AVS 63rd International Symposium & Exhibition
    Plasma Science and Technology Tuesday Sessions
       Session PS-TuM

Invited Paper PS-TuM1
Translational and Vibrational Energy in Cl2 and O2 Plasmas Probed by Innovative Optical Diagnostics

Tuesday, November 8, 2016, 8:00 am, Room 104B

Session: Plasma Diagnostics, Sensors and Control
Presenter: Jean-Paul Booth, LPP-CNRS, Ecole Polytechnique, France
Authors: J.-P. Booth, LPP-CNRS, Ecole Polytechnique, France
D. Marinov, LPP-CNRS, Ecole Polytechnique, France
M. Foucher, LPP-CNRS, Ecole Polytechnique, France
O.Y.N. Guaitella, LPP-CNRS, Ecole Polytechnique, France
C. Drag, Laboratoire Aime Cotton, CNRS-U. Paris-Sud, France
A. Agarwal, Applied Materials Inc.
S. Rauf, Applied Materials Inc.
Correspondent: Click to Email
A common assumption for “Low-temperature” plasmas is that neutral molecules and atoms in the system are in thermal equilibrium with the surrounding ambient (room) temperature, and only charged particles, which can acquire energy from applied electric fields, have higher mean energies. In reality, energy can be transferred from electrons or ions to the neutral gas, increasing the gas translational temperature. Furthermore, non-equilibrium vibrational or rotational distributions can occur in molecular gas plasmas. This can have significant effects on the plasma dynamics. Firstly, since most plasma reactors operate in a pressure-controlled regime, high gas temperatures will cause a considerable decrease in gas density (and therefore in electron-neutral collision rates). Secondly, the rates of activated processes may be significantly increased by translational energy. Vibrational excitation can lead to large increases in the rates of electron dissociative attachment and neutral dissociation. We have developed a new, unambiguous technique to measure gas translational temperature of atoms, using Doppler-resolution Two-Photon Absorption Laser Induced Fluorescence (HR-TALIF) employing a specially-built narrow-bandwidth tuneable pulsed UV laser. Initial results have been obtained on oxygen atoms, where a measurement precision of ±10K is readily obtained. In a DC glow discharge in pure O2 the gas temperature up to 550K are observed. The technique will be extended to the study of lower-pressure inductively-coupled plasmas, where higher temperatures are expected, and to chlorine atoms.

In order to investigate vibrational distributions, we have developed a high-sensitivity ultra-broadband ultraviolet absorption spectrometer. This employs a highly-stable laser-plasma light source and achromatic optics, allowing absorption spectra over a 250nm range to be measured with a baseline stability of the order 10-5. In pure O2 discharges (both DC glow and in a low-pressure ICP reactor) we were able to observe oxygen molecules in vibrationally-excited levels up to v=18 (more than half-way to dissociation), with a “tail” vibrational temperature of 7000K. Vibrational excitation was also detected in Cl2 molecules in a pure Cl2 ICP. However, Cl2 appears to be close to thermal equilibrium with the gas translational temperature, which nevertheless approaches 2000K in this case.

This work was performed within the LABEX Plas@par project, and received financial state aid managed by the Agence Nationale de la Recherche, as part of the programme "Investissements d'avenir" under the reference ANR-11-IDEX-0004-02 and ANR project CleanGRAPH ((ANR-13-BS09-0019).It was also supported by the Applied Materials University Research Partnership Program