Lithography system comprising a measurement device

Abstract

The invention relates to a lithography system, in particular an EUV lithography system, said lithography system comprising: a radiation source for generating useful radiation (16); a preferably reflective optical element (M1) having a surface (25a) designed to be exposed to the useful radiation (16); and a measurement device (26) for measuring a heat input, in particular a temperature (TM), at the surface (25a) of the optical element (M1), which measurement device has a detection device (27) for detecting radiation (28) emitted from the surface (25a) of the optical element (M1), and a lock-in amplifier (30). The lithography system comprises a heating device (39) having at least one heating unit (38a, b) for directing heating radiation (37a, b) onto the surface (25a) of the optical element (M1). The heating device (39) is designed to modulate the heating radiation (37a, b), that is directed by the heating unit (38a, b) onto the surface (25a) of the optical element (M1), in a preferably timed, in particular pulsed, manner.

Description

[0001] Lithography system with a measuring device

[0002] Reference to related application

[0003] This application claims priority from German patent application DE102023208308.6 filed on August 30, 2023, the entire disclosure of which is incorporated by reference into this application.

[0004] Background of the invention

[0005] The invention relates to a lithography system, in particular an EUV lithography system, comprising: a radiation source for generating useful radiation, an optical element, preferably a reflective optical element, with a surface intended for irradiation with the useful radiation, and a measuring device for measuring a heat input, in particular a temperature, at the surface of the optical element, wherein the measuring device has a detection device for detecting radiation emanating from the surface of the optical element and a lock-in amplifier.

[0006] The lithography system can, for example, be a projection exposure system for microlithography, which is used for the photolithographic production of semiconductor components and other structured parts, or a part of such a lithography system, for example an illumination system or projection optics of such a lithography system. However, the lithography system can also be another optical arrangement for lithography, for example an inspection system, e.g. for inspecting masks, wafers, or the like used in lithography. The lithography system can, in particular, be an EUV lithography system, which is operated with useful radiation in the EUV wavelength range between approximately 5 nm and approximately 20 nm or 30 nm.

[0007] Modern lithography optics or optical elements of projection exposure systems expose structures at the wafer level with a resolution of a few nanometers. The requirements for such optical elements are very high, especially with regard to the fit or surface shape of the optical elements. This is especially true for lithography optics or optical elements for useful radiation in the EUV wavelength range, which require a particularly precise surface shape, partly because all optical elements operate in reflection mode in the EUV wavelength range.

[0008] During operation of an EUV lithography system, the optical elements operated in reflection mode are exposed to the useful radiation in the EUV wavelength range and typically heat up, which can result in deformation of the optical elements. Therefore, materials with the lowest possible thermal expansion coefficient are used to manufacture the base bodies of optical elements for the EUV wavelength range. This coefficient is minimized at a temperature known as the zero crossing temperature (TZC). The zero crossing temperature of the material of the base body of an optical element can be adjusted within certain limits during its manufacture. The zero crossing temperature is typically selected to correspond to a desired operating temperature of the optical element.At temperatures above or below the desired operating temperature, a respective reflective optical element therefore produces imaging errors.

[0009] To determine the actual temperature on the surface of a reflective optical element, which for simplicity will be referred to as a mirror, temperature sensors can be inserted into holes in the mirror base body. The temperature sensors can be brought within a few millimeters of the rear of the mirror surface. However, it is not possible to determine the actual temperature on the mirror surface using such temperature sensors. In principle, it is possible to calculate the actual temperature on the mirror surface, but due to the distance between the temperature sensor and the mirror surface, this only occurs with a corresponding time delay. Therefore, it is practically impossible to determine short, near-surface temperature signals without a time delay using such temperature sensors.

[0010] However, determining the exact temperature at the mirror surface is useful in order to be able to precisely set or regulate the desired operating temperature of the mirror, for example by direct, variable cooling of the mirror base body using a liquid cooling system integrated into the mirror base body or by additionally irradiating the surface of the mirror with heat radiation.

[0011] DE102020203750A1 describes a device for detecting a temperature on a surface of an optical element for semiconductor lithography. The device comprises a temperature recording device which is designed as an infrared camera and detects a wavelength range between 1 pm and 15 pm or between 2 pm and 5 pm. When detecting the temperature or the temperature distribution on a reflective surface, the temperature recording device described in DE102020203750A1 not only registers the thermal radiation emitted by the mirror surface, but also the thermal radiation reflected by the mirror surface in the direction of the temperature recording device. For example, an infrared or thermal imaging camera on a mirror surface made of aluminum (emissivity s approx. 0.05) at an operating temperature of the mirror of, for example, approx. 30°C at a temperature of the housing or the surrounding walls of approx.22°C essentially register the mirror image of the enclosure seen from the temperature recording device, but due to their low emissivity hardly register the temperature distribution of the mirror surface itself.

[0012] To prevent this effect, DE102020203750A1 uses an element configured for temperature control. The element is arranged such that the thermal radiation detected by the temperature recording device and reflected by the surface of the optical element is emitted by the element. The element can be cooled to maximize the ratio of the radiation reflected by the surface of the optical element to the thermal radiation emitted by the element.

[0013] DE 102020203753A1 describes a projection exposure system for semiconductor lithography, which has an optical element and a temperature recording device for detecting a temperature on a surface of the optical element using electromagnetic radiation emanating from the surface of the optical element. The temperature recording device has an infrared camera and a lock-in amplifier to amplify a clocked signature of the useful radiation of the projection exposure system used for imaging. The lock-in amplifier amplifies only the signal component that has the clocked signature of the useful radiation. The background signal that the temperature recording device detects from components in the vicinity of the surface that are located in the camera's field of view therefore has little or no influence on the signal emitted by the lock-in amplifier.

[0014] Object of the invention

[0015] The object of the invention is to provide a lithography system which enables a contact-free measurement of a heat input, in particular a temperature, at the surface of an optical element even in the case that a heat input is generated at the surface which is not attributable to the useful radiation or the background radiation.

[0016] Subject of the invention

[0017] This object is achieved by a lithography system of the type mentioned at the outset, which has a heating device with at least one heating unit for irradiating heating radiation onto the surface of the optical element, wherein the heating device is designed for preferably clocked, in particular pulsed modulation of the heating radiation irradiated by the heating unit onto the surface of the optical element.

[0018] The lithography system described here comprises a heating device with at least one heating unit, which serves to irradiate heating radiation - in addition to the useful radiation used in the lithography system - onto the surface and in doing so generates an additional heat input at the surface. The heating unit(s) are generally used to bring the surface of the optical element to the desired operating temperature or to maintain it at the desired operating temperature. For this purpose, for example, a control of the heating radiation irradiated onto the surface of the optical element can be provided. The heating unit(s) can also be used to heat the optical element before commissioning of the lithography system, i.e. before irradiating the optical element with the useful radiation.

[0019] The wavelength or wavelength range of the heating radiation typically differs from the wavelength or wavelength range of the useful radiation. The wavelength range of the heating radiation is generally in the infrared wavelength range with wavelengths greater than approximately 1 pm, for example, between approximately 1 pm and 1.6 pm.

[0020] By modulating the heating radiation, the portion of the blackbody radiation emanating from the surface, detected by the detection device, and generated by the surface temperature that is attributable to the heating radiation can be amplified using the lock-in amplifier and distinguished from a portion attributable to background radiation from the environment of the optical element, which is reflected or deflected by the surface of the optical element toward the measuring device or the detection device. This makes it possible to determine the heat input or the temperature at the surface of the optical element when only the heating radiation, but no useful radiation, is radiated onto the surface of the optical element, which can be the case, for example, during downtimes of the lithography system.

[0021] The basic purpose of a lock-in amplifier is to specifically filter the measurement signal from a measurement signal that contains a large background signal and a small measurement signal, and to remove the background signal. In this case, the modulation of the heating radiation, which in this case corresponds to the measurement signal, occurs in such a way that the lock-in amplifier is able to specifically filter the measurement signal, i.e. the heating radiation detected by the detection device, and to remove the background signal detected by the detection device, which is caused by the background radiation from the environment and possibly by the useful radiation, from the measurement signal. In this case, the measurement signal corresponds to the radiation detected by the detection device.

[0022] Modulation is achieved using a deterministic modulation signal. Modulation can be clocked, for example, i.e., using a modulation signal with a constant frequency, such as a sinusoidal or rectangular modulation signal. If the modulation signal is a clock signal, it is switched or modulated between two or possibly more than two states. It is possible that the surface of the optical element is not irradiated with the heating radiation during periodically recurring time intervals, known as clock pauses. In this case, modulation is pulsed, i.e., the heating radiation is switched on and off in a clocked manner.

[0023] When modulating the heating radiation, the power or intensity of the heating radiation is typically modulated, for example by switching a heating light source for generating the heating radiation of the heating unit on and off in a clocked manner or by modulating its power. In principle, it is also possible to modulate other properties of the heating radiation, provided that the modulation of the respective property enables the lock-in amplifier to specifically amplify the modulation signal or, if applicable, an intrinsic modulation of the heating radiation. If a clocked modulation signal is used, only the portion of the measurement signal that has the set modulation frequency and / or phase of the modulation signal is amplified in the lock-in amplifier, which is why the background signal has little or no influence on the amplified signal output by the lock-in amplifier.

[0024] The deterministic modulation of the heating radiation described here differs from a variation of the heating radiation radiated onto the surface in the form of a control, in which the heating power of the heating radiation radiated by the heating unit is changed in a non-deterministic manner depending on an instantaneous temperature at the surface of the optical element that is not known in advance.

[0025] In one embodiment, the radiation source is designed for preferably clocked, in particular pulsed, modulation of the generated useful radiation. The useful radiation can be clocked or modulated with a predetermined pulse frequency. As described in more detail below, the modulation signal of the useful radiation or, if appropriate, a signal detected by a radiation sensor can be fed to the lock-in amplifier as a reference signal. The thermal radiation caused by the useful radiation can thus be amplified with the help of the lock-in amplifier, and the influence of the background radiation can be eliminated.

[0026] If, in addition to the useful radiation amplified by the lock-in amplifier, heating radiation is also radiated onto the surface of the optical element, the signal from the lock-in amplifier, which is attributable to the useful radiation, cannot be used alone to measure the heat input at the surface, as the heating radiation also generates heat input into the surface, which must be taken into account for the measurement. If the heating radiation is not modulated, it cannot be considered when determining the heat input, as the heating radiation cannot be distinguished from the background radiation in this case, which reduces the accuracy of the heat input or temperature measurement.

[0027] If the lock-in amplifier amplifies both the portion of the radiation emanating from the surface and detected by the detection device that is attributable to the useful radiation and the portion of the detected radiation that is attributable to the heating radiation, the total heat input at the surface of the optical element can be measured or determined using these portions. For example, the sum of the two signals of the useful radiation and the heating radiation amplified by the lock-in amplifier can be used to determine the total heat input. It is not absolutely necessary for the modulation of the heating radiation to differ from the modulation of the useful radiation when measuring the heat input. The modulation frequency of the useful radiation can, for example, be on the order of 50 Hz.In the case of a radiation source for generating useful radiation in the EUV wavelength range, the useful radiation is typically pulsed when generating the useful radiation or otherwise, ie it is not continuous radiation, but rather no useful radiation is generated by the radiation source at predetermined time intervals or the generated radiation is filtered in the radiation source.

[0028] In a further development of this embodiment, the modulation, for example the modulation frequency, of the useful radiation of the radiation source differs from the modulation, for example the modulation frequency, of the heating radiation of the heating unit. In the event that the modulation of the useful radiation and the heating radiation is selected to be identical, the entire heat input at the surface can be measured (see above). However, despite the generally significantly different wavelength ranges of the heating radiation and the useful radiation, in the spatially resolved detection of (heat) radiation emanating from the surface of the optical element, a heat input at the surface that is attributable to the heating radiation and a heat input that is attributable to the useful radiation cannot be distinguished from one another, at least in those parts of the surface of the optical element that are irradiated with both the useful radiation and the heating radiation.By modulating the heating radiation differently from the useful radiation differently, the heat input of the heating radiation at the surface can be differentiated from the heat input of the useful radiation at the surface, which can be advantageous, for example, for controlling the temperature at the surface. Alternatively or in addition to the modulation frequencies, the useful radiation and the heating radiation can also be differentiated by selecting or modulating their phases differently.

[0029] In one embodiment, the heating device is configured to supply a modulation signal used for modulating the heating radiation to the lock-in amplifier as a reference signal. In this embodiment, the modulation signal used for irradiating the heating radiation, which is supplied to the heating unit or the heating light source for modulation, is also supplied to the lock-in amplifier as a reference signal. If a clocked modulation signal is used, the lock-in amplifier amplifies only the portion of the measurement signal that has the modulation frequency and / or the phase of the modulation signal or correlates with it.

[0030] For the purposes of this application, the modulation signal supplied to the lock-in amplifier is also understood to be a signal that is correlated with the modulation signal and contains information about the modulation frequency and / or the phase of the modulation signal. It is understood that the radiation source can also be configured to supply the lock-in amplifier with a modulation signal used for modulating the useful radiation as a reference signal.

[0031] In a further embodiment, the lithography system comprises a radiation sensor for detecting an intensity of the heating radiation radiated onto the surface by the heating unit, wherein the radiation sensor is designed to supply the lock-in amplifier with a signal dependent on the intensity of the detected heating radiation as a reference signal. The radiation sensor can be, for example, a photodiode or the like, which generates a signal in the form of a photocurrent that is proportional to the intensity of the heating radiation incident on the photodiode. It is understood that the lithography system can also have a radiation sensor for detecting an intensity of the useful radiation radiated onto the surface by the radiation source, which is designed to supply the lock-in amplifier with a signal dependent on the intensity of the detected useful radiation as a reference signal.

[0032] In a further embodiment, the heating device has a first heating unit and a second heating unit for irradiating heating radiation onto the surface of the optical element, wherein the heating device is designed for preferably clocked, in particular pulsed modulation of the heating radiation of the first heating unit and for preferably clocked, in particular pulsed modulation of the heating radiation of the second heating unit, and wherein the modulation of the heating radiation of the first heating unit differs from the modulation of the heating radiation of the second heating unit. Due to the different modulation, the heat input or the heat signature of the first and second heating units on the surface can be differentiated from one another. This is advantageous in order to set or regulate the temperature on the surface as precisely as possible with the aid of the heating units.The heating radiation emitted by the heating units is typically infrared radiation with wavelengths of more than 1 pm, e.g. with wavelengths around approximately 1070 nm.

[0033] The heating radiation from a heating unit can be radiated onto the entire surface of the optical element; frequently, the heating radiation from one heating unit is only radiated or directed onto a partial area or sector of the surface. In particular, if the heating radiation from the two heating units is radiated onto different partial areas or sectors of the surface, different modulation may be unnecessary in order to differentiate between the heat inputs. In this case, for example, the surface can be detected using a spatially resolving bolometer in the measuring device. To detect the surface, the measuring device can also have a plurality of photodiodes, each of which is preceded by an imaging optic that detects only a sector of the surface or, if necessary, small partial areas of the surface in the form of pixels.In principle, a thermal imaging camera can also be used, provided it has sufficient temporal resolution for lock-in thermography. This allows the heat input from a heating unit to be assigned to a specific subsection of a thermal image of the surface, possibly with high spatial resolution.

[0034] The heating units themselves can have a heating radiation source, which is generally designed to generate heating radiation in the infrared wavelength range. However, it is also possible for the heating radiation to be generated by an external heating radiation source and supplied to the heating units, for example, via optical fibers or the like. It is understood that the heating device can also have more than two heating units, which serve to radiate heating radiation onto the surface of the optical element.

[0035] In a further embodiment, the lithography system has a further preferably reflective optical element with a further surface provided for irradiation with the useful radiation, wherein the heating device has at least one further heating unit for radiating further heating radiation onto the surface of the further optical element, and wherein the heating device is designed for preferably clocked, in particular pulsed modulation of the further heating radiation of the further heating unit.

[0036] In this embodiment, the lithography system can have a further measuring device designed to measure a heat input, preferably a temperature, on the further surface of the further optical element. The further measuring device can have a further detection device for detecting further radiation emanating from the further surface of the further optical element, and a further lock-in amplifier with which the heat input or the heat signature of the further heating radiation radiated by the further heating unit onto the further surface can be determined. It is understood that the heating device can have two or more further heating units for radiating further heating radiation onto the further surface of the further optical element.

[0037] In a further development of this embodiment, the modulation of the heating radiation of the heating unit differs from the modulation of the further heating radiation of the further heating unit. In particular, if the optical element is a reflective optical element, it is generally located in a highly reflective environment. In this case, part of the heating radiation from a heating unit that is radiated onto a reflective optical element is reflected off the surface of the reflective optical element and, through multiple reflection, reaches the surfaces of other reflective optical elements, where it usually leads to an unwanted heat input. This phenomenon is also referred to as crosstalk.

[0038] If the heating units of different reflective optical elements are modulated or pulsed, lock-in thermography or the respective lock-in amplifier can be used to determine how much absorbed crosstalk originates from which reflective optical element. To identify crosstalk, it can be advantageous if the modulation of the heating units of all reflective optical elements of the lithography system, or possibly of a part of the lithography system, such as a projection lens, differs from one another. This does not necessarily mean that the heating units of a particular reflective optical element must have different modulations. However, if the modulation of the heating units of one and the same optical element differs, not only the optical element but also the heating unit generating the crosstalk can be identified.For the identification of crosstalk, it is advantageous if the modulation of the heating radiation of the heating unit(s) or the other heating unit(s) differs from the modulation of the useful radiation.

[0039] To identify crosstalk, for example, the modulation or a modulation signal from the additional heating unit or a signal from a sensor unit for detecting the intensity of the additional heating radiation can be fed to the lock-in amplifier of the measuring device of the optical element described above as a reference signal. In this case, the lock-in amplifier amplifies the portion of the detected radiation that is attributable to the crosstalk that the additional heating radiation of the additional optical element causes on the surface of this optical element.

[0040] In a further embodiment, the detection device for detecting the radiation emanating from the surface of the optical element is designed as a spatially resolving bolometer. In this case, the radiation detected by the detection device is thermal radiation in the infrared wavelength range. The spatially resolving bolometer enables near-surface, temporally and spatially high-resolution, non-contact detection of the thermal radiation emanating from the surface of the optical element and thus an instantaneous measurement of the heat input or the temperature at the surface of the optical element. The spatially resolving bolometer usually internally images the surface area in which the heat input or the temperature is to be measured onto a spatially resolving detector. The surface area can be the entire surface or a partial area of ​​the surface.The imaging is accomplished using optics suitable for the wavelength range of the detected radiation.

[0041] In a further embodiment, the detection device has at least one photodiode, preferably a plurality of photodiodes, for detecting the radiation emanating from the surface of the optical element. It is generally advantageous if the amplification by means of the lock-in amplifier takes place on the analog signal, i.e. before any digitization, which may reduce the resolution of the signal. It is therefore advantageous if a detector is used in the detection device which is not read out in digitized form, as would be the case with a CCD chip, for example, because the subsequent signal amplification is to be carried out in analog form. For example, a grid of photodiodes sensitive to the radiation to be detected, whose analog signal is accessible, can be used as the detector. If such a grid orIf the array is integrated as a detector into a detection device in the form of a thermal imaging camera, a respective partial area of ​​the surface of the optical element can be imaged onto a photodiode using suitable imaging optics.

[0042] The photodiodes can be controlled in such a way that a so-called photocurrent is measured at them, which is proportional to the intensity of the thermal radiation incident on the photodiode. The photocurrent of the photodiode is fed to the lock-in amplifier as an input signal. In addition, one or more reference signals that correlate with the modulation of the wanted signal are fed to the lock-in amplifier at a reference input. The reference signal can be, for example, the photocurrent of a radiation sensor that detects the intensity of the wanted signal, or the modulation signal can be used as a reference signal. In principle, it is possible that the radiation emanating from the surface is not detected with spatial resolution. In this case, the detection device can have, for example, a single photodiode as a detector, onto which the radiation emanating from the surface is imaged with the aid of suitable optics.An array of photodiodes, possibly in combination with upstream optics, can also be used as a detection device. Such an array of photodiodes also enables the spatially resolved detection of the heat input or temperature at the surface of the optical element, as described above.

[0043] The detection device is typically designed or sensitive to detecting electromagnetic radiation with wavelengths in the wavelength range between approximately 1 pm and approximately 5 pm, or possibly up to approximately 15 pm. By selecting the spectral range in the near infrared or, if necessary, up to the long-wave infrared detected by the detection device, the sensitivity and resolution of the detector used can be optimized for temperatures of, for example, between +20°C and +150°C. The Stefan-Boltzmann law can be used to determine the temperature based on the detected electromagnetic radiation, provided the emissivity of the surface is known, which can be determined in advance, for example, through calibration.

[0044] It is not mandatory for the detector to generate an analog signal for the lock-in amplifier if modern digital image processing is used. For the purposes of this application, a lock-in amplifier also includes a digital circuit or corresponding software that performs the function of an analog lock-in amplifier.

[0045] The measuring device can measure the respective heat input at the surface of the optical element based on the signal amplified by the lock-in amplifier, which is attributable to the useful radiation, and on the signal amplified by the lock-in amplifier, which is attributable to the heating radiation. For this purpose, the measuring device can have an evaluation device implemented in the form of suitable hardware and / or software. The amplification of the signals attributable to the useful radiation and the heating radiation can take place in parallel in the lock-in amplifier, particularly in the case where the useful radiation and the heating radiation are instantaneous and synchronized. In this case, the lock-in amplifier can measure the total heat input of the useful radiation and the heating radiation during a respective heat pulse.

[0046] The lock-in amplifier also filters a non-cyclic base value of the background signal, which corresponds to the background temperature at the surface. In the case where the temperature is determined via the Stefan-Boltzmann law, the signal from the lock-in amplifier is proportional to TB. 4 - TA 4 , where TB denotes the temperature during the (heating) pulse and TA denotes the background temperature of the surface. To determine the background temperature TA of the surface, a time-delayed temperature sensor in a hole in the base body of the mirror can be used, for example. If the temperature TA is known, the (instantaneous) temperature TB at the surface can be determined from the signal of the lock-in amplifier. It is also possible to use the signal from the lock-in amplifier, which only measures the height of the heat pulses, as a control signal for the heating unit(s) of the heating device.

[0047] For this measurement or determination of the temperature, the measuring device typically has an evaluation device which can be designed, for example, in the form of a digital or analog circuit. As described above, the contribution of the background radiation can be eliminated when determining the temperature using the lock-in amplifier. Amplification using the time-division multiplex method is also possible. In this case, the useful radiation and the heating radiation differ from one another in at least one property, for example the frequency or the phase. In this case, the heating radiation and the useful radiation can, for example, have different frequencies. In this case, it is advantageous if the frequencies are not harmonic to one another, i.e. they are not integer multiples of one another.The heat input from the heating radiation and the heat input from the useful radiation can also be differentiated if the heating radiation and the useful radiation have identical frequencies but different phases. In this case, different reference signals are used to measure the heat input from the useful radiation and the heat input from the heating radiation.

[0048] In a further embodiment, the detection device is designed to detect radiation emanating from the entire surface. In this case, the entire surface is detected, for example, with the aid of a spatially resolving bolometer, a plurality of photodiodes, each of which is preceded by an imaging optic, or optionally by means of a thermal imaging camera with high temporal resolution. In this way, the temperature can be measured at any location on the surface using the measuring device. In principle, it is also possible to use the detection device to detect only radiation emanating from part of the surface of the optical element. In the case of an optical element that has a reflective coating, the surface forms an interface between the reflective coating and the environment of the optical element.

[0049] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention.

[0050] drawing

[0051] Examples of embodiments are shown in the schematic drawing and are explained in the following description.

[0052] Fig. 1 shows a meridional section of a projection exposure system for EUV projection lithography,

[0053] Fig. 2 is a schematic representation of a DUV lithography system with an illumination device and a projection lens,

[0054] Fig. 3 is a schematic representation of a mirror of the projection exposure apparatus of Fig. 1 with a measuring device for measuring a heat input, in particular a temperature, on a reflecting surface of the mirror,

[0055] Fig. 4 is a schematic diagram of a lock-in amplifier to which a measurement signal containing a useful signal and a background signal as well as a reference signal are fed,

[0056] Fig. 5 is a schematic representation of another mirror of the projection exposure system of Fig. 1 with another measuring device,

[0057] Fig. 6 is a schematic representation of the time course of a pulsed reference signal and a useful signal, Fig. 7 is a schematic representation of the time course of the useful signal of Fig. 6 and the heat propagation in the mirror at three different times,

[0058] Fig. 8 schematic representations of the spectrum of the radiation emanating from the surface of the reflecting element at two different times, as well as

[0059] Fig. 9 is a schematic representation of three radiation sensors with three different wavelength filters detecting thermal radiation at two different times.

[0060] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0061] The following describes, by way of example, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography with reference to Fig. 1. The description of the basic structure of the projection exposure system 1 and its components is not intended to be limiting.

[0062] An embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3. A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.

[0063] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction in Fig. 1 runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0064] The projection exposure system 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0065] The radiation source 3 is an EUV radiation source. The radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example, an LPP source (laser produced plasma) or a DPP source (gas discharged produced plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser (FEL).

[0066] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0067] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0068] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Only a few of these facets 21 are shown in Fig. 1 as examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

[0069] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or indeed the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0070] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0071] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical system. The projection optical system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0072] The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

[0073] Fig. 2 shows a schematic view of a DUV projection exposure system 100, which comprises a beam shaping and illumination device 102 and a projection lens 104. DUV stands for "deep ultraviolet" (DUV) and denotes a wavelength of the working light between 30 nm and 370 nm. The DUV projection exposure system 100 has a DUV light source 106. The DUV light source 106 can be, for example, an ArF excimer laser, which emits radiation 108 in the DUV range at, for example, 193 nm.

[0074] The beam-shaping and illumination device 102 shown in Fig. 2 directs the DUV radiation 108 onto a photomask 129. The photomask 129 is designed as a transmissive optical element and can be arranged outside the beam-shaping and illumination device 102 and the projection lens 104. The photomask 129 has a structure that is imaged in a reduced size onto a wafer 124 or the like by means of the projection lens 104.

[0075] The projection lens 104 has a plurality of lenses 128, 140 and / or mirrors 130 for imaging the photomask 129 onto the wafer 124. Individual lenses 128, 140 and / or mirrors 130 of the projection lens 104 can be arranged symmetrically to the optical axis 126 of the projection lens 104. It should be noted that the number of lenses and mirrors of the DUV projection exposure system 100 is not limited to the number shown. More or fewer lenses and / or mirrors can also be provided. Furthermore, the mirrors are typically curved at their front side for beam shaping.

[0076] An air gap between the last lens 140 and the wafer 124 can be filled by a liquid medium 132 having a refractive index > 1. The liquid medium 132 can be, for example, ultrapure water. Such a setup is also referred to as immersion lithography and has increased photolithographic resolution. A temperature measurement on a mirror Mi of the projection system 10 of the projection exposure apparatus 1 of Fig. 1 is described below with reference to Fig. 3. It is understood that such a temperature measurement can also be performed on one of the optical elements of the illumination system 2 of the projection exposure apparatus 1 or on one of the optical elements of the DUV projection exposure apparatus 100 shown in Fig. 2, for example, on a lens 128, 140 or on a mirror 130 of the projection objective 104.

[0077] Fig. 3 shows a reflective optical element in the form of the first mirror M1 of the projection system 10 of the projection exposure apparatus 1 of Fig. 1. The mirror M1 has a base body 24 made of a material with a low thermal expansion coefficient. The mirror M1 has a highly reflective coating 25 applied to the base body 24. In the example shown, in which the useful radiation 16 of the radiation source 3 is EUV radiation at a wavelength of 13.5 nm, the highly reflective coating 25 comprises alternating layers of silicon and molybdenum. A surface 25a on the upper side of the reflective coating 25 forms an interface to the environment, onto which the useful radiation 16 is radiated during operation and from which the useful radiation 16 is reflected, as can be seen in Fig. 3.

[0078] In principle, it is possible to measure a temperature TM on the surface 25a of the mirror M1 by installing temperature sensors (not shown) at a short distance of a few millimeters from the surface 25a of the mirror M1 in the base body 24 of the mirror M1. With the help of such temperature sensors, however, the temperature TM on the surface 25a can only be measured with a time delay. This time delay in the temperature measurement leads to a significant error contribution in the surface figure deformation. For the spatially resolved measurement of a temperature TM on the surface 25a of the mirror M1, more precisely the reflective coating 25, in the example shown, the projection exposure system 1 of Fig. 1 has a measuring device 26. In the example shown, the measuring device 26 comprises a detection device 27, which is used to detect radiation 28 emanating from the surface 25a of the mirror M1 and which in Fig.3 is indicated by dashed arrows. The detection device 27 detects the radiation 28 that lies within a field of view 29 indicated by dashed lines, which, in the example shown, includes the entire surface 25a of the mirror M1. Alternatively, only a partial area of ​​the surface 25a of the mirror M1 can be detected by the detection device 27.

[0079] In the example shown, the detection device 27 is designed to detect radiation in the infrared wavelength range from approximately 1 pm to approximately 5 pm or possibly up to approximately 15 pm. In the example shown in Fig. 3, the detection device 27 is designed as a spatially resolving bolometer. The problem with measuring the temperature TM at the surface 25a of the mirror M1 is that the field of view 29 of the detection device 27 detects not only radiation 28 from the surface 25a itself, but also radiation from the highly reflective environment of the mirror M1, which is reflected by the surface 25a in the direction of the detection device 27.

[0080] In addition, the temperature TM to be measured on the surface 25a of the mirror M1, which can be in the value range between 22°C and 40°C, for example, does not differ significantly enough from the temperature in the vicinity of the mirror M1, which can be approximately 22°C, for example. The measuring device 26 cannot therefore differentiate precisely enough between the temperature of the environment reflected on the surface 25a and the temperature of the reflecting surface 25a in order to achieve a temperature measurement accuracy of approximately 0.1 K, for example. The measuring device 26 shown in Fig. 3 takes advantage of the fact that the useful radiation 16 is not generated continuously by the radiation source 2, but in a pulsed manner, ie the radiation source 2 generates the useful radiation 16 with a pulsed modulation with a modulation frequency of approximately 50 Hz, for example, with radiation pulses and pulse pauses alternating.In the example shown, the modulation of the useful radiation 16 is used to amplify only the portion of the detected radiation 28 that is attributable to the useful radiation 16 in a lock-in amplifier 30 of the measuring device 26 and to distinguish it from a portion that is attributable to the background radiation from the environment of the mirror M1, which is reflected on the reflecting surface 25a.

[0081] The lock-in amplifier 30 in Fig. 3 is designed for analog signal processing. It is therefore necessary that a spatially resolving detector 31 mounted in the detection device can be read out in analog form. For this purpose, in the example shown, the detector 31 has a plurality of photodiodes 31a arranged in a grid arrangement and controlled such that a photocurrent can be measured at them that is proportional to the intensity of the infrared radiation 28 incident on the respective photodiode 31a, which is detected by the detection device 27 in the form of the spatially resolving bolometer. The detection device 27 has an optical system (not shown) that images a respective partial area of ​​the surface 25a onto a photodiode 31a assigned to this partial area.

[0082] The photocurrent generated by the respective photodiode 31a forms a measurement signal 32, the temporal profile of which is illustrated in Fig. 4, which is fed to a first input of the lock-in amplifier 30. A reference signal 33 is fed to a second input of the lock-in amplifier 30. In the example shown, this reference signal 33 is a pulsed square-wave signal, the temporal profile of which, or rather, its amplitude A, is also illustrated in Fig. 4. The measurement signal 32 has a first component in the form of a useful signal 34 and a second component in the form of a background signal 35, which has a significantly greater amplitude A than the useful signal 34.

[0083] As can also be seen in Fig. 4, only the wanted signal 34, but not the background signal 35, is modulated at a constant frequency. Since the lock-in amplifier 30 is supplied with the reference signal 33 pulsed at the same frequency, the lock-in amplifier 30 only amplifies the wanted signal 34, which is modulated at the same frequency and, if applicable, with the same phase as the reference signal 33. The wanted signal 34 amplified by the lock-in amplifier 30 is shown on the right in Fig. 4.

[0084] In the example described here, the useful signal 34 is the portion of the detected radiation 28 in the infrared range attributable to the useful radiation 16, which is detected by the detection device 27. The reference signal 33 is formed by a modulation signal that is also provided to the radiation source 2 for the pulsed generation of the useful radiation 16. Since the background signal 35 attributable to the background radiation does not correlate with the frequency of the useful radiation 16, the portion of the measurement signal 32 attributable to the background radiation is not amplified by the lock-in amplifier 30.

[0085] The amplified wanted signal 34 is fed to an evaluation device 36 of the measuring device 26, and based on the amplified wanted signal 34, the evaluation device 36 determines the temperature TM at the surface 25a of the mirror M1. In addition to a rapidly fluctuating component, the background signal 35 also has a non-cyclic component (base component), which is also not amplified by the lock-in amplifier 30. The lock-in amplifier 30 thus measures the magnitude or intensity of the difference between the temperature of the respective heat pulse and the background temperature. To determine the temperature TM at the surface 25a of the mirror M1 using the Stefan-Boltzmann law, it is therefore necessary to additionally determine the non-cyclic component of the background signal 35, which corresponds to the temperature at the surface 25a, which is not attributable to the cyclic heat input.To determine this time-averaged temperature at the surface 25a, the evaluation device 36 can, for example, use the time-delayed temperature sensors described above, which are integrated into the base body 24 of the mirror M1. The measuring device 26 can also be measured and calibrated before the projection exposure system 1 is first put into operation, for which purpose, for example, pulsed heat sources with a known power can be used.

[0086] Using the lock-in thermography described above, even in the projection exposure system 1 of Fig. 1, which has uncooled walls located in the vicinity of the mirror M1 and correspondingly strong background radiation, the near-surface temperature TM generated by the useful radiation 16 can still be measured photometrically and non-contact with high accuracy. However, this only applies if the temperature TM at the surface 25a of the mirror M1 is attributable only to the heat input of the useful radiation 16 and not additionally to heat input into the surface 25a from other heat sources.

[0087] In the example shown in Fig. 3, in addition to the useful radiation 16, heating radiation 37a, 37b is radiated onto the surface 25a of the mirror M1 by a first heating unit 38a and a second heating unit 38b of a heating device 39. In the example shown, the heating units 38a, b are designed to direct the heating radiation 37a, b onto different subregions of the surface 25a of the mirror M1. The heating units 38a, b can be designed in different ways, for example, in the form of sector heaters or in the form of heating heads. The heating radiation 37a, b can be generated by a heating radiation source in a respective heating unit 38a, b, but it is also possible for the heating radiation 37a, b to be generated by one or more heating radiation sources of the heating device 39, which are arranged at a distance from the heating units 38a, b and are supplied to the heating units 38a, b, for example, via fiber optic cables or the like.

[0088] The heating units 38a, b can be used, for example, to anticipate the initial warm-up of the mirror M1 before the exposure operation of the projection exposure system 1 or to preheat the mirror M1 for the exposure operation. The heating units 38a, b can also serve to homogenize the inhomogeneous temperature distribution on the surface 25a of the mirror M1 caused by a respective illumination setting through complementary heating. It is also possible to compensate for temporal and spatial temperature fluctuations of the surface 25a of the mirror M1 by means of the heating units 38a, b, which arise, for example, from a change from a bright to a dark reticle.

[0089] As can also be seen in Fig. 3, the surface 25a has subregions that are heated only by the useful radiation 16, as well as subregions where the surface 25a is exposed to both the useful radiation 16 and the heating radiation 37a, 37b. It is also fundamentally possible for subregions of the surface 25a to be heated only by the heating radiation 37a, 37b, but not by the useful radiation 16.

[0090] Particularly in sub-regions of the surface 25a that are heated by both the useful radiation 16 and the heating radiation 37a, b, the temperature TM at the surface 25a depends not only on the intensity or power of the useful radiation 16, but also on the intensity or power of the heating radiation 37a, b radiated onto the surface 25a. In order to determine the temperature TM at the surface 25a in these sub-regions, the heating radiation 37a, b radiated onto the surface 25a by the heating units 38a, b is also modulated, or more precisely, pulsed. A modulation signal m1, m2 supplied to the heating units 38a, b for this purpose is, in the example shown in Fig. 3, also supplied to the lock-in amplifier 30 as a reference signal 33. The reference signal 33 described in connection with Fig.In the manner described in Figure 4, the lock-in amplifier 30 can generate an amplified useful signal 34 which corresponds to the portion of the detected radiation 28 which is attributable to the heating radiation 37a, b of the heating units 38a, b or a respective heating unit 38a, b.

[0091] The modulation signals m1, m2 and the modulation signal of the useful radiation 16 can be identical. In this case, although the temperature TM in a respective partial area of ​​the surface 25a struck by the useful radiation 16 and the heating radiation 37a, b can be determined by the evaluation device 36, the components of the useful radiation 16 and the heating radiation 37a, b cannot be distinguished from one another. Such a distinction is advantageous, however, for controlling the heating units 38a, b to generate a desired temperature distribution on the surface 25a of the mirror M1.

[0092] If the modulation of the heating radiation 37a, b differs from the modulation of the useful radiation 16, the respective components can be amplified separately by the lock-in amplifier 30 and thus differentiated from one another. For this purpose, it is sufficient if the two modulation signals M1, M2 of the heating units 38a, b differ from the modulation signal of the useful radiation 16. For example, the two modulation signals m1, m2 can have a modulation frequency that differs from the modulation frequency of the useful radiation 16, e.g. 50 Hz. Alternatively, the two modulation signals m1, m2 of the heating radiation 37a, b can be clocked with a different phase than the modulation signal of the useful radiation 16. It is advantageous if the modulation signals m1, m2 of the two heating units 38a, b differ from one another.In this case, the lock-in amplifier 30 can be used to distinguish between the heat inputs of the two heating units 38a, b at the surface 25a. This is particularly advantageous if, unlike as shown in Fig. 3, the heat radiation 37a, b of the two heating units 38a, b is radiated onto a common portion of the surface 25a. For this purpose, the two modulation signals m1, m2 can, for example, have different modulation frequencies.

[0093] Fig. 5 shows a further mirror in the form of the second mirror M2 of the projection optics 10 of the projection exposure system 1 of Fig. 1 with a further measuring device 26' for measuring a temperature TM' on a further reflective surface 25a' of the further mirror M2. The further measuring device 26' is designed like the measuring device 26 shown in Fig. 3. The heating device 39, which serves to heat all mirrors M1 to M6 of the projection optics 10, has, in the example shown, two further heating units 38a', 38b', which are designed to radiate further heating radiation 37a', 37b' onto the further surface 25a' of the further mirror M2. The two further heating units 38a', 38b' are also used by the heating device 39 for the pulsed modulation of the further heating radiation 37a' 37b', which is radiated onto the further surface 25a'.

[0094] As can also be seen in Fig. 5, a first radiation sensor 40a' is mounted on the lateral edge of the beam cross-section of the additional heating radiation 37a' emitted by the first additional heating unit 38a' for detecting an intensity h of the additional heating radiation 37a'. Similarly, a second radiation sensor 40b' is mounted on the lateral edge of the beam cross-section of the additional heating radiation 37b' emitted by the second additional heating unit 38b' for detecting an intensity h of the additional heating radiation 37b'. A third radiation sensor 40c' is mounted on the lateral edge of the beam cross-section of the useful radiation 16 and serves to detect the intensity h of the useful radiation 16 radiated onto the additional surface 25a'.The three radiation sensors 40a-c' are photodiodes that generate a signal S1, S2, S3 dependent on the respectively detected intensity h, I2, h in the form of a photocurrent proportional to the respective intensity h, I2, h, which is fed to the further lock-in amplifier 30' of the further measuring device 27' as a respective reference signal 33. It is understood that the reference signal(s) 33 of the lock-in amplifier 30 of Fig. 3 can be provided in an analogous manner by three radiation sensors (not shown).

[0095] The modulation of the further heating radiation 37a' of the first further heating unit 38a' differs from the modulation of the further heating radiation 37b' of the second further heating unit 38b' in order to be able to differentiate the heat input of the respective further heating units 38a', 38b' onto the further surface 25a'.

[0096] In addition, the modulation of the heating radiation 37a, 37b of the first and second heating units 38a, 38b, which impinges on the surface 25a of the first mirror M1 shown in Fig. 3, differs from the modulation of the further heating radiation 37a', 37b', which impinges on the further surface 25a' of the second mirror M2. In this way, further heating radiation 37a', 37b' of the further heating units 38a', 38b', which is reflected at the further surface 25a' of the second mirror M2 and reaches the surface 25a of the first mirror M1 by multiple reflection, can be detected or identified with the aid of the measuring device 26. For this purpose, ieTo determine the heat input generated at the surface 25a of the first mirror M1 by the heating radiation 37a', 37b' radiated onto the further surface 25a' of the second mirror M2, the signals S1, S2 generated by the first radiation sensor 40a' and by the second radiation sensor 40b' are fed to the lock-in amplifier 30 of the measuring device 26 as reference signals 33. It is understood that the procedure described in connection with Fig. 5 can be carried out accordingly for the four further mirrors M3 to M6 of the projection system 10 in order to determine the crosstalk from all other mirrors M2 to M6 at the surface 25a of the first mirror M1. It is further understood that this procedure can also be carried out accordingly for the other mirrors M2 to M6.It is also understood that the temperature measurement carried out in the manner described above can also be carried out on mirrors which, in addition to heating, are cooled by means of direct cooling in the base body of the respective mirror.

[0097] In the manner described above, a near-surface, temporally and spatially high-resolution, non-contact temperature measurement of the surfaces of optical elements of lithography systems 1, 100 can be performed. These elements are heated by useful radiation and additionally by—if necessary, controllable—heating radiation. The temperature measurement can be used to improve the temperature control at the surfaces of the mirrors. Using the lock-in thermography described above in combination with heating the optical elements with the aid of the heating radiation, the optical elements can also be used as manipulators, e.g., to correct the wavefront of the useful radiation 16.

[0098] Furthermore, by a suitable choice of the phase relationship between the reference signal 33 and the wanted signal 34 (cf. Fig. 6), different aspects of the wanted signal 34 can be detected, as will be described below with reference to Fig. 7. The course of the wanted signal 34 shown in Fig. 7, i.e. the heat radiation from the surface 25a which is not due to the background radiation, has a temporal relationship to the pulses of the intensity of the pulsed wanted radiation 16 or the pulsed heating radiation 37a, 37b. The amplitude A of the wanted radiation 34 shown in Fig. 7 is linked to the instantaneous, near-surface temperature TM, before the heat pulse of the wanted radiation 16 or the heating radiation 37a, b can be dissipated into the underlying material of the base body 25. The temporal drop in the waveform shown in Fig.The profile of the useful signal 34 shown in Figure 7 is linked to the temperature of the underlying layers and the thermal conduction, heat capacity, and heat radiation of the materials. The information on the behavior of the thermal conduction and heat capacity can be used to verify finite element models for heat conduction within the mirror M1. For example, the thermal resistance of the reflective coating 25 as well as the thermal resistance between the reflective coating 25 and the base body 24, which can be formed, for example, in the form of titanium-doped quartz glass, can be determined or quantified in this way.

[0099] Fig. 7 illustrates this temporal behavior using three different times t1, t2, ts. At the first time t1, a pulse of heating radiation 37a is radiated onto a partial area of ​​the surface 25a, heating a small volume below the surface 25a, represented by a rectangle. Due to heat conduction, the heated volume increases at the two later times t2, ts, while at the same time the temperature in the heated volume decreases. By varying the phase relative to the reference signal 33, the course of the useful signal 34 can in principle be measured, the temporal resolution being of the order of magnitude of the length of the pulses of the useful radiation 16 or the heating radiation 37a, 37b.

[0100] The temperature of the irradiated surface layers can be determined based on the Stefan-Boltzmann law, since the thermal radiation detected by the detection device 27 is proportional to the heat flow radiated at the surface 25a:

[0101] Q = s ä AT 4 , where s is the emissivity of the surface 25a, o = 5.67 10' 8 W / (m 2 K 4 ) denotes the Stefan-Boltzmann constant and A the area of ​​the radiating surface.

[0102] Fig. 8 and Fig. 9 describe a further possibility for measuring the temperature TM at the surface 25a of the mirror M1. The example shown in Fig. 8 and Fig. 9 takes advantage of the fact that the temperature TM at the surface 25a of the mirror M1 can also be measured via the position of the blackbody spectrum emitted by a body, for example from the surface 25a of the mirror M1, as a function of the wavelength A (Wien's law, Planck's radiation law). In order to determine the temperature TM in this way, in the example shown in Fig. 9, three radiation sensors 41 ac are provided in the vicinity of the mirror M1 in order to detect a respective portion of the radiation 28 emanating from the surface 25a of the mirror M1. In the example shown, the radiation sensors 41 ac each have a wavelength filter that allows a wavelength range AAi, AA2, AA3 to pass through to a photodiode of the radiation sensors 41 ac.Each of the three radiation sensors 41ac is therefore sensitive to a different wavelength range AA1, AA2, AA3 of the blackbody radiation. The relative intensity I or amplitude of the radiation detected in the respective wavelength range AA1, AA2, AA3 can therefore be used to determine the temperature TM at the surface 25a. To determine the temperature TM in this way, two radiation sensors 41a, 41b are generally sufficient; however, a larger number of radiation sensors is advantageous for increasing the accuracy of the temperature measurement.

[0103] Fig. 8 and Fig. 9 each show the mirror M1 on the left-hand side at a first time t1, at which heating radiation 37a is radiated onto the surface 25a of the mirror M1. The irradiated surface 25a of the mirror M1 has a very small penetration depth for thermal radiation that arises below the surface 25a of the mirror M1. However, for a few nanometers, the thermal radiation from the layers lying below the surface 25a also reaches the radiation sensors 41a-c. In principle, therefore, the thermal radiation 28 at the first time t1, at which the heating radiation 37a strikes the surface 25a, can come from a different depth than the thermal radiation 28 that strikes the radiation sensors 41a-c at a second, later time t2. This leads to a change in the emissivity of the surface 25a from the first time t1 to the second time t2, as can be seen from the illustrations in Fig. 8 left and in Fig. 8 right.Information about this change in the emissivity s of the surface 25a can also be determined via the phase variation at the lock-in amplifier 30. Based on the change in emissivity s, a layer degradation of the layers of the reflective coating 25 can be detected and, if necessary, quantified.

Claims

Patent claims 1. Lithography system (1, 100), in particular an EUV lithography system, comprising: a radiation source (2, 106) for generating useful radiation (16), a preferably reflective optical element (M1) with a surface (25a) provided for irradiation with the useful radiation (16), a measuring device (26) for measuring a heat input, in particular a temperature (TM), at the surface (25a) of the optical element (M1), which measuring device has a detection device (27) for detecting radiation (28) emanating from the surface (25a) of the optical element (M1), and a lock-in amplifier (30), characterized in that the lithography system (1, 100) has a heating device (39) with at least one heating unit (38a, b) for radiating heating radiation (37a, b) onto the surface (25a) of the optical element (M1), and that the Heating device (39) for the preferably clocked, in particular pulsed modulation of the heating unit (38a,b) heating radiation (37a, b) radiated onto the surface (25a) of the optical element (M1).

2. Lithography system according to claim 1, wherein the radiation source (2) is designed for preferably clocked, in particular pulsed modulation of the generated useful radiation (16).

3. Lithography system according to claim 2, wherein the modulation of the useful radiation (16) of the radiation source (2) differs from the modulation of the heating radiation (37a, b) of the heating unit (38a, b).

4. Lithography system according to one of the preceding claims, in which the heating device (39) is designed to supply the lock-in amplifier (30) with a to supply the modulation signal (ml , m2) used for modulating the heating radiation (37a, b) as a reference signal (33).

5. Lithography system according to one of the preceding claims, further comprising: a radiation sensor for detecting an intensity of the heating radiation (37a, 37b) radiated by the heating unit (38a, 38b) onto the surface (25a), wherein the radiation sensor is designed to supply the lock-in amplifier (30) with a signal dependent on the intensity of the detected heating radiation (37a, 37b) as a reference signal (33).

6. Lithography system according to one of the preceding claims, in which the heating device (39) has a first heating unit (38a) and a second heating unit (38b) for irradiating heating radiation (37a, 37b) onto the surface (25a) of the optical element (M1), wherein the heating device (39) is designed for preferably clocked, in particular pulsed modulation of the heating radiation (37a) of the first heating unit (38a) and for preferably clocked, in particular pulsed modulation of the heating radiation (37b) of the second heating unit (38b), and wherein the modulation of the heating radiation (37a) of the first heating unit (38a) differs from the modulation of the heating radiation (37b) of the second heating unit (38b).

7. Lithography system according to one of the preceding claims, further comprising: a further preferably reflective optical element (M2) with a further surface (25a') provided for irradiation with the useful radiation (16), wherein the heating device (39) has at least one further heating unit (38a', 38b') for irradiating further heating radiation (37a', 37b') onto the further surface (25a') of the further optical element (M2), wherein the heating device (39) is for preferably clocked, in particular pulsed Modulation of the further heating radiation (37a', 37b') of the further heating unit (38a', 38b').

8. Lithography system according to claim 7, wherein the modulation of the heating radiation (37a, 37b) of the heating unit (38a, 38b) differs from the modulation of the further heating radiation (37a', 37b') of the further heating unit (38a', 38b').

9. Lithography system according to one of the preceding claims, in which the detection device (27) for detecting the radiation (28) emanating from the surface (25a) of the optical element (M1) is designed as a spatially resolving bolometer.

10. Lithography system according to one of the preceding claims, in which the detection device (27) for detecting the radiation (28) emanating from the surface (25a) of the optical element (M1) has at least one photodiode, preferably a plurality of photodiodes (31a).

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

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