A Diagnostic Study of a Saddle Field Ion Source

Abstract

This thesis describes a diagnostic study of the spherical version of the saddle field ion source. It also includes a review of the development of the source and its importance in relation to other sources in various aspects of ion beam technology. It has been shown that the source is easy to construct and operate, and the properties of the source have been studied for argon, nitrogen, and helium. The source is characterised by three principal modes of operation: the glow, transition, and oscillating modes. Experimental studies and theoretical considerations have confirmed that the source efficiency is greatest in the oscillating mode. This occurs at low pressures when the mean free path for ionisation by electron impact is greater than the radius of the source. The source produces energetic ions, electrons, and fast neutrals. An electrostatic deflecting system and an energy analyser have been used to show that the electrons are of low energy and originate from the edge of the cathode aperture due to secondary emission by ion bombardment. These electrons surround the ion beam and do not appear to be important in the application of the source to ion bombardment studies. A simple probe technique has been developed to show that the fast neutrals are mainly confined to the centre of the beam. A crossed beam technique has been used to show that the neutrals have energies similar to that of the ions. A theoretical analysis supports the idea that the neutrals are produced by a resonance charge exchange process inside the source. The neutral/ion content varies from about 20% in the low pressure oscillating mode to as much as 90% in the glow mode. Advantage can be taken of this effect when it is required to etch electrically insulating materials.