A remote expanding thermal plasma operated on an Ar-H-2-N-2-SiH4 mixture has been studied by several plasma diagnostics to obtain insight into the plasma processes and the hydrogenated amorphous silicon nitride (a-SiNx:H) growth mechanism from the N-2-SiH4 reactant mixture. From Langmuir probe measurements, ion mass spectrometry, and threshold ionization mass spectrometry, it is revealed that the Ar-H-2-N-2 operated plasma source leads mainly to N and H radicals in the downstream region. The H radicals react with the SiH4 admixed downstream creating a high SiH3 density as revealed by cavity ringdown spectroscopy. By cavity ringdown measurements, it is also shown that Si and SiH have a much lower density in the downstream plasma and that these radicals are of minor importance for the a-SiNx:H growth process. The ground-state N radicals from the plasma source do not react with the SiH4 injected downstream leading to a high N density under the a-SiNx:H deposition conditions as revealed by threshold ionization mass spectrometry. From these results, it is concluded that N and SiH3 radicals dominate the a-SiNx:H growth process and the earlier proposed growth mechanism of a-SiNx:H from the N-2-SiH4 mixture [D. L. Smith et al., J. Vac. Sci. Technol. B 8, 551 (1-990)] can be refined: During deposition, an a-Si:H-like surface layer is created by the SiH3 radicals and at the same time this a-Si:H-like surface layer is nitridated by the N radicals leading to a-SiN,:H formation. This growth mechanism is further supported by the correlation between the SiH3 and N plasma density and the incorporation flux of Si and N atoms into the a-SiNx: H films as deposited under various conditions. (C) 2004 American Vacuum Society.