OPTICAL ELEMENT FOR THE VUV WAVELENGTH RANGE, OPTICAL ARRANGEMENT, AND METHOD FOR MANUFACTURING AN OPTICAL ELEMENT

Abstract

An optical element (7, 8) for the VUV wavelength range includes a substrate (7a, 8a), and a coating (15) applied to the substrate (7a, 8a). The coating (15) has at least one fluorine scavenger layer (17, 17a, . . . , 17n) having a fluoride material (M.sup.x+F.sub.x.sup.−) doped with at least one preferably metallic dopant ion (A.sup.x+). Also described are an optical arrangement that includes at least one such optical element (7, 8), as well as a method for producing such an optical element (7, 8).

Claims

1. An optical element for the very-ultraviolet (VUV) wavelength range, comprising: a substrate, and a coating applied to the substrate, wherein the coating has at least one fluorine scavenger layer comprising a fluoride material doped with at least one dopant ion, and wherein the coating has at least one fluoride layer and the fluorine scavenger layer is applied to a side of the fluoride layer remote from the substrate.

2. The optical element as claimed in claim 1, wherein the dopant ion is a metallic dopant ion.

3. The optical element as claimed in claim 1, wherein the fluoride material has a host lattice ion, and an ionic radius of the host lattice ion differs by not more than 20% from an ionic radius of the dopant ion.

4. The optical element as claimed in claim 1, wherein the fluoride material has a metallic host lattice ion, and an ionic radius of the host lattice ion differs by not more than 15% from an ionic radius of the dopant ion.

5. The optical element as claimed in claim 3, wherein the host lattice ion of the fluoride material has a same valency as the dopant ion.

6. The optical element as claimed in claim 1, wherein the dopant ion has an electron configuration with at least one unpaired valence electron.

7. The optical element as claimed in claim 6, wherein the dopant ion has an electron configuration with a half-filled orbital.

8. The optical element as claimed in claim 1, wherein the fluoride scavenger layer is transparent to radiation in the VUV wavelength range.

9. The optical element as claimed in claim 1, wherein the dopant ion is selected from the group consisting essentially of: Gd.sup.3+, Eu.sup.2+, Mn.sup.2+, Fe.sup.3+, Ru.sup.3+ and TI.sup.+.

10. The optical element as claimed in claim 1, wherein the fluoride material has a host lattice ion selected from the group consisting essentially of: Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+, Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Al.sup.3+, La.sup.3+ and Y.sup.3+.

11. The optical element as claimed in claim 1, wherein the doped fluoride material of the fluorine scavenger layer is selected from the group consisting essentially of: RbF:TI.sup.+, KF:TI.sup.+, MgF.sub.2:Mn.sup.2+, SrF.sub.2:Eu.sup.2+, BaF.sub.2:Eu.sup.2+, LaF.sub.3:Gd.sup.3+, YF.sub.3:Gd.sup.3+, AlF.sub.3:Fe.sup.3+.

12. The optical element as claimed in claim 1, wherein the dopant ion is present in a further fluoride material that forms a solid solution with the fluoride material.

13. The optical element as claimed in claim 1, wherein the fluorine scavenger layer forms a capping layer of the coating or wherein the fluorine scavenger layer forms a diffusion barrier between the fluoride layer and a further layer.

14. The optical element as claimed in claim 13, wherein the further layer consists essentially of a further fluoride layer.

15. The optical element as claimed in claim 1, wherein the coating forms a reflective coating or an antireflection coating for radiation in the VUV wavelength range.

16. An optical arrangement configured for operation in the VUV wavelength range and comprising: at least one optical element as claimed in claim 1, and configured as a wafer inspection system or as a VUV lithography apparatus.

17. A method for producing an optical element for the very ultraviolet (VUV) wavelength range, comprising: applying a coating to a substrate, wherein said applying of the coating comprises applying at least one fluorine scavenger layer, where the fluorine scavenger layer includes a fluoride material doped with at least one dopant ion, wherein the coating comprises at least one fluoride layer, and applying the at least one fluorine scavenger layer to a side of the fluoride layer remote from the substrate.

18. The method as claimed in claim 17, wherein the fluorine scavenger layer is applied by simultaneous deposition of the fluoride material and of a further fluoride material containing the dopant ion.

19. The method as claimed in claim 17, wherein the fluorine scavenger layer is applied by deposition of the fluoride material doped with the dopant ion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Working examples are shown in the schematic drawing and are explained in detail in the description that follows. The figures show:

[0048] FIG. 1 a schematic diagram of an optical arrangement for the VUV wavelength range in the form of a VUV lithography apparatus,

[0049] FIG. 2 a schematic diagram of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system,

[0050] FIGS. 3A and 3B schematic diagrams of a transmitting optical element having a coating with a fluorine scavenger layer as capping layer (FIG. 3A), and of a reflective optical element having a reflective coating with a multitude of fluorine scavenger layers (FIG. 3B), and

[0051] FIGS. 4A and 4B schematic diagrams of a substrate of an optical element on deposition of the fluorine scavenger layer either stoichiometrically (FIG. 4A) or simultaneously through co-evaporation (FIG. 4B).

DETAILED DESCRIPTION

[0052] In the description of the drawings that follows, identical reference signs are used for components that are the same or analogous or have the same function or the analogous function.

[0053] FIG. 1 shows a schematic of an optical arrangement 1 in the form of a VUV lithography apparatus, in particular for wavelengths in the range of between 100 nm and 200 nm or 160 nm. The VUV lithography apparatus 1 has, as primary components, two optical systems in the form of an illumination system 2 and a projection system 3. For the performance of an exposure process, the VUV lithography apparatus 1 has a radiation source 4 which may, for example, be an excimer laser which emits radiation 5 at a wavelength in the VUV wavelength range of, for example, 193 nm, 157 nm or 126 nm and may be an integral part of the VUV lithography apparatus 1.

[0054] The radiation 5 emitted by the radiation source 4 is processed with the aid of the illumination system 2 in a manner such that a mask 6, also called reticle, can be fully illuminated thereby. In the example shown in FIG. 1, the illumination system 2 has both transmitting and reflective optical elements. In a representative manner, FIG. 1 shows a transmitting optical element 7, which focuses the radiation 5, and a reflective optical element 8, which deflects the radiation 5, for example. In a known manner, in the illumination system 2, a wide variety of transmitting, reflecting or other optical elements can be combined with one another in any manner, even in a more complex manner.

[0055] The mask 6 has, on its surface, a structure which is transferred to an optical element 9 to be exposed, for example a wafer, with the aid of the projection system 3 in the context of production of semiconductor components. In the example shown, the mask 6 is designed as a transmitting optical element. In alternative embodiments, the mask 6 may also be designed as a reflective optical element. The projection system 2 has at least one transmitting optical element in the example shown. The example shown illustrates, in a representative manner, two transmitting optical elements 10, 11, which serve, for example, to reduce the structures on the mask 6 to the size desired for the exposure of the wafer 9. In the case of the projection system 3 as well, it is possible for reflective optical elements among others to be provided, and for any optical elements to be combined with one another as desired in a known manner. It should be pointed out that optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0056] FIG. 2 shows a schematic of an illustrative embodiment of an optical assembly in the form of a wafer inspection system 21. The explanations that follow are also analogously applicable to inspection systems for inspection of masks.

[0057] The wafer inspection system 21 has an optical system 22 with a radiation source 24, from which the radiation 25 is directed onto a wafer 29 by the optical system 22. For this purpose, the radiation 25 is reflected onto the wafer 29 by a concave mirror 26. In the case of a mask inspection system 2, it would be possible to dispose a mask to be examined in place of the wafer 29. The radiation reflected, diffracted and/or refracted by the wafer 29 is guided onto a detector 30 for further evaluation by a further concave mirror 28 that likewise forms part of the optical system 22 via a transmitting optical element 27. The radiation source 24 may, for example, be exactly one radiation source or a combination of multiple individual radiation sources, in order to provide an essentially continuous radiation spectrum. In modifications, it is also possible to use one or more narrowband radiation sources 24. Preferably, the wavelength or the wavelength band of the radiation 25 generated by the radiation source 24 lies in the range of between 100 nm and 200 nm, more preferably between 110 nm and 190 nm.

[0058] In the example shown in FIG. 1, the illumination system 2 has a housing 12 in which there is formed an interior 13 within which there are disposed the transmissive optical element 7 and the reflective optical element 8 in the form of the mirror. Correspondingly, the optical system 22 of the wafer inspection system 21 of FIG. 2 has a housing 32 in which there is formed an interior 33 in which there are disposed the two mirrors 26, 28 and the transmissive optical element 27.

[0059] The lithography apparatus 1 of FIG. 1 also has a gas inlet 14 that serves to feed an inert gas into the interior 13, for example in the form of a noble gas, i.e. in the form of He, Ne, Ar, Kr, Xe or in the form of nitrogen (N.sub.2). Correspondingly, the wafer inspection system 21 of FIG. 2 also has a gas inlet 34 that serves for supply of an inert gas into the interior 33 of the housing 32 of the optical system 22.

[0060] FIG. 3A shows, by way of example, the transmitting optical element 7 of FIG. 1 in a detail diagram. The transmitting optical element 7 has a substrate 7a of an ionic crystal, for example in the form of MgF.sub.2, and is irradiated with radiation 5 from the radiation source 4 that typically has a high intensity. For protection of the substrate 7a, for example from a rearrangement in the irradiation, a coating 15 is applied to a surface of the substrate 7a and, in the example shown, has a fluoride layer 16, and a fluorine scavenger layer 17 applied thereto.

[0061] In the case of the irradiation of the optical element 7 with the radiation 5 in the VUV wavelength range, on account of the high radiation intensity, there can be degradation of the fluoride layer 16 in which fluorine defects are generated and interstitial fluorine is formed. In order to counteract the diffusion of fluorine atoms through the surface of the fluoride layer 16 into the environment, in the example shown in FIG. 3A, it is possible to apply a fluorine scavenger layer 17 to the fluoride layer 16. The material of the fluoride layer 16 may, for example, be MgF.sub.2, AlF.sub.3, LiF, LaF.sub.3, GdF.sub.3, BaF.sub.2 or another transparent fluoride material.

[0062] The transmitting optical element 27 shown in FIG. 2 has a substrate 27a composed of an ionic crystal and a coating 15 (not shown pictorially), which differs from the coating 15 shown in FIG. 3A in that it has an individual layer in the form of a fluorine scavenger layer 17. The coating 15 of the transmitting optical element 27 thus consists of the fluorine scavenger layer 17, which may be formed in the same way as the fluorine scavenger layer 17 described in connection with FIG. 3A.

[0063] FIG. 3B shows, by way of example, the reflective optical element 8 of FIG. 1 in a detailed description. The reflective optical element 8 has a substrate 8a, for example composed of a fluoridic material or of silicon. A reflective coating 15 is applied to the substrate 8a for reflection of the VUV radiation 5. The reflective coating 15 comprises an aluminum layer 18 disposed adjacent to the substrate 8a, and a sequence of n pairs of layers, each having a fluoride layer 16a, . . . , 16n and a fluorine scavenger layer 17a, . . . , 17n applied to the respective fluoride layer 16a, . . . , 16n. It will be apparent that the coating may have just a single pair of layers 16a, 17a (n=1).

[0064] With the exception of the uppermost fluorine scavenger layer 17n, which forms the capping layer of the reflective coating 15, the fluorine scavenger layers 17a, . . . , 17m serve as diffusion barriers between every two adjacent fluoride layers 16b, . . . , 16n. The pairs of layers 16a, 17a, . . . , 16n, 17n serve firstly to protect the aluminum layer 18 from oxidation and secondly to increase the reflectivity of the coating 15 for the radiation 5 in the VUV wavelength range. Accordingly, the refractive indices of the respective pairs of layers 16a, 17a, . . . , 16n, 17n are matched to one another such that a high reflectivity is established within a desired wavelength range within the VUV wavelength range. It will be apparent that the reflective optical elements 26, 28 shown in FIG. 2 are also provided, or can be provided, analogously with a reflective coating 15.

[0065] In the case of the reflective optical element 8 shown in FIG. 3B, as in the case of the transmitting optical element 27, it is possible to apply just a single fluorine scavenger layer 17 to the aluminum layer 18, which serves as protective layer and as capping layer. It is thus not a requirement for the coating 15 of the reflective optical element 8 to have one or more fluoride layers 16a, . . . , 16n, as shown in FIG. 3B.

[0066] The pairs of layers 16a, 17a, . . . , 16n, 17n may also be applied to the transmitting optical element 7 shown in FIG. 3A, in order to form an antireflection coating rather than a reflective coating. For this purpose, the layer thicknesses and the pairs of materials of the respective pairs of layers 16a, 17a, . . . , 16n, 17n are adjusted suitably. It will be apparent that such an antireflection coating 15 does not have an aluminum layer, as is the case in the diagram shown in FIG. 3B.

[0067] In the case of the reflective optical element 8 shown in FIG. 3B as well, it is optionally possible to dispense with the aluminum layer 18, meaning that the reflective coating 15 may take the form of a dielectric multilayer coating having exclusively pairs of layers composed of a fluoride layer 16a, . . . , 16n, and a fluorine scavenger layer 17a, . . . , 17n applied to the respective fluoride layer 16a, . . . , 16n.

[0068] Rather than an antireflective effect, the coating 15 may also have a beam divider effect, i.e. transmit a first fraction of radiation and reflect a second fraction of radiation. The (partly) transmitting optical element 7, 27 in this case forms a beam divider.

[0069] The fluorine scavenger layer 17, 17a-17n shown in FIGS. 3A and 3B is formed from a fluoride material M.sup.x+F.sub.x.sup.+ as ionic host lattice, doped with at least one generally metallic dopant ion A.sup.x+. The doped fluoride material of the fluorine scavenger layer 17, 17a-17n typically has the following chemical structural formula:

M.sup.x+F.sub.x .sup.−:A.sup.x+

where M denotes the (generally metallic) atom of the host lattice ion M.sup.x+ of the fluoride material, A the dopant atom of the dopant ion A.sup.x+, and x the valency (ionic charge) of the metal atom or dopant atom.

[0070] Suitable materials for the dopant ion A.sup.x− for production of a fluorine scavenger layer 17, which potentially forms a stable layer having a fluorine scavenger effect, may be selected using the criteria detailed in the following paragraphs:

[0071] A necessary property for the dopant ion A.sup.x− is for it to have an ionic radius R.sub.D similar to the ionic radius R.sub.I of the (metallic) host lattice ion M.sup.x+ of the fluoride material M.sup.x+F.sub.x.sup.−. This means that the ionic radius R.sub.I of the metallic host lattice ion M.sup.x+ should differ by not more than 20%, preferably by not more than 15%, from the ionic radius R.sub.D of the dopant ion A.sup.x− (or vice versa).

[0072] The deviation between the ionic radius R.sub.I of the host lattice ion M.sup.x+ and the ionic radius R.sub.D of the dopant ion A.sup.x− is determined here by the following formula:

(R.sub.I−R.sub.D)/R.sub.I

[0073] A sufficient but not a necessary condition on the dopant ion A.sup.x− is that the host lattice ion M.sup.x+ of the fluoride material M.sup.x+F.sub.x.sup.− has the same valency x as the dopant ion A.sup.x+. It is favorable, although not absolutely essential, for the host lattice ion M.sup.x+ and the dopant ion A.sup.x− to have the same valency, i.e. the same ionic charge x.

[0074] For the fluorine scavenger layer 17, it is likewise favorable when the dopant ion A.sup.x− has an electron configuration with at least one unpaired valency electron. Unpaired valency electrons of the dopant ion A.sup.x− are generally required for complexation with the interstitial fluorine, which reduces the mobility of the fluorine species. In particular, an electron configuration with a half-filled orbital has been found to be advantageous: If the dopant ion A.sup.x− has a half-filled orbital, this constitutes a chemically particularly stable configuration.

[0075] The fluorine scavenger layer 17 and also the fluoride layer 16 are generally transparent to the radiation 5 in the VUV wavelength range. The dopant ions A.sup.x− should therefore not contain any material having high absorption in the VUV wavelength range.

[0076] Table 1 below gives suitable materials for host lattice ions M.sup.x+ and for dopant ions A.sup.x+, and the ionic radii, coordination and electron configuration thereof. The values reported for the ionic radii are taken from the following source: “http://abulafia.mt.ic.ac.uk/shannon/ptable.php”.

TABLE-US-00001 TABLE 1 Ionic radius in Electron Ion ångströms Coordination configuration Host lattice ion M.sup.x+ Li.sup.+ 0.76 VI [He] Na.sup.+ 1.02 VI [Ne] K.sup.+ 1.38 VI [Ar] Rb.sup.+ 1.52 VI [Kr] Mg.sup.2+ 0.72 VI [Ne] Ca.sup.2+ 1.00 VI [Ar] Sr.sup.2+ 1.18 VI [Kr] Ba.sup.2+ 1.35 VI [Xe] Al.sup.3+ 0.535 VI [Ne] La.sup.3+ 1.032 VI [Xe] Y.sup.3+ 1.04 VI [Kr] Dopant ion A.sup.x+ Gd.sup.3+ 0.938 VI [Xe] 4f.sup.7 Eu.sup.2+ 1.17 VI [Xe] 4f.sup.7 Mn.sup.2+ 0.81 VI (low spin) [Ar] 3d.sup.5 Fe.sup.3+ 0.55 VI (low spin) [Ar] 3d.sup.5 Ru.sup.3+ 0.68 VI [Kr] 3d.sup.5 TI.sup.+ 1.5 VI [Xe] 3d.sup.106s.sup.12p.sup.1

[0077] With regard to the difference in their ionic radii, suitable pairs of host lattice ions M.sup.x+ or fluoride materials and dopant ions A.sup.x− are given in table 2 below, in which possible combinations of fluoride materials for production of a respective fluorine scavenger layer 17, 17a, . . . , 17n by coevaporation (see below) are also described:

TABLE-US-00002 TABLE 2 Dopant Difference in Fluoride materials Fluoride ion ionic radius for coevaporation RbF Tl+ 0.01% RbF + TLF KF TI+ −13.6% KF + TlF MgF.sub.2 Mn.sup.2+ −12.5% MgF.sub.2 + MnF.sub.2 SrF.sub.2 Eu.sup.2+ 0.8% SrF.sub.2 + EuF.sub.2 BaF.sub.2 Eu.sup.2+ 13.3% BaF.sub.2 + EuF.sub.2 LaF.sub.3 Gd.sup.3+ 9.1% LaF.sub.3 + GdF.sub.3 YF.sub.3 Gd.sup.3+ 9.8% YF.sub.3 + GdF.sub.3 AlF.sub.3 Fe.sup.3+ −2.8% AlF.sub.3 + FeF.sub.3

[0078] Of the material combinations described above, the following have been found to be especially favorable: LaF.sub.3:Gd.sup.3+, MgF.sub.2:Mn.sup.2+, SrF.sub.2:Eu.sup.2+, BaF.sub.2:Eu.sup.2+, YF.sub.3:Gd.sup.3+, AlF.sub.3:Fe.sup.3+.

[0079] For all the combinations of materials described above for a fluoride scavenger layer 17, 17a, . . . , 17n, it is favorable when the dopant ion A.sup.x− has a concentration in the doped fluoride material (M.sup.x+F.sub.x.sup.−: A.sup.x+) of between 0.1 at % and 2.0 at %, especially between 0.2 at % and 1.0 at %.

[0080] If the concentration of the dopant ion A.sup.x− is increased further, the fluorine scavenger layer 17, 17a, . . . , 17n typically forms a pseudo-binary mixture or a solid solution composed of the fluoride material M.sup.x+F.sub.x.sup.− and a further fluoride material A.sup.x+F.sub.x.sup.− having the chemical composition

(M.sup.x+F.sub.x.sup.−).sub.y(A.sup.x+F.sub.x.sup.−).sub.1−y

where y may assume values of y=0 to y=1, preferably of y=0.1 toy=0.9. Such a solid solution may be formed, for example, by coevaporation (see below).

[0081] The optical elements 7, 8 described in connection with FIGS. 3A and 3B may be produced, for example, in the manner described hereinafter in connection with FIGS. 4A and 4B. In the production of the respective optical element 7, 8, the respective substrate 7a, 8a is introduced into a coating system (not shown pictorially) in which there are disposed two evaporator sources 19a, 19b, each designed for evaporation of a fluoride material which is deposited on the substrate 7a, 8a. The substrate 7a, 8a is rotated about its center axis during the deposition, as indicated in FIGS. 4A and 4B.

[0082] In the example shown in FIG. 4A, in a first evaporation step, the fluoride layer 16 is deposited on the substrate 7a in that the first evaporator source 19a of the material of the fluoride layer 16, LaF.sub.3 in the example shown, is evaporated. In the example shown in FIG. 4A, the host lattice material, LaF.sub.3 in the present example, is doped beforehand with the dopant ion, e.g. Gd.sup.3+, and introduced into the second evaporator source 19b. Once the fluoride layer 16 having the desired thickness has been applied to the substrate 7a, the second evaporator source 19b is activated in order to stoichiometrically deposit the doped material of the fluorine scavenger layer, for example in the form of LaF.sub.3:Gd.sup.3+, onto the fluoride layer 16.

[0083] In the example shown in FIG. 4B, the first fluoride layer 16a is deposited as described in connection with FIG. 4A, in that the first evaporator source 19a is activated. In the example shown in FIG. 4B, the second evaporator source 19b comprises a further fluoride material A.sup.x+F.sup.−.sub.x containing the dopant ion A.sup.x+ (in the present example: GdF.sub.3). In this case, the first fluorine scavenger layer 17a is deposited by simultaneously activating the two evaporator sources (coevaporation), forming a pseudo-binary mixture or solid solution (LaF.sub.3).sub.(1−x)(GdF.sub.3).sub.x with x=0 . . . 1 of the two fluoride materials from the two evaporator sources 19a, 19b. The coevaporation in the case of the materials described in table 2 can be effected analogously to the manner described in connection with FIG. 4A.

[0084] In summary, the fluorine scavenger layer(s) 17, 17a, . . . , 17n described above can increase the lifetime of the respective optical elements 7, 8, 26, 27, 28, since the degradation of the respective fluoride layer(s) 16, 16a, . . . , 16n can be prevented or distinctly slowed. In this way, it is possible to dispense with the frequent exchange of the respective optical elements 7, 8, 26, 27, 28. It is typically likewise possible to dispense with the applying of protective layers of other materials, for example of oxidic materials, which generally have very high absorption at wavelengths of less than 160 nm. It is also possible to prevent the supply of gases intended to potentially protect the optical elements 7, 8, 26, 27, 28 on irradiation, or it is generally possible to distinctly reduce the concentrations or partial pressures of such gases.