WO2014114436A1

WO2014114436A1 - Polarization measurement system for a projection exposure apparatus

Info

Publication number: WO2014114436A1
Application number: PCT/EP2014/000120
Authority: WO
Inventors: Ingo SÄNGER; Frank Schlesener
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2013-01-22
Filing date: 2014-01-17
Publication date: 2014-07-31
Also published as: DE102013200961A1

Abstract

A polarization measurement system (34) for a microlithographic projection exposure apparatus (10) is provided. The polarization measurement system comprises at least one polarization-modulating element (50; 148), which has a monolithic configuration and varying polarizer properties along a translation axis (58), and a polarization-selective element (56), which is arranged downstream of the polarization-modulating element (50; 148) and configured to select radiation with a specific polarization property from incident radiation (64).

Description

Polarization measurement system for a projection exposure apparatus

This application claims priority to the German Patent Application No. 10 2013 200 961.5 filed on January 22, 2013 and the U.S. Provisional Application No. 61/755,126 filed on January 22, 2013. The entire disclosure of this German patent application, and this U.S. Provisional Application are incorporated into the present application by reference. Background of the invention

The invention relates to a polarization measurement system for a microlithographic projection exposure apparatus, a microlithographic reticle with a polarization measurement system integrated therein and a method for measuring polarization in a microlithographic projection exposure apparatus.

In order to ensure an unchanging high quality of microlithographic projection exposure apparatuses during production operation, monitoring of the exposure properties is carried out at regular intervals. In so doing, e.g. the homogeneity of the exposure radiation in the exposure beam path, such as in the mask plane, etc., is monitored at regular intervals. As a result of ever-increasing demands on the quality of the projection exposure apparatuses, it is desirable also to monitor polarization properties in the exposure beam path at regular intervals. However, the problem with this is that a polarization measurement system suitable for this may only use up little installation space in order to be able to be used without much redesigning outlay during exposure pauses of the projection exposure apparatus. In so doing, housing the polarization measurement system in the reticle plane or in the wafer plane, for example, would be expedient. However, conventional polarization measurement techniques with measurement accuracy sufficient for the purpose of the application are unsuitable for installation-space reasons or would require a disproportionately large outlay. An example for such a polarization measurement technique known to the person skilled in the art is described in WO 2010/105757 A1. Here, a rotatably mounted λ/4 retardation plate is arranged upstream of a beam splitter cube. Underlying object

It is an object of the invention to provide a polarization measurement system and a method, by means of which the aforementioned problems are solved, and, in particular, which allow a polarization measurement to be carried out with high quality on a microlithographic projection exposure apparatus installed in a production environment without much redesigning outlay.

Solution according to the invention According to the invention, the above object can be achieved, for example, by a polarization measurement system for a microlithographic projection exposure apparatus which is configured as follows: the projection exposure apparatus comprises at least one polarization-modulating element, which has a monolithic configuration and varying polarizer properties along a translation axis. Furthermore, the projection exposure apparatus comprises a polarization- selective element, which is arranged downstream of the polarization-modulating element and configured to select radiation with a specific polarization property from incident radiation. Due to the configuration of the polarization-modulating element with varying polarizer properties along a translation axis, a linear displacement of the polarization measurement system can generate a varying effect on the polarization of radiation radiated thereon. It is therefore possible, for example, to use a displacement device, such as the reticle stage or the wafer stage, which is already present in the projection exposure apparatus for the polarization measurement, as a result of which the installation space required by the polarization measurement can be reduced. A monolithically configured element should be understood to mean that the element has not been assembled, i.e. it has been manufactured from one piece. The polarization-modulating element is therefore integral and, at the same time, has a homogeneous basic structure. In other words, the polarization-modulating element is a continuous workpiece from a microscopic point of view, i.e. from the view of the material structure. Idiomatically speaking, it could be said that the polarization-modulating element "is molded as one", wherein the unified workpiece, from which the element is manufactured, need not be made by a molding process but can also be a crystal, for example.

The polarization-modulating element has varying polarizer properties along a translation axis, and hence along a straight line. As a result, the polarization- modulating element has different effects on the polarization properties of incident radiation at different points along the translation axis. This leads to radiation radiated in punctiform manner on the polarization modulator to be modulated in respect of the polarization property thereof in the case of a displacement of the polarization modulator along the translation axis. In accordance with one embodiment, the polarization properties vary continuously along the translation axis and can be described by means of a continuous function in this case. In accordance with a further embodiment, the variation of the polarization properties can be described by a step function.

The aforementioned effects on the polarization properties of the incident radiation can relate to e.g. a rotation of the polarization direction and/or a change in the phase relationship between the orthogonally polarized wave components, such as s- and p-polarized wave components. A different effect at different points is present if, in particular, at least one polarization parameter varies by more than 1 %, in particular 5% or 10%, at the different points. Thus, for example, a rotation of the polarization direction at the different points brought about by the polarization-modulating element can vary by more than 1 °, in particular more than 5° or more than 10°, and/or the phase relationship between orthogonally polarized wave components at the different points can differ from one another by at least TT/100, corresponding to a path length difference of at least 1/200 of the wavelength, in particular by at least ττ/10 or at least ττ/2. The polarization-selective element is arranged downstream of the polarization- modulating element. In other words, in respect of the translation axis, the polarization-selective element is arranged offset transversely opposite to the polarization-modulating element in such a way that radiation, which has interacted with the polarization-modulating element, is incident, at least in part, on the polarization-selective element. Expressed differently, the polarization-selective element is arranged in an output beam path of the polarization modulator, and so incident radiation is incident on the polarization-selective element after interaction with the polarization modulator. By way of example, the polarization-selective element can be a beam splitter cube or a thin-film polarizer.

In accordance with one embodiment, the polarization measurement system furthermore comprises a projection optical unit arranged upstream of the polarization modulator. By way of example, such a projection optical unit can be configured as Fresnel lens or else comprise a two-dimensional array of lenses. Furthermore, the polarization measurement system can contain a radiation detector, e.g. in the form of a CCD camera, for evaluating the intensity of the radiation passing through the polarization-selective element, which radiation detector is arranged downstream of the polarization-selective element. However, the intensity measurement can also be performed using a sensor present on the wafer stage of the projection exposure apparatus.

In accordance with a further embodiment according to the invention, the polarization-modulating element comprises optically active material for rotating a polarization direction of incident polarized radiation, with the optically active material having a thickness variation along the translation axis. The optically active material can comprise levorotatory and/or dextrorotatory molecules, or else crystals with an asymmetric crystal structure. In accordance with a further embodiment according to the invention, the polarization-modulating element has a wedge-shaped configuration. In accordance with one variant, the polarization-modulating element has optically active material for rotating a polarization direction of incident polarized radiation.

In accordance with a further embodiment according to the invention, the polarization-modulating element is configured as a first wedge-shaped element and the polarization measurement system furthermore comprises a second wedge-shaped element, with the wedge-shaped elements being oriented in mutually opposite directions. In accordance with one variant, the wedge-shaped elements comprise optically active material for rotating a polarization direction of incident polarized radiation. In accordance with a further embodiment according to the invention, the polarization measurement system comprises two polarization-modulating elements, respectively having polarizer properties that vary along the translation axis, and a retardation plate arranged between the two polarization-modulating elements. One of the polarization-modulating elements is the aforementioned polarization-modulating element with a monolithic configuration, wherein the second polarization-modulating element can likewise have a monolithic configuration. A retardation plate, which is also referred to as a wave plate, is to be understood to mean a thin pane or film made of optically anisotropic material, which has different propagation speeds in different directions for light with different polarizations. Typically, such a retardation plate has a birefringent crystal with an appropriately selected thickness and alignment. An example of such a retardation plate is a λ/4 plate.

In accordance with a further embodiment according to the invention, the two polarization-modulating elements each comprise optically active material for rotating a polarization direction of incident polarized radiation, with the respective optically active material having a respective thickness variation along the translation axis which differs from polarization-modulating element to polarization- modulating element. In this context, the thickness of the optically active material is to be understood to mean the extent of the optically active material in the passage direction of the polarized radiation. The passage direction in particular extends perpendicularly to the translation axis.

In accordance with a further embodiment according to the invention, the polarization-modulating element has at least one portion with an extent of 0.1 mm along the translation axis, in which portion the polarizer properties vary by less than 5%, more particularly by less than 1 %. In accordance with a variant, the polarization properties vary by less than 5%, more particularly by less than 1 %, in any portion of the polarization-modulating element with an extent of 0.1 mm.

In accordance with a further embodiment according to the invention, the polarization-modulating element has arranged along the translation axis a plurality of segments with respectively uniform polarizer properties, with the polarizer properties varying from segment to segment. In other words, the polarization properties are uniform in the respective segment. In accordance with one variant, the extent of each segment along the translation axis is at least 0.1 mm, in particular at least 1 mm. In the case of such dimensions, it is possible to perform the polarization measurement in the pupil-resolved manner, i.e. dependent on the angle of incidence of radiation to be measured. Uniform polarizer properties are to be understood to mean that polarization properties vary by less than 1 %, more particularly by less than 0.1 %.

Furthermore, according to the invention, provision is made for a polarization measurement system for a microlithographic projection exposure apparatus, comprising at least one polarization-modulating element. The polarization- modulating element comprises arranged along a translation axis a plurality of segments having polarizer properties that vary from segment to segment, with the polarizer properties being respectively uniform within the segments and each of the segments having an extent in the direction of the translation axis of at least 0.1 mm, in particular of at least 0.5 mm or at least 1 mm.

In accordance with a further embodiment according to the invention, at least one of the segments has an optically active material for rotating a polarization direction of incident polarized radiation.

In accordance with a further embodiment according to the invention, at least two of the segments have birefringent material, with the birefringent material differing from segment to segment in at least one of the following parameters: orientation of a fast axis of the birefringent material and thickness of the birefringent material.

In accordance with a further embodiment according to the invention, the segments have an effect of λ/4 plates with differing orientations. In the case of λ/4 plates with differing orientations, the optical axes of the plates have different orientations.

In accordance with a further embodiment according to the invention, the polarization-modulating element comprises amorphous material, which has laser- induced birefringence in at least one portion. By way of example, an amorphous material can be fused silica. The aforementioned laser-induced birefringence can be produced, for example, as described in the article "Laser-induced birefringence in fused silica from polarized lasers", U. Neukirch et al., Proceedings of SPIE, vol. 5754 (SPIE, Bellingham, WA, 2005). In accordance with one variant, the portion having the laser-induced birefringence lies in a region which is optically used during a measurement process and the polarization-modulating element furthermore has an optically unused region with a reduced transmission. The optically unused region can adjoin the optically used region directly. An optically used region is to be understood to mean a region characterized in that radiation is evaluated for the polarization measurement after interaction with the optically used region. The opposite is the case in the optically unused region. The optically unused region serves for laser irradiation for generating the birefringence in the optically used region. The laser irradiation can involve a transmission reduction in the irradiated region.

Furthermore, provision according to the invention is made for a polarization measurement system for a microlithographic projection exposure apparatus, configured as follows. This projection exposure apparatus comprises at least one polarization-modulating element. The polarization-modulating element has continuously varying polarizer properties and/or continuously varying polarization properties along a translation axis. Furthermore, the projection exposure apparatus comprises a polarization-selective element, which is arranged downstream of the polarization-modulating element and configured to select radiation with a specific polarization property from incident radiation. Continuously varying polarization properties are to be understood to mean that the profile thereof can be described by means of a continuous function. The features described in the above-described embodiments or embodiment variants can be applied in an analogous manner to this polarization measurement system.

Furthermore, provision according to the invention is made for a reticle for a microlithographic projection exposure apparatus, which comprises, integrated therein, a polarization measurement system according to one of the embodiments or embodiment variants described above.

Moreover, a reticle for a microlithographic projection exposure apparatus is proposed according to the invention, which reticle comprises at least one polarization-modulating element. The polarization-modulating element has an integral configuration and varying polarizer properties along a translation axis.

In accordance with the invention, provision is furthermore made for a microlithographic projection exposure apparatus, which comprises a polarization measurement system according to one of the aforementioned embodiments or embodiment variants. In accordance with one embodiment according to the invention of the projection exposure apparatus, the polarization measurement system is arranged on a reticle stage or wafer stage, which can respectively be displaced along the translation axis, of the projection exposure apparatus. The reticle stage and the wafer stage are often also referred to as reticle table and wafer table, respectively, and are each configured as displacement stage for the reticle and the wafer, respectively.

In accordance with a further embodiment according to the invention, the projection exposure apparatus is configured as a scanner, in which both the reticle stage and the wafer stage are moved parallel to the scanning direction during an exposure process. Here, the polarization measurement system is arranged in such a way that the translation axis is aligned parallel to the scanning direction. In accordance with a further embodiment according to the invention, the projection exposure apparatus furthermore comprises a pinhole stop and a holder for holding the pinhole stop in a position above the polarization measurement system. Here, in accordance with one embodiment variant, the pinhole stop is arranged during the measurement operation in a stationary manner with respect to the illumination optical system of the projection exposure apparatus.

Furthermore, provision according to the invention is made for a method for measuring polarization in a microlithographic projection exposure apparatus, comprising an exposure beam path for guiding exposure radiation. In accordance with the method, at least one polarization modulator is arranged in the exposure beam path in such a way that output radiation is generated by interaction of the exposure radiation, having an input polarization state, with the polarization modulator at a measurement location in the exposure beam path. Furthermore, a polarization state of the output radiation is varied by displacing the polarization modulator parallel to a translation axis. Furthermore, the input polarization state is established by virtue of a profile of a polarization property of the output radiation being evaluated during the displacement of the polarization modulator. As a result, the measurement location remains unchanged when displacing the polarization modulator.

In accordance with embodiments according to the invention of the above- described method, the polarization modulator is part of a polarization measurement system in accordance with one of the above-described embodiments or embodiment variants.

In accordance with one embodiment, the projection exposure apparatus has a displacement device for displacing a substrate along the scanning direction and the polarization modulator is arranged on the displacement device, at least during the measurement process. The substrate can be a reticle or wafer.

The features specified in respect of the above-described embodiments, exemplary embodiments or embodiment variants, etc. of the polarization measurement system according to the invention can be transferred accordingly to the method according to the invention, the reticle according to the invention and the projection exposure apparatus according to the invention. The converse also applies. These and other features of the embodiments according to the invention will be explained in the claims and in the description of the figures. The individual features can be implemented either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments, which themselves are protectable and the protection for which is claimed, if appropriate, only during or after pendency of the application.

Brief description of the drawings

The above-described and further advantageous features of the invention will be illustrated in the following detailed description of exemplary embodiments according to the invention, with reference being made to the attached schematic drawings. In detail: shows a schematic view of a microlithographic projection exposure apparatus with a polarization reticle, which contains a polarization measurement system in an embodiment according to the invention, shows a further embodiment of the polarization measurement system for use in the polarization reticle as per Figure 1 , shows an illustration of an embodiment of a detection arrangement for use in the polarization measurement system, shows an illustration of the functionality of the polarization measurement system in one of the embodiments according to Figure 1 or Figure 2 for the case of linearly polarized input radiation, shows an illustration of the functionality of the polarization measurement system in one of the embodiments according to Figure 1 or Figure 2 for the case of circularly polarized input radiation, shows the intensity profile measured during a measurement process by a radiation detector using the polarization system in one of the embodiments according to Figure 1 or Figure 2 for the case of input radiation with the polarization state +S1 or -S1 , shows the intensity profile measured during a measurement process by a radiation detector using the polarization system in one of the embodiments according to Figure 1 or Figure 2 for the case of input radiation with the polarization state +S2 or -S2, shows the intensity profile measured during a measurement process by a radiation detector using the polarization system in one of the embodiments according to Figure 1 or Figure 2 for the case of input radiation with the polarization state +S3 or -S3, shows a sectional view of a further embodiment of the polarization measurement system for use in the polarization reticle as per Figure 1 , comprising a polarization-modulating element in the form of a segmented polarization optical unit, shows an illustration of the functionality of the polarization- modulating element as per Figure 9, shows a top view of the polarization-modulating element as per Figure 10 in a first embodiment and shows a top view of the polarization-modulating element as per Figure 11 in a further embodiment.

Detailed description of exemplary embodiments according to the invention In the exemplary embodiments or embodiments and embodiment variants described below, functionally or structurally similar elements are provided as far as possible with the same or similar reference signs. Therefore, in order to understand the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention.

In order to simplify the description, the drawing specifies a Cartesian xyz- coordinate system, from which the respective positional relationships of the components depicted in the figures emerge. In Figure 1 , the y-direction extends perpendicular to the plane of the drawing and into the latter, the x-direction extends to the right and the z-direction extends upward. Figure 1 illustrates a microlithographic projection exposure apparatus 10 in one embodiment according to the invention. The projection exposure apparatus 10 comprises an illumination system 12 for illuminating a mask, which is arranged on a reticle stage 26, with exposure radiation 16. The illumination is for the purpose of imaging the mask on a substrate, for example in the form of a wafer 30 or of a transparent so-called "flat-panel".

The illumination system 12 comprises an exposure radiation source 14 for generating the exposure radiation 16. Depending on the embodiment of the projection exposure apparatus 10, the wavelength of the exposure radiation 16 can lie in the UV wavelength range, e.g. at 248 nm or 193 nm, or else in the extreme UV wavelength range (EUV), for example at 13.5 nm or 6.8 nm. Depending on the exposure wavelength, the optical elements of the illumination system 12 and of the projection lens 22 are embodied as lens elements and/or as mirrors. In the following, the invention will be explained on the basis of a projection exposure apparatus 10 operated in the UV wavelength range.

The exposure radiation 16 generated by the exposure radiation source 14 passes through a beam processing optical unit 18 and is thereupon radiated into the mask plane by an illuminator 20. In the configuration shown in Figure 1 , the projection exposure apparatus 10 is in a measurement mode, in which a measurement reticle 32 is arranged in the mask plane. The measurement reticle 32 is held by the reticle stage 26. A pinhole stop 38 is arranged upstream of the measurement reticle 32.

In the exposure mode - not depicted in the drawing - a mask to be exposed is arranged on the reticle stage 26 in place of the measurement reticle. Furthermore, the pinhole stop 38 is no longer in the exposure beam path 36. The reticle stage 26 is displaceably mounted compared to a frame 24 of the projection exposure apparatus 10. For exposure purposes, the wafer 30 is arranged on a wafer stage 28, which is likewise displaceably mounted. During an exposure process, the reticle stage 26 is displaced in a scanning direction 60, which is aligned parallel to the x-coordinate axis in accordance with Figure 1. At the same time, there is synchronous displacement of the wafer stage 28 parallel to the scanning direction 60. In the measurement mode already mentioned above, the measurement reticle 32 is arranged on the reticle stage 26 in the mask plane, as shown in Figure 1. Furthermore, the pinhole stop 38 is positioned above the measurement reticle 32 by means of a holder 40, in a position stationary with respect to the illumination system 12. The position of the pinhole stop 38 defines a measurement location 42 for the polarization measurement performed in the measurement mode. The measurement location 42 corresponds to the location of the aperture of the pinhole stop 38. The exposure radiation 16 emitted by the illumination system 12 passes through the aperture of the pinhole stop 38 at the measurement location 42 as a divergent beam. The divergent beam serves as input radiation 44 during the polarization measurement. In order to measure different field points in the radiation field of the exposure radiation 16 radiated into the mask plane, the pinhole stop 38, and hence the measurement location 42, is displaced in the x/y- plane in a suitable manner. The measurement reticle 32 comprises a polarization measurement system 34 for measuring a polarization property of the exposure radiation 16 radiated into the mask plane. An embodiment of the polarization measurement system 34 is depicted together with the pinhole stop 38 arranged thereabove in the left-hand upper portion of Figure 1.

This polarization measurement system 34 has a height, i.e. an extent in the z- direction, of approximately 6 mm, which has been matched to the thickness of the measurement reticle 32. In the x-direction or y-direction, the dimensions of the polarization measurement system 34 approximately correspond to the extent of the exposure field radiated into the mask plane in the scanning direction 60, i.e. to the extent of the field illuminated by the exposure radiation 16 in the mask plane in the x-direction. As a result, the dimensions of the polarization measurement system 34 in the x-direction or y-direction can be approximately 22 mm in each case.

The polarization measurement system 34 in the embodiment as per Figure 1 comprises a projection optical unit 46, which serves to convert the input radiation 44 in the form of an expanding wave, which leaves the pinhole stop 38, into a plane wave. A polarization modulator 48 is arranged below the projection optical unit 46 and comprises a first polarization-modulating element 50, a second polarization-modulating element 54 and a retardation plate 52, which is arranged between the elements 50 and 54. The two polarization-modulating elements 50 and 54 each have a wedge-shaped configuration, wherein the wedges are oriented in mutually opposite directions. The wedge alignment in each case extends parallel to a translation axis 58, which is aligned parallel to the scanning direction 60. The wedge-shaped elements each have a monolithic configuration and therefore have a homogeneous basic structure, as already explained above.

The wedge-shaped polarization-modulating elements 50 and 54 have a thickness of less than 3 mm in accordance with one embodiment variant; by way of example, the retardation plate 52 can be embodied with a thickness of approximately 7 mm. In accordance with an embodiment (not depicted in the drawing), the elements 50, 54 and the retardation plate 52 are wringed onto one another. To this end, the elements 50 and 54 are slightly twisted compared to the positions shown in Figure 1 , and so these come into contact with the retardation plate 52. When wringing, the respectively adjoining elements are placed so closely together with the corresponding surfaces thereof that there is a secure connection between the elements.

The wedge-shaped polarization-modulating elements 50 and 54 are each manufactured from optically active material, which serves to rotate a polarization direction of the incident radiation. The angle of rotation depends on the thickness of the optically active material and therefore varies along the translation axis 58 in both elements 50 and 54 due to the wedge shape. Crystals with asymmetric crystal structures and materials comprising chiral substances can be used as optically active material.

The input radiation 44 with a specific input polarization state leaves the polarization modulator 48 after passing through the latter as output radiation 64 with an output polarization state that is dependent on the input polarization state of the input radiation 44 and on the position of the polarization measurement system 34 along the translation axis 58, as explained in more detail below with reference to Figures 4 and 5.

In the embodiment as per Figure 1 , the polarization measurement system 34 furthermore comprises a polarization-selective element 56 in the form of a polarizing beam splitter cube, which is arranged downstream of the polarization modulator 48. Alternatively, the polarization-selective element 56 can also be configured as e.g. thin-film polarizer. Only radiation components of the output radiation 64 with P-polarization are passed by the polarization-selective element 56 and pass through the projection lens 22 of the projection exposure apparatus 10 as polarization-selected radiation 66. A radiation detector 62 in the form of a CCD camera is integrated into an edge region of the wafer stage 28. In the measurement mode, the wafer stage 28 is positioned in such a way that the polarization-selected radiation 66 is incident on the radiation detector 62. Thereupon, the reticle stage 26 is moved in the scanning direction 60, i.e. parallel to the translation axis 58, while the wafer stage 28 remains stationary. During this movement, the profile of the intensity of the radiation 66 incident on the radiation detector 62 is recorded. The input polarization state of the input radiation 44 is thereupon determined from the recorded profile by means of an evaluation device 63, as explained in more detail below. Here, the polarization influence by the projection lens 22 is taken into account by calculation. If the measurement location 42 is situated on the edge of the illumination field generated by the illumination system 12, the polarization measurement system 34 is driven beyond the illumination field during the polarization measurement. This process is also referred to as "overscan".

In the case in which the radiation detector 62 is configured as a spatially resolving detector, an intensity distribution can be recorded at any time during the scanning movement. This intensity distribution corresponds to the pupil distribution of the input radiation 44 in relation to the pupil of the illumination system 16 and this means the angular distribution of the input radiation 44 at the measurement location 42. The input polarization state can therefore be determined in a pupil- resolved manner.

Figure 2 shows a further embodiment of a polarization measurement system 34 according to the invention. It differs from the embodiment shown in Figure 1 merely in that the radiation detector 62 is integrated into the polarization measurement system 34 and hence into the measurement reticle 32. As a result, there is no need for a radiation detector integrated into the wafer stage 28 for measurement purposes. In this case, a possibly interfering influence of the projection lens 22 on the measurement is avoided. Furthermore, it is also possible to arrange the polarization-selective element 56 outside of the polarization measurement system. Thus, the latter can be integrated with e.g. the radiation detector 62 in the wafer stage 28 of the polarization exposure apparatus 10. As an alternative to the arrangement in the mask plane, the polarization measurement system 34 as per Figure 1 , as per Figure 2 or as per other embodiments described in this application can also be positioned at other points in the exposure beam path 16 of the projection exposure apparatus 10. In this case, the polarization measurement system 34 is arranged separately at the corresponding position, i.e. without measurement reticle 32. Thus, the polarization measurement system 34 can be integrated into e.g. the wafer stage 28 and can therefore be used to measure the polarization property of the exposure radiation 16 in the wafer plane. Figure 3 shows a modified variant of evaluating the output radiation 64. In accordance with this variant, respectively one radiation detector 62 is arranged in both output beam paths of the polarization-selective element 56 in the form of a polarized beam splitter cube. The output radiation 64, incident on the polarization- selective element 56, with orthogonally polarized polarization portions S and P is split into two output beam paths by the beam splitter cube. The first output beam path contains the polarization-selected radiation 66 with P-polarization, while the second output beam path contains a polarization-selected radiation 68 with S- polarization. The intensities of the two radiation components are measured by the two radiation detectors 62. As a result, the polarization composition of the radiation 64 can be registered with higher measurement accuracy. From the sum of the intensity recorded by the two radiation selectors 62, the overall intensity of the radiation 64 emerges, as a result of which intensity normalization can be carried out. Furthermore, it is possible to undertake intensity normalization as a function of the pupil distribution, i.e. of the angular distribution of the radiation at the measurement location 42.

In the following, the functionality of the polarization measurement system 34 as per Figure 1 and Figure 2 is explained with reference to Figures 4 to 8. Figure 4 shows the modification of the polarization state of the input radiation 44 when passing through the individual elements of the polarization measurement system 34 at different positions (Pos) of the polarization measurement system 34 during a measurement process. In the example illustrated in Figure 4, the input radiation 44 is completely P-polarized. That is to say, the polarization state Pi of the input radiation 44 is the P-polarization state.

As already mentioned above, the polarization measurement system 34 as part of the measurement reticle 32 is displaced in the scanning direction 60 during a measurement process. Here, the polarization measurement system 34 successively passes through the positions denoted by Pos 1 to Pos 5 in Figure 4. At the position Pos 1 , the polarization measurement system 34 is in a position in which the input radiation 44 experiences a rotation in the polarization direction of 180° when passing through the first polarization-modulating element 50 in the form of a first wedge-shaped element W1. Here, the input radiation 44 passes through the thick end of the wedge-shaped element W1. In the positions Pos 2 to Pos 5, the polarization measurement system 34 is in each case successively shifted to the left, such that the input radiation 44 in each case passes through thinner sections of the wedge-shaped element W1. The angle of rotation CM therefore becomes smaller in steps of 45°. In the position Pos 2, the angle of rotation CM at the wedge-shaped element W1 is 135°, it is 90° in the position Pos 3, it is still 45° in the position Pos 4, and, by contrast, CM = 0° applies to the angle of rotation in the position Pos 5.

As already mentioned above, the input radiation 44 in the example illustrated in Figure 4 is P-polarized. This refers to a linearly polarized state, the polarization direction of which is parallel to the plane of incidence of the input radiation 44. The plane of incidence of the input radiation 44 is defined by the plane spanned by the propagation vector of the input radiation 44 and by the normal to the beam- splitting face of the polarization-selective element 56 in the form of the polarizing beam splitter cube BS. The P-polarized polarization state is also referred to as +S1. This represents an abbreviation for a Stokes vector with Stokes components S1 =+1 and S2=0 and S3=0.

The polarization state present after the input radiation 44 passes through the wedge-shaped element W1 is denoted by Pwi in Figure 4 and it is depicted for each of the positions Pos 1 to 5. The radiation with the polarization state Pwi thereupon passes through the retardation plate 52 in the form of a λ/4 plate. The polarization state PA/4 present thereafter for the positions Pos 1 to Pos 5 successively is: P-polarization, right-hand circular polarization, S-polarization, left- hand circular polarization and P-polarization. The radiation thereupon passes through the second polarization-modulating element 54 in the form of the second wedge-shaped element W2, which has an orientation in the opposite direction compared to the first wedge-shaped element W1 , leading to the circular polarization states at position Pos 1 and Pos 4 being respectively reversed in terms of their orientation and the S-polarization at the position Pos 3 being converted into P-polarization. The respective rotation brought about by the second wedge-shaped element W2 is denoted by the angle of rotation 02 in Figure 4. Thereupon, the radiation with the polarization state Pw2 passes through the polarizing beam splitter cube BS, with it being only the radiation components with P-polarization that pass through the latter without being deflected. This is the aforementioned polarization-selected radiation 66, the intensity of which is recorded by the detector 62. At the positions Pos 2 and Pos 4, at which the radiation incident on the beam splitter BS has circular polarization, the intensity of the radiation 66 is in each case merely 50% of the intensity of the radiation 66 at the positions Pos 1 , Pos 3 and Pos 5. The diagram depicted below the beam splitter cube BS in Figure 4 shows the intensity profile I of the radiation 66 in the case of continuous displacement of the polarization measurement system 34 from position Pos 1 to position Pos 5, as recorded at a location on the detection face by the radiation detector 62. The intensity profile is depicted dependent on the angle of rotation 02 of the second wedge-shaped element W2. As can be identified from the diagram, the recorded intensity profile has the form of a cosine function with an angular period of 90°.

Figure 5 depicts the change in the polarization states when the input radiation 44 passes through the polarization measurement system 34 in the case where the input radiation 44 has complete right-hand circular polarization. The corresponding polarization state Pi is also referred to as +S3, which serves as an abbreviation for the Stokes vector in which the following applies to the Stokes components S1 , S2 and S3: S1 =0, S2=0 and S3=+1. The depiction in Figure 5 is analogous to that in Figure 4 for the individual positions Pos 1 to Pos 5. As can be seen from the depiction, the polarization state PA/4 after passing through the λ/4 plate is the same for all positions, namely a linear polarization state rotated by 45° compared to the P-polarization. The intensity profile recorded by the radiation detector 62 during the movement from position Pos 1 to position Pos 5 corresponds to a sine function with an angular period of 180°.

For completion, Figures 6 to 8 show the intensity profiles recorded by the radiation detector 62 for input radiation 44 with the polarization states +S1 and -S1 (Figure 6), the polarization states +S2 and -S2 (Figure 7) and the polarization states +S3 and -S3 (Figure 8). Here, +S1 corresponds to the Stokes vector (1 , 1 , 0, 0), -S1 corresponds to the Stokes vector (1 , -1 , 0, 0), +S2 corresponds to the Stokes vector (1 , 0, 1 , 0), -S2 corresponds to the Stokes vector (1 , 0, -1 , 0), +S3 corresponds to the Stokes vector (1 , 0, 0, 1 ), and -S3 corresponds to the Stokes vector (1 , 0, 0, -1 ).

As emerges from the diagrams, each of the depicted input polarization states can be uniquely determined on the basis of the signature of the intensity profile assigned thereto. Thus, the intensity profile of the polarization state +S1 corresponds to a positive cosine function with an angular period of 90° and an intensity amplitude of 0.25 lo (lo is the intensity of the input radiation 44), with the zero crossing lying at 0.75 lo. The intensity profile of the polarization state -S1 corresponds to a negative cosine function with the same angular period and the same intensity amplitude as the intensity profile of the polarization state +S1 , but has a zero crossing which is lower by 0.5 lo. The respective intensity profile of the polarization states +S2 and -S2 is characterized by a positive or negative sine function with a respective angular period of 90° and an intensity amplitude of 0.25 lo and a zero crossing of 0.5 lo. In respect of the polarization states +S3 and -S3, the respective intensity profile has the form of a negative or positive sine function with an angular period of 180° and an intensity amplitude of 0.5 lo and a zero crossing of 0.5 lo.

In the general case, in which the input radiation 44 has an arbitrary polarization state, i.e. a polarization state which has any mixture of the above-described states +S1 , -S1 , +S2, -S2, +S3 and -S3, the intensity profile recorded by the radiation detector 62 has a corresponding complex structure. In the evaluation device 63, the recorded intensity profile is evaluated by means of a Fourier transform. On the basis of the Fourier transform, it is then possible to establish the individual polarization components allotted to the various polarization states S1 to S3 and hence it is possible to determine the input polarization state of the radiation 44 completely. The assignment of the individual polarization states S1 to S3 is performed in the evaluation device 63 by means of the known signatures of the individual polarization states as per Figure 6 to Figure 8.

Figure 9 shows a further exemplary embodiment according to the invention of a polarization measurement system 34, which, like the polarization systems 34 as per Figure 1 and Figure 2, can be integrated into the measurement reticle 32 or else can be arranged at other positions of the exposure beam path 36 of the projection exposure apparatus 10 as well, for example on the wafer stage 28. The polarization measurement system 34 as per Figure 9 merely differs from the polarization measurement system as per Figure 2 in that the polarization modulator, which is denoted by the reference sign 148 in Figure 9, has an integral configuration in the direction of the propagation direction of the inward radiating input radiation 44. In accordance with an embodiment, the whole polarization modulator 148 is an integral element and can therefore also be referred to as polarization-modulating element.

Figure 10 shows the polarization modulator 148 as such an element with integral configuration in a sectional view. The polarization modulator 148 comprises a plurality of segments 170-1 to 170-4 arranged along the translation axis 54, with polarization properties that vary from segment to segment. Within each of these segments 170-1 to 170-4, the polarization properties are uniform, i.e. the polarization effect of the polarization modulator 48 is characterized by a stepped profile corresponding to the segment subdivision along the translation axis 58. In accordance with one embodiment, at least two of the segments 170-1 to 170-4 have birefringent material, wherein the birefringent material differs from segment to segment in at least one of the following parameters: orientation of a fast axis of the birefringent material and thickness of the birefringent material. At least one of the segments 170-1 to 170-4 can also be embodied as λ/4 plate. In accordance with one variant, the polarization modulator 148 comprises a plurality of λ/4 plates with in each case different orientation of the optical axis in the segments thereof. In the embodiment illustrated in an exemplary manner in Figure 10, a first segment 170-1 of the polarization modulator 148 is configured as polarization-neutral glass, and so the input radiation 44 with linear P-polarization remains unchanged when passing therethrough. The second segment 170-2 has a birefringent material, which makes an elliptically polarized wave from a P- polarized wave. The segment 170-3 is made of a polarization-rotating material and the segment 170-4 is configured to convert irradiated P-polarization into circular polarization.

Figure 11 shows the polarization modulator 148 in a top view. Furthermore, the figure specifies the scanning direction 60, in which the polarization modulator 148 is displaced when carrying out a measurement process. Analogously to the evaluation described with reference to Figures 3 to 8, an intensity signal is also recorded by the radiation detector 62 when using the polarization modulator 148 as per Figure 10 and 11 , which intensity signal varies in a step-like manner dependent on the position of the polarization modulator 148 in the scanning process. This signal is likewise evaluated by the evaluation device 63. Here, the input polarization state of the input radiation 44 radiated thereon is determined due to advance knowledge of the polarization-changing properties of the individual segments 170-1 to 170-4.

The individual segments 170 of the polarization modulator 148 as per Figure 10 can be manufactured from independent workpieces in each case and thereupon be assembled to form the polarization modulator 148 with an integral configuration. In accordance with a further variant, the polarization modulator 148 has a monolithic configuration and is therefore manufactured from one piece. In this case, the polarization-modulating element 148 has a homogeneous basic structure. The polarization-modulating element 148 is then a continuous workpiece, not only from a macroscopic view, but also from a microscopic view.

An example of such a polarization modulator 148 with a monolithic configuration and with several polarization properties that vary from segment to segment is depicted in Figure 12. In this embodiment, the polarization modulator 148 is manufactured from a contiguous workpiece of amorphous material, with provision being additionally made for two optically unused regions 174 in addition to a region 172 optically used during the measurement process. The optically unused regions adjoin the optically used region 172 in a direction transverse to the scanning direction 60. In other words, the optically unused regions 174 are arranged above and below the optically used region 172.

The segments 170-1 to 170-4 in the optically used region 172 are each assigned portions 176-1 to 176-4 of the optically unused region 174, which adjoin the corresponding segments. During the production of the polarization modulator 148 as per Figure 12, laser radiation is applied to the portions 176-1 in the optically unused region 174. The application is performed appropriately matched to the polarization effect desired in the segments 170-1 to 170-4, for example by virtue of laser-induced birefringence being generated in the desired form in the individual segments 170-1 to 170-4. To this end, it is possible, for example, to use the method for generating a laser-induced birefringence which is described in the article "Laser-induced birefringence in fused silica from polarized lasers", U. Neukirch et al., Proceedings of SPIE, vol. 5754 (SPIE, Bellingham, WA, 2005). By irradiating the associated portions in the optically unused region 174, suitable birefringence is generated in the segments 170 of the optically used region 172. Due to the high irradiation intensity, the optically unused region 174 generally has a reduced transmission, which is why this region is generally unsuitable for use during the polarization measurement.

The polarization modulator 148 is depicted with merely four segments 170-1 to 170-4 in the exemplary Figures 10 to 12. The number of segments 170 may however be greater for the practical application, but each of the segments 170 should have a minimum extent of 0.1 mm in the direction of the translation axis 58. In accordance with further variants, the minimum extent of the segments 170 in the direction of the translation axis is 0.5 mm or 1 mm.

List of reference signs

10 Projection exposure apparatus

12 Illumination system

14 Exposure radiation source

16 Exposure radiation

18 Beam processing optical unit

20 Illuminator

22 Projection lens

24 Frame

26 Reticle stage

28 Wafer stage

30 Wafer

32 Measurement reticle

34 Polarization measurement system

36 Exposure beam path

38 Pinhole stop

40 Holder

42 Measurement location

44 Input radiation

46 Projection optical unit

48 Polarization modulator

50 First polarization-modulating element

52 Retardation plate

54 Second polarization-modulating element

56 Polarization-selective element

58 Translation axis

60 Scanning direction

62 Radiation detector

63 Evaluation device

64 Output radiation

66 Polarization-selected radiation Polarization selected radiation Polarization modulator-1 to 170-4 Segments

Optically used region

Optically unused region

Claims

1. A polarization measurement system for a microlithographic projection exposure apparatus, comprising:

- at least one polarization-modulating element, which has a monolithic configuration and varying polarizer properties along a translation axis, and

a polarization-selective element, which is arranged downstream of the polarization-modulating element and configured to select radiation with a specific polarization property from incident radiation.

2. The polarization measurement system according to Claim 1 ,

wherein the polarization-modulating element has optically active material for rotating a polarization direction of incident polarized radiation, with the optically active material having a thickness variation along the translation axis.

3. The polarization measurement system according to one of the preceding claims,

wherein the polarization-modulating element has a wedge-shaped configuration.

4. The polarization measurement system according to one of the preceding claims,

wherein the polarization-modulating element is configured as a first wedge- shaped element and the polarization measurement system furthermore comprises a second wedge-shaped element, with the wedge-shaped elements being oriented in mutually opposite directions.

5. The polarization measurement system according to one of the preceding claims,

which comprises two polarization-modulating elements, respectively having polarizer properties that vary along the translation axis, and a retardation plate arranged between the two polarization-modulating elements.

6. The polarization measurement system according to one of the preceding claims,

wherein the polarization-modulating element has at least one portion with an extent of 0.1 mm along the translation axis, in which portion the polarizer properties vary by less than 5%.

7. The polarization measurement system according to Claim 1 ,

wherein the polarization-modulating element has arranged along the translation axis a plurality of segments with respectively uniform polarizer properties, with the polarizer properties varying from segment to segment.

8. A polarization measurement system for a microlithographic projection exposure apparatus, comprising at least one polarization-modulating element, which has arranged along a translation axis a plurality of segments having polarizer properties that vary from segment to segment, with the polarizer properties being respectively uniform within the segments and each of the segments having an extent in the direction of the translation axis of at least 0.1 mm.

9. The polarization measurement system according to Claim 7 or 8,

wherein at least one of the segments has an optically active material for rotating a polarization direction of incident polarized radiation.

10. The polarization measurement system according to one of Claims 7 to 9, wherein at least two of the segments have birefringent material, with the birefringent material differing from segment to segment in at least one of the following parameters: orientation of a fast axis of the birefringent material and thickness of the birefringent material.

11. The polarization measurement system according to one of Claims 7 to 10, wherein the segments have an effect of λ/4 plates with differing orientations.

12. The polarization measurement system according to one of the preceding claims,

wherein the polarization-modulating element comprises amorphous material, which has laser-induced birefringence in at least one portion.

13. A polarization measurement system for a microlithographic projection exposure apparatus, comprising:

at least one polarization-modulating element, which has continuously varying polarizer properties along a translation axis, and

- a polarization-selective element, which is arranged downstream of the polarization-modulating element and configured to select radiation with a specific polarization property from incident radiation.

14. A reticle for a microlithographic projection exposure apparatus, comprising a polarization measurement system according to one of the preceding claims integrated therein.

15. A reticle for a microlithographic projection exposure apparatus, comprising at least one polarization-modulating element, which has an integral configuration and varying polarizer properties along a translation axis.

16. A microlithographic projection exposure apparatus, comprising a polarization measurement system according to one of Claims 1 to 13.

17. The projection exposure apparatus according to Claim 16,

wherein the polarization measurement system is arranged on a reticle stage or wafer stage, which can respectively be displaced along the translation axis, of the projection exposure apparatus.

18. The projection exposure apparatus according to Claim 16 or 17,

which furthermore comprises a pinhole stop and a holder for holding the pinhole stop in a position above the polarization measurement system.

19. A method for measuring polarization in a microlithographic projection exposure apparatus comprising an exposure beam path for guiding exposure radiation, wherein:

- at least one polarization modulator is arranged in the exposure beam path in such a way that output radiation is generated by interaction of the exposure radiation having an input polarization state with the polarization modulator at a measurement location in the exposure beam path,

a polarization state of the output radiation is varied by displacing the polarization modulator parallel to a translation axis, and

the input polarization state is established by virtue of a profile of a polarization property of the output radiation being evaluated during the displacement of the polarization modulator.