This paper reports the effects of oxygen in a Si substrate on dryetching damage during gate-electrode formation and side-wall formation. Different types of deep level traps were created in Czochralsi (CZ) and in p/p+-epitaxial wafers during side-wall formation. In comparison to the CZ wafers, a deep energy state and large hole capture cross section for the electrically active trap appears in the p/p+-epitaxial wafers. The depth profile of the deep trap in CZ wafers is shallower than that in p/p+-epitaxial wafers. This result indicates that the trap diffusivity in a CZ wafer is smaller than that in a p/p+-epitaxial wafer. Therefore, it suggests that creation of the trap in CZ wafers is influenced by the oxygen within the silicon. From results, the origin of the trap was identified to be carbon-oxygen complexes in the CZ wafer and carbon complexes in the p/p+-epitaxial wafer. Thus, the oxygen in the Si substrate plays a role in the formation of carbon-oxygen complexes and in suppressing the trap introduced into the Si substrate during SiO2 etching.