An Investigation into the Nature of Field Electron Emission Sites on Broad-Area Metallic Electrodes

Abstract

A wide ranging investigation has been carried out into the nature of the localised electron emission sites that give rise to the prebreakdown conduction between extended area electrodes separated by a small (< 5mm) high vacuum gap. A high resolution electron spectrometer has been used to measure the energy distribution of the electrons arising from sites on copper, stainless steel, titanium and gold specimens. At low currents these distributions are shifted 100-500meV below the metal Fermi level and have half-widths of between 300 and 500meV. An extensive series of measurements on copper electrodes has shown that the energy distributions shift to lower energies and become wider as the gap field and the associated site current increases; several sites have displayed multi-peaked spectra, where the relative magnitudes of the peaks is also found to be a function of the gap field. The effect of two forms of in situ surface treatment are reported: outgassing of the specimen is found to increase the stability of the spectrum, whilst ion etching firstly destabilises the emission and then for sufficient depths can eliminate emission sites. A further facility has been developed to obtain complementary optical spectra from electron emission sites. Finally a micropoint anode probe technique has been established to directly locate emission sites in a scanning electron microscope; insulating particles of micron dimensions have been observed within the identified areas at a resolution of ~ 4 microns. The findings of these experiments are interpreted in terms of a model in which the electrons are emitted through a typically micron sized insulating impurity; it is assumed that at a certain critical penetrating field the insulator switches to a high conductivity state with associated electron and optical emission.