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Metal-Assisted Chemical Etching of Silicon

Improving our control over the structure of materials at small scales has been an essential part of solving many technological and scientific questions, and this is particularly true in recent decades in the case of silicon. One promising recent addition to our nano-toolkit in this respect is a wet-etching process, known amongst other names as metal-assisted chemical etching (MACE). Nanowires, trenches and porous networks are just some examples of the kinds of complex structures that can be formed using MACE of silicon.

As the name implies, metal particles are an integral component of the etching process in MACE. Simply put, the silicon is only removed in a highly localised region around the metal particles, and this is what leads to the variety of resulting structures characteristic to MACE. Furthermore, it is this possibility for highly directional, or anisotropic, etching of silicon that sets MACE apart from other comparably low-cost wet chemical techniques.

Schematic of MACE
A schematic of MACE, showing its localised nature and how the metal moves through the substrate (e.g., silicon).


If we are to use MACE in a productive manner, for example, to form well-defined porous silicon nanowires or pore networks, it is essential to control the most important etching parameters, as well as to characterise the resulting structures in a high degree of detail. Towards this two-part goal, we are developing new ways to visualise previously hidden structures of the MACE process.


Schematic of AFEI
Representation of a silicon pore network formed using MACE, as revealed by the AFEI procedure (above), and an illustration of the scheme (below)

The ALD-Fill-Etch-Imaging (AFEI), is being developed by us to uncover buried 3D pore networks that are formed under certain MACE conditions in silicon. Atomic Layer Deposition (ALD) is first used to fill the pores with zinc oxide, and this is followed by a selective removal of the surrounding silicon in a plasma etcher. What is revealed is a complete inverted network, with high fidelity to the shape of the original, that can subsequently be studied using commonly-available Scanning Electron Microscopy (SEM).


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