The effects of single and double masks on focused ion beam (FIB) direct patterning and chlorine-based inductively coupled plasma reactive ion etching (ICP-RIE) were studied in order to determine the influence of twice-deposited mask layers on the verticality of the side wall of silicon-based nanostructures. When a single mask was used as the etching mask, an inclined plane with a large side angle on the top area was formed. When a double mask was used, the first mask layer of chromium (Cr) was deposited by RF (radio frequency) magnetron sputtering and then directly patterned by FIB. Then, the secondary mask layer of SiO2, which was deposited to protect the side wall in order to retard etching and prevent the formation of an inclined plane, was deposited by RF magnetron sputtering. However, the SiO2 on the top and bottom of the nanostructure was removed through anisotropic etching by ICP-RIE, and only SiO2 on the side wall was retained. The experimental results show that the SiO2 layer left on the side wall as an etching barrier can effectively maintain the verticality of the nanostructure. The measurement results show that the verticality and aspect ratio of the nanostructure are 90.8 degrees and 5.08 (depth: 310 nm, width: 61 nm), respectively. (C) 2012 Elsevier B. V. All rights reserved.