Selective molecular gas phase etching in layered high aspect-ratio nanostructures for semiconductor processing. II. Experiments and model validation

A simulation model for selective molecular gas etching in nanostructures has been described in Paper I [Z. Zajo et al. J. Vac. Sci. Technol. A 43, 013006 (2025)], in which the transport of molecules was modeled as Knudsen diffusion in the free-molecular flow regime and the surface reactions were modeled using (i) a simple linear model and (ii) a Langmuir adsorption based model. In this paper, we complete experiments on etching of stacked SiGe-Si structures by molecular F₂ and compare the results of experiments and the predictions from the model mentioned above. The results of our investigation show that the transport of F₂ in the nanostructures is in the nearly total re-emission regime for the range of process parameters and length scales involved in our experiments and that only a very small fraction of the incoming F₂ flux reacts with SiGe. This is evidenced by the small values of estimated sticking coefficients on SiGe (⁠ $∼ 10 − 6$– $10 − 3$⁠) from the linear model as well as the small values of the reaction rate constant on SiGe relative to the F₂ flux on an open surface, $k 2/J$ (⁠ $∼ 10 − 7$– $10 − 4$⁠) with the exact value being dependent on Ge% and the temperature at which the etching is performed. This enables the achievement of uniform etch rates across all layers in highly stacked nanostructures as required in the fabrication of gate-all-around nanotransistors. We also estimate the surface reaction rate constants as well as the activation energies as a function of Ge% for SiGe etching by F₂, and the results are consistent with the observed Ge composition dependence of etch selectivity of SiGe over Si.