Capacitively coupled radio frequency nitrogen plasma generated at two different exciting frequencies of 13.56 MHz and 40 MHz analyzed using Langmuir probe along with optical emission spectroscopy

Abstract

Capacitively coupled nitrogen plasma discharges driven by two different exciting radio frequencies of 13.56 MHz, and 40 MHz are investigated. Langmuir probe diagnostics along with optical emission spectroscopy are used for interpreting the discharges. The results of these diagnostics are not shown sufficiently in the literature for 40 MHz even though there are some for 13.56 MHz. The electron density ne and the effective electron temperature Teff are calculated from the measurements of the current – voltage characteristics of the discharges. These calculated parameters are correlated with the vibrational temperatures of the N2 second positive system C3Πu−B3Πg and the N2+ first negative system B2Σu+−X2Σg+ measured via optical emission spectroscopy. The population of the vibrational excitation particles plays a crucial role in the determination of the vibrational temperature which strongly depends on ne and Teff. The transition from collisionless stochastic heating mode to collisional Ohmic heating mode into the bulk plasma appears at lower pressure value for 40 MHz as compared to 13.56 MHz. This effect is observed effectively with increasing the RF input power due to the high energy electrons. It is noted that the vibrational temperatures of N2 and N2+ decreases at high-pressure region (>0.3 Torr for 13.56 MHz and >0.2 Torr for 40 MHz) due to a reduction in the relative population of the vibrationally excited particle. The measurements of the Langmuir probe are very consistent with the results of the optical emission spectroscopy.