Energy spread in liquid metal ion sources at low currents

In previous work, attempts to lower the energy spread in Liquid metal ion sources (LMIS) by resorting to low current operation have rarely been successful. We have found the energy spread of gallium LMIS to be approximate to 4.5 eV full width at half-maximum for emission currents from 0.45 mu A down to 13 nA. Our experimental results suggest that this occurs because emission (0.45 mu A is formed by pulses of height 0.45 mu A and an appropriate duty cycle. Several observations support this hypothesis: (1) For sources that had a minimum current, below which they could not be made to operate, the minimum current was always 0.45 mu A at room temperature, regardless of apex radius or surface roughness. (2) Emission less than or equal to 0.45 mu A from blunt sources at room temperature always showed pulses of height 0.45 mu A, and a clear transition to de at 0.45 mu A The pulse height and transition current rose with source temperature. (3) Emission less than or equal to 0.45 mu A from sharp sources always showed pulsing from the lowest currents up to the current at which the amplifier bandwidth of 1 MHz was exceeded. (4) The angular intensity of emission on axis was in constant proportion to emission current up to 0.45 mu A, and in declining proportion at higher currents. (C) 1996 American Vacuum Society.