Methods for characterizing the thermal expansion properties of an optical component in an optical system, as well as methods for operating an optical system

Abstract

The invention relates to a method for characterizing the thermal expansion properties of an optical component in an optical system, and to a method for operating an optical system. The optical component has a substrate made of a substrate material with a coefficient of thermal expansion whose temperature dependence exhibits zero crossing at a zero-crossing temperature (ZCT). Based on measurement data acquired at at least two different times during the operation of the optical component, a rate of change over time for the zero-crossing temperature is determined.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The invention relates to a method for characterizing the thermal expansion properties of an optical component in an optical system, and to a method for operating an optical system. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a projection exposure system, which includes an illumination unit and a projection lens. The image of a mask (= reticulum) illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens. This transfers the mask structure onto the photosensitive coating of the substrate.

[0003] In projection lenses designed for the EUV range, i.e. at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of availability of suitable translucent refractive materials.

[0004] One problem that arises in practice is that the EUV mirrors, among other things, experience heating (also known as "mirror heating") and a concomitant thermal expansion or deformation as a result of absorption of the radiation emitted by the EUV light source, which in turn can impair the imaging properties of the optical system or the projection exposure system.

[0005] Known approaches to avoiding surface deformations caused by heat input into an EUV mirror, and the associated optical aberrations, include using an ultra-low thermal expansion material as the mirror substrate, such as a titanium dioxide (TiO2)-silicon dioxide (SiO2) glass-ceramic marketed under the name ULE™ by Corning Inc., or a lithium aluminum-silicon oxide glass-ceramic marketed under the name Zerodur™ by Schott AG, and setting the so-called zero-crossing temperature (ZCT) in a region close to the optical surface. At this zero-crossing temperature, which, for example,For ULE™, at approximately ϑ = 30°C, the coefficient of thermal expansion exhibits a zero crossing in its temperature dependence, around which there is no or only a negligible dependence of the thermal expansion of the mirror substrate material on occurring temperature variations. Furthermore, heating and control concepts are known in which active mirror heating is achieved by using a heating arrangement (e.g., based on infrared radiation) during phases of comparatively low absorption of EUV useful radiation, with this active mirror heating being reduced accordingly as the absorption of the EUV useful radiation increases.

[0006] Given the high accuracy requirements in the microlithography process, and despite the availability of the aforementioned materials with ultra-low thermal expansion and common heating and control concepts, the sufficient avoidance of thermally induced aberrations presents a demanding challenge in practice for several reasons.

[0007] One problem that arises here is that the zero-crossing temperature varies locally within the respective mirror substrate material. This is due, for example, to locally differing mixing ratios of the respective material components (e.g., titanium dioxide and silicon dioxide in ULE™). Given the typical deviation of operating temperatures from the manufacturing temperature, this leads to alignment errors that correspond to the respective deviation of the locally established operating temperature from the locally present zero-crossing temperature. Known approaches to overcoming this problem include incorporating a corresponding correction pass (i.e., a "pass allowance") as well as including specific sensitivity or reference tables to compensate for thermally induced aberrations occurring during operation using manipulators present in the optical system.

[0008] In practice, however, the further problem arises that the zero-crossing temperature mentioned above is not stable over a longer period of time, but is subject to a drift over time. Fig. Figure 2 merely illustrates possible, qualitative time profiles of the zero-crossing temperature for different materials. This temporal drift of the zero-crossing temperature, in turn, means that the aforementioned heating and control concepts may be based on zero-crossing temperature values ​​that are no longer valid (i.e., "outdated"), which leads to insufficient aberration correction and thus to image defects and impaired performance in the lithography process.

[0009] For the state of the art, reference is made only to DE 10 2022 114 969 A1 and DE 10 2019 219 289 A1 by way of example. SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide a method for characterizing the thermal expansion properties of an optical component in an optical system and a method for operating an optical system which enables effective avoidance of surface deformations caused by heat input into the optical element and associated optical aberrations, while at least partially avoiding the problems described above.

[0011] This problem is solved according to the features of the dependent patent claims.

[0012] According to one aspect, the invention relates to a method for characterizing the thermal expansion properties of an optical component in an optical system, in particular for microlithography, wherein the optical component has a substrate made of a substrate material with a coefficient of thermal expansion whose temperature dependence has a zero crossing at a zero-crossing temperature (ZCT), wherein a rate of change over time for the zero-crossing temperature is determined based on measurement data acquired at at least two different times during the operation of the optical component.

[0013] The wording “based on [...] measurement data acquired during the operation of the optical component” is to be understood in the context of the present application as encompassing both the acquisition of measurement data for an optical system comprising the component (into which the component is already installed) and the acquisition of measurement data for the optical component before its installation in the optical system (in particular, for example, based on fit measurements already carried out on the optical component during the manufacturing stage).

[0014] According to one embodiment, the measurement data each comprise a residual image error distribution of the optical system. This residual image error distribution corresponds to a distribution of imaging errors, wherein the respective imaging error can be, in particular, a coefficient of the decomposition of the optical wavefront into Zernike polynomials, and / or an RMS (squared mean) value of this wavefront, and / or an image position error, and / or a distortion error, and / or a focus error, and / or an astigmatism contribution, and / or a coma contribution, and / or a feature size variation of a predetermined structure under a fixed illumination.

[0015] According to one embodiment, the measurement data each include the manipulator position distribution of at least one manipulator in the optical system.

[0016] The relevant times at which the aforementioned measurement data are recorded are preferably chosen such that at these times there is a substantially similar thermal load on the optical component.

[0017] The invention is based in particular on the concept of using measurement data generated during the operation of the optical system or the projection exposure system (which, as described below, can relate to wavefront errors or aberrations occurring during the operation of the respective optical system, or alternatively or additionally to travel paths of manipulators used in the optical system) at different times and then assigning to these measurement data a temporal drift in the respective zero-crossing temperature of an optical component that is at least partially causal with a high probability.

[0018] According to the invention, a rate of change for the zero crossing temperature can thus be derived from measurements of a residual image error distribution occurring during the operation of the respective optical system and / or from a manipulator position distribution occurring during the operation of the respective optical system at at least two different times with preferably essentially the same thermal load, which corresponds with a high probability to or is the cause of the measured distributions.

[0019] The invention takes advantage of the fact that the corresponding measurement data for the wavefront, used according to the invention for determining the drift of the zero-crossing temperature, and the manipulator travel paths required for the respective adjustment of the wavefront are already generated or known and therefore, according to the invention, only need to be selected at the appropriate, well-defined times.

[0020] By determining the rate of change over time for the zero crossing temperature according to the invention, aberrations can be avoided which would result from implementing a “correction concept” (e.g., a common heating and control concept) based on an outdated or no longer accurate value of the zero crossing temperature.

[0021] According to a further aspect of the invention, the measurement data for determining a rate of change over time for the zero crossing temperature can also be acquired at least partially by means of pass measurements on the optical component before its installation in the optical system.

[0022] According to this aspect, the determination of zero-crossing temperature drift can be carried out by performing at least two pass measurements on the respective optical component or mirror at a stage in which the optical component has not yet been delivered or integrated into the corresponding optical system or microlithographic projection exposure unit. For example, these pass measurements can be repeated at suitable intervals (e.g., more than 50 days, particularly more than 100 days), and the result can be assigned to a corresponding drift in the zero-crossing temperature. During these pass measurements, the optical component or mirror is preferably already of sufficiently good optical quality, with only a few processing steps, e.g., for fine roughness correction, remaining.In accordance with this aspect, the invention takes advantage of the fact that optimal access to the optical component is still possible at the aforementioned manufacturing stage, which is why this stage is particularly suitable for the determination of a drift in the zero crossing temperature according to the invention.

[0023] If the time-dependent rate of change for the zero-crossing temperature determined in this way exceeds a predetermined limit for which sufficient compensation of the drift is still assumed, the use of the optical component in question may be rejected, i.e., the installation of this component in the optical system may be avoided from the outset.

[0024] Furthermore, depending on the determined rate of change over time for the zero-crossing temperature, the component can also be selectively installed at a position in the optical system where there is a comparatively low sensitivity to changes over time in the zero-crossing temperature.

[0025] The rate of change over time for the zero crossing temperature determined according to the invention can be, for example, at least 0.1 K / year, in particular 0.2 K / year, and further, in particular, at least 0.5 K / year.

[0026] Once a rate of change or drift over time for the zero-crossing temperature has been determined according to the invention, common correction concepts (e.g., a common heating and control concept) can be adapted to the changed zero-crossing temperature accordingly, so that, as a result, a more precise avoidance of thermally induced aberrations is achieved.

[0027] The invention is not further limited with regard to the specific correction concepts applied. Rather, any suitable correction concepts known in the prior art can be used, e.g., based on the use of preheaters, sector heaters or cooling devices, as well as on the basis of locally deformable (adaptive) optical elements or mirrors, or on the basis of optical elements that can be displaced in the direction of light or perpendicular to it, as well as on the basis of optical elements that can be rotated about the direction of light or about axes perpendicular to it.

[0028] In one embodiment, for example, in the respective correction concept, the target value of a thermal control can be maintained at any time within a temperature band of 4 K, in particular within a temperature band of 2 K, and further, in particular within a temperature band of 1 K around the current zero crossing temperature using a thermal manipulator.

[0029] The invention further relates to a method for operating an optical system, wherein the optical system comprises at least one optical component, and wherein the method comprises the following steps: - Determining a rate of change over time for the zero-crossing temperature (ZCT) of the substrate material using a method with the features described above; and - Adjusting a correction of thermally induced aberrations performed in the optical system depending on the determined rate of change over time for the zero crossing temperature (ZCT).

[0030] According to one embodiment, the optical element is a mirror.

[0031] According to one embodiment, the optical element is designed for a working wavelength of less than 400 nm, in particular less than 250 nm, and further in particular less than 200 nm.

[0032] According to one embodiment, the optical element is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

[0033] Further embodiments of the invention can be found in the description and the dependent claims.

[0034] The invention is explained in more detail below with reference to exemplary embodiments shown in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] They show: Fig. 1 a schematic representation of the possible setup of a microlithographic projection exposure system designed for operation in the EUV; Fig. Two possible time profiles of the zero-crossing temperature for different materials. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0036] Fig. Figure 1 shows a schematic representation of a projection exposure system 1 designed for operation in the EUV, in which the invention can be implemented, for example.

[0037] According to Fig. The projection exposure system 1 comprises a lighting device 2 and a projection lens 10. The lighting device 2 serves to illuminate an object field 5 in an object plane 6 with radiation from a radiation source 3 via a lighting optic 4. A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be moved, particularly in a scanning direction, via a reticle displacement drive 9. Fig. Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. The scan direction runs in Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane. 6.

[0038] The projection lens 10 serves to image the object field 5 onto an image field 11 in an image plane 12. A structure on the reticulum 7 is imaged onto a light-sensitive layer of a wafer 13 located in the image field 11 within the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved, particularly along the y-direction, via a wafer transfer drive 15. The movement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized.

[0039] The radiation source 3 is an EUV radiation source. In particular, the radiation source 3 emits EUV radiation, which is also referred to below as useful radiation or illumination radiation. The useful radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). The illumination radiation 16 emanating from the radiation source 3 is focused by a collector 17 and propagated through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflecting mirror 19 and, downstream in the beam path, a first faceted mirror 20 (with facets 21 shown schematically) and a second faceted mirror 22 (with facets 23 shown schematically).

[0040] The projection lens 10 has a plurality of mirrors Mi (i = 1, 2, ...) which are numbered according to their arrangement in the beam path of the projection exposure system 1. In the Fig. In the example shown, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection lens 10 is a double-obscured optical system. The projection lens 10 has an image-side numerical aperture, which, for example, can be larger than 0.3, and in particular larger than 0.5, and furthermore, larger than 0.6.

[0041] During operation of the microlithographic projection exposure system 1, some of the electromagnetic radiation incident on the optical surface of the mirrors is absorbed, leading, as explained above, to heating and a concomitant thermal expansion or deformation, which in turn can impair the imaging properties of the optical system. To avoid thermally induced aberrations, a suitable correction concept (e.g., a common heating and control concept) is implemented in the projection exposure system 1. For example, a thermal manipulator in the form of a heating arrangement can be used to actively heat the mirrors during phases of comparatively low absorption of EUV radiation, with this active heating being reduced accordingly as the absorption of the EUV radiation increases. Fig.Figure 1 merely schematically shows a heating arrangement and is labelled "25", where this heating arrangement 25 serves in the example to introduce heating power into the mirror M3.

[0042] It is further assumed that at least one of the mirrors has a substrate made of a substrate material such as ULE™ or Zerodur™ with a coefficient of thermal expansion whose temperature dependence exhibits zero crossing at a zero-crossing temperature (ZCT). According to the invention, a rate of change over time for the zero-crossing temperature (ZCT) is determined for this mirror. If necessary, the correction concept implemented in the optical system can then be adjusted accordingly, depending on the determined rate of change over time for the zero-crossing temperature (ZCT).

[0043] According to one aspect, the invention, when determining the rate of change over time for the zero-crossing temperature (ZCT), is based on the consideration that changes in the zero-crossing temperature generate individual residual image error distributions depending on the operating state (in particular, depending on the local illumination according to a specific temperature pattern). Furthermore, depending on the operating state, individually different manipulator travel paths of manipulators present in the projection exposure system 1 are used.

[0044] The aforementioned effects can be determined through simulation. For this purpose, based on the hypothesis of a globally uniform change, it can first be simulated which aberration pattern results from a given temporal change in the zero-crossing temperature. Furthermore, the thermal load present during operation of the optical system can be estimated based on the set illumination, and the distribution of residual image errors and / or travel distances to be expected at a given time during operation for an assumed change in the zero-crossing temperature can be determined. This expectation can then be compared with the measured aberrations, the set manipulator positions, or the manipulator position distribution.

[0045] This approach preferably considers the difference between different operating states where the optical component is subject to essentially the same thermal load. This difference depends on the zero-crossing temperature and can therefore be predicted by simulation.

[0046] The adjustment can include the magnitude of the zero-crossing temperature change as a free parameter. If a value exists for the rate of change of the zero-crossing temperature over time at which the calculated residual image error and / or manipulator travel differences between two operating states are essentially the same as the actually observed values, it is assumed that the zero-crossing temperature has changed accordingly. This assumption regarding the rate of change of the zero-crossing temperature over time can be verified by repeated comparisons during the same or subsequent state changes and / or by comparison with previous system behavior.

[0047] According to a further aspect of the invention, the measurement data used to determine the rate of change of the zero-crossing temperature can also be acquired before the optical component in question is installed in the respective optical system or the microlithographic projection exposure system, and in particular by means of pass measurements on this optical component during its manufacturing stage. During manufacturing, a pass change between two thermally different states can be determined at intervals of preferably more than 50 days, and more preferably more than 100 days. For example, the mean temperature between the thermally different states can differ by more than 1 K, and more preferably more than 2 K, and more preferably more than 4 K.Based on these pass measurements, a rate of change of the zero crossing temperature of the material is then determined (if necessary after normalization to a similar temperature change).

[0048] Depending on whether the optical element can be cooled exclusively or heated exclusively, the (initial) zero-crossing temperature targeted during the manufacturing process can preferably be selected appropriately. In particular, the initial zero-crossing temperature targeted during the manufacturing process can be set above a maximum expected operating temperature if heating is the only option, and below a minimum expected operating temperature if cooling is the only option. This allows for regulation even if the zero-crossing temperature changes over time, regardless of whether it falls or rises compared to the initial value.

[0049] The knowledge of the rate of change of the zero crossing temperature obtained according to the invention can also be used to insert several (i.e. at least two) optical components or mirrors in the optical system in such a way that their respective drifts of the zero crossing temperature at least partially compensate each other in combination.

[0050] Even though the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, for example, through the combination and / or exchange of features of individual embodiments. Accordingly, it is understood to the person skilled in the art that such variations and alternative embodiments are included in the present invention, and that the scope of the invention is limited only to the extent of the appended claims and their equivalents. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2022 114 969 A1

[0009] DE 10 2019 219 289 A1

[0009]

Claims

[1] Method for characterizing the thermal expansion properties of an optical component in an optical system, in particular for microlithography, wherein the optical component has a substrate made of a substrate material with a coefficient of thermal expansion whose temperature dependence has a zero crossing at a zero-crossing temperature (ZCT), characterized by , that a rate of change over time for the zero crossing temperature is determined based on measurement data recorded at at least two different times during the operation of the optical component. [2] Method according to claim 1, characterized by that the measurement data each include a residual image error distribution of the optical system. [3] Method according to claim 1 or 2, characterized by that the measurement data each include the manipulator position distribution of at least one manipulator in the optical system. [4] Method according to any one of claims 1 to 3, characterized by that the measurement data are at least partially acquired by means of fit measurements on the optical component before its installation in the optical system. [5] Method according to any one of the preceding claims, characterized by , that depending on the determined rate of change over time for the zero crossing temperature (ZCT), a correction of thermally induced aberrations carried out in the optical system is adjusted. [6] Method for operating an optical system, wherein the optical system has at least one optical component, the method comprising the following steps: - Determining a rate of change over time for the zero-crossing temperature (ZCT) of the substrate material using a method according to any of the preceding claims; and - Adjusting a correction of thermally induced aberrations performed in the optical system depending on the determined rate of change over time for the zero crossing temperature (ZCT). [7] Method according to any one of the preceding claims, characterized by that the optical element is a mirror. [8] Method according to any one of the preceding claims, characterized by that the optical element is designed for an operating wavelength of less than 400 nm, in particular less than 250 nm, and further in particular less than 200 nm. [9] Method according to any one of the preceding claims, characterized by that the optical element is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

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