Laser assisted and wet chemical etching of silicon nanostructures

Etching of silicon can be accomplished in a number of ways: in aqueous solutions of fluoride or KOH, or by simultaneous exposure to a gas such as SF6 or HCl and high power laser irradiation. Here we review how laser assisted etching, chemically enhanced laser ablation, and stain etching can be used to create pillars, pores, porosified pillars, and macropores. Pillar dimensions can range from 10μm tall with ∼200nm tips to >100μm high with 10μm tips. Ordered arrays can be produced and sharpening can lead to aspect ratio approaching 104. Porous silicon with a visibly photoluminescent nanocrystalline structure can be made by etching in fluoride solutions. Acidic fluoride+oxidant solutions (stain etchants) allow us to form porous silicon on substrates of arbitrary topology. Moreover, these etchants can be used to control the photoluminescence spectrum of the resulting film, presumably through control of the size distribution of the nanocrystalline structures that are responsible for photoluminescence.