Control of Ion Species and Energy in High-Flux Helicon-Wave-Excited Plasma Using Ar/N₂ Gas Mixtures

The atomic nitrogen (N) ion flux and impacting ion energy are the two important parameters, which influence the performance of production of plasma nitridation applications such as N-doped graphene. In this paper, a novel method is described to control the flux and ion energy of atomic N ion (N⁺) and molecular N₂ ion (N₂⁺) using a helicon-wave-excited plasma (HWP) with Ar/N₂ gas mixtures. It shows that by varying the flow-rate ratio of N₂/(N₂+Ar) (α ), the ratio of [N⁺]/[N₂⁺] ( β ) can be controlled obviously, and β could be increased up to 1.2 at α = 0.5 , which is much higher than that in pure N₂ HWP discharge (β ∼ 0.2). The maximum density and flux of atomic N⁺ are obtained, which are 2.5× 10¹⁸ m^-3 and 8.6 × 10²¹, m^-2s^-1, respectively. The results show that the addition of Ar into N₂ plasma can be employed to remarkably increase the [N⁺]/[N₂⁺] due to electron-impact ionization involving the metastable state of Ar. The N⁺ ion beams are formed with a speed near to Mach 3, and the ion-beam energy is increased from 30 to 50 eV with increasing α to 0.75.