Ultra low energy gold cluster ion beams for SIMS applications

Abstract

The purpose of work was to obtain gold and possible other negative ion cluster beams where each constituent atom in the cluster carried an energy ≤ 1 keV, using the floating low energy ion gun (FLIGTM), and to test these in secondary ion mass spectrometry. The heavy negative metal cluster is worth researching because (i) there is the possibility of generating high secondary ion/sputter yields, and (ii) negative ions would be useful for insulators. The sputter cluster ion source (SCIS) developed in the University of Antwerp (Belgium) was selected because it can emit cluster ions of pure materials as well as compounds. Gold was chosen due to its high mass and thus high sputter rate. The source was retrofitted to a FLIG column by designing matched ion optics based on simulation, a new mass separator, vacuum devices and power supplies. The ion gun of SCIS-FLIG was then tested and characterised, and defects were improved. As the result, ultra low energy gold cluster ions were obtained for the first time. The ratio of IAu1-:IAu2-:IAu3- is about 13:1.2:1, and a maximum 1 keV Au1 – current density of 38 μA cm-2 was obtained at the sample, which is superior to any reported negative gold cluster ions source. Ions with higher energy (up to 3.7 keV) and positive ions were also obtained. A preliminary study of the usefulness of the gold cluster ions in SIMS depth profiling with a silicon sample was conducted afterwards. Nonlinear sputtering with E0 ≤ 1 keV was observed for the first time. The sputter rates of the ions are higher than that of O2 + and Cs+. Gold deposition due to Au3 – bombardment (E0 = 0.33 keV) was observed at a dose ≤ 0.54×1017 cm-2, otherwise sputtering can be achieved with dose ≥ 1×1017 cm-2. A peculiar observation is the time dependent evolution of the bombarded silicon surface during subsequent atmospheric exposure. Further work in obtaining a more reliable and long-lived source and a higher current density at the sample is recommended