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

In this paper, we present an optical study of the A(3)Sigma(u)(+) state of N-2 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 x 10(18) m(2) s(-1), from which a N-2(+) ion density of similar to1.5 x 10(9) cm(-3) was inferred. Diode laser cavity enhanced absorption spectroscopy (CEAS) was used to probe the A(3)Sigma(u)(+) state via the B(3)Pi(g)(nu = 3) <-- A(3)Sigma(u)(+)(nu = 0) band at 686 nm. P-33 band head spectra were used to determine both the translational (T-tr) and rotational (T-rot) temperatures of the molecules at the nu = 0 level. These were found to be in equilibrium but dependent on plasma parameters; in a 10 mTorr discharge, T-rot approximate to T-tr, varying from similar to300 K at 5 W to similar to450 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 nu = 0 level was found to be (1.19 +/- 0.07) x 10(10) 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 similar to10(-5). A kinetic scheme is presented to account for the pressure and power dependence of the A-state concentration in the nu = 0 level.