Controlling solvent diffusivity via architecture of nanopore structures in porous low-k films

Three kinds of porous silsesquioxane-based thin films with different pore structures and porosities were prepared in order to investigate the relationship between the behaviors of wet chemical diffusion and pore structure. The first model of the films contains only partially interconnected small mesopores (∼2 nm), the second includes fully interconnected large mesopores (10-20 nm) and the last porous film model embeds isolated macropores (60-90 nm). Dependence of the diffusion coefficient of solvent (toluene and methanol) on the pore size and pore structure of the porous films is discussed. The diffusion coefficient of the porous films was found to be dependent not only on porosity but also significantly on the pore structure such as pore size and pore interconnectivity. As porosity and pore interconnectivity increased, the diffusion coefficient of solvent increased as expected. In the case of isolated macroporous film (model III), the diffusion coefficients were almost constant with varying porosity. Moreover, the porous films with isolated macropores showed extremely low solvent diffusivity (<10^-8 cm²/s) and water affinity (D_Tol/D_MeOH ∼ 0.05).