Copper Catalyzed Anisotropic Dissolution of Silicon in HF–HCl Solutions: Metal‐Assisted Chemical Etching Without Additional Oxidant

In this work, we present a novel approach to metal‐assisted chemical etching (MACE) of silicon using copper(II) ions in mixtures of hydrofluoric acid (HF) and hydrochloric acid (HCl) without the addition of an oxidizing agent. While the standard redox potential of Cu 2+ is typically considered too low for silicon oxidation in HF, we observed anisotropic etching, yielding pyramidal and inverted pyramidal surface structures, with etching rates up to 14.18 µm h −1 . The presence of HCl is crucial as it prevents the significant copper film deposition seen in HCl‐free solutions. Our analysis, including cyclic voltammetry and XPS, reveals that the Cu 2+ ions act as a catalyst for silicon oxidation by dissolved oxygen, O 2 . The HCl stabilizes the reduced species, Cu + , as chloro‐complexes of copper(I), maintaining the copper in solution and enabling a divalent dissolution mechanism. This work demonstrates a unique MACE regime where the metal catalyst remains dissolved, providing new insights into the complex mechanisms of silicon dissolution.