Emission characteristics of the ZrO/W thermal field electron source

Abstract

The ZrO/W thermal field emitter (TFE) has proven to be a useful high brightness electron source for a variety of microprobe applications. Desirable source characteristics are: 1 to 2 mA/sr angular intensity, 1 to 1.5 eV energy spread for an emitter radius of 0.6 to 1 μm, and <0.5% noise level between 1 Hz and 25 kHz. In this study we examine the effect of the faceted shape1 of the ZrO/W TFE on the angular distribution of the emission and virtual source size. In particular the scwim computer program1 is employed to compare the emission characteristics of faceted and spherical shaped emitters. The computer derived results agree qualitatively with experimental angular emission distributions measured for various emitter radii. It is shown that the high angular intensity obtained from the ZrO/W TFE is due in part to a large angular demagnification caused by the faceted emitter region. Nevertheless, a comparison of the virtual source size of the faceted and the spherical shapes shows only a small source image increase due to faceting. The minimum values of virtual source size at an angular intensity of 1 mA/sr for the spherical and faceted emitters are 8 and 12 nm, respectively.