DE102020101041A1 - Mirror and procedure for its renewal - Google Patents

Abstract

A mirror (10) is described, comprising a substrate (1), a silver layer (2) and an adhesive layer (3) arranged on the silver layer (3), the adhesive layer (3) comprising an aluminum oxide. Furthermore, the mirror comprises a layer stack (4) arranged on the adhesive layer (3) which has 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 n1 second aluminum oxynitride layer (42) has an index of refraction n2 <n1. A method for renewing the mirror (10) is also specified.

Description

The invention relates to a mirror which is 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 in the broadest possible spectral range. Examples are applications in terrestrial and satellite-based astronomy and earth observation. Metals have a high reflectivity in a wide spectral range.

For mirrors, gold, aluminum or silver are generally used, depending on the spectral range. Gold is very inert and aluminum quickly forms a thin passivating aluminum oxide layer. Both are good prerequisites for consistently high levels of reflection. Silver, on the other hand, is susceptible to corrosion, in particular 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 their susceptibility to corrosion, mirrors are increasingly being coated with silver and covered with an additional protective layer. While there are also applications for aluminum and gold without an applied protective layer, 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 U.S. 5,514,476 A it is known, for example, to protect a silver layer by layers of Ni, NiCr and Si ₃ N _4.

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

One object to be achieved is thus to provide an improved mirror with a silver layer which is characterized by high reflection, good resistance and, in particular, the possibility of renewal with relatively little effort. Furthermore, a method for renewing the mirror should be specified.

These objects are achieved 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, for example an adhesive layer, is arranged between the substrate and the silver 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 reflectivity in the entire visible spectral range.

On the silver layer at the mirror, an adhesive layer is arranged which has or consists of _{an aluminum oxide, in particular A1 2} O _3. In particular, the adhesive layer is arranged directly on the silver layer. The adhesive layer is intended to improve the adhesion of a stack of layers to the silver layer.

A layer stack, which has at least one first aluminum oxynitride layer and at least one second aluminum oxynitride layer, is arranged in the mirror on the adhesive layer, in particular directly on the adhesive 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 , 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 refractive index layer and the second aluminum oxynitride layer is a low refractive index layer. Preference is given to n ₁ > 1.8 and n ₂ <1.8 in the visible spectral range.

The layer stack can have 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, a second aluminum oxynitride layer with the lower refractive index n _{2 being arranged} between two first aluminum oxynitride layers with the higher refractive index n _1. It is possible that the layer stack has more than three Has 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 again from the mirror in a comparatively simple manner. The aluminum oxynitride layers are characterized by a high resistance to polar liquids, but are easier to remove from the mirror in comparison to 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 further intensified in this way. The optimization of the individual layer thicknesses required for this depends on the wavelength or the wavelength range and can be carried out by suitable simulation programs known per se to the person skilled in the art.

In accordance with at least one embodiment, the at least one first aluminum oxynitride layer has a greater 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 particularly preferably 0.8 <y1 / (x1 + y1) 1. For example, y1 / (x1 + y1) = 0.9.

According to 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, that is to say 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 bonding. A high degree of resistance of the layer stack to polar liquids can be achieved through a high proportion of covalent bonding in the cover layer.

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

According to 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 specified. In the method for renewing the mirror, for example in the event of damage to or contamination of the mirror, the layer stack and the adhesive layer are detached by an etching solution. The stack of layers is advantageously completely removed in this way. The layer stack is then applied again. In this way, the protection of the mirror brought about 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.

According to 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 hazardous 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. A further 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)} become. 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 adhesive layer and the silver layer are removed with a single etching solution. This can be done with an etching solution that contains 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 explained below using an exemplary embodiment in connection with 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 to one another are not to be regarded as true to scale.

The in 1 illustrated mirror 10 contains a substrate 1 . The substrate 1 can for example have a glass, a glass ceramic, a plastic, a semiconductor material or a metal. The substrate particularly preferably has 1 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.

On the substrate 1 is a layer of silver 2 arranged, which acts as a reflective layer in the mirror. It is possible that between the substrate 1 and the silver layer 2 one or more further layers are arranged, for example adhesive or polishing layers.

On the silver layer 2 is an adhesive layer 3 arranged that the adhesion of a subsequent stack of layers 4th improved. 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 is on the adhesive layer 3 , especially directly on the adhesive layer 3 , a stack of layers 4th arranged, which consists of several aluminum oxynitride layers (AlO _x N _y ) 41, 42, 43. In the example shown, there is a layer stack 4th of three aluminum oxynitride layers, but other configurations with a different number of layers are also conceivable. The number of aluminum oxynitride layers is preferably between 2 and 10.

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

The first aluminum oxynitride layers 41 , 43 have a composition AlO _x1 N _y1 , where 0.2 <y1 / (x1 + y1) ≤ 1 applies. Preferably 0.5 <y1 / (x1 + y1) 1 and particularly 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 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 described herein 10 is that the one above the silver layer 2 arranged layers, ie the adhesive layer 3 and the layer stack 4th , can be removed in a comparatively simple manner. In particular, in a method for renewing the mirror, the aluminum oxide and aluminum oxynitride layers 3 , 41 , 42 , 43 by means of an etching solution that does not contain hydrofluoric acid from the silver layer 2 removed. One to peel off the adhesive layer 3 and the stack of layers 4th suitable etching solution can contain, for example, phosphoric acid (H ₃ PO ₄ ), potassium hydroxide (KOH) or sodium hydroxide (NaOH).

In a further step or at the same time, the silver layer can optionally also be used 2 wholly or partially from the substrate 1 removed when the silver layer is to be renewed. For peeling off the silver layer 2 If necessary, another etching solution is used. 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 stack of layers 4th , the adhesive layer 3 and the silver layer 2 removed with a single etching solution. This can be done with an etching solution that contains 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. Subsequently, the adhesive layer 3 and the layer stack 4th be reapplied, for example by sputtering.

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

The invention is not restricted by the description based on the exemplary embodiments. Rather, the invention encompasses every new feature and every combination of features, which in particular includes every combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 5514476 A [0004]

Claims (14)

Mirror (10), comprising - a substrate (1) - a silver layer (2), - an adhesive layer (3) arranged on the silver layer (3), the adhesive layer (3) comprising an aluminum oxide, and - an adhesive layer ( 3) arranged layer stack (4) which has 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 , the second aluminum oxynitride layer (42) having a refractive index n ₂ <n ₁ . Mirror after Claim 1 wherein the at least one first aluminum oxynitride layer (41, 43) has a greater nitrogen content 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 applies. Mirror after Claim 3 , where 0.8 <y1 / (x1 + y1) ≤ 1 applies. 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, wherein a cover layer of the layer stack (4) is formed by a first aluminum oxynitride layer (43). Mirror according to one of the preceding claims, wherein the adhesive layer (3) is from 1 nm to 20 nm thick. Mirror according to one of the preceding claims, wherein exclusively the adhesive layer (3) and the layer stack (4) are arranged over the silver layer (2), and wherein the layer stack consists exclusively of aluminum oxynitride layers (41, 42, 43). Procedure for renewing a mirror after one of the 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 according to Claim 10 , wherein the etching solution does not contain hydrofluoric acid. Method according to one of the Claims 10 or 11 wherein the etching solution comprises phosphoric acid, potassium hydroxide or sodium hydroxide. Method according to one of the Claims 10 until 12th , further comprising peeling off and reapplying the silver layer (2). Procedure according to 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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