Low-Temperature Processing of Sol-Gel Derived Metal Oxide Thin Films using Supercritical Carbon Dioxide Fluid

Abstract

ABSTRACTWe demonstrate a novel technique using supercritical carbon dioxide (scCO2) fluid for lowering processing temperature of sol-gel-derived metal oxide thin films. The film processing was performed in a hot-wall closed vessel filled with scCO2 fluid. The effects of fluid temperature and additives on the sol-gel synthesis reaction under scCO2 fluid were also investigated. Precursor films of titanium dioxide (TiO2) prepared on silicon wafer and silica glass by sol-gel coating using Ti-alkoxide were converted to crystalline TiO2 (anatase) films successfully by treatment in scCO2 without additive agent at a fluid pressure of 15 MPa and at a substrate temperature of above 250°C, which is significantly lower than the processing temperature of conventional sol-gel deposition. Furthermore, additive agents such as water (H2O) and nitrogen-oxygen mixture (N2-O2) promoted the decomposition and crystallization of precursor films in scCO2 fluid to form the crystalline TiO2 (anatase) films at a substrate temperature at as low as 200°C although it also produced surface absorbates consisted of hydroxides on the film surface. The experimental results suggested that the hydrolysis and polymerization reactions of Ti-alkoxide in the precursor films were proceeded by the scCO2 processing to form titanium-oxygen (Ti-O) networks and that byproducts such as alcohols were removed from the resulting films.