When a highly energetic heavy ion passes through a target material, the damaged region left in its wake often exhibits preferential etching over the bulk. The etching process can create very high aspect ratio channels of up to tens of microns in length, with pore diameters as small as a few nanometers. Using this technique, so called “ion track etched membranes” can be fabricated in a variety of materials including polymers and the thin films typically used in semiconductor processing such as silicon dioxide (SiO₂) and silicon nitride (Si₃N₄). Pores formed by this method are highly parallel with very narrow size distributions. The morphology of the pores can be cylindrical, conical or double conical, depending on the etching conditions.

Freestanding SiO₂ and Si₃N₄ membranes were irradiated with 185 MeV ¹⁷⁹Au ions at the ANU Heavy Ion Accelerator Facility. SiO₂, and Si₃N₄ films were irradiated with 1.1 GeV ¹⁷⁹Au ions at the GSI UNILAC accelerator in Darmstadt, Germany. Low irradiation fluences in the order of 1x10⁸ ions/cm² were chosen to avoid significant overlap of the pores during etching. This study focuses on the influence of the etching time and etchant concentration on the pore morphology for ion tracks etched in SiO₂. We have determined the track etch rate and cone angle as a function of these parameters. 

We are seeking to develop a better understanding of the ion track etching process and its dependence on the un-etched track structure through a unique combination of complementary characterisation techniques including synchrotron based small angle x-ray scattering (SAXS) combined with Monte-Carlo simulation, and advanced electron microscopy. The combination of these techniques enables an accurate reconstruction of the size and shape of the pores. This is necessary for reproducible fabrication of porous membranes for specific functions in the future.