WO2013060561A1

WO2013060561A1 - Optical system in an illumination device of a microlithographic projection exposure apparatus

Info

Publication number: WO2013060561A1
Application number: PCT/EP2012/069632
Authority: WO
Inventors: Ingo SÄNGER; Frank Schlesener
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2011-10-27
Filing date: 2012-10-04
Publication date: 2013-05-02
Also published as: DE102011085334A1; TW201333636A

Abstract

The invention relates to an optical system in an illumination device of a microlithographic projection exposure apparatus. According to one aspect, an optical system in an illumination device of a microlithographic projection exposure apparatus has at least one polarization-influencing element (110, 210, 310, 410, 510, 610), which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section, and at least one diffractive structure (130, 230, 430, 530, 630), which is arranged such that it and the polarization-influencing element directly follow one another in the light-propagation direction and, during the operation of the projection exposure apparatus, brings about a ray deflection of passing-through light, which ray deflection varies over the light-beam cross section, wherein the polarization-influencing element (110, 210, 310, 410, 510, 610) is produced from linearly or circularly birefringent material and has a geometry which, at least in regions, is wedge-shaped.

Description

Optical system in an illumination device of a

microlithographic projection exposure apparatus

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority of German Patent Application DE 10 201 1 085 334.0 and US 61/552,001 , both filed on October 27, 201 1 . The content of these applications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to an optical system in an illumination device of a microlithographic projection exposure apparatus.

State of the art Microlithographic projection exposure apparatuses are used to produce microstructured components such as e.g. integrated circuits or LCDs. Such a projection exposure apparatus has an illumination device and a projection lens. In the microlithographic process, the image of a mask (= reticle) illuminated with the aid of the illumination device is projected by means of the projection lens onto a substrate (e.g. a silicon wafer), which is coated with a light-sensitive layer

(photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure onto the light-sensitive coating of the substrate. During the operation of a microlithographic projection exposure apparatus, there is the need to set defined illumination settings, i.e. intensity distributions in a pupil plane of the illumination device, in a targeted fashion. To this end, the use of mirror arrangements is also known, e.g. from WO 2005/026843 A2, in addition to the use of diffractive optical elements (so-called DOEs). Such mirror arrangements comprise a multiplicity of micromirrors which can be set independently of one another.

Furthermore, various approaches are known for setting specific polarization distributions in the pupil plane and/or in the reticle in a targeted fashion in the illumination device in order to optimize the imaging contrast. In respect of the prior art, reference made is, for example, to WO 2005/069081 A2, WO 2005/031467 A2, US 6, 191 ,880 B1 , US 2007/0146676 A1 , WO 2009/034109 A2, WO 2008/019936 A2, WO 2009/100862 A1 , DE 10 2008 009 601 A1 and DE 10 2004 01 1 733 A1 .

DE 10 2007 007 907 A1 has disclosed a production method for a diffractive optical element, in which at least two different types of individual elements with different predefined beam-forming and polarizing effect are generated with high structure accuracy, for the purpose of which etching structures in particular are introduced into a polarization-forming substrate.

EP 2 1 17 034 A1 has disclosed, inter alia, the practice of producing a diffractive optical element from a multiplicity of base elements of optically active crystal material, with each of these base elements having a diffractive surface for obtaining a wanted ray deflection and with the thickness of these base elements varying for producing different polarization rotations.

However, the realization, ultimately taking place in the above-described approaches, of diffractive optical elements which, in addition to providing a wanted intensity distribution, also - as an additional functionality - allow a wanted polarization distribution to be set requires comparatively complex microstructuring and hence relatively high manufacturing complexity. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical system in an illumination device of a microlithographic projection exposure apparatus which, with comparatively little effort, enables the provision of a wanted intensity and polarization distribution.

According to a first aspect of the invention, an optical system in an illumination device of a microlithographic projection exposure apparatus has:

- at least one polarization-influencing element, which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section; and - at least one diffractive structure, which is arranged such that it and the polarization-influencing element directly follow one another in the light- propagation direction and, during the operation of the projection exposure apparatus, brings about a ray deflection of passing-through light, which ray deflection varies over the light-beam cross section; - wherein the polarization-influencing element is produced from linearly or circularly birefringent material and has a geometry which, at least in regions, is wedge-shaped.

Here, the wording according to which the diffractive structure is "arranged such that it and the polarization-influencing element directly follow one another in the light-propagation direction" should be understood in the sense that the diffractive structure can alternatively be arranged in front of or behind the polarization- influencing element. According to the aforementioned first aspect, the present invention is based on the concept of enabling the conversion of an originally typically constant linear polarization distribution into a multiplicity of different polarization states in a particularly simple fashion from a manufacturing point of view by providing, in a (linearly or circularly) birefringent polarization-influencing element, a profile which, at least in sections, is wedge-shaped, with said polarization states or the light rays to which these have been applied being - to a certain extent in a functionally separated fashion - directed by means of the diffractive structure into suitable angular ranges and hence to specific points in a pupil plane of the illumination device in a targeted fashion. Here - in contrast to the conventional approaches described above - there is no need for microstructuring, which is complicated from a manufacturing point of view, of the birefringent material contained in the polarization-influencing element. Moreover, in principle, there can also be a variation of the respectively generated polarized illumination setting, for example for the purposes of an adjustment, by displacing firstly the polarization-influencing element and secondly the diffractive structure relative to one another (e.g. in a direction perpendicular to the light-propagation direction).

In particular, the approach according to the invention proceeds from the idea that the thickness variation in (linearly or circularly) birefringent material required for providing a typically wanted (e.g. tangential or quasi-tangential) polarization distribution is typically at least two orders of magnitude greater than the substantially finer diffractive structures as required to obtain wanted ray deflections for generating a wanted intensity distribution in the pupil plane, and so the wedge- shaped geometry, selected according to the invention, for the polarization- influencing material is particularly advantageous firstly from a manufacturing point of view and secondly in view of avoiding unwanted shadowing effects.

According to one embodiment, at least two light rays, which pass through the polarization-influencing element and have polarization states that differ from one another after passing through the polarization-influencing element, are coherently superposed in a pupil plane of the illumination device.

According to this embodiment, the invention therefore makes use of the principle of coherent superposition in order, starting from the geometry, predetermined according to the invention, of the polarization-influencing element, to generate by way of coherent superposition further polarization states, which fit to the respectively wanted polarized illumination setting, from partial rays which are situated within the same coherence cell and have different polarization states impressed thereon by the polarization-influencing element. By way of example - as explained in more detail below - this is how a quasi-tangential illumination setting can be generated instead of a tangential illumination setting by virtue of the light rays belonging to opposite edges of a coherence cell being coherently superposed while in the process of generating a polarization state corresponding to the center of the coherence cell.

The concept of coherent superposition, explained above, is not restricted to the geometry of the polarization-influencing element, which, at least in sections, is wedge-shaped and is selected as per the first aspect. Rather, according to a further aspect of the present invention, the concept of coherent superposition can also be advantageous or be implemented independently of the geometry of the polarization-influencing element.

Hence, according to a further aspect, the invention also relates to an optical system in an illumination device of a microlithographic projection exposure apparatus, comprising

- at least one polarization-influencing element, which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section; and - at least one ray-deflecting structure, which, during the operation of the projection exposure apparatus, brings about a ray deflection of passing- through light, which ray deflection varies over the light-beam cross section;

- wherein at least two light rays, which pass through the polarization- influencing element and have polarization states that differ from one another after passing through the polarization-influencing element, are coherently superposed in a pupil plane of the illumination device. According to one embodiment, at least two light rays, which pass through the polarization-influencing element and have polarization states that differ from one another after passing through the polarization-influencing element, are incoherently superposed in a pupil plane of the illumination device.

Such an incoherent superposition (in which, in contrast to the coherent superposition, the respectively superposed electric fields are firstly squared individually and are only then added in order to obtain the ultimately resulting intensity) can generate unpolarized light, ultimately as a result of incoherent superposition, in particular from partial rays on which different polarization states were respectively impressed initially as a result of the effect of the polarization- influencing element, which unpolarized light can be advantageous in conjunction with specific reticle structures or wanted illumination settings. Moreover, according to the invention, it is also possible to realize illumination settings by combining the aforementioned effects, i.e. by partly coherent and partly incoherent superposition, which illumination settings, in specific regions of the pupil plane, have intermediate states in respect of the degree of polarization (with 0 < DOP < 1 ) with a predetermined preferred polarization direction. Here, DOP denotes the degree of polarization.

According to one embodiment, the polarization-influencing element forms a raster- shaped arrangement of regions which have different influences on the polarization state of incident light. This raster-shaped arrangement can generate at least two, more particularly at least three, even more particularly four different polarization rotation angles for passing-through, linearly polarized light rays, particularly depending on the point where the light passes through. These polarization rotation angles can respectively be in particular an integer multiple of 22.5°, more particularly an integer multiple of 45° (with, for example, it being possible to generate the polarization rotation angles of 0°, 45°, 90° and 135°).

According to one embodiment, the diffractive or ray-deflecting structure is spatially separated from the polarization-influencing element. The concept of this spatial separation can be realized independently of the above- described approaches. According to a further aspect, the invention therefore also relates to an optical system in an illumination device of a microlithographic projection exposure apparatus, comprising - at least one polarization-influencing element, which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section; and

- at least one diffractive structure, which is arranged such that it and the polarization-influencing element directly follow one another in the light- propagation direction and, during the operation of the projection exposure apparatus, brings about a ray deflection of passing-through light, which ray deflection varies over the light-beam cross section;

- wherein the diffractive structure is spatially separated from the polarization- influencing element; and

- wherein the polarization-influencing element is produced from linearly or circularly birefringent material and has a thickness profile which varies over the light-beam cross section. According to one embodiment, the diffractive or ray-deflecting structure can also be formed directly on the polarization-influencing element.

In embodiments of the invention, the polarization-influencing element is produced from crystalline quartz. Here, the polarization-influencing element can in particular have an optical crystal axis which is parallel to the light-propagation direction in order to use the optical activity provided by crystalline quartz in this arrangement for generating different polarization states.

According to a further embodiment, the polarization-influencing element can also have an optical crystal axis which is oriented perpendicular to the light-propagation direction. According to one embodiment, the polarization-influencing element is made up of a first partial element and a second partial element, the second partial element at least partly compensating a ray deflection generated by the first partial element. The invention furthermore relates to a method for microlithographic production of microstructured components.

Further refinements of the description can be gathered from the description and the dependent claims. The invention will be explained in more detail below on the basis of exemplary embodiments illustrated in the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Figures 1 -10 show schematic illustrations for explaining different embodiments of the present invention; and Figure 1 1 shows a schematic illustration of the design of a microlithographic projection exposure apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following text, a principle design of a microlithographic projection exposure apparatus with an optical system according to the invention is first of all explained with reference to Fig. 1 1 . The projection exposure apparatus has an illumination device 10 and a projection lens 20. The illumination device 10 serves for illuminating a structure-bearing mask (reticle) 30 with light from a light-source unit

1 , which for example comprises an ArF excimer laser for a work wavelength of 193 nm and a ray-shaping optical unit for generating a parallel light beam. In general, the illumination device 10 and the projection lens 20 are preferably designed for a work wavelength of less than 400 nm, more particularly for less than 250 nm, even more particularly for less than 200 nm.

The illumination device 10 has an optical unit 1 1 , which, in the illustrated example, inter alia comprises a deflection mirror 12. Downstream of the optical unit 1 1 in the light-propagation direction and in the beam path, there is a light mixing device (not illustrated), which can have, for example, in a manner known per se, an arrangement of micro-optical elements that is suitable for achieving light mixing, and also a lens element group 14, downstream of which is situated a field plane with a reticle masking system (REMA), which is imaged by an REMA lens 15 disposed downstream in the light-propagation direction onto the structure-bearing mask (reticle) 30 arranged in a further field plane and thereby delimits the illuminated region on the reticle. The structure-bearing mask 30 is imaged by the projection lens 20 onto a substrate 40, or a wafer, provided with a light-sensitive layer. The projection lens 20 can be designed, in particular, for immersion operation. Furthermore, it can have a numerical aperture NA of greater than 0.85, in particular greater than 1.1 .

As is possible to identify from Fig. 1 1 , a polarization-influencing element 1 10 and a diffractive structure 130, which is arranged directly following the former in the light- propagation direction, are situated at the entry of the illumination device 1 1 , wherein the function of these will be explained in more detail below with reference to Fig. Iff. According to Fig. 1 , the polarization-influencing element 1 10 as a double wedge is composed of a first partial element 1 1 1 and a second partial element 1 12, with the partial elements 1 1 1 , 1 12 each having a geometry which is wedge-shaped or wedge-shaped in sections. In the exemplary embodiment, the first partial element 1 1 1 is produced from crystalline quartz, with the optical crystal axis in the crystal material being oriented parallel to the light-propagation direction (running in the z- direction in the plotted coordinate system). Moreover, the double-headed arrows plotted in Fig. 1 should symbolize that the light incident on the polarization- influencing element 1 10 or the first partial element 1 1 1 thereof is linearly polarized, with the polarization direction running in the y-direction (with respect to the plotted coordinate system, i.e. perpendicular to the plane of the paper). Accordingly, the representation of the further polarization distributions below should also be understood as respectively being viewed in the z-direction or in the x-y-plane.

The second partial element 1 12 of the double wedge forming the polarization- influencing element 1 10 is produced from non-birefringent (optically isotropic) material, e.g. quartz glass (S1O2), and serves to compensate for the ray deflection by the first partial element 1 1 1 .

As illustrated schematically in Fig. 1 , there is a resultant rotation of the preferred polarization direction for the polarization-influencing element 1 10, which rotation depends on the respective material path through the first partial element 1 1 1 , as a result of the circular birefringence brought about by the optically active material of the first partial element 1 1 1 , wherein the (linear) polarization states, which result from a rotation of the original preferred polarization direction pointing in the y- direction by 0°, 45°, 90° or 135°, are plotted between the polarization-influencing element 1 10, or the second partial element 1 12 thereof, and the diffractive structure 130 in an exemplary fashion in Fig. 1 .

The diffractive structure 130 now has a ray deflection designed for the wanted polarized illumination setting in the pupil plane of the illumination device, wherein this ray deflection is in each case selected in the exemplary embodiment such that, overall, this results in an annular or ring-shaped illumination setting with a tangential polarization distribution. To this end, as likewise indicated schematically in Fig. 1 , a "sub-pupil" in the form of a dipole setting with a quasi-tangential polarization distribution is generated in each case for light with a specific polarization direction generated by the polarization-influencing element 1 10 such that the "sub-pupils" 141 -145 generated in the process ultimately result in the annular, tangentially polarized illumination setting 150 in addition in the pupil plane. These sub-pupils have, for clarification purposes, been illustrated in a greatly exaggerated manner in Fig. 1 and in the further figures and should in actual fact be understood to be infinitesimally small such that this, e.g. in the example of Fig. 1 , results in a continuous polarization rotation over the shown annular illumination setting. In respect of the suitable design of the first partial element 1 1 1 , it is possible to base this on the specific rotation a of approximately 323.1 mm if use is made of synthetic optically active crystalline quartz according to the exemplary embodiment in the case of a wavelength of approximately 193 nm and a temperature of 21 .6°C.

As indicated by the symbol in Fig. 1 , it is typically also possible to provide a multiplicity of periods of the polarization rotation brought about by the polarization- influencing element 1 10 or of the beam deflection, matched thereto, brought about by the diffractive structure 130 with the goal of setting a light distribution in the pupil which is independent of the laser-ray profile, with only one period being illustrated in Fig. 1 purely for simplification purposes. In the following text, reference is made to the ratio of the extent of the coherence cells of the illumination light generated by the laser-light source 1 to the aforementioned polarization period generated by the polarization-influencing element 1 10 and to the size of the ray-deflecting structures present in the diffractive structure 130, which ratio is relevant to the polarized illumination setting ultimately generated by means of the arrangement according to the invention. Here, the extent of the coherence cells is typically greater (optionally substantially greater, e.g. by at least one order of magnitude) than the polarization period, with this polarization period in turn being (typically likewise substantially, e.g. by at least one order of magnitude) greater than the size of the ray-deflecting structures in the diffractive structure 130.

As explained below with reference to Fig. 2, it is now possible to use a coherent superposition of different polarization states, which were generated within the same coherence cell and emerge in the propagation direction downstream of the polarization-influencing element 1 10, for generating a quasi-tangentially polarized quadrupole illumination setting 250 instead of the tangentially polarized illumination setting 150 described above on the basis of Fig. 1 . To this end, the diffractive structure 230 as per Fig. 2 is embodied such that the polarization states, which are generated by the polarization-influencing element 210 or the first partial element 21 1 thereof and are rotated by ±20° with respect to the x-direction, are coherently superposed again into a state polarized in the x-direction (of course this applies to any angle ±a about the target state with polarization orientation γ, i.e. partial rays with the polarization rotation angles γ+α and γ-α are coherently superposed to a polarization rotation angle γ). As a result, the wanted polarized illumination setting 210 can thus be set without loss in the degree of polarization (DOP value, DOP = "Degree of polarization"). The invention is not restricted to exploiting circular birefringence or optical activity by polarization-influencing element 1 10 or 210. Rather, as illustrated schematically in Fig. 3a, it is also possible to use linear birefringence in further embodiments, for the purpose of which the first partial element 310 is produced from linearly birefringent material with an orientation of the optical crystal axis (which runs in the y-direction in the example of Fig. 3a) that is perpendicular to the light-propagation direction. What emerge in this case, as indicated schematically in Fig. 3a, are polarization states which, downstream of the polarization-influencing element 310 or the first partial element 31 1 thereof (here the illustration of the second partial element, typically provided for compensating the ray deflection, was dispensed with) in light-propagation direction, once again vary over the extent of the polarization-influencing element 310 or over the light-beam cross section, with elliptical or circular polarization states now being generated as a result of the linear birefringence. For comparison purposes, Fig. 3b shows the realization with optically active crystal material, for example in the form of crystalline quartz with an alignment of the optical crystal axis parallel to the light-propagation direction, in a polarization-influencing element 320 or in the first partial element 321 thereof.

This multiplicity of polarization states can now, as illustrated in Fig. 4, in turn be used in conjunction with the following diffractive structure (not illustrated in Fig. 3a and 3b) - and optionally also once again using the concept of coherent superposition - in order to generate a respectively wanted polarized illumination setting. Here, in Fig. 4, elements which are analogous or substantially functionally equivalent to Fig. 2 are denoted by corresponding reference signs, the number of which has been increased by 200.

Fig. 5 shows a further exemplary embodiment of the invention, wherein, in contrast to the above-described embodiments, the polarization-influencing element 510 does not have a constant linear profile of the thickness profile, but rather it has regions of respectively constant thickness, between which transition regions with linearly increasing or decreasing thickness are formed (said transition regions in turn respectively have a wedge section-shaped geometry within the meaning of the present application). The (wedge) angles present in the thickness profile should merely be understood in an exemplary fashion and can, for example, assume both values of the order of one or more degrees (°) and also substantially smaller values (e.g. smaller than 0.1 mrad). As indicated in the right-hand part of Fig. 5, the polarization distribution resulting in such transition regions is once again used in an advantageous and targeted fashion and without DOP loss, using coherent superposition in conjunction with a correspondingly adapted diffractive structure 530, in order to generate an appropriately polarized "sub-pupil" 543 or a polarized illumination setting ultimately resulting from the "sub-pupils" 541 , 542, 543, ... .

Of course, the polarization-influencing element 510 in the arrangement of Fig. 5 can, analogously to the preceding embodiments, also have a polarization-neutral or optically isotropic partial element, which serves to compensate for the ray deflection and has a thickness profile that is complementary to the partial element

51 1 .

As illustrated schematically in Fig. 6, the aforementioned coherent superposition of polarization states or partial rays generated by the polarization-influencing element according to the invention can, in further embodiments, also be combined with an incoherent superposition of other polarization states or partial rays generated by the polarization-influencing element, with the latter superposition typically leading to unpolarized light. As a result, as illustrated schematically in Fig. 6, this is how an illumination setting 650, for example, can be generated, which has an unpolarized region (corresponding to a "sub-pupil" 641 generated by incoherent superposition of the light rays originating from different coherence cells by means of the diffractive structure 630), found centrally in the example, and linearly polarized partial regions (corresponding to the "sub-pupil" 642 generated by coherent superposition of partial rays situated within the same coherence cell by means of the diffractive structure 630). The unpolarized region can naturally also be generated at a different, non-central position. Moreover, the polarized and unpolarized region can be arranged or combined in any way in the pupil plane. Partial rays with mutually orthogonal polarization states are preferably used for the depolarization, which partial rays have a comparatively large spatial distance from one another (preferably significantly larger than the size of the coherence cell) in order to ensure that incoherent superposition does in fact take place. Using the principle of the combination of coherent and incoherent superposition, it is also possible to generate illumination settings in further embodiments which have intermediate states of the polarization distribution to the extent that one or more regions or poles of the intensity distribution in the pupil plane of the illumination device have a degree of polarization between zero and one (e.g. DOP= 0.5) with a specific preferred polarization direction (e.g. 22.7° with respect to the x-axis).

In further embodiments of the invention, the principle of coherent superposition of different polarization states can also be realized by virtue of the fact that a suitable polarization-influencing element with a design explained on the basis of Fig. 7-10 is arranged in the pupil plane of the illumination device. The corresponding position is illustrated in Fig. 1 1 for a polarization-influencing element 710, the design of which will be described below with reference to Fig. 7ff. According to Fig. 7a-b, the polarization-influencing element 710 has a raster- shaped arrangement of a multiplicity of regions with a different influence on the polarization state of incident light, with four such regions 701 , 702, 703 and 704 respectively being combined to form a cell 700 in the exemplary embodiment and with this cell 700 repeating periodically in x- and y-directions within the polarization-influencing element 710. The regions 701 -704 are once again produced from optically active crystalline quartz and, in terms of their thickness, designed such that the rotation of the polarization direction obtained for light incident with a polarization direction pointing in the y-direction is 0° in the region 701 , 45° in the region 702, 90° in the region 703 and 135° in the region 704.

The illumination of the polarization-influencing element 710 containing the raster- shaped arrangement can be realized in an analog fashion to the embodiments described above via a diffractive structure typically located at the entry of the illumination device or, alternatively, also via a mirror arrangement with a multiplicity of independently adjustable mirror elements (also abbreviated to MMA, MMA = "micro mirror array"). It goes without saying that the illustration in Fig. 7, like the one in the further figures as well, is merely schematic and exemplary, with provision typically being able to be made for a significantly greater number of cells 700 within the polarization-influencing element 710 for improving the resolution. As indicated in Fig. 8 for an appropriately designed polarization-influencing element 810, the extent of the utilized light spot is preferably less than a quarter of the area of the individual cells 700 such that the above-described polarization states (corresponding to a rotation of the polarization direction by 0°, 45°, 90° and 135°) can respectively substantially be set in a pure form.

Moreover, as illustrated schematically in Fig. 9a, it is possible to set any intermediate states by coherent superposition of different polarization states set by means of the polarization-influencing element 710. Moreover, likewise analogous to the concept described above, it is also possible to generate unpolarized light by means of incoherent superposition of partial rays originating from different coherence cells of the laser-light source 1 , wherein these partial rays, as described above, should be situated relatively far apart (in respect of the coherence cell size) and have mutually orthogonal polarization states. In the specific example of Fig. 9a, the coherent superposition of the polarization states with a polarization direction which is rotated by an angle of 0° or 45° to the x- direction leads to an intermediate state with a polarization direction at 22.5° with respect to the x-direction, whereas, in the case of incoherently superposed light spots, this results in unpolarized or (depending on the polarization direction of the superposed polarization states) only partly polarized light in the case of superposition if, like in the example, the regions of the polarization-influencing element 710 corresponding to the different polarization rotations are illuminated in approximately equal parts.

In the embodiment of Fig. 9a, all of the four polarization states that can be generated are superposed in order to obtain a complete depolarization, i.e. a degree of polarization (DoP) of 0%. Fig. 9b schematically illustrates a further exemplary embodiment, in which a complete depolarization can also be brought about by illuminating only two adjacent cells. By way of example, in the schematic illustration of Fig. 9b, a light spot "A" always has a polarization direction at 22.5° with respect to the y-direction, with this being with partial polarization in the case of incoherent superposition and with complete polarization in the case of coherent superposition. The light spot "B" has a complete depolarization in the case of incoherent superposition and a complete polarization with a polarization direction at 45° with respect to the y-direction in the case of coherent superposition.

According to Fig. 10, any intermediate state in respect of the polarization direction (between 0° and 180°) can also be obtained by varying the position of a light spot within a cell 700 of the raster-shaped arrangement of the polarization-influencing element 710. Since, in principle, it is possible to generate a light spot with complete coherence over the light-beam cross section, it is possible to set such intermediate states or corresponding wanted illumination settings without DOP loss, i.e. without losses in the degree of polarization.

In the examples, the assumption is made in each case that the size of the cell 710 is smaller than the size of the coherence cells of the laser-light source 1 and that, furthermore, the size of the cells 710 is greater than the extent of the light spot in the pupil plane, with, for example, the edge length of the cell 710 being able to correspond substantially to double the light-spot diameter.

In embodiments of the invention, the above-described polarization-influencing element, which contains the raster-shaped arrangement or is parceled out, can respectively be displaceable in the x-y plane (preferably in x- and y-directions). This enables an adjustment which, particularly if used in conjunction with a diffractive optical element (DOE), may be expedient or necessary for ensuring that the respective light spots are also incident on the wanted cell or parcel on the polarization-influencing element. Moreover, this also allows a readjustment of the illumination distribution set by the DOE. If the polarization-influencing element, which contains the raster-shaped arrangement or is parceled out, is used in conjunction with a mirror arrangement (MMA), each individual light-spot position can, in principle, also be readjusted by means of the mirror elements of the MMA such that the polarization-influencing element is hit at the wanted positions.

Even though the invention was described on the basis of specific embodiments, a number of variations and alternative embodiments, e.g. by combining and/or interchanging features from individual embodiments, are accessible to a person skilled in the art. Accordingly, a person skilled in the art understands that such variations and alternative embodiments are also comprised by the present invention, and the scope of the invention is only restricted within the meaning of the attached patent claims and the equivalents thereof.

Claims

Optical system in an illumination device of a microlithographic projection exposure apparatus, comprising

• at least one polarization-influencing element (1 10, 210, 310, 410, 510, 610), which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section; and

• at least one diffractive structure (130, 230, 430, 530, 630), which is arranged such that it and the polarization-influencing element (1 10, 210, 310, 410, 510, 610) directly follow one another in the light- propagation direction and, during the operation of the projection exposure apparatus, brings about a ray deflection of passing-through light, which ray deflection varies over the light-beam cross section;

• wherein the polarization-influencing element (1 10, 210, 310, 410, 510, 610) is produced from linearly or circularly birefringent material and has a geometry which, at least in regions, is wedge-shaped.

Optical system according to Claim 1 , characterized in that at least two light rays, which pass through the polarization-influencing element (1 10, 210, 310, 410, 510, 610) and have polarization states that differ from one another after passing through the polarization-influencing element (1 10, 210, 310, 410, 510, 610), are coherently superposed in a pupil plane of the illumination device.

• at least one polarization-influencing element (1 10, 210, 310, 410, 510, 610, 710), which, during the operation of the projection exposure apparatus, brings about a change in the polarization state of passing-through light, which change varies over the light-beam cross section; and • at least one ray-deflecting structure (130, 230, 430, 530, 630), which, during the operation of the projection exposure apparatus, brings about a ray deflection of passing-through light, which ray deflection varies over the light-beam cross section;

• wherein at least two light rays, which pass through the polarization- influencing element (1 10, 210, 310, 410, 510, 610, 710) and have polarization states that differ from one another after passing through the polarization-influencing element (1 10, 210, 310, 410, 510, 610, 710), are coherently superposed in a pupil plane of the illumination device.

Optical system according to one of Claims 1 to 3, characterized in that, furthermore, at least two light rays, which pass through the polarization- influencing element (1 10, 210, 310, 410, 510, 610, 710) and have polarization states that differ from one another after passing through the polarization-influencing element (1 10, 210, 310, 410, 510, 610, 710), are incoherently superposed in a pupil plane of the illumination device.

Optical system according to Claim 3 or 4, characterized in that the polarization-influencing element (710) forms a raster-shaped arrangement of regions which have different influences on the polarization state of incident light.

Optical system according to Claim 5, characterized in that this raster- shaped arrangement (710) can generate at least two, more particularly at least three, even more particularly four different polarization rotation angles for passing-through, linearly polarized light rays, depending on the point where the light passes through.

Optical system according to Claim 6, characterized in that these polarization rotation angles respectively are an integer multiple of 22.5°, more particularly an integer multiple of 45°.

Optical system according to one of the preceding claims, characterized in that the diffractive or ray-deflecting structure (130, 230, 430, 530, 630) is spatially separated from the polarization-influencing element (1 10, 210, 310, 410, 510, 610, 700, 810, 910, 950).

• wherein the diffractive structure (130, 230, 430, 530, 630) is spatially separated from the polarization-influencing element (1 10, 210, 310, 410, 510, 610); and

• wherein the polarization-influencing element (1 10, 210, 310, 410, 510, 610) is produced from linearly or circularly birefringent material and has a thickness profile which varies over the light-beam cross section.

10. Optical system according to one of Claims 1 to 7, characterized in that the diffractive or ray-deflecting structure is formed directly on the polarization- influencing element.

1 1 . Optical system according to one of the preceding claims, characterized in that the polarization-influencing element (1 10, 210, 310, 410, 510, 610, 700, 810, 910, 950) is produced from crystalline quartz.

12. Optical system according to one of the preceding claims, characterized in that the polarization-influencing element (1 10, 210, 410, 510, 610, 700, 810, 910, 950) has an optical crystal axis which is oriented parallel to the light- propagation direction.

13. Optical system according to one of Claims 1 to 1 1 , characterized in that the polarization-influencing element (310) has an optical crystal axis which is oriented perpendicular to the light-propagation direction.

14. Optical system according to one of the preceding claims, characterized in that the polarization-influencing element (1 10, 210, 410, 510, 610) is made up of a first partial element (1 1 1 , 21 1 , 41 1 , 51 1 , 61 1 ) and a second partial element (1 12, 212, 412, 612), the second partial element at least partly compensating a ray deflection generated by the first partial element.

15. Method for microlithographic production of microstructured components, comprising the following steps:

• providing a substrate (40) on which a layer of a light-sensitive material has been applied at least in regions;

• providing a mask (30) which has structures to be imaged;

• providing a microlithographic projection exposure apparatus which has an optical system according to one of Claims 1 to 14; and

• projecting at least part of the mask (30) onto a region of the layer with the aid of the projection exposure apparatus.