Ion beam lithograpy using a nano-aperture M.L. Taylor a, * , A. Alves a , P. Reichart b , R.D. Franich a , S. Rubanov b , P. Johnston a , D.N. Jamieson b a Applied Physics, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia b School of Physics, Microanalytical Research Centre, University of Melbourne, Victoria 3010, Australia Available online 14 February 2007 Abstract A high aspect ratio nano-aperture mask was used to perform sub-micrometre lithography with MeV ions. Such a mask can localise ion placement in ion machining or doping, including the fabrication of high aspect ratio structures from ion beam damaged regions by subsequent etching. Ion scattering and straggling is the main limiting factor for the transmission with respect to transmitted energy, lat- eral intensity and angular intensity distribution of the ions. These limits are investigated by a 3D Monte Carlo modelling program based on the SRIM code; simulation of 1.5 MeV He incident on nano-scale apertures of various diameters indicates that the masking is highly effective. The simulation results were tested by experiments on the transmission of 1.5 MeV He ions through apertures that were machined using a focused ion beam of keV Ga ions in a 10 lm thick Si-cantilever. Firstly, the aperture was placed in front of a 50 nm thick polymethyl methacrylate (PMMA) coated photodiode and scanned by a microprobe. The photodiode gives an energy spec- trum of the transmitted ions. Secondly, a nano-scale pattern was produced in an 800 nm thick PMMA sample by the step-and-repeat process, implementing a nanonics stage that facilitates precise three-dimensional orientation of the mask. The damaged PMMA was developed and the lateral distribution of ion impacts was imaged using non contact AFM. Ó 2007 Elsevier B.V. All rights reserved. PACS: 07.05.Tp; 02.70.Uu; 61.18.Bn; 81.16.Nd Keywords: Nanostencil; Step and repeat nanolithography; High energy ion lithography; Ion mask; Nano-aperture; Single ion machining and doping; Ion straggling 1. Introduction Nuclear microprobe analysis has reached a beam spot resolution limit of around 0.3–0.9 lm [1]. With the excep- tion of some work done in the low current regime (<1 pA), such as [2], no significant improvements in spatial resolution have been published over the past five years. Applications for high resolution beam spots are emerging though several restrictive factors exist, including (i) diffi- culty in manufacturing sufficiently accurate ion optic lenses, (ii) influences of vibrations and stray fields, (iii) focusing difficulties and (iv) brightness and chromaticity limitations [3]. With recent developments in the construction of high aspect ratio nano-scale apertures (for example [4–9]) comes the prospect of using such apertures as masks for high energy ions. A previous study employed detailed Monte Carlo (MC) modelling to evaluate the potential of such masks to enable sub-micron resolution [10]. Experimental verification of that model in this work involves ion beam lithography in polymethyl methacrylate (PMMA) applied through a nano-aperture drilled in a Si cantilever by focused ion beam (FIB) milling, followed by atomic force microscope (AFM) imaging of the etched hole. There are growing applications for ion beams requiring the precision delivery of few or single ions to high resolution, and similar techniques have been adopted by Schenkel et al. with highly charged keV ions [11–13] and by Luthi et al. for nano-stencil deposition [14]. The initial motivation for this 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.02.057 * Corresponding author. Tel.: +61 3 9925 2138; fax: +61 3 9925 5290. E-mail address: michael.taylor@rmit.edu.au (M.L. Taylor). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 260 (2007) 426–430 NIM B Beam Interactions with Materials & Atoms