A carbonaceous material is removed using a low energy focused ion beam in the presence of an etch-assisting gas. Applicant has discovered that when the beam energy of the FIB is lowered, an etch-assisting gas, such as O2, greatly increases the etch rate. In one example, polyimide material etched using a Xe+ plasma FIB with a beam energy from 8 keV to 14 keV and O2 as an etch-assisting gas, the increase in etch rate can approach 30× as compared to the default mill rate.