Metal assisted chemical etching

The semiconductor, covered with the metal, is then immersed in an etching solution containing an oxidizing agent and hydrofluoric acid.

The first attempts of MACE consisted of a silicon wafer that was partially covered with aluminum and then immersed in an etching solution.

[citation needed] Further research showed that a thin film of a noble metal deposited on a silicon wafer's surface can also locally increase the etching rate.

In particular, it was observed that noble metal particles sink down into the material when the sample is immersed in an etching solution containing an oxidizing agent and hydrofluoric acid (see image in the introduction).

[1] There is agreement that the reduction of the oxidizing agent is catalyzed by the noble metal particle (see figure to the left).

This means that the metal particle has a surplus of positive charge which is eventually transferred to the silicon substrate.

The weakened bonds can be attacked by a nucleophilic species such as HF or H2O, which in turn leads to the dissolution of the silicon substrate in close proximity to the noble metal particle.

In particular it is proposed, that some metal ions from the particle are dissolved and eventually are re-deposited at the silicon surface with a redox reaction.

Therefore, it is proposed that the formation of such a porous region beneath the straight pores depends on the type of barrier that is formed at the metal/silicon interface.

In the case of downward band-bending holes could escape into the bulk of the silicon substrate and eventually lead to etching there.

[21] Finally, after the deposition of the metal on the surface of silicon, the sample is immersed in an etching solution containing hydrofluoric acid and oxidizing agent.

MACE, in principle allows the fabrication of steep trenches but is still cheap compared to gas-phase etching methods.

Cross sectional view of a silicon wafer after metal assisted chemical etching (MACE). The dissolution rate of silicon is locally enhanced due to silver nanoparticles initially deposited at the surface. Eventually the silver nanoparticles "sink" into the wafer leaving behind columnar pores.
Depiction of the mechanism of MACE: The reduction of an oxidizing agent (in this case H 2 O 2 ) is catalyzed at the surface of a noble metal nanoparticle (in this case gold). Because of this, holes (h + ) are injected into the valence band of the silicon substrate. These holes weaken chemical bonds at the silicon/etching solution interface and thus dissolution of the substrate with HF takes place.
Silver nanoparticles at the surface of a silicon wafer after immersion into a solution containing silver nitrate and hydrofluoric acid.
Schematic depiction of a silicon substrate that is covered with a protective layer (red). 1: Wet chemical etching leads to isotropic etching 2: Etching methods that operate in the gas-phase allow anisotropic etching but are expensive.