Controlling the Wet-Etch Directionality in Nanostructured Silicon

Anisotropic wet etching of crystalline silicon (c-Si) is a key chemical process used in microelectronic device fabrication. Controlled fabrication of c-Si nanostructures requires an understanding of how crystal planes evolve during silicon etching. Here, by imaging KOH wet etching of c-Si nanowires, we show that it is possible to switch the fast-etching direction (i.e., the etch anisotropy) between the Si {100} and {110} crystal planes at will through mechanical agitation of the etchant. Based on molecular dynamics simulations, we attribute this switching to the higher affinity of the Si(OH)4 etch byproducts to the Si {110} planes. These surface-bound byproducts hinder etchant access to the {110} surfaces under stationary etching conditions and thus reduce the etch rate in ⟨110⟩ directions. Most importantly, by cycling through stirred and stationary modes of etching, we can obtain isotropic etch profiles, fabricating high-quality, round Si nanowires with sub-10 nm diameters. Our study provides an important insight into the nanoscale wet etching of Si and demonstrates a new level of control for enabling highly scalable, advanced nanoelectronic devices.