Silicon Etching Using Copper–Metal-Assisted Chemical Etching: Unveiling the Role of Cu₂O in Microscale Structure Fabrication

Achieving precise and cost-effective etching in the field of silicon three-dimensional (3D) structure fabrication remains a significant challenge. Here, we present the successful fabrication of microscale anisotropic Si structures with an etching anisotropy of 0.73 using Cu-metal-assisted chemical etching (Cu-MACE) and propose a mechanism to elucidate the chemical behavior of Cu within the MACE solution. Our study reveals the formation of cuprous oxide (Cu₂O) within Cu thin films in the presence of hydrogen peroxide (H₂O₂), which plays a key role in Si etching. We propose that the holes generated through the reduction of Cu₂O back to Cu are transferred to Si, promoting its etching through a galvanic reaction with Cu₂O. This Cu-Cu₂O cyclic redox process in the Si-Cu₂O galvanic cell under the right conditions enables continuous etching of Si and significantly improves the chemical stability of Cu-MACE. Building on this cyclic process mechanism, we demonstrate the catalytic potential of Cu₂O for oxide-assisted chemical etching (OACE) by directly using Cu₂O in both thin-film and particle forms, rather than starting from Cu. This study opens possibilities for the precise control of Cu-MACE, extends the existing MACE mechanism, and contributes to our understanding of transition metal oxide behavior in OACE.