Lithographic system comprising measuring device

Abstract

The invention relates to a lithographic system, in particular an EUV lithographic system, comprising: a radiation source for generating use radiation (16); a preferably reflective optical element (M1) having a surface (25a) designed to be exposed to use radiation (16); and a measuring device (26) for measuring a heat input, in particular a temperature (TM), at the surface (25a) of the optical element (M1), said measuring device comprising a detection device (27) for detecting radiation (28) emitted from the surface (25a) of the optical element (M1) and comprising a lock-in amplifier (30). The lithographic system comprises a heating device (39) having at least one heating unit (38a, b) for guiding heating radiation (37a, b) to a surface (25a) of the optical element (M1). The heating device (39) is designed to modulate, preferably clock-modulate, in particular pulse-modulate, the heating radiation (37a, b) guided by the heating unit (38a, b) to the surface (25a) of the optical element (M1).

Description

[0001] Citation of relevant applications

[0002] This application claims priority to German patent application DE 102023208308.6, filed on August 30, 2023, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a lithography system, particularly an EUV lithography system, comprising: a radiation source for generating radiation for use; an optical element, preferably a reflective optical element, having a surface provided for irradiation with the radiation for use; and a measuring device for measuring heat input, particularly temperature, at the surface of the optical element, the measuring device comprising a detection device for detecting radiation emitted from the surface of the optical element and comprising a lock-in amplifier. Background Technology

[0004] A lithography system can be, for example, a microlithography projection exposure apparatus for the photolithographic fabrication of semiconductor components and other structured parts, or a part of such a lithography apparatus, such as the illumination system or projection optics unit of such an apparatus. However, a lithography system can also be different optical devices used for lithography, such as inspection systems, for example, for inspecting masks, wafers, etc., used in lithography. A lithography system can be particularly specific to EUV lithography systems, which use radiation operation in the EUV wavelength range between about 5 nm and about 20 nm or 30 nm.

[0005] Modern photolithography optics, or projection exposure equipment, exposes structures in a wafer plane with a resolution of a few nanometers. The requirements for these optics are extremely stringent, particularly regarding their surface shape. This is especially true for photolithography optics or those used in the EUV wavelength range that utilize radiation, requiring exceptionally precise surface shapes, especially since all optics operate reflectively in the EUV wavelength range.

[0006] During the operation of an EUV lithography system, reflective optical elements are exposed to operating radiation in the EUV wavelength range and typically heat up, which can lead to deformation of the optical elements. Therefore, materials with the smallest possible coefficient of thermal expansion, and minimal at a specific temperature (the so-called zero-crossing temperature, TZC), are used to manufacture the bodies of EUV wavelength-range optical elements. The zero-crossing temperature of the body material used for the optical element can be set within certain limits during the production of the optical element. In doing so, the zero-crossing temperature is typically chosen such that it corresponds to the desired operating temperature of the optical element. Therefore, the corresponding reflective optical element produces imaging aberrations at temperatures above or below the desired operating temperature.

[0007] To determine the actual temperature at the surface of a reflecting optical element (hereinafter referred to as a mirror for simplicity), a temperature probe or temperature sensor can be inserted into a hole in the mirror body. From the rear, the temperature probe can be brought within a few millimeters of the mirror surface. However, using such a temperature sensor does not allow for determining the actual temperature on the mirror surface. Although in principle the actual temperature of the mirror surface can be calculated in reverse, it is affected by a corresponding time lag due to the distance between the temperature probe and the mirror surface. Therefore, by using such a temperature probe, it is practically impossible to determine a short-term temperature signal from near the surface without time lag.

[0008] However, determining the precise temperature at the surface of the mirror is useful for the ability to precisely set or control the desired operating temperature of the mirror, for example, by directly and variably cooling the mirror body using a liquid-based cooling system integrated into the mirror body, or by irradiating the mirror surface with additional heating radiation.

[0009] DE102020203750A1 describes an apparatus for detecting the temperature at the surface of an optical element in semiconductor lithography. The apparatus includes a temperature recording device in the form of an infrared camera, which detects wavelengths between 1 μm and 15 μm or between 2 μm and 5 μm.

[0010] When detecting the temperature or temperature distribution of a reflective surface, the temperature recording device described in DE102020203750A1 records not only the thermal radiation emitted by the reflector surface but also the thermal radiation reflected by the reflector surface in the direction of the temperature recording device. For example, given a reflector surface made of aluminum (emissivity ε is approximately 0.05) with an operating temperature of, for example, about 30°C, and a housing or surrounding wall temperature of about 22°C, an infrared or thermal imaging camera would essentially record a mirror image of the housing as seen from the temperature recording device, but due to its low emissivity, it would be difficult to record the temperature distribution of the reflector surface itself.

[0011] To avoid this effect, a component configured for temperature control is used in DE102020203750A1. This component is arranged such that thermal radiation detected by the temperature recording device and reflected by the surface of the optical element is emitted by the component. The component can be cooled to maximize the ratio of radiation reflected from the surface of the optical element to the thermal radiation emitted by the component.

[0012] DE102020203753A1 describes a projection exposure apparatus for semiconductor lithography, comprising an optical element and a temperature recording device for detecting the temperature of the surface of the optical element by means of electromagnetic radiation emitted from the surface of the optical element. The temperature recording device includes an infrared camera and a lock-in amplifier, which amplifies the clock characteristics of the radiation used in the projection exposure apparatus for the imaging process. The lock-in amplifier amplifies only the signal component containing the clock characteristics of the radiation used. Therefore, background signals from components surrounding the surface within the field of view of the camera, detected by the temperature recording device, have little effect on the signal output by the lock-in amplifier. Summary of the Invention

[0013] The problem solved by the present invention

[0014] The problem solved by the present invention is to provide a lithography system that enables non-contact measurement of heat input (especially temperature) at the surface of an optical element, even when heat input is generated at the surface that is not attributable to the use of radiation or background radiation.

[0015] Invention Theme

[0016] This problem is solved by a lithography system of the type described at the beginning, which includes a heating device having at least one heating unit for irradiating the surface of an optical element with heating radiation, wherein the heating device is designed to modulate, preferably clock-modulated, particularly pulse-modulated, the heating radiation from the heating unit irradiating the surface of the optical element.

[0017] The lithography system described herein includes a heating device having at least one heating unit for irradiating a surface with heating radiation (in addition to the operating radiation used in the lithography system) and generating additional heat input on the surface in the process. Typically, one or more heating units are used to bring the surface of an optical element to a desired operating temperature or to maintain the surface at a desired operating temperature. For this purpose, the heating radiation irradiating the surface of the optical element can be controlled, for example. One or more heating units can also be used to heat the optical element before the lithography system is put into operation (i.e., before irradiating the optical element with the operating radiation). The wavelength or wavelength range of the heating radiation is typically different from the wavelength or wavelength range of the operating radiation. The wavelength range of the heating radiation is typically in the infrared wavelength range, with wavelengths longer than about 1 μm, for example, between about 1 μm and 1.6 μm.

[0018] Due to the modulation of heating radiation, the portion of blackbody radiation attributable to heating radiation and emitted from the surface, detected by the detection device, and generated by the surface temperature can be amplified by a lock-in amplifier and distinguished from the portion attributable to background radiation from around the optical element and reflected or deflected towards the measurement or detection device on the surface of the optical element. Therefore, when the surface of the optical element is irradiated only with heating radiation and not with radiation irradiation, the heat input or temperature at the surface of the optical element can be determined, which may be the case during an interruption of lithography system operation.

[0019] In principle, a lock-in amplifier is used to selectively filter measurement signals and remove background signals from measurement signals containing both large and small background signals. In the current case, the heating radiation corresponding to the measurement signal is modulated, enabling the lock-in amplifier to selectively filter the measurement signal—specifically, the heating radiation detected by the detection device—and remove background signals detected by the detection device and caused by background radiation from the surrounding environment and optional usage radiation. In the current case, the measurement signal corresponds to the radiation detected by the detection device.

[0020] The modulation is achieved using a deterministic modulation signal. For example, modulation can be clock-controlled, i.e., using a constant-frequency modulation signal, such as a sine or square wave modulation signal. If the modulation signal is a clock signal, it switches or modulates between two states or optionally more than two states. During periodically repeating time intervals (so-called clock pauses), the surface of the optical element may not be irradiated by heating radiation. In this case, the modulation is pulsed, i.e., the heating radiation is switched on and off in a clockwise manner.

[0021] When modulating heating radiation, the power or intensity of the heating radiation is typically modulated, for example, by clockwise switching the heating source used to generate heating radiation from the heating unit or modulating its power. In principle, other characteristics of the heating radiation can also be modulated, provided that the modulation of the corresponding characteristic allows the lock-in amplifier to selectively amplify the modulating signal or optionally amplify the inherent modulation of the heating radiation. If a clock-modulated signal is used, only the portion of the measured signal with the set modulation frequency and / or phase of the modulated signal is amplified in the lock-in amplifier; this is why background signals have almost no effect on the amplified signal output by the lock-in amplifier.

[0022] The deterministic modulation of the heating radiation described here differs from the variation of the closed-loop control form of the heating radiation used to irradiate the surface, in which the heating power of the heating radiation emitted from the heating unit is modified in a nondeterministic manner based on the current temperature (unknown in advance) at the surface of the optical element.

[0023] In this embodiment, the radiation source is designed to modulate the generated radiation used, preferably clock-modulated, particularly pulse-modulated. The radiation used can be clock-controlled or modulated using a predetermined pulse frequency. As described in detail below, a modulation signal for the radiation used, or optionally a signal detected by a radiation sensor, can be provided to the lock-in amplifier as a reference signal. The thermal radiation caused by the radiation used can be amplified in this way by means of the lock-in amplifier, and the effects of background radiation can be eliminated.

[0024] If the surface of the optical element is also irradiated by heating radiation in addition to the operating radiation amplified by a lock-in amplifier, the signal from the lock-in amplifier (attributable to the operating radiation) cannot be used alone to measure the heat input at the surface, because the heating radiation also generates a heat input at the surface, which must be taken into account in the measurement range. If the heating radiation is not modulated, the latter cannot be considered during the determination of the heat input, because in this case the heating radiation cannot be distinguished from the background radiation, thus reducing the accuracy of the heat input or temperature measurement.

[0025] If a lock-in amplifier is used to amplify the portion of radiation emitted from the surface that is detected by the detection device and is attributable to the radiation of use, as well as the portion of radiation attributable to the radiation of heating, these portions can be used to measure or determine the total heat input to the surface of the optical element. For example, the sum of the two signals (amplified by the lock-in amplifier) ​​of the radiation of use and the radiation of heating can be used to determine the total heat input. Within the range of heat input measurement, the modulation of the heating radiation does not necessarily have to be different from the modulation of the radiation of use. For example, the modulation frequency of the radiation of use can be on the order of 50 Hz. In the case of radiation sources used to generate radiation of use in the EUV wavelength range, the radiation of use is usually pulsed, either during the generation of the radiation of use or caused in some other way, i.e., it is not continuous radiation, but radiation of use that is not generated during predetermined time intervals, or the generated radiation is filtered out within the radiation source.

[0026] In this embodiment, the modulation (e.g., modulation frequency) of the used radiation from the radiation source differs from the modulation (e.g., modulation frequency) of the heating radiation from the heating unit. If the modulations of the used radiation and the heating radiation are chosen to be the same (see above), the total heat input at the surface can be measured. However, although the wavelength ranges of the heating radiation and the used radiation are generally significantly different, it is impossible to distinguish the surface heat input attributable to the heating radiation and the heat input attributable to the used radiation within the range of spatially resolved detection of (thermal) radiation emitted from the surface of the optical element, at least in those portions of the optical element surface irradiated by the used radiation and the heating radiation. Different modulations of the heating radiation and the used radiation allow for differentiation of the heat input at the surface from the heat input at the surface caused by the heating radiation, which may, for example, be advantageous for controlling the temperature at the surface. As an alternative or supplement to the modulation frequency, the used radiation and the heating radiation can also be made different from each other by different selections or modulations of their phases.

[0027] In one embodiment, the heating device is designed to provide a modulation signal as a reference signal to the lock-in amplifier, the modulation signal being used to modulate the heating radiation. In this embodiment, the modulation signal used for irradiation with heating radiation and provided to the heating unit or heating light source for modulation is also provided to the lock-in amplifier as a reference signal. If this is a clock modulation signal, the lock-in amplifier amplifies only the portion of the measurement signal that includes the modulation frequency and / or phase of the modulation signal or is related to the modulation frequency and / or phase of the modulation signal.

[0028] Within the meaning of this application, the modulation signal provided to the lock-in amplifier is also understood to be a signal associated with the modulation signal and containing information about the modulation frequency and / or phase of the modulation signal. However, it should be understood that the radiation source may also be accordingly designed to provide the modulation signal as a reference signal to the lock-in amplifier for modulating the radiation used.

[0029] In another embodiment, the lithography system includes a radiation sensor for detecting the intensity of heating radiation irradiated onto the surface by a heating unit. The radiation sensor is designed to provide a signal as a reference signal to a lock-in amplifier, the signal being dependent on the detected intensity of the heating radiation. For example, the radiation sensor may be a photodiode, which generates a signal in the form of photocurrent proportional to the intensity of the heating radiation incident on the photodiode. It should be understood that the lithography system may also include a radiation sensor for detecting the intensity of operational radiation irradiated onto the surface by a radiation source, the radiation sensor being designed to provide a signal as a reference signal to the lock-in amplifier, the signal being dependent on the detected intensity of the operational radiation.

[0030] In another embodiment, the heating device includes a first heating unit and a second heating unit for irradiating the surface of an optical element with heating radiation. The heating device is designed to modulate, preferably clock-modulated, particularly pulse-modulated, the heating radiation from the first heating unit, and to modulate, preferably clock-modulated, particularly pulse-modulated, the heating radiation from the second heating unit, and the modulation of the heating radiation from the first heating unit differs from the modulation of the heating radiation from the second heating unit. Due to the different modulations, the heat input or thermal characteristics of the first and second heating units may differ from each other at the surface. This is advantageous for setting or controlling the temperature at the surface as precisely as possible using the heating units. The heating radiation from the heating units used for irradiation is typically infrared radiation with a wavelength longer than 1 μm, for example, a wavelength of about 1070 nm.

[0031] The entire surface of the optical element can be illuminated by heating radiation from the heating unit, but the heating radiation from the heating unit is typically directed only to a portion or sector of the surface, or only used to illuminate that portion or sector. Particularly when different portions or sectors of the surface are illuminated by heating radiation from two separate heating units, the different modulations used to distinguish the thermal input can be optionally omitted. For example, in this case, the surface can be measured using a spatially resolved calorimeter of the measuring device. For measuring the surface, the measuring device can also include multiple photodiodes, each with an imaging optical unit in front of it, which detects only a sector of the surface, or optionally a small portion, in pixels. In principle, a thermal imaging camera can also be used, provided it has sufficient temporal resolution for locking the thermal image. In this way, the thermal input from the heating unit can be associated with a corresponding portion of the thermal image of the surface (optionally, a spatially high-resolution thermal image).

[0032] The heating unit itself may include a heating radiation source, which is typically designed to generate heating radiation in the infrared wavelength range. However, the heating radiation may also be generated by an external heating radiation source and provided to the heating unit, for example, via optical fiber. It should be understood that the heating device may also include two or more heating units, which can be used to irradiate the surface of optical elements.

[0033] In another embodiment, the lithography system includes an additional, preferably reflective, optical element having an additional surface for irradiation with radiation, and the heating device includes at least one additional heating unit for irradiating the surface of the additional optical element with additional heating radiation, and the heating device is designed to modulate the additional heating radiation from the additional heating unit, preferably clock modulation, particularly pulse modulation.

[0034] In this embodiment, the lithography system may include additional measuring devices designed to measure the heat input, preferably temperature, at an additional surface of an additional optical element. The additional measuring devices may include additional detection devices for detecting additional radiation emitted from the additional surface of the additional optical element and an additional lock-in amplifier, which can be used to determine the heat input or thermal characteristics of additional heating radiation from additional heating units irradiating the additional surface. It should be understood that the heating device may include two or more additional heating units, which can be used to irradiate the additional surface of the additional optical element with additional heating radiation.

[0035] In this embodiment, the modulation of heating radiation from the heating unit differs from the modulation of heating radiation from other heating units. This is particularly true when the optical element is a reflective optical element, which is typically located in a highly reflective environment. In this case, some of the heating radiation from the heating unit illuminating the reflective optical element is reflected from the surface of the reflective optical element and, after multiple reflections, reaches the surface of other reflective optical elements, where it results in unwanted heat input. This phenomenon is also known as crosstalk.

[0036] If the heating elements of different reflective optics are modulated or pulsed, the amount of absorbed crosstalk originating from the respective reflective optics can be determined using locked thermal imaging or a corresponding locked amplifier. To identify crosstalk, it may be advantageous to vary the modulation of the heating elements for all reflective optics in the lithography system or optionally for a portion of the lithography system (e.g., the projection lens).

[0037] This does not necessarily mean that the heating units used for the corresponding reflective optical elements must differ in their modulation. However, if the modulation of the heating units used for the same optical element is different, not only can the optical element causing crosstalk be identified, but also the heating unit causing crosstalk. To identify crosstalk, it is advantageous that the modulation of the heating radiation from one or more heating units, or one or more additional heating units, differs from the modulation of the radiation used.

[0038] To identify crosstalk, for example, a modulation or modulated signal from another heating unit or a signal from a sensor unit used to detect the intensity of additional heating radiation can be provided as a reference signal to the lock-in amplifier of the measuring device for the aforementioned optical element. In this case, the lock-in amplifier amplifies a portion of the detected radiation, which can be traced back to crosstalk caused by additional heating radiation from the surface of the other optical element.

[0039] In another embodiment, the detection device for detecting radiation emitted from the surface of an optical element is designed as a spatially resolved calorimeter. In this case, the radiation detected by the detection device is thermal radiation in the infrared wavelength range. A spatially resolved calorimeter allows for non-contact detection of thermal radiation emitted from the surface of an optical element, whether near the surface or with high temporal and spatial resolution, thus allowing for instantaneous measurement of heat input or temperature at the surface of the optical element. Typically, a spatially resolved calorimeter internally images a surface area onto a spatially resolved detector, in which heat input or temperature should be measured. The surface area can be the entire surface or a portion of it. The imaging process is accomplished by means of an optical unit adapted to the wavelength range of the detected radiation.

[0040] In another embodiment, the detection device for detecting radiation emitted from the surface of an optical element includes at least one photodiode, preferably multiple photodiodes. It is generally advantageous to amplify the analog signal using a lock-in amplifier, i.e., before any potential digitization that might reduce signal resolution. Therefore, it is advantageous to use a detector that is not read in digital form, such as in the case of a CCD chip, since subsequent signal amplification should be performed in an analog manner. For example, a grid of photodiodes sensitive to the radiation to be detected and whose analog signals are accessible can be used as a detector. If such a grid or array is integrated as a detector into a detection device in the form of a thermal imaging camera, the corresponding portion of the surface of the optical element can be imaged onto the photodiodes by means of a suitable imaging optics unit.

[0041] Photodiodes can be controlled in such a way that a so-called photocurrent can be measured at them, which is proportional to the intensity of thermal radiation incident on the photodiode. The photocurrent from the photodiode is provided as an input signal to a lock-in amplifier. Furthermore, one or more reference signals (see above) related to the modulation of the signal used are provided to the lock-in amplifier at a reference input. For example, the photocurrent from a radiation sensor detecting the intensity of the signal used can be used as a reference signal, or the modulation signal can be used as a reference signal.

[0042] In principle, radiation emitted from a surface can be detected in a non-spatial-resolved manner. In this case, the detection device may include, for example, a single photodiode acting as a detector, onto which the radiation emitted from the surface is imaged by means of a suitable optical unit. The arrangement of the photodiodes, optionally combined with upstream optical units, can also serve as a detection device. This arrangement of the photodiodes also enables spatially resolved detection of heat input or temperature at the surface of the optical element, as further described above.

[0043] Detection devices are typically designed to detect electromagnetic radiation in the wavelength range of approximately 1 μm to approximately 5 μm, optionally up to approximately 15 μm, or are designed to be sensitive to such electromagnetic radiation. The sensitivity and resolution of the detector used can be optimized for temperatures, for example, between +20°C and +150°C, as a result of selecting the spectral range detected by the detection device to be in the near-infrared or optionally into the long-wave infrared. To determine the temperature based on the detected electromagnetic radiation, the Stefan-Boltzmann law can be used, provided that the surface emissivity is known; the latter can be determined in advance, for example, through calibration.

[0044] If modern digital image processing is used, the detector does not need to generate an analog signal for the lock-in amplifier. Within the meaning of this application, a lock-in amplifier is also understood to refer to digital circuitry or corresponding software that performs the function of an analog lock-in amplifier.

[0045] The measuring device can measure the corresponding thermal input at the surface of an optical element based on a signal attributable to the radiation used and a signal attributable to the heating radiation, both amplified by a lock-in amplifier. For this purpose, the measuring device can include an evaluation apparatus implemented in suitable hardware and / or software. The amplification of the signals attributable to the radiation used and the heating radiation can be achieved simultaneously in the lock-in amplifier, particularly when the radiation used and the heating radiation are instantaneous and clock-synchronized. In this case, the total thermal input of the radiation used and the heating radiation during the corresponding thermal pulse can be measured by the lock-in amplifier.

[0046] The lock-in amplifier also filters the non-cyclic baseline value of the background signal, which corresponds to the background temperature at the surface. If the temperature is determined by the Stefan-Boltzmann law, the signal from the lock-in amplifier is related to T. B 4 – T A 4 Proportional, where T B T represents the temperature during the (heating) pulse. A This represents the background temperature of the surface. For example, a slow-responding temperature probe in a hole in the body of a mirror can be used to determine the background temperature T of the surface. A If the temperature T A If the signal from the lock-in amplifier is known, then the (instantaneous) temperature T at the surface can be determined. B The signal from a lock-in amplifier that measures only the amplitude of the thermal pulse can also be used as a control signal for one or more heating units in the heating device.

[0047] For such measurement or determination of temperature, the measuring apparatus typically includes an evaluation device, which can be designed, for example, in a digital or analog circuit manner. As further described above, when determining temperature by means of a lock-in amplifier, the contribution of background radiation can be eliminated.

[0048] Amplification via time-division multiplexing is also possible. Here, the radiation used and the heating radiation differ from each other in at least one characteristic, such as frequency or phase. In this case, the heating radiation and the radiation used may, for example, have different frequencies. Advantageously, in this case, the frequencies are not harmonically correlated with each other, i.e., they do not represent integer multiples of each other. If the heating radiation and the radiation used have the same frequency but different phases, the heat input via the heating radiation and the heat input via the heating radiation can also be distinguished from each other. In this case, different reference signals are used to measure the heat input via the heating radiation and the heat input via the heating radiation.

[0049] In another embodiment, the detection device is designed to detect radiation emitted from the entire surface. Here, the entire surface can be measured, for example, by means of a spatially resolved pyrometry meter, multiple photodiodes (each with an imaging optics unit preceding it), or optionally by means of a thermal imaging camera with high temporal resolution. In this way, the temperature at any location on the surface can be measured by the measuring device. In principle, it is also possible to detect radiation emitted from only a portion of the surface of an optical element using the detection device. If this involves an optical element comprising a reflective coating, then that surface forms the boundary between the reflective coating and the surrounding environment of the optical element.

[0050] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention with reference to the accompanying drawings, which illustrate essential details of the invention and are obvious from the claims. In variations of the invention, each feature may be implemented individually in each case or as a plurality of features in any desired combination. Attached Figure Description

[0051] Exemplary embodiments are illustrated in the schematic diagrams and explained in the following description. In the drawings:

[0052] Figure 1 A schematic diagram showing the meridional section of a projection exposure device passing through EUV projection lithography.

[0053] Figure 2 A schematic diagram of a DUV lithography apparatus, including an illumination device and a projection lens, is shown.

[0054] Figure 3 Show Figure 1A schematic diagram of a reflector in a projection exposure apparatus, which includes a measuring device for measuring the heat input, particularly the temperature, at the reflective surface of the reflector.

[0055] Figure 4 A schematic diagram of a lock-in amplifier is shown, which is provided with a measurement signal containing a usage signal and a background signal, and a reference signal is also provided.

[0056] Figure 5 Show Figure 1 A schematic diagram of another reflector in the projection exposure equipment, which has a separate measuring device.

[0057] Figure 6 A schematic diagram showing the time curves of the pulse reference signal and the used signal is presented.

[0058] Figure 7 Showing from Figure 6 The time curve of the signal and a schematic diagram of heat propagation in the mirror at three different times are shown.

[0059] Figure 8 A schematic diagram showing the spectrum of radiation emitted from the surface of the reflective element at two different times is provided.

[0060] Figure 9 A schematic diagram of three radiation sensors with three different wavelength filters is shown when detecting thermal radiation at two different times.

[0061] In the following description of the figures, the same reference numerals are used for the same or functionally identical parts. Detailed Implementation

[0062] The following is for reference. Figure 1 The main components of an optical apparatus used for EUV lithography, in the form of a microlithography projection exposure apparatus 1, are described by way of example. The description of the basic setup of the projection exposure apparatus 1 and its components should not be construed as limiting.

[0063] In embodiments of the illumination system 2 of the projection exposure apparatus 1, in addition to the light source or radiation source 3, there is also an illumination optical unit 4 for illuminating the object field 5 in the object plane 6. In alternative embodiments, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0064] The mask master 7, arranged in the object field 5, is illuminated. The mask master 7 is held by the mask master holder 8. The mask master holder 8 can be moved in the scanning direction, in particular, by the mask master displacement driver 9.

[0065] As an explanation, Figure 1The Cartesian xyz coordinate system is shown. The x-direction extends perpendicularly to the plane in the attached figure. The y-direction extends horizontally, and the z-direction extends vertically. The scan direction is... Figure 1 It extends in the y-direction. The z-direction extends perpendicularly to the object plane 6.

[0066] The projection exposure apparatus 1 includes a projection system 10. The projection system 10 is used to image the object field 5 onto the image field 11 in the image plane 12. The structure on the mask master 7 is imaged onto the photosensitive layer of the wafer 13, which is disposed in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved by a wafer displacement driver 15, particularly in the y-direction. The mask master 7 is moved first by the mask displacement driver 9, and then the wafer 13 is moved by the wafer displacement driver 15, and the two can be synchronized with each other.

[0067] Radiation source 3 is an EUV radiation source. Radiation source 3 specifically emits EUV radiation 16, which is also referred to below as working radiation, illumination radiation, or illumination light. Working radiation particularly has wavelengths in the range of 5 nm to 30 nm. Radiation source 3 can be a plasma source, such as an LPP (laser-generated plasma) source or a GDPP (gas discharge-generated plasma) source. It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0068] Illumination radiation 16 emitted from radiation source 3 is focused by a concentrator mirror 17. The concentrator mirror 17 may be a concentrator mirror having one or more elliptical and / or hyperboloidal reflective surfaces. The illumination radiation 16 may be incident on at least one reflective surface of the concentrator mirror 17 at either grazing incidence (GI) (i.e., at an angle of incidence greater than 45°) or normal incidence (NI) (i.e., at an angle of incidence less than 45°). The concentrator mirror 17 may be structured and / or coated, primarily to optimize its reflectivity to the radiation used, and secondarily to suppress intrusive light.

[0069] Downstream of the light-collecting reflector 17, the illumination radiation 16 propagates through the intermediate focal point in the intermediate focal plane 18. The intermediate focal plane 18 can form a separation between the radiation source module with radiation source 3 and light-collecting reflector 17 and the illumination optical unit 4.

[0070] The illumination optics unit 4 includes a deflecting mirror 19 and a first faceted mirror 20 disposed downstream of it in the beam path. The deflecting mirror 19 may be a planar deflecting mirror, or alternatively a mirror with beam-affecting effects beyond pure deflection. Alternatively, the deflecting mirror 19 may be implemented as a spectral filter that separates the wavelength of the illumination radiation 16 used light from intrusive light with wavelengths deviating from that wavelength. The first faceted mirror 20 includes a plurality of individual first facets 21, which are also referred to below as field facets. Figure 1 Some of these facets 21 are shown only by way of example. In the beam path of the illumination optics unit 4, a second facet mirror 22 is disposed downstream of the first facet mirror 20. The second facet mirror 22 includes a plurality of second facets 23.

[0071] The illumination optics unit 4 thus forms a two-faceted system. This basic principle is also known as a fly-eye integrator. The second faceted mirror 22 is used to image the individual first facet 21 into the object field 5. The second faceted mirror 22 is the final beam-shaping mirror, or in effect, the last mirror reflecting the illumination radiation 16 in the beam path upstream of the object field 5.

[0072] The projection system 10 includes a plurality of reflectors Mi, which are numbered sequentially according to their arrangement in the beam path of the projection exposure device 1.

[0073] exist Figure 1 In the example shown, the projection system 10 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a channel opening for illumination radiation 16. The projection system 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture greater than 0.4 or 0.5, and may also be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0074] Just like the reflector of the illumination optics unit 4, the reflector Mi may have a highly reflective coating for illumination radiation 16.

[0075] Figure 2 A schematic diagram of a DUV projection exposure apparatus 100 is shown, which includes a beam shaping and illumination device 102 and a projection lens 104. In this case, DUV stands for "deep ultraviolet" and indicates the wavelength of working light between 30 nm and 370 nm. The DUV projection exposure apparatus 100 includes a DUV light source 106. For example, an ArF excimer laser that can provide radiation 108 emitting in the DUV range (for example, at 193 nm) can serve as the DUV light source 106.

[0076] Figure 2The beam shaping and illumination device 102 described herein directs DUV radiation 108 onto a photomask 129. The photomask 129 is designed as a transmissive optical element and can be disposed outside the beam shaping and illumination device 102 and the projection lens 104. The photomask 129 includes a structure that projects its reduced image onto a wafer 124 or the like through the projection lens 104.

[0077] The projection lens 104 includes multiple lens elements 128, 140 and / or mirrors 130 for imaging the photomask 129 onto the wafer 124. In this case, the individual lens elements 128, 140 and / or mirrors 130 of the projection lens 104 may be symmetrically arranged about the optical axis 126 of the projection lens 104. It should be noted that the number of lens elements and mirrors in the DUV projection exposure apparatus 100 is not limited to the number shown. More or fewer lens elements and / or mirrors may also be provided. Furthermore, for beam shaping purposes, the mirrors are typically bent on their front side.

[0078] The air gap between the last lens element 140 and the wafer 124 can be filled with a liquid medium 132 with a refractive index >1. The liquid medium 132 can be, for example, high-purity water. This setup is also known as immersion lithography and has increased lithographic resolution.

[0079] The following text is based on Figure 3 describe Figure 1 Temperature measurement at the reflector Mi in the projection system 10 of the projection exposure apparatus 1. It is understood that this temperature measurement can also be performed at one of the optical elements in the illumination system 2 of the projection exposure apparatus 1 or... Figure 2 The exposure is performed at one of the optical elements in the DUV projection exposure apparatus 100 shown, such as at the lens elements 128, 140 or the mirror 130 in the projection lens 104.

[0080] Figure 3 show Figure 1 The projection system 10 of the projection exposure apparatus 1 is a reflective optical element in the form of a first reflector M1. The reflector M1 comprises a body 24 formed of a material having a low coefficient of thermal expansion. The reflector M1 includes a highly reflective coating 25 applied to the body 24. In the example shown, where the radiation 16 from the radiation source 3 is EUV radiation with a wavelength of 13.5 nm, the highly reflective coating 25 comprises alternating layers of silicon and molybdenum. A surface 25a at the top side of the reflective coating 25 forms a boundary with the surrounding environment, and during operation, this surface 25a is irradiated with the radiation 16 and the surface reflects the radiation 16, as from... Figure 3 It is obvious from the text.

[0081] In principle, the temperature T at the surface 25a of the reflector M1 can be measured by a temperature sensor (not shown here) fixed in the body 24 of the reflector M1 at a small distance of a few millimeters from the surface 25a of the reflector M1. M However, this temperature sensor only allows for the measurement of the temperature T at surface 25a with a certain time lag. M This time lag during temperature measurement results in a significant error contribution to surface pattern deformation.

[0082] In order to spatially resolve the temperature T at surface 25a of the reflector M1 (more precisely, the reflective coating 25) in the example shown. M , Figure 1 The projection exposure apparatus 1 includes a measuring device 26. In the example shown, the measuring device 26 includes a detection device 27 for detecting radiation emitted from the surface 25a of the reflector M1 and... Figure 3 Radiation 28 is indicated by the dashed arrow. Detection device 27 detects radiation 28 located within the field of view 29, indicated by the dashed line. In the example shown, the entire surface 25a of the reflector M1 is located within the field of view 29. Alternatively, only a portion of the surface 25a of the reflector M1 can be detected by means of detection device 27.

[0083] In the example shown, the detection device 27 is designed to detect radiation in the infrared wavelength range of 1 μm to approximately 10 μm, optionally up to approximately 15 μm. Figure 3 In the example shown, the detection device 27 is in the form of a spatially resolved radiative thermal meter. When measuring the temperature T at surface 25a of the reflector M1... M The problem is that the field of view 29 of the detection device 27 detects not only the radiation 28 from the surface 25a itself, but also the radiation from the highly reflective surrounding environment of the reflector M1 and reflected from the surface 25a to the direction of the detection device 27.

[0084] In addition, the temperature T to be measured at surface 25a of mirror M1 M (It may be in the range of, for example, between 22°C and 40°C) is not significantly different from the temperature of the environment surrounding the reflector M1 (which is, for example, about 22°C). Therefore, the measuring device 26 cannot distinguish the temperature of the surrounding environment reflected from the surface 25a from the temperature of the reflecting surface 25a with sufficient accuracy to achieve a temperature measurement accuracy of, for example, about 0.1K.

[0085] exist Figure 3In the case of the measuring device 26 shown, the used radiation 16 is not continuously generated by the radiation source 2, but is generated in a pulsed manner. That is, the radiation source 2 generates used radiation 16 with pulse modulation, the pulse frequency of which is, for example, about 50 Hz, using which radiation pulses and pulse pauses alternate. In the example shown, the modulation of used radiation 16 is used to amplify only the portion of the detected radiation 28 attributable to used radiation 16 in the lock-in amplifier 30 of the measuring device 26, and to distinguish this portion from the portion of background radiation attributable to the environment around the reflector M1 and reflected from the reflective surface 25a.

[0086] exist Figure 3 In this configuration, the lock-in amplifier 30 is designed for analog signal processing. Therefore, the spatially resolved detector 31 attached to the detection device must be capable of analog readout. For this purpose, the detector 31 in the illustrated example includes a plurality of photodiodes 31a arranged in a grid and controlled to measure the photocurrent at each photodiode, which is proportional to the intensity of infrared radiation 28 incident on the respective photodiode 31a and detected by the detection device 27 in the form of a spatially resolved pyrometry. The detection device 27 includes an optical unit (not shown here) that images a corresponding portion of surface 25a onto the photodiode 31a associated with that portion.

[0087] The photocurrent generated by the corresponding photodiode 31a forms a measurement signal 32, the time curve of which is shown in... Figure 4 As shown, it is provided to the first input of the lock-in amplifier 30. A reference signal 33 is provided at the second input of the lock-in amplifier 30. In the example shown, the reference signal is a pulsed square wave signal, and the time variation curve or amplitude A of the reference signal is also shown. Figure 4 As shown in the figure. The measurement signal 32 includes a first part in the form of a usage signal 34 and a second part in the form of a background signal 35, the latter having a significantly larger amplitude A than the usage signal 34.

[0088] from Figure 4 It is also evident that only the use signal 34 (not the background signal 35) is modulated at a constant frequency. Since the reference signal 33, pulsed at the same frequency, is provided to the lock-in amplifier 30, the lock-in amplifier 30 amplifies only the use signal 34, which is modulated at the same frequency as the reference signal 33 and optionally with the same phase. Figure 4 The right side depicts the use signal 34 amplified by the lock-in amplifier 30.

[0089] In the example described here, the signal 34 is a portion of the detected radiation 28 in the infrared range, which is attributable to the used radiation 16 and detected by the detection device 27. The reference signal 33 is formed by a modulation signal, which is also provided to the radiation source 2 to pulse the used radiation 16. Since the background signal 35, which is attributable to background radiation, is not correlated with the frequency of the used radiation 16, a portion of the measurement signal 32, which is attributable to background radiation, is not amplified by the lock-in amplifier 30.

[0090] The amplified usage signal 34 is provided to the evaluation device 36 of the measuring device 26, and the evaluation device 36 infers the temperature T at the surface 25a of the reflector M1 based on the amplified usage signal 34. M In addition to the rapid fluctuation component, the background signal 35 also includes a non-cyclic component (baseline component), which is not amplified by the lock-in amplifier 30. Therefore, the lock-in amplifier 30 measures the magnitude or intensity of the difference between the temperature of the corresponding thermal pulse and the background temperature. This is done to determine the temperature T at surface 25a of the reflector M1 using the Stefan-Boltzmann law. M Therefore, it is necessary to further determine the non-cyclic component of the background signal 35, which corresponds to the temperature at surface 25a that is not attributable to cyclic thermal input. To determine the time-averaged temperature at surface 25a, the evaluation device 36 can, for example, use the aforementioned slow-response temperature sensor integrated into the body 24 of the reflector M1. Furthermore, the measurement device 26 can be calibrated and characterized before the projection exposure equipment 1 is put into operation, for which a pulsed heat source with known power can be used, for example.

[0091] Even in Figure 1 In the projection exposure apparatus 1, even with a non-cooled wall and correspondingly significant background radiation surrounding the reflector M1, the temperature T near the surface generated by the use of radiation 16 can still be measured very accurately by a photometric device. M This does not require the use of contact locking for thermal imaging as described above. However, this only applies to the temperature T at surface 25a of mirror M1. M The heat input can only be attributed to the use of radiation 16, and not to the heat input to surface 25a from other heat sources.

[0092] exist Figure 3In the example shown, in addition to radiation 16, the surface 25a of the reflector M1 is also irradiated by heating radiation 37a, 37b from the first heating unit 38a and the second heating unit 38b of the heating device 39. In the example shown, the heating units 38a, 38b are designed to direct the heating radiation 37a, 37b to different portions of the surface 25a of the reflector M1. The heating units 38a, 38b can be implemented in different ways, for example, in the form of a fan-shaped heater or a heating head. The heating radiation 37a, 37b can be generated by a corresponding heating radiation source in each heating unit 38a, 38b; however, the heating radiation 37a, 37b can also be generated by one or more heating radiation sources in the heating device 39, which are arranged at a distance from the heating units 38a, 38b and supplied to the heating units 38a, 38b via, for example, fiber optic cables.

[0093] For example, heating units 38a and 38b can be used to predict the initial heating of the reflector M1 before the exposure operation of the projection exposure device 1, or to preheat the reflector M1 for the exposure operation. Heating units 38a and 38b can also be used to homogenize the non-uniform temperature distribution on the surface 25a of the reflector M1 by supplemental heating, which is caused by the corresponding illumination setting. Heating units 38a and 38b can also be used to compensate for temporal and spatial temperature fluctuations on the surface 25a of the reflector M1, such as those caused by changes from a bright mask master to a dark mask master.

[0094] from Figure 3 It can also be clearly seen that surface 25a includes a portion heated only by radiation 16 and a portion where both radiation 16 and heating radiation 37a and 37b are incident on surface 25a. In principle, it is also possible that a portion of surface 25a is heated only by heating radiation 37a and 37b without using radiation 16.

[0095] Especially in the portion of surface 25a that is heated by radiation 16 and heating radiation 37a, 37b, the temperature T at surface 25a M The temperature T at surface 25a depends not only on the intensity or power of radiation 16, but also on the intensity or power of the heating radiation 37a and 37b irradiating surface 25a. M The heating radiation 37a and 37b from the irradiation surfaces 25a of the heating units 38a and 38b is also modulated, or more precisely, pulsed. Figure 3 In the example shown, the modulation signals m1 and m2 provided for this purpose to the heating units 38a and 38b are also provided as reference signal 33 to the lock-in amplifier 30. Figure 4In the manner described, the lock-in amplifier 30 can generate an amplified use signal 34, which corresponds to a portion of the detected radiation 28 attributable to heating radiation 37a, 37b from heating units 38a, 38b or from the respective heating units 38a, 38b.

[0096] The modulation signals m1, m2 and the modulation signal used for radiation 16 can be the same. Although in this case the temperature T in the corresponding portion of the surface 25a to which radiation 16 and heating radiations 37a, 37b are incident can be determined by the evaluation device 36. M However, the portions using radiation 16 and heating radiation 37a, 37b cannot be distinguished from each other. Nevertheless, this distinction is advantageous for controlling the heating units 38a, 38b in order to produce the desired temperature distribution at the surface 25a of the reflector M1.

[0097] If the modulation of heating radiations 37a and 37b differs from the modulation of radiation 16, then each part can be individually amplified by lock-in amplifier 30 and thus distinguished from one another. For this purpose, it is sufficient that the two modulation signals M1 and M2 used for heating units 38a and 38b are different from the modulation signal used for radiation 16. For example, the two modulation signals M1 and M2 can have modulation frequencies that deviate from, for example, the modulation frequency of radiation 16 by 50 Hz. Alternatively, the two modulation signals M1 and M2 used for heating radiations 37a and 37b can be controlled with a phase clock different from the modulation signal used for radiation 16.

[0098] Advantageously, the modulation signals m1 and m2 of the two heating units 38a and 38b are different from each other. In this case, the heat input of the two heating units 38a and 38b at surface 25a can be distinguished by means of the lock-in amplifier 30. If compared with Figure 3 The difference described herein, where heating radiation 37a, 37b from the two heating units 38a, 38b is used to irradiate the common portion of surface 25a, is particularly advantageous. For example, the two modulation signals m1, m2 can therefore have different modulation frequencies.

[0099] Figure 5 show Figure 1 The projection optics unit 10 of the projection exposure apparatus 1 has a second mirror M2, which has an additional measuring device 26' for measuring the temperature T at the additional reflecting surface 25a' of the additional mirror M2. M The additional measuring device 26' is implemented in a manner similar to... Figure 3The measuring device 26 shown. The heating device 39 for heating all the mirrors M1 to M6 in the projection optical unit 10 has two additional heating units 38a' and 38b' in the illustrated example, and these heating units are configured such that the additional surface 25a' of the additional mirror M2 can be irradiated with additional heating radiation 37a' and 37b'. The two additional heating units 38a' and 38b' are also used by the heating device 39 for pulse modulation of the additional heating radiation 37a' and 37b', using this additional heating radiation 37a' and 37b' to irradiate the additional surface 25a'.

[0100] from Figure 5 It can also be clearly seen that the first radiation sensor 40a', used to detect the intensity I1 of the additional heating radiation 37a', is fixed at the lateral edge of the beam cross-section of the additional heating radiation 37a' emitted by the first additional heating unit 38a'. Therefore, the second radiation sensor 40b', used to detect the intensity I2 of the additional heating radiation 37b', is fixed at the lateral edge of the beam cross-section of the additional heating radiation 37b' emitted by the second additional heating unit 38b'. The third radiation sensor 40c' is fixed at the lateral edge of the beam cross-section of the radiation 16 used, and is used to detect the intensity I3 of the radiation 16 irradiating the additional surface 25a'. The three radiation sensors 40a-c' are photodiodes, each generating signals S1, S2, S3, which depend on the corresponding detection intensities I1, I2, I3 and are in the form of photocurrent proportional to the corresponding intensities I1, I2, I3. These signals are provided as corresponding reference signals 33 to the additional lock-in amplifier 30' in the additional measuring device 27'. It should be understood that Figure 3 The reference signal 33 of the lock-in amplifier 30 can be similarly provided by three radiation sensors (not shown here).

[0101] The modulation of the additional heating radiation 37a' from the first additional heating unit 38a' is different from the modulation of the additional heating radiation 37b' from the second additional heating unit 38b', so that the heat input of the respective additional heating units 38a' and 38b' at the other surface 25a' can be distinguished from each other.

[0102] Furthermore, incident at Figure 3The modulation of the heating radiation 37a, 37b from the first and second heating units 38a, 38b on the surface 25a of the first reflector M1 is different from the modulation of the additional heating radiation 37a', 37b' incident on the other surface 25a' of the second reflector M2. In this way, the additional heating radiation 37a', 37b' from the other heating units 38a', 38b', which is reflected from the other surface 25a' of the second reflector M2 and reaches the surface 25a of the first reflector M1 by means of the measuring device 26. For this purpose, i.e., to determine the heat input generated at the surface 25a of the first reflector M1 by the heating radiation 37a', 37b' used to irradiate the other surface 25a' of the second reflector M2, signals S1, S2 generated by the first radiation sensor 40a' and the second radiation sensor 40b' are provided as reference signals 33 to the lock-in amplifier 30 in the measuring device 26.

[0103] Understandably, in combination Figure 5 The described process can be performed accordingly at the four additional mirrors M3 to M6 in the projection system 10 to determine the crosstalk of all other mirrors M2 to M6 at the surface 25a of the first mirror M1. It should also be understood that this procedure can be performed accordingly at the other mirrors M2 to M6. Furthermore, it should be understood that the temperature measurement performed in the above manner can also be performed on a mirror that is cooled, in addition to being heated, by means of direct cooling within the body of the respective mirror.

[0104] In the manner described above, when the surface is heated by radiation and additionally by (optionally controllable) heating radiation, non-contact temperature measurement (both near the surface and with high temporal and spatial resolution) can be performed on the surface of the optical elements in the photolithography systems 1 and 100. Temperature measurement can be used to improve temperature control at the surface of the mirror. As mentioned above, by using phase-locked thermal imaging in conjunction with heating the optical elements by heating radiation, the optical elements can also be used as manipulators, for example, for correcting the wavefront using radiation 16.

[0105] Furthermore, the appropriate selection of the phase relationship between the reference signal 33 and the used signal 34 (see [reference]). Figure 6 This allows for the detection of different aspects of signal 34, as follows: Figure 7 As described. Figure 7 The curve of the use signal 34 shown, i.e. the thermal radiation from surface 25a, which is not attributed to background radiation, has a time relationship with the intensity pulse of the pulsed use radiation 16 or the pulsed heating radiation 37a, 37b. Figure 7The amplitude A of radiation 34 shown in the figure can dissipate to the instantaneous temperature T near the surface of the underlying material of the body 25 before the thermal pulses of radiation 16 or heating radiation 37a, 37b are dissipated. M Related. Figure 7 The time decay of the curve for signal 34 shown is related to the temperature of the underlying layer and the thermal conductivity, heat capacity, and thermal radiation of the material. Information about the behavior of thermal conductivity and heat capacity can be used to verify a finite element model of thermal conduction within mirror M1. For example, the thermal resistance of the reflective coating 25 and the thermal resistance between the reflective coating 25 and the body 24 can be determined or quantified in this way, for example, in the form of titanium-doped quartz glass.

[0106] Figure 7 This temporal behavior is illustrated at three distinct times, t1, t2, and t3. At the first time, t1, a pulse of heating radiation 37a irradiates a portion of surface 25a, where a small volume, represented by a rectangle, beneath surface 25a is heated. As a result of heat conduction, the heated volume increases at the two later times, t2 and t3, while the temperature within the heated volume decreases. By changing the phase relative to the reference signal 33, the curve of the signal 34 used can, in principle, be measured, where the temporal resolution is on the order of the length of the pulses using radiation 16 or heating radiation 37a and 37b.

[0107] Since the thermal radiation detected by the detection device 27 is proportional to the heat flux emitted at the surface 25a, the temperature of the irradiated surface layer can be inferred according to the Stefan-Boltzmann law:

[0108] ,

[0109] Where ε represents the emissivity of surface 25a, and σ = 5.67 10 -8 W / (m 2 K 4 ) represents the Stefan-Boltzmann constant, and A represents the area of ​​the radiating surface.

[0110] Figure 8 and Figure 9 The temperature T at surface 25a of mirror M1 was described. M Another option. In Figure 8 and Figure 9 The example shown utilizes the fact that the temperature T at surface 25a of mirror M1 is... M It can also be measured based on the position of the blackbody spectrum emitted by an object (e.g., surface 25a of mirror M1), as a function of wavelength λ (Wien's law, Planck's radiation law). To determine temperature T in this way... M , Figure 9The example shown provides three radiation sensors 41a-c in the environment surrounding the reflector M1 to detect corresponding portions of radiation 28 emitted from surface 25a of the reflector M1. In the example shown, each of the radiation sensors 41a-c includes a wavelength filter that transmits the corresponding wavelength ranges Δλ1, Δλ2, Δλ3 to the photodiode of the radiation sensor 41a-c. Therefore, each of the three radiation sensors 41a-c is sensitive to different wavelength ranges Δλ1, Δλ2, Δλ3 of blackbody radiation. Therefore, the temperature T at surface 25a... M The relative intensity I or amplitude of the radiation detected in the corresponding wavelength ranges Δλ1, Δλ2, and Δλ3 can be inferred. In principle, the two radiation sensors 41a and 41b are therefore sufficient to determine the temperature T. M However, considering the need to improve the accuracy of temperature measurements, a greater number of radiation sensors would be advantageous.

[0111] Figure 8 and Figure 9 The left side shows the reflector M1 at time t1 in each case, where the surface 25a of the reflector M1 is irradiated by heated radiation 37a. The irradiated surface 25a of the reflector M1 has a very low reverse penetration depth for the thermal radiation generated below the surface 25a of the reflector M1. However, thermal radiation from a layer located a few nanometers below the surface 25a also reaches the radiation sensor 41a-c. Therefore, in principle, the thermal radiation 28 at the first time t1 when the heated radiation 37a is incident on the surface 25a can originate from a different depth than the thermal radiation 28 incident on the radiation sensor 41a-c at a later second time t2. This results in a change in the emissivity of the surface 25a from the first time t1 to the second time t2, as can be based on Figure 8 Middle left and Figure 8 The diagram on the right is used for identification. Information regarding this change in emissivity ε of surface 25a can also be determined by the phase change at the locked amplifier 30. Layer degradation of the reflective coating 25 can be detected and optionally quantified based on the change in emissivity ε.

Claims

1. A lithography system (1, 100), particularly an EUV lithography system, comprising: Radiation source (2, 106), which is used to generate radiation (16). A preferred reflective optical element (M1) has a surface (25a) provided for irradiation with the said radiation (16). Measuring device (26) for measuring the heat input, particularly the temperature (T), at the surface (25a) of the optical element (M1). M The measuring device includes a detection device (27) for detecting radiation (28) emitted from the surface (25a) of the optical element (M1), and the detection device includes a lock-in amplifier (30). Its features The photolithography system (1, 100) includes a heating device (39) having at least one heating unit (38a, 38b) for irradiating the surface (25a) of the optical element (M1) with heating radiation (37a, 37b), and The heating device (39) is designed to modulate, preferably clock-modulate, and in particular pulse-modulate, the heating radiation (37a, 37b) from the heating unit (38a, 38b) that irradiates the surface (25a) of the optical element (M1).

2. The photolithography system according to claim 1, wherein, The radiation source (2) is designed for use radiation (16) generated by modulation, preferably clock modulation, and especially pulse modulation.

3. The photolithography system according to claim 2, wherein, The modulation of the radiation (16) from the radiation source (2) is different from the modulation of the heating radiation (37a, 37b) from the heating unit (38a, 38b).

4. The photolithography system according to any one of the preceding claims, wherein, The heating device (39) is designed to provide modulation signals (m1, m2) to the lock-in amplifier (30) as reference signals (33), the modulation signals (m1, m2) being used to modulate the heating radiation (37a, 37b).

5. The photolithography system according to any one of the preceding claims further comprises: A radiation sensor is used to detect the intensity of the heating radiation (37a, 37b) irradiated by the heating units (38a, 38b) onto the surface (25a), the radiation sensor being designed to provide a signal to the lock-in amplifier (30) as a reference signal (33), the signal being dependent on the intensity of the detected heating radiation (37a, 37b).

6. The photolithography system according to any one of the preceding claims, wherein, The heating device (39) includes a first heating unit (38a) and a second heating unit (38b) for irradiating the surface (25a) of the optical element (M1) with heating radiation (37a, 37b). The heating device (39) is designed to modulate the heating radiation (37a) from the first heating unit (38a), preferably clock modulation, particularly pulse modulation, and is designed to modulate the heating radiation (37b) from the second heating unit (38b), preferably clock modulation, particularly pulse modulation, and the modulation of the heating radiation (37a) from the first heating unit (38a) is different from the modulation of the heating radiation (37b) from the second heating unit (38b).

7. The photolithography system according to any one of the preceding claims further comprises: Another preferred reflective optical element (M2) has an additional surface (25a') provided for irradiation with the said radiation (16). The heating device (39) includes at least one additional heating unit (38a', 38b') for irradiating the additional surface (25a') of the additional optical element (M2) with additional heating radiation (37a', 37b'), and the heating device (39) is designed to modulate, preferably clock-modulate, particularly pulse-modulate, the additional heating radiation (37a', 37b') from the additional heating unit (38a', 38b').

8. The photolithography system according to claim 7, wherein, The modulation of the heating radiation (37a, 37b) from the heating units (38a, 38b) is different from the modulation of the additional heating radiation (37a', 37b') from the other heating units (38a', 38b').

9. The photolithography system according to any one of the preceding claims, wherein, The detection device (27) for detecting radiation (28) emitted from the surface (25a) of the optical element (M1) is designed as a spatially resolved radiative thermal meter.

10. The photolithography system according to any one of the preceding claims, wherein, The detection device (27) for detecting radiation (28) emitted from the surface (25a) of the optical element (M1) comprises at least one photodiode, preferably multiple photodiodes (31a).

11. The photolithography system according to any one of the preceding claims, wherein, The detection device (27) is designed to detect radiation (28) emitted from the entire surface (25a).

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