Field emission, developments and possibilities

Abstract

SUMMARYThe overall performance of any electron microscope is to a great extent determined by the electron source. For example, the current acceptance of the STEM concept is due to the stimulation provided by field emission sources, which made it possible to image single atoms in the STEM.The field emission source remains the electron source with the highest brightness and the lowest energy spread. As a consequence, considerable research and engineering work has been and is still being conducted in a number of important areas. These areas include: (i) the mechanisms which determine the beam current stability; (ii) the electron‐electron interactions which lead to beam spreading and an increase of the energy spread; (iii) various types of emitters such as bare tungsten tips, oxygen processed tips, zirconium coated tips, carbon emitters, carbon coated tungsten emitters and even solid state emitters; (iv) the lifetime limiting factors, e.g. inclusions, ion bombardment and flashovers effects; (v) different types of optical systems for field emission guns, e.g. choice of magnetic and/or electrostatic lenses at high voltage potential in UHV systems, and special magnetic lenses just below the anode; (vi) field emission guns operating at higher kV's, up to 1·6 MeV.Because analytical work in the smallest possible volumes is a growing area of scientific interest, the demand for field emission sources which are reliable, easy to operate and stable will continue.