High aspect ratio mirror bodies and mirror substrates, and methods and means for producing the mirror substrates

Abstract

To provide a mirror substrate which has a high aspect ratio and small absolute thickness and also offers superior surface quality, dimensional precision and stability, and to provide a support body that can be used to produce the high aspect ratio mirror substrate and a mirror, and a method of manufacturing such mirror substrate and mirror.SOLUTION: A mirror substrate disclosed herein has a mean linear thermal expansion coefficient, a ratio of a lateral dimension to a maximum thickness, a weight per unit area, and a surface roughness, which are within specific conditional ranges.SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention relates generally to mirror bodies and mirror substrates, particularly high aspect ratio mirror bodies and mirror substrates, and methods and means for making the same.

[0002] prior art Specular substrates are used, for example, in so-called reflecting telescopes, also known as reflectors or reflectors, or other precision optical components, and serve as a substrate for applying a highly reflective specular layer.

[0003] To achieve consistently high quality mirror bodies, especially with regard to long-term stability, the following properties of the mirror substrate play an important role:

[0004] - Mass of mirror substrate If the mass of the mirror substrate is too large, for example, because the mirror substrate is a mirror body with a particularly large diameter, deformation may occur under the inherent mass of the mirror substrate, and the quality of the optical image may be reduced.However, even if the mirror substrate is small, it is generally advantageous to have a light weight because it improves handling.

[0005] - Surface quality of mirror-finished substrates In order to deposit a high-quality reflective layer, the mirror substrate must have a high surface quality. Of particular importance is the roughness of the mirror surface of the mirror substrate onto which the highly reflective layer is deposited.

[0006] - Additional Features Furthermore, it is desirable for the mirror substrate to be substantially composed of a material with high resistance, particularly high thermal resistance, and a low coefficient of thermal expansion. Suitable materials include glass, ceramic, and glass-ceramic, among others. Typically, the thermal expansion coefficient of a material is measured statically by measuring the length of a specimen at the beginning and end of a specific temperature interval, and calculating the coefficient of expansion α or CTE (coefficient of thermal expansion) from the difference between these lengths. CTE is reported as the average over the temperature interval; for example, for the temperature interval 0°C to 50°C, it is reported as CTE(0;50) or α(0;50). Well-known materials for precision components with zero expansion in the temperature range near room temperature are ceramics, Ti-doped fused silica (SiO2), and glass-ceramics. Glass ceramics are in particular lithium aluminum silicate glass ceramics (LAS glass ceramics), which are described, for example, in U.S. Pat. No. 4,851,372 (U.S. Pat. No. 4,851,372), U.S. Pat. No. 5,591,682 (U.S. Pat. No. 5,591,682), European Patent No. 587979 (EP 587979), U.S. Pat. No. 7,226,881 (U.S. Pat. No. 7,226,881), U.S. Pat. No. 7,645,714 (U.S. Pat. No. 7,645,714) and German Patent Application Publication No. 102004008824 (DE 102004008824 A1).

[0007] Furthermore, lithium aluminum silicate glass ceramics, for example those sold under the trademarks ZERODUR® or Clearceram®, or cordierite-based materials, or glasses with very low thermal expansion coefficients, for example synthetic fused silica doped with TiO2, for example the glass sold under the trademark ULE®, or ceramics comprising or consisting of cordierite or SiC, are known.

[0008] Typically, mirror-finished substrates, such as glass-ceramic substrates, are produced by first preparing a melt and then casting the material to create a glass-like material. Complex temperature control, particularly slow, controlled cooling, is required to prevent cracking and produce glass-ceramics from the glassy starting material. In the production of synthetic fused silica, so-called ingots or fairly large sheets are first produced by deposition of SiO2. If necessary, the diameter of the sheets can be increased by reducing the process.

[0009] Mechanical reworking is then performed to obtain the workpiece with the desired dimensions and quality, which may include drilling, grinding, and polishing, among others.

[0010] It is known to use supports in such reworking processes, which are placed under the workpiece or the mirror substrate, respectively, and the use of such supports can prevent damage to the mirror substrate.

[0011] It is known that it is particularly desirable to support high aspect ratio workpieces, such as high aspect ratio mirror substrates, over all surfaces of the workpiece during mechanical reworking, other reworking steps such as coating, and during transportation, etc. Such full support of the workpiece is particularly intended to maintain the dimensional consistency of the workpiece, i.e., the mirror substrate, so that it does not deform, for example, under the influence of gravity.

[0012] For example, German Patent Application Publication No. 102015112036 (DE 10 2015 112 036 A1) discloses a mineral-based casting support for full support of a workpiece for multiple workpieces with high aspect ratios.

[0013] However, problems have been found with such mineral-based casting supports when extensive polishing processes are required to achieve a particular surface quality, as a large amount of heat is generated in this case, which is undesirable due to the difference in thermal expansion between the support workpiece made of a low-expansion material and the mineral-based casting support, making it impossible to achieve the required precision in terms of the geometric dimensions of the mirror substrate and its surface quality.

[0014] EP 1 391 433 A2 discloses a keatite glass ceramic support, which can be used to mold glass or glass ceramics. However, keatite glass ceramic also has a high thermal expansion coefficient compared to the usual low-expansion materials used for mirror substrates. Therefore, this method cannot produce mirror substrates with the required precision in terms of geometric dimensions and surface quality.

[0015] These problems arise especially when dealing with new mirror-surface substrates, which have a particularly small thickness and a large diameter at the same time, which currently cannot be manufactured, do not have sufficient quality in terms of shape stability and surface quality, or even break during processing.

[0016] Therefore, there is a need for a high-aspect ratio mirror substrate and mirror body that has a small absolute thickness while simultaneously having high surface quality, dimensional accuracy, and stability. Furthermore, there is a need for a support that can be used to manufacture a high-aspect ratio mirror substrate and mirror body, and a method for manufacturing the mirror substrate and mirror body.

[0017] Object of the invention The object of the present invention is to provide a mirror substrate that overcomes or at least mitigates the known deficiencies of the prior art, as well as to provide a mirror body, particularly a mirror body based on said mirror substrate. There is also a need for methods and means for producing said mirror substrate and mirror bodies based on said mirror substrate.

[0018] The object of the invention is achieved by the subject matter of the independent claims. Preferred specific embodiments are defined by the dependent claims.

[0019] The mirror substrate according to the present invention has an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less. According to a particularly advantageous embodiment, the specular substrate further comprises a material having a surface roughness of 0.02×10 -6 / K or less, or even 0.01×10 -6 / K or less.

[0020] The mirror substrate has at least one of the following characteristics: The specular substrate has a ratio of its lateral dimension to its maximum thickness of at least 100, preferably at least 150, more preferably at least 200, most preferably 300 or greater.

[0021] - The mass per unit area of the mirror substrate is 100 kg / m 2 Less than 50 kg / m 2 Less than or equal to 30 kg / m 2 , most preferably 15 kg / m 2 The following is the result.

[0022] The mirror substrate has a maximum roughness R of 3.5 μm, ideally less than 1.2 μm. a The mirror surface of the mirror substrate is preferably ground and finished. a is the arithmetic mean value of the filtered roughness profile determined from the deviation from the centerline within the evaluation length. Root Mean Square RMS is defined as the root mean square (the arithmetic mean of the squares of a set of numbers). R a Profile roughness parameters such as RMS and RMS are included in ISO 4287:1997.

[0023] According to one embodiment of the present invention, the maximum thickness of the mirror substrate is 50 mm or less, in particular 20 mm or less, preferably 15 mm or less, more preferably 10 mm or less, most preferably 2 mm or less.

[0024] Preferably, the specular substrate has lateral dimensions of at least 200 mm and / or up to 4500 mm, however the invention is equally applicable to specular substrates having smaller or larger lateral dimensions.

[0025] According to a further embodiment of the present invention, the mirror substrate can be polished on a support such that the mirror substrate has a polished mirror surface with a root mean square (RMS) roughness of less than 2 nm, preferably less than 1 nm.

[0026] In the context of this application, the following definitions and terms apply.

[0027] A mirror-finish substrate material with a low thermal expansion coefficient is one with an average linear thermal expansion coefficient of 3 x 10 -6 / K or less.

[0028] The ratio of the lateral dimension of a mirror substrate to its maximum thickness is also called the aspect ratio. Therefore, the aspect ratio is a dimensionless parameter. Since mirror substrates are usually circular or nearly circular in shape, the lateral dimension is usually the diameter of the mirror substrate. If the lateral dimensions of the mirror substrate vary, for example, if the width of the mirror substrate differs from its length, an average value is calculated and used to calculate the aspect ratio.

[0029] The thickness of the mirror substrate can vary over the entire range of the mirror substrate. For example, a mirror substrate may be thinner at its outer edge than at its center. The maximum thickness is used to determine the aspect ratio.

[0030] A high aspect ratio typically means a low mass per unit area. In one embodiment of the present invention, this mass per unit area is greater than 100 kg / m 2 Less than 50 kg / m2 Below 30 kg / m, most preferably 2 The following is the result.

[0031] In this application, unless otherwise specified, the mean coefficient of thermal expansion α is in the range of 0°C to 50°C, but the invention also relates to materials characterized by low thermal expansion having expansion coefficients measured in different temperature ranges. This value is the nominal coefficient of mean linear thermal expansion according to ISO 7991, determined by static measurements. In the context of the present invention, the terms coefficient of expansion, coefficient of thermal expansion, mean coefficient of linear thermal expansion, and α are used interchangeably, unless otherwise specified.

[0032] In the present invention, the term "mirror substrate" is understood to mean a substrate to which a highly reflective layer (or mirror layer) is applied.The term "mirror substrate" also includes the workpiece to be processed or to be processed into a mirror substrate, i.e., for example, semi-finished products of mirror substrates, which have already been cut to the dimensions of the finished mirror substrate, but still need to be subjected to further reprocessing steps.

[0033] In the context of the present invention, the mirror body refers to a composite of a mirror substrate and a highly reflective layer on its functional surface. The surface of the mirror substrate on which the reflective layer is applied is referred to as the functional surface of the mirror substrate in this disclosure. The highly reflective layer is also referred to as a mirror layer in the context of the present invention.

[0034] Support is understood to mean a means which serves as a support during storage, transport and / or reworking of a workpiece placed thereon, such as a mirror substrate.

[0035] In the context of the present invention, when an article is described as comprising a particular material, this includes in particular when the article is mainly composed of that material, i.e. when it consists of more than 50% by weight of that material, and even when it is substantially composed of that material, i.e. when it consists of more than 90% by weight of that material. It can also include when the article consists of that material, i.e. when it consists entirely of that material.

[0036] In the context of the present invention, the mirror surface of a mirror substrate refers to the surface of the mirror substrate on which or to which a mirror layer is applied. This is therefore the quality surface of the mirror substrate, i.e., the surface on which specific requirements are imposed with respect to surface quality.

[0037] Therefore, the mirror substrate comprises a material with a low average linear thermal expansion coefficient. In particular, the mirror substrate can be mainly, i.e., more than 50% by weight, or even substantially, i.e., more than 90% by weight, or even completely, composed of such a material or a mixture of such materials. In particular, materials with such a low thermal expansion coefficient include glasses, glass ceramics, and ceramics, such as lithium aluminum silicate glass ceramics sold under the trademarks ZERODUR®, Astrosital®, or Clearceram®, or cordierite-based materials, or glasses with very low thermal expansion coefficients, such as synthetic fused silica doped with TiO2, such as those sold under the trademark ULE®, or ceramics containing cordierite or SiC, or ceramics composed of cordierite or SiC.

[0038] It has not been possible to provide a mirror substrate according to the present invention in the past. That is, the mirror substrate according to the present invention has a low thermal expansion coefficient, - have an aspect ratio of at least 100, preferably at least 150, more preferably at least 200, most preferably 300 or more; and / or - The mass per unit area of the mirror substrate is 100 kg / m 2 Less than 50 kg / m 2 Less than or equal to 30 kg / m 2 a material that is: The mirror substrate has a roughness R of at most 3.5 μm, better still less than 1.2 μm. a The mirror surface has a

[0039] In particular, this has not been possible so far for maximum thicknesses of the mirror substrate of 20 mm or less, preferably 15 mm or less, more preferably 10 mm or less, most preferably 2 mm or less.

[0040] This is especially true for mirror substrates with lateral dimensions of at least 200 mm. For mirror substrates with lateral dimensions of 1000 mm or more, a maximum thickness of 50 mm or less, in particular 40 mm or less, more preferably 30 mm or less, was not feasible.

[0041] That is, because the aspect ratio is large and the absolute thickness is small, the surface of the mirror substrate is processed to have a roughness R a It was impossible to achieve a roughness of 3.5 μm or even 1.2 μm. In particular, it was impossible to achieve such a roughness by grinding. In machining such mirror-finished substrates, the mirror-finished substrates were damaged during re-machining before such low roughness could be achieved.

[0042] According to one embodiment, the mirror surface shown has a roughness R of 3.5 μm or less, preferably 1.2 μm or less. a The functional surface of the mirror substrate is preferably ground to a finish.

[0043] This too was previously impossible to achieve.

[0044] The difficulties in manufacturing such mirror substrates include, in particular, the fact that the reworking processes, such as grinding, lapping, or polishing, used to achieve the low roughness contemplated herein, release a large amount of thermal energy. This, in turn, heats up both the mirror substrate and the support used in the reworking process. This heating can be so great that, in the processes required herein, thermomechanical stresses are created between the support and the mirror substrate bearing the load, especially when the support has a high thermal expansion coefficient.

[0045] According to a further embodiment of the present invention, the mirror substrate comprises a glass ceramic, preferably a lithium aluminum silicate glass ceramic, or a Ti-doped synthetic silica glass, and / or a ceramic, preferably a ceramic comprising cordierite and / or SiC, wherein said lithium aluminum silicate glass ceramic is preferably in the form of a high-quartz solid solution glass ceramic.

[0046] The specular substrate may in particular consist mainly, ie more than 50% by weight, or substantially, ie more than 90% by weight, or even entirely, of such materials.

[0047] Such materials not only typically exhibit a low coefficient of thermal expansion, but also typically exhibit good reworkability, such as good sandability and polishability.

[0048] According to a further aspect, the present invention also relates to a mirror body, in particular a mirror body comprising a mirror substrate according to an embodiment of the present invention. The mirror body comprises a highly reflective layer on the mirror surface of the mirror substrate.

[0049] Another aspect of the present invention relates to a support for supporting a mirror substrate, particularly a mirror substrate according to an embodiment of the present invention, disposed thereon, preferably over its entire surface, during its processing and / or its transportation. The support has an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less. According to a particular embodiment, the expansion coefficient of the support is even 0.02×10 -6 / K or less, even 0.01×10 -6 / K or less.

[0050] In particular, the support may consist mainly, i.e., more than 50% by weight, or substantially, i.e., more than 90% by weight, or even entirely, of this material or a mixture of such materials. The support preferably generally has lateral dimensions of at least 200 mm and at most 4500 mm.

[0051] Therefore, the support is suitable for processing and / or transporting mirror-surface substrates with large lateral dimensions. At the same time, the average linear thermal expansion coefficient is 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05×10 -6 / K or less (and, according to a particular embodiment, even 0.02 × 10 -6 / K or less, even 0.01×10 -6 / K or less) ensures that the support material exhibits low thermal expansion even when large amounts of heat are generated during reworking such as grinding, lapping, or polishing.

[0052] In this specification, the surface of the support refers to the surface of the support that is at least partially loaded by the mirror substrate during rework and / or transport.

[0053] According to a further embodiment of the support, the surface of the support has a curved shape. Preferably, the surface of said support is approximated to an envelope.

[0054] In this specification, the envelope may be understood as the predetermined shape or geometry of the surface of the support. The quality of the support is therefore also characterized by the existing deviation between the actual surface of the support and the described ideal envelope. The deviation that can still be tolerated between the envelope and the actual surface varies depending on the diameter of the workpiece or mirror-surface substrate.

[0055] Particularly preferably, the deviation of the support from the envelope is For mirror substrates having a lateral dimension of at least 4000 mm, it is 0.5 mm or less, preferably 0.1 mm or less, more preferably 0.05 mm or less, and according to some embodiments, even 0.025 mm or less; and / or For mirror substrates having a lateral dimension of at least 2000 mm, the thickness is 0.2 mm or less, preferably 0.05 mm or less, and more preferably 0.025 mm or less; and / or For specular substrates having a lateral dimension of at least 1200 mm, it is 0.1 mm or less, preferably 0.05 mm or less, more preferably 0.01 mm or less.

[0056] According to a further embodiment of the support, the surface of the support is at least partially covered with an intermediate material. The intermediate material may be a film, and in one embodiment, may be a polymer film. The intermediate material may comprise pitch, bitumen, and / or silicone. Preferably, the intermediate material, e.g., film, has a thickness at least as large as the deviation of the actual shape of the surface from the envelope. Particularly preferably, the intermediate material, e.g., film, has a maximum thickness of at most 200 μm, preferably at most 100 μm, more preferably at most 50 μm, or even most preferably at most 25 μm. Here, it is preferable that the shape of the support is very well adapted to the envelope of the mirror-surface substrate.

[0057] Polymer films may exhibit nonlinear elastic behavior, which may ultimately result in deviations in the shape of the reflective surface of the mirror substrate after grinding. Optionally, this can be remedied by using alternative or additional intermediate materials. Such intermediate materials may include metal foils or paste-like or gelatinous materials, as well as cured films.

[0058] The intermediate material may, among other things, serve to mitigate deviations of the surface shape of the support from an ideal predetermined surface shape that may be represented by the envelope.

[0059] However, alternatively or additionally, the intermediate material may also perform other functions: for example, it may also serve to reduce the adhesive forces between the support and the mirrored substrate that it is loading, so that the mirrored substrate can be easily, and in particular without damage, lifted from the support once processing or transport of the mirrored substrate is complete.

[0060] When the intermediate material is provided in the form of a polymer film, the polymer may comprise one or more of the group consisting of polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), one or more polyesters, polyetherketone (PEK), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) such as Teflon®, one or more tetrafluoroethylene copolymers (ETFE), polyvinyl chloride (PVC).

[0061] Preferably, the intermediate material comprises a material designed to exhibit a heat resistance of 150°C or greater.

[0062] According to a further embodiment of the invention, intermediate materials with the lowest possible coefficient of thermal expansion for these material classes are preferred.

[0063] When the intermediate material is in the form of a polymer film, it is preferable for the polymer to have a heat resistance of at least 150° C. Furthermore, in this case, it is preferable for the polymer to have as low a thermal expansion coefficient as possible.

[0064] According to another embodiment of the support, the intermediate material comprises polyvinyl chloride (PVC), PTFE, ETFE, PEK, and / or PET. For example, the intermediate material may be in the form of a polymer film comprising PVC, PTFE, ETFE, PEK, and / or PET. If the thermal load generated during the reprocessing process is relatively low, a polymer film with relatively low thermal stability, such as a film made of polyethylene or polypropylene, may be used.

[0065] Furthermore, thermoplastic polymer films are preferred, which tend to be formed to some extent at the high temperatures that can occur during mechanical reworking of mirror-surface substrates. Deviations of the surface of the support from the envelope can be better compensated for in this way.

[0066] The film may be applied to the surface of the substrate as a whole, integral unit, or may be applied to the surface of the substrate in spaced apart portions, for example, in the form of individual, relatively small pieces.

[0067] According to one embodiment, the film has "channels" that extend at least to the edges, so that when the mirrored substrate is placed on the support, the air is released to the outside and the mirrored substrate is pressed against the surface. If the film is applied as a whole, such channels can then be introduced into the film, for example, cut.

[0068] According to another embodiment of the support, the surface of the support is partially covered with an intermediate material, for example, a film, which is applied to the surface in the form of spaced apart portions. These spaced apart portions of the intermediate material can be arranged in a symmetrical pattern. One option in this regard is a radially symmetrical pattern, for example a four-element radially symmetrical pattern, of the portions of the intermediate material, for example, the portions of the film.

[0069] According to one embodiment, the portions have the shape of a circle or an oval.

[0070] This embodiment of the support allows the mechanical loads that occur during processing or transportation of the mirror substrate, for example, due to the inherent mass of the mirror substrate, to be absorbed particularly uniformly by the entire surface of the support. This can minimize the mechanical stress on the mirror substrate. Therefore, the stability of the mirror substrate on the support during processing is improved.

[0071] According to a variant of the invention, the support may be configured so that any residues that arise during reworking of the supported workpiece can be cleanly removed without damaging the workpiece itself.

[0072] For this reason, the support may be configured in such a way that residues that arise during processing of the workpiece are evacuated.

[0073] For this purpose, the surface of the support may, for example, be smooth or provided with grooves.

[0074] The surface of the support may further include at least one opening, for example, a drain port through which residues generated during reworking of the workpiece can be discharged. When one or more openings are provided, the intermediate material may be applied steadily or even continuously so that there are no channels at the outer edge of the mirror-surface substrate, as shown in Figure 3, allowing air and residues to dissipate through the openings in the support.

[0075] According to one embodiment of the present invention, the outlet is defined by a web-like groove made of radially extending circular elements.

[0076] Another aspect of the invention relates to an assembly comprising a support and a mirror substrate.

[0077] The support is preferably used to support the mirror substrate over its entire surface while applying a load thereto during processing and / or transportation, and has an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less, and according to special embodiments, even 0.02 × 10 -6 / K or less, or even 0.01×10 -6 This includes materials with a temperature of 0.15°C or less.

[0078] The mirror substrate has an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less, and according to special embodiments, even 0.02 × 10 -6 / K or less, or even 0.01×10 -6 This includes materials with a temperature of 0.15°C or less.

[0079] The mirror substrate preferably has a maximum thickness of 50 mm or less, in particular 20 mm or less, preferably 15 mm or less, more preferably 10 mm or less, most preferably 2 mm or less. The lateral dimension of the mirror substrate is preferably at least 200 mm.

[0080] The specular substrate has at least one of the following characteristics: The specular substrate has a ratio of its lateral dimension to its maximum thickness of at least 100, preferably at least 150, more preferably at least 200, most preferably 300 or greater.

[0081] - The mass per unit area of the mirror substrate is 100 kg / m 2 Less than 50 kg / m 2 Less than or equal to 30 kg / m 2 Below 15 kg / m, most preferably 2 The following is the result.

[0082] According to one embodiment of the assembly, the specular surface of the specular substrate has a roughness R of at most 3.5 μm, ideally less than 1.2 μm. a The mirror surface of the mirror substrate is preferably ground to a finish.

[0083] According to a further embodiment, the surface of the mirror substrate has an RMS roughness of less than 2 nm, preferably less than 1 nm, and the surface of the mirror substrate is preferably provided with a polished finish.

[0084] According to another embodiment of the assembly, the difference in absolute value between the thermal expansion coefficient of the material constituting the support and the thermal expansion coefficient of the material constituting the mirror substrate is less than 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, more preferably 0.05×10 -6 / K or less, most preferably 0.02 × 10 -6 / K or less.

[0085] Preferably, the surface of the support and the surface of the mirror substrate bearing against the surface of the support have complementary curvatures.

[0086] In other words, the surface of the support preferably has a convex curvature if the surface of the mirrored substrate bearing it has a concave curvature, and vice versa, so that the surface of the support and the support surface of the mirrored substrate have complementary shapes in the overlapping region.

[0087] According to one embodiment of the assembly, the specular substrate is laterally fixed to the support, and preferably the specular substrate is additionally fixed so that it cannot be detached from the support.

[0088] Yet another aspect of the present invention relates to a method for manufacturing a mirror substrate, in particular a method for manufacturing a mirror substrate according to any of the aforementioned embodiments, said method comprising the steps of: - providing a support for supporting, preferably over its entire surface, a specular substrate disposed on said support; - Thermal expansion coefficient is 1 x 10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 preparing a mirror substrate made of a material having a viscosity of 1 / K or less; - placing a specular substrate on a support; - mechanically reworking the surface of the specular substrate, in particular its specular surface, in particular by polishing, drilling, grinding or lapping, The difference in absolute value between the thermal expansion coefficient of the material that makes up the support and the thermal expansion coefficient of the material that makes up the mirror substrate is 1 x 10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, more preferably 0.05×10 -6 / K or less, most preferably 0.02 × 10 -6 / K or less.

[0089] According to a specific embodiment of the method, the thermal expansion coefficient of the material constituting the mirror substrate is further 0.02 × 10 -6 / K or less, even 0.01×10 -6 It may be less than / K.

[0090] The functional surface of the mirror substrate is located opposite the surface that applies load to the support. In a preferred embodiment, in order to provide safe processing and / or safe transportation of the mirror substrate, the mirror substrate is fixed to the support at least against lateral displacement relative to the support. Preferably, the mirror substrate is also fastened or fixed so as not to peel or detach from the support. This fixation can be achieved, in particular, by clamps, adhesives, or negative pressure between the support and the mirror substrate.

[0091] Another aspect of the invention relates to the use of mirror substrates according to embodiments of the invention in astronomy applications or in lithographic processes, such as LCD lithography and / or microlithography.

[0092] The invention will now be described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0093] [Figure 1] FIG. 1 illustrates an assembly including a mirror substrate and a support according to an embodiment of the present invention. [Figure 2] FIG. 1 illustrates an assembly including a mirror substrate and a support according to an embodiment of the present invention. [Figure 3] 1 is a diagram of a surface of a support according to one embodiment of the present invention; [Figure 4] 1A-1D illustrate process steps for manufacturing a mirror substrate.

[0094] 1 is a schematic, not-to-scale, illustration of an assembly 15 including a support 1 and a specular substrate 20 according to one embodiment. Here, the specular substrate 20 is designed so that the surface of the element 20, which bears load on the support 1, i.e., the support surface 22, has a convex curvature. In contrast, the surface 4 of the support 1 has a concave downward curvature to ensure the best possible support of the workpiece or glass, glass-ceramic, or ceramic element. Thus, to achieve full support of the thin specular substrate 20, the support surface 22 and the surface 4 have complementary shapes.

[0095] 2 is a schematic, not-to-scale, illustration of a further embodiment of an assembly 15 consisting of a support 1 and a specular substrate 20 bearing on it, the specular substrate 20 being designed so that the bearing surface 22 bearing on the support 1 has a concave shape. In contrast, the surface 4 of the support 1 has a convex upward curvature to ensure the best possible support of the specular substrate 20.

[0096] Overall, in the assembly 15 shown diagrammatically and not to scale in Figures 1 and 2, the support 1 serves to support the mirror substrate 20, which is subjected to a load, preferably over its entire surface during its processing and / or its transportation. The support 1 has an average linear thermal expansion coefficient of 1 x 10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less, or even 0.02×10 -6 / K or less, or even 0.01×10 -6 / K or less. The mirror substrate 20 also includes a material having an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less, or even 0.02×10 -6 / K or less, or even 0.01×10 -6 / K or less. Preferably, the maximum thickness of the mirror substrate is 50 mm or less, in particular 20 mm or less, preferably 15 mm or less, more preferably 10 mm or less, and most preferably 2 mm or less. Further preferably, the lateral dimension of the mirror substrate 20 is at least 200 mm, in particular up to 4500 mm. The mirror substrate 20 has one of the following characteristics: the specular substrate 20 has a ratio of its lateral dimension to its maximum thickness of at least 100, preferably at least 150, more preferably at least 200, and most preferably 300 or greater; and / or The mass per unit area of the mirror substrate 20 is 100 kg / m 2 Less than 50 kg / m 2 Less than or equal to 30 kg / m 2 Mass per unit area is 15 kg / m or less. 2 It is particularly preferred that:

[0097] According to one embodiment of the present invention, the mirror surface of the mirror substrate has a roughness R of at most 3.5 μm, ideally less than 1.2 μm. a The surface of the mirror substrate is preferably ground. When the mirror surface of the mirror substrate is polished, the RMS roughness is preferably less than 2 nm, and most preferably less than 1 nm.

[0098] According to yet another embodiment of the assembly 15, the difference in absolute value between the thermal expansion coefficient of the material constituting the support 1 and the thermal expansion coefficient of the material constituting the mirror substrate 20 is less than 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, more preferably 0.05×10 -6 / K or less, most preferably 0.02 × 10 -6 / K or less.

[0099] FIG. 3 is a schematic view, not drawn to scale, showing, by way of example, a support 1 according to an embodiment of the invention used in an assembly according to either one of FIG. 1 or FIG.

[0100] The support 1, which supports the mirror substrate 20 (not shown) that is loaded on the support 1, preferably over the entire surface of the mirror substrate during processing and / or transportation, has an average linear thermal expansion coefficient of 1×10 -6 / K or less, preferably 0.1 × 10 -6 / K or less, most preferably 0.05 × 10 -6 / K or less, and in certain embodiments, even 0.02×10 -6 / K or less, or even 0.01×10 -6 / K or less, and the support preferably has a lateral dimension of at least 200 mm and / or at most 4500 mm.

[0101] According to one embodiment, the surface 4 of the support 1 has a curved shape as shown in the drawing.

[0102] Preferably, the surface 4 of the support 1 is approximated to an envelope, - most preferably, the deviation of the support 1 from the envelope is less than or equal to 0.5 mm, preferably less than or equal to 0.025 mm, for mirror substrates 20 with lateral dimensions of at least 4000 mm; and / or - in the case of a mirror substrate 20 preferably having a lateral dimension of at least 2000 mm, it is equal to or smaller than 0.2 mm, preferably equal to or smaller than 0.025 mm; and / or less than or equal to 0.1 mm, preferably less than or equal to 0.01 mm, in the case of a mirror substrate 20 which preferably has a lateral dimension of at least 1200 mm.

[0103] According to yet another embodiment of the support 1, the surface 4 of the support 1 is at least partially covered with an intermediate material 13, for example a film 13, in particular a polymer film, The intermediate material 13, e.g., a film, preferably has a thickness at least as large as the deviation of the actual shape of the surface 4 from the envelope; The intermediate material 13 preferably has a maximum thickness of at most 200 μm, preferably at most 100 μm, more preferably at most 50 μm, or even most preferably at most 25 μm.

[0104] Besides the curved embodiment of the support described above, according to a further embodiment, for example for manufacturing a plane mirror, the support may also be flat.

[0105] According to another embodiment of the support, the intermediate material 13 comprises polyvinyl chloride.

[0106] Preferably, the surface 4 of the support 1 is only partially covered with the intermediate material 13. For example, as shown here in Figure 3, a film can be applied to the surface as intermediate material 13 in the form of small pieces, which can in particular be arranged in a regular pattern, for example a symmetrical or radially symmetrical pattern. In one embodiment, a three-, four- or multiple-element symmetrical body of film pieces is proposed, the film pieces preferably having the shape of a circle or an ellipse.

[0107] FIG. 4 shows, in several partial views, method steps for manufacturing the mirror substrate 20.

[0108] First, the mirror substrate and the support are prepared. As shown in subdivision (a), the surface 4 of the support 1 is shaped according to a predetermined envelope, which is concave in this case, using a grinding tool 5, such as a rotating grinding disk. Subdivision (b) shows the corresponding processing of the support surface 22 of the mirror substrate 20, which now has a curvature complementary to that of the surface 4. Once the surfaces 4, 22 have been shaped in this way, the mirror substrate 20 is placed on the support 1 and fixed, as shown in subdivision (c). In this case, an intermediate material 13 may again be used to compensate for any remaining surface differences and, optionally, to provide shock absorption during further processing of the mirror substrate 20. Fixation of the mirror substrate 20 on the support against lateral displacement and detachment can be achieved, for example, by adhesion with the intermediate material 13. In the assembly 15 thus obtained, the functional surface 21 can be processed with the grinding tool 5 while the mirror substrate 20 is substantially undeformed, as shown in subdivision (d). Since the support 1 can be further used for processing further mirror substrates, the step according to partial view (a) only needs to be carried out once when producing a series of identical mirror substrates 20.

[0109] Without being limited to the above examples, the manufacturing method of the mirror substrate 20 according to the preferred embodiment of the present invention described above is based on shaping the surface 4 and the support surface of the support 1 according to a predetermined shape or according to a predetermined surface profile by machining for material removal, forming the support surface 22 of the mirror substrate and the surface 4 of the support 1 in shapes complementary to each other at least in the support area, bringing the support surface 22 of the mirror substrate 20 and the surface 4 of the support 1 together, fixing the mirror substrate 20 to the support 1 to form an assembly 15, and then forming the functional surface 21 opposite the support surface 22 in the assembly 15 by machining for material removal.

[0110] Example Example 1: Mirror substrates were manufactured as shown in Table 1. First, a mirror blank of the specified diameter and thickness was placed on one or more layers of intermediate material placed on a block of support material. These intermediate materials had channels extending to the edge of the support material. The top surface of the mirror blank was machined to have a convex shape, and the mirror blank was placed with the machined blank on a block of support material (with the aforementioned intermediate material) whose surface had been ground to have a concave shape matching the convex shape of the mirror blank substrate. The concave shape was then formed in the mirror blank substrate by grinding, and the thickness of the mirror substrate was reduced as needed. A summary of the achieved surface roughness is shown in Table 1.

[0111] In Comparative Example 1, a mineral support material as described in DE 10 2015 112 036 A was used. The mirror blank broke during the second grinding step before the desired thickness of the substrate was achieved.

[0112] [Table 1]

[0113] It will be clear to those skilled in the art that the invention is not limited to the illustrated embodiment, but can be varied in many ways. For example, the order of processing surfaces 4 and 22 is not important; they can be formed simultaneously, or the support surface 22 of the mirror substrate 20 can be formed first, contrary to the sequence shown in the partial drawings. [Explanation of symbols]

[0114] 1 Support 5 Grinding tools 13 Intermediate materials 15 Assemblage 20 Mirror base material 21 20 Functional Surfaces 22 20 support surfaces 4. Surface of the support

Claims

[Claim 1] A method for manufacturing a mirror substrate for astronomy, LCD lithography, or microlithography, which is made of a material having an average linear thermal expansion coefficient of 1×10 −6 / K or less measured in the range of 0°C to 50°C, comprising: The mirror substrate has the following characteristics: - said specular substrate is circular in shape and has a ratio of its transverse dimension or diameter to its maximum thickness of at least 100; the mass per unit area of said specular substrate is less than or equal to 100 kg / m 2 ; and The mirror substrate has a mirror surface with a roughness R a of at most 3.5 μm; and The mirror substrate is made of more than 90% by mass of glass ceramic, Ti-doped synthetic silica glass, or ceramic; The method comprises: - preparing a support made of a material having an average linear thermal expansion coefficient of 1 × 10 -6 / K or less measured in the range of 0 ° C to 50 ° C, for supporting the mirror-surface substrate; - Average linear thermal expansion coefficient measured in the range of 0°C to 50°C is 1 x 10 －６ preparing the mirror substrate made of a material having a viscosity of 1 / K or less; - placing said specular substrate on said support; - mechanically reworking the surface of the specular substrate, The difference in absolute value between the average linear thermal expansion coefficient of the material constituting the support and the average linear thermal expansion coefficient of the material constituting the mirror-surface substrate is 1 × 10 －６ / K or less, and shaping the surface of the support and the support surface of the mirrored substrate according to a predetermined surface profile by machining for material removal, forming the support surface of the mirrored substrate and the surface of the support in shapes complementary to each other, bringing the support surface of the mirrored substrate and the surface of the support together, fixing the mirrored substrate to the support to form an assembly, and then forming a functional surface in the assembly opposite the support surface by machining for material removal.

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