Fundamental Limits To Ion Beam Currents in Magnetic Isotope Separators and Related Systems

The technology of using magnetic analysis of high current ion beams was developed initially for uranium enrichment, and has since provided isotope separators for research and for commercial isotope production. It is the heart for commercial ion implantation systems for semiconductor doping, of which many thousand are in use worldwide. Yet its original purpose proved uneconomic largely because of still unsolved beam stability problems. Continuing reduction of critical semiconductor dimensions generates a requirement for very significant reduction in ion beam energies, possibly to below 1 keV, while still providing currents in excess of 1 mA, and most importantly while continuing to achieve a resolving power greater than 50 to maintain required levels of dopant purity. Such beams have similar Poissance to the most intense individual Calutron beams, while their low energy aggravates several degradation mechanisms. Conventional wisdom has been that space-charge-neutralisation of ion beams is essentially unaffected by magnetic fields, and that the high currents used in ion beam milling systems, etc., can be extended to systems with ion beam analysis.