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

The size, dynamics, and stability of electric sheaths around substrates imm ersed in vacuum are plasmas were investigated using positively biased probe s. 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 substrat e. Its thickness scales approximately with I mm kV(-1) for the conditions i nvestigated but the data can also be fitted with the theoretical (bias volt age)(3/4) law. The sheath reaches very fast(< 3 <mu>s) its average quasi-st ationary position. The exact position of the sheath edge is subject to fast fluctuations. It is argued that these fluctuations are correlated with flu ctuations of the plasma density. For high plasma density, the electric fiel d 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 downstr eam side is much greater due to the wake effect. The observed sheath behavi or is explained in light of the sheath theory developed by Child, Langmuir and Bohm. The feature of a self-adjusting sheath thickness is stressed. The re is evidence that no pre-sheath exists on the upstream side because the B ohm condition is oversatisfied. Although plasma ions are supersonic, no bow shock was observed. (C) 2001 Elsevier Science B.V. All rights reserved.