Design of a focused electron beam column for ring-cathode sources

This paper describes focused electron beam columns designed for ring-cathode sources. Each cathode is intended to have a nano-size ring-edge thickness (typically around 50 nm), while its radius can measure hundreds of microns or larger, providing an area of emission that is several orders of magnitude greater than that of conventional field emission cathodes. Using methods of first-order and second-order geometric aberration correction, direct ray tracing simulations predict that these columns will be capable of combining high probe current (>1 μA) with high spatial resolution (<20 nm). Several simple column designs consisting of a ring-cathode gun unit combined with either a single magnetic or electric objective lens are presented. The ring-cathode concept is applicable to both field emission and photo-cathode guns.