Method and apparatus for characterizing the heating state of a mirror with respect to the time difference from a thermal equilibrium state

Abstract

The invention relates to a method and a device for characterizing the heating state of a mirror with respect to the time difference from a thermal equilibrium state, wherein the mirror (100, 300, 400) comprises a mirror substrate (110, 310, 410) and an optical surface (101, 301, 401). In a method according to the invention, a temperature profile over time is determined using at least one temperature sensor (145, 345, 445, 446, 447) arranged remotely from the optical surface (101, 301, 401), and the required time interval until a thermal equilibrium state is reached is determined from the at least one temperature profile.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The invention relates to a method and a device for characterizing the heating state of a mirror with regard to the time interval from a thermal equilibrium state. 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 a concomitant thermal expansion or deformation away from the desired state, among other things due to absorption of the radiation emitted by the EUV light source, which in turn can result in an impairment of the imaging properties of the optical system.

[0005] Various approaches are known to prevent surface deformations and the associated optical aberrations caused by heat input into an EUV mirror. Among them is the use of 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 the setting of the so-called zero-crossing temperature in a region close to the effective optical 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, in the vicinity of which no or only negligible thermal expansion of the mirror substrate material occurs.

[0006] Possible further approaches to avoid 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. With such a heating arrangement, active mirror heating can take place in phases of comparatively low absorption of EUV useful radiation, whereby this active mirror heating is reduced accordingly with increasing absorption of the EUV useful radiation. The active mirror heating can in particular be carried out with the aim of keeping the average mirror temperature at the optical effective surface close to the above-mentioned zero-crossing temperature. The current heating state of the EUV mirror is typically determined with the aid of one or more temperature sensors attached to the EUV mirror close to the optical effective surface. In particular, for the control of the corresponding (e.g.radiation-based) heating device, temperature sensors are placed near the immediate influence zone of the heating device serving as a thermal manipulator.

[0007] A particular problem that arises in practice is that, as shown by investigations and simulations conducted by the inventors, after transport or maintenance work, the optical elements or mirrors for use in the projection exposure system remain in a thermal state for a relatively long period of time (typically several hours), which leads to significant optical aberrations and thus a considerable impairment of the performance of the projection exposure system. The reason for this is, in particular, the largely undefined thermal environment of the optical elements or mirrors in question during said maintenance or transport processes, which, through the imprinting of a three-dimensional temperature pattern, results in mechanical stresses and a concomitant deformation contribution, also on the optical effective surface.

[0008] A complicating factor is that the use of larger mirrors, for example with increasing numerical aperture, also requires increasingly large mirror thicknesses for reasons of stiffness, with the result that even when the mirror substrate is manufactured from a material with a low thermal expansion coefficient (e.g. ULE™) outside the zero-crossing temperature, significant deformation contributions still arise due to the above-mentioned disturbing effects.

[0009] As a result, the problems described above can either impair the performance of the optical system or the projection exposure system or require relatively long waiting times before starting lithography operations in order to ensure that the thermal state of the optical elements or mirrors in question corresponds to a state acceptable with regard to optical aberrations.

[0010] In order to optimize said waiting times, particularly after maintenance breaks or transport, with a view to the desired high throughput, there is a need in practice to determine or estimate as reliably as possible the time at which a thermal state acceptable with regard to optical aberrations is reached in the scenarios described above. In particular, there is a need to be able to perform this estimation or determination without performing a wavefront measurement in the optical system in order to optimize the time at which the optical system or projection exposure system starts operating, taking the appropriate waiting time into account.

[0011] As an example, reference is made to DE 10 2014 212 691 A1, DE 10 2017 207 862 A1 and DE 10 2013 203 338 A1 regarding the state of the art. SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a method and a device for characterizing the heating state of a mirror with regard to the time interval from a thermal equilibrium state, which enable an estimation or optimization of a recovery time following maintenance breaks before commissioning of the optical system while still effectively avoiding thermally induced deformations.

[0013] This problem is solved according to the features of the independent patent claims.

[0014] According to one aspect, the invention relates to a method for characterizing the heating state of a mirror with respect to the time interval from a thermal equilibrium state, wherein the mirror has a mirror substrate and an optical effective surface, - wherein a temporal temperature profile is determined using at least one temperature sensor arranged remotely from the optical active surface; and - wherein a required time period until a thermal equilibrium state is reached is determined from the at least one temperature profile.

[0015] The invention is based in particular on the concept of arranging and using at least one temperature sensor in a mirror in such a way that the information or sensor signals provided by this temperature sensor are suitable for estimating or determining when (e.g. after maintenance work or transport) a state sufficiently close to the thermal equilibrium state or ensuring an optical image of high quality (ie sufficiently low aberrations) is reached.

[0016] In particular, the invention includes the principle of arranging at least one temperature sensor in such a way that the sensor signals supplied by this temperature sensor exhibit a correlation with the said aberrations, which in turn is achieved in embodiments of the invention by placing the temperature sensor(s) used to estimate the achievement of the thermal equilibrium state relatively far away from the immediate zone of influence of a thermal manipulator, if present.

[0017] In particular (but without the invention being limited thereto), the mirror can be thermally manipulated via a thermal manipulator. However, the three-dimensional temperature distribution is also relevant in application scenarios in which no thermal manipulator (for setting a target temperature) is present. For example, even an unheated mirror can be brought out of its thermal equilibrium state by maintenance work, etc. Without the presence of a thermal manipulator, the mirror then has to return to the desired original state - driven by the ambient temperature - only through heat exchange with the environment, i.e., convection and thermal radiation. This takes considerably longer than using a thermal manipulator, although information about the current thermal state would also be advantageous in such a scenario for determining waiting times and relaxation times.

[0018] In an exemplary typical application scenario with active thermal manipulation or mirror heating by coupling heating radiation onto the optical effective surface, in embodiments of the invention, for example, one or more temperature sensors are placed in the area of ​​the back of the mirror with the desired effect that the temperature sensors placed there are then largely insensitive to thermal fluctuations due to the thermal manipulator or the heating device and can instead provide information about the achievement (or not yet achievement) of the thermal equilibrium state for the entire mirror.

[0019] The invention is based in particular on the knowledge gained by the inventors in investigations such as simulations that the information supplied by temperature sensors arranged in the area of ​​the immediate thermal influence zone of the heating device (i.e. in the above example in the area of ​​the optical effective surface) is insufficient for a reliable assessment of the aberration level because the optical aberrations still vary considerably even over a relatively long period of time in which the temperature sensors arranged, for example, in the area of ​​the optical effective surface already supply essentially constant sensor signals corresponding to the respective target temperature.

[0020] In other words, the invention takes into account the fact that the temperature distribution present in the entire mirror volume (ie not only at the optical effective surface, but also “in depth”) and the thermal deformations induced thereby have a considerable influence on the aberration level.

[0021] With regard to the respective time constants of the “delay elements” present in the optical element or mirror, the present invention includes the principle of implementing one or more temperature sensors at locations with a comparatively large thermal time constant (e.g. of the order of several hours), with the result that these temperature sensors are then unsuitable for directly controlling the operation of the thermal manipulator or the heating device, but provide the desired correlation between the sensor signals supplied by the temperature sensors and the aberrations supplied and thus enable the estimation of when the thermal equilibrium state or a sufficiently low aberration level is reached.

[0022] According to one embodiment, the at least one temperature sensor is arranged at a distance from the optical active surface at which the time constant of the thermal relaxation is at least twice the value of the time constant of the thermal relaxation at the optical active surface.

[0023] According to one embodiment, the at least one temperature sensor is located on a rear side of the mirror facing away from the optical active surface.

[0024] According to one embodiment, a temperature profile is determined for different depths in the mirror substrate volume using a plurality of temperature sensors arranged at different distances from the optical active surface.

[0025] With one or more sensors at at least one additional depth within the mirror substrate volume, additional information about the "heating" state (in particular, information about temperature gradients) can be obtained through modeling. This additional depth does not necessarily have to correspond to the back of the mirror (i.e., the greatest depth), although this represents a preferred embodiment as a limiting case.

[0026] According to one embodiment, the time required to reach a thermal equilibrium state is determined based on a model.

[0027] According to one embodiment, the time required to reach a thermal equilibrium state is determined based on a lookup table determined in a pre-calibration.

[0028] According to one embodiment, this lookup table lists a value for a time period required to reach a thermal equilibrium state for different temperature profiles measured away from the optical effective surface.

[0029] According to one embodiment, a value for a required waiting time until commissioning of an optical system comprising the mirror is output on the basis of the estimate.

[0030] According to one embodiment, a timer for starting up an optical system comprising the mirror is controlled on the basis of the estimation of the time interval until the thermal equilibrium state is reached.

[0031] According to one embodiment, the optical active surface is heated via at least one thermal manipulator.

[0032] According to one embodiment, at least one radiant heater is used as the thermal manipulator for coupling in electromagnetic heating radiation.

[0033] According to one embodiment, the step of determining the temporal temperature profile and / or the heating of the optical active surface is carried out via at least one thermal manipulator in a time window following maintenance or transport of the mirror.

[0034] According to one embodiment, the step of determining the temporal temperature profile and / or the heating of the optical effective surface is carried out via at least one thermal manipulator before the start of a useful operation of the mirror.

[0035] According to one embodiment, at least one radiant heater is used as the thermal manipulator for coupling in electromagnetic heating radiation.

[0036] According to one embodiment, the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0037] According to one embodiment, the mirror is a mirror for a projection lens or an illumination device of a microlithographic projection exposure apparatus.

[0038] The invention further relates to a device for characterizing the heating state of a mirror with regard to the time interval from a thermal equilibrium state, wherein the mirror has a mirror substrate and an optical effective surface, - wherein at least one temperature sensor arranged remotely from the optical active surface is provided for determining a respective temporal temperature profile; and - wherein the device has an evaluation unit which is configured to determine from the at least one temperature profile a required period of time until a thermal equilibrium state is reached.

[0039] The device can be designed in particular to carry out a method with the features described above.

[0040] Further embodiments of the invention can be found in the description and the dependent claims.

[0041] The invention is explained in more detail below with reference to embodiments shown in the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] They show: Fig. 1 is a schematic diagram illustrating a possible embodiment of the invention; Fig. 2 to explain the concept underlying the invention, diagrams of a respective exemplary temporal course of the temperature in the region of the optical effective surface and in the region of the mirror back as well as respective temporal course of optical aberrations; Fig. 3-4 are schematic representations for explaining further embodiments of the invention; and Fig. 5 a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] 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 reflection layer system 120 forming an optical active surface 101 (which can be designed merely as an example in the form of a molybdenum (Mo)-silicon (Si) multilayer stack or also in the form of a single layer of e.g. ruthenium (Ru)).

[0044] According to Fig. 1, a thermal manipulator 130 is further provided, which in the specific embodiment (but again without the invention being limited thereto) is designed as a radiant heater for coupling infrared (IR) radiation into the mirror 100. In further embodiments, the thermal manipulator 130 can also be designed in another suitable manner, e.g., as an electrical resistance heater. Furthermore, in addition (as shown in FIG. Fig. 3) the heating or coupling of heating radiation can also take place via another surface of the mirror (which does not correspond to the optical effective surface).

[0045] According to Fig. 1, the mirror 100 has, on the one hand - in a manner known per se - one or more temperature sensors in the vicinity of the optical effective surface 101 or in the vicinity of a direct influence zone of the thermal manipulator 130, wherein in Fig. 1 shows, merely by way of example, such a temperature sensor 132 within an access channel 131 extending from the mirror back into the mirror substrate 110. As in Fig. 1, the operation of the thermal manipulator 130 can be controlled on the basis of the sensor signals or temperature values ​​supplied by the temperature sensor 132.

[0046] According to the invention, as in Fig. 1, one or more temperature sensors 145 are arranged away from the optical effective surface 101 or the immediate influence zone of the thermal manipulator 130. These further temperature sensors 145, of which in the embodiment of Fig. 1 only by way of example, three temperature sensors 145 are shown on the back of the mirror facing away from the optical effective surface 101, are not used to control the operation of the thermal manipulator 130 and the setting of the desired temperature at the optical effective surface 101, but rather to Fig. 1, the evaluation unit 140, also schematically indicated in FIG. 1, is used to determine a required time period until a thermal equilibrium state is reached based on the temperature profile measured by the temperature sensors 145. For this purpose, the temperature sensors 150 are arranged in such a way that they are not sensitive to comparatively short-term temperature fluctuations of the thermal manipulator or its immediate zone of influence, but rather provide information about the thermal state of the mirror "in depth" or in the mirror substrate volume, and thus also enable the determination of a time period by which this thermal state is still removed from a thermal equilibrium state, so that this time period can then be selected as the optimal waiting time until the associated optical system or the projection exposure system is commissioned.

[0047] In relation to the respective time constants of the temperature sensors 145 on the one hand and the temperature sensors 135 used (as described above for controlling the thermal manipulator 130) on the other hand, this means that the time constant of the thermal relaxation for the temperature sensors 145 (in the diagram described below from Fig. 2 as „ τ b “) is much larger than the (in the diagram of Fig. 2 as “τ a “) time constant of the thermal relaxation of the temperature sensors 132 or is substantially greater than the time constant of the thermal relaxation in the region of the optical effective surface 101.

[0048] The invention is based on the consideration that the information supplied by the temperature sensors 132 arranged in the region of the optical effective surface is suitable for regulating the operation of the thermal manipulator 130, but not for a reliable assessment of optical aberrations generated by the mirror 100 during operation, since, as already explained above, the temperature distribution present in the entire mirror volume, ie also "in depth", has a considerable influence on the thermal deformations induced in the mirror 100 and thus on the aberration level.

[0049] Fig. To illustrate this fact and the inventive concept, Figure 2 shows diagrams of the respective temporal course of the temperature in the area of ​​the optical effective surface, the temperature in the area of ​​the back of the mirror and the optical aberrations.

[0050] As from Fig. 2, the temperature in the region of the optical effective surface reaches an equilibrium state corresponding to the target temperature after a relatively short time period t1, with the result that the temperature sensor 135 measures essentially constant temperature values ​​from the time t1 is reached. In contrast, according to the middle diagram of Fig. 2 the temperature on the back of the mirror on a much longer time scale. The optical aberrations of the mirror 100 also do not reach an equilibrium state at time t1, but only after a much longer period of time or a much later time t2. As can also be seen from Fig. 2 shows only the temperature sensor 145 with a comparatively large time constant τ b provided, and not the information from the temperature sensor 132 with a comparatively short time constant τ aThe information provided by the temperature sensor 145 correlates with the optical aberrations. This means that only the information provided by the temperature sensor 145 (and not by the temperature sensor 135) is suitable for assessing the aberration level or the time period to be waited before commissioning the optical system containing the mirror, e.g., after transport or maintenance work.

[0051] Fig. 3 shows a schematic representation to explain another possible embodiment, wherein in comparison to Fig. 1 analogous or essentially functionally equivalent components are designated with reference numbers increased by “200”. Fig. 3 is different from Fig. 1 additionally a coupling of heating radiation via a further thermal manipulator 331 (provided in addition to the thermal manipulator 130) from the rear side of the mirror, wherein in this case the temperature sensor(s) 345 according to the invention for determining the required time period until a thermal equilibrium state is reached is arranged at a suitable distance from the respective immediate influence zones of both thermal manipulators 330, 331, ie preferably as shown approximately in the region of the mirror center.

[0052] Fig. 4 shows another possible embodiment, in which, compared to Fig. 1 analogous or essentially functionally equivalent components are designated by reference numerals increased by “300”. Fig. 4, several temperature sensors (in the example, temperature sensors 445, 446, 447) are located at different depths in the mirror substrate 410 or at different distances from the optical effective surface 401, whereby additional information can be provided for determining the required time period until a thermal equilibrium state is reached.

[0053] In further embodiments, in addition to the above-described use of temperature sensors located remote from the optical effective area or the immediate influence zone of a thermal manipulator with a correspondingly large time constant, a suitable model can also be used. Furthermore, a lookup table determined during a pre-calibration can also be used additionally or alternatively. The thermal state of the mirror can be determined using the model or lookup table based on the values ​​obtained from the temperature sensors.

[0054] In principle, the heating state of a mirror can also be characterized with regard to the time interval from a thermal equilibrium state (possibly even without the temperature sensors according to the invention with a long time constant) solely on the basis of preliminary simulations in which specific waiting times to be observed until the mirror is put into operation are defined for different scenarios (transport and maintenance work or scenarios of operational use).

[0055] Fig. 5 shows schematically in meridional section the possible structure of a microlithographic projection exposure system designed for operation in the EUV as a purely exemplary application of the present invention.

[0056] According to Fig. 5, 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.

[0057] 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 5. The x-direction is perpendicular to the drawing plane. The y-direction is horizontal, and the z-direction is vertical. The scanning direction is in Fig. 5 along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0058] 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.

[0059] 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).

[0060] 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 5, 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 can, merely by way of example, have an image-side numerical aperture that is greater than 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0061] The inventive concept can be particularly advantageously applied to any mirror of the microlithographic projection exposure apparatus 1 of Fig. 5. However, the invention is not limited to application to a mirror of a projection exposure system designed for operation in the EUV. In particular, the invention can also be advantageously applied to a mirror of a projection exposure system designed for operation in the DUV (i.e., at wavelengths less than 250 nm, in particular less than 200 nm) or to a mirror for another optical system.

[0062] 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] DE 10 2014 212 691 A1

[0011] DE 10 2017 207 862 A1

[0011] DE 10 2013 203 338 A1

[0011]

Claims

[1] Method for characterising the heating state of a mirror with regard to the time interval from a thermal equilibrium state, wherein the mirror (100, 300, 400) has a mirror substrate (110, 310, 410) and an optical effective surface (101, 301, 401), - wherein a temporal temperature profile is determined using at least one temperature sensor (145, 345, 445, 446, 447) arranged remotely from the optical active surface (101, 301, 401); and - wherein a required time period until a thermal equilibrium state is reached is determined from the at least one temperature profile. [2] Method according to claim 1, characterized bythat the at least one temperature sensor (145, 345, 445, 446, 447) is arranged at a distance from the optical active surface (101, 301, 401) at which the time constant of the thermal relaxation is at least twice the value of the time constant of the thermal relaxation at the optical active surface (101, 301, 401). [3] Method according to claim 1 or 2, characterized by that the at least one temperature sensor (145) is located on a rear side of the mirror (100) facing away from the optical active surface (101). [4] Method according to one of claims 1 to 3, characterized by that a temperature profile is determined for different depths in the mirror substrate volume using a plurality of temperature sensors (445, 446, 447) arranged at different distances from the optical active surface (401). [5] Method according to one of the preceding claims, characterized bythat the time required to reach thermal equilibrium is determined based on a model. [6] Method according to one of the preceding claims, characterized by that the time required to reach thermal equilibrium is determined based on a lookup table determined in a pre-calibration. [7] Method according to claim 6, characterized by that in this look-up table, for different temperature profiles measured away from the optical effective surface (101, 301, 401), a value is listed for a period of time required until a thermal equilibrium state is reached. [8] Method according to one of the preceding claims, characterized bythat on the basis of the estimate a value is output for a required waiting time until commissioning of an optical system having the mirror (100, 300, 400). [9] Method according to one of the preceding claims, characterized by that, on the basis of the estimation of the time interval until the thermal equilibrium state is reached, a timer is controlled for the commissioning of an optical system comprising the mirror (100, 300, 400). [10] Method according to one of the preceding claims, characterized by that the optical active surface (101, 301, 401) is heated via at least one thermal manipulator (130, 330, 430). [11] Method according to claim 10, characterized by that at least one radiant heater is used as the thermal manipulator (130, 330, 430) for coupling in electromagnetic heating radiation. [12] Method according to one of the preceding claims, characterized bythat the step of determining the temporal temperature profile and / or the heating of the optical active surface (101, 301, 401) is carried out via at least one thermal manipulator (130, 330, 430) in a time window following maintenance or transport of the mirror (100, 300, 400). [13] Method according to one of the preceding claims, characterized by that the step of determining the temporal temperature profile and / or the step of heating the optical active surface (101, 301, 401) is carried out via at least one thermal manipulator (130, 330, 430) before the start of a useful operation of the mirror (100, 300, 400). [14] Method according to one of the preceding claims, characterized by that the mirror (100, 300, 400) is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm. [15] Method according to one of the preceding claims, characterized bythat the mirror is a mirror for a projection lens (10) or an illumination device (2) of a microlithographic projection exposure apparatus (1). [16] Device for characterising the heating state of a mirror with regard to the time interval from a thermal equilibrium state, wherein the mirror (100, 300, 400) has a mirror substrate (110, 310, 410) and an optical effective surface (101, 301, 401), • wherein at least one temperature sensor (145, 345, 445, 446, 447) arranged remotely from the optical active surface (101, 301, 401) is provided for determining a respective temporal temperature profile; and • wherein the device has an evaluation unit (140, 340, 440) which is configured to determine from the at least one temperature profile a required period of time until a thermal equilibrium state is reached. [17] Device according to claim 16, characterized by that it is designed to carry out a method according to one of claims 1 to 15.

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