Temperature Distributions Across the Plasma Layer of Planar Low Pressure Microwave Plasmas — A Comparative Investigation by Optical Emission Spectroscopy

Abstract Nowadays low temperature non‐equilibrium plasmas received considerable attention in very different fields of plasma processing. The subject of the present paper is the comparative measurement of neutral gas temperature and optical excitation temperature to analyze the temperature distributions across the plasma layer of H 2 non‐equilibrium plasmas ( p = 0.2 – 1.5 kPa) with small admixtures of hydrocarbons in a novel planar microwave plasma source (2.45 GHz) used for plasmachemical deposition purposes by means of optical emission spectroscopy. Typical microwave power flux densities into the plasma lie within a range of 2 W cm −2 to 20 W cm −2 . Results of neutral gas temperature measurements derived from H α line Doppler profiles are compared with rotational temperatures of H 2 and N 2 molecules. The neutral gas temperature (800–1700 K) corresponds to the rotational temperature of the H 2 molecules (Fulcher band, R 0–0 branch) but shows a more distinct spatial gradient. The rotational temperature of admixtured N 2 molecules (2000–3000 K) is much more higher although Boltzmann distribution was ensured. The spatially resolved measured excitation temperature (1–3 eV) determined with the help of line intensity ratios of admixtured Ar well agrees with Langmuir probe measurements. The reported measurements as a whole demonstrate the feasibility of comparative investigations of different optically determined temperatures for expressive characterization of low pressure microwave plasmas.