A steady-state and high-flux helicon-wave excited N-2 plasma was used to oxynitride Si substrates for the synthesis of silicon oxynitride (SiON) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been extensively used to characterize surface quality of the SiON films, and it is found that a large amount of nitrogen (N) can be incorporated into the films. The result of XPS depth profiles shows that the N concentration is high near the surface and the oxide/Si interface. In the UPS spectra, absence of the reappearance of surface states suggests a resistance to clustering of the oxynitride layer. The N-2 flux and Ar mixture quantity can facilitate tuning of the dissociation characteristics in N-2 discharge. By modulating the N-2 fractions, the N+ density reaches maximum at a N-2/(N-2 + Ar) flow-rate ratio of 0.5, resulting in incorporation of more N atoms into the SiON films. Considering the easy control of N-2 plasma, our work opens up a new avenue for achieving high-yield SiON films at low temperature.