Nitrogen discharge characteristics and species kinetics in helicon plasma source

Nitrogen (N₂) helicon plasma is produced with radio frequency (RF) right-helical antenna at low pressures. Several wave modes and their transitions of N₂ helicon discharge are observed experimentally. Blue Core phenomenon is achieved at high magnetic fields and high RF powers, with strong local blue light emissions of N II lines and high electron temperature inside the core area. Based on actinometric ratio and pressure balance model, species kinetics of N₂ helicon plasma are analyzed. It is shown that about 79% of N₂ molecules are dissociated and about 49% of the neutrals are ionized inside the Blue core in high magnetic field of 850 G and RF power of 2200 W. Nearly 99% of N₂ molecules inside the core area are depleted considering the neutral density before and after discharge, from 7.3 × 10¹³ cm⁻³ to 6.5 × 10¹¹ cm⁻³. Serious neutral depletion of N₂ and N neutrals and high electron temperature are suggested to be the dominant causes for significant enhancement of central N II emissions (blue lights). Meanwhile, evolution of reaction processes indicates that N ionization and N⁺ excitation become dominant in BC mode. Besides, external magnetic field is an important factor to control the discharge mode transitions as well as the radial distributions of plasmas. From the calculated results of dispersion relation, the cavity mode resonance, rather than antenna coupling resonance, of helicon waves plays a dominant role on the wave mode formation and RF energy coupling between RF antenna and plasma. The mode transition results from excitation of helicon wave of higher axial eigenmode. N₂ helicon plasma shows different characteristics from argon in mode transition, spectral emission and Blue Core formation. It is due to the high dissociation energy of N₂ molecules (9.8 eV) and extensive dissociation and ionization processes. This results in a higher RF power as well as magnetic field for helicon wave coupled mode in N₂ helicon plasma than that in Ar plasma.