Paper SE+PS-TuA9
Strategy to Tailor the Composition of Silicon Oxynitride Thin Films Deposited by Reactive High Power Impulse Magnetron Sputtering using Nitrous Oxide as a Single-source Precursor

Tuesday, October 20, 2015, 5:00 pm, Room 212A

Silicon oxynitride (SiON) is a material with tunable optical and electrical properties. It is used as thin films in antireflection coatings, surface passivation layers, and optical waveguides. Commonly two reactive gases, O₂ and N₂, have been employed for the reactive magnetron sputter deposition of SiON. The two-gas approach is, however, limited by the non-linear target effects as a function of reactive gas flow rates, which makes an accurate control of the deposition process challenging.

In this study we present SiON thin films deposited by reactive high power impulse magnetron sputtering from a pure Si target in Ar/N₂O plasmas. The influence of the average target power while maintaining a fixed pulse frequency (1000–4000 W at 1000 Hz and 1200–3000 W at 600 Hz) and the frequency at a fixed energy per pulse (300–1000 Hz at 4 J) on the resulting film properties were investigated.

The ~ 300 nm thick films show a dense and featureless morphology when viewed in cross-sectional scanning electron microscopy. X-ray diffraction reveals that the films are X-ray amorphous and exhibit compressive residual stresses in the range of 500–1500 Mpa. The chemical bonding structure and the elemental composition of the films were studied with X-ray photoelectron spectroscopy. The spectra acquired from the as-deposited samples show mixed Si—O, Si—N, and Si—Si bonding. The ratio between these type of bonds correlates with the atomic concentrations of Si, O, and N in the films. A decreased average power at a fixed pulse frequency induces an increase in O content (from 13 at.% to 51 at.% at 1000 Hz and from 17 at.% to 43 at.% at 600 Hz) while the N concentration remains virtually unaffected. An exception are films with the highest O content, showing a decrease in N content from ~ 26 at.% to 9 at.% at 1000 Hz and to 16 at.% at 600 Hz. A similar behavior is observed when the pulse frequency is lowered at a constant pulse energy, the film O content increases from 13 at.% to 43 at.%, followed by a decrease in N concentration from ~ 26 at.% to 17 at.%. Film densities as measured by X-ray reflectivity are found to range between 2.6–2.9 g/cm³. The refractive indices and extinction coefficients at 633 nm, measured by variable-angle spectroscopic ellipsometry, show that the films with highest total O plus N content have the lowest refractive index (1.6–1.8) and extinction coefficient (~ 0) values. The refractive index values are comparable to those of SiO₂ (~ 1.45) and Si₃N₄ (~ 2.0), confirming that the optical properties of the studied films can be tailored to range between SiO₂ and Si₃N₄.