Ion Track Formation and Nanopore Etching in Polyimide : Possibilities in the MeV Ion Energy Regime

Polyimide films of thickness 7.5 µm are irradiated by a wide range of ions (1H to 197Au) with energies between 1.05 and 48 MeV. Irradiated samples are then chemically etched in sodium hypochlorite solution to investigate nanopore formation due to ion track etching. A threshold in terms of electronic stopping power, Set, needs to be surpassed to preferentially etch the ion tracks. Close to Set, intermittent tracks are formed where only part of the track is etchable. The fraction of these etchable parts increases as we move away from Set, toward higher stopping powers, eventually yielding continuous etchable tracks. Both intermittent and continuous track formation thresholds are observed to be velocity-dependent, yielding a decrease of the thresholds in the present work compared to previously reported thresholds for swift heavy ions. This finding leads the authors to suggest that electrostatic ion accelerators with terminal voltages of several MV are applicable for the production of ion track membranes up to ≈10–20 µm in thickness. Suitable ions for nanopores in 7.5 µm polyimide films include 42 MeV 59Co and 48 MeV 197Au. The growth mechanism for the pores during etching is discussed, relating it to the properties of the original ion track