DE102021201193A1 - Method for adjusting an optical system, in particular for microlithography - Google Patents

Abstract

The invention relates to a method for adjusting an optical system, in particular for microlithography, the optical system having a plurality of optical elements each provided with an optically effective layer system, the method having the following steps: determining a through the optical system during operation in system wavefront provided at a predetermined level; determining a system wavefront deviation between this determined system wavefront and a target system wavefront; and performing a layer manipulation on at least one of the optical elements in such a way that the determined system wavefront deviation is reduced.

Description

BACKGROUND OF THE INVENTION

field of invention

The invention relates to a method for adjusting an optical system, in particular for microlithography.

State of the art

Microlithography is used to manufacture microstructured components such as integrated circuits or LCDs. The microlithographic process is carried out in a so-called projection exposure system, which has an illumination device and a projection objective. The image of a mask (= reticle) illuminated by means of the illumination device is projected by means of the projection objective onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to project the mask structure onto the light-sensitive coating of the to transfer substrate.

Mask inspection systems are used to inspect reticles for microlithographic projection exposure systems.

In projection lenses or inspection lenses designed for the EUV range, i.e. at wavelengths of e.g. about 13.5 nm or about 6.7 nm, reflective optical elements are used as optical components for the imaging process due to the lack of availability of suitable transparent refractive materials.

In the course of the development of projection lenses with ever higher resolving power and the associated increasing accuracy requirements, the implementation of the respective adjustment process, in the course of which the respective optical system is "brought to specification" using the existing degrees of freedom or manipulators, poses an increasingly demanding challenge For the purposes of the present invention, "adjustment" means the iterative reduction of the optical effects of process errors that are associated with the manufacturing process of the optical system or the associated optical elements (e.g. grinding errors on lenses, screwing effects on optical elements or their mounts, etc .), Roger that.

The prior art is only given as an example U.S. 7,629,572 B2 , U.S. 4,533,449 , EP 3 286 595 B1 and WO 2017/125362 A1 referred.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for adjusting an optical system, in particular for microlithography, which makes it possible to achieve a wavefront effect that can be set as precisely as possible.

This object is solved by the method according to the features of independent patent claim 1 .

• - Ermitteln einer durch das optische System im Betrieb in einer vorgegebenen Ebene bereitgestellten Systemwellenfront;
• - Ermitteln einer Systemwellenfront-Abweichung zwischen dieser ermittelten Systemwellenfront und einer Soll-Systemwellenfront; und
• - Durchführen einer Schichtmanipulation an wenigstens einem der optischen Elemente derart, dass die ermittelte Systemwellenfront-Abweichung reduziert wird.

A method according to the invention for adjusting an optical system, in particular for microlithography, the optical system having a plurality of optical elements each provided with an optically effective layer system, has the following steps:

• - determining a system wavefront provided by the optical system in operation in a predetermined plane;
• - determining a system wavefront deviation between this determined system wavefront and a target system wavefront; and
• - Carrying out a layer manipulation on at least one of the optical elements in such a way that the determined system wavefront deviation is reduced.

For the purposes of the present application, the term “layer manipulation” is also to be understood as meaning the deposition of a layer on an initially uncoated optical element.

According to one embodiment, the optical element is coated in the step of determining the system wavefront. This coating can be a coating of the relevant element that does not yet correspond to the finished layer design (in this sense “partial”). Furthermore, a layer manipulation can also be carried out iteratively on different layers of a layer system, with a system wavefront characterization taking place between the individual iteration steps.

The present invention is based in particular on the concept of carrying out the adjustment of an optical system composed of a plurality of optical elements, in particular with regard to the system wavefront provided by this optical system in a predetermined plane during operation, in such a way that a layer manipulation carried out on at least one of these optical elements or the optical effect or wavefront contribution caused by the layer system on the relevant optical element itself is used as an adjustment degree of freedom.

In other words, the invention includes the principle, after a determination of the actual system wavefront in a predetermined plane at the beginning of the system adjustment, to determine how the layer manipulation treated as a degree of freedom in the system adjustment of at least one optical element (which during the at the beginning of the system adjustment already installed in the optical system after the actual system wavefront has been determined) is to be carried out so that the deviation between the actual system wavefront determined and the desired system wavefront ultimately aimed for is reduced.

The invention differs from conventional approaches, among other things, in that the layer manipulation of a determination of the actual system wavefront carried out at the beginning of the system adjustment is already built into the optical system and possibly already provided with its optically effective coating (i.e. already contributing to the measured actual system wavefront ) Optical element is used as an adjustment degree of freedom in order to improve the wavefront properties of the overall system and to reduce wavefront aberrations.

In particular, the concept according to the invention differs on the one hand from conventional methods in which a wavefront correction is carried out only by processing an element that has not yet been installed in the optical system at the beginning of the adjustment process. In this context, an example is given EP 3 286 595 B1 referred. Furthermore, the concept according to the invention also differs from conventional methods in which a modification is made to an individual optical element only to optimize the wavefront properties or the transmission properties of the relevant optical element (and not the overall system). In this context, an example is given U.S. 7,629,572 B2 referred.

According to one embodiment, the layer manipulation includes carrying out a locally varying deposition of a layer material on the at least one optical element.

According to a further embodiment, the layer manipulation comprises carrying out a locally varying layer removal from the at least one optical element.

According to a further embodiment, the at least one optical element has a final layer made of silicon dioxide (SiO ₂ ). As will be explained below, this is particularly advantageous in the case of layer manipulation taking place by layer removal such as, for example, ion beam processing.

According to a further embodiment, the layer manipulation includes performing a locally varying ion implantation in the at least one optical element.

The layer system present on the at least one optical element during the determination of the actual system wave front can be a single layer or a multi-layer system. Furthermore, the invention is not further restricted with regard to the specific local area of the optical element or layer system in which the layer manipulation takes place. In particular, said layer manipulation can alternatively be carried out on an inner layer within a multi-layer system or also on a top layer located on top of a multi-layer system or an individual layer.

In embodiments of the invention, the selection of the layer manipulation that is suitable in the specific case to reduce the system wavefront deviation between the determined system wavefront and the target system wavefront can be made using a previously determined lookup table (“lookup table”) and the layer manipulation in question there as well if necessary, further manipulators present in the optical system are laid down sensitivities. In this look-up table, the respective wavefront contribution to the system wavefront or the brought about wavefront change compared to the optical design can be listed for different layer manipulations of the relevant optical element or for different configurations of the layer system manipulated according to the invention.

The determination of a system wavefront deviation between this determined system wavefront and a target system wavefront and the reduction of this deviation can take place in an iterative process in particular.

According to one embodiment, layer manipulation is carried out on the at least one of the optical elements in such a way that the deviation between an actual value given before the layer manipulation and a target value is reduced for at least one further characteristic property of the optical system. This at least one further characteristic property of the optical system can in particular include the reflection behavior and/or the transmission behavior of the optical system.

According to one embodiment, layer manipulation is carried out on the at least one optical element in such a way that the polarization effect of the optical system is changed.

According to one embodiment, layer manipulation is carried out on the at least one optical element in such a way that the respective deviation between a prior layer manipulation is given for the reflection behavior of the optical system, for the transmission behavior of the optical system and for the polarization effect of the optical system. Value and a target value is reduced.

According to one embodiment, the at least one of the optical elements on which a layer manipulation is carried out is a lens.

According to one embodiment, the at least one of the optical elements on which a layer manipulation is carried out is a mirror.

The optical system can in particular be an imaging system. In this case, the predefined plane can be an image plane of this imaging system.

According to one embodiment, the optical system is designed for a working wavelength of less than 250 nm, in particular less than 200 nm.

According to a further embodiment, the optical system is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

The invention further relates to an optical system for microlithography, which is designed using a method having the features described above. The optical system can in particular be a projection objective of a microlithographic projection exposure system or also a projection objective of a mask inspection system.

Further configurations of the invention can be found in the description and in the dependent claims.

The invention is explained in more detail below with reference to exemplary embodiments illustrated in the attached figures.

character list

• 1 ein Flussdiagramm zur Erläuterung des möglichen Ablaufs eines erfindungsgemäßen Verfahrens;
• 2 eine schematische Darstellung zur Erläuterung des möglichen Aufbaus eines in einer beispielhaften Ausführungsform einer erfindungsgemäßen Schichtmanipulation unterzogenen optischen Elements;
• 3a-6b Diagramme zur Erläuterung von mit einer erfindungsgemäßen Schichtmanipulation erzielbaren Veränderungen der optischen Eigenschaften des in seinem Schichtsystem manipulierten optischen Elements von 2;
• 7 eine schematische Darstellung des möglichen Aufbaus einer für den Betrieb im DUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage; und
• 8 eine schematische Darstellung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage.

Show it:

• 1 a flowchart to explain the possible sequence of a method according to the invention;
• 2 a schematic representation to explain the possible structure of an optical element subjected to layer manipulation according to the invention in an exemplary embodiment;
• 3a-6b Diagrams to explain changes in the optical properties of the optical element whose layer system has been manipulated that can be achieved with layer manipulation according to the invention 2 ;
• 7 a schematic representation of the possible structure of a designed for operation in the DUV microlithographic projection exposure system; and
• 8th a schematic representation of the possible structure of a designed for operation in the EUV microlithographic projection exposure system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1 shows a flowchart to explain the possible sequence of a method according to the invention for adjusting an optical system.

The adjustment according to the invention takes place after provision of the relevant optical system with a plurality of optical elements arranged and mounted in the optical beam path, which are already provided with an optically effective coating, the respective coatings as well as the geometries and distances of the optical elements according to a predetermined optical design are set. The adjustment process is carried out with the aim of achieving the specifications prescribed for the specific application, in particular with regard to the system wavefront provided by the system during operation, which in turn takes place in an iterative process using the available degrees of freedom.

The optical system to be adjusted can in particular be one for microlithography and more particularly a projection objective of a microlithographic projection exposure system or a mask inspection system. Examples of a microlithographic projection exposure system (designed for operation in DUV or EUV) are described below with reference to FIG 7 and 8th described.

At the beginning of the adjustment method according to the invention, the (actual) system wavefront provided by the optical system in a predetermined plane (e.g. the imaging plane of a projection lens forming the optical system) is determined in step S110. In step S120, this actual system wavefront is compared with the desired system wavefront required according to the specified specification in order to determine a system wavefront deviation.

In step S130, a layer manipulation of at least one optical element of the optical system is now carried out in such a way that said system wavefront deviation is reduced. The layer manipulation can in particular include carrying out a locally varying layer removal from the relevant optical element, carrying out a locally varying deposition of a layer material on said element and/or a locally varying ion implantation (for example a plasma immersion ion implantation).

Said layer manipulation takes place in such a way that ultimately the required specification with regard to the system wavefront provided by the optical system during operation is achieved. For this purpose, a previously determined lookup table can be used to determine the appropriate layer manipulation, in which the respective wavefront contribution of this optical element for different configurations of the manipulated layer system of the relevant optical element and possibly other manipulators present in the optical system system wavefront element. In further embodiments, the currently set system wavefront can also be determined in an iterative process with repeated implementation of a layer manipulation and compared with the target system wavefront.

It is essential to the invention that during the adjustment of the optical system, the layer manipulation itself is used as the degree of freedom of adjustment. The optical element subjected to the layer manipulation is already installed in the optical system during the initial determination of the actual system wavefront, with the result that the wavefront contribution of said optical element is also taken into account during the adjustment right from the start.

In the following, the changes in the optical properties of the optical element whose layer system has been manipulated that can be achieved with layer manipulation according to the invention are initially described on the basis of a specific exemplary embodiment and with reference to the schematic illustration in FIG 2 as well as the diagrams of 3a-3b , 4a-4b , 5a-5b and 6a-6b described. Although in the following as an example on a specific and on the basis of 2 If reference is made to the layer design of an optical element manipulated according to the invention, which is described in more detail, the invention is not limited to the materials or layer thicknesses used in this exemplary embodiment. With regard to other suitable layer materials is exemplified US 10,642,167 B2 such as U.S. 5,963,365 referred.

Furthermore, the layer design selected in the exemplary embodiment corresponds to that of a lens which is designed for operation in the DUV or at a wavelength of approximately 193 nm. However, the invention is not limited to this either, but can also be implemented in other applications in an optical element in the form of a mirror, in particular for operation in the EUV (i.e. at wavelengths of less than 30 nm, in particular less than 15 nm).

In the embodiment of 2 For example, an optical element 200 has a layer system of layers 202-206 on a substrate 201, with the respective materials or layer thicknesses being specified in Table 1. Table 1: location material refractive index Thickness [nm] 201 CaF ₂ 1.50150 202 SiO ₂ 1.58854 28.22 203 MgF ₂ 1.43758 34.56 204 SiO ₂ 1.58854 30.28 205 MgF ₂ 1.43758 18.20 206 SiO ₂ 1.58854 12.00 medium Air 1

The above layer design according to Table 1 and 2 The selected configuration of the final layer 206 (arranged furthest away from the substrate 201) made of amorphous silicon dioxide (SiO ₂ ) is particularly advantageous in the case of layer manipulation taking place by layer removal, such as ion beam processing, with regard to the fact that in this case a directional dependence of the layer-removing processing (the would result when using a final layer with a non-amorphous or at least partially crystalline phase, eg with a columnar structure) is avoided and an isotropic layer removal is achievable. In further embodiments (particularly in the case of an additive layer manipulation by way of the deposition of a layer material), however, other materials such as crystalline magnesium fluoride (MgF ₂ ) can also be used as the material for said final layer. Table 2 shows a possible layer design with a substrate made of SiO ₂ and a top layer made of MgF ₂ merely as an example. Table 2: location material refractive index Thickness [nm] 1 (substrate) SiO ₂ 1.56 2 MgF ₂ 1.42 19.86 3 LaF ₃ 1.69 80.91 4 MgF ₂ 1.42 36.64 medium Air 1

Table 3 shows a possible layer design with a substrate made of SiO ₂ and a top layer made of SiO ₂ purely as an example. Table 3: location material refractive index Thickness [nm] 1 (substrate) SiO ₂ 1.56312 2 MgF ₂ 1.44551 21.81 3 LaF ₃ 1.72862 29.31 4 MgF ₂ 1.44551 01/19 5 SiO ₂ 1.58854 12.00 medium Air 1

The invention is not limited to the adjustment solely with regard to the wavefront properties of the optical system. Rather, the adjustment can also be carried out in such a way that further properties (in particular the reflection behavior and/or the transmission behavior) of the optical system are improved. Investigations carried out by the inventors have shown that said further properties (e.g. reflection or transmission) can be improved or optimized by the layer manipulation according to the invention in one and the same adjustment method.

In the diagrams discussed below to explain changes in the optical properties of the optical element 200 whose layer system has been manipulated that can be achieved with layer manipulation according to the invention, the diagrams in FIG 3a-3b and 4a-4b in each case the behavior for unpolarized light incident on the optical element 200 is considered, whereas in 5a-5b and 6a-6b the dependence on the polarization state is considered.

In 3a-3b is for the above embodiment of 2 and Table 1 the change in phase ( 3a ) or the reflectivity ( 3b ) applied. A negative sign for the change in thickness corresponds to a reduction in the layer thickness achieved by layer manipulation. Furthermore, an angle of incidence of 15° is assumed here and in the following.

How out 3a As can be seen, a change in the layer thickness of 2nm can result in a phase change of about 1.5nm. At the same time according to 3b the change in reflectivity associated with such a change in thickness of 2nm is about 0.15 percentage points in an acceptable range. Furthermore, by suitable modification or optimization of the layer design, it can be achieved that the change in reflectivity reacts less sensitively to a change in thickness.

According to 4a-4b the dependence of the phase or reflectivity change achieved by the layer manipulation according to the invention on the angle of incidence is plotted, with a thickness change of 1.5 nm being used as an example in each case. How from the in 4a As can be seen from the scale selected with regard to the phase values, the phase change achieved with said slice manipulation is almost independent of the angle of incidence. Regarding the according 4b achieved change in reflectivity, a change of sign depending on the angle of incidence must be observed.

How out 5a-5b As can be seen, the dependency of the phase change achieved according to the invention during layer manipulation on the change in thickness is essentially independent of the state of polarization of the electromagnetic radiation impinging on the relevant optical element. On the other hand, for the change in reflectivity achieved in each case, there is a certain difference in the course dependent on the change in thickness, depending on whether the electromagnetic radiation is s-polarized or p-polarized.

According to 6a-6b is with increasing angle of incidence with respect to phase change ( 6a ) or reflectivity change ( 6b ) achieved difference between s- and p-polarization more pronounced. From this it follows that for the realization of the layer manipulation according to the invention, in the case of a desired change in the state of polarization as well, the application to an optical element with a comparatively large angle of incidence in the optical beam path is advantageous.

7 shows a schematic representation of a possible structure of a microlithographic projection exposure system 700, which is designed for operation at wavelengths in the DUV range (ie for a Working wavelength of less than 250 nm, in particular less than 200 nm, eg approx. 193 nm) and has an illumination device 702 and a projection lens 708.

The illumination device 702, into which the light from a light source 701 enters, is symbolized in a highly simplified manner via lenses 703, 704 and a diaphragm 705. In the example shown, the working wavelength of the projection exposure system 700 is 193 nm when using an ArF excimer laser as the light source 701. However, the working wavelength can also be 248 nm when using a KrF excimer laser or 157 nm when using an F ₂ laser as the light source 701 . A mask 707 is arranged in the object plane OP of the projection objective 708 between the illumination device 702 and the projection objective 708 and is held in the beam path by means of a mask holder 706 . The mask 707 has a structure in the micrometer to nanometer range, which is reduced by a factor of 4 or 5, for example, and is imaged onto an image plane IP of the projection lens 708 by means of the projection lens 708 . The projection lens 708 includes a lens arrangement, which is also symbolized in a highly simplified manner by lenses 709, 710, 711, 712, 720, and which defines an optical axis OA.

A substrate 716, or a wafer, positioned by a substrate holder 718 and provided with a light-sensitive layer 715 is held in the image plane IP of the projection lens 708. Between the last optical element 720 of the projection objective 708 on the image plane side and the light-sensitive layer 715 is an immersion medium 750, which can be deionized water, for example.

8th shows a schematic meridional section of the possible structure of a microlithographic projection exposure system designed for operation in the EUV.

According to 8th the projection exposure system 1 has an illumination device 2 and a projection lens 10 . The illumination device 2 serves to illuminate an object field 5 in an object plane 6 with radiation from a radiation source 3 via an illumination optics 4 . In this case, a reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8 . The reticle holder 8 can be displaced in particular in a scanning direction via a reticle displacement drive 9 . In 8th a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs into the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. The scan direction is in 8th along the y-direction. The z-direction runs perpendicular to the object plane 6.

The projection lens 10 is used to image the object field 5 in an image field 11 in an image plane 12. A structure on the reticle 7 is imaged on 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 Wafer holder 14 held. The wafer holder 14 can be displaced in particular along the y-direction via a wafer displacement drive 15 . The displacement of the reticle 7 via the reticle displacement drive 9 on the one hand and the wafer 13 on the other hand via the wafer displacement drive 15 can be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation, which is also referred to below as useful radiation or illumination radiation. In particular, the useful radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source or a free-electron laser (“free-electron laser”, FEL). act. The illumination radiation 16, which emanates from the radiation source 3, is bundled by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflection mirror 19 and a first (field) facet mirror arranged downstream of this in the beam path 20 (with facets 21 indicated schematically) and a second (pupil) facet mirror 22 (with facets 23 indicated schematically).

The projection lens 10 has a plurality of mirrors Mi (i=1, 2, . . . ) which are numbered consecutively according to their arrangement in the beam path of the projection exposure system 1. At the in the 8th In the example shown, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16 . The projection objective 10 is a doubly obscured optics. The projection objective 10 has an image-side numerical aperture which, for example, can be greater than 0.5, in particular greater than 0.6, and which can be 0.7 or 0.75, for example.

The optical element subjected to the layer manipulation according to the invention can be, for example, one of the lenses 709-712, 720 of the projection objective 708 7 or around one of the mirrors M1 to M6 of the projection lens 10 8th act.

Although the invention has also been described on the basis of specific embodiments, numerous variations and alternative embodiments will become apparent to the person skilled in the art, e.g. by combining and/or exchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be encompassed by the present invention, and the scope of the invention is limited only in terms of the appended claims and their equivalents.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 7629572 B2 [0006, 0015]
• US4533449 [0006]
• EP 3286595 B1 [0006, 0015]
• WO 2017/125362 A1 [0006]
• US 10642167 B2 [0042]
• US5963365 [0042]

Claims (21)

Method for adjusting an optical system, in particular for microlithography, the optical system having a plurality of optical elements each provided with an optically effective layer system, the method having the following steps: a) determining a system wavefront provided by the optical system in operation in a predetermined plane; b) determining a system wavefront deviation between this determined system wavefront and a target system wavefront; and c) carrying out a layer manipulation on at least one of the optical elements in such a way that the determined system wavefront deviation is reduced. procedure after claim 1 , characterized in that the optical element is coated in step a) during the determination of the system wave front. procedure after claim 1 or 2 , characterized in that the layer manipulation comprises carrying out a locally varying deposition of a layer material on the at least one optical element. Procedure according to one of Claims 1 until 3 , characterized in that the layer manipulation comprises carrying out a locally varying layer removal from the at least one optical element. Method according to one of the preceding claims, characterized in that the at least one optical element has a final layer of silicon dioxide (SiO ₂ ). Method according to one of the preceding claims, characterized in that the layer manipulation comprises performing a locally varying ion implantation in the at least one optical element. Method according to one of the preceding claims, characterized in that the selection of a layer manipulation suitable for reducing the system wavefront deviation is carried out using a previously determined lookup table, in which for different layer manipulations of the layer system of the at least one optical element respective wavefront contribution of this optical element to the system wavefront is listed. Method according to one of the preceding claims, characterized in that the determination of a system wavefront deviation between this determined system wavefront and a target system wavefront takes place in an iterative process. Method according to one of the preceding claims, characterized in that layer manipulation is carried out on the at least one optical element in such a way that, for at least one further characteristic property of the optical system, the deviation between an actual value given before the layer manipulation and a target value value is reduced. procedure after claim 9 , characterized in that the at least one further characteristic property of the optical system comprises the reflection behavior and/or the transmission behavior of the optical system. Method according to one of the preceding claims, characterized in that layer manipulation is carried out on the at least one optical element in such a way that the polarization effect of the optical system is changed. Method according to one of the preceding claims, characterized in that layer manipulation is carried out on the at least one optical element in such a way that the respective deviation for the reflection behavior of the optical system, for the transmission behavior of the optical system and for the polarization effect of the optical system between an actual value given before the shift manipulation and a target value is reduced. Procedure according to one of Claims 1 until 12 , characterized in that the at least one optical element on which a layer manipulation is carried out is a lens. Procedure according to one of Claims 1 until 12 , characterized in that the at least one optical element on which a layer manipulation is carried out is a mirror. Method according to one of the preceding claims, characterized in that the optical system is an imaging system. procedure after claim 15 , characterized in that the predetermined plane is an image plane of this imaging system. Method according to one of the preceding claims, characterized in that the optical system is designed for a working wavelength of less than 250 nm, in particular less than 200 nm. Method according to one of the preceding claims, characterized in that the optical system is designed for a working wavelength of less than 30 nm, in particular less than 15 nm. Optical system for microlithography, which by carrying out a method according to any one of Claims 1 until 18 is trained. Optical system after claim 19 , characterized in that this is a projection lens of a microlithographic projection exposure system. Optical system after claim 20 , characterized in that this is a projection lens of a mask inspection system.

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