METHOD FOR PRODUCING A REFLECTIVE OPTICAL ELEMENT, REFLECTIVE OPTICAL ELEMENT, AND USE OF A REFLECTIVE OPTICAL ELEMENT

Abstract

The disclosure provides a method that includes filling a cavity in a substrate with a second material, wherein the substrate includes a first material. The method also includes using galvanic and/or chemical deposition of a third material to apply an overcoating to a first surface of the substrate in a region of the cavity. The method further includes removing the second material from the cavity. In addition, the method includes, before or after removing the second material from the cavity, applying a reflective layer to the overcoating. The disclosure also provides related optical articles and systems.

Claims

1. A method, comprising: filling a cavity in a substrate with a second material, the substrate comprising a first material; using galvanic and/or chemical deposition of a third material to apply an overcoating to a first surface of the substrate in a region of the cavity; removing the second material from the cavity; before or after removing the second material from the cavity, applying a reflective layer to the overcoating, wherein the method forms an article.

2. The method of claim 1, further comprising forming the article into a reflective optical element having an optically effective surface, wherein, before filling the cavity with the second material, the first surface of the substrate is preformed to form a surface shape corresponding to a basic shape of the optically effective surface of the reflective optical element.

3. The method of claim 1, wherein the second material is liquefiable by applying heat, and/or the second material is soluble in a solvent.

4. The method of claim 1, wherein the second material is electrically conductive.

5. The method of claim 3, wherein the second material comprises at least one material selected from the group consisting of a wax, a polymer and a salt.

6. The method of claim 1, further comprising, between filling the cavity and applying the overcoating, applying an electrically conductive layer to the first surface of the substrate.

7. The method of claim 1, further comprising, after applying the overcoating or after removing the second material from the cavity, fine-machining the surface of the overcoating in accordance with a fine form of an optically effective surface that is to be achieved.

8. The method of claim 7, further comprising smoothing the fine-machined surface of the overcoating.

9. The method of claim 7, further comprising correcting a shape of the fine-machined surface of the overcoating.

10. The method of claim 7, further comprising, while fine-machining the overcoating, subjecting the cavity to pressure.

11. The method of claim 1, wherein the overcoating is applied so that the overcoating has a nonuniform thickness distributed over the first surface of the substrate.

12. The method of claim 1, wherein the third material is varied continuously or incrementally during overcoating.

13. The method of claim 1, further comprising processing the article to provide an optical element comprising the substrate supporting an optically effective surface of a reflective layer.

14. An optical element, comprising: a substrate; an overcoating comprising a galvanically or chemically deposited layer on a first surface of the substrate; a cavity configured to receive a fluid; and a reflective layer comprising an optically effective surface, wherein: the cavity is near the first surface of the substrate; the overcoating extends over the cavity; the cavity is free of material of the overcoating; and the reflective layer is on a surface of the overcoating that faces away from the substrate.

15. The optical element of claim 14, wherein the cavity comprises at least one channel.

16. The optical element of claim 14, wherein cavity comprises a plurality of channels, and the channels have a width which is in the range from a few micrometers to around one millimeter.

17. The optical element of claim 14, wherein the cavity comprises an opening that leads into the optically effective surface of the reflective layer.

18. The optical element of claim 14, wherein the substrate comprises steel.

19. The optical element of claim 14, wherein the overcoating comprises copper, nickel and/or nickel with phosphorus.

20. The optical element of claim 14, wherein the materials of the substrate and of the overcoating have an at least approximately identical coefficient of thermal expansion.

21. The optical element of claim 14, wherein the material(s) of the overcoating (16) have a coefficient of thermal expansion which is greater than a coefficient of thermal expansion of the material(s) of the substrate.

22. The optical element of claim 14, wherein the overcoating has a uniform thickness, or wherein the overcoating has a thickness that varies over the first surface of the substrate.

23. An optical system, comprising: a mirror comprising the optical element of claim 14, wherein the optical system is configured to be used in the VUV, EUV or even shorter-wave spectral range or for material processing with high-intensity light.

24. A system, comprising: a collector mirror comprising the optical element of claim 14, wherein the system is an EUV microlithography system.

25. A system, comprising: an article comprising the optical element of claim 14, wherein the article comprises an optical compensator and/or manipulator, and the cavity is subjectable to variable pressure to compensate for an undesired deformation of the optically effective surface to produce a desired deformation in the optically effective surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Further advantages and features can be gathered from the following description and the attached drawings. It goes without saying that the abovementioned features and those yet to be explained below may be used not only in the respectively specified combination but also in other combinations or on their own, without departing from the scope of the present disclosure. Exemplary embodiments of the disclosure are illustrated in the drawing and described in more detail below with reference thereto. In the drawings:

[0058] FIG. 1 shows a sectional illustration of a reflective optical element;

[0059] FIG. 2 shows an initial step in a method for producing the reflective optical element in FIG. 1;

[0060] FIG. 3 shows a further step in the method for producing the reflective optical element in FIG. 1;

[0061] FIG. 4 shows yet a further step in the method for producing the reflective optical element in FIG. 1;

[0062] FIG. 5 shows yet a further step in the method for producing the reflective optical element in FIG. 1;

[0063] FIG. 6 shows yet a further step in the method for producing the reflective optical element in FIG. 1;

[0064] FIG. 7 shows yet a further step in the method for producing the reflective optical element in FIG. 1;

[0065] FIG. 8 shows a detail of the method step in FIG. 6;

[0066] FIG. 9 shows a modification of the method step in FIG. 8;

[0067] FIG. 10 shows a result of the method step in FIG. 9;

[0068] FIGS. 11 to 15 show different geometries of one or more cavities with which the reflective optical element in FIG. 1 can be configured, in a plan view of the substrate of the element without overcoating;

[0069] FIG. 16 shows an exemplary embodiment of a result of the method step in FIG. 7;

[0070] FIG. 17 shows an exemplary embodiment according to which the reflective optical element in FIG. 1 is used as a manipulator/compensator; and

[0071] FIG. 18 shows an exemplary embodiment of an optical system in which the reflective optical element in FIG. 1 is used.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0072] FIG. 1 shows a reflective optical element, provided with the general reference sign 10, in section. The reflective optical element 10 can be used in different optical systems and for various purposes, as will be explained in more detail below.

[0073] The reflective optical element 10 generally has a substrate 12, at least one cavity 14, an overcoating 16 and a reflective layer 18, wherein the latter has an optically effective surface 20 on which light or generally electromagnetic radiation, which is reflected by the optically effective surface 20, impinges during operation of the reflective optical element 10.

[0074] The at least one cavity 14 serves to receive a fluid, wherein such a fluid can be a cooling medium, a flushing medium and/or a medium for subjecting the at least one cavity 14 to pressure, which should also be understood to be an underpressure, wherein the latter medium is useful for use of the element 10 as a manipulator/compensator. It goes without saying that all three functions mentioned (cooling, flushing, manipulation/compensation) can be realized by one and the same fluid.

[0075] In the exemplary embodiment shown, the cavity 14 has a plurality of cavities in the form of channels 22, 24, 26, 28, 30, 32, 34, wherein these channels 22 to 34 can be separated from one another in a fluid-tight manner, or at least some of the channels or all of the channels can communicate with one another.

[0076] The at least one cavity 14 is arranged adjacent to a first surface 36 of the substrate 12, the surface 36 facing the optically effective surface 20. The overcoating 16 extends here over the at least one cavity 14, in this case the channels 22 to 34.

[0077] The overcoating 16 in this case closes off the channels 22 to 34 in a fluid-tight manner at the surface 36 of the substrate 12. In an alternative exemplary embodiment, however, the at least one cavity 14 can have at least one opening 38, as is shown for example by way of broken lines for the channel 28. In this case, the optically effective surface 20 can be flushed via the fluid circulating through the at least one cavity 14.

[0078] In FIG. 1, the thicknesses of the substrate 12, of the overcoating 16 and of the reflective layer 18 and the dimensions of the channels 22 to 34 are not illustrated true to scale. The channels 22 to 34 can have a width and/or depth of less than 1 mm, for example a few 10 μm.

[0079] The overcoating 16 can be several millimeters thick.

[0080] The overcoating 16 has at least one layer galvanically or chemically deposited on the first surface 36. The overcoating 16 can be constructed from a single layer or from several layers.

[0081] The overcoating 16 can have been applied directly to the first surface 36 of the substrate 12, or indirectly by an intermediate layer (not shown) having been applied between the overcoating 16 and the substrate 12.

[0082] For the substrate 12, the following materials are particularly suitable: aluminum-silicon with a silicon content of up to 35% (trade name Dispal®), austenitic or martensitic steel, in particular 1.4 steel, or a copper alloy, for example a copper alloy which is sold under the trade name GlidCop® or Elmedur®.

[0083] The overcoating 16 can be formed particularly from the following materials: copper, (galvanic) nickel in the case of the overcoating 16 being in the form of an galvanic overcoating, and/or nickel with phosphorus (NiP) (chemical nickel) in the case of the overcoating 16 being configured as a chemically deposited overcoating.

[0084] The material(s) of the substrate 12 and the material(s) of the overcoating 16 are in this case selected such that they have an at least approximately identical coefficient of thermal expansion, such that tensions do not arise between the substrate 12 and overcoating 16 when the reflective optical element 10 heats up, for example on account of light impingement.

[0085] The coefficients of thermal expansion of the material(s) of the substrate 12 differ from the coefficients of thermal expansion of the material(s) of the overcoating 16 preferably by less than 10 ppm/K, more preferably less than 5 ppm/K, more preferably less than 2 ppm/K.

[0086] The material(s) of the overcoating 16 can in this case have a coefficient of thermal expansion which is slightly greater than the coefficient of thermal expansion of the material(s) of the substrate 12.

[0087] In the exemplary embodiment shown, the overcoating 16 has a uniform thickness over the substrate 12, wherein, however, some other configuration, in which the overcoating 16 does not have a uniform thickness but a varying thickness over the substrate 12, can also be drawn into consideration, as is shown by way of example in FIG. 10 and will be described in more detail below.

[0088] A method for producing the reflective optical element 10 will be described in the following text with reference to FIGS. 2 to 7.

[0089] In an initial method step, the substrate 12 is provided in a raw state. In this case, the substrate 12 can consist of an electrically conductive material, as goes for the abovementioned substrate materials, although this is not absolutely necessary for the method. The substrate 12 can also consist of a non-electrically conductive material.

[0090] In FIG. 2, the first surface 36 of the substrate 12 is planar. If the reflective optical element to be produced is intended to have, deviating from a planar configuration, a curved, for example concave optically effective surface 20, as in the case of the reflective optical element 10 in FIG. 1, or a convex optically effective surface 20, in a method step according to FIG. 3, first of all the first surface 36 of the substrate 12 is formed into a surface shape which corresponds to the basic shape of the optically effective surface 20 to be achieved. In the present exemplary embodiment, the first surface 36 of the substrate 12 is correspondingly preformed in a concave manner. The preforming of the first surface 36 can in this case take place in particular by machining.

[0091] In a method step according to FIG. 4, the at least one cavity 14, in this case the channels 22 to 34, are introduced into the substrate 12, into the surface 36 preformed as per FIG. 3, from the first surface 36. The introduction of the channels 22 to 34, too, can take place by machining, in particular via milling, turning, drilling, erosion and/or laser ablation.

[0092] In a further step according to FIG. 5, the at least one cavity 14 is, or in this case the channels 22 to 34 are, filled with a material 40. The material 40 is a material that is able to be liquefied by application of heat and/or a material that is soluble in a solvent, in particular a wax, a polymer or a salt. The material 40 can be filled into the at least one cavity 14 particularly easily when it has been liquefied before filling and subsequently solidifies.

[0093] The material 40 is furthermore preferably electrically conductive, as can be achieved by admixture of graphite when wax is used. In the case of a polymer, too, the polymer can be rendered electrically conductive by admixture of electrically conductive particles, if the polymer is not itself electrically conductive.

[0094] Optionally, after the at least one cavity 14 has been filled with the material 40, the first surface 36 can be reworked, for example by machining via turning or milling, etc. In a further method step according to FIG. 6, the substrate 12 is now overcoated. If the substrate 12 and the material 40 with which the at least one cavity 14 is filled are electrically conductive, the overcoating can take place by deposition of one or more galvanic layers directly onto the surface 36. It goes without saying that the material(s) for the overcoating are also electrically conductive.

[0095] If the substrate 12 and/or the material 40 with which the at least one cavity 14 is filled is or are not electrically conductive, an electrically conductive layer (not shown) can first of all be applied to the first surface 36 of the substrate 12 when the overcoating is intended to take place galvanically.

[0096] If the overcoating is intended to take place by chemical deposition of one or more layers, it is not necessary for the substrate 12 or for the material 40 to be electrically conductive, and a possible electrical layer is not required on the first surface 36 of the substrate 12, either.

[0097] Materials which can be deposited galvanically at a comparatively high rate are preferably copper or galvanic nickel. Materials which are suitable for chemical deposition are chemical nickel (NiP), i.e. a nickel-phosphorus compound.

[0098] The overcoating of the first surface 36 of the substrate 12 takes place in this case also over the at least one cavity 14, in this case the channels 22 to 34. The material 40 in the at least one cavity 14 in this case prevents material of the overcoating 16 passing into the at least one cavity 14. The cavity 14 in the finished optical element 10 is thus free of material of the overcoating 16. The overcoating can take place in a one-stage or multi-stage process, i.e. it is possible for one or more layers to galvanically or chemically deposited.

[0099] According to FIG. 6, the overcoating 16 is deposited with a uniform thickness over the substrate 12. In the case of galvanic deposition of the overcoating 16, FIG. 8 shows, in this regard, an electrode arrangement made up of an electrode 42 and a counter electrode 44, wherein the latter is formed by the substrate 12 itself, with which the overcoating with a uniform thickness can be realized.

[0100] In the case of overcoating by galvanic deposition of copper on the substrate 12, the overcoating can take place at a high rate of growth, even when the overcoating 16 is deposited to a thickness of several millimeters.

[0101] In the overcoating according to FIG. 6, the roughly preformed basic shape of the first surface 36 of the substrate 12 is substantially retained, i.e. a surface 46 of the overcoating 16 has substantially the basic shape of the surface 36 of the substrate 12, at least when the overcoating 16 has an at least substantially uniform thickness over the substrate 12. During overcoating of the substrate 12, the material which is deposited on the surface 36 of the substrate 12 can be varied continuously or incrementally. For example, when the material which is deposited on the substrate 12 is an alloy, it is possible for the composition of the alloy to be varied continuously or incrementally for example during the deposition of the material.

[0102] In a further method step according to FIG. 7, the surface 46 of the overcoating 16 is fine-machined in accordance with the final shape of the optically effective surface 20 that is to be achieved. The fine machining is carried out subtractively, for example via a cutting tool 48, for example a diamond milling cutter.

[0103] If the overcoating 16 is constructed from several galvanically or chemically deposited layers, at least the layer applied last should consist of a readily cuttable material which is accessible for ultraprecise machining.

[0104] During the fine machining of the surface 46 of the overcoating 16, the latter is also finely formed, for example aspherized or provided with a free form.

[0105] The fine-machined surface 46 is furthermore also smoothed, in particular polished, if desired.

[0106] The shape of the surface 46 of the overcoating 16 can subsequently also be corrected, in particular via ion beam forming, as is illustrated by ion beams 50 indicated by lines.

[0107] Before or after the fine machining of the surface 46 of the overcoating 16, the material 40 is emptied out of the at least one cavity 14, in this case the channels 22 to 34. In the case of a material 40 that is able to be liquefied by application of heat, the substrate 12 is correspondingly heated, for example, and in the case of a material 40 that is soluble in a solvent, a solvent is passed into the channels 22 to 34.

[0108] A further measure which can be taken during the fine machining of the surface 46 of the overcoating 16 is that of putting the at least one cavity 14, in this case at least individual channels 22 to 34 or all of the channels 22 to 34, under a pressure P with a fluid, with the result that the surface 46 has an outward bulge in the region of the channels 22 to 34, depending on which of the channels has been put under pressure, the bulge being correspondingly removed in each case during fine machining. Thus, a surface deformation error or shape accuracy error of the optically effective surface 20 during operation of the reflective optical element 10 in which the channels 22 to 34 have likewise been put under pressure, and/or when the reflective optical element is operated under vacuum, is prevented.

[0109] When the surface 46 of the overcoating 16 has been fully machined, the reflective layer 18 is applied to the surface 46 of the overcoating 16, such that the reflective optical element 10 according to FIG. 1 is produced.

[0110] The at least one reflective layer 18 is for example a stack of molybdenum and silicon (MoSi) layers.

[0111] FIG. 9 shows a modification of the configuration of the method according to FIG. 8, according to which the overcoating 16 is applied to the substrate 12 with a nonuniform thickness over the substrate 12, as is shown by way of example in FIG. 10. For this purpose, FIG. 9 shows an electrode arrangement in which the electrode 42 is flat such that the electrode is at a variable distance from the surface 36 of the substrate as seen over the substrate 12. As a result of the greater distance of the electrode 42 from the surface 36 in the middle of the substrate 12, the overcoating 16 is applied there with a smaller thickness than at the edges of the substrate 12, at which the electrode 42 is at a correspondingly smaller distance from the surface 36 of the substrate 12.

[0112] In order to achieve other thickness profiles of the overcoating 16 than the one shown in FIG. 10, the geometry of the electrode 42 or the distance positioning of several electrodes can be selected accordingly, in order to apply the overcoating 16 to the substrate 12 with a desired thickness profile, for example an undulating thickness profile. FIGS. 11 to 15 show various exemplary geometries for the at least one cavity 14, wherein FIGS. 11 to 14 show plan views of the substrate 12 following the introduction of the at least one cavity 14.

[0113] FIG. 11 shows a continuous spiral cavity 14 with an inlet 51 for letting in fluid and an outlet 52 for letting the fluid out of the cavity 14. The circulation of the fluid in this case takes place in a spiral shape from the center toward the edge of the substrate 12. FIG. 12 shows a geometry of cavities 14 which are formed in this exemplary embodiment from two channels 22 and 24 which do not communicate with one another. The channel 22 has an inlet 21a and two outlets 52a and 52b, and the channel 24 has an inlet 51b and two outlets 52c and 52d.

[0114] FIG. 13 shows a geometry of an individual continuous cavity 14 which extends over the substrate 12 in a meandering manner from the inlet 51 to the outlet 52.

[0115] FIG. 14 shows a cavity 14 which has a circular channel 22 radially on the outside, a circular channel 24 at the center and a multiplicity of radial channels 26, 28, etc. which connect the channels 22 and 24 together. From the inlet 51, the fluid first of all passes into the radially inner channel 24 and via the spoke-like radial channels 26, 28 etc. into the outer channel 14, and from there into the outlet 52.

[0116] FIG. 15 shows a perspective view of the substrate 12 with the channels according to FIG. 14.

[0117] FIG. 16 shows a further configuration of the method for producing the reflective optical element 10 in FIG. 1, in which, during the fine machining of the surface 46 of the overcoating 16 according to FIG. 7, an optically effective structure 54 is worked into the surface 46 of the overcoating 16, in particular by machining. The optically effective structure 54 is a diffraction grating for example in this case. When the reflective layer 18 is applied, the optically effective structure 54 is maintained, i.e. the optically effective structure 54 is transferred substantially precisely to a corresponding optically effective structure of the optically effective surface 20.

[0118] In further modifications of the reflective optical element 10 and of the method for producing same, the at least one cavity 14, for example channels 22 to 34, can be introduced into the substrate 12 such that the channels 22 to 34 narrow toward the surface 36 of the substrate 12. This makes it possible to carry out the overcoating 16 with a smaller thickness, since, when the channels 22 to 34 are subjected to pressure, on account of the narrowing shape of the channels 22 to 34 there is less of a tendency for the optically effective surface 20 to bulge.

[0119] On the other hand, the bulging of the optically effective surface 20 can also be desired, specifically when the reflective optical element 10 is used as an optical manipulator, as shown in FIG. 17. In this case, the channels 22 to 34 are individually or jointly subjected to a positive pressure P in a targeted manner, wherein provision can also be made for each of the channels 22 to 34 to be able to be subjected to a different pressure P. As a result, a desired surface deformation of the optically effective surface 20 can be set and changed in a targeted manner, as is illustrated in FIG. 17 with individual wave peaks 60. This can serve to influence, in a targeted manner, the wavefront of the optical system in which the element 10 is used. It is also conceivable for the channels 22 to 34 to be subjected to a negative pressure, with the result that wave troughs can also be set in the optically effective surface 20, and it is also possible for individual channels to be subjected to a positive pressure and others to a negative pressure.

[0120] The use of the reflective optical element 10 as a manipulator for influencing an optical wavefront is possible particularly in that the cavities, or channels 22 to 34, are arranged close to the surface 20 of the element 10, and in that the overcoating 16 can be embodied in a thin manner.

[0121] The optical element 10 can also be used as a compensator, wherein, in this case, the cavities, or channels 22 to 34, are subjected to a variable pressure in order to compensate for undesired deformation of the optically effective surface 20.

[0122] The reflective optical element 10 can be a macroscopic mirror or a miniaturized mirror element of a one-dimensional or two-dimensional mirror array. Furthermore, the reflective optical element 10 can also be configured as a mixing element in a honeycomb or cylinder honeycomb embodiment.

[0123] Finally, the reflective optical element 10 can also be configured simply as a thin membrane, optionally with an associated sensor system or actuator system on that side of the substrate 12 that faces away from the optically effective surface 20.

[0124] FIG. 18 shows a use of the reflective optical element 10 in an optical system 100 for EUV applications.

[0125] In this case of the use of the reflective optical element 10, it is used as a collector mirror of an EUV light source S, wherein the EUV radiation is generated by a target 102. The optical system 100 is in particular a microlithographic projection exposure apparatus and has, downstream of the light source S, an illumination system IL, a reticle R, a projection lens PO and a wafer W.

[0126] Further uses of the reflective optical element 10 in optical systems for material processing with high-intensity light, for example for boring, ablation, melting via laser light.