Helical as well as cylindrical holes with diameters ranging from 20 to 200 nm were bored in silicon by wet chemical etching in a solution containing HF and H2O2 using Pt nanoparticles deposited on the silicon surface before the etching process as a catalyst. In a solution containing HF at a low concentration, cylindrical holes grew preferentially in the < 100 > direction at the centers of cone-shaped concavities filled with microporous silicon. On the other hand, in a solution containing HF at a high concentration, helical holes grew in various directions without forming a thick microporous silicon layer. (c) 2005 The Electrochemical Society.