Initial stages of growth of LPCVD polysilicon films. Effect of the surface "ageing"

Abstract
The initial stages of growth of polycrystalline silicon films by silane decomposition, at a pressure of 230 m Torr and a temperature of 610 °C in a conventional low pressure chemical vapor deposition reactor, have been studied. Depositions at times varying between 1 and 5 min have been made on two kinds of SiO₂ covered, (100) Si substrates : i) immediately after withdrawal from the oxidation furnace ("fresh") and ii) left in the clean room for 24 hours after oxidation ("aged" oxides). The microstructural characteristics of these deposits (grain size and roughness) were studied with atomic force microscopy. It was found that grain size and surface roughness were decreasing with deposition time, and that films grown on "fresh" oxides exhibited smaller grain size and lower roughness, than films grown on "aged" oxides. After the first 5 min of deposition, the state of the substrate surface ceased to influence the grain size which became identical for films grown on both kind of surfaces, while it continued to influence the roughness of the samples. Such behaviour has not been observed in thicker films grown on "fresh" and "aged" oxides that have been studied with transmission electronic microscopy. The above observations were attributed to the increase of "activated" sites coverage with deposition time, which in turn has caused an increase of the activation energy for the surface diffusion of adsorbed silicon atoms. Surface "ageing" causes a decrease of sites available to form bonds, due to the absorption of impurities from the clean room ambient