Today we are introducing the nanofabrication method adopted at INRiM to nanofabricate large arrays of Silicon Nanowires: nanospheres self-assembly and Metal Assisted Chemical Etching.
The approach is using the supramolecular self-assembly to obtain a nanosphere nanolitography on large area, coupled with Metal Assisted Chemical Etching in hydrofluoric acid and hydrogen peroxide.
STEP 1 - Deposition of nanospheres.
On a clean and functionalised silicon substrate, with piraña solution (H2SO4:H2O2 3:1) or oxygen plasma, in order to render hydrophilic the silicon surface, the colloidal suspension is spun, forming a monolayer of polystyrene nanospheres arranged in hexagonal close packed manner.
STEP 2 - Reduction and control of nanospheres.
After self-assembly by spinning, the original sphere diameter is reduced in oxygen plasma. The initial sphere diameter will determine the periodicity of the array, the final sphere diameter will determine the nanowires diameter.
STEP 3 - Metal deposition.
Once the nanospheres has been reduced, the deposition of a thin gold film (20 nm) by e-beam evaporation is performed, covering all the surface.
STEP 4 - Nanospheres lift-off.
After the evaporation, the colloidal mask of nanospheres is lifted off by soaking the sample in an ultrasonic bath of ethanol (EtOH).
The final result obtained is formed by a pattern on gold consisting of a two-dimensional array of holes in the original position of the spheres
STEP 5 - Metal Assisted Chemical Etching
After an holey gold mask consisting of a two-dimensional array of holes in the original position of the spheres, the metal mask is used for the MACE silicon etching process to extrude a matrix of ordered silicon nanowires:
The gold holey mask sinks into the silicon, extruding the nanowires.
For this process, the solution used for the silicon etching is a mixture of hydrofluoric acid, hydrogen peroxide and ethanol.
The sample is mounted inside a Teflon cell in which the silicon wafer is exposed. A controlled amount of etching solution is poured onto the sample. To stop the process, the sample is finally rinsed thoroughly with EtOH. In the figure above, the sample mounting sequence is shown.
Final sample mounting sequence. The back-side metal electrode is used only for electrochemical etching.
Top view of the Teflon cell with the HF exposed area of the sample.