Negative ion beam focusing in a plasma sputter-type negative ion source

Abstract

The plasma sputter-type negative ion source (PSNIS) facility of the Plasma Physics Laboratory is designed to grow thin films through ion beam deposition. In the PSNIS, a negatively biased metallic target, e.g. Zr, is immersed in an Ar plasma to produce Zr^- ions by sputtering. The Zr^- ions are extracted to form a beam that is collimated into a reaction chamber, where these ions will be combined with N₂ near the substrate area to form thin nitride films, e.g., ZrN.
So far, problems preventing thin film formation by the aforementioned mechanism have been encountered. One major problem is brought about by divergence of the ion beam. The ion beam is divergent because of the ions' transverse thermal velocities and mutual repulsion. Since the beam must travel the large distance separating the ion source and reaction chambers, beam current densities obtained at the reaction chamber are very low. This would lead to poor substrate coverage and non-uniformity of the deposited film. Thus, there is a pronounced need to focus the ion beam in order to increase the beam current density and have a beam spot size comparable to the surface dimension of the substrate.