The impact of light illumination on silicon nanostructure such as oxidation and microroughness induced in wet processes was investigated. It was revealed that light, such as fluorescent light used prevalently in the clean room, surprisingly affects the oxidation of silicon (photoinduced oxidation) and causes roughness (photoinduced roughness). In the absence of dissolved oxygen and light, the silicon surface was not oxidized. Large numbers of nanosize hillocks were observed on the silicon surface treated with alkaline solution or ultrapure water with light illumination. The results for p- and n-type were almost the same. This observation suggests that the mechanism is simply a chemical reaction between surface silicon and solution. The origin of the roughness was considered to be the large numbers of nano- or sub-nanosize oxides induced by the light illumination. Roughness could be improved by light shading, but we have found that very small hillocks were induced even in a dark ambient environment. In this case, dissolved oxygen in the solution has affected the roughness. Finally, the silicon surface is found to be barely roughened, even in such alkaline solutions as ammonia solutions, when processed in an environment without oxygen and light. (c) 2006 The Electrochemical Society.