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 b een 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 t his 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 t his hypothesis: (1) For sources that had a minimum current, below whic h 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 roug hness. (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 c lear transition to de at 0.45 mu A The pulse height and transition cur rent rose with source temperature. (3) Emission less than or equal to 0.45 mu A from sharp sources always showed pulsing from the lowest cur rents up to the current at which the amplifier bandwidth of 1 MHz was exceeded. (4) The angular intensity of emission on axis was in constan t proportion to emission current up to 0.45 mu A, and in declining pro portion at higher currents. (C) 1996 American Vacuum Society.