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 N
₂
ion (N
₂
⁺
) using a helicon-wave-excited plasma (HWP) with Ar/N
₂
gas mixtures. It shows that by varying the flow-rate ratio of N
₂
/(N
₂
+ Ar) (α), the ratio of [N
⁺
]/[N
₂
⁺
] (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 N
₂
HWP discharge (β ~ 0.2). The maximum density and flux of atomic N+ are obtained, which are 2.5 × 10
¹⁸
m
^-3
and 8.6 × 10
²¹
m
^-2
s
^-1
, respectively. The results show that the addition of Ar into N
₂
plasma can be employed to remarkably increase the [N
⁺
]/[N
₂
⁺
] 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.