Fabrication of Three-Dimensional Photonic Band Gap Material by Deep X-Ray Lithography

Yablonovitch et al. [1] constructed the “three cylinder” dielectric structure which exhibits a photonic band gap in the whole Brillouin zone. They implemented the diamond structure proposed by Ho et al. [2,3]. This structure can be fabricated mechanically by drilling three sets of holes 35° off vertical into the top of a solid dielectric. However, extremely small dimensions, which are required for high frequencies, cannot be achieved. This problem can be solved by using deep X-ray lithography, which allows the fabrication of structures that can be used up to the infrared range. Three irradiations were performed in which the tilted arrangement of mask and resist was rotated each time by 120°. PMMA resist layers with a thickness of 500 microns were irradiated at the DCI storage ring in Orsay, France. The lattice constants of the structures were 227 and 114 microns corresponding to midgap frequencies of 0.75 and 1.5 THz, respectively.Since the dielectric constant of the PMMA is not high enough for the formation of a photonic band gap, a moulding step must be applied. The holes in the resist structure were filled with a solution of polyvinylsilazane in tetrahydrofuran. After the evaporation of the solvent, the samples were pyrolyzed at 1100°C under N₂ atmosphere. The resist decomposes into CO₂, CH₄, CO, and H₂O, whereas polyvinylsilazane is transformed into a SiCN ceramic. A lattice of ceramic rods corresponding to the holes in the resist structure remained.