Insulating oxide film formation with acid catalyzed hydrolysis of alkoxide precursors in supercritical fluid carbon dioxide

Abstract

Insulating oxide films can be produced by hydrolysis of metal alkoxide precursors in the presence of an acid catalyst in supercritical fluid carbon dioxide (sc-CO₂). Using tetraethylorthosilicate (TEOS) as a precursor and acetic acid (HAc) as a catalyst, uniform SiO₂ films can be formed on surfaces of different substrates according to the reaction Si(OCH₂CH₃)₄ + 2H₂O → SiO₂ + 4CH₃CH₂OH. The quality of the SiO₂ film is controlled by the rate of hydrolysis of TEOS which is determined by the amount of water available in the system. In our sc-CO₂ reaction system, water involved in the TEOS hydrolysis is generated by the in situ esterification process CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. In the absence of acetic acid, the reaction proceeds very slowly. The acid catalyzed reaction probably involves proton coordination to the oxygen atoms of TEOS molecules that facilitates the hydrolysis. The acid-catalyzed hydrolysis reaction produces dense SiO₂ films instead of porous SiO₂ films formed by water added hydrolysis of TEOS in sc-CO₂. Formation of SiO₂ films via hydrolysis in sc-CO₂ is more rapid compared to the traditional hydrolysis reaction at room temperature. In general, metal alkoxide hydrolysis reactions carried out in a closed sc-CO₂ system is not affected by moisture in air compared with traditional open-air hydrolysis systems. Using sc-CO₂ as a reaction medium also eliminates undesirable organic solvents utilized in traditional alkoxide hydrolysis reactions.