METHOD OF TREATING A REFLECTIVE OPTICAL ELEMENT, REFLECTIVE OPTICAL ELEMENT AND OPTICAL ARRANGEMENT

Abstract

A method treats a reflective optical element for the VUV wavelength range, wherein the optical element has an aluminium surface. Treating the reflective optical element comprises irradiating the reflective optical element with a hydrogen plasma jet for removing an aluminium oxide layer formed on the aluminium surface. A reflective optical element for use in the VUV wavelength range treated by the method can be included in an optical arrangement.

Claims

1. A method of treating a reflective optical element for the VUV wavelength range, the reflective optical element comprising an aluminium surface, the method comprising: irradiating the reflective optical element with a hydrogen plasma jet to remove an aluminium oxide layer formed on the aluminium surface, thereby forming an exposed aluminium surface; and after removing the aluminium oxide layer, irradiating the exposed aluminium surface with a fluorine plasma jet.

2. The method of claim 1, wherein the hydrogen plasma jet comprises at least one gas selected from the group consisting of N.sub.2 and Ar.

3. The method of claim 1, wherein irradiating the exposed aluminium surface with the fluorine plasma jet forms an aluminium fluoride layer on the aluminium surface.

4. The method of claim 3, further comprising, before forming the aluminium fluoride layer on the aluminium surface, removing a protective layer applied to the aluminium surface.

5. The method of claim 4, wherein the protective layer comprises a material that reacts with fluorine to provide a volatile fluorine species.

6. The method of claim 4, wherein the protective layer comprises silicon or carbon.

7. The method of claim 4, comprising using the fluorine plasma jet to remove the protective layer.

8. The method of claim 1, wherein a protective layer comprising a metal fluoride is supported by the aluminium surface, and the method further comprises irradiating the protective layer with the fluorine plasma jet for post-fluorination.

9. The method of claim 1, further comprising, before irradiating the reflective optical element with the hydrogen plasma jet to remove the aluminium oxide layer formed on the aluminium surface, irradiating the reflective optical element with VUV radiation.

10. The method of claim 1, wherein: irradiating with the hydrogen plasma jet comprises moving the hydrogen plasma jet across the aluminium surface; and/or irradiating with the fluorine plasma jet comprises moving the plasma jet across the aluminium surface.

11. The method of claim 1, wherein irradiating with the hydrogen plasma jet and irradiating with the fluorine plasma jet are done under vacuum conditions.

12. The method of claim 1, wherein the fluorine plasma jet comprises at least one member selected from the group consisting of CF.sub.4, CHF.sub.3, C.sub.2F.sub.6, NF.sub.3, SF.sub.6 and F.sub.2.

13. The method of claim 1, wherein the fluorine plasma jet comprises a carrier gas.

14. A method of treating a reflective optical element for the VUV wavelength range, the reflective optical element comprising an aluminium surface, the method comprising: irradiating the reflective optical element with a fluorine plasma jet.

15. The method of claim 14, wherein the method comprises irradiating the aluminium surface with the plasma jet.

16. The method of claim 15, wherein irradiating the aluminium surface with the fluorine plasma jet forms an aluminium fluoride layer on the aluminium surface.

17. The method of claim 16, further comprising, before forming the aluminium fluoride layer on the aluminium surface, removing a protective layer applied to the aluminium surface.

18. The method of claim 14, wherein irradiating the reflective optical element with the fluorine plasma jet removes an aluminium oxide layer formed on the aluminium surface.

19. The method of claim 14, wherein: irradiating the reflective optical element with the fluorine plasma jet removes an aluminium oxide layer formed on the aluminium surface, thereby forming an exposed aluminium surface; the method further comprises, after removing the aluminium oxide layer, irradiating the exposed aluminium surface with a fluorine plasma jet.

20. The method of claim 19, further comprising, before forming the aluminium fluoride layer on the aluminium surface, removing a protective layer applied to the aluminium surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The figures show:

[0048] FIGS. 1A-1B schematic diagrams of a mirror for the VUV wavelength range with an exposed aluminium surface on and after irradiation with VUV radiation;

[0049] FIGS. 2A-2B schematic diagrams of the mirror from FIG. 1A-1B during and after treatment with a fluorine plasma jet to form an aluminium fluoride layer;

[0050] FIGS. 3A-3C schematic diagrams of the mirror of FIGS. 1A-1B during treatment with a hydrogen plasma jet and during and after treatment with a fluorine plasma jet;

[0051] FIGS. 4A-4B schematic diagrams of a mirror for the VUV wavelength range with a protective layer during and after treatment with a fluorine plasma jet to form an aluminium fluoride layer;

[0052] FIGS. 5A-5B schematic diagrams of a mirror for the VUV wavelength range with a protective layer of a metal fluoride during and after irradiation with VUV radiation;

[0053] FIGS. 6A-6B schematic diagrams of the mirror from FIG. 5A-5B during and after treatment with a fluorine plasma jet for post-fluorination of the protective layer;

[0054] FIG. 7 a schematic diagram of an optical arrangement for the VUV wavelength range in the form of a VUV lithography apparatus; and

[0055] FIG. 8 a schematic diagram of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system.

DETAILED DESCRIPTION

[0056] In the description of the drawings that follows, identical reference signs are used for components that are the same or have the same function.

[0057] FIGS. 1A, 1B show schematic diagrams of a reflective optical element for the VUV wavelength range in the form of a mirror 1. The mirror 1 has a substrate 2 of aluminium, having an aluminium surface 3 that serves as mirror surface. As apparent in FIG. 1A, the reflective optical element 1 is hit during operation in an optical arrangement by VUV radiation 5, which is reflected at the aluminium surface 3 of the mirror 1. The reflective aluminium surface 3 may alternatively be formed on an aluminium layer applied to a substrate, for example of quartz glass or of another material. As apparent in FIG. 1A, a native aluminium oxide layer 4 has been formed on the aluminium surface 3.

[0058] FIG. 1B shows the mirror 1 after a defined period of irradiation with the VUV radiation 5. As apparent in FIG. 1B, the irradiation with the VUV radiation 5 has increased the thickness of the aluminium oxide layer 4. In order to increase the reflectivity of the mirror 1 after irradiation with VUV radiation 5 to the value of about 80% for use in a wafer inspection system for example, the mirror 1, after a defined period of irradiation with the VUV radiation 5, is introduced into a process chamber 6 and subjected to a treatment with a fluorine plasma jet 7, as shown in FIG. 2A. The treatment with the fluorine plasma jet 7 can be performed periodically whenever a particular duration of irradiation with the VUV radiation 5 within the optical arrangement is attained. It may also be possible to conduct the treatment with the fluorine plasma jet 7 whenever the reflectivity of the mirror 1 goes below a threshold value.

[0059] In the treatment of the mirror 1 with the fluorine plasma jet 7, the latter is directed onto the aluminium surface 3 of the mirror 1 and moved across the surface 3 with the aid of a position control system (not shown pictorially). The fluorine plasma jet 7 is generated by a plasma source 8, which may, for example, be an RF plasma source or a microwave plasma source. The fluorine plasma jet 7 is a gas jet in which reactive fluorine species are present in an inert carrier gas, e.g. N.sub.2, He or Ar, Ne, Kr, etc. The reactive fluorine species, for example in the form of fluorine radicals, are produced from a reactive fluorine gas in the plasma source 8. The reactive fluorine gas may, for example, be a gas selected from the group comprising: CF.sub.4, CHF.sub.3, C.sub.2F.sub.6, NF.sub.3, SF.sub.6 and F.sub.2. Also possible are fluorine-containing gases such as XeF.sub.2, XeF.sub.4, XeF.sub.6, etc. It is also possible to add oxygen (O.sub.2) to the fluorine plasma jet 7.

[0060] The power of the plasma source 8 and other plasma parameters may be adjusted such that the aluminium oxide layer 4 is firstly removed by reactive (ion) etching, as described in the articles cited above or in the thesis by F. Kazemi. After the removal of the aluminium oxide layer 4, it is possible using the fluorine plasma jet 7 to oxidize the metallic aluminium at the aluminium surface 3, in order to form an aluminium fluoride layer 9 as shown in FIG. 2B.

[0061] Alternatively, the plasma parameters can be adjusted such that, in the course of irradiation with the fluorine plasma jet 7 in the aluminium oxide layer 4, the following chemical reaction proceeds:

##STR00003##

[0062] In the above reaction equation, F* denotes a reactive fluorine species.

[0063] The treatment with the fluorine plasma jet 7 in this case converts the aluminium oxide layer 4 to the aluminium fluoride layer 9 shown in FIG. 2B. The aluminium fluoride layer 9 forms a passivating protective layer and has distinctly lower absorption for the VUV radiation 5 than the aluminium oxide layer 4. The mirror 1 after the treatment, as shown in FIG. 2B, has sufficient reflectivity in order to be introduced (possibly reintroduced) into an optical arrangement and to be operated therein.

[0064] As described above, the treatment with the fluorine plasma jet 7 is effected in the process chamber 6 shown in FIG. 2A. In the example shown, the treatment is effected under vacuum conditions in order to minimize the risk of surface contamination. It is alternatively possible to conduct the treatment with the fluorine plasma jet 7 in the process chamber 6 at higher pressures, for example at atmospheric pressure.

[0065] The components present in the process chamber 6 withstand the attack of reactive fluorine species or of fluorine gas. Monitoring of the process chamber 6 for escaping fluorine gas or corresponding safety checking is generally likewise desirable. The temperature in the process chamber 6 should also be monitored and, if desired, adjusted or regulated.

[0066] FIGS. 3A-3C show an alternative approach for treating the mirror 1 from FIG. 1A for restoration of its reflectivity. In this treatment, in a first step shown in FIG. 3a, the aluminium oxide layer 4 is irradiated with a hydrogen plasma jet 10 in order to remove it from the aluminium surface 3. The hydrogen plasma jet 10 is a gas jet in which reactive hydrogen species are present in an inert carrier gas, e.g. N.sub.2, He or Ar, Ne, Kr, etc. The reactive hydrogen species are generated in the further plasma source 11, for example in that molecular hydrogen is moved past a heated filament. It will be apparent that the aluminium oxide layer 4 can also be removed from the aluminium surface 3, if appropriate, in a different way than with the aid of the hydrogen plasma jet 10.

[0067] In a second step of the treatment of the mirror 1, as shown in FIG. 3B, the exposed aluminium surface 3 is irradiated with the fluorine plasma jet 7, as described in association with FIG. 2A. The metallic aluminium at the surface 3 is oxidized here to aluminium fluoride, in which case, for example, the following chemical reaction can proceed:

##STR00004##

[0068] After the treatment described in FIGS. 3A-3B, the reflective optical element 1 has a passivating aluminium fluoride layer 9 (cf. FIG. 3C) and can be used in an optical arrangement. The treatment with the hydrogen plasma jet 10 and with the fluorine plasma jet 7 can be effected in one and the same process chamber 6, but it is also possible that the treatment is conducted in two different process chambers or in two different plasma treatment systems.

[0069] By varying the time duration in which the fluorine plasma jet 7 is directed to a specific location of the exposed aluminium surface 3, a passivating aluminium fluoride layer 9 with a location-dependent thickness profile can be obtained. For instance, when the time duration of the irradiation with the fluorine plasma jet 7 is varied in a rotationally-symmetric manner, a passivating aluminium fluoride layer 9 with a rotationally-symmetric thickness profile can be generated. Such a rotationally-symmetric thickness profile can be used e.g. as a grey filter to produce or to compensate apodization effects.

[0070] FIGS. 4A-4B show the treatment of a mirror 1 for the VUV wavelength range, in which a dedicated protective layer 12 consisting of at least one material M that forms a volatile fluorine species M.sub.aF.sub.b with fluorine F is applied to the aluminium surface 3. The protective layer 12 may consist, for example, of silicon or of carbon. The protective layer 12 has been deposited in a preceding step (not shown pictorially) in the process chamber 6 using a conventional coating method immediately after the deposition of the aluminium on the aluminium surface 3, in order to prevent the formation of a native aluminium oxide layer.

[0071] As shown in FIG. 4A, the protective layer 12 is removed by reactive etching using a fluorine plasma jet 7, and the aluminium fluoride layer 9 shown in FIG. 4B is formed simultaneously or subsequently. In the treatment of the protective layer 12 of silicon as shown in FIG. 4A, for example, the following chemical reactions may proceed:

##STR00005##

[0072] SiF.sub.4 is a volatile fluoride that does not remain on the aluminium surface 3, such that the protective layer 12 of silicon is removed with the aid of the fluorine plasma jet 7 until the aluminium surface 3 is exposed. Correspondingly, in the case of the protective layer 12 composed of carbon too, irradiation with a fluorine plasma jet 7 forms volatile CF.sub.4. In the irradiation, oxygen may additionally be added as a reactive gas in order to promote the formation of volatile CO.sub.2 and to increase the etch rate. The carbon is removed until the aluminium surface 3 is exposed. The reaction of the metallic aluminium at the exposed aluminium surface 3 with the activated fluorine species in the fluorine plasma jet 7 forms the passivating aluminium fluoride layer 9 on the mirror 1 as shown in FIG. 4B.

[0073] FIGS. 5A-5B show a mirror 1 where, by contrast with the mirror 1 shown in FIGS. 1A-1B, the irradiation with VUV radiation 5 was preceded by application of a protective layer 13 in the form of a metal fluoride layer. The metal fluoride may be, for example, LiF, MgF.sub.2 or AlF.sub.3. If the metal fluoride is AlF.sub.3, the protective layer 13 may have been formed by the treatment of the mirror 1 described above and may correspond to the aluminium fluoride layer 9.

[0074] The metal fluoride in the protective layer 13, on irradiation with the VUV radiation 5, reacted with an oxidizing gas in the environment of the mirror 1, for example with O.sub.2, or possibly with O.sub.3, H.sub.2O, N.sub.2O, O*, OH*, NO*, O (.sup.1D), etc., and formed a metal oxide. In addition, at the surface 3 that forms the interface between the protective layer 13 and the aluminium substrate 2, aluminium was also converted to aluminium oxide (Al.sub.2O.sub.3), as indicated in FIG. 5B. As likewise indicated in FIG. 5B, colour centres 14 are formed in the protective layer 13 on irradiation with the VUV radiation 5.

[0075] Both the conversion of the metal fluoride in the protective layer 13 to a metal oxide and the formation of the colour centres 14, and the oxidation of the metallic aluminium at the surface 3 to Al.sub.2O.sub.3, result in a distinct reduction in reflectivity of the mirror 1.

[0076] The mirror 1 shown in FIG. 5B is therefore subjected to a treatment using a fluorine plasma jet 7, as shown in FIG. 6A. The mirror 1, or more specifically the protective layer 13, is irradiated in a process chamber which is not shown in FIG. 6A. The process chamber or the plasma coating system may be designed as described above in association with FIG. 2A.

[0077] The power of the plasma source 8 and other plasma parameters are adjusted such that the following chemical reactions proceed in the protective layer 13 or at the surface 3:

##STR00006##

[0078] In the above reaction equations, MO denotes a metal oxide and MF a metal fluoride. As apparent from the reaction equations, the reaction with fluorine species F* results in refluorination of the protective layer 13 in that the metal oxide MO formed is converted to a metal fluoride MF. The treatment with the fluorine plasma jet 7 also converts aluminium oxide formed at the aluminium surface 3 to aluminium fluoride, which likewise leads to an increase in reflectivity.

[0079] The treatment with the fluorine plasma jet 7 additionally eliminates the colour centres 14, as apparent in FIG. 6B. These above-described processes lead to an increase in the VUV reflectance of the mirror 1 to up to 80%, and to an increase in lifetime of the mirror 1.

[0080] The mirror 1 that has been treated in the manner described above can be used in different optical arrangements for the VUV wavelength range.

[0081] FIG. 7 shows an optical arrangement for the VUV wavelength range in the form of a VUV lithography apparatus 21. The VUV lithography apparatus 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 additionally has a radiation source 24, which can be an excimer laser, for example.

[0082] The radiation 25 emitted by the radiation source 24 is conditioned with the aid of the illumination system 22 such that a mask 26, also called a reticle, is illuminated thereby. In the example shown, the illumination system 22 has a housing 32, in which there are disposed both transmissive and reflective optical elements. In a representative manner, the illustration shows a transmissive optical element 27, which focuses the radiation 25, and a reflective optical element 28, which deflects the radiation.

[0083] The mask 26 has, on its surface, a structure which is transferred to an optical element 29 to be exposed, for example a wafer, with the aid of the projection system 23 for the purpose of producing semiconductor components. In the example shown, the mask 26 is designed as a transmissive optical element. In alternative embodiments, the mask 26 can also be designed as a reflective optical element.

[0084] The projection system 22 has at least one transmissive optical element in the example shown. The example shown illustrates, in a representative manner, two transmissive optical elements 30, 31, which serve, for example, to reduce the structures on the mask 26 to the size desired for the exposure of the wafer 29.

[0085] Both in the illumination system 22 and in the projection system 23, a wide variety of transmissive, reflective or other optical elements can be combined with one another as desired, including in a more complex manner. Optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0086] FIG. 8 shows an optical arrangement for the VUV wavelength range in the form of a wafer inspection system 41, but it may also be a mask inspection system. The wafer inspection system 41 has an optical system 42 with a radiation source 54, from which radiation 55 is directed onto a wafer 49 via the optical system 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In the case of a mask inspection system, it would be possible to replace the wafer 49 with a mask to be examined. The radiation reflected, diffracted and/or refracted by the wafer 49 is directed onto a detector 50 for further evaluation by a further concave mirror 48, which is likewise associated with the optical system 42, via a transmissive optical element 47. The wafer inspection system 41 additionally has a housing 52, in which there are disposed the two mirrors 46, 48 and the transmissive optical element 47. The radiation source 54 may, for example, be exactly one radiation source or a combination of a plurality of individual radiation sources in order to provide a substantially continuous radiation spectrum. In modifications, one or more narrowband radiation sources 54 can also be used.

[0087] At least one reflective optical element 28 of the VUV lithography apparatus 21 shown in FIG. 7 and at least one of the reflective optical elements 46, 48 of the wafer inspection system 41 shown in FIG. 8 may have been treated in the manner described above and irradiated using a fluorine plasma jet 7. In particular, at least one of the reflective optical elements 28, 46, 48 may have a protective layer 13 in the form of a metal fluoride layer that has been post-fluorinated using the method described in association with FIGS. 6A-6B