WO2022179766A1

WO2022179766A1 - Optical system, in particular for euv lithography

Info

Publication number: WO2022179766A1
Application number: PCT/EP2022/050880
Authority: WO
Inventors: Thomas Schicketanz; Toralf Gruner; Markus Schwab; Joachim Hartjes
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2021-02-23
Filing date: 2022-01-17
Publication date: 2022-09-01
Also published as: TW202248761A; CN116868130A; DE102021201690A1; KR20230150277A

Abstract

The invention relates to an optical system, in particular for EUV lithography, comprising: an illumination source for illuminating an illumination surface with illumination radiation, a detector (35) having a detection surface (36) for the spatially resolved detection of the illumination radiation, a projection system configured to image the illumination surface onto the detection surface (36), and also an evaluation device (37) configured to deduce contaminants on optical elements (M6) in a beam path (29) between the illumination surface and the detection surface (36) on the basis of an intensity (I) of the illumination radiation at the detection surface (36). The optical system is configured to set different angular distributions of the illumination radiation passing from the illumination surface to the detection surface (36), and the evaluation device (37) is configured to deduce the contaminants on the optical elements (M6), in particular the position of the contaminants on the optical elements (M6), on the basis of the intensity (I) of the illumination radiation on the detection surface (36) for the different angular distributions.

Description

Optical system, in particular for EUV lithography Cross-Reference to Related Application

This application claims priority to German Patent Application 102021201690.1 filed February 23, 2021 , the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

Background of the invention

The invention relates to an optical system, in particular for EUV lithography, comprising: an illumination source for illuminating an illumination surface with illumination radiation, a detector having a detection surface for the spatially resolved detection of the illumination radiation, a projection system configured to image the illumination surface onto the detection surface, and also an evaluation device configured to detect contaminants on optical elements in a beam path between the illumination surface and the detection surface on the basis of an intensity of the illumination radiation at the detection surface.

The optical system can be for example a lithography system in the form of a projection exposure apparatus for the exposure of a semiconductor substrate (wafer). Such a projection exposure apparatus comprises a projection system in order to image structures on a mask (reticle) onto the semiconductor substrate. In order to achieve a high resolution especially of lithography optical units on such a lithography system, for a few years EUV light having a wavelength of 13.5 nm has been used, in comparison with predecessor systems having typical operating wavelengths of 365 nm, 248 nm or 193 nm. The step to the EUV range meant dispensing with refractive media, which are no longer able to be used meaningfully at this wavelength, and transitioning to pure mirror systems operating with either virtually normal incidence or grazing incidence. In DUV systems, a gas flow through the system is often implemented, e.g. by nitrogen or air being blown in and discharged. Said gas flow effects cooling of the optical elements, heated by the absorption of light, and the discharge of harmful substances that could otherwise accumulate in the optical system as a result of factory influences or outgassing. At EUV wavelengths, vacuum is employed in principle, although a low gas pressure, preferably of hydrogen, prevails, once again in order to contribute to photocleaning, and sometimes to realize a limited cooling effect.

In the vicinity of the semiconductor substrate, gas is introduced such that firstly a first partial flow in the direction of the semiconductor substrate reduces the consequences of the outgassing of the resist or other sources of contamination because this material migrates in the direction of the projection system only with greater difficulty in this way. At the same time, a second gas flow in the direction of the projection system arises, which serves for photocleaning and cooling and additionally makes contamination of the semiconductor substrate from the direction of the projection system unlikely.

It has been proposed to arrange a thin membrane between projection system and semiconductor substrate in order to filter out exposing and thermally disturbing wavelengths especially in the visible range or near the visible range.

In order that an acceptable transmission for the EUV used light remains, said membrane must be decidedly thin and typically has a thickness of the order of magnitude of 100 nm. As a result, however, the thin membrane simultaneously becomes sensitive to pressure differences or to thermal loads, which may be locally high, particularly at particles that have deposited on the membrane. The membrane may tear and detachment of constituents of said membrane, also referred to hereinafter as shreds, cannot be ruled out.

Whereas compact particles do not progress far in a weak gas flow because in customary geometries they fight against gravity and lose, the situation for thin membrane portions is different. Here it is possible for the shreds of the membrane to have little mass (since they are thin) with a large surface area and to sail up in the gas flow.

The formation of contaminating material can occur not only when a membrane ruptures, but also in other cases of disturbance, the spreading of said material in the optical system being undesired. Specifically, if the contaminating material, e.g. in the form of particles, reaches optical elements, then at the latter it generally influences the reflected, refracted or diffracted light in a disturbing manner. In the case of an optical element in the form of a mirror, the contaminating material often reduces the reflectivity and/or changes the phase angle of the impinging and reflected light. This takes place locally, which makes correction difficult without cleaning. Cleaning in turn can be complex, e.g. if the component is operated in a vacuum and has to be aligned highly accurately, and it also involves risks of damage for instance of layer(s) or of a coating on such a component.

As a result of the local reduction of the quantity of light or change in the phase angle, the interference of these contributions in the image is influenced, which often causes an undesired change in the size of the structures of the mask that are imaged by a projection system. That can result in short circuits, interruptions or deviations in the electrical properties of a semiconductor component fabricated with the aid of the lithography system, which restrict the functionality of the semiconductor component.

It is therefore advantageous to carry out supervision or monitoring of cleanness- critical regions of optical elements with regard to contaminants, e.g. in the form of contaminating material or particles, even for the case where the optical elements are installed in a housing of an optical system and the optical system is in operation at the end customer's premises. Such supervision ensures that no contaminants have deposited on the optically used surfaces of the optical elements during the installation of the optical elements in the optical system.

The accessibility of optical elements in the installed state is hampered, however, e.g. by housings etc. of the optical system, such that it is not straightforwardly possible to detect contaminants.

One approach for detecting contaminants on optical elements, to put it more precisely on optically used surfaces of optical elements, provides for using a self-luminous illumination source, e.g. an LED, in order to areally illuminate an illumination surface. Contaminants on the optical elements have an effect specifically according to the location where they arise in that a location- dependent difference in uniformity arises during the imaging of the illumination surface onto the detection surface. For a successful measurement, a calibration is useful in order to computationally eliminate illumination variations of the illumination source and also design-related intensity variations, e.g. as a result of layers on the optical elements which have nothing to do with contaminants.

EP 2064597 B1 has disclosed an illumination system for a projection exposure apparatus, said illumination system comprising a light source for illuminating an exit pupil plane and also a faceted element for altering the illumination of the exit pupil plane. The faceted element makes it possible to change at least one first illumination in the exit pupil plane into a second illumination in the exit pupil plane. The illumination system can comprise a storage unit that stores a multiplicity of calibration values for different illuminations of the exit pupil plane in a calibration table. The illumination system comprises a detector for making available a control signal depending on what illumination was set in the exit pupil plane, said control signal enabling a scan speed of the projection exposure apparatus to be set. The detector can be arranged at or in the vicinity of an object plane or the image plane of the projection exposure apparatus.

Object of the invention It is an object of the invention to provide an optical system which improves the detection of contaminants, in particular the position of contaminants, on optical elements in the installed state of the optical elements.

Subject matter of the invention

This object is achieved by means of an optical system which is configured to set different angular distributions of the illumination radiation passing from the illumination surface to the detection surface, and in which the evaluation device is configured to deduce the contaminants on the optical elements, in particular the position of the contaminants on the optical elements, on the basis of the intensity of the illumination radiation on the detection surface for the different angular distributions.

In the optical system according to the invention, the angular distribution with which the illumination radiation passes from the illumination surface to the detection surface is varied. Each emission point on the illumination surface with a given direction or with a given emission angle corresponds to an illumination beam through the optical system with individual surface passage points on the optical elements. If such an illumination beam impinges on a contaminant, it is absorbed and/or deflected and impinges with reduced intensity on the original image point. If the angular distribution or the illumination directions is or are thus varied, this alters the illumination beams which pass between a specific object point of the illumination surface and an - on account of the imaging - associated image point through the optical system or through the projection system. The locations at the optical elements which are interrogated by said illumination beams accordingly change as well.

In the case of registering contaminants with constant angular distribution or illumination, only the comparison between different field points can reveal intensity differences. A constant or completely calibrated areal brightness of the illumination source which illuminates the illumination surface is therefore required. The variation of the angular distribution makes it possible to relax or optionally completely dispense with this requirement in the case of a measurement with a plurality of illuminations or angular distributions.

Furthermore, the fact that the loss of intensity occurs for one or a plurality of known illuminations and corresponding emission angle distributions yields an indication of the cause of the fault or of the position/location at which the contamination occurs in the beam path between the illumination surface and the detection surface, as illustrated by the following example:

We will assume for simplification that the illumination surface emits only in a single illumination direction (coherent limiting case) and that the illumination of the illumination surface is varied between e.g. ten different illuminations corresponding to ten different illumination directions at the illumination surface.

We will subsequently concentrate on the light intensity at an individual fixed image point on the detection surface. In the example described here, the illumination beams do not see any contaminant in the beam path between the illumination surface and the detection surface in respect of the illuminations 1 to 9. In this case, the intensity of the illumination radiation is determined exclusively by the brightness or the intensity of the source location on the illumination surface (a one-to-one assignment between points on the illumination surface and points on the detection surface prevails, such that a given point of the detection surface is illuminated only by a single point of the illumination surface) and also by design-related variations of the system transmission because (reflective) layers on the optical elements are illuminated at different angles and therefore have a varying reflectivity or transmissivity.

This variation in the transmission of the optical system will turn out to be very small or is known at least on the basis of the optical design and also the layer design of the (generally reflective) layers on the optical elements and can accordingly be eliminated by means of a correction calculation.

For the illumination source we demand a sufficient temporal constancy of the brightness at points, not necessarily over the whole area. For this purpose, the illumination source can comprise for example one or a plurality of light-emitting diodes, optionally supplemented by scattering elements. In this case, identical illumination intensities at the image point on the detection surface should be expected (if appropriate after the correction mentioned above) for the illuminations 1-9. In this example, illumination 10 generates an illumination beam that impinges on a contaminant. As a result, the illumination beam in the case of illumination 10 loses brightness, which leads to a reduced detected intensity on the detection surface. In the example described here, the fault or the contaminant must occur in those partial regions on the optically used surfaces of the optical elements in the beam path which are seen by the illumination beam of the illumination 10, which delimits the region on each of the optical elements from a complete subaperture to virtually a point. In this case, the subaperture is given by that partial region of an optically used surface of an optical element which is illuminated by the relevant field point of the illumination surface. Particularly in the case of near-pupil optical elements, the subaperture comprises the majority of the optically used surface.

If the evaluation device combines the information about the decrease in intensity depending on the field location on the illumination surface with the known properties of the respectively set angular distribution of the illumination radiation, the contaminant can often be directly assigned to a small partial region of an optically used surface of an individual optical element in the beam path, i.e. the position of the contaminant on the optical element can be deduced. Other methods use for this purpose previously calculated sensitivity tables, tomographic methods or else they take account of the so-called migration speed of the decrease in brightness as a function of direction and field.

Deducing the position of the contaminants on the basis of the information about the decrease in intensity depending on the field location and the illumination is illustrated on the basis of the following example: If it is assumed that a given contaminant (fault) for a field point located on the left of the field centre on the detection surface is impinged on by an illumination beam from a direction on the right of the pupil centre, then said fault or said contaminant lies directly in the field centre in the pupil centre and migrates at a field position on the right thereof into the left pupil half. If e.g. a pole in the left pupil half forms a first illumination, a circular zone in the pupil centre forms a second illumination and a pole in the right pupil half forms a third illumination, then at the field point located on the left the illumination 3 will yield a reduced intensity compared with the illumination 1 and 2. In the field centre, by contrast, the illumination 2 decreases compared to the illuminations 1 and 3, while at the field point located on the right the illuminations 2 and 3 yield a higher intensity than the illumination 1.

If it is assumed in the example described further above that the optical system has a rotational symmetry or at least a mirror symmetry on the axis that divides "right" and "left", then the fault described above or the contaminant lies in the centre of an optical element lying between a field plane, e.g. the object plane, and a first pupil plane or else between a further field plane, e.g. an intermediate image plane, and the nearest pupil plane. If, on said optical element, a contaminant were located in the left partial region of the optical element, then it would only be possible to reduce the intensity of illumination 2 for the left field point and illumination 1 for the field centre and there would be no illumination- dependent variation of the right field point because the subaperture thereof does not overlap with this fault or this contaminant. It is often possible to achieve a finer subdivision when deducing the position of the contaminant by measuring further illuminations or illumination settings. In this case, a variable in the form of the ratio of pupil change to field change can be determined, for example, which is dependent on the position of the causative optical element (near-field, intermediate or near-pupil) and is characteristic of this optical element or of a small group of optical elements under consideration. At the same time it is possible to delimit on the respective optical element where the cause or where the contaminant lies.

There are various possibilities for setting different angular distributions or different illuminations of the areal illumination.

In one embodiment, the optical system comprises an illumination system configured to vary the angular distribution of the illumination radiation at the illumination surface. In this embodiment, the illumination system makes it possible to set different illumination settings that produce a varying angular distribution on the illumination surface. In a pupil plane complementary to the illumination surface, it is possible to realize for example a dipole illumination with two or more poles outside the pupil centre, an annular field illumination that involves illuminating an annular region in the pupil plane, a uniform illumination, etc. Illumination systems which make it possible to set different illumination settings or angular distributions are known from the prior art and are often used in optical systems in the form of microlithographic projection exposure apparatuses. Illumination systems are known, in particular, which provide a variable illumination with virtually any desired flexibility and enable a fine measurement of the optical system in this way. In order to set illumination settings with a varying degree of coherence, it is possible to use stops, for example, as is described in EP 2064597B1. In order to set different illumination settings, it is also possible to alter the assignment of the light channels in a doubly faceted illumination system, for example as is described in US2002136351 A1. The case may occur, however, that in the event of a measurement after the mounting of the optical elements in the projection system, an illumination system has not yet been integrated into the optical system, or that the measurement or the deduction of the contaminants is not intended to take place at the operating wavelength, such that in the worst case scenario a dedicated illumination system would have to be provided specifically for this measurement. This outlay might possibly reduce the attractiveness of the solution described here.

In a further embodiment, the optical system comprises at least one transmissive optical element having a transmission dependent on the angle of incidence, where the transmissive optical element is positionable either in the beam path between the illumination surface and the detection surface or outside the beam path between the illumination surface and the detection surface.

As an alternative or in addition to setting the angular distribution with the aid of the illumination system, it is possible to set the angular distribution with the aid of one or a plurality of transmissive optical elements, the transmission of which is dependent on the angle of incidence of the illumination radiation on the transmissive optical element. With the aid of such a transmissive optical element, two different angular distributions can be set, depending on whether or not the transmissive optical element is arranged in the beam path. In order to increase the number of settable angular distributions, the optical system preferably comprises two or more transmissive optical elements. It is possible for said transmissive optical elements to be mutually interchangeable, such that optionally a single one of the transmissive optical elements is arranged in the beam path in order to increase the number of settable angular distributions.

In one development of this embodiment, at least two transmissive optical elements are jointly positionable either in the beam path between the illumination surface and the detection surface or outside the beam path between the illumination surface and the detection surface. In addition or as an alternative to the possibility of interchanging the individual transmissive optical elements, in this case two or more transmissive optical elements can be arranged jointly - at different points - in the beam path.

In a further development of this embodiment, a distance between the at least two transmissive optical elements is settable. For the setting of different angular distributions, in this case the distance between the transmissive optical elements is set or altered. This is advantageous if the procedure of interchanging or introducing and taking away the transmissive optical elements during the measurement involves an excessively high outlay or entails a great risk of contamination on account of movable parts. In this embodiment, the transmissive optical elements - typically before the measurement - are introduced jointly into the beam path. The distance between the transmissive optical elements is varied during the measurement and the transmissive optical elements are removed again from the beam path after the measurement. For introducing and taking away the transmissive optical element(s), the optical system can comprise a transport device that can be moved with the aid of a drive.

In a further development, the at least one transmissive optical element is plate shaped and preferably has a thickness of less than 10 mm, in particular of less than 1 mm. The plate-shaped optical element can be a thin, generally coated plate or a membrane. The thickness of the plate-shaped optical element should be chosen so as not to be too large, in order to prevent the beam path between the illumination surface and the detection surface from changing appreciably when the transmissive optical element is introduced.

In a further embodiment, the plate-shaped transmissive optical element has a coating having a wavelength-dependent transmission on at least one side. The coating can be formed from silver, for example. The coating, i.e. one or a plurality of layers, forms a wavelength filter, typically in the form of an interference filter. The effect of said filter corresponds to angular filtering in the case of approximately monochromatic light. The reason for this is the phase condition in the case of thin layers. For constructive interference in reflection, i.e. a low transmission, the following holds true for example approximately for the layer thickness D:

wherein l is the wavelength, n is the refractive index and a is the angle of incidence.

The above condition is thus always met if is constant. Therefore, there is

a relationship between wavelength filtering with a fixed angle and angular filtering with a fixed wavelength.

By way of example, a coating composed of silver or some other suitable material can serve as wavelength filter or as interference filter. However, the coating can also comprise a plurality of layers composed of different materials which act as wavelength filter.

In one development, the plate-shaped optical element positioned in the beam path is mounted such that it is deformable, tiltable and/or rotatable about a rotation axis. As a result of the deformation or tilting of the plate-shaped optical element, the angle of incidence of the illumination radiation on the plate-shaped optical element is altered (optionally locally), thus resulting in a change in the transmission that is dependent on the angle of incidence and thus in the angular spectrum of the illumination radiation.

In a further embodiment, the at least one transmissive optical element is positionable in a region in the beam path at which an angle of incidence spectrum on the transmissive optical element exceeds at least 15°, preferably at least 20°, particularly preferably at least 25°. The effect of a wavelength or interference filter for angular filtering is effective particularly in the case of high angles of incidence or in the case of a comparatively large angle of incidence spectrum, such that the transmissive optical element is preferably positioned in a region with a large angle of incidence spectrum in the beam path. A large angle of incidence spectrum is advantageous in order, during the setting of the different angular distributions, to produce a maximum variation of the intensity of the illumination radiation impinging on the detection surface. In the case of an optical system in the form of a lithography system, such a region in the beam path typically exists between the last optical element of the projection system and the image plane, in which the wafer is arranged during exposure operation. The arrangement of the transmissive optical element(s) in this region has therefore proved to be advantageous.

In a further embodiment, the optical system is configured, in the setting of the different angular distributions, for at least one angle, to vary the intensity of the illumination radiation impinging on the detection surface by at least 5%, preferably by at least 20%, particularly preferably by at least 50%. The variation of the intensity is understood to mean the difference between the maximum intensity and the minimum intensity relative to the maximum intensity. The variation of the intensity of the illumination radiation in the range of values specified above is possible for at least one position on the detection surface; in particular, however, such a variation of the intensity can also be produced at all positions on the detection surface. The angular dependence produced with the coating applies to the entire detection surface provided that the coating is homogeneous, i.e. if the coating has a location-independent constant thickness and a constant refractive index.

The examples described further above represent possibilities for varying or altering the illumination such that a characteristic signal in the measurement set-up or during the measurement results depending on the position and constitution of a contaminant.

In a further embodiment, the illumination surface is formed in a first field plane of the projection system, and the detection surface is formed in a second field plane of the projection system, wherein the first field plane preferably forms an object plane of the projection system and wherein the second field plane preferably forms an image plane of the projection system, or vice versa. The illumination surface is preferably formed either in the object plane or in the image plane of the projection system. Correspondingly, the detection surface is formed either in the image plane of the projection system or in the object plane of the projection system if the illumination with the illumination radiation is effected from the image plane. In principle, however, it is also possible to form the illumination surface and/or the detection surface in other field planes, e.g. in intermediate image planes.

In one development, the illumination surface covers the beam path of the projection system in the first field plane, and the detection surface covers the beam path of the projection system in the second field plane. It is advantageous if the illumination surface and/or the detection surface cover(s) the entire beam path or the entire optically used region. For the case where the illumination surface forms the object plane of the projection system, the illumination surface covers the entire object field in this case. For the case where the detection surface forms the image plane of the projection system, the detection surface covers the entire image field of the projection system in this case. In principle, however, it is also possible for the illumination surface and/or the detection surface to cover only a partial region of the beam path in the respective first and/or second field plane.

In addition or as an alternative to the procedure described further above, optical elements in an optical system which are accommodated in a housing or protected by mechanical casings can also be checked for contaminants or for cleanness faults with the aid of a camera or a detector, which can be designed in endoscopic fashion. For this purpose, either a dedicated illumination can be directed at said optical element or the optical element can be illuminated by light in the used beam path (at the operating wavelength or deviating therefrom) and the camera or the detector can be positioned outside the used beam path, such that the camera or the detector is illuminated only by scattered light produced at contaminants.

At least one of the optical elements which are examined for contaminants with the aid of the camera or the detector preferably belongs to the three most critical optical elements in the rank order of the sensitivity of the image uniformity to contaminants of a given size. A gas flow can be directed onto or over the optical element being examined with regard to contaminants, said gas flow being directed for example counter to the used light direction and from the direction of a thin membrane in the used beam path.

For the case where an optical system for EUV lithography is involved, the stated measures can in particular also be implemented in regions which lie between an object plane or an image plane of the system and an optical element having a maximum of 50%, preferably a maximum of 20%, of the optically used area of the largest optically used element of the relevant optical system.

For the case where the optical system is an EUV lithography apparatus, the used beam path is typically almost completely surrounded by an enclosure in order to maintain a vacuum in the region of the optical elements. In this case, the endoscope camera can be introduced into the used beam path through one of the small number of openings present in the enclosure in such a way that no appreciable disturbance of the vacuum results. Alternatively, it is possible to use maintenance pauses for checking the optical element(s) for contaminants. This makes use of the fact that openings in the optical system can be provided permanently or temporarily, e.g. during maintenance pauses, through which openings an endoscopic optical unit can be guided, which records an image of the optically used area of interest or of the optical element of interest. In this case, through the same opening as the camera or through an additional opening, an illumination source, e.g. an LED, can be directed at said optically used area. At the same time, however, there is also the possibility of causing a light source to be luminous directly from the object plane (reticle) or from the image plane (substrate) of a projection system of the EUV lithography apparatus. It is also possible to position the detector or the camera outside the design or used beam path, such that it is correspondingly blind to the specular reflection at the optically used area. An optically used area that is perfect in terms of design will project no light in the direction of the detector, but rather guide the light along the used beam path provided. By contrast, particles or contaminants on the optically used area often have an irregular surface or at any rate a surface embodied such that unforeseen scattering angles are illuminated. Only the latter are seen by the contaminant detection optical unit, such that from the point of view thereof contaminants light up properly.

If the operating wavelength of the optical system is in the EUV wavelength range because the entirely normal used light is used in the measurement mode, the detector or the camera can be configured to convert the EUV light into a different wavelength e.g. by means of luminescence or a photodiode can be used, thus resulting in an electron flow during the EUV irradiation.

In known methods for in-situ observation of optically used areas of optical elements, a shape change detection is typically performed, such that the preferably endoscopic observation described here, without an interferometric set-up or stripe projection, would be unsuitable. The solution proposed here differs from a large number of known methods in that even small disturbances are measurable thanks to the high contrast (scattered light vis a vis darkness), but a surface shape is not detected with high accuracy. The latter is the source of the strength of many known methods, which however have only a limited resolution and cannot reliably resolve e.g. structures having a size of less than 10 mm, preferably 1 mm, more preferably less than 0.1 mm.

With the aid of the procedure described here, by contrast, it is possible to measure cleanness faults or contaminants having a largest extent of at most 10 mm, preferably of at most 1 mm, more preferably of at most 0.1 mm, on an optical element or on an optically used area of the optical system.

Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing showing details essential to the invention, and from the claims. The individual features can each be realized individually by themselves or as a plurality in any desired combination in one variant of the invention.

Drawing

Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

Fig. 1 schematically shows a meridional section of a projection exposure apparatus for EUV lithography,

Figs 2a-c show schematic illustrations of a membrane in the region of an image plane of such an optical system and of shreds of the membrane that are entrained in a gas flow,

Fig. 3 shows a schematic illustration of an endoscope camera that is directed at a reflective optical element via an opening in an enclosure, and Figs 4a, b show schematic illustrations of respectively one and two plate shaped optical elements arranged in a beam path of a projection system in order to set different angular distributions of an illumination radiation that passes from an object field to an image field of the projection system, and

Figs 5a, b show schematic illustrations of the transmission of the two plate shaped optical elements from Fig. 4b as a function of the angle of incidence for three different distances.

In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

The essential components of an optical system for EUV lithography in the form of a microlithographic projection exposure apparatus 1 are described by way of example below with reference to Fig. 1. The description of the basic set-up of the projection exposure apparatus 1 and the components thereof should not be understood as restrictive in this case.

An illumination system 2 of the projection exposure apparatus 1 has, besides an illumination source 3, an illumination optical unit 4 for the illumination of an object field, which forms an illumination surface 5, in an object plane 6. In this case, a reticle 7 arranged in the object field is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9, in particular in a scanning direction.

For purposes of explanation, a Cartesian xyz-coordinate system is depicted in Fig. 1. The x-direction runs perpendicularly to the plane of the drawing into the latter. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction runs along the y-direction in Fig. 1. The z-direction runs perpendicularly to the object plane 6. The projection exposure apparatus 1 comprises a projection system 10. The projection system 10 serves for imaging the object field or the illumination surface 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder

14. The wafer holder 14 is displaceable by way of a wafer displacement drive

15, in particular along the y-direction. The displacement on the one hand of the reticle 7 by way of the reticle displacement drive 9 and on the other hand of the wafer 13 by way of the wafer displacement drive 15 can take place in such a way as to be synchronized with one another.

The illumination source 3 is an EUV radiation source. The illumination source 3 emits, in particular, EUV radiation 16, which is also referred to below as used radiation or illumination radiation. In particular, the used radiation has a wavelength in the range of between 5 nm and 30 nm. The illumination source 3 can be a plasma source, for example an LPP (“laser produced plasma”) source or a GDPP (“gas discharged produced plasma”) source. It can also be a synchrotron-based illumination source. The illumination source 3 can be a free electron laser (FEL).

The illumination radiation 16 emanating from the illumination source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be impinged on by the illumination radiation 16 with grazing incidence (Gl), i.e. at angles of incidence of greater than 45°, or with normal incidence (Nl), i.e. at angles of incidence of less than 45°. The collector mirror 17 can be structured and/or coated, firstly, for optimizing its reflectivity for the used radiation and, secondly, for suppressing extraneous light. The illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18 downstream of the collector mirror 17. The intermediate focal plane 18 can represent a separation between a radiation source module, having the illumination source 3 and the collector mirror 17, and the illumination optical unit 4.

The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20. The first facet mirror 20 comprises a multiplicity of individual first facets 21 , which are also referred to as field facets below. Fig. 1 illustrates only some of said facets 21 by way of example. In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

The illumination optical unit 4 consequently forms a doubly faceted system. This basic principle is also referred to as fly's eye integrator. The individual first facets 21 are imaged into the object field with the aid of the second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror or else, in fact, the last mirror for the illumination radiation 16 in the beam path upstream of the object field.

The projection system 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.

In the example illustrated in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are likewise possible. The penultimate mirror M5 and the last mirror M6 each have a through opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical unit. The projection optical unit 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, and can be for example 0.7 or 0.75.

Just like the mirrors of the illumination optical unit 4, the mirrors Mi can have a highly reflective coating for the illumination radiation 16.

Figs 2a-c show the region of the image plane 12 with the semiconductor substrate or with the wafer 13 in a projection exposure apparatus 1 that differs from the projection exposure apparatus 1 shown in Fig. 1 essentially in that the projection system 10 is not a doubly obscured optical unit, but rather a singly obscured optical unit. Accordingly, only the last mirror M6, but not the penultimate mirror M5, has a through opening for the used radiation, which is not illustrated pictorially in Figs 2a-c. In this case, the used radiation passes firstly from the opening in the last mirror M6 to the penultimate mirror M5 and is reflected back from the latter onto the concavely curved mirror surface of the last mirror M6. The last mirror M6 reflects the illumination radiation, via a membrane 25 arranged in the vicinity of the image plane 12, onto the wafer 13 to be patterned.

The membrane 25 arranged between the projection system 10 and the wafer 13 serves to filter out exposing and thermally disturbing wavelengths especially in the visible range or near the visible range. In order that an acceptable transmission for the EUV used light remains, the membrane 25 has a thickness of approximately 100 nm. Since the membrane 25 absorbs light components other than EUV radiation to a considerable extent, it heats up. This can give rise to stresses that can result in the membrane 25 tearing. In this case, parts of the membrane 25, also called shreds hereinafter, can become detached, as is illustrated schematically in Figs 2b, c.

The shreds constitute a contaminating material 26 which is entrained in a gas flow 27 which serves for photocleaning and cooling and additionally makes contamination of the wafer 13 from the direction of the projection system 10 unlikely. The flow lines of said gas flow 27 in the direction of the projection system 10 are illustrated in a roughly simplified way in Figs 2a-c. It goes without saying that the gas flow 27 does not flow through the membrane 25, but rather around the membrane 25 or is introduced above the membrane 25. The gas flow 27 is a hydrogen gas flow, i.e. a flow of molecular hydrogen (Fh). The gas flow 27 is not restricted to the volume region 28 which is shown in Figs 2a-c and in which the flow lines are depicted; rather, the volume region 28 through which the gas flow 27 passes extends into the projection system 10, as is indicated by an arrow in Figs 2a-c.

Whereas compact particles do not progress far in the weak gas flow 27 because in customary geometries they fight against gravity and lose, this is different for the shreds 26 of the thin membrane 25: Here it is possible for the shreds 26 of the membrane 25 to have little mass (since they are thin) with a large surface area and to sail up in the gas flow 27 counter to the direction of gravity, since the upwardly directed force on the membrane shreds 26 exceeds the gravitational force in the gas flow 27. A corresponding shred 26 can be entrained in the gas flow 27 and sail in the direction of the projection system 10, as is illustrated in Fig. 2c.

A further shred 26 can be deflected laterally from the gas flow 27 in the direction of the penultimate mirror M5 and reach the optically used surface thereof. The penultimate mirror M5 cannot be sufficiently shielded in a straightforward way on account of the light volume to be kept free. If the shred 26 settles on the optically used surface, then the reflectivity decreases or changes locally there and produces undesired imaging effects such as uniformity aberrations (a local variation of the quantity of light) or phase aberrations, for example, which disturb the interference of the imaging and lead for instance to structure size deviations during the exposure of the wafer 13. Fig. 3 shows such a contamination in the form of a shred or particle 26 that has deposited on an optically used area 30 of the fourth mirror M4 of the projection system 10. As can be discerned in Fig. 3, the illumination radiation 16 is specularly reflected at the optically used area 30 of the fourth mirror M4 and forms a beam path 29 which, coming from the third mirror M3 leads to the fifth mirror M5. Radiation 31 scattered at the optically used area 30 is detected by a detector 32 in the form of an endoscope optical unit (with an integrated camera). The detector 32 is arranged outside the beam path 29 and is thus blind to the specularly reflected radiation 16. Only the radiation 31 scattered at the contaminant in the form of the particle 26, which has an irregular surface, is depicted by the detector 32. In this way, the brightness contrast is increased during the detection of the contaminants 26, such that even small disturbances can be identified. In this way, even contaminants 26 having a maximum extent of at most 10 mm, preferably of at most 1 mm, in particular of at most 0.1 mm, are detectable.

The detector 32 in the form of the endoscope camera is guided through an opening 33 in an enclosure 34 in the form of a vacuum housing. Substantially the entire beam path of the optical system 1 is encapsulated in the enclosure 34, as is described for example in WO 2008/034582 A2, which is incorporated by reference in its entirety in the content of this application. The enclosure 34 serves to maintain the vacuum in the vicinity of the optical elements, for example the mirrors M1 to M6.

Alternatively, for detecting the contaminants 26, the fourth mirror M4 can be illuminated by an illumination source, e.g. by an LED, which is guided through the same opening 33 or through a different opening in the enclosure 34. For the case where the illumination source 3 of the optical system 1 is used, which source generates EUV radiation, it can be advantageous if the detector 32 brings about a conversion of the wavelength into a different wavelength range, for example by means of luminescence. Alternatively, it is possible to use a photodiode as detector 32, which generates an electron flow during the EUV irradiation.

Figs 4a, b show an alternative possibility for detecting contaminants 26 on the optically used surfaces of the mirrors M1 to M6 of the projection system 10, or for deducing the presence and also the position of contaminants 26. In order to make this possible, in place of the wafer 13 shown in Fig. 1 and in Figs 2a-c, a detector 35 is arranged in the image plane 12, said detector having a detection surface 36 in order to detect, in a spatially resolved manner, the illumination radiation 16 that emanates from the object field of the projection system 10 that is illuminated with the aid of the illumination system 2. The object field forms an illumination surface 5 covering the entire beam path 29 of the projection system 10 in the object plane 6. Correspondingly, the detection surface 36 also covers the entire beam path 29 of the projection system 10 in the image plane 12. The detection surface 36 is understood to mean that partial region of a (possibly larger) detector surface which is arranged in the beam path 29 of the projection system 10.

The illumination surface 5 is imaged onto the detection surface 36 by the projection system 10, i.e. there is a one-to-one assignment between points on the illumination surface 5 and points on the detection surface 36. The intensity I of the illumination radiation 16 that passes from the illumination surface 5 to the detection surface 36 is detected in a spatially resolved manner and communicated to an evaluation device 37. The evaluation device 37 can be a computer or some other programmable hardware and/or software. The evaluation device 37 makes it possible, on the basis of the intensity I - detected in a spatially resolved manner - of the illumination radiation 16 at the detection surface 36 to deduce contaminants 26 on the mirrors M1 to M6 in the beam path 29 of the projection system 10 between the illumination surface 5 and the detection surface 36. For checking the projection system 10 with regard to contaminants 26, it is possible to exchange the illumination system 2 and the EUV radiation source 3 for a self-luminous illumination source, e.g. an LED, which illuminates the illumination surface 5 with a predefined intensity distribution - for example as homogeneously as possible -, but this is not absolutely necessary. In this case, contaminants 26 have an effect specifically according to the location where they arise by virtue of the fact that a location-dependent difference in the intensity I on the detection surface 36 arises. For a successful measurement, a calibration is expedient in order to computationally extract illumination variations of the illumination source 3 and also design related intensity variations, which occur e.g. as a result of different angles of incidence on reflective layers on the mirrors M1 to M6 or the like, which are not attributable to contaminants 26.

The accuracy during the detection of the contaminants 26 with the aid of the evaluation device 37 can be increased if the optical system 1 is configured to set or to vary the angular distribution of the illumination radiation 16 passing from the illumination surface 5 to the detection surface 36. There are various possibilities for varying the angular distribution:

By way of example, the illumination system 2 can be configured to produce varying angular distributions W1, W2, ... at the illumination surface 5. In the case of an optical system 1 in the form of a projection exposure apparatus for EUV lithography, it is customary for varying angular distributions ("settings") to be able to be produced at the illumination surface 5 with the aid of the illumination system 2. By way of example, it is possible to set a uniform illumination, a dipole illumination, a quadrupole illumination, an annular illumination or - depending on the type of illumination system 2 - a multiplicity of other types of angular distributions in the illumination surface 5.

Each emission point on the illumination surface 5 with a given emission direction corresponds to a beam that passes through the projection system 10 with individual surface passage points on the optically used areas of the mirrors M1 to M6. If such a beam impinges on a contaminant 26, it is absorbed and/or deflected and therefore impinges with reduced intensity I at the associated image point on the detection surface 36. If the angular distribution W1, W2, ... on the illumination surface 5 is varied, then the beams that pass between a respective object point on the illumination surface 5 and a respective image point on the detection surface 36 through the projection system 10 vary as well. The positions on the optically used surfaces of the mirrors M1 to M6 which are interrogated by the respective beams with regard to contaminations thus vary as well.

The evaluation device 37 combines the information concerning the reduction of the intensity I at a respective point on the detection surface 36 with the information concerning the respective (known) illumination setting or angular setting. In this way, it is not just possible to determine that the contaminant 26 is present, rather it is often also possible to identify the location or the position of the contaminant in the projection system 10. In this case, often it is not only possible to determine on which of the mirrors M1 to M6 the contaminant 26 occurs, rather it is also possible to delimit the position of the contaminant 26 to a partial region of the respectively optically used area of one of the six mirrors M1 to M6. Therefore, often it is not just possible for the evaluation device 37 to determine whether or not contaminants 26 are present on the mirrors M1 to M6; in many cases it is also possible to determine the position of the contaminants 26 on the mirrors M1 to M6 on the basis of the variation of the angular distributions W1, W2, ....

The variation of the angular distribution W1 , W2, ... on the illumination surface 5 necessitates an illumination system 2 which provides this functionality. Directly after the mounting of the projection system 10, however, it may be the case that an illumination system 2 is not yet available or has not yet been integrated into the projection exposure apparatus 1. Moreover, it is possible that the checking of the projection system 10 for contaminants 26 is not intended to take place at the operating wavelength in the EUV wavelength range, such that in the worst case scenario a dedicated illumination system would be required specifically for this checking.

For setting a variable illumination or angular distribution, alternatively or additionally one or more transmissive optical elements 38, 38a, b can be arranged in the beam path 29 between the illumination surface 5 and the detector surface 36, as is described below with reference to Figs 4a, b.

In the example described in association with Figs 4a, b, the illumination surface 5 is illuminated as homogeneously as possible not by an illumination source 3 in the form of the EUV radiation source described further above, but rather by some other illumination source, e.g. by an LED. In this case, the illumination source 3 is arranged in the region of the reticle 7 in the vicinity of the object plane 6 of the projection system 10. The illumination source 3 serves for illuminating the illumination surface 5 as homogeneously as possible and can generate wavelengths deviating from the wavelength of the used radiation, e.g. in the visible wavelength range, for example at a wavelength of approximately 500 nm.

In the example shown in Fig. 4a, the projection exposure apparatus 1 comprises one transmissive optical element 38, which can optionally be introduced into the beam path 29 of the projection system 10 between the illumination surface 5 and the detection surface 36 or be removed from the beam path 29. For this purpose, the projection exposure apparatus 1 comprises a transport device 41 , which can comprise for example a linear motor or the like in order to displace the transmissive optical element 38 and in this way to transport it out of or into the beam path 29. The transmissive optical element 38 is moved into the beam path 29 of the projection system 10 for the measurement of the projection system 10 with regard to contaminants 26. During exposure operation, by contrast, the transmissive optical element 38 is arranged outside the beam path 29. As can be discerned in Fig. 4a, the transmissive optical element 38 is embodied in plate-shaped fashion and has a thickness D of less than 10 mm, optionally of less than 1 mm, in order not to appreciably alter the beam path 29 during the measurement. Within the meaning of this application, a plate-shaped transmissive optical element 38 is also understood to mean a membrane.

The transmissive optical element 38 has a transmission T that is dependent on the angle a of incidence, wherein the angle a of incidence, as is generally customary, is measured relative to the direction of the normal of the plate shaped optical element 38. In the example shown in Fig. 4a, the transmission T that is dependent on the angle a of incidence is produced by a coating 39 applied to a first side of the plate-shaped transmissive optical element 38. It goes without saying that a corresponding coating 39 can also be applied to the second, opposite side. The coating 39 serves as a wavelength filter and has a transmission T(l) that is dependent on the wavelength l. With the use of an illumination source 3 that generates substantially monochromatic illumination radiation 16, the effect of the wavelength-selective coating 39 corresponds to angular filtering, i.e. to an attenuation of the transmission of specific angles a of incidence. In the example shown, the wavelength-selective coating 39 is silver, but it is also possible to use some other material or a combination of a plurality of materials or of a plurality of plies or layers which produce a wavelength- selective effect.

The angle a of incidence typically varies along the surface of the plate-shaped optical element 38. For the following considerations, the angle a of incidence is related to a position at the centre of the surface of the plate-shaped optical element 38. In order to produce the greatest possible variation of the transmission T, it is advantageous if the plate-shaped transmissive optical element 38 is arranged at a location in the beam path 29 at which an angle of incidence spectrum |OMAX - OMIN|, i.e. the difference between a minimum angle OMIN of incidence and a maximum angle OMAX of incidence, is as large as possible. It is advantageous if the following holds true for the difference | OMAX - OMIN |: | OMAX - OMIN | > 15°, preferably > 20°, in particular > 25°, holds true. Such a large angle of incidence spectrum | OMAX - OMIN | is generally present between the last mirror M6 of the projection system 10 and the image plane 12, for which reason the plate shaped transmissive optical element 28 in Fig. 4a is arranged at this location in the beam path 29.

As can be discerned in Fig. 4a, the projection exposure apparatus 1 comprises a magazine 42, in which a plurality of further plate-shaped transmissive optical elements 38' are stored. The transport device 41 is configured to exchange the transmissive optical element 38 arranged in the beam path 29 for one of the further plate-shaped transmissive optical elements 38' stored in the magazine 42. By exchanging the transmissive optical elements 38, 38' having in each case a different transmission T depending on the angle a of incidence, it is possible to set varying angular distributions W1 , W2, ... of the illumination radiation 16 passing from the illumination surface 5 to the detection surface 36.

As an alternative or in addition to the exchange of the plate-shaped transmissive optical element 38, for the purpose of setting varying angular distributions W1, W2, ..., the plate-shaped transmissive optical element 38 can be deformed or tilted with the aid of suitable components, not illustrated pictorially. It is likewise possible for the plate-shaped transmissive optical element 38 to be mounted such that it is rotatable about a rotation axis 40, wherein the rotation axis 40 is oriented substantially along a central light direction in the beam path 29 of the projection system 10 at the location of the plate-shaped transmissive optical element 38, as is indicated in Fig. 4a.

In the example shown in Fig. 4b, two transmissive plate-shaped optical elements 38a, b are positionable jointly in the beam path 29 between the illumination surface 5 and the detection surface 36 or outside the beam path 29. In the example shown in Fig. 4b, too, the projection exposure apparatus 1 comprises a transport device 41 for jointly transporting the plate-shaped optical elements 38a, b.

What has proved to be advantageous for setting varying angular distributions W1, W2, ..., however, is alteration of the distance d between the two plate shaped optical elements 38a, b aligned parallel, as is illustrated in Fig. 4b. In the example shown, the alteration of the distance d is performed with the aid of the transport device 41, which comprises a suitable drive (motor) for this purpose.

In the example shown in Fig. 4b, a variation of the distance d between the two plate-shaped optical elements 38a, b is possible only in a comparatively small interval in the micrometers range, specifically between d = 0.9 pm and d = 1.1 pm. It goes without saying, however, that optionally even a larger variation of the distance d between the two plate-shaped optical elements 38a, b is settable.

Figs 5a, b show the (combined) transmission T of the two plate-shaped optical elements 38a, b at a wavelength l of the illumination radiation 16 of approximately 500 nm as a function of the angle a of incidence for three different distances: d = 0.9 pm, d = 1.0 pm, and d = 1.1 pm.

In the example shown in Fig. 5a, the plate-shaped optical elements 38a, b are not coated, for which reason the swing or the difference in transmission between the two distances d = 0.9 pm and d = 1.1 pm remains limited to a maximum 25% (given an angle a of incidence of approximately 32°). In the example shown in Fig. 5b, the two plate-shaped optical elements 38a, b each have a (thin) coating of silver. In the example shown in Fig. 5b, the swing or the maximum difference between the transmission T for the two distances d = 0.9 pm and d = 1.1 pm therefore turns out to be larger and is approximately 60-70% and is attained for an angle a of incidence of approximately 15°. An additional optimization of the coating or of the use of further or other materials for the coating gives reason to expect higher differences in the transmission T between the maximum and minimum distances d of the plate-shaped optical elements 38a, b.

The vertical line shown in Figs 5a, b represents the maximum angle a of incidence in the object plane 6 of an - exemplary - projection exposure apparatus 1 . In the example shown in Fig. 5a, the arrangement of the plate shaped optical element 38 in or in the vicinity of the object plane 6 is unfavourable; the plate-shaped optical element 38 should rather be positioned at a location at which the angle of incidence spectrum or the maximum angle OMAX of incidence is larger. Since the numerical aperture in a typical projection system 10 increases proceeding from the object plane 6 on account of etendue maintenance, it is advantageous to position the plate-shaped optical element 38 shown in Fig. 5a in or in the vicinity of a reduced intermediate image or in particular in the vicinity of the image plane 12, at which a larger angle of incidence spectrum prevails, as is illustrated in in Fig. 5a.

In the example shown in Fig. 5b, by contrast, the plate-shaped optical elements 38a, b can or should be arranged at a location at which the angle of incidence variation is at least 15°. In the example shown in Fig. 5b, e.g. for an angle a of incidence of 15° the joint transmission T of the plate-shaped optical elements 38a, b - and thus the intensity I of the illumination radiation 16 that impinges on the detection surface 36 from the illumination surface 5 - can be varied not just by at least 5% or by at least 20%, but rather by at least 50%. In contrast to the description further above, the illumination surface 5 is not necessarily arranged in the object plane 6 and the detection surface 36 is not necessarily arranged in the image plane 12. The roles of object plane 6 and image plane 12 can be interchanged. Moreover, it is possible to form the detection surface 36 or the illumination surface 5 in a different field plane of the projection system 10, for example in an intermediate image plane or the like.

Claims

Patent claims

1. Optical system (1 ), in particular for EUV lithography, comprising: an illumination source (3) for illuminating an illumination surface (5) with illumination radiation (16), a detector (35) having a detection surface (36) for the spatially resolved detection of the illumination radiation (16), a projection system (10) configured to image the illumination surface (5) onto the detection surface (36), and also an evaluation device (37) configured to deduce contaminants (26) on optical elements (M1 to M6) in a beam path (29) between the illumination surface (5) and the detection surface (36) on the basis of an intensity (I) of the illumination radiation (16) at the detection surface (36), characterized in that the optical system (1) is configured to set different angular distributions (W1, W2, ...) of the illumination radiation (16) passing from the illumination surface (5) to the detection surface (36), and in that the evaluation device (37) is configured to deduce the contaminants (26) on the optical elements (M1 to M6), in particular the position of the contaminants (26) on the optical elements (M1 to M6), on the basis of the intensity (I) of the illumination radiation (16) on the detection surface (36) for the different angular distributions (W1, W2, ...).

2. Optical system according to Claim 1 , further comprising: an illumination system (2) configured to vary the angular distribution (W1, W2, ...) of the illumination radiation (16) at the illumination surface (5).

3. Optical system according to any of the preceding claims, further comprising: at least one transmissive optical element (38, 38a, b) having a transmission (T) dependent on the angle (a) of incidence, where the transmissive optical element (38) is positionable either in the beam path (29) between the illumination surface (5) and the detection surface (36) or outside the beam path (29) between the illumination surface (5) and the detection surface (36).

4. Optical system according to Claim 3, wherein at least two transmissive optical elements (38a, b) are jointly positionable either in the beam path (29) between the illumination surface (5) and the detection surface (36) or outside the beam path (29) between the illumination surface (5) and the detection surface (36).

5. Optical system according to Claim 4, wherein a distance (d) between the at least two transmissive optical elements (38a, b) is settable.

6. Optical system according to any of Claims 3 to 5, wherein the at least one transmissive optical element (38, 38a, b) is plate-shaped and preferably has a thickness (D) of less than 10 mm, in particular of less than 1 mm.

7. Optical system according to Claim 6, wherein the plate-shaped transmissive optical element (38, 38a, b) has a coating (39) having a wavelength- dependent transmission (T(l)) on at least one side.

8. Optical system according to any of Claims 3 to 7, wherein the plate-shaped optical element (38, 38a, b) positioned in the beam path (29) is mounted such that it is deformable, tiltable and/or rotatable about a rotation axis (40).

9. Optical system according to any of Claims 3 to 8, wherein the at least one transmissive optical element (38, 38a, b) is positionable in a region in the beam path (29) at which an angle of incidence spectrum (OMIN, OMAX) on the transmissive optical element (38, 38a, b) exceeds at least 15°, preferably at least 20°, particularly preferably at least 25°.

10. Optical system according to any of the preceding claims, which is configured, in the setting of the different angular distributions (W1, W2, ...), for at least one angle (a), to vary the intensity (I) of the illumination radiation (16) impinging on the detection surface (36) from the illumination surface (5) by at least 5%, preferably by at least 20%, particularly preferably by at least 50%.

11. Optical system according to any of the preceding claims, wherein the illumination surface (5) is formed in a first field plane (6) of the projection system (10), and wherein the detection surface (36) is formed in a second field plane (12) of the projection system (10), wherein the first field plane (6) preferably forms an object plane of the projection system (10) and wherein the second field plane (12) preferably forms an image plane of the projection system (10), or vice versa.

12. Optical system according to Claim 11, wherein the illumination surface (5) covers the beam path (29) of the projection system (10) in the first field plane (6), and wherein the detection surface (36) covers the beam path (29) of the projection system (10) in the second field plane (12).