Energy spectra of sputtered species under sub-keV ion bombardment: experiments and computer simulations

Energy distributions of neutral atoms sputtered from elemental Cu and two binary alloys (Cu0.53W0.47 and Ni0.92W0.08) under steady-state conditions were investigated both by experiments and by computer simulations. Ar+ ions with impact energies ranging from 65 to 1030 eV were used as bombarding species. The experimental data were recorded with a secondary-neutral mass spectrometer that combines electron-gas post-ionization with a high-transmission double focusing mass spectrometer. Due to the instrument's high detection sensitivity, the energy spectra could be determined over a large intensity range, covering up to five orders of magnitude. A comparable (or even) larger emission-energy range could be monitored in the computer simulations which were done with the dynamic TRIDYN code. The computed energy spectra are in qualitative agreement with the corresponding experimental distributions. Not surprisingly, the data from both approaches indicate that the inverse-square emission-energy dependence of the yield predicted by analytical sputtering theory is generally not valid for the low (near-threshold) impact energies used in this work and for emission energies approaching the maximum possible values. Rather, the energy spectra exhibit, over a considerable energy range, an exponentially decreasing yield with increasing emission energy. The large range of emission energies detected provided the opportunity to derive estimates of the maximum emission energy and its dependence on the primary-ion energy E for the various atomic species. These values amount to ∼0.55E for Cu and Ni atoms and to ∼0.35E for W atoms sputtered from the alloys and to ∼0.6E for Cu atoms sputtered from the pure Cu specimen; they are compared to simple estimates of the maximum emission energies in low-energy sputtering based on collision sequences leading to the ejection events that involve only a small number of atoms