Vertical silicon nanostructures via metal-assisted chemical etching

Metal-assisted chemical etching (MACE) is a top-down fabrication method based on wet chemical etching and has been successfully used for preparing a variety of vertical silicon nanostructures. MACE allows for not only facile control over the geometric factors of vertical silicon nanostructures, but also large-batch fabrication in a cost-effective manner. Therefore, vertical silicon nanostructures fabricated via MACE have been proposed for various applications, such as photovoltaics, sensors, and bio-interfaces. Unlike typical wet etching processes, MACE uses a mixture of an acid (e.g., HF) and an oxidizing agent (e.g., H₂O₂) in the presence of a thin layer of metal catalyst (e.g., Au, Ag, and Pt) pre-patterned on the target silicon substrate. During the vertical etching process, the oxidizing agent is reduced by electrons transferred from silicon through the metal catalyst layer. Thus, the silicon surface just beneath the patterned metal layer is oxidized to silicon dioxide, which is subsequently dissolved by hydrofluoric acid (HF) contained in the etchant mixture. Accordingly, the geometric parameters of the resulting vertical silicon nanostructures can be delicately controlled by varying metal catalyst type, metal patterns, etchant composition, and etching reaction conditions used in MACE. As many researchers across various fields could benefit from versatile fabrication and application of vertical silicon nanostructures, a number of remarkable studies have recently been published regarding the fundamental MACE mechanism, the fine control over silicon nanostructure properties (e.g., geometric parameters), and various applications demonstrated with these nanostructures. Therefore, we suppose that a comprehensive review of vertical silicon nanostructures fabricated via MACE should be very beneficial not only for scientists researching on the nanoscale silicon wet-etching, but also for engineers interested in practical applications of geometry-controlled vertical silicon nanostructures for a variety of purposes.