METHOD FOR FORMING NANOSTRUCTURES ON A SURFACE AND OPTICAL ELEMENT

Abstract

A method for forming in particular reflection-reducing nanostructures (5) on a preferably polished surface (3) of a crystalline, in particular ionic, substrate (1) for transmission of radiation in the FUV/VUV wavelength range. The method includes: providing a surface (3, 7), which surface is not oriented along a lattice plane having a minimum surface energy, on the substrate (1) or on a layer (6) applied to the substrate (1) by a coating method, in particular vacuum vapor deposition, and introducing an energy input (E) into the surface (7) for rearranging the surface (7) to form the nanostructures (5), wherein the energy input (E) is generated by irradiating the surface (7) with electromagnetic radiation (4). Also, an optical element for transmission of radiation in the FUV/VUV wavelength range.

Claims

1. A method for forming nanostructures on a surface of a crystalline substrate for transmission of radiation in the far-ultraviolet/vacuum-ultraviolet (FUV/VUV) wavelength range, comprising: providing a surface, which surface is not oriented along a lattice plane having a minimum surface energy, on the substrate or on a layer applied to the substrate by a coating method, in particular vacuum vapor deposition, and introducing an energy input into the surface for rearranging the surface to form the nanostructures, wherein the energy input is generated by irradiating the surface with electromagnetic radiation.

2. The method as claimed in claim 1, wherein the nanostructures are reflection-reducing nanostructures, the surface is a polished surface and the crystalline substrate is ionic.

3. The method as claimed in claim 1, wherein the irradiation of the surface is with electromagnetic radiation introduced in a pulsed manner.

4. The method as claimed in claim 3, wherein the pulsed electromagnetic radiation irradiating the surface has pulse durations of less than 300 ns.

5. The method as claimed in claim 1, wherein the irradiation of the surface is electromagnetic radiation in a Far-Ultraviolet (FUV)/vacuum ultraviolet (VUV) wavelength range.

6. The method as claimed in claim 5, wherein the electromagnetic radiation irradiating the surface is at wavelengths of less than 200 nm.

7. The method as claimed in claim 1, wherein the electromagnetic radiation irradiating the surface is in an infrared (IR) wavelength range.

8. The method as claimed in claim 7, wherein the electromagnetic radiation irradiating the surface is at wavelengths of more than 9

9. The method as claimed in claim 1, wherein said providing the surface comprises cutting the substrate along a lattice plane which does not correspond to the lattice plane having a minimum surface energy.

10. The method as claimed in claim 1, wherein said introducing of the energy input is carried out until the surface on which the nanostructures are formed has a reflectivity for radiation in the FUV/VUV wavelength range that is reduced by at least 0.03 by comparison with the surface prior to introducing the energy input, and/or has a reflectivity of less than 0.02 for radiation in the FUV/VUV wavelength range.

11. The method as claimed in claim 1, wherein the substrate and/or the layer applied by the coating method are/is formed from a fluoridic crystal.

12. The method as claimed in claim 11, wherein the coating method comprises vacuum vapor deposition, and the fluoridic crystal is selected from the group consisting essentially of: CaF.sub.2, MgF.sub.2, BaF.sub.2, SrF.sub.2, LaF.sub.3, YF.sub.3, LiF.

13. A method for forming nanostructures on a surface of a crystalline substrate for transmission of radiation in the FUV/VUV wavelength range, comprising: forming the nanostructures by three-dimensional island formation on a fluoridic mixed crystal produced by a coating method on the surface of the substrate formed from a metal fluoride.

14. The method as claimed in claim 13, wherein the nanostructures are reflection-reducing nanostructures, the surface is a polished surface and the crystalline substrate is ionic, and wherein the coating method comprises vacuum vapor deposition, and wherein the metal fluorideis CaF.sub.2.

15. The method as claimed in claim 1, further comprising: applying at least one protective layer to the nanostructures.

16. The method as claimed in claim 15, wherein the at least one protective layer is a conformal, fluoridic or oxidic, protective layer.

17. The method as claimed in claim 15, wherein the protective layer is applied by atomic layer deposition.

18. An optical element for transmission of radiation in the FUV/VUV wavelength range, comprising: a crystalline substrate, a surface of the substrate or a surface of a layer applied to the substrate by a coating method has nanostructures formed in accordance with a method as claimed in claim 1.

19. The optical element as claimed in claim 18, wherein the crystalline substrate is ionic, and wherein the coating method comprises vacuum vapor deposition.

20. The optical element as claimed in claim 18, wherein the surface having the nanostructures has a reflectivity of less than 0.02 for radiation in the FUV/VUV wavelength range.

21. The optical element as claimed in claim 18, which is embodied as a discharge chamber window for an excimer laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

[0041] FIGS. 1A-C show schematic illustrations of several steps of a method for forming nanostructures on a surface of a substrate composed of CaF.sub.2,

[0042] FIG. 2 shows schematic illustrations of the reflectivity of the surface before and after forming the nanostructures as a function of wavelength,

[0043] FIGS. 3A-C show schematic illustrations of several steps of a method for forming nanostructures on a layer composed of MgF.sub.2,

[0044] FIGS. 4A-C show schematic illustrations of several steps of a method for forming nanostructures in the form of three-dimensional islands on a mixed crystal on a surface of a substrate composed of CaF.sub.2, and

[0045] FIG. 5 shows a schematic illustration of an optical element in the form of a discharge chamber window of an excimer laser.

DETAILED DESCRIPTION

[0046] In the following description of the drawings, identical reference signs are used for identical or functionally identical component parts.

[0047] FIG. 1A shows a (mono-)crystalline substrate 1 having a plane surface 3 that is exposed visa vis an environment 2. In the example shown, the substrate 2 is CaF.sub.2 forming an ionic crystal. CaF.sub.2 is suitable for the transmission of radiation at wavelengths in the FUV/VUV wavelength range, i.e. it has a comparatively low absorption for radiation in this wavelength range, and so this material can be used as a substrate for a transmissive optical element. Instead of CaF.sub.2, it is also possible to use a different material having a sufficient transmission for radiation at wavelengths in the FUV/VUV wavelength range, for example MgF.sub.2. Other fluoridic crystals, for example BaF.sub.2, SrF.sub.2, LaF.sub.3, YF.sub.3, LiF can also be used as substrate materials.

[0048] The exposed surface 3 shown in FIG. 1A is a (100) lattice plane of the lattice structure of the substrate 1, said lattice structure not being illustrated pictorially. In order to expose the surface 3, the substrate 1 was cut along the (100) lattice plane. However, exposing the surface 3 of the substrate 1 along the (100) lattice plane can also be effected by a different form of (mechanical) processing. The (100) lattice plane has a surface energy γ.sub.100 of approximately 0.979 J/m.sup.2, which is greater than the surface energy γ.sub.111 of approximately 0.438 J/m.sup.2 of a (111) lattice plane, which has the minimum surface energy of all the lattice planes of the substrate 1. The (110) lattice plane has for example a surface energy γ.sub.110 of 0.719 J/m.sup.2, i.e. γ.sub.100>γ.sub.110>γ.sub.111 holds true. The exposed surface 3 of the substrate 1 as illustrated in FIG. 1A has in the FUV/VUV wavelength range between 100 nm and 280 nm a reflectivity R—illustrated in a dashed manner in FIG. 2—as a function of the wavelength γ, that is more than 0.035 over the entire FUV/VUV wavelength range, i.e. more than 3.5% of the radiation impinging on the exposed surface 3 is reflected at the latter.

[0049] In order to reduce the reflectivity R of the exposed surface 3, an energy input E is introduced into the exposed surface 3. In the example shown in FIG. 1B, for this purpose the exposed surface 3 is irradiated with electromagnetic radiation 4, the latter being radiation 4 in the FUV/VUV wavelength range. The intensity of the radiation 4 is chosen with a magnitude such that an activation energy for the rearrangement of atoms or of groups of atoms of the exposed surface 3 is exceeded, such that the exposed surface 3 changes its configuration and nanostructures 5 form on the latter, i.e. the exposed surface 3 is roughened, as is illustrated (on an exaggerated scale) in FIG. 1C. The rearrangement is fostered by the surface energy γ.sub.100 of the exposed surface 3 being significantly greater than the minimum surface energy γ.sub.111 along the (111) lattice plane.

[0050] In the example shown in FIGS. 1A-C, the irradiated surface 3 is a polished surface, the plane basic geometry of which in the example shown should as far as possible not change when the energy input E is introduced. Since maintaining the geometry of the surface 3 may not necessarily be ensured in the event of heat being input into the volume of the substrate 1, it is advantageous if the energy input E is limited as far as possible to a region near the surface.

[0051] In order that the substrate 1 is heated as far as possible only in the region of the surface 3, but not in the volume, use is made of FUV/VUV radiation in a wavelength range in which the CaF.sub.2 substrate has the greatest possible absorption. This is the case for irradiation with wavelengths of less than 200 nm, ideally of less than 100 nm. In order to generate such radiation 4, use can be made of an ArF excimer laser, for example, which emits at a wavelength of approximately 193 nm. Excimer lasers at other wavelengths, for example an F.sub.2 excimer laser, which emits at a wavelength of approximately 157 nm, can also be used for this purpose. Laser-excited plasma light sources that emit radiation e.g. in the extreme ultraviolet (EUV) wavelength range at wavelengths of approximately 10-100 nm can also optionally be used for this purpose. During irradiation with short-wave radiation at wavelengths of less than approximately 200 nm, there is the risk, however, of the CaF.sub.2 substrate being damaged by two-photon processes.

[0052] In addition to irradiation of the surface 3 at a wavelength at which the absorption of the irradiated substrate material is particularly high, the penetration depth of the radiation 4 can be reduced by the irradiation being effected in a pulsed manner. In particular, the penetration depth of the radiation 4 can be reduced if the pulse durations t.sub.P of the individual (laser) pulses are comparatively short and are e.g. less than 300 ns, preferably less than 100 ns. Such short pulse durations t.sub.P can be achieved if the corresponding (laser) light sources are operated in the short-pulse mode, for example using Q-switching or the like.

[0053] The nanostructures 5 shown in FIG. 1C result in a faceting of the surface 3, which has the effect that the reflectivity R of the surface 3 as illustrated in FIG. 2 decreases. The nanostructures 5 therefore act in the manner of an antireflection coating of the surface 3. The rearrangement of the exposed surface 3 during irradiation is not effected instantaneously, but rather at a rearrangement rate or at a rearrangement speed that is typically all the greater, the smaller the coverage of the surface 3 with adsorbates.

[0054] The irradiation of the surface 3 is typically carried out until the reflectivity R of the surface 3 as a result of the nanostructures 5 formed thereon, for a predefined wavelength γ or for a predefined wavelength range, for example the FUV/VUV wavelength range between approximately 100 nm and approximately 280 nm, falls below a predefined value. In the case of the reflectivity profile illustrated by a solid line by way of example in FIG. 2 the reflectivity R of the surface 3 on which the nanostructures 5 are formed is less than 0.02 over the entire FUV/VUV wavelength range. For the VUV wavelength range between approximately 100 nm and 230 nm, the reflectivity R is even lower still and is less than approximately 0.01.

[0055] As can likewise be discerned in FIG. 2, the irradiation reduces the reflectivity R—illustrated in a dashed manner—of the surface 3 prior to irradiation as illustrated in FIG. 1A in comparison with the reflectivity R of the surface 3 after irradiation as illustrated in FIG. 1C by an absolute value of at least 0.02, specifically over the entire FUV/VUV wavelength range. For the VUV wavelength range, the reduction of the reflectivity R is even greater still, i.e. in this wavelength range the reflectivity R is reduced by an absolute value of approximately 0.035.

[0056] In a departure from the process—shown in FIG. 1B—of introducing the energy input E into the surface 3 by irradiation with radiation 4 in the FUV/VUV wavelength range, it is also possible to introduce the energy input E by irradiation with IR radiation at wavelengths of more than approximately 9 μm. In this case, too, it is advantageous if the IR radiation 4 is at wavelengths within a wavelength range in which the material of the substrate 1 has a high absorption, such that the IR radiation 4 has the smallest possible penetration depth into the substrate 1 and the energy input E during irradiation is substantially concentrated on the surface 3.

[0057] In the case of CaF.sub.2as material of the substrate 1, which practically completely absorbs radiation 4 at wavelengths of more than approximately 14 it has proved to be advantageous if the irradiation is performed with radiation 4 at wavelengths of more than 9 μm. By way of example, CO.sub.2 lasers that emit laser radiation at a wavelength in the range of 9-11 μm can be used for this purpose. Such CO.sub.2 lasers first attain a sufficiently high power or intensity, which can be e.g. more than approximately 20 mJ/cm.sup.2, and secondly, in short-pulse operation (in the case of Q-switching using a Q-switch or an optical modulator), can generate pulse durations of less than 100 ns, which prevents the propagation of heat in the volume of the crystalline substrate 1 in a particularly effective manner.

[0058] Irradiation with wavelengths in the IR wavelength range has the advantage that, in contrast to irradiation with wavelengths in the FUV/VUV wavelength range, practically no damage to the substrate 1 occurs as a result of two-photon processes. Instead of an infrared or CO.sub.2 laser, it is also possible to use a different type of IR radiation source, for example an infrared lamp, for irradiation purposes. Such a radiation source, just like a CO.sub.2 laser, can emit radiation both in continuous wave (cw) operation or in a pulsed manner, the pulsed mode being advantageous owing to the risk of thermal deformation of the optical element.

[0059] As an alternative to the introduction of the energy input E or of heat into the surface 3 by irradiation, as shown in FIG. 1B, the energy input E can optionally also be effected by direct heat transfer, i.e. by conduction, or by convection.

[0060] As an alternative to producing the nanostructures 5 directly on the surface 3 of the crystalline substrate 1, it is also possible to produce the nanostructures 5 on a surface 7 of a layer 6 which was applied to the surface 3 of the substrate 1 by a coating method, to put it more precisely by vacuum vapor deposition or by epitaxial growth, as is illustrated below in association with FIGS. 3A-C. In the example shown in FIGS. 3A-C, the substrate 1 is MgF.sub.2 , in the case of which, in contrast to the example shown in FIGS. 1A-C, the (110) lattice plane has the minimum surface energy γ.sub.110. The exposed surface 3 of the MgF.sub.2 substrate 1 is the (001) lattice plane, which has the minimum surface energy γ.sub.001 in the case of the substrate 1 consisting of an MgF.sub.2 crystal. In the example shown in FIG. 3A, the layer 6 is applied to the surface 3 of the substrate 1 by homoepitaxial growth, i.e. the layer 6 likewise consists of MgF.sub.2 and has the same crystal structure as the MgF.sub.2 crystal of the substrate 1. Accordingly, the surface 7 of the MgF.sub.2 layer 6 also runs along the (001) lattice plane, the surface energy γ.sub.001 of which is greater than that of the (110) lattice plane having the minimum surface energy γ.sub.110.

[0061] In the example described in FIGS. 3A-C, too, a rearrangement of the surface 7 of the layer 6 to form nanostructures 5 (cf. FIG. 3C) can therefore be effected by the layer 7 being irradiated with electromagnetic radiation 4, as is illustrated in FIG. 3b. MgF.sub.2 also has a comparatively high absorption in the FUV/VUV wavelength range and in the IR wavelength range. Therefore, it is possible to carry out the irradiation with electromagnetic radiation 4, in particular with laser radiation, at the wavelengths described further above in association with FIGS. 1A-C, the irradiation in the IR wavelength range having proved to be advantageous. Moreover, the irradiation of the surface 7 can be effected in a pulsed manner in order to minimize the penetration depth of the radiation 5 into the volume of the substrate 1.

[0062] As an alternative to homoepitaxial growth, the layer 6 can also be applied to the surface 3 of the substrate 1 by heteroepitaxial growth, wherein a fluoridic crystal can be used as layer material and/or as a material of the substrate 1. In this case, too, care should be taken to ensure that during the growth of the layer 6 a surface 7 forms which is not oriented along a lattice plane having the minimum surface energy, since in this case it is not possible to rearrange the surface 7 with an energy input E. During heteroepitaxial layer growth, care should be taken to ensure that the lattice defect between the material of the growing layer 6 and the material of the substrate 1 is not excessively large.

[0063] FIGS. 4A-C show an example of the formation of nanostructures 5 which involves using the insufficient lattice match between the substrate 1, which is CaF.sub.2 in this case, and a heteroepitaxial growing layer 8 in order to form insular nanostructures 5. In the example shown in FIGS. 4A-C, the material of the growing layer 8 is a metal fluoride, to put it more precisely MgF.sub.2. Instead of MgF.sub.2, it is also possible to use other fluorides or a fluoridic mixed crystal as layer materials. Given suitable process parameters when applying the layer 8, in the system CaF.sub.2—LnF.sub.3 (Ln denotes an element of the lanthanum group) it is possible to form a mixed crystal having a fluorite crystal structure, as is described in reference [10], which is incorporated by reference in its entirety in the content of this application. As can be gathered from the phase diagrams illustrated therein, comparatively large concentration ranges exist in which mixed crystals having a fluorite crystal structure form in the system CaF.sub.2—LnF.sub.3.

[0064] As is known from [9], a lattice mismatch during heteroepitaxial growth results in 3D island growth. As is illustrated in FIG. 4B, the island growth, i.e. the formation of the insular nanostructures 5, is assisted by introducing an energy input E into the surface 3 or into the growing mixed crystal 8, but this is not absolutely necessary. In the example shown in FIG. 4B, introducing the energy input E is effected by irradiation with radiation 4 in the IR wavelength range, i.e. at wavelengths of more than 1000 nm. By suitably choosing process parameters, the size and the density of the nanostructures 5 produced can be adapted in order to achieve the best possible antireflective effect.

[0065] As is illustrated in FIG. 4C, a fluoridic or oxidic protective layer 9, for example, can be applied to the nanostructures 5. In the example illustrated in FIG. 4C, the protective layer 9 was deposited on the surface 3 of the substrate 1 by atomic layer deposition. This is advantageous since the protective layer 9 is applied conformally (atomic layer by atomic layer) and conformal coverage of the nanostructures 5 is thus made possible, as can be discerned in FIG. 4C. The protective layer 9 serves to seal the nanostructures 5 or the surface 3 or to prevent the resistance of the surface 3 during irradiation with powerful radiation. Oxides, for example Al.sub.2O.sub.3, SiO.sub.2 or fluorinated SiO.sub.2, are suitable as material for the protective layer 9. The protective layer 9 shown in FIG. 4C can likewise be applied to the nanostructures 5 shown in FIG. 1C, or to those shown in FIG. 3C.

[0066] FIG. 5 shows an optical element 10 in the form of a plane-parallel plate, this optical element having a substrate 1 composed of CaF.sub.2. The optical element 10 forms a discharge chamber window of an excimer laser 11, into the resonator section of which a gas mixture, for example containing fluorine, is introduced. As can be discerned in FIG. 5, the nanostructures 5 illustrated—with an exaggerated size—are formed on the inner side and on the outer side of the optical element 10, said outer side being situated outside the housing of the excimer laser 11. The laser radiation 12 of the excimer laser 11 having a wavelength of 157 nm passes through the optical element 12 with the nanostructures 5, which have an antireflective effect for the laser radiation 12. The protective layer 9 described in association with FIG. 4C is applied on the nanostructures 5 formed on the outer side of the optical element 10. The inner side of the optical element 10 is situated in a fluorine-containing atmosphere and is therefore significantly more resistant to radiation, and so no protective layer is required there. Optical elements other than the discharge chamber window shown in FIG. 5, for antireflective coating, can also be provided with nanostructures 5.

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