The process begins with the deposition of a metal catalyst, typically gold or silver, onto the substrate. The substrate is then exposed to a chemical etchant, often a solution containing hydrofluoric acid (HF) and an oxidizing agent. The metal catalyst facilitates the local oxidation of the substrate, making it more susceptible to the etchant. As the etching progresses, the metal sinks into the substrate, creating a vertical trench or pore. The depth and shape of the nanostructures can be controlled by varying the etching time, the concentration of the etchant, and the type of metal catalyst used.
