Mirror arrangement, projection lens and method for measuring temperature of mirror

Abstract

The invention relates to a mirror arrangement, in particular for a microlithographic projection exposure system, comprising a mirror (20), a sensor unit (26) and a control unit (38). The mirror (20) comprises a mirror body (23) and a reflective surface (24) provided on the mirror body (23). The sensor unit (26) is designed to detect infrared radiation emitted by the mirror body (23) in order to derive a temperature measurement therefrom and to send the temperature measurement to the control unit (38). The mirror (20) comprises a target (37) having an increased infrared radiation emissivity. The invention also relates to a method for measuring the temperature of a mirror (20).

Description

[0001] This patent application claims priority to German patent application DE 10 2022210 245.2, filed on September 28, 2022, which is incorporated by reference and the contents of which are incorporated herein in their entirety (“incorporated by reference”). Technical Field

[0002] The invention relates to a reflector device, in particular to a reflector device for a microlithography projection exposure device, a projection lens and a method for measuring the temperature of a reflector. Background Art

[0003] Microlithography projection exposure apparatuses are used for the production of integrated circuits with particularly small structures. A mask (=reticle) irradiated with very short-wave deep ultraviolet or extreme ultraviolet radiation (DUV or EUV radiation) is imaged onto the lithographic object in order to transfer the mask structure onto the lithographic object.

[0004] The projection exposure apparatus comprises a plurality of mirrors at which the radiation is reflected. The mirrors have a precisely defined shape and are positioned precisely so that the imaging of the mask onto the lithographic object is of sufficient quality.

[0005] During operation, the projection exposure apparatus is subject to influences which have an influence on the imaging quality. As an example, if thermal expansion leads to changes in the geometry of the mirrors, the wavefront of the radiation reflected at the mirrors changes. For correct operation of the projection exposure apparatus, it is helpful to have information about the temperature of the mirrors. The temperature information can be used, for example, to control a heating unit or a cooling unit so that the temperature of the mirrors is kept at a constant value, or to appropriately adjust the projection exposure apparatus after a temperature change.

[0006] DE 10 2012 201 410 A1 and DE 10 2020 205 752 A1 disclose detecting infrared radiation emitted by a reflector body in order to obtain information about the temperature of the reflector body. It has proven difficult to obtain temperature information with sufficient accuracy in this way. Summary of the invention

[0007] The object of the invention is to provide a reflector arrangement, a projection lens and a method for measuring the temperature of a reflector which avoid these disadvantages. This object is achieved by the features of the independent claims. Advantageous embodiments are given in the dependent claims.

[0008] The object is therefore achieved by a mirror arrangement, particularly suitable for a microlithography projection exposure apparatus, comprising a mirror, a sensor unit and a control unit. The mirror comprises a mirror body and a reflecting surface formed on the mirror body. The sensor unit is designed to detect infrared radiation emitted by the mirror body in order to derive temperature measurements therefrom and to send the temperature measurements to the control unit. The mirror body comprises a target with increased infrared radiation emissivity.

[0009] In the case of temperature measurements based on infrared radiation, it is not always easy to distinguish what proportion of the infrared radiation detected by the sensor unit is emitted by the reflector body and what proportion is background radiation. The background radiation can, for example, emanate from the frame structure of the reflector arrangement or from adjacent housings. The invention proposes to reduce this uncertainty by equipping the reflector with a target having an increased infrared radiation emissivity. The increased emissivity leads to an increased proportion of the relevant infrared radiation relative to the background radiation, as a result of which the quality of the measurement is improved.

[0010] The target has an increased emissivity compared to an adjacent region of the reflector. The emissivity of the target may be at least 20% higher, preferably at least 40% higher, more preferably at least 60% higher than the emissivity of an adjacent region of the reflector, with respect to a scale extending between an ideal black body radiator with an emissivity ε(T)=1 and an ideal reflector with an emissivity ε(T)=0. If an adjacent region has an emissivity of ε(T)=0.5, the emissivity of the target with ε(T)=0.6 is increased by 20%. The indications about the emissivity each relate to a wavelength range within the IR spectrum to which the sensor unit is sensitive.

[0011] The temperature measurement values ​​recorded by the sensor unit may relate to the temperature of the target. A temperature measurement value valid for the reflector body may be derived from the temperature of the target. If the reflector body has a temperature that is constantly distributed over the reflector body, the temperature measurement value applies to the entire reflector body. If the temperature varies within the reflector body, the temperature measurement value may apply to a local area within the reflector body. It is also possible to determine a temperature measurement value that corresponds to an average value over a plurality of local areas of the reflector body. The control unit may be designed to process the temperature measurement values ​​recorded by the sensor unit, or values ​​derived therefrom.

[0012] During operation of the projection exposure apparatus, energy is constantly fed to the mirrors, since part of the incident EUV / DUV radiation is absorbed. In one embodiment, the invention is implemented without introducing additional energy into the mirror arrangement for the purpose of the temperature measurement. Thus, the measurement can be based solely on the energy that is anyway fed to the mirrors during operation of the projection exposure apparatus. Thermographic measurements can be performed. In order to infer the temperature from the measured radiant power, a calibration can be realized with reference to an ideal black body radiator.

[0013] The sensor unit may comprise an infrared sensor in order to detect infrared radiation (IR radiation) emitted from the reflector body. The infrared sensor may be configured as an image sensor so that the IR radiation emitted from the reflector body can be detected in a spatially resolved manner. Depending on the sensitivity and the wavelength range, for example, bolometers, thermopiles or semiconductor sensors (InSb, HgCdTe) may be used as detector elements.

[0014] The reflective surface of the reflector can have a high reflectivity for EUV radiation and / or DUV radiation. The reflective surface of the reflector can be formed by a highly reflective coating. This can involve an optical layer system in the form of a multilayer coating, in particular a multilayer coating with alternating layers of molybdenum and silicon. Using such a coating, about 70% of the incident EUV radiation can be reflected. The remaining about 30% is absorbed and causes heating of the EUV reflector. The term EUV radiation refers to electromagnetic radiation in the extreme ultraviolet spectral range with a wavelength between 5nm and 100nm, in particular a wavelength between 5nm and 30nm. DUV radiation is in the deep ultraviolet spectral range and has a wavelength between 100nm and 300nm.

[0015] Therefore, a high reflectivity for EUV / DUV radiation is usually accompanied by a low emissivity for IR radiation. For an ideal blackbody radiator, ε(T)=1 applies to the emissivity and R=1-ε(T)=0 applies to the reflectivity. Real opaque bodies have an emissivity ε(T)<1 and therefore a reflectivity R-ε(T)>0. Low emissivity is accompanied by high reflectivity, as a result of which, for example, background radiation emanating from the frame structure of the mirror arrangement or from adjacent housings may be reflected by the mirror into the infrared sensor, which may corrupt the measurement result. In the context of the present invention, if IR radiation emanating from the body is detected for the purpose of temperature measurement, the area adjacent to the target preferably has an emissivity for IR radiation of at least 0.15, preferably at least 0.5. The indications about the emissivity each relate to a wavelength range within the IR spectrum to which the sensor unit is sensitive.

[0016] In one embodiment, the sensor unit is arranged in front of the reflective surface so that the IR radiation emitted from the reflector body can propagate straight to the sensor unit without the reflector body getting in the way. In order to obtain meaningful measurement results, the target may be a measurement field formed in the reflective surface. The measurement field may have a higher IR radiation emissivity than the reflective surface.

[0017] The measuring field can be arranged in the middle of the reflecting surface so that the measuring field is completely surrounded by the reflecting surface. The emission characteristics of the measuring field can be as close as possible to the emission characteristics of an ideal black body radiator. One possibility for producing a measuring field with a desired emissivity is to provide the reflecting surface with a coating in the region of the measuring field. It is also possible to apply a film to the reflecting surface in the region of the measuring field. In another variant, the region of the measuring field is cut out of the highly reflective coating of the reflecting surface so that the surface of the reflector and therefore the measuring field is formed by the material of the reflector body arranged underneath.

[0018] The reflector may comprise an active optical surface, the EUV radiation impinging on the active optical surface during operation of the reflector arrangement. The active optical surface may correspond to the reflective surface or may be smaller than the reflective surface. The reflector may have a target arranged on the surface of the reflector, the target being arranged outside the active optical surface. In one embodiment, the active optical surface completely surrounds the target. In one embodiment, the target is arranged outside the active optical surface but within the reflective surface. The target may be completely surrounded by the reflective surface. The target may be configured as a measurement field arranged on the surface of the reflector.

[0019] In order to be able to reflect a sufficient amount of EUV / DUV radiation, the reflector generally has a large reflective surface. By way of example, the reflective surface may have an area of ​​at least 500 square centimeters, preferably at least 2000 square centimeters, more preferably at least 10000 square centimeters. The target, which does not contribute relevantly to the reflection of EUV / DUV radiation, should be small relative to the reflective surface. By way of example, the target may have an area of ​​less than 5 square millimeters, preferably less than 2 square millimeters, more preferably less than 1 square millimeter. In particular, the size of the target may be less than 1 μm. 2 and 1mm 2 The ratio between the size of the reflecting surface and the size of the target may be at least 10 4 , preferably at least 10 6 , more preferably at least 10 8 . Targets can be configured as measurement fields.

[0020] The reflective surface may be provided with a plurality of targets, for example at least two, preferably at least five, more preferably at least twenty targets. The targets may be evenly distributed on the reflective surface. In particular, the largest circle within the reflective surface without a target has a surface area preferably not greater than 20%, preferably not greater than 10%, more preferably not greater than 5% of the reflective surface.

[0021] The sensor unit may be directed toward the target, i.e. may be arranged such that a measurement signal representative of the temperature of the mirror body is determined based on the IR radiation emitted from the target. Within the IR spectrum, the sensor unit should be sensitive to wavelength ranges in which the target has a high emissivity. If the measuring field is formed by a silicon dioxide compound, for example if a highly reflective coating is removed in the region of the measuring field, the infrared sensor is preferably sensitive to long-wave IR radiation having a wavelength between 7 μm and 14 μm.

[0022] In the projection lens, the mirror arrangement with the target arranged in the active optical surface preferably has a near pupil position. If the mirror is arranged near the pupil of the beam path, the target affects the entire field of the beam path to the same extent. In contrast, if the mirror is at a greater distance from the pupil, the target may adversely affect specific areas within the field of the beam path, which is undesirable in many cases.

[0023] In order that other influences have little adverse effect on the temperature measurement, it is advantageous if the conditions in the vicinity of the reflector remain as constant as possible. In this respect, the frame structure of the reflector arrangement and / or a housing adjacent to the reflector arrangement can be provided with a black surface, i.e. with a surface having a high emissivity for infrared radiation. The reflector arrangement can include a cooling system in order to keep the frame structure and / or the housing at a constant temperature.

[0024] The reflector arrangement may compromise a frame structure having a reflector body suspended therefrom. A movable suspension may be involved so that the position of the reflector body is adjustable relative to the frame structure. The reflector arrangement may include one or more actuators to change the position of the reflector body relative to the frame structure.

[0025] In addition to or as an alternative to the target on the surface of the reflector, the reflector device may include one or more targets arranged inside the reflector body. The emissivity of such a target is increased compared to the adjacent material of the reflector body. In one embodiment, the target is arranged in a cavity formed in the reflector body. The target can be formed, for example, by a coating applied to the material of the reflector body adjacent to the cavity. The reflector body can have a channel extending from the surface of the reflector body until the target. The wall of the channel can be provided with a coating, which has a lower emissivity than the material of the reflector body and therefore has a high reflectivity. In this way, the channel can form a kind of light guide for the infrared radiation emitted by the target, so that the infrared radiation emitted by the target is guided out of the reflector body towards the outside and can be detected there by the sensor unit.

[0026] Additionally or alternatively thereto, the reflector may comprise a target integrated into the material of the reflector body. The sensor unit may be sensitive to a wavelength range within the IR spectrum for which the material of the reflector body is transparent. This opens up the possibility of detecting IR radiation emitted by a target integrated into the material of the reflector body. For example, the infrared sensor may be sensitive to medium wave IR radiation and thus to a wavelength range for which the silicon dioxide compound is transparent.

[0027] In the reflector body, a target with increased IR radiation emissivity can be formed, i.e. its emissivity is higher than the emissivity of the surrounding reflector body material. The target can be, for example, a layer formed in the reflector body. The target layer can extend parallel to the reflective surface. If the reflector body comprises a main body, from which the layer structure of the optical layer system including the reflective surface is produced by additive manufacturing, the target layer can be arranged between the optical layer system and the main body. If the layer structure comprises a surface protection layer, the target layer can be arranged between the surface protection layer and the main body. If the reflector body consists of a main body and a second part body, wherein the layer structure is implemented on the second part body, the target layer can also be arranged between the main body and the second part body.

[0028] The target may also be formed by a cavity formed in the reflector body and filled with a liquid having a high emissivity for IR radiation. The liquid may be water. In one embodiment, the cavity is a cooling channel and the liquid is water flowing through the cooling channel.

[0029] The infrared sensor can be arranged so that the IR radiation emitted from the target can propagate straight through the material of the reflector body to the infrared sensor without other obstacles getting in the way. By way of example, the infrared sensor can be arranged adjacent to the rear side of the reflector body, which is opposite the reflective surface. The reflector body can also have a recess in which the infrared sensor is arranged. The path taken by the IR radiation through the material of the reflector body can be shortened in this way.

[0030] If the reflective surface is subjected to EUV / DUV radiation during operation of the projection exposure apparatus, energy is also discharged by the mirror body in the form of electrical charges. Within the optical layer system of the reflective surface, standing waves are formed when excited by the EUV / DUV radiation and lead to the release of electrons from the layer system. The electrons form free charge carriers on the reflective surface, which can be conducted away via an electrical connection towards the outside to ground. In a variant of the independent invention, the number of dissipated charge carriers can be determined by measuring the current between the reflective surface and ground.

[0031] The state of the standing wave changes as a function of the heating-induced change in the thickness of the layer system. This change has the effect that the number of electrons released from the layer system also changes, so that the number of dissipated charge carriers forms a measure of the temperature of the mirror body in the region of the reflecting surface. As the temperature changes, the periodic thickness of the optical layer system changes, and the field intensity at the surface of the optical layer system also changes as a result. The number of charge carriers is proportional to the field intensity, so that to a first approximation there is a linear relationship between the temperature change and the measured photocurrent. The temperature change can therefore be determined from the measurement of the photocurrent.

[0032] The optical layer system may be electrically conductive and may be electrically insulated from the material of the mirror body.The reflective surface may be provided with a plurality of electrical contacts which are distributed over the circumference and via which the electrical charges are dissipated.

[0033] In other independent inventive embodiments of the invention, the reflector device can also be shaped so that the sensor unit comprises a light source for directing a light signal to the reflector. A portion of the energy introduced into the reflector body by the light signal is emitted again by the reflector body and forms a quantity of energy from which the temperature of the reflector body can be inferred.

[0034] The mirror body can be equipped with an interference layer system arranged between the reflective surface of the mirror body and the main body, which acts as a thin-film interference filter. The layers have different refractive indices, wherein the transition of the refractive index between adjacent layers can occur continuously in the manner of a corrugated filter or discontinuously in the manner of a Bragg filter.

[0035] The light signal directed to the interference layer system is reflected wavelength-selectively at the interference layer system. If the temperature of the reflector body changes, the thickness of the interference layer system changes due to thermal expansion, as a result of which the wavelength of the reflected part of the light signal changes. This change in wavelength can be measured by a suitable light sensor. The temperature of the reflector body in the region of the interference layer system can be derived from the measured values.

[0036] The sensor unit with the light source and the light sensor can be arranged on the rear side or laterally relative to the reflector body so that the light signal can reach the interference layer system without previously impinging on the reflective surface. The angle of incidence can be between 0° and 60°. The light signal can have a wavelength for which the material of the reflector body is transparent. For example, the wavelength can be in the visible range. The wavelengths of the interference layer system and the light signal are coordinated with each other.

[0037] The temperature measurement can be carried out in a spatially resolved manner by individually evaluating the light signals from different areas of the interference layer system. For this purpose, for example, light from multiple light sources can be directed to the interference layer system and the reflected light signals can be evaluated by means of a sensor array. The surface of the interference layer system can also be scanned by a sensor unit.

[0038] In one variant, the light source of the sensor unit is arranged in front of the reflective surface so that the light signal is incident on the reflective surface. In this case, the optical layer system of the reflective surface itself can act as a thin-film interference filter. The light signal for measurement purposes should have the same wavelength as the EUV / DUV radiation reflected at the reflective surface during operation of the projection exposure apparatus. The light source can be configured as an EUV light source or a DUV light source, which emits radiation of the relevant wavelength. If the thickness of a layer within the optical layer system changes due to thermal expansion, the reflected part of the light signal changes, so that the temperature of the mirror body in the region of the reflective surface can be deduced from the measured values.

[0039] In another independent inventive embodiment, the reflector body is provided with a thermochromic layer. The thermochromic layer can be arranged between the optical layer system and the main body of the reflector body. If the reflector body consists of a main body and a second part body, the thermochromic layer can also be arranged between the main body and the second part body.

[0040] The thermochromic layer has the property of changing color when the temperature changes. For example, the thermochromic layer may include an inorganic compound rutile or zinc oxide, whose molecular structure or crystal structure changes under the condition of temperature change, thereby causing color change. The light source can guide a light signal of an appropriate wavelength through the transparent material of the reflector body to the thermochromic layer. Based on the change in the color of the reflected light part, the light sensor can derive temperature information and send it to the control unit.

[0041] In one embodiment, the thermochromic layer is designed such that a color change occurs when a certain temperature threshold is exceeded or fallen below. If the temperature threshold corresponds to an expected temperature of the mirror body during operation of the projection exposure apparatus, the temperature signal recorded by the light sensor can be used directly to control the temperature regulation of the mirror arrangement. In particular, a closed control loop can be provided so that the mirror body is locally heated or cooled depending on the temperature signal of the light sensor.

[0042] The invention further relates to a projection lens of a projection exposure apparatus, wherein a mask is imaged onto a lithographic object via a plurality of mirror arrangements, wherein at least one of the mirror arrangements is configured as a mirror arrangement according to the invention. The projection lens may comprise at least two, preferably at least three, more preferably at least five mirror arrangements according to the invention. The temperature measurement values ​​obtained by the sensor unit according to the invention may be used in a control system of the projection lens in order to control operating parameters of the projection lens. In particular, the operating parameters may be controlled in a closed control loop using the temperature measurement values. The invention further relates to a projection exposure apparatus comprising such a projection lens.

[0043] The invention also relates to a method for measuring the temperature of a reflector of a microlithography projection exposure apparatus. The reflector comprises a reflector body and a reflective surface formed on the reflector body. A sensor unit detects the amount of energy emitted by the reflector body in order to derive a temperature measurement value therefrom. The temperature measurement value is sent to a control system of the microlithography projection exposure apparatus. The reflector comprises a target with an increased infrared radiation emissivity.

[0044] The present disclosure encompasses improvements of the method having features described in the context of the mirror arrangement according to the invention. The present disclosure encompasses improvements of the mirror arrangement having features described in the context of the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The invention is described below by way of example based on advantageous embodiments with reference to the accompanying drawings, in which:

[0046] Figure 1 : A schematic diagram showing a projection exposure apparatus according to the present invention;

[0047] Figure 2 : A schematic diagram showing a reflector device according to the present invention;

[0048] Figure 3 : Shows the Figure 2 A plan view of a reflector of a reflector device;

[0049] Figure 4 : shows a schematically shown sensor unit Figure 3 A cross-sectional view of a reflector;

[0050] Figure 5-7 : shows an alternative embodiment of the present invention according to Figure 4 's view;

[0051] Figure 8 : A schematic diagram showing a comparative example;

[0052] Figure 9-11 : A schematic diagram showing a comparative example;

[0053] Fig.12 , 13 : shows an alternative embodiment of the present invention according to Figure 3 's view;

[0054] Fig.14 : shows another embodiment according to the present invention. DETAILED DESCRIPTION

[0055] Figure 1 A microlithography EUV projection exposure apparatus is schematically illustrated. The projection exposure apparatus comprises an illumination system 10 and a projection lens 22. By means of the illumination system 10 an object field 13 in an object plane 12 is illuminated.

[0056] The illumination system 10 comprises an exposure radiation source 14 which emits electromagnetic radiation in the EUV range, ie in particular with a wavelength between 5 nm and 30 nm. The illumination radiation emerging from the exposure radiation source 14 is firstly focused by a collector 15 into an intermediate focal plane 16 .

[0057] The illumination system 10 comprises a deflection mirror 17, by which the illumination radiation emitted by the exposure radiation source 14 is deflected onto a first facet mirror 18. A second facet mirror 19 is arranged downstream of the first facet mirror 18. The individual facets of the first facet mirror 18 are imaged into the object field 13 by the second facet mirror 19.

[0058] By means of a projection lens 22, the object field 13 is imaged into an image plane 21 using a plurality of mirrors 20. Arranged in the object field 13 is a mask (also called reticle) which is imaged onto a photosensitive layer of a wafer arranged in the image plane 9.

[0059] The various mirrors of the projection exposure apparatus that reflect the illumination radiation are configured as EUV mirrors. The EUV mirrors are provided with a highly reflective coating. This can involve a multilayer coating, in particular a multilayer coating with alternating layers of molybdenum and silicon. The EUV mirrors reflect approximately 70% of the incident EUV radiation. The remaining approximately 30% is absorbed and leads to heating of the EUV mirrors.

[0060] Figure 2 A mirror arrangement is shown, wherein a mirror body 23 of a mirror 20 is held on a frame structure 29 by means of an actuator 28. The actuator 28 can be used to change the position of the mirror 20 relative to the frame structure 29 for the purpose of aligning and positioning the mirror 20 within the rigid body degree of freedom. A reflective surface 24 is formed on the mirror body 23, at which incident EUV radiation is reflected.

[0061] The mirror arrangement is equipped with a cooling system, which comprises a coolant reservoir 33 filled with a cooling liquid and a pump 30. With the aid of the pump 30, the cooling liquid is drawn from the coolant reservoir 33 and guided via a first connecting line 35 to the cooling channel 27. The cooling channel 27 extends on the frame structure 29 to the mirror body 23. A closed cooling circuit is formed via a return line 32. The cooling liquid absorbs the heat generated by the absorbed EUV radiation and dissipates this heat from the mirror body 23. At the transition between the frame structure 29 and the mirror body 23, the connecting lines 32, 35 are embodied as flexible hose lines so as not to hinder the adjustment and alignment of the mirror.

[0062] The cooling channels 27 are shaped so that heat is dissipated both from the frame structure 29 and from the mirror body 23, and both are kept at a substantially constant temperature during operation of the projection exposure apparatus. Within the mirror body 23, the cooling channels 27 branch into a plurality of parallel channels, so that heat is dissipated uniformly from the reflective surface 24. Figure 1 In the case of a projection exposure apparatus, each of the reflective mirrors 20 of the projection lens 22 is configured according to Figure 2 Reflector device.

[0063] The mirror arrangement comprises a control unit 38 which performs various control tasks for the mirror arrangement. In particular, the control unit 38 controls the actuator 28 in order to bring the mirror body 23 into a desired position and orientation relative to the frame structure 29 and controls the pump 30 of the cooling system in order to adjust the cooling capacity. One of the input variables processed by the control unit 38 when determining the control command for the actuator 28 is a temperature measurement on the temperature of the mirror body 23, which the control unit 38 obtains from a sensor unit in the form of an IR camera 26 which is sensitive to IR radiation. The temperature measurement is used as a basis for controlling operating parameters of the mirror arrangement, such as, for example, the cooling capacity of the actuator 28 or the cooling system. The control can be implemented within a closed control loop.

[0064] according to Figure 4 , the IR camera 26 is directed toward the reflective surface 24 of the reflector 20. The reflective surface is provided with a plurality of targets in the form of a measurement field 37, which are shown in FIG. Figure 3 3 shows a measurement field 37 in an enlarged view. In practice, the measurement fields 37 each have an area of ​​about 1 square millimeter, while the horizontal extent of the reflection surface 24 is about 80 centimeters. The measurement field 37 has a high emissivity for long-wave IR radiation, which is in particular significantly higher than the emissivity of the reflection surface 24. The measurement field 37 is generated by the optical layer system 40 forming the reflection surface 24 being removed in the region of the measurement field 37 so that the silicon dioxide material of the mirror body 23 is freely accessible.

[0065] Fig.12An alternative embodiment is shown in which an active optical surface 51 is formed within the reflective surface 24, on which EUV radiation impinges during operation of the projection exposure apparatus. The region of the reflective surface 24 arranged outside the active optical surface 51 is not located within the EUV beam path of the projection exposure apparatus. A target 52 configured as a measurement field is arranged within the reflective surface 24, but outside the active optical surface 51. This has the advantage of preventing the EUV beam path from being adversely affected by the target 52.

[0066] exist Fig.13 In the case of another alternative embodiment in , the reflective surface 34 includes both a target 37 arranged inside the active optical surface 51 and a target 52 arranged outside the active optical surface 51. If the reflective surface 24 is densely populated by the targets 37, 52, it is easier to obtain spatially resolved temperature information from the surface of the reflector 20.

[0067] An IR camera 26, which is sensitive to long-wave IR radiation with a wavelength in the order of 10 μm, records the IR radiation emitted by the entire reflective surface 24. However, only those measurement values ​​relating to the measurement fields 37, 52 are included in the further evaluation. Based on a previously performed calibration that associates the measured radiation power with specific temperature measurement values, temperature measurement values ​​for each of the measurement fields 37, 52 are derived from the measurement values. The temperature measurement values ​​are sent to the control unit 38 and evaluated there for the purpose of controlling the mirror arrangement.

[0068] The significance of the temperature measurement depends on the fact that the radiant power recorded by the IR camera 26 is not corrupted by interfering background signals. Background radiation cannot be completely avoided, because every object emits a specific amount of IR radiation at a specific temperature. The present invention seeks a method to keep the background radiation constant. For this purpose, the components near the reflector 20 are kept at a constant temperature. Figure 2 This is shown on the basis of the example of a frame structure 29 which is cooled by means of cooling channels 27 . Figure 2 Components near the reflector 20 (eg housing etc.) not shown in the figure are also cooled in a similar manner. Furthermore, the surface of the component is designed such that it is black for long-wave IR radiation.

[0069] In the projection lens 22, the mirror arrangement preferably has a near pupil position. If the mirror 20 is arranged near the pupil of the beam path, the measurement field 37, 52 affects the entire field of the beam path to the same extent. In contrast, if the mirror 20 is at a greater distance from the pupil, the measurement field 37, 52 may adversely affect specific areas within the field of the beam path, which is undesirable in many cases.

[0070] In accordance with Figure 5In the case of an alternative embodiment of the present invention, an IR camera 26 is arranged on the rear side of the reflector body 23. The IR camera is sensitive to medium-wave IR radiation having a wavelength in the order of magnitude of 4 μm, for which the material of the reflector body 23 is transparent. In the vicinity of the reflecting surface 24, the reflector body 23 is provided with a target layer 25, which has a high emissivity for IR radiation of this wavelength. The IR radiation emitted by the target layer 25, which radiation represents the temperature of the reflector body 23 in the vicinity of the target layer 25, propagates through the material of the reflector body 23 to the IR camera 26. By evaluating the IR radiation emitted by the target layer 25, the IR camera 26 can determine a temperature measurement in a locally resolved manner and transmit it to the control unit 38.

[0071] Figure 6 An embodiment is shown in which the IR camera 26 is also directed toward the rear side of the reflector body 23 and is sensitive to medium-wave IR radiation. The cooling water in the cooling channel 27 is black for IR radiation of this wavelength, so that the IR radiation emitted by the cooling water represents the temperature of the cooling water. The temperature of the reflector body 23 in the vicinity of the cooling channel 27 can be deduced from the temperature of the cooling water. The IR radiation emitted by the cooling water propagates through the transparent material of the reflector body 23 to the IR camera 26, which determines the locally resolved temperature distribution along the length of the cooling channel 27 from the recorded radiation temperature measurements.

[0072] In accordance with Figure 7 In the case of the embodiment of the present invention, a cavity 36 is formed in the reflector body 23 in the region between the cooling channel 27 and the reflective surface 24. The cavity 36 is filled with water. Figure 6 In a manner comparable to the case of the exemplary embodiment of the present invention, the temperature of the water in the cavity 36 is determined by the IR camera 26 arranged on the rear side of the reflector body 23. Since the cavity 36 is located closer to the reflective surface 24, the temperature information is obtained precisely from the area of ​​the reflector body 23 that is particularly relevant to the control of the reflector device.

[0073] Fig.14 An alternative embodiment is shown, in which a target 53 is arranged in a cavity 54 of the reflector body 23. The cavity 54 extends from the target 53 up to the rear side of the reflector body 23 located opposite the reflective surface 24. The end face of the cavity 54 is covered by the target 53. The side surfaces of the cavity 54 are provided with a coating with a high reflectivity for infrared radiation. The infrared radiation emitted by the target 53 is guided along the cavity 54 towards the outside as in a light guide. The infrared radiation is recorded by the IR camera 26. A temperature measurement value representing the temperature of the target 53 in the cavity 54 is determined.

[0074] Figure 8The construction of a mirror 20 is schematically shown, comprising a mirror body 23 and an optical layer system 40 comprising alternating layers of molybdenum and silicon applied to the mirror body 23. The optical layer system 40 forms a reflective surface 24. Using such a layer system, it is possible to reflect approximately 70% of the incident EUV radiation.

[0075] exist Figure 8 In the schematic illustration in , the Z direction starting from the reflective surface 24 and extending into the depth of the mirror body 23 is plotted on the horizontal axis. The vertical axis shows the energy of the incident EUV radiation 42 as the amplitude of a sinusoid. The EUV radiation 42 forms a standing wave in the optical layer system 40, wherein the amplitude decreases with increasing penetration into the optical layer system 40. The interaction between the EUV radiation 42 and the optical layer system 40 causes electrons to be released from their bonds within the optical layer system 14. The released electrons form free charge carriers 41 on the surface of the optical layer system 40. The optical layer system 40 is insulated from the mirror body 23 so that the charge carriers cannot flow away into the mirror body 23.

[0076] An electrical contact is established with the ground 44 in order to conduct the charge carriers away. A measuring instrument 43 is arranged between the ground 44 and the optical layer system 40 and measures the current and thus the number of charge carriers 41 .

[0077] The interaction between the EUV radiation 42 and the optical layer system 40 depends on the thickness of the layers within the optical layer system 40. The thickness of the layers within the optical layer system 40 varies with temperature due to thermal expansion. The strength of the electric field at the surface of the optical layer system 40 is related to the thermal expansion of the optical layer system 40. Therefore, the number of charge carriers 41, which is proportional to the strength of the electric field, forms a measure of the temperature. After appropriate calibration, the measuring instrument 43 can derive temperature information from the number of charge carriers 41 and can send this information to the control unit 38.

[0078] Fig. 9 An embodiment is shown in which an interference layer system 45 is formed between the optical layer system 40 of the reflector body 23 and the reflective surface 24. The sensor unit comprises a light source 46 and a light sensor 48. The light source 46 emits a light signal 47 having a wavelength in the visible range. The light signal 47 is incident on the rear side of the reflector body 23 at an angle of incidence between 0° and 60° and passes through the transparent material of the reflector body 23 to the interference layer system 45. The interference layer system 45 is subject to thermal expansion, so that the layer thickness of the layers within the interference layer system 45 forms a measure of the temperature of the reflector body 23 in the region of the reflective surface 24.

[0079] The interference layer system 45 can be used as a Bragg filter with vertically alternating thicknesses or as a corrugated filter with a continuously varying refractive index. In the event of a temperature change relative to a reference state, there is a change in the thickness and refractive index of the material within the interference layer system 45. As a result, the transmission or reflection spectrum of the resulting filter shifts by Δλ. Fig. 9 In the exemplary embodiment of the invention, the light sensor 48 evaluates the reflection spectrum. Alternatively, the method can also be carried out with the evaluation of the transmission spectrum. After appropriate calibration, the temperature information is derived from the reflection spectrum and sent to the control unit 38.

[0080] The method can be performed with multiple wavelengths. The precision can be increased by selecting a wavelength range in which large reflection changes occur. By means of multiple applications at different locations, for example by means of a laser diode array or a scanning laser, the method also enables locally resolved temperature measurements to be obtained.

[0081] In accordance with Fig.10 In the case of a variant of the optical layer system 40, the optical layer system 40 acts as an interference filter. The wavelength of the radiation emitted by the light source 46 is within the functional wavelength range of the optical layer system 40. In the present exemplary embodiment, the light source 46 emits EUV radiation with a wavelength between 13 nm and 14 nm, which is incident on the surface of the optical layer system 40 at an angle of incidence 49 between 0° and 45°. Fig. 9 In a manner comparable to that in FIG. 4 , the light sensor 48 determines temperature information from the reflection spectrum and sends this information to the control unit 38 .

[0082] exist Fig.11 In the case of the exemplary embodiment in FIG. 4 , the reflector body 23 is provided with a thermochromic layer 50 arranged adjacent to the optical layer system 40. The thermochromic layer 50 has the property of changing color when the temperature changes. For example, the thermochromic layer 50 may include an inorganic compound rutile or zinc oxide, whose molecular structure or crystal structure changes when the temperature changes, resulting in a color change. The light source 46 can guide a light signal 47 of a suitable wavelength through the transparent material of the reflector body 23 to the thermochromic layer 50. Based on the change in the color of the reflected light portion, the light sensor 48 derives temperature information and sends it to the control unit 38. Using this method, spatially resolved temperature measurements as described above can also be obtained.

[0083] In one embodiment, the thermochromic layer 50 is designed such that a color change occurs when a certain temperature threshold is exceeded or fallen below. If the temperature threshold corresponds to an expected temperature of the mirror body 23 during operation of the projection exposure apparatus, the temperature signal recorded by the light sensor 48 can be used directly to control the temperature regulation of the mirror arrangement. In particular, a closed control loop can be provided so that the mirror body 23 is locally heated or cooled depending on the temperature signal of the light sensor 48.

Claims

1. A reflector arrangement, in particular for a microlithography projection exposure apparatus, comprising a reflector (20), a sensor unit (26) and a control unit (38), wherein the reflector (20) comprises a reflector body (23) and a reflective surface (24) formed on the reflector body (23), wherein the sensor unit (26) is designed to detect infrared radiation emitted by the reflector body (23) in order to derive temperature measurements from the infrared radiation and to send the temperature measurements to the control unit (38), wherein the reflector (20) comprises a target (37) with increased infrared radiation emissivity.

2. The mirror arrangement according to claim 1, wherein the sensor unit (26) is designed to detect the infrared radiation emitted by the mirror body (23) in a spatially resolved manner.

3. The reflector device according to claim 1 or 2, wherein: The sensor unit (26) is arranged in front of the reflective surface (24).

4. The reflector device according to any one of claims 1 to 3, wherein the reflector (20) comprises a target (37) formed in the reflective surface (24) and having a higher infrared radiation emissivity than the reflective surface (24).

5. The mirror arrangement according to claim 4, wherein the target (37) is cut out of an optical layer system (40) forming the reflective surface (24).

6. A reflector arrangement according to claim 4 or 5, wherein the ratio between the size of the reflecting surface (24) and the size of the target (37) is at least 10 4 , preferably at least 10 6 , more preferably at least 10 8 .

7. A reflector arrangement according to any one of claims 1 to 6, wherein the reflector (20) comprises a target (52) arranged outside the active optical surface (51).

8. The mirror arrangement according to any one of claims 1 to 7, wherein the sensor unit (26) is sensitive to long-wave infrared radiation having a wavelength between 7 μm and 14 μm.

9. A reflector arrangement according to any one of claims 1 to 8, wherein the reflector comprises a target (25, 27, 36) arranged within the reflector body (23).

10. The mirror arrangement according to claim 9, wherein the target (25, 27, 36) is arranged in a cavity formed in the mirror body (23).

11. The reflector device according to claim 9, wherein the target (53) is integrated in the material of the reflector body (23).

12. The reflector arrangement according to claim 11, wherein the sensor unit (26) is sensitive to a wavelength range within the infrared spectrum, the material of the reflector body (23) being transparent to said wavelength range.

13. A reflector arrangement according to any one of claims 1 to 12, comprising a cooling system (27, 30, 33) in order to keep a frame structure (29) of the reflector arrangement and / or a housing adjacent to the reflector arrangement at a constant temperature.

14. The reflector device according to any one of claims 1 to 13, wherein a frame structure (29) of the reflector device and / or a housing adjacent to the reflector device is provided with a surface having a high emissivity for infrared radiation.

15. A projection lens for a microlithography projection exposure device (10, 22), wherein a mask (13) is imaged onto a lithographic object (21) via a plurality of mirror arrangements (20), wherein at least one of the mirror arrangements is configured as a mirror arrangement as claimed in any one of claims 1 to 14.

16. A method for measuring the temperature of a reflector (20) of a microlithography projection exposure device, wherein the reflector (20) comprises a reflector body (23) and a reflective surface (24) formed on the reflector body (23), wherein a sensor unit (26) detects infrared radiation emitted by the reflector body (23) to derive a temperature measurement value from the infrared radiation, and wherein the temperature measurement value is sent to a control system of the microlithography projection exposure device, wherein the reflector (20) comprises a target (37) with an increased infrared radiation emissivity.

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