Monte Carlo simulation of nanoscale material focused ion beam gas-assisted etching: Ga+ and Ne+ etching of SiO2 in the presence of a XeF2 precursor gas.

Elucidating energetic particle-precursor gas-solid interactions is critical to many atomic and nanoscale synthesis approaches. Focused ion beam sputtering and gas-assisted etching are among the more commonly used direct-write nanomachining techniques that have been developed. Here, we demonstrate a method to simulate gas-assisted focused ion beam (FIB) induced etching for editing/machining materials at the nanoscale. The method consists of an ion-solid Monte Carlo simulation, to which we have added additional routines to emulate detailed gas precursor-solid interactions, including the gas flux, adsorption, and desorption. Furthermore, for the reactive etching component, a model is presented by which energetic ions/target atoms, and secondary electrons, transfer energy to adsorbed gas molecules. The simulation is described in detail, and is validated using analytical and experimental data for surface gas adsorption, and etching yields. The method is used to study XeF2 assisted FIB induced etching of nanoscale vias, using both a 35 keV Ga+, and a 10 keV Ne+ beam. Remarkable agreement between experimental and simulated nanoscale vias is demonstrated over a range of experimental conditions. Importantly, we demonstrate that the resolution depends strongly on the XeF2 gas flux, with optimal resolution obtained for either pure sputtering, or saturated gas coverage; saturated gas coverage has the clear advantage of lower overall dose, and thus lower implant damage, and much faster processing.

Simulating advanced focused ion beam nanomachining: a quantitative comparison of simulation and experimental results.

A simulation study of focused ion beam (FIB) sputtering in SiO2 is presented. The basis of this study is an enhanced version of the EnvizION Monte Carlo simulation program for FIB processing, which previously was restricted to targets composed of a single atom. A Monte Carlo method is presented for the simulation of FIB sputtering in SiO2 in three-dimensions, with ion implantation, to elucidate the complex dynamics of nanoscale milling of compound targets. This method is applied to the simulation of sputtering experiments using both Ne+ and Ga+ ion beams. We compare simulations using experimentally derived 'measured' beam profiles for each ion species, and 'effective' beam profiles which are chosen to reproduce experimental results. Simulations using the 'measured' beam profiles produce vias which are narrower than experiments, while the 'effective' beam profiles for both Ne+ and Ga+ are significantly wider than the 'measured' profiles. The difference between the 'measured' and 'effective' beam profiles is attributed to widening of the milling effects of the beam beyond its static dimensions, due to platform level artifacts such as vibrations and, possibly, charging. Simulations using the 'effective' beam profiles are found to accurately reproduce the depths and overall shape of experimental FIB sputtered vias in test cases, which vary in ion species, beam energy, total dose, and raster parameters. This comparison is the most extensive validation of the EnvizION simulation against experiments to date. However, the location of implanted ions in simulations is shallower than experiments, which is attributed to the fact that implanted species are required to find nearest neighbor vacancies and not allowed to occupy interstitial positions.