WO2018007008A1

WO2018007008A1 - Measurement system for determining a wavefront aberration

Info

Publication number: WO2018007008A1
Application number: PCT/EP2017/000792
Authority: WO
Inventors: Ulrich Wegmann
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2016-07-08
Filing date: 2017-07-05
Publication date: 2018-01-11
Also published as: DE102016212464A1; TW201807389A

Abstract

The invention relates to a measurement system (10) for determining a wavefront aberration of an optical imaging system (12), comprising an irradiation device (24) for passing measurement radiation (26) through the imaging system (12), an analysis grating (30) which, disposed downstream of the imaging system (12), is arranged in the beam path (40) of the measurement radiation in a manner dis- placeable transversely to an optical axis (20) of the imaging system (12), and a detection device (32) for recording a radiation distribution of the measurement radiation (26). The measurement system (10) is configured to produce respective interferograms (62), formed by means of the analysis grating (30), at a plurality of displacement positions of the analysis grating (30) for the purposes of being reeorded on the detection device (32). Furthermore, the measurement system (10) is configured to ascertain at least one positional information item (78) of the analysis grating (30) in at least one of the displacement positions by means of a control beam path (46) that passes through the optical imaging system (12).

Description

Measurement system for determining a wavefront aberration

This application claims priority to the German Patent Application No. 10 2016 212 464.1 filed on July 8, 2016. The entire disclosure of this patent application is incorporated into the present application by reference.

Background of the invention The invention relates to a measurement system for determining a wavefront aberration of an optical imaging system. Furthermore, the invention relates to a micro- lithographic projection exposure apparatus comprising a projection lens for imaging mask structures onto a wafer, and a method for determining a wavefront aberration of an optical imaging system.

By way of example, shearing interferometry is used for very precisely measuring optical imaging systems, such as e.g. a microlithographic projection lens. The shearing interferometry is a phase-shifting interferometry technique. For the purposes of determining a wavefront aberration of an optical imaging system, a co- herence mask is, for example, arranged in the object plane and a phase-shifting structure, such as e.g. a displaceable diffraction grating, also referred to as analysis grating below, is arranged in the image plane. The analysis grating is displaced transversely to the optical axis of the imaging system in small steps. The spatial derivative of the wavefront in the movement direction of the analysis grat- ing and, therefrom, the topography of the wavefront and, ultimately, a wavefront aberration of the optical imaging system can be ascertained from the interference patterns or shearograms that were captured by the detector.

WO 01/63233 A2 describes different measurement systems that are based on shearing interferometry for determining a wavefront in optical systems. In addition to the use of perforated masks with different two-dimensional aperture patterns as a coherence mask in the object plane of an imaging optical system, measurement systems with a simultaneous production of, in each case, a measurement beam for different field points of the object plane are also presented. By way of example, a multiplicity of focusing lens elements are arranged in the beam path to this end, said focusing lens elements each focusing some of the measurement radiation onto one of several apertures of a perforated mask in the object plane. Using such a multi-channel measurement system, the optical system can be measured simultaneously in respect of its imaging properties for a multiplicity of field points.

A problem of the above-described measurement systems using a shearing- interferometric technique lies in the necessary highly precise positioning of the coherence mask and in carrying out the step-wise displacement of the analysis grating. Positions of the analysis grating must be approached and maintained to within an accuracy of a few nanometres during a measurement. If this condition is not satisfied, there are, as a consequence, errors when determining the phase and hence measurement errors when determining a wavefront. As a rule, there are high demands in respect of the rigidity and control accuracy of the measurement system in the prior art for reducing measurement errors in order to obtain a positioning that is as accurate as possible of the analysis grating during individual lateral displacement steps and of the coherence mask in relation to the analysis grating.

A further problem lies in brightness variations of the measurement radiation.

These also lead to measurement errors when determining the phase using a time- serial phase-displacement method. Brightness variations should ideally be taken into account with spatial resolution over a cross section of the beam path. However, very complicated output coupling and a spatially resolved brightness determination of the measurement radiation are often dispensed with and, instead, a time-dependent mean brightness value is determined at best, as a result of which further inaccuracies arise when determining a wavefront. Underlying object

It is an object of the invention to provide a system and a method which solve the aforementioned problems and, in particular, lead to a reduction of measurement errors when determining a wavefront aberration of an optical imaging system.

Solution according to the invention According to the invention, the aforementioned object can be achieved, for example, using a measurement system, as described below, for determining a wavefront aberration of an optical imaging system. The measurement system according to the invention comprises an irradiation device for passing measurement radiation through the imaging system, an analysis grating which, disposed down- stream of the imaging system, is arranged in the beam path of the measurement radiation in a manner displaceable transversely to an optical axis of the imaging system, and a detection device for recording a radiation distribution of the measurement radiation. Here, the measurement system according to the invention is configured to produce respective interferograms, formed by means of the analysis grating, at a plurality of displacement positions of the analysis grating for the purposes of being recorded on the detection device and to ascertain at least one positional information item of the analysis grating in at least one of the displacement positions by means of a control beam path that passes through the optical imaging system.

As mentioned above, the measurement system according to the invention comprises an irradiation device for passing measurement radiation along a measurement beam path of the imaging system. Below, the measurement beam path is also referred to as measurement channel. Preferably, the irradiation device is em- bodied in such a way that the measurement radiation has a wavelength which corresponds to an operating wavelength of the optical imaging system. For this purpose, the use of an operating radiation source for the provision of the meas- urement radiation is possible. The measurement system may be suitably configured for a specific measurement radiation from the infrared range up to the x-ray range. By way of example, a measurement radiation with a wavelength of less than 100 nm, in particular with a wavelength of approximately 13.5 nm or approx- imately 6.8 nm, can be used in a projection lens for microlithography with EUV radiation (extreme ultraviolet radiation). Further, the irradiation device can contain a coherence mask with a one-dimensional or two-dimensional structure in the object plane of the optical imaging system or with focusing elements for respectively focusing some of the measurement radiation into field points of the object plane for multi-channel shearing interferometry.

Furthermore, the measurement system according to the invention comprises a diffractive analysis grating that is displaceably arranged transversely to an optical axis of the imaging system in the output-side measurement beam path of the im- aging system, and a detection device for recording the radiation distribution of the measurement radiation. By way of example, the analysis grating may be embodied as a phase grating, amplitude grating or with any other suitable diffraction grating type. The analysis grating can also be configured as a reflecting grating for measurement radiation with a very short wavelength. By way of example, the de- tection device comprises a spatially resolving CCD sensor with a capturing area which contains a two-dimensional arrangement of individual sensors.

The measurement system according to the invention is configured to produce respective interferograms, formed by means of the analysis grating, at a plurality of displacement positions of the analysis grating for the purposes of being recorded on the detection device.

By way of example, the interferograms are produced by interference of radiation of the zero order of diffraction with radiation of a higher order of diffraction, such as e.g. the first order of diffraction, said orders of diffraction respectively being formed at the analysis grating. There is a so-called "temporal phase shift" as a result of displacing the analysis grating. Here, the phase of the higher order of diffraction changes while the phase of the zero order of diffraction remains the same, as a result of which there is a change in the respective interferogram.

The analysis grating may have a different pattern, in particular a different grating period, in a first region, with which the interferogram is formed, than in a second region, which is assigned to the control beam path. In accordance with an embodiment that is explained in more detail below, this second region may serve to form a multi-fringe interference pattern. Below, a beam path, with which the interferograms are formed, is also designated as a "measurement channel" and the control beam path is also designated as a "monitoring channel". The designation "monitoring channel" arises on account of the function thereof of monitoring the precise displacement positions of the analysis grating.

Further, the measurement system according to the invention is configured to ascertain at least one positional information item of the analysis grating in at least one of the displacement positions by means of the control beam path. The positional information item of the analysis grating in one of the displacement positions may contain an absolute position of the analysis grating in the displacement position or else a relative position of the analysis grating in the displacement position in relation to the position of the analysis grating in another displacement position, and hence a positional difference between the arrangements in two different displacement positions.

A topography of the wavefront of the measurement radiation after passing along the measurement beam path of the imaging system can be determined from the interferograms recorded at the individual displacement positions using the at least one positional information item.

A deviation of the actual form of the wavefront from an intended wavefront which, for example, may have the form of a spherical wave or a plane wave can be ascertained from the topography of the wavefront of the measurement radiation determined by means of the measurement system according to the invention. Here, the topography of the wavefront can also be specified on the basis of a phase distribution of the measurement radiation along an area defined by the intended wavefront. In turn, one or more wavefront aberrations of the optical imaging system can be determined from the deviation, determined thus, of the actual form of the wavefront from the intended wavefront.

Positioning errors of the analysis grating at the individual displacement positions can effectively be removed from the determination of the topography of the wave- front by the provision of a control beam path and the ascertainment, following therefrom, of the at least one positional information item of the analysis grating. Using this, it is possible to substantially reduce measurement errors that occur when determining the wavefront aberration of the optical imaging system.

According to one embodiment, the at least one positional information item com- prises a position specification for the analysis grating in the lateral direction and/or in the axial direction in relation to the optical axis, and/or a specification in respect of a tilt position of the analysis grating. In particular, the positional information item comprises position specifications in at least one, at least two, at least three, at least four, at least five or in all six spatial positions. Six spatial positions should be understood to mean the position coordinates in the three spatial directions and the respective rotational/tilt positions in respect of the three spatial directions. In particular, positional information items are ascertained for several of the displacement positions, which describe a precise "path trajectory" of the analysis grating when passing through the individual displacement positions. The path trajectory, like e.g. the course of a hilly toboggan run, can be characterized not only by spatial coordinates but also by tilt coordinates.

In accordance with a further embodiment, the at least one positional information item comprises a difference in position between the displacement positions of the analysis grating in the lateral direction in respect of the optical axis. In accordance with a further embodiment, the measurement system is configured to ascertain the at least one positional information item of the analysis grating by means of at least two, in particular at least three, four or more different control beam paths that pass through the optical imaging system. Thus, for example, link- ing the measurements of a plurality of control beam paths facilitates e.g. the ascertainment of tilt positions, in a manner analogous to the procedure during a tri- angulation.

In accordance with a further embodiment, the measurement system furthermore comprises an evaluation device configured to determine a topography of the wavefront of the measurement radiation after passing along the measurement beam path of the imaging system from the interferograms recorded at the individual displacement positions, using the at least one ascertained positional information item. In accordance with one embodiment variant, the evaluation device is configured to carry out a discrete Fourier analysis when determining the topography of the wavefront of the measurement radiation.

In accordance with a further embodiment, the measurement system is configured to produce a respective multi-fringe interference pattern at the displacement posi- tions for the purposes of being recorded on the detection device, said multi-fringe interference pattern being produced by means of the analysis grating, wherein the multi-fringe interference pattern comprises at least one complete period of alternating fringes of maximum constructive interference and maximum destructive interference, and the measurement system is furthermore configured to ascertain the at least one positional information item of the analysis grating on the basis of the recorded multi-fringe interference patterns. The recorded multi-fringe interference patterns comprise at least one full period, in particular at least two, at least five or at least ten full periods of alternating fringes of maximum constructive interference and maximum destructive interference. A maximum constructive interfer- ence should be understood to mean an intensity value in the interference pattern which corresponds to the maximum intensity value that is achievable by means of the employed diffraction grating. By way of example, the maximum achievable intensity value of a diffraction grating, which is not operated in the multi-fringe mode, may be ascertained as follows: The diffraction grating is continuously displaced transversely to the incoming radiation; the intensity radiated onto a specific location of the detector that records the interference pattern varies multiple times between a maximum value and a minimum value with the movement of the diffraction grating. This maximum value now is the aforementioned maximum achievable intensity value which is present in fringes of maximum constructive interference in the multi-fringe interference pattern. The minimum value that occurs during the displacement of the diffraction grating corresponds to the intensity value that is present at the maximum destructive interference in the multi-fringe interference pattern.

In accordance with a further embodiment according to the invention, the measurement system is furthermore configured to undertake the ascertainment of the positional information item of the analysis grating, in particular the position difference between the displacement positions of the analysis grating, by determining the phase distributions that underlie the corresponding multi-fringe interference patterns, determining a difference distribution by forming the difference of the determined phase distributions and averaging a plurality of values from the differ- ence distribution. The phase distribution underlying a multi-fringe interference pattern should be understood to mean the local distribution of the phase difference of the interfering waves underlying the interference pattern. By way of example, the interfering waves may be formed, firstly, by a wave formed by the zero order of diffraction and, secondly, by a wave formed by a higher order of diffraction, for example the first order of diffraction, at the analysis grating. These waves interfere on a capturing area of the detection device and lead to the aforementioned phase distribution on account of their respective phase difference, which varies over the capturing area. By way of example, a multi-fringe interference pattern is captured at each displacement position and a local phase distribution is determined in a suitable region of the multi-fringe interference pattern. A difference can subsequently be formed between the determined phases for corresponding image points of two multi-fringe interference patterns of adjacent displacement positions. The position difference between two adjacent displacement positions can thus be determined, for example, from a mean phase shift between these positions, said mean phase shift being determined by averaging the difference phases over all image points.

In accordance with a further embodiment the measurement system is configured to irradiate radiation contained in the control beam path onto the analysis grating in a defocused condition.

In accordance with a further embodiment of the measurement system according to the invention, the measurement system is configured to produce the multi- fringe interference patterns in each case by defocussed radiation of the measurement radiation onto the analysis grating. The measurement radiation is prefer- ably only defocussedly radiated onto a region that is assigned to the multi-fringe pattern and referred to above as second region. Here, the measurement system can be embodied in such a way that, in the aforementioned first region of the analysis grating, the measurement radiation is focussed onto the analysis grating for producing interferograms for shearing interferometry. By way of example, the analysis grating is arranged in an image plane of the optical imaging system to this end. As a result of the defocussed radiation of the measurement radiation onto the analysis grating, the measurement radiation effectively impinges onto the analysis grating in a beam of measurement waves with different propagation directions. Each of the measurement waves contributes to the interference, and so a multi-fringe interference pattern with at least one full period of alternating fringes of maximum constructive interference and maximum destructive interference emerges.

In accordance with one embodiment of the invention, the measurement system comprises a defocussing optical element which is arranged in the imaging beam path of the optical imaging system for the defocussed radiation of the measurement radiation onto the analysis grating. The imaging beam path of the optical imaging system is understood to mean the beam path that lies between the image plane of the imaging system defined by the analysis grating and the object plane assigned to this image plane. By way of example, a refractive, diffractive or reflective optical element is used as a defocussing optical element. The defocussing optical element may be arranged in the input-side region of the imaging system, i.e. disposed upstream of the imaging system, or in the output-side region of the imaging system, i.e. disposed downstream of the imaging system. If the defocussing optical element is arranged in the input-side region of the imaging system, it may be part of the irradiation device. In an embodiment variant of a defocussing element disposed downstream thereof, said defocussing element may be securely connected to the analysis grating, in particular attached to the surface of the analysis grating.

In a further embodiment of the measurement system according to the invention, the irradiation device has a wave-forming coherence structure for the defocussed radiation of the measurement radiation onto the analysis grating, said coherence structure being arranged offset in relation to an object plane of the imaging system. Alternatively, or additionally, a region of the analysis grating that serves to produce a multi-fringe interference pattern is arranged offset in relation to an im- age plane assigned to the object plane. In other words, the wave-forming coherence structure is arranged on the input side with respect to the optical imaging system in accordance with this embodiment.

In accordance with one embodiment variant of the offset arrangement of the co- herence structure, the coherence structure may be part of a coherence mask embodied in a stepped manner, in which a region serving to produce the interfero- grams is arranged in the object plane and a region having the coherence structure is arranged offset from the object plane. In accordance with a further embodiment variant of the offset arrangement of the region serving to produce the multi-fringe interference pattern, the analysis grating may have a stepped embodiment such that a region serving to produce the inter- ferograms is situated in the image plane and the region serving to produce the multi-fringe pattern is arranged offset from the image plane.

According to a further embodiment of the measurement system according to the invention, the evaluation device is furthermore configured to determine defocus aberrations and/or astigmatic aberrations of the optical imaging system directly from the respective multi-fringe interference pattern. By way of example, the evaluation device to this end differentiates a phase distribution ascertained from one or more multi-fringe interference patterns and ascertains the focus or astigmatism from a linear tilt of the phase distribution determined from the differentiation.

Moreover, it is possible to take into account the interferograms of the temporal phase shift.

In accordance with a further embodiment of the measurement system according to the invention, the evaluation device is configured to carry out a discrete Fourier analysis when determining the topography of the wavefront of the measurement radiation. In contrast thereto, a Fast Fourier Transform (FFT) is conventionally carried out when evaluating interferograms recorded by means of a shearing interferometer. However, the Fast Fourier Transform is based on constant phase steps between the individual interferograms or constant position differences between the displacement positions. By means of the discrete Fourier analysis, it is possible to take into account varying phase steps that are determined on the basis of the at least one ascertained position difference. In particular, a discrete Fourier transform allows the direct use of interferograms of displacement positions with non-constant phase steps for determining the topography of the wavefront. Hence, it is no longer necessary to readjust the analysis grating for the purposes of achieving constant position differences.

One embodiment of the measurement system according to the invention is con- figured in such a way that the measurement radiation in the control beam path, in particular the measurement radiation for producing a multi-fringe interference pattern, passes through a smaller surface area of a pupil of the optical imaging sys- tern than the measurement radiation for producing one of the interferograms. Expressed differently, the beam path of a control beam path or monitoring channel is configured in such a way that it irradiates a smaller area on a capturing plane of the detection device than a measurement channel. In this way, the same detec- tion device can be used to capture a plurality of measurement channels simultaneously or to capture individual measurement channels more accurately over a larger area. In addition to a control beam path for determining the phase in a first direction, further embodiments also provide a control beam path for determining the phase in a direction that is orthogonal to the first direction or a plurality of monitoring channels for in each case different directions.

In accordance with a further embodiment of the measurement system according to the invention, the measurement radiation in the control beam path of a multi- fringe interference pattern, in particular the measurement radiation for producing a multi-fringe interference pattern, has a different wavelength than the measurement radiation for producing one of the interferograms. By way of example, provision is made of a first radiation source for producing measurement radiation with a first wavelength for measurement channels and of a second beam source for producing measurement radiation with a second wavelength for control beam paths or monitoring channels. Preferably, the wavelength of the first radiation source corresponds to an operating wavelength of the optical imaging system. By way of example, an operational beam source of the optical imaging system can be used as first beam source. It is easier to carry out a separate capture of multi-fringe interference patterns and interferograms using different wavelengths for meas- urement channels and monitoring channels.

In one embodiment of the measurement system according to the invention, the detection device has a colour-selective embodiment for separating multi-fringe interference patterns and interferograms with different wavelengths that are su- perposed on one another in a capturing area. By way of example, the detection device contains colour filters or a colour camera which are configured for a sepa- rate capture of the multi-fringe interference patterns at a first wavelength and the interferograms at a second wavelength.

In accordance with a further embodiment of the measurement system according to the invention, the wavelength of the measurement radiation for a multi-fringe interference pattern is selected in such a way that a chromatic aberration of the optical imaging system causes a defocussing of the measurement radiation that is suitable for producing the multi-fringe interference pattern. Preferably, the measurement radiation for producing the interferograms in this case has an operating wavelength of the optical imaging system. In this way, it is possible to realize de- focusing of the measurement radiation of a monitoring channel for producing a multi-fringe interference pattern even without a defocussing optical element. The optical imaging system to be measured itself brings about the defocussing. Here, both measurement beams may irradiate mutually superposing regions on a cap- turing plane and the detection device may have a colour-selective embodiment for the purposes of separating the measurement beams in one embodiment. In particular, the measurement beams for a measurement channel and for a monitoring channel may emanate from the same location on the object plane of the optical imaging system in one embodiment. Alternatively, the beam paths of measure- ment channels and monitoring channels may be configured in such a way that mutually separated regions are irradiated on a capturing plane of the detection device. In particular, there may, to this end, be a spatially separate provision of the measurement radiation for measurement channels and monitoring channels at the input side of the optical imaging system.

In a further embodiment of the measurement system according to the invention, a region of the analysis grating and/or of a coherence mask that is arranged on the input side at the optical imaging system comprises ring-shaped structures. By way of example, the ring-shaped structures are embodied as concentrically arranged circular structures. A shearing spacing is invariant in relation to the shearing direction in the case of a rotationally symmetric grating. A phase shift may occur in different directions, in particular in the case of a non-rectangular image field. Alterna- tively, provision can also be made of concentrically arranged elliptical ring structures. In this case, the grating period is dependent on the shearing direction. By way of example, this facilitates an accurate alignment of the coherence mask and of the analysis grating in relation to one another.

Further, the evaluation device is configured to determine a brightness variation of the measurement radiation prior to the latter's entrance into the optical imaging system by means of captured multi-fringe interference patterns in one embodiment of the invention. In particular, a brightness variation of the measurement ra- diation provided by the irradiation device is determined. By way of example, the evaluation device is embodied to ascertain a mean value over one or more periods of a multi-fringe interference pattern as a constant light portion for each image point of a capturing area of the detection device and a comparison of these constant light portions in a plurality of multi-fringe interference patterns which are cap- tured in succession. Further, provision may be made for determining a correction factor for eliminating brightness variations in successively captured interferograms on the basis of the comparison.

According to the invention, provision is furthermore made of a microlithographic projection exposure apparatus, comprising a projection lens for imaging mask structures onto a wafer, and the measurement system according to any one of the preceding embodiments or embodiment variants for determining a wavefront aberration of the projection lens. By way of example, use is made here of a radiation source of the projection exposure apparatus for providing the measurement radia- tion for producing interferograms. Furthermore, a wafer holder or wafer stage of the projection exposure apparatus may be used as a positioning device for the analysis grating. The use of a reticle stage as a holder and positioning device for a coherence mask is likewise possible. Furthermore, the object may be achieved according to the invention by the method, described below, for determining a wavefront aberration of an optical imaging system. The method comprises the following steps: passing measurement radia- tion along a measurement beam path of the imaging system, arranging a diffrac- tive analysis grating in the exit-side measurement beam path of the imaging system and displacing the analysis grating transversely to an optical axis of the imaging system, recording respective interferograms that are formed by means of the analysis grating on a detection device, at a plurality of displacement positions of the analysis grating, ascertaining at least one positional information item of the analysis grating in at least one of the displacement positions by means of a control beam path that passes through the optical imaging system, and determining a topography of the wavefront of the measurement radiation after passing along the measurement beam path of the imaging system from the interferograms recorded at the individual displacement positions, using the at least one ascertained positional information item.

In other words, in a manner analogous to the measurement system according to the invention, at least one positional information item of the analysis grating is determined very precisely by means of a method according to the invention in at least one of the displacement positions by means of a control beam path that passes through the optical imaging system. The positional information item determined thus is used when determining a topography of the wavefront on the ba- sis of the interferograms that are captured in succession at different displacement positions, in particular for phase shift or shearing interferometry.

The features specified with respect to the aforementioned embodiments, exemplary embodiments or embodiment variants, etc., of the measurement system ac- cording to the invention can be correspondingly applied to the method according to the invention, and vice versa. These and other features of the embodiments according to the invention are explained in the description of the figures and in the claims. The individual features can be implemented, either separately or in combination, as embodiments of the invention. Furthermore, they can describe advan- tageous embodiments which are independently protectable and protection for which is claimed if appropriate only during or after pendency of the application. Brief description of the drawings

The above and further advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the accompanying schematic drawings. In the figures:

Figure 1 shows, in a schematic visualization, a first exemplary embodiment of a measurement system according to the invention for determining a wavefront aber- ration of an optical imaging system, comprising a defocussing optical element arranged on the input side in relation to the imaging system,

Figure 2 shows, in a schematic visualization, a second exemplary embodiment of a measurement system comprising a defocussing optical element arranged on the output side in relation to the optical imaging system,

Figure 3 shows, in a schematic visualization, a third exemplary embodiment of a measurement system comprising a defocussing optical element fastened to the analysis grating,

Figure 4 shows, in a schematic visualization, a fourth exemplary embodiment of a measurement system comprising a defocussing coherence structure arranged offset to the object plane of an optical imaging system, Figure 5 shows, in a schematic visualization, a fifth exemplary embodiment of a measurement system comprising a region of an analysis grating arranged offset to the image plane of an optical imaging system,

Figure 6 shows a schematic illustration of an arrangement of measurement chan- nel and monitoring channel regions on a capturing area of a detection device in accordance with an exemplary embodiment of a measurement system, Figure 7 shows a schematic illustration of an arrangement of measurement channel regions and monitoring channel regions that are smaller in relation to the measurement channel regions, on a capturing area of a detection device in accordance with a further exemplary embodiment of a measurement system,

Figure 8 shows a schematic illustration of an arrangement of measurement channel and monitoring channel regions with different wavelengths on a capturing area of a detection device in accordance with a further exemplary embodiment of a measurement system,

Figure 9 shows, in a schematic visualization, a further exemplary embodiment of a measurement system with different wavelengths for measurement and monitoring channels and a colour-selective detection device, Figure 10 shows, in a schematic visualization, a further exemplary embodiment of a measurement system with different wavelengths for measurement and monitoring channels and mutually separated capturing regions on a capturing area,

Figure 1 shows a schematic illustration of circular ring structures for a monitoring channel,

Figure 12 shows a schematic illustration of elliptical ring structures for a monitoring channel, Figure 13 shows a further exemplary embodiment of a measurement system for measuring an optical imaging system that is operated in the EUV wavelength range, and

Figure 4 shows a schematic visualization of the design of an embodiment of an evaluation device contained in the measurement systems in accordance with Figures 1 to 5, 9, 10, 12 or 13. Detailed description of exemplary embodiments according to the invention

In the exemplary embodiments or embodiments or embodiment variants de- scribed below, elements which 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.

In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in some drawings, from which system the respective positional relationship of the components illustrated in the figures is evident. In Figure 1 , the y-direction runs perpendicular to and out of the drawing plane, the x-direction upwardly, and the z-direction toward the right.

Figure 1 schematically shows a first exemplary embodiment of a measurement system 10 for determining a wavefront aberration of an optical imaging system 12. The optical imaging system 12 serves to image field points of an object plane 14 into an image plane 16 that is assigned to the object plane 14 and, to this end, it comprises optical elements 18, of which only two are depicted in Figure 1 in an exemplary manner. Further, Figure 1 depicts an optical axis 20 of the imaging system 12 parallel to the z-direction and indicates an aperture stop 21 that is arranged in a pupil plane for the purposes of delimiting a pupil 22. The optical imag- ing system 12 is conventionally embodied for imaging that is as aberration-free as possible at an operating or used wavelength or a specific operating wavelength range. An example of such an optical imaging system is a microlithographic projection lens for imaging mask structures onto a wafer. By way of example, certain projection lenses are suitably configured for microlithography with EUV radiation (extreme ultraviolet radiation) with a wavelength of less than 100 nm, in particular with a wavelength of approximately 13.5 nm or approximately 6.8 nm. Accordingly, the measurement system 10 is suitably configured for an operating wavelength of the optical imaging system 12. In general, the measurement system 10 may be suitably embodied for a wavelength from the infrared range up to the x-ray range.

Multi-channel shearing interferometry can be carried out with the measurement system 10 for the purposes of determining a wavefront aberration of the optical imaging system 12. Such interferometry that is based on the principle of the phase shift is described in e.g. WO 01/63233. The measurement system 10 comprises an irradiation device 24 with a radiation source 23 for providing suitable measurement radiation 26 and a coherence mask 28 that is arranged in the re- gion of the object plane 14 of the optical imaging system 2. Furthermore, the measurement system 10 comprises a diffractive analysis grating 30 that is arranged in the region of the image plane 16, a detection device 32 that is arranged in the beam path downstream of the analysis grating 30, said detection device comprising a capturing area 34 for the spatially-resolved capture of measurement radiation 26, and an evaluation device 36.

The radiation source 23 provides measurement radiation 26 with sufficient intensity and coherence for measuring the optical imaging system 2. Here, the wavelength of at least some of the measurement radiation 26 corresponds to an oper- ating wavelength of the optical imaging system 12. By way of example, an operating radiation source for the optical imaging system 12 is used for producing the measurement radiation 26, for example a radiation source of an illumination system of a microlithographic projection exposure apparatus when measuring a projection lens.

As a person skilled in the art knows, a microlithographic projection exposure apparatus comprises an illumination system for producing exposure radiation 204, for example in the form of DUV radiation, i.e. radiation in the deep UV wavelength range with a wavelength of e.g. 248 nm or 193 nm, or EUV radiation (extreme ultraviolet radiation) with a wavelength of <100 nm, in particular with a wavelength of approximately 13.5 nm or approximately 6.8 nm. The exposure radiation 204 impinges on a lithography mask with mask structures to be imaged that are ar- ranged thereon. Here, the exposure radiation may be reflected at the lithography mask 206, as is often the case when using EUV radiation. Alternatively, the lithography mask may also be embodied as a transmission mask. In this case, the exposure radiation passes through the mask. Imaging the mask structures on a wa- fer arranged in an image plane is effected by means of the projection lens which comprises a multiplicity of optical elements.

In the exemplary embodiment shown in Figure 1 , the coherence mask 28 comprises a two-dimensional arrangement of pinhole stops 38 that extends in the ob- ject plane 14 and, optionally in addition thereto, focusing elements (not depicted here), which each focus a portion of the measurement radiation 26 onto a pinhole stop 38. In this way, measurement radiation is respectively provided simultaneously for a plurality of field points in the object plane 14, the respective beam path thereof being referred to as measurement channel 40 or measurement beam path of the optical imaging system 12 below. A beam path of one of these measurement channels 40 is depicted in an exemplary manner in Figure 1. Using such a multi-channel measurement system 10, it is possible to simultaneously measure imaging properties of the optical imaging system 12 for a multiplicity of field points by means of shearing interferometry. The beam path of a measurement channel 40 is preferably configured in such a way that the measurement radiation emerges from the pinhole stop 38 in diverging fashion with a spherical wavefront and said measurement radiation is imaged or focussed onto the image plane 16 by the optical imaging system 12 to be measured. Here, as indicated in Figure 1 , the measurement radiation illuminates the entire region of the aperture stop or pupil 22. A translation module (not depicted in Figure 1 ) may be provided for exactly positioning the coherence mask. In particular, it is possible to use a reticle stage of the projection exposure apparatus as a translation module when measuring a projection lens that is integrated into a microlithographic projection exposure apparatus.

In alternative embodiments, provision may be made of only one measurement channel with a pinhole stop that has an embodiment which is displaceable in the object plane 14. Moreover, a plurality of pinhole stops for a measurement channel may be contained in the coherence mask 28 in a symmetric two-dimensional arrangement next to one another and, in addition to circular apertures, there can also be apertures embodied in a polygonal manner, e.g. as squares or triangles, as pinhole stops. Furthermore, the coherence mask 28 may have a two- dimensional, symmetric structure of apertures which are adapted to an employed analysis grating 30 for suppressing interfering orders of diffraction of the analysis grating 30. In respect of further possible embodiments of coherence masks or irradiation apparatuses and the description thereof, reference is made, in particular, to WO 01/63233.

By way of example, the diffractive analysis grating 30 is embodied as a phase grating or amplitude grating or with any other suitable diffraction grating type, for example as a greyscale value grating or else, for very short wavelengths, as a reflecting grating. As a diffraction structure, the analysis grating 30 comprises a line grating, a cross grating, a chequerboard grating, a triangular grating or any other suitably periodic structure. For a phase shift within the scope of shearing interferometry, the analysis grating 30 can be displaced together with the capturing area 34 of the detection device 32 in a translation direction 42, which is aligned substantially parallel to the x- or y-direction and hence transversely to the optical axis 20. Further displacement directions perpendicular to the optical axis 20 and tilt axes may also be provided. A displacement is carried out step-by-step in one direction by means of a positioning module not depicted in the drawing. An interferogram 62 is produced on the capturing area 34 at each measurement channel 40 as a result of interference between radiation, formed at the analysis grating 30, of the zero order of diffraction and radiation of a higher order of diffraction, such as e.g. the first order of diffraction. There is a so-called "temporal phase shift" as a result of displacing the analysis grating 30. Here, the phase of the higher order of diffraction changes while the phase of the zero order of diffraction re- mains the same, as a result of which there is a change in the respective interferogram 62. In particular, the distance between two adjacent displacement positions is selected in such a way that a phase shift that is suitable for the shearing inter- ferometry occurs between these displacement positions. Typically, the distance is a fraction of the grating period of the analysis grating 30. The use of a wafer stage as translation module is possible when measuring a projection lens that is integrated into a microlithographic projection lens as optical imaging system 12.

The detection device 32 comprises the radiation-sensitive capturing area 34, which contains a two-dimensional arrangement of individual sensors and, for example, is embodied as a spatially resolving CCD sensor. An optical arrangement, which is not depicted in Figure 1 , for imaging an interferogram 62 or multi-fringe interference pattern 64 onto the capturing area 34 may be provided between the analysis grating 30 and the capturing area 34. The patterns 62 and 64 captured by the detection device 32 are transmitted to the evaluation device 36.

Furthermore, the measurement system 10 comprises a plurality of defocussing optical elements 44 which, according to the exemplary embodiment in accordance with Figure 1 , are part of the irradiation device 24. The defocussing optical elements 44 are each arranged between the coherence mask 28 and the optical imaging system 12 in the beam path of one of the above-described measurement channels 40 in such a way that the measurement radiation 26 impinges on the analysis grating 30 in a defocussed manner. For reasons of clarity, only one of these defocussing elements 44 is depicted in Figure 1. Each defocussing optical element 44 forms a beam path for some of the measurement radiation 26, the focus of which lies in front of, or behind, the analysis grating 30 and which is also referred to as control beam path or monitoring channel 46 below. On account of the defocussing, a multi-fringe interference pattern 64 is formed on the capturing area 34 by means of the analysis grating 30 instead of an interferogram for the shearing interferometry. To this end, the structure pattern of the analysis grating 30, provided for the shearing interferometry, may be used. Alternatively, the analysis grating 30 can also have a different pattern, in particular a different grating period or grating alignment, in a region of the monitoring channel. The defocussing optical elements 44 may be embodied as refractive elements, for example as lens elements or prisms, as reflective elements, e.g. as mirrors, as diffractive elements or as optical arrangements made of a plurality of these elements. Here, the defocussing optical elements 44 are configured and arranged in such a way that the multi-fringe interference pattern of each monitoring channel 46 comprises at least one full period, in particular at least two, at least five or at least ten full periods of alternating fringes of maximum constructive and maximum destructive interference. Alternatively, a defocussing optical element 44 may be configured to produce a plurality of monitoring channels. It is also possible to use only some of the radiation of a measurement channel for a monitoring channel.

The evaluation device 36 is configured to determine a distance between two displacement positions on the basis of captured multi-fringe interference patterns 64 of one or more monitoring channels 46 in the case of different displacement posi- tions of the analysis grating 30. Furthermore, with the aid of multi-fringe interference patterns 64, the evaluation device 36 ascertains a local and temporal brightness profile of the measurement radiation 26 that is caused by lacking stability of the radiation source 23, i.e. a local and temporal brightness profile prior to the entrance into the optical imaging system 12, in particular at the location of the co- herence mask 28. Taking into account the ascertained distances between the displacement positions and the captured interferograms of the measurement channels 40 at these positions, the evaluation device 36 determines a topography of the wavefront of the measurement radiation 26 after passing through the imaging system 12, specifically after passing through the measurement channels 40 of the imaging system 12, by means of discrete Fourier analysis. Here, it is likewise possible to take into account the local and temporal brightness profile of the measurement radiation 26. A wavefront aberration of the optical imaging system 12 emerges from a deviation of the determined topography of the wavefront from an intended wavefront. Additionally, the evaluation device 36 can be configured to determine the focus and the astigmatism of the optical imaging system 12 by means of captured multi-fringe interference patterns. For these purposes, the evaluation device 36 comprises e.g. an interface with a connection to the detection device 32 for receiving data received by the detection device 32, a memory for storing transmitted interferograms, multi-fringe interference patterns and other data, and also an electronic processing unit. Alternatively, provision may also be made for the captured interferograms and multi-fringe interference patterns to be stored in a memory of the detection device 32 for a later evaluation or transmission to an external evaluation appliance. The functionality and the design of the evaluation device 36 will be described in detail below in the context of an exemplary embodiment of a method according to the invention in conjunction with Figure 14.

Figure 2 and Figure 3 each show further exemplary embodiments of a measurement system 10 for determining a wavefront aberration of an optical imaging system 12. In the embodiment according to Figure 2, at least one defocussing optical element 50 is arranged at a fixed position between the optical imaging system 12 and the analysis grating 30 in the beam path of the measurement radiation 26. The embodiment in accordance with Figure 3 contains at least one defocussing optical element 52 immediately in front of the analysis grating 30 in the beam path of the measurement radiation 26, said defocussing optical element being moved together with the analysis grating 30 to various displacement positions. To this end, the defocussing optical element 52 may be fastened to the analysis grating 30 or may be displaced by a separate translation module. Thus, in contrast to the exemplary embodiment according to Figure 1 , the exemplary embodiments according to Figure 2 and Figure 3 each have at least one defocussing optical ele- ment 50 or 52 that is arranged on the image side in relation to the imaging system 12. The beam path of the monitoring channel 46 or the control beam path of the measurement radiation 26 for the defocussing optical element 50 or 52 therefore initially corresponds to the beam path of a measurement channel 40 and is only formed as a monitoring channel 46 by defocussing downstream of the optical im- aging system 12. In these exemplary embodiments too, the defocussing optical elements 50 and 52 are embodied as refractive optical elements, for example as lens elements or prisms, as reflective elements, like mirrors, as diffractive elements or as optical arrangements made of a plurality of these elements. Preferably, provision is made of a plurality of monitoring channels 46, each with a defocussing optical element 50 or 52. In alternative exemplary embodiments, a defocussing optical element 50 or 52 is configured to produce a plurality of monitoring channels 46, or only some of the radiation of a measurement channel 40 is used for a monitoring channel 46. While the beam path of the monitoring channel 46 can be influenced by the optical imaging system 12 and, in particular, by an area in a pupil through which radiation passes in the case of an object-side arrangement of a defocussing optical element 44, a simple change of the size of an area irradiated by a monitoring channel 46 on the capturing area 34 of the detection device 32 is possible in the case of an image-side arrangement of the defocussing optical element 50 or 52. In ac- cordance with a further exemplary embodiment, a defocussing optical element for a monitoring channel 46 may be provided both on the object side and on the image side.

In the embodiments above, both the coherence mask 28 and the analysis grating 30 have the same diffraction structures in regions for measurement radiation 26 for producing multi-fringe interference patterns 64 as in the regions for measurement radiation 26 for producing interferograms 62. In alternative embodiments, a different diffraction structure is provided at the coherence mask 28 or at the analysis grating 30 or at both for monitoring channels 46 than for measurement chan- nels 40. In particular, it is possible to use a different grating period, grating inclination or line structure.

Figure 4 schematically depicts a further exemplary embodiment of a measurement system 10. It differs from the exemplary embodiment in accordance with Figure 1 in that, in place of a defocussing optical element, at least one coherence structure region 54 of the coherence mask 28 is arranged in the beam path upstream of the object plane 14. The arrangement and the distance from the object plane 14 are configured in such a way that there is suitable defocussing of the measurement radiation 26 for the purposes of producing a multi-fringe interference pattern 64 on the capturing area 34. The coherence structure region 54 is embodied as part of the coherence mask 28, which, to this end, has a stepped surface. Here, the coherence structure region 54 is arranged on a step which, in relation to the optical axis 20, is offset in the axial direction in relation to a measurement coherence region 55 that is used for the measurement channels 40. Alternatively, the coherence mask 28 has a different suitable form for offsetting the coherence structure region 54 from the object plane 14, such as e.g. a wedge shape or ramp shape. Also, an arrangement of the coherence structure region 54 on a separate carrier element that is separated from the coherence mask 28 is possible. In a further embodiment, the coherence structure region 54 is arranged offset downstream of the object plane 14 in the beam path of the measurement radiation 26. Preferably, the measurement system 10 comprises a plurality of monitoring channels 46, each with a coherence structure region 54 that is offset from the object plane 14. A coherence structure region 54 for a monitoring channel 46 may have the same structure as a region of the coherence mask 28 for a measurement channel 40. Alternatively, a coherence structure region 54 comprises a different structure in relation to the regions for measurement channels 40, such as e.g. a different arrangement or embodiment of pinhole stops or a different grating structure.

Figure 5 shows an exemplary embodiment in which, in contrast to the exemplary embodiment in accordance with Figure 4, at least one grating region 56 of the analysis grating 30 is arranged offset from the image plane 16. The grating region 56 is arranged in the beam path downstream of the image plane 16 in such a way that suitable defocussing of the measurement radiation 26 for producing a multi- fringe interference pattern 64 on the capturing area 34 is achieved. To this end, the analysis grating 30 comprises a step-shaped, wedge-shaped or similarly con- figured surface for correspondingly receiving the grating region 56. Alternatively, the grating region 56 may be arranged on a surface that lies opposite to the remaining structures of the analysis grating 30 or it may be arranged separately from the analysis grating 30 on a separate carrier element. Preferably, the measurement system 10 contains a plurality of monitoring channels 46, each with a grating region 56 that is offset from the image plane 16. In further embodiments, at least one grating region is arranged in the beam path upstream of the image plane 6 or provision is made of an offset of a coherence structure region at the object plane 14 and an offset of a grating region at the image plane 16 for the same monitoring channel. A grating region 56 for a monitoring channel 46 may have the same diffraction structure as a region of the analysis grating 30 for a measurement channel 40. In accordance with other embodiments, at least one grating region comprises a different diffraction structure in relation to the regions of the analysis grating 30 for measurement channels 40, for example a different grating period, grating inclination or line structure. An offset of a grating region 56 only has an effect on the region of the capturing area 34 that is irradiated by the monitoring channel 46 while an offset of a coherence structure region 54 influ- ences the course of the monitoring channel 46 in the optical imaging system 12.

Figure 6 schematically shows a capturing area 34 of the detection device 32 of a measurement system 10 which, for example, corresponds to one of the exemplary embodiments listed here. A plurality of measurement channel regions 58 and monitoring channel regions 60 are depicted on the capturing area 34. Each measurement channel 40 irradiates a measurement channel region 58, in which an interferogram 62 for a phase shift evaluation is produced in each case with the aid of the analysis grating 30. Furthermore, as a result of the defocussing of the monitoring channels 46, a multi-fringe interference pattern 64 is formed in each monitoring channel region 60 that is irradiated by a monitoring channel 46. The arrangement of the measurement channel regions 58 and monitoring channel regions 60 on the capturing area 34 is substantially set by the coherence mask 28 and does not change for different displacement positions of the analysis grating 30. The defocussing by way of defocussing optical elements 44, 50, 52, coher- ence structure regions 54 or grating regions 56 is configured in such a way that the size of the monitoring channel regions 60 approximately corresponds to that of the measurement channel regions 58. Further, all multi-fringe interference pat- . terns 64 have the same fringe orientation.

Figure 7 depicts a capturing area 34 of the detection device 32 in a further meas- urement system 10, in which, in contrast to the measurement system 10, which is assigned to the channel region distribution as per Figure 6, the monitoring channel regions 60 are smaller than the measurement channel regions 58 as a result of appropriate defocussing. To this end, there can be, in particular, an appropriate embodiment and arrangement of object-side defocussing optical elements 44 or coherence structure regions 54 in such a way that the respective monitoring channel 46 only passes through part of the pupil 22 of the optical imaging system 12 while the measurement channels 46 irradiate the entire pupil 22 where possible. At least, a monitoring channel 46 passes through a smaller surface region of the pupil 22 of the optical imaging system 12 than a measurement channel 40. Here, the size of the monitoring channel regions 60 is selected in such a way that it suffices for determining the position of the analysis grating 30 and determining the brightness profile of the measurement radiation 26 provided by the irradiation device 24. More space is provided on the capturing area 34 for the measurement channel regions 58 by way of this measure and hence this facilitates a more pre- cise shearing interferometry result.

Furthermore, both monitoring channels 46 and measurement channels 40 may be configured for different displacement directions of the analysis grating 30 transversely to the optical axis 20. By way of example, to this end, the analysis grating 30 comprises line gratings with different alignments for measurement channels 40 and monitoring channels 46. In Figure 7, these different alignments are identifiable in the multi-fringe interference patterns 64, produced by interference, in the monitoring channel regions 60. In this way, it is possible to simultaneously capture interferograms 62 and multi-fringe interference patterns 64 for different displace- ment directions, for example for a displacement in the x- and in the y-direction (see Figure 1 ), by way of appropriate displacements of the analysis grating 30. Figure 8 shows a capturing area 34 of the detection device 32 of a further exemplary embodiment of a measurement system 10, in which second measurement radiation is provided for monitoring channels 46, said second measurement radiation having a different wavelength to the first measurement radiation 26, which is used for the measurement channels 40. By way of example, to this end, the measurement system 10 comprises a further radiation source or an optical arrangement for producing the second measurement radiation from the first measurement radiation 26. Furthermore, the capturing area 34 or the detection device 32 has a colour-selective embodiment. In another embodiment, use could be made instead of colour filters for selecting between the measurement radiation types for a separate capture by the detection device 32. Both the measurement channel regions 58 and the monitoring channel regions 60 are configured to be so large that they intersect on the capturing area 34. The superposing structures of the interferograms 62 and of the multi-fringe interference patterns 64 are separat- ed by way of the colour-sensitive detection device 32. In addition to a sufficiently high resolution of the interferograms 62 for a very accurate phase shifting method on account of the size of the measurement channel regions 58, a resolution which, in addition to highly precise determination of the position of the analysis grating 30 and, optionally, of the temporal and local brightness profile of the measurement radiation 26 provided by the irradiation device, also allows further evaluations is also achieved for the multi-fringe interference patterns 64 on account of the size of the monitoring channel regions. By way of example, an additional determination of focus or astigmatism may be carried out with the multi- fringe interference patterns 64 and said focus or astigmatism may be included when ascertaining a wavefront aberration of the optical imaging system 12.

Figure 9 and Figure 10 likewise depict embodiments of measurement systems 10, in which second measurement radiation 66 with a wavelength that differs from the wavelength of the measurement radiation 26 for the measurement channels 40 is used for the monitoring channels 46. In contrast to the preceding embodiments, the wavelength of the second measurement radiation 66 is selected in such a way that there already is suitable defocussing for the purposes of producing multi- channel fringe patterns on account of a chromatic aberration of the optical imaging system 12. Microlithographic projection lenses are an example of such imaging systems 12. As a rule, these are only optimized for one operating wavelength and have large aberrations at other wavelengths. The measurement system 10 can produce multi-fringe interference patterns for very precisely determining the position of the analysis grating 30 on the capturing plane 34 without dedicated defocussing optical elements, i.e. without e.g. the above-described optical elements 44, 50 or 52, and without the above-described coherence structure regions 54 or grating regions 56.

In the exemplary embodiment in accordance with Figure 9, a radiation source 123 produces measurement radiation with two different wavelengths in such a way that the produced measurement radiation comprises the first measurement radiation 26 and the second measurement radiation 66. The measurement radiation 26, 66 with two different wavelengths irradiates at least one region of the coherence mask 28. Hence, a measurement channel 40 is used simultaneously as a monitoring channel 46. There is defocussing of the second measurement radiation 66 on account of chromatic aberrations of the optical imaging system 12. The focus of the measurement radiation 66 lies upstream of the analysis grating 30 that is arranged in the image plane 16, while the measurement radiation for inter- ferograms remains focussed on the image plane 16. The detection device has a colour-selective embodiment, e.g. as a colour camera, or appropriate colour filters that can be swivelled into the beam path are provided for the purposes of a separate capture of the multi-fringe interference patterns and interferograms that over- lap in the capturing area 34.

The exemplary embodiment according to Figure 10 provides a spatially separated irradiation of the coherence mask 28 with first measurement radiation 26 for the measurement channels 40 and second measurement radiation 66 with a different wavelength for the monitoring channels 46. To this end, the measurement system 10 comprises e.g. a first radiation source 23 for providing the first measurement radiation 26 and a second radiation source 68 for providing the second measure- ment radiation 66. The irradiation device 24 is configured in such a way that the second measurement radiation irradiates different regions of the coherence mask 28 than the first measurement radiation 26. These regions are selected in such a way that monitoring channel regions 60 of the second measurement radiation 66 do not overlap with measurement channel regions 58 of the first measurement radiation 26 on the capturing area 34 of the detection device 32. The second measurement radiation 66 of the monitoring channels 46 is defocussed by chromatic aberrations when passing through the optical imaging system 12 and forms multi-fringe interference patterns 64 in the monitoring channel regions 60. The first measurement radiation 26 of the measurement channels 40 remains focussed on the image plane 16 by way of the analysis grating 30 and forms interferograms 62 for the shearing-interferometric evaluation in the measurement channel regions 58. As a result of the spatial separation of the measurement channel regions 58 from the monitoring channel regions 60 on the capturing area 34, it is possible to use a detection device 32 without colour selection.

In other exemplary embodiments of the measurement system 10, further auxiliary channels for determining various parameters are provided in addition to the above-described monitoring channels 46 for producing multi-fringe interference patterns. By way of example, auxiliary channels may be configured to determine a translation or rotation of the coherence mask 28 or of the analysis grating 30 for all spatial directions, determine brightness profiles or exactly align the coherence mask 28 in relation to the analysis grating 30 by appropriately embodied structures at the coherence mask 28, at the analysis grating 30 or at both.

As an example of such structures, Figure 11 shows a circular ring structure 70 of a coherence mask 28 and a circular ring structure 72 of an analysis grating 30 assigned to this structure for forming an auxiliary channel for carrying out radial shearing interferometry. The shearing distance is the same for all displacement directions on account of the rotational symmetry of the ring structure 72 that serves as a shearing grating. Such an auxiliary channel is particularly suitable for imaging systems with a non-rectangular image field, for example for microlitho- graphic projection lenses with a sickle-shaped object field. Hence, each stepwise displacement of the analysis grating during a measurement may be carried out in a different direction. In Figure 12, an elliptic ring structure 74 of a coherence mask 28 and an elliptic ring structure 76 of an analysis grating 30 that is assigned to this structure are depicted schematically as a further example. An auxiliary channel that is formed with these elliptical ring structures 74, 76 facilitates radial shearing interferometry, in which the shearing distance is different for each displacement direction. The grating period of the ring structure 76 depends on the direction or the angle of the displacement. The elliptic form of the ring structure 74 arranged on the coherence mask 28 facilitates an accurate alignment of the coherence mask 28 in relation to the analysis grating 30. Figure 13 elucidates a further embodiment of a measurement system 10 which is configured to measure an optical imaging system 12 in the form of a projection lens of a microlithographic EUV projection exposure apparatus. To this end, the projection lens comprises six optical elements 18-1 to 18-6 in the form of mirrors in the shown embodiment variant.

With the exception of the peculiarities described below, the measurement system 10 in accordance with Figure 13 has a configuration that is analogous to that of the measurement system in accordance with Figure 1. In accordance with one of the peculiarities, it is embodied analogously to the measurement system 10 in accordance with Figure 10 to the extent that the measurement channels 40 are irradiated with a different wavelength to the monitoring channels 46. Thus, the irradiation device 24 comprises a first radiation source 23 for producing first measurement radiation 26 for the measurement channels 40 and a second radiation source 68 for producing second measurement radiation 66 for the monitoring channels 46. By way of example, an LED source or a laser can be used as second radiation source 68. In the illustration of Figure 13, only one channel has been depicted in each case for the measurement channels 40 and the monitoring channels 46 in an exemplary manner. The measurement radiation 26 can be radiation at the operating wavelength of the projection lens to be measured, i.e. EUV radiation. In the case where the measurement system 10 is integrated into a projection exposure apparatus, the measurement radiation 26 may be identical to the exposure radiation of the projection exposure apparatus. The measurement radiation 26 is radiated onto the coherence mask 28 by means of an optical deflection element 82.

In the shown embodiment variant, the coherence mask 28 has an analogous em- bodiment to the coherence mask 28 as per Figure 4 with a stepped surface. Here, a coherence structure region 54 for the monitoring channel 46 is arranged on a step which, in relation to the optical axis of the optical imaging system 12, is offset in the axial direction in relation to the measurement coherence region 55 that is used for the measurement channel 40. In the shown embodiment variant, the off- set of the coherence structure region 54 is designed in such a way that the associated focal plane is arranged above the analysis grating 30. In the shown embodiment, the coherence mask 28 is embodied as a reflection mask; however, in principle, it may also be embodied as a transmission mask. The coherence mask 28 is held by a mask holder 29 which facilitates both a translation and rotation of the coherence mask 28 in respect of all spatial directions.

The detection device 32 is arranged on a detection stage 63 which, if the measurement system 10 is integrated into a projection exposure apparatus, may be formed by the wafer stage. The detection stage 63 is configured to be displacea- ble and tiltable in respect of all spatial directions. A grating holder 31 for holding the analysis grating 30 is arranged on the detection stage 63. The grating holder 31 facilitates displacements of the analysis grating in relation to the detection stage 63 in all spatial directions. The measurement system 10, shown in Figure 13, for measuring an EUV projection lens may, in further embodiment variants, be configured in accordance with the embodiments described with reference to Figures 1 to 12. Below, functionalities and interaction of the described components of the measurement system 10 are described together with an exemplary embodiment of a method according to the invention. For the purposes of determining a wavefront aberration of an optical imaging system 12, for example of a microlithographic projection lens, the coherence mask 28 with the radiation source 23 and the analysis grating 30 with the detection device 32 are initially arranged at the optical imaging system 12. To this end, the coherence mask 28 may be received by a reticle stage and the analysis grating 30, to- gether with the detection device 32, may be received by a wafer stage in the case of a microlithographic projection exposure apparatus. Here, an illumination system of the projection exposure apparatus can be used as irradiation device 24.

Subsequently, an intended defocussing of a defocussing optical element 44, 50, 52 that is provided at the coherence mask 28 or at the analysis grating 30 is set by means of positioning in the z-direction. If use is made of an offset coherence structure region 54 or a grating region 56, the intended defocussing is already set during the production of the coherence mask 28 or of the analysis grating 30. By way of example, 10 to 100 fringes are produced over a diameter of a pupil 22 in a multi-fringe interference pattern 64 in the case of the intended defocussing. A resolution of the detection device 32 may be taken into account when selecting the fringe density.

During a measurement process, the analysis grating 30 is displaced from an initial position stepwise in fractions of the grating period of the analysis grating 30 by means of a translation module. The individual displacements are embodied as equidistant to one another as possible. By way of example, there are initially displacements in one direction, for example the x-direction, and subsequently displacements in a direction that is orthogonal to the first direction, e.g. the y- direction. Alternatively, there may also be an alternating displacement in both directions. This procedure is suitable, in particular, for capturing temporal changes of properties of the optical imaging system 12 to be measured, which changes may occur on account of e.g. heating. Here, a time lag between measurements in mutually orthogonal directions should be as small as possible.

All interferograms 62 and multi-fringe interference patterns 64 that are produced on the capturing area are captured by the detection device 32 at each displacement position. The captured interferograms 62 and multi-fringe interference patterns 64 are transmitted to the evaluation device 36, which is depicted in detail in an exemplary embodiment in Figure 14, and stored in a memory 37 of the evaluation device 36. Alternatively, provision may also be made for storage in the detec- tion device 32 for a subsequent transmission to the evaluation device 36. If use is made of a two-dimensional analysis grating 30, for example with a chequerboard or cross structure, there may be an oscillation of the analysis grating 30 across the displacement direction at a displacement position during a capturing duration. In this manner, distrurbing interference patterns are suppressed by an integration time of the detection device 32.

As furthermore depicted in Figure 14, a first evaluation unit 36-1 in the evaluation device 36 respectively determines a positional information item 78 of the analysis grating 30 in each of the displacement positions from at least one multi-fringe in- terference pattern 64 from each displacement position.

In accordance with a first embodiment, the positional information item 78 is determined by ascertaining the precise positional difference between the respective displacement position and the respectively adjacent displacement position. Here, the positional difference in this case is the position difference in the direction that is lateral in relation to the optical axis 20, i.e. in the xy-plane. Expressed differently, the first evaluation unit 36-1 determines the precise phase difference or position difference between all adjacent displacement positions. To this end, the phase is initially determined for a plurality of image points or pixels of each multi- fringe interference pattern 64 according to a multi-fringe evaluation method that is known to a person skilled in the art. By way of example, in the case of horizontally extending fringes, phase values are ascertained by means of a Fourier analysis for one or more image points in a column for each vertically extending column of image points. As a result, a phase distribution is then available for a plurality of image points of each multi-fringe interference pattern 64. By way of example, such a phase distribution comprises phase values for image points of one or more lines of each multi-fringe interference pattern.

Now, a phase difference is calculated for the same image points with a known phase between multi-fringe interference patterns 64 of adjacent displacement positions. By way of example, a phase difference is determined pixel-by-pixel be- tween the image points of the same line of multi-fringe interference patterns 64 of adjacent displacement positions. Since the phase difference should be the same for all image points of multi-fringe interference patterns 64 of adjacent displacement positions, there finally is averaging over the phase differences of the difference distribution of adjacent displacement positions. In this way, a phase differ- ence or position difference in the lateral direction is ascertained very precisely between all adjacent displacement positions.

While the lateral position of the analysis grating 30 can be determined in a specific displacement position on the basis of the fringe position of the associated multi- fringe interference pattern 64, an axial position of the analysis grating 30, i.e. the position thereof in the z-direction, can be determined on the basis of the fringe density of the associated multi-fringe interference pattern 64. In accordance with a second embodiment, the positional information item 78 of the analysis grating 30 is determined three-dimensionally in the respective displacement position, i.e. in the form of x-, y-, and z-coordinates of the analysis grating in relation to an adjacent displacement position. Expressed differently, positional information items 78 are determined in the form of three-dimensional positional differences between the individual displacement positions. Furthermore, the evaluation of the 3-dimensional positional information item allows, in a manner analogous to the triangulation measurement method, the ascertainment of the tilt position of the analysis grating 30 in respect of all three spatial directions if use is made of at least three control beam paths in the form of monitoring channels 46. In accordance with a third embodiment, the positional information item 78 of the analysis grating 30 is determined in the respective displacement information item in all six spatial positions, i.e. in relation to the de- grees of translational freedom in the x-, y- and z-directions and in relation to the degrees of tilt freedom about the spatial axes in the x-, y- and z-directions. Here too, the positional information item of the analysis grating 30 in the six spatial directions is determined in relation to the respective adjacent displacement position. Expressed differently, positional information items 78 are determined in the form of six-dimensional positional differences between the individual displacement positions. As a result, it is possible to quantitatively capture twists/z-rotations of the analysis grating 30 as a result of, or during, the phase shifts. In particular, the path of the analysis grating 30, which may reflect a hilly toboggan run in the nm and sub-nm range, can be captured quantitatively during the shearing interferometry. Additionally, a temporal and local brightness variation of the measurement radiation 26 prior to the entrance thereof into the optical imaging system 12, in particular a temporal and local brightness variation of the measurement radiation at the location of the coherence mask, is determined in the evaluation device 36 by means of an optional second evaluation unit 36-2. To this end, for example, at least one multi-fringe interference pattern 64 is selected from each displacement position and a constant light portion is determined for each image point by an averaging of adjacent image points over one or more fringe periods. Then, a pixel- resolved distribution of the constant light portion and hence an illumination distribution of the pupil 22 are present for each multi-fringe interference pattern 64.

Subsequently, an average is formed pixel-by-pixel of the multi-fringe interference patterns 64 from all displacement positions. A mean constant light portion emerges for each image point therefrom. Subsequently, the difference between the constant light portion and the mean constant light portion is formed at each image point of a multi-fringe interference pattern and a correction factor is determined by means of this difference. By way of example, the correction factor corresponds to the quotient of difference of the constant light portion and mean value of the con- stant light portion. Finally, this correction factor that was determined pixel-by-pixel and for a displacement position is used to correct the corresponding image points of the interferogram 62 captured in the respective displacement position such that appropriate brightness-corrected interferograms 62k are produced by the second evaluation unit 36-2.

A third evaluation unit 36-3 of the evaluation device 36 determines, with the aid of a phase shift method, the spatial derivative of the wavefront of the measurement radiation 26 at the capturing area 34 in the used displacement directions using, for all displacement positions, the lateral positional differences contained in the ascertained positional information items 78 between adjacent displacement positions and the brightness-corrected interferograms 62k or the interferograms 62 in an embodiment variant without the second evaluation unit 32-2. Here, use is made of a discrete Fourier analysis or a cosine fit instead of a Fast Fourier Trans- form (FFT) on account of the non-equidistant displacement positions or phase differences. Then, the topography of the wavefront of the measurement radiation 26 after passing through the optical imaging system 12 is determined from the spatial derivatives. The wavefront aberration 80 of the optical imaging system 12 is determined by a comparison of the ascertained topography of the wavefront with an intended wavefront. The further degrees of freedom of the positional differences (z-direction, tilt positions), which are possibly present in addition to the lateral positional differences, may be used to calibrate systematic "track errors" of the displacement device that moves the analysis grating 30 and may optionally be used to take these deviations into account as correction values in the controller. Moreover, these positional deviations may also be taken into account in the evaluation algorithms of the phase measurement.

Additionally, the evaluation device 36 can directly determine a defocus aberration or astigmatic aberration of the optical imaging system 12 from the multi-fringe in- terference patterns 64. To this end, the evaluation device 36 carries out e.g. an x- and y-differentiation of the phase distribution of a multi-fringe interference pattern 64 from one displacement position and determines the focus as mean value of the x- and y-tilt of the derivatives or determines the astigmatism as difference between the x- and y-tilt. Since the interferograms of all displacement positions are taken into account when determining the focus and the astigmatism of the optical imaging system 12 by means of the phase shift method, a temporal drift of the imaging properties of the imaging system 12 may lead to aberrations. These may be ascertained and corrected by the direct determination from a multi-fringe interference pattern 64, which can be carried out quickly.

The present description of exemplary embodiments is to be understood to be by way of example. The disclosure effected thereby firstly enables the 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 the 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.

List of reference signs

10 Measurement system

12 Optical imaging system

14 Object plane

16 Image plane

18 Optical element

20 Optical axis

21 Aperture stop

22 Pupil

23 Radiation source

24 Irradiation device

26 Measurement radiation

28 Coherence mask

29 Mask holder

30 Analysis grating

31 Grating holder

32 Detection device

33 Detection stage

34 Capturing area

36 Evaluation device

36-1 First evaluation unit

36-2 Second evaluation unit

36-3 Third evaluation unit

37 Memory

38 Pinhole stop

40 Measurement channel

42 Translation direction

44 Defocussing optical eler

46 Monitoring channel

Image-side defocussing optical element Moving defocussing optical element 54 Coherence structure region

55 Measurement coherence region

56 Grating region

58 Measurement channel region

60 Monitoring channel region

62 Interferogram

63 Detection stage

64 Multi-fringe interference pattern

66 Second measurement radiation with a different wavelength 68 Second radiation source

70 Circular ring structure coherence mask

72 Circular ring structure analysis grating

74 Elliptic ring structure coherence mask

76 Elliptic ring structure analysis grating

78 Positional information item

80 Wavefront aberration

82 Deflection element

123 Radiation source

Claims

1. A measurement system for determining a wavefront aberration of an optical imaging system, comprising an irradiation device for passing measurement radia- tion through the imaging system, an analysis grating which, disposed downstream of the imaging system, is arranged in the beam path of the measurement radiation in a manner displaceable transversely to an optical axis of the imaging system, and a detection device for recording a radiation distribution of the measurement radiation, wherein the measurement system is configured to

produce respective interferograms, formed by means of the analysis grating, at a plurality of displacement positions of the analysis grating for the purposes of being recorded on the detection device, and

ascertain at least one positional information item of the analysis grating in at least one of the displacement positions by means of a control beam path that passes through the optical imaging system.

2. The measurement system according to Claim 1 ,

wherein the at least one positional information item comprises a position specification for the analysis grating in the lateral direction and/or in the axial direction in relation to the optical axis, and/or a specification in respect of a tilt position of the analysis grating.

3. The measurement system according to Claim 1 or 2,

wherein the at least one positional information item comprises a difference in posi- tion between the displacement positions of the analysis grating in the lateral direction in respect of the optical axis.

4. The measurement system according to any one of the preceding claims, which is configured to ascertain the at least one positional information item of the analysis grating by means of at least two different control beam paths that pass through the optical imaging system.

5. The measurement system according to any one of the preceding claims, which furthermore comprises an evaluation device configured to determine a topography of the wavefront of the measurement radiation after passing along the measurement beam path of the imaging system from the interferograms recorded at the individual displacement positions, using the at least one ascertained positional information item.

6. The measurement system according to Claim 5,

wherein the evaluation device is configured to carry out a discrete Fourier analysis when determining the topography of the wavefront of the measurement radiation.

7. The measurement system according to any one of the preceding claims, which is furthermore configured to produce a respective multi-fringe interference pattern at the displacement positions for the purposes of being recorded on the detection device, said multi-fringe interference pattern being produced by means of the analysis grating, wherein the multi-fringe interference pattern comprises at least one complete period of alternating fringes of maximum constructive interference and maximum destructive interference, and the measurement system is furthermore configured to ascertain the at least one positional information item of the analysis grating on the basis of the recorded multi-fringe interference patterns.

8. The measurement system according to Claim 7,

which is furthermore configured to undertake the ascertainment of the positional information item of the analysis grating by determining the phase distributions that underlie the corresponding multi-fringe interference patterns, determining a difference distribution by forming the difference of the determined phase distributions and averaging a plurality of values from the difference distribution.

9. The measurement system according to any one of the preceding claims, which is configured to irradiate radiation contained in the control beam path onto the analysis grating in a defocused condition.

10. The measurement system according to any one of the preceding claims, which comprises a defocussing optical element which is arranged in the imaging beam path of the optical imaging system for the defocussed radiation of the measurement radiation onto the analysis grating.

11. The measurement system according to any one of the preceding claims, wherein the irradiation device has a wave-forming coherence structure for the defocussed radiation of the measurement radiation onto the analysis grating, said coherence structure being arranged offset in relation to an object plane of the im- aging system, and/or a region of the analysis grating that serves to produce a multi-fringe interference pattern is arranged offset in relation to an image plane assigned to the object plane.

12. The measurement system according to any one of Claims 7 to 1 ,

which is furthermore configured to determine defocus aberration and/or astigmatic aberration of the optical imaging system directly from the respective multi-fringe interference pattern.

13. The measurement system according to any one of the preceding claims, which is configured in such a way that the measurement radiation in the control beam path passes through a smaller surface area of a pupil of the optical imaging system than the measurement radiation for producing one of the interferograms.

14. The measurement system according to any one of the preceding claims, wherein the measurement radiation in the control beam path of a multi-fringe interference pattern has a different wavelength than the measurement radiation for producing one of the interferograms.

15. The measurement system according to Claim 14,

wherein the wavelength of the measurement radiation for a multi-fringe interference pattern is selected in such a way that a chromatic aberration of the optical imaging system causes a defocussing of the measurement radiation that is suitable for producing the multi-fringe interference pattern.

16. The measurement system according to any one of the preceding claims, wherein a region of the analysis grating and/or of a coherence mask that is arranged on the input side in relation to the optical imaging system comprises ring- shaped structures.

17. The measurement system according to any one of Claims 7 to 16,

which is furthermore configured to determine a brightness variation of the measurement radiation prior to the latter's entrance into the optical imaging system by means of a multiplicity of the captured multi-fringe interference patterns.

18. A microlithographic projection exposure apparatus, comprising a projection lens for imaging mask structures onto a wafer, and the measurement system according to any one of the preceding claims for determining a wavefront aberration of the projection lens.

19. A method for determining a wavefront aberration of an optical imaging system, comprising the following steps:

- passing measurement radiation along a measurement beam path of the imaging system,

- arranging a diffractive analysis grating in the exit-side measurement beam path of the imaging system and displacing the analysis grating transversely to an opti- cal axis of the imaging system,

- recording respective interferograms that are formed by means of the analysis grating on a detection device, at a plurality of displacement positions of the analysis grating,

- ascertaining at least one positional information item of_. the analysis grating in at least one of the displacement positions by means of a control beam path that passes through the optical imaging system, and - determining a topography of the wavefront of the measurement radiation after passing along the measurement beam path of the imaging system from the inter- ferograms recorded at the individual displacement positions, using the at least one ascertained positional information item.