It is demonstrated that the formation of the atomic scale flattened Si/SiO2 interface is effective in reducing the Flicker noise in n-channel metal oxide semiconductor field effect transistors (n-MOSFETs). The atomic scale flattened Si/SiO2 interface is realized, the atomic scale flattened silicon surface is obtained by the HF/HCl wet-etching process, and then the silicon surface is oxidized by radicals generated in Kr/O2 mixed high-density microwave-excited plasma at 400°C. Applying these techniques, the trap density at the Si/SiO2 interface is markedly reduced since the surface roughness is minimized and Flicker noise is markedly reduced as compared with the conventional process.