High-density nanoetchpit-array fabrication on Si surface using ultrathin SiO₂ mask

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.