Bromine as a Preferred Etchant for Si Surfaces in the Supersaturation Regime: Insights from Calculations of Atomic Scale Reaction Pathways

Etching of semiconductors by halogens is of vital importance in device manufacture. A greater understanding of the relevant processes at the atomistic level can help determine optimal conditions for etching to be carried out. Supersaturation etching is a seemingly counterintuitive process where the coverage of the etchant molecules on the surface to be etched is >1. Here we use density functional theory computations of reaction pathways and barriers to suggest that supersaturation etching of Si(001) by Br₂ should be more effective than conventional etching by Br₂, as well as both conventional and supersaturation etching by Cl₂. Analysis of our results shows that this is due in part to the larger size of bromine atoms, and partly due to Br–Si bonds being weaker than Cl–Si bonds. We also show that, for both conventional and supersaturation etching, the barrier for the rate-limiting step of desorption of SiX₂ units is lower when the halogen X is Br rather than Cl. This contributes to the overall reaction barrier for supersaturation etching being lower for Br₂ than for Cl<sub>2