Measurement arrangement for determining the position of a movable component

Abstract

A measurement arrangement (10) for determining the position of a movable component (526) in a system (500) comprises an optical resonator (26) with two resonator mirrors (28, 30) that enclose a resonator cavity (32); a movable measurement mirror (14) that is assigned to the component, is arranged within the resonator cavity for the purpose of directing a measurement radiation (18) back and forth between the resonator mirrors and is movable out of a normal position; and a focusing lens element (34). In this case, the lens element (34) is arranged immovably within the resonator cavity (32) in such a way that the measurement mirror in the normal position is arranged in a cat's eye position of the lens element.

Description

[0001] Measurement arrangement for determining the position of a movable component

[0002] This application claims priority to the German Patent Application No. 10 2024 201 474.5 filed on February 19, 2024. The entire disclosure of this patent application is incorporated into the present specification by reference.

[0003] Background of the invention

[0004] The invention relates to a measurement arrangement for determining the position of a movable component in a system. The invention also relates to a photolithographic projection exposure apparatus, an illumination optical unit of a photolithographic projection exposure apparatus, a projection lens of a photolithographic projection exposure apparatus, an inspection apparatus and a coordinate measuring machine, each with at least one measurement arrangement of the aforementioned type.

[0005] Photolithography is used for producing microstructured components, such as integrated circuits or LCDs. In this context, the term "microstructured components" refers in particular to components with microstructures and / or nanostructures. Photolithography is often also referred to as "microlithography", wherein the latter may also be used for the production of nanostructures in particular. The microstructured components are produced using what is known as a projection exposure apparatus, which comprises an illumination device and a projection lens. In this context, the image of a mask situated on a reticle and illuminated by means of the illumination device is projected by means of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0006] During operation of such projection lenses, during which mask and wafer are usually moved relative to one another in a scanning process, the positions of the mirrors, which are movable in part in all six degrees of freedom, have to be set and maintained with high accuracy both with respect to one another and also with respect to mask and / or wafer in order to avoid or at least reduce aberrations and accompanying impairments of the imaging result. This determination of position may require length measurement accuracies in the order of picometres (pm) over a path length of one metre, e.g. in EUV lithography.

[0007] Various approaches for measuring the position of the individual lens mirrors and also of the wafer or the wafer stage and the reticle plane are known in the prior art. Besides interferometric measurement arrangements, frequency-based position measurement using an optical resonator is also known here. A set-up used in DE 10 2012 212 663 A1 to this end comprises a resonator in the form of a Fabry-Perot resonator with two resonator mirrors, of which the first resonator mirror is secured to a reference element in the form of a measurement frame fixedly connected to the housing of the projection lens of the projection exposure apparatus and the second resonator mirror (as what is known as the "measurement target") is secured to an EUV mirror to be measured with regard to the position thereof. The actual distance measurement equipment comprises a radiation source, which is tuneable with respect to its optical frequency and which creates input coupling radiation that passes through a beam splitter and is input coupled into the optical resonator. In that case, the radiation source is controlled by a coupling device in such a way that the optical frequency of the radiation source is tuned to the resonant frequency of the optical resonator and is thus coupled to said resonant frequency. Input coupling radiation output coupled via a beam splitter is analysed by means of an optical frequency measuring device which may comprise e.g. a frequency comb generator for highly accurate determination of the absolute frequency. If the position of the EUV mirror changes in the direction of extent of the resonator, then together with the distance between the resonator mirrors the resonant frequency of the optical resonator also changes and hence - owing to the coupling of the frequency of the tuneable radiation source to the resonant frequency of the resonator - the optical frequency of the input coupling radiation changes as well, which is in turn registered directly by the frequency measuring device.

[0008] What is essential for the functionality of an optical resonator during the distance measurement is, firstly, that the measurement beam within the optical resonator is able to accomplish the highest possible number of circulations within the resonator (without said measurement beam leaving the cavity formed by the resonator), in order that eigenmodes may form in the resonator. What is also essential, moreover, is the capability of coupling the external radiation field present at the input of the resonator path (= "input coupling field") to the mode field of the optical resonator (= "resonator field"). In this case, the coupling efficiency that is characteristic of said coupling is defined by the overlap integral between input coupling field and resonator field, such that in order to achieve a high coupling efficiency, input coupling field and resonator field must correspond as well as possible in all relevant parameters.

[0009] In practice, then, with the use of an optical resonator for distance measurement during the measurement of the position of a component or of a mirror, problems may emerge from the fact that movements of the measurement target arranged at the mirror may occur not only along the actual measurement direction, but also in other degrees of freedom from among the total of six degrees of freedom. Such (parasitic) movements that do not take place along the measurement direction, e.g. intended or unintended tiltings or lateral displacements of the measurement target, may have the effect that a "drift" of the chief ray, on which the modes of the resonator are "threaded" as it were, in terms of position and angle takes place with the consequence that sufficient coupling of the resonator field to the input coupling field is no longer provided.

[0010] In view of the stringent requirements to be made here of the beam direction deviation (which requirements may demand e.g. that angular deviations for the beam vector of the chief ray are less than 0.1 mrad), ensuring that tiltings or lateral displacements of the measurement target do not take effect during the position determination constitutes a demanding challenge.

[0011] Problem of interest

[0012] A problem addressed by the invention is that of providing a measurement arrangement whereby the aforementioned problems are solved, and in particular a highly accurate position determination is rendered possible with justifiable outlay.

[0013] Solution according to the invention

[0014] In accordance with a first aspect according to the invention, the aforementioned problem can for example be solved using a measurement arrangement for determining the position of a movable component in a system. The measurement arrangement comprises an optical resonator with two resonator mirrors that enclose a resonator cavity, and a measurement mirror that is assigned to the component, is arranged within the resonator cavity for the purpose of directing a measurement radiation back and forth between the resonator mirrors and is movable out of a normal position. Furthermore, the measurement arrangement comprises a focusing lens element that is arranged immovably within the resonator cavity in such a way that the measurement mirror in the normal position is arranged in a cat's eye position of the lens element. For example, the system in which the movable component is contained may be a photolithographic optical system.

[0015] The arrangement of the measurement mirror in the cat's eye position of the lens element should be understood to mean that, in this arrangement, a radiation radiated at the lens element in a manner parallel to the optical axis of the lens element is focused on the measurement mirror in such a way that the focus point of said lens element is on a mirror surface of the measurement mirror. That is to say that all individual rays incident on the lens element in a manner parallel to the optical axis meet on the mirror surface. In other words, the focal point of the lens element is located on the mirror surface. Expressed differently yet again, the lens element and the measurement mirror in the normal position represent a cat's eye arrangement. The lens element may also be referred to as a Fourier lens and the measurement mirror as a measurement target. According to one embodiment, the measurement mirror has the function of a folding mirror for folding the beam path of the measurement radiation within the resonator cavity. According to one embodiment, the movable component is an optical component, in particular a lens element or a mirror, for instance an EUV mirror, a wafer stage or a reticle stage of a photolithographic projection exposure apparatus.

[0016] The arrangement of the measurement mirror in the cat's eye position of the lens element has the consequence that the focal point of the lens element defines the starting and reference position for the measurement mirror, in such a way that the chief ray associated with the measurement radiation passes through said measurement mirror in the normal position.

[0017] The statement that the lens element is arranged immovably within the resonator cavity should be understood to mean that said lens element, in contrast with the movable measurement mirror, is arranged at a fixed position within the resonator cavity, for example in a fixed positional relationship with the first and / or second resonator mirror.

[0018] In accordance with the first aspect according to the invention, the invention is preferably based on the concept of repeatedly passing over the path in the optical resonator to be travelled by the measurement radiation as a result of the provision of the cat's eye arrangement that is formed by the lens element and the measurement mirror. Use of the principle of the invertibility of the light path thus ensures that lateral displacements or tiltings on the part of the component to be measured, which do not act solely in the measurement direction, do not or hardly take effect during the position determination or continue to have no noticeable effects on the measurement result.

[0019] In other words, what is achieved by the use according to the invention of the lens element / measurement mirror arrangement is that regardless of lateral displacements or tiltings of the measurement mirror assigned to the component to be measured, the measurement radiation is substantially reflected back on itself. The measurement radiation thus returns on substantially the identical path via the measurement mirror, with the consequence that variations in the degrees of freedom which do not act along the direction of the measurement arm (measurement axis) are completely eliminated or almost completely eliminated in terms of their effects on the measurement.

[0020] In comparison with for instance the use of a corner cube retroreflector or a complete cat's eye arrangement as a measurement target, the measurement arrangement according to the invention allows for a much more compact embodiment of the photolithographic optical system. The use of a complete cat's eye arrangement as a measurement target should be understood to mean an assignment of an arrangement of the lens element and mirror, which are fixed to one another, of the cat's eye arrangement to the movable component.

[0021] Due to the immovable arrangement within the resonator cavity of the focusing lens element that is arranged in the cat's eye position, as proposed by the invention, only the measurement mirror needs to be assigned to the movable component in the photolithographic optical system. In this context, the measurement mirror may for example be embodied as a plane mirror, which for instance can be arranged in a very space-saving manner on an EUV mirror of a projection exposure apparatus. Then, the focusing lens element can for example be attached to at least one of the resonator mirrors, for which no further installation space is required on the EUV mirror. By contrast, much more installation space on the EUV mirror would be required if a corner cube retroreflector or a complete cat's eye arrangement were to be used as a measurement target.

[0022] Furthermore, the measurement arrangement according to the invention prevents temperature fluctuations at the component of the photolithographic optical system, which may often be very large in the case of EUV mirrors, for example, from impairing the measuring accuracy of the measurement arrangement. Due to the immovable arrangement of the focusing lens element, the latter may be positioned far enough away from the movable component such that no appreciable temperature input occurs at said location. The measurement mirror, by contrast, may be designed to be very insensitive to temperature fluctuations, for instance due to its design as a plane mirror.

[0023] According to one embodiment, a working distance between a measuring head, which at least comprises the resonator mirrors, and the measurement mirror is at least 100 mm, in particular at least 200 mm or at least 500 mm.

[0024] According to a further embodiment, the lens element is furthermore configured, or a further lens element is provided, to deflect the measurement radiation coming from the second resonator mirror toward the measurement mirror such that the chief ray associated with the measurement radiation passes through the focal point of the lens element in the non-deflected state.

[0025] According to a further embodiment, the lens element is arranged such that the measurement radiation coming from the first resonator mirror passes through the lens element off centre. This deflects the beam comprising the measurement radiation as it passes through the lens element. Advantageously, the measurement radiation coming from the other resonator mirror also passes through the lens element off centre, namely in such a way that the beam comprising the measurement radiation is deflected when passing through said lens element. By preference, the direction of the deflection is opposite to the deflection direction of the beam coming from the first resonator mirror. According to a further embodiment, at least one of the resonator mirrors and the lens element form a connected optical module. According to one embodiment variant, both resonator mirrors and the lens element form the connected optical module. In particular, the connected module can be designed in one piece or in monolithic fashion.

[0026] According to a further embodiment, a distance of the lens element from at least one of the resonator mirrors is at least one order of magnitude smaller than the focal length of the lens element.

[0027] According to a further embodiment, a working distance between the one of the resonator mirrors and the measurement mirror is at least 100 mm, in particular at least 200 mm or at least 500 mm.

[0028] In accordance with a second aspect according to the invention, provision is made for a measurement arrangement for determining the position of a movable component in a photolithographic optical system. The measurement arrangement comprises an optical resonator having a first resonator mirror, which serves to input couple measurement radiation into a resonator cavity, and a second resonator mirror. Furthermore, the measurement arrangement comprises a lens element that is arranged within the resonator cavity in such a way that a length of a section of the resonator cavity located between the lens element and the second resonator mirror is between 0.5-times and 1 .0-times the value of a focal length of the lens element.

[0029] In particular, the lower limit for the length of the section located between the lens element and the second resonator mirror may be 0.7-times or 0.9-times the focal length of the lens element. Furthermore, the upper limit for the length of the section located between the lens element and the second resonator mirror may be 0.8-times or 0.6-times the focal length of the lens element in particular. By preference, the first and the second resonator mirror enclose the resonator cavity. According to one embodiment, the movable component is an optical component, in particular a lens element or a mirror, for instance an EUV mirror, a wafer stage or a reticle stage of a photolithographic projection exposure apparatus.

[0030] The parameter selection made in accordance with the second aspect of the invention (length of the section of the resonator cavity located between the lens element and the second resonator mirror being between 0.5-times and 1 .0-times the value of the focal length of the lens element) leads to an optimization of an input coupling efficiency for the input coupling of a measurement radiation into the resonator cavity. That is to say that the coupling losses during input coupling are minimized. This relationship can be understood using a model that is described in the following description. A highly accurate determination of the position of the movable component is rendered possible with justifiable outlay on account of the optimization of the input coupling efficiency.

[0031] According to an embodiment in accordance with the second aspect according to the invention, the length of the section of the resonator cavity located between the lens element and the second resonator mirror corresponds to the distance between the lens element and the second resonator mirror. That is to say that, in this embodiment, the resonator cavity is not folded between the lens element and the second resonator mirror.

[0032] According to a further embodiment in accordance with the second aspect of the invention, a distance of the lens element from the first resonator mirror is smaller than the focal length of the lens element.

[0033] According to a further embodiment in accordance with the second aspect of the invention, the first resonator mirror and the lens element form a connected optical module. According to one embodiment variant, the connected module is in one piece or monolithic. According to a further embodiment in accordance with the second aspect of the invention, a working distance between the lens element and the second resonator mirror, i.e. the working distance of the measurement arrangement, is no more than 150 mm, in particular no more than 100 mm or no more than 10 mm.

[0034] According to one embodiment in accordance with the first or second aspect of the invention, the measurement arrangement is designed for frequency-based length measurement. According to one embodiment variant, the connected optical module is designed in one piece or in monolithic fashion. According to a further embodiment, the measurement arrangement furthermore comprises a beam shaping optical unit that is assigned to the first resonator mirror and serves to input couple a measurement radiation into the optical resonator, wherein the connected optical module furthermore comprises the beam shaping optical unit. The beam shaping optical unit may have the function of a focusing lens element. According to a further embodiment, the measurement arrangement furthermore comprises a tuneable laser that is stabilized to a resonator mode of the resonator. A tuneable laser should be understood to be a laser whose optical frequency can be changed.

[0035] According to a further embodiment in accordance with the first or second aspect of the invention, the system is a photolithographic optical system.

[0036] According to a further embodiment in accordance with the first or second aspect of the invention, the system is a photolithographic projection exposure apparatus.

[0037] Furthermore, according to the invention, a photolithographic projection exposure apparatus is provided, the latter comprising at least one movable component and at least one measurement arrangement in an embodiment according to the first or second aspect of the invention for determining the position of the movable component. In particular, the projection exposure apparatus may have an operating wavelength in the EUV wavelength range, i.e. it is what is known as an EUV projection exposure apparatus. According to one embodiment, the movable component is an optical component, in particular a lens element or a mirror, for instance an EUV mirror, a wafer stage or a reticle stage.

[0038] Furthermore, according to the invention, an illumination system of a photolithographic projection exposure apparatus is provided, said illumination system comprising at least one movable component and at least one measurement arrangement in an embodiment according to the first or second aspect of the invention for determining the position of the movable component.

[0039] Furthermore, according to the invention, a projection lens of a photolithographic projection exposure apparatus is provided, said projection lens comprising at least one movable component and at least one measurement arrangement in an embodiment according to the first or second aspect of the invention for determining the position of the movable component.

[0040] Furthermore, according to the invention, an inspection apparatus for inspecting a surface of a substrate is provided, said inspection apparatus comprising at least one movable component and at least one measurement arrangement in an embodiment according to the first or second aspect of the invention for determining the position of the movable component. For example, the substrate might be a mask or a wafer.

[0041] The movable component may be a component in an optical system of the inspection apparatus. An example of such an inspection apparatus for the inspection of masks or wafers (without the measurement arrangement according to the invention) is known from document DE 102012205181 A1 , the entire content of which is incorporated by reference in the present application.

[0042] Furthermore, according to the invention, a coordinate measuring machine is provided, the latter comprising at least one movable component and at least one measurement arrangement in an embodiment according to the first or second aspect of the invention for determining the position of the movable component. The movable component may be a component in an optical system of the coordinate measuring machine. The coordinate measuring machine serves to determine a respective position deviation of one or more measurement points on a test component from a respective target position. An example of such a coordinate measuring machine (without the measurement arrangement according to the invention) is known from document DE10 2019 213 794A1 , the entire content of which is incorporated by reference in the present application.

[0043] The features specified in relation to the aforementioned embodiments, exemplary embodiments and embodiment variants, etc., of the measurement arrangement according to the invention are explained in the description of the figures and the claims. The individual features may be implemented, either separately or in combination, as embodiments of the invention. Furthermore, they may describe advantageous embodiments which are independently protectable and protection for which is claimed only during or after pendency of the application, as the case may be.

[0044] Brief description of the drawings

[0045] The aforementioned features and further advantageous features of the invention will be illustrated in the following detailed description of exemplary embodiments according to the invention or of embodiments with reference to the attached schematic drawings, in which:

[0046] Fig. 1 shows an embodiment of a measurement arrangement in accordance with a first aspect according to the invention, configured for determining the position of a movable component,

[0047] Fig. 2 shows a beam generation and evaluation device of the measurement arrangement according to Fig. 1 , Fig. 3 shows an embodiment of a measurement arrangement in accordance with a second aspect according to the invention, configured for determining the position of a movable component,

[0048] Fig. 4 shows a further embodiment of the measurement arrangement in accordance with the second aspect according to the invention,

[0049] Fig. 5 shows a section of a photolithographic projection exposure apparatus having a movable component, the position of which is determinable by means of a measurement arrangement according to any of Figures 1 , 3 and 4,

[0050] Fig. 6 shows an embodiment of the projection exposure apparatus according to Fig. 5, and

[0051] Fig. 7 shows an enlarged detailed view of the projection exposure apparatus according to Fig. 6 with a measurement arrangement according to any of Figures 1 , 3 and 4 integrated therein.

[0052] Detailed description of exemplary embodiments according to the invention

[0053] In the exemplary embodiments or embodiments or embodiment variants described below, elements which are functionally or structurally similar to one another are provided with the same or similar reference signs as far as possible. Therefore, for understanding the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention.

[0054] In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in the drawing, from which system the respective positional relationship of the components illustrated in the figures is evident. In Fig. 1 , the x-direction runs perpendicular and into the plane of the drawing, the z-direction toward the right, and the y-direction upwardly.

[0055] Fig. 1 illustrates an embodiment of a measurement arrangement 10 in accordance with a first aspect according to the invention. This measurement equipment 10 is configured to determine the position of a movable component of a photolithographic optical system 500, which is illustrated in sections in Fig. 5 in an exemplary embodiment. The measurement arrangement 10 according to Fig. 1 comprises a measuring head 12 and a measurement mirror 14, which may also be referred to as measurement target and which is fastened, and hence assigned, to the movable component. To determine the position of the movable component, the measurement arrangement is used to determine the distance between the measuring head 12 and the measurement mirror 14, as described in detail hereinafter.

[0056] Fig. 6 shows a simplified illustration of the optical system 500 in the form of a photolithographic projection exposure apparatus. Fig. 5 shows a section of the projection exposure apparatus according to Fig. 6 with a mirror 526, which in this case serves as the aforementioned movable component. In the illustrated exemplary embodiment, the movable component with the mirror 526 is a component of a projection lens 516 of the projection exposure apparatus. Alternatively, the movable component might also be a component of the illumination system 515 of the projection exposure apparatus.

[0057] As mentioned, the component in the present exemplary embodiment is the mirror 526 which is movably mounted on a support structure 502 depicted in Fig. 5 or on a housing of the optical system 500. The support structure 502 or the housing is also referred to as reference frame hereinafter. To monitor the position and / or the orientation of the component 526 in relation to the reference frame during ongoing operation, i.e. in situ, the distance of selected measurement points M from the support structure 502 is ascertained. According to the present exemplary embodiment, the position of six measurement points M1 to M6, in particular in a hexapod configuration as depicted by way of example in Fig. 5, is determined in relation to an associated reference point R1 to R6 on the support structure 502 in each case. Thus, six hexapod lengths L1 to L6 are ascertained. The ascertainment of the respective position of the six measurement points M1 to M6 is implemented by measuring the lengths L1 to L6, in each case by means of an embodiment of the aforementioned measurement arrangement 10 or an embodiment of the measurement arrangement 210 described below with reference to Figures 3 and 4.

[0058] The photolithographic projection exposure apparatus depicted in Fig. 6 and serving as optical system 500 is designed for operation with EUV exposure radiation. In this text, EUV radiation should be understood to mean electromagnetic radiation at a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. However, the present invention is not limited to the application in such an apparatus but is also realizable when measuring projection exposure apparatuses with different operating wavelengths, for example operating wavelengths in the VUV or DUV range. In further applications, the invention can also be realized in a different photolithographic optical system, for instance a mask inspection apparatus or a wafer inspection apparatus.

[0059] According to the exemplary embodiment in Fig. 6, the optical system 500 in the form of an EUV projection exposure apparatus comprises a field facet mirror 503 and a pupil facet mirror 504. The light from a light source unit comprising a plasma light source 506 and a collector mirror 508 is directed to the field facet mirror 503. A first telescope mirror 510 and a second telescope mirror 512 are arranged downstream of the pupil facet mirror 504 in the light path. Arranged downstream in the light path is a deflection mirror 514, which directs the radiation incident thereon to an object field in the object plane of the projection lens 516, which comprises six mirrors 518, 520, 522, 524, 526 and 528. The collector mirror 508, the field facet mirror 503, the pupil facet mirror 504, the two telescope mirrors 510 and 512 and the deflection mirror 514 together form the illumination system 515 of the projection exposure apparatus. The radiation from the plasma light source 506 passes through the illumination system 515 and is subsequently incident on the object field in the object plane, i.e. the illumination system 515 illuminates the object field.

[0060] A reflective structure-bearing mask 530 on a mask stage 532 is arranged at the location of the object carrier, said mask being imaged by way of the projection lens 516 into an image plane, in which a substrate 534 coated with a lightsensitive layer (photoresist) is located on a wafer stage 536.

[0061] Fig. 7 shows an enlarged detailed view of the projection exposure apparatus from Fig. 6, which serves as optical system 500, in the region of the mirror 526, which serves as movable component, in the projection lens 516. In this case, Fig. 7 shows in simplified fashion the measurement arrangement 10 or the measurement arrangement 210 shown in Figures 3 and 4 by way of example. For example, the mirror 526 is movably held on the housing or on the wafer stage 536 by the support structure 502 in this case and thus represents the movable component in this exemplary embodiment. For reasons of clarity, the support structure 502 has not been shown in detail in the present case. The measuring head 12 of the measurement arrangement 10 or the measuring head 212 of the measurement arrangement 210 is arranged stationarily on the housing or, for example, on the mask stage. The measurement mirror 14 or the corresponding measurement target of the measurement arrangement 210 is secured to the underside of the mirror 526. According to one exemplary embodiment, the mirror 526 is adjusted in its position depending on the result of the position measurement by the measurement arrangement 10 or 210 and further measurement arrangements 10 or 210 provided in particular in accordance with the configuration illustrated in Fig. 5.

[0062] The measurement arrangement 10 illustrated in Fig. 1 comprises a radiation generation and evaluation device 16 for generating and evaluating measurement radiation 18, optionally an optical fibre 20, a beam shaping optical unit in the form of an input coupling lens element 22, an optical module in the form of a resonator module 24 and the aforementioned measurement mirror 14. The measuring head 12 likewise already mentioned above comprises at least the optical resonator module 24 and the input coupling lens element 22. The beam generation and evaluation device 16 may likewise be part of the measuring head 12 or else be arranged outside of the latter, as illustrated in Fig. 1 .

[0063] An exemplary embodiment of the beam generation and evaluation device 16 is depicted in detail in Fig. 2. It is based on the principle whereby a laser 42 that is tuneable with regards to the optical frequency follows a frequency of the optical resonator 26 by means of a suitable control loop (according to the Pound-Drever- Hall method in the example illustrated), and so the length of the resonator 26 that is ultimately to be measured, and hence the distance between the measuring head 12 and the measurement mirror 14, is encoded as a frequency of the tuneable laser 42. The laser 42 serves as radiation source for the measurement radiation 18, which for example is located in the visible or infrared wavelength range.

[0064] The device 16 comprises a Faraday isolator 44, an electro-optic modulator 46, a polarization-optical beam splitter 48, a quarter wave plate 50, a photodetector 52 and a low-pass filter 54. The portion of the measurement radiation 18 that passes through the quarter wave plate 50 enters the measuring head 12 via the optical fibre 20 depicted in Fig. 1 . Referring back to Fig. 2, for the purpose of frequency measurement, a portion of the measurement radiation 18 emitted by the tuneable laser 42 is output coupled via a beam splitter 56 and fed to an analyser 58 for frequency measurement. The actual frequency measurement in the analyser 58 may be effected for example by way of the comparison with a frequency reference, e.g. an fs frequency comb of a femtosecond laser. The measurement radiation 18 that leaves the resonator module according to Fig. 1 via the measuring head 12 again re-enters the device 16 via the optical fibre 20 and is captured by the photodetector 52. For further details regarding the functionality of the device 16, reference is made to DE 10 2018 208 147 A1 . The current distance between the measuring head 12 and the measurement mirror 14, and hence the position of the movable component, is determined from the frequency that is measured in the analyser and that changes accordingly when the resonator length changes.

[0065] The resonator module 24 forms an optical resonator 26 in conjunction with the measurement mirror 14 serving as measurement target. In this case, the resonator module 24 comprises a first resonator mirror 28 and a second resonator mirror 30 of the resonator 26, which enclose a resonator cavity 32. In the embodiment shown, the first resonator mirror 28 has a curved mirror surface, and the second resonator mirror 30 has a flat mirror surface. The measurement mirror 14 is arranged to direct the measurement radiation 18 back and forth between the two resonator mirrors 28 and 30, which are both aligned in the positive z-direction and hence in the same direction. In other words, the measurement mirror 14 has the function of a folding mirror for folding the beam path of the measurement radiation 18 within the resonator cavity 32.

[0066] Furthermore, the optical resonator 26 comprises a focusing lens element 34, the diameter of which is sufficiently large so that both the beam path of the measurement radiation 28 in the region of the first resonator mirror 28 and the beam path of the measurement radiation 18 in the region of the second resonator mirror 30 are comprised by the focusing lens element 34. In other words, the focusing lens element 34 is configured in such a way that the measurement radiation 18 reflected off the first resonator mirror 28 and the measurement radiation 18 reflected off the second resonator mirror 30 pass through the focusing lens element 34. In this case, both the measurement radiation 18 coming from the first resonator mirror 28 and the measurement radiation 18 coming from the second resonator mirror 30 pass through the focusing lens element 34 off centre. A consequence of this is that the measurement beam 19 formed by the measurement radiation 18 within the optical resonator 26 is deflected each time it passes through the focusing lens element 34, to be precise in such a way that the chief ray 25 associated with the measurement beam 19 passes through a focal point 36 of the focusing lens element 34. This applies both to the chief ray 25 of the measurement radiation 18 emanating from the first resonator mirror 28 and to the chief ray 25 of the measurement radiation 18 emanating from the second resonator mirror 30. The focal point 36 of the focusing lens element 34 is understood to mean the focus point of all the individual rays that are radiated at the focusing lens element 34 in a manner parallel to the optical axis of the focusing lens element 34.

[0067] In other words, in the position shown in Fig. 1 , which is also referred to as the normal position, the measurement mirror 14 is arranged in a cat's eye position of the focusing lens element 34. The arrangement of the measurement mirror 14 in the cat's eye position of the focusing lens element 34 should be understood to mean that, in this arrangement, a radiation radiated at the focusing lens element in a manner parallel to the optical axis of the focusing lens element is focused on the measurement mirror 14 in such a way that the focus point of said focusing lens element is on a mirror surface of the measurement mirror 14. The arrangement of focusing lens element 34 and measurement mirror 14 may also be referred to as a cat's eye arrangement.

[0068] The optical resonator 26 is surrounded by a dotted rectangle in Fig. 1 . The functional structure of the optical resonator 26 is shown below and denoted by reference sign 26f. The optical elements of the resonator module 24, namely the resonator mirrors 28 and 39 and the focusing lens element, may be separate elements or, as shown in the specific embodiment of the optical resonator according to Fig. 1 , be configured connectedly. In the latter case, the resonator module 24 is a connected optical module, which in particular may be designed in one piece or in monolithic fashion. The course of the beam path, folded by the measurement mirror 14, of the measurement radiation 28 within the optical resonator 26 corresponds to that of a Gaussian beam, the waist of which is located at the second resonator mirror 30, wherein the Gaussian beam is modified in such a way by the influence of the focusing lens element 34 that the beam is deflected during the respective passage through the focusing lens element 34.

[0069] Due to the measurement radiation 18 that comes from the first resonator mirror 28 being deflected when passing through the focusing lens element 34, the focal point 36 of the focusing lens element 34 on the measurement mirror 14 is offset from the optical axis 29 of the first resonator mirror 28 by the distance u; this also applies analogously with respect to the optical axis of the second resonator mirror 30. The distance u is at least half the beam diameter of the measurement radiation 18 that is radiated at the focusing lens element 34 from the first resonator mirror 30.

[0070] In the specific embodiment according to Fig. 1 , the resonator module 24 is manufactured from a lens element material and has a curved surface 38 on its side directed into the interior of the resonator cavity 32, in order to impart the function of the focusing lens element 34 on the resonator module 24. On the side facing away from the resonator cavity 32, i.e. on an outer surface 40, the resonator module 24 has respective reflective coatings for providing the function of the resonator mirrors 28 and 30.

[0071] The focal length of the focusing lens element 34 is denoted by F and, taking into account the optical theory of thick lenses, indicates the distance between a second main plane H' associated with the curved surface 38 and the focus plane in the concrete configuration of the resonator module 24 according to Fig. 1 . The nominal distance between the focusing lens element 34, specifically the main plane H', and the measurement mirror 14 is denoted Bo (reference sign 33). The distance Bo represents a working distance of the measurement arrangement 10 and denotes the distance between the measuring head 12 and the measurement mirror 14 serving as measurement target during the measurement operation of the measurement arrangement 10. Due to the cat's eye functionality described above, the following applies: Bo = F. In the functional representation 26f, the focusing lens element 34 is represented by a lens element that is approximated as being infinitely thin, i.e. the two main planes H and H' coincide.

[0072] In measurement operation, the measurement mirror 14 is allowed to move from the position of the measurement mirror shown in Fig. 1 , which is also referred to in this text as the normal position, by ±d in the z-direction, where |d| is less than F by at least a factor of 5, in particular by at least a factor of 10 (|d| «F). The real distance B between the focusing lens element 34 and the measurement mirror 14 thus is Bo±d, and hence the following applies: B ~ F. The respective distance of the first resonator mirror 28 and the second resonator mirror 30 from the focusing lens element 34 is denoted by G' (reference sign 37') and G” (reference sign 37"), respectively, in the general case. In the specific embodiment according to Fig. 1 , in which the two resonator mirrors 28 and 30 are arranged equidistantly from the focusing lens element 34, the common distance is denoted G (reference sign 37). The distance G, or the distances G' and G”, is or are less than F (G «F) by at least a factor of 10, in particular by at least a factor of 50. The ratio between the distance G and focal length F is defined by a design parameter y (G=y F). The length of the optical resonator 26 is 2F + G' + G” or 2F+2G. The radius of curvature of the resonator mirror 28 is denoted by R (reference sign 27).

[0073] The decisive optical property that is indispensable for the functionality of an optical cavity suitable for distance measurement, such as the optical resonator 26, is that a beam within the cavity that is delimited by the size of its mirrors may perform a large number, in the limit case an infinite number, of round trips without leaving the cavity, regardless of the parasitic deflections adopted by the measurement target in the form of the measurement mirror 14. If this condition is met, the chief ray, on which the modes of the resonator are threaded as it were, must not additionally migrate to such an extent in terms of its position and angle as a result of the parasitic movements leading to the parasitic deflections that sufficient coupling to the incoming measurement radiation field is no longer given or the aforementioned coupling efficiency becomes too low.

[0074] So that a high coupling efficiency may be achieved, the measurement radiation field that is coupled into the optical resonator 26 at the entrance of the resonator section, i.e. at the first resonator mirror 30, must therefore correspond to the best possible extent with the mode field of the resonator 26, wherein the latter should vary or migrate as little as possible in terms of its chief ray as a result of parasitic movements. For modelling purposes, the parasitic movements are denoted by the two tilt angles 0x and 6y, i.e. the tilt of the measurement mirror 14 that serves as the measurement target, with respect to tilt axes that are aligned transversely to the surface normal of the measurement mirror 14, specifically the x-axis and the y-axis. In the model described below, maximum tilts of ±6max in 0x and 6y are fixed.

[0075] Furthermore, the coupling efficiency is influenced by the displacement ±d of the measurement mirror 14 in the direction of the surface normal of the measurement mirror 14 that occurs during the measurement process. For modelling, a maximum permissible value dmax is therefore fixed for the displacement d.

[0076] According to one embodiment, the modelling is carried out on the basis of the known formalism of ray transfer matrix analysis. In the process, the essential conditions for optical resonators, which are suitable for frequency-based distance measurement, are derived in the matrix formalism of paraxial optics.

[0077] As a result of the modelling, the maximum coupling losses Kmax may be represented as follows: S2

[0078] In this case, the following applies to the combined loss contributions Pos>maxand s s2ize, max-

[0079] Here, Spos and Ssize are quality measures with regard to the respective sensitivity of the optical resonator 26 to the parasitic interference movements caused by the change in the tilt angles 6x and 6y. Spos in this case specifies the sensitivity of the beam input coupling at the resonator mirror 28, which acts as input coupling mirror, as a result of position variations, caused by the parasitic interference movements, of the mode field of the optical resonator 26 at the location of the resonator mirror 28. Stilt denotes the sensitivity of the beam input coupling at the resonator mirror 28 as a result of inclination variations, caused by the parasitic interference movements, of the mode field of the optical resonator 26 at the location of the resonator mirror 28. Ssize, finally, specifies the sensitivity of the beam input coupling at the resonator mirror 28 as a result of beam size variations, caused by the parasitic interference movements, of the mode field of the optical resonator 26 at the location of the resonator mirror 28. The quality measures Spos and Ssize specify the aforementioned sensitivities in each case as a function of the design parameter q-

[0080] In the expressions (2), the parameter y denotes the ratio, explained above with reference to Fig. 1 , between the distance G and the focal length F. The parameter u denotes the distance, also explained above with reference to Fig. 1 , by which the focal point 36 on the measurement mirror 15 is offset relative to the optical axis 29 of the resonator mirror, i.e. the parameter quantifies a corresponding decentration. The wavelength of the measurement radiation 18 is denoted by Z. The parameter amrepresents a dimensionless auxiliary variable and is defined as follows:

[0081] Fig. 3 illustrates an embodiment of a measurement arrangement 210 in accordance with a second aspect according to the invention. This measurement arrangement differs from the measurement arrangement 10 according to Fig. 1 merely in terms of the configuration of the optical resonator, which is denoted by the reference sign 226 in the embodiment according to Fig. 3. The functional structure of the optical resonator 226 is shown below and denoted by reference sign 226f. The optical resonator 226 comprises an input mirror that takes the form of a resonator mirror 228. It differs from the curved resonator mirror 28 according to Fig. 1 in that it takes the form of a plane mirror. In a manner analogous to the embodiment according to Fig. 1 , the optical resonator 226 according to Fig. 3 comprises a second plane resonator mirror 230. A resonator cavity 232 is formed between the first resonator mirror 228 and the second resonator mirror 230.

[0082] Unlike in the embodiment according to Fig. 1 , the resonator mirrors 228 and 230 do not point in the same direction but face each other; a folding mirror is not present. Instead, the second resonator mirror 230 represents the measurement target. The distance between the two resonator mirrors 228 and 230 corresponds to the length L of the resonator 226 or resonator cavity 232.

[0083] In the measurement arrangement 210 according to Fig. 3, a focusing lens element 229 is additionally arranged at a distance G (reference sign 237) upstream of the resonator mirror 228 within the resonator cavity 232. The focusing lens element 229, which has the focal length F (reference sign 235), adopts the function provided by the curvature of the resonator mirror 228 as regards the formation of the Gaussian beam within the resonator cavity 232 between the first resonator mirror 228 and the second resonator mirror 230, which represents the measurement target. The distance G between the first resonator mirror 228 and the focusing lens element 229, which is approximated as being infinitely thin, is less than F (G < ) by at least a factor of 2, in particular by at least a factor of 5.

[0084] The functional structure of the optical resonator 226 is shown below and denoted by reference sign 226f. The distance between the focusing lens element approximated as infinitely thin and the second resonator mirror 230 is denoted by B, where BQ (reference numeral 233) is the nominal distance. Since a displacement of the second resonator mirror 230 in the z-direction by ±d is permitted during measurement operation, the real distance between the focusing lens element 229 and the second resonator mirror 230 is B=BQ±d.

[0085] The coupling lens element 22 forms a measuring head 212 together with the first resonator mirror 228 and the focusing lens element 229. The distance Bo between the focusing lens element 229 and the resonator mirror 230 represents a working distance of the measurement arrangement 210 and denotes the distance between the measuring head 212 and the resonator mirror 230 serving as measurement target during the measurement operation of the measurement arrangement 210. In this text, the working distance of the measurement arrangement 210 is also referred to as the working distance between the focusing lens element 229 and the resonator mirror 230.

[0086] Another embodiment of the measurement equipment 210 in accordance with the second aspect according to the invention, illustrated in Fig. 4, differs from the embodiment according to Fig. 3 in that the resonator mirror 228 and the focusing lens element 229 form a connected mirror module 239. This consists of a lens element material that transmits the measurement radiation 18, has an outwardly facing surface 241 opposite the input coupling lens element 22 and has an inwardly facing surface 243 (facing into the resonator cavity 232). The outwardly facing surface 241 comprises a reflective coating for providing the function of the first resonator mirror 228. The inwardly facing surface 241 is convexly curved for providing the function of the focusing lens element 229. The focal length F and the distance 5o are measured from the second main plane H' of the mirror module 239, which is assigned to the inwardly facing surface 243. The distance G, by contrast, is measured between the outwardly facing surface 241 and the first main plane H of the mirror module 239.

[0087] According to one embodiment of the measurement arrangement 210 according to the invention in accordance with Fig. 3 or Fig. 4, the distance B, in particular its nominal length BQ, is between 0.5-times and 1 .0-times the focal length F of the focusing lens element 229, i.e. the following relationship applies using a design parameter)?:

[0088] Bo= fi F where 0.5 < p < 1.0 . (4)

[0089] The selection of the design parameter in the value range of 0.5 to 1 .0 predetermined in accordance with the embodiment according to the invention yields an operating point for the optical resonator 226 at which the coupling efficiency for input coupling the measurement radiation 18 into the optical resonator 226 is in an optimal range. That is to say that the coupling losses during input coupling of the measurement radiation 18 are minimized. According to another embodiment, 0.6 < < 0.9 applies. The advantageous nature of the aforementioned value range for the design parameter can be understood with the aid of the coupling losses modelling described below.

[0090] The decisive optical property that is indispensable for the functionality of an optical cavity suitable for distance measurement, such as the optical resonator 226, is that a beam within the cavity that is delimited by the size of its mirrors may perform a large number, in the limit case an infinite number, of round trips without leaving the cavity, regardless of the parasitic deflections adopted by the measurement target in the form of the resonator mirror 230. If this condition is met, the chief ray, on which the modes of the resonator are threaded as it were, must not additionally migrate to such an extent in terms of its position and angle as a result of the parasitic movements leading to the parasitic deflections that sufficient coupling to the incoming measurement radiation field is no longer given or the aforementioned coupling efficiency becomes too low.

[0091] So that a high coupling efficiency may be achieved, the measurement radiation field that is coupled into the optical resonator 226 at the entrance of the resonator section, i.e. at the first resonator mirror 228, must correspond to the best possible extent with the mode field of the resonator 226, wherein the latter must vary or migrate as little as possible in terms of its chief ray as a result of parasitic movements. For modelling purposes, the parasitic movements are denoted by the two tilt angles 0x and 0y, i.e. the tilt of the resonator mirror 230 that serves as the measurement target, with respect to tilt axes that are aligned transversely to the surface normal of the resonator mirror 230, specifically the x-axis and the y-axis. In the model described below, maximum tilts of ±6max in 0x and 6y are fixed.

[0092] Furthermore, the coupling efficiency is influenced by the displacement ±d of the resonator mirror 230 in the direction of the surface normal of the resonator mirror 230 that occurs during the measurement process. For modelling, a maximum permissible value dmax is therefore fixed for the displacement d.

[0093] According to one embodiment, the modelling is carried out on the basis of the known formalism of ray transfer matrix analysis. In the process, the essential conditions for optical resonators, which are suitable for frequency-based distance measurement, are derived in the matrix formalism of paraxial optics.

[0094] As a result of the modelling, the maximum coupling losses Kmax may be represented as follows: where Spos and Ssize are quality measures as a function of the design parameter P, the focal length F of the wavelength Z of the measurement radiation 18 and another design parameter y, defined by G = y F, to which the following applies:

[0095] The quality measures Spos and Ssize denote the respective sensitivity of the optical resonator 226 to the parasitic interference movements caused by the change in the tilt angles 6x and 6y. Spos in this case specifies the sensitivity of the beam input coupling at the resonator mirror 228, which acts as input coupling mirror, as a result of position variations, caused by the parasitic interference movements, of the mode field of the optical resonator 226 at the location of the resonator mirror 228. Ssize specifies the sensitivity of the beam input coupling at the resonator mirror 228 as a result of beam size variations, caused by the parasitic interference movements, of the mode field of the optical resonator 226 at the location of the resonator mirror 228.

[0096] In the expressions (7), Wm denotes the self-consistently setting beam size, which is defined as follows:

[0097] Hence, amis defined as a dimensionless auxiliary variable as follows:

[0098] The above description of exemplary embodiments, embodiments or embodiment variants should be understood to be by way of example. The disclosure effected thereby firstly enables the person skilled in the art to understand the present invention and the advantages associated therewith, and secondly encompasses alterations and modifications of the described structures and methods that are also obvious in the understanding of the person skilled in the art. Therefore, all such alterations and modifications, insofar as they fall within the scope of the invention in accordance with the definition in the accompanying claims, and equivalents are intended to be covered by the protection of the claims.

[0099] List of reference signs

[0100] 10 Measurement arrangement

[0101] 12 Measuring head

[0102] 14 Measurement mirror

[0103] 16 Radiation generation and evaluation device

[0104] 18 Measurement radiation

[0105] 19 Measurement beam

[0106] 20 Optical fibre

[0107] 22 Input coupling lens element

[0108] 24 Resonator module

[0109] 25 Chief ray

[0110] 26 Optical resonator

[0111] 26f Functional representation of the optical resonator

[0112] 27 Radius of curvature R

[0113] 28 Resonator mirror

[0114] 29 Optical axis of the resonator mirror

[0115] 30 Resonator mirror

[0116] 32 Resonator cavity

[0117] 33 Working distance Bo

[0118] 34 Focusing lens element

[0119] 35 Focal length F

[0120] 36 Focal point

[0121] 37, 37', 37" Distance G, G' and G" respectively

[0122] 38 Curved surface

[0123] 40 Outer surface

[0124] 42 Laser

[0125] 44 Faraday isolator

[0126] 46 Electro-optic modulator

[0127] 48 Polarization-optical beam splitter

[0128] 50 Quarter wave plate

[0129] 52 Photodetector 54 Low-pass filter

[0130] 56 Beam splitter

[0131] 58 Analyser

[0132] 210 Measurement arrangement

[0133] 212 Measuring head

[0134] 226 Optical resonator

[0135] 226f Functional representation of the optical resonator

[0136] 228 Resonator mirror

[0137] 229 Focusing lens element

[0138] 230 Resonator mirror

[0139] 232 Resonator cavity

[0140] 233 Distance Bo

[0141] 235 Focal length F

[0142] 237 Distance G

[0143] 239 Mirror module

[0144] 241 Outwardly facing surface

[0145] 243 Inwardly facing surface

[0146] 500 Optical system

[0147] 502 Support structure

[0148] 503 Field facet mirror

[0149] 504 Pupil facet mirror

[0150] 506 Plasma light source

[0151] 508 Collector mirror

[0152] 510 First telescope mirror

[0153] 512 Second telescope mirror

[0154] 514 Deflection mirror

[0155] 515 Illumination system

[0156] 516 Projection lens

[0157] 518, 520, 522, 524, 526, 528 Mirrors of the projection lens

[0158] 530 Mask

[0159] 532 Mask stage

[0160] 534 Substrate 536 Wafer stage

[0161] 526 Movable component

Claims

Claims1 . A measurement arrangement (10) for determining the position of a movable component (526) in a system (500), comprising:- an optical resonator (26) with two resonator mirrors (28, 30) that enclose a resonator cavity (32),- a measurement mirror (14) that is assigned to the component, is arranged within the resonator cavity for the purpose of directing a measurement radiation (18) back and forth between the resonator mirrors and is movable out of a normal position, and- a focusing lens element (34) that is arranged immovably within the resonator cavity (32) in such a way that the measurement mirror in the normal position is arranged in a cat's eye position of the lens element.

2. The measurement arrangement according to Claim 1 , wherein a working distance between a measuring head (12), which at least comprises the resonator mirrors (28, 30), and the measurement mirror (14) is at least 100 mm.

3. The measurement arrangement according to Claim 1 or 2, wherein the lens element (34) is furthermore configured, or a further lens element is provided, to deflect the measurement radiation coming from the second resonator mirror (30) toward the measurement mirror (14) such that the chief ray associated with the measurement radiation passes through the focal point (36) of the lens element in the non-deflected state.

4. The measurement arrangement according to any of the preceding claims, wherein the lens element (34) is arranged such that the measurement radiation (18) coming from the first resonator mirror (28) passes through the lens element (34) off centre.

5. The measurement arrangement according to any of the preceding claims, wherein at least one of the resonator mirrors (28, 30) and the lens element form a connected optical module (24).

6. The measurement arrangement according to any of the preceding claims, wherein a distance (37, 37', 37") of the lens element (34) from at least one of the resonator mirrors (28, 30) is at least one order of magnitude smaller than the focal length (35) of the lens element.

7. The measurement arrangement according to any of the preceding claims, wherein a working distance (33) between one of the resonator mirrors (28, 30) and the measurement mirror (14) is at least 100 mm.

8. A measurement arrangement (210) for determining the position of a movable component (526) in a photolithographic system (500), comprising:- an optical resonator (226) having a first resonator mirror (228), which serves to input couple measurement radiation into a resonator cavity (232), and a second resonator mirror (230), and- a lens element (229) that is arranged within the resonator cavity in such a way that a length (233) of a section of the resonator cavity located between the lens element and the second resonator mirror is between 0.5-times and 1 .0-times the value of a focal length (235) of the lens element.

9. The measurement arrangement according to Claim 8, wherein the length of the section of the resonator cavity (132) located between the lens element (229) and the second resonator mirror (230) corresponds to the distance (233) between the lens element and the second resonator mirror.

10. The measurement arrangement according to Claim 8 or 9, wherein a distance (237) of the lens element (229) from the first resonator mirror (228) is smaller than the focal length (235) of the lens element.11 . The measurement arrangement according to Claim 9, wherein the first resonator mirror (228) and the lens element (229) form a connected optical module (239).

12. The measurement arrangement according to any of Claims 8 to 1 1 , wherein a working distance (233) between the lens element (229) and the second resonator mirror (230) is no more than 150 mm.

13. The measurement arrangement according to any of the preceding claims, designed for frequency-based length measurement.

14. The measurement arrangement according to any of the preceding claims, wherein the system is a photolithographic optical system.

15. The measurement arrangement according to any of the preceding claims, wherein the system is a photolithographic projection exposure apparatus (500).

16. A photolithographic projection exposure apparatus (500) having at least one movable component (526) and at least one measurement arrangement (10; 210) according to any of the preceding claims for determining the position of the movable component.

17. An illumination system (515) of a photolithographic projection exposure apparatus (500) having at least one movable component and at least one measurement arrangement (10; 210) according to any of Claims 1 to 15 for determining the position of the movable component.

18. A projection lens (516) of a photolithographic projection exposure apparatus (500) having at least one movable component and at least one measurement arrangement (10; 210) according to any of Claims 1 to 15 for determining the position of the movable component.

19. An inspection apparatus for inspecting a surface of a substrate, in particular a mask or a wafer, having at least one movable component and at least one measurement arrangement (10; 210) according to any of Claims 1 to 13 for determining the position of the movable component.

20. A coordinate measuring machine having at least one movable component and at least one measurement arrangement (10; 210) according to any of Claims 1 to 13 for determining the position of the movable component.

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