METHOD FOR APPLYING AN ANTIREFLECTION COATING TO INORGANIC OPTICALLY TRANSPARENT SUBSTRATES

Abstract: An antireflection coating having a high abrasion and scratching resistance can be applied to natural glass by sputtering, the substrates being positioned closer to the target than usual in order to obtain an increased density of the applied layers. An irregular deposition rate can be compensated by moving the substrate, thus allowing to ensure a regular thickness and therefore a high optical quality of the antireflection coating in these conditions as well. On the preferred sapphire watch glass, SiO2 and Si3N4 resp. AlN and Al2O3 can be used for the individual layers, in particular. These layers do not have a noticeable color effect, but on account of their high hardness, their abrasion and scratching resistance is comparable to that of sapphire glass.

Description: METHOD FOR APPLYING AN ANTIREFLECTION COATING TO INORGANIC OPTICALLY TRANSPARENT SUBSTRATES The present invention relates to a method for applying an antireflection coating according to the preamble of claim 1. The invention further relates to products obtained with the method. Antireflection coatings are known and are being industrially manufactured worldwide mainly on natural or synthetic eyeglass lenses and on lenses of all kinds for various applications in fine optics. Such antireflection coatings may comprise a single layer or multiple layers composed of pairs of materials having high and low refractive indices, such as Ti0 2 , Si0 2 , Hf0 2 , MgF 2 , etc., which are preferably applied currently by vacuum evaporation, or also by sputtering. The refractive indices of the cited dielectric materials are ideal if they are deposited on substrates whose own refractive indices are comprised between 1.5 and 2.1. The only disadvantage of vacuum-evaporated coatings is their relatively low hardness and therefore a quick abrasion and scratching of the coated surfaces in the case of mechanical interaction with the environment (a problem that is well known to wearers of glasses) . Antireflection coatings have been used in the watch industry since the 80s, but in the beginning only on natural glass watch-glasses, i. e. glasses made of inorganic or mineralic material, for professional wristwatches of the chronometer type. With the introduction of largely scratch-resistant watch glasses made of sapphire glass (monocrystalline aluminum oxide) , antireflection coatings have become more important since in the uncoated condition, the overall reflection of these glasses is 6.5% higher than that of natural glass. In the case of sapphire glass with its high optical density (refractive index) of 1.77, the reflection amounts to 7.7% on each side, i.e. to a total of 15.4%. The readability of the dial is therefore deteriorated by a factor of 2, which is being noticed and increasingly criticized by consumers. Today, the "safe" way of providing sapphire watch glasses with antireflection coatings is to apply them by vacuum evaporation to the inner side of the glass only, with the result that the total reflection is reduced from 15.4% to 7.7% and thus comprised in the order of an uncoated natural glass (8.4%). In this case, the outer, uncoated side conserves the high scratching resistance of the sapphire glass . Two-sided coatings are also applied. These allow an optimum readability of the time with a residual reflection of less than 1%, which represents a significant improvement over watch glasses which are not coated on both sides. However, an outer antireflection coating deposited by conventional high vacuum evaporation has a substantially smaller resistance to abrasion than sapphire glass. Yet, the sensitivity to scratching is greatly influenced by the shape of the glass. Thus, a flat glass will be scratched much less than a curved one since it has fewer exposed surfaces. Therefore, in practice, the construction of a watch and its design also enter into the decision whether to apply a double-sided coating or not. Even so, the basic disadvantage of the known antireflection coatings subsists, namely their substantially reduced resistance to abrasion from the underlying material. This is especially true in the case of sapphire glass, which is used for watches precisely because of its mechanical resistance. Among other properties, the known antireflection coatings distinguish themselves by their high optical quality ( low residual reflection, low dispersion, and no absorption) . Since the resistance of coatings of high optical quality has been considered as secondary, known coatings of this kind have a low mechanical resistance. Therefore, the main object of the present invention is to provide a method allowing to provide a natural glass substrate with an antireflection coating whose scratching resistance approximately corresponds to that of the substrate, more particularly of sapphire glass, and which offers a high optical quality (more particularly a low residual reflection) . Such a method is defined in claim 1. Preferred embodiments and products obtained with the method are defined in the dependent claims. A preferred application of the method is the application of antireflection coatings to sapphire glass since, for the first time, the method of the invention allows the application of an antireflection coating to sapphire glass without noticeably affecting its scratching resistance . In this context, the term "natural glass" generally designates optically transparent, inorganic materials. Accordingly, such a coating is essentially produced by sputtering. Sputtering techniques (PEPVD) are not yet used in thin film technology for watch industry related optics as the control of the layer thickness is more difficult than in conventional vacuum evaporation techniques. However, it has been found that the hardness and the density of the applied layers can be increased if the coated objects are positioned closer to the target and suitable coating materials are used. The invention will be further explained by means of an exemplary embodiment and with reference to the Figures. FIG. 1 schematically shows a sputtering installation; and FIG. 2 schematically shows the procedure allowing to determine the diameter in the case of non-circular targets . Layer materials It has been found that in order to obtain a high scratching resistance, the layers must consist of materials having the highest possible hardness. A pair which meets these requirements is Si0 2 /Si 3 N 4 (silicon dioxide/silicon nitride) . However, the difference of the refractive indices of this combination (Ξi0 2 : 1.46; Si 3 N 4 : 2.0) is smaller than that of conventional combinations (e.g. Ti0 2 : 2.35; Si0 2 : 1.46) and therefore requires thicker layers while its residual reflection is greater. Surprisingly, these thicker layers can also be produced in the necessary quality by sputtering. Another advantage of this combination is that only one sputtering target is required and that the type of layer can be selected by changing the reaction gas only. The sequence and the thickness of the layers required for an antireflection coating are determined according to the usual rules of thin film optics. Preferably, at least 2 layers of a material having a high refractive index and a material having a low refractive index are applied while the low-refractive layer is applied last. A further improved effect over the applicable spectral range is obtained e.g. with 4 layers, namely, starting from the substrate, Si 3 N4/Si0 2 /Si 3 N/Si0 2 . Furthermore, the chemical resistance of these layer materials is improved over that of conventional layers. However, the coating may still be removed by the action of chemical substances without damaging the substrate. A further increase of the resistance can be obtained by applying an additional thin protection layer. The protection layer is substantially thinner than any one of the layers of the antireflection coating and preferably no more than half as thick as the thinnest one. Another possible layer combination of a high hardness is A1N/A1 2 0 3 , whose refractive indices are equal to 2.08 and 1.67, respectively. The residual reflection is higher than that of conventional combinations, but the hardness of the top layer is optimum. Layer production Figure 1 schematically shows a sputtering installation 4. A substrate 3, e.g. of sapphire glass, is disposed relatively close to target 1. Hitherto, this area of the plasma 2 has been considered as unsuitable for coatings since it involves a high thermal load of the objects, on one hand, and since the coating is still very irregular as the density of the plasma strongly varies in function of the location. As a result, the layer growth will be higher in areas close to the center of target 1 than in more distant areas, i.e. closer to the edge 7 of target 1. It has been found that sapphire glass and also other kinds of natural glass are capable of resisting this thermal load, and that it is therefore possible to produce very dense and thus hard layers with sputtering techniques. In the sputtering process of the invention, the substrates reach temperatures between 300 °C and 400 °C, and in the case of isolated substrates, up to 600 °C. For a regular layer thickness, which is a necessary condition for a high optical quality, the object may be moved in front of the target, thereby allowing to equalize the layer growth. To this end, the substrates 3 are disposed on supports 8 which are rotatably mounted on a plate 10. During the coating process, plate 10 is rotated about axis 13 according to arrow 12, while supports 8 and thus substrates 3 are simultaneously rotated according to arrow 14. The rotational movement of supports 8 may be produced by a dedicated driving unit, or it may be derived from the rotation of plate 10 e.g. by a gear assembly. Drives and gear assemblies of this kind are known per se . Plate 10 preferably comprises as many supports 8 as possible, thus allowing the simultaneous coating of a great number of substrates . For an improved adhesion of the layers to the substrates, the latter are preheated prior to the sputtering process. Presumably, the advantageous effect of the preheating phase in the process according to the invention is due to the resulting reduction of the temperature difference between the plasma and the substrate especially at the beginning of the sputtering operation. The ratio of the distance d sτ 25 between the substrate and the target and of the diameter of the (circular) target q 15 may serve as a measure for determining the position of the substrate 3 with respect to the target 1. Thus, for example, d S τ/q = 1 is a rather large distance, while preferred values are in the vicinity of 1/2, preferably 1/3 or smaller. If a position other than above the center of the target is chosen, the distance may have to be reduced so that the substrate is positioned in an area where the plasma density is the same as or higher than if it were positioned above the center according to the cited rule. In the case of non-circular targets such as the rectangular target 16 of Figure 2, diameter q 15 is determined by the diameter of a disk of a size that still fits on target 16. In other words, it is equal to incircle 17 of target surface 18. For this purpose, incircle 17 or the equivalent disk must be located under substrate 3. In spite of the high initial costs, the sputtering technique offers important advantages with respect to such properties as hardness and abrasion resistance of the coating. Compared to conventionally evaporated coatings, the density and hardness of sputtered coatings are much higher. However, the hardness of the layers is almost impossible to determine in practice. On account of the low thickness of the layers, inter alia, current measuring procedures essentially measure the hardness of the substrate or else yield artifacts. Consequently, since the standard Bayer Test does not noticeably affect the layers, the mechanical resistance has been measured by a "tightened" Bayer Test of the abrasion and scratching resistance. In the Bayer Abrasion Test according to ASTM F735-94, the test substrates, e.g. of synthetic glass, are placed on the bottom of a metal trough and covered with a specified amount of quartz sand. By means of a shaking device, the trough is subjected to 100 to 600 shaking cycles ("strokes") of a specified frequency and amplitude. The increase in light dispersion as compared to the uncoated substrate constitutes a measure of the scratching resistance. Example 1 Respective lenses of sapphire glass and of natural glass are positioned at a distance of 60 mm from a target having a diameter of 125 mm, and provided with an antireflection coating composed of alternating layers of silicon nitride and silicon oxide by reactive sputtering of silicon with oxygen and nitrogen, respectively, at a process pressure of p = 5-10 "3 mBar, the first two layers having a physical thickness of 20 nm each, and the last two layers having a physical thickness of 90 and 120 nm, respectively. The abrasion resistance was measured with the mentioned Bayer Test according to ASTM F 735-94 with tightened test conditions as follows: Tightened Bayer-Test • corundum sand instead of quartz sand; • 13.500 strokes instead of 600; • stroke length 60 mm instead of 50 mm; • sand layer height 25 mm instead of 13 mm; and • frequency 450 min "1 instead of 300 min "1 . For comparison purposes, an uncoated substrate and a substrate with a conventional evaporated coating were also measured. The following results were obtained: Table 1: increase in light dispersion of the transparent substrate after the tightened Bayer Test Example 2 A sapphire glass lens is positioned at a distance of 75 mm from a target having a diameter of 125 mm, and provided with an antireflection coating composed of alternating layers of silicon nitride and silicon oxide by reactive sputtering of silicon with oxygen and nitrogen, respectively, at a process pressure of p = 5-10 "J mBar, the first two layers having a physical thickness of 20 nm each, and the last two layers having a physical thickness of 90 and 120 nm, respectively. The abrasion resistance was measured with the tightened Bayer Test (see Example 1) . The following results were obtained: Table 2: increase in light dispersion of the transparent substrate after the tightened Bayer Test Example 3 A sapphire lens is positioned at a distance of 80 mm from a target having a diameter of 125 mm, and provided with an antireflection coating composed of alternating layers of silicon nitride and silicon oxide by reactive sputtering of silicon with oxygen and nitrogen, respectively, at a process pressure of p = 5-10 "3 mBar, the first two layers having an optical thickness of 20 nm each, and the last two layers having an optical thickness of 90 and 120 nm, respectively. The abrasion resistance was measured with the tightened Bayer Test (see Example 1) . The following results were obtained: Table 3: increase in light dispersion of the transparent substrate after the tightened Bayer Test The coatings obtained according to Example 1 may be considered as largely scratch-resistant. The increase in light dispersion of 0.1 % with respect to pure sapphire glass, i.e. to the double of the value, is not visible by the naked eye. In contrast, the increase in light dispersion of the lens of Example 2 is already apparent. The results of Example 3 speak for themselves. However, it will be noted that the test objects are subject to substantially higher requirements in the applied tightened Bayer Test than in practice. Thus, depending on the requirements, a coating according to Example 3 may still be sufficient. On the other hand, an even smaller distance allows to obtain a further increased scratching resistance. On natural glass, the coating of the invention even allows an improvement of the scratching resistance over the uncoated substrate (Example 1). Example 4 In another practical example, a magnifying lens of sapphire glass for use on a watch glass was coated. The magnifying lens of a diameter of 7 mm has a relatively important curvature, which has to be taken into account in the coating procedure since a variation of 2% of the coating thickness is optically visible already. s mentioned in the introduction, the important curvature also leads to an increased sensitivity to mechanical wear. In spite of the important curvature, the examination of the coated magnifying lens showed a high optical quality without noticeable shortcomings in comparison to the more unproblematic lenses of Examples 1 to 3. From the description of the method of the invention and of the coatings produced with the method, modifications and adaptations are apparent to those skilled in the art without leaving the protective scope of the claims. Thus, inter alia, _ instead of sapphire glass, other kinds of natural glass can be used as substrates; other layers can be applied to natural glass substrates in order to produce an antireflection coating of a high hardness, the properties relevant for abrasion and scratching resistance being adjusted through the process parameters of the sputtering operation and the choice of the layer compositions; _ other layer combinations of base materials having a high hardness can be used, particularly also of materials of different absorption in the visible spectrum, thus allowing to obtain color effects, e.g. ZrN/Zr0 2 ; the antireflection coating may be composed of a different number of layers equal to or greater than two, also of an odd number of layers, e.g. 5; and _ layers of other material pairs and/or of several material pairs can be applied, e.g. two layers of Si0 2 /A1N, a succession of layers of Si0 2 /Si 3 N 4 /Al 2 0 3 /AlN, or another succession of different material pairs. Glossary PEPVD plasma-enhanced physical vapor deposition PVD physical vapor deposition

Claims: Claims 1. A method for applying an antireflection coating to a substrate of an optically transparent, inorganic material, wherein alternating layers of different refractive indices are applied to the substrate by means of a plasma-enhanced PVD process, more particularly by so-called sputtering, the distance between the target and the substrate being chosen such that the scratching resistance of the obtained layers is similar to or higher than that of the substrate. 2. The method of claim 1, using a substrate of natural glass or of sapphire glass. 3. The method of claim 1, wherein said layers are composed of at least two of the following materials: Si0 2 , Si 3 N 4 , A1 2 0 3 , A1N, ZrN, Zr0 2 , of which Si0 2 and Si 3 N are preferred. 4. The method of claim 1, wherein said layers are composed of at least one of the following pairs: Si0 2 and Si 3 N 4 , A1 2 0 3 and A1N, ZrN and Zr0 2 , Si0 2 /Si 3 N 4 being the preferred pair. 5. The method of claim 1, wherein the ratio of the distance d sτ between said substrate and the target and of the target diameter q is equal to 1 at the most if said substrate is positioned approximately opposite the center of the target, said diameter being determined in non-circular targets by the largest circle fitting on the target surface under said substrate, and if said substrate is positioned otherwise, said distance is chosen such that the plasma density at the location of said substrate is the same as or greater than in the case of said central positioning. 6. The method of claim 5, wherein said ratio is not greater than 1/2 (one half) , preferably no greater than 1/3 (one third) . 7. The method of claim 1, wherein said substrates are preheated prior to the coating operation in order to improve the adhesion of the layers. 8. The method of claim 1, wherein the light dispersion of the applied antireflection coating, measured in the "tightened Bayer Test", is no more than twice as high as that of the uncoated sapphire glass. 9. The method of claim 1, wherein a protection layer is applied as the last layer in order to increase the chemical stability, said protection layer being substantially thinner than and preferably half as thick as the thinnest one of the antireflection coating layers. 10. The method of claim 9, wherein said protection layer consists of one of the following materials: Si 3 N 4 , ZrN, A1 2 0 3 , Zr0 2 . 11. A watch glass of natural glass, preferably sapphire glass, wherein at least a part of the surface, preferably a magnifying lens, or the entire surface is provided with an antireflection coating produced according to the method of claim 1. 12. The watch glass of claim 11, wherein the scratching resistance of said antireflection coating is essentially equal to or higher than that of the watch glass material, and in the case of a multilayer antireflection coating, each one of the individual layers thereof has this property. 13. The watch glass of claim 11, wherein said antireflection coating is essentially color-neutral. 14. A watch provided with the watch glass of claim 11.

Inventors: WITTEL BIRGIT, ZIMMERMANN HEINRICH, WITTEL, BIRGIT, ZIMMERMANN, HEINRICH
Applicants: BLOESCH HOLDING AG
Application year: 2001
Main classification: C03C17
IPC codes: C03C17, C23C14