lens holder  108  such that they compensate for the difference in the coefficient of thermal expansion between the lens  107  and the lens holder  108 . Thus, at the second temperature situation T 2  as well, the lens  107  is held such that substantially not thermally induced radial disturbing forces are introduced into the lens  107 . 
     Second embodiment 
     In the following, a second preferred embodiment of an optical element module  3 . 4  will be described with reference to  FIGS. 1 and 5 .  FIG. 5  shows a schematic sectional representation of a part of the optical element module  3 . 4  of the optical element unit  3  at a first temperature situation T 1  of the optical element unit  3 . The first temperature situation T 1  is characterized by a certain temperature profile within the components of the optical element unit  3 . 
     The optical element  207  of the optical element module  3 . 4  is a rotationally symmetric lens having an optical axis  207 . 1 . The lens  207  is made of Quartz (SiO 2 ) and has a first coefficient of thermal expansion. 
     The lens  207  is usually positioned in space such that the optical axis  207 . 1  of the lens  207  is substantially collinear with the optical axis  3 . 2  of the optical element unit  3 . It should be noted that the position of the optical axis  207 . 1  of the lens  207  shown in  FIG. 5  is not to scale. In reality, the optical axis  207 . 1  is located at a distance from the outer circumference of the lens  207  that by far exceeds the distance shown in  FIG. 5 . 
     The lens  207  is held by an optical element holder in the form of a ring shaped lens holder  208  which, in turn, may be held by a ring shaped frame element similar to frame element  109  of  FIG. 2 . The lens holder has a first axis of symmetry  208 . 1  which coincides with the optical axis  207 . 1 . 
     The lens holder  208  is made of Invar that has a second coefficient of thermal expansion different from, namely larger than the first coefficient of thermal expansion of the lens  207 . The lens holder  208  holds the lens  207  in place via a plurality of first contact elements  210  and a plurality of second contact elements  211 . 
     In the embodiment shown, three first contact elements  210  and three second contact elements  211  are evenly distributed at the inner circumference of the lens holder  208 . However, it will be appreciated that, with other embodiments of the disclosure, a different number of first and/or second contact elements may be provided. Furthermore, only one type of contact elements may be provided. For example, only the lower first contact elements may be provided to support the lens from below. 
     The first contact element  210  is a cylindrical roller of circular cross section. The first contact element  210  is supported on a plane annular first platform  208 . 2  of the lens holder  208 . The plane of the platform  208 . 2  is substantially perpendicular to the axis  208 . 1  and, thus, perpendicular to the optical axis  207 . 1 . The first contact element  210  contacts a plane first contact surface  207 . 3  of the lens  207 , the first contact surface  207 . 3  being perpendicular to the optical axis  207 . 1 . 
     The second contact element  211  is also a cylindrical roller of circular cross section. The second contact element  211  contacts a plane second contact surface  207 . 5  of the lens  207 , the second contact surface  207 . 3  also being perpendicular to the optical axis  207 . 1 . The second contact element  211  furthermore contacts a plane annular second platform  208 . 3  formed on a contact ring  208 . 4  of the lens holder  208 . The plane of the second platform  208 . 3  is also substantially perpendicular to the axis  208 . 1  and, thus, perpendicular to the optical axis  207 . 1 . 
     The first contact element  210  and the second contact element  211 , in the situation shown in  FIG. 5 , are arranged such that they are properly aligned in an axial direction parallel to the optical axis  207 . 1 . The cylindrical surface of the first contact element  210  forms a curved third contact surface  210 . 3  that contacts the first contact surface  207 . 3  of the lens  207 . Furthermore, the cylindrical surface of the second contact element  211  forms a curved fourth contact surface  211 . 3  that contacts the second contact surface  207 . 5  of the lens  207 . 
     To clamp the lens  207  between the first contact element  210  and the second contact element  211 , a resilient clamping element  208 . 5  is connected to the lens holder  208 . The clamping element is designed in the manner of the second contact element  111  of  FIG. 2 . Thus it has a clamping nose  208 . 6  connected via resilient arms  208 . 7  to a base part  208 . 8 , which in turn is mounted to the lens holder  208 . 
     The clamping nose  208 . 6  protrudes in such a manner that the arms  208 . 7  are elastically deflected when the base part  208 . 8  is connected to the lens holder  208 . As a consequence, the clamping nose  208 . 6 , via the contact ring  208 . 4  and the second contact element  211 , exerts a clamping force F 1  onto the second contact surface  207 . 5  of the lens  207 , said clamping force F 1  being substantially perpendicular to the second contact surface  207 . 5 . The amount of the clamping force again may be adjusted by a spacer of suitable thickness placed between the base part  208 . 8  and the lens holder  208 . 
     Since the first contact surface  207 . 3  and the second contact surface  207 . 5  are substantially perpendicular to the optical axis  207 . 1  of the lens  207 , at the first temperature situation T 1 , substantially no radial forces directed radially towards the center of the lens  207  are introduced into the lens  207  by its holding mechanism. 
     At a transition to a second temperature situation T 2  different from the first temperature situation T 1 , the temperature within the components of the optical element unit  3  is raised by a given amount. As a consequence of the rising temperature, among others, the lens  207  and the lens holder  208  expand in a radial direction  216 . Since the second coefficient of thermal expansion of the lens holder  208  is higher than the first coefficient of thermal expansion of the lens  207 , the raise in the temperature causes a relative movement between the lens  207  and the lens holder  208  in the radial direction  216  such that the lens holder  208  radially moves away from the lens  207 . 
     As mentioned above, in conventional systems with a conventional clamping mechanism directly acting onto the lens, this would lead to a relative radial movement and a residual elastic deformation at the interface between the respective contact element and the lens, both leading to the introduction of stresses into the optical element radially propagating through the optically used area of the lens. As previously explained, such radial stresses lead to imaging errors such as stress induced birefringence. 
     However, the curved third contact surface  210 . 3  of the first contact element  210  and the curved fourth contact surface  211 . 3  of the second contact element  211 , at this thermally induced relative movement between the lens  207  and the lens holder  208 , both execute a rolling movement on the first contact surface  207 . 3  and the second contact surface  207 . 5 , respectively. The curved third contact surface  210 . 3  and the curved fourth contact surface  211 . 3  also execute a rolling movement on the first platform  208 . 2  and the second platform  208 . 3 , respectively. Since both contact elements  210  and  211  have the same diameter, the contact elements  210  and  211  perform a synchronous rotation such that they keep being aligned in a direction parallel to the optical axis  207 . 1 . 
     This rolling movement is associated with very low frictional forces introduced into the lens  207  and directed in the radial direction  216 . It will be appreciated that, in other words, the rolling movement is a substantially pure rolling movement with substantially no friction. The substantially negligible residual friction that occurs here results from the deformation induced deviation of the contact area from the ideal line contact of the cylindrical contact element  210 ,  211  with its respective contact partner. Thus, a considerable reduction of thermally induced radial disturbing forces is achieved with the disclosure compared to conventional systems without such rolling contact elements. Thus disturbing radial stresses leading to imaging errors such as stress induced birefringence may be reduced considerably with the disclosure. 
     It will be appreciated that, with other embodiments of the disclosure, the respective first and second contact element does not necessarily have to contact the lens directly. For example, intermediate elements may be connected to the lens and contact the respective contact element. These intermediate elements may also be a clamping element designed in the manner of the clamping element  208 . 5  as it has been described above. 
     As outlined above, certain considerably reduced thermally induced radial disturbing forces may be introduced into the lens  207  at a temperature situation variation. However, the disclosure also provides for a further reduction of these thermally induced radial disturbing by the following means. 
     First of all, as had been explained in the context of  FIG. 3 , a gravity compensation means similar to the gravity compensation means  117  is provided. This gravity compensation means has the effect that the normal reaction force acting between the first contact elements  210  and the lens  207  perpendicular to the first contact surface  207 . 3  does not have to include a component resulting from the balancing of the gravitational force acting onto the lens  207 . Thus, the first contact elements  210  only exert a reduced normal contact force only balancing the clamping force exerted by the associated second contact elements  211 . This reduced normal contact force has the advantage that upon any thermally induced radial relative movement between the lens  207  and the first contact elements  210  only an even further reduced frictional disturbing force acts in the radial direction  216  onto the lens  207 , said frictional disturbing force being a function of the normal contact force and the friction coefficient at the contact location. 
     A further reduction of the frictional disturbing force acting onto the lens  207  upon any thermally induced radial relative movement between the lens  207  and the contact elements  210  and  211  may be achieved by providing at least one of the lens  207  and the first and second contact elements  210  and  211  with a low friction coefficient contact surface, e.g. with a low friction coefficient coating at the respective contact surface, i.e. the first and second contact surface  207 . 3  and  207 . 5  and/or the third or fourth contact surface  210 . 3  and  211 . 3 . By this means as well, only an even further reduced frictional disturbing force acts in the radial direction  216  onto the lens  207  upon such a thermally induced radial relative movement, said frictional disturbing force being a function of the normal contact force and the friction coefficient at the respective contact location. 
     Finally, a further reduction of the frictional disturbing force acting onto the lens  207  upon any thermally induced radial relative movement between the lens  207  and the contact elements  210  and  211  may be achieved by providing a securing device  222 . This securing device  222  overall allows to reduce, under normal operating conditions, the holding forces exerted on the lens  207  and, thus, also the disturbing frictional forces introduced into the lens  207  at a thermally induced relative movement between the lens  207  and the lens holder  208  under normal operating conditions. 
     This solution is based on the concept that the holding forces usually counteract also the thermally induced relative movement between the lens  207  and the lens holder  208  and, thus, have an influence on the frictional forces introduced into the lens  207  at such a relative movement between the lens  207  and the lens holder  208 . Usually, due to the manufacture and mounting of the optical element unit  3  at a location different from the location of its later use, the holding forces provided for the lens  207  do not only account for the forces occurring under normal operating conditions of the optical system but also have to account for considerably higher abnormal forces occurring during transport of the optical element unit  3 , for example. Thus, in conventional systems, holding forces exerted onto the lens  207  are considerably higher than necessary in normal use. Due to the correlation between the holding forces and the disturbing forces outlined above, this obviously leads to increased disturbing forces introduced into the lens  207  at a thermally induced relative movement between the lens  207  and the lens holder  208 . 
     The securing device  222 , further reduces these disturbing forces by allowing a reduction of the holding forces exerted on the lens  207  under normal operating conditions. The securing device  222  is only activated under abnormal load conditions in order to hold the lens  207 . To this end, the securing device  222  is fixedly mounted to the lens holder  208  and provides a stop element  222 . 1  that is spatially associated to the resiliently mounted clamping nose  208 . 6  of the respective clamping element  208 . 5 . 
     The clamping element  208 . 5 , via the second contact element  211 , under normal operating conditions of the optical element unit  3 , i.e. under a normal load situation, exerts a first holding force onto the lens  207 . This first holding force ranges up to a holding force limit is a maximum force that is necessary (together with the holding forces of the other clamping elements  208 . 5 ) to hold the lens  207  substantially in place against normal displacement forces to be expected to act onto the lens  207  under said normal load situation. 
     As long as the displacement forces acting onto the lens  207  do not require exertion of this holding force limit, a small gap  223  is formed between the stop element  222 . 1  and the clamping nose  208 . 6 . As soon as the holding force limit is reached, the clamping nose  208 . 6  comes into tight contact with the substantially rigid stop element  222 . 1  such that the holding forces exerted in the lens  207  abruptly increase to hold the lens  207  in place against abnormal displacement forces exceeding the displacement forces acting under normal operating conditions of the optical element unit  3 . 
     It should be noted that the gap  223  shown in  FIG. 5 , for reasons of better visibility, is way out of scale. In reality, the gap  223  is sufficiently small to provide sufficient contact between the lens  207  and the contact elements  210 ,  211  under any load condition. 
     It will be appreciated that, with other embodiments of the disclosure, the securing device may adapted to contact any other movable part of the clamping element  208 . 5  or any other suitable part of the lens  207  or any other movable component in mechanical connection with the lens  207 . Furthermore, the clamping element may be of any other suitable design that is activated under abnormal load conditions only. For example, it may be an active device, e.g. a electrically, pneumatically or otherwise actuated device, that is actively brought into contact with the lens  207 , the clamping nose  208 . 6  or any other movable component in mechanical connection with the lens  207  under abnormal load situations. 
     It will be further appreciated that the securing device  222  may also be used in combination with the embodiment shown in  FIG. 2  leading to the above beneficial reduction in the necessary holding forces under normal operating conditions. 
     It will be further appreciated that, with other embodiments of the disclosure, the respective first and second contact element does not necessarily have to be a cylindrical element. It is only necessary that the respective contact element has a curved contact surface that executes, upon a temperature situation variation, a substantially purely rotational movement on an interface surface between the lens and the lens holder where the relative motion occurs. For example, the respective contact element may be a ball shaped element. In this case, the contact partner of the contact element does not necessarily have to provide one single planar contact surface. For example, it is also possible that the ball shaped contacts the two contact surfaces of a substantially V-shaped groove extending in the radial direction. The ball shaped contact element, upon a temperature situation variation, then may execute the substantially frictionless rolling movement along this V-shaped groove. Furthermore, the respective contact element may be fixedly connected to one of the lens  207  and the lens holder  208 . 
       FIGS. 6A and 6B  show different views of an example of a contact element  310  that may replace the first contact element  210  and/or the second contact element  211  of  FIG. 5 . The contact element  310  has a movable contact part  310 .  4  with a spherical contact surface  310 . 3 . Via a flexure  310 . 5 , the spherical contact part  310 . 4  is monolithically connected to a base part  310 . 6 . The base part  310 . 6  may be connected to the lens holder  208  or the lens  207 , such that the flexural axis  310 . 7  of the flexure  310 . 5  runs perpendicular to the radial direction  216  in a plane perpendicular to the optical axis  207 . 1  of the lens  207 . 
     It will be appreciated that the optical element unit  3 . 4  of  FIG. 5  may as well be used to perform a method of holding an optical element similar to the one as it has been described above with reference to  FIG. 4 . 
     The difference with respect to the method performed with the embodiment of  FIG. 2  lies within the fact that, upon any change in the temperature situation, i.e. in step  21 . 2  of  FIG. 4 , a relative motion takes place at the mechanical interface between the lens  207  and the lens holder  208 . However, this relative motion is a low friction motion, namely a rolling motion. 
     Third Embodiment 
     In the following, a third preferred embodiment of an optical element module  3 . 5  will be described with reference to  FIGS. 1 ,  7 A and  7 B.  FIGS. 7A and 7B  show representations of a part of the optical element module  3 . 5  of the optical element unit  3 . 
     The optical element  407  of the optical element module  3 . 5  is a rotationally symmetric lens having an optical axis which, when mounted in the optical element unit  3 , lies in a substantially horizontal plane. 
     The lens  407  is made of Quartz (SiO 2 ) and is held by an optical element holder in the form of a ring shaped lens holder  408 . The lens holder has a first axis of symmetry which coincides with the optical axis of the lens  407 . The lens holder  408  is made of Invar that has a second coefficient of thermal expansion different from, namely larger than the first coefficient of thermal expansion of the lens  407 . The lens holder  408  holds the lens  407  in place via a plurality of first contact elements  410  and a plurality of second contact elements  411  in a manner as it has been described above in the context of  FIG. 2 , such that it is here mainly referred to the above explanations. 
     The optical element module  3 . 5  also has a gravity compensation device  417 . This gravity compensation device  417 —as the gravity compensation device  117 —is adapted to exert a support force onto the lens  407  that substantially balances the gravitational force acting on the lens  407  due to its mass. 
     Due to the so called standing arrangement of the lens  407 , the gravity compensation device  417  comprises a flexible tension element  417 . 1  in the form of a rope or strap. The tension element  417 . 1  has a middle section  417 . 3  and two end sections  417 . 4  and  417 . 5 . Both end sections  417 . 4  and  417 . 5  are hung to the lens holder  408  at a location located above the center of gravity of the lens  407  such that the middle section  417 . 3  is wrapped around a lower part of the lens  407 . 
     The force exerted by the gravity compensation device  417  onto the lens  407  may be adjusted by adjusting the pretension of springs  417 . 6  acting via nuts  417 . 7  onto threaded bolts  417 . 8  connected to the tension element  417 . 1  at its respective end section  417 . 4  and  417 . 5 . The angle of wrap is about 170° such that the forces exerted by the gravity compensation device  417  onto the lens  407  are distributed over a wide area avoiding local stress concentrations. 
     The lens  407  may be located in a seat within the lens holder  408  precisely defining the position of the lens in the radial direction, i.e. in the vertical plane. Anyway, it will be appreciated that the forces exerted by the gravity compensation device  417  may slightly exceed the gravitational force acting on the lens  407  such that the lens  407  is pulled against a plurality of stops—preferably two stops—provided on the lens holder  408  at the upper circumference of the lens  407  to secure the position of the lens in the vertical plane. 
     Furthermore, one or several further stops may be provided at the lower part of the lens holder  408  which the lens  407  may contact in case of abnormal vertical loads acting onto the lens  407 , e.g. during transport of the optical element unit  3 . 
     Fourth Embodiment 
     In the following, a fourth preferred embodiment of an optical element module  3 . 6  will be described with reference to  FIGS. 1 ,  8 A and  8 B.  FIGS. 8A and 8B  show representations of a part of the optical element module  3 . 6  of the optical element unit  3 . 
     The optical element  507  of the optical element module  3 . 6  is a rotationally symmetric mirror having an optical axis which, when mounted in the optical element unit  3 , lies in a substantially horizontal plane. 
     The mirror  507  is held by an optical element holder in the form of a ring shaped mirror holder  508 . The mirror holder has a first axis of symmetry which coincides with the optical axis of the mirror  507 . The mirror holder  508  is made of a material that has a second coefficient of thermal expansion different from, namely larger than the first coefficient of thermal expansion of the mirror  507 . The mirror holder  508  holds the mirror  507  in place via a plurality of first contact elements  510  and a plurality of second contact elements  511  in a manner as it has been described above in the context of  FIG. 2 , such that it is here mainly referred to the above explanations. 
     The optical element module  3 . 6  also has a gravity compensation device  517 . This gravity compensation device  517 —as the gravity compensation devices  117  and  417 —is adapted to exert a support force onto the mirror  507  that substantially balances the gravitational force acting on the mirror  507  due to its mass. 
     Due to the so called standing arrangement of the mirror  507 , the gravity compensation device  517  comprises a plurality of resilient force exerting elements  517 . 1  in the form of leaf spring elements. The leaf spring elements  517 . 1  are formed by slots in an arc shaped base element  517 . 9  fixedly connected to the mirror holder  508  at a location located below the center of gravity of the mirror  507 . The base element  517 . 9  is arranges symmetrically with respect to the vertical axis of symmetry  507 . 6  of the mirror  507 . 
     The free ends of the leaf spring elements  517 . 1  contact the outer circumference of the mirror  507  over an angle of about 90° in a lower part of the mirror  507 . However, it will be appreciated that other angles may be chosen, where appropriate. The leaf spring elements  517 . 1  are adapted such that the force exerted onto the mirror by the respective leaf spring element  517 . 1  decreases with increasing distance from the vertical axis  507 . 6  of the mirror  507 . Thus, proper support corresponding to the mass distribution of the mirror is achieved. 
     Again, the mirror  507  may be located in a seat within the mirror holder  508  precisely defining the position of the mirror in the radial direction, i.e. in the vertical plane. Anyway, it will be appreciated that the forces exerted by the gravity compensation device  517  may slightly exceed the gravitational force acting on the mirror  507  such that the mirror  507  is pushed against a plurality of stops—preferably two stops—provided on the mirror holder  508  at the upper circumference of the mirror  507  to secure the position of the mirror in the vertical plane. 
     Furthermore, one or several further stops may be provided at the lower part of the mirror holder  508  which the mirror  507  or the leaf springs  517 . 1  may contact in case of abnormal vertical loads acting onto the mirror  507 , e.g. during transport of the optical element unit  3 . 
     Although, in the foregoing, embodiments of the present disclosure have been described where the optical element has a circular shape, it will be appreciated that, with other embodiments of the present disclosure, the optical element may have any other shape. The same applies for the optical element holder. 
     Furthermore, the present disclosure has been described mostly in the context of embodiments where refractive optical elements such as lenses and plane parallel plates are held by respective optical element holders. However, it will be appreciated that, with other embodiments of the present disclosure, other types of optical elements, such as reflective and/or diffractive optical elements, e.g. mirrors or gratings or the like, may be held by a corresponding optical element holder as it has been described above. 
     Furthermore, the present disclosure has been described in the context of an optical element unit incorporating different designs of optical element modules. However, it will be appreciated that the disclosure may also be used in the context of optical element units incorporating one single design or type of optical element module. 
     Furthermore, the present disclosure has been described in the context of an optical element unit having a folded optical axis. However, it will be appreciated that the disclosure may also be used in the context of optical element units having a straight optical axis or an arbitrarily often folded optical axis. 
     Finally, the present disclosure has been described in the context of embodiments for optical exposure processes. However, it will be appreciated that the disclosure may also be used in the context of any other optical application, where a relief of an optical element from stresses resulting from thermal expansion in the region of the respective optical element is required.