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 nanometres. Membranes formed by this method are ideal for many advanced applications including medical and bio-sensing, filtration and separation processes, nano-fluidics, and nano-electronic devices. The morphology of the etched channels can be cylindrical, conical or double conical, depending on the etching conditions. The resulting etched pores are highly parallel with extremely narrow size distributions. In polymers, the track etching technique is used for the commercial fabrication of filtration membranes.
The aim of this research is to develop track etched nano-pore membranes in inorganic materials, in particular SiO2 and Si3N4. Such membranes have high potential applications in molecular sensing and fabrication is largely compatible with standard semiconductor processing. For controlled fabrication of nano-pore membranes with size and shape-specific pores, a comprehensive understanding of the etching kinetics and the relationship between the un-etched ion tracks and the etched structures is required. The characterisation work was done using 2 mm thin layers of SiO2 on Si irradiated with 185, 89 and 54 MeV 197Au ions at the ANU Heavy Ion Accelerator Facility and with 1.1 GeV 197Au ions at the GSI UNILAC in Darmstadt, Germany. Some results in Si3N4 and on free-standing SiO2 membranes will also be presented. The irradiated material was subsequently etched in diluted hydrofluoric acid at several concentrations. To characterise the pores we have used small angle X-ray scattering (SAXS) performed at the Australian Synchrotron, supported by scanning electron microscopy (SEM) imaging to determine the track etch rate and pore shape as a function of etch time and etchant concentration. A full reconstruction of the scattering images resulting from the etched pores, which resemble conical shapes, enables detailed characterisation of the pore cone angles and sizes. The results indicate that the track etching depends on the ion irradiation conditions, and that at short etching times the extent of the track damage in the radial direction becomes significant. The results obtained as well as the SAXS analysis methodologies developed will enable us to accurately tailor pores for specific applications in free-standing membranes.