MEASURING THE TRANSMISSION FUNCTION OF AN OPTICAL SYSTEM USING AN INTERFEROMETER

Abstract

Method (V1, V2) for measuring a transfer function of an optical system (101) comprising a positioning drive (105) and an optical element (104), namely in particular an optical element (101) of a lithography system (1), by means of an interferometer device (102) which is configured to generate two interference signals that are phase-shifted by 90° to each other, wherein the interferometer device (102) is in particular a full reflection interferometer device or a diffraction grating interferometer device, comprising: exciting (S10) at least one oscillation (115) of the optical system (101), changing (S11) a nominal distance (117) between the optical element (104) and the interferometer device (102), measuring (S12) a profile of the actual distance (117) between the optical element (104) and the interferometer device (104) at different Nominal distances (117) during at least one oscillation (115),and determining (S13) the transfer function based on the measured course of the actual distance (117).,

Description

[0001] The present invention relates to a method for measuring the transfer function of an optical system comprising a positioning drive and an optical element, in particular an optical element of a lithography system, by means of an interference device configured to generate two interference signals that are phase-shifted by 90°. The present invention also relates to a computer program product. The present invention further relates to a measuring device for measuring the transfer function of the optical system by means of the interference device. The present invention further relates to a circuit comprising the measuring device. Finally, the present invention relates to a lithography system with such a circuit, a projection exposure system with such a circuit, an inspection system with such a circuit, and a coordinate measuring machine with such a circuit.

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits. The microlithography process is carried out using a lithography system, which includes an illumination system and a projection system. The image of a mask (reticule) illuminated by the illumination system is projected by the projection system onto a substrate, such as a silicon wafer, coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection system. This transfers the mask structure onto the photosensitive coating of the substrate.

[0003] Driven by the pursuit of ever smaller structures in the fabrication of integrated circuits, EUV lithography systems are currently being developed that use light with a wavelength in the range of 0.1 nm to 30 nm, particularly 13.5 nm. Since most materials absorb light of this wavelength, such EUV lithography systems must use reflective optics, i.e., mirrors, instead of the refracting optics, i.e., lenses, used previously.

[0004] Interference measurement, such as with a Michelson interferometer, is a common method for measuring a change in distance. However, to measure a distance, one must be able to determine not only the magnitude but also the direction of the change. This can be achieved by generating two interference signals that are 90° out of phase with each other, for example, using a beam splitter, a partially reflected mirror, and so on.

[0005] When high accuracy is required, cyclic errors are observed. Cyclic errors are nonlinearities along the measurement axis that repeat at half the wavelength of the interferometer's light. This means that one or both interference signals repeat a measurement error after each period. Cyclic errors are generally caused by nonlinear effects. These cyclic errors distort the result of the position or distance measurement using the respective interferometer.

[0006] The problem is known, for example from HU Pengcheng, ZHU Jinghao, GUO Xuanbiao and TAN Jiubin, “Compensation for the Variable cyclic Error in Homodyne Laser Interferometers”, “Sensors (Basel)”, January 30, 2015, DOI: 10.3390 / s150203090, NLM PMID 25647739. A solution presented therein is a method for compensating variable cyclic errors in a homodyne laser interferometer, which is based on arithmetic calculations with trigonometric functions.

[0007] US 7,616,322 B2 is also a method for compensating for cyclic errors in interferometry data. It calculates an error function from a large number of old distance measurements, which is then used to correct current measurements.

[0008] One measurement result of particular interest is the transfer function of an optical system. Here, the optical system refers to an optical element that is positioned by a positioning drive, for example, held in a specific location or moved according to a predefined pattern. The optical system is therefore a mechatronic system. Measuring the transfer function can be performed once or multiple times, for example, for quality assurance or diagnostic purposes. A cyclic error in the interference measurement would lead to a distorted and / or inaccurately determined transfer function.

[0009] Against this background, one object of the present invention is to provide a means for measuring a transfer function of such an optical element with reduced cyclic errors.

[0010] Accordingly, a method for measuring a transfer function of an optical system using an interferometer device is proposed. The optical system has a positioning drive and an optical element. The optical element is, in particular, an optical element of a lithography system. The interferometer device is configured to generate two interference signals that are 90° out of phase with each other, wherein the interferometer device is, in particular, a full reflection interferometer device or a diffraction grating interferometer device.The proposed method comprises the following steps: exciting at least one oscillation of the optical system; changing a nominal distance between the optical element and the interferometer device; measuring the profile of an actual distance between the optical element and the interferometer device at different nominal distances during the at least one oscillation; and determining the transfer function based on the measured profile of the actual distance.

[0011] The proposed method thus exploits the fact that, in the general case, determining a transfer function depends not on the absolute distance between the interferometer and the optical element, but rather on its change in response to the excitation. Therefore, the nominal distance can be varied so that the cyclic errors cancel each other out. This variation of the nominal distance can be determined in advance or with minimal effort during the measurement. Furthermore, only the simplest post-processing is required. Consequently, the transfer function can be determined with minimal computational effort, resulting in a minimal influence of the cyclic error. The proposed method thus yields a qualitatively superior result to known methods with minimal post-processing effort.

[0012] A transfer function, as used here, is understood to be the gain curve as a function of the frequency of a free oscillation of the optical element. The transfer function is typically derived as a sequence of discrete values. It is particularly influenced by the mechanical properties of the optical element and its coupling to the positioning device. Other influencing factors may include, for example, a cooling system coupled to the optical element and / or another mechanically coupled device.

[0013] A full-reflection interferometer is preferably understood to be an interferometer in which the light beam is completely reflected by the object being measured, in this case, the optical element. In technical language, a full-reflection interferometer is often simply referred to as an "interferometer." A diffraction grating interferometer is preferably understood to be an interferometer in which the light beam is diffracted by means of a grating at the object being measured and thus partially reflected. In technical language, a diffraction grating interferometer is often referred to as an "encoder." These types of interferometers are described in more detail, for example, in DE 102024104846 A1. If the lithography system is an EUV lithography system, a full-reflection interferometer is preferably used.If the lithography system is a DUV lithography system, a diffraction grating interferometer device is preferably used.

[0014] The term "distance" here refers specifically to the position of the optical system, particularly the optical element, along the propagation direction of interferometer radiation. This distance may refer to a reflector and / or a diffraction grating attached to the optical element. A nominal distance can be understood as a predetermined distance or a nominal value. An actual distance can be understood as a real or measured distance or an actual value of the distance.

[0015] A projection exposure system is preferably a lithography system and even more preferably an EUV lithography system or a DUV lithography system.

[0016] Excitation is preferably understood to mean the application of a force to the optical element and / or a forced movement, including deformation, of the optical element. Excitation can, for example, be an abrupt application or termination of a force. The excitation is generated, for example, as a control signal or drive signal and output to the positioning device.

[0017] Vibration refers specifically to the free oscillation of the optical element or its mode shape. Free oscillation typically results from a force impulse or an abrupt cessation of force applied to the optical element by the positioning device. "Force applied" in this context means, in particular, actuating the positioning device to exert a force in addition to the weight of the optical element. Vibration can also be understood as a mechanical response of the optical system to excitation or mechanical excitation. Finally, vibration can be understood as a forced oscillation of the optical element during continuous excitation.

[0018] Changing the nominal distance between the optical element and the lithography apparatus preferably refers to a gradual change in the distance. For example, the positioning device is controlled to exert a force on the optical element. Preferably, the positioning device is controlled to exert a force on the optical element that is at least one order of magnitude smaller—and preferably at least two orders of magnitude smaller—than the amplitude of an internal force on the optical element caused by vibration (such as a tensile force, a compressive force, and / or a shear force).

[0019] Measuring the current distance during at least one oscillation can be understood as measuring over a multitude of oscillation periods. The measurement may begin during excitation. It may begin immediately after excitation. It may begin after a settling-in period. It may last for a specific duration, such as 45 to 90 seconds, particularly 50 to 60 seconds. It may continue until a certain signal-to-noise ratio is reached. It may continue until a certain decay of the maximum amplitude per period relative to the maximum amplitude of an initial amplitude occurs.

[0020] The combined waveform of the two interference signals, which are 90° out of phase, describes the distance between the optical element and the interferometer device. Measuring the actual distance therefore refers specifically to recording and evaluating this combined waveform and / or a derived quantity, such as the distance itself. The actual distance is measured, in particular, after the free oscillation has settled.

[0021] There is the so-called Lissajous figure, which forms a circular, or at least nearly closed, line. The Lissajous figure is constructed by interpreting the two interference signals as coordinates of a two-dimensional coordinate system and plotting points, symbols, and / or points on a line at these coordinate pairs. The cyclic error(s) manifest in the Lissajous figure as a deviation from a circle. Reference will be made to the Lissajous figure repeatedly in the following discussion.

[0022] An exemplary Lissajous- Fig. L is in Fig. Figure 3 shows this. In this example, a curve of measured values ​​is shown in gray, and for easy comparison, an ideal circle K is shown in black. It is noticeable that the curve L of the measured values ​​deviates radially from the ideal circle K, namely upwards and to the right (arrow R). A) and becomes wider at the bottom left and narrower towards the top left and bottom right than the ideal circle K. This is an exemplary representation of a second-order cyclic error. The Fig. Example 3, used as an illustrative example, does not show a cyclic error of order 1 or higher than order 2. In the Fig. Figure 3 also shows three black areas M which obscure the measured values ​​shown in gray; these black areas M are part of a proposed solution option of the invention and are described in detail below.

[0023] Determining the transfer function involves, in particular, performing a Fourier transform on the current distance. For example, the FFT algorithm can be used. Preferably, the measurement process is tailored to the determination process, for example, with regard to the number and sampling frequency of the individual measurements (test points).

[0024] It is possible that a wavelength (λ L The wavelength of an interferometer radiation is selected such that one period of one of the two interference signals corresponds to a nominal distance that is at least four times, and preferably at least ten times, the maximum amplitude of the oscillation, particularly after a transient phase. The period of the signal is understood here to be a complete cycle of the interference, for example, from a minimum interference amplitude (e.g., light signal intensity) through a maximum interference amplitude to the next minimum interference amplitude, during linear motion of the optical element. By selecting the wavelength such that the amplitude of the optical element's oscillation is much smaller than the change in the nominal distance corresponding to one interference period, the oscillation can be easily and clearly detected in the measurement signal, which is advantageous.

[0025] Excitation is preferably understood to mean the application of a force to the optical element and / or a forced movement, including deformation, of the optical element. Excitation can, for example, be an abrupt application or termination of a force. The excitation is generated, for example, as a control signal or drive signal to the positioning device.

[0026] For example, synthetic noise can also be used as an excitation signal. Synthetic noise is a sequence of force signals from the positioning device acting on the optical element, where the force signals are distributed in a frequency band, in particular uniformly or nearly, or as uniformly as technically possible. In synthetic noise, the amplitudes of the force signals, and especially the resulting amplitudes, are approximately equal. One can say that synthetic noise is characterized by both its frequency band and its amplitude.

[0027] The frequency band ranges, for example, from half the natural frequency of the optical element to more than twice its natural frequency. An upper cutoff frequency of the synthetic noise preferably corresponds to a sampling frequency of the interferometer device.

[0028] For example, the synthetic noise can be distributed in a frequency range from 1 kHz to 10 kHz. The amplitude can be scaled, for example, to obtain a constant power spectral density over this frequency range. The amplitude can also be scaled, for example, such that a superimposed signal in the time domain has a (total) amplitude of up to 5 N, preferably 1-2 N.

[0029] The nominal distance can be changed or varied by any amount. However, the nominal distance is preferably changed by up to one period of one of the two interference signals. In other words, the nominal distance is preferably changed by an amount or distance that corresponds to one period of one of the two interference signals. The periods of both interference signals are of equal length. The term "period of one of the two interference signals" is used here simply to provide a clear definition.

[0030] The period of the interference signal corresponds to half the wavelength of the interferometer radiation. In other words, the nominal distance is reduced by up to half a wavelength λ. L the interferometer radiation is modified. For example, if a light with a wavelength of 640 nm is used as the radiation source of the interferometer device, the period of the interference signal is 320 nm.

[0031] It may be that the nominal distance for determining the transfer function at an operating point x A of the optical system at the operating point x A enclosing region with a width of half a wavelength λ L / 2 of an interferometer radiation is changed, whereby the operating point x A preferably symmetrical in the middle of the area x A - λ L / 4 ... x A + λ L / 4. As will be shown below, this area has the advantage that measuring points can be selected so that cyclic errors are completely eliminated.

[0032] According to a preferred embodiment of the invention, the actual distance is measured at several discrete nominal distances. Instead of "discrete nominal distances," one can say "different nominal distances" or "distinguishable nominal distances." By setting different nominal distances that are not exactly one period apart, cyclic errors can be avoided, or rather, measurements are taken with different cyclic errors. Thus, the change in the cyclic error during measurement at different positions can be used to reduce the cyclic error to zero on average.

[0033] Preferably, the discrete / different / distinguishable nominal spacings are uniformly distributed within the period of one of the two interference signals. The inventors have found that with nominal spacings uniformly distributed within the period, the effects of cyclic errors become particularly noticeable, identifiable, and reducible or even eliminated.

[0034] The discrete / different / distinguishable nominal distances are preferably evenly distributed within the Lissajous figure. Thus, preferably not only one period is considered, but the nominal distances are also evenly distributed, taking into account assumed nominal distances in the adjacent periods at the same positions within those periods.

[0035] Preferably, a first transfer function is determined for each of the discrete nominal spacings, and a second transfer function is determined as the frequency-dependent average of the first transfer functions. In other words, the gain at a given frequency of the second transfer function is calculated as the average of the gains at that frequency of the first transfer functions. This calculation is particularly easy to implement and can be performed quickly and efficiently. Furthermore, this calculation is easily understood by a person, for example, graphically.

[0036] It is possible for the nominal distance to be continuously changed during an oscillation, and the actual distance to be continuously measured during this time. This represents a preferred embodiment of the invention. The continuous changing of the nominal distance can be described as a continuous repositioning or movement of the optical element. Since the process takes place during an oscillation, it is particularly time-saving.

[0037] It may be that where a distance between the nearest nominal distance and the furthest nominal distance equals half a wavelength λ Lof the interferometer radiation. The nearest nominal distance is the nominal distance closest to the interferometer device along a beam path of the interferometer. The furthest nominal distance is the nominal distance furthest from the interferometer device along the beam path. One can also say that, with continuous changes to the nominal distance, the two extreme nominal distances are separated by a distance equal to half the wavelength of the interferometer radiation. In this way, cyclic errors up to any order can be canceled out particularly efficiently on average within the measurement.

[0038] The nominal intervals at which the course of the actual distance is recorded are preferably chosen so that the requirement or equation 1M∑m=1M∑i=1NAi sin(2π2λLixm+φi)=0 is fulfilled. This means: M is the number of nominal distances, N is a maximum order up to which cyclic errors occur in the interferometer device, A i an amplitude of an i-th order cyclic error, λ L the wavelength of the interferometer radiation, x m the m-th nominal distance, and φ i a phase of the i-th order cyclic error.

[0039] If the nominal distances x m If the above requirement or equation is fulfilled, the cyclic errors at the selected nominal distances will cancel each other out.

[0040] Furthermore, a computer program product is proposed which contains instructions that, when executed by a computer, cause it to execute the procedure described above according to one of the variants described herein. The computer program product thus possesses the process steps, features, and advantages of the executable procedure.

[0041] A computer program product, such as a computer program tool, can be provided or delivered from a server on a network, for example, as a storage medium such as a memory card, USB stick, CD-ROM, DVD, or as a downloadable file. This can be done, for example, in a wireless communication network by transmitting the corresponding file containing the computer program product or tool.

[0042] Furthermore, a measuring device is proposed which is configured to measure a transfer function of an optical system comprising a positioning drive and an optical element, namely, in particular, an optical element of or for a lithography system, by means of an interferometer device configured to generate two interference signals that are phase-shifted by 90° to each other, wherein the interferometer device is, in particular, a full reflection interferometer device or a diffraction grating interferometer device. The proposed measuring device is configured to perform the steps of the proposed method, including each variant described herein. Thus, the measuring device has the features and advantages of the executable method.

[0043] According to a preferred embodiment, the proposed measuring device comprises a control unit, a detection unit, and a processing unit. The control unit is configured to control the positioning drive. The detection unit is configured to detect the actual distance between an interferometer device and the optical element using the interferometer device, wherein the detection unit is part of the interferometer device or can be coupled to it.The processing unit is configured to perform the following steps: exciting at least one vibration of the mechatronic system by means of the control unit, changing a nominal distance between the optical element and the interferometer device during the vibrations by means of the control unit, measuring a progression of the actual distance between the optical element and the interferometer device at different nominal distances during the vibrations by means of the detection unit, and determining the transfer function based on the measured actual distances.

[0044] According to a further aspect of the invention, a circuit is proposed which includes: the proposed measuring device, the optical element, the interferometer device, and the positioning drive. The detection unit is part of the interferometer device or is coupled to it. The interferometer device is arranged to detect the actual distance between the interferometer device and the optical element. The positioning drive is coupled to the optical element and the control unit for positioning the optical element. A coupling can be understood, in particular, as an operative connection, such as a mutually influencing or unilaterally influencing connection. The proposed circuit has the features and advantages of the included measuring device.

[0045] According to another aspect of the invention, a lithography system is proposed, specifically an EUV lithography system. EUV stands for "Extreme Ultraviolet" and denotes a wavelength of the working light between 0.1 nm and 30 nm. The lithography system can also be a DUV lithography system. DUV stands for "Deep Ultraviolet" and denotes a wavelength of the working light between 30 nm and 250 nm. The proposed lithography system has at least the proposed optical system. Therefore, the lithography system also has the features and advantages of the optical system. Thus, the proposed lithography system can be used for the lithographic production of particularly fine structures.

[0046] According to another aspect of the invention, a projection exposure system is proposed which incorporates the proposed circuit. Therefore, the projection exposure system also possesses the features and advantages of the circuit. Thus, the projection exposure system can project particularly fine structures.

[0047] According to another aspect of the invention, an inspection system for inspecting the shape, position, and / or geometry of an optical element is proposed. The proposed inspection system is specifically designed for inspecting or verifying an optical element, a wafer stage, or a mask. The proposed inspection system incorporates the proposed circuitry and thus also possesses its features and advantages. Therefore, the inspection system can inspect particularly fine structures or perform particularly high-resolution inspections. An example of such an inspection system is shown in DE 102012220518A1.

[0048] According to another aspect of the invention, a coordinate measuring machine is proposed which incorporates the proposed circuitry. Therefore, the coordinate measuring machine is particularly suitable for the precise measurement of fine structures. Preferably, the measuring machine is specifically designed for use in manufacturing technology or industrial metrology in mechanical engineering, for example, in the automotive or aerospace industries. An example of such a coordinate measuring machine is shown in German patent application DE10 2019 213 794A1.

[0049] The term "one" here is not necessarily to be understood as restricting the number to exactly one element. Rather, it can also refer to multiple elements, such as two, three, or more. Similarly, every other counter used here should not be interpreted as restricting the number to the exact number stated. Instead, numerical deviations, both higher and lower, are possible unless otherwise specified.

[0050] Other possible implementations of the invention also include combinations of features or embodiments described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In such cases, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention.

[0051] Further advantageous embodiments and aspects of the invention are the subject of the dependent claims and the exemplary embodiments of the invention described below. The invention will be explained in more detail below with reference to preferred embodiments and the accompanying figures. Fig. Figure 1 shows a schematic meridional section of a projection exposure system for EUV projection lithography; Fig. Figure 2 shows a circuit containing an optical system having an optical element and a positioning drive, an interferometer device and a measuring device for measuring a transfer function of the optical system using the interferometer device; Fig. Figure 3 shows a Lissajous figure with a second-order cyclic error and discrete nominal distances; Fig. Figure 4 shows an exemplary transfer function of an optical system, comparing a recording noisy due to cyclic errors with a recording made using the proposed method; Fig. Figure 5 shows a flowchart of a method for measuring a transfer function of an optical system using an interferometer device according to an embodiment of the invention; and Fig. Figure 6 shows a flowchart of a procedure for measuring a transfer function of an optical system using an interferometer device according to a further embodiment of the invention.

[0052] In the figures, identical or functionally equivalent elements have been labelled with the same reference symbols, unless otherwise indicated. Furthermore, it should be noted that the representations in the figures are not necessarily to scale.

[0053] Fig. Figure 1 shows an embodiment of a projection exposure system 1 (in particular a lithography system), especially an EUV lithography system. One embodiment of the illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optic 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system 2. In this case, the illumination system 2 does not include the light source 3.

[0054] A reticule 7 arranged in the object field 5 is exposed. The reticule 7 is held by a reticule holder 8. The reticule holder 8 can be moved, particularly in a scanning direction, via a reticule displacement drive 9.

[0055] The projection exposure system 1 comprises a projection optic 10. The projection optic 10 serves to image the object field 5 onto an image field 11 in an image plane 12. The image plane 12 is parallel to the object plane 6. Alternatively, an angle other than 0° between the object plane 6 and the image plane 12 is also possible.

[0056] A structure on the reticulum 7 is imaged onto a photosensitive layer of a wafer 13 located in the image plane 12 within the image field 11. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved, particularly along the y-direction y, via a wafer transfer drive 15. The movement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized.

[0057] Light source 3 is an EUV radiation source. Light source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illuminating radiation, or illuminating light. The useful radiation 16 has, in particular, a wavelength in the range between 5 nm and 30 nm.

[0058] The illumination radiation 16 emanating from the light source 3 is focused by a collector 17. After the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The illumination optics 4 comprise a deflecting mirror 19 and, downstream of this in the beam path, a first faceted mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with an effect that influences the beam beyond the mere deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter. The first faceted mirror 20 comprises a plurality of individual first facets 21. Downstream of the first faceted mirror 20 in the beam path of the illumination optics 4 is a second faceted mirror 22. The second faceted mirror 22 comprises a plurality of second facets 23. The illumination optics 4, in the embodiment described in the Fig. As shown in Figure 1, there are exactly three mirrors after the collector 17, namely the deflecting mirror 19, the first faceted mirror 20 and the second faceted mirror 22. In a further embodiment of the lighting optics 4, the deflecting mirror 19 can also be omitted, so that the lighting optics 4 can then have exactly two mirrors after the collector 17, namely the first faceted mirror 20 and the second faceted mirror 22.

[0059] The projection optics 10 comprise a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0060] In the Fig. In the example shown, the projection optics 10 comprise six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The projection optics 10 is a doubly obscured optic. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection optics 10 has an image-side numerical aperture that is greater than 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0061] The reflective surfaces of the mirrors Mi can be designed as freeform surfaces without an axis of rotational symmetry. Alternatively, the reflective surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflective surface shape. The mirrors Mi, like the mirrors of the illumination optics 4, can have highly reflective coatings for the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0062] As already mentioned, the Fig. 2 a transfer function measurement circuit 100, which is formed by an optical system 101, an interferometer device 102 and a measuring device 103.

[0063] The optical system 101 has an optical element 104 and a positioning drive 105. The optical element 104 can, for example, be one of the mirrors of the projection exposure system 1. The optical element 104 can, for example, be one of the mirrors M1 to M6. The positioning drive 105 can, for example, be a Lorenz actuator. The positioning drive 105 is coupled to the optical element 104 and an inertial system (not shown), such as a machine frame, in a manner suitable for positioning the optical element 104. Typically, each optical element 104 of the projection exposure system 1 is assigned a plurality of positioning drives 105, namely two to six or even more positioning drives 105. The positioning drives 105 of the optical element 104 are arranged and configured, both individually and in combination, to precisely define a position of the optical element 104.Preferably, the at least one positioning drive 105 defines a position of the optical element 104 in the six spatial rigid body degrees of freedom. The optical element 104 may be statically overdetermined, so that, for example, a deformation of the optical element 104 can be controlled.

[0064] Many interferometer designs are possible. An exemplary interferometer design of the full-reflection interferometer type is briefly described below. The interferometer device 102 has a radiation source 106. The radiation source 106 emits, for example, a laser beam with an interferometer wavelength of 640 nm. The laser beam is then split by means of an optical circuit 107, which contains, for example, partially transparent elements 108 and reflective elements 109. Part of the interferometer radiation is reflected by the optical element 104 and then superimposed with another part of the interferometer radiation, resulting in interference. Subsequently, two interference signals 110 are generated, which are phase-shifted by 90°, for example, by means of a partially transparent beam splitter 108.Each of the interference signals 110 is an optical signal that is detected by its own optical detector 111. These two optical detectors 111 each generate a data signal indicative of the phase of the respective interference signal. The two data signals are then transmitted to the measuring device 103 and evaluated there. The interferometer device is preferably a homodyne interferometer device.

[0065] The measuring device 103 is configured to measure a transfer function of the optical system 101 using the interferometer device 102. For this purpose, the measuring device 103 has, for example, a detection unit 112, a processing unit 113, and a control unit 114. The measuring device can also have a different configuration; for example, an interface unit (not shown), such as a bus interface or a network interface, can perform the functions of the detection unit 112 and the control unit 114. Furthermore, the functions of the processing unit 113 can be performed by the detection unit 112 and / or the control unit 114. The units can be physically separate units, and / or they can be implemented partially or entirely as distributed units and / or as integrated units. For example, the Fig. 2. Two detection units 112, each assigned to a detector 111, for example by wiring. The division into units 112 to 114 can be said to be a functional division.

[0066] The detection unit 112 has the function of detecting the actual distance between the interferometer device 102 and the optical element 104 by means of the interferometer device 102. In particular, the detection unit 112 reads the data from the detectors 111. Specifically, the detection unit 112 detects a time series of the actual distance.

[0067] The processing unit 113 has the function of determining the transfer function of the optical system based on the recorded course of the actual distances. The transfer function characterizes a mode shape of the mechatronic system comprising the positioning drive 105 or multiple positioning drives 105 and the optical element 104.

[0068] Optical element 104 can, for example, have a mirror body and a mirror surface. Optical element 104 can, for example, contain cooling channels and connecting lines. Optical element 104 can, for example, contain active elements, such as integrated piezo actuators.

[0069] The control unit 114 has the function of controlling the positioning drive 105. There may be several control units 114, each controlling one positioning drive 105 of the same optical element 104. It is also possible that the control unit 114 controls a single positioning drive 105 of the optical element 104.

[0070] Next, an embodiment of a proposed method V1 for measuring a transfer function of the optical system 101 is described using the flowchart of the Fig. 5, the Lissajou figure of the Fig. 3 and the transfer functions of the Fig. 4 described. The described method is carried out by the measuring device 103 using the interferometer device 102.

[0071] The distance between the interferometer device 102 and the optical element 104 is hereinafter always referred to as a distance 117, regardless of whether it is a nominal distance or an actual distance. A nominal distance is a distance that is specified or approached by the positioning drive. The nominal distance is preferably not controlled in such a way that vibration suppression takes place. An actual distance is a distance that actually exists. In this method, the actual distance results from the superposition of the nominal distance with the natural frequency of the optical system.

[0072] In a first step S10, a vibration 115 of the optical system 103, 104, 105 is excited. For this purpose, the positioning drive 105 is controlled by the control unit 114, so that the positioning drive 105 exerts a force 116 on the optical element 104.

[0073] A synthetic noise is used as the excitation signal, which in particular means: as the course of the force 116 exerted by the positioning drive 105 on the optical element 104.

[0074] In a next step S11, a nominal distance 117 between the optical element 104 and the interferometer device 102 is changed during the oscillation 115.

[0075] Then, in a next step S12, a course of an actual distance 117 between the optical element 104 and the interferometer device 102 is recorded at different nominal distances 117 during at least one oscillation, i.e. during the one excited oscillation or during the several excited oscillations.

[0076] It should be noted that the interferometer device 102 or the radiation source 106 of the same is chosen such that at the wavelength λ Lof the interferometer radiation a period length of one of the two interference signals corresponds to a nominal distance 117, which is at least 4 times and preferably at least 10 times a maximum amplitude of the oscillation 115.

[0077] In the first embodiment, the actual distance 117 is measured at several discrete nominal distances 117. At each of these discrete nominal distances 117, an oscillation 115 is first excited, and then the course of the actual distance 117 during this oscillation 115 is measured. Steps S11 and S12 are thus repeated several times.

[0078] The discrete nominal spacings 117 are chosen such that they are evenly distributed within the period of one of the two interference signals.

[0079] In the Fig. Figure 3 is the Lissajous figure with additional black sections S. The width of each black section S represents the amplitude of the actual distance 117. The nominal distances 117 are each located angularly approximately in the middle of the black sections S. Fig. Figure 3 is for illustrative purposes only and is not to scale.

[0080] Preferably, as in the case of the illustrated embodiment, the nominal distances 117 are chosen such that the equation 1M∑m=1M∑i=1NAi sin(2π2λixm+φi)=0 is fulfilled.

[0081] Here, "M" represents the number of nominal intervals 117 that are evenly distributed within the period, taking into account assumed adjacent periods, or within the Lissajous figure. In the example of the Fig. 3 has the value "M" 3.

[0082] In the equation above, "N" represents the maximum order up to which cyclic errors occur in the interferometer device. In the example of the Fig. 3 are each two areas of the Lissajou- Fig. To identify L outside circle K and two areas inside circle K, it is therefore advisable in this example to choose the number M of nominal distances 117 in order to compensate for cyclic errors up to the 2nd order in the measured profiles of the actual distance 177.

[0083] In the equation above, “A” means i “An amplitude of an i-th order cyclic error. This amplitude A i is a difference in radius R K of the assumed circle K to the maximum radius R A the Lissajous figure. If several cyclic errors of different orders overlap, the amplitude must be determined separately for each order i.

[0084] In the equation above, λ means L the wavelength of the interferometer radiation.

[0085] In the equation above, “x” means m“the distance or the value of the m-th nominal distance 117. In the Lissajous- Fig. L corresponds to this distance as the angle between two black areas S.

[0086] In the equation above, “φ” means i “A phase of the i-th order cyclic error. In other words, it signifies an offset.”

[0087] The above equation can be derived as follows:

[0088] An averaged dynamic behavior G̅(jω) over all measurement positions m□ M or over all nominal distances 117 can be formulated as follows: G¯(jω)=1M∑m=1MGm(jω)

[0089] In this context, a G means m (jω) Transfer function, which maps an actuating force 116 to a distance 117. If the actuating force is denoted as F and the distance (actually the position) as X, then the transfer function G can be m (jω) can be calculated as follows: F(jω)=Gm(jω)X(jω) Gm(jω)=X(jω)F(jω)

[0090] The measured distance 117, here denoted as X, contains both a real movement X real as well as the cyclic errors C E : X=Xreal+CE

[0091] The cyclic error C E This can be formulated as: CE(x)=∑i=1NAi sin(2π2λix+φi) where A i the amplitude and φ i denote the phase of the cyclic error of the i-th order, λ denotes the wavelength of the interferometer radiation (e.g. 640 nm), and N is the maximum order considered.

[0092] These equations can be combined and transformed into: CE¯=1M∑m=1M∑i=1NAi sin(2π2λix+φi)

[0093] Thus, by setting the average cyclic error to zero CE¯ The above demand is made.

[0094] An exemplary solution to this requirement is xm□[−λL6,0,λL6] for any N, A i and φ. This solution is in the Fig. Figure 3 shows this solution. In the angular representation of the Lissajous figure, this solution corresponds to three measuring positions or nominal distances 117 of the optical element 104 to the interferometer device 102, each shifted by 120°.

[0095] Then, in step S13, a transfer function is determined based on the measured actual distances, whereby a first transfer function G1 to G3 is determined for each of the three discrete nominal distances 117, and a second transfer function G m is determined as the mean of the first transfer functions G1 to G3. The second transfer function G m is the averaged transfer function G m It no longer contains the cyclic errors up to the selected order.

[0096] One can calculate the average transfer function G mpreferably determined according to the following multi-stage procedure: measuring the course of the actual distances 117 at each of the discrete / different / distinguishable nominal distances; determining a first transfer function G1, G2, G3 of the optical system 101 for each of the discrete / different / distinguishable nominal distances 117, wherein the first transfer functions G1, G2, G3 each specify a gain V for the same frequencies f; and determining an averaged transfer function G m of the optical system 101 by forming, for each frequency of the first transfer function(s) G1, G2, G3, an average value of the corresponding gains V of the first transfer functions G1, G2, G3 as the gain V of the averaged transfer function G mat this frequency f, no trigonometric or other calculations are necessary other than repeated averaging. Therefore, this proposed method can be implemented with particularly low effort. Furthermore, the cyclic error C can be reduced by choosing N orders. E not only approximately, but completely eliminated up to the chosen order N.

[0097] The focus is on the presentation of the Fig. Figure 4 refers to the values ​​determined by the inventor in a test. It is immediately noticeable that the three first transfer functions G1, G2, and G3, shown in gray, are significantly noisier than the averaged transfer function G, shown in black. m . In particular, the irregularity near the maximum gain is clearly evident in the averaged transfer function.

[0098] Fig. Figure 6 shows a flowchart of a procedure V2 for measuring a transfer function G.m of the optical system 101 by means of the interferometer device 102 according to a further embodiment of the invention.

[0099] Using the method according to the first embodiment, one can determine the averaged transfer function, which is approximately ten times smoother than any recorded first transfer function. This requires only three measurements and averaging for each frequency. Since each measurement takes approximately one minute, the entire transfer function can be obtained in under five minutes. The method is therefore quick to perform and provides a precise result.

[0100] In this embodiment, the transfer function G m of the optical system at a continuously changing measuring position or at continuously changing nominal distances 117.

[0101] This change occurs relatively slowly compared to the oscillation 115. For example, the optical element 104 oscillates with an amplitude of approximately 15 nm. Relative to the frequency of the maximum gain, which is approximately 300 Hz, this roughly translates to a velocity of over 14 µm / s. In contrast, during a measurement period of, for example, 60 seconds, the optical element is continuously moved by half the wavelength of the interferometer radiation, for example, 320 nm, resulting in a velocity of approximately 5 nm / s.

[0102] It is particularly advantageous if the change in the nominal distance 117 or the displacement during the continuous measurement is exactly half a wavelength λ. L This corresponds to the interferometer radiation. This case can be considered a special case. limM→∞1M∑m=1M∑i=1NAi sin(2π2λixm+φi)=0 the requirement described above should be considered.

[0103] It can also be said that the continuous change of the nominal distance 117 should correspond overall to a path length (along the interferometer radiation) of a cyclic error of the first order.

[0104] The velocity of the optical element 104 along the interferometer radiation should therefore be vM=λL2tM where v M the speed or nominal speed of the optical element 104 and t M The duration of the measurement is denoted by the duration t. M For example, a suitable low-excitation speed can be determined based on a single preliminary test. The duration t M For example, the usable time between the start of the vibration 115 as a result of an excitation and the decay of the vibration 115 below a limit value can be determined based on a preliminary test.

[0105] For example, in the case described here, it can be shown that the cyclic error CE¯ is balanced: CE¯=∫xM−λL4xM+λL4∑i=1NAi sin(2π2λix+φi) dx =∑i=1N−AiλL4πi cos(2π2λix+φi)|xM−λL4xM+λL4 =∑i=1N−AiλL4πi[cos(4πiλ(xM+λL4)+φi)−cos(4πiλ(xM−λL4)+φi)] =∑i=1N−AiλL4πi[cos(πi+φi)−cos(πi+φi)] =0

[0106] It will often be the case that the transfer functions of the optical system 101 at a position x M This is interesting because it represents an operating position of the optical element 104. Preferably, the nominal distance 117 is then in the range x M -λ L / 4 to x M +λ L / 4 changed. This advantage applies to all embodiments.

[0107] It is possible that several positioning drives 105 are provided for the six degrees of freedom of the optical element 104, each controlled by a control unit 114. It is also possible that the several positioning drives 105 are controlled by a common control unit 114.

[0108] Returning to the projection exposure system 1 described at the beginning, this system has, for example, 6 mirrors M1 to M6. If each of the 6 mirrors has an interferometer device 102 for each of the 6 spatial degrees of freedom, then a total of 36 interferometer devices are installed. The transfer functions of mirrors M1 to M6 can be determined simultaneously over time.

[0109] Although the present invention has been described using exemplary embodiments, it can be modified in many ways. REFERENCE MARK LIST 1 Projection exposure system 2 Lighting system 3 light source 4 Lighting optics 5 object field 6 Object level 7 reticles 8 label holders 9 Reticle displacement drive 10 Projection optics 11 Image field 12 Image plane 13 wafers 14 wafer holders 15 wafer transfer drive 16 Lighting radiation 17 Collector 18 Intermediate focus plane 19 deflecting mirrors 20 first faceted mirror 21 first facet 22 second faceted mirror 23 second facet 100 Transfer function measurement circuit 101 optical system 102 Interferometer device 103 Measuring device 104 optical element 105 Positioning drive 106 Radiation source 107 optical circuit 108 semi-permeable element 109 reflective element 110 Interference signal 111 optical detector 112 recording units 113 processing units 114 Control unit 115 vibration 116 force 117 Distance, nominal distance, actual distance M1 mirror M2 mirrors M3 mirror M4 mirrors M5 mirror M6 mirrors L Lissajou figure K Circle R K Circle radius R A amplitude radius Section S G1 first transfer function G2 first transfer function G3 first transfer function G m averaged or second transfer function V1, V2 procedure

Claims

[1] Method (V1, V2) for measuring a transfer function (G M ) of an optical system (101) comprising a positioning drive (105) and an optical element (104), namely in particular an optical element (104) of a lithography system (1), by means of an interferometer device (102) which is configured to generate two interference signals that are phase-shifted by 90° to each other, wherein the interferometer device (102) is in particular a full reflection interferometer device or a diffraction grating interferometer device, comprising: Excitation (S10) of at least one oscillation (115) of the optical system (101), Changing (S11) a nominal distance (117) between the optical element (104) and the interferometer device (102), Measuring (S12) a progression of an actual distance (117) between the optical element (104) and the interferometer device (102) at different nominal distances (117) during at least one oscillation (115), and Determine (S13) the transfer function based on the measured course of the actual distance (117). [2] Method according to claim 1, wherein a synthetic noise is used as an excitation signal. [3] Method according to one of the preceding claims, wherein the nominal distance (117) is increased by up to half a wavelength (λ) L ) the interferometer radiation is altered. [4] Method according to claim 3, wherein the nominal distance (117) for determining the transfer function at an operating point (x A ) of the optical system (101) is changed so that the operating point (x A ) symmetrical in the middle of the area (x A -λ L / 4 ... x A + λ L / 4) is located. [5] Method according to one of the preceding claims, wherein the actual distance (117) is measured at several discrete nominal distances (117). [6] Method according to claims 4 and 5, wherein the discrete nominal distances (117) are uniformly distributed within one period of one of the two interference signals. [7] Method according to one of claims 5 to 6, wherein at each of the discrete nominal distances (117) a vibration (115) is first excited and then the course of the actual distance (117) is measured during this vibration (115). [8] Method according to any one of claims 5 to 7, wherein a first transfer function (G1 to G3) is determined for each of the discrete nominal distances (117), and wherein a second transfer function (G m ) is determined as the means of the first transfer functions (G1 to G3). [9] Method according to any one of claims 1 to 4, wherein the nominal distance (117) is continuously changed during a vibration (115) and the actual distance (117) is continuously measured during this time. [10] Method according to claim 9, wherein a distance between the nearest nominal distance (117) and the furthest nominal distance (117) is equal to half a wavelength (λ) L ) of the interferometer radiation. [11] Method according to one of the preceding claims, wherein the nominal distances (117) at which the course of the actual distance (117) is recorded are selected such that the requirement 1M∑m=1M∑i=1NAi sin(2π2ALixm+φi)=0 is fulfilled, whereby: M the number of nominal distances (117), N e a maximum order up to which cyclic errors occur in the interferometer device (102), A i an amplitude of an i-th order cyclic error, λ Lthe wavelength (λ L ) the interferometer radiation, x m the m-th nominal distance (117), and φ i a phase of the i-th order cyclic error. [12] Computer program product which contains instructions which, when the program is executed by a computer device, cause it to execute the method (V1, V2) according to one of the preceding claims. [13] Measuring device (103) for measuring a transfer function (G m) of an optical system (101) comprising a positioning drive (105) and an optical element (104), namely in particular an optical element (104) of a lithography system (1), by means of an interferometer device (102) which is configured to generate two interference signals that are phase-shifted by 90° to each other, wherein the interferometer device (102) is in particular a full reflection interferometer device or a diffraction grating interferometer device, comprising: a control unit (114) which is set up to control the positioning drive (105), a detection unit (112) for detecting an actual distance (117) between the interferometer device (102) and the optical element (104), wherein the detection unit (112) is part of the interferometer device (102) or can be coupled to it, a processing unit (113) that is set up to perform the following steps: of an excitation (S10) of at least one oscillation (115) of the optical system (101) by means of the control unit (114), a change (S11) of a nominal distance (117) between the optical element (104) and the interferometer device (102) during the vibrations (115) by means of the control unit (114), a measurement (S12) of the course of the actual distance (117) between the optical element (104) and the interferometer device (102) at different nominal distances (117) during the vibrations (115) by means of the detection unit (112), and a determination (S13) of the transfer function (G m ) based on the measured actual distances (117). [14] Circuit (100) comprising: the measuring device (103) according to claim 13, the optical element (104), the interferometer device (102), wherein the detection unit (112) is part of or coupled to the interferometer device (102), and wherein the interferometer device (102) is arranged to detect the actual distance (117) to the optical element (104), wherein the interferometer device (102) is in particular a full reflection interferometer device or a diffraction grating interferometer device, and the positioning drive (105) which is coupled to the optical element (104) and the control unit (114) for positioning the optical element (104). [15] Lithography system (1), in particular EUV lithography system, which includes at least one circuit (100) according to claim 14. [16] Projection exposure system comprising at least one circuit (100) according to claim 14. [17] Inspection system comprising a circuit (100) according to claim 14, wherein the inspection system is configured to inspect a shape, position or geometry of the optical element (104). [18] Coordinate measuring machine comprising at least one circuit (100) according to claim 14.

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