Neutral Sputtered Films of Metal Alloy Oxides

The present invention relates generally to the art of cathode sputtering of metal oxide films, and more particularly to the art of magnetic sputtering of multiple layer films of metal and metal oxide.

U.S. Pat. No. 4,094,763 to Gillery et al discloses producing transparent, electroconductive articles by cathode sputtering metals such as tin and indium onto refractory substrates such as glass at a temperature above 400.degree. F. in a low pressure atmosphere containing a controlled amount of oxygen.

U.S. Pat. No. 4,113,599 to Gillery teaches a cathode sputtering technique for the reactive deposition of indium oxide in which the flow rate of oxygen is adjusted to maintain a constant discharge current while the flow rate of argon is adjusted to maintain a constant pressure in the sputtering chamber.

U.S. Pat. No. 4,166,018 to Chapin describes a sputtering apparatus in which a magnetic field is formed adjacent a planar sputtering surface, the field comprising arching lines of flux over a closed loop erosion region on the sputtering surface.

U.S. Pat. No. 4,201,649 to Gillery discloses a method for making low resistance indium oxide thin films by first depositing a very thin primer layer of indium oxide at low temperature before heating the substrate to deposit the major thickness of the conductive layer of indium oxide by cathode sputtering at typically high cathode sputtering temperatures.

U.S. Pat. No. 4,327,967 to Groth discloses a heat-reflecting panel having a neutral-color outer appearance, comprising a glass pane, an interference film having a refractive index greater than 2 on the glass surface, a heat reflecting gold film over the interference film and a neutralization film of chromium, iron, nickel, titanium or alloys thereof over the gold film.

U.S. Pat. No. 4,349,425 to Miyake et al discloses d-c reactive sputtering of cadmium-tin alloys in argon-oxygen mixtures to form cadmium-tin oxide films having low electrical resistivity and high optical transparency.

U.S. Pat. No. 4,462,883 to Hart discloses a low emissivity coating produced by cathode sputtering a layer of silver, a small amount of metal other than silver, and an antireflection layer of metal oxide onto a transparent substrate such as glass. The antireflection layer may be tin oxide, titanium oxide, zinc oxide, indium oxide, bismuth oxide or zirconium oxide.

U.S. Pat. No. Re 27,473 to Mauer discloses a multilayer transparent article comprising a thin layer of gold or copper sandwiched between two layers of transparent material such as various metals, titanium oxide, lead oxide or bismuth oxide.

In the interest of improving the energy efficiency of double-glazed window units, it is desirable to provide a coating on one of the glass surfaces which increases the insulating capability of the unit by reducing radiative heat transfer. The coating therefore must have a low emissivity in the infrared wavelength range of the radiation spectrum. For practical reasons, the coating must have a high transmittance in the visible wavelength range. For aesthetic reasons, the coating should have a low luminous reflectance and preferably be essentially colorless.

High transmittance, low emissivity coatings as described above generally comprise a thin metallic layer, for infrared reflectance and low emissivity, sandwiched between dielectric layers of metal oxides to reduce the visible reflectance. These multiple layer films are typically produced by cathode sputtering, especially magnetron sputtering. The metallic layer may be gold or copper, but is generally silver. The metal oxide layers described in the prior art include tin oxide, indium oxide, titanium oxide, bismuth oxide, zinc oxide, zirconium oxide, and lead oxide. In some cases, these oxides incorporate small amounts of other metals, such as manganese in bismuth oxide, indium in tin oxide and vice versa, to overcome certain disadvantages such as poor durability or marginal emissivity. However, all of these metal oxides have some deficiency.

Although the coating may be maintained on an interior surface of a double-glazed window unit in use, where it is protected from the elements and environmental agents which would cause its deterioration, a durable effective coating able to withstand handling, packaging, washing and other fabrication processes encountered between manufacture and installation is particularly desirable. These properties are sought in the metal oxide. However, in addition to hardness which provides mechanical durability, inertness which provides chemical durability, and good adhesion to both the glass and the metal layer, the metal oxide should have the following properties as well.

The metal oxide must have a reasonably high refractive index, preferably greater than 2.0, to reduce the reflection of the metallic layer and thus enhance the transmittance of the coated product. The metal oxide must also have minimal absorption to maximize the transmittance of the coated product. For commercial reasons, the metal oxide should be reasonably priced, have a relatively fast deposition rate by magnetron sputtering, and be nontoxic.

Perhaps the most important, and most difficult to satisfy, requirements of the metal oxide film relate to its interaction with the metallic film. The metal oxide film must have low porosity, to protect the underlying metallic film from external agents, and low diffusivity for the metal to maintain the integrity of the separate layers. Finally, and above all, the metal oxide must provide a good nucleation surface for the deposition of the metallic layer, so that a continuous metallic film can be deposited with minimum resistance and maximum transmittance. The characteristics of continuous and discontinuous silver films are described in U.S. Pat. No. 4,462,884 to Gillery et al, the disclosure of which is incorporated herein by reference.

Of the metal oxide films in general use, zinc oxide and bismuth oxide are insufficiently durable, being soluble in both acid and alkaline agents, degraded by fingerprints, and destroyed in salt, sulfur dioxide and humidity tests. Indium oxide, preferably doped with tin, is more durable; however, indium sputters slowly and is relatively expensive. Tin oxide, which may be doped with indium or antimony, is also more durable, but does not provide a suitable surface for nucleation of the silver film, resulting in high resistance and low transmittance. The characteristics of a metal oxide film which result in proper nucleation of a subsequently deposited silver film have not been established; however, trial-and-error experimentation has been widely practiced with the metal oxides described above.

U.S. Pat. No. 4,610,771 to Gillery, the disclosure of which is incorporated herein by reference, provides a novel film composition of an oxide of a metal alloy, as well as a novel multiple-layer film of metal and metal alloy oxide layers for use as a high transmittance, low emissivity coating.

The present invention improves the durability of metal alloy oxide films, especially multiple layer films comprising metal alloy oxides and metals such as silver, by providing a titanium oxide layer which improves the adhesion between the metal and metal oxide layers and also produces a coated article which is visually neutral. Because the titanium oxide film of the present invention is thicker than conventional primer layers, it may be advantageous to deposit a slightly thinner underlying antireflective metal oxide layer.

FIG. 1 is a chromaticity diagram illustrating the chromaticity coordinates of a film of the prior art and films of the present invention as described herein and the theoretical color movements of various films of Zn.sub.2 SnO.sub.4 /TiO.sub.2 /Ag/TiO.sub.2 /Zn.sub.2 SnO.sub.4 with different thicknesses of Zn.sub.2 SnO.sub.4 and TiO.sub.2. Solid lines track color movement when the Zn.sub.2 SnO.sub.4 layers are maintained at a constant thickness, and double lines track color movement when Zn.sub.2 SnO.sub.4 layer thickness is decreased adjacent to the TiO.sub.2 layer to compensate the optical thickness.

In a three-layer induced transmission type coating where a silver, copper or gold layer is antireflected with two transparent oxide coatings, more silver can be incorporated without changing the color or reflectance of the coating by using oxide coatings with a higher refractive index. Unfortunately, most fast sputtering oxides such as those of tin, indium, zinc have refractive indices of about 2.0. The higher refractive index transparent oxides such as titanium oxide or zirconium oxide sputter too slowly to be commercially practical.

However, it has been discovered that relatively small amounts of, for example, titanium oxide, can be useful in creating the desired high refractive index effect if it is incorporated in the optimum location in the film sequence, specifically between the first deposited antireflective metal oxide layer and the infrared reflective metal, such as silver. Deposition of titanium oxide in other positions in the coating sequence is relatively ineffective. To prevent undesirable color changes in the multilayer coating it is usually necessary to remove optical thickness from the adjacent oxide layer; that is, to make the underlying antireflective metal oxide layer slightly thinner. The invention can be made clearer by reference to FIG. 1.

The color of a coating on the chromaticity diagram moves from the point A, where x=0.283, y=0.303 to the point B, where x=0.294, y=0.295 as the silver layer is increased in thickness from 95 Angstroms to 110 Angstroms. A change in the thickness of the oxide layers moves the color in directions about perpendicular to this direction. Thus, only by using a thin silver layer can the ideal optical properties of color and reflectance of the 95 Angstroms film point be retained. However, a thin silver layer usually implies inferior properties of the coating such as electrical conductivity, reduced solar heat reflectance or reduced infrared reflectance.

By incorporating 50 Angstroms of titanium oxide between the antireflective metal oxide and the silver as taught in the present invention, the chromaticity coordinates of the film incorporating 110 Angstroms of silver are moved from the point B, where x=0.294, y=0.295 back to the point C, where x=0.284, y=0.301. The first antireflective metal oxide layer in this case is decreased in thickness from 380 Angstroms to 320 Angstroms to compensate for the optical thickness of the titanium oxide.

A preferred antireflective metal oxide film composition comprising an oxide of a metal alloy is preferably deposited by cathode sputtering, particularly magnetron sputtering. Cathode targets are preferably prepared comprising the desired ratio of metal elements. The targets are then sputtered in a reactive atmosphere, preferably containing oxygen in order to deposit a metal alloy oxide film on a surface of a substrate.

A preferred metal alloy oxide in accordance with the present invention is an oxide of an alloy comprising zinc and tin. A zinc/tin alloy oxide film may be deposited in accordance with the present invention by cathode sputtering, preferably magnetically enhanced. Cathode sputtering is also a preferred method for depositing high transmittance, low emissivity films in accordance with the present invention. Such films typically comprise multiple layers, preferably a layer of a highly reflective metal such as gold or silver sandwiched between antireflective metal oxide layers such as indium oxide or titanium oxide, or preferably an oxide of an alloy of zinc and tin which preferably comprises zinc stannate.

While various metals may be sputtered to form metal alloy oxide films, in order to produce a preferred high transmittance, low emissivity multiple layer film in accordance with the present invention, alloys of tin and zinc are preferred. A particularly preferred alloy comprises zinc and tin, preferably in proportions of 10 to 90 percent zinc and 90 to 10 percent tin. A preferred zinc/tin alloy ranges from 30 to 60 percent zinc, preferably having a zinc/tin ratio from 40:60 to 60:40. A most preferred range is 46:54 to 50:50 by weight tin to zinc. A cathode of zinc/tin alloy reactively sputtered in an oxidizing atmosphere results in the deposition of metal oxide layer comprising zinc, tin and oxygen, preferably comprising zinc stannate, Zn.sub.2 SnO.sub.4.

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. At least one cathode target surface preferably comprises a metal alloy such as zinc/tin which is sputtered in a reactive atmosphere to form a metal alloy oxide film. Alternatively, separate targets of zinc and tin may be sputtered substantially simultaneously. The anode is preferably a symmetrically designed and positioned assembly as taught in U.S. Pat. No. 4,478,702 to Gillery et al, the disclosure of which is incorporated herein by reference.

In a preferred embodiment of the present invention, a multiple layer film is deposited by cathode sputtering to form a high transmittance, low emissivity coating. In addition to the metal alloy target, at least one other cathode target surface comprises a metal to be sputtered to form a reflective metallic layer. At least one additional cathode target surface comprises titanium for sputtering in an oxidizing atmosphere to deposit a titanium oxide layer. A durable multiple layer coating having a reflective metallic film in combination with an antireflective metal alloy oxide film is produced as follows, using a titanium oxide layer to improve the adhesion between the metal and metal oxide films while producing a neutral coated article.

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

After the initial layer of zinc/tin alloy oxide is deposited, a cathode having a target surface of titanium metal is sputtered to deposit a layer of titanium oxide over the zinc/tin alloy oxide layer. The titanium oxide layer is preferably about 50 to 100 Angstroms thick, significantly thicker than conventional primer layers. A cathode having a target surface of silver is then sputtered to deposit a reflective layer of metallic silver over the titanium oxide layer which improves the adhesion of the silver film to the underlying metal oxide film while producing a visually neutral multiple layer low emissivity coating. A thicker silver film may be deposited in accordance with the present invention without altering the spectral properties of the multilayer film. An additional primer layer is then deposited by sputtering a metal such as copper or titanium over the reflective silver layer to improve the adhesion between the silver film and the overlying metal oxide film subsequently deposited. Finally, a second layer of zinc/tin alloy oxide is deposited over the second primer layer under essentially the same conditions used to deposit the first zinc/tin alloy oxide layer.

In most preferred embodiments of the present invention, a protective overcoat is deposited over the final metal oxide film. The protective overcoat is preferably deposited by sputtering over the metal oxide film a layer of a metal such as disclosed in U.S. Pat. No. 4,594,137 to Gillery et al. Suitable metals for the protective overcoat include alloys of iron or nickel, such as stainless steel or Inconel. Titanium is a most preferred overcoat because of its high transmittance. In an alternative embodiment, the protective layer may be a particularly chemical resistant material such as titanium oxide as disclosed in U.S. Pat. No. 4,716,086 to Gillery et al, the disclosure of which is incorporated herein by reference.

The present invention will be further understood from the description of a specific example which follows. In the example, the zinc/tin alloy oxide film is referred to as zinc stannate although the film composition need not be precisely Zn.sub.2 SnO.sub.4.

A multiple layer film is deposited on a soda-lime-silica glass substrate to produce a high transmittance, low emissivity coated product. A stationary cathode measuring 5 by 17 inches (12.7 by 43.2 centimeters) comprises a sputtering surface of zinc/tin alloy consisting of 52.4 weight percent zinc and 47.6 percent tin. A soda-lime-silica glass substrate is placed in the coating chamber which is evaluated to establish a pressure of 4 millitorr in an atmosphere of 50/50 argon/oxygen. The cathode is sputtered in a magnetic field at a power of 1.7 kilowatts while the glass is conveyed past the sputtering surface at a rate of 110 inches (2.8 meters) per minute. A film of zinc stannate is deposited on the glass surface. Three passes produce a film thickness of about 320 Angstroms, resulting in a decrease in transmittance from 90 percent for the glass substrate to 82 percent for the zinc stannate coated glass substrate. A stationary cathode with a titanium target is then sputtered to produce a titanium oxide layer about 50 Angstroms thick over the zinc stannate, reducing the transmittance to about 78 percent. Next, a layer of silver is deposited over the titanium oxide layer by sputtering a silver cathode target in an atmosphere of argon gas at a pressure of 4 millitorr to deposit silver to a film thickness of about 110 Angstroms, decreasing the transmittance of the coated substrate to about 66 percent. A thin neutral titanium primer layer is sputtered over the silver layer to improve the adhesion and protect the silver layer before the final antireflective layer of zinc stannate is deposited. The titanium primer layer is about 10 Angstroms thick and in the metallic state reduces the transmittance to about 60 percent. However, the titanium primer layer oxidizes and the transmittance increases as the subsequent metal oxide layer is deposited. Finally, the zinc/tin alloy cathode target is sputtered in an oxidizing atmosphere to produce a zinc stannate film. Four passes at a rate of 110 inches (2.8 meters) per minute produce a film thickness of about 380 Angstroms, increasing the transmittance of the coated product to 85 percent. The final coated product has a surface resistance of 8 ohms per square and a neutral reflectance from both sides, with a luminous reflectance of 5 percent from the coated side and 6 percent from the uncoated side.

The above example is offered to illustrate the present invention. Various modifications of the product and the process are included. For example, other coating compositions are within the scope of the present invention. Depending on the proportions of zinc and tin when a zinc/tin alloy is sputtered, the coating may contain widely varying amounts of zinc oxide and tin oxide in addition to zinc stannate. The adhesion between a wide variety of metal and metal oxide films may be improved by means of titanium oxide layers to produce visually neutral coatings in accordance with the present invention. Since the process does not require very high temperatures, substrates other than glass, such as various plastics, may be coated. A scanning cathode may be used with a stationary substrate. Process parameters such as pressure and concentration of gases may be varied over a broad range. Primer layers may comprise other metals such as zirconium which produce oxides which provide neutral reflectance in the multiple layer low emissivity coatings of the present invention. The scope of the present invention is defined by the following claims.