Mechanical properties of dual ion beam deposited silicon nitride films

Abstract

Amorphous silicon nitride (SiNx)films with nitrogen-to-silicon ratio (x) varying from 0-1.36 were fabricated by dual ion beam deposition (DIBD) on Si substrates. The composition, structure and many physical properties (i.e. mechanical, etching, optical and electrical properties) of the films were characterized. The hardness and elastic modulus of the hydrogen-free DIBD SiNx films were found to vary from 12.2-21.5 GPa and 191-256 GPa respectively with increasing x. The friction coefficient dropped from 0.58 to 0.38. Owing to peening effect, all the films had compressive stress, which rose from 0.53 to 1.41 GPa with x. These data revealed that DLBD SiNx could be used for surface protective coatings. The etching rate the SiN₁.₃₆ film in buffered hydrofluoric acid (BHF) was above 7 nm min-¹. For KOH at 80°C, the etching rate of all the samples was negligibly small. A model was established to fit the optical absorption spectra and temperature dependence of electrical conductivity of the films with high density of mid-gap states. Results indicated that the optical band gap of DIBD SiNx films rose from 1.8 to 4.2 eV when x increased. Hopping of charge carriers dominated the transport properties of the films. Nanoindentation tests were done on microbridges made of CVD silicon nitride films. The load-unload curves of CVD microbridges were fitted numerically, and the elastic modulus, residual stress and bending strength of the films were derived. This experiment demonstrated the validity of this methodology.