Erosive-thermal transition in high-flux focused ion beam nanomachining of surfaces

Focused ion beams (FIB) are increasingly used for surface modification and fabrication with nanometer scale precision. In FIB, an energetic beam of ions strikes a surface and removes material, a process that is understood to depend upon the properties of the beam (e.g. beam flux, ion energy) and is thought to be due to ion induced sputter erosion. We show that the material removal rate is also strongly affected by the thermal properties of the material, sample temperature, and geometry. Furthermore, we deduce a dimensionless parameter, a ratio of incident power to thermally dissipated power (QFIB), which parameterizes a switch of the underlying mechanism of material removal. It predicts with remarkable accuracy a previously overlooked transition from slow erosive material removal to significantly accelerated thermal vaporization material removal. Its critical value explains an observed transition in data covering a range of beam fluxes, ion energies, spot sizes, film thicknesses, materials, ion species, and temperatures. Large-scale parallel molecular dynamics simulations support this transition.