Plume dynamics of iron disilicide studied by time-of-flight mass spectroscopy

KrF excimer laser ablation of iron disilicide in the α phase (α-FeSi2) has been investigated using time-of-flight (TOF) mass spectroscopy to characterize the dependence of ejected neutrals and native positive ions on laser fluence. The TOF mass spectra showed that the ejection behavior was classified into three parts at low, moderate and high fluence ranges. At laser fluences below 0.7 J/cm2, the presence of only Si, Fe and Si2 neutrals was confirmed; the arrival profile of Si atoms is well described by a Maxwell–Boltzmann (MB) distribution with a translational velocity of 1.6×103 m/s, corresponding to a kinetic energy of 0.35 eV, and a target surface temperature of 4.0×103 K, indicating a thermal emission of these neutral species. With increasing the fluence, nascent positive ions, such as singly- or doubly-charged Si and Fe ions appeared; the arrival profiles of these ions are observed to fit a shifted MB distribution. Their kinetic energies range from several eV at moderate fluences to over 100 eV at high fluences up to 2.5 J/cm2. These high kinetic energies cannot be explained in terms of the FeSi2 target surface reaching a temperature that would produce such energetic ions upon ablation, suggesting the ion acceleration in a dense plasma plume.