Ion and Electron Emission from Liquid Metal Sources

In liquid metal ion sources, the emission is located at the apex of a liquid cone (the often so-called Taylor cone), formed by electrostatic forces and surface tension. Reversal of the extraction voltage polarity results in electron emission from the liquid metal surface. For small apex radii, ≤ 1 μm, steady field emission of electrons has been observed, whereas for apex radii ≥ 10 μm, explosive pulsed emission occurs. Since the onset voltage for electron emission has been found to be considerably lower than the critical voltage for the formation of the Taylor cone, it has been concluded that dc (direct current) electron emission from a field stabilized cone is not possible. In situ high-voltage transmission electron microscopy observations seem to confirm this conclusion, although in one case, a field-stabilized liquid micro-cone during electron emission has been observed for a liquid-gallium-indium-tin source. The literature on liquid metal ion and electron sources is reviewed. From in situ transmission electron microscopy observations of the cone formation, relations for cone angle and jet length dependent on ion emission current are derived. Growth and emission of microdroplets at liquid indium ion sources have been imaged. In the case of electron field emission from liquid indium sources, no liquid cone formation has been observed