Metal-Assisted Chemical Etching

Metal assisted chemical etching is a promissory technique for the fabrication of large scale cheap nanostructures. In a few words, a silicon substrate is covered by a patterned thin metal layer, and when submerged in a solution of alcohol, hydrogen peroxide and hydrofluoric acid, the metal layer etches the silicon leaving imprinted the pattern. Strangely, the thin layer of metal does not detach from the silicon, being able to etch several microns and keeping the pattern during the etching. This phenomenon is used to create long silicon nanowires. Another interesting feature is that the etching has some preferential directions, that seem to align with some crystallographic axes. Upon rinsing and alcohol and submerging the sample in the solution again, the so called quenching, the etching direction switches to another preferential direction, so zig-zag like nano wires can be obtained.
In the recent paper Nano Lett. 2019, 19, 11, 7681-7690 I contribute with numerical and analytical calculations to determine the etching rate of gold in silicon with good agreement with experimental data. Also, with a phenomenological model explaining the kinking by the preferential accumulation of alcohol in the acute corner of the nanowire near the etching area during the quenching, calculating the shape of nanodrops by the phase field method, all with the COMSOL package. The attraction of the metal layer and substrate during the etching is a more difficult topic. Several forces could be responsible, but I believe that is simple capillary force due to the presence of gas bubbles.