TRANSPARENT CONDUCTIVE COATINGS

Abstract: An article having a transparent, conductive, infra-red reflective coating with improved stability, comprising a substrate having a coating comprising a metallic layer of a material such as silver, the coating having on at least one of its surfaces a dielectric metal oxide layer of a material such as tin oxide doped with a metal such as copper. Optionally the metallic layer is sandwiched between two such dielectric metal oxide layers.

Description: TRANSPARENT CONDUCTIVE COATINGS Technical field of the invention This invention relates to transparent, conductive, infrared reflective coatings and methods therefor. Background of the invention Thin coatings of noble metals such as silver, gold, and copper are often used as visible or solar light transparent, infrared light reflective, or electrically conductive coatings, or coatings combining a plurality of these characteristics. (As used hereinafter, "transparent" denotes substantial transparency to visible or solar light, and "conductive" denotes substantial electrical conductivity.) The visible or solar transmittance of such coatings can be increased dramatically by depositing dielectric anti- reflection layers on one or both sides thereof. Typical materials suitable for antireflection layers include indium oxide, indium tin oxide, tin oxide, titanium oxide, zinc sulfide, and tantalum oxide. These antireflection layers may additionally perform the function of environmentally protecting the metal coating, such as from corrosion or abrasion. Dielectric-metal-dielectric (DMD) multilayer coatings, in which a metal layer is sandwiched between two layers of dielectric material, are well known in the art. Berning et al., J __ Opt. Soc. Am. r H, 230 (1957) discloses a DMD coating having a silver layer and dielectric multilayer antireflection coatings on either side for use as band pass filters. Holland et al., Brit. J. Appl. Phys. , _&, 359 (1958) discloses a gold DMD coating which is transparent to visible light but infrared reflective. Additional publications related to DMD coatings are Plumat et al., CA 840,513 (1970) and Asahi Glass, GB 1,307,642 (1973) . Thin metal layers are also incorporated into more complicated optically selective multilayer coatings, such as Fabry-Perot filters wherein two or more thin metal layers are separated by dielectric spacers. Antireflection layers may additionally be placed on the exterior metal surfaces of such coatings to increase transmittance. Multilayer coatings are often used in architectural or automotive glazing applications, where high visible or solar light transmittance and infrared light reflectance are important, and in aircraft transparencies where high light transmittance and electrical conductivity are necessary. Further, it is very important that these coatings be environmentally stable, because of the harsh environments in which they are called to perform. However, the metal layers therein are usually very thin, often only 50-200 angstroms thick, and relatively fragile. Thin layers of certain metals such as silver are not particularly stable, especially at such extreme thinnesses, because of the metal's tendency to form sulfides or oxides (from reaction with chemicals in the environment) and to agglomerate. Antireflection layers may provide some protection by serving as corrosion barriers, but they themselves are also usually very thin, typically 200- 1000 angstroms thick, and cannot afford adequate protection, unless the multilayer coating is further protected by placing it in a laminate or other protective structure. Chiba et al, in US 4,166,876 (1979), discloses a DMD coating in which the metal layer is silver alloyed with up to 30% by weight of copper to improve its environmental stability. Yatabe, in US 4,234,654 (1980) similarly discloses alloying the silver with 3-30% gold in a DMD coating. Dietrich et al., in GB 2,135,697 A (1984), teaches a four- layer DMD coating in which the metal layer is composed of two layers: a silver layer and a second metal layer of aluminum, titanium, tantalum, chromium, magnesium, or zirconium. The second metal layer protects the silver layer during the subsequent deposition of a dielectric anti-reflection layer, and also improves mechanical strength and chemical resistance. The antireflection layer may be indium or tin oxide containing some lead oxide. A small amount of nickel (less than 1%) may be added to the silver. Hart, in US 4,462,883 (1984), discloses various heat mirror constructions in which a thin metal layer is placed over a silver layer to protect the latter during the subsequent deposition of a dielectric layer on top. The metal layer may be a transition metal, such as manganese, chromium, iron, platinum, copper, or gold, or a group Illa-Va element, such as bismuth, indium, or lead. Summary of the invention In this invention, it has been discovered that in a transparent, conductive, infra-red reflective coating comprising a metallic and dielectric layer, the environmental stability of the metal layer is enhanced if the dielectric layer is doped with a dopant metal. Accordingly, this invention provides an article having a transparent, conductive, infra-red reflective coating with improved stability, comprising a substrate having on a surface thereof a transparent, conductive, infra-red reflective coating comprising a metallic layer sandwiched between two dielectric metal oxide layers, at least one of which is doped with a dopant metal selected from the group consisting of copper, gold, iron, nickel, cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, aluminum, bismuth, lead, and alloys thereof. Such an article may be made be depositing on the substrate, preferably by sputtering, a first dielectric metal oxide layer, the metallic layer, and then a second dielectric metal oxide layer. In another embodiment of the invention there is provided an article having a transparent, conductive, infra-red reflective coating with improved stability, comprising a substrate having on a surface thereof a transparent, conductive, infra-red reflective coating comprising a metallic layer and a dielectric metal oxide layer doped with a dopant metal as defined above. This embodiment may be made by depositing on the substrate, preferably by sputtering, first a metallic layer and then the dielectric metal oxide layer, or alternatively, first a dielectric metal oxide layer and then the metallic layer. Brief description of the drawings Fig. la shows in cross-section an embodiment of the invention, comprising a substrate having a metallic layer protected on both sides. Fig. lb shows in cross-section another embodiment of the invention, comprising a substrate having a metallic layer protected on one side. Fig. 2 is an XPS depth profile of a prior art coating. Fig. 3 is an XPS depth profile of a coating according to this invention. Fig. 4 is an XPS depth profile of another coating according to this invention. Fig. 5 compares the reflectance and transmission characteristics of coatings of this invention and a prior art coating, before environmental stability testing. Fig. 6 compares the reflectance and transmission characteristics of coatings of this invention and a prior art coating, after environmental stability testing. pescript pn of preferred e bodiment? An embodiment of the invention is shown in Fig. la, which depicts in cross-section an article 1 comprising a substrate 2 having deposited thereon a DMD coating 3 comprising dielectric layers 4a and 4b and metallic layer 5. At least one of dielectric layers is doped with a dopant as described in more detail hereinbelow, improving corrosion resistance of the metallic layer. Both dielectric layers can be doped, although where the substrate is a relatively impervious material such as glass, the inner dielectric layer 4b need not be doped. Conversely, where the substrate is a relatively pervious material such as a thin polymeric film, it is preferred that both dielectric layers be doped. Yet another embodiment of the invention is shown in Fig. lb. Article 6 comprises a substrate 7 having deposited thereon a coating 8 comprising a dielectric layer 9 and metallic layer 10, the dielectric layer being doped as discussed above. Thus, this embodiment differs from the one shown in Fig. la in that there is no dielectric layer interposed between the substrate and the metallic layer. This embodiment may advantageously be used where the substrate is a relatively impervious material, so that there is a lesser threat of environmental attack from the substrate-facing side. Suitable dielectric materials for the dielectric layers include tin oxide, indium oxide, indium tin oxide, titanium oxide, zinc oxide and tantalum oxide. Preferred are tin, indium, or indium-tin oxide. While generally, as a matter of convenience, the dielectric layers are made of the same material, such is not a requirement. Suitable dopant metals include transition metals such as copper, gold, manganese, iron, nickel, cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, and aluminum; group Illa-Va elements, including bismuth, indium, tin, and lead; and alloys thereof. Copper and gold are preferred. The metallic layer which is to be protected can be made of various metals. Silver, copper, and gold, which are often used in visible/solar transparent but infrared reflective, conductive coatings, are especially benefitted. Other metals which can be protected include platinum, palladium, nickel, and rhodium. In a preferred embodiment, the dopant metal forms an oxide with a heat of formation greater than that of the metal oxide corresponding to the metal of the metallic layer but less than that of metal oxide of the dielectric layer. In other words, the heats of formation preferably satisfy the following inequality: dielectric metal oxide > oxide of dopant metal > oxide of metallic layer For example, if the metallic layer is silver and the dielectric layer is tin oxide, copper is a preferred dopant metal, as cupric oxide has a heat of formation of -167.5 kJ/mole, between that of silver oxide (-30.6 kJ/mole) and tin oxide (-580.7 kJ/mole) . While not wishing to be bound by theory, it is believed that in such an embodiment a corrosion protection effect is derived from the partial oxidation of the dopant metal in the dielectric layer. For the purposes of this invention, it is not necessary that the dielectric layer(s) be doped with the dopant metal across its entire thickness, although such an embodiment is not excluded. In a preferred embodiment of the invention, the dielectric layer(s) are doped only along those regions thereof near the metallic layer. This represents an effective compromise between providing a sufficient degree of protection without adversely affecting the transmissive qualities of the dielectric layer, and hence of the entire coating. Substantial variance in the amount of dopant metal used is permissible, as can readily be determined empirically by those skilled in the art. For a dopant metal such as copper, in a dielectric layer of tin oxide, doping levels of up to 28 atom % are possible, without seriously reducing the transmittance. With other dopants which form more transparent oxides than copper, such as aluminum, considerably higher doping levels, up to 50 atom %, are tolerated without serious reduction in transmittance, as shown in Ritchie et al., U.S. Pat. 4,710,441 (1987) . Statement of industrial utility The coatings of this invention have significantly improved environmental stability, but yet their functional properties (e.g., visible/solar transmittance and infrared reflectance) are not significantly impacted. These coatings are useful in visible/solar transmissive but infrared reflective conductive coatings, e.g, DMD or Fabry-Perot coatings. In a Fabry-Perot coating, plural metallic layers are sandwiched between dielectric layers, for example D1/M1/D2/M2/D3, D1/M1/D2/M2/D3/M3/D4, and so forth. Fabry- Perot coatings may be prepared by the alternating deposition of metallic and dielectric layers. The practice of this invention can be further understood by reference to the following examples, which are provided for the purposes of illustration and not of limitation. Example 1 This is a comparative example in which a conventional (undoped) coating not according to this invention is made. A conventional sputter roll coating machine was used to deposit a DMD coating on poly(ethylene terephthalate) (PET) film 4 mil thick. This DMD coating consisted of a silver layer 120 angstroms thick having antireflection layers of tin oxide 430 angstroms thick on either side. The silver oxide was deposited by sputtering from a silver target in an argon atmosphere, and the tin oxide layers were deposited from tin targets in an argon-oxygen atmosphere. An XPS depth profile of this coating is shown in Fig. 2. The properties of this conventional coating are compared against those of a coating according to this invention in a subsequent example. Exa le 2. In this example, an environmentally stabilized DMD coating according to this invention is made, in the preferred embodiment in which the dopant is not present throughout the dielectric layers, but only in those portions thereof near the metallic layer. Using the same apparatus as in Example 1, 4 mil PET was coated with a tin oxide layer 300 angstroms thick. Then, by reactive magnetron sputtering, a tin oxide layer doped with copper was deposited to thickness of 130 angstroms, using and 82:18 atom % Sn:Cu target in an argon/oxygen atmosphere. Next, a layer of silver 120 angstroms thick was deposited, as described above, followed by another layer of copper-doped tin oxide 130 angstroms thick, and finally followed by a 300 angstrom-thick tin oxide layer. An XPS depth profile of this coating, showing the distribution of the copper in the tin oxide dielectric layers, is shown in Fig. 3. Thus, a DMD coating having two dielectric layers of tin oxide sandwiching a conducting layer of silver 120 angstroms thick is prepared. Each of the tin oxide layers has a total thickness of 430 angstroms, of which the 130 angstroms nearest the silver layer are doped, while the regions further removed from the silver layer are not doped. Those skilled in the art will appreciate, herein, as a matter of convenience, the dielectric layers have been deposited stagewise, one doped and one undoped, but that such stage-wise deposition is not a requirement of this invention. Exam le 1 This example shows another DMD coating according to our invention, in the embodiment in which the dopant is distributed througout the dielectric layers. A dielectric layer 430 angstroms thick of tin oxide doped with copper was deposited on 4 mil PET by reactive magnetron sputtering in a sputter roll coater, using an 82:18 atom % Sn:Cu target in an argon/oxygen atmosphere. Next, an 120-angstrom silver layer was deposited, followed by another dielectric layer of copper-doped tin oxide the same as the first layer. An XPS depth profile of this coating is shown in Fig. 4. Example J. The environmental stability of the DMD coatings made in Examples 1 through 3 were compared. Pieces of each coated PET film were exposed to the outdoor environment of Los Angeles, California, for a period of 13 days in September by placing them, unprotected, on the roof of a building. The comparative results are provided in Table I below. -= •■ Table I DMD Coating Example 1 Example 2 Example 3 Initial Prooerties Resistance (ohm/sq) 10.0 8.5 16 VLT (%) {*) 83.6 81.9 81.1 Reflectance @ 2.5 μm 83.0 79.0 83.5 Visual appearance Good Good Good Properties after test Resistance (ohm/sq) 1200 84 265 VLT (%) (*) 55.7 68.7 68.1 Reflectance @ 2.5 μm 26.0 46.0 48.0 Visual appearance Many spots ;, Few spots, Few spots, mottled no mot¬ some mot¬ tling tling * VLT = visible light transmittance over 400-700 nm, weighted to the human eye's response The reflectance and transmission of these coatings are compared in Fig. 5 (before testing) and Fig. 6 (after testing) . These results show that the coatings of this invention have superior environmental stability, compared to the conventional coating. The outdoor exposure effectively destroyed the conventional coating, while the coatings of this invention still retained useful properties. It is cautioned that, where less severe tests were carried out, for example in an office ambient environment, or exposure to fumes in a chemical storage cabinet, or exposure in environmental chambers (e.g. 600 °C/95% relative humidity or 1000 °C dry heat) , the testing conditions may not be sufficient to differentiate between coatings of this invention and the conventional coating.

Claims: Claims What is claimed is : 1. An article having a transparent, conductive, infra-red reflective coating with improved stability, comprising a substrate having on a surface thereof a transparent, conductive, infra-red reflective coating comprising a metallic layer sandwiched between two dielectric metal oxide layers, at least one of which is doped with a dopant metal selected from the group consisting of copper, gold, iron, nickel, cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, aluminum, bismuth, lead, and alloys thereof. 2. An article according to claim 1, wherein both dielectric metal oxide layers are doped with a dopant metal. 3. An article according to claim 1 or 2, wherein said dielectric metal oxide layers are made of a material selected from the group consisting of tin oxide, indium oxide, indium tin oxide, titanium oxide, zinc sulfide, and tantalum oxide. 4. An article according to claim 3, wherein said dielectric metal oxide layers are made of a material selected from the group consisting of tin oxide, indium oxide, and indium tin oxide. 5. An article according to claim 1 or 2, wherein said dopant metal is selected from the group consisting of copper and gold. 6. An article according to claim 1 or 2, wherein said metallic layer is made of a metal selected from the group consisting of copper, gold, and silver. 7. An article according to claim 1 or 2, wherein said dopant metal is present in said dielectric metal oxide layers in the regions thereof near said metallic layer. 8. An article according to claim 1 or 2, wherein said coating is deposited by sputtering. 9. An article according to claim 1 or 2, wherein the heat of formation of the oxide of said dopant metal is greater than the heat of formation of the metal oxide of the metal of said metallic layer but lesser than the heat of formation of the metal oxide of said dielectric metal oxide layer. 10. An article according to claim 1 or 2, wherein said coating has deposited thereover at least one metallic and at least one dielectric layer, to form a Fabry-Perot coating. 11. A method of making an article having a transparent, conductive, infra-red reflective coating with improved stability, comprising the steps of providing a substrate; depositing on a surface of said substrate a first dielectric metal oxide layer; depositing on said first dielectric metal oxide layer a metallic layer; and depositing on said metallic layer a second dielectric metal oxide layer, thereby forming a transparent, conductive, infra-red reflective coating comprising metallic layer sandwiched between two dielectric metal oxide layers; at least one of said first and second dielectric metal oxide layers being deposited containing a dopant metal selected from the group consisting of copper, gold, iron, nickel, cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, aluminum, bismuth, lead, and alloys thereof. 12. A method according to claim 11, wherein both first and second dielectric metal oxide layers are deposited containing a dopant metal. 13. A method according to claim 11 or 12, wherein said dielectric metal oxide layers are made of a material selected from the group consisting of tin oxide, indium oxide, indium tin oxide, titanium oxide, zinc sulfide, and tantalum oxide. 14. A method according to claim 13, wherein said dielectric metal oxide layers are made of a material selected from the group consisting of tin oxide, indium oxide, and indium tin oxide. 15. A method according to claim 11 or 12, wherein said dopant metal is selected from the group consisting of copper and gold. 16. A method according to claim 11 or 12, wherein said metallic layer is made of a metal selected from the group consisting of copper, gold, and silver. 17. A method according to claim 11 or 12, wherein said dopant metal is deposited in said dielectric metal oxide layers in the regions thereof near said metallic layer. 18. A method according to claim 11 or 12, wherein said coating is deposited by sputtering. 19. A method according to claim 11 or 12, wherein the heat of formation of the oxide of said dopant metal is greater than the heat of formation of the metal oxide of the metal of said metallic layer but lesser than the heat of formation of the metal oxide of said dielectric metal oxide layer. 20. A method according to claim 11 or 12, wherein at least one further metallic layer and at least one further dielectric layer are deposited on said coating, to form s a Fabry-Perot coating. 21. An article having a transparent, conductive, infra-red reflective coating with improved stability, comprising a substrate having on a surface thereof o a transparent, conductive, infra-red reflective coating comprising a metallic layer and a dielectric metal oxide layer doped with a dopant metal selected from the group consisting of copper, gold, iron, nickel, 5 cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, aluminum, bismuth, lead, and alloys thereof. 0 22. An article according to claim 21, wherein the metallic layer is in contact with the substrate. 23. An article according to claim 21 or 22, wherein said dielectric metal oxide layer is made of a material 5 selected from the group consisting of tin oxide, indium oxide, indium tin oxide, titanium oxide, zinc sulfide, and tantalum oxide. 24. An article according to claim 23, wherein said dielectric metal oxide layer is made of a material selected from the group consisting of tin oxide, indium oxide, and indium tin oxide. 25. An article according to claim 21 or 22, wherein said dopant metal is selected from the group consisting of copper and gold. 26. An article according to claim 21 or 22, wherein said metallic layer is made of a metal selected from the group consisting of copper, gold, and silver. 27. An article according to claim 21 or 22, wherein said dopant metal is present in said dielectric metal oxide layer in the region thereof near said metallic layer. 28. An article according to claim 21 or 22, wherein said coating is deposited by sputtering. 29. An article according to claim 21 or 22, wherein the heat of formation of the oxide of said dopant metal is greater than the heat of formation of the metal oxide of the metal of said metallic layer but lesser than the heat of formation of the metal oxide of said dielectric metal oxide layer. 30. A method of making an article having a transparent, conductive, infra-red reflective coating with improved stability, comprising the steps of prpviding a substrate; depositing on a surface of said substrate a metallic layer; and depositing on said metallic layer a dielectric metal oxide layer containing a dopant metal selected from the group consisting of copper, gold, iron, nickel, cobalt, molybdenum, tungsten, platinum, vanadium, tantalum, titanium, chromium, magnesium, zirconium, nickel, aluminum, bismuth, lead, and alloys thereof, thereby forming a transparent, conductive, infra-red reflective coating comprising metallic layer and a dielectric metal oxide layer. 31. A method according to claim 30, wherein said dielectric metal oxide layers is made of a material selected from the group consisting of tin oxide, indium oxide, indium tin oxide, titanium oxide, zinc sulfide, and tantalum oxide. 32. A method according to claim 31, wherein said dielectric metal oxide layer is made of a material selected from the group consisting of tin oxide, indium oxide, and indium tin oxide. 33. A method according to claim 30, wherein said dopant metal is selected from the group consisting of copper and gold. 34. A method according to claim 30, wherein said metallic layer is made of a metal selected from the group consisting of copper, gold, and silver. 35. A method according to claim 30, wherein said dopant metal is deposited in said dielectric metal oxide layer in the region thereof near said metallic layer. 36. A method according to claim 30, wherein said coating is deposited by sputtering. 37. A method according to claim 30, wherein the heat of formation of the oxide of said dopant metal is greater than the heat of formation of the metal oxide of the metal of said metallic layer but lesser than the heat of formation of the metal oxide of said dielectric metal oxide layer.

Inventors: KITTLER WILFRED C, KITTLER, WILFRED, C.
Applicants: ANDUS CORP, ANDUS CORPORATION
Application year: 1990
Main classification: C03C17
IPC codes: B32B9, B32B15, C03C17