Advanced Optical Filters of Porous Silicon

Porous silicon is fabricated by electrochemical etching of monocrystalline silicon in hydrofluoric acid (HF). A multilayer dielectric interference filter of porous silicon is based on the effect that the index of refraction depends on the degree of porosity, which can be chosen individually for every layer by the fabrication process. The porosity, i.e. the ratio of void volume to total volume in the layer, depends on the current density during the layer formation by electrochemical etching. To fabricate an optical filter the etch current is varied over time according to the desired refractive index profile of the layer stack. With this technology, optical filters of porous silicon can be produced within minutes in a single process step that generates a modulated density (Fig. 6.1).