Progression of Focused Helium Ion Beam Milling in Gold Substrates

The focused helium ion beam-solid state material interaction volumes differ in thin membranes and in bulk substrates [1, 2]. While nanometer-scale structures with feature sizes of 5 nm or smaller have been machined in thin membranes [3-5], inconsistent results have been observed in bulk substrates where the irradiated region often swells rather than mills [6]. The depth and breadth of the interaction volume scales with beam energy, and the sputter yield also varies significantly. In this study, we seek to explore and understand these differences by observing the progression of focused helium ion beam milling. For this we used transmission electron microscope images of the beam-irradiated cross sections of 100-nm-thick gold membrane and 400-nm-thick (essentially bulk) gold substrates. We varied the ion beam landing energies and the dose as well.