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 b inary alloys (Cu0.53W0.47 and Ni0.92W0.08) under steady-state conditions we re investigated both by experiments and by computer simulations. Ari ions w ith impact energies ranging from 65 to 1030 eV were used as bombarding spec ies. The experimental data were recorded with a secondary-neutral mass spec trometer that combines electron-gas post-ionization with a high-transmissio n 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) lar ger emission-energy range could be monitored in the computer simulations wh ich 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 spu ttering theory is generally not valid for the low (near-threshold) impact e nergies used in this work and for emission energies approaching the maximum possible values. Rather, the energy spectra exhibit, over a considerable e nergy range, an exponentially decreasing yield with increasing emission ene rgy. The large range of emission energies detected provided the opportunity to derive estimates of the maximum emission energy and its dependence on t he primary-ion energy E for the various atomic species. These values amount to similar to 0.55E for Cu and Ni atoms and to similar to 0.35E for W atom s sputtered from the alloys and to similar to 0.6E for Cu atoms sputtered f rom the pure Cu specimen; they are compared to simple estimates of the maxi mum emission energies in low-energy sputtering based on collision sequences leading to the ejection events that involve only a small number of atoms. (C) 1999 Elsevier Science B.V. All rights reserved.