Survey of electron sources for high-resolution microscopy

An estimate is made of the characteristics of some cathodes (thermionic and field emission) in an ultra-high-spatial-resolution electron beam column. It is shown that thermionic cathodes provide an order of magnitude less current than a field emission or an extended Schottky cathode in a 100 keV system at 0.5 nm resolution. It follows that field or extended Schottky mission cathodes should be the cathodes of choice for ultra-high-resolution applications. A comparison of the properties of cold, thermal field and extended Schottky emission cathodes is given.