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

Abstract

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 Br-2 should be more effective than conventional etching by Br-2, as well as both conventional and supersaturation etching by Cl-2. 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 Br-2 than for Cl-2.