Silicon nitride thin films were prepared by low frequency plasma enhanced chemical vapour deposition in silane, nitrogen and helium mixtures. With different silane flow rates, various thin films with nearly stoichiometric composition but different chemical bonds were prepared with hydrogen content lower than 12%. The films were annealed up to 1100 degrees C and the hydrogen content was measured by elastic recoil detection analysis and compared to the infrared vibration of hydrogenated modes of silicon and nitrogen atoms. Two kinds of behaviour were observed in the chemical bonding with the annealing temperature : either NH and SiH bonds show a decrease, as is generally reported in the literature, or a strong increase of the SiH one is observed. There is also a dramatic discrepancy between the hydrogen content measured by nuclear technique and by infrared spectroscopy if constant oscillator strengths of the stretching modes of hydrogenated sites are used. As in amorphous silicon, these results confirm the difficulty of deducing the concentration of hydrogen bonded to the silicon atoms in silicon nitride. We suggest that these effects are closely correlated to the local microstructure of the films and specific arrangements around SiH dipoles.