PURE AND FLUORINE-DOPED SILICA FILMS DEPOSITED IN A HOLLOW-CATHODE REACTOR FOR INTEGRATED-OPTIC APPLICATIONS

Silica films have been deposited in a high density hollow cathode plas ma 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 infr ared (FTIR) spectroscopy, wavelength dispersive x-ray spectroscopy, ch emical etch rate (P etch), stress and refractive index measurements. T he pure silica films, though deposited at a high rate (over 1500 Angst rom/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-temp erature (1000 degrees C) annealing. Based on the thickness change meas urements, the higher refractive index was attributed to a higher densi ty of the deposited silica due to a smaller Si-O-Si bond angle, as sup ported by FTIR data. Fluorine doping results in a reduction in film st ress by a factor of 4 over pure silica, as well as a reduction in OH c ontent from about 1 at. % in pure silica to below the FTIR detection l imit (0.1 at. %). The refractive index initially decreases with CF4 fl ow rate, concomitant with an increase in fluorine content, but then ri ses 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 deficien t discharge resulting from oxygen consumption by the dissociation prod ucts of CF4. (C) 1996 American Vacuum Society.