Characterization of Plasma-deposited a-C:H:Si:F:N Films

Thin a-C:H:Si:F:N films were studied as a function of the partial pressure of SF₆ in plasma feed, R_SF, together with tetramethylsilane and N₂. Deposition rates varied from ~4 to ~19 nm.min^-1. Surface roughnesses were typically less than 35 nm. Surface contact angles with water droplets, measured using goniometry, were all around 90°. Scanning electron micrography revealed surface particles, probably formed in the gas phase, of typical diameters ~8 μm. As revealed by Fourier transform infrared spectroscopy and energy dispersive x-ray spectroscopy, the films are plasma polymers with a carbon and silicon network. Most of the films contain ~ 60 at.% C, ~ 10 at.% Si, 20 at.% O and ~5 to 14 at.% N. Film doping with F rises to ~2 at.% as R_SF is increased. The Tauc gap, calculated from ultraviolet-visible near infrared spectroscopic data, is controllable in the range of ~3.5 to 4.1 eV by a suitable choice of R_SF. Fluorination causes the films to be softer and less stiff. Total deformation and stored energies are reduced compared to those of the film deposited at R_SF = 0%. The modulus of dissipation increases from ~8% to a maximum of ~65% for the fluorinated films.

Keywords:
PECVD; a-C:H:Si:F:N; thin films; optical properties