Sheath Physics


Industrial plasma-processing research at Wright Patterson Air Force Base

Objective: To measure the metastable-ion populations in an inductively coupled plasma discharge and understand their role in controlling the sheath thickness.

Significance: Minute amounts of metastable ions have been found to alter significantly the chemistry associated with plasma processing.

Long Range Goal: To improve models used for industrial applications of plasma.

Collaborators: C.A. DeJoseph, Jr. (Wright Patterson Air Force Base) V.I. Demidov, J. Blessington, Mark Koepke.

The plasma sheath is the interface between the ambient plasma and the boundaries, such as the chamber walls. The size of the sheath can be dramatically influenced by what goes on quite a distance from the sheath, in the ambient plasma, and this would be considered a nonlocal effect. The sheath will adopt a thickness and a voltage drop that will result in a balance of electrons and ions passing through the sheath in equal numbers. The influence of a small minority of energetic electrons from the ambient plasma will result in a thickening of the sheath and an increased voltage drop of the sheath so that ion escape will be increased and electron escape will be decreased to compensate the energetic electrons contribution to the electron escape. If only the less-energetic 99.99% of the electrons participated in this balance, the sheath characteristics would be quite different and, consequently, the plasma processing capabilities would be fundamentally different.

Recent work (see reference and figure below) explores a novel approach for controlling the non-local parameters of near-wall plasma sheath. Non-locality refers to conditions at one point being strongly influenced by conditions elsewhere. Pulsed radio waves are responsible for ionizing argon, neon, and helium and the experiments are conducted immediately after the pulse has produced the plasma. In this instance, the energetic electrons emerge from collisions and from the decay of excited neon, argon, or helium atoms.

The long-range goal of the project is to optimize, well beyond the present state of the art, both near-wall sheath and plasma for the purposes of nanofabrication and plasma-processing. The additional dimension of research capability is broadening student research and educational opportunities in the physics, science and technology of plasmas.

This work was recently nominated to be highlighted in European Physics News, a semi-monthly journal covering the best in innovative scientific publications. The project has incorporated a new plasma-processing research component into the WVU Plasma Physics Laboratory in the WVU Department of Physics in the Eberly College of Arts and Sciences.

References

1.  C A DeJoseph Jr, V I Demidov, J Blessington and M E Koepke, Investigation of a radio-frequency inductive- coupled-plasma discharge afterglow in noble gases, 2007 J. Phys. B: At. Mol. Opt. Phys. 40 3823-3833