Investigation of Single and Dual RF Capacitively Coupled Nitrogen Plasma Discharges Using Optical Emission Spectroscopy

High-resolution optical emission spectroscopy and automated Langmuir probe are applied to examine the production of active species in the pure nitrogen plasma excited by single (40.68 MHz) and dual high-frequency (HF) radio frequency (RF) power sources (40.68/2.1 MHz). The emission intensities of the spectral bands correspond to the (0, 2) transition of the second positive system (SPS) of N₂ (λ =380.50 nm) and (0, 0) transition of the first negative systems (FNSs) of N₂⁺ (λ =391.40 and 427.81 nm have been measured and compared with plasma parameters using the Langmuir probe to investigate the dependence of their radiative states on operating conditions for both single and dual HF RF capacitively coupled plasma (CCP) discharge. It was found that the high power of the low-frequency (LF) source and the gas pressures are the most effective parameters in dual RF CCP discharge system. Thus, both SPS intensity decreases and FNS intensity increases with the increasing LF power at constant pressure. Furthermore, the emission intensity of the first negative band heads increases at least four times in dual RF mode in comparing with the single RF CCP discharge. In contrast, the intensity of the second band heads decreases dramatically to its half values. Likewise, it was found that the transition pressures for the electron heating mode (with increasing the gas pressure) and the transition from α to γ mode (with increasing the RF power) is close to 0.3 torr in single RF mode, but these transitions are about 0.5 torr for dual RF CCP mode.