Thin films of Si3 N4, SiO2, and of silicon oxynitrides with compositions in the range 0.3≤O/N≤3.6 were deposited on silicon substrates to thicknesses between 20 and 150 nm by low-pressure chemical vapor deposition. Auger parameters of the films were measured during the study of ion bombardment effects and depth profiling, and as a function of the O/N ratios.
The particular parameter used was based on the Si 2s photoelectron, and bremsstrahlung-excited Si KLL Auger lines. When normalized to constant ion dose, no significant differences in the effects of bombardment with argon ions in the energy range 0.5–4.0 keV could be found, and the equilibrium O/N ratio measured by x-ray photoelectron spectroscopy was close to that measured by Rutherford backscattering and elastic recoil detection analysis. During depth profiling no reduction of the oxynitride to elemental silicon was observed, in contrast to previous work using the low-energy Si LVV Auger peak with electron excitation; it is therefore recommended that to perform depth profiling of these materials while retaining artefact-free compositional and chemical information, the high kinetic energy Si spectral features should be used. At the oxynitride/silicon interfaces in some of the films additional features were found corresponding to intermediate Si chemical states. The widths of the Si 2s, 2p and KLL spectral features were measured at the stages in depth profiling at which equilibrium composition had been reached.
In every case there was an increase in width as a function of oxygen fraction O/(O+N), from Si3 N4, through a maximum at about O/(O+N)≊0.5, followed by a decrease to SiO2. The maximum increase for the KLL Auger peak was ≊0.9 eV, but for the 2s and 2p only ≊0.35 eV, and the peak broadening was uniformly distributed about the peak centroid. The broadening is attributed to the appearance of a number of additional chemical states of Si too closely spaced to be resolvable, and probably arising from a defective film structure. The Auger parameters of the various films, measured also once equilibrium composition had been reached during profiling, showed what appeared to be a monotonic dependence on O/(O+N), thus supporting the random bonding model in which nitrogen atoms are substituted randomly by oxygen in the SiNz O4−z (z=0, 1, 2, 3, 4) tetrahedra.
However, the Auger parameter versus composition plot was displaced slightly from the theoretical one in the direction of higher parameter than expected. When the Auger parameter was plotted against the function (n2−1 )/n2, proportional to the polarization energy, n being the measured refractive index of a film, a linear relationship was found within experimental error, but with a slope different from that for the standards SiO4/2 (silica), SiO6/3 (stishovite), Si3 N4, and Si. This divergence is interpreted in terms of a lower O ionic polarizability in the oxynitrides than in the oxides.
