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This invention was made at least in part with Government support under Air Force Office of Scientific Research grant number AFOSR-90-0279, Office of Naval Research grant number N00014-88-K-0331, and National Science Foundation grant number DMR 88 18 558-A02. The Government has certain rights in the invention.
Technical Field
This invention is directed at ceramic substrates coated with thin films having improved continuity, improved adhesion and improved electrical conductivity properties.
Background of the Invention
Thin film metallization of ceramics is important in at least four technologies. Firstly, in electronic packaging, metal lines are used in multilayer ceramic structures for electrical interconnects. Secondly, very thin metal films supported on ceramic substrates are used as catalysts, for example, in automobile catalytic converters. Thirdly, metal coated ceramic fibers are employed to fabricate metal matrix composites by liquid metal infiltration. Fourthly, devices that employ metal coated active or functional ceramic materials find application in optical, sensor and superconductor technologies.
In all these cases there is a need for providing thin continuous and/or patterned metal films on ceramics. In the case of electronic packaging, metal line density is limited because of the tendency of thin films to become discontinuous. In the case of catalysts, discontinuities in the film lowers efficiency. In the case of metal matrix composites fabricated from fibers, a discontinuous metal coating requires higher infiltration pressure on the liquid metal.
Kennefick, C. M. , et al, Acta Metall. Vol. 37, No. 11, pp. 2947-2952 (1989) discuss the problem of the instability of thin metal films when heated to a temperature ranging from 25% of the melting point expressed in degrees Kelvin up to just below the melting point. The instability manifests in the fragmentation of the metal film into islands, hereinafter referred to as beading of the thin film. The same phenomenon is important in all of the applications described above.
The terms "thin metal films" and "thin metal coatings" are used herein to indicate thickness normal to the substrate of at least about 10 nm and less than 10 microns.
Summary of the Invention
It is an object herein to overcome the beading problem by providing a novel composite composition which does not bead.
This novel composite composition is a ceramic substrate coated with a continuous or patterned film of thickness less than 10 micrometers comprising an intimate mixture of at least one transition element metal and at least one element different from said transition metal element which is from the same group of the Periodic Table of Elements as the metallic or semimetallic atom of the ceramic of the substrate.
The term "substrate" is used herein to mean supporting means or a layer coating on a different supporting means.
The distinction between continuous and patterned films as related to the instant invention is that a continuous film covers a full surface while a patterned film consists of interconnected metal lines.
The term "intimate mixture" is used herein to mean that the said one transition element metal and said at least one different element are admixed on an atomic level so as to be homogeneous on a scale of 5 nm.
Detailed Description of the Invention
The ceramic substrate can be formed of oxides, nitrides, carbides or borides. Suitable ceramic oxides include, for example, alumina, zirconia, beryllia, silica, magnesia, glasses and glass ceramics, and superconductor oxides, such as yttrium-barium-cuprate and superconducting oxides derived from bismuth, strontium, calcium, and thallium and perovskites, such as PZT, PLZT, PMN, barium titanate, potassium niobate, and lithium niobate. Suitable ceramic nitrides include, for example, silicon nitride, aluminum nitride and titanium nitride. Suitable ceramic carbides include, for example, silicon carbide. Suitable ceramic borides include, for example, titanium boride. The term "metallic or semimetallic atom of the ceramic" means the atom of the ceramic which is not oxygen in the oxides, which is not nitrogen in the nitrides, which is not carbon in the carbides and which is not boron in the borides.
The transition element metal component of the intimate mixture can be, for example, titanium, iron, nickel, copper, palladium, platinum, zirconium, niobium, gold, silver, tungsten and tantalum.
The different element component of the intimate mixture, as indicated above, is one that belongs to the same Periodic Table group as the metallic or semimetallic atom of the ceramic and preferably is the same as the metallic or semimetallic atom of the ceramic. For example, when alumina is the ceramic, said different element can be aluminum or boron, and when zirconia is the ceramic, said different element can be zirconium or titanium. Normally, said different element is selected from the group consisting of metals and semimetals from Groups II, III, IV, VIB and VIII of the Periodic Table, e.g., beryllium, magnesium, calcium, barium, aluminum, scandium, yttrium, cerium, thorium, uranium, silicon, zirconium, titanium, chromium, nickel, cobalt, iron or ruthenium.
The weight percentage of transition element metal in said intimate mixture can vary widely and can be as little as 0.1% or as much as 99.9%, with the remainder being said different element.
The film comprising said intimate mixture preferably has a thickness less than 150 nm. Normally, said film has a thickness of at least 15 nm but films having thickness of 10 nm or less can find application in some instances, e.g., in catalysts.
Turning now to methods for making the coated ceramic substrate herein, these methods preferably involve intimately mixing the constituents forming said intimate mixture. One way of accomplishing this is by premixing said constituents to form an alloy and applying this alloy as a coating by means of sputtering or physical vapor deposition. Another way of accomplishing this is to prepare said alloy and form it into a fine powder and apply it to the substrate using the well-established ink and screen printing techniques and sintering. Yet another way of accomplishing this is to codeposit said constituents by physical deposition on the substrate so that intimate mixing is achieved in situ.
The intimate mixture can be converted to an intermetallic compound and the benefits of the invention are preserved. This conversion is carried out, for example, by heating at 300° to

1200° C for 5 minutes to 50 hours in air or a reducing atmosphere (e.g., at an oxygen partial pressure ranging from 10-1 to 10~35 atmospheres) .

The invention is illustrated by the following specific example.
Coated ceramic substrates were formed by codepositing aluminum and palladium (50% of each by volume) by electron beam deposition in a vacuum chamber (approximately 10~6 torr.) on sapphire substrates (5 mm x 5 mm x 1 mm) to provide a coating of thickness of 120 nm.
In another case, coated ceramic substrates were made the same as above but the film deposited was of palladium only (120 nm thick) .
The relative stability, i.e., beading property of the two types of coated substrates, was evaluated by heat treating the coatings at 600° C and 750° C in air or in hydrogen having the dew point of liquid nitrogen, for two hours. The beading phenomenon was observed microscopically in a scanning electron microscope and also was characterized by measuring the electrical resistance of the metal coating using the well-known four point contact method.
The microscopic examination showed that the palladium only films beaded into separate islands.

These films exhibited a very high electrical resistance that ranged from 16 kiloohms to several megaohms. In contrast, the Pd-Al codeposited films were seen under microscopic examination to have remained continuous and yielded an electrical resistance of less than 25 ohms.
When the Pd:Al volume ratio is varied from 25:1 to 1:25, the same advantage of no beading is derived.
When platinum is substituted for all or part of the palladium, the same advantage of no beading is retained.
When glass or glass ceramic containing alumina and silica is substituted for the sapphire (alumina) in the above test, the same advantage of no beading is retained.
When zirconia is substituted for the sapphire (alumina) and zirconium or titanium is substituted for the aluminum, the same advantage of no beading is retained.
When aluminum nitride is substituted for the sapphire (alumina) , the same advantage of no beading is retained.
When barium titanate is substituted for the sapphire (alumina) , and titanium or zirconium is substituted for the aluminum, the same advantage of no beading is retained. When in this case, silver is substituted for the palladium, the same advantage of no beading is retained.

When PZT is substituted for the sapphire

(alumina) , and zirconium or titanium is used in conjunction with either silver or palladium or platinum, the same advantage of no beading is obtained.
When the ceramic substrate is silica and silicon is used in conjunction with nickel or palladium for intimate mixture which is applied to the substrate, the same advantage of no beading is obtained.
Variations will be obvious to those skilled in the art. Thus, the scope of the invention is defined in the claims.