Method for operating an EUV mirror system, EUV mirror system, and microlithographic projection exposure installation

Abstract

The invention relates to a method for operating an EUV mirror system, wherein heating radiation emitted from a radiation source (41) is directed onto an EUV mirror (M1-M6) by means of a heating device (29). Part of the heating radiation (32) is coupled out, and a sensor (39) is used to record measured values relating to the radiation power of the coupled-out component (38) of the heating radiation (32). A first comparison is carried out between an absolute value (47) of the measured radiation power and a first threshold value (44), a second comparison is carried out between a rate of change (48) of the measured radiation power and a second threshold value (46), and the first comparison and the second comparison are evaluated in order to determine a fault of the heating device (29). The invention also relates to an EUV mirror system and a microlithographic projection exposure installation.

Description

Method for operating an EUV mirror system, EUV mirror system, microlithographic projection exposure system

[0001] This patent application claims priority from German patent application DE 10 2024 202 666 . 2, filed on March 21, 2024, to which reference is made and the contents of which are incorporated herein in their entirety (“incorporation by reference”).

[0002] The invention relates to a method for operating an EUV mirror system, an EUV mirror system and a microlithographic projection exposure system.

[0003] Microlithographic projection exposure systems are used for the production of integrated circuits with particularly small structures. A photomask illuminated with very short-wavelength, extreme ultraviolet (EUV) radiation is projected onto a lithographic object to transfer the mask structure to the object.

[0004] The projection exposure system comprises several EUV mirrors, each with an optical surface that reflects the EUV radiation. The EUV mirrors have a precisely defined shape and are precisely positioned to ensure sufficient image quality of the mask onto the lithography object.

[0005] The EUV radiation incident on the optical surface of the mirror is partly reflected and partly absorbed. The absorbed part of the EUV radiation causes heating of the mirror body. Temperature changes of a body are generally accompanied by thermal deformation. This is undesirable for mirrors of a microlithographic projection exposure system, because a change in the The geometric shape of the mirror changes the wavefront of the EUV radiation reflected from the optical surface. This usually leads to a reduction in image quality.

[0006] To influence the temperature of an EUV mirror in a desired manner, the EUV mirror system can comprise a heating device designed to direct heating radiation onto the EUV mirror. It is known to extract a portion of the emitted heating radiation in order to determine the total power of the heating radiation from the extracted portion of the heating radiation. The extracted portion of the heating radiation can also be used to identify faults in the heating device (DE 10 2020 207 752 A1, DE 10 2021 206 203 A1). It has proven difficult to reliably detect faults based on measurements in the extracted portion of the heating radiation.

[0007] However, reliable error detection is of great importance because it can have serious consequences, such as damage to the product or the system or even the risk of fire, if heating power emitted within a microlithographic projection exposure system does not reach the place for which it is intended.

[0008] The invention is based on the object of presenting a method for operating an EUV mirror system, an EUV mirror system, and a microlithographic projection exposure system that mitigate these disadvantages. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0009] In the method according to the invention for operating a EUV mirror system is equipped with a heating device from a Heating radiation emitted by a radiation source is directed onto an EUV mirror. A portion of the heating radiation is coupled out. A sensor records measured values ​​of the radiant power of the coupled-out portion of the heating radiation. A first comparison is made between an absolute value of the measured radiant power and a first threshold. A second comparison is made between a rate of change of the measured radiant power and a second threshold. The first and second comparisons are evaluated to determine a fault in the heating device.

[0010] The invention is based on the realization that it is not always sufficient to compare an absolute value of the measured radiant power with an expected value in order to detect a fault in the heating device. The bandwidth around the expected value which is still considered error-free must not be set too narrow, because otherwise normal operating fluctuations in the heating device would lead to an error message. However, it has been shown that in view of the large bandwidth required, not all faults can be detected early enough. The invention therefore proposes not only comparing an absolute value of the measured radiant power with an expected value in a first comparison, but also comparing the rate of change of the measured radiant power in a second comparison. Certain faults can be detected more easily by the second comparison than by the first comparison.For example, a defect within an optical component in the heating beam path can lead to a large amount of heat being generated locally, while the absolute value of the measured radiant power has changed only slightly. Such defects often occur suddenly, so that at the moment of the defect, a high rate of change in the measured radiant power occurs.

[0011] The method can be carried out in such a way that a fault in the heating device is concluded if both the first threshold value is exceeded by the measured value and the second threshold value is exceeded by the change in the measured value. An error that directly affects both the first threshold value and the second threshold value could, for example, be a mechanical fault in an optical element through which all of the heating radiation passes. In such a case, the absolute value of the measured radiant power drops sharply, which is simultaneously accompanied by a high rate of change in the measured radiant power. The mirror system can be set up in such a way that in the event of such an error, in which both the first threshold value and the second threshold value are exceeded, the radiation source of the heating device is switched off directly.

[0012] Additionally or alternatively, the method can be carried out in such a way that a fault in the heating device is concluded if either the first threshold is exceeded or the second threshold is exceeded. This is based on the knowledge that there are certain types of faults in the heating device which only affect one of the thresholds, rather than both. If, for example, the heating radiation is guided through a bundle of parallel optical fibres, a break in one of the optical fibres does not always result in the absolute value of the measured radiant power changing significantly, which is why the first threshold is not necessarily exceeded. On the other hand, there is a significant rate of change the moment the optical fibre breaks, so that the second threshold is exceeded.Conversely, there are errors, such as gradual coking of an optical element in the heating beam path, where the rate of change. is small, but which, over a longer period, lead to a significant change in the absolute value of the measured radiant power. Such errors can therefore only be detected if the first threshold value is exceeded. Exceeding a threshold value includes exceeding a threshold value above a target value, and exceeding a threshold value below a target value, both upwards.

[0013] An expected value for the extracted portion of the heating radiation can be determined. The first threshold can be defined relative to the current expected value. For example, it can be considered an indication of an error if the radiant power measured at a given time is 10% lower than the expected value of the radiant power for that time.

[0014] If the expected value for the extracted portion of the heating radiation is determined as a time series, an expected rate of change can be derived from the time series of expected values. The second threshold can be defined relative to the expected rate of change. For example, it can be considered an indication of a fault in the heating system if the rate of change of the measured radiant power is twice as large as the expected rate of change.

[0015] An expected value for the extracted portion of the heating radiation can be determined using a mathematical model of the heating device. The mathematical model can be designed in such a way that it relates input variables, which are known or can be measured, to the expected value for the extracted radiant power. One aspect within the mathematical model can be the relationship between the extracted radiant power and the The total power of the heat radiation transmitted by the heating device. This ratio is preferably a constant ratio.

[0016] The input power of the radiation source used to operate the heating device can be used as an input variable for the mathematical model, or one of several input variables for the mathematical model. Typically, there is a direct relationship between the input power of the radiation source and the measured value of the output radiation power.

[0017] The EUV mirror system can comprise a control unit which is designed to continuously determine an expected value for the outcoupled radiant power during operation of the EUV mirror system. The control unit can derive an expected rate of change from the temporal progression of the expected value. The control unit can be designed to process measured values ​​of the outcoupled radiant power and to compare them with the first threshold value and also with the second threshold value. If a threshold value is exceeded, the control unit can output an error message or go into error mode. In the event of error mode, the control unit can, for example, be designed to send a switch-off signal to the radiation source of the heating device. Once the radiation source is switched off, no further radiant power is supplied to the heating device, so that there is no risk of excessive heating.

[0018] The heating device may comprise a plurality of parallel heating channels through which heating radiation is directed from the radiation source toward the EUV mirror. The heating radiation transmitted via a heating channel may be separately adjustable. This may apply to each of the heating channels. The heating device can be configured such that a portion of the radiation output is extracted from several of the parallel heating channels and directed to a sensor. The heating device can comprise an associated sensor for each of the channels. It is also possible for the number of sensors to be fewer than the number of channels. In one embodiment, a portion of the heating radiation is extracted from several channels and superimposed on a sensor. In the EUV mirror system, sensors associated with a single channel of the heating device can be combined with sensors associated with several channels of the heating device.

[0019] For each of the sensors in the EUV mirror system, an expected absolute value of the measured radiant power and an expected rate of change can be determined. It can be considered sufficient indication of a fault if the first or second threshold is exceeded for one of the sensors.

[0020] The EUV mirror system can be configured such that the portion of the heating beam path decoupled from a channel of the heating device is directed directly to a sensor. For example, a beam splitter can be arranged in the beam path of the channel and the sensor can be positioned such that the sensor is struck by the decoupled portion of the heating radiation. In addition to this heating radiation directed directly onto the sensor, other portions of the heating radiation can also strike the sensor. These can be portions, for example, that were previously reflected once or multiple times by the EUV mirror or other components of the microlithographic projection exposure system. The heating radiation can come from the heating channel in which the beam splitter is arranged or from another heating channel of the heating device.

[0021] The fact that the radiant power measured with a sensor can be influenced by heating radiation from several channels of the heating device can be taken into account by applying a weighting factor. The method can be carried out in such a way that, based on weighting factors, a relationship is established between the input power of a heating channel and an expected measured value of a sensor. The method can be carried out in such a way that, using a plurality of weighting factors, a relationship is established between the input power of several heating channels and an expected measured value of a single sensor. In one embodiment, a weighting factor is determined for all pairs of a heating channel and a sensor. A heating channel of a heating device is characterized in that the heating power transmitted via the heating channel can be adjusted separately.

[0022] A sensitivity matrix can be created from the weighting factors, in which a weighting factor is assigned to combinations of a heating device channel and a sensor. The sensitivity matrix can be used as part of the mathematical model used to establish a relationship between the input power of the heating device's radiation source and the expected value of the radiation output by a sensor.

[0023] The heating beam path can extend from a radiation source of the heating device to the EUV mirror. The heating beam path can be aligned such that the heating radiation partially or entirely hits the optical surface of the EUV mirror. The optical surface is the surface of the EUV mirror at which the EUV radiation is reflected during operation of the microlithographic projection exposure system. The optical surface of the mirror body can be formed by a highly reflective coating. It can be a multilayer coating, in particular a multilayer coating with alternating layers of molybdenum and silicon.

[0024] A light guide through which the heating radiation passes can be arranged between the radiation source and the EUV mirror. A lens or a combination of lenses can be arranged between an exit end of the light guide and the EUV mirror, through which the heating radiation passes and with which the heating beam is directed to the desired surface areas of the EUV mirror.

[0025] Additionally or alternatively, a beam-shaping element can be arranged between the exit end of the light guide and the EUV mirror to adjust the intensity distribution of the heating radiation onto the EUV mirror as desired. The beam-shaping element can be designed, for example, as a diffractive optical element (DOE) or a refractive optical element (ROE).

[0026] A beam splitter can be arranged in the heating beam path, with which a portion of the heating radiation is coupled out. The beam splitter can be aligned such that the coupled-out portion of the heating radiation is directed to a sensor designed to detect the radiant power of the coupled-out portion of the heating radiation. The beam splitter is preferably designed such that the smaller portion of the heating radiation is coupled out and the larger portion of the heating radiation is directed to the EUV mirror. The coupled-out portion of the heating radiation can constitute less than 20%, preferably less than 15%, more preferably less than 10% of the heating radiation. The beam splitter can have a partially reflecting surface through which a portion of the heating radiation passes, while another portion of the heating radiation is reflected. It is also possible to of the heating beam path by directing part of the heating beam path onto a mirror surface, while another part of the heating beam path does not hit the mirror surface.

[0027] The heating device can comprise a plurality of parallel heating channels through which the heating radiation is guided between the radiation source and the EUV mirror. Each of the heating channels can have the aforementioned features of a heating beam path according to the invention individually or in combination. Several heating channels of the heating device can be bundled in a heating head. A heating head is a structural unit in which several heating channels are connected to one another. The heating device can comprise a plurality of heating heads, with each heating head having a plurality of heating channels.

[0028] An acousto-optical modulator can be arranged between the exit end of a heating head and the EUV mirror. In a first state, this modulator allows the heating radiation to pass straight through and, in a second state, changes the propagation direction of the heating radiation. The acousto-optical modulator can be used to guide the heating radiation into a light trap while the radiation source is switched on or off. This can reduce the risk of damage to the heating device, which could otherwise occur due to power peaks when switching the radiation source on or off.

[0029] The radiation source of the heating device can be designed so that heat radiation can be fed into each of the parallel heating channels. The radiation source can be configured so that the power fed into the individual heating channels can be varied independently of one another. can. The term "radiation source" does not imply any restriction with regard to a specific structural design of the radiation source. The radiation source can be designed as a structural unit from which several heating channels are fed. A design in which several sub-units of the radiation source are structurally separated from one another is also possible. The radiation source can be configured to emit electromagnetic radiation in the infrared wavelength range, in particular with a wavelength between 780 nm and 2000 nm. The heating radiation is then infrared radiation.

[0030] The control unit can be designed to control the radiation source so that a predetermined amount of heat radiation is fed into the heating channels. The radiation source can be controlled within a closed control loop. Within the closed control loop, the control unit can process information, in particular a measured value about the temperature of the EUV mirror, to generate a control command with which the radiation source is controlled.

[0031] The information used in the mathematical model regarding the input power of the heating device can be derived from the control commands used to control the radiation source. Each of the sensors of the EUV mirror system can transmit a measured value regarding the output heating radiation to the control unit. The control unit can process the input power of the heating device and the measured values ​​regarding the radiation power of the output heating radiation in order to make the inventive comparisons with the first threshold value and the second threshold value. The mathematical model can be processed in the control unit. Weighting factors can be processed, which can be stored in particular in a sensitivity matrix. which represent the relationship between individual heating channels and individual sensors.

[0032] The invention also relates to an EUV mirror system having an EUV mirror, a control unit, and a heating device. The heating device defines a heating beam path extending from a radiation source to the EUV mirror, wherein heating radiation is guided onto the EUV mirror via the heating beam path. The control unit is designed to process measured values ​​relating to the radiant power of an outcoupled portion of the heating radiation in order to perform a first comparison between an absolute value of the measured radiant power and a first threshold value, and to perform a second comparison between a rate of change of the measured radiant power and a second threshold value. The control unit is further designed to evaluate the first comparison and the second comparison in order to determine a fault in the heating device.

[0033] The invention further relates to a microlithographic projection exposure system comprising an illumination system and a projection lens, wherein the illumination system directs EUV radiation onto an object field in an object plane, and wherein the projection lens images the object field into an image plane. The projection lens is equipped with an EUV mirror system according to the invention.

[0034] The disclosure includes further developments of the method with features described in connection with the EUV mirror system according to the invention. The disclosure includes further developments of the EUV mirror system with features described in connection with the method according to the invention.

[0035] The invention is described below by way of example with reference to the accompanying drawings using advantageous embodiments. They show: Fig. 1: a schematic representation of a microlithographic projection exposure system; Fig. 2: a schematic representation of a first embodiment of an EUV mirror system according to the invention; Fig. 3: an exemplary temporal course of the expected absolute value of the radiated power; Fig. 4: a time course of the expected rate of change of the radiant power corresponding to Fig. 3; Fig. 5: a schematic representation of the time course of the measured absolute value of the radiated power; Fig. 6: a schematic representation of the time course of the measured rate of change of the radiant power corresponding to Fig. 5; Fig. 7, 8: the representation according to Fig. 5, 6 with another Course of the method according to the invention; Fig. 9: a schematic representation of a second embodiment of an EUV mirror system according to the invention; Fig. 10: a representation of a sensitivity matrix according to the invention; Fig. 11: the view according to Fig. 10 in an alternative Embodiment of the invention; Fig. 12: a schematic representation of a heating beam path; Fig. 13: the view according to Fig. 12 in an alternative Embodiment of the invention.

[0036] Fig. 1 schematically illustrates a microlithographic EUV projection exposure system. The projection exposure system comprises an exposure beam source 14, an illumination system 10, and a projection lens 22, which are operated together in a vacuum chamber 23.

[0037] The exposure beam source 14 generates electromagnetic radiation in the EUV range, i.e., in particular, with a wavelength between 5 nm and 30 nm. The exposure beam emanating from the exposure beam source 14 is focused into an intermediate focal plane 16 by a collector 15. Exposure beam emanating from the intermediate focal plane 16 is guided into an object plane 12 by the illumination system 10, so that an object field in the object plane 12 is illuminated with uniform radiation intensity.

[0038] The illumination system 10 comprises a deflecting mirror 17, with which the exposure radiation is deflected onto a first facet mirror 18. A second facet mirror 19 is arranged downstream of the first facet mirror 18. The second facet mirror 19 images the facets of the first facet mirror 18 onto the object plane 12.

[0039] In the object plane 12, a photomask 13 is arranged, which is imaged into an image plane 21 via a plurality of mirrors M1-M6 of the projection lens 22. A The structure formed on photomask 13 is transferred to a radiation-sensitive layer of a wafer 20 arranged in the image plane 21. The photomask 13 is suspended from a first scanning device 24, and the wafer 20 rests on a second scanning device 25, so that the wafer 20 can be exposed in a scanning process in which the photomask 13 and the wafer 20 are moved synchronously with one another.

[0040] Each of the EUV mirrors M1-M6 comprises a mirror body 30 (Fig. 2) which is made of a mirror substrate material with ultra-low thermal expansion ("ultra-low expansion material"). An example of such a material is a titanium silicate glass sold under the name ULE™ by Corning Inc. which has a so-called zero crossing temperature (= "zero crossing temperature"). At this zero crossing temperature, which for example for ULE™ is around 30 °C, the thermal expansion coefficient has a zero crossing in its temperature dependence, in the vicinity of which no or only negligible thermal expansion of the mirror substrate material occurs.

[0041] A multilayer coating system with alternating layers of molybdenum and silicon is applied to the mirror body 30, forming an optical surface 31 of the EUV mirror. The optical surface 31 is optimized for the reflection of EUV radiation in the extreme ultraviolet spectral range with wavelengths between 5 nm and 30 nm. With such a coating, approximately 70% of the incident EUV radiation can be reflected. The remaining approximately 30% is absorbed and leads to heating of the EUV mirror. The heat distribution across the optical surface 31 depends on the illumination setting and can vary considerably depending on the illumination setting.

[0042] An EUV mirror system according to the invention comprises a heating device 29, with which heating radiation 32 in the form of infrared radiation can be directed onto the optical surface 31 of the EUV mirror. A heating beam path 33 extends from a radiation source 41 via a bundle of several optical fibers 35 to a heating head 34. The heating radiation 32 exits the heating head 34 and propagates from there to the optical surface 31 of the EUV mirror. By suitably adjusting the heating power and directing the heating radiation 32 onto suitable surface areas of the EUV mirror, an inhomogeneous temperature distribution across the optical surface 31 can be counteracted.

[0043] The EUV mirror system comprises a control unit 40, which is supplied with measured values ​​of the temperature of the mirror body 30 by a temperature sensor 42. Depending on the measured temperature values ​​and other input variables, the control unit 40 controls the radiation source 41 of the heating device 29 in a closed control loop. The control objective is to achieve a desired temperature distribution across the mirror body 30 or the optical surface 31.

[0044] A mirror element 37 is arranged between the heating head 34 and the EUV mirror. The mirror element's surface area is significantly smaller than the cross-section of the heating beam path 33 in this section of the beam path. A portion of the heating radiation 32 is coupled out with the mirror element 37 and directed to a sensor 39. The sensor 39 measures the radiant power of the coupled-out portion 38 of the heating radiation 32. There are other ways to couple out a portion of the heating radiation, such as arranging a diffractive optical element (DOE) in the heating beam path.

[0045] The measured value of sensor 39 is fed to the control unit 40 as an absolute value of the coupled radiation power for the respective point in time. From a temporal sequence of such measured values, the control unit 40 determines a corresponding rate of change.

[0046] Fig. 3 shows a curve over time T in relative units, in which the heating device 29 is switched on between times TI and T2, while the heating device 29 is switched off before time TI and after time T2. When the radiation source 41 of the heating device 29 is controlled in this way, the absolute value of the radiant power measured by the sensor 39 follows an expected curve, as shown schematically in curve 43. Before time TI and after time T2, the expected absolute value of the radiant power is zero. At time TI, a sharp increase in the radiant power measured by the sensor 39 is expected, and at time T2, a sharp decrease in the radiant power. Between times TI and T2, the power of the radiation source 41 is increased once. It is to be expected that the radiant power measured by the sensor 39 will also increase at this time.

[0047] A tolerance band 44 extends above and below the curve 43 and defines the range within which the absolute values ​​measured with the sensor 39 should lie during proper operation of the EUV mirror system. The tolerance band 44 provides a first threshold value within the meaning of the invention for each point in time. By comparing a measured value of the output radiation power with the first threshold value, it can be determined whether the absolute value of the measured radiation power lies within the expected range.

[0048] In Fig. 4, the expected time course of the rate of change of the radiant power is shown in relative units, with curve 45 being indicated in simplified form as a plurality of peaks. A tolerance band 46 for curve 45 is indicated by a dashed line. Tolerance band 46 provides a second threshold value within the meaning of the invention for each point in time. By comparing the measured rate of change with the second threshold value, it can be determined whether the rate of change of the measured radiant power lies within the expected range.

[0049] Fig. 5 shows a measured absolute value 47 of the coupled-out radiation power over time. The absolute value 47 remains within the first tolerance band 44 throughout the entire time period shown. This is determined in the control unit 40 by performing a comparison for each point in time with the first threshold value, which is derived from the first tolerance band 44.

[0050] Fig. 6 shows the rate of change 48 of the output radiation power corresponding to Fig. 5 over time. The rate of change 48 remains within the second tolerance band 46 throughout the entire time period shown. This is determined in the control unit 40 by performing a comparison for each point in time with the second threshold value, which is derived from the second tolerance band 46.

[0051] Since neither the absolute value 47 nor the rate of change 48 of the coupled-out radiation power exceeds the threshold value resulting from the tolerance bands 44, 46, the control unit 40 receives no indication of a fault in the heating device 29 from Figs. 5, 6.

[0052] Figs. 7 and 8 also show a related temporal progression of the absolute value 47 and the rate of change 48. Shortly before time T2, the rate of change 48 falls below the second threshold value, which is derived from the second tolerance band 46. The control unit 40 detects the violation of the second threshold value and switches off the heating device 29.

[0053] One fault pattern that could explain the temporal progression shown in Figs. 7 and 8 is a break in one of the multiple optical fibers 35, via which the heating radiation 32 is transmitted from the radiation source 41 to the heating head 34. A fault in only one of the optical fibers does not have a significant impact on the absolute value 47 of the decoupled portion 38 of the heating radiation 32. On the other hand, a break in an optical fiber leads to a sudden change in the measured radiation output, which is reflected in a violation of the second tolerance band 46. If the fault were not detected, it could lead to significant heat development in the area of ​​the broken optical fiber, which could endanger the operational reliability of the microlithographic projection exposure system.

[0054] Fig. 9 shows an alternative embodiment in which the radiation source 41 supplies a plurality of heating heads 34, which are connected to the radiation source 41 via a plurality of optical fiber bundles 35. Each of the three heating beam paths 33 is assigned a mirror element 37 and a sensor 39. The measured values ​​of the sensors 39 are fed to a control unit 40 (not shown in Fig. 9) in order to perform error identification.

[0055] Due to the large number of heating heads 34, the radiation from several heating heads 34 is superimposed on each of the sensors 39. The largest part of the measured with the sensors 39 Radiant power represents the radiation directly reflected by the mirror elements 37 and guided to the sensors 39. A smaller portion of the radiation incident on the sensors results from the heating radiation emitted by the heating heads 34 being reflected or scattered by the optical surface 31 of the EUV mirror, or from multiple reflections by other components of the microlithographic projection exposure system. This superposition of various components of the heating radiation is indicated in Fig. 9 by dashed arrows 49.

[0056] Fig. 10 shows a first sensitivity matrix 50 which, based on weighting factors for four different heating heads LI, L2, L3, L4 and four different sensors SI, S2, S3, S4, each in pairs, represents the relationship between the heating radiation emitted by the heating heads and the measured values ​​recorded by the sensors. The second sensor S2 is assigned to the first heating head LI, so that the decoupled part of the heating radiation is directed directly to the second sensor S2. The sensitivity matrix 50 shows that 1.1% of the heating radiation from the first heating head LI hits the second sensor S2. Due to reflections, an additional 0.1% of the heating radiation from the second heating head L2 and 0.2% of the heating radiation from the fourth heating head L4 hits the second sensor. In contrast, the third heating head L3 is positioned such that the emitted heating radiation does not hit the second sensor S2.Corresponding relationships exist between the other sensors Sl, S3, S4 and the heating heads LI, L2, L3, L4.

[0057] The sensitivity matrix 50 can be processed in the control unit 40 of the EUV mirror system to determine expected measured values ​​for the sensors S1, S2, S3, S4. This results in improved error detection. For example, if there is a significant drop in the sensor S2 and a slight drop in the radiant power measured with the first sensor S1 and the third sensor S3, this is an indication of a fault in the first heating head LI.

[0058] The method can be refined with a larger number of heating heads and a larger number of sensors. The method can be applied analogously if the heating heads each comprise several independently adjustable heating channels.

[0059] It is not absolutely necessary for the number of sensors to match the number of heating heads. Fig. 11 shows a sensitivity matrix 51 for an example in which the number of sensors SI, S2, S3 is smaller than the number of heating heads LI, L2, L3, L4. There is a direct assignment between the first three sensors SI, S2, S3 and the first three heating heads LI, L2, L3. The fourth heating head L4 is not assigned its own sensor. An indication of a fault in the fourth heating head L4 arises if a drop in the measured radiant power occurs simultaneously on all three sensors SI, S2, S3. The expected drop is greatest for the first sensor S1 and smallest for the second sensor S2.

[0060] Fig. 12 schematically shows the course of a channel of a heating beam path. The heating radiation emitted by the radiation source 41 is fed into a light guide 52. A lens 53 is arranged at the exit end of the light guide, which concentrates the heating radiation and directs it toward the EUV mirror. A diffractive optical element (DOE) 54 is arranged between the lens 53 and the EUV mirror, which is used to achieve a desired intensity distribution of the heating radiation.

[0061] Damage to the DOE 54 can occur if the heating radiation passing through it has too high a power. It has been found that damaging power peaks can occur at the DOE 54, particularly when the radiation source 41 is switched on or off. The alternative embodiment in Fig. 13 therefore proposes arranging an acousto-optical modulator 55 in front of the exit end of the light guide 52, which in its basic state just allows the heating radiation to pass through. A light trap 56, which absorbs the incoming heating radiation, is arranged behind the acousto-optical modulator 55.

[0062] Once the switch-on process is complete and the radiation source 41 has transitioned to a stable state, the acousto-optical modulator 55 is activated, resulting in a change in the propagation direction of the passing heating radiation. The heating radiation is shaped in the DOE 54 in a manner appropriate for heating the EUV mirror. Since no power peaks occur during continuous operation of the radiation source 41, the risk of damage to the DOE 54 is low.

Claims

Patent claims 1. Method for operating an EUV mirror system, in which heating radiation emitted by a radiation source (41) is directed onto an EUV mirror (M1-M6) by means of a heating device (29), in which a part of the heating radiation (32) is coupled out, in which measured values ​​about the radiation power of the coupled-out part (38) of the heating radiation (32) are recorded by means of a sensor (39), in which a first comparison is carried out between an absolute value (47) of the measured radiation power and a first threshold value (44), in which a second comparison is carried out between a rate of change (48) of the measured radiation power and a second threshold value (46), and wherein the first comparison and the second comparison are evaluated in order to determine an error in the heating device (29).

2. Method according to claim 1, wherein a fault in the heating device (29) is inferred if the threshold value (44, 46) has been exceeded in the first comparison or in the second comparison.

3. The method according to claim 1 or 2, wherein the first comparison and the second comparison are carried out in a control unit (40) and wherein the control unit (40) is designed to switch off the radiation source (41) when an error has been detected.

4. Method according to one of claims 1 to 3, wherein an expected value (43) for the coupled-out portion (38) of the heating radiation (32) is determined.

5. The method according to claim 4, wherein the expected value (43) is determined using a mathematical model of the heating device (29).

6. The method according to claim 5, wherein the input power of the radiation source (41) is used as input variable of the mathematical model.

7. The method according to claim 5 or 6, wherein an expected rate of change (45) is inferred from a temporal course of the expected value (43).

8. Method according to one of claims 1 to 7, wherein the heating device (29) has a plurality of parallel heating channels and wherein a decoupled portion of the heating radiation (32) is guided from several of the heating channels to a sensor (39).

9. The method according to claim 8, wherein the number of sensors (39) is smaller than the number of heating channels.

10. The method according to one of claims 1 to 9, wherein a relationship between the input power of a heating channel (LI, L2, L3, L4) and an expected measured value of a sensor (SI, S2, S3, S4) is established using weighting factors (51, 52).

11. The method according to claim 10, wherein a relationship between the input power of several heating channels (L1, L2, L3, L4) and an expected measured value of an individual sensor (S1, S2, S3, S4) is established using a plurality of weighting factors (51, 52).

12. EUV mirror system, with an EUV mirror (M1-M6), with a control unit (40) and with a heating device (29), wherein the heating device (29) has a heating beam path (33) which extends from a radiation source (41) to the EUV mirror (M1-M6), wherein heating radiation is guided onto the EUV mirror (M1-M6) via the heating beam path (33), and wherein the control unit is designed to process measured values ​​relating to the radiant power of an outcoupled portion (38) of the heating radiation (32) in order to carry out a first comparison between an absolute value (47) of the measured radiant power and a first threshold value (44), and to carry out a second comparison between a rate of change (48) of the measured radiant power and a second threshold value (46), and to evaluate the first comparison and the second comparison in order to determine a fault in the heating device (29).

13. Microlithographic projection exposure system, comprising an illumination system (10) and a projection lens (22), wherein the illumination system (10) directs EUV radiation onto an object field in an object plane (12) and wherein the object field is imaged into an image plane (21) by the projection lens (22), wherein the projection lens comprises an EUV mirror system according to claim 12.

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