TW201807389A - Measurement system for determining a wavefront aberration - Google Patents

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 displaceable 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 recorded 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

Measuring system for determining wavefront aberration [Related patent reference]

The present application claims priority to German Patent Application No. 10 2016 212 464.1, filed on Jul. 8, 2016. The entire content of this patent application is incorporated herein by reference.

The present invention relates to a measurement system that determines the wavefront aberration of an optical imaging system. Moreover, the present invention is directed to a lithographic projection exposure apparatus including a projection lens for imaging a reticle structure to a wafer, and a method for determining wavefront aberrations of an optical imaging system.

For example, shear interferometry is used to measure optical imaging systems very accurately, such as, for example, a lithographic projection lens. Shearing interferometry is a phase shifting interferometry technique. To determine the wavefront aberration of the optical imaging system, the coherent reticle is disposed, for example, at the object plane, and a phase shifting structure (eg, a displaceable diffraction grating, hereinafter also referred to as an analytic grating) is disposed in the image plane. The analysis grating is displaced in a small step transverse to the optical axis of the imaging system. The interference pattern or shear map captured from the detector determines the spatial derivative of the wavefront in the direction of motion of the analysis grating and thereby determines the topology of the wavefront and ultimately determines the wavefront aberration of the optical imaging system.

WO 01/63233 A2 describes different measuring systems based on shearing interferometry for determining the wavefront of an optical system. In addition to using a porous reticle having a two-dimensional aperture pattern as a coherent reticle on the object plane of the imaging optical system, a measuring beam having different field points for the plane of the object simultaneously formed in each case is also proposed. Measurement system. For example, for this A plurality of focusing lens elements are disposed in the beam path, each of the focusing lens elements focusing certain measurement radiation to one of a plurality of apertures of the porous reticle in the object plane. Using this multi-channel metrology system, the optical system can be simultaneously measured for its imaging characteristics for multiple field points.

A problem with the above described measurement system using shear interferometry is that the coherent reticle requires highly accurate positioning and progressive displacement of the analytical grating. The position of the analysis grating must be close to and maintained within a few nanometers of accuracy during the measurement. If this condition is not met, there will be errors in determining the phase and thus the measurement error in determining the wavefront. Therefore, the prior art has high requirements on the rigidity and control accuracy of the measurement system to reduce the measurement error, so as to obtain the most accurate analysis grating positioning and the coherent reticle relative analysis grating during the individual lateral displacement steps. Positioning.

Another problem is measuring the change in brightness of the radiation. These problems can also cause measurement errors when using time series phase shift methods to determine phase. Ideally, the spatial resolution in the cross section of the beam path should take into account the change in brightness. However, the very complex output coupling and the spatially resolved brightness decision of the measured radiation are usually omitted, and at most the time dependent average brightness value is determined, with the result that other inaccuracies will occur in determining the wavefront.

It is an object of the present invention to provide a system and method for solving the aforementioned problems, and in particular to reduce measurement errors in determining wavefront aberrations of an optical imaging system.

In accordance with the present invention, the foregoing objects can be achieved, for example, using a metrology system for determining the wavefront aberration of an optical imaging system as described below. The metrology system according to the present invention includes an illumination device for passing the measurement radiation through the imaging system, disposed downstream of the imaging system, and disposed in a beam path transverse to the optical axis of the imaging system. An analysis grating and a detecting device for recording the radiation distribution of the measured radiation. Here, the metrology system according to the invention is configured to generate an analysis at a plurality of displacement positions of the analysis grating A corresponding interferogram formed by the grating is recorded on the detecting device and configured to determine at least one positional information item of the analysis grating at at least one of the displacement positions by a control beam path through the optical imaging system.

As previously mentioned, the metrology system according to the present invention includes an illumination device for passing the measurement radiation along the path of the measurement beam of the imaging system. In the following, the measuring beam path is also referred to as a measuring channel. Preferably, the illumination device is implemented such that the measurement radiation has a wavelength corresponding to the operating wavelength of the optical imaging system. For this purpose, it is possible to use an operational radiation source for providing the measured radiation. The metrology system can also be suitably configured for specific measurement radiation from the infrared range to the x-ray range. For example, a measuring radiation having a wavelength of less than 100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm, can be used for a projection lens having lithography of EUV radiation (extreme ultraviolet radiation). Furthermore, the illumination device may comprise a coherent reticle having a one- or two-dimensional structure in the object plane of the optical imaging system or having a focusing element for focusing certain measured radiation to a field point of the object plane, respectively. For multi-channel shearing interferometry.

Furthermore, the metrology system according to the present invention comprises a diffraction analysis grating configured to be displaceable transversely to the optical axis of the imaging system in the exit side measurement beam path of the imaging system, and to include a radiation distribution for recording the measurement radiation a detection device. For example, the analysis grating can be implemented as a phase grating, an amplitude grating or with any other suitable diffraction grating type. The analysis grating can also be configured as a reflection grating for measuring radiation with very short wavelengths. For example, the detection device includes a spatially resolved CCD sensor having a capture area that includes a two-dimensional configuration of individual sensors.

The metrology system according to the present invention is configured to generate a corresponding interferogram formed by the analysis grating at a plurality of displacement locations of the analysis grating for recording on the detection device.

For example, the interferogram is produced by interference with zero order diffraction and interference with higher order diffraction (eg, first order diffraction, which is formed at the analysis grating, respectively). The result of the displacement analysis grating is a so-called "time phase shift". Here, the phase of the higher order diffraction changes, while the phase of the zeroth order diffraction remains the same, with the result that there is a change in the corresponding interferogram.

Analyzing the grating in the first region (which forms an interferogram) may have a second region Different patterns (which are assigned to the control beam path), especially different grating periods. This second region can be used to form a multi-strip interference pattern, according to a particular embodiment, which will be explained in more detail below. Hereinafter, the beam path forming the interferogram is also referred to as a "measurement channel" and the control beam path is also referred to as a "monitor channel." The name of the "monitor channel" is due to its ability to monitor the precise displacement position of the analysis grating.

Furthermore, the metrology system according to the invention is configured to determine at least one positional information item of the analysis grating at at least one of the displacement positions by controlling the beam path. A position information item of the analysis grating at at least one of the displacement positions may comprise an absolute position of the analysis grating at the displacement position, or a relative position of the analysis grating at the displacement position relative to the position of the analysis grating at the other displacement position, and thus comprises The difference in position between configurations at two different displacement positions.

The topology of the wavefront of the measured radiation after passing along the measurement beam path of the imaging system can be determined from the interferogram recorded at the individual displacement locations using at least one positional information item.

The deviation of the actual form of the wavefront from the expected wavefront, which may for example be in the form of a spherical wave or a plane wave, may be determined from the topology of the wavefront of the measured radiation as determined by the metrology system according to the invention. Here, the topology of the wavefront can also be specified based on the phase distribution of the measured radiation along the area defined by the expected wavefront. Accordingly, one or more wavefront aberrations of the optical imaging system can also be determined from the deviation of the actual form of the wavefront determined thereby from the expected wavefront.

By providing a control beam path and subsequent determination of at least one positional information item of the analysis grating, the positioning error of the analysis grating at the individual displacement locations can be effectively removed from the topology decision of the wavefront. Using this method, it will be possible to substantially reduce the measurement error that occurs when determining the wavefront aberration of the optical imaging system.

According to a specific embodiment, the at least one positional information item comprises a positional specification of the analysis grating relative to the optical axis in the lateral direction and/or in the axial direction, and/or a specification relating to an oblique position of the analysis grating. In particular, the location information item includes location specifications in at least one, at least two, at least three, at least four, at least five, or at all six spatial locations. Six spatial positions are understood to mean position coordinates in three spatial directions and corresponding rotational/tilt positions in three spatial directions. In particular, position information items are determined for a number of displacement positions that describe the precise "path trajectory" of the analysis raster as it passes through the individual displacement positions. Path trajectory (for example, more The route of the slope slide can be characterized not only by the space coordinates but also by the tilt coordinates.

According to another specific embodiment, the at least one positional information item comprises a positional difference between the position of the displacement of the analysis grating relative to the optical axis in the lateral direction.

According to another specific embodiment, the metrology system is configured to determine at least one positional information item of the analysis grating by at least two, in particular at least three, four or more different control beam paths of the optical imaging system . Thus, for example, the measurement of the plurality of control beam paths will facilitate determining the tilt position, for example, in a manner similar to the procedure of the triangulation process.

In accordance with another embodiment, the metrology system further includes an evaluation device configured to determine the measurement radiation along the imaging system from the interferogram recorded at the individual displacement locations using at least one determined position information item A topology of the wavefront after the beam path passes. According to a variant of a particular embodiment, the evaluation unit is configured to perform a discrete Fourier analysis when determining the topology of the wavefront of the measured radiation.

In accordance with another embodiment, the metrology system is configured to generate a corresponding multi-striped interference pattern at the displacement location for recording on the detection device, the multi-strip interference pattern being generated by the analysis grating, wherein the multi-strip interference pattern comprises a maximum The constructive stem involves at least one complete cycle of alternating fringes of maximum destructive interference, and the metrology system is further configured to determine at least one positional information item of the analytical grating based on the recorded multi-striped interference pattern. The recorded multi-strip interference pattern comprises at least one complete period of maximum constructive interference involving alternating interference of maximum destructive interference, in particular at least two, at least five or at least ten complete periods. The maximum constructive interference is understood to mean the intensity value in the interference pattern corresponding to the maximum intensity value that can be achieved by the diffraction grating used. For example, the maximum achievable intensity value of the diffraction grating (which is not operating in multi-strip mode) can be determined as follows: the diffraction grating is continuously transverse to the incident radiation displacement; as the diffraction grating moves, the recording interference The intensity of the radiation at a particular location of the pattern detector varies between maximum and minimum values several times. This maximum is now the maximum achievable intensity value that occurs in the aforementioned stripe of maximum constructive interference in the multi-strip interference pattern. The minimum value that occurs during the displacement of the diffraction grating corresponds to the intensity value that occurs with the most destructive interference in the multi-strip interference pattern.

According to another embodiment, the measurement system is further configured to be determined by The phase distribution under the multi-strip interference pattern, the difference in the phase distribution determined by the formation determines a difference distribution, and the average number of values from the difference distribution is averaged to determine the position information item of the analysis grating, in particular the displacement of the grating. The difference in position between locations. The phase distribution under the multi-strip interference pattern is understood to mean a local distribution of the phase difference of the interference wave under the interference pattern. For example, the interfering wave may first be formed by a wave formed by zero-order diffraction and then by a higher order diffraction (eg, first-order diffraction) at the analysis grating. These waves interfere on the capture area of the detection device and result in the aforementioned phase distribution due to their corresponding phase differences, which vary over the extraction area.

For example, a multi-strip interference pattern is taken at each displacement location and a local phase distribution is determined in a suitable region of the multi-strip interference pattern. A difference may then be formed between the determined phases of the respective image points of the two multi-strip interference patterns adjacent to the displaced position. The difference in position between two adjacent displacement positions can thus be determined, for example, from the average phase offset between these positions, which is determined by averaging different phases on all image points.

According to another specific embodiment, the metrology system is configured to illuminate the radiation contained in the control beam path onto the analysis grating in a defocused state.

According to a further embodiment of the measuring system according to the invention, the measuring system is configured to generate a multi-strip interference pattern in each case by measuring the defocused radiation of the radiation on the analysis grating. The measuring radiation is preferably only defocusedly radiated on a region assigned to the multi-strip pattern and referred to herein as the second region. Here, the metrology system can be implemented such that in the first region of the aforementioned analysis grating, the measurement radiation is focused on the analysis grating to produce an interferogram for shear interferometry. For example, the analysis grating is configured for this purpose in the image plane of the optical imaging system. Since the measurement radiation is defocused radiation on the analysis grating, the measurement radiation effectively strikes the analysis grating in the beam of the measurement wave having different propagation directions. Each measurement wave contributes to interference, and thus a multi-strip interference pattern having at least one full cycle of alternating stripes of maximum constructive dryness involving maximum destructive interference occurs.

In accordance with an embodiment of the present invention, the metrology system includes a defocusing optical element disposed in the imaging beam path of the optical imaging system for defocusing radiation of the measured radiation on the analysis grating. The imaging beam path of an optical imaging system is understood to represent the presence of light in the analysis The beam path between the image plane of the imaging system defined by the grid and the plane of the object assigned to this image plane. For example, refractive, diffractive or reflective optical elements are used as defocusing optical elements. The defocused optical element can be disposed in the input side region of the imaging system (ie, disposed upstream of the imaging system) or in the output side region of the imaging system (ie, disposed downstream of the imaging system). If the defocused optical element is disposed in the input side region of the imaging system, it can be part of the illumination device. In a specific embodiment variant of the defocusing element arranged downstream thereof, the defocusing element can be firmly connected to the analysis grating, in particular to the surface of the analysis grating.

In another embodiment of the metrology system according to the invention, the illumination device has a wave-forming coherent structure for defocusing radiation of the measured radiation on the analysis grating, the coherent structure being configured relative to the imaging system An object plane is offset. Alternatively or additionally, an area of the analysis grating used to generate the multi-strip interference pattern is configured to be offset relative to an image plane assigned to the object plane. In other words, with respect to the optical imaging system according to this embodiment, the wave forming coherent structure is disposed on the input side.

According to a specific embodiment variant of the offset arrangement of the coherent structure, the coherent structure may be part of a coherent reticle implemented in a stepwise manner, wherein a region for generating an interferogram is arranged in the plane of the object and has a coherent structure The area is configured to deviate from the plane of the object.

According to another embodiment variant of the offset arrangement of the region for generating the multi-strip interference pattern, the analysis grating can have a stepped embodiment such that an area for generating the interferogram is in the image plane and is used The area that produces the multi-striped pattern is configured to deviate from the image plane.

In another specific embodiment of the metrology system according to the present invention, the evaluation device is further configured to determine defocus and/or astigmatism of the optical imaging system directly from the corresponding multi-strip interference pattern. For example, the evaluation device calculates a differentiation of a phase distribution determined by one or more multi-striped coherent patterns for this purpose, and determines a focus or astigmatism by a linear tilt of the phase distribution determined by the differentiation. In addition, it is possible to consider an interferogram of time phase shifts.

In another embodiment of the metrology system according to the invention, the evaluation device is configured to perform a discrete Fourier analysis when determining the topology of the wavefront of the measured radiation. In contrast, when evaluating interferograms recorded by shearing interferometry, fast Fourier transforms are traditionally performed. (FFT). However, fast Fourier transform is based on a fixed position difference between fixed phase steps or displacement positions between individual interferograms. By discrete Fourier analysis, it is possible to consider varying phase steps that are determined based on at least one determined position difference. In particular, discrete Fourier transform allows the use of an interferogram with displacement positions of non-constant phase steps to determine the topology of the wavefront. Therefore, it is no longer necessary to readjust the analysis grating to achieve a constant positional difference.

A specific embodiment of the metrology system according to the present invention is configured to measure radiation in the control beam path (in particular to generate a multi-strip interference pattern) compared to the measurement radiation used to generate one of the interferograms The measurement of radiation is through a small surface area of a pupil of the optical imaging system. In other words, the beam path or monitoring channel configuration of the control beam path is such that it illuminates a smaller area on the capture plane of the detection device than the measurement channel. In this way, the same detection device can be used to simultaneously capture multiple measurement channels or more accurately capture individual measurement channels over a larger area. In addition to the control beam path for determining the phase in the first direction, other embodiments provide a control beam path for determining the phase in a direction orthogonal to the first direction or in each case Multiple monitoring channels for different directions.

In another embodiment of the metrology system according to the present invention, the measurement radiation in the control beam path of the multi-strip interference pattern (especially the measurement radiation used to generate the multi-strip interference pattern) has One of the interferograms measures a different wavelength of radiation. For example, a first radiation source for generating a measurement radiation having a first wavelength for a measurement channel and a measurement radiation for generating a second wavelength for controlling a beam path or a monitoring channel are provided Two beam sources. Preferably, the wavelength of the first radiation source corresponds to the operating wavelength of the optical imaging system. For example, an operational beam source of an optical imaging system can be used as the first beam source. For the measurement channel and the monitoring channel, it is easier to use different wavelengths for single-stripe interference pattern and interference image acquisition.

In another embodiment of the metrology system according to the present invention, the detecting means has a color selection embodiment for separating multi-strip interference patterns and interferograms having different wavelengths superimposed on each other in the capture area. For example, the detection device includes a color filter or a color camera configured for separate capture of the multi-strip interference pattern at the first wavelength and the interferogram at the second wavelength.

In accordance with another embodiment of the metrology system of the present invention, the wavelength of the measurement radiation for the multi-strip interference pattern is selected such that the chromatic aberration of the optical imaging system causes defocusing of the measurement radiation suitable for generating the multi-strip interference pattern. . Preferably, the measurement radiation used to generate the interferogram has in this case the operating wavelength of the optical imaging system. In this way, even without the defocusing optics, it is possible to achieve defocusing of the measured radiation of the monitoring channel used to create the multi-strip interference pattern. The optical imaging system to be measured itself causes defocusing. Here, in one embodiment, the two measuring beams may illuminate areas that overlap each other on the capturing plane, and the detecting means may have a color selection to separate the measuring beam. In particular, in one embodiment, the metrology beam for the measurement channel and for the monitoring channel may be from the same location on the object plane of the optical imaging system. Alternatively, the beam paths of the measurement channel and the monitoring channel can be configured such that mutually separated regions illuminate on the capture plane of the detection device. In particular, measuring radiation for the measuring channel and the monitoring channel can be provided spatially separately on the input side of the optical imaging system for this purpose.

In another embodiment of the metrology system according to the invention, the analysis grating and/or a region of a coherent reticle disposed on the input side of the optical imaging system comprises a ring structure. For example, the annular structure is implemented as a circular structure that is concentrically arranged. In the case of a rotationally symmetric grating, the shear spacing is constant with respect to the shearing direction. Phase shifts can occur in different directions, especially in the case of non-rectangular image fields. Alternatively, a concentric configuration of elliptical ring structures can be provided. In this case, the grating period depends on the cutting direction. This, for example, facilitates accurate alignment of the coherent reticle with the analysis grating relative to each other.

Moreover, in a particular embodiment of the invention, the evaluation device is further configured to determine a change in brightness of the measured radiation prior to entering the optical imaging system by the multi-strip interference pattern captured. In particular, the change in brightness of the measured radiation provided by the illumination device is determined. For example, the evaluation device is implemented to determine an average of one or more periods of the multi-strip interference pattern as a constant portion of each image point of the capture area of the detection device and to determine a plurality of consecutively captured Comparison of these constant bright portions in the fringe interference pattern. Furthermore, a correction factor to eliminate the change in brightness in the continuously captured interferogram can be determined based on the comparison.

According to the present invention, there is further provided a lithographic projection exposure apparatus comprising a projection lens for imaging a reticle structure on a wafer, and according to the foregoing specific embodiment or implementation A measurement system for determining the wavefront aberration of the projection lens of any of the variations. For example, a radiation source of a projection exposure apparatus is used herein to provide a measurement radiation for generating an interferogram. Further, a wafer holder or wafer stage of the projection exposure apparatus can be used as a positioning means for analyzing the grating. It is also possible to use a mask table as a holder and a positioning device for the coherent reticle.

Moreover, the objects of the present invention can be achieved by a method for determining the wavefront aberration of an optical imaging system as described below. The method comprises the steps of: transmitting a measurement radiation along a measurement beam path of the imaging system, configuring a diffraction analysis grating on the exit side of the imaging system to measure the beam path and transversely analyzing an optical axis of the imaging system to analyze the grating, and analyzing the grating Recording, at a plurality of displacement positions, a corresponding interferogram formed by the analysis grating on a detecting device, determining at least one position information item of the analysis grating at the at least one displacement position by a control beam path of the optical imaging system, And determining, by using at least one determined position information item, an interferogram recorded at the individual displacement position to determine a topography of the wavefront after the measurement of the radiation path along the measurement beam path of the imaging system.

In other words, in a manner similar to the measuring system according to the invention, at least one positional information item of the analysis grating is determined very accurately in at least one displacement position by the method according to the invention by the control beam path of the optical imaging system. Therefore, when determining the topology of the wavefront based on the interferogram continuously captured at different displacement positions, the determined position information item is used, particularly for phase shifting or shearing interferometry.

Features indicated in relation to the above-described specific embodiments, exemplary embodiments or specific embodiment variations of the measuring system according to the invention may be applied correspondingly to the method according to the invention and vice versa. These and other features in accordance with the present invention will be explained in the description of the drawings and in the scope of the claims. Individual features may be implemented individually or in combination as a specific embodiment of the invention. Furthermore, it may describe advantageous embodiments that may be independently protected, and the protection thereof is claimed only if or after the pending period of the application.

10‧‧‧Measurement system

12‧‧‧Optical imaging system

14‧‧‧ object plane

16‧‧‧Image plane

18‧‧‧Optical components

18-1-18-5‧‧‧Optical components

20‧‧‧ Optical axis

21‧‧‧ aperture diaphragm

22‧‧‧Light

23‧‧‧radiation source

24‧‧‧Irrigation device

26‧‧‧Measured radiation

28‧‧‧Coherent reticle

29‧‧‧Photomask holder

30‧‧‧Analytical grating

31‧‧‧Grating holder

32‧‧‧Detection device

34‧‧‧Drawing area

36‧‧‧Evaluation device

36-1‧‧‧First Evaluation Unit

36-2‧‧‧Second evaluation unit

36-3‧‧‧ Third evaluation unit

37‧‧‧ memory

38‧‧‧ pinhole aperture

40‧‧‧Measurement channel

42‧‧‧Translation direction

44‧‧‧ Defocused optical components

46‧‧‧Monitoring channel

50‧‧‧ Defocused optical components

52‧‧‧ Defocused optical components

54‧‧‧Coherent structure area

55‧‧‧Measure coherent area

56‧‧‧Grating area

58‧‧‧Measurement channel area

60‧‧‧Monitoring channel area

62‧‧‧Interferogram

62k‧‧‧Brightness Correction Interferogram

63‧‧‧Detection station

64‧‧‧Multi-strip interference pattern

66‧‧‧Second measurement radiation

68‧‧‧second source of radiation

70‧‧‧ ring structure coherent reticle

72‧‧‧ ring structure analysis grating

74‧‧‧Oval ring structure coherent reticle

76‧‧‧Elliptical ring structure analysis grating

78‧‧‧Location Information Items

80‧‧‧ wavefront aberration

82‧‧‧ deflecting elements

123‧‧‧radiation source

The above and other advantageous features of the present invention will be described in the following detailed description of exemplary embodiments of the invention. In the schema: 1 is a schematic view showing a first exemplary embodiment of a metrology system according to the present invention for determining wavefront aberrations of an optical imaging system, comprising a defocused optical component disposed on the input side relative to the imaging system, 2 shows in schematic view a second exemplary embodiment of a metrology system comprising a defocused optical element disposed on the output side relative to the optical imaging system, and FIG. 3 shows in schematic view a defocused optical element comprising a fastening to the analysis grating A third exemplary embodiment of a measurement system, FIG. 4 is a schematic diagram showing a fourth exemplary embodiment of a measurement system including a defocus coherent structure configured to offset the object plane of the optical imaging system, 5 is a schematic diagram showing a fifth exemplary embodiment of a metrology system including an area of an analysis grating configured to shift the image plane of the optical imaging system, and FIG. 6 shows an example of a specific embodiment according to the measurement system. Schematic diagram of the measurement channel area and the monitoring channel area on the capture area of the measuring device, FIG. 7 shows another exemplary embodiment according to the measuring system Schematic diagram of the measurement channel area on the capture area of the detection device and the monitoring channel area on the measurement channel area. FIG. 8 shows the capture area of the detection device according to another exemplary embodiment of the measurement system. FIG. 9 is a schematic diagram showing another exemplary embodiment of a measurement system and a color selection detection device having different wavelengths for measuring and monitoring channels, wherein the measurement channel region and the monitoring channel region have different wavelengths. Figure 10 is a schematic view showing another exemplary embodiment of a measurement system having different wavelengths for measuring and monitoring channels and a capture area separated from each other in the extraction area, and Figure 11 shows a ring structure for monitoring the channels. Schematic diagram, FIG. 12 shows a schematic diagram of an elliptical ring structure for monitoring a channel, and FIG. 13 shows another exemplary embodiment of a measurement system for measuring an optical imaging system operating in the EUV wavelength range, and FIG. 14 shows The amount included in FIG. 1 to FIG. 5, FIG. 9, FIG. 10, FIG. 12 or FIG. A schematic diagram of the design of a specific embodiment of an evaluation device of the measurement system.

In the exemplary embodiments or the specific embodiment variations described below, elements that are functionally or structurally similar to each other are provided with the same or similar element symbols as much as possible. Therefore, in order to understand the features of the specific elements of the specific example embodiments, reference should be made to the description of the specific example embodiments or the general description of the invention.

For ease of description, a Cartesian xyz coordinate system is indicated in some of the figures, and the corresponding positional relationship of the components shown in the drawings is clearly indicated by the system. In Figure 1, the y-direction is perpendicular and leaves the drawing plane, with the x-direction being upward and the z-direction pointing to the right.

FIG. 1 schematically shows a first exemplary embodiment of a metrology system 10 for determining wavefront aberrations of optical imaging system 12. The optical imaging system 12 is used to image the field points of the object plane 14 to the image plane 16 assigned to the object plane 14, and for this purpose comprises an optical element 18, which is illustrated by way of example only in which two optical elements 18 are . In addition, FIG. 1 illustrates the optical axis 20 of the imaging system 12 parallel to the z-direction and shows an aperture stop 21 disposed in the pupil plane for defining the aperture 22. Optical imaging system 12 is typically implemented to image as much as possible without aberrations at operating or use wavelengths or over a particular range of operating wavelengths. An example of such an optical imaging system is a lithographic projection lens for imaging a reticle structure onto a wafer. For example, some projection lenses are suitable for configuration for lithography having EUV radiation (extreme ultraviolet radiation) having a wavelength of less than 100 nm (especially having a wavelength of about 13.5 nm or about 6.8 nm). System 10 is adapted to configure an operating wavelength for optical imaging system 12. In general, metrology system 10 can be adapted to implement wavelengths from the infrared range to the x-ray range.

Multi-channel shearing interferometry can be implemented by the metrology system 10 used to determine the wavefront aberrations of the optical imaging system 12. Such interferometry based on the phase shift principle is described, for example, in WO 01/63233. The metrology system 10 includes an illumination device 24 having a radiation source 23 for providing appropriate measurement radiation 26 and a coherent reticle 28 disposed in the region of the object plane 14 of the optical imaging system 12. In addition, the measurement system 10 includes a diffraction analysis grating 30 disposed in the image plane 16 region, a detection device 32 disposed in the beam path downstream of the analysis grating 30, and an evaluation device 36. The detection device includes a capture area 34 for measuring the spatial resolution of the radiation 26 .

Radiation source 23 provides measurement radiation 26 having sufficient intensity and coherence to measure optical imaging system 12. Here, the wavelength of at least some of the measured radiation 26 corresponds to the operating wavelength of the optical imaging system 12. For example, the operational radiation source for optical imaging system 12 is used to generate measurement radiation 26, such as a radiation source of an illumination system of a lithographic projection exposure apparatus when measuring a projection lens.

As is known to those skilled in the art, lithographic projection exposure apparatus includes an illumination system for generating exposure radiation, such as DUV radiation (i.e., radiation having a deep UV wavelength range of, for example, 248 nm or 193 nm wavelength) or EUV radiation (extreme ultraviolet light). Radiation, which has a form of <100 nm, in particular having a wavelength of about 13.5 nm or about 6.8 nm. The exposure radiation impinges on a lithographic mask having a reticle structure to be imaged thereon. Here, the exposure radiation can be reflected at the lithographic reticle, as is the case with EUV radiation. Alternatively, the lithographic mask can also be implemented as a transmissive reticle. In this case, the exposure radiation passes through the reticle. Imaging the reticle structure onto a wafer disposed in the image plane is accomplished by a projection lens comprising a plurality of optical elements.

In the exemplary embodiment illustrated in FIG. 1, the coherent reticle 28 includes a two-dimensional configuration of pinhole apertures 38 extending in the object plane 14, in addition to selectively including focusing elements (not depicted herein) Each of them focuses a portion of the measurement radiation 26 onto the pinhole stop 38. In this manner, a plurality of field points in the object plane 14 are simultaneously provided with measurement radiation, the respective beam paths of which are referred to hereinafter as the measurement channel 40 or the measurement beam path of the optical imaging system 12. Figure 1 shows the beam path of one of these measurement channels 40 in an illustrative manner. Using this multi-channel metrology system 10, it is possible to simultaneously measure the imaging characteristics of the optical imaging system 12 for multiple field points by shear interferometry. The beam path of the measurement channel 40 is preferably configured such that the measurement radiation is emitted from the pinhole stop 38 in a diverging manner with a spherical wavefront, and the measurement radiation is imaged or focused by the optical imaging system 12 to be measured to the image. On the plane 16. Here, as shown in FIG. 1, the entire area of the radiation illumination aperture stop or diaphragm 22 is measured. A translation module (not shown in Figure 1) can also be provided to accurately position the coherent reticle. In particular, when the equivalent measurement is integrated into the projection lens of the lithographic projection exposure apparatus, it is possible to use the mask stage of the projection exposure apparatus as the translation module.

In other embodiments, only one amount having a pinhole stop can be provided A channel having a specific embodiment that is displaceable in the plane of the object. In addition, a plurality of pinhole apertures for the measurement channel may be included in the coherent reticle 28 in a symmetric two-dimensional configuration adjacent to each other, and in addition to the circular aperture, there may be apertures implemented in a polygonal manner (eg, square or Triangle) as a pinhole stop. In addition, the coherent reticle 28 can have a two-dimensional, symmetrical aperture structure that is suitable for use with the analysis grating 30 to suppress interference diffraction steps of the analysis grating 30. Further possible embodiments of the coherent reticle or illumination device and their description may be specifically referred to WO 01/63233.

For example, the diffraction analysis grating 30 is implemented as a phase grating or an amplitude grating or as any other suitable diffraction grating type (eg, a gray scale grating, etc.) for very short wavelengths. As a diffraction structure, the analysis grating 30 comprises a line grating, a cross grating, a checkerboard grating, a triangular grating or any other suitable periodic structure. For the phase offset in the context of shearing interferometry, the analysis grating 30 can be displaced in the translational direction 42 together with the capture area 34 of the detection device 32, the translational direction 42 being substantially parallel to the x or y direction and thus transverse to Optical shaft 20. Other directions of displacement perpendicular to the optical axis 20 and the tilting axis can also be provided. The displacement is performed stepwise in one direction by a positioning module not shown in the drawings. An interferogram 62 is generated on the capture area 34 at each measurement channel 40 due to zero-order diffracted radiation (formed between the analysis grating 30) and higher order diffraction (eg, first order diffraction). The interference produced. There is a so-called "time phase shift" due to the displacement analysis grating 30. Here, the phase of the higher order diffraction changes, while the phase of the zeroth order diffraction remains unchanged, with the result that there is a change in the corresponding interferogram 62. In particular, the distance between the two adjacent displacement positions is chosen such that a phase offset suitable for shearing interferometry occurs between these displacement positions. In general, the distance is a fraction of the grating period of the analysis grating 30. When the equivalent measurement is integrated into the lithographic projection lens as the projection lens of the optical imaging system 12, it is possible to use the wafer stage as a translation module.

The detection device 32 includes a radiation-sensitive extraction area 34 that includes a two-dimensional configuration of individual sensors and is implemented, for example, as a spatially resolved CCD sensor. An optical configuration (which is not shown in FIG. 1) for imaging the interferogram 62 or the multi-strip interference pattern 64 to the capture area 34 may be provided between the analysis grating 30 and the capture area 34. The patterns 62 and 64 captured by the detecting device 32 are transmitted to the evaluation device 36.

In addition, the measurement system 10 includes a plurality of defocused optical elements 44 that are in accordance with A portion of illumination device 24 in the exemplary embodiment of FIG. The defocused optical elements 44 are each disposed between the coherent reticle 28 in the beam path of one of the measurement channels 40 and the optical imaging system 12 such that the measurement radiation 26 impinges on the analysis in a defocused manner On the grating 30. For the sake of clarity, only one of these defocusing elements 44 is shown in FIG. Each defocused optical element 44 forms a beam path of some of the measured radiation 26 with a focus position before or after the analysis of the grating 30 and which is also referred to hereinafter as a control beam path or monitoring channel 46. Due to the defocus, the multi-strip interference pattern 64 is formed on the capture area 34 via the analysis grating 30, rather than the interferogram of the shearing interferometry. For this purpose, a structural pattern of the analysis grating 30 provided for shear interferometry can be used. Alternatively, the analysis grating 30 can have different patterns in the region of the monitoring channel, in particular different grating periods or grating alignments.

The defocused optical element 44 can be implemented as a refractive element (such as a lens element or iridium), as a reflective element (such as a mirror), as a diffractive element, or as an optical configuration formed by a plurality of these elements. Here, the defocused optical element 44 is configured and arranged such that the multi-strip interference pattern of each monitoring channel 46 contains at least one complete cycle of alternating stripes of maximum constructive and maximum destructive interference, in particular comprising at least two, at least Five or at least ten full cycles. Alternatively, defocusing optical element 44 can be configured to generate a plurality of monitoring channels. It is also possible to use only some of the radiation of the measurement channel for monitoring the channel.

The evaluation device 36 is configured to determine the distance between the two displacement positions based on the extracted multi-strip interference pattern 64 of the one or more monitoring channels 46, with different displacement positions of the analysis grating 30. Moreover, with the aid of the multi-strip interference pattern 64, the evaluation device 36 determines the local and temporal brightness distribution of the measured radiation 26 due to insufficient stability of the radiation source 23, i.e., prior to entering the optical imaging system 12. And the temporal brightness distribution, particularly at the location of the coherent reticle 28. Taking into account the determined distance between the displaced positions and the captured interferogram of the measuring channel 40 at these locations, the evaluation device 36 determines by the discrete Fourier analysis that the measured radiation 26 passes through the imaging system 12 (especially at The topography of the wavefront after the channel 40 is measured by the imaging system 12. Here, it is equally possible to measure the local and temporal brightness distribution of the radiation 26 . The wavefront aberration of optical imaging system 12 is derived from the deviation between the determined wavefront topology and the expected wavefront. Additionally, the evaluation device 36 can be configured to determine the focus of the optical imaging system 12 by the multi-strip interference pattern captured. And astigmatism.

For these purposes, the evaluation device 36 includes, for example, a memory coupled to the detection device 32 for receiving data received by the detection device 32, a memory for storing the transmitted interferogram, a multi-strip interference pattern, and other materials, and an electronic Processing unit. Alternatively, the captured interferogram and the multi-strip interference pattern may be stored in the memory of the detecting device 32 for subsequent evaluation or transmission to an external evaluation device. The functionality and design of the evaluation device 36 will be described in detail below in the context of an exemplary embodiment of the method of the present invention in conjunction with FIG.

2 and 3 each show other example embodiments of the measurement system 10 for determining the wavefront aberration of the optical imaging system 12. In the particular embodiment illustrated in FIG. 2, at least one defocused optical element 50 is disposed at a fixed location between the optical imaging system 12 and the analysis grating 30 in the beam path of the measurement radiation 26. The embodiment according to Fig. 3 comprises at least one defocused optical element 52 located in front of the analysis grating 30 in the beam path of the measurement radiation 26, which is moved together with the analysis grating 30 to the respective displacement positions. For this purpose, the defocused optical element 52 can be fastened to the analysis grating 30 or can be displaced by a separate translation module. Thus, in contrast to the exemplary embodiment illustrated in FIG. 1, each of the example embodiments of FIGS. 2 and 3 has at least one defocused optical element 50 or 52 disposed on the image side of the imaging system 12. The beam path of the monitoring channel 46 for the defocusing optical element 50 or 52 or the control beam path of the measuring radiation 26 thus initially corresponds to the beam path of the measuring channel 40 and is formed only by defocusing the downstream of the optical imaging system 12. To monitor channel 46.

In these exemplary embodiments, the defocused optical elements 50 and 52 are also implemented as refractive optical elements (eg, lens elements or turns), reflective elements (such as mirrors), diffractive elements, or a plurality of such elements. Optical configuration. Preferably, a plurality of monitoring channels 46 are provided, each having a defocused optical element 50 or 52. In other exemplary embodiments, defocusing optical element 50 or 52 is configured to generate a plurality of monitoring channels 46, or only certain of the radiation of metrology channel 40 is used to monitor channel 46. Although the beam path of the monitoring channel 46 can be affected by the optical imaging system 12, particularly by a region of the pupil that is radiated through the object side configuration of the defocused optical element 44, in the defocused optical element 50 or 52 In the case of the image side arrangement, a simple change in the area size illuminated by the monitoring channel 46 on the capture area 34 of the detection device 32 is possible. root According to another exemplary embodiment, the defocused optical element for the monitoring channel 46 can be disposed on the object side and on the image side.

In the above-described embodiment, the coherent reticle 28 and the analysis grating 30 have regions in the region for the measurement radiation 26 used to generate the multi-strip interference pattern 64 and in the region for the measurement radiation 26 used to generate the interference pattern 62. The same diffraction structure in the middle. In other embodiments, the monitoring channel 46 is provided with a diffractive structure different from the measurement channel 40 at the coherent reticle 28 or at the analysis grating 30 or both. In particular, it is possible to use different grating periods, grating tilts or line structures.

FIG. 4 schematically illustrates another exemplary embodiment of the measurement system 10. The difference from the exemplary embodiment shown in FIG. 1 is that at least one coherent structure region 54 of the coherent reticle 28 is disposed in the beam path upstream of the object plane 14 in place of the defocused optical element. The configuration and distance from the object plane 14 are configured such that the measurement radiation 26 has an appropriate defocus to create a multi-strip interference pattern 64 on the capture area 34. The coherent structure region 54 is embodied as a partial coherent reticle 28 which has a stepped surface for this purpose. Here, the coherent structure region 54 is arranged on a step which is offset in the axial direction relative to the optical axis 20 relative to the measured coherence region 55 for the measurement channel 40. Alternatively, the coherent reticle 28 has a different suitable form for offsetting the coherent structure region 54 from the object plane 14, such as a wedge or ramp shape. Furthermore, it is also possible for the coherent structure region 54 to be disposed on a separate carrier element that is separate from the coherent reticle 28. In another specific embodiment, the coherent structure region 54 is configured to be offset downstream of the object plane 14 in the beam path of the measurement radiation 26. Preferably, the measurement system 10 includes a plurality of monitoring channels 46 each having a coherent structure region 54 that is offset from the object plane 14. The coherent structure region 54 for the monitoring channel 46 can have the same structure as the region of the coherent reticle 28 used to measure the channel 40. Alternatively, the coherent structure region 54 includes different structures associated with the regions used to measure the channel 40, such as different configurations of pinhole apertures or specific embodiments or different grating structures.

FIG. 5 shows an exemplary embodiment in which, in contrast to the exemplary embodiment illustrated in FIG. 4, at least one of the grating regions 56 of the analysis grating 30 is configured to be offset from the image plane 16. The grating region 56 is disposed in the beam path downstream of the image plane 16 to achieve proper defocusing of the metrology radiation 26 used to create the multi-strip interference pattern 64 over the capture area 34. To this end, analyze the grating 30 package A stepped, wedge or similarly configured surface is provided for receiving the grating region 56 accordingly. Alternatively, the grating region 56 can be disposed on a surface opposite the remaining structure of the analysis grating 30 or it can be disposed separately from the analysis grating 30 on a separate carrier element. Preferably, the metrology system 10 includes a plurality of monitoring channels, each of which has a grating region 56 that is offset from the image plane 16. In other embodiments, at least one of the grating regions is disposed in a beam path upstream of the image plane 16 or provides an offset of the coherent structure at the object plane 14 and a bias of the grating region at the image plane 16 for the same monitoring channel. shift. The grating region 56 for the monitoring channel 46 may have the same diffraction structure as the region of the analysis grating 30 used to analyze the channel 40. According to other embodiments, at least one of the grating regions comprises a different diffractive structure than the region of the analysis grating 30 used to measure the channel 40, such as a different grating period, grating tilt or line structure. The offset of the grating region 56 only affects the region of the capture area 34 illuminated by the monitoring channel 46, while the offset of the coherent structure region 54 affects the path of the monitoring channel 46 in the optical imaging system 12.

FIG. 6 schematically shows the capture area 34 of the detection device 32 of the metrology system 10, which corresponds, for example, to one of the example embodiments listed herein. A plurality of measurement channel regions 58 and monitoring channel regions 60 are depicted on the capture area 34. Each measuring channel 40 illuminates a measuring channel region 58 in which an interferogram 62 for phase offset evaluation is generated in each case with the aid of an analysis grating 30. Furthermore, due to the defocus of the monitoring channel 46, a multi-strip interference pattern 64 is formed in each of the monitoring channel regions 60 illuminated by the monitoring channel 46. The configuration of the measurement channel region 58 and the monitoring channel region 60 on the capture area 34 is substantially set by the coherent reticle 28 and is constant for different displacement positions of the analysis grating 30. The defocusing configuration via the defocused optical elements 44, 50, 52, the coherent structure region 54, or the grating region 56 causes the size of the monitoring channel region 60 to substantially correspond to the size of the metrology channel region 58. Further, the multi-strip interference pattern 64 has the same stripe direction.

Figure 7 illustrates the capture area 34 of the detection device 32 in another measurement system 10, wherein the monitoring channel 60 is due to proper defocusing, as opposed to the measurement system 10 assigned to the channel region distribution according to Figure 6. Less than the measurement channel area 58. To this end, in particular, there may be a suitable implementation and configuration of the object side defocusing optical element 44 or the coherent structure region 54 such that the respective monitoring channel 46 passes only a portion of the aperture 22 of the optical imaging system 12, while the measurement channel 40 The entire aperture 22 is illuminated where possible. At least, compared to the measurement channel 40, the monitoring channel 46 passes through the optical imaging system The smaller surface area of the aperture 22 of the system 12. Here, the size of the monitoring channel region 60 is selected such that it is sufficient to determine the position of the analysis grating 30 and to determine the brightness distribution of the measurement radiation 26 provided by the illumination device 24. From this measurement, more space is provided on the capture area 34 for measuring the channel area 58, thus facilitating more accurate shearing interferometric results.

Furthermore, both the monitoring channel 46 and the measuring channel 40 can be configured for different displacement positions of the analysis grating 30 transverse to the optical axis 20. For example, to this end, the analysis grating 3 comprises line gratings having different alignments for the measurement channel 40 and the monitoring channel 46. In FIG. 7, these different alignments can be identified in the multi-strip interference pattern 64 produced by the interference in the monitor channel region 60. As such, it is possible to simultaneously extract the interference pattern 62 and the multi-strip interference pattern 64 for different displacement directions by analyzing the appropriate displacement of the grating 30, for example, for displacement in x and in the y-direction (refer to FIG. 1).

8 shows a capture area 34 of a detection device 32 of another exemplary embodiment of the measurement system 10, wherein a second measurement radiation is provided for monitoring the channel 46, the second measurement radiation having and using The first measurement radiation 26 of the channel 40 is measured at a different wavelength. For example, to this end, the measurement system 10 includes another radiation source or optical configuration for generating a second measurement radiation from the first measurement radiation 26. In addition, the capture area 34 or detection device 32 has a color selection embodiment. In another embodiment, instead of a color filter, a choice between the types of measurement radiation can be selected for a separate capture by detection device 32. Both the measurement channel area 58 and the monitoring channel area 60 are configured to be large enough to intersect on the capture area 34. The superimposed structure of the interferogram 62 and the multi-strip interference pattern 64 is separated by the color-sensitive detecting device 32. In addition to the sufficiently high resolution of the interferogram 62 for a very accurate phase shift method based on the size of the measurement channel region 58, the multi-strip interference pattern 64 can also be implemented in addition to the analysis grating 30 based on the size of the monitored channel region. The location and (optionally) the timing of the measured radiation 26 provided by the illumination device and the high degree of accuracy of the local luminance distribution also allow for further evaluation of the resolution. For example, the multi-strip interference pattern 64 can be used to make additional decisions for focus or astigmatism, and can include the focus or astigmatism when determining the wavefront aberration of the optical imaging system 12.

9 and 10 also illustrate a particular embodiment of the metrology system 10 in which a second gauge 66 having a different wavelength than the wavelength of the metrology radiation 26 used to measure the channel 40 is used. Monitor channel 46. In contrast to the previous embodiments, the wavelength of the second measurement radiation 66 is selected such that based on the chromatic aberration of the optical imaging system 12, appropriate defocus has been used to create a multi-channel fringe pattern. A lithographic projection lens is an example of such an imaging system 12. In general, these are optimized for only one operating wavelength and have large aberrations at other wavelengths. The metrology system 10 can produce a multi-strip interference pattern for very accurately determining the position of the analysis grating 30 on the capture plane 34 without dedicated defocusing optics, ie without optical elements 44, 50 or 52, such as described above. There is no coherent structure region 54 or grating region 56 as described above.

In the exemplary embodiment illustrated in FIG. 9, radiation source 123 produces measurement radiation having two different wavelengths such that the generated measurement radiation includes first measurement radiation 26 and second measurement radiation 66. The measurement radiation 26, 66 having two different wavelengths illuminates at least a region of the coherent reticle 28. Therefore, the measurement channel 40 is simultaneously used as the monitoring channel 46. The defocus of the second measurement radiation 66 is due to the chromatic aberration of the optical imaging system 12. The focus of the measurement radiation 66 is upstream of the analysis grating 30 disposed in the image plane 16, while the measurement radiation for the interferogram remains focused on the image plane 16. The detection device has a color selection embodiment (e.g., a color camera) or a suitable color filter that can be rotated into the beam path for separate capture of the multi-strip interference pattern or interferogram that overlaps in the capture area 34.

The spatially independent illumination of the coherent reticle 28 is provided in accordance with the exemplary embodiment of FIG. 10 with the first measurement radiation 26 for the measurement channel 40 and the second measurement radiation 66 having a different wavelength for the monitoring channel 46. To this end, the measurement system 10 includes, for example, a first radiation source 24 for providing a first measurement radiation 26 and a second radiation source 68 for providing a second measurement radiation 66. The illumination device 24 is configured such that the second measurement radiation and the illumination of the first measurement radiation 26 cohere to different regions of the reticle 28. These regions are selected such that the monitored channel region 60 of the second gauge radiation 66 does not overlap the measurement channel region 58 of the first gauge radiation 26 on the capture area 34 of the detection device 32. The second measurement radiation 66 of the monitoring channel 46 is defocused by chromatic aberration as it passes through the optical imaging system 12 and forms a multi-strip interference pattern 64 in the surveillance channel region 60. The first measurement radiation 26 of the measurement channel 40 remains focused on the image plane 16 via the analysis grating 30 and forms an interference pattern 62 for shear interference evaluation in the measurement channel region 58. Since the measurement channel area 58 and the monitoring channel area 60 are spatially separated on the capture area 34, it is possible to use the detection device 32 without color selection.

In other exemplary embodiments of the metrology system 10, in addition to the monitoring channel 46 described above for generating a multi-strip interference pattern, other auxiliary channels for determining various parameters are also provided. For example, the auxiliary channel can be configured to determine the translation or rotation of the coherent reticle 28 or analysis grating 30 for all spatial directions, to determine the brightness distribution, or by appropriate at the coherent reticle 28, at the analysis grating 30, or both. The structure is implemented to accurately align the coherent reticle 28 with respect to the analysis grating 30.

As an example of such a structure, Figure 11 shows the annular structure 70 of the coherent reticle 28 and the annular structure 72 assigned to the analytical grating 30 of the structure to form an auxiliary channel for performing radial shearing interferometry. Due to the rotational symmetry of the annular structure 72 used as the shear grating, the shear distance is the same for all displacement directions. This auxiliary channel is particularly suitable for imaging systems having a non-rectangular image field, such as a lithographic projection lens having a sickle shaped object field. Therefore, each stepwise displacement of the analysis grating during the measurement can be performed in different directions.

In Fig. 12, an elliptical ring structure 74 of the coherent reticle 28 and an elliptical ring structure 76 of the analysis grating 30 assigned to the structure are schematically shown as another example. The auxiliary channels formed by these elliptical ring structures 74, 76 contribute to radial shearing interferometry, wherein the shearing distance in each direction of displacement is different. The grating period of the annular structure 76 depends on the direction or angle of the displacement. The elliptical form of the annular structure 74 disposed in the coherent reticle 28 facilitates accurate alignment of the coherent reticle 28 relative to the analytical grating 30.

Figure 13 illustrates another embodiment of the metrology system 10 configured to measure the optical imaging system 12 in the form of a projection lens of a lithographic EUV projection exposure apparatus. To this end, in the particular embodiment variant shown, the projection lens comprises six optical elements 18-1 to 18-6 in the form of mirrors.

In addition to the features described below, the metrology system 10 according to Fig. 13 has a configuration similar to that of the metrology system of Fig. 1. According to one of the features, an aspect similar to that achieved by the metrology system 10 of FIG. 10 is that the metrology channel 40 is illuminated at a different wavelength than the surveillance channel 46. Accordingly, illumination device 24 includes a first radiation source 23 for generating first measurement radiation 26 for measurement channel 40 and a second radiation source 68 for generating second measurement radiation 66 for monitoring channel 46. . For example, an LED source or laser can be used as the second radiation source 68. In the description of Figure 13, by way of example In each case, only one channel is shown for the measurement channel 40 and the monitoring channel 46. The measurement radiation 26 can be radiation at the operating wavelength of the projection lens to be measured, ie EUV radiation. In the case where the metrology system 10 is integrated into the projection exposure apparatus, the measurement radiation 26 can be the same as the exposure radiation of the projection exposure apparatus. The measurement radiation 26 is illuminated on the coherent reticle 28 via an optical deflecting element 82.

In the particular embodiment variation shown, the coherent reticle 28 has a similar embodiment to the coherent reticle 28 having a stepped surface as shown in FIG. Here, the coherent structure region 54 for the monitoring channel 46 is disposed on a step that is offset relative to the optical axis of the optical imaging system 12 in the axial direction relative to the measured coherence region 55 for the measurement channel 40. shift. In the particular embodiment variant shown, the offset structure of the coherent structure region 54 is such that the associated focal plane is disposed above the analysis grating 30. In the particular embodiment shown, the coherent reticle 28 is embodied as a reflective reticle; however, it can also be implemented substantially as a transmissive reticle. The coherent reticle 28 is held by the reticle holder 29, which assists in translation and rotation of the coherent reticle 28 relative to all spatial directions.

The detecting device 32 is disposed on a detecting station 63. If the measuring system 10 is integrated into the projection exposure device, the detecting station 63 can be formed by the wafer table. The detection station 63 is configured to be displaceable and tiltable with respect to all spatial directions. The grating holder 31 for holding the analysis grating 30 is disposed on the detection stage 63. The grating holder 31 helps to analyze the displacement of the grating relative to the detection stage 63 in all spatial directions. In other specific variations, the metrology system 10 for measuring EUV projection lenses shown in FIG. 13 can be configured in accordance with the specific embodiments described with reference to FIGS. 1 through 12.

In the following, the functionality and interaction of the components of the metrology system 10 will be described in conjunction with exemplary embodiments of the method according to the invention.

In order to determine the wavefront aberration of the optical imaging system 12 (e.g., a lithographic projection lens), the coherent reticle 28 having the radiation source 23 and the analysis grating 30 having the detection device 32 are initially disposed in the optical imaging system 12. To this end, in the case of a lithographic projection exposure apparatus, the coherent reticle 28 can be received by a masking station, and the analysis grating 30 along with the detecting means 32 can be received by a wafer station. Here, the illumination system of the projection exposure apparatus can be used as the illumination apparatus 24.

Next, the desired defocus of the defocused optical elements 44, 50, 52 disposed in the coherent reticle 28 or the analysis grating 30 is set by positioning in the z-direction. If offset coherent structure region 54 or grating region 56 is used, defocusing is expected during production of coherent reticle 28 or analytic grating 30 Has been set. For example, in the case of defocusing is expected, 10 to 100 stripes are produced in the multi-striped coherent pattern 64 on the diameter of the aperture 22. The resolution of the detection device 32 can be considered when selecting the stripe density.

During the metrology procedure, the analysis grating 30 is progressively displaced from the initial position by the translation module to analyze the fraction of the grating period of the grating 30. Individual displacements are implemented as equidistant from each other as possible. For example, displacement is initially initiated in one direction (eg, the x-direction) and then displaced in a direction orthogonal to the first direction (eg, the y-direction). Alternatively, there may be alternating displacements in both directions. This procedure is particularly suitable for taking time variations of the characteristics of the optical imaging system 12 to be measured, which may occur due to, for example, heating. Here, the time lag between the measurements in mutually orthogonal directions should be as small as possible.

All interferograms 62 and multi-strip interference patterns 64 produced on the capture area are captured by the detection device 32 at each displacement location. The captured interferogram 62 and multi-strip interference pattern 64 are transmitted to an evaluation device 36 (which will be illustrated in detail in the exemplary embodiment of FIG. 14) and stored in memory 37 of evaluation device 36. Alternatively, storage in the detection device 32 may also be provided for subsequent delivery to the evaluation device 36. If a two-dimensional analysis grating 30 is used (e.g., having a checkerboard or cross-over structure), the analysis grating 30 may oscillate in the displacement direction at the displacement position during the capture. In this way, the interference pattern of interference will be suppressed by the integration time of the detection device 32.

As further depicted in FIG. 14, the first evaluation unit 36-1 in the evaluation device 36 accordingly determines the position information item 78 of the analysis grating 30 at each displacement position from at least one multi-strip interference pattern 64 from each displacement position.

According to a first embodiment, the position information item 78 is determined by determining an accurate position difference between each displacement position and a corresponding adjacent displacement position. Here, the position difference in this case is a positional difference in a direction transverse to the optical axis 20 (ie, in the xy plane). In other words, the first evaluation unit 36-1 determines an accurate phase difference or position difference between all adjacent displacement positions. To this end, the phase is determined for the pixels of the plurality of image points or each of the multi-strip interference patterns 64 initially, according to a multi-strip evaluation method known to those skilled in the art. For example, in the case of horizontally extending stripes, the phase value is determined by Fourier analysis of one or more image points in a column of each vertically extending image dot column. Therefore, each multi-strip interference pattern can be obtained The phase distribution of a plurality of image points of 64. For example, the phase distribution includes phase values for image points of one or more lines of each multi-strip interference pattern.

Now, the phase difference is calculated for the same image point of a known phase between the multi-strip interference patterns 64 having adjacent displacement positions. For example, the phase difference is determined one by one between the image points of the same line of the multi-strip interference pattern 64 at adjacent displacement positions. Since the phase differences should be the same for all image points of the multi-strip interference pattern 64 adjacent to the displacement position, the phase differences of the different distributions of adjacent displacement positions are finally averaged. In this way, the phase difference or position difference in the lateral direction is determined very accurately between all adjacent displacement positions.

Although the lateral position of the analysis grating 30 can be determined at a particular displacement position based on the stripe position of the associated multi-strip interference pattern 64, the axial position of the analysis grating 30 can be determined based on the stripe density of the associated multi-strip interference pattern 64 (ie, it is in the z-direction) Position on). According to a second embodiment, the positional information item 78 of the analysis grating 30 is determined three-dimensionally at the respective displacement position, i.e., to analyze the form of the x, y, and z coordinates of the grating relative to the adjacent displacement position. In other words, the position information item 78 is determined in the form of a three-dimensional position difference between the individual displacement positions.

Furthermore, if at least three control beam paths in the form of monitoring channels 46 are used, the evaluation of the 3-dimensional positional information items allows the tilt position of the analysis grating 30 relative to all three-dimensional spatial directions to be determined in a manner similar to triangulation. According to a third embodiment, the position information item 78 of the analysis raster 30 is determined in the corresponding displacement information items of all six spatial positions, ie with respect to the translational degrees of freedom in the x, y and z directions and related to x, y. And the degree of freedom of inclination to the spatial axis in the z direction. Here again, the positional information items of the analysis grating 30 in the six spatial directions are determined relative to the respective adjacent displacement positions. In other words, the location information item 78 can be determined in the form of a six-dimensional position difference between the individual displacement positions. Therefore, it is possible to quantitatively take the torsion/z rotation of the analysis grating 30 due to phase shift or phase shift. In particular, the path of the analysis grating 30 can be quantitatively taken during the shearing intervention, which can reflect the slopes in the nano or sub-nano range.

Furthermore, the time and local brightness variation of the measurement radiation 26 before entering the optical imaging system 12, in particular the position of the measurement radiation at the coherent reticle, is determined in the evaluation device 36 by the selective optical second evaluation unit 36-2. Time and local brightness changes. To this end, for example, at least one multi-strip interference pattern 64 is selected from each displacement position, and by pairing one or more The average of adjacent image points of the fringe period determines the constant light portion of each image point. Next, the pixel resolution distribution of the constant light portion appears for each multi-strip interference pattern 64 and thus the illumination distribution of the pupil 22 occurs.

Next, the average of the multi-strip interference pattern 64 is formed pixel by pixel from all the displacement positions. For each image point, an average constant light portion appears. Next, a difference between the constant light portion and the average constant light portion is formed at each image point of the multi-strip interference pattern, and the difference is determined by the difference. For example, the correction factor corresponds to the quotient of the difference between the constant light portion and the average value of the constant light portion. Finally, the correction factor determined pixel by pixel and determined for a displacement position is used to correct the corresponding image point of the interference pattern 62 captured at the corresponding displacement position, so that the appropriate brightness correction interferogram is generated by the second evaluation unit 36-2. 62k.

The third evaluation unit 36-3 of the evaluation device 36, with the aid of the phase shifting method, determines the spatial derivative of the wavefront of the measurement radiation 26 in the displacement direction 34 in the use of the displacement direction, which is in the absence of the second evaluation unit 36. In a particular embodiment variant of -2, the lateral position difference included in the determined position information item 78 between the adjacent displacement position and the brightness corrected interference pattern 62k or the interferogram 62 is used for all displacement positions. Here, discrete Fourier analysis or cosine fitting is used instead of Fast Fourier Transform (FFT) due to non-equidistant displacement positions or phase differences. Next, the topology of the wavefront after the radiation 26 is passed through the optical imaging system 12 is determined from the spatial derivative. The wavefront aberration 80 of the optical imaging system 12 is determined by the comparison of the determined topology of the wavefront with the expected wavefront. Other degrees of freedom (z-direction, tilt position) of positional differences that may exist in addition to lateral position differences may be used to correct the system "trajectory error" of the displacement device that moves the analysis grating 30 and may be selectively used to bias these deviations As a correction value in the controller. In addition, these positional deviations can also be considered in the evaluation algorithm of the phase measurement.

Moreover, the evaluation device 36 can directly determine the defocus or astigmatic aberration of the optical imaging system 12 from the multi-strip interference pattern 64. To this end, the evaluation device 36 performs, for example, x and y differentiation of the phase distribution of the multi-strip interference pattern 64 from a displacement position, and determines the average of the x and y tilts of the derivative as the derivative or determines the astigmatism between x and y tilt. difference. The time drift of the imaging characteristics of the imaging system 12 may result in aberrations due to the decision to determine the focus of the optical imaging system 12 and the interference of all displacement positions when determining the astigmatism by the phase shift method. These can be obtained from multi-striped interferograms The direct decision of Case 64 is to determine and correct, which can be performed quickly.

The description of specific embodiments of the invention is to be understood as illustrative. The disclosure thus will be apparent to those skilled in the art that the present invention and its related advantages are obvious, and the changes and modifications of the structure and method described are obvious to those skilled in the art. Therefore, all such variations and modifications and equivalents are intended to be covered by the scope of the appended claims.

Claims (19)

A metrology system for determining a wavefront aberration of an optical imaging system, including an illumination device for passing measurement radiation through the imaging system, disposed downstream of the imaging system, and transverse to the imaging system An optical grating is disposed in an analysis grating in the beam path of the measuring radiation, and a detecting device for recording a radiation distribution of the measuring radiation, wherein the measuring system is configured to: Generating a corresponding interferogram formed by the analysis grating at a plurality of displacement positions of the analysis grating for recording on the detecting device, and by controlling a beam path through the optical imaging system, at least one of the At least one positional information item of the analysis grating is determined at the displacement location. The measurement system of claim 1, wherein the at least one location information item comprises a positional specification of the analysis grating relative to the optical axis in a lateral direction and/or in an axial direction, and/or A specification of an oblique position of the grating is analyzed. The measurement system of claim 1 or 2, wherein the at least one position information item comprises a positional difference between the analysis grating and a displacement position of the optical axis in the lateral direction. A measurement system according to any one of the preceding claims, configured to determine the at least one positional information item of the analysis grating by at least two different control beam paths through the optical imaging system . A measuring system according to any one of the preceding claims, Further comprising an evaluation device configured to use the at least one determined position information item to determine from the interferogram recorded at the individual displacement position that the measurement radiation passes through the measurement beam path along the imaging system After the top of the wavefront. The measurement system of claim 5, wherein the evaluation unit is configured to perform a discrete Fourier analysis when determining the topology of the wavefront of the measurement radiation. A measuring system according to any one of the preceding claims, further configured to generate a corresponding multi-striped interference pattern at the displaced position for recording on the detecting device, the multi-strip interference pattern being An analysis grating generation, wherein the multi-strip interference pattern comprises at least one complete period of maximum constructive dry alternating fringes involving maximum destructive interference, and the metrology system is further configured to determine the analysis based on the recorded multi-strip interference pattern The at least one location information item of the raster. The measurement system of claim 7, further configured to determine a difference distribution by determining a phase distribution under the corresponding multi-strip interference pattern, and determining a difference in phase distribution determined by the formation, And determining the location information item of the analysis raster by averaging a plurality of values from the difference distribution. A measurement system according to any one of the preceding claims, configured to illuminate radiation contained in the control beam path onto the analysis grating in a defocused state. A measurement system according to any one of the preceding claims, comprising a defocused optical element disposed in the imaging beam path of the optical imaging system for the measurement radiation on the analysis grating The defocused radiation. A measurement system according to any one of the preceding claims, wherein the illumination device has a wave forming coherent structure of the defocused radiation for the measurement radiation on the analysis grating, the coherent structure An area of the analysis grating configured to be offset relative to the imaging system and/or to generate a multi-strip interference pattern is configured to be offset relative to an image plane assigned to the object plane. The measurement system of any one of clauses 7 to 11, further configured to determine a defocus aberration of the optical imaging system directly from the corresponding multi-stripion interference pattern and/or Or astigmatic aberrations. A measurement system according to any one of the preceding claims, configured such that the measurement in the control beam path is compared to the measurement radiation used to generate one of the interferograms Radiation passes through a smaller surface area of a pupil of the optical imaging system. A measurement system according to any one of the preceding claims, wherein the measured radiation in the control beam path of a multi-strip interference pattern has an amount that is used to generate one of the interferograms Measuring a different wavelength of radiation. The measurement system of claim 14, wherein the wavelength of the measurement radiation for a multi-strip interference pattern is selected such that a color difference of the optical imaging system is adapted to produce the multi-strip interference pattern. The measurement measures a defocus of the radiation. A measurement system according to any one of the preceding claims, wherein the analysis grating and/or a region of a coherent reticle disposed on an input side relative to the optical imaging system comprises a ring structure. The measurement system of any one of clauses 7 to 16, further configured to enter the optical imaging at the measurement radiation by a plurality of the multi-strip interference patterns captured The system determines a change in brightness of the measured radiation. A lithographic projection exposure apparatus comprising a projection lens for imaging a reticle structure on a wafer, and determining a wavefront aberration of the projection lens according to any one of the preceding claims The measurement system. A method for determining a wavefront aberration of an optical imaging system, comprising the steps of: transmitting a measurement radiation along a measurement beam path of the imaging system, and configuring a diffraction analysis grating at an exit side of the imaging system Measure the beam path and move the analysis grating transversely to an optical axis of the imaging system. At a plurality of displacement positions of the analysis grating, record a corresponding interferogram formed by the analysis grating on a detecting device, by passing the optical A control beam path of the imaging system determines at least one positional information item of the analysis grating at at least one of the displacement locations, and using the at least one determined position information item to determine from the interferogram recorded at the individual displacement position The measurement radiation is a topology of the wavefront after passing along the measurement beam path of the imaging system.