Pure and fluorine-doped silica films deposited in a hollow cathode reactor for integrated optic applications

Abstract

Silica films have been deposited in a high density hollow cathode plasma deposition system from silane and oxygen gas mixtures. Additions of carbon tetrafluoride (CF₄) were used to fluorine dope the silica. The deposited films were characterized by means of Fourier transform infrared (FTIR) spectroscopy, wavelength dispersive x‐ray spectroscopy, chemical etch rate (P etch), stress and refractive index measurements. The pure silica films, though deposited at a high rate (over 1500 Å/min), exhibit a P‐etch rate only 1.3 times that of thermal oxide. The refractive index of the as‐deposited silica is higher than that of thermal oxide, but reduces to the thermal oxide value after high‐temperature (1000 °C) annealing. Based on the thickness change measurements, the higher refractive index was attributed to a higher density of the deposited silica due to a smaller Si–O–Si bond angle, as supported by FTIR data. Fluorine doping results in a reduction in film stress by a factor of 4 over pure silica, as well as a reduction in OH content from about 1 at. % in pure silica to below the FTIR detection limit (0.1 at. %). The refractive index initially decreases with CF₄ flow rate, concomitant with an increase in fluorine content, but then rises above the refractive index of pure silica. This increase has been found to be due to the deposition of silicon‐rich oxide at the higher CF₄ flow rates, which is attributed to an increasingly oxygen deficient discharge resulting from oxygen consumption by the dissociation products of CF₄.