EMISSION CHARACTERISTICS OF A CONICAL FIELD-EMISSION GUN

Characteristics of a conical field emission gun (FEG) are measured and discussed. The field emission current from a W emitter is found to ob ey the Fowler-Nordheim (FN) equation. An expression was given describi ng the dependence of the mean work function on the curvature radius at the vertex of the emitter. Using scanning electron microscopy, a radi us of 1500 Angstrom was measured with which the slope of the FN plot y ielded a mean work function of 4.8 eV. The initial value of the emissi on current, obtained after cleaning the tip by flashing, decays with t ime due to an increase of the work function caused by the adsorption o f residual gases. The measured time dependent mean emission current wa s interpreted as a weighted superposition of local field emissions fro m the W(310), (100), (110), (111), and (112) planes. This interpretati on resulted in a mean work function of 4.6 eV in good agreement with t he value determined from the EN plot. For an accelerating voltage of 1 5 keV and an emission current of 8 mu A after flashing, the dependence of the probe current on the extraction voltage was measured. For V-ac c/V-ex = 1, an angular current density of (41 +/- 4) mu A/sr was found . Similar measurements of the dependence of the probe current on the r atio V-acc/V-ex (accelerating voltage/extraction voltage) made 3 h aft er flashing yielded (20 +/- 6) mu A/sr for V-acc = V-ex. The transfer efficiency defined as the ratio between probe current and emission cur rent was found to be 0.4%-0.3%. (C) 1998 American Vacuum Society.