Dynamic sheath expansion in a non-uniform plasma with ion drift

Dynamic sheath expansion in front of a target biased with a negative, high-voltage pulse is investigated in a non-uniform plasma, taking into account the influence of ion drift, which is inevitable in diffusive plasmas with a non-uniform density profile. Temporal evolutions of a sheath edge and a rarefaction wave measured in a low-pressure argon plasma diffusing towards the target agree well with numerical predictions of their transient behaviors as obtained using a dynamic sheath model for non-uniform plasmas with ion drift. It is found that the thickness of the expanding sheath edge is reduced considerably by the ion drift velocity when compared with the thickness without ion drift. Moreover, because the ion drift prevents the propagation of the rarefaction wave significantly, the phase velocity of the wave is observed to be much less than the Bohm speed. The propagating characteristics of the rarefaction wave confirm that the ion drift velocity plays an important role in the dynamic sheath expansion in non-uniform plasmas with ion drift. The results are expected to be useful in analyzing the dose and energy of implanted ions as well as understanding the sheath dynamics in a real plasma source ion implantation system in which the plasma sheath commonly evolves in a non-uniform plasma with ion drift.