Fundamental study on the selective etching of SiGe and Si in ClF3 gas for nanosheet gate-all-around transistor manufacturing: A first principle study

We conduct an atomic-level investigation on how a Ge atom impacts on the SiGe etching rate. The plasmaless dry-etching process in ClF3 gas is considered in this study. We perform the density functional theory to model the elementary reactions of an etchant molecule fluorinating Si/Ge atom. Based on the modeling results, the activation energy (Ea) of a single-F-transfer breaking Ge-Ge bond is 0.4 eV lower than the Si-Si bond, with the Ea of Si-Ge cases fall between. The overall smaller Ea suggests that the relatively active fluorination reaction enabled by a Ge atom facilitates the selective etching. In addition, a unique double-F-transfer from ClF3 is identified, which simultaneously fluorinates two adjacent Ge atoms. The reaction enhances selective etching with the drastically lowered Ea and the more negative total energy change. We modeled different locations of a Ge atom with respect to the fluorinated Si atom. The results show that the effect of the Ge atom on lowering the Si fluorination Ea has a long-range nature. The calculation predicts a reduced Ea even when the Ge atom is on the second-nearest-neighbor site to the fluorinated Si atom. The finding implies that the Ge-induced Ea reduction can continuously assist the selective etching with a Ge-percentage as low as 6 at. %. Details of the surface chemical reactions and byproduct formation are discussed in the report.