A highly cross-linked porous silica dielectric (PoSiO) film was fabricated at a low temperature of 350 degrees C. PoSiO films were derived by the sol -gel method and their pore surface silanol groups were silylated with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) by vapor phase treatment. To promote the degree of siloxane cross-linkage of the film, cesium (Cs) was added to the precursor solution with the amount of 0, 5, 15, and 30 wt ppm as a base catalyst. Then, the amount of methyl -silicon -three oxygen (Me -Si T-type) and hydrogen -silicon -three oxygen (H -Si T-type) bridged structures of the chemisorbed TMCTS were increased, and the amount of surface silanol groups was decreased markedly with the increasing amount of Cs concentration. Leakage current and dielectric constant were measured under various humidity conditions, which were hardly degraded for the highly cross-linked PoSiO owing to its small amount of residual silanol groups and adsorbed water. It was also shown that the amount of mobile protons originated from the silanol groups became negligible. Time zero dielectric breakdown field strength was improved to 6.7 MV/cm and a projected time-dependent dielectric breakdown lifetime satisfied 10 years for Cs 30 ppm-doped PoSiO under a stress condition of 220 degrees C and vertical bar E vertical bar = 1 MV/ cm. (C) 2008 The Electrochemical Society.