Properties of nanoporous silica thin films determined by high-resolution x-ray reflectivity and small-angle neutron scattering

A new methodology based on a novel combination of a high-resolution specular x-ray reflectivity and small-angle neutron scattering has been developed to evaluate the structural properties of low-dielectric-constant porous silica thin films about one micrometer thick supported on silicon wafer substrates. To complement these results, film composition was determined by high-energy ion scattering techniques. For the example thin film presented here, the overall film density was found to be (0.55 +/- 0.01) g/cm(3) with a pore wall density of (1.16 +/- 0.05) g/cm(3) and a porosity of (53 +/- 1)%. The characteristic average dimension for the pores was found to be (65 +/- 1) Angstrom. It was determined that (22.1 +/- 0.5)% of the pores had connective paths to the free surface. The mass fraction of water absorption was (3.0 +/- 0.5)% and the coefficient of thermal expansion was (60 +/- 20) x 10(-6)/degrees C from room temperature to 175 degrees C. Lastly, model fitting of the specular x-ray reflectivity data indicated the presence of a thin surface layer with an increased electron density compared to the bulk of the film as well as an interfacial layer with a reduced electron density. (C) 2000 American Institute of Physics. [S0021-8979(00)00703-9].