Modeling molecules responsible for the sidewall protection during the chemical dry etching of silicon related materials using F$_{2} + $ NO$_{x} \to $ F $+$ FNO$_{x}$

We have been investigating the chemical dry etching of Si related materials using the reaction of F$_{2} + $ NO$_{x}$ (X $=$ 1, 2) $\to $ F $+$ FNO$_{x}$. In our previous study, we found that this chemical dry etching technique generated anisotropic etching profile when the substrate temperature was maintained at \textless 60$^{\circ}$C. In this study, we evaluated the cause of the anisotropic etching by measuring the molecules present in the gas phase by Fourier Transform Infrared Spectroscopy (FTIR) followed by calculating the chemical bonding structure formed on the Si surface by density functional theory (DFT). First the reaction between SiF$_{4}$ and molecules generated by the reaction between F$_{2}$, NO$_{x}$, and adsorbed H$_{2}$O such as F$_{2}$, NO$_{x}$, FNO$_{x}$, H$_{2}$O, OH, and HF, measured by FTIR, were calculated by DFT to identify key molecules that can present in the gas phase. We found that the chemical reaction between SiF$_{4}$ and multiple FNO may play a key role to prevent the sidewall etching. The modeling at the surface reaction using DFT is in progress.