Properties and applications of saddle-field ion sources

Abstract

The operation of these dc-energized cold cathode sources is based on the effect that electrons describe long oscillatory paths in the presence of an electrostatic saddle potential field. A high probability thus exists of ionizing any gas present in the source, to maintain a discharge at considerably lower pressures (10−4–10−3 Torr) than in conventional cold cathode tubes without a magnetic field. Depending on the symmetry of the saddle-field, beams of various cross-sections are obtained: an axial field produces a high-intensity pencil beam (2 mm diameter), planar fields give rise to divergent beams (e.g.,extending to 25 cm diameter at 10 cm from the source operating at 20 mA, 2 keV). The beam contains a high proportion of energetic neutrals, or may be fully neutralized on emerging from the cathode aperture, depending on source geometry and applied field. Neutralization may be due to the recombination of ions with slow secondary electrons from the edges of the aperture. The operating pressure is compatible with filament and electron beam gun evaporation. Evaporation on a surface exposed to a saddle-field beam combines rapid deposition with strong adhesion characteristic of sputtered films. Other applications include preparation of specimens for TEM and SEM, cleaning and profiling for surface analysis, ion beam machining and sputtering. At present there appears no obstacle to further increasing the size and output of saddle-field ion sources.