Paper PS2-ThA3
Influence of Argon Metastables on the Rotational Temperature of Nitrogen in Inductively Coupled Ar/N₂ Plasmas

Thursday, November 12, 2009, 2:40 pm, Room B2

In low-pressure discharges commonly used for materials processing, neutral gas temperature are routinely determined from the rovibronic structure of N₂ inserted as a tracer. This is realized by comparing the measured emission intensities of the bandhead and the violet-degraded tail of the second positive system of N₂ (C³Π_u ν’ -> B³Π_g ν’’) to the prediction of a model with the rotational temperature, T_rot, as the adjustable parameter. Such measurements are usually performed using the most intense (ν’,ν’’)=(0,0) and (0,2) bands at 337.1 and 380.5 nm. However, in argon-containing plasmas, these bands typically yield temperatures larger than those obtained from other methods such as Doppler-shifted laser-induced fluorescence (D-LIF). Hypotheses for such discrepancy vary; either the emitting C³Π_un’=0 level (11.026 eV above the ground state) could be populated by the ³P₀ and ³P₂ argon metastables (11.723 and 11.548 eV above the ground state) or the spatial non-uniformity of the plasma could skew D-LIF measurements. In this work, we examined the influence of Ar metastables on the rotational temperature of N₂. We compared T_rot values obtained from different N₂ bands, notably (0,0), (0,2), (1,0), and (4,2) to a less conventional plasma sampling mass spectrometry (PSMS) technique in which the Ar plasma on-to-plasma off signal intensity ratio is linked to the gas temperature. These measurements were performed in an inductively coupled 98%Ar/2%N₂ plasma as a function of pressure and absorbed power. We show that N₂ bands with ν’≤2 generated much higher T_rot values than the (4,2) band or the PSMS. For example, for a 20 mTorr, 1000 W Ar plasma, the (0,0), (0,2), (1,0), (4,2), and PSMS yielded temperatures of 973, 715, 920, 485, and 415 K, respectively. We then computed the reaction rates for excitation of the C³Π_u ν’=0 level by collisions with Ar metastables, R-Ar^m, and by electron-impact, R-e, from the ground state. The densities required as inputs for those calculations were measured by Langmuir probe for electrons and, for the metastables, were determined from a global model. We found that that the ratio of the temperatures obtained from the (0,0) and (1,0) bands to that of the (4,2) band increased quasi-linearly with the R-Ar^m-R-e ratio, going from 1.5 to 2 as the Ar metastable-to-electronic excitation rate ratio increased from 0.01 to 0.1. Since Ar metastables can have a strong influence on rotational temperatures even for R-Ar^m-R-e ratio as low as 0.01, accurate gas temperature measurements in mostly Ar plasmas thus require analysis of bands for which ν’≥3 as these levels (e.g. 11.74 eV for C³Π_u ν’=4) are above the ³P₀ Ar metastable level.