Inverse-Woodpile Photonic Band Gap Crystals with a Cubic Diamond-like Structure Made from Single-Crystalline Silicon

Abstract

Three dimensional photonic band gap crystals with a cubic diamond-like symmetry are fabricated. These so-called inverse-woodpile nanostructures consist of two perpendicular sets of pores in single-crystal silicon wafers and are made by means of complementary metal oxide–semiconductor (CMOS)-compatible methods. Both sets of pores have high aspect ratios and are made by deep reactive-ion etching. The mask for the first set of pores is defined in chromium by means of deep UV scan-and-step technology. The mask for the second set of pores is patterned using an ion beam and carefully placed at an angle of 90° with an alignment precision of better than 30 nm. Crystals are made with pore radii between 135–186 nm with lattice parameters a = 686 and c = 488 nm such that a/c = √2; hence the structure is cubic. The crystals are characterized using scanning electron microscopy and X-ray diffraction. By milling away slices of crystal, the pores are analyzed in detail in both directions regarding depth, radius, tapering, shape, and alignment. Using optical reflectivity it is demonstrated that the crystals have broad reflectivity peaks in the near-infrared frequency range, which includes the telecommunication range. The strong reflectivity confirms the high quality of the photonic crystals. Furthermore the width of the reflectivity peaks agrees well with gaps in calculated photonic band structures.