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 maskless 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 degrees using CF4/O-2 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 degrees. Further reduction of the RF bias power led to an unprecedented large taper angle of 39 degrees (at the cost of greatly reduced etching rate), which is even higher than the angle obtained by anisotropic wet etching of silicon. (C) 2014 American Vacuum Society.