Characteristics of oxide thin films from carboxylate precursors
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Oxide thin films on Si substrates were prepared from carboxylate precursors by the reaction of the metal nitrates and ammonium trimethylacetate. Precursor salts were characterized with respect to purity, structure, thermal pyrolysis, and phase development during calcination. A solvent system, based on carboxylic acid/amine mixture, was developed to dissolve the synthesized precursors, resulting in increased solubility, viscosity, and stability. Smooth, fine-grained ZrO 2 , Y2O3, and YSZ films were obtained on Si wafers by spin-coating and subsequent heat treatment above 500 °C. Films heat treated below 700 °C were generally adherent, amorphous, or microcrystalline, while YSZ and ZrO 2 showed (111) preferred orientation above 700 °C. These oxide films show promise as protective or buffer layers on Si wafers.
I. INTRODUCTION
nRCOONH 4 ( s ) + M ( N 0 3 ) n ( o , )
Oxide thin films for device applications have traditionally been prepared by chemical vapor deposition as well as by various sputtering or electrodeposition techniques. Although good quality films are prepared by these techniques, high facility cost, low deposition rate, sample geometry limitation, materials constraint, and sometimes difficulties in stoichiometry control may be disadvantages. Recently, the solution deposition method has proven to be an alternative route in preparing high quality thin films for optical, electronic, and protective applications.1^1 Liquid precursor methods offer the advantage of atomic scale mixing of chemical species and close control of the deposition process. Preparation methods for the chemically derived ceramic films include the use of sol-gel, metal salts, and carboxylate precursors. Metal alkoxides are generally used as precursors in the sol-gel method, which then undergo hydrolysis and polycondensation reactions to form a three-dimensional network structure. Control of water content and pH is required for specific applications,5 and since most alkoxides are moisture sensitive, humidity must be controlled during processing. Film densification is usually achieved at temperatures markedly below those needed for bulk solid state sintering. The metal carboxylates (such as neodecanoate, neooctoate, and neopentanoate) represent another precursor route for thin film preparation.6 Equations (1) and (2) show how these metal carboxylates can be synthesized. RCOOH (1) + NH 4 OH ( a q ) => RCOONH4(S)
H2O(1) (1)
1736 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 8, Aug 1991
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>M(RC00)n(ppt) + nNH4NO3(aq)
(2) where R = C9H19, C7H15, C 4 H 9 and M = metal ions. Typically, these compounds are not moisture sensitive and can be dissolved in suitable solvents. Since hydrolysis and polycondensation reactions are not involved in the solution stage of preparation, shelf-life can be extensive. Gelation, however, can be achieved by modifying the surface charge on the solute particles and by controlling the ion concentration in the solution. Polymerization reactions have been reported to occur during t
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