Mirror, optical system and method for operating an optical system

Abstract

This invention discloses a method for operating an optical system, a mirror, and an optical system, particularly for lithography exposure apparatuses. The mirror (particularly for lithography exposure apparatuses, wherein the mirror has an optically effective surface) comprises a mirror substrate (101, 201, 301, 401, 501) and a plurality of cavities (110, 210, 310, 410, 411, 510) disposed in the mirror substrate, and wherein each cavity is applicable to a fluid, wherein deformation is transmitted to the optically effective surface by changing the fluid pressure in the cavities (110, 210, 310, 410, 411, 510).

Description

[Technical Field]

[0001] This patent application claims priority to German Patent Application No. DE 10 2021 200 790.2, filed on January 28, 2021. The contents of that patent application are incorporated herein by reference.

[0002] This invention relates to a reflector, an optical system, and a method for operating the optical system, particularly for a photolithography exposure apparatus. [Previous Technology]

[0003] Lithography is used to produce microstructure components, such as integrated circuits or LCDs. The lithography process is performed in a known projection exposure apparatus, which has an illumination device and a projection lens. The image of a photomask (= magnified photomask) illuminated by the illumination device is projected onto a substrate (e.g., a silicon wafer), the substrate is coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens, and the photomask structure is transferred to the photosensitive coating of the substrate by the projection lens.

[0004] In projection lenses designed for the EUV range (i.e., at wavelengths of about 13 nm or about 7 nm), mirrors are used as optical components for imaging processing due to the lack of suitable available light-transmitting and refractive materials.

[0005] In practice, the problem is that due to the absorption of radiation emitted by the EUV light source and other reasons, the EUV mirror will heat up and undergo related thermal expansion or deformation, which will have a negative impact on the imaging characteristics of the optical system.

[0006] Several methods are known to be used to avoid surface deformation and associated optical aberrations caused by heat input into EUV mirrors. Among other things, it is known to use materials with ultra-low thermal expansion ("ultra-low expansion materials"), such as titanate glass sold by Corning Inc. under the name ULETM, as mirror substrate materials, and to set a so-called zero-point crossover temperature in the region near the optically effective surface. At the zero-point crossover temperature, for example for ULETM, the zero-point crossover temperature is approximately ϑ = 30°C, the coefficient of thermal expansion has a zero-point crossover in its temperature dependence, and no thermal expansion of the mirror substrate material occurs in its vicinity, or only negligible thermal expansion occurs.

[0007] Other methods for avoiding surface deformation caused by heat input in EUV mirrors include active direct cooling. However, ensuring sufficiently effective heat dissipation while maintaining high precision in the mirror's optics presents a very demanding challenge as the power of the light source increases.

[0008] In particular, a problem that arises in practice is that during the operation of an optical system or mirror, the cooling channels through which the cooling fluid flows may themselves provide a parasitic contribution to the deformation of the optically effective surface of the mirror. This contribution may first arise from the temperature gradient formed in the mirror substrate (and is particularly significant in cases where the mirror substrate material has low thermal conductivity), and ultimately lead to deformation of the optically effective surface (depending on the geometry of the cooling channels) through thermal expansion in the mirror substrate material. Furthermore, the mechanical pressure transmitted from the flowing cooling fluid through the cooling channel walls to the mirror substrate may also cause elastic expansion of the mirror substrate material, which provides a parasitic deformation benefit to the optically effective surface (depending on the geometry of the cooling channels). The aforementioned problems become increasingly severe with increasing source power, because in this case, the cooling power required to avoid thermal deformation and to be input to the individual mirrors increases simultaneously. [Summary of the Invention]

[0009] One object of the present invention is to provide a reflector, an optical system and a method for operating the optical system, particularly for a lithography exposure apparatus, which helps to effectively avoid thermal deformation and at least reduce the effects of the aforementioned problems.

[0010] This objective can be achieved according to the features of the alternative independent items. According to one embodiment, the present invention relates to a mirror, particularly for use in a lithography projection exposure apparatus, wherein the mirror has an optically effective surface and comprises - a mirror substrate; and - a plurality of cavities disposed in the mirror substrate and capable of supplying fluid to each cavity; - wherein deformation can be transferred to the optically effective surface by changing the fluid pressure in the cavity.

[0011] According to this configuration, the present invention incorporates the aforementioned benefits of using fluid pressure and, as a result, forces acting on the reflector substrate via individual channel walls and causing elastic deformation of the reflector substrate, ultimately achieving a targeted deformation of the optically effective surface. This is the ideal effect, thus providing additional degrees of freedom in setting the system wavefront generated by the optical system including the reflector, in order to provide an adaptable reflector. In this case, the present invention starts from the discovery that a suitable configuration of the cavities (which will be described in more detail below), particularly in terms of their size and their distance from the optically effective surface, allows for the production of deformation profiles that, firstly, still vary locally in a targeted manner by applying independent pressure to each cavity, but secondly, allow for the production of quasi-continuous deformation profiles even when adjacent cavities sufficiently "overlap" with each other in terms of their deformation benefits.

[0012] According to a specific example, at least a subset of the cavity has the same distance to the optically effective surface.

[0013] According to a specific example, a plurality of cavities have pairs of cavities stacked on top of each other in the direction of the optically effective surface. By applying different fluid pressures to the same pair of cavities, the component force acting along the optically effective surface can generate the effect of deformation of the optically effective surface.

[0014] According to this method, it is also possible (as an alternative or supplement to a suitable size for a single cavity) to achieve a quasi-continuous deformation profile (within the meaning of avoiding the locally strictly defined deformation effect of a single cavity in each case), wherein the invention also utilizes the principle based on the "bimetallic effect" in this case, i.e., deformation is generated due to the different expansions of two parts fixed to each other in each case.

[0015] According to a specific example, the fluid is a cooling fluid that flows through the cavity and is used to absorb the heat generated in the reflector substrate by electromagnetic radiation incident on the optically effective surface.

[0016] According to this method, the fluid used to provide the desired deformation of the optically effective surface in the adaptive mirror according to the invention also serves as a cooling fluid. However, the invention is not limited thereto, and therefore also includes specific examples without the additional (cooling) function of the associated fluid.

[0017] The present invention also relates to an optical system for a reflector having the aforementioned features. The optical system may in particular be a projection lens or illumination device of a lithography exposure apparatus.

[0018] According to another embodiment, the present invention also relates to a method for operating an optical system, wherein the optical system has at least one mirror having an optically effective surface and a mirror substrate, wherein at least one cooling channel is disposed in the mirror substrate; - wherein a cooling fluid having a variable cooling fluid temperature and a variable cooling fluid pressure flows through the cooling channel, the purpose of which is to absorb heat generated in the mirror substrate by electromagnetic radiation generated by a light source and incident on the optically effective surface; - wherein the cooling fluid temperature and cooling fluid pressure vary according to the power of the light source; and - wherein such variation causes a first parasitic effect on the deformation of the optically effective surface caused by the temperature gradient generated by the cooling fluid in the mirror substrate, and a second parasitic effect on the deformation of the optically effective surface caused by the mechanical pressure transmitted from the cooling fluid to the mirror substrate, to at least partially compensate for each other.

[0019] In particular, according to this configuration, the present invention is based on the concept that in an optical system including a mirror, the mirror is actively cooled by at least one cooling channel through which a cooling fluid flows, thereby avoiding or at least reducing the undesirable effects of the cooling channel, and in particular the geometry of the cooling channel, on the final optically effective surface of the mirror. This is achieved by appropriately changing the two parameters of "cooling fluid temperature" and "cooling fluid pressure", thereby "balancing" the aforementioned parasitic effects (i.e., firstly, the effect of the temperature gradient formed in the mirror substrate, and secondly, the effect of the mechanical pressure applied by the flowing cooling fluid through the cooling channel wall) according to the individual source power (i.e., the light source power).

[0020] Here, the present invention uses the following idea as a starting point: the thermally induced surface deformation of a mirror that is subjected to electromagnetic (e.g., EUV) radiation impact during operation and is actively cooled by cooling fluid flow through at least one cooling channel is ultimately determined by three parameters: "source power", "cooling fluid temperature" and "cooling fluid pressure". The minimization of the thermally induced surface error of the optically effective surface of the mirror and the wavefront aberration of the optical system can be obtained by different appropriate combinations of the parameters of source power, cooling fluid temperature and cooling fluid pressure (i.e., different "triads").

[0021] For example, if the increased source power requires a lower cooling fluid temperature, then a suitable additional adjustment of the cooling fluid pressure can be achieved so that the individual parasitic effects of surface deformation on the cooling fluid pressure and cooling fluid temperature are just balanced relative to each other, thus maintaining minimal thermal disturbance or deformation of the optically effective surface.

[0022] According to a specific example, the changes in cooling fluid temperature and cooling fluid pressure are at least partially based on a preliminary calibration, within which, for the purpose of generating a lookup table, individual combinations of light source power, cooling fluid temperature and cooling fluid pressure suitable for the compensation are determined.

[0023] In other words, the changes in cooling fluid temperature and cooling fluid pressure can be based on previously recorded characteristics, where a triplet of different parameter values ​​for light source power, cooling fluid temperature, and cooling fluid pressure provides a variable characteristic (e.g., as an RMS value) for specific individual residual interferences or surface errors. Then, if it is necessary to change the two parameters (e.g., light source power and cooling fluid temperature) during optical system or mirror operation, the value to be selected for the individual residual parameter (e.g., cooling fluid pressure) can be directly determined based on this characteristic, on the basis of the initial calibration, to set the surface error to a value close to zero.

[0024] According to another specific example, the determination is based at least in part on wavefront measurements and / or interferometric measurements of the mirror surface shape in the optical system.

[0025] According to another specific example, this determination of the cooling fluid temperature and cooling fluid pressure is at least partially based on simulation.

[0026] According to a specific example, the variations in cooling fluid temperature and cooling fluid pressure are based at least in part on measurements of the current wavefront characteristics performed during continuous operation of the optical system.

[0027] According to a specific example, the mirror is designed for operating wavelengths less than 30 nm, particularly less than 15 nm.

[0028] According to a specific example, the optical system is a projection lens or illumination device of a photolithography exposure device.

[0029] Furthermore, the present invention also relates to an optical system comprising: - at least one mirror having an optically effective surface and a mirror substrate, wherein at least one cooling channel is disposed in the mirror substrate, wherein a cooling fluid having a variable cooling fluid temperature and a variable cooling fluid pressure can flow through the cooling channel, the purpose of which is to absorb heat generated in the mirror substrate by electromagnetic radiation generated by a light source and incident on the optically effective surface; and - a device for changing the cooling fluid temperature and cooling fluid pressure according to the power of the light source, such that a first parasitic effect on the deformation of the optically effective surface caused by the temperature gradient generated by the cooling fluid in the mirror substrate, and a second parasitic effect on the deformation of the optically effective surface caused by the mechanical pressure transmitted from the cooling fluid to the mirror substrate, are at least partially mutually compensated.

[0030] According to a specific example, the device is configured to change the cooling fluid temperature and cooling fluid pressure based on a lookup table, which contains different combinations of individual values ​​of the light source power, cooling fluid temperature and cooling fluid pressure.

[0031] According to a specific example, the device is configured to change the cooling fluid temperature and cooling fluid pressure based on features obtained through simulation and / or measurement or calibration. These features specifically relate to individual composite deformations of the optically effective surface of the mirror with respect to different combinations of parameter values ​​for the light source power, cooling fluid temperature, and cooling fluid pressure. The optical system may in particular include a memory or storage device in which the recorded features (or feature maps) are stored.

[0032] Further configurations of the invention can be understood from the embodiments and appendices.

[0033] The invention will now be explained in more detail with reference to the exemplary specific examples illustrated in the accompanying drawings.

Implementation Method

[0035] Figure 6 first schematically shows a radial cross-section of a possible configuration of a lithography projection exposure apparatus designed for operation in EUV.

[0036] According to FIG. 6, the projection exposure apparatus 1 includes an illumination device 2 and a projection lens 10. A specific example of the illumination device 2 of the projection exposure apparatus 1 has a light source or radiation source 3 and an illumination optical unit 4 for illuminating the object field 5 within the object plane 6. In an alternative specific example, the light source 3 may also be configured as a module separate from the other illumination devices. In this case, the illumination device does not include the light source 3.

[0037] In this text, the magnifying mask 7, disposed in the object field 5, is exposed. The magnifying mask 7 is held by a magnifying mask holder 8, which can be displaced by a magnifying mask displacement driver 9, particularly along the scanning direction. For ease of explanation, a Cartesian xyz coordinate system is depicted in Figure 6. The x-direction is perpendicular to the drawing plane and enters into it, the y-direction is along the horizontal direction, and the z-direction is along the vertical direction. The scanning direction in Figure 6 is along the y-direction. The z-axis is perpendicular to the object plane 6.

[0038] The projection lens 10 is used to image the object field 5 within the image field 10 of the image plane 12. The structure on the magnifying mask 7 is imaged onto the photosensitive layer of the wafer 13, wherein the wafer is arranged within the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14, which can be moved by a wafer displacement driver 15, particularly along the y-direction. The movement of the magnifying mask 7 by the magnifying mask displacement driver 9 and the movement of the wafer 13 by the wafer displacement driver 15 can occur synchronously.

[0039] Radiation source 3 is an EUV radiation source. Radiation source 3 emits EUV radiation, which is also referred to below as operational radiation or illumination radiation. In particular, the operational radiation has a wavelength in the range of 5 nm to 30 nm. Radiation source 3 can be, for example, a plasma source, a synchrotron type radiation source, or a free electron laser (FEL). Illumination radiation 16 emitted from radiation source 3 is focused by light collector 17 and propagates through the intermediate focal point in intermediate focal plane 18 to illumination optical unit 4. Illumination optical unit 4 includes deflector 19; and a first facet mirror 20 (with schematically indicated small facet 21) and a second facet mirror 22 (with schematically indicated small facet 23) disposed downstream of the deflector in the beam path.

[0040] The projection lens 10 includes a plurality of mirrors Mi (i = 1, 2, ...), which are numbered according to their arrangement order in the optical path of the projection exposure apparatus 1. In the example shown in FIG. 6, the projection lens 10 includes six mirrors M1 to M6. Alternatives using four, eight, ten, twelve, or any other number of mirrors Mi are also possible. Each of the penultimate mirror M5 and the last mirror M6 has a through-hole for illumination radiation 16. The projection lens 10 is a double-shielded optical unit. The image-side numerical aperture of the projection lens 10 is greater than 0.5, and may also be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0041] During the operation of the lithography exposure apparatus 1, electromagnetic radiation incident on the optically effective surface of the mirror is partially absorbed, and as explained in the description, this results in heating and associated thermal expansion or deformation, which in turn leads to impaired imaging characteristics of the optical system. Therefore, the concept according to the invention can be particularly advantageously applied to any mirror of the lithography exposure apparatus 1 in FIG. 6.

[0042] The present invention is not limited to its application in projection exposure apparatuses designed for operation in the EUV range. In particular, the present invention can also be advantageously applied in projection exposure apparatuses designed for operation in the DUV range (i.e., at wavelengths less than 250 nm, especially less than 200 nm), or in different optical systems.

[0043] FIG1 is only a schematic diagram showing a possible exemplary specific example of a reflector 100 according to the present invention. The reflector 100 has a reflector substrate 101 (e.g., made of ULETM) and a reflective layer system (e.g., in the form of a molybdenum (Mo)-silicon (Si) multilayer stack), not shown in FIG1. ​​Within the reflector substrate 101 are a plurality of cavities 110 (in the exemplary specific example, substantially shaped as containers), again, the cavities are independent of each other, and in each case, fluid can be supplied to them via fluid inlets 110a and fluid outlets 110b that guide to the outer region of the reflector substrate 101. The size and individual distance of each cavity 110 or container are appropriately selected according to the size of the reflector, such that by applying fluid pressure individually to each cavity 110, firstly, sufficient spatial resolution variation can still be obtained on the surface profile of the reflector 100, but secondly, a continuous deformable profile can still be obtained, as a result of the configuration of the cavities 110 within the reflector substrate 101 being sufficiently "deep" from the optically effective surface.

[0044] In an exemplary specific example, the distance between each cavity 110 or container and the optically effective surface can be in the range of 2 mm to 100 mm, particularly 3 mm to 50 mm, and more particularly 5 mm to 20 mm. Furthermore, for example, the lateral dimension of the cavity 110 or container can be in the range of 5 mm to 150 mm. Also in a purely exemplary manner (and the invention is not limited thereto), the lateral dimension of the cavity 110 or container can be selected according to the mirror size, for example, in a manner such that approximately 80% of the lateral cross-sectional area of ​​the mirror 100 is "covered" by the cavity or container, thus the remaining 20% ​​of the lateral mirror area corresponds to the gap between the cavities 110 or containers.

[0045] The present invention does not impose further limitations on the geometry of the various cavities 110 or containers; however, in order to avoid undesirable mechanical stress peaks in the reflector substrate material, and from a manufacturing point of view, the circular structure illustrated in FIG1 in an exemplary manner is preferred.

[0046] In a specific embodiment of the invention, each cavity 110 or container may have the same distance to the optically effective surface in each case (such that the arrangement of the cavity 110 in the depth of the mirror substrate 101 follows the surface form or profile of the optically effective surface).

[0047] Preferably, the mirror 100 is manufactured in such a manner that the mirror substrate 101 is assembled from separate mirror substrate components, and the boundary surface of the cavity 110 formed in the finished mirror is then incorporated into these components.

[0048] The fluid applied to the cavity 110 or container in the adaptive mirror 100 according to the invention may be, in particular, a cooling fluid, as not limited thereto, wherein the individual fluid temperature may be set, for example, according to the source power in the optical system, to avoid or reduce undesirable thermal deformation of the mirror 100 due to the impact of electromagnetic radiation. However, the invention is not limited thereto. In this regard, FIG2 shows a specific example (very similar to FIG1 in other respects) in which the fluid applied to the respective cavities or containers 210 does not have an additional cooling function. Therefore, since it is not necessary to flow through the cavity 210, the respective cavities 210 or containers according to FIG2 only have a fluid inlet 210a (and no additional fluid outlet).

[0049] To simplify the manufacturing process, the fluid inlet 210a of cavity 210 according to FIG. 2 or the cooling fluid inlet and outlet 110a, 110b according to FIG. 1 can be configured at the same level or depth as the associated cavities 210 and 110, and for this purpose can be incorporated into the corresponding substrate components during manufacturing. However, in other specific examples, feeder configurations in different planes can also be achieved, as shown purely schematically in FIG. 3. Thus, in the example of FIG. 3 (which shows a mirror 300 with a mirror substrate 301 and a reflective layer system 302 in a very simplified manner), the central feeder 310a (each cavity has a fluid feeder 310a) in the configuration of the three cavities 310 is guided to the outside (i.e., into the outer region of the mirror substrate 301) at a greater depth or greater distance from the optically effective surface compared to the feeders 310a guiding adjacent cavities 310. In this case, by introducing fluid pressure in the feeder 310a, the potential undesirable effects on the deformed profile produced by supplying fluid to the cavity 310 are avoided, while accepting the increased manufacturing costs.

[0050] Figures 4a-4c show schematic diagrams illustrating the structure and functionality of an adaptable mirror 400 according to another specific embodiment of the invention. According to this specific embodiment, in each case, pairs of cavities 410, 411 stacked on top of each other in the direction of the optically effective surface are disposed within a mirror substrate 401, wherein the respective cavities 410, 411 can be subjected to impacts independently of each other (via fluid inlets not illustrated in any of Figures 4a-4c but configured in a manner similar to the aforementioned specific embodiment). To separate the cavities 410, 411 stacked on top of each other in each case, a plate made of, for example, a metallic material and having an exemplary thickness of approximately 1 mm can be disposed therebetween. In this case, the invention can particularly take advantage of the fact that, during operation of the optical system, effective cooling exists in the region of the plate when there is flow through the cavities 410, 411, thus selectively avoiding the use of materials with ultra-low thermal expansion, such as ULETM (which can also be used in principle).

[0051] As shown in Figure 4b, the impacts of the same pair of cavities 410 and 411, which belong to different fluid pressures, now also result in deformation of the optically effective surface, thus similar to the known bimetallic effect shown in Figure 4c. However, since this deformation, compared to the aforementioned specific examples based on Figures 1 to 3, is initially caused by a component force acting along the optically effective surface (corresponding to the different extensions of cavities 410 and 411 in the transverse or along the optically effective surface direction, as shown in Figure 4b), it can further assist in the formation of a basically ideal continuous deformation profile (within the meaning of avoiding the effect of strictly separated deformation of the individual cavities).

[0052] Regarding the cavity where fluid is applied and located within the reflector substrate, the present invention is not limited to the container geometry selected according to Figures 1 to 3 and Figures 4a to 4c. In particular, the cavity can also be constructed in the form of a channel, wherein multiple separate channel portions can again be formed to achieve the purpose of multiple regions, wherein fluid pressure can be applied to multiple regions independently of each other. In this regard, Figure 5 again shows only one exemplary configuration in which multiple separate channel portions are provided, in which each channel portion has a substantially meandering geometry and each is connected to a fluid inlet, and a fluid outlet is provided as a cavity 510 within the reflector substrate 501 in the reflector 500. Such geometry of the channel portions forming the cavity 510 may be advantageous, particularly in specific instances where the fluid used is a cooling fluid, to avoid introducing unwanted time-varying vibrations into the reflector 500 due to the flow (cooling) fluid.

[0053] According to another aspect of the invention, when the fluid is configured as a cooling fluid flowing through the reflector substrate of the reflector, firstly, the appropriate variation of the cooling fluid temperature and secondly, the cooling fluid pressure depends on the individual source power of the source that generates electromagnetic radiation incident on the reflector. Thus, firstly, a parasitic effect of a temperature gradient is formed within the reflector substrate, and secondly, the mechanical pressure exerted by the flowing cooling fluid through the cooling channel wall balances this effect. In other words, appropriate adjustment of the cooling fluid pressure can prevent changes in the cooling fluid temperature (e.g., necessary due to increased source power) from causing undesirable parasitic deformation due to the formation of a temperature gradient within the reflector substrate.

[0054] In this case, appropriate characteristics can be pre-recorded (by simulation and / or measurement or calibration), particularly in specific embodiments of the invention, for different combinations of parameter values ​​of source power, cooling fluid temperature, and cooling fluid pressure, to determine the individual synthetic perturbations or deformations of the optically effective surface in a particular mirror. Therefore, even at high source power, this effectively avoids thermally induced deformation because it effectively avoids the parasitic effects of the cooling channels through which the cooling fluid flows in each case, even when an increase in cooling power is introduced for individual mirrors.

[0055] Although the invention has been described using specific examples, those skilled in the art will be able to see various variations and alternative examples, such as by combining and / or substituting features of the various examples. Therefore, those skilled in the art will understand that the invention also covers these variations and alternative examples, and that the scope of the invention is limited only to the appended claims and their equivalents. [Simplified Explanation of the Diagram]

[0034] In the figures: Figure 1 shows a schematic diagram for explaining a possible structure of a mirror according to one specific embodiment of the present invention; Figures 2-3 show schematic diagrams for explaining a possible structure of a mirror according to another specific embodiment; Figures 4a-4c show schematic diagrams for explaining the structure and functionality of a mirror according to another specific embodiment; Figure 5 shows a schematic diagram for explaining a possible structure of a mirror according to another specific embodiment; Figure 6 shows a schematic diagram of a possible structure of a lithography projection exposure apparatus designed for operation in EUV.

Claims

1. A mirror comprising: an optically effective surface; a mirror substrate comprising a substrate material having a plurality of cavities disposed therein, each cavity being separated from the other cavities by the substrate material; and a plurality of fluid inlets and a plurality of fluid outlets; wherein each of the cavities includes a fluid conduit; each of the cavities has a fluid supplied therein; deformation is transmitted to the optically effective surface by changing the fluid pressure in the cavities; each of the plurality of cavities has a corresponding fluid inlet and a corresponding fluid outlet; and each of the fluid inlets and each of the fluid outlets is located in an outer periphery of the substrate.

2. The reflector as claimed in claim 1 further includes a cooling fluid in the cavities, wherein the cooling fluid flows through the cavities to absorb heat generated in the reflector substrate due to electromagnetic radiation incident on the optically effective surface during use of the reflector.

3. An optical system comprising a mirror as described in claim 1, wherein the optical system is a lithography optical system.

4. A method of using a photolithography exposure apparatus, wherein the photolithography exposure apparatus includes a projection lens and an illumination device, the method comprising: using the illumination device to illuminate a photomask in an object plane of the projection lens; and using the projection lens to image the illuminated photomask onto a photosensitive material in an image plane of the projection lens; wherein the photolithography exposure apparatus includes a mirror as described in claim 1.

5. A method for operating an optical system, wherein the optical system includes a mirror having an optically effective surface and a mirror substrate, wherein the mirror substrate has a cooling channel disposed therein, the method comprising: flowing a cooling fluid having a variable cooling fluid temperature and a variable cooling fluid pressure through the cooling channel to absorb heat generated in the mirror substrate due to electromagnetic radiation impacting the optically effective surface; varying the cooling fluid temperature and the cooling fluid pressure according to a power of a light source generating the electromagnetic radiation, such that a first parasitic effect and a second parasitic effect on the optically effective surface are at least partially mutually compensated; The first parasitic effect is caused by a temperature gradient generated by the cooling fluid in the cooling channel, and the second parasitic effect is caused by a mechanical pressure transmitted from the cooling fluid to the reflector substrate; the optical system is a projection lens of a lithography exposure apparatus; and the variation of the cooling fluid temperature and the cooling fluid pressure is at least partially based on a preliminary calibration, the determination of which includes generating a lookup table containing multiple combinations of values ​​of the power of the light source, the cooling fluid temperature, and the cooling fluid pressure suitable for at least partial compensation of the first and second parasitic effects.

6. The method as described in claim 5, wherein the determination includes multiple wavefront measurements using the projection lens.

7. The method as described in claim 5, wherein the determination includes using a simulation.

8. The method as described in claim 5 further includes measuring multiple wavefront characteristics of the projection lens during operation of the projection lens, wherein varying the cooling fluid temperature and the cooling fluid pressure includes using the wavefront characteristics measured with the projection lens.

9. The method as described in claim 5, wherein the electromagnetic radiation has a wavelength of less than 30 nm.

10. The mirror as claimed in claim 1, wherein the plurality of cavities comprises a plurality of pairs of cavities stacked on top of each other in one direction of the optically effective surface, such that a component force acting along the optically effective surface can be generated by applying different fluid pressures to the cavities of the same pair of cavities.

11. The method as described in claim 5, wherein the determination comprises multiple interferometric measurements using one facet of the mirror.

12. The mirror as claimed in claim 1, wherein each of the plurality of cavities is container-shaped.

13. The mirror as claimed in claim 1, wherein each of the plurality of cavities is defined by a wall formed of the substrate material.

14. The reflector as claimed in claim 1, wherein the substrate comprises a plurality of substrate portions that separate the cavities from each other, and each cavity is defined by a plurality of substrate portions of the plurality of substrate portions.

15. The reflector as claimed in claim 1, wherein for each cavity: the fluid conduit is located in an outer periphery of the substrate; and the cavity chamber is larger than the fluid conduit along a direction parallel to the outer periphery of the substrate.

Images