Diffusion of solvents in thin porous films

Porous films are used nowadays as dielectrics with low dielectric constant (so-called low-k dielectrics). Knowing the porous structure of such films is important for successful integration in the semiconductor manufacturing technology. We developed a simple characterization method based on diffusion of solvents inside a porous film. In our experiments toluene as a non-polar solvent and ethanol as a polar solvent were used. A porous film is covered with a barrier impermeable for solvent so the solvent only can enter the film from the side. The barrier is transparent that allows observation of diffusion as a color change of the film. Measuring diffused distance as a function of diffusion time, diffusion coefficients can be calculated. It was found that the diffusion coefficients strongly depend on the porous structure of the films. Small pores (with the size comparable with the solvent molecule size) show low diffusion coefficients with high activation energies comparable with the enthalpy of evaporation. Bigger pores exhibit high diffusion coefficients with low activation energies corresponding to the activation energy of the solvent viscosity. Measuring diffusion coefficients as functions of porosity/pore size it is possible to evaluate the interconnectivity of the porous structure of a film in question.