Ion Transport in Ion-Ion Plasmas

When strongly electronegative source gases are used, the LAPPS e-beam-generated plasma device at NRL$^{\ast }$ is capable of producing a steady-state nearly electron-free positive ion / negative ion plasma. It is commonly thought that ion flux to a substrate, in such a plasma, is limited by thermal diffusion of ions in the core plasma. We present a new theoretical treatment which shows that strong ion flux occurs even in the limit of \textit{zero} ion temperature, if a DC bias is applied to the substrate. We show how this ion current scales with source strength, bias voltage, ion temperature, and device geometry. The theory will be used to interpret experimental results from the LAPPS experiment.$^{\ast }$ *See adjacent paper, ``Etching with Electron Beam-Generated Ion-Ion Plasmas,'' by S. G. Walton