Why are hydrogen ions best for MeV ion beam lithography?

The exposure characteristics of poly-(methyl methacrylate) (PMMA) for 2 MeV ¹H ⁺, 3 MeV ⁴He ²⁺ and 6 MeV ¹²C ³⁺ have been investigated. The samples were characterised using Atomic Force Microscopy (AFM), optical microscopy and Raman spectroscopy. Development was carried out using a 7:3 propan-2-ol:H ₂O mixture to determine clearing and cross-linking fluences. It was found that protons had a considerably wider tolerance to exposure variations and a smaller span of doses within the ion track. Furthermore, the void formation and consequent stress-induced surface roughening were smaller for protons. For all ions, the CC bond Raman signal increased continuously with dose and fluence, even well above that required for total cross linking.