DE102015114877B4 - Scratch-resistant anti-reflective coating and mobile electronic device - Google Patents

Abstract

Disk-shaped element (1) with a disk-shaped substrate (3) which is transparent in the visible spectral range and an anti-reflective coating (5) deposited on the substrate (1), the anti-reflective coating (5) - four successive layers (51, 52, 53, 54 ), wherein adjoining layers differ in their refractive index, such that the refractive index increases and decreases alternately from layer to layer and layers (52, 54) with a lower refractive index alternate with layers (51, 53) with a higher refractive index, and wherein - the lowest (51) of the four layers (51, 52, 53, 54) is a layer with a higher refractive index and has a higher refractive index than the adjacent second lowest layer (52), - wherein the four layers (51, 52 , 53, 54) have at least three different refractive indices, such that the refractive index of the lowest layer (51) closest to the substrate (3) with a higher refractive index is lower than the refractive index the further layer (53) with a higher refractive index, wherein the refractive index of the lowest layer (51) with a higher refractive index is between 1.665 and 1.795 and wherein the lowest layer (51) with a higher refractive index is formed from an oxygen-containing material, whereby this disc-shaped element ( 1) shows in particular a slight change in the color location with fluctuations in layer thickness.

Description

The invention relates generally to optical interference coatings. In particular, the invention relates to anti-reflective or anti-reflective coatings with high scratch resistance.

Chemically toughened glasses for displays of mobile electronic devices such as smartphones or tablet PCs are known. As a rule, aluminosilicate glasses are used as cover glasses for these displays. These aluminosilicate glasses typically have a refractive index greater than 1.5 at a wavelength of 550 nm.

It is also known and customary to chemically toughen cover glasses for optical displays. Due to the chemical tempering, in which the Na ions of the glass are exchanged for potassium ions from a salt bath, the potassium accumulates in a zone of several micrometers near the surface. The potassium content is in the range up to a few percent by mass. The chemical toughening increases the flexural strength and the scratch resistance. Nevertheless, after a short period of use of a typical product, damage to the surface in the form of scratches can be seen. Chemical toughening prevents the glass from breaking easily when it is subjected to bending, as the compressive toughening prevents or delays the propagation of a crack, for example caused by a scratch - but scratches cannot be avoided.

An anti-fingerprint coating (AFP), which is either oleophobic or hydrophobic or amphiphobic, can be applied to the glass so that fingerprints do not arise or can at least be cleaned off very easily. Today's products typically have an AFP that is not permanently resistant, so that the effect of easy cleanability or reduced fingerprint conspicuity is no longer given after a few months. The AFP is generally a partially organic layer that is applied very thinly so that it is not optically active. The reflection of the product is therefore primarily given by the glass surface. At the top of the glass, about 4 percent of the light is reflected at a perpendicular incidence, which is particularly annoying in bright lighting, such as sunshine, and restricts the readability of the content of the display.

Anti-reflective coatings for anti-reflective coating of glass surfaces are known. Depending on the embodiment, they consist of several layers that are applied to one or both sides of the glass substrate. Depending on the field of application of the product, a mechanically stable, i.e. damage-resistant or scratch-resistant design of the anti-reflective system is desirable.

The pamphlet DE 10 2014 108 058 A1 discloses an anti-reflective coating with two different refractive indices and preferably with a silicon nitride-containing bottom layer and wherein a layer of chain-like organofluorine molecules is arranged on the anti-reflective coating, the molecules being bound at the end to the surface of an optical element.
The pamphlet DE 600 31 629 T2 discloses a transparent substrate which comprises on at least one side a three-layer anti-reflective coating which is made from a structure of thin layers and comprises at least one of the thin layers with a high refractive index titanium oxide which has been applied by sputtering and chemically modified by the incorporation of nitrogen in this way that its refractive index has been reduced to a value of at most 2.4 at a wavelength of 580 nm. The DE 10 2008 054 139 A1 as well as the DE 10 2012 002 927 A1 to watch.

The general goal of an optical anti-reflective coating is primarily to suppress the reflection of the visible part of the electromagnetic spectrum evenly so that there is as little color conspicuousness as possible due to uneven residual reflection. An example of such a color-neutral, scratch-resistant anti-reflective coating is in US 2012/0212826 A1 specified.

In the course of the cell phone cover manufacturing process, said glasses can be provided with a decorative print on the side facing away from the user, the so-called back of the cover, which is typically black or white. Often the company name of the manufacturer or the product name is printed in silver-colored lettering.

Antireflection systems usually consist of more than one layer, especially when anti-reflective coating is to be applied evenly over a large part of the visual spectrum and not just at a certain wavelength. Two consecutive layers have different indices of refraction. It layers with high and low refractive index alternate. The wavelength-dependent reflection curve is the result of the interaction of the individual reflections at the interfaces. A uniform, color-neutral reflection is the result of an exactly coordinated sequence of optical layer thicknesses, ie the product of the refractive index and layer thickness. The change of one or more layer thicknesses changes the reflection conditions and thus also the reflection result, in particular also the color location of the reflection.

Process-related fluctuations in layer thickness, such as those that occur, for example, at different positions within a sample holder (carrier) or with fluctuations from production lot to production lot, can therefore lead to a strong change in the color locus. Depending on the strength of the layer thickness fluctuations, there are very noticeable variations in the color location, which reduce the production yield.

The invention is therefore based on the object of providing an anti-reflective system that has good adhesion to the glass substrate, has high scratch resistance, represents a good basis for an anti-fingerprint coating and, in particular with typical process-related fluctuations in layer thickness, has only a slight change in the color location. The invention specifically relates to a transparent element having a four-layer anti-reflective coating. According to the above-mentioned object, this anti-reflective coating is designed in such a way that the color location of reflected light has a reduced dependence on fluctuations in layer thickness.

The anti-reflective interference layer system according to the invention comprises a sequence of alternating high and low index layers. Progressing from layer to layer, the refractive index is therefore alternately higher and lower. The layer system comprises at least four layers. A high level of scratch resistance is achieved by using a transparent hard material for at least one high-index layer.

It turns out that the layer thickness-dependent color locus fluctuation can be achieved with a four-layer anti-reflective coating by using more than two materials. The first high-index layer applied to the substrate should have a refractive index n _m which is between the refractive index of the further high-index layer n _h and the refractive index of the low-index layer n _n .

• - einem im sichtbaren Spektralbereich transparenten scheibenförmigen Substrat und
• - einer auf dem Substrat abgeschiedenen Antireflexbeschichtung vor, wobei die Antireflexbeschichtung
• - vier aufeinanderfolgende Lagen umfasst, bei welcher aneinander angrenzende Lagen sich jeweils in ihrem Brechungsindex unterscheiden, derart, dass der Brechungsindex von Lage zu Lage abwechselnd ansteigt und abnimmt und sich Lagen mit niedrigerem Brechungsindex mit Lagen mit höherem Brechungsindex abwechseln.

Accordingly, the invention provides a disk-shaped element

• a disk-shaped substrate that is transparent in the visible spectral range and
• an anti-reflective coating deposited on the substrate, the anti-reflective coating
• - comprises four successive layers in which adjoining layers differ in their refractive index in such a way that the refractive index increases and decreases alternately from layer to layer and layers with a lower refractive index alternate with layers with a higher refractive index.

The lowest of the four layers is a layer with a higher refractive index and consequently has a higher refractive index than the neighboring second lowest layer.

The four layers now have at least three different refractive indices, such that the refractive index of the lowest layer with a higher refractive index closest to the substrate is lower than the refractive index of the further layer with a higher refractive index, the refractive index of the lowest layer with a higher refractive index between 1.665 and 1.795. The lowest layer with a higher refractive index is formed from an oxygen-containing material. As a result of these features, the disc-shaped element shows, in particular, a slight change in the color locus with fluctuations in layer thickness.

The lowest layer of the anti-reflective coating closest to the substrate is preferably an oxynitride layer or an oxide layer. Oxynitride layers with silicon or aluminum or with a mixture of both materials are particularly suitable because these materials are very transparent and at the same time hard, whereby the refractive index can be set and adapted well via the oxygen content during the coating process. In general, several components can also be used for the composition of the layer. According to one embodiment of the invention, it is provided that the bottom layer contains at least three different components in oxynitridic or oxidic form, which contribute more than 1 atomic percent to the total composition.

The use of oxynitrides facilitates the production of a multi-layer anti-reflective layer system in that silicon oxynitride or aluminum nitride, aluminum oxynitride or a nitride or oxynitride made from a mixture of aluminum and silicon can also be used for the further hard material layer with a higher refractive index. Since their refractive index should be higher, the oxygen content is correspondingly lower. According to one embodiment of the invention, it is therefore provided that both layers with a higher refractive index are silicon oxynitride layers or aluminum oxynitride layers or a mixture of silicon and aluminum oxynitride, the lowest layer with a higher refractive index having a higher oxygen content than the further layer with a higher refractive index.

If a purely oxidic layer system is to be produced via a sputtering process, only oxygen can be used as reactive gas. A mixture of the reactive gas is eliminated. Nevertheless, it is possible to produce an anti-reflective layer system according to the invention by using an alloy target which consists of at least two materials in such a way that the oxide of one material is high-index and the oxide of the other material is low-index. By cleverly choosing the composition of the alloy target, a mixed oxide can now be produced via the sputtering process in an oxygen-containing process atmosphere, which meets the above-mentioned requirements with regard to a higher refractive index, but which is lower than the refractive index of the further high-refraction layer. A target made of silicon with an admixture of zirconium or a target made of aluminum with an admixture of zirconium may be mentioned here as an example without limitation to further possibilities.

As already said above, more than two components can be involved in the construction of the lowest layer. In particular, at least three different components can be contained in oxynitridic or oxidic form, which contribute more than 1 atomic percent to the total composition. For example, in addition to Si and Al, another component with one or more atomic percent can be present and incorporated in oxidic or oxynitridic form.

The sequence of the four layers of the antireflective coating is particularly preferably applied directly to the substrate. Accordingly, the substrate surface directly adjoins the sequence of the four layers of the anti-reflective coating.

According to a particularly preferred development of the invention, an organofluorine layer can be applied to the antireflective coating. In particular, such a layer can also be designed as a monolayer of organofluorine molecules, preferably with a layer thickness of 1 nm to 20 nm, particularly preferably with a layer thickness of 1 nm to 10 nm. The organofluorine layer can be, for example, an oleophobic coating

It has been shown that a layer system in which an additional fluorinated organic layer is added to an anti-reflective coating with a thick, upper hard material layer, in particular with the greatest layer thickness of the anti-reflective coating, is not only effective in reducing abnormalities in fingerprints or in making it easy to clean but that the chemically toughened glass substrate coated in this way, and thus the element according to the invention, in particular supports the avoidance of scratches.

This is attributed to the fact that the additional fluorine-organic layer presumably reduces the coefficient of friction of the surface in such a way that damage to the surface is less.

In order to deposit the anti-reflective coating according to the invention, sputtering, in particular reactive sputtering, is preferably used.

The invention is explained in more detail below with reference to the figures and with reference to exemplary embodiments. In the figures, the same reference symbols denote the same or similar layers.

• 1 ein Substrat mit einer vierlagigen Antireflexbeschichtung,
• 2 eine Variante der in 1 gezeigten Ausführungsform,
• 3 eine Weiterbildung der Erfindung mit einer lichtabsorbierenden Rückseitenbeschichtung,
• 4 Farbortänderungen an verschiedenen Elementen abhängig vom Aufbau der Antireflexbeschichtung und dem Vorhandensein oder Fehlen einer lichtabsorbierenden Rückseitenbeschichtung,
• 5 Farbortänderungen an Antireflexschichtsystemen mit SiO₂/TiO₂-Lagen.

Show it:

• 1 a substrate with a four-layer anti-reflective coating,
• 2 a variant of the in 1 embodiment shown,
• 3 a further development of the invention with a light-absorbing back coating,
• 4th Changes in color location on various elements depending on the structure of the anti-reflective coating and the presence or absence of a light-absorbing rear-side coating,
• 5 Changes in color location on anti-reflective coating systems with SiO ₂ / TiO ₂ layers.

1 shows an example of a disk-shaped element according to the invention. The element 1 includes generally and without limitation to the specific example shown, a substrate that is transparent in the visible spectral range 3 and one on the substrate 1 on a side face 30th deposited anti-reflective coating 5 .

The anti-reflective coating 5 comprises four consecutive layers 51 , 52 , 53 , 54 . Neighboring layers differ in their refractive index. In particular, the refractive index changes from position to position so that it alternately increases and decreases. Going through the layers from top to bottom to the substrate 3 through, so the refractive index increases from the top layer 54 to the second from the top 53 on, sinks to the next layer 52 and rises to the lowest layer 51 again, being below the level of the refractive index of the location 53 remains.

The lowest of the four layers 51 , 52 , 53 , 54 (i.e. location 51 ) is a layer with a higher refractive index and consequently has a higher refractive index than the adjacent second lowest layer 52 .

Without being limited to the specific embodiment of FIG 1 is in any case such a sequence of four layers in an anti-reflective coating according to the invention 51 , 52 , 53 , 54 available. The coating preferably contains 5 only these four layers are optically effective layers. However, it is not ruled out that further layers follow this sequence above and / or below, or on the substrate side.

The four layers 51 , 52 , 53 , 54 have at least three different refractive indices, such that the refractive index of the lowest, the substrate 3 next location 51 with a higher refractive index is lower than the refractive index of the further layer 53 with a higher refractive index. The lowest layer is 51 with a higher refractive index, preferably formed from an oxygen-containing material.

The terms “lower refractive index” and “higher refractive index” apply relative to the adjacent layers. A layer with a lower refractive index consequently adjoins a layer (if the low-refractive-index layer completes the layer system) or two layers (if the low-refractive-index layer adjoins other layers of the layer system at both interfaces) with a higher refractive index.

The substrate 3 is particularly preferably an inorganic, oxidic substrate, in particular a glass substrate. The anti-reflective layer system according to the invention is particularly suitable for chemically toughened glass substrates.

It turns out that a good connection of the first hard material layer to the substrate can be achieved if it is an oxidic hard material layer (for example ZrO ₂ or a Zr-Si mixed oxide, ie a mixture of ZrO ₂ and SiO ₂ or a mixed oxide with ZrO ₂ and Al ₂ O ₃ ) or if the first hard material layer is an oxygen-containing nitride in the form of an oxynitride. Suitable oxynitrides are, for example, silicon oxynitride (SiO _x N _y ) or aluminum oxynitride (AlO _x N _y ) or silicon-aluminum oxynitride (Si AlO _x N _y ).

These mentioned materials, ZrO ₂ or a mixture of ZrO ₂ and SiO ₂ or Al ₂ O ₃ ) and oxynitrides of silicon or aluminum or a mixture of aluminum and silicon are particularly preferred for the lowermost layer with a higher refractive index. ZrO ₂ can, for example, be combined with a TiO2 layer as a further or upper layer with a higher refractive index. Another possibility is to use an oxide containing titanium, for example a mixed oxide with titanium and a further element, for the lower layer with a higher refractive index. One possibility is a titanium / silicon mixed oxide. A preferred variant of an anti-reflective system according to the invention provides for the use of a ZrO ₂ layer as the top layer with a higher refractive index in combination with a mixed oxide of zirconium and silicon or zirconium and aluminum or zirconium and silicon and aluminum as the lower layer 51 with a higher refractive index.

According to a preferred embodiment of the invention, the lowest one is the substrate 3 next layer 51 the anti-reflective coating 5 an oxynitride or oxide layer. This layer is in particular made of oxide or oxynitride at least one of the elements silicon or aluminum or from their Mixture formed. An oxide layer as the lower layer 51 with a higher refractive index can be formed from an oxide of titanium and / or zirconium in a mixture with aluminum and / or silicon. A particularly good adhesion is generally also achieved on chemically toughened glass substrates.

In any case, the two layers are 51 , 53 Although layers with a higher refractive index differ in that the refractive index of the two layers, as explained above, is different.

Both layers can be used 51 , 53 contain exactly the same ingredients, but in different compositions. If the constituents are at least partially the same, the production can be made easier because both layers can in principle be deposited using the same production process, with only the process parameters being adapted.

A preferred embodiment of the invention is based on the fact that both layers 51 , 53 with higher refractive index are layers containing silicon, nitrogen and oxygen, with the lowest layer 51 with a higher refractive index has a higher oxygen content than the further layer 53 with a higher refractive index.

In the case of an oxynitride layer, it can be seen that sufficient adhesion of the layer can be achieved even with a low oxygen content of about 5-10 atomic percent compared to nitrogen. In other words, according to this embodiment of the invention, the content of oxygen to nitrogen is in the lowest layer 51 at least 5 atomic percent, but below 10 atomic percent. According to a further embodiment, the ratio of the contents of oxygen to nitrogen of the oxynitride (the contents indicated in atomic percent), preferably of a silicon oxynitride, is in the range from 0.41 to 1.02.

In general, without being restricted to the exemplary embodiments, all silicon-containing layers can contain not only silicon but also aluminum. Preferably it is in the top three layers, i.e. the layers 52 , 52 , 54 the proportion of aluminum is less than the proportion of silicon, in each case given in atomic percent. The ratio of the amount of substance of aluminum to silicon is preferably greater than 0.05, preferably greater than 0.08, but the amount of substance of silicon outweighs the amount of substance of aluminum. The ratio of the amounts of aluminum to silicon is preferably about 0.075 to 0.5, particularly preferably about 0.1.

For the lowest layer 51 with a higher refractive index, the ratio of the amounts of substance given above can also be used, but any ratio of the amounts of substance of Al and Si is also suitable.

The scratch resistance of the anti-reflective coating can be further improved by applying an anti-reflective coating according to the invention 5 an additional organofluorine layer 6th is upset. This organofluorine layer is also effective in reducing abnormal fingerprints or making it easy to clean.

The increased scratch resistance is attributed to the fact that the additional organofluorine layer presumably reduces the coefficient of friction of the surface so that there is less damage to the surface.

2 shows a variant of the in 1 embodiment shown. Either 1 , as well as 2 are examples of an embodiment of the invention in which the top layer with a higher refractive index is the second top layer 53 the four-layer anti-reflective coating 5 forms, has a greater layer thickness than the layers 51 and 52 .

In the embodiment of 2 but is the layer thickness of the second top layer 53 again significantly larger than the in 1 shown example. In general, layer designs can be found in which one of the higher refractive index layers is very thick and which nevertheless have good anti-reflective properties. The great layer thickness of one of the hard, higher refractive index layers, preferably the second from the top 53 leads to a high scratch resistance of the layer system. However, it turns out to be beneficial if the layer thickness of the second layer from the top 53 no more than half of the total layer thickness of all four layers 51 , 52 , 53 , 54 amounts. According to another in the example of 2 also realized embodiment shows the situation 52 , i.e. the layer with a lower refractive index, which is between the two layers 51 , 53 with a higher refractive index is the smallest layer thickness of the four layers. It turns out that this layer should actually preferably be selected to be as thin as possible in order to achieve a comparatively minimal fluctuation in the color location when the layer thickness varies.

As layers 52 , 54 SiO ₂ layers with a lower refractive index can generally be used, without being restricted to the specific exemplary embodiments. In other words, with a four-layer anti-reflective coating 5 then the top layer 54 and the second from the bottom 52 formed from SiO ₂ . Due to its relatively low refractive index, silicon oxide is particularly suitable for obtaining high refractive index jumps at the interfaces of the anti-reflective coating. These silicon oxide layers can preferably contain a small proportion of Al ₂ O ₃ , the ratio of the contents in atomic percent of aluminum to silicon in the range from 0.05 to 0.3, preferably in the range 0.1. Such a small addition of Al ₂ O ₃ to SiO ₂ only leads to a very small increase in the refractive index.

In the embodiment of 2 Yet another embodiment of the invention is implemented. According to this embodiment, the layer thickness is the lowest layer 51 greater than the layer thickness of the second lowest layer 52 . This general relationship between the layer thicknesses, in conjunction with the design of the refractive indices, also leads to an improved stability of the antireflection effect in the case of production-related fluctuations in layer thickness. This embodiment is independent of the specific in 2 layer thicknesses shown.

3 shows a further development of the invention. According to this embodiment is generally on the side 31 of the substrate 3 which side 30th with the anti-reflective coating 5 opposite, a light-absorbing coating 7th , preferably a black coating applied. This decorative coating is preferably used as an opaque decoration and typically frames a display area 32 one, within which a display, for example an LCD or OLED display, is visible to a viewer. The opaque decoration hides components, such as an edge attachment of the substrate 3 or connections on the edge of the display.

A cover glass with an antireflective coating known from the prior art with the layer sequence glass / Si ₃ N ₄ / SiO ₂ / Si ₃ N ₄ / SiO ₂ serves as an example as a starting point to illustrate the invention. Accordingly, on a sheet of glass as a substrate 3 first a silicon nitride layer as a layer 51 , then as a layer 52 a SiO ₂ layer, then another silicon nitride layer 53 and thereon SiO ₂ layer as the top layer 54 the anti-reflective coating 5 deposited. As in 1 shown can be on the top layer 54 the anti-reflective coating 5 an organofluorine layer can also be applied.

4th shows changes in color location on various elements 1 depending on the structure of the anti-reflective coating 5 and the presence or absence of a light-absorbing or light-blocking coating 7th on the website 31 of the substrate 3 . The measured values are individually designated with reference symbols in brackets.

An optimal case of uniform reflection is, for example, when for the substrate 3 an aluminosilicate glass is used and the layer thicknesses from the glass are as follows: Location (51) Si ₃ N ₄ 15 nm Location (52) SiO ₂ 32 nm Location (53) Si ₃ N ₄ 132 nm Location (54) SiO ₂ 87 nm

In this case, a color location according to CIE-Lab can be applied to the pure glass without a decoration or without a light-absorbing coating 7th determine with the coordinates a and b of 1.0 and 3.7 (measured value 40 ).

If one deviates by + 1nm in the layer thickness (16nm (position 51 ), 33nm (location 52 ), 133nm (position 53 ), 88nm (location 54 )), the chromaticity coordinates 1.5 and 2.8 result (measured value 41 ). If the layer thickness is reduced by 1 nm (14nm, 31nm, 131nm, 86nm), a and b result in 0.1 and 4.7. The three measurements discussed above 40 , 41 , 42 are in 4th drawn in with triangular symbols.

With a black decor, or in the area one like in 3 drawn light-absorbing coating 7th on the back of the cover glass, the chromaticity values fluctuate significantly more with the same coating. The corresponding measured values 43 , 44 , 45 are marked with diamond symbols. Stronger color locus fluctuations also occur when the display is switched off and then dark, and also in dark areas of the display when it is switched on, for example with a dark screen background.

• a = -0,2, b = 2,1 (Messwert 43); bei +1 nm Schichtdicke:
  • a = 1,7 und b = -0,7 (Messwert 44); bei -1 nm Schichtdicke a= -2,6 und b = 4,6 (Messwert 45).

The color values of the optimal case are here:

• a = -0.2, b = 2.1 (measured value 43 ); at +1 nm layer thickness:
  • a = 1.7 and b = -0.7 (measured value 44 ); at -1 nm layer thickness a = -2.6 and b = 4.6 (measured value 45 ).

This greater fluctuation and dependence on the layer thickness can be attributed to the rear-side reflex that is masked out by the decor. In the case of pure glass, its rear side reflects about 4% (evenly over the wavelength range of visible light) and thus contributes about ten times as much to the total reflection as the anti-reflective front side (about 0.5%). The color location variations caused by the fluctuations in layer thickness are "outshone" by the back and are less noticeable. In the case of a black backside decoration, the backside reflex is almost suppressed. Thus the color point is almost exclusively due to the anti-reflective coating 5 Determined and caused by fluctuations in the layer thickness, changes in color location are more significant.

The readings 60 to 62 (Cross symbols) and 63 to 65 (square symbols) are color locus changes calculated for comparison on antireflective coatings according to the invention 5 , which have the same layer thickness fluctuations.

The first layer of the anti-reflective coatings of the invention 5 is an oxygen-containing silicon nitride layer. The ratio of oxygen to nitrogen, O / N, is set in such a way that the result is a refractive index at 550 nm of approximately 1.71. With higher refractive indices in the direction of Si ₃ N ₄ , the color locus fluctuations increase. If there is too much oxygen in the SiO _x N _y , there are no longer any good antireflective conditions for a four-layer antireflective coating. Without limitation to the composition of the lowest layer 51 is the refractive index of the lowest layer 51 with a higher refractive index between 1.665 and 1.795. The refractive index is preferably less than 1.790, particularly preferably less than 1.785.

The readings 60 - 62 concern a glass substrate without decoration and are therefore with the measured values 40 , 41 , 42 to compare. It can be seen that the color locations in a layer system according to the invention are significantly closer together with the same layer thickness fluctuations than in an anti-reflective coating with similar higher refractive index layers. Even with an anti-reflective coating according to the invention 5 with different higher refractive index layers in terms of material and refractive index 52 , 53 increases in the area of a light-absorbing coating 7th the color locus fluctuation (measured values 63 - 65 ). Here, too, the color locus fluctuations are much smaller than in the case of an anti-reflective coating with higher refractive index layers of the same type in terms of material and refractive index 52 , 53 .

A specific example of an antireflection system according to the invention with AR systems built up compared to the prior art with lower color locus fluctuations with layer thickness fluctuations is listed in the following table: Location (51) SiO _X N _Y (n = 1.77) 33 nm Location (52) SiO ₂ 24 nm Location (53) Si ₃ N ₄ 132 nm Location (54) SiO ₂ 87 nm

5 shows color locations of the light reflections from substrates with TiO ₂ / SiO ₂ -based anti-reflective coatings. The color locations were again for one with a rear light-absorbing coating 7th provided disc-shaped element 1 calculated.

These anti-reflective coatings too 5 each have four layers. For the locations 51 , 53 with a higher refractive index, the in 5 entered color locus values 46 , 47 , 48 Titanium dioxide layers (TiO ₂ ) used. The chromaticity coordinates 66 , 67 , 68 belong to an anti-reflective coating according to the invention 5 , in which the four layers 51 , 52 , 53 , 54 of the coating have at least three different refractive indices, such that the refractive index of the lowest, the substrate 3 next location 51 with a higher refractive index is lower than the refractive index of the further layer 53 with a higher refractive index, with the lowest layer 51 is formed with a higher refractive index from an oxygen-containing material.

In the present case, for the lowest layer 51 a SiO _x N _y , that is to say an oxygen-containing silicon nitride, in which the refractive index is approximately 1.77. In contrast, pure silicon nitride has a refractive index of around n = 2. The refractive index of the layer is 1.77 51 between the values of SiO ₂ (ie the refractive index of the layers 52 , 54 ), and TiO ₂ (i.e. the refractive index of the layer 53 ).

A possible example for the construction of an AR system according to the invention from the aforementioned materials SiO _x N _y , SiO ₂ and TiO ₂ is given in the following table: Location (51) SiO _X N _Y (n = 1,733) 61 nm Location (52) SiO ₂ 10 nm Location (53) TiO ₂ 102 nm Location (54) SiO ₂ 89 nm

The picture is similar to that of the examples in 4th . When using only two different refractive indices in the 4-layer AR system (color locus values 46 , 47 , 48 ), variations in the layer thicknesses of the layers by ± 1 nm lead to a greater fluctuation in the color locus than when using three refractive indices (color locus values 66 , 67 , 68 ), if, as described above, they run in the order of n _medium , n _low , n _high , n _low as seen from the substrate. The in 2 In the example shown, the proportions of the layer thicknesses correspond to the values in the table above.

Good anti-reflective properties can also be achieved if the layer thicknesses given above differ by 10%. According to one embodiment of the invention, the layer thicknesses for the bottom layer are therefore 51 61 ± 6.1 nm, for the second lowest position 52 10 ± 1 nm, for the upper, higher index layer 53 102 ± 10.2 nm and for the top layer 54 89 ± 8.9 nm.

These examples make it clear that it is beneficial if there is a certain minimum difference in the refractive indices of the two layers 51 , 53 with a higher refractive index is present. In general, without limitation to the examples described with reference to the figures, a further development of the invention provides that the refractive index of the lowest layer 51 of the four layers 51 , 52 , 53 , 54 the anti-reflective coating 5 less than 1.79 is to be selected and at the same time to be set in such a way that the adjacent position 52 has a layer thickness between 1 and 20 nm, preferably between 5 and 15 nm. A thin layer of the layer 52 proves to be particularly effective in reducing the sensitivity of the AR system to color locus fluctuations when the layer thicknesses vary. According to a further development of the invention, it is therefore provided that the layer thickness of the layer 52 with a lower refractive index, which is between the two layers 51 , 53 with a higher refractive index, or the second lowest layer is below 20 nm, preferably below 18 nm, particularly preferably below 15 nm.

This difference is limited towards the bottom by the refractive index of the layers with the lower refractive index. Depending on the materials used in the structure of the anti-reflective system, here are for the bottom layer 51 Refractive indices from about 1.66 are preferred.

The refractive index of the layer 51 is therefore preferably in the range between 1.66 and 1.79.

6th shows a typical application for the invention. According to one embodiment of the invention, without being limited to the example shown, there is a mobile electronic device 20th with an electronic display 21st , preferably a matrix display or a pixel-controlled display is provided, the electronic device 20th a disk-shaped element according to the invention 1 has which the electronic display 21st covers, with the side 30th of the disc-shaped element 1 with the anti-reflective coating 5 is turned outwards. This way the anti-reflective coating can 5 exercise their effect in the reduction of disruptive light reflections on the one hand and protection against scratching by external mechanical influences on the other.

The example shown is a mobile device in the form of a tablet PC. The invention is equally suitable for mobile telephones, in particular so-called smart phones, mobile navigation devices and mobile media playback devices, such as in particular music and video playback devices, and for smartwatches.

Particular mechanical loads occur with such devices in particular when the devices are equipped with a user interface with a touch-sensitive screen, so that the side 30th with the anti-reflective coating 5 of the element according to the invention 1 at the same time also represents a user interface of the interface. Particularly in this embodiment of the invention, there is also a further coating in the form of the one mentioned above and in the example of 1 organic fluorine layer shown 6th .

Often, the cover glass of the electronic display should not be completely transparent, but, as also based on the 3 explained, certain areas are covered. This is done using a light-absorbing coating 7th provided which the edge areas of the element 1 covered and the electronic display 21st framed. Especially in this in 6th Black-drawn edge areas provided with a light-blocking coating are particularly clearly visible, as can be seen from the examples in FIG 4th and 5 was shown. The light reflections are due to the anti-reflective coating 5 in itself only very weak. However, this is countered by the fact that the dark background ensures a high contrast due to the light-absorbing coating, so that even very weak light reflections are noticeable. With the design of the anti-reflective coating according to the invention 5 it is now ensured that the remaining visible light reflections appear uniformly colored.

It is evident to the person skilled in the art that the invention is not restricted to the exemplary embodiments as shown in the figures, but rather can be varied in many ways within the scope of the subject matter of the patent claims. In particular, the features of the individual exemplary embodiments can also be combined with one another. So can for a mobile electronic device 20th both a coating according to 1 , as well as with the other features of the example of the 2 be used. An organic fluorine coating 6th may be in any of the illustrated embodiments. This option is also expressly preferred.

Claims (16)

Disc-shaped element (1) with - A disk-shaped substrate (3) and transparent in the visible spectral range - An anti-reflective coating (5) deposited on the substrate (1), the anti-reflective coating (5) - comprises four successive layers (51, 52, 53, 54), adjacent layers each differing in their refractive index, such that the refractive index alternately increases and decreases from layer to layer and layers (52, 54) with a lower refractive index alternate with layers (51, 53) with a higher refractive index, and where - The lowest (51) of the four layers (51, 52, 53, 54) is a layer with a higher refractive index and has a higher refractive index than the adjacent second lowest layer (52), - The four layers (51, 52, 53, 54) having at least three different refractive indices, such that the refractive index of the lowest layer (51) closest to the substrate (3) is lower with a higher refractive index than the refractive index of the further layer (53) with a higher refractive index, the refractive index of the lowermost layer (51) with a higher refractive index being between 1.665 and 1.795 and the lowermost layer (51) with a higher refractive index being formed from an oxygen-containing material, whereby this disc-shaped element (1) in particular shows a slight change in color location with fluctuation in layer thickness Disc-shaped element (1) according to the preceding claim, characterized in that the substrate (3) is a glass substrate, in particular a chemically toughened glass substrate. Disc-shaped element (1) according to one of the two preceding claims, characterized in that the lowest layer (51) of the anti-reflective coating (5) closest to the substrate (3) is an oxynitride layer or an oxide layer. Disc-shaped element (1) according to the preceding claim, characterized in that both layers (51, 53) with a higher refractive index are silicon oxynitride layers, the lowest layer (51) with a higher refractive index having a higher oxygen content than the further layer (53) with a higher refractive index. Disc-shaped element (1) according to one of the two preceding claims, characterized in that oxygen in the lowermost layer (51) is contained in the range of 5 to 10 atomic percent, or that the ratio of the contents in atomic percent of oxygen to nitrogen of the oxynitride in atomic percent ranges from 0.41 to 1.02. Disc-shaped element (1) according to one of the preceding claims, characterized in that the top layer with a higher refractive index, which forms the second top layer of the four-layer anti-reflective coating (5), is the layer with the greatest layer thickness among the four layers (51, 52, 53 , 54) is. Disc-shaped element (1) according to one of the preceding claims, characterized in that an organofluorine layer 6 is applied to the anti-reflective coating (5). Disc-shaped element according to one of the preceding claims, characterized in that a light-absorbing coating (7) is applied to the side (31) of the substrate (3) which is opposite the side (30) with the anti-reflective coating (5). Disc-shaped element (1) according to one of the preceding claims, characterized in that the layers (52, 54) with a lower refractive index are SiO ₂ layers. Disc-shaped element (1) according to one of the preceding claims, characterized in that the layers (52, 54) with a lower refractive index are SiO ₂ layers which additionally contain Al ₂ O _{3 in} such a way that the ratio of the Al / Si contents in Atomic percent is in the range from 0.05 to 0.3, preferably about 0.1. Disc-shaped element (1) according to one of the preceding claims, characterized in that the layer thickness of the second lowest layer (52) is below 20 nm, preferably below 18 nm, particularly preferably below 15 nm. Disc-shaped element (1) according to one of the preceding claims, characterized in that the layer thickness of the lowest layer (51) is greater than the layer thickness of the second lowest layer (52). Disc-shaped element (1) according to one of the preceding claims, characterized in that the anti-reflective coating (5) has the following layer thicknesses: 61 ± 6.1 nm for the bottom layer (51) of the four successive layers, 10 ± 1 nm for the second bottom Position (52) of the four successive layers, 102 ± 10.2 nm for the upper, higher refractive index layer (53) of the four successive layers and 89 ± 8.9 nm for the top layer (54) of the four successive layers. Disc-shaped element according to one of the preceding claims, characterized in that the substrate surface directly adjoins the sequence of the four layers (51, 52, 53, 54) of the anti-reflective coating (5). Mobile electronic device (20) with an electronic display (21), wherein the electronic device (20) comprises a disc-shaped element (1) according to one of the preceding claims, which covers the electronic display (21), wherein the side (30) of the disc-shaped element (1) with the anti-reflective coating (5) facing outwards. Mobile electronic device (20) according to the preceding claim, characterized by a user interface with a touch-sensitive screen, wherein the side (30) of the element (1) with the anti-reflective coating (5) represents a user interface of the interface.

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