REFLECTIVE OPTICAL ELEMENT

Abstract

A reflective optical element, in particular for a microlithographic projection exposure apparatus or a mask inspection apparatus. According to one aspect, the reflective optical element has an optically effective surface, a substrate (405, 505), a reflection layer system (410, 510) and at least one porous outgassing layer (450, 550), which at least intermittently releases particles adsorbed in the outgassing layer (450, 550) when the optically effective surface (400a, 500a) is irradiated by electromagnetic radiation.

Claims

1. A reflective optical element, comprising: an optically effective surface; a substrate; a reflection layer system; and at least one porous outgassing layer, which at least intermittently releases particles adsorbed in the outgassing layer when the optically effective surface is irradiated by electromagnetic radiation, and which is arranged on a side of the reflection layer system facing the substrate.

2. An optical apparatus, comprising at least one of a microlithographic projection exposure apparatus or a mask inspection apparatus, wherein the at least one apparatus comprises a reflective optical element as claimed in claim 1.

3. The reflective optical element as claimed in claim 1, wherein the outgassing layer has a first porous outgassing layer and a second porous outgassing layer configured to at least intermittently adsorb particles released by the first outgassing layer when the optically effective surface is irradiated by electromagnetic radiation.

4. The reflective optical element as claimed in claim 1, further comprising at least one heat radiation layer, wherein, in comparison with an analogously structured reflective optical element without the heat radiation layer, the reflective optical element increases a radiation of heat induced by the electromagnetic radiation in the reflective optical element when the optically effective surface is irradiated by electromagnetic radiation.

5. The reflective optical element as claimed in claim 4, wherein the heat radiation layer is arranged on a side arranged on a side of the reflection layer system facing the substrate.

6. The reflective optical element as claimed in claim 4, wherein the heat radiation layer is arranged on a side arranged on a side of the reflection layer system facing the optically effective surface.

7. A reflective optical element, comprising: an optically effective surface; a substrate; a reflection layer system; and at least one heat radiation layer, wherein, in comparison with an analogously structured reflective optical element without the heat radiation layer, the reflective optical element increases a radiation of heat induced by the electromagnetic radiation in the reflective optical element when the optically effective surface is irradiated by electromagnetic radiation, and wherein the heat radiation layer is arranged on a side of the reflection layer system facing the optically effective surface.

8. An optical apparatus, comprising at least one of a microlithographic projection exposure apparatus or a mask inspection apparatus, wherein the at least one apparatus comprises a reflective optical element as claimed in claim 7.

9. The reflective optical element as claimed in claim 7, further comprising a heat insulation layer arranged between the substrate and the reflection layer system.

10. The reflective optical element as claimed in claim 9, wherein the heat insulation layer comprises quartz.

11. The reflective optical element as claimed in claim 7, further comprising a Peltier element arranged between the substrate and the reflection layer system.

12. A reflective optical element, comprising: an optically effective surface; a substrate; a reflection layer system; and a Peltier element arranged between the substrate and the reflection layer system.

13. The reflective optical element as claimed in claim 12, further comprising a heat buffer layer arranged between the substrate and the reflection layer system.

14. The reflective optical element as claimed in claim 12, configured for an operating wavelength of less than 30 nm.

15. The reflective optical element as claimed in claim 1, configured as a mirror.

16. An optical system comprising at least one reflective optical element as claimed in claim 1.

17. The optical system as claimed in claim 16, further comprising a heat sink arranged on a side of the substrate facing away from the reflection layer stack.

18. The optical system as claimed in claim 16, configured as a microlithographic projection exposure apparatus.

19. The optical system as claimed in claim 16, configured as an optical system of a mask inspection apparatus.

20. A microlithographic projection exposure apparatus comprising an illumination device and a projection lens, wherein the projection exposure apparatus has a reflective optical element as claimed in claim 1.

21. A mask inspection apparatus comprising an illumination device and an inspection lens, wherein the mask inspection apparatus comprises a reflective optical element as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In the figures:

[0048] FIG. 1 shows a schematic illustration for explaining the construction of a reflective optical element according to a first embodiment of the invention;

[0049] FIGS. 2-5 show schematic illustrations for explaining the construction of a reflective optical element according to further embodiments of the invention;

[0050] FIG. 6 shows a diagram for explaining the mode of operation of a reflective optical element according to a further embodiment of the invention;

[0051] FIGS. 7-8 show schematic illustrations for explaining the construction of a reflective optical element according to further embodiments of the invention; and

[0052] FIG. 9 shows a schematic illustration for explaining the possible construction of a microlithographic projection exposure apparatus designed for operation in EUV.

DETAILED DESCRIPTION

[0053] FIG. 1 shows a schematic illustration for describing the construction of a reflective optical element according to the invention in a first embodiment of the invention.

[0054] The reflective optical element 100 comprises, in particular, a substrate 105, which is made from any desired suitable (mirror) substrate material. Suitable substrate materials are e.g. quartz glass doped with titanium dioxide (TiO.sub.2), with, merely by way of example (and without the invention being restricted thereto), materials that are usable being those sold under the trade names ULE (by Corning Inc.) or Zerodur (by Schott AG).

[0055] Furthermore, the reflective optical element 100 has, in a manner known per se in principle, a reflection layer system 110, which, in the embodiment illustrated comprises merely by way of example a molybdenum-silicon (MoSi) layer stack. Without the invention being restricted to specific configurations of this layer stack, one suitable construction that is merely exemplary can comprise approximately 50 plies or layer packets of a layer system comprising molybdenum (Mo) layers having a layer thickness of 2.7 nm in each case and silicon (Si) layers having a layer thickness of 3.3 nm in each case. Optionally, further functional layers, such as, e.g., a capping layer (cap layer), a substrate protection layer (SPL=substrate protection layer) and/or diffusion barriers, can be provided.

[0056] In particular, the reflective optical element 100 can be a reflective optical element or a mirror of an optical system, in particular of the projection lens or of the illumination device of a microlithographic projection exposure apparatus or of the inspection lens of a mask inspection apparatus. Moreover, the optical element or the optical system can be designed, in particular, for the operation in the EUV.

[0057] The impingement of an optically effective surface 100a of the reflective optical element 100 with electromagnetic EUV radiation (indicated by an arrow in FIG. 1) that occurs during the operation of the optical system has as a consequence that there is an increase in the volume of both the reflection layer system 110 and the substrate 105, wherein this volumetric increase may extend in inhomogeneous fashion over the optically effective surface 100a depending on the intensity distribution of the incident electromagnetic radiation (thus, in particular, depending on the set illumination setting in the case of a near-pupil reflective optical element).

[0058] In order now to at least partly reduce the deformation of the reflective optical element 100 overall caused by said irradiation of the optically effective surface 100a by electromagnetic radiation and, in particular, the deformation of the optically effective surface 100a and a wavefront effect consequently accompanying this during the operation of the reflective optical element 100, the reflective optical element 100 has a heat radiation layer 120 which, according to FIG. 1, is situated on the side of the reflection layer system 110 facing away from the optically effective surface 100a.

[0059] This heat radiation layer 120 is distinguished by a comparatively high emissivity for infrared (IR) radiation such that heat dissipation occurs through the substrate 105 by way of the heat radiation layer 120. On account of this heat dissipation, the correspondingly dissipated radiation-induced heat is no longer available for producing a thermally caused deformation, in particular of the substrate 105 or of the optically effective surface 100a, and so, overall, such a deformation is avoided or at least reduced.

[0060] FIG. 2 shows a further embodiment, wherein components analogous or substantially functionally identical to FIG. 1 are designated by reference numerals increased by 100. The embodiment of FIG. 2 differs from that of FIG. 1 in that the heat radiation layer 220 is arranged on the side of the reflection layer system 210 facing the optically effective surface 200a. Preferably, the heat radiation layer 220 is configured in such a way in this embodiment that, although it has an emissivity that is as high as possible for the IR radiation to be dissipated, only a low emissivity (as this is synonymous to a low absorption capacity) is present for the electromagnetic used radiation or EUV radiation incident on the reflective optical element 200 during operation.

[0061] Suitable materials for the heat radiation layer 220 and/or 120 are, for example, niobium oxide (NbO), silicon nitride (SiN), zirconium oxide (ZrO) or amorphous carbon (C).

[0062] In embodiments, the heat radiation layer 220 and/or 120 can also have doping (of a silicon (Si) or molybdenum (Mo) layer, for example) with one or more of the aforementioned materials (for example, doping with 10% carbon atoms). As a result of this, it is possible, as desired, to obtain a comparatively negligible influence on the emission properties of the reflection layer system for the EUV radiation forming the used light, while the emissivity for IR radiation is significantly increased.

[0063] Typical thicknesses of the heat radiation layer 220 and/or 120 can lie in the range of 5 nm to 100 nm, for example, depending on the material, wherein, in principle, the placement of the heat radiation layer 120 on the side of the reflection layer system 110 facing the substrate 105, realized in FIG. 1, facilitates comparatively larger thicknesses and also a greater flexibility in respect of the material selection (since there is no need to take account of the radiation or emission properties for EUV radiation, which are irrelevant in this case).

[0064] FIG. 3 shows a further embodiment of a reflective optical element 300 according to the invention, wherein components analogous or substantially functionally identical to FIG. 2 are designated by reference numerals increased by 100.

[0065] The embodiment of FIG. 3 differs from that of FIG. 2 in that, according to FIG. 3, an additional heat insulation layer 330 is arranged between the reflection layer system 310 and the substrate 305. This achieves a longer stay of the radiation-induced heat in the reflection layer system 310, with the consequence that a longer time interval is also available for the heat dissipation by the IR radiation by the heat radiation layer 320.

[0066] A suitable material for the heat insulation layer 330 is amorphous fused silica (SiO.sub.2), with the thickness thereof in view of the placement on the side of the reflection layer system 310 facing away from the optically effective surface 300a being comparatively uncritical and being able to be, purely by way of example, in the region of several 10 nm or several 100 nm.

[0067] FIG. 4 shows a further embodiment of a reflective optical element 400 according to the invention, wherein components once again analogous or substantially functionally identical to FIG. 3 are designated by reference numerals increased by 100.

[0068] In contrast to the embodiments described above, the reflective optical element 400 according to FIG. 4 has a porous outgassing layer 440, which is arranged between the reflection layer system 410 and the substrate 405 in the exemplary embodiment. The thickness of the porous outgassing layer 440 can be, purely by way of example, in the range from 1 m to 10 m.

[0069] The outgassing layer 440 serves to temporarily store particles or molecules (e.g., water or a noble gas such as argon (Ar), for example). Releasing these stored or absorbed particles using the heat introduced into the reflective optical element 400 when light pulses are incident on the optically effective surface 400a has as a consequence that correspondingly less energy is available in the surface-near region of the reflective optical element 400 (i.e., in the vicinity of the optically effective surface 400a) and hence a temperature increase, in particular of the reflection layer system 410, also turns out to be lower in comparison with an analogous construction without the outgassing layer 440.

[0070] Although the previously desorbed particles are once again stored in the outgassing layer 440 between successive light pulses, with absorption energy being released, a reduction in the maximum peak temperatures occurring and consequently a protection of, in particular, temperature-sensitive reflection layer systems 410 can be obtained. This is indicated in FIG. 6 on the basis of a comparison of two typical curves for the time dependence of the increase in temperature T in the surface-near region without (curve A) and with (curve B) the presence of the outgassing layer 440.

[0071] It is further clear from FIG. 4 that the reflective optical element 400 further has defects 460 in the exemplary embodiment, said defects facilitating an emergence of the released or desorbed particles. Such defects 460 can be provided in any suitable way, for example in the form of carbon nanotubes, by way of a suitable etching method, etc.

[0072] FIG. 5 shows a further embodiment of a reflective optical element 500 according to the invention, wherein components analogous or substantially functionally identical to FIG. 4 are designated by reference numerals increased by 100.

[0073] The embodiment of FIG. 5 differs from that of FIG. 4 in that a further outgassing layer 570 is provided in addition to the outgassing layer 550 on the side of the latter facing the substrate 505. The particles that were driven out of the outgassing layer 550 as described above can be buffered in this further outgassing layer 570 such that, where applicable, it is possible in this embodiment to dispense with the defects 460 present in the reflective optical element 400 of FIG. 4.

[0074] There can be active cooling between successive light pulses in each case in the embodiments of FIGS. 4 and 5 in order to correspondingly dissipate heat that arises with the release of absorption energy.

[0075] FIG. 7 shows a corresponding further embodiment of a reflective optical element 700 according to the invention, wherein components analogous or substantially functionally identical to FIG. 4 are designated by reference numerals increased by 100. Arranged between the substrate 705 and the reflection layer system 710 in this embodiment is a Peltier element 780, which is constructed from Peltier layers 781, 782 and to which an electric current is appliable by way of electrodes (not plotted here).

[0076] In this way, it is possible to obtain cooling of the substrate 705 at a cost of heating of the reflection layer system 710, with, once again, the circumstances already explained at the outset that, depending on the specific construction of the reflective optical element, a heat influx into the reflection layer system is comparatively unproblematic in comparison with a heat influx into the substrate being exploited.

[0077] The use of a Peltier element 780 described above can also, in particular, be advantageously combined with the presence of a heat radiation layer 220 and/or 320, for example according to FIG. 2 or FIG. 3, since the heat additionally introduced into the reflection layer system 210 and/or 310 by way of the Peltier element 780 can be effectively dissipated in this way.

[0078] FIG. 8 shows a further embodiment of a reflective optical element 800 according to the invention, wherein components analogous or substantially functionally identical to FIG. 7 are designated by reference numerals increased by 100.

[0079] According to FIG. 8, the reflective optical element 800 according to the invention has a heat buffer layer 890, which can be arranged between the reflection layer system 810 and substrate 805 and which has a comparatively high heat capacity. What this can achieve is that the heat introduced into the reflective optical element 800 while light pulses are incident thereon is transferred comparatively quickly from the reflection layer system 810 to the heat buffer layer 890, and so a pronounced temperature increase can be avoided on the part of the reflection layer system 810 (and, once again analogous to FIGS. 4 to 6, for example, and where applicable, it is possible to protect temperature-sensitive layers and/or layer systems). On account of this configuration, it is possible to take account of the circumstances that typical substrate materials, as a rule, have a comparatively low thermal conductivity, and so heat accumulation caused thereby within the reflection layer system can lead to damage of the layer construction in the case of temperature-sensitive layers.

[0080] The heat can be transferred from the heat buffer layer 890 to the substrate 805 in time intervals between the light pulses. As a consequence of the heat dissipation by way of the heat buffer layer 890 described above, it is possible to achieve a lowering of the peak temperatures occurring in the region of the reflection layer system 810. By way of example, the heat buffer layer 890 can have a material which exhibits a phase transition at the operating temperature, wherein particularly efficient buffering of the heat can be obtained in this case on account of the phase transformation occurring here.

[0081] Even though the embodiments described above in each case show a mirror with a reflection layer system in the form of a multiple layer system or layer stack (made of molybdenum (Mo) and silicon (Si) layers, for example), the invention is not restricted thereto. In further embodiments, the reflection layer system can also be an individual layer, for example a ruthenium (Ru) layer of a mirror designed for operation under grazing incidence (also referred to as a GI Mirror; GI=grazing incidence).

[0082] FIG. 9 shows a schematic illustration of an exemplary projection exposure apparatus which is designed for operation in the EUV and in which the present invention can be realized. According to FIG. 9, an illumination device in a projection exposure apparatus 900 designed for EUV comprises a field facet mirror 903 and a pupil facet mirror 904. The light from a light source unit comprising a plasma light source 901 and a collector mirror 902 is directed onto the field facet mirror 903. A first telescope mirror 905 and a second telescope mirror 906 are arranged in the light path downstream of the pupil facet mirror 904. A deflection mirror 907 is arranged downstream in the light path, said deflection mirror directing the radiation that is incident thereon onto an object field in the object plane of a projection lens comprising six mirrors 951-956. At the location of the object field, a reflective structure-bearing mask 921 is arranged on a mask stage 920, said mask being imaged with the aid of the projection lens into an image plane in which a substrate 961 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 960.

[0083] Out of the mirrors 951-956 of the projection lens, it is in particular the mirrors 951 and 952arranged in the starting area of the projection lens with respect to the optical beam paththat can be designed in the manner according to the invention, since the effect attained according to the invention is particularly noticeable as a result of the at said mirrors 951, 952on the basis of the still comparatively low summed reflection losses and hence relatively high light intensities. However, the invention is not restricted to application to these mirrors 951, 952, and so, in principle, other mirrors can also be designed in the manner according to the invention.

[0084] Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are apparent to a person skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly, such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the accompanying patent claims and equivalents thereof.