Ultra-thin, nanoporous silica membranes have the potential to be used in high temperature gas separation processes (notably combustion gases at temperatures above 750K). Amorphous silica layers formed via chemical vapor deposition (CVD) have been shown to exhibit high permselectivities at elevated temperatures, but the influence of the process conditions on the microscopic details of the membrane formed needs to be better understood.

In this work we investigate the thermally induced deposition of silica layers from TEOS (tetraethoxysilane) on realistic amorphous nonporous silica substrates via chemical vapour deposition techniques. To model this process we use a hybrid kinetic Monte-Carlo (KMC) algorithm. Lattice KMC is used for the elementary reactions and an off-lattice method is employed for silica network relaxation and bond switching moves.

This kinetic model involves 12 different surface reactions which are considered to play a primary role in the build-up the SiOx layer on the substrate surface. Lattice KMC is used for these elementary reactions which include direct TEOS deposition and a variety of dissociation reactions which to take place within the layers as they form. All of the reactions have been carried out using reported reaction rate constants for this system.

The characterization of the deposited structure (density, pore size distribution and composition) and effects of the deposition temperature, initial concentration of surface reactive sites and water vapour pressure will be presented.