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.