Characterization of an inductively coupled N2 plasma using sensitive diode laser spectroscopy

Abstract

In this paper, we present an optical study of the state of N2 produced in an inductively coupled plasma. The operation of the discharge was characterized using ion flux measurements and broadband optical emission, and a clear change from capacitively to inductively coupled behaviour was observed with increasing applied power. The typical ion flux at 100 W and 10 mTorr was found to be 1.8 × 1018 m2 s−1, from which a ion density of ∼1.5 × 109 cm−3 was inferred. Diode laser cavity enhanced absorption spectroscopy (CEAS) was used to probe the state via the band at 686 nm. P33 band head spectra were used to determine both the translational (Ttr) and rotational (Trot) temperatures of the molecules at the v = 0 level. These were found to be in equilibrium but dependent on plasma parameters; in a 10 mTorr discharge, Trot ≈ Ttr, varying from ∼300 K at 5 W to ∼450 K at 400 W applied power. Absolute number densities in individual spin–rotation states were determined by calibrating the CEAS technique using the cavity ringdown time to measure the mirror reflectivity. The overall population in the v = 0 level was found to be (1.19 ± 0.07) × 1010 cm−3 under typical conditions of 100 W radio frequency power and 10 mTorr pressure, corresponding to a discharge efficiency for the production of this level of ∼10−5. A kinetic scheme is presented to account for the pressure and power dependence of the A-state concentration in the v = 0 level.