Ion beam lithograpy using a nano-aperture

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, lateral 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 μm 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 spectrum 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.