CH718382A2 - Process for applying an anti-reflective coating by PECVD. - Google Patents

Abstract

The invention relates to a method of applying as well as a system for depositing an antireflection coating on a substrate (5) of optically transparent inorganic material, characterized in that alternating layers of different refractive indices are applied to the substrate (5) using a plasma enhanced chemical vapor deposition technique. The invention also relates to a watch glass, preferably made of sapphire, coated according to such a process.

Description

Field of the invention

The present invention relates to a method of applying an antireflection coating to a substrate of an optically transparent inorganic material. The invention further relates to a glass obtained with such a process and to a watch comprising such a glass. The invention also relates to a plasma enhanced chemical vapor deposition system.

Background of the invention

[0002] Anti-reflective coatings are known and are manufactured industrially throughout the world mainly on natural or synthetic spectacle lenses and on lenses of all types for various precision optical applications. Such anti-reflective coatings may comprise a single layer or multiple layers composed of pairs of materials having high and low refractive indices, such as Ta2O5, TiO2, SiO2, HfO2, MgF2, etc., which are preferably commonly applied by vacuum evaporation. or also by sputtering. The refractive indices of the aforementioned dielectric materials are ideal if they are deposited on substrates whose refractive index is between 1.4 and 2.3. The only drawback of vacuum-evaporated coatings is their relatively low hardness and hence rapid abrasion and scratching of coated surfaces in the event of mechanical interaction with the environment (a problem which is well known to eyeglass wearers).

[0003] Anti-reflective coatings have been used in watchmaking since the 1980s, but initially only on watch crystals made of natural glass, that is to say lenses made of inorganic or mineral material, for professional wristwatches in the chronometer type. With the introduction of largely scratch resistant watch crystals made of sapphire (single crystal aluminum oxide) glass, anti-reflective coatings have become increasingly important since, in the uncoated state, the overall reflection of these glasses is 6.5% higher than that of natural glass. In the case of sapphire crystal with its high optical density (refractive index) of 1.77, the reflection amounts to 7.7% on each side, or a total of 15.4%. The readability of the dial is therefore deteriorated by a factor of 2, which is noticed and increasingly criticized by consumers.

[0004] Still currently, the "safe" way to provide sapphire watch crystals with anti-reflective coatings consists in applying them by vacuum evaporation on the inner side of the glass only, with the consequence that the total reflection is reduced by 15, 4% to 7.7%, which is on the order of uncoated natural glass (8.4%). In this case, the uncoated outer side retains the high scratch resistance of sapphire crystal.

[0005] Coatings on both sides are also applied. These allow for optimal readability of the time with less than 1% residual reflection, which is a significant improvement over watch crystals that are not coated on both sides. Nevertheless, an exterior anti-reflective coating deposited by conventional high vacuum evaporation has a significantly lower abrasion resistance than sapphire crystal.

[0006] Despite this, the basic drawback of known antireflection coatings remains, in this case their substantially reduced resistance to abrasion from the underlying material. This is all the more true in the case of sapphire glass, which is used for watches precisely because of its mechanical resistance.

[0007] Among other properties, known antireflection coatings are distinguished by their high optical quality (low residual reflection, low dispersion and absence of absorption). Since the strength of high optical quality coatings has been considered secondary, known coatings of this type have low mechanical strength.

Summary of the invention

Therefore, the main object of the present invention is to provide a method for providing a natural glass substrate with an anti-reflective coating whose scratch resistance corresponds approximately to that of the substrate, more particularly sapphire glass , and which has a high optical quality (more particularly a low residual reflection).

This object is achieved by a method according to claim 1, wherein alternating layers having different indices of refraction are applied to the substrate by means of a plasma enhanced chemical vapor deposition technique. The gas ratio between a carrier gas and a reactive gas and the treatment and removal temperature are chosen so that the scratch resistance of the layers obtained is greater than or similar to that of the substrate. The scratch resistance of the layers obtained is several orders of magnitude higher than that of the conventional evaporation technique so that the light scattering of the applied anti-reflective coating, measured by the „tight Bayer test“ defined in the description , i.e. at least four times lower.

[0010] Preferred embodiments of the method are defined in the dependent claims. This PECVD (plasma-enhanced chemical vapor deposition) technique presents a good opportunity to have very little pinhole in/on the coating layers compared to other PVD techniques ( physical vapor deposition).

[0011] Consequently, such a coating is essentially produced by PECVD. These techniques (PECVD) have not yet been used in thin film technology in watchmaking optics because controlling the thickness of the film is more difficult than in conventional vacuum evaporation techniques. Nevertheless, the applicant has found that the hardness and density of the applied layers can be increased if suitable coating deposition materials are used. The use of PECVD technology makes it possible to have a specific refractive index, not only that which comes from stoichiometric materials, to obtain specific optical properties (high and low refractive index with very low absorption) associated with high resistance mechanical.

[0012] The invention also relates to a glass produced with such a process and a watch comprising such a glass.

The invention also relates to the plasma-enhanced chemical vapor deposition system configured to carry out such a process, the system comprising a substrate holder moving in front of the plasma. Preferred embodiments of the system are defined in the dependent claims.

Brief description of the drawings

The invention will be described in detail below by means of exemplary embodiments and with reference to the accompanying drawings in which: FIG. 1 schematically illustrates a PECVD system according to a first embodiment of the invention, FIG. 2 schematically illustrates a PECVD system according to a second embodiment of the invention, FIG. 3 is a table presenting comparative results obtained with the standard evaporation technique and the process according to the invention, and FIG. 4 represents a typical stack of layers forming an antireflection coating on a transparent substrate.

Detailed description of the invention

Figures 1 and 2 schematically describe two embodiments of a type 1.10 PECVD system associated with the method. Each system 1, 10 comprises an enclosure 2, in which mixed gases from gas inlets 8, 9 are activated in contact with a plasma generator 3, 13 to produce a plasma 6. In Figure 1, the generator Plasma generator 3 comprises a plate and, in Figure 2, the plasma generator 13 comprises two tubes. The figures show two pairs of inlets 8, 9 at the top of enclosure 2 for two types of gas. The plasma generator 3, 13 produces a plasma 6 thanks to the gases coming from the inlets, and the plasma 6 is diffused towards a substrate holder 4. The gas mixture is ionized near the plasma generators. Unconsumed gases are pumped through an outlet 7 at the bottom of enclosure 2.

[0016] A preferred application of the method is the application of anti-reflective coatings on sapphire glass. In this context, the term "natural glass" generally refers to optically transparent inorganic materials. Substrates 5 to be coated consist, for example, of glass, and are held in front of the plasma 6. For a uniform layer thickness, which is a necessary condition for high optical quality, the substrates 5 are preferably moved in front of the plasma 6, thus standardizing the layer growth. The substrate holder 4 is configured to move in front of the plasma 6 so that layers are deposited on the substrates 5. The substrate holder 4 supporting the substrates 5 moves in front of the plasma generator 3, 13 at a predefined speed. The substrate carrier 4 moves one or more times to reach the desired optical layer thickness. The substrate holder 4 preferably moves in a linear motion, preferably back and forth, as indicated by the arrows in Figures 1 and 2. The substrate holder 4 is configured to accept all types of conventional forms of substrate 5 thereon.

The different types of deposited materials are selected by changing the mixtures of gases coming from the different inlets 8, 9. To obtain good deposit homogeneity, the typical gas ratio (reactive gas flow divided by vector gas flow) is in a range of 3 to 10, the ratio depending on the optical and physical properties required.

[0018] With regard to the layer materials, it has been found that, in order to obtain a high scratch resistance, the layers must be composed of materials having the greatest possible hardness. Preferably, a combination of SiOx and SiNy (silicon oxide/silicon nitride) meets these requirements.

[0019] An advantage of this combination is that only one carrier gas is needed and the type of layer can be selected by changing only the reactive gases. The sequence and thickness of the layers required for an antireflection coating are determined according to the usual rules of thin film optics. Preferably, at least two layers are coated, one having a high index of refraction, the other having a low index of refraction. The low refractive layer is applied last. A further improved effect with respect to the applicable spectral range is obtained for example with four layers, in this case, starting from the substrate, SiNy/SiOx/SiNy/SiOx. Furthermore, the coating can always be removed by the action of chemical substances without damaging the substrate.

[0020] A further increase in strength can be achieved by applying an additional thin protective layer over the anti-reflective layers. Said protective layer is substantially, or similar to, the thinnest of the anti-reflective coating layers.

[0021] During the PECVD process of the invention, the substrates reach temperatures between 200°C and 450°C. It has been found that a sapphire glass as well as other types of natural glass are able to withstand this thermal load, and therefore it is possible to produce very dense and therefore very hard layers with PECVD techniques.

[0022] For improved adhesion of the layers to the substrates, the latter are preheated before the PECVD process.

The substrate carrier 4 is preferably preheated. To deepen this field, the use of specific ceramic materials for the substrate holder 4, either in bulk or in a coating thereon, comprising for example silicon carbide, yttrium oxide, zirconia or aluminum oxide achieves a very high coating quality.

[0024] Despite the high initial costs, the PECVD technique has significant advantages with respect to properties such as hardness and abrasion resistance of the coating. Compared to conventionally evaporated coatings, the density and hardness of PECVD coatings are much greater.

To demonstrate the improvement, by this technique, of the mechanical resistance of the final product, two main measurements are carried out: a “tightened” Bayer test and a nano scratch test. It is possible to measure the hardness of such a stack composed of very thin layers, but the effect of the substrate and/or of the underlying layer can affect the result.

[0026] In the Bayer abrasion test according to ASTM F735-94, the test substrates, for example synthetic glass, are placed at the bottom of a metal tank and covered with a specified quantity of quartz sand. By means of a stirring device, the vessel is subjected to 100 to 600 cycles of stirring („strokes“) of a specified frequency and a specified amplitude. The increase in light scattering relative to the uncoated surface is a measure of scratch resistance. The proposed "tight Bayer test" is mainly performed with the following parameters: corundum sand instead of quartz sand, 300 races instead of 600, stroke length of 30 mm instead of 50 mm, sand layer height of 25 mm instead of 13 mm, frequency of 180 min-1 instead of 300 min-1.

[0027] The results of the mechanical strength of the tightened Bayer test on standard sapphire crystals treated with anti-reflective coating by different processing techniques are indicated in the table below. Each has four stacked anti-reflective layers, and the result of an uncoated sapphire is also shown for comparison. Sapphire 0.3% Sapphire coated with evaporation technique 2.5 to 3.5% Sapphire coated according to the invention 0.3 to 0.6%

[0028] The results of the nano scratch test are shown in figure 3, the test parameters for the comparative test being: - sphero-conical indenter type - indenter radius 5 µm - progressive loading type - scanning load 0 .5 mN - initial load 0.5 mN - final load 100 mN - loading rate 199 mN/min - scratch length 0.5 mm - scratch speed 1 mm/min

As shown in Figure 3, the sapphire coated according to the invention is less scratched than a sapphire coated with PVD.

[0030] As an example of a typical product made with this technique, a sapphire crystal is placed on an aluminum oxide substrate holder and provided with an anti-reflective coating composed of alternating layers of silicon nitride and silicon oxide produced by PECVD. The process uses carrier gas SiH4 and reactive gases NH3, respectively N2O, with a gas ratio of 6 for all layers, at a process pressure of p = 0.2 mbar. Therefore, as shown in Figure 4, on sapphire crystal 15, the first three layers of SiNy 16, SiOx 17, and SiNy 18 each have a physical thickness of 20 nm, and the last layer of SiOx 19 has a physical thickness of 90 nm.

Claims (17)

1. Method of applying an antireflection coating to a substrate (5, 15) of an optically transparent inorganic material, characterized in that alternating layers (16, 17, 18, 19) of different refractive indices are applied to the substrate (5, 15) using a plasma enhanced chemical vapor deposition technique. 2. Method according to claim 1, wherein said layers (16, 17, 18, 19) comprise at least two of the following materials: SiO2, Si3N4, SiOxNy, preferably in their entirety. 3. Method according to claim 1 or 2, wherein said substrates (5, 15) are preheated before the coating operation in order to improve the adhesion of the layers (16, 17, 18, 19). 4. Method according to any one of the preceding claims, in which the substrate (5) is arranged on a substrate holder (4) moving in front of the plasma. 5. A method according to any preceding claim, wherein the light scattering of the applied anti-reflective coating layers (16, 17, 18, 19), measured in the „tight Bayer test“, is not more three times that of uncoated sapphire crystal (15). 6. Method according to any one of the preceding claims, in which a protective layer is applied to the last layer (19) in order to increase the chemical stability and/or the cleaning and/or the mechanical resistance. 7. Method according to claim 5, wherein said protective layer consists of one of the following materials: organofluorine compounds or composition of thin film materials with higher mechanical resistance. 8. A method according to any preceding claim, wherein the substrate (5, 15) comprises natural glass or sapphire glass, preferably in its entirety. 9. A method according to any preceding claim, wherein a substrate holder (4) of the substrate (5) comprises a ceramic material as follows: silicon carbide, yttrium oxide, zircon oxide or aluminum oxide , preferably in its entirety. 10. A method according to any preceding claim, wherein the gas ratio of reactant gas flow to carrier gas flow is in a range of 3 to 10. 11. Watch glass made of natural glass, preferably sapphire glass, in which at least part of the surface or the entire surface is provided with an anti-reflective coating produced according to the method according to claims 1 to 10, so that the light scattering of the anti-reflective coating is at least four times lower than that of the anti-reflective coating produced by the conventional evaporation technique in a scratch resistance test. 12. Watch fitted with the watch glass according to claim 11. 13. A plasma enhanced chemical vapor deposition system (1, 10) associated with the process according to claims 1 to 10, the system comprising a substrate carrier (4) moving past the plasma (6). 14. System according to claim 13, in which the substrate holder (4) is configured to accept all types of conventional shapes of watch glass, for example flat or spherical. 15. The system of claim 13 or 14, configured to perform single-sided and double-sided deposition. 16. System according to any one of claims 13 to 15, configured to achieve a very high efficiency compared to machines with the conventional evaporation technique, for example by several orders of magnitude. 17. A system according to any one of claims 13 to 16, configured to effect full or partial etch removal of the layer stack.