Formation of neutral and positively charged clusters (Ag n and Ag n+; n ⩽ 4) during sputtering of silver

Abstract

The emission of neutral and positively charged silver clusters during sputtering of a polycrystalline silver target by 5 keV Ar + ion bombardment has been studied and the sputter ejected silver flux has been characterized. While the sputtered ions Ag n + were measured directly using secondary ion mass spectrometry, appropriate postionization techniques (50 eV electron impact ionization; two-photon ionization at λ = 248 nm) were applied for the simultaneous measurement of the emitted neutral clusters Ag n . By the combination of these techniques, the contribution of Ag n + and Ag n in the sputte emitted silver flux is determined quantitatively. As a result, the silver flux is found to be strongly dominated by neutral clusters rather than cluster ions (flux ratio: Ag n + Ag n ⩽ 1% for n = 1–4). The contribution of neutral clusters in the overall silver flux decreases rapidly and monotonic with increasing cluster size n and decreases, in addition, with decreasing bombarding energy (⩽ 5 keV) in agreement with a statistical model for the emission of sputtered neutral metal clusters. The well known alternation of the secondary ion intensities of Ag n + as a function of cluster size (higher intensities for odd n) is found to be correlated with the effective “ionization potentials the corresponding sputtered neutral clusters, which were additionally measured.