Stainless Steel Overcoat for Sputtered Films

The present invention relates generally to the art of sputtering transparent metallic films for solar energy control, and more particularly to the art of controlling the color and durability of sputtered films.

Transparent films of metals such as silver, copper, nickel and cobalt are readily produced by electroless deposition methods. U.S. Pat. No. 3,457,138 to Miller discloses an electroless (wet chemical) method for producing transparent copper films over a thin transparent silver film on glass. Similarly produced wet chemical silver/copper films on glass have achieved commercial success as architectural products for solar energy control. The coating effectively reflects solar heat energy while presenting an aesthetically appealling low reflectance pinkish colored surface to the observer.

However, advances in coating technology provide alternative processes for producing transparent metallic films for solar energy control, such as chemical vapor deposition, vacuum evaporation deposition, and cathode sputtering. Cathode sputtering is particularly desirable for producing metallic films in a method that is fast and efficient on a large scale. U.S. Pat. No. 4,166,018 to Chapin discloses a particularly useful sputtering process and apparatus which employ a magnetic field to improve cathode sputtering conditions. A sputtering apparatus is described in which a magnetic field is formed adjacent a planar sputtering surface. The field comprises arching lines of flux over a closed loop erosion region of the cathode surface.

In attempting to reproduce the properties of a wet chemical two-layer silver/copper film on glass by cathode sputtering, using known cathode sputtering techniques to produce two-layer silver/copper films at various thicknesses and thickness ratios, the desirable low reflectance pinkish colored appearance of the wet chemical two-layer silver/copper film is not obtained. Rather, typical sputtering conditions result in two-layer silver/copper films that range in appearance from highly reflective bright silver to highly reflective bright copper depending on the silver/copper ratio and the orientation of the sample.

U.S. Pat. No. 4,462,884 to Gillery et al entitled "Low Reflectance, Low Emissivity Sputtered Films" discloses a method for producing transparent two-layer silver/copper films having a low reflectance pinkish colored appearance by cathode sputtering. The method disclosed therein requires the deposition of a first layer of silver in a discontinuous agglomerated state. The deposition of a continuous copper film over the discontinuous agglomerated silver film produces the desired low reflectance pinkish colored appearance characteristic of the wet chemical product. The deposition of silver in a discontinuous agglomerated state by cathode sputtering requires conditions which allow the silver arriving at the substrate surface sufficient time to agglomerate. This can be accomplished by either slowing the arrival rate of silver or increasing its mobility at the substrate surface. Slowing the arrival rate of the sputtered silver may be achieved by either lowering the power applied to the sputtering cathode or by increasing the gas pressure in the vacuum chamber in which the sputtering process is being performed. Increasing the mobility of the silver may be achieved by raising the temperature or decreasing the surface energy of the substrate.

Unfortunately, the discontinuous silver layer applied by cathode sputtering as described above does not have as good adhesion to glass as is desired for a commercial window unit. Moreover, the degree of agglomeration of the silver is difficult to control. The deposition of the silver layer is affected by the condition of the glass surface and the method used to clean it. The color and adhesion of a dual layer coating of silver and copper are particularly difficult to optimize if the glass substrate has been in storage and has stain or incipient stain on the surface to be coated.

U.S. Pat. No. 4,512,863 to Criss et al provides a method for achieving improved adhesion and color control of a sputtered film of a metal such as silver or copper by means of a primer layer of a metal such as stainless steel. The sputtering of a stainless steel primer layer is particularly useful in the production of a desirable silver/copper two-layer sputtered film on glass. The stainless steel primer layer may be sputtered onto a substrate surface to a thickness on the order of an atomic layer to provide a suitable surface for the sputtering of more sensitive metals such as silver. Thicker primer layers are also disclosed, limited only by the desired final transmittance of the coated article. Two-layer silver/copper films are preferably stabilized by treatment with passivating agents.

The present invention provides a method for achieving improved stability and durability of a sputtered film of a metal such as silver or copper by means of an overcoat of a metal such as stainless steel. The sputtering of a stainless steel protective overcoat is particularly useful in the production of a desirable silver/copper two-layer sputtered film on glass. The stainless steel overcoat may be sputtered onto the metal film to a thickness of at least about 20 Angstroms. Thicker overcoats are also useful, limited only by the desired final transmittance of the coated article.

Nonmetallic substrates, preferably transparent substrates such as glass, are coated with various metal and metal oxide films by cathode sputtering. The stability and durability of some metallic films, notably silver and copper, are not as good as desired for a commercial product.

In accordance with the present invention, the stability and durability problems of the prior art are solved by means of an overcoat of a durable, stable metal, such as stainless steel, chromium or titanium, which forms a dense oxide surface. Preferably, a stainless steel cathode is installed in a sputtering chamber. In a particularly preferred embodiment of the present invention, the stainless steel overcoat is applied in conjunction with a stainless steel primer layer as disclosed in U.S. Pat. No. 530,553 to Criss et al entitled "Stainless Steel Primer For Sputtered Films", the disclosure of which is incorporated herein by reference. A clean, transparent substrate is preferably coated with a very thin primer layer of stainless steel prior to the deposition of other compositions such as silver and/or copper. A stainless steel overcoat is then sputtered over the silver and/or copper film to provide stability and durability to the exposed surface of the film. A thin layer of stainless steel, about 20 Angstroms thick, is sufficient to provide the desired stability and durability. Thicker overcoats are also useful, limited in thickness only by the desired transmittance of the final coated article.

In a particularly preferred embodiment, glass substrates to be coated with a two-layer silver/copper film by cathode sputtering are first coated with a stainless steel primer layer. The glass substrates are preferably first washed in a conventional washing operation to remove any accumulated dirt and debris. The glass may be of any composition, but soda-lime-silica float glass is preferred. Typical float glass substrates have a transmittance of about 90 percent in the visible wavelength range. A clean glass substrate is placed in a sputtering chamber equipped with a stainless steel cathode. Sputtering is carried out to form a very thin primer layer of stainless steel on the glass surface. A silver layer is then sputtered onto the primer layer, and a copper layer sputtered over the silver layer. Finally, a stainless steel overcoat is sputtered over the copper. The thickness of the stainless steel overcoat may be as little as about 20 Angstroms, which decreases the transmittance by only about 3 percent. Such a stainless steel overcoat is sufficient to produce the desired stability and durability of the underlying sputtered films. Thicker overcoats, on the order of 30 to 40 Angstroms, are generally preferred.

In a most preferred embodiment, wherein a two-layer film of silver and copper is to be sputtered, it is noted that a discontinuous silver film is not produced at useful thicknesses when silver is sputtered onto the stainless steel primer layer. As a result of the formation of a continuous silver layer, the resultant coated article may have a more highly reflective appearance and a more silvery or coppery color than is desired. In accordance with the present invention, a desired pinkish colored silver/copper coating can be formed by increasing the thickness of the stainless steel primer layer, typically to about 20 to 25 Angstroms. If the thicker primer layer produces a lower reflectance than is desired, the reflectance can be raised by increasing the amount of silver relative to the amount of copper in the overlying two-layer film. Since the stainless steel overcoat further reduces the reflectance and transmittance, the amount of silver may be further increased relative to the amount of copper to achieve the desired appearance.

While the present invention is described in detail with regard to a stainless steel primer layer on glass, an overlying silver/copper coating deposited by cathode sputtering, and a stainless steel overcoat, various other embodiments are included within the scope of the invention. For example, other metals which form a dense oxide surface are suitable protective overcoats, such as titanium, chromium, vanadium, aluminum, tin, tantalum and nickel alloys such as Inconel. The underlying sputtered metal may include other sensitive metals such as gold, platinum and palladium. Nonmetallic substrates other than glass may be coated, and need not be transparent. The present invention will be further understood from the descriptions of specific examples which follow.

Clear float glass sheets measuring 3 by 2 feet (91.times.61 centimeters) and 6 millimeters thick are washed in a conventional washer. The transmission of the clear glass measured at 550 nanometers is about 90 percent. Clean glass sheets are coated using an in-line coating facility available from Airco-Temescal. The coater is a semi-continuous architectural coating facility capable of coating glass in sizes up to 7 by 12 feet (2.1 by 3.7 meters) on a conveyor while employing up to nine stationary cathodes which are 13 by 106 inches (0.3 by 2.7 meters). Adjacent the planar sputtering surfaces is created a magnetic field comprising curved lines of magnetic flux which form a closed loop erosion region on the sputtering surface. This erosion region has a slightly irregular "oval racetrack" shape.

Stainless steel, silver and copper cathodes are installed in the coater in sequence. Each cathode is powered by a standard 120 kilowatt power supply operating under power control at various percentages of the rated power. The speed of the conveyor and number of cathodes may be varied to obtain the desired film thickness. The atmosphere in the sputtering chamber is argon at a pressure of 3 millitorr. In this example, the line speed is about 80 inches (about 2 meters) per minute for a single pass under stainless steel, silver and copper cathodes in sequence. A transmission photometer which reads luminous transmission is installed on-line to monitor the transmission of the film after each stage of coating.

A glass substrate is conveyed under the stainless steel cathode, which comprises 19 percent chromium, 9 percent nickel, no more than 0.08 percent carbon and the balance iron, at an applied voltage of 360 volts. A stainless steel primer layer is sputtered onto the glass surface, decreasing the transmittance from about 90 percent to about 77 percent, corresponding to a stainless steel primer layer thickness of about 21 Angstroms. The stainless steel primer coated glass is then conveyed under the silver cathode. A continuous silver layer is sputtered, at an applied voltage of 325 volts, over the stainless steel primer layer, decreasing the transmittance of the coated glass to about 63 percent, corresponding to a silver layer thickness of about 27 Angstroms. The silver coated, stainless steel primed glass is conveyed under the copper cathode, at an applied voltage of 424 volts, sputtering a layer of copper over the silver layer, decreasing the transmittance to about 16 percent, corresponding to a copper layer thickness of about 257 Angstroms. Finally, the stainless steel overcoat is sputtered at an applied voltage of 389 volts, decreasing the transmittance to 12 percent, corresponding to a stainless steel overcoat thickness of about 34 Angstroms.

Although thinner or thicker stainless steel layers are effective to improve the adhesion, stability and durability of the intermediate sputtered films, the stainless steel layers of this example, in combination with preferred silver and copper layer thicknesses, duplicate a desired low reflectance pinkish colored appearance. The final coated article has a luminous reflectance of about 37.5 percent and chromaticity coordinates of x=0.3408 and y=0.3363 with respect to Illuminant D-65. The improved stability of the silver/copper film with the stainless steel overcoat is evaluated by means of a heat soak test wherein coated samples are placed in an oven at 275.degree. F. (135.degree. C.) for 5 hours. Without the overcoat, untreated silver/copper films undergo significant visible color changes. Silver/copper films treated with a passivating agent do not significantly change color, but become nonuniform, e.g., streaked. However, when the silver/copper film is overcoated with stainless steel in accordance with the present invention, as in this example, there is no significant visible change in the film, in either the color or the uniformity.

The above example is offered to illustrate the present invention. Various modifications, such as the use of other overcoat metals which form a dense oxide at the surface, the sputtering of other sensitive metals, and the coating of nonmetallic substrates other than glass, which need not be transparent, are included within the scope of the present invention as defined by the following claims.