Nanopattern transfer to SiO₂ by ion track lithography and highly selective HF vapor etching

The authors present a method for high aspect ratio nanopatterning of high density (10¹⁰ pores/cm²) self-assembled porous alumina membrane pattern into thermally grown SiO₂ on silicon. The pattern transfer is accomplished by irradiating through 2 μm thick porous alumina membrane with swift heavy ions (4 MeV Cl²⁺). Ions passing through the nanopores in the mask at a fairly high fluence (typically 10¹⁴ ions/cm²) are impinging on the substrate and creating a continuous volume of overlapping ion tracks of damage. The damage is sufficient to be selectively etched by HF vapor from an aqueous HF solution. From an alumina mask with pores of 70 nm diameter, a pattern of pores of 77 nm in diameter and the same distance of 100 nm between the centers of the pores was transferred. The deepest observed etched pores were 355 nm, giving an aspect ratio of 5, which is up to 40 times larger compare to earlier work where HF wet etching was used. This ion track lithography technique shows a potential to produce nanostructures with even higher aspect ratios.