The influence of plasma boundary shape on the transport of energetic ion beams

The effect of plasma boundary shape on the characteristics of medium energy (10-40 keV) ion beams has been studied. A tetrode extraction lens is used to separate the processes of ion extraction and acceleration during the formation of both Ar and BF_x beams from an Applied Materials Bernas-type ion source. Perveance measurements reveal that the system operates in a bimodal manner that is set by the magnitude of the extraction field. The two modes are characterized by perveance values that are consistent with the plasma boundary shapes being concave in one case and convex in the other. Beams are simulated with these two plasma shapes using the charge density method. Predictions that narrower beams will be formed from convex plasma boundaries are in agreement with measurements of the beam profiles, made 880 mm downstream from the source. Each beam type is passed around a 400 mm analyzing magnet and profiled both 170 mm upstream and 280 mm down-stream from the focal point of the magnetic lens. The beam extracted from the convex plasma boundary maintains its shape between these two points while the beam sourced from the concave plasma displays aberrations, suggesting that convex plasma modes may be well suitable for use in long-path implant systems