High aspect ratio silicon pillars fabricated by electrochemical etching and oxidation of macroporous silicon

A technique for fabricating sub-micron free-standing silicon pillars has been developed. The silicon pillars have a high packing density, and aspect ratios over 50:1 can easily be achieved. Silicon wafers of 3-5 Omega cm resistivity is patterned with regular windows on a silicon nitride layer by conventional photolithography. Inverted pyramids are formed within the windows after orientation-dependent KOH etching. During photo-assisted electrochemical etching in 2.5% ethanoic hydrofluoric acid, the photo-generated holes are collected at the sharp tips of the inverted pyramids where the directional etching of macropores is localized. The bulk silicon left between the close-packed macropores is oxidised. Free-standing pillars are then formed by subsequent wet etching of the silicon dioxide.