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 (E-a) of a single-F-transfer breaking Ge-Ge bond is 0.4 eV lower than the Si-Si bond, with the E-a of Si-Ge cases fall between. The overall smaller E-a 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 E-a 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 E-a has a long-range nature. The calculation predicts a reduced E-a even when the Ge atom is on the second-nearest-neighbor site to the fluorinated Si atom. The finding implies that the Ge-induced E-a 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.