Low Reflectance, Highly Saturated Colored Coating for Monolithic Glazing

The present invention relates generally to the art of sputtering metal-containing films on non-metallic substrates, and more particularly to the art of magnetic sputtering of multiple-layer metal/dielectric transparent films on glass.

U.S. Pat. No. 3,990,784 to Gelber discloses a coated architectural glass system comprising a transparent substrate and a multilayer coating comprising first and second metal layers with a dielectric layer between them, wherein the first and second metal layers have a thickness ratio so that the transmission of the coating can be changed independent of its reflection properties by varying the thickness of the metal layers while maintaining the ratio constant. The dielectric has a thickness such that the reflection from the coating is not strongly colored.

U.S. Pat. No. 4,022,947 to Grubb et al discloses a transparent panel capable of transmitting a desired portion of visible radiation while reflecting a large portion of incident solar radiation and a method of preparing same, by sputtering an iron, nickel and chromium alloy to obtain a transparent metal film, and reactively sputtering the same or a similar alloy in the presence of oxygen to form an oxide film. In one preferred embodiment, the metal film lies between the substrate and the metal oxide film. In another preferred embodiment, the metal oxide film lies between the substrate and the metal film.

U.S. Pat. No. 4,534,841 to Hartig et al discloses solar-control glazing produced by applying first an oxide layer having an optical thickness of 20 to 280 nanometers to a transparent substrate by cathodic evaporation in an oxygen-containing atmosphere, and second a chromium nitride layer having a geometric thickness of 10 to 40 nanometers applied in an atmosphere consisting of an inert gas, such as argon, and nitrogen. An optical third dielectric layer may be applied to the second layer. The oxide layer is selected from oxides of tin, titanium and aluminum.

Architectural glass products with metallic and/or metal oxide films are growing in importance as energy demands for heating and cooling become increasingly expensive. Coated glass architectural products generally fall into two categories, solar energy control and high transmittance, low emissivity coated products.

Solar energy control glass products are generally glass substrates, often tinted, coated with a low visible transmittance colored film which reduces solar energy transmittance through the windows into the building interior, thereby reducing air conditioning costs. These products are most effective in warm climates and are most often seen in commercial construction. In areas where heating costs are of greater concern, and particularly in residential construction, high transmittance, low emissivity coatings are desirable in order to allow high transmittance of visible light into the interior while reflecting infrared radiation to retain heat inside the building. High transmittance, low emissivity coatings are typically multiple layer films wherein an infrared reflecting metal such as silver, gold or copper is sandwiched between anti-reflective metal oxide layers such as bismuth, indium and/or tin oxides. Solar energy control films, on the other hand, are typically single layer films of one or more metals or oxides of metals such as cobalt, iron, chromium, nickel, copper, etc.

The simplest method of producing a colored reflective coating is to use the body color of the metal. Thus, depositing copper produces a copper colored coating, gold produces a gold color, nickel a gray color, and so on. A more sophisticated coating technique is to modify the metal color or create a new color by introducing an absorption-colored coating in front of the metal. The interference effect is usually weak, but if reasonably large and equal reflections are made to interfere, the effect can be quite strong. This can be done by using dielectrics of high refractive index, or by enhancing the reflectance of the dielectric layer with metal layers. This involves using the two-layer sequence of metal oxide dielectric/metal or the three layer sequence of metal/metal oxide dielectric/metal.

Early well known films were of the body color variety. Wet chemical methods for producing such films for solar energy control are well known from U.S. Pat. Nos. 3,846,152; 4,091,172; 3,723,158; and 3,457,138. When vacuum coating technologies became commercially available, those existing colored coatings could be duplicated using essentially the same metals as the originals. Sputtering technologies for producing metallic solar energy control films are disclosed in U.S. Pat. No. 4,512,863 and 4,594,137. Disadvantages of metallic coatings are their lack of color variety and intensity, and the large number of materials and target sequences needed to make them. A desirable new series of coatings should provide a maximum number of colors from a minimum number of materials. For this to be accomplished, the interference color type of coating is ideal since its color can be changed by changing the thickness of the dielectric layer, and its transmittance and reflectance by changing the thickness of the metal layer or layers. The other desirable properties of a new series of coatings are intense saturated color, low reflectance, and second surface monolithic durability; i.e., the coating should be hard and chemically durable. In addition, the materials used should be inexpensive, nontoxic, and fast-sputtering. The reflectance color of a material is described by its luminous reflectance spectrophotometric curve. The intense colors are produced by large amplitudes of the curve, and low reflectance requires the curve minimum to approach zero. Second order interference colors are more intense than first order, but third order interference colors become less intense. In general, high refractive index materials produce high reflectances or large curve amplitudes. High refractive index dielectric materials also give a more intense color for double layer coating. For a metal/dielectric/metal sequence of layers, it appears that a lower refractive index dielectric material may give acceptable results.

U.S. patent application Ser. No. 125,437 filed Nov. 25, 1987, by F. H. Gillery discloses multilayer films with relatively saturated colors wherein a tin/antimony oxide film is deposited in combination with a metallic film such as chromium nitride. The disclosure is incorporated herein by reference.

The present invention provides a novel and superior dielectric film for use in a wide variety of multiple-layer architectural coatings on glass. The present invention involves sputtering zinc and tin simultaneously in an atmosphere comprising oxygen in order to deposit a coating comprising zinc/tin oxide. The zinc/tin oxide film of the present invention is deposited in combination with a metallic film such as chromium nitride to form a variety of colored multilayer coatings with relatively saturated colors.

The present invention provides the capability of making a series of colored coatings with a minimum of layers and materials. The coating system of the present invention has relatively low reflection, high color saturation and monolithic durability.

It is known that a color series can be made with first and second metal layers surrounding a layer of a transparent dielectric material; the color being varied by changing the thickness of the dielectric layer. However, previously known dielectric materials are deficient in one or more of the requisite properties of high index of refraction, fast sputtering and monolithic durability. The zinc/tin oxide of the present invention has the above properties, as well as the ability, in combination with a suitable metallic film, to produce intensely colored architectural coatings. For example, zinc/tin oxide in combination with a chromium-based film can be used to make a range of attractive colors with excellent durability.

Using reflectance circle diagrams and computer calculations, it is determined that a two layer combination of metal and dielectric has an optimum thickness for both layers which gives the minimum reflectance and highest color saturation combination. The higher the refractive index of the dielectric, the lower the transmission of the coating at the optimum, and the higher the color saturation. Metals with low n and high k, where n and k are the real and complex parts of the complex refractive index, defining the propagation of electromagnetic radiation in the medium, tend to give the lowest transmission and highest saturation.

If the thickness of the metal is increased in an attempt to lower the transmission, the reflectance increases and the color intensity is diminished. Depositing a thin metallic layer prior to the deposition of the dielectric layer can decrease the reflectance and give a more intense saturated color. If the thickness of the primary metallic layer is increased in combination with the deposition of a very thin metal layer, a low transmittance, low reflectance, highly colored coating can be produced. If two primary metallic layers are used, a low refractive index dielectric in combination with a low n, high k metal gives the most attractive appearance. The calculations show that at 20 percent light transmittance, adequate saturation can be obtained using a metal in combination with a dielectric with a refractive index of 2.3. For a lower refractive index dielectric, a metal-dielectric-metal system is preferred.

With the zinc/tin oxide of the present invention, many metal or metal alloy films can be used to provide a multiple-layer coating with good properties. Preferred films include metals such as chromium, metallic coatings such as slightly oxidized chromium and, most preferably, chromium nitride. Other preferred metallic films include titanium and titanium nitride, as well as metal alloys such as stainless steel and nickel alloys such as Inconel.

In a conventional magnetron sputtering process, a substrate is placed within a coating chamber in facing relation with a cathode having a target surface of the material to be sputtered. Preferred substrates in accordance with the present invention include glass, ceramics and plastics which are not detrimentally affected by the operating conditions of the coating process.

The cathode may be of any conventional design, preferably an elongated rectangular design, connected with a source of electrical potential, and preferably employed in combination with a magnetic field to enhance the sputtering process. The anode is preferably a symmetrically designed and positioned assembly as taught in U.S. Pat. No. 4,478,702 by Gillery et al, the disclosure of which is incorporated herein by reference.

A clean glass substrate is placed in a coating chamber which is evacuated, preferably to less than 10.sup.-4 torr, more preferably less than 2.times.10.sup.-5 torr. A selected atmosphere of reactive gases, preferably argon and oxygen, is established in the chamber to a pressure between about 5.times.10.sup.-4 and 10.sup.-2 torr. A cathode having a target surface of zinc and tin is operated over the surface of the substrate to be coated. The target metals are sputtered, reacting with the atmosphere in the chamber to deposit a zinc/tin oxide coating layer on the glass surface.

After the initial layer of zinc/tin oxide is deposited, the coating chamber is evacuated, and an inert atmosphere such as pure nitrogen is established at a pressure between about 5.times.10.sup.-4 and 10.sup.-2 torr. A cathode having a target surface of metal or metal alloy, preferably chromium, is operated over the zinc/tin oxide coated surface. The target is sputtered to deposit a metallic layer on the zinc/tin oxide coated glass surface. While chromium metal, and chromium in a slightly oxidized state, are useful in accordance with the present invention, chromium nitride is preferred. However, titanium, titanium nitride, stainless steel and nickel alloys also produce suitable films.

In most preferred embodiments of the present invention, a metal film is deposited under, as well as over, the zinc/tin oxide film. As in the case of a two layer film, the dominant wavelength of the reflected color from the uncoated surface depends almost totally on the thickness of the zinc/tin oxide layer. The thickness of the top metal layer is varied until the transmission has approximately the required value, then the thickness of the underlying metallic layer is varied until the desired reflection from the uncoated side of the article is attained. Final modification of the top metallic film thickness may be required to obtain the optimum final transmittance. Within the thickness range of interest, increasing the thickness of the top metallic film decreases the transmittance and increases the reflectance from the uncoated side of the coated article. Increasing the thickness of the bottom metallic film within the thickness range of interest decreases the transmittance and decreases the reflectance from the uncoated side.

The coating derives its color from the interference effect of the dielectric layer, depending on its thickness. A chromium nitride-zinc/tin oxide-chromium nitride film in accordance with the present invention is especially attractive in a deep blue form. The zinc/tin oxide layer is preferably deposited by sputtering a zinc/tin alloy cathode. The proportions of zinc and tin are preferably between 10 and 90 percent of each, more preferably 30 to 70 percent tin and the balance zinc, and most preferably about 50 percent each of zinc and tin.

A multiple layer coating of zinc/tin oxide and chromium nitride is deposited on a glass substrate under the following conditions. A clean glass substrate is maintained in a vacuum chamber in an atmosphere of pure nitrogen at a pressure of 2.2 millitorr. With a cathode powered at 7.5 kilowatts at 377 volts, a chromium target size of 15 by 106 inches (about 0.4 by 2.7 meters) and a line speed of 100 inches (about 2.54 meters) per minute, a chromium nitride layer 35 Angstroms thick is deposited, reducing the transmittance of the glass to 70 percent. The oxide layer is applied from four targets of 50/50 zinc/tin running at voltages between 320 and 340 volts at a total power of 76 kilowatts in an atmosphere of 50/50 argon/oxygen at a pressure of 3 millitorr and the same glass speed. The transmission is reduced to 58 percent by the zinc/tin oxide layer. A final layer of chromium nitride is deposited from a single chromium cathode energized at 492 volts and 54 kilowatts in an atmosphere of pure nitrogen at 3 millitorr. At the same glass speed, a single pass reduces the transmittance to 22 percent. The coating has a deep blue color with chromaticity coordinates of Y=9.28, x=.2325 and y=.2460 in reflectance from the uncoated glass surface. The coating is extremely hard, increasing in transmittance by only 1.6 percent when subjected to 80 cycles of a CS10F wheel on a Taber Abrasor. The coating also survives immersion in 30 percent nitric acid or 20 percent hydrochloric acid for 30 minutes, at least one month in the Cleveland (condensing humidity) Test at 150.degree. F. (about 66.degree. C.), and one month in a 5 percent salt spray.

For these examples, the chromium nitride films are deposited in pure nitrogen and the zinc/tin oxide films are deposited in 50/50 argon/oxygen as in the previous example. The following table shows the transmittances after deposition of each layer, and the reflected color in terms of color coordinates and visual observation from the uncoated glass surface of the coated article.

The above examples are offered to illustrate the present invention. Various metal-containing layers may be used on one or both sides of the zinc/tin oxide dielectric layer of the present invention in a wide range of thicknesses depending on the desired transmittance and reflectance properties. The thickness of the zinc/tin oxide dielectric film itself is varied to produce the desired color. The ranges of sputtering parameters such as pressure, atmosphere, power, voltage, line speed and so on are dependent on the sputtering apparatus employed and the desired throughput. The scope of the present invention is defined by the following claims.