Width, structure and stability of sheaths in metal plasma immersion ion implantation and deposition: measurements and analytical considerations

Abstract

The size, dynamics, and stability of electric sheaths around substrates immersed in vacuum arc plasmas were investigated using positively biased probes. The conditions are applicable to metal plasma immersion ion implantation and deposition (MePIIID). It was found that due to the high plasma density and velocity, the sheath is very thin at the upstream side of the substrate. Its thickness scales approximately with 1 mm kV −1 for the conditions investigated but the data can also be fitted with the theoretical (bias voltage) 3/4 law. The sheath reaches very fast (<3 μs) its average quasi-stationary position. The exact position of the sheath edge is subject to fast fluctuations. It is argued that these fluctuations are correlated with fluctuations of the plasma density. For high plasma density, the electric field strength at the substrate surface can exceed the critical field strength that is known to cause explosive plasma formation. This can cause breakdown (short-circuit) of the sheath voltage. The sheath thickness on the downstream side is much greater due to the wake effect. The observed sheath behavior is explained in light of the sheath theory developed by Child, Langmuir and Bohm. The feature of a self-adjusting sheath thickness is stressed. There is evidence that no pre-sheath exists on the upstream side because the Bohm condition is oversatisfied. Although plasma ions are supersonic, no bow shock was observed.