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

Nitrogen dual-frequency capacitively coupled plasmas (DF-CCPs) with different frequency configurations, i.e., 60/2 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 probe-measured 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.