Optical system and method of operating an optical system

Abstract

The invention relates to an optical system, in particular for microlithography, and to a method for operating an optical system. According to an aspect of the invention, the optical system comprises at least one mirror (100, 500, 600) having an optically effective surface (101, 501, 601) and a mirror substrate (110, 510, 610), wherein at least one cooling channel (115, 515, 615) in which a cooling fluid can flow is arranged in the mirror substrate for dissipating heat generated in the mirror substrate as a result of the absorption of electromagnetic radiation incident on the optically effective surface from a light source, and an adjustment unit (135, 535, 635) for adjusting the temperature and / or flow rate of the cooling fluid as a function of a measured variable characterizing the thermal loading in the mirror substrate or as a function of an estimated thermal loading expected in the mirror substrate at a given power of the light source.

Description

Technical Field

[0001] This invention relates to optical systems, particularly optical systems for microlithography, and to methods for operating optical systems. Background Technology

[0002] Microlithography is used to produce microstructured components, such as integrated circuits or LCDs. The microlithography process is performed in a so-called projection exposure apparatus, which has an illumination device and a projection lens. In this case, the image of a mask (mask master) illuminated by the illumination device is projected onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens, thereby transferring the mask structure onto the photosensitive coating of the substrate.

[0003] In projection lenses designed for the EUV range (i.e., wavelengths of approximately 13 nm or 7 nm), mirrors are used as optical components in the imaging process due to the lack of suitable transmissive and refractive materials. One problem that arises in practice is that EUV mirrors heat up and thus deform, particularly due to absorption of radiation emitted by the EUV source, which in turn negatively impacts the imaging characteristics of the optical system. Known methods for mitigating this problem include, for example, using mirror substrate materials exhibiting a so-called zero-crossing temperature, where the coefficient of thermal expansion has zero crossing in its relationship with temperature, resulting in no or only negligible thermal expansion. Suitable mirror substrate materials include, for example, titanium dioxide-doped (TiO2) quartz glass, for example, trade names... Materials sold by Corning Incorporated. Another suitable mirror substrate material is, for example, lithium-aluminum-silicon oxide-glass ceramic, for example, under the trade name... The materials are sold by (Schott AG). Therefore, it is desirable to keep the mirror at this zero cross temperature to minimize its sensitivity to thermal effects and achieve high imaging performance of the optical system.

[0004] However, in practice, another problem arises here: during the operation of the microlithography projection exposure equipment, the EUV mirror is exposed to an environment with constantly changing intensity of incident electromagnetic radiation, particularly localized variations, such as due to the use of illumination settings with varying intensity on the optically effective surface of the corresponding EUV mirror, as well as temporal variations, where the relevant EUV mirror is typically heated from a relatively low temperature to its operating temperature reached during the microlithography process, especially at the beginning of the microlithography exposure process.

[0005] One known method for avoiding surface deformation and associated optical aberrations caused by varying heat introduction to the EUV mirror involves using a preheater, for example, based on infrared radiation. With such a preheater, active mirror heating can occur during a phase of relatively low absorption of the EUV radiation used, where the active mirror heating decreases accordingly as the absorption of the EUV radiation increases. To account not only for the temporal variation of heat introduction to the EUV mirror (e.g., during the initial phase of the microlithography exposure process) but also for the spatial variation of heat introduction to the EUV mirror (which could be due to the use of certain illumination settings), such a preheater can also be designed to be spatially controllable.

[0006] However, while such a preheater may be largely effective considering the temporal and / or spatial variations in heat introduction to the EUV mirror, further problems may arise as the power values ​​of the (EUV-) light source used in the lithography process increase. One reason for using (EUV-) light sources with enhanced power (e.g., exceeding 500W, especially exceeding 800W) is the accompanying use of less sensitive photoresist materials, which again may benefit from reducing the relative noise effect between the photon number and the total photon count.

[0007] For reference to existing technologies, see, for example, WO 2018 / 177649 A1 and US 10,324,383B2. Summary of the Invention

[0008] The purpose of this invention is to provide an optical system, particularly for microlithography, and a method for operating the optical system, which can at least reduce unwanted heat-induced mirror deformation and associated optical performance degradation, even at higher power values ​​of the light source used in the optical system.

[0009] This objective can be achieved, for example, by an optical system according to the features of independent claim 1 and a method according to the features of independent claim 13.

[0010] According to one aspect of an embodiment of the present invention, an optical system includes:

[0011] At least one reflector having an optically effective surface and a reflector substrate, wherein at least one cooling channel through which cooling fluid can flow is disposed in the reflector substrate to dissipate heat generated in the reflector substrate due to the absorption of electromagnetic radiation incident from a light source onto the optically effective surface; and

[0012] The adjustment unit is used to adjust the temperature and / or flow rate of the cooling fluid based on a measured value that characterizes the heat load in the reflector substrate or based on an estimated heat load expected in the reflector substrate at a given power of the light source.

[0013] The mirror may be specifically designed for use in microlithography projection exposure equipment. However, the invention is not limited thereto. The mirror according to the invention may also be used or utilized in other applications, such as in systems for mask metrology.

[0014] The embodiments of the present invention are particularly based on the concept that in an optical system including a mirror, the mirror can be cooled by a cooling fluid flowing in a cooling channel during operation of the optical system, such that the temperature and / or flow rate of the cooling fluid is suitable for the source power value of the light source used in the optical system. This in particular provides the possibility of considering an increase in the value of the source power while attempting to avoid surface deformation and associated optical aberrations caused by the introduction of heat into the mirror during system operation.

[0015] More specifically, by adjusting the temperature and / or flow rate of the cooling fluid according to the expected thermal load in the mirror substrate for the source power, embodiments of the present invention will be able to reduce the temperature of the cooling fluid (and / or increase the flow rate of the cooling fluid) at higher source power, thereby bringing the average temperature of the mirror closer to the zero-crossing temperature, while reducing the influence of material variations on that zero-crossing temperature (also referred to below). Figures 7a-7f (To be explained in more detail).

[0016] The embodiments of the present invention are particularly based on the following consideration: if the average mirror temperature is far from the zero cross temperature, the temperature gradient in the mirror substrate (which may be due, for example, to uneven illumination of the mirror during system operation under certain illumination settings, and / or to different light absorption in the object plane due to the use of different mask masters) has an increasingly greater negative impact on unwanted surface deformation and related optical aberrations.

[0017] Here, embodiments of the present invention intentionally take extra effort to adjust or control the temperature and / or flow rate of the cooling fluid in exchange for the advantageous effect of bringing the average mirror temperature close to the zero cross temperature while reducing the influence of material changes.

[0018] Regarding the adjustment or control of the cooling fluid temperature, different concepts may exist according to embodiments of the invention: According to one option, feedforward control can be implemented based on a pre-estimation of the anticipated thermal load in the mirror substrate, wherein this pre-estimation can be performed through calibration measurements (performed for different source power values) or through simulation. According to a further option, feedback control can be implemented, wherein the cooling fluid temperature is controlled (in the sense of closed-loop control) based on measurements of the mirror temperature during system operation (or temperature-related characteristics or magnitudes characterizing the thermal load in the mirror substrate). Such measurements can be achieved, for example, by using a temperature sensor or by using an infrared camera. A further option may include the use of so-called sub-resolution auxiliary features in the mask master, which produce a diffraction angle exceeding the numerical aperture of the optical system and directs light to a mirror region outside the optical usage area, such that a detector present in this external region can be used to estimate the incident light and thermal load on the mirror.

[0019] According to one embodiment, the optical system further includes a heater for heating the mirror. This heater may be specifically configured to introduce heat into the mirror in a spatially variable manner. Thus, in a preferred embodiment (but not limited thereto), the inventive concept of adapting the temperature of the cooling fluid to the source power is combined with the presence of a heater for heating the mirror (more specifically, a heater configured to introduce heat into the mirror in a spatially variable manner). Here, the invention is based on the further consideration that such heaters (although they offer the possibility of non-uniformly introducing heat into the mirror, thereby primarily compensating for undesirable deformation effects due to inhomogeneities in mirror illumination or substrate material properties) are ineffective or unresponsive to increased source power (or in other words, unstable to changes in source power). This is because such heaters may only introduce additional heat into the mirror substrate without actively reducing the mirror substrate temperature in response to increased source power.

[0020] In contrast, the aforementioned concept of the present invention, which combines such a heater with specific adjustments or control of the cooling fluid temperature and / or cooling fluid flow rate, allows for the combination of the advantageous effects of a non-uniform heater (considering or compensating for local variations in heat generation in the mirror substrate due to illumination settings or the effects of heat generation in the mirror substrate due to material inhomogeneity on deformation and optical aberrations) with fluid temperature variations (used to bring the average mirror substrate temperature close to the zero cross temperature, which will be referred to...). Figures 7a-7f (To be explained in more detail) and / or the beneficial effects of changes in cooling fluid flow rate.

[0021] According to one embodiment, the light source has a power of at least 500W, particularly at least 800W, and more particularly at least 1kW.

[0022] According to one embodiment, the temperature of the cooling fluid is variably set to at least 0.1K, particularly at least 0.2K, and more particularly at least 0.5K.

[0023] According to one embodiment, the average zero-crossing temperature of the mirror substrate material (at which the coefficient of thermal expansion has zero crossover in its temperature dependence) is substantially equal to the manufacturing temperature at which the optically effective surface of the mirror is formed. The advantageous effects of this embodiment will be discussed below. Figures 7a-7f This is derived from further explanation.

[0024] According to one embodiment, the optical system is designed for operating wavelengths less than 250 nm, particularly less than 200 nm, and even more particularly less than 160 nm.

[0025] According to one embodiment, the optical system is designed for operating wavelengths less than 30 nm, particularly less than 15 nm.

[0026] According to one embodiment, the optical system is an optical system for microlithography.

[0027] The present invention relates more to a microlithography projection exposure apparatus having an illumination device and a projection lens, wherein the projection exposure apparatus has an optical system as defined above.

[0028] This invention relates more to a method for operating an optical system, wherein the optical system has at least one mirror having an optically effective surface and a mirror substrate, wherein at least one cooling channel is disposed in the mirror substrate.

[0029] Cooling fluid flows within the cooling channel to dissipate heat generated in the mirror substrate due to the absorption of electromagnetic radiation incident from the light source onto the optically effective surface; and

[0030] The temperature and / or flow rate of the cooling fluid are adjusted based on the measured value of the heat load in the characteristic mirror substrate or based on the estimated heat load expected in the mirror substrate at a given power of the light source.

[0031] According to one embodiment, the adjustment is performed such that the average mirror temperature is maintained within a predefined temperature range.

[0032] According to one embodiment, the zero-crossing temperature of the reflector substrate material (at which the coefficient of thermal expansion has zero crossover in its temperature dependence) is within the predefined temperature band.

[0033] According to one embodiment, the adjustment of the temperature and / or flow rate of the cooling fluid includes feedforward control based on a pre-estimated thermal load in the reflector substrate for different power values ​​of the light source.

[0034] According to one embodiment, the preliminary estimate of the expected thermal load in the reflector substrate for different power values ​​of the light source is based on calibration measurements.

[0035] According to one embodiment, the preliminary estimate of the expected thermal load in the reflector substrate for different power values ​​of the light source is based on simulation.

[0036] According to one embodiment, the adjustment of the temperature and / or flow rate of the cooling fluid includes feedback control based on measurements of the magnitude of the thermal load on the characteristic mirrors during operation of the optical system.

[0037] According to one embodiment, the adjustment of the temperature and / or flow rate of the cooling fluid includes intervening in the feedback control within a time interval of less than 120 seconds, particularly less than 60 seconds, and even more particularly less than 20 seconds.

[0038] Further configurations of the invention can be found in the detailed description and dependent claims.

[0039] The invention will now be explained in more detail based on exemplary embodiments shown in the accompanying drawings. Attached Figure Description

[0040] In the diagram:

[0041] Figure 1 A schematic diagram illustrating the construction of a reflector according to an embodiment of the present invention is shown;

[0042] Figure 2 A flowchart is shown to explain the possible operation of the optical system in an exemplary embodiment of the present invention;

[0043] Figure 3 and Figure 4 A flowchart is shown to explain the possible operation of the optical system in a further exemplary embodiment of the present invention;

[0044] Figure 5 A schematic diagram illustrating the construction of a reflector according to an embodiment of the present invention is shown, wherein a segmented heating configuration is additionally provided;

[0045] Figures 6a-6b A schematic diagram illustrating the construction of a reflector according to another embodiment of the present invention is shown, wherein a segmented heating configuration is additionally provided;

[0046] Figures 7a-7f A diagram is shown to illustrate the advantageous effects achieved according to the present invention;

[0047] Figure 8 A schematic diagram showing a possible configuration of a microlithography projection exposure apparatus designed for operation in EUV is displayed; and

[0048] Figure 9 A schematic diagram of a possible configuration for a microlithography projection exposure apparatus designed for operation in DUV is shown. Detailed Implementation

[0049] Different embodiments of the reflector are described below. These embodiments share a common feature: a cooling device is provided in which the temperature and / or flow rate of the cooling fluid can be specifically adjusted to reduce unwanted heat-induced reflector deformation and associated optical performance degradation, even when the light source used in the optical system has a high power value.

[0050] Figure 1 A schematic diagram illustrating the construction of a mirror according to the invention in one embodiment is shown. The mirror 100 can be an EUV mirror of an optical system, particularly a microlithography projection exposure device (see below for example). Figure 8 The projection lens or illumination device of the projection exposure apparatus 800 is described in further detail, but the invention is not limited thereto.

[0051] The mirror 100, having an optically effective surface 101, specifically includes a mirror substrate 110, which is made of any suitable mirror substrate material desired. Suitable mirror substrate materials are, for example, titanium dioxide-doped (TiO2) quartz glass, such as those marketed under trade names... Materials sold by Corning Incorporated. Another suitable mirror substrate material is, for example, lithium-aluminum-silicon oxide-glass ceramic, for example, under the trade name... Materials sold by (Schott AG). The reflector 100 further comprises a reflective layer stack 120 (e.g., a multilayer system made of molybdenum and silicon layers). The invention is not limited to a specific configuration of this layer stack; merely as an example, a suitable construction may comprise about fifty layers or groups of layers, wherein the layer system comprises a molybdenum (Mo) layer with a thickness of 2.4 nm and a silicon (Si) layer with a thickness of 3.4 nm. In a further embodiment, the reflector may also be configured for so-called grazing incidence. In this case, the reflective layer system may comprise, for example, particularly only a single layer, which is composed of ruthenium (Ru) having an exemplary thickness of, for example, 30 nm.

[0052] During the operation of the optical system, electromagnetic EUV radiation (by...) Figure 1 (The arrow in the image indicates that) an impact on the optically effective surface 101 of the reflector 100 may cause non-uniform volume changes in the reflector substrate 110 due to the temperature distribution resulting from the absorption of radiation that impacts the optically effective surface 101 non-uniformly.

[0053] according to Figure 1The reflector 100 includes at least one cooling channel (or multiple cooling channels) 115 disposed in the reflector substrate 110 near its boundary facing the reflective layer system 120. Cooling fluid (e.g., water) flows through the cooling channel 115 and is fed into the cooling channel 115 via a fluid supply 125. In an exemplary embodiment, the distance between each cooling channel 115 and the boundary facing the reflective layer system 120 may be less than 20 mm, particularly less than 10 mm. Furthermore, the cooling power of the cooling channel 115 may be at least 0.1 W, particularly greater than 0.5 W, particularly greater than 1 W.

[0054] In addition, according to Figure 1 An adjustment unit 135 is provided for adjusting the temperature and / or flow rate of the cooling fluid supplied to the cooling channel 115 via the supply unit 125. In particular, the goal of this specific adjustment may be to maintain the average temperature of the reflector substrate within a predefined temperature range, although the power value of the light source may vary.

[0055] See below for reference Figure 2-4 The flowchart provided may explain that, for this specific adjustment of the temperature and / or flow rate of the cooling fluid, different embodiments may exist. Furthermore, as referred to... Figure 5 and Figures 6a-6b The above concept can be advantageously used in conjunction with a heater for heating a reflector (especially to introduce heat into the reflector in a spatially variable manner).

[0056] In some embodiments of the present invention, according to Figure 1 The adjustment unit 135 includes a feedforward control unit, which sets the temperature and / or flow rate of the cooling fluid based on a prior estimate of the expected thermal load in the reflector substrate. This prior estimate can be made by performing calibration measurements for different source power values ​​beforehand, or by simulation.

[0057] Figure 2 A flowchart is shown to explain the operation of the optical system in an exemplary embodiment. According to Figure 2 In step S210, the actual value of the source power is provided, and in step S220, a target value for the (average) mirror temperature is defined, which may specifically be the zero-crossing temperature of the mirror. In the next step S230, a value for the cooling fluid temperature and / or the cooling fluid flow rate is determined, which is suitable for achieving heat dissipation capable of maintaining the target mirror temperature defined in step S220. In step S240, the determined values ​​of the cooling fluid temperature and flow rate are used to cool the mirror.

[0058] If the actual source power provided in step S210 changes during the operation of the optical system, other temperature and / or flow rate values ​​for the cooling fluid may be appropriate to still maintain the target mirror temperature (e.g., zero cross temperature). Therefore, in step S240... Figure 1 The adjustment unit 135 has the capability to set these different values ​​to specifically adjust the temperature and / or flow rate of the cooling fluid. In an embodiment, a lookup table can be used to perform this adjustment, wherein the lookup table is determined by prior calibration measurements for different heat loads or by simulation.

[0059] Figure 3 A flowchart illustrating another embodiment of the invention is shown. Although Figure 3 The flowchart initially contained with Figure 2 The same steps (this step is in) Figure 3 The specifications are labeled S310-S340, but further steps S350 and S360 are added to additionally achieve feedback control of the temperature and / or flow rate of the cooling fluid. More specifically, according to Figure 3 In step S350, the actual temperature of the reflector is measured and compared with the target temperature of the reflector. Next, in step S360, an appropriate value for the temperature and / or flow rate of the cooling fluid is determined to achieve heat dissipation capable of maintaining or reaching the target reflector temperature, and (by returning to step S340) utilizing… Figure 1 Unit 135 in the middle uses the determined temperature and / or flow rate values ​​of the cooling fluid to cool the reflector.

[0060] While the above embodiments explicitly include temperature measurement at the mirror using one or more temperature sensors, other embodiments are also possible to determine the actual thermal load on the mirror during operation of the optical system to achieve this feedback control. Such embodiments may, for example, include the use of an infrared camera or one or more intensity detectors outside the optical operating area, wherein light can be directed to the intensity detector using a sub-resolution auxiliary feature in a mask master that produces a diffraction angle exceeding the numerical aperture of the optical system.

[0061] Figure 4 A flowchart illustrating yet another embodiment of feedback control is shown. According to... Figure 4 The method (initially without defining the actual source power) begins by defining a target mirror temperature (e.g., average zero-crossing temperature). Next, in step S420, the actual temperature of the mirror is determined and compared to the defined target temperature. In the following step S430, appropriate values ​​for the temperature and / or flow rate of the cooling fluid are determined to maintain or reach the target mirror temperature, and in step S440, this temperature and / or flow rate of the cooling fluid is used to cool the mirror (again utilizing...). Figure 1Adjustment unit 135 in the middle).

[0062] Figure 5 and Figures 6a-6b A schematic diagram illustrating the construction of a reflector according to other embodiments of the invention is shown. These embodiments share the common feature of providing a heater or segmented heating configuration configured to thermally induce localized variable deformation of the optically effective surface.

[0063] refer to Figure 5 The mirror 500 is shown only in a very simplified manner and has an optically effective surface 501. The mirror 500 includes (similar to) Figure 1 At least one cooling channel 515 is disposed in the mirror substrate 510 near its facing reflective layer system. Figure 5 (Not shown in the image) The boundary is designed to dissipate heat from the reflector 500. Furthermore, it is similar to... Figure 1 A supply unit 535 is provided to adjust the temperature and / or flow rate of the cooling fluid supplied to the cooling channel 515 via the supply unit 525. The objective of this particular adjustment may be to maintain the average temperature of the reflector substrate 510 within a predefined temperature range, although the power value of the light source may vary.

[0064] In addition, according to Figure 5 The reflector 500 includes a heater 580 implemented in a segmented heating configuration with multiple irradiation sources 581 configured to irradiate the reflector substrate 510 using electromagnetic radiation, thereby thermally inducing the deformation of the optically effective surface. Depending on the operation of the individual irradiation sources 581, which can be controlled independently of each other, the irradiation results in heating that causes localized changes on the reflector surface. The wavelength of the electromagnetic radiation (which may be, for example, infrared radiation) makes the material of the reflector substrate 510 substantially transparent in the corresponding wavelength region. The irradiation sources 581 are preferably designed and configured such that the irradiation does not (or at least does not significantly) interfere with the cooling channel 515.

[0065] Figures 6a-6b A schematic diagram illustrating the construction of a reflector according to another embodiment of the present invention is shown.

[0066] Similar to Figure 1 and Figure 5 In one embodiment, the reflector 600 includes at least one cooling channel 615 disposed in the reflector substrate 610 near its boundary facing the reflective layer system 620. Furthermore, similar to... Figure 1 and Figure 5A unit 635 is provided to adjust the temperature and / or flow rate of the cooling fluid supplied to the cooling channel 615 via the supply unit 625. The objective of this particular adjustment may be to maintain the average temperature of the reflector substrate 610 within a predefined temperature range, although the power value of the light source may vary.

[0067] In addition, according to Figures 6a-6b The reflector 600 includes a heater 680 configured with a plurality of electrodes 681, which are electrically actuated or capable of having a selectively set current applied to them via electrical leads 682. Furthermore, the reflector 600 includes a conductive layer 685. Figure 6a In this context, "665" indicates a smooth and insulating layer, which specifically ensures that the electrodes 681 of the electrode configuration are electrically insulated from each other and can be made of, for example, quartz glass (SiO2). It can also be provided in the layered structure of the reflector 600. Figure 6a Additional functional layers not shown in the diagram (such as diffusion barrier layers, adhesion reinforcement layers, etc.).

[0068] During operation of the reflector 600, different potentials can be applied to the individual electrodes 681 of the electrode configuration, and the voltage generated between the electrodes 681 thereby produces a current via the conductive layer 685. Depending on the potential applied to each electrode 681, the heat generated by this current causes localized heating of the reflector surface. Figure 6a The embodiments are not limited to a specific geometric configuration of the electrode arrangement. The electrode 681 can be arranged in any suitable distribution (e.g., Cartesian grid, hexagonal configuration, etc.). In a further embodiment, the electrode 681 may also be located only in a specific region. Figure 6b An example of the geometric configuration of the electrode configuration is shown.

[0069] According to the present invention, in the case of the reflector 600, the combined use of the electrode configuration and the conductive layer 685 (although the electrode configuration is relatively coarse) allows the power input to the reflector to be continuously varied, while the coupling of thermal power (compared to, for example, the use of an infrared (IR) heating device) is limited to the reflector itself. Due to the choice of materials, there is a relatively high resistance in the conductive layer 685, causing a voltage drop there, while due to the relatively significantly higher conductivity in the lead 682, there is no voltage or thermal drop in the lead 682, and in this respect, a fine structure is not required to generate high resistance.

[0070] In the following text, see references Figures 7a-7f The diagrams shown illustrate the advantageous effects of the invention. These diagrams demonstrate the dependence of the surface deformation ΔL of the mirror on the temperature change ΔT under different conditions.

[0071] refer to Figure 7aThe three different curves shown in the ΔL vs. ΔT graph represent the fact that the substrate material of the mirror exhibits inhomogeneity, which leads to different values ​​of the zero-crossing temperature on the mirror substrate. It is advantageous to choose the average zero-crossing temperature (the solid curve between the two dashed lines) such that it substantially corresponds to the manufacturing temperature during the formation of the optically effective surface of the mirror. However, due to the aforementioned inhomogeneity in the substrate material and the accompanying variation in the zero-crossing temperature on the substrate, Figure 7a In this state, the deviation between the actual shape of the entire optical surface of the reflector substrate and the target shape (hereinafter referred to as "shape deviation" and generally determined using interferometry) is not zero, but varies by a quantity denoted by "D". However, according to Figure 7b These shape deviations can be almost corrected to zero through the final shaping of the mirror, so that the final manufactured mirror can be provided and delivered in this state.

[0072] As discussed above and in Figure 7c The illustration shows that during the operation of an optical system, non-uniform illumination from a mirror causes local temperature variations, which in turn lead to shape deviations that depend on local zero-crossing temperatures (which have different values ​​at different locations on the mirror).

[0073] Figure 7d This indicates if a heater is used (e.g., according to...) Figure 5 and Figures 6a-6b Achievable states can be achieved by introducing heat into the mirror in a spatially variable manner. By applying an appropriate heating profile (e.g., complementary to the heat load imposed due to the illumination light in the actual lighting setup), a uniform temperature can be achieved across the entire mirror, such as... Figure 7d As shown by the thick vertical line in the image. However, the surface deformation is still caused by this constant temperature exceeding the zero-crossing temperature (and the temperature at which the mirror is formed).

[0074] Figure 7e The diagram illustrates the state achievable if the aforementioned localized heating is combined with "global cooling" (i.e., combined with mirror cooling using a fixed cooling fluid temperature and flow rate). Through this cooling, the mirror's temperature can be transferred to the temperature at which the mirror was formed, i.e., the zero-crossing temperature. Due to the low temperature dependence within the zero-crossing temperature region, small temperature errors have a relatively small impact on shape deviations.

[0075] However, if the thermal load or absorption intensity of the reflector increases due to increased source power or increased mask master reflectivity, then without additional countermeasures, according to Figure 7fThis will result in relatively large shape deviations. If the temperature of the reflector is reduced to the temperature at which the reflector was formed (ideally corresponding to the zero-crossing temperature), this undesirable effect can now be avoided. This can be achieved through the following (or by...). Figure 7f As can be seen from the above: enhancing the heating power provided by the heater and enhancing the cooling power provided by the cooling fluid (i.e., by changing the temperature and / or flow rate of the cooling fluid according to the invention).

[0076] Figure 8 A schematic diagram of an exemplary projection exposure apparatus designed for operation in EUV and in which the present invention can be implemented is shown. According to Figure 8 The illumination device in the projection exposure apparatus 800 designed for EUV includes a field plane mirror 803 and a pupil plane mirror 804. Light from a light source unit including a plasma light source 801 and a light collector mirror 802 is guided onto the field plane mirror 803. A first telescope mirror 805 and a second telescope mirror 806 are disposed in the optical path downstream of the pupil plane mirror 804. A deflecting mirror 807 is disposed downstream of the optical path, which guides the radiation incident on it onto the object field in the object plane of a projection lens including six mirrors 851-856. A mask 821 with a reflective structure on a mask stage 820 is disposed at the location of the object field, and the mask is imaged onto the image plane by means of the projection lens, wherein a substrate 861 coated with a photosensitive layer (photoresist) on a wafer stage 860 is disposed in the image plane.

[0077] Figure 9 A schematic diagram of an exemplary projection exposure apparatus designed for operation in DUV and in which the present invention can be implemented is shown. The projection exposure apparatus 900 includes a beam shaping and illumination system 910 and a projection lens 920. In this case, DUV stands for “deep ultraviolet light” and indicates that the wavelength of the working light is between 30 nm and 250 nm. The beam shaping and illumination system 910 and the projection lens 920 may be configured in a vacuum enclosure and / or surrounded by a machine room with corresponding drive units. The projection exposure apparatus 900 has a DUV light source 901. For example, an ArF excimer laser emitting radiation 902 in the 193 nm DUV range can be provided as the DUV light source 901.

[0078] Figure 9 The beam shaping and illumination system 910 shown directs DUV radiation 902 onto a mask 905. The mask 905 is implemented as a transmission optical element and can be configured outside the beam shaping and illumination system 910 and the projection lens 920. The mask 905 has a structure that images onto a substrate or wafer 930 in a reduced manner via the projection lens 920. The projection lens 920 has multiple lens elements (…). Figure 9Three lens elements 921-923 and at least one reflector are shown schematically and exemplary. Figure 9 Two mirrors 924 and 925 are schematically and exemplaryly shown for imaging a mask 905 onto a wafer 930. In this case, the individual lens elements 921-923 of the projection lens 920 and / or mirrors 924 and 925 may be symmetrically arranged relative to the optical axis OA of the projection lens 920. It should be noted that the number of lens elements and mirrors in the DUV microlithography apparatus 900 is not limited to the number shown in the figure. More or fewer lens elements and / or mirrors may also be provided. Furthermore, the front side of the mirrors is typically curved for beam shaping. The air gap between the last lens element 923 and the wafer 930 may be replaced by a liquid medium 926 with a refractive index greater than 1. For example, the liquid medium 926 may be high-purity water. This configuration is also known as immersion microlithography and has higher optical lithography resolution.

[0079] Even though the invention has been described based on specific embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example, through combinations and / or exchanges of features of the various embodiments. Therefore, it is beyond doubt that the invention also includes such variations and alternative embodiments, and that the scope of the invention is limited only to the scope of the appended claims and their equivalents.

Claims

1. An optical system comprising: At least one reflector having an optically effective surface and a reflector substrate, wherein at least one cooling channel formed within the reflector substrate is configured to receive cooling fluid adapted to flow within the reflector substrate to dissipate heat generated in the reflector substrate as a heat load due to the absorption of electromagnetic radiation incident from a light source on the optically effective surface. A heater is arranged to heat the reflector in a spatially variable manner; The controller is configured to adjust the temperature and / or flow rate of the cooling fluid based on a measured value of the heat load in the reflector substrate characterized or an estimated heat load determined for the reflector substrate based on a given power for the light source, and is configured to control the heater to heat two or more spatial regions of the optically effective surface differently to cause local deformation of the optically effective surface, and simultaneously control the cooling channel to globally cool the reflector substrate.

2. The optical system as described in claim 1, characterized in that, The controller includes a feedforward control unit that controls the temperature and / or flow rate of the cooling fluid based on a pre-estimated thermal load determined by the reflector substrate (110, 510, 610) for different power values ​​of the light source.

3. The optical system as described in claim 1, characterized in that, The controller includes a feedback control unit that controls the temperature and / or flow rate of the cooling fluid based on measurements of the magnitude that characterizes the thermal load in the reflector substrate.

4. The optical system as claimed in claim 1, characterized in that, The light source has a power of at least 500W.

5. The optical system as described in claim 4, characterized in that, The light source has a power of at least 1kW.

6. The optical system as claimed in claim 1, characterized in that, The temperature of the cooling fluid is set to vary in increments of at least 0.1 K.

7. The optical system as claimed in claim 1, characterized in that, The average zero-crossing temperature of the reflector substrate material is essentially equal to the manufacturing temperature at which the optically effective surface of the reflector is formed. At this average zero-crossing temperature, the coefficient of thermal expansion of the reflector substrate material has a zero crossover in temperature dependence.

8. The optical system as claimed in claim 1, characterized in that, This optical system is designed for operating wavelengths less than 250 nm.

9. The optical system as claimed in claim 1, characterized in that, This optical system is designed for operating wavelengths less than 30 nm.

10. The optical system as claimed in claim 1, characterized in that, The heater includes a plurality of electrode heaters disposed between the substrate and the optically effective surface, or a plurality of radiation sources arranged to guide radiation having wavelengths to which the substrate is substantially transparent.

11. The optical system as claimed in claim 1, characterized in that, The optical system is designed as a microlithography optical system.

12. A microlithography projection exposure apparatus comprising an illumination device and a projection lens, characterized in that, At least one of the lighting device and the projection lens includes the optical system as described in claim 11.

13. A method for operating an optical system, wherein the optical system has at least one mirror having an optically effective surface and a mirror substrate, the method comprising: At least one cooling channel is provided in the reflector substrate; A heater is provided, which is arranged to heat the reflector in a spatially variable manner; Cooling fluid is circulated in the cooling channel to dissipate the heat generated in the reflector substrate as a heat load due to the absorption of electromagnetic radiation incident from the light source onto the optically effective surface. as well as The temperature and / or flow rate of the cooling fluid are adjusted according to the estimated thermal load determined for the reflector substrate based on the given power for the light source, and the heater is operated simultaneously to locally deform the optically effective surface.

14. The method as described in claim 13, characterized in that, This adjustment keeps the average mirror temperature within a predefined temperature range.

15. The method as described in claim 14, characterized in that, The zero-crossing temperature of the reflector substrate material is within the predefined temperature range, and at this zero-crossing temperature, the coefficient of thermal expansion has zero crossover in temperature dependence.

16. The method as described in claim 13, characterized in that, The adjustment of the temperature and / or flow rate of the cooling fluid includes feedforward control based on a pre-estimated thermal load determined for the reflector substrate for different power values ​​of the light source.

17. The method as described in claim 16, characterized in that, The preliminary estimate of the thermal load for the reflector substrate for different power values ​​of the light source is based on calibration measurements.

18. The method as described in claim 16, characterized in that, The preliminary estimate of the thermal load on the reflector substrate for different power values ​​of the light source is based on simulation.

19. The method as described in claim 13, characterized in that, The adjustment of the temperature and / or flow rate of the cooling fluid includes feedback control based on measurements of the magnitude that characterizes the thermal load on the mirror substrate during operation of the optical system.

20. The method as described in claim 19, characterized in that, The adjustment of the temperature and / or flow rate of the cooling fluid is included in the intervention of the feedback control within a time interval of less than 120 seconds.

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