Oxidation Rate Effect on the Direction of Metal-Assisted Chemical and Electrochemical Etching of Silicon

Assisted by noble metal particles, non-(100) Si substrates were etched in solutions with different oxidant concentrations at different temperatures. The etching directions of (110) and (111) Si substrates are found to be influenced by the concentration of oxidant in etching solutions. In solutions with low oxidant concentration, the etching proceeds along the crystallographically preferred (100) directions, whereas the etching occurs along the vertical direction relative to the surface of the substrate in solutions with high oxidant concentration. These phenomena are found for both and p-type substrates as well as in experiments with different oxidants. The experiments on metal-assisted chemical etching are complemented by additional experiments on metal-assisted electrochemical etching of (111) Si substrates with different current densities. As a function of current density, a change of etching directions is observed. This shows that the change of the etching directions is mainly driven by the oxidation rate. On the basis of these phenomena, we have demonstrated fabrication of Si nanopores with modulated orientations by periodically etching a (111) substrate in solutions of low and high oxidant concentrations.