Ellipsometric study of a-C:H films

The optical constants, porosity and thickness of amorphous hydrogenated carbon (a-C:H) films were determined by multiple-angle ellipsometry at an optical wavelength of 632.8 nm. The films were produced by a plasma-activated CVD process in a de glow discharge of acetylene, toluene and octane. The results indicate that the refractive index of the a-C:H films can be changed over the interval 2.35-1.55 by increasing the deposition rate and an appropriate choice of hydrocarbon precursor. A reduction of the refractive index correlates with a decrease in the extinction index in the range 0.3-0.01. The influence of the chemical nature of the hydrocarbon precursor on optical constants in the visible region is found from the results of the ellipsometric measurements. The variations of the experimental data are reviewed in view of the a-C:H structure model proposed previously. The porosity of the a-C:H films has been determined by a new technique based on the ellipsometric measurements.