High-field ion sources

Abstract

High electric fields may be used to create ions either by field evaporation, as in the case of liquid metal ion sources (LMISs) or by field ionization, as in the case of gas-phase field ionization sources (GFISs). LMISs have now reached a mature state of development and have found an increasing number of focused beam applications, ranging from submicron mask repair to maskless ion implantation. They may be fabricated from either pure elements, or from alloys, provided that the element or alloy vapor pressure is low enough to avoid excessive evaporation. In the case of alloys, it is necessary to employ a mass filter, typically a Wien filter, to remove unwanted ion species from the ion beam. Angular intensities, at the threshold of source operation, range from 1 μA/cm2 for alloy LMIS ion species to 20 μA/cm2 for elemental LMISs; under the same operating conditions, the ion energy spreads are ∼5 eV. Recent advances in the development of GFISs raise the possibility that GFISs may be nearing the stage when they also might be used in fine focus applications. By special shaping of the metal emitter in the apex region and by cooling the emitter to ∼20 K; it has been found possible to cause confinement of the ion emission to ∼0.5° half-angle, thereby increasing angular intensities of beams of He+ or H+2 to values similar to those obtained from LMISs.