2D hybrid simulation of a dual-frequency capacitively coupled nitrogen discharge: effects of gas pressure and low-frequency voltage on generation and distribution of species

Abstract The effect of the gas pressure and the low frequency (LF) voltage, Vlf, on the spatial distributions of electron energy distribution function (EEDF), electron and neutral densities in a dual-frequency (DF 2/60 MHz) driven capacitive discharge in nitrogen (N2) is investigated by employing a two-dimensional fluid model with electron collision processes treated by Monte Carlo model. It is found that by increasing the pressure the region of strong ionization shrinks immensely towards the sheath edge of the powered electrode and the ionization maximum occurs below the edge of the powered electrode, leading to edge-high density profiles of the electron and excited-state nitrogen molecules at a higher pressure, while nitrogen atom density remains spatially uniform, due to higher transport coefficient. By increasing Vlf, the discharge becomes more symmetrical in axial direction, the ionization rate changes from a radial uniform to an edge-high profile, leading to an edge-high profile of electron density. The EEDF is found to exhibit a dip at 2.35 eV, due to depletion of electron via the electron-impact vibrational excitation of nitrogen molecule. This dip of the EEDF is most significant at the discharge center, and become less pronounced towards the sheath edge and the sidewall, due to low-enerngy electrons being heated by local strong electric field. Besides, at a higher Vlf, the dip becomes less significant, due to enhanced electron heating.