Cryogenic etching of positively tapered silicon pillars with controllable profiles

Fabrication of high aspect ratio silicon nanopillars is challenging for various applications. A cryogenic silicon etching process using SF6 and O2 plasma is investigated to create silicon nanopillars with 10 μm height and tens of nanometers apex. In the process, fluorine radicals react with silicon atoms, releasing volatile SiFx byproducts and then oxygen atoms interact with SiFx and deposit a SiOxFy film acting as an inhibitor. By adjusting the O2 concentration and the forward radio frequency power, this process modifies the formation of the SiOxFy passivation film and adjusts the bombardment of ions onto the inhibitor, resulting in the desired positive taper angles of silicon pillars. Two etching steps, with higher and lower O2 concentrations, are consecutively combined to create a sharp apex and a wide base. The results demonstrate the high etching rate and controllability of cryogenic etching to obtain high aspect ratio silicon pillars with desired profiles.