Investigation of Rb$^+$ Milling Rates using an Ultracold Focused Ion Beam

Abstract: Several ion source alternatives for current focused ion beam (FIB) systems have been studied to achieve higher brightness, including cold atom ion sources. However, a study of ultracold ions interacting with often used materials is seldom reported. Here we investigate milling on several typical samples in a prototype ultracold Rb FIB system at 8.5 keV beam energy. For polycrystalline metallic substrates, such as Cu and Au, patterns milled by Rb$^+$ ions are observed to have reduced surface roughness, but still high milling rates compared with those milled by Ga$^+$ ions. Rb$^+$ also shows similar sputter rates as 30 keV Ga$^+$ on semiconductor substrates GaAs and InP. Special cases for Rb$^+$ milling show that the Rb$^+$ ion beam has a $2.6 \times$ faster sputter rate on diamond but a $3 \times$ slower sputter rate on Al compared with a normal 30 keV Ga$^+$ ion beam. Generally, a Rb$^+$ ion beam is shown to be suitable for nanostructuring of several basic materials.