DE102020101041B4 - Mirror and procedure for its renewal - Google Patents

Abstract

Mirror (10), comprising - a substrate (1), - a silver layer (2), - an adhesive layer (3) arranged on the silver layer (3), the adhesive layer (3) having an aluminum oxide, and- an adhesive layer ( 3) arranged layer stack (4) having at least one first aluminum oxynitride layer (41, 43) and at least one second aluminum oxynitride layer (42), wherein the first aluminum oxynitride layer (41, 43) has a refractive index n1 Claim 8, second aluminum oxynitride layer (42) has a refractive index n2<n1.

Description

The invention relates to a mirror intended in particular for applications in precision optics and to a method for renewing the mirror.

Numerous applications of precision optical components require optical elements with a mirror layer that is highly reflective over as wide a spectral range as possible. Examples are applications in terrestrial and satellite-based astronomy and earth observation. Metals exhibit high reflectivity over a wide spectral range.

Depending on the spectral range, gold, aluminum or silver are generally used for mirrors. Gold is very inert and aluminum quickly forms a thin passivating aluminum oxide layer. Both are good prerequisites for consistently high degrees of reflection. Silver, on the other hand, is susceptible to corrosion, especially to H ₂ S from the ambient air. On the other hand, of the metals, silver has the highest reflectivity from the visible to the infrared spectral range. Therefore, despite the susceptibility to corrosion, mirrors are increasingly coated with silver and covered with an additional protective layer. While there are also applications for aluminum and gold without applied protective layers, silver mirrors are practically always provided with a protective layer.

The production of protected silver mirrors for front-side mirrors, which should have maximum reflectivity in a broad spectral range, is particularly demanding, since the protective layers influence the optical function of the layers. Certain dielectric materials are used for the protective layer in order to keep the negative influence on the degree of reflection as low as possible. From the pamphlet US 5,514,476A it is known, for example, to protect a silver layer by layers of Ni, NiCr and Si ₃ N ₄ .

The pamphlet WO 99/64900 A1 describes a multilayer mirror with reflective layers of aluminum and silver. Furthermore, a protective layer is provided which has, for example, nickel, a chromium alloy or a nitride.

The article by Anjum, F. et al. entitled "Improving silver mirrors with aluminum oxynitride protection layers: variation in refractive index with controlled oxygen content by radiofrequency magnetron sputtering" in Journal of Astronomical Telescopes, Instruments, and Systems, Vol. 4, 2018, no. 4, p. 044004-1 - 044004-10 describes a method in which specific amounts of oxygen are intentionally introduced into an AlN protective layer in order to obtain AlON protective layers with a specific refractive index.

Previous approaches to protecting mirrors with a silver layer pursue a maximum service life of the protected mirror. Due to the desired resistance and the combination of materials, a wet-chemical removal of the reflector layer system is usually not possible or only possible with hydrofluoric acid. However, hydrofluoric acid is very hazardous to health and therefore very difficult to handle for occupational safety reasons.

One problem to be solved is therefore to specify an improved mirror with a silver layer, which is distinguished by high reflection, good durability and, in particular, the possibility of renewal with relatively little effort. Furthermore, a method for renewing the mirror is to be specified.

These objects are solved by a mirror and a method for its renewal according to the independent patent claims. Advantageous refinements and developments of the invention are the subject matter of the dependent claims.

In accordance with at least one embodiment, the mirror comprises a substrate and a silver layer arranged on the substrate. It is not excluded that at least one further layer is arranged between the substrate and the silver layer, for example an adhesive layer. The substrate can be a planar substrate or a curved substrate. The silver layer serves as a reflector layer in the mirror. Silver is characterized in particular by a high level of reflectivity in the entire visible spectral range.

In the case of the mirror, an adhesive layer is arranged on the silver layer, which has or consists of an aluminum oxide, in particular Al ₂ O ₃ . The adhesive layer is in particular arranged directly on the silver layer. The adhesion layer is intended to improve the adhesion of a layer stack to the silver layer.

On the adhesive layer, in particular directly on the adhesive layer, a layer stack is arranged in the mirror, which has at least a first aluminum oxynitride layer and at least a second aluminum oxynitride layer. The at least one first aluminum oxynitride layer has a refractive index n ₁ and the at least one second aluminum oxynitride layer has a refractive index n ₂ on, where n ₂ < n ₁ . The at least one first aluminum oxynitride layer thus has a higher refractive index than the at least one second aluminum oxynitride layer. In other words, the at least one first aluminum oxynitride layer is a high-index layer and the second aluminum oxynitride layer is a low-index layer. In the visible spectral range, n ₁ >1.8 and n ₂ <1.8 are preferred.

The layer stack may include one or more of the first aluminum oxynitride layers and one or more of the second aluminum oxynitride layers. For example, the layer stack can consist of three aluminum oxynitride layers, with a second aluminum oxynitride layer having the lower refractive index n ₂ being arranged between two first aluminum oxynitride layers having the higher refractive index n ₁ . It is possible for the layer stack to have more than three aluminum oxynitride layers. In this case, the first aluminum oxynitride layers and the second aluminum oxynitride layers are advantageously arranged alternately in the layer stack. The top layer of the layer stack is preferably a first aluminum oxynitride layer with the higher refractive index n ₁ .

The mirror described here makes use in particular of the idea that the layer stack protecting the mirror layer can be removed from the mirror again in a comparatively simple manner. The aluminum oxynitride layers are notable for their high resistance to polar liquids, but are easier to detach from the mirror than known protective layers. In particular, it is possible to remove the aluminum oxynitride layers with an etching solution that does not contain hydrofluoric acid.

The thicknesses of the aluminum oxynitride layers in the layer stack are advantageously optimized in such a way that maximum reflection is achieved at a wavelength provided for the mirror or in a wavelength range provided for the mirror. The already high reflection of the silver layer can be increased even further in this way. The optimization of the individual layer thicknesses required for this purpose depends on the wavelength or the wavelength range and can be carried out using suitable simulation programs known per se to those skilled in the art.

In accordance with at least one embodiment, the at least one first aluminum oxynitride layer has a larger proportion of nitrogen than the at least one second aluminum oxynitride layer. In this way it is achieved that the first aluminum oxynitride layer has a higher refractive index than the second aluminum oxynitride layer. The refractive index of aluminum oxynitride increases with increasing nitrogen content.

In accordance with at least one embodiment, the at least one first aluminum oxynitride layer has the composition AlO _x1 N _y1 , where 0.2<y1/(x1+y1)≦1 applies. Preferably 0.5<y1/(x1+y1)≦1 and more preferably 0.8<y1/(x1+y1)≦1. For example, y1/(x1+y1)=0.9.

In accordance with at least one embodiment, the at least one second aluminum oxynitride layer has the composition AlO _X2 N _y2 , where 0≦y2/(x2+y2)≦0.2. Preferably 0 ≤ y1 /(x1 + y1) ≤ 0.1. In this way it is achieved that the second aluminum oxynitride layer has a lower refractive index than the first aluminum oxynitride layer.

According to at least one embodiment, a cover layer of the layer stack is formed by a first aluminum oxynitride layer, ie by an aluminum oxynitride layer with the higher refractive index n ₁ . The first aluminum oxynitride layers are characterized in particular by a high proportion of covalent bonds. A high level of covalent bonding in the cover layer means that the layer stack can be highly resistant to polar liquids.

According to at least one embodiment, the adhesion layer is from 1 nm to 20 nm thick. In this case, due to its low thickness, the adhesive layer only has a minor influence on the optical properties of the mirror.

In accordance with at least one embodiment, only the adhesive layer and the layer stack are arranged above the silver layer, the layer stack consisting exclusively of aluminum oxynitride layers. No further layers are arranged above the silver layer in the mirror. This simplifies the detachment of the layer stack when it is to be removed for renewal.

A method for renewing the mirror is also provided. In the process for renewing the mirror, for example in the event of damage or contamination of the mirror, the layer stack and the adhesive layer are detached using an etching solution. The layer stack is advantageously completely removed in this way. The layer stack is then applied again. In this way, the protection of the mirror provided by the layer stack can be renewed. The method for renewing the mirror is particularly advantageous when the mirror has an expensive substrate. In this case, the substrate advantageously does not have to be renewed.

In accordance with at least one embodiment, the etching solution does not contain any hydrofluoric acid. Since the aluminum oxynitride layers of the layer stack can also be removed by etching agents that are less harmful to health, the use of hydrofluoric acid can advantageously be dispensed with. The aluminum oxynitride layers are preferably removed with phosphoric acid (H ₃ PO ₄ ). Alternatively, the etching solution can be an aqueous alkaline solution. For example, the etching solution contains KOH or NaOH.

According to at least one embodiment, the method for renewing the mirror also includes removing and reapplying the silver layer. Another etching solution suitable for etching silver can be used to remove the silver layer from the substrate. For example, silver can be etched with nitric acid (HNO ₃ ). Other etching solutions suitable for etching silver are an iodine-potassium iodide solution (I ₂ /KI ₂ ) or an NH ₃ /H ₂ O ₂ /CH ₃ OH solution.

In a preferred variant, the layer stack, the adhesion layer and the silver layer are removed with a single etching solution. This can be done with an etching solution containing phosphoric acid (H ₃ PO ₄ ) and nitric acid (HNO ₃ ).

• 1 zeigt eine schematische Darstellung eines Querschnitts durch einen Spiegel gemäß einem Ausführungsbeispiel.

The invention is based on an embodiment in connection with the 1 explained in more detail.

• 1 shows a schematic representation of a cross section through a mirror according to an embodiment.

The components shown and the proportions of the components among one another are not to be regarded as true to scale.

the inside 1 The mirror 10 shown contains a substrate 1. The substrate 1 can have, for example, a glass, a glass ceramic, a plastic, a semiconductor material or a metal. The substrate 1 particularly preferably has a glass ceramic with a particularly low coefficient of thermal expansion. The substrate 1 can be flat or curved. The substrate 1 can in particular be an optical element of precision optics, for example an optical element for terrestrial or satellite-based astronomy or earth exploration.

A silver layer 2 is arranged on the substrate 1 and acts as a reflective layer in the mirror. It is possible for one or more further layers to be arranged between the substrate 1 and the silver layer 2, for example adhesive or polishing layers.

An adhesion layer 3 is arranged on the silver layer 2 and improves the adhesion of a subsequent layer stack 4 . The adhesive layer has an aluminum oxide (AlO _x ), in particular Al ₂ O ₃ , and is preferably between 1 nm and 20 nm thick.

To protect the silver layer 2 from corrosion, a layer stack 4 is arranged on the adhesive layer 3, in particular directly on the adhesive layer 3, which consists of several aluminum oxynitride layers (AlO _x N _y ) 41, 42, 43. In the example shown, the layer stack 4 consists of three aluminum oxynitride layers, but other configurations with a different number of layers are also conceivable. Preferably the number of aluminum oxynitride layers is between 2 and 10.

In the example, the mirror 10 has an aluminum oxynitride layer 42 with a refractive index n ₂ which is arranged between two further aluminum oxynitride layers 41, 43 with a higher refractive index n ₁ >n ₂ . In the visible spectral range, n ₁ is preferably >1.8 and n ₂ is <1.8. The different refractive indices are achieved by different proportions of oxygen (O) and nitrogen (N) in the aluminum oxynitride layers 41,42,43.

The first aluminum oxynitride layers 41, 43 have a composition AlO _x1 N _y1 , where 0.2<y1/(x1+y1)≦1. Preferably 0.5<y1/(x1+y1)≦1 and more preferably 0.8<y1/(x1+y1)≦1. For example, y1/(x1+y1)=0.9. The second aluminum oxynitride layer 42 has the composition AlO _X2 N _y2 , where 0≦y2/(x2+y2)≦0.2. Preferably 0 ≤ y1 /(x1 + y1) ≤ 0.1.

The layer thicknesses of the aluminum oxynitride layers 41, 42, 43 are preferably optimized in such a way that the reflectivity of the mirror 10 is at a maximum in a predetermined wavelength range, for example the visible spectral range.

The mirror 10 is characterized by good resistance, in particular to polar liquids, and at the same time high reflection. A particular advantage of the mirror 10 described herein is that the layers arranged above the silver layer 2, ie the adhesion layer 3 and the layer stack 4, can be removed in a comparatively simple manner. In particular, in a mirror renewal process, the aluminum oxide and aluminum oxynitride layers 3, 41, 42, 43 can be removed from the silver layer 2 by means of an etching solution which does not contain hydrofluoric acid. An etching solution suitable for detaching the adhesive layer 3 and the layer stack 4 can be phosphoric acid, for example (H ₃ PO ₄ ), potassium hydroxide (KOH) or sodium hydroxide (NaOH).

In a further step or at the same time, the silver layer 2 can optionally also be completely or partially removed from the substrate 1 if the silver layer is also to be renewed. If necessary, another etching solution is used to remove the silver layer 2 . Suitable etchants for silver are, for example, nitric acid (HNO ₃ ), iodine-potassium iodide solution (I ₂ /KI ₂ ) or an NH _{3 /} H ₂ O ₂ /CH ₃ OH solution. In a preferred variant, the layer stack 4, the adhesion layer 3 and the silver layer 2 are removed with a single etching solution. This can be done with an etching solution containing phosphoric acid (H ₃ PO ₄ ) and nitric acid (HNO ₃ ). In this case, the silver layer is subsequently renewed, for example by renewed application by sputtering. The adhesive layer 3 and the layer stack 4 can then be applied again, for example by sputtering.

The comparatively simple possibility of replacing the mirror is particularly advantageous for applications in which particularly high-quality mirrors are used, for example in the case of mirrors in astronomy or earth exploration.

Claims (14)

Mirror (10), comprising - a substrate (1) - a silver layer (2), - an adhesion layer (3) arranged on the silver layer (3), the adhesion layer (3) having an aluminum oxide, and - an adhesion layer ( 3) arranged layer stack (4) having at least one first aluminum oxynitride layer (41, 43) and at least one second aluminum oxynitride layer (42), the first aluminum oxynitride layer (41, 43) having a refractive index n ₁ claim 8 , second aluminum oxynitride layer (42) has a refractive index n ₂ <n ₁ . mirror after claim 1 , wherein the at least one first aluminum oxynitride layer (41, 43) has a larger proportion of nitrogen than the at least one second aluminum oxynitride layer (42). Mirror according to one of the preceding claims, wherein the at least one first aluminum oxynitride layer (41, 43) has the composition AlO _x1 N _y1 , with 0.2 < y1 /(x1 + y1) ≤ 1. mirror after claim 3 , where 0.5 < y1 /(x1 + y1) ≤ 1. mirror after claim 3 , where 0.8 < y1 /(x1 + y1) ≤ 1. Mirror according to one of the preceding claims, wherein the at least one second aluminum oxynitride layer (42) has the composition AlO _X2 N _y2 , with 0 ≤ y2 /(x2 + y2) ≤ 0.2. Mirror according to one of the preceding claims, in which a cover layer of the layer stack (4) is formed by a first aluminum oxynitride layer (43). A mirror as claimed in any preceding claim, wherein the adhesion layer (3) is from 1nm to 20nm thick. Mirror according to one of the preceding claims, wherein only the adhesive layer (3) and the layer stack (4) are arranged over the silver layer (2) and the layer stack consists exclusively of aluminum oxynitride layers (41, 42, 43). Method for renewing a mirror according to any one of Claims 1 until 9 , comprising the steps: - detaching the layer stack (4) and the adhesive layer (3) with an etching solution and - reapplying the adhesive layer (3) and the layer stack (4) to the silver layer (2). procedure after claim 10 , wherein the etching solution does not contain hydrofluoric acid. Procedure according to one of Claims 10 or 11 , wherein the etching solution comprises phosphoric acid, potassium hydroxide or sodium hydroxide. Procedure according to one of Claims 10 until 12 , further comprising the detachment and renewed application of the silver layer (2). procedure after Claim 13 wherein the etching solution contains nitric acid, an iodine-potassium iodide solution, an NH ₃ /H ₂ O ₂ /CH ₃ OH solution, or a mixture comprising nitric acid and phosphoric acid.

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