In order to understand the chemical vapor deposition (CVD) reaction mechanism, forming a thin film from the gas phase, it is important to identify the intermediate chemical active species (radicals) for each reaction. Radicals generally have very short lifetimes; therefore, it is very difficult to detect them. Molecular beam sampling (MBS) is a method that can extract radicals in the gas phase using free jet expansion. A vacuum system using a MBS method was designed to analyze the CVD reaction mechanism in the gas phase near the surface of the wafer. The system consists of a thermal flow‐through type CVD reactor and three differential pumping vacuum chambers with a quadrupole mass analyzer. The performance of the MBS system was tested with various gases. The system was applied to detect radicals produced in the region near the wafer in a thermal TEOS/O3 (tetraethylorthosilicate) CVD reaction to deposit SiO2 thin films. It is proved that the radicals, from which ethoxy bases of TEOS are extracted, are formed in the gas phase and can play an important role in the thermal TEOS/O3 CVD reaction.

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