Minimum emission current of liquid metal ion sources

The inability of liquid metal ion sources (LMIS) to operate at low de emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum de emission current (I-min) and also predicts I-min as a function of the temperature and the properties of the liquid metal. The model predicts that I-min=217+0.744T(nA) for gallium LMIS, where T is the temperature (K). Measurements of I-min for gallium LMIS between 30 and 890 degrees C are in reasonable agreement with the model. A better fit to this data, however, is given by I-min = 1187 exp(-0.026/kT)(nA) where k is Boltzmann's constant (eV/K). Below 30 degrees C, I-min drops precipitously-values as low as 380 nA have been measured at temperatures as low as 25.8 degrees C. This drop is attributed to a supercooling effect that is not accounted for in the model. I-min is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 mu A for aluminum, I-min is measured to be much lower for bismuth LMIS than for gallium LMIS, as predicted by the model, although difficulties with the bismuth LMIS have allowed only an upper limit of I(min)less than or equal to 77 nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating I-min. (C) 1997 American Vacuum Society.