Analysis of the Particles Produced by Saddle Field Ion Sources

Abstract

Review of the literature concerned with the production of ion beams is given and the development of saddle field ion sources and their importance in relation to other devices is also discussed. In this work, two forms of the saddle field ion source, one with cylindrical and the other with spherical geometry, have been studied for argon, nitrogen, and helium, and a comparison between them has been made in terms of their characteristics and applications. It has been confirmed that in both sources there are three principal modes of operation - the oscillating, transition, and glow modes. These have been explained in terms of the mean free path for electron-molecular collisions inside the source and its relation to the dimensions of the source and show that the source efficiency is greatest in the low-pressure oscillating mode. A retarding field energy analyser has been developed to measure the energy distribution of the ions. The results show that the energy spectra of both sources are dominated by a high energy peak which arises from ions being accelerated through a potential equal to that between the saddle point and the cathode. Lower energy ions are also produced but these are mainly confined to the edges of the ion beam. The proportion of these low energy ions increases as the mode changes from the oscillating to the glow mode. The energetic neutral content of the beam has been studied using an electrostatic analyser. These experiments indicate that the percentage of energetic neutrals can vary from 10% to 80% with different gases and modes of operation. These results have been confirmed by etching copper films and are explained in terms of a charge exchange process between a fast ion and a slow neutral occurring inside the source