Sheath expansion in a drifting, nonuniform plasma

A model is proposed for the sheath expansion into a nonuniform plasma with substantial drift velocity. We refer in particular to the streaming metal p lasma produced by a vacuum are discharge; this kind of plasma is characteri zed by its spatial nonuniformity and high ion drift velocity. It is shown t hat for a nonuniform plasma and high substrate bias voltage (>10 kV), the s heath thickness is significantly smaller than in a uniform plasma. High dri ft velocity also leads to a decrease in sheath thickness. For plasma immers ion ion implantation carried out in a nonuniform plasma, the increase in io n implantation current with increasing voltage is different from that of a uniform plasma where the ion current is constant with voltage. This effect was found to be quantitative agreement with experiment. The decrease in she ath thickness with increasing plasma drift velocity can explain qualitative ly the experimentally observed existence of a stable high-voltage sheath fo r a relatively long time. (C) 1999 American Vacuum Society. [S0734-211X(99) 01106-3].