In-situ observation of thermal oxidation on the Si surface by RHEED-AES CL

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The initial stage of thermal oxidation on the Si(001) surface with dry O_2 was investigated by a realtime monitoring method of Auger electron spectroscopy combined with reflection high energy electron diffraction (RHEED-AES) in order to clarify the growth model of very thin SiO_2 layers for the gate insulator of metal-oxide-semiconductor field effect transistor (MOSFET). The RHEED-AES method made it possible to measure simultaneously the oxide coverage and etching rate during Si thermal oxidation. The time evolution of O KLL Auger electron intensity discriminated clearly three oxidation schemes of Langmuir-type oxidation, two-dimensional oxide island growth and active oxidation, and gave a phase diagram of the oxidation scheme as a function of O_2 pressure and temperature. In the Langmuir-type oxidation region, emission of Si atoms from oxides was suggested by the time evolution of half-order RHED spot intensities, which was interpreted in terms of the interfacial strain due to volume expansion associated with oxidation. In the two-dimensional oxide island growth and active oxidation region, the half-order RHEED spot intensities showed a oscillatory behavior, the oscillation period of which was independent of temperature, oxide coverage and oxidation scheme, but decreased in proportion to O_2 pressure. This means that the etching reaction is not rate-limited by SiO desorption but by O_2 adsorption. Based on the experimental results, a surface reaction model of two-dimensional oxide island growth was proposed, in which collisions between desorption precursors of SiO migrating on the surface lead to nucleation and growth of oxide islands and therefore etching of the surface originates from oxide growth as well as SiO desorption. As a result, it was concluded that the emission and surface migration of Si atoms should be considered in the surface reaction model of all oxidation schemes appearing at the initial Si thermal oxidation.