Fabrication of silicon nanostructures with large taper angle by reactive ion etching

Micro- and nanostructures with a tapered sidewall profile are important for antireflection and light trapping applications in solar cell, light emitting diode, and photodetector/imager. Here, the authors will show two etching processes that offer a large taper angle. The first process involved a mask-less etching of pre-etched silicon structures having a vertical profile, using a recipe that would give a vertical profile when masked. The authors obtained a moderate taper angle of 14° using CF4/O2 etching gas. The second process involved a one-step etching step with Cr as mask using a recipe that was drastically modified from a nonswitching pseudo-Bosch process that gives a vertical profile. The gas flow ratio of C4F8/SF6 was greatly increased from 38/22 to 59/1 to result in a taper angle of 22°. Further reduction of the RF bias power led to an unprecedented large taper angle of 39° (at the cost of greatly reduced etching rate), which is even higher than the angle obtained by anisotropic wet etching of silicon.