Insulator/Si interface flattening effects and gate oxide breakdown characteristics are evaluated for the oxygen radical oxidation and the wet oxidation at 750 °C. The radical oxidation is confirmed to exhibit a superior flattening effect than the wet oxidation. To obtain atomically flat top surface and interface to Si for oxides, radical oxidation on atomically flattened surfaces is indispensable. When the oxides are formed by radical oxidation on conventional flat Si surfaces with Ra > 0:12 nm, early breakdowns occur more frequently than wet oxides. These early breakdowns are eliminated when surfaces with Ra < 0:06nm are employed before oxidation. It is suggested that the early breakdowns occur at local spots that induce excess electric field due to the flattening of micro-roughness by the radical oxidation. To apply the radical reaction based insulator formation technology to the gate insulator formation, the surface before gate insulator formation must be sufficiently flattened.