Ion tracks in amorphous SiO2 irradiated with low and high energy heavy ions

Latent ion tracks induced by heavy ions in amorphous silicon dioxide (a-SiO2) were studied by scanning electron microscopy after chemical etching in an aqueous HF solution. The evolution of etched pore diameter and etching efficiency was studied as a function of the electronic energy loss for each ion species. Etching was possible only above a threshold of (dE/dx)e ≈ 1.5 keV/nm when using energies below 1 MeV/u, as demonstrated both from pore size and etching yield results. In a transition regime 1.5 keV/nm < (dE/dx)e < 4.0 keV/nm only a fraction of all pores were revealed by etching. For (dE/dx)e > 4.0 keV/nm all tracks were revealed due to the formation of continuous tracks. Above the etching threshold the pore diameter increased with energy and electronic stopping power. A comparison with ions having energies above 1 MeV/u shows that the etching threshold depends on the energy regime. Furthermore, the pore diameter reached a maximum value before the electronic stopping power maximum of the respective ion was attained.