This report deals with the application-oriented research of molecular plasmas. Different discharge types have been developed in the past few decades and have reached industrial maturity. Low pressure plasmas often play an important role and the plasma chemistry of the neutral particles has a particularly key function.
The plasma processes, which are mainly electronically induced, start complete chains of various secondary chemical reactions that comprise the entire group of materials of the original molecules. Almost always there is an immeasurable number of compounds, stable molecules, and neutral radicals.
New findings are visible in detailed models of possible chemical reactions and the measurement of neutral specimen.
The application-oriented optimization requires a strong understanding of the processes involved. The formed molecules have to be identified and quantified with the highest selectivity and sensitivity, even across a wide, dynamic range of concentration measurement.
Almost without comparison, high resolution infrared absorption spectroscopy using tunable lead salt laser diodes (TDLAS) fulfills the demand for the concentration measurement of stable and transient molecules: The majority of plasma particles – neutral molecules in their ground state – are experimentally accessible under plasma conditions and in situ without taking samples or doing external calibration.
A paper was recently published on the successful collaboration between the Laboratoire d’Engéniere des Materiaux et des Haute Pression at the University of Paris and the Institut für Niedertemperatur- Plasmaphysik e. V. (Institute for Low Temperature Plasma Physics) in Greifswald. [1]
The fundamental diagnostic method used in diamond-forming hydrogen methane microwave plasmas (f=2.45 GHz) was carried out at a pressure range of 2.5 to 12 kPa. The goal: To compare a thermochemical model for the optimization of diamond deposition processes with experimentally achieved absolute concentrations of the involved hydrocarbon molecules. In this way, not only the methyl radical but also related molecules such as methane, ethane, and ethyne could be quantified. The visible line character of the measurement was accounted for with a special one-dimensional model. For the plasma chemical model 28 specimen and 131 reactions were processed. The correlation of the experimental data to the data gathered from the model makes it possible to better describe the chemical processes in plasma (i.e. not only in the plasma volume itself but in close proximity to the substrate).
[1] G. Lombardi, K. Hassouni, G. D. Stancu, L. Mechold, J. Röpcke and A. Gicquel, Plasma Sources Science and Technology 14, 440-450 (2005).
