Silicon nitride thin films were prepared by low frequency plasma enhan
ced chemical vapour deposition in silane, nitrogen and helium mixtures
. With different silane flow rates, various thin films with nearly sto
ichiometric composition but different chemical bonds were prepared wit
h hydrogen content lower than 12%. The films were annealed up to 1100
degrees C and the hydrogen content was measured by elastic recoil dete
ction analysis and compared to the infrared vibration of hydrogenated
modes of silicon and nitrogen atoms. Two kinds of behaviour were obser
ved in the chemical bonding with the annealing temperature: either NH
and SiH bonds show a decrease, as is generally reported in the literat
ure, 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 strength
s of the stretching modes of hydrogenated sites are used. As in amorph
ous silicon, these results confirm the difficulty of deducing the conc
entration of hydrogen bonded to the silicon atoms in silicon nitride.
We suggest that these effects are closely correlated to the local micr
ostructure of the films and specific arrangements around SiH dipoles.