Concentration Dependent Ionization Dynamics in Torr Pressure N2-NH3 CCPs

The formation of ions in moderate and high-pressure capacitively coupled plasma (CCP) discharges often involves multi-step processes where one species, generated by electron-neutral collisions, interacts further with other neutral species to form ions. Two such processes are the N₂(a’)+N₂(a’) -> N₄⁺ + e^- in N₂ plasmas, and NH₃⁺ + NH₃ -> NH₄⁺ + NH₂ in NH₃ plasmas. In torr pressure pure N₂ and pure NH₃ CCPs, these and similar processes result in N₄⁺ and NH₄⁺ being the predominant ion species. However, mixtures often have ionization processes that are different than their pure components. For example, addition of small amounts of NH₃ to N₂ significantly reduces the N₄⁺ production mechanism due to the presence of competing inelastic loss and ionization processes at similar 7-10 eV energies that change the electron energy distribution function. Addition of NH₃ to N₂ gas results in the near complete absence of N₄⁺ ions, greater presence of N₂⁺ ions, and significant sub-RF-cycle fluctuations in NH₃⁺ ion densities as these species are quickly converted to NH₄⁺ in collisions with neutrals. In this work, these and other qualitative changes in ionization mechanism are explored as a function of concentration and pressure using a 1D fluid CCP model.