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 F 2, XeF 2, and ClF 3 etchants interacting with the surface bonds in SiGe (Si Layer1 –Si Layer2, Si Layer1 –Ge Layer2, Ge Layer1 –Si Layer2, and Ge Layer1 –Ge Layer2 ) and Si (Si Layer1 –Si Layer2 ). The SiGe to Si (SiGe:Si) selectivity in F 2 etching arises from the lower activation energy required for F 2 to fluorinate the surface Si in SiGe and break the Si Layer1 –Ge Layer2 bonds compared to etching the Si Layer1 –Si Layer2 bond in pure Si. The XeF 2 chemistry follows a multietchant chain-reaction mechanism, initiated by the dissociative chemisorption of XeF 2 and facilitated by condensation. While XeF 2 and XeF· preferentially etch Si Layer1 –Ge Layer2 bonds in a manner similar to that of F 2, F· primarily targets the Ge Layer1 –Si Layer2 bonds. Therefore, by adjusting the amounts of XeF 2, 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 ClF 3 chemistry follows a similar multietchant chain-reaction mechanism to XeF 2 . However, the lower generation of byproduct Cl·, the smaller amount of condensation from ClF 3 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 XeF 2 . 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.