CHEMICAL-BONDS AND MICROSTRUCTURE IN NEARLY STOICHIOMETRIC PECVD ASI(X)N(Y)H(Z)

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.