OPTICAL ELEMENT HAVING A PROTECTIVE COATING, METHOD FOR THE PRODUCTION THEREOF AND OPTICAL ARRANGEMENT

Abstract

An optical element includes: a substrate, a reflective coating, applied to the substrate, for reflecting radiation in a first wavelength range (Δλ.sub.1) between 100 nm and 700 nm, preferably between 100 nm and 300 nm, more preferably between 100 nm and 200 nm, and a protective coating applied to the reflective coating. The substrate is formed from a material which is transparent to the radiation in the first wavelength range (Δλ.sub.1). The reflective coating is applied to a rear face of the substrate and is structured to reflect radiation that passes through the substrate to the reflective coating. Also disclosed are an optical arrangement with at least one such optical element and a method of producing such an optical element.

Claims

1. An optical element, comprising: a substrate, a reflective coating, applied to a rear face of the substrate, for reflecting radiation in a first wavelength range (Δλ.sub.1) between 100 nm and 300 nm, and a protective coating applied to the reflective coating, wherein the substrate is formed from a fluoridic material which is transparent to the radiation in the first wavelength range (Δλ.sub.1), wherein the reflective coating is structured to reflect radiation that passes through the substrate to the reflective coating, and wherein the protective coating comprises at least one layer of a material non-transparent to the first wavelength range (Δλ.sub.1).

2. The optical element of claim 1, wherein the first wavelength range (Δλ.sub.1) is between 100 nm and 200 nm.

3. The optical element of claim 1, wherein the protective coating has a thickness of at least 50 nm.

4. The optical element of claim 1, wherein the protective coating has at least one layer of an oxidic material which is selected from the group consisting of: Al.sub.2O.sub.3, SiO.sub.2, MgO, BeO, HfO.sub.2, Sc.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3 and combinations thereof.

5. The optical element of claim 1, wherein the reflective coating consists essentially of aluminium or an aluminium alloy.

6. The optical element of claim 1, wherein the reflective coating comprises a multilayer coating having a plurality of alternating layers composed of materials having different refractive indices (n.sub.a, n.sub.b).

7. The optical element of claim 6, wherein the multilayer coating has at least one layer of a fluoridic material which is selected from the group consisting of: AlF.sub.3, LiF, BaF.sub.2, NaF, MgF.sub.2, CaF.sub.2, LaF.sub.3, GdF.sub.3, HoF.sub.3, YbF.sub.3, YF.sub.3, LuF.sub.3, ErF.sub.3, Na.sub.3AlF.sub.6, Na.sub.5Al.sub.3F.sub.14, ZrF.sub.4, HfF.sub.4 and combinations thereof.

8. The optical element of claim 6, wherein at least one layer of aluminium or an aluminium alloy is applied to the multilayer coating.

9. The optical element of claim 6, wherein the protective coating takes the form of a multilayer coating having a plurality of alternating layers of materials having different refractive indices.

10. The optical element of claim 1, further comprising a further substrate on which a surface is formed, which is bonded to a surface of the protective coating by a direct bond, wherein the surface bonded to the surface of the protective coating is formed atop a coating applied to the further substrate.

11. The optical element of claim 10, wherein the substrate has a thickness (D) of less than 5 mm.

12. The optical element of claim 10, wherein the substrate, the further substrate, the protective coating, the reflective coating and the coating of the further substrate are transparent in a second wavelength range (Δλ.sub.2) different than the first wavelength range (Δλ.sub.1), wherein the second wavelength range (Δλ.sub.2) comprises wavelengths greater than wavelengths of the first wavelength range (Δλ.sub.1) and comprises wavelengths between 200 nm and 2000 nm.

13. The optical element of claim 10, wherein a coefficient of thermal expansion (α.sub.1) of the substrate and a coefficient of thermal expansion (α.sub.2) of the further substrate differ by not more than 5*10.sup.−6K.sup.−1.

14. The optical element of claim 10, wherein the further substrate is formed from a fluoridic material selected from the group consisting of: CaF.sub.2, MgF.sub.2, LiF, LaF.sub.3, BaF.sub.2 and SrF.sub.2.

15. An optical arrangement of a wafer inspection device, comprising: a radiation source for generating radiation in a first wavelength range (Δλ.sub.1) between 100 nm and 700 nm; and an optical element, comprising: a substrate, a reflective coating, applied to a rear face of the substrate, for reflecting radiation in a first wavelength range (Δλ.sub.1) between 100 nm and 300 nm, and a protective coating applied to the reflective coating, wherein the substrate is formed from a fluoridic material which is transparent to the radiation in the first wavelength range (Δλ.sub.1), and wherein the reflective coating is structured to reflect radiation that passes through the substrate to the reflective coating; wherein the optical arrangement is structured to direct the radiation from the radiation source onto a front face of the substrate.

16. The optical arrangement of claim 15, wherein the radiation source or a further radiation source is structured to generate further radiation in a second wavelength range (Δλ.sub.2) different than the first wavelength range (Δλ.sub.1), wherein the second wavelength range (Δλ.sub.2) comprises wavelengths greater than wavelengths of the first wavelength range (Δλ.sub.1) and comprises wavelengths between 200 nm and 2000 nm, and wherein the optical arrangement is structured to direct the further radiation in the second wavelength range (Δλ.sub.2) onto the front face or onto the rear face of the substrate.

17. A method of producing a reflective optical element, comprising: applying a reflective coating to a rear face of a substrate formed from a fluoridic material, wherein the reflective coating is structured to reflect radiation in a first wavelength range (Δλ.sub.1) between 100 nm and 300 nm, and to transmit further radiation in a second wavelength range (Δλ.sub.2) different than the first wavelength range (Δλ.sub.1), which passes through the substrate to the reflective coating, and wherein the substrate is formed from a material transparent to the radiation in the first wavelength range (Δλ.sub.1) and to the further radiation in the second wavelength range (Δλ.sub.2), and applying a protective coating to the reflective coating which has a thickness (d) of at least 50 nm.

18. The method of claim 17, further comprising directly bonding a surface of the protective coating to a surface formed on a further substrate.

19. The method of claim 18, wherein the protective coating is formed from an oxidic material, and wherein the surface formed on the further substrate comprises the same material formed on the surface of the protective coating.

20. The method of claim 17, further comprising removing material on a front face of the substrate to reduce a thickness (D) of the substrate.

21. The method of claim 17, wherein the protective coating is applied to the reflective coating by atomic layer deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Working examples are shown in the schematic drawing and are elucidated in the description that follows. The figures show:

[0049] FIG. 1A illustrates a schematic diagram of a first optical element for reflection of radiation in the VUV wavelength range, which has a protective coating and a reflective coating on its rear side, according to an example embodiment,

[0050] FIG. 1B illustrates a schematic diagram of a second optical element for reflection of radiation in the VUV wavelength range, which has a protective coating and a reflective coating on its rear side, according to an example embodiment,

[0051] FIG. 1C illustrates a schematic diagram of a third optical element for reflection of radiation in the VUV wavelength range, which has a protective coating and a reflective coating on its rear side, according to an example embodiment,

[0052] FIG. 1D illustrates a schematic diagram of a fourth optical element for reflection of radiation in the VUV wavelength range, which has a protective coating and a reflective coating on its rear side, according to an example embodiment,

[0053] FIG. 2A is a first schematic diagram of two steps of the production of an optical element, in which the protective coating is bonded to a carrier substrate, according to an example embodiment,

[0054] FIG. 2B is a second schematic diagram of two steps of the production of an optical element, in which the protective coating is bonded to a carrier substrate, according to an example embodiment,

[0055] FIG. 3A is a first schematic diagram of the optical element of FIGS. 2A and 2B with a reflective coating transparent to radiation in a second wavelength range, according to an example embodiment,

[0056] FIG. 3B is a second schematic diagram of the optical element of FIGS. 2A and 2B with a reflective coating transparent to radiation in a second wavelength range, according to an example embodiment,

[0057] FIG. 4A is a graph of the reflectance of the optical element of FIGS. 1B, D and of FIG. 3A, B as a function of the wavelength, according to an example embodiment,

[0058] FIG. 4B is a graph of the transmittance of the optical element of FIGS. 1B, D and of FIGS. 3A, B as a function of the wavelength, according to an example embodiment, and

[0059] FIG. 5 is a diagram of a wafer inspection device with two optical elements for reflection of radiation in the VUV wavelength range.

DETAILED DESCRIPTION

[0060] In the description of the drawings that follows, identical reference numbers are used for components that are the same or have the same function.

[0061] FIGS. 1A-D show an optical element 1 having a substrate 2 formed from a material transparent to radiation 5 within a broad wavelength range between 100 nm and 1000 nm. The material of substrate 2 may be, for example, CaF.sub.2, MgF.sub.2, LiF, LaF.sub.3, BaF.sub.2 or SrF.sub.2. Applied to a rear face 2b of the substrate 2 is a reflective coating 3 which is designed to reflect radiation 5 in a first wavelength range Δλ.sub.1 between 100 nm and 200 nm, which enters the substrate 2 at a front face 2a and passes through the substrate 2 to the reflective coating 3. The reflective coating 3 is typically what is called a highly reflective coating having a reflectance of more than 60% for the radiation 5 in the first wavelength range Δλ.sub.1.

[0062] Applied to the reflective coating 3, on its face or surface remote from the substrate 2, is a protective coating 4 that protects the reflective coating 3 from oxidation, inter alia. Owing to the fact that the radiation 5 does not have to penetrate the protective coating 4 applied to the rear face 2b of the substrate 2, the protective coating 4 may in principle have a high thickness d. In order to achieve a sufficient protective effect for the reflective coating 3 that covers the protective coating 4, it has been found to be beneficial when the protective coating 4 has a thickness d of at least 50 nm, preferably of at least 90 nm, and in particular of at least 120 nm.

[0063] In the examples shown in FIGS. 1A-C, the protective coating 4 is composed of a layer 4 of an oxidic material, specifically of aluminium oxide (Al.sub.2O.sub.3). Alternatively, the protective coating may have one or more layers of another oxidic material, for example of SiO.sub.2 or of MgO. The protective coating 4 may have at least one layer of a material which is non-transparent to the first wavelength range Δλ.sub.1, i.e., to wavelengths between 100 nm and 200 nm. A material non-transparent to the first wavelength range Δλ.sub.1 is understood to mean a material which, given a thickness of 100 nm, has a transmittance of less than 30% for radiation 5 in the first wavelength range Δλ.sub.1. Accordingly, a material transparent to the first wavelength range Δλ.sub.1 is understood to mean a material which, given a thickness of 100 nm, has a transmittance of more than 60% for radiation 5 in the first wavelength range Δλ.sub.1.

[0064] In the optical element 1 shown in FIG. 1A, the reflective coating 3 is composed of a metallic material, more specifically of aluminium. The reflective coating 3 may alternatively be formed from another metallic material, for example from an alloy, e.g., an aluminium alloy.

[0065] Rather than a reflective coating 3 of a metallic material, the reflective coating 3 may be formed from dielectric materials. FIG. 1B shows such a reflective coating 3 that forms a multilayer coating having a plurality of pairs, for example about ten pairs, of alternating layers 6a and 6b of materials having different refractive indices n.sub.a and n.sub.b, respectively. In order to generate high reflectivity in the first wavelength range Δλ.sub.1 between 100 nm and 200 nm, it has been found to be beneficial when the materials of the reflective coating 3 are fluoridic materials, for example AlF.sub.3, LiF, BaF.sub.2, NaF, MgF.sub.2, CaF.sub.2, LaF.sub.3, GdF.sub.3, HoF.sub.3, YbF.sub.3, YF.sub.3, LuF.sub.3, ErF.sub.3, Na.sub.3AlF.sub.6, Na.sub.5Al.sub.3F.sub.14, ZrF.sub.4, HfF.sub.4 and combinations thereof.

[0066] FIG. 1C shows an optical element 1 in which the reflective coating 3 is a dielectrically enhanced metallic coating. The reflective coating 3 has a multilayer coating 3a to which a metallic layer 3b of, for example, aluminium is applied. The reflective coating 3 shown in FIG. 1C thus constitutes a combination of the reflective coating shown in FIG. 1A and that shown in FIG. 1B.

[0067] In the optical element 1 shown in FIG. 1D, the reflective coating 3 takes the form of a multilayer coating as in FIG. 1B. In addition, the protective coating 4 also takes the form of a multilayer coating and has a plurality of pairs of layers 7a and 7b, for example about ten pairs of layers 7a and 7b, with different refractive indices n.sub.a, and n.sub.b, respectively. In this case, the protective layer 4 enables an increase in the reflectance R of the optical element 1 for the radiation in the first wavelength range Δλ.sub.1. The table below gives one example each for the layer sequences and layer thicknesses of the layers of the reflective coating 3 and the protective coating 4 of the optical element of FIG. 1B and of FIG. 1D.

TABLE-US-00001 Reflective coating with Reflective coating with single protective layer multilayer protective coating # Material Layer thickness Material Layer thickness First substrate — First substrate — MgF.sub.2 MgF.sub.2  1 BaF.sub.2 25.1 BaF.sub.2 25.1  2 LiF 28.0 LiF 28.0  3 BaF.sub.2 25.1 BaF.sub.2 25.1  4 LiF 28.0 LiF 28.0  5 BaF.sub.2 25.1 BaF.sub.2 25.1  6 LiF 28.0 LiF 28.0  7 BaF.sub.2 25.1 BaF.sub.2 25.1  8 LiF 28.0 LiF 28.0  9 BaF.sub.2 25.1 BaF.sub.2 25.1  10 LiF 28.0 LiF 28.0  11 BaF.sub.2 25.1 BaF.sub.2 25.1  12 LiF 28.0 LiF 28.0  13 BaF.sub.2 25.1 BaF.sub.2 25.1  14 LiF 28.0 LiF 28.0  15 BaF.sub.2 25.1 BaF.sub.2 25.1  16 LiF 28.0 LiF 28.0  17 BaF.sub.2 25.1 BaF.sub.2 25.1  18 LiF 28.0 LiF 28.0  19 BaF.sub.2 25.1 BaF.sub.2 25.1  20 LiF 28.0 LiF 28.0  21 BaF.sub.2 25.1 BaF.sub.2 25.1  22 LiF 28.0 LiF 28.0  23 BaF.sub.2 25.1 BaF.sub.2 25.1  24 LiF 28.0 LiF 28.0  25 BaF.sub.2 25.1 BaF.sub.2 25.1  26 LiF 28.0 LiF 28.0  27 BaF.sub.2 25.1 BaF.sub.2 25.1  28 LiF 29.9 LiF 29.9  29 BaF.sub.2 26.9 BaF.sub.2 26.9  30 LiF 29.9 LiF 29.9  31 BaF.sub.2 26.9 BaF.sub.2 26.9  32 LiF 29.9 LiF 29.9  33 BaF.sub.2 26.9 BaF.sub.2 26.9  34 LiF 29.9 LiF 29.9  35 BaF.sub.2 26.9 BaF.sub.2 26.9  36 LiF 29.9 LiF 29.9  37 BaF.sub.2 26.9 BaF.sub.2 26.9  38 LiF 29.9 LiF 29.9  39 BaF.sub.2 26.9 BaF.sub.2 26.9  40 LiF 29.9 LiF 29.9  41 BaF.sub.2 26.9 BaF.sub.2 26.9  42 LiF 29.9 LiF 29.9  43 BaF.sub.2 26.9 BaF.sub.2 26.9  44 LiF 29.9 LiF 29.9  45 BaF.sub.2 26.9 BaF.sub.2 26.9  46 LiF 29.9 LiF 29.9  47 BaF.sub.2 26.9 BaF.sub.2 26.9  48 LiF 29.9 LiF 29.9  49 BaF.sub.2 26.9 BaF.sub.2 26.9  50 LiF 29.9 LiF 29.9  51 BaF.sub.2 26.9 BaF.sub.2 26.9  52 LiF 29.9 LiF 29.9  53 BaF.sub.2 26.9 BaF.sub.2 26.9  54 LiF 29.9 LiF 29.9  55 BaF.sub.2 26.9 BaF.sub.2 26.9  56 LiF 29.9 LiF 29.9  57 BaF.sub.2 26.9 BaF.sub.2 26.9  58 LiF 29.9 LiF 29.9  59 BaF.sub.2 26.9 BaF.sub.2 26.9  60 LiF 31.2 LiF 31.2  61 BaF.sub.2 28.1 BaF.sub.2 28.1  62 LiF 31.2 LiF 31.2  63 BaF.sub.2 28.1 BaF.sub.2 28.1  64 LiF 31.2 LiF 31.2  65 BaF.sub.2 28.1 BaF.sub.2 28.1  66 LiF 31.2 LiF 31.2  67 BaF.sub.2 28.1 BaF.sub.2 28.1  68 LiF 31.2 LiF 31.2  69 BaF.sub.2 29.2 BaF.sub.2 29.2  70 LiF 32.5 LiF 32.5  71 BaF.sub.2 29.2 BaF.sub.2 29.2  72 LiF 32.5 LiF 32.5  73 BaF.sub.2 29.2 BaF.sub.2 29.2  74 LiF 32.5 LiF 32.5  75 BaF.sub.2 29.2 BaF.sub.2 29.2  76 LiF 32.5 LiF 32.5  77 BaF.sub.2 29.2 BaF.sub.2 29.2  78 LiF 32.5 LiF 32.5  79 BaF.sub.2 29.2 BaF.sub.2 29.2  80 LiF 32.5 LiF 32.5  81 BaF.sub.2 29.2 BaF.sub.2 29.2  82 LiF 32.5 LiF 32.5  83 BaF.sub.2 29.2 BaF.sub.2 29.2  84 LiF 32.5 LiF 32.5  85 BaF.sub.2 29.2 BaF.sub.2 29.2  86 LiF 32.5 LiF 32.5  87 BaF.sub.2 29.2 BaF.sub.2 29.2  88 LiF 32.5 LiF 32.5  89 BaF.sub.2 29.2 BaF.sub.2 29.2  90 LiF 32.5 LiF 32.5  91 BaF.sub.2 29.2 BaF.sub.2 29.2  92 LiF 32.5 LiF 32.5  93 BaF.sub.2 29.2 BaF.sub.2 29.2  94 LiF 32.5 LiF 32.5  95 BaF.sub.2 29.2 BaF.sub.2 29.2  96 LiF 32.5 LiF 32.5  97 BaF.sub.2 29.2 BaF.sub.2 29.2  98 LiF 32.5 LiF 32.5  99 Al.sub.2O.sub.3 120 Al.sub.2O.sub.3 26.5 100 Environment or — SiO.sub.2 32.2 further substrate 101 Al.sub.2O.sub.3 26.5 102 SiO.sub.2 32.2 103 Al.sub.2O.sub.3 26.5 104 SiO.sub.2 32.2 105 Al.sub.2O.sub.3 26.5 106 SiO.sub.2 32.2 107 Al.sub.2O.sub.3 26.5 108 SiO.sub.2 32.2 109 Al.sub.2O.sub.3 26.5 110 SiO.sub.2 32.2 111 Al.sub.2O.sub.3 26.5 112 SiO.sub.2 32.2 113 Al.sub.2O.sub.3 26.5 114 SiO.sub.2 32.2 115 Al.sub.2O.sub.3 26.5 116 SiO.sub.2 32.2 117 Al.sub.2O.sub.3 26.5 118 SiO.sub.2 32.2 119 Al.sub.2O.sub.3 26.5 120 SiO.sub.2 32.2 121 Al.sub.2O.sub.3 26.5 122 SiO.sub.2 32.2 123 Al.sub.2O.sub.3 26.5 124 SiO.sub.2 32.2 125 Al.sub.2O.sub.3 26.5 126 SiO.sub.2 32.2 127 Al.sub.2O.sub.3 26.5 100 Environment or — further substrate

[0068] In the example given in the table above, the reflective coating 3 has alternating layers 6a and 6b of LiF (n.sub.a=1.425 at 180 nm) and BaF.sub.2 (n.sub.b=1.583 at 180 nm), respectively, which have respective thicknesses of 32.5 nm to 28 nm and of 29.2 nm to 25.1 nm. The protective layer coating 4 in the example of the optical element 1 shown in FIG. 1B, has a single layer of Al.sub.2O.sub.3 having a thickness of 120 nm. In the example shown in FIG. 1D, the protective coating 4, by contrast, has alternating layers 7a and 7b of Al.sub.2O.sub.3 (n.sub.a=1.84 at 200 nm) and SiO.sub.2 (n.sub.b=1.554 at 200 nm), respectively, which have respective thicknesses of about 26.5 nm and of about 32.2 nm. In the example shown in FIG. 1D, the reflective coating 3 or the protective coating 4 is a multilayer coating 3, 4 which is periodic (within the respective subranges), but it will be apparent that it is also possible to use aperiodic multilayer coatings 3, 4 in order to further increase the reflectance R of the optical element 1 if appropriate. F

[0069] FIG. 4A, shows, as a dotted line, the reflectance R of the optical element 1 of FIG. 1B as a function of the wavelength λ without the complex protective coating 4, i.e., solely with the protective coating 4 with a 120 nm-thick layer of Al.sub.2O.sub.3 as specified on the left-hand side of the table.

[0070] FIG. 4A shows, as a solid line, the reflectance R of the optical element 1 of FIG. 1D with the multilayer protective coating 4 as specified on the right-hand side of the table. The example in the left-hand column of the table and of FIG. 4A is designed for the wavelength range of 160 nm to 190 nm. The example in the right-hand column of the table and of FIG. 4B is designed for the wavelength range of 160 nm to 205 nm. Adjustment to the first wavelength range Δλ.sub.1 between 100 nm and 200 nm is possible in the same way with the specified materials.

[0071] As apparent from a comparison of the reflectance R of the optical element 1 of FIG. 1B as shown in FIG. 4A and the reflectance of the optical element 1 of FIG. 1D as shown in FIG. 4B, the protective coating 4 shown in FIG. 1D can increase the reflectance R of the optical element 1 within a subrange of the first wavelength range Δλ.sub.1. In order to achieve this, the materials of the protective coating 4 are oxidic materials, for example Al.sub.2O.sub.3, SiO.sub.2, MgO, BeO, HfO.sub.2, Sc.sub.2O.sub.3, Y.sub.2O.sub.3 or Yb.sub.2O.sub.3.

[0072] For production of the optical element 1 of FIGS. 1A-D, the reflective coating 3 is first applied with a PVD or CVD process to the rear face 2b of the substrate 2. In a subsequent step, the protective layer coating 4 is applied to the reflective coating 3. If the material of the protective layer coating 4 is an oxidic material, for example aluminium oxide, it may be beneficial when the protective layer coating 4 is applied by an ALD process, since it is possible in this case to achieve a high density of the protective layer coating 4 which enhances the protective effect thereof.

[0073] FIG. 2A shows a further method step in which a surface 4a of the protective coating 4 is bonded to a surface 8a of a further layer 8 applied to a further substrate 9 (hereinafter: carrier substrate 9). The material of the further layer 8 is the same material as that of protective layer 4, i.e., Al.sub.2O.sub.3. This facilitates bonding of the two surfaces 4a, 8a to one another by direct bonding, i.e., establishment of a bond that does not need any bonding agent, for example without any adhesive or the like. Direct bonding can be effected, for example, in the way described in the article “Novel hydrophilic SiO.sub.2 wafer bonding using combined surface-activated bonding technique” by Ran He et al., Jpn. J. Appl. Phys. 54, 030218 (2015) which is cited above, and is incorporated herein by reference in its entirety.

[0074] FIG. 2B shows the optical element 1 after a process step in which material has been removed from the front face 2a of the substrate 2 in order to reduce the thickness D of the substrate 2. The reduction in thickness D of the substrate 2 to a value of, for example, D=5 mm or D=1 mm or less can reduce the absorption losses due to the passage of the radiation 5 twice through the substrate 2 (i.e., the incident and reflected light each passes through substrate 2) to a negligible value. The material can be removed from the front face 2a of the substrate 2 by lapping and polishing, during which the front face 2a of the substrate 2 is simultaneously converted to a desired shape. It will be apparent that the removal of material of the substrate 2 is not necessary, but merely that the substrate 2 may already have the desired thickness D on bonding to the carrier substrate 9.

[0075] In principle, the thickness D of the substrate 2, by virtue of the bonding to the carrier substrate 9, may have a lower thickness D than is the case for an optical element 1 without the carrier substrate 9. The carrier substrate 9 generally has a greater thickness D′ than the substrate 2, which may, for example, be more than about 10 mm.

[0076] In the example shown in FIGS. 2A, B, the material of the substrate 2 has a coefficient of thermal expansion α.sub.1 that differs from a coefficient of thermal expansion α.sub.2 of the further substrate 9 by not more than 5*10.sup.−6K.sup.−1. In this way, it is possible to reduce deformation of the substrates 2, 9 secured to one another by virtue of different expansion of the substrate materials. This criterion may be fulfilled when the two substrates 2, 9 are manufactured from the same material. However, also possible are combinations of different materials that fulfil this criterion, for example MgF.sub.2 (as substrate 2) and MgO (as further substrate 9).

[0077] FIGS. 3A, B show the optical element 1 of FIG. 2B, in which radiation 5 in the first wavelength range Δλ.sub.1 between 100 nm and 200 nm is directed onto the front face 2a of the substrate 2, and in which further radiation 5a in a second wavelength range Δλ.sub.2 between 200 nm and 1000 nm is directed onto the front face 2a of the substrate 2. In the examples shown in FIGS. 3A, B, the reflective coating 3 is transparent to radiation 5 in the second wavelength range Δλ.sub.2.

[0078] Such a reflective coating 3 may, for example, be as described above with reference to FIG. 1B or FIG. 1D, meaning that it may take the form of a reflective multilayer coating 3. In this case, the dielectric materials of the reflective multilayer coating 3 may be selected so as to not have too high an absorption for wavelengths in the second wavelength range Δλ.sub.2. FIG. 4B shows the transmittance T of the reflective multilayer coating 3 as a function of the wavelength λ. The dotted line here shows the transmittance T of an optical element 1 having a protective layer 4 with a single layer, in this case 120 nm of Al.sub.2O.sub.3. The optical element 1 thus corresponds to the embodiment shown in FIG. 1B and on the left in the table. The solid line shows the spectral transmittance T of an optical element 1 as shown in FIG. 1D and on the right in the table.

[0079] In the examples shown in FIGS. 3A, B, the protective coating 4, the carrier substrate 9 and the coating 8 applied to the carrier substrate 9 are transparent to further radiation 5a within the second wavelength range Δλ.sub.2.

[0080] The transparency of the optical element 1 to the further radiation 5a in the second wavelength range Δλ.sub.2 can be utilized advantageously in different ways. In the example shown in FIG. 3A, the optical element 1 serves as a beam divider device that reflects the radiation 5 in the first wavelength range Δλ.sub.1 that hits the front face 2a of the substrate 2 and transmits the further radiation 5a in the second wavelength range Δλ.sub.2 that likewise hits the front face 2a of the substrate 2. The further radiation 5a transmitted by the optical element 1 may, for example, be trapped and absorbed in a beam trap (not shown). The radiation 5 and the further radiation 5a may be generated by one and the same radiation source or, if appropriate, by multiple radiation sources (not shown in FIG. 3A).

[0081] In the example shown in FIG. 3B, the radiation 5 in the first wavelength range is directed to the front face 2a of the substrate 2 and reflected at the reflective coating 3. The further radiation 5a in the second wavelength range Δλ.sub.2, in the example shown in FIG. 3B, is generated by a further radiation source 10 which directs the further radiation 5a onto the rear face of the optical element 1, more specifically onto the rear face of the carrier substrate 9b. In particular if the second wavelength range Δλ.sub.2 is at greater wavelengths than the first wavelength range Δλ.sub.1, for example in the Near Infrared (NIR) wavelength range at more than 800 nm, control of the temperature of the substrate 2 can be realized by the further radiation 5a. The further radiation 5a in this case may serve as heating radiation, for example in order to generate a homogeneous temperature distribution in the substrate 2. For this purpose, the further radiation source 10 may be designed to direct the further radiation 5a with an adjustable radiation intensity or radiant power that varies in a location-dependent manner onto the rear face 9b of the carrier substrate 9.

[0082] It will be apparent that the optical elements 1 having no carrier substrate 9 that are shown in FIG. 1B or in FIG. 1D can also fulfil the functionality shown in association with FIGS. 3A, B. It is also possible for the geometry of the optical element 1 to differ from the concave geometry shown in FIGS. 1A-D to FIGS. 3A, B. In particular, the substrate 2 may have a planar geometry, i.e., take the form of a planar sheet.

[0083] The optical element 1 designed in the manner described above may be used in different optical arrangements. FIG. 5 shows an illustrative design of such an optical arrangement in the form of a wafer inspection system 20. The elucidations that follow are also analogously applicable to inspection systems for inspection of masks.

[0084] The wafer inspection device 20 has a radiation source 21, from which the VUV radiation 5 in the first wavelength range Δλ.sub.1 is directed at a wafer 25 by an optical system 22. For this purpose, the radiation 5 is reflected onto the wafer 25 by a concave mirror 24. In the case of a mask inspection device, one possible arrangement would have a mask to be examined in place of the wafer 25.

[0085] The radiation reflected, diffracted and/or refracted by the wafer 25 is directed at a detector 27 for further evaluation by a further concave mirror 26, which is likewise associated with the optical system 22. The optical system 22 of the wafer inspection device 20 comprises a housing 27, in the interior 27a of which are disposed the two reflective optical elements or mirrors 24, 26. In the example shown in FIG. 5, a respective mirror 24, 26 is one of the optical elements 1 shown above in association with FIGS. 1A-D or FIGS. 3A, B.

[0086] The radiation source 21 may be exactly one radiation source or a combination of multiple individual radiation sources to provide an essentially continuous radiation spectrum. In other examples, it is also possible to use one or more narrowband radiation sources 21. Preferably, the wavelength band of the radiation 15 generated by the radiation source 21 is in the VUV wavelength range Δλ.sub.1 between 100 nm and 200 nm.

[0087] It is also possible, though not required, for the radiation source 21 to be designed to generate further radiation 5a in a second wavelength range Δλ.sub.2, which is preferably between 200 nm and 1000 nm. In one such example, the second wavelength range Δλ.sub.2 does not directly adjoin the first wavelength range Δλ.sub.1; instead, there is generally a wavelength range of at least 100 nm between the two wavelength ranges Δλ.sub.1, Δλ.sub.2. In other words, the two wavelength ranges Δλ.sub.1, Δλ.sub.2 are spaced apart on the spectrum.

[0088] The optical element 1 described above may also be used advantageously in other optical arrangements, for example in a lithography system, such as a VUV lithography system, or the like.