Mechanistic insights into the acidic wet-chemical silicon etching in HF-HX-X₂ solutions, X = Cl, Br, I

Alkaline, wet-chemical silicon etching is an essential part in the manufacturing of silicon solar cells, because the anisotropy of the etching process enables texturization of the wafer surface. Anisotropic etching in alkaline systems is traditionally attributed to the nucleophilic attack of OH^- on Si-H, forming a pentacoordinated Si-atom as a transition state. This leads to (111) crystal planes being etched much slower, thus generating pyramidal structures on (100) wafer surfaces. In aqueous HF solutions with halogens as oxidants, similar surface structures are observed, even though the reactants are completely different from the alkaline solutions. The data show that in HF-HCl-Cl₂ solutions, the presence of Cl₃ ^- plays a decisive role. Trihalide ions and hydroxide ions are very similar: both are negatively charged, nucleophilic and relatively small. This suggests a similar reaction behavior. In this article, it is shown that trihalides in acidic wet-chemical silicon etching systems represent a functional analogy to hydroxide ions in alkaline systems and are responsible for anisotropic etching. The findings are supported by spectroscopic and surface analytical data.