Influence of the field frequency on the nitrogen atom yield in the remote plasma of an N2 high frequency discharge

An N-2 discharge sustained by an electromagnetic surface wave is investigated in order to develop an efficient nitrogen atom source. We take advantage of the flexibility of surface-wave discharges (SWDs) in terms of operating frequency to examine the influence of the field frequency (f = 13.56, 40.68, 440 and 2450 MHz) on the nitrogen atom concentration in the discharge spatial afterglow. The effects of absorbed power P-A (up to 200 W), pressure p (1 to 8 Torr) and discharge tube diameter phi (4.5 and 15 mm) are also considered. The N atom concentration is determined through emission spectroscopy from the afterglow following validation by NO titration. We find that: (i) the N atom concentration increases with P-A but saturates past a certain power, the value of which decreases with increasing f, (ii) the N atom saturation concentration is 1.5 x 10(15) cm(-3) at 2450 MHz but only 7 x 10(14) cm(-3) at f = 13.56 MHz (p = 4 Torr); (iii) the vibrational 'temperature' of the N-2 (C(3)Pi(u)) state in the discharge varies in the same way as the N atom concentration with respect to f and P-A; (iv) whatever f, saturation of the N atom concentration occurs at a threshold value T-r = 500-600 K (p = 4 Torr) of the N-2 molecule rotational 'temperature' in the discharge, suggesting that too high a gas temperature is the limiting factor of the N atom yield (v) the N atom concentration increases with increasing p and decreasing phi.