Characterization of single ion tracks in PMMA created by light and heavy ion microprobes

Abstract

The ultimate resolution in ion beam lithography can be achieved by etching the material modified by a single ion impact which has a diameter in the order of only 10 nm. The sensitivity of the resist needs to be as high as possible but also keeping the bulk etch rate at a minimum. Our study is focused on poly methyl methacrylate (PMMA) with a small bulk etch rate compared to other track detectors (e.g. CR-39). We investigated the sensitivity using 3 MeV H, 8 MeV F, 71 MeV Cu and 88 MeV I generated by ion microprobes. Precise ion counting into a spin coated PMMA film on top of an active substrate enables us to control the exact dose and fluence delivered to the PMMA from a homogenous dose down to separated single ion tracks. With homogenous irradiation, the sensitivity limit was found to be 1-2 eV/nm^3, hence the damage radius and LET determines the etchability of a single ion track. Analysis of the track radii was performed using non-contact AFM. A high aspect ratio Si nanowhisker tip was used to image the openings to the etched tracks. It was found that for up to 8 min development in a water:IPA solution holes were created for the F, Cu and I ions. The undamaged bulk PMMA is also etched at a significantly lower rate than the ion tracks and contributed to a roughening of the PMMA surface. At 16 min developing time the PMMA film was completely removed. This was attributed to a mechanical failure in the film and not gradual film etching. Hole diameter and depth has been quantitatively analysed in an effort to determine the relationship between the LET of the incident ion, the development time and the resulting hole diameter.