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Argon Gas Cluster-Ion Beam sources have become widely-used on x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) instruments in recent years, but there is little reference data on sputter yields in the literature as yet. Total sputter yield reference data is needed in order to plan, and later calibrate the depth scale, of XPS or SIMS depth profiles. We previously published a semi-empirical Threshold equation for estimating cluster total sputter yield from the energy-per-atom of the cluster and the effective monatomic sputter threshold of the material. This has already been shown to agree extremely well with sputter yield measurements on a range of organic and inorganic materials for clusters of around a thousand atoms. Here we use the molecular dynamics (MD) approach to explore a wider range of energy and cluster size than is easy to do experimentally to high precision. We have performed MD simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) parallel MD code on high-performance computer (HPC) systems at Cincinnati and Newcastle. We performed 1,150 simulations of individual collisions with a silicon (100) surface as an archetypal inorganic substrate, for cluster sizes between 30 and 3,000 argon atoms and energies in the range 5 to 40eV per atom. This corresponds to the most important regime for experimental cluster depth-profiling in SIMS and XPS. Our MD results show a dependence on cluster size as well as energy-per-atom. Using the exponent previously suggested by Paruch et al, we modified the Threshold model equation published previously to take this into account. The modified Threshold equation fits all our MD results extremely well, building on its success in fitting experimental sputter yield measurements. |
