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

Abstract

The invention relates to a measuring device (10) for the frequency-based determination of the position of a movable component (226) in an optical system (200) for microlithography, comprising two optical resonators (16-1; 16-2) each having 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 comprising a common retroreflector (14) for the two resonators, which retroreflector is configured to guide respective measurement radiation (24-1; 24-2) back and forth between the respective resonator mirrors of the two resonators, wherein the measuring device is configured such that, while the position of the movable component is being determined, an influence of a refractive index of a medium within the resonator cavities is prevented from affecting the result of the position determination.

Description

[0001] Measuring device for frequency-based position determination of a movable component

[0002] This application claims priority from German patent application 10 2024 203 642.0 filed on April 19, 2024. The entire disclosure of this patent application is incorporated by reference into the present description.

[0003] Background of the invention

[0004] The invention relates to a measuring device for frequency-based position determination of a movable component in an optical system for microlithography and to a projection exposure system with at least one measuring device of the aforementioned type.

[0005] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. This is done using a so-called projection exposure system, which comprises an illumination device and a projection lens. The image of a mask located on a reticle and illuminated by the illumination device is projected by 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] When operating such projection lenses, where the mask and wafer are typically moved relative to each other during a scanning process, the positions of the mirrors, some of which are movable in all six degrees of freedom, must be adjusted and maintained with high precision, both relative to each other and relative to the mask or wafer, to avoid or at least reduce aberrations and the associated impairment of the imaging result. For this position determination, for example, in E UV lithography, length measurement accuracies in the picometer (pm) range may be required over a path length of 1 meter.

[0007] Various approaches are known in the prior art for measuring the position of the individual objective mirrors as well as the wafer or wafer stage and the reticle 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 resonator in the form of a Fabry-Perot resonator with two resonator mirrors, of which the first resonator mirror is attached to a reference element in the form of a measurement frame firmly connected to the housing of the projection objective of the projection exposure system, and the second resonator mirror (as a so-called "measurement target") is attached to an EUV mirror whose position is to be measured.

[0008] The actual distance measuring device comprises a radiation source whose optical frequency is tunable, generating input 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 the optical frequency of the radiation source 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. Input radiation coupled out via a beam splitter is analyzed with an optical frequency measuring device, which may include, for example, 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 - due to the coupling of the frequency of the tunable radiation source to the resonance frequency of the resonator - also the optical frequency of the coupling radiation, which in turn is directly registered with the frequency measuring device.

[0009] In interferometric or frequency-based position measurement, the optical phase or its change is recorded along an optical path defined by mirrors. The actual measured value therefore does not correspond to the desired geometric path length S. geo , but the optical path length modified by the refractive index n of the usually gaseous medium

[0010] ' <'?opt according to S op tn S geo .

[0011] 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.

[0012] In interferometric systems for length measurement, a reference distance with a known geometric length can be used to measure the refractive index for the purpose of correcting for atmospheric influences. During atmospheric correction, the measured optical length S ref opt the reference distance and the knowledge of its geometric length S ref geo the refractive index n ref according to n ref = S refi0pt / S ref ge0 Assuming that the refractive indices of the measurement and reference sections match n ref = n meas , applies to the desired geometric length S meaSigeo = Smeas,op t Sref,geo / Sref,opt > where S meaSiOptstands for the measured optical length of the measuring section.

[0013] However, due to space-related or cost-related limitations, this measuring section is not always – or even rarely – located in the immediate vicinity of the measuring sections of interest. As a result, differences in gas densities (gas density gradients and striae) lead to differences in the refractive indices An = n meas - n ref and thus to falsifications of the extracted geometric information according to AS meaSigeo = To S meaSiOpt . Underlying task

[0014] It is an object of the invention to provide a measuring device of the type mentioned at the outset, whereby 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.

[0015] Inventive solution

[0016] The aforementioned object can be achieved 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, which each enclose a resonator cavity, wherein the resonator cavities have different lengths. Furthermore, the measuring device comprises a common retroreflector for both resonators, which is configured to direct a respective measuring radiation back and forth between the respective resonator mirrors of the two resonators. The measuring device is configured to exclude any influence of a refractive index of a medium within the resonator cavities from the position determination result when determining the position of the movable component. This means that the same retroreflector is used in both optical resonators.

[0017] In other words, by eliminating the influence of the refractive index from the position determination result using the measuring device, the refractive index can be compensated. In this text, a retroreflector is understood to be a reflector that reflects an incident light beam back into itself, i.e., reverses the direction of an incident light beam, whereby the light beam usually experiences a beam offset. The beam offset only does not occur if the light beam has a certain beam diameter and is irradiated perfectly centrally onto the retroreflector. In this case, however, partial rays of the light beam experience a respective beam offset. A cube corner, a cat's eye, or even crossed 90° reflectors, for example, can be used as a retroreflector.

[0018] In this text, an optical resonator is understood to be an arrangement of mirrors designed to reflect light back and forth as frequently as possible. According to one embodiment, the optical resonators of the measuring device are each configured such that the measurement radiation radiated into the respective optical resonator retains at least 90%, advantageously at least 99%, of the radiated intensity after at least five passes, preferably after at least ten passes, through the optical resonator.According to a further embodiment, the optical resonators of the measuring device are each configured to reflect the irradiated measuring radiation back and forth multiple times, for example, at least 10 times, at least 100 times, or at least 1000 times, before the measuring radiation leaves the optical resonator again. Leaving the optical resonator means that the intensity of any measuring radiation remaining in the resonator is less than 50% of the intensity of the irradiated measuring radiation. According to a further embodiment, the optical resonators of the measuring device each have a finesse of at least 100, advantageously of at least 1000.

[0019] The inventive configuration of the measuring device with two optical resonators and a shared retroreflector makes it possible to determine the position of the movable component and thereby exclude or calculate out the influence of a fluctuating refractive index from the result. This can be done with high accuracy; the shared retroreflector prevents a tilt of the measurement target to which the movable component is attached from leading to a falsification of the measurement result. This is made possible by the main effect of a retroreflector, which is to reflect an incident beam back into itself with a parallel offset, whereby the distance traveled is independent of the parallel offset of the incoming beam.In comparison, in a comparative example of a measuring device in which each of the optical resonators has its own retroreflector, which is then rigidly coupled to the other retroreflector to form a measuring target, a tilt of the measuring target leads to a considerable falsification of the measurement result, as will be explained in more detail later in this text.

[0020] According to one embodiment, the measuring device for frequency-based position determination comprises a radiation source for each of the two optical resonators whose optical frequency can be tuned to generate a respective measuring radiation. Advantageously, the measuring module has a coupling module for each of the optical resonators, 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, i.e., the optical frequency follows the resonant frequency.

[0021] Advantageously, the measuring device further comprises a frequency measuring device for measuring the optical frequency of the respective coupled radiation source. The length of the optical resonator has a functional dependence on the measured optical frequency, i.e., the current 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 comprises a respective computing 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 respective optical resonator and the respective measurement target.

[0022] According to one embodiment, the measuring device further comprises an evaluation device which is configured to determine a change in length of the resonator cavities from measurements of respective resonance frequencies of the resonator cavities.

[0023] According to a further 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.

[0024] According to a further embodiment, the evaluation device is further configured to determine the refractive index of the medium within the resonator cavities from the measurements of the respective resonance frequencies of the resonator cavities.

[0025] According to a further embodiment, the retroreflector serves as a measurement target associated with the movable component. The retroreflector is common to both cavities and can be present 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 a lens or mirror design, and as other retroreflector designs familiar to those skilled in the art. According to a further embodiment, the measuring device further comprises a measurement mirror associated with the component, which is also arranged within both resonator cavities for deflecting the respective measurement radiation back and forth between the respective resonator mirrors of the two resonators.

[0026] According to a further embodiment, the optical resonators each contain at least four beam folds. These optical resonators are thus so-called 4-pass resonators, in which the measuring radiation passes four times through a measuring section of the measuring device. The measuring section extends between a measuring head and a measuring mirror. A pass is understood to mean that the measuring radiation passes back and forth within the measuring section.

[0027] According to a further embodiment, the optical resonators each contain at least two, preferably at least four, and 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. If a plane mirror is used as the measurement target, the number of folds is advantageously 4.

[0028] According to a further embodiment, the measuring mirror is configured as a stepped plane mirror to effect the different lengths of the resonator cavities.

[0029] According to a further embodiment, the measuring device further comprises a polarizing beam splitter cube for coupling the retroreflector to the beam paths of the optical resonators.

[0030] According to a further embodiment, the two resonator cavities are rigidly coupled to each other. According to a further 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 length difference is in the range of 20 mm to 50 mm.

[0031] In a modification, the resonator cavities can each be formed as open cavities with propagating waves, and the measuring device can be designed as an interferometer. This can be achieved by modifying a respective coupling mirror, which is highly reflective in an optical resonator (closed cavity with standing waves), with an at least partially transmitting element. The interferometer preferably comprises a beam splitter for generating a measurement path and a reference path.

[0032] 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 embodiment variants for determining the position of the component.

[0033] The features mentioned with regard to the above-mentioned embodiments, exemplary embodiments, or variant embodiments, etc., are explained in the description of the figures 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 protectable and whose protection may be claimed only during or after the filing of the application.

[0034] Brief description of the drawings

[0035] The above and other 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 attached schematic drawings.

[0036] They show:

[0037] Fig. 1 shows an embodiment of a measuring device for frequency-based position determination of a movable component, which comprises two optical resonators, each with a beam generation and evaluation device,

[0038] Fig. 2 an embodiment of the beam generation and evaluation device,

[0039] Fig. 3 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0040] Fig. 4 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0041] Fig. 5 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0042] Fig. 6 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0043] Fig. 7 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0044] Fig. 8 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators, Fig. 9 shows a further embodiment of a measuring device for frequency-based position determination of a movable component with two optical resonators,

[0045] 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

[0046] Fig. 11 shows an embodiment of a projection exposure system for microlithography with a movable component, the position of which can be determined by means of the measuring device according to Fig. 1 or according to one of the figures 3 to 10.

[0047] Detailed description of embodiments according to the invention

[0048] In the embodiments or variants described below, functionally or structurally similar elements are provided with the same or similar reference numerals wherever possible. Therefore, to understand the features of the individual elements of a particular embodiment, reference should be made to the description of other embodiments or the general description of the invention.

[0049] To facilitate the description, a Cartesian xyz coordinate system is shown in the drawing, from which the respective positional relationship of the components shown in the figures results. In Fig. 1, the x-direction runs perpendicular to the plane of the drawing, the z-direction to the right, and the y-direction upwards. Fig. 1 shows a first exemplary embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. Fig. 11 illustrates, in a simplified representation, such an optical system in the form of a projection exposure system 200 for microlithography, which is described in more detail later in the text. A mirror 226 of a projection objective 216 of the projection exposure system 200 serves, for example, as the aforementioned component to be measured.The position of this component can change during operation of the projection exposure system 200, therefore the component can also be referred to as a movable component.

[0050] 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 associated therewith. 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 by means of the measuring device 10, as described in detail below. In other words, in this exemplary embodiment, the position ZT of the measuring target 23 is determined as a function of time t, i.e., zi-(t).

[0051] In this text, a retroreflector is understood to be a reflector that reflects an incident light beam back into itself, i.e. it reverses the direction of an incident light beam, whereby the light beam usually experiences a beam offset. The beam offset only does not occur if the light beam has a certain beam diameter and is irradiated perfectly centrally onto the retroreflector. In this case, however, partial rays of the light beam experience a respective beam offset. In the embodiment according to Fig. 1, the retroreflector 14 is shown as a cube corner and thus represents a hollow retroreflector. Alternatively, a cat's eye or crossed 90° reflectors can be used as the retroreflector 14. 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 comprise a resonator cavity 18-1 and 18-2, respectively, which are each surrounded by two resonator mirrors 20-.

[0052] 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 measuring radiation 24-1 or 24-2 is radiated 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 measuring radiation 24-1 or 24-2 forms a

[0053] 2 each emit a Gaussian beam whose waist is located on the mirror surface of the second resonator mirror 24-1 or 24-2, which is designed as a plane mirror.

[0054] The aforementioned retroreflector 14 serves as a folding mirror for both optical resonators 16-1 and 16-2, ie, it represents 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 extend via the retroreflector 14 between the respective resonator mirrors 20-1 and 22-1, or 20-2 and 22-2. The beam paths of the optical resonators 16-1 and 16-2 are designated (1) and (2) in Fig. 1, respectively. Thus, the beam path (1 ) runs within the first optical resonator 16-1 from the first resonator mirror 20-1 via the retroreflector 14 to the second resonator mirror 22-1 and from there again via the retroreflector 14 back to the first resonator mirror 20-1.Accordingly, the beam path (2) runs within the second optical resonator 16-2 from the first resonator mirror 20-2 via the retroreflector 14 to the second resonator mirror 22-2 and from there again via the retroreflector 14 back to the first resonator mirror 20-2. The path extending from the measuring head 12 to the retroreflector 14 serving as the measuring target 23 is referred to as the measuring path 26 of the measuring device 10. Specifically, in the embodiment according to Fig. 1, the measuring path 26 extends from the z-coordinate of the first resonator mirrors 20-1 and 20-2 in the measuring head to the z-coordinate ZT of the cube tip 15 (rear corner) of the retroreflector 14. The length of the measuring path is L. With a time (t)-dependent displacement of the retroreflector 14 in the measuring operation in the z-direction by. Starting from a length Lo of the measuring section 26 in the ground state, the length L of the

[0055] Measuring section 26 accordingly, so that: L = Lo +

[0056] The second resonator mirror 22-2 of the second resonator cavity 18-2 is arranged at the same z-position as the two first resonator mirrors 20-1 and 20-2. The resonator cavity 18-1 therefore has a length of 2 ■ L due to the 2-fold beam folding by means of the retroreflector in the embodiment according to Fig. 1. For the general case of NF-fold beam folding, the length of the resonator cavity 18-2 is NF - L in each case. The second resonator mirror 22-1 of the first resonator cavity 18-1 is offset by a distance D from 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, 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 thus has a length of 2 ■ L + D.

[0057] The free spectral ranges FSRi(n, L) and FSR2(n, L) of the two resonators 16-1 and 16-2 are, depending on the refractive index n of a medium within the resonator cavities 18-1 and 18-2 and the length L: CQ

[0058] FSR2(L)

[0059] 2N F nL + 2nD (2) where co is the speed of light in vacuum.

[0060] The frequencies of the resonator modes resulting from the standing wave condition, hereinafter referred to as resonance frequencies fi of the first optical resonator 16-1 and f2 of the second optical resonator 16-2, are: fi(ki)= FSRi (L) ■ ki and f2(k2)= FSR2(L) ■ k2 where ki and k2 are integer mode indices. For small changes in length L and small changes of the refractive index n, after a few elementary calculation steps, the linear relationship for the relative changes in frequencies results

[0061] After further elementary calculation steps, the refractive index n is expressed as: and for the relative geometric change in length the expression:

[0062] The measuring device 10 illustrated in Fig. 1 comprises, for each of the optical resonators 16-1 and 16-2, a radiation generation and evaluation device 28-1 or 28-2 for generating and evaluating the aforementioned measuring radiation 24-1 or 24-2. Furthermore, the measuring device 10 optionally comprises, for each of the resonators 16-1 and 16-2, an optical fiber 30-1 or 30-2, as well as, if appropriate, a respective beam-shaping optic in the form of a coupling lens (not shown in the drawing). The beam generation and evaluation devices 28-1 and 28-2 can also be part of the measuring head 12 or can be arranged outside it, as illustrated in Fig. 1.

[0063] The beam generation and evaluation device 28-1 or 28-2 is shown in detail in Fig. 2 in an exemplary embodiment. This is based on the principle according to which a laser 32 tunable with regard to the 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 ultimately to be measured is encoded as the frequency of the tunable laser 32. The laser 32 serves as a radiation source for the measuring radiation 24-1 or 24-2, which lies, for example, in the visible or infrared wavelength range.

[0064] In the illustrated embodiment according to Fig. 2, the device 28-1 or 28-2 comprises a Faraday isolator 34, an electro-optical 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 measuring radiation 24-1 or 24-2 passing through the lambda / 4 plate 40 enters the measuring head 12 via the optical fiber 30-1 or 30-2 shown in Fig. 1. Referring again to Fig. 2, for frequency measurement, a portion of the measuring 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 fs frequency comb of a femtosecond laser. The measurement radiation 24-1 or 24-2 leaving the resonator module via the measuring head 12 as shown in Fig. 1.24-2 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, please refer to DE 10 2018 208 147 A1.

[0065] As a result of the frequency measurement in the analyzer 48, the device 28-1 or 28-2 provides the respective current resonance frequency fi or f2 (reference numerals 50-1 or 50-2) of the optical resonator 16-1 and 16-2 to an evaluation device 48 (see Fig. 1). The evaluation device 48 determines the relative geometric length change based on the above equation (8). of the measuring section 26. Based on the previously precisely measured length L of the measuring section 26, it determines the absolute change in length the measuring section 26. The evaluation device 48 thus implicitly determines the change in length of the resonator cavities 18-1 and 18-2, which results from multiplying the beam folding number NF by SL.

[0066] From the change in length SL, the evaluation device 48 in turn determines the z-coordinate zi-(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. Since in equation (8) the relative change in length ÖL / Lwithout specifying the refractive index n, the influence of the refractive index n is excluded from the position determination result when determining the position of the movable component 226. In other words, the refractive index is compensated for during the position determination. According to one exemplary embodiment, the evaluation device 48 further determines the refractive index n within the resonator cavities 18-1 and 18-2 using the above equation (7). Due to the use of a common retroreflector 14 for both optical resonators 16-1 and 16-2, here as the measurement target 23, the measuring device 10 is robust against a tilt of the measurement target 23. In other words, a tilt of the measurement target 23 does not lead to a falsification of the measurement result for the position 54.

[0067] To understand this effect, the main effect of a retroreflector is important: it reflects the beam back into itself with a parallel offset, whereby the distance traveled is independent of the parallel offset to the main ray. The main ray is defined by the straight line that runs parallel to the propagation direction and pierces the cube apex of the retroreflector. Fig. 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 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 ZT (t) remains unaffected.

[0068] The situation is different in a comparative example 110 illustrated in Fig. 10. The measuring device 110 differs from the exemplary embodiment 10 according to the invention according to Fig. 1 essentially in that each of the two optical resonators 18-1 and 18-2 has its own retroreflector 114-1 or 114-2 for reflecting the respective measuring radiation 24-1 or 24-2 back and forth between the respective resonator mirrors 20-1 and 22-1 or 20-2 and 22-2. 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 0, the length Li of the measurement path of the first optical resonator 20-1 increases in the example shown, while the length L2 of the measurement path of the second optical resonator 20-2 decreases. Thus, formula (8), which assumes a uniform length L for the measurement path, no longer applies with the required accuracy.Therefore, with the measuring device 110 according to Fig. 10, the measurement result zi- (t) can only be determined with a reduced accuracy compared to the measuring device 10 according to Fig. 1.

[0069] Fig. 4 illustrates a further exemplary embodiment according to the invention of a measuring device 10 for determining the position of a component in an optical system for microlithography. This differs from the exemplary embodiment according to Fig. 1 in that the measuring target 23 is formed by a measuring mirror 56 configured as a plane mirror, which is arranged at the same location as the retroreflector 14 according to Fig. 1 and, like the latter, is provided for both optical resonators 16-1 and 16-2. This means that the measuring mirror 56 serves to reflect both the measuring radiation 24-1 and the measuring radiation 24-2. The retroreflector 14 is provided in the exemplary embodiment according to Fig. 4 for further folding the beam paths within the resonator cavities 18-1 and 18-2. The respective measuring radiation 24-1 or 24-2 thus travels in the exemplary embodiment according to Fig. 4, starting from the first resonator mirror 20-1 or20-2, 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 section 26 four times, thus making the optical resonators 16-1 and 16-2 so-called 4-pass resonators. In other words, the convolution number in this embodiment is 4.

[0070] In the exemplary embodiment according to Fig. 4, the use of the common retroreflector 14 also ensures that the measurement result for the position ZT (t) is not distorted if the measurement target 23, which here is formed by the measurement mirror 56, is tilted. In this configuration, a tilt of the measurement 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 in each case, so that the resonance frequencies fi and f2 change in a manner adapted to one another that does not distort the result determined for SL from equation (8).

[0071] Fig. 5 illustrates a further exemplary 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 configured analogously to the measuring device 10 according to Fig. 1 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 rather next to one another (i.e., offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This provides a compact embodiment of the measuring device 10 according to Fig.

[0072] 1. As shown in Fig. 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 allows drift effects to be avoided. According to one variant, these two assemblies can also be configured contiguously, ie, again as a monolithic assembly.

[0073] Fig. 6 illustrates a further exemplary 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 configured analogously to the measuring device according to Fig. 4 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 rather next to one another (i.e., offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This provides a compact embodiment of the measuring device 10 according to Fig.

[0074] 4. As shown in Fig. 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 makes it possible to avoid drift effects. According to a variant, these two assemblies can also be configured contiguously, i.e. again as a monolithic assembly. Fig. 7 illustrates a further exemplary embodiment according to the invention of a measuring device 10 for determining the position of a component in an optical system for microlithography. This is configured analogously to the measuring device 10 according to Fig. 4 and differs therefrom only in that the measuring mirror 56 serving as the measuring target 23 is configured as a stepped plane mirror 58.The stepped plane mirror 58 has a recess in a central region that is set back by a distance D from the edge region, so that the different lengths of the resonator cavities 18-1 and 18-2 are caused by the stepping of the plane mirror 58. Therefore, in this embodiment, the second resonator mirrors 22-1 and 22-2 are arranged at the same z-position, 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.

[0075] Fig. 8 illustrates another exemplary embodiment of a measuring device 10 according to the invention for determining the position of a component in an optical system for microlithography. This measuring device is configured analogously to the measuring device according to Fig. 4 and differs 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 a quarter-wave plate 62 associated therewith, 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 tilting of the measuring mirror 56 present in the embodiment according to Fig. 4 can be dispensed with, ie the measuring mirror is oriented perpendicular to the incident measuring radiation 24-1 or 24-2 (vertical 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. Fig. 9 illustrates a further exemplary 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 configured analogously to the measuring device 10 according to Fig. 8 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 one another (i.e. offset in the x-direction). The same applies to the second resonator mirrors 22-1 and 22-2. This provides a compact embodiment of the measuring device 10 according to Fig.

[0076] 8. As shown in Fig. 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 allows drift effects to be avoided. According to one variant, these two assemblies can also be configured contiguously, ie, again as a monolithic assembly.

[0077] Fig. 11 shows, in a simplified representation, the above-mentioned projection exposure system 200 for microlithography with the mirror 226, which serves as a component for the measurement by means of the measuring device 10 according to Fig. 1 or one of Figs. 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.

[0078] The projection exposure system 200 according to Fig. 11 is designed for operation with EUV exposure radiation 201. In this text, EUV radiation is understood to mean 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 optics 205 and a projection lens 216.

[0079] According to the embodiment of Fig. 11, the illumination optics 205 comprises a field facet mirror 202, a pupil facet mirror 204, and two telescope mirrors 210 and 212. The exposure 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 downstream of the pupil facet mirror 204. A deflecting mirror 214 is arranged downstream in the radiation path, which deflects the radiation incident on it onto an object field in an object plane of the projection lens 216, which comprises six mirrors 218, 220, 222, 224, 226 and 228.

[0080] At the location of the object carrier, a reflective structure-bearing mask 230 is arranged on a mask table 232, which is imaged by means of the projection lens 216 into an image plane 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.

[0081] The principle underlying the exemplary embodiments described above with reference to the figures can be described as follows: Two measuring sections designed as cavities jointly use a retroreflector. By introducing an offset, the cavity lengths, which would be identical without the offset, become different in size, so that the geometric length of interest of the shared cavity as well as the refractive index in the cavity can be determined from the two measured optical lengths, knowing the geometric offset length. The above description of exemplary embodiments, embodiments, or design variants is to be understood as exemplary.The disclosure herein will enable one skilled in the art to understand the present invention and the associated advantages, and will also encompass obvious variations and modifications of the described structures and methods within the understanding of one skilled in the art. Therefore, all such variations and modifications, insofar as they fall within the scope of the invention as defined in the appended claims, and equivalents, are intended to be covered by the claims.

[0082]

[0083] 10 Measuring device

[0084] 12 measuring head

[0085] 14 Retroreflector

[0086] 15 dice tip

[0087] 16-1 , 16-2 optical resonators

[0088] 18-1 , 18-2 Resonator cavities

[0089] 20-1 , 20-2 first resonator mirror

[0090] 22-1 , 22-2 second resonator mirror

[0091] 23 Measurement target

[0092] 24-1 , 24-2 Measuring radiation

[0093] 26 measuring section

[0094] 28-1 , 28-2 Beam generation and evaluation device

[0095] 30-1 , 30-2 optical fiber

[0096] 32 tunable lasers

[0097] 34 Faraday insulator

[0098] 36 electro-optical modulator

[0099] 38 polarization optical beam splitters

[0100] 40 Lambda / 4 plate

[0101] 42 Photodetector

[0102] 44 low-pass filters

[0103] 46 beam splitters

[0104] 48 Evaluation device

[0105] 50-1 , 50-1 resonance frequencies

[0106] 52 Change in length of the measuring section

[0107] 54 Position of the movable component

[0108] 56 measuring mirrors

[0109] 58 stepped plane mirror

[0110] 60 polarizing beam splitter cubes

[0111] 62 quarter-wave plate

[0112] 114-1 first retroreflector -2 second retroreflector

[0113] Measurement target

[0114] Projection exposure system for microlithography

[0115] Exposure radiation

[0116] Field facet mirror

[0117] Pupillary facet mirror

[0118] Lighting optics

[0119] Plasma light source

[0120] Collector mirror first telescope mirror second telescope mirror

[0121] Deflecting mirror

[0122] Projection lens

[0123] Exposure beam path, 220, 222, 224, 228 Mirror of the projection lens serving as the component to be measured Mirror of the projection lens

[0124] mask

[0125] Mask table

[0126] Substrat

[0127] Wafer table

Claims

Claims 1. A 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, 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, wherein the measuring device is configured to exclude an influence of a refractive index of a medium within the resonator cavities from the result of the position determination when determining the position of the movable component.

2. Measuring device according to claim 1, which further comprises an evaluation device (48) which is configured to determine a change in length (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 the refractive index of the 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 a measuring target (23) which is assigned to the movable component (226).

6. Measuring device according to one of claims 1 to 4, which further comprises a measuring mirror (56) assigned to 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) for effecting 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 one another.

11. Measuring device according to one of the preceding claims, wherein the lengths of the resonator cavities (18-1, 18-2) differ from one another by at least 10 mm.

12. Projection exposure system (200) for microlithography with 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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