Diagnostics of N2 dissociation in RF plasmas by vacuum ultraviolet emission and absorption spectroscopy

Abstract

The N2 dissociation degrees in a 50 MHz inductively coupled plasma (ICP) are evaluated by a procedure using vacuum ultraviolet optical emission spectroscopy (VUVOES). The number density of N 4S° atoms (nN) is evaluated from the N2 dissociation degree by determining the N2 rotational temperature that is in equilibrium with the gas temperature. The nN evaluated by VUVOES is crosschecked by the nN measured by vacuum ultraviolet absorption spectroscopy (VUVAS). The comparison reveals a fair proportionality between the nNs evaluated by VUVOES and those measured by VUVAS, suggesting that VUVOES is applicable to the monitoring of N2 dissociation in low-pressure, high-density N2 plasmas. The number density of N 2D° metastables is also measured by VUVAS and found to be comparable to the number density of the ground-level N atoms (4S°). The dissociative recombination of with electrons is suggested as a possible mechanism of N 2D° metastable generation that makes the N 2D° density comparable to the N 4S° density in the pulsed N2 ICP during power-off periods.