Focused ion beams (FIB) are a common tool in nanotechnology for surface analysis, sample preparation for electron microscopy and atom probe tomography, surface patterning, nanolithography, nanomachining and nanoprinting. For many of these applications, a precise control of the ion-beam induced processes is essential. The effect of contaminations on these processes has not been explored thoroughly but can often be substantial, especially for ultralow impact energies in the sub-keV range. In this paper we investigate by molecular dynamics (MD) simulations how one of the most commonly found residual contaminations in vacuum chambers, i.e. water, adsorbed on a silicon surface, influences sputtering by 100 eV argon ions. The incidence angle was changed from normal incidence close to grazing incidence. For the simulation conditions used in this work, the adsorption of water favours the formation of defects in silicon by mixing of the hydrogen and oxygen atoms into the substrate. The sputtering yield of silicon is not changed significantly by the contamination, but the fraction of hydrogen and oxygen atoms that is sputtered depends largely on the incidence angle. This fraction is largest for incidence angles between 70 and 80 degrees defined with respect to the sample surface. Overall, it changes from 25% to 65%.

Ion beam processes related to focused ion beam (FIB) milling, surface patterning and secondary ion mass spectrometry (SIMS) require precision and control, the quality and cleanliness of the sample being a detrimental factor. Furthermore, several domains of nanotechnology and industry use nano-scaled samples that need to be controlled to an extreme level of precision. To reduce the irradiation-induced damage and to limit the interactions of the ions with the sample, low energy ion beams are used due to their low implantation depths. Yet, low energy ion beams come with a variety of challenges. Indeed, for such low energies, the residual gas molecules in the instrument chamber can adsorb on the sample surface and impact the ion beam processes. In this paper we pursue an investigation on the effects of the most common contaminant, water, sputtered by ultra-low energy ion beams, ranging from 50 to 500 eV and covering the full range of incidence angles, using Molecular Dynamics (MD) simulations with the ReaxFF potential. From this study we show that the expected sputtering yields trends are respected down to the lowest sputtering yields, a region of interest with low damage being obtained for incidence angles around 60 to 75°. We also demonstrate that higher energies induce a larger removal of the water contaminant and at the same time induce an increased amorphization, which produces a trade-off between sample cleanliness and damage.