mirror

Abstract

The invention relates to a mirror (1) comprising a substrate (4), a polished layer (3) comprising amorphous silicon material, and a reflective layer (2). Said mirrors having no or minor imperfections between the surface of its substrate (4) and the polishing layer (3), also in the case when an adhesion layer (5) is applied between the surface of its substrate (4) and the polishing layer (3); and said mirrors having improved reflectivity. Additionally, it relates to a method for producing said a mirror wherein the application of the polishable layer (3') is carried out by DC magnetron sputtering.

Description

[0001] Title: Mirror

[0002] Field of the Invention

[0003] The present invention relates to a mirror comprising a reflective layer (2) configured to reflect radiation in a wavelength of more than 25 nm that exhibits improved reflection properties and longer lifetime and to a process for producing said mirror. It also relates to a satellite or drone or flight propelled device or a telescope or a spectrometer or a microscope or a telecommunications device or solar energy system or a lighting system or a medical device or a thermal imaging device comprising said mirror.

[0004] Background

[0005] Mirrors with high reflectivity are very useful and of great importance in applications like for example in telecommunications, optical instruments, and laser technology. Some developments have been carried out in the field of EUV lithography. For example US8976927B2 discloses an apparatus comprising a substrate for a mirror configured to reflect extreme ultraviolet (EUV) radiation, the substrate comprising: a main body produced from an aluminum alloy, a polishing layer having a thickness of less than 10 pm and a root-mean-square roughness of less than 0.5 nm; and a reflective layer arranged on the polishing layer of the substrate, the reflecting layer comprising a multilayer structure configured to reflect EUV radiation. According to US8976927B2 the root-mean square roughness of a maximum of approximately 0.5 nm of the polishing layer of the substrate has a positive impact on the reflectivity of the mirror. However, the invention in US8976927B2 does not provide the possibility to use different substrate materials as it is limited to aluminum alloy substrates.

[0006] Risse S et al. ’’Ultra-precise Optical Mirrors with Thick Amorphous Silicon Layer”. 1 May 2011 (2011 -05-01 ), XP093219975, discloses a process chain for metal mirrors fabrication where a layer of amorphous silicon is applied on AI6061 and AIS i-40 substrates. The thickness of the amorphous silicon layers being 1 pm or 3 pm or 6 pm or 10 pm. The application of the amorphous silicon layer on the substrate was carried out by Mid-frequency Dual Magnetron Sputtering by using a multi-layer stack approach of several 0.5 pm to 1 pm layers with interruptions to get to the final amorphous silicon layer (for example to get to a 10 pm amorphous silicon layer). This to reduce the film stress and the temperature of the substrate during the deposition of the amorphous silicon layer. The amorphous silicon layer showing a root-mean-square roughness of 0.73 nm after being polished.

[0007] However, Risse S et al. do not mention a specific advantage of the thickness of the amorphous silicon layer, and they mention that the silicon layers show for higher film thickness an increasing porosity. This increment of porosity can affect the surface quality of the silicon layer and as a result, can also affect the quality of the resulting layer after polishing. Additionally, the application of the amorphous silicon layer by using a multi-layer stack approach has some drawbacks, for example: process complexity, time-consuming as it takes more time than depositing a single layer, high costs due to the additional processing steps, defect propagation as defects in individual layers can propagate and affect the overall quality of the stack, layer interfaces formation that can cause issues during polishing and result in bad surface quality. Additionally, Risse S et al. are mainly focused on substrates made of AI6061 and AISi-40 alloy.

[0008] Generally, the possibility to have low root-mean square roughness on the polished layer of a mirror is strongly related to the type of substrate that is used, generally this is possible on very good polishable substrates. This means that having a polished layer with low root-mean square roughness and without defects on other type of substrate cannot be achieved. This limits, the type of substrates to be used in the production of the mirrors; for example, substrates that are difficult to be mechanically processed or substrates that are less polishable. The term “less polishable” means that the substrate has a surface with many defects and / or has a surface that is very difficult to be polished to a roughness having low values of root-mean-square roughness (for example of less than 1 nm) without defects or with minor defects and / or when the surface is polished, it takes long polishing time to achieve low values of root-mean-square roughness without defects or with minor defects. Another drawback of the current mirrors is that normally the polishable layer (layer to be polished to a polished layer) is thin because it is generally applied on very good polishable substrates. For example, US2020150315 discloses an optical device comprising a substrate, an amorphous silicon layer and a layer of material comprising an optical coating, applied over the amorphous silicon layer. This amorphous silicon layer being a “low-stress” layer since it is thin enough to create little or no stress on the shape of the underlying substrate. For example, the amorphous silicon layer having a thickness such as about 0.5 pm or less. As the amorphous silicon layer is thin, the surface of the substrate should be smoothed before applying the amorphous silicon layer on the surface of the substrate, which restricts the type of substrate to be used.

[0009] This means that if a substrate that is difficult to be mechanically processed or a substrate that is less polishable is used, the thin polishable layer cannot completely cover the imperfections of the surface of the substrate, creating an incomplete and non- uniform adhesion of the subsequent layer to be applied on the polishing layer, for example of the reflective layer to be applied on the polishing layer; during the production of the mirror. This creates the formation of some imperfections or holes between the polishing layer and the subsequent layer (for example between the polishing layer, and the reflective layer) resulting in areas of the mirror with poor or no reflection, affecting the overall reflective properties of the mirror. This also limits, the type of substrates to be used in the production of the mirrors; for example, substrates that are difficult to be mechanically processed or substrates that are less polishable.

[0010] In the production of some mirrors an adhesive layer is applied on the surface of the substrate before the application of the polishable layer. However, this adhesive layer replicates the condition of the surface of the substrate, also replicating the defects of the surface of the substrate. Therefore, the same drawbacks as mentioned above also apply to mirrors having an adhesive layer applied on the surface of the substrate before the application of the polishable layer.

[0011] Additionally, in the case of concave or convex mirrors, when a thin polishable layer is applied on the substate, it is not possible to achieve a proper centering of the mirror curve to the middle point of the outer diameter of the of the mirror. This is a disadvantage in the production of good quality mirrors.

[0012] Generally, the application of the polishable layer on a substrate is carried out by vacuum coating methods or by using dispersion layers or by chemical vapor deposition. However, some of these methods have limitations with respect to the substrate where the polishable layer is applied. Additionally, due to the method used for applying the polishable layer, the polishing of said polishable layer takes long time to get the required polished layer (polishing layer).

[0013] Therefore, there is the need of mirrors that does not have the above-mentioned drawbacks and also of mirrors made on a substrate that is not limited by its surface quality. Said mirrors having no or minor imperfections between the surface of its substrate and the polishing layer, and / or on the polished layer. Also, in the case when an adhesive layer is present, said mirrors having no or minor imperfections between the surface of its substrate and the adhesive layer and / or on the surface of the polished layer. Said mirrors having improved reflectivity.

[0014] It has to be understood that the term “polishing layer” is the same as “polished layer”.

[0015] Additionally, there is the need of a method for producing a mirror wherein the application of the polishable layer is carried out by an uncomplicated, faster and therefore less costly process. Additionally, there is the need that the resulting polishable layer is suitable to be polished faster and more accurately and without limitation on the type of substrate used in the production of said mirror.

[0016] Summary of the Invention

[0017] In order to solve the above-mentioned problems, the present invention provides a mirror comprising a reflective layer (2) configured to reflect radiation in a wavelength of more than 25 nm, a polished layer (3), a substrate (4), wherein the polished layer (3) is arranged between the reflective layer (2) and the substrate (4) and comprises an amorphous silicon material, characterized in that the polished layer (3) is the result of a polishable layer (3') previous to be polished having a thickness in the range of from 20000 nm to 150000 nm, that was applied to the surface of the substrate (4) by DC magnetron sputtering.

[0018] Unexpectedly, the mirror according to the invention has a high reflection rate of light waves compared to current mirrors. The mirror according to the invention has no or minor imperfections on the surface of the polished layer and as a result of that the reflectivity of the mirror improves.

[0019] Additionally, as the reflection rate is high, fewer light waves are absorbed by the mirror surface and the heating rate of the mirror decreases significantly. Therefore, the use of cooling systems to cool down the mirror to lower temperatures, is decreased. Therefore, the risk that the mirror according to the invention breaks due to overheat is significantly decreased and the lifetime of the mirror significantly increases. This is very advantageous in applications where the risk of generating high temperatures in the mirror is high, for example, in laser applications.

[0020] Additionally, the mirror according to the invention can be made of different types of substrates including less polishable substrates.

[0021] In one embodiment of the invention the mirror comprises a reflective layer (2) configured to reflect radiation in a wavelength of more than 25 nm, a polished layer (3), a substrate (4), wherein the polished layer (3) is arranged between the reflective layer (2) and the substrate (4) and comprises an amorphous silicon material, characterized in that the polished layer (3) is the result of a polishable layer (3') previous to be polished having a thickness in the range of from 20000 nm to 150000 nm, that was applied to the surface of the substrate (4) by DC magnetron sputtering. Generally, the reflective layer (2) comprised in the mirror according to the invention, is configured to reflect radiation in a wavelength of more than 25 nm, preferably in the range to from 100 nm to 10600 nm, more preferably in the range of from 120 nm to 2000 nm.

[0022] Generally, the polished layer (3) comprised in the mirror according to the invention, has a root-mean-square roughness of below 1 nm. Preferably the root-mean-square roughness of the polished layer (3) is of from 0.5 nm to 0.8 nm. This is advantageous because it decreases the number of light waves that are absorbed or scattered by the mirror surface resulting in a significant decrease of the heating rate of the mirror. Therefore, the risk that the mirror according to the invention breaks due to overheat is significantly decreased and the lifetime of the mirror significantly increases.

[0023] The root-mean-square roughness of the polished layer is measured by Atomic Force Microscopy (AFM) with a scan-Area of 10x10 pm with 512 x 512 measurement points and calculated according to ISO4287.

[0024] The polished layer (3) is the result of a polishable layer (3') that has generally been applied to the surface of the substrate (4) in a step during the preparation of the mirror according to the invention.

[0025] The polishable layer (3') is generally applied to the surface of the substrate (4) by DC magnetron sputtering. After the application of the polishable layer (3') on the surface of the substrate (4), said layer (3') is then polished to create the polished layer (3). The polishable layer (3') applied by DC magnetron sputtering method is advantageous because it can be polished to a polished layer (3) having a root-mean-square roughness of less than 1 nm, with no or minor imperfections, which is difficult or not possible to be achieved with polishable layers applied by other coating methods. Additionally, these coating methods have limitations with regards to the type of substrate to be coated. DC magnetron sputtering (Direct Current magnetron sputtering) method is also advantageous because the polishable layer (3') applied by this method can be polished to a polished layer (3) faster compared to polishable layers applied by other methods, for example compared to chemical vapor deposition method. The person skilled in the art understands the DC magnetron sputtering method and how it works.

[0026] According to the present invention the polishable layer is not limited to be applied only on the surface of the substrate (4). The polishable layer (3') can also be applied on other surface or surfaces which has / have previously been applied on the surface of the substrate (4).

[0027] More detailed information about the method for preparing the mirror according to the invention and other embodiments of the method can be found below in the section “method for preparing the mirror according to the invention”.

[0028] The polished layer (3) comprised in the mirror according to the invention comprises an amorphous silicon material or is made of an amorphous silicon material. The application of the amorphous silicon material on the substrate (4) by DC magnetron sputtering method is very advantageous as it allows to polish the polishable layer (3') to a polished layer (3) having a root-mean-square roughness of below 1 nm and having no or minor imperfections on the polished layer (3).

[0029] In one embodiment of the present invention, the mirror further comprises an adhesion layer (5) arranged on the surface of the substrate (4) and between the surface of the substrate (4) and the polished layer (3).

[0030] The presence of the adhesion layer (5) in the mirror according to the invention is advantageous because it creates a strong bonding between the surface of the substrate (4) and the polishable layer (3'). This strong bonding prevents that the polishable layer (3') breaks during the polishing of said layer to a polished layer (3).

[0031] In another embodiment of the present invention the mirror further comprises a transition layer (5a) arranged between the adhesion layer (5) and the polished layer (3).

[0032] Generally, the transition layer (5a) comprises a mixture of the compounds comprised in the adhesion layer (5) and in the polished layer (3) as this transition layer (5a) is applied by DC magnetron sputtering method so that it transitions from the adhesion layer (5) to the polishable layer (3'). The presence of the transition layer (5a) in the mirror according to the invention is advantageous as it improves the adhesive properties of the adhesion layer (5) creating a stronger bonding between the substrate (4) and the polished layer (3).

[0033] Generally, the composition of the transition layer (5a) transitions from 0 % of the composition of the polished layer (3) to 100 % of the composition of the polished layer (3) along the thickness of said transition layer (5a). When the transition layer (5a) has transitioned to 100 % of the composition of the polished layer (3) the polished layer (3) starts.

[0034] Generally, the adhesion layer (5) comprises at least one material selected from the group consisting of ruthenium, chromium, platinum, iridium, copper, silver, gold, nickel, nickel-phosphorus, tantalum, titanium, zirconium, tungsten, molybdenum, niobium. According to the present invention any combination of the above-mentioned materials is also possible to be comprised in the adhesion layer (5).

[0035] In another embodiment of the present invention the adhesion layer (5) has a thickness in the range of from 10 nm to 10000 nm, preferably in the range of from 50 nm to 1000 nm, more preferably in the range of from 50 nm to 100 nm. This is advantageous as it improves the adhesive properties of the adhesion layer (5) creating a stronger bonding between the substrate (4) and the polished layer (3).

[0036] Generally, the transition layer (5a) has a thickness in the range of from 10 nm to 5000 nm, preferably in the range of from 50 nm to 1000 nm, more preferably in the range of from 100 nm to 500 nm. This ranges are independent from the thickness of the adhesion layer.

[0037] In another embodiment of the present invention the thickness of the polished layer (3) is in the range of from 250 nm to 100000 nm, preferably in the range of from 400 nm to 75000 nm, more preferably in the range of from 500 to 50000 nm.

[0038] In another embodiment of the present invention the thickness of the polishable layer (3') is in the range of from 20000 nm to 150000 nm, preferably in the range of from 25000 to 75000 nm. This thickness is advantageous as it allows to polish the polishable layer (3'), to a polished layer (3) without defects or imperfections or with minor defects or imperfections on said polished layer (3). Additionally, it increases the use of different types of substrates to be used in the production of the mirrors; for example, the use of substrates that are difficult to be mechanically processed or substrates that are less good polishable, as a thicker polishable layer covers the imperfections of the surface of the substrates. This eliminates the formation of imperfections or holes between the polished layer (3) and the subsequent layer resulting in a mirror with improved reflection properties.

[0039] Additionally, in the case of concave or convex mirrors, a thick polishable layer (3') allows achieving a proper centering of the mirror curve to the middle point of the outer diameter of the of the mirror, resulting in mirrors with improved quality.

[0040] In another embodiment of the present invention the substrate (4) comprises at least one or is made of at least one material selected from the group consisting of metal, polymer- based material, glass, ceramic, metalloid or composite material.

[0041] In one specific embodiment, as the substrate (4) can comprise at least one of the above-mentioned materials, the substrate (4) can be made of one of the above- mentioned materials, said material being coated or precoated with at least one of the above-mentioned materials.

[0042] Examples of metals are aluminum, aluminum alloy, copper, copper alloy, beryllium, steel, magnesium, magnesium alloy.

[0043] Examples of ceramic materials are silicon, silicon carbide, fused silica, reinforced silicon carbide, sapphire.

[0044] Example of glass material is ultra-low expansion glass, optical glass, borosilicate glass.

[0045] Examples of polymers are engineering polymers, reinforced polymers.

[0046] Example of metalloid is germanium.

[0047] In another embodiment of the present invention the reflective layer (2) comprises at least one metallic coating and / or at least one dielectric coating.

[0048] In a further embodiment of the present invention the reflective layer (2) comprises one metallic coating and more than one dielectric coatings. Examples of metallic coatings are aluminum, gold, silver, rhodium, copper, platinum, chrome.

[0049] Examples of dielectric coatings are oxide materials, fluoride materials, sulfide materials and selenide materials.

[0050] Examples of oxide materials are titanium dioxide, hafnium dioxide, tantalum pentoxide, silicon dioxide, yttrium oxide.

[0051] Examples of fluoride materials are magnesium fluoride, barium fluoride, yttrium fluoride.

[0052] Examples of sulfide materials and selenide materials are zinc sulfide and zinc selenide.

[0053] In another embodiment of the present invention the mirror further comprises at least one protective overcoat layer (6) arranged on the top of the reflective layer (2).

[0054] Examples of protective overcoat layer (6) are dielectric coatings like silicon oxide or magnesium fluoride, aluminum oxide, ruthenium. However, other types of protective layers can be used.

[0055] The present invention also relates to a substrate (4) having a polishable layer (3') having a thickness in the range of from 20000 nm to 150000 nm, preferably of from 25000 to 75000.

[0056] Method for preparing the mirror according to the invention.

[0057] The present invention also relates to a process for producing a mirror according to the invention, comprising the steps of a. applying, by DC magnetron sputtering, a polishable layer (3') on the surface of a substrate (4) or on the surface of an adhesion layer (5), b. applying a reflective layer (2) on a polished layer (3) wherein the polished layer (3) is the result of the polishable layer (3') after being polished.

[0058] The process for producing a mirror according to the invention generally comprises a step where a polishable layer (3') is applied on the surface of a substrate (4). The polishable layer (3') is applied by DC magnetron sputtering method. The person skilled in the art understands the DC magnetron sputtering method and how it works.

[0059] The application of the polishable layer by DC magnetron sputtering method is advantageous as it allows the deposition of the polishable layer (3') having a thickness between 20000 nm to 150000 nm. It is also advantageous as the resulting polishable layer (3') can be polished to a polished layer (3) faster than polishable layers deposed by using other deposition or coating methods.

[0060] The mirror according to the invention generally has a high reflection rate of light waves compared to current mirrors. Additionally, as the reflection rate is high, fewer light waves are absorbed by the mirror surface and the heating rate of the mirror decreases significantly

[0061] The presence of the polishable layer (3') is very important because it creates the anchor for the reflective layer (2) of the mirror according to the invention.

[0062] The polishable layer (3') can be polished to create a polished layer (3) having a root- mean-square roughness of below 1 nm. The root-mean-square roughness of the polished layer is measured by Atomic Force Microscopy (AFM) with a scan-Area of 10x10 pm with 512 x 512 measurement points and calculated according to ISO4287.

[0063] Once the polished layer (3) having a root-mean-square roughness of below 1nm has been created, a reflective layer (2) can be applied on said polished layer (3).

[0064] The polishing of the polishable layer (3') is generally carried out by one of the following methods: diamonds turning, grinding, polishing, milling, chemical mechanical polishing (CMP), ion beam figuring (IBF), magneto rheological finishing (MRF), laser polishing. However, any other method that provides a surface polishing suitable to be used in mirrors is also applicable.

[0065] The person skilled in the art can understand the concept of polishing in the context of the invention. The application of the reflective layer (2) on the polished layer (3) is not limited to DC magnetron sputtering or other physical vapor deposition method as other methods suitable for this purpose can be used.

[0066] Examples of method for the application of the reflective layer (2) on the polished layer (3) are thermal evaporation, physical vapor deposition, chemical vapor deposition, atomic layer deposition.

[0067] In another embodiment, the method according to the invention further comprises the step of applying an adhesion layer (5) on the surface of the substrate (4) previous to the application of the polishable layer (3'). In this embodiment, once the adhesion layer has been applied to the surface of the substrate (4) the polishable layer (3') is then applied on the adhesion layer (5).

[0068] In another embodiment, the method according to the invention comprises the step of applying at least one metallic coating and / or at least one dielectric coating on the reflective layer (2). Generally, these coatings are applied on the reflective layer (2) and are very useful as they provide a protective coating to the mirror according to the invention. When at least one metallic coating and / or at least one dielectric coating is applied on the reflective layer (2), this means that they are applied one on the top of the other at least one metallic coating and / or at least one dielectric coating on the reflective layer (2), creating a multilayer structure on the reflective layer (2).

[0069] The present invention also provides a satellite or drone or flight propelled device or a telescope or a spectrometer or a microscope or a telecommunications device or solar energy system or a lighting system or a medical device or a thermal imaging device comprising at least one mirror according to the invention.

[0070] The present invention also provides a laser system comprising at least a mirror according to the invention. Examples of laser systems are a laser cutting device, a laser welding device, a medical laser, a CO2 laser resonator and a manufacturing laser system .

Claims

Claims1- A mirror comprising a reflective layer (2) configured to reflect radiation in a wavelength of more than 25 nm, a polished layer (3), a substrate (4), wherein the polished layer (3) is arranged between the reflective layer (2) and the substrate (4) and comprises an amorphous silicon material, characterized in that the polished layer (3) is the result of a polishable layer (3') previous to be polished having a thickness in the range of from 20000 nm to 150000 nm, that was applied to the surface of the substrate (4) by DC magnetron sputtering.2- The mirror according to claim 1 , wherein the polished layer (3) has a thickness in the range of 400 nm to 75000 nm.3- The mirror according to claim 1 or 2, wherein the surface of the polished layer (3) has a root-mean-square roughness of below 1 nm as measured by Atomic Force Microscopy (AFM) with a scan-Area of 10x10 pm with 512 x 512 measurement points and calculated according to ISO4287.4- The mirror according to anyone of claims 1 to 3, further comprising an adhesion layer (5) arranged on the surface of the substrate (4) and a transition layer (5a) arranged between the adhesion layer (5) and the polished layer (3).5- The mirror according to claim 4, wherein the composition of the transition layer (5a) transitions from 0 % of the composition of the polished layer (3) to 100 % of the composition of the polished layer (3) along the thickness of said transition layer (5a).6- The mirror according to any one of claims 4 to 5, wherein the adhesion layer (5) comprises at least one material selected from the group consisting of ruthenium, chromium, platinum, iridium, copper, silver, gold, nickel, nickel-phosphorus, tantalum, titanium, zirconium, tungsten, molybdenum or niobium.7- The mirror according to anyone of claims 4 to 6, wherein the adhesion layer (5) has a thickness in the range of from 10 nm to 10000 nm.8- The mirror according to anyone of claims 1 to 7, wherein the substrate (4) comprises at last one or is made of at least one material selected from the group consisting of metal, polymer-based material, glass, ceramic, metalloid or composite material.9- The mirror according to claim 8, wherein the substrate (4) is made of a material according to claim 8, said material being coated or precoated with at least one of the materials according to claim 8.10-The mirror according to anyone of claims 1 to 9, wherein the substrate (4) is made of magnesium, magnesium alloy or a mixture thereof.11- The mirror according to anyone of claims 1 to 10, wherein the reflective layer (2) comprises at least one metallic coating and / or at least one dielectric coating.12-The mirror according to anyone of claims 1 to 11 , further comprising at least one protective overcoat layer (6) arranged on the top of the reflective layer (2).13- A process for producing a mirror according to anyone of claims 1 to 12, comprising the steps of a. applying, by DC magnetron sputtering, a polishable layer (3') on the surface of a substrate (4) or on the surface of an adhesion layer (5), b. applying a reflective layer (2) on a polished layer (3) wherein the polished layer (3) is the result of the polishable layer (3') after being polished.14- The process according to claim 12 comprising the step of applying at least one metallic coating and / or at least one dielectric coating on the reflective layer (2).15- A satellite or drone or flight propelled device or a telescope or a spectrometer or a microscope or a telecommunications device or solar energy system or a lighting system or a medical device or a thermal imaging device or a laser system comprising at least one mirror according to anyone of claims 1 to 12.

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