wMesoporous organically modified silicon oxides were fabricated using multi-step sol-gel technique with tetraethyl orthosilicate (TEOS) and methyltriethoxysilane MTES) as precursors. The characterization of the dielectric films with Fourier transform infrared, thermogravimetric (TG) and differential thermal analysis (DTA) as well as dielectric analyzer showed the effect of annealing temperature on the development of porosity and dielectric properties of the films. The pore size was measured by small angle X-ray scattering (SAXS) and was found that the annealing temperature did not affect the pore size very much during the experimental temperature range. An ultra-low dielectric constant of about 2.1 was realized for about 57% porosity in the silica film. It was shown that the method employed is useful for preparing new kinds of films used as interlayer dielectrics. (C) 2004 Elsevier B.V. All rights reserved.