Use of sol-gels in the application of ceramic oxide thin films

Abstract

<p>Metal oxide sol-gels were produced through a controlled hydrolysis-condensation reaction of the metal alkoxide in alcohol using nitric acid as a catalyst. In obtaining sols of niobia, tantala, and titania, increasing either the water content or the system concentration increased the rate of gelation. However, while a sufficient amount of nitric acid was required to promote the condensation of the metal oxide and inhibit the precipitation, increasing the amount of nitric acid beyond that point slowed the gelation rate. A sufficient excess of water also lead to precipitation. These systematic parameter investigations were used to obtain sols of bismuth oxide, germania, yttria, and zirconia;The single component sols were characterized and used to make thin films on silicon wafers. The empirical equation, d = (2000 log([mu]) + 350) (w)[superscript]-0.5, was obtained, which adequately describes the thickness of the sintered films, independent of the ceramic oxide, solvent, concentration, sol age, and reaction conditions;The single component sols were used to make binary oxide sols--barium titanate, lithium niobate, and lithium tantalate. A barium solution can be added to the titania sol to produce a barium titanate compound of the desired composition, including Ba[subscript]2Ti[subscript]9O[subscript]20 and BaTi[subscript]5O[subscript]11. Mixing a lithium solution into either a niobia or a tantala sol yielded the desired compound along with a heavy metal oxide (either lithium niobate with niobium oxide or lithium tantalate with tantalum oxide). However, dissolving lithium methoxide with the ethoxide of niobium or tantalum in ethanol, prior to hydrolysis, did produce the desired binary oxide (lithium niobate or lithium tantalate) without any uncombined metal oxide;By altering the stoichiometric ratios of the constituent cations, different compounds were produced for dielectric characterization with an AC bridge circuit. In the case of the barium titanates, these calculated constants ranged from a high of 26.0 for TiO[subscript]2, to a low of 11.9 for BaTi[subscript]5O[subscript]11. The range for lithium tantalate was from a high of 21.6 for a stoichiometry of Li[subscript]7TaO[subscript]6, to a low at 8.4 for LiTaO[subscript]3. Lithium niobate ranged from a high at 21.1 for Nb[subscript]2O[subscript]5, to a low at 8.9 for Nb[subscript]2O[subscript]5 at a stoichiometry of Li[subscript]2Nb[subscript]32O[subscript]81.</p>