Method for operating a projection exposure apparatus and projection exposure apparatus

Abstract

The present invention relates to a method of operating a projection exposure apparatus for microlithography, which includes a step of heating an optical element by irradiating a surface (30) of the optical element with heating radiation during an operation pause in which the surface of the optical element is not irradiated with exposure radiation. During heating during the operation pause, a non-uniform temperature distribution (33a to 33c) that reduces the aberration of the projection exposure apparatus is formed in at least one portion (TBa to TBc) of the surface (30) of the optical element, and this non-uniform temperature distribution (33a to 33c) is formed by irradiating the portion (TBa to TBc) with heating radiation in at least one continuous heating radiation profile formed by a beam shaping element. The present invention also relates to a projection exposure apparatus for microlithography, which includes at least one optical element and a heating device that irradiates the surface (30) of the optical element with heating radiation, and the heating device is designed to irradiate the surface (30) of the optical element with heating radiation during an operation pause in which the surface of the optical element is not irradiated with exposure radiation. In order to reduce the aberration of the projection exposure apparatus, the heating device is designed to form a non-uniform temperature distribution (33a to 33c) in at least one portion (TBa to TBc) of the surface (30) of the optical element during heating during the operation pause. For this purpose, the heating device includes at least one heating radiation source that generates heating radiation and at least one beam shaping element that forms a continuous heating radiation profile.

Description

Technical Field

[0001] [CROSS - REFERENCE TO RELATED APPLICATIONS] This application claims the priority of German Patent Application No. 102022205814.3, filed on June 8, 2022, and incorporates its entire disclosure herein by reference.

[0002] The present invention relates to a method for operating a projection exposure apparatus for microlithography, particularly for EUV lithography, which includes a step of heating an optical element by irradiating a heating radiation onto the surface of the optical element during an idle operation in which the exposure radiation does not irradiate the surface of at least one optical element, preferably at least one mirror, of the projection exposure apparatus. The present invention also relates to a projection exposure apparatus including at least one optical element, particularly at least one mirror, and a heating device for irradiating a heating radiation onto the surface of the optical element, preferably the mirror, and being designed to irradiate the heating radiation onto the surface of the optical element during an idle operation in which the exposure radiation does not irradiate the surface of the optical element.

Background Art

[0003] A microlithography projection exposure apparatus is used for manufacturing fine - structure or nanostructure components for microelectronics or microsystem technology. In order to manufacture components having structures with extremely small dimensions in nanometer and micrometer units as accurately as possible, the corresponding projection exposure apparatus must be able to accurately image the structures arranged on a mask (reticle) onto a substrate, for example, a wafer.

[0004] In order to obtain the highest possible resolution, especially in lithographic optical units, EUV radiation with wavelengths in the EUV wavelength range has been used in recent years. This wavelength is generally 13.5 nm, whereas the operating wavelengths commonly used in previous systems were 365 nm, 248 nm, or 193 nm. The transition to the EUV wavelength range results in the abandonment of the use of refractive media that would not make sense even when used at these wavelengths, and a shift to a pure mirror system that operates at virtually normal incidence or grazing incidence. In the case of grazing incidence, approximately 1 / 3 of the incident radiation is absorbed by each mirror (depending on a specific angle-of-incidence spectrum), whereas in normal incidence, the normal absorption value is 1 / 4 or 1 / 5. For comparison, in the case of a refractive medium with an anti-reflection coating, the intensity absorbed is approximately 1 / 1000. This explains that the temperature change in the EUV optical unit is more significant, approximately several kelvins, compared to previous systems where the temperature change was at most a few tenths of a kelvin.

[0005] The temperature gradients within the material of the optical element due to the coefficient of thermal expansion of the material of the optical element lead to surface defects or deformations on the surface of the optical element irradiated with the use radiation or exposure radiation. Therefore, these temperature gradients, especially in the case of mirrors, result in significant optical aberrations that deteriorate the imaging characteristics of the projection exposure apparatus with respect to the use radiation. Therefore, the mirrors for EUV lithography, more precisely their substrates, are manufactured from materials having an especially low coefficient of thermal expansion, such as Zerodur® or ULE® (“ultra-low expansion” materials).

[0006] These zero-expansion materials interact components or phases having positive and negative coefficients of thermal expansion. As a result, a virtually non-linear relationship between thermal expansion and temperature is obtained, and there is only one temperature value at which thermal expansion disappears or is least affected by temperature changes, which is known as the zero-crossing temperature (ZCT).

[0007] During the exposure operation of a projection exposure apparatus, the radiation intensity received by the mirror of the projection optical unit changes locally due to different illumination and diffraction structures on the mask and temporally due to different operating states. However, the average temperature of the mirror or mirror material should be near the zero-crossing temperature so that the aberration resulting from surface deformation due to the temperature gradient is minimized as much as possible.

[0008] To achieve this, it is known to use a heating radiation source that emits heating radiation onto the surface of each mirror. The heating radiation source typically operates in the infrared wavelength range ("IR heater") and can be used during the exposure operation when the surface of each mirror is irradiated with the use radiation and / or during the operation pause outside the exposure operation. In the latter case, it is used for preheating each mirror before the exposure operation. To keep the heat input during the exposure operation as constant as possible, the heating radiation source can emit with high heating power when each mirror does not absorb or only absorbs a very small part of the exposure radiation, and can reduce the heating power as the heat input by the exposure radiation increases. Since the IR heating radiation source can usually irradiate only a part of the entire surface of the optical element, it does not provide a uniform temperature or temperature distribution throughout the three-dimensional body of the optical element.

[0009] Patent Document 1 describes a heating arrangement for heating an optical element, comprising a plurality of IR emitters that irradiate IR radiation onto the optically effective surface of the optical element, which are independently switchable on and off for the purpose of variably setting different heating profiles of the optical element, and at least one beam shaping unit that shapes the beam of IR radiation directed from the IR emitter towards the optically effective surface. The beam shaping unit may include a plurality of beam shaping segments for irradiating different regions of the optically effective surface of the optical element. For example, the beam shaping segments can be different regions of a diffractive optical element (DOE).

[0010] Patent Document 2 and Patent Document 3 describe an optical system and a lithographic apparatus for a lithographic apparatus, comprising an optical element having an optical surface, and a temperature control unit configured to supply heat to and / or dissipate heat from the optical element in order to keep or control the deformation of the optical element constant 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, and the temperature control point can be designed, for example, in the form of an infrared light emitting diode.

[0011] During exposure of the optical surface, the temperature control unit can be used to (actively) control the deformation of the optical element such that the optical aberration with respect to the actinic light is minimized when additional aberration-related factors are included. The optical aberration can be measured using a sensor. If the coefficient of thermal expansion of the substrate of the optical element changes for manufacturing reasons, the temperature control unit can be configured to supply heat to the optical element according to the measured coefficient of thermal expansion profile. In this case, the coefficient of thermal expansion profile of the optical element can be determined and stored in advance. During the exposure operation, the temperature control unit can control the heat supply based on the stored coefficient of thermal expansion profile.

[0012] Patent Document 4 describes a method for manufacturing a mirror of an EUV lithography apparatus in which the predicted heat flux distribution of the mirror is confirmed in the first step. In the second step, a plurality of heating zones are formed on the mirror based on the confirmed heat flux distribution. In the third step, each heating zone is provided with a heating device for heating the heating zone based on the measured temperature of the heating zone or the predicted heat flux distribution of the mirror. In this way, in order to ensure a temperature distribution that is as constant as possible within the mirror volume, the temperature should be kept constant or substantially constant in all heating zones of the mirror. The heating zones can be preheated before the exposure operation, and the degree of preheating can be varied for each heating zone. At least one heating device can heat the corresponding heating zone in a pattern that can include, for example, a ring profile or a part of a ring profile. Since the pattern can exactly correspond to the heat flux distribution, the temperature can be kept constant within the heating zone. The heating device can be a resistance heating heater and / or a thermal emitter that emits radiation in the infrared region, more precisely an IR heater head.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0014] An object of the present invention is to provide a method for operating a projection exposure apparatus and a projection exposure apparatus that reduces aberration.

Means for Solving the Problem

[0015] According to a first aspect, this object is achieved by the method of the aforementioned type, which forms a non-uniform temperature distribution that reduces, preferably minimizes, the aberration of the projection exposure apparatus at least at one part (the “heating zone”) of the surface of the optical element during heating while the operation is paused (defining a target). The irradiated portion can be the entire surface of the optical element designed to reflect the exposure radiation and including a reflective coating for this purpose. However, this portion may also form a partial area of the optically used area on the surface of the optical element. The reduction of the aberration of the projection exposure apparatus can be related to various application examples listed below. In particular, the non-uniform temperature distribution can help reduce the aberration of the projection exposure apparatus during the pause in operation, i.e., when the optical element is not irradiated with exposure radiation and is irradiated with heating radiation.

[0016] Basically, two operating states can be distinguished during the operation of the projection exposure apparatus. There are an exposure operation in which the optical element, usually in the form of a mirror, is irradiated with exposure radiation for exposing the substrate, and a non-exposure operation in the form of a pause in operation where no exposure radiation is present in the projection exposure apparatus. Usually, the exposure operation follows after the heating during the above-mentioned pause in operation, i.e., each optical element is preheated during the pause in operation.

[0017] The following situations (phases) can occur during each operating state (exposure operation or pause in operation).

[0018] 1) When there is no exposure radiation, i.e., during the pause in operation, and no additional heating radiation is used, the performance of the projection exposure apparatus is (very) good, i.e., the aberration is small.

[0019] 2) When using exposure radiation, if individual or some optical elements are not preheated using heating radiation, the performance of the projection exposure apparatus is (very) poor. This is because, for example, when the exposure radiation is turned on, the temperature distribution of the optical elements changes significantly over time (in some cases by several Kelvin; see above), and thus the aberration also changes significantly over time. In particular, these changes over time occur more rapidly or may cause other aberrations than can be corrected in a timely manner by other correction options of the projection exposure apparatus (e.g., by rigid body movement). Furthermore, due to inappropriate material properties, such as variations in the average zero-crossing temperature for each optical element due to manufacturing reasons, aberrations can occur.

[0020] In order to improve the imaging quality of the projection exposure apparatus, for example, for good imaging of extremely small structures using exposure radiation, it may be advantageous to already perform preheating by heating radiation in phase 1) as well, as described, for example, in Patent Document 4 cited above and incorporated herein by reference in its entirety, in such a way as to reduce the temperature change over time in phase 2), and thus find a compromise point between the two phases 1) and 2).

[0021] Patent Document 4 proposes to keep the temperature constant or substantially constant in all heating zones of the mirror during the exposure operation in order to ensure a temperature distribution in the mirror volume that is as constant as possible. The heating zones can be preheated before the exposure operation so that a substantially zero-crossing temperature is set or acquired in each heating zone. In particular, in order to reduce the temporal change in the average temperature of each heating zone when switching the operating mode, in this way, each heating zone should be kept as permanently as possible at or near the zero-crossing temperature. It has been confirmed by both simulation and measurement that the use of such preheating of each optical element in each heating zone can significantly improve the imaging quality of the projection exposure apparatus.

[0022] However, it has been found that by generating a uniform (constant) temperature in each heating zone by preheating, undesirable degradation of the imaging quality of the projection exposure apparatus can occur during phase 2), i.e., during the exposure operation. These degradations have been tolerated in the past and may in particular be due to the fact that it is generally necessary to preheat the optical elements in a non-uniform manner in order to generate a uniform temperature in each heating zone. Even when the temperature distribution in each heating zone is uniform, there is still a spatially dependent deformation of the optical element, although it is significantly reduced compared to the case where the temperature distribution in each heating zone is non-uniform. Furthermore, the individual characteristics of the optical element that may be due to material and manufacturing variations during the manufacture of the optical element are not taken into account during preheating using a uniform temperature distribution and may contribute to the aberration in phase 2).

[0023] It has also been observed that the deformation of the optical element strongly depends on how different parts of the optical element are heated. Usually, the entire surface area of the optical element is not externally accessible for heating, and in any case, it is not possible to heat the entire volume of the optical element to a uniform temperature in three dimensions. In general, uniform heating of a part of the optical element, for example, the entire optical use surface area, is not optimal for the imaging quality of the projection exposure apparatus. It may be much more advantageous to provide a non-uniform temperature distribution in each part.

[0024] Therefore, the present invention proposes to set or form a non-uniform temperature distribution that passes through the target instead of setting or forming a uniform temperature distribution in the relevant part irradiated with the heating radiation.

[0025] Preferably, the non-uniform temperature distribution of at least one part reduces the aberration of the projection exposure apparatus as compared to the uniform temperature distribution of at least one part. When comparing the non-uniform temperature distribution with the uniform temperature distribution, the uniform temperature distribution is assumed to have a constant temperature over that part and to correspond to the temperature of the non-uniform temperature distribution averaged over that part. In this case, the non-uniform temperature distribution is selected such that the aberration of the projection exposure apparatus is reduced as compared to the heating of that part at the uniform temperature distribution. The reduced aberration can in particular be a wavefront aberration. The reduced aberration or wavefront aberration can be caused, for example, by manufacturing variations, its own preheating, the non-uniform spatial distribution of the exposure radiation expected during the next exposure operation, etc. The aberration or wavefront aberration to be corrected is known, for example, from measurements or can be estimated using predictions (for example, in the case of the expected spatial distribution of the exposure radiation).

[0026] Various effects can be better corrected by the non-uniform temperature distribution as desired than by the uniform temperature distribution. In particular, each optical element can be pre-temperature-controlled (suitable for phase 2) without significantly impairing its performance in phase 1). Furthermore, by the non-uniform preheating as desired, the performance of the projection exposure apparatus, i.e., the imaging quality, can also be improved compared to the above-described state without using a heating radiation source in phase 1).

[0027] In a variant of the method, the average temperature of the non-uniform temperature distribution of the above part of the surface is such that the deviation from the zero-crossing temperature of the optical element is ±1.5 K or less, preferably ±0.5 K or less. The zero-crossing temperature of the optical element is understood to mean the zero-crossing temperature of the material of the body or substrate forming the optical element. In this case, the substrate or body material is a zero-crossing material, for example, doped quartz glass, specifically titanium-doped quartz glass commercially available under the trade name ULE (registered trademark), or glass ceramic, for example, Zerodur (registered trademark).

[0028] Typically, since the zero-crossing temperature of the material is substantially constant within the volume of the substrate, it is also constant at the above-mentioned portion on the surface of the substrate, that is, the spatially dependent deviation is small. In the considerations described below, since the spatially dependent deviation is generally negligible, the zero-crossing temperature of the above-mentioned portion of the optical element or the entire optical use surface is considered to be constant in the considerations described below. As described above, since the optical element is least affected by the zero-crossing temperature or the temperature change near the zero-crossing temperature, it is advantageous if the heating portion is heated to approximately the zero-crossing temperature by preheating. Similarly, as described above, by generating a uniform temperature that is constant across the above-mentioned portion and (substantially) corresponds to the zero-crossing temperature, it is not always the case that the aberration of the projection exposure apparatus during operation pause is minimized, and it may be advantageous to deviate from the uniform temperature distribution as desired in the above-mentioned portion in order to bring about the target deformation. The allowable deviation of the average temperature of the non-uniform temperature distribution from the zero-crossing temperature varies particularly depending on the heating power of the incident EUV radiation and the material characteristics of the optical element. The specified values of 1.5 K and 0.5 K are for reference. In specific applications, it may be advantageous in some cases to have a deviation larger than the specified value.

[0029] In yet another variant, the method includes a step of determining a non-uniform temperature distribution that reduces or minimizes the aberration of the projection exposure apparatus, and when determining the non-uniform temperature distribution, it is preferable that the individual material-specific characteristics of the optical element in which the non-uniform temperature distribution is formed are taken into account.

[0030] In principle, the non-uniform temperature distribution can be determined for each application example either in advance or during the operation of the projection exposure apparatus ("in real time").

[0031] For example, possible application examples are as follows.

[0032] a) The projection exposure apparatus already has a specific aberration when there is no exposure radiation nor heating radiation (e.g., for pre-temperature control to approximately the zero-crossing temperature. See the above reference).

[0033] b) The projection exposure apparatus already has specific aberrations when there is no exposure radiation but heating radiation is already being used (e.g., for pre-temperature control to a substantially zero-crossing temperature; see above).

[0034] c) The projection exposure apparatus has specific aberrations during operation with exposure radiation. In some cases, even during the exposure operation, there may be a temperature input by heating radiation in addition to the exposure radiation.

[0035] d) The projection exposure apparatus has specific aberrations that are different from other projection exposure apparatuses of the same type (variations between tools), regardless of the presence or absence of a heat load.

[0036] Regarding this, the above application examples partially coincide with the above phases 1) and 2), where a) corresponds to phase 1), c) corresponds to phase 2), and b) represents an intermediate state. Further application examples may be advantageous.

[0037] A non-uniform temperature profile that reduces or minimizes the relevant aberrations (e.g., distortion or wavefront aberration) can be determined for each of the above application examples. It is also possible to add a plurality of aberrations and determine a non-uniform temperature profile that reduces or minimizes the sum of the aberrations. For example, in order to correct both the aberrations of a projection exposure apparatus that may be caused by manufacturing errors during the manufacture of a plurality of projection exposure apparatuses of the same type and the aberrations that occur similarly in all projection exposure apparatuses of the same type, application example d) can be combined with one of application examples a) to c). In particular, the individual material-specific characteristics of the substrate of the optical element, which are caused by manufacturing variations during the manufacture of the optical element and distinguish the optical element of the projection exposure apparatus from another optical element of the same type, can be considered in the case of d).

[0038] For example, the non-uniform temperature distribution can be determined by simulation, in some cases including the individual material-specific properties of the associated optical elements, such as the averaged distribution of the zero-crossing temperature of the body or substrate of the optical element in which the non-uniform temperature profile is formed. The distribution of the zero-crossing temperature can also be determined by spatial decomposition as another option. Generally, in application example c), a feed-forward model is required for the spatially dependent radiation distribution expected during the exposure operation.

[0039] The non-uniform temperature distribution can also be determined experimentally, for example, by changing different heating powers during irradiation of the optical element with heating radiation. In that case, the heating power can also be changed spatially dependently, or the optical element can be irradiated with different heating radiation profiles, and the aberrations generated in the process are measured. Instead of measuring the (wavefront) aberration using an appropriate measuring device, it is also possible to perform measurements of the temperature distribution of the optical element using, for example, an IR camera, or measurements of the surface deformation of the optical element, and use these measurements for the determination of the non-uniform temperature profile.

[0040] For the correction of the aberrations (especially wavefront aberrations) described in the above application examples, it is not always necessary to generate aberrations by heating the optical element that generates the wavefront aberration precisely and non-uniformly with heating radiation. The targeted heating to form a non-uniform temperature profile can also be carried out (although not necessarily required) with other optical elements (using the compensation effect in a projection exposure apparatus). In this case, the aberrations to be corrected may be caused by thermal effects, but may also be caused by other material deformations or position changes of components, especially those originating from non-optical components.

[0041] In one deformation mode, the non-uniform temperature distribution reduces or minimizes the aberration in the form of wavefront aberration that occurs in the above-mentioned portion of the surface when the optical element is heated during the standby operation. This deformation mode corresponds to the above application example b). As explained there, when the above-mentioned portion is heated to an average temperature that substantially corresponds to the zero-crossing temperature of the optical element, an aberration that can be compensated for by the non-uniform temperature distribution occurs. The preheating of the optical element is usually carried out from a reference temperature that can be, for example, 22°C to an average temperature that substantially corresponds to the zero-crossing temperature. In this case, the wavefront aberration is caused by, for example, the non-uniform heat flux distribution of the optical element, material variations during the manufacture of the optical element, manufacturing effects during the manufacture of the optical element, etc. The non-uniform temperature distribution can help to significantly correct or minimize these wavefront aberrations.

[0042] It is understood that the non-uniform temperature distribution can further be designed to reduce or minimize the aberration or wavefront aberration of the existing projection exposure apparatus or optical element without irradiating the optical element with heating radiation. This corresponds to the above application example a). A non-uniform temperature distribution suitable for this purpose can be determined as described above, for example, by measuring the (individual) wavefront aberration of the optical element. In this case, the wavefront aberration described in case d) that may be caused by material variations or manufacturing effects of the (glass) blank ("boule") can also be measured and taken into account. In some cases, the manufacturing effects on each mirror of different projection exposure apparatuses of the same type are similar, and as a result, the required non-uniform temperature distribution of the optical element is known at an early stage. Thus, in order to form a non-uniform temperature distribution with an appropriate heating radiation profile, the beam shaping element can be appropriately designed early. When the beam shaping element includes sufficient degrees of freedom, for example, in the form of a plurality of separate controllable segments, there is a certain flexibility anyway within the range of forming the desired non-uniform temperature distribution. As described above, the wavefront aberration generated in the optical element may also be completely or partially corrected by (at least) one other optical element.

[0043] In one variant, the non-uniform temperature distribution is formed by irradiating the above-mentioned portion with the heating radiation of at least one continuous heating radiation profile formed by at least one beam shaping element. The continuous heating radiation profile can be formed using a suitable beam shaping element, for example a diffractive optical element, or a part (segment) of a diffractive optical element. The diffractive optical element can individually match the material properties of an optical element used in a projection exposure apparatus and irradiated with heating radiation, for example a mirror. Different non-uniform temperature distributions, and thus different diffractive optical elements, can be used in different projection exposure apparatuses of the same type. However, in some cases the manufacturing effects during the production of each mirror are the same (see above), and thus in some cases the same diffractive optical element can be used as the beam shaping element of different projection exposure apparatuses of the same type, so this is not essential. Usually, each beam shaping element is designed to form a fixedly pre-defined heating radiation profile. The heating radiation profile is understood to mean the heat flux density distribution formed by each beam shaping element in combination with the heating radiation source.

[0044] It is possible to use a single continuous heating radiation profile to irradiate the above-mentioned portion in order to form a non-uniform temperature distribution. However, it is also possible that the irradiation of the above-mentioned portion of the surface is carried out with two or more heating radiation profiles formed by two or more beam shaping elements. In this case, each region larger than the above-mentioned portion of the surface can be irradiated, for example with the heating radiation profiles, and these profiles overlap at the above-mentioned portion of the surface to form a non-uniform temperature profile.

[0045] Since the above-mentioned application examples can occur gradually during the use of the projection exposure apparatus, the set non-uniform temperature distribution can also change over time (within the same projection exposure apparatus), that is, different non-uniform temperature profiles can be set according to the application examples. When switching between application examples, it may be advantageous to keep the heating power of the heating radiation or the non-uniform temperature distribution constant, as described below.

[0046] In one variant, the heating power of the heating radiation for forming a non-uniform temperature distribution during the pause in operation is maintained during the next exposure operation of the projection exposure apparatus, and during this exposure operation, exposure radiation is incident on the surface of the optical element. During heating during the pause in operation, a steady state is reached after a certain time, and a (steady) non-uniform temperature distribution (preheat temperature) is formed on the optical element. The heating power introduced into the optical element during the pause in operation in one of the two application examples a) and b) is fixed in this case, i.e., it is maintained even if an additional heat load occurs in that optical element (e.g., in the form of exposure radiation). Generally, by fixing or maintaining the heating power, the complexity of the control during the exposure operation is reduced, and a steady state is reached again relatively quickly during a further pause in operation after the exposure operation, and a non-uniform temperature distribution is obtained again.

[0047] In an alternative variant, in order to (as far as possible) maintain the non-uniform temperature distribution of the above-mentioned part of the surface of the optical element, the heating power of the heating radiation during the pause in operation is changed during the next exposure operation in which the surface is irradiated with exposure radiation. It may be advantageous to maintain the non-uniform temperature distribution formed or set during the pause in operation also during the exposure operation. To achieve this, generally, since there is heat input into the optical element by the exposure radiation, it is necessary to change the heating power of the heating radiation. In this case, the heating power of the heating radiation is changed using the degrees of freedom available during the provision of the heating radiation or the available heating radiation source (see below) such that the sum of the total heat loads of the optical element corresponds as far as possible to the non-uniform temperature distribution formed during preheating during the pause in operation. For example, this can be achieved by keeping the average temperature of the entire surface as constant as possible, or by supplying less heating radiation to the regions of the surface heated by the exposure radiation than in the case of preheating. In order to enable this, the corresponding regions heated by the exposure radiation must have sufficient pre-temperature control during preheating.

[0048] If such a reduction in 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 appropriately designed towards a "common" profile of the non-uniform temperature distribution, i.e., it should include sufficient heating in different regions of the surface of the optical element.

[0049] Another aspect of the present invention is a projection exposure apparatus of the type described above, wherein, in order to reduce the aberration of the projection exposure apparatus, the heating device is designed or programmed to form a non-uniform temperature distribution at at least one portion of the surface of the optical element during heating while the operation is suspended, and the non-uniform temperature distribution of the at least one portion preferably reduces the aberration of the projection exposure apparatus as compared with the uniform temperature distribution of the at least one portion. The present invention relates to a projection exposure apparatus.

[0050] The projection exposure apparatus can be an EUV lithography apparatus, but can also relate to a UV lithography apparatus operating with UV radiation in a wavelength range of less than about 370 nm. The heating device includes at least one heating radiation source that generates heating radiation or irradiates the above-mentioned portion of the surface. The heating radiation source can be designed to generate a predetermined heating power at startup, but the heating power of the heating radiation source may also be continuously adjustable. The heating device is designed to control at least one heating radiation source to activate at least one heating radiation source during the suspension of operation in order to form a non-uniform temperature distribution at at least one portion of the surface, or includes a programmable controller that controls it.

[0051] In one embodiment, the heating device is designed or programmed to produce an average temperature of the non-uniform temperature distribution of the above-mentioned portion of the surface where the deviation from the zero-crossing temperature of the optical element is ±1.5 K or less, preferably ±0.5 K or less. As described above, in order to reduce the aberration during the exposure operation, it is advantageous to preheat the optical element to approximately the zero-crossing temperature of the optical element before the exposure operation.

[0052] In yet another embodiment, the heating device for forming the non-uniform temperature distribution includes at least one heating radiation source that generates heating radiation and at least one beam shaping element that forms a continuous heating radiation profile. For example, the use of a continuous heating radiation profile formed by a beam shaping element in the form of a diffractive optical element is advantageous for forming a non-uniform temperature distribution. The heating radiation source can be an IR radiation source, such as an IR laser, an IR diode, or the like.

[0053] The heating device can be designed in various ways. For example, the heating device can be designed as in Patent Document 1 cited above and incorporated herein by reference. For example, the heating device can include a plurality of heating light sources in the form of IR emitters that can be switched on and off independently of each other to form different heating radiation profiles. As described in Patent Document 1, a plurality of beam shaping elements or beam shaping segments of the same diffractive optical element can be used to form a non-uniform temperature distribution individually adapted at different parts of the surface of the optical element. Alternatively, a plurality of heater heads can be used that each form only one heating radiation profile of the heating radiation irradiated on each part of the surface of the optical element. However, the use of a heating device or heater head capable of forming a plurality of heating radiation profiles is generally more advantageous because the installation space is much smaller for the same number of available degrees of freedom.

[0054] As a result of the heating device being designed to irradiate the heating radiation generated by two or more beam shaping elements on a spatially overlapping (common) part of the surface of the optical element, there is also a possibility that the heating radiation profiles overlap spatially. As a result of the common part being irradiated with the heating radiation sequentially or simultaneously, different heating radiation profiles can be formed there. The (non-uniform) temperature distribution of each part results from the combination of the heating radiation profile, i.e., the intensity distribution of the heating radiation on the surface of the optical element in each part, and the material properties of the optical element and the thermal boundary conditions of the entire projection exposure apparatus.

[0055] The above heating device is used not only during the pause in operation but also during the exposure operation to optimize not only the average value of the temperature of each part but also the non-uniform temperature distribution with respect to aberration or wavefront aberration, and to adapt the non-uniform temperature distribution to different operation modes of the projection exposure apparatus in which different thermal loads occur in each part, and thus the thermal load can be compensated.

[0056] In principle, it is possible to irradiate the optically used portion of the optical element using a heating device having only one heating radiation source that generates heating radiation and only one beam shaping element that forms only one heating radiation profile. In this case, it is particularly advantageous if the boundary conditions of each projection exposure apparatus are highly adaptable to, for example, the individual expansion behavior of the optical element, such as its zero-crossing temperature, the EUV and IR reflectivity of each optical element, the pressure conditions in the projection exposure apparatus, etc., usually in the form of a diffractive optical element, a beam shaping element. However, the correction options in this case are strongly tied to the boundary conditions and the predetermined aberrations to be corrected. In contrast, due to its higher degree of freedom, the heating device described in Patent Document 1 can also somewhat accommodate changes in the boundary conditions or changes in the aberrations to be corrected even during the exposure operation of the projection exposure apparatus, and for this purpose, it is not necessary to replace the heating device or its components, such as the beam shaper.

[0057] In principle, the heating device can further include at least one heater designed to form a non-uniform heating radiation profile, i.e., one heater designed to form a uniform heat flux density distribution on the above-mentioned portion of the surface of the optical element.

[0058] In yet another embodiment, the heating device is designed to maintain the heating power of the heating radiation used to form the non-uniform temperature distribution during the rest period during the next exposure operation or to change the heating power of the heating radiation used to form the non-uniform temperature distribution during the rest period during the next exposure operation in order to maintain the non-uniform temperature distribution of the above-mentioned portion of the surface during the exposure operation of the projection exposure apparatus. As described above, it may be advantageous to maintain the heating power of the heating radiation that resulted in the non-uniform temperature distribution during the rest period during the exposure operation. Alternatively, the heating power can be changed during the exposure operation so that the non-uniform temperature distribution formed during the rest period is also maintained during the exposure operation.

[0059] Further features and advantages of the present invention will become apparent from the following description of embodiments of the invention with reference to the figures which show details essential to the invention, and from the claims. Individual features can be implemented separately or in any combination of a plurality in a variant form of the invention.

[0060] Exemplary embodiments are shown in schematic diagrams and described in the following description.

Brief Description of the Drawings

[0061]

Figure 1

Figure 2a

Figure 2b

Figure 3a

Figure 3b

Figure 4

Mode for Carrying Out the Invention

[0062] In the following description of the drawings, the same reference numerals are used for the same components or components having the same function.

[0063] Essential components of an optical apparatus for EUV lithography in the form of a projection exposure apparatus 1 for microlithography are described as examples below with reference to FIG. 1. The description of the basic configuration of the projection exposure apparatus 1 and its components should not be regarded as limited here.

[0064] One embodiment of the illumination system 2 of the projection exposure apparatus 1 has an illumination optical unit 4 that illuminates the object field of view 5 of the object plane 6 in addition to the light source or radiation source 3. 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 include the light source 3.

[0065] The reticle 7 disposed in the object field of view 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by a reticle displacement drive 9, particularly in the scanning direction.

[0066] For the sake of explanation, a Cartesian xyz coordinate system is shown in FIG. 1. The x direction extends perpendicular to the plane of the figure. The y direction extends horizontally, and the z direction extends vertically. In FIG. 1, the scanning direction extends in the y direction. The z direction extends perpendicular to the object plane 6.

[0067] The projection exposure apparatus 1 includes a projection system 10. Using the projection system 10, the object field of view 5 is imaged onto the image field of view 11 of the image plane 12. The structure on the reticle 7 is imaged onto the photosensitive layer of the wafer 13 disposed in the region of the image field of view 11 of the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by a wafer displacement drive 15, particularly along the y direction. On the one hand, the displacement of the reticle 7 by the reticle displacement drive 9 and, on the other hand, the displacement of the wafer 13 by the wafer displacement drive 15 can be synchronized.

[0068] The radiation source 3 is an EUV radiation source. The radiation source 3 emits EUV radiation 16, which is also particularly referred to hereinafter as the used radiation, illumination radiation, or illumination light. In particular, the used radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 can be a plasma source, for example, an LPP (laser-produced plasma) source or a GDPP (gas discharge plasma) source. This can also be a synchrotron-based radiation source. The radiation source 3 can be a free electron laser (FEL).

[0069] The illumination radiation 16 emitted from the radiation source 3 is focused by the collector mirror 17. The collector mirror 17 can be a collector mirror having one or more elliptical reflecting surfaces and / or hyperbolic reflecting surfaces. The illumination radiation 16 can be incident on at least one reflecting surface of the collector mirror 17 at an oblique incidence (GI), i.e., at an incident angle greater than 45°, or at a normal incidence (NI), i.e., at an incident angle less than 45°. The collector mirror 17 can be structured and / or coated, firstly, to optimize the reflectivity for the useful radiation and, secondly, to suppress the ambient light.

[0070] Downstream of the collector mirror 17, the illumination radiation 16 propagates through the intermediate focus of the intermediate focal plane 18. The intermediate focal plane 18 can form a separation between the radiation source module including the radiation source 3 and the collector mirror 17 and the illumination optical unit 4.

[0071] The illumination optical unit 4 comprises a deflection mirror 19 and, downstream thereof in the beam path, a first facetted mirror 20. The deflection mirror 19 can be a planar deflection mirror or a mirror having a beam influencing effect exceeding the pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be designed as a spectroscopic filter that separates the useful light wavelength of the illumination radiation 16 from the ambient light of different wavelengths. The first facetted mirror 20 includes a plurality of individual first facets 21, which are also referred to below as field facets. FIG. 1 shows only some of the facets 21 as an example. In the beam path of the illumination optical unit 4, a second facetted mirror 22 is arranged downstream of the first facetted mirror 20. The second facetted mirror 22 includes a plurality of second facets 23.

[0072] Accordingly, the illumination optical unit 4 forms a double facetted system. This basic principle is also referred to as a fly-eye integrator. Using the second facetted mirror 22, the individual first facets 21 are imaged onto the object field of view 5. The second facetted mirror 22 is the last beam shaping mirror or actually the final mirror for the illumination radiation 16 in the beam path upstream of the object field of view 5.

[0073] The projection system 10 includes a plurality of mirrors Mi, which are assigned consecutive numbers according to their arrangement in the beam path of the projection exposure apparatus 1.

[0074] In the example shown in FIG. 1, the projection system 10 includes six mirrors M1 to M6. Replacements with four, eight, ten, twelve, or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have a passage aperture for the illumination radiation 16. The projection system 10 is a double shielding optical unit. The projection optical unit 10 has a numerical aperture on the image side that is greater than 0.3 or 0.5, may be greater than 0.6, and can be, for example, 0.7 or 0.75.

[0075] Similar to the mirrors of the illumination optical unit 4, the mirrors Mi can have a highly reflective coating for the illumination radiation 16.

[0076] FIGS. 2a and 2b show, by way of example, one of the six mirrors Mi of the projection optical unit 10 from the projection exposure apparatus 1 of FIG. 1 and the heating device 24. In the example shown, the heating device 24 includes three heating radiation sources 27a to 27c, which are designed as IR lasers and are connected to the heater head 26 of the heating device 24 via respective fibers 28a to 28c. Three beam shapers in the form of diffractive optical elements 29a to 29c, shown only in FIGS. 2a and 2b, are arranged on the heater head 26. The heater head 26 serves to irradiate the surface 30 of the mirror Mi, more precisely the mirror Mi formed on the upper side of the substrate 29 of the mirror Mi, with heating radiation 31. For example, the heating device 24 shown in FIGS. 2a and 2b can be designed as described in Patent Document 1.

[0077] In the example shown, the substrate 29 consists of titanium-doped quartz glass, more precisely ULE (registered trademark), which has a zero-crossing temperature T ZC For the sake of simplicity, the zero-crossing temperature T ZC of the substrate 29 is considered to be constant throughout the volume of the substrate 29 below. However, this generally has variations related to mirror manufacturing in the volume of the substrate 29.

[0078] FIG. 2a shows the optical element Mi during the standby P in which the surface 30 of the substrate 29 is not irradiated with EUV radiation. FIG. 2b shows the optical element Mi during the exposure operation B of the projection exposure apparatus 1 in which the EUV radiation 16 is irradiated onto the surface 30 of the substrate 29. Although not shown in FIGS. 2a and 2b, a high-reflection coating that functions to reflect the EUV radiation 16 is applied on the upper side of the substrate 29. In the illustrated example, the reflective coating includes a plurality of alternating layers of silicon and molybdenum.

[0079] As is apparent from FIG. 3a, the surface 30 of the three portions TBa to TBc is irradiated with the heating radiation 31, and the outer contour of the above portions is circular in the illustrated example shown in FIG. 2 and corresponds to the region of the surface 30 of the mirror Mi that is irradiated with the EUV radiation 16. In the illustrated example, the three portions TBa to TBc are designed as sectors and each extend at an angle of 120°. This is not essential, and it is obvious that the portions TBa to TBc may have different geometric shapes and, in particular, do not have to be of the same size. Since the size and shape of the region of the surface 30 irradiated with the exposure radiation in the form of the EUV radiation 16 can vary according to the illumination setting of the projection exposure apparatus 1, it is also possible for the three portions TBa to TBc together to cover an area larger or smaller than the region of the surface 30 irradiated with the EUV radiation 16.

[0080] Figure 3a very schematically shows the intensity distribution of the heating radiation 31 from the heater head 26 that irradiates the surface 30 of the mirror Mi, or the isointensity lines of each continuous heating radiation profile 32a - 32c. In this case, each heating radiation profile 32a - 32c is formed by one of three beam shaping elements 29a - 29c housed in the heater head 26 and assigned to each heating radiation source 27a - 27c. The three heating radiation sources 27a - 27c can be switched on and off independently of each other. A controller of a heating device 24 (not shown) serves for control purposes and can be, for example, appropriate hardware and / or software in the form of a control computer. By switching the heating radiation sources 27a - 27c on and off, as is apparent from Figure 3a, the three sub - areas TBa - TBc can be irradiated with different heating radiation profiles 32a - 32c independently of each other. In this case, each heating radiation profile 32a - 32c is fixedly pre - defined by each beam shaping element 29a - 29c and usually cannot be changed any further during the operation of the projection exposure apparatus 1.

[0081] The non - uniform temperature distributions 33a - 33c of each sub - area TBa - TBc shown in Figure 3b result from the combination of the heating radiation profiles 32a - 32c, i.e., the intensity distribution of the heating radiation 31 on the surface 30 of the mirror Mi in each sub - area TBa - TBc, and the material properties of the mirror Mi and the thermal boundary conditions of the entire projection exposure apparatus 1. As is apparent from Figure 3b, which shows exemplary isotherms, the non - uniform temperature distributions 33a - 33c of each sub - area TBa - TBc are different from the irradiated heating radiation profiles 32a - 32c shown in Figure 3a for the reasons described above.

[0082] During the pause operation P, the non-uniform temperature distributions 33a to 33c formed in the respective parts TBa to TBc are designed or selected to enable reduction, preferably minimization, of the aberrations of the projection exposure apparatus 1. In the illustrated example, by being able to set the heating powers Pa, Pb, and Pc of the respective heating radiation sources 27a to 27c, the non-uniform temperature distributions 33a to 33c can be changed in the respective parts TBa to TBc. Hereinafter, it is assumed that a steady operation state is set during the pause operation P in which the heating powers Pa, Pb, and Pc of the three heating radiation sources 27a to 27c are constant or kept constant and the non-uniform temperature distributions 33a to 33c do not change over time.

[0083] The non-uniform temperature distributions 33a to 33c of the respective parts TBa to TBc can be set or optimized for various application examples that can reduce or, in an ideal case, minimize the aberrations of the projection exposure apparatus 1 during the pause operation P and / or during the exposure operation B.

[0084] In the examples shown in FIGS. 3a and 3b, the non-uniform temperature distributions 33a to 33c are selected or defined such that the aberrations of the projection exposure apparatus 1 already generated by the mirror Mi are reduced when neither the heating radiation 31 nor the exposure radiation 16 is radiated to the mirror Mi in the respective parts TBa to TBc. Further, the aberrations generated by heating the mirror Mi from the reference temperature, usually room temperature (22°C), to the average temperature T Ma 、T Mb 、T Mc averaged in the respective parts TBa to TBc are also reduced.

[0085] By appropriate design of the heating device 24, the average temperatures T Ma 、T Mb 、T Mc of the non-uniform temperature distributions 33a to 33c of the respective parts TBa to TBc of the surface 30 are defined such that the deviation from the zero-crossing temperature T ZC of the substrate 29 of the optical element Mi is 1.5 K or less, preferably 0.5 K or less. The average temperatures T Ma 、T Mb 、T Mc of the non-uniform temperature distributions 33a to 33c are the zero-crossing temperature T ZCIt is not essential to be within a specified interval in the vicinity. The non-uniform temperature distributions 33a to 33c shown in Fig. 3b further enable correction of the wavefront aberration of this mirror Mi, which can be attributed to the individual material properties of the mirror Mi. These are wavefront aberrations that occur only with this mirror Mi and not with other mirrors of the same type used in other projection exposure apparatuses 1 of the same type.

[0086] The non-uniform temperature distributions 33a to 33c formed in each part TBa to TBc can reduce the wavefront aberration compared to the uniform temperature distribution of each part TBa to TBc. In the case of a uniform temperature distribution, each part TBa to TBc is always heated to the average temperature T Ma 、T Mb 、T Mc of the non-uniform temperature distributions 33a to 33c.

[0087] Fig. 4 shows the aberration in the form of wavefront aberration averaged over three parts TBa to TBc, represented in the form of Zernike coefficients Z2 and Z3. The left side of Fig. 4 shows the wavefront aberrations Z2 and Z3 that occur in the case of a uniform temperature distribution of the three parts TBa to TBc, while the right side of Fig. 4 shows the wavefront aberrations Z2 and Z3 that occur in the case of the non-uniform temperature distribution 33a shown in Fig. 3b. It is clear that the non-uniform temperature distribution 33a significantly reduces the wavefront aberrations Z2 and Z3 compared to the uniform temperature distribution of the three parts TBa to TBc.

[0088] In the above example, the wavefront aberrations Z2 and Z3 are optimized for the steady operating state during the standby P in which the mirror Mi is heated on average to the desired preheating temperature that approximately corresponds to the zero-crossing temperature T ZC . It is clear that additional aberrations occur during the exposure operation B due to the irradiation of the projection radiation 16. Aberrations that occur during the exposure operation B can also be reduced using the heating device 24. There are various options in this regard.

[0089] During the next exposure operation B of the projection exposure apparatus 1, it is possible to maintain the heating powers Pa, Pb, and Pc of the heating radiation 31 for forming the non-uniform temperature distributions 33a to 33c during the operation pause P. The heating radiation profiles 32a to 32c shown in FIG. 3a are formed in the respective portions TBa to TBc also during the exposure operation B. However, during the exposure operation B, since there is heat input by the exposure radiation 16, the temperature profiles of the respective portions TBa to TBc do not correspond to the non-uniform temperature profiles 33a to 33c shown in FIG. 3b.

[0090] Alternatively, in order to ideally maintain each of the specified non-uniform temperature distributions 33a to 33c in the portions TBa to TBc of the surface 30 during the operation pause P, the heating powers Pa, Pb, and Pc of the heating radiation 31 that led to the formation of the non-uniform temperature distributions 33a to 33c during the operation pause P may be changed by the heating device 24 during the next exposure operation B (see FIG. 3b). The three heating powers Pa, Pb, and Pc of the heating radiation sources 27a to 27c can be appropriately changed for this purpose. In either case, the aberration of the projection exposure apparatus 1 can also be reduced by the heating device 24 during the exposure operation B.

[0091] The non-uniform temperature distributions 33a to 33c for reducing the aberration of the projection exposure apparatus 1 can be determined in different ways for each of the portions TBa to TBc. For example, the non-uniform temperature distributions 33a to 33c may include, depending on the case, the individual material properties of the associated mirror Mi, such as the zero-crossing temperature T of the substrate 29 of the mirror Mi. ZCIt can be determined by simulation considering the averaged distribution, and the distribution of the zero-crossing temperature can also be determined by spatial decomposition as another option. The non-uniform temperature distributions 33a to 33c can also be determined experimentally, for example, by changing different heating powers Pa, Pb, and Pc when irradiating the heating radiation 31 to the mirror Mi. In that case, in some cases experimentally, the heating radiation 31 is irradiated to the mirror Mi with different heating radiation profiles, and the aberration generated in the process is measured. Instead of measuring the (wavefront) aberrations Z2, Z3,... by using an appropriate measuring device, for example, a Shack-Hartmann sensor, it is also possible to measure the temperature distribution of the mirror Mi using an IR camera, or measure the surface deformation of the mirror Mi, and use them to determine the non-uniform temperature distributions 33a to 33c.

[0092] The non-uniform temperature distributions 33a to 33c can also serve to reduce the aberration of the projection exposure apparatus 1 that may be caused by other effects rather than resulting from the mirror Mi itself (use of the compensation effect in the projection exposure apparatus). In this case, the aberration to be corrected may be caused by thermal effects, other material deformations, or changes in the positions of components, and may particularly occur from non-optical components.

[0093] The number of portions TBa, TBb, TBc,... present on the surface 30 of the mirror Mi may increase or decrease, and it is clear that the heating radiation 31 is irradiated to each of the above portions with individual heating radiation profiles 32a, 32b, 32c,.... In particular, the heating device 24 may include only one heating radiation source that generates the heating radiation 31 having a single heating radiation profile formed on the surface 30 of the mirror Mi. Alternatively, the heating device 24 may be designed to form a plurality of heating radiation profiles 32a, 32b, 32c,... that partially overlap or overlap each other on the surface 30 of the mirror Mi.

Claims

1. A method for operating a projection exposure apparatus (1) for microlithography, particularly for EUV lithography, The step of heating the optical element by irradiating the surface (30) of the optical element with heating radiation (31) during a pause (P) in which exposure radiation (16) is not irradiated onto the surface (30) of the optical element, preferably at least one mirror (Mi). A method comprising the above, wherein, during heating during the pause in operation (P), a non-uniform temperature distribution (33a to 33c) that reduces aberrations (Z2, Z3, ...) of the projection exposure apparatus (1) is formed on at least one portion (TBa to TBc) of the surface (30) of the optical element, and the non-uniform temperature distribution (33a to 33c) is formed by irradiating the portion (TBa to TBc) with heating radiation (31) using at least one continuous heating radiation profile (32a to 32c) formed by beam shaping elements (29a to 29c).

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

3. The method according to claim 1 or 2, wherein the average temperature (TMa to TMc) of the non-uniform temperature distribution (33a to 33c) of the portion (TBa to TBc) of the surface (30) is ±1.5 K or less, preferably ±0.5 K or less, from the zero-crossing temperature (TZC) of the optical element (Mi).

4. A method according to claim 3, wherein the non-uniform temperature distribution (33a to 33c) reduces aberrations in the form of wavefront aberrations (Z2, Z3, ...) that occur in the portion (TBa to TBc) of the surface (30) when the optical element is heated to the average temperature (TMa to TMc) during the operation pause (P).

5. A method according to claim 1 or 2, further comprising the step of determining a non-uniform temperature distribution (33a to 33c) that reduces the aberrations (Z2, Z3, ...) of the projection exposure apparatus (1), wherein, when determining the non-uniform temperature distribution (33a to 33c), the individual material-specific properties of the optical element (Mi) on which the non-uniform temperature distribution (33a to 33c) is formed are preferably taken into consideration.

6. The method according to claim 1 or 2, wherein the heating power (Pa, Pb, Pc) of the heating radiation (31) for forming the non-uniform temperature distribution (33a to 33c) during the pause (P) is maintained during the next exposure operation (B) of the projection exposure apparatus (1).

7. The method according to claim 1 or 2, wherein the heating power (Pa, Pb, Pc) of the heating radiation (31) for forming the non-uniform temperature distribution (33a to 33c) of the portion (TBa to TBc) of the surface (30) is changed during the next exposure operation (B) in order to maintain the non-uniform temperature distribution (33a to 33c) during the operation pause (P).

8. A projection exposure apparatus (1) for microlithography, particularly for EUV lithography, At least one optical element, in particular at least one mirror (Mi), A heating device (24) that irradiates the surface (30) of the optical element, preferably the mirror (Mi), with heating radiation (31), and the heating device (24) is designed to irradiate the surface (30) of the optical element with heating radiation (31) during a period of inactivity (P) when the surface (30) of the optical element is not irradiated with exposure radiation (16). In a projection exposure apparatus (1) equipped with, To reduce the aberrations (Z2, Z3, ...) of the projection exposure apparatus (1), the heating device (24) is designed to form a non-uniform temperature distribution (33a to 33c) in at least one portion (TBa to TBc) of the surface (30) of the optical element during heating in the operation pause (P), and the heating device (24) for forming the non-uniform temperature distribution (33a to 33c) comprises at least one heating radiation source (27a to 27c) that generates heating radiation (31) and at least one beam shaping element (29a to 29c) that forms a continuous heating radiation profile (32a to 32c), and the non-uniform temperature distribution (33a to 33c) of the at least one portion (TBa to TBc) preferably reduces the aberrations of the projection exposure apparatus (1) compared to a uniform temperature distribution of the at least one portion (TBa to TBc).

9. A projection exposure apparatus according to claim 8, wherein the heating device (24) is designed to produce an average temperature (TMa to TMc) of the non-uniform temperature distribution (33a to 33c) of the portion (TBa to TBc) of the surface (30) such that the deviation from the zero-crossing temperature (TZC) of the optical element is ±1.5K or less, preferably ±0.5K or less.

10. A projection exposure apparatus according to claim 8 or 9, wherein the heating device (24) is designed to maintain the non-uniform temperature distribution (33a to 33c) of the portion (TBa to TBc) of the surface (30) during the exposure operation (B) of the projection exposure apparatus (1) by maintaining the heating power (Pa, Pb, Pc) of the heating radiation (31) used to form the non-uniform temperature distribution during the operation pause (P) during the next exposure operation (B), or by changing the heating power (Pa, Pb, Pc) of the heating radiation (31) used to form the non-uniform temperature distribution (33a to 33c) during the operation pause (P) during the next exposure operation (B).

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