  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.
