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Metal-Assisted Chemical Etching of Silicon

Structured silicon is an increasingly important part of our modern lives – from the array of sensors in our pockets to the solar panels on our roofs, we don’t have to look far to find it. But is cost-effective miniaturisation really as inevitable as recent history might lead us to believe? The answer is, of course, completely dependent on our ability to come up with new ways to structure silicon – controllably and efficiently. One promising addition to our structuring toolkit is Metal-Assisted Chemical Etching, or “MACE.”

As a wet-chemical technique, MACE lends itself to low-cost batch processing – a clear advantage in the age of mass production. What makes it unique is that MACE combines these benefits of wet chemistry with directional and high aspect ratio etching, but without the limitations or standard anisotropic dry etching processes. Nanowires, trenches, porous networks, and black silicon are just some of the scientifically and technologically interesting structures that we can form as a result.


To see how this concept works, let’s look at the diagram below. Four main components are always present in MACE – a substrate (Si), a metal (Au, Ag, Cu, etc.), an oxidant (e.g. H2O2), and an etchant (HF). Simply stated, the metal catalyses the oxidation of silicon, leaving it open to attack by the etchant. That is the origin of MACE’s usefulness – etching gets localised to the area close to the metal.
 

 

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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.

ALD-Fill-Etch-Imaging

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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