Superior insulating performance is found for dense silicon dioxide ultra‐thin films (∼3 nm) grown by UV photo‐oxidation of silicon. Density profile obtained by glazing incidence x‐ray reflectivity shows that the high density (2.32 g/cm3) SiO2is formed on Si(100) surface at much lower temperature (<450 °C) than thermal oxidation, using 126 nm photons. The sharp and flat interface within 1–2 monolayers is revealed by high resolution transmission electron microscopy. The film density is strongly dependent on wavelength around 172–126 nm, suggesting that the specific excited species of oxygen is involved in the growth mechanism. Unique properties of photo‐oxidized silicon dioxide are related to the modified Si‐O network structure and ring statistics. © 2004 American Institute of Physics