Selective molecular gas phase etching in layered high aspect-ratio nanostructures for semiconductor processing. I. Modeling framework and simulation

The need for precise control of nanoscale geometric features poses a challenge in manufacturing advanced gate-all-around nanotransistors. The high material selectivity required in fabricating these transistors makes thermal gas etching much more appealing in comparison to liquid phase and plasma-based etching techniques. The selective thermal etching by F₂ of silicon–germanium (SiGe) stacks comprised of alternating layers of silicon (Si) and SiGe is explored in this context for semiconductor manufacturing applications. We propose and develop computer simulations as a tool to predict the etch profile evolution over time in such an etching process. The tool is based on a mathematical model that considers the transport processes and surface interactions involved in the gas phase etching process—which at the nanoscale is primarily Knudsen diffusion in the free molecular flow regime. Thus, the transport model is formulated as a boundary integral equation, which takes into account the direct flux of etchant molecules that any given point on the exposed surface receives from the bulk gas phase as well as the re-emission flux from other parts of the structure itself. We compared the applicability of two different surface reaction models—a model where the local etch rate is linear in the flux at a point and a Langmuir adsorption/reaction model—to connect the net flux received at a point on the surface to the local etch rate. This paper precedes Paper II of this series, which describes the experimental methods and comparison with model predictions of F₂ etching in high aspect ratio Si–SiGe stacked nanostructures.