DE102016212464A1 - Measuring device for determining a wavefront error - Google Patents

Abstract

The invention relates to a measuring device (10) for determining a wavefront aberration of an optical imaging system (12) having an irradiation device (24) for irradiating the imaging system (12) with a measuring radiation (26), an analysis grating (30), the imaging system (12 ), in the beam path (40) of the measuring radiation transversely to an optical axis (20) of the imaging system (12) is arranged displaceably, and a detection device (32) for recording a radiation distribution of the measuring radiation (26). The measuring device (10) is configured to generate respective interferograms (62) formed by the analysis grating (30) at a plurality of displacement positions of the analysis grating (30) for recording on the detection device (32). Furthermore, the measuring device (10) is configured to determine at least one position information (78) of the analysis grid (30) in at least one of the displacement positions by means of a control beam path (46) passing through the optical imaging system (12).

Description

Background of the invention

The invention relates to a measuring device for determining a wavefront error of an optical imaging system. Furthermore, the invention relates to a projection exposure apparatus for microlithography with a projection objective for imaging mask structures onto a wafer and to a method for determining a wavefront error of an optical imaging system.

For high-precision measurement of optical imaging systems, such as a projection objective for microlithography, for example, shear interferometry, also referred to as "shearing interferometry", is used. Scher interferometry is a phase-shifting interferometry technique. For a determination of a wavefront error of an optical imaging system, e.g. a coherence mask in the object plane and a phase-shifting structure in the image plane, such as, for example, a displaceable diffraction grating, also referred to below as analysis grids. The analysis grid is moved in small steps across the optical axis of the imaging system. From the interference patterns or shearograms detected by the detector, the location derivative of the wavefront in the direction of movement of the analysis grid and therefrom the topography of the wavefront and finally a wavefront error of the optical imaging system can be determined.

The WO 01/63233 A2 describes various shear interferometry-based measuring devices for determining a wavefront in optical systems. In addition to using shadow masks with different two-dimensional aperture patterns as a coherence mask in the object plane of an imaging optical system, measuring devices with a simultaneous generation of one measuring beam each for different field points of the object plane are also shown. For this purpose, for example, a plurality of focusing lenses are arranged in the beam path, which each focus a part of the measuring radiation on one of a plurality of openings of a shadow mask in the object plane. With such a multi-channel measuring device, the optical system can be measured simultaneously for a plurality of field points with regard to its imaging properties.

One problem with the shear interferometric technique described is the need for high accuracy positioning of the coherence mask and the stepwise translation of the analysis grid. During a measurement, positions of the analysis grid must be exactly approached and maintained down to a few nanometers. If this condition is not met, errors in the phase determination and thus measurement errors in a wavefront determination will follow. In order to reduce measurement errors, the prior art generally places high demands on the rigidity and control accuracy of the measuring device in order to achieve the most accurate possible positioning of the analysis grid for individual lateral displacement steps and the coherence mask in relation to the analysis grid.

Another problem is brightness fluctuations of the measuring radiation. These also lead to measurement errors in a phase determination with a time-series phase shift method. A consideration of brightness fluctuations would ideally take place in a spatially resolved manner over a cross section of the beam path. However, a very complex decoupling and spatially resolved determination of the brightness of the measuring radiation is often dispensed with and instead at most a time-dependent mean brightness value is determined, which results in further inaccuracies in a determination of a wavefront.

Underlying task

It is an object of the invention to provide an apparatus and a method whereby the above-mentioned problems are solved and in particular a reduction of measurement errors in a determination of a wavefront error of an optical imaging system is achieved.

Inventive solution

The above object can be achieved according to the invention, for example, with a measuring device described below for determining a wavefront error of an optical imaging system. The measuring device according to the invention comprises an irradiation device for irradiating the imaging system with a measuring radiation, an analysis grating, which is disposed displaceably in the beam path of the measuring radiation, transverse to an optical axis of the imaging system, and a detection device for recording a radiation distribution of the measuring radiation. In this case, the measuring device according to the invention is configured to generate respective interferograms formed by the analysis grid at a plurality of displacement positions of the analysis grid for recording on the detection device, and to determine at least one position information of the analysis grid in at least one of the displacement positions by means of a control beam path passing through the optical imaging system.

As mentioned above, the measuring device according to the invention comprises an irradiation device for irradiating a measuring beam path of the imaging system with a measuring radiation. The measuring beam path is also referred to below as the measuring channel. Preferably, the irradiation device is designed so that the measuring radiation has a wavelength which corresponds to an operating wavelength of the optical imaging system. For this purpose, it is possible to use an operating beam source to provide the measuring radiation. The measuring device can be suitably configured for a specific measuring radiation from the infrared to the x-ray range. For example, in the case of a projection objective for microlithography with EUV radiation (extreme ultraviolet radiation), measuring radiation having a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm, may be used. The irradiation device may further include a coherence mask having a one- or two-dimensional structure in the object plane of the imaging optical system, or focusing elements for focusing a respective part of the measurement radiation into field points of the object plane for a multichannel shear interferometry.

Furthermore, the measuring device according to the invention comprises a diffractive analysis grating arranged displaceably transversely to an optical axis of the imaging system in the output-side measuring beam path of the imaging system, and a detection device for recording a radiation distribution of the measuring radiation. The analysis grid can be designed, for example, as a phase grating, amplitude grating or with another suitable diffraction grating type. For a measuring radiation with a very short wavelength, the analysis grid can also be configured as a reflective grid. The detection device comprises, for example, a spatially resolving CCD sensor with a detection surface, which contains a two-dimensional arrangement of individual sensors.

The measuring device according to the invention is configured to generate respective interferograms formed by means of the analysis grid at a plurality of displacement positions of the analysis grid for recording on the detection device

The interferograms are e.g. generated by interference of each formed at the analysis grating radiation zeroth diffraction order with radiation of a higher diffraction order, such as the first diffraction order. By moving the analysis grid, a so-called "temporal phase shift" occurs. In this case, the phase of the higher diffraction order changes, while the phase of the zeroth diffraction order remains the same, whereby the respective interferogram changes.

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 konstroll beam path. This second region, according to an embodiment explained in more detail below, can serve to form a multi-strip interference pattern. Hereinafter, a beam path with which the interferograms are formed is also referred to as a "measurement channel" and the control beam path is also referred to as a "monitoring channel". The term "monitoring channel" is due to its function of monitoring the exact displacement positions of the analysis grid.

Furthermore, the measuring device according to the invention is configured to determine at least one position information of the analysis grid in at least one of the displacement positions by means of the control beam path. The position information of the analysis grid in one of the displacement positions may include an absolute position of the analysis grid of the displacement position or a relative position of the analysis grid in the displacement position with respect to the position of the analysis grid in another displacement position and thus a position difference between the positions in two displacement positions.

From the interferograms recorded at the individual displacement positions, using the at least one position information, a topography of the wavefront of the measurement radiation can be determined after passing through the measurement beam path of the imaging system.

From the topography of the wavefront of the measuring radiation determined by means of the measuring device according to the invention, a deviation of the actual shape of the wavefront from a desired wavefront, which is e.g. may be in the form of a spherical wave or a plane wave. In this case, the topography of the wavefront can also be specified on the basis of a phase distribution of the measurement radiation along a surface defined by the desired wavefront. From the thus determined deviation of the actual shape of the wavefront from the desired wavefront can be or can in turn determine one or more wavefront errors of the optical imaging system.

By providing a control beam path and the resulting determination of the at least one position information of the analysis grid in at least one of the displacement positions, positioning errors of the analysis grid at the individual displacement positions can effectively be determined from the topography determination of the wavefront be calculated out. Thus, measurement errors occurring in the determination of the wavefront error of the optical imaging system can be significantly reduced.

According to one embodiment, the at least one position information comprises a position specification of the analysis grid in lateral and / or axial direction with respect to the optical axis and / or an indication with respect to a tilted position of the analysis grid. In particular, the position information comprises position information in at least one, at least two, at least three, at least four, at least five or in all six spatial positions. The six spatial positions are to be understood as meaning the position coordinates in the three spatial directions as well as the respective rotational / tilt positions with respect to the three spatial directions. In particular, position information for a plurality of the displacement positions are determined, which describe an exact "trajectory" of the analysis grid when passing through the individual displacement positions. The course of the course, like the course of a hilly toboggan run, can be characterized not only by spatial coordinates but also by tilting coordinates.

According to a further embodiment, the at least one position information comprises a positional difference between the displacement positions of the analysis grid in a lateral direction with respect to the optical axis.

According to a further embodiment, the measuring device is configured to determine the at least one position information of the analysis grid by means of at least two, in particular at least three, four or more, different, the optical imaging system traversed control beam paths. Thus, e.g. a combination of the measurements of several control beam paths analogous to the procedure in a triangulation e.g. the determination of tilt positions.

According to a further embodiment, the measuring device further comprises an evaluation device which is configured to determine a topography of the wavefront of the measuring radiation after passing through the measuring beam path of the imaging system from the interferograms recorded at the individual displacement positions using the at least one determined position information. According to one embodiment, the evaluation device is configured to perform a discrete Fourier analysis when determining the topography of the wavefront of the measurement radiation.

According to another embodiment, the measuring device is configured to generate a respective multi-strip interference pattern generated by the analysis grid at the displacement positions for recording on the detection device, the multi-strip interference pattern comprising at least one full period of alternating strips of maximum constructive and maximum destructive interference, and the measuring device is further configured to determine the at least one location information of the analysis grid from the recorded multi-strip interference patterns. The recorded multi-stripe interference patterns comprise at least one full period, more particularly at least two, at least five or at least ten full periods of alternating strands of maximum constructive and maximum destructive interference. Maximum constructive interference is an intensity value in the interference pattern that corresponds to the maximum intensity value that can be achieved by means of the diffraction grating used. The maximum achievable intensity value of a diffraction grating, which is not operated in the multi-strip mode, can be determined, for example, as follows: The diffraction grating is continuously displaced transversely to the incoming radiation, the intensity irradiated at a specific location of the detector recording the interference pattern varies several times with the movement of the diffraction grating between a maximum value and a minimum value. This maximum value is now the o.g. maximum achievable intensity value present in the stripe of maximum constructive interference in the multi-stripe interference pattern. The minimum value occurring in the displacement of the diffraction grating corresponds to the intensity value which is present at the maximum destructive interference in the multi-strip interference pattern.

According to a further embodiment according to the invention, the measuring device is further configured to determine the positional information of the analysis grid, in particular the position difference between the displacement positions of the analysis grid, by determining the phase distributions underlying the corresponding multiple-strip interference patterns, determining a difference distribution by subtraction of the determined phase distributions and averaging make several values from the difference distribution. The phase distribution underlying a multi-strip interference pattern is to be understood as meaning the local distribution of the phase difference of the interfering waves on which the interference pattern is based. The interfering waves may be e.g. on the one hand formed by a wave formed by the zeroth order of diffraction and on the other hand by a higher diffraction order, such as the first diffraction order, the analysis grid formed wave. These waves interfere on a detection surface of the detection device and due to their respective phase difference, which varies over the detection surface, lead to the o.g. Phase distribution.

For example, at each shift position, a multi-strip interference pattern is detected and a local phase distribution is determined in a suitable region of the multi-strip interference pattern. A subtraction can then take place between the specific phases for corresponding pixels of two multi-strip interference patterns of adjacent shift positions. The position difference between two adjacent displacement positions can be determined for example from a mean phase shift between these positions, which is determined by an averaging of the difference phases over all pixels.

According to a further embodiment of the measuring device according to the invention, the measuring device is configured to generate the multi-strip interference pattern in each case by defocused irradiation of the measuring radiation on the analysis grid. The defocused irradiation of the measuring radiation preferably takes place only in a region assigned to the multi-strip pattern and designated above as the second region. In this case, the measuring device can be designed so that the measuring radiation is focused on the analysis grid for generating interferograms for a shear interferometry, also referred to as "shearing interferometry" in the above-mentioned first region of the analysis grid. For example, the analysis grid for this purpose is arranged in an image plane of the optical imaging system. As a result of the defocused irradiation of the measurement radiation onto the analysis grid, the measurement radiation effectively impinges on the analysis grid in a bundle of measurement waves of different propagation directions. Each of the measurement waves contributes to interference, resulting in a multi-stripe interference pattern having at least one full period of alternating strands of maximum constructive and maximum destructive interference.

According to one embodiment of the invention, the measuring device comprises a defocusing optical element, which is arranged in the imaging beam path of the optical imaging system for defocused irradiation of the measurement radiation on the analysis grid. The imaging beam path of the optical imaging system is understood to mean the beam path lying between the image plane of the imaging system defined by the analysis grid and the object plane assigned to this image plane. As a defocusing optical element, for example, a refractive, diffractive or reflective optical element is used. The arrangement of the defocussing optical element may be in the input side region of the imaging system, i. upstream of the imaging system, or on the output side of the imaging system, i. downstream of the imaging system. In the case of the arrangement of the defocusing optical element in the input-side region of the imaging system, this part may be the irradiation device. In one embodiment variant of a defocusing element arranged downstream, it can be fixedly connected to the analysis grid, in particular attached to the surface of the analysis grid.

In a further embodiment of the measuring device according to the invention, the irradiation device has a wave-forming coherence structure for the defocused irradiation of the measurement radiation on the analysis grid, which is arranged offset relative to an object plane of the imaging system. Alternatively or additionally, a region of the analysis grid serving to generate the multi-strip interference pattern is offset relative to an image plane assigned to the object plane. In other words, according to this embodiment, the wave-forming coherent structure is arranged on the input side with respect to the imaging optical system.

According to one embodiment variant of the staggered arrangement of the coherence structure, the coherence structure can be part of a step-shaped coherence mask, in which an area serving to generate the interferograms is arranged offset in the object plane and a region having the coherence structure is offset relative to the object plane.

According to a further embodiment of the staggered arrangement of the region generating the multi-strip interference pattern, the analysis grating may be step-shaped such that an area serving to produce the interferograms is located in the image plane and the area serving to generate the multi-stripe pattern is offset from the image plane.

According to a further embodiment of the measuring device according to the invention, the evaluation device is further configured to determine focus errors and / or astigmatism errors of the optical imaging system directly from the respective multi-strip interference pattern. For example, the evaluation device for this purpose performs a derivation of a phase distribution determined from one or more multi-strip interference patterns and determines the focus or astigmatism from a linear tilting of the phase distribution determined from the derivation. In addition, consideration of the interferograms of the temporal phase shift is possible.

According to a further embodiment of the measuring device according to the invention, the evaluation device is configured to be used in the determination To perform a discrete Fourier analysis of the topography of the wavefront of the measurement radiation. In contrast, a fast Fourier transform (FFT) is conventionally performed in the evaluation of interferograms recorded by a shear interferometer. However, the fast Fourier transformation is based on constant phase steps between the individual interferograms and constant position differences between the displacement positions. By means of the discrete Fourier analysis, it is possible to take account of varying phase steps determined on the basis of the at least one determined position difference. In particular, with a discrete Fourier transformation, interferograms of displacement positions with non-constant phase steps can be used directly for a determination of the wavefront topography. An adjustment of the analysis grid to achieve constant position differences is therefore no longer necessary.

An embodiment of the inventive measuring device is configured such that the measuring radiation in the control beam path, in particular the measuring radiation for generating a multi-strip interference pattern, passes through a smaller area of a pupil of the optical imaging system than the measuring radiation for generating one of the interferograms. In other words, the beam path of a control beam path or monitoring channel is configured so that a smaller area is irradiated on a detection plane of the detection device than through a measurement channel. In this way, more measuring channels can be detected more accurately simultaneously or individual measuring channels over a larger area with the same detection device. In further embodiments, a control beam path for phase determination in a direction orthogonal to the first direction or a plurality of monitoring channels for respectively different directions is provided in addition to a control beam path for phase determination in a first direction.

According to a further embodiment of the measuring device according to the invention, the measuring radiation in the control beam path, in particular the measuring radiation for generating a multi-strip interference pattern, a different wavelength than the measuring radiation for generating one of the interferograms. For example, a first radiation source for generating a measurement radiation having a first wavelength for measurement channels and a second radiation source for generating a measurement radiation having a second wavelength for control beam paths or monitoring channels is provided. Preferably, the wavelength of the first radiation source corresponds to an operating wavelength of the optical imaging system. As the first radiation source, for example, an operation beam source of the imaging optical system can be used. With different wavelengths for measurement channels and monitoring channels, separate detection of multiple-strip interference patterns and interferograms is easier to perform.

In one embodiment of the inventive measuring device, the detection device is designed to be selective in color for the purpose of separating multi-strip interference patterns and interferograms with different wavelengths superposed in a detection surface. For example, the detection device includes color filters or a color camera, which are configured for separate detection of the multi-strip interference patterns at a first wavelength and the interferograms at a second wavelength.

According to a further embodiment of the measuring device according to the invention, the wavelength of the measuring radiation for a multi-strip interference pattern is selected so that a chromatic aberration of the optical imaging system causes a defocusing of the measuring radiation suitable for generating the multi-strip interference pattern. In this case, the measuring radiation for generating the interferograms preferably has an operating wavelength of the optical imaging system. In this way, even without a defocusing optical element, a defocusing of the measurement radiation of a monitoring channel for generating a multiple-strip interference pattern can be realized. The optical imaging system to be measured itself causes the defocusing. In one embodiment, both measuring beams can irradiate mutually overlapping regions on a detection plane, and the detection device can be designed to be color-selective for separating the measuring beams. In particular, in one embodiment, the measurement beams for a measurement channel and a monitoring channel may originate from the same location in the object plane of the optical imaging system. Alternatively, the beam paths of measurement channels and monitoring channels can be configured so that separate regions are irradiated on a detection plane of the detection device. In particular, spatially separate provision of the measuring radiation for measuring channels and monitoring channels at the input side of the optical imaging system can take place.

In a further embodiment of the measuring device according to the invention, a region of the analysis grid and / or a coherence mask arranged on the input side of the optical imaging system has annular structures. For example, the annular structures are formed as concentrically arranged circular structures. In a rotationally symmetric grid is a shear distance invariant to the shear direction. Phase shifting can take place in different directions, in particular in the case of a non-rectangular image field. Alternatively, concentrically arranged elliptical ring structures may also be provided. In this case, the grating period is dependent on the shear direction. This allows, for example, an exact alignment of the coherence mask and the analysis grid to each other.

Furthermore, in one embodiment of the invention, the evaluation device for determining a brightness fluctuation of the measuring radiation is configured prior to its entry into the optical imaging system by means of detected multi-strip interference patterns. In particular, a brightness fluctuation of the measuring radiation provided by the irradiation device is determined. For example, the evaluation device is designed to determine an average over one or more periods of a multi-strip interference pattern as a constant light component for each pixel of a detection surface of the detection device and a comparison of these constant light components in a plurality of successively detected multi-strip interference patterns. Further, based on the comparison, a determination of a correction factor for eliminating brightness fluctuations in successively detected interferograms may be provided.

According to the invention, a projection exposure apparatus for microlithography with a projection objective for imaging mask structures onto a wafer and the measuring apparatus according to one of the preceding embodiments or variants for determining a wavefront error of the projection objective are furthermore provided. For example, a radiation source of the projection exposure apparatus is used to provide the measurement radiation for generating interferograms. Furthermore, a wafer holder or wafer stage of the projection exposure apparatus can be used as the positioning means of the analysis grid. Likewise, it is possible to use a reticle stage as a holder and positioning device of a coherence mask.

Furthermore, the object can be achieved according to the invention by the method described below for determining a wavefront error of an optical imaging system. The method comprises the following steps: irradiating a measuring beam path of the imaging system with a measuring radiation, arranging a diffractive analysis grating in the output-side measuring beam path of the imaging system and moving the analysis grating transversely to an optical axis of the imaging system, recording respective interferograms formed by the analysis grating on a detection device at several Displacement positions of the analysis grid, determining at least one position information of the analysis grid in at least one of the displacement positions by means of a, the optical imaging system continuous control beam path, and determining a topography of the wavefront of the measuring radiation after passing through the measuring beam path of the imaging system from the recorded at the individual shift positions interferograms using the at least a determined position information.

In other words, analogously to the measuring device according to the invention with a method according to the invention, at least one position information of the analysis grid in at least one of the displacement positions is determined very accurately by means of a control beam path passing through the optical imaging system. The position information thus determined is used in determining a topography of the wavefront on the basis of the interferograms acquired in succession at different displacement positions, in particular for a phase shift or shear interferometry.

The features specified with respect to the above-mentioned embodiments, exemplary embodiments or variants, etc. of the measuring device according to the invention can be correspondingly transferred 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 the claims. The individual features can be realized either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments which are independently protectable and whose protection is possibly claimed only during or after pending the application.

Brief description of the drawings

The foregoing and other advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the accompanying diagrammatic drawings. It shows:

1 a first embodiment of a measuring device according to the invention for determining a wavefront error of an optical imaging system with a defocusing optical element arranged on the input side with respect to the imaging system in a schematic illustration,

2 a second embodiment of a measuring device with a defocusing optical element arranged on the output side with respect to the optical imaging system in a schematic illustration,

3 A third embodiment of a measuring device with a defocusing optical element attached to the analysis grid in a schematic illustration,

4 A fourth exemplary embodiment of a measuring device with a defocusing coherence structure arranged offset to the object plane of an optical imaging system in a schematic illustration,

5 A fifth exemplary embodiment of a measuring device with a region of an analysis grid arranged offset from the image plane of an optical imaging system, in a schematic illustration,

6 FIG. 2 a schematic representation of an arrangement of measuring channel and monitoring channel regions on a detection surface of a detection device according to an embodiment of a measuring device, FIG.

7 FIG. 2 a schematic representation of an arrangement of measuring channel regions and monitoring channel regions smaller than the measuring channel regions on a detection surface of a detection device according to a further exemplary embodiment of a measuring device, FIG.

8th FIG. 2 a schematic representation of an arrangement of measuring channel and monitoring channel regions with different wavelengths on a detection surface of a detection device according to a further exemplary embodiment of a measuring device, FIG.

9 a further embodiment of a measuring device with different wavelengths for measuring and monitoring channels and a color-selective detection device in a schematic illustration,

10 a further embodiment of a measuring device with different wavelengths for measuring and monitoring channels and separate detection areas on a detection surface in a schematic illustration,

11 a schematic representation of circular ring structures for a monitoring channel,

12 a schematic representation of elliptical ring structures for a monitoring channel,

13 a further embodiment of a measuring device for measuring an operating in the EUV wavelength range optical imaging system, and

14 a schematic illustration of the structure of an embodiment of a in the measuring devices according to the 1 to 5 . 9 . 10 . 12 or 13 included evaluation

Detailed description of inventive embodiments

In the embodiments or embodiments or design variants described below, functionally or structurally similar elements are as far as possible provided with the same or similar reference numerals. Therefore, for the understanding of the features of the individual elements of a particular embodiment, reference should be made to the description of other embodiments or the general description of the invention.

To facilitate the description, in some drawings a Cartesian xyz coordinate system is given, from which the respective positional relationship of the components shown in the figures results. In 1 the y-direction is perpendicular to the drawing plane out of this, the x-direction up and the z-direction to the right.

1 schematically shows a first embodiment of a measuring device 10 for determining a wavefront error of an optical imaging system 12 , The optical imaging system 12 serves to image field points of an object plane 14 into one of the object levels 14 assigned image plane 16 and includes optical elements 18 of which in 1 only two are shown by way of example. Furthermore, in 1 an optical axis 20 of the imaging system 12 shown parallel to the z-direction and arranged in a pupil plane aperture 21 to limit a pupil 22 indicated. The optical imaging system 12 is usually designed for possible aberration-free imaging at an operating or Nutzwellenlänge or a certain operating wavelength range. An example of such an optical imaging system is a microlithography projection objective for imaging mask patterns onto a wafer. Certain projection lenses are, for example, for microlithography with EUV radiation (extreme ultraviolet radiation) having a wavelength of less than 100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm, suitably configured. Accordingly, the measuring device 10 for an operating wavelength of the imaging optical system 12 suitably configured. Generally, the measuring device 10 be designed suitable for a wavelength from the infrared to the x-ray range.

With the measuring device 10 can be used for a determination of a wavefront error of the optical imaging system 12 perform a multichannel shear interferometry. Such based on the principle of phase shifting interferometry is, for example, in the WO 01/63233 described. The measuring device 10 includes an irradiation device 24 with a radiation source 23 to provide a suitable measuring radiation 26 as well as one in the area of the object plane 14 of the optical imaging system 12 arranged coherence mask 28 , Furthermore, the measuring device comprises 10 one in the area of the image plane 16 arranged diffractive analysis grid 30 , one in the beam path behind the analysis grid 30 arranged detection device 32 with a detection area 34 for the spatially resolved detection of measuring radiation 26 and an evaluation device 36 ,

The radiation source 23 provides a measuring radiation 26 with a sufficient intensity and coherence for a measurement of the optical imaging system 12 ready. The wavelength of at least part of the measuring radiation 26 corresponds to an operating wavelength of the optical imaging system 12 , For example, an operating radiation source for the optical imaging system 12 for generating the measuring radiation 26 used, such as a radiation source of a lighting system of a microlithography projection exposure apparatus in a measurement of a projection lens.

As is known to those skilled in the art, a microlithography projection exposure apparatus includes an illumination system for generating exposure radiation 204 eg in the form of DUV radiation, ie radiation in the deep UV wavelength range with a wavelength of eg 248 nm or 193 nm or EUV radiation (extreme ultraviolet radiation) with a wavelength of <100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm. The exposure radiation 204 encounters a lithography mask with mask structures to be imaged thereon. In this case, the exposure radiation and the lithography mask 206 reflected, as is often the case when using EUV radiation. Alternatively, the lithography mask can also be designed as a transmission mask. In this case, the exposure radiation passes through the mask. The image of the mask structures on a wafer arranged in an image plane takes place by means of the projection objective, which comprises a multiplicity of optical elements.

The coherence mask 28 includes in the in 1 shown embodiment in the object plane 14 extensive, two-dimensional arrangement of pinholes 38 and possibly additionally not shown focusing elements, which each have a portion of the measuring radiation 26 on a pinhole 38 focus. In this way, at the same time for a plurality of field points of the object plane 14 each provided a measuring radiation whose respective beam path hereinafter referred to as a measuring channel 40 or measuring beam path of the optical imaging system 12 referred to as. In 1 is exemplarily a beam path of one of these measurement channels 40 shown. With such a multi-channel measuring device 10 At the same time imaging properties of the optical imaging system can be 12 measured for a plurality of field points by shear interferometry. The beam path of a measuring channel 40 is preferably configured so that the measuring radiation with a spherical wavefront diverging from the pinhole 38 exit and from the optical imaging system to be measured 12 on the picture plane 16 is shown or focused. The measuring radiation illuminates as in 1 indicated the entire area of the aperture or pupil 22 out. For exact positioning of the coherence mask, an in 1 not shown translation module can be provided. In particular, in a measurement of a projection lens integrated in a projection exposure apparatus for microlithography, it is possible to use a reticle stage of the projection exposure apparatus as a translation module.

In alternative embodiments, only one measuring channel with one in the object plane 14 slidably formed pinhole be provided. Also, a plurality of apertured apertures for a measuring channel in a symmetrical two-dimensional arrangement to one another and in addition to circular also angular, such as squares or triangles, formed openings as pinholes in the coherence mask 28 be included. Furthermore, the coherence mask 28 have a two-dimensional, symmetrical structure of openings, which to a used analysis grid 30 for the suppression of interfering diffraction orders of the analysis grid 30 are adjusted. For further possible embodiments of coherence masks or irradiation devices and their description is particularly to the WO 01/63233 directed.

The diffractive analysis grid 30 is designed, for example, as a phase grating, amplitude grating or with another suitable diffraction grating type, for example as a gray-level grating or, for very small wavelengths, as a reflective grating. When Diffraction structure includes the analysis grid 30 a line grid, a cross grid, a checkerboard grid, a triangular grid, or other suitable periodic structure. The analysis grid 30 can be used for a phase shift in the context of shear interferometry together with the detection surface 34 the detection device 32 in a translation direction 42 which are substantially parallel to the x or y direction and thus transverse to the optical axis 20 is aligned, move. Further displacement directions perpendicular to the optical axis 20 Tilting axles may also be provided. A shift takes place stepwise in one direction with the aid of a positioning module which is not shown in the drawing. By interference from at the analysis grid 30 formed radiation of zeroth diffraction order with radiation of a higher diffraction order, such as the first diffraction order, on the detection surface 34 at each measuring channel 40 an interferogram 62 generated. By moving the analysis grid 30 a so-called "temporal phase shift" takes place. The phase of the higher diffraction order changes while the phase of the zeroth diffraction order remains the same, which causes the respective interferogram 62 changed. The distance between two adjacent displacement positions is in particular selected such that a phase shift suitable for shear interferometry occurs between these displacement positions. Typically, the distance is a fraction of the grating period of the analysis grid 30 , When measuring a projection lens integrated into a projection objective for microlithography as an optical imaging system 12 is a use of a wafer stage as a translation module possible.

The detection device 32 includes the radiation-sensitive detection surface 34 which contains a two-dimensional arrangement of individual sensors and is designed for example as a spatially resolving CCD sensor. Between analysis grid 30 and detection area 34 can an in 1 not shown optical arrangement for imaging an interferogram 62 or multi-strip interference pattern 64 on the detection surface 34 be provided. That of the detection device 32 recorded pattern 62 and 64 are sent to the evaluation device 36 transmitted.

Furthermore, the measuring device comprises 10 several defocusing optical elements 44 , which according to the embodiment according to 1 Part of the irradiation facility 24 are. The defocusing optical elements 44 are each such in the beam path of one of the measurement channels described above 40 between the coherence mask 28 and the optical imaging system 12 arranged that the measuring radiation 26 defocused on the analysis grid 30 incident. In 1 for the sake of clarity, only one of these defocusing elements 44 shown. Any defocusing optical element 44 forms a beam path for a part of the measuring radiation 26 whose focus is in front of or behind the analysis grid 30 and in the following also as a control beam path or monitoring channel 46 referred to as. Due to the defocusing is done with the analysis grid 30 instead of an interferogram for shear interferometry, a multi-stripe interference pattern 64 on the detection surface 34 educated. For this purpose, provided for the Scherinterferometrie structural pattern of the analysis grid 30 be used. Alternatively, the analysis grid 30 in the region of a monitoring channel also have a different pattern, in particular a different grating period or grid alignment.

The defocusing optical elements 44 may be formed as refractive elements, such as lenses or prisms, as reflective elements, such as mirrors, as diffractive elements or as optical arrangements of several of these elements. These are the defocusing optical element 44 configured and arranged such that the multi-stripe interference pattern of each monitoring channel 46 at least one full period, in particular at least two, at least five or at least ten full periods, of alternating strips of maximum constructive and maximum destructive interference. Alternatively, a defocusing optical element 44 be configured to generate multiple monitoring channels. It is also possible to use only part of the radiation of a measuring channel for a monitoring channel.

The evaluation device 36 is configured to determine a distance between two shift positions based on detected multi-band interference patterns 64 from one or more monitoring channels 46 at different displacement positions of the analysis grid 30 , Furthermore, the evaluation determined 36 using multi-band interference patterns 64 a lack of stability of the radiation source 23 Conditional local and temporal brightness course of the measuring radiation 26 ie one before entering the optical imaging system 12 , especially at the location of the coherence mask 28 , present local and temporal brightness course. Taking into account the determined distances between the shift positions and the detected interferograms of the measurement channels 40 The evaluation device determines in these postions 36 by discrete Fourier analysis a topography of the wavefront of the measuring radiation 26 after passing through the imaging system 12 , specifically after passing through the measuring channels 40 of the imaging system 12 , In this case, the local and temporal brightness curve of the measuring radiation 26 also be taken into account. A wavefront error of the optical imaging system 12 results from a deviation the particular topography of the wavefront from a nominal wavefront. In addition, the evaluation device 36 for determining the focus and astigmatism of the optical imaging system 12 be configured by means of detected multi-strip interference pattern. For these purposes, the evaluation includes 36 For example, an interface with a connection to the detection device 32 for receiving from the detection device 32 captured data, a memory for storing transmitted interferograms, multi-striped interference patterns and other data, and an electronic processing unit. Alternatively, it is also possible to store the detected interferograms and multi-strip interference patterns in a memory of the detection device 32 be provided for later evaluation or transmission to an external evaluation device. The mode of operation and the structure of the evaluation device 36 will be in connection with an embodiment of a method according to the invention in conjunction with 14 described in detail.

2 and 3 each show further embodiments of a measuring device 10 for determining a wavefront error of an optical imaging system 12 , In the execution after 2 is at least one defocusing optical element 50 at a fixed position in the beam path of the measuring radiation 26 between the optical imaging system 12 and the analysis grid 30 arranged. The execution according to 3 contains in the beam path of the measuring radiation 26 immediately before the analysis grid 30 at least one defocusing optical element 52 , which together with the analysis grid 30 is moved to different displacement positions. The defocusing optical element 52 can do this at the analysis grid 30 be attached or moved by a separate translation module. In contrast to the embodiment according to 1 prove the embodiments 2 and 3 So in each case at least one image side to the imaging system 12 arranged defocusing optical element 50 respectively. 52 on. The beam path of the monitoring channel 46 or control beam path of the measuring radiation 26 for the defocusing optical element 50 respectively. 52 thus initially corresponds to the beam path of a measuring channel 40 and only after the optical imaging system 12 by defocusing as a monitoring channel 46 educated.

Also in these embodiments, the defocusing optical elements 50 respectively. 52 as refractive optical elements, for example lenses or prisms, as reflective elements, such as mirrors, as diffractive elements or as optical arrangements of a plurality of these elements. Preferably, several monitoring channels 46 each with a defocusing optical element 50 respectively. 52 intended. In alternative embodiments, a defocussing optical element 50 respectively. 52 for generating several monitoring channels 46 configured, or it will only a part of the radiation of a measuring channel 40 for a monitoring channel 46 used. While in an object-side arrangement of a defocusing optical element 44 the beam path of the monitoring channel 46 through the optical imaging system 12 and in particular a transirradiated surface in a pupil can be influenced, is in an image-side arrangement of the defocusing optical element 50 respectively. 52 a mere change in the size of one of a monitoring channel 46 irradiated area on the detection surface 34 the detection device 32 possible. According to a further exemplary embodiment, a defocusing optical element for a monitoring channel can be provided both on the object side and on the image side 46 be provided.

Both the coherence mask 28 as well as the analysis grid 30 have in the above statements in areas for measuring radiation 26 for generating multi-band interference patterns 64 the same diffraction structures as in the areas for measuring radiation 26 for generating interferograms 62 on. In alternative designs is for monitoring channels 46 a different diffraction structure than for measuring channels 40 at the coherence mask 28 or the analysis grid 30 or at both. In particular, another grating period, grating pitch, or line structure may be used.

In 4 is another embodiment of a measuring device 10 shown schematically. This differs from the embodiment according to 1 in that instead of a defocusing optical element at least one coherence structure region 54 the coherence mask 28 in the beam path in front of the object plane 14 is arranged. The arrangement and the distance to the object plane 14 are configured to provide a suitable defocusing of the measuring radiation 26 for generating a multi-striped interference pattern 64 on the detection surface 34 takes place. The coherence structure area 54 is as part of the coherence mask 28 formed, which has a step-shaped surface. The coherence structure area 54 is arranged on a step, which with respect to the optical axis 20 in the axial direction opposite one for the measuring channels 40 used measurement coherence range 55 is offset. Alternatively, the coherence mask has 28 another suitable form for displacing the coherence structure region 54 from the object level 14 such as a wedge or ramp shape. Also, an arrangement of the coherence structure region 54 on a separate, from the coherence mask 28 separate carrier element possible. In another embodiment, the coherence structure region is 54 in the beam path the measuring radiation 26 behind object level 14 staggered. Preferably, the measuring device comprises 10 several monitoring channels 46 with one to the lens level 14 shift coherence structure area 54 , A coherence structure area 54 for a monitoring channel 46 can have the same structure as an area of the coherence mask 28 for a measuring channel 40 exhibit. Alternatively, it includes a coherence structure region 54 one opposite the areas for measuring channels 40 another structure, such as a different arrangement or formation of pinhole or another grid structure.

5 shows an embodiment in which, in contrast to the embodiment according to 4 at least one grid area 56 of the analysis grid 30 offset to the image plane 16 is arranged. An arrangement of the grid area 56 takes place in the beam path behind the image plane 16 in that a suitable defocusing of the measuring radiation 26 for generating a multi-striped interference pattern 64 on the detection surface 34 is reached. The analysis grid 30 For this purpose, a step-shaped, wedge-shaped or similarly configured surface for corresponding reception of the grid area is included 56 , Alternatively, the grid area 56 on one of the other structures of the analysis grid 30 opposite surface or on a separate support member separated from the analysis grid 30 be arranged. Preferably, the measuring device contains 10 several monitoring channels 46 with one to the image plane 16 offset grid area 56 , In other embodiments, at least one grating area in the beam path in front of the image plane 16 arranged or a displacement of a coherence structure region at the lens level 14 and a displacement of a grating area at the image plane 16 intended for the same monitoring channel. A grid area 56 for a monitoring channel 46 can have the same diffraction structure as an area of the analysis grid 30 for a measuring channel 40 exhibit. According to other embodiments, at least one grating region comprises one opposite to the regions of the analysis grating 30 for measuring channels 40 other diffraction structure, for example another grating period, grating pitch or line structure. A displacement of a grid area 56 only affects that of the monitoring channel 46 irradiated area of the detection area 34 during a displacement of a coherency structure area 54 the course of the monitoring channel 46 in the optical imaging system 12 affected.

6 schematically shows a detection surface 34 the detection device 32 a measuring device 10 which corresponds for example to one of the embodiments listed here. On the detection surface 34 are several measuring channel areas 58 and monitoring channel areas 60 shown. Each measuring channel 40 irradiates a measuring channel area 58 in which with the help of the analysis grid 30 one interferogram each 62 is generated for a phase shift evaluation. By defocusing the monitoring channels 46 will continue in each of a monitoring channel 46 irradiated monitoring channel area 60 a multi-strip interference pattern 64 educated. The arrangement of the measuring channel areas 58 and monitoring channel areas 60 on the detection surface 34 is essentially the coherence mask 28 and changes for different shift positions of the analysis grid 30 Not. Defocusing by defocusing optical elements 44 . 50 . 52 , Coherence structure areas 54 or grid areas 56 is configured so that the size of the monitoring channel areas 60 approximately that of the measuring channel areas 58 equivalent. Further, all multi-band interference patterns 64 the same strip orientation on.

In 7 becomes a detection area 34 the detection device 32 another measuring device 10 shown, in contrast to the measuring device 10 which corresponds to the channel area distribution according to 6 is assigned, the monitoring channel areas 60 by a corresponding defocusing smaller than the measuring channel areas 58 are. For this purpose, in particular a corresponding design and arrangement of object-side defocusing optical elements 44 or coherence structure areas 54 done in such a way that the respective monitoring channel 46 only part of the pupil 22 of the optical imaging system 12 goes through while the measurement channels 46 if possible the whole pupil 22 radiate. At least one monitoring channel radiates through 46 a smaller area of the pupil 22 of the optical imaging system 12 as a measuring channel 40 , Here is the size of the monitoring channel areas 60 chosen so that they are used for a position determination of the analysis grid 30 and a determination of the brightness profile of the irradiation device 24 provided measuring radiation 26 sufficient. With this measure is on the detection area 34 more space for the measuring channel areas 58 provided and thus allows a more precise Scherinterferometrieergebnis.

Furthermore, both monitoring channels 46 as well as measurement channels 40 for different displacement statements of the analysis grid 30 transverse to the optical axis 20 be configured. For this purpose, the analysis grid includes 30 For example, line grids with different orientation for measuring channels 40 and monitoring channels 46 , These different orientations are in 7 in the interference-generated multi-band interference patterns 64 in the monitoring channel areas 60 recognizable. In this way can be by appropriate shifts of the analysis grid 30 at the same time interferograms 62 and multi-striped interference patterns 64 for various displacement corrections, for example for a shift in the x and y directions (see 1 ) to capture.

8th shows a detection area 34 the detection device 32 a further embodiment of a measuring device 10 in the case of monitoring channels 46 a second measuring radiation is provided, which has a different wavelength than the first measuring radiation 26 which, for the measuring channels 40 is used. The measuring device 10 includes for example a further radiation source or an optical arrangement for generating the second measurement radiation from the first measurement radiation 26 , Furthermore, the detection area 34 or the detection device 32 color selectively formed. In another embodiment, color filters may instead be used to select between the measurement radiations for separate detection by the detection means 32 be used. The measuring channel areas 58 as well as the monitoring channel areas 60 are configured so large that they are on the detection surface 34 about divorce. A separation of the overlapping structures of the interferograms 62 and the multi-strip interference pattern 64 takes place with the color-sensitive detection device 32 , In addition to one due to the size of the measuring channel areas 58 sufficiently high resolution of the interferograms 62 for a very accurate phase shift method, due to the size of the monitoring channel areas, also for the multi-strip interference patterns 64 achieved a resolution, which in addition to a highly accurate determination of the position of the analysis grid 30 and, if appropriate, the temporal and spatial brightness curve of the measuring radiation provided by the irradiation device 26 also allows further evaluations. For example, with the multi-strip interference patterns 64 an additional focus or astigmatism determination is performed and in the determination of a wavefront error of the optical imaging system 12 received.

9 and 10 also provide embodiments of measuring devices 10 in which for the monitoring channels 46 a second measuring radiation 66 is used with a wavelength which differs from the wavelength of the measuring radiation 26 for the measuring channels 40 different. In contrast to the previous statements, the wavelength of the second measuring radiation 66 selected so that already by a chromatic aberration of the optical imaging system 12 a suitable defocusing for generating multi-channel strip patterns takes place. An example of such imaging systems 12 are projection objectives for microlithography. These are usually optimized for one operating wavelength only and have large aberrations at other wavelengths. The measuring device 10 can without its own defocusing optical elements, ie without, for example, the optical elements described above 44 . 50 respectively. 52 , as well as without the coherence structure regions described above 54 or grid areas 56 Multi-strip interference pattern for high-precision position determination of the analysis grid 30 at the capture level 34 produce.

In the embodiment according to 9 generates a radiation source 123 Measuring radiation with two different wavelengths, such that the generated measuring radiation, the first measuring radiation 24 as well as the second measuring radiation 66 includes. The measuring radiation 25 . 66 with two different wavelengths irradiated at least a portion of the coherence mask 28 , A measuring channel 40 thus becomes a monitoring channel at the same time 46 used. By chromatic aberrations of the optical imaging system 12 Defocusing of the second measuring radiation takes place 66 , The focus of the measuring radiation 66 lies before in the picture plane 16 arranged analysis grid 30 while measuring radiation for interferograms on the image plane 16 remains focused. For separate detection of itself in the detection area 34 overlapping multi-strip interference pattern and interferograms, the detection device is color selective, for example, as a color camera, formed or are provided corresponding color filter pivotable in the beam path.

The embodiment according to 10 sees a spatially separated irradiation of the coherence mask 28 with a first measuring radiation 26 for the measuring channels 40 and a second measuring radiation 66 with different wavelength for the monitoring channels 46 in front. This includes the measuring device 10 for example, a first radiation source 23 for providing the first measuring radiation 26 and a second radiation source 68 for providing the second measuring radiation 66 , The irradiation device 24 is configured so that the second measuring radiation is at other areas of the coherence mask 28 as the first measuring radiation 26 irradiated. These areas are selected so that monitoring channel areas 60 the second measuring radiation 66 on the detection surface 34 the detection device 32 not with measuring channel areas 58 the first measuring radiation 26 overlap. The second measuring radiation 66 the monitoring channels 46 when passing through the optical imaging system 12 Defocused by chromatic aberrations and forms in the monitoring channel areas 60 More fringe interference pattern 64 , The first measuring radiation 26 the measuring channels 40 stays on the picture plane 16 with the analysis grid 30 focuses and forms in the measurement channel areas 58 interferograms 62 for the shear interferometric evaluation. Due to the spatial separation of the measuring channel areas 58 from the monitoring channel areas 60 on the detection surface 34 may be a detection device 32 to be used without color selection.

In other embodiments of the measuring device 10 are in addition to the monitoring channels described above 46 to generate multi-band interference patterns, further auxiliary channels are provided for determining various parameters. For example, auxiliary channels can be formed by correspondingly formed structures in the coherence mask 28 , at the analysis grid 30 or both for determining translation or rotation of the coherence mask 28 or the analysis grid 30 for all spatial directions, of brightness gradients or for exact alignment of the coherence mask 28 to the analysis grid 30 be configured.

11 shows as an example of such structures a circular ring structure 70 a coherence mask 28 and a circular ring structure associated with this structure 72 an analysis grid 30 for forming an auxiliary channel for performing a radial shear interferometry. Due to the rotational symmetry of serving as a shear lattice ring structure 72 the shear distance is the same for all displacement directions. Such an auxiliary channel is particularly suitable for imaging systems with non-rectangular image field, for example for projection objectives of microlithography with a crescent-shaped object field. Each stepwise shift of the analysis grid during a measurement can thus be carried out in a different direction.

In 12 As another example, it becomes an elliptic ring structure 74 a coherence mask 28 and an elliptical ring structure associated with this structure 76 an analysis grid 30 shown schematically. One with these elliptical ring structures 74 . 76 trained auxiliary channel allows a radial shear interferometry, in which the shear distance for each shift direction is different. The grating period of the ring structure 76 depends on the direction or the angle of the shift. The elliptical shape of the on the coherence mask 28 arranged ring structure 74 allows accurate alignment of the coherence mask 28 to the analysis grid 30 ,

In 13 is another embodiment of a measuring device 10 which illustrates the measurement of an optical imaging system 12 is configured in the form of a projection objective of an EUV projection exposure apparatus for microlithography. The projection lens comprises in the embodiment shown, six optical elements 18-1 to 18-6 in the form of mirrors.

The measuring device 10 according to 13 with the exception of the features described below is analogous to the measuring device according to 1 configured. According to one of the special features, it is analogous to the measuring device 10 according to 10 trained that the measuring channels 40 with a different wavelength than the monitoring channels 46 be irradiated. Thus, the irradiation device comprises 24 a first radiation source 23 for generating a first measuring radiation 26 for the measuring channels 40 and a second radiation source 68 for generating a second measuring radiation 66 for the monitoring channels 46 , As second radiation source 68 For example, an LED source or a laser may be used. In the presentation of 13 are for the measurement channels 40 as well as the monitor channels 46 only one channel is shown as an example in each case. In the measuring radiation 26 this can be radiation with the operating wavelength of the projection objective to be measured, ie, EUV radiation. In case of integration of the measuring device 10 in a projection exposure system, the measuring radiation 26 be identical with the exposure radiation of the projection exposure system. The measuring radiation 26 is by means of an optical deflecting element 82 on the coherence mask 28 irradiated.

The coherence mask 28 In the embodiment shown, it is analogous to the coherence mask 28 according to 4 formed with a stepped surface. Here is a coherence structure area 54 for the monitoring channel 46 arranged on a stage, which with respect to the optical axis of the optical imaging system 12 in the axial direction relative to that for the measuring channel 40 used measurement coherence range 55 is offset. The displacement of the coherence structure area 54 is designed in the embodiment shown such that the associated focal plane above the analysis grid 30 is arranged. The coherence mask 28 is executed in the embodiment shown as a reflection mask, but in principle can also be configured as a transmisson mask. The coherence mask 28 is from a mask holder 29 which contains both a translation and a rotation of the coherence mask 28 with respect to all spatial directions.

The detection device 32 is on a detection table 63 arranged, which in integration of the measuring device 10 can be formed in a projection exposure system through the wafer stage. The detection table 63 is configured with respect to all spatial directions displaceable and tiltable. On the detection table 63 is a grid holder 31 for holding the analysis grid 30 arranged. The grid holder 31 allows displacements of the analysis grid with respect to the detection table 63 in all directions. In the 13 shown measuring device 10 for measuring an EUV projection lens can in further embodiments according to the in relation to 1 to 12 be configured described embodiments.

In the following, functions and interaction of the described components of the measuring device 10 described together with an embodiment of a method according to the invention.

For determining a wavefront error of an optical imaging system 12 , For example, a projection lens for microlithography, first the coherence mask 28 with the radiation source 23 and the analysis grid 30 with the detection device 32 in the optical imaging system 12 arranged. In a microlithographic projection exposure machine, the coherence mask can do this 28 from a reticle day and the analysis grid 30 together with the detection device 32 be picked up by a Waferstage. It can be used as irradiation device 24 a lighting system of the projection exposure system can be used.

Subsequently, a setpoint defocusing is set in the coherence mask 28 or at the analysis grid 30 provided defocusing optical element 44 . 50 . 52 by means of a positioning in the z-direction. When using a staggered coherence structure region 54 or a grid area 56 the target defocusing already occurs when the coherence mask is produced 28 or the analysis grid 30 set. For example, in the target defocusing 10 to 100 Strip over a diameter of a pupil 22 in a multi-band interference pattern 64 generated. In the selection of the strip density, a resolution of the detection device 32 be taken into account.

During a measurement process, the analysis grid is shifted 30 by means of a translation module starting from an initial position stepwise in fractions of the grating period of the analysis grid 30 , The individual displacements are formed as equidistant from each other as possible. For example, first displacements in one direction, such as the x-direction, and then displacements in a direction orthogonal to the first direction, eg the y-direction. Alternatively, an alternating displacement in both directions can take place. This approach is particularly useful for detecting temporal changes in properties of the optical imaging system to be measured 12 , which can occur for example by heating, suitable. Here, a time interval between measurements in mutually orthogonal directions should be as small as possible.

At each shift position, all of the detection area is recorded 34 generated interferograms 62 and multi-striped interference patterns 64 by the detection device 32 , The recorded interferograms 62 and multi-striped interference patterns 64 will be sent to the in 14 in an exemplary embodiment, the evaluation device shown in detail 36 transmitted and in a memory 37 the evaluation device 36 saved. Alternatively, a storage in the detection device 32 for a later transmission to the evaluation device 36 be provided. When using a two-dimensional analysis grid 30 For example, with a checkerboard or cross structure, during a detection period at a shift position, an oscillation of the analysis grid 30 take place transversely to the direction of displacement. In this way, by an integration time of the detection device 32 disturbing interference pattern suppressed.

As continues in 14 represented, determined in the evaluation device 36 a first evaluation unit 36-1 from at least one multi-strip interference pattern 64 each location information from each shift position 78 of the analysis grid 30 in each of the shift positions.

According to a first embodiment, the determination of the position information takes place 78 by determining the exact. Position difference of the respective displacement position to the respective adjacent displacement position. The position difference in this case is the position difference with respect to the optical axis 20 lateral direction, ie in the xy plane. In other words, the first evaluation unit determines 36-1 the exact phase difference or position difference between all adjacent shift positions. For this purpose, first a determination of the phase for a plurality of pixels or pixels of each multi-strip interference pattern 64 according to a multi-strip evaluation method known to the person skilled in the art. For example, in the case of horizontally extending strips, phase values for one or more pixels of the column are determined for each vertically extending column of pixels by means of a Fourier analysis. As a result, there is then a phase distribution for several pixels of each multi-striped interference pattern 64 in front. Such a phase distribution includes, for example, phase values for pixels of one or more lines of each multi-striped interference pattern.

Between multi-striped interference patterns 64 adjacent shift positions is now calculated for the same pixels with a known phase, a phase difference. For example, pixel by pixel becomes a phase difference between pixels of the same row of multi-band interference patterns 64 of adjacent shift positions. Since the phase difference for all pixels of multi-strip interference patterns 64 Finally, an averaging over the phase differences of the difference distribution of adjacent displacement positions takes place. On In this way, a phase difference or position difference in the lateral direction between all adjacent shift positions is determined very accurately.

While the lateral position of the analysis grid 30 in a certain displacement position based on the strip position of the associated Mehrstreifeninterenzmusters 64 can be determined, an axial position of the analysis grid 30 ie, its position in the z-direction based on the stripe density of the associated multi-strip interference pattern 64 be determined. According to a second embodiment, the determination of the position information takes place 78 of the analysis grid 30 in the respective displacement position three-dimensional, ie in the form of x, y, and z coordinates of the analysis grid with respect to an adjacent displacement position. In other words, location information 78 determined in the form of three-dimensional position differences between the individual displacement positions.

Furthermore, the evaluation of the 3-dimensional position information allows using at least three control beam paths in the form of monitoring channels 46 analogous to the measuring method of triangulation, the determination of the tilt position of the analysis grid 30 with respect to all three spatial directions. According to a third embodiment, the determination of the position information takes place 78 of the analysis grid 30 in the respective displacement information in all six spatial positions, ie with respect to the three translational degrees of freedom in the x, y or z direction as well as with respect to the tilt degrees of freedom about the spatial axes in the x, y and z directions. The determination of the position information of the analysis grid 30 in the six spatial directions also takes place here in relation to the respective adjacent displacement position. In other words, location information 78 determined in the form of six-dimensional position differences between the individual displacement positions. This can be twists / z rotations of the analysis grid 30 be detected quantitatively by or during the phase shift. In particular, the course of the analysis grid, which is the same in the nm and sub-nm range of a hilly toboggan run, can 30 are recorded quantitatively during shear interferometry. Additionally takes place in the evaluation 36 by means of an optional second evaluation unit 36-2 a determination of a temporal and local brightness variation of the measuring radiation 26 before entering the optical imaging system 12 , in particular a temporal and local brightness variation of the measuring radiation at the location of the coherence mask. For this purpose, for example, of each shift position, at least one multi-strip interference pattern 64 selected and determined for each pixel by averaging adjacent pixels over one or more stripe periods a DC component. It then lies for each multi-striped interference pattern 64 a pixel-resolved distribution of the direct light component and thus an illumination distribution of the pupil 22 in front.

This is followed pixel by pixel via the multi-strip interference pattern 64 all shift positions an averaging. This results in a middle steady light component for each pixel. For each pixel of a multi-strip interference pattern, the difference between the direct light component and the middle direct light component is subsequently formed and a correction factor is determined by means of this difference. For example, the correction factor corresponds to the quotient of the difference between the direct light component and the mean value of the constant light component. This pixel-by-pixel and for a shift position determined correction factor is finally used to correct the corresponding pixels of the detected in the respective shift position interferogram 62 used so by the second evaluation unit 36-2 corresponding brightness-corrected interferograms 62k be generated.

With the in the determined situation information 78 contained lateral position differences between adjacent displacement positions and the brightness-corrected interferograms 62k , or in a variant without the second evaluation unit 32-2 the interferograms 62 , for all displacement positions, a third evaluation unit determines 36-3 the evaluation device 36 with the help of a phase shift method, the local derivative of the wavefront of the measuring radiation 26 at the detection area 34 in the used displacement statements. In this case, a discrete Fourier analysis or a cosine fit is used instead of a fast Fourier transformation (FFT) because of the non-equidistant displacement positions or phase differences. The topography of the wavefront of the measurement radiation then becomes the location derivative 26 after passing through the optical imaging system 12 certainly. A comparison of the determined topography of the wavefront with a desired wavefront determines the wavefront error 80 of the optical imaging system 12 , The additional degrees of freedom in the position differences (z-direction, tilting positions) that may be present in addition to the lateral position differences can be used to systematically "track errors" of the analysis grid 30 To calibrate moving displacement device and, if necessary, to take these deviations as correction values in the control. In addition, these positional deviations can also be taken into account in the evaluation algorithms of the phase measurement.

In addition, the evaluation device 36 a focus error or astigmatism error of the imaging optical system 12 directly from the multi-strip interference patterns 64 determine. This is done by the evaluation device 36 for example, an x and y-derivative of the phase distribution of a multi-band interference pattern 64 from a shift position and determines the focus as the average of the x- and y-tilt of the derivatives or the astigmatism as the difference between the x- and y-tilt. As in determining the focus and astigmatism of the optical imaging system 12 By means of the phase shift method, the interferograms of all shift positions can be taken into account, a temporal drift of the imaging properties of the imaging system 12 lead to errors. These can be determined by the fast feasible direct determination of a multi-strip interference pattern 64 determine and correct.

The above description of exemplary embodiments is to be understood by way of example. The disclosure thus made makes it possible for the skilled person, on the one hand, to understand the present invention and the associated advantages, and on the other hand, in the understanding of the person skilled in the art, also encompasses obvious modifications and modifications of the structures and methods described. It is therefore intended that all such alterations and modifications as fall within the scope of the invention as defined by the appended claims, as well as equivalents, be covered by the scope of the claims.

LIST OF REFERENCE NUMBERS

10

measuring device

12

optical imaging system

14

object level

16

image plane

18

optical element

20

optical axis

21

aperture

22

pupil

23

radiation source

24

irradiation device

26

measuring radiation

28

coherence mask

29

headgear

30

analysis grid

31

grid support

32

detection device

33

detection table

34

detecting surface

36

evaluation

36-1

first evaluation unit

36-2

second evaluation unit

36-3

 third evaluation unit

37

Storage

38

pinhole

40

measuring channel

42

Translation direction

44

defocusing optical element

46

monitoring channel

50

image-defocusing optical element

52

moving defocusing optical element

54

Coherence structure area

55

Measuring coherence area

56

grating region

58

Measuring channel area

60

Monitoring channel region

62

interferogram

63

detection table

64

More fringe interference pattern

66

second measuring radiation with a different wavelength

68

second radiation source

70

circular ring structure coherence mask

72

circular ring structure analysis grid

74

elliptical ring structure coherence mask

76

elliptical ring structure analysis grid

78

Location information

80

Wavefront error

82

deflecting

123

radiation source

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 01/63233 A2 [0003]
• WO 01/63233 [0059, 0063]

Claims (19)

Measuring device ( 10 ) for determining a wavefront error of an optical imaging system ( 12 ) with an irradiation device ( 24 ) for irradiating the imaging system ( 12 ) with a measuring radiation ( 26 ), an analysis grid ( 30 ), which, downstream of the imaging system, in the beam path ( 40 ) of the measuring radiation transversely to an optical axis ( 20 ) of the imaging system ( 12 ) is arranged displaceably, as well as a detection device ( 32 ) for recording a radiation distribution of the measuring radiation ( 26 ), wherein the measuring device ( 10 ) is configured to be read by the analysis grid ( 30 ) formed interferograms ( 62 ) at several displacement positions of the analysis grid ( 30 ) for recording on the detection device ( 32 ), and by means of one, the optical imaging system ( 12 ) continuous control beam path ( 46 ) at least one position information ( 78 ) of the analysis grid ( 30 ) in at least one of the shift positions. Measuring device according to claim 1, wherein the at least one position information ( 78 ) a position specification of the analysis grid ( 30 ) in lateral and / or axial direction with respect to the optical axis and / or an indication relating to a tilted position of the analysis grid. Measuring device according to claim 1 or 2, wherein the at least one position information ( 78 ) a position difference between the displacement positions of the analysis grid ( 30 ) in relation to the optical axis ( 20 ) lateral direction. Measuring device according to one of the preceding claims, which is configured to store the at least one position information ( 78 ) of the analysis grid ( 30 ) by means of at least two different, the optical imaging system ( 12 ) to determine the transmitted control beam paths. Measuring device according to one of the preceding claims, which further comprises an evaluation device which is configured to display a topography of the wavefront of the measuring radiation ( 26 ) after passing through the measuring beam path ( 40 ) of the imaging system ( 12 ) from the interferograms recorded at the individual displacement positions ( 62 ) using the at least one determined position information ( 78 ). Measuring device according to Claim 5, in which the evaluation device ( 36 ) is configured to determine the topography of the wavefront of the measuring radiation ( 26 ) perform a discrete Fourier analysis. Measuring device according to one of the preceding claims, which is further configured to use a respective, by means of the analysis grid ( 30 ) generated multi-strip interference pattern ( 64 ), at the displacement positions for recording on the detection device ( 32 ), the multi-band interference pattern ( 64 ) comprises at least one full period of alternating strips of maximum constructive and maximum destructive interference, and the measuring device is further configured to provide the at least one location information ( 78 ) of the analysis grid ( 30 ) based on the recorded multi-band interference patterns ( 64 ) to investigate. The measuring device of claim 7, further configured to determine the location information of the analysis grid ( 30 ) by determining the corresponding multi-striped interference patterns ( 64 ), determination of a difference distribution by subtraction of the determined phase distributions and averaging of several values from the difference distribution. Measuring device according to claim 7 or 8, which is configured to detect the multi-strip interference patterns ( 64 ) in each case by defocused irradiation of the measuring radiation ( 26 ) on the analysis grid ( 30 ) to create. Measuring device according to claim 9, which comprises a defocussing optical element ( 44 . 50 . 52 ), which in the imaging beam path of the optical imaging system ( 12 ) for the defocused irradiation of the measuring radiation ( 26 ) on the analysis grid ( 30 ) is arranged. Measuring device according to Claim 9, in which the irradiation device ( 24 ) a waveshaping coherence structure ( 54 ) for the defocused irradiation of the measuring radiation ( 26 ) on the analysis grid ( 30 ), which are opposite an object plane ( 14 ) of the imaging system ( 12 ) and / or one of the generation of the multi-strip interference pattern ( 64 ) serving area ( 56 ) of the analysis grid ( 30 ) with respect to one of the object levels ( 14 ) associated image plane ( 16 ) is arranged offset. Measuring device according to one of claims 7 to 11, which is further configured to focus error and / or astigmatism error of the optical imaging system ( 12 ) directly from the respective multi-strip interference pattern ( 64 ). Measuring device according to one of the preceding claims, which is configured such that the measuring radiation in the control beam path has a smaller surface area of a pupil ( 22 ) of optical imaging system ( 12 ) radiates as the measuring radiation for generating one of the interferograms ( 62 ). Measuring device according to one of the preceding claims, in which the measuring radiation ( 66 ) in the control beam path of a multi-strip interference pattern ( 64 ) a different wavelength than the measuring radiation ( 26 ) for generating one of the interferograms ( 62 ) having. Measuring device according to Claim 14, in which the wavelength of the measuring radiation ( 66 ) for a multi-strip interference pattern ( 64 ) is selected so that a chromatic aberration of the optical imaging system ( 10 ) a defocusing of the measuring radiation suitable for generating the multi-strip interference pattern ( 66 ) causes. Measuring device according to one of the preceding claims, wherein a portion of the analysis grid ( 30 ) and / or an input side with respect to the optical imaging system ( 12 ) arranged coherence mask annular structures ( 70 . 72 . 74 . 78 ) having. Measuring device according to one of claims 7 to 16, which is further configured to a brightness variation of the measuring radiation ( 26 ) before it enters the optical imaging system ( 12 ) by means of a plurality of the detected multi-strip interference patterns ( 64 ). Projection exposure apparatus for microlithography with a projection objective for imaging mask structures on a wafer and the measuring device ( 10 ) according to any one of the preceding claims for determining a wavefront error of the projection lens. Method for determining a wavefront error of an optical imaging system ( 12 ) comprising the steps: - irradiation of a measuring beam path ( 40 ) of the imaging system ( 12 ) with a measuring radiation ( 26 ), - arranging a diffractive analysis grid ( 30 ) in the output-side measuring beam path of the imaging system ( 12 ) and moving the analysis grid ( 30 ) transverse to an optical axis ( 20 ) of the imaging system ( 12 ), - recording respective, by means of the analysis grid ( 30 ) on a detection device ( 32 ) formed interferograms ( 62 ) at several displacement positions of the analysis grid ( 30 ), - determining at least one position information ( 78 ) of the analysis grid ( 30 ) in at least one of the displacement positions by means of one, the optical imaging system ( 12 ) continuous control beam path, and - determining a topography of the wavefront of the measuring radiation ( 26 ) after passing through the measuring beam path ( 40 ) of the imaging system ( 12 ) from the interferograms recorded at the individual displacement positions ( 62 ) using the at least one determined position information ( 78 ).

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