Method for operating an optical system, as well as optical system

Abstract

The invention relates to an optical system and a method for operating an optical system, particularly in a microlithographic projection exposure system, wherein the optical system comprises at least one mirror (100) with an optical surface (101) and a mirror substrate (110) made of a mirror substrate material, wherein at least one cooling channel (131-136) through which a cooling fluid with a variably adjustable cooling fluid temperature flows is provided in the mirror substrate. In a method according to the invention, the cooling fluid temperature is set based on an existing deviation between an actual value (ZCT2) of the mean zero-crossing temperature of the coefficient of thermal expansion of the mirror substrate material and a predetermined setpoint value (ZCT1) of this mean zero-crossing temperature.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The invention relates to a method for operating an optical system and an optical system, in particular in a microlithographic projection exposure system. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure system, which has an illumination device and a projection lens. The image of a mask (= reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0003] In projection lenses designed for the EUV range, ie at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of availability of suitable light-transmitting refractive materials.

[0004] A problem that occurs in practice is that the EUV mirrors experience heating and associated thermal expansion or deformation, partly due to absorption of the radiation emitted by the EUV light source, which in turn can impair the imaging properties of the optical system. Various approaches are known to prevent surface deformations and the associated optical aberrations caused by heat input into an EUV mirror.

[0005] Among other things, it is known to use a material with ultra-low thermal expansion ("ultra-low expansion material") as the mirror substrate material, e.g., a titanium quartz glass marketed under the name ULE™ by Corning Inc., and to set the so-called zero-crossing temperature in a region close to the optical effective surface. At this zero-crossing temperature, which for ULE™, for example, is approximately ϑ= 30°C, the thermal expansion coefficient exhibits a zero crossing in its temperature dependence, around which the sensitivity to temperature changes or local temperature variations is low.

[0006] Possible additional approaches to prevent surface deformations caused by heat input into an EUV mirror include active direct cooling or the use of a heating arrangement, e.g., based on infrared radiation. Such a heating arrangement can be used to actively heat the mirror during phases of comparatively low absorption of useful EUV radiation, with this active mirror heating being reduced accordingly as the absorption of the useful EUV radiation increases. Active mirror heating can be used, in particular, with the aim of keeping the average mirror temperature close to the aforementioned zero-crossing temperature.

[0007] In practice, however, further problems arise, including the fact that the actual zero-crossing temperature of the mirror substrate material used generally deviates from the originally desired or requested zero-crossing temperature due to the manufacturing process or only matches it within a manufacturing-related tolerance, which can ultimately result in a significant contribution to the thermally induced wavefront aberrations. A further problem is that, in addition to the absorption of the radiation emitted by the EUV light source, the mirror in question is also subject to thermal loads caused by components (e.g., actuators) or structures (e.g., support frames) located elsewhere in the optical system that may be inadequately cooled.

[0008] For the prior art, reference is made to WO 2022 / 161659 A1 and WO 2022 / 028710 A1 by way of example only.

[0009] It is an object of the present invention to provide a method for operating an optical system and an optical system which enable effective prevention of thermally induced deformations while at least mitigating the problems described above.

[0010] This problem is solved according to the features of the independent patent claims.

[0011] According to one aspect, the invention relates to a method for operating an optical system, wherein the optical system has at least one mirror with an optical effective surface and a mirror substrate made of a mirror substrate material, wherein at least one cooling channel through which a cooling fluid with a variably adjustable cooling fluid temperature can flow is provided in the mirror substrate.

[0012] The method is characterized in that the cooling fluid temperature is adjusted on the basis of an existing deviation between an actual value (ZCT2) of the mean zero-crossing temperature of the thermal expansion coefficient of the mirror substrate material and a predetermined target value (ZCT1) of this mean zero-crossing temperature.

[0013] The invention is based on the consideration that even in the case of approaches widely used in practice to use mirror substrate materials with ultra-low thermal expansion with the aim of operating the mirrors in question at the zero crossing temperature (ZCT = “Zero Crossing Temperature”), the actual (actual) value of this zero crossing temperature for the mirror substrate material used does not correspond to the actually expected or “assumed” (e.g. the one ordered from the manufacturer) target value of the zero crossing temperature, e.g. due to production-related tolerances or deviations, with the result that without suitable measures the performance of the associated optical system is significantly impaired.

[0014] Based on this consideration, the invention is now based in particular on the concept of transferring the mirror in question into a thermal state by suitably adjusting the cooling fluid temperature of the cooling fluid flowing through the at least one cooling channel, which is adapted to the mirror substrate material actually used or its actual zero-crossing temperature, in which, in other words, the existing deviation between the actual and desired value of the zero-crossing temperature of the mirror substrate material is at least partially compensated in its influence on the optical performance of the optical system.Ideally, as a result of the adjustment of the cooling fluid temperature according to the invention, the mirror in question is then in a temperature range which corresponds to the temperature band optimal for the actual zero-crossing temperature of the mirror substrate material used, with the result that thermally induced deformations or aberrations occurring in the mirror in question during operation of the optical system can again be minimized in the desired manner.

[0015] In particular, the present invention includes the principle of accepting the presence of manufacturing-related deviations in the zero-crossing temperature of the mirror substrate material as such, but at the same time actively utilizing the cooling fluid flowing in the mirror substrate material or its cooling fluid temperature in order to at least partially eliminate the undesirable effects of these deviations on the performance of the optical system.

[0016] According to one embodiment, the cooling fluid temperature is adjusted in such a way that a contribution to thermally induced wavefront aberrations during operation of the optical system caused by the deviation between the actual value (ZCT2) and the desired value (ZCT1) of the mean zero-crossing temperature of the thermal expansion coefficient of the mirror substrate material is at least partially compensated.

[0017] According to one embodiment, the cooling fluid temperature is adjusted taking into account a predetermined usage scenario of the optical system, in particular with regard to a reticle used, an illumination setting used and / or a source power used of a light source.

[0018] According to one embodiment, heating energy is additionally coupled into the mirror via a heating device. The combination of the inventive adjustment of the cooling fluid temperature with the known approach of actively heating the mirror by coupling heating energy from a heating device (which in turn can be realized, for example, via infrared radiation heaters or resistive heating elements that can be supplied with electrical current) has the advantage - again based on the generally unavoidable deviations between the actual and target values of the zero-crossing temperature of the mirror substrate material used - that the heating device is "relieved" of the burden of taking these deviations into account or compensating for them.Since the optimal thermal state of the mirror (so to speak as a "ground state"), taking into account the actual zero-crossing temperature of the mirror substrate material used, can already be set via the inventive adjustment of the cooling fluid temperature, only the respective "stroke" needs to be adjusted via the said heating device in order to achieve the effects desired by the active mirror heating (typically a spatial and / or temporal homogenization of the temperature profile of the mirror to avoid thermally induced aberrations).The heating power of the respective heating device required to achieve the desired effect during active mirror heating can therefore be significantly reduced, since, within the scope of the inventive concept, the problem of existing material deviations of the mirror substrate material with regard to the zero-crossing temperature is taken into account by adjusting the cooling fluid temperature.

[0019] According to one embodiment, the cooling fluid temperature is controlled during operation of the optical system. This control can be based, in particular, on the determination of a variable characteristic of the heat load acting on the mirror or a variable characteristic of the current heating state of the mirror, in particular based on measured values of the cooling fluid temperature or based on sensor values provided by at least one temperature sensor provided on the mirror. Furthermore, the control can be based on a feedforward model.

[0020] By also regulating the cooling fluid temperature during operation of the optical system in addition to the inventive adjustment of the cooling fluid temperature for adaptation to existing material deviations of the mirror substrate material with regard to the zero crossing temperature, it is possible to take into account in particular the fact that during operation of the associated optical system the exposure of the mirror in question to useful light (e.g. EUV radiation) is also accompanied by a heat input, which can be taken into account via the inventive adjustment or regulation of the cooling fluid temperature.

[0021] According to one embodiment, the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0022] According to one embodiment, the optical system is an illumination device or a projection lens of a microlithographic projection exposure apparatus.

[0023] The invention further relates to an optical system comprising at least one mirror with an optical effective surface and a mirror substrate, wherein at least one cooling channel through which a cooling fluid with a variably adjustable cooling fluid temperature can flow is provided in the mirror substrate. The optical system is characterized in that a device is provided for adjusting the cooling fluid temperature based on an existing deviation between an actual value (ZCT2) of the average zero-crossing temperature of the thermal expansion coefficient of the mirror substrate material and a predetermined target value (ZCT1) of this average zero-crossing temperature.

[0024] According to one embodiment, the optical system is designed to carry out a method with the features described above.

[0025] Further embodiments of the invention can be found in the description and the dependent claims.

[0026] The invention is explained in more detail below with reference to embodiments shown in the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] They show: Fig. 1 is a schematic diagram illustrating the possible structure of a mirror according to an embodiment of the invention; Fig. 2 is a diagram for explaining a principle underlying the present invention; Fig. 3 a schematic representation to illustrate the mode of operation of the invention; and Fig. 4 a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] In the following, exemplary embodiments of the invention are described with reference to the merely schematic representations or diagrams of Fig. 1-3. These embodiments have in common that, during operation of a mirror or an optical system comprising such a mirror, the cooling fluid temperature of a cooling fluid flowing through at least one cooling channel within the mirror substrate is adjusted to account for an existing, typically production-related deviation of the mean zero crossing temperature (ZCT) from the actually desired (e.g., the "ordered" from the manufacturer) target value.

[0029] Fig. 1 shows a purely schematic representation of a possible embodiment of a mirror 100 according to the invention. The mirror 100 has a mirror substrate 110 (e.g. made of ULE™) and a reflective layer system 120 (e.g. in the form of a molybdenum (Mo)-silicon (Si) multilayer stack). As also shown in Fig. As indicated in Figure 1, at least one cooling channel through which a cooling fluid (e.g., cooling water) with a variably adjustable cooling fluid temperature can flow is located in the mirror substrate 110. In the exemplary embodiment (although the invention is not limited thereto), a plurality of cooling channels 131-136 are provided. In further embodiments, only one cooling channel (e.g., a meandering one) can be provided. "140" denotes a device for adjusting the cooling fluid temperature in the at least one cooling channel. The cooling channel arrangement can be used, in particular in a manner known per se from the prior art, to counteract mirror heating caused by useful light (e.g., EUV radiation) impinging on the mirror 100 or its optical active surface designated "101."

[0030] According to the inventive concept already mentioned above, the cooling channel arrangement is now additionally used to, as shown in the diagram of Fig. 2, the mirror 100 is to be transferred into a thermal state which is adapted to the mirror substrate material used or its actual zero-crossing temperature ZCT2 - which, as already mentioned, generally deviates from the actually desired zero-crossing temperature or its setpoint ZCT1 due to manufacturing reasons.

[0031] In this case, Fig. 2 "A" denotes the range of average temperatures associated with the said target value ZCT1 of the zero-crossing temperature, which are established due to the absorption of the useful light (e.g. EUV radiation) incident on the optical effective surface 101. The Fig. 2 The discernible deviation between the said target value ZCT1 and the range A is due, among other things, to the fact that, depending on the illumination setting or intensity distribution of the effective radiation incident on the mirror 100, significant location-dependent variations in the heat load (e.g. EUV-related) on the mirror 100 may occur, whereby the zero-crossing temperature is then selected, for example, such that the thermal expansion is minimal in the range of the highest intensities or maximum occurring temperatures. This "pair" consisting of the range A of expected average temperatures and the value ZCT1 of the zero-crossing temperature is now, as already mentioned, shown in the diagram of Fig. 2 insofar as the actual (actual) value of the zero crossing temperature ZCT2 as well as the corresponding range “B” of average temperatures for which the zero crossing temperature ZCT2 would be optimal are at comparatively higher temperatures.

[0032] For this purpose, the mirror 100 or the mirror substrate 110 is transferred into the corresponding thermal state by means of the inventive adjustment of the cooling fluid temperature, which is given by the fact that the average temperatures arising due to incident useful radiation (e.g. EUV radiation) are in the range "B" (corresponding to the actual zero-crossing temperature ZCT2 of the mirror substrate material) and not in the range "A" (corresponding to the actually expected, but not actually given zero-crossing temperature ZCT1).

[0033] Fig. 3 shows a schematic representation to illustrate the mode of operation of the invention. For each of three exemplary quantities (“key parameters”) KP1-KP3, each representative of the optical properties or performance of the optical system having a mirror according to the invention, the left-hand bar represents the maximum across the tolerance band in the case without the inventive adaptation of the cooling fluid temperature to the actual zero-crossing temperature ZCT2 of the mirror substrate material, whereas the right-hand bar applies to the case of the inventive adaptation to the actual zero-crossing temperature ZCT2 of the mirror substrate material. In the example shown, the performance in the characteristic quantities KP1-KP3 is improved by a factor of approximately 2 to 3 in each case.

[0034] In the embodiment described above, the existing deviation between the target and actual value of the zero-crossing temperature of the mirror substrate material is taken into account by adjusting the cooling fluid temperature, i.e. the cooling fluid temperature is set on the basis of the previously determined deviation between the target and actual value of the zero-crossing temperature and then maintained at the corresponding value. However, the invention is not limited to such a one-time adjustment of the cooling fluid temperature. In further embodiments, during operation of the mirror 100 or the optical system comprising it, the cooling fluid temperature can also be regulated in addition to the mere adjustment, wherein in particular the heat input of the useful radiation (e.g. EUV radiation) incident on the mirror or its optical effective surface can be taken into account.In other words, the heat load occurring during operation of the optical system, which typically varies over time, can be accounted for by controlling the cooling fluid temperature. This control, in turn, can be based, for example, on measurement data from a temperature sensor determining the cooling fluid temperature at the cooling channel outlet, measurement data from temperature sensors in or on the mirror substrate or in the environment, based on a feedforward model for the temperatures in the mirror substrate and in the cooling fluid, or on measurement data concerning the optical aberrations occurring in the optical system (e.g., the wavefront provided by the optical system in a specific plane).

[0035] When adjusting the cooling fluid temperature according to the invention, in addition to the deviation between the actual and target value of the zero-crossing temperature, other factors or parameters of the respective current usage scenario, in particular the source power of the existing light source in the optical system, a currently set illumination setting and / or a reticle used in the optical system, can also be taken into account.

[0036] The adjustment or control of the cooling fluid temperature according to the invention can, but does not necessarily have to, be based on a specific value of the cooling fluid temperature at the cooling channel inlet (as a target value or control variable). In further embodiments, the adjustment or control can also be based on a different temperature, e.g., a temperature measured by a sensor on the mirror or in the mirror substrate or in the cooling fluid lines, an average temperature over a specific area (e.g., the optical effective area) or a specific volume, etc., whereby the implementation can be carried out in a simulation or using a thermal imaging camera.

[0037] In further embodiments, the inventive concept can also be implemented in a mirror that can be heated in a manner known per se via a heating device (e.g., radiation-based or with resistive heating elements), typically to account for or compensate for temporal and / or local variations in the heat load or the temperature distribution developing in the mirror. This configuration has the advantage that the corresponding heating device then only has to apply the desired "mirror heating stroke" for the aforementioned compensation of temporal and / or local variations in the heat load, whereas the inventive transition of the mirror to the thermal "ground state" according to the actual zero-crossing temperature ZCT2 of the mirror substrate material already takes place via the adjustment of the cooling fluid temperature.

[0038] In further embodiments, the adjustment or variation of the cooling fluid temperature within the scope of the invention can also serve to at least partially compensate for external heat sources. This makes it possible to take into account the fact that, in addition to the heat load caused by incident useful light (e.g., EUV radiation), the temperature of the mirror in question and thus also thermally induced deformations and resulting wavefront aberrations are influenced by other components in the optical system, e.g., actuators, heating devices of other mirrors, mounting elements, apertures, etc., which generally also increase the mirror temperature. This effect can also be at least partially compensated according to the invention by reducing the cooling fluid temperature.

[0039] Fig. Figure 4 initially shows schematically in meridional section the possible structure of a microlithographic projection exposure system designed for operation in the EUV.

[0040] According to Fig. 4, the projection exposure system 1 comprises an illumination device 2 and a projection lens 10. One embodiment of the illumination device 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a module separate from the other illumination device. In this case, the illumination device does not include the light source 3.

[0041] A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced via a reticle displacement drive 9, in particular in a scanning direction. Fig. For illustrative purposes, a Cartesian xyz coordinate system is shown in Figure 4. The x-direction is perpendicular to the plane of the drawing. The y-direction is horizontal, and the z-direction is vertical. The scanning direction is in Fig. 4 along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0042] The projection lens 10 is used to project the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is projected onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0043] Radiation source 3 is an EUV radiation source. Radiation source 3 emits, in particular, EUV radiation, which is also referred to below as useful radiation or illumination radiation. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). The illumination radiation 16 emanating from the radiation source 3 is bundled by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflection mirror 19 and, downstream of this in the beam path, a first facet mirror 20 (with schematically indicated facets 21) and a second facet mirror 22 (with schematically indicated facets 23).

[0044] The projection lens 10 has a plurality of mirrors Mi (i= 1, 2, ...), which are numbered according to their arrangement in the beam path of the projection exposure system 1. In the Fig. In the example shown in Figure 4, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection lens 10 is a doubly obscured optical system. The projection lens 10 has an image-side numerical aperture, which, purely by way of example (and without the invention being limited thereto), can be greater than 0.5, in particular greater than 0.6 or greater than 0.7.

[0045] During operation of the microlithographic projection exposure system 1, the electromagnetic radiation impinging on the optical effective surface of the mirrors is partially absorbed and, as explained above, leads to heating and a concomitant thermal expansion or deformation, which in turn can result in an impairment of the imaging properties of the optical system. The inventive concept can be particularly advantageously applied to any mirror of the microlithographic projection exposure system 1 of Fig. 4 may be applied.

[0046] The invention is not limited to application in a projection exposure system designed for operation in the EUV range. In particular, the invention can also be advantageously applied in a projection exposure system designed for operation in the DUV range (i.e., at wavelengths less than 250 nm, in particular less than 200 nm) or in another optical system.

[0047] Although the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments will become apparent to those skilled in the art, e.g., by combining and / or interchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are encompassed by the present invention, and the scope of the invention is limited only by the appended claims and their equivalents. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2022 / 161659 A1

[0008] WO 2022 / 028710 A1

[0008]

Claims

[1] Method for operating an optical system, wherein the optical system comprises at least one mirror (100) with an optical effective surface (101) and a mirror substrate (110) made of a mirror substrate material, wherein at least one cooling channel (131-136) through which a cooling fluid with a variably adjustable cooling fluid temperature can flow is provided in the mirror substrate (110), characterized by that the cooling fluid temperature is set on the basis of an existing deviation between an actual value (ZCT2) of the mean zero-crossing temperature of the thermal expansion coefficient of the mirror substrate material and a predetermined setpoint value (ZCT1) of this mean zero-crossing temperature. [2] Method according to claim 1, characterized bythat the cooling fluid temperature is adjusted in such a way that a contribution to wavefront aberrations thermally induced during operation of the optical system caused by the deviation is at least partially compensated. [3] Method according to claim 1 or 2, characterized by that the cooling fluid temperature is adjusted taking into account a given usage scenario of the optical system, in particular with regard to a reticle used, an illumination setting used and / or a source power used of a light source. [4] Method according to one of claims 1 to 3, characterized by that additional heating energy is coupled into the mirror via a heating device. [5] Method according to one of the preceding claims, characterized by that the cooling fluid temperature is regulated during operation of the optical system. [6] Method according to claim 5, characterized bythat this control is carried out on the basis of the determination of a variable characteristic of the heat load acting on the mirror or of the current heating state of the mirror, in particular on the basis of measured values ​​of the cooling fluid temperature or on the basis of sensor values ​​supplied by at least one temperature sensor provided on the mirror. [7] Method according to claim 5, characterized by that this regulation is based on a feedforward model. [8] Method according to one of the preceding claims, characterized by that the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm. [9] Method according to one of the preceding claims, characterized by that the optical system is an illumination device (2) or a projection lens (10) of a microlithographic projection exposure system (1). [10] An optical system comprising at least one mirror (100) with an optical effective surface (101) and a mirror substrate (110), wherein at least one cooling channel (131-136) through which a cooling fluid with a variably adjustable cooling fluid temperature can flow is provided in the mirror substrate (110), characterized by that a device (140) is provided for adjusting the cooling fluid temperature on the basis of an existing deviation between an actual value (ZCT2) of the mean zero-crossing temperature of the thermal expansion coefficient of the mirror substrate material and a predetermined target value (ZCT1) of this mean zero-crossing temperature. [11] Optical system according to claim 10, characterized by that it is designed to carry out a method according to one of claims 1 to 9.

Images