Measuring device for determining the position of a component

Abstract

A measuring device (10) for determining the position of a component (226) in a system (200) comprises a radiation generator (24) configured to generate a measuring radiation (22e) with at least two discrete radiation frequencies (36; 136), which are selected comb frequencies (38) of a frequency comb structure (37), wherein the frequency comb structure is defined by at least two comb parameters, of which the first comb parameter comprises a uniform frequency spacing (42) of the comb frequencies and the second comb parameter comprises a frequency offset of the frequency comb structure from a reference frequency, an optical resonator (16) comprising resonator frequencies (17) and a measuring target (14) assigned to the component and configured to measure distances by irradiating the measuring target with the measuring radiation, and a coupling module (32),which is configured to couple the at least two discrete radiation frequencies to two of the resonator frequencies and for this purpose comprises a radiation detector (50) for detecting a radiation sample (22r) of the measuring radiation branched off from the optical resonator and a control device (54), wherein the control device (54) is configured to generate a control signal (56) for the radiation generator from the radiation sample detected by the radiation detector, which control signal comprises a specification (42v) for at least one of the comb parameters.

Description

Background of the invention

[0001] The invention relates to a measuring device for determining the position of a component in a system. Furthermore, the invention relates to a projection exposure system for photolithography, an illumination optic for a projection exposure system for photolithography, a projection lens for a projection exposure system for photolithography, a projection exposure system for photolithography, an inspection system, and a coordinate measuring machine, each comprising at least one measuring device of the type mentioned.

[0002] Photolithography is used to fabricate microstructured components, such as integrated circuits or LCDs. In this context, "microstructured components" refers specifically to components with microstructures and / or nanostructures. Photolithography is often also called "microlithography," and it can be used particularly for the fabrication of nanostructures. The fabrication of microstructured components is carried out 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 coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens (e.g., a microfibre).a silicon wafer) to transfer the mask structure onto the light-sensitive 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 measurement setups, 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, and the second resonator mirror (as a so-called "measurement 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 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. Radiation coupled out of the optical resonator 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] For many applications, it is important to be able to measure over large distances and / or at high rates of distance change. Changing the resonator length leads to a change in the frequency of the individual resonant frequencies. Continuous measurement according to the principle described above is only possible within the frequency range in which the laser frequency can be continuously tuned.

[0007] The continuously tunable frequency range of laser oscillators is ultimately limited by the gain spectrum of the laser medium, but in low-noise lasers, it is usually also limited by internal filters and mode-hopping dynamics. The laser frequency can also be detuned outside the laser oscillator, for example, using acousto-optic or electro-optic modulators.

[0008] In this case, the tunable frequency range is limited by the bandwidth of the modulators or the modulation-generating electronics. The limitation of the continuously tunable frequency range of the laser radiation poses a problem for measurements over large distances. If a measurement is to be performed outside the distance range defined by the extent of the tunable frequency range, a new laser frequency may need to be coupled to a different resonator frequency. However, stabilizing the frequency control at the new target frequency is time-consuming; that is, the stabilization time is considerable, thus reducing the achievable measurement dynamics. Consequently, the permissible distance change rate at which position determination can be performed continuously is severely limited.

[0009] Another problem with existing measuring devices is measurement uncertainties caused by standing waves occurring on the beam path between the radiation source, the optical resonator, and the radiation detector of the coupling device. Avoiding such standing waves can pose a significant challenge in the optical transmission lines of real measuring devices. Underlying task

[0010] It is an object of the invention to provide a measuring device of the type mentioned at the outset, which solves the aforementioned problems and preferably enables position determination over larger distance ranges with high measurement dynamics and low measurement uncertainties. Inventive solution

[0011] The aforementioned problem can be solved according to the invention, for example, with a measuring device for determining the position of a component in a system with a radiation generator configured to generate a measurement radiation with at least two discrete radiation frequencies, which are selected comb frequencies of a frequency comb structure, wherein the frequency comb structure is defined by at least two comb parameters, the first of which comprises a uniform frequency spacing of the comb frequencies and the second of which comprises a frequency offset of the frequency comb structure. The measuring device further comprises an optical resonator, which includes resonator frequencies and a measurement target associated with the component, and is configured for distance measurement by irradiating the measurement target with the measurement radiation.Furthermore, the measuring device comprises a coupling module configured to couple at least two discrete radiation frequencies to two of the resonator frequencies. This module includes a radiation detector for detecting a radiation sample of the measurement radiation diverted from the optical resonator, as well as a control unit. The control unit is configured to generate a control signal for the radiation generator from the radiation sample detected by the radiation detector. This control signal includes at least one setting for at least one of the comb parameters. The system in which this component is incorporated can, for example, be an optical system for photolithography.

[0012] In this text, an optical resonator is understood to be an arrangement of mirrors designed to reflect light back and forth as often as possible. According to one embodiment, the optical resonator of the measuring device is configured such that, after at least five passes, preferably at least ten passes, the measurement radiation emitted into the optical resonator retains at least 90%, and advantageously at least 99%, of its original intensity.According to a further embodiment, the optical resonator of the measuring device is 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 having an intensity of less than 50% of the intensity of the incident measurement radiation remaining in the resonator. According to a further embodiment, the optical resonator of the measuring device has a fineness of at least 100, advantageously at least 1000.

[0013] The control signal can, for example, include a digitally transmitted input, such as in the form of one or more digital values, an analog signal, such as in the form of one or more analogously transmitted radio frequencies, or an analog control signal for an actuator designed to influence a comb parameter.

[0014] Position determination refers to the process of determining the component's position in at least one coordinate direction. This determined position can be an absolute position in space or a relative position with respect to an initial position, i.e., a change in position.

[0015] According to one embodiment, the radiation sample detected by the radiation detector is represented by a detector signal, from which the control unit then generates the control signal for the radiation generator. The coupling of the discrete radiation frequencies to the resonator frequencies means that the radiation frequencies are aligned with the resonator frequencies. If the length of the resonator changes, the discrete radiation frequencies follow the resonator frequencies. The frequency offset of the frequency comb structure from the reference frequency is the frequency difference between a given comb frequency of the frequency comb structure and the reference frequency. The uniform frequency spacing of the comb frequencies means that the comb frequencies have the same frequency spacing with each other, i.e., that adjacent comb frequencies have the same frequency spacing.

[0016] If the control signal contains only a specification for one of the comb parameters, then, according to one embodiment, the radiation generator uses the previously used value for the comb parameter not provided by the radiation generator, i.e., this comb parameter is left unchanged.The inventive configuration of the radiation generator for generating the measurement radiation with the at least two radiation frequencies in the form of selected comb frequencies of a frequency comb structure with a uniform frequency spacing, the provision of the coupling module for coupling the at least two discrete radiation frequencies to respective resonator frequencies, and the configuration of the control device for generating a specification for the at least one comb parameter makes it possible to maintain the frequency coupling of the measurement radiation to the optical resonator without time loss, even when reaching the limit of the tunable frequency range of the radiation generator, hereinafter also referred to as the generator spectrum, by one of the discrete radiation frequencies.

[0017] Due to the continuously updated specification for at least one comb parameter provided by the control unit, the discrete radiation frequency that reaches the limit of the generator spectrum can be replaced by another comb frequency from the frequency comb structure that lies within the generator spectrum at a sufficient distance from the aforementioned limit. Because of the currently available specification for at least one comb parameter, all comb frequencies of the frequency comb structure are adapted to the resonator frequencies. Therefore, the newly selected discrete radiation frequency, which is also a comb frequency, is already adapted to the corresponding resonator frequency, and time-consuming frequency stabilization can be avoided.

[0018] In comparison to a solution where the radiation frequencies are not tuned to a frequency comb structure, but rather each is individually controlled or coupled to a specific resonator frequency, and where the frequency reaching the edge of the generator spectrum undergoes a frequency jump that subsequently needs to be stabilized with respect to the nearest resonator frequency, the solution according to the invention is more efficient. The stabilization time can be avoided or significantly reduced, thus achieving a considerably higher permissible distance change rate.

[0019] In the solution according to the invention, when the beam frequency changes, it is briefly unavailable for control signal calculation. However, the control signal can still be determined during this period from the still-active beam frequency, albeit with reduced accuracy. The resulting uninterrupted stabilization of the changed beam frequency minimizes the duration of its unavailability for control signal calculation.

[0020] Furthermore, the solution according to the invention offers the advantage that the control signal can be generated over a large number of resonator frequencies. This allows measurement uncertainties that follow a spectral distribution independent of the resonator frequencies to be averaged and thus reduced.

[0021] According to one embodiment, the specification encompassed by the control signal includes a specification for the frequency offset.

[0022] According to another embodiment, the specification generated by the control signal includes a specification for the frequency spacing.

[0023] According to another embodiment, the setpoint included in the control signal contains a setpoint for a combination of the frequency offset and the frequency spacing. In other words, the frequency offset and the frequency spacing can be determined from the setpoint, but they are not contained separately in the setpoint included in the control signal.

[0024] According to another embodiment, the control device is configured to determine the target frequency spacing from the calculated frequency offset, taking into account a fixed-point frequency of the optical resonator. This determination is based, according to one embodiment, on the concept that when the frequency spacing of the frequency comb structure changes, the comb frequencies are shifted proportionally to the distance of the respective comb frequency from the fixed-point frequency. According to this concept, the shifting of the comb frequencies occurs as if pulling on a rubber band fixed at the fixed-point frequency, to which the comb frequencies are arranged.

[0025] According to a further embodiment, the radiation generator comprises a reference radiation source for generating reference radiation at the reference frequency and a frequency-shifting module configured to generate the measurement radiation with at least two discrete radiation frequencies from the reference radiation by frequency shifting. According to one embodiment, the reference frequency is defined by a highly stable optical reference resonator. According to a further embodiment, the highly stable optical reference resonator is located in the vicinity of the optical resonator and is exposed to the same environmental conditions, in particular the same atmospheric fluctuations in the refractive index. According to a further embodiment, the frequency-shifting module is configured to generate the measurement radiation with the at least two discrete radiation frequencies by modulating the reference radiation.According to one embodiment, the frequency shift module comprises at least two frequency modulators. The frequency shift to be generated is specified to the modulator modules, which then add it to the optical frequency of the reference radiation. Such a frequency shift module is known to those skilled in the art, for example, as an IQ modulator.

[0026] According to a further embodiment, the radiation generator can only produce radiation frequencies lying within a limited generator spectrum, and the measuring device further comprises a frequency-hopping module configured to replace, when a limit of the generator spectrum is reached by one of the coupled radiation frequencies during the generation of the measurement radiation in the radiation generator, the radiation frequency in question with another comb frequency of the frequency comb structure. The frequency-hopping module can be part of the radiation generator or part of the control device.

[0027] According to another embodiment, the radiation generator comprises a frequency comb generator and is configured to generate a frequency comb which includes the measurement radiation with all comb frequencies within a limited generator spectrum.

[0028] According to another embodiment, the radiation generator comprises a reference radiation source for generating a reference radiation with the reference frequency in continuous wave operation, and the frequency comb generator is configured to generate the frequency comb from the reference radiation.

[0029] According to a further embodiment, the frequency comb generator comprises at least one modulation unit for periodic modulation of the reference radiation, which is configured such that a frequency spacing of the generated frequency comb can be adjusted by varying the modulation period of the modulation unit.

[0030] According to one embodiment, the at least one modulation unit comprises at least one phase modulator and / or at least one intensity modulator. The phase modulator is advantageously configured such that the frequency spacing of the generated frequency comb can be adjusted by varying the modulation period of the phase modulator. For this purpose, the phase modulator is preferably implemented as an electro-optical phase modulator. The intensity modulator is advantageously configured to generate a periodic pulse train from the reference radiation, and the intensity modulator may include at least one downstream phase modulator for broadening the generator spectrum. Preferably, the intensity modulator is configured as an electro-optical intensity modulator.

[0031] According to a further embodiment, the frequency comb generator comprises an electro-optical intensity modulator for generating a periodic pulse train from the reference radiation, wherein the frequency spacing of the generated frequency comb is adjustable by varying the pulse repetition rate of the intensity modulator.

[0032] According to a further embodiment, the frequency comb generator comprises an electro-optic phase modulator for modulating the reference radiation, which is configured such that a frequency offset of the generated frequency comb can be produced by a phase shift induced regularly during phase modulation. For example, the phase shift can occur between each pair of pulses generated by the electro-optic phase modulator. According to one embodiment, a value of n · 2π is subtracted or added to the newly set phase value after the phase shift, where n is an integer and is chosen such that the phase value lies within the finite phase range of the phase modulator.

[0033] According to another embodiment, the radiation generator comprises a reference transmitter for generating a reference frequency in the form of a radio frequency, and the frequency comb generator includes a pulsed mode-locked laser for generating the frequency comb with a frequency offset from the reference frequency. The frequency comb generator can be configured, for example, as a femtosecond laser. The frequency spacing of the generated frequency comb can be manipulated, for example, by changing the optical path length of the laser oscillator. The temperature of the gain medium in the laser oscillator can be changed to manipulate the frequency offset of the frequency comb.

[0034] According to a further embodiment, the radiation generator comprises at least one comb manipulator, which can be configured to adjust one of the comb parameters or to adjust both comb parameters in a predetermined ratio and is configured to receive the control signal.

[0035] According to another embodiment, the coupling module is configured to couple the comb frequencies generated by the radiation generator to one of the respective resonator frequencies, wherein the coupling of a comb frequency newly appearing in the generator spectrum due to a shift of the resonator frequencies takes place due to the already existing coupling of the other comb frequencies.

[0036] According to another embodiment, the control device is configured to determine the control signal from a detector signal representing the detected radiation sample without separating the signal components attributable to the individual discrete radiation frequencies. That is, the detector signal is not isolated with respect to the discrete radiation frequencies to determine the control signal. In other words, the control signal is determined cumulatively with respect to the discrete radiation frequencies.

[0037] According to another embodiment, the control device is configured to generate the control signal using a phase modulation signal to create sideband frequencies of the discrete radiation frequencies by means of the Pound-Drever-Hall method, wherein the same phase modulation signal is used for the at least two discrete radiation frequencies. Thus, the sideband frequencies of the two radiation frequencies have a uniform frequency separation.

[0038] The following describes the operation of the Pound-Drever-Hall method in its conventional embodiment and an embodiment for use in the measuring device according to the invention: In the conventional embodiment of the Pound-Drever-Hall method, a radiation generator produces measurement radiation with an operating frequency BF. In the present case, the measurement radiation has at least two operating frequencies BF1 and BF2, namely the two discrete radiation frequencies. The measurement radiation passes through a phase modulator, to which an RF oscillator supplies a phase modulation signal in the form of a modulation frequency f. PDH Given, the result of the modulation in the phase modulator, in the conventional embodiment of the Pound-Drever-Hall method, is the generation of two sidebands with the frequencies BF ± f PDH, in the present case the generation of at least two pairs of sidebands with the frequencies BF1 ± f PDH as well as BF2 ± f PDH .

[0039] The modulated measurement radiation is directed into a resonator cavity with at least one resonant frequency, hereinafter referred to as resonator frequency RF. In this case, the resonator cavity is formed by the optical resonator, which has several resonator frequencies. Measurement radiation reflected from the resonator cavity is detected by a radiation detector. Frequencies within the finite linewidth of the resonator frequencies are reflected with a phase and intensity modified by the resonator. The detector signal emitted by the radiation detector exhibits temporal modulation.By comparing the detector signal with the signal of the RF oscillator using a mixer and separating the high-frequency signal component contained in the mixed signal with a low-pass filter, an error signal is generated, which in the conventional embodiment represents a measure of the deviation of the operating frequency BF from the resonator frequency RF.

[0040] In this case, the error signal contains at least the information about the average of the individual deviations from the operating frequencies BF1 and BF2 from the resonator frequencies within whose linewidths they lie. The error signal is fed to a controller, which, possibly with the aid of a converter, transforms it into a control signal for the radiation generator. In the conventional embodiment, this signal serves to correct the operating frequency BF generated by the radiation generator and thus align it with the resonator frequency RF. In this case, the control signal serves to align the operating frequencies with assigned resonator frequencies.

[0041] According to a further embodiment, the radiation generator is also configured to generate, in addition to the at least two discrete radiation frequencies, two sideband frequencies with a uniform frequency separation from the radiation frequency in question in the measurement radiation.

[0042] According to another embodiment, the system is an optical system for photolithography.

[0043] Furthermore, according to the invention, a projection exposure system for photolithography is provided with at least one component and at least one measuring device in one of the embodiments or variants described above for determining the position of the component.

[0044] Furthermore, according to the invention, an illumination optic for a projection exposure system for photolithography is provided, which comprises at least one component and at least one measuring device in one of the embodiments or variants described above for determining the position of the component.

[0045] Furthermore, according to the invention, a projection lens of a projection exposure system for photolithography is provided, which comprises at least one component and at least one measuring device in one of the embodiments or variants described above for determining the position of the component.

[0046] Furthermore, according to the invention, an inspection system for inspecting the surface of a substrate is provided, comprising at least one component and at least one measuring device in one of the embodiments or variants described above for determining the position of the component. The substrate can, for example, be a mask or a wafer.

[0047] The movable component can be a component in an optical system of the inspection system. An example of such an inspection system for mask or wafer inspection (without the measuring device according to the invention) is known from publication DE 102012205181A1, the entire contents of which are incorporated into the present application by reference.

[0048] Furthermore, according to the invention, a coordinate measuring machine is provided which comprises at least one component and at least one measuring device in one of the embodiments or variants described above for determining the position of the component.

[0049] The movable component can be a component in an optical system of the coordinate measuring machine. The coordinate measuring machine serves to determine the respective positional deviation of one or more measuring points on a test component from a respective target position. An example of such a coordinate measuring machine (without the measuring arrangement according to the invention) is known from publication DE10 2019 213 794A1, the entire content of which is incorporated into the present application by reference.

[0050] 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

[0051] 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 determining the position of a component in an optical system for photolithography, comprising a radiation generator for generating a measuring radiation with at least two discrete radiation frequencies, wherein the radiation generator comprises a frequency shift module, Fig. 2 Another embodiment of a measuring device for determining the position of a component in an optical system for photolithography, comprising a radiation generator for generating a measuring radiation with at least two discrete radiation frequencies, wherein the radiation generator comprises a frequency comb generator, Fig. 3 the frequency comb generator according to Fig. 2 in a first version variant, Fig. 4 the frequency comb generator according to Fig. 2 in a further variant version, Fig. 5 an embodiment of a control device of the measuring device according to Fig. 1 or Fig. 2, as well as Fig. 6 An embodiment of a projection exposure system for photolithography with a movable component in the form of a mirror of the projection lens, the position of which is determined by means of a measuring device according to Fig. 1 or Fig. 2 can be determined. Detailed description of embodiments according to the invention

[0052] 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.

[0053] 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.

[0054] In Fig. Figure 1 shows an embodiment of a measuring device 10 for determining the position of a component in an optical system for photolithography. Position determination refers to determining the position of the component in at least one coordinate direction, here in the z-direction. The determined position can be an absolute position in space or a relative position with respect to an initial position, i.e., a change in position. Fig. Figure 6 illustrates, in a simplified representation, such an optical system in the form of a projection exposure unit 200 for photolithography, which will be described in more detail later in this 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 operation of the projection exposure unit 200; therefore, the component can also be referred to as a movable component. Alternatively, the component to be measured could be a component of an illumination optic 205 of the projection exposure unit 200 or another component of the projection exposure unit 200.

[0055] The measuring device 10 according to Fig. The device comprises a measuring head 12 and a measuring mirror, which can also be referred to as a measuring target 14, and which is attached to and thus associated with the movable component. To determine the position of the movable component, the measuring device measures the distance between the measuring head 12 and the measuring target 14 in the z-direction, as described in detail below. In other words, in this embodiment, the position z is determined at one or more points in time.

[0056] The measuring device 10 comprises an optical resonator 16. This resonator includes a resonator cavity 18, which is enclosed by two resonator mirrors of the resonator 16. The first resonator mirror is formed by a coupling mirror 20, through which a measurement beam 22 is directed into the resonator cavity 18. In the present embodiment, the second resonator mirror is formed by the measurement target 14. In the illustrated embodiment, the coupling mirror 22 has a curved mirror surface, while the measurement target 14 has a flat mirror surface. Thus, the measurement beam 22 forms a Gaussian beam in the resonator cavity 18, the waist of which is located on the mirror surface of the measurement target 14. In a further embodiment, not shown in the drawing, the optical resonator 18 can include one or more folded mirrors in addition to the two resonator mirrors, and one of the folded mirrors can function as the measurement target.In a variant where only one folding mirror is used, the coupling mirror and the second resonator mirror are oriented in the same direction.

[0057] In the exemplary embodiment, the resonator cavity 18 has the following features: Fig. In a ground state, the resonator cavity 18 has a length L0. During measurement operation, if the measurement target 14 is shifted in the z-direction by ±d(t) as a time (t) factor, the length L(t) of the resonator cavity 18 changes accordingly, such that: L(t) = L0 ± d(t). The length L of the resonator cavity 18 determines the resonance frequencies of the optical resonator 16, referred to below as resonator frequencies RF. q (reference symbol 17). The index q serves to number the vibrational orders corresponding to the various resonance states of the optical resonator 16. In the upper section of Fig. Figure 1 contains a frequency space representation 33. This shows, for example, the resonator frequencies RF. qThe frequency range of the radiation generator 24 is shown in a tunable frequency range, i.e., in a frequency range in which the radiation generator 24 is able to adjust the frequency of the measurement radiation 22. This frequency range is also referred to in this text as the limited generator spectrum 34.

[0058] In the frequency space representation 33 according to Fig. 1 is the resonator frequency with the index i, i.e. RF, which lies approximately in the middle of the generator spectrum 34. i , denoted. The distance between the respective resonator frequencies RF q is uniform and is known in the scientific community as the free spectral range f FSR (FSR - English: free spectral range) of the optical resonator 16.

[0059] If the length L of the resonator cavity 18 is changed, the free spectral range f changes. FSR Accordingly. The distribution of frequencies RF, which can be described to a first approximation as a comb, is used.q accordion-like around a fixed-point frequency ffixR (Reference symbol 21) stretched or compressed, i.e., the distances between adjacent frequency modes become uniformly larger (when the length L decreases) or smaller (when the length L increases) by a uniform factor. To the left of the fixed-point frequency ffixR arranged resonator frequencies RF q They move to the left when the frequency ridge is stretched, while to the right of the fixed-point frequency ffixR arranged resonator frequencies RF q Move accordingly to the right.

[0060] In the present embodiment, the fixed-point frequency is ffixR close to the zero frequency point (f=0Hz), so that the resonator frequencies RF arranged in the generator spectrum 34, i.e. those in the tunable frequency range of the radiation generator 24, i-5 to RF i+4As the length L of the optical resonator 16 decreases over time, it moves to the right, as shown in the frequency space diagram 33 with arrows 19 to illustrate the direction of dRFqdt shown in the depicted frequency range.

[0061] The in Fig. The illustrated measuring device 10 comprises a radiation generator 24 for generating the measurement radiation 22, optionally an optical fiber 26, a beam shaping optic in the form of a coupling lens 28, the aforementioned optical resonator 16, a circulator 30, and a coupling module 32. The measuring head 12, also mentioned above, comprises at least the coupling mirror 20 and the coupling lens 28 for collimating the measurement radiation 22 emerging from the optical fiber 26 before it enters the coupling mirror 20. The radiation generator 24 and / or the coupling module 32 can also be part of the measuring head 12 or, as shown in Fig. 1 is illustrated, arranged outside of it and connected to it via the optical fiber 26.

[0062] The radiation generator 24 is in the exemplary embodiment according to Fig. 1 configured to generate the measurement radiation 22 with at least two discrete radiation frequencies f1 and f2 (reference symbols 36), which select comb frequencies KF q (Reference symbol 38) of a frequency comb structure 37. The frequency comb structure 37 comprises a plurality of comb frequencies 38 that have a uniform frequency spacing f. rep (reference symbol 42), i.e., the frequency spacing between adjacent comb frequencies 38 is uniform. The frequency comb structure 37 exhibits a frequency offset Δf (reference symbol 40) from a reference frequency f generated and thus defined in the radiation generator 24, possibly also a previously known reference frequency. R (Reference 43) on.

[0063] The frequency offset Δf is the frequency difference between a given comb frequency 38 of the frequency comb structure 37 and the reference frequency f. R to understand. In the example according to Fig. 1 is the specified comb frequency 38, the comb frequency KF0, and the frequency offset Δf is positive. The comb frequency KF0 is the comb frequency of the frequency comb structure 37 which corresponds to the reference frequency f at the depicted time. R is nearest. Due to the coupling of the discrete radiation frequencies f1 and f2 to associated resonator frequencies RF, described in more detail below. q The frequency offset Δf changes when the resonator length L changes, while the reference frequency f R remains constant. Therefore, the given comb frequency KF0, with respect to which the frequency offset Δf is determined, is no longer the same as the reference frequency f when the resonator length L is changed accordingly. R nearest comb frequency.

[0064] In the exemplary embodiment according to Fig. 1 are the comb frequencies 38 of the frequency comb structure 37, numbered with reference to the comb frequency KF0, namely comb frequencies lying to the left of KF0 with descending negative count values ​​(KF -1 , KF -2 , ...) and comb frequencies to the right of KF0 with ascending positive count values ​​(KF +1 , KF +2 , ...). In the exemplary embodiment according to Fig. 1 around the comb frequencies KF +1 (radiation frequency f1) and KF -2 (radiation frequency f2).

[0065] The radiation generator 24 according to Fig. 1 comprises a reference radiation source 44 in the form of a frequency-stable laser for generating a reference radiation 46 with the reference frequency f RThe frequency-stable laser can be, for example, a femtosecond laser generating an femtosecond frequency comb, or another frequency-stabilized laser. For instance, it could be a laser stabilized on a reference resonator. The reference resonator can be an optical reference resonator located in the vicinity of the optical resonator 16, which is subject to the same density and temperature fluctuations as the beam-guiding medium. Furthermore, the radiation generator 24 includes a frequency-shifting module 48, which is configured to generate the measurement radiation 22 with the two discrete radiation frequencies f1 and f2 from the reference radiation 46 by frequency shifting.

[0066] Furthermore, the frequency shift module 48 generates two signals for each of the radiation frequencies f1 and f2, each with a uniform frequency spacing f. PDH(reference symbol 41) sideband frequencies 39 spaced apart from the relevant radiation frequency f1 or f2, namely f1SB±=f1±fPDH and f2SB±=f2±fPDH. These sideband frequencies 39 are used for frequency coupling of the radiation frequencies f1 and f2 to associated resonator frequencies 17, as explained in more detail below, based on the Pound-Drever-Hall method known to those skilled in the art. According to one embodiment, the frequency shift module 48 comprises a frequency modulator for each of the frequencies to be generated. According to one embodiment, the frequency shift to be generated by the frequency modulators is specified to each frequency modulator by a radio frequency generated in the frequency shift module 48; this radio frequency is added by the respective frequency modulator to the optical frequency of the reference radiation 46.The function of the aforementioned frequency modulators can be taken over by a so-called IQ modulator for the joint generation of the frequencies to be generated from the reference frequency by frequency shifting (singlesideband modulation) or by an IQ modulator for the generation of the frequencies f1 and f2 by frequency shifting, as well as a phase modulator for the generation of the symmetrical PDH sidebands.

[0067] The circulator 30 can be implemented, for example, using a Faraday rotator. It is also possible to implement the circulator 30 using an optical coupler. The circulator 30 directs the measurement radiation 22 coming from the radiation generator 24, also referred to as incoming measurement radiation 22e, via the optical fiber 26 to the measuring head 12. After interaction at the optical resonator 16, the circulator 30 directs the measurement radiation returning from the measuring head 12, which in this text is also referred to as measurement radiation branched off from the optical resonator 16 or as measurement radiation 22r reflected from the optical resonator 16, into the coupling module 32.

[0068] When operating the measuring device 10 according to Fig. 1 The incoming measurement radiation 22e generated by the radiation generator 24 with the two discrete radiation frequencies f1 and f2 as well as the sideband frequencies f1SB±,f1SB−,f2SB± and f2SB− The radiation 22r reflected from the optical resonator 16, also referred to in this text as the radiation sample, is detected in the coupling module 32 by a radiation detector 50. Depending on the intensity of the detected radiation, the radiation detector 50 generates a detector signal 52 in the form of an electrical signal. For this purpose, the radiation detector 50 can, for example, be configured as a photodiode. The detector signal 52 is forwarded to a control unit 54 of the coupling module 32. The control unit 54 generates a control signal 56 from the received detector signal 52 and forwards this to the frequency shift module 48 of the radiation generator 24.

[0069] For this purpose, the control unit 54 can be purely digital, for example as a field-programmable gate array (FPGA). The analog signal from the radiation detector 50 must be converted into a digital signal either by the FPGA or by an analog-to-digital converter (ADC) not shown. The analog control signal required for controlling the frequency shift module 48, in the form of an alternating voltage, can be generated by a DAC conversion within the frequency shift module 48. In simplified terms, the operating principle of the [function / system] is based on... Fig. 1 illustrated measuring device 10 to shift the radiation frequencies f1 and f2 by means of the frequency shift module 48 so that they correspond to the respective resonator frequencies RF q of the optical resonator 16 and put it into a resonance state.

[0070] By appropriately adjusting the radiation frequencies f1 and f2 when the resonator frequencies RF are changed q The radiation frequencies f1 and f2 are aligned to the respective resonator frequencies RF. q coupled. In the exemplary embodiment according to Fig. 1. The coupled resonator frequencies are RF. i+3 (coupled with the radiation frequency f1) and RF i-3 (coupled with the radiation frequency f2).

[0071] In order to enable the radiation generator 24 to match the radiation frequencies f1 and f2 to the respective resonator frequencies RF q To track the frequency offset, the control signal 56 includes at least one specification Δf* (reference symbol 40v) for the frequency offset 40. In the embodiment according to Fig. 1 the control signal 56 still includes a specification frep* (Reference sign 42v) for the frequency spacing 42. As below with reference to Fig. As explained in more detail in section 5, in this embodiment the control device 54 determines the specification frep* using the previously determined setpoint Δf*. In an embodiment where the control signal 56 only includes the setpoint Δf* for the frequency offset 40, this functionality can be used to determine the setpoint. frep* for example, it may also be integrated into the radiation generator 24.

[0072] From the specifications transmitted by the control unit 54 to the radiation generator 24 frep* and Δf* determines the discrete radiation frequencies to be generated 36 as follows: f1=fR+Δf*+n1⋅frep*, as well as f2=fR+Δf*+n2⋅frep*, where n1 and n2 are integers. In the embodiment according to the frequency space representation 33 of Fig. 1. n1 = +1 and n2 = -2. Based on the determined discrete radiation frequencies f1 and f2, the frequency shift module 48 is instructed to adjust these frequencies.

[0073] Since the reference frequency f R exhibiting high stability, the frequency offset indicates shifts in the resonator frequencies RF. q of the optical resonator 16 with high accuracy. The values ​​of the comb parameters Δf (frequency offset) and f set in the radiation generator 24. rep (Frequency spacing) are transmitted to an evaluation unit 58. According to the described embodiment, these are the respective digital values ​​of the specifications Δf* and frep* of the two comb parameters. The specifications Δf* and provided by the control unit 54. frep* Depending on the design variant, these could be analog signals or already include the digital values.

[0074] According to alternative implementation variants, only Δf or f can be used. rep to the evaluation unit 58, whereby the other quantity is determined using the fixed-point frequency. ffixR The evaluation unit 58 determines the time-dependent displacement d(t) of the measurement target 14 of the optical resonator 16 in real time, i.e., the displacement d at different times. From the displacement d(t), the position z (reference symbol 15) of the measurement target 14, and thus of the movable component, is derived.

[0075] In other words, this approach uses the reference frequency f R the reference radiation source 44 was kept unchanged with high precision and the tuning at both resonant frequencies RF was maintained. qThe optical resonator 16 is modified by varying its frequency shift using the frequency shift module 48. When the length L of the optical resonator 16 is changed, the resulting change in the resonance frequencies RF is q The change in length L is determined from the required tracking of the frequency shift of the optical resonator 16 by the frequency shift module 48.

[0076] As mentioned above, the radiation generator 24 is only capable of generating radiation frequencies that are in the Fig. The generator spectrum 34 shown as an example is located in 1. If one of the radiation frequencies f1 and f2 or one of the associated sideband frequencies 39 now reaches a certain value due to a change in the length of the optical resonator 16 and the coupling of the radiation frequencies to the associated resonator frequencies RF described above, qIf a limit of the generator spectrum 34 is reached, the corresponding radiation frequency is replaced by another comb frequency 38 of the frequency comb structure 37. This frequency substitution is also referred to as a frequency jump.

[0077] In the frequency space representation 33 of Fig. Figure 1 illustrates such a frequency jump, here for the case in which the length L of the optical resonator 16 is reduced to such an extent that the radiation frequency f1, shifting to the right, reaches the right boundary of the generator spectrum 34. Reaching the right boundary of the generator spectrum 34 means that the radiation frequency f1 is so close to the boundary that it could no longer be generated with sufficient intensity if shifted further to the right, or that the right sideband frequency would then be affected. f1SB+ The radiation frequency f1 could no longer be generated with sufficient intensity. If the radiation frequency f1 has now reached the right boundary of the generator spectrum 34, where, as in Fig. 1 shows the radiation frequency f1 from the original comb frequency KF +1 , which previously corresponded to the resonator frequency RF i+3 was coupled to the new comb frequency KF -3 , which of the resonator frequency RFi -5 This corresponds to the set value. This is done by assigning a new value to n1, which is referred to as n1' in the frequency space representation 33, and the resulting new radiation frequency as f1'. In the example shown, the old value n1 = 1 and the new value n1' = -3.

[0078] To initiate a frequency jump, the radiation generator 24 includes a frequency jump module 60. This module provides the frequency shift module 48 with the values ​​for n1 and n2 for equations (1) and (2) to determine f1 and f2. The frequency jump module 60 receives as input the specifications determined by the control unit 54. frep* and Δf** and corrects n1 or n2 accordingly as soon as one of the radiation frequencies reaches a limit of the generator spectrum 34. The frequency hopping module 60 can, instead of being part of the radiation generator 24, also be integrated into the control unit 54.

[0079] During the execution of the frequency jump, the radiation frequency f1 falls for a short period of time for the calculation of the control signal 56 and thus for the frequency coupling of the measurement radiation 22e to the resonator frequencies RF. qaway. However, since during this period the further beam frequency f2 continues to correspond to the associated resonator frequency RF i-3 When coupled, the control signal 56 can still be determined by the control device 54.

[0080] This may be done with slightly lower accuracy due to the omission of f1, but it is sufficient to couple the frequency of the measurement radiation 22e to the resonator frequencies RF. q to prevent the interruption, especially since the aforementioned period with reduced frequency coupling is only short. This is because, due to the prior adjustment of the frequency spacing 42 of the frequency comb structure 37 to the resonator frequencies RF, q Since the process has been carried out with high accuracy, the new radiation frequency f1' set after the frequency jump is already very precisely matched to the new resonator frequency RF. i-3 set up so that time-consuming frequency stabilization can be avoided.

[0081] Since the period with reduced frequency coupling is only short, it is ensured that even with rapid changes in the length of the optical resonator 16, at least one radiation frequency always coincides with one of the resonator frequencies RF. q is coupled. This means that even if, due to the rapid change in length, the radiation frequency f2 also reaches the right limit of the generator spectrum 34 a short time later, it is ensured that, at the frequency jump that then follows for the radiation frequency f2, the radiation frequency f1 is already available again for determining the control signal 56.

[0082] As mentioned above, the coupling of the radiation frequencies f1 and f2 to assigned resonator frequencies 17 is based on the Pound-Drever-Hall method. The operation of the Pound-Drever-Hall method in its conventional embodiment and in an embodiment for use in the present measuring device 10 is described below: In the conventional embodiment of the Pound-Drever-Hall method, a radiation generator produces measurement radiation with only one discrete radiation frequency, which is referred to below as the operating frequency BF.

[0083] In the present case, the measurement radiation 22e has at least the two radiation frequencies f1 and f2. The measurement radiation 22 with the radiation frequencies f1 and f2 can be generated in the frequency shift module 48 from the reference radiation 46 by controlling an acousto-optic modulator or an IQ modulator with a radio frequency signal generated in the frequency shift module 48. For this purpose, the corresponding offsets Δf1 and Δf2 of the radiation frequencies f1 and f2 from the reference frequency f are present in the radio frequency signal. R contain.

[0084] The measurement radiation 22 with the two radiation frequencies f1 and f2 passes through a modulator in the frequency shift module 48, which receives a phase modulation signal in the form of the modulation frequency f from an RF oscillator 62. PDH Given the specified parameters, the result of the modulation is the generation of the two pairs of sidebands. f1SB± and f2SB± with the frequencies f1SB±=f1±fPDH as well as f2SB±=f2±fPDH. In other words, the modulation of both radiation frequencies f1 and f2 is based on the same phase modulation signal, i.e., the sidebands. f1SB± and f2SB± exhibit the same absolute band gap (F) PDH ) from the respective radiation frequency f1 or f2. The sidebands can be generated in the same modulator by adding the sideband frequencies. f1SB± or Δf2SB± The sidebands can be generated from the radio frequency signal. Alternatively, they can be generated by a separate phase modulator (e.g., an EO modulator) with a modulation frequency f. PDH can be modulated, with the phase modulator being positioned in front of or behind the IQ modulator in the optical beam path.

[0085] In the conventional embodiment of the Pound-Drever-Hall method, however, only a pair of sidebands with the frequencies BF ± f are used. PDHgenerated. As mentioned above, the modulated measurement radiation 22e is introduced into the resonator cavity 18 of the optical resonator 16 with several resonator frequencies RF. q The radiation reflected from the optical resonator 16 is detected by the radiation detector 50. Frequencies within the finite linewidth of the resonator frequencies RF are detected. q The radiation is reflected with a phase and intensity modified by the resonator. The detector signal 52 emitted by the radiation detector 50 exhibits temporal modulation.

[0086] In one embodiment of the control device 54, which is located in Fig. As shown in detail in Figure 5, by comparing the detector signal 52 with the signal of the RF oscillator 62 using a mixer 64 and separating the high-frequency signal component contained in the mixed signal 66 with a low-pass filter 68, an error signal 70 is generated, which in the conventional embodiment represents a measure of the deviation of the operating frequency BF from the resonator frequency RF.

[0087] In the present case, the error signal 70 contains at least the information about the average deviation of the two discrete radiation frequencies f1 and f2 from the respective nearest resonator frequencies RF. q , which in the exemplary embodiment according to Fig. 1 the six times the free spectral range (6x f FSR ). The error signal 70 is fed to a controller 72, which converts it into a correction signal 74, which is then converted in a converter 76 into the control signal 56 for the radiation generator 24.

[0088] In the conventional embodiment, the correction signal can already be used as a control signal. In the conventional embodiment, this signal serves to correct the operating frequency BF generated by the radiation generator and thus to align it with the resonator frequency RF. In the present case, the control signal 56 serves to align the discrete radiation frequencies f1 and f2 together with the assigned resonator frequencies RF. i+3 as well as RF i-3 to align.

[0089] The converter 76 includes in the Fig. In the embodiment shown in Figure 5, two conversion units 78 and 80 are used. The first conversion unit 78 is configured to convert the correction signal 74 into the specification Δf* (reference 40vv) for the frequency offset. In another embodiment of the converter 76, the first conversion unit 78 can optionally be omitted, and the correction signal 74 can be used directly as the specification 42v for the frequency offset.

[0090] The second conversion unit 80 is configured to derive the specified frequency difference from the given Δf*. frep* (42v) to determine the frequency offset.

[0091] This is done using the fixed-point frequency. ffixR (Reference numeral 21) of the optical resonator 16, which is determined beforehand and, if necessary, subsequently at regular intervals using a suitable calibration device. This determination is based on the previously explained concept that a change in the free spectral range f FSR The resonator frequencies are, to a first approximation, proportional to the distance of the respective resonator frequency from the fixed-point frequency. ffixR The resonator frequencies are shifted. According to this concept, the shifting of the resonator frequencies occurs as if pulling on a rubber band fixed at the fixed-point frequency 21, on which the resonator frequencies are arranged, so that the resonator frequencies are stretched or compressed around the fixed-point frequency 21 in an accordion-like manner.

[0092] As can be seen from the preceding description, the control device 54 is configured to determine the control signal 56 from a detector signal 52, which represents the detected radiation sample 22r, without separating the signal components attributable to the individual discrete radiation frequencies 36. In other words, the detector signal 52 is not isolated with respect to the discrete radiation frequencies 36 for the purpose of determining the control signal 56; that is, the determination of the control signal 56 is carried out with respect to the discrete radiation frequencies 36 as a whole, i.e., cumulatively.

[0093] The fixed-point frequency of the optical resonator 16 can also be interpreted as the optimal carrier envelope offset frequency of a pulse train circulating in the optical resonator 16. It is influenced by the Gouy phase of the resonator, the dispersion of the resonator mirrors 20 and 14, and the dispersion of the medium filling the resonator cavity 18. The fixed-point frequency ffixR The frequency of optical resonator 16, as mentioned above, is close to zero. As mentioned above, the fixed-point frequency ffixR determined with a suitable calibration device and, in the case where the dispersion of the resonator exhibits significant temporal variability, for example in the presence of a changing medium, possibly by changes in the free spectral range f FSR adjusted.

[0094] By jointly controlling the frequency spacing f repand the frequency offset Δf of the frequency comb structure 37 with a fixed proportionality factor, the fixed point of this virtual controlled variable can be set to the fixed point frequency ffixR the optical resonator 16 can be adjusted. With high stability of the fixed-point frequency (for example, in a vacuum), or a small bandwidth of the comb, or a small change in distance, all discrete radiation frequencies 36 can thus be kept in resonance via a common control variable. With a variable but only slowly changing fixed-point frequency ffixR (For example, due to pressure fluctuations) the proportionality factor, and thus the fixed point of the virtual controlled variable, can be recalibrated as needed. Alternatively, if required, the comb parameters can be controlled via two independent control loops.

[0095] The maximum possible bandwidth over which lines of a comb can be simultaneously brought into resonance with the optical resonator 16 is limited by the intraresonator dispersion (in high-finesse resonators in vacuum, dominated by the dispersion of the multilayer mirrors) and decreases with increasing resonator finesse. The bandwidth can be optimized through optimized multilayer designs.

[0096] In implementations of the measurement principle with high resonator finesse and a large utilized bandwidth of the generator spectrum, resonator dispersion, in combination with the field build-up time, can lead to distance measurement uncertainties that depend on the distance change rate. These uncertainties can be mitigated by using optical filters to direct only a narrow central region of the available spectrum of the measurement radiation to the radiation detector. In this case, the outer regions of the spectrum serve only to "fill" the resonances without contributing to the error signal themselves.

[0097] In Fig. Figure 2 shows a further embodiment of a measuring device 10 for determining the position of a component in an optical system for photolithography. The embodiment according to Fig. 2 differs from the embodiment according to Fig. 1 only in the configuration of the radiation generator 24. This is in the embodiment according to Fig. 2 configured to do so, not only how the radiation generator 24 according to Fig. 2, to generate two discrete radiation frequencies f1 and f2 of a frequency comb structure 37, but instead to generate all comb frequencies 36 of the frequency comb structure 37. The frequency comb structure 37 comprises only comb frequencies 36 that lie within the generator spectrum 34. In the exemplary embodiment, the frequency comb structure is generated according to Fig. 2 also simply referred to as frequency comb 137.

[0098] The radiation generator 24 according to Fig. 2 includes a reference generator 144, which is used to generate the reference frequency f R(reference numeral 43) is configured, as well as a frequency comb generator 148, which is configured to generate the frequency comb 137, which includes the measurement radiation 22e with all comb frequencies 38 within the limited generator spectrum 34. In other words, the frequency comb generator 148 generates the following comb frequencies: fk=fR+Δf+k⋅frep, where k are integers in a limited range corresponding to the generator spectrum 34. In the frequency space representation 33 according to Fig. In the example shown, k includes the values ​​-3, 2-, -1, 0 and +1. In other words, the frequency comb generator 148 generates the discrete radiation frequencies f therein. -3 , f -2 , f -2 , f0 and f1, each by f rep are spaced apart from each other and where f0 is the frequency offset Δf from the reference frequency f R exhibits. Furthermore, the frequency comb generator 148 generates for each of the discrete frequencies fk a pair of sideband frequencies fkSB± with a distance of f each PDH of the relevant discrete radiation frequency f k : fkSB±=fk±fPDH

[0099] According to one embodiment, the frequency difference f rep in the range between 5 MHz and 50 GHz.

[0100] The coupling of the discrete radiation frequencies f k to the resonator frequencies RF q This is done analogously to, with reference to the measuring device 10 according to Fig. 1 described, functioning of the coupling module 32. In contrast to the measuring device according to Fig. 1. The frequency jump does not need to be triggered by a frequency jump module 60, as will become clear from the following explanation: If the radiation frequencies f shift kFor example, if the length L of the optical resonator 16 decreases over time in the frequency space representation 33 to such an extent that the radiation frequency f1 exceeds the right boundary of the generator spectrum 34, then the radiation frequency f1 is no longer contained in the measurement radiation 22e. Simultaneously, however, a new radiation frequency (in this case, the radiation frequency f) migrates from the left. -3 or the radiation frequency f that follows it -4 ) into the generator spectrum 34, so that the missing radiation frequency f1 is compensated for by it. In other words, a sufficient number of comb frequencies 38, in this case at least three or four comb frequencies 38, are automatically always contained in the measurement radiation 22e, so that the coupling to the resonator frequencies RF q remains upheld.

[0101] The frequency space representation according to Fig. 2 new radiation frequency f entering the generator spectrum 34 -3 is due to the continuous adjustment of the frequency spacing f rep and the frequency offset Δf of the frequency comb 137 on the free spectral range f FSR of the optical resonator 16 and the absolute position of the resonator frequencies RF q without a dedicated frequency stabilization to the resonator frequency RF i-6 adapted. In other words, the coupling is achieved by shifting the resonator frequencies RF. q new comb frequency KF appearing in the generator spectrum q due to the existing coupling of the other comb frequencies.

[0102] In other words, the signal strength, or gain, of the error signal is proportional to the intraresonator field strength. At high distance change rates, the resonances move rapidly through the frequency range of the comb spectrum. The delayed build-up of the intraresonator field means that the resonances and comb lines moving into the comb spectrum contribute to the gain of the cumulative error signal with a delay. This results in a decrease in the cumulative gain at high distance change rates. The larger the frequency range of the comb spectrum, the smaller the gain dip, and the higher the distance change rates that can be tolerated.

[0103] The frequency comb generator 148 according to Fig. 2 can be configured in two different versions. In the first version, 148a, which is in Fig. As shown in Figure 3, the frequency comb generator comprises a modulation unit 149 for the periodic modulation of a reference radiation 146, which is derived from the embodiment shown in Figure 3. Fig. Reference radiation is generated by the reference transmitter 144 configured as reference radiation source 144a. The reference radiation source 144a generates the reference radiation 146 in continuous wave mode and can be operated analogously to the reference radiation source 44 according to Fig. 1 be configured.

[0104] The modulation unit 149 can comprise acousto-optical and / or electro-optical modulators. In this context, the modulation unit 149 may include one or more intensity modulators, one or more phase modulators, one or more Mach-Zehnder modulators, or IQ modulators. Advantageous implementations include the use of a single IQ modulator, as well as the cascaded use of intensity modulators and / or IQ modulators in combination with phase modulators. The modulation signals can be generated digitally (for example, by an FPGA and / or DDS) or by analog radio frequency sources. Frequency multipliers can also be used to generate the modulation signals.

[0105] The frequency difference f repThe frequency comb generated 137 can be freely and continuously adjusted by varying the modulation period of the modulator 149 within the available modulation bandwidth. To achieve the resonant frequencies RF q Furthermore, to enable tracking over large ranges of different resonator lengths, the frequency offset Δf is flexibly adjustable. According to one embodiment, the modulator 149 is configured such that the frequency offset Δf can be generated by a phase shift induced regularly and synchronously with the modulation period. For example, the phase shift can occur with each pulse generated by the modulator 149, resulting in a phase deviation that increases successively from pulse to pulse.

[0106] This is possible using the technique described by Gotti, R. et al., “Comb-locked frequency-swept synthesizer for high precision broadband spectroscopy”, Sci Rep. 2020 Feb 13, 10(1):2523 (see especially Fig. 1 of the publication). In the time domain, the frequency comb 137 corresponds to a periodically modulated electric field, where the repetition rate corresponds to the modulation period. A non-zero offset frequency corresponds to a discrete phase shift occurring with each repetition period (cf. Fig. 1c of the publication). The frequency with which the resulting, time-increasing phase shift traverses the interval of 2π corresponds to the frequency offset Δf. The increasing phase shift can be folded into an interval of 2π, so that arbitrary frequency offsets can be generated even with the finite modulation depth of a modulator. The periodicity of the frequency offset with respect to the repetition rate is obtained in the time domain by aliasing the discrete phase shifts (see ). Fig. 1c of the publication).

[0107] Fig. Figure 4 illustrates a second embodiment 148b of the frequency comb generator 148 according to Fig. 2. This frequency comb generator 148a comprises a pulsed mode-locked laser 151 for generating the frequency comb 137, which can be implemented as a femtosecond laser. The frequency offset Δf from the reference frequency f is set... R The reference frequency f is provided to the mode-locked laser 151 by the reference transmitter 144, which is configured here as a radio frequency generator 144b. R transmitted in the form of a radio signal 146b. In the mode-locked laser 52, the frequency spacing f rep The generated frequency comb 137 can be manipulated, for example, by changing the optical path length of the laser oscillator. To manipulate the frequency offset Δf of the frequency comb, the temperature of the amplification medium in the laser oscillator can be changed. The specifications Δf* and frep* The control signal 56, which includes the frequency offset and the frequency spacing, can, for example, comprise a digitally transmitted specification, such as in the form of one or more digital values, an analog signal, such as in the form of one or more analogously transmitted radio frequencies, or an analog control signal for an actuator designed to influence a comb parameter. The control signal 56 can contain the specifications Δf* and frep* also by specifying a combination of the frequency offset Δf and the frequency spacing f rep provide to the frequency comb generator 148b.

[0108] Fig. Figure 6 shows in a simplified representation the aforementioned projection exposure system 200 for photolithography with the mirror 226, which is shown as an example component for measurement using the measuring device 10 according to Fig. 1 or Fig. 2. Furthermore, other mirrors of the projection exposure system can also be measured with the measuring device 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.

[0109] The projection exposure system 200 according to Fig. 6 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 photolithography, 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 the illumination optics 205 and the projection lens 216.

[0110] 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.

[0111] 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.

[0112] 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 measuring targets 15 Position z(t) of the measurement target 16 optical resonator 17 Resonator frequency RF q 18 Resonator cavity 19 Direction of movement of the resonator frequencies 20 coupling mirrors 21 Fixed-point frequency of the resonator 22 Measurement radiation 22e incoming measurement radiation 22r reflected measurement radiation 24 Radiation generator 26 optical fibers 28 coupling lens 30 circulator 32 Coupling module 33 Frequency space representation 34 Generator spectrum 36 discrete radiation frequencies of the measurement radiation 37 Frequency comb structure 38 Comb frequency 39 sideband frequencies 40 Frequency offset 40V specification for the frequency offset 41 Sideband frequency spacing 42 Frequency spacing 42V specification for the frequency spacing 43 Reference frequency 44 Reference radiation source 46 Reference radiation 48 Frequency shift module 50 radiation detector 52 Detector signal 54 Control unit 56 Control signal 58 Evaluation unit 60 Frequency Jump Module 62 RF oscillator 64 mixers 66 mixed signal 68 Low-pass filters 70 Error signal 72 controllers 74 Correction signal 76 converters 78 first conversion unit 80 second conversion unit 136 discrete radiation frequencies of the measurement radiation 137 Frequency comb 144 reference providers 144a Reference radiation source 144b Radio frequency generator 146a Reference radiation 146b Radio signal 148 Frequency comb generator 148a first version of the frequency comb generator 148b second version of the frequency comb generator 149 electro-optical phase modulator 151 mode-locked laser 200 projection exposure system for photolithography 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 102012205181A1

[0047] DE 10 2019 213 794A1

[0049] Cited non-patent literature

[0000] Gotti, R. et al., “Comb-locked frequency-swept synthesizer for high precision broadband spectroscopy”, Sci Rep. 2020 Feb 13, 10(1):2523

[0106]

Claims

[1] Measuring device (10) for determining the position of a component (226) in a system (200) with: - a radiation generator (24) configured to generate a measurement radiation (22e) with at least two discrete radiation frequencies (36; 136) which are selected comb frequencies (38) of a frequency comb structure (37), wherein the frequency comb structure is defined by at least two comb parameters, the first of which comprises a uniform frequency spacing (42) of the comb frequencies and the second of which comprises a frequency offset of the frequency comb structure from a reference frequency, - an optical resonator (16) comprising resonator frequencies (17) and a measurement target (14) associated with the component, and configured for distance measurement by irradiating the measurement target with the measurement radiation, as well as - a coupling module (32) configured to couple the at least two discrete radiation frequencies to two of the resonator frequencies and comprising a radiation detector (50) for detecting a radiation sample (22r) of the measurement radiation diverted from the optical resonator and a control device (54), wherein the control device (54) is configured to generate a control signal (56) for the radiation generator from the radiation sample detected by the radiation detector, which includes a specification (40v, 42v) for at least one of the comb parameters. [2] Measuring device according to claim 1, wherein the setpoint encompassed by the control signal includes a setpoint (40v) for the frequency offset. [3] Measuring device according to claim 1 or 2, wherein the setpoint generated by the control signal includes a setpoint (42v) for the frequency spacing. [4] Measuring device according to claim 1, wherein the specification encompassed by the control signal includes a specification for a combination of the frequency offset and the frequency spacing. [5] Measuring device according to claim 2, wherein the control device (54) is configured to determine the setpoint (40v) for the frequency offset from the determined frequency difference (42v) taking into account a fixed-point frequency (21) of the optical resonator. [6] Measuring device according to one of the preceding claims, in which the radiation generator (24) comprises a reference radiation source (44) for generating a reference radiation (43) with the reference frequency (43) and a frequency shift module (48) which is configured to generate the measurement radiation (22e) with the at least two discrete radiation frequencies from the reference radiation by frequency shifting. [7] Measuring device according to claim 6, in which the frequency shift module (48) is configured to generate the measuring radiation (22e) with the at least two discrete radiation frequencies by modulating the reference radiation (43). [8] Measuring device according to one of the preceding claims, wherein only radiation frequencies (36) lying in a limited generator spectrum (34) can be generated by the radiation generator (24) and the measuring device further comprises a frequency hopping module (60) which is configured to replace the radiation frequency (36) in question with another comb frequency (38) of the frequency comb structure (37) when a limit of the generator spectrum is reached by one of the coupled radiation frequencies during the generation of the measurement radiation in the radiation generator. [9] Measuring device according to any one of claims 1 to 5, wherein the radiation generator (24) comprises a frequency comb generator (148) and is configured to generate a frequency comb (137) which includes the measurement radiation with all comb frequencies (36) within a limited generator spectrum (34). [10] Measuring device according to claim 9, wherein the radiation generator (24) comprises a reference radiation source (144a) for generating a reference radiation (146a) with the reference frequency (43) in continuous wave operation and the frequency comb generator (148a) is configured to generate the frequency comb (137) from the reference radiation. [11] Measuring device according to claim 10, wherein the frequency comb generator (148a) comprises at least one modulation unit (149) for periodic modulation of the reference radiation (146). [12] Measuring device according to claim 11, wherein the at least one modulation unit comprises at least one phase modulator and / or at least one intensity modulator. [13] Measuring device according to one of claims 10 to 12, in which the frequency comb generator (148a) comprises an electro-optical phase modulator (149) for modulating the reference radiation (146a), which is configured such that a frequency offset (40) of the generated frequency comb (137) can be generated by a phase jump regularly induced during phase modulation. [14] Measuring device according to claim 9, wherein the radiation generator (24) comprises a reference transmitter (144b) for generating a reference frequency in the form of a radio frequency and the frequency comb generator (148a) comprises a pulsed mode-locked laser (151) for generating the frequency comb with a frequency offset from the reference frequency. [15] Measuring device according to one of claims 9 to 14, wherein the coupling module (32) is configured to couple the comb frequencies (136) generated by the radiation generator (24) to one of the respective resonator frequencies (38), wherein the coupling of a comb frequency newly appearing in the generator spectrum due to a shift of the resonator frequencies is carried out due to the already existing coupling of the other comb frequencies. [16] Measuring device according to one of the preceding claims, wherein the control device (54) is configured to determine the control signal (56) from a detector signal (52) representing the detected radiation sample (22r) without separating signal components attributable to the individual discrete radiation frequencies (36; 136). [17] Measuring device according to one of the preceding claims, wherein the control device is configured to generate the control signal (56) using a phase modulation signal (41) to generate sideband frequencies (39) of the discrete radiation frequencies by means of the Pound-Drever-Hall method, wherein the same phase modulation signal (41) is used for the at least two discrete radiation frequencies. [18] Measuring device according to one of the preceding claims, wherein the radiation generator (24) is further configured to generate, in addition to the at least two discrete radiation frequencies (36; 136), two sideband frequencies (39) with a uniform frequency separation (41) to the radiation frequency in question in the measuring radiation (22e). [19] Measuring device according to one of the preceding claims, wherein the system is an optical system for photolithography. [20] Projection exposure system (200) for photolithography 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. [21] Illumination optics (205) of a projection exposure system (200) for photolithography with at least one component and at least one measuring device (10) according to one of claims 1 to 19 for determining the position of the component. [22] Projection lens (216) of a projection exposure system (200) for photolithography with at least one component and at least one measuring device (10) according to one of claims 1 to 19 for determining the position of the component. [23] Inspection system for inspecting a surface of a substrate, in particular a mask or a wafer, comprising at least one component and at least one measuring device (10) according to any one of claims 1 to 18 for determining the position of the component. [24] Coordinate measuring device with at least one component and at least one measuring device (10) according to any one of claims 1 to 18 for determining the position of the component.

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