Silicon oxide decomposition and desorption during the thermal oxidation of silicon

The thermal oxidation of silicon is normally considered to occur via two different routes. At higher O-2 pressures and lower temperature SiO2(s) film growth occurs ("passive" oxidation), while at lower O-2 pressures and higher temperature SiO(g) is desorbed in an etching process ("active" oxidation). We have measured the yield of SiO into the gas phase in a wide range of dry O-2 pressures (10(-7)-10(-5) Torr) and Si substrate temperatures (620-870 degrees C) in the passive as well as the active oxidation regimes. A phase diagram for silicon oxidation in this pressure-temperature region is obtained. We have found evidence for small but measurable yields of SiO(g) desorbing from the nascent oxide film during the initial stages of passive oxidation, even when the oxide film continuously covers the surface. A sensitive method for detecting volatile products based on condensation of desorbed species is described.