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

The initial stages of growth of polycrystalline silicon films by silane dec omposition, at a pressure of 230 mTorr and a temperature of 610 degrees C i n 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 withd rawal from the oxidation furnace ("fresh") and ii) left in the clean room f or 24 hours after oxidation ("aged" oxides). The microstructural characteri stics 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 exh ibited smaller grain size and lower roughness, than films grown on "aged" o xides. After the first 5 min of deposition, the state of the substrate surf ace ceased to influence the grain size which became identical for films gro wn 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 elec tronic microscopy. The above observations were attributed to the increase o f "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 fo rm bonds, due to the absorption of impurities from the clean room ambient.