Figure 2.

Etching mechanism and effects of the ICP-RIE processing parameters on the nanowire height and sidewall angle. (a) Scanning electron microscopy (SEM) image illustrating chemical species involved in the cryogenic dry etching process. Oxygen (O^*) and fluorine (F^*) form a passivation layer (SiO_xF_y) on the nanowire sidewall as well as at the bottom part of Si at cryogenic temperatures. The physical bombardment of SF_x ions triggered by the applied bias results in the removal of the bottom passivation layer. F radicals can react chemically with Si atoms at the bottom part forming volatile SiF_x species, which are detached from the crystal, resulting in anisotropic etching. In this case, the mask at the top of the nanowire (in red color) acts as a shielding layer to prevent both physical and chemical etching processes. Various parameters, namely (b) temperature, (c) oxygen content, (d) pressure, (e) radio-frequency (RF) power, and (f) ICP power affect the height and sidewall angle of Si nanowires.