Recent tests of negative electron affinity photocathodes as source for electron lithography and microscopy

Abstract

We report recent results from tests of laser driven and modulated negative electron affinity photocathodes as high-brightness electron sources. Preliminary tests used an electron column that could image the cathode surface with a resolution of less than 100 nm. We measured a current density of 2.4(±0.2) A/cm2 from an emission area with a diameter [full width at half maximum (FWHM)] of 3.5 μm. However, unwanted field emission in the gun (aided by the presence of Cs) limited the extraction field to about 1 kV/mm and negatively affected quantum efficiency and emission stability. A curve fit of the source angle with accelerating voltage gave a typical transverse energy spread of 44(±2) meV, for an extrapolated brightness at 50 kV of 1.7×106 (±0.2) A/cm2 sr. Indications that space charge above the flat cathode limited the current density are presented; a possible model is discussed. The electron gun has been redesigned for electric fields up to 7 kV/mm (where space-charge effects are unlikely) with no field emission. Lack of access to an imaging column has prevented subsequent measurements of current density and brightness. In a nonimaging column an emission current of 200(±0.4) nA, from a single emission area, has been maintained for more than a week, with no reapplication of cesium. As with other photocathodes, variations of quantum efficiency after sudden changes in illumination intensity are usually present. We suggest that competing effects of Cs mobility and electron trapping explain this behavior. Transverse energy spreads (FWHM) have been studied as a function of extracted current; 20(±5) meV at small currents (<10 nA) and 70 meV at large currents (700 nA), both from an illumination area of less than 2 μm diameter, are typical.