The Physical Origin of Pre-Breakdown Electron Emission From Broad-Area High Voltage Electrodes
An investigation has been undertaken into the nature of the localised electron emission sites which cause pre-breakdown conduction between broad area electrodes separated by short ultra high vacuum gaps. A new automated specimen scanner and an electron imaging technique have shown that emission sites are composed of many distinct electron sources (sub-sites) which in total may extend over areas as large as 1mm2. The energy distributions of the electrons emitted from individual sub-sites on copper electrodes have been measured using a high resolution spectrometer. The spectra invariably have single peaks which are shifted at least 200 meV below the metal Fermi level and have half widths > 300 meV. Detailed analyses of the field dependence of the various spectral parameters have provided sufficient data to enable a quantitative model of the emission process to be constructed. This assumes that thin (<1 um) contaminated insulating layers provide the structures necessary to initiate electron emission. Conduction through the insulator, in the form of narrow filaments, then leads to local field intensificatim and to a "hot electron" mechanism at the vacuum interface which allows emission over the potential barriers The model provides relationships between previously unconnected observations, particularly the amount of spectral shift and the degree of non-linearity of a Fowler-Nordheim plot, and allows the dimensions and properties of the conducting filaments to be estimated. The validity of the model is supported by two types of in situ surface treatments: argon ion etching,which not only removes emitting sites but allows others to be initiated, and thick oxidation, which has yielded important information on the nature of the conduction processes occuring in the insulating layer.
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