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A- macroporous silicon
The macroporous etching technology originally developed by Lehmann et al., is naturally adapted to the fabrication of regular lattices of air holes in silicon. The etching process is as follows. First, a silica mask of the photonic lattice pattern is obtained on a low doped (100) N type silicon wafer by lithography. We use different lithography methods depending on the required lattice period. A hot KOH etch gives a lattice of pyramids pointing to the rear side of the wafer. Each pyramid created at the surface of the wafer is the starting point for a macro pore. The wafer is then used as the anode in an electrochemical cell. The electrolyte used is 2-5% hydrofluoric acid. A back side illumination oh the wafer enables to control the dissolution current and therefore the cylinder diameters. With this etching technique pores with a variable radius, more than 100 um deep following the vertical (100) silicon crystal direction are obtained on 1 cm of silicon. A thermal oxidation followed by HF dip in order to remove the oxidized silicon layer is sometimes used to enlarge the pore radius.
The photonic crystals are then obtained by cutting parallelepiped slices from the bulk macroporous with a rotative diamond saw. The slices are cut to obtain a photonic crystal interface parallel to the cylinders axes and perpendicular to one or the other of the photonic crystal directions. When only reflection characterizations are performed the interfaces can be obtained simply by cleaving the bulk macroporous silicon (as long an the photonic crystal planes are parallel to the silicon crystal planes)
