Strut and optical system

Abstract

The invention relates to a strut (134, 136, 200) for an optical system (100), having a plurality of flexure hinges (234) and a compensation force generating device (300A, 300B, 300C), wherein the strut (134, 136, 200) can be brought from an undeflected state (Z1) into a deflected state (Z2) with the aid of a deformation of the plurality of flexure hinges (234), and wherein the compensation force generating device (300A, 300B, 300C) is designed to generate a compensation force (F-) when the strut (134, 136, 200) is brought from the undeflected state (Z1) into the deflected state (Z2), which compensation force (F-) compensates for a parasitic force (F+) resulting from the deformation of the plurality of flexure hinges (234).

Description

[0001] STRUT AND OPTICAL SYSTEM

[0002] The present invention relates to a strut for an optical system and an optical system comprising such a strut.

[0003] The content of the priority application DE 10 2024 201 139.8 is incorporated in its entirety by reference.

[0004] Microlithography is used to manufacture microstructured components, such as integrated circuits. The microlithography process is carried out using a lithography system equipped with an illumination system and a projection system. The image of a mask (reticle) illuminated by the illumination system is projected by the projection system onto a substrate, such as a silicon wafer, coated with a highly sensitive layer (photoresist) and positioned in the image plane of the projection system. This transfers the mask structure to the light-sensitive coating of the substrate.

[0005] Driven by the pursuit of ever smaller structures in the production of integrated circuits, EUV lithography systems are currently being developed that use light with a wavelength in the range of 0.1 nm to 30 nm, particularly 13.5 nm. Due to the high absorption of light at this wavelength by most materials, such EUV lithography systems require the use of reflective optics, i.e., mirrors, instead of the previously used refractive optics, i.e., lenses.

[0006] To measure such a mirror in a measuring machine, so-called bipods can be used to support or mount the mirror. Such a bipod comprises two struts arranged at an angle to each other. Such a strut has a monolithic joint that locks exactly one degree of freedom and exhibits a high and defined degree of compliance in the remaining degrees of freedom. For example, this can be achieved using elements made of wire, folded leaf springs, or a combination of two swivel joints with two universal joints per strut.

[0007] With the help of three bipods or six struts, the position of such a mirror can be defined, for example, on a measuring table of a measuring machine. A locked degree of freedom per strut is achieved by free cuts that prevent force transmission in any direction other than the longitudinal direction of the respective strut. Such a strut can also be referred to as a single degree of freedom joint, especially a monolithic one.

[0008] If a force is applied obliquely or perpendicularly to the longitudinal direction, the strut deflects. This deflection of the strut is made possible by elastically deformable solid-state joints. However, since the spring stiffness of the solid-state joints cannot be chosen arbitrarily small, the deformation of the solid-state joints results in parasitic forces and moments that can be introduced into the mirror. These parasitic forces and moments can lead to undesirable deformations (surface figure deformation, SFD) of an optically effective surface of the mirror.

[0009] Against this background, it is an object of the present invention to provide an improved strut for an optical system.

[0010] Accordingly, a strut for an optical system is proposed. The strut comprises a plurality of solid-state joints and a compensation force generating device. The strut can be moved from an undeflected state to a deflected state by deforming the plurality of solid-state joints. The compensation force generating device is configured to generate a compensation force upon moving the strut from the undeflected state to the deflected state, which compensates for a parasitic force resulting from the deformation of the plurality of solid-state joints.

[0011] Because the compensating force generating device compensates for the parasitic force with the aid of the compensating force, it is possible, for example, to prevent the parasitic force from being introduced into the optical element when the strut is used to support an optical element. This prevents unwanted deformation of an optically effective surface of the optical element. This prevents the optical properties of the optically effective surface from being adversely affected.

[0012] In this context, a "strut" is understood to mean a component that has its greatest extent in one spatial direction, for example, in a longitudinal direction of the strut. In two other spatial directions, the strut has significantly smaller geometric dimensions than in the spatial direction mentioned above. The strut can be assigned a coordinate system with a first spatial direction or x-direction, a second spatial direction or y-direction, and a third spatial direction or z-direction. The directions are perpendicular to each other. The longitudinal direction of the strut is preferably oriented along the z-direction. The strut is preferably assigned a central or symmetry axis, to which the strut is rotationally symmetrical. The longitudinal direction runs along the axis of symmetry. The longitudinal direction can also be referred to as the axial direction.The strut can also be referred to as a single degree of freedom joint or IDoF joint (EnglJ 1 Degree of Freedom), especially if it is monolithic.

[0013] The strut can also be called a socket strut. The strut can

[0014] It can be part of a mount for an optical element of the optical system. For example, two struts can be combined to form a so-called bipod. The optical element can be mounted using several such bipods. The strut can also be part of a manipulator or actuator for actuating the optical element. However, the strut can also be part of a gravity compensator. Such a gravity compensator can counteract the weight of the optical element and thus compensate for it.

[0015] The strut preferably has exactly one degree of freedom. This means, in particular, that the strut can only transmit forces along its longitudinal direction. In directions deviating from the longitudinal direction, the strut can transmit no forces or only minimal forces. This is achieved by ensuring that the axial spring stiffness of the strut oriented along the longitudinal direction is significantly greater than the lateral spring stiffness of the strut oriented perpendicular to the longitudinal direction.

[0016] Any component, such as the optical element mentioned above, has six degrees of freedom, namely three translational degrees of freedom along the x-direction, the y-direction, and the z-direction, as well as three rotational degrees of freedom about the x-direction, the y-direction, and the z-direction. This means that a position and an orientation of the component can be determined or described using the six degrees of freedom. The "position" of the component refers in particular to its coordinates with respect to the x-direction, the y-direction, and the z-direction. The "orientation" of the component refers in particular to its tilt with respect to the three spatial directions. This means that the component can be tilted about the x-direction, the y-direction, and / or the z-direction.

[0017] This results in six degrees of freedom for the position and / or orientation of the component. A "position" of the component encompasses both its position and its orientation. The term "position" can therefore be replaced by the phrase "position and orientation," and vice versa. Accordingly, the component can be supported, for example, with six such struts. A hexapodic arrangement can be realized with six struts. Such a hexapodic arrangement, due to its overall topology, results in the blocking of all six degrees of freedom. For example, a pair of struts is required to block a tilting degree of freedom. The position of the component can thus be defined using the six struts.

[0018] The strut can have any number of flexural joints. The strut preferably comprises a strut housing on which the flexural joints are provided. At least two flexural joints are provided. The strut housing can have multiple sections, for example, a first connection section, a second connection section, and a central section arranged between the two connection sections, which are pivotably or tiltably connected to one another by means of the flexural joints. For this purpose, the flexural joints are elastically deformable, particularly spring-elastic.

[0019] A "flexible joint" is generally understood to mean an area, for example, a cross-sectional constriction or thinning, of a component that enables relative movement between two rigid body regions of the component through bending or torsion, or a combination thereof. By adjusting the spring stiffness of the flexible joint, it can be adapted to any application. Such a flexible joint can, for example, be web-shaped or spring-shaped, in particular leaf spring-shaped. The flexible joints can be implemented by cutouts provided in the aforementioned strut housing. A "cutout" is understood to mean an opening that extends from an outer side or outer surface of the strut housing to an inner side or inner surface of the strut housing. The cutouts can be slot-shaped.

[0020] Every flexural joint can deform elastically from an undeformed state to a deformed state and back again. To do this, a force is applied to the respective flexural joint. When this force is no longer applied, the respective flexural joint deforms independently or spontaneously from the deformed state back to the undeformed state. This aforementioned deformation of the flexural joint from the undeformed state to the deformed state results in the parasitic force or forces that counteract the force or forces required to deform the flexural joint.

[0021] If the strut is moved from the undeflected state to the deflected state, the flexure joints deform. The strut can also be moved from the deflected state back to the undeflected state by further deformation of the flexure joints. For example, the strut is moved from the undeflected state to the deflected state by applying a force. This is the case, for example, if a force acting on the strut does not act along the longitudinal direction of the strut, but rather perpendicular or oblique to it. This results in the strut deforming from the undeflected state to the deflected state, whereby the strut cannot transmit the force oriented obliquely to the longitudinal direction.

[0022] The deflected state of the strut differs from the undeflected state in that the flexure joints are in the deformed state in the deflected state. The flexure joints can thus return the strut from the deflected state to the undeflected state. For example, in the deflected state, one of the previously mentioned connection sections of the strut housing is tilted relative to the central section, whereas the corresponding connection section is not tilted relative to the central section in the undeflected state.

[0023] The compensation force generating device generates the compensation force or the compensation forces counteracting the parasitic force when the strut is moved from the undeflected state to the deflected state. This prevents the parasitic force or the parasitic forces from being transmitted to a component, for example, to an optical element as mentioned above, which is operatively connected to the strut in the deflected state. In particular, the plurality of flexure joints have a positive spring stiffness. The compensation force generating device, in contrast, has a negative spring stiffness. The positive spring stiffness and the negative spring stiffness compensate each other. As a result, the parasitic forces can be compensated within a deflection path of the strut.

[0024] In this case, the fact that the compensating force and the parasitic force "compensate" or "cancel" each other means, in particular, that the compensating force and the parasitic force are equal in magnitude and oppositely oriented. "Compensating" or "cancelling" can also be understood to mean that the parasitic force is compensated, preferably only by 80%, more preferably only by 90%, more preferably only by 95%, more preferably only by 99%, more preferably exactly by 100%. This means, in particular, that "compensating" can also be understood to mean that small parasitic forces can still occur. In other words, the compensating force or the compensating forces do not necessarily have to completely cancel or compensate the parasitic force or the parasitic forces.This means in particular that the compensation force generated by the compensation force generating device at least partially compensates or cancels out the parasitic force resulting from the deformation of the solid-state joints.

[0025] According to one embodiment, the strut has a strut housing, wherein the compensation force generating device is arranged within the strut housing.

[0026] The strut housing is also constructed rotationally symmetrically to the axis of symmetry. The strut housing can be hollow-cylindrical or tubular. Therefore, the strut housing can also be referred to as a strut tube. The strut housing preferably has an outer surface as mentioned above and an inner surface facing away from the outer surface as mentioned above. The outer surface and the inner surface can be cylindrical. The strut housing encloses an interior space within which the compensation force generating device is arranged. In other words, the strut housing encloses or encapsulates the compensation force generating device. The strut housing can be open at its end. The compensation force generating device is preferably set back with respect to the end surfaces of the strut housing.

[0027] According to a further embodiment, the strut housing has a first connection section, a second connection section and a middle section arranged between the first connection section and the second connection section, wherein the first connection section is connected to the middle section by means of several solid joints, and wherein the second connection section is connected to the middle section by means of several solid joints.

[0028] The strut housing is preferably a one-piece component, in particular a one-piece component made of a single material. "One-piece" or "single-piece" here means that the strut housing is not constructed from different sub-components, but rather that the first connection section, the second connection section, and the central section form a common component, namely the strut housing. "One-piece material" here means that the strut housing is made entirely of the same material. For example, the strut housing can be made of a metallic material. Stainless steel, for example, is used as the material for the strut housing. Preferably, the first connection section is connected to the central section with four flexural joints. Accordingly, the second connection section is also preferably connected to the central section with the aid of four flexural joints.The central section is arranged between the first connection section and the second connection section, viewed along the longitudinal direction of the strut.

[0029] According to a further embodiment, when the strut is moved from the undeflected state to the deflected state, the first connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes of the first connection section, wherein the second connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes of the second connection section when the strut is moved from the undeflected state to the deflected state.

[0030] The joint axes are preferably oriented perpendicular to the central axis. Each of the two joint axes of the first connection section is preferably assigned exactly two flexural joints. These two flexural joints of each joint axis are offset by 180° from one another. The first connection section is assigned a first joint axis and a second joint axis oriented perpendicular to the first joint axis. The two joint axes of the first connection section form a first universal joint of the strut or strut housing. The two flexural joints of the first joint axis of the first connection section and the two flexural joints of the second joint axis of the first connection section are each offset by 90° from one another. The joint axes preferably intersect in the strut's axis of symmetry. The two joint axes of the second connection section are also each assigned two flexural joints.The flexural joints of a joint axis are arranged offset by 180° from one another. The first connection section can be tilted or pivoted about both joint axes simultaneously relative to the central section. The second connection section is also assigned a first joint axis and a second joint axis oriented perpendicular to the first joint axis. The two joint axes of the second connection section form a second universal joint of the strut or strut housing. The two flexural joints of the first joint axis of the second connection section and the two flexural joints of the second joint axis of the second connection section are positioned offset by 90° from one another. The joint axes of the second connection section intersect each other, in particular in the axis of symmetry. The second connection section can be tilted or pivoted about both joint axes simultaneously relative to the central section.The first joint axis of the first connection section and the first joint axis of the second connection section preferably run parallel to each other and spaced apart from each other. Accordingly, the second joint axis of the first connection section and the second joint axis of the second connection section preferably also run parallel to each other and spaced apart from each other.

[0031] According to a further embodiment, the plurality of solid-state joints have a positive spring stiffness, wherein the compensating force generating device has a negative spring stiffness, and wherein the positive spring stiffness and the negative spring stiffness compensate for each other. The positive spring stiffness and the negative spring stiffness "compensate" or "cancel" each other is understood here to mean, in particular, that the strut has zero spring stiffness in a direction oblique or perpendicular to the longitudinal direction or the axis of symmetry. This means, in particular, that the strut can be moved from the undeflected state to the deflected state without force or without force within a deflection range of the strut as mentioned above."Forceless" or "force-free" means that no parasitic force or forces result from moving the strut from the undeflected state to the deflected state. The deflection range in which the strut exhibits the aforementioned zero spring stiffness can, for example, be ± 100 mm to ± 2 mm. The size of this deflection range can be adjusted within a wide range depending on the application by designing the flexure joints and the compensating force generating device. The term "positive spring stiffness" in this context specifically means that the force required to deflect the flexure joints becomes increasingly larger with an increasing deflection path. In contrast, the term "negative spring stiffness" in this context means that the force required to deflect the compensating force generating device becomes increasingly smaller with an increasing deflection path.In other words, with positive spring stiffness, a reaction force of the flexure joints will act opposite to a direction of movement to deflect the flexure joints. Thus, the movement works against the reaction force. With negative spring stiffness, the opposite occurs. If the compensating force generating device is deflected, a reaction force is generated in the same direction as a direction of movement to deflect the compensating force generating device. According to a further embodiment, the compensating force generating device is spring-loaded along a longitudinal direction of the strut.

[0032] The spring preload is preferably generated by compressive forces acting on the compensating force generating device. With the help of the spring preload, it is possible to achieve the aforementioned negative stiffness Fe of the compensating force generating device. This is achieved by the fact that when the compensating force generating device is deflected, the spring preload causes the force required to deflect the compensating force generating device to become increasingly smaller as the deflection path increases, since the compensating force generating device is automatically deflected further and further by the spring preload.

[0033] According to a further embodiment, when the strut is moved from the undeflected state to the deflected state, the compensation force generating device automatically moves from an unstable rest state to a compensation force generating state with the aid of the spring preload in order to generate the compensation force.

[0034] In particular, the spring preload ensures that the compensating force generating device is moved from the unstable rest state to the compensating force generating state exclusively with the help of the spring preload and without the application of an external force. However, it is also possible for the compensating force generating device to be moved from the unstable rest state to the compensating force generating state with the help of a weight force. The term "unstable" in the present case means, in particular, that upon a slight deflection of the compensating force generating device from the unstable rest state toward the compensating force generating state, the spring preload moves or moves the compensating force generating device from the unstable rest state further toward the compensating force generating state. An external force is no longer required for this after the slight deflection.

[0035] According to a further embodiment, the compensation force generating device for generating the spring preload has a first spring element and a second spring element, wherein the first spring element applies a first compressive force to the compensation force generating device, and wherein the second spring element applies a second compressive force to the compensation force generating device.

[0036] The first spring element and the second spring element are preferably tension springs. The first spring element and the second spring element can be designed as cylindrical springs. Another possibility is the use of spiral springs. The first spring element and the second spring element generate the spring preload so that the compensation force generating device can be brought from the unstable rest state into the compensation force generating state with the aid of the first spring element and with the aid of the second spring element. Thus, the compensation force or the compensation forces are generated with the aid of the first spring element and / or the second spring element. The compensation force is or the compensation forces are preferably a force oriented perpendicular to the longitudinal direction or forces oriented perpendicular to the longitudinal direction, which result or result from the applied compressive forces.

[0037] According to a further embodiment, the first spring element and the second spring element move the compensation force generating device from the unstable rest state to the compensation force generating state when the strut is moved from the undeflected state to the deflected state.

[0038] In other words, the first spring element and the second spring element automatically pull or move the compensating force generating device into the compensating force generating state upon a slight deflection from the unstable rest state. The compensating force generating state differs from the unstable rest state in that the compensating force generating device does not generate a compensating force in the unstable rest state.

[0039] According to a further embodiment, the compensation force generating device has a base section, a first rod section pivotably connected to the base section, which is subjected to the first compressive force by the first spring element, and a second rod section pivotably connected to the base section, which is subjected to the second compressive force by the second spring element, wherein the base section is arranged between the first rod section and the second rod section.

[0040] In particular, the base section is disc-shaped. The base section is firmly connected to the strut housing. In particular, the base section can be glued, soldered, and / or welded to the strut housing. With respect to the longitudinal direction, the base section is preferably positioned centrally in the strut housing. The first rod section is preferably connected to the base section by means of a proximal joint section. Accordingly, the second rod section is also connected to the base section by means of a proximal joint section. "Proximal" here means facing the base section. In contrast, "distal" means facing away from the base section. The aforementioned joint sections can be wire-shaped. The joint sections are resiliently deformable and enable pivoting of the rod sections relative to the base section. The joint sections function as solid-state joints.Each joint section is preferably assigned a pivot point about which the respective rod section can be pivoted or tilted relative to the base section.

[0041] According to a further embodiment, the first rod section is pivotably connected to a first fastening section of the compensation force generating device, wherein the second rod section is pivotably connected to a second fastening section of the compensation force generating device.

[0042] In particular, the first rod section is pivotally connected to the first fastening section by means of a distal joint section. Accordingly, the second rod section is also connected to the fastening section by means of a distal joint section. The joint sections are in particular wire-shaped. The joint sections are designed as solid-state joints. Each joint section is preferably assigned a pivot point about which the respective rod section can be pivoted or tilted relative to the respective fastening section. The first fastening section is connected to the strut housing by means of a first fastening element. Accordingly, the second fastening section is connected to the strut housing by means of a second fastening element. The fastening elements can be spring-shaped, in particular leaf spring-shaped. Each fastening element can have two strut sections arranged perpendicular to one another.A first strut section can run parallel to the first joint axes of the connection sections of the strut housing, and a second strut section can run parallel to the second joint axes of the strut housing. Alternatively, the fastening elements can also be designed as resiliently deformable membranes. According to a further embodiment, the first rod section has a proximal subsection facing the base section and a distal subsection facing away from the base section, which is pivotably connected to the proximal subsection of the first rod section, wherein the second rod section has a proximal subsection facing the base section and a distal subsection facing away from the base section, which is pivotably connected to the proximal subsection of the second rod section.

[0043] Accordingly, the base section is arranged between the two proximal subsections. The proximal subsections are fixedly connected to the base section so that they cannot pivot relative to the base section. The subsections of the first rod section and the subsections of the second rod section are each connected to one another by means of wire-shaped joint sections. The joint sections are solid-state joints. A pivot point formed by the joint section of the first rod section preferably lies on the joint axes of the first connection section of the strut housing. Accordingly, a pivot point formed by the joint section of the second rod section preferably lies on the joint axes of the second connection section of the strut housing. The distal subsections of the two rod sections are preferably connected to the aforementioned fastening sections, which are coupled to the strut housing.

[0044] According to a further embodiment, the compensating force generating device comprises a first receiving tube, within which the first spring element is arranged, and a second receiving tube, within which the second spring element is arranged, wherein the first spring element applies the first compressive force to the first receiving tube, and wherein the second spring element applies the second compressive force to the second receiving tube. Preferably, the first receiving tube and the second receiving tube are structurally identical to the strut housing. However, the first receiving tube and the second receiving tube are smaller than the strut housing, so that the first receiving tube and the second receiving tube can be placed within the strut housing. The two receiving tubes are coupled to a base section of the compensating force generating device, as mentioned above. The base section is connected to the strut housing.The base section is arranged between the first receiving tube and the second receiving tube. The first spring element is connected, in particular, to the base section and to the first receiving tube. Accordingly, the second spring element is connected to the base section and to the second receiving tube.

[0045] According to a further embodiment, the first receiving tube has a proximal connection section, a distal connection section, and a central section arranged between the proximal connection section and the distal connection section, wherein the proximal connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes, wherein the distal connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes, wherein the second receiving tube has a proximal connection section, a distal connection section, and a central section arranged between the proximal connection section and the distal connection section, wherein the proximal connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes,and wherein the distal connection section is pivotable relative to the central section about two mutually perpendicular and intersecting joint axes. The proximal connection section, the distal connection section, and the central section of the first receiving tube are connected to one another by means of solid-state joints as mentioned above. Accordingly, the proximal connection section,The distal connection section and the middle section of the second receiving tube are connected to each other by means of solid-state joints. The first receiving tube and the second receiving tube are preferably structurally identical. Each of the aforementioned joint axes of the receiving tubes is preferably formed by two solid-state joints offset by 180° from each other. The proximal connection sections of the receiving tubes are preferably firmly connected to the base section of the compensation force generating device. The distal connection sections of the receiving tubes are connected to the strut housing by means of fastening elements as mentioned above. The spring elements are attached to the distal connection sections.

[0046] Furthermore, an optical system, in particular a measuring machine or projection optics, is proposed. The optical system comprises an optical element and at least one such strut, wherein the strut is connected to the optical element.

[0047] The optical system can have any number of struts. The optical system can also have any number of optical elements. The optical system is preferably a projection optics system of a projection exposure system. However, the optical system can also be an illumination system. Furthermore, the optical system can also be a measuring machine, in particular a measuring machine for interferemetric measurement of the optical element. The projection exposure system can be an EUV lithography system. EUV stands for "Extreme Ultraviolet" and refers to a wavelength of the working light between 0.1 nm and 30 nm. The projection exposure system can also be a DUV lithography system. DUV stands for "Deep Ultraviolet" and refers to a wavelength of the working light between 30 nm and 250 nm.

[0048] "One" in this case is not necessarily limited to a single element. Rather, multiple elements, such as two, three, or more, may also be included. Any other counting term used here should not be understood as implying a limitation to the exact number of elements mentioned. Rather, numerical deviations upwards and downwards are possible, unless otherwise stated.

[0049] The embodiments and features described for the strut apply accordingly to the proposed optical system and vice versa.

[0050] Further possible implementations of the invention also include combinations of features or embodiments described above or below with respect to the exemplary embodiments that are not explicitly mentioned. In this case, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention.

[0051] Further advantageous embodiments and aspects of the invention are the subject of the dependent claims and the exemplary embodiments of the invention described below. The invention will be explained in more detail below using preferred embodiments with reference to the accompanying figures.

[0052] Fig. 1 shows a schematic meridional section of a projection exposure system for EUV projection lithography!

[0053] Fig. 2 shows a schematic view of an embodiment of an optical system for the projection exposure apparatus according to Fig. 1; Fig. 3 shows a schematic plan view of the optical system according to Fig. 2;

[0054] Fig. 4 shows a schematic side view of an embodiment of a strut for the optical system according to Fig. 2;

[0055] Fig. 5 shows the detailed view V according to Fig. 4;

[0056] Fig. 6 shows again the detailed view V according to Fig. 4;

[0057] Fig. 7 shows a schematic plan view of the strut according to Fig. 4;

[0058] Fig. 8 shows a schematic sectional view of the strut according to the section line VIII-VIII of Fig. 6;

[0059] Fig. 9 shows a schematic view of an embodiment of a compensation force generating device for the strut according to Fig. 4;

[0060] Fig. 10 shows a further schematic view of the compensation force generating device according to Fig. 9;

[0061] Fig. 11 shows a force-displacement diagram of the strut according to Fig. 4;

[0062] Fig. 12 shows a schematic view of another embodiment of a compensation force generating device for the strut according to Fig. 4; and

[0063] Fig. 13 shows a schematic view of another embodiment of a compensating force generating device for the strut according to Fig. 4. In the figures, identical or functionally identical elements are provided with the same reference numerals unless otherwise indicated. Furthermore, it should be noted that the illustrations in the figures are not necessarily to scale.

[0064] Fig. 1 shows an embodiment of a projection exposure system 1 (lithography system), in particular an EUV lithography system. One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system 2. In this case, the illumination system 2 does not include the light source 3.

[0065] A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced via a reticle displacement drive 9, in particular in a scanning direction.

[0066] For illustrative purposes, Fig. 1 shows a Cartesian coordinate system with an x-direction (x), a y-direction (y), and a z-direction (z). The x-direction (x) runs perpendicular to the drawing plane. The y-direction (y) runs horizontally, and the z-direction (z) runs vertically. The scanning direction in Fig. 1 runs along the y-direction (y). The z-direction (z) runs perpendicular to the object plane (6).

[0067] The projection exposure system 1 comprises projection optics 10. The projection optics 10 serve to image the object field 5 into an image field 11 in an image plane 12. The image plane 12 runs parallel to the object plane 6. Alternatively, an angle other than 0° between the object plane 6 and the image plane 12 is also possible. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced via a wafer displacement drive 15, in particular along the y-direction y. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and of the wafer 13, on the other hand, via the wafer displacement drive 15 can be synchronized with one another.

[0068] The light source 3 is an EUV radiation source. The light source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation 16 has, in particular, a wavelength in the range between 5 nm and 30 nm. The light source 3 can be a plasma source, for example an LPP source (Laser Produced Plasma, plasma generated with the aid of a laser) or a DPP source (Gas Discharged Produced Plasma, plasma generated by gas discharge). It can also be a synchrotron-based radiation source. The light source 3 can be a free-electron laser (FEL).

[0069] The illumination radiation 16 emanating from the light source 3 is focused by a collector 17. The collector 17 can be a collector with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0070] After the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the light source 3 and the collector 17, and the illumination optics 4.

[0071] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 16 from stray light of a different wavelength. If the first facet mirror 20 is arranged in a plane of the illumination optics 4 that is optically conjugate to the object plane 6 as the field plane, it is also referred to as a field facet mirror. The first facet mirror 20 comprises a plurality of individual first facets 21, which can also be referred to as field facets. Only a few of these first facets 21 are shown in Fig. 1 as examples.

[0072] The first facets 21 can be designed as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or partially circular edge contour. The first facets 21 can be designed as flat facets or, alternatively, as convexly or concavely curved facets.

[0073] As is known, for example, from DE 10 2008 009 600 A1, the first facets 21 themselves can also be composed of a plurality of individual mirrors, in particular a plurality of micromirrors. The first facet mirror 20 can, in particular, be designed as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 A1.

[0074] Between the collector 17 and the deflecting mirror 19, the illumination radiation 16 runs horizontally, i.e. along the y-direction y.

[0075] In the beam path of the illumination optics 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. If the second facet mirror 22 is arranged in a pupil plane of the illumination optics 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged at a distance from a pupil plane of the illumination optics 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US 2006 / 0132747 A1, EP 1 614 008 B1, and US Pat. No. 6,573,978.

[0076] The second facet mirror 22 comprises a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

[0077] The second facets 23 can also be macroscopic facets, which can, for example, be round, rectangular, or hexagonal, or alternatively facets composed of micromirrors. Reference is also made to DE 10 2008 009 600 A1 in this regard.

[0078] The second facets 23 can have flat or, alternatively, convex or concave curved reflection surfaces. The illumination optics 4 thus forms a double-faceted system. This basic principle is also referred to as a honeycomb condenser (English fly's eye integrator).

[0079] It may be advantageous to arrange the second facet mirror 22 not exactly in a plane that is optically conjugate to a pupil plane of the projection optics 10. In particular, the second facet mirror 22 can be arranged tilted relative to a pupil plane of the projection optics 10, as described, for example, in DE 10 2017 220 586 A1.

[0080] With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror or actually the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0081] In a further embodiment of the illumination optics 4 (not shown), a transmission optics can be arranged in the beam path between the second facet mirror 22 and the object field 5, which transmission optics contributes in particular to the imaging of the first facets 21 into the object field 5. The transmission optics can have exactly one mirror, but alternatively also two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 4. The transmission optics can in particular comprise one or two mirrors for normal incidence (Ni mirrors, normal incidence mirrors) and / or one or two mirrors for grazing incidence (GF mirrors, grazing incidence mirrors).

[0082] In the embodiment shown in Fig. 1, the illumination optics 4 has exactly three mirrors after the collector 17, namely the deflecting mirror 19, the first facet mirror 20, and the second facet mirror 22. In a further embodiment of the illumination optics 4, the deflecting mirror 19 can also be omitted, so that the illumination optics 4 can then have exactly two mirrors after the collector 17, namely the first facet mirror 20 and the second facet mirror 22.

[0083] The imaging of the first facets 21 by means of the second facets 23 or with the second facets 23 and a transmission optics into the object plane 6 is usually only an approximate imaging.

[0084] The projection optics 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0085] In the example shown in Fig. 1, the projection optics 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The projection optics 10 is a doubly obscured optics. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection optics 10 has an image-side numerical aperture that is greater than 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0086] Reflection surfaces of the mirrors Mi can be designed as freeform surfaces without a rotational symmetry axis. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one rotational symmetry axis of the reflection surface shape. The mirrors Mi, like the mirrors of the illumination optics 4, can have highly reflective coatings for the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon. The projection optics 10 has a large object-image offset in the y-direction y between a y-coordinate of a center of the object field 5 and a y-coordinate of the center of the image field 11. This object-image offset in the y-direction y can be approximately as large as a z-distance between the object plane 6 and the image plane 12.

[0087] The projection optics 10 can, in particular, be anamorphic. It has, in particular, different image scales ßx, ßy in the x and y directions x, y. The two image scales ßx, ßy of the projection optics 10 are preferably (ßx, ßy) = (+ / - 0.25, + / - 0.125). A positive image scale ß means an image without image inversion. A negative sign for the image scale ß means an image with image inversion.

[0088] The projection optics 10 thus leads to a reduction in the ratio 4 in the x-direction x, i.e. in the direction perpendicular to the scanning direction.

[0089] The projection optics 10 leads to a reduction of 84 in the y-direction y, i.e. in the scanning direction.

[0090] Other magnifications are also possible. Magnifications with the same sign and absolutely identical in the x and y directions (x, y), for example, with absolute values ​​of 0.125 or 0.25, are also possible.

[0091] The number of intermediate image planes in the x- and y-directions x, y in the beam path between the object field 5 and the image field 11 can be the same or can be different, depending on the design of the projection optics 10. Examples of projection optics with different numbers of such intermediate images in the x- and y-directions x, y are known from US 2018 / 0074303 A1. Each of the second facets 23 is assigned to exactly one of the first facets 21 to form a respective illumination channel for illuminating the object field 5. This can, in particular, result in illumination according to the Köhler principle. The far field is divided into a plurality of object fields 5 with the aid of the first facets 21. The first facets 21 generate a plurality of images of the intermediate focus on the second facets 23 assigned to them.

[0092] The first facets 21 are each imaged onto the reticle 7 by an associated second facet 23, superimposed on one another, to illuminate the object field 5. The illumination of the object field 5 is, in particular, as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

[0093] By arranging the second facets 23, the illumination of the entrance pupil of the projection optics 10 can be geometrically defined. By selecting the illumination channels, in particular the subset of the second facets 23 that guide light, the intensity distribution in the entrance pupil of the projection optics 10 can be adjusted. This intensity distribution is also referred to as the illumination setting or illumination pupil fill.

[0094] A likewise preferred pupil uniformity in the area of ​​defined illuminated sections of an illumination pupil of the illumination optics 4 can be achieved by redistributing the illumination channels.

[0095] Further aspects and details of the illumination of the object field 5, and in particular of the entrance pupil of the projection optics 10, are described below. The projection optics 10 can, in particular, have a homocentric entrance pupil. This can be accessible. It can also be inaccessible.

[0096] The entrance pupil of the projection optics 10 cannot usually be precisely illuminated with the second facet mirror 22. When imaging the projection optics 10, which telecentrically images the center of the second facet mirror 22 onto the wafer 13, the aperture rays often do not intersect at a single point. However, a surface can be found in which the pairwise determined spacing of the aperture rays is minimized. This surface represents the entrance pupil or a surface conjugate to it in spatial space. In particular, this surface exhibits a finite curvature.

[0097] It is possible that the projection optics 10 have different entrance pupil positions for the tangential and sagittal beam paths. In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second facet mirror 22 and the reticle 7. With the help of this optical element, the different positions of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

[0098] In the arrangement of the components of the illumination optics 4 shown in Fig. 1, the second facet mirror 22 is arranged in a surface conjugate to the entrance pupil of the projection optics 10. The first facet mirror 20 is arranged tilted to the object plane 6. The first facet mirror 20 is arranged tilted to an arrangement plane defined by the deflection mirror 19. The first facet mirror 20 is arranged tilted to an arrangement plane defined by the second facet mirror 22. Fig. 2 shows a schematic view of an embodiment of an optical system 100. Fig. 3 shows a schematic plan view of the optical system 100. Reference is made below to Figs. 2 and 3 simultaneously.

[0099] The optical system 100 can be a measuring machine for interferometrically measuring an optical element 102. The optical element 102 can be, for example, one of the mirrors M1 to M6. The optical element 102 comprises a substrate 104 and an optically effective surface 106, for example, a mirror surface. The substrate 104 can also be referred to as a mirror substrate. The substrate 104 can comprise glass, ceramic, glass-ceramic, or other suitable materials.

[0100] The optically effective surface 106 is provided on a front side 108 of the substrate 104. The optically effective surface 106 can be realized with the aid of a coating applied to the front side 108. The optically effective surface 106 is a mirror surface. The optically effective surface 106 is suitable for reflecting illumination radiation 16, in particular E UV radiation, during operation of the optical system 100. The optically effective surface 106 can have an oval or elliptical geometry in the plan view according to Fig. 3. The optical element 102 or the substrate 104 can have a triangular geometry. In principle, however, the geometry is arbitrary.

[0101] Facing away from the optically effective surface 106 or the front side 108, the optical element 102 has a rear side 110. The rear side 110 has no defined optical properties. This means, in particular, that the rear side 110 is not a mirror surface and therefore does not have any reflective properties.

[0102] A plurality of mirror bushings 112, 114, 116 are provided on the rear side 110. A first mirror bushing 112, a second mirror bushing 114, and a third mirror bushing 116 are provided. In other words, the optical element 102 comprises exactly three mirror bushings 112, 114, 116. The mirror bushings 112, 114, 116 can be geometrically identical. The mirror bushings 112, 114, 116 are cylindrical and extend from the underside of the rear side 110 in the orientation shown in Fig. 2. The mirror bushings 112, 114, 116 form the corners of an imaginary triangle.

[0103] The optical element 102 or the optically effective surface 106 has six degrees of freedom, namely three translational degrees of freedom along the first spatial direction or x-direction x, the second spatial direction or y-direction y, and the third spatial direction or z-direction z, as well as three rotational degrees of freedom about the x-direction x, the y-direction y, and the z-direction z. This means that a position and an orientation of the optical element 102 or the optically effective surface 106 can be determined or described using the six degrees of freedom.

[0104] The "position" of the optical element 102 or the optically effective surface 106 refers in particular to its coordinates or the coordinates of a measuring point provided on the optical element 102 with respect to the x-direction x, the y-direction y, and the z-direction z. The "orientation" of the optical element 102 or the optically effective surface 106 refers in particular to its tilt with respect to the three spatial directions x, y, and z. This means that the optical element 102 or the optically effective surface 106 can be tilted about the x-direction x, the y-direction y, and / or the z-direction z.

[0105] This results in six degrees of freedom for the position and / or orientation of the optical element 102 or the optically effective surface 106. A "position" of the optical element 102 or the optically effective surface 106 encompasses both its position and its orientation. The term "position" can therefore be replaced by the phrase "position and orientation," and vice versa.

[0106] The optical element 102 is mounted using three bipods 118, 120, 122, which are shown only very schematically in Fig. 2. Each mirror socket 112, 114, 116 is assigned a bipod 118, 120, 122. This means, in particular, that exactly three bipods 118, 120, 122 are provided. With the three bipods 118, 120, 122, all of the aforementioned degrees of freedom can be locked. Thus, with the three bipods 118, 120, 122, the position of the optical element 102 is defined with respect to all six degrees of freedom.

[0107] A first bipod 118 is assigned to the first mirror socket 112. A second bipod 120 is assigned to the second mirror socket 114. A third bipod 122 is assigned to the third mirror socket 116. The bipods 118, 120, and 122 are identical in design. Therefore, only the first bipod 118 and the first mirror socket 112 will be discussed below, which will be referred to simply as bipod 118 and mirror socket 112. All subsequent statements regarding the bipod 118 apply to the bipods 120, 122, and vice versa.

[0108] The bipod 118 is coupled to the mirror socket 112 via a connection point 124. Furthermore, the bipod 118 is coupled to a fixed world 130 via two additional connection points 126, 128. The fixed world 130 can be a measuring stage of the optical system 100 or another immovable structure. In addition to the fixed world 130, the optical system 100 has a measuring head 132 for interferometrically measuring the optical element 102, in particular the optically effective surface 106.

[0109] The bipod 118 has two struts 134, 136. The six degrees of freedom of the optical element 102 are defined by means of all struts 134, 136 of all bipods 118, 120, 122. The struts 134, 136 can be referred to in particular as mirror struts or support struts. Both struts 134, 136 can be connected to the mirror socket 112 at the common connection point 124. Furthermore, the struts 134, 136 are connected to the fixed world 130 via the connection points 126, 128.

[0110] However, the optical system 100 can be not only a measuring machine, but also a projection optics system 10 as explained above or part of such a projection optics system 10. Therefore, the optical system 100 can also be referred to as a projection optics system. However, the optical system 100 can also be an illumination system 2 as explained above or part of such an illumination system 2. Therefore, the optical system 100 can alternatively also be referred to as an illumination system. The optical system 100 is suitable for EUV lithography. However, the optical system 100 can also be suitable for DUV lithography.

[0111] In the case where the optical system 100 is a projection optics 10 as mentioned above, the fixed world 130 can be a support frame (engineering force frame). Furthermore, the fixed world 130 can also be a manipulator frame that can be deflected to adjust the optical element 102. Struts 134, 136 as mentioned above can be used to mount the optical element 102, for example, on the manipulator frame.

[0112] Furthermore, the optical element 102 can also be assigned gravity compensators for compensating for the weight of the optical element 102. In this case, a strut 134, 136 as mentioned above can be part of such a gravity compensator. Furthermore, the struts 134, 136 can also be parts of actuators or manipulators for adjusting the optical element 102. The struts 134, 136 can be used for all applications in which exactly one degree of freedom is to be defined or in which forces are to be transmitted in exactly one direction.

[0113] Fig. 4 shows a schematic side view of an embodiment of a strut 200. Fig. 5 and Fig. 6 each show the detailed view V according to Fig. 4. Fig. 7 shows a schematic plan view of the strut 200. Fig. 8 shows a schematic sectional view of the strut according to the section line VIII-VIII of Fig. 7. In the following, reference is made simultaneously to Figs. 4 to 8.

[0114] The strut 200 can also be referred to as a, particularly monolithic, single degree of freedom joint or IDoF joint (1 Degree of Freedom). The strut 200 can be part of a bipod 118, 120, 122 as previously explained. Two struts 200 can each form a bipod 118, 120, 122. In particular, the strut 200 can be one of the previously explained struts 134, 136.

[0115] The strut 200 is part of the optical system 100. In particular, the strut 200 can be part of an actuator or manipulator for actuating the optical element 102. The strut 200 can also be part of a gravity compensator.

[0116] The strut 200 has a central or symmetry axis 202, to which the strut 200 is rotationally symmetrical. As previously mentioned, the strut 200 is assigned a coordinate system with an x-direction x, a y-direction y, and a z-direction z. The x, y, and z directions are oriented perpendicular to each other. The symmetry axis 202 runs along the z-direction z or is oriented parallel to the z-direction z. The strut 200 is assigned a longitudinal direction L, which runs along the symmetry axis 202 or along the z-direction z.

[0117] The strut 200 comprises a strut housing 204, which is rotationally symmetrical to the axis of symmetry 202. The strut housing 204 is hollow-cylindrical or tubular and can therefore also be referred to as a strut tube. The strut housing 204 has an outer side or outer surface 206 and an inner side or inner surface 208 facing away from the outer surface 206. The strut housing 204 encloses an interior space 210. The strut housing 204 further has two annular end surfaces 212, 214. The strut housing 204 is divided into a central section 216 and two connecting sections 218, 220. The central section 216 is positioned between a first connecting section 218 and a second connecting section 220, viewed along the longitudinal direction L. The first connection section 218 can be coupled to the optical element 102 by means of the connection point 124 (not shown).The second connection section 220 can be coupled to the fixed world 130 by means of the connection point 126 (not shown).

[0118] The first connection section 218 can be pivoted about a first joint axis 222 relative to the central section 216. The first joint axis 222 coincides with the x-direction or is arranged parallel to it. In addition, the first connection section 218 can be pivoted about a second joint axis 224 relative to the central section 216. The second joint axis 224 coincides with the y-direction or is arranged parallel to it. The two joint axes 222, 224 are thus oriented perpendicular to each other.

[0119] The two joint axes 222, 224 thus enable movement of the first connection section 218 relative to the central section 216 in two degrees of freedom, namely both tilting about the first joint axis 222 and tilting about the second joint axis 224. The joint axes 222, 224 preferably intersect each other in the axis of symmetry 202. This means, in particular, that the joint axes 222, 224 also intersect the axis of symmetry 202. The two joint axes 222, 224 form a first universal joint 226 of the strut housing 204 or the strut 200. The second connection section 220 can also be pivoted about a first joint axis 228 relative to the central section 216. The first joint axis 228 coincides with the x-direction x or is arranged parallel to it. Thus, the first joint axes 222, 228 are oriented parallel to each other.In addition, the first connection section 218 can be pivoted about a second joint axis 230 relative to the central section 216. The second joint axis 230 coincides with the y-direction y or is arranged parallel to it. Thus, the second joint axes 224, 230 are also oriented parallel to each other. The two joint axes 228, 230 are oriented perpendicular to each other.

[0120] The two joint axes 228, 230 thus enable movement of the second connection section 220 relative to the central section 216 in two degrees of freedom, namely both tilting about the first joint axis 228 and tilting about the second joint axis 230. The joint axes 228, 230 preferably intersect each other at the axis of symmetry 202. This means, in particular, that the joint axes 228, 230 also intersect the axis of symmetry 202. The two joint axes 228, 230 form a second universal joint 232 of the strut housing 204.

[0121] The joint axes 222, 224, 228, 230 are implemented using flexural joints 234, as shown in Fig. 5 and Fig. 6 using the first joint axis 222 of the first connection section 218. A "flexural joint" is generally understood to mean an area, for example, a cross-sectional constriction or thinning, of a component, in this case the strut housing 204, which enables relative movement between two rigid body regions of the component through bending or torsion. By adjusting the spring stiffness of the flexural joint 234, the strut housing 204 can be adapted to any desired application. In this context, "stiffness" is generally understood to mean the resistance of a body, in this case the flexure joint 234, to an elastic deformation imposed by an external load and conveys the relationship between the load on the body and its deformation.The stiffness is determined by the material of the body and its geometry.

[0122] The first joint axis 222 is assigned two such flexure joints 234, which are arranged spaced apart from one another along the x-direction. Accordingly, the second joint axis 224 is also assigned two such flexure joints 234 (not shown), which are arranged spaced apart from one another along the y-direction. The flexure joints 234 of the joint axes 222, 224 are arranged in pairs and offset by 180° from one another.

[0123] In the following, however, reference is made to only one solid-state joint 234. The solid-state joint 234 is web-shaped and connects the first connection section 218 to the central section 216 in one piece, in particular in one piece of material. "Integral" or "one-piece" here means that the first connection section 218, the central section 216, and the solid-state joint 234 form a common component, namely the strut housing 204, and are not composed of different subcomponents. "Integral material" here means that the strut housing 204 is made entirely of the same material, for example, a stainless steel alloy.

[0124] As previously mentioned, the flexure joint 234 is web-shaped. The flexure joint 234 is implemented by cutouts 236, 238 provided in the strut housing 204. A "cutout" is understood here as an opening extending from the outer surface 206 to the inner surface 208. The cutouts 236, 238 are slot-shaped. The cutouts 236, 238 can be introduced into the strut housing 204, for example, using an EDM process. Using the cutouts 236, 238, the first connecting section 218 is separated from the central section 216 such that the first connecting section 218 is connected to the central section 216 only by means of the flexure joint 234.

[0125] In Fig. 5 and Fig. 6, the cutouts 236, 238 are each shown very schematically as running along the y-direction y. In fact, the cutouts 236, 238 have a curved geometry that allows the four flexure joints 234 of the two joint axes 222, 224 of the first connection section 218 to be arranged such that the two joint axes 222, 224 intersect both each other and the axis of symmetry 202.

[0126] The strut 200 can be moved from an initial state or undeflected state ZI shown in Fig. 5, in which the connecting sections 218, 220 are not pivoted or tilted about the joint axes 222, 224, 228, 230 relative to the central section 216, into a deflected state Z2 shown in Fig. 6, in which the first connecting section 218 and / or the second connecting section 220 are pivoted or tilted about at least one of the joint axes 222, 224, 228, 230 relative to the central section 216, and vice versa. In the tilted state Z2, the first connection section 218 and / or the second connection section 220 can be tilted about any number or all of the joint axes 222, 224, 228, 230. However, the two states Z1, Z2 are explained below only with reference to the first joint axis 222 of the first connection section 218.The following statements concerning the first connection section 218 and the first joint axis 222 are correspondingly applicable to the second connection section 220 and / or the other joint axes 224, 228, 230.

[0127] If the first connection section 218 is now tilted, for example, about the first joint axis 222 relative to the central section 216, the flexural joint 234 deforms elastically from an undeformed state Z10 shown in Fig. 5 to a deformed state Z20 shown in Fig. 6. For this purpose, a force is applied to the first connection section 218. If this force is no longer applied, the flexural joint 234 deforms independently or automatically from the deformed state Z20 back to the undeformed state Z10. The same applies to the second joint axis 224.

[0128] The previous statements regarding the first connection section 218 apply correspondingly to the second connection section 220. This means, in particular, that the second connection section 220 is also connected to the central section 216 in an articulated manner by means of the previously mentioned solid joints 234. Two such solid joints 234 are assigned to both the first joint axis 228 and the second joint axis 230 of the second connection section 220.

[0129] As shown in Fig. 8, the strut 200 has a compensation force generating device 300A. Fig. 9 and Fig. 10 each show a schematic view of the compensation force generating device 300A. In the following, reference is made simultaneously to Figs. 8 to 10.

[0130] The compensation force generating device 300A is arranged within the strut housing 204, in particular in the interior space 210. The compensation force generating device 300A is slightly recessed relative to the end faces 212, 214. The compensation force generating device 300A is essentially rotationally symmetrical to the axis of symmetry 202. "Essentially" means that some parts of the compensation force generating device 300A can be rotationally symmetrical to the axis of symmetry 202, while other parts are not rotationally symmetrical to the axis of symmetry 202. The compensation force generating device 300A comprises a disc-shaped base section 302 that is firmly connected to the strut housing 204. For example, the base section 302 is glued into the strut housing 204. The base section 302 is placed centrally between the end faces 212, 214.The base section 302 is made of a metallic material.

[0131] The base section 302 is positioned between a first rod section 304 and a second rod section 306. The rod sections 304, 306 can each have a circular cross-section. The rod sections 304, 306 extend along the longitudinal direction L or along the axis of symmetry 202. The rod sections 304, 306 are preferably constructed identically.

[0132] The first rod section 304 is connected to the base section 302 by means of a proximal joint section 308 and to a first fastening section 312 by means of a distal joint section 310. "Proximal" means facing the base section 302. "Distal" means facing away from the base section 302. The joint sections 308, 310 are resiliently deformable and enable pivoting of the first rod section 304 relative to the base section 302 and relative to the first fastening section 312. The joint sections 308, 310 are wire-shaped and have a smaller cross-sectional area than the first rod section 304. The joint sections 308, 310 thus function as solid-state joints.

[0133] The second rod section 306 is connected to the base section 302 by means of a proximal joint section 314 and to a second fastening section 318 by means of a distal joint section 316. The joint sections 314, 316 are resiliently deformable and enable pivoting of the second rod section 306 relative to the base section 302 and relative to the second fastening section 318. The joint sections 314, 316 are wire-shaped and have a smaller cross-sectional area than the second rod section 306. The joint sections 314, 316 thus function as solid-state joints.

[0134] A first spring element 320 is assigned to the first rod section 304. The first spring element 320 is a tension spring. The first rod section 304 is guided through the first spring element 320. The first spring element 320 is connected to the base section 302 at a proximal connection point 322 and to the first fastening section 312 at a distal connection point 324. The first spring element 320 applies a first compressive force Fl to the first rod section 304 and the joint sections 308, 310.

[0135] A second spring element 326 is assigned to the second rod section 306. The second spring element 326 is a tension spring. The second rod section 306 is guided through the second spring element 326. The second spring element 326 is connected to the base section 302 at a proximal connection point 328 and to the second fastening section 318 at a distal connection point 330. The second spring element 326 applies a second compressive force F2 to the second rod section 306 and the joint sections 314, 316. The compressive forces F1, F2 are equal.

[0136] The compensation force generating device 300A is connected to the fastening sections 312, 318 by means of a first fastening element 332 to a first end face 212 of the strut housing 204 and by means of a second fastening element 334 to a second end face 214 of the strut housing 204. The fastening elements 332, 334 have an S-shaped curved contour. As shown in Fig. 6 with reference to the first fastening element 332, this has two strut sections 336, 338 arranged perpendicular to one another. The strut sections 336, 338 connect the first fastening section 312 to the first end face 212. A first strut section 336 runs parallel to the first joint axes 222, 228 and a second strut section 338 runs parallel to the second joint axes 224, 230. Alternatively, the fastening elements 332, 334 can also be designed as membranes.

[0137] The functionality of the strut 200 and the compensation force generating device 300A is explained below. When the optical element 102 is mounted with bipods 118, 120, 122 as explained with reference to Fig. 2 and Fig. 3, parasitic forces and moments that can lead to undesirable deformations of the optically effective surface 106 can be introduced into the optical element 102. This is undesirable.

[0138] In order to avoid or at least reduce the aforementioned parasitic forces, it is necessary to adjust the lateral spring stiffness Ci aterai of the strut 200 or the solid joints 234 as small as possible, while at the same time the axial spring stiffness c axiai the strut 200 or the solid joints 234 significantly larger than the lateral spring stiffness Ci ateraiThe "lateral spring stiffness" in this case refers to the spring stiffness of the strut 200 or the flexure joints 234 perpendicular to the axis of symmetry 202 or perpendicular to the longitudinal direction L, whereas the "axial spring stiffness" refers to the spring stiffness of the strut 200 or the flexure joints 234 along the axis of symmetry 202 or along the longitudinal direction L.

[0139] To determine the lateral spring stiffness Ci ateraiTo reduce the spring stiffness, the respective cross-sectional area of ​​the flexural joints 234 can be reduced to reduce their spring stiffness. This allows the flexural joints 234 to be deformed with less force. However, this reduction in the cross-sectional area can result in the flexural joints 234 becoming increasingly more complex to manufacture. In addition to making them more difficult to manufacture, a further reduction in the cross-sectional area of ​​the flexural joints 234 can also lead to undesirable buckling.

[0140] In addition, for example, in the case where the optical system 100 is a measuring machine, it is desirable to determine the axial spring stiffness c axiaiof the strut 200, as this can increase the measurement accuracy. This can be achieved by increasing the spring stiffness of the flexural joints 234 and thus by increasing the cross-sectional areas of the flexural joints 234. In summary, it is therefore not possible to make the flexural joints 234 infinitely soft in order to thereby increase the lateral spring stiffness c iaterai to be significantly reduced. A ratio of the axial spring stiffness c axiai to the lateral spring stiffness Ci aterai of greater than 10,000.

[0141] If the strut 200 is now deformed from the undeflected state Z 1 to the deflected state Z2, for example by pivoting the first connection section 218 about the first joint axis 222 relative to the central section 216, the lateral spring stiffness Ci results in ateraithe solid-state joints 234, which are assigned to the first joint axis 222, a parasitic force, designated F+ in Fig. 8, which should not be introduced into the optical element 102.

[0142] To prevent the parasitic force F+ from being introduced into the optical element 102, the compensation force generating device 300A generates a compensation force F- oriented opposite to the parasitic force F+, as will be explained below. The parasitic force F+ and the compensation force F- are oriented opposite to one another and are equal in magnitude or at least almost equal in magnitude. Thus, the parasitic force F+ and the compensation force F- cancel each other out, and the parasitic force F+ is not introduced into the optical element 102. The generation of the compensation force F- with the aid of the compensation force generating device 300A is explained below. Figure 9 shows the compensation force generating device 300A in a neutral state or rest state Z100. Figure 10 shows the compensation force generating device 300A in a compensation force generating state Z200. The rest state Z100 is unstable.In the present case, the fact that the rest state Z100 is "unstable" is to be understood as meaning that the compensation force generating device 300A moves independently or automatically from the rest state Z100 to the compensation force generating state Z200 as soon as the compensation force generating device 300A is slightly deflected from the rest state Z100 in the direction of the compensation force generating state Z200.

[0143] As shown in Fig. 9 and Fig. 10, moving the strut 200 from the undeflected state Z1 to the deflected state Z2 results, for example, in the first rod section 304 being pivoted relative to the base section 302 about a proximal pivot point 340 formed by the proximal joint section 308. The deflection of the first rod section 304 amounts to only a few μrad. In Fig. 10, the deflection of the first rod section 304 is shown greatly exaggerated. The first rod section 304 can also be pivoted relative to the first fastening section 312 about a distal pivot point 342 formed by the distal joint section 310. The joint axes 222, 224 of the first connection section 218 are arranged between the two pivot points 340, 342 as viewed along the longitudinal direction L.

[0144] The same applies to the second rod section 306, which is also assigned a proximal pivot point 344 and a distal pivot point 346. The pivot points 344, 346 are realized with the help of the joint sections 314, 316 of the second rod section 306. The joint axes 228, 230 of the second connection from section 220 are arranged between the two pivot points 344, 346, viewed along the longitudinal direction L.

[0145] Since the compressive forces F1, F2 generated by the spring elements 320, 326 act on the rod sections 304, 306, this results in the first rod section 304 being deflected further and further, so that the compensation force generating device 300A is moved from the rest state Z100 into the compensation force generating state Z200. In other words, the first spring element 320 pulls the compensation force generating device 300A from the rest state Z100 into the compensation force generating state Z200. The further the first rod section 304 is deflected, the smaller the force required for deflection becomes. The compensation force F- is a partial force of the first compressive force F1 oriented opposite to the y direction y. The compensation force F- becomes increasingly larger with increasing deflection of the first rod section 304. The compensation force F- is oriented opposite to the parasitic force F+ resulting from the deformation of the flexure joints 234.

[0146] Fig. 11 shows a force-displacement diagram of the solid joints 234 of the first joint axis 222.

[0147] However, the following explanations are applicable to all other joint axes 224, 228, 230. In Fig. 11, a deflection path w of the strut 200 or the compensation force generating device 300A is plotted on the abscissa axis or right axis, and a force F for deforming the two solid-state joints 234 of the first joint axis 222 or for deforming the compensation force generating device 300A, in particular the first rod section 304, is plotted on the ordinate axis or vertical axis.

[0148] The deflection of the two solid-state joints 234 of the first joint axis 222 is shown in Fig. 11 with a dashed line 348. In other words, line 348 represents the spring stiffness of the two solid-state joints 234 of the first joint axis 222. The deflection of the compensation force generating device 300A is shown in Fig. 11 with a dotted line 350. In other words, line 350 represents the spring stiffness of the compensation force generating device 300A. The superposition of the spring stiffnesses is shown with a dash-dotted line 352. In other words, line 352 represents the spring stiffness of the strut 200 comprising the strut housing 204 and the compensation force generating device 300A.

[0149] The two flexure joints 234 of the first joint axis 222 exhibit positive spring stiffness. With reference to Fig. 11, the term "positive spring stiffness" is understood to mean that the force F required to deflect the two flexure joints 234 of the first joint axis 222 increases with increasing deflection distance w.

[0150] In contrast, the compensation force generating device 300A has a negative spring stiffness. With reference to Fig. 11, the term "negative spring stiffness" is to be understood as meaning that the force F for deflecting the compensation force generating device 300A becomes increasingly smaller with an increasing deflection distance w. This results from the aforementioned movement of the compensation force generating device 300A from the rest state Z100 to the compensation force generating state Z200 with the aid of the spring elements 320, 326.

[0151] As previously explained, the parasitic force F+ and the compensation force F- compensate for each other. The positive spring stiffness of the two flexure joints 234 of the first joint axis 222 and the negative spring stiffness of the compensation force generating device 300A also compensate for each other, as shown in Fig. 11 by line 352. The strut 200 thus has zero spring stiffness at least in a deflection range Aw of the strut 200. This means, in particular, that the strut 200 can be moved from the undeflected state ZI to the deflected state Z2 in the deflection range Aw without force or without force. "Without force" or "without force" means that no parasitic force F+ results from moving the strut 200 from the undeflected state ZI to the deflected state Z2.

[0152] The deflection range Aw, in which the strut 200 has the aforementioned zero spring stiffness, can be, for example, ± 100 jun to ± 1 mm. The size of the deflection range Aw can be adjusted within a wide range depending on the application by designing the flexure joints 234 and the compensation force generating device 300A. The zero spring stiffness is achieved in particular in the lateral direction, i.e., perpendicular to the axis of symmetry 202. The lateral spring stiffness Ci aterai thus approaches zero. A ratio of the axial spring stiffness c axiai to the lateral spring stiffness Ci aterai of significantly greater than 10,000.

[0153] Fig. 12 shows a schematic view of another embodiment of a compensation force generating device 300B for the strut 200.

[0154] The compensation force generating device 300B essentially corresponds in its structure and functionality to that of the compensation force generating device 300A. Therefore, only differences between the two embodiments of the compensation force generating device 300A, 300B will be discussed below.

[0155] The compensation force generating device 300B differs from the compensation force generating device 300A in that the two rod sections 304, 306 are multi-part. The first rod section 304 has a proximal subsection 354, which is fixedly connected to the base section 302. In addition to the proximal subsection 354, the first rod section 304 has a distal subsection 356, which is connected to the first fastening section 312 by means of a distal joint section 310, as mentioned above. The two subsections 354, 356 are connected by means of a joint section 358, which forms a pivot point 360. The pivot point 360 lies on the joint axes 222, 224 of the first connection section 218.

[0156] The same applies to the second rod section 306. This means, in particular, that the second rod section 306 has a proximal subsection 362 that is firmly connected to the base section 302. In addition to the proximal subsection 362, the second rod section 306 has a distal subsection 364 that is connected to the second attachment section 318 by means of a distal joint section 316, as mentioned above. The two subsections 362, 364 are connected by means of a joint section 366, which forms a pivot point 368. The pivot point 368 lies on the joint axes 228, 230 of the second connection section 220.

[0157] Fig. 13 shows a schematic view of another embodiment of a compensation force generating device 300C for the strut 200.

[0158] The compensation force generating device 300C essentially corresponds in its structure and functionality to that of the compensation force generating device 300A. Therefore, only differences between the two embodiments of the compensation force generating device 300A, 300C will be discussed below.

[0159] The compensation force generating device 300C differs from the compensation force generating device 300A in that the compensation force generating device 300C does not have rod sections 304, 306, but rather a first receiving tube 370 in which the first spring element 320 is received, and a second receiving tube 372 in which the second spring element 326 is received. The first spring element 320 is connected to the base section 302 at the proximal connection point 322 and to the first receiving tube 370 at the distal connection point 324. The second spring element 326 is connected to the base section 302 at the proximal connection point 328 and to the second receiving tube 372 at the distal connection point 330. The receiving tubes 370, 372 are constructed rotationally symmetrically to the axis of symmetry 202.

[0160] The first receiving tube 370 comprises a proximal connection section 374, which is fixedly connected to the base section 302, a middle section 376, which is connected to the proximal connection section 374 by means of previously mentioned solid joints 234 (not shown), and a distal connection section 378, which is connected to the middle section 376 by means of previously mentioned solid joints 234 (not shown). The middle section 376 is positioned between the two connection sections 374, 378. The distal connection point 324 is provided at the distal connection section 378.

[0161] The proximal connection section 374 and the central section 376 are pivotable relative to one another about a first joint axis 380 running along the x-direction x. In addition, the proximal connection section 374 and the central section 376 are pivotable relative to one another about a second joint axis 382 running along the y-direction y. The joint axes 380, 382 run perpendicular to one another and intersect at the axis of symmetry 202. The joint axes 380, 382 can coincide with the joint axes 222, 224 of the first connection section 218.

[0162] The distal connection section 378 and the central section 376 are pivotable relative to one another about a first joint axis 384 running along the x-direction x. In addition, the distal connection section 378 and the central section 376 are pivotable relative to one another about a second joint axis 386 running along the y-direction y. The joint axes 384, 386 run perpendicular to one another and intersect at the axis of symmetry 202. The joint axes 380, 382, ​​384, 386 are implemented by solid-state joints 234 (not shown).

[0163] The second receiving tube 372 comprises a proximal connection section 388, which is fixedly connected to the base section 302, a middle section 390, which is connected to the proximal connection section 388 by means of previously mentioned solid joints 234 (not shown), and a distal connection section 392, which is connected to the middle section 390 by means of previously mentioned solid joints 234 (not shown). The middle section 390 is positioned between the two connection sections 388, 392. The distal connection point 330 is provided at the distal connection section 392.

[0164] The proximal connection section 388 and the central section 390 are pivotable relative to one another about a first joint axis 394 running along the x-direction x. In addition, the proximal connection section 388 and the central section 390 are pivotable relative to one another about a second joint axis 396 running along the y-direction y. The joint axes 394, 396 run perpendicular to one another and intersect at the axis of symmetry 202. The joint axes 394, 396 can coincide with the joint axes 228, 230 of the second connection section 220.

[0165] The distal connection section 392 and the central section 390 are pivotable relative to one another about a first joint axis 398 running along the x-direction x. In addition, the distal connection section 392 and the central section 390 are pivotable relative to one another about a second joint axis 400 running along the y-direction y. The joint axes 398, 400 run perpendicular to one another and intersect at the axis of symmetry 202. The joint axes 394, 396, 398, 400 are implemented by solid-state joints 234 (not shown). The compressive forces F1, F2 act on opposite end faces 402, 404 of the receiving tubes 370, 372. The end faces 402, 404 are annular. The end faces 402, 404 of the receiving tubes 270, 272 are recessed relative to the end faces 212, 214. Although the present invention has been described using exemplary embodiments, it is capable of numerous modifications.

[0166] LIST OF REFERENCE SYMBOLS

[0167] 1 projection exposure system

[0168] 2 lighting system

[0169] 3 Light source

[0170] 4 Lighting optics

[0171] 5 Object field

[0172] 6 Object level

[0173] 7 reticles

[0174] 8 reticle holders

[0175] 9 Reticle displacement drive

[0176] 10 Projection optics

[0177] 11 Image field

[0178] 12 Image plane

[0179] 13 wafers

[0180] 14 wafer holders

[0181] 15 Wafer relocation drive

[0182] 16 Illumination radiation

[0183] 17 Collector

[0184] 18 Intermediate focal plane

[0185] 19 Deflecting mirrors

[0186] 20 first facet mirror

[0187] 21 first facet

[0188] 22 second facet mirror

[0189] 23 second facet

[0190] 100 optical system

[0191] 102 optical element

[0192] 104 Substrat

[0193] 106 optically effective area

[0194] 108 Front 110 Back

[0195] 112 Mirror socket

[0196] 114 Mirror socket

[0197] 116 Mirror socket

[0198] 118 Bipod

[0199] 120 Bipod

[0200] 122 Bipod

[0201] 124 Connection point

[0202] 126 Connection point

[0203] 128 Connection point

[0204] 130 solid world

[0205] 132 measuring head

[0206] 134 Strut

[0207] 136 Strut

[0208] 200 strut

[0209] 202 axis of symmetry

[0210] 204 strut housing

[0211] 206 exterior area

[0212] 208 interior surface

[0213] 210 Interior

[0214] 212 frontal area

[0215] 214 frontal area

[0216] 216 Middle section

[0217] 218 Connection section

[0218] 220 connection section

[0219] 222 Joint axis

[0220] 224 Joint axis

[0221] 226 Cardan joint

[0222] 228 Joint axis

[0223] 230 Joint axis 232 Cardan joint

[0224] 234 Fixed joint

[0225] 236 free cut

[0226] 238 free cut

[0227] 300A compensation force generating device

[0228] 300B Compensating force generating device

[0229] 3000 Compensation force generating device

[0230] 302 Base section

[0231] 304 Bar Section

[0232] 306 Bar section

[0233] 308 Joint section

[0234] 310 Joint section

[0235] 312 Fastening section

[0236] 314 Joint section

[0237] 316 Joint section

[0238] 318 Fastening section

[0239] 320 spring element

[0240] 322 Connection point

[0241] 324 Connection point

[0242] 326 spring element

[0243] 328 Connection point

[0244] 330 Connection point

[0245] 332 Fastener

[0246] 334 Fastener

[0247] 336 struts section

[0248] 338 Strut section

[0249] 340 pivot point

[0250] 342 pivot point

[0251] 344 Pivot point

[0252] 346 Pivot point 348 Line

[0253] 350 Line

[0254] 352 Line

[0255] 354 Subsection

[0256] 356 Subsection

[0257] 358 Joint section

[0258] 360 pivot point

[0259] 362 Subsection

[0260] 364 Subsection

[0261] 366 joint section

[0262] 368 pivot point

[0263] 370 receiving tube

[0264] 372 receiving tube

[0265] 374 Connection section

[0266] 376 middle section

[0267] 378 Connection section

[0268] 380 joint axis

[0269] 382 joint axis

[0270] 384 Joint axis

[0271] 386 joint axis

[0272] 388 Connection section

[0273] 390 middle section

[0274] 392 connection section

[0275] 394 Joint axis

[0276] 396 joint axis

[0277] 398 Joint axis

[0278] 400 joint axis

[0279] 402 frontal area

[0280] 404 Frontal area F Force

[0281] F 1 compressive force

[0282] F2 compressive force

[0283] F+ parasitic force

[0284] F- compensation force

[0285] L longitudinal direction

[0286] ml mirror

[0287] M2 mirror

[0288] M3 mirror

[0289] M4 mirror

[0290] M5 mirror

[0291] M6 Mirror w Deflection path x x- direction y y- direction z z- direction

[0292] ZI condition

[0293] Z2 condition

[0294] Z10 condition

[0295] Z20 condition

[0296] Z100 sleep mode

[0297] Z200 Compensation force generation state

[0298] Aw deflection range

Claims

PATENT CLAIMS 1. Strut (134, 136, 200) for an optical system (100), comprising a plurality of solid joints (234), and a compensation force generating device (300A, 300B, 300C), wherein the strut (134, 136, 200) can be brought from an undeflected state (Z1) into a deflected state (Z2) by means of a deformation of the plurality of solid joints (234), and wherein the compensation force generating device (300A, 300B, 300C) is designed to generate a compensation force (F-) when bringing the strut (134, 136, 200) from the undeflected state (Z1) into the deflected state (Z2), which compensation force is a force resulting from the deformation of the plurality of solid joints (234) resulting parasitic force (F+) is compensated.

2. Strut according to claim 1, comprising a strut housing (204), wherein the compensation force generating device (300A, 300B, 300C) is arranged within the strut housing (204).

3. Strut according to claim 2, wherein the strut housing (204) has a first connection section (218), a second connection section (220) and a middle section (216) arranged between the first connection section (218) and the second connection section (220), wherein the first connection section (218) is connected to the middle section (216) by means of a plurality of solid joints (234), and wherein the second connection section (220) is connected to the middle section (216) by means of a plurality of solid joints (234).

4. Strut according to claim 3, wherein the first connection section (218) is bent by two mutually perpendicular and intersecting joint axes (222, 224) of the first connection section (218) is pivotable relative to the central section (216), and wherein the second connection section (220) is pivotable relative to the central section (216) about two mutually perpendicular and intersecting joint axes (228, 230) of the second connection section (220) when the strut (134, 136, 200) is moved from the undeflected state (Z1) to the deflected state (Z2).

5. Strut according to one of claims 1-4, wherein the plurality of solid joints (234) have a positive spring stiffness, wherein the compensating force generating device (300A, 300B, 300C) has a negative spring stiffness, and wherein the positive spring stiffness and the negative spring stiffness compensate each other.

6. Strut according to one of claims 1 - 5, wherein the compensation force generating device (300A, 300B, 300C) is under a spring preload along a longitudinal direction (L) of the strut (134, 136, 200).

7. Strut according to claim 6, wherein the compensation force generating device (300A, 300B, 300C) automatically moves from an unstable rest state (Z100) into a compensation force generating state (Z200) with the aid of the spring preload when the strut (134, 136, 200) is moved from the undeflected state (Z1) to the deflected state (Z2) in order to generate the compensation force (F-).

8. Strut according to claim 6 or 7, wherein the compensation force generating device (300A, 300B, 300C) for generating the spring preload comprises a first spring element (320) and a second spring element (326), wherein the first spring element (320) applies a first compressive force (Fl) to the compensation force generating device (300A, 300B, 300C), and wherein the second Spring element (326) applies a second compressive force (F2) to the compensation force generating device (300A, 300B, 300C).

9. Strut according to claim 8, wherein the first spring element (320) and the second spring element (326) move the compensation force generating device (300A, 300B, 300C) from the unstable rest state (Z100) to the compensation force generating state (Z200) when the strut (134, 136, 200) is moved from the undeflected state (Z1) to the deflected state (Z2).

10. Strut according to claim 8 or 9, wherein the compensation force generating device (300A, 300B) has a base section (302), a first rod section (304) pivotally connected to the base section (302), which first spring element (320) applies the first compressive force (F1) to, and a second rod section (306) pivotally connected to the base section (302), which second spring element (326) applies the second compressive force (F2), and wherein the base section (302) is arranged between the first rod section (304) and the second rod section (306).

11. Strut according to claim 10, wherein the first rod portion (304) is pivotally connected to a first attachment portion (312) of the compensating force generating device (300A, 300B), and wherein the second rod portion (306) is pivotally connected to a second attachment portion (318) of the compensating force generating device (300A, 300B).

12. Strut according to claim 10 or 11, wherein the first rod section (304) has a proximal subsection (354) facing the base section (302) and a distal subsection (356) facing away from the base section (302) and pivotally connected to the proximal subsection (354) of the first rod section (304), and wherein the second rod section (306) has a proximal subsection (362) facing the base section (302) and a a distal subsection (364) facing away from the base section (302) which is pivotally connected to the proximal subsection (362) of the second rod section (306).

13. Strut according to claim 8 or 9, wherein the compensation force generating device (300C) comprises a first receiving tube (370), within which the first spring element (320) is arranged, and a second receiving tube (372), within which the second spring element (326) is arranged, wherein the first spring element (320) applies the first compressive force (F1) to the first receiving tube (370), and wherein the second spring element (326) applies the second compressive force (F2) to the second receiving tube (372).

14. Strut according to claim 13, wherein the first receiving tube (370) has a proximal connection section (374), a distal connection section (378) and a middle section (376) arranged between the proximal connection section (374) and the distal connection section (378), wherein the proximal connection section (374) is pivotable relative to the middle section (376) about two mutually perpendicular and intersecting joint axes (380, 382), wherein the distal connection section (378) is pivotable relative to the middle section (376) about two mutually perpendicular and intersecting joint axes (384, 386), wherein the second receiving tube (372) has a proximal connection section (388), a distal connection section (392) and a between the proximal connection section (388) and the distal connection section (392),wherein the proximal connection section (388) is pivotable relative to the central section (390) about two mutually perpendicular and intersecting joint axes (394, 396), and wherein the distal connection section (392) is pivotable relative to the central section (390) about two mutually perpendicular and intersecting joint axes (398, 400).

15. Optical system (100), in particular measuring machine or projection optics (10), comprising an optical element (102), and at least one strut (134, 136, 200) according to one of claims 1 - 14, wherein the strut (134, 136, 200) is connected to the optical element (102).

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