Ion sheath dynamics in a plasma for plasma-based ion implantation

Summary form only given. Spatial and temporal growth and collapse of ion sheath around an electrode of a negative high-voltage pulse (voltage: -10 kV, pulse duration: 10 /spl mu/s) have been studied in a plasma for plasma-based ion implantation. A spherical electrode of 1.9 cm in a diameter is immersed in a nitrogen plasma with the plasma density range of 10/sup 9/ to 10/sup 10/ cm/sup -3/, the electron temperature of 1.4 eV and the gas pressure of 8/spl times/10/sup -4/ Torr. The transient sheath dynamics was observed by the measurement of electron saturation current to a Langmuir probe, where a depletion of electron saturation current indicates the arrival time of sheath edge at the probe position. The expanding speed of sheath edge is higher than the ion acoustic speed until the sheath length reaches the steady-state extent determined by Child-Langmuir law. In the region beyond the steady-state extent, the rarefying disturbance produced by sheath expansion continues to propagate into the plasma at the ion acoustic speed. After the pulse voltage is returned to zero (more exactly, the floating potential), the electron current begins to recover. When the pulse fall time is shorter than the plasma transit time, the electron saturation current overshoots the steady-state saturation current at once, resulting in an excess of plasma density which propagates like a tidal wave into the plasma at the ion acoustic speed.