Method of operating projection exposure system and projection exposure system

Abstract

The invention relates to a method of operating a projection exposure apparatus for microlithography, comprising heating at least one optical element of the projection exposure apparatus by irradiating a surface (30) of the optical element with heating radiation during an interruption of operation, during which the surface of the optical element is not irradiated by exposure radiation. During the heating in the interruption operation, a non-uniform temperature distribution (33a-c) is generated on at least one portion (TBa-c) of the surface (30) of the optical element that reduces aberration of the projection exposure apparatus, wherein the non-uniform temperature distribution (33a-c) is produced by irradiating a portion (TBa-c) with heating radiation having at least one continuous heating radiation profile formed by a beam shaping element. The invention also relates to a projection exposure apparatus for microlithography, comprising at least one optical element, a heating device for irradiating a surface (30) of the optical element with heating radiation, which heating device is designed to irradiate the surface (30) of the optical element with heating radiation during an operating interruption, during the interruption, the surface (30) of the optical element is not irradiated by the exposure radiation. In order to reduce aberrations of the projection exposure apparatus, the heating device is designed to produce a non-uniform temperature distribution (33a-c) on at least a portion (TBa-c) of the surface (30) of the optical element during heating in an operational interruption. For this purpose, the heating device comprises at least one heating radiation source for generating heating radiation and at least one beam shaping element for generating a continuous heating radiation profile.

Description

[0001] Citation of Related Applications

[0002] This application claims the priority of German patent application DE102022205814.3 filed on June 8, 2022, the entire disclosure of which is incorporated by reference into this application. Background Art

[0004] Microlithography projection exposure apparatuses are used to produce microstructured or nanostructured components for microelectronics or microsystems technology. In order to be able to produce components with structures of extremely small dimensions (on the nanometer and micrometer scale) as precisely as possible, the corresponding projection exposure apparatus must be able to image the structures arranged on the mask (reticle) precisely onto a substrate (e.g. a wafer).

[0005] EUV radiation at wavelengths in the EUV wavelength range has now been used for several years in order to obtain the highest possible resolution, in particular for lithography optical units; the wavelength is typically 13.5 nm, while earlier systems used typical operating wavelengths of 365 nm, 248 nm or 193 nm. The consequence of the step into the EUV wavelength range must be managed without refractive media, which can no longer be used meaningfully at these wavelengths, and the transition to pure mirror systems operating at virtual normal incidence or grazing incidence. In the case of almost normal incidence, about one third of the incident radiation is absorbed at each mirror (depending on the specific incidence angle spectrum); in the case of grazing incidence, typical absorption values ​​are one quarter or one fifth. For comparison: in a refractive medium with an antireflection coating, the intensity of the absorption is of the order of a few parts per thousand. This explains the significantly more significant changes in temperature in EUV optical units, which are of the order of a few Kelvin, compared to earlier systems, where the temperature changes were at most a few tenths of a Kelvin.

[0006] Since temperature gradients within the material of the optical element (due to the material's coefficient of thermal expansion) are converted into surface defects or deformations on the surface of the optical element irradiated by the used or exposure radiation, these temperature gradients (especially in the mirrors) lead to significant optical aberrations, which are dependent on the wavelength used, leading to a degradation of the imaging properties of the projection exposure apparatus. Mirrors used for EUV lithography, more precisely their substrates, are therefore made of materials with a particularly low coefficient of thermal expansion, such as or ("Ultra-low expansion" material).

[0007] These zero expansion materials interact with constituent parts or phases having positive and negative thermal expansion coefficients. This results in an effectively nonlinear relationship between thermal expansion and temperature, where there is exactly one temperature value at which thermal expansion vanishes or is least sensitive to temperature changes; specifically, this is the so-called zero crossing temperature (ZCT).

[0008] During exposure operation of the projection exposure apparatus, the mirrors from the projection optical unit are exposed to varying radiation intensities locally due to different illumination and diffraction structures on the mask and temporally due to different operating states. However, the average temperature of the mirrors or the mirror material should be kept close to the zero crossing temperature so that as few aberrations as possible occur which are the result of surface deformations caused by temperature gradients.

[0009] In order to achieve this, it is known practice to use a heating radiation source which radiates heating radiation onto the surface of the respective reflector. The heating radiation source usually operates in the infrared wavelength range ("IR heater") and can be used during an exposure operation, wherein the surface of the respective reflector is irradiated by the radiation used, and / or during an operation interruption outside the exposure operation, wherein the heating radiation source in the latter case serves to preheat the respective reflector before the exposure operation. In order to keep the heat input as constant as possible during the exposure operation, the heating radiation source can radiate with a high heating power when the respective reflector does not absorb the exposure radiation or absorbs only a small part of the exposure radiation, and the heating power can be reduced when the heat input of the exposure radiation increases. The IR heating radiation source can usually irradiate only a part of the entire surface of the optical element and therefore does not provide a uniform temperature or temperature distribution throughout the three-dimensional body of the optical element.

[0010] DE 10 2019 219 289 A1 describes a heating device for heating an optical element, comprising: a plurality of IR emitters for irradiating an optically active surface of the optical element with IR radiation, the IR emitters being able to be switched on and off independently of one another in order to variably set different heating curves in the optical element, and at least one beam shaping unit for shaping the beam of IR radiation directed from the IR emitters to the optically active surface. The beam shaping unit may comprise a plurality of beam shaping segments for irradiating different segments on the optically active surface of the optical element with IR radiation. For example, the beam shaping segments may be different areas on a diffractive optical element (DOE).

[0011] DE 10 2014 212 691 A1 and DE 10 2015 203 267 A1 describe an optical system for a lithographic apparatus and a lithographic apparatus, comprising: an optical element having an optical surface and a temperature control unit, the temperature control unit being configured to supply heat to the optical element and / or to dissipate heat from the optical element in order to keep a deformation of the optical element constant or to control a deformation of the optical element during exposure of the optical surface. In one example, the temperature control unit is configured to project a temperature control point onto the optical surface, the temperature control point being designed, for example, in the form of an infrared light emitting diode.

[0012] During exposure of the optical surface, the temperature control unit can be used to (actively) control the deformation of the optical element so that, when further aberration-related factors are included, optical aberrations associated with the operating light are minimized. The optical aberrations can be measured by means of a sensor. If the thermal expansion coefficient of the substrate of the optical element varies for manufacturing-related reasons, the temperature control unit can be configured to supply heat to the optical element in a manner that depends on the measured thermal expansion coefficient curve. In this case, the thermal expansion coefficient curve of the optical element can be predetermined and stored. During the exposure operation, the temperature control unit can control the heat supply based on the stored thermal expansion coefficient curve.

[0013] DE 10 2015 224 281 A1 describes a method for manufacturing a mirror for an EUV lithography apparatus, wherein in a first step an expected heat flux distribution on the mirror is determined. In a second step, a plurality of heating zones are formed on the mirror based on the determined heat flux distribution. In a third step, the respective heating zones are provided with respective heating devices for heating the respective heating zones based on the measured temperature of the respective heating zones or the expected heat flux distribution on the mirror. In this way, the temperature should remain constant or almost constant in all heating zones of the mirror in order to ensure a temperature distribution in the volume of the mirror that is as constant as possible. The heating zones can be preheated before the exposure operation, wherein different heating zones can be preheated to different degrees. At least one heating device can heat the corresponding heating zone with a pattern, which pattern can include, for example, an annular contour or a part of an annular contour. The pattern can correspond exactly to the heat flux distribution, so that the temperature can remain constant within the heating zone. The heating device can be a resistive heater and / or a heat emitter, which emits radiation in the infrared range, or more precisely an IR heater head. Summary of the invention

[0014] It is an object of the present invention to provide a method for operating a projection exposure apparatus and a projection exposure apparatus with reduced aberrations.

[0015] Subject matter of the invention

[0016] According to a first aspect, this object is achieved by a method of the type mentioned at the outset, wherein during heating in an interruption in operation, a non-uniform temperature distribution is generated (in a targeted manner) on at least one portion of the surface of the optical element ("heating zone"), which reduces the aberrations of the projection exposure apparatus, preferably minimizes the aberrations of the projection exposure apparatus. The irradiated portion can be the entire surface of the optical element, which is designed for reflecting exposure radiation and comprises a reflective coating for this purpose. However, it is also possible that the portion forms a partial area of ​​the optical use area on the surface of the optical element. Reducing the aberrations of the projection exposure apparatus can involve different applications listed below. In particular, the non-uniform temperature distribution can be used to reduce the aberrations of the projection exposure apparatus during an interruption in operation, i.e. when the optical element is not irradiated by exposure radiation and thermal radiation is radiated onto the optical element.

[0017] Basically, a distinction can be made between two operating states during operation of the projection exposure apparatus: exposure operation, during which an optical element, usually in the form of a mirror, is irradiated with exposure radiation for exposing a substrate, and non-exposure operation, in the form of an operating interruption, during which no exposure radiation is present in the projection exposure apparatus. The above-mentioned heating during an operating interruption is usually followed by exposure operation, i.e. preheating of the corresponding optical element during the operating interruption.

[0018] The following situations (phases) may occur during the corresponding operating state (exposure operation or interruption operation):

[0019] 1) The performance of the projection exposure apparatus is (very) good, ie with low aberrations, without exposure radiation, ie during interruptions in operation, and without additional use of heating radiation.

[0020] 2) The performance of the projection exposure apparatus in the presence of exposure radiation but without preheating of individual or several optical elements using heating radiation is (very) poor, for example because the temperature distribution of the optical elements varies significantly over time (optionally by several Kelvin, see above) and thus also the aberrations vary significantly over time when the exposure radiation is switched on. In particular, these changes over time may occur more rapidly or lead to other aberrations than those which can be corrected in time by other correction options of the projection exposure apparatus (e.g. by rigid body movements). Furthermore, aberrations may arise due to unsuitable material properties, for example variations in the mean zero-crossing temperature from optical element to optical element for production-related reasons.

[0021] In order to improve the imaging quality of the projection exposure device, for example in order to better image very small structures using the exposure radiation, it may be advantageous to find a compromise between the two stages 1) and 2), for example in the form of preheating by heating radiation also taking place already in stage 1), and in this way reducing the temperature change over time in stage 2), as described, for example, in DE 10 2015 224 281 A1 cited at the outset, the entire content of the latter being incorporated into the present application by reference.

[0022] DE 10 2015 224 281 A1 proposes to keep the temperature constant or almost constant in all heating zones of the mirror during the exposure operation in order to ensure a temperature distribution of the mirror volume that is as constant as possible. The heating zones can be preheated before the exposure operation so that a zero-crossing temperature is approximately set or obtained at the respective heating zone. The respective heating zone should be kept as permanently as possible at or near the zero-crossing temperature in this way, in particular in order to reduce the temporal variation of the average temperature in the respective heating zone in the case of alternating operating modes. Both simulations and measurements have shown that such preheating of the respective optical element in the respective heating zone can significantly improve the imaging quality of the projection exposure apparatus.

[0023] However, it has been shown that the generation of a uniform (constant) temperature in the respective heating zone by preheating can lead to undesirable deteriorations in the imaging quality of the projection exposure apparatus during stage 2), i.e. during the exposure operation. These deteriorations were accepted in the past; in particular, they can be traced back to the fact that the optical element usually needs to be preheated non-uniformly in order to generate a uniform temperature in the respective heating zone. Even in the case of a uniform temperature distribution in the respective heating zone, the heating zone still has a spatially dependent deformation of the optical element, although this deformation is significantly reduced in comparison with a non-uniform temperature distribution in the respective heating zone. In addition, by means of a uniform temperature distribution, individual properties of the optical element which can be traced back to material and manufacturing variations during the production of the optical element and which can contribute to aberrations in stage 2) are not taken into account during preheating.

[0024] It has also been observed that the deformation of the optical element depends strongly on the way in which different parts of the optical element are heated. It is usually not the case that all surface areas of the optical element are accessible from the outside for heating purposes; in any case the entire volume of the optical element cannot be heated to a three-dimensionally uniform temperature. Often, parts of the optical element that are heated uniformly, for example the entire optically used surface area, are not optimal for the imaging quality of the projection exposure apparatus. It may be significantly more advantageous to provide the corresponding parts with a non-uniform temperature distribution.

[0025] The invention therefore proposes not to set or generate a uniform temperature distribution in the respective portion irradiated by the heating radiation, but instead to set or generate a targeted non-uniform temperature distribution.

[0026] Preferably, a non-uniform temperature distribution over at least one portion reduces aberrations of the projection exposure apparatus relative to a uniform temperature distribution over at least one portion. A typical assumption made during the comparison between a non-uniform temperature distribution and a uniform temperature distribution is that the uniform temperature distribution has a temperature that is constant over the portion and corresponds to the temperature when the non-uniform temperature distribution is averaged over the portion. In this case, the non-uniform temperature distribution is selected such that aberrations of the projection exposure apparatus are reduced - compared to the heating of the portion with a uniform temperature distribution. The reduced aberrations can in particular be wavefront aberrations. The reduced aberrations or wavefront aberrations can be traced back, for example, to manufacturing variations, the preheating itself, a non-uniform spatial distribution of the exposure radiation to be expected during a subsequent exposure operation, etc. The aberrations or wavefront aberrations to be corrected are, for example, known from measurements or can be estimated by means of predictions (for example in the case of a spatial distribution of the exposure radiation to be expected).

[0027] By means of a targeted non-uniform temperature distribution, various effects can be corrected better than by means of a uniform temperature distribution. In particular, individual optical elements can be temperature-controlled in advance (good for stage 2)) without (too much) compromising the performance in stage 1). Furthermore, targeted non-uniform preheating allows the performance of the projection exposure apparatus (i.e. the imaging quality) to also be improved over the above-described situation in which no heating radiation source is used in stage 1).

[0028] In a variant of the method, the average temperature of the inhomogeneous temperature distribution in the portion of the surface deviates from the zero-crossing temperature of the optical element by no more than ±1.5 K, preferably by no more than ±0.5 K. The zero-crossing temperature of the optical element is understood to mean the zero-crossing temperature of the material forming the body or substrate of the optical element. In this case, the substrate or body material is a zero-expansion material, for example doped quartz glass, in particular titanium-doped quartz glass, such as is available under the trade name Commercially available, or glass ceramics, e.g.

[0029] Typically, the zero-crossing temperature of the material is substantially constant within the volume of the substrate and therefore also constant in the portion on the surface of the substrate, i.e., the spatially dependent deviations are small. For the considerations explained below, the spatially dependent deviations are generally negligible, and therefore for the considerations explained below, the zero-crossing temperature in the portion of the optical element or over the entire optically used surface is considered to be constant. As described above, it is advantageous if the heated portion is heated to approximately the zero-crossing temperature by preheating, because the optical element is least sensitive to temperature changes at or near the zero-crossing temperature. As also described above, the generation of a uniform temperature that is constant over the portion and (substantially) corresponds to the zero-crossing temperature does not necessarily lead to minimal aberrations of the projection exposure apparatus during interrupted operation; on the contrary, it may be advantageous to deviate from the uniform temperature distribution in the portion in a targeted manner in order to achieve a targeted deformation. The extent to which the average temperature of the non-uniform temperature distribution is allowed to deviate from the zero-crossing temperature depends in particular on the heating power of the incident EUV radiation and the material properties of the optical element. The specified values ​​of 1.5 K and 0.5 K are guidelines. In certain applications, it may be optionally advantageous to allow larger deviations than the specified values.

[0030] In a further variant, the method comprises determining an inhomogeneous temperature distribution which reduces or minimizes aberrations of the projection exposure apparatus, wherein individual material-specific properties of the optical element on which the inhomogeneous temperature distribution is generated are preferably taken into account when determining the inhomogeneous temperature distribution.

[0031] In principle, the inhomogeneous temperature distribution can be determined in advance or for different applications during operation of the projection exposure apparatus (“real time”).

[0032] For example, possible applications are:

[0033] a) When neither exposure radiation nor heating radiation is present (eg for preliminary temperature control to approximately the zero crossing temperature, see above), the projection exposure apparatus already has certain aberrations.

[0034] b) When no exposure radiation is present but heating radiation has been used (eg for preliminary temperature control to approximately the zero crossing temperature, see above), the projection exposure apparatus already has certain aberrations.

[0035] c) Projection exposure apparatuses have certain aberrations when operating with exposure radiation. Optionally, in addition to the exposure radiation, even during the exposure operation, there may be a temperature input by means of heating radiation.

[0036] d) The projection exposure apparatus has certain aberrations (with or without thermal load) that differ from other projection exposure apparatus of the same type (tool-to-tool variations).

[0037] In this context, the above applications partially correspond to the above stages 1) and 2), wherein a) corresponds to stage 1) and c) corresponds to stage 2); b) represents an intermediate state. Further applications may be advantageous.

[0038] A non-uniform temperature profile which reduces or minimizes the relevant aberrations, such as deformations or wavefront aberrations, can be determined for each of the above-mentioned applications. It is also possible to add a plurality of aberrations and determine a non-uniform temperature profile which reduces or minimizes the sum of the aberrations. For example, application d) can be combined with one of applications a) to c) in order to correct aberrations of the projection exposure apparatus which are traceable to manufacturing inaccuracies when producing a plurality of projection exposure apparatuses of the same type and aberrations which occur in the same way in all projection exposure apparatuses of the same type. In particular, individual material-specific properties of the substrate of the optical element can be taken into account in case d), which material-specific properties can be traced back to manufacturing variations during the production of the optical element and distinguish the optical element of the projection exposure apparatus from another optical element of the same type.

[0039] For example, a non-uniform temperature distribution can be determined by simulation, optionally including individual material-specific properties of the relevant optical element, such as an average distribution of zero-crossing temperatures of the bulk or substrate of the optical element on which the non-uniform temperature profile is created, in an optional alternative, said distribution of zero-crossing temperatures also being determined with spatial resolution. As a rule, a feed-forward model is required in application c) for the spatially dependent radiation distribution expected during the exposure operation.

[0040] The inhomogeneous temperature distribution can also be determined experimentally, for example by varying different heating powers when the optical element is irradiated with heating radiation, wherein the heating power can also be varied in a spatially dependent manner, or the optical element is irradiated with different heating radiation profiles and the aberrations generated in the process are measured. In addition to the measurement of the (wavefront) aberrations with the aid of suitable measuring devices, it is also possible to carry out measurements of the temperature distribution of the optical element, for example with the aid of an IR camera, or measurements of the surface deformation of the optical element and use these measurements to determine the inhomogeneous temperature profile.

[0041] The correction of aberrations, in particular wavefront aberrations, described in the above applications does not necessarily require targeted inhomogeneous heating of one or more optical elements generating the wavefront aberrations by means of aberration-generating heating radiation. Targeted heating for generating an inhomogeneous temperature profile can also (but need not) be performed on other optical elements (using compensation effects in the projection exposure apparatus). In this case, the aberrations to be corrected may arise due to thermal effects, but may also arise due to other material deformations or changes in the position of components and may in particular also originate from non-optical components.

[0042] In one variant, the non-uniform temperature distribution reduces or minimizes aberrations in the form of wavefront aberrations, which arise on a portion of the surface when the optical element is heated during an interruption in operation. This variant corresponds to application b) above: as described herein, aberrations that can be compensated by the non-uniform temperature distribution occur when the portion is heated to an average temperature approximately corresponding to the zero-crossing temperature of the optical element. Preheating of the optical element is typically carried out from a reference temperature (which can be, for example, 22° C.) to an average temperature approximately corresponding to the zero-crossing temperature. In this case, wavefront aberrations arise due to a non-uniform heat flux distribution in the optical element, due to material changes during the production of the optical element, due to manufacturing effects during the production of the optical element, etc. The non-uniform temperature distribution can be used to correct or minimize these wavefront aberrations to a large extent.

[0043] It will be appreciated that the non-uniform temperature distribution can additionally be designed to reduce or minimize aberrations or wavefront aberrations of the projection exposure apparatus or of the optical element, which are present when the optical element is not irradiated with heating radiation; this corresponds to application a) above. A non-uniform temperature distribution suitable for this purpose can be determined in the manner described above, for example by means of measuring the (individual) wavefront aberrations of the optical element. In this case, the wavefront aberrations described in case d) can also be measured and taken into account, which can be traced back to material variations in the (glass) blank ("blank") or manufacturing influences. Optionally, the manufacturing influences on the individual mirrors of different projection exposure apparatuses of the same type are similar, with the result that the desired non-uniform temperature distribution of the optical element is known at an early stage. In this way, the beam shaping element can be appropriately designed at an early stage in order to create the non-uniform temperature distribution by means of an appropriate heating radiation profile. As long as the beam shaping element comprises a sufficient number of degrees of freedom, for example in the form of a plurality of individually controllable segments, there is in any case a certain flexibility within the scope of creating the desired non-uniform temperature distribution. As mentioned above, wavefront aberrations generated at one optical element may also be fully or partially corrected at (at least) one other optical element.

[0044] In a variant, the non-uniform temperature distribution is generated by irradiating the portion with heating radiation having at least one continuous heating radiation profile formed by at least one beam shaping element. The continuous heating radiation profile can be generated by means of a suitable beam shaping element, for example by means of a diffractive optical element or by means of a part (segment) of a diffractive optical element. The diffractive optical element can be matched on an individual basis to the material properties of an optical element (e.g. a mirror) which is used in a projection exposure apparatus and irradiated with the heating radiation: different non-uniform temperature distributions and therefore different diffractive optical elements can also be used for different projection exposure apparatuses of the same type. However, this is not mandatory, since the manufacturing influences during the production of the individual mirrors are optionally similar (see above), so that the same diffractive optical element is optionally used as a beam shaping element for different projection exposure apparatuses of the same type. Typically, the respective beam shaping element is designed to create a fixedly predefined heating radiation profile. A heating radiation profile is understood to mean the heat flux density distribution generated by the respective beam shaping element in combination with the heating radiation source.

[0045] In order to generate the non-uniform temperature distribution, the portion may be irradiated using a single continuous heating radiation profile. However, it is also possible that the portion of the surface is irradiated with two or more heating radiation profiles generated by two or more beam shaping elements. In this case, for example, respective areas that are larger than the portion of the surface may be irradiated by the heating radiation profiles and these areas overlap in the portion of the surface in order to generate the non-uniform temperature profile.

[0046] Since the above-mentioned applications can take place gradually during the use of the projection exposure apparatus, the non-uniform temperature distribution to be set can in each case also be varied over time (within the same projection exposure apparatus), i.e. different non-uniform temperature profiles can be set depending on the application. When switching between applications, it may be advantageous to keep the heating power of the heating radiation or the non-uniform temperature distribution constant, as described below.

[0047] In a variant, the heating power of the heating radiation used to generate the inhomogeneous temperature distribution during the interruption in operation is maintained during a subsequent exposure operation of the projection exposure apparatus, wherein the exposure radiation irradiates the surface of the optical element during the exposure operation. During the heating in the interruption in operation, a stationary state is reached after a certain amount of time, wherein a (stationary) inhomogeneous temperature distribution (preheating temperature) is generated on the optical element. In this case, in one of the two applications a) and b), the heating power introduced into the optical element during the interruption in operation is fixed, i.e. the heating power is maintained even if an additional heat load (e.g. in the form of exposure radiation) is generated at the respective optical element. In general, fixing or maintaining the heating power reduces the control complexity during the exposure operation and leads to a stationary state being reached again relatively quickly during a further interruption in operation after the exposure operation, in which the inhomogeneous temperature distribution is again reached.

[0048] In an alternative variant, the heating power of the heating radiation during the interruption in operation is modified during the subsequent exposure operation (during which the exposure radiation irradiates the surface of the optical element) in order to maintain (as much as possible) the non-uniform temperature distribution in parts of the surface. It is also advantageous to maintain the non-uniform temperature distribution generated or set during the interruption in operation during the exposure operation. In order to achieve this, it is usually necessary to modify the heating power of the heating radiation due to the heat input into the optical element due to the exposure radiation. In this case, the heating power of the heating radiation is modified using the degrees of freedom that exist when providing the heating radiation or the degrees of freedom from the available heating radiation sources (see below) so that the sum of all heat loads on the optical element corresponds to the best possible degree of non-uniform temperature distribution generated during the preheating during the interruption in operation. This can be achieved, for example, by keeping the average temperature of the entire surface as constant as possible or by supplying less heating radiation to the surface area heated by the exposure radiation than was the case during the preheating. In order to achieve this, the corresponding area heated by the exposure radiation must be subjected to a sufficient amount of preliminary temperature control during the preheating.

[0049] If such a reduction of the heating power of the heating radiation is possible for a plurality of different operating modes and exposure radiation distributions on the surface of the optical element, the preheating should be designed accordingly for a "common" profile of the non-uniform temperature distribution, i.e. it should involve sufficiently large heating in different areas of the surface of the optical element.

[0050] Another aspect of the invention relates to a projection exposure device of the aforementioned type, wherein, in order to reduce the aberrations of the projection exposure device, the heating device is designed or programmed to produce a non-uniform temperature distribution on at least a portion of the surface of the optical element during heating in an operation interruption, wherein the non-uniform temperature distribution on at least a portion preferably reduces the aberrations of the projection exposure device compared to a uniform temperature distribution on at least a portion.

[0051] The projection exposure apparatus may be an EUV lithography apparatus, but may also be a UV lithography apparatus operating with UV radiation in a wavelength range of less than about 370 nm. The heating device comprises at least one heating radiation source for generating heating radiation or irradiating a portion of the surface. The heating radiation source may be designed to generate a predefined heating power when activated; however, the heating power of the heating radiation source may also be continuously adjustable. The heating device is designed as or comprises a programmable controller for controlling the at least one heating radiation source so as to activate the heating radiation source (also) during interruptions in operation so as to generate a non-uniform temperature distribution on at least a portion of the surface.

[0052] In one embodiment, the heating device is designed or programmed to generate an average temperature of the non-uniform temperature distribution in the portion of the surface which deviates from the zero-crossing temperature of the optical element by no more than ±1.5 K, preferably by no more than ±0.5 K. As mentioned above, in order to reduce aberrations in the exposure operation, it is advantageous to preheat the optical element to a temperature close to the zero-crossing temperature of the optical element before the exposure operation.

[0053] In a further embodiment, the heating device for creating a non-uniform temperature distribution comprises at least one heating radiation source for creating the heating radiation and at least one beam shaping element for creating a continuous heating radiation profile. For example, the use of a continuous heating radiation profile generated by a beam shaping element in the form of a diffractive optical element is advantageous for generating a non-uniform temperature distribution. The heating radiation source may be an IR radiation source, such as an IR laser, an IR diode, etc.

[0054] The heating device can be designed in different ways. For example, the heating device can be designed as in DE 10 2019 219 289 A1 cited at the beginning, the entire content of the latter being incorporated into the present application by reference. For example, the heating device can include a plurality of heating light sources in the form of IR emitters, which can be switched on and off independently of one another in order to generate different heating radiation profiles. As described in DE 10 2019 219 289 A1, a plurality of beam shaping elements or beam shaping segments of the same diffractive optical element can be used to generate corresponding individually adapted non-uniform temperature distributions on different parts of the surface of the optical element. Alternatively, a plurality of heater heads can be used, each of which generates only one heating radiation profile of the heating radiation, with which the corresponding part of the surface of the optical element is irradiated. However, it is generally more advantageous to use a heating device or heater head that is capable of generating a plurality of heating radiation profiles, since this requires much less installation space for the same number of available degrees of freedom.

[0055] It is also possible that the heating device is designed to irradiate spatially overlapping (common) portions on the surface of the optical element with heating radiation generated by two or more beam shaping elements, with the result that the heating radiation profiles overlap in space. As a result of the common portions being irradiated sequentially or simultaneously with the heating radiation, different heating radiation profiles can be generated there. The (non-uniform) temperature distribution in the respective portions results from the heating radiation profile (i.e. the intensity distribution of the heating radiation on the surface of the optical element in the respective portions) in combination with the material properties of the optical element and the thermal boundary conditions of the entire projection exposure apparatus.

[0056] The above-mentioned heating device can be used not only during interruptions in operation but also during exposure operation in order to optimize the non-uniform temperature distribution in the corresponding parts (i.e. not only the average value of the temperature) with regard to aberrations or wavefront aberrations and to adapt the non-uniform temperature distribution to different operating modes of the projection exposure device, wherein in each case different heat loads are generated in the corresponding parts which can therefore be compensated.

[0057] In principle, the entire optically used part of the optical element can be irradiated using a heating device which comprises only one heating radiation source for generating the heating radiation and only one beam shaping element for generating exactly one heating radiation profile. In this case, it is particularly advantageous if the beam shaping element, which is typically in the form of a diffractive optical element, has a very significant adaptability to the boundary conditions of the respective projection exposure apparatus, such as to the individual expansion behavior of the optical element, such as its zero-crossing temperature, to the EUV and IR reflectivity of the respective optical element, to the pressure conditions within the projection exposure apparatus, etc. However, the correction options in this case are strongly coupled to the boundary conditions and the predefined aberrations to be corrected. In contrast, due to the greater number of degrees of freedom, the heating device described in DE 10 2019 219 289 A1 allows a certain degree of response to changes in the boundary conditions or to changes in the aberrations to be corrected even during exposure operation of the projection exposure apparatus, without the need to replace the heating device or parts thereof, such as the beam shaping device, for this purpose.

[0058] In principle, the heating arrangement may additionally also comprise at least one piece of heating equipment designed to generate a uniform heating radiation profile, ie a piece of heating equipment designed to generate a uniform heat flux density distribution in a portion of the surface of the optical element.

[0059] In a further embodiment, the heating device is designed to maintain the heating power of the heating radiation used to generate the non-uniform temperature distribution during the operation interruption during a subsequent exposure operation of the projection exposure apparatus, or to modify the heating power of the heating radiation used to generate the non-uniform temperature distribution during the operation interruption during a subsequent exposure operation, so as to maintain the non-uniform temperature distribution in the portion of the surface during the exposure operation. As described above, it may be advantageous to maintain the heating power of the heating radiation during the exposure operation, which results in a non-uniform temperature distribution during the operation interruption. Alternatively, the heating power may be modified during the exposure operation, so that the non-uniform temperature distribution generated during the operation interruption is also maintained during the exposure operation.

[0060] Other features and advantages of the invention will become apparent from the following description of a working embodiment of the invention, with reference to the accompanying drawings showing details essential to the invention, and from the claims. The individual features can be realized in a variant of the invention either individually or in any combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Exemplary embodiments are shown in the schematic diagram and explained in the following description. In detail:

[0062] Figure 1 Schematically shows a meridional cross section of a projection exposure apparatus for EUV projection lithography,

[0063] Figure 2a , 2b Shown from Figure 1 schematic diagram of an optical element of a projection exposure apparatus and a heating device for heating the optical element during interruptions in operation and during exposure operation,

[0064] Figure 3a , 3b Schematic diagram showing three sections of a surface of an optical element with three heating radiation profiles and three inhomogeneous temperature distributions emerging from the heating radiation profiles,

[0065] Figure 4 Schematic diagram showing the wavefront aberration of an optical element in the case of a uniform temperature distribution and in the case of a non-uniform temperature distribution in one of the parts. DETAILED DESCRIPTION

[0066] In the following description of the drawings, the same reference numerals are used for components that are the same or have the same function.

[0067] The following description is made with reference to 1 by way of example of the basic components of an optical arrangement for EUV lithography in the form of a projection exposure apparatus 1 for microlithography. The description of the basic arrangement of the projection exposure apparatus 1 and its components should not be considered restrictive here.

[0068] An embodiment of an illumination system 2 of a projection exposure apparatus 1 has, in addition to a light source or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided in the form of a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.

[0069] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable via a reticle displacement drive 9, in particular in a scanning direction.

[0070] For the purpose of explanation, a Cartesian xyz coordinate system is depicted in 1. The x direction extends vertically into the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. The scanning direction is Figure 1 The z direction extends perpendicular to the object plane 6.

[0071] The projection exposure apparatus 1 comprises a projection system 10. The projection system 10 is used to image an 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. The displacement of the reticle 7 by the reticle displacement drive 9 on the one hand and the displacement of the wafer 13 by the wafer displacement drive 15 on the other hand can be synchronized.

[0072] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, which is hereinafter also referred to as used radiation, illumination radiation or illumination light. In particular, the used radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 may be a plasma source, such as an LPP (laser produced plasma) source or a GDPP (gas discharge produced plasma) source. It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).

[0073] The illumination radiation 16 emitted from the radiation source 3 is focused by a collector reflector 17. The collector reflector 17 may be a collector reflector having one or more elliptical and / or hyperbolic reflective surfaces. The illumination radiation 16 may be incident on at least one reflective surface of the collector reflector 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence less than 45°. The collector reflector 17 may be constructed and / or coated, firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0074] Downstream of the collector reflector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may constitute a separation between a radiation source module, which comprises the radiation source 3 and the collector reflector 17, and the illumination optics unit 4.

[0075] The exposure optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 downstream of the deflection mirror 19 in the beam path. The deflection mirror 19 can be a plane deflection mirror or, alternatively, a mirror with a beam-influencing effect that goes beyond a pure deflection effect. In an alternative or in addition thereto, the deflection mirror 19 can be designed as a spectral filter, which separates the used light wavelengths of the illumination radiation 16 from extraneous light of different wavelengths. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Figure 1 Only some of the facets 21 are shown by way of example. In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

[0076] The illumination optical unit 4 thus forms a two-facet system. This basic principle is also known as a fly's eye integrator. The individual first facets 21 are imaged into the object field 5 using a second facet mirror 22. The second facet mirror 22 is the last beam shaping mirror or in fact the last mirror in the beam path upstream of the object field 5 for the illumination radiation 16.

[0077] The projection system 10 comprises a plurality of mirrors Mi which are numbered consecutively according to their arrangement in the beam path of the projection exposure apparatus 1 .

[0078] In the example shown in 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives 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 passage opening for the illumination radiation 16. The projection system 10 is a doubly shielded optical unit. The projection optical unit 10 has an image side numerical aperture which is greater than 0.3 or 0.5 and may also be greater than 0.6 and may be, for example, 0.7 or 0.75.

[0079] Just like the reflector of the illumination optical unit 4 , the reflector Mi can have a highly reflective coating for the illumination radiation 16 .

[0080] Figure 2a , Figure 2bBy way of example, one of the six reflectors Mi of the projection optical unit 10 of the projection exposure apparatus 1 in 1 and a heating device 24 are shown. In the example shown, the heating device 24 comprises three heating radiation sources 27a-c, which are designed as IR lasers and are connected to a heater head 26 of the heating device 24 via corresponding optical fibers 28a-c. Three pieces of beam shaping devices in the form of diffractive optical elements 29a-c (which are only Figure 2a , 2b The heater head 26 is arranged in a heater head 26. The heater head 26 is used to irradiate the mirror Mi, more precisely the surface 30 of the mirror Mi formed on the top side of the substrate 29 of the mirror Mi, with heating radiation 31. For example, Figure 2a , 2b The heating device 24 shown in can be designed as described in DE 10 2019 219289 A1.

[0081] In the example shown, the substrate 29 consists of titanium-doped quartz glass, more precisely of It has a zero-crossing temperature T ZC To simplify the problem, the zero-crossing temperature T of the substrate 29 ZC In the following it is considered to be constant over the volume of substrate 29 ; however, it typically has small manufacturing-related variations in the volume of substrate 29 .

[0082] Figure 2a The optical element Mi is shown during an operating interruption P, during which the surface 30 of the substrate 29 is not irradiated by EUV radiation. Figure 2b The optical element Mi is shown during an exposure operation B of the projection exposure apparatus 1, wherein EUV radiation 16 irradiates a surface 30 of a substrate 29. Figure 2a , 2b 2, but a highly reflective coating for reflecting EUV radiation 16 has been applied to the top side of the substrate 29. In the example shown, the reflective coating comprises a plurality of alternating layers of silicon and molybdenum.

[0083] As from Figure 3aIt is obvious that three portions TBa-c of the surface 30 are irradiated by the heating radiation 31, wherein the outer contour of the portions corresponds to the area of ​​the surface 30 of the mirror Mi (shown in FIG. 2 and circular in the example shown), which is irradiated by EUV radiation 16. In the example shown, the three portions TBa-c are designed as circular sectors and extend over an angle of 120° in each case. It goes without saying that this is not mandatory and that the portions TBa-c may have different geometries and, in particular, do not need to have the same size. It is also possible that the three portions TBa-c together cover an area that is larger or smaller than the area of ​​the surface 30 irradiated by EUV radiation 16, in particular because the size and shape of the area of ​​the surface 30 irradiated by the exposure radiation in the form of EUV radiation 16 may vary depending on the illumination settings of the projection exposure apparatus 1.

[0084] Figure 3a Very schematically, the intensity distribution of the heating radiation 31 or the lines of equal intensity of the corresponding continuous heating radiation profiles 32a-c from the surface 30 of the irradiation mirror Mi of the heater head 26 is shown. In this case, the corresponding heating radiation profiles 32a-c are generated by one of three beam shaping elements 29a-c accommodated in the heater head 26 and assigned to the corresponding heating radiation sources 27a-c. The three heating radiation sources 27a-c can be switched on and off independently of one another. A controller of the heating device 24, not shown here, can be used for control purposes; the controller can be suitable hardware and / or software, for example in the form of a control computer. Switching the heating radiation sources 27a-c on and off allows the three parts TBa-c to be irradiated independently of one another and in each case by a different heating radiation profile 32a-c, as is evident from FIG. 3a. In this case, the corresponding heating radiation profiles 32a-c are fixedly predefined by the corresponding beam shaping elements 29a-c and can generally no longer be modified during operation of the projection exposure apparatus 1.

[0085] exist Figure 3b The non-uniform temperature distribution 33a-c in the respective portion TBa-c depicted in FIG. 1 results from the heating radiation profile 32a-c, i.e. the intensity distribution of the heating radiation 31 on the surface 30 of the mirror Mi in the respective portion TBa-c, in combination with the material properties of the mirror Mi and the thermal boundary conditions of the entire projection exposure apparatus 1. Figure 3b As shown, exemplary equal temperature lines are shown. Due to the above reasons, the non-uniform temperature distribution 33a-c in the corresponding parts TBa-c is different from Figure 3a The radiated heating radiation profiles 32a-c shown in FIG.

[0086] The non-uniform temperature distribution 33a-c generated in the respective parts TBa-c during the operating interruption P is designed or selected such that aberrations of the projection exposure apparatus 1 can be reduced, preferably minimized. In the example shown, the heating powers Pa, Pb, Pc of the respective heating radiation sources 27a-c can be set, whereby the non-uniform temperature distribution 33a-c can be modified in the respective parts TBa-c. It is assumed hereinafter that the heating powers Pa, Pb, Pc of the three heating radiation sources 27a-c are constant or remain constant and that a stationary operating state has been set during the operating interruption P in which the non-uniform temperature distribution 33a-c does not change over time.

[0087] The non-uniform temperature distribution 33a-c on the respective part TBa-c can be designed or optimized for different applications, wherein aberrations of the projection exposure apparatus 1 can in each case be reduced or ideally minimized during the operating interruption P and / or during the exposure operation B.

[0088] exist Figure 3a , 3b In the example shown in , a non-uniform temperature distribution 33a-c is selected or defined in the respective portion TBa-c such that when neither heating radiation 31 nor exposure radiation 16 is radiated onto the mirror Mi, these aberrations of the projection exposure apparatus 1 which have been generated by the mirror Mi are reduced. Furthermore, the mirror Mi is heated from a reference temperature, typically room temperature (22° C.), to a mean temperature T averaged over the respective portion TBa-c. Ma 、T Mb 、T Mc The aberration produced is also reduced.

[0089] Due to a suitable design of the heating device 24, the average temperature T of the non-uniform temperature distribution 33a-c in the corresponding portion TBa-c of the surface 30 is Ma 、T Mb 、T Mc is defined such that it deviates from the zero-crossing temperature T of the substrate 29 of the optical element Mi. ZC The average temperature T of the non-uniform temperature distribution 33a-c does not exceed ±1.5 K, and usually does not exceed ±0.5 K. Ma 、T Mb 、T Mc Not forced to be at zero temperature T ZC Within the specified interval around. Figure 3b The non-uniform temperature distribution 33a-c shown in also allows correcting wavefront aberrations of the mirror Mi, which can be traced back to precisely the individual material properties of this mirror Mi; these are wavefront aberrations which occur only for this mirror Mi and not for other mirrors of the same type used in other projection exposure devices 1 of the same type.

[0090] The non-uniform temperature distribution 33a-c generated on the corresponding portion TBa-c allows the wavefront aberration to be reduced compared to the uniform temperature distribution in the corresponding portion TBa-c, wherein in the case of the uniform temperature distribution, the corresponding portion TBa-c is constantly heated to the average temperature T of the non-uniform temperature distribution 33a-c. Ma 、T Mb 、T Mc .

[0091] Figure 4 Aberrations are shown in the form of wavefront aberrations which are expressed in the form of Zernike coefficients Z2, Z3 and which are averaged over the three parts TBa-c in the example shown. Figure 4 The left side shows the wavefront aberrations Z2 and Z3 generated in the case of uniform temperature distribution on the three parts TBa-c, while Figure 4 The right side shows the Figure 3b Wavefront aberrations Z2 and Z3 generated in the case of the non-uniform temperature distribution 33a are shown. Obviously, the non-uniform temperature distribution 33a significantly reduces the wavefront aberrations Z2 and Z3 compared with the uniform temperature distribution in the three parts TBa-c.

[0092] In the example described further above, the wavefront aberrations Z2, Z3 are optimized during the operating interruption P for a stationary operating state, in which the mirror Mi is heated on average to a temperature approximately corresponding to the zero-crossing temperature T ZC The desired preheating temperature. It is obvious that additional aberrations occur during the exposure operation B due to the exposure radiation 16 being radiated in. The heating device 24 can also be used to reduce the aberrations occurring during the exposure operation B. There are various options in this regard:

[0093] The heating powers Pa, Pb, Pc of the heating radiation 31 can be maintained for generating the non-uniform temperature distribution 33a-c during an operation interruption P during a subsequent exposure operation B of the projection exposure apparatus 1. During the exposure operation B, a non-uniform temperature distribution 33a-c is also generated in the corresponding parts TBa-c. Figure 3a However, in the exposure operation B, due to the heat input of the exposure radiation 16, the temperature profile on the corresponding portion Tba-Tbc no longer corresponds to Figure 3b The non-uniform temperature profiles 33a-33c are depicted in FIG.

[0094] Alternatively, it is possible that the heating powers Pa, Pb, Pc of the heating radiation 31 which lead to the generation of the non-uniform temperature distribution 33a-c during the operating interruption P are modified by the heating device 24 during the subsequent exposure operation B in order to ideally maintain the corresponding non-uniform temperature distribution 33a-c (see FIG. 3b ) as specified during the operating interruption P in the parts TBa-c of the surface 30. The three heating powers Pa, Pb, Pc of the heating radiation sources 27a-c can be appropriately modified for this purpose. In both cases, the aberrations of the projection exposure apparatus 1 can also be reduced by the heating device 24 during the exposure operation B.

[0095] The non-uniform temperature distribution 33a-c which reduces the aberrations of the projection exposure apparatus 1 can be determined in the respective part TBa-c in different ways. For example, the non-uniform temperature distribution 33a-c can be determined by simulations, optionally including the respective material-specific properties of the relevant mirror Mi, for example taking into account the zero-crossing temperature T of the substrate 29 of the mirror Mi. ZC The average distribution of the zero-crossing temperature 33a-c can also be determined experimentally, for example by varying different heating powers Pa, Pb, Pc when the reflector Mi is irradiated with heating radiation 31, wherein, optionally, it is determined experimentally that the reflector Mi is irradiated with heating radiation 31 with different heating radiation profiles and the aberrations generated in the process are measured. Instead of measuring the (wavefront) aberrations Z2, Z3, ... by means of a suitable measuring device (for example by means of a Shack-Hartmann sensor), it is also possible to carry out a measurement of the temperature distribution of the reflector Mi, for example by means of an IR camera, or to carry out a measurement of the surface deformation of the reflector Mi and use said measurement to determine the non-uniform temperature profile 33a-c.

[0096] The non-uniform temperature distribution 33a-c can also be used to reduce aberrations of the projection exposure apparatus 1 which are not due to the mirror Mi itself, but can be traced back to other effects (use of compensation effects within the projection exposure apparatus). In this case, the aberrations to be corrected may be due to thermal effects, due to other material deformations or due to changes in the position of components and may in particular also originate from non-optical components.

[0097] Obviously, a greater or lesser number of portions TBa, TBb, TBc, ... may be present on the surface 30 of the reflector Mi, wherein each of said portions is irradiated with a heating radiation 31 having a separate heating radiation profile 32a, 32b, 32c, ... In particular, the heating device 24 may comprise only one heating radiation source for generating heating radiation 31 having a single heating radiation profile, which is generated on the surface 30 of the reflector Mi. Alternatively, it is possible that the heating device 24 is designed to generate a plurality of heating radiation profiles 32a, 32b, 32c, ... on the surface 30 of the reflector Mi, said heating radiation profiles overlapping one another or partially overlapping one another.

Claims

1. A method for operating a projection exposure apparatus (1) for microlithography, in particular for EUV lithography, comprising: heating of at least one optical element, preferably at least one mirror (Mi), of the projection exposure apparatus (1) by irradiating a surface (30) of the optical element with heating radiation (31) during an operating interruption (P), wherein the surface (30) of the optical element is not irradiated by the exposure radiation (16), Features During heating in an interrupted operation (P), a non-uniform temperature distribution (33a-c) is generated on at least one portion (TBa-c) of the surface (30) of the optical element, which reduces aberrations (Z2, Z3, ...) of the projection exposure apparatus (1), wherein the non-uniform temperature distribution (33a-c) is generated by irradiating the portion (TBa-c) with heating radiation (31) having at least one continuous heating radiation profile (32a-c) formed by a beam shaping element (29a-c).

2. The method according to claim 1, wherein: The non-uniform temperature distribution (33a-c) over the at least one portion (TBa-c) reduces aberrations (Z2, Z3, ...) of the projection exposure apparatus (1) compared to a uniform temperature distribution over the at least one portion (TBa-c).

3. The method according to claim 1 or 2, wherein: The average temperature (TMa-c) of the non-uniform temperature distribution (33a-c) in the portion (TBa-c) of the surface (30) is proportional to the zero-crossing temperature (T ZC ) deviates by no more than ±1.5K, preferably by no more than ±0.5K.

4. The method according to claim 3, wherein: The non-uniform temperature distribution (33a-c) reduces aberrations in the form of wavefront aberrations (Z2, Z3, ...) generated on the portion (TBa-c) of the surface (30) when the optical element is heated to the mean temperature (TMa-c) during the operating interruption (P).

5. The method according to any one of the preceding claims, further comprising: A non-uniform temperature distribution (33a-c) is determined which reduces the aberrations (Z2, Z3, ...) of the projection exposure device (1), wherein when determining the non-uniform temperature distribution (33a-c), the individual material-specific properties of the optical element (Mi) on which the non-uniform temperature distribution (33a-c) is preferably taken into account.

6. A method as claimed in any one of the preceding claims, wherein: A heating power (Pa, Pb, Pc) of the heating radiation (31) for generating the non-uniform temperature distribution (33a-c) during the interruption operation (P) is maintained during a subsequent exposure operation (B) of the projection exposure apparatus (1).

7. The method according to any one of claims 1 to 5, wherein: The heating power (Pa, Pb, Pc) of the heating radiation (31) used to generate the non-uniform temperature distribution (33a-c) during the operation interruption (P) is modified during a subsequent exposure operation (B) in order to maintain the non-uniform temperature distribution (33a-c) in the portion (TBa-c) of the surface (30).

8. A projection exposure apparatus (1) for microlithography, in particular for EUV lithography, comprising: at least one optical element, in particular at least one mirror (Mi), a heating device (24) for irradiating a surface (30) of the optical element, preferably the reflector (Mi), with heating radiation (31), The heating device (24) is designed to irradiate the surface (30) of the optical element with heating radiation (31) during an operating interruption (P) in which the surface (30) of the optical element is not irradiated with exposure radiation (16), It is characterized in that In order to reduce the aberrations (Z2, Z3, ...) of the projection exposure apparatus (1), the heating device (24) is designed to generate a non-uniform temperature distribution (33a-c) on at least one portion (TBa-c) of the surface (30) of the optical element during the heating in the operation interruption (P), wherein the heating device (24) for generating the non-uniform temperature distribution (33a-c) comprises at least one heating radiation source (27a-c) for generating heating radiation (31) and at least one beam shaping element (29a-c) for generating a continuous heating radiation profile (32a-c), and the non-uniform temperature distribution (33a-c) on the at least one portion (TBa-c) preferably reduces the aberrations of the projection exposure apparatus (1) compared to a uniform temperature distribution on the at least one portion (TBa-c).

9. The projection exposure apparatus according to claim 8, wherein: The heating device (24) is designed to generate a mean temperature (TMa-c) of the non-uniform temperature distribution (33a-c) in the portion (TBa-c) of the surface (30), the mean temperature (TMa-c) being in direct correspondence with the zero-crossing temperature (T ZC ) deviates no more than ±1.5K, preferably no more than ±0.5K.

10. The projection exposure apparatus according to claim 8 or 9, wherein: The heating device (24) is designed to maintain the heating power (Pa, Pb, Pc) of the heating radiation (31) used to produce the non-uniform temperature distribution (33a-c) during the interruption operation (P) during a subsequent exposure operation (B) of the projection exposure device (1), or to modify the heating power (Pa, Pb, Pc) of the heating radiation (31) used to produce the non-uniform temperature distribution (33a-c) during the interruption operation (P) during the subsequent exposure operation (B) so as to maintain the non-uniform temperature distribution (33a-c) in the portion (TBa-c) of the surface (30) during the exposure operation (B).

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