A uniform, large-area thermionic cathode as a high emittance electron source for the SCALPEL(R) tool

The column of the SCALPEL electron beam lithography exposure tool consists of an illumination system and projection system, working in conjunction to expose wafer features as small as 0.1 /spl mu/m and below. In the illumination system a 1/spl times/1 mm wide high energy (100 kV) electron beam illuminates a thin membrane mask. After passing through the mask, electrons are focused by means of a projection doublet onto the silicon wafer with a 4/spl times/ demagnification. This illumination system possesses an unusually high depth of focus (at least 10/sup 11/) reproducing nonuniformity of emission at the cathode surface at the mask plane and subsequently by the projector system at the wafer plane, thus downgrading the exposure quality.