The effects of the nitrogen dilution and pressure on the structural and optical properties of hydrogenated microcrystalline silicon films deposited at 250 C by the radiofrequency magnetron sputtering method have been studied in the (0-1%) range of nitrogen dilution percentage in the gas phase mixture (Ar 70%-H-2 30%, total pressure 5 Pa). A combination of mass spectrometry, Raman spectroscopy, infrared absorption, UV-visible transmission and photothermal deflection spectroscopies has been used to characterize the films. Mass spectrometry revealed that the SiH2/SiH ratio decreased when increasing the nitrogen partial pressure. This change has been found to be related to the microstructural transition from microcrystalline to amorphous by Raman spectroscopy, while infrared spectroscopy showed an increase of Si-N bonds. The structural change has been explained in terms of reaction of SiH2 crystallization precursors with nitrogen. The very small nitrogen partial pressure leads to an important nitrogen enrichment of the films, which causes the decrease of absorption coefficient and refractive index and the increase of the optical gap. This suggests that the films turned into an amorphous hydrogenated silicon nitride alloy. (C) 2003 Elsevier B.V. All rights reserved.