The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography andatomic layer deposition

Abstract

Patterned nanoscale materials with controllable characteristic feature sizes andperiodicity are of considerable interest in a wide range of fields, with various possibleapplications ranging from biomedical to nanoelectronic devices. Block-copolymer(BC)-based lithography is a powerful tool for the fabrication of uniform, densely spacednanometer-scale features over large areas. Following this bottom-up approach,nanoporous polymeric films can be deposited on any type of substrate. Thenanoporous periodic template can be transferred to the underlying substrate bydry anisotropic etching. Nevertheless the physical sizes of the polymeric maskrepresent an important limitation in the implementation of suitable lithographicprotocols based on BC technology, since the diameter and the center-to-centerdistance of the pores cannot be varied independently in this class of materials.This problem could be overcome by combining block copolymer technology withatomic layer deposition (ALD): by means of BC-based lithography a nanoporousSiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequentlyused as a backbone for the growth of perfectly conformal thin oxide films by ALD. In thiswork polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etchingare used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thickSiO2 matrix. By ALDdeposition of Al2O3 thin filmsonto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of theAl2O3 film the dimension of the pores in the oxide films is progressively reduced, withnanometer precision, from the original size down to complete filling of the pores,thus providing a simple and fast strategy for the fabrication of nanoporousAl2O3 surfaces with well-controllable feature size.