Inductively coupled plasma sources for atmospheric pressure applications
Usual plasma processing requires low pressure, where cold plasmas can be easily achieved. However, the need for a vacuum chamber restricts the number of applications and the low density of the gas leads to long processing times and high energy consumption. Moreover, the standard discharge between two electrodes (corresponding to the capacitive coupling) generates a large space-charge region (sheath) that limits the plasma (free electron) density. This disadvantage can be eliminated by inductive coupling, i.e., generating currents in the plasma by magnetic induction. Moreover, when exciting the plasma with high-frequency fields (e.g., in the microwave range) the ions virtually do not move, while electrons succeed on a short mean free path to create a high number of ions. This feature allows the generation of a stable cold plasma at atmospheric pressure.
The new miniature source developed at FBH combines the advantages of the inductive coupling and the microwave excitation. Among the many prospective applications there are industrial ones (e.g., thin-film deposition) as well as medical ones (e.g. skin treatment).
But the magnetic excitation has also disadvantages. The currents do not flow on a predefined path like in the winding of a transformer. Therefore many modes may be excited. We study these modes using microwave “Hot-S” parameter measurements and optical emission spectroscopy, and correlate them with photographs of the plasma.
In the course of a DFG project with Bochum University we have shown for the first time that an inductively coupled microwave plasma can be excited at atmospheric pressure. Moreover, the estimated efficiency is better than 60 %, i.e., the most part of the incident microwave power is absorbed in plasma, while comparably source at lower frequencies achieve much lower efficiencies.
I. Stefanović, N. Bibinov, H.E. Porteanu, M. Klute, R.P. Brinkmann, P. Awakowicz, "Optical characterization of a novel miniature microwave ICP plasma source in nitrogen flow", Plasma Sources Sci. Technol. 27, 12LT01 (2018).
H.E. Porteanu, I. Stefanović, N. Bibinov, M. Klute, P. Awakowicz, R.P. Brinkmann, W. Heinrich, "Correlated Mode Analysis of a microwave driven ICP source", Plasma Sources Sci. Technol., vol. 28, no. 03, pp. 035013 (2019).