Electrochemical anodization of bulk silicon has applications in many micromachining processes. However, its use for silicon photonics is limited because silicon-on-insulator (SOI) wafers cannot be anodized using a conventional process because of the buried oxide. We overcome this using an alternating potential to induce an ac across an SOI wafer, treating it as a capacitative structure. The resultant surface roughness is comparable to that obtained using conventional anodization, and uniform etching across a 6 mm exposed surface is obtained with a minimum patterned linewidth of 2.5 mu m in the device layer. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3431038] All rights reserved.