Nanoetchpit arrays (NEPAs) with a density of 1.56 T(1012)pit/in2 were artificially fabricated on Si surfaces. The key to the success of high-density-NEPA fabrication is through utilization of ultrathin SiO2 layer as an Si etching mask and N2H4 (hydrazine) solution as an Si etchant. The NEPAs were fabricated in only three steps: (1) focused electron-beam (EB) irradiation onto SiO2 mask, (2) SiO2 mask development using an HF-based solution, and (3) Si etching using hydrazine. The enhanced etching phenomenon of EB-exposed SiO2 in an HF-based solution was applied to pattern the SiO2 mask. Thin SiO2 layers with a thickness of 8 nm at the initial stage (4 nm after the development) were used as Si etching mask to suppress both spreading of the EB-exposed region by the forward scattering of the electrons and lateral extension of the SiO2 etching region during the development. Si substrates were etched through 4-nm-thick SiO2 mask by dipping in a hydrazine solution with an extremely high etching selectivity for Si/SiO2. By using this simple process, 8-nm-diameter NEPAs with a 20 nm pitch were successfully arranged on Si surfaces.