The interactions of moisture with spin-on porous methylsilsesquioxane (p- MSQ) low- k dielectric films are investigated by on-line and real-time measurement of the rates of moisture uptake and removal. A process model is developed that provides information on the dynamics of moisture adsorption and desorption processes. The process model can be used to find optimum purge temperature and gas purity condition for cleaning and drying of low- k films. The cured p- MSQ films are compared with the partially etched and N2H2 ashed films. The results show that these two films have similar moisture solubilities; however, the moisture diffusivity in the patterned film is considerably higher. Transmission electron microscope results show that the etching and ashing processes not only decrease the overall film thickness but also increase the film porosity. The process model also provides information on moisture distribution within the film; this information is important in characterizing residual moisture and interfacial adhesion. The results for a wide range of diffusivity show that thin cap layers with low moisture solubility effectively block moisture penetration into the p- MSQ film without slowing down the outgassing and moisture removal during the purge. (c) 2007 The Electrochemical Society.