The liquid metal ion source (LMIS) is an extremely important ion source technologically. In an appropriately designed optical column the small virtual source size and the relatively high angular intensity of the LMIS allows an ion beam to be focused to a spot as small as ≈ 5 nm (FWHM) containing 1 – 10 pA current, producing a current density J ~ 10 A cm-2. When used as a sputtering or machining tool, material can be removed from a surface at a rate of the order of 1 μm3 per nA of ion current, depending on the ion species used and the material being sputtered. Ion beams produced by LMIS-based focused beam systems have proved to be of enormous importance to the semiconductor industry, allowing failure analysis and circuit editing functions to be performed on a ~ 10 nm scale. This has made the LMIS an important enabling technology. As well, it has proved to be of great value for materials research in general.
A LMIS consists of a blunt field emitter substrate coated with a liquid metal having suitable properties, two of the most important being adequate surface tension so the liquid can survive exposure to an intense electric field without disintegrating, and vapor pressure at the melting point low enough that the liquid will last a reasonable time at normal ion currents. When an electric potential of sufficient magnitude is applied to the liquid metal the liquid deforms into a conical shape under the opposing forces of electrostatic field stress and surface tension. As the metal approaches a conical shape the field strength at the apex of the cone becomes high enough to support field evaporation followed by ionization; ions are then accelerated rapidly away from the cone apex by the high field. The flow of the liquid metal that replenishes the material lost by ionization results in a further deformation of the liquid into an elongated jet near the cone apex. At low total emission currents (1 - 2 mA) the source has very useful ion optical properties including a small virtual source size ≈ 50 nm, corresponding to an emittance εx= εy ≈ 5 x 10-7 m rad V1/2 , an angular intensity of ≈ 20 μA sr-1, and a reasonable energy spread ≈ 5 eV. Focused beam currents of 1 pA - 50 nA are routinely achieved. In consideration of the forces involved and the high rate of flow of the liquid metal, the source is remarkably stable: the noise in the ion beam is only ≈ 2X the shot-noise level. Unlike an electron field emitter, the virtual or apparent source size is much larger than the physical source size because of space charge effects (statistical beam broadening). The current density just above the emitting area is ~ 106 A cm-2, as the diameter of the region where ion emission takes place is ~ 5 nm. With a proper source configuration the LMIS is capable of long life (~ 1500 μA-hours for Ga metal).
