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Usually, etching of n‐type silicon in the dark produces “break down” mesopores. In this letter, by etching with aqueous and strong oxidizing electrolytes respectively, heavily branched macropores with diameters up to 4 μm and depths up to 200 μm were produced without illumination on low doped n‐type silicon. Isolated silicon islands were obtained due to the interconnection of the main pores. The heavily branched macropores on the micron scale could be filled with smooth macropores with diameters up to 500 nm. Due to a small primary length scale no more than 250 nm in their rather short nucleation phases, the main pores should be secondary macropores. Based on these findings, a single trench with long branches, which might facilitate potential applications, was fabricated with prestructured linear nuclei. Current‐burst‐model combined with breakdown mechanism was finally found to be responsible for the formation of heavily‐branched macropores. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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