<title>Multiphoton photochemical reactions during laser-induced deposition of silicon thin films</title>

Abstract

Silicon deposition via decomposition of SiH4 molecules induced by excimer laser irradiation has been previously reported to be controlled by a two photon absorption process. This behavior was found at a total pressure lower than 5 Torr. At higher pressures the absorbed intensity depended linearly on the incident intensity. However, a multiphoton chemical reaction is indicated by a non-linear dependence of the deposition rate on laser energy. A different non-linear dependence of the deposition rate on laser energy was found in our work during the investigation of the mechanism of silicon thin films deposition by laser irradiation of a silane gas diluted with argon (15% SiH4/Ar). The Si films were deposited by a focused laser beam irradiating in parallel to silicon and silicon dioxide substrates at a gas flow rate of 20 SCCM, total pressure of 60 Torr and a repetition rate of 15 Hz. At laser energies higher than 160 mJ/cm2 the deposition rate was almost independent of the incident laser energy, while at a lower energy the deposition rate depended strongly on the laser energy. A 3/2 power was found in absorption measurements carried out at the same pressure under flow conditions and for several repetition rates at average laser intensity above 300 mW. This kind of behavior is typical of a multiphoton absorption process involving saturation effects caused by focusing of the laser beam. Below 300 mW the power dependence indicated a two-photon absorption process. From the observed photochemical yield we found the value of 5.7 X 10-44 cm4 sec. molec.-1 for the two- photon absorption cross section.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.