Investigation of a tin liquid metal ion source

In view of its importance in materials research, tin is a metal worth studying in a liquid metal ion source configuration, even if results complement or extend previous work. This is the more so if the new work corrects misconceptions of the past and adds to current thinking. We, therefore, prepared a Sn liquid metal ion source employing a Ni needle to anchor the liquid, cone-shaped, emitter. Source properties, such as the current-voltage curve, the mass spectra of the beam and the energy spread of the main ionic species, were studied in detail. The mass spectra show a considerable amount of Sn clusters, apart from the dominant species, Sn⁺ and Sn⁺⁺. The source was stable down to 1-μA emission current, corresponding to an energy spread for the singly charged ions of 7 eV. Theoretical arguments, involving the peak energy deficit of the ion-energy distribution, strongly suggest that both Sn⁺ and Sn⁺⁺ are emitted by direct field evaporation from the liquid surface. The same conclusion is reached from a careful examination of the beam mass spectra of the source.