Hydrofluoric acid etching indicates that ultra thin silicon dioxide film made by high purity ozone on Si(100)2 x 1 between 300 and 700 degrees C has the same film density as that of thermally grown silicon dioxide for device use on Si(100) at 750 degrees C in a wet environment. Rate of oxide film growth > 6 Angstrom on Si(100)2 x 1 by ozone is, however, much lower at the substrate temperature between 300 and 500 degrees C than at 700 degrees C. This is indicating different kinetics and mechanism of oxide film growth > 6 Angstrom by high purity ozone.