Abnormal Enhancement of N2+ Emission Induced by Lower Frequency in N2 Dual-Frequency Capacitively Coupled Plasmas

Nitrogen dual-frequency capacitively coupled plasmas (DF-CCPs) with different frequency configurations, i.e., 60/2 MHz and 60/13.56 MHz, are investigated by means of optical emission spectroscopy (OES) and a floating double probe. The excited nitrogen molecule ion N2+(B) is monitored by measuring the emission intensity of the (0,0) bandhead of the first negative system (FNS) at 391.44 nm. It is shown that in the discharge with 60/13.56 MHz, the N2+ emission intensity decreases with the increase in pressure. In the discharge with 60/2 MHz, however, an abnormal enhancement of N2+ emission at higher pressure is observed when a higher power of 2 MHz is added. Variation in the ion density shows a similar dependence on the gas pressure. This indicates that in the discharge with 60/2 MHz there is a mode transition from the alpha to gamma type when a higher power of 2 MHz is added at high pressures. Combining the measurements using OES and double probe, the influence of low frequency on the discharge is investigated and the excitation route of the N2+(B) state in the discharge of 60/2 MHz is also discussed.