A numerical model of silicon nitride deposition from an NH3-SiH4 mixture in a large-sized radiofrequency plasma reactor has been developed. A bidimensional treatment of transport phenomena, of major importance in this type of reactor, was used. Analysis of the modelling results showed that aminosilane radicals were the main deposition precursors. The steep decrease in deposition rate in the flow direction that was observed can be explained by examining the calculated concentration profiles.