We demonstrate that extremely high aspect ratio patterns can be created at a submicrometre scale using focused-ion-beam etching in a porous material. Thanks to a preliminary microscopic structuration of the etching material, the limiting effects usually observed in bulk material ion-beam etching are strongly reduced. Holes with an overall aspect ratio of 50 are obtained. In combination with the photo-electrochemical etching, this method is successfully used to fabricate three-dimensional silicon photonic crystals up to five periods thick at a submicrometre scale.
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Wang et al. (2000) studied this question.
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