Method for smoothing a surface of a substrate

Abstract

The invention relates to a method for smoothing a surface (2) of a substrate (1) made of a brittle, hard material, in particular glass or a glass ceramic, comprising: smoothing the surface (2) with a brush tool (3). The brush tool (3) can have diamond-coated filaments (4), which are preferably formed from an elastic material.

Description

BACKGROUND OF THE INVENTIONThe invention relates to a method for smoothing a surface of a substrate made of a brittle-hard material, in particular made of glass or of a glass ceramic.A brittle-hard material is understood to mean a material which has a very low absorption for light, as is the case, for example, with glasses or with glass ceramics. The surfaces of such materials can be processed until they have a surface quality which makes the surface transparent to the passage of light, so that interferometric measurement can be carried out on the substrate.Currently, transparent interferometrically measurable surfaces of substrates are produced in a processing process that includes multiple steps, namely a grinding step, a fine grinding step, and a subsequent polishing step. The polishing step builds on the quality of the finish grinding step and is dependent on the process control of the finish grinding step, i.e. on the outlay and on the quality of the finish grinding step. In the finish grinding step, depth damage is introduced into the material of the substrate, which damage is dependent on the finish grinding process, in particular on the machining parameters and on the tool design.Depending on the grinding quality or on the depth damage in the finish grinding step, a subsequent etching step up to a depth of 500 μm may be required on the ground surface in order to eliminate the depth damage induced in the material. As a result of the inhomogeneous etching removal, errors can occur with respect to dimensional and shape tolerances, which can result in renewed reworking of the surface by a further fine grinding step. This finish grinding step or finish grinding process introduces again depth damage which must be kept as low as possible. Owing to the depth damage, the production of optical surfaces causes enormous outlay. In addition, the grinding and polishing steps are generally not implementable on a machine.Object of the InventionIt is an object of the invention to provide a method for smoothing a surface of a substrate made of a brittle-hard material, which method produces the smallest possible depth damage.The Invention Subject MatterThis object is achieved by a method of the type mentioned at the beginning, in which the surface of the substrate is smoothed with a brush tool.The inventors have recognized that, by smoothing with a brush tool, the acting process forces are reduced and no or only very little depth damage is produced in the brittle-hard material of the substrate. During smoothing with the aid of a brush tool, the depth damage produced in the substrate in a preceding machining step, for example in a precision grinding step, can therefore be reduced by the amount of removal by the brush tool.Using brush tools, contours, etched surfaces and optical surfaces of brittle-hard materials such as glasses or glass ceramics can be smoothed. In the smoothing with the brush tool, fine texture of the surface is improved. The outlay of a possibly subsequent polishing step is reduced on account of the improved fine surface structure and can optionally be completely omitted (see below).Filament tools in the form of brushes are generally used for deburring ceramic indexable inserts in the tool industry. Their use for achieving cosmetic effects on surfaces is likewise known. The use of brush tools for reducing depth damage or for producing transparent, interferometrically measurable surfaces on substrates made of brittle-hard materials is, on the other hand, not known.In one variant, the brush tool has filaments which are preferably formed from an elastic material. The resilient material is typically a plastic material that deflects upon smoothing as the brush tool is moved across the surface. However, it can also be a metallic material which deflects in the event of a suitable formation of the filaments. The bending of the filaments results in a statistically random drag of the filaments, thereby breaking the peaks in the topography of the surface, causing a smoothing effect.In a further development of this variant, the filaments are covered with at least one material which has a Mohs hardness of at least 8. In this refinement, the filaments generally have grains of at least one material with a Mohs hardness of at least 8 on their surface in order to bring about the removal of material and thus the smoothing of the surface. The material with the Mohs hardness of at least 8 can be, for example, metallic carbides, nitrides, corundums (e.g. zirconium corundums).In one development, the filaments are diamond-filled. The filaments have diamond grains on their surface which allow the hard material to be removed. Diamond has a Mohs hardness of 10.In a further variant, during the smoothing, the brushing tool is positioned at a predefined distance from the surface, which is preferably selected such that the elastic filaments flex. The brush tool receives a predetermined feed during surface processing, whereby the elastic filaments result in bending and thus in a follow-up of the diamond grains of the diamond-filled filaments. As described above, this statistically random drag results in a smoothing effect.In a development of this variant, the brush tool is moved over the surface at the predefined distance along a predefined path curve. In this further development, the contour of the surface is followed by the brush tool at a constant distance, so that the brushes or the filaments always have a constant contact pressure. The surface to be smoothed can be a planar surface. In this case, the predetermined delivery of the brush tool can be maintained when the brush tool is moved over the surface. In the case that it is a curved surface, for example a free-form surface, an adjustment of the delivery of the brush tool is typically required in order to maintain the constant distance from the surface.In a further variant, the brush tool is moved over the surface in a rotating, oscillating, pulsing and / or brushing or towing manner during smoothing. The brush tool typically travels the surface along a predetermined path once or more times to produce the desired smoothing effect. As an alternative to the variant described further above, the distance of the brush tool perpendicular to the surface can be changed when the brush tool is moved over the surface; for example, the brush tool can execute a, for example, pulsating or oscillating movement perpendicular to the surface.In addition to the design of the base body of the brush tool and the diamond grains, the structure and the composition of the filaments can also vary depending on the surface to be processed or smoothed. The filaments may be formed of plastic or metal. Likewise, the process parameters such as the speed, the depth of penetration or the distance between the brush tool and the surface, as well as the trajectory, can vary depending on the intended application.In a further variant, a cooling lubricant is used during the smoothing of the surface. In addition to the cooling effect, the cooling lubricant also binds particles which form during the smoothing of the surface.In an alternative variant, particles formed during the smoothing are blown off from the surface. In this variant, the smoothing is carried out "dry", i.e. without the use of a cooling lubricant. The particles formed during the smoothing are typically blown off from the surface by means of a gas stream, for example by means of an air stream.In a further variant, the surface is processed in a finish grinding process before the smoothing. As described further above, in the finish grinding process depth damage is introduced into the material of the substrate, which damage is reduced by the smoothing, since material is removed during the smoothing, but practically no additional depth damage is produced. In this way, the depth damage can be reduced by the amount of removal by the brush tool and the fine surface structure can be improved. The polishing amount on the surface of the substrate required in the subsequent polishing can be reduced in this way. The surface of the substrate may be an optical surface or a non-optical surface.In a further variant, the surface is processed in an etching process before the smoothing. As described above, the etching process may reduce the depth damage of the material of the substrate caused by the previous finish grinding process. As has likewise been described further above, an inhomogeneous etching removal is produced during the etching process, which etching removal can cause dimensional and shape tolerances, which can possibly result in a renewed fine grinding process. The smoothing can improve the surface fine structure of the surface, so that typically a further fine grinding process after the etching can be dispensed with.In a further variant, the smoothing is carried out on an optical surface of the substrate until the optical surface can be measured interferometrically. In this variant, the smoothing is carried out until a fine surface structure or a surface quality is achieved, in which the optical surface of the substrate is transparent and can be measured interferometrically. In this variant, the optical surface can thus be measured directly interferometrically without a subsequent polishing step being required.The method described further above makes it possible to simplify the process of producing transparent, interferometrically measurable optical surfaces on substrates made of brittle-hard material, and to shorten the processing times for producing such surfaces.Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, on the basis of the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be realized individually or severally in any combination in a variant of the invention.DRAWINGExemplary embodiments are illustrated in the schematic drawing and are explained in the following description. It shows FIGS. 1 a, b are schematic representations of a brush tool having a pot shape, in the unloaded state and under load with deformation of the filaments, and FIGS. 2 a, b are schematic representations analogous to FIGS. 1 a, b in the case of a brush tool in the form of a grinding wheel.In the following description of the drawings, identical reference numerals are used for identical or functionally identical components.FIGS. 1 a, b show a workpiece in the form of a substrate 1 which is formed from a brittle-hard material. In the example shown, the brittle-hard material is quartz glass, but it can also be another type of glass or a glass ceramic. In the example shown, the substrate 1 has a planar surface 2 which is provided for the passage of light or radiation. In order to achieve a surface quality on the surface 2 that allows the passage of the radiation and makes the surface 2 interferometrically measurable, a production process is carried out that comprises a plurality of steps:First, a grinding process and subsequently a finish grinding process of the surface 2 are carried out. In the finish grinding process, depth damage is introduced into the material of the substrate 1, which damage is to be eliminated in a subsequent etching process. Since errors with regard to dimensional and shape tolerances can occur as a result of inhomogeneous etching removal in the etching process, the etching process is stopped in the production process described in FIGS. 1 a, bbefore the depth damage has been completely eliminated.In order to eliminate the depth damage and to improve the surface fine structure after the finish grinding process and the etching process, a brush tool 3 is used in a subsequent smoothing step, which has a pot shape in the example shown in FIGS. 1 a, b. The brush tool 3 has a multiplicity of elastic filaments 4 which are fastened to a planar end face of a main body 5 of the brush tool 3. The filaments 4 consist in one aspect of the invention of a plastic material. In a further aspect of the invention, the filaments 4 consist of a metallic material which deflects when the filaments are suitably formed. The filaments 4 are diamond-filled, i.e. diamond grains are attached to their surfaces. Alternatively or additionally, the filaments 4 can also be populated with grains of at least one other material which is particularly hard and has a Mohs hardness of at least 8, for example metallic carbides, nitrides or corundums, in particular zirconium corundums.In the example shown in FIGS. 1 a, b, the brush tool 3 can rotate about a vertical axis of rotation 6 perpendicular to the surface 2 with the aid of a drive, not shown in the drawing. The brush tool 3 unloaded in FIG. 1 ais brought into contact with the surface 2 for the smoothing process and advanced at a predetermined distance A from the surface 2. The distance A is selected such that the elastic filaments 4 flex, as can be seen in FIG. 1 b. The bending results in the diamond grains of the filaments 4 being dragged along as the brush tool 3 rotates, and this statistically random drag results in a smoothing effect in which peaks are broken in the topography of the surface 2, resulting in the surface 2 being smoothed. Due to the comparatively low process forces acting on the surface 2, practically no depth damage is generated in the material of the substrate 1 by the brush machining. Alternatively or additionally to the rotating movement of the brush tool 3, the latter can execute an oscillating, pulsating and / or brushing or towing movement.FIGS. 2 a, b show a smoothing process by brush machining, in which instead of a cup-shaped brush tool 3 a grinding wheel is used, which is rotatable about an axis of rotation 6 oriented perpendicular to the plane of the drawing. As in Figs. 1a,b, the rotating brush tool 3 is advanced at a predetermined distance A from the surface 2 chosen such that the elastic filaments 4 flex and are dragged, causing the smoothing effect described above. It goes without saying that the brush tool 3 can also be formed in a different manner than is illustrated in FIGS. 1 a, band in FIGS. 2 a, b. For example, the brush tool 3 can also be designed as a ball section tool.For the smoothing, the brush tool 3 is moved along a predefined path over the surface 2, wherein the movement can take place once or several times, i.e. a respective position on the surface 2 can be traveled over a single time or several times by the brush tool 3 during the smoothing. The process parameters during smoothing, i.e. the rotational speed of the brush tool 3, the depth of penetration or the distance A and the predefined trajectory can vary depending on the intended application. The design of the base body 5, the filaments 4 and the diamond grains, for example their size or grain size, can also vary depending on the surface 2 to be machined. The smoothing can be carried out "dry" or using a cooling lubricant. In the first case, particles which are produced during the smoothing process can be blown off from the surface 2.The polishing effort required to achieve a high surface quality or a high degree of polishing in a subsequent polishing step is reduced by the smoothing effect achieved during filament processing and improved fine structure of the surface 2. Ideally, the smoothing can result in a fine surface structure which produces a transparent surface 2 which is suitable for interferometric measurement. In this case, a subsequent polishing step may be omitted. However, it is also possible for the smoothing step described here to be followed by a polishing step in order to produce a transparent surface suitable for interferometric measurement. In this case, the smoothing reduces the polishing removal required for the subsequent polishing step.The substrate 1 with the surface 2 which can be interferometrically measured can serve, for example, for producing a mirror for use in a system for semiconductor technology, e.g. for an EUV lithography system. In this case, a reflective coating is applied to the surface 2. The use of the substrate 1 as a base body of a transmissive optical element, for example in the form of a lens for a system of semiconductor technology, is also possible. The system of semiconductor technology can be, for example, a projection exposure system, a mask inspection system or a wafer inspection system. It is understood that the smoothing process described here is not limited to the smoothing of optically used surfaces 2, but can also be applied to non-optically used surfaces of the substrate 1 in order to smooth these or their contours.

Claims

Method for smoothing a surface (2) of a substrate (1) made of a brittle-hard material, in particular made of glass or a glass ceramic, comprising: smoothing the surface (2) with a brush tool (3).Method according to claim 1, wherein the brush tool (3) comprises filaments (4) preferably formed from an elastic material.Method according to claim 2, wherein the filaments (4) are populated with a material having a Mohs hardness of at least 8, which is preferably selected from the group comprising: metallic carbides, nitrides, corundums, in particular zirconium corundums.Method according to claim 2 or 3, wherein the filaments (4) are diamond-filled.Method according to one of the preceding claims, in which, during the smoothing, the brushing tool (3) is positioned at a predefined distance (A) from the surface (2), which is preferably selected such that the elastic filaments (4) are bent.Method according to claim 5, wherein the brush tool (3) is moved over the surface (2) at the predefined distance (A) along a predefined trajectory.Method according to one of the preceding claims, in which during smoothing the brush tool (3) is moved over the surface (2) in a rotating, oscillating, pulsing and / or brushing manner.Method according to one of the preceding claims, in which a cooling lubricant is used in the smoothing of the surface (2).Method according to one of Claims 1 to 7, in which particles formed during the smoothing are blown off from the surface (2).Method according to one of the preceding claims, in which, before the smoothing, the surface (2) is processed in a precision grinding process.Method according to one of the preceding claims, in which, before the smoothing, the surface (2) is processed in an etching process.Method according to one of the preceding claims, in which the smoothing is carried out on an optical surface (2) of the substrate (1) until the optical surface (2) can be measured interferometrically.

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