New method for making porous SiO2 thin films

A new method based on the conventional base-catalyzed Sol-Gel process has been proposed for making porous SiO2 thin films in this paper. Silicon alkoxide (TEOS) was hydrolyzed and condensed in water under the action of the base catalyst (NH4OH) without ethanol, and then acetic acid was added to the SiO2 gel solution to slow down the speed of condensation and delay the growth of SiO2 gel particles. Polyvinyl alcohol (PVA) was added to the solution to coat SiO2 gel particles in order to obtain a stable solution and to restrict the growth of gel particles. Using this gel solution, uniform, crack-free and porous films with a thickness of 3.6 mu m and a porosity of 50% were successfully achieved by spin coating after annealing at 550 degrees C for 30 min. The silica film fabricated by this new method maintains its amorphous structure and high porosity after treatment at high temperatures. (C) 1999 Elsevier Science S.A. All rights reserved.