In this paper, atomic layer deposition of SiO2 thin films was investigated with Si2Cl6 and O3/O2 (400 g m-3). O3/O2 is not preferred for high-temperature (>400 °C) processes due to its lower decomposition temperature, especially in a furnace-type chamber. However, with Si oxidation test using a cold-wall chamber, we have demonstrated the reactivity of O3/O2 up to 800 °C in comparison with O2 and H2O. The ALD of SiO2 films was examined at deposition temperatures from 500 °C to 700 °C. The growth rate at 600 °C was saturated to 0.03 nm/cycle with Si2Cl6 exposure over 1.2 × 105 L. O3/O2 also showed ALD-like saturation behaviors for exposures over 2.4 × 106 L. The ALD films deposited at 600 °C exhibited relatively smooth surface roughness (<0.18 nm) and low wet etch rate (1.6 nm min-1, 500:1 HF) that are comparable with PECVD SiO2 deposited at 250 °C and LPCVD SiO2 deposited at 450 °C.