The fabrication of macropores in crystalline silicon by photoelectrochemical etching in a hydrofluoric acid electrolyte is investigated. It is shown that the dimensional constraints on the pore diameters, which, in previous literature, are considered to depend on substrate doping, can be significantly relaxed. We show that it is possible to fabricate arrays of square section macropores with sides ranging from 2 to 15 m using the same n-doped ͑2.4-4 ⍀ cm͒ silicon substrate. Moreover, we demonstrate that macropore arrays with pitch variation up to 100% ͑8 and 16 m͒ can be simultaneously grown on the same silicon sample. The same process is used to fabricate arrays of silicon walls with different spacing and pitch as well. A simple model, based on the coalescence in a single pore of multiple stable pores, is proposed to explain the experimental data.Electrochemical etching of silicon in hydrofluoric acid ͑HF͒ electrolyte is a well known technique for the formation of porous silicon. 1,2 The dissolution of silicon in the electrolyte is activated by holes. In p-silicon electrodes holes are the majority carriers, so that it is possible to produce porous silicon simply by etching the sample in an HF-based solution, even in the dark. For n-type silicon, the majority carriers are electrons, so that it is necessary to generate holes in order to produce porous silicon. Depending on the silicon doping and on the type of the anodized silicon substrate, different pore morphologies can be obtained. In fact, microporous layers characterized by random pores with nanometric dimensions can be easily grown on p-type substrates, 3 while macroporous layers with micrometric straight pores can be obtained from n-type substrates. 4 In the last case, by illuminating the rear surface of the wafer with sufficiently energetic photons, holes can be generated in the substrate by a process of photon absorption. Under anodic biasing conditions, the photogenerated holes move to the front surface and silicon dissolution takes place. Initially, the electric field concentrates at sharp defects on the flat wafer surface. Surface defects therefore act as seeding points for macropore formation. As the etch progresses, the electric field still concentrates at the pore tips, where most of the injected holes react with the electrolyte. Fewer holes are then available for the dissolution of the sidewalls, that are therefore protected against dissolution. 5 By prepatterning the wafer surface with defect sites it is possible to determine where macropores will form. KOH etching after a standard photolithographic step can be used to create pyramidal notches in the required positions which can act as an array of defects. Both random 5,6 and prepatterned 7 macropore arrays with high aspect ratio ͑up to 250͒ were fabricated through the wafer thickness 8 and on the whole wafer. 9-11 The macropore morphology significantly depends on the anodization conditions, such as current density, etching time, HF concentration, temperature, and bias, as well as on substrate pro...
