Method and device for determining the heating state of an optical element of a microlithography optical system

Abstract

The invention relates to a method and a device for determining a heating state of an optical element in a microlithography optical system, wherein at least one non-contact sensor (330, 430, 531, 532, 533, 534, 631, 632) is used, the at least one non-contact sensor being based on receiving electromagnetic radiation from the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601), and wherein the range of the radiation captured by the sensor (330, 430, 531, 532, 533, 534, 631, 632) varies to determine the temperature distribution in the optical element.

Description

[0001] This application claims the priority of German patent application DE 10 2018 212 400.0 filed on July 25, 2018. The contents of this DE application are incorporated into the text of the present application by reference. Background of the Invention Technical Field

[0003] The invention relates to a method and a device for determining a heating state of an optical element in a microlithography optical system. Background Art

[0004] Microlithography is used, for example, for the production of microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a facility called a projection exposure apparatus, which comprises an illumination device and a projection lens. In this case, an image of a mask (=reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g. a silicon wafer) which is coated with a photosensitive layer (photoresist) and is arranged in the image plane of the projection lens, in order to transfer the mask structure to the photosensitive coating of the substrate.

[0005] 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 available light-transmitting refractive materials. One problem that arises in practice is that, due to, for example, absorption of radiation emitted by the EUV light source, the EUV mirrors heat up and are subject to associated thermal expansion or deformation, which in turn may have a negative impact on the imaging properties of the optical system.

[0006] One known method of taking this effect into account is to use materials with ultra-low thermal expansion ("ultra-low expansion materials"), such as those sold by Corning under the name ULE TM Titanium silicate glass is used as the mirror substrate material and the so-called zero-crossing temperature is set in the area near the optically active surface. At this zero-crossing temperature (e.g. for ULE TM The thermal expansion coefficient has a zero crossing in its temperature dependence, near which the reflector substrate material does not expand thermally or only undergoes negligible thermal expansion.

[0007] In practice, however, the problem arises here that the EUV mirrors are exposed during operation of the microlithography projection exposure apparatus to locally and temporally varying intensity of the incident electromagnetic radiation, in particular this local variation being due to the use of an illumination setup with an intensity which varies over the optically active surface of the respective EUV mirror, wherein the relevant EUV mirrors are typically heated, in particular at the start of the microlithography exposure process, from a relatively low temperature to the operating temperature which they reach during the lithography process.

[0008] One way to overcome the above problems and in particular to avoid surface deformations caused by varying the amount of heat introduced into the EUV mirror and the associated optical aberrations consists in using a pre-heater, for example based on infrared radiation. By means of such a pre-heater, active mirror heating can take place during phases in which the absorption of useful EUV radiation is relatively low, wherein the active mirror heating decreases correspondingly as the absorption of useful EUV radiation increases.

[0009] The regulation of the operation of such a preheater with the goal of maintaining the mirror temperature as constant as possible (typically the above-mentioned zero-crossing temperature) requires knowledge of the radiation power incident on the relevant mirror in each case, so that the preheating power can be adjusted accordingly. For this purpose, temperature sensors are used (in addition to infrared cameras, which are not always practical due to installation space), for example in the form of thermocouples or resistance-based (e.g. NTC) temperature sensors, which can typically be mounted in a force-matched or adhesively bonded manner at different locations on the respective mirror.

[0010] However, by installing such thermocouples, undesirable mechanical stresses may firstly be induced in the mirror substrate, wherein moreover - in particular when a large number of temperature sensors are required to determine the spatially varying temperature distribution within the mirror - the manufacturing complexity is significantly increased and the mechanical stability of the mirror may be impaired.

[0011] With regard to the prior art, reference is made purely by way of example to DE 36 05 737 A1 and DE 10 2005 004 460 A1. Summary of the invention

[0012] The object of the present invention is to provide a method and a device for determining the heating state of an optical element in a microlithography optical system, which can grasp the heating state as accurately as possible and at the same time avoid the above-mentioned problems.

[0013] This object is achieved by a method according to the features of independent patent claim 1 and by a device according to the features of the associated patent claim 13 .

[0014] In the method according to the invention for determining the heating state of an optical element in a microlithography optical system, at least one contactless sensor is used which is based on receiving electromagnetic radiation from the optical element, wherein the range of the radiation captured by the sensor is varied in order to achieve the purpose of determining the temperature distribution in the optical element.

[0015] In particular, the invention is based on the idea of ​​using a contactless sensor to achieve a contactless determination of the heating state of a lithographic optical element, which is based on receiving electromagnetic or thermal radiation from the optical element. Here, according to the invention, an accurate temperature measurement - for example, a thermocouple attached to a mirror or an NTC sensor can be used - is deliberately omitted, and furthermore, possible increased costs in terms of equipment and increased requirements for installation space (for example, the use of a pyrometer) are accepted.

[0016] In return, firstly, according to the invention, the introduction of mechanical stresses into the optical element or the reflector which is accompanied by the conventional use of, for example, the described thermocouples or NTC sensors is avoided. Secondly, as will be explained in more detail below, the concept according to the invention of using a non-contact, radiation-based sensor and a variation of the radiation range captured by the sensor - achievable in different embodiments - can also be used to determine the (typically locally variable) temperature distribution in the optical element, wherein because of the described "area sensing" of the optical element, this temperature distribution can already be determined by one or only a few non-contact sensors (e.g. achievable optically and / or mechanically as described below).

[0017] Due to the use of a contactless radiation-based sensor according to the invention, in principle all mechanical manipulation of the optical element whose heating state is to be measured can be omitted. However, in other embodiments of the invention, the geometry of one or more (in particular refractive) surfaces on the optical element (for example on the back side of the reflector) can also be additionally designed in a targeted manner in order, for example, in combination with a lateral and / or axial movement of the contactless radiation-based sensor relative to the reflector, to additionally assist in an appropriate manner the area sensing of the reflector. In particular, according to the invention, at least one cutout or recess and / or at least one elevated structure can be provided in the optical element on the side facing the sensor, as a result of which, for example, in combination with a lateral movement of the contactless radiation-based sensor, (heat) radiation from different "heights" of the optical element reaches the sensor.

[0018] In principle, the signal captured by the contactless sensor according to the invention represents a superimposed signal of contributions from different areas of the optical element - in contrast to the substantially precise information obtained by, for example, thermocouples or NTC temperature sensors. However, according to the invention, the spatial temperature distribution in the optical element with a high spatial resolution can be determined based on simulations and / or by comparison with previously carried out calibration measurements and based on a model.

[0019] According to one embodiment, a beam shaping optical unit located between the optical element and the sensor is used to achieve a change in the area of ​​radiation captured by the sensor. By this change, area sensing of the optical element can be achieved.

[0020] According to one embodiment, the beam shaping optical unit comprises a zoom axicon system. In other embodiments of the beam shaping optical unit, one or more optical elements with a suitable (e.g. wedge-shaped or cylindrical) geometry arranged to be displaceable and / or rotatable (in particular also relative to each other) may be used between the optical element and the contactless sensor for the purpose of varying the radiation area captured by the sensor.

[0021] According to one embodiment, the variation of the radiation area captured by the sensor comprises a manipulation of the relative position between the optical element and the sensor.

[0022] According to one embodiment, the optical element is a mirror.

[0023] According to one embodiment, the optical element is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0024] Based on the determination of the heating state, according to one embodiment, there is a preheating of the optical element for at least partially compensating for changes in the heating state of the optical element over time, which changes occur during operation of the optical system. In other embodiments, compensation of optical aberrations caused by the heating state in the optical system can also be performed by suitable manipulators (e.g. adaptive mirrors). Alternatively or additionally, a corresponding compensation of changes in gas pressure, radiation intensity, radiation wavelength and / or illumination settings in the corresponding optical system can also be performed.

[0025] According to one embodiment, the determination of the heating state is performed during operation of the optical system, eg a microlithography projection exposure apparatus.

[0026] The invention also relates to a device for determining the heating state of a mirror in an optical system, in particular in a microlithography projection exposure device, wherein the device comprises at least one contactless sensor based on receiving electromagnetic radiation from an optical element, and means for changing the radiation area captured by the sensor.

[0027] In particular, the device can be configured to carry out a method having the features described above. With regard to advantages and preferred configurations of the device, reference is made to the above explanations in conjunction with the method according to the invention.

[0028] Further configurations of the invention can be gathered from the description and the dependent claims.The invention is explained in more detail below based on exemplary embodiments illustrated in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings show:

[0030] Figure 1 A schematic diagram of a possible configuration for a microlithography projection exposure apparatus designed for operation in EUV; and

[0031] 2-6 are schematic diagrams for explaining possible embodiments of the method or device according to the present invention. DETAILED DESCRIPTION

[0032] Figure 1 Shown is a schematic diagram of a projection exposure apparatus 100 which is designed for operation in EUV and in which the present invention is implementable as an example.

[0033] according to Figure 1 , the illumination device of the projection exposure apparatus 100 comprises a field facet mirror 103 and a pupil facet mirror 104. Light from a light source unit comprising an EUV light source (plasma light source) 101 and a collector mirror 102 is directed onto the field facet mirror 103 in this example. A first telescopic mirror 105 and a second telescopic mirror 106 are arranged in the optical path downstream of the pupil facet mirror 104. A deflection mirror 107 is arranged downstream in the optical path, which deflection mirror directs the radiation incident thereon onto an object field in the object plane of a projection lens comprising six mirrors 121-126. At the location of the object field, a mask 131 carrying a reflective structure is arranged on a mask stage 130, the mask being imaged into an image plane by means of a projection lens, wherein a substrate 141 coated with a photosensitive layer (photoresist) is located on a wafer stage 140.

[0034] During operation of the microlithography projection exposure device 100, electromagnetic radiation incident on the optically active surface or entrance surface of the mirrors present can be partially absorbed and, as explained in the introduction, lead to heating and associated thermal expansion or deformations, which in turn can lead to impairment of the imaging properties.

[0035] The method or device according to the invention for determining the heating state of an optical element can be used, for example, in particular in Figure 1 On any desired mirror of the microlithography projection exposure apparatus 100.

[0036] In the following, different embodiments of the method and device according to the invention are described with reference to the schematic diagrams in Figures 2 to 6. These embodiments have in common that at least one contactless, radiation-based sensor is used in each case to determine the heating state.

[0037] Even though the non-contact sensor is implemented as a pyrometer in each exemplary embodiment described below, the present invention is not limited thereto. Other suitable non-contact radiation-based sensors, such as IR photodiodes or photomultiplier tubes, may also be used in other embodiments.

[0038] Furthermore, in the embodiments described below based on FIGS. 2 to 6 , a change in the radiation area captured by the relevant sensor is implemented in each case in combination with the non-contact radiation-based sensor used according to the invention in order to determine the typically locally varying temperature distribution within the optical element by area sensing of the optical element whose heating state is to be measured.

[0039] The above changes in the radiation area can be seen from Figures 2 to Figure 3a-3b The embodiments of the present invention are realized optically by a suitable upstream optical unit and, in addition or alternatively, can be realized mechanically or by manipulating the optical element with a contactless radiation-based sensor (according to FIGS. 4 to Figure 6 ) is achieved by the relative position between them.

[0040] The present invention is not limited to the embodiments illustrated in Figures 2 to 6; in particular, it should also be considered that the present invention covers the following configurations, in which the change of the radiation area captured by the non-contact sensor according to the present invention is achieved in a different way (in particular by any suitable optical and / or mechanical actuator), or in which such a change is completely omitted.

[0041] Again, refer to Figures 2 to Figure 3a-3b In a first exemplary embodiment, a zoom axicon system 210 is located between an optical element 301 in the form of a reflector (whose heating state is to be measured) and a non-contact radiation-based sensor 330 (configured as a pyrometer in the exemplary embodiment), and the zoom axicon system ( Figure 3a The following effects are achieved: Figure 3a The incident (heat) radiation at the non-contact sensor or pyrometer 330 originates from different areas of the optical element 301 (depending on Figure 2b ), where for different settings of the zoom axicon system these areas are Figure 2b In the figure, it is represented by 1 to 4 or 1' to 4'.

[0042] Due to the above-mentioned functionality of the upstream optical unit formed by the zoom axicon system, the use of a single non-contact sensor 330 (or at least the use of only a few sensors) already facilitates the area sensing of the optical element 301 and thus the determination of a typical spatially varying temperature distribution in this element 301, since the above-mentioned upstream optical unit facilitates a continuous variation of the radiation area captured by the sensor. In other words, what is achieved according to the invention is that during a scanning process, the heating state or the corresponding emitted thermal radiation of different areas of the optical element 301 is captured using one and the same sensor 330 and is projected onto the sensor 330.

[0043] In other embodiments, the variable (upstream) optical unit used between the optical element 301 and the contactless sensor 330 for varying the radiation area captured by the sensor 330 may also be implemented in any other suitable manner, for example by one or more displaceable and / or rotatable optical elements (so-called "Alvarez" elements) having a suitably curved optical refractive surface (e.g. in a wedge-shaped or cylindrical geometry). Furthermore, the element may also be combined with a displaceable and / or rotatable aperture (in the Figure 2a ) in order to limit the area of ​​the optical element 301 captured with respect to the detected radiation.

[0044] In accordance with Figure 3a In an exemplary embodiment of the present invention, the optical element 301 or the reflector has a tapered hole on its back side facing the sensor 330. Other embodiments of such holes, recesses or depressions on the optical element will be explained based on FIGS. 4 to 6. However, the present invention is not limited to its specific configuration or even to the presence of holes, recesses or depressions on the optical element.

[0045] Even in Figure 3a In the embodiment of the invention, the radiation captured by the sensor 330 is incident parallel to the optical axis (extending along the z-axis in the depicted coordinate system) on both the zoom axicon system 210 and the sensor 330, but the invention is not limited thereto. Therefore, in other embodiments, deviated beam profiles, such as divergent beam profiles, may also be achieved by appropriately adjusting the components of the zoom axicon system 310.

[0046] Figure 3b Other possible configurations of the zoom axicon system 310 are shown, in which a conical hole or an otherwise angled back side of the optical element is omitted. For this purpose, there is a targeted deviation from the mutually complementary form of the two elements 310a, 310b of the zoom axicon system 310 by different angles of the apex or by an adapted curvature of the cross section of at least one of the elements 310a, 310b of the zoom axicon system 310.

[0047] According to Figure 2 and Figure 3a-3b The described embodiment, due to the above-described functionality of the variable (upstream) optical unit, does not in principle require a displacement of the contactless sensor 330 relative to the optical element 301 for the purpose of determining a spatially varying temperature distribution. However, in addition to or as an alternative to the above-described optical realization of a variation of the radiation area captured by the contactless sensor, a manipulation of the relative position between the corresponding optical element and the contactless sensor may be employed. To this end, FIGS. 4 to 5 are schematically shown in FIG. Figure 6 Possible embodiments are shown.

[0048] according to Figures 4a to 4c, in terms of the optical elements 401 to 403 whose heating state is to be measured, there is a corresponding lateral or lateral movement of the contactless sensor 430 (in the x-direction relative to the depicted coordinate system), wherein the embodiments differ from one another in terms of the specific configuration of the (back) side of the respective optical element or mirror 401-403 facing the sensor 430. In particular, the optical element 401 has an oblique hole 401a, the optical element 402 has an oblique milled groove 402a, and the optical element 403 has a circular cutout 403a.

[0049] Figures 5a to 5d Other exemplary embodiments of relative movement between the non-contact sensor 531, 532, 533 or 534 and the optical element 501, 502, 503, 504 are shown. Figure 5a , the sensor 531 is displaced parallel to the back side of the optical element 501 facing the sensor 531 (in the x-direction relative to the depicted coordinate system); according to Figure 5b , the sensor 532 tilted relative to the x direction is displaced in the x direction; according to Figure 5c In the optical element 503, the sensor 533 is displaced parallel to the surface 503b of the recessed portion present in the optical element 503; and according to Figure 5d , the sensor 534 rotates about an axis parallel to the back side of the optical element 504 or in the y-direction relative to the depicted coordinate system.

[0050] In other embodiments, radiation emitted from the optical element whose heating state is to be measured may additionally or alternatively be captured by one or more side non-contact sensors (ie, non-contact sensors on the back side of the optical element not facing the reflector).

[0051] Figure 6 Schematic diagrams showing other possible embodiments.

[0052] according to Figure 6 In the case of an optical element 601, the heating state of which is to be measured, two contactless sensors 631, 632 according to the invention are provided at a housing 640, which is implemented as a packaged unit, so as to be individually axially displaceable by an electromechanical actuator 635. The sensors 631, 632 (which are according to Figure 6 The displacement of the optical element or mirror 601 is realized in each case in a hole 601a, 601b, connected to the electronic board 631a), wherein the radiation area captured by the corresponding sensor 631 or 632 (indicated by "605") can be varied by the axial displacement. This mechanical variation of the captured radiation area can be realized alternatively or in addition to using an optical variation (for example, based on the upstream optical unit described in Figures 2 to 3). According to Figure 6The upstream optical unit 610 may also have a static configuration (ie, not shown in FIG. Figure 3a-3b adjustment possibilities provided in the zoom-axicone system).

[0053] In fact, in all the above-described embodiments, a simulation can also be preliminarily carried out, which generates corresponding (superimposed) signals expected at the non-contact sensor or pyrometer for certain use scenarios of the optical element (e.g. certain radiation situations of the mirror). Based on the simulation, the heating state or certain radiation situations can be inferred from the measurement signal actually obtained at the non-contact sensor. Then, a preheating of the optical element can be carried out based on the determination of the heating state, in order to at least partially compensate for the change of the heating state of the optical element over time.

[0054] Even though the present invention has been described based on specific embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example by combining and / or exchanging the features of the various embodiments. Therefore, it goes without saying to those skilled in the art that the present invention also encompasses such variations and alternative embodiments, and that the scope of the present invention is limited only to the scope provided by the attached patent claims and their equivalents.

Claims

1. A method for determining a heating state of an optical element in a microlithography optical system, wherein at least one contactless sensor (330, 430, 531, 532, 533, 534, 631, 632) is used, the at least one contactless sensor being based on receiving electromagnetic radiation from the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601), wherein a variation of the radiation range captured by the sensor (330, 430, 531, 532, 533, 534, 631, 632) for the purpose of determining the temperature distribution in the optical element, wherein the optical element is a reflector, the optical element having at least one cutout or recess and / or at least one raised structure on the back side facing the sensor, and the variation is achieved using a beam shaping optical unit located between the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601) and the sensor (330, 430, 531, 532, 533, 534, 631, 632), and Wherein, the method comprises: i) varying the beam shaping optical unit to vary the area of ​​the optical element from which the non-contact sensor receives electromagnetic radiation; and ii) After i), using the electromagnetic radiation received from the optical element by the non-contact sensor to determine a temperature distribution in the optical element.

2. The method according to claim 1, characterized in that The variation enables regional sensing of the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601) with respect to the electromagnetic radiation emitted from the element.

3. The method according to claim 1, characterized in that: The beam shaping optical unit comprises a zoom axicon system (210, 310).

4. The method according to any one of the preceding claims, characterized in that The variation includes manipulation of the relative position between the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601) and the sensor (330, 430, 531, 532, 533, 534, 631, 632).

5. The method according to any one of claims 1 to 3, characterized in that A pyrometer is used as the sensor ( 330 , 430 , 531 , 532 , 533 , 534 , 631 , 632 ).

6. The method according to any one of claims 1 to 3, characterized in that The optical elements are designed for operating wavelengths less than 30 nm.

7. The method according to claim 6, characterized in that The operating wavelength is less than 15 nm.

8. The method according to any one of claims 1 to 3, characterized in that Based on the determination of the heating state, there is a pre-heating of the optical element for at least partially compensating for a change over time of the heating state of the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601), which change occurs during operation of the optical system and / or during compensation of optical aberrations in the optical system caused by the heating state.

9. The method according to any one of claims 1 to 3, characterized in that The heating state is determined during operation of the optical system.

10. The method according to any one of claims 1 to 3, characterized in that The optical system is a microlithography projection exposure apparatus.

11. A method for determining a heating state of an optical element in a microlithography optical system, wherein at least one non-contact sensor (330, 430, 531, 532, 533, 534, 631, 632) is used, the at least one non-contact sensor being based on receiving electromagnetic radiation from the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601), wherein the range of radiation captured by the sensor (330, 430, 531, 532, 533, 534, 631, 632) is varied for the purpose of determining the temperature distribution in the optical element, wherein the optical element is a reflector and the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601) has at least one cutout or recess and / or at least one raised structure on the back side facing the sensor (330, 430, 531, 532, 533, 534, 631, 632), Wherein, the method comprises: i) changing the relative position of the optical element and the contactless sensor to change the area of ​​the optical element from which the contactless sensor receives electromagnetic radiation; and ii) After i), using the electromagnetic radiation received from the optical element by the non-contact sensor to determine a temperature distribution in the optical element.

12. An optical system for microlithography, comprising an apparatus for determining a heating state of an optical element, wherein the apparatus comprises: at least one contactless sensor (330, 430, 531, 532, 533, 534, 631, 632) based on receiving electromagnetic radiation from the optical element (301, 401, 402, 403, 501, 502, 503, 504, 601); as well as means for varying the range of radiation captured by said sensor (330, 430, 531, 532, 533, 534, 631, 632); The device is configured to carry out the method according to any one of claims 1 to 11.

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