WO2012072090A1

WO2012072090A1 - Method of determining a border of an intensity distribution

Info

Publication number: WO2012072090A1
Application number: PCT/EP2010/007219
Authority: WO
Inventors: Helmut Haidner; Rainer Hoch
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2010-11-29
Filing date: 2010-11-29
Publication date: 2012-06-07

Abstract

A method of determining a border (50) of an intensity distribution (48) on a detection surface (42) is provided, the intensity distribution (48) being generated by electromagnetic radiation after having passed through an optical system (12) and being recorded by a wave front measuring apparatus (10) for measuring a wave front distribution of the radiation. The intensity distribution (48) is blurred in the region of the border (50) due to spurious radiation (32b, 32c, 32d). The method comprises the steps of: identifying a parameter relating to the configuration of the wave front measuring apparatus (10) and influencing the generation of the spurious radiation, determining an influence of the identified parameter on the blurring of the recorded intensity distribution (48), and determining the border (50) of the intensity distribution from the recorded intensity distribution (48) taking the determined influence on the blurring into account.

Description

Method of determining a border of an intensity distribution

Background of the invention

The invention relates to a method of determining a border of an intensity distribution on a detection surface, which intensity distribution is generated by electromagnetic radiation after having passed through an optical system and being recorded for measuring a wave front distribution of the radiation.

In several areas of technology optical imaging systems are used, in which requirements on the imaging quality are becoming tighter and tighter. One example is the photolithographic production of semiconductor elements and other finely structured components, in which structures in the submicrometer range are produced by means of high performance projection objectives. Another example are photo objectives of all kind, for which typically less stringent requirements apply regarding imaging quality. Imaging optics often comprise a multitude of optical elements, which usually makes it impossible to deduct the optical properties from theoretical calculations. Therefore, the optical properties of the imaging systems have to be measured reliably. The accuracy of the testing procedures used for measuring aberrations of these imaging systems are typically adapted to the imaging accuracy of the optical systems. Often interferometric measurement procedures are used for such measurements. For example, a measurement apparatus operating according to the principle of shearing interferometry can be used. In this case a mask is arranged in the object plane of the optical system to be tested and illuminated by incoherent light. The mask comprises a two-dimensional pattern. In the imaging plane of the optical system a diffraction grating acting as a reference pattern is arranged. By superposition of the waves generated by diffraction at the diffraction grating superposition patterns in form of interferograms are generated and recorded by a suitable detector.

From the respective intensity distributions of the interferograms the wave front distribution of the radiation after having passed through the optical system is determined.

The wave front distribution is evaluated with respect to deviations from a target distribution. From the wave front deviation determined this way aberrations of the optical system under test are determined. In order to determine the aberrations, the borders of the intensity distributions have to be known with good accuracy. As the resolution limits in microlithography are moved lower and lower, the requirements for the accuracy with which the borders of the intensity distributions have to be known are becoming tighter and tighter as well. However, due to blurring effects and measurement uncertainties the border of the intensity distribution cannot always be determined with the required accuracy.

It is therefore an object of the invention to solve the above mentioned problems and provide a method of determining a border of an intensity distribution generated on the detector of a wave front measuring apparatus, like a shearing interferometer, to be detected with improved accuracy.

Summary of the invention

According to the invention a method of determining a border of an intensity distribution on a detection surface is provided. The intensity distribution is generated by electromagnetic radiation after having passed through an optical system and is recorded by a wave front measuring apparatus, e.g. an interferometer or a Hartmann-Shack-sensor, for measuring a wave front distribution of the radiation. The intensity distribution is blurred in the region of the border due to spurious radiation. The method according to the invention comprises the steps of: identifying a parameter relating to the configuration of the wave front measuring apparatus and influencing the generation of the spurious radiation, determining an influence of the identified parameter on the blurring of the recorded intensity distribution, and determining the border of the intensity distribution from the recorded intensity distribution taking the determined influence on the blurring into account.

The border of the intensity distribution in the above context can be an outer border of the intensity distribution, e.g. having the shape of a disk. Further, the border can also be an inner border, which can appear on the detection surface in case the optical system has an obscured pupil. As explained in the following in more detail, a parameter relating to the configuration of the wave front measuring apparatus, which e.g. can be a shearing interferometer, and influencing the generation of the spurious radiation, which in turn causes the border of the intensity distribution to be blurred, can for example be the thickness of a substrate of a diffraction grating, a distance between the diffraction grating and the detection surface and/or a size of a coherence pattern imaged onto the diffraction grating during the wave front measurement. According to the invention the influence of the identified parameter on the blurring of the recorded intensity distribution is determined. This can for example be done by measurement and/or by simulation using a mathematical model. The influence determined can be in form of a spurious intensity distribution relating to the identified parameter and contained in the measured radiation. Subsequently, the border of the intensity distribution is determined taking the determined influence on the blurring into account. This determined influence on the blurring can for example be taken into account by extracting a respective spurious intensity distribution from the recorded intensity distribution. The above method according to the invention allows the border of the intensity distribution to be identified with improved accuracy. According to an embodiment of the invention the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, wherein the spurious radiation is split off from radiation having passed through the diffraction grating, and the identified parameter relates to the configuration of the shearing interferometer.

According to an embodiment of the invention the detection surface is arranged in a plane conjugate to a pupil plane of the optical system. The border of the intensity distribution determined according to the invention can therefore be referred to as a border of the pupil.

According to a further embodiment of the invention the influence of the identified parameter on the blurring of the recorded intensity distribution is determined by measuring the parameter and calculating the resulting influence from a mathematical model. As already mentioned above, according to a further embodiment of the invention the wave front apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, which diffraction grating comprises a substrate, and the parameter measured is a thickness of the substrate. According to a further variation the thickness of the substrate is minimized in order to minimize spurious radiation.

According to a further embodiment of the invention the influence of the identified parameter on the blurring of the recorded intensity distribution is determined by measuring test intensity distributions for different values of the parameter, extrapolating the intensity distribution at a parameter value of minimized blurring, and subtracting the extrapolated intensity distribution from the recorded intensity distribution.

According to a further formulation of the invention the wave front measuring apparatus comprises a shearing interferometer comprising a diffraction grating, the shearing interferometer further comprises a mask comprising a coherence pattern, which is imaged onto the diffraction grating, the size of the coherence pattern is identified as the parameter influencing the generation of the spurious radiation, the test intensity distributions are measured for coherence patterns of different sizes, and the intensity distribution is extrapolated for a coherence pattern of infinitely small size.

According to a further embodiment of the invention the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, the distance between the diffraction grating and the detection surface is identified as the parameter influencing the generation of the spurious radiation, the test intensity distributions are measured for different settings of the distance between the diffraction grating and the detection surface, and the intensity distribution is extrapolated for a distance setting of minimal blurring in the intensity distribution.

According to a further embodiment of the invention the wave front measuring apparatus comprises a shearing interferometer comprising a diffraction grating, and the spurious radiation is caused due to single rays of the radiation originating from the diffraction grating respectively being split up into several rays before striking the detection surface.

According to a further embodiment of the invention the detection surface is covered by a wavelength conversion layer for converting the wavelength of the radiation, the wavelength conversion layer is identified as the parameter influencing the generation of spurious radiation, and the influence of the wavelength conversion layer on the blurring of the recorded intensity distribution is determined. This can be done e.g. by imaging a known pattern onto the wavelength conversion layer, measuring the imaged pattern and comparing the same with a target pattern. According to a further embodiment of the invention the surface of the diffraction grating facing towards the detection surface is coated with a reflection suppressing layer. This way the generation of spurious radiation from radiation split off at the above identified surface of the diffraction grating is suppressed. According to a further variation the grating structures contain black chrome. This measure reduces the generation of spurious radiation by reflection at the grating structures.

According to a further embodiment of the invention the the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a coherence mask having coherence patterns used for measuring the wave front distribution and a test structure, which test structure is of smaller size than the coherence patterns used, and the test structure is used for determining the radius and/or the center point of the border of the intensity distribution. The test structure can be configured as a pinhole.

According to the invention a further method of determining a border of an intensity distribution on a detection surface of a two-dimensionally resolving detector is provided. The intensity distribution is generated by electromagnetic radiation after having passed through an optical system and is recorded for measuring a wave front distribution of the radiation, the detector comprises an array of detection cells, each detection cell being configured to detect the intensity of a radiation impinging onto a respective associated collection area and the detector being configured to record the intensity distribution from the radiation intensities detected by the detection cells. The method according to the invention comprises the steps of arranging the detector in at least two different positions, which are shifted relative to each other laterally with respect to the propagation direction of the radiation by a fraction of the size of a single collection area, and recording the intensity distribution in each position. Further according to the method of the invention a border recognition criterion is applied to the recorded intensity distributions and thereby respective border data sets are obtained. Further, the border data sets are evaluated and thereby a measurement result of the border is determined.

According to the inventive method the detector is arranged in a first position and the intensity distribution is recorded in this position. Thereafter, the detector is shifted laterally by a fraction of the size of a single collection area. For example a CCD-camera having detection pixels forming the collection areas can be used as the detector. In this case the shift performed is a sub-pixel shift, i.e. the shift distance is smaller than the lateral dimension of each pixel. Subsequently, the intensity distribution is recorded again in the shifted position. Further sub-pixel shifts and corresponding recordals of the respective intensity distribution can follow.

Subsequently, a border recognition criterion is applied to each recorded intensity distribution to determine respective border data sets representing respective border measurements. The border recognition criterion can e.g. comprise an intensity or a contrast threshold, according to which it can be decided whether a specific collection area is considered to be inside or outside the intensity distribution. The border data sets obtained this way are then evaluated to determine a final measurement result of the border. The evaluation is performed in an appropriate way, e.g. by averaging the borders represented by the border data sets and/or by selecting one or a subset of border data sets based on plausibility considerations. The described method allows the border of the intensity distribution to be determined with improved accuracy, as effectively the resolution, with which the border is measured is driven beyond the resolution given by the size of the collection areas of the detector. As already mentioned above, according to an embodiment of the invention the evaluating of the border data sets comprises an averaging of the borders represented by the data sets. According to a further embodiment the evaluating of the border data sets comprises selecting one or a subset of the border data sets and determining the measurement result from the selected border data sets.

According to a further embodiment the border data sets are obtained in at least three, at least four or at least five different detector positions. According to a further embodiment the detector is arranged in positions shifted relative to each other in two lateral dimensions.

According to a further embodiment a center point of the intensity distribution is determined from the determined border. This can be done by use of mathematical algorithms known to the skilled person.

These and other features of embodiments of the invention are described in the claims as well as in the specification and the drawings. The individual features may be implemented either alone or in combination as embodiments of the invention, or may be implemented in other fields of application. Further, they may represent advantageous embodiments, that are protectable in their own right, for which protection is claimed during pendency of this application and/or continuing applications.

Brief description of the drawings

The foregoing, as well as other advantageous features of the invention, will be more apparent from the following detailed description of exemplary embodiments of the invention with reference to the following diagrammatic drawings, wherein:

Fig. 1 illustrates a measurement apparatus for measuring an imaging aberration of an optical system comprising a shearing interferometer; Fig. 2 illustrates an intensity distribution recorded by a detection camera of the shearing interferometer, which intensity distribution is delimited by a circular border; Fig. 3 illustrates the formation of the intensity distribution from radiation components of different diffraction orders; Fig. 4 illustrates an exemplary embodiment of an optical system to be measured by the measurement apparatus according to Fig. 1 , which optical system has an obscured pupil;

Fig. 5 schematically illustrates an intensity distribution recorded by the detection camera of the shearing interferometer when testing the optical system according to Fig. 4;

Fig. 6 illustrates an exemplary ray path in the shearing interferometer according to Fig. 1 including rays of spurious radiation;

Fig. 7 illustrates a further exemplary ray path in the shearing interferometer according to Fig. 1 also including rays of spurious radiation;

Fig. 8 illustrates an effect of the spurious radiation on the border of the intensity distribution on the detection camera being subject to a correction measure according to the invention;

Fig. 9 illustrates a method according to the invention directed at precisely measuring the border of the intensity distribution by moving the detector of the shearing interferometer in sub-pixel increments;

Figures 10a and 10b illustrate the method visualized in Fig. 9 further;

Fig. 11 illustrates a further method according to the invention for determining the border of the intensity distribution; Fig. 12 shows an insert containing a reference structure to be inserted into a pupil plane of the optical system for calibrating the intensity distribution on the detector of the shearing interferometer; and Fig. 13 shows an attenuator to be inserted into the pupil plane of the optical system according to an embodiment of the invention.

Detailed description of exemplary embodiments

In the exemplary embodiments of the invention described below, components that are alike in function and structure are designated as far as possible by the same or alike reference numerals. Therefore, to understand the features of the individual components of a specific embodiment, the descriptions of other embodiments or the summary of the invention should be referred to.

Fig. 1 illustrates a measurement apparatus 10 in an embodiment according to the invention configured for measuring an imaging aberration of an optical system 12. The measurement apparatus 10 according to Fig. 1 is further configured for performing a method according to the invention for determining a border of an intensity distribution recorded during measurement of the imaging aberration.

The measurement apparatus 10 is in the illustrated embodiment configured as a shearing interferometer and comprises an illumination module 14 arranged above the optical system 12 and a detection module 16 arranged below the optical system 12 according to Fig. 1. The illumination module 14 comprises a radiation source 17 for generating incoming radiation 18 of a wavelength adapted to the operation wavelength of the optical system 12 under test. The optical system 12 under test can be a projection objective of a protection exposure system for microlithography. Depending on the type of lithography system the operation wavelength can be in the ultraviolet wavelength range, for example at 248 nm or 193 nm. Further, the wavelength can be in the extreme ultraviolet wavelength range (EUV), in particular at a wavelength of less than 100 nm, for example of 13,5 nm or 6,8 nm. The optical system 12 typically comprises a number of optical elements 26, two of which are shown in Fig. 1 for illustration purposes. The optical elements 26 can be configured as lenses, as shown in Fig. 1 , or as reflective elements in the form of mirrors. In case the operational wavelength is in the EUV- wavelength range all optical elements 26 are configured as mirrors.

An embodiment of such an optical system 12 comprising only mirrors is shown in Fig. 4. The pupil of this optical system 12 comprises a central obscured area. The design of the optical system shown in Fig. 4 is an exemplary design of a system with an obscured pupil comprising only two reflective elements 26-1 and 26-2. However, the optical system may also comprise a larger number of optical elements and in particular be designed as a projection objective for EUV- lithography.

The illumination module 14 further comprises a coherence mask 22 arranged in an object plane 24 of the optical system 12. The coherence mask 24 comprises a two-dimensional pattern, which can be provided in form of an array of pinholes or openings. The illumination module 14 further comprises a focussing device 20 configured for focussing the radiation 18 onto a small area of the coherence mask 24.

The detection module 16 is configured for determining the wave front of the radiation 18, after having passed through the optical system 12, also referred to as outgoing radiation 32. For this purpose, the shearing interferometer 16 comprises a grating module 34 comprising a diffraction grating 35 arranged in the image plane 36 of the optical system 12. Underneath the grating module 34 a detection camera 40 having a detection surface 42 is arranged in a plane conjugate to a pupil plane 28 of the optical system 12. In the pupil plane 28 an aperture 30 is disposed for limiting the angle of incidence of radiation entering the image plane 36. The diffracting grating 34 generates waves of different diffraction orders, in particular of minus first, zeroth and plus first diffraction order, as illustrated in Fig. 3. By superposition of these three waves an interferogram is formed on the detection surface 42, subsequently also referred to as intensity distribution 48. In order to obtain a two-dimensional phase distribution of the radiation 32 at the image plane 36, several interferograms having different phase settings are generated.

This is done by shifting the diffraction grating 35 along the image plane 36, in the embodiment according to Fig. 1 along the x-axis of the coordinate system shown in the figure. For this purpose, the grating module 34 is held by a translation device 38. Alternatively, the variation of the phase setting can be achieved by shifting the coherence mask 22 in the object plane 24. The interferograms generated this way on the detection surface 42 are recorded by the detection camera 40 and the wave front is calculated by an evaluation device 46 from the recorded interferograms.

According to an embodiment, the shearing detection module 16 comprises a translation device 44, which is configured for moving the detection camera 40 laterally with respect to the propagation direction of the radiation 32. Referring to the coordinate system of Fig. 1 , the translation device 44 is configured to move the detection camera 40 in x- and y-direction. The translation device 44 can be used for performing a border recognition procedure described subsequently with reference to Figures 9, 10a, and 10b.

The measuring apparatus 10 shown in Fig. 1 is only an example of a measuring apparatus for determining aberrations of an optical test system using a shearing interferometer. Other examples of measurement apparatuses of this kind are known to the skilled person and can for example include a measurement apparatus extracting reference radiation from the illumination module, bypassing the optical system and superimposing the reference radiation on the detection side of the measurement apparatus to form an interferogram. An example of such a measuring apparatus is described in US 7,333,216 B2.

As mentioned above, for measuring the wave front distribution of the radiation 32 having passed through the optical system 12, an intensity distribution in form of an interferogram is generated on the detection surface 42 of the detection camera 40. This intensity distribution is schematically shown in Fig. 2 for the optical system of Fig.1 and designated with the reference numeral 48. The detection camera 40 is arranged in a plane conjugate to the pupil plane 28. Therefore the intensity distribution 48 is an image of the pupil of the optical system 12. The border 50 of the intensity distribution is roughly circular. As however the intensity distribution 48 is composed of waves of plus first, minus first and zeroth diffraction order, the overall border 50 is somewhat blurred due to the respective borders 50a, 50b, and 50c of the different diffraction order waves being shifted relative to each other, as illustrated in Fig. 3.

The intensity distribution 48 comprises areas, in which all diffraction orders contribute to the formation of the interferogram (three beam region), and crescent moon shaped regions near the border of the interferogram, which include only two diffraction orders (two-beam regions). Next to the two-beam regions further crescent moon shaped regions exist, which are only formed by -1. or +1 diffraction orders and in which no superposition with a further diffraction order is present. These regions differ from each other in intensity and contrast. The border 50 of the intensity distribution 48 is determined by the evaluation device 46 by applying a border recognition criterion to the recorded intensity distribution 50. The border recognition criterion can include a threshold criterion, based for example on an intensity or a contrast threshold.

Fig. 5 shows the intensity distribution 48 generated on the detection camera 40 by the optical system 12 according to Fig. 4 having an obscured pupil. The intensity distribution 48 therefore comprises an obscured area 54, which makes the border 50 to be composed of an outer border 50o and an inner border 50i. As mentioned above, the intensity distribution 48 is blurred in the region of the border 50, which complicates determining the aberrations of the optical system 12 from the measured wave front. One of the reasons for the blurring of the border 50 is due to a lateral shift of the different diffraction orders, explained above with reference to Fig. 3. A further cause for the blurring can be related to spurious radiation being split off from the radiation 32 after it has passed through the diffraction grating 35. Figures 6 and 7 illustrate different effects causing the generation of such spurious radiation. Fig. 6 shows an exemplary imaging ray 32a of the radiation 32 passing through the diffraction grating 35 and entering a substrate 58 holding the diffraction grating 35 on its top surface. The imaging ray 32a then traverses a gap 56 between the grating module 34 and the detection camera 40. In the shown embodiment of the detection camera 40 a fluorescent layer 60 is arranged on the detection surface 42 of the camera 40. The fluorescent layer 60 has the function of converting the wavelength of the radiation 32 into a wavelength, which can be easily detected by the camera 40. According to one example, the fluorescent layer 60 can be configured to convert EUV-radiation into visible light, which can be detected for example by a CCD camera.

The radiation of the imaging ray 32a impinging onto the fluorescent layer 60 mostly enters the fluorescent layer 60. A portion of the radiation, however, is split off in reflection forming the split-off ray 32b of spurious radiation, which travels back to the substrate 58. At a bottom surface 59 of the substrate 58 further a portion of the radiation of ray 32b is split off in reflection forming the split-off ray 32b'. The split-off ray 32b' enters into the fluorescent layer 60. The remaining radiation in ray 32b is passes back into the substrate 58, is reflected on the diffraction grating 35 and subsequently passes back through the substrate 58, the gap 56 and thereupon enters into the fluorescent layer 60. From rays 32a, 32b and 32b' entering the fluorescent layer 60 further spurious radiation in the form of split-off rays 32c is generated. The rays 32c are generated by scattering, when the radiation enters the fluorescent layer 60.

Taking the above described influences from spurious radiation on the blurring of the intensity distribution 48 into account, the intensity distribution I on the detection surface 42 can be described as follows:

i = (i₀ + ci₀ ® (K_sub + K_grating)) ® K_fluor ® K_mask _ ₍₁₎

wherein:

- I₀ is the intensity distribution in the pupil plane 28 of the optical system 12, that means the intensity distribution, which would be displayed on the detection surface 42 without blurring effects,

- c is the ratio between the intensity of the radiation impinging on the detection surface 42 on direct path, i.e. the intensity of the radiation of ray 32a impinging onto the detection surface 42 and the intensity of the radiation reaching the detection surface 42 on other paths, i.e. the total intensity of the radiation of rays 32b, 32b' and 32c,

- fiuor is the convolution kernel describing the scattering in the fluorescent layer 60,

- Ksub is a convolution kernel describing the reflection on the bottom surface 59 of the substrate 58, - Kgrating is a convolution kernel describing the reflection at the diffraction grating 35, and - Kmask is the convolution kernel describing the influence of the size of the coherence pattern 23 imaged onto the diffraction grating 35 on the blurring of the intensity distribution. K_SUb and Kgrating are mainly determined from the beam path shown in Fig. 7. Knuor is dependent on the material and the thickness of the fluorescent layer 60. K_maSk becomes insignificant, in case the size of the coherence pattern 23 is infinitely small. Fig. 7 illustrates a further effect causing spurious radiation to be generated. In this case the imaging ray 32a is partially reflected back at the bottom surface 59 of the substrate 58 generating a split-off ray 32d. The ray 32d is reflected again at the diffraction grating 35, passes through the substrate 58 and the gap 56, and enters the fluorescent layer 60 causing further rays 32c to be split off. All of the split-off radiation 32b, 32c, 32d, and potentially other split-off radiation not illustrated here cause a blurring in the region of the border 50 of the intensity distribution 48.

The extent of the blurring can be influenced by the size of the coherence pattern 23 on the coherence mask 22 imaged onto the diffraction grating 35 during the interferometric measurement. In other words, the size of the area of the coherence mask 22 illuminated by the incoming radiation 18 during the measurement influences the blurring of the border 50.

The detection module 16 can also have a different configuration than shown in Fig. 1. It can for example comprise transfer optics, comprise an arrangement of grating patches and/or be configured without a fluorescent layer 60. Also such a configuration may show a blurring of the intensity distribution similar to the blurring illustrated by means of Figures 6 and 7. The blurring of the intensity distribution 48, also referred to as the blurring of the pupil, causes an enlargement of the intensity distribution 50, which causes the radius of the border 50 to be measured too large. Fig. 8 illustrates this effect. The intensity distribution 62 in the pupil plane 28 is convoluted with any of the above described convolution kernels K. This results in an intensity distribution 48, which is enlarged as compared to the intensity distribution 62 in the pupil plane. Put in different words, the pupil is enlarged by the effect of the convolution kernel.

Further, the blurring or the intensity distribution 62 results in larger errors when determining the border 50 using a threshold criterion. According to an embodiment of the invention one or several of the parameters relating to the configuration of the measurement apparatus 10 and influencing the generation of the spurious radiation 32b, 32c, and 32d are identified, the influence of the identified parameter on the blurring of the recorded intensity distribution 48 is determined and the border 50 of the intensity distribution 48 is determined from the recorded intensity distribution 48 taking the determined influence on the blurring into account.

According to one embodiment the thickness d_s of the substrate 58 is identified as parameter influencing the generation of spurious radiation. The thickness d_s is measured and the resulting convolution kernel defining the influence of the thickness d_s on the blurring of the intensity distribution I is calculated. The calculated influence on the blurring is taken into account when determining the border 50 from the recorded intensity distribution 48. This is done by the evalutaion device 46, which uses the calculated convolution kernel to calculate a corrected intensity distribution by extracting the blurring caused by the measured thickness d_s from the measured intensity distribution 48. Thereafter the border 50 of the intensity distribution 48 is determined from the corrected intensity distribution. This can be done by applying the above described threshold criterion.

According to a further embodiment the distance d_a between the grating module 34 and the detection camera 40 is identified as the parameter influencing the generation of the spurious radiation. Thereafter, the influence of the distance d_a on the blurring of intensity distribution is experimentally determined. This is done by varying the distance d_a and measuring the resulting shape of the recorded intensity distribution 48. From this data the blurred portion of the intensity distribution, which correlates with the distance d_a is determined and a corrected intensity distribution is calculated by extracting the blurred portion from the recorded intensity distribution. Thereafter, the border 50 of the intensity distribution 48 is determined from the corrected intensity distribution.

According to a further embodiment, a blurring portion in the intensity distribution being caused by the fluorescent layer 60 is determined experimentally in a separate measurement. This information is used to calculate a corrected intensity distribution by extracting the blurred portion from the recorded intensity distribution. Thereafter, the border 50 of the intensity distribution 48 is determined from the corrected intensity distribution.

According to a further embodiment the size of the coherence pattern 23 on the coherence mask 22 imaged onto the diffraction grating 35 is identified as the parameter influencing the generation of spurious radiation. Thereafter, the wave front measurement is performed using coherence patterns 23 of different sizes and resulting borders of the recorded intensity distributions 48 on the detection surface 42 are determined respectively. From this the respective border 50 of the intensity distribution 48 for a hypothetical infinitely small coherence pattern 23 is calculated by extrapolation. According to a variation only the diameters of the intensity distributions 48 are determined for the different sizes of coherence patterns 23 and therefrom the resulting diameter of a hypothetical infinitely small coherence pattern 23 is calculated. According a further embodiment of the invention measures are taken to minimize the blurring in the recorded intensity distribution. According to a first measure the bottom surface 59 of the substrate 58 of the grating module 34 is coated with a reflection suppressing layer 66, as shown in Fig. 7. This way the generation of the split-off radiation, illustrated by ray 32d, is minimized. According to a further measure the diffraction grating 35 is made from black chrome reducing the reflection of spurious radiation 32d impinging onto the grating. According to a further embodiment the distance d_a between the grating module 34 and the detection camera 40 is reduced to a value smaller than the desired detection accuracy of the border 50. The same can be done regarding the thickness d_s of the substrate 58. A reduction of d_a and d_s results in a smaller lateral displacement of the split-off rays 32b and 32d and therefore minimizes the blurring. According to a further measure a pinhole having a small opening is provided in the coherence mask 22 and is used for determining the radius and the center of the intensity distribution 48. The various parameters described above influencing the generation of the spurious radiation can be taken into account separately or in any combination.

Fig. 9 illustrates a further cause for a blurring in the region of the border 50 of the recorded intensity distribution 48 as well as a method according to the invention for reducing this blurring effect. Fig. 9 shows the surface 42 of the detection camera 40 for recording the intensity distribution 48 in three positions, denoted by pos. 1 , pos. 2 and pos. 3. The detection camera 40 comprises an array of detection cells, each of which is associated to a respective collection area 52 on the detection surface 42. According to an embodiment a CCD camera is used as the detection camera 40. The detection pixels of the CCD camera form the respective collection areas 52.

The detection accuracy of the border 50 of the intensity distribution 48 is limited by the size of the collection areas 52. When applying a threshold criterion to the intensity detected by a single detection cell the measurement result for a detection cell in the region of the border 50 may depend on the precise position of the detection camera 40. A displacement of the detection camera 40 by a fraction of the size of a single collection area 52 may result in the respective measurement to switch from "1" to "0", wherein "1" identifies a pixel considered inside the border 50 and "0" a pixel considered outside the border 50. Therefore, the measurement of the border 50 is characterized by a significant uncertainty, which effectively contributes to the blurring of the border 50. According to an embodiment of the invention a method is provided in order to obtain a more precise measurement of the border 50. According to this method the detection camera 40 is shifted laterally with respect to the propagation direction of the radiation 32, that means in the x-y plane according to the coordinate system of Fig. 1 between single measurements. The shift distance between two measurements is only a fraction of the size of a single collection area 52. This is illustrated in Fig. 9 for three positions shifted relative to each other in the x-direction.

Figures 10a and 10b illustrate the measurement signals obtained by a set of four horizontally arranged detection cells crossing the right side border 50 of the intensity distribution 48. These detection cells are associated to the collection areas 52-1 , 52-2, 52-3, and 52-4 shown in Figures 10a and 10b for exemplary illustration. The collection area 52-2 is also indicated in Fig. 9. Fig. 10a contains a graph showing the course of the intensity I in the region of the border 50. The exact position of the border in the depicted sectional plane is denoted by Ro. Ro exactly defines the position, at which the intensity curve intersects with a threshold intensity l_t.

As can be seen from the drawing underneath the graph, which shows the collection areas 52-1 to 52-2 in position 1 , Ro is located in a right portion of the collection area 52-2. When applying a threshold criterion to the respective total intensity received by the single collection areas 52-1 to 52-4, the collection areas 52-1 and 52-2, lying above the threshold, turn out the result "1" and the collection areas 52-3 and 52-4, lying below the threshold, turn out the detection result "0". Accordingly, the location of the border for position 1 of the detection camera 40 turns out to lie between the collection areas 52-2 and 52-3, which location is denoted by Ri. The border locations for position 1 are recorded along the entire border 50 shown in Fig. 9.

Thereafter, the detection camera 40 is moved to position 2 by shifting the same by a distance Δχ to the right, as illustrated in Fig. 10b, Δχ being a fraction of the extension of a single collection area 52 in the x- dimension. The border R2 is determined in position 2, which lies in the illustrated case again between the collection areas 52-2 and 52-3. Subsequently the position of the detection camera 40 is moved to position 3 by shifting the same again by Δχ. The border, now denoted as R3_, is determined also in this position. In the illustrated example R3 is located between collection areas 52-1 and 52-2.

When comparing the obtained measurement results Ri , R2 and R3 with the actual location of the border Ro, as illustrated in the respective graph of Figures 10a and 10b, it is noted that Ri and R2 lie on the right side and R3 on the left side of Ro- According to the inventive method, further measurements at positions shifted respectively by Δχ and also in the y-direction may be collected for the measuring the entire border 50. The recorded border data sets are then evaluated to extract a measurement result of the border, which represents the real border 50 better than any of the single measurements.

The evaluation can be performed for example by averaging the border data sets obtained for the different positions. According to another variation the single border data sets are evaluated by an algorithm, a subset of the border data sets are selected from the data sets according to plausibility considerations, and the measurement result is determined from the selected border data sets.

Fig. 11 illustrates a further embodiment according to the invention for determining the border 50 of the intensity distribution 48 more precisely. As already illustrated in Fig. 3, the intensity distribution 48 is generated by waves of different diffraction orders, especially a wave of zero diffraction order generating an intensity distribution with the border 50b, a wave of plus first diffraction order generating an intensity distribution with the border 50a and a wave of minus first diffraction order generating an intensity distribution with the border 50c.

According to an embodiment of the invention several measurements are taken at different shearing distances. The shearing distance is varied by varying a period p of the diffraction grating 35. Further, several measurements are taken at different shearing directions. The shearing directions are varied by rotating the coherence mask 22 or the detection module 16 with respect to the surface normal of the diffraction grating 35. Alternatively, the coherence mask 22 may contain rotated coherence patterns 23. Further, the grating module 34 can also be provided with rotated diffraction gratings 35. The data sets obtained by varying the shearing distance and the shearing direction are then evaluated to determine a more precise measurement result of the border 50. Fig. 12 illustrates a further embodiment according to the invention for more precisely determining the border 50 of the intensity distribution 48. According to this embodiment an aperture insert 68 is inserted into the aperture 30. The insert 68 comprises a reference structure 70, for example in form of a grating. The reference structure 70 is used to calibrate the angular distribution of the rays generating the intensity distribution 48 on the detection surface 42.

Fig. 13 illustrates a further embodiment according to the invention. According to this embodiment an attenuator 72 is inserted into a plane conjugate to the plane of the detection surface 42. The border 50 of the intensity distribution 48 recorded in the measurement apparatus 10 according to Fig. 1 has a relatively small intensity compared to the center of the intensity distribution. Therefore, the noise level at the border 50 is relatively high, leading to errors in calculating the radius and the center of the intensity distribution 48. By inserting the attenuator 72 into the plane conjugate to the detection surface 42, for example into a pupil plane of the illumination module 14 or into the pupil plane 28 of the optical system 12, the dynamics of the detection camera 40 can be used for enlarging the signal at the border 50. This way the border 50 can be determined more accurately.

The above description of exemplary embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.

List of reference signs

10 measurement apparatus

12 optical system

14 illumination module

16 detection module

17 radiation source

18 incoming radiation

20 focussing device

22 coherence mask

23 coherence pattern

24 object plane

26 optical element

28 pupil plane

30 aperture

32 outgoing radiation

32a imaging ray

32b, 32b', 32c, 32d split-off ray

34 grating module

35 diffraction grating

36 image plane

38 translation device

40 detection camera

42 detection surface

44 translation device

46 evaluation device

48 intensity distribution

50 border of intensity distribution

50a, 50b, 50c border of radiation component 50o outer border

50i inner border

52 collection area obscured area

gap

substrate

bottom surface

fluorescent layer

pupil-intensity distribution convolution kernel reflection suppressing layer aperture insert

reference structure attenuator

Claims

1. Method of determining a border of an intensity distribution on a detection surface, the intensity distribution being generated by electromagnetic radiation after having passed through an optical system and being recorded by a wave front measuring apparatus for measuring a wave front distribution of the radiation, wherein the intensity distribution is blurred in the region of the border due to spurious radiation, and the method comprises the steps of:

- identifying a parameter relating to the configuration of the wave front measuring apparatus and influencing the generation of the spurious radiation,

- determining an influence of the identified parameter on the blurring of the recorded intensity distribution, and

- determining the border of the intensity distribution from the recorded intensity distribution taking the determined influence on the blurring into account.

2. Method according to claim 1 ,

wherein the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, the spurious radiation is split off from radiation having passed through the diffraction grating, and the identified parameter relates to the configuration of the shearing interferometer.

3. Method according to claim 1 or 2,

wherein the detection surface is arranged in a plane conjugate to a pupil plane of the optical system.

4. Method according to any one of the previous claims,

wherein the influence of the identified parameter on the blurring of the recorded intensity distribution is determined by measuring the parameter and calculating the resulting influence from a mathematical model.

5. Method according to claim 4,

wherein the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, which diffraction grating comprises a substrate, and the parameter measured is a thickness of the substrate.

6. Method according to any one of claims 1 to 3,

wherein the influence of the identified parameter on the blurring of the recorded intensity distribution is determined by measuring test intensity distributions for different values of the parameter, extrapolating the intensity distribution at a parameter value of minimized blurring, and subtracting the extrapolated intensity distribution from the recorded intensity distribution.

7. Method according to claim 6,

wherein the wave front measuring apparatus comprises a shearing interferometer comprising a diffraction grating, the shearing interferometer further comprises a mask comprising a coherence pattern, which is imaged onto the diffraction grating, the size of the coherence pattern is identified as the parameter influencing the generation of the spurious radiation, the test intensity distributions are measured for coherence patterns of different sizes, and the intensity distribution is extrapolated for a coherence pattern of infinitely small size.

8. Method according to claim 6 or 7,

wherein the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a diffraction grating arranged between the optical system and the detection surface, the distance between the diffraction grating and the detection surface is identified as the parameter influencing the generation of the spurious radiation, the test intensity distributions are measured for different settings of the distance between the diffraction grating and the detection surface and the intensity distribution is extrapolated for a distance setting of minimal blurring in the intensity distribution.

9. Method according to any one of the previous claims,

wherein the wave front measuring apparatus comprises a shearing interferometer comprising a diffraction grating, and the spurious radiation is caused due to single rays of the radiation originating from the diffraction grating respectively being split up into several rays before striking the detection surface.

10. Method according to any one of the previous claims,

wherein the detection surface is covered by a wavelength conversion layer for converting the wavelength of the radiation, the wavelength conversion layer is identified as the parameter influencing the generation of the spurious radiation, and the influence of the wavelength conversion layer on the blurring of the recorded intensity distribution is determined.

1 1. Method according to any one of the previous claims,

wherein the surface of the diffraction grating facing towards the detection surface is coated with a reflection suppressing layer.

12. Method according to any one of the previous claims,

wherein the wave front measuring apparatus comprises a shearing interferometer, which shearing interferometer comprises a coherence mask having coherence patterns used for measuring the wave front distribution and a test structure, which test structure is of smaller size than the coherence patterns used, and the test structure is used for determining the radius and/or the center point of the border of the intensity distribution.

13. Method of determining a border of an intensity distribution on a detection surface of a two-dimensionally resolving detector, the intensity distribution being generated by electromagnetic radiation after having passed through an optical system and being recorded for measuring a wave front distribution of the radiation, the detector comprising an array of detection cells, each detection cell being configured to detect the intensity of a radiation impinging onto a respective associated collection area and the detector being configured to record the intensity distribution from the radiation intensities detected by the detection cells, wherein the method comprises the steps of:

- arranging the detector in at least two different positions, which are shifted relative to each other laterally with respect to the propagation direction of the radiation by a fraction of the size of a single collection area, and recording the intensity distribution in each position,

- applying a border recognition criterion to the recorded intensity distributions and thereby obtaining respective border data sets, and

- evaluating the border data sets and thereby determining a measurement result of the border.

14. Method according to claim 13,

wherein the evaluating of the border data sets comprises an averaging of the borders represented by the data sets.

15. Method according to claim 13 or 14,

wherein the evaluating of the border data sets comprises selecting one or a subset of the border data sets and determining the measurement result from the selected border data sets.

16. Method according to any one of claims 13 to 15,

wherein border data sets are obtained in at least three different detector positions.

17. Method according to any one of claims 13 to 16,

wherein the detector is arranged in positions shifted relative to each other in two lateral dimensions.

18. Method according to any one of the previous claims,

wherein a center point of the intensity distribution is determined from the determined border.