Silicon Etching Using\nCopper–Metal-Assisted\nChemical Etching: Unveiling the Role of Cu<sub>2</sub>O in Microscale\nStructure Fabrication

Achieving precise and cost-effective etching in the field\nof silicon\nthree-dimensional (3D) structure fabrication remains a significant\nchallenge. Here, we present the successful fabrication of microscale\nanisotropic Si structures with an etching anisotropy of 0.73 using\nCu-metal-assisted chemical etching (Cu-MACE) and propose a mechanism\nto elucidate the chemical behavior of Cu within the MACE solution.\nOur study reveals the formation of cuprous oxide (Cu<sub>2</sub>O)\nwithin Cu thin films in the presence of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), which plays a key role in Si etching. We propose\nthat the holes generated through the reduction of Cu<sub>2</sub>O\nback to Cu are transferred to Si, promoting its etching through a\ngalvanic reaction with Cu<sub>2</sub>O. This Cu–Cu<sub>2</sub>O cyclic redox process in the Si–Cu<sub>2</sub>O galvanic\ncell under the right conditions enables continuous etching of Si and\nsignificantly improves the chemical stability of Cu-MACE. Building\non this cyclic process mechanism, we demonstrate the catalytic potential\nof Cu<sub>2</sub>O for oxide-assisted chemical etching (OACE) by directly\nusing Cu<sub>2</sub>O in both thin-film and particle forms, rather\nthan starting from Cu. This study opens possibilities for the precise\ncontrol of Cu-MACE, extends the existing MACE mechanism, and contributes\nto our understanding of transition metal oxide behavior in OACE.