Method for heating optical element and optical system

Abstract

The invention relates to: a method of heating an optical element in an optical system, in particular in a microlithographic projection exposure system; and an optical system. In the method according to the invention, a thermal manipulator (210) is used to introduce a heating power into the optical element (200) in order to produce a thermally induced deformation, in which the heating power is adjusted relative to a desired state of the optical element, in which the heating power is adjusted relative to the desired state of the optical element, prior to starting an operation of the optical system in which light is incident on the optical element. The first optical aberration is at least partially compensated, and wherein, after starting the operation of the optical system, the heating power is adjusted to a desired state as a function of the thermal load of the useful light incident on the optical element, and wherein the heating power is adjusted such that the average temperature of the optical element (200) remains constant up to a maximum deviation of 0.5 K, in particular at most 0.2 K.

Description

[0001] This application claims the priority of German patent applications DE 10 2022 114 974.9 filed on June 14, 2022 and DE 10 2022 131 353.0 filed on November 28, 2022. The contents of these DE applications are incorporated into the text of the present application by reference. Technical Field

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

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

[0004] 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 available suitable light-transmitting refracting materials.

[0005] One problem that arises in practice is that, due to manufacturing fluctuations of the mirrors and the limited precision of mounting and / or alignment processes performed during assembly of the projection exposure apparatus, there are unavoidable optical aberrations due to deviations from the ideal optical design. In order to correct such optical aberrations (also called "cold aberrations"), one approach - in addition to the targeted actuation of the respective mirrors in their rigid body degrees of freedom - is to apply a suitable heating profile to the mirrors in a targeted manner using a heating device (which, for example, couples infrared radiation) in order to achieve a correction of the optical cold aberrations by means of the thermally induced deformation obtained in this way.

[0006] Another problem that arises in practice is that, in particular due to the absorption of radiation emitted by the EUV light source, the EUV mirrors heat up (also called "mirror heating") and experience associated thermal expansion or deformation, which in turn may lead to impairment of the imaging properties of the optical system. In order to avoid surface deformations caused by the heat input into the EUV mirrors and the accompanying optical aberrations, it is known to use mirror substrate materials in the form of ultra-low expansion materials, such as those marketed by Corning Inc. under the trade name ULE TMTitanium silicate glasses are marketed and set in the region near the optically active surface a so-called zero crossing temperature (ZCT). At this zero crossing temperature (e.g. for ULE TM At about θ=30°C), the coefficient of thermal expansion has a zero crossing in its temperature dependence, near which the thermal expansion of the mirror substrate material has no or only a negligible dependence on the temperature changes that occur. Furthermore, due to the use of a heating arrangement (e.g. based on infrared radiation), active mirror heating can take place in phases of relatively low absorption of the radiation used by EUV, said active mirror heating being reduced accordingly as the absorption of the radiation used by EUV increases.

[0007] Another problem that now arises in practice is that the above-mentioned methods for firstly correcting "cold aberrations" and secondly for avoiding deformations caused in the corresponding mirrors during operation of the optical system due to the impingement of EUV or the light used (i.e. "mirror heating") comprise in this respect opposite or conflicting requirements, since Figure 3a As shown in the schematic diagram in , the temperature range or target value that is advantageous or beneficial for each of the two methods is different from each other. In this regard, in order to avoid optical aberrations induced by EUV radiation heat during operation of the optical system, a temperature window in the region of the above-mentioned zero crossing temperature (=ZCT) is desirable, see Figure 3a The target value of 300 in MH is favorable with respect to "mirror heating" (MH) and at which the thermally induced deformations that occur are as insensitive as possible to local temperature variations at the different mirror positions. On the contrary, for the correction effect sought for "cold aberrations", it is necessary to heat the individual mirrors to a temperature range, see Figure 3a 302 in represents a target value which is favorable with respect to cold aberrations and at which the deformation of the mirror is sufficiently sensitive to additional thermal radiation, however, as a result, the aberrations caused by the impact of EUV light during operation and the effect of the above-mentioned "mirror heating" are then enhanced.

[0008] Even when the heating power introduced into the mirror by the thermal manipulator is set to a target state, in which the thermal load acting on the mirror due to the incident light used during operation of the optical system is already taken into account in addition to the cold aberration to be compensated, and in this respect (e.g. Figure 3b The heating power generated by the thermal manipulator is initially optimized with respect to both cold aberrations and "mirror heating" aspects, as indicated in FIG. 1 based on the optimized target value for the fan heater specified by 304. A further problem arises: due to the substantial deviation from the above-mentioned zero crossing temperature (ZCT), the temperature increase accompanying EUV loading during operation inevitably leads to transient aberrations, or aberrations that vary with time from the start of operation (i.e., the start time), see FIG. Figure 3b The MH-induced aberrations highlighted as 306 in the figure are enhanced, with the result that the performance of the optical system or the projection exposure apparatus is impaired.

[0009] With regard to the prior art, reference is made merely by way of example to DE 10 2019 219 289 A1. Summary of the invention

[0010] It is an object of the present invention to provide a method for heating an optical element, in particular in a microlithography projection exposure apparatus, and an optical system which are able to effectively avoid surface deformations and the accompanying optical aberrations caused by heat input in the optical element.

[0011] This object is achieved by a method and an optical system, respectively, according to the features of the alternative independent patent claims.

[0012] According to one aspect, the invention relates to a method for heating an optical element in an optical system, in particular an optical element in a microlithography projection exposure apparatus,

[0013] wherein a thermal manipulator is used to introduce heating power into the optical element in order to produce a thermally induced deformation;

[0014] - wherein, before starting operation of the optical system, wherein the light used is incident on the optical element, said heating power is set relative to a target state of the optical element, in which the first optical aberration is at least partially compensated; and

[0015] - wherein, after the operation of the optical system has started, the heating power is controlled to the target state depending on the thermal load of the light used incident on the optical element, wherein the control is carried out in such a way that the average temperature of the optical element remains constant up to a maximum deviation of 0.5 K, in particular up to a maximum deviation of 0.2 K.

[0016] The first optical aberration may in particular be caused at least in part by manufacturing or alignment. However, the invention is not limited thereto, so that the first optical aberration may also be caused by other fault sources.

[0017] In an embodiment, the invention firstly starts from the following approach: in view of the fundamental problem (explained in the introduction) of the opposite or conflicting requirements firstly regarding the temperature range favourable for the correction of cold aberrations and secondly regarding the temperature range favourable for compensating the effects of "mirror heating", the two aspects "correction of cold aberrations" and "compensation of the effects of mirror heating" are already included simply by defining a suitable set of target values ​​of the heating power introduced into the optical element by the thermal manipulator. This is done by virtue of the fact that during the setting of the heating power, in addition to the correction of the above-mentioned "cold aberrations", the corresponding effects of the heating power on the optical aberrations generated due to the used light incident during the actual operation of the optical element or of an optical system having the element are also taken into account (in particular by "co-optimization").

[0018] With regard to the specific configuration of the thermal manipulator, i.e. the manner in which the heating power is introduced into the optical element, the invention is not further restricted. By way of example only, the heating power can be introduced, for example, by means of infrared (IR) emitters in a manner known per se, wherein in particular the individual sectors can also be subjected to IR radiation by setting a corresponding heating curve. For this purpose, for example, the heating device described in DE 10 2019 219 289 A1 can be used. Alternatively, the heating power can also be introduced via electrodes to which a voltage can be applied and which are arranged on the optical element or mirror to be heated.

[0019] In this case, the expression "set of target values" is intended to express the fact that the thermal manipulator can also optionally subject individual sectors to IR radiation in a targeted manner by setting a corresponding heating curve (for example, using the heating device described in DE 10 2019 219 289 A1). In this respect, the "set of target values" for the heating power of the thermal manipulator optionally includes a value for each sector (in the form of a vector with multiple components).

[0020] The first optical aberration (eg, caused by manufacturing or alignment) may be caused by the optical element itself or elsewhere in the optical system (eg, by some other optical element).

[0021] Starting from the method described above, the present invention is now based on a further concept, namely that during operation of the optical system, the heating power of the thermal manipulator set at the beginning of the operation (in particular with regard to the correction of cold aberrations as described above) is not kept constant over time, for example, but is readjusted depending on the thermal load of the used light incident on the optical element.

[0022] In this case, in an embodiment, the concept according to the invention differs from conventional preheating methods in particular in that, during operation of the optical system, for example, a uniform or constant temperature distribution within the optical element or mirror is not sought, but rather existing cold aberrations are taken into account, a constant wavefront effect is maintained, which in turn can be achieved by generating a correspondingly inhomogeneous heating distribution within the mirror and optionally also by maintaining this inhomogeneous heating distribution under the load used by readjusting the heating power during operation of the optical system.

[0023] Furthermore, according to the invention, the heating power is intended to be controlled in such a way that the average temperature of the optical element remains constant up to a maximum deviation of 0.5 K, in particular up to 0.2 K. In this case, the invention particularly comprises the further concept of ensuring that the average temperature of the optical element remains substantially constant over the entire time period from the initial state, i.e. the state before the impact of the light used (e.g. EUV) to the state during the operation of the optical system during the incidence of the light used (e.g. EUV), in order to avoid or at least reduce "overshoot" in this respect. In this case, this condition of maintaining the average temperature of the optical element over time can influence the control of the heating power as a constraint. Furthermore, different illumination settings can also be considered in this case. In other words, for each illumination setting, in the control of the heating power, a corresponding set of target values ​​for the heating power of the thermal manipulator can be selected so that in each case the average temperature of the optical element is maintained over the time period from the initial state or the state before the impact of the light used (e.g. EUV) during the operation of the optical system to the state during the impact of the light used (e.g. EUV) during the operation of the optical system.

[0024] However, according to more general aspects of the disclosure, the control may also be implemented differently, in particular without or without continuously limiting deviations of the average temperature of the optical element to a certain maximum value, or limiting deviations of the average temperature to higher maximum values ​​than the abovementioned 0.5 K and 0.2 K.

[0025] The readjustment according to the invention of the heating power of the thermal manipulator can be realized according to the invention in various ways, as described below. In this respect, the invention firstly comprises embodiments with a temperature-based control concept, which in turn can be based on the determination of an average temperature at the optically active surface, or alternatively also on a temperature distribution at the optically active surface (which involves optionally taking different temperature values ​​for different sectors at the optically active surface as a basis). Furthermore, the invention also comprises embodiments in which the readjustment according to the invention is realized based on wavefront effects of the optical element or the associated optical system (using at least one wavefront sensor and / or estimated in a model-assisted manner).

[0026] According to one embodiment, the optical system has at least one further mirror actuatable in multiple degrees of freedom, in particular multiple further mirrors each actuatable in multiple degrees of freedom, wherein the totality of these degrees of freedom serves for at least partial compensation of the first optical aberration.

[0027] According to one embodiment, the heating power is set before the start of operation of the optical system by additionally taking into account the respective influence of the heating power on the second optical aberration caused by the used light incident on the optical element during the subsequent operation of the optical system. In other words, before the start of operation of the optical system, the heating power is set with the aim of jointly optimizing both with respect to the cold aberration and with respect to the "mirror heating" occurring during the subsequent operation due to the EUV load, as already mentioned.

[0028] According to one embodiment, the target state is defined by the thermal state of the optical element.

[0029] According to one embodiment, the target state is defined by a wavefront provided by the optical system in the image plane.

[0030] According to one embodiment, after operation of the optical system has started, the heating power is controlled based on at least one temperature measured using at least one temperature measuring device.The temperature measuring device may be configured in any suitable manner, for example as a temperature sensor or a thermal imaging camera.

[0031] According to one embodiment, after start of operation of the optical system, the heating power is controlled based on at least one average temperature at the optically active surface of the optical element estimated using at least one temperature measuring device.

[0032] According to another embodiment, after the operation of the optical system has started, the heating power is controlled based on a temperature distribution at the optically active surface of the optical element estimated using one or more temperature measuring devices.

[0033] According to one embodiment, the temperature distribution at the optically active surface of the optical element is estimated from the measurement signal provided by the temperature measuring device based on a model using an observer.

[0034] According to one embodiment, the heating power is controlled after the start of operation of the optical system based on an estimation of a wavefront effect of the optical system, in particular based on a feedforward model.

[0035] According to one embodiment, said estimation of the wavefront effect of the optical element is realized using at least one wavefront sensor.

[0036] According to one embodiment, the estimation of the wavefront effect of the optical element is performed based on a target value of the heating power set by the thermal manipulator.

[0037] According to one embodiment, the estimation of the wavefront effect of an optical element is achieved based on a combination of wavefront and temperature measurements.

[0038] According to one embodiment, information about the used reticle, the used illumination settings and / or information from intensity measurements are used in the feedforward model. In this case, the information about the used reticle may in particular relate to information about the optical far field, which is generated during operation of the optical system by diffraction of the light used (e.g. EUV) at structures of the reticle and which may be determined by optical forward simulation. The information from intensity measurements may in particular relate to the intensity of the radiation of a thermal manipulator (e.g. an infrared laser) used to generate the heating power.

[0039] According to one embodiment, the control of the heating power is performed instantaneously based on a feedforward model. In particular here, a temperature distribution in the optical element suitable for setting a target state or a desired wavefront can be set instantaneously at different times during operation of the optical system or be re-determined in each case.

[0040] According to one embodiment, this transient control based on the feedforward model is performed as a model predictive control to take into account changes in the reticle used and / or the illumination settings used. In other words, changes in the reticle used and / or the illumination settings used can be anticipated so that overshoots after the change are avoided in this respect.

[0041] According to one embodiment, a combination of multiple mirrors is used in the control, wherein the multiple mirrors include at least one mirror in which a heating curve complementary to the temperature distribution caused by the used light incident on the mirror is generated by a heating device and at least one mirror is actively deformed for wavefront manipulation.

[0042] If implemented, in addition to the above control, the above control variants may also preferably be performed to achieve the maximum permissible deviation of the average temperature of the optical element. However, in the context of the general aspects of the present disclosure, the above control variants may also be implemented independently of the limitation of the maximum average temperature deviation, either individually or in combination.

[0043] According to one embodiment, the optical element is a mirror.

[0044] According to one embodiment, the optical system is designed for an operating wavelength of less than 400 nm, in particular less than 250 nm, more in particular less than 200 nm.

[0045] According to one embodiment, the optical element is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0046] The present invention further relates to an optical system, in particular an optical system in a microlithography projection exposure apparatus, comprising:

[0047] - at least one optical element,

[0048] - a thermal manipulator for heating the optical element; and

[0049] - a control unit for controlling the heating power introduced into the optical element by the thermal manipulator,

[0050] - wherein the control is carried out based on a target state in which the first optical aberration is at least partially compensated and in dependence on the thermal load of the used light incident on the optical element, wherein the heating power is controlled such that the average temperature of the optical element remains constant up to a maximum deviation of 0.5 K, in particular up to 0.2 K.

[0051] However, according to more general aspects of the present disclosure, in the case of optical systems, the control can also be implemented differently, in particular without or without continuously limiting the deviations of the mean temperature of the optical elements to a certain maximum value, or by limiting the deviations of the mean temperature to maximum values ​​higher than the above-mentioned 0.5 K and 0.2 K.

[0052] According to one embodiment, the first optical aberration is at least partially caused by manufacturing or alignment.

[0053] According to one embodiment, the optical system is configured to perform a method having the above-described features. With regard to advantages and further preferred configurations of the optical system, reference is made to the above explanations associated with the method according to the invention.

[0054] Further configurations of the invention can be gleaned from the description and the dependent claims.

[0055] The invention is explained in more detail below based on the exemplary embodiments shown in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In the attached picture:

[0057] Figure 1 Schematic diagram showing a possible setup of a microlithography projection exposure apparatus designed for operation in EUV;

[0058] Figure 2 shows a schematic diagram for explaining a possible implementation of the method according to the invention; and

[0059] Figure 3a-3b A diagram is shown for explaining the basic problem on which the invention is based. DETAILED DESCRIPTION

[0060] Implementation

[0061] Figure 1 Firstly, a schematic diagram of a projection exposure apparatus 1 is shown, which is designed for operation in EUV and in which the present invention can be implemented by way of example.

[0062] according to Figure 1 , the projection exposure apparatus 1 comprises an illumination device 2 and a projection lens 10. The illumination device 2 is used to illuminate an object field 5 in an object plane 6 with radiation from a radiation source 3 via an illumination optical unit 4. What is exposed here is a mask 7 arranged in the object field 5. The mask 7 is held by a mask holder 8. The mask holder 8 can be displaced by a mask displacement drive 9, in particular in the scanning direction. For the purpose of explanation, Figure 1 A Cartesian xyz coordinate system is depicted in FIG. The x direction extends perpendicular to the plane of the drawing into the drawing. The y direction extends horizontally and the z direction extends vertically. The scanning direction is along Figure 1 The z direction extends perpendicular to the object plane 6.

[0063] The projection lens 10 serves to image the object field 5 into an image field 11 in an image plane 12. The structures on the reticle 7 are imaged on a photosensitive 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 in the y direction, by a wafer displacement drive 15. Firstly the displacement of the reticle 7 by the reticle displacement drive 9 and secondly the displacement of the wafer 13 by the wafer displacement drive 15 can be synchronized with each other.

[0064] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation, which is also referred to as used radiation or illumination radiation in the following text. In particular, the used radiation has a wavelength in the range between 5 nm and 30 nm. The 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 emitted from the radiation source 3 is focused by a light collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optical unit 4. The illumination optical unit 4 comprises a deflection mirror 19, and a first facet mirror 20 (with a schematically shown facet 21) and a second facet mirror 22 (with a schematically shown facet 23) arranged downstream of the deflection mirror 19 in the beam path.

[0065] The projection lens 10 comprises a plurality of mirrors Mi (i=1, 2, ...), which are numbered consecutively according to their arrangement in the beam path of the projection exposure apparatus 1. Figure 1In the example shown, the projection lens 10 comprises six mirrors M1 to M6. Alternative solutions with four, eight, ten, twelve or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a through-opening for the illumination radiation 16. The projection lens 10 is a doubly shielded optical unit. The projection lens 10 has an image-side numerical aperture which, by way of example only, may be greater than 0.3, in particular also greater than 0.5, more in particular greater than 0.6.

[0066] During operation of the microlithographic projection exposure apparatus 1, electromagnetic radiation incident on the optically active surfaces of the mirrors is partially absorbed and, as explained in the introduction, leads to heating and associated thermal expansions or deformations, which in turn can lead to impairment of the imaging properties of the optical system. By means of a thermal manipulator in the form of a heating device as described in the introduction, active mirror heating can then be carried out in each case in phases of relatively low absorption of the radiation used for EUV, said active mirror heating being correspondingly reduced as the absorption of the radiation used for EUV increases.

[0067] exist Figure 1 A heating device is depicted only schematically and is indicated by "25" in the example, which serves to introduce heating power into the reflector M3. In this case, the invention does not further restrict the manner in which the heating power is introduced or the configuration of the heating device used for this purpose. Purely by way of example, the heating power can be introduced in a manner known per se via an infrared emitter or via an electrode to which a voltage can be applied and which is arranged on the optical element or the reflector to be heated.

[0068] Furthermore, the invention has no further restrictions with regard to the number of optical elements or mirrors to be heated, with the result that the control according to the invention can be applied either only to the heating of a single optical element or to the heating of a plurality of optical elements.

[0069] The embodiments described below have in common that the heating power is not only introduced into the optical element or the mirror by (initially) setting to a target state which takes into account optical aberrations to be compensated due to manufacturing or alignment faults and optionally also takes into account the influence of the heating power on secondary optical aberrations caused by "mirror heating" during subsequent operation, but that, in addition, the heating power is also controlled as a function of the thermal load of the used light (i.e. EUV radiation) incident on the optical element after starting operation of the optical system or the used light irradiating the optical element.

[0070] In particular, it is therefore contemplated according to the invention that in a projection exposure apparatus, during a microlithographic exposure process, in addition to the heating power introduced into the mirror by IR radiation, the light used, for example in the form of EUV radiation, also affects the mirror surface, with the result that the mirror temperature at the optically active surface increases locally and, depending on the selected illumination settings, in comparison with the temperature profile in the absence of EUV radiation, as already described with reference to Figure 3b Explained.

[0071] In order to take this effect into account, various control concepts according to the present invention are described below, first referring to Figure 2 The temperature-based control concept is explained. For this purpose, in the relevant optical element or mirror 200, in a manner known per se, one or more temperature measuring devices 203a, 203b (e.g. temperature sensors) are arranged in the inlet channel 202 located in the mirror substrate 201, which temperature measuring devices 203a, 203b are used to estimate the average temperature or the temperature distribution at the optically active surface of the mirror 200.

[0072] In addition, Figure 2 In the embodiment, a thermal manipulator in the form of an infrared heating device is indicated by "210" and EUV radiation (= used light) acting on the mirror 200 during operation is indicated by "220". Furthermore, a plurality of sectors in the area of ​​the optically active surface of the mirror are indicated by "211" to "214", wherein in an embodiment, given the presence of a sufficient number of temperature measuring devices 203a, 203b and optionally also using an observer 230 and based on a model, an estimation of the local temperature distribution at the optically active surface of the mirror 200 can be performed.

[0073] In a first embodiment of the temperature-based control concept according to the invention, the heating power fed to the mirror 200 by the thermal manipulator 210 after starting operation can be controlled based on an estimated average temperature at the optically active surface (i.e., first still dispensing with a spatially resolved determination of the temperature distribution over a plurality of sectors). The method is based on the following observation: In simplified considerations, the EUV radiation 220 absorbed by the mirror 200 leads to an increase in the average mirror temperature:

[0074]

[0075] It will now be assumed in the following that the thermal manipulator 210 is configured as a “sector heater”, provided that it enables heating power to be introduced in a targeted manner into different sectors of the mirror 200 (e.g., according to Figure 2 For this purpose, for example, the heating device described in DE 10 2019 219 289 A1 can be used.

[0076] According to a first embodiment, after estimating the average temperature at the optically active surface of the mirror 200, the heating power can then be controlled to the effect that said average temperature remains constant during operation and when EUV radiation 220 acts on the mirror 200. This can alternatively be done first in such a way that the thermal manipulator 210 or the IR radiation heater achieves a correspondingly uniform heating of the optically active surface, which can be correspondingly controlled downwards during the increase of the average mirror temperature associated with the EUV radiation 220. Alternatively, the heating power can also be subtracted in such a way that a reduction in the average temperature at the optically active surface is achieved overall, while maintaining the inhomogeneous heating profile introduced by the thermal manipulator 210 or the IR radiation heater. This can be achieved, for example, by a global scaling factor S according to the following equation:

[0077]

[0078] Alternatively, a linear combination of the heating powers introduced per sector can also be determined which produces a temperature increase that is as uniform as possible and can then be used for the control according to the invention.Instead of a global scaling factor, other suitable mathematical methods can also be used to reduce the average temperature at the optically active surface.

[0079] In order to implement the above-described temperature-based control concept based on the average temperature at the optically active surface, a sufficiently large offset of the average temperature at the optically active surface is preferably already available during the initial setting of the heating power of the thermal manipulator 210 (by co-optimizing the corresponding merit function both with respect to cold aberrations and with respect to "mirror heating"). In other words, in each sector, the (average) temperature at the optically active surface that is initially generated according to the corresponding "co-optimized" target value of the heating power should generally be greater than the temperature caused by the UV radiation 220 maximum in this sector. In individual cases, in particular depending on the specific expansion behavior of the optical element (position of the ZCT), it may be advantageous to achieve heating at a temperature that is less than the temperature caused by the EUV radiation 220 maximum.

[0080] As already mentioned, in further embodiments, the temperature-based control concept can be extended to the extent to which the temperatures of the individual sectors 211, 212, 213, 214 at the optically active surface remain constant over time relative to the corresponding initial values ​​(i.e. the temperature values ​​set by the thermal manipulator 210 at the start of operation). As a result, not only does the average temperature of the mirror 200 remain constant over time, as opposed to a first approximation, but also the spatial temperature distribution required for correcting cold aberrations remains constant. Since the number of temperature measuring devices 203a, 203b, ... that can be integrated in the mirror 200 is limited, usually due to boundary conditions in the optical system, and sometimes N>K applies (where N denotes the number of sectors 211, 212, ... and K denotes the number of temperature measuring devices 203a, 203b, ...), in embodiments of the present invention, preferably a control method according to the invention is additionally used. Figure 2 An observer 230 is provided to estimate the temperature distribution in the individual sectors 211, 212, ... based on the temperature measurements and additionally based on the process model. The quality of the estimation that can be achieved here depends both on the quality of the process model and on the knowledge of the input signals. In order to improve the estimation quality, the EUV load (as a significant thermal load) can also be fed to the observer 230.

[0081] In a further embodiment, the control according to the invention of the heating power coupled into the optical element or mirror 200 by the thermal manipulator 210 during operation can also be implemented in a wavefront-based manner, i.e. the heating curve set by the thermal manipulator 210 (again in particular configured as a sector heater) is optimized directly with respect to the wavefront generated by the mirror 200 or the associated optical system. In this case, a wavefront sensor (for example arranged in the region of the wafer stage) and preferably additionally a feedforward model can be used to capture or estimate the current wavefront effect, respectively. Since this wavefront-based approach does not pursue the goal of keeping the initial temperature distribution (i.e. the temperature distribution set at the beginning of operation or at the beginning of the impact of the light or EUV radiation 220 used on the mirror 200) constant according to a target value of the heating power set in particular for correcting cold aberrations, but rather pursues the goal of directly keeping the wavefront effect of the mirror constant, significant deviations from the initial temperature distribution can occur.

[0082] The corresponding wavefront-based optimization can be achieved by minimizing the merit function, where, for a predefined disturbance S, the position and orientation of the optical elements The wavefront effect Further wavefront effects of thermal manipulators or sector heaters and (heating power) amplitude In the case of the thermal manipulator settings, a scalar value is assigned by a weighted metric D. The wavefront-based optimization then corresponds to the minimization of this merit function according to:

[0083]

[0084] Then, the present invention predicts the wavefront effect and aberrations To extend this merit function for transient, wavefront-based “co-optimization”, the wavefront effect Aberrations introduced by thermal manipulators or sector heaters Caused by "mirror heating" due to EUV load:

[0085]

[0086] In this case, the disturbance S can in particular be determined by an initial wavefront measurement and can be repeatedly updated based on further wavefront measurements. Alternatively, the disturbance S can also be determined by simulation. Delays in the heating and cooling behavior of the sector heaters can be taken into account in the optimization. The feedforward model can also be used to predict the corresponding wavefront effects between corresponding wavefront measurements. Furthermore, the feedforward model can also be used for model-based predictive control so that in the event of an impending setup change, the heating power introduced by the thermal manipulator or the sector heater is already prepared for the new EUV load.

[0087] Although the present invention has been described based on specific embodiments, many variations and alternative embodiments are obvious to those skilled in the art, such as by combining and / or exchanging the features of the various embodiments. Therefore, it goes without saying to those skilled in the art that these variations and alternative embodiments are simultaneously included in the present invention, and the scope of the present invention is limited only to the meaning of the attached patent claims and their equivalents.

Claims

1. A method for heating an optical element in an optical system, in particular in a microlithography projection exposure apparatus, -in, Using a thermal manipulator (210), heating power is introduced into the optical element (200) to produce thermally induced deformation; - wherein, before the operation of the optical system with the light used being incident on the optical element (200) begins, the heating power is set relative to a target state of the optical element in which the first optical aberration is at least partially compensated; and - wherein, after the operation of the optical system has started, the heating power is controlled to the target state depending on the thermal load of the used light (220) incident on the optical element, wherein the heating power is controlled such that the average temperature of the optical element (200) remains constant up to a maximum deviation of 0.5 K, in particular up to 0.2 K.

2. The method according to claim 1, characterized in that: The first optical aberration is at least partially caused by manufacturing or alignment.

3. The method according to claim 1 or 2, characterized in that: The optical system has at least one further mirror actuatable in a plurality of degrees of freedom, in particular a plurality of further mirrors each actuatable in a plurality of degrees of freedom, wherein the totality of these degrees of freedom serves to at least partially compensate the first optical aberration.

4. The method according to any one of claims 1 to 3, characterized in that Before operation of the optical system begins, the heating power is set by additionally taking into account a corresponding influence of the heating power on a second optical aberration caused by the used light incident on the optical element (200) during subsequent operation of the optical system.

5. The method according to any one of claims 1 to 4, characterized in that The target state is defined by the thermal state of the optical element (200).

6. The method according to any one of claims 1 to 4, characterized in that The target state is defined by a wavefront provided by the optical system in an image plane.

7. The method according to any one of the preceding claims, characterized in that After the operation of the optical system has started, the heating power is controlled based on at least one temperature measured using at least one temperature measuring device (203a, 203b).

8. The method according to any one of the preceding claims, characterized in that After the operation of the optical system has started, the heating power is controlled based on at least one average temperature at an optically active surface of the optical element (200) estimated using at least one temperature measuring device (203a, 203b).

9. The method according to any one of the preceding claims, characterized in that After operation of the optical system begins, the heating power is controlled based on a temperature distribution at an optically effective surface of the optical element (200) estimated using one or more temperature measuring devices (203a, 203b).

10. The method according to claim 9, characterized in that Based on a model using an observer (230), the temperature distribution at the optically active surface of the optical element (200) is estimated from the measurement signals provided by the temperature measurement devices (203a, 203b).

11. The method according to any one of the preceding claims, characterized in that After the operation of the optical system has started, the heating power is controlled based on an estimation of a wavefront effect of the optical system, in particular based on a feedforward model.

12. The method according to claim 11, characterized in that The estimation of the wavefront effect of the optical element (200) is achieved using at least one wavefront sensor.

13. The method according to claim 11 or 12, characterized in that: The estimation of the wavefront effect of the optical element (200) is achieved based on a target value of the heating power set by the thermal manipulator.

14. The method according to claims 11 to 13, characterized in that An estimation of the wavefront effect of the optical element (200) is achieved based on a combination of wavefront and temperature measurements.

15. The method according to any one of claims 11 to 14, characterized in that Information about the used reticle, the used illumination settings and / or information from intensity measurements is used in the feed-forward model.

16. The method according to any one of claims 11 to 15, characterized in that The controlling of the heating power is performed transiently based on the feedforward model.

17. The method according to claim 16, characterized in that This transient control based on the feed-forward model is performed as a model predictive control to take into account variations of the used reticle and / or the used illumination settings.

18. A method according to any one of the preceding claims, characterised in that A combination of multiple mirrors is used in the control, wherein the multiple mirrors include at least one mirror in which a heating curve complementary to the temperature distribution caused by the used light incident on the mirror is generated by a heating device and at least one mirror is actively deformed for wavefront manipulation.

19. The method according to any one of the preceding claims, characterized in that The optical element (200) is a reflector.

20. The method according to any one of the preceding claims, characterized in that The optical element (200) is designed for an operating wavelength of less than 400 nm, in particular less than 250 nm, more particularly less than 200 nm.

21. The method according to any one of the preceding claims, characterized in that The optical element (200) is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

22. An optical system, in particular an optical system in a microlithography projection exposure apparatus, comprising: - at least one optical element (200); - a thermal manipulator (210) for heating the optical element (200); and - a control unit for controlling the heating power introduced into the optical element (200) by the thermal manipulator (210); - wherein the control is carried out based on a target state that at least partially compensates for the first optical aberration and depends on the thermal load of the used light incident on the optical element (200), wherein the heating power is controlled such that the average temperature of the optical element (200) remains constant up to a maximum deviation of 0.5 K, in particular up to 0.2 K.

23. The optical system according to claim 22, characterized in that The first optical aberration is at least partially caused by manufacturing or alignment.

24. The optical system according to claim 22 or 23, characterized in that The optical system is configured to perform the method according to any one of claims 1 to 21 .

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