Holes with diameters of tens of nanometers were bored in Si(100) in aqueous solutions containing hydrofluoric acid and hydrogen peroxide utilizing silver nanoparticles as catalysts. The holes grew deeply in the [100] direction when the concentration of hydrogen peroxide was about 0. 18 M. In addition to these vertical holes, holes were generated horizontal to the surface in the (100) directions near the surface of the sample. We found that the silver particles making the deep holes in the [100] direction were more spherical than those making the horizontal holes near the surface. These results indicate that the shape of the silver particles is an important factor controlling the direction of the holes. When the hydrogen peroxide concentration was increased to 1.8 M, as well as the vertical and horizontal holes, a microporous layer was formed on the top surface region. On the other hand, when the hydrogen peroxide concentration was lowered to 0.00 18 M, holes did not show any preference for growing direction and were crooked. Even in HF solutions that did not contain hydrogen peroxide, holes were generated at a very slow rate in random directions if the solutions contained oxygen. (C) 2007 Elsevier Ltd. All rights reserved.