Structural evolution of nanoporous silica thin films studied by positron annihilation spectroscopy and Fourier transform infrared spectroscopy

Three series of silica thin films with thicknesses in the 300 nm range were deposited by spin coating on Si substrates using different compositions of the sol precursors. Film samples were thermally treated in static air at temperatures ranging from 300 to 900 degrees C. The effect of sol precursors and thermal treatment temperature on the film porosity was analysed by Fourier transform infrared ( FTIR) spectroscopy, depth profiling with positron annihilation spectroscopy ( DP- PAS) and the analysis of the capacitance - voltage ( C - V) characteristic. The maximum of the total porosity was found to occur at a temperature of 600 degrees C when removal of porogen and OH groups was completed. Film densification due to the collapsing of the pores was observed after drying at 900 degrees C. DP- PAS provides evidence that the increase in the total porosity is related to a progressive increase in the pore size. The increase in the pore size never gives rise to the onset of connected porosity. In the silica film samples prepared using a low acidity sol precursor, the pore size is always lower than 1 nm. By increasing the acid catalyst ratio in the sol, larger pores are formed. Pores with size larger than 2.3 nm can be obtained by adding porogen to the sol. In each series of silica film samples the shift of the antisymmetric Si - O - Si transversal optical ( TO3) mode upon thermal treatment correlates with a change of the pore size as evidenced by DP- PAS analysis. The pore microstructure of the three series of silica films is different at all the examined treatment temperatures and depends on the composition of the precursor sol.