Method and apparatus for determining a heating state of an optical element in an optical system

Abstract

The invention relates to a method and to a device for determining a heating state of an optical element in an optical system, more specifically in a microlithographic projection exposure system, wherein, during operation of the optical system, electromagnetic radiation impinges on an entrance surface (401) of the optical element (400), wherein, by using calibration parameters, an average temperature at the entrance surface is estimated on the basis of temperature measurements performed by at least one temperature sensor (420) located at a distance from the entrance surface (401), and wherein the calibration parameters are selected differently depending on an illumination setting set in the optical system.

Description

[0001] This application claims priority to German patent application DE 10 2021 100 995.2, filed January 19, 2021. The contents of that DE application are incorporated herein by reference. Technical Field

[0002] The present invention relates to a method and apparatus for determining the heating state of optical elements in an optical system, particularly in a microlithography projection exposure apparatus. Background Technology

[0003] Microlithography is used to produce microstructured components, such as integrated circuits or liquid crystal displays. The microlithography process is performed in a so-called projection exposure apparatus, which has an illumination device and a projection lens. In this case, an image of a mask (=mask master) illuminated by the illumination device is projected through the projection lens onto a substrate (e.g., a silicon wafer). This substrate is coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection lens, thereby transferring the mask structure onto the photosensitive coating of the substrate.

[0004] In projection lenses designed for the EUV range, that is, at wavelengths of approximately 13 nm or 7 nm, mirrors are used as optical components in the imaging process due to the lack of suitable transmissive and refractive materials available. One problem that arises in practice is that, among other things, EUV mirrors heat up and undergo associated thermal expansion or deformation due to the absorption of radiation emitted by the EUV source, which negatively impacts the imaging characteristics of the optical system.

[0005] One known approach to account for this effect is to use a material with ultra-low thermal expansion (“ultra-low expansion material”) as the mirror substrate material, such as the ULE material sold by Corning Inc. TM Titanium silicate glass, and sets a so-called zero cross temperature in the region near the optically effective surface. For example, for ULE TM For example, the zero-crossing temperature is located at approximately θ = 30°C, at which the coefficient of thermal expansion has a zero-crossing point in its temperature dependence. Near this zero-crossing point, the mirror substrate material does not undergo thermal expansion or only undergoes negligible thermal expansion.

[0006] However, in practice, the problem that arises here is that during the operation of the microlithography projection exposure equipment, the EUV mirrors are exposed to varying intensities of incident electromagnetic radiation, particularly locally (because, for example, illumination settings are used where the intensity varies on the optically effective surface of each EUV mirror) and temporarily, wherein the associated EUV mirrors typically heat up from a relatively low temperature at the beginning of the microlithography exposure process to their operating temperature reached during the lithography process.

[0007] In this context and below, "illumination setup" is understood to refer to the intensity distribution in the pupil plane of an optical system. Typical illumination setups in microlithography projection exposure equipment include, for example, dipole illumination or quadrupole illumination.

[0008] Methods to overcome the above problems, particularly to avoid surface deformation and related optical aberrations caused by varying heat input to the EUV mirror, include, for example, active cooling or the use of a preheater (which enables active mirror heating during the relatively low absorption phase of EUV radiation, and which can be reduced accordingly as the absorption of EUV radiation increases).

[0009] To maintain the mirror temperature as constant as possible (typically the aforementioned zero-crossing temperature), the operation of this preheater is controlled in a closed-loop manner. This requires knowing the radiant power incident on the relevant mirror under each condition, thus allowing the preheating power to be adjusted accordingly. For this purpose (infrared cameras are not always practical due to installation space constraints, except in the case of infrared cameras), a temperature sensor is typically attached to the back of the mirror at an appropriate distance from the optically effective surface of the respective mirror.

[0010] However, in practice, another problem arises in this case: if the three-dimensional temperature distribution set on the volume of the reflector is not spatially uniform, the temperature value measured by such a temperature sensor will be sensitively dependent on the corresponding sensor location.

[0011] Figure 5 A-5d shows a schematic diagram illustrating this problem, where the effective optical surface of the mirror-type optical element is indicated by "501", and the temperature sensor by "520". Furthermore, "TS" represents the temperature measured by the temperature sensor 520, while "T" is the average temperature sought at the incident surface 501. In each case, the shading indicates whether a uniform or non-uniform temperature distribution is assumed within the optical element.

[0012] Only according to Figure 5 Under the condition of a uniform temperature distribution, the temperature T measured by temperature sensor 520 S This corresponds to the average temperature T sought at the incident surface 501, regardless of the sensor position. In the case of non-uniform temperature distribution within the optical element, the temperature sensor 520 provides an "incorrect" value, that is, a value T deviating from the average temperature T sought at the incident surface 501. S Among them, specifically according to Figure 5 Placing the temperature sensor 520 near the minimum of an uneven temperature distribution will cause the measured temperature T to... SIt is too low compared to the actual temperature T, and according to Figure 5 Placing the temperature sensor 520 near the maximum value of an uneven temperature distribution will cause the measured temperature T to... S Too high.

[0013] in this case, Figure 5 a-5c only shows a simplified two-dimensional view of the problem. For example... Figure 5 As shown in d, the temperature sensor position actually has three degrees of freedom, which makes it more difficult to draw conclusions about the average temperature T sought at the incident surface 501 in the case of non-uniform temperature distribution within the optical element.

[0014] For example, in the case of the aforementioned preheating concept, incorrectly determining the heating state based on the temperature measured by one or more temperature sensors can cause the corresponding closed-loop control to react inappropriately, thus failing to properly adjust the preheating power. This, in turn, can lead to thermal deformation, resulting optical aberrations, and related damage to the imaging characteristics of the optical system or projection exposure equipment. Summary of the Invention

[0015] One object of the present invention is to provide a method and apparatus for determining the heating state of optical elements in an optical system, particularly in a microlithography projection exposure apparatus, which allows for more reliable, temperature sensor-based characterization of the heating state.

[0016] This objective is achieved by a method according to the features of independent patent claim 1 and an apparatus according to the features of alternative independent patent claim 16.

[0017] The present invention relates to a method for determining the heating state of an optical element in an optical system, particularly in a microlithography projection exposure apparatus, wherein electromagnetic radiation is incident on the incident surface of the optical element during operation of the optical system.

[0018] Specifically, the average temperature at the incident surface is estimated using calibration parameters based on temperature measurements performed using at least one temperature sensor positioned at a distance from the incident surface; and

[0019] The calibration parameter is selected differently depending on the illumination settings set in the optical system.

[0020] This invention is based, in particular, on the concept of estimating the desired surface temperature of an optical element, for example, based on temperature values ​​measured at different locations of the optical element, but not merely on a single relationship between the sought surface temperature and a sensor-based measured temperature (which is the same for all possible applications and is fixed in this sense), by selecting different calibration parameters to estimate the sought surface temperature of an optical element, such as a mirror, based on the corresponding application scenario, particularly based on the illumination settings set in the optical system, to convert the measured value determined during temperature measurement.

[0021] According to the present invention, in order to avoid the problems described at the beginning and the problems associated with the incorrect determination of the heating state (in particular the potential inappropriate adjustment of the preheating power controlled based on the determination of the heating state), a greater effort, which may be accompanied by more complex calibration, has been intentionally accepted.

[0022] The specific embodiments of calibration according to the invention can be carried out in different ways, as described in more detail below, in particular using existing knowledge about the thermal behavior of optical systems or optical elements for a predetermined reference illumination setting.

[0023] According to one embodiment, calibration parameters for the currently set illumination settings in the optical system are thus determined based on reference calibration parameters previously determined for reference illumination settings.

[0024] According to one embodiment, calibration parameters are determined based on at least one measurement or simulation of a variable of the currently set illumination settings in the optical system, the variable depending on the thermal state of the optical element.

[0025] According to one embodiment, this variable, which depends on the thermal state of the optical element, is the intensity distribution generated in a plane downstream of the optical element relative to the optical path during operation of the optical system. In this case, the invention can particularly utilize the fact that, for example in projection exposure equipment, the intensity setting in the wafer planar region is typically measured in any case using an intensity sensor present there during operation, and therefore this information is generally already available and can be used for calibration according to the invention.

[0026] According to another embodiment, the variable depending on the thermal state of the optical element is the wavefront generated during operation of the optical system in a plane downstream of the optical element relative to the optical path. In this case, the invention can particularly utilize the fact that, based on optical design knowledge and the application of tomographic imaging methods known per se, this wavefront determination can be used to draw conclusions about changes in the optical element (particularly its thermally induced deformation). In this case, reference may be made, for example, to DE 102019 213 233 A1.

[0027] According to one embodiment, the distance of the temperature sensor from the incident surface is also taken into account when determining the calibration parameters.

[0028] In other words, geometric information related to the distance of the sensor from the incident surface is used here as additional information for calibration according to the invention. As described below, this can be combined with time-resolved temperature measurements, particularly to achieve a relatively reliable characterization of the heating state of optical elements or mirrors immediately after the optical system has been put into operation, and, for example, to reliably protect optical elements from thermally induced deformation already in the start-up phase when used for the aforementioned preheating concept.

[0029] According to one embodiment, temperature measurements are therefore repeatedly performed using a temperature sensor to determine a time curve.

[0030] According to one embodiment, a temperature sensor is arranged in an access channel extending from the side of the optical element away from the incident surface into the optical element.

[0031] According to one embodiment, the estimated average temperature is used as an input signal for closed-loop control of at least one parameter characterizing the optical element and / or optical system.

[0032] According to one embodiment, the preheating of the optical element is controlled based on the estimated average temperature, thereby at least partially compensating for changes in the heating state of the optical element over time during the operation of the optical system.

[0033] According to one embodiment, the optical element is a reflector.

[0034] According to one embodiment, the optical element is designed for operating wavelengths less than 30 nm, particularly less than 15 nm.

[0035] According to one embodiment, the optical system is a microlithography projection exposure device.

[0036] According to one embodiment, calibration parameters are selected differently depending on the mask master used in the projection exposure apparatus.

[0037] According to one embodiment, the above steps are performed during the operation of the microlithography projection exposure apparatus.

[0038] The present invention also relates to an apparatus for determining the heating state of a mirror in an optical system, particularly in a microlithography projection exposure apparatus, wherein the apparatus is configured to perform a method having the above-described features. For advantages and preferred configurations of the apparatus, refer to the above explanation relating to the method according to the invention.

[0039] Other embodiments of the invention will be apparent from the description and dependent claims.

[0040] The invention will now be explained in more detail with reference to the exemplary embodiments shown in the accompanying drawings. Attached Figure Description

[0041] In the attached diagram:

[0042] Figure 1 A schematic diagram of a possible structure for a microlithography projection exposure apparatus designed for operation in EUV is shown.

[0043] Figure 2 A graph illustrating, by way of example, different time curves of calibration parameters caused by different lighting settings and determined and used in the method according to the invention;

[0044] Figure 3 A schematic diagram showing the time curves of the corresponding measurement signals at different sensor locations compared to the actual average temperature at the surface of the optical element is presented.

[0045] Figure 4 A schematic diagram illustrating a possible structure for explaining a reflector, which allows the method according to the invention to be implemented in an exemplary manner; and

[0046] Figure 5 a-5d shows a schematic diagram used to explain the problems encountered in conventional methods for determining the thermal state of a mirror. Detailed Implementation

[0047] Figure 1 A schematic diagram of a projection exposure apparatus 1 is shown, which is designed for operation in EUV, and in which the present invention can be implemented by way of example. The description of the basic structure and components of the projection exposure apparatus 1 should not be construed as limiting thereto.

[0048] One embodiment of the illumination system 2 of the projection exposure apparatus 1 includes an illumination optics unit 4, in addition to a light source or radiation source 3, for illuminating the object field 5 in the object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0049] Here, the mask master 7, arranged in the object field 5, is exposed. The mask master 7 is held by the mask master support 8. The mask master support 8 can be moved by the mask master displacement driver 9, especially in the scanning direction. For illustrative purposes, in Figure 1 The diagram illustrates a Cartesian xyz coordinate system. The x-direction extends perpendicularly to the drawing plane. The y-direction extends horizontally, and the z-direction extends vertically. The scanning direction is along... Figure 1 It extends in the y-direction. The z-direction extends perpendicular to the object plane.

[0050] Projection lens 10 is used to image the object field 5 onto the image field 11 in the image plane 12. The structure on the mask master 7 is imaged onto the photosensitive layer of the wafer 13, which is 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 moved by a wafer displacement driver 15, specifically longitudinally relative to the y-direction. The movement of the mask master 7 by the mask master displacement driver 9 and the movement of the wafer 13 by the wafer displacement driver 15 can be synchronized.

[0051] Radiation source 3 is an EUV radiation source. Radiation source 3 specifically emits EUV radiation, which is also referred to hereinafter as working radiation or illumination radiation. In particular, the working radiation has a wavelength in the range of 5 nm to 30 nm. Radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). Illumination radiation 16 emitted from radiation source 3 is focused by light collector 17 and propagates through an intermediate focal point in intermediate focal plane 18 to illumination optical unit 4. Illumination optical unit 4 includes a deflector 19, and a first facet mirror 20 (with facet 21 schematically shown) and a second facet mirror 22 (with facet 23 schematically shown) disposed downstream of deflector mirror 19 in the optical path.

[0052] The projection lens 10 includes multiple mirrors Mi (i = 1, 2, ...), which are sequentially numbered according to their arrangement in the optical path of the projection exposure device 1. Figure 1 In the example shown, the projection lens 10 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have through-holes for illumination radiation 16. The projection lens 10 is a double-shielded optical unit. The image-side numerical aperture of the projection lens 10 is greater than 0.5, and may also be greater than 0.6, for example, 0.7 or 0.75.

[0053] During the operation of the microlithography projection exposure apparatus 1, electromagnetic radiation incident on the optically effective surface of the mirror is partially absorbed, and as explained in the introduction, this leads to heating and associated thermal expansion or deformation, which in turn impairs the imaging characteristics of the optical system. The temperature of the mirror can now be properly controlled by a suitable correction device (e.g., a preheater).

[0054] Reference Figures 2 to 4 The following describes embodiments of the method according to the invention, which is used to estimate the heating state of the mirror in question accordingly and may be used to control a suitable correction device, such as a preheater. The concept according to the invention for determining the heating state can be particularly advantageously applied here. Figure 1 Any desired reflector in the microlithography projection exposure apparatus 1.

[0055] Figure 4 A possible arrangement of the temperature sensor 420 at an optical element 400 in the form of a mirror is shown in purely schematic form. According to... Figure 4 (However, the invention is not limited thereto), the temperature sensor 420 is located in a drilled hole that serves as an access channel 411 leading from the back of the reflector into the substrate 410 of the reflector. “405” indicates the reflective layer system, and “401” indicates the optically effective surface or incident surface of the reflector for electromagnetic radiation.

[0056] The common feature of the embodiments described below is that, for all use cases or operating states in the optical system, the calibration parameter used to estimate the average temperature present on the incident surface of the optical element or mirror based on sensor-based temperature measurements is not chosen to be constant and fixed, but rather is selected differently depending on the currently set illumination settings in the optical system (and possibly also depending on the mask master used in each case). If the calibration parameter is represented by "q", the average temperature sought at the incident surface of the optical element or mirror is represented by "T", and the temperature measured by means of a sensor is represented by "TS", then the following formula applies:

[0057] T = q·T S (I)

[0058] According to the first embodiment, the calibration parameters for the currently set illumination setting in the optical system can be determined based on reference calibration parameters previously determined for the reference illumination setting. In this case, if a single temperature measurement occurring at a single point in time is initially taken as the starting point, and if it is assumed that the average temperature T sought at the incident surface and the associated sensor-based measured temperature To are... S If the reference lighting setup is known, then the reference lighting setup (corresponding to a previously known use case) and the lighting setup currently set in the optical system (corresponding to an unknown use case) can be related to each other, as described below, wherein, for example, the combined power behind the mask master can be used for both use cases.

[0059] If the power is used for reference lighting settings or for a previously known usage scenario, it is provided by This indicates that the power is used for the currently set lighting configuration or for unknown usage scenarios. This means that, under the assumption of a linear relationship, the following situation occurs:

[0060]

[0061] This results in the following calibration parameters for the currently set lighting settings or unknown usage scenarios:

[0062]

[0063] Therefore, depending on the current lighting settings or usage scenario, different calibration parameters q will appear. U .

[0064] In this regard, Figure 2 Only the calibration parameter q is shown for three different use cases (represented by UC1, UC2, and UC3, respectively). U An exemplary possible time curve. For use case UC1, the value of the calibration parameter is always q. U <1. For application scenario UC2, the calibration parameter q U The value of q deviates from 1 only for a relatively short period after the optical system has been put into operation. In use case UC3, q U <1 is suitable for relatively short periods of time, q U >1 is suitable for relatively long periods of time, and the calibration parameter q U (Corresponding to "overshoot") It increases temporarily and relatively violently on a medium time scale.

[0065] In embodiments of the invention, temperature measurements can also be performed using at least one temperature sensor 420 to determine a time profile. Thus, it can be considered that, depending on the position of the temperature sensor, temperature changes occurring on the incident surface of the optical element or mirror become "visible" on the temperature sensor at different rates; that is, the corresponding temperature sensor "sees" the temperature changes with a delay.

[0066] to this end, Figure 3 A graph showing the time curves of the corresponding measurement signals plotted for two different sensor locations, compared to the actual average temperature at the incident surface of the optical element. According to... Figure 3 For example, after the optical system is turned on, the average temperature T sought at the incident surface rises immediately, while sensors located at different distances from the incident surface transmit measurement signals with corresponding delays. The following heat conduction equation is used:

[0067]

[0068] Utilizing thermal conductivity and source For example, under simplified boundary conditions (e.g., assuming an infinitely large optical element with a surface parallel to the xy plane, and assuming a uniform optical load), the corresponding sensor-based measurement temperature T can be specified. S The distance r between the temperature sensor and the incident surface Z The following correlations (only as examples of analytical relationships, and the invention is not limited thereto):

[0069]

[0070] The result is that the temperature T measured by the temperature sensor based on its location... S Therefore, it can be determined as a function of time and used to determine calibration parameters, thereby providing a reliable characterization of the heating state of optical elements or mirrors, and is used immediately after the optical system is turned on for the aforementioned startup phase.

[0071] In a further embodiment, the calibration parameter q can be performed. U The numerical values ​​are determined by using, in this case, an optical system model that is as realistic as possible as possible, in addition to knowing the current lighting setup and mask master. Based on this, the temperature distribution settings within the optical elements can be predicted, and the temperature T that the temperature sensor will measure at its corresponding location can be calculated. S Based on this, the calibration parameter q can be determined using formula (1). U .

[0072] If the lighting settings currently in use are not specifically known, you can also select the lighting settings that are closest to the desired settings or the lighting settings currently in use from the previously created lighting settings library (lookup table).

[0073] In a further embodiment, temperature measurement or simulation can be replaced by wavefront measurement or simulation, thereby using a wavefront (e.g., a wavefront determined in a region of the wafer plane) to draw conclusions about thermal changes in the optical element or mirror. In this case, the calibration parameter q can also be determined by classifying the currently used lighting setup by comparing the deformation determined based on the measured wavefront with the deformation of a pre-calculated lighting setup. U .

[0074] Although the invention has been described based on specific embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example, through combinations and / or exchanges of features of the various embodiments. Therefore, it will be understood by those skilled in the art that such variations and alternative embodiments are also included in the invention, and that the scope of the invention is limited only to the meaning of the appended claims and their equivalents.

Claims

1. A method for determining the heating state of an optical element in an optical system, wherein, During operation of the optical system, electromagnetic radiation is incident on the incident surface (401) of the optical element (400). - Wherein, the average temperature present at the incident surface (401) is estimated using calibration parameters based on temperature measurements performed using at least one temperature sensor (420), which is arranged at a certain distance from the incident surface (401); and - Wherein, the calibration parameters are selected differently depending on the illumination settings set in the optical system. - Wherein, the calibration parameters are determined based on at least one measurement or simulation of a variable of the currently set illumination setting in the optical system, the variable depending on the thermal state of the optical element (400), and - in, The variable, which depends on the thermal state of the optical element (400), is the intensity distribution generated in a plane downstream of the optical element (400) relative to the optical path during operation of the optical system; or The variable that depends on the thermal state of the optical element (400) is the wavefront generated in a plane located downstream of the optical element (400) relative to the optical path during the operation of the optical system.

2. The method as described in claim 1, characterized in that, The calibration parameters are determined based on reference calibration parameters previously established for the reference lighting setup.

3. The method as described in claim 1 or 2, characterized in that, The variable, which depends on the thermal state of the optical element (400), is the intensity distribution generated in a plane located downstream of the optical element (400) relative to the optical path during the operation of the optical system.

4. The method as described in claim 1 or 2, characterized in that, The variable that depends on the thermal state of the optical element (400) is the wavefront generated in a plane located downstream of the optical element (400) relative to the optical path during the operation of the optical system.

5. The method as described in claim 1 or 2, characterized in that, The calibration parameters are also determined by taking into account the distance between the temperature sensor (420) and the incident surface (401).

6. The method as described in claim 1 or 2, characterized in that, Temperature measurements are repeatedly performed using the temperature sensor (420) to determine the time curve.

7. The method as described in claim 1 or 2, characterized in that, The temperature sensor (420) is arranged in an access channel (411) extending from the side of the optical element (400) away from the incident surface (401) into the optical element (400).

8. The method as described in claim 1 or 2, characterized in that, The estimated average temperature is used as an input signal for closed-loop control of at least one parameter characterizing the optical element (400) and / or the optical system.

9. The method as described in claim 1 or 2, characterized in that, The preheating of the optical element (400) is controlled based on the estimated average temperature, thereby at least partially compensating for changes in the heating state of the optical element (400) over time during the operation of the optical system.

10. The method as described in claim 1 or 2, characterized in that, The optical element (400) is a reflector.

11. The method as described in claim 1 or 2, characterized in that, The optical element (400) is designed for operating wavelengths less than 30 nm.

12. The method as described in claim 11, characterized in that, The operating wavelength is less than 15 nm.

13. The method as described in claim 1 or 2, characterized in that, The optical system is a microlithography projection exposure device.

14. The method as described in claim 13, characterized in that, The calibration parameters are selected differently depending on the mask master used in the microlithography projection exposure equipment.

15. The method as described in claim 13, characterized in that, The average temperature is estimated during the operation of the microlithography projection exposure apparatus.

16. An apparatus for determining the heating state of a mirror in an optical system, characterized in that, The apparatus is configured to perform the method as described in any one of the preceding claims.

17. The apparatus as claimed in claim 16, characterized in that, The optical system is a microlithography projection exposure device.

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