On Ceramic Thin Film Formation from Gels: Evolution of Stress, Cracks and Radiative Striations

There are still several problems to be scientifically solved for the ``conventional''sol-gel ceramic coating technique to be utilized in industrial scale. Based on the recognition, the author's group studied the stress evolution and crack formation in gel-derived films and the evolution of radiative striations in spin-coating films. Possible modification of the sol-gel technique has also been made for realizing thicker films via non-cycled deposition. First, the residual stress was measured on alkoxide-derived silica and titania films deposited on silica glass substrate. The residual stress increased with increasing heat-treatment temperature due to densification. It was demonstrated that the stress in films is not a simple function of the difference in thermal expansion coefficient between the film and the substrate, but is greatly affected by the processing parameters including the amount of water for hydrolyzing alkoxides and the kinds of the chelating agents. Second, in situ observation was conducted on silica and titania gel films deposited on single crystal silicon substrates. Macroscopic cracking occurred in the heating-up stage, suggesting that the cracks are formed by tensile stress generated in films due to densification. Also, such cracking was found to occur at lower temperatures when the films are thick or heated at lower rates. Third, expecting that organic polymers with amide groups like polyvinylpyrrolidone (PVP) can promote structural relaxation and can suppress stress evolution on heating, PVP-assisted sol-gel thin film deposition was proposed. Incorporating PVP in coating solutions, crack-free ceramic thin films including BaTiO3, PZT and BaBi4Ti4O15 films over submicron or micron in thickness could be prepared via non-cycled deposition. Finally, radiative striations were quantitatively evaluated by surface roughness measurement, and the effects of the processing parameters on striation evolution were studied. The height and spacing of striations were found to be increased with decreasing spinning rate and with increasing sol viscosity. Striations were also found to be formed even without the rotation of the substrate, suggesting that the substrate rotation is not a necessary condition for evolution of striations. On the assumption that the mechanism of the evolution of striations is the same irrespective of the presence or absence of the substrate rotation, in situ observation of the evolution of striations was conducted on a sol layer placed on a stationary substrate. It was observed that striations are formed on solvent evaporation after the sol was spread, not on proceeding of the sol front towards the outer.