Development of a negative helium ion source with non-metallic charge exchange

Negative helium ion (He-) beams are required for tandem accelerators used at research centers and implanter facilities. The common production method of such beams involves charge exchange between a positive helium ion (He+) source or beam, with a low pressure alkali metal vapour. However, the efficiency of this reaction is poor (only a small percentage) and the alkali metal vapour increases the potential flammability of the interior surfaces of the reaction chamber, as well as contributing to unwanted electrostatic discharge and target contamination downstream. In this paper we describe initial results from experiments using a focused He+ ion microscope (HIM) to measure charge uptake efficiencies after transmission through carbon membranes. The He+ beam (15 keV to 30 keV, 50 fA to 10 pA) collides with a non-metallic foil, and transmitted particles (He+, Hey, and He-) are separated electrostatically into discrete beam spots and detected using a radiation camera (AdvaCam MiniPIX). A He- percentage uptake of 0.05 was observed using a 20 nm exfoliated graphite membrane consistent with previous reports. Future work will investigate the efficiency of charge exchange through other suitable membrane materials.