Measurement apparatus for determining the position of a component

Abstract

A measurement apparatus (10) for determining the position of a component (226) in a system (200) comprises a radiation generator (24) that is configured to generate a measurement radiation (22e) with at least two discrete radiation frequencies (36; 136) that 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) between 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) that comprises resonator frequencies (17) and a measurement target (14) assigned to the component and is configured to carry out a distance measurement by irradiating the measurement target with the measurement radiation, and a coupling module (32) that 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 measurement radiation, said radiation sample having been 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, said control signal comprising a specification (42v) for at least one of the comb parameters.

Description

[0001] Measurement apparatus for determining the position of a component

[0002] This application claims priority to the German Patent Application No. 10 2024 203 804.0 filed on April 23, 2024. The entire disclosure of this patent application is incorporated into the present specification by reference.

[0003] Background of the invention

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

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

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

[0007] Various approaches for measuring the position of the individual lens mirrors and also of the wafer or the wafer stage and the reticle plane are known in the prior art. Besides interferometric measurement arrangements, frequency-based position measurement using an optical resonator is also known here. A set-up used in DE 10 2012 212 663 A1 to this end comprises a resonator in the form of a Fabry- Perot resonator with two resonator mirrors, of which the first resonator mirror is secured to a reference element in the form of a measurement frame and the second resonator mirror (as what is known as the “measurement target”) is secured to an EUV mirror to be measured with regard to the position thereof.

[0008] The actual distance measurement apparatus comprises a radiation source, which is tuneable with respect to its optical frequency and which generates input coupling radiation that is input coupled into the optical resonator. In this case, the radiation source is controlled by a coupling device in such a way that the optical frequency of the radiation source is tuned to a resonant frequency of the optical resonator, which is also referred to as resonator frequency in this text, and is thus coupled to this resonant frequency. Radiation coupled out of the optical resonator is analysed by an optical frequency measurement device that may comprise 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 extent of the resonator, then together with the distance between the resonator mirrors the resonant frequency of the optical resonator also changes and hence - owing to the coupling of the frequency of the tuneable radiation source to the resonant frequency of the resonator - the optical frequency of the input coupling radiation changes as well, which is in turn registered directly by the frequency measurement device.

[0009] For many applications, it is important here to be able to measure over large distance ranges and / or at high distance rates of change. Changing the resonator length results in a change in the frequency position of the individual resonant frequencies. Continuous measurement in accordance with the principle described above is possible only within the frequency range in which the laser frequency is continuously tuneable.

[0010] The continuously tuneable frequency range of laser oscillators is ultimately limited by the gain spectrum of the laser medium, but usually additionally by internal filters and mode jump dynamics in the case of low-noise lasers. The laser frequency may likewise be tuned outside the laser oscillator, for example via acousto-optical or electro-optical modulators.

[0011] In this case, the tuneable frequency range is limited by the bandwidth of the modulators or the modulation-generating electronics. Limiting the continuously tuneable frequency range of the laser radiation is a problem when measuring over long distance ranges. Specifically, if a measurement is to be carried out outside the distance range specified by the extent of the tuneable frequency range, then it may be necessary to couple a new frequency of the laser radiation with a different resonator frequency. However, stabilizing the frequency control to the new target frequency is time-consuming, that is to say the stabilization period is considerable, and thus reduces the measurement dynamics able to be achieved. The permissible distance rate of change at which the position determination is able to be carried out continuously is thus highly limited.

[0012] Another problem with pre-existing measurement apparatuses is measurement uncertainties caused by standing waves occurring on the beam path between the ra- diation source, the optical resonator and the radiation detector of the coupling device. Avoiding such standing waves in the light supply lines of real measurement apparatuses may constitute a considerable challenge.

[0013] Problem of interest

[0014] One object of the invention is to provide a measurement apparatus of the type mentioned at the outset by way of which the abovementioned problems are solved and using which it preferably becomes possible to determine positions over longer distance ranges with high measurement dynamics and low measurement uncertainties.

[0015] Solution according to the invention

[0016] The abovementioned object may for example be achieved, according to the invention, by way of a measurement apparatus for determining the position of a component in a system having a radiation generator that is configured to generate a measurement radiation with at least two discrete radiation frequencies that are selected comb frequencies of a frequency comb structure, wherein the frequency comb structure is defined by at least two comb parameters, of which the first comb parameter comprises a uniform frequency spacing between the comb frequencies and the second comb parameter comprises a frequency offset of the frequency comb structure. The measurement apparatus furthermore comprises an optical resonator that comprises resonator frequencies and a measurement target assigned to the component and is configured to carry out a distance measurement by irradiating the measurement target with the measurement radiation. The measurement apparatus furthermore comprises a coupling module that 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 for detecting a radiation sample of the measurement radiation, said radiation sample having been branched off from the optical resonator, and a control device. The control device is configured to generate a control signal for the radiation generator from the radiation sample detected by the radiation detector, said control signal comprising at least one specification for at least one of the comb parameters. For example, the system in which the component is contained may be a photolithographic optical system.

[0017] In this text, an optical resonator should be understood to mean an arrangement of mirrors that serves to reflect light back and forth as often as possible. According to one embodiment, the optical resonator of the measurement apparatus is configured such that measurement radiation irradiated into the optical resonator still has at least 90%, advantageously still has at least 99%, of the irradiated intensity after at least five passes, preferably after at least ten passes, through the optical resonator. According to a further embodiment, the optical resonator of the measurement apparatus is configured to reflect the irradiated measurement radiation back and forth multiple times, for example at least 10 times or at least 100 times or at least 1000 times, before the measurement radiation leaves the optical resonator again, wherein leaving the optical resonator should be understood to mean that the intensity of a measurement radiation remaining in the resonator is less than 50% of the intensity of the irradiated measurement radiation. According to a further embodiment, the optical resonator of the measurement apparatus has a finesse of at least 100, advantageously of at least 1000.

[0018] The control signal may for example comprise a digitally transmitted specification, for instance in the form of one or more digital values, an analogue signal, for instance in the form of one or more radio frequencies transmitted in analogue form, or else an analogue control signal for an actuator that is designed to influence a comb parameter.

[0019] Position determination should be understood to mean determining the position of the component in at least one coordinate direction. The determined position may in this case be an absolute position in space or a relative position in relation to a starting position, that is to say a change in position.

[0020] According to one embodiment, the radiation sample detected by the radiation detector is represented by a detector signal, from which the control device then generates the control signal for the radiation generator. Coupling the discrete radiation frequencies to the resonator frequencies should be understood to mean 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 should be understood to mean the frequency spacing between a specified comb frequency of the frequency comb structure and the reference frequency. The uniform frequency spacing between the comb frequencies should be understood to mean that the comb frequencies each have the same frequency spacing from one another, that is to say in each case mutually adjacent comb frequencies each have the same frequency spacing.

[0021] If the control signal contains only a specification for one of the comb parameters, then, according to one embodiment, the radiation generator uses, for the comb parameter not provided by the radiation generator, the value used previously for this purpose, that is to say this comb parameter is left unchanged.

[0022] By virtue of 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, it becomes possible to maintain the frequency coupling of the measurement radiation to the optical resonator even when one of the discrete ra- diation frequencies reaches the limit of the tuneable frequency range of the radiation generator, hereinafter also referred to as generator spectrum, without loss of time.

[0023] On account of the current specification for the at least one comb parameter, said specification being provided continuously by the control device, it is possible to replace the discrete radiation frequency that reaches the limit of the generator spectrum with another comb frequency of the frequency comb structure that lies within the generator spectrum with a sufficient spacing from the abovementioned limit. All comb frequencies of the frequency comb structure are adapted to the resonator frequencies on account of the currently available specification for the at least one comb parameter. The newly selected discrete radiation frequency, which is likewise a comb frequency, is thus already adapted to the corresponding resonator frequency, and time-consuming frequency stabilization may be avoided.

[0024] In comparison to a solution in which the radiation frequencies are not matched to a frequency comb structure but rather are each matched to a relevant resonator frequency through separate control or coupling, and in which the frequency that reaches the limit of the generator spectrum makes a frequency jump that then has to be stabilized with respect to the nearest resonator frequency, the solution according to the invention is more efficient. The stabilization period may be avoided or greatly reduced, thus achieving a significantly higher permissible distance rate of change.

[0025] In the solution according to the invention, when the beam frequency is changed, said stabilization period is dispensed with for a short period for the control signal calculation. However, the control signal may be determined during this period from the beam frequency, which is still active, with a lower accuracy but in principle nevertheless. The uninterrupted stabilization of the jumped beam frequency thereby made possible minimizes the time for which this beam frequency is unavailable for control signal calculation. Furthermore, the solution according to the invention provides the advantage that the control signal can be generated over a large number of resonator frequencies. Measurement uncertainties that follow a spectral distribution independent of the resonator frequencies may thereby be averaged and thus reduced.

[0026] According to an embodiment, the specification included in the control signal comprises a specification for the frequency offset.

[0027] According to a further embodiment, the specification generated from the control signal contains a specification for the frequency spacing.

[0028] According to a further embodiment, the specification included in the control signal contains a specification for a combination of the frequency offset and the frequency spacing. In other words, it is possible to determine the frequency offset and the frequency spacing from the specification, but these are not contained separately in the specification included in the control signal.

[0029] According to a further embodiment, the control device is configured to ascertain the specification for the frequency spacing from the ascertained frequency offset taking into account a fixed point frequency of the optical resonator. According to one embodiment, this ascertainment is based on the idea that, in the event of a change in the frequency spacing of the frequency comb structure, the comb frequencies are shifted proportionally to the spacing between the relevant comb frequency and the fixed point frequency. The comb frequencies are shifted, according to this idea, in the same way as if pulling on a rubber band that is fixed to the fixed point frequency and on which the comb frequencies are arranged.

[0030] According to a further embodiment, the radiation generator comprises a reference radiation source for generating a reference radiation with the reference frequency and a frequency shift module that is configured to generate the measurement radiation with the at least two discrete radiation frequencies from the reference radiation by carrying out frequency shifting. According to one variant embodiment, the reference frequency is specified by a highly stable optical reference resonator. According to a further embodiment, the highly stable optical reference resonator is arranged in the vicinity of the optical resonator and is exposed to the same ambient conditions, in particular the same atmospheric fluctuations of the refractive index.

[0031] According to a further embodiment, the frequency shift module is configured to generate the measurement radiation with the at least two discrete radiation frequencies by modulating the reference radiation. According to one variant embodiment, the frequency shift module comprises at least two frequency modulators for this purpose. The frequency shift to be generated is specified to the modulator modules, and is added up by the relevant modulator module 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.

[0032] According to a further embodiment, only radiation frequencies that lie in a limited generator spectrum are able to be generated by the radiation generator and the measurement apparatus furthermore comprises a frequency jump module that is configured to replace the relevant radiation frequency with another comb frequency of the frequency comb structure when one of the coupled radiation frequencies reaches a limit of the generator spectrum during the generation of the measurement radiation in the radiation generator. The frequency jump module may in this case be part of the radiation generator or part of the control device.

[0033] According to a further embodiment, the radiation generator comprises a frequency comb generator and is configured to generate a frequency comb that comprises the measurement radiation with all comb frequencies within a limited generator spectrum.

[0034] According to a further 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.

[0035] According to a further embodiment, the frequency comb generator comprises at least one modulation unit for periodically modulating the reference radiation, which modulation unit is configured such that a frequency spacing of the generated frequency comb can be set by varying the modulation period of the modulation unit.

[0036] According to a variant 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 a frequency spacing of the generated frequency comb is able to be set by varying the modulation period of the phase modulator. For this purpose, the phase modulator is preferably designed as an electro-optical phase modulator. The intensity modulator is advantageously configured to generate a periodic pulse train from the reference radiation, wherein the intensity modulator may comprise at least one downstream phase modulator for widening the generator spectrum. The intensity modulator is preferably configured as an electro-optical intensity modulator.

[0037] 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 able to be set by varying the pulse repetition rate of the intensity modulator.

[0038] According to a further embodiment, the frequency comb generator comprises an electro-optical phase modulator for modulating the reference radiation, which electro-optical phase modulator is configured such that a frequency offset of the generated frequency comb is able to be generated by a phase jump induced regularly in the phase modulation. The phase jump may thus for example take place between any two pulses generated by the electro-optical phase modulator. According to one embodiment, a value of n ■ 2TT is subtracted from or added to the phase specification newly set after the phase jump, wherein n is an integer and is selected such that the phase specification lies within the finite phase deviation of the phase modulator.

[0039] According to a further embodiment, the radiation generator comprises a reference generator for generating a reference frequency in the form of a radio frequency and the frequency comb generator has a pulsed mode-coupled laser for generating the frequency comb with a frequency offset from the reference frequency. The frequency comb generator may for instance be configured as a femtosecond laser. The frequency spacing of the generated frequency comb may for instance be manipulated by changing the optical path length of the laser oscillator. The temperature of the gain medium in the laser oscillator may be changed in order to manipulate the frequency offset of the frequency comb.

[0040] According to a further embodiment, the radiation generator comprises at least one comb manipulator that may be designed to adapt one of the comb parameters or to adapt both comb parameters in a specified ratio and is configured to receive the control signal.

[0041] According to a further embodiment, the coupling module is configured to couple the comb frequencies generated by the radiation generator to a respective one of the resonator frequencies, wherein the coupling of a comb frequency that newly appears in the generator spectrum due to a shift of the resonator frequencies takes place as a result of the pre-existing coupling of the other comb frequencies.

[0042] According to a further embodiment, the control device is configured to ascertain the control signal from a detector signal representing the detected radiation sample without separating signal components that stem from the individual discrete radiation frequencies. This means that the detector signal is not split up with regard to the discrete radiation frequencies in order to ascertain the control signal. In other words, the control signal is ascertained as a whole or cumulatively in relation to the discrete radiation frequencies.

[0043] According to a further embodiment, the control device is configured to generate the control signal using a phase modulation signal so as to generate sideband frequencies of the discrete radiation frequencies by way of the Pound-Drever-Hall method, wherein the same phase modulation signal is used for the at least two discrete radiation frequencies. The sideband frequencies of the two radiation frequencies thus have a uniform frequency spacing.

[0044] The operation of the Pound-Drever-Hall method in its conventional embodiment and an embodiment for use in the measurement apparatus according to the invention is described below: In the conventional embodiment of the Pound-Drever- Hall method, measurement radiation with an operating frequency BF is generated by a radiation generator. 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, a phase modulation signal in the form of a modulation frequency fpDH is specified to said phase modulator by an RF oscillator, and 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 ± fpDH, in the present case the generation of at least two pairs of sidebands, specifically with the frequencies BF1 ± fpDH and BF2 ± fpDH.

[0045] The modulated measurement radiation is irradiated into a resonator cavity with at least one resonant frequency, which is referred to hereinafter as resonator frequency RF. In the present case, the resonator cavity is formed by the optical resonator, which has multiple resonator frequencies. Measurement radiation reflected from the resonator cavity is detected by a radiation detector. In this case, frequencies that are in the range of the finite line width of the resonator frequencies are reflected with a phase and intensity modified by the resonator. The detector signal output by the radiation detector has a temporal modulation. Comparing the detector signal with the signal from the RF oscillator by way of a mixer and separating the high-frequency signal component contained in the mixed signal using a low- pass filter generates an error signal that, in the conventional embodiment, represents a measure of the deviation of the operating frequency BF from the resonator frequency RF.

[0046] In the present case, the error signal contains at least the information about the mean of the individual deviations of the operating frequencies BF1 and BF2 from the resonator frequencies within the line widths of which they are located. The error signal is supplied to a controller, which converts it, optionally with the addition of a converter, into a control signal for the radiation generator, which, in the conventional embodiment, is used 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 is used to align the operating frequencies with assigned resonator frequencies.

[0047] According to a further embodiment, the radiation generator is furthermore configured to generate, in addition to the at least two discrete radiation frequencies, in each case two sideband frequencies with a uniform frequency spacing from the relevant radiation frequency in the measurement radiation.

[0048] According to a further embodiment, the system is a photolithographic optical system.

[0049] Furthermore, a photolithographic projection exposure apparatus is provided according to the invention, having at least one component and at least one measurement apparatus in one of the embodiments or variant embodiments described above for determining the position of the component. Furthermore, an illumination optical unit of a photolithographic projection exposure apparatus is provided according to the invention, comprising at least one component and at least one measurement apparatus in one of the embodiments or variant embodiments described above for determining the position of the component.

[0050] Furthermore, a projection lens of a photolithographic projection exposure apparatus is provided according to the invention, comprising at least one component and at least one measurement apparatus in one of the embodiments or variant embodiments described above for determining the position of the component.

[0051] Furthermore, an inspection apparatus for inspecting a surface of a substrate is provided according to the invention, comprising at least one component and at least one measurement apparatus in one of the embodiments or variant embodiments described above for determining the position of the component. For example, the substrate might be a mask or a wafer.

[0052] The movable component may be a component in an optical system of the inspection apparatus. One example of such an inspection apparatus for the inspection of masks or wafers (without the measurement apparatus according to the invention) is known from document DE 102012205181 A1 , the entire content of which is incorporated by reference in the present application.

[0053] Furthermore, a coordinate measuring machine is provided according to the invention, comprising at least one component and at least one measurement apparatus in one of the embodiments or variant embodiments described above for determining the position of the component.

[0054] The movable component may be a component in an optical system of the coordinate measuring machine. The coordinate measuring machine serves to determine a respective position deviation of one or more measurement points on a test component from a respective target position. One example of such a coordinate measuring machine (without the measurement arrangement according to the invention) is known from document DE10 2019 213 794A1 , the entire content of which is incorporated by reference in the present application.

[0055] The features mentioned in relation to the abovementioned embodiments, exemplary embodiments and variant embodiments, etc. are explained in the description of the figures and the claims. The individual features may be implemented, either separately or in combination, as embodiments of the invention. Furthermore, they may describe advantageous embodiments that are independently protectable and protection for which is claimed only during or after pendency of the application, as the case may be.

[0056] Brief description of the drawings

[0057] The abovementioned features and further advantageous features of the invention will be illustrated in the following detailed description of exemplary embodiments according to the invention or of embodiments or variant embodiments with reference to the attached schematic drawings. In the figures:

[0058] Fig. 1 shows one exemplary embodiment of a measurement apparatus for determining the position of a component in a photolithographic optical system with a radiation generator for generating a measurement radiation with at least two discrete radiation frequencies, wherein the radiation generator comprises a frequency shift module,

[0059] Fig. 2 shows another exemplary embodiment of a measurement apparatus for determining the position of a component in a photolithographic optical system with a radiation generator for generating a measurement radiation with at least two discrete radiation frequencies, wherein the radiation generator comprises a frequency comb generator, Fig. 3 shows the frequency comb generator according to Fig. 2 in a first variant embodiment,

[0060] Fig. 4 shows the frequency comb generator according to Fig. 2 in a further variant embodiment,

[0061] Fig. 5 shows one exemplary embodiment of a control device of the measurement apparatus according to Fig. 1 and Fig. 2, and

[0062] Fig. 6 shows one exemplary embodiment of a photolithographic projection exposure apparatus having a movable component in the form of a mirror of the projection lens, the position of which is able to be determined by way of a measurement apparatus according to Fig. 1 and Fig. 2.

[0063] Detailed description of exemplary embodiments according to the invention

[0064] In the exemplary embodiments or embodiments or variant embodiments described below, elements that are functionally or structurally similar to one another are provided with the same or similar reference signs as far as possible. Therefore, for understanding the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention.

[0065] In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in the drawing, from which system the respective positional relationship of the components illustrated in the figures is evident. In Fig. 1 , the x-direction runs perpendicular and into the plane of the drawing, the z-direction runs to the right, and the y-direction runs upwardly. Fig. 1 illustrates an exemplary embodiment of a measurement apparatus 10 for determining the position of a component in a photolithographic optical system. Determining the position should be understood here to mean determining the position of the component in at least one coordinate direction, in this case in the z-di- rection. The determined position may in this case be an absolute position in space or a relative position in relation to a starting position, that is to say a change in position. Fig. 6 illustrates, in a simplified illustration, such an optical system in the form of a photolithographic projection exposure apparatus 200, which will be described in more detail later on in the text. A mirror 226 of a projection lens 216 of the projection exposure system 200 serves for example as the abovementioned component to be measured. The position of this component may change during operation of the projection exposure apparatus 200, and the component may therefore also be referred to as a movable component. As an alternative, the component to be measured may be a component of an illumination optical unit 205 of the projection exposure apparatus 200 or another component of the projection exposure apparatus 200.

[0066] The measurement apparatus 10 according to Fig. 1 comprises a measuring head 12 and a measurement mirror, which may also be referred to as measurement target 14 and which is fastened, and hence assigned, to the movable component. To determine the position of the movable component, the measurement apparatus is used to determine the distance between the measuring head 12 and the measurement target 14 in the z-direction, as described in detail hereinafter. In other words, in this exemplary embodiment, the position z is determined at one point in time or at different points in time.

[0067] The measurement apparatus 10 comprises an optical resonator 16. Said optical resonator comprises a resonator cavity 18 that is enclosed by two resonator mirrors of the resonator 16. In this case, the first resonator mirror is formed by an input coupling mirror 20, through which a measurement radiation 22 is irradiated into the resonator cavity 18. The second resonator mirror is formed, in the present exemplary embodiment, by the measurement target 14. The input coupling mirror 22 has a curved mirror surface in the illustrated exemplary embodiment, while the measurement target 14 has a plane mirror surface. The measurement radiation 22 in the resonator cavity 18 thus forms a Gaussian beam the waist of which is located on the mirror surface of the measurement target 14. In a further exemplary embodiment not illustrated in the drawings, the optical resonator 18 may, in addition to the two resonator mirrors, comprise one or more folding mirrors, and one of the folding mirrors may act as a measurement target. In a variant in which only one folding mirror is used, the input coupling mirror and the second resonator mirror are oriented in the same direction.

[0068] In the exemplary embodiment according to Fig.1 , the resonator cavity 18 has a length Lo in a basic state. In the event of a time (t)-dependent shift of the measurement target 14 in measuring mode in the z-direction by ±d(t), the length L(t) of the resonator cavity 18 changes accordingly, such that: L(t) = Lo ± d(t). The length L of the resonator cavity 18 determines the resonant frequencies of the optical resonator 16, hereinafter referred to as resonator frequencies RFq(reference sign 17). The index q is used here to number the vibration orders corresponding to the different resonance states of the optical resonator 16. The upper section of Fig. 1 contains a frequency space representation 33. This illustrates, by way of example, the resonator frequencies RFqin a tuneable frequency range of the radiation generator 24, that is to say in a frequency range in which the radiation generator 24 is able to set the frequency of the measurement radiation 22. This frequency range is also referred to as a limited generator spectrum 34 in this text.

[0069] In the frequency space representation 33 according to Fig. 1 , the resonator frequency that lies approximately in the middle of the generator spectrum 34 is denoted with the index i, that is to say RFi. The spacing between the respective resonator frequencies RFqis uniform and is referred to by experts as the free spectral range (FSR) fFSR of the optical resonator 16.

[0070] If the length L of the resonator cavity 18 is changed, then the free spectral range fFSR changes accordingly. In the process, the distribution, which may be described as a comb in a first approximation, of the frequencies RFqis stretched or com- pressed about a fixed point frequency jfix(reference sign 21 ) in the manner of a harmonica, that is to say the spacings between mutually adjacent frequency modes become uniformly greater (when the length L decreases) or less (when the length L increases) by a uniform factor. Resonator frequencies RFqarranged to the left of the fixed point frequency jfixmove to the left when the frequency comb is stretched, while resonator frequencies RFqarranged to the right of the fixed point frequency jfixaccordingly move to the right.

[0071] In the present exemplary embodiment, the fixed point frequency f^ixis close to the frequency zero point (f = 0 Hz), such that the resonator frequencies RFi-s to RFi+4 arranged in the generator spectrum 34, that is to say arranged in the tuneable frequency range of the radiation generator 24, move to the right when the length L of the optical resonator 16 decreases over time, as shown in the fre- dRFa quency space representation 33 by arrows 19 illustrating the direction of — - in dt the illustrated frequency range.

[0072] The measurement apparatus 10 illustrated in Fig. 1 comprises a radiation generator 24 for generating the measurement radiation 22, optionally an optical fibre 26, a beam-shaping optical unit in the form of an input coupling lens 28, the above- mentioned optical resonator 16, a circulator 30 and a coupling module 32. The measuring head 12, likewise already mentioned above, comprises at least the input coupling mirror 20 and also the input coupling lens 28 for collimating the measurement radiation 22 leaving the optical fibre 26 before it enters the input coupling mirror 20. The radiation generator 24 and / or the coupling module 32 may likewise be part of the measuring head 12 or else, as illustrated in Fig. 1 , be arranged outside it and connected thereto via the optical fibre 26.

[0073] In the exemplary embodiment according to Fig. 1 , the radiation generator 24 is configured to generate the measurement radiation 22 with at least two discrete ra- diation frequencies fi and f2 (reference sign 36) that are selected comb frequencies KFq(reference sign 38) of a frequency comb structure 37. The frequency comb structure 37 comprises a multiplicity of comb frequencies 38 having a uniform frequency spacing frep(reference sign 42), that is to say the frequency spacing between adjacent comb frequencies 38 is uniform. The frequency comb structure 37 has a frequency offset Af (reference sign 40) from a reference frequency fR (reference sign 43) that is generated in the radiation generator 24 and is thus defined, and possibly also known beforehand.

[0074] The frequency offset Af should be understood here to mean the frequency spacing between a specified comb frequency 38 of the frequency comb structure 37 and the reference frequency fR. In the example according to Fig. 1 , the specified comb frequency 38 is the comb frequency KFo and the frequency offset Af is positive. In this case, the comb frequency KFo is the comb frequency of the frequency comb structure 37 that is closest to the reference frequency fR at the time illustrated. On account of the coupling, described in more detail below, of the discrete radiation frequencies fi and f2 to assigned resonator frequencies RFq, the frequency offset Af changes when the resonator length L changes, while the reference frequency fR remains constant. This means that the specified comb frequency KFo with respect to which the frequency offset Af is determined is no longer the comb frequency closest to the reference frequency fR when the resonator length L changes accordingly.

[0075] In the exemplary embodiment according to Fig. 1 , the comb frequencies 38 of the frequency comb structure 37 are numbered with reference to the comb frequency KFo, namely comb frequencies that lie to the left of KFo are numbered with increasing negative count values (KF-i , KF-2, etc.) and comb frequencies that lie to the right of KFo are numbered with increasing positive count values (KF+i, KF+2, etc.). In the exemplary embodiment according to Fig. 1 , the radiation frequencies fi and f2 of the measurement radiation 22 are the comb frequencies KF+i (radiation frequency fi) and KF-2 (radiation frequency f2). 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 fR. The frequency-stable laser may be for example a femtosecond laser that generates a fs frequency comb, or another frequency-stabilized laser. By way of example, it may be a laser stabilized to a reference resonator. The reference resonator may be an optical reference resonator arranged in the vicinity of the optical resonator 16 and that is exposed to the same density and temperature fluctuations of the beam-guiding medium. The radiation generator 24 furthermore comprises a frequency shift module 48 that is configured to generate the measurement radiation 22 with the two discrete radiation frequencies fi and f2 from the reference radiation 46 by carrying out frequency shifting.

[0076] The frequency shift module 48 furthermore generates, for each of the radiation frequencies fi and f2, two sideband frequencies 39 that are each spaced from the relevant radiation frequency fi and f2 by the uniform frequency spacing fpDH (refer- CD I * D I ence sign 41 ), namely ~ = f + fPDHand f2- = f2+ fpDH- These sideband frequencies 39 are used for the frequency coupling, explained in more detail below, of the radiation frequencies fi and f2 to assigned resonator frequencies 17 on the basis of 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 the frequency modulators in each case by a radio frequency generated in the frequency shift module 48, this being added up to the optical frequency of the reference radiation 46 by the relevant frequency modulator. The function of said frequency modulators may be taken on by what is known as an IQ modulator for jointly generating the frequencies to be generated from the reference frequency by carrying out frequency shifting (single-sideband modulation) or by an IQ modulator for generating the frequencies fi and f2 by carrying out frequency shifting and a phase modulator for generating the symmetric PDH sidebands. The circulator 30 may be implemented for example by way of a Faraday rotator. It is likewise also possible to replicate the circulator 30 by way of an optical coupler. The circulator 30 conducts the measurement radiation 22 coming from the radiation generator 24, also referred to as incoming measurement radiation 22e, via the optical fibre 26 to the measuring head 12. After interaction at the optical resonator 16, the circulator 30 guides measurement radiation returning from the measuring head 12, also referred to in this text as a measurement radiation that has been branched off from the optical resonator 16 or as a measurement radiation 22r reflected from the optical resonator 16, into the coupling module 32.

[0077] During operation of the measurement apparatus 10 according to Fig. 1 , the incoming measurement radiation 22e, generated by the radiation generator 24, with the two discrete radiation frequencies fi and f2 and the sideband frequencies fiB+’ fiB~ ’ f2SB+ ar|d f2SB~ is fed into the optical resonator 16. The measurement radiation 22r reflected from the optical resonator 16, also referred to in this text as a radiation sample, is detected in the coupling module 32 by a radiation detector 50. In this case, the radiation detector 50 generates a detector signal 52 in the form of an electrical signal depending on the intensity of the detected radiation. For this purpose, the radiation detector 50 may be designed for example as a photodiode. The detector signal 52 is forwarded to a control device 54 of the coupling module 32. The control device 54 generates a control signal 56 from the received detector signal 52 and forwards it to the frequency shift module 48 of the radiation generator 24.

[0078] For this purpose, the control device 54 may be designed in purely digital form, for example as a field-programmable gate array (FPGA for short). The analogue signal from the radiation detector 50 must, for this purpose, be converted into a digital signal either by the FPGA or by an A / D converter, not illustrated in the drawings. The analogue control signal, required to control the frequency shift module 48, in the form of an AC voltage may be generated by way of a D / A conversion in the frequency shift module 48. Illustrated in simplified form, the operating principle of the measurement apparatus 10 illustrated in Fig. 1 is based on shifting the radiation frequencies fi and f2 by way of the frequency shift module 48 such that they match the respective resonator frequencies RFqof the optical resonator 16 and put it into a resonance state.

[0079] By tracking the radiation frequencies fi and f2 appropriately in the event of a change in the resonator frequencies RFq, the radiation frequencies fi and f2 are coupled to the relevant resonator frequencies RFq. In the exemplary embodiment according to Fig. 1 , the coupled resonator frequencies are RFi+3 (coupled with the radiation frequency fi) and RFi-3 (coupled with the radiation frequency f2).

[0080] To enable the radiation generator 24 to track the radiation frequencies fi and f2 to the relevant resonator frequencies RFq, the control signal 56 comprises at least one specification Af* (reference sign 40v) for the frequency offset 40. In the exemplary embodiment according to Fig. 1 , the control signal 56 furthermore comprises a specification fr*ep(reference sign 42v) for the frequency spacing 42. As explained in more detail below with reference to Fig. 5, in this exemplary embodiment, the control device 54 ascertains the specification fr*epusing the previously ascertained specification Af*. In one exemplary embodiment in which the control signal 56 comprises only the specification Zlf*for the frequency offset 40, this functionality for ascertaining the specification fr*epmay also for example be integrated in the radiation generator 24.

[0081] From the specifications fr*epand Af* transferred from the control device 54 to the radiation generator 24, it ascertains the discrete radiation frequencies 36 to be generated as follows: / 2= fR+ 4 / ’ + n2- f^p , (2) where m and n2 are integers. In the exemplary embodiment in accordance with the frequency space representation 33 of Fig. 1 , the following applies: m = +1 and n2 = -2. Based on the ascertained discrete radiation frequencies fi and f2, the frequency shift module 48 is instructed to adjust these frequencies.

[0082] Since the reference frequency fR has a high stability, the frequency offset reflects shifts of the resonator frequencies RFqof the optical resonator 16 with a high accuracy. The values of the comb parameters Af (frequency offset) and frep(frequency spacing) set in the radiation generator 24 are transferred to an evaluation device 58. According to the described embodiment, these values are the respective digital values of the specifications f* and fr*epof the two comb parameters. The specifications Zlf*and fr*epprovided by the control device 54 may, depending on the variant embodiment, be analogue signals or already comprise the digital values.

[0083] According to alternative variant embodiments, only Af or frep may also be transferred to the evaluation device 58, wherein the respective other variable may be ascertained using the fixed point frequency jfix. The evaluation device 58 ascertains from this the time-dependent shift d(t) of the measurement target 14 of the optical resonator 16 in real time, that is to say the shift d at different points in time. The shift d(t) in turn gives the position z (reference sign 15) of the measurement target 14 and thus of the movable component.

[0084] In other words, in this procedure, the reference frequency fR of the reference radiation source 44 is maintained unchanged with high precision and the tuning to both resonant frequencies RFqof the optical resonator 16 is achieved by varying the frequency shift using the frequency shift module 48. If the length L of the optical resonator 16 is changed, the accompanying change in the resonant frequencies RFqof the optical resonator 16 is compensated for by the frequency shift module 48 and the change in the length L is ascertained from the tracking of the frequency shift required for this purpose. As already mentioned above, the radiation generator 24 is only capable of generating radiation frequencies that lie in the generator spectrum 34 illustrated by way of example in Fig. 1. If one of the radiation frequencies fi and f2 or one of the assigned sideband frequencies 39 then reaches a limit of the generator spectrum 34 due to a change in length of the optical resonator 16 and the above-described coupling of the radiation frequencies to the assigned resonator frequencies RFq, then the relevant radiation frequency is replaced by another comb frequency 38 of the frequency comb structure 37. This frequency replacement is also called a frequency jump.

[0085] Such a frequency jump is illustrated by way of example in the frequency space representation 33 of Fig. 1 , here for the case in which the length L of the optical resonator 16 is reduced to the extent that the radiation frequency fi that moves to the right as a result reaches the right-hand limit of the generator spectrum 34. Reaching the right-hand limit of the generator spectrum 34 should be understood here to mean that the radiation frequency fi lies so close to the limit that, in the event of a further shift to the right, it would no longer be able to be generated with a sufficient intensity, or that the right sideband frequency fB+of the radiation frequency fi would no longer be able to be generated with a sufficient intensity. If the radiation frequency fi has then reached the right-hand limit of the generator spectrum 34, then, as illustrated in Fig. 1 , the radiation frequency fi from the original comb frequency KF+i, which was previously coupled to the resonator frequency RFi+3, is set to the new comb frequency KF-3, which corresponds to the resonator frequency RFi-s. This is done by assigning a new value for m, which is referred to in the frequency space representation 33 as value m‘, and the resulting new radiation frequency is referred to as fi‘. In the illustrated example, the old value is m=1 and the new value is m‘=-3.

[0086] To initiate a frequency jump, the radiation generator 24 comprises a frequency jump module 60. This specifies the values for m and n2 for equations (1) and (2) to the frequency shift module 48 in order to determine fi and f2. The frequency jump module 60 receives, as input, the specifications fr*epand 21 f** ascertained by the control device 54 and corrects m and n2 accordingly as soon as one of the radiation frequencies reaches a limit of the generator spectrum 34. The frequency jump module 60 may, instead of being part of the radiation generator 24, also be integrated into the control device 54.

[0087] In the embodiment of the frequency jump, the radiation frequency fi is dispensed with for a short period for the calculation of the control signal 56, and thus for the frequency coupling of the measurement radiation 22e to the resonator frequencies RFq. However, since the other beam frequency f2 is still coupled with the assigned resonator frequency RFi-3 in this period, the control signal 56 may still be ascertained by the control device 54.

[0088] Although this may take place with slightly less accuracy due to fi being dispensed with, this is sufficient to allow the frequency coupling of the measurement radiation 22e to the resonator frequencies RFqto be maintained, especially since said period with reduced frequency coupling is only short. Since the adaptation, carried out beforehand, of the frequency spacing 42 of the frequency comb structure 37 to the resonator frequencies RFq was carried out with high accuracy, the new radiation frequency fi ‘ set after the frequency jump is already set very precisely to the new resonator frequency RFi-3, and so time-consuming frequency stabilization may be avoided.

[0089] Since the period with reduced frequency coupling is thus only short, this ensures that, even in the event of a rapid change in length of the optical resonator 16, at least one radiation frequency is always coupled with one of the resonator frequencies RFq. This means that, even if the radiation frequency f2 also reaches the right-hand limit of the generator spectrum 34 a short time later due to the rapid change in length, it is guaranteed that the radiation frequency fi is already available again for ascertaining the control signal 56 in the frequency jump that then follows for the radiation frequency f2. As already mentioned above, the radiation frequencies fi and f2 are coupled to assigned resonator frequencies 17 on the basis of the Pound-Drever-Hall method. For this purpose, the operation of the Pound-Drever-Hall method in its conventional embodiment and one for use in the present measurement apparatus 10 is described below: In the conventional embodiment of the Pound-Drever-Hall method, measurement radiation with only a discrete radiation frequency is generated by a radiation generator, this radiation frequency being referred to below as operating frequency BF.

[0090] In the present case, the measurement radiation 22e has at least the two radiation frequencies fi and f2. The measurement radiation 22 with the radiation frequencies fi and f2 may be generated in the frequency shift module 48 from the reference radiation 46 by controlling an acousto-optical modulator or an IQ modulator with a radiofrequency signal generated in the frequency shift module 48. For this purpose, the radiofrequency signal contains the corresponding spacings Afi and Af2between the radiation frequencies fi and f2 and the reference frequency fR.

[0091] The measurement radiation 22 with the two radiation frequencies fi and f2 passes through a modulator in the frequency shift module 48, with a phase modulation signal in the form of the modulation frequency fpDH being specified for said modulator by an RF oscillator 62, and the result of the modulation is the generation of CD_|_ D-L * rj I the two pairs of sidebands ~ and f2~ with the frequencies ~ = j + CD I fPDHand f2- = f2+ fPDH. In other words, both radiation frequencies fi and f2 are modulated on the basis of the same phase modulation signal, that is to say the sidebands ~ and f2~ have the same absolute bandgap (fpDH) from the relevant radiation frequency fi and f2. The sidebands may in this case be gener- ated in the same modulator by adding the sideband frequencies A^ - and to the radiofrequency signal. As an alternative, the sidebands may be modulated by a separate phase modulator (for example EO modulator) with the modulation frequency fpDH, wherein the phase modulator may be arranged upstream or downstream of the IQ modulator in the optical beam path. In the conventional embodiment of the Pound-Drever-Hall method, on the other hand, only one pair of sidebands with the frequencies BF ± fpDH is generated. As already mentioned above, the modulated measurement radiation 22e is irradiated into the resonator cavity 18 of the optical resonator 16 with multiple resonator frequencies RFq. Measurement radiation 22r reflected from the optical resonator 16 is detected by the radiation detector 50. In this case, frequencies that are in the range of the finite line width of the resonator frequencies RFqare reflected with a phase and intensity modified by the resonator. The detector signal 52 output by the radiation detector 50 has a temporal modulation.

[0092] In an exemplary embodiment of the control device 54, which is illustrated in detail in Fig. 5, comparing the detector signal 52 with the signal from the RF oscillator 62 by way of a mixer 64 and separating the high-frequency signal component contained in the mixed signal 66 using a low-pass filter 68 generates an error signal 70 that, in the conventional embodiment, represents a measure of the deviation of the operating frequency BF from the resonator frequency RF.

[0093] In the present case, the error signal 70 contains at least the information about the averaged deviation of the two discrete radiation frequencies fi and f2 from the respectively closest resonator frequencies RFq, which, in the exemplary embodiment according to Fig. 1 , corresponds to six times the free spectral range (6 x fFSR). The error signal 70 is supplied 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.

[0094] In the conventional embodiment, the correction signal may possibly already be used as a control signal. In the conventional embodiment, this is used 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 is used to jointly align the discrete radiation frequencies fi and f2 with the assigned resonator frequencies RFi+3 and RFi-3. In the exemplary embodiment illustrated in Fig. 5, the converter 76 comprises two conversion units 78 and 80. The first conversion unit 78 is configured to convert the correction signal 74 to the specification Af* (reference sign 40vv) for the frequency spacing. In another exemplary embodiment of the converter 76, the first conversion unit 78 may possibly be dispensed with and the correction signal 74 may be used directly as a specification 42v for the frequency offset.

[0095] The second conversion unit 80 is configured to ascertain the specification fr*ep(42v) for the frequency offset from the specification Z\f*for the frequency spacing. This is done using the fixed point frequency f^ix(reference sign 21 ) of the optical resonator 16, which is determined beforehand and, if necessary, subsequently at regular time intervals using a suitable calibration device. This ascertainment is based on the idea, already explained above, that, in the event of a change in the free spectral range fpsR, the resonator frequencies are shifted proportionally, as a first approximation, to the spacing between the relevant resonator frequency and the fixed point frequency jfix. The resonator frequencies are shifted, according to this idea, in the same way as if pulling on a rubber band that is fixed to the fixed point frequency 21 and on which the resonator frequencies are arranged, such that the resonator frequencies are stretched or compressed about the fixed point frequency 21 in the manner of a harmonica.

[0096] As is clear from the above description, the control device 54 is configured to ascertain the control signal 56 from a detector signal 52 representing the detected radiation sample 22r without separating signal components that stem from the individual discrete radiation frequencies 36. In other words, the detector signal 52 is not split up with regard to the discrete radiation frequencies 36 in order to ascertain the control signal 56, that is to say the control signal 56 is ascertained as a whole or cumulatively with respect to the discrete radiation frequencies 36. The fixed point frequency of the optical resonator 16 may 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 f^ixof the optical resonator 16 is, as already mentioned above, close to the frequency zero. As mentioned above, the fixed point frequency f^ixis determined using a suitable calibration device and, in the case in which the dispersion of the resonator exhibits significant temporal variability, for example in the presence of a changeable medium, adapted, if necessary, to changes in the free spectral range fpsR.

[0097] Jointly controlling the frequency spacing frep and the frequency offset Af of the frequency comb structure 37 with a fixed proportionality factor makes it possible to set the fixed point of this virtual control variable to the fixed point frequency jfixof the optical resonator 16. With a high stability of the fixed point frequency (for example in a vacuum), or a low bandwidth of the comb, or a small distance change, all discrete radiation frequencies 36 may thus be kept in resonance via a common control variable. In the event of a fixed point frequency f^ixthat is variable but only slowly changeable (for example due to pressure fluctuation), the proportionality factor and thus the fixed point of the virtual control variable may be recalibrated as required. If required, the comb parameters may alternatively be controlled via two independent control loops.

[0098] The maximum possible bandwidth over which lines of a comb are able to be brought simultaneously into resonance with the optical resonator 16 is limited by intra-resonator dispersion (in high-finesse resonators in a vacuum dominated by the dispersion of the multilayer mirrors) and decreases as resonator finesse increases. Optimized multilayer designs make it possible to optimize bandwidth. In embodiments of the measuring principle with a high resonator finesse and a large used bandwidth of the generator spectrum, the resonator dispersion in combination with the field establishment time may give rise to a distance measurement uncertainty dependent on the distance rate of change. This may be limited by using optical filters to supply 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 themselves contributing to the error signal.

[0099] Fig. 2 illustrates another exemplary embodiment of a measurement apparatus 10 for determining the position of a component in a photolithographic optical system. The exemplary embodiment according to Fig. 2 differs from the exemplary embodiment according to Fig. 1 only in terms of the configuration of the radiation generator 24. In the exemplary embodiment according to Fig. 2, this is configured not only to generate two discrete radiation frequencies fi and f2 of a frequency comb structure 37, like the radiation generator 24 according to Fig. 2, but instead to generate all comb frequencies 36 of the frequency comb structure 37. The frequency comb structure 37 in this case comprises only comb frequencies 36 that lie within the generator spectrum 34. In the exemplary embodiment according to Fig. 2, the frequency comb structure is also referred to simply as a frequency comb 137.

[0100] The radiation generator 24 according to Fig. 2 comprises a reference generator 144 that is configured to generate the reference frequency fR (reference sign 43), and a frequency comb generator 148 that is configured to generate the frequency comb 137 that comprises 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 ~ fn + f + k ■ frep(3) wherein k are integers in a limited range corresponding to the generator spectrum 34. In the example illustrated in the frequency space representation 33 according to Fig. 2, k comprises the values -3, -2, -1 , 0 and +1 . In other words, the frequency comb generator 148 generates therein the discrete radiation frequencies f-3, f-2, f-2, fo and fi, which are each spaced from one another by frepand wherein fo has the frequency offset Af from the reference frequency fR. Furthermore, the frequency comb generator 148 generates, for each of the discrete frequencies fk, a pair of sideband frequencies fk~ each with a spacing of fpDH from the respective discrete radiation frequency fk: fkB~=fk i fpDH (4)

[0101] According to one exemplary embodiment, the frequency spacing frep is in the range between 5 MHz and 50 GHz.

[0102] The discrete radiation frequencies fk are coupled to the resonator frequencies RFqanalogously to the operation of the coupling module 32 as described with reference to the measurement apparatus 10 according to Fig. 1 . In contrast to the measurement apparatus according to Fig. 1 , the frequency jump does not have to be triggered specifically by a frequency jump module 60, as may be seen from the following explanation: If the radiation frequencies fk move, for instance when the length L of the optical resonator 16 decreases over time, to the right in the frequency space representation 33 to the extent that the radiation frequency fi exceeds the right-hand limit of the generator spectrum 34, then the radiation frequency fi is no longer contained in the measurement radiation 22e. At the same time, however, a new radiation frequency moves into the generator spectrum 34 from the left (in this case the radiation frequency f-3 or the radiation frequency f-4 that follows it), such that the missing radiation frequency fi is compensated for thereby. In other words, enough comb frequencies 38, in the present case at least three or four comb frequencies 38, are automatically always contained in the measurement radiation 22e for the coupling to the resonator frequencies RFqto be maintained. The radiation frequency f-3 that newly runs into the generator spectrum 34 in the frequency space representation according to Fig. 2 is adapted to the resonator frequency RFi-6 due to the continuous tuning of the frequency spacing frepand the frequency offset Af of the frequency comb 137 to the free spectral range fFSR of the optical resonator 16 and the absolute position of the resonator frequencies RFq, without any separate frequency stabilization being required for this purpose. In other words, the coupling of a comb frequency KFqthat newly appears in the generator spectrum due to a shift of the resonator frequencies RFqtakes place as a result of the pre-existing coupling of the other comb frequencies.

[0103] In other words, the signal strength or gain of the error signal is proportional to the intra-resonator field strength. At a high distance rate of change, the resonances move at high speed through the frequency range of the comb spectrum. The delayed establishment of the intra-resonator field means that the resonances and comb lines moving in the region of the comb spectrum contribute to the gain of the cumulative error signal in a delayed manner. This reduces cumulative gain at high distance rates of change. The greater the frequency range of the comb spectrum, the lower the gain drop and the higher the distance rates of change able to be tolerated.

[0104] The frequency comb generator 148 according to Fig. 2 may be configured in two different variant embodiments. In the first variant embodiment 148a, which is illustrated in Fig. 3, the frequency comb generator comprises a modulation unit 149 for periodically modulating a reference radiation 146, which is generated by the reference generator 144, which is configured as a reference radiation source 144a in the variant embodiment according to Fig. 3. The reference radiation source 144a generates the reference radiation 146 in continuous wave operation and may be configured in the same way as the reference radiation source 44 according to Fig. 1 . The modulation unit 149 may comprise acousto-optical and / or electro-optical modulators. In this case, the modulation unit 149 may comprise for instance one or more intensity modulators, one or more phase modulators, one or more Mach- Zehnder modulators or IQ modulators. Advantageous implementations comprise the use of a single IQ modulator, and also the cascaded use of intensity modulators and / or IQ modulators in combination with phase modulators. The modulation signals may be generated digitally (for example by an FPGA and / or DDS) or by analogue radiofrequency sources. Frequency multipliers may also be used to generate the modulation signals.

[0105] The frequency spacing frepof the generated frequency comb 137 may be set freely and continuously by varying the modulation period of the modulator 149 within the available modulation bandwidth. In order to be able to follow the resonant frequencies RFqover large ranges of different resonator lengths, the frequency offset Af is also able to be set flexibly. In this case, the modulator 149 is configured, according to one embodiment, such that the frequency offset Af is able to be generated by a phase jump induced regularly in synchronicity with the modulation period. For example, the phase jump may thus occur at each pulse generated by the modulator 149, so as to give rise to 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 in particular Figure 1 of the publication). In the time domain, the frequency comb 137 corresponds to a periodically modulated electric field, wherein the repetition rate corresponds to the modulation period. An offset frequency not equal to zero corresponds to a discrete phase jump that occurs with each repetition period (cf. Figure 1 c of the publication). The frequency at which the resulting phase jump, which increases over time, passes through the interval of 2TT corresponds to the frequency offset Af. The increasing phase jump may be folded into an interval of 2TT, meaning that arbitrary frequency offsets may be generated even with the finite modulation depth of a modulator. The periodicity of the frequency offset in relation to the repetition rate is achieved in the time domain by aliasing the discrete phase jumps (cf. Figure 1 c of the publication).

[0107] Fig. 4 illustrates a second variant embodiment 148b of the frequency comb generator 148 according to Fig. 2. This frequency comb generator 148a comprises a pulsed mode-coupled laser 151 for generating the frequency comb 137, which may be designed for instance as a femtosecond laser. To set the frequency offset Af of the reference frequency fR, the reference frequency fR is transmitted, in the form of a radio signal 146b, to the mode-coupled laser 151 by the reference transmitter 144, which is configured here as a radiofrequency generator 144b. In the mode-coupled laser 52, the frequency spacing frepof the generated frequency comb 137 may be manipulated for instance by changing the optical path length of the laser oscillator. To manipulate the frequency offset Af of the frequency comb, the temperature of the gain medium in the laser oscillator may be changed. The control signal 56 comprising the specifications Zlf*and fr*epfor the frequency offset and the frequency spacing may for example comprise a digitally transmitted specification, for instance in the form of one or more digital values, an analogue signal, for instance in the form of one or more radio frequencies transmitted in analogue form, or else an analogue control signal for an actuator that is designed to influence a comb parameter. The control signal 56 may also provide the specifications f* and fr*epthrough a specification for a combination of the frequency offset Af and the frequency spacing frep to the frequency comb generator 148b.

[0108] Fig. 6 shows a simplified illustration of the abovementioned photolithographic projection exposure apparatus 200 having the mirror 226, which is used for example as a component for the measurement carried out by the measurement apparatus 10 according to Fig. 1 and Fig. 2. Other mirrors of the projection exposure apparatus may furthermore also be measured using the measurement apparatus 10. The mirror 226 is mounted on a support structure, not illustrated in the drawings, for instance in the form of a reference frame or a housing of the projection lens 216 of the projection exposure apparatus 200. The projection exposure apparatus 200 according to Fig. 6 is designed to operate with EUV exposure radiation 201. In this text, EUV radiation should be understood to mean electromagnetic radiation at a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. However, the present invention is not limited to application in such an apparatus, but may also be implemented when measuring projection exposure apparatuses with different operating wavelengths, for example operating wavelengths in the VUV or DUV range. In further applications, the invention may also be implemented in a different photolithographic optical system, for instance a mask inspection apparatus or a wafer inspection apparatus. The projection exposure apparatus 200 has an exposure beam path 217 in which the exposure radiation 201 is guided through the illumination optical unit 205 and the projection lens 216.

[0109] According to the exemplary embodiment of Fig. 6, the illumination optical unit 205 comprises a field facet mirror 202, a pupil facet mirror 204 and two telescopic 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 initially steered onto the field facet mirror 202 and, from there, onto the pupil facet mirror 204. The first telescopic mirror 210 and the second telescopic mirror 212 are arranged downstream of the pupil facet mirror 204 in the beam path. Arranged downstream in the beam path is a deflection mirror 214, which steers the radiation incident thereon onto an object field in the object plane of the projection lens 216, which comprises six mirrors 218, 220, 222, 224, 226 and 228.

[0110] A reflective structure-bearing mask 230 on a mask stage 232 is arranged at the location of the object carrier, said mask being imaged by way of the projection lens 216 into an image plane, in which a substrate 234 in the form of a wafer, coated with a radiation-sensitive layer (photoresist), is located on a wafer stage The above description of exemplary embodiments, embodiments or variant embodiments should be understood to be by way of example. The disclosure effected thereby firstly enables a person skilled in the art to understand the present invention and the advantages associated therewith, and secondly encompasses alterations and modifications of the described structures and methods that are also obvious in the understanding of a person skilled in the art. Therefore, all such alterations and modifications, insofar as they fall within the scope of the invention in accordance with the definition in the accompanying claims, and equivalents are intended to be covered by the protection of the claims.

[0111] List of reference signs

[0112] 10 Measurement apparatus

[0113] 12 Measuring head

[0114] 14 Measurement target

[0115] 15 Position z(t) of the measurement target

[0116] 16 Optical resonator

[0117] 17 Resonator frequency RFq

[0118] 18 Resonator cavity

[0119] 19 Movement direction of the resonator frequencies

[0120] 20 Input coupling mirror

[0121] 21 Fixed point frequency of the resonator

[0122] 22 Measurement radiation

[0123] 22e Incoming measurement radiation

[0124] 22r Reflected measurement radiation

[0125] 24 Radiation generator

[0126] 26 Optical fibre

[0127] 28 Input coupling lens

[0128] 30 Circulator

[0129] 32 Coupling module

[0130] 33 Frequency space representation

[0131] 34 Generator spectrum

[0132] 36 Discrete radiation frequencies of the measurement radiation

[0133] 37 Frequency comb structure

[0134] 38 Comb frequency

[0135] 39 Sideband frequencies

[0136] 40 Frequency offset

[0137] 40v Specification for the frequency offset

[0138] 41 Sideband frequency spacing

[0139] 42 Frequency spacing

[0140] 42v Specification for the frequency spacing

[0141] 43 Reference frequency 44 Reference radiation source

[0142] 46 Reference radiation

[0143] 48 Frequency shift module

[0144] 50 Radiation detector

[0145] 52 Detector signal

[0146] 54 Control device

[0147] 56 Control signal

[0148] 58 Evaluation device

[0149] 60 Frequency jump module

[0150] 62 RF oscillator

[0151] 64 Mixer

[0152] 66 Mixed signal

[0153] 68 Low-pass filter

[0154] 70 Error signal

[0155] 72 Controller

[0156] 74 Correction signal

[0157] 76 Converter

[0158] 78 First conversion unit

[0159] 80 Second conversion unit

[0160] 136 Discrete radiation frequencies of the measurement radiation

[0161] 137 Frequency comb

[0162] 144 Reference transmitter

[0163] 144a Reference radiation source

[0164] 144b Radiofrequency generator

[0165] 146a Reference radiation

[0166] 146b Radio signal

[0167] 148 Frequency comb generator

[0168] 148a First variant embodiment of the frequency comb generator

[0169] 148b Second variant embodiment of the frequency comb generator

[0170] 149 Electro-optical phase modulator

[0171] 151 Mode-coupled laser

[0172] 200 Photolithographic projection exposure apparatus 201 Exposure radiation

[0173] 202 Field facet mirror

[0174] 204 Pupil facet mirror

[0175] 205 Illumination optical unit 206 Plasma light source

[0176] 208 Collector mirror

[0177] 210 First telescopic mirror

[0178] 212 Second telescopic mirror

[0179] 214 Deflection mirror 216 Projection lens

[0180] 217 Exposure beam path

[0181] 218, 220, 222, 224, 228 Mirrors of the projection lens

[0182] 226 Mirror of the projection optical unit, used as component to be measured

[0183] 230 Mask 232 Mask stage

[0184] 234 Substrate

[0185] 236 Wafer stage

Claims

Claims1. A measurement apparatus (10) for determining the position of a component (226) in a system (200), comprising:- a radiation generator (24) that is configured to generate a measurement radiation (22e) with at least two discrete radiation frequencies (36; 136) that 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) between 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) that comprises resonator frequencies (17) and a measurement target (14) assigned to the component and is configured to carry out a distance measurement by irradiating the measurement target with the measurement radiation, and- a coupling module (32) that 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 measurement radiation, said radiation sample having been 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, said control signal comprising a specification (40v, 42v) for at least one of the comb parameters.

2. The measurement apparatus according to Claim 1 , wherein the specification included in the control signal contains a specification (40v) for the frequency offset.

3. The measurement apparatus according to Claim 1 or 2, wherein the specification generated from the control signal contains a specification (42v) for the frequency spacing.

4. The measurement apparatus according to Claim 1 , wherein the specification included in the control signal contains a specification for a combination of the frequency offset and the frequency spacing.

5. The measurement apparatus according to Claim 2, wherein the control device (54) is configured to ascertain the specification (40v) for the frequency offset from the ascertained frequency spacing (42v) taking into account a fixed point frequency (21 ) of the optical resonator.

6. The measurement apparatus 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) that is configured to generate the measurement radiation (22e) with the at least two discrete radiation frequencies from the reference radiation by carrying out frequency shifting.

7. The measurement apparatus according to Claim 6, in which the frequency shift module (48) is configured to generate the measurement radiation (22e) with the at least two discrete radiation frequencies by modulating the reference radiation (43).

8. The measuring apparatus according to any of the preceding claims, wherein only radiation frequencies (36) that lie in a limited generator spectrum (34) are able to be generated by the radiation generator (24) and the measurement apparatus furthermore comprises a frequency jump module (60) that is configured to replace the relevant radiation frequency (36) with another comb frequency (38) of the frequency comb structure (37) when one of the coupled radiation frequencies reaches a limit of the generator spectrum during the generation of the measurement radiation in the radiation generator.

9. The measurement apparatus according to one of Claims 1 to 5,in which the radiation generator (24) comprises a frequency comb generator (148) and is configured to generate a frequency comb (137) that comprises the measurement radiation with all comb frequencies (36) within a limited generator spectrum (34).

10. The measurement apparatus according to Claim 9, in which 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 . The measurement apparatus according to Claim 10, in which the frequency comb generator (148a) comprises at least one modulation unit (149) for periodically modulating the reference radiation (146).

12. The measurement apparatus according to Claim 11 , in which the at least one modulation unit comprises at least one phase modulator and / or at least one intensity modulator.

13. The measurement apparatus 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) that is configured such that a frequency offset (40) of the generated frequency comb (137) is able to be generated by way of a phase jump induced on a regular basis during the phase modulation.

14. The measurement apparatus according to Claim 9, in which the radiation generator (24) comprises a reference generator (144b) for generating a reference frequency in the form of a radio frequency and the frequency comb generator (148a) has a pulsed mode-coupled laser (151 ) for generating the frequency comb with a frequency offset from the reference frequency.

15. The measurement apparatus according to one of Claims 9 to 14, in which the coupling module (32) is configured to couple the comb frequencies (136) generated by the radiation generator (24) to a respective one of the resonator frequencies (38), wherein the coupling of a comb frequency that newly appears in the generator spectrum due to a shift of the resonator frequencies takes place as a result of the pre-existing coupling of the other comb frequencies.

16. The measurement apparatus according to one of the preceding claims, in which the control device (54) is configured to ascertain the control signal (56) from a detector signal (52) representing the detected radiation sample (22r) without separating signal components that stem from the individual discrete radiation frequencies (36; 136).

17. The measurement apparatus according to one of the preceding claims, in which the control device is configured to generate the control signal (56) using a phase modulation signal (41 ) so as to generate sideband frequencies (39) of the discrete radiation frequencies by way of the Pound-Drever-Hall method, wherein the same phase modulation signal (41 ) is used for the at least two discrete radiation frequencies.

18. The measurement apparatus according to one of the preceding claims, in which the radiation generator (24) is furthermore configured to generate, in addition to the at least two discrete radiation frequencies (36; 136), in each case two sideband frequencies (39) with a uniform frequency spacing (41 ) from the relevant radiation frequency in the measurement radiation (22e).

19. The measurement apparatus according to one of the preceding claims, in which the system is a photolithographic optical system.

20. A photolithographic projection exposure apparatus (200) having at least one component (226) and at least one measurement apparatus (10) according to one of the preceding claims for determining the position of the component.

21. An illumination optical unit (205) of a photolithographic projection exposure apparatus (200) having at least one component and at least one measurement apparatus (10) according to one of Claims 1 to 19 for determining the position of the component.

22. A projection lens (216) of a photolithographic projection exposure apparatus (200) having at least one component and at least one measurement apparatus (10) according to one of Claims 1 to 19 for determining the position of the component.

23. An inspection apparatus for inspecting a surface of a substrate, in particular a mask or a wafer, having at least one component and at least one measurement apparatus (10) according to one of Claims 1 to 18 for determining the position of the component.

24. A coordinate measuring machine having at least one component and at least one measurement apparatus (10) according to one of Claims 1 to 18 for determining the position of the component.

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