Laser-induced chemical vapor deposition of silicon nitride films: Film and process characterizations

We have deposited hydrogenated amorphous silicon-nitride (a-SixN1−x:H) films from NH3-SiH4-Ar gas mixtures, heated by gas-phase absorption of CO2 laser radiation. For the first time, stoichiometric (Si/N=0.75) a-Si3N4:H films were obtained for NH3/SiH4 flow ratios of the order of 1000 and substrate temperatures Ts of about 500 °C. Growth rates as high as 13 Å/min were observed, depending on the partial pressure of SiH4, P(SiH4), the gas temperature Tg and Ts. Tg was calculated from a derived energy-balance equation, which depends sensitively on process parameters. To model the deposition process, the experimental film growth rate is separated into the Si growth rate G(Si) and the N growth rate G(N). Film stoichiometry is interpreted as the ratio G(Si)/G(N). The rate-limiting steps for Si growth are the gas-phase decomposition of SiH4 for Tg below about 750 °C and the SiH4 flow rate at higher Tg. G(N) is affected by both Ts and Tg. The NH3/SiH4 flow ratio must be kept large to ensure a sufficient concentration of N to attain stoichiometry. The reactions are likely to occur both in the gas phase and on the film surface.