Characterisation of micropores in plasma deposited SiO x films by means of positron annihilation lifetime spectroscopy

The effect of average incorporated ion energy and impinging atomic oxygen flux on the structure and permeability of SiO(x)thin films by a microwave driven low-pressure discharge with additional radio frequency bias is studied by means of positron annihilation lifetime spectroscopy (PALS) and complementary analytical approaches. The film growth and structure were controlled by the particle fluxes. A correlation between the pore sizes and pore size distribution as measured by PALS and the adjusted plasma parameters was established. The corresponding barrier performance was measured by oxygen transmission rate and could be explained by the pore size distribution. The dominant pore size characteristic for dangling bonds within the SiOx-network was found to be in the range of 0.8 nm. The chemical composition and morphology were analysed by means of x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy diffuse reflectance measurements and atomic force microscopy. It was observed that a combination of both an increase in incorporated energy per deposited Si atom and low oxygen to silicon ratio resulted in an enhanced cross-linking of the SiO(x)network and thereby led to a decrease in micropore density and to a shift of the pore size distribution function to lower values.