ABSTRACT: The inability of liquid metal ion sources (LMIS) to operate at low dc emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum dc emission current (I<sub> min </sub>) and also predicts I<sub> min </sub> as a function of the temperature and the properties of the liquid metal. The model predicts that I<sub> min </sub>=217+0.744T( nA ) for gallium LMIS, where T is the temperature (K). Measurements of I<sub> min </sub> for gallium LMIS between 30 and 890 °C are in reasonable agreement with the model. A better fit to this data, however, is given by I<sub> min </sub> =1187 exp (-0.026/kT)( nA ) where k is Boltzmann’s constant (eV/K). Below 30 °C, I<sub> min </sub> drops precipitously—values as low as 380 nA have been measured at temperatures as low as 25.8 °C. This drop is attributed to a supercooling effect that is not accounted for in the model. I<sub> min </sub> is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 μA for aluminum. I<sub> min </sub> 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<sub> min </sub>≤77 nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating I<sub> min </sub>. © 1997 American Vacuum Society.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/1997; · 1.34 Impact Factor