Measuring device for frequency-based position determination of a movable component

Abstract

A measuring device (10) for frequency-based position determination of a movable component (226) in an optical system (200) for microlithography comprises two optical resonators (16-1; 16-2), each with two resonator mirrors (20-1, 22-1; 20-2, 22-2), which each enclose a resonator cavity (18-1; 18-2), wherein the resonator cavities have different lengths, and a common retroreflector (14) for both resonators, which is configured to direct a respective measuring radiation (24-1; 24-2) back and forth between the respective resonator mirrors of the two resonators.

Description

Background of the invention

[0001] The invention relates to a measuring device for frequency-based position determination of a movable component in an optical system for microlithography and a projection exposure system with at least one measuring device of the aforementioned type.

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. This is achieved using a projection exposure system, which includes an illumination unit and a projection lens. The image of a mask located on a reticle and illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens, in order to transfer the mask structure onto the photosensitive coating of the substrate.

[0003] In the operation of such projection lenses, where the mask and wafer are typically moved relative to each other in a scanning process, the positions of the mirrors, which are sometimes movable in all six degrees of freedom, must be set and maintained with high accuracy both relative to each other and to the mask or wafer, in order to avoid or at least reduce aberrations and the associated impairment of the image result. For this position determination, accuracies in the picometer (pm) range may be required, for example, in EUV lithography over a distance of 1 meter.

[0004] Various approaches are known in the prior art for measuring the position of individual objective mirrors, as well as the wafer or wafer stage and the reticulate plane. In addition to interferometric measuring arrangements, frequency-based position measurement using an optical resonator is also known. A setup used for this purpose in DE 10 2012 212 663 A1 comprises a Fabry-Perot resonator with two resonator mirrors. The first resonator mirror is attached to a reference element in the form of a measuring frame rigidly connected to the housing of the projection objective of the projection exposure system, and the second resonator mirror (as a so-called "measuring target") is attached to an EUV mirror whose position is to be measured.

[0005] The actual distance measuring device comprises a radiation source whose optical frequency can be tuned. This source generates coupling radiation that passes through a beam splitter and is coupled into the optical resonator. The radiation source is controlled by a coupling device such that its optical frequency is tuned to a resonant frequency of the optical resonator, also referred to as the resonator frequency in this text, and thus coupled to this resonant frequency. The coupling radiation extracted via the beam splitter is analyzed by an optical frequency measuring device, which may, for example, include a frequency comb generator for highly accurate determination of the absolute frequency.If the position of the EUV mirror changes in the direction of extension of the resonator, the resonance frequency of the optical resonator also changes with the distance between the resonator mirrors, and thus – as a result of the coupling of the frequency of the tunable radiation source to the resonance frequency of the resonator – the optical frequency of the coupling radiation also changes, which in turn is directly registered by the frequency measuring device.

[0006] In interferometric or frequency-based position measurement, the optical phase or its change along an optical path defined by mirrors is recorded. The actual measured quantity therefore does not correspond to the desired geometric path length S. geo , but rather the optical path length S modified by the refractive index n of the usually gaseous medium opt according to S opt = n S geo .

[0007] Fluctuations in the refractive index and / or the uncertainty of its knowledge limit, in most cases, the achievable accuracies in optical (interferometric or frequency-based) length measurement.

[0008] In interferometric systems for length measurement, a reference path with a known geometric length can be used to measure the refractive index for the purpose of correcting for atmospheric influence. During atmospheric correction, the measured optical length S is then used to calculate the ref,opt the reference line and the knowledge of its geometric length S ref,geo the refractive index n ref according to n ref = s ref,opt / s ref,geo determined. Provided that the refractive indices of the measuring and reference sections are the same n ref = n meas , applies to the desired geometric length S meas,geo = S meas,opt S ref,geo / S ref,opt , where S meas,optstands for the measured optical length of the measuring distance.

[0009] However, due to space constraints or cost limitations, this measuring section is not always—or even rarely—located in the immediate vicinity of the measuring sections of interest. As a consequence, differences in gas densities (gas density gradients and schlieren) lead to differences in the refractive indices Δn = n meas - n ref and thus to distortions of the extracted geometric information according to ΔS meas,geo = Δn S meas,opt . Underlying task

[0010] It is an object of the invention to provide a measuring device of the type mentioned above, with which the aforementioned problems are solved, and in particular the position of the movable component can be reliably measured with high accuracy even when the refractive index of the medium in the optical resonator fluctuates. Inventive solution

[0011] The aforementioned problem can be solved according to the invention, for example, with a measuring device for frequency-based position determination of a movable component in an optical system for microlithography. The measuring device comprises two optical resonators, each with two resonator mirrors, each enclosing a resonator cavity, wherein the resonator cavities have different lengths. Furthermore, the measuring device includes a common retroreflector for both resonators, which is configured to direct a respective measurement beam back and forth between the respective resonator mirrors of the two resonators. That is, the same retroreflector is used in both optical resonators.

[0012] In this text, a retroreflector is understood to be a reflector that reflects an incident light beam back onto itself, i.e., reverses the direction of the incident light beam, with the beam typically experiencing a beam offset. This beam offset does not occur only if the light beam has a certain diameter and is perfectly centered on the retroreflector. In this case, however, individual rays of the light beam will experience their own beam offsets. Examples of retroreflectors include a cube corner, a cat's eye reflector, or crossed 90° reflectors.

[0013] In this text, an optical resonator is understood to be an arrangement of mirrors designed to reflect light back and forth as many times as possible. According to one embodiment, the optical resonators of the measuring device are each configured such that, after at least five passes, preferably at least ten passes, the measurement radiation emitted into the respective optical resonator retains at least 90%, and advantageously at least 99%, of its original intensity.According to a further embodiment, the optical resonators of the measuring device are each configured to reflect the incident measurement radiation back and forth multiple times, for example at least 10 times, at least 100 times, or at least 1000 times, before the measurement radiation leaves the optical resonator. Leaving the optical resonator is defined as the intensity of any measurement radiation remaining in the resonator being less than 50% of the intensity of the incident measurement radiation. According to a further embodiment, the optical resonators of the measuring device each have a fineness of at least 100, advantageously at least 1000.

[0014] The inventive configuration of the measuring device with two optical resonators and a common retroreflector makes it possible to determine the position of the movable component while eliminating or computing for the influence of a fluctuating refractive index. This can be achieved with high accuracy, as the common retroreflector prevents tilting of the measuring target to which the movable component is attached from distorting the measurement result. This is made possible by the primary function of a retroreflector, which is to reflect an incident beam back onto itself with a parallel offset, whereby the distance traveled is independent of the parallel offset of the incoming beam.In contrast, in a comparative example of a measuring device where each of the optical resonators has its own retroreflector, which is then rigidly coupled to the other retroreflector to form a measuring target, a tilting of the measuring target leads to a significant distortion of the measurement result, as will be explained in more detail later in this text.

[0015] According to one embodiment, the measuring device for frequency-based position determination comprises a radiation source, tunable with respect to its optical frequency, for each of the two optical resonators to generate a respective measurement beam. Advantageously, the measuring module for each of the optical resonators includes a coupling module, which is configured to couple the optical frequency of the respective radiation source to a resonant frequency of the respective optical resonator. Coupling the optical frequency to the resonant frequency means that the optical frequency is aligned with the resonant frequency. In other words, the optical frequency of the respective radiation source is tuned to the resonant frequency of the respective optical resonator; that is, the optical frequency follows the resonant frequency.

[0016] Advantageously, the measuring device further includes a frequency measuring unit for measuring the optical frequency of the respective coupled radiation source. The length of the optical resonator exhibits a functional dependence on the measured optical frequency; that is, the actual length of the optical resonator can be determined from the measured optical frequency. In other words, the length of the optical resonator is encoded as the optical frequency of the tunable radiation source. Advantageously, the measuring module further includes a processing unit for determining the length of the respective optical resonator from the measured optical frequency and thus for determining the distance between a respective coupling mirror of the optical resonator and the measurement target.

[0017] According to one embodiment, the measuring device further comprises an evaluation unit which is configured to determine a change in the length of the resonator cavities from measurements of the respective resonance frequencies of the resonator cavities.

[0018] According to another embodiment, the evaluation device is configured to determine the position of the movable component from the change in length of the resonator cavities. For this purpose, the position of a measuring mirror contained in the two resonator cavities is determined.

[0019] According to another embodiment, the evaluation device is further configured to determine a refractive index of a medium within the resonator cavities from the measurements of the respective resonance frequencies of the resonator cavities.

[0020] According to another embodiment, the retroreflector serves as a measuring target assigned to the movable component. The retroreflector is common to both cavities and can be in various embodiments: as a cube-corner retroreflector, as a distributed retroreflector, e.g., consisting of two crossed roof-edge mirrors, as a cat's-eye retroreflector in lens or mirror form, and as other retroreflector designs familiar to those skilled in the art.

[0021] According to a further embodiment, the measuring device further comprises a measuring mirror assigned to the component, which is also arranged within both resonator cavities to deflect the respective measuring radiation back and forth between the respective resonator mirrors of the two resonators.

[0022] According to another embodiment, the optical resonators each contain at least four beam folds. These optical resonators are therefore so-called 4-pass resonators, in which the measuring radiation undergoes four passes through a measuring section of the measuring device. The measuring section extends between a measuring head and a measuring mirror. A pass means that the measuring radiation travels back and forth through the measuring section.

[0023] According to another embodiment, the optical resonators each contain at least two, preferably at least four, but in any case an even number of folds. If the retroreflector is the measurement target, then a double fold is advantageously present. The number of folds corresponds to the number of reflections at the measurement target for one revolution in the resonator. For a plane mirror as the measurement target, the number of folds is advantageously 4.

[0024] According to another embodiment, the measuring mirror is configured as a stepped plane mirror to achieve the different lengths of the resonator cavities.

[0025] According to another embodiment, the measuring device further comprises a polarizing beam splitter cube for coupling the retroreflector to the beam paths of the optical resonators.

[0026] According to another embodiment, the two resonator cavities are rigidly coupled to each other.

[0027] According to another embodiment, the lengths of the resonator cavities differ from each other by at least 10 mm, in particular by at least 20 mm. For example, the difference in length is in the range of 20 mm to 50 mm.

[0028] In one modification, the resonator cavities can each be configured as open cavities with propagating waves, and the measuring device can be designed as an interferometer. This can be achieved by modifying a coupling mirror, which is highly reflective in an optical resonator (closed cavity with standing waves), by using an at least partially transmitting element. The interferometer preferably includes a beam splitter for generating a measurement path and a reference path.

[0029] Furthermore, according to the invention, a projection exposure system for microlithography is provided, which comprises at least one component and at least one measuring device according to one of the preceding embodiments or variants for determining the position of the component.

[0030] The features mentioned in relation to the aforementioned embodiments, exemplary embodiments, or variants, etc., are explained in the figure description and the claims. The individual features can be implemented either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently patentable and whose protection may be claimed only during or after the filing of the application. Brief description of the drawings

[0031] The foregoing, as well as further advantageous features of the invention, are illustrated in the following detailed description of exemplary embodiments or embodiments or variants of the invention with reference to the accompanying schematic drawings. These show: Fig. 1 an embodiment of a measuring device for frequency-based position determination of a movable component, comprising two optical resonators, each with a beam generation and evaluation unit, Fig. 2 an embodiment of the beam generation and evaluation device, Fig. 3 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 4 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 5 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 6 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 7 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 8 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 9 Another embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 10 a comparative example of a measuring device for frequency-based position determination of a movable component with two optical resonators, as well as Fig. 11 an embodiment of a projection exposure system for microlithography with a movable component, the position of which is determined by means of the measuring device according to Fig. 1 or according to one of the Fig. 3 to 10 can be determined. Detailed description of embodiments according to the invention

[0032] In the exemplary embodiments or variants described below, functionally or structurally similar elements are, as far as possible, provided with the same or similar reference numerals. Therefore, to understand the features of the individual elements of a particular exemplary embodiment, reference should be made to the description of other exemplary embodiments or to the general description of the invention.

[0033] To facilitate description, a Cartesian xyz coordinate system is shown in the drawing, from which the respective positional relationships of the components depicted in the figures can be derived. Fig. 1. The x-direction runs perpendicular to the plane of the drawing, into it, the z-direction to the right, and the y-direction upwards.

[0034] In Fig. Figure 1 shows a first embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. Fig. Figure 11 illustrates, in a simplified representation, such an optical system in the form of a projection exposure unit 200 for microlithography, which will be described in more detail later in the text. A mirror 226 of a projection lens 216 of the projection exposure unit 200 serves, for example, as the component to be measured. The position of this component can change during the operation of the projection exposure unit 200; therefore, the component can also be referred to as a movable component.

[0035] The measuring device 10 according to Fig. 1 comprises a measuring head 12 and a measuring target 23, which is attached to the movable component and thus assigned to it. The measuring target 23 according to Fig. 1 is configured as a retroreflector 14. To determine the position of the movable component, the distance between the measuring head 12 and the measuring target 23 in the z-direction is determined using the measuring device 10, as described in detail below. In other words, in this embodiment, the position z T of the measurement target 23 as a function of time t, i.e. z T (t) determined.

[0036] In this text, a retroreflector is understood to be a reflector that reflects an incident light beam back onto itself, i.e., reverses the direction of the incident light beam, whereby the light beam typically experiences a beam displacement. This beam displacement does not occur only if the light beam has a certain beam diameter and is perfectly centered on the retroreflector. In this case, however, partial rays of the light beam experience their own beam displacement. In the embodiment according to Fig. The retroreflector 14 is shown as a cube corner and thus represents a hollow retroreflector. Alternatively, a cat's eye reflector or crossed 90° reflectors can also be used as retroreflector 14.

[0037] The measuring device 10 comprises two optical resonators 16-1 and 16-2, and can therefore also be referred to as a twin resonator. The optical resonators 16-1 and 16-2 are rigidly coupled to each other and each comprises a resonator cavity 18-1 or 18-2, which is enclosed by two resonator mirrors 20-1 or 20-2 and 22-1 or 22-2 of the respective resonator 16-1 or 16-2. The first resonator mirror 20-1 or 20-2 is formed by a coupling mirror through which a measurement beam 24-1 or 24-2 is directed into the resonator cavity 18-1 or 18-2. The second resonator mirror 22-1 or 22-2 is formed by a plane mirror. In the illustrated embodiment, the first resonator mirrors 20-1 and 20-2 each have a curved mirror surface. Thus, the measurement radiation 24-1 and 24-2, respectively, forms a reflection in the respective resonator cavity 18-1 and 24-2.18-2 each emits a Gaussian beam, the waist of which is located on the mirror surface of the second resonator mirror 24-1 or 24-2, which is designed as a plane mirror.

[0038] The aforementioned retroreflector 14 serves as a folding mirror for both optical resonators 16-1 and 16-2; that is, it constitutes a common retroreflector for both resonators 16-1 and 16-2. In other words, the beam paths of both optical resonators 16-1 and 16-2 pass through the retroreflector 14 between the respective resonator mirrors 20-1 and 22-1, and 20-2 and 22-2, respectively. The beam paths of the optical resonators 16-1 and 16-2 are in Fig. 1 is denoted by (1) and (2), respectively. Thus, the beam path (1) within the first optical resonator 16-1 runs from the first resonator mirror 20-1 via the retroreflector 14 to the second resonator mirror 22-1 and from there back via the retroreflector 14 to the first resonator mirror 20-1. Similarly, the beam path (2) within the second optical resonator 16-2 runs from the first resonator mirror 20-2 via the retroreflector 14 to the second resonator mirror 22-2 and from there back via the retroreflector 14 to the first resonator mirror 20-2.

[0039] The distance extending from the measuring head 12 to the retroreflector 14, which serves as the measuring target 23, is referred to as the measuring section 26 of the measuring device 10. Specifically, in the embodiment according to Fig. 1 the measuring section 26 from the z-coordinate of the first resonator mirrors 20-1 and 20-2 in the measuring head to the z-coordinate z Tthe cube tip 15 (rear corner) of the retroreflector 14. The length of the measuring section is L. With a time (t)-dependent displacement of the retroreflector 14 in the z-direction by δL during measurement operation, starting from a length L0 of the measuring section 26 in the ground state, the length L of the measuring section 26 changes accordingly, such that: L = L0 + δL.

[0040] The second resonator mirror 22-2 of the second resonator cavity 18-2 is arranged at the same z-position as the first two resonator mirrors 20-1 and 20-2. Therefore, due to the double beam folding by means of the retroreflector in the exemplary embodiment according to Fig. 1 a length of 2 · L. For the general case of an N F With -fold beam folding, the length of the resonator cavity is 18-2 N each. FThe second resonator mirror 22-1 of the first resonator cavity 18-1 is offset by a distance D relative to the resonator mirror 22-2 of the second resonator cavity 18-2 in the negative z-direction. According to one embodiment, the distance D is at least 10 mm, and in particular at least 20 mm. Thus, the resonator cavities 18-1 and 18-2 have different lengths. While the resonator cavity 18-2, as already stated above, has a length of 2 · L, the resonator cavity 18-1 is longer by D and therefore has a length of 2 · L + D.

[0041] The free spectral ranges FSR1(n, L) and FSR2(n, L) of the two resonators 16-1 and 16-2 are given as a function of the refractive index n of a medium within the resonator cavities 18-1 and 18-2 and the length L: FSR1(L)=c02NFnL and FSR2(L)=c02NFnL+2nD, where c0 denotes the speed of light in a vacuum.

[0042] The frequencies of the resonator modes resulting from the standing wave condition, hereinafter referred to as the resonance frequencies f1 of the first optical resonator 16-1 and f2 of the second optical resonator 16-2, are: f1(k1)=FSR1(L)⋅k1 as well as f2(k2)=FSR2(L)⋅k2 where k1 and k2 denote integer mode indices. For small changes δL of length L and small changes δn of refractive index n, the following linear relationship is obtained for the relative changes in frequencies after a few elementary calculation steps. (δf1 / f1δf2 / f2)=−(11NFL / (NFL+D)1)(δL / Lδn / n)

[0043] After further elementary calculation steps, the following expression is obtained for the refractive index n: n=1+δn=1+NFLDδf1 / f1−(1+NFLD)δf2 / f2 and for the relative geometric change in length, the expression: δL / L=(1+NFLD)(δf2 / f2−δf1 / f1)

[0044] The in Fig. The illustrated measuring device 10 comprises, for each of the optical resonators 16-1 and 16-2, a radiation generation and evaluation unit 28-1 and 28-2, respectively, for generating and evaluating the aforementioned measurement radiation 24-1 and 24-2, respectively. Furthermore, the measuring device 10 optionally comprises, for each of the resonators 16-1 and 16-2, an optical fiber 30-1 and 30-2, respectively, and optionally, a respective beam shaping optic in the form of a coupling lens (not shown in the drawing). The beam generation and evaluation units 28-1 and 28-2 can also be part of the measuring head 12 or arranged outside of it, as shown in Fig. 1 illustrates.

[0045] The beam generation and evaluation unit 28-1 or 28-2 is in Fig. Figure 2 shows an exemplary embodiment in detail. This embodiment is based on the principle that a laser 32, tunable with respect to its optical frequency, follows a frequency of the optical resonator 16-1 or 16-2 via a suitable control loop (in the illustrated embodiment, according to the Pound-Drever-Hall method), so that the length L of the resonator 16-1 or 16-2 to be measured is encoded as the frequency of the tunable laser 32. The laser 32 serves as the radiation source for the measurement radiation 24-1 or 24-2, which lies, for example, in the visible or infrared wavelength range.

[0046] The device 28-1 or 28-2 comprises, in the illustrated embodiment according to Fig. 2 a Faraday isolator 34, an electro-optic modulator 36, a polarization-optical beam splitter 38, a lambda / 4 plate 40, a photodetector 42 and a low-pass filter 44. The portion of the measurement radiation 24-1 or 24-2 passing through the lambda / 4 plate 40 emerges via the in Fig. 1. The optical fiber 30-1 or 30-2 shown is inserted into the measuring head 12. Referring again to Fig. 2. For frequency measurement, a portion of the measurement radiation 24-1 or 24-2 emitted by the tunable laser 32 is coupled out via a beam splitter 46 and fed to an analyzer 48 for frequency measurement. The actual frequency measurement in the analyzer 48 can be performed, for example, by comparison with a frequency reference, e.g., an femtosecond frequency comb of a femtosecond laser. The resonator module according to Fig. The measurement radiation 24-1 or 24-2 exiting the measuring head 12 re-enters the respective device 28-1 or 28-2 via the optical fiber 30-1 or 30-2 and is detected by the photodetector 42. For further details regarding the operation of the device 28-1 or 28-2, reference is made to DE 10 2018 208 147 A1.

[0047] As a result of the frequency measurement in the analyzer 48, the device 28-1 or 28-2 provides the respective current resonance frequency f1 or f2 (reference symbols 50-1 or 50-2) of the optical resonator 16-1 and 16-2 to an evaluation device 48 (see Fig. 1) The evaluation unit 48 determines the relative geometric change in length δL / L of the measuring section 26 using the equation (8) above. Based on the previously precisely measured length L of the measuring section 26, it then determines the absolute change in length δL of the measuring section 26. The evaluation unit 48 thus implicitly determines the change in length of the resonator cavities 18-1 and 18-2, which can be calculated by multiplying the beam folding number N. F with δL.

[0048] From the change in length δL, the evaluation unit 48 in turn determines the z-coordinate z. T (t) of the cube tip 15 of the retroreflector 14 and thus the position of the movable component 226 as a function of time t. According to one embodiment, the evaluation device 48 further determines the refractive index n within the resonator cavities 18-1 and 18-2 based on the above equation (7).

[0049] Due to the use of a common retroreflector 14 for both optical resonators 16-1 and 16-2, here as the measuring target 23, the measuring device 10 is robust against a tilting of the measuring target 23. In other words, a tilting of the measuring target 23 does not lead to a distortion of the measurement result for position 54.

[0050] To understand this effect, the main function of a retroreflector is important: it reflects the beam back onto itself with a parallel offset, whereby the distance traveled is independent of the parallel offset to the main beam. The main beam is defined by the straight line that runs parallel to the direction of propagation and pierces the apex of the retroreflector. Fig. Figure 3 shows the measuring device 10 according to Fig. 1, in which the retroreflector 14 is tilted about the cube apex 15 by an angle θ. As can be seen therein, such a tilt has no influence whatsoever on the respective optical path length in the two optical resonators 16-1 and 16-2. In other words, a tilt θ of the retroreflector 14 about its cube apex 15 does not change the respective length of the optical resonators 18-1 and 18-2, so that the measurement result z T (t) remains unaffected by this.

[0051] The situation is different in the case of one in Fig. 10 illustrated comparative example 110. The measuring device 110 differs from the embodiment 10 according to the invention. Fig. 1. This is achieved essentially by each of the two optical resonators 18-1 and 18-2 having its own retroreflector 114-1 and 114-2, respectively, for reflecting the measurement radiation 24-1 and 24-2 back and forth between the respective resonator mirrors 20-1 and 22-1 and 20-2 and 22-2, respectively. The two retroreflectors 14-1 and 14-2 are rigidly coupled to each other and together form the measurement target 123. When the measurement target 123 is tilted by the angle θ, the length L1 of the measurement path of the first optical resonator 20-1 increases in the illustrated example, while the length L2 of the measurement path of the second optical resonator 20-2 decreases. Therefore, formula (8), which assumes a uniform length L for the measurement path, no longer holds with the necessary accuracy. Therefore, the measuring device 110 can be used according to Fig. 10 the measurement result z T (t) compared to measuring device 10 according to Fig. 1 can only be determined with reduced accuracy.

[0052] Fig. Figure 4 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This differs from the embodiment according to Figure 4 in that it differs from the embodiment shown in Figure 4 in that it is not a further embodiment of a measuring device 10 for determining the position of a component in an optical system for microlithography. Fig. 1, that the measuring target 23 is formed by a measuring mirror 56 configured as a plane mirror, which is located at the same place as the retroreflector 14 according to Fig. 1 is arranged and how it is provided for both optical resonators 16-1 and 16-2. That is, the measuring mirror 56 serves to reflect both the measuring radiation 24-1 and the measuring radiation 24-2. The retroreflector 14 is arranged according to the exemplary embodiment. Fig. 4 is provided for further folding of the beam paths within the resonator cavities 18-1 and 18-2. The respective measurement radiation 24-1 or 24-2 thus travels in the exemplary embodiment according to Fig. 4, starting from the first resonator mirror 20-1 or 20-2, via the measuring mirror 56 and the retroreflector 14, and then again via the measuring mirror 56 to the second resonator mirror 22-1 or 22-2. From there, the respective measuring radiation 24-1 or 24-2 travels the reverse path back to the first resonator mirror 20-1 or 20-2. The respective measuring radiation 24-1 or 24-2 thus passes through the measuring path 26 four times; therefore, the optical resonators 16-1 and 16-2 are so-called 4-pass resonators. In other words, the convolution number in this embodiment is 4.

[0053] Even in the exemplary embodiment according to Fig. 4. The use of the common retroreflector 14 leads to the fact that if the measuring target 23, which is formed here by the measuring mirror 56, is tilted, the measurement result for position z T(t) is not distorted. In this configuration, a tilting of the measuring mirror 56 can indeed cause a change in the optical path length in the respective optical resonator 18-1 or 18-2, but to the same extent, so that the resonance frequencies f1 and f2 change in such a way that the result obtained from equation (8) for δL is not distorted.

[0054] In Fig. Figure 5 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This is analogous to the measuring device 10 according to Figure 5. Fig. 1 is configured and differs only in that the first resonator mirrors 20-1 and 20-2 are not arranged one above the other (i.e., offset in the y-direction), but next to each other (i.e., offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This results in a compact embodiment of the measuring device 10 according to Fig. 1 provided. As in Fig. As shown in Figure 5, both the first resonator mirrors 20-1 and 20-2 and the second resonator mirrors 22-1 and 22-2 can each be configured as a monolithic assembly. This avoids drift effects. According to one variant, these two assemblies can also be configured together, i.e., again as a monolithic assembly.

[0055] In Fig. Figure 6 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This is analogous to the measuring device according to Figure 6. Fig. 4 is configured and differs only in that the first resonator mirrors 20-1 and 20-2 are not arranged one above the other (i.e., offset in the y-direction), but next to each other (i.e., offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This results in a compact embodiment of the measuring device 10 according to Fig. 4 provided. As in Fig. As shown in Figure 6, both the first resonator mirrors 20-1 and 20-2 and the second resonator mirrors 22-1 and 22-2 can each be configured as a monolithic assembly. This avoids drift effects. According to one variant, these two assemblies can also be configured together, i.e., again as a monolithic assembly.

[0056] In Fig. Figure 7 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This is analogous to the measuring device 10 according to Figure 7. Fig. 4 is configured and differs from it only in that the measuring mirror 56, which serves as the measuring target 23, is configured as a stepped plane mirror 58. The stepped plane mirror 58 has a depression in a central area, which is set back from the edge area by a distance D, so that the different lengths of the resonator cavities 18-1 and 18-2 are caused by the stepping of the plane mirror 58.

[0057] The second resonator mirrors 22-1 and 22-2 are therefore arranged at the same z-position in this embodiment, i.e., without axial offset. However, in this variant, the geometric path length and the refractive index cannot be determined without additional knowledge of the tilt angle of the measuring mirror 56.

[0058] In Fig. Figure 8 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This is analogous to the measuring device according to Figure 8. Fig. 4 is configured and differs from it only in that it comprises a polarizing beam splitter cube 60 arranged in the beam paths of the optical resonators 16-1 and 16-2, with an associated quarter-wave plate 62, which serves to couple the retroreflector 14 to the beam paths of the optical resonators 16-1 and 16-2. As a result of this coupling, the following can be observed in the exemplary embodiment according to Fig. 4. The existing tilting of the measuring mirror 56 is dispensed with, i.e., the measuring mirror is oriented perpendicular to the incident measuring radiation 24-1 or 24-2 (perpendicular incidence) and thus, in particular, also parallel to the resonator mirrors 20-1, 20-2, 22-1 or 22-2. The polarizing beam splitter cube 60 implements the principle of the polarization-optical switch, the basic principle of which is explained in DE 10 2018 208 147 A1.

[0059] In Fig. Figure 9 illustrates a further embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This is analogous to the measuring device 10 according to Figure 9. Fig. 8 is configured and differs only in that the first resonator mirrors 20-1 and 20-2 are not arranged one above the other (i.e., offset in the y-direction), but next to each other (i.e., offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This results in a compact embodiment of the measuring device 10 according to Fig. 8 provided. As in Fig. As shown in Figure 9, both the first resonator mirrors 20-1 and 20-2 and the second resonator mirrors 22-1 and 22-2 can each be configured as a monolithic assembly. This avoids drift effects. According to one variant, these two assemblies can also be configured together, i.e., again as a monolithic assembly.

[0060] Fig. Figure 11 shows in a simplified representation the aforementioned projection exposure system 200 for microlithography with the mirror 226, which serves as a component for measurement using the measuring device 10 according to Fig. 1 or one of the Fig. 3 to 10. The mirror 226 is mounted on a support structure not shown in the drawing, for example in the form of a reference frame or a housing of the projection lens 216 of the projection exposure system 200.

[0061] The projection exposure system 200 according to Fig. The device 11 is designed for operation with EUV exposure radiation 201. In this text, EUV radiation refers to electromagnetic radiation with 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 use in such a system, but can also be implemented in the measurement of projection exposure systems with other operating wavelengths, for example, operating wavelengths in the VUV or DUV range. In further applications, the invention can also be implemented in another optical system for microlithography, such as a mask inspection system or a wafer inspection system. The projection exposure system 200 has an exposure beam path 217 in which the exposure radiation 201 is guided through an illumination optic 205 and a projection lens 216.

[0062] According to the embodiment of Fig. The illumination optics 205 comprise a field facet mirror 202, a pupil facet mirror 204, and two telescope mirrors 210 and 212. The illumination radiation 201, which is generated by an EUV radiation source comprising a plasma radiation source 206 and a collector mirror 208, is first directed onto the field facet mirror 202 and from there onto the pupil facet mirror 204. The first telescope mirror 210 and the second telescope mirror 212 are arranged in the radiation path after the pupil facet mirror 204. A deflecting mirror 214 is arranged downstream in the radiation path, which directs the radiation striking it onto an object field in an object plane of the projection objective 216, which comprises six mirrors 218, 220, 222, 224, 226 and 228.

[0063] At the location of the microscope slide, a reflective structure-bearing mask 230 is arranged on a mask table 232, which is projected into an image plane by means of the projection lens 216, in which a substrate 234 coated with a radiation-sensitive layer (photoresist) in the form of a wafer is located on a wafer table 236.

[0064] The principle underlying the embodiments described above with reference to the figures can be represented as follows: Two measuring sections designed as cavities share a retroreflector. By introducing an offset, the cavity lengths, which are identical without the offset, become different, so that, from the two measured optical lengths and with knowledge of the geometric offset length, the geometric length of interest of the shared cavity as well as the refractive index in the cavity can be determined.

[0065] The foregoing description of exemplary embodiments, embodiments, or variants is to be understood as illustrative. The disclosure thereby enables the person skilled in the art to understand the present invention and its associated advantages, and also encompasses, in the understanding of the person skilled in the art, obvious modifications and alterations of the described structures and methods. Therefore, all such modifications and alterations, insofar as they fall within the scope of the invention as defined in the appended claims, as well as equivalents, are to be covered by the protection of the claims. Reference symbol list 10 Measuring device 12 Measuring head 14 Retroreflector 15 dice tips 16-1, 16-2 optical resonators 18-1, 18-2 resonator cavities 20-1, 20-2 first resonator mirror 22-1, 22-2 second resonator mirror 23 measuring target 24-1, 24-2 Measurement radiation 26 measuring section 28-1, 28-2 Beam generation and evaluation unit 30-1, 30-2 optical fiber 32 tunable lasers 34 Faraday insulator 36 electro-optical modulator 38 polarization-optical beam splitter 40 Lambda / 4 plate 42 Photodetector 44 Low-pass filters 46 beam splitters 48 Evaluation unit 50-1, 50-1 Resonance frequencies 52 Change in the length of the measuring section 54 Position of the movable component 56 measuring mirrors 58-tiered plan grid 60 polarizing beam splitter cubes 62 Quarter wave plate 114-1 first retroreflector 114-2 second retroreflector 123 measuring target 200 projection exposure system for microlithography 201 Exposure radiation 202 Field Facet Mirror 204 pupil facet mirrors 205 Lighting optics 206 Plasma light source 208 Collector mirrors 210 first telescope mirror 212 second telescope mirror 214 Deflection mirrors 216 Projection lens 217 Exposure beam path 218, 220, 222, 224, 228 Mirror of the projection lens 226 Mirror of the projection lens serving as a component to be measured 230 mask 232 Mask table 234 Substrat 236 Wafer table QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2012 212 663 A1

[0004] DE 10 2018 208 147 A1 [0046, 0058]

Claims

[1] Measuring device (10) for frequency-based position determination of a movable component (226) in an optical system (200) for microlithography, comprising: - two optical resonators (16-1; 16-2) each with two resonator mirrors (20-1, 22-1; 20-2, 22-2), each enclosing a resonator cavity (18-1; 18-2), wherein the resonator cavities have different lengths, as well as - a common retroreflector (14) for both resonators, which is configured to direct a respective measurement radiation (24-1; 24-2) back and forth between the respective resonator mirrors of the two resonators. [2] Measuring device according to claim 1, which further comprises an evaluation device (48) which is configured to determine a length change (52) of the resonator cavities from measurements of respective resonance frequencies (50-1, 50-1) of the resonator cavities. [3] Measuring device according to claim 2, wherein the evaluation device (48) is configured to determine the position (54) of the movable component (226) from the change in length of the resonator cavities. [4] Measuring device according to claim 2 or 3, wherein the evaluation device (48) is further configured to determine a refractive index of a medium within the resonator cavities from the measurements of the respective resonance frequencies of the resonator cavities (18-1, 18-2). [5] Measuring device according to one of the preceding claims, wherein the retroreflector (14) serves as the measuring target (23) which is associated with the movable component (226). [6] Measuring device according to one of claims 1 to 4, which further comprises a measuring mirror (56) associated with the component, which is also arranged within both resonator cavities (18-1, 18-2) for deflecting the respective measuring radiation (24-1, 24-2) back and forth between the respective resonator mirrors of the two resonators. [7] Measuring device according to claim 6, wherein the optical resonators (16-1, 16-2) each contain at least four beam folds. [8] Measuring device according to claim 6 or 7, wherein the measuring mirror (56) is configured as a stepped plane mirror (58) to effect the different lengths of the resonator cavities (18-1, 18-2). [9] Measuring device according to claim 7 or 8, which further comprises a polarizing beam splitter cube (60) for coupling the retroreflector (14) to the beam paths of the optical resonators. [10] Measuring device according to one of the preceding claims, wherein the two resonator cavities (18-1, 18-2) are rigidly coupled to each other. [11] Measuring device according to one of the preceding claims, wherein the lengths of the resonator cavities (18-1, 18-2) differ from each other by at least 10 mm. [12] Projection exposure system (200) for microlithography comprising at least one component (226) and at least one measuring device (10) according to one of the preceding claims for determining the position of the component.

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