EUV MIRROR SYSTEM, METHOD FOR OPERATING AN EUV MIRROR SYSTEM, PROJECTION LENS FOR A MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS, AND COMPUTER PROGRAM PRODUCT

Abstract

The invention relates to an EUV mirror system comprising an EUV mirror, a support structure (31) and a control unit (34), wherein the EUV mirror has a mirror body (38) and an optical surface (32) formed on the mirror body (38), and wherein the support structure (31) supports the EUV mirror. The EUV mirror system comprises a cooling system for cooling a system component (31, 38) of the EUV mirror system, wherein the cooling system comprises a pump (33) and a cooling channel (37). The pump is designed to convey a cooling liquid along the cooling channel (37). An input variable representing a heat load acting on the system component (31, 38) is fed to the control unit (34). The control unit (34) processes the input variable and a thermal model of the system component (31, 38) in order to determine a target value for the volumetric flow rate of the cooling liquid. The control unit (34) derives a control command for the cooling system from the target value for the volumetric flow rate, such that the volumetric flow rate is changed and a predefined maximum rate (75) of change of the volumetric flow rate is not exceeded. The cooling system is controlled using the control command. The invention also relates to a method for operating an EUV mirror system, to a projection lens for a microlithographic projection exposure apparatus, and to a computer program product.

Description

[0001] EUV mirror system, method for operating an EUV mirror system, projection lens for a microlithographic projection exposure system, computer program product

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

[0003] This patent application claims priority from German patent application DE 10 2023 212 527 . 7, filed on December 12, 2023, to which reference is made and the contents of which are incorporated herein in their entirety ( "incorporation by reference").

[0004] 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 imaged onto a lithographic object to transfer the mask structure to the object.

[0005] The projection exposure system comprises several EUV mirrors, each with an optical surface that reflects the radiation. The mirrors have a precisely defined shape and are precisely positioned to ensure sufficient image quality from the photomask onto the lithography object.

[0006] During operation of the projection exposure system, heat is supplied to the mirror body of an EUV mirror, so that the mirror body heats up. The heat supply results, among other things, from the fact that the EUV radiation incident on the mirror body is not completely reflected, but is largely absorbed. A support structure carrying the mirror body also heats up, for example through heat exchange with the mirror body or through the supply of infrared radiation. Temperature changes of a body are generally accompanied by thermal deformation. This is undesirable for EUV mirrors in a microlithographic projection exposure system because a change in the geometric shape of the mirror changes the wavefront of the EUV radiation reflected at the optical surface.Thermal deformation of the mirror support structure is also undesirable because it changes the position of the supported mirrors. This usually leads to a reduction in image quality.

[0007] A cooling system can be used to keep the temperature of a system component of the EUV mirror system, in particular the temperature of the mirror body and / or the temperature of the support structure, as constant as possible. A pump of the cooling system can be used to convey a cooling liquid along a cooling channel formed in the system component. The cooling liquid cools the system component by transferring heat from the system component to the cooling liquid in the cooling channel. A higher volume flow of the cooling liquid leads to increased cooling performance and vice versa. Keeping the volume flow of the cooling liquid constant has the advantage that turbulence in the cooling liquid within the cooling channel can be reduced.Turbulence of the cooling liquid within the cooling channel is undesirable because it can be transferred to the mirror body as vibrations, which usually also leads to a reduction in the image quality.

[0008] The heat load acting on the mirror body can change over time, for example, when no EUV radiation hits the EUV mirror during dark phases or when the power of other heat sources changes. If the heat supply changes while the volume flow of the cooling fluid remains constant, an undesirable change in the temperature of the mirror body occurs.

[0009] It is possible to monitor the temperature of the mirror body with a temperature sensor and to control the cooling performance by changing the volume flow of the cooling liquid based on a detected temperature change. This has the disadvantage, however, that the change in the volume flow can only occur with a time delay, namely after the temperature of the mirror body has already changed. In this case, the change in the volume flow must also occur quickly so that the undesirable temperature change is not too great. A rapid change in the volume flow has the disadvantage that it can cause undesirable turbulence. It is not possible to satisfactorily keep the temperature of the mirror body as constant as possible while at the same time avoiding undesirable turbulence in the cooling liquid in this way.

[0010] The invention is based on the object of presenting an EUV mirror system, a method for operating an EUV mirror system, a projection lens for a microlithographic projection exposure system, and a computer program product with which these disadvantages are mitigated. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0011] An EUV mirror system according to the invention comprises an EUV mirror, a support structure, a cooling system for cooling a system component of the EUV mirror system and a control unit. The EUV mirror has a mirror body and an optical surface formed on the mirror body. The support structure carries the EUV mirror. The cooling system comprises a pump and a cooling channel. The pump is designed to pump a cooling liquid along the cooling channel. An input variable representing a heat load acting on the system component is supplied to the control unit. The control unit processes the input variable and a thermal model of the system component in order to determine a setpoint value for the volume flow of the cooling liquid.The control unit derives a control command for the cooling system from the setpoint for the volume flow, so that the volume flow is changed and a specified maximum rate of change of the volume flow is not exceeded. The cooling system is controlled by the control command, so that the cooling system influences the volume flow based on the control command.

[0012] The invention proposes not to wait until a temperature change of the system component has already occurred to react, but to link it to a change in a heat load acting on the system component. The influence of the change on the temperature of the system component is predicted using a thermal model and the volume flow of the coolant is set based on the prediction. The heat load acting on a system component can change suddenly, for example when the EUV radiation is switched off to change the wafer or the photomask. Conventional control can result in a correspondingly rapid change in the volume flow. The invention has recognized that the time gained by the prediction makes it possible to spread the change in the volume flow over time without there being an excessive change in the temperature of the system component.This leads to the inventive proposal of specifying a maximum rate of change for the volume flow and controlling the cooling system in such a way that this maximum rate of change is not exceeded. This makes it possible to react to a change in the thermal state of the mirror body without inducing undesirable vibrations of the mirror body.

[0013] In one embodiment, the system component is the mirror body of the EUV mirror. The cooling channel can extend within the mirror body. In another embodiment, the system component is the support structure that supports the EUV mirror. The cooling channel can extend within the support structure. The invention also encompasses embodiments in which the mirror body is provided with a cooling channel and the support structure is provided with a cooling channel, and in which the volume flow in the cooling channel of the mirror body and the volume flow in the cooling channel of the support structure are adjusted according to the invention.

[0014] The heat source of the heat load acting on the mirror body can be the EUV radiation impinging on the optical surface of the mirror, which is partially absorbed. The change in the heat load acting on the mirror body can then result from the fact that no EUV radiation impinges on the optical surface of the mirror body during a dark phase.

[0015] Another heat source can be a heating device with which thermal energy is supplied to the mirror body. The heating device can, for example, be designed to conduct suitable electromagnetic radiation, in particular infrared radiation, onto the mirror body. The heating device can heat the mirror body evenly, for example in a preheating phase before operation of the projection exposure system. It is also possible for the heating device to be designed as a sector heater that heats only certain areas of the mirror body (sectorally). A sector heater can, for example, be used to supply heat to areas of the mirror body that are only heated to a lesser extent by EUV radiation.

[0016] A change in the heat load acting on the mirror body can then result from the heating device being switched on or off, from the heating device switching between uniform and sectoral heating of the mirror body, or from a change in the power of the electromagnetic radiation that the heating device directs onto the mirror body. The support structure carrying the mirror body, which is in heat exchange with the mirror body, can constitute a further heat source. The change in the heat load acting on the mirror body can then result from a change in the heat absorbed or released by the support structure itself.

[0017] A change in the heat load acting on the supporting structure can result, for example, from a change in the thermal radiation reflected by other system components or from a change in the amount of heat emitted by another system component. For example, if the EUV radiation is switched off, the mirror body radiates less heat. It is also possible that the output of a heating device directed at the supporting structure changes.

[0018] The input variable, which represents a heat load acting on the system component, can be a binary value about the activity of a heat source. For example, the value 0 can mean that the heat source does not transfer any heat energy to the system component and the value 1 can mean that the heat source does transfer heat energy to the system component. In another embodiment, the input variable is a value about the amount of heat energy transferred from the heat source to the system component. The input variable can be time-dependent and / or a value for a specific point in time. For example, the input variable can include information about a future point in time at which a heat load acting on the system component changes, i.e. at which, for example, the EUV radiation incident on the optical surface of the mirror will cease. The input variable can also include several such values.

[0019] The thermal model of the mirror body can be selected such that it establishes a relationship between the heat load acting on the system component and the temperature of the system component, so that future temperature values ​​of the mirror body can be predicted. The thermal model can, for example, take into account the density, thermal conductivity and / or specific heat capacity of the material of the system component. The thermal model can, for example, be obtained using a machine learning method. The thermal model can take into account heat transfer between the mirror body and the supporting structure.

[0020] In one embodiment, the control unit processes the input variable and the thermal model to determine a future temperature value of the system component, assuming that the volume flow of the coolant is kept constant at its current value. In other words, the control unit then predicts the future course of the temperature of the mirror body at a constant volume flow. The temperature value can be above or below a current temperature of the mirror body, or equal to it. Based on the difference between the future temperature value determined in this way and a setpoint value for the temperature of the system component, the control unit can determine a setpoint value for the cooling power that is necessary to keep the temperature of the mirror body constant. The control unit then determines the setpoint curve for the volume flow based on the setpoint value for the cooling power of the cooling system.For this purpose, the control unit can, for example, access information such as the nature of the coolant or a relationship between the pump speed and the coolant flow rate. There are several variants for the practical implementation of the method. For example, the control unit can be designed to perform the relevant calculations online before a new control command is transmitted. It is also possible to store relationships once determined in a table and generate the control commands based on values ​​read from the table.

[0021] The maximum rate of change refers to the change in the volume flow with which the volume flow is brought to the setpoint. Starting from an actual value of the volume flow, a transition phase follows in which the volume flow changes. At the end of the transition phase, the volume flow corresponds to the setpoint. During the transition phase there is no point in time at which the rate of change in the volume flow is greater than the specified maximum rate of change. The transition phase extends over a period of time which can be between 10 s and 500 s, for example, and preferably between 30 s and 300 s. The duration of the transition phase ensures that a sufficiently smooth control trajectory can be specified along which the volume flow is controlled from the actual value to the setpoint of the volume flow without exciting undesirable vibrations of the mirror body.

[0022] The maximum rate of change of the volume flow can be specified in such a way that, on the one hand, the setpoint value of the volume flow is reached as quickly as possible, while on the other hand the transition phase is long enough that no significant oscillations are excited in the system component. The maximum rate of change can also result from the form of a specified control trajectory for the transition from the actual value of the volume flow to the setpoint value of the volume flow. The control trajectory is advantageous if it does not contain any frequencies that lead to undesirable oscillations of the mirror body. It is also advantageous if the control trajectory fulfills the system dynamics so that the volume flow is physically capable of satisfying the control trajectory. The value for the maximum rate of change resulting from these requirements can be determined, for example, by tests or simulations.The predetermined maximum rate of change of the volume flow can, for example, be less than 30% / min, preferably less than 20% / min, more preferably less than 10% / min.

[0023] In one embodiment, the transition from the actual value of the volume flow to the setpoint value of the volume flow is carried out at a constant rate of change that corresponds to the specified maximum rate of change. The specified maximum rate of change then defines a type of ramp along which the volume flow is transferred from the actual value to the new setpoint value. Embodiments are also possible in which the rate of change changes during the transition. The transition can include sections in which the rate of change is smaller than the specified maximum rate of change, but no sections in which the rate of change exceeds the specified maximum rate of change. The transition can be carried out in such a way that the second derivative of the rate of change does not exceed a specified maximum value. This can make a further contribution to avoiding flow-induced oscillations.In one embodiment, a control trajectory is specified for the transition from the actual value of the volume flow to the setpoint value of the volume flow. The control trajectory can be non-linear. If the control trajectory is non-linear, this has the advantage that sudden changes in the volume flow can be avoided, particularly at the beginning and end of the control process. The control trajectory can, for example, correspond to a sigmoid or S-curve. If the volume flow follows the control trajectory, the specified maximum rate of change of the volume flow is not exceeded. For example, the control trajectory is selected such that its steepest point corresponds to a change in the volume flow that is less than or equal to the specified maximum rate of change.The control trajectory can be selected to account for the dead time required for the cooling system to reach the flow rate setpoint. For example, the dead time may originate from the inertia of the pump or the cooling fluid. In other words, the boundary conditions of the maximum rate of change and the dead time can limit the selection of possible control trajectories.

[0024] Control commands for the cooling system pump can be derived from a planned transition between the actual flow rate and the setpoint flow rate, so that the pump is controlled according to the desired transition. For example, the pump speed can be regulated based on the control trajectory, thus influencing the flow rate accordingly. The cooling fluid can be water or other industrially standard cooling fluids.

[0025] The mirror system can include a temperature sensor to determine a temperature measurement from the system component. The temperature sensor can be formed on the system component. The temperature measurement can be fed to the control unit. For this purpose, the temperature sensor can be connected to the control unit via an information line. The temperature measurement can be processed in the control unit to detect and compensate for control errors. The control error can result, for example, from the difference between a predicted temperature value and a temperature measurement value.

[0026] The control unit can include a memory block in which, among other things, the thermal model and the maximum rate of change of the volume flow can be stored. It is also possible for the memory block to store relationships between specific changes in a heat load and the associated control commands for the pump in tabular form.

[0027] The cooling system can influence the flow rate by changing the pump speed. For this purpose, the pump can be controlled by the control unit via a control line with a control command. It is also possible for the cooling system to include valves or slides controlled by the control unit to influence the flow rate.

[0028] The cooling system can comprise a first cooling channel which is formed in the mirror body, and a second cooling channel which is formed in a support structure carrying the mirror body. The cooling system can also comprise a first cooling structure of a plurality of cooling channels which is formed in the mirror body, and a second cooling structure of a plurality of cooling channels which is formed in a support structure carrying the mirror body. The second cooling channel can be connected to the first cooling channel. The invention can also be used to keep the temperature of the support structure carrying the mirror body as constant as possible. The invention further relates to a projection objective having a plurality of EUV mirrors with which a photomask is imaged in an image plane. One or more EUV mirrors of the projection objective can be designed as a component of an EUV mirror system according to the invention.The invention further relates to a microlithographic projection exposure system with such a projection lens.

[0029] The invention also relates to a method for operating a mirror system with an EUV mirror, wherein the EUV mirror has a mirror body and an optical surface formed on the mirror body, and with a support structure carrying the EUV mirror. A pump of a cooling system conveys a cooling liquid along a cooling channel formed within a system component of the EUV system in order to cool the system component. An input variable representing a heat load acting on the system component is supplied to a control unit.In the control unit, the input variable and a thermal model of the system component are processed in order to determine a setpoint value for the volume flow of the cooling liquid, whereby the control unit derives a control command for the cooling system from the setpoint value for the volume flow so that the volume flow is changed and a predetermined maximum rate of change of the volume flow is not exceeded, and whereby the cooling system is controlled with the control command.

[0030] The invention also relates to a computer program product or a set of computer program products, comprising program parts which, when loaded into a computer or into interconnected computers connected to a device according to the invention, are designed to carry out the method according to the invention. The disclosure includes developments of the EUV mirror system with features that are described in connection with the method according to the invention. The disclosure includes developments of the method with features that are described in connection with the EUV mirror system according to the invention.

[0031] The invention is described below by way of example with reference to the accompanying drawings using advantageous embodiments. They show:

[0032] Fig. 1: an embodiment of a projection exposure system according to the invention;

[0033] Fig. 2: a schematic representation of an EUV mirror system according to the invention;

[0034] Fig. 3: a schematic representation of another EUV mirror system according to the invention;

[0035] Fig. 4: a schematic plot of the temperature of a mirror body of the EUV mirror system over time at a constant volume flow of the cooling liquid;

[0036] Fig. 5: a schematic plot over time of a control trajectory according to the invention;

[0037] Fig. 6: a temperature profile of the mirror body over time as desired according to the invention;

[0038] Fig. 7: a schematic representation of a further embodiment of a mirror system according to the invention;

[0039] Fig. 8: a schematic representation of another EUV mirror system according to the invention. Fig. 1 schematically shows 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. A vacuum is present in the vacuum chamber 23 during operation of the EUV projection exposure system.

[0040] 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.

[0041] 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 projects the facets of the first facet mirror 18 into the object plane 12.

[0042] A photomask 13 is arranged in the object plane 12 and is imaged into an image plane 21 by a plurality of mirrors M1-M6 of the projection objective 22. A structure formed on the 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, 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. Fig. 2 shows a mirror device in which a mirror body 38 of a mirror M1-M6 is held on a support structure 31 by actuators 30. The position of the mirror body 38 relative to the support structure 31 can be changed via the actuators 30 in order to align and position the mirror body 38.An optical surface 32, at which EUV radiation is reflected, is formed on the mirror body 38. In the projection exposure system of Fig. 1, each of the mirrors M1-M6 of the projection lens 22 is designed as a mirror device according to Fig. 2. It is also possible to equip only some of the mirrors M1-M6 in this way.

[0043] A cooling channel 37 is formed inside the mirror body 38, which in this embodiment extends along a serpentine path through the mirror body 38. The cooling channel 37 belongs to a cooling system in which a pump 33 pumps a cooling liquid along a closed cooling circuit.

[0044] In the embodiment shown, the cooling circuit extends from the pump 33 via a supply line 35 to the cooling channel 37 and via a return line 36 back to a reservoir 41, from which the cooling liquid is sucked in by the pump 33. The supply line 35 is connected to the mirror body 38 via an inlet connection 39, and the return line 36 is connected to the mirror body 38 via an outlet connection 40. The supply line 35 and the return line 36 are sufficiently flexible so that the adjustment and alignment of the mirrors are not hindered. The cooling liquid absorbs the heat generated by the absorbed EUV radiation and dissipates it from the mirror body 38.

[0045] The mirror system comprises a control unit 34 which controls the operation of the mirror system as a function of various input variables, influencing, among other things, the thermal state of the mirror body 38 using control commands. One aspect of this is that the control unit 34 controls the pump 33 of the cooling system using control commands, so that the pump 33 influences the volume flow of the cooling liquid based on the control command by changing its speed.

[0046] If the cooling capacity of the cooling system corresponds to the supplied heat capacity, the temperature of the mirror body 38 remains constant. If a heat load is removed while the state of the cooling system remains unchanged, the mirror body 38 cools down. Fig. 4 shows a schematic plot of the temperature 61 of the mirror body, plotted against a time axis 60 for this case. At time TO a change in a heat load acting on the mirror body occurs. More precisely, in the present example the heat load acting on the mirror body due to the EUV radiation is removed at time TO, so that less heat is supplied to the mirror body overall. Because the volume flow of the cooling liquid remains unchanged, a future temperature value 71 at a future time TI is below the temperature value 70 at time TO when the heat load is removed.In other words, the cooling system cools the mirror body after the heat load is removed, which leads to an undesirable temperature change 72 .

[0047] To counteract this, the control unit 34 in the present exemplary embodiment processes information that a heat load will be eliminated at time T0 as an input variable and predicts the future temperature value 71 based on a thermal model. The thermal model is stored in a memory block 43. The control unit 34 then determines a setpoint for the cooling capacity of the cooling system based on the predicted temperature value 71. The setpoint for the cooling capacity is determined such that the temperature of the mirror body 38 is kept as constant as possible. In this example, the cooling capacity must be reduced in order to compensate for the elimination of the heat load. The control unit 34 then establishes a relationship between the setpoint for the cooling capacity and a resulting setpoint for the volume flow of the cooling liquid in order to determine a control command for the pump 33.In this example, the volume flow of the cooling liquid must be reduced in order to reduce the cooling capacity.

[0048] In the present exemplary embodiment, the determined control command causes the speed 62 of the controlled pump 33 to follow a control trajectory 73. Fig. 5 shows a schematic plot of the control trajectory 73, plotted against a time axis 60. In this example, the control trajectory 73 is a non-linear S-curve, by means of which the speed 62 of the pump 33 is reduced over time. Because the control trajectory is non-linear, sudden changes in the speed 62 of the pump 33 are avoided, particularly at the beginning and end of the control process, and thus undesirable turbulence in the cooling liquid is prevented. As a result of the control trajectory 73, the reduction in the speed 62 of the pump 33 already begins in a period 74 shortly before the time TO at which the heat load is eliminated. This is possible because the control unit 34 has processed the information as input that the heat load will be eliminated at time TO.This makes it possible to select a flatter S-curve as the control trajectory and to adhere to a predetermined maximum rate of change 75, which results from a predetermined maximum rate of change in the volume flow, in order to avoid undesired turbulence. In other words, the time period 74 is used to begin control before the time TO. The control trajectory 73 also takes into account the inertia of the pump, i.e. a dead time in the cooling system, with the time period 74. Fig. 6 shows an idealized plot of the temperature 61 of the mirror body with inventive control of the volume flow, plotted against a time axis 60. At the time TO there is a change in a heat load acting on the mirror body. By controlling the volume flow using the control trajectory 73 the temperature 61 remains constant over time, so that no undesired temperature change occurs.

[0049] Fig. 3 shows a schematic representation of another EUV mirror system according to the invention, in which the cooling channel 37 extends through the support structure 31. In this case, the control unit 34 influences the thermal state of the support structure 31 with the control commands. The control unit 34 determines the control commands in a similar manner to the previous embodiment.

[0050] Fig. 7 shows a schematic representation of a mirror system with feedback. The feedback consists in a temperature sensor 51 determining a temperature measurement from the mirror body and transmitting it to the control unit 34 via an information line 52. The control unit 34 detects a control error based on the temperature measurement and adjusts the determined control trajectory 73 to compensate for the control error.

[0051] Fig. 8 shows a schematic representation of a further EUV mirror system according to the invention with a first cooling channel 37 which is formed in the mirror body 38, a second cooling channel 80 which is formed in a support structure 31 carrying the mirror body, and a third cooling channel 81 which is formed in a further support structure 31 carrying the mirror body. The first cooling channel 37 is connected to the third cooling channel 81. A second pump 83 is assigned to the second cooling channel 80. The second pump 83, like the pump 33, is controlled by control commands which are determined by the control unit 34.

Claims

Patent claims 1. An EUV mirror system comprising an EUV mirror and a support structure (31), wherein the EUV mirror has a mirror body (38) and an optical surface (32) formed on the mirror body (38), and wherein the support structure (31) supports the EUV mirror, with a cooling system for cooling a system component (31, 38) of the EUV mirror system, wherein the cooling system comprises a pump (33) and a cooling channel (37), wherein the pump (33) is designed to convey a cooling liquid along the cooling channel (37), and with a control unit (34), wherein the control unit (34) is supplied with an input variable that represents a heat load acting on the system component (31, 38), wherein the control unit (34) is designed to process the input variable and a thermal model of the system component (31, 38) in order to determine a setpoint value for the volume flow of the cooling liquid, wherein the control unit (34) is designed toto derive a control command for the cooling system from the setpoint value for the volume flow, so that the volume flow is changed and that a predetermined maximum rate of change (75) of the volume flow is not exceeded, and wherein the control unit (34) is designed to control the cooling system with the control command.

2. EUV mirror system according to claim 1, wherein the system component is the mirror body (38) and wherein the cooling channel (37) extends within the mirror body (38).

3. EUV mirror system according to claim 1, wherein the system component is the support structure (31) and wherein the cooling channel (37) extends within the support structure (31).

4. EUV mirror system according to one of the preceding claims, wherein the thermal model of the system component (31, 38) provides a relationship between the heat load acting on the system component (31, 38) and the temperature (61) of the system component (31, 38), and wherein the thermal model is applied to determine a future temperature value (71) of the system component (31, 38).

5. EUV mirror system according to one of the preceding claims, wherein the control unit (34) determines the setpoint value for the volume flow from a setpoint value for the cooling capacity of the cooling system.

6. EUV mirror system according to one of the preceding claims, wherein the input variable represents a future change in a heat load acting on the system component (31, 38).

7. EUV mirror system according to one of the preceding claims, wherein the thermal model takes into account heat transfer between the mirror body (38) and the support structure (31).

8. EUV mirror system according to one of the preceding claims, wherein the cooling system influences the volume flow based on the control command, so that the volume flow follows a control trajectory (73).

9. EUV mirror system according to claim 8, wherein the control trajectory (73) is non-linear.

10. EUV mirror system according to claim 8 or 9, wherein the steepest point (75) of the control trajectory (73) corresponds to a change in the volume flow that is less than or equal to the predetermined maximum rate of change (75).

11. EUV mirror system according to one of the preceding claims, wherein a heat source of the heat load acting on the mirror body (38) is an EUV radiation impinging on the optical surface (32) of the mirror and / or a heating device and / or the support structure (31).

12. EUV mirror system according to one of the preceding claims, characterized by a temperature sensor (51) for determining a temperature measurement value from the system component (31, 38), wherein the temperature measurement value is provided to the control unit (34) to detect a control error.

13. Projection lens of a microlithographic projection exposure system, with a plurality of EUV mirrors (M1-M6) with which a photomask is imaged in an image plane, wherein one or more of the EUV mirrors (M1-M6) are part of an EUV mirror system according to one of claims 1 to 12.

14. Method for operating an EUV mirror system with an EUV mirror, wherein the EUV mirror has a mirror body (38) and an optical surface (32) formed on the mirror body (38), and with a support structure (31) carrying the EUV mirror, in which method a cooling liquid is conveyed along a cooling channel (37) formed within a system component (31, 38) of the EUV mirror system by means of a pump (33) of a cooling system in order to cool the system component (31, 38), in which method an input variable is supplied to a control unit (34) which represents a heat load acting on the system component (38), in which method the control unit (34) processes the input variable and a thermal model of the system component (31, 38) in order to determine a target value for the volume flow of the cooling liquid, in which method the control unit (34) determines from the target value value for the volume flow, a control command for the cooling system is derived, so that the volume flow is changed and so that a predetermined maximum rate of change (75) of the volume flow is not exceeded, and in which the cooling system is controlled with the control command.

15. Computer program product or set of computer program products, comprising program parts which, when loaded into a computer or into networked computers connected to an EUV mirror system according to one of claims 1 to 13, are designed to carry out the method according to claim 14.

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