A First-Principles Mechanistic Study of Thermal Selective Etching of SiGe and Si with Fluorine-Containing Etchants

The advancement and scaling of semiconductor devices have driven the development of processes with atomic-scale precision, such as thermal selective etching. Thermal etching utilizes neutral gases and thermal energy in two primary steps: physisorption and dissociative chemisorption. We performed first-principles calculations and quantum chemistry modeling to study the detailed reaction pathways for F2, XeF2, and ClF3 etchants interacting with the surface bonds in SiGe (SiLayer1–SiLayer2, SiLayer1–GeLayer2, GeLayer1–SiLayer2, and GeLayer1–GeLayer2) and Si (SiLayer1–SiLayer2). The SiGe to Si (SiGe:Si) selectivity in F2 etching arises from the lower activation energy required for F2 to fluorinate the surface Si in SiGe and break the SiLayer1–GeLayer2 bonds compared to etching the SiLayer1–SiLayer2 bond in pure Si. The XeF2 chemistry follows a multietchant chain-reaction mechanism, initiated by the dissociative chemisorption of XeF2 and facilitated by condensation. While XeF2 and XeF· preferentially etch SiLayer1–GeLayer2 bonds in a manner similar to that of F2, F· primarily targets the GeLayer1–SiLayer2 bonds. Therefore, by adjusting the amounts of XeF2, XeF·, and F· on the surface, a more effective removal of Si and Ge from a SiGe surface, relative to Si removal from a Si surface (i.e., a higher SiGe:Si selectivity), can be achieved while maintaining similar etch rates across varying Ge concentrations. The ClF3 chemistry follows a similar multietchant chain-reaction mechanism to XeF2. However, the lower generation of byproduct Cl·, the smaller amount of condensation from ClF3 and byproduct ClF, and the surface chlorination from ClF result in a lower overall etch rate and reduced the ability to maintain consistent etch rates across varying Ge concentrations, compared to XeF2. Each of these etchants demonstrates key differences in their reaction pathways, physisorption behavior, thermodynamics, and kinetics, which is likely to result in different process operating regimes to gain optimal etching performance.