The present invention relates generally to the preparation of metal oxide thin films and more particularly to the preparation of solution-derived metal oxide thin films from metal diketonate or carboxylate precursors.
Metal oxide thin films have applications ranging from catalysts to capacitors. For example, ruthenium oxide materials can be used in electrochemical capacitors and nickel-, cobalt- and rhodium-oxide thin films are useful in catalysis of hydrocarbon oxidation reactions.
Solution routes are widely used for the production of thin films through spin-casting or dip-coating methodologies. These methods are typically used due to the flexibility in the stoichiometry of precursor solutions, the ease of altering processing variables, cost effectiveness (inexpensive), and the reduction of the sintering temperatures.
Sol-gel methods are often used to produce thin films but sol-gel processing generally suffers from limitations due to the relative solubilities of the various metals salts and metal alkoxides. Therefore, the starting materials are often modified to permit adequate solubility, adding processing steps to the method. These methods generally require synthesis of novel starting materials, relatively long mixing times, and/or heating during preparation of the desired precursor solutions. Miller et al., in U.S. Pat. No. 5,116,643, issued on May 26, 1992, as well as in U.S. Pat. No. 4,946,710 issued on Aug. 7, 1990, and in U.S. Pat. No. 5,028,455 issued on Jul. 2, 1991, describe a sol-gel method for producing ferroelectric thin films using alcohol and acid solvents in proportions sufficient to ensure equal reaction rates. However, Miller et al. also require heating to drive off the solvent, the addition of further reagents to quench reactivity and the introduction of water to hydrolyze the produced precursors.
Hampden-Smith, in U.S. Pat. No. 5,308,601 issued on May 3, 1994, describes a metallo-organic decomposition method for making metal oxides at low temperatures. Again, because of the problems associated with achieving adequate solubility, the method of Hampden-Smith requires synthesis of the starting materials as well as acid modification of all precursors and the addition of water.
One standard method of producing metal oxide solutions has been to decompose or hydrolyze metal halide compounds to yield the metal oxide. For example, Zheng et al. (U.S. Pat. No. 5,851,506, issued on Dec. 22, 1998) and Jow et al. (U.S. Pat. No. 5,600,535, issued on Feb. 4, 1997) also discuss a method for making ruthenium oxide by hydrolyzing ruthenium compounds, and more particularly ruthenium halide compounds, in an aqueous solution at an elevated temperature with the addition of a hydroxide compound. Zheng et al. utilized this process to eliminate the need to form the precursor metal oxide solution by decomposing ruthenium chloride at elevated temperatures of 300-400.degree. C. as was previously done in prior art.
Tomita (U.S. Pat. No. 5,256,443, issued on Oct. 26, 1993) discuss a method of preparing thin films from a sol solution containing a noble metal alkoxide in an organic solvent for use in sensor applications. Tomita starts with one or more metal alkoxides in an alcohol solvent with acetic acid as a catalyzing agent to form a solution that can be deposited as a thin film on a substrate.
Boyle et al. (U.S. Pat. No. 5,858,451, issued on Jan. 12, 1999) present a solution-route method for synthesizing ferroelectric material thin films of the general formula (Pb,La)(Nb,Sn,Zr,Ti)O.sub.3 using multiple metal precursor solutions dissolved in amine and other solvents where the metal precursor compounds can be metal acetates or metal acetylacetonates.
Improvements to the preparation of metal-oxide thin film materials by these methods would be to reduce the preparation or synthesis time, to prepare the materials using commercially-available starting materials, to reduce the concentration of impurities in the final film, and to reduce the severity of the preparation conditions, particularly the severity of the heating conditions. Once the precursor solutions have been prepared, either thin films or powders can be prepared by standard methods.