Optical system, and method for operating an optical system

Abstract

The invention relates to an optical system and to a method for operating an optical system, in particular in a microlithographic projection exposure apparatus. An optical system according to the invention has at least one optical element and a heating device for heating the optical element, the heating device comprising a plurality of heating sections to which an electric current can be applied in order to generate heat, wherein the continuous thermally induced deformation profile of the optically effective surface having a deformation amplitude of at least 1 lambda can be adjusted by means of the heating section such that the integral of the Fourier decomposition over at least one decimal spatial wavelength range is less than 10 m lambda.

Description

[0001] This application claims priority to German Patent Application DE 10 2022 211 636.4, filed on November 4, 2022. The content of the DE application is incorporated herein by reference. Field of the Invention

[0002] The present invention relates to an optical system and a method for operating an optical system, in particular in a microlithography projection exposure apparatus. Background Art

[0003] Microlithography is used for the production of microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which comprises an illumination device and a projection lens. An image of a mask (= mask blank) illuminated by the illumination device is in this case projected onto a substrate (e.g., a silicon wafer) by means of the projection lens, which substrate is coated with a photosensitive layer (photoresist) and is arranged in the image plane of the projection lens in order to transfer the mask structure onto the photosensitive coating on the substrate.

[0004] In projection lenses designed for the EUV range, i.e., at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components of the imaging process due to the lack of availability of suitable transmissive refractive materials.

[0005] A problem that arises in practice is that, in particular as a result of the absorption of radiation emitted as an EUV light source, EUV mirrors become hot and undergo associated thermal expansion or surface deformation, which in turn can lead to impairment of the imaging characteristics of the optical system. Various methods are known for avoiding such surface deformation and associated optical aberrations.

[0006] In particular, it is known to use materials with ultra-low thermal expansion ("ultra-low expansion materials"), such as titanium silicate glass sold by Corning Inc. under the name ULE TM as the mirror substrate material, and to set a so-called zero-crossing temperature in the region near the optically effective surface. At this zero-crossing temperature (which is, for example, about TM for ULE ), the coefficient of thermal expansion has a zero-crossing in its temperature dependence, near which no thermal expansion or only negligible thermal expansion of the mirror substrate material occurs. Another method for avoiding surface deformation caused by heat input into the EUV mirror includes active direct cooling through cooling channels formed in the mirror substrate and through which a cooling fluid can flow.

[0007] Another known method involves indirect heating using a heating device, such as based on infrared radiation or having a resistive heating element to which an electric current can be applied. With such a heating device, active mirror heating can be achieved during a phase of relatively low absorption of the radiation used in EUV, and the active mirror heating decreases correspondingly as the absorption of the radiation used in EUV increases.

[0008] In fact, a further problem occurs: during operation of a microlithography projection exposure apparatus, the EUV mirror is also exposed from a spatial perspective to a varying intensity of incident electromagnetic radiation, for example due to the use of an illumination setting having a varying intensity on the optically effective surface of the respective EUV mirror.

[0009] Considering that such scenarios are a demanding challenge in practice, for example in the case of radiation-based heating, thermalization is only possible with a relatively very limited spatial resolution and may additionally lead to an undesired coupling of stray light into the respective optical system. In contrast, the use of resistive heating enables a higher spatial resolution, but when heating individual sectors, the finally set deformation profile has undesired steps through thermalization, which impair the compensation or correction action of the finally set and thus the optical properties of the respective optical system.

[0010] Regarding the prior art, reference is made by way of example only to WO 2018 / 177649 A1 and DE 10 2017 207862 A1. Summary of the Invention

[0011] It is an object of the present invention to provide an optical system and a method for operating an optical system, in particular in a microlithography projection exposure apparatus, which can effectively avoid surface deformations and associated optical aberrations caused by heat input into the optical elements.

[0012] This object is achieved by an optical system and a method having the features according to the appended independent claims.

[0013] An optical system according to the invention, in particular in a microlithography projection exposure apparatus, having a predefined operating wavelength λ, comprises:

[0014] - at least one optical element having an optically effective surface; and

[0015] - a heating device for heating the optical element, wherein the heating device has a plurality of heating segments to which an electric current can be applied to generate heat;

[0016] - wherein the heating segments are capable of being used to set a continuous thermally induced deformation profile of the optically effective surface having a deformation amplitude of at least 1 λ such that the integral of the Fourier analysis in at least one decimal spatial wavelength range is less than 10 mλ.

[0017] The generation of the heating curve according to the present invention can be achieved, in particular, to avoid the deformation of the optically effective surface of the optical element that ultimately occurs during the operation of the optical system. However, in another application, the generation of the heating curve can also be achieved for the purpose of active manipulation, i.e., the targeted deformation of the optically effective surface in the context of actuation (e.g., for correcting aberrations introduced elsewhere in the optical system).

[0018] The present invention is particularly based on the concept that starting from the use of an electric heating device, a relatively smoother gradient is achieved in the finally set heating curve by avoiding undesired sharp transitions or edges between different heating zones. In this case, as described below based on various embodiments, the achievement of such a relatively smooth gradient can be realized in different ways.

[0019] Here, according to the present invention, the integral of the Fourier analysis of the set deformation profile is used as a measure of the waviness of the deformation profile.

[0020] For example, in the spatial wavelength range from 0.1 mm to 1 mm, this can be a measure of the stray light induced in the system, which has an adverse effect on the performance of the optical system. According to the present invention, a deformation profile with a significant stroke of at least 1 λ is settable, where the integral of the Fourier analysis of the deformation profile set in the decimal spatial wavelength range (e.g., from 0.1 mm to 1 mm) is less than 10 mλ.

[0021] In an embodiment of the present invention, the heating device is designed such that at least two heating zones generated by different heating segments have a sufficiently large overlap. Here, the present invention includes the concept of preventing the problem of forming an undesired stepped profile described in the introduction by generating heating zones that significantly overlap each other in certain parts by using the heating device according to the present invention. In this case, as described below based on various exemplary embodiments, the achievement of such sufficient overlap can again be realized in different ways.

[0022] Compared with radiation-based heating (e.g., by IR emitters), implementing the heating curve in the optical element based on the heating segments to which current can be applied is advantageous in many aspects according to the present invention. On the one hand, according to the present invention, the stray light effect usually associated with radiation-based heating systems is avoided. In addition, regarding the settable heating curve, a significantly higher spatial resolution can be achieved compared with radiation-based heating systems. Contrary to radiation-based heating, the direct coupling provided according to the present invention between the individual heating segments and the optical element to be heated also makes subsequent control for the case of position changes (e.g., tilting) of the optical element under discussion redundant.

[0023] Another advantage is that the heating section according to the invention can also be used simultaneously as a temperature sensor, with the result that the thermal state of the optical element in question can be closed-loop controlled, thereby detecting and influencing the temperature distribution or the thermally induced deformation occurring in the optical element in a targeted manner without additional temperature measurement design work. In particular, changes in the illumination settings implemented during the operation of the optical system can also be quickly recognized and form the basis for a dynamic adaptation of the set of heating curves according to the invention.

[0024] According to one embodiment, a continuous thermally induced deformation profile with a deformation amplitude of at least 1 λ can be set such that the integral of the Fourier analysis over at least one decade of spatial wavelength ranges is less than 5 mλ, in particular less than 3 mλ.

[0025] According to one embodiment, the at least one decade of spatial wavelength ranges includes spatial wavelengths of less than 100 μm.

[0026] According to one embodiment, the at least one decade of spatial wavelength ranges includes spatial wavelengths from 100 μm to 1 mm.

[0027] According to one embodiment, at least one decade of spatial wavelength ranges includes spatial wavelengths from 1 mm to 10 mm.

[0028] According to one embodiment, the operating wavelength is less than 250 nm, in particular less than 200 nm.

[0029] According to one embodiment, the operating wavelength is less than 30 nm, in particular less than 15 nm.

[0030] According to one embodiment, at least two heating zones generated by different heating sections partially overlap each other.

[0031] According to one embodiment, the heating sections are arranged in at least two planes, which are different from each other and at different distances from the optically effective surface.

[0032] According to one embodiment, the heating sections are joined to each other in some parts. The heating sections can in particular together form an arrangement that is staggered at least in some regions.

[0033] According to one embodiment, the heating sections are in the form of electrical conductor tracks, which form a branched arrangement and / or vary in their width, their relative distance from each other or their material in order to obtain a locally variable heating capacity.

[0034] According to one embodiment, the heating sections are configured as layers or layer segments.

[0035] According to one embodiment, the heating sections can be selectively actuated independently of each other for variable setting of different thermally induced deformation profiles in the optical element.

[0036] According to one embodiment, such selective actuation of the heating sections comprises transmitting actuation signals of different frequencies to different heating sections via a common lead.

[0037] According to one embodiment, the optical system comprises control means for changing the thermally induced deformation profile generated by the heating means in the optical element according to an illumination setting set in the optical system.

[0038] According to one embodiment, the optical element is a mirror.

[0039] The invention also relates to a method for operating an optical system, in particular an optical system in a microlithographic projection exposure apparatus, wherein the optical system has a predefined operating wavelength λ and comprises at least one optical element having an optically effective surface and heating means for heating the optical element, the heating means having a plurality of heating sections to which an electric current can be applied to generate heat, wherein the heating sections are used to set a continuous thermally induced deformation profile of the optically effective surface having a deformation amplitude of at least 1 λ such that the integral of the Fourier analysis is less than 10 mλ in at least one decimal spatial wavelength range.

[0040] According to one embodiment, the thermally induced deformation profile set by the heating sections varies according to the illumination setting set in the optical system.

[0041] According to one embodiment, the continuous thermally induced deformation profile is set such that the deformation of the optical element is at least partially compensated for, the deformation being associated with the application of electromagnetic radiation to the optical element during operation of the optical system.

[0042] According to one embodiment, the continuous thermally induced deformation profile is set such that the optical aberrations occurring during operation of the optical system are at least partially compensated for.

[0043] The present invention also relates to a method for operating an optical system, wherein the optical system comprises an optical element and a heating device for heating the optical element, the heating device having a plurality of heating segments to which an electric current can be applied to generate heat, and wherein a heating curve provided by these heating segments is selected according to the illumination setting used in the optical system. Here, the present disclosure includes the concept of avoiding or at least reducing thermally induced deformation of an optical element (such as a mirror) in the optical system by providing a heating curve in the optical element that is adapted to the illumination setting currently used during operation of the optical system. For example, if the illumination setting is a dipole setting with horizontally arranged illumination poles, then the heating device according to the invention can be used to generate a heating curve on the optically effective surface of the optical element or mirror, which heating curve is complementary to this illumination setting or to the temperature distribution thereby generated in the optical element, so as to thereby obtain as locally uniform a temperature distribution as possible in the optical element and thus effectively avoid thermally induced deformation.

[0044] For further preferred refinements and advantages of the method, reference is made to the above statements relating to the heating device according to the invention.

[0045] Further refinements of the invention can be obtained from the description and the dependent claims.

[0046] The invention will be explained in more detail below on the basis of exemplary embodiments shown in the drawings. Description of the Drawings

[0047] In the drawings:

[0048] Figure 1 A schematic diagram showing a possible structure of a microlithographic projection exposure apparatus designed for operation in EUV;

[0049] Figure 2 a - 2b show schematic diagrams for explaining possible embodiments of a heating device according to the invention having branched conductor tracks;

[0050] Figures 3-4 A schematic diagram showing a possible savings situation according to the invention for explaining cable leads;

[0051] Figure 5 a - 7 show schematic diagrams for explaining possible embodiments of a heating device according to the invention, wherein a dielectric layer with a relatively low electrical conductivity is used;

[0052] Figure 8 a - 8b show schematic diagrams for explaining possible embodiments of a heating device according to the invention based on energy - based inductive feeding;

[0053] Figures 9-10bShows a schematic diagram for explaining a possible implementation of a heating device with overlapping heating zones according to the present invention;

[0054] Figure 11 Shows a schematic diagram for explaining a possible minimization of the number of channels or leads in a heating device according to the present invention;

[0055] Figure 12 Shows a schematic diagram of another embodiment of a heating device according to the present invention;

[0056] Figures 13a-13d Shows a schematic diagram for explaining possible additional temperature measurements in the case of a heating device according to the present invention;

[0057] Figure 14 Shows a graph for illustrating Fourier analysis in a logarithmic plot of the spatial wavelength axis of an exemplary surface structure;

[0058] Figure 15 a - 18b shows graphs of different temperature curves generated by individual heating segments and their superposition; and

[0059] Figure 19 Shows a graph for illustrating the influence of lateral heat conduction on the overlap of temperature curves of adjacent heating segments. Detailed Description

[0060] Figure 1 Schematically shows in a meridional section a possible structure of a microlithographic projection exposure apparatus designed for operation in EUV. The present invention is not limited to use in a projection exposure apparatus designed for operation in EUV. In particular, the present invention can also be advantageously used in a projection exposure apparatus designed for operation in DUV (i.e., at wavelengths less than 250 nm, especially less than 200 nm) or also in another optical system.

[0061] According to Figure 1 , the projection exposure apparatus 101 includes an illumination device 102 and a projection lens 110. In addition to a light source or radiation source 103, one embodiment of the illumination device 102 of the projection exposure apparatus 101 also has an illumination optical unit 104 for illuminating the object field 105 in the object plane 106. In an alternative embodiment, the light source 103 can also be provided as a module separate from the rest of the illumination device. In this case, the illumination device does not include the light source 103.

[0062] Exposed herein is a mask blank 107 arranged in the object field 105. The mask blank 107 is held by a mask blank holder 108. The mask blank holder 108 can be displaced, especially in the scanning direction, by a mask blank displacement drive 109. For purposes of explanation, Figure 1A Cartesian xyz coordinate system is depicted. The x-direction extends into the drawing perpendicular to the plane of the drawing. The y-direction extends horizontally and the z-direction extends vertically. The scanning direction extends along the Figure 1 y-direction in

[0063] The projection lens 110 serves to image the object field 105 into the image field 111 in the image plane 112. The structure on the mask blank 107 is imaged onto the photosensitive layer of the wafer 113 in the region of the image field 111 arranged in the image plane 112. The wafer 113 is held by a wafer holder 114. The wafer holder 114 can be displaced, in particular in the y-direction, by a wafer displacement drive 115. The displacement of the mask blank 107 by the mask blank displacement drive 109 and, secondly, the displacement of the wafer 113 by the wafer displacement drive 115 can be synchronized with one another.

[0064] The radiation source 103 is an EUV radiation source. The radiation source 103 emits, in particular, EUV radiation, which is also referred to hereinafter as the radiation used or illumination radiation. In particular, the radiation used has a wavelength in the range between 5 nm and 30 nm. The radiation source 103 can be, for example, a plasma source, a synchrotron-based radiation source or a free electron laser (FEL). The illumination radiation 116 emitted from the radiation source 103 is focused by the condenser 117 and propagated through an intermediate focus in the intermediate focal plane 118 into the illumination optical unit 104. The illumination optical unit 104 comprises a deflecting mirror 119 and a first faceted mirror 120 (with a facet 121 indicated schematically) and a second faceted mirror 122 (with a facet 123 indicated schematically) arranged downstream of it in the beam path.

[0065] The projection lens 110 comprises a plurality of mirrors Mi (i = 1, 2, …), which are numbered consecutively according to their arrangement in the beam path of the projection exposure apparatus 101. In Figure 1 the example shown, the projection lens 110 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or a different number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a through-opening for the illumination radiation 116. The projection lens 110 is a double-masking optical unit. The projection lens 110 has an image-side numerical aperture greater than 0.5, and it can also be greater than 0.6 and can be, for example, 0.7 or 0.75.

[0066] During the operation of the microlithographic projection exposure apparatus 101, the electromagnetic radiation incident on the optically effective surfaces of the mirrors is partially absorbed and, as explained in the introduction, causes heating and associated thermal expansion or deformation, which in turn can lead to impairment of the imaging properties of the optical system.

[0067] The concept for heating a mirror according to the invention can be particularly advantageously applied to any desired mirror of a microlithographic projection exposure apparatus 101 from Figure 1 . This can be achieved to avoid or compensate for thermally induced deformations of the mirror in question (e.g., compensating for the spatial distribution of the zero-crossing temperature), or to provide additional degrees of freedom in setting the wavefront characteristics of the entire optical system, that is, with or without the correction action implemented by the mirror in question.

[0068] The invention then particularly includes the following concept: starting from the use of an electrical heating device for heating a mirror, a relatively smoother gradient is achieved in the finally set heating curve by avoiding undesired sharp transitions or edges between different heating zones. To this end, embodiments for achieving such a relatively smooth gradient in different ways are described below.

[0069] The heating device has a plurality of heating segments to which an electric current can be applied to generate heat. According to the quantitative criterion for the relatively smooth gradient set according to the invention, in the deformation profile of the optically effective surface finally set by the heating segments, the deformation profile can be set to have a continuous thermally induced deformation profile with a deformation amplitude of at least 1 λ, such that the integral of the Fourier analysis within at least one decimal spatial wavelength range is less than 10 mλ.

[0070] To illustrate an arbitrary surface structure, Figure 14 the Fourier analysis of the deformation profile is shown in a logarithmic diagram of the spatial wavelength axis, showing the spatial wavelength range from 0.001 mm to 10 mm. For an operating wavelength of λ = 13.5 nm, the above quantitative criterion means that for the integral of the Fourier analysis, for example, within the spatial wavelength range from 0.01 mm to 0.1 mm, a value less than 10 mλ is achieved, that is, a value less than 0.01 * 135 nm = 135 pm. Thus, the deformation profile has a relatively low waviness within this spatial wavelength range.

[0071] To achieve as smooth a temperature curve as possible, the shape of the temperature curves generated by the individual heating segments and their overlap are both relevant. Figure 15 a-15b, Figure 16 a-16b, Figure 17 a-17b and Figure 18 a-18b schematically show four different types of temperature curves (referred to as "temperature curve 1" to "temperature curve 4" in Figure 15 a-18a), and as an example, five temperature curves of the individual heating segments are referred to as "A" (corresponding to the temperature curve of heating segment A) to "E" (corresponding to the temperature curve of heating segment E) in Figure 15 b-18b. The (normalized) superposition of the heating profiles is calculated as 1*T A+2*T B +3*T C +2*T D +1*T E 。In the case of "Temperature Curve 1", a distinct waviness of the temperature superposition can be seen. In the case of "Temperature Curve 2", the waviness is reduced due to a greater overlap between the temperature curves of the individual heating sections "A" - "E". Due to a large enough overlap, the superimposed waves are no longer visible in "Temperature Curve 3". "Temperature Curve 4" corresponds to the superposition of a top-hat-shaped temperature curve and has a distinct stepped appearance, which can also be interpreted as waviness.

[0072] The overlap of the temperature curves of adjacent heating sections is promoted by lateral heat conduction present towards the respective adjacent heating sections. Figure 19 This is intended to be illustrated. If a temperature is applied at the center of an object, the thermal energy runs into the low-temperature regions, and as a result, these regions are heated and the temperature curve overall becomes wider. At time t 1 there is a surface temperature T 1 on the object, and at a later time t 2 there is a surface temperature T 2 and so on.

[0073] Figure 2 a shows a schematic illustration for explaining a possible improvement of a heating device according to the invention, and in Figure 2 b, a corresponding equivalent circuit diagram is shown to explain the function. According to Figure 2 a, the heating device according to the invention in a first embodiment includes electrically conductive tracks arranged in a branched manner, where in the present case - but the invention is not limited thereto - a spiral arrangement is formed. Specifically, here, the conductive tracks extending from the inside of the spiral first radially divide into two conductive tracks according to a spiral path and then into three conductive tracks, assuming a constant cross-section of the conductive tracks according to the Figure 2 equivalent circuit diagram of b, which is associated with the current flowing through the respective conductive tracks first decreasing to half and then to one-third. It is also assumed that the cross-section of the conductive tracks is constant, and the specific heat capacity per line length is proportional to the square of the current, with the result that the resulting heating capacity from the radial inside to the radial outside is reduced to one-third.

[0074] Then, the above improvement not only leads to a local change in the thermal distribution introduced into the respective optical element to be heated (such as an EUV mirror), but also has the advantage of avoiding an undesired stepped profile or a final thermally induced deformation profile in thermalization compared to a conventional sector heating with clearly demarcated heating zones, for example.

[0075] As described below, in order to smooth the heating or deformation profile, it is additionally or alternatively also possible to achieve a partial overlap of the heating zones generated by different heating segments. For this purpose, the corresponding heating segments can be present in different planes or in a staggered arrangement (formed in the same plane).

[0076] Figure 3 and Figure 4 shows a schematic illustration of an advantageous improvement in the actuation of different heating segments of a heating device according to the invention, wherein the associated wiring effort is limited in each case. According to this concept, the heating segments in question are selectively actuated by actuation signals of different frequencies, and according to Figure 3 , this actuation signal is achieved using band-stop filters B-1, B-2, … (having heating resistors H-1, H-2, …) assigned to the respective heating segments. Thus, actuation signals of different frequencies can be conducted via the same lead, since only the actuation signal having the frequency blocked by the first band-stop filter B-1 flows via the first heating resistor H-1, and only the actuation signal having the frequency blocked by the second band-stop filter B-2 flows via the heating resistor H-2, and so on. Thus, overall only two leads are required for the heating device, resulting in a significant reduction in the number of lines required.

[0077] In a further embodiment according to Figure 5 a-5b, when heating an optical element, the desired heating curve according to the invention can also be achieved using a dielectric layer having a relatively low electrical conductivity. According to Figure 5 a, such a dielectric layer is denoted by “510”, and electrodes denoted by “511”, “512”, “513”, … are used to apply a locally variably settable voltage to the dielectric layer for resistive heating. The finally set heating curve can additionally be predefined by appropriate structuring of the electrodes.

[0078] Figure 5 b schematically shows another exemplary embodiment, in which a dielectric layer 520 with a relatively low electrical conductivity is used in combination with a spiral conductor track, such that, as shown in the cross-section according to Figure 5 b and in the plan view according to Figure 6a , in each case the dielectric layer 520 enables a partial short circuit between the radially adjacent portions 531, 532, 533 of the spiral conductor track 530. In this case, in Figure 6a , the voltage source for generating the voltage applied to the spiral conductor track 530 is denoted by “540”. According to the equivalent circuit diagram shown in Figure 6b and also in the figure shown in Figure 6b , for the gradient of the heating capacity from the inside to the outside of the spiral, as via the dielectric layer 520 (which in Figures 6a-6bAs a result of partial current flow (represented by the ohmic resistor in the figure), the heating capacity locally introduced into the corresponding optical element decreases radially outward. Since the heating curve is affected by both the path of the conductor track and the dielectric layer, the number of degrees of freedom in designing the desired heating curve increases. The dielectric layer 520 can also be implemented in a structured manner to influence the heating curve.

[0079] Figure 7 The schematic diagram of another possible embodiment is shown. To heat an optical element in the form of a mirror having a mirror substrate 705 and a reflective layer system 740, a heating layer 720 having a relatively high specific resistivity is used here. The heating layer 720 is disposed between a first electrode layer 730 on its side facing the reflective layer stack 740 and a second electrode layer 710 on its side facing the mirror substrate 705. The electrode layers 710, 730 can be designed in a structured manner suitable for generating the desired heating curve. Contrary to the above embodiment, according to Figure 7 , the current flow for generating the heating curve occurs in the z direction with respect to the depicted coordinate system. The resistance of the heating layer 720 is preferably at least 100 times higher, more preferably at least 1000 times higher, and further preferably at least 10000 times higher than the resistance of the leads or the electrode layers 710, 730.

[0080] In another embodiment, locally selective or controllable heat input can also be achieved by an alternating magnetic field for inducing eddy currents in at least one conductive induction layer. Figure 8 Figures a - 8b show the schematic diagrams of the corresponding embodiments. In Figure 8 Figures a - 8b, the induction layers are represented by "812" and "822" respectively, the insulating layers are represented by "811" and "821" respectively, and the mirror substrates are represented by "810" and "820" respectively. In the illustrated embodiments (but the present invention is not limited thereto), coil arrays 813 and 823 are respectively used to provide a plurality of effective heat sources and generate the desired temperature distribution. In addition, the formation of eddy currents and thus the heat input can also be affected by the appropriate configuration of the conductive regions within the induction layers 812 and 822. When applying this principle to a mirror as shown in Figure 8 Figures a - 8b, the coil or coil array can be arranged at the rear side of the mirror (see Figure 8 Figure a) or within the layer structure of the mirror or near the optically effective surface of the mirror (see Figure 8 Figure b, where the optically effective surface itself is not shown).

[0081] In the above-described embodiments, the following principle is utilized, according to which heat is introduced into the conductive material of the induction layer due to eddy current losses occurring at corresponding positions of the current, which can be used to deform the optical element or its optically effective surface. Since the penetration depth of such eddy currents depends on the frequency of the alternating magnetic field, the position of the corresponding heat input or its distance from the optically effective surface can be controlled by appropriately selecting the frequency of the alternating magnetic field. Here, alternatively, a conductive induction layer of sufficient thickness or a plurality of discrete induction layers can be provided.

[0082] In the implementation of the generation of a spatially resolved heat input or a correspondingly suitable heating curve with local variations according to the invention, the following fact can also be utilized here: the spatial extent of the induction eddy currents parallel to the optically effective surface is also frequency-dependent, since in the case of relatively high frequencies, the eddy currents are concentrated more in the direction of the coil and thus a concentrated heat input is generated. Furthermore, the shape of the eddy current formation can be manipulated by the shape of the coil for generating the alternating magnetic field and also by using a suitable ferromagnetic material (such as iron or ferrite), which increases the magnetic flux density due to its high magnetic permeability. In addition, conductive regions can be provided purposefully in which eddy currents can be formed. When using ferroelectric materials, the saturation properties of the ferroelectric materials can also be utilized by applying a static magnetic field for field shaping or eddy current formation (in the sense of an iron magnetic induction layer).

[0083] Furthermore, for the heat input according to the invention, a ferromagnetic layer can also be used (utilizing the magnetic reversal losses occurring in ferromagnetic materials). In addition, as an alternative to a coil array, the scanning operation can also be implemented by using one or more displaceable coils. In this case, the magnetic field generated by a plurality of coils can also be used to saturate the ferromagnetic layer anywhere except at the field-free point or region, and this field-free region then generates heat during magnetic reversal, so that the scanning heat source can be realized by the displacement of this field-free region.

[0084] The coil for generating the magnetic field can be formed in any desired manner (for example, by a coating process or by winding a wire). In addition, the positions of both the coil and the conductive or induction layer can be optimized in a manner depending on the installation space for maximum effective heating or minimum influence on other components in the optical system.

[0085] In a further embodiment, the heating of the optical element or mirror according to the invention can be achieved by a heating wire or a conductor track, which can be realized by a coating and structuring process and, when applied to the mirror, can be arranged on the rear side of the mirror. In this case, the coating can be designed such that the conductor track has a constant heating per unit length. In addition, a local variation in the heating capacity can also be achieved by a change in the corresponding cross-section of the conductor track or by using different materials with different specific resistances.

[0086] Figure 9 A schematic diagram for explaining an embodiment is shown, in which, in order to smooth the heating or deformation profile, the heating sections are implemented in different planes or in multiple layers in the form of heating layers 910, 920, 930. In this case, the mirror substrate is denoted by "905" and the reflective layer system is denoted by "940". Due to the overlap between the heating curves generated in different planes, an effective reduction in the waviness caused by the superposition of the heating curves can be achieved, and thus a more uniform gradient of the resulting temperature curve can be achieved.

[0087] Figures 10a-10b A schematic diagram of a possible embodiment of a heating device according to the present invention is shown, in which - again for the purpose of partial overlap of the generated heating zones and associated smoothing of the heating or deformation profile - different heating sections are provided in a staggered or interlaced arrangement. In this case, the interlaced or staggered heating sections for generating overlapping heating zones are located in the same plane, and are denoted by "1011" and "1012" in Figure 10a and are denoted by "1021" and "1022" in Figure 10b

[0088] According to Figure 11 , in order to reduce the number of conductor tracks or channels and leads, the current path can also be controlled by diodes so as to switch on or off individual heating resistors in a targeted manner. According to Figure 11 , by way of example only, current flows through resistor R2 only when switches S1 and S4 are closed. By means of fast switching (with a typical switching time in the range of ms), different channels can be selected in order to achieve spatially selective heating. In this example, a total of nine heating filaments require six switches and leads. The circuit discussed is particularly advantageous in the case of a large number of heating resistors, for example, in the case of one hundred heating resistors, twenty switches and leads are sufficient. The heating resistors can be present at the intersection points of the corresponding conductor tracks. In another embodiment, the heating filaments can also be separated by a layer of relatively low electrical conductivity, and the flow of current occurs transversely to this layer.

[0089] According to Figure 12 , a conductive layer 1200 having external contacts for heating according to the present invention can also be used. Due to the ohmic resistance of the layer, the generation of a magnetic field provides additional possibilities for manipulating the current path, and thus additional possibilities for manipulating the corresponding location of the heat input.

[0090] ​The aforementioned arrangement of the heating wire on the rear side of the mirror is particularly advantageous in the range where parasitic deformation effects of (EUV) light incident during operation can be avoided. In addition, the mirror substrate can also be made of different mirror substrate materials in order to maintain the temperature in the region of the zero-crossing temperature on a part of the optically effective surface, preferably by means of cooling channels for example, and to set a target deformation on the rear side of the mirror by means of the heating wire.

[0091] In a further embodiment, the heating resistor according to the invention can additionally be used for temperature measurement, wherein the current temperature detected at the location in question can be used as the basis for a corresponding temperature control.

[0092] Figures 13a-13d A circuit of a possible embodiment for explaining the concept is shown. The temperature determination here is based on the fact that the ohmic resistance can be determined from the voltage drop across the respective heating resistor and the current flowing through the respective heating resistor, wherein the temperature dependence of this ohmic resistance is known and thus the average temperature of the heating resistor in question can be inferred. In addition, the product of the voltage drop across the heating resistor and the current flowing through the heating resistor corresponds to the heating capacity, which can therefore also be determined and used as the basis for control or regulation.

[0093] Figure 13a A possible embodiment is shown in which a heating resistor 1302 for heating an optical element or mirror 1301 operates together with a voltage source 1303, and wherein an ammeter 1304 measures the current flowing through the heating resistor 1302. It should be noted that the determination accuracy of the ohmic resistance and thus the determination accuracy of the temperature are impaired due to the fact that only the voltage drop across the entire circuit (i.e., the internal resistors of the wire, the heating resistor and the ammeter) is known and not the voltage drop across the heating resistor.

[0094] Figure 13b Another possible embodiment is shown in which a heating resistor 1312 for heating an optical element or mirror 1311 operates together with a current source 1313, and here the voltage is measured by a voltmeter 1314. Since, here, the voltage drop in the electrical circuit cannot be separated from the voltage drop across the heating resistor 1312, the determination accuracy of the ohmic resistance and thus the determination accuracy of the temperature are also impaired here.

[0095] To avoid or reduce the above-mentioned accuracy losses, according to Figure 13c (the heating resistor 1322 is similar to that through a current source 1323 Figure 13bOperation), the voltage can be tapped at two points in the region of the heating resistor 1322 through two additional voltage measurement lines 1324a, 1324b, and the voltage is measured using a voltmeter 1324. To achieve the highest possible measurement accuracy, the corresponding voltage taps are preferably implemented at the beginning and end of the conductor track forming the heating resistor 1322 under discussion (i.e., at the transition from the lead to the heating section).

[0096] Figure 13d Another possible embodiment is shown, in which, in order to achieve a higher spatial resolution of the temperature determination, a plurality of voltage taps are implemented within the conductor track forming the corresponding heating resistor 1332, with the result that a plurality of temperature measurement regions 1335a, 1335b, 1335c are achieved. In this case, the corresponding voltmeters for voltage measurement are denoted by "1334a", "1334b", and "1334c".

[0097] Based on the above Figures 13a through 13d The described temperature measurement can be carried out in parallel with the corresponding heating operation. As an alternative, the heating can also be interrupted during the duration of the corresponding temperature measurement. Here, then a defined current is applied to the corresponding heating resistor in order to determine the associated voltage drop.

[0098] The above embodiments can also be advantageously combined with the direct cooling of the optical element or mirror under discussion.

[0099] Although the present invention has been described based on specific embodiments, those skilled in the art can recognize many variations and alternative embodiments, for example, by combining and / or exchanging the features of the individual embodiments. Therefore, those skilled in the art can understand that these variations and alternative embodiments are also included in the present invention, and the scope of the present invention is limited only in the sense of the appended claims and their equivalents.

Claims

1. An optical system, in particular in a microlithography projection exposure apparatus, having a predefined operating wavelength λ, comprising: • at least one optical element having an optically effective surface; and • heating means for heating the optical element, wherein the heating means has a plurality of heating segments to which an electric current can be applied to generate heat; • wherein the heating segments can be used to set a continuous thermally induced deformation profile of the optically effective surface with a deformation amplitude of at least 1 λ such that the integral of the Fourier analysis over at least one decade of spatial wavelengths is less than 10 mλ.

2. The optical system according to claim 1, characterized in that the continuous thermally induced deformation profile having a deformation amplitude of at least 1 λ can be set in such a way that the integral of the Fourier analysis over at least one decade of spatial wavelengths is less than 5 mλ, in particular less than 3 mλ.

3. The optical system according to claim 1 or 2, characterized in that the at least one decade of spatial wavelengths includes spatial wavelengths less than 100 μm.

4. The optical system according to any one of claims 1 to 3, characterized in that the at least one decade of spatial wavelengths includes spatial wavelengths from 100 μm to 1 mm.

5. The optical system according to any one of the preceding claims, characterized in that the at least one decade of spatial wavelengths includes spatial wavelengths from 1 mm to 10 mm.

6. The optical system according to any one of the preceding claims, characterized in that the operating wavelength is less than 250 nm, in particular less than 200 nm.

7. The optical system according to any one of the preceding claims, characterized in that the operating wavelength is less than 30 nm, in particular less than 15 nm.

8. The optical system according to any one of the preceding claims, characterized in that at least two heating zones generated by different heating segments overlap each other partially.

9. The optical system according to any one of the preceding claims, characterized in that the heating segments are arranged in at least two planes which are different from each other and at different distances from the optically effective surface.

10. The optical system according to any one of the preceding claims, characterized in that the heating segments are joined to each other in some parts.

11. The optical system according to any one of claims 1 to 10, characterized in that the heating segments are in the form of electrical conductor tracks which, in order to obtain a locally variable heating capacity, form a branched arrangement and / or vary in their width, their relative distance from each other or their material.

12. The optical system according to any one of claims 1 to 10, characterized in that the heating segments are designed as layers or layer segments.

13. The optical system according to any one of the preceding claims, characterized in that the heating segments can be selectively actuated independently of each other to variably set different thermally induced deformation profiles in the optical element.

14. The optical system according to claim 13, characterized in that This selective actuation of the heating sections includes transmitting actuation signals of different frequencies to different heating sections via a common lead.

15. The optical system according to any one of the preceding claims, characterized in that the system includes control means for changing the thermally induced deformation profile generated in the optical element by the heating means in accordance with an illumination setting established in the optical system.

16. The optical system according to any one of the preceding claims, characterized in that the optical element is a mirror.

17. A method for operating an optical system, in particular an optical system in a microlithography projection exposure apparatus, wherein the optical system has a predefined operating wavelength λ and includes at least one optical element having an optically effective surface and a heating means for heating the optical element, the heating means having a plurality of heating sections to which an electric current can be applied to generate heat, wherein the heating sections are for setting a continuous thermally induced deformation profile of the optically effective surface, the profile having a deformation amplitude of at least 1 λ such that the integral of the Fourier analysis over at least one decade of spatial wavelengths is less than 10 mλ.

18. The method according to claim 17, characterized in that the thermally induced deformation profile set by the heating sections varies in accordance with an illumination setting established in the optical system.

19. The method according to claim 17 or 18, characterized in that the continuous thermally induced deformation profile is set such that at least partially compensates for the deformation of the optical element associated with the application of electromagnetic radiation to the optical element during operation of the optical system.

20. The method according to claim 17 or 18, characterized in that the continuous thermally induced deformation profile is set such that at least partially compensates for the optical aberrations occurring during operation of the optical system.

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