In order to fabricate deep silicon (Si) microstructures with possibly vertical sidewalls by wet etching, macroporous Si was formed in a Si substrate by anodization using a hydrofluoric-acid (HF)-based solution with backside illumination. The shape of microstructures such as fixed–fixed beams and cantilevers were defined by boron (B) diffusion, and the B-diffused microstructures were left by a p++ etch stop after etching the macroporous Si in tetramethyl ammonium hydroxide. The B-diffused microstructures were fabricated, but the sidewalls were not as vertical as expected, but were inclined at 40°–60° to the substrate surface, due to undercutting during anodization. The reason for the undercutting is discussed. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.