Thick etch-through macroporous Si membrane from p- and n-Si, and fast pore etching and tuning the pore size from n-Si

A thick macroporous silicon (macro PS) membrane has been fabricated by electrolytic etching through either n- or p-type silicon wafers. The microscopic structure of the membrane has been investigated by scanning electron microscopy. Three different electrolytes were investigated: (i) an aqueous solution of hydro fluoric acid (HF) with ethanol (EtOH); (ii) an organic solution of HF with dimethyl sulfoxide (DMSO); (iii) an organic solution of HF with dimethyl formamide (DMF). The concentration and the applied current in the electrolytic cell during etching have been varied and been optimized with respect to etching rate and suitability of the resulting structure for different applications. For the p-type case etched with the organic electrolytes, smooth and straight pores were realized but only when the conditions are near optimized. Outside that parameter space, one can get too thin pore walls on one side of the wafer, for example by increasing the HF concentration, etching for excessive times or omitting to refresh the HF during etching. For the n-type case, a fast pore etching up to 10 μm/min was achieved for the first time for high HF concentration and a high current density. The pore size has been tuned over a large range by varying the Si doping concentration; the lower the doping, the larger the pore size and the faster the etch rate. The macropores in the n-type wafers were highly branched and the morphology could be varied for the electrolytes with different oxidizing agents. Suitability of the different membrane structures for specific applications is briefly discussed.