Abstract
A method for producing a substrate (10) for an optical element (11) includes: introducing a starting material, preferably a metal or a semimetal, into a container and melting the starting material, producing a material body having a quasi-monocrystalline volume region (8) by directionally solidifying the molten starting material proceeding from a plurality of monocrystalline seed plates arranged in the region of a base of the container, and producing the substrate by processing the material body to form an optical surface (12). An associated reflective optical element (11), in particular for reflecting EUV radiation (14) includes: a substrate having an optical surface on which a reflective coating (13) is applied. The substrate is typically produced in accordance with the associated method and has a quasi-monocrystalline volume region (8).
Claims
1. Method for producing a reflective optical element, comprising: introducing a starting material into a container and melting the starting material, producing a material body having a quasi-monocrystalline volume region by directionally solidifying the molten starting material proceeding from a plurality of monocrystalline seed plates arranged in a region of a base of the container, producing the substrate by processing the material body to form an optical surface, and applying a reflective coating for reflecting extreme ultraviolet (EUV) radiation onto the optical surface for producing the reflective optical element.
2. Method according to claim 1, wherein the starting material is a metal or a semimetal.
3. Method according to claim 1, wherein the optical surface is formed while processing the material body at one of the seed plates.
4. Method according to claim 3, wherein the optical surface is formed while processing the material body at one of the seed plates, which is produced using a Czochralski method.
5. Method according to claim 1, wherein the optical surface is formed while processing at the quasi-monocrystalline volume region of the material body.
6. Method according to claim 1, wherein the substrate is formed both from a quasi-monocrystalline volume region of the material body and from a polycrystalline volume region of the material body.
7. Method according to claim 6, wherein the polycrystalline volume region forms an edge region of the substrate that projects laterally beyond the quasi-monocrystalline volume region at least at one side of the quasi-monocrystalline volume region.
8. Method according to claim 6, wherein the polycrystalline volume region extends at least partly below the optical surface formed at the quasi-monocrystalline volume region.
9. Method according to claim 5, wherein the processing of the material body comprises separating off a volume region containing the seed plates.
10. Reflective optical element for reflecting EUV radiation, comprising: a substrate having an optical surface), and a reflective coating applied onto the optical surface of the substrate, wherein the substrate comprises a quasi-monocrystalline volume region.
11. Reflective optical element according to claim 10, wherein the substrate further comprises a monocrystalline volume region, wherein the optical surface is formed at the monocrystalline volume region.
12. Reflective optical element according to claim 10, wherein the optical surface is formed at the quasi-monocrystalline volume region.
13. Reflective optical element according to claim 10, wherein the substrate further comprises a polycrystalline volume region, which projects laterally beyond the quasi-monocrystalline volume region at least at one lateral side of the quasi-monocrystalline volume region.
14. Reflective optical element according to claim 13, wherein the polycrystalline volume region extends at least partly below the optical surface formed at the quasi-monocrystalline volume region.
15. Reflective optical element according to claim 11, wherein the substrate is formed from silicon or from germanium.
16. Reflective optical element according to claim 13, wherein the substrate is formed from silicon or from germanium.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:
[0032] FIGS. 1A and 1B show a schematic illustration of a container with a plurality of seed plates before (FIG. 1A) and after (FIG. 1B) being filled with a starting material in the form of granules,
[0033] FIGS. 2A-2B show schematic illustrations of a material body having a (quasi-) monocrystalline and a polycrystalline volume region, wherein the material body is shown after melting and directionally solidifying the starting material (FIG. 2A) and after the monocrystalline region with the seed plates removed (FIG. 2B),
[0034] FIGS. 3A-3C show schematic illustrations of three optical elements having substrates which were produced from the material body shown in FIG. 2B, wherein FIG. 3A shows a substrate formed exclusively from the quasi-monocrystalline volume region, FIG. 3B shows a substrate formed from both quasi-monocrystalline and polycrystalline volume regions, and FIG. 3C shows a substrate in which the polycrystalline volume region extends partly below the quasi-monocrystalline volume region, and
[0035] FIGS. 4A-4B show a schematic illustration of a material body having a large seed plate (FIG. 4A), and also a substrate for an optical element that is produced from the material body (FIG. 4B).
DETAILED DESCRIPTION
[0036] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.
[0037] FIG. 1A shows a container 1 in the form of a large crucible, which, in the example shown, has a base 2 having a square basic area of approximately 130 cm130 cm, for example. The base 2 is adjoined by four side walls 3 extending in the vertical direction, wherein the side walls are formed integrally with the base 2 in the example shown. The container 1 has a height of approximately 100 cm in the example shown. For the production of a (quasi-) monocrystalline material body made from silicon, the base 2 of the container 1 is covered, in a manner exhibiting substantial surface area coverage, with monocrystalline seed plates 4 composed of silicon, which can have a thickness of approximately 2-5 cm, for example. The monocrystalline seed plates 4 are referred to as seed plates or seed boards because they form the seeds for the later growth of the quasi-monocrystalline daughter material.
[0038] The container 1 is subsequently filled with starting material 5 in the form of polycrystalline silicon granules of varying granulation with the highest possible filling factor, as is shown in FIG. 1B. In a special furnace (not illustrated pictorially), the starting material 5 is melted and a temperature gradient is set in such a way that a melting front 6 extends from above down to a few centimeters into the seed plates 4, as is illustrated in FIG. 1B. Through targeted temperature control, the molten starting material 5 situated above the melting front 6 is subsequently crystallized starting from the seed plates 4 with a solidification front that is as horizontal as possible.
[0039] After the walls 3 and the base 2 of the container 1 have been removed, a material body 7 shown in FIG. 2A arises, which also contains the partly melted seed plates 4. In general, a volume region 7b having the partly melted seed plates 4 is separated off from the material body 7, specifically along a horizontal cutting line indicated in FIG. 2A. After the separating off, a material body 7a having a (quasi-)monocrystalline volume region 8 and a polycrystalline volume region 9 surrounding the latter in a ring-shaped manner remains, said material body being illustrated in FIG. 2B. In this case, the quasi-monocrystalline volume region 8 of the material body 7a substantially corresponds to a volume region above the seed plates 4 shown in FIG. 1B in which the (quasi-) monocrystalline silicon is formed in the course of solidifying the melt of the starting material 5.
[0040] In the region of a wall gap R between the side walls 3 of the container 1 and the side edges of the seed plates 4, the polycrystalline volume region 9 in the form of a seam of polycrystalline silicon is formed around the (quasi-)monocrystalline volume region 8 of the material body 7, 7a. The material body 7a shown in FIG. 2B can have comparatively large dimensions of approximately 130 cm130 cm50 cm, for example, wherein the quasi-monocrystalline volume region 8 forms a proportion of generally more than approximately 90% of the total volume of the material body 7a.
[0041] For the production of substrates 10 for (reflective) optical elements 11, such as are illustrated in FIGS. 3A-3C, it is necessary for the material body 7a shown in FIG. 2B to be suitably processed. The processing comprises a mechanical processing that involves bringing the material body 7a to a desired three-dimensional shape. The mechanical processing also involves smoothing and polishing the substrate 10 at an optical surface 12 in order to change the surface constitution thereof, in particular the roughness thereof, and to prepare application of a reflective coating 13. In order to ensure the highest possible surface quality at the optical surface 12, the optical surface 12 is typically formed at that side of the material body 7a which faces the seed plates 4. In the examples shown in FIGS. 3A-3C, the reflective coating 13 is configured to reflect the EUV radiation 14 incident on the reflective coating 13.
[0042] In the example shown, the reflective coating 13 comprises alternating individual layers (not illustrated pictorially) composed of a high refractive index material and a low refractive index material, which are molybdenum and silicon in the example shown. The combination of these materials makes it possible to reflect EUV radiation 14 having a wavelength of approximately 13.5 nm. Other material combinations of the individual layers such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and B.sub.4C are likewise possible. The respective reflective optical element 11 is configured for reflecting EUV radiation 14 incident on the reflective coating 13 with normal incidence, i.e. at comparatively small angles of incidence relative to the surface normal of the optical surface 12. In order to reflect EUV radiation 14 incident on the optical surface 12 with grazing incidence, the reflective coating 13 can be embodied in a different way and comprise for example just a single individual layer or, if appropriate, two or three individual layers.
[0043] In the case of the reflective optical element 11 shown in FIG. 3A, the substrate 10 is exclusively formed from the quasi-monocrystalline volume region 8 of the material body 7a. In the case of the example shown in FIG. 3B, the substrate 10 additionally has a polycrystalline volume region 9 surrounding the quasi-monocrystalline volume region 8 in a ring-shaped manner. In the case of the reflective optical element 11 shown in FIG. 3C, the polycrystalline volume region 9 extends partly below the optical surface 12 with the reflective coating 13; to put it more precisely, the polycrystalline volume region 9 extending circumferentially in a ring-shape manner has a radially inwardly projecting partial volume extending below the optical surface 12.
[0044] As can likewise be discerned in FIG. 3C, the quasi-monocrystalline volume region 8 extends downwards to a distance A of at least approximately 1 cm from the optical surface 12, i.e. the polycrystalline volume region 9 is spaced apart from the optical surface 12 far enough not to negatively influence the quality of the silicon material at the optical surface 12. The polycrystalline volume region 9 projecting radially outwards beyond the optical surface 12 and shown in FIGS. 3B and 3C can serve for example for linking or for securing the substrate 10 of the reflective optical element 11 for example to a mount or the like.
[0045] FIG. 4A shows a material body 7 which, analogously to the material body 7 shown in FIG. 2A, still comprises the seed plates 4. Unlike in the examples described further above in association with FIGS. 3A-3C, in the example shown in FIG. 4A and in FIG. 4B, the seed plates 4 are not separated from the material body 7 for the production of the substrate 10. Rather, in the case of the substrate 10 shown in FIG. 4B, a central seed plate which was produced by the Czochralski method and accordingly has a large diameter of approximately 30 cm, for example, is mechanically processed in order to form the optical surface 12 at the side of said seed plate facing away from the quasi-monocrystalline volume region 8. In addition to the quasi-monocrystalline volume region 8, the substrate 10 shown in FIG. 4B has a polycrystalline volume region 9 forming a circumferential edge of the substrate 10.
[0046] The diameter of the optical surface 12 of the substrate 10 illustrated in FIG. 4B substantially corresponds to the diameter of the seed plate 4 produced using the Czochralski method, but by virtue of the quasi-monocrystalline volume region 8 and additionally by virtue of the polycrystalline volume region 9 the diameter of the (integral) substrate 10 can be increased by comparison with an ingot produced using the Czochralski method. Moreover, by virtue of the quasi-monocrystalline volume region 8, formed below the seed plate 4 forming a monocrystalline volume region, it is possible to increase the thickness of the substrate 10 without the need to use monocrystalline silicon for this purpose, the production of monocrystalline silicon being more complex and thus more expensive than the production of quasi-monocrystalline silicon.
[0047] In order to produce an optical element, in the case of the substrate 10 shown in FIG. 4B, a reflective coating is applied on the optical surface 12 of the monocrystalline volume region formed by the seed plate 4, as has been described further above in association with FIGS. 3A-3C. A reflective coating 13 having a high reflectivity for wavelengths other than for wavelengths in the EUV wavelength range can also be applied on the optical surface 10. Applying a reflective coating can be dispensed with, if appropriate, if the substrate 10 is used for producing an optical element operated in transmission, for example.
