METHOD FOR PRODUCING AN OPTICAL SYSTEM

The present invention relates to a method for producing an optical system, in particular for a lithography apparatus, and to the use of the produced optical system in a lithography apparatus.

The contents of the priority apphcation DE 10 2020 203 027.8, filed on March 10, 2020, is incorporated by reference in its entirety.

Microlithography is used for producing microstructured components, for example integrated circuits. The microlithography process is performed using a lithogra phy apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated by means of the illumination system is in this case projected by means of the projection system onto a substrate, for exam ple a silicon wafer, which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system in order to transfer the mask structure to the light-sensitive coating of the substrate.

Hydrophobic properties of specific surfaces of optical systems of a hthography ap paratus may be desirable both in the production of the lithography apparatus and during its operation.

One example in which hydrophobic properties of specific surfaces of optical sys tems are desired relates to the cleaning of optical components in the production of an optical system. After the optical component has been cleaned, the cleaning liquid should be removed from the component as much as possible. Comphcated methods for cleaning and drying are known, for example multi-stage wash cas cades and vacuum drying, wherein large amounts of deionized water and energy are needed for the drying.

An example in which hydrophobic properties of specific surfaces of optical sys tems are desired relates to operation of the hthography apparatus. During opera tion of the lithography apparatus, the adhesion of liquids to components of the li thography apparatus is disadvantageous because temperature changes of the component may occur due to the evaporation cooling caused by the evaporation of the adhered hquid. Such a temperature change of the component can adversely affect the function thereof. In the case of mechanical components, the tempera ture change can cause an undesirable change in length, for example. In the case of optical components, the temperature change can lead to errors in the imaging properties of the optical component.

It is known to apply a hydrophobic material to the relevant surface to prevent such thermal effects due to the adhesion of a liquid. Such a material is typically attached to the component by means of an adhesive. Owing to the ageing of both the material itself and of the adhesive, in particular caused by short-wave DUV radiation during DUV lithography, the material needs to be replaced regularly. This causes production downtimes.

DE 10 2016 203 714 Al discloses the provision of a surface of an optical compo nent with a nanostructure and/or microstructure to bring about a hydrophobic property of the surface.

Against this background, an object of the present invention is to provide an im proved method for producing an optical system for a lithography apparatus and the use of the optical system.

Accordingly, a method for producing an optical system, in particular for a lithog raphy apparatus, is proposed. The method includes the step of interference-pat terning a first surface of the optical system for producing nanostructures and/or microstructures in order to counter any wetting of the first surface by a liquid.

Since the nanostructures and/or microstructures are produced on the first surface of the optical system by way of interference patterning, the nanostructures and/or microstructures can be produced faster than with conventional methods. The process time can thus be reduced, in particular by a great deal. Furthermore, large surfaces can also be structured easily, rehably and with a higher processing speed.

For example, the first surface can be interference-patterned at a speed of greater than 0.1 m ² /minute, preferably greater than 0.3 m ² /minute, more preferably greater than 0.5 m ² /minute, even more preferably greater than 0.7 m ² /minute, even more preferably greater than 0.9 m ² /minute and even more preferably at 1.0 m ² /minute or faster.

In addition, uniform nanostructures and/or microstructures can be produced on the first surface by way of interference patterning. In particular, periodic nanostructures and/or microstructures are produced on the first surface. In partic ^¬ ular, a period of the interference structure introduced into the first surface, and thus of the produced nanostructures and/or microstructures, is constant or approx ^¬ imately constant.

In particular, an adhesively bonded hydrophobic coating can be dispensed with. In this way, replacement of such adhesively bonded hydrophobic coatings that can lead to production downtimes can be avoided.

Owing to the described production of nanostructures and or microstructures on the first surface, a liquid can better roll off the first surface (lotus effect). For example, liquid droplets can better roll off an inclined surface. The first surface becomes in particular hydrophobic or superhydrophobic and/or a hydrophobic property of the first surface is amplified owing to the nanostructures and/or microstructures. For example, a self-cleaning property of the first surface is produced or amplified. In particular, a contact angle of the first surface can be increased owing to the inter ^¬ ference patterning of the surface. As a result, it is possible to counteract undesira ^¬ ble adhesion of the liquid to the first surface.

Firstly, it may be advantageous to prevent or reduce undesirable adhesion of the liquid to the first surface during the operation of the optical system. For example, a surface contamination of the first surface can be reduced during operation. For example, a liquid, for example an immersion liquid, can be prevented from accu ^¬ mulating on other surfaces of the optical system at which they are unwanted (e.g. the first surface). The interference-patterned surface prevents accumulation of the liquid and thus evaporation on the interference-patterned surface. An undesired temperature change that is due to coohng by evaporation and could disadvanta- geously change the optical properties of the system can thus be avoided. Secondly, it may be advantageous to also prevent or reduce adhesion of the liquid to the first surface during the production method itself. For example, an interfer ence-patterned, and thus hydrophobic, surface can be advantageous for complete removal of a cleaning liquid in a cleaning step in the production method.

The optical system can be an optical system of a lithography apparatus. The li thography apparatus can be a DUV or EUV hthography apparatus. The optical system can be, for example, an illumination system or a projection system of the hthography apparatus.

The first surface can be planar or curved.

The nanostructures and/or microstructures have, for example, a shape that is similar to that of corrugated roofs or similar to a columned hall. The spacings be tween the wave peaks (in the case where a shape is similar to that of corrugated roofs) or between the columns (in the case where a shape is similar to a columned hall) are selectable, in particular freely selectable, in the method in the region of micro /nanostructures. Said spacings correspond to the period of the interference structure.

The microstructures typically have structure sizes (spacings between the struc tures) of approximately 10 pm or above, for example up to approximately 50 pm or 100 pm. The nanostructures have smaller structure sizes (spacings between the structures), for example of less than 10 pm, less than 1 pm, less than 500 nm, less than 200 nm, less than 100 nm, less than 50 nm, less than 20 nm and/or less than 10 nm. The nanostructures for example have structure sizes of between 2 pm and 200 nm or between 5 nm and 10 nm. The first surface can be structured in particular such that both nanostructures and microstructures are produced. For example, the first surface has hierarchic micro-/nanostructures.

The liquid is for example a polar liquid, in particular water, for example (highly) purified water. The hquid has, for example, a refractive index > 1. The liquid is, for example, an immersion hquid. The hquid is, for example, a cleaning hquid. The hquid can include, for example, tin, since tin drops are generated in an EUV light source when generating EUV radiation and said tin drops can pass into the vacuum environment of the EUV lithography apparatus, where they can deposit on components.

According to an embodiment, the first surface is a surface of an optical component or of a mechanical component of the optical system.

The mechanical component is in particular a non-optical component.

According to a further embodiment, the first surface is arranged outside a beam path of the optical system.

This prevents the beam path of the optical system being disturbed by the nanostructures and/or microstructures.

In particular, the used radiation of the optical system does not come into contact with the first surface. In particular, the first surface is not an optically active sur face. For example, the first surface is arranged outside an optically used region (optically free diameter) of a last lens element of a projection system.

According to a further embodiment, an interference pattern is applied to the first surface by means of a laser during the interference -patterning step.

In particular, the nanostructures and/or microstructures are produced by locally changing the first surface in regions of interference maxima of the interference pattern. For example, material removal, material melting, a phase transformation, a photopolymerization, a change in the chemical properties and/or another local change of the first surface takes place in the region of the interference maxima. In particular, the energy density of the laser radiation applied in the interference maxima on the first surface is selected appropriately (for example between 0.1 and 10 J/cm ^2> ) to effect the relevant surface change. In regions of interference minima, the first surface is in particular not changed, or changed to a lesser extent than in regions of interference maxima.

In particular, the nanostructures and or microstructures on the first surface are produced by means of direct interference pattern generation using laser (“Direct Laser Interference Patterning", DLIP), as is described for example in DE 10 2012 011 343 Al. Laser radiation is generated here in particular with at least one laser. The laser radiation has, for example, (ultrashort) laser pulses. In particular, the laser radiation of the at least one laser is focused by means of a focusing arrange ment that is arranged in the beam path of the laser. The laser radiation from the at least one laser is in particular divided into at least two partial beams (bundles of rays) by means of a splitting arrangement that is arranged in the beam path of the laser and is directed onto the first surface such that the two partial beams (bundles of rays) superpose and interfere on the first surface. The splitting ar rangement includes, for example, a beam splitter. Owing to the described superpo sition of the at least two partial beams on the first surface, an interference pattern is generated within the entire superposition zone of the two partial beams.

The laser radiation is focused with the focusing arrangement in particular in a first spatial direction, and beam splitting with the splitting arrangement (in par ticular also very large beam expansion of up to, for example, approximately 20 to 60 cm) is effected in a second spatial direction. In particular, the period (the spac ing between the generated lines of the interference pattern) in the second spatial direction depends only on the wavelength of the laser and on the angle (interfer ence angle) between the two partial beams (bundles of rays) radiated onto the first surface. In this way, periods in the nanometre range up to in the range of a few hundred micrometres can be produced. Thus, the nanostructures and/or micro structures can be produced on the first surface. In addition, the method can be used to produce processing extents in the second spatial direction of 1 mm or a few mil limetres up to 40 cm, 100 cm or more. In this way, interference patterning can be effected on large areas in a single process step.

The superposition zone of the at least two partial beams comprises, for example, the entire first surface, with the result that the interference pattern of the at least two partial beams is applied to the entire first surface. As a result, the entire first surface can be interference-patterned in one process step.

Alternatively, the first surface is interference-patterned in multiple process steps, in particular if it is (strongly) curved. For example, a first interference pattern is applied in a first superposition zone on a first partial region of the first surface, and a second interference pattern is applied in a second superposition zone on a second partial region of the first surface.

The surface that has the nanostructures and/or microstructures after the pro cessing, that is to say the first surface, can be formed on the material of the (optical or mechanical) component that has the first surface itself. Alternatively, the nanostructures and/or microstructures can be formed on a layer that is applied on the component.

According to a further embodiment, a layer is applied to a further surface of the optical system before the interference-patterning step, and the first surface is a surface of the layer that has been apphed.

For example, in the case of an optical component, at least one layer on whose sur face the nanostructures and/or microstructures are formed can be applied to the component. The layer can form a part of a coating that is built from a plurality of layers and/or from a plurahty of materials. The coating may contain, for example, protective layers, barrier layers, reflection -reducing coats, anti -reflective layers, highly reflective layers and/or adhesion-promoting layers. At least the layer pro vided with the nanostructures and/or microstructures is typically made from a ma terial that has a stronger liquid-repellent effect than the base material of the com ponent (e.g. quartz glass) itself. A surface that is more capable of letting the liquid roll off than the base material can be produced by way of the nanostructures and/or microstructures on the layer.

According to a further embodiment, the first surface includes quartz, Zerodur, ul tra-low expansion glass, metal, aluminium and/or steel.

The first surface is for example a surface of an optical component made from quartz glass (e.g. a lens element and/or last lens element of a projection system).

The first surface is for example a surface of an optical component made from Zero dur or ultra-low expansion glass (ULE) (e.g. of a mirror). The first surface is, for example, a surface of a mechanical component made of metal, for example aluminium or steel (e.g. a carrier part or a retaining device).

The first surface is for example a surface of a metal layer (e.g. aluminium or steel) applied to an optical component.

The surface that has been interference-patterned with the method described (first surface) can firstly allow a hquid to roll off better from said surface during opera tion of the optical system (e.g. the lithography apparatus). In addition, the method described can allow a liquid to roll off better from the surface during the production method itself, that is to say before the optical system (e.g. the lithography appa ratus) is put into operation for the first time. The following text describes an ap plication during which the first surface allows a liquid to roll off better during op eration of the optical system (e.g. the hthography apparatus).

According to a further embodiment, the first surface is interference-patterned at a site of operation of the hthography apparatus.

In this way, interference patterning for the purposes of producing a hydrophobic surface that is advantageous during the operation of the lithography apparatus can be performed at the site of the final customer. Interference patterning of the first surface can be performed, for example, after the remaining optical system has been produced. For example, interference patterning of the first surface can also be performed during a temporary production stop of a lithography apparatus that has already been put into operation. For example, rather than replacing an aged, adhesively bonded hydrophobic coating, the aged hydrophobic coating can be re moved and the underlying surface can be interference-patterned in situ.

According to a further embodiment, the first surface is set up to be arranged adja cent to a liquid, in particular an immersion liquid, during operation of the lithog raphy apparatus.

Accumulation of liquid, in particular immersion liquid, on the first surface can thereby be prevented or reduced. In this way, surface contamination of the first surface can be prevented or reduced. In addition, evaporation of (immersion) liquid on the first surface, and thus a temperature change of the component having the first surface, can be avoided. Consequently, extent changes (which can result in imaging changes in the case of optical components) due to temperature changes of the component are avoided.

The first surface during operation of the lithography apparatus is in particular adjacent to a liquid reservoir, for example an immersion liquid reservoir.

According to a further embodiment, the optical system has a second surface adja cent to the first surface, the second surface is set up to be wetted at least partially with an immersion liquid during operation of the lithography apparatus, and the nanostructures and/or microstructures are produced on the first surface to counter wetting by the immersion hquid.

In this way, contamination by the immersion liquid of other areas than the sur faced) that is/are intended to be wetted with the immersion liquid can be avoided in the case of immersion lithography.

The second surface is in particular a surface that is provided during operation of the lithography apparatus for being wetted with the immersion liquid. The second surface is, for example, an optically active surface.

The first surface is, for example, a surface of the same component directly adjoining the second surface. The first surface is, for example, a surface of the same compo nent that does not directly adjoin the second surface. The first surface is, for ex ample, a surface of another adjacent component.

According to a further embodiment, the second surface is a surface of a transmis sive optical element made from a material that is transparent to wavelengths in the UV range.

According to a further embodiment, the transmissive optical element forms a last optical element of a projection lens. The last optical element is in particular a last lens element of the projection lens. The projection lens is in particular part of a lithography apparatus.

The above text described an example of an application during which the first sur face allows a hquid to roll off better during operation of the optical system (e.g. the lithography apparatus). In the following text, an example of a further application is described, in which the first surface allows a liquid to roll off better from the surface during the production method itself, that is to say before the optical system (e.g. the lithography apparatus) is put into operation for the first time.

According to a further embodiment, the method includes, after the step of interfer ence-patterning the first surface, a step of cleaning the first surface with the liquid.

Since the first surface was interference-patterned before being cleaned with the liquid and was thus rendered hydrophobic, the liquid can run off better from the first surface at the end of the cleaning, and dirt, cleaning agent residues, and/or contaminations can be more easily removed. In this way, post-flushing and/or dry ing can be simplified or avoided. For example, post-flushing and/or drying steps can be omitted.

In this case, the hydrophobic property of the interference-patterned surface is used during the production method itself, that is before the finished optical sys tem (e.g. the lithography apparatus) is put into operation for the first time.

Cleaning the first surface with the liquid is, for example, cleaning the component having the first surface with the liquid. Cleaning the first surface by means of the liquid means in particular mechanical removal, in particular rinsing or wash ing, of contaminations and/or agents used during cleaning (e.g. cleaning agents, solvents).

According to a further embodiment, the liquid is a cleaning liquid.

According to a further embodiment, in the cleaning step, the liquid is applied to the first surface by means of spray nozzles and or the liquid is applied to the first surface by washing and or immersion of the component having the first surface. By using spray nozzles, the liquid can be applied to the first surface in a pressur ized manner. In this way, the first surface can be cleaned better.

Immersing the component having the first surface is, for example, immersing the component in a liquid bath having the liquid, for example the cleaning liquid. For example, the component is partially or completely covered by the liquid during the immersion.

Washing the component having the first surface is, for example, rinsing off the component with the hquid.

According to a further aspect, the use of an optical system produced as described above in a lithography apparatus is proposed.

“A(n); one” in the present case should not necessarily be understood as restrictive to exactly one element. Rather, a plurahty of elements, such as, for example, two, three or more, can also be provided. Any other numeral used here, too, should not be understood to the effect that there is a restriction to exactly the stated number of elements. Rather, numerical deviations upwards and downwards are possible, unless indicated to the contrary.

The embodiments and features described for the production method apply corre spondingly to the proposed use, and vice versa.

Further possible implementations of the invention also comprise not explicitly mentioned combinations of features or embodiments that are described above or below with respect to the exemplary embodiments. In this case, a person skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the invention.

Further advantageous configurations and aspects of the invention are the subject matter of the dependent claims and also of the exemplary embodiments of the in vention described below. In the text that follows, the invention is explained in more detail on the basis of preferred embodiments and with reference to the ac companying figures.

Fig. 1A shows a schematic view of one embodiment of an EUV hthography appa ratus;

Fig. IB shows a schematic view of one embodiment of a DUV lithography appa ratus with a projection system;

Fig. 2 shows a schematic view of a last optical element of the projection system from Fig. IB, wherein the last optical element has a hydrophobic surface;

Fig. 3 shows a flow chart illustrating the steps of a method for producing an opti cal system according to a first embodiment, wherein the optical system produced with the method is for example the projection system from Fig. IB, and the hy drophobic surface shown in Fig. 2 is produced in the method, for example;

Fig. 4 shows a schematic view of an optical component of the EUV lithography apparatus from Fig. 1A or the DUV lithography apparatus from Fig. IB during a cleaning step that is part of a method for producing an optical system according to a second embodiment, wherein the optical component has a hydrophobic sur face and is cleaned in a liquid bath;

Fig. 5 shows a similar view to Fig. 4, wherein the optical component having the hydrophobic surface is cleaned by means of spray nozzles;

Fig. 6 shows a flow chart illustrating the steps of the method for producing the optical system according to the second embodiment, wherein the optical system produced with the method is, for example, an optical system of the EUV lithogra phy apparatus from Fig. 1A or of the DUV lithography apparatus from Fig. IB; and

Fig. 7 shows an interference pattern for producing a hydrophobic surface. Identical elements or elements having an identical function have been provided with the same reference signs in the figures, unless indicated to the contrary. It should also be noted that the illustrations in the figures are not necessarily true to scale.

Fig. 1A shows a schematic view of an EUV lithography apparatus 100A, which comprises a beam shaping and illumination system 102 and a projection system 104. In this case, EUV stands for “extreme ultraviolet” and denotes a wavelength of the working hght of between 0.1 nm and 30 nm. The beam shaping and illumi ^¬ nation system 102 and the projection system 104 are respectively provided in a vacuum housing (not shown), wherein each vacuum housing is evacuated with the aid of an evacuation apparatus (not shown). The vacuum housings are sur ^¬ rounded by a machine room (not shown), in which drive apparatuses for mechani ^¬ cally moving or setting optical elements are provided. Moreover, electrical con ^¬ trollers and the like can also be provided in this machine room.

The EUV lithography apparatus 100A has an EUV light source 106A. A plasma source (or a synchrotron) which emits radiation 108A in the EUV range (extreme ultraviolet range), that is to say for example in the wavelength range of 5 nm to 20 nm, can for example be provided as the EUV light source 106A. In the beam shaping and illumination system 102, the EUV radiation 108A is focused and the desired operating wavelength is filtered out from the EUV radiation 108A. The EUV radiation 108A generated by the EUV light source 106A has a relatively low transmissivity through air, for which reason the beam guiding spaces in the beam shaping and illumination system 102 and in the projection system 104 are evacuated.

The beam shaping and illumination system 102 illustrated in Fig. 1A has five mirrors 110, 112, 114, 116, 118. After passing through the beam shaping and illu ^¬ mination system 102, the EUV radiation 108A is guided onto a photomask (reti ^¬ cle) 120. The photomask 120 is likewise embodied as a reflective optical element and can be arranged outside the systems 102, 104. Furthermore, the EUV radia ^¬ tion 108A can be directed onto the photomask 120 by means of a mirror 122. The photomask 120 has a structure which is imaged onto a wafer 124 or the hke in a reduced fashion by means of the projection system 104. The projection system 104 (also referred to as a projection lens) has six mirrors Ml to M6 for imaging the photomask 120 onto the wafer 124. In this case, indi ^¬ vidual mirrors Ml to M6 of the projection system 104 can be arranged symmetri ^¬ cally in relation to an optical axis 126 of the projection system 104. It should be noted that the number of mirrors Ml to M6 of the EUV lithography apparatus 100A is not restricted to the number shown. A greater or lesser number of mir ^¬ rors Ml to M6 can also be provided. Furthermore, the mirrors Ml to M6 are gen ^¬ erally curved on their front sides for beam shaping purposes.

Fig. IB shows a schematic view of a DUV lithography apparatus 100B which comprises a beam shaping and illumination system 102 and a projection system 104. In this case, DUV stands for “deep ultraviolet” and denotes a wavelength of the working light of between 30 nm and 250 nm. As has already been described with reference to Fig. 1A, the beam shaping and illumination system 102 and the projection system 104 can be arranged in a vacuum housing and/or be sur ^¬ rounded by a machine room with corresponding drive devices.

The DUV lithography apparatus 100B has a DUV light source 106B. By way of example, an ArF excimer laser that emits radiation 108B in the DUV range at 193 nm, for example, can be provided as the DUV light source 106B.

The beam shaping and illumination system 102 illustrated in Fig. IB guides the DUV radiation 108B onto a photomask 120. The photomask 120 is embodied as a transmissive optical element and can be arranged outside the systems 102, 104. The photomask 120 has a structure which is imaged onto a wafer 124 or the like in a reduced fashion by means of the projection system 104.

The projection system 104 has a plurality of lens elements 128, 130 and/or mir ^¬ rors 132 for imaging the photomask 120 onto the wafer 124. In this case, individ ^¬ ual lens elements 128, 130 and or mirrors 132 of the projection system 104 can be arranged symmetrically relative to an optical axis 126 of the projection system 104. It should be noted that the number of lens elements 128, 130 and mirrors 132 of the DUV lithography apparatus 100B is not restricted to the number shown. A greater or lesser number of lens elements 128, 130 and/or mirrors 132 can also be provided. Furthermore, the mirrors 132 are generally curved on their front sides for beam shaping purposes.

An air gap between the last lens element 130 and the wafer 124 can be replaced by a liquid medium 134 which has a refractive index > 1. The liquid medium 134 can be highly purified water, for example. Such a construction is also referred to as immersion lithography and has an increased photolithographic resolution. The liquid medium 134 can also be referred to as an immersion liquid.

The last lens element 130, which is adjacent to the image plane 136, is designed in the example shown as a planoconvex lens element. The last lens element 130 has a conical lens element part 138 with a planar end face 140 that faces the im ^¬ age plane 136 and through which the photomask 120 is imaged onto the wafer 124.

The planar end face 140 of the last lens element 130 forms a surface region for coming into contact with the immersion liquid 134. In particular, a space 142 formed between the planar end face 140, a peripheral ring-shaped element 144 and the wafer 124 is filled with the immersion hquid 134. Using a feed device 146 and a suction extraction device 148, the immersion hquid 134 is correspondingly fed into the space 142 and extracted therefrom by suction. As a result, the im ^¬ mersion hquid 134 stays inside the space 142, for example even during a move ^¬ ment of the wafer 124 (a wafer stage 150) synchronously with the photomask 120. The suction extraction device 148 generally works with a negative pressure with respect to the environment, so that the immersion hquid 134 can be easily taken up. In this way, a continuous (and dynamic) exchange of the immersion hq ^¬ uid 134 can take place. The immersion hquid 134 in the example shown is a polar hquid, in particular highly purified water.

One problem that arises in immersion lithography is that the immersion hquid can also precipitate, for example due to splashing, on surfaces that are not in ^¬ tended to be wetted with the immersion hquid. This can result in contaminations and/or temperature changes due to evaporation. To avoid such undesirable ef ^¬ fects, surfaces that are not intended to be wetted with the immersion hquid can be rendered hydrophobic. The following text describes a method according to a first embodiment for produc ^¬ ing an optical system (here a projection system 104 of the DUV lithography appa ^¬ ratus 100B, Fig. IB) with a hydrophobic surface.

Fig. 2 shows the last lens element 130 of the projection system 104 and the im ^¬ mersion liquid 134 of Fig. IB in more detail. The surface 140 (surface of the pla ^¬ nar end face 140) of the last lens element 130 is intended to be wetted with the immersion liquid 134 in order to achieve increased photolithographic resolution of the DUV lithography apparatus 100B. However, a lateral surface 152 of the last lens element 130 directly adjoining the surface 140 (surface of the planar end face 140) is not intended to be wetted and/or splashed with the immersion hquid 134. This is because adhesion of the immersion liquid 134 to the lateral surface 152 can result in heat being extracted from the lateral surface 152 in the case of evaporation and thus in a temperature change of the last lens element 130. A temperature change of the last lens element 130, however, can change the imag ^¬ ing property of the last lens element 130 and thus decrease the imaging accuracy of the DUV lithography apparatus 100B.

To avoid adhesion of immersion liquid 134 to the lateral surface 152, the lateral surface 152 is coated in the present example and the applied metallic layer 154 is rendered hydrophobic by way of interference patterning.

The lateral surface 152 and the metallic layer 154 are arranged outside the beam path of the projection system 104 extending through the planar end face 140 of the conical lens element part 138. The metallic layer 154 has a surface 156 on a side facing away from the last lens element 130. The surface 156 in the example shown has nanostructures and microstructures. The nanostructures and micro ^¬ structures are produced by interference patterning the surface 156 by means of a laser. In this process, an interference pattern is apphed to the surface 156. In re ^¬ gions of interference maxima of the interference pattern, a material of the metal ^¬ lic layer 154 is changed at the surface 156, for example locally remelted or par ^¬ tially evaporated. The structuring of the surface 156 thus produced results in it having (super)hydrophobic properties. As is shown in Fig. 7, the interference pattern 400 applied to a surface to be structured, such as the surface 156, is for example a periodic line structure with interference maxima 401 and interference minima 402. A stripe-shaped structur ing of the surface is obtained using such an interference pattern 400 with a peri odic hne structure. If, in addition to this first interference pattern 400 with a pe riodic hne structure, a second interference pattern with a periodic line structure (not shown) is applied to the surface such that the second interference pattern is rotated by 90° with respect to the first interference pattern 400, column-shaped structuring of the surface can be attained.

In order to structure a curved area, such as the surface 156, interference patterns can be applied to partial regions of the surface, for example trapezoidal partial regions. Interference patterns in trapezoidal partial regions can be produced for example with the aid of a stop.

When producing the structures, the component whose surface is intended to be rendered hydrophobic can be placed on a support that is as free as possible from shocks and the laser can be guided over the component. Alternatively, the laser can be fixed and the component with the surface to be processed can be guided past the laser, for example using a linear stage or a robot.

In the example shown in Fig. 2, of the surfaces of the last lens element 130 that are not intended to be wetted with the immersion hquid, only the lateral surface 152 is provided with a metallic layer 154 and the surface 156 thereof is interfer ence-patterned. In addition (although not shown in Fig. 2), it is also possible for a planar surface 158 of the last lens element 130 that adjoins the conical lens ele ment part 138 and surrounds the latter in the shape of a ring to be designed to be hydrophobic. In particular, the planar surface 158 can also be coated with a me tallic layer (similar to the layer 154) and nanostructures and microstructures can be formed therein by way of interference patterning in order to produce (su- per)hydrophobicity.

The last lens element 130 in the example shown is attached to a retaining device in the form of a mount 160. In embodiments, the mount 160, which typically con sists of a metallic material, can also be provided with nanostructures and/or microstructures in the manner described above in order to renter them (super)hy- drophobic at least in a section facing the immersion bquid 134. This can prevent a deterioration of the retaining device 160, for example due to corrosion. Other components, such as stops for blocking scattered light, which are arranged adja ^¬ cent to the last lens element 130 can be provided with nanostructures and/or mi ^¬ crostructures by either being provided with nanostructures and/or microstruc ^¬ tures directly or by applying a metallic layer and structuring the latter.

Other optical and mechanical components that are provided in the EUV lithogra ^¬ phy apparatus 100A of Fig. 1A or the DUV lithography apparatus 100B of Fig.

IB can also be provided with a (sup er)hy drophobic surface by producing nanostructures and/or microstructures by way of interference patterning.

Fig. 3 shows a flow chart illustrating the steps of the method for producing the optical system according to the first embodiment. The optical system produced with the method according to the first embodiment is, for example, the projection system 104 from Fig. IB. Furthermore, in the method according to the first em ^¬ bodiment, the surface 156 shown in Fig. 2 for example is rendered hydrophobic by way of interference patterning.

In a first step S100 of the production method according to the first embodiment, the metallic layer 154 is applied to the lateral surface 152 of the last lens element 130 (Fig. 2).

In a second step S102 of the production method according to the first embodi ^¬ ment, the surface 156 of the metallic layer 154 is provided with nanostructures and/or microstructures by way of interference patterning.

For example, the projection system 104 for a DUV hthography apparatus 100B (Fig. IB) is produced using the method having at least the steps S100 and S102.

If a projection system 104 for a DUV lithography apparatus 100B produced in this way is put into operation, immersion liquid droplets of the immersion hquid 134 that are splashed against the surface 156 or condense thereon can easily roll off from the surface 156 during operation of the lithography apparatus 100B. As a result, they cannot adhere to the surface 156 and contaminate it or evaporate there.

Furthermore, a method for producing an optical system, in particular for a lithog ^¬ raphy apparatus, according to a second embodiment is proposed. In this case, the hydrophobic property of an interference-patterned surface is already advanta ^¬ geously utilized during the production method itself.

During the production of optical systems, for example for the EUV hthography apparatus 100A (Fig. 1A) or the DUV lithography apparatus 100B (Fig. IB), it may be necessary to clean components. For example, the mirrors 110-118, 122, M1-M6, 132 of the EUV or DUV lithography apparatus 100A, 100B may need to be cleaned. The cleaning liquid should be removed completely from the compo ^¬ nent once the cleaning has finished, for example so that hthography lenses will not become contaminated.

Fig. 4 shows an optical element in the form of a mirror 200 during cleaning in a liquid bath 202 using a cleaning hquid 204. The mirror 200 is, for example, one of the mirrors 110-118, 122, M1-M6, 132 from Figures 1A, IB. The cleaning liquid 204 is held in a container 206.

The mirror 200 has an optically active surface 208 and optically non-active sur ^¬ faces 210. The optically non-active surfaces 210 have nanostructures and micro ^¬ structures that were introduced into the surfaces 210 as part of the method by way of interference patterning. The surfaces 210 thus structured have a hydro- phobic property and allow the cleaning liquid 204, which was apphed to the sur ^¬ faces 210 upon immersion in the liquid bath 202, to easily roll off. As a result, a drying step is no longer necessary or a drying step can become shorter.

In the method according to the example of Fig. 4, the mirror 200 is transported by means of a transport apparatus 212, for example in the form of a robot or crane. The transport apparatus 212 has a wash frame 214 for holding the mirror 200. The mirror 200 held in the wash frame 214 is moved horizontally by means of the transport apparatus 212 such that it is transported to a location above the receptacle 206. The mirror 200 is then lowered into the receptacle 206 by means of the transport apparatus 212 and immersed in the liquid bath 202 having the cleaning liquid 204. The mirror 200 is subsequently lifted out of the pool 206 by means of the transport apparatus 212. As the mirror 200 is being lifted, the cleaning liquid 204 easily rolls off the surfaces 210 of said mirror due to their hy drophobic property.

Fig. 5 shows a variant of the cleaning step shown in Fig. 4 for cleaning a mirror 300. In the variant according to Fig. 5, a cleaning liquid 302 is applied to surfaces 306, 308 of the mirror 300 by means of spray nozzles 304. In this case, the sur face 306 is an optically active surface of the mirror 300 and the surfaces 308 are optically non-active surfaces of the mirror 300. The optically non-active surfaces 308 of the mirror 300 were first - similar to the optically non-active surfaces 210 of the mirror 200 from Fig. 4 - rendered hydrophobic by way of interference pat terning.

In the method, the mirror 300 is transported by means of a transport apparatus 310 and a wash frame 312 to a pool 314 and lowered into it. The pool 314 has at its inner walls 316 a plurality of the spray nozzles 304 for cleaning the mirror 300 with the cleaning liquid 302. The mirror 300 is sprayed with the cleaning liq uid 302 provided by the plurality of spray nozzles 304. The cleaning liquid thus applied to the surfaces 308 of the mirror 300 easily rolls off the surfaces 308 due to their hydrophobic property.

The cleaning liquid 204, 302 in Figures 4 and 5 is, for example, partly desali nated water (highly purified water). The cleaning liquid can also be a surfactant- containing cleaning liquid.

Fig. 6 shows a flow chart illustrating the steps of the method for producing the optical system according to the second embodiment. The optical system produced with the method is, for example, an optical system of the EUV lithography appa ratus from Fig. 1A or the DUV lithography apparatus from Fig. IB. The optical system produced with the method has, for example, the mirror 200 from Fig. 4 or the mirror 300 from Fig. 5. In a first step S104 of the production method according to the second embodi ^¬ ment, the surfaces 210 of the mirror 200 (Fig. 4) or the surfaces 308 of the mirror 300 (Fig. 5) are provided with nanostructures and microstructures by way of in ^¬ terference patterning. As a result the surfaces 210 or 308 have hydrophobic prop ^¬ erties.

In a second step S 106 of the production method according to the second embodi ^¬ ment, the mirror 200 or 300 is cleaned by being immersed into a hquid bath 202 having a cleaning liquid 204 (Fig. 4) or by being sprayed with a cleaning liquid 302 by means of spray nozzles 304 (Fig. 5). In this case, the cleaning liquid 204, 302 in each case easily rolls off the hydrophobic surfaces 210 or 308, as a result of which a drying step can be dispensed with or become shorter.

During the production method according to the second embodiment, it is alterna ^¬ tively also possible before step S104 to first coat a non-optically active surface of the mirror (similar to step S100 of the production method according to the first embodiment), for example with a metal layer. In this case, a surface of the ap ^¬ plied (metal) layer is then provided in step S104 with nanostructures and micro ^¬ structures by way of interference patterning.

Using the method according to the second embodiment including at least steps 104 and 106, for example an optical system having an optical element (mirror 200 or 300) for an EUV or DUV lithography apparatus 100A, 100B from Fig. 1A, IB is produced.

Although the present invention has been described on the basis of illustrative embodiments, it is modifiable in diverse ways. LIST OF REFERENCE SIGNS

100A EUV lithography apparatus

100B DUV lithography apparatus

102 Beam shaping and ihumination system

104 Projection system

106A EUV light source

106B DUV hght source

108A EUV radiation

108B DUV radiation

110 Mirror

112 Mirror

114 Mirror

116 Mirror

118 Mirror

120 Photomask

122 Mirror

124 Wafer

126 Optical axis

128 Lens element

130 Last lens element

132 Mirror

134 Liquid

136 Image plane

138 Conical lens element part

140 End face

142 Space

144 Ring-shaped element

146 Feed device

148 Suction extraction device

150 Wafer stage

152 Lateral surface

154 Layer

156 Surface

158 Surface 160 Holding device

200 Mirror

202 Liquid bath

204 Cleaning liquid

206 Container

208 Surface

210 Surface

212 Transport apparatus

214 Wash frame

300 Mirror

302 Cleaning liquid

304 Spray nozzle

306 Surface

308 Surface

310 Transport apparatus

312 Wash frame

314 Pool

316 Inner wall

400 Interference pattern

401 Interference maxima

402 Interference minima

Ml Mirror

M2 Mirror

M3 Mirror

M4 Mirror

M5 Mirror

M6 Mirror

S100 Method step

S102 Method step

S104 Method step S106 Method step