CO2 laser CVD of a-Si:H:: in situ gas analysis and model calculations

The gas phase chemistry of monosilane SiH4 and disilane Si2H6 induced by continuous wave CO2 laser irradiation has been investigated under the conditions of chemical vapor deposition (CVD) of amorphous hydrogenated silicon a-Si:H. At the very position of depositing the thin film the stationary chemical composition of the processing gas is probed in situ by an effusive molecular beam which passes through a differential pumping stage into a quadrupole mass spectrometer (QMS). With SiH4 as educt and SF6 as a sensitizer, SiH4 and Si2H6 are found in the processing gas while Si3H8 or higher silanes are lacking. Si2H6 and SF6 lead to SiH4, Si2H6, and Si3H8, but higher silanes are missing. The experimentally determined composition of the processing gas is semiquantitatively reproduced by model calculations based on the assumption of stationary local equilibrium conditions and applying thermodynamic and spectroscopic data (molecular statistics). The mass balance of the processing gas entering and leaving the CVD chamber states an atomic ratio Si:H of 1:2 for the gas phase species forming the solid deposit on the reactor walls. This finding together with theoretical considerations indicates the intermediate Si2H4 to be the dominating gas phase species forming the a-Si:H thin films.