Si-based nanowires with high aspect ratios have been fabricated using an inductively coupled plasma reactive ion etching (ICP-RIE) with a continuous processing gas mixture of fluorine-based SF(6):C(4)F(8) combined with a thermal oxidation technique. The subsequent thermal oxidation further reduced the nanowire diameter utilizing the self-limiting oxidation effect below the lithographic dimensions. Transmission electron microscopy analysis of the completed nanostructures revealed the total oxide thickness and the consumption of the Si core which determines the inner nanowire diameter. The final dimensions of the inner Si nanowire are about 600 nm tall and less than 25 nm wide using top-down processing techniques. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3455498]