Dopant cross-contamination reduction with a 3-position beamstop

A new multi-position beamstop has been designed for the Precision Implant 9000 series of ion implanters, to reduce species cross-contamination when switching between B⁺, As⁺ and P⁺ implants. The beamstop features a rotatable flange with three graphite plates mounted on it, each of which is dedicated to one ion species. This prevents re-deposition of the previous beam through back-sputtering or evaporation from the beamstop. Contamination performance of the new beamstop has been evaluated using dynamic and O-leak SIMS, with most emphasis on P and B in As⁺ implants. Reductions of up to 50% have been observed, improving device performance and reducing the need for `pre-season´ implants. The rate of clean-up with repeated implants has been studied, and some contamination is attributed to re-sputtering from the wheel spokes and beam-line. Levels are also found to depend strongly on beam widths and system cleanliness. The effect of venting the system between tests, to accelerate the clean-up rate, has also been investigated