LIQUID-METAL ION SOURCES - THE SHAPE AND SIZE OF THE ION EMITTING AREA

Using the potential profile around the cone-shaped protrusion at the end of the Taylor cone of the liquid metal ion sources, we investigate the shape and size of the ion emitting region and the ion formation mechanisms analytically. We find that the field evaporation is the main current-generating mechanism. Contribution of the field ionization mechanism is around 0.01 muA, independent of the applied voltage. The ion emitting area is usually confined within a few tens of angstroms from the protrusion apex. The most important parameter that determines the emission current is found to be the apex angle of the protrusion cone. The apex angle near the ignition voltage is close to the Taylor angle. At this voltage, the ions are emitted from the very vicinity of the cone apex. Above the ignition voltage, a sharp protrusion cone develops of which the apex angle decreases as the applied voltage increases. In other words, sharpness of the cone apex is the measure of the field strength responsible for the field evaporation. At currents about 10 muA, the apex half angle becomes about 36-degrees, and the ion emitting region expands to about 20 angstrom from the apex. At currents above 100 muA, the apex half angle reduces to less than 28-degrees, and the emission region is still confined within 100 angstrom from the apex. These results are found to be well-fitted to the previously reported microscope pictures taken by G. Benassayag [Ultramicroscopy 16, 1 (1985)].