Chemical structure and optical properties of a-SiNC coatings synthesized from different disilazane precursors with the RF plasma enhanced CVD technique - a comparative study

Silicon carbonitride (SiNC) coatings are characterized by high refractive index, which makes them suitable for a manufacture of optical filters. In this work, thin SiNC films synthesized with the plasma enhanced chemical vapor deposition (PE CVD) method from liquid organosilicone precursors, namely tetramethyldisilazane and hexamethyldisilazane, are presented. Depositions were carried out either under nitrogen (N-2) atmosphere, or under that of ammonia (NH3), or else in a presence of nitrogen/ammonia (N-2/NH3) mixture. Negative self-bias potential was used as another glow discharge operational parameter. Both composition of the reactive atmosphere and self-bias potential were optimized with a use of refractive index, extinction coefficient and thickness data obtained from variable angle spectroscopic ellipsometry. UV-vis transmittance of the coatings was also determined and the values of the optical gap (E-g) were calculated from the absorption data. For the coatings of the best performance, elemental composition and chemical bonding were established with the help of x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. In addition, their surface topography was assessed with atomic force microscopy. Depending on deposition conditions, thickness of the coatings changed from 140 nm to 800 nm, their refractive index varied between 1.60 and 2.29, and the value of the optical band-gap E-g altered from 2.38 eV to 3.33 eV. The roughness parameters R-a and R-q of the coating surface were also found to depend on both operational parameters of the discharge, with their values changing from 0.299 nm to 0.99 nm for R-a and from 0.404 nm to 1.44 nm for R-q.