DE102019122451B4 - Process for producing a porous silicon oxide layer - Google Patents

Abstract

A method for producing a porous silicon oxide layer (2A), comprising the steps of A) depositing a silicon oxide layer (2), B) applying a water-insoluble and water-permeable layer (3) to the silicon oxide layer (2), and C) creating pores in the silicon oxide layer (2) by means of a heat treatment in a water bath.

Description

The invention relates to a method for producing a porous silicon oxide layer which is characterized in particular by a reduced refractive index. The porous silicon oxide layer can be provided in particular as a reflection-reducing layer or as part of a reflection-reducing layer sequence.

For the antireflection coating of surfaces, reflection-reducing interference layer systems are usually used, which contain several alternating layers of high-index and low-index materials. MgF ₂ with n=1.38 is often used as a material with a particularly low refractive index in the visible spectral range. The reflection-reducing effect of conventional dielectric layer systems could be improved if materials with a lower refractive index were available.

In the pamphlet WO 2015/110546 A1 it is proposed to produce an inorganic porous layer with a refractive index n<1.38 by oblique vapor deposition.

From the publication G. B. Alexander, W. M. Heston and R. K. Iler, "THE SOLUBILITY OF AMORPHOUS SILICA IN WATER", Journal of Physical Chemistry 58 (6), 1954, 453-455, it is known that silicon oxide can be dissolved in water, the Solubility of silicon oxide in water depends in particular on the temperature and pH of the water bath. The partial dissolution leads to pore formation in the silicon oxide. However, the dissolution process has so far been difficult to control. Treatments in water randomly remove larger amounts of material from the surface. The layer thickness changes uncontrollably and the layer becomes uneven. So far it has not been possible to set the layer properties precisely, but this is of essential importance for the technical use of the layer, in particular as an optical interference layer.

In the pamphlet DE 10 2006 001 078 A1 discloses an antireflection coating comprising a dense layer and a porous silica aerosol gel layer in this order on a substrate.

The pamphlet JP 2009 168 852 A describes an anti-reflective coating with seven layers, where the seventh layer is a porous material.

One problem to be solved is to specify an improved method for producing a porous layer, with which, in particular, a porous layer with a reduced refractive index can be produced.

This object is achieved by a method for producing a porous silicon oxide layer according to the independent patent claim. Advantageous refinements and developments of the invention are the subject matter of the dependent claims.

According to at least one embodiment of the method, a silicon oxide layer, in particular an SiO ₂ layer, is deposited in a step A). The silicon oxide layer can be deposited on a substrate, in particular on a glass substrate, or on a layer or layer sequence already deposited on a substrate. In a further step B) of the method, a water-insoluble and water-permeable layer is applied to the silicon oxide layer. In particular, the water-insoluble and water-permeable layer comprises a material other than silicon oxide which does not dissolve in a water bath but is permeable to water. In a subsequent step C) of the method, pores are produced in the silicon oxide layer by means of a temperature treatment in a water bath, and the porous silicon oxide layer is produced in this way. During the temperature treatment in the water bath, water can penetrate through the water-insoluble and water-permeable layer into the underlying silicon oxide layer and partially dissolve the silicon oxide, resulting in pores.

The method is based in particular on the finding that the process of pore formation in the silicon oxide layer can be controlled by applying the water-insoluble and water-permeable layer. In particular, the water-insoluble and water-permeable layer prevents the silicon oxide layer from quickly dissolving in water and becoming uneven. After the temperature treatment in the water bath, the silicon oxide layer is preferably at most 10% thinner than before the treatment. Furthermore, the porous silicon oxide layer preferably has an rms roughness of less than 20 nm.

The porous silicon oxide layer that can be produced using the method is characterized in particular by the fact that the refractive index is reduced in comparison to a non-porous silicon oxide layer, in particular an SiO ₂ layer, due to the pores. The porous silicon oxide layer can advantageously be produced with comparatively little production effort, in particular by a coating method without oblique vapor deposition and without an etching process. The porous silicon oxide layer can be environmentally friendly with the method, can be produced quickly and inexpensively. The porous silicon oxide layer is characterized in particular by high stability with respect to laser radiation and high transparency in the UV range. Furthermore, the pores cause only minor scattering losses, in particular if the pores have an average pore size of less than 50 nm.

According to at least one embodiment, the water bath has a temperature of between 70° C. and 100° C., preferably between 80° C. and 95° C., during the temperature treatment. It has been found that the solubility of silicon oxide in water depends in particular on the temperature and pH of the water bath. A temperature treatment in a water bath in this temperature range can efficiently cause pore formation in the silicon oxide layer. The duration of the temperature treatment is preferably less than 10 minutes, particularly preferably less than 4 minutes. The duration of the temperature treatment can be between 2 minutes and 10 minutes, for example.

According to at least one embodiment, the water bath has a pH between 7 and 10. A pH in this range is advantageous for the formation of pores in the silicon oxide layer. The pH of the water bath can be specifically adjusted using a suitable buffer, for example a sodium bicarbonate buffer.

According to at least one embodiment, the water-insoluble and water-permeable layer has TiO ₂ , Ta ₂ O ₅ , HfO ₂ or Al ₂ O ₃ . These materials do not dissolve during the temperature treatment in the water bath, so they form a partially permeable barrier for water, which ensures that the silicon oxide layer does not dissolve immediately. On the other hand, these materials can be sufficiently permeated by water that the permeated water causes pore formation in the silicon oxide layer. This is the case when the water-insoluble and water-permeable layer is a very thin layer.

In accordance with at least one embodiment, the water-insoluble and water-permeable layer has a thickness of between 0.3 nm and 3 nm. In this thickness range, so much water can penetrate the water-insoluble and water-permeable layer during the temperature treatment in the water bath that the desired pore formation takes place effectively.

In accordance with at least one embodiment, the silicon oxide layer has a density of between 2.23 g/cm ³ and 2.33 g/cm ³ before the heat treatment. It has been found that the property of the silicon oxide layer to partially dissolve in water with the formation of pores depends in particular on the density of the silicon oxide layer. Silicon oxide layers that have a density of less than 2.23 g/cm ³ after production dissolve completely in water very quickly, while silicon oxide layers that have a density of more than 2.33 g/cm ³ after production show little or no pore formation. The goal of a porous silicon oxide layer can be achieved particularly well with a density that is between 2.23 g/cm ³ and 2.33 g/cm ³ after production. The initial density of the silicon oxide layer can be influenced by the manufacturing process. Suitable manufacturing processes are, in particular, ion-assisted evaporation, electron beam evaporation, sputtering and atomic layer deposition.

In accordance with at least one embodiment, the silicon oxide layer is deposited by an ion-assisted coating method, for example by ion-assisted vapor deposition. In the ion-assisted coating process, the ion energies during the deposition of the silicon oxide layer are preferably between 80 eV and 120 eV. The ion energy can be adjusted by a suitable acceleration voltage (BIAS voltage). In particular, the density of the deposited silicon oxide layer can be influenced by the ion energy in the ion-assisted coating process. It has been found that silicon oxide layers with the preferred density between 2.23 g/cm ³ and 2.33 g/cm ³ can be produced with an ion energy in the range from 80 eV to 120 eV.

In accordance with at least one embodiment, the porous silicon oxide layer has an effective refractive index neff in the range 1.05≦neff≦1.38, preferably in the range 1.11≦neff≦1.30. The effective index of refraction is the index of refraction averaged over the layer, which due to the pores is lower than the index of refraction of the bulk material, which is approximately n=1.46. The porous silicon oxide layer acts optically as an effective medium that, due to the admixture of air with a refractive index of about n=1, can have a very low refractive index that cannot be easily achieved with conventional layers. The effective refractive index depends on the proportion of pores in the silicon oxide and can be specifically adjusted by the proportion of pores. A low effective refractive index in the range 1.05≦neff≦1.38 is advantageous if the porous silicon oxide layer is a reflection-reducing layer or part of a reflection-reducing layer system.

According to at least one embodiment, in the method above the porous silicon oxide layer produces at least one further porous silicon oxide layer by repeating steps A) to C) at least once. It is also possible for steps A) to C) to be repeated several times in order, for example, to arrange three or four or even more porous silicon oxide layers one on top of the other. In this embodiment of the method, in particular, two or more porous silicon oxide layers can be arranged one on top of the other, which differ from one another in their porosity and thus in their effective refractive indices. In a preferred variant, the additional porous silicon oxide layer has a lower effective refractive index than the underlying porous silicon oxide layer. In particular, the method can be used to produce a plurality of porous silicon oxide layers arranged one on top of the other, the effective refractive index of which decreases in steps starting from a substrate. In this way, a layer system with a refractive index gradient can be realized.

In the method, the silicon oxide layer can be applied to a substrate or to a layer or layer sequence arranged on a substrate. For example, the silicon oxide layer is applied directly to a substrate and converted into a porous silicon oxide layer in the process. In this case, the porous silicon oxide layer can function as a reflection-reducing layer for the substrate due to its comparatively low refractive index. It is also possible for a layer or a sequence of layers to be applied to the substrate first, with the porous silicon oxide layer being applied to the layer or sequence of layers, for example as a covering layer. For example, a dielectric interference layer system consisting of alternating layers with a lower and higher refractive index is applied to the substrate, followed by the porous silicon oxide layer as the top layer. In this case, the dielectric interference layer system can in particular be a reflection-reducing layer sequence, the reflection-reducing effect of which can be further improved by the porous silicon oxide layer as the cover layer. When using the porous silicon oxide layer as a reflection-reducing layer or as part of a reflection-reducing layer sequence, a broadband and essentially angle-independent recording can advantageously be achieved.

Furthermore, it is possible with the method to produce alternating layer systems from alternating silicon oxide layers of higher density and lower density. In this case, the silicon oxide layers of lower density are each produced by the temperature treatment in the water bath. The higher density silicon oxide layers can be produced by a conventional coating method without the thermal treatment according to the method described herein. An alternating layer system produced in this way can form a reflection-reducing layer sequence overall. Such an alternating layer system, which apart from the thin water-permeable and water-insoluble layers preferably has no other materials than silicon oxide, is advantageously characterized by high laser stability and high transparency in the UV spectral range.

In a variant of the method, a protective layer can be applied over the porous silicon oxide layer. The protective layer is preferably a dense layer of silicon oxide which is not treated in a water bath. Alternatively, an aluminum oxide layer, for example, can be applied as a protective layer. The protective layer is preferably less than 20 nm thick, particularly preferably less than 10 nm thick.

In the method, the substrate is preferably a glass, in particular quartz glass. The substrate can in particular be an optical element, for example a lens. By producing the porous silicon oxide layer or a layer sequence with the porous silicon oxide layer, the optical element can in particular be antireflective.

The invention is described below using exemplary embodiments in connection with the 1 until 5 explained in more detail.

• 1A bis 1C eine schematische Darstellung eines Beispiels des Verfahrens anhand von Zwischenschritten,
• 2 eine grafische Darstellung der Absorption A in Abhängigkeit von der Wellenzahl v bei einem Beispiel der Siliziumoxidschicht,
• 3 eine schematische Darstellung eines Querschnitts durch ein Beispiel eines mit dem Verfahren herstellbaren Schichtsystems,
• 4 eine schematische Darstellung eines Querschnitts durch ein weiteres Beispiel eines mit dem Verfahren herstellbaren Schichtsystems, und
• 5 eine grafische Darstellung der Reflexion R in Abhängigkeit von der Wellenlänge A des Schichtsystems bei dem Beispiel der 4.

Show it:

• 1A until 1C a schematic representation of an example of the method based on intermediate steps,
• 2 a graphical representation of the absorption A as a function of the wave number v for an example of the silicon oxide layer,
• 3 a schematic representation of a cross section through an example of a layer system that can be produced with the method,
• 4 a schematic representation of a cross section through a further example of a layer system that can be produced with the method, and
• 5 a graphical representation of the reflection R as a function of the wavelength A of the layer system in the example of FIG 4 .

Components that are the same or have the same effect are each provided with the same reference symbols in the figures. The components shown and the proportions of the components among themselves are not to be regarded as true to scale.

in a 1A In the illustrated first step A) of an example of the method, a silicon oxide layer 2 has been deposited on a substrate 1 . The thickness of the silicon oxide layer can be between 40 nm and 350 nm, for example. The substrate 1 is, for example, a glass substrate, which in particular can have quartz glass. The substrate 1 is, for example, an optical element that is to be provided with a reflection-reducing layer. In the example shown, the silicon oxide layer 2 is applied directly to the substrate 1 . Alternatively, it is possible for one or more further layers to be applied to the substrate 1 before the silicon oxide layer 2 is applied.

The silicon oxide layer 2 is preferably deposited by a vacuum deposition process such as vapor deposition, in particular ion-assisted vapor deposition, electron beam vapor deposition, sputtering or atomic layer deposition (ALD). The parameters of the coating process are preferably set in such a way that the silicon oxide layer 2 has a density of between 2.23 g/cm ³ and 2.33 g/cm ³ after the deposition. This can be achieved, for example, by electron beam evaporation at a working pressure <2*10 ^-4 mbar, a coating rate of 0.1 nm/s to 0.5 nm/s and a temperature between 25 °C and 60 °C. Alternatively, the silicon oxide layer 2 can be deposited, for example, by ion-assisted vapor deposition at a working pressure of 4.1 to 4.6*10 ^-4 mbar, a coating rate of 0.2 nm/s to 1 nm/s, a temperature of 25° C. to 60 °C and ion energies in the range from 80 eV to 120 eV.

It has been found that when an ion-assisted coating process is used to produce the silicon oxide layer 2, the ion energy in particular influences the density of the silicon oxide layer 2. The ions that are accelerated during the ion-assisted coating process in the direction of the growing layer can be, for example, oxygen ions or noble gas ions such as argon ions. By setting the ion energy in the range between 80 eV and 120 eV, it is possible to produce a silicon oxide layer with a density of between 2.23 g/cm ³ and 2.33 g/cm ³ , which is advantageous for the process. Silicon oxide layers with a density in this range have physisorbed water in already existing pores as a characteristic but not sufficient criterion. This can be seen in infrared absorption spectra.

An example is in 2 an infrared absorption spectrum of an example of the silicon oxide film is shown. The example of the silicon oxide layer shows an OH vibration band in the infrared range with a maximum at a wave number of about 3400 cm ^-1 to 3500 cm ^-1 , which indicates physisorbed water in the pores. In particular, this vibrational band is more pronounced than for layers with a higher density, which typically do not have such a vibrational band or a vibrational band with a maximum at about 3550 cm ^-1 to 3650 cm ^-1 , which is indicative of chemisorbed OH groups.

Referring to 1B a thin water-insoluble and water-permeable layer 3 is applied to the silicon oxide layer 2 in a further step of the method. The water-insoluble and water-permeable layer 3 preferably has a thickness of only 0.3 nm to 3 nm. Particularly suitable materials for the water-insoluble and water-permeable layer 3 are metal oxides such as TiO ₂ , Ta ₂ O ₅ , HfO ₂ or Al ₂ O ₃ . Like the silicon oxide layer 2, the water-insoluble and water-permeable layer 3 is formed by a vacuum deposition method such as vapor deposition, electron beam evaporation, magnetron sputtering or atomic layer deposition. The thin, water-insoluble and water-permeable layer 3 is preferably produced by vapor deposition with a low growth rate of less than 0.1 nm/s.

After the thin, water-insoluble and water-permeable layer 3 has been applied, the previously produced layer system is subjected to a thermal treatment in a hot water bath. This step is outlined in 1C shown. For example, the substrate 1 with the layer system of the silicon oxide layer 2 and the water-insoluble and water-permeable layer 3 is immersed in the hot water bath, the water temperature being between 70° C. and 100° C., preferably between 80° C. and 95° C. The pH of the water bath is preferably between 7 and 10 and can optionally be adjusted with a buffer, for example a sodium bicarbonate buffer. During the temperature treatment in the water bath, water can penetrate into the silicon oxide layer 2 and partially dissolve the silicon oxide with the formation of pores. In this way, a porous silicon oxide layer 2A is formed. The porosity of the silicon oxide layer 2A can be influenced in particular by the duration of the heat treatment. The duration of the temperature treatment is preferably about 2 minutes to 10 minutes.

The porous silicon oxide layer 2A produced in this way is particularly distinguished characterized by a low effective refractive index, for which 1.05≦neff≦1.38 and particularly preferably 1.11≦neff≦1.30 applies.

In 3 An example of a layer system that can be produced using the method is shown, in which a dielectric layer system 4 was applied to the substrate 1 before the application of the porous silicon oxide layer 2A, which can in particular be a reflection-reducing layer system. The dielectric layer system 4 has, for example, alternating first layers with a lower refractive index n ₁ , in particular n ₁ <1.6, and second layers 42 with a higher refractive index, in particular n ₂ >1.6. Apart from the thin, water-insoluble and water-permeable layer 3, which is essentially negligible from an optical point of view, the porous silicon oxide layer 2 functions in this case as the cover layer of the layer system. The reflection-reducing effect of the dielectric layer system 4 can be improved by the cover layer with a particularly low refractive index.

In a further variant of the method, a layer system is produced which alternately has silicon oxide layers produced in a conventional manner and porous silicon oxide layers. An example of a layer system that can be produced with the method is in 4 shown. The thin water-insoluble and water-permeable layers 3 used to produce the porous silicon oxide layers 2A are in 4 not shown for the sake of simplicity, although they remain in the finished layer system because they are essentially negligible from an optical point of view due to their small thickness of less than 3 nm. A first silicon oxide layer 21 is arranged on a quartz glass substrate 1 and has a thickness of, for example, 177 nm and a refractive index n=1.46 (corresponding to the bulk material). This is followed by a first porous silicon oxide layer 2A with a thickness of, for example, 71 nm and an effective refractive index neff = 1.30. Above this are a second silicon oxide layer 22 with a thickness of, for example, 10 nm and a refractive index n = 1.46, another porous silicon oxide layer 2B with a thickness of, for example, 127 nm and an effective refractive index neff = 1.15 and a third silicon oxide layer 23 with a Thickness of, for example, 5 nm and a refractive index n = 1.46 arranged. The top layer 23 of the layer system forms a protective layer and is therefore a dense silicon oxide layer.

Such an alternating layer system consisting of alternating silicon oxide layers 21, 22, 23 with a conventional density and porous silicon oxide layers 2A, 2B is particularly suitable for achieving antireflection coating over a wide range of wavelengths and angles of incidence. For clarification is in 5 the reflection R of the layer system according to 4 as a function of the wavelength A for angles of incidence of 0° and 45°. It is found that the reflection R at these angles of incidence in the wavelength range from 300 nm to 1000 nm is less than 0.5% on average.

The invention is not limited by the description based on the exemplary embodiments.

Claims (13)

A method for producing a porous silicon oxide film (2A), comprising the steps of - A) depositing a silicon oxide layer (2), - B) applying a water-insoluble and water-permeable layer (3) to the silicon oxide layer (2), and - C) Creation of pores in the silicon oxide layer (2) by means of a heat treatment in a water bath. procedure after claim 1 , whereby the water bath has a temperature between 70 °C and 100 °C during the temperature treatment. Method according to one of the preceding claims, in which the water bath has a pH value of between 7 and 10 during the temperature treatment. Method according to one of the preceding claims, in which the water-insoluble and water-permeable layer (3) comprises TiO ₂ , Ta ₂ O ₅ , HfO ₂ or Al ₂ O ₃ . Method according to one of the preceding claims, in which the water-insoluble and water-permeable layer (3) has a thickness of between 0.3 nm and 3 nm. Method according to one of the preceding claims, in which the silicon oxide layer (2) has a density of between 2.23 g/cm ³ and 2.33 g/cm ³ before the heat treatment. Method according to one of the preceding claims, in which the silicon oxide layer (2) is deposited by an ion-assisted coating method with ion energies of between 80 eV and 120 eV. Method according to one of the preceding claims, in which the porous silicon oxide layer (2A) has an effective refractive index neff in the range 1.05 ≤ neff ≤ 1.38. Method according to one of the preceding claims, wherein at least one further porous silicon oxide layer (2B) is produced over the porous silicon oxide layer (2A) by repeating steps A) to C) at least once. procedure after claim 9 , wherein the further porous silicon oxide layer (2B) has a lower effective refractive index than the porous silicon oxide layer (2A). Method according to one of the preceding claims, in which the silicon oxide layer (2) is applied to a substrate (1) or to a layer or layer sequence (4) arranged on a substrate (1). procedure after claim 11 , wherein the substrate (1) comprises a glass. procedure after claim 11 or 12 , wherein the substrate (1) is an optical element.

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