The Use of Field Emission Cathodes in Electron Optical Systems: Emission Localization to Small Solid Angles

Abstract

In this chapter several approaches to the fabrication of high-brightness field emission (FE) electron sources are discussed. Many electron optical systems require that the electron beam be formed into a very small spot, of 10 nm or less. The current density per unit solid angle, or brightness β, of the electron source, is limited by the Langmuir equation to a value B_max.

$$ B_{\max } = jeV/(\pi k T), $$

((7.1))

where j is the current density at the cathode, T is the cathode temperature, k is the Boltzmann’s constant, and e is the electron charge. To exceed this Langmuir limit, the size of the cathode must be reduced, and the current density increased. Field electron emission meets these requirements. Unique features of FE electron sources are:

• 1. very high-energy efficiency;

• 2. available steady-state FE current densities approaching 10⁵–-10⁶ A/cm²;

• 3. a very small (∼0.4 eV) electron energy spread;

• 4. a high beam coherence due to the narrow energy spread and the use of small (atomic dimension) emission areas.