Characterization of CVD carbon films for hermetic optical fiber coatings

Thin carbon films are being studied as hermetic coatings for optical fibers used in harsh environment applications. This work details an experimental study of pyrolytic carbon films grown in a cold walled chemical vapor deposition (CVD) reactor. The films are grown from methane, acetylene, propane and butane precursors on stationary 3-mm quartz rods. Results are presented at 15 and 600 torr, total pressure for substrate temperatures between 1050 and 1750 K. Hydrocarbon precursor gas concentrations are varied between 10 and 100% in N2 dilutant. Auger electron spectroscopy (AES) shows mostly carbon composition, with trace levels of Cr and O2 impurities. X-Ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) reveal small amounts of SiC at the carbon–glass interface, but could not resolve a continuous transition layer. The thickness, microstructure and surface roughness are analyzed using environmental scanning electron microscopy (ESEM). ESEM reveals an increase in deposition rate with temperature and concentration for all hydrocarbon species and a noticeable change in surface morphology with deposition temperature and hydrocarbon species. Raman spectroscopy is used to relate film microstructure to deposition conditions, providing non-intrusive evaluation of crystalline size and hermetic properties.