Optical system, especially for microlithography, and methods for operating an optical system

Abstract

The invention relates to an optical system, in particular for microlithography, comprising at least one optical element (100, 200, 300, 400) which has a substrate (105, 205, 305, 405) and at least one optically active layer system (110, 210, 310, 311, 410) located on this substrate (105, 205, 305, 405), and at least one monitoring unit (120, 220, 240, 320, 340) for monitoring a degradation state of this layer system (110, 210, 310, 311, 410), wherein the monitoring unit (120, 220, 240, 320, 340) has a detector (121, 221, 241, 321, 341) for detecting non-directional radiation (130, 230, 330, 430) which is emitted by the optical element (100, 200, 300, 400) during the operation of the optical system.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The invention relates to an optical system, particularly for microlithography, and to a method for operating an optical system. The invention is advantageously applicable, among other things, in a laser light source with an optical pulse stretcher, and particularly for use in a microlithographic projection exposure system. State of the art

[0002] Microlithography is used to manufacture microstructured electronic components. 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] During the operation of a microlithographic projection exposure system, degradation or defects can form in the optically active layer systems of optical elements (e.g., in highly reflective or partially reflective layers of mirrors or beam splitters, or in antireflective coatings of lenses) due to the energy input from incident electromagnetic radiation. These include, for example, changes in the respective layer materials through chemical reactions (e.g., oxidation) or the incorporation of contaminants (e.g., hydrocarbons), as well as partial delamination of layer materials. As a result, such degradation or defects lead to a significant impairment of the optical properties (e.g., reduced reflectivity, spectral shift, or increased absorption or transmission). This, in turn, can lead to a deterioration in the optical performance of the respective optical element.leading to a complete failure of the (overall) system, up to and including a failure of an optical system containing this element.

[0004] In a projection exposure system designed for operation in the DUV range (e.g., at operating wavelengths of less than 250 nm, especially less than 200 nm), laser light sources in the form of excimer lasers are typically used, particularly krypton fluoride excimer lasers at an operating wavelength of 248 nm or argon fluoride excimer lasers at an operating wavelength of 193 nm. The optical elements exposed to the aforementioned high radiation levels can be, in particular, mirrors or beam splitters used in such a laser light source. This can occur, for example, in an optical pulse stretcher or as deflecting or output coupler mirrors within such a laser light source.

[0005] One possible approach to preventing degradation-related system failure, for example in the aforementioned laser light source or a microlithographic projection exposure system, involves the preventive replacement of optical elements during regular service intervals and / or monitoring the overall system performance. However, in addition to the risk of delayed replacement and the resulting system failure, there is also the risk of prematurely and unnecessarily replacing still-functional optical elements. Furthermore, monitoring the overall system performance does not allow for the targeted attribution of any performance degradation to individual optical elements, and therefore does not enable their selective replacement.

[0006] For the state of the art, reference is made only by way of example to DE 10 2010 006 326 A1, WO 2010 / 047768 A1, as well as the publications J. Heber et al.: “Deep UV laser induced fluorescence in fluoride thin films”, Appl. Phys. A 76, 123-128 (2003), DOI: 10.1007 / s00339-002-1502-9, Ch. Mühlig et al.: “Laser induced fluorescence and absorption measurements for DUV optical thin film characterization”, Proc. of SPIE Vol. 7101 (2008), 71011R-1 to 71011R-9 and W. Triebel: “Characterization of DUV optical materials by direct absorption measurements and LIF”, Proc. of SPIE Vol. 5991, 59911P-1 to 59911P-16, referenced. SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide an optical system, in particular for microlithography, and a method for operating an optical system, which enables the detection or monitoring of a degradation state existing within the optical system while at least partially avoiding the problems described above.

[0008] This problem is solved by the features of independent patent claims.

[0009] According to the invention, an optical system, particularly for microlithography, comprises: - at least one optical element comprising a substrate and at least one optically active layer system located on that substrate; and - at least one monitoring unit to monitor a degradation state of this layer system, - wherein the monitoring unit has a detector for detecting non-directional radiation emitted by the optical element during the operation of the optical system.

[0010] For the purposes of this application, the term "non-directional radiation" includes both electromagnetic radiation that has the same intensity in all directions emanating from the optical element and radiation whose intensity varies depending on the direction. In particular, for the purposes of this application, the term "non-directional radiation" includes radiation that deviates from the main optical path of an optical system containing the optical element.

[0011] The invention is based in particular on the concept of detecting or monitoring a degradation state existing in an optical system or in an optical element located therein, based on the detection of non-directional radiation emitted by the optical element in question during operation. In particular—but without limiting the invention to this—this non-directional radiation can be fluorescence radiation. The invention takes advantage of the fact that such fluorescence radiation can be caused by defects in the coating material and that, accordingly, the intensity and spectral position of a detected fluorescence signal can be representative of existing impurities, as well as their type and concentration.

[0012] In further embodiments, alternatively or additionally to the detection of fluorescence radiation, other non-directional radiation, in particular in the form of scattered light, can be detected.

[0013] The present invention, as described in more detail below, comprises both embodiments in which the aforementioned non-directional radiation (in particular, fluorescence radiation) is caused by the layer degradation to be detected or by associated defects, and embodiments in which the aforementioned non-directional radiation or fluorescence radiation is generated otherwise (in particular by a fluorescence layer specifically incorporated for this purpose into the layer system of the optical element) and then altered or becomes visible (possibly to an increased degree) upon degradation. In particular, a fluorescence layer can also be used that initially fluoresces but then, upon increased irradiation, no longer emits light (e.g., as a result of layer degradation on the optical element) (e.g., a fluorophore that degrades significantly through a two-photon process or upon contact with oxygen, etc.).

[0014] The specific placement of the fluorescent layer can be suitably chosen depending on the operating wavelength of the optical system and the material combination of the optical element's layer system. In particular, the fluorescent layer can be incorporated as an intermediate layer within the optically active layer system, or, in other embodiments, it can be placed, for example, on the back side of the optical element. Furthermore, the fluorescent layer can be incorporated as an additional layer to the actual layer structure of the optical element or can replace a layer that is standardly included in the optical element's layer structure (i.e., be provided in place of that layer).

[0015] The monitoring of layer degradation according to the invention, based on the detection of non-directional radiation, has the particular advantage that a detector used for this purpose can be placed outside the optical beam path of the useful light passing through the optical system during operation. As a result, the actual operation of the optical system is not disturbed, and monitoring during operation ("in operando") can be implemented.

[0016] According to the invention, the disadvantage that the detected non-directional radiation has relatively low intensities compared to directional radiation is accepted. However, this disadvantage is deliberately accepted in order to achieve the aforementioned advantages, particularly those of "in operando" monitoring. Furthermore, embodiments are described below in which the comparatively low intensities of the non-directional radiation are compensated for by a particularly advantageous placement of the detector (e.g., on the back side of a substrate transparent to said radiation).

[0017] According to one embodiment, the monitoring unit is designed to detect temporal changes in the non-directional radiation. These temporal changes can relate to the intensity (optionally also the intensity normalized to a current source power in the optical system) of the non-directional radiation, or additionally or alternatively to other parameters characteristic of this radiation. In particular, spectral changes, such as a transition from one fluorescence state to another, can be detected and monitored.

[0018] According to one embodiment, the non-directional radiation includes fluorescence radiation.

[0019] According to one embodiment, the non-directional radiation includes scattered light.

[0020] According to one embodiment, the non-directional radiation is caused by defects that are generated when the layer system interacts with useful light present during the operation of the optical system.

[0021] According to one embodiment, the layer system comprises a fluorescent layer which emits fluorescent radiation when interacting with the useful light present during operation of the optical system. According to the invention, a loss of useful light associated with such a fluorescent layer, as well as an increased complexity of the layer structure, are deliberately accepted in order to enable a particularly reliable diagnosis of incipient layer degradation.

[0022] According to one embodiment, the monitoring unit has a spectral filter designed according to a wavelength or wavelength range of the non-directional radiation.

[0023] According to one embodiment, the detector is arranged outside the optical beam path of the useful light passing through the optical system during operation.

[0024] According to one embodiment, the substrate is made of a material that is transparent to non-directional radiation.

[0025] According to one embodiment, the detector is arranged on the side of the substrate facing away from the layer system.

[0026] According to one embodiment, the optical system is designed for an operating wavelength of less than 250 nm, in particular less than 200 nm. However, the invention is not limited to this, and the optical system can also be designed for higher operating wavelengths (e.g., also in the visible wavelength range).

[0027] According to one embodiment, the optical element is a mirror, wherein the optically effective layer system is a reflective layer system of this mirror.

[0028] According to one embodiment, the optical element is a lens or an output coupler, wherein the optically effective layer system is an antireflective layer of this lens or output coupler.

[0029] According to one embodiment, the optical element is a beam splitter, wherein the optically effective layer system is an antireflective layer or a partially reflective layer of this beam splitter.

[0030] According to one embodiment, the optical system has a laser light source, wherein the optical element is arranged in this laser light source.

[0031] According to one embodiment, the optical system is a microlithographic projection exposure system.

[0032] The invention further relates to a method for operating an optical system, wherein the optical system comprises at least one optical element with a substrate and at least one optically effective layer system located on this substrate, wherein non-directional radiation emanating from the optical element during operation of the optical system is detected by at least one monitoring unit for monitoring a degradation state of the layer system.

[0033] According to one embodiment, the monitoring is carried out during ongoing operation of the optical system.

[0034] According to one embodiment, the optical element is replaced depending on a change in the detected non-directional radiation over time. For example, the optical element can be replaced if a predetermined threshold for the intensity of the non-directional radiation is exceeded.

[0035] The optical system can be designed with the features described above.

[0036] Further embodiments of the invention can be found in the description and the dependent claims.

[0037] The invention is explained in more detail below with reference to an embodiment shown in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] They show: Fig. 1-4 schematic representations to illustrate exemplary embodiments of an optical system according to the invention; Fig. 5 a diagram to illustrate the mode of operation of the invention in an exemplary embodiment; Fig. 6 a schematic representation of the possible basic structure of a laser light source as an application example of the present invention; and Fig. 7 a schematic representation of the possible basic structure of a microlithographic projection exposure system designed for operation in DUV. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0039] The following, with reference to Fig. The embodiments described in sections 1-5 have in common that the detection and monitoring of layer degradation occurring within an optical system or on an optical element located therein is based on the measurement of non-directional radiation (in particular, fluorescence radiation and / or scattered light). This advantageously allows monitoring to be carried out, especially during the ongoing operation of the optical system, with the result that any necessary replacement of a significantly degraded component can be initiated precisely and at the appropriate time – i.e., neither too late (i.e., after the optical system has already failed) nor too early (i.e., while the optical performance or functional optical element is still sufficient).

[0040] Knowing the temporal progression of the intensity of non-directional radiation also allows for the planning of future maintenance work.

[0041] Fig. Figure 1 shows, in a purely schematic representation, an optical element 100 with a substrate 105 and an optically active layer system 110 located on this substrate 105. In the exemplary embodiment of Fig. 1. The optical element 100 is a mirror in which the layer system 110 is designed as a reflective layer system. The mirror, in turn, can be used in an exemplary application in a laser light source (e.g., with the one in Fig. The optical element 100 can be arranged in the schematically shown and further described structure shown in Figure 6. In other applications, it can also be a beam splitter (e.g., for the optical pulse stretcher of a laser light source) or a lens (in which the optically effective layer system 110 is designed as an antireflective layer), e.g., in the microlithographic projection exposure system with the one described in Figure 6. Fig. 7 schematically shown and described in more detail structure.

[0042] The layer system 110 of the optical element 100 in the form of a mirror according to Fig. 1 can, for example, in a multilayer structure, comprise a plurality of alternatingly arranged individual layers in a manner known per se, wherein these individual layers can, for example, comprise a fluoride material. In one embodiment, the layer structure can consist of alternating lanthanum fluoride (LaF3) and magnesium fluoride (MgF2) layers. In embodiments, the material of the respective individual layers can be selected from the group consisting of AlF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3, SiO2, Al2O3, TiO2, ZrO, HfO2, Ta2O5, and Si3N4.

[0043] The number of individual layers can typically be at least twenty, in particular at least thirty, and further, in particular, at least forty (without the invention being limited thereto). Furthermore, functional layers, e.g., in the form of barrier layers or adhesive layers, can also be provided in the layer structure.

[0044] During operation, electromagnetic radiation or useful light (in) strikes the optical element 100 or the layer system 110. Fig. 1 designated with “101”). This can (without limiting the invention to this) in particular be DUV radiation with a wavelength of less than 250 nm.

[0045] With "120" in Fig. 1 a monitoring unit which has a detector 121 for detecting non-directional radiation 130 emitted by the optical element 100 during operation of the optical system. In the exemplary embodiment of Fig. 1 furthermore a spectral filter 122 for selecting the spectral range detected by the detector 121 and a converging lens 123. Both the spectral filter 122 and the converging lens 123 are optional and can also be omitted in further embodiments of the invention.

[0046] The non-directional radiation 130 can be, in particular, fluorescence radiation generated by defects arising in the layer system 110 of the optical element 100 during operation and associated with the layer degradation to be detected. The monitoring unit 120 can detect changes in the intensity of this non-directional radiation 130 over time in order to identify significant degradation and a corresponding impending failure of the optical element 100 at an early stage. For example, if a predefined threshold for the intensity of the non-directional radiation 130 is exceeded, the optical element 100 can be replaced precisely.

[0047] In addition to or as an alternative to the detection of fluorescence radiation, other generated, non-directional radiation, especially in the form of scattered light, can also be detected.

[0048] Fig. Figure 2 schematically shows another embodiment, wherein in comparison to Fig. 1 analogous or essentially functionally identical components are designated with reference digits increased by “100”.

[0049] According to Fig. In addition to the monitoring unit 220, a further monitoring unit 240 (again with detector 241, spectral filter 242 and converging lens 243) is provided (in other embodiments not shown, possibly also as an alternative), which is placed on the (rear) side of the substrate 205 or the optical element 200 facing away from the layer system 210. The substrate 205 is made of a material transparent to said radiation 230 to enable the detector 241 to detect non-directional (e.g., fluorescence) radiation 230. The embodiment of Fig. 2 has compared to that of Fig. 1. In particular, the advantage of detecting a larger solid angle range for the non-directional radiation 230 is achieved, thus taking into account the potentially comparatively low intensity of this radiation 230. In further embodiments, the monitoring unit according to the invention can also be placed only on the (rear) side of the substrate 205 or the optical element 200 facing away from the layer system 210.

[0050] The detector of the monitoring unit according to the invention can be a photodetector, or in further embodiments, a CCD camera or a multi-diode or quadrant diode for detecting and analyzing the two-dimensional intensity distribution. The detector can also be placed directly on the back of the optical element or mirror. A possible criterion for replacing the optical element could then be, for example, a local increase in light intensity. Reliable monitoring can thus be ensured even with a (intended or undesired) variation in the intensity of the primary radiation. Furthermore, automatic calibration of the monitoring system according to the invention can be implemented.

[0051] Fig. Figure 3 schematically shows another embodiment, in comparison to Fig. Two analogous or essentially functionally identical components are designated with reference numerals increased by "100". In the embodiment of Fig. In contrast to the optical element 300, the optical element 300 is... Fig. 1 and Fig. 2 a beam splitter which has an optically active layer system 310 or 311 on opposite sides of the substrate 305. This beam splitter can, by way of example only, be an optical pulse stretcher of Fig. 6 existing beam splitters.

[0052] Fig. Figure 4 shows a schematic representation to illustrate a further embodiment. The embodiment of Fig. 4 differs from those from Fig. 1-3 in particular by the fact that a fluorescence layer 450 (in the example instead of a layer 410a) is incorporated into the layer structure of the layer system (designated by “410” and comprising a plurality of alternating layers 410a, 410b), which can be made, for example, of doped or modified SiO2 and emits fluorescence radiation as non-directional radiation 430 when interacting with useful light present during operation of the optical system. In the embodiment according to Fig. 4 (however, without limiting the invention thereto), the layer material of layers 410a is lanthanum fluoride (LaF3) and the layer material of layers 410b is magnesium fluoride (MgF2). In further examples, other layer materials and, if necessary, more than two different layer materials may be provided in the layer structure.

[0053] The – incidentally analogous to the embodiments of Fig. 1 to Fig. 3 detected - non-directional radiation 430 is as in Fig. 5 indicates that layer degradation (in the example, detachment of part of the upper six layers) is modified with regard to its intensity, thus allowing for the early detection of layer degradation. In contrast to the embodiments of Fig. 1-3 will be according to Fig. 4-5 The non-directional radiation 430, or fluorescence radiation, is thus generated by the fluorescence layer 450, which is specifically incorporated for this purpose in the layer system 410 of the optical element 400, and becomes visible (possibly to an increased degree) upon degradation. In a variation of the example of Fig. 5. Layer degradation can also affect more or fewer layers, whereby these layers may each be partially or completely degraded (e.g., detached or modified in their composition).

[0054] In further embodiments, the fluorescent layer can also be designed in such a way that it initially fluoresces, but then no longer glows under increased irradiation (e.g., as a result of layer degradation on the optical element) (e.g., a fluorophore that degrades significantly through a two-photon process or upon contact with oxygen, etc.).

[0055] The specific placement of the fluorescent layer 450 can be suitably selected depending on the operating wavelength of the optical system and the material combination of the optical element. In particular, the fluorescent layer 450 can be positioned according to the embodiment shown in Fig. 4 is incorporated into the optically active layer system, but in other embodiments it can also be placed, for example, on the back side of the optical element. Furthermore, the fluorescent layer 450 can be incorporated as a layer provided in addition to the actual layer structure of the optical element or – as in the exemplary embodiment of Fig. 4 - replace one layer in the layer structure of the optical element (in the example, one layer 410a made of lanthanum fluoride (LaF3)).

[0056] The fluorescence layer 450 can optionally – as explained below – also be used for adjustment, for example in an optical pulse stretcher, e.g., with the following reference to Fig. The structure described in section 6 can be used.

[0057] In the schematic representation of Fig. The layer system designated "510" corresponds to the undegraded state, whereas "510" symbolizes the layer system in a state that is at least spatially locally degraded. The graphic illustrates, using a simulation of a LaF3 / MgF2 multilayer system as an example, that during degradation, the intensity of the ambient light striking the inserted fluorescent layer is subject to corresponding variation. The distribution of the ambient light's electric field strength (plotted in arbitrary units) is spatially resolved along the layer system. A comparison of this distribution in layer systems 510 and 510' clearly shows the increase in electric field strength within the layer system during degradation, particularly the increase within the fluorescent layer 550. In this example, degradation thus has the effect of the uppermost layers "disappearing" (e.g.,locally through the formation of holes in the layer).

[0058] Fig. Figure 6 shows a schematic and simplified representation of the possible basic structure of a laser light source 600 as a possible application of the present invention. The laser light source 600 comprises, in particular, a seed laser 601 and an amplification stage 620 with a beam inversion module 621, which is only indicated. Also shown in Figure 6 is a simplified representation of the possible basic structure of a laser light source 600 as a possible application of the present invention. Fig. Figure 6 indicates a relay optic 605 for adapting or aligning the output signal of the seed laser 601 to the amplification stage 620. "606" designates a unit for coupling out a portion of the laser beam. "610" denotes an optical pulse stretcher, which includes a beam splitter 615 and a plurality of mirrors 611-614 for generating a multitude of light pulses (of which in Fig. 6 (for the sake of simplicity, only four mirrors are shown). As a result of the in Fig. Following the repeated reflections of the light component coupled out by the beam splitter 615 (dashed arrows) at the respective optical surfaces of the mirrors 611-614, this coupled-out light component experiences a time delay relative to the light component transmitted by the beam splitter 615, in a manner known per se, before said coupled-out light component follows the transmitted light component after reflection at the beam splitter 615. For the sake of simplicity, further details have been omitted in Fig. 6. In this context, reference is made only by way of example to WO 2010 / 047768 A1 and DE 10 2022 005 007 A1 as examples of the state of the art.

[0059] The monitoring for layer degradation according to the invention can be carried out in particular at a mirror (e.g., deflecting mirror or output coupling mirror) or a beam splitter of the laser light source 600 described above, e.g., in the optical pulse stretcher 610 (at one or more of the mirrors 611-614 or at the beam splitter 615), in the relay optics 605, or also in the unit 606 for coupling out a portion of the laser beam. In this way, when implementing the invention, analogous to the embodiments according to Fig. 1-3 or analogous to the embodiment according to Fig. 4-5 (i.e., using an additional fluorescent layer 450). In the latter case, the fluorescent layer 450 can optionally also be used for adjustment. This takes advantage of the fact that, when useful light is incident or the optical element is correctly adjusted, the fluorescent layer 450 may already emit fluorescence radiation (albeit with comparatively low intensity) even if the layer system 410 of the respective optical element 400 is intact. Thus, the correct alignment of the optical elements in the optical beam path can be determined during adjustment based on the fluorescence signal.

[0060] Fig. Figure 7 shows a possible basic structure of a microlithographic projection exposure system 700 designed for operation in DUV as an application example of the invention.

[0061] The projection exposure system 700 according to Fig.7 comprises a lighting device 710 and a projection lens 720. The lighting device 710 serves to illuminate a structure-bearing mask (reticule) 715 with light from a light source unit 705, which includes a laser light source, for example in the form of an ArF excimer laser for a working wavelength of about 193 nm (or also in the form of an XeF excimer laser for a working wavelength of about 351 nm, in the form of a KrF excimer laser for a working wavelength of about 248 nm or in the form of an F2 excimer laser for a working wavelength of about 157 nm) and a beam shaping optic that generates a parallel light beam.

[0062] The lighting device 710 has an optical unit 711, which, in the illustrated example, includes a deflecting mirror 712. The optical unit 711 can, for example, include a diffractive optical element (DOE) and a zoom-axicon system to generate different lighting settings (i.e., intensity distributions in a pupil plane of the lighting device 710). Downstream of the optical unit 711, a light mixing device (not shown) is located in the beam path, which, for example,The instrument may, in a manner known per se, have an arrangement of micro-optical elements suitable for achieving light mixing, as well as a lens group 713, behind which a field plane with a reticle masking system (REMA) is located. This system is imaged by a REMA lens 714, which follows in the direction of light propagation, onto the structure-bearing mask (reticle) 715 arranged in a further field plane, thereby limiting the illuminated area on the reticle. The structure-bearing mask 715 is imaged by the projection lens 720 onto a lens substrate or a wafer 730 provided with a photosensitive layer (photoresist). The projection lens 720 can be designed, in particular, for immersion operation, in which case an immersion medium is located in front of the wafer or its photosensitive layer with respect to the direction of light propagation. Furthermore, it can, for example, have a numerical aperture NA greater than 0.85, especially greater than 1.1.

[0063] 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 2010 006 326 A1

[0006] WO 2010 / 047768 A1 [0006, 0058] DE 10 2022 005 007 A1

[0058]

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