Quantum chemical investigation for Chemical dry etching by flowing NF$_{3 }$into H$_{2}$ down flow plasma

The molecular orbital calculations were carried out in order to clarify the reaction schemes of the chemical dry etching using H$_{2}$ down flow plasma and NF$_{3}$ flowing. It was found that not only HF formation but also F atom generation takes place. And this F atom generation mechanism is very important to realize the highly selective SiO$_{2}$ etching process. Probably F formation is suppressed under the three-body reaction regime and the higher gas flow rate of H$_{2}$ than that of NF$_{3}$. So, careful control of the pressure is a key factor. The examined reaction schemes do not only take place in the vapor phase but also in the condensed phase on the wafer surface. As a result, complex (NH$_{4})_{2}$SiF$_{6}$ is formed on the SiO$_{2}$ surface, and SiO$_{2}$ film is removed. This complex molecule is decomposed by elevating the wafer temperature to produce SiF$_{4}$ and the white powder based on NH$_{3}$-HF, which may be composed mainly by stratified n(NH$_{4}$F)$_{3}$ with C$_{3}$ symmetry axis. Increasing the wafer temperature moreover, up to 500 K, this white powder also decomposes to HF+NH$_{3}$.