Reaction Mechanism Analysis of Si Selective Etching for Gate-All-Around Transistors by Molecular Simulations

The selective etching of Si in multi-stacked Si and SiGe structures is a key process to fabricate the next-generation of FET, Gate-all-around FET. In this study, we investigated the mechanism of wet chemical etching process at the molecular level for two common ionic solutions, potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH). One of the important factors in the etching process is the reaction rate between the water and Si surface. Therefore, the water dynamics in i) bulk system and ii) Si wall confined system were mainly analyzed using molecular dynamics simulations. As a feature of bulk, TMAH showed the larger hydration structure around cation and the lower mobility of water in the hydration shell compared to KOH. In the Si wall confined system, the water and ion dynamics on the OH-terminated Si surface were distinctive. TMAH showed the lower mobility of water as in bulk system. Furthermore, the concentration and long stay of cation near the Si surface were observed in TMAH. This behavior of cation may directly prevent water from contacting surface. These characteristics of TMAH may slow down the Si etching process. However, if the blocking effect for etching depends on the surface composition, it will be useful for selective etching.