Emission characteristics of a conical field emission gun

Abstract

Characteristics of a conical field emission gun (FEG) are measured and discussed. The field emission current from a W emitter is found to obey the Fowler–Nordheim (FN) equation. An expression was given describing the dependence of the mean work function on the curvature radius at the vertex of the emitter. Using scanning electron microscopy, a radius of 1500 Å was measured with which the slope of the FN plot yielded a mean work function of 4.8 eV. The initial value of the emission current, obtained after cleaning the tip by flashing, decays with time due to an increase of the work function caused by the adsorption of residual gases. The measured time dependent mean emission current was interpreted as a weighted superposition of local field emissions from the W(310), (100), (110), (111), and (112) planes. This interpretation resulted in a mean work function of 4.6 eV in good agreement with the value determined from the FN plot. For an accelerating voltage of 15 keV and an emission current of 8 μA after flashing, the dependence of the probe current on the extraction voltage was measured. For Vacc/Vex=1, an angular current density of (41±4) μA/sr was found. Similar measurements of the dependence of the probe current on the ratio Vacc/Vex (accelerating voltage/extraction voltage) made 3 h after flashing yielded (20±6) μA/sr for Vacc=Vex. The transfer efficiency defined as the ratio between probe current and emission current was found to be 0.4%–0.3%.