Method for improving an imaging quality of an arrangement of optical elements, and corresponding arrangement of optical elements

Abstract

Method for improving an imaging quality of an arrangement of optical elements (23-28), in particular for use in an apparatus pertaining to semiconductor technology (20), wherein a first optical element (23) of the arrangement of optical elements (23-28) is subject to an expected change in the course of the time profile The expected change is suitable for influencing an optical property (W) of the arrangement. According to the invention, the following steps are carried out: - ascertaining an expected time profile of the expected change; - equipping at least one of the optical elements (23-28) with a compensation region (51a, 60b, 62a) subject to an intentional change in the course of the time profile. The time profile of the intentional change and the expected time profile of the expected change have opposite effects on the optical property (W) of the arrangement. By virtue of the opposite effects, the expected change can possibly be completely compensated for or else a maintenance interval can be significantly lengthened.

Description

Method for improving an imaging quality of an arrangement of optical elements, and corresponding arrangement of optical elements

[0001] The present application claims the priority of the German patent application DE 10 2024 203 044.9, filed on 03 April 2024, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for improving an imaging quality of an arrangement of optical elements, in particular for use in an apparatus pertaining to semiconductor technology, and a corresponding arrangement of optical elements.

[0003] Microlithography is used for producing microstructured components, such as for example integrated circuits. The microlithography process is carried out in what is known as a projection exposure apparatus, which has an illumination system and a projection system. The image of a mask (also called "reticle") illuminated by means of the illumination system is projected in this case with the aid of the projection system onto a substrate, for example a silicon wafer, that is coated with a light-sensitive layer (so- called "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. In subsequent production steps, the transferred structure is implemented in the substrate, e.g. by etching or material deposition.

[0004] Apparatuses pertaining to semiconductor technology such as for example projection exposure apparatuses, mask inspection apparatuses or wafer inspection apparatuses need to have a high imaging quality owing to the advancingminiaturization in the semiconductor field and the transition in the wavelength during exposure from DUV (e.g. 193 nm) to the EUV (e.g. 13.5 nm) . In the case of projection exposure apparatuses in the EUV field (5-20 nm) , exclusively reflective optical elements are used here for imaging a mask in the object plane onto a substrate in the image plane, e.g. with a reduction factor of 8:1. Corresponding reflective optical elements generally comprise a substrate, to the surface of which, at least in the regions provided as actual mirror surface, is applied a layer or multilayer arrangement configured for reflecting the radiation of the wavelength (such as e.g. 13.5 nm) provided for the exposure, or in a corresponding wavelength range.

[0005] In order to achieve a high imaging quality, it is necessary, besides aligning the optical elements as accurately as possible, for the optical surfaces of the individual optical elements to correspond to their respective target shape as accurately as possible. Prior to application with the actually reflective coating, the substrate of a reflective optical element is fabricated with very high accuracies and very low tolerances, in principle, at least in the region of the actual mirror surface. Application of the reflective coating also generally takes place with high precision .

[0006] Despite the high-precision fabrication, in the course of the time profile changes in shape of the reflective optical elements and hence deviations from the ideal target shape may occur which, despite a small size, may nevertheless lead to a significant deterioration in the imaging quality of an optical element or of a projection device composed of a plurality of optical elements.

[0007] In order to minimi ze age-related changes in the optical elements , the prior art discloses , in principle , having a targeted ef fect on the optical element in order to avoid optical changes in the course of the time profile . By way of example , DE 10 2018 211 596 Al discloses producing a protective layer that is intended to improve the dimensional stability of the optical element in the course of the time profile . It is also known, in principle , to bring about an arti ficial pre-ageing, for example by way of a targeted heat input into the optical element , in order as far as possible to anticipate changes in shape . Even i f a certain dimensional stabili zation can thereby be achieved, changes in an optical element in the course of the time profile cannot be entirely avoided .

[0008] The unavoidable changes in the optical elements may lead to a deterioration in the imaging quality of a proj ection system in the course of the time profile , such that said proj ection system is no longer suitable for the envisaged purpose . All that then remains is the possibility of correcting one or more optical elements by way of rework or possibly a renewed adj ustment or, i f that is not possible , exchanging one or more optical elements . However, the associated maintenance work is very time- and cost-intensive .

[0009] It is an obj ect of the present invention to provide a method for improving an imaging quality of an arrangement of optical elements , and a corresponding arrangement of optical elements which enable the disadvantages explained above to be avoided or at least to occur to a lesser extent . This obj ect is achieved by the features of the independent claims . Advantageous embodiments are described in the dependent claims .

[0010] Accordingly, the invention relates to a method for improving an imaging quality of an arrangement of optical elements , in particular for use in a microlithographic proj ection system . A first optical element of the arrangement of optical elements is subj ect to an expected change in the course of the time profile , wherein the expected change is suitable for influencing an optical property of the arrangement . According to the invention, the following steps are carried out :- ascertaining an expected time profile of the expected change ;- equipping at least one of the optical elements with a compensation region subj ect to an intentional change in the course of the time profile , wherein the time profile of the intentional change and the expected time profile of the expected change have opposite ef fects on the optical property of the arrangement .

[0011] Firstly, some terms used in the context of the present disclosure are explained . The term arrangement of optical elements should be understood broadly in so far as the arrangement can also consist of a single optical element . Preferably, the arrangement comprises a plurality of optical elements . The number of optical elements of the arrangement can be between 2 and 20 , for example . The arrangement of optical elements can comprise reflective , refractive or di f fractive optical elements . I f an optical element is subj ect to an expected change in the course of the time profile , this means that an optical property of the optical element changes in an expected manner in the course of the time profile . The changing optical property can be the shape of an optical surface , for example . On account of the expected change , therefore , a corresponding change in shape of the optical surface can occur . In thepresent case , such a change in shape is also referred to as a change in the figure of the optical element . In the case of a refractive optical element , the changing optical property can also be a local refractive index, for example .

[0012] The fact that optical elements can change in the course of the time profile is known from the prior art , in principle . By way of example , it is known that the cumulated heat input into a reflective optical element that is associated with the exposure by means of EUV radiation, over the course of time ( i . e . independently of a momentary thermal expansion) , can lead to a change in shape of the optical surface and thus to a change in figure . Moreover, over the course of time , changes in volume at speci fic regions of the substrate and / or within a layer system applied to the substrate can lead to influencing of the optical surface . Gravity, too , can lead to a change in figure in the course of the time profile . In addition, it has been observed that the optical surfaces in the case of speci fic types of optical elements , in the course of the time profile , are subj ect to changes in shape whose cause has not been conclusively clari fied hitherto , but which are reproducible and thus predictable , in principle .

[0013] In order to ascertain the time profile of an expected change , it is possible for example to have recourse to previously empirically gathered data or already existing theoretical models which were obtained on the basis of comparable optical elements and from which the expected change can be derived . It is also possible to carry out , for a speci fic first optical element , a series of experiments that makes possible a prediction about the expected time profile of the expected change . The expected change can result in particular from a change in the shape of the surface of the first optical element which cannot be avoided by means of known dimensional stabili zation measures .

[0014] The optical property of the arrangement can be in particular one or more wavefront parameters of an output beam path of the arrangement of optical elements , the output beam path resulting from a predefined input beam path that has passed through the arrangement of optical elements in a defined manner . The determination of one or more wavefront parameters of an arrangement of optical elements can take place in a manner known from the prior art and therefore need not be explained in more speci fic detail in the present case . An imaging quality of the arrangement is improved i f an optical property of the arrangement has been brought closer to an optimum value of the optical property .

[0015] Equipping at least one of the optical elements with a compensation region subj ect to an intentional change in the course of the time profile af fords a totally new possibility of countering a deterioration in the imaging quality of the arrangement . While previous approaches aimed in particular to avoid as far as possible changes in shape of the optical elements by means of targeted measures , now at least one of the optical elements is deliberately equipped with a compensation region that intentionally changes in the course of the time profile . Said compensation region is constituted such that an ef fect of the compensation region on an optical property of the arrangement in the course of the time profile is opposite to an ef fect of the expected change in the first optical element . As a result of the opposite ef fect in the course of the time profile , a potential deterioration in the imaging quality of the arrangement which would have taken place without the compensation region as a result of the expected change is at least partly compensated for in the course of the time profile . A deterioration in the imaging quality is thus actively counteracted, such that it is possible to achieve a lasting improvement in the imaging quality for a longer period of time . An exchange or a correction of an optical element istherefore not required until signi ficantly later or - in the best case - is no longer required at all within an envisaged period of use .

[0016] It is possible for the first optical element to be equipped with a compensation region . In this case , a compensation of the expected change takes place within the optical element that is subj ect to the expected change . In this case , it is not absolutely necessary for the arrangement to have a plurality of optical elements , rather the method can already be reali zed i f the arrangement of optical elements comprises only a single optical element (namely the first optical element ) . By way of example , the expected change can be configured in such a way that at least one portion of the optical surface of the first optical element moves upwards in a vertical direction in the course of the time profile , provided that a compensation region is not present . In this case , the compensation region can be produced below the moving portion of the optical surface in such a way that it is subj ect to a reduction of volume ( also referred to hereinafter as compaction) in the course of the time profile in order to produce an opposite , downwardly directed movement which at least partly compensates for the upwardly directed movement or even completely compensates therefor . In this case , the optical surface of the first optical element deviates from a target shape to a lesser extent in the course of the time profile or even corresponds to the target shape during an entire planned period of use .

[0017] As an alternative thereto , a di f ferent optical element from the first optical element can also be equipped with a compensation region . In this case , the change in the first optical element is accepted and the ef fect of this change is compensated for by a di f ferent optical element being equipped with a compensation region . It is also possible for both the first optical element and one or more other optical elementsto be equipped with a compensation region . I f a plurality of optical elements have a compensation region, the ef fects on the optical property of the arrangement that are exerted by the respective compensation region add up, in which case a cumulated total ef fect of the compensation regions in the course of the time profile is then opposite to the ef fect of the expected change on the optical property .

[0018] It is not ruled out that besides the first optical element , even further optical elements are subj ect to an unavoidable expected change which can likewise be compensated for with the aid of the above-described method in order to further improve the imaging quality of the arrangement .

[0019] In one embodiment , a hypothetical change in the optical property of the arrangement over time is determined from the expected change in the first optical element . A hypothetical change in the optical property of the arrangement over time can be determined by determining the change in the optical property for the case where no compensation region is provided . The hypothetical change in the optical property over time can be used to determine requirements in respect of the intentional change in the compensation region . I f the expected ef fects of the expected change on the optical property of the arrangement are known, it is possible in particular to derive therefrom what influence on the optical property the compensation region ( s ) must have in the course of the time profile in order that a compensation - and hence an improvement in the imaging quality - occurs .

[0020] As already mentioned above , the first optical element can have in particular a reflective optical surface . The expected change can be suitable for bringing about a change in shape of the reflective optical surface .

[0021] The optical element equipped with the compensation region can have a substrate and a coating system applied thereto , wherein at least one portion of the coating system forms a reflective optical surface . The intentional change in the compensation region can comprise in particular a change in shape of the compensation region, which influences a shape of the optical surface .

[0022] By way of example , one embodiment can provide for the compensation region to be produced within the substrate . The production of the compensation region can comprise an irradiation of the substrate using a particle beam . Alternatively or additionally, the production of the compensation region can comprise an irradiation of the substrate using electromagnetic radiation . Alternatively or additionally, the production of the compensation region can comprise a heating of the substrate or of a partial region of the substrate . It is known in principle from the prior art ( see for example DE 10 2018 211 596 Al ) that a change in shape , in particular a compaction or decompaction ( increase in volume ) , can be attained within the substrate with the aid of the methods mentioned above . However, for such methods the prior art has proposed attaining a static change in the optical surface , and consideration has always been given to avoiding as far as possible changes in shape of the substrate that take place in the course of the time profile . By contrast , in the context of the present disclosure , it is proposed to deliberately utili ze the stated methods for treatment of the substrate in order to create a compensation region subj ect to an intentional change in the course of the time profile .

[0023] One embodiment provides for the compensation region to be produced within the coating system . By way of example , it is possible to deliberately add to the coating system a layer that is known to be subj ect to a certain change in shape inthe course of the time profile . Alternatively, at least one partial region of an existing coating system can be processed ( in a manner known in principle ) in such a way that the partial region is subj ect to an intentional change in the course of the time profile .

[0024] In one embodiment , the imaging quality of the arrangement of optical elements is improved by an arrangement being newly created which contains the at least one optical element that is equipped with the compensation region . In particular, in the course of the planning and design of the optical elements , in one or more of the optical elements it is possible to provide a compensation region which at least partly compensates for the ef fect of the expected change with the aid of the opposite time profile .

[0025] It is also possible to improve the imaging quality of the arrangement of optical elements by virtue of the fact that the at least one optical element that is equipped with the compensation region replaces an optical element already present in an arrangement . In this way, an optical arrangement already present can be retrofitted with an optical element having a compensation region coordinated with the expected change . In particular, it can be provided that during maintenance of the optical arrangement that is required anyway, an optical element equipped with the compensation region is introduced into the optical arrangement in order to replace one of the optical elements . In this case , the optical element introduced can replace the first optical element subj ect to the expected change or another of the optical elements .

[0026] In the course of producing the compensation region, it can happen that an immediate static change in the optical surface occurs in addition to the intentional change in the course of the time profile . By way of example , a particleirradiation of a substrate region can lead to compaction of the substrate region without a time delay, and a relaxation associated with a decompaction follows the initial compaction in the course of the time profile . While the decompaction can be utili zed in the context of the invention in order to attain the intentional change , the initial compaction may be undesirable . In one embodiment of the method described here , therefore , at least one of the optical elements is equipped with an allowance . An allowance results in an optical property of the optical element being changed in a static manner . This means that the change in the optical property occurs immediately and does not have any component changing in the course of the time profile . In particular, provision can be made for the allowance to at least partly compensate for a static change in the optical element produced in the course of producing the compensation region . The allowance can be provided in the same optical element in which the compensation region is also produced, or in another of the optical elements .

[0027] In certain cases , despite compensation region it is not possible to completely compensate for the expected change for an entire planned period of use . By way of example , it is conceivable for a speci fic type of optical elements to be subj ect to an expected change which leads to a substantially linear change in an optical property in the course of the time profile , wherein on the other hand with the aid of the above-described measures for producing a compensation region in many cases only degressive intentional changes are attained in the course of the time profile , i . e . ones where the change in the optical property decreases in the course of the time profile . This scenario can have the consequence that compensation of the expected change cannot be achieved for a time period of arbitrary length . In this case , the optical property of the arrangement can be subj ect to a temporal dri ft despite compensation region .

[0028] On the basis of this insight , one embodiment provides for at least one of the optical elements to be equipped with an allowance which shi fts the optical property of the arrangement in such a way that said optical property has a sub-optimum value at the beginning of a planned period of use , wherein the optical property passes through an optimum value of the optical property proceeding from the sub-optimum value during the planned period of use on account of the temporal dri ft . This measure makes it possible to signi ficantly lengthen a period of time during which the optical property is close to the optimum value ( and in particular does not exceed a maximum permitted deviation from the optimum value ) . By contrast , i f the optical property were already optimal at the beginning of the planned period of use , the value of the optical property would move away from the optimum value within a signi ficantly shorter time on account of the temporal dri ft during the planned period of use , such that a maximum permitted deviation of the optical property would be exceeded within a shorter time period . A maintenance interval which after having elapsed necessitates a correction or an exchange of optical elements because a maximum permitted deviation of the optical property has been exceeded can be lengthened further in this way .

[0029] A si ze of the allowance can be determined taking into account a total change in the optical property of the arrangement that is expected during a planned period of use . The total change results from an addition of the ef fects of the expected change and the intentional change on the optical property . The allowance can be selected in such a way that its effect on the optical property amounts to a proportion of between 30% and 70% , in particular between 40% and 60% , of the ef fect on the optical property that is brought about by the total change .

[0030] The concept of providing an allowance which shi fts the optical property of the arrangement in such a way that said optical property has a sub-optimum value at the beginning of a planned period of use , wherein the optical property passes through an optimum value of the optical property proceeding from the sub-optimum value during the planned period of use on account of a temporal dri ft , has independent inventive content . In this case , therefore , it is not absolutely necessary to equip one of the optical elements with a compensation region subj ect to an intentional change in the course of the time profile .

[0031] The allowance can be produced by an irradiation of the optical element using a particle beam and / or by an irradiation of the optical element using electromagnetic radiation and / or by a targeted heating of at least one partial region of the optical element and / or by applying an ablating treatment to an optical surface of the at least one optical element .

[0032] In one embodiment , the at least one compensation region is coordinated with the expected change in such a way that the optical property assumes an extreme value within the planned period of use , at which extreme value a rate of change of the optical property reverses its sign . The optical property thus approaches the optimum value in a first time period immediately adj acent to the extreme value , and it moves away from the optimum value within a second time period immediately adj acent to the extreme value . I f exactly one extreme value is present , it can fall approximately in the middle of the planned use period . By way of example , proceeding from the planned start of use the extreme value can be assumed after a time has elapsed which is between 20% and 80% , preferably between 30% and 70% , of the planned period of use . What can be achieved by means of this measure , too , is that the opticalproperty does not exceed a maximum permitted deviation for a time period that is as long as possible .

[0033] The invention furthermore relates to an arrangement of optical elements , in particular for use in an apparatus pertaining to semiconductor technology . The arrangement comprises a first optical element subj ect to an expected change in the course of the time profile , wherein the expected change is suitable for influencing an optical property of the arrangement . According to the invention, one of the optical elements has a compensation region subj ect to an intentional change in the course of the time profile , wherein the time profile of the intentional change and the expected time profile of the expected change have opposite ef fects on the optical property of the arrangement . The arrangement can be produced in particular with the aid of a method according to the invention . The arrangement can be developed by further features from among those described above in connection with the method according to the invention . The advantages described above in connection with the method are equally af forded for the arrangement according to the invention .

[0034] The invention is described by way of example below on the basis of advantageous embodiments with reference to the accompanying drawings , in which :Figure 1 : shows a schematic illustration of a proj ection exposure apparatus for EUV microlithography with various optical elements ;Figure 2A: shows a schematic lateral sectional view of an exemplary optical element at a first point in time ;Figure 2B : shows a schematic lateral sectional view of the exemplary optical element from Figure 2A at a second point in time ;Figure 3A: shows a schematic lateral sectional view of an alternative optical element , equipped with a compensation region, at a first point in time ;Figure 3B : shows a schematic lateral sectional view of the alternative optical element from Figure 3A at a second point in time ;Figure 4 : shows an illustration of the time profile of a figure parameter P of the optical element shown in Figure 3 and also the contributions of di f ferent partial regions of the optical element to the figure parameter ;Figure 5A: shows a schematic lateral sectional view of an arrangement of two optical elements at a first point in time ;Figure 5B : shows a schematic lateral sectional view of the arrangement of two optical elements from Figure 5A at a second point in time ;Figure 6A: shows a schematic lateral sectional view of an alternative arrangement of two optical elements at a first point in time ;Figure 6B : shows a schematic lateral sectional view of the arrangement of two optical elements from Figure 6A at a second point in time ;Figure 7 : shows an illustration of the time profile of a deviation of a wavefront parameter from an optimumvalue , determined from the arrangement shown in Figure 1 with the optical elements from Figure 6 integrated therein;Figure 8A: shows a schematic lateral sectional view of an alternative arrangement of two optical elements at a first point in time ;Figure 8B : shows a schematic lateral sectional view of the arrangement of two optical elements from Figure 8A at a second point in time ;Figure 9 : shows an illustration of the time profile of a deviation of a wavefront parameter from an optimum value , determined from the arrangement shown in Figure 1 with the optical elements from Figure 8 integrated therein;Figure 10 : shows an illustration of the time profile of a deviation of a wavefront parameter from an optimum value , determined from the arrangement shown in Figure 1 with an alternative arrangement of optical elements integrated therein .

[0035] Figure 1 illustrates a schematic meridional section of a microlithographic proj ection exposure apparatus 1 . In this case , the proj ection exposure apparatus 1 comprises an illumination system 10 and a proj ection system 20 .

[0036] An obj ect field 11 in an obj ect plane or reticle plane 12 is illuminated with the aid of the illumination system 10 . For this purpose , the illumination system 10 comprises an exposure radiation source 13 , which, in the illustrated exemplary embodiment , emits illumination radiation at least comprising used light in the EUV range , i . e . inparticular with a wavelength of between 5 nm and 30 nm, in particular of 13 . 5 nm .

[0037] The illumination radiation emerging from the exposure radiation source 13 is initially focused in a collector 14 .

[0038] Downstream of the collector 14 , the illumination radiation propagates through an intermediate focus in an intermediate focal plane 15 . I f the illumination system 10 is constructed in a modular design, the intermediate focal plane 15 can be used, in principle , for the separation - including the structural separation - of the illumination system 10 into a radiation source module , comprising the exposure radiation source 13 and the collector 14 , and the illumination optical unit 16 described below . In the case of a corresponding separation, radiation source module and illumination optical unit 16 then j ointly form a modularly constructed illumination system 10 .

[0039] The illumination optical unit 16 comprises a deflection mirror 17 . The deflection mirror 17 can be a plane deflection mirror or, alternatively, a mirror with a beam-influencing ef fect that goes beyond the purely deflecting effect . Alternatively or additionally, the deflection mirror 17 can be embodied as a spectral filter separating a used light wavelength of the illumination radiation from extraneous light having a wavelength that deviates therefrom .

[0040] The deflection mirror 17 is used to deflect the radiation emanating from the exposure radiation source 13 to a first facet mirror 18 . I f - as in the present case - the first facet mirror 18 is arranged in a plane of the illumination optical unit 16 which is optically conj ugate to the reticle plane 12 as a field plane , this facet mirror is also referred to as a field facet mirror . The first facet mirror18 is a microelectromechanical system (MEMS system) having a multiplicity of individually pivotable micromirrors 18' , as also described in DE 10 2008 009 600 Al, for example.

[0041] A second facet mirror 19 (also called "pupil facet mirror") is arranged downstream of the first facet mirror 18 in the beam path of the illumination optical unit 16, with the result that this yields a doubly faceted system, the fundamental principle of which is also referred to as a fly's eye integrator. The second facet mirror 19, too, comprises - as illustrated - a microelectromechanical system having a multiplicity of individually pivotable micromirrors 19' .

[0042] The facets of the first facet mirror 18 are imaged overlaid on one another by way of a respective assigned facet of the second facet mirror 19, for the purpose of illuminating the object field 11 as homogeneously as possible.

[0043] By selecting the ultimately used illumination channels, which is possible without problems by way of a suitable setting of the micromirror 18' of the first facet mirror 18, it is still possible to set the intensity distribution in the entrance pupil of the projection system 20 described below. This intensity distribution is also referred to as illumination setting.

[0044] The object field 11 in the reticle plane 12 is transferred to the image field 21 in the image plane 22 with the aid of the projection system 20. For this purpose, the projection system 20 comprises a plurality of optical elements configured as mirrors 23-28. In the example illustrated in Figure 1, the projection system 20 comprises six mirrors 23- 28. Alternatives with four, eight, ten, twelve or any other number of mirrors are likewise possible. The penultimatemirror 27 and the last mirror 28 each have a passage opening for the illumination radiation, as a result of which the illustrated projection system 20 is a doubly obscured optical unit. The projection system 20 has an image-side numerical aperture that is greater than 0.3 and can also be greater than 0.6, and can be for example 0.7 or 0.75.

[0045] The reflection surfaces of the mirrors 23-28 can be embodied as freeform surfaces without an axis of rotational symmetry. Just like the mirrors of the illumination optical unit 16, the mirrors 23-28 can have reflective coatings for the illumination radiation. These reflective coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0046] A reticle 30 (also referred to as mask) arranged in the object field 11 is exposed by the illumination system 10 and transferred by the projection system 20 onto the image plane 22. The reticle 30 is held by a reticle holder 31. The reticle holder 31 is displaceable by way of a reticle displacement drive 32, in particular in a scanning direction. In the exemplary embodiment illustrated, the scanning direction runs in the y-direction.

[0047] A structure on the reticle 30 is imaged onto a lightsensitive layer of a wafer 35 arranged in the region of the image field 21 in the image plane 22. The wafer 35 is held by a wafer holder 36. The wafer holder 36 is displaceable by way of a wafer displacement drive 37, in particular along the y-direction. The displacement, firstly, of the reticle 30 by way of the reticle displacement drive 32 and, secondly, of the wafer 35 by way of the wafer displacement drive 37 can be implemented so as to be synchronized with one another.

[0048] The mirrors 23-28 form an arrangement of optical elements according to the invention . An imaging quality of the arrangement can be determined in a manner known in principle , for example by a defined input beam path being radiated into the arrangement and an output beam path of the arrangement being measured . The measurement of the output beam path makes it possible to determine one or more wavefront parameters of the arrangement , which can constitute optical properties of the arrangement in the context of the present disclosure . Such a measurement is known in principle in the prior art and is therefore not explained in detail in the present case . The arrangement has an optimum value for each wavefront parameter . I f a wavefront parameter deviates from its optimum value , this adversely af fects the imaging quality of the arrangement of optical elements .

[0049] In order to explain the method according to the invention, firstly the construction of the mirror 23 is explained below with reference to Figure 2A. The mirror 23 is subj ect to an expected change in the course of the time profile , said change being evident from a comparison of Figures 2A and 2B . Figure 2A shows the mirror 23 at a point in time tO , and Figure 2B shows the mirror 23 at a later point in time tl . The mirror 23 comprises a substrate 50 , which is fixedly connected to a substructure 52 . A coating system 51 is applied to the substrate 50 . At least one portion of the coating system 51 forms an optical surface 53 , shown as a plane surface in Figure 2A. For the sake of simplicity, it is assumed that the optical surface 53 corresponds to its target shape in Figure 2A, i . e . at the point in time tO .

[0050] Figure 2B reveals that the mirror 23 was subj ect to a change in a partial region 55 in the course of the time profile , i . e . upon transition from the point in time tO to the point in time tl . In the present case , the change consistsin the fact that the optical surface 53 has moved upwards in the partial region 55 and a corresponding change in figure has occurred . It is assumed here by way of example that the change was caused by a change in shape of the substructure 52 , as a result of which the substrate 50 and the coating 51 situated thereon in the partial region 55 were correspondingly displaced upwards , this being indicated schematically in Figure 2B .

[0051] Within the arrangement of mirrors 23-28 shown in Figure 1 , the change in figure of the mirror 23 would have caused a deviation of a wavefront parameter of the arrangement which would have led to a deterioration in the imaging quality . A description is given below of various embodiments of the method according to the invention which make it possible to prevent such a deterioration in the imaging quality .

[0052] Figures 3A and 3B show schematic, lateral sectional views of an alternative optical element that was equipped with a compensation region in order to carry out the method . Elements that have already been described in association with Figures 2A and 2B bear the same reference signs in Figures 3A and 3B . Analogously to Figures 2A and 2B, Figures 3A and 3B also show the mirror at two di f ferent points in time tO and tl . The partial region 52a is subj ect to a change in the same way as described in association with Figure 2 , as a result of which change the overlying regions 50a and 51a of the substrate 50 and of the coating system 51 , respectively, are displaced upwards in the course of the time profile . In contrast to the embodiment in Figure 2 , before the point in time tO a compensation region is produced by the substrate 50 being treated using a particle beam in the partial region 55 . This results in a compaction of a region 50a of the substrate 50 ( see Figure 3A) .

[0053] In order to prevent the region 50a from decompacting again in the course of the time profile , the region 50a is additionally treated using electromagnetic radiation in order to produce a protective layer, which can stabili ze the compacted shape of the region 50a . Furthermore , a region 51a of the coating 51 that is situated above the compacted region 50a is produced with a larger layer thickness and using a material which is subj ect to a compaction in a predictable manner in the course of the time profile . An intentional compaction of the region 51a occurs in such a way that the change in shape of the region 52a of the substructure 52 is completely compensated for . At the point in time tl , the optical surface 53 therefore still corresponds to its target shape and a change in figure was avoided . In the context of the present disclosure , the region 51a can also be referred to as a compensation region . An ef fect of the region 51a on a wavefront parameter is opposite to an ef fect of the change in shape of the region 52a in the course of the time profile .

[0054] In order to improve the imaging quality of the arrangement of mirrors 23-28 , the mirror 23 described above can be integrated into the arrangement shown in Figure 1 as early as during the production thereof . It is also possible to replace a mirror already present in the arrangement with the mirror 23 equipped with the compensation region 51a .

[0055] Figure 4 shows a graph in which a figure parameter P and also the contributions of the regions 51a and 52a to the figure parameter P of the mirror 23 are plotted against time t . A target value of the figure parameter Pois additionally illustrated . I f the figure parameter P deviates from the target value Po, this means that the optical surface deviates from its target shape in a vertical direction in the partial region 55 . The contributions of the regions 51a and52a are designated by AP(51a) and AP(52a) , respectively, in Figure 4. The contribution AP(52a) rises degressively in the time profile, while the contribution AP(51a) falls repressively in the time profile. In total, the contributions AP(51a) and AP(52a) at each point in time add up in such a way that the total figure P of the optical element in the partial region 55 in the time window considered constantly corresponds to the target value Po (see the horizontal dashed line in Figure 4) .

[0056] An alternative embodiment of the method according to the invention is described below with reference to Figures 5A and 5B . Figures 5A and 5B show a schematic lateral sectional view of an arrangement of two mirrors 23 and 24 at two different points in time tO and tl. The mirror 23 corresponds to the mirror 23 already described in association with Figure 2. In addition to the mirrors 23, 24, two partial beams 81, 82 of a beam path are illustrated. The partial beams 81, 82 are reflected at the mirror 23 and subsequently impinge on the mirror 24 so as to be reflected again at the latter.

[0057] As already described above in association with Figure 2, the mirror 23 comprises a partial region 55 in which, in the course of the time profile, on account of a change in shape of the substructure 52, a change in figure occurs which is suitable for changing a wavefront parameter of the arrangement of the mirrors in the course of time and thus adversely affecting the imaging quality. In contrast to the method described with reference to Figure 3, the change in figure of the mirror 23 is not corrected on the mirror 23 itself, rather a compensation region 62a is produced within the mirror 24, said compensation region being subject to a compaction in the course of the time profile. Just like the region 51a described in association with Figure 3, theregion 62a comprises a material which is subj ect to a compaction in a predictable manner in the course of the time profile . An intentional change in shape of the region 62a occurs in such a way that the change in shape of the region 52a of the substructure 52 is completely compensated for . In the present case , an ef fect of the change in shape of the region 52a on a wavefront parameter is opposite to the effect of the change in shape of the region 62a on the wavefront parameter, such that the wavefront parameter considered remains unchanged or at least changes to a lesser extent in the course of the time profile .

[0058] A further alternative embodiment of the method is illustrated by way of example with reference to Figure 6 . The illustrations in Figures 6A and 6B correspond to those in Figures 5A and 5B, and elements that have already been explained in association with Figures 5A and 5B bear the same reference signs in Figures 6A and 6B . Only the di f ferences with respect to the embodiment in Figures 5A and 5B are explained below . In the embodiment in Figure 6 , the mirror 23 comprises a substructure 52 having a partial region 52b subj ect to a change in shape in the course of the time profile , as a result of which change the optical surface 53 is moved vertically downwards ( see Figure 6B ) . In order to counteract a change in a wavefront parameter and an attendant deterioration in the imaging quality, in the embodiment in Figure 6 , in the mirror 24 a particle irradiation is applied to a region 60b of the substrate , as a result of which a compaction of the region 60b is attained ( see Figure 6A) . In order to avoid a deviation of the optical surface 63 from its target shape at the point in time tO , a layer thickness of the coating 62 in the region 62b is increased during the production of the mirror 24 . In contrast to the embodiment in Figure 5 , the coating 62 is not subj ect to a change in shape in the region 62b .

[0059] However, the previously performed compaction of the substrate 60 in the region 60b relaxes in a predictable manner in the course of the time profile, which leads to an intentional decompaction of the region 60b. As a result of the decompaction of the region 60b, the partial region 62b is displaced together with a portion 63b of the optical surface. The displacement of the portion 63b of the optical surface 63 compensates for a portion of the change in shape of the optical surface 53 in the course of the time profile.

[0060] Figure 7 shows a graph in which a deviation AW of a wavefront parameter W from an optimum value of the wavefront parameter is plotted against time t. The wavefront parameter can be determined from the arrangement of the mirrors 23-28 illustrated in Figure 1, wherein the mirrors 23 and 24 are the mirrors illustrated in Figure 6. In contrast to the case explained in Figure 4, the region 52b shown in Figure 6 is subject to a linear change which has a linear effect on the considered wavefront parameter in the course of the time profile. This effect is represented by the contribution AW (52b) in Figure 7. In the course of the time profile, the compaction of the region 62a has an opposite but decreasing (degressive) influence on the wavefront parameter, which is illustrated on the basis of the contribution AW(60b) in Figure 7. Besides the two contributions AW(52b) and AW (60b) , the resulting time profile of the wavefront parameter AW is illustrated in Figure 7. In addition, a maximum permitted deviation AWmaxfrom the optimum value of the wavefront parameter is depicted as a horizontal dashed line. If the deviation AW exceeds this maximum value, it is necessary to correct or exchange one or more of the mirrors.

[0061] The change in the wavefront parameter that is caused by the contribution AW(52b) is partly compensated for on account of the compensation region 60b. Without thecompensation region 60b, the deviation AW would exceed the maximum value AWmaxas early as at the point in time t2 . On account of the compensation region 60b, the wavefront parameter does not exceed the maximum value Wmaxuntil at a later point in time t3 . A maintenance interval can be signi ficantly lengthened as a result .

[0062] A further alternative embodiment of the method is illustrated by way of example with reference to Figure 8 . The illustrations in Figures 8A and 8B correspond to those in Figures 6A and 6B, and elements that have already been explained in association with Figures 6A and 6B bear the same reference signs in Figures 8A and 8B . Only the di f ferences with respect to the embodiment in Figures 8A and 8B are explained below . In contrast to the embodiment in Figure 6 , at the point in time tO the mirror 24 is equipped with an allowance by virtue of a partial section 63b of the optical surface 63 being deliberately permitted to deviate from its target shape at the point in time tO ( see Figure 8A) . It is only in the course of the time profile that the partial section 63b of the optical surface gets closer to its target shape on account of the decompaction of the partial region 62b, in which case it corresponds to the target shape at the point in time tl ( see Figure 8B ) .

[0063] Figure 9 shows a graph in which a deviation AW of a wavefront parameter W from an optimum value of the wavefront parameter is plotted against time t . The wavefront parameter can be determined from the arrangement of the mirrors 23-28 illustrated in Figure 1 , wherein the mirror is 23 and 24 are the mirror is illustrated in Figure 8 . The profile of the contributions AW ( 52b ) and AW ( 60b ) shown in Figure 9 corresponds to the profile already illustrated in Figure 7 . The allowance introduced in the region 62b leads to an additional contribution AW ( 62b ) to the wavefront parameter,which contribution is constant in the time profile . The total deviation AW of the wavefront parameter results from an addition of the contributions AW ( 52b ) , AW ( 60b ) and AW ( 62b ) .It is evident that in contrast to the case illustrated in Figure 7 , the wavefront parameter deviates from the optimum value (AW=0 ) at the point in time t=0 and approaches the optimum value in a time period between t=0 and t4 , with the optimum value being traversed at the point in time t4 . It is not until at the point in time t3 that the wavefront parameter exceeds the maximum permitted deviation AWmax. The allowance thus enables a maintenance interval to be lengthened further .

[0064] A further exemplary embodiment of the method is illustrated with reference to Figure 10 . Figure 10 shows a graph in which a deviation AW of a wavefront parameter W from an optimum value of the wavefront parameter is plotted against time t . The wavefront parameter W can be determined from the arrangement of the mirrors 23-28 illustrated in Figure 1 , wherein the mirrors 23-28 are configured in accordance with an alternative embodiment . In this embodiment , a first mirror 23 is subj ect to an expected change in the course of the time profile , which change yields the contribution AW1 to the wavefront parameter as illustrated in Figure 10 . The contribution AW1 rises in the time profile . Furthermore , one of the mirrors 23-28 comprises a partial region subj ect to an intentional change in the course of the time profile , which change yields the contribution AW2 to the wavefront parameter as illustrated in Figure 10 .At the point in time t=0 the contribution AW2 leads to a change AW0in the wavefront parameter, wherein the contribution AW2 decreases and asymptotically tends towards zero in the time profile . In addition, a static allowance was produced inone of the optical elements , and af fects the wavefront parameter by virtue of the constant contribution AW3 = -AW0in the time profile . The allowance is coordinated with the partial region subj ect to the intentional change in such a way that the contributions AW2 and AW3 completely cancel one another out at the point in time t=0 . Since the expected change in the first mirror 23 also does not yet make a contribution to the change in the wavefront parameter at the point in time t=0 (AW1=O at the point in time t=0 ) , the wavefront parameter W assumes its optimum value at the point in time t=0 . The influence of the two contributions AW1 and AW2 subsequently until a point in time t* has the ef fect that the wavefront parameter moves away from its optimum value , wherein it approaches the optimum value again between the points in time t* and t* * and reaches the optimum value at the point in time t* * . At the point in time t* the optical property thus assumes an extreme value , a minimum in the present case . In the present case , the planned period of use of the arrangement of optical elements 23-28 starts at the point in time t=0 . The minimum is assumed after approximately 30% of the planned period of use . By virtue of the wavefront parameter firstly moving away from the optimum value and subsequently approaching it again, the deviation AW of the wavefront parameter from the optimum value can be minimi zed overall for a longer period of time . A maintenance interval which after having elapsed necessitates a readj ustment or an exchange of optical elements can be lengthened in this way . What can be achieved in the best case is that maintenance is no longer required within the planned period of use .

Claims

Patent Claims1. Method for improving an imaging quality of an arrangement of optical elements (23-28) , in particular for use in an apparatus pertaining to semiconductor technology (20) , wherein a first optical element of the arrangement of optical elements (23-28) is subject to an expected change in the course of the time profile, wherein the expected change is suitable for influencing an optical property (W) of the arrangement, characterized by the following steps:- ascertaining an expected time profile of the expected change;- equipping at least one optical element of the arrangement of optical elements (23-28) with a compensation region (51a, 60b, 62a) subject to an intentional change in the course of the time profile, wherein the time profile of the intentional change and the expected time profile of the expected change have opposite effects on the optical property (W) of the arrangement .

2. Method according to Claim 1, characterized in that the first optical element of the arrangement of optical elements (23-28) is equipped with a compensation region (51a) .

3. Method according to either of Claims 1 and 2, characterized in that at least one of the optical elements of the arrangement of optical elements (23-28) which is different from the first optical element is equipped with a compensation region (60b, 62a) .

4. Method according to any of Claims 1 to 3, characterized in that a hypothetical change in the optical property of thearrangement over time is determined from the expected change in the first optical element of the arrangement of optical elements (23-28) , wherein the hypothetical change in the optical property over time is used to determine requirements in respect of the intentional change in the compensation region (51a, 60b, 62a) .

5. Method according to any of Claims 1 to 4, in which the first optical element of the arrangement of optical elements (23-28) has a reflective optical surface (53) , wherein the expected change is suitable for bringing about a change in shape of the reflective optical surface (53) .

6. Method according to any of Claims 1 to 5, characterized in that the optical element (23, 24) equipped with the compensation region (51a, 60b, 62a) has a substrate (50, 60) and a coating system (51, 61) applied thereto, wherein at least one portion of the coating system (51, 61) forms a reflective optical surface (53, 63) , wherein the intentional change in the compensation region (51a, 60b, 62a) comprises a change in shape of the compensation region (51a, 60b, 62a) , which influences a shape of the reflective optical surface ( 53 , 63 ) .

7. Method according to Claim 6, characterized in that the compensation region (60b) is produced within the substrate (60) , wherein the production of the compensation region (60b) preferably comprises an irradiation of the substrate using a particle beam and / or an irradiation of the substrate using electromagnetic radiation and / or a heating of the substrate or of a partial region of the substrate.

8. Method according to Claim 6 or 7, characterized in that the compensation region (51a, 62a) is produced within the coating system.

9. Method according to any of Claims 1 to 8, characterized in that the imaging quality of the arrangement of optical elements (23-28) is improved by an arrangement being newly created which contains the at least one optical element of the arrangement of optical elements (23-28) that is equipped with the compensation region (51a, 60b, 62a) .

10. Method according to any of Claims 1 to 9, characterized in that the imaging quality of the arrangement of optical elements (23-28) is improved by virtue of the fact that the at least one optical element of the arrangement of optical elements (23-28) that is equipped with the compensation region (51a, 60b, 62a) replaces an optical element (23, 24) already present in the arrangement.

11. Method according to any of Claims 1 to 10, characterized in that at least one of the optical elements (24) is equipped with an allowance which at least partly compensates for a static change in the optical element (24) produced in the course of producing the compensation region.

12. Method according to any of Claims 1 to 11, characterized in that the optical property (W) of the arrangement is subject to a temporal drift despite compensation region (51a, 60b, 62a) , wherein at least one of the optical elements (24) is equipped with an allowance which shifts the optical property (W) of the arrangement in such a way that said optical property has a sub-optimum value at the beginning of a planned period of use, wherein the optical property (W) passes through an optimum value of the optical property (W) proceeding from the sub-optimum value during the planned period of use on account of the temporal drift.

13. Method according to Claim 11 or 12, characterized in that the allowance is produced by an irradiation of the opticalelement (23, 24) using a particle beam and / or by an irradiation of the optical element (23, 24) using electromagnetic radiation and / or by a targeted heating of at least one partial region of the optical element (23, 24) and / or by applying an ablating treatment to an optical surface of the at least one optical element (23, 24) .

14. Method according to any of Claims 1 to 13, characterized in that the at least one compensation region (51a, 60b, 62a) is coordinated with the expected change in such a way that the optical property (W) assumes an extreme value within a planned period of use, at which extreme value a rate of change of the optical property reverses its sign, wherein proceeding from a planned start of use of the arrangement of optical elements (23-28) the extreme value is preferably assumed after a time has elapsed which is between 20% and 80%, more preferably between 30% and 70%, of the planned period of use.

15. Arrangement of optical elements (23-28) , in particular for use in an apparatus pertaining to semiconductor technology (20) , comprising a first optical element (23) subject to an expected change in the course of the time profile, wherein the expected change is suitable for influencing an optical property of the arrangement, characterized in that one of the optical elements (23, 24) has a compensation region (51a, 60b, 62a) subject to an intentional change in the course of the time profile, wherein the time profile of the intentional change and the expected time profile of the expected change have opposite effects on the optical property of the arrangement.

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