Fundamental studies on mechanical properties of functional ceramic thin films prepared by sol-gel method

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Gel films were deposited on single-crystal Si wafers and heated at a constant rate where in-plane stress in films was measured in situ in the course of heating. The effects of the amount of water for alkoxide hydrolysis, methyltriethoxysilane, chelating agents and polyvinylpyrrolidone (PVP) were investigated.Tetraethoxysilane (TEOS) was hydrolyzed under acidic condition, followed by deposition of silica gel thin films. The in-plane stress generating in the course of heating was found to be increased with increasing H_2O/TEOS mole ratios in solutions. The partial substitution of methyltriethoxysilane (MTES) for TEOS and the addition of PVP in solutions resulted in suppression of stress evolution during heating.Titania gel films were prepared from titanium tetraisopropoxide solutions. The addition of PVP and chelating agents like acetic acid and acetylacetone in solutions was found to suppress the stress evolution.It has been known that cracking is suppressed and uncracking critical thickness of silica coatings increases when smaller amount is water for silicon alkoxides or partial substitution of MTES. It is also known for gel-derived ceramic coatings that chelating agents and PVP are effective in increasing the uncracking critical thickness. Suppressed cracking and increased uncracking critical thickness are now concluded to be caused by the reduced stress evolution.