Coating with photo-induced hydrophilicity

0001The present invention relates to a coating of a layer system for anti-reflection of particular transparent substrates, which also acts self-cleaning. Also for heat protection, this coating is suitable.

0002Recently, self-cleaning surfaces based on hydrophilic, water-attracting coatings have gained in importance. For self-cleaning surfaces titanium dioxide is usually TiO₂ used, which has a photocatalytic activity. The photocatalytic activity means that upon irradiation with light of suitable wavelength, a surface of titanium dioxide becomes hydrophilic, that is attracting water. This results in a self-cleaning effect under the influence of water. This effect of the photoinduced hydrophilic effect has already been described in Nature "Light-induced amphilic surfaces" Vol. 388 (1997), page 431. Upon irradiation of light having a photon energy larger than the band gap of titanium dioxide, the surface thereof becomes water-attracting to form a smooth water film. This effect can be exploited to create self-cleaning surfaces. The band gap of titanium dioxide is approx. 3.4 eV corresponding to a wavelength of 340 nm, so that the self-cleaning effect can usually be induced by irradiation with UV light.

0003Such self-cleaning surfaces can be realized by the application of thin titanium dioxide layers on the desired substrates. The application can be carried out by known coating methods. Examples of these are vacuum coating methods, such as reactive or non-reactive magnetron sputtering, thermal or electron beam-assisted vapor deposition, or plasma-assisted vapor deposition.

0004To achieve a hydrophilic surface, titanium dioxide must be applied as the uppermost layer, since otherwise direct contact with water and thus the self-cleaning effect under the influence of water can not be guaranteed.

0005A disadvantage, however, is that titanium dioxide has a high refractive index, whereby a high reflection is effected. For example, when transparent substrates such as glass surfaces coated with such a titanium dioxide layer, the resulting coated glass substrate has a much higher reflection than the pure glass surface. Due to this high reflectivity, water-repellent coatings based on titanium dioxide are not suitable for many applications, in particular for applications on transparent substrates such as glass or plastic as used for spectacles or architectural glass.

0006Traditional antireflective systems are based on applying a material with a smaller refractive index to a material with a higher refractive index, with a layer thickness at which interference effects are minimal. Thus, a glass surface with a refractive index of n = 1.5 can be antireflected with a layer with a refractive index of 1.22 (see, for example, McLeod, "Thin-film optical filters" Institute of Physics Publishing, London 2001).

0007In the general case, a material having a refractive index n is passed through a layer having a refractive index n_layer with n_layer = √n and a thickness d = λ₀/ 4n_layer with λ₀ = Central wavelength of the antireflection coating, antireflected against air (n = 1). Naturally, this is the anti-reflection to the central wavelength λ₀ limited and therefore only narrowband.

0008For practical applications, however, broadband antireflective systems are desirable.

0009According to the above equations, a broadening of the bandwidth of the antireflection coating could be achieved by materials with the smallest possible refractive index. The classical coating methods, however, do not allow the suitable production of thin layers having a refractive index of less than about 1.46 (silica, SiO 2)₂) or 1.35 (magnesium difluoride, MgF₂). Materials with a smaller refractive index are technically difficult to achieve and if at all with only insufficient layer properties.

0010In order to achieve a broadband antireflection coating, a multilayer system is usually used. A classic glass broadband antireflective system is a system according to Equation 1:<maths id="math0001" num=""><math display="block"><mrow><mtext>Equation 1:</mtext><mspace linebreak="newline" /><msub><mrow><mtext>Glass - TiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> (11nm) - SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> (40nm) - TiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> (110nm) - SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mtext> (85nm) - air.</mtext></mrow></math><img file="EP1291331A2_D0001.tif" /></maths>

0011With such a multilayer anti-reflection system, a reflection of less than 0.5% in the visible spectral range can be achieved.

0012Since glass typically has a refractive index in a range of 1.4 to 1.6 (depending on the type of glass), titania with a relatively high refractive index n = 2.4 to 2.7 at 550 nm, depending on the manufacturing conditions, as an anti-reflection layer is unsuitable for materials such as glass, since, as stated above, to achieve an antireflection coating, the refractive index of the antireflection coating must be less than the refractive index of the substrate to be antireflected. According to<maths id="math0002" num=""><math display="block"><mrow><mtext>R = </mtext><mfrac><mrow><msub><mrow><mtext>(n</mtext></mrow><mrow><mtext>TiO2</mtext></mrow></msub><msup><mrow><mtext>-1)</mtext></mrow><mrow><mtext>2</mtext></mrow></msup></mrow><mrow><msub><mrow><mtext>(n</mtext></mrow><mrow><mtext>TiO2</mtext></mrow></msub><msup><mrow><mtext>+1)</mtext></mrow><mrow><mtext>2</mtext></mrow></msup></mrow></mfrac></mrow></math><img file="EP1291331A2_D0002.tif" /></maths> results for TiO₂ against air (n = 1) a reflection of well over 10%.

0013On the other hand, to exploit the hydrophilic properties of titanium dioxide, it is necessary for the titanium dioxide layer to form the uppermost layer.

0014One way to solve this problem could be to apply a porous layer with a lower refractive index to the titanium dioxide layer. The porosity of this lower refractive index layer ensures sufficient access of water to the titanium dioxide layer. The disadvantage here, however, that such porous layers have only insufficient mechanical stability.

0015It was an object of the present invention to provide an antireflection coating for in particular transparent substrates with photo-induced hydrophilicity, which has a self-cleaning surface and at the same time low reflection.

0016This object is achieved by an antireflection coating with self-cleaning effect, wherein the antireflection coating has a layer system of at least three layers and the upper, photocatalytically active layer is titanium dioxide.

0017According to the invention, an antireflection coating is thus provided which consists of a layer system with photoinduced hydrophilicity. The antireflection coating has, as the upper layer, a titanium dioxide layer which becomes water-attracting and thus hydrophilic by light irradiation of a suitable wavelength. Due to this photoinduced hydrophilicity, the titanium dioxide layer acts as a self-cleaning surface. Furthermore, the antireflection coating according to the invention has at least two further layers in addition to the topmost titanium dioxide layer. These further layers are used for neutralization or Reduction of high reflection caused by the upper layer of titanium dioxide.

0018With the antireflective coating according to the invention, transparent substrates such as glass can be provided with a self-cleaning surface without having to accept any impairment of the reflection.

0019Thus, the antireflection coating according to the invention with photo-induced hydrophilicity is suitable for coating optical glasses such as spectacles and for the architectural glass sector such as, for example, windows.

0020In principle, the present invention is suitable for coating transparent substrates of any materials such as glasses or transparent plastics. The refractive index of glass or plastic varies depending on the type and manufacturing process. Typically, glass has a refractive index in a range of about 1.4 to 1.6 and plastic of about 1.5 to 1.7.

0021The layers used according to the invention are thin layers with a layer thickness in the nanometer range which are permeable to light and thus do not impair the transparency of the underlying substrate.

0022It has also been observed that the antireflective coating of the present invention can reduce the emissivity of the substrates. Thus, the present invention is also suitable for low-energy applications. In addition, the solar radiation in the near infrared can be reduced, so that a combined sun and heat protection coating ("sun control") can be realized.

0023Show it<dl id="dl0001"><dt><b>FIG. 1</b></dt><dd>a graph of the calculated Y-norm color value of a TiO₂Layer on glass,</dd><dt><b>FIG. 2</b></dt><dd>the layer structure of an antireflection coating according to the invention,</dd><dt><b>FIG. 3</b></dt><dd>a diagram of the reflection and transmission of an embodiment according to the invention for an antireflection coating according to Figure 2,</dd><dt><b>FIG. 4</b></dt><dd>a further embodiment of an antireflection coating according to the invention,</dd><dt><b>FIG. 5</b></dt><dd>a diagram of the reflection and transmission of the antireflection coating according to Figure 4,</dd><dt><b>FIG. 6</b></dt><dd>a further embodiment of the antireflection coating according to the invention,</dd><dt><b>FIG. 7</b></dt><dd>a diagram with the reflection and transmission of the antireflection coating according to Figure 6,</dd><dt><b>FIG. 8</b></dt><dd>a diagram with a comparison of the reflection of different thicknesses of TiO₂Layers and an antireflective coating according to the invention,</dd><dt><b>FIG. 9</b></dt><dd>a diagram of the reflection of another embodiment of an antireflection coating according to the invention,</dd><dt><b>FIG. 11</b></dt><dd>a diagram with the reflection spectrum of a further embodiment of the antireflection coating according to the invention,</dd><dt><b>FIG. 12</b></dt><dd>the layer structure of a further embodiment for an antireflection coating according to the invention,</dd><dt><b>FIG. 13</b></dt><dd>a diagram with the reflection spectrum of the antireflection coating according to Figure 12,</dd><dt><b>FIG. 14</b></dt><dd>the layer structure of another embodiment of an antireflection coating according to the invention, and</dd><dt><b>FIG. 15</b></dt><dd>a diagram of the reflection spectrum of an antireflection coating according to FIG. 14.</dd></dl>

0024The Y standard color, reflection and transmission in Figures 1, 3, 8, 9, 10 and 11 were obtained according to Nature "Light-induced amphiphilic surfaces", Vol. 388, (1997), page 431. In Fig. 8, "AR" means anti-reflection coating.

0025The uppermost layer of the self-cleaning antireflective coating according to the invention with photo-induced hydrophilicity consists of TiO₂, TiO₂ may have a refractive index of n = 2.4 to 2.7 at 550 nm, depending on the manufacturing conditions. For the present invention, TiO₂ be used in its anatase or rutile modification. The anatase modification is preferred because of its easier-to-induce hydrophilicity and thus better self-cleaning effect.

0026As already stated, the band gap of titanium dioxide is about 3.4 eV, which corresponds to a wavelength of 340 nm. This can be induced by irradiation with UV light, the hydrophilicity.

0027According to the invention, the titanium dioxide used can be doped with suitable dopants. By doping, the band gap and thus the required photon energy for inducing the hydrophilicity can be influenced. Suitable dopants are, for example, Fe₂O₃ or Nb₂O₃,

0028It has been generally believed that for optimum self-cleaning effect, the titanium dioxide layer should have a thickness of at least 100 to 200 nm. However, such thick titanium dioxide layers are unsuitable for certain applications such as for example for the coating of spectacles or in the field of architecture. According to the invention, it has now been found that sufficient cleaning effects can be achieved even with thinner titanium dioxide layers. For example, for spectacles, antireflective coating systems according to the invention having a titanium dioxide layer with a thickness of 20 nm or less and for architectural glasses having a thickness of 50 nm or less, in particular of about 40 nm or less, can be used.

0029In <b>FIG. 1</b> Figure 4 is a graph showing the reflectance spectrum of a titanium dioxide layer on glass as a function of titanium dioxide layer thickness. Here, the Y standard color value, which is a measure of the spectrally averaged reflection of the surface, is calculated as a function of the titanium dioxide layer thickness. It can be seen that the Y standard color value is an oscillatory function of the layer thickness. The reason for this is interference effects, which usually occur in the case of thin transparent layers on glass. For a pure glass surface without titania coating, the minimum value for Y is 4.2. The second minimum with Y = 6.4 results for a titanium dioxide layer with a thickness of 110 nm and further minima with Y = 10.9 for a titanium dioxide layer thickness of 225 nm and Y = 15.2 for a titanium dioxide layer thickness of 340 nm.

0030According to the invention, by combining the titanium dioxide layer with suitable further layers, the increased reflection of a substrate coated therewith by the titanium dioxide layers is neutralized or reduced.

0031In a first embodiment, in this case, the titanium dioxide layer can be combined with an oxide layer having a high refractive index and a metal layer. In this case, the oxide layer with a high refractive index forms the lowest layer in the coating, i. H. the layer that rests directly on the substrate. The metal layer is disposed between this lower oxide layer having a high refractive index and the upper titanium dioxide layer, which is photocatalytically active. By using the metallic intermediate layer, an anti-reflection of the upper high-index titanium dioxide layer is possible.

0032For the lower oxide layer of high refractive index, any oxide having a refractive index which is typically in a range of 1.9 to 2.5 or more can be used. For the lower oxide layer with high refractive index can also TiO₂ be used. Examples of other suitable materials are SnO₂, ZnO, Si₃N₄, Bi₂O₃, ZrO₂ and Ta₂O₅, The lower oxide layer having a high refractive index typically has a thickness in a range of 20 to 50 nm, especially 25 to 40 nm. If necessary, the thickness may be lower or higher.

0033Between the lower oxide layer having a high refractive index and the upper titanium dioxide layer is an intermediate layer of a metal. By combining with this intermediate layer, the increased reflection caused by the upper titanium dioxide layer can be neutralized or lowered. Examples of suitable metals are silver, gold or titanium. The thickness of the metallic intermediate layer varies depending on the metal used. The lower and upper limits are determined by the properties to be achieved. If the layer is too thin, sufficient anti-reflection of the upper titanium dioxide layer is not obtained. If, on the other hand, the layer is too thick, the transmission of the light may be impaired. A typical value range for the metallic intermediate layer is between 5 nm to 20 nm, in particular 8 nm and 20 nm. For example, layer thicknesses in a range from 8 nm to 20 nm have proved particularly suitable for Ag.

0034According to a further embodiment, the coating system of the invention comprises a titanium oxide layer having a metallic intermediate layer and a lower oxide layer having a high refractive index and a blocking layer. This blocker layer is applied to the metallic intermediate layer and serves to prevent oxidation of the metallic intermediate layer during the subsequent coating processes, for example for application of the upper titanium dioxide layer. Examples of suitable materials for the blocking layer are stoichiometric TiOx and NiCr. The thickness of the blocking layer is typically in the range of 1 nm to 6 nm.

0035In <b>FIGS. 2, 4 and 6</b> Examples of antireflective coatings according to the invention with a metallic intermediate layer are shown. The substrate used was glass in each case.

0036The embodiment according to <b>FIG. 2</b> consists of a glass substrate (1) (1 mm), which is coated with a coating according to the invention with a lower oxide layer (3) of TiO₂ in the anatase modification (36.12 nm), a metallic intermediate layer (4) of Ag (12.88 nm), a blocking layer (5) of a substoichiometric NiCrOx (2.0 nm) and an upper layer (2) TiO₂ in the anatase modification (33.77 nm).

0037The reflection and transmission spectrum of the layer system according to FIG. 2 is shown in FIG. For comparison, in Figure 3, the reflection or Transmission of glass shown with an absorption index k = 0 and a refractive index n = 1.5 for both sides. It can be seen from FIG. 3 that the layer system according to FIG. 2 has good antireflection properties. It is possible to achieve a transmission in the visible spectral range between 85 and 95%, the absorption losses being in the range of 3 to 10%. Due to this low absorption, such a layer system is excellent for the antireflective z. B. Eyeglasses.

0038In addition, it has been found that the emissivity of this layer system is low, so that it is also suitable as a low-energy coating (low-e coating), as it is preferred for example for the architectural glass sector.

0039According to a further embodiment, a repetition of the layer structure of the antireflection coating is possible, whereby a further improvement of the low-energy properties ("double-low-e") or else a combined solar and heat protection ("sun control") can be effected.

0040The emissivity of the coating according to the invention and thus the k value of a glass composite of the coating according to the invention can be influenced by the thickness and by the electrical conductivity of the metallic intermediate layer. For example, when using silver at layer thicknesses of 13 nm, particularly small k values ​​(for example 1.1 in double-glazing with Ar filling) can be achieved.

0041In <b>FIG. 4</b> is another example of an antireflection coating according to the invention with metallic intermediate layer (4) on a glass substrate (1) shown. Here, on the glass substrate (1) (1 mm), a lower oxide layer having a refractive index of 2.6 (3) (43.28 nm), a metallic intermediate layer (4) of platinum (7.00 nm), and an upper layer (2) of TiO₂ applied in the anatase modification (45.46 nm). The upper titanium dioxide layer (2) is here chosen to be thicker than in the example using silver as the metallic intermediate layer (4), since the use of platinum as a metallic intermediate layer, the system has a different refractive index. A refractive index of 2.6 for the lower oxide layer (3) can be achieved, for example, by TiO₂ in rutile modification or Bi₂O₃ realize.

0042Tabulated values ​​for refractive indices and absorption indices of silver can be found in Palik (ED Palik (ed), Handbook of Optical Constants of Solids, Vol. II, Academic Press, New York (1991)). and for platinum in ED Palik, Handbook of Optical Constants of Solids Volume I, Academic Press, New York (1985), p. 275th

0043The values ​​given there can serve as a basis for selecting suitable layer thicknesses for the metallic intermediate layer.

0044The reflection and transmission spectrum of the embodiment according to FIG. 4 is shown in FIG.

0045Another example of an anti-reflective coating with metallic interlayer according to the invention is shown in FIG. Here, on a glass substrate (1) (4,000,000 nm), a lower oxide layer (3) of high refractive index of TiO₂ in the anatase modification with a refractive index of 2.45 (53.25 nm), a metallic intermediate layer (4) of titanium (5.00 nm) and an upper layer (2) of TiO₂, which is also present in the anatase modification applied.

0046The reflection and transmission spectrum of the antireflective coating according to <b>FIG. 6</b> is in <b>FIG. 7</b> shown.

0047According to a further embodiment, a multilayer system consisting of different thin dielectric layers is applied between the substrate surface and the upper titanium dioxide layer for the antireflection coating according to the invention. The refractive index and the thicknesses of the individual layers are selected here in order to minimize the reflection but without impairing the self-cleaning photocatalytic surface.

0048Preferably, the multilayer system consists of a combination in which layers of high refractive index alternate with layers of low refractive index.

0049As materials for high-refractive-index layers, the same materials as those exemplified above for the high-refractive-index lower oxide layer may be used. For this embodiment, preferred materials for the high refractive index layers are materials whose refractive index is 2.2 or greater than 2.2.

0050As layers with a low refractive index, typically layers are selected whose refractive index is less than 2 and in particular &lt;1.7. Examples are SiO₂(n = 1.46), MgF₂ (n = 1.35) and Al₂O₃ (n = 1.6-1.7) The most suitable material for sputter coatings is SiO₂,

0051Hereinafter, this embodiment of an antireflection coating according to the present invention will be explained by way of some examples. The layer thickness of the upper titanium dioxide layer was set at about 100 nm in the following examples, resulting in minimal reflection, as can be seen from FIG.

0052The multilayer system consisting of thin, dielectric layers can contain layers with the same or different materials, the same or different layer thicknesses and the same or different refractive indices. For example, the layer system may be constructed alternately of a high-refractive-index layer and a low-refractive-index layer, with the same material being used for the high-refractive-index layers and the low-refractive-index layers, respectively. The layer structure can vary as desired in terms of thickness, material and refractive index, provided that this minimizes the reflection is achieved.

0053A first example of an antireflective coating according to the invention with a multilayer system of dielectric layers is shown in Table 1. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="left">#</entry><entry namest="col2" nameend="col2" align="left">Physical. thickness</entry><entry namest="col3" nameend="col3" align="left">Optical thickness</entry><entry namest="col4" nameend="col4" align="left">QWOT</entry><entry namest="col5" nameend="col5" align="left">material</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">1</entry><entry namest="col2" nameend="col2" align="right">2.85</entry><entry namest="col3" nameend="col3" align="right">7.70</entry><entry namest="col4" nameend="col4" align="char" char=".">0.061617</entry><entry namest="col5" nameend="col5" align="left">RUTIL</entry></row><row><entry namest="col1" nameend="col1" align="left">2</entry><entry namest="col2" nameend="col2" align="right">39.35</entry><entry namest="col3" nameend="col3" align="right">57.45</entry><entry namest="col4" nameend="col4" align="char" char=".">0.459595</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="left">3</entry><entry namest="col2" nameend="col2" align="right">100.59</entry><entry namest="col3" nameend="col3" align="right">271.61</entry><entry namest="col4" nameend="col4" align="char" char=".">2.172851</entry><entry namest="col5" nameend="col5" align="left">RUTIL</entry></row><row><entry namest="col1" nameend="col1" align="left">4</entry><entry namest="col2" nameend="col2" align="right">96.36</entry><entry namest="col3" nameend="col3" align="right">140.68</entry><entry namest="col4" nameend="col4" align="char" char=".">1.125441</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="left">5</entry><entry namest="col2" nameend="col2" align="right">1.81</entry><entry namest="col3" nameend="col3" align="right">4.88</entry><entry namest="col4" nameend="col4" align="char" char=".">0.039041</entry><entry namest="col5" nameend="col5" align="left">RUTIL</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">6</entry><entry namest="col2" nameend="col2" align="right">107.76</entry><entry namest="col3" nameend="col3" align="right">269.39</entry><entry namest="col4" nameend="col4" align="char" char=".">2.155154</entry><entry namest="col5" nameend="col5" align="left">TiO₂surface</entry></row></tbody></tgroup></table></tables>

Where:

0054Optical thickness: layer thickness x refractive index QWOT = λ₀/ 4n_layerThe layer 1 is the first layer on the substrate, the layer 6 the topmost layer of the stack.

0055This example according to Table 1 is an antireflection coating with a multilayer system of alternating high refractive index and low refractive index materials, using as high refractive index material TiO2 in the rutile modification and as low refractive index material silica.

0056Layer number 6 is the top layer (2) of titanium dioxide which is in the anatase modification.

0057In <b>FIG. 8</b> a comparison of the reflections of a 114 nm thick and a 224 nm thick titanium dioxide layer on glass and the antireflective coating according to the invention according to Table 1 is also shown on a glass substrate.

0058For the anti-reflection coating according to the invention shown in FIG. 8, a Y-standard color value of 3.8 was calculated. It can be seen from FIG. 8 that not only a lowering of the reflection but also a widening of the wavelength range of the reduced reflection can be achieved for the antireflection coating according to the invention in comparison to glass with a pure titanium dioxide coating. In addition, it follows from FIG. 8 that the minimum reflection wavelength region for titanium dioxide-coated glass decreases as the thickness of the titanium dioxide layer increases.

0059In <b>FIG. 9</b> the reflectance spectrum for a further example of an antireflective coating according to the invention with multilayer system of dielectric layers on a glass substrate is shown. The antireflection coating shows the following composition of the layers starting from the glass substrate: SiO₂ (35 nm), TiO₂Anatase (107 nm), SiO₂ (93 nm) and TiO₂Anatase (114 nm) as the upper self-cleaning layer. SiO₂ Here is the low refractive and TiO₂ the high-index material. The thickness of the antireflective coating is 350 nm and the total thickness including the glass substrate is 550 nm. With this antireflection coating, it was possible to achieve a Y standard color value of 2.2 which is significantly below the Y standard color value of glass with 4.5 according to FIG

0060Another example of an antireflective coating according to the invention comprising a multilayer system of dielectric materials is shown in Table 2. In the example of Table 2, the top layer of titanium dioxide is layer number 13.<tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">#</entry><entry namest="col2" nameend="col2" align="center">Physical. thickness</entry><entry namest="col3" nameend="col3" align="center">Optical thickness</entry><entry namest="col4" nameend="col4" align="center">QWOT</entry><entry namest="col5" nameend="col5" align="center">material</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">1</entry><entry namest="col2" nameend="col2" align="right">16.90</entry><entry namest="col3" nameend="col3" align="right">42.25</entry><entry namest="col4" nameend="col4" align="char" char=".">0.338004</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">2</entry><entry namest="col2" nameend="col2" align="right">28.61</entry><entry namest="col3" nameend="col3" align="right">41.77</entry><entry namest="col4" nameend="col4" align="char" char=".">0.334145</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="center">3</entry><entry namest="col2" nameend="col2" align="right">8:00 pm</entry><entry namest="col3" nameend="col3" align="right">50.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.400000</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">4</entry><entry namest="col2" nameend="col2" align="right">6:00 am</entry><entry namest="col3" nameend="col3" align="right">8.76</entry><entry namest="col4" nameend="col4" align="char" char=".">0.070080</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="center">5</entry><entry namest="col2" nameend="col2" align="right">8:00 pm</entry><entry namest="col3" nameend="col3" align="right">50.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.400000</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">6</entry><entry namest="col2" nameend="col2" align="right">8:00</entry><entry namest="col3" nameend="col3" align="right">11.68</entry><entry namest="col4" nameend="col4" align="char" char=".">0.093440</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="center">7</entry><entry namest="col2" nameend="col2" align="right">8:00 pm</entry><entry namest="col3" nameend="col3" align="right">50.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.400000</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">8th</entry><entry namest="col2" nameend="col2" align="right">10:00</entry><entry namest="col3" nameend="col3" align="right">14.60</entry><entry namest="col4" nameend="col4" align="char" char=".">0.116800</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="center">9</entry><entry namest="col2" nameend="col2" align="right">8:00 pm</entry><entry namest="col3" nameend="col3" align="right">50.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.400000</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">10</entry><entry namest="col2" nameend="col2" align="right">12:00</entry><entry namest="col3" nameend="col3" align="right">17:52</entry><entry namest="col4" nameend="col4" align="char" char=".">0.140160</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="center">11</entry><entry namest="col2" nameend="col2" align="right">97.11</entry><entry namest="col3" nameend="col3" align="right">242.77</entry><entry namest="col4" nameend="col4" align="char" char=".">1.942133</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row><row><entry namest="col1" nameend="col1" align="center">12</entry><entry namest="col2" nameend="col2" align="right">50.00</entry><entry namest="col3" nameend="col3" align="right">73.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.584000</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">13</entry><entry namest="col2" nameend="col2" align="right">8.98</entry><entry namest="col3" nameend="col3" align="right">22:46</entry><entry namest="col4" nameend="col4" align="char" char=".">0.179663</entry><entry namest="col5" nameend="col5" align="left">TiO₂ anatase</entry></row></tbody></tgroup></table></tables>

0061In the antireflective coating according to Table 2, the multilayer system consists of an alternating sequence of high-index titanium dioxide layers in the anatase modification (n = 2.45) and low-refraction silicon dioxide layers. The total thickness of the antireflective coating according to Table 2 including glass substrate is 317 nm, whereby a Y-standard color value of only 0.37 was measured.

0062In <b>FIG. 10</b> the reflectance spectrum of the antireflective coating is shown in Table 2.

0063The construction of another example of an antireflective coating with multi-layer system of dielectric layers according to the invention is shown in Table 3. Layer No. 5 is the upper layer of titanium dioxide. Layer # 1 is the lowermost layer adjacent to the substrate.<tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">#</entry><entry namest="col2" nameend="col2" align="center">Physical. thickness</entry><entry namest="col3" nameend="col3" align="center">Optical thickness</entry><entry namest="col4" nameend="col4" align="center">QWOT</entry><entry namest="col5" nameend="col5" align="center">material</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">1</entry><entry namest="col2" nameend="col2" align="char" char=".">14.74</entry><entry namest="col3" nameend="col3" align="char" char=".">35.37</entry><entry namest="col4" nameend="col4" align="char" char=".">0.282976</entry><entry namest="col5" nameend="col5" align="left">ANATAS</entry></row><row><entry namest="col1" nameend="col1" align="left">2</entry><entry namest="col2" nameend="col2" align="char" char=".">29.08</entry><entry namest="col3" nameend="col3" align="char" char=".">42.45</entry><entry namest="col4" nameend="col4" align="char" char=".">0.339600</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row><entry namest="col1" nameend="col1" align="left">3</entry><entry namest="col2" nameend="col2" align="char" char=".">80.45</entry><entry namest="col3" nameend="col3" align="char" char=".">193.08</entry><entry namest="col4" nameend="col4" align="char" char=".">1.544636</entry><entry namest="col5" nameend="col5" align="left">ANATAS</entry></row><row><entry namest="col1" nameend="col1" align="left">4</entry><entry namest="col2" nameend="col2" align="char" char=".">50.00</entry><entry namest="col3" nameend="col3" align="char" char=".">73.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.584000</entry><entry namest="col5" nameend="col5" align="left">LOW (SiO₂)</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">5</entry><entry namest="col2" nameend="col2" align="char" char=".">10:00</entry><entry namest="col3" nameend="col3" align="char" char=".">25.00</entry><entry namest="col4" nameend="col4" align="char" char=".">0.200000</entry><entry namest="col5" nameend="col5" align="left">TiO₂ surface</entry></row></tbody></tgroup></table></tables>

0064The antireflective coating according to Table 2 contains a multilayer system of alternating high-index and low-refraction layers of titanium dioxide in the anatase modification or silicon dioxide. The top titanium dioxide layer is in the anatase modification.

0065<b>FIG. 11</b> shows the reflection spectrum of the antireflective coating according to Table 3 on a glass substrate with a total thickness (including glass substrate) of 180 nm and Y = 0.29.

0066The antireflection coating according to the invention with a multilayer system of dielectric layers is not limited to comparatively thick titanium dioxide layers as the upper layer as used in the examples according to Tables 1 to 3. Excellent antireflection coating can also be achieved with such an antireflective coating with thinner titanium dioxide layers as the top layer. Thus, according to a further embodiment of the antireflection coating according to the invention, the upper titanium dioxide layer can be produced significantly thinner, for example in a thickness of 20 nm or less.

0067An example of the structure of such an anti-reflection coating is in <b>FIG. 12</b> shown. Here, on a glass substrate, a multilayer system of dielectric layers with alternating high (titanium dioxide in the anatase modification) and low (SiO₂) Refractive index applied. The structure is as follows: glass substrate, anatase (14.72 nm), SiO 2₂ (33.08 nm), anatase (121.84 nm), SiO 2₂ (42.78 nm) and anatase (15.00 nm) as the upper layer of titanium dioxide.

0068The reflection spectrum of the antireflection coating on glass shown in FIG. 12 is in FIG <b>FIG. 13</b> shown as lower graph. The upper graph is the reflection spectrum of a pure glass surface. It follows from FIG. 13 that the antireflection coating according to the invention, as shown, for example, in FIG. 12, can significantly reduce the reflection from a purely glass surface. The reflection is not only significantly reduced but is also reduced over a broader wavelength range.

0069In the example according to FIG. 12, the upper silicon dioxide layer with approximately 43 nm is significantly thinner compared to the upper silicon dioxide layer in a classical anti-reflection system according to equation 1 shown above with approximately 85 nm. Nevertheless, with the construction according to FIG lower reflection achieved.

0070In the antireflection coating shown in Figure 12, the thickness of the individual layers may vary over a wide range. Thus, the lower anatase layer may have a thickness between 10 and 20 nm. The overlying low refractive index layer may have a thickness between 20 and 50 nm. The overlying titanium dioxide layer in the anatase modification may have a thickness between 80 and 200 nm. Also, the thickness of the upper titanium dioxide layer in anatase modification may vary. For example, a thickness of up to 20 nm is possible in order to achieve a reduction of the reflection.

0071As a low-refractive material, not only SiO₂but other materials may also be used, such as MgF₂, Al₂O₃, Si₃N₄ or mixtures thereof.

0072Another example of an antireflective coating according to the invention comprising a multilayer system of dielectric layers with a thin top titanium dioxide layer is shown in FIG <b>FIG. 14</b> shown. The antireflection coating shown in Figure 14 is composed of three layers. The antireflective coating is deposited on a glass substrate and consists of a lower, high refractive layer of anatase (26.00 nm), a low refractive layer (68.00 nm) and an upper titanium dioxide layer in the anatase modification (15.00 nm).

0073Again, the low-refractive layer may consist of any low-refractive material, but preferably of silicon dioxide.

0074The reflection spectrum of the antireflection coating according to FIG. 14 is in FIG <b>FIG. 15</b> shown. In Fig. 15, the upper graph is the reflection spectrum of the anti-reflection coating having three layers in Fig. 14, and the lower graph is the reflection spectrum of the antireflection coating of Fig. 12 having five layers. The figure shows that there is a higher reflectance compared to the five-layer antireflective coating.

0075A comparison of the reflection spectra of the embodiments according to Tables 1 to 3 with the reflection spectra of the embodiments according to Figures 12 and 14 with thin TiO₂Shows that the antireflective coatings with comparatively thick upper titanium dioxide layers (2) on the one hand have a low reflection over a narrower wavelength range and on the other hand, the antireflection coatings with thin upper titanium dioxide layers have a slightly worse reflection but over a wider wavelength range. By varying the thickness of the upper titanium dioxide layer (2), it is thus possible to influence the reflection and the wavelength range of the reflection.

0076The titanium dioxide layers used according to the invention can in principle be obtained with coating methods known for the production of such thin layers. For example, vacuum deposition techniques such as reactive or nonreactive magnetron sputtering or thermal or electron beam assisted vapor deposition may serve as the coating process. Plasma-assisted vapor deposition can also be used here. The coating preferably takes place in such a way that the preferred anatase modification results. For reactive magnetron sputtering, higher temperatures between 100 and 250 ° C have proven to be helpful.

0077The layers of the antireflective coating of the present invention can generally be obtained by methods known and described per se for the preparation of such thin layers. For example, the same methods can be used as described above for the preparation of the titanium dioxide layer.

LIST OF REFERENCE NUMBERS

0078<dl id="dl0002" compact="compact"><dt>1</dt><dd>substratum</dd><dt>2</dt><dd>Upper layer of TiO₂</dd><dt>3</dt><dd>Lower layer with high refractive index</dd><dt>4</dt><dd>Metallic interlayer</dd><dt>5</dt><dd>blocker layer</dd></dl><dl id="dl0003" compact="compact"><dt>6</dt><dd>TiO₂ (Anatase)</dd><dt>7</dt><dd>low-breaking layer</dd><dt>8th</dt><dd>TiO₂ (Anatase)</dd><dt>9</dt><dd>low-breaking layer</dd><dt>10</dt><dd>TiO₂ (Anatase)</dd><dt>11</dt><dd>low-breaking layer</dd></dl>