A Study of Electrostatic Ion Sources

Abstract

Ion sources have important applications in metallurgy, solid state physics and biology, where the controlled removal of materials is required. The present thesis is aimed at increasing the understanding of "saddle-field" electrostatic ion sources in which an oscillating electron beam produces a narrow beam of ions without the aid of a magnetic field. The characteristics of the cylindrical source, incorporating a cylindrical cathode and two anode rods, have been investigated. The main objectives were the improvement of the intensity and focussing of the emerging ion beam so as to increase the ion etching rate. The characteristics of a new source, which incorporates an annular anode instead of the two anode rods and two hemispherical cathodes instead of a cylindrical cathode, have also been investigated. It was found that this source produces an intense narrow central beam which is very suitable for ion-thinning of small specimens. The energy of the emerging ions have been measured for both sources. It was found that the energy distribution for both sources is broad but exhibits a number of peaks at different energies. The shape of the energy spectrum depends upon the gas pressure, the nature of the gas, the discharge conditions and the size of the ion exit aperture. The charge state of the ions is important for nuclear applications. A magnetic analyzer has been used to determine the charge states of ions produced by the cylindrical source. Using the appropriate gas, the predominant ions were found to be H+2, He+, N+ and A2+. Higher charge states such as N3+ and A3+ were also observed at lower abundance. However, the relative abundance of these states increases as the input power to the source is increased. It would appear that practical forms of "saddle-field" electrostatic ion sources described above can find wide application in materials analysis and nuclear physics.