The extraction of negative carbon ions from a volume cusp ion source

Acetylene and carbon dioxide gases are used in a filament-powered volume-cusp ion source to produce negative carbon ions for the purpose of carbon implantation for gettering applications. The beam was extracted to an energy of 25 keV and the composition was analyzed with a spectrometer system consisting of a 90° dipole magnet and a pair of slits. It is found that acetylene produces mostly $C2−$ ions (up to 92 µA), while carbon dioxide produces mostly O⁻ with only trace amounts of C⁻. Maximum $C2−$ current was achieved with 400 W of arc power and, the beam current and composition were found to be highly dependent on the pressure in the source. The beam properties as a function of source settings are analyzed, and plasma properties are measured with a Langmuir probe. Finally, we describe testing of a new RF H⁻ ion source, found to produce more than 6 mA of CW H⁻ beam.