In a Liquid metal ion source (LMIS), a metal is heated to the liquid state. A Taylor cone is then formed under the application of a strong electric field. As the cone's tip gets sharper, the electric field becomes stronger, until ions are produced by field evaporation. These ion sources are particularly used in ion implantation or in focused ion beam instruments.
Typically gallium is preferred for its low melting point, low vapor pressure, its relatively unreactive nature, and because the gallium ion is heavy enough for ion milling.
The LMIS technique originated in the development of colloid thrusterspacecraftpropulsion systems. Research beginning in the early 1960s showed that liquid metal can generate large numbers of ions. By the early 1970s, these results spawned the development of LMIS ion microprobes. Initially, in the development of this technique, the liquid metal was supplied by a capillary tube. This method can be difficult to control at low emission currents. The technique of "blunt-needle" LMIS was discovered by accident in the early 1970s. For this method a thin-film of liquid metal is allowed to flow to the apex of a sharp needle (see, Orloff 2009).